JP3421742B2 - Optical oscillation method and oscillation device - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical oscillation method and an oscillation device. In particular, the present invention relates to an oscillating method and an oscillating device that can self-stabilize oscillation light whose oscillation output changes abruptly with respect to a change in optical length by a 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, and is provided with a feedback loop control circuit for feedback to the piezoelectric element by using a frequency discriminator. The optical length of the resonator was controlled by feeding back the voltage applied to the piezoelectric element according to the above, and the oscillation was maintained. As a method of creating an error signal, Hansch-Cou
There are an illaud method and a Pound-Drever method for performing FM modulation on input light (pump light). here,
The conventional technique will be described by a method based on the Hansch-Couilleud 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 is a pump light generator. Reference numeral 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 is a lens, which narrows the pump light and makes it enter the optical oscillator. 86 is an optical oscillator. A beam splitter 85 reflects the return light from the optical oscillator 86 toward the λ / 4 wavelength plate 92. 87 and 88 are concave mirrors, which are resonance reflecting mirrors for optical oscillators. 89 is a piezoelectric element,
The thickness changes depending on the voltage. 90 is an optical crystal, for example, a non-linear optical crystal such as MgO: LiNbO ₃ . Reference numerals 91 and 91 'are heaters for keeping the crystal at a constant temperature for phase matching.

Reference numeral 92 denotes a λ / 4 wavelength plate, which makes the elliptically polarized light reflected by the beam splitter 85 incident and makes it linearly polarized. The λ / 4 wavelength plate 92 converts the elliptically polarized light into vertical linearly polarized light or horizontal elliptically polarized light depending on the rotation direction. Reference numeral 93 denotes an optical separator, which is a λ / 4 wave plate 9
The linearly polarized light converted in 2 is separated according to the polarization direction (vertical polarized light or horizontal polarized light). Reference numerals 94 and 95 denote optical-electrical conversion elements which convert an optical signal into an electric signal. Reference numeral 96 denotes a difference signal calculation unit that generates a frequency discrimination error signal by calculating the output difference between the opto-electric conversion element 94 and the opto-electric conversion element 95. 9
Reference numeral 7 is a negative feedback amplifier, which amplifies the output of the difference signal calculation unit 96.

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

Pump light is output from the YAG laser pump light generator 81 by its second harmonic. The pump light passes through the isolator 82, is attenuated and adjusted by the attenuator 83, and is further rotated and adjusted by the λ / 2 wavelength plate 83 ′ to be incident on the lens 84. After that, the light is focused by the lens 84 and enters the optical oscillator 86. In the process of repeating 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 after passing through a concave mirror 88 having a partially reflective 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 reflection 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 rotates depending on whether it is larger or smaller than the resonance optical length. The directions are different. Further, the return reflected light passes through the λ / 4 wave plate 92 to become linearly polarized light in the vertical direction or the horizontal direction depending on the rotation direction. The light separator 93 separates the linearly polarized light in either direction of the photoelectric conversion element 94 and the photoelectric conversion element 95 according to the vertical polarization and the horizontal polarization. Difference signal calculation unit 9
Reference numeral 6 represents the output difference between the opto-electric conversion element 94 and the opto-electric conversion element 95 to generate a frequency discrimination error signal. 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 extends, and when the optical length is longer than the resonance point, the optical length shrinks. A voltage is applied to the piezoelectric element.

Conventionally, as described above, optical oscillation is sustained 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 resonant optical length. .

FIG. 9 is an example of the angular frequency discrimination error signal of the optical oscillator, the vertical axis is the error signal output (arbitrary unit),
The horizontal axis represents the angular frequency (the angular frequency corresponding to the resonator optical length). A is the angular frequency at the resonance point. 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 1 and a positive error signal voltage is generated when the angular frequency is higher than the angular frequency of the resonance point A is shown.

The conventional optical length control is based on the optical length (angular frequency)
The negative feedback control of the voltage applied to the piezoelectric element is performed by setting the target value of point A to the angular frequency of point A and using the fact that the error signal changes positively or negatively depending on whether the optical length is larger or smaller than the resonance optical length. It was done by.

[0012]

The oscillation sustaining control of the conventional optical oscillation device such as an optical parametric oscillator is complicated and requires a piezoelectric element and a feedback loop.

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

[0014]

The optical oscillator of the present invention includes an optical crystal, and the present invention utilizes the characteristic that the oscillation output changes abruptly with respect to the change of the optical length. Oscillation is self-stabilized by the change in optical length caused by the heat generated in the optical crystal.

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

In FIG. 1A, 4 is an optical crystal and 5 and 5'are partially reflective films.

The optical oscillation operation of FIG. 1A is as follows.

The pump light is transmitted through the partial reflection film 5 and repeatedly reflected between the partial reflection films 5 and 5 ′ to resonate, so that optics such as bistable optical parametric oscillation due to three-wave resonance occur in the optical crystal 4. Oscillation light having a characteristic in which the oscillation output changes sharply with respect to a change in length is generated. The oscillated light is a partial reflection film 5 '.
And is output to the outside.

FIGS. 1 (b) and 1 (c) are explanatory views of the principle of the operation in which oscillation is self-stabilized and automatically continued by 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 the temperature rise of the optical crystal 4 is ΔLc, and the refractive index increasing with the temperature rise is Let ΔNc.

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

In FIG. 1 (c), M is the same as in FIG. 1 (a).
Resonance characteristics are shown, and the resonance angular frequency is ω ₀Is. N is optical
Resonance characteristics when the temperature of the crystal 4 rises and the optical length becomes long
And the resonance angular frequency ω₁Is. Temperature of optical crystal 4
Indicates that the resonance characteristic is detuned by increasing
(Hereinafter, detuning is referred to as detuning).

The principle of the present invention will be described below with reference to FIGS. 1 (b) and 1 (c).

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

Due to the increase in the optical length, the resonance angular frequency is lowered and the detuning state is set.

The detuning reduces the oscillation energy, lowers the temperature of the optical crystal 4, shortens the length of the optical crystal 4, and lowers the refractive index, which shortens the optical length.

Since the optical length is shortened, the tuning state (resonance state) is returned to and oscillation is continued.

In the case of oscillated light whose oscillation output characteristic changes abruptly according to the change in optical length, a large temperature detuning causes a large detuning. Can be sustained.

According to the present invention, as described above, the resonance of the resonator can be continuously maintained without the 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 reflecting mirror of the resonator cannot be increased. Therefore, the conventional optical parametric oscillator has a drawback that the resonance characteristic of the resonator is deteriorated and it is difficult to obtain oscillation light having a steep resonance characteristic. However, by using the present invention and using bistable optical parametric oscillation, it is possible to obtain oscillation light having steep oscillation characteristics even if the oscillation characteristics of the oscillator are poor.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention can be applied to an optical oscillator having a characteristic in which the oscillation output changes sharply with respect to a change in optical length. An embodiment will be described of an optical oscillator that produces bistable optical parametric oscillation in a state where 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 resonant optical parametric oscillator in which pump light, signal light, and idler light resonate, a bistable oscillating light is generated depending on the detuning condition of the resonator (the optical length is deviated from the resonance point). Occurs (oscillation light has a hysteresis characteristic with respect to changes in pump light intensity).
The three-wave resonant optical parametric oscillator has a characteristic that the oscillation output changes sharply with respect to the change of the optical length. Here, a case where the three-wave resonant optical parametric oscillator outputs signal light and idler light having substantially the same angular frequency will be described, but the present invention is not limited thereto. Hereinafter, the sinagle 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. 2 (b), P is the resonance characteristic of the pump light, and the central angular frequency ω ₀ and the angular frequency half-width half-width γ
It is ₀ . S is a signal light having a central angular frequency ω ₁ and an angular frequency half-value half width γ ₁ . The degree of detuning standardized by the half-width at half maximum of the angular frequency of the resonator is Δ ₀ = (2ω−ω ₀ ) / γ ₀ for the pump light mode.

When Δ ₁ = (ω-ω ₁ ) / γ _{1 is set} for the signal light mode, and bistable optical parametric oscillation is 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 a condition, where Δ ₀ =
The bistable oscillation output characteristics obtained by changing Δ ₁ are shown at −1.2.

It has been shown that at point C which satisfies Δ ₀ Δ ₁ > 1, oscillation starts with the jump, which is a characteristic of bistability. At this time, the state of the resonator is set so that Δ ₀ becomes a predetermined value, and the optical length of the resonator is swept and continuously changed to sweep Δ 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 of is smaller than that of Δ ₁ . Note _that Δ ₀ can be set by the crystal temperature.

In the case of the bistable optical parametric oscillator, FIG.
It has a characteristic that the oscillation output changes abruptly with respect to the change of the optical length like AB of (a). The present invention uses the oscillation in the AB range where the oscillation characteristic is sharp.

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

In FIG. 3, reference numeral 1 denotes a pump light generator, which outputs the second harmonic of the YAG laser.
An optical oscillator 2 generates an optical parametric oscillation light by three-wave resonance. Reference numeral 4 denotes an optical crystal, which is, 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. Reference numerals 5 and 5'represent partial reflection films. Reference numeral 6 is a piezoelectric element, which 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 detuning state. 10
Is an isolator. 11 is an attenuator. 1
2 is a λ / 2 wave plate. Reference numeral 13 is a lens. The roles of the isolator 10, the attenuator 11, the λ / 2 wavelength plate 12, and the lens 13 are the same as those of the isolator 82, the attenuator 83, the λ / 2 wavelength plate 83 ′, and the lens 84 of FIG. Reference numeral 20 denotes an oscillation detecting means, which detects the start of optical oscillation in the optical oscillator 2. Reference numeral 21 is a beam splitter, which is, for example, a reflecting mirror formed of a partially reflecting film.

In the configuration of FIG. 3, the optical length of the optical oscillator 2 is made longer than the pump light, for example, longer than the resonance optical length,
Keep in detuned state. This can be set to Δ ₀ = −1.2 by adjusting the temperature of the optical crystal 4, for example. The characteristic of bistable oscillating light is that the rate of change of the oscillation output with respect to the optical length at the start of oscillation 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 generator 1 is a second YAG laser.
Output harmonics. After passing through the isolator 10, the amount of light is attenuated and adjusted by the attenuator 11, and the λ / 2 wave plate 12
The rotation angle of the polarized light incident on the optical crystal is adjusted by. 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 in the optical crystal 4 due to the optical parametric process, a bistable optical parametric oscillation of three-wave resonance can be obtained. Time point C when bistable optical parametric oscillation starts
At a time point (a time point corresponding to a point A and a point B in FIG. 2) after a lapse of a little more time, the oscillation detecting means 20 stops the sweep by the sweep applied voltage of the piezoelectric element 6. After that, FIG. 5 (a)
As shown in, the voltage at the start of oscillation (time T) is held.

On the other hand, when the optical oscillator 2 starts bistable optical parametric oscillation, the temperature of the optical crystal rises, the length of the optical crystal 4 becomes longer, and the refractive index also becomes larger.
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 detuning increases and oscillation energy sharply decreases and the temperature of the optical crystal decreases. Therefore, the optical length of the optical oscillator 2 is shortened, the original oscillation conditions are again approached, 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 decreases due to the same operation as described above, the temperature of the optical crystal decreases, and the oscillation continues. This series of operations is repeated, and oscillation is sustained in a self-stabilizing manner due to the negative feedback effect.

FIG. 4 shows a configuration example 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. Reference numeral 20 is an oscillation detecting means.

In the oscillation detecting means 20, reference numeral 31 is an opto-electrical conversion section 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. A sweep stop signal generator 35 generates a sweep stop signal for stopping the sweep signal generated by the voltage sweep signal generator 36 in response to an instruction from the oscillation determiner 33. Three
A voltage sweep signal generator 6 generates a sweep voltage to be applied to the piezoelectric element 6. Reference numeral 36 'denotes an applied voltage holding unit that holds the applied voltage when the oscillation starts.

The operation of FIG. 4 will be described with reference to FIG. FIG. 5A is a sweep signal and shows the sweep voltage applied to the piezoelectric element. FIG. 5B shows the relationship between the oscillation output and Δ ₁ . FIG. 5C shows the relationship between the oscillation output and time. FIG. 5D shows the 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 oscillated by the beam splitter 21 to detect oscillation.
Reflect 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
The optical length of the resonator 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 point, and the sweep stop signal generation unit 35 causes the delay stop signal (not shown) or the like to start the optical oscillation at a timing slightly delayed (for example, at 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. 5).
(Signal like (d)). The voltage sweep signal generator 36 stops the sweep when the sweep stop signal is input. Then, the applied voltage holding unit 36 ′ holds the applied voltage to the piezoelectric element 6. As described above, when the oscillation starts, the crystal temperature rises and the optical length of the optical crystal becomes long, and the absolute value of Δ ₁ becomes large (see B in FIG. 5 (b)).
Moves to the left of the point), 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 negative feedback automatically occurs, and the thermal effect of the optical crystal 4 causes the oscillation to continue in a self-stable manner.

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

Δ ₁ is swept with Δ ₀ = -1.2. The horizontal axis is Δ ₁ .

FIG. 7 is a waveform showing a state in which oscillation is automatically maintained by a thermal effect. Oscillation starts at time T,
After that, it is shown that the oscillation automatically shifts to the self-stable state and continues due to the thermal effect of the optical crystal 4.

In the above description, the start of the bistable optical oscillation is automatically detected by using the oscillation detecting means, but the circuit such as the oscillation detecting means is not provided and the operation is manually performed. , The voltage applied to the piezoelectric element is changed,
Even if the oscillation start point is set by detecting the time when the oscillation starts and manually stopping the change in the applied voltage, the oscillation can be self-stabilized.

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

[0058]

According to the present invention, an optical oscillation device and method capable of sustaining self-stable oscillation by the thermal effect of the optical crystal itself with respect to oscillation light having a characteristic that the oscillation output is steep 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 continuous oscillation control in the conventional coherent light oscillation controller. As a result, according to the present invention, it becomes possible to provide an optical oscillator such as an optical parametric oscillator with a simple configuration at low cost. Moreover, 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 bistable oscillation, and it is possible to sustain oscillation with a simple configuration.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of 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 oscillation detecting means of the present invention.

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

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

FIG. 7 is an observed waveform showing the duration 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 sustaining 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)

(57) [Claims] 1. A method of oscillating an optical oscillator including an optical crystal, wherein light for which an oscillation output characteristic is steep with respect to a change in optical length generated when a resonator optical length of the optical oscillator is detuned from a resonance point, An optical oscillation method, characterized in that oscillation is self-stabilized by a change in optical length based on a thermal effect of the optical crystal caused by the optical oscillation. 2. The oscillated light is parametric oscillated light generated by an optical parametric process by injecting pump light into an optical crystal, the optical length of which is deviated from a resonance point and normalized by a half width at half maximum of resonance of the pump light. Detuning degree
₀ , assuming that the detuning degree specified by the half-width at half maximum of resonance of signal light or idler light is Δ ₁ , the oscillation light has resonance Δ ₀ Δ ₁ > by changing the resonator optical length of the optical oscillator.
The optical oscillation method according to claim 1, wherein the optical oscillation method is one that oscillates under the condition 1. 3. An optical oscillation device comprising an optical resonator including an optical crystal, wherein the oscillation output characteristic is steep with respect to a change in optical length generated when the optical length of the resonator of the optical oscillator is shifted from the resonance point. An optical oscillating device, characterized in that, with respect to light, oscillation is self-stabilized by a change in optical length due to a thermal effect of the optical crystal generated by the optical oscillation. 4. The optical oscillator is for making pump light incident on an optical crystal and performing parametric oscillation by an optical parametric process. The optical oscillator includes means for changing an optical length, and is normalized by a half width at half maximum of resonance of the pump light. When the detuning degree is Δ ₀ and the detuning degree specified by the half-width at half maximum of resonance of signal light or idler light is Δ ₁ , the oscillation light oscillates under the condition of Δ ₀ Δ ₁ > 1. Item 3. The optical oscillation device according to item 3. 5. An optical oscillator that oscillates light whose oscillation output characteristic is steep with respect to a change in optical length, a means for adjusting a resonator optical length of the optical oscillator, and an oscillation detection means for detecting the start of oscillation of the optical oscillator. The oscillation detecting means detects the time when the oscillation is started by changing the optical length, and sets the operating point at a steep point between the oscillation output and the peak of the oscillation output at the start of the oscillation. 2. An optical oscillator device, which continuously oscillates in a self-stabilizing manner by automatically adjusting the resonator optical length of the optical oscillator by the thermal effect of 1.