JPH06318759A - Narrow-line width frequency stabilizing light source - Google Patents

Abstract

PURPOSE:To provide a new narrow-line width frequency stabilizing light source having a long-term frequency stability and a reproducibility, which are improved higher than those of a narrow-line width frequency stabilizing light source using an optical resonator manufactured by a prior art by a method wherein the oscillation frequency of a semiconductor laser is stabilized using a frequency discriminator and at the same time, an emitted laser beam is fed back to the laser to generate a self-injection synchronism. CONSTITUTION:A narrow-line width frequency stabilizing light source is formed into a constitution, wherein the oscillation frequency of a semiconductor laser 1 is stabilized in a slope part having the frequency characteristics of a frequency discriminator 3 by a differential type frequency stabilizing method. At the same time, one part of an emitted laser beam from the laser 1 is fed back to the laser 1 to apply a selfinjection synchronism and an oscillation spectral line width is made narrow. At this time, in order to generate stably this self-injection synchronism, a feedback optical path length regulator 12 is controlled so as to reduce the amplitude strength of frequency noise made an FM/AM conversion by the slope part having the frequency characteristics of the discriminator 3 and the position of a light feedback means 11 is adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow linewidth frequency stabilized light source using a semiconductor laser. More specifically, for example, an absorption cell enclosing atoms or molecules is used to detect the linear absorption spectrum line of the atoms or molecules, and the oscillation frequency of the semiconductor laser is stabilized in the slope portion of this linear absorption spectrum line. At the same time, the self-injection locking effect is generated by returning the emitted laser light emitted from the frequency-stabilized semiconductor laser to the inside of the semiconductor laser again, and the narrow linewidth frequency stabilized light source in which the oscillation spectrum linewidth is narrowed. Regarding INDUSTRIAL APPLICABILITY The present invention is used for various optical measurement light sources, high-accuracy optical frequency reference light sources, stabilized light sources for physical / chemical measurements, light sources for coherent optical communication, and the like.

[0002]

2. Description of the Related Art As a prior art, "Narrow spectrum line width semiconductor laser capable of frequency modulation" by Motoichi Otsu (Technical Research Report of the Institute of Electronics, Information and Communication Engineers OQE87-135, p.
p. 40) This is a confocal resonator (confocal r
esonator) to stabilize the oscillation frequency of the semiconductor laser and narrow the oscillation spectral line width.
That is, the laser light emitted from the semiconductor laser is made incident on the confocal resonator, and an optical system is assembled so that only the resonated laser light is returned to the semiconductor laser. When a semiconductor laser is injected with laser light from the outside, the oscillation frequency is pulled into the optical frequency of the injected laser light, and at the same time, a phenomenon called self-injection locking in which the oscillation spectrum line width is narrowed occurs. The oscillation frequency of the semiconductor laser can be stabilized at the resonance frequency of the confocal resonator, and the oscillation spectrum line width can be narrowed.

At this time, it is necessary to adjust the optical path length of the returned laser light in order to make the phase of the light inside the semiconductor laser and the phase of the returned laser light equal. By detecting the amount of transmitted light from the confocal resonator,
This is done by monitoring the deviation of the phase conditions, changing the position of the corner mirror placed in the optical path of the returned laser light by a piezo element, and providing a mechanism that can automatically adjust the optical path length.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a narrow linewidth frequency stabilized light source that solves the following problems in the prior art. (1) When the laser light resonated by the confocal resonator is fed back to the semiconductor laser, fluctuations in the constriction degree of the oscillation spectrum line width of the semiconductor laser due to self-injection locking are reduced due to mechanical deterioration, and at the same time, confocal resonance The optical system is complicated and expensive due to the use of a container. (2) When stabilizing the oscillation frequency of the semiconductor laser to the resonance frequency of the confocal resonator, the resonance frequency of the confocal resonator, which is the frequency reference, changes due to ambient temperature changes, vibration, or mechanical deterioration. Therefore, long-term frequency stability and reproducibility cannot be obtained at the oscillation frequency of the semiconductor laser.

[0005]

In order to solve the above-mentioned problems (1) and (2), the present invention takes the following technical means. (1) A first feedback system for stabilizing the oscillation frequency of the semiconductor laser is adopted by using the slope portion of the frequency characteristic of the frequency discriminator. (2) A second feedback system using a position-modulated plane mirror is adopted, which receives the emitted laser light emitted from the semiconductor laser and detects the amplitude intensity of the FM noise at a specific band frequency. In order to position-modulate the plane mirror so as to suppress the noise most and stabilize the self-injection locking system, the position of the plane mirror can be detected by phase-detecting the amplitude intensity of FM noise with the modulation signal emitted from the modulation signal source. The control means to control is adopted.

[0006]

[Advantages] By adopting these technical means, it has the following unique action. (1) By stabilizing the oscillation frequency of the semiconductor laser using the slope portion of the frequency characteristic of the frequency discriminator as a frequency reference, it becomes possible to use a plane mirror instead of the confocal resonator. The tolerance for ambient temperature change, vibration, or mechanical deterioration is increased, and the fluctuation of the narrowing degree of the oscillation spectrum line width of the semiconductor laser due to self-injection locking is reduced, and at the same time, the optical system becomes simple and inexpensive. (2) By adopting an optical system that returns the laser light into the semiconductor laser by using a plane mirror, it is possible to achieve a long-term oscillation frequency of the semiconductor laser like an absorption cell in which atoms or molecules are enclosed as a frequency discriminator. It has become possible to adopt a frequency reference that provides frequency stability and reproducibility.

[0007]

FIG. 1 shows an embodiment of a narrow linewidth frequency stabilized light source according to the present invention. Of the laser light emitted by the semiconductor laser 1 in two directions a and b, the laser light emitted in the a direction is branched by the first branching means 2 into two directions c and d. c
The laser light branched in the direction enters a frequency discriminator 3 which converts the frequency fluctuation of the laser light emitted from the semiconductor laser 1 into a light intensity fluctuation.

As the frequency discriminator 3, an apparatus or an etalon utilizing the absorption characteristics of atoms or molecules is used. In this embodiment, as the frequency discriminator 3, an absorption cell in which atoms or molecules that absorb laser light in a specific frequency band are enclosed is used. The laser light transmitted through this absorption cell is converted into an electric signal in the photodetector 4, and a linear absorption spectrum line a of the atom or molecule as shown in FIG. 2 is output.

The laser light emitted in the d direction is converted into an electric signal V by a photodetector used as the reference signal generating means 6. The converted electric signal V is used as a reference voltage value signal corresponding to a specific frequency fs for stabilizing the oscillation frequency of the semiconductor laser 1, and is input to the differential amplifier 5.
The electric signal output by the photodetector 4 is input to the differential amplifier 5. The semiconductor laser driving current source 7 is electrically negatively fed back to control the injection current to the semiconductor laser 1 so that the difference between the voltage values of the two signals input to the differential amplifier 5 becomes zero. The oscillation frequency of 1 is stabilized at the slope portion of the linear absorption spectrum line of the atom or molecule enclosed in the absorption cell.

As described above, the first frequency stabilizing means 8 including the frequency discriminator 3, the photodetector 4, and the reference signal generating means 6 for stabilizing the oscillation frequency of the semiconductor laser 1 is included. The frequency control band of the control system 9 is about DC to 10 kHz, and such a method for stabilizing the oscillation frequency of the semiconductor laser 1 is called a differential frequency stabilization method.

Further, the laser beam emitted from the semiconductor laser 1 in the b direction is emitted in the two directions e by the second branching means 10.
It is branched to f. The laser light emitted in the e direction is reflected on the same optical path by the optical feedback means 11 and returned to the inside of the semiconductor laser 1. Normally, a plane mirror is used as the optical feedback means 11. The laser light branched in the f direction is
It is used as output light.

At this time, the optical frequency noise FM / AM converted by the slope portion of the linear absorption spectrum line of the atom or molecule is output at the photodetector 4, but the second branching means 10 is provided by the optical feedback means 11. When the phase of the laser light fed back into the semiconductor laser 1 through is adjusted, self-injection locking occurs, the optical frequency noise is suppressed, and the oscillation spectrum line width of the semiconductor laser 1 can be narrowed.

In this way, the semiconductor laser 1 and the optical feedback means 11 constitute a self-injection locking system. Regarding the phase adjustment of the laser light fed back into the semiconductor laser 1,
Describe in detail. The feedback optical path length adjuster 12 is attached to the rear of the optical feedback means 11, and the feedback optical path length adjuster controller 13 controls the feedback optical path length adjuster 12 to thereby provide the optical feedback means 1.
1 is moved in the laser optical axis direction to change the distance between the semiconductor laser 1 and the optical feedback means 11.

Usually, a piezo element is used for the feedback optical path length adjuster 12, and a piezo element controller is used for the feedback optical path length adjuster controller 13. When the optical feedback means 11 is swept, the amplitude intensity of optical frequency noise is periodically increased and decreased in the photodetector 4. The output of this photodetector 4 is shown in FIG.
When self-injection locking is applied by adjusting the position of the optical feedback means 11, the oscillation spectrum line width is narrowed. This state corresponds to the portion where the amplitude intensity of the optical frequency noise in FIG. The deviated state corresponds to a large part of the amplitude intensity of the optical frequency noise. Therefore, it is necessary to control the position of the optical feedback means 11 in order to maintain the self-injection locked state.

In the present invention, the output of the photodetector 4 is branched into two, one of which is input to the above-mentioned differential amplifier 5 for stabilizing the laser frequency, and the other of which is input to the detection means 14. The detection means 14 is a frequency band 1M capable of detecting a change in amplitude intensity between when the self-injection locking is applied and when the self-injection locking is out of the optical frequency noise, with a good S / N ratio.
The filter 15 is configured to transmit only an electric signal corresponding to the optical frequency noise of Hz to 100 MHz, and the level meter 16 for outputting a value corresponding to the amplitude intensity of the optical frequency noise transmitted through the filter 15.

The optical frequency noise of a specific band transmitted through the filter 15 enters the level meter 16, and the amplitude intensity of the optical frequency noise is obtained as the level meter output VL. Here, if the filter 15 is not used, the level meter 16 detects the total optical frequency noise amount of the semiconductor laser 1, but this total optical frequency noise amount is before and after the oscillation spectrum line width of the semiconductor laser 1 is narrowed. Since the amounts are the same, it is not possible to maintain a state in which self-injection locking is applied and optical frequency noise is most reduced. Therefore, the filter 15 is an indispensable component in the detection means 14.

FIG. 4A shows the position of the optical feedback means 11, FIG. 4B shows the optical frequency noise with respect to the position of the optical feedback means 11, and FIG. 4C shows the output by the level meter 16. Amplitude strength VL of the optical frequency noise corresponding to the position of the optical feedback means 11, FIG. 4D is a modulation signal from the modulation signal source 17 for performing position modulation on the optical feedback means 11, and the amplitude strength of the optical frequency noise is phased. By detecting the phase detector 18
The differential value output VR of the level meter output VL obtained by As can be seen from FIG. 4, the feedback optical path length adjuster controller 13 is electrically negative so that the differential value output VR of the phase detector 18 becomes zero (corresponding to D1, D2, D3 in FIG. 4D). When feedback is applied, optical frequency noise is most suppressed. The phase detection will be described in detail below.

When the position Dh of the optical feedback means 11 is modulated by the modulation signal output from the modulation signal source 17, the level meter 16 displays the positions as shown in FIGS. 5 (a), 5 (b) and 5 (c). The output Vh is obtained. That is, the level meter 16
When the position Dh of the optical feedback unit 11 is on the right side of the position D1 where the output of the level meter is minimum,
As shown in FIG. 5A, when the position Dh of the optical feedback unit 11 is on the left side of the position D1 at which the level meter output is minimum, the result is as shown in FIG. 5B. When the position Dh of the optical feedback unit 11 matches D1, the result is as shown in FIG.

The output Vh of the level meter 16 is phase-detected by the modulation signal from the modulation signal source 17 and the phase detector 18, and two kinds of different amplitudes with respect to the center position Dh of the position-modulated optical feedback means 11 are detected. When the amplitude difference of the peaks is monitored, the phase detector 18 can obtain the differential value output of the amplitude intensity of the optical frequency noise with respect to the position change of the optical feedback unit 12 output by the level meter 16. Position D of optical feedback means 11
When h is in the vicinity of the position D1 at which the level meter output is minimum, the difference between the position Dh of the optical feedback unit 11 and the position D1 at which the level meter output is minimum and the phase detector output VR are: There is a linear relationship, and if the phase detector output VR is electrically negatively fed back to the feedback optical path length adjuster controller 13 so that the phase detector output VR becomes zero, the position Dh of the optical feedback means 11 becomes. Position D where the level meter output becomes the minimum
It will be stabilized at 1, and the optical frequency noise will be suppressed most.

As described above, the control means 1 including the feedback optical path length adjuster controller 13, the modulation signal source 17, and the phase detector 18
9 makes it possible to suppress the optical frequency noise of the semiconductor laser 1. A second control system 20 including this control means 19
Thus, if the optical path length L from the semiconductor laser 1 to the optical feedback means 11 is controlled, frequency noise from DC to C / 2L [Hz] (C represents the speed of light in air) can be suppressed. In such a self-injection locking system, the oscillation spectrum line width of the semiconductor laser 1 becomes narrower as the optical path length L from the semiconductor laser 1 to the optical feedback means 11 is increased.

Further, in this embodiment, the laser light emitted from the semiconductor laser 1 was reflected by the optical feedback means 11 and returned to the inside of the semiconductor laser 1. However, as shown in FIG. It is also possible to make the light incident on the inside of the fiber 21 whose one end is mirror-coated, to be reflected by this coating surface and to be returned to the inside of the semiconductor laser 1. In this case, the fiber 21 is wound around the return optical path length adjuster 12, and the return optical path length adjuster 12
By controlling the feedback optical path length adjuster controller 13,
Adjust the optical path length with the waveguide in the fiber 21 as the optical path,
By adjusting the phase of the laser light to be fed back, self-injection locking can be generated and the oscillation spectrum line width of the semiconductor laser 1 can be narrowed.

As described above, the laser frequency of the semiconductor laser 1 is stabilized at the slope portion of the linear absorption spectrum line of the atom or molecule, and at the same time, the frequency-stabilized semiconductor is stabilized at the slope portion of the linear absorption spectrum line of the atom or molecule. By returning the laser light emitted from the laser 1 to the semiconductor laser 1, a narrow linewidth frequency stabilized light source with a narrow oscillation spectrum linewidth was realized.

[0023]

As described above, the narrow linewidth frequency stabilizing light source according to the present invention serves as a frequency discriminator for converting the frequency fluctuation amount of laser light into the light intensity fluctuation amount, for example, an absorption line of an atom or a molecule. Since it can also be used, when an optical frequency is stabilized on such an absorption line, and when the frequency-stabilized emitted laser light is fed back to the semiconductor laser, an optical feedback means is provided at a position where optical frequency noise is most suppressed. The self-injection locking is generated by matching and the oscillation spectrum line width of the semiconductor laser is narrowed, so that the following effects are obtained. (1) By stabilizing the oscillation frequency of the semiconductor laser by using the slope portion of the frequency characteristic of the frequency discriminator as the frequency reference, it becomes possible to use a plane mirror instead of the confocal resonator. The tolerance for the temperature change, vibration, or mechanical deterioration is increased, and the fluctuation of the narrowing degree of the oscillation spectrum line width of the semiconductor laser due to self-injection locking is reduced, and at the same time, the optical system becomes simple and inexpensive. (2) By adopting an optical system that returns laser light into the semiconductor laser using a plane mirror, long-term frequency stability against the oscillation frequency of the semiconductor laser, such as an absorption cell in which atoms or molecules are enclosed as a frequency discriminator It has become possible to adopt a frequency reference that gives good performance and reproducibility.

[Brief description of drawings]

FIG. 1 shows an embodiment of the configuration of a narrow linewidth frequency stabilized light source according to the present invention.

FIG. 2 shows the linear absorption spectral line of an atom or molecule encapsulated in an absorption cell output at a photodetector.

FIG. 3 shows the FM / AM due to the slope portion of the linear absorption spectrum line of atoms or molecules when the position of the optical feedback means is changed.
A state of the converted optical frequency noise is shown.

FIG. 4A shows the position of an optical feedback unit.

FIG. 4B shows optical frequency noise with respect to the position of the optical feedback unit.

FIG. 4C shows the amplitude intensity of optical frequency noise.

FIG. 4 (d) shows a differential waveform of the level meter output obtained by the phase detector.

FIG. 5 shows a level meter output waveform when the position of the optical feedback unit is modulated by the modulation signal output from the modulation signal source.

FIG. 6 shows an optical system in which a laser beam emitted from a semiconductor laser is incident on a fiber, reflected by a coating surface, and then re-incident inside the semiconductor laser to narrow the oscillation spectrum line width of the semiconductor laser.

[Explanation of symbols]

 1: Semiconductor laser 2: First branching means 3: Frequency discriminator (absorption cell) 4: Photodetector 5: Differential amplifier 6: Reference signal generating means 7: Semiconductor laser driving current source 8: Frequency stabilizing means 9 : First control system 10: Second branching means 11: Optical feedback means 12: Returning optical path length adjuster 13: Returning optical path length adjuster controller 14: Detection means 15: Filter 16: Level meter 17: Modulation signal source 18: Phase detector 19: Control means 20: Second control system 21: Fiber

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[Procedure amendment]

[Submission date] May 13, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 shows an embodiment of the configuration of a narrow linewidth frequency stabilized light source according to the present invention.

FIG. 2 shows the linear absorption spectral line of an atom or molecule encapsulated in an absorption cell output at a photodetector.

FIG. 3 shows the FM / AM due to the slope portion of the linear absorption spectrum line of atoms or molecules when the position of the optical feedback means is changed.
A state of the converted optical frequency noise is shown.

FIG. 4A shows the position of an optical feedback unit. (B) Represents optical frequency noise with respect to the position of the optical feedback means. (C) Represents the amplitude strength of optical frequency noise. (D) Shows the differential waveform of the level meter output obtained by the phase detector.

FIG. 5 shows a level meter output waveform when the position Dh of the optical feedback unit is on the right side of the position D1 at which the output of the level meter is minimized.

FIG. 6 shows a level meter output waveform when the position Dh of the optical feedback unit is on the left side of the position D1 where the output of the level meter is minimum.

FIG. 7 shows a level meter output waveform when the position Dh of the optical feedback unit coincides with the position D1 where the output of the level meter becomes minimum.

FIG. 8 shows an optical system for narrowing the oscillation spectrum line width of a semiconductor laser by making laser light emitted from a semiconductor laser incident on a fiber, reflecting it by a coating surface, and making it enter the inside of the semiconductor laser again.

[Description of Reference Signs] 1 semiconductor laser 2 first branching means 3 frequency discriminator (absorption cell) 4 photodetector 5 differential amplifier 6 reference signal generating means 7 semiconductor laser drive current source 8 frequency stabilizing means 9 first Control system 10 Second branching means 11 Optical feedback means 12 Feedback optical path length adjuster 13 Feedback optical path length adjuster controller 14 Detection means 15 Filter 16 Level meter 17 Modulation signal source 18 Phase detector 19 Control means 20 Second control System 21 fiber

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 8]

[Figure 4]

[Figure 5]

[Figure 6]

[Figure 7]

Claims (2)

[Claims] 1. A semiconductor laser (1), a frequency discriminator (3) for stabilizing the oscillation frequency of the semiconductor laser, and converting a frequency fluctuation of at least laser light emitted from the semiconductor laser into a light intensity fluctuation, and Photodetector that converts the laser light intensity transmitted through the frequency discriminator into an electrical signal and outputs it (4)
A frequency stabilizing means (8), a light feedback means (11) for receiving a part of the laser light emitted from the semiconductor laser and returning the laser light to the semiconductor laser, and emitting the light from the semiconductor laser. A feedback optical path length adjuster (12) for varying the optical path length of the reflected laser light before returning to the semiconductor laser through the optical feedback means, and an electric signal of a desired band from the electric signal output from the photodetector. A detection means (14) for transmitting only a signal and measuring the amplitude intensity of the transmitted electric signal, and a control means for receiving the output of the detection means and controlling the feedback optical path length adjuster to reduce the amplitude intensity. (19) A narrow linewidth frequency-stabilized light source comprising: 2. The narrow linewidth frequency stabilized light source according to claim 1, wherein the frequency discriminator (3) is an absorption cell enclosing atoms or molecules exhibiting absorption at a predetermined frequency.