JP7265771B2 - WAVELENGTH CONVERSION DEVICE AND LASER EQUIPMENT USING THE SAME - Google Patents

Description

The present invention relates to a wavelength conversion device and a laser apparatus using the same.

Research on nonlinear optics has advanced with the invention of lasers, and technologies such as wavelength conversion, optical pulse technology, and nonlinear spectroscopy using nonlinear optical effects have become widespread. In particular, wavelength conversion is a useful technique for realizing a coherent light source in a wavelength range in which direct laser oscillation cannot be obtained.

Attempts have been proposed to manipulate or design a nonlinear optical process by controlling the phase relationship between a plurality of electromagnetic fields involved in the process in which the nonlinear optical phenomenon progresses (see, for example, Patent Document 1). In this proposal, one or more transparent dispersive media are placed in a nonlinear optical medium, and the nonlinear optical process is manipulated by adjusting the position and effective thickness of the dispersive media.

When the birefringence of the nonlinear optical medium is large, wavelength conversion using angular phase matching causes a walk-off phenomenon in which the incident laser light and the beam generated by wavelength conversion are separated, making it impossible to obtain sufficient conversion efficiency. . In order to compensate for walk-off, a walk-off compensation structure is known in which β-BBO (BaB ₂ O ₄ : barium borate) flakes are bonded at room temperature (for example, see Non-Patent Document 1).

Republished patent WO2015/170780

Shuhei Koyama, "Study on high efficiency of walk-off compensated β-BaB2O4 device using room temperature junction", Chuo University Master's thesis summary (2016)

Known techniques for manipulating nonlinear optical processes involve placing a dispersive medium in a nonlinear optical medium and controlling the effective thickness of the dispersive medium within the nonlinear optical medium. The thickness of the dispersive medium with respect to the light wave can be adjusted by forming the dispersive medium into a wedge-shaped pair and changing the insertion thickness of one, or by using a plate-like dispersive medium and changing the inclination with respect to the optical axis. However, in order to control the insertion thickness and inclination of the dispersive medium in the nonlinear optical medium, a mechanism for driving and controlling the dispersive medium becomes large-scale.

SUMMARY OF THE INVENTION It is an object of the present invention to achieve highly efficient wavelength conversion by controlling the relative phase relationship of light waves (electromagnetic fields) involved in nonlinear optics with a simple configuration.

In the present invention, a dispersive medium is arranged between solid nonlinear optical media divided into two or more pieces, and the nonlinear optical media are separated so as to obtain a desired phase relationship between a plurality of frequency components transmitted through the nonlinear optical media. The optical length of the dispersion medium intervening in the optical axis direction is adjusted.

The optical length of the dispersion medium in the optical axis direction can be adjusted by controlling at least one of the (effective) thickness and refractive index of the dispersion medium.

Specifically, the wavelength conversion device is
a solid nonlinear optical medium divided into two or more;
a dispersive medium interposed between the nonlinear optical media;
has
The length of each of the nonlinear optical media is designed to have an interaction length that requires phase manipulation to obtain a desired result in the process of progressing the nonlinear optical phenomenon,
The optical length of the dispersive medium in the optical axis direction has a desired relative phase relationship between one or more frequency components incident on the nonlinear optical medium and new frequency components generated in the nonlinear optical medium. is variably adjusted so that

In a good configuration example, the nonlinear optical media divided into two or more are arranged so that their crystal axes are alternately reversed.

With a simple configuration, the relative phase relationship between multiple light waves or electromagnetic fields involved in nonlinear optics is controlled to realize various wavelength conversions with high efficiency.

1 is a schematic diagram of a laser device using a wavelength conversion device of an embodiment; FIG. It is a figure which shows the example of position control of a nonlinear optical medium. FIG. 10 is a schematic diagram of a laser apparatus using a modified wavelength conversion device; FIG. 11 is a schematic diagram of a laser apparatus using a wavelength conversion device of still another modification; FIG. 11 is a schematic diagram of a laser apparatus using a wavelength conversion device of still another modification; FIG. 4 is a diagram for explaining propagation of each frequency component between nonlinear optical media; This is a configuration example in which the configuration of FIG. 1 is provided with a walk-off compensation function. It is a figure explaining walk-off compensation. It is actual measurement data in the arrangement configuration of FIG. FIG. 4 is a schematic diagram of another laser device used to measure walk-off compensation; FIG. 10 is a conceptual diagram of walk-off compensation in the device of FIG. 9; 10 is an output profile of an optical parametric amplifier in the device of FIG. 9; It is a figure which shows the effect of an Example.

FIG. 1A is a schematic diagram of a laser device 1 using a wavelength conversion device 10 of an embodiment. A laser device 1 has a light source 2 and a wavelength conversion device 10 . The light source 2 generates laser light containing one or more frequency components. The one or more frequency components may be, for example, discrete spectral components when a frequency comb light source is used as the light source 2, or pulsed discrete spectra using optical parametric resonance. Alternatively, the light source 2 may be formed by one or more laser diodes with different output wavelengths.

The wavelength conversion device 10 has a solid nonlinear optical medium 11 divided into two or more pieces disposed within a dispersive medium 12 . In FIG. 1A, nonlinear optical media 11a, 11b, and 11c are arranged in dispersion medium 12, but the number of nonlinear optical media 11 is an arbitrary number of two or more. When a light wave having sufficient intensity is incident on the nonlinear optical medium 11, nonlinear (secondary or higher) polarization occurs inside the nonlinear optical medium 11 with respect to the incident electric field. A frequency component is generated.

The dispersion medium 12 is a medium whose refractive index depends on frequency or wavelength, and may be gas, liquid, or solid. In the example of FIG. 1A, the dispersion medium 12 is air.

In the example of FIG. 1A, light waves having angular frequencies of ω _o and ω ₁ are incident from the light source 2 on the first nonlinear optical medium 11a of the wavelength conversion device 10, for example. These light waves have sufficient intensity to interact with the nonlinear optical medium 11 to produce a response (polarization) that is not linearly proportional to the electromagnetic field.

Wavelength conversion device 10 utilizes nonlinear optical effects, where wavelength conversion may include sum or difference frequency generation, harmonic generation, optical parametric amplification, optical parametric oscillation, and the like. By using the nonlinear optical medium 11 divided into two or more, it is possible to reinforce new frequency components generated in the course of the electromagnetic wave interacting with the nonlinear optical medium 11 .

The nonlinear optical media 11 a , 11 b , 11 c are arranged along the optical axis in the dispersion medium 12 . The positions of the nonlinear optical media 11a, 11b, and 11c in the optical axis direction are determined by one or more frequency components incident on these nonlinear optical media 11a to 11c and new frequency components generated in the nonlinear optical media 11a to 11c. is adjusted so that the relative phase relationship between is a desired relationship. Adjusting the positions of the nonlinear optical media 11a to 11c in the optical axis direction means that the optical lengths t1 and t2 of the dispersion medium 12 in the optical axis direction are adjusted. The optical lengths t1 and t2 of the dispersion medium 12 in the optical axis direction are effective optical path lengths of light passing through the dispersion medium 12, and can also be adjusted by controlling the refractive index of the dispersion medium 12. FIG.

The lengths L1, L2, and L3 of the nonlinear optical media 11a to 11b in the optical axis direction should have a desired phase relationship between a plurality of frequency components in order to obtain desired results in the process of progressing the nonlinear optical phenomenon as described later. is set to the length to be manipulated to the value of The length L of the nonlinear optical medium 11 in the optical axis direction also indicates the effective optical path length of light passing through the nonlinear optical medium 11 .

In the example of FIG. 1A, a dispersive medium 12 such as air exists between the divided nonlinear optical media 11 . The adjustment of the optical length t of the dispersion medium 12 in the optical axis direction can be performed by any means, such as manual adjustment, mechanical and/or electrical drive adjustment, and optical control.

FIG. 1B is a diagram showing an example of position control of the nonlinear optical medium 11. As shown in FIG. A wavelength conversion device 10A of a laser device 1A has a plurality of nonlinear optical media 11a to 11c divided into two or more and a dispersive medium 12 that fills the space between adjacent nonlinear optical media 11. FIG. In this example, the positions of the nonlinear optical media 11a-11c of the wavelength conversion device 10A can be controlled by the position adjustment mechanism 15. FIG. As an example of the position adjustment mechanism 15, rails extending in a direction parallel to the optical axis are provided with supports for individually holding the nonlinear optical media 11a, 11b, and 11c, and the position of each support is adjusted by a translational actuator, motor, MEMS, or the like. may be controlled by the drive means of

By independently controlling the positions of the nonlinear optical media 11a, 11b, and 11c in the optical axis direction, the interval between the adjacent nonlinear optical media 11 (that is, the optical length t of the dispersion medium 12 in the optical axis direction) is adjusted. enable. The length L of the nonlinear optical medium 11 in the optical axis direction is designed to be a length that requires manipulation to achieve a desired relative phase relationship.

The optical length t1 in the optical axis direction of the dispersive medium 12 interposed between the nonlinear optical media 11a and 11b and the optical length t2 in the optical axis direction of the dispersive medium 12 interposed between the nonlinear optical media 11b and 11c are the wavelength conversion The phase matching condition is maintained throughout the device 10A. Alternatively, it may be adjusted so that a desired relative phase relationship is obtained at a desired position of the wavelength conversion device 10A.

The optical lengths t1 and t2 of the dispersion medium 12 in the optical axis direction can be changed by adjusting the positions of at least some of the nonlinear optical media 11a, 11b, and 11c in the optical axis direction with the position adjusting mechanism 15. FIG. The configuration of FIG. 1B can be realized by simply holding the nonlinear optical media 11a to 11c with a simple position adjustment mechanism 15 in air.

In order for new frequency components generated inside the nonlinear optical medium 11 to have sufficient intensity, there must be a phase difference between one or more incident light components and/or between the incident light and the newly generated frequency components. Must be consistent. It is desirable that this phase matching condition is maintained even at the time of incidence on the next nonlinear optical medium 11 . The optical length t, which is the effective length of the dispersion medium 12 in the optical axis direction, is adjusted with high accuracy by the position adjustment mechanism 15, and the phase matching condition is satisfied at the next incident position to the nonlinear optical medium 11. By controlling in such a manner, high conversion efficiency can be achieved in the wavelength conversion device 10A.

Furthermore, by setting the length L of the nonlinear optical medium 11 in the optical axis direction to an appropriate value, it is possible to design the phase relationship between a plurality of frequency components to have a desired relationship at an appropriate interaction length. . It is possible to freely design the phase relationship at which position of the wavelength conversion device 10A, and obtain a desired frequency profile as the final output of the wavelength conversion device 10A as shown in FIGS. 1A and 1B. be able to.

FIG. 2 is a schematic diagram of a laser device 1B using the wavelength conversion device 10B. In the wavelength conversion device 10B, solid dispersion media 12a and 12b are used for at least part of the dispersion medium 12 arranged between adjacent nonlinear optical media 11. FIG. The solid dispersion media 12a, 12b are, for example, glass, silicate, calcium fluoride, magnesium fluoride, etc., transparent to the wavelength used.

In the example of FIG. 2, the dispersive medium 12a placed between the nonlinear optical media 11a and 11b is composed of a pair of wedge-shaped parts 12a ₁ and 12a ₂ . A dispersive medium 12b placed between the nonlinear optical media 11b and 11c is composed of a pair of wedge-shaped parts 12b ₁ and 12b ₂ . Either one of the dispersion media 12a and 12b may be an air layer.

In order to variably control the optical length t1 of the solid dispersion medium 12a along the optical axis, at least one of the parts 12a ₁ and 12a ₂ is moved along the ridgeline of the wedge. The position of the dispersion medium 12a ₁ and/or 12a ₂ may be manually adjusted, or may be connected to a position adjustment mechanism 15 as shown in FIG. 1B. Similarly, to variably control the optical length t2 of the dispersion medium 12b along the optical axis, at least one of the parts 12b ₁ and 12b ₂ is moved along the ridgeline of the wedge. When connecting to the position adjusting mechanism 15 as shown in FIG. 1B, the dispersion medium 12a and the dispersion medium 12b may be movable independently of each other. The amount of movement of the parts 12a ₁ and/or 12a ₂ in the ridgeline direction and the amount of movement of the parts 12b ₁ and/or 12b ₂ in the ridgeline direction are determined by the refractive index difference between the nonlinear optical medium 11 and air, and the difference between the air and the solid. It may be calculated in consideration of the refractive index difference with the dispersion media 12a and 12b.

FIG. 3 is a schematic diagram of a laser device 1C using a wavelength conversion device 10C. In the wavelength conversion device 10C, cuboid or plate-shaped dispersion media 12c and 12d are used for at least a part of the dispersion medium 12 arranged between adjacent nonlinear optical media 11. FIG. The solid dispersion media 12c, 12d are, for example, glass, silicate, calcium fluoride, magnesium fluoride, etc., transparent to the wavelength used. Either one of the dispersion media 12c and 12d may be an air layer.

When the optical length t1 along the optical axis of the solid dispersion medium 12c and/or the optical length t2 along the optical axis of the dispersion medium 12d are variably controlled, the inclination of the dispersion medium 12c and/or dispersion medium 12d with respect to the optical axis or change the angle of rotation. The inclinations of the dispersion media 12c and 12d with respect to the optical axis may be adjusted using the position adjustment mechanism 15 as shown in FIG. 1B. The amount of tilt or rotation of the dispersion media 12c and 12d may be calculated in consideration of the refractive index difference between the nonlinear optical medium 11 and air and the refractive index difference between the air and the dispersion media 12c and 12d.

FIG. 4 is a schematic diagram of a laser device 1D using the wavelength conversion device 10D. In the wavelength conversion device 10D, a liquid or a gas other than air is used as the dispersive medium 12 that fills the space between adjacent nonlinear optical media 11 . When a gas other than air or a liquid is used as the dispersion medium 12, the entire wavelength conversion device 10A is placed inside a container 21 provided with transmission windows 22 and 23 on the incident side and the outgoing side, respectively.

As the dispersion medium 12 for gases other than air, an inert gas such as nitrogen or argon, or a mixture of air and an inert gas such as nitrogen or argon can be used. When a liquid is used as the dispersion medium 12, an organic solvent, an optical gel, or the like whose refractive index depends on the frequency or wavelength can be used.

When the container 21 is filled with liquid or gas other than air, the position adjustment mechanism 15 arranged outside the container 21 is used to control the positions of the nonlinear optical media 11a to 11c. may be adjusted.

Alternatively, by changing the mixing ratio of an inert gas such as nitrogen or argon mixed with air or the mixing ratio of two or more liquids to change the refractive index of the dispersion medium 12, the optical axis direction of the dispersion medium 12 is changed. The optical lengths t1 and t2 may be adjusted. Further, the optical lengths t1 and t2 of the dispersion medium 12 may be adjusted by changing the pressure of the dispersion medium 12 to change the refractive index.

FIG. 5 is a diagram for explaining propagation of each frequency component between a plurality of nonlinear optical media 11. FIG. In the wavelength conversion device 10, the position of incidence on the first nonlinear optical medium 11a is position I, the position immediately after incidence on the nonlinear optical medium 11a is position II, and the position on the exit surface of the nonlinear optical medium 11a is position III. . Position IV is the position of incidence on the second nonlinear optical medium 11b, and position V is the position immediately after incidence on the nonlinear optical medium 11b.

The propagation length in the optical axis direction of the nonlinear optical medium 11a is L. The distance between the nonlinear optical media 11a and 11b, that is, the propagation length (optical length) of the dispersive medium 12 in the optical axis direction is t. Consider wavelength conversion in which frequency components with angular frequencies of ω1 and ω2 are incident on the wavelength conversion device 10 to generate frequency components ω3=ω1+ω2.

The phase of the light wave with angular frequency ω1 at position I is φ _I,1 , the phase of the light wave with angular frequency ω2 at position I is φ _I,2 , and the phase of the light wave with angular frequency ω1 at position II is φ _{II,1 .} , the phase of the light wave with angular frequency ω2 at position II is φ _II,2 .

At position II, the interaction of the incident light at ω1 and ω2 with the nonlinear optical medium 11a initiates the generation of the frequency component at ω3. If the phase of ω3 at position II is φ _II,3 , then φ _II,3 =φ _I,1 +φ _I,2 +π/2. Since ω3 has not yet occurred at the interface between position I and position II, nonlinear polarizations ω3−ω2 and ω3−ω1 have not occurred, and φ _I,1 =φ _II,1 and φ _I,2 =φ _{II, 2} . The phase difference Δφ _II between light waves at position II immediately after incidence is
Δφ _II =φ _II,3 −(φ _II,1 +φ _II,2 +π/2)=0
is.

When the frequency components ω1, ω2, and ω3 are propagated by the optical path length L to position III, the phase of each frequency component at position III is as follows.

φ _III,1 = k ₁ L + φ _II,1
φ _III,2 = k ₂ L + φ _II,2
φIII _,3 = _k3L +φII _,3
Here, k ₁ , k ₂ , and k ₃ are wavenumbers of each frequency component in the nonlinear optical medium 11a. Assuming that the light waves are phase-matched at the position III, that is, at the exit surface of the nonlinear optical medium 11a, the following is obtained.

Δφ _III =[k ₃ −(k ₁ +k ₂ )]L+φ _II,3 −(φ _II,1 +φ _II,2 +π/2)
= ΔkL + Δφ _II = 0
When the frequency components ω1, ω2, and ω3 are propagated through the dispersive medium 12 with wavenumbers k ₁ ', k ₂ ', and k ₃ ' respectively from the position III to the position IV by a distance d, the second nonlinear optical medium 11b enters The phases corresponding to Δφ _II and Δφ _III at plane position IV are as follows.

φ _IV,1 =k ₁ 'd+φ _III,1
φ _IV,2 =k ₂ 'd+φ _III,2
φ _IV,3 =k ₃ 'd+φ _III,3
The phase difference of each frequency component at position IV is as follows.

Δφ _IV =[k ₃ '-(k ₁ '+k ₂ ')]d+Δφ _III =[k ₃ '-(k ₁ '+k ₂ ')]d
Considering the polarization of ω3=ω1+ω2 for the light wave propagating through the nonlinear optical medium 11b at the position V immediately after incidence on the nonlinear optical medium 11b, the phase of each frequency component at the position V is as follows.

φ _V,3 =φ _IV,1 +φ _IV,2 +π/2
At this time, the phase difference between the frequency component ω3 generated in the nonlinear optical medium 11a and the frequency component ω3 newly generated in the nonlinear optical medium 11b is
Δφ _V =φ _IV,3 −φ _V,3 =φ _IV,3 −(φ _IV,1 +φ _IV,2 +π/2)
=Δφ _IV =[k ₃ '−(k ₁ '+k ₂ ')]d
becomes.

If the desired purpose is to generate high intensity sum frequency ω3 without phase shift over a long interaction length, the phase difference Δφ _V at position V is 2mπ(m is an integer). At this time, the sum frequency ω3 can be added without causing a phase shift (while maintaining the phase matching condition). At this time, the distance between the nonlinear optical media 11a and 11b, that is, the optical length t in the optical axis direction of the dispersive medium 12 is determined so as to satisfy Δφ _V =2mπ (m is an integer).

Since the relationship among wave number k, wavelength λ, and refractive index n is k=2πn/λ, phase difference Δφ _V at position V is expressed by equation (1).

分散媒質１２を空気または窒素とする場合、下記の空気と窒素の分散式を用いて、位相整合が得られる、すなわちΔφ_V＝２ｍπ（ｍは整数）となる距離ｄ、すなわち分散媒質１２の光学長ｔを計算する。

If the dispersive medium 12 is air or nitrogen, the following dispersion equations for air and nitrogen are used to obtain phase matching, i.e., a distance d such that Δφ _V =2mπ (where m is an integer), i.e., the optical Compute the length t.

非線形光学媒質１１ａ、１１ｂが第２高調波発生用の結晶の場合、基本波を７４８．８８ｎｍとすると、位相整合条件Δφ_V＝２π（ｍ＝１）を満たす結晶間の距離、すなわち分散媒質１２の光軸方向の光学長ｔは、空気（２０℃、１気圧）の場合で４２．０４ｍｍ、窒素（２０℃、１気圧）の場合で４４．３３ｍｍである。

When the nonlinear optical media 11a and 11b are crystals for generating the second harmonic, and the fundamental wave is 748.88 nm, the distance between the crystals that satisfies the phase matching condition Δφ _V =2π (m=1), that is, the dispersive medium 12 is 42.04 mm for air (20° C., 1 atmosphere) and 44.33 mm for nitrogen (20° C., 1 atmosphere).

For a crystal that mixes the fundamental wave (748.88 nm) and the second harmonic (374.44 nm) to generate the third harmonic (249.63 nm), the phase matching condition Δφ _V =2π (m=1) The distance between the crystals that satisfies , that is, the optical length t in the optical axis direction of the dispersion medium 12 is 12.335 mm for air and 13.519 mm for nitrogen.

By controlling the spacing of the nonlinear optical media 11 used in the wavelength conversion device 10, that is, the optical length t of the dispersive medium 12 in the optical axis direction, the entire wavelength conversion device 10 maintains phase matching and high conversion efficiency. be able to.

<Walk-off compensation>
FIG. 6 is a configuration diagram in which the wavelength conversion device 10E of the embodiment is provided with a walk-off compensation function. The optical axis or propagation axis of incident light is the Z axis, the plane on which the wavelength conversion device 10 is arranged is the YZ plane, and the height direction of the crystal is the X axis. In FIG. 6, the nonlinear optical medium 11 is arranged such that its C-axis lies in a plane parallel to the YZ plane.

Walk-off refers to a phenomenon in which an incident laser beam and a beam generated by wavelength conversion separate as they propagate due to the birefringence of the nonlinear optical medium 11 . Walk-off occurs with light components that have an angular dependence with respect to the crystallographic axis. Walk-off distorts the beam profile at the output stage of the wavelength conversion device 10 and reduces the conversion efficiency.

In FIG. 6, the directions of the C-axes of the nonlinear optical media 11a to 11d used in the wavelength conversion device 10A are alternated in order to compensate for walk-off. For example, by alternately arranging the crystals upside down, the C-axis of the crystals is oriented as indicated by the arrow in the figure. By arranging in this way, the direction of walk-off can be reversed as shown in FIG. 7, and the deviation of the wavelength-converted light from the fundamental light can be reduced on the exit surface. This configuration is effective when the nonlinear optical medium 11 has a large birefringence.

<Experimental example>
In the arrangement configuration of FIG. 7, LBO (LiB ₃ O ₅ : lithium triborate) having the same thickness (optical length L in the optical axis direction = 6 mm) is arranged as the nonlinear optical media 11a and 11b, and the wavelength is 787 nm and the energy is 14.5 mJ. Using the nanosecond pulsed laser beam as the fundamental wave, a double wave is generated.

FIG. 8 shows the observation results obtained. The horizontal axis is the distance (mm) corresponding to the crystal spacing t1, and the vertical axis is the double wave output energy (mJ) immediately after the crystal 11b. The black circles are the measured values, and the solid line is the fitting curve. Air (22° C., 1 atm) is used as the dispersion medium 12 in this experiment.

As can be seen in FIG. 8, the observed double wave output energy has a maximum value of 1.35 mJ and a minimum value of 0 mJ. The double wave generation energy immediately after the first crystal 11a is 0.4 mJ.

Air dispersion changes the relative phase relationship between the fundamental wave (787 nm) and the double wave (393.5 nm). , 0 to 1.35 mJ, which varies periodically with a width of 0 to 100%. The observed period of the crystal spacing t1 was 50.5 mm, which was confirmed to match the calculated value (50.2 mm) expected from the dispersion of the air, which is the dispersion medium 12, within the range of measurement accuracy.

FIG. 9 is a schematic diagram of another laser device 1 used to measure walk-off compensation. The laser device 1 constitutes an optical parametric amplifier as the wavelength conversion device 10 . KTP (KTiOPO ₄ : potassium titanyl phosphate) is used as the nonlinear optical medium 11 .

The first KTP has a length L1 in the optical axis direction of 2.5 mm, the second KTP has a length L2 of 5 mm in the optical axis direction, the third KTP has a length L3 of 5 mm in the optical axis direction, and the fourth KTP has a length L3 of 5 mm in the optical axis direction. The length L4 of the KTP in the optical axis direction is 2.5 mm. The dispersion medium 12 is air (20° C., 1 atmosphere).

The distance between the first KTP and the second KTP (or the optical length t1 of the dispersion medium 12) is 493.4 mm, or a distance sufficiently smaller than 493.4 mm (for example, 20 mm or less). The distance between the second KTP and the third KTP (or the optical length t2 of the dispersion medium 12) is 493.4 mm, or a distance sufficiently smaller than 493.4 mm (for example, 20 mm or less). The distance between the fourth KTPs (or the optical length t3 of the dispersion medium 12) is 493.4 mm, or a distance sufficiently smaller than 493.4 mm (for example, 20 mm or less).

If the purpose is to spatially overlap the beam generated by the incident laser light and the wavelength-converted light over a long interaction length by incorporating a walk compensation mechanism to induce strong optical parametric amplification, then the optical length t1 to t3 are set so that the phase matching condition is satisfied as described above. The theoretical value of the interval (optical length t) when satisfying the phase matching condition Δφ _V =2π (m=1) is 493.4 mm. is too long, it is set to a value sufficiently smaller than 493.4 mm (a value small enough to ignore the phase matching condition shift) to maintain the intensity of the light incident on the crystal.

The four KTPs used as the nonlinear optical media 11a to 11d are arranged such that the directions of the C-axes (indicated by arrows in the figure) in the YZ plane alternately face opposite directions, as in FIG. Placement is flipped. When walk-off is compensated for by staggering the directions of the crystal axes, it is desirable to use an even number of nonlinear optical media 11 and arrange them symmetrically with respect to the midpoint in the optical axis direction.

A signal light pulse with a wavelength of 1201 nm and a pump light pulse with a wavelength of 801 are made incident on the wavelength conversion device 10 from the light source 2 . The pump light pulse is for inducing a large nonlinear polarization in the nonlinear optical medium 11 . A signal light pulse of 1201 nm is a 1200 nm band CW (continuous wave) laser light oscillated by an external cavity diode laser (ECDL) and periodically poled lithium niobate (PPLN). ) is pulsed and generated by a crystal-based optical parametric resonator.

This wavelength conversion device 10 generates idler light with a wavelength of 2403 nm.

As shown in FIG. 10, by alternately reversing the orientation of the C-axis of the KTP, the walk-off of the converted light with a wavelength of 2403 nm is compensated for, at the output face of the wavelength conversion device 10, the beam of idler light from the fundamental wave. deviation is minimal.

The distance between the first nonlinear optical medium 11a and the second nonlinear optical medium 11b, that is, the optical length t of the dispersive medium 12 is the phase matching condition Δφ=[k ₃ −(k ₁ +k ₂ )] It is set sufficiently smaller than the value that satisfies t=2π (m=1).

Here, _k3 is the wavenumber of the pump light, _k1 is the wavenumber of the signal light, and _k2 is the wavenumber of the idler light. The idler light incident on the second nonlinear optical medium 11b and the pump light and signal light are phase-matched, and the idler light works in a constructive direction to form a new idler with sufficient intensity in the nonlinear optical medium 11b. Light is produced. The third and subsequent nonlinear optical media 11 are also arranged so that the phase matching condition is satisfied.

FIG. 11 is an output profile from the wavelength conversion device 10 of FIG. FIG. 11A shows the profile in the direction (x-axis direction) perpendicular to the plane on which the C-axis of the crystal exists, and FIG. 11B shows the profile in the Y-axis direction. The XY plane is a plane orthogonal to the propagation direction Z. By arranging the nonlinear optical media 11a and 11b so that the directions of the crystal axes are alternately reversed and arranging them so that the phase matching condition is satisfied throughout the wavelength conversion device 10, walk-off is compensated and the x-axis A highly symmetrical beam profile is obtained in both the direction and the y-axis direction. In addition, idler light of sufficient intensity can be obtained, and conversion efficiency can be maintained high.

FIG. 12 is a diagram showing the effect of the embodiment. The horizontal axis is the number of crystals, and the vertical axis is the conversion efficiency (%). The conversion efficiency is determined by the ratio of the generated energy of the double wave to the energy of the fundamental wave (generated energy of the double wave/energy of the fundamental wave).

Here, seven crystals are arranged along the optical axis to generate a double wave (375 nm wavelength) of a single-frequency nanosecond pulsed laser light (fundamental wave) with a wavelength of 750 nm. The generation of the double wave may be considered as the case of 2×ω3=ω1=ω2 in the sum frequency generation described with reference to FIG.

The crystal used for generating the double wave is a type 1 LBO with a length (L) of 6 mm in the optical axis direction. Seven LBO crystals of the same shape are arranged with the C-axis alternately reversed to cancel walk-off (see FIG. 6), and the conversion efficiency is measured. The optimum crystal spacing under this condition, that is, the optical length t of the air layer as the dispersion medium in the optical axis direction is 43.1 mm. The optimum crystal spacing calculated from the above formula (1) and the air dispersion formula is 42.9 mm, but the crystal spacing obtained with a period of 2π of the measured value as shown in FIG. 8 is 43.1 mm. Matches within the measurement accuracy.

Line A shows the conversion efficiency when the seven LBO crystals are arranged at optimum positions, that is, when the interval between adjacent crystals or the optical length of the dispersion medium in the optical axis direction is optimized. Line B is the conversion efficiency when all seven LBO crystals are placed at positions shifted by 1 mm from the optimum position. Line C is the conversion efficiency when 7 LBO crystals are appropriately spaced at 4-5 cm without controlling the crystal spacing.

When the arrangement positions of the nonlinear optical crystals are not controlled, if the number of crystals is about 3 as in line C, a conversion efficiency of up to about 50% can be obtained, as in the case of strict position control. When the number of crystals exceeds 3, the conversion efficiency of line C decreases, and when the number of crystals exceeds 5, the phase mismatch accumulates and the difference with line A increases.

Line B when shifted by 1 mm from the optimum position shows the same tendency as line A up to 6 crystals, and the conversion efficiency simply increases, but the conversion efficiency is lower than that of line A. If the number of crystals exceeds 6, the phase matching condition cannot be maintained, and the conversion efficiency decreases.

When a plurality of divided nonlinear optical crystals are arranged along the optical axis, the conversion efficiency increases as the number of crystals increases. By arranging the LBO crystals so that the phase matching condition is maintained on the incident surfaces of all the LBO crystals from the second onwards, the second harmonic wave component is output without attenuation. A very high energy conversion efficiency of 82% is obtained at the output stage of the seventh LBO crystal.

The length of the LBO crystal in the optical axis direction is preferably a length that satisfies the phase matching condition on the output surface. Then, by controlling the optical length of the dispersion medium, it is possible to generate sum frequency, difference frequency, second harmonic, third harmonic, etc. with high energy conversion efficiency. A wavelength conversion device can be realized simply by using a plurality of mass-produced nonlinear optical crystals having the same shape and arranging them at optimum positions.

Although the present invention has been described above based on specific configuration examples, the present invention is not limited to the above-described embodiments. For example, a walk-off compensation configuration can be applied to wavelength conversion device 10B of FIG. 2 and wavelength conversion device 10C of FIG. In this case, an even number of nonlinear optical media 11 are arranged symmetrically with respect to the midpoint in the optical axis direction, and the directions of the C-axes are alternately reversed.

As the nonlinear optical medium 11, in addition to LBO, KTP, β-BBO, and PPLN, lithium niobate (LiNbO3), lithium cesium borate (CLBO), potassium aluminum borate (KABO), potassium dihydrogen phosphate (KDP ) and the like may be used.

The laser device of the embodiment can be suitably applied to semiconductor lithography light sources, short wavelength processing lasers, remote environment measurement lasers, optical microscopes, and the like.

1, 1A to 1D laser device 2 light source 10, 10A to 10E wavelength conversion device 11, 11a to 11d nonlinear optical medium 12, 12a, 12b dispersion medium

Claims (6)

a solid nonlinear optical medium of the same material or composition divided into two or more;
a dispersive medium interposed between the nonlinear optical media;
In a wavelength conversion device having
a container containing the nonlinear optical medium and the dispersion medium;
Each length of the nonlinear optical medium is an interaction length in which a phase operation is performed to determine which position and which phase relationship a plurality of frequency components generated in the course of the progress of the nonlinear optical phenomenon are brought to in the wavelength conversion device. is designed to
The optical length of the dispersive medium in the optical axis direction is such that a phase matching condition between one or more frequency components incident on the nonlinear optical medium and a new frequency component generated in the nonlinear optical medium is satisfied. is variably adjusted to
The dispersion medium is a gas or liquid excluding air, and the pressure of the gas or liquid filling the space between the two or more divided nonlinear optical media inside the container is adjusted so as to satisfy the phase matching condition. A wavelength conversion device characterized by: 2. The wavelength conversion device according to claim 1, wherein the optical length of the dispersion medium is adjusted by setting a predetermined interval between the nonlinear optical media inside the container. The dispersion medium is a mixed gas of two or more gases or a mixed liquid of two or more liquids. 2. The wavelength conversion device according to claim 1, wherein the optical length is adjusted. 4. The wavelength conversion device according to any one of claims 1 to 3, wherein the nonlinear optical media divided into two or more are arranged so that the directions of crystal axes are alternately reversed. the nonlinear optical medium divided into two or more includes a first nonlinear optical medium and a second nonlinear optical medium arranged adjacent to each other;
The distance between the first nonlinear optical medium and the second nonlinear optical medium is the one or more frequency components incident on the first nonlinear optical medium on the plane of incidence of the second nonlinear optical medium. , and the new frequency component generated in the first nonlinear optical medium are set so that the phase matching condition is satisfied. The wavelength conversion device according to . a wavelength conversion device according to any one of claims 1 to 5;
a light source that outputs light containing the one or more frequency components incident on the wavelength conversion device;
A laser device having

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