JPS6355991A - Wavelemgth stabilizer of semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a compact laser wavelength stabilizer characterized by simple adjustment, by inputting part of the output light of a semiconductor laser into an optical fiber passing a standard material, and utilizing the absorption of the part of the light propagating through the fiber. CONSTITUTION:The output of a temperature controlled laser LD1 is propagated through a fiber FB1 and split in a fiber coupler CP1. One beam is sent to the outside as output light 11. The other beam is inputted into a waveguide type acoustooptic modulator UM2 through a fiber FB3. The modulated light is propagated through FB4 and passes an absorbing cell CL2. The propagating light leaks out of a core (a) of the FB4 in the cell. The electric field at this part mutually reacts with Cs gas at the outer surface, and absorption occurs at a specified wavelength. When the output of the fiber FB4 is detected by PD1, an absorption signal is obtained. The signal is fed back to the laser LD1 through a lock-in amplifier LA1. The oscillating frequency of the laser can be controlled in the vicinity of the center of the absorption. Position alignment is not required, adjustment is simple and the compact configuration can be implemented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) 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) FIG. 4 is a structural block diagram showing a prior art (Japanese Patent Application No. 11894/1988) of a semiconductor laser wavelength stabilizing device that has been filed by the present applicant.

LDI is a semiconductor laser, PE1 is this semiconductor laser LD
TSL is a temperature sensor that measures the temperature of the semiconductor laser; TSL is a temperature sensor that measures the temperature of the semiconductor laser; and TSL is a temperature sensor that drives the Bertier element PEI based on the measured value of this temperature sensor to keep the temperature of the semiconductor laser LDI constant. 881 is a beam splitter that separates the output light of the semiconductor laser LD1 into two directions, and UMI is the output light of one of the beam splitters 881. The acousto-optic modulator CL1 constitutes a modulation means, and CL1 is an absorption cell P which receives the diffracted light output of the acousto-optic modulator UM1 and encapsulates a standard material (C5 in this case) that absorbs light of a specific wavelength.
CI is a photodetector such as a photodiode that receives the transmitted light of this absorption cell CL1, A1 is an amplifier that inputs the output electric signal of this photodetector l5PDI, and LAI is this amplifier A.
A lock-in amplifier C70 inputs an electric output of 1, and C70 is a PI that inputs the output of this lock-in amplifier LAI and constitutes a control means for controlling υ1 part of the current of the semiconductor laser LDI.
D controller, SWl is the acousto-optic modulator tJM1
A switch, SG1, whose one end is connected to
), SG2 is the switch S
Frequency fo (e.g. 80Mt) connected to the other end of W1
-1z) second signal generator.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be explained below. Semiconductor laser LD1 is in constant temperature bath TBI
The temperature is controlled to a constant temperature via the Bertier element PE1 by a control means CTI which inputs a detection signal from a temperature sensor TSI. The output light of the semiconductor laser LD1 is separated into two directions by the beam splitter BS1, the reflected light becomes output light to the outside, and the transmitted light enters the acousto-optic modulator UMI. When the switch SW1 is on, the acousto-optic modulator 5UM1 is driven by the output of the frequency fo from the signal generator SG2, so most of the incident light with the frequency ν0 is diffracted and undergoes a frequency (Doppler) shift, and becomes the first-order diffracted light. Frequency νo+f
o light enters the absorption cell CL1. When switch SW1 is off, all incident light is O-order diffracted light with frequency ν0.
and enters the absorption cell CLI. Since the switch SW1 is driven by the clock of the frequency f1 of the signal generator SGI,
The light incident on the absorption cell CL1 is subjected to frequency modulation with a modulation frequency f and a modulation depth fD. Absorption cell CL
When light modulated by the acousto-optic modulator UM1 is incident on I, the amount of transmitted light is modulated only at the location of the absorption signal of the Cs atoms, and a signal appears at the output, as shown in the operational diagram of FIG. If this signal is converted into an electrical signal by the photodetector PD1 and synchronously rectified at a frequency f by the lock-in amplifier LAI via the amplifier A1, the waveform for several machines as shown in the frequency characteristic curve diagram in Fig. 6 will be obtained. 1q is received. The current of the semiconductor laser LDI is controlled by the PID controller CT2,
If the output of the lock-in amplifier LA1 is locked (controlled) to the center of the waveform for the several devices mentioned above, the output light of the semiconductor laser will be νs
This results in a stable frequency of fo/2.

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 (O-th order diffracted light) increases and the signal intensity decreases, but the center wavelength is not affected.

(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. Alignment is required, and there are problems such as interference with returned light or reflected light at the input end or output end of the absorption cell.

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. A part of the output light from a semiconductor laser is incident on an optical fiber passing through a standard material, and absorption in the evanescent wave portion of the light propagating through the optical fiber is utilized.

(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. FBI is a single mode optical fiber into which the output light of semiconductor laser LD1 is input, and CPI is this optical fiber F.
A fiber coupler into which the output light of B1 is input, Fe2 is a single mode optical fiber into which one output light of this fiber coupler CP1 is input, and Fe3 is the fiber coupler C.
A single mode optical fiber into which the other output light of P1 is input, 0M2 is a waveguide type acousto-optic modulator into which the output light of this optical fiber FB3 is input, and Fe2 is into which the output light of this acousto-optic modulator LIM2 is input. C10 is a single-mode optical fiber that outputs light to the photodetector PD1, C10 is an absorption cell filled with a standard material (Cs in this case) that absorbs light of a specific wavelength, and the optical fiber FB4 passes through the absorption cell; This is a portion of the fiber FB4 with the cladding portion removed to leave only the core portion.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be explained below. Temperature-controlled semiconductor laser LD1
The output light propagates through the optical fiber FBI and is split into two directions by the fiber coupler CP1, one output becomes the output light to the outside, and the other output is transmitted through the optical fiber FB3 to the waveguide type acousto-optic modulator LIM2. incident on . Light modulated by the acousto-optic modulator LIM2 in the same manner as in the conventional example propagates through the optical fiber FB4 and passes through the absorption cell CL2.

In 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 CS gas, causing absorption at a specific wavelength. . Therefore, if the output of the optical fiber FB4 is detected by the photodetector PD1, an absorption signal can be obtained, and if it is fed back to the semiconductor laser LD1 via the lock-in amplifier LA1 etc. as in the conventional case, the absorption signal will be near the center of absorption. The oscillation frequency of the semiconductor laser can be controlled.

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.
The fiber is not limited to this, and may be a multimode fiber.

Further, the acousto-optic modulator UM2 used as the modulation means may be any modulator as long as it can perform frequency modulation, for example, a phase modulator using an electro-optic element may be used. For example, a vertical modulator.

There are horizontal modulators, traveling wave modulators, etc. (AmnOn
Yar+'r: Optoelectronics l1il (round neck), p247-1) 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 frequency fTIL is used as the reference frequency of the lock-in amplifier LA1, but a frequency that is an integral multiple of the modulation frequency fTIL may be used.

In addition to Cs, for example, Rb, NH3, H20, etc. may be used as the standard substance for the absorption cell CL2.

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 be controlled.

Further, the excitation frequency of the acousto-optic modulator may be modulated using a multiplier instead of the switch SW1.

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 0QE82-116)
) is a block diagram illustrating a main part configuration of a semiconductor laser wavelength stabilizing device using a semiconductor laser wavelength stabilizing device.

Figure 3 is a modification of part 1 in Figure 1, where Fe5 is a single mode optical fiber that propagates the output light of the acousto-optic modulator IJM2, and CF2 is a fiber coupler to which this optical fiber FB5 is connected at one end. , Fe6 is a single mode optical fiber connected to the other end of this fiber coupler CP2, b is a portion of the optical fiber FB6 in which the cladding portion is removed to leave only the core portion within the absorption cell OL2, and 2 is a half mirror of the FB6. -・Coated end face, PD
2 is a first photodetector P that detects the transmitted light of this end surface 2;
D3 is a second photodetector A2 that detects the reflected light from the end face 2 of the optical fiber FB6 via the fiber coupler CP2.
is photodetector PD2. This is a differential amplifier that inputs the electrical output of PD3 and outputs it to lock-in amplifier LAI.

The output light of the acousto-optic modulator UM2 enters the fiber coupler CP2 via the optical fiber B5, and then enters the fiber coupler CP2 through the optical fiber F86.
The evanescent wave propagated outside the core portion serves as pump light and saturates the light absorption of a nearby standard material (for example, Cs). Most of the light (e.g. 90%) propagated through the optical fiber FB6 enters the photodetector PD2 via the end face 2, but a portion (e.g. 10%) is reflected from the end face 2 and travels through the optical fiber FB6 in the opposite direction. The evanescent wave is absorbed as a probe light superimposed on the pump light.This probe light is guided to an optical fiber FB7 by a fiber coupler CP2 and enters a photodetector PD3.

Photodetector PD2. The output of PD3 is filtered by differential amplifier A2 to eliminate the absorption signal due to Doppler spread, and is output to the lock-in amplifier as a saturated absorption signal having a sharp absorption spectrum. By using a feedback loop similar to that shown in FIG. 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 device shown in FIG.

In the above embodiment, the end face 2 is coated with a half-mirror coating, but the coating is not limited to this.
A half mirror may be inserted in the middle of 6. .

(Effects of the Invention) As described above, according to the present invention, it is possible to realize a compact 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. FIG. 4 is a configuration block diagram showing another embodiment of the semiconductor laser wavelength stabilization device, FIG. 4 is a configuration block diagram showing a conventional semiconductor laser wavelength stabilization device, and FIG. 5 is a diagram for explaining the operation of the device shown in FIG. FIG. 6 is a characteristic curve diagram for explaining the operation of the device shown in FIG. 4. LDl... semiconductor laser, FB4. FB6...Optical fiber, UM2...Modulation means, C10...Absorption cell, PDl, PD2. PD3...Photodetector, LAl...
・Lock-in amplifier, Cr2...control means, a, b・
...Optical fiber core. Tail 2 figure Atsushi 3 figure

Claims (3)

[Claims] (1) In 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 material, a part of the output light of the semiconductor laser is input into an optical fiber that passes through the standard material. . A semiconductor laser wavelength stabilization device, characterized in that it utilizes absorption in an evanescent wave portion of light propagating in the optical fiber. (2) An acousto-optic modulator that modulates the frequency by inputting a part of the output light of a semiconductor laser, and an optical fiber that is encapsulated with a standard material that causes absorption at a specific wavelength and is installed to pass through the standard material. An absorption cell into which the output light of the acousto-optic modulator is input, a photodetector which converts the transmitted light of the absorption cell into an electrical signal, and an electrical signal related to the output electrical signal of the photodetector is input. a lock-in amplifier that performs synchronous rectification at the modulation frequency of the modulation means or a frequency that is an integral multiple thereof; and a control means that controls the current or temperature of the semiconductor laser so that the output of the lock-in amplifier becomes a constant value. A semiconductor laser wavelength stabilizing device according to claim 1. (3) The semiconductor laser wavelength stabilizing device according to claim 1, which uses R_b or C_s as a standard substance.