JP2002055371A - Light oscillating method and oscillation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an oscillation to be continued self-stably with respect to an oscillated light whose oscillation output is changed steeply to the change of an optical length with simple constitution as to a light oscillating method and an oscillation device. SOLUTION: This device has constitution for making the oscillation to be continued self-stably with the change of an optical length based on the thermal effect of the optical crystal which is generated by the oscillation of the light with respect to the light which is generated in a state in which the optical length of a light oscillator is separated from its resonance point and whose oscillation output characteristic is steep to the change of the optical length.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical oscillation method and an oscillation device. In particular, the present invention relates to an oscillation method and an oscillation device capable of sustaining oscillation of oscillation light whose oscillation output changes steeply in response to a change in optical length in a self-stabilized manner by the thermal effect of an optical crystal.

[0002]

2. Description of the Related Art Conventionally, a coherent light oscillating device has a piezoelectric element mounted on the back surface of a concave mirror of an optical oscillator, a feedback loop control circuit for feeding back to the piezoelectric element using a frequency discriminator, and an error signal of the discriminator. The optical length of the resonator is controlled by feeding back the voltage applied to the piezoelectric element according to the above, so that the oscillation is maintained. As a method of producing an error signal, Hansch-Cou
There are an illaud method and a pound-driver method for performing FM modulation on input light (pump light). here,
The prior art will be described by a method based on the Hansch-Couillaud method.

FIG. 8 shows a system configuration of a conventional optical oscillation device, which performs optical parametric oscillation by three-wave resonance.

In FIG. 8, reference numeral 81 denotes a pump light generator. 82 is an isolator and 83 is an attenuator. The attenuator 83 attenuates and adjusts the pump light incident on the optical oscillator 86, and the isolator 82 prevents the reflected light from the optical oscillator 86 from returning to the pump light generator 81. Reference numeral 83 ′ denotes a λ / 2 wavelength plate for rotating and adjusting the polarization of the coherent light generated by the pump light generation unit 81. Reference numeral 84 denotes a lens, which narrows down the pump light and enters the optical oscillator. 86 is an optical oscillator. Reference numeral 85 denotes a beam splitter, which reflects the return light from the optical oscillator 86 in the direction of the λ / 4 wavelength plate 92. Reference numerals 87 and 88 denote concave mirrors, which are resonance reflectors for an optical oscillator. 89 is a piezoelectric element,
The thickness changes according to the voltage. Reference numeral 90 denotes an optical crystal, for example, a non-linear optical crystal such as MgO: LiNbO ₃ . Reference numerals 91 and 91 'denote heaters for keeping the crystal at a constant temperature for phase matching.

[0005] Reference numeral 92 denotes a λ / 4 wavelength plate for making the elliptically polarized light reflected by the beam splitter 85 incident and converting it into linearly polarized light. The λ / 4 wavelength plate 92 converts the elliptically polarized light into a linearly polarized light in a vertical direction or an elliptically polarized light in a horizontal direction according to the rotation direction of the elliptically polarized light. 93 is an optical separator, which is a λ / 4 wavelength plate 9
2 separates the linearly polarized light converted in accordance with the polarization direction (vertically polarized light or horizontal polarized light). Reference numerals 94 and 95 denote optical-electrical conversion elements for converting optical signals into electric signals. Reference numeral 96 denotes a difference signal calculation unit which generates a frequency discrimination error signal by calculating an output difference between the photoelectric conversion element 94 and the photoelectric conversion element 95. 9
Reference numeral 7 denotes a negative feedback amplifier, which amplifies the output of the difference signal calculator 96.

The operation of the configuration shown in FIG. 8 will be described.

A pump light is output from a pump light generator 81 using a YAG laser by the second harmonic. The pump light passes through the isolator 82 and is attenuated and adjusted by the attenuator 83, and the rotation of the polarization plane is further adjusted by the λ / 2 wavelength plate 83 ′ to be incident on the lens 84. After that, it is stopped down by the lens 84 and enters the optical oscillator 86. In the process of repeating the reflection between the concave mirror 87 and the concave mirror 88 in the optical oscillator 86, parametric oscillation due to three-wave resonance occurs in the optical crystal 90. The parametric oscillation light is output through a concave mirror 88 provided with a partially reflecting film. Also,
A part of the return reflected light of the pump light from the optical oscillator 86 is reflected by the beam splitter 85 and is incident on the λ / 4 wavelength plate 92.

The return reflected light of the pump light from the optical oscillator 86 becomes elliptically polarized light when the optical length between the concave mirrors 87 and 88 is larger or smaller than the resonance condition, and is rotated according to whether the optical length is larger or smaller than the resonance optical length. The directions are different. Further, the return reflected light passes through the λ / 4 wavelength plate 92 and becomes linearly polarized light in the vertical or horizontal direction depending on the rotation direction. The light separator 93 separates the linearly polarized light in either direction of the light-to-electricity conversion element 94 or the light-to-electricity conversion element 95 according to the vertical polarization and the horizontal polarization. Difference signal calculator 9
Reference numeral 6 denotes a frequency discrimination error signal by taking the output difference between the optical-electrical conversion element 94 and the optical-electrical conversion element 95. The negative feedback amplifier 97 amplifies the error signal and applies it to the piezoelectric element 89. When the optical length of the resonator is shorter than the resonance point, a voltage is applied to the piezoelectric element so that the optical length of the resonator is extended, and when the optical length is longer than the resonance point, the voltage is reduced so that the optical length is reduced. A voltage is applied to the piezoelectric element.

Conventionally, as described above, by controlling the optical length between the concave mirror 87 and the concave mirror 88 by the piezoelectric element 89 and the feedback loop so as to be the resonance optical length, light oscillation is maintained. .

FIG. 9 is an example of the angular frequency discrimination error signal of the optical oscillator, and the vertical axis is the error signal output (arbitrary unit).
The horizontal axis is the angular frequency (angular frequency corresponding to the optical length of the resonator). A is the angular frequency of the resonance point. FIG. 9 shows the resonance point A
A case where a negative error signal voltage is generated when the angular frequency is lower than the angular frequency of the resonance point A and a case where a positive error signal voltage is generated when the angular frequency is higher than the angular frequency of the resonance point A

In the conventional optical length control, the optical length (angular frequency)
Is set to the angular frequency of the point A, and the voltage applied to the piezoelectric element is negatively controlled by utilizing the fact that the error signal changes to positive or negative depending on whether the optical length is larger or smaller than the resonance optical length. It was done by.

[0012]

Oscillation sustaining control of a conventional optical oscillating device such as an optical parametric oscillator is a complicated one requiring a piezoelectric element and the like and a feedback loop.

An object of the present invention is to provide an optical oscillation method and apparatus capable of sustaining oscillation with a simple configuration utilizing a thermal effect without a complicated control circuit such as a feedback loop. .

[0014]

The optical oscillator according to the present invention includes an optical crystal. The present invention utilizes the characteristic that the oscillation output changes steeply with respect to the change in optical length. Oscillation is continued in a self-stabilizing manner due to a change in optical length due to heat generated by the generated optical crystal.

FIG. 1 shows the basic configuration and operation principle of the present invention.

In FIG. 1A, reference numeral 4 denotes an optical crystal, and reference numerals 5 and 5 'denote partially reflecting films.

The light oscillation operation shown in FIG. 1A is as follows.

The pump light is transmitted through the partial reflection film 5, and is repeatedly reflected between the partial reflection films 5 and 5 ′ to resonate. In the optical crystal 4, optical characteristics such as bistable optical parametric oscillation due to three-wave resonance are obtained. Oscillation light having the characteristic that the oscillation output changes steeply with the change in length is generated. Oscillation light is partially reflected film 5 '
And is output to the outside.

FIGS. 1B and 1C are explanatory diagrams of the principle of the operation in which the oscillation is self-stably and automatically continued due to the thermal effect of the present invention.

The length of the optical crystal 4 is Lc, the optical refractive index of the optical crystal 4 is Nc, the elongation of the crystal length caused by a rise in the temperature of the optical crystal 4 is ΔLc, and the refractive index that increases with the temperature rise is Lc. ΔNc.

The amount of increase in optical length ΔL that increases as the temperature of the optical crystal 4 increases is ΔL = Nc · ΔLc + ΔNc · Lc> 0.

In FIG. 1 (c), M is the same as in FIG.
Represents the resonance characteristics, and the resonance angular frequency is ω ₀It is. N is optical
Resonance characteristics when the temperature of crystal 4 rises and optical length becomes longer
And the resonance angular frequency ω₁It is. Temperature of optical crystal 4
Shows that the resonance characteristic is detuned by increasing
(Hereinafter, detuning is referred to as detuning).

Referring to FIGS. 1B and 1C, the principle of the present invention will be described as follows.

In the optical crystal 4, when optical resonance starts, the temperature of the optical crystal 4 increases due to oscillation energy, the length and refractive index of the crystal increase, and the optical length increases.

Due to the increase in the optical length, the resonance angular frequency decreases, and a detuning state occurs.

Oscillation energy is reduced by detuning, the temperature of the optical crystal 4 is reduced, the length of the optical crystal 4 is shortened, and the optical length is shortened by reducing the refractive index.

As the optical length is shortened, the tuning state (resonance state) is restored and the oscillation is continued.

In the case of oscillating light whose oscillation output characteristics change steeply in accordance with the change in optical length, a large temperature change causes a large detuning. Can be maintained.

According to the present invention, as described above, the resonance of the resonator can be continuously maintained without a feedback loop for controlling the optical length of the resonator. Therefore, oscillation can be stably maintained with a simple configuration.

In the case of an optical parametric oscillator, in order to extract the output practically, the reflectance of the output mirror of the resonator cannot be increased. For this reason, the conventional optical parametric oscillator has a disadvantage that the resonance characteristics of the resonator deteriorate, and it is difficult to obtain oscillation light having sharp resonance characteristics. However, if the present invention is used and bistable optical parametric oscillation is used, it is possible to obtain oscillation light having steep oscillation characteristics even if the oscillator has poor oscillation characteristics.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is applicable to an optical oscillation device having a characteristic in which an oscillation output sharply changes with a change in optical length. The following is an example of such an oscillation light. An embodiment will be described for an optical oscillator that generates bistable optical parametric oscillation when the optical length of the resonator is detuned from the resonance point.

FIG. 2 is an explanatory diagram of a bistable optical parametric oscillator targeted by the present invention.

In a three-wave resonance type optical parametric oscillator in which pump light, signal light, and idler light resonate, bistable oscillation light depends on the detuning condition of the resonator (the optical length is shifted from the resonance point). (The oscillation light has a hysteresis characteristic with respect to the change in the pump light intensity).
The three-wave resonance type optical parametric oscillator has a characteristic that the oscillation output changes sharply with the change of the optical length. Here, the case where the three-wave resonance type optical parametric oscillator outputs the signal light and the idler light having substantially the same angular frequency will be described, but the present invention is not limited thereto. Hereinafter, the synagogue light will be described (the present invention is also applicable to idler light).

FIG. 2A is a waveform showing the relationship between the detuning condition and the oscillation light output.

FIG. 2B is an explanatory diagram of detuning according to the present invention.

In FIG. 2B, P is a resonance characteristic of the pump light, and has a central angular frequency ω ₀ , an angular frequency half width at half maximum γ
It is ₀ . S is a signal light, which has a center angular frequency ω ₁ and an angular frequency half width at half maximum γ ₁ . The degree of detuning normalized by the half-width at half maximum of the angular frequency of the resonator is Δ ₀ = (2ω−ω ₀ ) / γ ₀ with respect to the pump light mode.

When Δ ₁ = (ω−ω ₁ ) / γ _{1 is set} for the signal light mode, a bistable optical parametric oscillation can be obtained when the condition of Δ ₀ Δ ₁ > 1 is satisfied.

FIG. 2A is a simulation result of the oscillation output waveform obtained under such conditions, and Δ ₀ =
When -1.2, showing a bistability of the oscillation output characteristic obtained by changing the delta _1.

It is shown that oscillation starts at a point C satisfying Δ ₀ Δ ₁ > 1 with a jump which is a characteristic of bistability. In this, delta ₀ is allowed to set the state of the resonator to a predetermined value, by sweeping the optical length of the resonator, by continuously changing, sweeping the delta _1. At this time, since the half width at half maximum γ ₀ is larger than the half width at half maximum γ ₁ , Δ ₀
The rate of change with respect to the optical length is smaller than the change in the delta _{1. Incidentally,} delta ₀ is one that can be set by the crystal temperature.

FIG. 2 shows a bi-stable optical parametric oscillator.
It has a characteristic that the oscillation output changes abruptly in response to a change in the optical length like AB in FIG. The present invention uses the oscillation in the range of AB having such steep oscillation characteristics.

FIG. 3 shows a system configuration of the present invention.

In FIG. 3, reference numeral 1 denotes a pump light generator for outputting a second harmonic of a YAG laser.
Reference numeral 2 denotes an optical oscillator which generates optical parametric oscillation light by three-wave resonance. Reference numeral 4 denotes an optical crystal, for example, a non-linear optical crystal such as KTP, which generates optical parametric oscillation light by an optical parametric process. 4 'is a Peltier element for controlling the crystal temperature. 5, 5 'are partial reflection films. A piezoelectric element 6 changes the optical length of the optical oscillator by applying a voltage. In this embodiment, it is used to determine the oscillation optical length in the detuned state. 10
Is an isolator. 11 is an attenuator. 1
Reference numeral 2 denotes a λ / 2 wavelength plate. 13 is a lens. The roles of the isolator 10, attenuator 11, λ / 2 wavelength plate 12, and lens 13 are the same as those of the isolator 82, attenuator 83, λ / 2 wavelength plate 83 ', and lens 84 in FIG. Reference numeral 20 denotes an oscillation detecting means for detecting the start of optical oscillation in the optical oscillator 2. Reference numeral 21 denotes a beam splitter, which is a reflecting mirror formed of, for example, a partially reflecting film.

In the configuration shown in FIG. 3, the optical length of the optical oscillator 2 is set to be longer than, for example, the resonance optical length with respect to the pump light.
Leave in detuned state. This can be set to Δ ₀ = −1.2 by adjusting the temperature of the optical crystal 4, for example. A characteristic of the bistable oscillation light is that the rate of change of the oscillation output with respect to the optical length at the time when the oscillation starts is steep. That is, there is a feature that the inclination between A and B in FIG.

The operation of the configuration shown in FIG. 3 will be described.

The pump light generation unit 1 is provided with a second YAG laser.
Outputs harmonics. After passing through the isolator 10, the light amount is attenuated and adjusted by the attenuator 11, and the λ / 2 wavelength plate 12
Adjusts the angle of polarization incident on the optical crystal with. The pump light is focused by the lens 13 and is incident on the optical crystal 4 of the optical oscillator. If the detuning condition satisfies the bistability condition by the optical parametric process in the optical crystal 4, a three-wave resonance bistable optical parametric oscillation can be obtained. Time C at which bistable optical parametric oscillation starts
At a point in time when a short time has elapsed (a point corresponding to a point between points A and B in FIG. 2), the oscillation detecting means 20 stops the sweeping of the piezoelectric element 6 by the applied sweep voltage. Thereafter, FIG.
As shown in (2), the voltage at the time of starting the oscillation (time T) is held.

On the other hand, when the bistable optical parametric oscillation starts in the optical oscillator 2, the temperature of the optical crystal rises, the length of the optical crystal 4 increases, and the refractive index increases.
Therefore, the optical length increases and the resonance angular frequency decreases. At this time, in the case of bistable oscillation, the oscillation output characteristic with respect to detuning is steep, so that the detuning increases, the oscillation energy sharply decreases, and the temperature of the optical crystal decreases. For this reason, the optical length of the optical oscillator 2 is shortened, the oscillation condition becomes close to the original oscillation condition, and the oscillation energy is increased again. As a result, the oscillation continues and the temperature of the optical crystal rises again, but the oscillation energy is reduced by the same operation as described above, the temperature of the optical crystal decreases, and the oscillation continues. This series of operations is repeated, and the oscillation is self-stably maintained by the negative feedback action.

FIG. 4 shows an example of the configuration of the oscillation detecting means of the present invention.

In FIG. 4, 2 is an optical oscillator, 3 is a concave mirror, 4 is an optical crystal, and 6 is a piezoelectric element. 20 is an oscillation detecting means.

In the oscillation detecting means 20, reference numeral 31 denotes an optical-to-electrical conversion unit which converts an optical signal from a photodiode or the like into an electric signal. An amplifier 32 amplifies an electric signal. An oscillation determination unit 33 detects the start of oscillation in the optical oscillator 2. Reference numeral 35 denotes a sweep stop signal generator which generates a sweep stop signal for stopping the sweep signal generated by the voltage sweep signal generator 36 in accordance with an instruction from the oscillation determiner 33. 3
Reference numeral 6 denotes a voltage sweep signal generator, which generates a sweep voltage applied to the piezoelectric element 6. Reference numeral 36 'denotes an applied voltage holding unit for holding the applied voltage when the oscillation starts.

The operation of FIG. 4 will be described with reference to FIG. FIG. 5A shows a sweep signal and shows a sweep voltage applied to the piezoelectric element. 5 (b) shows the relationship between the oscillation output and delta _1. FIG. 5C shows the relationship between the oscillation output and time. FIG. 5D shows a sweep stop signal.

In FIG. 4, for example, the temperature of the optical crystal is adjusted to set Δ ₀ = −1.2. A part of the output of the optical oscillator 2 is output by the beam splitter 21 to the oscillation detecting means 2.
Reflects to the 0 side.

In the oscillation detecting means 20, the sweep signal generated by the voltage sweep signal generator 36 is amplified by the amplifier 37 and applied to the piezoelectric element 6. Optical oscillator 2 with piezoelectric element 6
Is controlled, and at time C, the optical oscillator 2 starts bistable optical parametric oscillation. The oscillation determination unit 33 detects the oscillation at that time, and the sweep stop signal generation unit 35 generates a timing slightly delayed from the point A at which the optical oscillation is started by a delay circuit (not shown) (for example, the point A and the peak). At a point B between points P), a sweep stop signal is output so that the sweep signal stops (for example, FIG.
(Signal as in (d)). The voltage sweep signal generator 36 stops the sweep when the sweep stop signal is input. Then, the applied voltage to the piezoelectric element 6 is held by the applied voltage holding unit 36 '. As described above, the oscillation is the optical length of the optical crystal is long by the crystal temperature to be initiated is increased, the absolute value of delta ₁ increases (B in FIG. 5 (b)
Point to the left), and the oscillation output decreases. Therefore, the crystal temperature decreases, the optical length of the optical crystal 4 shrinks, and Δ
The absolute value of ₁ decreases, and the oscillation output recovers. The above-described negative feedback occurs automatically, and the oscillation is self-stably maintained due to the thermal effect of the optical crystal 4.

FIG. 6 is a diagram showing an example of observation of oscillation output during detuning of the bistable optical parametric oscillator used in the method of the present invention.

Δ ₁ is swept as Δ ₀ = −1.2. The horizontal axis is delta _1.

FIG. 7 is a waveform showing how the oscillation is automatically maintained by the thermal effect. Oscillation starts at time T,
Thereafter, the oscillation automatically shifts to the self-stabilized state due to the thermal effect of the optical crystal 4 and continues.

In the above description, the start of the bistable optical oscillation is automatically detected by using the oscillation detecting means. However, without providing a circuit such as the oscillation detecting means, manual operation is performed. , By changing the voltage applied to the piezoelectric element,
Even if the oscillation start point is detected and the change in the applied voltage is manually stopped to set the oscillation start point, the oscillation can be sustained in a self-stable manner.

In the above embodiment, KTP has been described as an example of an optical crystal. However, in the present invention, LiNbO ₃ , KDP, Ba ₂ NaNb ₅ O ₁₅ , CO 2
(NH ₂ ) ₂ , BBO and the like can be used. Also, the case where the pump light source uses a YAG laser device has been described. However, depending on the nonlinear optical crystal used, Nd ³⁺ : C
aWO ₄ , Nd ³⁺ : glass, ruby, Ar ⁺ , T
i: A laser device such as Sapphire can be used.

[0058]

According to the present invention, there is provided an optical oscillation apparatus and method capable of sustaining oscillation in a self-stabilized manner by the thermal effect of the optical crystal itself with respect to oscillation light having a characteristic that the oscillation output has a steep characteristic with respect to a change in optical length. Can be provided. for that reason,
A complicated control circuit such as a piezoelectric element and a feedback loop is not required for oscillation continuous control in a conventional coherent light oscillation control device. As a result, according to the present invention, an optical oscillator such as an optical parametric oscillator can be provided at a low cost with a simple configuration. In addition, even if the resonance characteristic of the oscillator is poor, it is possible to obtain oscillation light having a steep oscillation output characteristic by utilizing the bistable oscillation, and the oscillation can be maintained with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of a bistable optical parametric oscillation.

FIG. 3 is a diagram showing a system configuration of the present invention.

FIG. 4 is a diagram showing an embodiment of the oscillation detecting means of the present invention.

FIG. 5 is an explanatory diagram of the operation of the system configuration of the present invention.

FIG. 6 is a diagram showing an example of observation of oscillation output during detuning of the bistable optical parametric oscillator used in the present invention.

FIG. 7 is an observation waveform showing the continuation of oscillation of the bistable optical parametric oscillator according to the present invention.

FIG. 8 is a diagram showing a system configuration of an optical parametric oscillator having a conventional oscillation continuous feedback loop.

FIG. 9 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 4: Optical crystal 5: Partial reflection film 5 ': Partial reflection film M: Resonance characteristic of optical oscillator N: Resonance characteristic of optical oscillator when temperature of optical crystal rises

Claims (5)

[Claims] 1. A method for oscillating an optical oscillator including an optical crystal, wherein light having a steep oscillating output characteristic with respect to a change in optical length generated when the optical length of a resonator of the optical oscillator is detuned from a resonance point. An optical oscillation method characterized in that oscillation is self-stably maintained by a change in optical length based on a thermal effect of the optical crystal caused by the optical oscillation. 2. The oscillation light is a parametric oscillation light generated by an optical parametric process when the pump light is incident on an optical crystal. The oscillation length is shifted from a resonance point, and is normalized by a half width at half maximum of resonance of the pump light. Δ
_0, when the detuning degree obtained by normalizing with the half width at half maximum of the resonance of the signal light or idler light was delta _1, emitting oscillating light, the resonance is delta ₀ delta ₁ by changing the cavity optical length of the optical oscillator>
2. The light oscillation method according to claim 1, wherein the light oscillation is performed under the condition (1). 3. An optical oscillation device provided with an optical resonator including an optical crystal, wherein an oscillation output characteristic with respect to a change in optical length generated when the resonator optical length of the optical oscillator is shifted from a resonance point is steep. An optical oscillation device characterized in that oscillation of light is self-stably maintained by a change in an optical length due to a thermal effect of the optical crystal caused by the optical oscillation. 4. The optical oscillator according to claim 1, wherein said optical oscillator emits pump light into an optical crystal and oscillates parametrically by an optical parametric process. The optical oscillator has means for changing an optical length, and is standardized by a half width at half maximum of resonance of the pump light. the detuning delta _0, when the detuning degree obtained by normalizing was delta ₁ in the half width at half maximum of the resonance of the signal light or idler light, claims, characterized in that an oscillation light oscillated in the condition is Δ ₀ Δ _{1> 1} Item 4. An optical oscillator according to Item 3. 5. An optical oscillator which oscillates light having a steep oscillation output characteristic with respect to an optical length change, means for adjusting the resonator optical length of the optical oscillator, and oscillation detecting means for detecting the start of oscillation of the optical oscillator. The oscillation detecting means detects a point in time when oscillation starts by changing the optical length, sets an operating point at a steep point between the oscillation output at the start of oscillation and the peak of the oscillation output, and An optical oscillation device characterized in that oscillation is self-stably continued by automatically adjusting the optical length of the resonator of the optical oscillator by the thermal effect of (1).