JPS5842638B2 - Laser oscillation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser oscillation device, and provides a laser oscillation device having a laser output monitoring function necessary for controlling and stabilizing laser output.

Control and stabilization of laser output is done by monitoring the laser output during oscillation (often by converting it into voltage).
The output is compared with a reference voltage, which is a control input signal, and the amount of deviation is passed through an arithmetic circuit and then negatively fed back to the laser excitation device.

Of these technologies, the present invention particularly relates to improvements in laser light monitoring technology, and other parts use well-known technologies, so in the following description, only the parts related to laser output monitoring will be discussed. I will omit that part.

As shown in Figure 1, the conventional method of output monitoring is to replace the reflecting mirror, which is normally a total reflecting mirror, on the opposite side of the laser's output coupling mirror 12 with a partially transmitting mirror 13 with a low output coupling ratio (about 1%). The output detector 17 is used to measure the output 16 through the rays.

Although this method has the advantage of a simple device configuration, it has the following disadvantages.

Firstly, when the front and back surfaces of the mirror 13 have sufficient parallelism, a two-beam interferometer is constructed, and the apparent transmittance becomes large (as a result of simple calculation, partial transmission Mirror 13
It has been found that 10% amplitude modulation can occur in the output light 16 of the device, and this has been confirmed by our experience).

The second problem is a change in the spectral characteristics of the interference filter of the partially transmitting mirror 13 due to fluctuations in the emission frequency. It sways.

In the figure, reference numeral 14 indicates an output beam, and reference numeral 18 indicates a monitor signal obtained by the output detector 17.

FIG. 2 shows an example of the magnitude of this fluctuation in the case of a CO2 laser.

In the curve in the figure, the horizontal axis is the wave number, and the vertical axis is the spectral transmission characteristic of the partially transmitting mirror 13 shown in Figure 1.For a CO2 laser, it is 10.6μ, but it is 943crrL-1. A design is made to have a desired transmittance T.

(In the case of the same figure, T=0.001 rice cake.

) However, laser light oscillates due to transitions between different rotational levels of the CO2 molecule, and although PI3 and P26 are shown in the figure, normally it oscillates while irregularly fluctuating between different P lines within this range. do.

At this time, JT=(T2-T1)/T1=13.6. Even if the intensity of the beam 15 inside the laser tube 11 in FIG. 1 is constant, the output of the monitor beam 16 is 13.6.
It will show instability within a range of 6%.

It is sufficient if JT=0 holds at the oscillation wavelength, but this is not realistic as it requires excessive precision in the multilayer film deposition technique for the mirror surface.

In addition, in the case of the output coupling mirror 12, the value of T itself is expressed as the number 10.
Since the value is so large that even if some JT exists, JT
/T is 0, and this instability can be ignored, and only in the case of a minute output monitor where T is 0, this instability cannot be ignored.

As mentioned above, the method of controlling the output by monitoring a minute amount proportional to the laser output through a partially transmitting mirror is a desirable method from the viewpoint of both energy efficiency and the size and shape of the device, but at least sufficient accuracy is required. This has not been realized at present.

The present invention monitors a part of the output beam through a laser synchronizer or a beam splitter installed externally, compares the measured amount with a reference voltage that is a control signal, and negative feedback of the deviation amount to the laser excitation device. By this, the laser output is controlled.

The wavelength dependence of transmittance and reflectance described above is difficult to prevent as long as a dielectric multilayer filter is used. Therefore, in the present invention, the wavelength dependence of transmittance and reflectance is stabilized by using the material-specific reflectance given by the Fresnel reflection coefficient. There used to be.

In the example of using the partially transmitting mirror 13 shown in Fig. 1, since the monitor output needs to be as small as possible, it is necessary to use a mirror of 1 in R, and since it must be transparent to laser light, It was not easy to select a material that satisfies these conditions.

In contrast, in the present invention, the output is monitored by a beam splitter installed outside the oscillator, which requires 1 in T, that is, R, =:=O, and the incident angle is selected near Brewster's angle. By doing so, this condition can be satisfied.

At this time, since the beam is naturally incident on the beam splitter in an oblique direction, the instability of the output value due to the three-beam interference described above can be prevented.

In the case of oblique incidence, there is instability in the output value due to fluctuations in the deflection surface, but this can also be prevented according to the present invention.

According to the present invention, even if there is fluctuation in the oscillation wavelength, it is possible to detect an amount proportional to the laser output, and this is said to be the optimal output control method for lasers whose oscillation wavelength fluctuates easily, such as CO2 lasers. I can do it.

FIG. 3 shows an embodiment of the present invention.

In the figure, 31 is a linearly polarized laser tube, 32 is an output coupling mirror, 33 is a total reflection mirror, 34 and 34' are laser tube optical windows, and 35 is a laser tube provided outside the laser oscillator whose incident angle is set to Brewster's angle. Beam splitter nearby and with both sides non-parallel; 36 is a detector for output monitoring; 37
is a laser excitation power source, 38 is a laser beam in the resonator, 39
is the laser output beam, 310 is the beam splitter 35
The laser beam after passing, 311 is a beam for output monitoring, 312 is an output from an output monitor detector, 313 is a laser output control signal, and 314 is a laser excitation power output.

In the present invention, a monitor beam 311 proportional to the laser beam 39 is measured by a monitor detector 36, and its output 3
12 and the control signal 313, and the amount of deviation is negatively fed back to the excitation power source 37 for control.

Next, a specific configuration example of the present invention will be explained for each necessary element.

The beam splitter 35 must be made of a material that is transparent to the laser beam.

When the following explanation is given with reference to a C02 laser, the following substances can be used.

What is written in parentheses is the refractive index of each of these materials, which is an important value for the following analysis.

The beam splitter 35 needs to have as high an overall transmittance as possible for the laser beam.

Moreover, for the reasons mentioned above, an antireflection film, which is a type of dielectric multilayer interference filter, cannot be used.

The present invention solves this problem by adopting the Brewster angle of incidence.

Therefore, the laser used in this Yoriaki must be a linearly polarized laser that incorporates a polarizing element such as a Brewster window as shown in FIG.

This is the first component of the invention. In the following description, the plane of polarization imposed on the external beam splitter is designated by the subscript 11 because it is common to the plane of polarization of this laser.

FIG. 4 shows the relationship between the incident angle and the reflectance R11 for Zn5e, which is an optical material often used in CO2 lasers.

From this, in the case of Zn5e, if an incident angle near Brewster's angle, which is 67.38°, is selected, the transmittance can be made as close to 1 as possible depending on the accuracy.

An accurate method for selecting this angle of incidence is based on the concept described later.

Since the coherence length of the laser beam is sufficiently large compared to the size of the optical component, if a beam splitter 51 with parallel front and back surfaces as shown in FIG. Since the reflected beam 55 from the front surface and the reflected beam 56 from the back surface enter the monitor detector 52, the output of the detector 52 exhibits fluctuations as shown in both figures as a result of two-beam interference.

In addition, 54 in a shows the laser beam after passing through the beam splitter 51;

is the beam intensity of reflected beams 55 and 56 of a.

δ on the horizontal axis is the phase difference between the beams 55 and 56, and when the beam splitter plate thickness T is constant, the incident beam 53
fluctuates according to changes in wavelength.

That is, the factor that changes δ is the oscillation wavelength, that is, the same frequency ν.

Letting one period of ν be AνT, then 1. It is known from the interference theory that C is the speed of light and n is the refractive index of the beam splitter, and in the case of Zn5e with T23n, equation (2) gives JνT = 21 GHz.

On the other hand, CO2 lasers without frequency stabilization are
The frequency changes randomly within the range of 320 GHz from P26 to PI3, which is the transition between rotational levels of the molecule.

At this time, fluctuations corresponding to 15 cycles shown in FIG. 5 are exhibited, and it must be said that the instability experienced by the detector 52 is extremely large.

In order to prevent instability due to this interference, the second component of the present invention is to configure the beam splitter in a prism-like configuration whose front and back surfaces are not parallel.

FIG. 6 shows an optical path diagram regarding a general prism type beam splitter 61.

62 is an incident beam, 63 is a transmitted beam, 64 is a first reflected beam, 65 is a second reflected beam, and 66 is a higher order beam.

The prism apex angle is φ, and the incident angle of 62 is θ. , refraction angle θ1,
The angle of incidence on the second surface is ψ+θ1, the angle of refraction θ2, the angle of incidence of the second reflected beam 65 is 2φ+θ1, and the angle of refraction is θ3.

As a result of geometrical optical calculations, the following relationship holds between them.

[Declination angle between incident beam and exit beam]

Deflection angle between the first and second reflected beams [beam splitter - the first and second wide reflected beam exit spatial distance on the first surface is the beam splitter plate thickness] In the case of Zn5e, the incident angle θ.

is chosen to be the Brewster angle θβ expressed by the following equation (9), that is, θβ = 67.38°, and the deflection angle Jθ between the incident beam and the exit beam is the polarization between the primary and secondary reflected beams. When the angle Aθ' is calculated as a function of the apex angle ψ, the results are as shown in Table 1.

The same table also shows the deviation Aθ'' from Brewster's angle when injecting from the back side.

These relationships are illustrated in FIG.

In the figure, Aθ and Aθ”, that is, the quantity related to the transmitted beam is φ.

It can be calculated for the following ψ φ〉φ.

For the above φ, total reflection occurs and no transmitted beam exists.

Since the transmitted beam is a necessary element for the beam splitter of the present invention, the apex angle φ is φ<φ.

(This must be selected. □ Next, with the beam splitter angle 'i' of this principle, it is possible to prevent the occurrence of three-beam interference by preventing the secondary reflected beam from being emitted to the incident surface side. .

That is, the quantity lθ' related to the secondary reflected beam does not exist for φ of 94 or more shown in the figure, and at this time, the secondary reflected beam can be prevented from exiting the beam splitter by total reflection.

Therefore, by selecting the apex angle ψ such that ψ>φd, three-beam interference will not occur.

Therefore, in this example, ψ.

The apex angle ψ is selected so that 〉φ〉ψd, and φ.

For φ of 〉φ〉φd, there is a transmitted beam, and the second reflected beam is captured in the beam splitter by total internal reflection, so three-beam interference no longer occurs, so the operation of the beam splitter is highly stable. It can be said that it is ideal.

ψ. As a result of geometrical optical calculation, it can be seen that φd and φd are given by the following equation.

Table 2 shows ψ calculated for various materials.

and the value of φd.

In general, Aθ" is not 0, so if the second plane incidence is made at Brewster's angle, the primary reflected beam 6°j becomes negligible, and the secondary reflected beam 65 remains as the reflected beam, and is substantially In this case, three-beam interference no longer occurs.

-′j5Jθ”20 If θ is selected, the reflection at the second surface becomes O, and only the first surface reflection occurs, thereby preventing instability due to three-beam interference.

Moreover, this configuration is more convenient because it makes it easier to align the monitor detector until the position of the reflected beam is easily determined.

This is the second embodiment of the invention.

When φ is given, β becomes θ22θβ (Q, β Nybrewster angle).

is given by the following equation as a result of geometrical optical calculation.

(12) θ.

When using , the secondary reflected beam no longer exists, so φ is equal to ψ as described above.

There is no need to select 〉φ〉ψd.

Rather, it is better to determine ψ so that the ratio of primary reflection becomes an optimum value.

This optimum value depends on the level of laser output, and for a medium-power CO2 laser with an output of about ioow, it is judged that about 1 cup is a desirable value from both the calorie making sensitivity and the loss of main output.

For the case of Zn5e, these values are expressed in the equation (
3)-(12) can be calculated and designed as follows.

That is, the angle of incidence on a Zn5e prism-shaped beam splitter with an apex angle of 1° is 62. By selecting r·5°, it is possible to prevent second surface reflection and eliminate instability caused by three-beam interference by 1.

Generally speaking, even if the second surface reflection is eliminated by the above method, there will be manufacturing errors in the apex angle φ or the incident angle θ.

It is highly conceivable that due to a setting error of , the second surface reflection becomes an unreasonable amount and three-beam interference occurs.

Therefore, ψ is the above-mentioned φ.

It is practical to select the relationship 〉ψ〉φd to be at least φ=φd.

Further, as a third example, θ.

, ψ, etc. do not satisfy the above relationship, Jθ〈 is sufficiently large and the primary and secondary beams have a beam diameter φ at the light receiving surface of the monitor detector.

If the beams are separated by a distance, no beam interference will occur.

The conditions for this are.

The advantage of this method is that the manufacturing angle error and the incident angle setting error of the beam splitter are relatively loose, and the disadvantage is that energy loss is slightly increased due to reflection at both the first and second surfaces.

As described above, the present invention extracts a part of the output using a prism-shaped beam splitter installed outside the linearly polarized laser and whose incident angle is approximately close to Brewster's angle, and uses this for measurement. In a laser oscillation device that negatively feeds back the excitation power source by the amount of deviation, the second surface is Monitor the first and second reflected beams to see if the reflected light is trapped inside the beam splitter by total internal reflection, or if the second surface reflection is performed accurately at the Brewster angle of incidence to reduce the amount of reflection sufficiently. The beam splitter has a wider range than the conventional configuration.
This enables high controllability with a main laser beam consumption of 1 or less using materials.

Brief Explanation of the Drawings ' Figure 1 is a schematic diagram showing the conventional method of output monitoring;
The figure is a diagram showing the transmission spectral characteristics of a low transmittance dielectric multilayer interference filter for monitoring minute output, and Figure 3' is a schematic configuration diagram of a laser oscillation device in an embodiment of the present invention.
Figure 4 shows CO whose polarization plane is parallel to the plane of incidence in Z n S e.
Figure 5a is a diagram showing the relationship between the incident angle (horizontal axis) and reflectance (vertical axis) when two laser beams are incident, and Figure 5a is a diagram showing the incident, reflected, and transmitted beam optical paths in a beam splitter that is a parallel plate. , Figure 5 is a diagram showing the three-beam interference phenomenon between two beams in the same figure a, Figure 6 is a diagram showing the optical path in a prism type beam splitter, and Figure 1 is a diagram showing the Brewster angle in the Zn5e beam splitter. The apex angle φ (horizontal axis) of the beam splitter when the beam is incident, the declination angle Jθ′ between the incident beam and the transmitted beam, and the declination angle Jθ” (Jθ, Jθ′, Jθ” is the vertical axis)
FIG.

31... Laser tube, 32... Output coupling mirror, 33... Total reflection mirror, 34... Laser tube optical window, 34'... ...Brewster window, 3
5... Beam splitter, 36... Output monitor detector, 37... Laser excitation power supply,
38...Intra-cavity laser beam, 39...
... Output beam, 310 ... Beam splitter-transmitted beam, 311 ... Output monitor beam, 312 ... Monitor detector output, 313 ...
... Control signal, 314 ... Laser power output, 51 ... Beam splitter, 52 ...
... Output monitor detector, 53 ... Incident beam, 54 ... Transmitted beam, 55 ... First surface reflected beam, 56 ... Second surface reflected beam, 6
1...Prismatic beam splitter, 62...
...Incoming beam, 63...Transmitted beam, 6
4...Primary reflected beam, 65...Second reflected beam, 66...High order reflected beam.

Claims (1)

[Claims] 1. A laser resonator, a beam splitter installed outside the laser resonator, the beam splitter, a nine-output detector for introducing reflected light from the ivy, and a laser excitation means. In the laser oscillation device, the laser resonator is a linearly polarized laser resonator, and the laser resonator is a linearly polarized laser resonator, and the laser resonator is a linearly polarized laser resonator, and the laser resonator is a linearly polarized laser resonator, and the laser resonator is a linearly polarized laser resonator. A laser oscillation device characterized in that a splitter has a prism shape that is transparent to a laser beam. 2 Beamsp IJ is constructed so that the angle of incidence and refraction at the entrance and exit surfaces are approximately equal to the Brewster's angle to prevent total reflection at the exit surface, and the beam is reflected at the exit surface and exits to the entrance surface side. 2. The laser oscillation device according to claim 1, wherein the beam splitter is configured such that the second reflected beam is captured within the beam splitter by total internal reflection. 3. The laser oscillation device according to claim 1, wherein the beam splitter is configured such that the refraction angle at the exit surface of the beam splitter coincides with the Brewster angle. 4. The beam splitter is configured such that the distance between the beam centers of the first and second reflected beams reflected at the incident surface of the beam splitter on the output detector surface is separated by at least a beam diameter. A laser oscillation device according to claim 1.