JP2000151003A - Three-mode frequency stabilized laser using square device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the use in a wide field with a device easy to obtain, by a method wherein low frequency component is picked up after an electric signal of a photo detector is squared with a square device, and resonator length of a laser tube is so controlled that a frequency of a sine wave contained in the low frequency component is made constant. SOLUTION: Polarized output light of a laser tube 1 outputting three or four line-spectrums are superposed by using a polarizer 2 and converted to electric signals with a photo detector 3. The spectrums are turned into two or three optical beats. When three optical beats are obtained, a control loop is closed. Signals of the photo detector 3 are amplified with an amplifier 4 and multiplied in a mixer 5, and five components are obtained. When the signals are made to pass a low-pass filter 6, a component having an angular frequency (ω1-ω2) and a DC component only are obtained. The DC component is eliminated, and the frequency (ω1-ω2) is converted to a voltage with a frequency-voltage converter 7. Current of a heater 9 is controlled with a power amplifier 8, and resonator length is so controlled that a value of the frequency (ω1-ω2) is made constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention does not use an expensive and cumbersome VCO and does not use the unstable nonlinear characteristics of an APD (avalanche photodiode). The present invention relates to a three-mode laser that performs frequency stabilization by a method.

[0002]

2. Description of the Related Art An orthogonal three-mode laser cannot be realized by a conventional frequency stabilization method, that is, a polarization intensity stabilization method or a Zeeman frequency stabilization method, and can be realized only by utilizing a frequency pulling phenomenon. VC for that
There have been means of using heterodyne technology with O as the local oscillator or using the non-linear characteristics of the detector. However, the former has a problem that the VCO of the local oscillator is expensive and difficult to obtain, and the latter has a severe selection in order to obtain a convenient product because the non-linearity of the detector is out of specification.

[0003]

SUMMARY OF THE INVENTION The present invention uses a VCO as a local oscillator and does not rely on the nonlinear characteristics of a detector, but provides a low-cost, reliable (selectable) signal equivalent to the signal obtained when using them. And the like).

[0004]

When the number of longitudinal modes of a laser is three, the frequency interval between one high-frequency side and one low-frequency side viewed from the center one is a frequency attraction phenomenon. Have different values for This is because when these three optical spectra are superposed by polarizers of appropriate orientations and converted into electrical signals by a photodetector, an optical beat having a frequency equal to the frequency difference of the light is obtained. There are two spectra, and the difference in the frequency interval means that the frequencies of the two optical beat spectra do not match and the two are separated.

[0005] In a general HeNe laser, the frequency of two optical beats is about 430 MHz on average, and the amount of separation is several hundred KHz. This separation amount of several hundred KHz is a signal necessary for control for frequency stabilization.

[0006] The amount of separation of several hundred KHz is extracted from the electric signal of the detector, and the length of the laser tube is controlled so that the value becomes constant. it can.

In the present invention, the output electric signal is squared to obtain the separation amount of the output electric signal of the photodetector, that is, the difference between the frequencies of the two optical beat spectra of the output electric signal. A signal for control is extracted by extracting a frequency component.

That is, it is now assumed that the heights of the two optical beat spectra are equal, and the vibrations are respectively cos ω1
t, cos ω2t, the square S of the output electric signal is

It is expressed as in Equation 1. ω1 and ω2 are the angular frequencies of the spectrum, respectively.

(Equation 1) The first term of is a constant, except for the last term, as described above.
Since it has a frequency of 0 MHz to 800 MHz, if it is removed by a filter having a low-pass characteristic, the signal passing through the filter becomes

(Equation 2) become that way.

[0009] (ω1-ω2) of the signal SL obtained here.
Is an amount that changes with the change in the length of the laser resonator due to the frequency pulling phenomenon. By controlling the length of the resonator so that this amount is constant, the frequency of the three-mode laser can be stabilized. become.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A laser having a condition in which the polarizations of adjacent modes are orthogonal and the number of spectra is effectively three is such that the number of spectra is between four and three due to a change in the cavity length. And one that moves between three and two.

FIG. 1 shows the former. Practically, the one that shifts between four and three lines has a high output. The ease of determining the start point of control is greatly improved.

FIG. 1 (a) shows the spectrum at the time of four lines, and FIG. 1 (b) shows the spectrum at the time of three lines. Assuming that the angular frequency of the difference between the low frequency side and the center spectrum at the time of three lines is ω1 and the high frequency side and the center is ω2, the spectrum of the optical beat output from the detector is the spectrum shown in FIG. Becomes
Both ω1 and ω2 (ω1−ω2) change as the resonator length changes. In the present invention, the lowest frequency (ω1−ω2) among the three is used. Then, the laser cavity length is controlled so that the value of (ω1−ω2) becomes constant.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Polarization of output light of a laser tube 1 which emits three or four spectra is superposed on a polarizer 2 having an appropriate direction, and when converted into an electric signal by a photodetector 3, two or more spectra are obtained. There are three light beats. The point at which the number of light beats becomes three is selected, and the control loop is closed. Detector 3
Are amplified by the amplifier 4 and multiplied by the mixer 5, (ω1 + ω2), 2ω1, 2ω2, (ω1−ω
2), a DC component is obtained, and when passing through a low-pass filter 6, a component having an angular frequency of (ω1−ω2) and D
C only. Therefore, the DC is removed, the frequency (ω1-ω2) is converted into a voltage by the frequency-voltage converter 7, and the current of the heater 9 wound around the laser tube is controlled with the help of the power amplifier 8 to control the frequency (ω1-ω2). ) Is controlled so that the value of ()) becomes constant.

[0012]

The field of use of the three-mode laser is wide, but its frequency stabilization has not been simple so far. According to the present invention, by using a readily available device to stabilize the frequency of a three-mode laser with high reliability, the use of the three-mode laser in a wide field can be facilitated.

[0013]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a spectrum of a laser of the present invention.

FIG. 2 is an embodiment of the present invention.

[Explanation of symbols]

 1, laser tube 2, polarizer 3, photodetector 4, amplifier 5, mixer 6, low-pass filter 7, frequency-voltage converter 8, power amplifier 9, heater

Claims (1)

[Claims] 1. A method for passing light from a laser tube having a condition that the polarization directions of adjacent longitudinal mode spectra are orthogonal to each other and the number of all spectra is effectively three passes through a polarizer having an appropriate direction. After that, it is detected by a broadband photodetector, and the electric signal of the detector is squared by a squarer operating at several hundred megahertz. A three-mode frequency stabilized laser, wherein the resonator length of the laser tube is controlled so that the frequency of the waveform becomes constant.