JPH07193310A - Laser adjusting device - Google Patents

Abstract

PURPOSE:To provide the title laser adjusting device capable of phase matching of a non-linear optical crystal to the optical axis of a laser beam with high precision. CONSTITUTION:The title laser adjusting device is provided with a non-linear optical crystal l in the phase matching direction X' to the optical axis 10 of an input beams having the birefringence for the frequency conversion of the input beams and a supporting body 3 adjusting the inclination of the non-linear optical crystal l in the plane orthogonal to the plane capable of tilting said crystal l in the the phase matching the phase matching direction X; and the crystal axis Z making an angle not exceeding 90' with said direction X' so as to adjust the relative position of the non-linear optical crystal l to the input beams.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a laser adjusting device for adjusting the phase matching direction of a nonlinear optical crystal that frequency-converts the laser light as an input light with respect to the optical axis of the laser light.

[0002]

2. Description of the Related Art Conventionally, there has been known a laser adjusting device for performing frequency conversion of input light by utilizing nonlinear polarization of a nonlinear optical crystal. In this laser adjustment device, it is necessary to satisfy the phase matching condition in order to improve the conversion efficiency. Here, the phase matching condition refers to a condition of a phase that needs to be maintained between each component wave when performing light mixing, generation of optical harmonics, etc. (Source: Optical terminology dictionary; Publisher: Ohmsha) ).

Light propagating in a uniaxial or biaxial nonlinear optical crystal includes an ordinary ray whose propagation velocity does not vary depending on the traveling direction of the light and an extraordinary ray whose propagation velocity varies depending on the traveling direction of the light.

Now, let us consider the phase matching condition when the laser light is made incident on the nonlinear optical crystal as input light and is emitted as output light from the nonlinear optical crystal. In this case, the input light is an ordinary ray having the first frequency and the output light is an extraordinary ray having the second frequency.

Here, in the phase matching, the angular frequencies of the input light are ω11 and ω12, and the angular frequency after frequency conversion is ω2,
The refractive indices of the nonlinear optical crystal with respect to the input light are n11 and n1.
2. If the refractive index of the nonlinear optical crystal for output light after frequency conversion is n2, the phase matching direction is known as a direction that satisfies the following two expressions.

Ω11 + ω12 = ω2 (1) (n11 × ω11) + (n12 × ω12) = (n2 × ω2) (2) Laser so that the conditions of the above equations (1) and (2) are satisfied. Phase adjustment can be achieved by adjusting the incident direction of light relative to the nonlinear optical crystal. This phase matching is performed by adjusting the angle of the nonlinear optical crystal with respect to the laser light.

As the angle adjusting mechanism of this type of optical system, those generally shown in FIGS. 10 and 11 are known. In FIG. 10, reference numeral 57 is a housing, and the housing 57 is tubular. A base 56 is attached to the tip of the housing 57. A support portion 53 is integrally provided on the base 56, and an annular diaphragm portion 54 is provided between the base 56 and the support portion 53. Support part 5
An annular flange 55 is provided at 3. The optical crystal 52 as the component to be adjusted is attached to the support portion 53. As shown in FIG. 11, a screw 51 is provided in the circumferential direction on the annular flange portion 55. By tightening or loosening the screw 51 to elastically deform the diaphragm portion 54, the optical crystal is moved in three axial directions. Adjust (JP-A-60
-47486 gazette).

[0008]

However, since the nonlinear optical crystal is provided with an optical adjustment mechanism for each crystal axis without paying special attention to its characteristics,
There is a problem that the adjustment work for achieving the phase matching with respect to the optical axis of the laser light is complicated. In addition, in this type of laser device, it is also known that the nonlinear optical crystal is incorporated after adjusting the phase matching of the nonlinear optical crystal with respect to the optical axis of the laser light externally. If the phase matching of the light with respect to the optical axis deviates, it becomes extremely difficult to perform the phase matching adjustment.
On the other hand, if the phase matching of the nonlinear optical crystal with respect to the optical axis of the laser light is not performed, there is a problem that the conversion efficiency is lowered as described above.

The present invention has been made in view of the above circumstances, and its object is to provide a small and simple structure.
An object of the present invention is to provide a laser adjustment device that can perform phase matching adjustment of a nonlinear optical crystal with respect to the optical axis of laser light with good accuracy.

[0010]

In order to solve the above-mentioned problems, a laser adjusting device according to a first aspect of the present invention provides:
A non-linear optical crystal having a phase matching direction with respect to the optical axis of the input light and having birefringence for performing frequency conversion of the input light, and the phase matching direction and an angle of less than 90 degrees with respect to the phase matching direction. And a support capable of inclining the non-linear optical crystal in a plane including a crystal axis that forms the crystal axis and controlling the inclination of the non-linear optical crystal in a plane orthogonal to the plane.

In order to solve the above problems, the laser adjusting device according to claim 3 of the present invention has a phase matching direction with respect to the optical axis of the input light and has a birefringence for performing frequency conversion of the input light. A non-linear optical crystal having optical properties and adjustment of the non-linear optical effect to a direction having a strong angle dependence in the vicinity of phase matching, so that the non-linear optical crystal can be tilted in a direction having a strong angle dependence and the angle dependence And a support body that restricts an inclination in a direction orthogonal to the strong direction.

[0012]

In the laser adjusting device according to the first and third aspects of the present invention, when the support is tilted, the phase matching adjustment is performed only in the plane including the direction in which the phase matching condition is severe.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a laser adjusting device according to the present invention will be described below with reference to the drawings.

The phase matching condition of the non-linear optical crystal with respect to the optical axis of the laser light is shown again.

Ω11 + ω12 = ω2 (1) (n11 × ω11) + (n12 × ω12) = (n2 × ω2) (2) ω11 and ω12 are the angular frequencies of the input light, and ω2 is the output light after the frequency conversion. Angular frequencies, n11 and n12 are the refractive indices of the nonlinear optical crystal with respect to the input light, and n2 is the refractive index of the nonlinear optical crystal with respect to the output light after frequency conversion.

In the above equations (1) and (2), assuming that the propagation speed of the input light is v11, v12 and the propagation speed of the output light is v2, (1 / v11) + (1 / v12) = (1 / v2) (1) '(n11 / v11) + (n12 / v12) = (n2 / v2) (2)' Especially, when the angular frequency ω11 and the angular frequency ω12 are equal, the angular frequency ω2 of the output light is , Ω2 = 2ω11 (3) When this equation (3) is used, the equation (2) is: n11 + n12 = 2 × n2 (4) In the equations (1) and (2), the angular frequency ω11 and the angular frequency ω12 are When the phase matching is performed so that the two are equal to each other, the second harmonic can be generated as is apparent from the equation (4).

An adjusting mechanism of the nonlinear optical crystal for generating the second harmonic will be described below with reference to FIG.

FIG. 1 is a sectional view of a semiconductor laser resonator for generating a second harmonic wave in which an adjusting mechanism of a nonlinear optical crystal is incorporated, and FIG. 2A is a perspective view of the adjusting mechanism. In FIG. 1, the laser resonator has a semiconductor laser 6 as an excitation source, a condenser lens 7, a laser medium 2, a nonlinear optical crystal 1, and an output mirror 5. In this embodiment, the nonlinear optical crystal 1 is a KTP that is often used in a YAG laser or the like.
(KTiPO4) crystal. The nonlinear optical crystal 1 is provided on the support 3.

As shown in FIGS. 2A and 2B, the support 3 has a horizontal mounting portion 3a, a bending portion 3b, a vertical support portion 3c, and a positioning portion 3e. The vertical support 3c has a reference surface 3c '.
Are formed. The reference plane 3c 'is perpendicular to the laser optical axis 10 described later. The vertical support portion 3c has a through hole 3d. The laser medium 2 is fixed to the vertical support portion 3c. The nonlinear optical crystal 1 is fixed to the horizontal mounting portion 3a. At the bottom of the horizontal mounting portion 3a, the support 3
The tip of the screw 4 provided on the above is abutted. By rotating the screw 4, the bending amount of the bending portion 3b is adjusted. The angle of the horizontal mounting portion 3a with respect to the reference surface 3c 'is adjusted by the amount of bending of the bending portion 3b.

The laser medium 2 has a total reflection surface 2a.
The output mirror 5 has a semi-transmissive surface 5a. The semi-transmissive surface 5a constitutes a concave surface. The excitation laser light 8 emitted from the semiconductor laser 6 is condensed by the condenser lens 7, guided to the laser medium 2 through the through hole 3d, and absorbed. The laser medium 2 is excited by the exciting laser beam 8. The laser medium 2 generates a laser beam 9 as an input beam by the excitation laser beam 8.

The nonlinear optical crystal 1 is arranged so that the phase matching direction X'of the nonlinear optical crystal 1 substantially coincides with the laser optical axis 10 when the nonlinear optical crystal 1 is supported by the horizontal mounting portion 3a. The crystal block of the nonlinear optical crystal 1 is cut out.

Now, consider the case where the nonlinear optical crystal 1 is a uniaxial crystal. In this case, the laser light incident on the nonlinear optical crystal 1 is branched into an ordinary ray whose refractive index does not change depending on the propagation direction and an extraordinary ray whose refractive index changes depending on the propagation direction. At this time, the refractive indices of the ordinary ray and the extraordinary ray of the fundamental wave which are the input light are respectively no1, eo1,
FIG. 3 shows the refractive index surfaces when the refractive indexes of the ordinary ray and the extraordinary ray of the higher harmonics which are the output light are no2 and eo2. This refractive index plane has a crystal axis X, as shown in FIG.
In the coordinate system represented by Y and Z, the optical axis is made to coincide with the Z axis, and Y is set in the direction orthogonal to the optical axis on the paper surface.
The axis and the direction orthogonal to the paper surface are shown as the X axis.

The directions in which the refractive indices of the ordinary ray and the extraordinary ray of the fundamental wave and the harmonic wave are equal to each other (no1 = ne1, no2)
= Ne2) is called the optical axis of the crystal.

In this case, the following two cases can be considered as the phase matching direction satisfying the phase matching condition.

A case where the laser light is incident in a direction in which the refractive index of the ordinary ray of the fundamental wave and the refractive index of the extraordinary ray of the harmonic become equal (type I).

When laser light is incident in a direction in which the average refractive index of the ordinary ray of the fundamental wave and the refractive index of the extraordinary ray of the fundamental wave are equal to the refractive index of the extraordinary ray of the harmonic wave (type II) .

The phase matching angle with the optical axis Z indicating the type I phase matching direction is θm.

The ordinary rays no1 and no2 are spherical waves, and the extraordinary rays ne1 and ne2 are represented by spheroids having a major axis in the Z-axis direction and a minor axis in the Y-axis direction. The phase matching angle θm is a spheroid of a spherical surface (a circle indicated by reference numeral no1 in FIG. 3) representing the refractive index of a fundamental wave of an ordinary ray and a fundamental wave (an ellipse indicated by reference numeral ne2 in FIG. 3) of an extraordinary ray. It is determined by the intersection point Fo with.

Now, consideration will be given to the optical axis and the adjustment in the direction orthogonal to the optical axis.

Adjustment around the optical axis Z does not break the phase matching condition. This is because even if the nonlinear optical crystal is rotated about the optical axis Z, the phase matching direction always exists in the rotation direction. That is, the phase matching direction is
This is because it is determined by the line of intersection between the spheroid and the spherical surface, and the line of intersection draws a circle cir with the optical axis Z as the center.

In the adjustment around the optical axis Y, the difference between the refractive index of the ordinary ray of the fundamental wave and the refractive index of the extraordinary ray of the harmonic wave is ±.
It becomes maximum only when rotated 90 degrees.

On the other hand, in the case of the adjustment in the plane including the optical axis Z and the phase matching direction, that is, the adjustment around the optical axis X (in FIG. 3, the direction perpendicular to the paper surface is the X axis). .), The angle θm formed by the optical axis Z and the phase matching direction is 9
Since it is less than 0 degree, and the difference in refractive index is maximized here by the rotation of the angle θm, in the case of the adjustment around the optical axis X, more precise adjustment is required as compared with the adjustment around the remaining optical axes Z and Y. Required. This is because, for example, on the optical axis Z, the point F1 indicates the refractive index of the ordinary ray of the fundamental wave, the point F2 indicates the refractive index of the extraordinary ray of the fundamental wave, and the difference of the refractive index (F1- This is because F2) becomes maximum.

Next, the uniaxial crystal "KDP (KH2
PO4) ”, a ruby pulse laser (λ = 0.6
The phase matching angle θm for 94 μm) is 51 degrees.
Therefore, in the rotation around the optical axis X, the difference between the refractive index of the ordinary ray of the fundamental wave and the refractive index of the extraordinary ray of the harmonic wave is maximized by changing the phase matching angle θm which is an angle less than 90 degrees. Become. That is, it is understood that the adjustment around the optical axis X requires more precise adjustment than the adjustment around the remaining two axes Z and Y.

Next, the case of a biaxial crystal will be described.

In the case of a biaxial crystal, it propagates as two wavefronts having different propagation velocities in directions other than the two optical axis directions. In the plane formed by the two principal axes (optical axes) of the crystal, an ordinary ray having the same refractive index regardless of the traveling direction and an extraordinary ray having a different refractive index depending on the traveling direction propagate. A ray that travels on a plane other than the plane formed by the two optical axes behaves as an extraordinary ray having a different refractive index depending on the traveling direction.

Therefore, in the case of the biaxial crystal, there are two cases of the phase matching directions that satisfy the phase matching condition, which will be described below.

When laser light is incident in a direction in which the refractive index of the fundamental wave with respect to one wavefront is equal to the refractive index of the harmonic wavefront with which the harmonic wave vibrates in a direction orthogonal to the vibration direction of the one wavefront of the fundamental wave. (Type I).

A case in which laser light is incident in a direction in which the average refractive index of the respective refractive indexes of the wavefronts of the fundamental wave orthogonal to each other and the refractive index of one of the harmonic wavefronts are equal (type II).

An index ellipsoid in a general biaxial crystal is shown in FIG. This index ellipsoid has crystal axes X, Y, Z
At the refractive index of n ₁ , n ₂ , n ₃ (n ₁ <n ₂ <
n ₃ ). A plane wave traveling in an arbitrary direction K can be considered as two plane polarized lights in which orthogonal planes vibrate in parallel to a cutting plane (elliptical plane) that cuts an index ellipsoid. The lengths of the major axis and the minor axis of the cut surface represent the refractive index for the wavefront vibrating in parallel with the respective axes.

In the biaxial crystal, the cross section of the index ellipsoid for the fundamental wave is a solid line, the cross section of the index ellipsoid for the harmonic is a dotted line, and the cross section of the XZ plane is shown in FIG.
The cross section of the YZ plane is shown in FIG. 5B, and the cross section of the XY plane is shown in FIG.
It shows in (c). Further, the relationship between two types of phase matching, type I and type II, is shown by an arrow in FIG. Figure 5
In, the subscript “I” indicates type I, and the subscript “I”
"I" indicates type II.

FIG. 6 shows the relationship with the normal velocity.

Here, the normal velocity of the wavefront of the fundamental wave is a solid line, the normal velocity of the wavefront of the harmonic is a dotted line, the type I phase matching direction is a dotted arrow, and the type II phase matching direction is a dashed line. Shown as an arrow. In addition, in FIG.
In order to explain the type II phase matching condition, the crystal axis XY
The average locus of the normal velocities with respect to two wavefronts in which the fundamental wave on each surface formed by Z is separated by birefringence is partially shown by a chain line.

Further, in FIG. 7, the biaxial crystal K
In the case of TP crystal, Nd: YAG with λ = 1.064 μm
The effective non-linear optical constants at that time are shown by dotted lines as solid lines of the type I and type II phase matching directions (the tilt angles from the crystal axis X and the crystal axis Z are φ and θ, respectively) with respect to the harmonics of the laser. ing.

In this case, the type II has a value of the nonlinear optical constant about 10 times larger than that of the type I, and the phase matching direction (the crystal axis X and the phase matching direction) in which the nonlinear optical constant becomes maximum. It is practical to adopt a phase matching angle φ of 24 ° and a phase matching angle θ of about 90 ° between the crystal axis Z and the phase matching direction.

In this biaxial crystal, since the angle formed by the phase matching direction and the crystal axis X or the crystal axis Y is less than 90 °, the phase matching in the plane including the phase matching direction (rotation around the Z axis). The adjustment in the angular direction requires higher accuracy than the adjustment in the direction orthogonal thereto.

This will be considered from the viewpoint of the angle dependence of the nonlinear optical effect.

8 and 9 show the fundamental wave 106 of the KTP crystal.
The angle dependence of the second harmonic wave 532 nm with respect to 4 nm is shown, and the output change due to the deviation from φ = 24 ° and θ = 90 ° is shown. Also, KT regarding the polarization direction
The output change due to the shift of the P crystal in the φ direction shows the characteristic according to FIG. 7. From this, it can be seen that the angle dependency in the φ direction is about four times higher in sensitivity than in the θ and φ directions.

Therefore, specifically regarding the KTP crystal, the incident direction of the laser beam is the rotation direction about the Z axis (rotation in the plane perpendicular to the crystal axis Z), and particularly approaches the X axis. Even if it shifts slightly in the direction, the phase matching condition will be broken. In this direction, depending on the cutting accuracy of the crystal and the mounting accuracy on the supporting portion 3, it does not fall within the range of ± 0.5 ° where sufficient phase matching can be achieved. Requires at least a precision adjustment mechanism with a sensitivity of 0.5 ° or less.

On the other hand, the other two directions (X axis and Y axis directions)
With respect to, the laser optical axis 10 (the incident direction of the laser light) is sufficiently phase-matched with respect to the phase-matching direction depending on the cutting-out accuracy of the crystal and the mounting accuracy on the support body 3 without providing an adjusting mechanism. It is possible to set it within a range of about ± 2 °, which allows for the removal.

In FIG. 2B, the support body 3 has the horizontal mounting portion 3a integrated with the thin plate-shaped bending portion 3b as described above. The bending portion 3b is plastically or elastically deformed by the rotation of the adjusting screw 4, and the horizontal mounting portion 3a is thereby tilted with the DD 'axis (crystal axis Z) as the rotation axis. Further, the rotation in the direction orthogonal to the rotation axis is restricted. The nonlinear optical crystal 1 is mounted on the horizontal mounting portion 3a such that the surface including the optical axis and the phase matching direction coincides with the tilt direction.

Therefore, by adjusting the adjusting screw 4, D-
The non-linear optical crystal 1 can be tilted in the plane including the optical axis and its phase matching direction with the D ′ axis (crystal axis Z) as the rotation axis, and ±± with respect to the phase matching direction required around the crystal axis Z. Fine adjustment is possible within 0.5 °.

Although the embodiments have been described above, the present invention is not limited to KTP crystals, but can be applied to all nonlinear optical crystals even if the adjustment sensitivity values are different.

If the tilts of the nonlinear optical crystal 1 in the θ direction and the φ direction with respect to the laser optical axis 10 are limited within the effective range of matching, the support 3 is nonlinear with the vertical support portion 3c of the laser medium. It may be divided by a plane E-E 'between the optical crystal and the horizontal mounting portion 3a.

The excitation of the laser medium 2 may be side excitation, and the excitation light source is not limited to the semiconductor laser, but may be, for example, a lamp, and the laser medium is not limited to solid but may be gas or liquid. Further, the support may be provided outside the laser resonator. Further, the supporting body 3 is composed of separate parts of a horizontal supporting portion 3a of the nonlinear optical crystal and a vertical supporting portion 3c of the laser medium, and the bending portion 3 is plastically or elastically deformed.
It is also possible to adopt a configuration in which they are connected by b.

In the above embodiments, the description has been made assuming that the input light has a single wavelength. However, when a plurality of light beams having different wavelengths are used as the input light, that is, in the case of sum frequency oscillation, difference frequency oscillation, etc. The present invention can be applied.

In this case, the angular frequency of the signal wave is ω ₁₁ , the angular frequency of the pump wave is ω ₁₂ , the frequencies of the output wave are ω _2, and the refractive indices for the respective lights are n ₁₁ , n ₁₂ , n _2. If the equations (1) and (2) are satisfied, the phase matching condition is satisfied.

Ω ₁₁ + ω ₁₂ = ω ₂ (1) (n ₁₁ × ω ₁₁ ) + (n ₁₂ × ω ₁₂ ) = (n ₂ × ω ₂ ) ... (2)

[0058]

[Effect] Since the present invention is configured as described above,
The adjustment of the nonlinear optical crystal is easier and smaller than the one having a biaxial or more adjusting mechanism or the built-in type, and it is less likely to change over time, and the phase matching can be performed with good accuracy. The effect is that you can do it.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a semiconductor laser resonator for generating a second harmonic, which incorporates a nonlinear optical crystal adjusting mechanism according to the present invention.

FIG. 2 is a perspective view showing only an adjusting mechanism of the nonlinear optical crystal shown in FIG.

FIG. 3 is a diagram showing a refractive index of an uniaxial nonlinear optical crystal with respect to an ordinary ray and an extraordinary ray.

FIG. 4 is a diagram showing an index ellipsoid of a biaxial crystal.

5 is a diagram showing a cross section of a refractive index ellipsoid with respect to a fundamental wave of a biaxial crystal as a solid line and a cross section of a refractive index ellipsoid with respect to higher harmonics as a dotted line. FIG.
FIG. 5B shows a cross section of the (a) and YZ planes, and FIG. 5C shows a cross section of the XY plane.

FIG. 6 is a diagram showing normal velocity planes of wavefronts of a biaxial nonlinear optical crystal that vibrate in directions orthogonal to each other, respectively.

FIG. 7 is a diagram showing phase matching directions of type I and type II and an effective nonlinear optical constant at that time when a Nd: YAG laser of λ = 1.064 μm is incident on a KTP crystal which is a biaxial crystal. is there.

FIG. 8 is a diagram showing the angle dependence of the SHG intensity in the phase matching direction of the second harmonic 532 nm with respect to the fundamental wave 1064 nm of the KTP crystal and the phase matching direction (φ = 24 °) in the plane including the optical axis. .

FIG. 9 shows the angle dependence of the SHG intensity in the phase matching direction of the second harmonic 532 nm with respect to the fundamental wave 1064 nm of the KTP crystal and the phase matching direction (θ = 90 °) in the plane orthogonal to the plane including the optical axis. It is a figure.

FIG. 10 is a sectional view showing an example of a conventional technique.

11 is a front view of the supporting portion of FIG. 10 viewed from the direction of the arrow.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Nonlinear optical crystal 2 ... Laser medium 2a ... Total reflection mirror 3 ... Support body 3a ... Horizontal mounting part 3b ... Bending part 3c ... Vertical support part 4 ... Adjusting screw 5 ... Output mirror 6 ... Semiconductor laser 7 ... Lens 8 ... Excitation light 9 ... Laser beam trajectory 10 ... Laser optical axis

Claims (3)

[Claims] 1. A nonlinear optical crystal having a phase matching direction with respect to an optical axis of input light and having birefringence for performing frequency conversion of the input light, and the phase matching direction and the phase matching direction. A support capable of inclining the nonlinear optical crystal in a plane including a crystal axis forming an angle of less than 90 degrees and restricting inclination of the nonlinear optical crystal in a plane orthogonal to the plane, A laser adjusting device for adjusting the relative position of the nonlinear optical crystal to light. 2. The phase matching direction is relative to a refractive index with respect to one wavefront of the input light and an output light obtained by frequency-converting the input light, and with respect to a vibration direction of the one wavefront of the input light. The average refractive index of each refractive index of the input light of two or more kinds of the input light having a direction in which the refractive index with respect to the wavefront of the output light in the orthogonal vibration direction matches, or the vibration directions are different from each other, and the output light of the frequency-converted output light 2. The laser adjusting device according to claim 1, wherein the directions are the same as the refractive index. 3. A non-linear optical crystal having a phase matching direction with respect to the optical axis of the input light and having birefringence for performing frequency conversion of the input light, and an angle dependence of the non-linear optical effect near the phase matching. For adjustment to a strong direction, it is possible to tilt the non-linear optical crystal in a direction having a strong angle dependence, and a support for restricting a tilt in a direction orthogonal to the direction having a strong angle dependence is provided, A laser adjusting device for adjusting the relative position of the nonlinear optical crystal with respect to the input light.