JP7611577B2 - Laser Equipment - Google Patents

Description

The present invention relates to a laser device.

For example, Patent Document 1 describes a Littrow type laser device. Non-Patent Document 1 describes a Littman type laser device. In both of the devices described in Patent Document 1 and Non-Patent Document 1, laser light emitted from a light source is deflected by a deflector and then incident on a diffraction grating, and the light that is diffracted by the diffraction grating and then returns to the light source via the deflector is used to obtain resonant light of the desired wavelength.

JP 2017-207428 A

Takamasa Suzuki, Ryu-ichi Nagai, Osami Sasaki, Samuel Choi. Rapid wavelength scanning based on acousto-optically tuned external-cavity laser diode. Optics Communications 284. pp. 4615-4618. 2011.

In the devices described in Patent Document 1 and Non-Patent Document 1, the light emitted from the deflector is diffracted by a diffraction grating with a fixed grating period before returning to the deflector, resulting in a decrease in the intensity of the light returning to the light source. A diffraction grating with a fixed grating period has a wavelength-dependent diffraction efficiency of up to about 70%, so when the round trip of light is taken into account, at least about 50% of the light that reaches the deflector is lost. In addition, these devices have a complex configuration because they use a diffraction grating that requires alignment adjustment.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a laser device that can suppress a decrease in the intensity of light returning to the light source with a simple configuration.

In order to achieve the above object, the laser device according to the present invention comprises:
A light source that emits laser light;
a deflector that deflects the incident laser light in response to an applied signal and emits the laser light as diffracted light having a wavelength in response to the signal;
a mirror that reflects the diffracted light that is emitted from the deflector and enters the deflector without passing through a diffraction grating along an optical path opposite to that of the diffracted light when the diffracted light enters the deflector ;
an adjustment unit capable of adjusting a position of the mirror in a direction along the optical axis of the diffracted light ,
The diffracted light, which is reflected by the mirror and then returns to the light source via the deflector, is resonated with the laser light and is output as resonant light.

The deflector may be an AOD (Acousto-Optic Deflector).

The present invention makes it possible to suppress a decrease in the intensity of light returning to the light source with a simple configuration.

FIG. 1 is a diagram showing the configuration of a laser device according to an embodiment of the present invention. FIG. 13 is a diagram showing theoretical calculation results of scanning wavelengths by the laser device according to the embodiment, together with results of a conventional example. FIG. 2 is a diagram showing the configuration of a measurement device including the laser device according to the embodiment. FIG. 13 is a diagram showing a configuration of a laser device according to a modified example.

One embodiment of the present invention will be described with reference to the drawings.

The laser device 1 shown in FIG. 1 is configured as an external cavity laser (ECL) and includes a light source 2, a lens 3, a beam splitter (BS) 4, and a wavelength selection unit 5.

The light source 2 emits laser light L containing various wavelengths according to a drive current supplied from a power source (not shown). The light source 2 is composed of, for example, an LD (Laser Diode) with an AR (Anti Reflection) coating. The lens 3 is a collimator lens that converts the laser light L from the light source 2 into parallel light. The BS4 splits the laser light L incident through the lens 3, and emits one of the split lights to the wavelength selection unit 5 and the other split light to the outside.

The wavelength selection unit 5 includes a deflector 50, a driver 51, and a mirror 52.

The laser light L that has passed through the BS4 is incident on the deflector 50. The deflector 50 in this embodiment is an AOD (Acousto-Optic Deflector) that utilizes the acousto-optic (AO) effect. The driver 51 is an ultrasonic driver that applies an ultrasonic signal of frequency f to the deflector 50. The deflector 50 transmits the incident laser light L, deflects it according to the signal applied from the driver 51, and emits it as diffracted light L1 having a wavelength according to the signal. The diffracted light L1 in this embodiment is first-order diffracted light. If the AOD is designed appropriately, the first-order diffracted light can be made to have maximum efficiency.

The crystal structure and refractive index of the AOD change depending on the frequency of the applied acoustic wave. The material of the AOD can be selected arbitrarily depending on the purpose, and is not limited. For example, gallium phosphide, tellurium dioxide, indium phosphide, chalcogenite glass, fused quartz, etc. can be used as the material of the AOD.

The laser light L incident on the deflector 50 (AOD) to which ultrasonic waves of frequency f are applied from the driver 51 is emitted as zero-order light L0 and diffracted light L1 (first-order diffracted light) as shown in FIG. 1. At this time, θ=λ·f/V holds. Here, θ is the angle of the optical axis of the diffracted light L1 with respect to the normal to the emission surface (surface from which light is emitted) of the deflector 50, V is the speed of ultrasonic waves, and λ is the wavelength of light in air. The AOD allows beam deflection and intensity modulation at the same time, and the frequency of the laser beam shifts by the same amount as the frequency f of the applied ultrasonic waves. In other words, by using the deflector 50 (AOD), it is possible to select not only the degree of deflection of the diffracted light L1 but also the wavelength of the diffracted light L1 by adjusting the frequency f of the ultrasonic waves applied from the driver 51.

The mirror 52 is a highly reflective mirror with a reflectance of, for example, 99.8% or more, and is made of a dielectric multilayer mirror or a metal mirror using Al, Au, Ag, or the like. Here, unlike the prior art, no diffraction grating is provided between the deflector 50 and the mirror 52 in the laser device 1. Therefore, the diffracted light L1 from the deflector 50 is incident on the mirror 52 without passing through a diffraction grating.

The mirror 52 is installed so that the diffracted light L1 is perpendicularly incident on its reflecting surface 52a. Therefore, the normal of the reflecting surface 52a is parallel to the optical axis of the diffracted light L1. As a result, the mirror 52 reflects the incident diffracted light L1 along the optical path opposite to that of the diffracted light L1. That is, only the diffracted light L1 having a wavelength (hereinafter referred to as a specific wavelength) that is emitted from the deflector 50 and corresponds to the applied frequency f is reflected by the mirror 52 toward the deflector 50 and returns (feeds back) to the light source 2 via the deflector 50. The diffracted light L1 of the specific wavelength that has returned to the light source 2 in this way resonates with the light of the specific wavelength among the laser light L containing various wavelengths. As a result, the light source 2 outputs resonant light in which the diffracted light L1 and the laser light L resonate. The laser device 1 is equipped with a control unit (not shown) that is composed of a computer that controls the overall operation of the laser device 1. The control unit controls the operation of each of the light source 2 and the driver 51. The laser device 1 can output resonant light of the desired wavelength under the control of the control unit.

In this embodiment, the configuration in which the BS4 is used to output the resonant light to the outside of the laser device 1 is shown as an example, but the configuration for outputting the resonant light is not limited to this example and can be designed as desired. For example, a configuration can be adopted in which an SOA (Semiconductor Optical Amplifier) is used as the light source 2, and the resonant light is output to the outside via a half mirror formed on the end face of the light source 2 opposite the lens 3.

Here, in conventional laser devices constituting an external cavity laser, a diffraction grating with a fixed grating period was always used, based on the fixed idea that a wavelength selection element was necessary in the path that returned the laser light. However, the inventor of this application came to the realization that, from a different perspective, the deflector 50 itself functions as a diffraction grating with wavelength selectivity, and can also be considered as a variable diffraction grating whose period can be controlled by an external signal. This led to the invention of the laser device 1, which does away with the fixed diffraction grating.

The laser device 1 described above includes a light source 2 that emits a laser beam L, a deflector 50, and a mirror 52. The deflector 50 deflects the incident laser beam L in response to an applied signal, and emits the laser beam L as diffracted beam L1 having a wavelength in response to the signal. The mirror 52 reflects the diffracted beam L1 that is emitted from the deflector 50 and enters the mirror 52 without passing through a diffraction grating, following an optical path opposite to that taken when the diffracted beam L1 entered the mirror 52. The laser device 1 outputs resonant beam produced by resonating the diffracted beam L1 that is reflected by the mirror 52 and returns to the light source 2 via the deflector 50, and the laser beam L.
According to this configuration, since there is no need to provide a diffraction grating between the deflector 50 and the mirror 52, the configuration is simple. Furthermore, since the reflectance of the mirror 52 is far superior to that of the diffraction grating, it is possible to suppress a decrease in the intensity of the light returning to the light source 2. In other words, compared to the conventional configuration, it is possible to suppress a decrease in the intensity of the light in the external resonance route that is emitted from the light source 2 and returns to the light source 2 again via the deflector 50 and the mirror 52. More specifically, the laser device 1 provides the following effects (1) to (5).

(1) As an optical element that returns light to the light source 2, the mirror 52, which is less expensive than a diffraction grating, can be used.
(2) Since the decrease in the light intensity in the external resonator can be suppressed as described above, the optical resonance is stabilized and wavelength scanning over a wider band can be realized.
(3) Since the configuration does not require a diffraction grating, which requires time-consuming alignment adjustment, the configuration is simple and mechanically stable.
(4) Since only the light vertically reflected by the mirror 52 is always fed back to the light source 2, the optical resonator length does not change with wavelength scanning, so that favorable optical resonance conditions can be maintained.
(5) The laser device 1 can be regarded as a Littrow type that does not have a fixed diffraction grating, and therefore, it goes without saying that the output is superior to the Littman type, in which the output is limited by the laser light being diffracted twice by the diffraction grating. In addition, the laser device 1 has less loss of light returning to the light source 2 than the conventional Littrow type, and can achieve even higher output.

Figure 2 shows the results of theoretical calculations that reveal the effect of (2) above. In the figure, the "Example" is a configuration equivalent to laser device 1, and the "Conventional Example" is a configuration equivalent to the laser device described in Patent Document 1 (a Littrow type using a fixed diffraction grating) by the same inventor as the present application. These calculation results were obtained with both examples set to the same conditions except for the mirror 52 and the diffraction grating. Looking at Figure 2, it can be seen that the Example can perform wavelength scanning over a wider band than the conventional example.

From here, an example of using the laser device 1 will be described with reference to FIG. 3. The measurement device 100 includes the laser device 1, a BS (Beam Splitter) 6, a reference mirror 7, and a photodetector 8. The measurement device 100 is configured as an OCT (Optical Coherence Tomography) device, and enables imaging of the internal microstructure of a sample 9. This measurement device 100 is configured using the SS-OCT (Swept Source-OCT) method. The sample 9 may be any type of sample, such as biological tissue or an industrial sample, as long as it is an object that can be measured by OCT.

The light output from the laser device 1 (resonant laser light whose wavelength has been scanned as described above) is split by the BS 6 and irradiated onto the sample 9 and the reference mirror 7. From the light irradiated onto the sample 9, reflected light according to the structure returns to the BS 6 as signal light. The signal light and the reference light from the reference mirror 7 are combined again by the BS 6, causing interference, and are detected by the photodetector 8. The photodetector 8 is composed of, for example, a balanced photodetector using a PD (Photo Diode), and is connected to a computer (not shown). This computer performs a Fourier transform on the interference signal detected by the photodetector 8 to obtain an image in the depth direction of the sample 9 and image it. In order to improve the performance (measurement accuracy, measurement time) of this OCT measurement technology, a wavelength scanning with a wide achievable wavelength scanning width and a fast scanning speed is required, and therefore the laser device 1 described above is useful.

The present invention is not limited to the above-described embodiment, modified examples, and drawings. Appropriate modifications (including the deletion of components) may be made to the embodiment without departing from the spirit of the present invention.

(Modification)
The laser device 1 described above may further include an adjustment unit 53 capable of adjusting the position of the mirror 52 in the optical axis direction of the diffracted light L1, as shown in FIG. 4. The adjustment unit 53 is, for example, a piezoelectric ceramic using lead zirconate titanate (PZT). When the wavelength scanning width becomes large, there is a possibility that good optical resonance conditions cannot be realized. In that case, the adjustment unit 53 can finely adjust the position of the mirror 52 in the optical axis direction of the diffracted light L1 in response to the change in wavelength. This makes it easy to control the optical resonator length and the phase control. Since the wavelength of the laser light output by the light source 2 is temperature dependent, the adjustment unit 53 is useful, for example, when the wavelength deviates from the desired wavelength due to the environmental temperature. The configuration of the adjustment unit 52 can be arbitrarily selected, and a piezoelectric element of a type different from that exemplified may be used, or a mechanism for driving the mirror 52 other than a piezoelectric element may be used.

Furthermore, the deflector 50 is not limited to an AOD. The configuration of the deflector 50 can be changed as desired as long as it can deflect the incident laser light L in response to an applied signal and emit the diffracted light L1 having a wavelength in response to the signal. Furthermore, the signal is not limited to a signal whose frequency changes, and may be a signal whose voltage changes.

The laser device 1 may further include a position adjustment mechanism capable of adjusting the position of the mirror 52 so that the diffracted light L1 emitted from the deflector 50 at various deflection angles in response to the application of a signal is perpendicularly incident on and perpendicularly reflected by the mirror 52. In addition, while the above example shows the diffracted light L1 being first-order diffracted light, the diffracted light L1 may be second-order or higher diffracted light. The use of the laser device 1 is not limited to OCT and may be any use.

In the above explanation, explanations of well-known technical matters have been omitted as appropriate to facilitate understanding of the present invention.

This invention is open to various embodiments and modifications without departing from the broad spirit and scope of the invention. Furthermore, the above-described embodiments are intended to explain the invention and do not limit the scope of the invention. In other words, the scope of the invention is indicated by the claims, not the embodiments. Furthermore, various modifications made within the scope of the claims and within the scope of the meaning of the invention equivalent thereto are considered to be within the scope of the invention.

Reference Signs List 1: Laser device, 2: Light source, 3: Lens, 4: BS
5...wavelength selection unit, 50...deflector, 51...driver, 52...mirror, 52a...reflecting surface, 53...adjustment unit, L...laser light, L0...zeroth order light, L1...diffracted light, 100...measuring device, 6...BS, 7...reference mirror, 8...photodetector, 9...sample

Claims (2)

A light source that emits laser light;
a deflector that deflects the incident laser light in response to an applied signal and emits the laser light as diffracted light having a wavelength in response to the signal;
a mirror that reflects the diffracted light that is emitted from the deflector and enters the deflector without passing through a diffraction grating along an optical path opposite to that of the diffracted light when the diffracted light enters the deflector ;
an adjustment unit capable of adjusting a position of the mirror in a direction along the optical axis of the diffracted light ,
outputting resonant light in which the diffracted light, which is reflected by the mirror and then returns to the light source via the deflector, and the laser light resonate with each other;
Laser device. The deflector is an AOD (Acousto-Optic Deflector).
2. The laser device of claim 1.