JPH0637384A - Light frequency stabilizing light source apparatus - Google Patents

Abstract

PURPOSE:To reduce the frequency fluctuation of a laser beam further by a method wherein a wide band is employed as the control band of the feedback system of a light frequency stabilizing light source apparatus which detects the frequency fluctuation of the output light of a laser light source by the absorption peak of a light absorbing cell and feeds the detected result to the light source to constrict the output light. CONSTITUTION:A laser light source 1 and a photodetector 3 are provided on the incident side (I) of a light absorbing cell 2 and a light reflecting means 13 is provided on the emitting side (E) of the light absorbing cell 2. A light emitted from the laser light source 1 enters the light absorbing cell 2. The light absorbed and transmitted by the cell 2 is reflected by the light reflecting means 13 and enters the light absorbing cell 2 again. The light is again absorbed and transmitted by the cell 2 and detected by the photodetector 3 and converted into an electric signal. The electric signal is inputted to a current supply 12 which controls and drives the laser light source 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects the frequency fluctuation of the output light of a laser light source by the absorption peak peculiar to atoms and molecules enclosed in a light absorption cell, and the detected signal is fed through a feedback circuit to the laser light source. The present invention relates to an optical frequency stabilizing light source device that suppresses the frequency fluctuation of light by feeding back to the optical frequency stabilizing light source device, and particularly to an optical frequency stabilizing light source device that narrows the laser light by widening the control band of the feedback system.

The present invention is used for a high-accuracy optical frequency standard or a pumping light source that requires high energy resolution.
For example, the reference frequency light source of the ultra-precision laser length measurement system applying the Michelson interferometer, the standard light source of the optical heterodyne frequency meter, and the transmitting station in the optical heterodyne communication which is being researched as the next generation communication system. It can be used as a master light source for a receiving station.

[0003]

2. Description of the Related Art As a method for stabilizing the frequency of oscillated light by removing frequency noise (frequency fluctuation) contained in the oscillated light of a laser light source, conventionally, atoms and molecules are enclosed and absorption characteristics at a specific frequency are measured. There has been a method in which the light absorption cell of the cell is used as an optical frequency reference. For example, there was an invention "frequency-stabilized light source" by the same applicant disclosed in Japanese Patent Laid-Open No. 2-234484. FIG. 4 shows the configuration of the present invention. In FIG. 4, the laser light emitted from the laser light source 1 enters the light absorption cell 2. The light absorption cell 2 has a substance (for example, rubidium (R
b), cesium (Cs), acetylene (C2H2), etc.)
Has an absorption peak at a specific frequency determined by, and this absorption peak is used as an optical frequency reference for stabilizing the optical frequency.

FIG. 5 is a diagram showing an example of this absorption peak. As is clear from the figure, as the optical frequency (horizontal axis) changes, the transmitted light intensity (vertical axis) of the laser light that passes through the light absorption cell 2 also changes correspondingly. That is,
The optical frequency can be detected by detecting the transmitted light intensity of the laser light emitted from the light absorption cell 2 with the photodetector 3. The laser light that has entered the light absorption cell 2 and has been absorbed and transmitted by the absorption peaks of the atoms and molecules is received by the photodetector 3. The photodetector 3 outputs an electric signal (transmitted light intensity signal) corresponding to the transmitted light intensity, that is, the optical frequency. The laser light source 1 and the photodetector 3 are respectively arranged on the incident side and the emitting side of the light absorption cell 2, and the laser light emitted from the laser light source 1 and incident on the light absorption cell 2 is The light passes through the absorption cell 2 once in one direction and enters the photodetector 3. The transmitted light intensity signal output from the photodetector 3 is input to the first feedback circuit 6 and the second feedback circuit 8.

The first feedback circuit 6 is composed of a reference voltage source 5, a comparator 4 and a drive current source 7, and drives the laser light source 1 in order to suppress frequency fluctuations of low frequency components contained in the laser light. . In the reference voltage source 5, a voltage value (Vr in FIG. 5) of the photodetector output (transmitted light intensity signal) corresponding to an optical frequency to be fixed (for example, fr in FIG. 5) is set. The comparator 4 compares the output value (Vr) of the reference voltage source 5 with the transmitted light intensity signal (for example, V1 in FIG. 5) actually output from the photodetector 3 to fix the optical frequency (fr) to be fixed. And the optical frequency that is actually oscillating (f1 in Fig. 5)
Voltage (Vr-V1) corresponding to the deviation (fr-f1)
Is detected, amplified, and fed back to the drive current source 7. The drive current source 7 controls the injection current of the laser light source 1. As a result, the first feedback system (having a band of about DC to 10 kHz) is formed, and the frequency fluctuation of the low frequency component of the laser light source is suppressed. Then, the optical frequency of the laser light is stabilized at the optical frequency to be fixed within the absorption peak.

The second feedback circuit 8 is composed of a wide band amplifier 9, a phase compensator 10 and a capacitor 11, and drives the laser light source 1 in order to suppress frequency fluctuations of high frequency components contained in the laser light. . Wideband amplifier 9
Are amplified by the transmitted light intensity signal output from the photodetector 3 and input to the phase compensator 10 that compensates for the phase delay of the laser light source 1 and the feedback system. The output of the phase compensator 10 is superimposed on the injection current from the drive current source 7 and fed back to the laser light source 1 via the capacitor 11 for cutting off the low frequency band. As a result, the second feedback system (having a band of about 10 kHz to 100 MHz) is formed, and the frequency fluctuation of the high frequency component is removed. Then, the spectral line width of the laser light is narrowed.

[0007]

In the above-mentioned conventional example, the spectral line width of the narrowed laser light is 5
The frequency is about 00 kHz, which cannot be said to be sufficient for suppressing frequency fluctuations of high frequency components. In order to suppress the frequency fluctuation of this high frequency component and further narrow the spectral line width of the laser light, the frequency band of the second feedback system is widened (resulting in a smaller phase delay), It is necessary to increase the feedback gain.

To this end, the optical path length of the optical system (from the laser light source 1 through the light absorption cell 2 to the photodetector 3) and the electric circuit system (from the photodetector 3 through the second feedback circuit 8) It is most effective to shorten the electrical length (up to the laser light source 1) and widen the frequency band of the feedback system.
However, the conventional example has the following problems, which cannot be realized.

In order to shorten the optical path length, the light absorption cell 2
It is possible to shorten the length of the light absorption cell 2 (about 5 cm), but since the absorption peak by the light absorption cell 2 occurs due to the interaction between gas molecules (atoms) and light, the length of the light absorption cell 2 is 5 cm. Degree is necessary and cannot be shortened any further.

In order to shorten the electrical length, it is necessary to reduce the size of the second feedback circuit 8 by using a hybrid IC or the like, shorten the connection wiring length between the circuits, and the like. However, even if the size of the second feedback circuit 8 is reduced, the photodetector 3 is arranged because the laser light source 1 and the photodetector 3 are arranged on the incident side and the emitting side with the light absorption cell 2 interposed therebetween. As the electric length of the electric signal output from the laser light source 1 and returned to the laser light source 1, at least the connection wiring length corresponding to the length of the light absorption cell 2 (about 5 cm) is required. Therefore, the length of the light absorption cell 2 is not shortened. An object of the present invention is to solve the above problems and provide an optical frequency stabilizing light source device in which the laser light is narrowed.

[0011]

In order to solve the above problems, according to the present invention, the facts grasped by trial experiments and the like, that is, shortening the optical length is several times shorter than shortening the optical path length. The above has focused on the fact that the frequency band of the feedback system can be widened. In other words,
It is possible to widen the frequency band by shortening the electrical length even if the optical path length is lengthened. For this reason, a photodetector is arranged on the incident side of the light absorption cell, similarly to the laser light source, and further, the laser light incident on the light absorption cell from the laser light source reciprocates at least once in the light absorption cell. The light reflecting means is provided so as to form an optical path reaching the detector.

[0012]

Since both the laser light source and the photodetector are arranged on the incident side of the light absorption cell, the electrical length of the electric signal output from the photodetector and returned to the laser light source is at least the light absorption cell. Is shortened and the frequency band is widened. Further, since the laser light incident on the light absorption cell from the laser light source makes at least one round trip in the light absorption cell by the light reflecting means, the absorption amount of the laser light can be increased, and as a result, the feedback gain is reduced. growing.

[0013]

1 is a block diagram showing an embodiment of the present invention. The same components as those of the conventional example are designated by the same symbols. The laser light emitted from the laser light source 1 provided on the incident side of the light absorption cell 2 enters the light absorption cell 2 and is absorbed therein by the absorption peaks of atoms and molecules. Then, the laser light transmitted through the light absorbing cell 2 is reflected by the reflecting means 13 provided on the emission side of the light absorbing cell 2 and enters the light absorbing cell 2 again. In this embodiment, a corner mirror is used as the reflecting means 13.

The laser light which is incident on the light absorption cell 2 and is again absorbed by the absorption peaks of the atoms and molecules and transmitted therethrough is received by the photodetector 3 provided on the incident side of the light absorption cell 2. The photodetector 3 outputs an electric signal (transmitted light intensity signal) corresponding to the transmitted light intensity, that is, the optical frequency. The transmitted light intensity signal output from the photodetector 3 is input to the current source 12 for driving the laser light source 1. The current source 12 has the same configuration and function as those of the conventional example.
The feedback circuit 6 and the second feedback circuit 8 of FIG.

The first feedback circuit 6 forms the first feedback system as shown in the conventional example, and suppresses the frequency fluctuation of the low frequency component of the laser light source. Then, the optical frequency of the laser light is stabilized at the optical frequency to be fixed within the absorption peak. The frequency band of the first feedback system is DC to 10 kHz, as in the conventional example. Second
The feedback circuit 8 of (1) forms a second feedback system as shown in the conventional example, and removes the frequency fluctuation of the high frequency component of the laser light source. Then, the spectral line width of the laser light is narrowed.

In the optical frequency stabilizing light source device configured as described above, the photodetector 3 is similar to the laser light source 1 in that
Since the light absorption cell 2 is arranged on the incident side, the connection wiring length from the current source 12 to the laser light source 1 is shortened by at least the length of the light absorption cell 2. For example, if the same light absorbing cell 2 as the conventional example is used, the length is shortened by about 5 cm. As a result, the electrical length of the electrical signal output from the photodetector 3 and fed back to the laser light source 1 is shortened, so that the frequency band of the second feedback system, which has a high frequency band in particular, is widened, and conventionally, 10 kHz. The frequency band, which was about -100 MHz, has been expanded to about 10 kHz-300 MHz. Further, since the light absorbing cell 2 is transmitted twice (one round trip), the absorption amount of the laser light can be doubled, and as a result, the feedback gain can be increased.

As described above, since the frequency band of the second feedback system can be widened and the feedback gain can be increased, the frequency fluctuation of the high frequency component of the laser light can be suppressed more than the conventional example. And as a result,
The spectral line width of laser light can be narrowed. As shown in FIG. 2, when the laser light is transmitted through the same light absorption cell 2 (about 5 cm) as the conventional example four times (two round trips), the narrowed spectrum line of the laser light The width is about 150 kHz, which is about one third of the conventional example (about 500 kHz).

In the above embodiment, the laser light transmitted through the light absorption cell 2 is received by one photodetector 3, and the transmitted light intensity signal output from this photodetector 3 is divided into two. Although it is input to the first feedback circuit 6 and the second feedback circuit 8, the present invention is not limited to this. For example, after dividing the laser light from the light absorption cell 2 into two by a beam splitter or the like. , Each of them is received by two photodetectors, and the transmitted light intensity signals output from the two photodetectors are input to the first feedback circuit 6 and the second feedback circuit 8. Good. In this case, since the two photodetectors are used, photodetectors having characteristics suitable for the respective purposes can be used in the first and second feedback systems. That is, a photodetector with a narrow frequency band and little noise is used for the first feedback system, and a photodetector with a wide frequency band is used for the second feedback system.

Further, although the corner mirror is used as the reflecting means 13 in the above embodiment, the present invention is not limited to this, and a plate mirror may be used as shown in FIG.
Further, in the embodiment of FIGS. 1 to 3, the light absorption cell 2 and the reflection means 13 are configured independently of each other, but they may be configured integrally.

[0020]

As described above, in the optical frequency stabilizing light source device according to the present invention, the laser light source and the photodetector are arranged on the incident side of the light absorption cell, and further the laser light source is incident on the light absorption cell. By providing the light reflecting means for forming the optical path in which the laser light makes at least one round trip in the light absorption cell and reaches the photodetector, the following effects are obtained. The electrical length of the electrical signal output from the photodetector and returned to the laser light source via the current source can be shortened, and the laser light can be transmitted multiple times through one light absorption cell. Therefore, the frequency band of the feedback system output from the photodetector and fed back to the laser light source can be widened, and the feedback gain can also be increased. As a result, the spectral line width of the laser light output from the laser light source can be narrowed to about one third of the conventional one.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a case where a light beam makes two round trips in a light absorption cell 2.

FIG. 3 is a configuration diagram showing another embodiment of the present invention.

FIG. 4 is a configuration diagram showing a conventional example.

FIG. 5 is a diagram showing an example of an absorption peak of the light absorption cell 2.

[Explanation of symbols]

 1 Laser light source. 2 Light absorption cell. 3 Photodetector. 4 comparator. 5 Reference voltage source. 6 First feedback circuit. 7 Drive current source. 8 Second feedback circuit. 9 Wide band amplifier. 10 Phase compensator. 11 capacitors. 12 Current source. 13 Reflection means.

Claims (1)

[Claims] 1. A laser light source (1), a light absorption cell (2) having absorption characteristics at a specific frequency and receiving a laser beam emitted from the laser light source, and a laser beam transmitted through the light absorption cell. (3) for converting into an electric signal and outputting the electric signal, and a current source (12) for controlling the laser light source controlled by the electric signal output from the photodetector (3)
In the optical frequency stabilized light source device having, the laser light source and the photodetector are arranged on the incident side of the light absorption cell, further, the light incident on the light absorption cell from the laser light source is A light reflection means (1) that forms an optical path that travels at least once in the light absorption cell to reach the photodetector.
An optical frequency stabilizing light source device characterized by comprising 3).