JPH0482070B2 - - Google Patents

Description

[Detailed Description of the Invention] <<Industrial Application Field>> The present invention relates to improving the characteristics of a quantum standard 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. 10 is a block diagram showing a conventional semiconductor laser wavelength stabilizing device. semiconductor laser LD
The oscillation wavelength of the laser output is modulated by superimposing a modulation signal of frequency m on the current of beam splitter BS.
One of the separated lights is incident on the absorption cell CL, which contains a standard substance that causes absorption at a specific wavelength. The other light separated by the beam splitter BS is reflected by a mirror M and becomes output light. The light emitted from the absorption cell CL is converted into an electrical signal by the photodetector PD,
It is synchronously rectified by lock-in amplifier LA. By controlling the current of the semiconductor laser LD using the control means CT so that the output of the lock-in amplifier LA, that is, the first-order differential signal with respect to the frequency, becomes 0, the wavelength of the semiconductor laser LD is adjusted to the absorption line of the atoms in the absorption cell CL. It can be locked.

<<Problems to be Solved by the Invention>> However, in the semiconductor laser wavelength stabilizing device configured as described above, the average frequency of the output light of the semiconductor laser is locked to the absorption line of the standard material and becomes stable; Since the frequency always fluctuates at the frequency m, the instantaneous value of the oscillation frequency is not stable and there is a problem that the spectral width widens.

The present invention has been made to solve these problems, and an object of the present invention is to realize a semiconductor laser wavelength stabilization device that has a stable wavelength and a narrow spectrum width.

<Means for Solving the Problems> The present invention relates to a semiconductor laser wavelength stabilizing device that stabilizes the wavelength by controlling the wavelength of a semiconductor laser according to the absorption spectrum line of a substance. A shift separation means that separates the output light of the laser into two optical outputs having a predetermined frequency difference, and an absorption cell in which the two output lights from the shift separation means are input and a substance that causes absorption at a specific wavelength is sealed. The wavelength of the semiconductor laser is controlled so that the difference between the detection outputs of the two output lights of the absorption cell becomes zero.

"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. LD is a semiconductor laser, LS is a lens that converts the output light of this semiconductor laser LD into parallel light, and S1
is a shift separation means that inputs the output light of this lens LS. In the shift separation means S1,
PBS1 is a first polarizing beam splitter into which the output light from the lens LS is incident, P1 is a quarter-wave plate into which the light reflected by this polarizing beam splitter PBS1 is incident, and AOM is this quarter-wave plate P1. M1 is a mirror that receives the output light of this acousto-optic modulator AOM, P
2 is a 1/4 wavelength plate through which the light reflected by this mirror M1 enters through the acousto-optic modulator AOM, 1/4 wavelength plate P1, and the polarizing beam splitter PBS1, and M2 is this 1/4 wavelength plate. This is a mirror on which the light that has passed through P1 is incident. CL means that the light reflected by the mirror M2 of this shift separation means S1 is transmitted to the quarter wavelength plate P2 and the polarizing beam splitter PBS1.
PBS2 is a second polarizing beam splitter that receives the light transmitted from this absorption cell CL, and PBS2 is a polarizing beam splitter that receives the light transmitted from this absorption cell CL. , BS1 is a beam splitter into which the transmitted light of this polarizing beam splitter PBS2 is incident, and PD1 and PD2 are the polarizing beam splitter PBS2 and beam splitter BS.
A is a differential amplifier that inputs the electrical outputs of these photodetectors PD1 and PD2, OP is an arithmetic circuit that inputs the output of this differential amplifier A, and DR is this arithmetic circuit OP. A drive circuit inputs the output of the semiconductor laser LD to drive the current of the semiconductor laser LD, V is the frequency of the acousto-optic modulator AOM.
It is a driving source driven by m.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be described in detail below. semiconductor laser
The output light of the frequency o of the LD is converted into parallel light by the lens LS, and then separated into two directions by the polarizing beam splitter PBS1. Polarizing beam splitter PBS1
The reflected light (with a plane of polarization perpendicular to the plane of the paper) is the second
As shown in the operation diagram in the figure, it becomes circularly polarized light by the quarter-wave plate P1, is diffracted by the acousto-optic modulator AOM, and undergoes a frequency shift of m due to the Doppler effect.
This diffracted light is reflected by the mirror M1 and returns to the original direction, and is again subjected to a frequency shift of m by the acousto-optic modulator AOM, so that it finally becomes light with a frequency of o+2m that has undergone a frequency shift of 2m. When this light passes through the quarter-wave plate P1 again, it becomes polarized with a plane parallel to the plane of the paper, and is transmitted through the polarizing beam splitter PBS1. This transmitted light is made into circularly polarized light by a 1/4 wavelength plate P2,
After being reflected by mirror M2, it is polarized again by 1/4 wavelength plate P2 with a polarization plane perpendicular to the plane of the paper.
When it enters PBS1, it is all reflected. This reflected light enters the absorption cell CL, and the transmitted light is completely reflected by the polarizing beam splitter PBS2 and then passes through the light receiving element PD.
1. On the other hand, as shown in Fig. 3, the light whose plane of polarization is horizontal to the plane of the paper that first passes through the polarizing beam splitter PBS1 passes through the absorption cell CL, and then
The light passes through the polarizing beam splitter PBS2, is partially reflected by the beam splitter BS1, and enters the light receiving element PD2. The light transmitted through the beam splitter BS1 is extracted to the outside as output light. Photodetector PD
The light incident on PD2 and the light output to the outside have the same oscillation frequency o of the semiconductor laser, but the light incident on photodetector PD1 has been shifted in frequency by 2m and has a frequency of o+2m, so the oscillation occurs. The output of the light receiving element when the frequency o is swept is as shown in FIG. 4, and the centers of absorption of the output of the light receiving element PD1 of A and the output of the light receiving element PD2 of B are shifted by a frequency of 2 m. Differential amplifier A calculates the difference between the outputs of light receiving elements PD1 and PD2, and the output is as shown in C. The arithmetic circuit OP and the drive circuit DR control the oscillation wavelength of the semiconductor laser LD to the zero cross point (point where it intersects with 0) of the output of the differential amplifier A. The oscillation frequency of the semiconductor laser LD when the wavelength is controlled to the zero crossing point is _o =
It becomes stable as _R −m. However, the resonance frequency _R
Assume that the spread of the Doppler shift for is symmetric.

According to the semiconductor laser wavelength stabilizing device having such a configuration, the wavelength is controlled to the zero cross point without using lock-in up, so the output light is not modulated and the spectral width can be narrowed.

Furthermore, since it is controlled to the zero-crossing point, it is highly stable, and even if the absorption amount changes due to changes in the temperature of the absorption cell, the wavelength at the zero-crossing point does not change.

Note that in the above embodiments, the oscillation wavelength of the semiconductor laser is changed by controlling the current, but the present invention is not limited to this.
Temperature, etc. may also be controlled.

Further, as a standard substance for the absorption cell CL, in addition to Cs, for example, Rb, NH ₃ , H ₂ O, etc. may be used.

Alternatively, the saturation absorption method described later may be used.

FIG. 5 is a block diagram showing a second embodiment of the semiconductor laser wavelength stabilizing device according to the present invention. The same parts as in the apparatus of FIG. 1 are given the same symbols and explanations are omitted. In the shift separation means S2,
BS2 is a beam splitter that separates the output light of the semiconductor laser LD into two directions. BS3 inputs the transmitted light of this beam splitter BS2, uses the transmitted light as external output light, and the reflected light enters the acousto-optic modulator AOM. It is a beam splitter. Photodetector PD1,
PD2 is a cell that absorbs the reflected light from the beam splitter BS2 and the transmitted light from the acousto-optic modulator AOM, respectively.
Incident via CL. Light having an oscillation frequency o of the semiconductor laser LD is incident on the light receiving element PD1, and light having a shifted frequency o+m is incident on the light receiving element PD2.

According to the device having such a configuration, the structure is simplified because the light is directly irradiated onto the absorption cell without passing through the acousto-optic modulator AOM twice.

FIG. 6 is a block diagram showing a third embodiment of the present invention using saturated absorption (References regarding saturated absorption: 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). The same parts as in FIG. 5 are given the same symbols and the explanation will be omitted. Shift separation means S2
The reflected light from the beam splitter BS2 enters the absorption cell CL as pump light, and the transmitted light enters the mirror M3.
The probe light passes through the absorption cell CL from the opposite direction so as to overlap with the pump light, and the light receiving element PD1 detects the sharp absorption light without Doppler spread. Similarly, the output diffracted light of the acousto-optic modulator AOM enters the absorption cell CL as a pump light, and the transmitted light is reflected by the mirror M4 and passes through the absorption cell CL again as a probe light. Detects absorbed light. As shown in FIG. 7A, the frequency characteristics of the outputs of the light-receiving elements PD1 and PD2 both exhibit a lamb dip, and have a frequency m deviation from each other. Differential amplifier A at this time
The output characteristic is as shown in B, and the zero crossing point has a sharp slope, which can further improve frequency stability.

FIG. 8 is a block diagram showing a fourth embodiment of the semiconductor laser wavelength stabilizing device of the present invention. A part of the output light from the semiconductor laser LD is reflected by the beam splitter BS4 and enters the acousto-optic element AOM of the shift separation means S3. acousto-optic element
The -1st-order diffracted light of the AOM undergoes a frequency shift of o-m, and the +1st-order diffracted light undergoes a frequency shift of o+m, and then pass through the absorption cell CL to the light receiving element.
It enters PD1 and PD2. Figure 9A is the light receiving element
The frequency characteristics of the outputs of PD1 and PD2 are shown, and the differential amplifier A output, which is the difference between them, has a zero-crossing point that coincides with the resonant frequency R of the absorption cell, as shown in FIG. 9B. Therefore, even if the value of the modulation frequency m changes, the zero crossing point does not change. Also, even if the diffraction efficiency of the acousto-optic modulator AOM changes, PD1 and PD2
Since the output changes in the same way, the zero crossing point does not move. Further, the saturated absorption method can also be used as in the apparatus shown in FIG.

Note that in each of the above embodiments, the wavelength of the semiconductor laser is controlled so that the output of the differential amplifier circuit A becomes 0, but any value can be used as long as a decrease in the stability of the zero-crossing point is tolerated. .

<<Effects of the Invention>> As described above, according to the present invention, a semiconductor laser wavelength stabilizing device having a stable wavelength and a narrow spectrum width can be realized with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are operation explanatory diagrams for explaining the operation of the apparatus shown in FIG. 1, and FIG. 4 is an illustration of the operation of the apparatus shown in FIG. A characteristic curve diagram for explanation; FIG. 5 is a block diagram showing a second embodiment of the present invention; FIG. 6 is a block diagram showing a third embodiment of the present invention;
7 is a characteristic curve diagram for explaining the operation of the device shown in FIG. 6, FIG. 8 is a block diagram showing a fourth embodiment of the present invention, and FIG. 9 is a diagram for explaining the operation of the device shown in FIG. 8. FIG. 10 is a block diagram showing a conventional semiconductor laser wavelength stabilizing device. LD...Semiconductor laser, S1, S2, S3...Shift separation means, CL...Absorption cell.

Claims (1)

[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 substance, the output light of the semiconductor laser is divided into two optical outputs having a predetermined frequency difference. A shift separation means for separating the two output lights from the shift separation means, and an absorption cell sealed with a substance that causes absorption at a specific wavelength upon input of the two output lights from the shift separation means, the detection output of the two output lights of the absorption cell A semiconductor laser wavelength stabilizing device characterized by controlling the wavelength of a semiconductor laser so that the difference between the two becomes zero. 2. The semiconductor laser wavelength stabilization device according to claim 1, wherein the shift separation means includes an acousto-optic element that frequency-shifts the output light of the semiconductor laser. 3. The semiconductor laser wavelength stabilizing device according to claim 1, which uses Rb or Cs as the substance.