JPH04307983A - Multiwavelength stabilizing laser - Google Patents

Abstract

PURPOSE:To make laser beams from semiconductor lasers incident respectively on stabilized optical interferometers and after that, to select signals of prescribed wavelengths to stabilize the oscillation wavelengths of the lasers and at the same time, to make it possible to settle respectively the oscillation wavelengths of a plurality of lasers to an arbitrary wavelength. CONSTITUTION:The oscillation wavelength of a semiconductor laser 11 is stabilized into the absorption wavelength lambda1 of a wavelength reference medium 14 by a lock-in amplifier 15. Then, the emitted light of the laser 11 is transmitted to a ring resonator 13 via an optical coupler 110 and a signal only of the wavelength lambda1 is selected by a wavelength filter 115. Then, the optical path length of the resonator 13 is stabilized by a lock-in amplifier 16. On the other hand, the emitted light of a semiconductor laser 12 is subjected to frequency modulation by a frequency modulator 19, the modulated light is transmitted to the coupler 110 and the resonator 13 and a signal of a wavelength lambda2 is selected in a wavelength filter 116. The oscillation wavelength of the laser 12 is stabilized into the wavelength lambda2 by a photodetector 118, an oscillator 114 and a lock-in amplifier 17.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is based on the resonance lines and absorption lines of atomic or molecular gases and the wavelength of an optical interferometer, for use as a light source in, for example, large-capacity coherent optical frequency division multiplexing optical communication and optical measurement. , relates to a multi-wavelength stabilized laser device constructed by synchronizing a plurality of laser beams with the reference.

0002

2. Description of the Related Art FIG. 10 is a block diagram showing the configuration of a conventional wavelength stabilized laser device. In FIG. 10, code 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. Semiconductor laser 1
is directly modulated by the oscillator 5 by a minute amount. This modulated light 6 emitted from one end face of the semiconductor laser 1 (the light 7 emitted from the other end face is used as a light source light to be supplied to the outside) passes through a wavelength reference medium 2 and is photoelectrically converted in a light receiver 3. can be converted into an electrical signal. This electrical signal is compared with the output signal of the oscillator 5 and processed by the lock-in amplifier 4 to obtain an error signal. This error signal is generated by the semiconductor laser 1
By changing the drive current of the semiconductor laser 1 based on the error signal, the center oscillation wavelength of the semiconductor laser 1 is synchronized with the reference wavelength of the wavelength reference medium 2 described above, and the laser beam is stabilized. ing. Here, as the above-mentioned wavelength reference medium, a resonance line of an atom such as krypton, an absorption line of a molecule such as ammonia, an optical interferometer such as an optical Fabry-Perot interferometer, etc. are used.

0003

[Problems to be Solved by the Invention] However, in such conventional wavelength-stabilized laser devices, when atomic resonance lines are used, the wavelength spacing of the atomic resonance lines is generally large. Only a few absorption lines are available in the wavelength range. Furthermore, although there are many absorption lines of molecules with relatively small intervals, it is not possible to arbitrarily set multiple absorption lines at the same time. Furthermore, Fabry-Perot interferometers can be set to any wavelength by setting the resonator length, but the optical path length changes with changes in ambient temperature.
The resonance characteristics will fluctuate. Therefore, there is a problem in terms of stability compared to atomic resonance lines and molecular absorption lines.

[0004] The present invention was made to solve these problems, and it is possible to obtain stability similar to that obtained by using molecular absorption lines and to be able to stabilize at multiple wavelengths simultaneously. The purpose is to provide a practical multi-wavelength stabilized laser device.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a reference laser section consisting of a wavelength stabilized laser whose oscillation wavelength is stabilized using a medium that absorbs only light of a predetermined wavelength as a wavelength reference. , an interferometer section comprising an optical interferometer having a plurality of resonant wavelength peaks and whose optical path length is stabilized using the emitted light of the wavelength stabilized laser, and a semiconductor laser;
a wavelength filter that transmits a laser beam from the semiconductor laser through the interferometer and then optically selects a signal having a wavelength of one of a plurality of resonant wavelength peaks of the interferometer from the laser beam; and a plurality of stabilizing laser sections having means for stabilizing the oscillation wavelength of the semiconductor laser based on the selected signal.

[0006]

[Operation] According to the present invention, for example, using absorption lines of molecules,
By stabilizing an optical interferometer and using the stabilized optical interferometer, it is possible to simultaneously adjust multiple oscillation wavelengths of semiconductor lasers at arbitrary multiple wavelengths between the absorption lines of molecules, which could not be stabilized so far. Stabilize.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Embodiment 1] FIG. 1 is a block diagram showing a first embodiment of a multi-wavelength stabilized laser device according to the present invention. As shown in FIG. 1, this embodiment includes a reference laser section 8 that oscillates at the absorption wavelength of a light absorption medium, an interferometer section 9 that stabilizes the optical path length of the optical interferometer, and a It mainly consists of a stabilized laser section 10 that oscillates at any wavelength of the resonance peak of the interferometer.

The reference laser section 8 includes a semiconductor laser 11 and
A wavelength reference medium 14 that absorbs only light of a predetermined wavelength, a photodetector 111, and an oscillator 11 for directly modulating the laser.
3 and a lock-in amplifier 15. The interferometer section 9 includes a ring resonator (optical interferometer) 13 that can vary the optical path length, a temperature controller (temperature controller) 18 as an optical path length adjusting means, a photodetector 117, and a lock-in amplifier 16. , wavelength filter 115. The stabilized laser section 10 includes a semiconductor laser 12, a lock-in amplifier 17, an optical (frequency) modulator 19 for frequency modulating the laser beam from one end face of the semiconductor laser 12, and a device for driving the optical modulator 19. It consists of an oscillator 114, a photodetector 118, and a wavelength filter 116. The optical output from each end face of the laser 11 of the reference laser section 8 and the laser 12 of the stabilized laser section 10 passes through each optical fiber, is superimposed by an optical coupler 110, and is sent to the interferometer section 9.

Next, the operation of the multi-wavelength stabilized laser device of this embodiment will be explained with reference to FIG. The light emitted from one end surface of the semiconductor laser 11 is transmitted through a wavelength reference medium 14 that absorbs only light of a predetermined wavelength, and the transmitted light is transmitted to the light receiver 11.
1 is photoelectrically converted. FIG. 3(a) shows its absorption characteristics. At this time, the semiconductor laser 11 is connected to the oscillator 1.
A lock-in amplifier 15 compares the signal from the optical receiver 111 and the signal from the oscillator 113 to determine the deviation of the oscillation frequency of the semiconductor laser 11 from the absorption wavelength of the wavelength reference medium 14. The error signal is detected and fed back to the semiconductor laser 11, and the driving current of the semiconductor laser is controlled based on this error signal, thereby stabilizing the oscillation wavelength of the semiconductor laser 11 to the absorption wavelength of the wavelength reference medium 14. . The stabilized light passes through an optical fiber via an optical coupler 110 and then reaches the ring resonator 1.
3.

The ring resonator 13 has absorption characteristics for each frequency corresponding to its resonator length. FIG. 3(b) shows the absorption characteristics of the ring resonator 13. One of these absorption lines is matched to the wavelength of the absorption line in FIG. 3(a). The output light of the semiconductor laser 11 stabilized at wavelength λ1 is transmitted through the ring resonator 13, and the transmitted light passes through the optical coupler 112 (via the optical fiber until it is received by the light receiver 117), and then passes through the wavelength filter. Only that signal is selected by the photodetector 115, photoelectrically converted by the photoreceiver 117, and sent to the lock-in amplifier 16.

FIG. 4(a) shows the transmission characteristics of the wavelength filter 115, and FIG. 4(b) shows the transmission characteristics when the absorption line (λ1) is selected. (a)
This corresponds to the superposition of . At this time, the semiconductor laser 1
1 is directly modulated by the oscillator 113, so the laser beam is frequency modulated, and the lock-in amplifier 16 combines the signal from the oscillator 113 with the light receiver 11.
By comparing the signal from the receiver 117 with the signal from the receiver 117
The deviation of the signal from the absorption line λ1 (ring resonator 13 output) from the absorption line λ1 is detected as an error signal. This error signal is sent to the temperature controller 18 of the ring resonator 13, which changes the ambient temperature of the ring resonator 13 to control the optical path length of the ring resonator 13. ) as in λ
1.

The oscillation wavelength of the second semiconductor laser 12 is stabilized using this stabilized ring resonator 13. The light emitted from one end face of the semiconductor laser 12 is frequency-modulated using an external frequency modulator 19 in the same way as the first semiconductor laser (the light emitted from the other end face is used as light source light to be supplied to the outside). . The modulated light passes through an optical fiber via an optical coupler 110 and enters the ring resonator 13. At this time, the frequency of the oscillator 114 is set to be different from the frequency of the oscillator 113. ring resonator 1
The light emitted from the ring resonator 13 passes through an optical fiber via an optical coupler 112, and a wavelength filter 116 selects a signal corresponding to one wavelength (for example, λ2) of the resonant wavelength peak of the ring resonator 13. Photoelectrically converted. The deviation of the signal from the photoreceiver 118 from the resonance frequency of the ring resonator 13 is detected by the lock-in amplifier 17 with reference to the signal from the oscillator 114, and the error signal is fed back to the semiconductor laser 12 (for example, as shown in FIG. 3). b) The oscillation wavelength of the semiconductor laser 12 is stabilized to the wavelength of λ2. Therefore, by changing the selected frequency by the wavelength filter 116, the resonant frequency λ of FIG.
Any resonance point other than 1 can be stabilized. The wavelength filters 115 and 116 can have the same configuration and will be described in detail below.

FIG. 5 shows a wavelength filter 115 (or 116).
) is shown. Reference numeral 31 designates a pair of non-reflective coating layers, and inside thereof a pair of glass substrates 35, a pair of transparent electrode layers 32 made of indium tin oxide (ITO) or tin oxide, a pair of mirrors 33, and a pair of mirrors 33. Alignment films 34 made of polyimide or the like are sequentially arranged, and a liquid crystal layer 36 is placed inside them.
Place. 37 is a potential difference V between a pair of transparent electrode layers 32.
This is the wiring for applying . 38 and 39 are a pair of optical fibers for light input and output, 310 and 311 are a pair of collimating lenses, 312 and 313 are a pair of birefringent crystals, and 31
4,315 are a pair of half-wave plates, each of which is placed outside a pair of coating layers.

First, the operating principle of the wavelength filter will be explained. FIGS. 6A and 6B illustrate the orientation of liquid crystal molecules when no electric field is applied to the liquid crystal. Here, 41 is a liquid crystal molecule, 42 is an alignment film, and the arrow shown on the alignment film 42 is the alignment direction. For example, if you use a nematic liquid crystal as the liquid crystal and a polymer film with a thickness of several tens to 1000 angstroms as the alignment film and perform alignment treatment (rubbing, etc.), or use an obliquely vapor-deposited SiO film with a similar thickness, there will be no electric field. In the added state, the liquid crystal molecules are aligned parallel to the substrate in the alignment direction of the alignment film. This is because the potential energy of the liquid crystal plus the interaction energy with the alignment film becomes minimum when the liquid crystal molecules are aligned in the alignment direction. However, when an electric field is applied perpendicular to the substrate, interaction with the electric field occurs because the liquid crystal molecules have a dipole moment, and this system becomes energetically stable as the liquid crystal rotates in the direction of the electric field. At this time, since the alignment directions of the opposing alignment films are opposite to each other, the rotation direction of the liquid crystal molecules is uniquely determined, and disclination, which is a cause of light scattering, can be avoided. Here, consider the case where parallel light polarized in the alignment direction of the liquid crystal is incident perpendicularly on the substrate. As expected from the highly anisotropic shape, liquid crystal molecules have extremely high optical anisotropy. Therefore, when an electric field is applied, the refractive index (n) in the direction of the polarization plane changes greatly as the liquid crystal molecules rotate. The refractive index change reaches a maximum of more than ten percent. n is a function of the electric field E.

n(E)=n0+Δn(E)

(1) Here, n0 is the refractive index when no electric field is applied.

In the wavelength filter 115, such a liquid crystal layer 36 and a pair of alignment films 34 are disposed on a pair of opposing mirrors 33.
An optical resonator is constructed by sandwiching the two. When the reflectance of the mirror 33 is γ, the wavelength of the incident light is λ, and the interval between the mirrors 33 is L, the transmittance T and resonance wavelength λres of this optical resonator are expressed by the following equation.

T=1/[1+Fsin2 (2πn(E)L/λ
)] (2) F=4γ
/(1-γ)2
(3
) λres = m/2n(E)L (m=1
, 2,...) (4) Here, F is a figure of merit called finesse.

As is clear from equation (4), the resonance peak moves as the refractive index of the liquid crystal layer 36 changes when an electric field is applied. That is, by controlling the potential difference V between the pair of transparent electrode layers 32 on the outside of the resonator, the resonant wavelength of the resonator can be controlled and the resonator can be operated as a variable wavelength filter. If the upper and lower limits of the operating wavelength of the wavelength filter 115 are λmax and λmin, respectively, then the upper limit of the value of m is given by the following equation. (Condition that only one resonance peak exists in the operating wavelength range) m<1+λmax/(λmax −λmin)

(5) For example, if the reflectance of the mirror is 99.0% and the cavity length is 15 μm, the half width is about 50 nm at 30 GHz.
A wavelength sweep of m is possible. Furthermore, by increasing the finesse, it is possible to achieve a half width of about 5 GHz.

Furthermore, in the wavelength filter using liquid crystal,
Since it strongly depends on the polarization characteristics of the incident light, as shown in Figure 5,
The incident light through the collimating lens 310 is divided into a p-polarized light component and an s-polarized light component by one birefringent crystal 312, and the half-wave plates 314 and 315 are appropriately rotated to cause the light of the two polarized light components to undergo optical resonance. When passing through the vessel, one polarized component becomes the other.
Adjust the polarization component so that it receives the same effect as the other polarization component. Thereafter, the other birefringent crystal 313 is used to overlap the light beams again, and the light beams are emitted through the collimating lens 311. In this way, by using the configuration shown in FIG. 5, polarization dependence can be eliminated.

For example, in the device configuration shown in FIG. 1, the semiconductor lasers 11 and 12 are InGaAsP distributed feedback semiconductor lasers (DFB) that oscillate at a wavelength of 1.5500 μm.
Type LD) was used. In the absorption cell 14 with a cell length of 5 cm,
Isotopically substituted acetylene gas (13C2
H2) was sealed at 10 Torr. FIG. 7 is a diagram showing the absorption characteristics of acetylene gas and isotope-substituted acetylene gas. The cell length is 10 cm and the pressure is 760 Torr. Among them, the absorption line of 1.54949 μm (full width at half maximum 80
The wavelength of the semiconductor laser 11 was synchronized to the absorption line using a wavelength of 0 MHz and an absorption intensity of 10%. The frequency of the oscillator 113 is 10kHz, and the semiconductor laser 1 is
The fluctuation in the center oscillation wavelength of No. 1 was suppressed to 1×10 −5 nm (1 MHz in terms of optical frequency) or less.

This wavelength stabilized light is transferred to the optical fiber coupler 1.
10 and entered the ring resonator 13. The ring resonator 13 has a diameter of 13 mm and a loss of 0.04 dB/
A quartz-based waveguide ring resonator with a finesse of 30 cm and a finesse of 30 cm was used. The resonant frequency spacing is 5 GHz. The refractive index change due to temperature is 10-5/℃, and 1.8GHz at 1℃
It is possible to shift the resonance peak of Semiconductor laser 11
The 10 KHz modulated light component was demultiplexed by a wavelength filter 115, lock-in was detected by a lock-in amplifier 16, and the resonator length of the ring resonator 13 was stabilized.

By the way, the light emitted from the ring resonator 13 is distributed by the optical coupler 112, and a part of the light is transmitted to the wavelength filter 11.
5. At this time, the voltage applied to the wavelength filter 115 is adjusted so that only the laser beam from the reference laser section 8 is selected. In this configuration, one of the resonance peaks of the ring resonator 13 was aligned with an absorption line having a wavelength of 1.54949 μm (optical frequency: 193612 GHz). The frequency stability of the resonance peak was 1 MHz.

Furthermore, the output light from one end facet of the semiconductor laser 12 was input to an acousto-optic modulator 19, frequency modulated at 100 kHz with a modulation width of 120 MHz, and input into the stabilized ring resonator 13. Semiconductor laser 12 of 10
The 0kHz modulated light component is separated by the wavelength filter 116, locked-in is detected by the lock-in amplifier 17, and one resonance peak of the ring resonator 13, wavelength 1.54953 μm is detected.
(Optical frequency is 193,607 GHz). Using this configuration system, it was possible to suppress fluctuations in the center oscillation wavelength of the semiconductor laser 12 to 1×10 −5 nm (1 MHz in terms of optical frequency) or less. With this diameter, the oscillation wavelength of the semiconductor laser can be stabilized at any frequency of 5 GHz. Furthermore, by arbitrarily selecting the resonator length of the ring resonator 13, stabilization can be achieved at any wavelength.

In particular, by using the acetylene gas and the isotope-substituted acetylene gas shown in FIG. 7, it is possible to stabilize the continuous wavelength between absorption lines localized over a wide range of 1.50 μm to 1.56 μm. Compared to the case where one absorption line is used, this makes it possible to eliminate frequency errors due to the influence of the wavelength dispersion characteristics of the ring resonator. Even if ordinary acetylene gas, ammonia gas, methane gas, carbon dioxide, etc. are used as the light-absorbing gas, the oscillation wavelength can be stabilized by the same operating principle as the above function.

In FIG. 1, direct modulation is used to modulate the semiconductor laser 11, and an acousto-optic modulator 19 is used to modulate the semiconductor laser 12, but the present invention can be realized using either of them. Note that similar effects can be obtained by using modulators with other configurations, such as a LiNbO3 modulator or an electro-optic modulator, instead of the acousto-optic modulator. Further, by connecting a plurality of stabilizing laser sections 10, 10', 10'', . . . as shown in FIG. 8, a plurality of lasers can be stabilized at the same time.

[Embodiment 2] FIG. 2 is a block diagram showing a second embodiment of the multi-wavelength stabilized laser device of the present invention. As shown in FIG. 2, this embodiment uses a Mach-Zehnder interferometer 119 as a wavelength filter. Light from the ring resonator 13 is incident on this interferometer 119 . Further, the Mach-Zehnder interferometer 119 is configured on the same quartz waveguide as the ring resonator 13. Therefore, it is possible to make the system extremely compact. Like the ring resonator 13, a heater is embedded in one of the optical paths of the Mach-Zehnder interferometer 119, and the wavelength is selected by changing the applied voltage. By setting the frequency interval to 10 GHz, 20 GHz, and 40 GHz using the 5 GHz frequency interval of the ring resonator 13 as the minimum unit, light of an arbitrary frequency can be selected. The light from this interferometer 119 is transmitted to receivers 117 and 1.
given to 18. The other configurations are the same as in the first embodiment.

Furthermore, by stacking the Mach-Zehnder interferometers 119 in multiple stages and connecting a plurality of stabilizing laser sections as shown in FIG. 9, a plurality of lasers can be stabilized at the same time.

Although the present invention has been specifically explained above based on examples, it goes without saying that the present invention is not limited to the above-mentioned examples and can be modified in various ways without departing from the gist thereof. .

[0030]

As explained above, according to the present invention, the oscillation wavelengths of a plurality of semiconductor lasers can be simultaneously synchronized and stabilized to an arbitrary wavelength with extremely high precision. It has the advantage of being usable as a standard light source or a light source for optical measurement.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of a multi-wavelength stabilized laser device of the present invention.

FIG. 2 is a block diagram showing the configuration of the second embodiment.

FIG. 3 is a diagram showing the light intensity of light transmitted through a light-absorbing gas and a ring resonator.

FIG. 4 is a diagram showing transmission characteristics of a wavelength filter.

FIG. 5 is a configuration diagram of a wavelength filter using liquid crystal.

FIG. 6 is a configuration diagram of liquid crystal molecules.

FIG. 7 is a diagram showing the light absorption characteristics of acetylene gas and isotope-substituted acetylene gas.

FIG. 8 is a block diagram showing the configuration of another embodiment of the wavelength stabilized laser device of the present invention.

FIG. 9 is a block diagram showing the configuration of another embodiment.

FIG. 10 is a block diagram showing the configuration of a wavelength stabilized laser device using a conventional light absorption cell.

[Explanation of symbols]

1, 11, 12 Semiconductor laser 2, 14 Absorption cell 3, 111, 117, 118 Light receiver 4, 15, 16
, 17 Lock-in amplifier 5, 113, 114 Oscillator 8 Reference laser section 9 Interferometer section 10 Stabilizing laser section 13 Ring resonator 18 Temperature controller 19 External modulator 110, 112 Optical coupler 115, 116 Wavelength filter 119 Mach-Zehnder interferometer

Claims (3)

[Claims] 1. A reference laser section comprising a wavelength stabilized laser whose oscillation wavelength is stabilized using a medium that absorbs only light of a predetermined wavelength as a wavelength reference; an interferometer section consisting of an optical interferometer whose optical path length is stabilized using emitted light from a laser; a semiconductor laser; and a laser beam from the semiconductor laser that transmits the laser beam through the interferometer and then transmits the interference from the laser beam. a wavelength filter that optically selects a signal having one wavelength among a plurality of resonant wavelength peaks of the laser, and a means for stabilizing the oscillation wavelength of the semiconductor laser based on the signal selected by the wavelength filter. A multi-wavelength stabilized laser device comprising: a stabilized laser section; 2. The wavelength filter includes a pair of glass substrates, a pair of transparent electrode layers sequentially laminated inside the pair of glass substrates, a pair of reflective mirrors, and a pair of alignment films. ,
2. The multi-wavelength stabilized laser device according to claim 1, further comprising a liquid crystal. 3. A reference laser section comprising a wavelength stabilized laser whose oscillation wavelength is stabilized using a medium that absorbs only light of a predetermined wavelength as a wavelength reference, a plurality of semiconductor lasers, and a laser beam from the reference laser section. The light and the laser beams from the plurality of semiconductor lasers are incident on an optical interferometer having a plurality of resonance peaks, and the output of the optical interferometer is connected in multiple stages to optically detect any wavelength of the plurality of resonance peaks. means for stabilizing the optical path length of the optical interferometer based on the light emitted from the reference laser section from any one of the plurality of wavelength filters; A multi-wavelength stabilized laser device comprising: means for stabilizing the oscillation wavelength of each of the plurality of semiconductor lasers based on a signal selected by each of the filters.