WO2013102307A1 - 用4h碳化硅晶体制造的非线性光学器件 - Google Patents

用4h碳化硅晶体制造的非线性光学器件 Download PDF

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WO2013102307A1
WO2013102307A1 PCT/CN2012/070097 CN2012070097W WO2013102307A1 WO 2013102307 A1 WO2013102307 A1 WO 2013102307A1 CN 2012070097 W CN2012070097 W CN 2012070097W WO 2013102307 A1 WO2013102307 A1 WO 2013102307A1
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silicon carbide
laser
crystal
nonlinear optical
carbide crystal
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PCT/CN2012/070097
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English (en)
French (fr)
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陈小龙
王顺冲
彭同华
王刚
刘春俊
王文军
金士锋
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中国科学院物理研究所
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Priority to PCT/CN2012/070097 priority Critical patent/WO2013102307A1/zh
Priority to JP2014550604A priority patent/JP5898341B2/ja
Priority to EP12864638.7A priority patent/EP2801860A4/en
Priority to US14/370,510 priority patent/US9500931B2/en
Publication of WO2013102307A1 publication Critical patent/WO2013102307A1/zh

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G02F1/3551—Crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G02F1/361—Organic materials
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
    • G02F1/392—Parametric amplification
    • 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
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0064—Anti-reflection devices, e.g. optical isolaters
    • 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
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0092—Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
    • 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
    • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
    • H01S3/2383—Parallel arrangements
    • H01S3/2391—Parallel arrangements emitting at different wavelengths

Definitions

  • the present invention relates to a nonlinear optical device fabricated using 4H silicon carbide crystal, belonging to the field of materials or laser technology.
  • the mid-infrared band (3-5 ⁇ ) is an important window of the atmosphere.
  • the laser in this band has strong permeability to large fog, smoke and the like in the air, so the band laser can be used for laser guidance in the military. Photoelectric countermeasures and target detection.
  • most hydrocarbon gases and other toxic gas molecules are strongly absorbed in the 3-5 ⁇ band. Therefore, mid-infrared lasers are also widely used in gas detection, atmospheric remote sensing, and environmental protection.
  • nonlinear frequency conversion such as optical parametric oscillation, optical parametric amplification and difference frequency are the main means of generating mid-infrared laser.
  • nonlinear optical crystals include LiNb0 3 , KTiOP0 4 , A g GaS 2 , and ZnGeP 2 .
  • the above nonlinear crystal has a large nonlinear coefficient, its laser damage threshold is very low.
  • the laser damage threshold of LiNb0 3 is about 120MW/cm 2 (1.064 ⁇ , 30ns)
  • the laser damage threshold of KTiOP0 4 is about It is (1.064 ⁇ , 30ns)
  • AgGaS laser damage threshold 150MW / cm 2 2 ZnGeP 2 were approximately 25MW / cm 2 (1.064 ⁇ , 35ns ) and 3MW / cm 2 (1.064 ⁇ , 30ns )
  • Dmitriev Handbook of Nonlinear Optical Crystals Springer, Berlin, 1999, p. 118. Therefore, the above-mentioned mid-infrared nonlinear optical crystal is limited by the laser damage threshold, and is not widely used in many cases.
  • Silicon carbide crystals have more than 250 crystal forms, the most common of which are 3C silicon carbide, 4H silicon carbide and 6H silicon carbide. Among them, 4H and 6H silicon carbide have non-zero second-order nonlinear optical coefficients and have the following characteristics:
  • High laser damage threshold (6 ⁇ and 4 ⁇ silicon carbide laser damage thresholds are greater than 80GW / cm 2 (1.064 ⁇ , 10ns)) (see: Niedermeier et al. "Second-harmonic generation in silicon carbide polytypes" , Applied Physics Letter. 75, 618 (1999);
  • the crystal growth process is mature, and the optical quality of the crystal is high.
  • Both 4H and 6H silicon carbide crystals are positive uniaxial crystals (n. ⁇ n e ).
  • Accurate measurement of crystal refractive index (n. and ) is an important prerequisite for studying nonlinear optical properties.
  • the refractive index data of the crystal at a certain temperature uniquely determines whether the crystal satisfies the phase matching condition of the nonlinear optical frequency conversion in the light transmission range. Only when phase matching is achieved, nonlinear frequency conversion has higher efficiency, which leads to practical application.
  • Thibault In 1944, Thibault first measured the refractive index of 6H silicon carbide in the visible range (0.4047-0.6708 ⁇ ) using the minimum deflection angle method (see: Thibault's "Morphological and structural crystallography and optical properties of silicon carbide (SiC)", The american Mineralogist 29, 327 (1944) ), the accuracy of the test is about 3 ⁇ 10- 4. In 1968, Choyke et al. measured the 0-light refractive index ( n .) of 6H silicon carbide using Newton's isosceles interferometry and gave n.
  • the absorption spectrum of the silicon carbide crystal in this patent shows that the crystal has a shortest transmission wavelength of 0.4 ⁇ m, corresponding to the band gap G.OeV of 6 ⁇ silicon carbide; the test data of the refractive index also advances.
  • One point indicates that the crystal is 6H silicon carbide.
  • the patent proposes to use 6H silicon carbide as a nonlinear optical crystal for angular frequency matching, which can be used for frequency conversion such as frequency doubling and optical parametric, and the beam ⁇ participating in nonlinear optical frequency conversion, at least one laser has a wavelength greater than 1 ⁇ m.
  • the method of extrapolating the longer wavelength index based on the dispersion formula of the short-wavelength refractive index fit results in a large deviation in the refractive index data.
  • the new refractive index data provided by the inventors of the present invention indicates that 6 ⁇ silicon carbide crystals cannot be used for laser frequency doubling and optical parameters in the infrared band, that is, the inventions claimed by Smgh et al. and the inventions related to the same family patents. Content is impossible to achieve, see below for details.
  • the inventors of the present invention measures the refractive index of the 6H silicon carbide crystals in the visible and infrared light (0.4358-2.325 ⁇ ) of ( ⁇ . And n e), accuracy of about 3> ⁇ 5 through 10_ angle or minimum deviation, and intends The dispersion equation of the 6H silicon carbide crystal is combined.
  • the results of the inventors of the present invention are very close to the refractive index data of the prior literature in the visible light band, and there are significant differences in the infrared light band.
  • the experimental data of the inventors of the present invention show that the 6H silicon carbide crystal has a large dispersion in the infrared light band, and the refractive index of the 1971 Shaffer and the 1972 US patent (patent number: US3676695) extrapolated by the dispersion formula is in the infrared light.
  • the band dispersion is small.
  • the inventors of the present invention further calculated the phase matching of the nonlinear frequency conversion of the 6H silicon carbide crystal.
  • the 6H silicon carbide crystal point group is 6mm, and there is only a second type of angular phase matching.
  • For multiplier if angle phase matching is achieved, it should satisfy: ! ⁇ +! ⁇ nowadays ⁇ . (Pushed by the sine of the phase matching angle less than 1).
  • n le are the 0- and e-light refractive indices of the fundamental light, respectively. It is the 0-light refractive index of the frequency-doubled light.
  • the phase matching condition is: n 3 .
  • the technical problem to be solved by the present invention is to provide a nonlinear optical device fabricated using 4H silicon carbide crystal.
  • a nonlinear optical device comprising at least one nonlinear optical crystal for changing at least one laser beam having a specific frequency to generate at least one The beam is different from the laser of another specific frequency of the frequency, the nonlinear optical crystal being a 4H silicon carbide crystal.
  • a tunable mid-infrared laser includes a first pump light source and a second pump light source, the first pump light source and the second pump light source emitting different laser frequencies
  • the invention comprises a 4H silicon carbide crystal, wherein the laser light emitted by the first pump source and the second pump source is incident on the 4H silicon carbide crystal collinearly to perform a difference frequency to emit a mid-infrared laser.
  • an optical parametric amplification apparatus comprising a third pump light source, a broadband signal light laser, further comprising a 4H silicon carbide crystal, and a laser generated by the third pump light source and a broadband signal light laser generated The signal light is incident on the 4H silicon carbide crystal, and after the optical parametric amplification, a mid-infrared laser is emitted.
  • a broadband tunable mid-infrared laser comprising a fourth pump light source, which is a broadband pulse laser, further comprising a 4H silicon carbide crystal, and a pump outputted by the fourth pump light source
  • the high-frequency component and low-frequency component of Puguang are in the 4H silicon carbide crystal, and then the broadband infrared-infrared laser is emitted after filtering the light.
  • Figure 1 is a dispersion curve of 6H silicon carbide crystal no and a comparison with the previous literature no data.
  • Figure 2 is the dispersion curve of the 6H silicon carbide crystal ne and its comparison with the previous literature ne data.
  • Figure 3 is a transmission curve of a 4H silicon carbide crystal.
  • Figure 4 is a dispersion curve of a 4H silicon carbide crystal.
  • Figure 5 is a schematic view showing the structure of Embodiment 1 of the present invention.
  • Figure 6 is a schematic view showing the structure of Embodiment 2 and Embodiment 3 of the present invention.
  • Fig. 7 is a second type of difference frequency phase matching tuning curve of the first pumping source and the second pumping source of the second embodiment of the present invention.
  • Figure 8 is a second type of difference frequency phase matching tuning curve of the first pumping source and the second pumping source of Embodiment 3 of the present invention.
  • Figure 9 is a schematic view showing the structure of Embodiment 4 of the present invention.
  • Figure 10 is an optical parametric amplifying phase matching tuning curve of a third pumping light source according to Embodiment 4 of the present invention.
  • Figure 11 is a schematic view showing the structure of Embodiment 5 of the present invention.
  • Fig. 12 is a view showing the angle of incidence of the incident laser light and the angle of the main section of the crystal in the fifth embodiment of the present invention.
  • Figure 13 is a spectrum chart of a fourth pumping light source of Embodiment 5 of the present invention.
  • Fig. 14 is a view showing a mid-infrared laser spectrum obtained by the difference frequency of the fifth embodiment of the present invention.
  • the nonlinear optical crystal provided by the invention is 4H silicon carbide, and its chemical formula is ffl-SK ⁇ 4H.
  • the effective second-order nonlinear optical polarization coefficient of the silicon carbide crystal is d efi ⁇ d 15 sme, due to the point group of the 4H-SiC crystal.
  • phase II phase matching that is, the polarization directions of the two incident lights are inconsistent, one beam is 0 light and the other beam is e light, and this phase matching method is called the third phase matching
  • ⁇ is the phase matching angle
  • the 4H silicon carbide crystal does not have a symmetry center, belongs to a hexagonal system, and has a space group of P6 3 mc, wherein each unit cell contains four layers of carbon silicon atoms arranged in an ABCB manner.
  • the growth method of the 4H silicon carbide crystal includes a physical vapor transport method, a high temperature chemical vapor deposition method, or a liquid phase method.
  • high purity 4H silicon carbide crystal can be obtained by controlling the purity of the silicon carbide raw material and the growth chamber consumable; or the crystal transmittance can be improved by artificial doping.
  • n-type impurities (nitrogen) in a crystal for example, by p-type doping (aluminum or boron doping), or by doping deep-level vanadium to compensate for shallow-level donors (nitrogen) or acceptors (boron or Aluminum), etc., can also compensate for shallow-level donors or acceptors by introducing point defects to achieve high transmittance of 4H silicon carbide crystals.
  • the preparation method of the 4H silicon carbide crystal includes a physical vapor transport method, a high temperature chemical vapor deposition method or a liquid phase method.
  • the inventors of the present invention have grown a high-permeability 4H silicon carbide crystal by a physical vapor phase transfer method, and the transmittance spectrum thereof is shown in Fig. 3.
  • the high-temperature chemical vapor deposition method and the liquid phase method can also obtain a high-transmittance 4H silicon carbide crystal by the above principle.
  • the inventors of the present invention tested the refractive index of the 4H silicon carbide crystal in the visible and infrared light bands (0.4047-2.325 ⁇ ) with a minimum deflection angle method with an accuracy of about 3 X 10 - 5 , and measured 4 ⁇ by any deflection method.
  • the SiO refractive index of the silicon carbide crystal in the mid-infrared band (3-5 ⁇ m) is fitted to its dispersion equation.
  • the refractive index of the 4 ⁇ silicon carbide crystal measured by the minimum deflection angle method in the visible and infrared light bands, and the 0 ⁇ refractive index of the 4 ⁇ silicon carbide crystal measured by any deflection method in the mid-infrared band are shown in Table 1. , where ⁇ .
  • the 0-ray refractive index of 4 ⁇ silicon carbide is the e-light refractive index of 4 ⁇ silicon carbide.
  • the unit of the wavelength ⁇ is micrometer.
  • the dispersion curve of the 4 ⁇ silicon carbide crystal is shown in Fig. 4.
  • 4 ⁇ silicon carbide has a larger birefringence than 6 ⁇ silicon carbide, which makes it possible for 4 ⁇ silicon carbide crystals to achieve phase matching of nonlinear optical frequency conversion.
  • the inventors of the present invention have found that a 4 ⁇ silicon carbide crystal can realize a nonlinear optical frequency change of an output mid-infrared laser beam.
  • the phase matching makes the 4H silicon carbide crystal better meet the practical application requirements in the tunable output of 3.4-7.1 ⁇ infrared laser, which has significant practical application value.
  • Example 1 A specific embodiment of a tunable mid-infrared laser fabricated using a 4 ⁇ silicon carbide crystal is described below.
  • Example 1 A specific embodiment of a tunable mid-infrared laser fabricated using a 4 ⁇ silicon carbide crystal is described below.
  • Example 1 A specific embodiment of a tunable mid-infrared laser fabricated using a 4 ⁇ silicon carbide crystal is described below.
  • Example 1 A specific embodiment of a tunable mid-infrared laser fabricated using a 4 ⁇ silicon carbide crystal is described below.
  • a nonlinear optical device includes at least one laser beam as incident light, and after passing through at least one nonlinear optical crystal, generates at least one laser output having a frequency different from the wavelength of the incident light.
  • the nonlinear optical crystal is a 4 ⁇ silicon carbide crystal.
  • the optical device realizes the output of the tunable medium-infrared laser by optical parametric amplification, optical parametric oscillation or difference frequency techniques.
  • FIG. 5 is a schematic view showing the operation of Embodiment 1 of the present invention, in which a laser (11) emits an incident laser beam (12) which sequentially passes through a silicon carbide crystal (13) and a filter (15), (14). Is a laser beam that is emitted by a process of optical parametric amplification, optical parametric oscillation or difference frequency by a 4 ⁇ silicon carbide crystal (13), and the filter (15) functions to filter out the wavelength of the incident laser beam (12).
  • Reference numeral (16) is an outgoing mid-infrared laser.
  • the 4 ⁇ silicon carbide crystal used is a 4 ⁇ silicon carbide crystal as described above, and
  • the wavelength of the incident light of the laser ranges from 0.38 to 5.5 ⁇ m, and the transmittance of the silicon germanium crystal is greater than 10% in the wavelength range of 0.38-5.5 ⁇ m and 6.7-6.9 ⁇ m.
  • the incident light wavelength is preferably 0.8 ⁇ m, and at this time, the light transmittance is more than 40%.
  • the present invention is not limited thereto, and the incident light wavelength may be any value between 0.38 ⁇ m, 0.55 ⁇ m, or 0.38-5.5 ⁇ m.
  • phase matching method of the nonlinear optical frequency conversion of the 4 ⁇ silicon carbide crystal is the first ⁇ phase matching
  • phase matching of the 4 ⁇ silicon carbide crystal may be achieved by adjusting the crystal temperature to achieve critical phase matching
  • the at least one side of the 4 ⁇ silicon carbide crystal is optically polished
  • the surface of the 4 ⁇ silicon carbide crystal is plated with an anti-reflection film, a high-reflection film, and/or a semi-permeable film.
  • Example 2 The surface of the 4 ⁇ silicon carbide crystal is plated with an anti-reflection film, a high-reflection film, and/or a semi-permeable film.
  • Embodiment 2 of the present invention is a tunable mid-infrared laser fabricated from 4H silicon carbide crystal.
  • the structure is schematically shown in FIG. 6.
  • the tunable mid-infrared laser includes a first pumping source (21) and a second pump.
  • the laser light emitted by the first pumping source passes through the polarizing plate (24), the isolator (26), the mirror (28), the dichroic mirror (29), the converging lens (210), and the 4H silicon carbide crystal (211);
  • the laser light emitted from the second pumping source passes through the polarizing plate (25), the isolator (27), the dichroic mirror (29), the converging lens (210), and the 4H silicon carbide crystal (211).
  • the first pump source has a wavelength of 0.8-0.9 ⁇ m and the second pump source has a wavelength of 1.064 ⁇ m.
  • the first pumping source and the second pumping source may be a mode-locked laser or a Q-switched laser; the mode-locking mode may be active mode-locking, passive mode-locking or self-locking mode; the Q-switching mode may be active electro-optic Q-switching, sounding Light Q or passive 0.
  • the first pump source may be a tunable titanium sapphire laser and the second pump source may be a Nd:YAG laser.
  • the pumping mode of the first pumping source and the second pumping source may be xenon pumping, semiconductor laser pumping or solid state laser pumping.
  • the gain medium of the second pumping source may be Nd:YAG, Nd:YV0 4 or Nd:YLF.
  • the convergent lens is collinear under the condition of satisfying the specified phase matching. It is incident on the 4H silicon carbide crystal for difference frequency, and then passes through the filter (212) to emit the infrared laser.
  • the 4H silicon carbide crystal used is the 4H silicon carbide crystal as in Example 1, and
  • the wavelength of the incident light of the laser is in the range of 0.7-0.9 ⁇ m and 1.064 ⁇ m, and the transmittance of the 4 ⁇ silicon carbide crystal is greater than 10% in the wavelength range of 0.38-5.5 ⁇ m and 6.7-6.9 ⁇ .
  • the incident light wavelength is preferably 0.838 ⁇ m, and the light transmittance thereof is greater than 40%.
  • the present invention is not limited thereto, and the incident light wavelength may be any value between 0.38 ⁇ m, 0.55 ⁇ m, or 0.38-5.5 ⁇ m.
  • phase matching method of the nonlinear optical frequency conversion of the 4 ⁇ silicon carbide crystal is the second type phase matching
  • phase matching of the 4 ⁇ silicon carbide crystal may be achieved by adjusting the crystal temperature to achieve critical phase matching
  • the at least one side of the 4 ⁇ silicon carbide crystal is optically polished
  • the surface of the 4 ⁇ silicon carbide crystal is plated with an antireflection film, a high reflective film and/or a semipermeable film.
  • the cutting angle of the 4 ⁇ silicon carbide crystal is ⁇ , that is, the angle between the crystal light passing direction and the crystal optical axis.
  • the tunable output of the mid-infrared difference frequency light is achieved by adjusting the output wavelength of the first pump light and the angle of the 4 ⁇ silicon carbide crystal.
  • Fig. 7 is a view showing the relationship between the wavelength of the first pumping light source and the wavelength of the difference frequency light and the corner angle of the second embodiment.
  • the value of the crystal cutting angle ⁇ is generally 79°-89°, for example, 82°, and the wavelength of the first pumping source may be 0.8 ⁇ -0.9 ⁇ , preferably 0.838 ⁇ , the wavelength range of the difference frequency light. It is 3.6-5.3 ⁇ m, preferably 3.945 ⁇ m.
  • Example 3
  • the tunable mid-infrared laser of Example 3 was constructed in the same manner as in Example 2, and the same was used for the 4 ⁇ silicon carbide crystal. The difference is that the first pump source and the second pump source are both Ti:Sapphire lasers.
  • Figure 8 shows the wavelength of the mid-infrared light and the crystal of the difference frequency output when the wavelengths of the first pump source are 0.7, 0.72, 0.74, 0.76, and 0.78 ⁇ m, and the wavelength of the second pump source is 0.7-0.9 ⁇ m. The relationship between the cutting angles.
  • the mid-infrared light having a difference frequency output has a wavelength range of 3.6 to 7 ⁇ m, preferably 4.0 ⁇ m, and a crystal cut angle ⁇ of 73 to 89, preferably 81.5.
  • Example 4
  • Embodiment 4 of the present invention is an optical parametric amplifying device manufactured using 4H silicon carbide crystal.
  • the optical parametric amplifying device includes a third pumping light source (41), a broadband signal light laser (42), A dichroic mirror (43), a converging lens (44), a 4H silicon carbide crystal (45), and a filter (46), wherein the third pumping source is a 532 nm laser.
  • the laser beam emitted by the third pumping source (41) sequentially passes through the dichroic mirror (43), the converging lens (44), and the 4H silicon carbide crystal (45).
  • the signal light emitted by the broadband signal light laser (42) passes through the dichroic mirror (43), the converging lens (44) and the 4H silicon carbide crystal (45) in sequence.
  • the 532 nm laser as the third pump source is multiplied by a 1.064 ⁇ laser, and the doubling crystal is BBO, LBO, KDP, KTP or CLBO.
  • the 532nm laser can be a mode-locked pulsed laser or a Q-switched laser.
  • the Q-switching method can be active electro-optic Q-switching, acousto-optic Q-switching or passive Q-switching.
  • the pumping method is xenon lamp pumping, semiconductor laser pumping or solid state laser pumping.
  • the signal light generated by the 532 nm pump light and the broadband signal light laser (42) passes through the converging lens (44) and is incident on the 4H silicon carbide crystal (45). After the optical parametric amplification, the mid-infrared light is generated, and then the filter is filtered. (46) After obtaining a mid-infrared laser.
  • the 4H silicon carbide crystal was the same as the 4H silicon carbide crystal used in Examples 1-3.
  • Figure 10 shows the relationship between the wavelength of the mid-infrared light and the angle of the pupil generated by optical parametric amplification when the pump wavelength is 532 nm.
  • the value of the crystal cut angle ⁇ is generally 72°-88°, preferably 78°, and the mid-infrared light range is 4.3-7 ⁇ m, preferably 4.756 ⁇ m.
  • Embodiment 5 of the present invention is a broadband tunable mid-infrared laser fabricated by using 4 ⁇ silicon carbide crystal.
  • the structure is shown in FIG. 11.
  • the broadband tunable mid-infrared laser includes a fourth pump source (51) and a converging lens. (54), 4 ⁇ silicon carbide crystal (55) and filter (56).
  • the laser light emitted by the fourth pumping source passes through the converging lens (54), the 4 ⁇ silicon carbide crystal (55), and the filter (56).
  • the pump light (52) emitted by the fourth pumping source (51) is linearly polarized light, and the polarization direction (53) has an angle ⁇ with the main section of the crystal, and satisfies 0 ⁇ 90°, and ⁇ is preferably 4 ⁇ .
  • reference numeral (58) denotes the intersection of the main section of the crystal and the plane of the paper.
  • the fourth pumping source is an amplifying femtosecond titanium sapphire laser having a repetition frequency of IKHz, a pulse width of 20 fs, and a spectral range of 500-1000 nm
  • FIG. 13 is the output of the fourth pumping source (51) of the fifth embodiment.
  • Supercontinuous laser spectroscopy The pump light (52) output from the fourth pump source simultaneously generates 0 light and e light in the 4H silicon carbide crystal, using high frequency components and low frequency components in the supercontinuum broad spectrum femtosecond pulse in the 4H silicon carbide crystal (55) In the direct difference frequency, after passing through the filter (56), the infrared laser is emitted.
  • the 4H silicon carbide crystal used is the 4H silicon carbide crystal as in Example 1, and
  • the wavelength of the incident light of the laser ranges from 0.38 to 1.0 ⁇ m, and the transmittance of the silicon germanium crystal is greater than 10% in the wavelength range of 0.38-5.5 ⁇ m and 6.7-6.9 ⁇ m.
  • phase matching method of the nonlinear optical frequency conversion of the 4 ⁇ silicon carbide crystal is the second type phase matching
  • phase matching of the 4 ⁇ silicon carbide crystal may be achieved by adjusting the crystal temperature to achieve critical phase matching
  • the at least one side of the 4 ⁇ silicon carbide crystal is optically polished
  • the surface of the 4 ⁇ silicon carbide crystal is plated with an antireflection film, a high reflective film and/or a semipermeable film.
  • the cutting angle of the 4 ⁇ silicon carbide crystal is ⁇ , that is, the angle between the crystal light passing direction and the crystal optical axis.
  • the value of the crystal cut angle ⁇ is generally 79°-89°, and the wavelength of the difference frequency light is 3.6-7 ⁇ m.
  • Fig. 14 is a view showing the broadband mid-infrared difference frequency spectrum obtained when the crystal cutting angle of the embodiment 5 is 82°.
  • the above embodiment of the present invention uses a 4 ⁇ silicon carbide crystal to realize a mid-infrared nonlinear optical frequency conversion, which is higher than the existing nonlinear optical device due to the 4 ⁇ silicon carbide crystal.

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Abstract

提供一种用4H碳化硅晶体制造的非线性光学器件。该非线性光学晶体用于改变具有特定频率的至少一束激光（12），产生至少一束不同于所述频率的另一特定频率的激光（16），所述非线性光学晶体为4H碳化硅晶体（13）。由于4H碳化硅晶体具有很高的激光损伤阙值、较宽的透光范围（0.38-5.9μm及6.6-7.08μm）、较大的二阶非线性光学系数（d15=6.7pm/V）、较大的双折射、高热导率（490Wm-1Κ-1）以及高化学稳定性等特点，使得本发明的非线性光学器件在输出高功率、高光束质量的中红外激光方面更好地满足实际应用需求，具有显著地实际应用价值。

Description

用 4H碳化硅晶体制造的非线性光学器件 技术领域 本发明涉及一种用 4H碳化硅晶体制造的非线性光学器件， 属于材 或和激光技术领域。
背景技术 中红外波段 （3-5μπι) 是大气的一个重要窗口， 该波段的激光对空 气中的大雾、 烟尘等具有很强的透过能力， 因而该波段激光在军事上可 用于激光制导、 光电对抗及目标探测等。 另外， 多数的碳氢气体及其它 有毒的气体分子在 3-5μπι波段有很强的吸收， 因此， 中红外激光在气体 探测、 大气遥感和环境保护等领域也有着广泛的应用。
由于缺乏直接的激光增益介质， 非线性频率变换如光参量振荡、 光 参量放大及差频等是产生中红外激光的主要手段。 在 3-5μπι波段， 一般 采用的非线性光学晶体有 LiNb03、 KTiOP04、 AgGaS2及 ZnGeP2等。 上 述非线性晶体虽然具有较大的非线性系数， 但其激光损伤阈值都很低， 其中， LiNb03的激光损伤阈值约为 120MW/cm2 ( 1.064μπι， 30ns ) , KTiOP04的激光损伤阈值约为 150MW/cm2 ( 1.064μπι， 30ns) , AgGaS2 及 ZnGeP2的激光损伤阈值约分别为 25MW/cm2 ( 1.064μπι， 35ns) 和 3MW/cm2 ( 1.064μπι， 30ns ) (详见： Dmitriev 等人的 Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1999, p.118 )。因此， 上述中红 外非线性光学晶体受到激光损伤阈值的限制， 在很多场合得不到广泛的 应用。
碳化硅晶体具有 250多种晶型， 其中最常见的有 3C碳化硅、 4H碳 化硅和 6H碳化硅，其中 4H和 6H碳化硅具有非零的二阶非线性光学系 数， 并具有以下特点：
1.具有较大的二阶非线性光学系数4H碳化硅： d15=6.7pm/V; 6H 碳化 ¾έ:
Figure imgf000003_0001
(详见： Sato等人的 "Accurate measurements of second-order nonlinear optical coefficients of 6H and 4H silicon carbide", Journal of the Optical Society of America B 26, 1892 (2009) )； 2.在可见和红外光区有较高的透过率 （4H 碳化硅透光范围为 0.38-5.5μπι， 6Η碳化硅透光范围为 0.4-5.5μπι);
3.具有较高的激光损伤阈值（6Η和 4Η碳化硅的激光损伤阈值均大 于 80GW/cm2 ( 1.064μπι, 10ns ) ) (详见： Niedermeier 等人的 " Second-harmonic generation in silicon carbide polytypes "， Applied Physics Letter. 75， 618 (1999》；
4.热导率高（6H和 4H 的热导率均为 490Wm— i 1 ) ,化学稳定性好， 不潮解；
5.晶体生长工艺成熟， 晶体光学质量较高。
4H和 6H碳化硅晶体均为正单轴晶体 （n。<ne)， 精确测量晶体的折 射率 （n。及 ) 是研究其非线性光学性质的重要前提。 一定温度下晶体 的折射率数据唯一决定了该晶体在透光范围内是否满足非线性光学频 率变换的相位匹配条件。 只有实现相位匹配时， 非线性频率转换才有较 高的效率， 进而得到实际应用。
1944年， Thibault采用最小偏向角法率先测量了 6H碳化硅在可见 光波段 （0.4047-0.6708μπι) 的折射率 (详见： Thibault的 "Morphological and structural crystallography and optical properties of silicon carbide (SiC) "， The American Mineralogist 29， 327 (1944))， 测试的精度约为 3 χ 10— 4。 1968年， Choyke等人采用牛顿等倾干涉法测量了 6H碳化硅的 0光折射率（n。）， 并把 n。扩展到紫外和红外波段， 测量精度约为 2x 10— 3 (详见： Choyke等人的 "Refractive index and low-frequency dielectric constant of 6H SiC"， Journal of the Optical Society of America 58， 377 (1968) ) o 1971 年， Shaffer 测量了 4H 和 6H 碳化硅在可见光波段 (0.467-0.691μπι) 的折射率， 并拟合了它们的色散方程， 测量精度约为 1 X 1CT3 (详见： Shaffer的 "Refractive index, dispersion, and birefringence of silicon carbide polytypes", Applied Optics 10， 1034 (1971))。
1972年的美国专禾 [I "Nonlinear optical devices utilizing substantially hexagonal silicon carbide " (专利号： US3676695 ) 及其同族专利 （CA 962755， NL 7210039， SE 3676695， IT 964758， GB 1375638， FR 2147103， DE 2235800及 BE 786555 )通过最小偏向角法测试了一个六方结构的碳 化硅在 6个波长下（0.488， 0.5017， 0.5145， 0.5321， 0.6328及 1.064μπι) 的折射率。 该专利中碳化硅晶体的吸收谱显示， 该晶体的最短透过波长 为 0.4μπι， 对应于 6Η碳化硅的带隙 G.OeV); 其折射率的测试数据也进 一歩表明该晶体为 6H碳化硅。该专利提出采用 6H碳化硅作为非线性光 学晶体通过角度相位匹配可用于倍频及光参量等频率变换， 而且参与非 线性光学频率变换的光束屮， 至少有一束激光的波长大于 1μπι。 上述专 利的发明人 Singh等人在随后发表的文章中指出 6H碳化硅在基频光波 长大于 2μπι时可以实现倍频相位匹配， 特别是当基频光波长为 2.128μπι 时， 倍频相位匹配角约为 75° (详见： Singh等人的 "Nonlinear optical properties of hexagonal silicon carbide", Applied Physics Letters 19， 53 (1971))。 值得注意的是， 该专利测试 6H碳化硅折射率所使用的光源的 最长波长仅为 1.064μπι， 而非线性光学频率变换的波长涉及到波长较长 的红外光波段（如 2.128μπι)， 通过基于短波长的折射率拟合的色散公式 外推较长波长折射率的方法会导致折射率数据有很大的偏差。 本发明的 发明人提供的新的折射率数据表明， 6Η碳化硅晶体不可能在红外波段 内用于激光倍频及光参量， 即 Smgh等人所申请的专利及其同族专利中 所涉及的发明内容是不可能实现的， 详见下述内容。
1985年 Choyke等人在文献中提供的 6H碳化硅的折射率 n。数据大 部分取自于上述的 1944年 Thibault, 1968年 Choyke等人及 1971 年 Shaffer等人文献中的数据，并且只是将这些文献中的折射率 n。的数据进 行简单的堆砌； 由于这三篇文献测试折射率时采用不同的测试方法， 使 得 Choyke等人的文献提供的折射率 n。数据存在许多矛盾的地方； 根据 现有知识可知， 随着波长的增加， 折射率 n。的数值应当是减小的， 然而 Choyke 等人的文献中报道的折射率 n。的数据并不如此： 比如波长为 0.4959μπι 的 η。数值为 2.684， 反而小于 0.498μπι 的 η。数值（2.687 )等
(详见： Choyke 等人的 Handbook of Optical Constants of Solids， Academic, New York, 1985， p.593 )。 2003年， Baugher等人测量了 6H 碳化硅晶体的双折射 （ne-n。） 数值， 想当然地采用 1985年 Choyke等在 文献中报道的折射率 n。的数据， 计算指出 6H碳化硅晶体可以满足光参 量振荡的相位匹配条件（详见： Baugher等人的 "Temperature dependence of the birefringence of SiC"， Optical Materials 23 , 519 (2003))。 Baugher 等人仅测量了 6H碳化硅晶体的双折射， 然而引用了不正确的折射率数 据， 因此实际上光参量振荡的相位匹配条件是无法实现的。
由此可见， 现有文献对 6H碳化硅晶体折射率的测试大都集中在可 见光波段， 在波长更长的红外光波段， 折射率数据极其缺乏； 而 6H碳 化硅晶体的非线性光学频率变换主要涉及到红外光波段， 为了减小通过 色散公式外推折射率引起的误差， 精确测试 6H碳化硅在红外光波段的 折射率显得十分重要。
本发明的发明人通过最小偏向角法测量了 6H碳化硅晶体在可见及 红外光波段 (0.4358-2.325μπι) 的折射率 （η。及 ne)， 精度约为 3>< 10_5， 并拟合了 6H碳化硅晶体的色散方程。 通过与上述文献的折射率数据对 比发现（如图 1、 图 2所示）， 本发明的发明人的结果在可见光波段与之 前文献的折射率数据十分接近， 而在红外光波段有显著差异。 本发明的 发明人的实验数据表明 6H碳化硅晶体在红外光波段有较大的色散， 而 1971年 Shaffer及 1972年美国专利 （专利号： US3676695 ) 通过色散公 式外推得到的折射率在红外光波段色散较小。
本发明的发明人进而计算了 6H碳化硅晶体非线性频率变换的相位 匹配情况。 6H碳化硅晶体点群为 6mm, 只存在第二类角度相位匹配。 对于倍频， 若实现角度相位匹配， 则应满足： ！^+！^〉?!^。 （由相位匹配 角的正弦值小于 1推得）， 。和 nle分别为基频光的 0光与 e光折射率， 。为倍频光的 0光折射率。 由于 6H碳化硅在红外光波段色散较大而双 折射相对较小， 计算结果表明， 6H碳化硅晶体在透光波段（0.4-5.5μπι) 不能实现倍频相位匹配。 对于光参量或差频等非线性频率变换， 相位匹 配条件为： n3。(D3-nle(e) ωι= η2οω2 , 其中， ω3和 ωι为泵浦光的频率， ω2 为红外光的频率； η3。和 η2。分别为泵浦光 0)3和红外光 ω2的 ο光折射率， nle^为与光轴夹角为 Θ方向上的泵浦光 ωι的 e光的折射率。通过计算， 6H碳化硅在透光范围内光参量或差频也不能实现角度相位匹配。 美国 专利 （专利号： US3676695 )及 2003年 Baugher等人的计算采用了错误 的折射率数据， 得出了 6H碳化硅晶体可以实现中红外非线性频率变换 相位匹配的错误结论。
1971 年， Shaffer 测量了 4H 碳化硅在可见光波段的折射率
(0.467-0.691μπι) (详见： Shaffer的 "Refractive index, dispersion, and birefringence of silicon carbide polytypes " , Applied Optics 10， 1034 (1971))o 但迄今为止， 尚未见到有关 4H碳化硅晶体非线性光学性能及 将 4H碳化硅晶体用于制作非线性光学器件的报道。 发明内容
(一） 要解决的技术问题
本发明要解决的技术问题是提供一种用 4H碳化硅晶体制造的非线 性光学器件。
(二） 技术方案
为解决上述技术问题， 根据本发明的第一方面， 提供一种非线性光 学器件， 包括至少一个非线性光学晶体， 该非线性光学晶体用于改变具 有特定频率的至少一束激光， 产生至少一束不同于所述频率的另一特定 频率的激光， 所述非线性光学晶体为 4H碳化硅晶体。
根据本发明的第二方面， 提供一种可调谐中红外激光器， 包括第一 泵浦光源、 第二泵浦光源， 该第一泵浦光源与该第二泵浦光源发射的激 光频率不同， 还包括一个 4H碳化硅晶体， 其中第一泵浦光源和第二泵 浦光源发射的激光共线入射到该 4H碳化硅晶体上进行差频， 以出射一 中红外激光。
根据本发明的第三方面， 提供一种光参量放大装置， 包括第三泵浦 光源、 宽带信号光激光器， 还包括 4H碳化硅晶体， 并且第三泵浦光源 发射的激光和宽带信号光激光器产生的信号光入射到 4H碳化硅晶体 上， 经过光参量放大后， 出射一中红外激光。
根据本发明的第四方面， 提供一种宽带可调谐中红外激光器， 包括 第四泵浦光源， 该泵浦光源为宽带脉冲激光器， 还包括 4H碳化硅晶体， 利用第四泵浦光源输出的泵浦光的高频成分和低频成分在 4H碳化硅晶 体中差频， 再经过滤光片后出射宽带中红外激光。 (三） 有益效果
本发明的用 4H碳化硅晶体制造的非线性光学器件采用 4H碳化硅晶 体实现中红外非线性光学频率变换， 与现有的非线性光学器件相比， 由 于 4H碳化硅晶体具有很高的激光损伤阈值、 较宽的透光范围 （0.38-5.9 μ πι及 6.6-7.08 μ πι)、 较大的二阶非线性光学系数 （dl5=6.7pm/V)、 较 大的双折射、 高热导率 （490Wm-lK-l ) 以及高化学稳定性等特点， 使 得本发明的非线性光学器件在输出高功率、 高光束质量的中红外激光方 面更好地满足实际应用需求， 具有显著地实际应用价值。
附图说明 图 1是 6H碳化硅晶体 no的色散曲线及与之前文献 no数据的对比。 图 2是 6H碳化硅晶体 ne的色散曲线及与之前文献 ne数据的对比。 图 3是 4H碳化硅晶体的透过率曲线。
图 4是 4H碳化硅晶体的色散曲线。
图 5是本发明实施例 1的结构示意图。
图 6是本发明实施例 2与实施例 3的结构示意图。
图 7是本发明实施例 2的第一泵浦光源与第二泵浦光源的第 II类差 频相位匹配调谐曲线。
图 8是本发明实施例 3的第一泵浦光源与第二泵浦光源的第 II类差 频相位匹配调谐曲线。
图 9是本发明实施例 4的结构示意图。
图 10是本发明实施例 4的第三泵浦光源的光参量放大相位匹配调 谐曲线。
图 11是本发明实施例 5的结构示意图。
图 12是本发明实施例 5的入射激光偏振方向与晶体主截面夹角示 意图。
图 13是本发明实施例 5的第四泵浦光源的光谱曲线。
图 14是本发明实施例 5的差频所得的中红外激光光谱图。
具体实施方式 为使本发明的目的、 技术方案和优点更加清楚明白， 以下结合具体 实施例， 并参照附图， 对本发明进一歩详细说明。 本发明提供的非线性光学晶体为 4H碳化硅， 其化学式为 ffl-SK^ 4H 碳化硅晶体的有效二阶非线性光学极化系数为 defi^d15sme， 由于 4H-SiC 晶体的点群为 6mm， 所以只存在第 II类相位匹配 （即入射的两 束光的偏振方向不一致， 一束光为 0光， 另一束为 e光， 此种相位匹配 方式称为第 Π类相位匹配）， Θ为相位匹配角度。
所述 4H碳化硅晶体不具有对称中心，属六方晶系，空间群为 P63mc， 其中每个晶胞内含有四层碳硅原子层， 按 ABCB方式排列而成。
4H碳化硅晶体的生长方法包括物理气相传输法、 高温化学气相沉 积法或液相法。 为使 4H碳化硅晶体拥有较高的透过率， 可以通过控制 碳化硅原料和生长室内耗材的纯度， 获得高纯的 4H碳化硅晶体； 或者 通过人为掺杂的方式来提高晶体的透过率： 例如通过 p型掺杂 （掺铝或 硼） 来补偿晶体中的 n型杂质 （氮）， 或通过掺杂深能级的钒来补偿浅 能级的施主 （氮） 或者受主 （硼或铝） 等， 也可以通过引入点缺陷的方 式来补偿浅能级的施主或受主， 实现 4H碳化硅晶体的高透过率。
所述 4H碳化硅晶体的制备方法包括物理气相传输法、 高温化学气 相沉积法或液相法。 本发明的发明人通过物理气相传输法生长出了高透 过率的 4H碳化硅晶体， 其透过率光谱如图 3所示。 但是， 需要说明的 是， 高温化学气相沉积法以及液相法采用上述原理同样可以获得高透过 率的 4H碳化硅晶体。
本发明的发明人采用最小偏向角法测试了 4H碳化硅晶体在可见及 红外光波段 (0.4047-2.325μπι) 的折射率， 精度约为 3 X 10—5, 同时通过 任意偏折法测量了 4Η碳化硅晶体在中红外波段（3-5μπι)的 ο光折射率， 并拟合了其色散方程。
通过最小偏向角法测量的 4Η碳化硅晶体在可见以及红外光波段的 折射率， 以及通过任意偏折法测量的 4Η碳化硅晶体在中红外波段的 0 光折射率的测试结果如表 1所示， 其中 η。为 4Η碳化硅的 0光折射率， 为 4Η碳化硅的 e光折射率。
表 1 : 室温下 4H碳化硅晶体的折射率测试结果
波长 (μιη) 0.4047 2.75980 2.82891
0.4358 2.72794 2.79222
0.48 2.69479 2.75447
0.5461 2.66131 2.71665
0.5875 2.64634 2.69984
0.6438 2.63085 2.68251
0.7065 2.61792 2.66811
0.8521 2.59832 2.64638
1.014 2.58290 2.63203
1.53 2.56448 2.60938
1.8 2.55736 2.60173
1.97 2.55325 2.59735
2.325 2.54479 2.58837
3.0 2.5288
3.5 2.5131
4.0 2.4932
4.5 2.4704
5.0 2.4440
体的折射率:
0.20075/12 5.5486U2 35.65066/1
1 +— +— +■
A2 + 12.07224 A2 - 0.02641 A2 - 1268.24708
η2 (λ) = 6.79485 + 0 15558—— 0.02296/12
A2 - 0.03535
其中波长 λ的单位为微米。 4Η碳化硅晶体的色散曲线如图 4所示。 4Η碳化硅与 6Η碳化硅相比有较大的双折射， 这使得 4Η碳化硅晶 体有可能实现非线性光学频率变换的相位匹配。 本发明的发明人实验发 现， 4Η碳化硅晶体可以实现输出中红外波段激光的非线性光学频率变 换的相位匹配， 使得 4H碳化硅晶体在可调谐输出 3.4-7.1 μπι中红外激 光方面更好地满足实际应用需求， 具有显著地实际应用价值。
以下介绍用 4Η碳化硅晶体制造的可调谐中红外激光器的具体实施 方式。 实施例 1:
根据本发明的实施例 1， 一种非线性光学器件包括采用至少一束激 光作为入射光， 通过至少一块非线性光学晶体后， 产生至少一束频率不 同于入射光波长的激光输出。其中的非线性光学晶体为 4Η碳化硅晶体。 所述光学器件通过光参量放大、 光参量振荡或差频技术实现可调谐中红 外激光的输出。
图 5是本发明实施例 1的工作原理图， 其中激光器 （11 ) 发射一入 射激光束（12)， 该入射激光束依次通过 4Η碳化硅晶体（13 )和滤光片 ( 15)， ( 14) 是通过 4Η碳化硅晶体 （13 ) 而经光参量放大、 光参量振 荡或差频等过程后出射的激光束， 滤光片 （15 ) 的作用是滤去入射激光 束 （12) 的波长， 附图标记 （16) 是出射的中红外激光。
在该实施例 1中，所采用的 4Η碳化硅晶体是如前所述的 4Η碳化硅 晶体， 并且
所述激光器入射光波长范围为 0.38-5.5μπι， 所述 4Η碳化硅晶体在 0.38-5.5μπι及 6.7-6.9μπι波长范围内透过率大于 10%。
在该实施例 1中，入射光波长优选为 0.8μπι，此时其透光率大于 40%。 但本发明并不限于此， 入射光波长也可为 0.38μπι、 0.55μπι 或者 0.38-5.5μπι之间的任意值。
此外， 所述 4Η碳化硅晶体实现非线性光学频率变换的相位匹配方 式为第 Π类相位匹配；
所述 4Η碳化硅晶体相位匹配的方式可以是通过调节晶体温度实现 临界相位匹配；
所述 4Η碳化硅晶体至少有一面为光学抛光；
所述 4Η碳化硅晶体表面镀有增透膜、 高反膜和 /或半透膜。 实施例 2:
本发明的实施例 2是一种用 4H碳化硅晶体制造的可调谐中红外激 光器， 结构示意图如图 6所示， 该可调谐中红外激光器包括第一泵浦光 源 （21 )、 第二泵浦光源 （22)、 激光同歩装置 （23 )、 偏振片 （24)、 偏 振片（25)、 隔离器（26)、 隔离器（27)、 反射镜（28)、 二向色镜（29)、 会聚透镜（210)、 4H碳化硅晶体（211 )、 滤光片（212)。 第一泵浦光源 发射的激光依次经过偏振片 （24)、 隔离器 （26)、 反射镜 （28)、 二向 色镜 （29)、 会聚透镜 （210)、 4H碳化硅晶体 （211 ); 第二泵浦光源出 射的激光依次经过偏振片 （25 )、 隔离器 （27 )、 二向色镜 （29)、 会聚 透镜 （210)、 4H碳化硅晶体 （211 )。
第一泵浦光源的波长为 0.8-0.9μπι， 第二泵浦光源的波长为 1.064μπι。 第一泵浦光源及第二泵浦光源可以是锁模激光器或调 Q激光 器； 锁模的方式可以是主动锁模、 被动锁模或自锁模； 调 Q方式可以是 主动电光调 Q、 声光调 Q或被动调0。
第一泵浦光源可以是可调谐钛宝石激光器， 第二泵浦光源可以是 Nd:YAG激光器。
第一泵浦光源及第二泵浦光源的泵浦方式可以是氙灯泵浦、 半导体 激光泵浦或固态激光泵浦。 第二泵浦光源的增益介质可以是 Nd:YAG、 Nd:YV04或 Nd:YLF。
第一泵浦光源和第二泵浦光源经同歩装置同歩后， 分别经过偏振 片、 隔离器、 反射镜及二向色镜后， 在满足规定的相位匹配条件下， 由 会聚透镜共线入射到 4H碳化硅晶体上进行差频， 再经过滤光片 （212) 后出射中红外激光。
在该实施例 2中，所采用的 4H碳化硅晶体是如实施例 1的 4H碳化 硅晶体， 并且
所述激光器入射光波长范围为 0.7-0.9μπι及 1.064μπι，所述 4Η碳化 硅晶体在 0.38-5.5μπι及 6.7-6.9μπι波长范围内透过率大于 10%。 在该实施例 2中， 入射光波长优选为 0.838μπι， 此时其透光率大于 40%。 但本发明并不限于此， 入射光波长也可为 0.38μπι、 0.55μπι或者 0.38-5.5μπι之间的任意值。
此外，
所述 4Η碳化硅晶体实现非线性光学频率变换的相位匹配方式为第 II类相位匹配；
所述 4Η碳化硅晶体相位匹配的方式可以是通过调节晶体温度实现 临界相位匹配；
所述 4Η碳化硅晶体至少有一面为光学抛光；
所述 4Η碳化硅晶体表面镀有增透膜、 高反膜和 /或半透膜。
其中 4Η碳化硅晶体切割角为 Θ, 即晶体通光方向与晶体光轴的夹 角。
通过调整第一泵浦光的输出波长及 4Η碳化硅晶体的角度， 实现中 红外差频光的可调谐输出。
图 7所示为本实施例 2的第一泵浦光源的波长及差频光波长与 Θ角 的关系。
在本实施例 2中， 晶体切割角 Θ的值一般为 79°-89°， 例如 82°, 第 一泵浦光源的波长可取 0.8μπι-0.9μπι， 优选为 0.838μπι， 差频光的波长 范围为 3.6-5.3μπι， 优选为 3.945μπι。 实施例 3:
实施例 3的可调谐中红外激光器采用与实施例 2相同的结构， 采用 的 4Η碳化硅晶体也相同。 所不同的是： 第一泵浦光源与第二泵浦光源 均为钛宝石激光器。
图 8所示为第一泵浦光源的波长分别为 0.7、 0.72、 0.74、 0.76、 及 0.78μπι、 第二泵浦光源的波长为 0.7-0.9μπι时， 差频输出的中红外光波 长与晶体切割角 Θ的关系。
在本实施例 3中， 差频输出的中红外光波长范围为 3.6-7μπι， 优选 为 4.0μπι， 晶体切割角 Θ为 73°-89°， 优选为 81.5。。 实施例 4:
本发明的实施例 4是一种用 4H碳化硅晶体制造的光参量放大装置， 如图 9所示， 该光参量放大装置包括第三泵浦光源 （41 )、 宽带信号光 激光器 （42)、 二向色镜 （43 )、 会聚透镜 （44)、 4H碳化硅晶体 （45) 及滤光片（46)，其中第三泵浦光源为 532nm激光器。第三泵浦光源（41 ) 发射的激光束依次经过二向色镜（43 )、 会聚透镜（44)及 4H碳化硅晶 体（45)。宽带信号光激光器（42)发射的信号光依次经过二向色镜（43 )、 会聚透镜 （44) 及 4H碳化硅晶体 （45)。
作为第三泵浦光源的 532nm激光器由 1.064μπι激光器倍频得到，倍 频晶体为 BBO、 LBO、 KDP、 KTP或 CLBO。 532nm激光器可以是锁模 脉冲激光器或调 Q激光器， 调 Q方式可以是主动电光调 Q、 声光调 Q 或被动调0。 泵浦方式是氙灯泵浦、 半导体激光泵浦或固态激光泵浦。
532nm泵浦光与宽带信号光激光器 （42)产生的信号光通过会聚透 镜 （44) 后入射到 4H碳化硅晶体 （45 ) 上， 经过光参量放大后， 产生 中红外光， 再经过滤光片 （46) 后得到中红外激光。
其中 4H碳化硅晶体与实施例 1-3中采用的 4H碳化硅晶体相同。 图 10所示为泵浦光波长为 532nm时， 光参量放大产生的中红外光 波长与 Θ角的关系。
在本实施例 4中， 晶体切割角 Θ的值一般为 72°-88°， 优选为 78°, 中红外光范围为 4.3-7μπι， 优选为 4.756μπι。
实施例 5:
本发明的实施例 5是一种用 4Η碳化硅晶体制造的宽带可调谐中红 外激光器， 结构示意图如图 11 所示， 该宽带可调谐中红外激光器包括 第四泵浦光源 （51 )、 会聚透镜 （54)、 4Η碳化硅晶体 （55 ) 及滤光片 (56)。 第四泵浦光源发射的激光依次经过会聚透镜 （54)、 4Η碳化硅 晶体 （55 ) 及滤光片 （56)。 第四泵浦光源（51 )发射的泵浦光（52)为线偏振光，偏振方向（53 ) 与晶体的主截面有一个夹角 α， 且满足 0< α <90°, α优选 4Γ ， 如图 12所示， 附图标记 （58) 表示晶体主截面与纸面的交线。
第四泵浦光源为重复频率为 IKHz , 脉宽为 20fs， 光谱范围为 500-1000nm的放大飞秒钛宝石激光器， 图 13所示为本实施例 5的第四 泵浦光源（ 51 )输出的超连续激光光谱。第四泵浦光源输出的泵浦光（ 52 ) 在 4H碳化硅晶体中同时产生 0光和 e光， 利用超连续宽谱飞秒脉冲中 的高频成分和低频成分在 4H碳化硅晶体 （55) 中直接差频， 再经过滤 光片 （56) 后出射中红外激光。
在该实施例 5中，所采用的 4H碳化硅晶体是如实施例 1的 4H碳化 硅晶体， 并且
所述激光器入射光波长范围为 0.38-1.0μπι， 所述 4Η碳化硅晶体在 0.38-5.5μπι及 6.7-6.9μπι波长范围内透过率大于 10%。
此外，
所述 4Η碳化硅晶体实现非线性光学频率变换的相位匹配方式为第 II类相位匹配；
所述 4Η碳化硅晶体相位匹配的方式可以是通过调节晶体温度实现 临界相位匹配；
所述 4Η碳化硅晶体至少有一面为光学抛光；
所述 4Η碳化硅晶体表面镀有增透膜、 高反膜和 /或半透膜。
其中 4Η碳化硅晶体切割角为 Θ, 即晶体通光方向与晶体光轴的夹 角。
在本实施例 5中， 晶体切割角 Θ的值一般为 79°-89°，差频光的波长 范围为 3.6-7μπι。
图 14所示为本实施例 5的晶体切割角为 82° 时所得到的宽带中红 外差频光光谱。 本发明的上述实施例采用 4Η碳化硅晶体实现中红外非线性光学频 率变换， 与现有的非线性光学器件相比， 由于 4Η碳化硅晶体具有很高 的激光损伤阈值、 较宽的透光范围（0.38-5.9μπι及 6.6-7.08μπι)、 较大的 二阶非线性光学系数 （dl5=6.7pm/V )、 较大的双折射、 高热导率 (490Wm-1K-1 ) 以及高化学稳定性等特点， 使得本发明的非线性光学器 件在输出高功率、 高光束质量的中红外激光方面更好地满足实际应用需 求， 具有显著地实际应用价值。
以上所述的具体实施例， 对本发明的目的、 技术方案和有益效果进 行了进一歩的详细说明， 所应理解的是， 以上所述仅为本发明的具体实 施例而已， 并不用于限制本发明， 凡在本发明的精神和原则之内， 所做 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

权利要求
1、 一种非线性光学器件， 包括至少一个非线性光学晶体， 该非线 性光学晶体用于改变具有特定频率的至少一束激光 （12 )， 产生至少一 束不同于所述频率的另一特定频率的激光 （16)， 其特征在于：
所述非线性光学晶体为 4H碳化硅晶体 （13 )。
2、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4H碳 化硅晶体 （13 ) 的空间群为 P63mc， 其中每个晶胞内含有四层碳硅原子 层， 按 ABCB方式排列而成。
3、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4H碳 化硅晶体（ 13 )在 0.38-5.5μπι及 6.7-6.9μπι波长范围内的透过率大于 10%。
4、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4Η碳 化硅晶体 （13 ) 实现非线性光学频率变换的相位匹配方式为第 II类相位 匹配。
5、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4Η碳 化硅晶体 （13 ) 实现非线性光学频率变换的相位匹配的方式可以是通过 调节晶体温度实现临界相位匹配。
6、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4Η碳 化硅晶体 （13 ) 至少有一面为光学抛光。
7、 如权利要求 1所述的非线性光学器件， 其特征在于， 所述 4Η碳 化硅晶体 （13 ) 表面镀有增透膜、 高反膜和 /或半透膜。
8、 如权利要求 1 所述的非线性光学器件， 其特征在于， 还包括至 少一激光器 （11 )， 用于产生所述具有特定频率的激光 （12 ) 使之入射 到所述 4Η碳化硅晶体 （13 ) 上。
9、 如权利要求 8所述的非线性光学器件， 其特征在于， 所述入射 的激光的波长范围为 0.38-5.5μπι。
10、 如权利要求 9所述的非线性光学器件， 其特征在于， 所述入射 的激光的波长范围为 0.5-2μπι。
11、 一种可调谐中红外激光器， 包括第一泵浦光源 （21 )、 第二泵 浦光源 （22)， 该第一泵浦光源 （21 ) 与该第二泵浦光源 （22) 发射的 激光频率不同， 其特征在于：
还包括一个 4H碳化硅晶体 （211 )， 其中
第一泵浦光源 （21 ) 和第二泵浦光源 （22) 发射的激光共线入射到 该 4H碳化硅晶体上进行差频， 以出射一中红外激光。
12、 如权利要求 11 所述的可调谐中红外激光器， 其特征在于， 出 射的激光的波长为 3.5-7.08μπι。
13、 如权利要求 11 所述的可调谐中红外激光器， 其特征在于， 其 中 4Η碳化硅晶体 （211 ) 的切割角为 θ， Θ 为相位匹配角度， 并满足 50。<θ<90。。
14、 一种光参量放大装置， 包括第三泵浦光源 （41 )、 宽带信号光 激光器 （42)， 其特征在于：
还包括 4Η碳化硅晶体 （45)， 并且
第三泵浦光源 （41 ) 发射的激光和宽带信号光激光器 （42) 产生的 信号光入射到 4Η碳化硅晶体 （45 ) 上， 经过光参量放大后， 出射一中 红外激光。
15、 如权利要求 14所述的光参量放大装置， 其特征在于： 第三泵浦光源 （41 ) 为 532nm激光器。
16、 如权利要求 14所述的光参量放大装置， 其特征在于， 其中 4H 碳化硅晶体 （45 ) 的晶体切割角为 Θ, 且满足 68°<θ<88°。
17、 如权利要求 14 所述的光参量放大装置， 其特征在于， 出射的 激光的波长为 4.3-7μπι。
18、 一种宽带可调谐中红外激光器， 包括第四泵浦光源 （51 )， 该 泵浦光源为宽带脉冲激光器， 其特征在于还包括一个 4Η碳化硅晶体
(55)， 并且
利用第四泵浦光源 （51 ) 输出的泵浦光 （52) 中的高频成分和低频 成分在 4Η碳化硅晶体 （55 ) 中差频， 再经过滤光片 （56) 后出射宽带 中红外激光。
19、 如权利要求 18所述的宽带可调谐中红外激光器， 其特征在于， 第四泵浦光源 （51 ) 为超连续宽谱脉冲钛宝石激光器。
20、 如权利要求 18所述的宽带可调谐中红外激光器， 其特征在于， 第四泵浦光源的偏振方向 （53 ) 与晶体的主截面有一个夹角 α， 且满足 0< α <90。。
21、 如权利要求 18所述的宽带可调谐中红外激光器， 其特征在于， 所述 4Η碳化硅晶体 （55 ) 的晶体切割角为 Θ, 且满足 70°<θ<89°。
22、 如权利要求 18所述的宽带可调谐中红外激光器， 其特征在于， 出射的激光的波长为 3.5-7.08μπι。
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