JPH1117265A - Single mode high power hene laser device with simple constitution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To output a single mode laser beam by taking out frequency components of less than specified frequency from an output current of an avalanche photodiode and controlling the length of a laser oscillator so that the frequency of an a-c electric signal is const. SOLUTION: Utilizing nonlinear characteristics of both avalanche photodiode 3 and detection system enhanced by an impedance 5 added to a reverse bias power source 4, a secondary beat is taken out and converted to a square wave signal through a comparator 9 after being amplified by an amplifier 6 having a response of 1 MHz or less, a frequency divider circuit 8 divides the frequency of a crystal oscillator 7 to generate a reference pulse signal, a frequency different measuring circuit 10 measures the difference from the square wave signal to thereby control the current of a heater 13 wound round a laser tube through a frequency-voltage converter circuit 11 and current amplifier 12 so that the square wave frequency is equal to the reference frequency and a polarizer 14 allowing only a polarized wave at a center mode to pass is used to take out a single mode light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode light output HeNe laser device using a frequency pulling phenomenon which can be manufactured at a low cost.

[0002]

2. Description of the Related Art In a conventional single mode HeNe laser using a frequency pulling phenomenon, a secondary beat generated between beats between modes of a plurality of oscillation lights uses a voltage controlled oscillator (hereinafter referred to as VCO) as a local oscillator. It was determined using an electric heterodyne system. This heterodyne method is excellent in that a second beat can be obtained with a high signal-to-noise ratio, and therefore, a frequency-stabilized laser using this method has high stability. (Rev. Sci. Ins
trum. 66 2788 (1995)) has had a drawback of requiring VCO which oscillates at a specific frequency range is difficult to obtain and expensive.

A method of obtaining a secondary beat using a PIN photodiode having a small nonlinearity has also been reported. However, since the magnitude of the secondary beat is determined by the magnitude of the nonlinearity of the detection system, the signal-to-noise ratio is low, and thus the signal-to-noise ratio is low. Stability is low.

[0004]

SUMMARY OF THE INVENTION The present invention provides an expensive VCO
And a single-mode stabilization having an appropriate stability without obtaining a second beat under the condition that a signal for control does not have a low signal-to-noise ratio as in the case of using a PIN photodiode. The task is to obtain a laser.

[0005]

The beat between modes of oscillating light is obtained by the square characteristic that "the square of the electric field of light becomes a photocurrent" which is present in any photodetector. It can be extracted in the form of a signal. On the other hand, when there are a plurality of inter-mode beats, they appear as an envelope waveform whose frequency is the difference between the inter-mode beats caused by the frequency pulling phenomenon. I do.

The above-mentioned He having four and three longitudinal modes
In the case of the Ne laser, the frequency of the carrier wave whose envelope is the secondary beat is the average frequency of the longitudinal mode beat frequency.
It is about 0 MHz.

Although the rectification detection of 430 MHz is performed by an electric circuit, it is not much lower than the heterodyne method in view of the high-frequency circuit element.

Accordingly, in the present invention, AP is used as a detector.
By using D, the non-linear characteristic of the APD, and the enhanced non-linear characteristic of the APD by adding a circuit element that is larger than the non-linear characteristic, can be used.
The secondary beat, that is, the above-mentioned envelope is extracted by using the power characteristic (second-order term in the case where the nonlinear function is expanded). The frequency of the secondary beat, that is, the frequency of the envelope is 1 MHz or less, and if the length of the resonator monotonously changes, it periodically changes due to the frequency pulling phenomenon.

Therefore, if the length of the resonator is controlled so that the frequency of the secondary beat becomes constant, the frequency of the laser beam can be made constant. Further, if control is performed so that one mode is located at the center of the gain curve of the laser, the number of oscillation modes becomes three, and the polarization directions of adjacent modes are orthogonal. By taking out the mode, laser light of a single frequency can be obtained.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The matters necessary for the description of the embodiments and the embodiments will be described with reference to the drawings. FIG. 1 is an explanatory diagram of three and four vertical modes. If L is the length of the resonator, the value of C / 2L [Hz] is slightly smaller than 1/3 of the width of the gain curve.
Depending on the value of, the vertical mode becomes three (a) or four (b) as shown in FIG. Now, when the number of beats becomes three, the frequency of beats between modes is fb1 and fb1.
2, the frequencies are different from each other due to the frequency pulling phenomenon, and λ
/ 2 periodically.

The beat frequency between the modes is 430 MH
z, but the difference in beat frequency between modes is 1
MHz or lower, and when observed with a measurement system having a sufficiently high frequency response, a carrier wave of 430 MHz as shown in FIG.
You should see a waveform with an envelope of frequency (fb1-fb2) below MHz.

The output current i is shown in FIG.
When the detection having the non-linearity as shown in (1) is used, the waveform of the envelope, that is, the second beat can be extracted by the non-linear rectification. This non-linear form is APD
Although it depends on itself, it can be changed by inserting an impedance into the reverse bias. A in FIG.
Is the form when the impedance is small and B is large.

FIG. 4 shows an embodiment of the present invention. The light behind the internal cavity laser 1 having three and four longitudinal modes passes through the polarizer 2 having an azimuth of approximately 45 ° to the polarization azimuth of the laser. You get the beat.

After the secondary beat is taken out by the nonlinear characteristic of the APD 3 and the nonlinear characteristic of the detection system emphasized by the impedance 5 added to the reverse bias power supply 4, the secondary beat is amplified by an amplifier 6 having a response of 1 MHz or less. The signal is converted by the comparator 9 into a square wave signal. On the other hand, the frequency of the crystal oscillator 7 is divided by the frequency dividing circuit 9 to produce a reference pulse signal of an arbitrary frequency, and the difference between this frequency and the frequency of the square wave signal is obtained by the frequency difference measuring circuit 10, and the frequency-voltage The current of the heater 13 wound around the laser tube through the conversion circuit 11 and the current amplifier 12 is controlled so that the frequency of the square wave becomes equal to the reference frequency. The oscillation frequency of the reference frequency is shown in FIG.
The value is set such that one central spectrum is located at the center of the gain curve as shown in FIG.

Since the polarization directions of adjacent longitudinal modes of this type of laser are orthogonal, single-mode light can be extracted by using the polarizer 14 that passes only the polarization direction of the central mode. .

[0016]

According to the present invention, a HeNe laser having three and four longitudinal modes is frequency-stabilized in three modes, and when a central mode is taken out, a practical single-mode HeNe laser having the highest output on the ground is obtained. can get. (Compound resonators provide even higher power, but this is not practical.) Such single-mode lasers can be realized without the use of VOCs, which are somewhat difficult to obtain and effective.

[0017]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram relating to a longitudinal mode and a beat between modes of a HeNe laser. FIG. 2 is an explanatory diagram related to the secondary beat. FIG. 3 is a diagram illustrating the nonlinear characteristics of the APD. FIG. 4 shows an embodiment of the present invention.

[0017]

[Explanation of symbols]

 1, internal resonator type HeNe laser tube 2, 14 polarizer 3, APD 4, bias high voltage power supply 5, high frequency impedance 6, amplifier 7 having a bandwidth of 1 MHz or less 7, crystal oscillator 8, frequency divider 9, comparator 10, frequency Difference measurement circuit 11, frequency-voltage conversion circuit 12, current amplifier 13, heater

Claims (1)

[Claims] In an uncontrolled mode, after passing oscillation light of an internal resonator type HeNe laser having four and three longitudinal mode spectra through a polarizing element, it has a frequency characteristic of 400 MHz or more and an input. Avalanche photodiode (hereinafter referred to as A) having a nonlinear characteristic between light intensity and output current.
PD), and extracts a frequency component of 1 MHz or less of the output current of the APD, and controls the resonator length of the laser so that the frequency of the extracted AC electric signal of 1 MHz or less becomes constant. A frequency stabilized HeNe laser device characterized by the above-mentioned.