WO2025124197A1 - Semiconductor laser device capable of achieving frequency self-locking - Google Patents

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

The present invention claims priority to the Chinese patent application with application number 202311696167.1 filed with the Patent Office of China on December 12, 2023, and with invention name “Semiconductor laser device capable of achieving frequency self-locking”. The entire contents of the application are incorporated herein by reference.

Technical Field

The 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

External cavity feedback based on high-quality factor etalon (standard) or FP (Fabry-Perot) optical cavity is an effective way to achieve ultra-narrow linewidth semiconductor lasers. Usually, etalon or FP cavities require that the incident light is perpendicular to the reflection surface to effectively excite the fundamental mode, reduce losses and competition with high-order transverse modes. One difficulty this brings is that the first cavity mirror reflection surface of the etalon or FP cavity produces strong reflection injection into the semiconductor laser, competing with the light emitted from the cavity. What we hope is that the light energy emitted from the cavity is fed back and injected into the semiconductor laser, because only in this way can the narrowband filtering effect of the optical cavity be used to narrow the linewidth of the laser. Therefore, for this type of external cavity narrow linewidth laser with this architecture, how to eliminate the interference of the reflected light from the first cavity mirror with the semiconductor laser is a key issue.

In the disclosed Chinese patent application No. CN102709811A, a method based on polarization prism c and quarter wave plate is used. When linear polarized light passes through the first quarter wave plate d, it becomes circular polarized light. If the center wavelength of the light wave does not match the transmission wavelength of the F-P etalon e, the circular polarized light will be reflected by the first end face of the F-P etalon e, and after passing through the first quarter wave plate d again, it becomes linear polarized light with a polarization direction perpendicular to the original one. Therefore, after passing through the polarization prism c, the light path changes by 90 degrees and is emitted, and will not be injected into the semiconductor laser a. There is no light output at this time; if the central wavelength of the light wave is aligned with the transmission wavelength of the F-P etalon e, the circularly polarized light passes through the F-P etalon e, passes through the second 1/4 wave plate f, becomes linearly polarized light perpendicular to the original polarization direction, and then reflects after passing through the front cavity surface reflector g, returns along the original light path, and passes through the second 1/4 wave plate f, the F-P etalon e and the first 1/4 wave plate d for multiple times, becomes linearly polarized light, and is consistent with the original vibration direction, passes through the polarization prism c and the collimating lens b, and is injected into the semiconductor laser a, which can produce an ultra-narrow laser line width effect. The disadvantages of using this solution include at least:

(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 PBS and wave plate is shown in Figure 1. The formula is derived as shown in Formula (1). (1)

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 Respectively couple the optical cavity resonant mode of a specific polarization.

(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 the two 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 a 1/4 wave plate, it becomes circularly polarized light, and half of the energy is transmitted through the PBS again. Then, for the light emitted from the laser, 1/2×1/2×1/2=1/8 of the energy is fed back to the laser light source after a round trip. 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 percents. This requires the incident laser to be attenuated again, 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.

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

The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention and should not be regarded as an acknowledgment 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

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

In order to achieve the above object, a technical solution provided by a specific embodiment of the present invention is as follows:

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 arranged in sequence along an optical path, 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.

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.

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°.

Preferably, in the above-mentioned semiconductor laser device capable of realizing frequency self-locking, 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 on two opposite ends of the birefringent effect material along an optical path direction.

Preferably, in the above-mentioned semiconductor laser device capable of realizing 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.

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

Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, the birefringence FP cavity comprises 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.

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

Preferably, in the above-mentioned semiconductor laser device capable of achieving frequency self-locking, 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.

Preferably, in the above-mentioned semiconductor laser device capable of realizing frequency self-locking, a phase shifter is further included which is arranged between the collimating lens and the polarization beam splitter.

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

Compared with the prior art, the present invention adopts a Faraday rotator combined with a polarization beam splitter to cut off the light directly reflected by the first cavity mirror, and combines it with a 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

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 creative work.

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;

FIG2 is a schematic diagram of the principle of a laser device in one embodiment of the present invention;

FIG3a is a schematic diagram showing the relationship between the energy coupled into the optical cavity and the angle in one embodiment of the present invention;

FIG3 b is a schematic diagram showing the relationship between the energy fed back from the optical cavity and the angle in one embodiment of the present invention;

FIG. 4 is a graph showing the relationship between the injection capability of the FP cavity, the feedback energy and the angle θ in one embodiment of the present invention.

DETAILED DESCRIPTION

In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in 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 creative work should fall within the scope of protection of the present invention.

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 satisfy 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 can not only increase the energy coupled into the optical cavity to nearly 100%, but also cut 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 a part of the polarized light is fed back 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.

Specifically, as shown in FIG2 , a semiconductor laser device capable of achieving frequency self-locking in one embodiment of the present invention comprises 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.

The seed light source 1 is an optical cavity with gain, and a semiconductor laser can be selected, which can produce a seed beam with a wider 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 band, and the filter can select the laser wavelength within the gain spectrum range of the gain chip.

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.

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 light path in parallel.

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

Polarization beam splitter 3 is used to transmit linear polarized light of a specific polarization direction of the seed light beam. In the optical path from the seed light source to the birefringent FP cavity, when the linear polarized light in various directions passing through the collimating lens 2 passes through the polarization beam splitter 3, the P parallel polarization component in the seed light beam can be transmitted, and the S vertical polarization component is reflected to other directions. In the optical path from the birefringent FP cavity to the seed light source, the polarization beam splitter 3 will also pass only the linear polarized light of a specific polarization direction and feed it back to the active area of the seed light source 1, and at the same time reflect the light directly reflected by the first cavity mirror 51.

The Faraday rotator 4 rotates the linearly polarized light passing through and then outputs it. The Faraday rotator 4 uses a 45 degree or 45+90N Faraday rotator, where N is an integer greater than or equal to 1.

The birefringent FP cavity is essentially a high-Q optical cavity. The laser after feedback from the FP cavity can form self-injection locking and can narrow the original line width of the laser. The birefringent FP cavity can be a hollow FP cavity or a solid FP cavity. The hollow FP cavity can be a parallel cavity, a plano-concave cavity or a concave-concave cavity. Specifically, the birefringent FP cavity includes a first cavity mirror 51 and a second cavity mirror 52 arranged in sequence along the optical path.

The present application adopts a Faraday rotator 4 to replace 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 FIG. 3a and FIG. 3b, the light behavior is as follows:

(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 so as not to be fed back to the seed light source 1.

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 .

(2) When the light in the birefringent FP cavity is emitted again through the original optical path, it follows the The linearly polarized light is rotated 45 degrees by Faraday again, and forms 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 the two ends of the optical cavity are symmetrical and emit half of the energy, the maximum energy of the feedback injected laser is .

It can be seen that the advantage of using the Faraday rotator in this embodiment is that the energy injected into the birefringent optical 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%.

The change of the angle θ can be achieved by changing the rotation of the optical cavity along the optical axis. In another embodiment, a 1/2 wave plate 6 can also be set between the polarization beam splitter 3 and the birefringent FP cavity, and the angle θ can be continuously changed by rotating the 1/2 wave plate 6. In one embodiment, the 1/2 wave plate 6 is set between the Faraday rotator 4 and the birefringent FP cavity; in another embodiment, the 1/2 wave plate 6 can also be set between the polarization beam splitter 3 and the Faraday rotator 4.

FIG4 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%.

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 cavity coupling injection energy ratio and feedback ratio can be continuously changed. A smaller angle is usually selected to maximize the energy injected into the cavity while providing a small amount of feedback energy.

In some embodiments, materials with nonlinear effects can be introduced into the birefringent FP cavity, for example, materials that can produce N-fold frequency, combined frequency or difference frequency, or materials with 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.

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).

In the first embodiment, in the birefringent FP cavity, a birefringent film is disposed on the first cavity mirror 51 or the second cavity mirror 52. The birefringent film itself has a birefringent effect and is an anisotropic film with birefringent properties. For example, a cavity mirror substrate with anisotropic thermal expansion is used for coating, or the surface of the substrate is artificially modified, such as a super surface structure, to make the coating produce a birefringent effect.

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

The stress applying device applies mechanical stress on the first cavity mirror or the second cavity mirror to generate a birefringence effect, which can cause a linear 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.

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

In the fourth embodiment, the birefringent FP cavity includes a birefringent effect material, and two opposite ends of the birefringent effect material are respectively coated. The birefringent effect material may be, for example, yttrium vanadate (YVO ₄ ).

In some embodiments, an optical cavity frequency adjustment module is also included to perform frequency modulation on the self-injection locked laser and other wavelength narrow linewidth lasers 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 can 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.

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

It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description 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 may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims (10)

A semiconductor laser device capable of realizing frequency self-locking, characterized in that it comprises a seed light source, a collimating lens, a polarization beam splitter, a Faraday rotator and a birefringent FP cavity arranged in sequence 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. The semiconductor laser device capable of achieving frequency self-locking according to claim 1 is characterized in that 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, wherein 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. The semiconductor laser device capable of achieving self-frequency locking according to claim 2 is characterized in that 0°＜θ≤30°, or 1°≤θ≤10°, or 1°≤θ≤5°, or 1°≤θ≤3°. The semiconductor laser device capable of realizing frequency self-locking 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 on two opposite ends of the birefringent effect material along an optical path direction. The semiconductor laser device capable of achieving frequency self-locking 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. The semiconductor laser device capable of achieving frequency self-locking according to claim 5, characterized in that in the birefringent FP cavity, the first cavity mirror and/or the second cavity mirror is 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. The semiconductor laser device capable of achieving frequency self-locking according to claim 5, characterized in that the birefringent FP cavity comprises a wave plate arranged between the first cavity mirror and the second cavity mirror. The semiconductor laser device capable of achieving frequency self-locking according to claim 1 is characterized in that it also includes an optical cavity frequency adjustment module, wherein 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. The semiconductor laser device capable of achieving frequency self-locking according to claim 1, further comprising a phase shifter disposed between the collimating lens and the polarization beam splitter. The semiconductor laser device capable of achieving frequency self-locking according to claim 1, characterized in that the seed light source is a semiconductor laser.