JPH0459797B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to improving the characteristics of a semiconductor laser wavelength stabilizing device that stabilizes the wavelength of a semiconductor laser by controlling it to the absorption line of atoms or molecules.

<<Prior Art>> FIG. 11 is a structural block diagram showing a prior art of a semiconductor laser wavelength stabilizing device described in Japanese Patent Application No. 11894/1989 by the present applicant.

LD1 is a semiconductor laser, PE1 is a Peltier element that cools or heats this semiconductor laser LD1, and CT
1 is a temperature control means for controlling the temperature of the semiconductor laser LD1 to be constant by driving the Peltier element PE1, TB1 is a constant temperature bath for storing these and reducing temperature fluctuations, and BS1 is for controlling the output light of the semiconductor laser LD1. Beam splitter that separates into two directions,
UM1 is an acousto-optic modulator that receives the output light from one side of the beam splitter BS1 and constitutes a modulation means, and CL1 is a standard material that receives the diffracted light output of the acousto-optic modulator UM1 and absorbs light of a specific wavelength. PD1 is a photodetector that receives the transmitted light of this absorption cell CL1,
A1 is an amplifier that inputs the output electric signal of this photodetector PD1, LA1 is a lock-in amplifier that inputs the electric output of this amplifier A1, and CT2 inputs the output of this lock-in amplifier LA1 to control the current of the semiconductor laser LD1. Configure the control means to control
PID controller, SW1 is the acousto-optic modulator
A switch whose one end is connected to UM1, SG1 is a signal generator whose output turns the switch SW1 on and off at a frequency fm (e.g. 2kHz), and SG2 is a signal generator whose output is a frequency f _D (e.g. 80MHz) connected to the other end of SW1. is the second signal generator.

The operation of the semiconductor laser wavelength stabilizing device configured as described above will be explained next. Semiconductor laser LD1 is a control means that inputs a temperature detection signal in thermostatic chamber TB1.
The temperature is controlled to be constant by CT1 via Peltier element PE1. The output light of the semiconductor laser LD1 is separated into two directions by the beam splitter BS1.
The reflected light becomes output light to the outside, and the transmitted light enters the acousto-optic modulator UM1. When the switch SW1 is on, the acousto-optic modulator UM1 is driven by the output of the signal generator SG2 at the frequency f _D , so most of the incident light with the frequency ν _O is diffracted and undergoes a frequency (Doppler) shift. Light with a frequency ν _O +f _D enters the absorption cell CL1 as the next diffracted light. When the switch SW1 is off, all incident light enters the absorption cell CL1 as 0th-order diffracted light at a frequency ν _O.

Switch SW1 is the frequency fm of signal generator SG1
Therefore, the light incident on the absorption cell CL1 is subjected to frequency modulation with a modulation frequency fm and a modulation depth _fD . When this modulated light enters the absorption cell CL1, as shown in the operational diagram of FIG. 12, the amount of transmitted light is modulated only at the location of the absorption signal of the Cs atoms, so a signal appears in the output. If this signal is converted into an electric signal by the photodetector PD1 and then synchronously rectified at the frequency fm by the lock-in amplifier LA1 via the amplifier A1, a first-order differential waveform as shown in the frequency characteristic curve diagram in Fig. 13 is obtained. . The current of the semiconductor laser LD1 is controlled by the PID controller CT2 to create a locking amplifier.
If the output of LA1 is locked (controlled) to the center of the first-order differential waveform, the output light of the semiconductor laser becomes ν _s −f _D /
This results in a stable frequency of 2. Here, ν _s indicates the center frequency of absorption of Cs atoms.

According to the semiconductor laser wavelength stabilizing device having such a configuration, since the oscillation frequency of the laser is not modulated, it becomes an extremely stable light source even momentarily.

<<Problems to be Solved by the Invention>> However, each element of the semiconductor laser wavelength stabilization device having the above structure is composed of a single component, so it has the disadvantage that the structure is complicated and large. .

The present invention has been made to solve these problems, and an object of the present invention is to miniaturize and realize a semiconductor laser wavelength stabilization device with a stable oscillation frequency.

<<Means for Solving the Problems>> The present invention relates to a semiconductor laser wavelength stabilizing device that stabilizes the wavelength by controlling the wavelength of a semiconductor laser according to the absorption spectrum line of a standard substance. A semiconductor laser LD2, and the output light of this semiconductor laser is passed through a first optical waveguide 3.
an acousto-optic modulator that enters through 1 and modulates the frequency.
UM2 and a first signal generator that generates a first frequency signal
SG3, a second signal generator SG4 that generates a second frequency signal, whose output is frequency modulated by the output of the first signal generator, and drives the acousto-optic modulator with this modulated signal; an absorption section CL2 in which the first-order diffraction light and the zero-order diffraction light output from the acousto-optic modulation section enter through the second optical waveguide 32 and enclose a standard substance that causes absorption at a specific wavelength; and the adjacent absorption section. a photodetection unit that converts the first-order diffraction light and zero-order diffraction light transmitted through the
PD2; synchronous detection means LA2 for synchronously detecting the output electrical signal of the photodetector at the first frequency (fm) or an integral multiple thereof, and controlling the oscillation wavelength of the semiconductor laser based on the detected output; and are formed on the same substrate 1.

≪Example≫ The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. Reference numeral 1 denotes an optical integrated circuit (optical IC) substrate made of, for example, GaAs, etc. Below, we will explain what is formed on this optical IC substrate 1. LD 2 is a semiconductor laser section, and 31 is an input device for inputting the output light of this semiconductor laser section. The optical waveguide UM2 is an acousto-optic modulator (ultrasonic light modulator) into which the light emitted from the optical waveguide 31 enters;
32 is an optical waveguide into which the output light of this acousto-optic modulation unit UM2 is input, CL2 is an absorption unit filled with a standard substance (Cs in this case) that absorbs light of a specific wavelength into which the output light of this optical waveguide 32 is input; PD2 is a light receiving section into which the light emitted from the absorption section CL2 is input, and 2 is a control section into which the output electric signal of the light receiving section PD2 is input. In the control section 2, LA2 is this light receiving section.
A lock-in amplifier circuit in which the output of PD2 is connected to the input, CT3 is a PID controller circuit, etc., in which the output of the lock-in amplifier circuit LA2 is connected to the input, and its output is connected to the injection current input of the semiconductor laser section LD2. The control circuit, SG3, is a signal generation circuit (oscillation circuit) with a frequency fm (for example, 2kHz), one of whose outputs is the reference signal input of the lock amplifier circuit LA2, and SG4 is this signal generation circuit.
Frequency f _D (e.g. 80kHz) which is modulated by the output of SG3 and whose output is connected to the acousto-optic modulator UM2.
This is a second signal generation circuit (oscillation circuit).

The operation of the device having the above configuration is similar to that of the semiconductor laser wavelength stabilizing device shown in FIG.

The semiconductor laser wavelength stabilizing device having such a configuration can be integrated into one chip, so it has the advantage of being compact, mass-producible, and easy to adjust.

Furthermore, even if the diffraction efficiency of the acousto-optic modulator UM2 changes, the light component that does not contribute to modulation (0th-order diffracted light)
increases and the signal strength decreases, but does not affect the center wavelength.

Furthermore, since a low modulation frequency fm and a high frequency deviation _fD can be simultaneously obtained, the S/N ratio for detecting the deviation of the laser wavelength from the absorption center is high, and the stability of the laser output is excellent.

Furthermore, due to integration, the output light of the semiconductor laser LD2 is absorbed by the absorption part CL2 via the waveguide 31, the acousto-optic modulator UM2, and the waveguide 32, and the transmitted light directly enters the adjacent light receiving part PD2. The number of reflective surfaces is reduced compared to when these components are used alone, and interference noise due to reflected light is significantly reduced.

In addition, the synchronous detection means LA2 and the signal generation section SG
By integrating 3, SG4, etc., the phase delay of the circuit is reduced, and the phase stability and frequency stability are improved.

Further, by integrating the acousto-optic modulator UM2, it becomes possible to lower the power, and since the effective diameter becomes smaller, the diffraction efficiency becomes higher and the S/N ratio improves.

In the above embodiment, the lock-in amplifier LA2
Although the modulation frequency fm is used as the reference frequency, a frequency that is an integral multiple of the modulation frequency fm may be used.

Further, as a standard substance for the absorption cell CL2, in addition to Cs, for example, Rb, NH ₃ , H ₂ O, etc. may be used.

Further, in the above embodiment, an acousto-optic modulator is used as the modulation means, but the present invention is not limited to this, and a phase modulator using an electro-optic element may be used, for example.

These include, for example, vertical modulators, horizontal modulators, traveling waveform modulators, etc. (Amnon Yarif: Fundamentals of Optical Electronics (Maruzen), p.247-p.253).

Further, in the above embodiment, the current of the semiconductor laser is controlled by the output of the control section, but the present invention is not limited to this, and the temperature of the semiconductor laser may also be controlled.

FIG. 2 is a table showing a concrete implementation method of each component of the apparatus shown in FIG. For example, if the electrical circuit part is made of silicon substrate, it will have a monolithic structure,
In other cases, it is a hybrid configuration. This will be explained in more detail below using specific examples.

Figure 3 shows a semiconductor laser in the apparatus shown in Figure 1.
FIG. 2 is a perspective view of a main part showing a specific example in which an LD 2 is monolithically realized on an optical IC board 1. FIG.

4 and 5 are a perspective view and a sectional view of the main parts of a specific example of a hybrid configuration in which the semiconductor laser LD2 is externally attached to the apparatus shown in FIG. /501).
Figure 4 shows a configuration in which the output of the semiconductor laser LD2 is directly irradiated onto the end face of the waveguide 31 formed on the optical IC substrate 1, and Figure 5 shows the configuration in which the output light of the semiconductor laser LD2 is introduced into the waveguide 31 via a prism PR. It is configured to do this.

FIG. 6 shows the apparatus shown in FIG. 1, in which the absorption section CL2 forms a glass film 4 by glass coating or thermal oxidation in a recess provided by etching or the like on the surface of the optical IC substrate 1, and after putting a standard substance therein, the glass plate 5 is attached. FIG. 3 is a cross-sectional view showing a specific example of a structure that is fused and sealed.

FIG. 7 is a sectional view showing another specific example of the absorption section _CL2 in the device shown in FIG. The standard substance sealed in the lid 51 above the waveguide 32 absorbs the output light from the waveguide 32 by an evanescent effect. Compared to the configuration shown in FIG. 6, this has the advantage of being easier to manufacture.

In each of the above specific examples, the photodetector can have a monolithic or hybrid configuration (see the above-mentioned "Optical Communication Handbook" 433/434).

FIG. 8 is a structural plan view showing a second embodiment of the semiconductor laser wavelength stabilizing device, which is the device shown in FIG. 1 with a narrower spectrum. an optical branching part OB1 that branches part of the output light of the semiconductor laser LD2; an optical resonator FP1 made of a Fabry-Perot etalon into which the branched output light of the optical branching part OB1 is incident;
A second photodetector PD3 into which the output light of the optical resonator FP1 is incident, and a broadband amplifier A2 which amplifies the electrical output of the photodetector PD3 and feeds it back to the current injected into the semiconductor laser LD2 are connected to the optical IC substrate 1. The wideband amplification section A2 is provided in addition to the above.
The control unit 21 is provided in the control unit 21 (shown in a simplified manner in the figure). After adjusting the resonance curve (position shifted from the center frequency) of the optical resonator FP1 to the oscillation wavelength of the semiconductor laser LD2 and converting the phase noise contained in the output light of the semiconductor laser LD2 into an amplitude modulation signal, the photodetector PD3 The phase noise of the semiconductor laser LD2 is suppressed by detecting the electrical output and negatively feeding back the electrical output to the drive current (injected current) of the semiconductor laser LD2 via a broadband amplifier A2 having a band wider than the spectral width of the semiconductor laser light. , which aims to narrow the spectrum. (Reference: Furutashima et al.; IEICE Technical Report, OQE84-1303 (1984)) Figure 9 is a perspective view of the main part showing a specific example of the Fabry-Perot resonator FP1 provided on the optical IC board 1 in the device shown in Figure 8. 9A and 9B) and a plan view of the main part (FIG. 9C). In FIG. 9A, a hole 7 is provided in a part of the waveguide 61 provided on the substrate 1.
A resonator is formed by coating two opposing surfaces 81 of a resonator with a reflective film, and FIG. A resonator formed by forming a reflective film on the opposing end surfaces 82 of 62,
FIG. 9C shows a resonator 83 formed by doping a part of the waveguide 63 provided on the substrate 1 with a high refractive index material.

Figure 10 shows the optical resonator in the device shown in Figure 9C.
This is a perspective view showing a specific example of a means for adjusting the resonance frequency of the FP 1. Electrodes 9 are provided on both sides of a resonance section 83 on the substrate 1, and the current applied between the two electrodes 9 causes the resonance section 83 to change. Resonant part 8 by changing the refractive index
This changes the effective resonator length of No. 3.

Another method for adjusting the resonant frequency is to form a heater thin film resistor near the optical resonator FP1 on the substrate and change the resonator length by thermal expansion.

There is also a method in which a ferroelectric material is doped as a high refractive index material in a structure similar to that shown in FIG. 10C, and the refractive index is changed by an applied electric field.

Further, when controlling the semiconductor laser LD2 and the optical resonator FP1 to a predetermined temperature, a thin film resistor or the like is used as a heater, respectively, but it is preferable that both heaters are placed as far away as possible so that they do not interfere with each other.

In each of the above embodiments, the linear absorption method is used to control the wavelength of the semiconductor laser to the absorption line of atoms or molecules, but instead, a part of the light emitted from the acousto-optic modulator UM2 is converted into pump light. A saturation absorption method (Hori, Tsunoda, Kitano, Yabusaki, Ogawa:: Frequency stabilization of semiconductor lasers using saturated absorption spectroscopy, IEICE Technical Report OQE82-116)
By using this, a more stable semiconductor laser wavelength stabilizing device can be realized.

<<Effects of the Invention>> As described above, according to the present invention, it is possible to miniaturize and integrate a semiconductor laser wavelength stabilizing device that has a stable oscillation frequency and is easy to adjust.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of the structure of an embodiment of a semiconductor laser wavelength stabilizing device according to the present invention, and FIG.
3 to 7 are diagrams for explaining specific examples of the components of the device shown in FIG. 1. FIG.
FIGS. 9 and 10 are configuration plan views showing examples of the
Figure 8 shows a specific example of the optical resonator part FP1 of the device, Figure 11 is a block diagram showing a conventional example of a semiconductor laser wavelength stabilizing device, and Figure 12 shows this first example.
FIG. 1 is an operation explanatory diagram for explaining the operation of the apparatus, and FIG. 13 is a characteristic curve diagram for explaining the operation of the apparatus shown in FIG. 1...Substrate, 2...Control unit, 31, 32...Waveguide, LD2...Semiconductor laser, CL2...Absorption unit, PD2...Photodetection unit, UM2...Acousto-optic modulation unit, SG3...No. 1 signal generator, SG4...second signal generator, LA2...synchronous detection means, fm...
...first frequency.

Claims (1)

[Scope of Claims] 1. A semiconductor laser wavelength stabilization device that stabilizes the wavelength by controlling the wavelength of a semiconductor laser according to the absorption spectrum line of a standard substance, comprising: a semiconductor laser LD2; Optical waveguide 3
an acousto-optic modulator that enters through 1 and modulates the frequency.
UM2 and a first signal generator that generates a first frequency signal
SG3, a second signal generator SG4 that generates a second frequency signal, whose output is frequency modulated by the output of the first signal generator, and drives the acousto-optic modulator with this modulated signal; An absorption section CL2 in which the first-order diffraction light and the zero-order diffraction light output from the acousto-optic modulation section enter through the second optical waveguide 32 and enclose a standard substance that causes absorption at a specific wavelength, and the adjacent absorption section. a photodetection unit that converts the transmitted first-order diffraction light and zero-order diffraction light into electrical signals;
synchronously detecting the output electrical signal of the photodetector at the first frequency fm or an integral multiple thereof;
A semiconductor laser wavelength stabilizing device characterized in that a synchronous detection means LA2 for controlling the oscillation wavelength of the semiconductor laser based on the detection output thereof is formed on the same substrate 1. 2. The semiconductor laser wavelength stabilizing device according to claim 1, which uses Rb or Cs as a standard substance. 3 Forming a glass film by glass coating or thermal oxidation in the recesses provided in the substrate surface by the absorption part,
2. A semiconductor laser wavelength stabilizing device according to claim 1, wherein a standard substance is put in and sealed with a glass plate. 4. The semiconductor according to claim 1, wherein the absorption section has a waveguide provided on a glass substrate, and is configured to absorb light related to the output light of a semiconductor laser by means of an evanescent effect using a standard substance on the waveguide. Laser wavelength stabilizer. 5. An optical branching section that branches part of the output light of the semiconductor laser, an optical resonating section that receives the branched output light of this optical branching section, and a second photodetecting section that receives the output light of this optical resonating section. 2. The semiconductor laser wavelength stabilizing device according to claim 1, further comprising: a broadband amplifying section for amplifying the electrical output of the photodetecting section and feeding it back to the injection current of the semiconductor laser on the substrate.