Semiconductor laser device capable of achieving frequency self-locking

Abstract

Disclosed is a semiconductor laser device capable of achieving frequency self-locking, comprising a seed light source, a collimating lens, a polarizing beam splitter, a Faraday rotator and a birefringent FP cavity which are sequentially arranged in the direction of an optical path. According to the present invention, the Faraday rotator is used in combination with the polarizing beam splitter to block the light directly reflected by a first surface cavity mirror, and additionally, by combining with the birefringent FP cavity, the energy injected into an optical cavity can be as high as nearly 100%; in this way, the generation of nonlinear effects in the cavity is facilitated, and the energy fed back to the light source can be controlled to be relatively low.

Description

Semiconductor laser device capable of self-locking frequency

[0001] This invention claims priority to Chinese patent application No. 202311696167.1, filed with the Patent Office of China on December 12, 2023, and entitled “Semiconductor laser device capable of achieving frequency self-locking”. The entire contents of this application are incorporated herein by reference. Technical Field

[0002] The present invention belongs to the field of laser technology, and in particular relates to a semiconductor laser device capable of realizing frequency self-locking. Background Art

[0003] External cavity feedback based on a high-quality etalon (standard) or Fabry-Perot (FP) cavity is an effective approach to achieving ultra-narrow linewidth semiconductor lasers. Typically, etalon or FP cavities require incident light perpendicular to the reflecting surface to effectively excite the fundamental mode, minimizing losses and competition with higher-order transverse modes. This presents a challenge: the first-surface mirror reflective surface of the etalon or FP cavity produces strong reflections that inject into the semiconductor laser, competing with the light emitted from the cavity. However, the desired effect is to feed the energy emitted from the cavity back into the semiconductor laser, as this is the only way to exploit the narrowband filtering effect of the cavity to narrow the laser linewidth. Therefore, for external cavity narrow-linewidth lasers with this architecture, eliminating interference from the first-surface mirror reflective light is a key issue.

[0004] The disclosed Chinese patent application CN102709811A utilizes a method based on a polarizing prism c and a quarter-wave plate. When linearly polarized light passes through the first quarter-wave plate d, it becomes circularly polarized. If the center wavelength of the light wave does not match the transmission wavelength of the FP etalon e, the circularly polarized light will be reflected by the first end face of the FP etalon e. After passing through the first quarter-wave plate d again, it will become linearly polarized light with a polarization direction perpendicular to the original direction. As a result, after passing through the polarizing prism c, the light path is altered by 90 degrees and will not be injected into the semiconductor laser a. At this time, there is no light output; if the center wavelength of the light wave is aligned with the transmission wavelength of the FP etalon e, the circularly polarized light passes through the FP etalon e, passes through the second 1 / 4 wave plate f, and becomes linearly polarized light perpendicular to the original polarization direction. It then reflects after passing through the front cavity surface reflector g and returns along the original optical path. After passing through the second 1 / 4 wave plate f, the FP etalon e, and the first 1 / 4 wave plate d multiple times, it becomes linearly polarized light with the original vibration direction consistent. After passing through the polarizing prism c and the collimating lens b, it is injected into the semiconductor laser a, which can produce an ultra-narrow laser linewidth effect. The disadvantages of using this solution include at least:

[0005] (1) The light passing through the first quarter wave plate is circularly polarized light. If the optical cavity has a birefringence effect, for example, it has resonance modes with horizontal and vertical polarization directions, then only half of the incident light energy can be coupled to each polarization resonance mode. The polarization decomposition diagram of light through the PBS and wave plate is shown in Figure 1. The formula is derived as shown in Formula (1). (1)

[0006] Among them, the polarization after passing through PBS is along the horizontal direction , decomposed into the wave plate optical axis and , is the incident light field intensity. If the resonant mode of the birefringent cavity is also along the 45-degree direction, that is, and , then at most only half the energy The optical cavity resonant modes of a specific polarization are coupled respectively.

[0007] (2) The energy fed back from the cavity to the laser chip may be too high. For an FP cavity with the same reflectivity on both cavity surfaces, ideally, 50% of the energy coupled to the incident cavity is emitted from the first cavity surface and is linearly polarized light. After passing through the 1 / 4 wave plate, it becomes circularly polarized light, and half of the energy is transmitted through the PBS again. Then, after the round trip, 1 / 2×1 / 2×1 / 2=1 / 8 of the energy of the light emitted from the laser is fed back into the laser light source. However, the appropriate ratio required for external cavity injection feedback is usually much lower than this value, ranging from a few thousandths to a few percent. This requires further attenuation of the incident laser, such as adding a wave plate before the PBS to produce beam splitting, or inserting a non-polarizing beam splitter or attenuation plate. This further reduces the energy of the incident cavity.

[0008] The above two problems both reduce the energy coupled into the optical cavity, which is very disadvantageous for applications requiring nonlinear effects in the cavity, because nonlinear effects usually require the energy injected into the optical cavity to be as large as possible.

[0009] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art. Summary of the Invention

[0010] The object of the present invention is to provide a semiconductor laser device capable of achieving frequency self-locking, which can simultaneously solve the technical problems of low energy coupled into the optical cavity and high energy of the feedback injected laser light source in the prior art.

[0011] In order to achieve the above object, a specific embodiment of the present invention provides the following technical solutions:

[0012] In one or more embodiments of the present invention, a semiconductor laser device capable of achieving frequency self-locking includes a seed light source, a collimating lens, a polarization beam splitter, a Faraday rotator, and a birefringent FP cavity sequentially arranged along an optical path, wherein:

[0013] A seed light source outputs a seed light beam;

[0014] A collimating lens, for collimating the seed beam;

[0015] Polarization beam splitter, which transmits linearly polarized light of a specific polarization direction of the seed beam;

[0016] Faraday rotator, which rotates the linearly polarized light before outputting it;

[0017] The light directly reflected by the birefringent FP cavity passes through the Faraday rotator again and is reflected by the polarization beam splitter and cannot be fed back to the seed light source. The light coupled to the birefringent FP cavity and then returned is fed back into the seed light source.

[0018] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, a 1 / 2 wave plate is arranged between the polarization beam splitter and the birefringent FP cavity, and the 1 / 2 wave plate is rotated to change the size of θ, where θ is the angle between the polarization direction of the output light of the Faraday rotator and the polarization direction of the resonant mode of the birefringent FP cavity.

[0019] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, 0°<θ≤30°, or 1°≤θ≤10°, or 1°≤θ≤5°, or 1°≤θ≤3°.

[0020] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, the birefringence FP cavity is provided with a nonlinear crystal, or

[0021] The birefringence FP cavity includes a birefringence effect material and a first reflection film and a second reflection film respectively coated on two opposite ends of the birefringence effect material along an optical path direction.

[0022] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, in the birefringent FP cavity, the birefringent FP cavity comprises a first cavity mirror and a second cavity mirror arranged along the direction of the optical path.

[0023] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, in the birefringent FP cavity, the first cavity mirror and / or the second cavity mirror are provided with a birefringent film.

[0024] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, the birefringence FP cavity includes a stress applying device, and the stress applying device acts on the first cavity mirror or the second cavity mirror to generate a birefringence effect.

[0025] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, the birefringent FP cavity includes a wave plate arranged between the first cavity mirror and the second cavity mirror.

[0026] Preferably, the above-mentioned semiconductor laser device capable of achieving frequency self-locking further includes an optical cavity frequency adjustment module, which is an electrically controlled displacement module or a thermally controlled refractive index module, and the electrically controlled displacement module is assembled on at least one optical component of the birefringent FP cavity.

[0027] Preferably, the above-mentioned semiconductor laser device capable of achieving frequency self-locking further comprises a phase shifter disposed between the collimating lens and the polarization beam splitter.

[0028] Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, the seed light source is a semiconductor laser.

[0029] Compared with the existing technology, the present invention uses a Faraday rotator combined with a polarization beam splitter to cut off the light directly reflected by the first cavity mirror. At the same time, combined with the birefringent FP cavity, the energy injected into the optical cavity can be as high as close to 100%, which is conducive to the generation of nonlinear effects in the cavity. At the same time, the energy fed back to the light source can be controlled to be relatively low. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] FIG1 is a schematic diagram of polarization decomposition of light passing through a PBS and a wave plate in a laser of the prior art;

[0032] FIG2 is a schematic diagram of the principle of a laser device according to an embodiment of the present invention;

[0033] FIG3a is a schematic diagram showing the relationship between the energy coupled into the optical cavity and the angle according to one embodiment of the present invention;

[0034] FIG3 b is a schematic diagram showing the relationship between the energy fed back from the optical cavity and the angle according to an embodiment of the present invention;

[0035] FIG4 is a graph showing the relationship between the injection capability of the FP cavity, the feedback energy, and the angle θ according to an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0037] In order to simultaneously overcome the problems faced in the prior art of low energy coupled into the optical cavity, excessive energy fed back into the light source, and the need to meet self-injection locking, in one aspect, the present application adopts a Faraday rotator to replace the 1 / 4 wave plate in the prior art, which not only increases the energy coupled into the optical cavity to nearly 100%, but also cuts off the light directly reflected by the first cavity mirror; in another aspect, the present application adopts a birefringent FP cavity to generate two intrinsic polarization directions in the cavity, and feeds back a portion of the polarized light into the semiconductor laser to achieve self-injection locking, produce an ultra-narrow laser linewidth effect, and at the same time control the amount of energy fed back into the light source.

[0038] Specifically, as shown in FIG2 , a semiconductor laser device capable of achieving frequency self-locking in one embodiment of the present invention includes a seed light source 1, a collimating lens 2, a polarization beam splitter 3, a Faraday rotator 4, and a birefringent FP cavity arranged in sequence along the optical path.

[0039] The seed light source 1 is an optical cavity with gain. A semiconductor laser can be used, which can produce a seed beam with a wide line width. A combination of a gain chip and a filter can also be used. The gain chip has a higher gain for a specific wavelength band, and the filter can select the laser wavelength within the gain spectrum range of the gain chip.

[0040] In one embodiment, the semiconductor laser includes two end faces, one of which can be coated with a reflective film and the other end face can be coated with an anti-reflection film. This not only allows the light generated by the laser to be output as much as possible, but also allows the feedback light to enter the active area of ​​the semiconductor laser as much as possible.

[0041] The collimating lens 2 collimates the light beam of the seed light source so that the light emitted by the seed light source passes through each optical element in the optical path in parallel.

[0042] In one embodiment, both end surfaces of the collimating lens 2 may be coated with an anti-reflection film to reduce reflectivity.

[0043] Polarization beam splitter 3 is used to transmit linearly polarized light of a specific polarization direction in the seed light beam. In the optical path from the seed light source to the birefringent FP cavity, when the linearly polarized light in various directions passes through collimating lens 2 and then passes through polarization beam splitter 3, the P parallel polarization component in the seed light beam is transmitted, while the S perpendicular polarization component is reflected in other directions. In the optical path from the birefringent FP cavity to the seed light source, polarization beam splitter 3 also transmits only the linearly polarized light of a specific polarization direction and feeds it back to the active area of ​​seed light source 1, while simultaneously reflecting light directly reflected by first cavity mirror 51.

[0044] The Faraday rotator 4 rotates the linearly polarized light and outputs the light. The Faraday rotator 4 uses a 45-degree or 45+90N Faraday rotator, where N is an integer greater than or equal to 1.

[0045] A birefringent FP cavity is essentially a high-Q optical cavity. Laser light fed through the FP cavity can achieve self-injection locking, while also narrowing the laser's original linewidth. The birefringent FP cavity can be either a hollow or solid cavity. Hollow FP cavities can be parallel, plano-concave, or concave-concave. Specifically, the birefringent FP cavity comprises a first cavity mirror 51 and a second cavity mirror 52, arranged sequentially along the optical path.

[0046] This application uses a Faraday rotator 4 instead of a quarter wave plate to cut off the light directly emitted by the first cavity mirror 51. When used in conjunction with the polarization beam splitter 3 and the birefringent FP cavity, as shown in Figures 3a and 3b, the light behavior is as follows:

[0047] (1) P-parallel polarized light of the seed beam The light is transmitted through the polarization beam splitter 3 and rotated after passing through the Faraday rotator 4. If the central wavelength of the light wave does not match the transmission wavelength of the birefringent FP cavity, the linearly polarized light is directly reflected by the first cavity mirror 51. After passing through the Faraday rotator 4 for the second time, it is still linearly polarized light, and the polarization is rotated 90 degrees to become vertically polarized light. , and then reflected by the polarization beam splitter 3 and cannot be fed back to the seed light source 1.

[0048] Referring to Figure 3a, assuming that the polarization state of the birefringent FP cavity resonant mode forms an angle θ with the 45-degree direction, the maximum energy that can be coupled into the optical cavity can reach .

[0049] (2) When the light in the birefringent FP cavity is emitted back through the original optical path, it is along the The linearly polarized light is rotated 45 degrees by Faraday again, forming an angle θ with the vertical direction as shown in Figure 3b. A part of the horizontally polarized light can be fed back into the seed light source 1. Assuming that half of the energy is emitted from each end of the optical cavity, the maximum energy of the feedback injected laser is .

[0050] It can be seen from this that the advantage of using a Faraday rotator in this embodiment is that the energy injected into the birefringent cavity can reach a maximum of When θ is relatively small, such as about 3 degrees, the energy injected into the FP cavity can be close to 100%, which is conducive to the generation of nonlinear effects in the cavity, while the energy fed back to the seed light source 1 can still reach about 5%.

[0051] The angle θ can be changed by rotating the optical cavity along the optical axis. In another embodiment, a half-wave plate 6 can be placed between the polarization beam splitter 3 and the birefringent FP cavity. By rotating the half-wave plate 6, the angle θ can be continuously changed. In one embodiment, the half-wave plate 6 is placed between the Faraday rotator 4 and the birefringent FP cavity; in another embodiment, the half-wave plate 6 can also be placed between the polarization beam splitter 3 and the Faraday rotator 4.

[0052] Figure 4 shows the relationship curve between the FP cavity injection energy, the energy injected by the seed light source and θ. It can be seen from the figure that when θ is 0-5 degrees, the corresponding FP cavity injection energy is close to 100%. When θ is 3 degrees, the energy injected by the seed light source is still about 5%; when θ is 10 degrees, the corresponding FP cavity injection energy can reach more than 95%, and the energy injected by the seed light source is close to 20%; when θ is 15 degrees, the corresponding FP cavity injection energy is still more than 90%, and the energy injected by the seed light source is about 25%; when θ is 20 degrees, the corresponding FP cavity injection energy is about 90%, and the energy injected by the seed light source is about 30%; when θ is 25 degrees, the corresponding FP cavity injection energy exceeds 80%, and the energy injected by the seed light source is less than 40%.

[0053] In this technical solution, by changing the polarization direction of the cavity resonant mode relative to the polarization direction of the linearly polarized light after passing through the Faraday rotator, the ratio of cavity coupling injection energy and feedback energy can be continuously varied. A small angle is typically selected to maximize the energy injected into the cavity while still providing a small amount of feedback energy.

[0054] In some embodiments, materials with nonlinear effects can be introduced into the birefringent FP cavity, such as materials that can produce N-fold frequency, combined frequency, or Kerr nonlinearity. While self-injection locking narrows the linewidth of the seed semiconductor laser, the injected laser energy can produce narrow-linewidth lasers of other wavelengths with nonlinear changes through the cavity enhancement effect.

[0055] Preferably, the material with nonlinear effect can be selected from lithium triborate (LBO), barium metaborate (BBO), cesium lithium borate (CLBO), periodically poled lithium niobate (PPLN), periodically poled lithium tantalate (PPSLT), and periodically poled potassium titanyl phosphate (PPKTP).

[0056] In the first embodiment, a birefringent FP cavity is provided with a birefringent film on the first cavity mirror 51 or the second cavity mirror 52. The birefringent film itself exhibits a birefringent effect and is an anisotropic film with birefringent properties. For example, the birefringent film can be coated on a cavity mirror substrate with anisotropic thermal expansion, or the substrate surface can be artificially modified, such as with a metasurface structure, to induce the birefringent film to produce the birefringent effect.

[0057] In the second embodiment, the birefringent FP cavity includes a stress applying device, which acts on the first cavity mirror 51 or the second cavity mirror 52 to generate a birefringence effect.

[0058] The stress-applying device applies mechanical stress to the first cavity mirror or the second cavity mirror, thereby generating a birefringence effect. This birefringence effect can cause a linearly polarized light to output two perpendicular polarization frequencies. In one embodiment, the stress-applying device can be a screw, and the magnitude of the stress can be adjusted by adjusting the tightness of the screw.

[0059] In the third embodiment, the birefringent FP cavity includes a wave plate 53 disposed between the first cavity mirror 51 and the second cavity mirror 52 . The wave plate 53 is preferably a quarter wave plate.

[0060] In the fourth embodiment, the birefringence FP cavity includes a birefringence effect material, and the two opposite ends of the birefringence effect material are respectively coated. The birefringence effect material can be, for example, yttrium vanadate (YVO4).

[0061] In some embodiments, an optical cavity frequency adjustment module is also included to perform frequency modulation on the self-injection locked laser and other narrow linewidth lasers of other wavelengths generated by its nonlinear effect. The optical cavity frequency adjustment module is an electrically controlled displacement module or a thermally controlled refractive index module, and the electrically controlled displacement module is assembled on at least one optical component of the birefringent FP cavity. For example, the electrically controlled displacement module can be installed on the back of the second cavity mirror 52, and of course it can also be installed on the side or front of the second cavity mirror 52. The electrically controlled displacement module can specifically be a piezoelectric ceramic PZT or a voice coil motor. Those skilled in the art will understand that under the control of an electrical signal, the piezoelectric ceramic PZT or the voice coil motor can accurately move the position of the second cavity mirror 52, thereby adjusting the cavity length. In other words, the resonant frequency can be adjusted by changing the cavity length through the electrically controlled displacement module.

[0062] In some embodiments, a phase shifter 7 is further included and is disposed between the collimating lens 2 and the polarization beam splitter 3 .

[0063] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0064] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A semiconductor laser device capable of achieving self-frequency locking, characterized in that: It includes a seed light source, a collimating lens, a polarization beam splitter, a Faraday rotator and a birefringent FP cavity which are sequentially arranged along the optical path direction, wherein: A seed light source outputs a seed light beam; A collimating lens, for collimating the seed beam; A polarization beam splitter transmits linearly polarized light of a specific polarization direction of the seed beam; Faraday rotator, which rotates the linearly polarized light passing through and then outputs it; The light directly reflected by the birefringent FP cavity passes through the Faraday rotator again and is reflected by the polarization beam splitter and cannot be fed back to the seed light source. The light coupled to the birefringent FP cavity and then returned is fed back and injected into the seed light source.

2. The semiconductor laser device capable of achieving self-frequency locking according to claim 1, characterized in that: A 1 / 2 wave plate is arranged between the polarization beam splitter and the birefringent FP cavity. The 1 / 2 wave plate is rotated to change the value of θ, where θ is the angle between the polarization direction of the output light of the Faraday rotator and the polarization direction of the resonant mode of the birefringent FP cavity.

3. The semiconductor laser device capable of self-locking frequency according to claim 2, characterized in that: 0°<θ≤30°, or 1°≤θ≤10°, or 1°≤θ≤5°, or 1°≤θ≤3°.

4. The semiconductor laser device capable of self-locking frequency according to claim 1, characterized in that: The birefringent FP cavity is provided with a nonlinear crystal, or The birefringent FP cavity comprises a birefringent effect material and a first reflective film and a second reflective film respectively coated at two opposite ends of the birefringent effect material along an optical path direction.

5. The semiconductor laser device capable of self-locking frequency according to claim 1, characterized in that: The birefringent FP cavity comprises a first cavity mirror and a second cavity mirror arranged along the optical path direction.

6. The semiconductor laser device capable of self-locking frequency according to claim 5, characterized in that: In the birefringent FP cavity, the first cavity mirror and / or the second cavity mirror are provided with a birefringent film, or The birefringent FP cavity comprises a stress applying device, which acts on the first cavity mirror or the second cavity mirror to generate a birefringent effect.

7. The semiconductor laser device capable of self-locking frequency according to claim 5, characterized in that: The birefringent FP cavity includes a wave plate disposed between the first cavity mirror and the second cavity mirror.

8. The semiconductor laser device capable of self-locking frequency according to claim 1, characterized in that: It also includes an optical cavity frequency adjustment module, which is an electrically controlled displacement module or a thermally controlled refractive index module, and the electrically controlled displacement module is assembled on at least one optical component of the birefringent FP cavity.

9. The semiconductor laser device capable of achieving self-frequency locking according to claim 1, characterized in that: Also included is a phase shifter disposed between the collimating lens and the polarization beam splitter.

10. The semiconductor laser device capable of self-locking frequency according to claim 1, characterized in that: The seed light source is a semiconductor laser.

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