JP3023725B2 - Harmonic generator - Google Patents

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 or a third harmonic 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 reflection surfaces, and a fundamental wave is confined in the resonator and amplified, so that harmonics are efficiently generated.

[0003] As a resonator, a monolithic resonator in which a reflection film is provided on an end face of a nonlinear optical material and resonates inside the resonator and a plurality of mirrors are arranged to form a resonator. And an external resonator in which a nonlinear optical material is disposed. In recent years, the mainstream has shifted from an external resonator type to a monolithic type that resonates a fundamental wave inside a nonlinear optical material in order to reduce the size of the device and improve the conversion efficiency to harmonics. I am doing it.

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

The second harmonic generation device 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 a KNbO ₃ crystal or the like. The LD 2 emits a fundamental wave 6 having a wavelength of 860 nm, for example. The left and right surfaces of the nonlinear optical material 5 in the figure are polished into a spherical shape.

[0006] Of these, the left surface in the figure forms an incident surface of the fundamental wave 6, on which a spherical mirror 8 partially transmitting the fundamental wave 6 and reflecting the second harmonic 7 is formed. I have. In addition, the surface on the right side in the drawing constitutes an emission surface of the second harmonic 7, and a spherical mirror 9 that reflects the fundamental wave 6 and transmits the second harmonic 7 is formed on this surface. Further, the lower surface in the figure of the nonlinear optical material 5 forms a plane mirror 10 that reflects both the fundamental wave 6 and the second harmonic 7.

In the above configuration, the fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 is collimated by the collimator 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 shifted to the crystal axis a 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.
At a specific angle θ. This fundamental wave 6
The light is reflected and amplified in a ring shape at points A, B, and C in a ring resonator constituted by two spherical mirrors 8, 9 and a plane mirror 10. And the fundamental wave 6 is the nonlinear optical material 5
When passing through the inside in the direction of the crystal axis a,
It is converted into the second harmonic 7 of 30 nm, and B
Emitted from the point.

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

[0009]

However, in the above-mentioned 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 conform to the phase matching condition, the temperature may be changed. As a result, there is a problem that the thermal expansion of the nonlinear optical material 5 and a change in the refractive index occur, and the optical path length in the resonator changes. That is,
Even if the temperature condition is kept optimal, the optical path length in the resonator changes, which deviates from the resonance condition, and the second harmonic cannot be stably obtained. Therefore, an object of the present invention is to provide a harmonic generation device capable of preventing a change in an optical path length due to a temperature change and efficiently and stably generating a harmonic.

[0010]

To achieve the above object, the present invention provides a harmonic generator using a monolithic resonator, wherein the monolithic resonator comprises a nonlinear optical material and an optical glass. And the optical path length in the nonlinear optical material and the optical glass so that the total optical path length in the monolithic resonator does not substantially change even if the temperature of the monolithic resonator changes. The ratio with the optical path length is determined.

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

[0012]

L ₁ / L ₂ ≒ − (α ₂ n ₂ + N ₂ ) / (Α ₁ n ₁ + N ₁ )

[0013] (but, alpha ₁ is thermal expansion coefficient of the nonlinear optical material, alpha ₂ is the thermal expansion coefficient of optical glass, n ₁ is the refractive index of the nonlinear optical material, n ₂ is the refractive index of the optical glass, N ₁ Is the amount of change in the refractive index of the nonlinear optical material due to temperature change, N ₂ Is the amount of change in the refractive index of the optical glass due to temperature change. )

[0014]

The harmonic generation device according to the present invention employs a monolithic resonator constituted by joining a nonlinear optical material and an optical glass. Even if the temperature of the resonator changes, the total optical path length in the resonator is reduced. The non-linear optical material and the optical glass are joined 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 a change in the temperature of the resonator due to thermal expansion or the like, the optical path length in the nonlinear optical material and the optical glass length change. The non-linear optical material and the optical glass are joined at a predetermined length ratio such that the total of the optical path lengths is always constant.

Here, when the resonator is constituted only by 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 a fundamental wave of 860 nm is n, and the temperature is n. Assuming that the amount of change of the refractive index n due to the change is N , the amount of change ΔL of the optical path length with respect to the temperature change ΔT is approximately expressed by the following equation (3).

[0017]

ΔL = (1 + α · ΔT) · (n + N · ΔT) · L−nL ≒ (α · n + N ) · L · ΔT

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

[0019]

## EQU4 ## ΔL / ΔT = −470 nm / ° C.

That is, for the fundamental wave light of 860 nm,
When the temperature changes by 1.83 ° C., 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 thermal expansion coefficients of the nonlinear optical material and the optical glass are α
₁ , α ₂ , the refractive index are n ₁ and n ₂ , respectively, and the amount of change in the refractive index due to temperature change is N ₁ , N ₂ Assuming that the optical path lengths are L ₁ and L ₂ , the following Equation 5 can be derived from Equation 3.

[0022]

ΔL / ΔT ≒ (α ₁ n ₁ + N ₁ ) L ₁ + (α ₂ n ₂ + N ₂₎ ) L ₂

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

[0024]

L ₁ / L ₂ ≒ − (α ₂ n ₂ + N ₂ ) / (Α ₁ n ₁ + N ₁ )

Here, KNbO _{3 is} used as a nonlinear optical material.
Crystals are used as optical glass, for example, BK7 ( borosilicate
Crown glass: α ₂ = 7.1 × 10 ⁻⁶ / ° C., n ₂ =
1.51 and N ₂ = 2.5 × 10 ⁻⁶ / ° C.)
The following equation 7 is obtained.

[0026]

L ₁ / L ₂ ≒ 1.32 × 10 ⁻⁵ /3.36×10 ⁻⁵ = 1 / 2.54

Therefore, the optical path length in the optical glass and KN
If the resonator is configured such that its ratio to 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, generation of harmonics can be stabilized even if a temperature change occurs.

[0028]

FIG. 1 shows an embodiment in which the present invention is applied to a second harmonic generator. It should be noted that the present invention
The present invention is not limited to the harmonic generator, and can be applied to a third harmonic generator and the like.

The second harmonic generator 11 is configured by sequentially arranging an LD 13 as a laser light source, a collimator lens 15, a mode matching lens 17, and a monolithic resonator 19.

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

The monolithic resonator 19 is formed by bonding a non-linear optical crystal 21 between two optical glasses 23a and 23b, and an antireflection film 29 is formed at a joint between these. In this embodiment, a KNbO ₃ crystal is used as the nonlinear optical crystal 21, and BK7 (borosilicate crown glass) is used as the optical glasses 23a and 23b.

The end surface of one of the optical glasses 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 deposited on this surface, and the spherical mirror 31 is formed. Have been. The end face of the optical glass 23b located on the emission side of the second harmonic 27 is similarly formed in a spherical shape, and the fundamental wave 25 is applied to this face by 99.9%.
A spherical mirror 33 is formed by depositing a reflective film that transmits 90% of the reflected and second harmonics 27. Further, the lower surface in the figure of the nonlinear optical crystal 21 is cut into a plane along the crystal axis a to form a plane mirror 35 that totally reflects the fundamental wave 25 and the second harmonic 27 together.

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

[0034 ]

Using this second harmonic generator 11, an LD
When the fundamental wave 25 having a wavelength of 860 nm is emitted from the light source 13, the fundamental wave 25 is converted into a parallel beam by the collimator lens 15, and then condensed by the mode matching lens 17. Incident inside.

The fundamental wave 25 incident on the resonator 19 includes the optical glass 23a, the nonlinear optical crystal 21, and the optical glass 2a.
3b, propagates along the crystal axis a, is reflected at the point B of the opposing spherical mirror 33, goes to the point C of the plane mirror 35, is reflected at the point C of the plane mirror 35, and is reflected at the point C of the plane mirror 35.
Returning to point A propagates I again along the crystal axis a is reflected at point A, the resonance of the traveling wave overlaps with the original light is made. In this manner, the fundamental wave 25 resonates and is amplified in the monolithic resonator 19 by taking a triangular ring-shaped resonance path.

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

The nonlinear optical crystal 21 and the optical glass 2
3a and 23b are joined at a ratio such that the ratio of their optical path lengths is 1: 2.54, as described above, so that the total optical path length in the resonator 19 changes due to a temperature change. There is no. That is, when the temperature of the resonator 19 changes, the optical path length in the nonlinear optical crystal 21 and the optical path length in the optical glass 23 change respectively due to thermal expansion, a change in the refractive index, and the like. 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.

For this reason, even if the temperature of the resonator 19 is adjusted by a Peltier element or the like (not shown) to perform phase matching, the entire optical path length in the resonator 19 is always kept constant. Can do it.

FIG. 2 shows another embodiment in which the present invention is applied to a second harmonic generator. Incidentally, denoted by the Nos. Marks the device substantially the same parts of the embodiment of FIG. 1, the description thereof is omitted.

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

That is, the optical glass 45 has both end surfaces formed in a spherical shape, and a U-shaped notch formed in the center of the upper surface. Then, a rectangular parallelepiped nonlinear optical crystal 47 is fitted and attached to the notch. An anti-reflection film 49 is formed on the joint surface between the optical glass 45 and the nonlinear optical crystal 47 in the same manner as described above. Also, the spherical surface of the optical glass 45 on the incident side of the fundamental wave 25
% Reflecting film is deposited to form a spherical mirror 51,
The spherical surface on the emission side of the second harmonic 27 converts the fundamental wave 25 into 99.
A spherical mirror 53 is formed by depositing a reflection film that reflects 9% and transmits 90% of the second harmonic 27.

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

As the nonlinear optical crystal 47, a KNbO ₃ crystal is used, and as the optical glass 45, BK7 (borosilicate crown glass) is used. The nonlinear optical crystal 47 and the optical glass 45 are
And the ratio of the optical path length in the optical glass 45 to the optical path length in 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 direction of the crystal axis a is 4.2.
mm, the length of the optical glass 45 in the direction of the crystal axis a is 3.0 mm including the incident and outgoing sides, and the lengths of EF and FD
7.7 mm in total, and the radius of curvature of the spherical surface 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 into 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 opposing spherical mirror 53, is further reflected at the point F of the plane mirror 55, and returns to the point D. The fundamental wave 25 is thus resonated and amplified in a triangular ring shape. When the light passes through the nonlinear optical crystal 47 in the direction of the crystal axis a, a part of the light is converted into the second harmonic 27 having a wavelength of 430 nm and emitted from the spherical mirror 53.

Also in this embodiment, the nonlinear optical crystal 4
7 and the optical path length in the optical glass 45 are joined so that the ratio becomes 1: 2.54. Therefore, 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 kept constant, and the second harmonic can be obtained stably.

[0047]

As described above, according to the present invention,
A monolithic resonator is formed by joining a nonlinear optical material and optical glass, and the optical path in the nonlinear optical material is controlled so that the total optical path length in the resonator does not substantially change even if the temperature changes. Since the ratio between the optical path length and the optical path length in the optical glass is determined, the entire optical path length in the resonator can be kept constant even if the temperature changes due to temperature adjustment by a Peltier element, etc. It can be obtained stably.

[Brief description of the drawings]

FIG. 1 is a side view showing an embodiment of a harmonic generation device according to 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]

 Reference Signs List 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 plane mirror

Claims (2)

(57) [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 changes even when the temperature of the monolithic resonator changes. A harmonic generator wherein the ratio between the optical path length in the nonlinear optical material and the optical path length in the optical glass is determined so that the total optical path length in the type resonator does not substantially change. Wherein the optical path length L ₂ in the optical glass with an optical path length L ₁ in said nonlinear optical material, harmonic generation device according to claim 1, wherein so as to satisfy the following equation 1. L ₁ / L ₂ ≒ − (α ₂ n ₂ + N ₂ ) / (Α ₁ n ₁ + N ₁ ) (However, alpha ₁ is thermal expansion coefficient of the nonlinear optical material, alpha ₂ is the thermal expansion coefficient of optical glass, n ₁ is the refractive index of the nonlinear optical material, n ₂ is the refractive index of the optical glass, N ₁ Is the amount of change in the refractive index of the nonlinear optical material due to temperature change, N ₂ Is the amount of change in the refractive index of the optical glass due to temperature change. )