JPH02244782A - Frequency stabilized semiconductor laser driver - Google Patents

Abstract

PURPOSE:To narrow the band of light spectra and stabilize a frequency by a method wherein the output light of a semiconductor laser is applied to an absorbing cell and a Fabry-Perot interferrometer and the semiconductor laser is controlled by a signal obtained by applying the transmitted light intensity signal of the absorbing cell to a low-pass filter and a signal obtained by applying the transmitted light intensity signal of the Fabry-Perot interferrometer to a high-pass filter. CONSTITUTION:The output light of a semiconductor laser 1 is divided into four lights by half-mirrors 5, 6 and 7 and the lights are applied to a Fabry-Perot interferrometer 14, an absorbing cell 8 and a photodetector 17 and also outputted to the outside. The high frequency component of the output of a first photodetector 11 which detects the transmitted light intensity of the absorbing cell 8 is cut off by a low-pass filter 12 and the low frequency component of the output of a second photodetector 15 which detects the transmitted light intensity of the Fabry-Perot interferrometer 14 is cut off by a high-pass filter 16. The semiconductor laser 1 is controlled by the summation signal of those output signals. With this constitution, noise produced by the absorbing cell 8 and the influence of the fluctuation of transmission characteristics caused by the fluctuation of the distance between the semi-transmitting mirrors of the Fabry-Perot interferrometer 14 and the refractive index of the air can be avoided.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a frequency-stabilized semiconductor laser device that controls a semiconductor laser to stabilize the frequency of its output light, and particularly to a frequency-stabilized semiconductor laser device that controls a semiconductor laser to stabilize the frequency of its output light. The present invention relates to a frequency-stabilized semiconductor laser device that can achieve band narrowing at the same time.

<Prior Art> Semiconductor lasers have the characteristics of being small and highly reliable, but have the disadvantage that the frequency of their output changes depending on the surrounding conditions and the half-width of the optical spectrum is wide.

With reference to FIG. 6, a wavelength stabilizing device for improving such drawbacks of semiconductor lasers will be explained. In FIG. 6, the semiconductor laser LD is placed in a constant temperature bath, and its ambient temperature is controlled. The constant temperature bath is controlled by a temperature controller 1°C. The output light of this semiconductor laser LD is focused by a lens L,
It is split into two by half mirror HM. Half mirror H
The transmitted light of M is outputted to the outside as output light, and the reflected light is 1
/2λ wavelength plate Z and polarizing beam splitter PBS
The beam is input into the , and is split into two.

The intensity of one of the branched lights is converted into an electrical signal by the first photoelectric conversion element PDI, and the first #! It is amplified by an I amplifier G1, and the output of this first amplifier G1 is input to an integrator B. The other light split by the polarizing beam splitters PB and S is input to a wavelength selection element E such as a Fabry-Perot interferometer, and the transmitted light is converted into an electrical signal by a second charging conversion element PD2 and input to an integrator B. be done. Integrator B integrates the difference between the input signals of the two photoelectric conversion elements.

The output of the integrator B is amplified by the second amplifier G2, and the output controls the drive current of the semiconductor laser LD to change the wavelength of the output light. In this configuration, the wavelength of the output light of the semiconductor laser LD is controlled to be at the slope of the wavelength characteristic of the transmitted light intensity of the wavelength selection element E, so that the wavelength can be stabilized. In addition, semiconductor laser LD
Since the output light of the semiconductor laser LD is branched into two and the semiconductor laser LD is controlled by the difference between the light that passes through the wavelength selection element E and the light that does not pass through the wavelength selection element E, the operating point remains unchanged even if the output light intensity of the semiconductor laser LD changes. It also has the feature of not shifting.

<Problems to be Solved by the Invention> However, such wavelength stabilizing devices for semiconductor lasers have the following problems. As a wavelength selection element, a Fabry-Perot interferometer or an absorption cell containing a standard substance that absorbs only light of a specific wavelength, such as Rb, is used, but when using a Fabry-Perot interferometer, it is not possible to narrow the optical spectrum. However, since the intensity characteristics of the transmitted light are determined by the distance between the two semi-transparent mirrors, it has been difficult to stabilize it over the long term. Also, since the speed of light changes depending on the refractive index of the air, it was not possible to stabilize the frequency. On the other hand, absorption cells utilize the absorption characteristics of atoms, so they have sufficient long-term stability, but the absorption cells themselves generate noise, making it impossible to narrow the spectrum of output light.

<Object of the Invention> An object of the invention is to provide a frequency-stabilized semiconductor laser device that can simultaneously stabilize the frequency and narrow the spectrum of output light.

Means for Solving the Problems> In order to solve the above problems, the present invention makes the output light of a semiconductor laser enter an absorption cell and a Fabry-Perot interferometer, and calculates the intensity of the transmitted light of the absorption cell and Fabry-Perot interferometer. A first photodetector and a second photodetector detect the semiconductor laser, and a control means controls the frequency of the output light of the semiconductor laser based on the outputs of these detectors,
The output of the second photodetector is limited to a low frequency band and input to the control means, and the output of the second photodetector is limited to a high frequency band and input to the control means. be.

<Embodiment> FIG. 1 shows an embodiment of a frequency-stabilized semiconductor laser device according to the present invention. In FIG. 1, 1 is a semiconductor laser, which is placed in a thermostatic oven 2. This constant temperature'
! The temperature inside R2 is precisely controlled by the temperature controller 3. 4 is an isolator, into which the output light of the semiconductor laser 1 collimated by a lens is inputted.

Isolator 1 blocks the return light and semiconductor laser 1
Used to ensure stable operation. 5 is a half mirror tear, the transmitted light of isolator 4 is incident, and this light is
Do one trick every minute. 6 is a half mirror, into which the light transmitted through the half mirror 5 is incident, and this light is split into two. The light transmitted through the half mirror 6 is output to the outside as output light. 7 is a half mirror, into which the light reflected by the half mirror 6 is incident, and this light is split into two. 8 is an absorption cell, into which the light transmitted through the half mirror 7 is incident.

A standard substance such as I'N b that absorbs light of a specific frequency is sealed inside the absorption cell 8 . Absorption cell 8
is a constant temperature I so that its absorption intensity does not change depending on the temperature.
! 9 to keep its temperature constant. This constant temperature bath 9 has a temperature controller 104. ,: more controlled. 11 is a first photodetector, into which the transmitted light of the absorption cell 8 is incident;
The light intensity is converted into an electrical signal. 12 is a low-pass filter to which the output of the first photodetector 11 is input.

13 is a control means to which the output of the low-pass filter 12 is input. The output of the control means 13 is input to the semiconductor laser 1 by AC coupling, and controls the injection current. 1
4 is a Fabry-Perot interferometer, into which the light reflected by the half mirror 5 is incident. 15 is a second photodetector;
Transmitted light from the Fabry-Perot interferometer 14 is incident, and its light intensity is converted into an electrical signal. 16 is a bypass filter to which the output of the second photodetector 15 is input. The output of the bypass filter 16 is input to the control means 13. 17
is a photodetector, into which the light reflected by the half mirror 7 is incident, and the light intensity is converted into an electrical signal.

The output of the photodetector 17 is input to the control means 13.

The control means 13 controls the semiconductor laser 1 based on a signal obtained by subtracting the output of the photodetector 17 from the signal obtained by adding the outputs of the low-pass filter 12 and the bypass filter 16.

Next, the operation of this embodiment will be explained. Semiconductor laser 1
The output light is split into four by half mirrors 5, 6, and 7,
They are incident on the Fabry-Perot interferometer 14, the absorption cell 8, and the photodetector 17, and are output to the outside. The absorption cell 8 absorbs only light of a specific frequency, and the Fabry-Herot interferometer 14 transmits only light of a specific frequency, so the injection current of the semiconductor laser 1, etc. is controlled so that the intensity of these transmitted lights is constant. By changing the frequency of the output light, the frequency is controlled to be constant.

In this example, the first
The output of the second photodetector 11 is filtered by a low-pass filter 12 to remove its high frequency components, and the output of the second photodetector 15, which represents the transmitted light intensity of the Fabry-Bero interferometer 14, is filtered by a bypass filter 16 to remove its low frequency components. Since the semiconductor laser 1 is controlled by the sum signal of these signals, short-period control is performed by the Fabry-Perot interferometer 14.
Long-period control is performed by the absorption cell 8. For that reason,
It is possible to eliminate the noise generated by the absorption cell 8 and the coloring caused by variations in transmission characteristics due to changes in the spacing between the semi-transparent mirrors of the Fabry-Perot interferometer 14 and the refractive index of air, narrowing the optical spectrum and changing the frequency. For example, in order to achieve a frequency stability of 10-8 using only a Fabry-Perot interferometer, a Fabry-Perot interferometer 14
Even if a material with a low expansion coefficient such as ZERODUR is used as a spacer, temperature fluctuations must be kept below ±0.2°C, and air gap etalons have a temperature fluctuation of ±0.01° in order to suppress fluctuations in the refractive index of air. However, in this embodiment, the long-term stability is controlled by the absorption cell 8, so that the Fabry-Perot interference print 14 has a peak of transmitted light 18 shown in FIG.
Since it only needs to be at the shoulder part, Δf corresponding to the shoulder part
Some degree of variation is allowed.In a normal Fabry-Perot-T interpolator, this Δf' is about 400 MH7, so 1.
It is sufficient to have a stability of 1xio'. For this, low v6
If a material with a capacitance is used, a temperature fluctuation of about ±21°C is allowed, and even with an air gap etalon, it is sufficient to control the temperature to within ±1°C.

FIG. 3 shows a semiconductor laser 1, a first photodetector 11, and a second photodetector 11.
．． 15. An example of a specific configuration of the photodetector 17, low-pass filter 12, bypass filter 16, and control means 13 is shown. The same elements as in FIG. do. In FIG. 3, the output of the photodetector 17 is inverted and amplified by the amplifier 19 and added to the output of the first photodetector 11. The low pass filter 12 is an amplifier 20
It consists of a resistor and a capacitor connected to the feedback side. In addition, the output of the second photodetector 15 is passed through a bypass filter 16 composed of an amplifier 21 and a capacitor.
is man-powered. The outputs of the bypass filter 16 and the low-pass filter 12 are input to a control means 13 made up of two transistors, which controls the injection current of the semiconductor laser 1. The semiconductor laser 1 is driven by the combined output of the low-pass filter 12 and the bypass filter 16. FIG. 4 shows yet another embodiment. The output of the photodetector 17 is sent to the amplifier 1
The signal is inverted and amplified at 9, and is input to the low-pass filter 12 together with the output of the first photodetector 11.

The low frequency component of the output of the second photodetector 15 is cut off by a capacitor 22, added to the output of the low-pass filter 12 by an amplifier 23, and input to the control means 13.

As shown in FIG. 5, when the monotonically changing peak 24 of the transmitted light characteristic of the absorption cell is locked to the frequency of the output light of the semiconductor laser 1, the change in frequency is converted into a change in light intensity. Therefore, the frequency can be stabilized with a simple device without using a device such as a lock-in amplifier.

In addition, a low-pass filter 12, a bypass filter 16,
The control means 13 may be constructed in one piece.

<Effects of the Invention> 1 = As specifically explained based on the embodiment, in this invention, the output light of a semiconductor laser is incident on an absorption cell and a Fabry-Perot interferometer, and the transmitted light intensity of the absorption cell is 18
The semiconductor laser was controlled by a signal obtained by passing a signal passed through a bus filter and a signal obtained by passing a transmitted light intensity signal of a two-fold bellows interferometer through a bypass filter. Therefore, there is an effect that the optical spectrum can be narrowed and the frequency can be stabilized at the same time.

Furthermore, since there is no need to strictly control the temperature of the Fabry-Perot interferometer, there is also the effect that the configuration of the device is simplified.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a frequency-stabilized semiconductor laser device according to the present invention, FIG. 2 is a characteristic curve diagram showing the characteristics of a Fabry-Perot interferometer, and FIGS. 3 and 4 are specific configuration diagrams. FIG. 5 is a characteristic curve diagram showing the characteristics of an absorption cell, and FIG. 6 is a configuration diagram showing the configuration of a conventional wavelength-stabilized semiconductor laser device. 1... Semiconductor laser, 5-7... Half mirror 8
...Absorption cell, 11...First photodetector, 12...
-Low pass filter, 13...control means, 14...
Fabry-Perot interferometer, 15...second photodetector, 1
6... Bypass filter, 17... Photodetector. Figure 3', ','E 2 T! Stabilized frequency by

Claims (1)

[Claims] A semiconductor laser, an absorption cell into which the output light of the semiconductor laser is incident, a first photodetector which detects the intensity of the transmitted light of the absorption cell, and a Fabry-Perot interferometer into which the output light of the semiconductor laser is incident. and a second photodetector that detects the intensity of the transmitted light of the Fabry-Perot interferometer.
a control means to which the output of the second photodetector and the first photodetector is input and controls the frequency of the output light of the semiconductor laser; A frequency stabilized semiconductor laser device characterized in that the output of the second photodetector is limited to a high frequency band and input to the control means, and the output of the second photodetector is limited to a high frequency band and input to the control means. .