Picosecond optical parametric oscillator for realizing near transformation limit output and design method thereof

Abstract

The invention belongs to the related technical field of picosecond laser regulation and control, and discloses a picosecond optical parametric oscillator for realizing near transformation limit output and a design method thereof, the oscillator comprises a pumping source laser, an optical resonant cavity, a nonlinear crystal and a dispersion element; the pumping source laser is used for providing pulse pumping light for the optical resonant cavity; the nonlinear crystal is arranged on a light path in the optical resonant cavity and is used for converting pump light into signal light and idler frequency light, the optical resonant cavity is used for realizing oscillation of the signal light, and the dispersion element is arranged on the light path in the optical resonant cavity and is used for introducing group velocity dispersion for the signal light; signal light and idler frequency light of the near conversion limit pulse are output from the optical resonant cavity, and the dispersion amount of the dispersion element meets the near conversion limit output requirement. The optical parametric oscillator provided by the invention not only can output near-conversion limit pulses, but also has the advantages of wide wavelength tuning range, high conversion efficiency, low cost and low loss.

Description

Technical Field

[0001] The present invention belongs to the technical field related to laser control, and more specifically, relates to a picosecond optical parametric oscillator that achieves near-conversion limit output and a design method thereof. Background Art

[0002] Near-conversion-limited pulses are pulses whose product of time width and spectral width (time-bandwidth product) approaches the theoretical minimum. Both frequency-domain width and time-domain width must be as narrow as possible. Picosecond lasers with near-conversion-limited pulse output capabilities have broad applications in many fields. For example, in biophotonics, near-conversion-limited near-infrared lasers can be used to improve the resolution of coherent anti-Stokes microscopy (CARS). In trace gas sensing, near-conversion-limited mid-infrared picosecond lasers can significantly improve detection resolution.

[0003] Optical parametric oscillators are based on the second-order nonlinear effect of nonlinear crystals. They achieve frequency conversion through optical parametric processes, breaking through the limitations of the energy level structure of traditional laser gain media and achieving a very wide wavelength tuning range.

[0004] In order to take into account wide-range wavelength tuning and achieve near-conversion-limited output, there have been reports in recent years on the realization of near-conversion-limited output in optical parametric oscillators. It is usually necessary to introduce additional optical elements into the optical parametric oscillator system. For example, researchers have obtained near-conversion-limited pulses by introducing narrow-bandwidth birefringent filters into the optical parametric oscillator or adding volume-chirped Bragg gratings into the resonant cavity. If a birefringent filter is introduced, it will lead to large optical losses, and the filter configuration needs to be changed synchronously while the wavelength is being tuned, which is not conducive to achieving low-loss fast tuning. If a volume-chirped Bragg grating is introduced, the tuning range of the optical parametric oscillator will be limited, and its cost is relatively high.

[0005] Therefore, there is currently a lack of a picosecond optical parametric oscillator that is low-cost, low-loss, has wide tunability, and can output near the conversion limit. Summary of the Invention

[0006] In response to the above-mentioned defects or improvement needs of the prior art, the present invention provides a picosecond optical parametric oscillator that achieves near-conversion limit output and a design method thereof, the purpose of which is to reduce device cost and loss while achieving near-conversion limit, and to ensure that the device has a wide wavelength tuning capability.

[0007] To achieve the above objectives, according to one aspect of the present invention, a picosecond optical parametric oscillator (OPO) that achieves near-conversion-limited output is provided, comprising a pump source laser, an optical resonant cavity, a nonlinear crystal, and a dispersive element, wherein the dispersive element is a prism pair or a bulk dispersive material for introducing group velocity dispersion into signal light.

[0008] The pump source laser is used to provide picosecond pulse pump light to the optical resonant cavity; the nonlinear crystal is placed on the optical path within the optical resonant cavity, and is used to convert the pump light frequency into signal light and idler light through a second-order nonlinear effect. The optical resonant cavity is used to achieve oscillation of the signal light. The dispersion element is inserted into the optical resonator at a Brewster angle and is placed on the optical path within the optical resonant cavity, and is used to introduce group velocity dispersion into the signal light so as to output near-conversion-limited pulse signal light and idler light from the optical resonant cavity. The dispersion amount of the dispersion element meets the requirements for achieving near-conversion-limited output.

[0009] Optionally, the dispersion element has an optimal dispersion amount, and under the optimal dispersion, the output pulse is closest to the conversion limit.

[0010] Optionally, the material used for the bulk dispersion material is selected from SiO2, Si or ZnSe.

[0011] Optionally, the dispersion element is coated with an anti-reflection film.

[0012] Optionally, the optical resonant cavity comprises a first curved mirror, a second curved mirror, a plane dielectric mirror and a plane output coupling mirror, wherein the first curved mirror, the second curved mirror and the plane dielectric mirror all transmit the pump light and the idler light and reflect the signal light, and the plane output coupling mirror transmits a portion of the signal light in proportion and reflects the remaining signal light;

[0013] After the pump source laser emits pump light, the pump light enters the optical resonant cavity through the first curved mirror, and after passing through the nonlinear crystal, idler light and signal light are obtained. The idler light and the remaining pump light are output from the optical resonant cavity through the second curved mirror. The signal light is reflected by the second curved mirror to the plane dielectric mirror, then reflected by the plane dielectric mirror to the second curved mirror, then reflected by the second curved mirror to the first curved mirror, then reflected by the first curved mirror to the dispersion element, where it undergoes group velocity dispersion and is then transmitted to the plane output coupling mirror. Part of the signal light is output from the optical resonant cavity through the plane output coupling mirror, and the remaining signal light is reflected by the plane output coupling mirror, then returns along the original path to the first curved mirror to enter the next oscillation.

[0014] Optionally, a translation stage is further included, and the plane dielectric mirror is placed on the translation stage, and the translation stage can control the cavity length of the optical resonant cavity.

[0015] Optionally, the nonlinear crystal is periodically poled lithium niobate or barium metaborate.

[0016] Optionally, the tuning range of the picosecond optical parametric oscillator covers 1.4 μm-4 μm.

[0017] Optionally, a pump focusing mirror is further included, which is used to focus the pump light emitted by the pump source laser and then input it into the optical resonant cavity.

[0018] According to another aspect of the present invention, a method for designing a picosecond optical parametric oscillator that achieves near-conversion-limited output is provided, comprising:

[0019] Constructing a simulation model of a picosecond optical parametric oscillator, the picosecond optical parametric oscillator comprising a pump source laser, an optical resonant cavity, a nonlinear crystal, and a dispersive element, wherein the dispersive element is a prism pair or a bulk dispersive material for introducing group velocity dispersion into signal light; the pump source laser is used to provide picosecond pulsed pump light to the optical resonant cavity; the nonlinear crystal is placed in an optical path within the optical resonant cavity and is used to convert the pump light frequency into signal light and idler light through a second-order nonlinear effect; the optical resonant cavity is used to achieve oscillation of the signal light; the dispersive element is inserted into the optical resonator at a Brewster angle and is placed in an optical path within the optical resonant cavity and is used to introduce group velocity dispersion into the signal light so as to output the signal light and idler light from the optical resonant cavity;

[0020] The dispersion amount of the dispersion element is simulated according to the simulation model to obtain the dispersion amount that achieves near-transform-limited pulse output.

[0021] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0022] The present invention has discovered that by optimizing the group velocity dispersion within the resonant cavity, near-conversion-limited pulses can be achieved, and therefore applies this method to optical parametric oscillators. Compared to the method of inserting a birefringent filter, the present invention inserts a prism pair or a block of dispersive material at the Brewster angle to minimize intracavity losses. During wavelength tuning, the position does not need to be adjusted, reducing the complexity of the tuning process. Moreover, the loss is insensitive to the center wavelength and does not affect the original wavelength tuning range of the optical parametric oscillator. Therefore, after inserting the dispersion element proposed by the present invention, the optical parametric oscillator still has a wide wavelength tuning range. Compared to the method of inserting a volume-chirped grating pair, the present invention effectively reduces the application cost and does not affect the original wavelength tuning range of the optical parametric oscillator. In summary, the optical parametric oscillator mentioned in the present invention can output near-conversion-limited pulses, has a wide wavelength tuning range, and also has the advantages of low cost and low loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic structural diagram of a picosecond optical parametric oscillator in one embodiment of the present invention;

[0024] Figure 2This is a diagram showing the relationship between the time domain pulse width of a signal pulse at different central wavelengths and the multiples of the conversion limit pulse width in a numerical simulation based on the parameters in one embodiment of the present invention;

[0025] Figure 3 Figure 1 is a graph showing the time-domain pulse autocorrelation and frequency-domain spectrum of a picosecond optical parametric oscillator output obtained under the given parameter conditions of an embodiment of the present invention, where (a) is the measurement result when the signal light wavelength is 1560 nanometers, and (b) is the measurement result when the signal light wavelength is 1920 nanometers. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining 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 may be combined with each other as long as they do not conflict with each other.

[0027] The present invention provides a picosecond optical parametric oscillator that achieves near-conversion-limited output, such as Figure 1 FIG2 is a schematic structural diagram of a picosecond optical parametric oscillator in an embodiment of the present invention, which includes a pump source laser, an optical resonant cavity, a nonlinear crystal, and a dispersion element.

[0028] A pump source laser is used to provide picosecond-level pump light to the optical resonator. A nonlinear crystal is placed in the optical path of the optical resonator and is used to convert the pump light frequency into signal light and idler light through the second-order nonlinear effect. The optical resonator is used to achieve oscillation of the signal light. The dispersive element is a prism pair or a bulk dispersive material used to introduce group velocity dispersion into the signal light. The dispersive element is placed in the optical path of the optical resonator and is used to introduce group velocity dispersion into the signal light, thereby outputting near-conversion-limited pulses (time-bandwidth product less than 1) of signal light and idler light from the optical resonator. The dispersion of the dispersive element meets the requirements for achieving near-conversion-limited output.

[0029] The working principle of the above-mentioned picosecond optical parametric oscillator is as follows: pump light enters the optical resonant cavity and is frequency-converted into signal light and idler light through a nonlinear crystal. The signal light has the desired target central wavelength and oscillates in the optical resonant cavity. During the oscillation process, it passes through a dispersive element. The dispersive element introduces sufficient group velocity dispersion to the signal light. That is, different wavelengths of light are output from the dispersive element at different times. The purpose of introducing sufficient group velocity dispersion to the signal light by the dispersive element is to achieve near-conversion-limited pulse output. The dispersion amount of the dispersive element is a key factor in whether near-conversion-limited pulse output can be achieved. The optimal dispersion amount for achieving near-conversion-limited pulse output can be determined through experiments or simulations, thereby achieving near-conversion-limited pulse output through the dispersive element.

[0030] The core innovation of this invention lies in the discovery that near-conversion-limited pulses can be achieved simply by manipulating the dispersion of a dispersive material block, and its application to optical parametric oscillators (OPOs). In previous research, our research group considered using a phase mismatch scheme (see Chinese Patent CN114204390B) to achieve near-conversion-limited pulses from an OPO. However, this invention eliminates the need for a phase mismatch mechanism by directly inserting a dispersive material block and manipulating its dispersion to achieve near-conversion-limited pulses. Compared to inserting a birefringent filter, this invention minimizes intracavity losses by inserting a prism pair or a dispersive material block at the Brewster angle. This position does not need to be adjusted during wavelength tuning, reducing the complexity of the tuning process. Furthermore, the prism pair or dispersive material block does not affect the original wavelength tuning range of the OPO. Therefore, even with the dispersive element proposed in this invention, the OPO maintains a wide wavelength tuning range. Compared to inserting a volume-chirped grating pair, this invention significantly reduces application costs without compromising the optical parametric oscillator's inherent wavelength tuning range. In summary, the optical parametric oscillator described in this invention can output near-conversion-limited pulses while maintaining a wide wavelength tuning range, all while offering the advantages of low cost and low loss.

[0031] It should be noted that the dispersion of the dispersive element can be normal dispersion or anomalous dispersion. The required dispersion amount is related to the incident pump light pulse width and the configuration of the resonant cavity. The optimal dispersion amount can be obtained through simulation. For example, when the input pulse width is 6ps, the absolute value of the dispersion can be 0.4ps. 2 .

[0032] In one embodiment, a dispersion adjustment element can also be provided to adjust the dispersion of the dispersion element based on the operating parameters of the pump source laser and the central wavelength of the signal light, thereby achieving near-conversion-limited pulse output. This dispersion adjustment element can pre-store the dispersion values ​​under different parameter configurations. When the parameter configuration changes, it searches for the corresponding dispersion value and adjusts the dispersion element, thereby quickly adapting to the current configuration and continuously outputting near-conversion-limited pulses.

[0033] Specifically, one dispersive element may be provided, or a plurality of dispersive elements may be provided, and each dispersive element is inserted into the optical resonant cavity in a Brewster angle manner.

[0034] In one embodiment, the dispersive material block may be a silicon block, which can cause the signal light to generate time walk-off. It is understandable that other dispersive materials may also be used, such as glass SiO2, zinc selenide ZnSe, etc.

[0035] It can be understood that the nonlinear crystal is a key element for the optical parametric oscillator to realize the optical parametric process. Its type can be determined by the pump source and the output signal light wavelength range, and can be periodically poled lithium niobate PPLN, barium metaborate BBO, etc. By regulating the position of the nonlinear crystal and the cavity length of the optical resonant cavity (the length of the optical signal oscillating in the resonant cavity), signal light with different central wavelengths can be output. Therefore, the nonlinear crystal and the cavity length can be regulated according to the central wavelength of the required signal light. Among them, the cavity length of the optical resonant cavity should match the cavity length of the pump source laser to achieve synchronous pumping. For example, the nonlinear crystal is periodically poled lithium niobate (PPLN), type0 phase matching, and the output wavelength is tuned by changing the polarization period of the PPLN and the cavity length of the optical resonant cavity.

[0036] In one embodiment, an optical resonant cavity includes a first curved mirror (curved mirror 1), a second curved mirror (curved mirror 2), a dielectric mirror, and a planar output coupling mirror. The first curved mirror and the second curved mirror both transmit pump light and idler light and reflect signal light. The dielectric mirror reflects the signal light, and the planar output coupling mirror proportionally transmits a portion of the signal light and reflects the remaining signal light. When a pump source laser emits pump light, the pump light enters the optical resonant cavity through the first curved mirror, passes through a nonlinear crystal, and generates idler light and signal light. The idler light and the remaining pump light are output from the optical resonant cavity through the second curved mirror. The signal light is reflected from the second curved mirror to the dielectric mirror, then from the dielectric mirror to the second curved mirror, then from the second curved mirror to the first curved mirror, then from the first curved mirror to the dispersion element, where it undergoes time walk-off and is subsequently transmitted to the planar output coupling mirror. Part of the signal light is output from the optical resonant cavity through the planar output coupling mirror, and the remaining signal light is reflected from the planar output coupling mirror and returns along the original path to the first curved mirror for the next oscillation.

[0037] In one embodiment, the optical parametric oscillator also includes a translation stage, on which a dielectric mirror is placed. The translation stage can control the movement of the dielectric mirror along the direction of incident light to change the cavity length of the optical resonant cavity, thereby outputting signal light with different central wavelengths. It should be noted that adjusting the cavity length also ensures that the oscillator cavity length matches the pump source laser.

[0038] In one embodiment, the optical parametric oscillator further includes a pump focusing mirror for focusing the pump light emitted by the pump source laser and then inputting the pump light into the optical resonant cavity.

[0039] The present invention also provides a design method for a picosecond optical parametric oscillator that achieves near-conversion-limited output, which includes:

[0040] Constructing a simulation model of a picosecond optical parametric oscillator, the picosecond optical parametric oscillator comprising a pump source laser, an optical resonant cavity, a nonlinear crystal, and a dispersive element, wherein the dispersive element is a prism pair or a bulk dispersive material for introducing group velocity dispersion into signal light; the pump source laser is used to provide picosecond pulsed pump light to the optical resonant cavity; the nonlinear crystal is placed in an optical path within the optical resonant cavity and is used to convert the pump light frequency into signal light and idler light through a second-order nonlinear effect; the optical resonant cavity is used to achieve oscillation of the signal light; the dispersive element is inserted into the optical resonator at a Brewster angle and is placed in an optical path within the optical resonant cavity and is used to introduce group velocity dispersion into the signal light so as to output the signal light and idler light from the optical resonant cavity;

[0041] The dispersion amount of the dispersion element is simulated according to the simulation model to obtain the dispersion amount that achieves near-transform-limited pulse output.

[0042] The following is an explanation using a specific example.

[0043] This embodiment uses a pair of bulk silicon materials as dispersive elements. The specific parameters of the pump source laser are: output average power 5.15W, central wavelength 1036nm, spectral linewidth 0.39nm, and pulse width 6.8ps. The nonlinear crystal in the optical parametric oscillator is periodically poled lithium niobate (PPLN), type 0 phase matching.

[0044] as follows Figure 2 The figure shows the effect of adjusting the dispersion on the time-bandwidth product of the output signal light. It can be seen that too small or too large a dispersion is not conducive to the generation of near-transform-limited pulses. Therefore, the dispersion needs to be regulated to achieve the optimal value for achieving near-transform-limited pulses, thereby generating near-transform-limited pulses. It is particularly important to emphasize that in traditional practices, there are solutions to achieve narrow linewidth (narrow spectrum width) pulses by adjusting the dispersion. However, a narrow pulse width in the frequency domain does not mean that the near-transform limit has been achieved. The near-transform limit requires that the product of the pulse's time width and the spectrum width (time-bandwidth product) be less than 1. A narrow spectrum width alone does not necessarily achieve the near-transform limit, as Figure 2 As shown, the greater the dispersion, the narrower the spectrum width. However, when the dispersion exceeds a certain value, although the spectrum width narrows, the time-bandwidth product actually increases. Therefore, although some studies have proposed the possibility of achieving narrow-linewidth pulses by adjusting dispersion, this solution is the first to achieve near-conversion-limited pulses by adjusting dispersion. It can provide an effective solution for widely tunable near-conversion-limited picosecond pulse laser sources in many fields.

[0045] Through simulation, in the above embodiment, in order to achieve the near conversion limit, the dispersion amount that each block of silicon material needs to provide is 0.2ps 2In order to achieve this dispersion, the length of each silicon block is 10 cm and inserted into the resonant cavity at the Luster angle, as shown in Figure 3 The figure shows the experimental results of the time-domain pulse autocorrelation and frequency-domain spectrum of the picosecond optical parametric oscillator output obtained under the given parameter conditions of this embodiment, where (a) is the measurement result when the signal light wavelength is 1560 nanometers, and the left and right figures correspond to the signal light and the idler light, respectively; (b) is the measurement result when the signal light wavelength is 1920 nanometers, and the left and right figures correspond to the signal light and the idler light, respectively. This figure records the specific spectral shapes of the signal light and the idler light and the corresponding measurement autocorrelation results. It can be found that the time-bandwidth product of the signal light and the idler light is less than 0.8, achieving near-conversion-limited pulse output. In addition, the tuning range width of this solution covers 1.4μm-4μm, and near-conversion-limited pulse output can be achieved throughout the entire tuning range. At the same time, the optical parametric oscillator conversion efficiency is greater than 60%.

[0046] The technical features of the above embodiments can be combined in any manner. To simplify the description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. It should be noted that the phrases "in one embodiment", "for example", "and another example", etc. of the present invention are intended to illustrate the present invention and are not intended to limit the present invention.

[0047] The above embodiments merely illustrate several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A picosecond optical parametric oscillator that achieves near-conversion-limited output, characterized in that: The invention comprises a pump source laser, an optical resonant cavity, a nonlinear crystal and a dispersive element, wherein the dispersive element is a prism pair or a bulk dispersive material for introducing group velocity dispersion into the signal light; The pump source laser is used to provide picosecond pulse pump light to the optical resonant cavity; the nonlinear crystal is placed on the optical path within the optical resonant cavity, and is used to convert the pump light frequency into signal light and idler light through a second-order nonlinear effect. The optical resonant cavity is used to achieve oscillation of the signal light. The dispersion element is inserted into the optical resonator at a Brewster angle and is placed on the optical path within the optical resonant cavity, and is used to introduce group velocity dispersion into the signal light so as to output near-conversion-limited pulse signal light and idler light from the optical resonant cavity. The dispersion amount of the dispersion element meets the requirements for achieving near-conversion-limited output.

2. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The dispersion element has an optimal dispersion amount. Under the optimal dispersion, the output pulse is closest to the conversion limit.

3. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The material used for the bulk dispersion material is selected from SiO2, Si or ZnSe.

4. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The dispersion element is coated with an anti-reflection film.

5. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The optical resonant cavity comprises a first curved mirror, a second curved mirror, a plane dielectric mirror and a plane output coupling mirror, wherein the first curved mirror, the second curved mirror and the plane dielectric mirror all transmit pump light and idler light and reflect signal light, and the plane output coupling mirror transmits a portion of the signal light in proportion and reflects the remaining signal light; After the pump source laser emits pump light, the pump light enters the optical resonant cavity through the first curved mirror, and after passing through the nonlinear crystal, idler light and signal light are obtained. The idler light and the remaining pump light are output from the optical resonant cavity through the second curved mirror. The signal light is reflected by the second curved mirror to the plane dielectric mirror, then reflected by the plane dielectric mirror to the second curved mirror, then reflected by the second curved mirror to the first curved mirror, then reflected by the first curved mirror to the dispersion element, where it undergoes group velocity dispersion and is then transmitted to the plane output coupling mirror. Part of the signal light is output from the optical resonant cavity through the plane output coupling mirror, and the remaining signal light is reflected by the plane output coupling mirror, then returns along the original path to the first curved mirror to enter the next oscillation.

6. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 5, wherein: It also includes a translation stage, the plane dielectric mirror is placed on the translation stage, and the translation stage can control the cavity length of the optical resonant cavity.

7. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The nonlinear crystal is periodically poled lithium niobate or barium metaborate.

8. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: The tuning range of the picosecond optical parametric oscillator covers 1.4 μm-4 μm.

9. The picosecond optical parametric oscillator achieving near-conversion-limited output according to claim 1, wherein: It also includes a pump focusing mirror for focusing the pump light emitted by the pump source laser and then inputting it into the optical resonant cavity.

10. A design method for a picosecond optical parametric oscillator achieving near-conversion-limited output, characterized in that: include: Constructing a simulation model of a picosecond optical parametric oscillator, the picosecond optical parametric oscillator comprising a pump source laser, an optical resonant cavity, a nonlinear crystal, and a dispersive element, wherein the dispersive element is a prism pair or a bulk dispersive material for introducing group velocity dispersion into signal light; the pump source laser is used to provide picosecond pulsed pump light to the optical resonant cavity; the nonlinear crystal is placed in an optical path within the optical resonant cavity and is used to convert the pump light frequency into signal light and idler light through a second-order nonlinear effect; the optical resonant cavity is used to achieve oscillation of the signal light; the dispersive element is inserted into the optical resonator at a Brewster angle and is placed in an optical path within the optical resonant cavity and is used to introduce group velocity dispersion into the signal light so as to output the signal light and idler light from the optical resonant cavity; The dispersion amount of the dispersion element is simulated according to the simulation model to obtain the dispersion amount that achieves near-transform-limited pulse output.

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