JP2001274495A - Autolock method for laser beam oscillation frequency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an autolock method for a laser beam oscillation frequency in which an erroneous lock is prevented without counting the number of saturable absorption lines from a mode hop when an arbitray saturable absorption line is detected. SOLUTION: A modulation laser beam which is passed through an iodine cell and which is modulated by a modulation means is detected electrically so as to obtain a modulation signal, and the modulation signal is fed back. When the laser beam oscillation frequency is locked to the saturable absorption line of iodine, the generation sequence of the saturable absorption lines and the interval between the saturable absorption lines are used as a judgment reference when the saturable absorption line is specified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for automatically locking a laser beam oscillation frequency, and more particularly to a method for automatically locking a laser beam oscillation frequency to a specific saturated absorption line of iodine.

[0002]

2. Description of the Related Art In an iodine-stabilized He-Ne laser, the oscillation frequency of a He-Ne laser is locked to a specific saturated absorption line of iodine having a highly accurate frequency to stabilize the oscillation frequency. Since it can emit laser light with a very accurate frequency, it is used as a reference for length.

Since iodine has a large number of saturated absorption lines, such an iodine-stabilized He—Ne laser needs to lock the oscillation frequency to a desired saturated absorption line. What is done is the auto lock function. In this auto-lock, it is necessary to perform two operations of controlling the oscillation frequency of the laser and specifying the saturated absorption line of iodine.

[0004] As is well known, the oscillation frequency of a He-Ne laser is controlled by moving one position of a mirror placed at both ends of a laser tube using a linear piezo.

In the modulation for detecting a saturated absorption line of iodine, an AC waveform is applied to the other modulation piezo to modulate the position of the other mirror. Synchronous detection is used for demodulation of the modulated wave.

In the auto-locking method, a He-Ne laser is modulated with an AC waveform, and a harmonic detected from a demodulated signal obtained by demodulating the modulated wave is checked to specify and lock a desired saturated absorption line of iodine. .

Although the relationship between the voltage applied to the linear piezo and the laser oscillation frequency is proportional, there is no reproducibility even with the same applied voltage. Therefore, normally, the harmonics of the demodulated signal are detected while changing the voltage applied to the linear piezo. And locked to the saturated absorption line.

FIG. 2 shows the states of the fundamental wave (1f), the second harmonic (2f), and the third harmonic (3f) of the demodulated signal with respect to the applied voltage when a voltage is applied to the linear piezo. I have.

Conventionally, when an iodine-stabilized He—Ne laser is used to automatically lock to an arbitrary saturated absorption line, a mod hop of the laser is detected, and the number of saturated absorption lines is counted based on the detected hop to perform locking. I was

[0010]

However, the characteristics near the mode hop differ from laser device to laser device, and the signal change near the mode hop is not the same (because there are a plurality of signals). For this reason, a mode hop point detection error occurs, and erroneous locking of the saturated absorption line occurs, thereby impairing the reliability of the locking operation.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and it is not necessary to count the number of saturated absorption lines from a mode hop for detecting an arbitrary saturated absorption line. It is another object of the present invention to provide a laser light frequency auto-locking method that prevents erroneous locking and enables highly accurate auto-locking.

[0012]

In order to solve the above-mentioned problems, a method of auto-locking a laser oscillation frequency according to the present invention employs the following characteristic configuration.

(1) A modulated signal obtained from a laser beam modulated by a modulating means and passed through an iodine cell is demodulated, and the demodulated signal is fed back to reduce a laser beam oscillation frequency to a specific saturated absorption line of iodine. In the method of auto-locking the laser light oscillation frequency using a saturated absorption line of iodine molecules locked to, the order in which the saturated absorption lines are generated and the interval between the saturated absorption lines are determined when locking to the specific saturated absorption line. An auto-lock method of the laser light oscillation frequency as a criterion

(2) The method of (1) above, wherein the signal level demodulated at the third harmonic frequency of the modulation signal is locked so as to become a peak.

(3) The automatic locking method of the laser light oscillation frequency according to (2), wherein the signal level demodulated at the third harmonic frequency of the modulation signal is locked so as to be zero.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a method for automatically locking a laser beam frequency according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration of an embodiment of a method for automatically locking a laser beam frequency according to the present invention. In the figure, mirrors (half mirrors) 2 and 6 are disposed on both sides of a laser tube 4 that generates laser light. An iodine cell 5 filled with iodine gas is disposed between the laser tube 4 and the mirror 6.
The iodine molecules are excited by the laser light passing through the inside, causing absorption. The generated laser light is output from the mirror 6.

The modulation piezo 3 drives the mirror 2 to modulate the laser light. The modulation piezo 3 is a modulation oscillator 11 based on a fundamental wave f generated from a controller 12.
Driven by the modulation signal from Linear Piezo 8
This is for adjusting the oscillation frequency of the laser light, and the frequency is adjusted by changing the position of the mirror 6.

The linear piezo 8 is driven by an s-scan signal and a lock signal which are switched and output via a switch 9. The scan signal and the lock signal are used to "scan" and "lock" the frequency. When the switch 9 is set to the "scan" side, a signal from the linear piezo applied voltage 10 is applied to the linear piezo 8, When 9 is set to the "lock" side, the laser operates so as to lock the oscillation frequency of the laser to the saturated absorption spectrum of iodine.

That is, in the automatic lock operation, when the oscillation frequency of the laser is locked to the saturated absorption line of iodine, the switch 9 is set to the "lock" side.

As a measurement procedure, first, the switch 9 is set to the "scan" side to change the voltage of the linear piezo applied voltage 10 from the sweep oscillator 10, and the 2f signal is observed to find a peak point of a desired saturated absorption line. . When the second harmonic (2f) signal is at the peak point, the 3f signal becomes zero volt. Therefore, when the third harmonic (3f) signal becomes zero volt (or substantially zero volt), the switch 9 is turned on.
To the “lock” side. Then, by the function of the feedback loop including the laser tube 4, the detector 1, the synchronous detector 17, and the linear piezo 8, the feedback loop is fixed, that is, locked to the frequency of the saturated absorption line.

The laser light is modulated by applying an AC waveform to the modulation piezo 3, and the modulated laser light is converted into an electric signal by the detector 1. This electric signal is amplified to a predetermined signal level by the amplifier 13 and applied to one input of the synchronous detectors 14 to 17 connected to the next stage. Different reference signals are applied to the other input of the synchronous detector.

The synchronous detector 14 detects the fundamental wave component (1f), and receives a 1f signal from the controller 12 as a reference signal. This 1f signal has the same frequency as the modulation signal. Unnecessary components are removed from the output of the synchronous detector 14 by a low-pass filter (LPF) 18.
The fundamental wave component (1f) is output to the fundamental wave (1f) output terminal 29.

The synchronous detectors 15 and 16 detect the second harmonic (2f) of the modulated signal.

The reason why the two synchronous detectors 15 and 16 are used is to detect only the amplitude component of the second harmonic (2f). This is because in the conventional method using only one synchronous detector, the DC level of the output of the second harmonic (2f) changes as the position of the linear piezo 8 changes (that is, the oscillation frequency of the laser beam changes). . Therefore, when a saturated absorption line is specified (identified) in an electronic circuit using the 2f signal having the fluctuation of the DC level, the circuit causes saturation or the like, and the measurement is inconvenient.

Therefore, in order to eliminate such inconvenience, the two synchronous detectors 15 and 16 are driven by a reference signal (described later) having a phase difference of 90 degrees to thereby obtain the amplitude of the second harmonic (2f). Only components are detected. Unnecessary components are removed by low-pass filters (LPFs) 19 and 20, and the respective components are squared by squaring circuits 22 and 23. This 2
The raised signal is passed through an adder circuit 24 to be added, and the added signal is passed through a square root circuit 25 to obtain a square root. As a result, only the amplitude component of the 2f signal is output to the 2f output terminal 30. The reference signal used here is supplied from 2f (0 °) and 2f (90 °) of the controller 12, and the former and the latter have a phase difference of 90 degrees.

With such a configuration, only the amplitude component can be detected, and the DC component does not fluctuate due to the phase change. For this reason, it is possible to prevent problems associated with the electronic circuit when performing spectrum identification using the electric signal of the 2f output terminal 30.

The synchronous detector 17 detects the third harmonic (3f).
The f signal is applied. An unnecessary component is removed from the output of the synchronous detector 17 by a low-pass filter (LPF) 21, and a third harmonic (3f) is obtained at a 3f output terminal 31.

Since the 3f component is also used as a signal for locking the oscillation frequency of the He—Ne laser to the saturated absorption line of iodine, the “lock” of the switch 9 is passed through an integrator including an operational amplifier 26, a resistor 27 and a capacitor 28. "Side. When locked to the saturated absorption line, this 3f signal goes to zero volts.

FIG. 2 shows the outputs 1f, 2f and 3f described above. The horizontal axis represents the applied voltage of the linear piezo 8, and the voltage is higher toward the right. The vertical axis indicates the voltage value of each output. (A) is fundamental wave component 1
f, (b) is the second harmonic 2f, and (c) is the third harmonic 3f.
Are respectively shown.

As an example, the second harmonic (b) will be described. When the voltage applied to the linear piezo 8 is increased, mode hop 1 appears first. As the applied voltage is increased, [abc] in which three saturated absorption lines form a group appears. When the applied voltage is further increased, [defg], [hij], and [klmn], in which a plurality of saturated absorption lines form a group, appear in order, and then mode hop 2 appears.

The saturated absorption lines [defg] and [klmn] have four saturated absorption lines.
g] is determined from the fact that the distance between the respective saturated absorption lines is substantially the same.

Hereinafter, the method of auto-locking the laser light frequency according to the present invention will be described. First, the general operation is as follows.

In this apparatus, the voltage applied to the linear piezo 8 is controlled by changing the voltage between about 50 V and 60 V, which is slightly wider than twice the interval at which mode hops occur.

When the sequence of the auto lock function is started, first, the applied voltage of the linear piezo 8 is changed to the center voltage (about 5 V).
5V).

Next, the applied voltage of the linear piezo 8 is changed from the center voltage, and it is checked that the intensity of the laser beam is within a specified range.

The direction in which the applied voltage is changed differs depending on the setting of the saturated absorption line to be locked. When the saturated absorption line to be locked is [abc] or [def
g], the applied voltage of the linear piezo 8 is increased from the center voltage toward the maximum voltage (60 V).

When the saturation absorption line setting is [hij] or [kl
mn], the minimum voltage (50 V) from the center voltage
, The applied voltage of the linear piezo 8 is decreased.

Next, the harmonic contained in the modulated wave signal is detected while changing the applied voltage of the linear piezo 8.
At the time of detection, it is calculated whether or not the applied voltage values of the linear piezos 8 at the time of the past four detections are at equal intervals.

When the saturated absorption line [defg] is specified, the voltage applied to the linear piezo 8 is changed based on the specified value, and
Detects harmonics in the modulated signal and locks to the set lock absorption line.

FIG. 3 shows a sequence corresponding to a saturated absorption line to be locked together with sequence numbers # 1 to # 16 in the figure.

(1) When locking to the saturated absorption line [abc]: (# 1) Apply the voltage applied to the linear piezo 8 to the center voltage (55
V). (# 2) The voltage applied to the linear piezo 8 is increased from the center voltage to the maximum voltage (60 V). (# 3) The voltage applied to the linear piezo 8 is increased to the maximum voltage (60
The saturation absorption line [defg] is searched while decreasing from V). (# 4) Further, the harmonic component (2f) of the modulated wave signal is observed while further reducing the voltage applied to the linear piezo 8. The first in the case of the saturated absorption line c, the second in the case of b,
In the case of a, the lock operation is performed when the third is detected. (# 5) Further, the third harmonic signal (3f) of the modulated wave is set to zero volt (locked state) while reducing the voltage applied to the linear piezo 8.

(2) When locking to the saturated absorption line [defg]: (# 1) The voltage applied to the linear piezo 8 is set to the center voltage (55
V). (# 2) The voltage applied to the linear piezo 8 is increased from the center voltage to the maximum voltage (60 V). (# 6) The voltage applied to the linear piezo 8 is increased to the maximum voltage (60
Search for [defg] while decreasing from V). After searching, the applied voltage is reduced by a predetermined value and the voltage is passed once. (# 7) Again, the harmonic component of the modulated wave signal is observed while increasing the voltage applied to the linear piezo 8. The lock operation is performed when the first is detected for the saturated absorption line d, the second for e, the third for f, and the fourth for g. (# 8) While further increasing the voltage applied to the linear piezo 8, the third harmonic of the modulation signal is set to zero volt (locked state).

(3) When locking to the saturated absorption line [hij]: (# 1) The voltage applied to the linear piezo 8 is set to the center voltage (55
V). (# 9) The voltage applied to the linear piezo 8 is reduced from the center voltage to the minimum voltage (50 V). (# 10) The voltage applied to the linear piezo 8 is reduced to the minimum voltage (5
[Defg] while increasing from 0V). (# 11) While further increasing the voltage applied to the linear piezo 8, a harmonic component of the modulation signal is searched for. Saturated absorption line h
The lock operation is performed when the first is detected in the case of i, the second in the case of i, and the third in the case of j. (# 12) While further increasing the voltage applied to the linear piezo 8, the third harmonic of the modulation signal is set to zero volt (locked state).

(4) When locking to the saturated absorption line [klmn]: (# 1) Centering the voltage applied to the linear piezo 8 (55 V)
Set to. (# 9) The voltage applied to the linear piezo 8 is reduced from the center voltage to the minimum voltage (50 V). (# 13) The voltage applied to the linear piezo 8 is reduced to the minimum voltage (5
[Defg] while increasing from 0V). (# 14) Search for [hij] while increasing the voltage applied to the linear piezo 8. (# 15) While further increasing the voltage applied to the linear piezo 8, a harmonic component of the modulation signal is searched for. Saturated absorption line k
The lock operation is performed when the first is detected in the case of 1, the second in the case of 1, the third in the case of m, and the fourth in the case of n. (# 16) Further, while increasing the voltage applied to the linear piezo 8, the third harmonic of the third harmonic signal of the modulated wave signal is set to zero volt (locked state).

In the above description, the peak of the saturated absorption line is detected from the peak of the 2f signal. However, the peak of the saturated absorption line may be detected by observing the 3f signal.

Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and various changes and extensions can be considered.

[0048]

As described above, in the laser auto-locking method according to the present invention, in order to detect an arbitrary saturated absorption line, the harmonic generation pattern of the modulation signal (the order of generation of the saturation absorption line, the saturation absorption line). ), A saturated absorption line, for example, “d, e, f, g” is specified, and locked to an arbitrary saturated absorption line based on that. Therefore, even if there is a difference in the mode hop characteristics due to the individual difference of the laser used, it is possible to correctly lock to the desired saturated absorption line without performing an erroneous locking operation.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an iodine-stabilized laser device using an automatic locking method of a laser light oscillation frequency according to an embodiment of the present invention.

FIG. 2 is a diagram showing a state of a fundamental wave and second and third harmonics with respect to a voltage applied to a linear piezo 8;

FIG. 3 is a diagram showing a procedure of a method for specifying a saturated absorption line in the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Detector 2, 6 Mirror 3 Modulation piezo 4 Laser tube 5 Iodine cell 7 Laser light 8 Linear piezo 9 Changeover switch 10 Linear piezo application voltage 11 Modulation transmitter 12 Controller 13 Amplifier 14-17 Synchronous detector 18-21 LPF (Low-pass filter) 22, 23 square circuit 24 adder circuit 25 square root circuit 26 operational amplifier 27 resistor 28 feedback capacitance

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroaki Iwao 6-3-20 Tsunashimahigashi, Kohoku-ku, Yokohama F-term in NF Corporation circuit design block (reference) 5F072 AA01 HH02 HH05 JJ20 KK01 KK30 MM12 MM16

Claims (3)

[Claims] A demodulated signal obtained from a laser beam modulated by a modulating means and passed through an iodine cell is demodulated, and the demodulated signal is fed back, so that the laser beam oscillation frequency is changed to a specific saturated absorption line of iodine. In an automatic locking method of a laser light oscillation frequency using a saturated absorption line of an iodine molecule to be locked, the generation order of the saturated absorption lines and the interval between the saturated absorption lines are determined when locking to the specific saturated absorption line. An automatic locking method of a laser beam oscillation frequency, which is used as a reference. 2. The method according to claim 1, wherein the operation is performed such that a signal level of the modulated signal demodulated at the second harmonic frequency becomes a peak. 3. The method of auto-locking a laser light oscillation frequency according to claim 2, wherein said signal is locked so that a signal level demodulated at a third harmonic frequency of said modulated signal becomes a zero level.