All-solid-state laser and longitudinal mode selection method

Abstract

The all-solid-state laser comprises a pumping source, a polarization splitting prism, a lambda / 2 wave plate and an optical annular resonant cavity, a gain medium nonlinear optical crystal is arranged in the optical annular resonant cavity, and pumping light emitted by the pumping source sequentially passes through the polarization splitting prism and the lambda / 2 wave plate and then enters the optical annular resonant cavity. One part of the pump light is absorbed by the gain medium and converted into oscillation laser propagating in the forward direction and the reverse direction, and the pump light which is not absorbed by the gain medium continues to propagate, is reflected and then is focused in the nonlinear optical crystal together with the oscillation laser propagating in the same direction to be subjected to nonlinear optical frequency conversion so as to be inhibited; oscillation laser transmitted reversely is output from the optical annular resonant cavity due to the fact that the oscillation laser is not restrained, the laser runs in a one-way traveling wave mode, and single-longitudinal-mode laser output is achieved. The pump light of the laser system is used as the fundamental frequency light to replace an external light source device to carry out difference frequency or sum frequency with oscillation laser to realize longitudinal mode selection, and the device has the characteristics of low cost, high stability and simple device.

Description

Technical Field

[0001] The invention relates to the technical field of all-solid-state lasers, and in particular to a longitudinal mode selection method of an all-solid-state laser. Background Art

[0002] All-solid-state lasers with single longitudinal mode output are an important laser light source with the characteristics of high power, narrow linewidth, high quality, and low noise. They are widely used in laser radar, precision measurement, laser medical treatment, and optical frequency standards. The commonly used longitudinal mode selection method for all-solid-state lasers is to place a magneto-optical crystal and a half-wave plate in the laser resonant cavity to form a isolator, and use the non-reciprocity of the magneto-optical crystal to force the oscillating laser in the resonant cavity to operate in one direction, eliminating the spatial hole burning effect caused by the standing wave field to achieve the longitudinal mode selection of the laser. The defect of this method is that the magneto-optical crystal has a certain absorption coefficient for the laser, and the longer the wavelength, the more significant the absorption. Especially for the infrared band, the absorption loss of the magneto-optical crystal seriously limits the output power of the laser. Currently commonly used magneto-optical crystals, such as terbium gallium garnet (TGG) and terbium scandium aluminum garnet (TSAG), have such problems.

[0003] Based on the problem of large absorption loss of magneto-optical crystals in the infrared band, another longitudinal mode selection technology has been developed. This method places an additional nonlinear optical crystal in the ring resonator of the laser, and injects laser into the resonator through another laser outside the resonator to match the oscillating laser in the same propagation direction in the resonator in the nonlinear optical crystal to achieve difference frequency or sum frequency, thereby consuming the oscillating laser in the propagation direction to achieve unidirectional traveling wave operation, eliminating the spatial hole burning effect caused by the standing wave field, and realizing the longitudinal mode selection of the laser (Optics Letters 49.2 (2024): 407-410). The advantage of this method is that the absorption loss of nonlinear optical crystals is much smaller than that of magneto-optical crystals, especially in the infrared band. The disadvantage of this method is that an additional laser needs to be set outside the resonator, resulting in high technical cost and complex optical path, which is not conducive to the miniaturization, cost control and stability improvement of all-solid-state lasers. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of complicated devices in the existing nonlinear optical mode selection technology, and propose an all-solid-state laser, which uses the pump light of the laser system as the fundamental frequency light to replace the external light source device and the oscillating laser to perform difference frequency or sum frequency to achieve longitudinal mode selection, and has the characteristics of low cost, high stability and simple device. In addition, a longitudinal mode selection method of the all-solid-state laser is proposed.

[0005] To achieve the purpose of the present invention, the present invention provides an all-solid-state laser, comprising a pump source, a polarization beam splitter prism, a λ / 2 wave plate and an optical ring resonator, wherein the optical ring resonator comprises an incident mirror, a reflector and an output mirror, wherein the incident mirror has high transmittance to the pump light and high reflectance to the oscillation laser, the output mirror has high transmittance to the pump light and the oscillation laser, and the reflector has high reflectance to the pump light and the oscillation laser; a gain medium with polarization light selective absorption characteristics and a nonlinear optical crystal are arranged in the optical ring resonator, the pump light emitted by the pump source is sequentially incident into the optical ring resonator after passing through the polarization beam splitter prism and the λ / 2 wave plate, a part of the pump light is absorbed by the gain medium and converted into oscillation laser light propagating in both forward and reverse directions, the pump light not absorbed by the gain medium continues to propagate and after reflection, is focused together with the oscillation laser light propagating in the same direction in the nonlinear optical crystal for nonlinear optical frequency conversion and is suppressed, and the oscillation laser light propagating in the reverse direction is output from the optical ring resonator because it is not suppressed, so that the laser operates in one direction and realizes single longitudinal mode laser output.

[0006] In this all-solid-state laser, when the forward-propagating pump light and the oscillating laser injected into the nonlinear optical crystal satisfy phase matching or quasi-phase matching in the nonlinear optical crystal, the two produce difference frequency or sum frequency and are consumed, and the reverse-propagating oscillating laser is not suppressed because it does not satisfy phase matching and quasi-phase matching and there is no loss, resulting in a gain difference between the oscillating lasers propagating in the forward and reverse directions.

[0007] The present invention also discloses a method for causing the oscillating laser propagating in the forward and reverse directions to produce a gain difference. Specifically, the polarization angle of the pump light is controlled by rotating the λ / 2 wave plate to adjust the absorption coefficient of the gain medium to the pump light, thereby changing the energy distribution ratio of the pump light between the gain medium and the nonlinear optical crystal, and further adjusting the gain difference produced by the oscillating laser propagating in the forward and reverse directions.

[0008] In the all-solid-state laser of the present invention, phase matching is achieved by adjusting the incident angle of the forward propagating pump light and the oscillating laser or the temperature of the nonlinear optical crystal. In addition, when the nonlinear optical crystal is a periodically polarized nonlinear optical crystal, quasi-phase matching is achieved by the periodically polarized nonlinear optical crystal.

[0009] In addition, the present invention also claims protection for a longitudinal mode selection method for an all-solid-state laser, which is characterized by comprising the following steps: Step 1: construct the aforementioned all-solid-state laser, select an anisotropic gain medium with selective absorption characteristics for polarization direction according to the target laser wavelength, and select a suitable nonlinear optical crystal to be inserted into the resonant cavity according to the laser wavelength and the pump light wavelength; Step 2, by adjusting the angle of the λ / 2 wave plate to change the polarization direction of the pump light, the absorption coefficient of the gain medium to the pump light is reduced, and the power of the pump light absorbed by the nonlinear optical crystal through the gain medium is increased; Step 3, forwardly injecting the pump light and the oscillating laser into the nonlinear optical crystal to achieve phase matching or quasi-phase matching in the nonlinear optical crystal, consuming the pump light and the oscillating laser propagating in the same direction, so that the forward and reverse oscillating lasers produce a gain difference; Step 4: If it is detected that stable single longitudinal mode selection has been achieved, it means that the laser has achieved stable single longitudinal mode operation, otherwise jump to step 2 to continue adjusting the λ / 2 wave plate angle.

[0010] Furthermore, if the nonlinear optical crystal is a periodically polarized nonlinear optical crystal, the inherent characteristics of the periodically polarized nonlinear optical crystal are utilized in step 3 to achieve quasi-phase matching between the forward pump light and the oscillating laser, so that a gain difference is generated between the forward and reverse oscillating lasers; otherwise, the phase matching between the forward pump light and the oscillating laser in the nonlinear optical crystal is completed by adjusting the incident angle of the forward-direction pump light and the oscillating laser incident on the nonlinear optical crystal or the temperature of the nonlinear optical crystal.

[0011] The technical solution of the present invention is to place a nonlinear optical crystal in a ring resonant cavity composed of three or more reflectors, and the gain medium adopts an anisotropic crystal with polarization spectrum selective absorption characteristics. The pump light is polarized by a polarization beam splitter prism and then enters the optical resonant cavity through a λ / 2 wave plate and is focused in the gain medium. The absorption coefficient of the gain medium to the pump light is adjusted by rotating the λ / 2 wave plate to change the polarization angle of the pump light. A portion of the pump light is absorbed by the gain medium and converted into an oscillating laser in the resonant cavity. The unabsorbed pump light is reflected by the cavity mirror and focused together with the oscillating laser in the resonant cavity with the same propagation direction at the center of the nonlinear optical crystal. When the phase matching condition is met, a difference frequency or sum frequency process occurs to generate a third light wave. Since the oscillating laser that meets the phase matching condition is consumed and causes loss, the laser oscillation threshold in this propagation direction is increased. Since the oscillating laser that propagates in the reverse direction does not meet the phase matching condition and has no loss, the output laser is introduced into a Fabry-Perot cavity interferometer for scanning and monitoring the output transmission peak. The λ / 2 wave plate angle is further adjusted according to the stability of the longitudinal mode of the laser to change the pump light power injected into the nonlinear optical crystal until the gain difference between the forward and reverse oscillating lasers meets the condition that the laser is in a stable single longitudinal mode operation and then stops.

[0012] At present, the most commonly used method for all-solid-state lasers is to realize unidirectional traveling-wave operation by forming a isolator with a magneto-optical crystal for longitudinal mode selection. However, due to the lack of low-loss magneto-optical crystals suitable for the infrared band, the power loss caused by the mode selection process is serious. For example, the nonlinear optical crystal MgO:PPLN used in the present invention to realize unidirectional traveling-wave operation of the resonant cavity has an absorption rate of only 0.05% / cm for 1.7-micron laser, while the bismuth iron garnet crystal, a mode-selective magneto-optical crystal commonly used in 1.5-micron all-solid-state lasers in the prior art, has an absorption efficiency of no less than 5.9% / cm for 1.7-micron laser.

[0013] Although the nonlinear optical mode selection technology solves the loss problem of magneto-optical crystals, it requires another laser light source as the fundamental frequency light to participate in the nonlinear process, which introduces a complex optical path and additional costs. Compared with the existing nonlinear optical mode selection technology (Optics Letters 49.2 (2024): 407-410), the present invention does not need to set up other laser light sources and optical paths outside the resonant cavity to provide the fundamental frequency light participating in the nonlinear process. The pump source, which is a component of the laser, is used instead of the fundamental frequency light to realize the nonlinear process, which simplifies the experimental device and cost, while improving stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the optical path of the all-solid-state laser according to an embodiment of the present invention.

[0015] Figure 2 This is a flow chart of the longitudinal mode selection method of the all-solid-state laser according to an embodiment of the present invention.

[0016] The accompanying figures are as follows: 1- pump source; 2- polarization beam splitter; 3- λ / 2 wave plate; 4- incident mirror; 5- laser crystal; 61- first reflector; 62- second reflector; 7- nonlinear optical crystal; 8- output mirror. DETAILED DESCRIPTION

[0017] The present invention is further explained below in detail with reference to the accompanying drawings so that those skilled in the art can more deeply understand the present invention and be able to implement it. However, the following reference examples are only used to explain the present invention and are not intended to limit the present invention.

[0018] according to Figure 1 As shown, a solid-state laser is constructed, including a pump source 1, a polarization beam splitter prism 2, a λ / 2 wave plate 3, an optical ring resonator, a gain medium 5 placed in the optical ring resonator, and a nonlinear optical crystal 7. The pump source 1 uses a laser diode with a central wavelength of 976nm; the gain medium 5 uses a piece of a-cut Er;Yb:YAB crystal. In addition, the gain medium 5 can also use 1.5 micron erbium-doped solid laser, 2 micron thulium-doped solid laser, 2.7 micron, 2.9 micron erbium-doped solid laser, etc. The nonlinear optical crystal 7 uses magnesium oxide-doped periodically poled lithium niobate crystal (MgO:PPLN crystal). The periodically poled nonlinear optical crystal can achieve quasi-phase matching of two frequency lights in the same direction, thereby suppressing them and causing the oscillating lasers in two directions to produce a gain difference. The nonlinear optical crystal 7 can also be periodically poled potassium titanyl phosphate, or non-periodically poled nonlinear optical crystals, such as potassium titanyl phosphate, barium metaborate crystal, lithium triborate, potassium dihydrogen phosphate, potassium titanyl phosphate, barium metaborate crystal, ammonium dihydrogen phosphate, potassium dideuterium phosphate, sodium barium niobate, etc.

[0019] like Figure 1 In the embodiment shown, the optical ring resonator is an "8"-shaped ring cavity structure composed of an incident mirror 4, a first reflector 61, a second reflector 62 and an output mirror 8 (in addition, it can also be a three-mirror ring cavity or a six-mirror ring cavity), the second reflector 62 is located below the incident mirror 4, and the output mirror 8 is located below the first reflector 61. The gain medium 5 is located on the optical path between the incident mirror 4 and the first reflector 61, and the nonlinear optical crystal 7 is located on the optical path between the second reflector 62 and the output mirror 8. Among them, the incident mirror 4 has high transmittance to the pump light and high reflectance to the oscillating laser, the output mirror 8 has high transmittance to the pump light and the oscillating laser, and the reflector has high reflectivity to the pump light and the oscillating laser. The incident mirror is coated with HT@976nm&HR@1.5μm; the reflector is coated with HR@976nm&HR@1.5 μm; the output mirror is coated with HT@976nm&T=2.5%@1.5 μm. HT represents high transmittance, and HR represents high reflectivity. The output and input mirrors must not reflect the pump light.

[0020] The 976 nm pump light emitted by the pump source 1 is polarized after passing through the polarization beam splitter prism 2, and the remaining P-polarized pump light continues to propagate, and is incident into the optical ring resonator after passing through the λ / 2 wave plate 3. A part of the pump light is absorbed by the gain medium 5 and converted into a 1.5 μm oscillating laser that propagates in both forward and reverse directions. At this time, there is an oscillating laser that propagates in both forward and reverse directions in the ring resonator, and the 976 nm pump light that is not absorbed by the gain medium 5 continues to propagate and after reflection, it is focused together with the 1.5 μm oscillating laser that propagates in the same direction into the nonlinear optical crystal 7 (a periodically polarized nonlinear optical crystal in this example) for nonlinear optical frequency conversion and is suppressed. Specifically, after meeting the quasi-phase matching condition, the 976 nm pump light and the 1.5 μm oscillating laser that propagate in the same direction can generate a laser with a wavelength of 2.6 μm by frequency difference in the nonlinear optical crystal 7 (theoretically deduced). At the same time, the 976 nm pump light and the 1.5 μm oscillating laser are suppressed by nonlinear loss, and the 1.5 μm oscillating laser that propagates in the reverse direction is suppressed by frequency difference. The μm oscillation laser is not suppressed and is output from the optical ring resonator, making the laser unidirectional traveling wave operation and realizing single longitudinal mode laser output.

[0021] As an alternative, the nonlinear optical crystal uses a non-periodic polarized crystal. For example, when the nonlinear optical crystal 7 is one of potassium titanyl phosphate, barium metaborate crystal, and lithium triborate, the phase matching of the co-propagating 976 nm pump light and the 1.5 μm oscillating laser can be achieved by adjusting the temperature of the nonlinear optical crystal 7, thereby suppressing the phase matching. When the nonlinear optical crystal 7 is one of potassium dihydrogen phosphate, potassium titanyl phosphate, barium metaborate crystal, ammonium dihydrogen phosphate, and potassium dideuterium phosphate, the phase matching of the co-propagating 976 nm pump light and the 1.5 μm oscillating laser can be achieved by adjusting the incident angle of the forward-propagating 976 nm pump light and the 1.5 μm oscillating laser (just adjust the reflector), thereby suppressing the phase matching of the co-propagating 976 nm pump light and the 1.5 μm oscillating laser.

[0022] In the all-solid-state laser of the embodiment of the present invention, when the forward propagating 976 nm pump light and the 1.5 μm oscillating laser light injected into the nonlinear optical crystal 7 meet the quasi-phase matching (or phase matching), the two generate a difference frequency (which may also be a sum frequency in different systems) and are consumed, and the reverse propagating 1.5 μm oscillating laser light does not meet the quasi-phase matching (or phase matching) condition and does not have loss and is not suppressed, so that the oscillating laser light propagating in the forward and reverse directions generates a gain difference. This gain difference can be controlled. Specifically, the polarization angle of the 976 nm pump light can be controlled by rotating the λ / 2 wave plate to adjust the absorption coefficient of the gain medium 5 to the 976 nm pump light, thereby changing the energy distribution ratio of the pump light between the gain medium 5 and the nonlinear optical crystal, and then adjusting the gain difference generated by the 1.5 μm oscillating laser light propagating in the forward and reverse directions.

[0023] This embodiment also relates to a longitudinal mode selection method for an all-solid-state laser. See the flow chart for Figure 2 , the longitudinal mode selection method includes the following steps: Step 1, construct the above-mentioned all-solid-state laser, select anisotropic gain medium 5 with selective absorption characteristics for polarization direction according to the target laser wavelength, and select a suitable nonlinear optical crystal to be inserted into the resonant cavity according to the laser wavelength and the pump light wavelength; Step 2: By adjusting the angle of the λ / 2 wave plate to change the polarization direction of the pump light, the absorption coefficient of the gain medium 5 to the pump light is reduced, and the power of the pump light absorbed by the nonlinear optical crystal 7 through the gain medium 5 is increased; Step 3, forwardly injecting the pump light and the oscillating laser into the nonlinear optical crystal 7 (using a periodically polarized nonlinear optical crystal) to achieve quasi-phase matching in the nonlinear optical crystal 7, consuming the pump light and the oscillating laser propagating in the same direction, so that the forward and reverse oscillating lasers produce a gain difference; Step 4: If it is detected that stable single longitudinal mode selection has been achieved, it means that the laser has achieved stable single longitudinal mode operation, otherwise jump to step 2 to continue adjusting the λ / 2 wave plate angle.

[0024] When the nonlinear optical crystal 7 is a non-periodically polarized crystal, the phase matching between the forward pump light and the oscillating laser in the nonlinear optical crystal 7 can be achieved by adjusting the incident angle of the forward pump light and the oscillating laser entering the nonlinear optical crystal 7 or the temperature of the nonlinear optical crystal 7.

[0025] The specific implementation scheme described above further describes in detail the purpose, technical scheme and beneficial effects of the present invention. It should be understood that the above is only a specific implementation scheme of the present invention and is not intended to limit the scope of the present invention. Any equivalent changes and modifications made by any technician in the field without departing from the concept and principle of the present invention should fall within the scope of protection of the present invention.

Claims

1. An all-solid-state laser, comprising a pump source (1), a polarization beam splitter (2), a λ / 2 wave plate (3) and an optical ring resonator, wherein the optical ring resonator comprises an incident mirror (4), a reflector and an output mirror (8), wherein the incident mirror (4) has a high transmittance to the pump light and a high reflectance to the oscillating laser, the output mirror (8) has a high transmittance to the pump light and the oscillating laser, and the reflector has a high reflectance to the pump light and the oscillating laser; and the optical ring resonator is provided with a gain medium (5) having a polarization selective absorption characteristic and a nonlinear optical Crystal (7), pump light emitted by a pump source (1) is sequentially incident on an optical ring resonator after passing through a polarization beam splitter prism (2) and a λ / 2 wave plate (3), a portion of the pump light is absorbed by a gain medium (5) and converted into an oscillating laser light that propagates in both forward and reverse directions, the pump light that is not absorbed by the gain medium (5) continues to propagate and after reflection is focused together with the oscillating laser light that propagates in the same direction in the nonlinear optical crystal (7) for nonlinear optical frequency conversion and is suppressed, the oscillating laser light that propagates in the reverse direction is not suppressed and is output from the optical ring resonator, so that the laser operates in a unidirectional traveling wave manner and single longitudinal mode laser output is achieved.

2. The all-solid-state laser according to claim 1, characterized in that: When the forward propagating pump light and the oscillating laser light injected into the nonlinear optical crystal (7) satisfy phase matching or quasi-phase matching in the nonlinear optical crystal (7), the two generate a difference frequency or a sum frequency and are consumed, and the reverse propagating oscillating laser light is not suppressed because it does not satisfy phase matching and quasi-phase matching and does not suffer loss, resulting in a gain difference between the oscillating laser light propagating in the forward and reverse directions.

3. The all-solid-state laser according to claim 1, characterized in that: The polarization angle of the pump light is controlled by rotating the λ / 2 wave plate to adjust the absorption coefficient of the gain medium (5) to the pump light, thereby changing the energy distribution ratio of the pump light between the gain medium (5) and the nonlinear optical crystal, and further adjusting the gain difference generated by the oscillating laser propagating in the forward and reverse directions.

4. The all-solid-state laser according to claim 1, characterized in that: The gain medium (5) is located on the optical path between the incident mirror (4) and the reflecting mirror, and the nonlinear optical crystal (7) is located on the optical path between the reflecting mirror and the output mirror (8).

5. The all-solid-state laser according to claim 4, characterized in that: The optical ring resonator is a three-mirror ring cavity, a four-mirror ring cavity or a six-mirror ring cavity. When the optical ring resonator is a four-mirror ring cavity, the optical ring resonator has an "8"-shaped ring cavity structure, comprising an incident mirror (4), a first reflector (61), a second reflector (62) and an output mirror (8), wherein the second reflector (62) is located below the incident mirror (4), the output mirror (8) is located below the first reflector (61), the gain medium (5) is located on the optical path between the incident mirror (4) and the first reflector (61), and the nonlinear optical crystal (7) is located on the optical path between the second reflector (62) and the output mirror (8).

6. The all-solid-state laser according to claim 2, characterized in that: The phase matching is achieved by adjusting the incident angles of the forward-propagating pump light and the oscillating laser light or the temperature of the nonlinear optical crystal (7).

7. The all-solid-state laser according to claim 6, characterized in that: When the nonlinear optical crystal (7) is one of potassium titanyl phosphate, barium metaborate crystal, and lithium triborate, the phase matching is achieved by adjusting the temperature of the nonlinear optical crystal (7); when the nonlinear optical crystal (7) is one of potassium dihydrogen phosphate, potassium titanyl phosphate, barium metaborate crystal, ammonium dihydrogen phosphate, and potassium dideuterium phosphate, the phase matching is adjusted by adjusting the incident angles of the forward propagating pump light and the oscillating laser.

8. The all-solid-state laser according to claim 2, characterized in that: The nonlinear optical crystal (7) is a periodically polarized nonlinear optical crystal, and quasi-phase matching between forward propagating pump light and oscillating laser light is achieved through the periodically polarized nonlinear optical crystal.

9. A method for selecting longitudinal modes of an all-solid-state laser, characterized in that The following steps are involved: Step 1, constructing the all-solid-state laser as claimed in claim 1, selecting an anisotropic gain medium (5) having selective absorption characteristics for polarization direction according to the target laser wavelength, and selecting a suitable nonlinear optical crystal to be inserted into the resonant cavity according to the laser wavelength and the pump light wavelength; Step 2, by adjusting the angle of the λ / 2 wave plate to change the polarization direction of the pump light, the absorption coefficient of the gain medium (5) to the pump light is reduced, and the power of the pump light absorbed by the nonlinear optical crystal (7) through the gain medium (5) is increased; Step 3, forwardly injecting the pump light and the oscillating laser into the nonlinear optical crystal (7) to achieve phase matching or quasi-phase matching in the nonlinear optical crystal (7), consuming the pump light and the oscillating laser propagating in the same direction, so that a gain difference is generated between the forward and reverse oscillating lasers; Step 4: If it is detected that stable single longitudinal mode selection has been achieved, it means that the laser has achieved stable single longitudinal mode operation, otherwise jump to step 2 to continue adjusting the λ / 2 wave plate angle.

10. The longitudinal mode selection method of the all-solid-state laser according to claim 9, characterized in that: When the nonlinear optical crystal (7) is a periodically polarized nonlinear optical crystal, the inherent characteristics of the periodically polarized nonlinear optical crystal are utilized in step 3 to achieve quasi-phase matching between the forward pump light and the oscillating laser, so that a gain difference is generated between the forward and reverse oscillating lasers; otherwise, the phase matching between the forward pump light and the oscillating laser in the nonlinear optical crystal (7) is achieved by adjusting the incident angle of the forward pump light and the oscillating laser incident on the nonlinear optical crystal (7) or the temperature of the nonlinear optical crystal (7).

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