System and method for improving output power of mid-infrared laser generated by intra-pulse difference frequency

Abstract

The invention discloses a system and a method for improving output power of mid-infrared laser generated by intra-pulse difference frequency, and belongs to the technical field of ultrafast optics. The system comprises a seed source, a first pulse amplification module, a spectrum broadening module, a second pulse amplification module, a synchronous control module and a self difference frequency module. The first pulse amplification module comprises a first isolator, a first semiconductor laser, a first wavelength division multiplexer, a first gain fiber, a second wavelength division multiplexer and a second semiconductor laser which are arranged in sequence. And the second pulse amplification module comprises a second isolator, a third semiconductor laser, a third wavelength division multiplexer, a second gain optical fiber and an optical fiber collimator which are arranged in sequence. The laser amplifier is used for directly amplifying the wavelength component required by the difference frequency after spectrum spreading, the time delay of the pump light and the signal light is accurately controlled by the birefringent crystal, the infrared laser generation power in the intra-pulse difference frequency is improved, and the device has the advantages of being high in conversion efficiency, simple in light path structure, stable and reliable in system and the like.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast optical technology and relates to a system and method for improving the output power of a mid-infrared laser generated by intra-pulse difference frequency. Background Art

[0002] The mid-infrared (2-20μm) band covers many vibration characteristic absorption peaks of organic and inorganic molecules, and the absorption intensity is at least one order of magnitude higher than that of the near-infrared band. Therefore, lasers in this band have important application prospects in the fields of molecular spectroscopy and biomedicine. In many application scenarios such as long-distance spectral measurement and laser diagnosis and treatment, high-power mid-infrared lasers are required.

[0003] At present, the main technologies for obtaining mid-infrared lasers are direct emission of doped ions, quantum cascades, and parametric down-conversion. Direct emission of doped ions is limited by factors such as the absorption and emission spectrum range of the gain medium, the working wavelength and bandwidth of the mode-locked device, and the radiation wavelength of the pump source. The wavelength range and pulse width of the output mid-infrared laser are subject to certain limitations. Quantum cascade lasers can achieve a wide spectral tuning range, but the output spectrum bandwidth is narrow, and the output laser pulse width is limited to the picosecond level or even wider. Parametric down-conversion uses the three-wave mixing effect in nonlinear crystals to generate mid-infrared laser output when the phase matching conditions are met. There is no heat accumulation in this process, and broadband phase matching can achieve a wider spectral range and higher energy mid-infrared laser output. The intra-pulse difference frequency technology, one of the parametric down-conversion technologies, optically differs the short-wave and long-wave components in a single pulse to achieve frequency down-conversion. It has the advantages of stable carrier envelope phase and no need for complex high-precision time synchronization and spatial adjustment of pump light and signal light. It is a key technology for generating short-cycle, broadband, and phase-stable mid-infrared laser pulses. The generation of intra-pulse difference frequency requires a single pulse to have a wide spectral range, at least to ensure that the two wavelength points required by the self-difference frequency are covered at the same time. Since the emission spectrum width of the gain medium is limited, it is usually necessary to broaden the spectrum of a single pulse directly emitted by the laser. The commonly used method to broaden the spectrum of a single pulse is to use a high peak power mode-locked laser as the driving source, broaden the spectrum by self-phase modulation through a highly nonlinear optical fiber, or generate Raman solitons through an anomalous dispersion optical fiber to shift the wavelength to the required spectral range. This structure allows only a small part of the energy of the driving source to be transferred to the two effective wavelength points generated by the mid-infrared, and most of the energy of the driving source is wasted on unnecessary wavelength components, and the mid-infrared generation efficiency is extremely low. In order to increase the output power of the mid-infrared laser generated by the intra-pulse difference frequency, the current commonly used approach is to increase the peak power of the driving source to increase the power of the two effective difference frequency wavelength components generated by the spectrum expansion, which also faces the problem of extremely low mid-infrared conversion efficiency. At the same time, due to the limitation of conversion efficiency, a significant increase in the peak power of the driving source will greatly increase the complexity of the system, reduce the stability and reliability of the system, and limit its application in more scenarios. Summary of the invention

[0004] In order to solve the problems of low conversion efficiency, reduced stability or reliability of mid-infrared laser generated by difference frequency generation, which is caused by increasing the effective wavelength component power generated by spectrum expansion by increasing the peak power of the driving source in the existing intra-pulse difference frequency process, and the problems of reduced stability or reliability, the purpose of the present invention is to provide a system and method for improving the output power of mid-infrared laser generated by intra-pulse difference frequency, which uses a laser amplifier to directly amplify the wavelength component required for the difference frequency after spectrum expansion, and uses a birefringent crystal to accurately control the time delay of the pump light and the signal light, thereby realizing the improvement of the power generated by the intra-pulse difference frequency mid-infrared laser. The present invention also has the advantages of high conversion efficiency, simple optical path structure, and stable and reliable system.

[0005] The purpose of the present invention is achieved through the following technical solutions.

[0006] On the one hand, the present invention discloses a system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency, including a seed source, a first pulse amplification module, a spectrum broadening module, a second pulse amplification module, a synchronization control module, and a self-difference frequency module.

[0007] The seed source is preferably a mode-locked solid laser or a mode-locked fiber laser.

[0008] The working wavelength of the seed source is within the near infrared wavelength range.

[0009] The first pulse amplification module includes a first isolator, a first semiconductor laser, a first wavelength division multiplexer, a first gain fiber, a second wavelength division multiplexer, and a second semiconductor laser which are arranged in sequence.

[0010] The spectrum broadening module is preferably a highly nonlinear optical fiber, which is used to generate the effective wavelength component required for the difference frequency.

[0011] The second pulse amplification module includes a second isolator, a third semiconductor laser, a third wavelength division multiplexer, a second gain fiber, and a fiber collimator which are arranged in sequence.

[0012] The third wavelength division multiplexer in the second pulse amplification module is preferably a broadband wavelength division multiplexer. The components of the broadband wavelength division multiplexer are replaceable, and the requirements of the self-difference frequency for lasers of different wavelengths can be met by replacing the components of the broadband wavelength division multiplexer.

[0013] The synchronous control module comprises an achromatic half-wave plate, a first dichromatic half-wave plate, a birefringent material, and a second dichromatic half-wave plate which are arranged in sequence.

[0014] The first two-color half-wave plate and the second two-color half-wave plate in the synchronous control module are preferably multi-stage half-wave plates, which are used to produce an odd-numbered delay of half a wavelength for one effective wavelength component required for the self-difference frequency and an even-numbered delay of half a wavelength for the other effective wavelength component.

[0015] The self-difference frequency module comprises a first off-axis parabolic mirror, a nonlinear crystal, a second off-axis parabolic mirror, and a filter which are arranged in sequence.

[0016] The nonlinear crystal in the self-difference frequency module is preferably a quasi-phase matching nonlinear crystal, such as a periodically poled lithium niobate crystal or an orientation-patterned gallium phosphide crystal.

[0017] On the other hand, the present invention discloses a method for improving the output power of a mid-infrared laser generated by a pulse difference frequency, which is implemented based on the system for improving the output power of a mid-infrared laser generated by a pulse difference frequency. A method for improving the output power of a mid-infrared laser generated by a pulse difference frequency comprises the following steps:

[0018] The seed source, the first pulse amplification module, the spectrum broadening module, the second pulse amplification module, the synchronization control module and the self-difference frequency module are arranged in sequence.

[0019] The near-infrared seed laser emitted by the seed source is nonlinearly amplified by the first pulse amplification module, and the spectrum is broadened by the spectrum broadening module to generate the wavelength laser component required by the intra-pulse self-difference frequency. The frequencies of the laser components are respectively recorded as f a and f b The second pulse amplification module amplifies the relatively low-power laser components (such as f b ) for power amplification.

[0020] The polarization direction of the amplified wide-spectrum laser is controlled by the achromatic half-wave plate in the synchronous control module, so that the polarization direction of the laser is aligned with the polarization direction of the nonlinear crystal; the laser component f is aligned with the nonlinear crystal by the first two-color half-wave plate. b The polarization direction is rotated 90° around the transmission direction, so that the laser component f a The polarization direction remains unchanged; the laser components f a and f b By using birefringent materials, a time delay Δt=L / c·(n a -n b ), which is used to precisely control the time delay between the two laser components before they self-difference in the nonlinear crystal, where L is the length of the birefringent material, c is the speed of light in vacuum, and n a and n b The laser components f a and f b The refractive index in the birefringent material. The laser pulse component f after time delay is precisely controlled by the second two-color half-wave plate. b The polarization direction is reversed by 90°, so that the laser component f a and f b The polarization directions coincide with and are aligned with the polarization direction of the nonlinear crystal.

[0021] The laser pulse with precise time delay control is focused into the nonlinear crystal by the first off-axis parabolic mirror in the self-difference frequency module, and a wavelength of f is generated by self-difference frequency. a ·f b / |f a -f b |The high-power mid-infrared laser is collimated by the second off-axis parabolic mirror and the residual near-infrared laser is filtered out by the filter to output the mid-infrared laser.

[0022] Beneficial effects:

[0023] 1. The present invention discloses a system and method for improving the output power of mid-infrared laser generated by intra-pulse difference frequency, which uses a laser amplifier to directly amplify the wavelength component required by the difference frequency after spectrum expansion, uses a birefringent crystal to quantitatively control the time delay of the pump light and the signal light, and uses a two-color wave plate to optimize the polarization direction of the pump light and the signal light so as to best match the polarization direction of the nonlinear crystal, thereby achieving the improvement of the power generated by the intra-pulse difference frequency mid-infrared laser. Therefore, the present invention can provide a technical solution with high conversion efficiency, simple optical path structure, and stable and reliable system for the problem of improving the power of mid-infrared laser based on intra-pulse difference frequency.

[0024] 2. The present invention discloses a system and method for improving the output power of mid-infrared laser generated by intra-pulse difference frequency. The nonlinear crystal in the self-difference frequency module is a quasi-phase matching nonlinear crystal, such as periodically poled lithium niobate and oriented patterned gallium phosphide crystal. Compared with other phase matching methods, it has the advantages of high conversion efficiency and wide tuning range, and can flexibly design the polarization period to meet different mid-infrared wavelength requirements.

[0025] 3. The present invention discloses a system and method for improving the output power of a mid-infrared laser generated by an intra-pulse difference frequency, which improves the output power of the mid-infrared laser generated by the intra-pulse difference frequency by amplifying the wavelength component required for the difference frequency after spectrum expansion. The present invention does not depend on the wavelength of the near-infrared seed source used in the embodiment, and is therefore suitable for near-infrared light sources in various optical bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of a system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency according to the present invention;

[0027] Among them: 11-seed source, 21-first isolator, 22-first semiconductor laser, 23-first wavelength division multiplexer, 24-first gain fiber, 25-second wavelength division multiplexer, 26-second semiconductor laser, 31-polarization-maintaining high nonlinear fiber, 41-second isolator, 42-third semiconductor laser, 43-third wavelength division multiplexer, 44-second gain fiber, 45-fiber collimator, 51-achromatic half-wave plate, 52-first dichroic half-wave plate, 53-birefringent material, 54-second dichroic half-wave plate, 61-first off-axis parabolic mirror, 62-nonlinear crystal, 63-second off-axis parabolic mirror, 64-filter. DETAILED DESCRIPTION

[0028] In order to better illustrate the purpose and advantages of the present invention, the invention is further described below with reference to the accompanying drawings and examples.

[0029] Embodiment 1:

[0030] The present embodiment discloses a system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency, which is composed of a seed source 1, a first pulse amplification module 2, a spectrum broadening module 3, a second pulse amplification module 4, a synchronization control module 5, and a self-difference frequency module 6.

[0031] The seed source 1 is a mode-locked fiber laser with a central wavelength of 1560 nm.

[0032] The first pulse amplification module 2 includes a first isolator 21, a first semiconductor laser 22, a first wavelength division multiplexer 23, a first gain fiber 24, a second wavelength division multiplexer 25, and a second semiconductor laser 26 which are arranged in sequence.

[0033] The first gain fiber 24 in the first pulse amplification module 2 is a polarization-maintaining erbium-doped gain fiber.

[0034] The spectrum broadening module 3 is preferably a polarization-maintaining highly nonlinear optical fiber 31, which is used to generate the effective wavelength component required for the difference frequency.

[0035] The second pulse amplification module 4 includes a second isolator 41 , a third semiconductor laser 42 , a third wavelength division multiplexer 43 , a second gain fiber 44 , and a fiber collimator 45 , which are arranged in sequence.

[0036] The third wavelength division multiplexer 43 in the second pulse amplification module 4 is a broadband wavelength division multiplexer, and the transmission spectrum range is preferably 1000-1600nm.

[0037] The second gain fiber 44 in the second pulse amplification module 4 is a polarization-maintaining ytterbium-doped gain fiber.

[0038] The synchronization control module 5 includes an achromatic half-wave plate 51 , a first dichroic half-wave plate 52 , a birefringent material 53 , and a second dichroic half-wave plate 54 which are arranged in sequence.

[0039] The working wavelength range of the achromatic half-wave plate 51 in the synchronization control module 5 is preferably 900-2000nm.

[0040] The first two-color half-wave plate 52 and the second two-color half-wave plate 54 in the synchronization control module 5 are preferably multi-stage half-wave plates, which produce an odd-numbered delay of half the wavelength for the 1030nm laser required for self-difference frequency and an even-numbered delay of half the wavelength for the 1560nm laser.

[0041] The birefringent material 53 in the synchronous control module 5 is preferably magnesium fluoride (MgF 2 ) crystal.

[0042] The self-difference frequency module 6 includes a first off-axis parabolic mirror 61 , a nonlinear crystal 62 , a second off-axis parabolic mirror 63 , and a filter 64 , which are arranged in sequence.

[0043] The nonlinear crystal 62 in the self-difference frequency module 6 is preferably a periodically poled lithium niobate crystal in a quasi-phase matching nonlinear crystal.

[0044] This embodiment also discloses a method for improving the output power of a mid-infrared laser generated by intra-pulse difference frequency, which is used in the above-mentioned system for improving the output power of a mid-infrared laser generated by intra-pulse difference frequency. The specific implementation steps are as follows:

[0045] A seed source 1, a first pulse amplification module 2, a spectrum broadening module 3, a second pulse amplification module 4, a synchronization control module 5, and a self-difference frequency module 6 are arranged in sequence.

[0046] The seed laser with a central wavelength of 1560nm emitted by the seed source 1 is nonlinearly power amplified by the first pulse amplification module 2, and the spectrum is broadened by the polarization-maintaining high nonlinear optical fiber 31 in the spectrum broadening module 3 to generate a wavelength laser component of 1030nm required for the intra-pulse self-difference frequency, and the relatively low-power 1030nm laser component is power amplified by the second pulse amplification module 4.

[0047] The polarization direction of the amplified wide-spectrum laser is controlled by the achromatic half-wave plate 51 in the synchronous control module 5, so that the polarization direction of the laser is aligned with the polarization direction of the periodically polarized lithium niobate crystal of the nonlinear crystal 62; the polarization direction of the 1030nm wavelength laser component is rotated 90° around the transmission direction by the first two-color half-wave plate 52, so that the polarization direction of the 1560nm wavelength laser component remains unchanged; the 1030nm and 1560nm laser components with mutually perpendicular polarization directions are passed through the birefringent material 53 magnesium fluoride (MgF2) crystal to obtain a time delay Δt=L / c·(ne -n o ), which is used to precisely control the time delay between the two laser components before they self-difference in the nonlinear crystal, where L is the length of the birefringent material, c is the speed of light in vacuum, and n e and n o are the refractive indices of the laser components 1030nm and 1560nm in the birefringent material. Since MgF2 crystal is a positive uniaxial crystal (n e >n o ), so the birefringent material 53MgF2 crystal realizes precise control of the time delay of the 1030nm and 1560nm laser components in the negative chirp pulse. The polarization direction of the 1030nm laser component after precise time delay control is reversely rotated by 90° through the second two-color half-wave plate 54, so that the polarization directions of the 1030nm and 1560nm laser components coincide and are aligned with the polarization direction of the periodically polarized lithium niobate crystal of the nonlinear crystal 62.

[0048] The laser pulse after precise time delay control is focused into the nonlinear crystal 62 through the first off-axis parabolic mirror 61 in the self-difference frequency module 6, and a high-power mid-infrared laser with a wavelength of 3031.7nm is generated through self-difference frequency. After being collimated by the second off-axis parabolic mirror 63 and the remaining near-infrared laser is filtered out by the filter 64, the mid-infrared laser is output.

[0049] The terms “first” and “second” mentioned in the present invention are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features.

[0050] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency, characterized in that: It includes a seed source, a first pulse amplification module, a spectrum broadening module, a second pulse amplification module, a synchronization control module, and a self-difference frequency module; The working wavelength of the seed source is within the near-infrared wavelength range; The first pulse amplification module includes a first isolator, a first semiconductor laser, a first wavelength division multiplexer, a first gain fiber, a second wavelength division multiplexer, and a second semiconductor laser arranged in sequence; The spectrum broadening module is used to generate the effective wavelength component required for the difference frequency; The second pulse amplification module includes a second isolator, a third semiconductor laser, a third wavelength division multiplexer, a second gain fiber, and a fiber collimator arranged in sequence; The third wavelength division multiplexer in the second pulse amplification module is preferably a broadband wavelength division multiplexer, and the requirements of the self-difference frequency for lasers of different wavelengths are met by replacing the components of the broadband wavelength division multiplexer; The synchronous control module includes an achromatic half-wave plate, a first dichromatic half-wave plate, a birefringent material, and a second dichromatic half-wave plate which are arranged in sequence; The first two-color half-wave plate and the second two-color half-wave plate in the synchronization control module are preferably multi-stage half-wave plates, which are used to generate an odd-numbered delay of half a wavelength for one effective wavelength component required for the self-difference frequency, and generate an even-numbered delay of half a wavelength for the other effective wavelength component; The self-difference frequency module comprises a first off-axis parabolic mirror, a nonlinear crystal, a second off-axis parabolic mirror, and a filter which are arranged in sequence.

2. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 1, characterized in that: The seed source is preferably a mode-locked solid laser or a mode-locked fiber laser.

3. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 1, characterized in that: The spectrum broadening module is selected as a highly nonlinear optical fiber.

4. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 1, characterized in that: The third wavelength division multiplexer in the second pulse amplification module is selected as a broadband wavelength division multiplexer.

5. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 1, characterized in that: The nonlinear crystal in the self-difference frequency module is selected as a quasi-phase matching nonlinear crystal.

6. A system for improving the output power of mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 5, characterized in that: The quasi-phase matching nonlinear crystal includes a periodically poled lithium niobate crystal and an orientation-patterned gallium phosphide crystal.

7. A method for improving the output power of a mid-infrared laser generated by intra-pulse difference frequency, based on the system for improving the output power of a mid-infrared laser generated by intra-pulse difference frequency as claimed in claim 1 or 2, characterized in that: The following steps are included: Arrange the seed source, the first pulse amplification module, the spectrum broadening module, the second pulse amplification module, the synchronization control module, and the self-difference frequency module in sequence; The near-infrared seed laser emitted by the seed source is nonlinearly power amplified by the first pulse amplification module, and the spectrum is broadened by the spectrum broadening module to generate the wavelength laser component required by the intra-pulse self-difference frequency. The frequencies of the laser components are respectively recorded as f a and f b , amplifying the power of the laser component with relatively low power by a second pulse amplification module; The polarization direction of the amplified wide-spectrum laser is controlled by the achromatic half-wave plate in the synchronous control module, so that the polarization direction of the laser is aligned with the polarization direction of the nonlinear crystal; the laser component f is aligned with the nonlinear crystal by the first two-color half-wave plate. b The polarization direction is rotated 90° around the transmission direction, so that the laser component f a The polarization direction remains unchanged; the laser components f a and f b By using birefringent materials, a time delay Δt=L / c·(n a -n b ), which is used to precisely control the time delay between the two laser components before they self-difference in the nonlinear crystal, where L is the length of the birefringent material, c is the speed of light in vacuum, and n a and n b The laser components f a and f b The refractive index in the birefringent material; the laser pulse component f after precise time delay control is b The polarization direction is reversed by 90°, so that the laser component f a and f b The polarization direction coincides with and is aligned with the polarization direction of the nonlinear crystal; The laser pulse with precise time delay control is focused into the nonlinear crystal by the first off-axis parabolic mirror in the self-difference frequency module, and a wavelength of f is generated by self-difference frequency. a ·f b / |f a -f b |The high-power mid-infrared laser is collimated by the second off-axis parabolic mirror and the residual near-infrared laser is filtered out by the filter to output the mid-infrared laser.

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