JPS63179587A - Oscillation frequency controller for laser device - Google Patents

Abstract

PURPOSE:To simultaneously select and stabilize a frequency by using as a frequency stabilizer a high finesse optical resonator and as a frequency selector a low finesse optical resonator. CONSTITUTION:Not only a high finesse optical resonator 8 but a low finesse optical resonator 10 are simultaneously used as a frequency discriminator, the resonator 8 stabilizes the frequency, and the resonator 10 selects the frequency. Accordingly, the longitudinal mode of the resonator 8 which cannot be discriminated solely by the resonator 8 can be discriminated, and a laser device oscillation frequency is switched to be stabilized as one of the frequencies for providing the peak of the longitudinal mode of the resonator 8. The switching and the stabilizing of the frequency of a laser device emitting light can be simultaneously performed easily by the combination of the two resonators 8, 10.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a device that switches and stabilizes the frequency of light emitted from a laser device to one of frequencies at a certain fixed interval.

(Prior Art) A conventional oscillation frequency control device for a laser device includes a laser device, an optical resonator having a high finesse that transmits part or all of the light emitted from the laser device, and a light emitted from the optical resonator. It is characterized by being configured to include a laser drive circuit control device that generates a control signal to be given to the laser drive circuit from the emitted light, and a laser drive circuit, and the resonant frequency or transmittance of the optical resonator with high finesse The operating principle was to stabilize the laser device oscillation frequency to a frequency corresponding to the 50% point. That is, the method is to detect a deviation of the laser device oscillation frequency from the above frequency using the frequency discrimination element, and apply a control signal generated from this error to the laser drive circuit so that the error becomes zero. It has been confirmed that this method stabilizes the frequency with extremely good followability.

(Problem to be solved by the invention) However, in this method, since the resonance peaks of each longitudinal mode of the optical resonator are exactly the same, it is impossible to select one mode and switch to that frequency. It is possible. Furthermore, for the same reason, it is conceivable that the frequency to be stabilized becomes a frequency for another mode separated by a fixed frequency interval determined by the optical resonator.

(Means for Solving the Problems) The laser device oscillation frequency control device of the present invention has a device for branching the emitted light from the laser whose oscillation frequency is to be controlled into at least two lights, first and second lights. an optical splitter,
A low finesse optical resonator provided for passing the first light among the lights branched by the optical splitter and switching the oscillation frequency of the laser to a certain value; The oscillation frequency of the laser is stabilized at the resonant frequency by passing a second light among the second lights and converting the deviation of the oscillation frequency of the laser from the resonant frequency into an electric signal and returning it to the laser. a high finesse optical resonator provided for controlling the laser beam, and a laser drive circuit control device that outputs a control signal to keep the oscillation frequency of the laser constant using at least the electric signals obtained from the first and second lights. , and a laser drive circuit that controls the magnitude of the current injected into the laser according to a control signal from the laser drive circuit control device.

(Function) In this device, not only a high finesse optical resonator but also a low finesse optical resonator are used as frequency discriminating elements.The high finesse optical resonator is used for frequency stabilization, and the low finesse optical resonator is used for frequency selection. By using this method, the longitudinal modes of the high finesse optical resonator, which could not be distinguished using the high finesse optical resonator alone, can be distinguished from each other. As a result, the laser device oscillation frequency is switched to one of the frequencies giving the peak of each longitudinal mode of the high finesse optical resonator and stabilized.

By combining these two optical resonators, switching and stabilization of the frequency of light emitted by the laser device can be easily performed at the same time.

(Example) Hereinafter, a laser device oscillation frequency control device will be described with reference to an illustrated example. FIG. 1 is a block diagram of a first embodiment of the present invention. 1.55 for a semiconductor laser
A μm DFB laser 1 is used. 1.55μm DF
The light emitted from the B laser 1 is transmitted through the isolator 2 optical fiber 3.
The light beam passes through the beam splitter 4 and is divided into three optical paths. The laser drive circuit control device 40 includes first, second, and third photodetectors 5, 9, and 9 as shown surrounded by dotted lines in FIG.
11, a system consisting of first and second divider circuits 6.7, and first and second differential amplifiers 12.13 is used.

The light emitted from the first optical path 19 is converted into an electrical signal by the first photodetector 5, and the first and second dividing circuits 6.
It is input to the denominator input of 7. The emitted light from the second optical path 20 passes through the waveguide ring resonator 8 of the finesse 50 used as a high finesse optical resonator, and is converted into an electrical signal by the second photodetector 9, and is converted into an electrical signal by the second dividing circuit 7 at the subsequent stage. is input to the molecule input of The emitted light from the third optical path 21 passes through the Fapley-Perot resonator 10 of the finesse 5 used as a low finesse optical resonator, is converted into an electric signal by the third photodetector 11, and is converted into an electrical signal by the third photodetector 11. entered into the input. The frequency characteristics of the output of the first division circuit 6 are shown in Fig. 2 (
a), where the Fries vector range of the waveguide ring resonator 8 is 5GH2, and the Fries vector range of the Fapley-Perot resonator 1o is 125G11. is set to . The output of the first divider circuit 6 is compared with a variable reference voltage by a first differential amplifier 12, and the output of the second divider circuit 7 is compared with a constant reference voltage by a second differential amplifier 13. Ru. The frequency characteristics of the output of the second division circuit 7 are shown in FIG. 2(b). First and second differential amplifiers 12
．． 13 output from these first and second differential amplifiers 12
．． 13 is input to the laser drive circuit 14 so that it becomes zero.

Wavelength 1.55 mounted on copper heat sink 1B
The μm DFB laser 1 is supplied with current from the laser drive circuit 14 and oscillates. The semiconductor laser temperature control circuit 41 includes a Bertier element 17 and a PID controller 1.
5. A system consisting of a Peltier element drive circuit 16 is used. The temperature of the Vertier element 17 attached to the copper heat sink 18 is detected by a sensor, and a voltage signal corresponding to the temperature is sent to the PID.
Input to controller 15. A control signal such that the difference between the temperature of the Bertier element 17 set in the PID controller 15 and the current temperature of the Bertier 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 Vertier element 17 according to its input, and the temperature of the copper sink 18 is changed by the Vertier element 17. By controlling the current flowing to the Bertier element 17 in this manner, the temperature of the 1.55 μm DFD laser 1 is stabilized at a certain value.

FIG. 3 is a block diagram of a second embodiment according to the present invention. A 1.55 μm DFB laser 23 is used as the semiconductor laser. The emitted light from the 1.55 μm DFB laser 23 passes through an isolator 27 arranged to prevent reflected light from returning to the 1.55 μm DFB laser 23, and then enters the optical fiber 2B. The light propagating through the optical fiber 28 is split into two by an optical splitter 29 provided midway. As the laser drive circuit control device 42,
The first, second, third
photodetector 3θ, 33.34, first and second dividing circuits 3
5.36, a system constituted by first and second differential amplifiers 37 and 38 is used. One of the lights branched by the optical splitter 29 is output from the optical fiber and then converted into an electrical signal by the first photodetector 30. The other light is transmitted through a Finesse 5 Fabry-Perot resonator 31 used as a low finesse optical resonator and a finesse 50 used as a high finesse optical resonator.
The emitted light is successively incident on the waveguide ring resonator 32, and the emitted light is converted into an electrical signal by the second photodetector 33.

Here, the Fries vector range of the waveguide type ring resonator is set to 5 GH, and the Fries vector range of the Fapley-Perot resonator is set to 125 GH. A part of the light exiting the Fapley-Perot resonator 31 is branched before entering the waveguide ring resonator 32 and sent to the third photodetector 34.
and is converted into an electrical signal. 1st, 2nd, 3rd
The electrical signals from the photodetectors 30, 33, and 34 are input to the first and second divider circuits 35, 36 as shown in FIG.
The fluctuation in the amplitude of the emitted light of the laser 23 itself is corrected. The first and second divider circuits 35 have frequency characteristics corresponding to the characteristics of the waveguide ring resonator 32 and the Fapley-Perot resonator 31.
．． The frequency characteristics of the output of the second divider circuit 36 are shown in FIGS. 4(a) and 4(b).
B, it is compared with a variable reference voltage corresponding to the desired frequency, and the difference is fed back to the laser drive circuit 24, and the second
The output of the differential amplifier 38 is controlled to be zero.

Further, the output of the first dividing circuit 35 is transmitted to the first differential amplifier 37.
is compared to a variable reference voltage corresponding to the desired frequency. By changing the values of the two variable reference voltages, one of the peak frequencies shown in FIG. 4(b) is selected, and the oscillation frequency of the 1.55 μm DFB laser 23 is stabilized at this frequency.

The outputs of the first and second differential amplifiers 37 and 38 are fed back to the laser drive circuit 24 so that their values become zero. Note that the semiconductor laser temperature control circuit 43 is the same as that in the first embodiment, so a description thereof will be omitted. The second embodiment is basically the same as the first embodiment.
If the finesse of the waveguide ring resonator is infinite, the stability obtained from the first and second embodiments is exactly the same. However, since the finesse of a waveguide ring resonator is actually finite, the smaller the peak of each longitudinal mode shown in Fig. 4(b), the larger the full width at half maximum, and the frequency discrimination characteristic is as shown in Fig. 2(b). It is worse than the case shown in a). Therefore, the first embodiment has higher frequency stability after switching than the second embodiment.

The first and second embodiments have been described above. In these embodiments, a DFB semiconductor laser is used as the laser device, but other semiconductor lasers such as a Fabry-Perot type semiconductor laser may also be used. Furthermore, not only semiconductor lasers but also other types of laser devices such as gas lasers may be used. In this example, a waveguide type ring resonator is used as a high finesse optical resonator, and a Fapley-Perot resonator is used as a low finesse optical resonator. However, the invention is not limited to this, and either a ring resonator or a Fapley-Perot resonator may be used as the low finesse or high finesse optical resonator.

(Effects of the invention) As mentioned above, in the past, only high finesse optical resonators were used, so only frequency stabilization could be performed, but in this control method, frequency stabilization is achieved by using high finesse optical resonators, and frequency Since selection is performed using a low finesse optical resonator, frequency selection and stabilization can be performed simultaneously. Furthermore, the present control method improves both the conventionally narrow tuning range and tuning instability. In other words, the tuning range is determined by the ratio of the finesse of the two optical resonators, so it is easy to widen it, and the optical resonator with high finesse is used as a frequency discriminator for frequency stabilization. High frequency stability can be obtained.

[Brief explanation of the drawing]

Figures 1 and 3 are block diagrams of an embodiment of the frequency control method for the emitted light from a laser device, Figure 2 is the frequency characteristic of the output of the divider circuit in Figure 1, and (a) is a waveguide type ring. Figure 4 shows the frequency characteristics of the output of the divider circuit in Figure 3, and (a) shows the calibrated characteristics of the resonator when it is passed through the Fapley-Perot resonator. The characteristics of the resonator are calibrated, and (b) is the calibrated characteristic when passing through a Fapley-Perot resonator and a waveguide ring resonator in sequence. In the figure, 1.23 is a 1.55μm DFB laser, 2:2
7 is an isolator, 3.28 is an optical fiber, 4 is a three-branch optical splitter, 5, 9, 11, 30, 33.34 is a photodetector, 6, 7, 35.36 is a dividing circuit, 8. 32 is a waveguide ring resonator, 10.31 is a Fapley-Perot resonator,
12.13.37.38 is a differential amplifier, 14.24 is a laser drive circuit, 15.25 is a PID controller, 16.26
is a Beltier element drive circuit, 17.39 is a Beltier element,
29 is a two-branch optical splitter, 18.22 Figure 1 1--1.55pm DFB laser 15-PID
Controller 2-11 Isolator 1
6---Peltier element drive circuit 3-optical fiber
17---Peltier element 4-optical splitter (
3 branches) 18-Copper heat sink 5.9.11=-Photodetector 40--Laser drive circuit controller 6.
7=-divider circuit 41-m-semiconductor laser temperature 8--Waveguide type 1-rug resonance story
Control circuit 10-m-77 Brie-Perot resonator 12.13-1 differential amplifier 14=-Laser drive circuit Fig. 2Frequency 22-Copper heat sink 35.36-・
-Divider circuit 23---1.55pm OFB laser
37, 38-Differential amplifier 24--Laser drive circuit 39--Peltier element 25-PID controller 42-Laser drive circuit 26-
Peltier element drive circuit Control device 29 - Optical splitter 30, 33, 34 - Photodetector 31 - Fabry-Perot resonator 32 - Waveguide type ring resonance

Claims (1)

[Claims] 1. An optical splitter for branching the emitted light from the laser whose oscillation frequency is to be controlled into at least two lights, a first and a second light, and the first light branched by the optical splitter. A low finesse optical resonator is provided for passing the light of the laser and switching the oscillation frequency of the laser to a certain value, and a second light of the light branched by the optical splitter is passed, and the resonance a high finesse optical resonator provided for stabilizing the oscillation frequency of the laser at its resonant frequency by converting the deviation of the oscillation frequency of the laser from the frequency into an electrical signal and feeding it back to the laser;
a laser drive circuit control device that outputs a control signal that keeps the oscillation frequency of the laser constant using electrical signals obtained from at least the first and second lights; 1. An oscillation frequency control device for a laser device, comprising a laser drive circuit that controls the magnitude of current injected into the laser.