JPH0462477B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to a semiconductor laser stabilizing device that stabilizes the wavelength of semiconductor laser output light. In particular, the present invention relates to improvements in quantum standard semiconductor laser stabilization devices that fix the wavelength of a semiconductor laser to the absorption spectrum of a metal gas.

<<Prior Art>> Semiconductor lasers are used in a wide range of fields due to their features such as small size, low power operation, normal temperature operation, easy modulation at high frequencies, wide frequency adjustment range, and low cost. On the other hand, however, the oscillation frequency of a semiconductor laser largely depends on the laser temperature and laser current, so even minute changes in these will cause the oscillation frequency to change quickly, resulting in very poor stability. This drawback becomes a major problem when semiconductor lasers are used for various measurements and high-resolution spectroscopy.
Furthermore, in optical communications, frequency stabilization of semiconductor lasers will be an important condition for the future development of frequency multiplexing systems.

One method of frequency stabilization is to use absorption lines of atoms (or molecules, the same applies hereinafter).
When laser light is absorbed by a standard gas, only a wide absorption spectrum line with Doppler broadening is obtained due to the thermal motion of the atoms.

However, in saturated absorption spectroscopy, laser light is input as pump light from one direction of the absorption cell, and while standard gas atoms are excited at a specific velocity, probe light of the same frequency and weak intensity is input from the opposite direction. Since the probe light is not absorbed only when the laser frequency matches the resonance frequency of the atoms, it is not affected by the velocity distribution due to the thermal motion of the atoms, and a sharp absorption line without Doppler broadening can be obtained. If this is used as a frequency standard, frequency stability can be greatly improved. (T. Yabuzaki, A.
Hori, M. Kitano, and T. Ogawa: Frequency
Stabilization of Diode Lasers Using Doppler
−Free Atomic Spectra，Proc.Int.Conf.Laser´s
83/Hori, Kadota, Kitano, Yabusaki, Ogawa: Frequency stabilization of semiconductor lasers using saturation absorption spectroscopy, IEICE Technical Report
OQE82-116) <<Problems to be Solved by the Invention>> However, the semiconductor laser wavelength stabilizing device having the above configuration has the drawbacks that the configuration is complicated, the device is large, and the adjustment of the optical system is troublesome.

The present invention has been made to solve these problems, and an object of the present invention is to realize a small-sized semiconductor laser wavelength stabilizing device with a simple configuration.

<Means for Solving the Problems> The semiconductor laser wavelength stabilization device according to the present invention relates to a semiconductor laser wavelength stabilization device that stabilizes the wavelength by controlling the wavelength of the semiconductor laser to the absorption spectrum line of a standard substance. Its features include a polarizing beam splitter that receives the output light from a semiconductor laser, an absorption cell filled with a standard material that receives a portion of the output light from one of the polarizing beam splitters, and a 1/4 a wavelength plate, a mirror into which the light that has passed through the absorption cell and the quarter-wave plate is incident, and the light reflected by this mirror is transmitted to the quarter-wave plate;
A first light receiving element enters through the absorption cell and the polarization beam splitter, and the other part of the one output light of the polarization beam splitter receives light related to the light transmitted through the absorption cell. a second light-receiving element into which the light enters, the outputs of the two light-receiving elements are calculated, and the absorption spectrum line in the absorption cell is controlled by driving the current or temperature of the semiconductor laser in accordance with the calculated output. The point is that it is configured to control the oscillation frequency.

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

FIG. 1 is a block diagram showing an embodiment of a semiconductor laser wavelength stabilizing device according to the present invention.
LD1 is a semiconductor laser element, LS1 is a lens that focuses the output light of the semiconductor laser element LD1, BS1 is a polarizing beam splitter that receives the light that has passed through this lens LS1, and CL1 is a polarizing beam splitter that receives the light that has passed through this polarizing beam splitter BS1. The absorption cell P1 is filled with metal gases such as R _b and C _s .
A 1/4 wavelength plate into which a part of the light passing through P1 (the upper part in the figure) enters, F1 is an ND filter into which the output light of this 1/4 wavelength plate P1 enters, and M1 is the output of this ND filter F1. The mirror PD1 receives the light, and the reflected light from the mirror M1 passes through the filter F1, 1/
A light-receiving element receives light through a four-wavelength plate P1, an absorption cell CL1, and a beam splitter BS1, and BS2 is a beam splitter that receives another part of the light (lower part in the figure) that passes through the absorption cell CL1. PD2 is the second light receiving element into which the light reflected by the beam splitter BS2 enters, and OP1 is the light receiving element PD.
1. An arithmetic circuit DR1 that calculates the output of PD2 is a drive circuit that inputs the output of this arithmetic circuit OP1 and controls the injection current of the semiconductor laser LD1.

Next, the operation of the apparatus configured as described above will be explained in detail. The light output from the semiconductor laser LD1 is focused by the lens LS1, and then enters the polarizing beam splitter BS1. At this time, the semiconductor laser
The plane of polarization of LD1 is such that its output light is completely transmitted through polarization beam splitter BS1. The light transmitted through the polarizing beam splitter BS1 enters the absorption cell CL1 as saturated light (pump light) and is absorbed by atoms within the absorption cell CL1. Absorption cell CL
A part of the light that passed through 1 (upper half) is passed through 1/4 wavelength plate P
1 and becomes circularly polarized light, which is reflected by mirror M1 via ND filter F1. The reflected light from mirror M1 is again passed through ND filter F1 and 1/4 wavelength plate P.
1 and enters the absorption cell CL1 as probe light in the opposite direction to the pump light. At this time, absorption cell CL
The optical axes of the saturated light and the probe light are adjusted in the same way. Since the probe light output from the absorption cell CL1 has its plane of polarization rotated by 90° while passing through the quarter-wave plate P1 twice, it is reflected by the polarizing beam splitter BS1 and enters the light receiving element PD1. The other part (lower half) of the light that has passed through the absorption cell CL1 enters the beam splitter BS2, the transmitted light becomes output light to the outside, and the reflected light enters the light receiving element PD2 as reference light. Photodetector PD
The output of 1 and the output of photodetector PD2 are the arithmetic circuit OP1.
The output is input to the drive circuit DR1. By driving the current of the semiconductor laser with the output of the drive circuit DR1, the oscillation frequency of the semiconductor laser is controlled to a saturated absorption spectrum signal of the reference gas in the absorption cell CL1.

In the above device, the light incident on the absorption cell CL1 acts as pump light to excite atoms and bring them into a saturated state. As a result, the output of the light receiving element PD2 that detects the light emitted from the absorption cell CL1 is linear absorption (absorption with Doppler spread) as shown in 1 in Figure 2.
receive. The upper output light of the absorption cell CL1 is sufficiently attenuated when passing through the ND filter twice, and then enters the absorption cell CL1 from the opposite direction as pump light and undergoes saturated absorption. In other words, as mentioned above, the probe light is not absorbed only when the laser frequency matches the resonance frequency of the atoms in the absorption cell.
Sharp absorption lines without Doppler spread can be obtained. As a result, the output of the first light receiving element PD1 has a saturated absorption characteristic as shown by the solid line 2 in FIG. For example, the arithmetic circuit OP1 adjusts the output of the photodetector PD2, and aligns it with the broken line 3 in Fig. 3.
By subtracting from the output of PD1 and amplifying it, the characteristics shown in Fig. 4 are obtained. The current of the semiconductor laser LD1 is controlled via the PID circuit in the drive circuit DR1 so as to reach the peak value of this characteristic.

According to the semiconductor laser wavelength stabilization device having such a configuration, standard gases such as R _b and C _s or NH ₃ ,
Since the frequency is fixed to the saturated absorption spectrum of H ₂ O, etc., the oscillation wavelength of the semiconductor laser becomes stable.

Furthermore, there is no optical path parallel or perpendicular to the absorption cell, making the optical system simpler and more compact than conventional devices. Further, adjustment of the optical axis, etc. becomes easier.

Furthermore, since the reference light is not orthogonal to the pump light or probe light and is not subject to disturbance, accuracy and stability are good.

In the above embodiment, a lock-in amplifier may be used in the drive circuit or the like to control the frequency to the zero-crossing point of the first-order differential waveform of the saturation absorption signal.

Furthermore, the ND filter F1 can be made unnecessary by reducing the reflectance of the mirror M1.

Further, contrary to the above embodiment, the polarizing beam splitter BS1 may reflect the output of the semiconductor laser LD1 and guide it to the absorption cell CL1, and transmit the probe light and guide it to the light receiving element PD1.

Further, in the above embodiment, the quarter-wave plate P1 may be placed in front of the absorption cell CL1. Further, even if it is behind the absorption cell CL1, it may be placed in front of the mirror M1.

In addition, the polarization plane of the emitted light from the semiconductor laser LD1 is not completely aligned with the polarizing beam splitter BS1, but a portion is reflected as output light to the outside, and a photodetector PD2 is placed at the same position instead of the beam splitter BS2. By doing so, it is possible to omit one beam splitter and obtain output light that is not affected by disturbances such as absorption.

FIG. 5 is a perspective view of the main part of the second embodiment of the present invention. This device is different from the device shown in FIG. 1 in that the 1/4 wavelength plate P2, ND filter F2, and mirror M2 are arranged at the center of the output light of the absorption cell, and the light at the center is used for the saturated absorption signal. The operation is similar to that in FIG. On the contrary, FIG. 6 shows a device in which peripheral light is used as a saturated absorption signal by using a doughnut-shaped quarter-wave plate P3, an ND filter F3, and a mirror M3. P4, F4 and M4 are a 1/4 wavelength plate, an ND filter and a mirror, respectively, as in the case of FIG.

FIG. 7 is a block diagram showing the main part of a third embodiment of the present invention. Unlike the device shown in Figure 1,
The incident light is divided into 3 without using the beam splitter BS2.
The light reflected by the mirror M5 is used as a reference light, and a part of the output light of the absorption cell CL1 is directly extracted as output light to the outside.

In each of the above embodiments, a case is shown in which gas is sealed in the absorption cell, but the absorption line is not limited to gas, and liquid or solid may also be used.

<<Effects of the Invention>> As described above, according to the present invention, it is possible to realize a highly accurate and compact semiconductor laser wavelength stabilizing device with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing an embodiment of a semiconductor laser wavelength stabilizing device according to the present invention, and FIGS.
FIG. 4 is a characteristic curve diagram for explaining the operation of the device shown in FIG. FIG. 7 is a block diagram showing a main part configuration of a third embodiment of the invention. LD1...Semiconductor laser, BS1...Polarizing beam splitter, CL1...Absorption cell, P1...1/4 wavelength plate, M1...Mirror, PD1...First light receiving element, PD2...Second light receiving element element.

Claims (1)

[Scope of Claims] 1. A semiconductor laser wavelength stabilization device that stabilizes wavelength by controlling the wavelength of a semiconductor laser to the absorption spectrum line of a standard material, comprising: a polarizing beam splitter into which output light from the semiconductor laser is incident; An absorption cell filled with a standard material and a quarter-wave plate into which a part of the output light from one side of the polarizing beam splitter is incident;
a mirror on which the light that has passed through the wavelength plate is incident; a first light-receiving element on which the light reflected by the mirror is incident through the quarter-wave plate, the absorption cell, and the polarization beam splitter; and the polarization beam splitter. and a second light-receiving element that receives light related to the light that the other part of the output light of the one of the plurality of output lights has transmitted through the absorption cell, and calculates the outputs of the two light-receiving elements, and calculates the output of the two light-receiving elements. A semiconductor laser wavelength stabilizing device, characterized in that the oscillation frequency of the absorption spectrum line in the absorption cell is controlled by driving the current or temperature of the semiconductor laser in accordance with the output.