JPH0286179A - Method for preventing polarization inversion of internal mirror type he-ne laser having 534-nm oscillating wavelength - Google Patents

Abstract

PURPOSE:To prevent reflection of polarized light by impressing a specific static magnetic field upon the inside of a laser thin tube so as to make polarizing directions of adjacent longitudinal modes linear-orthogonal to each other. CONSTITUTION:Magnets 20 are arranged around a laser tube 10 so that a static magnetic field can be impressed upon the inside of the laser thin tube 11 of the laser 10 perpendicularly to the optical axis of the thin 11. The magnitude of the static magnetic field is adjusted so that the Zeeman frequency between pi and + or -alpha transition can become equal to the longitudinal-mode interval when the magnetic field is impressed. The direction of the static magnetic field is perpendicular to the axis of the laser thin tube 11 and the angle theta between the inherent polarizing direction C1 of the laser tube 10 and the direction H of the static magnetic field is set to about 30-42 deg.. In addition, the magnetic field is almost uniform in the length direction of the thin tube 11. When the static magnetic field is impressed under such conditions, the longitudinal mode repeatedly changes from (a) to (f) through (b), (c), (d), and (e) shown in the figure and the polarizing direction of each longitudinal mode does not change. Therefore, the frequency can be stabilized by a two mode method and occurrence of inversion of polarization can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is directed to a laser whose oscillation wavelength is '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. This invention relates to a method for preventing polarization reversal.

[Technology background]

The oscillation wavelength of an internal mirror type helium-neon laser is conventionally known to be 633 nm (red).

Means for stabilizing the oscillation frequency of this internal mirror type helium-neon laser (hereinafter also referred to as r633nm He-Ne laser) with an oscillation wavelength of 633 nm include, for example, the two-mode method, the Lamb-Dip method, the iodine absorption cell method,
Many control methods have already been established, such as the longitudinal Zeeman method, the transverse Zeeman method, and the magnetic modulation method.
43 nm (green) internal mirror type helium-neon laser (hereinafter also referred to as "543 nm He-Ne laser").

) is a relatively new eraser whose oscillation was first reported by Berry in 1970 (References: D,
L, Perry, 1EiE[EJ.

Quantumε1ectron, QB-7 (1971
) 102).

This 543nm He-Ne radar is a 633nm H
Since the oscillation wavelength is shorter than that of the e-Ne laser, 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 gain of 633 nm He-
It is extremely small, about 1/15 to 1/17, compared to Ne lasers, so it has long been difficult to put it into practical use.

Recently, however, due to improved performance of laser mirrors, it has become possible to produce 543 nm He with a cavity length of about 40 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 performed using a 43 nm He-Ne laser were reported (Reference: Applie
cl 0ptics, vol 126. No, 14.
P2705. (1987).

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

Therefore, when stabilizing the frequency of a 543 nm He-Ne laser by applying, for example, a 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 (
This is a method in which 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, currently commercially available laser devices have a resonator length of about 43 cm. This causes 54 oscillations.
When observing the longitudinal mode of a 3nm He-Ne laser,
As shown in FIG. 1, a 543 nm He-Ne laser typically oscillates in a range of 3 to 4 longitudinal modes. When the power is turned on, the laser tube thermally expands and the resonator length increases, so the longitudinal mode is
)-(b)-(C)-(d)-(a)--... C1 and C2 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 a 543 nm He-Ne laser, (1) adjacent longitudinal modes are not necessarily orthogonally polarized, but there is always a pair of parallel polarized lights, and (2) gain competition between modes becomes intense. In the case of such a longitudinal mode arrangement, that is, in the case of Fig. 1 (a) or (d), polarization reversal occurs in which the polarization direction of each mode suddenly changes by 90°. 633nm is becoming increasingly popular
It is significantly different from a He-Ne laser.

Therefore, for example, by applying the two-mode method as is, 543n
When stabilizing the frequency of mHe-Ne laser, the following problems arise.

■ The frequency stabilization range is narrow.

■ Since the frequency can only be stabilized with the mode arrangement and polarization state shown in Figure 1 (a) or (C), one polarization component has two frequency components whose 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 circumstances, and its purpose is to provide a method for preventing polarization reversal that can effectively prevent polarization reversal of 543 nm He-Ne L/-. be.

[A means of promise to solve problems]

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 preventing polarization reversal 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 prevent polarization of adjacent longitudinal modes of a laser having an oscillation wavelength of 543 nm; It is characterized by linearly polarized light whose directions are orthogonal to each other.

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 satisfying the above conditions [1] to [3] is applied inside the laser 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°.

In this way, in the 543 nm He-Ne laser,
Since polarization reversal can be effectively prevented, 633nm He-
It is expected that the frequency will be stabilized with the same performance as the Ne laser.

[Specific structure of the invention]

The present invention will be specifically explained 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

That is, by applying a specific static magnetic field inside the 543 nm He-Na laser tube, each longitudinal mode can be arranged symmetrically with respect to the gain center, even if the longitudinal modes are orthogonally linearly polarized, adjacent We found that the longitudinal mode stably maintains the orthogonal linear polarization state 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 Lede gain curve.

The present invention was made based on such knowledge, and
By employing a configuration in which a specific static magnetic field is applied to the laser tube in a 543 nm He-Ne laser, the polarization directions of adjacent longitudinal modes are set to mutually orthogonal linear polarization to prevent polarization reversal.

FIG. 2(a) is an explanatory diagram showing an example of application of a static magnetic field. A magnet 20 surrounds the outer periphery of the laser tube 10.
is placed, and by this magnet 20, the laser capillary ll
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 a direction in which the angle θ formed with H is about 30 to 42 degrees.

Here, in the present invention, the intrinsic polarization directions C1 and C
2, the direction in which the peak light intensity is weak when the output is monitored through a linear polarizing plate is defined as C, and the direction in which peak light intensity is strong is defined as 02.

The static magnetic field is substantially 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 longitudinal mode is (a) - et al) → (C) → (d) - (e
)-(f)→(a)... and so on. That is, while moving along the laser gain curve, the polarization direction of each longitudinal mode does not change.

Therefore, (b), (c), (e), (f
), if you use the mote arrangement, 543 nm
Similarly to the 633 nm He-Ne laser, the frequency of the Ha-Ne laser can be stabilized using the two-mode method.

As described above, according to the present invention, the occurrence of polarization reversal can be effectively prevented. Therefore, there are the following advantages.

(1) The range in which the frequency can be stabilized is approximately six times wider than that of the method of T. Fulman et al.

(2) The autorotter circuit can be configured extremely easily. By the way, an autorotator circuit is a circuit that automatically performs mode locking.

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-LGR-171, ME
(manufactured by LLES GR-10T), 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=l(1/1 ・Total pressure crab 186.2 pa (1.4 Torr) ・
Resonator length: 402mm (longitudinal mode interval 373MHz)・
Diameter of laser tube: l, 5mm Length of laser tube: 330mm ・Radius of curvature of output side mirror: 800+n+n ・Radius of curvature of high reflection side mirror: ■ Figure 4 shows longitudinal mode observation using a Fabry-Bérot 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 directly I-polarized, but the 633n
Unlike the case of the red He-Na laser of 1.0 m, adjacent longitudinal modes are not necessarily orthogonally polarized but may be parallel polarized.

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

When the intensity of each longitudinal mode becomes a nearly symmetrical mode arrangement as shown in (a) and (d) of the same figure, polarization reversal occurs in which the polarization direction of each longitudinal mode suddenly changes by 90°.

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 the peak light intensity is parallel to the weak intrinsic polarization direction CI, 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 C2 up to the moment when C2 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, which has a weak peak light intensity, was set at 45°, and a polarizing plate D~2 (1M
A broadband amplifier sensitive to Hz 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 understood from the figure, a beat was observed near 50 MHz. Note that during the observation, a beat was momentarily observed near loOMHz, but its intensity was about 1/10 or less compared to the beat at 50 kHz.

FIG. 6(b) is a diagram schematically showing the observation results in a mode arrangement corresponding to FIG. 4(b). As understood from the figure, two or more complex changes in spectra were observed near 5QkHz.

As described above, in zero magnetic field, oscillation occurs in three to four longitudinal modes, and its intrinsic polarization direction and beat are also resonator detuned (c
No characteristics were found that showed complex changes in the frequency detuning (avity detuning) and could be used for frequency stabilization.

Therefore, in order to stabilize the frequency in this laser device, it is necessary to avoid the polarization inversion region, which narrows the region in which it can be stabilized.

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
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 heterodyne 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 the 6331m transition, the Landeg factors of both the upper and lower units are g.

=1.295, g, =1.301, which are almost equal, and the Zeeman separation frequency Fz is: Fz=μm g-B/h...(1)(
μ, : Bohr magneton 1 B = magnetic flux density), but
For the 543 nm transition gb = 1.984 and g6
There is a difference that cannot be ignored.

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. .

This is directly related to the coupling between π and σ transitions that share the upper or lower level. 5431m transition, the π and σ transitions sharing the lower level are m.

=1, transition from 0 to m, =1, m, =O, -1 to m
There are two types of transitions to b=l, and the frequency difference between these σ and π transitions is given by the above equation (1).

On the other hand, the π and σ transitions that share the upper level also transition from m4 = 1 to mb = l, Q, m, = 1 to mb = Q,
There are two types of transitions to −1, and the Zeeman separation frequency F'z between these σ and π transitions is F″2=μa gb B
/h...(2). That is, 5
In the 43 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 horizontal magnetic field H is a ferrite magnet on a 6-stop thick iron plate.
(FB4B material, made by TDK) 30X40X10mffl
were fixedly arranged at appropriate intervals. The magnetic flux density is 20% in about 78% of the laser tube length (330+I1m).
A -like transverse magnetic field of 2±86 auss (uniformity ±4%) was applied.

Note that the case where the angle θ between the direction of the transverse magnetic field H and the intrinsic polarization direction CI 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. Note that 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 5QkHz. 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. The beat spectrum disappeared when the linear polarizing plate LP was rotated to match its orientation with the intrinsic polarization orientation C3 or C2.

In addition, the Fabry-Perot interferometer FP is
When longitudinal modes are observed with I, amplitude modulation is seen in each longitudinal mode, and the polarization direction of the linear polarizer LP is fixed to CI or C2.
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 C1, 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 present 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. At this time, Zeeman beat in the longitudinal mode is not observed no matter which eigenpolarization direction the linear polarizer LP is set to. It is a figure which shows the result of longitudinal mode observation without using LP.As 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 FPr and observe the mode with its orientation matching the intrinsic polarization orientation CI, where the peak light intensity is weak, we find that the 11th
As shown in Figures (a) and (b), the adjacent modes disappeared, resulting in two longitudinal mode oscillations separated by twice the longitudinal mode interlayer.

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 an accumulation-type onroscope the period during which the linearly polarized light with the characteristic polarization direction C, where the peak light intensity is weak, is oscillated in a single longitudinal mode, and the single mode region is , approximately 591 MHz.

Figure 14 shows how the laser output is transferred to the polarizing beam splitter PBS.
FIG. 3 is a diagram showing the results of separating component light into respective directions of intrinsic polarization directions CI 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 31m transition, frequency stabilization using the two-mode method is possible for the 543 nm transition without polarization reversal.

In this case, compared to the method of T. Ferman et al., it is disadvantageous in that it requires 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 single longitudinal mode oscillation.

Note that the phenomenon in which the Zeeman beat in the longitudinal mode disappears when θ≠0° is already known in the 633nlTIHe=Ne laser. In other words, the reason why the beat disappears is
It is understood that this is because the oscillation in the longitudinal mode is completely polarized, unlike the case where θ=0°. 543n
It can be understood that the beat disappears in the m 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 out 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.

The area indicated by 8 in FIG. 14 is another area that can be used for frequency stabilization. In this region A, the component of the intrinsic polarization direction C2 causes a concavity in the output near the center of the gain width, and oscillates in a single longitudinal mode. Therefore, similarly to the Lamb dip method, the resonator length is modulated, the polarization beam splitter PBS extracts only the polarization direction C2 component, phase-sensitive detection is performed, and this is used as an error signal to change the laser oscillation frequency to the center of the depression. It is possible to stabilize the

Next, with θ=33°, the intrinsic polarization directions C1 and C
When we set the linear polarizing plate LP so that each component light of 2 passes through and observed the beat, we found that when the longitudinal mode arrangement corresponds to points A, B, X, and Y in FIG. A single beat spectrum appeared near 100 MHz. Points A, B, X, and Y in FIG. 14 correspond to the mode configurations in FIGS. 12(C) and (f). In other words, this beat appears while the vertical mode maintains three lines, and its change range is approximately 90MH.
z~100 MHz.

The intensity of this beat is higher when the central longitudinal mode is the characteristic polarization direction C2 component with a strong peak light intensity, that is, when the mode arrangement corresponds to points A and B in Fig. 14. It was about 4 to 5 times larger than the opposite case.

Considering that this beat occurs only in the case of trimodal oscillation, it is different from the Zeeman beat in the case of θ = 06, and is considered to be a difference frequency spectrum between the two longitudinal mode intervals that occurs during trimodal oscillation. .

This beat characteristic is also useful from the viewpoint of frequency stabilization. For example, the frequency can be stabilized by detecting this beat using leakage light from a high-reflection mirror and controlling the resonator length using a heater or the like based on the frequency-voltage converted output.

In this case, by arranging a linear polarizing plate LP or a polarizing beam splitter PBS on the output mirror side, only the central longitudinal mode can be selectively extracted. In this case,
Modulation of the resonator length as in the regularization method is not required.

〔Effect of the invention〕

As described above in detail, according to the present invention, in a laser having an oscillation wavelength of 5431 m that is emitted from an internal mirror type helium neon laser device having a structure in which a laser tube is arranged inside the laser tube, By applying a specific static magnetic field, the polarization directions of adjacent longitudinal modes are set to mutually orthogonal linear polarization, so that polarization reversal can be effectively prevented.

[Brief explanation of the drawing]

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-Perot 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 the mode arrangement corresponding to Figures 4(a) and (C);
), FIG. 7 is an explanatory diagram showing the Ne 543 nm transition in a transverse magnetic field, and FIG. 8 is an explanatory diagram showing the outline of the laser experimental setup.
Figure 9 shows the longitudinal mode arrangement, beat spectrum and beat waveform when θ = 0° in a characteristic transverse magnetic field, and Figure 10 shows linearly polarized light when θ = 33° in a characteristic transverse magnetic field. Diagram showing the longitudinal mode arrangement observed without using a plate, No. 11
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 the linearly polarized light with the intrinsic polarization direction CI is in single longitudinal mode oscillation using a storage oscilloscope. Intrinsic polarization direction C
FIG. 15 is a diagram showing the results of separating component lights in each direction of I and C2 and simultaneously recording their temporal changes. FIG. 15 is a diagram showing a state in which a single beat spectrum is generated. IO...Laser tube 11...Laser tube 2
0... Magnet C3... Inherent polarization direction C2 with weak light intensity... Inherent polarization direction LP with strong light intensity... Linear polarizing plate FD... PIN photodiode VA... Video amplifier O8...・Oscilloscope SA...Spectrum analyzer PH...Pinhole FPI...Fabry-Bello interferometer ST...Storage type oscilloscope +12 Figure 1 (b) (C) (d) (e) (f) +13 Figure 15 Figure 5゜ (KHz)

Claims (1)

[Claims] (1) A method for preventing polarization reversal 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: A method for preventing polarization reversal, which comprises applying a static magnetic field that satisfies [1] to [3] so that the polarization directions of adjacent longitudinal modes of a laser having an oscillation wavelength of 543 nm are linearly polarized orthogonal to each other. 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.