WO2020056655A1 - 一种近红外激光器及激光产生方法 - Google Patents

一种近红外激光器及激光产生方法 Download PDF

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WO2020056655A1
WO2020056655A1 PCT/CN2018/106624 CN2018106624W WO2020056655A1 WO 2020056655 A1 WO2020056655 A1 WO 2020056655A1 CN 2018106624 W CN2018106624 W CN 2018106624W WO 2020056655 A1 WO2020056655 A1 WO 2020056655A1
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light
crystal
continuous
fundamental frequency
frequency
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PCT/CN2018/106624
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English (en)
French (fr)
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徐庆扬
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烟台龙驰光电技术有限公司
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Priority to PCT/CN2018/106624 priority Critical patent/WO2020056655A1/zh
Priority to CN201880096275.6A priority patent/CN112640231B/zh
Publication of WO2020056655A1 publication Critical patent/WO2020056655A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering

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  • the invention relates to a laser structure and principle method for generating a single wavelength or a tunable wavelength in a near-infrared band, and belongs to the field of nonlinear optics and laser.
  • Near-infrared lasers such as 800-1200nm wavelength range
  • lasers with different wavelengths are required as light sources for treatment.
  • Infrared lasers used for laser thermal treatment are mainly laser crystals that emit using specific wavelengths, for example, Nd: YAG and ND: YVO 4 emitting 1064 nm wavelength, Nd: YLF emitting 1047 nm wavelength, and the like.
  • Laser crystals with a wavelength of 700-900 nm are also doped with titanium sapphire crystals.
  • solid-state lasers can be used for both pulsed and continuous operation, which are well known to those skilled in the art.
  • near-infrared laser generation methods such as lasers using highly nonlinear photonic crystal fibers, which use the third-order nonlinear effect of photonic crystal fibers to generate 700-1200nm wavelengths for pump pulsed light.
  • This method requires a picosecond pump source and a highly non-linear optical fiber, and has a complex structure.
  • YVO 4 solid-state green laser to synchronously pump parametric oscillation of a nonlinear crystal to generate a picosecond pulse with a wide wavelength range that can be adjusted.
  • semiconductor lasers such as AlGaAs, InGaAs, and InGaNAs materials, can achieve 800-1200nm coverage by adjusting the material composition and quantum well structure. Common wavelengths are 785nm, 808nm, 830nm, 915nm, 976nm, 1064nm, etc. However, semiconductor lasers It is not easy to generate high-energy pulses.
  • the present invention provides a near-infrared laser and a laser generating method thereof, which have the characteristics of compact structure, wide wavelength selectable range, tunable wavelength, and high conversion efficiency.
  • the technical solution adopted by the present invention is as follows:
  • a pulsed near-infrared laser including: a front cavity mirror (1), a laser crystal (2), a Q-switched crystal (3), a quasi-phase matching crystal (4), and a fundamental frequency.
  • the fundamental frequency light reflector (5) is highly reflective to the pulsed fundamental frequency light (8) and highly transparent to other wavelengths; the pulsed frequency doubled light ( 9) Pulsed fundamental frequency light (8) is generated by the frequency doubling process in the quasi-phase matching crystal (4) and propagates forward; pulsed signal light (10) and / or idle light (11) is generated by pulsed frequency doubled light (9) ) Generated by a parametric process in the quasi-phase matching crystal (4) and resonant output on the (S6) plane and the (S10) plane of the rear cavity mirror (7); (S10) facing the pulse signal light (10) and / or left idle The light (11) is partially reflected, (S6) faces the pulsed fundamental frequency light (8), is highly transparent, and highly reflective to other wavelengths; the wavelength selection crystal (6) specifies the pulse signal light (10) and the pulse idle light (11) Wavelength selection output or tunable wavelength output.
  • the pulsed fundamental frequency light mirror (5) is replaced by an end face (S71) of a quasi-phase matching crystal (4); (S71) faces the pulse
  • the fundamental frequency light (8) is highly reflective and highly transparent to other wavelengths.
  • a continuous near-infrared laser including: a front cavity mirror (1), a laser crystal (2), a quasi-phase matching crystal (4), and a fundamental frequency-octave optical mirror ( 51), wavelength-selective crystal (6), and rear cavity mirror (7); of which: continuous fundamental frequency light (8 ') is generated by laser crystal (2) and is on the (S1) plane and fundamental frequency of front cavity mirror (1) -Resonance between frequency-doubled light reflector (51), fundamental frequency-frequency-doubled light reflector (51) is highly reflective to continuous fundamental frequency light (8 ') and continuous frequency-doubled light (9'), and highly transparent to other wavelengths ; Continuous frequency doubling light (9 ') is generated by continuous fundamental frequency light (8') in a quasi-phase matching crystal (4) through a frequency doubling process and on the (S6) plane and the fundamental frequency-octave light reflector (51) Resonance between; continuous signal light (10 ') and / or continuous idle light (11')
  • a continuous near-infrared laser in which the fundamental frequency-octave light reflecting mirror (51) is replaced by an end face (S72) of a quasi-phase matching crystal (4); a (S72) face It is highly reflective to continuous fundamental frequency light (8 ') and continuous frequency doubled light (9'), and highly transparent to other wavelengths.
  • the quasi-phase-matching crystal (4) is a periodically polarized crystal, including periodically polarized lithium niobate, periodically polarized lithium tantalate, and periodically polarized potassium potassium phosphate, And periodic polarization doped with magnesium oxide lithium niobate and periodic polarization doped with lithium magnesium oxide tantalate;
  • the polarization period is uniform period, erbium period and multi-region period;
  • Q-switched crystal (3) includes active Q-switched crystal and passive tuning The Q crystal;
  • the wavelength selection crystal (6) includes a birefringent filter and a Fabry-Perot etalon;
  • the fundamental frequency light mirror (5) and the fundamental frequency-octave light mirror (51) are volume Bragg gratings.
  • a method for generating a pulsed laser which includes: using a cascade frequency-parametric oscillation mode in a laser cavity, and a cascade second-order nonlinearity in a quasi-phase matching crystal (4) Effect is equivalent to a third-order non-linear effect, resulting in a broadband parametric gain; the pulsed fundamental frequency light (8) is on the (S1) plane of the front cavity mirror (1) and the fundamental frequency light reflector (5) (or (S71) plane) Resonance between; pulsed frequency doubled light (9) is generated by pulsed fundamental frequency light (8) in a quasi-phase matching crystal (4) through a frequency doubling process and propagates forward; pulsed signal light (10) and / or pulsed idle light (11) Generated by pulsed frequency doubling light (9) in a quasi-phase matching crystal (4) through a parametric process and resonating between the (S6) plane and the (S10) plane of the rear cavity mirror (7); through the cavity resonance
  • the method of pulsed fundamental frequency is on the (S1) plane of the front cavity
  • a method for generating a continuous laser which includes: using a cascade frequency-parametric oscillation mode in a laser cavity, and a cascade second-order nonlinearity in a quasi-phase matching crystal (4)
  • the effect simulates a third-order nonlinear effect, resulting in a broadband parametric gain;
  • the continuous fundamental frequency light (8 ') is on the (S1) plane of the front cavity mirror (1) and the fundamental frequency-octave light reflector (51) (or ( S72) surface)
  • continuous frequency doubling light (9 ') is generated by continuous fundamental frequency light (8') in the quasi-phase matching crystal (4) through the frequency doubling process, and the end surface (S6) and the fundamental frequency-times Resonance between the frequency light reflector (51) / end face (S72).
  • Continuous signal light (10 ') and / or continuous idle light (11') are generated by continuous frequency doubling light (9 ') in a quasi-phase matching crystal (4) through a parametric process and on the surface of the quasi-phase matching crystal (4) (S6) and the (S10) plane of the rear cavity mirror (7); by simultaneously increasing the optical power density of the continuous fundamental frequency light (8 ') and the continuous frequency doubled light (9') in the resonant cavity, The gain of the parametric continuous signal light (10 ') and / or the continuous idle light (11') in the cavity is further enhanced to further reduce the parametric oscillation threshold to achieve continuous parametric oscillation.
  • the present invention has the following beneficial effects: If the pump wavelength uses a common 1064nm wavelength, a broadband gain of 800-1200nm can be generated, and then the wavelength selection element in the cavity can be used to generate the required range of 800-1200nm. Wavelength or tunable wavelength output.
  • the structure of the invention can realize continuous wave and pulse output, which greatly simplifies the structure of the laser cavity, which is very beneficial to the miniaturization and efficiency of the laser.
  • FIG. 1a is a schematic diagram of a cascade frequency-parametric oscillation principle of the present invention
  • FIG. 1b is a simulation curve diagram of the cascade frequency-parameter process gain spectrum of the present invention.
  • 2a is a schematic diagram of a pulsed infrared laser according to a first embodiment of the present invention
  • 2b is a schematic diagram of another pulsed infrared laser according to the first embodiment of the present invention.
  • 3a is a schematic diagram of a structure of a continuous infrared laser according to a second embodiment of the present invention.
  • FIG. 3b is a schematic diagram of another continuous infrared laser according to the second embodiment of the present invention.
  • Figure 1 gives a schematic diagram of the principle of cascaded frequency-parametric oscillation, that is, the frequency-doubling and parametric processes are realized simultaneously in a periodically polarized crystal.
  • the cascaded frequency doubling-parameter process of a quasi-phase-matching crystal is: when the pump light ⁇ P (ie, the fundamental frequency light) enters a periodic polarization crystal with a period ⁇ , a frequency-doubled light ⁇ SHG is generated, and the frequency is doubled
  • the light ⁇ SHG simultaneously undergoes a parametric process in the periodically polarized crystal, generating signal light gains and idle light gains with bandwidths of - ⁇ to + ⁇ , respectively.
  • the gain bandwidth is determined by the refractive index dispersion characteristics of the quasi-phase matching crystal.
  • the cascade frequency doubling-parameter process needs to be performed in the laser resonant cavity; at the same time, the wavelength selection element in the cavity and the resonance characteristics of the laser cavity are used to generate the required signal light in the range from - ⁇ to + ⁇ / Idle light wavelength output. It should be noted that this method will lose the wavelength of a small part of the frequency band ⁇ Pump ′ occupied near the pump light, as shown in FIG. 1a.
  • Fig. 1b shows the cascade frequency-parameter process gain spectrum simulation curve of a 1cm long, 5.3um period-polarized periodic polarization doped 5% lithium magnesia niobate (PPMgO: LN) under a 1064nm optical pump.
  • PPMgO lithium magnesia niobate
  • a 1 cm long period of polarized lithium niobate produces a parametric gain bandwidth of about 40 nm.
  • the crystal length can be shortened; or erbium-polarized lithium niobate with a linearly varying period, such as a periodic variation from 5.1um to 5.5um; or the periodicity of the polarization crystals can be separated in series, such as uniform Cycles of 5.1um, 5.2um, 5.3um, 5.4um, 5.5um and 2mm in length are made in series.
  • the simulation data shows that the parametric gain bandwidth can reach more than 400nm, covering a range of about 200nm each with a pump center wavelength of about 1064nm.
  • FIG. 2a the laser structure of the first embodiment of the present invention is shown in FIG. 2a.
  • This structure is used to generate a pulsed light output.
  • a pulsed light output Including front cavity mirror (1), laser crystal (2), Q-switched crystal (3), quasi-phase matching crystal (4), fundamental frequency light reflector (5), wavelength selection crystal (6) and rear cavity mirror (7 ).
  • Fig. 2a also shows the oscillation path of the pulsed fundamental frequency light (8), the pulsed frequency doubled light (9), the pulsed signal light (10) and the pulsed idle light (11) in the laser cavity.
  • the pulsed fundamental frequency light (8) resonates between the (S1) surface of the front cavity mirror (1) and the fundamental frequency light reflector (5), and is generated through a frequency doubling process in the quasi-phase matching crystal (4).
  • the pulse signal light (10) and / or the idle light (11) are generated by a pulsed frequency doubling light (9) in a quasi-phase matching crystal (4) through a parametric process and on the (S6) plane and the rear cavity mirror (7) ( S10) Surface resonance output.
  • the light energy is converted from pulsed fundamental frequency light (8) to pulsed frequency doubled light (9), and then converted to pulsed signal light (10) and / or pulsed idle light (11).
  • the quasi-phase crystal (4) is a uniformly periodic polarized crystal with a period ⁇ , such as periodically polarized lithium niobate, periodically polarized lithium tantalate, periodically polarized KTP, and various doped crystals, such as Periodic polarization is doped with 5% lithium magnesium oxide niobate, periodic polarization is doped with 5% lithium magnesium tantalate, periodic polarization is the same as lithium niobate in chemical ratio, and periodic polarization is the same as lithium tantalate in chemical ratio.
  • a periodically polarized doped 5% magnesium oxide lithium niobate crystal is used.
  • the crystal can also be a linear ⁇ -polarized crystal, for example, the polarization period linearly changes from 5 microns to 5.6 microns.
  • the crystal may also be a multi-region polarized periodic crystal having a period from ⁇ 1 to ⁇ N.
  • the design wavelength is: a pulse fundamental frequency wavelength of 1064 nm, a frequency doubling wavelength of 532 nm, and a signal light wavelength of 900 nm.
  • the fundamental frequency light reflector (5) is preferably a volume Bragg grating, with a reflectance of> 99%, preferably greater than 99.8%, and a reflection wavelength bandwidth of ⁇ 10nm, preferably less than 1nm, in order to minimize the occupation of signal light and / or idleness.
  • the frequency band of light is shown in the table below.
  • the selection of the oscillation wavelength of the pulse signal light (10) / pulse idle light (11) is performed by adjusting the wavelength selection crystal (6).
  • the wavelength-selective crystal (6) is preferably a birefringent filter and a Fabry-Perot etalon.
  • the structure of FIG. 2a can also be used to continuously tune the wavelength selection crystal (6) to form a tunable light source with a wavelength between 800-1060nm.
  • the conditions of (S6)-(S10) surface coating are as follows :
  • the (S71) surface coating on the quasi-phase matching crystal (4) is used to ensure high reflection of the pulsed fundamental frequency light (9).
  • the pulsed fundamental frequency light (8) resonates between the (S1) and (S71) surfaces of the front cavity mirror (1), and the generated pulsed frequency doubled light (9) propagates forward in the quasi-phase matching crystal (4).
  • the pulse signal light (10) and / or pulse idle light (11) are quasi-phase-matched crystals (4) and resonate between the (S6) plane and the (S10) plane of the rear cavity mirror (7).
  • the light energy is converted from pulsed fundamental frequency light (8) to pulsed frequency doubled light (9), and then converted to pulsed signal light (10) and / or pulsed idle light (11).
  • the fundamental frequency is 1064 nm and the frequency doubling wavelength is 532 nm. It is necessary to generate signal light with a wavelength of 900 nm.
  • the coating conditions of the (S71) surface are as follows:
  • the structure of FIG. 2b can also form a tunable light source with a wavelength between 800-1060nm by continuously tuning the wavelength selection crystal (6).
  • the coating is as follows:
  • the method and structure of the present invention can also generate continuous near-infrared light output, that is, the method of resonating the continuous frequency doubled light (9 ') further reduces the threshold of parametric oscillation to achieve continuous parametric oscillation Output.
  • Figure 3a it includes a front cavity mirror (1), a laser crystal (2), a quasi-phase matching crystal (4), a fundamental frequency-octave light reflecting mirror (51), a wavelength selection crystal (6), and a rear cavity mirror (7 ).
  • Fig. 3a also shows the oscillation paths of continuous fundamental frequency light (8 '), continuous frequency doubled light (9'), continuous signal light (10 ') and continuous idle light (11') in the cavity.
  • the continuous frequency-doubled light (9 ') resonates between the (S6) plane and the fundamental frequency / frequency-doubled light reflector (51), so that the power density of the continuous frequency-doubled light (9') in the cavity is greatly enhanced, so the parametric oscillation is greatly increased.
  • Conversion efficiency of continuous signal light (10 ') and / or continuous idle light (11') thereby reducing the oscillation threshold to achieve the purpose of continuous wave output.
  • the fundamental frequency-octave light reflecting mirror (51) is preferably a volume Bragg grating. Unlike the fundamental frequency light reflector (5), which only forms a high reflection on the pulsed fundamental frequency light (8), the fundamental frequency-doubler light reflector (51) needs to be in the continuous fundamental frequency light (8 ') and the continuous frequency doubler light at the same time. At (9 '), a high reflection occurs simultaneously. At this time, the modulation depth of the refractive index period ⁇ B of the Bragg grating needs to be increased, that is, the refractive index modulation distribution must be changed from a sinusoidal form to a rectangular form, so that the second-order grating also generates high reflection at the frequency-doubled light wavelength.
  • This method enables the reflection bandwidth of the Bragg grating to the fundamental frequency light (8 ′) to be widened, preferably from 0.1 nm to 10 nm.
  • the thickness of the volume Bragg grating can be increased to achieve high reflection of the frequency-doubled light wavelength.
  • the fundamental frequency-octave light reflector (51) is replaced by (S72) surface coating, and the reflectance of (S72) for continuous frequency-doubled light (9 ') also reaches 99. % Or more, preferably 99.8% or more, to increase the optical power density of the continuous frequency-doubled light (9 ') in the cavity.
  • the fundamental frequency is 1064 nm and the frequency doubling wavelength is 532 nm. Signal light with a wavelength of 900 nm needs to be generated.
  • the coating of the mirror (S72) is as follows:
  • the reflection bandwidth is relatively large. (S72)
  • the reflection bandwidth is about 10-100nm at 1064nm and 532nm, so it occupies a certain range of wavelength for generating the required signal light / idle light.
  • the structure of FIG. 3b can also form a tunable light source with a wavelength between 800-1060nm by continuously tuning the wavelength selection crystal (6).
  • the coating is as follows:
  • the method of the invention can also produce shorter light pulses such as sub-nanosecond or even picosecond pulse outputs.
  • passive Q-switched crystals such as chromium-doped garnet (Cr: YAG), gallium arsenide (GaAs) crystals, or semiconductor saturable absorption mirrors (SESAM), or bonded erbium-doped garnet-chromium-doped garnet ( Nd: YAG-Cr: YAG) crystals
  • the doping ratio, crystal size and cavity structure can produce high repetition frequency sub-nanosecond 1064nm pulse output, in conjunction with the intra-cavity cascade frequency-parametric oscillation Method and structure can realize sub-nanosecond pulse output in the range of 800-1200nm.
  • the cavity length can be extended to suit the picosecond mode-locking mode.
  • the repetition frequency of the picosecond mode-locking pulse output is designed to be a standard 76MHz by the laser cavity
  • the quasi-phase matching crystals preferably used in the present invention include lithium niobate and lithium tantalate, and the transparent range is 250nm to 6um
  • the near-infrared laser structure and generation method of the present invention are not limited to near-infrared wavelengths. Any other wavelength in the transparent range of the quasi-phase-matched crystal can be produced using the method and structure of the present invention. For example, pumping an infrared laser near a wavelength of 1550 nm produces an arbitrary wavelength of 1400-1600 nm and a tunable wavelength.
  • the present invention uses a concave cavity as an example, but various laser cavity structures can be used, such as a flat concave cavity, a flat flat cavity, a folding cavity, etc., which does not violate the spirit of the present invention.

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Abstract

一种近红外激光器，包括前腔镜、激光晶体、调Q晶体、准相位匹配晶体、波长选择晶体以及后腔镜。采用腔内级联倍频-光学参量振荡(cSHG-OPO)方法，利用周期极化晶体的准相位匹配，最大限度的利用非线性晶体的二阶非线性系数d 33。级联的二阶非线性效应模拟了三阶非线性效应，在泵浦波长周边产生了类似三阶非线性的波混频效应，从而产生宽带参量增益；通过增强谐振腔内的基频光以及倍频光功率密度的方法，使得腔内参量增益大大增强，从而降低参量振荡的阈值；再利用腔内的波长选择元件产生特定波长的参量振荡激光输出。本发明的结构及方法使得宽波长范围的近红外激光器结构大为简化，非常有利于激光器的小型化和高效化。

Description

一种近红外激光器及激光产生方法 技术领域
本发明涉及一种产生近红外波段的单波长或可调谐波长的激光器结构及原理方法，属于非线性光学及激光领域。
背景技术
近红外激光例如800-1200nm波长范围在生物成像、激光医疗等领域有着广泛应用。例如在激光热治疗领域，由于不同生物组织对近红外光的吸收不同，需要不同波长的激光器作为治疗光源。用于激光热治疗的红外激光器主要是使用特定波长发射的激光晶体，例如，发射1064nm波长的Nd：YAG和ND：YVO 4，发射1047nm波长的Nd：YLF等等。发射700-900nm的波长的激光晶体有还掺钛蓝宝石晶体。这些固体激光器都可以用作脉冲和连续工作，这已为该领域的专业人员所熟知。目前也有一些新型的近红外激光产生方法出现，例如使用了高非线性光子晶体光纤的激光器，利用光子晶体光纤的三阶非线性效应对泵浦脉冲光产生700-1200nm的波长。该方法需要皮秒泵浦源以及高非线性光纤，结构复杂。也有使用传统Nd:YVO 4固体绿光激光器同步泵浦非线性晶体的参量振荡的方法，产生宽波长范围可调的皮秒脉冲。该方法需要基频激光腔和参量振荡腔且严格同步，光学结构复杂，调节困难。另外，半导体激光器，例如AlGaAs，InGaAs以及InGaNAs材料可以通过调节材料组分及量子阱结构实现800-1200nm的覆盖，常见的波长有785nm，808nm，830nm，915nm，976nm，1064nm等等，然而半导体激光器不易产生高能量脉冲。
发明内容
针对现有技术中存在的不足，本发明提供一种近红外激光器及其激光产生方法，具有结构紧凑、波长可选择范围宽、波长可调谐、转换效率高的特点。为了实现上述发明目的，本发明采用的技术方案如下：
根据本发明的第一个方面，提供了一种脉冲近红外激光器，包括：前腔镜(1)、激光晶体(2)、调Q晶体(3)、准相位匹配晶体(4)、基频光反射镜(5)、波长选择晶体(6)以及后腔镜(7)；其中：脉冲基频光(8)由激光晶体(2)和调Q晶体(3)产生并在前腔镜(1)的(S1)面和基频光反射镜(5)之间谐振，基频光反射镜(5)对脉冲基频光(8)高反，对其他波长高透；脉冲倍频光(9)由脉冲基频光(8)在准相位匹配晶体(4)中通过倍频过程产生并向前传播；脉冲信号光(10)和/或闲置光(11)由脉冲倍频光(9)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面谐振输出；(S10)面对脉冲信号光(10)和/或闲置光(11)部分反射，(S6)面对脉冲基频光(8)高透，对其他波长高反；波长选择晶体(6)对脉冲信号光(10)、脉冲闲置光(11)进行特定波长选择输出或者可调谐波长输出。
根据本发明的第一个方面，提供了另一种脉冲近红外激光器，其中脉冲基频光反射镜(5)由准相位匹配晶体(4)的端面(S71)代替；(S71)面对脉冲基频光(8)高反射，对其他波长高透。
根据本发明的第二个方面，提供了一种连续近红外激光器，包括：前腔镜(1)、激光晶体(2)、准相位匹配晶体(4)、基频-倍频光反射镜(51)、波长选择晶体(6)以及后腔镜(7)；其中：连续基频光(8’)由激光晶体(2)产生并在前腔镜(1)的(S1)面和基频-倍频光反射镜(51)之间谐振，基频-倍频光反射镜(51)对连续基频光(8’)和连续倍频光(9’)高反，对其他波长高透；连续倍频光(9’)由连续基频光(8’)在准相位匹配 晶体(4)中通过倍频过程产生并在(S6)面和基频-倍频光反射镜(51)之间谐振；连续信号光(10’)和/或连续闲置光(11’)由连续倍频光(9’)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面谐振输出；(S10)面对连续信号光(10’)和/或连续闲置光(11’)部分反射，(S6)面对连续基频光(8’)高透，对其他波长高反；波长选择晶体(6)对连续信号光(10’)、连续闲置光(11’)进行特定波长选择输出或者可调谐波长输出。
根据本发明的第二个方面，提供了另一种连续近红外激光器，其中基频-倍频光反射镜(51)由准相位匹配晶体(4)的端面(S72)代替；(S72)面对连续基频光(8’)和连续倍频光(9’)高反，对其他波长高透。
根据本发明第一和第二方面的近红外激光器，其中准相位匹配晶体(4)为周期极化晶体，包括周期极化铌酸锂、周期极化钽酸锂、周期极化磷酸钛钾，以及周期极化掺氧化镁铌酸锂，周期极化掺氧化镁钽酸锂；极化周期为均匀周期、啁啾周期以及多区周期；调Q晶体(3)包括主动调Q晶体和被动调Q晶体；波长选择晶体(6)包括双折射滤波片和法布里-珀罗标准具；基频光反射镜(5)与基频-倍频光反射镜(51)为体布拉格光栅。
根据本发明的第三个方面，提供了一种产生脉冲激光的方法，包括：采用激光腔内的级联倍频-参量振荡方式，准相位匹配晶体(4)中的级联二阶非线性效应等效三阶非线性效应，从而产生宽带参量增益；脉冲基频光(8)在前腔镜(1)的(S1)面和基频光反射镜(5)(或(S71)面)之间谐振；脉冲倍频光(9)由脉冲基频光(8)在准相位匹配晶体(4)中通过倍频过程产生并前向传播；脉冲信号光(10)和/或脉冲闲置光(11)由脉冲倍频光(9)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面之间谐振；通过腔内谐振脉冲基频光(8)的方法增强光功率密度，使得腔内参量脉冲信号光(10)和/或脉冲闲置光(11)的光增益大大增 强，从而降低参量振荡的阈值；
根据本发明的第四个方面，提供了一种产生连续激光的方法，包括：采用激光腔内的级联倍频-参量振荡方式，准相位匹配晶体(4)中的级联二阶非线性效应模拟了三阶非线性效应，从而产生宽带参量增益；连续基频光(8’)在前腔镜(1)的(S1)面和基频-倍频光反射镜(51)(或(S72)面)之间谐振，连续倍频光(9’)由连续基频光(8’)在准相位匹配晶体(4)中通过倍频过程产生并在端面(S6)和基频-倍频光反射镜(51)/端面(S72)之间谐振。连续信号光(10’)和/或连续闲置光(11’)由连续倍频光(9’)在准相位匹配晶体(4)中通过参量过程产生并在准相位匹配晶体(4)的面(S6)和后腔镜(7)的(S10)面之间谐振；通过同时增强谐振腔内的连续基频光(8’)和连续倍频光(9’)的光功率密度的方法，使得腔内参量连续信号光(10’)和/或连续闲置光(11’)的增益进一步增强从而进一步降低参量振荡阈值，以达到连续参量振荡。
与现有技术相比，本发明具有如下有益效果：如果泵浦波长使用常见的1064nm波长，可以产生在800-1200nm的宽带增益，进而通过腔内的波长选择元件产生800-1200nm范围内所需波长或者可调谐波长输出。本发明的结构可以实现连续波和脉冲输出，使得激光腔结构大为简化，非常利于激光器的小型化和高效化。
附图说明
图1a是本发明的级联倍频-参量振荡原理示意图；
图1b是本发明的级联倍频-参量过程增益谱模拟曲线图；
图2a是本发明第一实施例的脉冲红外激光器示意图；
图2b是本发明第一实施例的另外一种脉冲红外激光器示意图；
图3a是本发明第二实施例的连续红外激光器结构的示意图；
图3b是本发明第二实施例的另外一种连续红外激光器示意图。
具体实施方式
下面结合具体实施例对本发明做进一步说明。
图1给出了级联倍频-参量振荡原理示意图，即在一个周期极化的晶体中，同时实现倍频和参量过程。
参照图1a，准相位匹配晶体的级联倍频-参量过程为：在泵浦光λ P(即基频光)进入周期为Λ的周期极化晶体中，产生倍频光λ SHG，倍频光λ SHG同时在周期极化晶体中发生参量过程，分别产生带宽为-Δλ到+Δλ的信号光增益和闲置光增益。增益带宽由准相位匹配晶体的折射率色散特性决定。为了增强参量增益，级联倍频-参量过程需要放到激光谐振腔中进行；同时配合腔内波长选择元件以及激光腔的谐振特性，产生从-Δλ到+Δλ范围内所需要的信号光/闲置光波长输出。需要注意的是，该方法将损失泵浦光附近所占用的小部分频段Δλ Pump’的波长，如图1a所示。
图1b给出1cm长、极化周期Λ为5.3um的周期极化掺5％氧化镁铌酸锂(PPMgO：LN)在1064nm光泵浦下的级联倍频-参量过程增益谱模拟曲线。在如图的示例中，1cm长周期极化铌酸锂产生的参量增益带宽约为40nm。为了增大增益带宽，可以将晶体长度缩短；或者设计周期线性变化的啁啾极化铌酸锂，例如周期变化从5.1um到5.5um；或者将极化晶体的周期分立串联的方式，例如均匀周期5.1um、5.2um、5.3um、5.4um、5.5um各2mm长串联制作在一起。模拟数据表明参量增益带宽可达400nm以上，覆盖泵浦中心波长1064nm左右各约200nm范围。
实施例一
参考图2a，本发明第一实施例的激光器结构如图2a所示。该结构用于产生脉冲光输出。包括前腔镜(1)、激光晶体(2)、调Q晶体(3)、准相位 匹配晶体(4)、基频光反射镜(5)、波长选择晶体(6)以及后腔镜(7)。同时图2a也示出了脉冲基频光(8)、脉冲倍频光(9)、脉冲信号光(10)和脉冲闲置光(11)在激光腔中的振荡路径。
具体而言，脉冲基频光(8)在前腔镜(1)的(S1)面和基频光反射镜(5)之间谐振，在准相位匹配晶体(4)中通过倍频过程产生向前传播的脉冲倍频光(9)。脉冲信号光(10)和/或闲置光(11)由脉冲倍频光(9)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面谐振输出。光能量从脉冲基频光(8)转化到脉冲倍频光(9)，进而转化到脉冲信号光(10)和/或脉冲闲置光(11)。
根据本发明，准相位晶体(4)为周期为Λ的均匀周期极化晶体，例如周期极化铌酸锂，周期极化钽酸锂，周期极化KTP，以及各种掺杂的晶体，例如周期极化掺5％氧化镁铌酸锂，周期极化掺5％氧化镁钽酸锂，周期极化同化学比铌酸锂，周期极化同化学比钽酸锂。作为示例，使用周期极化掺5％氧化镁铌酸锂晶体。该晶体也可以为线性啁啾极化晶体，例如极化周期从5微米线性渐变到5.6微米。该晶体也可以为周期从Λ 1至Λ N的多区极化周期晶体。
本发明的各个光学元件需要配合适当的光学介质膜，用于形成该发明实施例的腔结构。参照图2a，在一个优选的实施例中，设计波长为：脉冲基频波长1064nm，倍频波长532nm，信号光波长900nm。基频光反射镜(5)优选采用体布拉格光栅，反射率>99％，优选大于99.8％，反射波长带宽<10nm，优选小于1nm，以最小程度的占用所需要产生的信号光和/或闲置光的频段。元件膜系反射率见下表。
Figure PCTCN2018106624-appb-000001
脉冲信号光(10)/脉冲闲置光(11)的振荡波长选择通过调节波长选择晶体(6)进行。波长选择晶体(6)优选双折射滤波片和法布里-珀罗标准具。在另一个优选的实施例中，图2a的结构也可以通过连续调谐波长选择晶体(6)形成波长在800-1060nm之间可调谐的光源，此时(S6)-(S10)面镀膜情况如下：
S6：增透，T>99.8％@1064nm；高反，R>99.8％@532nm&800-1060nm
S7，S8，S9：高透，T>99.8％@800-1064nm
S10：半反，R<90％@800-1060nm
根据本发明的另一个具体实施例，参照图2b，通过在准相位匹配晶体(4)上的(S71)面镀膜来保证对脉冲基频光(9)的高反。脉冲基频光(8)在前腔镜(1)的(S1)面和(S71)面之间谐振，产生的脉冲倍频光(9)在准相位匹配晶体(4)中向前传播。脉冲信号光(10)和/或脉冲闲置光(11)准相位匹配晶体(4)并在(S6)面和后腔镜(7)的(S10)面之间谐振。光能量从脉冲基频光(8)转化到脉冲倍频光(9)，进而转化到脉冲信号光(10)和/或脉冲闲置光(11)。
在一个优选实施例中，基频波长1064nm，倍频波长532nm，需要产生波长为900nm的信号光，(S71)面的镀膜情况如下：
S71：高反，R>99.8％@1064nm&532nm；高透，T>99.8％@900nm此结构节省了元件。由于通过镀介质膜的方式达到高反，反射带宽比较大。例如在优选的1064nm波长制作的高反介质膜，反射带宽约20-100nm，因此在一定程度上占用了产生所需要的脉冲信号光(10)和/或脉冲闲置光(11)的波长范围。
在另一个优选的实施例中，图2b的结构也可以通过连续调谐波长选择晶体(6)形成波长在800-1060nm之间可调谐的光源，此时(S71)镀膜情况如下：
S71：高反，R>99.8％@1064nm&532nm；高透，T>99.8％@800-1060nm
实施例二
根据本发明的第二实施例，本发明的方法及结构也可以产生连续近红外光输出，即通过对连续倍频光(9’)谐振的方法进一步降低参量振荡的阈值，以达到连续参量振荡输出。根据图3a，包括前腔镜(1)、激光晶体(2)、准相位匹配晶体(4)、基频-倍频光反射镜(51)、波长选择晶体(6)以及后腔镜(7)。同时图3a也示出了连续基频光(8’)、连续倍频光(9’)、连续信号光(10’)和连续闲置光(11’)在腔中的振荡路径。具体而言，参照图3a，采用的基频-倍频光反射镜(51)对连续基频光(8’)和连续倍频光(9’)的反射率都达到99.8％以上。连续倍频光(9’)在(S6)面和基频/倍频光反射镜(51)之间谐振使得腔内连续倍频光(9’)的功率密度大大增强，因此大大增加参量振荡连续信号光(10’)和/或连续闲置光(11’)的转换效率，从而降低振荡阈值，以达到连续波输出的目的。
基频-倍频光反射镜(51)优选采用体布拉格光栅。不同于基频光反射镜(5)仅对脉冲基频光(8)形成高反射，基频-倍频光反射镜(51)需要同时在连续基频光(8’)和连续倍频光(9’)处同时产生高反射。此时的布拉格光栅的折射率周期Λ B的调制深度需要增大，即折射率调制分布要从正 弦形式变为为矩形形式，以便使其二阶光栅在倍频光波长处也产生高反射。该方法使得布拉格光栅对基频光(8’)的反射带宽要加宽，优选0.1nm-10nm。另外可以通过增加体布拉格光栅厚度的方法来达到对倍频光波长产生高反射。
参照3b，为了进一步简化腔结构，类似图3a，基频-倍频光反射镜(51)使用(S72)面镀膜替代，(S72)对连续倍频光(9’)的反射率也达到99％以上，优选达到99.8％以上，以提高腔内连续倍频光(9’)的光功率密度。作为优选实施例，基频波长1064nm，倍频波长532nm，需要产生波长为900nm的信号光，镜面(S72)的镀膜情况如下：
S72：高反，R>99.8％@1064nm&532nm；高透，T>99.8％@900nm
同样的，由于通过镀介质膜的方式达到高反，反射带宽比较大。(S72)在1064nm和532nm反射带宽约10-100nm，因此在一定程度上占用了产生所需要的信号光/闲置光的波长范围。
在另一个优选的实施例中，图3b的结构也可以通过连续调谐波长选择晶体(6)形成波长在800-1060nm之间可调谐的光源，此时(S72)镀膜情况如下：
S72：高反，R>99.8％@1064nm&532nm；高透，T>99.8％@800-1060nm
本发明的方法也可以产生更短的光脉冲例如亚纳秒甚至皮秒脉冲输出。例如，使用被动调Q晶体如掺铬石榴石(Cr：YAG)、砷化镓(GaAs)晶体、或者半导体可饱和吸收镜(SESAM)，或者键合的掺铷石榴石-掺铬石榴石(Nd：YAG-Cr：YAG)晶体，通过设计掺杂比例、晶体尺寸以及腔结构，都可以产生高重复频率的亚纳秒的1064nm脉冲输出，配合本发明的腔内级联倍频-参量振荡方法及结构，可以实现800-1200nm范围内的亚纳秒脉冲输出。通过使用折叠腔，可以将腔长拉长以适应皮秒锁模工作模式，通过激光腔长的设计使得皮秒锁模脉冲输出的重复频率在标准的76MHz。
需要说明的是，由于本发明优选使用的准相位匹配晶体包括铌酸锂和钽酸锂，透明范围为250nm至6um，因此本发明的近红外激光结构及产生方法并不局限于近红外波长，在准相位匹配晶体透明范围内的其他任何波长完全可以利用本发明的方法和结构产生。例如，泵浦在1550nm波长附近的红外激光产生1400-1600nm的任意波长及可调谐波长。
另外，本发明使用凹凹腔作为示例的说明，然而各种激光腔结构都可以使用，例如平凹腔，平平腔，折叠腔等等，这并不违背本发明的精神。
以上所述仅为本发明的较佳实施例，并不用以限制本发明，凡在本发明的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。

Claims (10)

  1. 一种脉冲近红外激光器，其特征在于：包括：前腔镜(1)、激光晶体(2)、调Q晶体(3)、准相位匹配晶体(4)、基频光反射镜(5)、波长选择晶体(6)以及后腔镜(7)；其中：
    脉冲基频光(8)由激光晶体(2)和调Q晶体(3)产生并在前腔镜(1)的(S1)面和基频光反射镜(5)之间谐振，基频光反射镜(5)对脉冲基频光(8)高反，对其他波长高透；
    脉冲倍频光(9)由脉冲基频光(8)在准相位匹配晶体(4)中通过倍频过程产生并向前传播；
    脉冲信号光(10)和/或闲置光(11)由脉冲倍频光(9)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面谐振输出；(S10)面对脉冲信号光(10)和/或闲置光(11)部分反射，(S6)面对脉冲基频光(8)高透，对其他波长高反；
    波长选择晶体(6)对脉冲信号光(10)、脉冲闲置光(11)进行特定波长选择输出或者可调谐波长输出。
  2. 根据权利要求1所述的脉冲近红外激光器，其特征在于：脉冲基频光反射镜(5)由准相位匹配晶体(4)的端面(S71)代替；(S71)面对脉冲基频光(8)高反，对其他波长高透。
  3. 一种连续近红外激光器，其特征在于：包括前腔镜(1)、激光晶体(2)、准相位匹配晶体(4)、基频-倍频光反射镜(51)、波长选择晶体(6)以及后腔镜(7)；其中：
    连续基频光(8’)由激光晶体(2)产生并在前腔镜(1)的(S1)面和基频-倍频光反射镜(51)之间谐振，基频-倍频光反射镜(51)对连续基频光(8’)和连续倍频光(9’)高反，对其他波长高透；
    连续倍频光(9’)由连续基频光(8’)在准相位匹配晶体(4)中通过倍频过程产生并在(S6)面和基频-倍频光反射镜(51)之间谐振；
    连续信号光(10’)和/或连续闲置光(11’)由连续倍频光(9’)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面谐振输出；(S10)面对连续信号光(10’)和/或连续闲置光(11’)部分反射，(S6)面对连续基频光(8’)高透，对其他波长高反。
    波长选择晶体(6)对连续信号光(10’)、连续闲置光(11’)进行特定波长选择输出或者可调谐波长输出。
  4. 根据权利要求3所述的连续近红外激光器，其特征在于：基频-倍频光反射镜(51)由准相位匹配晶体(4)的端面(S72)代替；(S72)面对连续基频光(8’)和连续倍频光(9’)高反，对其他波长高透。
  5. 根据权利要求1-4任一项所述的近红外激光器，其特征在于：准相位匹配晶体(4)为周期极化晶体，包括周期极化铌酸锂、周期极化钽酸锂、周期极化磷酸钛氧钾，以及周期极化掺氧化镁铌酸锂、周期极化掺氧化镁钽酸锂；极化周期为均匀周期、啁啾周期以及多区周期。
  6. 根据权利要求1或2所述的脉冲近红外激光器，其特征在于：调Q晶体包括主动调Q晶体和被动调Q晶体。
  7. 根据权利要求1-4任一项所述的近红外激光器，其特征在于：波长选择晶体(6)包括双折射滤波片和法布里-珀罗标准具。
  8. 根据权利要求1或3所述的近红外激光器结构，其特征在于：基频光反射镜(5)与基频-倍频光反射镜(51)为体布拉格光栅。
  9. 一种采用权利要求1或2所述的近红外激光器产生脉冲激光的方法，其特征在于：采用激光腔内的级联倍频-参量振荡方式，准相位匹配晶体(4)中的级联二阶非线性效应等效三阶非线性效应，从而产生宽带参量增益；脉冲基频光(8)在前腔镜(1)的(S1)面和基频光反射镜(5)(或(S71) 面)之间谐振；脉冲倍频光(9)由脉冲基频光(8)在准相位匹配晶体(4)中通过倍频过程产生并前向传播；脉冲信号光(10)和/或脉冲闲置光(11)由脉冲倍频光(9)在准相位匹配晶体(4)中通过参量过程产生并在(S6)面和后腔镜(7)的(S10)面之间谐振；通过腔内谐振脉冲基频光(8)的方法增强光功率密度，使得腔内参量脉冲信号光(10)和/或脉冲闲置光(11)的光增益大大增强，从而降低参量振荡的阈值。
  10. 一种采用权利要求3或4所述的近红外激光器产生连续激光的方法，其特征在于：采用激光腔内的级联倍频-参量振荡方式，准相位匹配晶体(4)中的级联二阶非线性效应模拟了三阶非线性效应，从而产生宽带参量增益；连续基频光(8’)在前腔镜(1)的(S1)面和基频-倍频光反射镜(51)(或(S72)面)之间谐振，连续倍频光(9’)由连续基频光(8’)在准相位匹配晶体(4)中通过倍频过程产生并在端面(S6)和基频-倍频光反射镜(51)/端面(S72)之间谐振。连续信号光(10’)和/或连续闲置光(11’)由连续倍频光(9’)在准相位匹配晶体(4)中通过参量过程产生并在准相位匹配晶体(4)的面(S6)和后腔镜(7)的(S10)面之间谐振；通过同时增强谐振腔内的连续基频光(8’)和连续倍频光(9’)的光功率密度的方法，使得腔内参量连续信号光(10’)和/或连续闲置光(11’)的增益进一步增强从而进一步降低参量振荡阈值，以达到连续参量振荡。
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