JPH06338651A - Wavelength conversion light source within external resonator - Google Patents

Abstract

PURPOSE:To stabilize light feedback by a method wherein distortion of a phase modulation signal, which is included in an external resonator transmitted light signal, is used as an error signal and the current of a semiconductor laser is controlled in such a way that this error signal is made to nil. CONSTITUTION:A light feedback stabilizing circuit FS detects an error signal for controlling the single oscillation wavelength of a semiconductor laser LD from a detection signal of resonator transmitted light, which is received by a photodetector PD, of a fundamental wave and control the current of the LD in such a way that the output of this error signal is made to zero. By returning one part of resonance light in a ring resonator ER to the laser LD, the laser LD is turned to a mode wavelength of the resonator ER in the vicinity very close to the mode wavelength and oscillates. In order to make this coincide with the mode wavelength of the resonator ER, adjustment of the phase of the returning light is necessary. This is performed in such a way that an installed mirror MM is moved and the length between the laser LD and the resonator ER is changed. A controller P1 is used for controlling this mirror MM.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention resonates a fundamental wave in an external resonator provided independently of a semiconductor laser (hereinafter, simply referred to as an LD) that outputs a fundamental wave, resulting in a large power. The present invention relates to a wavelength conversion light source in an external resonator in which a wavelength different from that of a fundamental wave is obtained by using a parametric oscillator or generation of a second harmonic.

[0002]

2. Description of the Related Art FIG. 5 is a diagram showing an example of a conventional light source for wavelength conversion (second harmonic generation) in an external resonator. The device shown in FIG. 5 injects the LD light, which is the fundamental wave, into the ring resonator to cause resonance, and highly efficiently obtains the second harmonic by the large power generated inside.

However, in order to resonate, it is necessary to match the single oscillation wavelength λ _{LD of the LD} with the wavelength λ _CAV of the resonator. Therefore, it is possible to adopt a configuration in which the resonant light in the resonator is slightly returned to the LD. For example, it uses optical feedback that can automatically tune the LD at the wavelength λ _CAV of the resonator. In the ring type resonator, a counterclockwise resonance light is generated due to a slight scattered light from the mirror. Part of this becomes the return light to the LD. This optical feedback tuning oscillation is a Fabry-Perot LD
As described above, a laser having a wide line width can be coupled to a resonator having a large Q value with high efficiency, and the tuning oscillation range is widened.

However, in order to cause optical feedback, the LD
Single oscillation wavelength λ_LDThe wavelength of the resonator λ _CAVNumber of 100MH
It is necessary to bring it close to z (about 0.0005 nm).
In order to cause this with good reproducibility, LD and resonator
Temperature control is required, and the light source becomes large.
Furthermore, LD must have a strict temperature stability of several mK °.
And pay attention to changes over time in LDs and temperature sensors
There is a need.

[0005]

Conventionally, the LD temperature / current and the resonator temperature are set so that the LD single-oscillation wavelength λ _LD is _{close to} the lock range of the resonator wavelength λ _CAV (range where optical feedback occurs). Then, the reproducibility and stability were obtained by absolute value control.

In the present invention, the transmission signal of the resonator is used to
By controlling the temperature or current of the _LD and keeping the LD's single-oscillation wavelength λ _LD and the resonator's wavelength λ _CAV always the same, a compact and stable external resonator wavelength conversion that stabilizes the optical feedback. The purpose is to provide a light source.

[0007]

The structure of the present invention for solving the above-mentioned problems is a semiconductor laser which outputs a fundamental wave,
An external resonator to which the output light of this semiconductor laser is input,
A wavelength conversion element arranged in the external resonator, a photodetector for detecting transmitted light of the external resonator, a mirror for varying the phase of return light appearing in the transmitted light, and a phase of the return light In an external cavity wavelength conversion light source consisting of a modulator for optimization control, a lock-in amplifier, and a controller,
By controlling the temperature or current of the semiconductor laser such that the distortion of the phase modulation signal included in the transmitted light signal of the external resonator detected by the photodetector is used as an error signal and the error signal becomes zero. It is characterized in that it is provided with a means for stabilizing the optical feedback.

[0008]

In the present invention, in order to stabilize the external resonator tuning oscillation of the LD by optical feedback, the phase of the returning light is controlled and
Locking out of the lock range is prevented by controlling the _LD single-oscillation wavelength λ _LD so as to follow changes in the resonator mode. Distortion of the phase modulation signal included in the resonator transmission signal is used as the error signal, and strict temperature control of the resonator and LD is not required, and the device is downsized and stabilized.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a wavelength conversion light source in an external resonator of the present invention. In FIG. 1, LD is a semiconductor laser that outputs a fundamental wave and has a wavelength of about 860 nm. This light is a ring type resonator ER composed of three mirrors.
Enters through a stationary mirror MM that can be finely adjusted by the mode match lens ML and PZT. The mode matching lens ML is a lens for performing mode matching for causing the light of the semiconductor laser LD to resonate in the ring resonator ER with high efficiency. C is a wavelength conversion element, which is a nonlinear optical crystal (for example, KNbO ₃ ) for generating the second harmonic wave of 430 nm from the fundamental wave. The mirror constituting the ring resonator ER has a high reflectance for the fundamental wave (about 860 nm) and a high transmission for the second harmonic light (430 nm).

In order to optimize the phase of the returning light,
The stationary mirror MM is modulated by the modulator OSC, the resonator transmitted light of the fundamental wave is received by the photodetector PD, and the error signal is obtained by the lock-in amplifier LIA. Using this error signal, the stationary mirror MM is adjusted via the controller PI so that the intracavity power is maximized.

The optical feedback stabilizing circuit FS detects an error signal for controlling the single oscillation wavelength of the semiconductor laser LD from the detection signal received by the photodetector PD of the resonator transmitted light of the fundamental wave, and the LD drive circuit. Are in control. This optical feedback stabilization circuit FS detects (V _p −V _q ) shown in FIG.
It is used as an error signal. The error signal output (V _p -V _q)
However, the LD current is controlled so as to become zero.

In such a structure, the semiconductor laser L
D is made to resonate with the ring resonator ER, and the ring resonator E
The optical power of several to several tens of times the input power inside R (λ = 860n
m) is confined, and the nonlinear optical crystal C performs highly efficient wavelength conversion. In order to resonate, the condition that λ _LD (single oscillation wavelength of semiconductor laser LD) = λ _ER (mode wavelength of ring resonator ER) is required, and optical feedback is used for that purpose. By returning a part of the resonance light in the ring resonator ER to the semiconductor laser LD, the semiconductor laser LD tunes and oscillates in the vicinity of the mode wavelength λ _ER of the ring resonator ER. In order to match this with the mode wavelength λ _ER of the ring resonator _ER , it is necessary to adjust the phase of the return light, which moves the stationary mirror MM and the distance between the semiconductor laser LD and the ring resonator ER. And change it. A modulator OSC, a lock-in amplifier LIA, and a controller PI are used for controlling the stationary mirror MM.

Here, the installation mirror MM is shown in FIG.
When modulated as described above, the signals 1 to 3 shown in FIG. 3 are generated in the photodetector PD according to the difference between λ _OSC (tuning oscillation wavelength) and the mode wavelength λ _ER of the ring resonator ER. When this is lock-in detected by the lock-in amplifier LIA, an error signal is obtained as shown in FIG. 4, and the error signal is controlled to control the mounting mirror MM to the optimum position.

However, in order to cause the tuned oscillation due to the optical feedback, the single oscillation wavelength λ _LD of the semiconductor laser LD needs to be sufficiently close to the mode wavelength λ _ER of the ring resonator ER, which is within this lock range. For example, tuned oscillation occurs, and by the above control, oscillation occurs at the mode wavelength λ _ER of the ring resonator ER, but the temperature of the semiconductor laser LD and the ring resonator ER changes, and the single oscillation wavelength λ _LD of the semiconductor laser LD When it goes out of the lock range, the tuning oscillation stops and the semiconductor laser L
D oscillates at the single oscillation wavelength λ _LD of the semiconductor laser LD.

Here, by controlling the single oscillation wavelength λ _LD of the semiconductor laser LD so as to follow the mode wavelength λ _ER of the ring resonator ER, the lock range deviation is prevented,
Stabilize optical feedback.

Due to the return light phase control, a signal having a frequency of 2f as shown in FIG. 2B is generated in the photodetector PD, which is generated when λ _LD ≠ λ _ER . It has distortion as shown in (c). In the case of λ _LD ≠ λ _ER , this distortion is due to the change in tuning oscillation wavelength with respect to the change in return optical phase.
It occurs because of saturation.

Here, (V _p −V _q ) in FIG.
Is used as an error signal to control the current of the semiconductor laser LD, thereby maintaining λ _LD = λ _ER and stabilizing the optical feedback. This error signal (V _p −V _q ) matches the sign of (λ _LD −λ _ER ) and changes from negative → 0 → positive. In addition, the error signal (V _p −
Specifically, the detection circuit of V _q ) has a circuit configuration as shown in FIG. 4, and samples and holds the PD signal at the timing of the positive peak and the negative peak of the MM modulation signal shown in FIG. Then, the error signal (V _p −V _q ) is output by the next differential amplifier.

As described above, λ_LDRock range
Stopping optical feedback tuning oscillation due to disconnection is a big problem,
Generally, precise temperature control is performed on the LD and the resonator.
However, it was difficult to stabilize it completely. Also,
The device was upsized due to precise temperature control, but
In the clear, the LD's single-oscillation wavelength λ _LD
The optical feedback can be completely stabilized. Well
Also, there is no need for special circuits other than the optical feedback stabilization circuit,
The device can be miniaturized and simplified.

[0019]

The effects of the present invention have been specifically described above together with the embodiments.
As described above, according to the present invention, by using the transmission signal of the resonator, L
LD's temperature or current is controlled, and LD's single oscillation is always
Wavelength λ _LDAnd the wavelength of the resonator λ_ERKeep it to match
And, a small and stable external cavity wave with stabilized optical feedback.
A long conversion light source can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a wavelength conversion light source in an external resonator of the present invention.

FIG. 2 is a diagram showing signals detected by a photodetector of the apparatus shown in FIG.

3 is a diagram showing a single oscillation wavelength lambda _LD and photodetector signal of the semiconductor laser LD.

FIG. 4 is a diagram showing a specific example of a stabilized signal detection circuit.

FIG. 5 is a configuration diagram showing an example of a conventional wavelength conversion light source in an external resonator.

[Explanation of symbols]

 LD Semiconductor laser ER External cavity C Wavelength conversion element PD Photodetector MM Fixed mirror OSC modulator LIA Lock-in amplifier PI controller FS Optical feedback stabilization circuit

Claims (1)

[Claims] 1. A semiconductor laser that outputs a fundamental wave, an external resonator into which the output light of this semiconductor laser is input, a wavelength conversion element arranged in this external resonator, and transmitted light of the external resonator. An external resonator including a photodetector for detecting the light, a mirror for varying the phase of the return light appearing in the transmitted light, a modulator for optimizing the phase of the return light, a lock-in amplifier, and a controller. In the wavelength conversion light source, the distortion of the phase modulation signal included in the transmitted light signal of the external resonator detected by the photodetector is used as an error signal, and the temperature or current of the semiconductor laser is adjusted so that the error signal becomes zero. A wavelength conversion light source in an external resonator, characterized in that it is configured to have means for stabilizing optical feedback by controlling the.