WO2005053119A1

WO2005053119A1 - An injection-seeded self-adaptive optical resonant cavity and a method of generating coherent light

Info

Publication number: WO2005053119A1
Application number: PCT/AU2004/001635
Authority: WO
Inventors: Yabai He; Brian John Orr
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-11-25
Filing date: 2004-11-25
Publication date: 2005-06-09
Anticipated expiration: 2006-05-25

Abstract

An apparatus and a method are described for providing a pulsed beam of narrowband coherent light. The apparatus (210) includes an optical gain medium (212) disposed inside an optical cavity (230) formed by a reflector (214) and a second optical medium (216). The cavity (230) is injection seeded by an input seed beam (220) of coherent light that is injected through a side of the second optical medium (216) opposite a side that faces the optical cavity (230), whereby the second optical medium (216) becomes partially reflective to light in the optical cavity (230) that has a wavelength which is about the same as the wavelength of the input seed beam and that overlaps spatially and temporally, in the second optical medium (216), with the input seed beam (220), and whereby the optical cavity (230) becomes resonant in respect of such light in the optical cavity (230).

Description

An injection-seeded self-adaptive optical resonant cavity and a method of generating coherent light

Technical Field The present invention relates to optics and its applications. More particularly, the invention relates to pulsed, tunable coherent light sources such as optical parametric oscillator (OPO) devices or lasers, of which the output wavelengths can be controlled. Pulsed, tunable coherent light sources enable spectroscopic diagnostic sensing of chemical substances in industrial, clinical, or environmental situations. Other examples of the use of such light sources are in remote sensing of the atmosphere (e.g., by lidar), or in basic scientific measurements.

Background of the Invention

Tunable coherent light sources (lasers and their nonlinear-optical counterparts) play a vital role in spectroscopic sensing of chemical processes, in industrial and environmental diagnostics and in basic science. One such form of tunable coherent light source is the pulsed optical parametric oscillator (OPO) [1 - 4].

Early tunable nanosecond-pulsed OPOs were difficult to operate and were damage-prone, largely because intra-cavity losses from gratings and etalons caused the operating threshold to approach the damage threshold of optical materials such as lithium niobate. A variety of nanosecond-pulsed, continuously tunable OPOs, with wavelength-control strategies that are useful for numerous spectroscopic sensing applications [4 - 6], are known to the applicant. Among the most recent advances [5 - 8] is a class of devices capable of generating single-longitudinal-mode (SLM) tunable coherent light in a pulsed OPO system comprising an actively controlled ring cavity with a quasi-phase-matched (QPM) nonlinear-optical material (such as periodically poled lithium niobate, PPLN) as the OPO gain medium, and a continuous-wave (cw), SLM tunable diode laser to injection-seed the OPO. Such a tunable diode-laser-seeded, quasi-phase-matched OPO is pumped by a pulsed Nd: YAG laser which is either a high-performance SLM system [7, 8] or a compact, low-cost multimode system [5, 8]. A novel narrowband tunable pulsed OPO design with wavelength-selective optical feedback has also been developed [9]. OPOs typically involve coherent three-wave nonlinear-optical processes in non- centrosymmetric solid-state media [1 - 6, 10], with a single coherent input wave (also referred to as a 'pump' wave having a frequency ω_P) and two coherent output waves (also referred to as a 'signal' wave having a frequency ω_s and an 'idler' wave having a frequency ω_l5 where ω_s > ω_τ). These three waves obey both energy-conservation and phase-matching conditions: ω_P = ω_s + ω ; and k_P - k_s - k - Δk = 0 (1) wherein k; is a wave vector with the subscript j denoting pump light (P), signal light (S) or idler light (I), respectively. The magnitude of k_j equals n_j cθ_j / c (= 2π n_j / λ_j), where n_j is the refractive index at vacuum wavelength λ:, and c is the speed of light. The phase- mismatch increment Δk must be minimised to optimise OPO conversion efficiency and thereby to control the output signal and idler wavelengths, λ_s and λ . Equation (1) applies to conventional birefringently phase-matched nonlinear-optical media, in which phase matching entails adjusting the angle and/or temperature of a birefringent nonlinear-optical crystal via its ordinary- and extraordinary-ray refractive indices [10], resulting in a birefringently phase-matched OPO. Various high-quality bulk OPO materials (such as lithium niobate, beta barium borate and potassium titanyl phosphate) may be used. However, different cuts of OPO crystal are required for different spectral regions [10]. A more recently implemented alternative, for which a slightly modified version of equation (1) is required, is to use quasi-phase-matched media tailored for specific wavelengths by periodic optical structuring; periodically poled lithium niobate (PPLN) is a prominent example [5 - 8]. Other promising quasi-phase-matched OPO materials include periodically poled potassium titanyl phosphate (PPKTP), and periodically poled rubidium titanyl arsenate (PPRTA). All of these quasi-phase-matched OPO media offer higher nonlinear-optical coefficients, lower operating thresholds, and smaller size than birefringently phase-matched OPO materials. Another recently developed quasi-phase- matched nonlinear-optical material is orientation-patterned gallium arsenide (GaAs), which offers (as yet unrealised) OPO tunability up to about 16 μm in the far-infrared wavelength range with a conveniently short pump wavelength at about 1 μm [6].

Various types of optical cavity design may be used for pulsed OPOs, as illustrated schematically in Figures 1(a) to 1(c). At one extreme of operational simplicity are 'free- running' pulsed OPOs, with no intra-cavity wavelength-selective components. Such pulsed OPOs often employ a simple two-mirror optical cavity (resonant at either λ_s or λ ) and are usually pumped at λ_P by a pulsed monochromatic coherent source (e.g., harmonics of a Nd:YAG laser). Free-running pulsed OPOs, as in Figure 1(a), yield broadband tunable output light and may be based on nonlinear-optical materials [10] that are either birefringently phase-matched or quasi-phase-matched. Optical bandwidths of the pulsed OPO output signal and idler light are typically 5 - 50 cm^"1 [1 - 6]. The output of such a free-running pulsed OPO (with no wavelength-selective mechanism) is therefore broadband, comprising many frequency components determined by the resonance frequencies of the OPO cavity. The output from such a light source is suitable for low-resolution or multiplex spectroscopy, but unsuitable for use in many spectroscopic applications or in most optical communications situations. For many higher-resolution spectroscopic applications, or in many optical communications situations, it is necessary to use additional ways to narrow the optical bandwidth and control the output wavelengths. The conventional approach, at the other extreme of operational complexity, is to employ intra-cavity gratings and/or etalons [3 - 6], as in Figure 1(b). Various continuously tunable nanosecond-pulsed OPO designs of this type have been used to generate SLM signal or idler output light of narrow optical bandwidth for a variety of high-resolution spectroscopic applications. A disadvantage of this approach is that intra-cavity wavelength-selection elements such as gratings and etalons tend to introduce optical losses that cause the threshold for OPO operation to approach the optical damage threshold of the nonlinear-optical medium. Such complications were particularly prominent during the first twenty years of OPO development. They are still problematic now, although they have been diminished by improvements in pump-laser design, in nonlinear-optical materials, in mirror coatings, and in tunable OPO cavity design.

Within the last 15 years, a popular alternative approach to narrowband nanosecond-pulsed OPO tuning has been injection seeding by a dye laser, a single-mode tunable diode laser, or other form of tunable coherent light source [5, 6], as shown in Figure 1(c). Injection seeding has the advantage of eliminating optical losses due to intra-cavity wavelength- selection elements such as gratings and etalons. However, it is limited by the need for suitable tunable lasers or other coherent light sources for injection-seeding. Such injection-seeding sources are often more expensive than intra-cavity gratings and etalons and, moreover, they are generally more limited in their optical tuning range. High-resolution spectroscopic applications require narrow optical bandwidth and high spatial beam quality. Active control of the length of an injection-seeded OPO cavity is generally necessary for stable, continuously tunable SLM operation, as in Figures 1(b) or 1(c). This has been achieved by actively varying the length of the OPO cavity synchronously with the wavelength scan of the seed source, using some form of optoelectronic feedback to stabilise the process. High-resolution spectra were recorded with this type of configuration, by actively tuning output light from an injection-seeded nanosecond-pulsed OPO (either continuously or in fine wavelength steps). However, these devices suffer from the disadvantage of having to actively and synchronously vary the length of the optical cavity.

Optical oscillator devices based on nonlinear-optical phase conjugation, in particular photorefractive materials, have been reviewed [11]. Although the nonlinear-optical phase conjugation and photorefractive effect is well understood [12], there is only limited applications to generating narrowband tunable OPO coherent light.

There have been two reports [13, 14] on the use of photorefractive crystals in an OPO. An intra-cavity photorefractive crystal in a pulsed KTP OPO grating has been used [13] to achieve self-adaptive longitudinal mode selection and consequent spectral narrowing. However, this system has all the disadvantages associated with Figure 1 (b). In addition, it is difficult to tune the optical cavity of that apparatus. The use of a permanent photorefractive grating, written with an ultraviolet light beam, has been described [14] for distributed-feedback operation of a pulsed PPLN OPO; however, this has similar disadvantages to, and is less flexible than, the apparatus described in ref. [13]. Current narrowband tuning strategies for optical parametric oscillators and lasers entail inclusion of wavelength-selective elements, such as gratings, filters or etalons, in the cavity. Injection seeding by an independent source of coherent tunable is a useful alternative approach. However, both of these approaches require intricate active optoelectronic feedback control of the cavity length and component orientations. This limits ease of operation, portability and ruggedness. There is accordingly a need for an optical cavity design that comprises fewer or no adjustable optical tuning elements, that is simpler in construction and that is more rugged in construction and operation than conventional designs. Objects of the Invention

It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages.

It is another object of the present invention to simplify the wavelength control of pulsed coherent light from a laser or nonlinear-optical system (e.g., an optical parametric oscillator).

It is a further object of the present invention to simplify the continuous tunability of pulsed coherent light from a laser or nonlinear-optical system (e.g., an optical parametric oscillator).

Summary of the Invention

According to a first aspect of the invention, there is provided an apparatus for providing a pulsed beam of narrowband coherent light, the apparatus including:

-a first optical medium selected from the group consisting of a laser gain medium and a nonlinear-optical gain medium, the first optical medium being capable of generating coherent output light, in use, from pulsed pump light; -a first reflector located on one side of the first optical medium;

-a second optical medium located on an opposite side of the first optical medium, the first reflector and the second optical medium together defining an optical cavity, the second optical medium in use becoming at least partially reflective to light in the optical cavity; - a coherent light source selected from the group consisting of pulsed, quasi-continuous- wave and continuous-wave, for injecting into the optical cavity, in use, an input seed beam of coherent light, said input seed beam being injected through the second optical medium, from a side of the second optical medium opposite a side that faces the optical cavity, whereby the second optical medium becomes partially reflective to light in the optical cavity that has a wavelength which is about the same as the wavelength of the input seed beam and that overlaps spatially and temporally, in the second optical medium, with the input seed beam, and whereby the optical cavity becomes resonant in respect of such light in the optical cavity; - a pulsed light source for providing, in use, pulsed pump light to pump the first optical medium, whereby the first optical medium is caused to generate coherent output light of which at least a portion has a wavelength that is the same as the wavelength of the input seed beam; - a first coupler for coupling the input seed beam into said second optical medium;

- a second coupler for coupling the pulsed pump light into the first optical medium; and

- a decoupler for decoupling the coherent output light from the apparatus.

According to a second aspect of the invention, there is provided a method of providing a pulsed beam of coherent output light, the method including the steps of:

- locating a first optical medium between a first reflector and a second optical medium, the first optical medium being selected from the group consisting of a laser gain medium and a nonlinear-optical gain medium, and being capable of generating coherent output light, in use, the first reflector and the second optical medium together defining an optical cavity, in use, the second optical medium being at least partially reflective to light in the optical cavity, when in use;

- passing through the second optical medium, into the optical cavity, from a side of the second optical medium opposite a side that faces the optical cavity, an input seed beam of coherent light selected from the group consisting of pulsed, quasi-continuous- wave and continuous- wave, whereby the second optical medium becomes partially reflective to light in the optical cavity that has a wavelength which is about the same as the wavelength of the input seed beam and that overlaps spatially and temporally, inside the second optical medium, with the input seed beam, and whereby the optical cavity becomes resonant in respect of such light in the optical cavity; and - injecting pulsed pump light into the first optical medium to pump the first optical medium, causing the first optical medium to generate coherent output light of which at least a portion has a wavelength that is the same as the wavelength of the input seed beam. The input seed beam of coherent light preferably has a narrowband spectrum of wavelengths, more preferably a single longitudinal mode. The light in the optical cavity may be visible or invisible.

In the event that the apparatus is configured to operate as an OPO, the wavelength of the input seed beam may be the same as the wavelength of either a signal component or an idler component of the coherent output light generated by the first optical medium. In one embodiment of the invention, the optical cavity includes a second reflector arranged such that the first reflector, the second reflector and the second optical medium are located at the corners of a triangle, so that light in the optical cavity is reflected between the first reflector and the second reflector, between the second reflector and the second optical medium and between the second optical medium and the first reflector. In this embodiment of the invention, the first optical medium may be located between the first reflector and the second optical medium, or between the second reflector and the second optical medium, or between the first reflector and the second reflector. Also, in this embodiment of the invention, an uncharacterised sample of interest may, in use, be located between two corners of the triangle other than those between which the first optical medium is located. In addition, a characterised reference sample may, in use, be located between two corners of the triangle, which may be corners other than those between which the first optical medium and the sample are located. In other embodiments of the invention, the optical cavity includes three, four or more reflectors, arranged in such a way as to reflect light from one to the other reflector in ring, bow-tie or zig-zag fashion.

In other embodiments of the invention, the optical cavity includes deflectors and/or reflectors. The deflector(s) may be a prism(s) having at least two surfaces disposed at a first angle relative to each other, so as to cause the prism to deflect a beam of optical radiation through a second angle. Where deflectors are used, an optical cavity may be configured that has a simple mechanical construction and that may improve the mecahnical stability of the apparatus. Also, a deflector may provide a means for decoupling desired radiation out of the optical cavity and for coupling input radiation into the optical cavity.

In this specification, the expression "laser gain medium" is to be understood as referring to an optical medium which is capable of generating an optical gain, by a laser process, at the wavelength of an input seed beam, whereas the expression "nonlinear-optical gain medium" is to be understood as referring to an optical medium which causes optical gain of an input seed beam by a nonlinear-optical interaction such as the interaction taking place in an optical parametric oscillator.

The second optical medium may be dominantly transparent when not in use and dominantly reflective towards the optical cavity when in use. In this context, dominantly reflective may mean more than about 50% reflective, preferably more than about 60 % reflective, still more preferably more than about 70% reflective towards the optical cavity when in use.

The absorptivity of the second optical medium is prefeably as low as possible so as to minimise losses. The second optical medium may have an internal structure of axes. Its axis orientation in respect to the input seed beam and the optical path of light in the optical cavity may be such as to enable it to perform the desired self-adaptive reflection. The geometry and orientation of the second optical medium may conveniently be such as to avoid reflections, from one or more of the surfaces thereof, along the optical path of the input seed beam, or along the optical path of the light in the optical cavity. The angle of the surface may thus be cut so as to face the incident light beam (being the input seed beam and/or the light in the cavity) at a non-zero incidence angle varing from about 1° to about 60°. The surfaces of the second optical medium may be anti-reflection coated to reduce reflection losses.

The resonance between the light passing through the optical cavity and the injection-seed beam may be self-adaptive and may be independent of the wavelength within the response wavelength range of the medium used for the second optical medium. The intensities of the input seed beam and the pump pulse are important for the efficient operation of the apparatus in accordance with the invention.

The seed beam may be either a continuous-wave beam or a pulsed coherent beam, preferably of narrow optical bandwidth. It may be very weak, for example of a few mW average intensity in the event that a photorefractive-type second optical medium is used. In other cases, a pulsed seed beam of a high peak power of several hundreds of kW may be required, for example, where it is necessary to perform nonlinear-optical phase conjugating interactions in the second optical medium (e.g. four-wave mixing, stimulated Brillouin scattering).

The interaction, in the second optical medium, between the input seed beam and the intra- cavity light may be such that the optical cavity automatically stays at a maximum energy- density state. A nonlinear-optical phase conjugating interaction in the second optical medium (e.g. four-wave mixing or stimulated Brillouin scattering) is an example of a type of interaction which could achieve a constructive interference of the input seed beam in the cavity. The second optical medium may thus become a reflector or mirror which may be based on a photorefractive effect, and which may be controlled self-adaptively by continuous- wave (cw) or pulsed light from the injection seeder. The injection seeder may be tunable. The second optical medium or self-adaptive reflector or mirror may be made of a suitable photorefractive material which may be or which may comprise Rh-doped BaTiO

(rhodium-doped barium titanate).

The photorefractive effect is a phenomenon whereby the local index of refraction is modified by spatial variations of the light intensity. It is strongly observed when coherent beams interfere with each other in a photorefractive material, which forms a spatially varying pattern of illumination.

The basic mechanism of the effect is the excitation and redistribution of charge carriers inside a crystal upon non-uniform illumination. The redistributed charges give rise to a non-uniform internal electric field and thus spatial variations in the refractive index of the crystal through the electro-optic effect. Significant nonlinearity can be induced by relatively weak (~ mW) laser light.

The photorefractive effect leads to a variety of optical phenomena, such as photorefractive Bragg grating and self-pumped phase conjugation, in certain types of crystals.

Available photorefractive crystals include the following: barium titanate (BaTiO₃) with or without a suitable dopant, such as Ce:BaTiO₃ and Rh:BaTiO₃; strontium barium niobate with or without a suitable dopant, including SBN60, SBN75 and the like; potassium sodium strontium barium niobate with or without a suitable dopant, including KNSBN, Ce:KNSBN, Cu:KNSBN, etc.; potassium niobate (KN) with or without a suitable dopant, including Fe:KNbO₃, Mn:KNbO₃, Rh:KNbO₃, Ni:KNbO₃; iron-doped lithium niobate (Fe:LiNbO₃); and vanadium-doped cadmium telluride (V:CdTe).

BaTiO₃ (barium titanate) has been found to have particularly good photorefractive properties for the visible wavelength range (430 nm - 700 nm). Rhodium-doped barium titanate (Rh:BaTiO₃) has been found to be suitable for expanding the wavelength range of the photorefractive effect into the near-infrared portion of the spectrum (630 nm - 1064 nm).

Vanadium-doped cadmium telluride (V:CdTe) has been found to be suitable for expanding the near-infrared wavelength range of the photorefractive effect beyond 1064 nm into the "communications band" portion of the spectrum (out to 1600 nm).

In one embodiment of the invention, the apparatus is in the form of an OPO, and the first optical medium is a nonlinear gain medium. The injection-seed light may be continuous-wave (cw) and its wavelength may be tunable (e.g., from a single-mode tunable diode laser at a wavelength of -850 nm). It may be capable of writing a photorefractive grating in the second optical medium, to selectively enhance the optical reflectivity thereof at an injection-seed wavelength which may be varied. The photorefractive grating may require some time to form. The time may vay from a few miliseconds to several minutes.

When pulsed pump light (e.g., at a wavelength of 532 nm from a frequency-doubled Nd:YAG laser) enters an OPO cavity in accordance with the invention, the output signal and idler wavelengths may each be adaptively controlled to a single narrowband wavelength by the second optical medium, in a manner consistent with equation (1).

If the cw injection-seed wavelength is tuned slowly from shot to shot of the OPO pump laser, the grating in the second optical medium adjusts itself automatically and ensures that signal and idler output wavelengths automatically track the injection-seed tuning. Such a controlled OPO requires no moving parts in the optical cavity. This is in contrast to conventional injection-seeded OPOs, which require intricate active optoelectronic devices for feedback control of the cavity length and component orientations. The resonance between the optical cavity and the injection-seed beam may be self- adaptive, and may be independent of the wavelength of the seed beam within the response wavelength range of the second optical medium. The interaction between the input seed beam and the intra-cavity light may be constructive, and may cause the optical cavity to automatically stay at an elevated energy state. The elevated energy state may be at maximum.

The finesse of the optical cavity may conveniently be optimised at the seed wavelength. It has been found that, because of the interaction between the input seed light and the intracavity light in the second optical medium, there is no need to adjust the length of the optical cavity when the input seed beam wavelength is varied, as a new grating, which relates to the varied wavelength, is formed in the second optical medium when the wavelength of the said beam is varied. The apparatus according to the invention may be configured as a narrowband tunable optical parametric oscillator (OPO) or as a laser. Such OPOs and lasers are useful in optical sensing applications in areas such as medical diagnostics, atmospheric monitoring, industrial process control and scientific measurements. The apparatus according to the invention may be provided in a modular form as individual units or, alternatively, it may be provided as a complete, assembled system. The complete system may comprise the items necessary to form an injection-seeded self-adaptive resonant cavity, in use, a suitable optical gain medium and an associated controller for controlling its operational parameters (e.g. temperature, orientation and position), an injection seed source and an optical pump source. The aforementioned items may be supplied as a kit or they may supplied fully assembled and ready to use. Alternatively, some of the components of the apparatus according to invention may be supplied as an add-on accessory to an existing pump laser system. Higher output power (which may be required for applications such as atmospheric lidar sensing [6] or nonlinear-optical wavelength conversion [5, 6] or molecular state preparation [5]) may be attainable by means of one or more additional optical parametric amplifier (OP A) stages [5 - 7], or by means of a second OPO using the coherent output of the first OPO to inject the second OPO. The OPO-based spectroscopic systems described above may be pumped by high- performance nanosecond-pulsed solid-state lasers from suppliers such as Continuum, Positive Light, Quantel, Spectra-Physics, and Spectron. The pump laser may for example be a 1.064-μm flashlamp-pumped, Q-switched Nd:YAG oscillator/amplifier system that is equipped with an injection seeder for SLM operation, special cavity optics to yield a quasi-Gaussian beam profile, and nonlinear-optical stages to generate harmonics at 532 nm and 355 nm. Typical operating parameters may be: pulse duration, about 8 ns; repetition rate, 10 Hz; pulse energies: >1 J at 1.064 μm, >0.7 J at 532 nm, >400 mJ at 355 nm. The optical bandwidth of one such SLM pulsed laser has been measured with a confocal Fabry-Perot etalon to be 45 ± 5 MHz (0.0015 ± 0.0002 cm^"1) fwhm. The pump laser may alternatively be a simple, compact, multimode Nd:YAG laser. A Continuum Minilite II laser, delivering pulses at about 50 mJ per pulse at a wavelengtli of about 1.064 μm and a repetition rate of about 10 Hz, has been found to work well. Such a laser may be used to pump a nanosecond-pulsed periodically poled lithium niobate first optical medium, where the apparatus is an OPO. The multimode Nd:YAG laser may oscillate on several longitudinal modes, yielding an optical bandwidth of about 1 cm^" and a rapidly modulated temporal profile (about 6 ns fwhm). It may be driven off regular mains power and it may be air-cooled, thereby facilitating field-based OPO applications. It may conveniently use the resonance properties of the OPO cavity to constrain a resonated wave to a single longitudinal mode of the OPO cavity and to ensure that it is continuously tunable without mode hops as the cavity length and tunable-diode-laser injection-seeding wavelength are scanned. The idler output may remain multimode, consistent with the characteristics of the multimode pump laser. The pump laser may alternatively be an all-solid-state (e.g., diode-pumped) nanosecond- pulsed laser. Such lasers are able to offer high repetition rates ( kHz) and may be readily transportable.

Injection-seeding may be arranged for simultaneous generation of two or more adjustable output wavelengths. The corresponding injection-seeding wavelengths may be controlled by spectroscopic tailoring, for instance, to match on- and off-resonance wavelengths of a spectrum of interest. Dual-wavelength injection seeding of an OPO according to the invention may be used for atmospheric remote sensing, whereby simultaneous monitoring of characteristic on-resonance and off-resonance wavelengths may be used as a spectroscopically tailored OPO system. An OPO in accordance with the invention may thus be used for thermometric sensing of nitrogen gas in furnace air by coherent anti-Stokes spectroscopy (CARS).

Narrowband tunable OPOs and lasers in accordance with the invention have a wide variety of applications in areas such as medical diagnostics, atmospheric monitoring, industrial process control and scientific measurements. The optical cavity may include three or more optically interconnected reflectors. Each reflector may have a reflective surface or may comprise a mirror.

One or more of the reflectors may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s), in order to decouple coherent light from the cavity. One or more of the reflectors may be highly reflective. The decoupler may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s).

In the event that the first optical medium is a nonlinear-optical gain medium, signal and idler beams having different wavelengths will be generated in the optical cavity. The output may be selected from either the signal or the idler light resulting from the input seed beam. In the embodiment in which the first optical medium is a laser gain medium, the pump pulse may pump the same or a larger region of the first optical medium. It may pump it from the side, from a direction other than that of the input seed beam. By using more than one reflector or mirror to form a ring or a bow-tie shaped optical cavity, various options may be provided for arranging the intra cavity optical gain media, for coupling the pump beam into the cavity or for decoupling intracavity light. Dichroic mirrors may form the optical cavity. It is possible to use more than one intra-cavity optical gain medium, and each optical gain medium may be independently pumped. An additional dichroic beam combiner and/or a beam splitter may be required for steering, coupling and decoupling the pump beams. The different media may be made of the same or different materials. Alternatively, they may comprise crystals cut at different angles or crystals having different internal structures or, alternatively, having uniform structures. Optionally, different regions of the same crystal may be used as different media. As still another option, the different media may be operated at different operating conditions such as temperature. These variations in the construction of the optical cavity may be employed to extend the range of wavelengths that can be handled by the apparatus. The reflectivity formed in the second optical medium, by the interaction between the input seed beam and light from the optical cavity, may continue to exist for some time in the absence of the input seed beam, after the injection-seeding by the seed beam is terminated. For a photorefractive-type of second optical medium, the time may vay from a few miliseconds to several minutes. This memory effect provides a different operational mode for the apparatus, that is, to pump the first optical medium by the pulsed pump laser in the absence of the input seed beam. This creates a free-running OPO or laser operation. The optical bandwidth of the coherent output radiation in this event is predominantly determined by the reflection bandwidth of the second optical medium. Therefore, the apparatus may provide outputs of two different optical bandwidths. For continuous-wave seed sources, the cw input seed beam may be blocked mechanically by a beam flag. For a pulsed seed source, the input seed beam may be either blocked by a beam flag or it may be controlled by a timing circuit controlling the timing of the pulsed seed and pump sources. As a nonlinear-optical gain medium PPKTP (periodically poled potassium titanyl phosphate, KTiOPO₄), which may be quasi-phase-matched, may be used.

Alternatively, a multi-grating quasi-phase-matched optical element for instance, a suitable periodically poled lithium niobate element, may be used. The medium may have a set of eight parallel quasi-phase-matched gratings of varying periodicity disposed on a single substrate [10]. At a fixed OPO crystal temperature, a single periodically poled lithium niobate grating generates broadband signal and idler output light spread over about 5 cm^-1 (about 150 GHz) fwhm. With 1.064-μm pump light, this combination of eight periodically poled lithium niobate grating periods and temperature variation over about 50 °C provides uninterrupted quasi-phase-matched OPO tuning ranges from about 1.45 μm to about 1.55 μm for the signal output and from about 4.0 μm to about 3.4 μm for the idler output [7, 8].

As an example of a suitable quasi-phase-matched nonlinear-optical medium, a single periodically poled lithium niobate grating (on a multi-grating substrate [10]) allows continuous tunability over the following ranges in the 1.5-μm region: about 400 GHz

(about 13 cm"l) at constant temperature; about 7.5 THz (about 250 cm~l) with additional temperature tuning.

While the preferred form of nonlinear-optical medium for the OPO embodiment is quasi- phase-matched, (i.e., a crystal with alternating structural domains), it is also possible to use a birefringently phase-matched nonlinear-optical medium (i.e., a homogeneous, non- centrosymmeric crystal). Another option is to use more than one nonlinear-optical crystal, such as a matched pair birefringently phase-matched crystals that are aligned to minimise nonlinear-optical walk-off effects. The apparatus according to the invention may be configured for spectroscopic applications requiring a coherent source that simultaneously generates two or more adjustable output wavelengths, for instance, to match on- and off-resonance wavelengths of a spectrum of interest [6].

Spectroscopic tailoring of OPO output by multi-wavelength injection seeding is most readily implemented with a birefringently phase-matched medium in an OPO cavity that is slightly misaligned to reduce its finesse. It is also possible in quasi-phase-matched media with grating channels wide enough to allow different non-collinear phase-matching angles for each of the OPO output wavelengths. Conventional multi-wavelength OPOs require a source of coherent seed light that can simultaneously generate a structured set of discrete injection-seeding wavelengths; an array of tunable diode lasers is a suitable injection-seeding source for such a purpose.

The geometric arrangements for nonlinear-optical phase-matching are typically collinear, with the pump beam, injection-seeding beam and coherent output beam co-propagating in the gain medium. As an alternative, noncollinear phase matching may be employed, in which the pump beam, injection-seeding beam and coherent output beam are each in different directions determined by the phase-matching conditions as in Equation (1); this can be advantageous in increasing the optical bandwidth of a free-running OPO configured according to the invention, to yield a wider range of wavelengths and hence to facilitate multi-wavelength injection-seeded operation of the OPO.

One advantage of the apparatus according to the invention is that it provides a self- adaptive, tunable optical cavity.

Another advantage is that the resonance between the optical light entering the second optical medium from the optical cavity and the injection-seed beam, is automatic, self- adaptive and independent of the wavelength of the injection-seed beam, within the response wavelength range of the material used for the self-adaptive reflector. The cavity finesse may be optimised at the seed wavelength. There is accordingly no need to adjust the cavity when the injection-seed wavelength is varied. There are accordingly no moving parts in the optical cavity, allowing a robust, compact system design.

Also, by elimination of active optoelectronic feedback control of the optical cavity, the optical pathlength of the cavity may be matched to the wavelength of the injection-seed light.

The apparatus according to the invention does not require wavelength-selective elements, such as gratings, filters or etalons, to be located in the cavity. The optical loss of the cavity is thereby reduced and the damage and oscillation thresholds may be more widely separated. The invention thus makes it possible to avoid or at least reduce the need for intra-cavity wavelength-selective elements or the active optoelectronic control of the optical cavity length, or both. This enhances the ease of operation, portability and ruggedness of a narrowband tunable optical parametric oscillator or laser configured according to the invention. Brief Description of the Drawings

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein:

Figure 1(a) is a schematic representation of one alternative design of a prior art OPO system showing a free OPO with no active wavelength control;

Figure 1(b) is a schematic representation of another alternative design of a prior art OPO system which includes an intra-cavity tuning element;

Figure 1(c) is a schematic representation of yet another alternative design of a prior art

OPO system which is injection seeded, with no wavelength control; Figure 2 shows a diagrammatic representation of one embodiment of a self-adaptive optical resonant cavity in accordance with the present invention operated as an OPO;

Figure 3 shows a diagrammatic representation of another embodiment of a self-adaptive optical resonant cavity in accordance with the present invention, in the form of a pulsed, injection seeded tunable laser system; Figure 4 is a graph showing the idler output pulse energies of the seed, signal and idler wavelengths of the embodiment of the invention shown in Figure 2;

Figure 5 is a composite graph showing a pair of etalons output fringes recorded for each of a series of seed light inputs, each having a different wavelength, injected into the embodiment of the invention shown in Figure 2; Figure 6 is a graph showing lower and upper bounds of idler input energy as a function pump pulse energy; and

Figure 7 is a graph showing an absorption spectrum of carbon dioxide gas obtained according to the cavity ringdown spectroscopy technique by using frequency-scanned coherent output light from an OPO apparatus as shown in Figure 2; Figure 8 is a diagrammatic representation of another embodiment of a self-adaptive optical resonant cavity in accordance with the present invention, operated as an OPO;

Figure 9 is a graphical representation of the intensity of beam reflected from a photorefractive grating formed inside the second optical medium, in use; and

Figure 10 as a graphical representation of the intensity of light reflected from the photorefractive grating of the second optical medium, as a function of the wavelength of a probe beam injected into the second optical medium. Detailed Description of the Preferred Embodiments

Figures 1(a) to 1(c) schematically show three alternative designs of prior art ns-pulsed OPO systems. Figure 1(a) depicts a free-πmning OPO (with no active wavelength control). Figure 1(b) depicts an OPO with an intra-cavity tuning element (T), such as a grating, filter or etalon. Figure 1(c) depicts an injection-seeded OPO. Each of the alternative designs shown in Figures 1(a) to 1(c) uses an optical cavity defined between two mirrors (Mi, M₂) and a nonlinear-optical medium C (with susceptibility χ⁽²⁾). Input light is respectively indicated by a solid line arrow P (denoting pulsed pump light) and a dotted line arrow B (indicating seed light). Output light consists of a signal component S and an idler component I, both being indicted by solid line arrows. The optical frequencies and the wave vectors of the respective light obey equation 1. The pump light P (left-hand solid arrow) in each of Figures 1(a), 1(b) and 1(c) is obtained from a pulsed laser (not shown). The nonlinear-optical medium C generates two output beams (represented by the right-hand arrows in Figures 1(a) to 1(c) having different wavelengths. Narrowband tunability is achieved either by inserting an intra-cavity tuning element T {as in Figure 1(b)} or by injecting seed light (represented by the dashed arrow B) from another laser as in Figure 1(c).

In the devices of each of Figures 1(a) to 1(c), the OPO resonator is defined by two mirrors

In Figure 1(c), narrowband seed light from one or more external tunable light source is used to control the OPOs output wavelength(s). It requires no intra-cavity tuning element T. However, it requires an active optoelectronic controller (not shown in Figure 1(c)) to match the optical cavity length to the seed wavelength for good injection-seeding operation. The present invention is an improvement of the injection-seeded OPO approach shown in Figure 1(c). In the apparatus according to one aspect of the invention, the mirror M_Ϊ is replaced by a self-adaptive photorefractive Bragg grating mirror (PCM), which is adaptively created and controlled by the tunable seed light. This greatly simplifies pulsed, tunable OPO design. . _ _ Figure 2 depicts one embodiment of an apparatus in accordance with the invention, in the form of a pulsed, injection-seeded tunable OPO system 210, which is self-adaptive. The OPO system 210 includes a nonlinear-optical medium 212, typically a crystal of periodically-poled potassium titanyl phosphate (PPKTP); one regular reflective mirror 214 (also designated M2); and one photorefractive crystal 216 (also designated PCMl) which is able to serve as a Bragg grating mirror.

A (cw or pulsed) injection seeder 218 generates injection seeding light 220 of narrow optical bandwidth, preferably single-longitudinal-mode, which passes through an optical isolator 222, then through the photorefractive crystal 216 (also designated PCMl), a dichroic mirror 224 (also designated M4), and the nonlinear-optical medium 212. In the arrangement shown in Figure 2, the injection seeding light 220 is reflected through 90° by each of two reflectors 226 and 228 as shown. The mirrors 214 and 216 together define an optical cavity 230. A portion of the seeding light 220 passes through the nonlinear-optical medium 212, and is reflected back into the nonlinear-optical medium 212 by the reflective mirror 214 (also designated M2). The portion of the seed beam 220 that is reflected by the mirror 214 (M2) overlaps with the incident seed beam at the photorefractive crystal 216 (also designated PCMl). The resulting interaction between the overlapping incident and reflected seed beams with the photorefractive crystal 216 (PCMl) forms a Bragg grating that acts as a wavelength-selective reflector.

The formation of a Bragg grating at photorefractive crystal 216 (PCMl) is not instantaneous. Depending on the materials used, it typically takes an interval of time ranging from less than one second to several minutes or even longer. After the Bragg grating has been formed, the photorefractive crystal 216 (PCMl) becomes partially reflective at and around the wavelength of the injection seeding light 220. The OPO cavity 230 therefore becomes selectively resonant at the wavelength of the injection seeding light 220. The nonlinear-optical medium 212 is pumped by a pulsed, coherent pump light beam 232 obtained from a pump source 234. The pump source 234 is typically a pulsed, single- longitudinal-mode, frequency-doubled Nd:YAG laser. The coherent pump light beam 232 is aligned to overlap with the seed beam 220 by steering mirrors 236 and 224 (respectively also designated M3 and M4). In the event that the injection seeder 218 provides pulsed light, it is desirable to use a control unit (not shown in Figure 2) to synchronise the pump pulse 232 and the seed pulse 220 to ensure a temporal overlap at the nonlinear-optical medium 212 to injection seed the OPO process. The mirror 224(M4) is a dichroic mirror that is highly reflective at the wavelength of the pump light beam 232, but transparent at the wavelength of the seed beam 220. By controlling the temperature, quasi-phase-matched grating spacing and orientation of the nonlinear-optical crystal 212, an OPO process is caused to take place in the optical cavity 230. This OPO process is phase-matched at the wavelength of the seed beam 220. The signal output of the OPO process is typically at the wavelength of the seed beam 220. The reflector 214 (M2) is also dichroic and is transparent at the OPO wavelength of the idler output beam, so that the idler light of the OPO process exits the optical cavity 230 through the reflector 214 (M2). The mirror 214 (M2) may be transparent at the pump wavelength. Alternatively, the mirror 214 (M2) may be chosen to be reflective at the pump wavelength to direct the pump light beam 232 back to the nonlinear-optical medium 212 for a second time. However, for this alternative the pump source 234 is preferably either protected against the back-reflected residual pump light beam, or is insensitive thereto.

The signal output of the OPO process may be coupled out of the system at any one or more of several points, such as at the optical isolator 222, or at an inner surface 216.1 of the photorefractive crystal 216 (PCMl). Alternatively or additionally, if the reflector 214 (M2) is chosen to be a partial reflector at the OPO signal wavelength, then part of the OPO signal output may also exit the optical cavity 230 through the reflector 214 (M2). Figure 3 depicts another embodiment of an apparatus in accordance with the invention, in the form of a pulsed, injection-seeded, tunable laser system 310, which is self-adaptive. The laser system 310 includes a laser gain medium 312, one regular reflective mirror 314 (also designated M2); and one photorefractive crystal 316 (also designated PCMl) which is able to serve as a self-adaptive Bragg grating mirror.

A (cw or pulsed) injection seeder 318 generates injection seeding light 320 of narrow optical bandwidth, preferably single-longitudinal-mode, which passes through an optical isolator 322, then through the photorefractive crystal 316 (PCMl), and the laser gain medium 312. As in the case of the embodiment of the apparatus shown in Figure 2, the injection seeding light 320 is reflected through 90° twice by each of two reflectors 326 and 328 as shown. The embodiment of the apparatus shown in Figure 3 does not include a dichroic mirror similar to the mirror 224 (M4) of Figure 2.

The photorefractive crystal 316 (PCMl) and the mirror 314 together define an optical cavity 330. A portion of the seeding light 320 passes through the laser gain medium 312, and is reflected back into the laser gain medium 312 by the reflective mirror 314 (M2). The portion of the seed beam 320 that is reflected by the mirror 314 (M2) overlaps with the incident seed beam 320 at the photorefractive crystal 316 (PCMl). The resulting interaction between the overlapping incident and reflected seed beams with the photorefractive crystal 316 (PCMl) forms a self-adaptive Bragg grating that acts as a wavelength-selective reflector.

The formation of a self-adaptive Bragg grating at the photorefractive crystal 316 (PCMl) is not instantaneous. Depending on the materials used, it typically takes an interval of time ranging from less than one second to several minutes or even longer.

After the Bragg grating has been formed, the photorefractive crystal 316 (PCMl) becomes reflective at and around the wavelength of the injection seeding light 320. The optical cavity 330 therefore becomes selectively resonant at the wavelength of the injection seeding light 320. The laser gain medium 312 is pumped from its side by a pulsed, pump light beam 332 obtained from a pump source 334. The pump source 334 is typically a pulsed light source emitting light capable of exiting the laser gain medium to the desired exited state. The pump light beam 332 does not require steering as it is introduced into the laser gain medium 312 directly, from its side, along a different optical path as the one in which the seed light beam 320 travels.

The seed beam is chosen to have the desired wavelength of the coherent output light within the wavelength range of the laser gain. In the event that the injection seeder 318 is pulsed, it is desirable to use a control unit (not shown in Figure 3) to synchronise the pump pulse 332 and the seed pulse 320 to ensure a temporal overlap at the laser gain medium 312 to injection seed the laser process.

The output of the optical cavity 330 may be coupled out of the apparatus 310 at any one or more of several points, such as at the optical isolator 322, or at an inner surface 316.1 of the photorefractive crystal 316 (PCMl). Alternatively or additionally, if the reflector 314 (M2) is chosen to be a dichroic, partial jeflector at the output wavelength of the optical cavity 330 of the apparatus 310, then at least a part of the output may also exit the optical cavity 330 through the reflector 314 (M2).

Figures 4 to 7 depict various performance characteristics, performed, in Example 2, on an apparatus in accordance with the invention as shown in Figure 8. Figure 4 shows the variation of 1.42-μm idler output energy for a series of 0.85-μm seed (and signal) wavelengths in the vicinity of the OPO gain bandwidth, at a constant PPKTP temperature (98 °C) and with a PPKTP grating pitch of 9.35 μm; this shows that the output is sensitive to detuning from the free-running OPO centre wavelength. Figure 5 shows etalon fringes (from both a coarse etalon and a fine etalon of a Burleigh WA-4500 pulsed wavemeter) recorded for a series of ~0.852-μm seed wavelengths that are varied in fine steps of -0.025 nm, indicating that the OPO output can be tuned continuously without mode hops. In Figure 6, the upper and lower bounds of idler output energy are plotted as a function of pump pulse energy.

Figure 7 shows an absorption spectrum of carbon dioxide gas. The spectrum was obtained in Example 1 by recording the cavity ringdown rate, according to the cavity ringdown spectroscopy technique, of frequency-scanned coherent output light obtained from an OPO constructed in accordance with Figure 8. Figure 8 depicts another embodiment of an apparatus in accordance with the invention, in the form of a pulsed, injection-seeded tunable OPO system 810, which is self-adaptive. The OPO system 810 includes a nonlinear-optical medium 812, typically a crystal of periodically-poled potassium titanyl phosphate (PPKTP); one regular reflective mirror 814 (also designated M2); and one photorefractive crystal 816 (also designated PCMl) which is able to serve as a Bragg grating mirror.

A (cw or pulsed) injection seeder 818 generates injection seeding light 820 of narrow optical bandwidth, preferably single-longitudinal-mode, which passes through an optical isolator 822, then through the photorefractive crystal 816 (also designated PCMl), a dichroic mirror 824 (also designated M4), and the nonlinear-optical medium 812. In the arrangement shown in Figure 8, the injection seeding light 820 is reflected through 90° by each of two reflectors 826 and 828 as shown. The mirrors 814 and 816 together define an optical cavity 830.

A portion of the seeding light 820 passes through the nonlinear-optical medium 812, and is reflected back into the nonlinear-optical medium 812 by the reflective mirror 814 (also designated M2). The portion of the seed beam 220 that is reflected by the mirror 814 (M2) overlaps with the incident seed beam at the photorefractive crystal 816 (also designated PCMl). The resulting interaction between the overlapping incident and reflected seed beams with the photorefractive crystal 816 (PCMl) forms a Bragg grating that acts as a wavelength-selective reflector.

The formation of a Bragg grating at photorefractive crystal 816 (PCMl) is not instantaneous. Depending on the materials used, it typically takes an interval of time ranging from less than one second to several minutes or even longer.

After the Bragg grating has been formed, the photorefractive crystal 816 (PCMl) becomes reflective at and around the wavelength of the injection seeding light 820. The OPO cavity 830 therefore becomes selectively resonant at the wavelength of the injection seeding light 820. The nonlinear-optical medium 812 is pumped by a pulsed, coherent pump light beam 832 obtained from a pump source 834. The pump source 834 is typically a pulsed, single- longitudinal-mode, frequency-doubled Nd:YAG laser. The coherent pump light beam 832 is aligned to overlap with the seed beam 820 by steering mirrors 836 and 824(respectively also designated M3 and M4). In the event that the injection seeder 818 is pulsed, it is desirable to use a control unit (not shown in Figure 8) to synchronise the pump pulse 832 and the seed pulse 820 to ensure a temporal overlap occurs at the nonlinear-optical medium 812 to injection seed the OPO process.

The mirror 824 (M4) is a dichroic mirror that is highly reflective at the wavelength of the pump light beam 832, but transparent at the wavelength of the seed beam 820. By controlling the temperature, quasi-phase-matched grating spacing and orientation of the nonlinear-optical crystal 812, an OPO process is caused to take place in the optical cavity 830. This OPO process is phase-matched at the wavelength of the seed beam 820. The signal output of the OPO process is typically at the wavelength of the seed beam 820. The reflector 814 (M2) is also dichroic and is transparent at the OPO wavelength of the idler output beam, so that the idler light of the OPO process exits the optical cavity 830 through the reflector 814 (M2).

The signal output of the OPO process may be coupled out of the system at any one or more of several points, such as at the optical isolator 822, or at an inner surface 816.1 of the photorefractive crystal 816 (PCMl). Alternatively or additionally, if the reflector 814 (M2) is chosen to be a partial reflector at the OPO signal wavelength, then part of the OPO signal output may also exit the optical cavity 830 through the reflector 814 (M2). Example 1

An OPO was constructed in accordance with the layout shown in Figure 2. The frequency-scanned coherent OPO idler output light is used to record an absorption spectrum feature at ~1.439 μm of 20 Torr pure carbon dioxide gas by a cavity ringdown spectroscopy measurement technique. The frequency scan of the OPO idler output is achieved by scanning the seed wavelength over a small range around 844.6 nm. These performance data were achieved without any adjustment of the OPO. The data points were well fitted by a Lorentian profile with a bandwidth of 507 MHz (0.017 cm^"1), consistent with a Doppler- and pressure-broadened spectrum at room temperature. Example 2

In this example, an embodiment of the apparatus, such as the one described with reference to Figure 8, was used. Two types of tunable single-longitudinal-mode diode laser with output intensity of up to -10 mW were used as injection seeders. The first one was an external-cavity diode laser with a large wavelength tuning range from 834 nm to 851 nm (manufacturer: New Focus, Inc., Model 6316); and the second was a distributed-feedback diode laser, output wavelength -851.5 nm, (Manufacturer: SDL, Inc., Model 5702-H1). The output of the injection seeder was coupled into a 5 -meter-long silica single-mode optical fiber (cut off wavelength <780 nm) by the lens L3. Light emerging out of the other end of the optical fiber was slightly focused by the lens L4 and was propagated through the optical isolator 822, formed by two polarizers 822.1, 822.2 and a 45-degree Faraday rotator 822.3. The polarisation of the light before it entered the optical isolator was optimised for maximum transmission. This was achieved by forming the fibre into small loops and by twisting the plane of each loop. The light passing through the optical isolator 822 was horizontally polarised. The mirrors M5 and M6 of the apparatus 810 were used to direct the light through another lens LI, which focused the light on to the nonlinear-optical periodically-poled KTiOPO₄ (PPKTP) crystal 812. This light beam was used as the guiding optical axis of the OPO cavity 830. Lens LI had a focal length of 250 mm. It focuses the input seed beam 820 to the centre of the nonlinear-optical crystal 812. The nonlinear-optical crystal 812 had dimensions of 1 mm thick x 2 mm wide x 20 mm long (Z x Y x X axes), and a periodicity of ferroelectric domain structure along the X axis of 9.35 microns. The input seed beam 820 propagated along the X axis of the PPKTP crystal. The polarisation of the input seed beam 820 was parallel to the Z-axis of the crystal. The temperature of the PPKTP crystal has controlled by means of a temperature control unit 838, which had the ability of controlling the temperature within a range of ±0.05 °C of a set temperature. The signal wavelength of the free-running OPO was measured to be 877 nm at a PPKTP crystal temperature of 20 °C, and to be 818 nm at 200 °C. The OPO signal wavelength depended quasi linearly on the crystal temperature within this temperature range. The corresponding OPO idler wavelengths were 1354 nm at 20 °C and 1524 nm at 200 °C. The OPO cavity was formed by a Rh:BaTiO₃ (rhodium-doped barium titanate) photorefractive crystal 816 and the mirror 814 (M2). The distance from the center of the PPKTP crystal 812 to the mirror M2 was 45 mm, to the photorefractive crystal 816 was 80 mm, and to the lens LI was 180 mm, respectively.

The Rh:BaTiO₃ crystal 816 had a rhodium concentration of ~6 ppm. The dimensions of the crystal 816 were 7.5 mm x 5 mm x 5 mm for the corresponding c x a x a axes, respectively. All six facets of the crystal 816 were optically polished, but not anti- reflection coated. The c-axis of the crystal 816 was in the horizontal plane. The vertical surface of the crystal 816 formed by the c- and a-axes was rotated vertically by an angle of -10 degrees away from normal to the horizontal optical axis of the OPO cavity, to avoid overlap of multiple-reflections at the crystal facets of injection-seed beam 820. Anti-reflection coatings at the seed wavelength at the two vertical surfaces formed by the c- and a-axes were not used, but would have been advantageous to reduce reflection. The antireflection coatings are preferably at the seed wavelength and is preferably applied on the two vertical surfaces formed by the c-and a-axes.

Mirror 814 (M2) was a plano-concave mirror. The curvature of M2 was 20 cm. Both sides of M2 were anti-reflection coated for a wavelength of 532 nm. Two mirror reflectivities, 80% and 100%), for the OPO signal wavelength range, were tested. Both reflectivities were found to be suitable.

Mirror M2 was adjusted to retro-reflect the input seed beam 820. The incoming beam geometry was optimised by lenses LI and L4 for the mirror M2, so that the beam which was retroreflected by the mirror M2 matched the initial incoming beam geometry. The two counter-propagating seed beams interacted with the photorefractive crystal 816 to form a photorefractive Bragg grating within a few seconds. After the photorefractive grating was formed, the photorefractive crystal 816 became partially reflecting for a narrowband of wavelengths centred at the wavelength of the seed beam 820. The maximum reflectivity obtained was about 60%. The photorefractive grating adapted itself to seed wavelength changes so that it maintained a maximum light density in the cavity between the photorefractive crystal 816 and M2. The pump laser 834 was a pulsed, single-longitudinal-mode, frequency-doubled Nd:YAG laser (-10 ns @ 532 nm, 10 Hz). A half waveplate 840 and a thin-film-polariser 842 combination selected a portion (typically around 100 μJ) of pump laser output at desired polarisation orientation to pump the nonlinear-optical gain medium (PPKTP crystal) 812. The pump pulse 832 was combined collinearly by the mirror 836 (M3) and a dichroic beam combiner 824 (M4) with the injection-seed beam 820. The polarisation of the pump beam is same as that of the seed beam in the horizontal plane. The beam combiner (M4) was highly transmissive at the OPO signal wavelength and was highly reflective at the pump wavelength. A lens L2 was provided, that matched the pump beam geometry to that of the injection-seed beam 820, so as to ensure that good overlapping would be achieved. The OPO idler output exited the cavity 830 through the mirror 814 (M2). The OPO signal output exited the optical cavity 830 where it is deflected by the optical isolator 822. Additinally or as an option, the OPO signal output may exite the cavity through the mirror 814 M2 when the mirror 814 M2 is coated partially reflective and partially transmissive at the OPO signal wavelength. The OPO signal output may additionally exit the optical cavity 830 when it is reflected from a portion of the surface of the medium 816 if it has not been coated with an anti-reflection coating. Example 3

The time response of forming photorefractive gratings in the photorefractive crystal PCMl by the seed beam was measured using an apparatus as shown in Figure 8. The dichroic beam combiner 824 (M4) and the PPKTP crystal 812 were removed. The pump laser 834 and its associated optical components λ/2, TFP, L2 and M3 were not needed. The injection seeder 818 delivered single-longitudinal-mode light at 851.5 nm. The seed beam intensity was measured between the lens LI and the photorefractive crystal 816 (PCMl). Throughout the example it was maintained at 1.7 mW. The mirror 814 (M2) was a highly reflective reflector at the wavelength of the seed beam 820.

An opaque object (not shown in the Figure 8) was inserted temporarily between the mirror 814 (M2) and the second optical medium 816 (PCMl) to block the propagation and return of the seed beam inside the cavity. The reflected seed beam intensity was monitored at a position indicated by OUTPUT 1, by means of a photodetector (not shown in Figure 8). The photodector output was determined, and is displayed in Figure 9. The cavity was firstly blocked by an opaque object and then the opaque object was removed. After the opaque object was removed,

5 the photodetector received light reflected back from the mirror 814 (M2) and its output increased to a level of about 2 arbitrary units, as indicated by a horizontal dashed line in the Figure 9. The output of the photodetector increased further as the photorefractive grating was forming in the photorefractive crystal PCMl. It took about 3.2 seconds for the photorefractive grating to reach 63% of its final reflectivity. When the cavity waso blocked by an opaque object afterwards, the photorefractive crystal was exposed only to the incoming seed beam 820, but not to the reflected beam seed from the mirror 814 (M2). About 63% of the photorefractive grating vanished in about 2.1 seconds, causing the photorefractive crystal 816 to lose its reflectivity. Example 4s In a separate experiment, carried out with the apparatus as used for Example 3, it was determined that, when the seed beam 820 was blocked off, so that the photorefractive crystal 816 was no longer exposed to any seeding, the photorefractive grating formed previously inside the photorefractive crystal 816 continued to exist for more than 70 seconds.0 Example 5 Example 3 was repeated to determine the effect of the wavelength of the incoming seed beam 820 as a very weak probe beam on the reflectivity of the photorefractive crystal 816. After a photorefractive grating was formed, the seed beam 820 was turned off. The cavity5 830 was blocked by inserting an opaque object into the cavity 830. Instead of the seed beam 820, a very weak probe beam of less than 25 μW was sent to probe the reflectivity of the photorefractive grating in the photorefractive crystal 816. The frequency of the probe beam was adjustable relative to the seed beam frequency. The reflected probe beam intensity was monitored at position OUTPUT 1 by a photodetector. Theo photodetector output is displayed in Figure 10 as a measurement of the reflection of the photorefractive crystal 816, as a function of the wavelength of the probe beam. The measured bandwidth (full width at half maximum) of the photorefractive grating is about 14 GHz. The dimension of the photorefractive crystal 816 along the beam path is about 5 mm. The bandwidth of the photorefractive grating is inversely proportional to the beam path length at weak interaction. A longer (along the beam path) photorefractive crystal 816 is expected to reduce the reflection bandwidth accordingly.

References

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Claims

1. An apparatus for providing pulsed beam of narrowband coherent light, said apparatus including: - a first optical medium selected from the group consisting of a laser gain medium and a nonlinear-optical gain medium, the first optical medium being capable of generating coherent output light, in use, from pulse pump light; - a first reflector located on one side of the first optical medium; - a second optical medium located on an opposite side of the first optical medium, the first reflector and the second optical medium together defining an optical cavity, the second optical medium in use becoming at least partially reflective to light in the optical cavity; - a coherent light source selected from the group consisting of pulsed, quasi- continuous-wave and continuous-wave, for injecting into the optical cavity, in use, an input seed beam of coherent light, said input seed beam being injected through the second optical medium, from a side of the second optical medium opposite a side that faces the optical cavity, whereby the second optical medium becomes partially reflective to light in the optical cavity that has a wavelength which is about the same as the wavelength of the input seed beam and that overlaps spatially and temporally, in the second optical medium, with the input seed beam, and whereby the optical cavity becomes resonant in respect of such light in the optical cavity; - a pulsed light source for providing, in use, pulsed pump light to pump the first optical medium, whereby the first optical medium is caused to generate coherent output light of which at least a portion has a wavelength that is the same as the wavelength of the input seed beam; - a first coupler for coupling the input seed beam into said second optical medium; - a second coupler for coupling the pulsed pump light into the first optical medium; and - a decoupler for decoupling the coherent output light from the apparatus.

2. An apparatus as claimed in claim 1, configured to operate as an optical parametric oscillator.

3. An apparatus as claimed in claim 2, wherein the wavelength of the seed beam is the same as the wavelength of either a signal component or an idler component of the coherent output light generated by the first optical medium.

4. An apparatus as claimed in claim 1, wherein the optical cavity includes a second reflector, in addition to the first reflector and the second optical medium.

5. An apparatus as claimed in claim 4, wherein the second reflector is arranged such that the first reflector, the second reflector and the second optical medium are located at the corners of a triangle, so that light in the optical cavity is reflected between the first reflector and the second reflector, between the second reflector and the second optical medium and between the second optical medium and the first reflector.

6. An apparatus as claimed in claim 4, wherein the first optical medium is located between the first reflector and the second optical medium, or between the second reflector and the second optical medium, or between the first reflector and the second reflector.

7. An apparatus as claimed in claim 1, wherein the optical cavity includes three, four or five reflectors.

8. An apparatus as claimed in claim 7, wherein the reflectors are arranged in such a way as to reflect light from one to the other reflector in ring, bow-tie or zig-zag fashion.

9. An apparatus as claimed in claim 1, wherein the apparatus includes a second intracavity optical gain medium selected from the group consisting of a laser gain medium and a nonlinear-optical gain medium.

10. An apparatus as claimed in claim 1, wherein the optical cavity includes a deflector.

11. An apparatus as claimed in claim 10, wherein the deflector is a prism having at least two surfaces disposed at a first angle relative to each other, so as to cause the prism to deflect a beam of optical radiation through a second angle.

12. An apparatus as claimed in claim 10, wherein the deflector includes a means for decoupling desired radiation out of the optical cavity.

13. An apparatus as claimed in claim 10, wherein the deflector includes a means for coupling input radiation into the optical cavity.

14. An apparatus as claimed in claim 1, wherein the input seed beam is a continuous- wave beam.

15. An apparatus as claimed in claim 1, wherein the input seed beam is a pulsed coherent beam.

16. An apparatus as claimed in claim 1, wherein the input seed beam is of narrow optical bandwidth.

17. An apparatus as claimed in claim 1, wherein the wavelength of the input seed beam is tunable.

18. An apparatus as claimed in claim 1, wherein the interaction in the second optical medium between the input seed beam and the intra-cavity light achieves a constructive interference of the seed beam in the cavity.

19. An apparatus as claimed in claim 1, wherein the interaction, in the second optical medium between the input seed beam and the intra-cavity light causes the optical cavity to automatically stay at an elevated energy state.

20. An apparatus as claimed in claim 1, wherein the interaction, in the second optical medium between the input seed beam and the intra-cavity light, is a nonlinear- optical phase conjugating interaction.

21. An apparatus as claimed in claim 1, wherein the interaction in the second optical medium between the input seed beam and the intra-cavity light is based on a photorefractive effect.

22. An apparatus as claimed in claim 1, wherein the second optical medium is made of a photorefractive material.

23. An apparatus as claimed in claim 2, wherein the first optical medium is a nonlinear-optical gain medium.

24. An apparatus as claimed in claim 1, wherein the apparatus is configured as a laser.

25. An apparatus substantially as herein described with reference to the accompanying drawings.

26. A method of providing a pulsed beam of coherent output light, the said method including the steps of: - locating a first optical medium between a first reflector and a second optical medium, the first optical medium being selected from the group consisting of a laser gain medium and a nonlinear-optical gain medium, and being capable of generating coherent output light, in use, the first reflector and the second optical medium together defining an optical cavity, in use, the second optical medium being at least partially reflective to light in the optical cavity, when in use; - passing through the second optical medium, into the optical cavity, from a side of the second optical medium opposite a side that faces the optical cavity, an input seed beam of coherent light selected from the group consisting of pulsed, quasi-continuous-wave and continuous-wave, whereby the second optical medium becomes partially reflective to light in the optical cavity that has a wavelength which is about the same as the wavelength of the input seed beam and that overlaps spatially and temporally, inside the second optical medium, with the input seed beam, and whereby the optical cavity becomes resonant in respect of such light in the optical cavity; and - injecting pulsed pump light into the first optical medium to pump the first optical medium, causing the first optical medium to generate coherent output light of which at least a portion has a wavelength that is the same as the wavelength of the input seed beam.

27. A method of providing a pulsed beam of coherent output light substantially as herein described with reference to the accompanying drawings.