Common external cavity line width compression system based on transmission grating

Abstract

The invention belongs to the technical field of blue light semiconductor laser, and discloses a common external cavity line width compression system based on a transmission grating, which comprises N blue light modules, a common external cavity module, an output coupling module and an optical fiber focusing output module, the common external cavity module comprises a first cubic optical splitter, a transmission grating and a first reflector; a blue light beam output by any blue light module is divided into reflected light and transmission light after passing through the first cubic optical splitter; the reflected light passes through the transmission grating, then returns along an original light path through the first reflector and is split again at the first cubic optical splitter, one part of light beams return to any blue light module along the original light path, and the other part of light beams are transmitted to the other blue light modules respectively; the laser beam and a light beam returned along an original light path in any blue light module jointly form littman external cavity feedback of any blue light module so as to realize line width compression; according to the invention, high-power and narrow-linewidth laser output can be realized, the expandability of the system is improved, and the structural complexity of the system is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of blue light semiconductor lasers, and more specifically, relates to a common external cavity line width compression system based on a transmission grating. Background Art

[0002] High-brightness blue laser is one of the important light sources for future advanced manufacturing, and has urgent application needs in power batteries, aerospace, power systems, large ship heat exchangers, and thick copper welding in new energy flat wire motors. Through line width compression technology, semiconductor laser output with high power, high power density and high beam quality can be obtained.

[0003] At present, the linewidth compression technology of semiconductor lasers is mainly divided into two categories, namely light injection method and light feedback method. The light feedback method can be divided into internal cavity method and external cavity method. The internal cavity method is mostly used in the manufacturing process of semiconductor lasers. The external cavity method is to couple light into the external cavity, split it by a wavelength selection device, and only reflect the useful components back to the resonant cavity of the laser to achieve specific light feedback. During the reflection process of the external cavity, the laser will produce a certain phase shift, resulting in the phase of the reflected light being different from the phase of the light field in the resonant cavity. Only the mode with a wavelength consistent with or close to the longitudinal mode wavelength of the external cavity feedback can achieve in-phase feedback, thereby gaining an advantage in mode competition.

[0004] The effect of grating external cavity technology on the line width narrowing of semiconductor lasers is mainly reflected in two aspects: the laser output line width is inversely proportional to the square of the photon lifetime in the resonant cavity, which is proportional to the laser cavity length, so the line width is also inversely proportional to the square of the laser cavity length; for grating external cavity semiconductor lasers, the external cavity length is much longer than the internal cavity length of the LD single tube, so the grating external cavity has a significant effect on the line width narrowing of semiconductor lasers. However, the existing solutions based on the combination of grating and FP cavity to achieve narrow line width laser output have the following problems:

[0005] 1. The system structure is complex, including multiple optical isolators, half-wave plates, gratings and FP cavities, etc., and requires precise feedback control equipment to maintain the narrow line width. This not only leads to complex optical path design, difficult operation and high cost, but also makes adjustment and maintenance difficult.

[0006] 2. The system mainly relies on a single power amplifier and a complex feedback system to achieve high-power and narrow-linewidth laser output, lacks modular design, and has poor scalability. The complex optical path and numerous optical components further reduce the efficiency of optical path allocation and limit the increase in output power. Summary of the invention

[0007] In view of the above defects or improvement needs of the prior art, the present invention provides a common external cavity linewidth compression system based on a transmission grating, the purpose of which is to achieve high-power and narrow-linewidth laser output without complex feedback control, improve the scalability of the system, and reduce the complexity of the system structure.

[0008] To achieve the above-mentioned object, according to a first aspect of the present invention, a common external cavity linewidth compression system based on a transmission grating is provided, comprising: N blue light modules, N≥2, a common external cavity module, an output coupling module and a fiber focusing output module; the common external cavity module comprises a first cubic beam splitter, a transmission grating and a first reflector;

[0009] Each blue light module simultaneously outputs a blue light beam of the same wavelength, and the blue light beam output by any blue light module is divided into reflected light and transmitted light after passing through the first cubic beam splitter; the reflected light is diffracted by the transmission grating to generate diffracted light, and the diffracted light is reflected by the first reflector and returns along the original light path, and is split again at the first cubic beam splitter; a part of the light beam after the second splitting returns to any blue light module along the original light path, and another part of the light beam is transmitted to other N-1 blue light modules respectively, and together with the light beam in any blue light module that returns along the original light path constitutes the Littman external cavity feedback of any blue light module to achieve line width compression; the transmitted light of N blue light modules is coupled into a beam of light through the output coupling module, and then output through the optical fiber focusing output module.

[0010] Furthermore, the N blue light modules are arranged in sequence;

[0011] The common external cavity linewidth compression system further includes N-1 second cubic beam splitters corresponding to the remaining N-1 blue light modules except the first blue light module, and used to split the blue light beam output by the corresponding blue light module into reflected light and transmitted light;

[0012] The blue light beams inputted into the common external cavity module by the remaining N-1 blue light modules are reflected lights after passing through the corresponding second cubic beam splitter;

[0013] The transmitted light of the N blue light modules includes the transmitted light of each blue light module after passing through the first cubic beam splitter and the transmitted light of the remaining N-1 blue light modules after passing through their corresponding second cubic beam splitter.

[0014] Furthermore, the blue light beam of the same wavelength output by each blue light module is a P-polarized blue light beam;

[0015] The transmission grating is a grating whose diffraction efficiency for S-polarized light in the blue light band reaches a preset threshold or above, and the diffracted light is +1-order diffracted light;

[0016] The first cubic beam splitter and the second cubic beam splitter are cubic beam splitters that reflect and transmit the P-polarized blue light beam;

[0017] The common external cavity module further comprises a first half-wave plate arranged between the first cubic beam splitter and the transmission grating along the optical path direction, and is used for transforming the polarization state of the blue light beam.

[0018] Furthermore, the blue light beam of the same wavelength output by each blue light module is an S-polarized blue light beam;

[0019] The transmission grating is a grating whose diffraction efficiency for P-polarized light in the blue light band reaches a preset threshold or above, and the diffracted light is +1-order diffracted light;

[0020] The first cubic beam splitter and the second cubic beam splitter are cubic beam splitters that reflect and transmit the S-polarized blue light beam;

[0021] The common external cavity module further comprises a first half-wave plate arranged between the first cubic beam splitter and the transmission grating along the optical path direction, and is used for transforming the polarization state of the blue light beam.

[0022] Furthermore, when N>2, the output coupling module includes a cube polarization beam splitter, N-1 second half-wave plates and second reflectors corresponding one-to-one to the remaining N-1 blue light modules, and a beam combining unit;

[0023] The transmitted light after passing through the second cubic beam splitter undergoes polarization state conversion through the corresponding second half-wave plate, and then undergoes optical path direction change through the corresponding second reflector, and then undergoes beam combining unit to combine N-1 light beams to obtain a combined light beam;

[0024] The combined light beam and the transmitted light after passing through the first cubic beam splitter are coupled into one light beam through the cubic polarization beam splitter, and the coupled light beam is used as the output total light beam.

[0025] Further, when N=2, the output coupling module includes a cube polarization beam splitter, a second half-wave plate and a second reflector;

[0026] The transmitted light beam after passing through the second cubic beam splitter undergoes polarization state conversion through the second half-wave plate, and then changes the direction of the light path through the second reflector, and is coupled into a beam of light with the transmitted light after passing through the first cubic beam splitter through the cubic polarization beam splitter, and the coupled light beam is used as the output total light beam.

[0027] Furthermore, a ratio of reflectivity to transmittance of the first cubic beam splitter and the second cubic beam splitter is 2:8.

[0028] Furthermore, it also includes a third reflector corresponding to the last blue light module;

[0029] The third reflector is used to reflect the transmitted light beam of the blue light beam output by the last blue light module after passing through the common external cavity module and then passing through its corresponding second cubic beam splitter again, so that the reflected light beam passes through its corresponding second cubic beam splitter again.

[0030] Furthermore, each blue light module includes, in sequence along the optical path direction: a blue light single tube array, a fast axis collimator lens group, a slow axis collimator lens group, a 45° reflector lens group and a 45° reflector lens;

[0031] The blue light beam generated by the blue light single tube array passes through the fast axis collimating lens group and the slow axis collimating lens group in sequence to be collimated in the fast axis direction and the slow axis direction; the collimated light beam passes through the 45° reflector group to be shaped into a beam of light, and then passes through the 45° reflector to change the direction of the light path and output the required blue light beam.

[0032] Further, the optical fiber focusing output module includes a focusing lens and an optical fiber connector;

[0033] The light beam output by the output coupling module is focused by the focusing lens and then coupled into the optical fiber connector.

[0034] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0035] (1) The transmission grating-based common external cavity linewidth compression system of the present invention adopts the design of N blue light modules in the common external cavity, and uses only one transmission grating as an optical feedback element. Without increasing the complexity and cost of the system, it can efficiently achieve linewidth compression, improve the utilization rate of the grating and the stability of linewidth compression. By flexibly adjusting the number N of blue light modules, the required power and narrow linewidth laser output can be achieved. This modular design makes it possible for the system to achieve high-power and narrow linewidth laser output without relying on a single power amplifier and a complex feedback system. It has good flexibility and scalability, reduces the complexity of the system structure, improves the efficiency of optical path allocation, and is easier to achieve an increase in output power.

[0036] (2) Compared with single-module feedback, the feedback beam (target longitudinal mode) received by each blue light module not only includes the feedback light of the current blue light module, but also includes the feedback light transmitted to the current blue light module from other blue light modules, which makes the target longitudinal mode have more energy, which is conducive to the target longitudinal mode gaining an advantage in the mode competition, thereby making the optical system more stable.

[0037] (3) The present invention uses a Littman grating external cavity structure. The laser is incident on the transmission grating and undergoes the first diffraction. The diffracted light no longer directly returns to the active area (blue light module), but is incident on the first reflector and reflected back to the transmission grating to undergo the second diffraction. The diffracted light returns to the laser resonant cavity along the original optical path. Due to the two diffractions, compared with the single grating Littrow structure, light of a smaller wavelength range is fed back to the active area, and theoretically a narrower output line width can be obtained.

[0038] (4) Preferably, the third reflector corresponding to the last blue light module is further included, which is used to reflect the transmitted light beam of the blue light beam output by the last blue light module after passing through the common external cavity module and then passing through its corresponding second cube spectrometer, so that the reflected light beam passes through the corresponding second cube spectrometer again to avoid energy leakage.

[0039] In summary, the N blue light modules of the present invention share an external cavity feedback system (common external cavity module) which not only optimizes the beam quality, but also improves the optical efficiency of the grating. The modular design improves the reliability and maintainability of the system, while providing convenience for future power expansion. By increasing the number of modules, the linear increase in output power can be achieved while maintaining optical performance to meet the needs of different application scenarios. Compared with the traditional single power amplifier solution, the present invention can more effectively disperse the heat load and improve the long-term stability and service life of the system. In addition, the multi-module design also provides the possibility of realizing more complex functions (such as multi-wavelength output, fast wavelength switching, etc.), further expanding the application prospects of this technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of a common external cavity linewidth compression system based on a transmission grating according to an embodiment of the present invention.

[0041] Figure 2 It is a schematic diagram of a blue light single tube array arrangement beam combining module according to an embodiment of the present invention.

[0042] Figure 3 1 is a principle diagram of transmission grating characteristics according to an embodiment of the present invention.

[0043] Figure 4 This is a schematic diagram of the first blue light module external cavity feedback path of an embodiment of the present invention.

[0044] Figure 5 Schematic diagram of the external cavity feedback path of the second blue light module according to an embodiment of the present invention.

[0045] Figure 6 Schematic diagram of dual-module output coupling according to an embodiment of the present invention.

[0046] Figure 7Schematic diagram of light focusing output according to an embodiment of the present invention.

[0047] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0048] 1- blue light single tube array, 2- fast axis collimator lens group, 3- slow axis collimator lens group, 4- 45° reflector group, 5- 45° reflector, 6, 7- blue light module, 8- first cube beam splitter, 9- first half-wave plate, 10- transmission grating, 11- first reflector, 12- second cube beam splitter, 13- third reflector, 14- second half-wave plate, 15- second reflector, 16- cube polarization beam splitter, 17- focusing lens, 18- optical fiber connector, 19- blue light module 6 external cavity feedback path, 20- blue light module 7 external cavity feedback path, 21- output coupling module, 22- optical fiber focusing output module. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0050] In the present invention, the terms "first", "second", etc. in the present invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0051] like Figure 1 As shown, the present invention provides a common external cavity linewidth compression system based on transmission grating, which mainly includes: N blue light modules, N≥2, a common external cavity module, an output coupling module 21 and a fiber focusing output module 22;

[0052] Each blue light module is used to output a corresponding blue light beam of the same wavelength. In the embodiment of the present invention, since the adopted transmission grating 10 is a grating with a diffraction efficiency of more than 94% for S light in the blue light band, the blue light beam is a P-polarized blue light beam as an example for description.

[0053] The common external cavity module includes a first cubic beam splitter 8, a first half-wave plate 9, a transmission grating 10 and a first reflector 11;

[0054] Each blue light module simultaneously outputs a blue light beam of the same wavelength. After the blue light beam output by the i-th (i is 1, 2, ... or N) blue light module passes through the first cubic beam splitter 8 in the common external cavity module, the P-polarized blue light beam is divided into reflected light and transmitted light; after the reflected light passes through the first half-wave plate 9 to convert the P-polarized blue light beam into an S-polarized blue light beam, it is diffracted by the transmission grating 10 to generate diffracted light, and then the first-order diffracted light is returned along the original optical path through the first reflector 11, and is split again at the first cubic beam splitter 8; a part of the light beam after splitting again is returned to the i-th blue light module along the original optical path, and the other part of the light beam is transmitted to the remaining N-1 blue light modules respectively, and together with the light beam returned along the original optical path in the current i-th blue light module, it constitutes the Littman external cavity structure feedback of the current i-th blue light module to achieve line width compression;

[0055] The transmitted light of N blue light modules is coupled into a beam of light by the output coupling module 21 and then outputted by the optical fiber focusing output module 22 .

[0056] Specifically, N blue light modules are arranged in sequence; the common external cavity linewidth compression system in the embodiment of the present invention also includes N-1 second cubic beam splitters 12 corresponding to the remaining N-1 blue light modules except the first blue light module, which are used to split the blue light beam output by the corresponding blue light module into reflected light and transmitted light. In the embodiment of the present invention, the first cubic beam splitter 8 and the N-1 second cubic beam splitters 12 corresponding to the remaining N-1 blue light modules are both devices that only reflect and transmit P polarization, wherein reflection: transmission = 2:8.

[0057] In the remaining N−1 blue light modules, the blue light beam input into the common external cavity module is the reflected light after passing through the corresponding second cubic beam splitter 12 .

[0058] Specifically, after the blue light beam output by the first blue light module passes through the first cubic beam splitter 8, the first half-wave plate 9, the transmission grating 10 and the first reflector 11 in the common external cavity module in sequence, the generated first-order diffraction light is returned along the original optical path and split again at the first cubic beam splitter 8; a part of the light beam after splitting again returns to the first blue light module along the original optical path, and the other part of the light beam returns to the remaining N-1 blue light modules along the second cubic beam splitter 12 corresponding to the remaining N-1 blue light modules. After the blue light beam output by a certain blue light module among the remaining N-1 blue light modules passes through the corresponding second cubic beam splitter 12 to be divided into reflected light and transmitted light, the reflected light passes through the first cubic beam splitter 8, the first half-wave plate 9, the transmission grating 10 and the first reflector 11 in the common external cavity module in sequence, and the generated +1-order diffraction light is returned along the original optical path and split again at the first cubic beam splitter 8; a part of the light beam after splitting again is returned to the certain blue light module along the original optical path, and the other part of the light beam is transmitted to the first blue light module and to the remaining N-2 blue light modules along the second cubic beam splitter 12 corresponding to the remaining N-2 blue light modules outside the certain blue light module.

[0059] Specifically, the transmitted light of the N blue light modules includes the transmitted light beam of each blue light module after passing through the first cubic beam splitter 8 and the transmitted light beams of the remaining N−1 blue light modules after passing through their corresponding second cubic beam splitter 12 .

[0060] Preferably, a third reflector 13 corresponding to the last blue light module is further included, which is used to reflect the transmitted light beam of the blue light beam output by the last blue light module after passing through the common external cavity module and then passing through its corresponding second cube spectrometer 12, so that the reflected light beam passes through the corresponding second cube spectrometer 12 again to avoid energy leakage.

[0061] Specifically, when N>2, the output coupling module 21 includes a cube polarization beam splitter 16, N-1 second half-wave plates 14, N-1 reflectors 15, and a beam combining unit; wherein the N-1 second half-wave plates 14 and the second reflectors 15 correspond one-to-one to the remaining N-1 blue light modules;

[0062] The transmitted light beam after passing through the second cubic beam splitter 12 corresponding to the remaining N-1 blue light modules is converted into S-polarized light after passing through the second half-wave plate 14 corresponding to the remaining N-1 blue light modules, and then changes the direction of the light path through the second reflector 15 corresponding to the remaining N-1 blue light modules, and then passes through the beam combining unit to combine the N-1 light beams to obtain S-polarized light after combining, and couples with the transmitted light beam in the P-polarized state after passing through the first cubic beam splitter 8 through the cubic polarization beam splitter 16 to obtain the output total light beam; wherein the cubic polarization beam splitter 16 is a beam splitting device that transmits P light and reflects S light.

[0063] When N=2, no beam combining unit is required, and the output coupling module 21 includes a cube polarization beam splitter 16, a second half-wave plate 14 and a second reflector 15. The cube polarization beam splitter 16 is used to couple the transmitted light beam in the P polarization state after passing through the first cube beam splitter 8 with the light in the S polarization state after the optical path direction is changed by the second reflector 15 to obtain the output total light beam.

[0064] The optical fiber focusing output module 22 includes a focusing lens 17 and an optical fiber connector 18 ; the total light beam output by the output coupling module 21 is focused by the focusing lens 17 and then coupled into the optical fiber connector 18 .

[0065] like Figure 1 and Figure 2 As shown, in the embodiment of the present invention, taking N=2 as an example, among the two blue light modules 6 and 7, the blue light module 6 is the first blue light module, and the blue light module 7 is the second blue light module, and each blue light module includes in sequence along the optical path direction: a blue light single tube array 1, a fast axis collimator lens group 2, a slow axis collimator lens group 3, a 45° reflector group 4 and a 45° reflector 5;

[0066] Among them, the blue light single tube array 1 includes multiple blue light single tubes, each of which is used to generate blue light of required wavelength and power; in the embodiment of the present invention, the blue light single tube array 1 includes 10 blue light single tubes with an output wavelength of about 444nm and an output power of about 3W, and the 10 blue light single tubes are evenly arranged in a line.

[0067] The blue light beam output by the blue light single tube array 1 is collimated in the fast axis direction and the slow axis direction by the fast axis collimator group 2 and the slow axis collimator group 3 in turn to reduce the fast axis and slow axis divergence angles of each light beam; the collimated blue light beam passes through the 45° reflector group 4 to shape each light beam into a beam of light; then the light path direction is changed by the 45° reflector 5 to obtain a 30w blue light beam with a wavelength of 444nm, which is freely output and is a P-polarized blue light beam.

[0068] In the embodiment of the present invention, the transmission grating 10 used is a grating with a diffraction efficiency of more than 94% for S light in the blue light band, that is, the +1 order diffraction efficiency at 444nm is about 94%, and there are only 0th and +1st order diffracted lights, and the 0th and +1st order diffracted lights of the grating exist in both transmission and reflection. Figure 3 As shown, for the grating, when the incident light is in the P polarization state, the 0th order diffraction transmission light accounts for 91% of the total energy, and when the incident light is in the S polarization state, the +1st order diffraction transmission light accounts for 94% of the total energy. Therefore, before the light enters the grating, the first half-wave plate 9 is used to convert the light into the S polarization state, and the first reflector 11 is used to select the +1st order transmission light. In other embodiments, other gratings whose diffraction efficiency for S light in the blue light band reaches a preset threshold or above can also be used.

[0069] like Figure 4 and Figure 6 As shown, the blue light module 6 sends a blue light beam, which is divided into 20% reflected light and 80% transmitted light after passing through the first cubic beam splitter 8; the 20% reflected light is reflected upward, and after passing through the first half-wave plate 9 to convert the P-polarized blue light beam into an S-polarized blue light beam, it is diffracted by the transmission grating 10 to generate +1-order diffracted light, and then the +1-order diffracted light is returned along the original optical path through the first reflector 11, and is split again at the first cubic beam splitter 8; a part of the light beam after splitting again is returned to the blue light single tube array 1 of the blue light module 6 along the original optical path, and together with the light beam of the blue light module 7 that is split again at the first cubic beam splitter 8 and is transmitted to the blue light module 6, constitutes the Littman external cavity structure feedback of the blue light module 6, thereby realizing line width compression. This process is the external cavity feedback of the blue light module 6; the other part of the light beam after splitting again is transmitted downward along the second cubic beam splitter 12 of the blue light module 7 to the blue light single tube array 1 of the blue light module 7. 80% of the transmitted light is incident on the cube polarization beam splitter 16 toward the left, and is subsequently coupled with the light beam from the blue light module 7 that is incident on the cube polarization beam splitter 16 after passing through the second reflector 15 .

[0070] like Figure 5 and Figure 6 As shown, the blue light module 7 sends a blue light beam, which is divided into 20% of reflected light and 80% of transmitted light after passing through the second cubic beam splitter 12; the 20% of reflected light passes through the first cubic beam splitter 8 upwards and is divided into reflected light and transmitted light again, the transmitted light is incident on the cubic polarization beam splitter 16 to the left together with the transmitted light beam of the blue light module 6, and the reflected light passes through the first half-wave plate 9 upwards and changes from the P polarization state to the S polarization state, and then obtains +1-order diffraction light after diffraction by the transmission grating 10, and the +1-order diffraction light is reflected by the first reflector 11, returns along the original light path, and is split again at the first cubic beam splitter 8; a part of the light beam split again at the first cubic beam splitter 8 is sent along the original light path. The optical path returns to the blue light single tube array 1 of the blue light module 7, and together with the light beam of the blue light module 6 that is split again after passing through the first cubic beam splitter 8 and transmitted to the blue light module 7 along the second cubic beam splitter 12 of the blue light module 7, constitutes the Littman external cavity structure feedback of the blue light module 7, thereby achieving line width compression. This process is the external cavity feedback of the blue light module 7; another part of the light beam that is split again at the first cubic beam splitter 8 is transmitted to the right into the blue light module 6, and together with the light beam of the blue light module 6 that is split again after passing through the first cubic beam splitter 8 and returned to the blue light module 6 along the original optical path, constitutes the Littman external cavity structure feedback of the blue light module 6. For details, see Figure 1The blue light module 6 external cavity feedback path 19 and the blue light module 7 external cavity feedback path 20. 80% of the transmitted light passes through the second half-wave plate 14 to the left to transform the P polarization state light beam transmission into the S polarization state, and after being reflected by the second reflector 15, it is coupled with the 80% transmitted light of the blue light module 6 above after passing through the first cubic beam splitter 8 at the cube polarization beam splitter 16 to output the total light beam. In the embodiment of the present invention, Figure 1 , Figure 4 and Figure 5 The black solid arrow in the figure indicates the direction of the output light path, and the arrow filled with diagonal lines indicates the direction of the feedback light path.

[0071] A third reflector 13 is disposed below the second cubic beam splitter 12 to reflect the transmitted light beam that passes through the common external cavity module and then passes through the corresponding second cubic beam splitter 12 again to the second cubic beam splitter 12 to avoid energy leakage.

[0072] like Figure 6 As shown, the light beam passing through the cube polarization beam splitter 16 includes two parts, the first part is the transmitted light beam after passing through the first cube beam splitter 8, and this part of the light beam includes 80% of the transmitted light beam of the blue light module 6 after passing through the first cube beam splitter 8, and the reflected light beam of the blue light module 7 after passing through the second cube beam splitter 12 and passing through the first cube beam splitter 8 again and then to the left; the second part is 80% of the transmitted light beam of the blue light module 7 after passing through the second cube beam splitter 12. Both parts of the light beam are P polarization state transmitted light beams. Since the cube polarization beam splitter 16 reflects the S polarization state light beam and transmits the P polarization state light beam, the 80% transmitted light beam of the blue light module 7 after passing through the second cube beam splitter 12 is transmitted through the second half-wave plate 14 to convert the P polarization state light beam transmission into the S polarization state, and after being reflected by the second reflector 15, it is coupled with the transmitted light beam after passing through the first cube beam splitter 8 at the cube polarization beam splitter 16 to output the total light beam.

[0073] It should be noted that if N>2, the second part of the light beam coupled through the cube polarization splitter 16 includes 80% of the transmitted light beams of the remaining N-1 blue light modules after passing through their corresponding second cube splitter 12. These N-1 transmitted light beams are P-polarized state transmitted light beams. The P-polarized state light beams are transmitted and converted into S-polarized state through the corresponding second half-wave plate 14. After being reflected by the corresponding second reflectors 15, they are combined by the beam combining unit. The combined light beam is coupled with the transmitted light beam of the first part after the first cube splitter 8 at the cube polarization splitter 16 to output the total light beam.

[0074] like Figure 7 As shown, after the beam combination is completed, the light beam is focused by the focusing lens 17 and then coupled into the optical fiber connector 18.

[0075] It should be noted that the above embodiment is described by taking the transmission grating 10 as a grating with a diffraction efficiency of more than 94% for S light in the blue light band, and the blue light beam output by the blue light module is P polarized as an example. In other embodiments, the transmission grating 10 can also be a grating with high diffraction efficiency for P light in the blue light band, and the blue light output by the blue light single tube in each blue light module is an S polarized blue light beam, the first cubic beam splitter 8 and the second cubic beam splitter 12 are cubic beam splitters that reflect and transmit the S polarized blue light beam, and the other components remain unchanged. The polarization characteristics of the blue light output by the blue light module are determined specifically according to the polarization characteristics of the transmission grating 10.

[0076] In addition, whether the first half-wave plate 9 and the second half-wave plate 14 are needed to change the polarization state can be selected according to the characteristics of the cube polarization splitter 16. For example, when the blue light beam output by the blue light module is S-polarized, the cube polarization splitter 16 is a splitter device that transmits S light and reflects P light, and there is no need for a corresponding half-wave plate to change the polarization state.

[0077] The number of blue light tubes in each blue light module and the number of blue light modules are determined according to the actual power line width requirements; and the number, structure and size of other components in the fast-axis collimator lens group, slow-axis collimator lens group and other devices can also be changed as needed to achieve line width compression of multiple modules.

[0078] The transmission grating-based common external cavity linewidth compression system in the embodiment of the present invention adopts the design of N blue light modules in the common external cavity, and uses only one transmission grating as an optical feedback element. Without increasing the complexity and cost of the system, the linewidth compression is efficiently achieved, and the utilization rate of the grating and the stability of the linewidth compression are improved. By flexibly adjusting the number N of blue light modules, the required power and narrow linewidth laser output can be achieved. This modular design makes it possible for the system to achieve high-power and narrow linewidth laser output without relying on a single power amplifier and a complex feedback system. It has good flexibility and scalability, reduces the complexity of the system structure, improves the efficiency of optical path allocation, and is easier to achieve an increase in output power.

[0079] Compared with single-module feedback, the feedback beam (target longitudinal mode) received by each blue light module not only includes the feedback light of the current blue light module, but also includes the feedback light transmitted to the current blue light module from other blue light modules, which makes the target longitudinal mode have more energy, which is beneficial for the target longitudinal mode to gain an advantage in the mode competition, thereby making the optical system more stable.

[0080] The transmission grating is used as an optical feedback element. The high-efficiency diffraction characteristics of the transmission grating enable the system to obtain a narrower laser linewidth and do not require complex feedback control equipment, thereby simplifying the system design and operation. This design is closer to the classic external cavity diode laser (ECDL) structure, which is conducive to achieving a wider wavelength tuning range. Compared with the blazed grating, the transmission grating has lower diffraction loss, higher mechanical stability, and is not easily affected by temperature changes.

[0081] The present invention uses a transmission grating for line width compression. An ideal diffraction grating can be considered to be composed of a group of equally spaced infinitely long and infinitely narrow slits, and the spacing between the slits is d, which is called the grating constant. When a plane wave with a wavelength of λ is incident vertically on the grating, the points on each slit play the role of a secondary wave source; the light emitted from these secondary wave sources propagates in all directions (i.e., spherical waves). Since the slits are infinitely long, only the situation on the plane perpendicular to the slits can be considered, that is, the slits are simplified to a row of points on the plane. Then the light field along a certain direction on the plane is formed by the coherent superposition of the light emitted from each slit. When interference occurs, since the phases of the light emitted from each slit at the interference point are different, they will partially or completely cancel each other out. However, when the optical path difference of the light emitted from two adjacent slits to the interference point is an integer multiple of the wavelength of the light, the two beams of light have the same phase, and interference enhancement will occur. When a plane wave is incident at an angle of θ i At incident light, the grating equation is written as:

[0082] d(sinθ m +sinθ i )=mλ

[0083] Where d is the slit spacing, i.e., the grating constant, and m is an integer with values ​​of 0, ±1, ±2, ...; θ m is the grating diffraction angle.

[0084] The diffraction angle of the grating is related to the wavelength of the incident light. The emission spectrum of a semiconductor laser has a certain width, and the laser emitted by the laser contains multiple wavelengths. Therefore, when the laser is incident on the grating, except for the zeroth order, the longer the wavelength of the diffracted light of the same order, the larger the diffraction angle, that is, dispersion occurs. The use of higher-order diffracted light can also bring about greater angular dispersion, but in general, the diffraction efficiency of higher-order diffracted light is very small, which means that there is not enough energy to be fed back into the active area of ​​the laser resonant cavity. Therefore, in the grating external cavity technology, +1 (or -1) order diffracted light is often used. In the embodiment of the present invention, +1 order diffracted light is used.

[0085] The present invention uses a Littman grating external cavity structure. The laser is incident on the transmission grating 10 and undergoes the first diffraction. The +1-order diffraction light no longer directly returns to the active area (blue light module), but is incident on the first reflector 11 and reflected back to the transmission grating 10 for the second diffraction. The +1-order diffraction light diffracted this time returns to the laser resonant cavity along the original optical path. Due to the two diffractions, compared with the single grating Littrow structure, light of a smaller wavelength range is fed back to the active area, and theoretically a narrower output line width can be obtained.

[0086] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A common external cavity linewidth compression system based on transmission grating, characterized in that: include: N blue light modules, N≥2, a common external cavity module, an output coupling module (21) and a fiber focusing output module (22); the common external cavity module comprises a first cubic beam splitter (8), a transmission grating (10) and a first reflector (11); Each blue light module simultaneously outputs a blue light beam of the same wavelength. The blue light beam output by any blue light module is divided into reflected light and transmitted light after passing through the first cubic beam splitter (8); the reflected light is diffracted by the transmission grating (10) to generate diffracted light, and the diffracted light is reflected by the first reflector (11) and returns along the original light path, and is split again at the first cubic beam splitter (8); a part of the light beam after the split again returns along the original light path to any blue light module, and another part of the light beam is transmitted to other N-1 blue light modules respectively, and together with the light beam in any blue light module that returns along the original light path, constitutes the Littman external cavity feedback of any blue light module, so as to achieve line width compression; the transmitted light of the N blue light modules is coupled into a beam of light through the output coupling module (21), and is output through the optical fiber focusing output module (22).

2. The common external cavity linewidth compression system according to claim 1, characterized in that: The N blue light modules are arranged in sequence; The common external cavity linewidth compression system further comprises N-1 second cubic beam splitters (12) corresponding one-to-one to the remaining N-1 blue light modules except the first blue light module, and used for splitting the blue light beam output by the corresponding blue light module into reflected light and transmitted light; The blue light beams inputted into the common external cavity module by the remaining N-1 blue light modules are reflected lights after passing through the corresponding second cubic beam splitter (12); The transmitted light of the N blue light modules includes the transmitted light of each blue light module after passing through the first cubic beam splitter (8) and the transmitted light of the remaining N-1 blue light modules after passing through their corresponding second cubic beam splitter (12).

3. The common external cavity linewidth compression system according to claim 2, characterized in that: The blue light beam of the same wavelength output by each blue light module is a P-polarized blue light beam; The transmission grating (10) is a grating whose diffraction efficiency for S-polarized light in the blue light band reaches above a preset threshold, and the diffracted light is +1-order diffracted light; The first cubic beam splitter (8) and the second cubic beam splitter (12) are cubic beam splitters that reflect and transmit P-polarized blue light beams; The common external cavity module further comprises a first half-wave plate (9) arranged between the first cubic beam splitter (8) and the transmission grating (10) along the optical path direction, and is used for transforming the polarization state of the blue light beam.

4. The common external cavity linewidth compression system according to claim 2, characterized in that: The blue light beam of the same wavelength output by each blue light module is an S-polarized blue light beam; The transmission grating (10) is a grating whose diffraction efficiency for P-polarized light in the blue light band reaches above a preset threshold, and the diffracted light is +1-order diffracted light; The first cubic beam splitter (8) and the second cubic beam splitter (12) are cubic beam splitters that reflect and transmit S-polarized blue light beams; The common external cavity module further comprises a first half-wave plate (9) arranged between the first cubic beam splitter (8) and the transmission grating (10) along the optical path direction, and is used for transforming the polarization state of the blue light beam.

5. The common external cavity linewidth compression system according to any one of claims 2 to 4, characterized in that: When N>2, the output coupling module (21) comprises a cube polarization beam splitter (16), N-1 second half-wave plates (14) and second reflectors (15) corresponding one-to-one to the remaining N-1 blue light modules, and a beam combining unit; The transmitted light after passing through the second cubic beam splitter (12) undergoes polarization state conversion through a corresponding second half-wave plate (14), and then undergoes optical path direction change through a corresponding second reflector (15), and then undergoes beam combining unit to combine N-1 light beams to obtain a combined light beam; The combined light beam and the transmitted light after passing through the first cubic beam splitter (8) are coupled into one light beam through the cubic polarization beam splitter (16), and the coupled light beam serves as the output total light beam.

6. The common external cavity linewidth compression system according to any one of claims 2 to 4, characterized in that: When N=2, the output coupling module (21) comprises a cube polarization beam splitter (16), a second half-wave plate (14) and a second reflector (15); The transmitted light beam after passing through the second cubic beam splitter (12) undergoes polarization state conversion through the second half-wave plate (14), and then changes the direction of the light path through the second reflector (15), and is coupled with the transmitted light after passing through the first cubic beam splitter (8) through the cubic polarization beam splitter (16) to form a beam of light, and the coupled light beam serves as the output total light beam.

7. The common external cavity linewidth compression system according to any one of claims 2 to 4, characterized in that: The ratio of the reflectivity to the transmittance of the first cubic beam splitter (8) and the second cubic beam splitter (12) is 2:

8.

8. The common external cavity linewidth compression system according to any one of claims 2 to 4, characterized in that: It also includes a third reflector (13) corresponding to the last blue light module; The third reflector (13) is used to reflect the transmitted light beam of the blue light beam output by the last blue light module after passing through the common external cavity module and then passing through its corresponding second cubic beam splitter (12), so that the reflected light beam passes through its corresponding second cubic beam splitter (12) again.

9. The common external cavity linewidth compression system according to any one of claims 1 to 4, characterized in that: Each blue light module comprises, in sequence along the optical path direction: a blue light single tube array (1), a fast axis collimating lens group (2), a slow axis collimating lens group (3), a 45° reflector group (4) and a 45° reflector (5); The blue light beam generated by the blue light single tube array (1) passes through the fast axis collimating lens group (2) and the slow axis collimating lens group (3) in sequence to be collimated in the fast axis direction and the slow axis direction; the collimated light beam passes through the 45° reflector group (4) to be shaped into a beam of light, and then passes through the 45° reflector (5) to change the direction of the light path, and then outputs the required blue light beam.

10. The common external cavity linewidth compression system according to any one of claims 1 to 4, characterized in that: The optical fiber focusing output module (22) comprises a focusing lens (17) and an optical fiber connector (18); The light beam output by the output coupling module (21) is focused by the focusing lens (17) and then coupled into the optical fiber connector (18).

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