JP2501484B2 - Wavelength stabilization laser device - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength-stabilized laser device suitable for use as a wavelength reference in, for example, coherent optical communication or optical measurement, and particularly to wavelengths of resonance lines and absorption lines of atomic or molecular gas and optical interferometer The present invention relates to a wavelength-stabilized laser device capable of oscillating laser light synchronized with the reference wavelength. [Prior Art] FIG. 4 is a block diagram showing a configuration of a conventional wavelength stabilized laser device. In FIG. 4, reference numeral 1 is a semiconductor laser, 2 is a wavelength reference medium, 3 is a light receiver, 4 is a lock-in amplifier, and 5 is an oscillator. A small amount of the semiconductor laser 1 is directly modulated (frequency modulated) by the oscillator 5. The modulated emitted light 6 from one end surface of the semiconductor laser 1 passes through the wavelength reference medium 2 and is photoelectrically converted by the light receiver 3 to be converted into an electric signal. This electric signal is compared with the output signal of the oscillator 5 by the lock-in amplifier 4 and processed to obtain an error signal. This error signal is fed back to the semiconductor laser 1, and the drive current of the semiconductor laser 1 is changed based on the error signal, whereby the above-mentioned wavelength reference medium 2 is supplied.
The central oscillation wavelength of the semiconductor laser 1 is synchronized with the reference wavelength of 1 to stabilize the laser light. Here, as the above-mentioned wavelength reference medium, resonance lines of atoms such as krypton, absorption lines of molecules such as ammonia, and optical interferometers such as an optical Fabry-Perot interferometer are used.

[Problems to be Solved by the Invention]

However, in such a conventional wavelength-stabilized laser device, when a resonance line of an atom having a large wavelength interval is generally used as a wavelength reference medium, several resonances are used within a limited wavelength band used in optical communication and the like. There was a drawback that only lines could be used. Further, when utilizing the absorption spectrum of a molecule having a large number of absorption lines, the wavelength interval is relatively large and the wavelength of the laser beam cannot be set to an arbitrary wavelength in the absorption spectrum. There was also. Furthermore, when using a Fabry-Perot interferometer, it can be set to any wavelength by setting the resonator length, but the optical path length changes due to disturbance such as changes in ambient temperature, and the resonance point shifts. It had the drawback of changing. When using an interferometer like this,
There is a problem with stability as compared with the case of using the resonance line of atoms or the absorption line of molecules. An object of the present invention is to provide a wavelength stabilized laser device capable of stabilizing laser light at an arbitrary wavelength in order to solve the above technical problem.

[Means for Solving the Problems]

The present invention relates to a first semiconductor laser, a first optical frequency modulation means for frequency-modulating a part of light emitted from the first semiconductor laser, and a predetermined one of frequency-modulated light by the optical frequency modulation means. A medium that absorbs only light of a wavelength, a first photodetector that photoelectrically converts light that has passed through the medium, an input signal from the photodetector, and a reference signal from the first optical frequency modulation means. And a first lock-in amplifier for synchronizing the wavelength of the emitted light of the first semiconductor laser with the absorption wavelength of the medium by controlling the first optical frequency modulation means based on An optical interferometer having a plurality of resonance wavelength peaks and receiving light emitted from the first semiconductor laser synchronized with the absorption wavelength of the medium; a means for adjusting an optical path length of the optical interferometer; The second that photoelectrically converts the transmitted light from the optical interferometer The optical path length adjusting means is controlled on the basis of a detector, an input signal from the photodetector, and a reference signal from the first optical frequency modulating means of the reference laser section, and the optical path length adjusting means is controlled. An interferometer unit including a second lock-in amplifier that synchronizes any one of the resonance wavelength peaks of the optical interferometer with the wavelength of the emitted light;
Optical semiconductor laser, second optical frequency modulating means for frequency-modulating a part of light emitted from the second semiconductor laser to a frequency different from that of the first optical frequency modulating means, and optical interference of the interferometer section. Based on the input signal obtained by photoelectrically converting the remaining part of the light emitted from the second semiconductor laser that has passed through the meter by the second photodetector and the reference signal from the second optical frequency modulation means. A third optical frequency modulating means is controlled to synchronize the wavelength of the emitted light from the second semiconductor laser with one of the resonance wavelength peaks of the optical interferometer of the interferometer section. It is characterized by having a lock-in amplifier. [Operation] In the present invention, the optical interferometer is stabilized by using the absorption line of molecules, and the stabilized optical interferometer is used in the conventional method.
The oscillation wavelength of the semiconductor laser can be stabilized at an arbitrary wavelength within the resonance wavelength peak of the optical interferometer, which could not be stabilized. [Examples] Hereinafter, the present invention will be described in detail based on Examples shown in the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the wavelength stabilized laser device of the present invention. As shown in FIG. 1, the wavelength-stabilized laser device of this example has a reference laser section 8 that oscillates a laser beam at the absorption wavelength of a light absorbing medium and an optical interference between the wavelength of the laser beam from the reference laser section 8. An interferometer unit 9 that synchronizes and stabilizes any one of the resonance wavelength peaks of the meter and maintains the optical path length by adjusting the temperature of the optical interferometer to stabilize the resonance wavelength peak of the optical interferometer.
And a stabilizing laser unit 10 that oscillates laser light at any wavelength of the resonance wavelength peaks of the stabilized optical interferometer. The reference laser unit 8 photoelectrically converts the semiconductor laser 11, a wavelength reference medium 14 that absorbs only light of a predetermined wavelength λ ₁ out of the light emitted from the semiconductor laser 11, and the transmitted light that has passed through the wavelength reference medium 14. A photodetector 111 for conversion, an oscillator 113 for directly modulating the semiconductor laser 11 described above, and the photodetector 1 described above.
The electric signal from the oscillator 11 is compared with the reference signal from the oscillator 113 so that the wavelength of the emitted light from the semiconductor laser 11 becomes the absorption wavelength λ ₁ of the wavelength reference medium 14 described above. And a lock-in amplifier 15 for controlling the. As the wavelength reference medium 14 described above, a light absorbing gas such as ordinary acetylene gas, ammonia gas, methane gas, carbon dioxide, or isotope-substituted acetylene gas is preferably used. The oscillation wavelength can be stabilized by using any of the gases. The interferometer unit 9 has a ring resonator 13 having a plurality of resonance wavelength peaks and receiving the emitted light from the semiconductor laser 11 via the optical coupler 110, and adjusts the temperature environment of the ring resonator 13. A temperature controller (optical path length adjusting means) 18 for adjusting the optical path length of the ring resonator 13, and a photodetector 112 for photoelectrically converting the transmitted light transmitted through the ring resonator 1 into an electric signal. Among the electric signals from the photodetector 112, the electric filter 115 that selects only the electric signal corresponding to the wavelength of the emitted light from the semiconductor laser 11 described above.
Then, the electric signal selected by the electric filter 115 is compared with the reference signal from the oscillator 113 of the standard laser unit 8 and one of the resonance wavelength peaks of the ring resonator 13 is emitted from the semiconductor laser 11. The temperature controller 18 is controlled so as to be synchronized with the wavelength λ ₁ of the emitted light, and the ring resonator 13 is controlled by this control.
The lock-in amplifier 16 stabilizes the optical path length of the. Further, the stabilizing laser unit 10 includes a second semiconductor laser 12
An external modulator 19 for frequency-modulating the laser light from the semiconductor laser 12, an oscillator 114 having a different frequency from the oscillator 113 for driving the external light modulator 19, and the second semiconductor laser 12 for the above. The light emitted from the optical coupler
It corresponds to the wavelength λ ₂ in the resonance wavelength peak of the ring resonator 13 other than the wavelength λ _{1 in} the electric signal obtained by photoelectrically converting the transmitted light through the ring resonator 13 via 110. An electric filter 116 for selecting only electric signals, an electric signal selected by the electric filter 116, and the oscillator 114 described above.
The wavelength of the light emitted from the semiconductor laser 12 is compared with the reference signal from the
The third lock-in amplifier 17 controls the oscillator 114 so as to synchronize with the _second lock-in amplifier 2. The light output from both ends of the reference laser section 8 and the stabilization laser section 10 are superposed by the optical coupler 110 as described above and sent to the interferometer section 9. Next, the operation of the wavelength stabilized laser device having the above-mentioned configuration will be described according to the first. The semiconductor laser 11 is directly modulated by the oscillator 113, and the emitted light is frequency-modulated. A part of the emitted light 118 passes through the wavelength reference medium 14, and the transmitted light absorbs only the light having the wavelength λ ₁ as shown in FIG. 2 (a).
This transmitted light is photoelectrically converted by the light receiver 111, and its electric signal is sent to the lock-in amplifier 15 as an input signal. On the other hand, the lock-in amplifier 15 is supplied with the reference signal from the oscillator 113. The lock-in amplifier 15 compares the above-mentioned input signal with the reference signal, and detects the deviation from the wavelength reference absorption line as an error signal. This error signal is fed back to the semiconductor laser 11, and the oscillation wavelength is stabilized so that the emitted light has the wavelength λ ₁ . The stabilized light 119 having a wavelength of λ ₁ obtained in this way is incident on the ring resonator 13 from the semiconductor laser 11 via the optical fiber and the optical fiber coupler 110. The ring resonator 13 has an absorption characteristic for each frequency corresponding to the resonator length, as shown in FIG. Here, one of the plurality of absorption lines is adjusted to the wavelength λ ₁ of the absorption line of FIG. 2 (a). That is,
The output light 119 of the semiconductor laser 11 stabilized at the wavelength λ ₁ is transmitted through the ring resonator 13, the transmitted light is photoelectrically converted by the light receiver 112, and only the signal is selected by the electric filter 115, and the lock-in amplifier 16 Sent to. On the other hand, the lock-in amplifier 16 is supplied with the output signal from the above-mentioned oscillator 113 as a reference signal. Therefore, in the lock-in amplifier 16, the deviation from the absorption line in FIG.
Lock-in is detected as an error signal. This error signal is sent from the lock-in amplifier 16 to the temperature controller 18 of the ring resonator 13, and one wavelength of the absorption line shown in FIG. 2 (a) becomes the wavelength λ ₁ shown in FIG. As ring resonator 13
The optical path length is controlled and stabilized by adjusting the temperature environment of. The ring resonator 13 thus stabilized is used to stabilize the oscillation wavelength of the second semiconductor laser 12. That is, the light emitted from one side of the semiconductor laser 12 is frequency-modulated in the same manner as in the case of the first semiconductor laser 11 by using the external frequency modulator 19 driven by the oscillator 114 having a frequency different from that of the oscillator 113. This modulated light passes through the optical coupler 110 and enters the ring resonator 13. The transmitted light from the ring resonator 13 is photoelectrically converted by the light receiver 112, and the signal thereof is selected by the electric filter 116 only the signal corresponding to one wavelength λ ₂ of the resonance wavelength peaks of the ring resonator 13 described above. It The deviation between the selected signal and the resonance frequency of the ring resonator 13 is lock-in detected by the lock-in amplifier 17 as an error signal. This error signal is fed back to the semiconductor laser 12, whereby the oscillation wavelength of the semiconductor laser 12 is synchronized and stabilized with the wavelength λ ₂ of FIG. 2B, for example. Further, it is possible to stabilize at any resonance point of the resonance frequency shown in FIG. In the embodiment described above, as shown in FIG.
Although the direct modulation method by the oscillator 113 was used for the modulation of 11 and the external light modulation method was used for the modulation of the semiconductor laser 12, the object and effect of the present invention can be achieved by using any modulation method for each semiconductor laser. be able to. The same effect can be obtained by using a modulator such as an acousto-optic modulator, a LiNbO ₃ modulator, or an electro-optic modulator as the external light modulator. Further, in the above-mentioned embodiment, the temperature controller 18 is used as the optical path length adjusting means of the ring resonator 13, but not limited to this,
Any means that can adjust the optical path length of the ring resonator 13 can be used. Further, in the above-described embodiment, the one-sided optical outputs of the reference laser unit 8 and the stabilizing laser unit 10 are superposed by the optical coupler 110 and sent to the interferometer unit 9. However, these three components are mutually connected. The optical coupling means is not limited to the above-mentioned optical coupler, and other optical coupling means such as an optical fiber may be used for coupling. (Experimental Example) In the device shown in FIG. 1, as the semiconductor lasers 11 and 12, InGaAsP-based distributed feedback semiconductor lasers (DFB LDs) oscillating at a wavelength of 1.5500 μm were used. Cell length 5
An isotope-substituted acetylene gas ( ¹³ C ₂ H ₂ ) as a light absorption medium was filled in a cm absorption cell 14 at 10 Torr. FIG. 3 is a diagram showing absorption characteristics of acetylene gas and isotope-substituted acetylene gas, respectively. Cell length 10 cm, pressure 760 Tor
It was r. Of the absorption lines of isotope-substituted acetylene gas,
Absorption line with wavelength 1.54949μm (full width at half maximum 800MHz, absorption intensity 10
%) To the wavelength of the semiconductor laser 11. Oscillator 11
The frequency of 3 was set to 10 kHz, and the fluctuation of the central oscillation wavelength of the semiconductor laser 11 was adjusted to 1 × 10 ^-5 cm (optical frequency of 1 MHz) or less by using the configuration system of FIG. The wavelength-stabilized light thus obtained was made incident on the ring resonator 13 via the optical fiber coupler 110. Here, as the ring resonator 13, a silica-based waveguide ring resonator having a direct line of 13 mm, a loss of 0.04 dB / cm, and a finesse of 30 was used. The resonance frequency interval of this ring resonator is 5 GHz. Also,
Change in refractive index with temperature is ^-5 GHz at 10 ^-5 / ° C per 1 ° C
It is possible to shift the resonance wavelength peak of. Semiconductor laser
The 11-kHz modulated light component of 11 was passed through an electric filter 115 having a 10-kHz bandpass characteristic, and lock-in detection was performed by a lock-in amplifier 16 to stabilize the resonator length of the ring resonator 13. In this system, one of the resonance peaks of the ring resonator 13 was aligned with the absorption line having a wavelength of 1.54949 μm (optical frequency: 193612 GHz). The frequency stability of the resonance peak was 1MHz. Further, the one-sided output light of the semiconductor laser 12 was made incident on the acousto-optic modulator 19, frequency-modulated with a modulation width of 120 NHz at 100 kHz, and made incident on the stabilized ring resonator 13. The 100 kHz modulated light component of the semiconductor laser 12 is passed through an electric filter 116 having a 100 kHz bandpass characteristic, and the lock-in amplifier 1
Lock-in detection was performed at 7, and stabilization was performed at one resonance peak of the ring resonator 13, at a wavelength of 1.54953 μm (optical frequency 193607 GHz). By using this configuration system, the fluctuation of the central oscillation wavelength of the semiconductor laser 12 could be suppressed to 1 × 10 ⁻⁵ cm (optical frequency of 1 MHz) or less. In this experimental example, the resonance frequency interval of the ring resonator is 5G.
Since the frequency is set to Hz, it is possible to stabilize the oscillation wavelength of the semiconductor laser at any frequency of 5 GHz. Furthermore, by arbitrarily selecting the resonator length of the ring resonator,
Stabilization at any wavelength is also possible. In particular, if the acetylene gas shown in FIG. 3 is used as the light absorbing medium in addition to the isotope-substituted acetylene gas, 1.
Stabilization is possible with continuous wavelengths between absorption lines localized over a wide range from 50 μm to 1.56 μm. As a result, it is possible to eliminate the frequency error due to the influence of the chromatic dispersion characteristic of the ring resonator, which occurs when using one absorption line. Although the present invention has been specifically described based on the above examples, the present invention is not limited to the above examples,
It goes without saying that various modifications can be made without departing from the spirit of the invention. [Effects of the Invention] As described above, according to the present invention, the oscillation wavelength of the semiconductor laser can be synchronized with an arbitrary wavelength with extremely high accuracy and can be stabilized, so that a wavelength standard light source in coherent optical communication or There is an advantage that it can be used as a light source in optical measurement.

[Brief description of drawings]

FIG. 1 is a structural block diagram showing an embodiment of a wavelength stabilized laser device of the present invention, FIG. 2 is a graph showing the light intensity of light transmitted through a light absorbing gas and a ring resonator, and FIG. 3 is acetylene. A graph showing the light absorption characteristics of gas and isotope-substituted acetylene gas, and FIG. 4 is a configuration block diagram showing a wavelength-stabilized laser device using a conventional light absorption cell. 1 ... Semiconductor laser, 2 ... Absorption cell (wavelength reference medium), 3 ... Photoreceiver, 4 ... Lock-in amplifier, 5 ...
Oscillator, 8 ... Reference laser section, 9 ... Interferometer section, 10 ...
Stabilizing laser unit, 11 ... first semiconductor laser, 12 ... second semiconductor laser, 13 ... ring resonator (optical interferometer),
14 ... Absorption cell (wavelength reference medium), 15 ... First lock-in amplifier, 16 ... Second lock-in amplifier, 17 ... Third lock-in amplifier, 18 ... Temperature controller (optical path length) Adjusting means), 19 ... External modulator (optical frequency modulating means), 110
...... Optical coupler, 111 …… Receiver (first photodetector), 1
12 ... Photoreceiver (second photodetector), 113 ... Oscillator (optical frequency modulation means), 114 ... Oscillator (optical frequency modulation means), 115,116 ... Electrical filters.

Claims (1)

(57) [Claims] 1. A first semiconductor laser, a first optical frequency modulation means for frequency-modulating a part of light emitted from the first semiconductor laser, and a predetermined one of frequency-modulated light by the optical frequency modulation means. Medium for absorbing only the light of the wavelength, a first photodetector for photoelectrically converting the light transmitted through the medium, an input signal from the photodetector, and a reference signal from the first optical frequency modulation means. And a first lock-in amplifier for synchronizing the wavelength of the emitted light of the first semiconductor laser with the absorption wavelength of the medium on the basis of An optical interferometer having a plurality of resonance wavelength peaks and receiving emitted light from the first semiconductor laser synchronized with the absorption wavelength of the medium, and means for adjusting an optical path length of the optical interferometer,
A second photodetector for photoelectrically converting the transmitted light from the optical interferometer, an input signal from the photodetector, and a reference signal from the first optical frequency modulating means of the reference laser section. An interferometer including a second lock-in amplifier that controls the optical path length adjusting means to synchronize any one of the resonance wavelength peaks of the optical interferometer with the wavelength of the emitted light of the first semiconductor laser. Section, a second semiconductor laser, a second optical frequency modulation means for frequency-modulating a part of light emitted from the second semiconductor laser to a frequency different from that of the first optical frequency modulation means, and the interference. Reference from the second optical frequency modulator and an input signal obtained by photoelectrically converting the remaining part of the emitted light from the second semiconductor laser that has passed through the optical interferometer of the measuring section by the second photodetector. And the second optical frequency change based on the signal A third lock-in amplifier for controlling the adjusting means to synchronize the wavelength of the light emitted from the second semiconductor laser with one of the resonance wavelength peaks of the optical interferometer of the interferometer section. A wavelength-stabilized laser device characterized by the above.