A Method for Reducing the Repetition Rate of a Synchronously Pumped Optical Parametric Oscillator

Abstract

The present invention discloses a method for reducing the repetition rate of a synchronously pumped optical parametric oscillator. The method is to make multiple transverse modes oscillate in the cavity of the synchronously pumped optical parametric oscillator, and through the coherent superposition between different transverse modes, the spot energy flows periodically in space, and only a part of the oscillating light spot is output, so as to achieve an integer multiple reduction of the repetition rate of the synchronously pumped optical parametric oscillator. The present invention can solve the problem that the repetition rate of the synchronously pumped optical parametric oscillator cannot be lower than that of the pump source without relying on any active control elements, and provides an effective method for reducing the repetition rate of the synchronously pumped optical parametric oscillator.

Description

Technical Field

[0001] The present invention relates to the field of ultrafast laser technology, and particularly to a method for reducing the repetition rate of a synchronously pumped optical parametric oscillator. Background Art

[0002] High-power ultrafast lasers have attracted extensive attention as important tools for ultrafast science and industrial applications. Compared with mode-locked lasers, synchronously pumped optical parametric oscillators of ultrafast lasers can generate broadband tunable ultrafast coherent light sources in the ultraviolet to mid-infrared range, and thus have important application values in fields such as gas detection, free-space optical communication, laser medicine, material processing, and optoelectronic countermeasures.

[0003] The synchronously pumped optical parametric oscillator needs to satisfy the synchronization condition between the oscillating pulse and the pumping pulse. Therefore, generally, the repetition rate of the synchronously pumped optical parametric oscillator is the same as that of the pumping source. If the cavity length of the optical parametric oscillator is set to be M / N times the cavity length of the pumping source (M and N are relatively prime positive integers), the repetition rate of the output pulse can be increased to N times that of the pumping source. However, no matter how the cavity length of the optical parametric oscillator is set, the repetition rate of the output pulse cannot be lower than that of the pumping source. If the repetition rate of the output pulse is to be lower than that of the pumping source, currently it depends on the cavity dumping technology, but the cavity dumping system is bulky, has a complex structure, and is not yet mature in the mid-infrared band, which limits its application. In addition, currently there is no method for reducing the repetition rate of a synchronously pumped optical parametric oscillator without relying on the cavity dumping technology. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for reducing the repetition rate of a synchronously pumped optical parametric oscillator, which can solve the problem that the repetition rate of the synchronously pumped optical parametric oscillator cannot be lower than that of the pumping source without relying on any active control elements.

[0005] To achieve the above purpose, the present invention provides the following solution:

[0006] A method for reducing the repetition rate of a synchronously pumped optical parametric oscillator, the method is to make multiple transverse modes oscillate in the cavity of the synchronously pumped optical parametric oscillator, and through the coherent superposition between different transverse modes, make the spot energy flow periodically in space, and only output a part of the oscillating light spot, so as to realize an integer multiple down-regulation of the repetition rate of the synchronously pumped optical parametric oscillator.

[0007] Further, the method specifically includes the following steps:

[0008] S1, according to the ultrashort pulse of the required wavelength band, select the pumping light wavelength, the oscillating light wavelength, and the corresponding nonlinear crystal;

[0009] S2, shape the pump light spot so that the size of the pump light spot matches the size of the highest-order mode expected for the oscillation light spot;

[0010] S3, observe the output spot of the oscillation light. If there is only one transverse mode in the oscillation light, increase the pump light power until the oscillation light contains at least two transverse modes;

[0011] S4, intercept a part of the oscillation light spot at the output end of the optical parametric oscillator, and only output the bright spot with the strongest light intensity in the oscillation light spot;

[0012] S5, finely adjust the distance between the lenses of the resonator while observing the output pulse train, so as to obtain the desired pulse output with a repetition frequency lower than that of the pump source.

[0013] Furthermore, when the fractional part of the ratio of the transverse mode frequency interval to the longitudinal mode frequency interval oscillating in the cavity is equal to M / N, the repetition frequency of the output pulse drops to 1 / N of the repetition frequency of the pump source, where M and N are relatively prime positive integers.

[0014] Furthermore, the synchronously pumped optical parametric oscillator includes: a pump source, a first half-wave plate, a Faraday isolator, a second half-wave plate, a first cylindrical lens, a second cylindrical lens, a first plano-concave mirror, a nonlinear crystal, a second plano-concave mirror are sequentially arranged along the laser output direction of the pump source. A plane-parallel mirror is arranged in the reflection direction of the second plano-concave mirror, and an output coupling mirror and a slit are sequentially arranged in the reflection direction of the first plano-concave mirror. The slit is used to intercept the central bright spot of the idler light.

[0015] Furthermore, the pump source outputs laser with a pulse width of 800 fs, a repetition frequency of 84.2 MHz, and a central wavelength of 1030 nm.

[0016] Furthermore, the focal lengths of the first cylindrical lens and the second cylindrical lens are 50 mm and 100 mm respectively, and are used to focus the pump light into the nonlinear crystal.

[0017] Furthermore, the nonlinear crystal uses a 2-mm-long periodically poled lithium niobate doped with 5 mol% magnesium oxide, and the poling period is 31 μm, which is used to generate signal light and idler light.

[0018] Furthermore, the radius of curvature of the first plano-concave mirror and the second plano-concave mirror is 300 mm, which form a resonator and focus.

[0019] According to the specific embodiments provided by the present invention, the following technical effects are disclosed: The method for reducing the repetition frequency of the synchronously pumped optical parametric oscillator provided by the present invention enables multiple transverse modes to oscillate in the cavity of the synchronously pumped optical parametric oscillator. Through the coherent superposition between different transverse modes, the spot energy flows periodically in space. By only outputting a part of the oscillating light spot, an integer multiple reduction of the repetition frequency of the synchronously pumped optical parametric oscillator can be achieved, providing an effective method for reducing the repetition frequency of the synchronously pumped optical parametric oscillator. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0021] Figure 1 is a schematic structural diagram of the synchronously pumped optical parametric oscillator according to an embodiment of the present invention;

[0022] Figure 2 is the variation of the idler output power and spot pattern with the pump power according to an embodiment of the present invention;

[0023] Figure 3 is the variation of the idler light spot with time under different transverse mode frequency intervals obtained by theoretical calculation according to an embodiment of the present invention;

[0024] Figure 4 is the spot pattern before and after intercepting the central bright spot with a slit according to an embodiment of the present invention;

[0025] Figure 5 is the pulse sequence with different repetition frequencies output when the distance between two plano-concave mirrors is adjusted after the idler light passes through the slit according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0027] The object of the present invention is to provide a method for reducing the repetition frequency of a synchronously pumped optical parametric oscillator, which can solve the problem that the repetition frequency of the synchronously pumped optical parametric oscillator cannot be lower than the repetition frequency of the pump source without relying on any active control elements.

[0028] To make the above objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] The method for reducing the repetition frequency of a synchronously pumped optical parametric oscillator provided by the present invention is to make multiple transverse modes oscillate in the cavity of the synchronously pumped optical parametric oscillator, and through the coherent superposition between different transverse modes, the spot energy flows periodically in space, and only a part of the oscillating light spot is output, thereby realizing an integer multiple reduction of the repetition frequency of the synchronously pumped optical parametric oscillator. When the fractional part of the ratio of the transverse mode frequency interval to the longitudinal mode frequency interval in the cavity is equal to M / N, the repetition frequency of the output pulse is reduced to 1 / N of the repetition frequency of the pump source, where M and N are relatively prime positive integers.

[0030] Among them, the method specifically includes the following steps:

[0031] S1. According to the ultrashort pulse of the required wavelength band, select the pump light wavelength, the oscillating light wavelength, and the corresponding nonlinear crystal;

[0032] S2. Shape the pump light spot so that the size of the pump light spot matches the size of the highest-order mode expected for the oscillating light spot;

[0033] S3. Observe the output spot of the oscillating light. If there is only one transverse mode in the oscillating light, increase the pump light power until at least two transverse modes are included in the oscillating light;

[0034] S4. Intercept a part of the oscillating light spot at the output end of the optical parametric oscillator, and only output the bright spot with the strongest light intensity in the oscillating light spot;

[0035] S5. Fine-tune the distance between the lenses of the resonant cavity, and at the same time observe the output pulse sequence, so as to obtain the desired pulse output with a repetition frequency lower than that of the pump source.

[0036] As Figure 1 shown, the synchronously pumped optical parametric oscillator includes: a pump source 1, a first half-wave plate 2, a Faraday isolator 3, a second half-wave plate 4, a first cylindrical lens 5, a second cylindrical lens 6, a first plano-concave mirror 7, a nonlinear crystal 8, a second plano-concave mirror 9 are sequentially arranged along the laser output direction of the pump source 1. A plane-plane mirror 10 is arranged in the reflection direction of the second plano-concave mirror 9, and an output coupling mirror 11 and a slit 12 are sequentially arranged in the reflection direction of the first plano-concave mirror 7. The slit 12 is used to intercept the central bright spot of the idler light.

[0037] The pump source 1 is a master oscillator power amplifier system based on a ytterbium-doped fiber, and outputs laser with a pulse width of 800 fs, a repetition frequency of 84.2 MHz, and a central wavelength of 1030 nm.

[0038] The first half-wave plate 2 is used to rotate the polarization direction of the laser to the transmission direction of the Faraday isolator 3.

[0039] The Faraday isolator 3 is used to prevent the laser from feeding back to the pump source 1.

[0040] The second half-wave plate 4 is used to control the polarization direction of the laser incident on the nonlinear crystal 8, so as to achieve the best phase matching.

[0041] The first cylindrical lens 5 and the second cylindrical lens 6, with focal lengths of 50 mm and 100 mm respectively, are used to focus the pump light into the nonlinear crystal 8.

[0042] The nonlinear crystal 8 is a 2-mm-long periodically poled lithium niobate (MgO:PPLN) doped with 5 mol% magnesium oxide, with a poling period of 31 μm, and is used to generate signal light and idler light. This crystal is coated with an antireflection film in the pump light and idler light bands, with a transmittance greater than 99.5% in the wavelength range of 2.0 μm to 2.65 μm and greater than 95% in the wavelength range of 1.02 μm to 1.08 μm.

[0043] The first plano-concave mirror 7 and the second plano-concave mirror 9, with a radius of curvature of 300 mm, form a resonant cavity and focus. Their reflectance is greater than 99.5% in the wavelength range of 2.06 μm to 2.5 μm, and their transmittance is greater than 90% in the wavelength range of 1.02 μm to 1.04 μm.

[0044] The plane-plane mirror 10 is used to form one end mirror of the optical parametric oscillator. The plane-plane mirror 10 is fixed on a one-dimensional precision translation stage, and the cavity length of the optical parametric oscillator can be precisely changed to achieve cavity length matching with the pump source. Its reflectance is greater than 99.5% in the wavelength range of 2.06 μm to 2.5 μm.

[0045] The output coupler mirror 11 is used to form the other end mirror of the optical parametric oscillator and output a part of the idler light, and its transmittance for the wavelength of 2.1 μm to 2.6 μm is 9%.

[0046] The slit 12, with a width of about 1 mm, is used to intercept the central bright spot of the idler light.

[0047] Calibrate the resonant cavity, gradually increase the pump power, reach the threshold condition of parametric oscillation, adjust the cavity length of the optical parametric oscillator to be precisely matched with the cavity length of the pump source, and make the idler light start to oscillate in the cavity. The output power of the idler light and the corresponding spot pattern as a function of the pump light power are shown in Figure 2 . It can be seen that when the pump light power is low, the idler light is a fundamental mode Gaussian beam, and when the pump light power exceeds 16 W, the idler light presents the Hermite-Gaussian 00 mode (HG 00mode) and Hermite-Gaussian 04 mode (HG 04 mode) mixed-mode state.

[0048] When HG 00 mode and HG 04 mode oscillate simultaneously, the change of the idler light spot over time under different transverse mode frequency intervals obtained through theoretical calculation can be referred to Figure 3 where Δω T represents the transverse mode frequency interval between HG 00 mode and HG 04 mode, Δω L represents the adjacent longitudinal mode frequency interval, a is a natural number, and T c represents the cavity period of the optical parametric oscillator, and its value is equal to the pump pulse sequence period. It can be seen that the coherent superposition of two transverse modes in space will cause the spot energy to flow periodically in space. When the fractional part of the ratio of the transverse mode frequency interval to the longitudinal mode frequency interval is equal to M / N (M and N are relatively prime positive integers), the spot energy flow period is N times the cavity period.

[0049] When the pump light power is 23 W, a slit 12 with a width of about 1 mm is placed behind the output coupling mirror 11 to allow the central bright spot of the idler light to pass through the slit. The spot patterns before and after passing through the slit can be referred to Figure 4 .

[0050] After the idler light passes through the slit, on the premise of keeping the cavity length of the optical parametric oscillator unchanged, the distance between the first plano-concave mirror 7 and the second plano-concave mirror 9 is adjusted, and the measured idler light output pulse sequence can be referred to Figure 5 where L 7,9 represents the distance between the first plano-concave mirror 7 and the second plano-concave mirror 9. It can be seen that the repetition frequency of the output pulse is reduced to 1 / N of the pump source repetition frequency, and adjusting the distance between the first plano-concave mirror 7 and the second plano-concave mirror 9 can adjust the reduction ratio of the repetition frequency.

[0051] The main idea of the present invention is to make the spot energy flow periodically in space through the coherent superposition between multiple transverse modes, and only output a part of the oscillating light spot to realize the integer multiple down-regulation of the repetition frequency of the synchronously pumped optical parametric oscillator.

[0052] Although the present invention has been described to a certain extent, obviously, under the condition of not departing from the spirit and scope of the present invention, appropriate changes can be made to each condition, such as making more transverse modes oscillate in the optical parametric oscillator cavity, using the coating method to output the central bright spot from the output coupling mirror, and reflecting the two sides of the spot back into the cavity by the output coupling mirror, etc. Therefore, it can be understood that the present invention is not limited to the described embodiments, and any changes should be covered within the scope of the claims of the present invention without departing from the spirit and scope of the technical solution of the present invention.

[0053] In this article, specific examples are used to elaborate on the principles and implementation modes of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation modes and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for reducing the repetition rate of a synchronously pumped optical parametric oscillator, characterized in that, the method is to make multiple transverse modes of the idler light oscillate in the cavity of the synchronously pumped optical parametric oscillator, and through the coherent superposition between different transverse modes, the spot energy flows periodically in space, and only a part of the oscillating light spot is output, so as to achieve an integer multiple reduction of the repetition rate of the synchronously pumped optical parametric oscillator; the oscillating light spot is the idler light spot; wherein, a part of the oscillating light spot is the central bright spot.

2. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 1, characterized in that, the method specifically includes the following steps: S1, according to the ultrashort pulse of the required wavelength band, select the pump light wavelength, the oscillating light wavelength and the corresponding nonlinear crystal; S2, shape the pump light spot so that the size of the pump light spot matches the size of the highest-order mode expected for the oscillating light spot; S3, observe the output spot of the oscillating light. If there is only one transverse mode in the oscillating light, increase the pump light power until the oscillating light contains at least two transverse modes; S4, intercept a part of the oscillating light spot at the output end of the optical parametric oscillator, and only output the bright spot with the strongest light intensity in the oscillating light spot; S5, finely adjust the distance between the lenses of the resonant cavity, and at the same time observe the output pulse train, so as to obtain the desired pulse output with a repetition rate lower than that of the pump source.

3. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 1, characterized in that, When the fractional part of the ratio of the transverse mode frequency interval to the longitudinal mode frequency interval of the intracavity oscillation is equal to M / N , the repetition frequency of the output pulse drops to 1 / of the repetition frequency of the pump source N， where M 、 N are relatively prime positive integers.

4. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 1, characterized in that, the synchronously pumped optical parametric oscillator includes: a pump source (1), a first half-wave plate (2), a Faraday isolator (3), a second half-wave plate (4), a first cylindrical lens (5), a second cylindrical lens (6), a first plano-concave mirror (7), a nonlinear crystal (8), a second plano-concave mirror (9) are arranged in sequence along the laser output direction of the pump source (1), a plane-plane mirror (10) is arranged in the reflection direction of the second plano-concave mirror (9), and an output coupler (11) and a slit (12) are arranged in sequence in the reflection direction of the first plano-concave mirror (7), and the slit (12) is used to intercept the central bright spot of the idler light.

5. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 4, characterized in that, the pump source (1) outputs laser with a pulse width of 800 fs, a repetition rate of 84.2 MHz, and a central wavelength of 1030 nm.

6. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 4, characterized in that, the first cylindrical lens (5) and the second cylindrical lens (6) have focal lengths of 50 mm and 100 mm respectively, and are used to focus the pump light into the nonlinear crystal (8).

7. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 4, characterized in that, the nonlinear crystal (8) uses periodically poled lithium niobate doped with 5 mol% magnesium oxide with a length of 2 mm and a poling period of 31 μm, and is used to generate signal light and idler light.

8. The method for reducing the repetition rate of a synchronously pumped optical parametric oscillator according to claim 4, characterized in that, the radius of curvature of the first plano-concave mirror (7) and the second plano-concave mirror (9) is 300 mm, forming a resonant cavity and focusing.

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