JPH027587A - Variable frequency light source - Google Patents

Abstract

PURPOSE:To assure a variable frequency light source excellent in frequency accuracy and long-time stability with a simplified structure by changing an oscillation frequency of a semiconductor laser by changing a frequency of an oscillator. CONSTITUTION:A switch 14 is shifted from the side (a) to the side (b) when the oscillation frequency of a semiconductor laser makes coincidence with one frequency at a point P among peaks of transmission characteristics of a Fabry-Perrot interferometer 6. Modulation frequency f1 is here assumed f1L. An output from an optical detector 9 is subjected to synchronous detection with a frequency f1 signal in a synchronous rectifier circuit 131. The output is proportional to a frequency difference between FM sub-carriers 22, 23 and the transmission peak through the Fabry-Perrot interferometer 6. Accordingly, with such a difference, a control section 132 controls an injection current into the semiconductor laser 1 such that said difference is zero. Hereby, the emission frequency of the semiconductor laser 1 is changed and the transmission peak of the Fabry-Perrot interferometer 6 is forced to follow the just- mentioned change of the emission frequency by first control means 10. Thus, FSR makes coincidence with the modulation frequency f1. In the above operation, the number (m) of modes of the Fabry-Perrot interferometer 6 is evaluated according to the formula I and the laser oscillation frequency f0 is continuously changed according to the formula II by changing f1.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention [Field of Industrial Application] The present invention relates to an improvement of a variable frequency laser light source that can change the oscillation frequency of a semiconductor laser.

[Conventional technology]

FIG. 4 is a principle diagram showing a first conventional example of a variable frequency light source. The output light of the optical amplifying section 41 enters the diffraction grating 43 via the condensing lens 42, and the first-order diffracted light 45 enters the optical amplifying section 41.
Return to When the zero diffraction grating 44, which is the O-order diffraction light, is rotated, the wavelength of the first-order diffraction light returning to the optical amplification section 41 changes, so that the oscillation wavelength can be controlled.

FIG. 5 is a principle diagram showing a second conventional example of a variable frequency light source. Instead of rotating the diffraction grating as shown in FIG. 4, the oscillation wavelength of the output light from the optical amplifying section 51 is controlled by changing the incident angle to the diffraction grating 53 using the acousto-optic element 52.

[Problem that the invention seeks to solve]

However, the variable wavelength laser light source configured as described above has the following problems. That is, in the method shown in FIG. 4, the diffraction grating is moved mechanically, so high precision is required, the response is poor, and it is susceptible to changes over time.

In addition, in the method shown in Figure 5, the wavelength of the light returning to the optical amplification section is slightly shifted due to Doppler shift, so stable oscillation cannot be obtained and the oscillation spectrum width becomes wide. Since the absolute value of the frequency is not known, calibration is necessary.

The present invention was made to solve the above-mentioned problems,
The purpose of this invention is to realize a variable frequency light source with good frequency accuracy and excellent long-term stability with a simple configuration.

``Structure of the Invention'' [Means for Solving Problems] A variable frequency light source according to the present invention includes a semiconductor laser, an oscillator with a highly stable output frequency, and a part of the output light of the semiconductor laser incident on the variable frequency light source. a phase modulator whose frequency of transmitted light is modulated by the output of the oscillator; a Fabry-Perot interferometer into which the output light of the phase modulator is incident;
a photodetector that receives the transmitted light of the Perot interferometer and converts its intensity into an electrical signal; a first control means that controls the transmission frequency of the Fabry-Perot interferometer to the oscillation frequency of the semiconductor laser; and a second control means for controlling the free spectral region of the Perot interferometer to the output frequency of the oscillator, and the oscillation frequency of the semiconductor laser is changed by changing the frequency of the oscillator.

[Effect]

The first control means causes the transmission frequency of the Fabry-Perot interferometer to follow the oscillation frequency of the semiconductor laser, and when the output frequency of the oscillator is changed, the second control means causes the Fabry-Perot interferometer to follow the oscillation frequency of the semiconductor laser.
The oscillation frequency of the semiconductor laser changes so that the free spectral range of the Perot interferometer follows this.

〔Example〕

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a block diagram showing an embodiment of a variable frequency light source according to the present invention. 1 is a semiconductor laser, 2 and 3 are beam splitters that separate the light emitted from the semiconductor laser 1 into two directions, and 4 is an electro-optic crystal such as LiNb0° (lithium niobate) that modulates the phase of the transmitted light from the beam 1 ritter 3. A phase modulator, 5, modulates the phase modulator 4 with a modulating frequency f.

6 is a Fabry-Perot interferometer into which the output light of the phase modulator 4 is input; 7 is a Fabry-Perot interferometer in which the mirror spacing of the Fabry-Perot interferometer 6 is minutely changed, for example, on the wavelength order. PZT mounted on one side
8 is a vacuum chamber that stores the Fabry-Perot interferometer 6, and 9 is a light receiving element 10 such as a photodiode that detects the intensity of light passing through the Fabry-Perot interferometer 6 and converts it into an electric signal. is a first control means which inputs the output of the light receiving element 9 and controls the transmission frequency of the Fabry-Perot interferometer 6 to the oscillation frequency of the semiconductor laser 1. In the first control means 10, 101 is a synchronous rectifier circuit that inputs the output of the light receiving element 9, and 102 is a control section that inputs the output of the synchronous rectifier circuit 101. 11 is an adder which receives the output of the control unit 102 as one input; 12 is a second oscillator whose output is the other input of the synchronous rectifier circuit 101 and the adder 11; and 13 receives the output of the light receiving element 9. This is second control means for controlling the free spectral range of the Fabry-Perot interferometer 6 to the output frequency of the oscillator 5. The second control means 13 has a synchronous rectifier port 1131 which has the output of the light receiving element 9 as one input and the output of the oscillator 5 as the other input.
and a control unit 132 that inputs the output of the synchronous rectification circuit 131.
It consists of Reference numeral 14 denotes a changeover switch which inputs the output of the control section 132 at one end and whose output becomes the injection current of the semiconductor laser 1. 15 is a third control means that inputs the reflected light from the beam splitter 2 and controls the oscillation frequency of the semiconductor laser 1 to the reference frequency; 151 is an acousto-optic modulator that inputs the reflected light from the beam splitter 2 and modulates the frequency. , 15
2 is an absorption cell in which the output light of the acousto-optic modulator 151 is input and standard substances such as Rb and NHZ are sealed to absorb light of a specific wavelength; 153 is an absorption cell that detects the transmitted light of the absorption cell 152 and converts it into an electrical signal. 154 is a synchronous rectifier circuit that inputs the output of the light receiving element 153, and 155 is a frequency of 3 (
a third oscillator (e.g. 80MHz), 156 is oscillator 1
A switch 157 inputs the output of 55 and outputs it to the acousto-optic modulator 151, and 157 has one output connected to this switch 156.
A fourth oscillator with a frequency of 14, whose output is turned on and off and whose other output serves as a reference signal for the synchronous rectifier circuit 154; 158 inputs the output of the synchronous rectifier circuit 154, and its output is connected to the other input @a of the changeover switch 14; The output light from the six-piece device, which is a control section, is extracted to the outside as reflected light from the beam splitter 3.

The operation of the variable frequency light source configured as described above will be explained next.

(a) First, the switch 14 is set to the a side to control the oscillation frequency fO of the semiconductor laser 1 to the absorption line frequency of the absorption cell 152. That is, a part of the output light of the semiconductor laser is reflected by the beam splitter 2 and enters the acousto-optic modulator 151. When the switch 156 is on, the optical modulator 151 is driven by the output of the oscillator 155 at the frequency of Light enters absorption cell 152. When the switch 156 is off, all incident light enters the absorption cell 152 as zero-order light at a frequency f.

Since the switch 156 is driven by the taro clock of the frequency f4 of the oscillator 157, the light incident on the absorption cell 152 is subjected to frequency modulation of modulation frequency fd+modulation depth f. The frequency-modulated light from the acousto-optic modulator 151 is modulated in the amount of transmitted light at the absorption signal location of the absorption cell 152, and a signal is output. This signal is converted into an electrical signal by a photodetector 153 and synchronously rectified at a frequency of 4 by a synchronous rectifier circuit 154, thereby obtaining a second-order differential signal.
The control unit 158 controls the injection current of the semiconductor laser 1 so that this signal has a predetermined value, and the oscillation frequency fo can be controlled to the center fa of the absorption wavelength. As a result, an instantaneously stable output light whose oscillation frequency is not modulated can be obtained from the semiconductor laser 1. Also, the acousto-optic modulator 1
Both the first-order light and the zero-order light diffracted by the light receiving element 1
53, it is not affected by fluctuations in the amount of light from the semiconductor laser 1.

(b) Next, the transmission frequency of the Fabry-Perot interferometer 6 is locked to the laser frequency f. At this time, the phase modulator is not operated, and the output light from the semiconductor laser 1 enters the Fabry-Perot interferometer 6 as it is, is modulated at a frequency of 2, and is detected by the light receiving element 9.

The output of the light receiving element 9 is synchronously rectified by a synchronous rectifier circuit 101 at a reference frequency f2, and is fed back as an applied voltage to the piezoelectric actuator via a control unit 102, and the peak of the transmission frequency of the Fabry-Perot interferometer 6 is set to the laser oscillation frequency. control to fa.

(c) Next, the FSR (Fre) of the Fabry-Perot interferometer 6
e 5spectral Ranoe (free spectral region) to the phase modulation frequency f. First, oscillator 5
The phase modulator 4 is operated by turning on the output of
As shown in B), subcarriers 22 and 23 are generated around the frequency fa of the laser beam 21.

By sweeping 1 at the modulation frequency and moving the subcarriers 22 and 23 left and right on the frequency axis, point P coincides with one frequency of the peak of the transmission characteristic of the Fabry-Perot interferometer 6 shown in Figure 2 (A). when the switch 14 is turned from side a to side b
Switch to lll. The value of modulation frequency f1 at this time is f
Let it be LL. As a result of this switching, the loop that locks the laser oscillation frequency to the absorption frequency is separated, and a new loop that locks the FSR of the Fabry-Perot interferometer 6 to the modulation frequency f1 is formed. In other words, the output of the light receiving element 9 is synchronously detected using a signal with a frequency of 1 in the synchronous rectifier circuit 131, and the output is output from the FM subcarrier 22°23 and the Fabry subcarrier 22°23.
Since it is proportional to the frequency deviation from the transmission peak of the Perot interferometer 6, if there is a frequency deviation, the control unit 132 controls the injection current of the semiconductor laser 1 so that this becomes zero, and changes the frequency of the semiconductor laser 1. First control means 10
Therefore, the transmission peak of the Fabry-Perot interferometer 6 follows this, and as a result, the FSR coincides with the modulation frequency f.

In the above operation, the number of modes m of the Fabry-Perot interferometer 6 is m = fa / f + L - (1
), so the laser oscillation frequency f is f□=m
-f.

=fa − fl /f1 L = (2), and by changing fl, the output frequency fO can be changed continuously. Here, assuming that the piezoelectric actuator 7 can sweep the mirror up to 2 μm, Δfo
=2.6GHz (wavelength λ=1°557zm, fFSR
= IGH2), so if Δf is set to 2.6 GHz or more, the number of modes m may be changed stepwise from m+l to m+2.

According to the variable frequency light source with such a configuration, the frequency f1 of the first oscillator 5 is highly stable, so as is clear from equation (2), a variable frequency laser with good single frequency accuracy and excellent long-term stability can be obtained. realizable.

Furthermore, as is clear from equation (2), by changing the modulation frequency f1, the output frequency f can be changed in an analog manner.

Since only one semiconductor laser is used, miniaturization is possible.

By selecting the material to be filled in the absorption cell, it is possible to realize a variable frequency light source in various wavelength bands.

In the above embodiment, in order to highly stabilize the modulation frequency f, the output of an oscillator such as an external Cs beam may be used as a reference.

Moreover, instead of feedback to the current injected into the semiconductor laser, feedback may be applied to the temperature.

Further, the control means 15 stabilizes the oscillation frequency of the semiconductor laser.
It is not limited to this method.

In addition, the FSR of the Fabry-Perot interferometer 6 is determined by the modulation frequency f
Instead of locking to 1, as shown in Figure 3, n-FS
R = f, and subcarrier 3 with f, which is an integer multiple of FSR.
It may be locked to 2.33.

C. "Effects of the Invention" As is clear from the explanation in E, according to the claimed invention,
Good frequency accuracy and excellent long-term stability: A variable frequency light source can be realized with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a variable frequency light source according to the present invention, FIG. 2 is a diagram for explaining the operation of FIG. 1, and FIG. 3 is an operation of a modification of the device shown in FIG. 1. FIGS. 4 and 5 are explanatory diagrams showing conventional examples of variable frequency light sources. 1... Semiconductor laser, 4... Phase modulator, 5...
Oscillator, 6... Fabry-Perot interferometer, 9... Photodetector, 4th factor → ; Figure 5

Claims (1)

[Claims] a semiconductor laser, an oscillator with a highly stable output frequency, and a phase modulator into which a portion of the output light of the semiconductor laser enters and the frequency of the transmitted light is modulated by the output of the oscillator;
A Fabry-Perot interferometer into which the output light of this phase modulator enters, a photodetector which receives the transmitted light of this Fabry-Perot interferometer and converts its intensity into an electrical signal, and a transmitted light of the Fabry-Perot interferometer. A first control means for controlling the frequency to the oscillation frequency of the semiconductor laser, and a second control means for controlling the free spectral region of the Fabry-Perot interferometer to the output frequency of the oscillator, and changing the frequency of the oscillator. A variable frequency light source characterized by being configured to change the oscillation frequency of a semiconductor laser by changing the oscillation frequency of a semiconductor laser.