JP2898720B2 - Optical nonlinearity generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to light obtained by converting the frequency of a laser beam that is mainly used by a laser light source by utilizing the properties of a nonlinear optical element, for example, obtaining a second harmonic. The present invention relates to a nonlinearity generator.

[Conventional technology]

Conventionally, by using a non-linear optical element, the second harmonic (laser light having a wavelength half that of the laser light output from the laser light source) having a frequency twice as high as that of the laser light output from the laser light source is obtained. It is known to be obtained.

Note that the laser light (hereinafter referred to as ω) supplied to the nonlinear optical element
Called waves. ) Is ω, and in order to efficiently generate the second harmonic (hereinafter referred to as 2ω wave) output from the nonlinear optical element, the phase in which the ω wave travels and the phase in which the 2ω wave travels must be changed. Must match.

Then, as the condition, it is necessary to determine the phase matching angle theta _m, in the phase matching angle theta _m phase matching, called Type I, It can be expressed as.

Here, the ordinary refractive index n _o and an extraordinary ray refractive index n _e, so varies with the wavelength of the temperature and ω wave of the nonlinear optical element,
To determine the phase matching angle theta _m, for example when using a semiconductor laser device in the occurrence of ω waves, temperature and current of the semiconductor laser device, it is necessary to know the temperature of the nonlinear optical element.

Then, by calculation using each value can be determined the phase matching angle theta _m, the phase matching angle theta _m
When the crystal axis of the nonlinear optical element is arranged such that
ω waves can be generated.

FIG. 7 is a block diagram showing a configuration of a conventional optical nonlinearity generator.

7, reference numeral 1 denotes a semiconductor laser device as a laser light source, 2 denotes an output control circuit for controlling the output of the ω-wave generated by the semiconductor laser device 1 to a constant value, and 3 denotes a light source temperature for detecting the temperature of the semiconductor laser device 1. A detector 4, a first Peltier element for maintaining the semiconductor laser device 1 at a predetermined temperature;
Reference numeral 5 denotes a light source temperature control circuit.
Controls the supply of current to the first Peltier element 4 for maintaining the semiconductor laser device 1 at a predetermined temperature based on the output of the light source temperature detector 3.

Reference numeral 6 denotes a first coupling lens for converging an ω wave output from the semiconductor laser device 1 to a non-linear optical element 7 described later. Reference numeral 7 denotes a non-linear optical element. The non-linear optical element 7 passes through the first coupling lens 6. The supplied ω wave and a 2ω wave obtained by converting a part of the supplied ω wave into a double frequency are output.

The crystal axis of the nonlinear optical element 7, so as to convert the ω waves efficiently 2ω waves are arranged at a phase matching angle theta _m with respect to the main ray of ω waves.

Reference numeral 8 denotes a second coupling lens that converts a 2ω wave output from the nonlinear optical element 7 into a parallel 2ω wave, and 9 denotes a nonlinear optical element 7.
, A second Peltier element for keeping the temperature of the nonlinear optical element 7 at a predetermined temperature, 11 an element temperature control circuit, and an element temperature control circuit 11 which is an element temperature control circuit. Based on the output of the detector 9, control is performed to supply a current for keeping the nonlinear optical element 7 at a predetermined temperature to the second Peltier element 10.

Reference numeral 12 denotes a half mirror, which passes or partially reflects the ω wave and the 2ω wave from the second coupling lens 8.

13 is a selection filter that passes only the 2ω wave reflected by the half mirror 12, and 14 is 2ω that has passed through the selection filter 13.
2 shows a photodetector that detects the light.

 Next, the operation will be described.

First, the semiconductor laser device 1 is controlled by the output control circuit 2 to output a constant ω wave.

Then, the light source temperature control circuit 5
The semiconductor laser device 1 is maintained at a predetermined temperature by controlling the first Peltier element 4 based on the output of the light source temperature detector 3 in order to keep the frequency of the ω wave generated by the laser beam constant.

Therefore, the semiconductor laser device 1 has a constant output,
Further, it outputs an ω wave having a constant frequency.

In addition, the element temperature control circuit 11 controls the second Peltier element 10 based on the output of the element temperature detector 9 to control the nonlinear optical element 7 in order to keep the 2ω wave output from the nonlinear optical element 7 constant. Temperature.

Therefore, the nonlinear optical element 7 can efficiently output the 2ω wave.

However, when the wavelength or temperature of the semiconductor laser device 1 changes due to aging, or the temperature of the nonlinear optical element 7 changes, the crystal axis of the nonlinear optical element 7 changes the phase matching angle θ.
_m , the nonlinear optical element 7 does not efficiently output the 2ω wave.

Therefore, in such a case, so that the output of the photodetector 14 is maximized, and the slope of the nonlinear optical element 7 is changed by the manually crystal axes again installed such that the phase matching angle theta _m Will be.

[Problems to be solved by the invention]

Conventional optical nonlinearity generator, which is configured as described above, as described above, the crystal axis of the nonlinear optical element 7, such as by aging is deviated from the phase matching angle theta _m, the photodetector 14 so that the output is maximized, the inclination of the crystal axis of the nonlinear optical element 7 is changed by hand, because the crystal axes again installed such that the phase matching angle theta _m, disadvantageously takes a long time to adjust there were.

The present invention has been made to solve the above-described inconvenience, and automatically sets a crystal axis of a nonlinear optical element to a phase matching angle with respect to a principal ray of an ω wave output from a laser light source. The present invention provides an optical nonlinearity generator capable of performing the above-described operations.

[Means for solving the problem]

An optical nonlinearity generating device according to the present invention includes a nonlinear optical element displacement means for changing an angle and a direction of a crystal axis of a nonlinear optical element with respect to a principal ray of laser light output from a laser light source, and only a light whose frequency is converted. And the intensity of light that is output from the nonlinear optical element when the crystal axis of the nonlinear optical element is set to the phase matching angle with respect to the principal ray of the laser beam, and the frequency of the light that has passed through the selective filter is converted. At least two or more photodetectors for detecting distribution, and a shift amount and a shift direction from a phase matching angle of a crystal axis with respect to a principal ray of laser light based on outputs of the two or more photodetectors. And a driving means for operating the nonlinear optical element displacement means so as to correct the shift amount and the shift direction based on the output of the calculation means.

(Operation)

In the optical nonlinearity generating apparatus according to the present invention, the shift amount and the shift direction from the phase matching angle of the crystal axis of the nonlinear optical element with respect to the principal ray of the laser beam are calculated by the arithmetic means based on the outputs of the two or more photodetectors. Then, the non-linear optical element displacement means is operated by the driving means so as to correct the shift amount and the shift direction of the crystal axis of the non-linear optical element with respect to the principal ray of the laser beam calculated by the calculation means.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of an optical nonlinearity generating apparatus according to an embodiment of the present invention, and the same or corresponding parts as in FIG.

In FIG. 1, reference numeral 21 denotes a non-linear optical element displacing portion composed of, for example, a piezoelectric element, an electromagnet, etc., which displaces the crystal axis of the non-linear optical element 7 with respect to the principal ray of the ω wave by a supplied current. It is.

Reference numerals 22A and 22B denote first and second selection filters, and 2ω from the nonlinear optical element 7 via the second coupling lens 8.
It allows only waves to pass.

Reference numerals 23A and 23B denote first and second photodetectors. The first photodetector 23A detects a 2ω wave that has passed through the first selection filter 22A, and the second photodetector 23B includes a second photodetector 23B. Selection filter 22B
It is used to detect the 2ω wave which has passed through the, etc. on a concentric circle centered on the 2ω wave of a principal ray output from the nonlinear optical element 7 when the crystal faces of the nonlinear optical element 7 is a phase matching angle theta _m They are arranged at intervals.

Numeral 24 denotes a subtractor, which subtracts the output of the second photodetector 23B from the output of the first photodetector 23A to obtain a phase matching angle θ.
_This is output as the amount (angle) of deviation of the crystal axis from _m .

Reference numeral 25 denotes a control unit having the above-mentioned element temperature control circuit 11 and a driving means. Based on the output of the element temperature detector 9, a current for keeping the nonlinear optical element 7 at a predetermined temperature is supplied to the second Peltier element 10. Control and supply to the subtractor
So that the output of the subtracter 24 becomes zero based on the output of 24, i.e., non-linear optical element displacement unit 21 so that the phase matching angle theta _m crystal axes with respect to the main ray of ω wave of the nonlinear optical element 7 Is to operate.

FIG. 2 is a partial block diagram for explaining the relationship between the ω wave and the 2ω wave and each photodetector, and FIG. 3 is 2ω with respect to the principal ray of the 2ω wave when the crystal axis of the nonlinear optical element is set to a phase matching angle. FIG. 4 (a) to FIG.
(C) is an explanatory diagram showing the output of each photodetector according to the amount of deviation of the crystal axis of the nonlinear optical element from the phase matching angle, and FIG. 5 is an error and subtraction of the error of the crystal axis of the nonlinear optical element with respect to the phase matching angle. FIG. 6 is a characteristic diagram showing a relationship between outputs of the containers.

In FIG. 2, 7a is the crystal axis of the nonlinear optical element 7, P ₁
(Ω) is the principal ray of the ω wave incident on the crystal axis 7a of the nonlinear optical element 7 at an angle θ, and P ₂ (ω) and P ₃ (ω) are the incident rays (ω
P ₁ (2ω) is the principal ray of a 2ω wave corresponding to the principal ray P ₁ (ω) output from the nonlinear optical element 7, and P ₂ (2ω) is nonlinear A light beam corresponding to the light beam P ₂ (ω) output from the optical element 7 and a light beam P ₃ (2ω) corresponding to the light beam P ₃ (ω) output from the nonlinear optical element 7 are shown.

In FIG. 4, A indicates an output value detected and output by the first photodetector 23A, and B indicates an output value detected and output by the second photodetector 23B.

3 and 4, the asymmetry of the intensity distribution of the ω wave is ignored.

 Next, an operation different from the conventional example will be described.

First, as shown in FIG. 2, with respect to the principal ray P ₁ (ω) in a state in which the crystal axis 7a of the nonlinear optical element 7 is disposed at an angle theta, to the angle theta between the phase matching angle theta _m By
As shown in FIG. 3, it has been confirmed that a 2ω wave is output from the nonlinear optical element 7.

The first and second selection filters 22A and 22B are 2
Since the ω wave having an intensity of 10 times or more of the ω wave is cut off, the intensity distribution of the 2ω wave excluding the influence of the ω wave is detected by the first and second photodetectors 23A and 23B, so that the crystal axis is reduced. it can be calculated the amount of deviation from the phase matching angle theta _m of 7a.

That is, the intensity distribution of the 2ω wave becomes as shown in FIGS. 4 (a) to 4 (c) in accordance with the amount of deviation of the crystal axis 7a from the phase matching angle θ _m , so that the first and second photodetectors 23A, 23B
Are output values A and B corresponding to the shaded portions.

Therefore, if the value output from the subtractor 24 is zero, that is, if the state shown in FIG.
Is at the phase matching angle θ _m , and if the value output from the subtractor 24 is positive, that is, the state shown in FIG. 4B, (if the angle θ in FIG. 2 is (θ _m + Δ)) If the crystal axis
7a is shifted in the clockwise direction, and if the value output from the subtractor 24 is negative, that is, the state shown in FIG. 4C, (when the angle θ in FIG. 2 is (θ _m −Δ), If present, crystal axis
Since 7a is shifted in the counterclockwise direction, the control unit 25
As the output of the subtracter 24 becomes zero in response to the output of the subtractor 24, i.e. the non-linear optical element 7 so that the angle theta by controlling the non-linear optical element displacement unit 21 is a phase matching angle theta _m
Is displaced.

Since the control unit 25 controls the non-linear optical element displacement unit 21, the crystal axes 7a is a phase matching angle theta _m, because it is automatically set, adjustment of the crystal axes 7a and phase matching angle theta _m Is unnecessary, and the 2ω wave can be efficiently output from the nonlinear optical element 7.

FIG. 6 is a partial block diagram showing a modification of the embodiment shown in FIG. 1, and the same parts as those in FIG.

In FIG. 6, reference numeral 26 denotes a light splitting element for splitting the supplied parallel light in a plurality of directions, such as a diffraction grating or a roof prism.

According to this embodiment, since the parallel light can be split in a plurality of directions by the light splitting element 26, the function of the first and second coupling lenses 6, 8 can be achieved by the light splitting element 26.

In the above-described embodiment, an example in which the semiconductor laser device 1 is used as a laser light source has been described.
Needless to say, a solid-state laser, a dye laser or the like can also be used.

If the laser light source outputs an ω wave with a small frequency (wavelength) change, the light source temperature detector 3 is unnecessary,
If the laser light source has a constant output, the output control circuit 2
Is also unnecessary.

Further, although an example has been described in which an ω wave converging to the nonlinear optical element 7 is supplied, an ω wave that diffuses to the nonlinear optical element 7 may be supplied.

Furthermore, the configuration in which the amount of displacement in one direction is obtained by the subtractor 24 based on the outputs of the first and second photodetectors 23A and 23B has been described. However, the number of photodetectors is three or more, and the three or more photodetectors are used. Since the displacement amount and the displacement direction can be obtained by computing the output of the photodetector by the computing means, the displacement of the nonlinear optical element displacement means in three dimensions and the displacement of the crystal axis 7a in three dimensions. Can be.

In this case, three or more photodetectors are connected to the nonlinear optical element 7.
Are preferably arranged at equal intervals on a concentric circle centered on the principal ray P ₁ (2ω) output by the, but since the intensity distribution of the 2ω wave only needs to be known, it is not necessarily centered on the principal ray P ₁ (2ω). It is not necessary to arrange them at equal intervals on concentric circles.

Although the so-called one-pass type in which the ω wave passes through the nonlinear optical element 7 only once has been described in the embodiment of FIG. 1, a Fabry-Perot resonator in which the first and second coupling lenses 6 and 8 are constituted by concave mirrors is described. , Or a so-called ring resonator, it is possible to increase the efficiency of the output 2ω.

Furthermore, in the case it has been described in the phase matching angle theta _m to output the 2ω wave from the nonlinear optical element 7, the other of the converted light from the nonlinear optical element 7 (third harmonic, fourth harmonic, etc.) to output Needless to say, this can also be applied.

〔The invention's effect〕

As described above, according to the present invention, the crystal axis of the nonlinear optical element is calculated by the calculating means based on the outputs of the two or more photodetectors that detect the intensity distribution of the light whose frequency is output by the nonlinear optical element. The deviation amount and the deviation direction from the phase matching angle are determined, and the crystal axis is adjusted to the phase matching angle via the nonlinear optical element displacement means by the driving means, so that the crystal axis is automatically adjusted to the phase matching angle. Can be set.

Therefore, it is not necessary to adjust the phase matching angle of the crystal axis, and it is possible to efficiently output light whose frequency is converted from the nonlinear optical element.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an optical nonlinearity generating apparatus according to one embodiment of the present invention, and FIG. 2 is a partial block diagram for explaining the relationship between laser light and a second harmonic and each photodetector. FIG. 3 is a characteristic diagram showing the intensity distribution of the second harmonic with respect to the principal ray of the second harmonic when the crystal axis of the nonlinear optical element is set to the phase matching angle, and FIGS. 5) is an explanatory diagram showing the output of each photodetector according to the amount of deviation of the crystal axis of the nonlinear optical element from the phase matching angle. FIG. 6 is a partial block diagram showing a modification of the embodiment of FIG. 1, and FIG. 7 is a block diagram showing a configuration of a conventional optical nonlinearity generating device. 1 ... Semiconductor laser device, 2 ... Output control circuit, 3 ...
Light source temperature detector, 4 first Peltier element, 5 light source temperature control circuit, 6 first coupling lens, 7 nonlinear optical element, 7a crystal axis, 8 second Coupling lens,
9: element temperature detector, 10: second Peltier element, 11
…… Element temperature control circuit, 21 …… Non-linear optical element displacement part,
22A, 22B ... first and second selection filters, 23A, 23B ...
... First and second photodetectors, 24... Subtracter, 25... Controller, θ _m ... Phase matching angle.

Claims (1)

(57) [Claims] 1. An optical nonlinearity generator having a nonlinear optical element for converting the frequency of laser light output from a laser light source, wherein the angle and direction of a crystal axis of the nonlinear optical element with respect to a principal ray of the laser light are displaced. A nonlinear optical element displacing means, a selection filter for passing only the light whose frequency has been converted, and a non-linear optical element when the crystal axis of the nonlinear optical element is set to a phase matching angle with respect to the principal ray. At least two or more photodetectors for detecting the intensity distribution of the light that has been output and passed through the selection filter and has been converted in frequency, and the laser based on the outputs of the two or more photodetectors Calculating means for calculating a shift amount and a shift direction of the crystal axis from the phase matching angle with respect to the principal ray of light; A driving means for operating the non-linear optical element displacement means so as to correct the deviation direction, light nonlinearity generator, characterized in that a.