JPH0286180A - Method for stabilizing frequency of internal mirror type he-ne laser having 543-nm oscillating wavelength - Google Patents

Abstract

PURPOSE:To stabilize the frequency of the title laser within a wide range by impressing a specific static magnetic field upon the inside of the laser thin tube and, at the same time, feedback controlling the resonator length by utilizing the intensity difference or intensity ratio between adjacent longitudinal modes as an error signal after detecting the difference or ratio. CONSTITUTION:The 543-nm He-Ne laser light oscillated from a laser tube LA is separated into light components respectively having inherent polarizing directions C1 and C2 at a polarization beam splitter PBS and the components are respectively detected and amplified by means of PIN photodiodes PD1 and PD2 and amplifiers AM1 and AM2 as DCs. Outputs of the amplifiers AM1 and AM2 are inputted to a differential amplifier DA where an intensity difference between the light components is detected. The intensity difference is outputted from the amplifier DA as an error signal after amplification and the collector current of a transistor Tr2 is controlled from the amplified error signal. Then the electric power supplied to a heater HE is controlled by the controlled collector current and the resonator length of the laser tube LA is corrected. Therefore, the longitudinal mode can be stabilized to a fixed frequency position.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to a laser having an oscillation wavelength of 543 nm, which is emitted from an internal mirror type helium-neon laser device having a structure in which a laser tube is disposed within a laser tube. Regarding the stabilization method.

[Technology background]

Conventionally, the oscillation wavelength of an internal mirror type helium neon laser is 633 nm (red).This internal mirror type helium neon laser (hereinafter also referred to as a 633 nm He-Ne laser) has an oscillation wavelength of 633 nm.

), many control means have already been established, such as the two-mode method, the Lamb-Dip method, the iodine absorption cell method, the longitudinal Zeeman method, the transverse Zeeman method, and the magnetic modulation method.

On the other hand, an internal mirror type helium neon laser (hereinafter referred to as "543nmH") with an oscillation wavelength of 543nm (green)
Also called "e-Ne laser". ) is a relatively new laser whose oscillation was first reported by Berry in 1970.
[E.J.

Quantum Electron 08-7(19
71) 102).

This 543nm He-Ne laser is a 633nm H
Since the oscillation wavelength is shorter than that of the e-Ne radar, improvement in measurement accuracy can be expected when applied to precision measurements.

However, on the other hand, this 543 nm He-Ne laser has a transition (3S2 → 2 pro) gain of 633 nm He-Ne laser.
-Ne laser is extremely small, about 1/15 to 1/17, and therefore it has been difficult to put it into practical use for a long time.

However, due to the recent improvement in the performance of laser mirrors, it has become possible to generate 543 nm He with a cavity length of about 4 Q cm.
It has become possible to increase the oscillation output of the -Ne laser to a practical level.

In 1987, T. Ferman et al.
Experimental results of frequency stabilization conducted using a 43 nω He-Ne laser were reported (References; ^ppled
0ptics, vol126. No, 14. P2
705. (1987).

According to this report, the polarization direction of each orthogonal linearly polarized longitudinal mode suddenly changes by 90 degrees near the point where the longitudinal mode is arranged symmetrically with respect to the gain center, which is called polarization flipping.
ng) (hereinafter also referred to as "polarization reversal").

Therefore, when stabilizing the frequency of a 543 nm He-Ne laser using the two-mode method, it is necessary to avoid the region where polarization inversion occurs (hereinafter referred to as "polarization inversion region").

Here, the two-mode method utilizes the property that the polarized light of adjacent longitudinal modes is always orthogonally linearly polarized, and the polarized light of adjacent longitudinal modes is, for example, polarized by a polarizing beam splitter (
In this method, the intensity difference or intensity ratio is used as an error signal for the laser oscillation frequency to stabilize the frequency.

However, in order to increase the laser output to a practical level, the laser device currently on the market has a resonator length of about 49 cm. Therefore, 543 oscillates
When the longitudinal modes of the nm He-Na laser are observed, as shown in FIG. 1, the 543 nm He-Ne laser normally oscillates in a range of 3 to 4 longitudinal modes. and,
When the power is turned on, the laser tube thermally expands and the resonator length increases, so the longitudinal mode changes as follows in Figure 1: (a)→ra)→(C)→(d)→(a)...・Changes repeatedly like this. C1 and C3 are the unique polarization directions of the laser tube, where C1 is the direction where the light intensity is weak and C2 is the direction where the light intensity is strong.

[Problem to be solved by the invention]

However, in the 543 nm He-Ne laser,
(1) Adjacent longitudinal modes are always orthogonally polarized (and there is always a pair of parallel polarizations); (2) In the case of a longitudinal mode arrangement where gain competition between modes becomes intense,
In other words, in the case of (a) or (d) in Figure 1, a polarization reversal occurs in which the polarization direction of each mode suddenly changes by 90 degrees.
It is significantly different from a He-Ne laser.

Therefore, by applying the two-mode method as is,
Stabilizing the frequency of the e-Na laser causes the following problems.

(1) The range in which frequency can be stabilized is narrow.

(2) Since the frequency can only be stabilized with the mode arrangement and polarization state shown in (a) or (C) of Figure 1, for one polarization component, the frequency difference is the longitudinal mode spacing.
This is disadvantageous when used as a light source for a polarization interferometer.

The present invention has been made based on the above-mentioned circumstances, and its purpose is to produce 543 nm He-N
The object of the present invention is to provide a frequency stabilization method that can sufficiently stabilize the oscillation frequency of an a-laser.

[Means to solve the problem]

In order to achieve the above object, the present invention provides an oscillation wavelength of 543 nm oscillated from an internal mirror type helium neon laser device having a structure in which a laser tube is disposed within a laser tube.
A method for stabilizing the frequency of a laser having an oscillation wavelength of 543 nm, the method comprising applying a static magnetic field satisfying the following conditions [1] to [3] within the laser tube to stabilize the polarization of adjacent longitudinal modes of a laser having an oscillation wavelength of 543 nm. The frequency of the laser with an oscillation wavelength of 543 nm is stabilized by setting the orientations to mutually orthogonal linearly polarized light, detecting the intensity difference or intensity ratio between adjacent longitudinal modes, and using this as an error signal to feedback control the resonator length. It is characterized by becoming

Condition ■ The direction of the static magnetic field is perpendicular to the axis of the laser tube, and the angle between the direction of the static magnetic field and the laser tube's intrinsic polarization direction is 30 to 4.
Must be 2°.

Condition (1) The magnitude of the static magnetic field is such that the Zeeman frequency between π and ±σ transitions and the longitudinal mode spacing are equal when the static magnetic field is applied.

Condition ■ The magnitude of the static magnetic field is almost uniform along the longitudinal direction of the laser tube.

[Effect]

When a specific static magnetic field that satisfies the above conditions [1] to [3] is applied inside the Rede tube, each longitudinal mode becomes symmetrically arranged with respect to the gain center, as will be understood from the experimental results described in detail later. Even if the light is orthogonally linearly polarized in the vicinity, the polarization direction of each longitudinal mode will not suddenly change by 90°.

Therefore, by applying the two-mode method, that is, by detecting the intensity difference or intensity ratio of adjacent longitudinal modes and using this as an error signal to feedback control the cavity length, the 543 nm HeNe laser can be oscillated over a wide range. The frequency can be stabilized.

[Specific structure of the invention]

The present invention will be explained in detail below.

While investigating the polarization characteristics in a static magnetic field of a laser with an oscillation wavelength of 543 nm emitted from an internal mirror type helium-neon laser device having a structure in which a laser tube is arranged within a laser tube, the inventor found that discovered the phenomenon of

In other words, by applying a specific static magnetic field inside the 543 nm He-NIL laser tube, even if each longitudinal mode is located near the symmetrical arrangement with respect to the gain center and is orthogonally linearly polarized, We found that adjacent longitudinal modes stably maintain orthogonal linear polarization states without polarization reversal.

The static magnetic field at this time satisfied the following conditions [1] to [3].

Condition ■ The direction of the static magnetic field is perpendicular to the axis of the laser tube, and the angle between the direction of the static magnetic field and the laser tube's intrinsic polarization direction is 30 to 4.
Must be 2°.

Condition (1) The magnitude of the static magnetic field is such that the Zeeman frequency between π and ±σ transitions and the longitudinal mode spacing are equal when the static magnetic field is applied.

Condition ■ The magnitude of the static magnetic field is almost uniform along the longitudinal direction of the laser tube.

It has also been found that when the static magnetic field is applied, two longitudinal modes oscillate over a wide range of the laser gain curve.

The present invention was made based on such knowledge, and
In the 543 nm He-Ne radar, a configuration is adopted in which a specific static magnetic field is applied inside the laser tube to prevent polarization reversal by setting the polarization directions of adjacent longitudinal modes to mutually orthogonal linear polarization. The intensity difference or intensity ratio of the longitudinal mode is detected and used as an error signal to feedback control the resonator length, thereby increasing the oscillation wavelength to 54.
This stabilizes the frequency of the 3 nm laser.

FIG. 2(a) is an explanatory diagram showing an example of means for applying a static magnetic field. A magnet 20 surrounds the outer periphery of the laser tube 10.
, and by this magnet 20, the laser thin tube 11
A static magnetic field in a direction perpendicular to the tube axis of the laser tube 11, that is, the optical axis, is applied within the laser tube 11.

The magnitude of this static magnetic field is π when the static magnetic field is applied.
and the magnitude at which the Zeeman frequency between ±σ transitions and the longitudinal mode spacing are equal.

The direction of the static magnetic field is perpendicular to the tube axis of the laser tube 11, and as shown in FIG.
This is the direction in which the angle θ between the two ends is 30 to 42 degrees.

Here, in the present invention, the intrinsic polarization directions C and C2
When the output is monitored through a linear polarizing plate, the direction where the peak light intensity is weak is defined as C, and the strong direction is defined as 02.

The static magnetic field is uniform along the longitudinal direction of the laser tube 11.

The magnet 20 for forming the static magnetic field may be, for example, a ferrite permanent magnet or an electromagnet.

FIG. 3 is a diagram showing an example of a change in the longitudinal mode when a static magnetic field is applied under the conditions shown in FIG. 2. In Fig. 3, the vertical mode is (a)-(b)-(C)-(d)→(
It changes repeatedly as e)→(f)→(a)... That is, while moving along the laser gain curve, the polarization direction of each longitudinal mode does not change.

Therefore, if we use the mode configurations of (b), (C), (e), and (f) in Figure 3, 543 nm He-
Similarly to the 633 nm He--Ne laser, the frequency of the Ne laser can be stabilized using the two-mode method.

By effectively preventing the occurrence of polarization reversal in this way, the following advantages can be obtained.

(1) The range in which frequencies can be stabilized is expanded approximately six times compared to the method of T. Ferman et al.

(2) The autorotter circuit can be configured extremely easily. Note that the auto-lock circuit refers to a circuit for automatically performing mode lock.

Next, experiments conducted to prove the effectiveness of the present invention will be explained.

[Experiment 1] A commercially available internal mirror type helium-neon laser device (05-LGR171, MBL) with an oscillation wavelength of 543 nm and having a structure in which a laser tube is placed inside a laser tube.
(manufactured by LES GRIOT), the polarization characteristics of the laser were investigated in a state where no static magnetic field was applied (zero magnetic field).

The main features of the above laser device are as follows, according to the analysis results of the present inventor.

'He/22Ne = 10/1 ・Total pressure = 186.2 pa (1.4 Torr) ・Resonator length + 402 mm (longitudinal mode interval 373 MHz) ・
Diameter of laser tube: 1°5 mm ・Length of laser tube: 330 mm ・Radius of curvature of output side mirror + 800 + 1101 ・Radius of curvature of high reflection side mirror; ■ Figure 4 shows longitudinal mode observation using a Fabry-Perot interferometer. The results are shown schematically. The figure shows a typical mode arrangement and its polarization state. It always oscillates in three to four longitudinal modes, and each mode is linearly polarized, but 633n
Unlike the case of the red He--Ne laser of 100 m, there are cases in which adjacent longitudinal modes are not necessarily orthogonally polarized, but parallel polarized.

When the power is turned on and the laser device is driven, the resonator thermally expands, so the longitudinal mode moves to the lower frequency side within the gain width, and the mode arrangement is as shown in Fig. 4 (a) → et al) → ( C
)→(d)→(a)... and so on repeatedly.

When the intensity of each longitudinal mode has a nearly symmetrical mode arrangement, as shown in FIG.

In fact, when we conducted separate experiments using two commercially available laser devices of the same type as described above, we were able to observe polarization reversal in both of the two laser devices.

Figure 5 shows a linear polarizing plate placed in front of the Fabry-Perot interferometer so that it is parallel to the characteristic polarization direction C1 where the peak light intensity is weak, and after one polarization reversal occurs, the next polarization reversal occurs. This diagram schematically shows the results of tracking the movement of the longitudinal mode in the eigenpolarization direction C1 up to the moment when eigenpolarization direction C1 occurs, using a storage oscilloscope, and corresponds to the change in mode arrangement from (C) to (d) in Figure 4. This corresponds to the polarization inversion in (d).

The two modes on the left were caused by polarization reversal, and at this point storage in the storage oscilloscope was stopped. In this experiment, the range in which the polarization direction is stable from one polarization reversal to the next polarization reversal is approximately 93 MHz to the right.
Met.

Next, we attempted to observe beats due to the coupling between longitudinal modes.

For observation, we used a linear polarizing plate whose angle with the unique polarization direction C1, where the peak light intensity is weak, was set at 45°, and a 0 to 20 MH
A broadband amplifier sensitive to z was used.

FIG. 6(a) is a diagram schematically showing the observation results in a mode arrangement corresponding to FIGS. 4(a) and (C). As can be understood from the figure, a beat was observed near 50 MHz. Note that during the observation, beats were momentarily observed in the vicinity of 100 MHz, but the intensity was about 1/10 or less compared to the 50 kHz beat.

FIG. 6 et al.) are diagrams schematically showing observation results in a mode arrangement corresponding to FIG. 4(b). As understood from the figure, complex changes in two or more spectra were observed near 50 kHz.

As described above, in zero magnetic field, oscillation occurs in three to four longitudinal modes, and its intrinsic polarization direction and beat are also the resonator capacity m(c
We could not find any characteristics that showed complex changes in frequency detuning (avity detuning) and that could be used to stabilize the frequency.

Therefore, in order to stabilize the frequency by applying the two-mode method in this laser device, it is necessary to avoid the polarization inversion region, and the region in which it can be stabilized becomes narrower.

Furthermore, the longitudinal mode of one eigenpolarization direction necessarily includes two frequency components whose frequency difference is the longitudinal mode interval, which is disadvantageous when used as a light source for a polarization interferometer.

[Experiment 2] Using the same laser device as in Experiment 1, an experiment was conducted to examine the polarization characteristics when a static magnetic field perpendicular to the tube axis (transverse magnetic field) was applied inside the laser tube.

At the Ne 633nm transition, as a means of frequency stabilization,
The two-mode method, Lamb-Deep method, longitudinal Zeeman method, transverse Zeeman method, etc. have already been put into practical use.

In particular, in the transverse Zeeman method, the cavity length is relatively long (approximately 26
~28cm) It is known that the laser stabilized by this method can be applied to a laser device and is useful as a light source for optical heterogain measurement.

Therefore, in order to confirm whether the transverse Zeeman method is also applicable to the Ne 543 nm transition, an experiment was conducted to examine the polarization characteristics in a transverse magnetic field.

Figure 7 schematically shows the Ne 543nm transition in a transverse magnetic field.
Expression % transitions are prohibited. On the other hand, in the case of 633 nm transition, the Landeg factors of both the upper and lower levels are g1
= 1.295, g, = 1.301, which are almost equal, and the Zeeman separation frequency Fz is: Fz = μa g-B/h (1)
(μB: Bohr magneton, B: magnetic flux density), but in the case of 543 nm transition, gb = 1.984,
There is a non-negligible difference from g6.

In the transverse Zeeman method, a single longitudinal mode is achieved by applying a magnetic field with Fz equal to the longitudinal mode spacing, that is, a characteristic transverse magnetic field to strengthen the coupling (gain competition) between the three longitudinal modes. There is.

This is directly related to the coupling between π and σ transitions that share the upper or lower level. For the 543 nm transition, the π and σ transitions that share the lower level are mal, 0 to m, =
There are two types of transitions: a transition to 1, and a transition from m, =Q, -1 to m, = -1, and the frequency difference between these σ and π transitions is given by the above equation (1).

On the other hand, π and σ transitions that share the upper level are also m.

Transition from =1 to mb=1.0, ma=1 to m,
There are two types of transitions to =Q and -1, and the Zeeman separation frequency F'z between those σ and π transitions is Fz=μBgb
B/h - (2). That is, 54
In the 3 nm transition, unlike the 633 nm transition, the characteristic transverse magnetic field has two values. In this experiment, we chose the former. In this case, the magnetic flux density providing the characteristic transverse magnetic field was 206 Gauss.

FIG. 8 is an explanatory diagram showing an outline of the experimental apparatus.

Both ends of the laser head have been removed by cutting, so that the end of the laser tube on the output side (anode side) can be seen visually, and the laser beam can also be extracted from the high-reflection mirror side at the rear of the laser tube. It has been remodeled.

The transverse magnetic field H was formed by fixing 30 x 40 x 10 mm ferrite magnets (FB4B material, manufactured by TDK) at appropriate intervals on a 6 mm thick iron plate. Laser tube length (
The magnetic flux density is 202± in about 78% of the area (330m+n).
AUSS-like transverse magnetic field (uniformity ±4%) was applied to 8.

Note that the case where the angle θ between the direction of the transverse magnetic field H and the intrinsic polarization direction C3 having a weak peak light intensity is 0° or 90° corresponds to the case of the transverse Zeeman method.

In this Figure 8, LP is a linear polarizing plate and PD is a PIN.
A photodiode, VA a video amplifier, O8 an oscilloscope, SA a spectrum analyzer, PH a pinhole, FPI a Fabry-Perot interferometer, and ST a storage oscilloscope.

(1) When θ=0° FIG. 9 is a diagram showing experimental results when θ=0°.

The upper row shows a typical longitudinal mode arrangement, and the middle and lower rows show the beat spectrum and beat waveform at the longitudinal mode position. On the right, the angle between the linear polarizing plate LP and the intrinsic polarization direction C1 is set to 45 degrees.

As can be understood from Fig. 9, when a transverse magnetic field is applied, the number of longitudinal modes decreases by one to 2 to 3 compared to the case of zero magnetic field.
oscillating in mode.

On the other hand, the number of beat spectra corresponding to the number of longitudinal modes appeared near 50 kHz. However, in the case of three-mode oscillation, the intensity of the longitudinal mode at both ends of FIG. 9(C) is weak, so the beat spectrum mainly corresponds to the longitudinal mode in the center, and it looks as if it were one line. Rotate the linear polarizing plate LP and change its orientation to the intrinsic polarization orientation C1 or C2
When matched to , the beat spectrum disappeared.

In addition, the Fabry-Bérot interferometer FP is
When longitudinal modes are observed at
When parallel to , the modulation waveform disappeared. However, in this case, the vertical mode itself did not disappear.

From the above experimental results, these beat spectra can be considered to be Zeeman beats within the longitudinal mode caused by σ and π transitions.

Further, as understood from FIG. 9, the low frequency side longitudinal mode corresponds to the low frequency side beat spectrum. When the longitudinal mode moved within the gain width, the change in beat frequency was several kHz or less. Note that the beat waveform in the lower row of FIG. 9 can be explained as a synthesis of the beat spectra in the middle row.

As described above, when θ=0゛, Zeeman beat within the same longitudinal mode was observed, and although the number of longitudinal modes was reduced by one compared to the case of zero magnetic field, a single longitudinal mode could not be achieved.

This is due to the difference in the coupling coefficient between σ-π transitions due to the difference in the number of J in the lower level when compared with the 633 nm transition,
It is thought that differences in values, differences in mirror quality (differences in in-plane anisotropy), differences in resonator length, etc. are closely related.

Note that, as shown in FIG. 9(C), in the case of three-mode oscillation, there is a tendency for oscillation to concentrate in the central longitudinal mode. Therefore, it is possible to realize a single longitudinal mode by lowering the excitation intensity of the laser.

However, if the excitation intensity of the laser is lowered, although a single longitudinal mode is possible, the laser output becomes very small, which is inconvenient for practical use.

(2) Case of θ≠0° Next, we investigated the case where the characteristic polarization direction CI, where the peak light intensity is weak, is at a general angle that does not match the direction of the transverse magnetic field in the characteristic transverse magnetic field.

As a result, the inventor found that in the range of θ approximately 36°±6°, oscillation occurs in two to three longitudinal modes, and the polarization of adjacent longitudinal modes is always orthogonal linear polarization, and within the gain width. It was found that no polarization reversal occurs while the longitudinal mode moves. Moreover, no matter which eigenpolarization direction the linear polarizing plate LP was set at at this time, no Zeeman beat in the longitudinal mode was observed.

FIG. 10 is a diagram showing the results of longitudinal mode observation in a characteristic transverse magnetic field with θ=33° and without using a linear polarizing plate LP. As can be understood from the figure, it is 2-3 mode oscillation.

Next, when we insert a linear polarizing plate LP in front of the Fabry-Perot interferometer FPI and observe the mode with its orientation aligned with the intrinsic polarization orientation C3 where the peak light intensity is weak, we find that the 11th
As shown in Figures (a) and (a), the adjacent modes disappeared, resulting in two longitudinal mode oscillations separated by twice the longitudinal mode interval.

At this time, when the linear polarizing plate LP was rotated by 90 degrees, the longitudinal mode that had disappeared until then appeared, and conversely, the longitudinal mode that had appeared until then disappeared. In other words, the adjacent longitudinal mode is 2
orthogonal linear polarization along two unique polarization directions;
It was confirmed that Zeeman beat did not occur in the longitudinal mode.

FIG. 12 is a diagram schematically showing the movement of a series of modes and the polarization state due to thermal expansion of the resonator. In the same figure, (
The mode change is repeated with a)-(b)-(C)-(d)-(e)-(f) as one cycle.

FIG. 13 is a diagram showing the results of recording on a storage oscilloscope the period during which the linearly polarized light with the characteristic polarization direction c1 with a weak peak light intensity is oscillated in a single longitudinal mode, and the single mode region is The frequency was approximately 591 MHz.

Figure 14 shows how the laser output is transferred to the polarizing beam splitter PBS.
FIG. 4 is a diagram showing the results of separating component light into respective directions of intrinsic polarization directions C6 and C2 using the method and simultaneously recording their temporal changes. At this time, the movement in the longitudinal mode was also monitored at the same time by detecting leaked light from the high-reflection mirror side.

As a result, it was confirmed that each component light was in a single longitudinal mode except for the regions corresponding to X and W in FIG. 14. Therefore, if we use the straight slope section in Fig. 14, 63
As in the case of the 3 nm transition, the frequency can be stabilized by the two-mode method in the 543 nm transition without polarization reversal.

In this case, compared to the method of T. Ferman et al., it has the disadvantage of requiring a transverse magnetic field, but since the occurrence of polarization reversal can be prevented, the range in which the frequency can be stabilized is widened, and each unique polarization direction component is It has the advantage of oscillating in one longitudinal mode.

Note that the phenomenon in which the Zeeman piet in the longitudinal mode disappears when θ≠06 is already known in the 633 nm He-Ne laser. That is, it is understood that the reason why the beat disappears is that the oscillation in the longitudinal mode is completely polarized, unlike the case where θ=0°. 543nm
It can be understood that the beat disappears in the transition for the same reason.

In other words, by considering that the birefringence anisotropy of the resonator mirror and the birefringence anisotropy induced in the laser medium by the magnetic field are canceled by adjusting θ, it is possible to sufficiently polarize the longitudinal mode safely. This can be fully explained, and the disappearance of the beat can also be fully explained.

〔Example〕

Examples of the present invention will be described in detail below, but the present invention is not limited to these examples.

FIG. 15 schematically shows a laser device suitable for implementing the present invention. In the figure, LA is an internal mirror type helium neon laser tube, MG is a magnet, PBS is a polarizing beam splitter, PDI, PD2 are PIN photodiodes, AMI, AM2 are amplifiers (LP356), DA is a differential amplifier, BA is a buffer amplifier, DB is a DC bias voltage, PR is a pen recorder connection terminal, R is a resistor, D is a diode, Tri and Tr2 are transistors, HE is a heater that controls the resonator length, E is a heater power supply, PH is a pin Hall and LP are linear polarizing plates, FPI is a Fabry-Perot interferometer, and ST is a storage oscilloscope.

The heater HE is installed in the glass outer tube at the rear of the laser tube LA.
It is composed of a bifilar-wound nichrome wire of 8 mm width and a diameter of 0.26 mm (resistance 20 Ω), on which a layer of Teflon tape is wrapped to fix the nichrome wire.

The 5430 mHe-Ne laser oscillated from the laser tube LA is separated by the polarization beam splitter PBS into respective component lights with unique polarization directions CI and C2, and these component lights are sent to the PIN photodiodes PDI and PD2, the amplifiers AMI and AM2 detects and amplifies the signal using direct current.

The outputs from amplifiers AMI and AM2 are connected to differential amplifier D
The difference in output corresponding to each component light (
intensity difference) is detected.

This intensity difference is amplified as an error signal by the differential amplifier DA, and the collector current of the transistor Tr2 is controlled based on this amplified error signal. Therefore, the power supplied to the heater HE is controlled by the controlled collector current, thereby correcting the cavity length of the laser tube LA, so that the longitudinal mode of the 543 nm He-Ne laser has a constant frequency position. stabilized at

Using the laser device with the above configuration, a test was conducted to stabilize the frequency of the 543 nm He-Ne radar in a room where the room temperature was not controlled, and the output of the buffer amplifier BA was recorded using a pen recorder. The results are shown in Figure 16. was gotten. The conversion to the width of laser frequency change on the vertical axis is:
This was done by simultaneously observing the output of the buffer amplifier BA and the movement within the gain width of the longitudinal mode.

As can be understood from FIG. 16, in a room where the room temperature is not controlled, approximately 2. lX10-' (58 minutes)
A frequency stability of .

Although the case where the intensity difference of each component light is detected and used as an error signal has been described above, a configuration may be adopted in which the intensity ratio of each component light is detected and this is used as an error signal. Specifically, for example in FIG. 15, the differential amplifier DA may be replaced with a divider circuit.

〔Effect of the invention〕

As explained in detail above, according to the present invention, polarization reversal can be effectively prevented by applying a specific static magnetic field within the laser tube of an internal mirror type helium neon laser device with an oscillation wavelength of 543 nm. , detects the intensity difference or intensity ratio between adjacent longitudinal modes and uses this as an error signal to feedback control the resonator length.
The frequency of neon laser can be stabilized.

[Brief explanation of drawings]

Figure 1 is a diagram showing the longitudinal mode of a conventional 543 nm He-Ne laser, Figure 2 is an explanatory diagram showing an example of a means for applying a static magnetic field, and Figure 3 is an example of changes in the longitudinal mode when a static magnetic field is applied. Figure 4 is a diagram showing the longitudinal mode observed using a Fabry-Bérot interferometer without applying a static magnetic field, and Figure 5 is a diagram showing the longitudinal mode observed after a polarization reversal occurs until the moment when the next polarization reversal occurs. Figure 6 (
a) is a diagram showing the observation results of beats in a mode arrangement corresponding to Fig. 4 (a) and (C), and Fig. 6 0)) is a diagram showing the observation results of beats in a mode arrangement corresponding to Fig. 4 et al. FIG. 7 is an explanatory diagram showing the Ne 543 nm transition in a transverse magnetic field. FIG. 8 is an explanatory diagram showing the outline of the laser experimental equipment.
Figure 9 shows the longitudinal mode arrangement, beat spectrum, and beat waveform when θ is set to 0° in a characteristic transverse magnetic field. Figure 11 showing the longitudinal mode arrangement observed without using
The figure shows the longitudinal mode arrangement when observed with a linear polarizer inserted and its orientation matched to the intrinsic polarization direction C1. Figure 12 shows the mode movement and polarization state due to thermal expansion of the resonator. Figure 13 shows the results of recording the period during which linearly polarized light with the intrinsic polarization direction C1 is in single longitudinal mode oscillation using a storage oscilloscope. Intrinsic polarization direction C
FIG. 15 is an explanatory diagram showing an outline of a laser device suitable for carrying out the present invention; FIG.
The figure is a diagram showing the results of frequency stabilization in an example of the present invention. 10... Laser tube 11... Laser thin tube 2
0...Magnet CI...Intrinsic polarization direction with weak light intensity C2...Polarization direction with strong light intensity LP...Linear polarizing plate PD PDI, PD2...PIN Photodiode VA...Video Amplifier O3...Oscilloscope SA...Spectrum analyzer PH...Pinhole FET...Fabry-Perot interferometer ST...Storage type oscilloscope MG...Magnet LA...Laser tube AMI
, AM2...Amplifier DA...Differential amplifier BA...Buffer amplifier DB
...DC bias voltage PR...Connection terminal of pen recorder R...Resistance D...Diode Tri
, Tr2...Transistor HE...Heater E...Heater power supply +2 CG) (b) 4971+Figure 8+Figure 4+Figure 10+1 Figure (b) Figure 19 (b) (C) (d)

Claims (1)

[Claims] (1) A method for stabilizing the frequency of a laser with an oscillation wavelength of 543 nm emitted from an internal mirror type helium-neon laser device having a structure in which a laser tube is arranged within the laser tube, the method comprising: By applying a static magnetic field that satisfies [1] to [3], the polarization directions of adjacent longitudinal modes of a laser with an oscillation wavelength of 543 nm are set to mutually orthogonal linear polarization, and the intensity difference or intensity ratio of these adjacent longitudinal modes is calculated. The oscillation wavelength is 543 nm, which is characterized by stabilizing the frequency of a laser with an oscillation wavelength of 543 nm by detecting it and using it as an error signal to feedback control the resonator length.
A method for stabilizing the frequency of a nm internal mirror type helium-neon laser. Condition [1] The direction of the static magnetic field is perpendicular to the axis of the laser tube, and the angle between the direction of the static magnetic field and the intrinsic polarization direction of the laser tube is 30 to 4.
Must be 2°. Condition [2] The magnitude of the static magnetic field is such that the Zeeman frequency between π and ±σ transitions and the longitudinal mode interval are equal when the static magnetic field is applied. Condition [3] The magnitude of the static magnetic field is approximately uniform along the longitudinal direction of the laser tube.