JPH0523512B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to an improvement in the structure of a semiconductor laser wavelength stabilizing device that stabilizes the wavelength of a semiconductor laser by controlling it to the absorption line of atoms or molecules.

《Prior art》 Figure 4 shows the prior art of a semiconductor laser wavelength stabilizing device (Japanese Patent Application No. 11894-1983) filed by the present applicant.
FIG. LD1
is a semiconductor laser, PE1 is this semiconductor laser LD1
TS1 is a temperature sensor that measures the temperature of the semiconductor laser;
CT1 drives the Peltier element PE1 based on the measured value of this temperature sensor to drive the semiconductor laser LD.
Temperature control means for controlling the temperature of TB1 to a constant value, TB1
is a constant temperature bath that stores these and stabilizes the temperature.
BS1 is a beam splitter that separates the output light of the semiconductor laser LD1 into two directions, UM1 is an acousto-optic modulator that receives the output light from one of the beam splitters BS1 and constitutes a modulation means, and CL1 is the acousto-optic modulator. PD1 is a photodetector such as a photodiode that receives the diffracted light output from the absorption cell UM1 and contains a standard substance ( _CS in this case) that absorbs light of a specific wavelength. ,
A1 is an amplifier that inputs the output electric signal of this photodetector PD1, LA1 is a lock-in amplifier that inputs the electric output of this amplifier A1, and CT2 inputs the output of this lock-in amplifier LA1 to control the current of the semiconductor laser LD1. Configure the control means to control
PID controller, SW1 is the acousto-optic modulator
A switch whose one end is connected to UM1, SG1 is a signal generator whose output turns the switch SW1 on and off at a frequency m (e.g. 2kHz), and SG2 is a signal generator whose output is a signal generator with a frequency _D (e.g. 80MHz) connected to the other end of the switch SW1. 2 signal generator.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be explained below. Semiconductor laser LD1
is controlled to a constant temperature through a Peltier element PE1 by a control means CT1 which inputs a detection signal from a temperature sensor TS1 in a constant temperature bath TB1. The output light of semiconductor laser LD1 is transmitted through beam splitter BS.
1, the reflected light becomes output light to the outside, and the transmitted light enters the acousto-optic modulator UM1. When switch SW1 is on, the acousto-optic modulator
Since UM1 is driven by the output of frequency _D from signal generator SG2, most of the incident light with frequency ν ₀ is diffracted and undergoes a frequency (Doppler) shift, and the light with frequency ν ₀ + _D is the first-order diffracted light. The light enters the absorption cell CL1. When switch SW1 is off, all incident light is absorbed by absorption cell CL1 at frequency ν ₀ as 0th order diffracted light.
The switch SW1 input to the absorption cell is driven by the clock of frequency m of the signal generator SG1.
The light incident on CL1 has a modulation frequency m and a modulation depth _D
will be subjected to frequency modulation. Absorption cell CL
When light modulated by the acousto-optic modulator UM1 is incident on the acousto-optic modulator UM1, the amount of transmitted light is modulated only at the absorption signal of the _CS atom, and a signal appears at the output, as shown in the operational diagram of FIG. This signal is transmitted to photodetector PD1
If the signal is converted into an electric signal by the amplifier A1 and then synchronously rectified at the frequency m in the lock-in amplifier A1, a first-order differential waveform as shown in the frequency characteristic curve diagram of FIG. 6 is obtained. If the current of the semiconductor laser LD1 is controlled by the PID controller CT2 and the output of the lock-in amplifier LA1 is locked (controlled) to the center of the first-order differential waveform, the output light of the semiconductor laser becomes stable with ν _S − _D /2. becomes the frequency.

According to the semiconductor laser wavelength stabilizing device having such a configuration, since the oscillation frequency of the laser is not modulated, it becomes an extremely stable light source even momentarily.

Furthermore, even if the diffraction efficiency of the acousto-optic modulator UM1 changes, the light component that does not contribute to modulation (0th-order diffracted light)
increases and the signal strength decreases, but does not affect the center wavelength.

<<Problems to be solved by the invention>> However, in the semiconductor laser wavelength stabilizing device configured as described above, the optical circuit is constructed by spatial propagation using beam splitters, lenses, etc. In addition, interference with returned light and reflected light at the input end and output end of the absorption cell was a problem.

The present invention was made to solve these problems, and an object of the present invention is to realize a small-sized semiconductor laser wavelength stabilizing device that is easy to adjust.

<Means for Solving the Problems> The present invention relates to a semiconductor laser wavelength stabilization device that stabilizes the wavelength by controlling the wavelength of a semiconductor laser according to the absorption spectrum line of a standard substance. , an acousto-optic modulator that modulates the frequency by inputting a part of the output light of a semiconductor laser, and an optical fiber core that is encapsulated with a standard material that causes absorption at a specific wavelength and is provided to pass through the standard material. The absorption cell into which the output light of the acousto-optic modulator enters and the electric field of the evanescent wave portion generated in the optical fiber core of this absorption cell interact with the surrounding reference material to cause absorption at a specific wavelength. a photodetector that converts the transmitted light of the absorption cell from which the absorbed portion has been removed into an electrical signal;
A lock-in amplifier inputs an electric signal related to the output electric signal of the photodetector and performs synchronous rectification at the modulation frequency of the modulation means or an integral multiple thereof; and a control means for controlling the current or temperature of the semiconductor laser.

"Example" The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. The same parts as in FIG. 4 are given the same symbols and explanations are omitted. FB1 is a single mode optical fiber into which the output light of semiconductor laser LD1 is input;
CP1 is a fiber coupler that inputs the output light of this optical fiber FB1, and FB2 is this fiber coupler CP.
FB3 is a single mode optical fiber into which the other output light of the fiber coupler CP1 is input, and UM2 is a waveguide type into which the output light of this optical fiber FB3 is input. The acousto-optic modulator FB4 is a single-mode optical fiber that receives the output light of the acousto-optic modulator UM2 and outputs it to the photodetector PD1.
CL2 encloses a standard substance ( _CS in this case) that absorbs light of a specific wavelength, and inserts it into the optical fiber FB.
4 passes through the absorption cell, and a is the portion of the optical fiber FB4 in which the cladding portion is removed to leave only the core portion.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be explained below. The output light of the temperature-controlled semiconductor laser LD1 propagates through the optical fiber FB1 and is split into two directions by the fiber coupler CP1.
One output is output light to the outside, and the other output is input to the waveguide type acousto-optic modulator UM2 via the optical fiber FB3. The light modulated by the acousto-optic modulator UM2 in the same manner as the conventional example is transmitted through the optical fiber.
It propagates through FB4 and passes through absorption cell CL2. As shown in FIG. 2, within the absorption cell CL2, a portion where the propagating light leaks out of the core portion a of the optical fiber FB4, that is, an evanescent wave is generated, and the electric field in this portion interacts with the surrounding C _S gas. As a result, absorption occurs at specific wavelengths. In addition, since the evanescent wave propagates as it seeps out, it does not scatter inside the absorption cell CL2 and go somewhere.
If there is no absorption, all the light enters the photodetector PD1 via the optical fiber FB4. In other words, when absorption occurs, the absorbed amount decreases, and all the rest is left by the optical fiber.
The light enters the photodetector PD1 via FB4. Therefore, the output of optical fiber FB4 is detected by photodetector PD1.
If detected, an absorption signal is obtained, and if it is fed back to the semiconductor laser LD1 via the lock-in amplifier LA1 etc. as in the conventional example, the oscillation frequency of the semiconductor laser can be controlled to be near the center of absorption.

According to the semiconductor laser wavelength stabilizing device having such a configuration, in addition to having the same advantages as the conventional example shown in FIG. 4, the optical system can be constructed entirely of optical fibers, so alignment is unnecessary and adjustment is easy. , and can be made smaller.

In the above embodiment, a single mode fiber is used as the optical fiber FB4 passing through the absorption cell CL2, but the present invention is not limited to this, and a multimode fiber may be used.

Also, an acousto-optic modulator is used as a modulation means.
The UM2 may be any modulator that can perform frequency modulation; for example, a phase modulator using an electro-optical element may be used. This includes, for example, a vertical modulator,
There are horizontal modulators, traveling waveform modulators, etc. (Amnon Yarif: Fundamentals of Optoelectronics (Maruzen), p.247-p.253). Alternatively, the injection current of the semiconductor laser may be modulated using an acousto-optic modulator without external modulation.

Further, in the above embodiment, the modulation wave number m is used as the reference frequency of the lock-in amplifier LA1, but a frequency that is an integer multiple thereof may also be used.

Further, as a standard substance for the absorption cell CL2, in addition to C _S , for example, R _b , NH ₃ , H ₂ O, etc. may be used.

Further, in the above embodiment, the current of the semiconductor laser is controlled by the output of the control means, but the present invention is not limited to this, and the temperature of the semiconductor laser may also be controlled.

Further, a multiplier may be used instead of the switch SW1 to modulate the excitation frequency of the acousto-optic modulator.

Figure 3 shows the second embodiment of the present invention using the saturation absorption method (Reference: Hori, Tsunoda, Kitano, Yabusaki, Ogawa: Frequency stabilization of semiconductor lasers using saturation absorption spectroscopy, IEICE Technical Report OQE82-116) FIG. 2 is a block diagram showing a main part configuration of a semiconductor laser wavelength stabilizing device using a semiconductor laser wavelength stabilizing device. Third
The figure is a modified version of part 1 in Figure 1, FB5
is a single mode optical fiber that propagates the output light of the acousto-optic modulator UM2, CP2 is a fiber coupler to which this optical fiber FB5 is connected to one end,
FB6 is a single mode optical fiber connected to the other end of this fiber coupler CP2, and b is an absorption cell CL2.
2 is a half-mirror coated end face of the optical fiber FB6, and PD2 is a first photodetector for detecting light transmitted through this end face 2;
PD3 is a second photodetector that detects the reflected light from the end face 2 of the optical fiber FB6 via the fiber coupler CP2, and A2 is a differential that inputs the electrical outputs of the photodetectors PD2 and PD3 and outputs them to the lock-in amplifier LA1. It is a dynamic amplifier.

The output light of the acousto-optic modulator UM2 is an optical fiber.
The evanescent wave enters the fiber coupler CP2 via the FB5, propagates through the optical fiber FB6, and is generated outside the core portion b, serving as pump light to saturate the optical absorption of a nearby standard substance (for example, C _S ). Most of the light propagated through optical fiber FB6 (e.g. 90%)
is incident on the photodetector PD2 via the end surface 2, but
A portion (for example, 10%) is reflected by the end face 2 and propagates in the opposite direction through the optical fiber FB6, and the evanescent wave is absorbed as a probe light that overlaps with the pump light. This probe light is fiber coupler CP
2 to the optical fiber FB7, and the photodetector PD
3. The outputs of the photodetectors PD2 and PD3 are subtracted in a differential amplifier A2 to eliminate absorption signals due to Doppler spread, and are output to a lock-in amplifier as a saturated absorption signal having a sharp absorption spectrum. With a feedback loop similar to that shown in Figure 1,
The oscillation frequency of the semiconductor laser LD1 can be controlled to the peak of the saturated absorption spectrum with higher stability than in the case of the apparatus shown in FIG.

In the above embodiment, the end face 2 is coated with a half mirror, but the present invention is not limited to this. For example, a half mirror may be inserted in the middle of the optical fiber FB6.

<<Effects of the Invention>> As described above, according to the present invention, it is possible to realize a small-sized semiconductor laser wavelength stabilizing device that does not require alignment of an optical system, is easy to adjust, and has a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing an embodiment of the semiconductor laser wavelength stabilizing device according to the present invention, FIG. 2 is an explanatory diagram for explaining the operation of the device shown in FIG. 1, and FIG.
The figure is a block diagram showing another embodiment of the semiconductor laser wavelength stabilizing device according to the present invention, FIG. 4 is a block diagram showing a conventional semiconductor laser wavelength stabilizing device, and FIG. 5 is a block diagram of the device shown in FIG. FIG. 6 is a characteristic curve diagram for explaining the operation of the apparatus shown in FIG. 4. LD1...Semiconductor laser, FB4, FB6...Optical fiber, UM2...Modulation means, CL2...Absorption cell, PD1, PD2, PD3...Photodetector, LA1
... Lock-in amplifier, CT2 ... Control means, a,
b... Optical fiber core.

Claims (1)

[Scope of Claims] 1. A semiconductor laser wavelength stabilization device that stabilizes the wavelength by controlling the wavelength of a semiconductor laser on the absorption spectrum line of a standard substance, comprising the steps of frequency modulating a part of the output light of the semiconductor laser by inputting it to the absorption spectrum line of the standard substance. an acousto-optic modulator that encapsulates a standard material that causes absorption at a specific wavelength, and an absorption cell that injects the output light of the acousto-optic modulator into an optical fiber core that is provided so as to pass through the standard material; The electric field of the evanescent wave portion generated in the optical fiber core of the absorption cell interacts with the surrounding standard material to cause absorption at a specific wavelength, and the transmitted light of the absorption cell from which the absorbed portion has been removed is converted into an electric field. a photodetector for converting into a signal; a lock-in amplifier for inputting an electric signal related to the output electric signal of the photodetector and synchronously rectifying it at the modulation frequency of the modulation means or a frequency that is an integral multiple thereof; 1. A semiconductor laser wavelength stabilizing device, comprising: control means for controlling the current or temperature of the semiconductor laser so that the output of the amplifier becomes a constant value. 2. The semiconductor laser wavelength stabilizing device according to claim 1, wherein Rb or _CS is used as the standard substance.