JPH05323403A - Harmonic generator - Google Patents

Abstract

(57) [Summary] [Object] To provide a harmonic generation device capable of realizing stable wavelength conversion by keeping the optical path length in the resonator constant even if the temperature of the resonator changes. [Structure] An LD 13, a collimator lens 15, a mode matching lens 17, and a monolithic resonator 19 constitute a second harmonic generation device 11. The monolithic resonator 19 includes a nonlinear optical crystal 21 and optical glasses 23a, 2
Constructed by joining a 3b, the optical path length L ₁ and the optical glass 23a of the nonlinear optical crystal 21, the ratio between the optical path length L ₂ in _{23b, L 1 / L 2 ≒} - (α 2 n 2 + Δn 2 ) / (Α ₁
n ₁ + Δn ₁ ) is satisfied. Even if the temperature changes, the total optical path length in the monolithic resonator 19 is maintained constant, so that harmonics can be stably obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generator for converting a fundamental wave emitted from a laser light source into a harmonic in a resonator.

[0002]

2. Description of the Related Art In recent years, various devices have been proposed for obtaining a second harmonic wave or a third harmonic wave whose wavelength is converted by passing a fundamental wave emitted from a semiconductor laser or the like through a nonlinear optical material. In these devices, a nonlinear optical material is arranged in a resonator composed of a plurality of reflecting surfaces, and a fundamental wave is confined in the resonator to be amplified so that harmonics are efficiently generated.

As the resonator, a reflection film is provided on the end face of the nonlinear optical material, and a monolithic resonator for resonating inside thereof and a plurality of mirrors are arranged to form the resonator. An external resonator in which a non-linear optical material is arranged is known. Recently, in order to downsize the device and improve the conversion efficiency to higher harmonics, the mainstream is shifting from the external resonator type to the monolithic type that resonates the fundamental wave inside the nonlinear optical material. I am doing it.

FIG. 3 shows a second harmonic generation device using a monolithic resonator as an example of a conventional harmonic generation device.

This second harmonic generator 1 is composed of a semiconductor laser (hereinafter referred to as LD) 2, a collimating lens 3, a mode matching lens 4, and a nonlinear optical material 5 made of KNbO ₃ crystal or the like. The LD 2 emits the fundamental wave 6 having a wavelength of 860 nm, for example. Nonlinear optical material 5
In the figure, the left and right two surfaces are polished into a spherical shape.

Of these, the surface on the left side of the drawing is an incident surface for the fundamental wave 6, and a spherical mirror 8 for partially transmitting the fundamental wave 6 and reflecting the second harmonic wave 7 is formed on this surface. There is. The surface on the right side of the drawing is an exit surface for the second harmonic wave 7, and a spherical mirror 9 that reflects the fundamental wave 6 and transmits the second harmonic wave 7 is formed on this surface. Furthermore, the lower surface of the nonlinear optical material 5 in the figure forms a plane mirror 10 that reflects both the fundamental wave 6 and the second harmonic wave 7.

In the above structure, the fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 is collimated by the collimating lens 3, passes through the mode matching lens 4, and passes from the point A of the spherical mirror 8 of the nonlinear optical material 5. Incident. At this time, the fundamental wave 6 is incident on the crystal axis a at a specific angle θ so that the fundamental wave 6 incident on the point A travels in parallel with the crystal axis a of the nonlinear optical material 5. The fundamental wave 6 is reflected in a ring shape at points A, B, and C in the ring resonator composed of the two spherical mirrors 8 and 9 and the plane mirror 10 to be amplified. When the fundamental wave 6 passes through the nonlinear optical material 5 in the direction of the crystal axis a, a part of the fundamental wave 6 is converted into a second harmonic wave 7 having a wavelength of 430 nm and emitted from the point B of the spherical mirror 9.

The nonlinear optical material 5 is temperature-controlled by a Peltier element or the like in order to meet the phase matching condition and stabilize the conversion efficiency to higher harmonics. By using such a harmonic generator, the fundamental wave can be efficiently converted into a harmonic.

[0009]

However, in the above-described conventional second harmonic generator, if the temperature of the nonlinear optical material 5 is controlled by a Peltier element or the like in order to meet the phase matching condition, the temperature change will result. Then, there has been a problem that the thermal expansion of the nonlinear optical material 5 and the change of the refractive index occur, and the optical path length in the resonator changes. That is,
Even if the temperature condition is kept optimum, the optical path length in the resonator changes, so that the resonance condition is not satisfied and the second harmonic wave cannot be stably obtained. Therefore, it is an object of the present invention to provide a harmonic generator capable of preventing a change in optical path length due to a change in temperature and efficiently and stably generating a harmonic.

[0010]

In order to achieve the above object, a harmonic generator of the present invention is a harmonic generator using a monolithic resonator, wherein the monolithic resonator is a nonlinear optical material and an optical glass. And an optical path length in the nonlinear optical material and an optical path length in the optical glass so that the total optical path length in the monolithic resonator does not change even if the temperature of the monolithic resonator changes. It is characterized in that the ratio with

In a preferred aspect of the present invention, the optical path length L ₁ in the nonlinear optical material and the optical path length L ₂ in the optical glass are set so as to satisfy the following expression 2.

[0012]

[Formula 2] L ₁ / L ₂ ≈− (α ₂ n ₂ + Δn ₂ ) / (α ₁ n ₁ + Δn ₁ )

(Where α ₁ is the coefficient of thermal expansion of the nonlinear optical material, α ₂ is the coefficient of thermal expansion of the optical glass, n ₁ is the refractive index of the nonlinear optical material, n ₂ is the refractive index of the optical glass, and Δn ₁ is the temperature Amount of change in refractive index of nonlinear optical material due to change, Δn ₂
Is the change amount of the refractive index of the optical glass due to the temperature change. )

[0014]

The harmonic generator of the present invention employs a monolithic resonator constructed by joining a non-linear optical material and an optical glass, and the total optical path length in the resonator is maintained even if the temperature of the resonator changes. The non-linear optical material and the optical glass are bonded at a ratio that does not change.

That is, even if the optical path length in the nonlinear optical material and the optical path length in the optical glass change due to thermal expansion etc. due to the temperature change of the resonator, the optical path length in the nonlinear optical material and the optical glass The non-linear optical material and the optical glass are bonded at a predetermined ratio so that the sum of the optical path lengths of the optical glass and the optical path length is always constant.

Here, when the resonator is composed of only the nonlinear optical material, the total optical path length in the resonator at a certain temperature is L, the thermal expansion coefficient is α, the refractive index for the fundamental wave of 860 nm is n, and the temperature is If the change amount of the refractive index n due to the change is Δn,
The change amount ΔL of the optical path length with respect to the temperature change ΔT is approximately represented by the following expression 3.

[0017]

[Formula 3] ΔL = (1 + α · ΔT) · (n + Δn · ΔT) · L−nL ≈ (α · n + Δn) · L · ΔT

When KNbO ₃ crystal is used as the nonlinear optical material, α = 5 × 10 ^{−6 /} ° C., n = 2.278, Δn
= −4.5 × 10 ⁻⁵ / ° C. Therefore, assuming that the actual optical path length in the resonator is L = 14 mm, the following formula 4 is obtained.

[0019]

[Formula 4] ΔL / ΔT = −470 nm / ° C.

That is, for the fundamental wave light of 860 nm, 1.83
If the temperature changes, the optical path length changes by one wavelength.

On the other hand, when a non-linear optical material and optical glass are joined to form a monolithic resonator,
The coefficient of thermal expansion of the nonlinear optical material and the optical glass are α
₁ , α ₂ and n ₁ and n ₂ , respectively, and Δn ₁ and Δn ₂ , respectively, the change amounts of the refractive index due to temperature change, and the optical path lengths L ₁ and L ₂ , respectively, from the above formula 3, 5
Can be guided.

[0022]

[Formula 5] ΔL / ΔT≈ (α ₁ n ₁ + Δn ₁ ) L ₁ + (α ₂ n ₂ + Δn ₂ ) L ₂

In this equation, the optical path lengths L ₁ and L ₂ that satisfy ΔL / ΔT = 0 are given by the following equation 6.

[0024]

[Equation 6] L ₁ / L ₂ ≈− (α ₂ n ₂ + Δn ₂ ) / (α ₁ n ₁ + Δn ₁ )

Here, KNbO _{3 is} used as the nonlinear optical material.
For example, BK7 (α ₂ =
7.1 × 10 ^-6 / ° C, n ₂ = 1.51 and Δn ₂ = 2.5 × 10 ^-6 /
(° C) is used, the following expression 7 is obtained.

[0026]

[Equation 7] L ₁ / L ₂ ≈1.32 × 10 ^-5 /3.36×10 ^-5 = 1 / 2.54

Therefore, the optical path length and KN in the optical glass are
If the resonator is constructed so that the ratio with the optical path length in the bO ₃ crystal is 2.54: 1, the total optical path length in the resonator can be kept constant even if the temperature of the resonator changes. As a result, the generation of harmonics can be stabilized even if the temperature changes.

[0028]

FIG. 1 shows an embodiment in which the present invention is applied to a second harmonic generation device. The present invention is the second
The present invention is not limited to the harmonic generator, but can be applied to a third harmonic generator or the like.

The second harmonic generator 11 comprises an LD 13 as a laser light source, a collimator lens 15, a mode matching lens 17, and a monolithic resonator 19 which are sequentially arranged.

In this embodiment, the LD 13 has a wavelength of 860 nm.
A single longitudinal and single transverse mode that emits a fundamental wave with little astigmatism is used. As the light source, light emitted from a solid-state laser medium such as YAG or YLF excited by an LD can be used. The collimator lens 15 converts the fundamental wave 25 emitted from the LD 13 into a parallel beam, and the mode matching lens 17 narrows the beam to match the resonance mode in the monolithic resonator 19 with the incident beam.

The monolithic resonator 19 is constructed by laminating a non-linear optical crystal 21 between two optical glasses 23a and 23b, and an antireflection film is formed at the junction of these. In this embodiment, a KNbO ₃ crystal is used as the nonlinear optical crystal 21, and the optical glass 2
BK7 (borosilicate crown glass) as 3a and 23b
Is used.

The end face of one optical glass 23a located on the incident side of the fundamental wave 25 is formed in a spherical shape, and a reflective film for reflecting the fundamental wave 25 by 93% is vapor-deposited on this surface to form a spherical mirror 31. Has been done. The end surface of the optical glass 23b located on the emission side of the second harmonic 27 is also formed in a spherical shape, and the fundamental wave 25 is 99.9% on this surface.
A spherical mirror 33 is formed by depositing a reflective film that reflects and transmits 90% of the second harmonic 27. Further, the lower surface of the nonlinear optical crystal 21 in the figure is a plane mirror 35 that is cut into a plane along the crystal axis a and totally reflects both the fundamental wave 25 and the second harmonic 27.

The nonlinear optical crystal 21 constituting the monolithic resonator 19 and the optical glasses 23a and 23b are the optical path length in the nonlinear optical crystal 21 and the optical glasses 23a and 2b.
The size of each part is determined so that the ratio with the optical path length within 3b is 1: 2.54. That is, the nonlinear optical crystal 21 is composed of a rectangular parallelepiped block having a length of 2.0 mm in the crystal axis a direction, and the optical glasses 23a and 23b have a length of 2.5 mm in the crystal axis a direction and a spherical radius of curvature of 5. 0 mm
It is said to be a block.

The lower surface of the nonlinear optical crystal 21 of the monolithic resonator 19 in the figure is a plane mirror 35 which is cut into a plane along the crystal axis a and totally reflects both the fundamental wave 25 and the second harmonic 27.

Using this second harmonic generator 11, LD
When the fundamental wave 25 with a wavelength of 860 nm is emitted from 13, the fundamental wave 25 is made into a parallel beam by the collimator lens 15 and then condensed by the mode matching lens 17, and the monolithic resonator 19 from the point A of the spherical mirror.
Incident on the inside.

The fundamental wave 25 that has entered the resonator 19 has the optical glass 23a, the nonlinear optical crystal 21 and the optical glass 2.
3b propagates along the crystal axis a, is reflected at the point B of the opposing spherical mirror 33, is directed to the point C of the plane mirror 35, is reflected at the point C of the plane mirror 35, and is reflected by the spherical mirror 31.
Returning to the point A, the light is reflected at the point A, propagates along the crystal axis a again, and overlaps with the original light to cause traveling wave type resonance. In this way, the fundamental wave 25 resonates in the triangular ring-shaped resonance path in the monolithic resonator 19 and is amplified.

When the fundamental wave 25 thus amplified propagates through the nonlinear optical crystal 21 in the direction of the crystal axis a, a part of it is converted into the second harmonic wave 27 having a wavelength of 430 nm.
The harmonic 27 is emitted from the spherical mirror 33. Therefore, when an information reading device for an optical recording medium is constructed by using the harmonic generating device of the present invention as a light source for information detection, a device with high recording density can be obtained.

Further, the nonlinear optical crystal 21 and the optical glass 2
As described above, since the optical path lengths of 3a and 23b are joined to each other at a ratio of 1: 2.54, the total optical path length in the resonator 19 changes due to temperature change. There is no. That is, when the temperature of the resonator 19 changes, the optical path length inside the nonlinear optical crystal 21 and the optical path length inside the optical glass 23 respectively change due to thermal expansion, changes in the refractive index, etc. Since the sum with the optical path length in the nonlinear optical crystal 21 is always the same, the total optical path length in the resonator 19 is always constant.

Therefore, even if the temperature of the resonator 19 is adjusted by a Peltier element (not shown) to perform phase matching, the total optical path length in the resonator 19 is always kept constant, so that stable wavelength conversion is performed. Can be done.

FIG. 2 shows another embodiment in which the present invention is applied to the second harmonic generation device. Note that the substantially same parts as those of the apparatus of the embodiment of FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

This second harmonic generator 41 has basically the same structure as the device of the embodiment shown in FIG.
3, collimator lens 15, mode matching lens 1
7. The monolithic resonator 43 is sequentially arranged. However, in this embodiment, the shapes of the optical glass 45 and the non-linear optical crystal 47 and the bonding mode thereof are different.

That is, both ends of the optical glass 45 are formed in a spherical shape, and a U-shaped notch is formed in the central portion of the upper surface. Then, a rectangular parallelepiped nonlinear optical crystal 47 is fitted and attached to the notch. An antireflection film 49 is formed on the bonding surface between the optical glass 45 and the nonlinear optical crystal 47, as described above. In addition, the spherical surface of the optical glass 45 on the incident side of the fundamental wave 25 has the fundamental wave 25 of 93
% A reflective film is evaporated to form a spherical mirror 51,
The output side spherical surface of the second harmonic 27 transmits the fundamental wave 25 to 99.
A reflective film that reflects 9% and transmits 90% of the second harmonic 27 is deposited to form a spherical mirror 53.

Further, the lower surface of the optical glass 45 in the figure is a plane mirror 55 which is cut into a plane along the crystal axis a of the nonlinear optical crystal 47 and totally reflects both the fundamental wave 25 and the second harmonic 27.

KNbO ₃ crystal is used as the nonlinear optical crystal 47, and BK7 (borosilicate crown glass) is used as the optical glass 45. The nonlinear optical crystal 47 and the optical glass 45 are
The ratio of the optical path length inside the optical glass 45 to the optical path length inside the optical glass 45 is 1:
They are joined at a ratio of 2.54. That is, the length of the nonlinear optical crystal 47 in the crystal axis a direction is 4.2.
The length of the optical glass 45 in the direction of the crystal axis a is 3.0 mm in total on the incident and outgoing sides, and the radius of curvature of the spherical surfaces at both ends is 5.0 mm.

The fundamental wave 25 emitted from the LD 13 passes through the collimator lens 15 and the mode matching lens 17 and enters the monolithic resonator 43 from the point D of the spherical mirror 51. Fundamental wave 2 incident on the resonator 43
5 passes through the nonlinear optical crystal 47 in the direction of the crystal axis a on the way, is reflected at the point E of the spherical mirror 53 which faces it, is further reflected at the point F of the plane mirror 55, and returns to the point D. The fundamental wave 25 thus resonates in a triangular ring shape and is amplified. When passing through the nonlinear optical crystal 47 in the direction of the crystal axis a, a part of the second harmonic 27 having a wavelength of 430 nm is used.
And is emitted from the spherical mirror 53.

Also in this embodiment, the nonlinear optical crystal 4 is used.
Since the optical path length in 7 and the optical path length in the optical glass 45 are bonded together at a ratio of 1: 2.54, even if the temperature of the monolithic resonator 43 changes due to temperature adjustment by a Peltier element or the like, The total optical path length can be maintained constant, and the second harmonic can be stably obtained.

[0047]

As described above, according to the present invention,
A monolithic resonator is constructed by joining a non-linear optical material and optical glass, and the optical path in the non-linear optical material is made so that the total optical path length in the resonator does not change substantially even if the temperature changes. Since the ratio of the length to the optical path length in the optical glass is set, the total optical path length in the resonator can be kept constant even if the temperature changes due to temperature adjustment by the Peltier element etc. It can be stably obtained.

[Brief description of drawings]

FIG. 1 is a side view showing an embodiment of a harmonic generator of the present invention.

FIG. 2 is a side view showing another embodiment of the harmonic generator of the present invention.

FIG. 3 is a side view showing an example of a conventional second harmonic generator.

[Explanation of symbols]

 11 Second Harmonic Generator 13 LD 15 Collimator Lens 17 Mode Matching Lens 19 Monolithic Resonator 21 Nonlinear Optical Crystal 23a Optical Glass 23b Optical Glass 25 Fundamental Wave 27 Second Harmonic 31 Spherical Mirror 35 Plane Mirror 33 Spherical Mirror 43 Monolithic -Type resonator 45 Optical glass 47 Nonlinear optical crystal 51 Spherical mirror 53 Spherical mirror 55 Planar mirror

Claims (2)

[Claims] 1. A harmonic generator using a monolithic resonator, wherein the monolithic resonator is composed of a non-linear optical material and optical glass, and the monolithic resonator is constructed even if the temperature of the monolithic resonator changes. A harmonic generation device, characterized in that the ratio of the optical path length in the nonlinear optical material and the optical path length in the optical glass is set so that the total optical path length in the cavity is not substantially changed. 2. The harmonic generator according to claim ₁ , wherein the optical path length L ₁ in the nonlinear optical material and the optical path length L ₂ in the optical glass satisfy the following expression 1. ## EQU1 ## L ₁ / L ₂ ≈− (α ₂ n ₂ + Δn ₂ ) / (α ₁ n ₁ + Δn ₁ ) (where α ₁ is the coefficient of thermal expansion of the nonlinear optical material, and α ₂ is the thermal expansion of the optical glass. Coefficient, n ₁ is the refractive index of the nonlinear optical material, n ₂ is the refractive index of the optical glass, Δn ₁ is the amount of change in the refractive index of the nonlinear optical material due to temperature changes, and Δn ₂ is the change in the refractive index of the optical glass due to temperature changes It is the amount.)