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WO1994010729A1 - Laser et dispositif d'enclenchement du verrouillage de mode d'un faisceau laser - Google Patents

Laser et dispositif d'enclenchement du verrouillage de mode d'un faisceau laser Download PDF

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Publication number
WO1994010729A1
WO1994010729A1 PCT/GB1992/002026 GB9202026W WO9410729A1 WO 1994010729 A1 WO1994010729 A1 WO 1994010729A1 GB 9202026 W GB9202026 W GB 9202026W WO 9410729 A1 WO9410729 A1 WO 9410729A1
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Prior art keywords
laser
pump
laser beam
medium
main
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Wilson Sibbett
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BTG International Ltd
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British Technology Group Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1112—Passive mode locking
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
    • H01S3/09415—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08018—Mode suppression
    • H01S3/0804—Transverse or lateral modes
    • H01S3/0805—Transverse or lateral modes by apertures, e.g. pin-holes or knife-edges
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/0811—Construction or shape of optical resonators or components thereof comprising three or more reflectors incorporating a dispersive element, e.g. a prism for wavelength selection
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/09408—Pump redundancy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/109—Frequency multiplication, e.g. harmonic generation

Definitions

  • This invention relates in general terms to a laser and to a device for initiating mode-locking of a laser beam.
  • the invention relates more particularly to a laser which is powered by an arrangement of one or more semiconductor lasers ⁇ or "laser diodes") and which is capable of producing a frequency-tunable
  • Lasers capable of producing pulses in the femto second regime are of use in time-domain-laser spectroscopy (for example, photophysics, photochemistry and photobiology). They are also of use in radar, range-finding, communications and optical or optoelectronic data processing applications, as well as in many engineering and medical applications.
  • the present inventor is author or co-author of a number of publications disclosing techniques for self-mode-locking lasers.
  • a paper by Spence, D.E., Kean, P.N. and Sibbett, W. entitled "60-fsec pulse generation from a self-mode-locked Ti .sapphire laser" (Optics Letters, 1991, Vol. 16, No. 1, p. 2) a self-mode-locked large frame Argon-ion laser pumped Ti .sapphire laser was demonstrated which was capable of generating 60-fs pulses at peak powers of 90kW.
  • Self-mode-locking was initiated by tapping one of the laser mirrors of the near-concentric extended resonator.
  • an initial pulse be generated by suitable initiating means within the laser cavity, of sufficient intensity to access a ⁇ 3 -type (or Optical Kerr-type) non-linearity in the laser gain medium or additional intracavity non-linear medium.
  • a sufficient intracavity initial pulse intensity has been established then a focussed beam in the gain medium (or additional non-linear medium) gives rise to self-phase-modulation and self-focussing (optical Kerr-lens) effects.
  • the spectral expansion due to self- phase-modulation and the spatial beam changes due to self- focussing, together with controlled intracavity Group Velocity Dispersion (GVD) lead to substantial pulse shortening and pulse shaping processes.
  • Mode-locked lasers by Oain, S.C. st &]_. (Optical Engineering, 1992, Vol. 31, No. 6, p.1287).
  • the self-mode-locked lasers disclosed in the papers by the present inventor referred to above were Argon-ion laser pumped Ti:sapphire lasers.
  • large frame Argon-ion lasers are expensive, bulky (some metres in length), cumbersome (they require water cooling) and inefficient (about 0.5% efficiency).
  • they can consume of the order of 40kW of electricity, and require a three-phase power supply.
  • they suffer from beam intensity variability arising from electrical ripple in their power supplies and plasma instabilities in the ion-laser tubes. Such variability can give rise to long-term timing jitter.
  • a solution to these problems would be to pump the main laser with a semiconductor laser arrangement (for instance, a high power, broad stripe or multi-stripe laser diode array) rather than the Argon-ion pump.
  • Semiconductor lasers are relatively cheap, comparatively small (a few millimetres in length), easily handled and efficient (about 507. efficiency). Further, they consume relatively small amounts of electricity (typically less than 1 kW) and do not require a three-phase supply. In addition, they suffer less markedly from beam intensity variability. If such a semiconductor laser pump arrangement were combined with a Ti.sapphire or other similar solid-state vibronic laser medium, a cheap and compact all-solid-state laser could be produced.
  • a self-mode-locked, semiconductor laser pumped Nd.YLF laser has also been disclosed in a paper by Malcolm, G.P.A. et al. entitled "Self-mode locking of a diode-pumped Nd.YLF laser” (Optics Letters, Vol. 16, No. 24, p.1967, published on 15th December 1991).
  • the pump power was provided by two high power laser diodes, producing a combined pump output power of about 5W.
  • Self-mode-locking was initiated and maintained not in the gain medium but in a separate non-linear medium in the cavity. Pulse durations of 6ps were reported. This arrangement has the disadvantages firstly that the addition of a separate non-linear medium renders the cavity arrangement more complicated and cumbersome, and secondly that the duration of the pulses is insufficiently short for many purposes.
  • a laser comprising a pump laser for generating a pump laser beam and a main laser arranged to be pumped by the pump laser beam, the beam pointing of the pump laser beam being stable to within ⁇ 30 ⁇ rad, preferably to within ⁇ 10 ⁇ rad, more preferably to within ⁇ 5 ⁇ rad.
  • This aspect of the invention arises from the discovery pursuant to the present invention that strict control of the beam pointing (or directionality) of the pump beam can be important. It has been discovered that if the beam pointing is controlled to within at least ⁇ 30 ⁇ rad, then the gain mode volume in the laser medium in the main laser can be spatially defined with high precision. This in turn can assure that the laser beam generated by the main laser is accurately spatially confined so that the propagation path followed by the intensity pulses in traversing the main laser cavity is accurately reproduced and stable and thus so that the laser cavity frequency is precisely defined. If the cavity frequency is precisely defined, the longitudinal mode separation can be maintained constant. By applying suitable modulation (for example, amplitude modulation), there can thus be coherent communication between the longitudinal modes, so that the modes can be phase coupled over enlarged bandwidths. In this way the spectral broadening necessary for pulse shortening can be enhanced.
  • suitable modulation for example, amplitude modulation
  • a laser comprising a pump laser for generating a pump laser beam and a main laser arranged to be pumped by the pump laser beam, the pump laser beam being within 30%, preferably within 20%, more preferably within 10%, even more preferably within 5% of the diffraction limit.
  • a near-diffraction limited pump beam can be an important measure in ensuring the successful initiation and maintenance of self-mode-locking of the main laser beam. It has been discovered that, unless the pump laser beam performs to within 30% (or less) of the diffraction limit, the pump laser beam may not be confined to a sufficiently small volume in the main laser medium or provide sufficiently good spatial overlap with the main laser beam to permit intensity-induced non-linearities to be accessed by that main beam. However, if the beam is near-diffraction limited, then the power levels in the laser medium can become sufficiently high to access these non-linearities.
  • a closely related aspect of the invention provides a laser comprising a pump laser for generating a pump laser beam and a main laser including a laser medium arranged to be pumped by the pump laser beam, the minimum cross-sectional area of the pump laser beam in the laser medium being less than 3000 ⁇ m 2 , preferably less than lOOO ⁇ m 2 , more preferably less than 300 ⁇ m 2 , even more preferably less than lOO ⁇ m 2 , the pump laser beam confocal parameter being at least 50%, preferably at least 75%, and more preferably being roughly 100% of the operational length of the laser medium.
  • the areas of 3000, 1000 and 300 ⁇ m 2 correspond to beam waists (diameters) of roughly 60, 35 and 20 ⁇ m. Since the cross- sectional area of the beam may vary with time, the values given above would usually be considered as time-averaged values.
  • the confocal parameter is defined as twice the distance from the beam location of minimum cross-sectional area to the beam location of twice that minimum area.
  • the article by Malcolm, G.P.A. ei al. entitled "Self-mode locking of a diode-pumped Nd.YLF laser” (published 15th December 1991), and referred to earlier, discloses a pump beam waist in the gain medium of 270 ⁇ m. A relatively low confocal parameter would obtain due to the type of optics used for focussing.
  • the beam waist for the main laser beam (not the pump beam) is disclosed as being 30 ⁇ m in the additional intra-cavity non-linear medium. It is to be noted that the pump beam does not impinge on this non-linear medium.
  • This aspect of the invention arises from the discovery, alluded to in relation to the preceding aspect of the invention, of the importance of a near-diffraction limited pump beam, especially at relatively low pump output powers.
  • the particular values for the cross-sectional area and confocal parameter limits have been derived from a series of experiments.
  • the confocal parameter specifically, it has been found to be important that the pump beam remains focussed for a significant proportion of the operating length of the laser medium.
  • the minimum cross-sectional area of the pump laser beam in the gain medium itself is preferably less than 3000 ⁇ m 2 , more preferably less than lOOO ⁇ 2 , even more preferably less than 300 ⁇ m 2 , and more preferably still less than lOO ⁇ m 2 . This can ensure that self-mode-locking occurs in the gain medium rather than (or as well as) in the non-linear medium.
  • the advantage of self-mode-locking occurring in the gain medium is that beneficial self-amplitude and gain guiding effects can occur there.
  • a laser comprising a pump laser for generating a pump laser beam and a main laser arranged to be pumped by the pump laser beam, the pump laser being arranged to operate in a single longitudinal mode.
  • This aspect of the invention arises from the discovery pursuant to the present invention that operation of the pump laser in a single longitudinal mode can be an important measure in ensuring the initiation and maintenance of self-mode-locking.
  • mode beating or gain competition
  • the self-mode-locking process which relies on intensity-induced non-linear effects, can be stable so that ultrashort (high coherence) intensity pulses can be reliably and regularly produced.
  • a laser comprising a pump laser for generating a pump laser beam and a main laser arranged to be pumped by the pump laser beam, the intensity of the pump laser beam being stable to within 1%, preferably to within 0.1% of its root mean square value.
  • This aspect of the invention is closely related to the previous, near-diffraction limited pump beam aspect in that it has been found pursuant to the present invention that a pump beam having the intensity stability qualities as aforesaid can reliably produce ultrashort (high coherence) intensity pulses in the main laser. Such intensity stability qualities can also serve to minimize long-term timing jitter.
  • a laser comprising a pump laser, including a radiation source comprising a semiconductor laser arrangement, for generating a pump laser beam, a main laser including a laser medium arranged to be pumped by the pump laser beam to generate a main laser beam, and means for matching the cross-sectional shape, and preferably also the size, of the pump and main laser beams in the laser medium.
  • This aspect of the invention arises from the discovery pursuant to the invention that the cross-sectional shapes of the pump and main laser beams must be closely matched in the laser medium if self-mode-locking is to be successfully initiated and maintained at low intra-cavity power thresholds. It has been found that unless close matching occurs there, sufficiently efficient lasing cannot occur. With a semiconductor laser arrangement as the radiation source, the two beams will not usually have matching shapes; the output beam from a semiconductor diode is usually highly elliptical, whereas the main laser beam is usually circular or near-circular.
  • the matching means comprises means (such as an anamorphic optical device, for example a cylindrical lens) for circularising the beam output by the semiconductor laser arrangement.
  • means such as an anamorphic optical device, for example a cylindrical lens
  • a pump beam may be produced from a semiconductor laser arrangement, of sufficiently high quality to initiate and maintain self-mode-locking, is to injection lock the semiconductor laser or lasers. Injection locking may, for instance, be achieved by a master laser or external cavity arrangement.
  • a laser consisting of an injection-locked semiconductor laser is disclosed in an article by Goldberg, L. £ ⁇ al- entitled "Injection locking of coupled-stripe diode laser arrays" (Appl . Phys. Lett., Vol. 46, No. 3, 1985, p.236). There is no disclosure of the use of this laser as a pump laser.
  • a laser comprising a pump laser for generating a pump laser beam in a pump laser cavity and a main laser arranged to be pumped by the pump laser beam, the pump laser including means for converting the frequency of the pump laser beam, the converting means being mounted within the pump laser cavity.
  • This aspect of the invention arises from the discovery pursuant to the present invention that the quality of the pump beam, and especially its intensity stability, may be insufficient to initiate and maintain self-mode-locking if the frequency conversion means is mounted external to the pump laser cavity, and that an intracavity location is a convenient solution to this problem.
  • Mounting the frequency conversion means intra-cavity has the advantage that the efficiency of the harmonic generation process is enhanced through the higher level of power available inside the cavity rather than outside it.
  • a laser comprising means for generating a laser beam having at least two distinct frequency components, and means for initiating mode-locking of the beam.
  • the laser would usually generate the two or more distinct frequency components simultaneously and preferably in time synchronisation.
  • the distinct frequency components may also be spatially distinct at various locations.
  • This invention is based on the surprising discovery that a laser can be arranged to produce two or more distinct frequency components whilst in a mode-locked (for example, self-mode-locked) configuration. Hitherto, it had only been believed possible to produce distinct frequency components whilst in a continuous wave configuration.
  • Operation at two or more distinct frequencies in a mode-locked configuration has several advantages. For example, in many types of photophysical , photochemical or photobiological investigations it may be useful to have pulses at different frequencies.
  • One particularly important advantage, provided that the two distinct frequency components are temporally synchronised, is that these components (with one or both of the frequencies possibly being frequency converted) can be mixed in a non-linear device to provide a difference frequency which can (and usually will) be quite different from the fundamental frequencies.
  • the short duration, high peak power pulses produced by the mode-locking process can ensure that the difference frequency beam can be generated relatively efficiently by the non-linear device.
  • the main laser medium may include a gain medium and a separate non-linear medium, and, indeed, where the gain medium is not extended (for example, a colour-centre crystal or dye jet) a separate non-linear medium may be essential.
  • the laser medium consists only of gain medium. This can improve the functioning of the self-mode-locking process since self-amplitude-modulation and gain guiding effects (which can only occur in the gain medium) would be exploited to the full. Additionally, removal of the non-linear medium can render the whole laser more compact, especially since this would remove the need for the cavity optics required with the non-linear medium.
  • the invention extends to the laser as aforesaid including means for initiating mode-locking of the main laser beam.
  • means for initiating mode-locking of the main laser beam may in some rather limited circumstances not be necessary; mode-locking may, for instance, in certain circumstances be initiated simply by tapping one of the mirrors of the laser cavity.
  • a laser comprising means for generating a laser beam and means for initiating mode-locking of the laser beam, the initiating means being arranged to initiate an intensity pulse of duration less than 50ps, preferably less than 20ps, more preferably less than lOps, even more preferably less than 5ps.
  • the acousto-optic modulator employed as the initiating means in the Argon-ion pumped laser yielded a relatively slow amplitude (intensity) modulation with a period of approximately 12 ns.
  • the initial pulse is relatively long (of the order of some hundreds of picoseconds to one nanosecond). It has been discovered pursuant to the present invention that, especially in the circumstances of low to moderate pump power (as opposed to the relatively high power of the Argon-ion pump laser), non-linearities in the gain medium (or additional intracavity non-linear medium) may not be adequately accessible by initial pulses of such duration due to their insufficient peak powers.
  • this aspect of the invention arises from the discovery pursuant to the invention that the duration of the initial (or enabling) pulses produced by the initiating means needs to be sufficiently short before self-mode-locking can be successfully initiated and maintained, and that, all other circumstances being equal, the lower the pump laser power, the shorter the duration needs to be.
  • the duration may need to be especially short if the radiation source for the pump laser is a semiconductor laser arrangement of relatively low power (for example, 1 to 3W).
  • the initiating means includes a saturable absorbing medium having an operating thickness less than lOO ⁇ m, preferably less than 75 ⁇ m. It has been discovered pursuant to the present invention that such an absorbing medium is capable of ultrafast (less than 50ps, of the order of 20 to 40 ps) recovery. Such a recovery time leads to the production of an intial pulse having a duration in the 10 to 15ps region.
  • the initiating means comprises a support element having cavities (usually a large number of preferably volumetric micro-cavities or inclusions), and a saturable absorbing medium contained in the cavities.
  • cavities usually a large number of preferably volumetric micro-cavities or inclusions
  • a saturable absorbing medium contained in the cavities.
  • the support element consists of a sol-gel material.
  • sol-gel material is meant a material made by a sol-gel process.
  • a sol-gel material is advantageous because it can be manufactured at temperatures sufficiently low to avoid decomposition of the saturable absorbing medium. High temperatures, by contrast, are incompatible with molecular or microcrystallite stability requirements.
  • a further advantage stemming from the low temperature processing and consequent absence of surface annealing is that the existence of a higher surface density of states can lead to pronounced fast-response non-linear characteristics.
  • the initiating means includes means, preferably an acousto-optic modulator, for modulating a property of the beam, the modulation being effected at a frequency higher than 150MHz, preferably higher than 200MHz and more preferably higher than 250MHz.
  • a modulator can afford a suitably fast response.
  • the depth of modulation is at least 20%, so that a strong discrimination can exist between the continuous and mode-locked operational status.
  • the pump laser beam is itself mode-locked. This can afford a satisfactory alternative means of initiating mode-locking of the main laser beam. It will of course be understood that any of the above described aspects of the invention may be provided in any appropriate combination one with another.
  • Figure 1 illustrates the far-field intensity distribution from a typical semiconductor laser diode
  • Figure 2 is a schematic representation of a first variant of a first embodiment of laser according to the present invention
  • Figure 3 is a schematic representation of a pump laser for use in the first variant
  • Figures 4a and 4b are respectively schematic plan and side view representations of a pump laser for use in a second such variant
  • Figure 5 is a block diagram of a second embodiment of laser according to the present invention.
  • Figure 6 is a more detailed schematic representation of the second embodiment
  • Figure 7 shows two plots illustrating the beam pointing stability of a pump laser for use in the second embodiment
  • Figure 8 demonstrates the characteristics of a saturable absorber suitable for use in the present invention
  • Figure 9 is an embodiment of laser according to the present invention for generating a laser beam having two distinct frequency components.
  • FIG. 10 illustrates the tuning range of this laser.
  • the first embodiment illustrates two different schemes (the first embodiment).
  • Figure 1 illustrates the far-field intensity distribution from a typical broad stripe high-power laser diode 100.
  • the laser beam 102 emitted by the diode is nowhere near its diffraction limit and, as explained previously, is hence unsuitable for creating and sustaining self-mode-locked conditions in a laser medium. Indeed most such diodes yield multiple lobe problems in the far-field.
  • a dual lobe pattern
  • the laser shown in Figure 2 includes a main laser cavity 110 comprising a relatively extended Cr:LiSAF (Chromium:Lithium-Strontium-Aluminium-Fluoride) laser medium 112 (which absorbs light at 450-670 nm and emits at 750 - 1000 nm), optical elements 114 which are, where appropriate, reflecting and transmitting at the appropriate wavelengths of light to allow lasing to occur, an optional aperture 116 for tuning purposes (and to assist gain guiding at higher-power levels), a prism pair 118 to compensate for second and higher order group velocity dispersion effects, and an acousto-optic modulator 120 or other means for initiating the self-mode-locking process.
  • Such means are described in more detail later in the section entitled "Means for initiating self-mode-locking".
  • the main laser cavity 110 is pumped in this embodiment by four MOPA's (Master Oscillator, Power-Amplified diode laser), MOPA 1 to MOPA 4, producing high quality laser beams around 670 nm wavelength, via two PBC's (Polarisation Beam Combiners), PBC 1 and PBC 2 and via focussing elements of the optical elements 114.
  • MOPA's will be described later in more detail in relation to Figure 3.
  • This embodiment of laser functions as follows. Horizontally polarised light is injected into the laser cavity 110 from MOPA 1 and MOPA 3 whilst vertically polarised light is injected from MOPA 2 and MOPA 4.
  • the Cr:LiSAF laser medium 112 has an absorption coefficient which varies significantly in dependence on the polarisation of light, so that the PBC's provide a convenient way of combining light from the sources. Because of the high beam quality of the pump beams entering the main laser cavity the non-linearities in the laser medium can be accessed even though the MOPA's are of relatively low power (0.25 to 0.5W). Self-mode-locking of the laser beam is initiated by means of the acousto-optic modulator 120 and is sustained by means of the high beam quality of the pump beams. Picosecond or even femtosecond duration pulses can be produced which are output via optical element 114c (or 114d if preferred).
  • Cr:LiSAF Chromium:Lithium-Calcium-Aluminium-Fluoride
  • Cr:LiSCAF Chromium:Lithium-Strontium-Calcium-Aluminium- Fluoride
  • the MOPA comprises an AlGalnP (Aluminium-Gallium-Indium-Phosphide) dual-mode slave laser diode 130 (which emits at 670nm), a comparatively low power (5mW), high beam quality, short cavity length semiconductor master laser 132 operating in a single longitudinal mode at 670nm, times 20 magnification microscope objective lenses 134 and 136, which are anti-reflective at 670nm, interposed between the slave and master lasers, a cylindrical lens 138 of 25mm focal length and a uni-directional device 140 interposed between the lenses 134 and 136.
  • AlGalnP AlGalnP (Aluminium-Gallium-Indium-Phosphide) dual-mode slave laser diode 130 (which emits at 670nm), a comparatively low power (5mW), high beam quality, short cavity length semiconductor master laser 132 operating in a single longitudinal mode at 670nm, times 20 magnification microscope objective lenses 134 and 136, which are anti-reflect
  • the master laser 132 is of high beam quality due largely to its short cavity length and low power. It acts to "injection seed" (or injection lock) the dual-mode slave laser diode 130 in such a way that the slave laser produces a single longitudinal mode, high quality output beam which is matched to and controlled by the wavelength of the master laser. In other words, the output beam is both spatially and spectrally enhanced so that it is near-diffraction limited. If the power of the slave laser is 250mW (as is often the case), the output power will typically be on the order of 200mW or higher.
  • the cylindrical lens 138 functions to circularise the output beam, since otherwise the output beam would be highly non-circular (see Figure 1), and hence unsuitable for producing an optimal spatial distribution of gain within the main laser medium.
  • the optimal spatial distribution is provided when the pump beam in the laser medium is matched to the transverse mode of the main laser beam.
  • the laser diode is powered from an electrical power supply via an electrical smoothing unit (not shown) to assist the MOPA in producing a constant intensity output.
  • FIG. 4 A second variant of the first embodiment of laser according to the present invention is now described with particular reference to Figures 4.
  • the basic configuration is the same as that with the first variant (see Figure 2) but, instead of the four MOPA's, four corresponding external-cavity injection locked laser diode pump arrangements are provided.
  • FIGS. 4a and 4b Plan and side views of one of these pump arrangements are shown in Figures 4a and 4b.
  • the arrangement includes an AlGalnP laser diode 142.
  • a portion of the laser diode output is "injected" back into the diode by means of an external cavity 144 comprising a times 20 magnification microscope objective lens 146 which is anti-reflective at 670nm and serves to focus the laser diode output, a 25mm focal length cylindrical lens 148, and a mirror 150 which is highly reflective at 670nm.
  • the spatial beam quality of the pump output beam (which is output via the mirror 150) can be significantly enhanced. In fact, output beams of only 1.2 times the diffraction limit have been produced pursuant to the present invention. However, unlike the first variant (using a master laser), no control of the wavelength of the beam is effected, so that there is no spectral enhancement.
  • the pump laser is itself either actively, passively or self-mode-locked. This is explained later in the section entitled "Means for initiating self-mode-locking" in relation to the third embodiment of initiating means.
  • one way of self-mode-locking the pump laser is to use an intracavity ultrafast saturable absorber.
  • Such an absorber is described in more detail in relation to the first embodiment of initiating means.
  • laser stable beam pointing is assured by use of the injection locking technique (whether a master oscillator or a coupled cavity is utilised).
  • laser diodes in general have higher beam pointing stability than Argon-ion lasers; this of itself can make a significant contribution to the beam pointing stability of the laser.
  • the fact that the laser medium 112 is end-pumped ensures that the advantages of the high beam pointing stability are exploited.
  • the second embodiment of laser according to the present invention is now described with reference to Figures 5 to 7.
  • semiconductor laser diodes of higher power than those discussed in relation to the first embodiment are commercially available, but these only emit at higher wavelengths, for example 810nm (for an AlGaAs laser diode) as compared with 670nm (for an AlGalnP laser diode).
  • the second embodiment utilises such higher power diodes. Pump beam quality is assured by arranging that the diodes pump an intermediate pump laser medium. Compatibility with the absorption spectrum of the pumped, possibly vibronic, main laser medium can be achieved by the use of intracavity frequency doubling in the pump optical cavity.
  • the second embodiment of laser is illustrated in broad terms with reference to Figure 5.
  • a laser diode 200 pumps a pump laser comprising a pump microlaser cavity 202, a microlaser medium 204 and an optional intracavity frequency doubling crystal 206.
  • the pump laser in turn pumps a main laser comprising a main laser cavity 208 and a vibronic laser medium 210.
  • the diode 200 may be AlGaAs at 810nm for pumping a Nd:YAG (Neodymium:Yttrium- Aluminiurn-Garnet) microlaser medium 204 or AlGaAs at 795nm for pumping a Nd:YLF (Neodymium:Yttrium-Lithiurn-Fluoride) microlaser medium.
  • the frequency doubling crystal may be a KTP (Potassium- Titanyl-Phosphate) or LB0 (Lithiu -Triborate) non-linear crystal, and is employed if required to render compatible the emission wavelength of the laser diode 200 and the absorption spectrum of the vibronic medium 210.
  • KTP Potassium- Titanyl-Phosphate
  • LB0 Lithiu -Triborate
  • the frequency doubling crystal is required if, for instance, the vibronic medium 210 is Ti :A1 2 O 3 (Titanium:Sapphire), pumped around 530nm (frequency doubled 1064nm Nd:YAG, or 1053, 1047nm Nd:YLF), or if it is Cr:LiSAF, Cr:LiCAF, or Cr:LiSCAF, pumped around 660nm (frequency doubled 1320nm Nd:YAG or Nd:YLF).
  • the vibronic medium 210 is Ti :A1 2 O 3 (Titanium:Sapphire)
  • pumped around 530nm frequency doubled 1064nm Nd:YAG, or 1053, 1047nm Nd:YLF
  • Cr:LiSAF, Cr:LiCAF, or Cr:LiSCAF pumped around 660nm (frequency doubled 1320nm Nd:YAG or Nd:YLF).
  • the frequency doubling crystal is not required if vibronic media such as Cr 4+ :Forsterite, Cr 4+ :YAG, KC1:T1 (Potassium Chloride:Thallium) or NaCl :0H " colour-centre crystals are employed, since these media have absorption bands which are directly compatible with the output of a Nd:YAG (or Nd:YLF) microlaser.
  • vibronic media such as Cr 4+ :Forsterite, Cr 4+ :YAG, KC1:T1 (Potassium Chloride:Thallium) or NaCl :0H " colour-centre crystals are employed, since these media have absorption bands which are directly compatible with the output of a Nd:YAG (or Nd:YLF) microlaser.
  • Co-doped ions for example Cr ions in a Nd:YAG medium
  • the microlaser is either arranged to operate in a single longitudinal mode or is itself mode-locked.
  • a pair of high power AlGaAs laser diodes 200 pumps a microlaser medium 204 consisting of a Brewster-angled Nd:YAG slab.
  • the pump cavity 202 comprises plane mirrors 212a and 212b which are highly reflecting at 1064nm, optical elements 214a and 214b which are highly reflecting at 1064nm and highly transmitting at 532nm and which have a radius of curvature of -100mm and a separation of 100mm, an LB0 frequency doubling crystal 206, and a unidirectional device 216.
  • the output of the pump cavity 202 is passed to the main laser cavity 208 via optical element 214a.
  • the main optical cavity 208 includes a Ti:sapphire vibronic laser medium 210 and some means, such as an acousto-optic modulator 218, for initiating self-mode-locking of the output laser beam.
  • the laser diode pair 202 is arranged for balanced optical pumping of the Nd:YAG slab by being located on opposite sides of the slab. This permits control of thermal lensing effects in the slab so that the beam pointing stability (beam directionality) from the pump cavity 202 can be ultrastable.
  • the pump cavity 202 is configured as a ring cavity and is rendered unidirectional by the unidirectional device 216. This arrangement ensures that the pump laser beam operates in a single longitudinal mode, by avoiding standing wave effects. As mentioned previously, it is important for the initiation and maintenance of self-mode-locking that the pump laser beam operates in a single longitudinal mode (unless it provides a mode-locked output). This ensures that a good and stable fundamental spectral intensity is available at the output of the pump cavity. Operation in a single longitudinal mode is particularly important if a frequency doubling crystal is used, since the quadratic intensity dependence of the frequency- doubling process has the effect of exacerbating any unwanted variations in the intensity of the fundamental-frequency radiation.
  • operation in a single longitudinal mode can alternatively be achieved in a standing wave cavity with the gain (laser) medium located near one end of the cavity.
  • the frequency doubling crystal 206 could alternatively be mounted outside the pump cavity 202, it is preferably mounted inside the cavity because beam intensity there is generally higher.
  • Figure 7 shows the beam pointing stability achieved in practice with the microlaser described in relation to Figure 6.
  • ⁇ x and ⁇ 2 are the pointing angles in the two orthogonal beam output directions. It can be seen that the beam pointing is stable to within roughly ⁇ 10 ⁇ rad.
  • the microlaser is itself either actively, passively, or self-mode-locked. This is explained later in more detail in the section entitled “Means for initiating self-mode-locking" in relation to the third embodiment of the initiating means.
  • the uni-directional ring cavity can produce a near-diffraction limited pump beam due to the extended length of the cavity and the operation of the cavity in a fundamental transverse mode (that is, a Gaussian beam).
  • a near-diffraction limited pump beam can be of sufficiently small cross-sectional focussed area and high confocal parameter to permit the initiation and maintenance of self-mode-locking.
  • the laser for the synchronous pumping of an external cavity containing non-linear crystals for the purposes of frequency conversion (for example, frequency doubling) or parametric oscillation.
  • Such circumstances might be the provision of doubly or triply resonant cavities for pump, signal and idler waves.
  • high beam pointing stability of the pump laser and operation by the pump laser in a single longitudinal mode may be vital to ensure the reproducibility of beam paths and thus avoid intra-crystal intensity variations.
  • an asymmetrical main laser cavity may be used to reduce the beam waist and mode volume still further.
  • the theory underlying the initiation of self-mode-locking is that a suitably intense initial (or "enabling") pulse must be established so that the non-linear effects of such self-mode-locking phenomena as self-phase- modulation, self-amplitude-modulation and self-focussing (preferably in combination with controlled Group Velocity Dispersion compensation) can lead to substantial pulse shortening (typically into the femto second regime).
  • Most of the practically feasible initiating means generate the Initial pulse by applying a modulation to the laser beam at the cavity frequency (or a multiple or sub-multiple thereof). Such a modulation generates sidebands which can be precisely matched to the longitudinal modes of the cavity.
  • a short duration initial pulse may be generated. Once established, the initial pulse can access the non-linearities in the laser medium if it is suitably intense so that self-mode-locking occurs. It will be understood that the initiation means serves only to generate the initial pulse, and generally not to maintain the self-mode-locking process.
  • a first embodiment of initiating means is a passive, loss modulation scheme consisting of a saturable absorbing medium having an ultrafast (less than 50ps) recovery time so that short pulses can be produced.
  • the absorber in order that the absorbing medium can function under low intra-cavity power levels, the absorber has a low optical density, is relatively weak and is thus readily saturated.
  • One particular medium having these characteristics is a thin slice ( ⁇ 50 ⁇ m thickness) of semiconductor-doped glass (for example, Schott (trade mark) glass colour edge-filter, RG830 containing a CdS ⁇ e. ⁇ microcrystallite dopant). The characteristics of this glass are shown in Figure 8 for various thicknesses of glass ( ⁇ 90 ⁇ m, 400 ⁇ m and 3mm). It can be seen from Figure 8 that a thin ( ⁇ 90 ⁇ m) absorber can permit a substantial frequency tuning range in the laser.
  • a suitable saturable absorber consists of a high optical quality sol-gel material, doped either with semiconductor microcrystallites, silver particles or a fast recovery dye or cocktail of dyes.
  • a sol-gel material is one made by the "sol-gel" process. Suitable materials for such processing are glass or other polymers.
  • a sol-gel material is amorphous and sponge-like, and has a large number of volumetric, pervasive micro-cavities in which the saturable absorbing medium is contained. If a dye dopant is employed, the dye species can be very stable because it is protected in the micro-cavities within the sol-gel glass.
  • Such an absorbing medium permits a wide range of absorber spectral ranges and absorption characteristics to be made available.
  • a further suitable saturable absorbing medium is a low-temperature-grown epitaxial polycrystalline semiconductor structure such as AlGaAs (which is suitable for part of the wavelength range of Ti-Sapphire lasers).
  • a second embodiment of initiating means is an active, amplitude modulation scheme involving acousto-optic modulation (see the Spence et a/L "Regeneratively initiated self-mode-locked Ti:sapphire laser" paper).
  • the acousto-optic modulator is operated at a significantly higher frequency than that disclosed in the Spence et al- paper.
  • a typical frequency is 250MHz.
  • a third embodiment of initiating means is a "synchronous", gain modulation scheme in which the pump laser itself is mode-locked.
  • Mode-locking of the pump laser can be achieved by any suitable mode-locking scheme (including self-mode-locking). Because of the high peak powers of the pulses from the pump laser, the mode-locked pump beam may be easily frequency doubled if appropriate.
  • the mode-locked pump beam is applied to the main, vibronic laser medium to provide a synchronous amplitude modulation in this medium.
  • a sufficiently short ( ⁇ 10ps) initial pulse can evolve even at modest average pump power levels ( ⁇ 2W).
  • ⁇ 2W modest average pump power levels
  • the laser medium 112 in this case is a Ti:sapphire medium and is pumped by a focussed beam from a semiconductor laser diode arrangement 300 at a suitable pump frequency.
  • the prism pair 118 of Figure 2 is replaced by a prism triplet 302 which serves both to provide Group Velocity Dispersion compensation and also to split the beam into two components ("1" and "2") according to their frequency.
  • Pairs of apertures 116-- and 116 2 , for frequency tuning purposes, and optical elements 114d* * and 114d 2 are provided, one each for the respective beam components.
  • Output** and 0utput 2 are the respective output beam components at the two distinct frequencies.
  • self-mode-locking of both beam components is effected with an acousto-opti modulator 120.
  • a variety of means for initiating mode-locking of the beam components may be utilised, such as those described in the preceding section.
  • both components may be self-mode-locked, or, for example, one component may operate in a self-mode-locked femto second regime whilst the other may operate in a purely regeneratively mode-locked pico second regime.
  • the cavity periods for the two frequencies should be arranged to be equal so that the two pulse sequences can be precisely synchronised.
  • the laser shown in Figure 9 with the modification that it is Argon-ion laser rather than laser diode pumped, has been demonstrated as being capable of generating a self-mode-locked laser beam operating simultaneously in two different wavelength regimes, ⁇ -- and ⁇ 2 .
  • the tuning range of this laser over these wavelength regimes has been determined experimentally and is shown in Figure 10.
  • a relatively short tuning range is available for the ⁇ -* regime, around 850nm, whereas a relatively long range is available for the ⁇ 2 regime, between approximately 700 and 800nm.
  • a self-mode-locked vibronic laser for example, Ti :A1 2 0 3 or Cr:LiSAF
  • Ti :A1 2 0 3 or Cr:LiSAF operating simultaneously at 750 nm and 850 nm
  • the corresponding difference-frequency is then 6375 nm (or 6.375 ⁇ m) in the mid-infrared.
  • ⁇ - is taken to be 850nm then as ⁇ 2 is varied from 800nm to 700nm the corresponding mid-infrared tuning range will be 3967-13600nm (that is, 3.967-13.6 ⁇ m).
  • Another attribute of the two-frequency operation of the self-mode-locked vibronic laser is that one of the beam components can be frequency upconverted (for example, doubled) before being difference-frequency mixed in a non-linear crystal.
  • ⁇ -- 950nm and ⁇ 2 frequency-doubled from 750nm to 375nm gives a difference-frequency output at 619nm which is shorter than the short wavelength directly available from either Ti:Al 2 ⁇ 3 or Cr:LiSAF gain media.
  • ⁇ -- is frequency-doubled from 960nm to 480nm and ⁇ 2 is 720nm then the difference-frequency output is at 1396nm which exceeds considerably the long wavelength limit from such media.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
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  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

Un laser comprend un laser de pompage (202, 204, 206) pour générer un faisceau de laser de pompage, ainsi qu'un laser principal (208, 210) servant à générer un faisceau de laser principal et conçu pour être pompé par le faisceau laser de pompage. Diverses mesures sont prises pour permettre l'enclenchement et le maintien du verrouillage en mode automatique du faisceau principal du laser, même à des puissances de laser de pompage dont le seuil est peu élevé. On décrit également un dispositif d'enclenchement du verrouillage de mode du faisceau laser principal.
PCT/GB1992/002026 1992-11-03 1992-11-03 Laser et dispositif d'enclenchement du verrouillage de mode d'un faisceau laser Ceased WO1994010729A1 (fr)

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US11552442B2 (en) 2018-01-25 2023-01-10 Max-Planck-Gesellschaft Zur Foerderung Der Wissenschaften E.V. Device and method for generating laser pulses by Kerr lens based mode locking with a loss-modulation device as a Kerr medium
US12014918B2 (en) 2021-05-24 2024-06-18 Hamamatsu Photonics K.K. Laser-driven light source with electrodeless ignition
US12144072B2 (en) 2022-03-29 2024-11-12 Hamamatsu Photonics K.K. All-optical laser-driven light source with electrodeless ignition
US12156322B2 (en) 2022-12-08 2024-11-26 Hamamatsu Photonics K.K. Inductively coupled plasma light source with switched power supply
US12165856B2 (en) 2022-02-21 2024-12-10 Hamamatsu Photonics K.K. Inductively coupled plasma light source

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6141359A (en) * 1998-01-30 2000-10-31 Lucent Technologies, Inc. Modelocking laser including self-tuning intensity-dependent reflector for self-starting and stable operation
CN1099741C (zh) * 1998-10-06 2003-01-22 中国科学院西安光学精密机械研究所 一种全固体自锁模飞秒激光器
US9609732B2 (en) 2006-03-31 2017-03-28 Energetiq Technology, Inc. Laser-driven light source for generating light from a plasma in an pressurized chamber
WO2009046717A3 (fr) * 2007-10-09 2009-05-28 Univ Danmarks Tekniske Système de lidar cohérent basé sur un laser à semi-conducteur et un amplificateur
US8891069B2 (en) 2007-10-09 2014-11-18 Windar Photonics A/S Coherent LIDAR system based on a semiconductor laser and amplifier
JP2015092184A (ja) * 2007-10-09 2015-05-14 ウインダー フォトニクス エー／エスＷｉｎｄａｒ Ｐｈｏｔｏｎｉｃｓ Ａ／Ｓ 半導体レーザと増幅器とに基づくコヒーレントライダーシステム
US11552442B2 (en) 2018-01-25 2023-01-10 Max-Planck-Gesellschaft Zur Foerderung Der Wissenschaften E.V. Device and method for generating laser pulses by Kerr lens based mode locking with a loss-modulation device as a Kerr medium
US12014918B2 (en) 2021-05-24 2024-06-18 Hamamatsu Photonics K.K. Laser-driven light source with electrodeless ignition
US12176200B2 (en) 2021-05-24 2024-12-24 Hamamatsu Photonics K.K. Laser-driven light source with electrodeless ignition
US12165856B2 (en) 2022-02-21 2024-12-10 Hamamatsu Photonics K.K. Inductively coupled plasma light source
US12144072B2 (en) 2022-03-29 2024-11-12 Hamamatsu Photonics K.K. All-optical laser-driven light source with electrodeless ignition
US12156322B2 (en) 2022-12-08 2024-11-26 Hamamatsu Photonics K.K. Inductively coupled plasma light source with switched power supply

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