JPH04155878A - Frequency stabilizing device for semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a frequency stabilizing device high in frequency stability by a method wherein an SHG element which is formed of material that induces both a nonlinear optical effect and an electro-optic effect and outputs second harmonics of light emitted from a semiconductor laser 2 basing on the light concerned is provided. CONSTITUTION:The output light 1.5mum in wavelength of a semiconductor laser 2 is made to penetrate an SHG element 4 so as to obtain a second harmonic of a certain wavelength coincident to the absorption line of a rubidium cell 8. The SHG element 4 is formed of material which induces an electro-optic effect, an electrode 6 is mounted on the SHG element concerned, and a modulation signal of low frequency is applied to the electrode 6 concerned to modulate the second harmonics in frequency. By this setup, the oscillation frequency of a semiconductor laser can be controlled so as to have a prescribed relation to the absorption wavelength of rubidium. In this case, a modulation current is not required to overlap the drive carnet of a semiconductor laser, so that a light source can be protected against deterioration in frequency stability.

Description

Detailed Description of the Invention Overview Regarding a frequency stabilization device for a semiconductor laser, the purpose of the present invention is to provide the above device with high frequency stability, and to provide a semiconductor laser that oscillates at a wavelength in the 1.5 μm band, a nonlinear optical effect, and an electro-optic effect. an SHG element that is made of a material that produces a
An electrode mounted on the SHG element, a rubidium cell that transmits the second harmonic, and an optical-to-electrical converter for the second harmonic transmitted through the cell to generate electricity according to the intensity of the second harmonic. a photodetector that outputs a signal; and a low-frequency modulation signal is applied to the electrode to frequency-modulate the second harmonic output from the SHG element by the electro-optic effect, and the output of the photodetector due to the modulation. and a control circuit for controlling parameters on which the oscillation frequency of the semiconductor laser depends so that the fluctuation is constant.

Industrial applications The present invention relates to a frequency stabilizing device for a semiconductor laser.

In recent years, in the field of optical communications, research into coherent optical communication technology has become active with the aim of increasing communication capacity and long-distance transmission. Further, studies are also underway on optical switching technology that performs direct switching or cross-connection in the state of optical signals. In these systems, it is being considered to increase communication capacity and identify information during exchange by preparing a large number of light sources with controlled wavelengths (frequency) and multiplexing them in the optical frequency domain. . The light source used must have a sufficiently stabilized output light frequency, but the oscillation frequency of semiconductor lasers, which are common as light sources, is extremely sensitive to drive current and temperature, so these must be adjusted precisely. need to be controlled.

Furthermore, even if the driving current and temperature are precisely controlled, the frequency may change due to aging of the semiconductor laser.
In order to manage this, we must set an absolute standard for optical frequency in some way, and then set and set the wavelength (frequency) of each light source based on this.
Management is required.

Conventional technology Conventionally, as a frequency stabilizing device that creates an absolute frequency standard for semiconductor lasers, devices that utilize atomic or molecular absorption lines and stabilize the oscillation frequency by controlling it at the center of the absorption line have been considered. has been done. Since there is no absorption line due to electronic transition near 1550 nm, which is the wavelength band for optical communication, the methods proposed so far are ■H
, O, N H3゜Those that utilize absorption lines based on the rotational and vibrational motion of molecules such as HCN, ■Rb (rubidium) that generates the second harmonic of a laser beam that oscillates at 1560 nm and has an absorption at 780 nm. There is something that stabilizes the absorption line of . Of these, for the latter technique of stabilizing the Rb absorption line, the technique applied to the atomic microwave frequency standard using an Rb cell can be applied almost as is, so the accuracy of the standard of Rb, It has extremely high practical utility, as stability has been studied extensively and absolute standards for electric pulses and light can be obtained simultaneously.

FIG. 6 is a diagram showing the configuration of a conventional frequency stabilizing device for a semiconductor laser using an Rb absorption line.

In this conventional example, a low-frequency modulation signal is superimposed on the drive current of the semiconductor laser, and a phase comparison is made between the detection signal of the intensity of light transmitted through the absorption cell and the modulation signal.
The oscillation frequency of the semiconductor laser is stabilized at the top position of the absorption peak.

Problems to be Solved by the Invention In conventional frequency stabilizing devices, a modulation signal is superimposed on the drive current of the semiconductor laser that is the light source.In other words, the semiconductor laser is frequency modulated, so the light source There was a problem that the frequency stability of the

The present invention was created in view of these circumstances, and aims to realize a frequency stabilizing device with high frequency stability.

Means to solve problems FIG. 1 is a block diagram of the principle of the present invention.

The semiconductor laser frequency stabilizing device of the present invention has a
a semiconductor laser 2 that oscillates at an m-band wavelength; an SHG element 4 that is made of a material that produces a nonlinear optical effect and an electro-optic effect and outputs a second harmonic wave based on the light from the semiconductor laser 2; An electrode 6 mounted on the element 4, a rubidium cell 8 that transmits the upper and second harmonics,
A photodetector 1 that converts the second harmonic transmitted through the cell 8 into electricity and outputs an electrical signal according to the intensity of the second harmonic.
0 and a low frequency modulation signal is applied to the upper electrode 6 to generate the SHG! by the electro-optic effect. The second output from child 4
The photodetector 10 frequency-modulates harmonics and uses the modulation
A control circuit 1 that controls a parameter on which the oscillation frequency of the semiconductor laser 2 depends so that the output fluctuation of the semiconductor laser 2 is constant.
2.

Function: In the present invention, since an SHG element made of a material that produces a nonlinear optical effect is used, the SHG element has 1.
By transmitting the semiconductor laser output light having a wavelength in the 5 μm band, a second harmonic having a wavelength matching the absorption line of rubidium can be obtained. In addition, an SHG element made of a material that produces an electro-optic effect is used, and this SHG element is
Since the HG element is loaded with electrodes, it is possible to frequency modulate the second harmonic by applying a low frequency modulation signal to these electrodes, thereby making the oscillation wavelength of the semiconductor laser different from the absorption wavelength of rubidium. The wavelength can be controlled to have a predetermined relationship. In this case, unlike conventional devices, there is no need to superimpose a modulation current on the drive current of the semiconductor laser, so there is no problem of impairing the frequency stability of the light source, and it is possible to provide a frequency stabilizing device with high frequency stability. It becomes possible.

Embodiment FIG. 2 is a block diagram of a semiconductor laser frequency stabilizing device showing a first embodiment of the present invention. In this embodiment, the control circuit includes an oscillator 22 that oscillates at a low frequency;
Adding a modulation signal to the electrode 6 based on the low frequency signal from S
An HG drive circuit 24, a phase comparator 26 that compares the phase of the signal from the photodetector 10 and the above-mentioned low frequency signal and outputs an error signal according to the phase difference, and an LD drive that supplies a drive current to the semiconductor laser 2. a circuit 28, and a PID calculator 3 that performs PID control on the self-driving current so that the error signal becomes zero.
0. 32 is an optical filter provided between the Rb cell 8 and the photodetector 10, and this optical filter removes the fundamental wave and only the second harmonic is transmitted to the photodetector 1.
It functions to be incident on 0. 34 is an amplifier that amplifies the output signal of the photodetector 10 and inputs it to the phase comparator 26. 31 is a condensing lens provided between the semiconductor laser 2 and the SHG element 4. Note that the rubidium cell 8 is constructed by sealing rubidium vapor having an absorbed atomic beam at a wavelength of around 0.78 μm in a gas container made of quartz or the like at a predetermined pressure together with a neutral buffer gas as the case may be.

FIG. 3 is an explanatory diagram of the operation of the phase comparator 26, in which the vertical axis represents the output of the photodetector lO, and the horizontal axis represents the frequency of the second harmonic. 101 represents an absorption curve resulting from absorption of the second harmonic in the rubidium cell 8. The frequency fo at which the absorption is maximum is absolutely constant, and the frequency of the second harmonic is stabilized at this frequency. Now, if the frequency of the second harmonic is lower than fo and a modulation signal (frequency ft) as shown at 102 is given, the output signal of the photodetector is modulated as shown at 102'. The signal has a phase opposite to that of the signal and has a frequency fLO.

Also, the frequency of the second harmonic is f. , and when a modulation signal as shown at 103 is applied, the output signal of the photodetector is in phase with the modulation signal and has a frequency f, as shown at 103'. When the frequency of the second harmonic matches fo and a modulation signal as shown at 104 is given, the output signal of the photodetector has a component of frequency fL as shown at 104'. It does not include the frequency 2fL component. In order to compare the phases of the modulation signal and the output signal of the photodetector, it is preferable to perform synchronous detection of these signals.

FIG. 4 is an explanatory diagram of an error signal obtained by synchronous detection. In this example, a positive error signal is obtained when the modulation signal and the photodetector output signal are out of phase, a negative error signal is obtained when they are in phase, and the level of the error signal is within a given frequency range. The difference is proportional to the phase difference. Therefore, by performing normal feedback control so that this error signal becomes equal to zero, the frequency of the second harmonic can be stabilized to the frequency of the absorption peak. In addition to the driving current of the semiconductor laser, the temperature of the semiconductor laser can be selected as the object to be controlled by the PID control circuit 30.

By realizing the SHG element 4 as an optical waveguide, high second harmonic conversion efficiency can be obtained. This is because the fundamental wave can be efficiently confined within a narrow optical waveguide and the second harmonic can be efficiently generated. An example of the material of the optical waveguide that produces the nonlinear optical effect and the electro-optic effect is utium niobate. In this case, by forming an electrode made of a conductor on or near the optical waveguide using a normal pattern forming technique, it is possible to control the refractive index of the guide waveguide with a relatively low voltage. Since the generated second harmonic only needs to be incident on the light-receiving surface of the photodetector that converts light to electricity, there is no need to narrow it down to a minute area. Therefore, the Cerenkov type can be used as a phase matching method. can. In this case, phase matching between the fundamental wave and harmonic components is automatically performed, so the modulation index of the phase modulation to the SHG element for frequency stabilization is set high.
Even if the S/N ratio of the detection signal is improved, high second harmonic conversion efficiency can be maintained.

FIG. 5 is a block diagram of a frequency stabilizing operation of a semiconductor laser showing a second embodiment of the present invention. In this example, SHG
Reflective films 36 and 38 having high reflectivity for the fundamental wave are formed at the input end and the output end of the element 4, respectively. By configuring the resonator-type SHG generator in this way, it is possible to improve the conversion efficiency of the fundamental wave into the second harmonic.

In this case, the space between the reflective films formed on both ends of the SHG element becomes a Fabry-Perot resonator, which limits the wavelength of incidence on the SHG element, but the original purpose was to stabilize the frequency accurately to the absorption line of Rb. Therefore, when the frequency stabilization operation of the semiconductor laser is working well, the frequency fluctuation width is sufficiently smaller than the bandwidth of the resonant state of the Fabry-Perot resonator, and there is no problem.

However, there is a possibility that the resonant frequency of the Fabry-Perot resonator may fluctuate due to changes in the temperature of the SHG # element, so the SHG element 4 must be It is preferable to control the temperature or the voltage applied to the electrode 6. A specific example will be explained with reference to FIG. In the same figure, 4o is a wavelength of 1.5
This is a demultiplexer that separates a fundamental wave of 6 μm and a second harmonic with a wavelength of 0.78 μm, and the second harmonic that passes through this demultiplexer 40 is transmitted through the rubidium cell 8 and optically detected as in the previous embodiment. The light enters the vessel 10. On the other hand, the fundamental wave reflected by the demultiplexer 40 enters the photodetector 42, which is a component of a feedback loop that controls the resonant frequency of the Fabry-Perot resonator to be constant. In this example, since the demultiplexer 40 is provided, the optical filter 32 in the previous embodiment is unnecessary. The output signal of the photodetector 42 is amplified by an amplifier 44 and input to a phase comparator 46. 48 is an oscillator that oscillates at a low frequency different from that of the oscillator 22, and its output signal is input to the phase comparator 46 and the SHG drive circuit 24. Therefore, S.H.
The G element 4 is frequency modulated based on the low frequency signal from the oscillator 48 even at a frequency different from the frequency fL. The error signal from the phase comparator 46 is sent to the drive circuit 2.
4 and a temperature control device 50 that controls the temperature of the constant temperature bath 52 of the SHG element 4. By forming such a feedback loop, fluctuations in the resonance peak of the Fabry-Perot resonator described above due to changes in the temperature of the SHG element 4 can be effectively prevented. In addition, the phase comparator 45
The error signal may be manually input to either the SHG drive circuit 24 or the temperature control device 50.

According to this embodiment, since the conversion efficiency of the fundamental wave into the second harmonic can be sufficiently increased, the laser light output used for frequency stabilization of the semiconductor laser can be reduced.

As described in detail, the present invention has the advantage that it is possible to provide a frequency stabilizing device for a semiconductor laser with high frequency stability.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the principle of the present invention, Fig. 2 is a block diagram of a frequency stabilizing device for a single conductor laser showing a first embodiment of the present invention, and Fig. 3 is a block diagram of the frequency stabilizing device for a single conductor laser showing a first embodiment of the present invention. FIG. 4 is an explanatory diagram of an error signal in an embodiment of the present invention; FIG. 5 is a block diagram of a semiconductor laser frequency stabilizing device according to a second embodiment of the present invention; FIG. FIG. 2 is an explanatory diagram of a prior art. 2... Semiconductor laser, 4... SHG element, 6... Electrode, 8... Rubidium cell (Rb cell), 10... Photodetector, 12... Control circuit.

Claims (1)

[Claims] Semiconductor laser (2) that oscillates at a wavelength in the 1 and 1.5 μm band
and a material that produces a non-linear optical effect and an electro-optic effect, and based on the light from the semiconductor laser (2), the second
An SHG element (4) that outputs harmonics, and the SHG element (
4) mounted on the electrode (6), a rubidium cell (8) that transmits the second harmonic, and the second harmonic transmitted through the cell (8) undergoes optical-to-electrical conversion to convert the second harmonic into the second harmonic. a photodetector (10) that outputs an electrical signal according to the intensity of the harmonic; 2
The harmonics are frequency modulated, and the photodetector (1
The above semiconductor laser (
2) A control circuit (12) for controlling a parameter on which the oscillation frequency depends. 2. The frequency stabilization device for a semiconductor laser according to claim 1, wherein the control circuit (12) includes: an oscillator (22) that oscillates at a low frequency; (
6) an SHG drive circuit (24) that applies a modulation signal to the signal, and a phase comparator that compares the phase of the signal from the photodetector (10) and the low frequency signal and outputs an error signal according to the phase difference. (26), an LD drive circuit (28) that provides a drive current to the semiconductor laser (2), and a PID calculator (30) that performs PID control of the drive current so that the error signal becomes zero. A semiconductor laser frequency stabilizing device characterized by: 3. In the frequency stabilizing device for a semiconductor laser according to claim 1 or 2, the SHG element (4) has reflective films (36, 38) at the input end and the output end that have a high reflectance for the fundamental wave. A frequency stabilizing device for a semiconductor laser, characterized in that: 4. In the frequency stabilizing device for a semiconductor laser according to claim 3, the temperature of the SHG element (4) or the electrode ( 6) A frequency stabilizing device for a semiconductor laser, characterized in that the voltage applied to the semiconductor laser is controlled.