JPH03129891A - Laser oscillation wavelength stabilizing device - Google Patents

Abstract

PURPOSE:To obtain an stable optical output by a method wherein laser rays are communicated between the components of an optical system through an optical fiber. CONSTITUTION:Light emitted from a semiconductor laser 1a is guided to an optical fiber, inputted into a light frequency modulation means 4a propagating through the optical fiber, and modulated in frequency as prescribed. The frequency-modulated light is guided to an optical fiber FB1 and inputted into an optical absorbent 5a as propagating, and the transmitted light is guided to an optical fiber FB2, propagates, detected by a photodetector 6a, and photoelectrically converted. In succession, a difference between the absorption wavelength peak value of the light absorbent 5a and the central wavelength of the inputted light is detected through a feedback circuit 7, the oscillation wavelength of the semiconductor laser 1a is made to synchronize with the absorption frequency concerned basing on the difference signal obtained by detection. By this setup, a stabilized optical output can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention aims to stabilize the oscillation wavelength of laser light, which is used as a wavelength standard in optical communication and optical measurement, with reference to the wavelength of the resonant absorption line of atoms or gas molecules. The present invention relates to a laser oscillation wavelength stabilizing device that can be used for various purposes.

(Prior art) Figure 2 is a block diagram of a conventional laser oscillation wavelength stabilization device (Reference: Yanagawa et al., Applied Physics Letters, Vol. 47, pp. 1036-1038, 19
(see 1985).

In Figure 2, 1 is a semiconductor laser that oscillates at a predetermined wavelength, 2.3 is a lens, 4 is an optical frequency modulator, and 5 is an absorber consisting of a glass tube with a length of about 10 cm to 1 m filled with acetylene gas, etc. 6 is a light receiver, and 7 is a feedback circuit for stabilizing the oscillation wavelength.

In such a configuration, light emitted from one end surface of the semiconductor laser 1 is converted into parallel light by the lens 2 and input to the optical frequency modulator 4 . This input light is subjected to predetermined frequency modulation in the optical frequency modulator 4 with reference to the signal from the feedback circuit 7, and is output as frequency modulated light.

Next, this frequency modulated light is input to the absorption cell 5.

In the absorption cell 5, a change in the frequency of light is converted into a change in the intensity of light due to the frequency dependence of its transmittance. That is, the frequency difference between the center frequency (wavelength) of the frequency modulated light and the peak frequency (wavelength) of the absorption cell 5 is detected.

The output light of this absorption cell 5 is received by a light receiver 6, converted from an optical signal to an electrical signal, and outputted to a feedback circuit 7.

The feedback circuit 7 generates an error signal according to the above-mentioned frequency difference, and supplies the semiconductor laser 1 with an injection current based on this error signal, that is, an injection current controlled so that the oscillation wavelength follows the wavelength reference. . As a result, stabilized output light SO is output from the other end surface of the semiconductor laser 1.

(Problem to be Solved by the Invention) However, in the above-mentioned conventional device, the optical path is disturbed due to minute positional changes of the lenses 2 and 3 and the absorption cell 5, and the output light from the optical frequency modulator 4 cannot be accurately transferred to the absorption cell 5. It becomes difficult to allow the light to pass through the inside and to receive the transmitted light with good sensitivity at the light receiver 6.

That is, there is a problem in that it is not possible to input light of stable intensity to the absorption cell 5, which hinders stabilization of the oscillation wavelength of the semiconductor laser 1.

In addition, in order to solve this problem, it may be possible to install the lens 2° 3, optical frequency modulator 4, absorption cell 5, and light receiver 6 on a strong fixed plate, but this would reduce the size of the device. There were disadvantages in the device configuration, such as increased weight and weight.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser oscillation wavelength stabilizing device that has an excellent optical system stability and can obtain stable optical output.

(Means for Solving the Problems) In order to achieve the above object, the present invention includes a semiconductor laser, an optical frequency modulation means for frequency modulating the laser light from the semiconductor laser, and a predetermined wavelength modulation unit for the frequency modulated light. In a laser oscillation wavelength stabilizing device that includes an optical system including a light absorption medium that absorbs only light, and oscillates the semiconductor laser in synchronization with the absorption wavelength of the light absorption medium, the optical system has a light absorption medium that absorbs only light. Laser light is transmitted and received via an optical fiber.

(Function) According to the present invention, the light emitted from the semiconductor laser is guided to the end fiber, propagated through the optical fiber, and then input to the optical frequency modulation means. The light input to the optical frequency modulation means is subjected to predetermined frequency modulation. This frequency modulated light is guided into an optical fiber, and after propagating through the optical fiber,
input into a light absorbing medium.

The light input to the light absorption medium is absorbed only with a predetermined wavelength, and the error between the peak absorption wavelength of the medium and the center frequency of the input light is detected, and based on the error signal accompanying this detection. By synchronizing the oscillation wavelength of the semiconductor laser with the absorption wavelength, stabilized optical output can be obtained.

(Embodiment) FIG. 1 is a block diagram showing a first embodiment of a laser oscillation wavelength stabilizing device according to the present invention, and the same components as those in FIG. 2 showing a conventional example are denoted by the same reference numerals. That is, 1a is a semiconductor laser module (hereinafter simply referred to as a semiconductor laser), 4a is an optical frequency modulator, 5a is an absorption cell for optical wavelength reference, 6a is a light receiver, 7 is a feedback circuit for stabilizing the oscillation wavelength,
8 is a fiber coupler (hereinafter referred to as an optical coupler) F
BI and FB2 are optical fibers. Note that an optical system is composed of a semiconductor laser la, an optical frequency modulator 4a, an absorption cell 5as, and a light receiver 6a.

The semiconductor laser 1a is composed of an In1CaAsP distributed feedback semiconductor laser (DFB type LD) that oscillates at a wavelength of 1 and 5500 μm, for example. Furthermore, an optical fiber output terminal (not shown) is provided so that output can be taken out through an optical fiber.

The optical frequency modulator 4a applies predetermined frequency modulation to the light emitted from the semiconductor laser 1a. It is also provided with an optical fiber input/output terminal (not shown) so that input/output can be performed using a fiber end.

For example, the optical wavelength reference absorption cell 5a has a cell length of 5 (up to)
, isotope-substituted acetylene gas ("C2
It is composed of a cell with an absorption line of 1.5495 μm (full width at half maximum: 800 MHz, absorption intensity: 10%) in which H2) is sealed at 10 Torr. Further, an optical fiber input/output terminal (not shown) is provided so that input/output can be performed using an optical fiber μ.

Further, FIG. 3 is a graph showing the transmitted light intensity characteristics with respect to the input optical frequency of the absorption cell 5a, in which the horizontal axis represents the optical frequency (wavelength) and the vertical axis represents the transmitted light intensity.

The light receiver 6a receives the output light of the absorption cell 5a, converts it into an electrical signal, and outputs this signal to the feedback circuit 7. Also,
An optical fiber input terminal (not shown) is provided so that input can be performed using an optical fiber.

The feedback circuit 7 inputs the electric signal from the photoreceiver 6a, generates an error signal in response to the electric signal, and supplies the semiconductor laser 1a with an injection current controlled so that the oscillation wavelength of the semiconductor laser 1a follows the wavelength reference. .

The optical coupler 8 connects the semiconductor laser 1a and the optical frequency modulator 4.
The input end is connected to the output terminal of the semiconductor laser 1a, and the negative output end is connected to the input terminal of the optical frequency modulator 4a, respectively, and the emitted light from the semiconductor laser 1a is split into two.

One end of the optical fiber FBI is connected to the output terminal of the optical frequency modulator 4a, and the other end is connected to the input terminal of the absorption cell 5a.

Similarly, the optical fiber FB2 has one end connected to the output terminal of the absorption cell 5a, and the other end connected to the input terminal of the light receiver 6a.

Next, the operation of the above configuration will be explained.

First, the light emitted from the semiconductor laser 1 oscillated at a predetermined wavelength (1,5500 μm) is input to the optical coupler 8 and split into two, and the - branched light is input to the optical frequency modulator 4a. This input light is subjected to predetermined frequency modulation in the optical frequency modulator 4a with reference to the signal from the feedback circuit 7, and is guided to the optical fiber FBI as so-called frequency modulated light.

Next, the frequency modulated light propagated through the optical fiber FBI is input to the absorption cell 5a. In the absorption cell 5a, light absorption occurs at a certain specific frequency, and the intensity of light transmitted through the absorption cell 5a exhibits characteristics as shown in FIG.

The transmitted light of the absorption cell 5a is transmitted through the optical fiber F B 21::
After being guided and propagating through the optical fiber FB2, the light receiver 6a
The electric signal received by the sensor and photoelectrically converted there is output to the feedback circuit 7.

Feedback circuit 7 generates an error signal according to the input electrical signal. This error signal corresponds to the first-order differential form of the absorption cell 5a. By controlling the current injected into the semiconductor laser 1a by the feedback circuit 7 based on this error signal, the oscillation wavelength of the semiconductor laser 1a is synchronized with the absorption line peak of the absorption cell 5a, and the oscillation wavelength of the semiconductor laser 1a is stabilized at this wavelength.

This wavelength-stable light is split into two by the optical coupler 8, and output from the other output end as the output light SO of the device.

In fact, in the configuration shown in FIG. 1, it is possible to suppress fluctuations in the central oscillation wavelength of the semiconductor laser 1a to less than 1X10-' nm (I MHz in terms of optical frequency).

Further, in the configuration shown in FIG. 1, the temperature of the gas filled in the absorption cell 5a was changed by changing the ambient temperature of the absorption cell 5a. At this time, the full width at half maximum of the absorption line changed by several tens of percent, but the measurement accuracy was 1
No changes were observed within the 0 MHz range.

As explained above, according to the present embodiment, between each element constituting the optical system, specifically, between the output terminal of the semiconductor laser 1a, the input terminal of the optical frequency modulator 4a, and the optical frequency modulator 4,
The output terminal of a and the input terminal of the absorption cell 5a, and the output terminal of the absorption cell 5a and the input terminal of the light receiver 6a are connected to an optical fiber 5-fiber coupler 8 and an optical fiber FBI, FB, respectively.
2, the intensity of light input to the absorption cell 5a can be stabilized without increasing the size and weight of the device. Therefore, it is possible to increase the light transmission intensity in the absorption cell 5a, obtain high light receiving sensitivity, and realize a laser oscillation wavelength stabilizing device that can stabilize the oscillation wavelength of the semiconductor laser 1a with high precision.

In this example, isotope-substituted acetylene gas was used as the gas to be filled in the absorption cell 5a, but the gas is not limited to this, and ordinary acetylene gas, ammonia gas, methane gas, carbon dioxide, etc. may be used. However, the same effects as described above can be obtained.

FIG. 4 is a configuration diagram showing a second embodiment of the laser oscillation wavelength stabilizing device according to the present invention. The difference between the embodiment of Section 2 and the first embodiment is that the absorption cell 5a and the light receiver 6a
Instead of connecting the two with an optical fiber, the two are integrated.

Other configurations and effects are similar to those of the first embodiment.

(Effects of the Invention) As explained above, according to the present invention, there is provided a semiconductor laser, an optical frequency modulation means for frequency modulating the laser light from the semiconductor laser, and a semiconductor laser that modulates the frequency of the laser light from the semiconductor laser. A laser oscillation wavelength stabilizing device that includes an optical system including a light absorption medium and oscillates the semiconductor laser in synchronization with the absorption wavelength of the light absorption medium, wherein the optical system connects each component with an optical fiber. Since laser light is transmitted and received through the semiconductor laser, fluctuations in the intensity of light input to the absorption cell can be suppressed without increasing the size or weight of the device. The present invention has the advantage of being able to provide a laser oscillation wavelength stabilizing device that can stabilize the oscillation wavelength of the laser with high precision.

Further, as described above, since it has effects such as stability of the optical system and miniaturization of the device, it has the advantage that it can be used as a wavelength standard light source in coherent optical communication and a light source in optical measurement.

[Brief explanation of the drawing]

FIG. 1 shows the first part of the laser oscillation wavelength stabilizing device according to the present invention.
FIG. 2 is a configuration diagram of a conventional laser oscillation wavelength stabilizing device, FIG. 3 is an input/output characteristic diagram of an absorption cell according to the present invention, and FIG. 4 is a diagram showing a laser oscillation according to the present invention. It is a block diagram which shows the 2nd Example of a wavelength stabilization device. In the figure, 1a... Semiconductor laser, 4a... Optical frequency modulator, 5i... Absorption cell for optical wavelength reference, 6a... Light receiver, 7... Feedback circuit, 8... Optical fiber. Coupler, FBl, FB2...optical fiber.

Claims (1)

[Claims] An optical system comprising a semiconductor laser, an optical frequency modulation means for frequency modulating laser light from the semiconductor laser, and a light absorption medium that absorbs only light of a predetermined wavelength of the frequency modulated light. In the laser oscillation wavelength stabilizing device that oscillates the semiconductor laser in synchronization with the absorption wavelength of the light absorption medium, the optical system transmits and receives laser light between each component via an optical fiber. A laser oscillation wavelength stabilizing device characterized by: