JPH0716069B2 - Oscillation frequency control method for laser device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oscillation frequency control method for a laser device, which stabilizes by switching the frequency of light emitted from the laser device to one of frequencies of a certain fixed interval. It is a thing.

[Conventional technology]

Heretofore, it has been attempted to stabilize the oscillation frequency of a laser device by using only an optical resonator having a high finesse as a frequency discriminating element or at a frequency corresponding to the resonance frequency or the point of 50% transmittance. That is, the deviation of the oscillation frequency of the laser device from the frequency is detected by the frequency discriminating element, and the control signal generated from this error is applied to the laser drive circuit so that the error becomes zero. It has been confirmed that this method has very good followability and the frequency is stabilized.

[Problems to be solved by the invention]

However, in the above-mentioned conventional method, since the resonance peaks of the longitudinal modes of the optical resonator are exactly the same, it is impossible to select a certain mode and switch to that frequency. Further, for the same reason, there is a problem in that the stabilized frequency may be a frequency for another mode separated by a constant frequency interval determined by the optical resonator.

[Means for solving problems]

An oscillation frequency control method for a laser device according to the present invention is a first optical resonator having a small free spectrum range and a high finesse, which is used as an optical frequency discriminating element by branching emitted light from a laser device whose oscillation frequency is to be controlled. A second optical resonator having a large free spectral range and a low finesse is separately incident on the second optical resonator, and the output voltage signal of the light receiving element that receives the optical output through the first optical resonator is used as the first optical resonance. The output voltage signal of the light receiving element for receiving the optical output through the second optical resonator is compared with the constant reference voltage corresponding to the output at the resonance frequency of the resonator so that the difference signal becomes zero. Compared with a variable reference voltage corresponding to the resonator output for a desired frequency that matches the resonance frequency, the difference signal becomes zero, The oscillation frequency is controlled, and the emitted light from the laser device whose oscillation frequency is to be controlled is branched and made incident on the first optical resonator having a large free spectrum range and a low finesse to be used as an optical frequency discriminating element. A part of the optical output is branched and made incident on the light receiving element, and its output voltage signal is compared with a variable reference voltage corresponding to the output at the resonance frequency of the first optical resonator so that the difference signal becomes zero. In addition, the other branched component of the light output from the first optical resonator is made incident on the second optical resonator having a smaller free spectrum range and high finesse, and the light output is received. The output voltage signal of the light receiving element is compared with the variable reference voltage corresponding to the resonator output for a desired frequency that matches the resonance frequency, and the difference signal becomes zero. And controlling the oscillation frequency of sea urchin the laser device.

An oscillation frequency control device for a laser device according to the present invention is an optical branching device for branching emitted light from a laser whose oscillation frequency is controlled into at least two first and second lights, and the optical branching device. An optical resonator having a large free spectrum range and a low finesse, which is provided to pass the first light among the light branched by the above, and switch the oscillation frequency of the laser to a certain value, and the optical branching device. The second oscillation light of the light branched at the resonance frequency is converted into the electric signal by converting the deviation of the oscillation frequency of the laser from the resonance frequency into an electric signal and returning the electric signal to the laser. An optical resonator having a small free spectral range and a high finesse, which is provided for stabilizing the optical path, and an electric signal obtained from at least the first and second lights. A laser drive circuit controller that outputs a control signal that keeps the oscillation frequency of the laser constant, and a laser drive circuit that controls the magnitude of the injection current to the laser according to a control signal from the laser drive circuit controller. It is characterized by being composed of.

[Action]

In the method of the present invention, not only the high finesse optical resonator but also the low finesse optical resonator are used at the same time as the frequency discriminating element, the high finesse optical resonator is used for frequency stabilization, and the low finesse optical resonator is used for frequency selection. As a result, the longitudinal modes of the high finesse optical resonator, which could not be discriminated by the high finesse optical resonator alone, can be discriminated from each other. As a result, the laser device oscillation frequency is switched and stabilized at one of the frequencies giving the peaks of the longitudinal modes of the high finesse optical resonator.

By combining these two optical resonators, it is possible to easily switch and stabilize the frequency of light emitted from the laser device at the same time.

〔Example〕

 Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the first method of controlling the oscillation frequency of the laser device according to the present invention.
It is a block diagram showing an example of.

In the figure, the emitted light from the 1.55 μm DFB laser 1 passes through the isolator 2 and the optical fiber 3 and is split into three optical paths by the optical branching device 4. The light emitted from the first optical path 19 is converted into an electric signal by the first photodetector 5, and the first, second
It is input to the denominator inputs of the division circuits 6 and 7. Second optical path 20
The emitted light is converted into an electric signal by the second photodetector 9 through the waveguide ring resonator 8 having the finesse 50, and is input to the molecular input of the second division circuit 7 in the subsequent stage. Third optical path
The emitted light of 21 is converted into an electric signal by the third photodetector 11 through the Fabry-Perot resonator 10 of the finesse 5 and input to the numerator input of the first division circuit 6 in the subsequent stage. Here, the free spectrum range of the waveguide ring resonator 8 is 5 GHz.
In addition, the free spectral range of the Fabry-Perot resonator 10 is set to 125 GHz. The transmission characteristics of the waveguide ring resonator 8 and the Fabry-Perot resonator 10 are shown in FIGS. 2 (a) and 2 (b), respectively. The output of the first division circuit 6 is compared with the variable reference voltage by the first differential amplifier 12, and the output of the second division circuit 7 is compared with the constant reference voltage with the second differential amplifier 13. It First and second differential amplifier 12,1
The differential signals of the outputs of 3 are input to the laser drive circuit 14 so that each becomes zero.

1.55 μm wavelength mounted on copper heat sink 18
The DFB laser 1 is supplied with a current from the laser drive circuit 14 and oscillates as a laser. A Peltier element 17 is attached to the copper heat sink 18, the temperature of this element is detected by a sensor (not shown), and a voltage signal corresponding to the temperature is input to the PID controller 15. A control signal such that the difference between the temperature of the Peltier element 17 set in the PID controller 15 and the current temperature of the Peltier element 17 becomes zero is output from the PID controller 15 and input to the Peltier element drive circuit 16. . The Peltier element drive circuit 16 changes the current flowing through the Peltier element 17 according to the input, and changes the temperature of the copper heat sink 18 by the Peltier element 17. By controlling the current flowing to the Peltier element 17 in this way, the temperature of the 1.55 μm DFB laser 1 is stabilized at a certain value.

Next, FIG. 3 is a block diagram showing a second embodiment of the present invention.

The emitted light from the 1.55 μm DFB laser 23 is 1.55 μmD in reflected light.
After passing through an isolator 27 arranged to prevent returning to the VB laser 23, it is incident on an optical fiber 28. The light propagating in the optical fiber 28 is branched into two by an optical branching device 29 provided on the way. One of them is emitted into an optical fiber and then converted into an electric signal by the first photodetector 30. The other light is the Fabry-Perot resonator 3 with finesse 5.
The incident light is successively incident on the waveguide type ring resonator 32 having the finesse 50, and the emitted light is converted into an electric signal by the second photodetector 33. Here, the free spectrum range of the waveguide ring resonator is set to 5 GHz, and the free spectrum range of the Fabry-Perot resonator is set to 125 GHz. A part of the light emitted from the Fabry-Perot resonator 31 is a waveguide ring resonator.
Before being incident on 32, it is branched and incident on the third photodetector 34 to be converted into an electric signal. First, second and third photodetector 3
The electric signals of 0, 33, 34 are inputted to the first and second division circuits 35, 36 as shown in FIG. 3, that is, the division circuit 35 receives the electric signals from the photodetectors 33, 34. However, the division circuit 36 has a photodetector 3
A light source for optical frequency discrimination when the electrical signals of 0 and 33 are input.
The output light amplitude fluctuation of the 1.55 μm DFB laser 23 itself is corrected.
First and second division circuits 35, 3 having frequency characteristics corresponding to the characteristics of the waveguide ring resonator 32 and the Fabry-Perot resonator 31.
The frequency characteristics of the output of 6 are shown in FIGS. 4 (a) and (b).
The output of the second division circuit 36 is compared with the variable reference voltage corresponding to the desired frequency in the second differential amplifier 38, and the difference is fed back to the laser drive circuit 24, and the second differential amplifier 38 is fed.
The output of is controlled to zero. The output of the first division circuit 35 is compared with a variable reference voltage corresponding to a desired frequency in the first differential amplifier 37. By changing the values of the above two variable reference voltages, one of the peak frequencies shown in FIG. 4 (b) is selected, and 1.55μ is selected for this frequency.
The oscillation frequency of the mDFB laser 23 is stabilized. The outputs of the first and second differential amplifiers 37 and 38 are fed back to the laser drive circuit 24 so that the values thereof become zero. The temperature control unit is the same as that of the first embodiment, so its explanation is omitted.

Although the DFB semiconductor laser is used as the laser device in the first and second embodiments described above, other semiconductor lasers such as Fabry-Perot type semiconductor laser may be used. In addition to the semiconductor laser, other types of laser devices such as a gas laser may be used. Further, in this embodiment, the waveguide type ring resonator is used as the high finesse optical resonator and the Fabry-Perot resonator is used as the low finesse optical resonator, but the present invention is not limited to this, and the ring is used as both the low finesse and high finesse optical resonators. Either a resonator or a Fabry-Perot resonator may be used.

Further, in order to detect the difference between the resonance frequency of each optical resonator and the incident light frequency, the optical resonator output light level was compared with a certain reference level in the present embodiment, but the present invention is not limited to this, and fine modulation is performed. Change of the optical resonator output light level with respect to the incident light,
Phase modulation may be performed by applying a modulation signal.

〔The invention's effect〕

As is clear from the above description, since only the high finesse optical resonator was used in the past, only frequency stabilization could be performed, but in the control method of the present invention, frequency stabilization is performed using the high finesse optical resonator, Since frequency selection is performed using a low finesse optical resonator, frequency selection and stabilization can be performed simultaneously. Further, according to the control method of the present invention, both the tuning range and the tuning instability, which were conventionally narrow, are improved. That is, since the tuning range is determined by the ratio of the finesses of the two optical resonators, it is easy to widen the tuning range. Also, since the optical resonator having a high finesse is used as a frequency discriminating element for frequency stabilization, high frequency stability is achieved. You get a degree.

[Brief description of drawings]

1 and 3 are block diagrams showing the first and second embodiments of the frequency control method of the laser device of the present invention, respectively, and FIG. 2 is a diagram showing the frequency characteristics of the output of the division circuit in FIG. (A) is a calibrated waveguide ring resonator characteristic,
(B) is a calibrated characteristic when passing through a Fabry-Perot resonator, FIG. 4 is a diagram showing frequency characteristics of the output of the division circuit in FIG. 3, and (a) is calibrated characteristic of the Fabry-Perot resonator. FIG. 3B is a diagram in which the characteristics when the Fabry-Perot resonator and the waveguide ring resonator are sequentially passed are calibrated. 1,23 …… 1.55μm DFB laser, 2,27 …… isolator, 3,
28 …… Optical fiber, 4 …… 3-branch optical splitter, 5, 9, 11, 3
0,33,34 …… Photodetector, 6,7,35,36 …… Division circuit, 8,32…
… Waveguide ring resonator, 10,31 …… Fabry-Perot resonator, 12, 13, 37, 38 …… Differential amplifier, 14, 24 …… Laser drive circuit, 15, 25 …… PID controller, 16 , 26 …… Peltier element drive circuit, 18,39 …… Peltier element, 29 …… Two-branch optical splitter, 18,22 …… Copper heat sink.

Claims (3)

[Claims] 1. A small free spectral range, a first optical resonator having a high finesse and a large free spectral range, and a low finesse, in which light emitted from a laser device whose oscillation frequency is to be controlled is branched and used as an optical frequency discriminating element. To the output at the resonance frequency of the first optical resonator, the output voltage signal of the light receiving element which receives the optical output through the first optical resonator separately. It is desired that the difference signal is zero compared with the corresponding constant reference voltage, while the output voltage signal of the light receiving element that receives the optical output through the second optical resonator matches the resonance frequency. Is controlled so that the difference signal between the variable reference voltage corresponding to the output of the resonator and the difference signal becomes zero. Oscillation frequency control method of the laser device. 2. Light emitted from a laser device whose oscillation frequency is to be controlled is branched and made incident on a first optical resonator having a large free spectral range and a low finesse used as an optical frequency discriminating element, and its optical output Part of the light is branched and made incident on the light receiving element, and its output voltage signal is compared with a variable reference voltage corresponding to the output at the resonance frequency of the first optical resonator so that the difference signal becomes zero. ,
The output of the light receiving element that receives the optical output of the second optical resonator having a smaller free spectral range and a higher finesse by making the other branched component of the optical output that has passed through the first optical resonator enter. The oscillation frequency of the laser device is controlled so that the voltage signal is compared with a variable reference voltage corresponding to the resonator output for a desired frequency that matches the resonance frequency and the difference signal becomes zero. Oscillation frequency control method for laser device. 3. An optical branching device for branching light emitted from a laser whose oscillation frequency is controlled into at least two first and second lights, and among the lights branched by the optical branching device. An optical resonator having a large free spectrum range and a low finesse, which is provided to pass the first light and switch the oscillation frequency of the laser to a certain value;
Oscillation frequency of the laser by passing the second light of the light branched by the optical branching device, converting the deviation of the oscillation frequency of the laser from its resonance frequency into an electric signal and feeding back to the laser. An optical resonator having a small free spectral range and a high finesse, which is provided for stabilizing the optical resonance frequency of the optical resonator.
And a laser drive circuit controller for outputting a control signal for keeping the oscillation frequency of the laser constant by using an electric signal obtained from the second light, and injection into the laser according to the control signal from the laser drive circuit controller An oscillation frequency control device for a laser device, comprising: a laser drive circuit that controls the magnitude of a current.