JPH0763102B2 - Variable wavelength light source - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement of a variable wavelength light source capable of changing the oscillation wavelength of a laser.

<< Prior Art >> Conventionally, there is the following as a variable wavelength light source using a laser.

I. Rotating Diffraction Grating (Fig. 6) The output light of the optical amplifier LD is transmitted through the condenser lens LS to the diffraction grating DG.
And the first-order diffracted light 61 returns to the optical amplification section LD. 62 is the 0th order diffracted light. When the diffraction grating DG is rotated, the first-order diffracted light returning to the optical amplification section LD changes, so that the oscillation wavelength can be controlled.

B. By acousto-optic element (Fig. 7) Instead of rotating the diffraction grating as shown in (a), acousto-optic element UM
The oscillation wavelength is controlled by changing the angle of incidence on the diffraction grating DG.

<< Problems to be Solved by the Invention >> However, the variable wavelength light source configured as described above has the following problems.

Since b) mechanically moves the diffraction grating, it is difficult to achieve high precision, the response is poor, and it is weak against changes over time.

In B, since the wavelength of the light returning to the optical amplification section LD is slightly shifted by the Doppler shift, stable oscillation cannot be obtained and the oscillation spectrum width becomes wide.

In either case, the absolute value of the wavelength is unknown, so calibration is necessary.

The present invention has been made to solve such problems, and an object thereof is to realize a tunable wavelength light source having excellent long-term stability and good wavelength accuracy.

<< Means for Solving Problems >> The variable wavelength light source according to the present invention controls the transmission wavelength of the Fabry-Perot etalon to the wavelength of the output light of the reference wavelength light source,
The output wavelength of the semiconductor laser is controlled to be one of the transmission wavelengths of the Fabry-Perot etalon so that the output wavelength can be changed stepwise by inputting the current or temperature of the semiconductor laser. And

<< Operation >> The above object can be achieved by controlling the output wavelength of the semiconductor laser to a plurality of transmission wavelengths of the Fabry-Perot etalon and moving the output wavelength by temperature input or current input.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration block diagram showing an embodiment of a variable wavelength light source according to the present invention. In the figure, the optical path is represented by a band-shaped signal line to distinguish it from an electric signal. Reference numeral 1 is a reference wavelength light source having a very stable wavelength, which uses a semiconductor laser whose wavelength is controlled by an atomic absorption line such as Cs or Rb. Reference numeral 2 is a polarization beam splitter for injecting the output light of the reference wavelength light source 1. ,
Reference numeral 3 is a beam splitter which makes the output light of the polarization beam splitter 2 incident, 4 is the Fabry-Perot etalon which makes the output light of the beam splitter 3 incident, and 5 is polarization which makes the output light of the Fabry-Perot etalon 4 enter. A beam splitter, 6 is a light receiving element such as a photodiode that receives the transmitted light of the polarization beam splitter 5, 7 is a control circuit including a lock-in amplifier that inputs the electric output of the light receiving element 6, and 8 is this control. An oscillator for giving a reference signal to the circuit 7, a piezoelectric element 9 such as a PZT for driving the Fabry-Perot etalon 4 by inputting a signal obtained by adding the output of the oscillator 8 as a modulation signal to the output of the control circuit 7, 10 is a semiconductor A laser, 11 is a lens for collimating the output light of the semiconductor laser 10, and 13 is an output light of the lens 11 for a beam splitter 12 and a polarization beam splitter. A light receiving element such as a photodiode, which is incident through the shutter 2, the beam splitter 3, the Fabry-Perot etalon 4 and the polarization beam splitter 5,
Reference numeral 14 is a control circuit which receives the electric output of the light receiving element 13, controls the injection current of the semiconductor laser 10 with the output, and is composed of a lock-in amplifier which uses the output of the oscillator 8 as a reference signal. A counter (counter) for inputting the output of 16 receives a reflected light from the beam splitter 3 and receives a light such as a photodiode for electrically outputting a beat signal of the output light of the reference wavelength light source 1 and the output light of the semiconductor laser 10. An element, 17 is a bandpass filter for inputting the electric output of the light receiving element 16, and 18 is a detector such as a diode bridge for inputting the output of the bandpass filter 17 and applying the output to the reset input terminal of the counter 15. As the reference wavelength light source 1,
For example, an apparatus described in Japanese Patent Application No. 61-11894 by the same applicant can be used.

The output light of the reference wavelength light source 1 is the polarization beam splitter 2
And Fabry-Perot via beam splitter 3
It is incident on the etalon 4. Fabry Perot Etalon 4
Since the resonator length is periodically changed by the modulation signal from the oscillator 8, the transmitted light intensity is modulated. The transmitted light is detected by the light receiving element 6 via the polarization beam splitter 5, synchronously rectified by the control circuit 7, returned to the applied voltage of the piezoelectric element 9, and transmitted by any one of the transmitted wavelengths of the Fabry-Perot etalon 4. The peak is controlled to the oscillation wavelength of the reference wavelength light source 1. FIG. 2 is a characteristic curve diagram showing the transmission signal intensity of the Fabry-Perot etalon 4 with respect to frequency. For example, the peak aO of the transmission wavelength is matched with the oscillation wavelength of the reference wavelength light source 1. The output light of the semiconductor laser 10 becomes parallel light by the lens 11, passes through the beam splitter 12, is reflected by the polarization beam splitter 2, enters the same optical path as the reference wavelength light, and passes through the beam splitter 3 to the Fabry-Perot etalon 4 And is modulated in the same manner as the reference wavelength light, and the transmitted light is reflected by the polarization beam splitter 5 to be separated from the reference wavelength light and enters the light receiving element 13. By arranging the polarization beamsplitters 2 and 5 so that the output light of the reference wavelength light source 1 and the output light of the semiconductor laser 10 are perpendicular to each other and the former is transmitted and the latter is reflected, both lights are combined. , Separation is possible. The output of the light receiving element 13 is synchronously rectified by the control circuit 14 and then fed back to the injection current of the semiconductor laser 10 to change the oscillation wavelength of the semiconductor laser 10 to one of the transmission wavelengths of the Fabry-Perot etalon 4, for example, as shown in FIG. Control to peak a1. A part of the output light of the semiconductor laser 10 is reflected by the beam splitter 12 and becomes the variable wavelength light output of this device. If the temperature of the semiconductor laser 10 is changed in this state, while the control loop for controlling the wavelength of the semiconductor laser 10 is operating, the wavelength change due to temperature is negatively fed back to the current to perform the correction operation, so that the wavelength is constant. However, if the control loop saturates and the loop breaks, Fabry-Perot
The wavelength jumps to the next transmission wavelength peak of etalon 4. By repeating this operation, the wavelength can be changed stepwise by the laser temperature as shown in FIG. Here, the flat part of the wavelength is the transmission wavelength peak of Fabry-Perot Etalon 4, which is the peak a in Fig. 2.
Corresponding to 1, a2, a3, the peak interval is determined by the FSR (Free Spectrum Range) of Fabry-Perot Etalon 4, and since it is based on the absorption line of a standard substance such as Rb, the accuracy is very excellent. Has been. The counter 15 is the light receiving element 13
The output of is input and the number of peaks of the transmission characteristics of the Fabry-Perot etalon 4 is counted. The light receiving element 16 detects the coincidence of the oscillation wavelengths of the semiconductor laser 10 and the reference wavelength light source 1,
The counter 15 is reset via the bandpass filter 17 and the detection circuit 18. That is, when the power is turned on, the oscillation wavelength of the semiconductor laser 10 is controlled to the oscillation wavelength of the reference wavelength light source 1 (peak aO in FIG. 2) and the counter 15 is reset.
The oscillation wavelength of the semiconductor laser 10 is changed by changing the laser temperature, and the transmission wavelength of the Fabry-Perot etalon 4 (peak a in FIG.
1, a2, a3, ...) can be controlled in steps
15 shows the number of steps, that is, the order from the reference transmission wavelength (peak aO in FIG. 2), and the absolute wavelength of the output light can be accurately read from this value.

According to the variable wavelength light source having such a configuration, since the reference wavelength light source is a quantum standard, it is possible to realize a variable wavelength light source that is excellent in long-term stability and that can be varied stepwise with wavelength accuracy.

FIG. 4 is a block diagram showing a modification of the apparatus shown in FIG. 1 and showing a case where the reference wavelength light source is a modulation output. The same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. Circuits related to the counter are similar to those in FIG. 1, but are omitted in the figure. Unlike the case of FIG. 1 in which the reference wavelength light source 1 has an unmodulated output, since the reference wavelength light source 1 is modulated by the output of the oscillator 8, the Fabry-Perot etalon 4 is not modulated and the central transmission wavelength is set to the reference wavelength. The oscillation wavelength of the light source 1 can be controlled. To modulate the reference wavelength light source 1, a modulation signal may be superimposed on the injection current of the internal output semiconductor laser or the output light may be passed through a modulator. When the oscillation wavelength of the semiconductor laser 10 is controlled to the transmission wavelength of the Fabry-Perot etalon 4, since the Fabry-Perot etalon 4 is not modulated, the amplifier 19 is fed back instead of the synchronous detection. As a result, as shown by b1, b2, b3, ... In FIG. 5, the shoulder portion of the transmission signal is controlled. The position of the shoulder is defined by the comparison set point of the amplifier 19.

In each of the above embodiments, the laser injection current is fed back to form the control loop, and the oscillation temperature is changed by inputting the laser temperature. On the contrary, the laser temperature is fed back to form the control loop, and the laser loop is formed. The oscillation wavelength may be changed by using the injected current as an input.

Further, instead of using the piezoelectric element for changing the resonator length of the Fabry-Perot etalon 4, an electro-optical element may be arranged between the mirrors of the Fabry-Perot etalon 4 to feed back the voltage.

<Effects of the Invention> As described above, according to the present invention, excellent long-term stability,
A variable wavelength light source with good wavelength accuracy can be realized with a simple configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a variable wavelength light source according to the present invention, FIGS. 2 and 3 are explanatory views for explaining the operation of the apparatus, and FIG. 4 is FIG. FIG. 5 is a configuration block diagram showing a modified example of the device, FIG. 5 is an explanatory diagram for explaining the operation of the device, and FIGS. 6 and 7 are configuration block diagrams showing a conventional example of a variable wavelength light source. 1 ... Reference wavelength light source, 4 ... Fabry-Perot etalon,
10 ... Semiconductor laser.

Claims (3)

[Claims] 1. A transmission wavelength of a Fabry-Perot etalon is controlled to a wavelength of an output light of a reference wavelength light source,
A variable structure characterized in that by controlling the output wavelength of the semiconductor laser to one of the transmission wavelengths of the Perot etalon, the output wavelength can be changed stepwise by inputting the current or temperature of the semiconductor laser. Wavelength light source. 2. The variable wavelength light source according to claim 1, wherein a semiconductor laser whose wavelength is controlled by an atomic absorption line of Cs or Rb is used as the reference wavelength light source. 3. A structure in which a signal corresponding to a step change in the output wavelength of a semiconductor laser is counted by a counter, and the counter is reset by a beat signal of the output light of a reference wavelength light source and the output light of a semiconductor laser. Range 1st
Variable wavelength light source according to the item.