JP2006114642A - Infrared coherent light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the conventional middle infrared laser has a high output but has the drawback of having no wavelength variability and on the contrary, a middle infrared optical parametric oscillator has a wavelength variability but has the drawback of having difficulty obtaining a high output. <P>SOLUTION: When constructing an infrared light source by arranging crystals by difference frequency generation inside an optical parametric oscillator, a polarization inversion period is designed in consideration of the relationship on wavelength dispersion among individual crystals and the temperature expansion of each crystal, by using a pseudo-phase matching technology to make it possible to remarkably increase a temperature range of phase matching, and eventually to obtain a high output and a wavelength variability at the same time. By this technology, an infrared light source having a larger wavelength variability and a high output can be fabricated. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a technique for realizing an infrared light source having a light output and a variable wavelength. More specifically, the present invention relates to periodic polarization of a quasi-phase-matched crystal in a broad wavelength tunable band when generating high-efficiency mid-infrared light using inter-cavity difference frequency mixing by quasi-phase matching. The present invention relates to inversion structure control technology.

Lasers broaden the oscillation material into solids, gases, and liquids, expanding the oscillation wavelength from the vacuum ultraviolet region to the ultraviolet, visible, infrared, and far infrared regions. Currently, lasers are coherent and highly oriented. It has been applied in a wide range of fields such as communication, measurement, and medical care due to its excellent features such as lightness, high light concentration, and monochromaticity. In particular, since the wavelength of light called the mid-infrared region (3 mm to 30 mm) matches the vibration of molecules, it is expected to be applied to medical, biological and environmental measurements. As a light source for generating the mid-infrared light, there are a gas laser, a solid-state laser, and the like, but a weak point is that the oscillation wavelength is fixed. Therefore, optical parametric oscillation (Optical using non-linear optical effect as a wavelength tunable technology)
Parametric Oscillation (OPO) is noted.

However, in the parametric process, oscillation does not occur efficiently without the initial values of Signal and Idler. Therefore, oscillation is performed by feeding back the output obtained in the parametric process with a resonator. Here, it is conceivable that output light can be obtained with high efficiency by constructing a resonator with a highly reflective mirror for the output light, but in reality the efficiency of OPO is limited by Manley-Rowe law and back conversion Will be. As a result, there is a limit to the high output of high strength pump in OPO.
JP 2002-303906 A

  The present invention solves the above-mentioned problems, greatly expands the temperature range of phase matching, and makes it possible to construct a high-power infrared light source having wavelength variability. Wavelength tunability cannot be obtained with conventional high-power infrared lasers, and high output and wavelength tunability can be obtained simultaneously with the present invention, although it is difficult to achieve high output with conventional wavelength-tunable infrared parametric oscillators.

  According to the present invention, in the infrared coherent light source, the intra-cavity difference frequency mixing unit in which the parametric generation unit and the difference frequency mixing unit are monolithically produced in the same crystal, and the temperature adjustment unit that simultaneously adjusts the crystal temperature are provided. As a result, an infrared coherent light source characterized by high output and wavelength tunability can be obtained.

Further, according to the present invention, high output by determining by said a polarization inversion period lambda _DFG of the difference frequency mixing section and the polarization inversion period lambda _OPO parametric generator, desired high efficiency oscillation is obtained relationship And an infrared coherent light source characterized in that it is possible to obtain both wavelength conversion and wavelength tunability at the same time.

Further, according to the present invention, the relationship in which the desired high-efficiency oscillation is obtained is obtained by using the signal inversion frequency Λ _OPO of the parametric generation unit as a parameter, the signal wave λs, the idler wave λi, and the difference frequency in the difference frequency mixing unit. Due to the temperature dependence of λd phase shift Δk = ks-ki-kd and wave number compensation 2π / Λ _DFG by quasi-phase matching, the difference between the two curves in the temperature region is related to the desired high-efficiency oscillation. High output and wavelength tunability can be obtained at the same time by obtaining the polarization inversion period Λ _OPO of the parametric generation part where the absolute value is below a certain value and the polarization inversion period Λ _DFG of the difference frequency mixing part. An infrared coherent light source is obtained.

Further, according to the present invention, the relationship in which the desired high-efficiency oscillation is obtained is obtained by using the signal inversion frequency Λ _OPO of the parametric generation unit as a parameter, the signal wave λs, the idler wave λi, and the difference frequency in the difference frequency mixing unit. The phase shift at the temperature at which the intersection of each curve is obtained from the temperature dependence characteristic diagram of each curve of λd phase shift Δk = ks-ki-kd and wave number compensation 2π / Λ _DFG by pseudo phase matching with temperature High output and wavelength _{tunability by making} the difference between the curve Δk = ks-ki-kd and the wave number compensation curve 2π / Λ _DFG be π / L (where L is the crystal length of the difference frequency mixing section) Infrared coherent light sources characterized in that can be obtained simultaneously.

Further, according to the present invention, there is provided an infrared coherent light source characterized in that high output and wavelength tunability can be obtained simultaneously by using a nonlinear optical crystal capable of periodically polarization reversal in the intracavity difference frequency mixing unit. can get. More specifically, the non-linear optical crystal capable of reversing the periodic polarization includes CLN (melting point coincidence lithium niobate), MgO-doped CLN (magnesium oxide added melting point coincidence lithium niobate), SLN (stoichiometric composition niobic acid). Lithium), MgO-doped SLN (magnesium oxide-added stoichiometric composition lithium niobate), CLT (melting point-matched lithium titanate), SLN (stoichiometric composition lithium titanate), MgO-doped CLT (magnesium oxide-added melting point-matched titanate) Lithium), MgO-doped SLT (magnesium oxide addition stoichiometric composition lithium titanate) and KLN (potassium lithium niobate) can be selected to achieve high output and wavelength tunability at the same time. A light source is obtained.

  Hereinafter, a quasi phase matching device design method according to the present invention will be specifically described with reference to the drawings.

  The present invention controls the periodic polarization inversion structure of a quasi-phase-matched crystal when generating high-efficiency mid-infrared light using intra-cavity difference frequency mixing by quasi-phase matching. It was made clear that this can be achieved. As a result, phase matching by difference frequency mixing can be realized by rough temperature control. Hereinafter, the phase matching according to the design of the periodically poled structure according to the present invention will be described in detail.

  FIG. 1 is a diagram for explaining the principle of the intra-resonator difference frequency mixing unit that is the basis of the present invention. FIG. 1 (a) schematically shows an intra-resonator difference frequency mixing device, and FIG. 1 (b) shows an energy diagram of the relationship of nonlinear wavelength conversion in intra-resonator difference frequency mixing. The intra-resonator difference frequency mixing device has a structure in which a parametric generator (OPO) and a difference frequency mixer (ic-DFG) are arranged in the resonator, and the resonator generates a signal wave λs generated by the parametric generator. Resonate with it. When the pump wave λp is incident on the intracavity difference frequency mixing device, a signal wave λs and an idler wave λi are generated from the pump wave λp.

At this time, the relationship between the frequency wp of the pump wave λp, the frequency ws of the signal wave λs generated in the parametric generator, and the frequency wi of the idler wave λi is given by equation (1).
wp = ws + wi (1)

The difference frequency λd is generated by combining the signal wave λs and the idler wave λi generated by the parametric generation unit by the difference frequency mixing unit. At this time, the relationship between the frequency ws of the signal wave λs, the frequency wi of the idler wave λi, and the frequency wd of the difference frequency λd is given by equation (2).
wd = ws-wi (2)

When the intracavity difference frequency mixing device is constructed using a quasi phase matching crystal, the phase matching condition in the parametric generator is given by equation (3).
Δk _OPO = kp-ks-ki-2π / Λ _OPO (3)
(Δk _OPO is the phase mismatch in the parametric generator, kp is the wave number of the pump wave λp, ks is the wave number of the signal wave λs, ki is the wave number of the idler wave λi, Λ _OPO is the quasi-phase matched crystal used in the parametric generator Periodic polarization reversal period)

Similarly, the phase matching condition in the difference frequency mixing unit is given by equation (4).
Δk _DFG = ks-ki-kd-2π / Λ _DFG (4)
(Δk _DFG is the phase mismatch in the difference frequency mixing section, kd is the wave number of the difference frequency λd, and Λ _DFG is the periodic polarization inversion period of the quasi phase matching crystal used in the difference frequency mixing section)

Figure 2 shows the use of a periodically poled lithium niobate crystal (PPLN) as the parametric generator, and the fundamental wave of the Nd: YAG laser (1064) as the pump wave λp.
nm) is a diagram showing the wavelengths of the signal wave λs and the idler wave λi generated in the parametric generator.

Wavelength conversion in the parametric generator is performed with high efficiency when equations (1) and (3) are satisfied simultaneously. The wavelengths of the generated signal wave λs and idler wave λi are determined by the compensation 2π / Λ _OPO of the phase matching condition by the chromatic dispersion and quasi phase matching of the crystal. The chromatic dispersion relationship of the crystal is a function of temperature, and the polarization inversion period Λ _OPO of the crystal of the parametric generator changes with temperature due to the thermal expansion of the quasi-phase-matched crystal. The wavelengths of the wave λs and the idler wave λi are functions of temperature.

Figure 3 shows the difference between the wavelength λd and the signal wave of the difference frequency generated by the difference frequency mixing unit when PPLN is used for the parametric generation unit and the fundamental wave (1064 nm) of the Nd: YAG laser is used as the pump wave λp. It is a figure which shows the wavelength relationship of wavelength (lambda) s and idler wave (lambda) i. Wavelength difference frequency λd is, by determining the polarization inversion period lambda _OPO and the operating temperature of the parametric generator crystals, is uniquely determined according to equation (2).

Fig. 4 shows the case where the parametric generator and the difference frequency mixer are made monolithically in the same crystal with Λ _OPO = 28.9 mm and the crystal temperature is adjusted simultaneously (the temperature adjuster is not shown in Fig. 1). FIG. 5 is a diagram showing the temperature dependence of a phase shift Δk = ks−ki−kd of a signal wave λs, an idler wave λi, and a difference frequency λd in a frequency mixing unit.

  A simple and reliable device avoiding the complexity of adjusting the crystal angle and controlling each temperature separately by manufacturing the crystal monolithically and simultaneously controlling the temperature of the parametric generator and the difference frequency mixer. Can be built. The value of Δk is determined by the dispersion relationship between the wavelength of the three waves of the signal wave λs, the idler wave λi, and the difference frequency λd and the wavelength of the crystal.

Phase matching by quasi phase matching is satisfied by canceling the value of Δk with the wave number component 2π / Λ _DFG of the polarization inversion period Λ _DFG of the crystal of the difference frequency mixing unit. The polarization inversion period Λ _DFG of the crystal of the difference frequency mixing portion required for phase matching at 50 ° C., 100 ° C., 150 ° C., and 200 ° C. can be obtained by Λ _DFG = 2π / Δk. At this time, it is necessary to determine the polarization inversion period Λ _DFG of the crystal in the difference frequency mixing unit in consideration of the change in the inversion period of the crystal in the polarization difference frequency mixing unit due to the thermal expansion of the crystal.

Fig. 5 shows the parametric generator's polarization inversion period Λ _OPO = 28.9 mm, the parametric generator and the difference frequency mixing unit are monolithically fabricated in the same crystal, and the crystal temperature is adjusted simultaneously. is a graph showing the temperature dependency of the compensation 2 [pi / lambda _DFG wavenumber by phase shift Δk = ks-ki-kd and quasi-phase matching of the signal wave λs and the idler wave λi and difference frequency .lambda.d. 2π / Λ _DFG becomes a slightly inclined curve due to the thermal expansion of the crystal, and this inclination is determined by the thermal expansion characteristics of the crystal.

At the intersection of .DELTA.k and 2 [pi / lambda _DFG, the phase matching condition _{Δk DFG = ks-ki-kd} -2π / Λ DFG = 0 next to the difference frequency mixing unit, the phase matching is satisfied.

Fig. 6 (a) shows the difference frequency mixing when the parametric generator and the difference frequency mixer are monolithically fabricated in the same crystal with the polarization inversion period Λ _OPO = 28.9 mm of the parametric generator and the crystal temperature is adjusted simultaneously. FIG. 5 is a diagram showing the temperature dependence of a phase matching condition Δk _DFG = ks-ki-kd-2π / Λ _DFG in the section. The phase matching can be satisfied at an arbitrary temperature by designing the polarization inversion period Λ _DFG of the crystal in the difference frequency mixing section.

Fig. 6 (b) shows the crystal inversion period Λ _OPO = 28.9mm of the parametric generator, (1) temperature control of the parametric generator and the difference frequency mixer at the same time, and (2) fixing the temperature of the parametric generator. (3) Phase matching condition Δk _DFG = ks-ki-kd-2π / Λ _{DFG in} each case where the temperature of the parametric generator is controlled by fixing the temperature of the difference frequency mixer It is the figure which showed the temperature dependence. By controlling the temperature of the parametric generator and the difference frequency mixer at the same time, the temperature range in which phase matching can be achieved is expanded.

FIG. 7 shows the signal wave λs, idler wave λi, and difference frequency λd in the difference frequency mixing unit when the polarization inversion period Λ _OPO of the parametric generator is changed from 28.9 mm to 29.6 mm and the crystal temperature is adjusted simultaneously. FIG. 6 is a graph showing the temperature dependence of the phase shift Δk = ks−ki-kd and the wave number compensation 2π / Λ _DFG by pseudo phase matching. The phase shift Δk = ks-ki-kd of the signal wave λs, idler wave λi, and difference frequency λd is a curve with a minimum value _{depending on} the dispersion relationship of the wavelength of the crystal and the period of the polarization inversion period Λ _OPO of the parametric generator. Become. On the other hand, 2 [pi / lambda _DFG is that a curved line having a slope slightly due to thermal expansion of the crystal, the two curves will have two points intersection by condition. The phase matching condition Δk _DFG = ks-ki-kd-2π / Λ _DFG of the difference frequency mixing unit is satisfied when the absolute value of the difference between the two curves is small. That is, if the polarization inversion period Λ _OPO of the parametric generation unit and the polarization inversion period Λ _DFG of the difference frequency mixing unit are designed so that Δk _DFG = ks-ki-kd-2π / Λ _DFG becomes small in a wide range, An infrared light source that satisfies phase matching in the temperature range can be constructed.

FIG. 8 shows the signal wave λs, idler wave λi, and difference frequency λd in the difference frequency mixing unit when the crystal inversion period Λ _OPO is changed from 28.9 mm to 29.6 mm and the crystal temperature is adjusted simultaneously. FIG. 6 is a diagram obtained by differentiating each curve of the phase shift Δk = ks−ki-kd and the wave number compensation 2π / Λ _DFG by pseudo phase matching with temperature. From the intersection of the Δk = ks-ki-kd temperature differential curve and the 2π / Λ _DFG temperature differential curve, the temperature at which the Δk = ks-ki-kd and 2π / Λ _DFG curve contact is obtained.

In order to maximize the temperature range at which phase matching can be _achieved , if the difference between the phase shift curve Δk = ks-ki-kd and the wave number compensation curve 2π / Λ _DFG is p / L histological the range value of the phase matching condition Δk = ks-ki-kd- 2π / Λ DFG is equal to or less than p / L is maximized (crystal length where L is the difference frequency mixing unit). At this time, the phase shift curve Δk = ks-ki-kd is a value determined by the dispersion relation of the crystal and the polarization inversion period Λ _OPO of the crystal of the parametric generation part, but the curve 2π / Λ _DFG is the value of the frequency mixing part. Since it can be adjusted by the polarization inversion period Λ _DFG of the crystal, it is possible to design the polarization inversion period Λ _OPO of the crystal in the parametric generation part that satisfies the above geometric relationship and the polarization inversion period Λ _DFG of the crystal in the frequency mixing part Become.

FIG. 9 shows a phase shift Δk = ks-ki− between the signal wave λs and the idler wave λi and the difference frequency λd in the difference frequency mixing portion when the polarization inversion period Λ _OPO of the crystal of the parametric generation portion is 29.5 mm. FIG. 6 is a diagram showing the temperature dependence of kd and wave number compensation 2π / Λ _DFG by pseudo phase matching. When the crystal length of the difference frequency mixing part is 20 mm, p / L = 157m ^-1 and the difference between the two curves at the temperature at which the two curves Δk = ks-ki-kd and 2π / Λ _{DFG meet.} The inversion period Λ _DFG of the crystal in the frequency mixing part where is 157 m ⁻¹ is 33.75 mm. At this time, as shown in FIG. 9, the temperature range in which the difference between the two curves becomes 157 m ⁻¹ becomes the maximum, and the temperature width that satisfies the phase matching becomes the maximum.

FIG. 10 is a diagram showing the temperature dependence of the phase matching condition. Comparing the phase-matching conditions designed by the prior art and the design optimized to maximize the phase matching temperature range, the polarization inversion period and frequency mixing section of the crystal of the parametric generator designed by this technology In the case of using the crystal inversion period, the phase matching temperature range is greatly expanded. The temperature range in which the p / L = 157m ^-1 in the case of a crystal length of difference frequency mixing section and 20 mm, respectively 9 ° C. in the case designed by the conventional art, 14 ° C., whereas it is 25 ° C., the technology In the case of using the design method according to, it will be expanded to 137 ℃. As a result, the wavelength tunable range can be greatly increased with an increase in the output of infrared light due to intracavity difference frequency mixing.
In the description of the present embodiment, a periodically poled lithium niobate crystal (PPLN) is used for the intra-cavity difference frequency mixing unit. However, the present invention is not limited to this. The same effect can be obtained even with non-linear optical crystals capable of polarization reversal, for example, MgO-doped CLN
(Magnesium oxide added melting point matched lithium niobate), SLN (stoichiometric lithium niobate), MgO doped SLN (magnesium oxide added stoichiometric lithium niobate), CLT (melting point matched lithium titanate), SLN (constant Specific composition lithium titanate), MgO doped CLT (magnesium oxide added melting point coincidence lithium titanate), MgO doped SLT (magnesium oxide added stoichiometric composition lithium titanate), KLN (potassium lithium niobate) may be used.

By realizing a 3-micron band broadband wavelength tunable light source with a mid-infrared wavelength conversion quantum efficiency exceeding 100% according to the present invention, products such as a dental medical light source, a spectroscopic light source, and an atmospheric observation light source are expected.

Fig. 1 (a) is a schematic representation of the intra-cavity difference frequency mixing device.

Fig. 1 (b) shows an energy diagram of the relationship of nonlinear wavelength conversion in intra-cavity difference frequency mixing.
Signal generated in the parametric generator when a periodically poled lithium niobate crystal (Periodically Poled Lithium Niobate: PPLN) is used in the parametric generator and the fundamental wave (1064 nm) of the Nd: YAG laser is used as the pump wave λp It is a figure which shows the wavelength of wave (lambda) s and idler wave (lambda) i. When PPLN is used for the parametric generator and the fundamental wave of Nd: YAG laser (1064 nm) is used as the pump wave λp, the wavelength of the difference frequency λd generated by the difference frequency mixer, the signal wave λs, and the idler wave λi It is a figure which shows the relationship of the wavelength. When Λ _OPO = 28.9 mm, the parametric generator and the difference frequency mixer are monolithically fabricated in the same crystal and the crystal temperature is adjusted simultaneously, the signal wave λs, idler wave λi and difference frequency λd at the difference frequency mixer FIG. 6 is a diagram showing the temperature dependence of a phase shift Δk = ks−ki−kd. When Λ _OPO = 28.9 mm, the parametric generator and the difference frequency mixer are monolithically fabricated in the same crystal and the crystal temperature is adjusted simultaneously, the signal wave λs, idler wave λi and difference frequency λd at the difference frequency mixer FIG. 6 is a diagram showing the temperature dependence of the phase shift Δk = ks−ki-kd and the wave number compensation 2π / Λ _DFG by pseudo phase matching. FIG. 6A shows a phase matching condition Δk in the difference frequency mixing unit when Λ _OPO = 28.9 mm and the parametric generation unit and the difference frequency mixing unit are monolithically fabricated in the same crystal and the crystal temperature is adjusted simultaneously. It is the figure which showed the temperature dependence of _DFG = ks-ki-kd-2π / Λ _DFG . Fig. 6 (b) shows the parametric generator's polarization inversion period Λ _OPO = 28.9 mm, temperature control of the parametric generator and the difference frequency mixer at the same time, and fixing the temperature of the parametric generator to control the temperature of the difference frequency mixer. control is a diagram showing the temperature dependence of the phase-matching condition _{Δk DFG = ks-ki-kd} -2π / Λ DFG in each case of controlling the temperature of the parametric generator to fix the temperature difference frequency mixing unit. When the polarization inversion period Λ _OPO of the parametric generator is changed from 28.9 mm to 29.6 mm and the crystal temperature is adjusted at the same time, the phase shift Δk = of the signal wave λs, the idler wave λi and the difference frequency λd in the difference frequency mixing part It is the figure which showed ks-ki-kd and the temperature dependence of 2π / Λ _DFG of wave number compensation by quasi phase matching. When the polarization inversion period Λ _OPO of the parametric generator is changed from 28.9 mm to 29.6 mm and the crystal temperature is adjusted at the same time, the phase shift Δk = of the signal wave λs, the idler wave λi and the difference frequency λd in the difference frequency mixing part and ks-ki-kd, which is a diagram obtained by differentiating at a temperature of each curve of quasi-phase matching according to the wave number of the compensation 2π / Λ _DFG. Phase shift Δk = ks-ki-kd of signal wave λs, idler wave λi and difference frequency λd in the difference frequency mixing part when the polarization inversion period Λ _OPO of the parametric generator is 29.5 mm, and quasi phase matching FIG. 5 is a graph showing the temperature dependence of 2π / Λ _DFG for wave number compensation by. It is the figure which showed the temperature dependence of phase matching conditions.

Claims (6)

  Infrared coherent light source, characterized in that it has an intracavity difference frequency mixing unit in which a parametric generation unit and a difference frequency mixing unit are monolithically fabricated in the same crystal, and a temperature adjustment unit that simultaneously adjusts the crystal temperature. Infrared coherent light source. 2. The infrared according to claim 1, wherein the polarization inversion period Λ _OPO of the parametric generation unit and the polarization inversion period Λ _DFG of the difference frequency mixing unit are determined in a relationship that provides desired high-efficiency oscillation. Coherent light source. The relationship for obtaining the desired high-efficiency oscillation is that the phase shift Δk = of the signal wave λs, the idler wave λi and the difference frequency λd in the difference frequency mixing unit using the polarization inversion period Λ _OPO of the parametric generation unit as a parameter. Due to the temperature dependence of ks-ki-kd and wave number compensation 2π / Λ _DFG due to quasi-phase matching, the absolute value of the difference between the two curves in the temperature region is less than a certain value due to the desired high-efficiency oscillation. 3. The infrared coherent light source according to claim 1, wherein a polarization inversion period Λ _OPO of the parametric generation unit and a polarization inversion period Λ _DFG of the difference frequency mixing unit are obtained. The phase shift Δk = ks between the signal wave λs, the idler wave λi, and the difference frequency λd in the difference frequency mixing unit using the polarization inversion period Λ _OPO of the parametric generation unit as a parameter because the desired high-efficiency oscillation is obtained. -ki-kd and compensation of wave number by quasi-phase matching 2π / Λ _DFG curves are temperature dependent characteristics, and the phase shift curve Δk = ks-ki-kd at the temperature where the intersection of each curve is obtained _4. The infrared ray according to claim 1, wherein a difference between a compensation curve of 2 and a wave number 2π / Λ _DFG is π / L (where L is a crystal length of a difference frequency mixing unit). Coherent light source.   5. The infrared coherent light source according to claim 1, wherein a nonlinear optical crystal capable of reversing periodic polarization is used in the intracavity difference frequency mixing unit. 6. The non-linear optical crystal capable of reversing the periodic polarization is MgO-doped CLN
(Magnesium oxide added melting point matched lithium niobate), SLN (stoichiometric lithium niobate), MgO doped SLN (magnesium oxide added stoichiometric lithium niobate), CLT (melting point matched lithium titanate), SLN (constant Specific composition Lithium titanate), MgO-doped CLT (magnesium oxide added melting point coincidence lithium titanate), MgO-doped SLT (magnesium oxide-added stoichiometric composition lithium titanate) and KLN (potassium lithium niobate) The infrared coherent light source according to claim 5.

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