Single-frequency pulse laser with high spectral purity

Abstract

The invention discloses a single-frequency pulse laser with high spectral purity. The single-frequency pulse laser comprises a pump light processing system, an active resonant cavity, a gain medium, a light pulse generating device and a nonlinear optical modulator, the gain medium is arranged on an emergent light path of an input mirror of the active resonant cavity; the optical pulse generating device and the nonlinear optical modulator are both placed in the active resonant cavity, the optical pulse generating device is used for adjusting the loss of the active resonant cavity so as to form pulsed light, and the nonlinear optical modulator is used for rapidly increasing the loss difference between different modes in the resonant cavity so as to generate high-spectral-purity single-frequency pulse laser; the nonlinear optical modulator comprises a nonlinear crystal, and the nonlinear crystal is arranged on an optical path between the gain medium and the output mirror of the active resonant cavity. According to the invention, the nonlinear crystal, the optical pulse generation device and the gain medium are utilized to enable a secondary mode to be rapidly annihilated within a short time, so that the single-frequency pulse laser output with high spectral purity is realized.

Description

Technical Field

[0001] The present application relates to the field of laser technology, and more specifically, to a single-frequency pulse laser with high spectral purity. Background Art

[0002] All-solid-state single-frequency pulse lasers have the advantages of narrow line width, low noise, good coherence, high conversion efficiency, and compact structure. They have been widely used in laser weapons, laser radar, space laser communication, coherent optical communication, high-precision spectral measurement, gravitational wave detection and other fields. However, with the continuous deepening of scientific research, these fields have put forward higher requirements for all-solid-state single-frequency pulse lasers, requiring not only higher output power of the laser, but also better spectral purity, that is, the line width of the laser should be as narrow as possible. This is because the higher the output power of the laser, the greater the intensity of the action, and the longer the propagation distance; and the higher the spectral purity, the better the temporal coherence of the output laser, the lower the phase noise, and the smaller the dispersion during the transmission process, which can effectively improve the resolution of the interferometer and the number of channels of optical fiber communication, thereby achieving ultra-high precision, ultra-long distance and extremely weak signal measurement. However, since the gain of a pulsed laser is much greater than its loss, it cannot achieve single-frequency output by operating in a gain saturation state and utilizing the long-term mode competition effect like a continuous laser. Instead, it must continuously increase the proportion of the main mode by increasing the mode loss difference and the number of laser round trips during the laser pulse establishment process to achieve higher spectral purity.

[0003] Therefore, in order to achieve the generation of single-frequency pulsed lasers, people generally use three methods: the first is to extend the practice of single-frequency continuous lasers, inserting standard tools and other mode selection elements in the resonant cavity to increase the mode loss difference, but the mode loss difference brought by this method is not enough to suppress the generation of sub-modes at high power, so that high spectral purity pulsed lasers cannot be obtained, and it will also bring very large linear losses. The second is to use saturated absorbers to passively Q-select modes, and extend the mode competition process of the main mode and the sub-mode by increasing the establishment time of the laser pulse, so as to achieve the generation of single-frequency pulsed lasers. However, the saturated absorber will bleach when the high-power laser is output, and lose the function of increasing the mode competition time, thereby reducing the spectral purity. At the same time, in the experiment, the saturated absorber needs to be combined with the short cavity method to obtain single-frequency output; the third is external seed light injection. This method improves the output power of the single-frequency pulsed laser to a certain extent, but under a certain seed light power, the laser spectral purity will also decrease with the increase of output power. At the same time, this method requires another laser as a seed light, which increases the complexity of the system.

[0004] In general, the above methods are mainly based on linear modulation of loss or laser round trip times when the pulse is established to continuously improve the spectral purity of the output laser. There are problems such as long action time and insufficient introduced mode loss difference, which makes it difficult to obtain high spectral purity for pulse lasers whose generation process itself is very short. Summary of the invention

[0005] The present application provides a single-frequency pulse laser with high spectral purity, which utilizes the property that the nonlinear effect of the nonlinear crystal in the nonlinear optical modulator is proportional to the square of the laser light intensity and the sum frequency process occurring in the nonlinear crystal to enlarge the loss difference between the main mode and the secondary mode, and increase the proportion of the main mode photon number. At the same time, the higher oscillation light peak power brought by the optical pulse generating device and the mode competition of the gain medium cause the secondary mode to be rapidly annihilated in a shorter time, thereby realizing single-frequency pulse laser output with high spectral purity.

[0006] The present application provides a single-frequency pulse laser with high spectral purity, comprising a pump light processing system, an active resonant cavity, a gain medium, an optical pulse generating device and a nonlinear optical modulator;

[0007] The gain medium is arranged on the outgoing light path of the input mirror of the active resonant cavity, and the pump light processing system is used to couple the pump light into the gain medium;

[0008] The optical pulse generator and the nonlinear optical modulator are both placed in the active resonant cavity. The optical pulse generator is used to adjust the loss of the active resonant cavity to form pulsed light, and the nonlinear optical modulator is used to quickly increase the loss difference between different modes in the resonant cavity, thereby generating a single-frequency pulsed laser with high spectral purity.

[0009] The nonlinear optical modulator includes a nonlinear crystal, which is arranged in an optical path between a gain medium and an output mirror of an active resonant cavity.

[0010] Preferably, the nonlinear optical modulator further comprises a precision temperature controller, and the precision temperature controller is used to control the temperature of the nonlinear crystal.

[0011] Preferably, the optical pulse generating device is a Q-switched element.

[0012] Preferably, the active resonant cavity is a traveling wave cavity.

[0013] Preferably, the Q-switching element in the traveling wave cavity includes an acousto-optic crystal, an acousto-optic driver and a first signal generator. The acousto-optic crystal is arranged on an optical path between the gain medium and the first output mirror of the active resonant cavity. The acousto-optic driver is used to provide a radio frequency signal to the acousto-optic crystal. The first signal generator is used to control the acousto-optic driver to turn on and off.

[0014] Preferably, the pump light processing system comprises a pump light providing device and a pump light coupling system both arranged outside the active resonant cavity.

[0015] Preferably, a first plane mirror is provided on the output optical path of the active resonant cavity, and the first plane mirror is coated with a high reflection film for fundamental frequency light and a high transmission film for double frequency light and sum frequency light.

[0016] Preferably, the active resonant cavity is a standing wave cavity.

[0017] Preferably, the pump light processing system is arranged in the active resonant cavity.

[0018] Preferably, relative to the optical pulse generating device, the nonlinear optical modulator is closer to the second output mirror of the active resonant cavity, and a second plane mirror is provided between the optical pulse generating device and the nonlinear optical modulator, and the second plane mirror is coated with a high-transmittance film for fundamental frequency light and a high-reflection film for doubled frequency light and sum frequency light.

[0019] Other features and advantages of the present application will become apparent from the following detailed description of exemplary embodiments of the present application with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the application and, together with the description, serve to explain the principles of the application.

[0021] Figure 1 A structural diagram of a first embodiment of a single-frequency pulsed laser with high spectral purity provided by the present application;

[0022] Figure 2 A structural diagram of the second embodiment of the high spectral purity single-frequency pulse laser provided by the present application

[0023] The following are marked in the figure:

[0024] 1-pump light providing device; 2-pump light coupling system; 21-first plano-convex mirror; 22-second plano-convex mirror; 3-active resonant cavity; 31-third plano-convex mirror; 32-fourth plano-convex mirror; 33-third plane mirror; 34-fourth plane mirror; 35-gain medium; 36-isolator; 4-optical pulse generating device; 41-acousto-optic crystal; 42-acousto-optic drive; 43-first signal generator; 5-nonlinear optical modulator; 51-nonlinear crystal; 52-precision temperature controller; 6-first plane mirror; 7-frequency-doubled light and sum-frequency light; 8-fundamental frequency light; 9-active resonant cavity; 91-fifth plano-convex mirror; 92-fifth plane mirror; 93-polarizer; 94-sixth plane mirror; 95-pump light processing system; 951-gain medium; 952-semiconductor laser array; 10-optical pulse generating device; 101-Pockels cell; 102-quarter wave plate; 103-high voltage drive; 104-signal generator; 5-nonlinear optical modulator; 11-second plane mirror; 12-fundamental frequency light; 13-frequency-doubled light and sum-frequency light. DETAILED DESCRIPTION

[0025] Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. It should be noted that unless otherwise specifically stated, the relative arrangement of components and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present application.

[0026] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the present application, its application, or uses.

[0027] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0028] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0029] The present application provides a single-frequency pulse laser with high spectral purity, which utilizes the property that the nonlinear effect of the nonlinear crystal in the nonlinear optical modulator is proportional to the square of the laser light intensity and the sum frequency process occurring in the nonlinear crystal to enlarge the loss difference between the main mode and the secondary mode, and increase the proportion of the main mode photon number. At the same time, the higher oscillation light peak power brought by the optical pulse generating device and the mode competition of the gain medium cause the secondary mode to be rapidly annihilated in a shorter time, thereby realizing single-frequency pulse laser output with high spectral purity.

[0030] The single-frequency pulse laser with high spectral purity provided in the present application comprises a pump light processing system, an active resonant cavity, a gain medium, an optical pulse generating device and a nonlinear optical modulator.

[0031] The gain medium is arranged on the output optical path of the input mirror of the active resonant cavity, and the pump light processing system is used to couple the pump light into the gain medium. The optical pulse generator and the nonlinear optical modulator are both placed in the active resonant cavity. The optical pulse generator is used to adjust the loss of the active resonant cavity to form pulsed light, and the nonlinear optical modulator is used to quickly increase the loss difference between different modes in the resonant cavity, thereby generating a single-frequency pulsed laser with high spectral purity.

[0032] In the present application, the cavity structure of the active resonant cavity can be a simple two-mirror resonant cavity or a multi-mirror folded cavity. The active resonant cavity can be a standing wave cavity or a traveling wave cavity.

[0033] In the present application, the structure of the gain medium may be a rod, a slab, a disk, a single crystal fiber or an optical fiber.

[0034] As an embodiment, the gain medium is a laser crystal.

[0035] like Figure 1 As shown, the nonlinear optical modulator 5 includes a nonlinear crystal 51 and a precision temperature controller 52. The nonlinear crystal 51 is arranged in the optical path between the gain medium and the output mirror of the active resonant cavity to introduce nonlinear loss and realize stable single-frequency operation of the laser. The precision temperature controller 52 is used to control the temperature of the nonlinear crystal 51. When performing nonlinear frequency conversion, the phase matching condition of the nonlinear crystal 51 is regulated by the precision temperature controller 52, so that the sub-mode can be quickly annihilated in a short time, thereby realizing single-frequency pulse laser output.

[0036] Optionally, the nonlinear crystal 51 includes, but is not limited to, lithium triborate crystal (LBO), barium metaborate (BBO), potassium titanyl phosphate (KTP), and periodically poled crystal.

[0037] As an embodiment, the nonlinear crystal 51 is a lithium triborate crystal with a rod-like structure and a size of 3×3×20 mm. 3 .

[0038] As an embodiment, the optical pulse generating device is a Q-switching element, and the Q-switching method may be acousto-optic Q-switching, electro-optic Q-switching, saturated absorber Q-switching or mechanical structure Q-switching.

[0039] Figure 1 FIG. 1 is a structural diagram of an embodiment of a high spectral purity single-frequency pulse laser of the present application. Figure 1As shown, the first active resonant cavity 3 is a traveling wave cavity, and the first optical pulse generating device 4 is an acousto-optic Q-switching element. The pump light processing system includes a pump light providing device 1 and a pump light coupling system 2, both of which are arranged outside the first active resonant cavity 3. The pump light providing device 1 is used to provide pump light, and the pump light coupling system 2 is used to couple the pump light into the first gain medium 35 of the laser resonant cavity.

[0040] The pump light providing device 1 is a laser, including but not limited to a semiconductor laser, a solid laser, a gas laser, a dye laser and an excimer laser.

[0041] As an embodiment, the pump light coupling system 2 is an end-pump module, including a first plano-convex mirror 21 and a second plano-convex mirror 22. The first plano-convex mirror 21 and the second plano-convex mirror 22 form a telescope system for achieving mode matching between the pump light and the cavity mode of the active resonant cavity.

[0042] Figure 1 In the single-frequency pulse laser with high spectral purity shown, the first active resonant cavity 3 includes a third plano-convex mirror 31, a fourth plano-convex mirror 32, a third plane mirror 33, and a fourth plane mirror 34 arranged in sequence along the optical path, the third plano-convex mirror 31 is the first input mirror, and the fourth plane mirror 34 is the first output mirror. The first gain medium 35 is arranged on the optical path between the third plano-convex mirror 31 and the fourth plano-convex mirror 32, and a isolator 36 is placed in the center of the optical path between the third plane mirror 33 and the fourth plane mirror 34. A first plane mirror 6 is arranged on the output optical path of the first active resonant cavity 3, and the first plane mirror 6 is coated with a high reflection film for fundamental frequency light and a high transmittance film for double frequency light and sum frequency light. The fourth plane mirror 34 is coated with a film with a certain transmittance for fundamental frequency light and a high transmittance film for double frequency light and sum frequency light. Therefore, after part of the fundamental frequency light, all the double frequency light and sum frequency light are transmitted through the fourth plane mirror 34, the fundamental frequency light is reflected by the first plane mirror 6 to form a first light beam 8, thereby obtaining a single-frequency pulse laser with high spectral purity. The frequency-doubled light and the sum frequency light are transmitted through the first plane mirror 6 to form a second light beam 7 .

[0043] Preferably, the isolator 36 is composed of an optical rotator and a half-wave plate.

[0044] As an example, in this embodiment, the first gain medium 35 is a rod-shaped structure with a size of 3×3×20 mm. 3 .

[0045] like Figure 1 As shown, the first optical pulse generating device 4 and the nonlinear optical modulator 5 are arranged on the optical path between the third plane mirror 33 and the fourth plane mirror 34 , the first optical pulse generating device 4 is close to the third plane mirror 33 , and the nonlinear optical modulator 5 is close to the fourth plane mirror 34 .

[0046] As an example, Figure 1As shown, the first optical pulse generating device 4 includes an acousto-optic crystal 41, an acousto-optic driver 42 and a first signal generator 43. The acousto-optic crystal 41 is arranged on the optical path between the third plane mirror 33 and the isolator 36. The acousto-optic driver 42 is used to provide a radio frequency signal to the acousto-optic crystal 41. The first signal generator 43 is used to control the acousto-optic driver 42 to turn on and off.

[0047] like Figure 1 As shown, the nonlinear crystal 51 is arranged on the optical path between the isolator 36 and the fourth plane mirror 34 .

[0048] It can be understood that, in this embodiment, the positions of the first optical pulse generating device 4 and the nonlinear optical modulator 5 can be interchanged.

[0049] Figure 2 FIG. 1 is a structural diagram of another embodiment of a single-frequency pulsed laser with high spectral purity of the present application. Figure 2 As shown, the second active resonant cavity 9 is a standing wave cavity, and the second optical pulse generating device 10 is an electro-optical Q-switching element. The pump light processing system is arranged in the active resonant cavity.

[0050] Specifically, the second active resonant cavity 9 includes a fifth plano-convex mirror 91, a fifth plane mirror 92, a polarizer 93, and a sixth plane mirror 94, which are sequentially arranged along the optical path. The fifth plano-convex mirror 91 is a second input mirror, and the sixth plane mirror 94 is a second output mirror. The fifth plane mirror 92 and the polarizer 93 play a light guiding role.

[0051] like Figure 2 As shown, the pump light processing system is a semiconductor laser array 952, which is arranged on the optical path between the fifth plano-convex mirror 91 and the fifth plane mirror 92. The semiconductor laser array 952 and the second gain medium 951 are packaged together, and the semiconductor laser array 952 surrounds the second gain medium 951 for transverse pumping.

[0052] Compared with the second optical pulse generating device 10, the nonlinear optical modulator 5 is closer to the second output mirror of the second active resonant cavity 9. Figure 2 As shown, the second optical pulse generating device 10 comprises a Pockels cell 101, a quarter wave plate 102, a polarizer 93, an electro-optical driver 103 and a second signal generator 104. The quarter wave plate 102 and the Pockels cell 101 are arranged in sequence on the optical path between the polarizer 93 and the sixth plane mirror 94, the electro-optical driver 103 is used to control the voltage value applied to the Pockels cell 101, and the second signal generator 104 is used to control the electro-optical driver 103 to turn on and off.

[0053] Among them, the polarizer 93 is not only a light guide mirror of the active resonant cavity, but also constitutes the optical part of the electro-optical Q-switched element together with the Pockels cell 101 and the quarter-wave plate 102. The linearly polarized light reflected by the polarizer 93 is reflected by the sixth plane mirror 94 and then passes through the quarter-wave plate 102 and the Pockels cell 101 twice. At this time, the polarization direction of the light beam incident on the polarizer 93 can be regulated by the voltage applied to the Pockels cell 101, so that the Q value of the active resonant cavity can be actively controlled.

[0054] like Figure 2 As shown, a second plane mirror 11 is provided between the second optical pulse generating device 10 and the nonlinear optical modulator 5. The second plane mirror 11 is coated with a high transmittance film for fundamental frequency light and a high reflectance film for double frequency light and sum frequency light. The double frequency light and sum frequency light are reflected by the second plane mirror 11 to form a third light beam 13. The sixth plane mirror 94 is coated with a partial transmittance film for fundamental frequency light. Therefore, the fundamental frequency light is outputted by the sixth plane mirror 94 to form a fourth light beam 12, thereby obtaining a single-frequency pulse laser with high spectral purity.

[0055] Here’s how this application works:

[0056] After the pump light is injected into the active resonant cavity (3 or 9), it generates high-power oscillating light through the gain medium. The resonant cavity will simultaneously contain oscillating fundamental frequency light and non-oscillating double frequency light and sum frequency light. The oscillating light generated by the active resonant cavity (3 or 9) is Q-switched by the optical pulse generator (4 or 10) to form pulsed light. In the process of forming pulsed light, by adjusting the temperature (or angle) of the nonlinear crystal 51 in the nonlinear optical modulator 5 to achieve the phase matching condition, since the nonlinear effect of the nonlinear crystal 51 is proportional to the square of the laser light intensity, the nonlinear loss of the resonant cavity will increase sharply with the establishment of the oscillating light in the resonant cavity. At the same time, the optical frequency conversion occurring in the nonlinear crystal 51 causes a great difference in the loss of photons of different oscillation modes in the resonant cavity. Specifically, when the sub-mode photons and the main mode photons pass through the nonlinear crystal 51 at the same time, since the frequencies of the main mode photons and the sub-mode photons are almost the same (relative to the acceptance bandwidth of the nonlinear crystal 51), the two have the same nonlinear optical conversion efficiency, so the number of photons consumed when the photons of different modes are sum-frequency converted in the nonlinear crystal 51 is the same. However, since the cardinality of the number of main mode photons is greater than that of sub-mode photons, the proportion of the number of main mode photons after the nonlinear crystal sum frequency will further increase, while the proportion of the number of sub-mode photons will gradually decrease, that is, the sum frequency effect between different modes in the nonlinear crystal 51 will further widen the loss difference between the main mode and the sub-mode. For the optical pulse generator (4 or 10), the peak power of the oscillating light in the resonant cavity can reach the kilowatt level or even megawatt within a few nanoseconds. At this time, the nonlinear effect caused is already very strong, thereby further ensuring the efficiency of increasing the loss difference between the main and sub-modes by using the nonlinear effect. At the same time, when the mode loss difference caused by the nonlinear effect causes the proportion of the number of main mode photons to continue to increase, there is also an obvious mode competition effect in the gain medium. For mode competition, the energy extracted by the main mode photons in the gain medium will also be greater than that of the sub-mode photons, thereby further increasing the proportion of the main mode photons. Finally, by utilizing the uniqueness of the nonlinear effect of the nonlinear crystal 51 being proportional to the square of the laser light intensity, combined with the higher peak power of the optical pulse generator, the weaker secondary mode can be rapidly annihilated in a shorter time through the sum frequency process between the main mode and the secondary mode, thereby achieving high spectral purity single-frequency pulse laser output.

[0057] The present application utilizes the characteristics of large mode loss difference and fast response time caused by nonlinear loss, thereby avoiding problems such as low output power of the laser caused by inserting a mode selection element in the resonant cavity, and low spectral purity caused by factors such as ambient temperature and mechanical vibration. It overcomes the problem of low spectral purity when high-power pulsed laser is output, greatly improves the spectral purity and stability of high-power single-frequency pulsed laser, and has high practical value.

[0058] Although some specific embodiments of the present application have been described in detail by way of example, it should be understood by those skilled in the art that the above examples are only for illustration, not for limiting the scope of the present application. It should be understood by those skilled in the art that the above embodiments may be modified without departing from the scope and spirit of the present application. The scope of the present application is defined by the appended claims.

Claims

1. A single-frequency pulsed laser with high spectral purity, characterized in that: It includes a pump light processing system, an active resonant cavity, a gain medium, an optical pulse generating device and a nonlinear optical modulator; The gain medium is arranged on the outgoing light path of the input mirror of the active resonant cavity, and the pump light processing system is used to couple the pump light into the gain medium; The optical pulse generator and the nonlinear optical modulator are both placed in the active resonant cavity, the optical pulse generator is used to adjust the loss of the active resonant cavity to form pulsed light, and the nonlinear optical modulator is used to quickly increase the loss difference between different modes in the resonant cavity, thereby generating a single-frequency pulsed laser with high spectral purity; The nonlinear optical modulator includes a nonlinear crystal, which is arranged on an optical path between the gain medium and an output mirror of the active resonant cavity.

2. The high spectral purity single-frequency pulse laser according to claim 1, characterized in that: The nonlinear optical modulator further comprises a precision temperature controller, and the precision temperature controller is used to control the temperature of the nonlinear crystal.

3. The high spectral purity single-frequency pulse laser according to claim 1, characterized in that: The optical pulse generating device is a Q-switched element.

4. The high spectral purity single-frequency pulse laser according to claim 3, characterized in that: The active resonant cavity is a traveling wave cavity.

5. The high spectral purity single-frequency pulse laser according to claim 4, characterized in that: The Q-switching element in the traveling wave cavity includes an acousto-optic crystal, an acousto-optic driver and a first signal generator. The acousto-optic crystal is arranged on an optical path between the gain medium and the first output mirror of the active resonant cavity. The acousto-optic driver is used to provide a radio frequency signal to the acousto-optic crystal. The first signal generator is used to control the acousto-optic driver to turn on and off.

6. The high spectral purity single-frequency pulse laser according to claim 4, characterized in that: The pump light processing system includes a pump light providing device and a pump light coupling system both arranged outside the active resonant cavity.

7. The high spectral purity single-frequency pulse laser according to claim 4, characterized in that: A first plane mirror is arranged on the output optical path of the active resonant cavity, and the first plane mirror is coated with a high reflection film for fundamental frequency light and a high transmission film for double frequency light and sum frequency light.

8. The high spectral purity single-frequency pulse laser according to claim 3, characterized in that: The active resonant cavity is a standing wave cavity.

9. The high spectral purity single-frequency pulse laser according to claim 8, characterized in that: The pump light processing system is disposed in the active resonant cavity.

10. The high spectral purity single-frequency pulse laser according to claim 8, characterized in that: Relative to the optical pulse generating device, the nonlinear optical modulator is closer to the second output mirror of the active resonant cavity, and a second plane mirror is arranged between the optical pulse generating device and the nonlinear optical modulator, and the second plane mirror is coated with a high transmittance film for fundamental frequency light and a high reflective film for double frequency light and sum frequency light.

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