JPS5841794B2 - Laser oscillation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser oscillation device, and an object of the present invention is to provide a laser oscillation device that has a laser output control function and a laser output monitoring function necessary for stabilization.

Control and stabilization of laser output is performed by monitoring the laser output during oscillation and converting it into voltage in many cases).
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 the improvement of laser light monitoring technology, and since other parts use well-known technologies, the following description will focus only on the parts related to laser output monitoring. Other parts will be omitted.

As shown in Fig. 1, the conventional output monitor technology replaces the reflector, which is normally a total reflection mirror, on the opposite side of the output coupling mirror 2 of the laser with a partially transmitting mirror 3 with a low output coupling ratio (about 1 cup). A measure is taken to measure the output 6 through it using an output detector 7.

Here, 4 indicates the output beam, and 8 indicates the electrical signal from the output detector 7.

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 partially transmitting mirror 30 have sufficient parallelism, a two-beam interferometer is constructed and the apparent transmittance changes greatly (as a result of simple calculations). It has been found that 100% amplitude modulation can occur in the output light 6 from the partially transmitting mirror 3, and this has been confirmed by the inventor's experience).

The second problem is a change in the spectral characteristics of the interference filter of the partially transmitting mirror 3 due to fluctuations in the oscillation frequency.At this time, even if the intensity of the beam 5 in the laser tube 1 is constant, the intensity of the monitor beam 6 fluctuates. Let's do it.

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

In the curve in the same figure, the horizontal axis is the wave number, and the vertical axis is the spectral transmission characteristic of the partially transmitting mirror 30 in Figure 1. If it is a CO2 laser, it is 10
．． A design has been made to have the desired transmittance T at 6 μ = 943 cm ′.

(In the case of the figure, T=0.001%.

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

At this time, JT=(T2-T1)/T□=13.6%, so even if the intensity of the intra-resonator beam 5 in Fig. 1 is constant, the output of the monitor beam 6 is within the range of 13.6%. This indicates instability.

It is sufficient if AT = 0 holds at the oscillation wavelength, but this requires excessive precision in the mirror surface multilayer film deposition technology and is not realistic.In addition, in the output coupling mirror 2, the value of T itself is Since it is a large value of several tens of tens, even if there is some JT, JT/T ± 0, and this instability can be ignored.
This instability cannot be ignored only in the case of a small output monitor with T*0.

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 the preferred method from both energy efficiency and device dimensions, but at least sufficient accuracy is required. This has not been realized today.

The present invention monitors a part of the output beam through a beam splitter installed outside the laser resonator, compares the measured amount with a reference voltage that is a control signal, and feeds the deviation amount back to the laser excitation device. It is designed to control output.

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 3 in Fig. 1, it is necessary to make the monitor output as small as possible, so it is necessary to use a mirror with R÷1, and it must be transparent to the laser beam. Therefore, it was not easy to select a material that satisfies those conditions.

In contrast, in the present invention, the output is monitored by a beam splitter installed outside the oscillator, where T÷1 or R÷0 is required, and n±1 materials such as ionic crystals are used. Can be used.

Furthermore, since the beam is incident on the beam splitter in an oblique direction, the instability of the output value due to the two-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 polarization plane Q, but 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. It can be said that this is the optimal output control method for lasers whose oscillation wavelength fluctuates easily, such as CO2 lasers.

An embodiment of the present invention is shown in FIG. 3 below.

In this embodiment, a laser with a fixed polarization plane, that is, a laser having a Brewster window 14' will be described.

In the figure, 11 is a laser tube, 12 is an output coupling mirror, 1
3 is a total reflection mirror, 14 and 14' are laser tube optical windows, 15
is a beam splitter installed outside the laser oscillator,
16 is a detector for output monitoring, 17 is a laser excitation power source,
18 is a laser beam inside the resonator, 19 is a laser output beam, 20 is a laser beam after passing through a beam splitter, 21
is the output monitoring beam, 22 is the output from the output monitoring detector, 23 is the laser output control signal, and 24 is the laser excitation power output. In this embodiment, the monitoring beam 21 proportional to the laser beam 19 is detected by the monitor. The output 22 is compared with the control signal 23, and the amount of deviation is negatively fed back to the excitation power source 17 for control.

Next, specific embodiments of the present invention will be explained for each necessary element.

In the present invention, the material of the beam splitter is determined based on two characteristics: (1) it must be transparent to the laser beam, and (iD) R in equation (1) must be as small as possible, that is, n must be as close to 1 as possible. selected.

For example, for a CO2 laser, NaC1 (n =
1.45R=3.4%), KCl (n=1.52.R=
4.3%) KB r (n=1.47, R=3.
6%) is appropriate.

The object of the present invention can be achieved by applying these materials to the beam splitter of the present invention.

On the other hand, not only the reflection on the first surface of the beam splitter but also the reflection on the second surface may become a problem.

In other words, in addition to the reflection from the first surface, two beams having approximately the same intensity are reflected from the beam splitter.

This reduces the overall i-transmittance of the beam splitter and causes apparent reflectance instability due to two-beam interference.

In order to prevent these two types of undesirable phenomena, an antireflection coating may be provided on the second surface of the beam splitter.

At the same time, it also serves as a protective film to prevent ionic crystals from becoming deliquescent.

An example of the configuration of a beam splitter provided with this antireflection film is shown in FIG.

In the same figure, 25 is a beam splitter, 126 is an anti-reflection coating provided on the second surface to cover the laser wavelength, 16 is a monitor beam detector, 19 is an incident beam to the beam splitter, and 20 is the transmitted beam. A beam 21 is reflected light from the first surface of the beam splitter and is directed to a detector 16.

Since this type of beam splitter has only one reflected beam, instability due to two-beam interference does not occur.

In the absence of an antireflection film, a phenomenon as shown in FIG. 5 appears.

That is, as shown in Figure a, when the input beam 19 is incident on the beam splitter 15, in addition to the transmitted light 20, a reflected beam 21 on the first surface and a reflected beam 27 on the second surface are generated, whose intensity is determined by the reflectance R. When the amount is minute, it may be considered to be approximately the same level.

According to simple geometrical optics calculations, the interval between these two beams is given by the following equation, where θi is the incident angle at which the refractive index of the beam splitter 15 is applied to the first surface of n1, and T is the splitter plate thickness.

Now, the diameter of the laser beam is φ. When this happens, these two beams overlap almost completely, and the beam intensity (2) delivered to the detector 16 exhibits the fluctuations shown in FIG. 5, as is well known from the theory of interference.

■ in the same figure. are the beam intensities indicated by 21 and 27 in a.

Further, δ on the horizontal axis is the phase difference between the beams 21 and 27,
When the thickness T of the beam splitter 15 is constant, the thickness T of the incident beam 190 changes as the wavelength changes.

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

It is known that one period of ν is Jν1 and A. From the interference theory, C is the speed of light. Zn5 with T = 3 mm.
Calculating equation 4 for the case of e, AνT=21G
Become a horse.

On the other hand, a CO2 laser without circular wavenumber stabilization
P(26), which is the rotational level transition of the molecule, P(14)
The frequency varies randomly within the range of the AC320GH2 over time.

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 16 is extremely large.

To prevent instability due to this interference, the distance between the two beams is determined by the laser beam diameter φ.

Just make it bigger and separate the two.

For this purpose, using equation (2), it is sufficient to set the beam splitter plate thickness T, and in this case, it can be seen that the two beams do not cause interference and the output of the detector 16 shows complete stability. .

φ of formula 5. It makes a big difference even if the inner diameter φ of the laser tube is used instead.

FIG. 6 shows another embodiment of the present invention, in which a prism-type beam splitter having an apex angle φ is used.

In this case, the two beams can be effectively separated with a smaller plate thickness than the parallel plate described above.

In the figure, 31 is a prism type beam splitter with an apex angle ψ, 132 is an incident beam, 33 is an exit beam, and 34
35 is the reflected beam from the first surface, 35 is the reflected beam from the second surface, and 36 is the high-order reflected light within the prism.

Various relations of the resultant method of geometrical optical calculations are derived.

[Beam splitter - split on the first surface Distance between two beam injection points〕 In this case, the conditions for preventing interference are And φ.

For closing, the inner diameter of the laser tube may be used as described above.

In this example, the shape of the beam splitter can be determined using the relational expressions (6) to (10) so as to satisfy (ii), and KC1#n=1.46 and T =1
Table 1 shows an example where φ=100.

This Ijcosθ.

, the parameters of the beam splitter configuration can be determined by comparing the values of the button and the inner diameter of the laser tube.

The above explanation has been based on a device having a Brewster window as shown in Fig. 3, in which the output beam is linearly polarized in a specific direction, but the invention is not limited to this, and the plane of polarization is The same applies to lasers that are not fixed.

Next, regarding lasers whose polarization plane is not fixed, there is a problem of fluctuations in the monitor output due to fluctuations in the polarization plane, but a feature of the present invention is that it is possible to prevent fluctuations in the monitor output due to fluctuations in the polarization plane. one of.

If the beam splitter is used with vertical incidence, the monitor beam will return to the laser oscillator, making measurement difficult, so it is used with oblique incidence.

In this case, since the reflectance of the P component whose polarization plane is horizontal to the incident plane and the S component whose polarization plane is perpendicular to the plane of incidence are different, fluctuations in the polarization plane cause instability of the monitor output.

Figure 7 shows the angle of incidence for the case of K(l) and the angle of incidence, pi- and
The reflectance of each portion and its ratio are shown.

In a laser whose polarization plane is not fixed, the Rp/Rs ratio is 1.
Beam splitters can only be used in areas close to .

In the figure, if the allowable measurement accuracy is 5% or less, the incident angle of the laser beam on the first surface of the beam splitter must be 100 or less.

When considering the second beam from the second surface of a five-way beam splitter, for example, in the case of the prism type beam splitter shown in Fig. 6, the second beam is Since it is more difficult to maintain the above angle of incidence condition since a larger angle of incidence is experienced in the optical path, it is better to receive the second beam with a beam stopper and prevent it from entering the monitor detector.

This configuration is shown in FIG.

In the figure, 41 is a laser tube, 42 is an output coupling mirror,
43 is a total reflection mirror, 44 is a beam splitter's first reflected beam, 45 is its second reflected beam, 46 is a beam splitter, 147 is an emitted laser beam, 48 is a monitor detector,
Reference numeral 49 is a beam stopper, 50 is a laser excitation power source, 51 is a monitor output, 52 is a control signal, and 53 is an excitation power source output.Since the operating principle is the same as described above, a description thereof will be omitted.

Furthermore, when the antireflection film shown in FIG. 4 is employed, since the second beam does not exist, it is of course unnecessary to install a beam stopper.

This problem of fluctuations in the monitor output due to fluctuations in the plane of polarization is a problem specific to lasers whose plane of polarization is not fixed.If you use a laser whose laser light is linearly polarized in a specific direction, this problem can be solved. It will be resolved.

That is, when the laser beam is linearly polarized in a certain specific direction, this type of instability does not exist, so there is no restriction on the incident angle, and the degree of freedom in the laser configuration increases.

Since the laser shown in FIG. 3 has a Brewster window 4', the E vector is polarized in the plane of incidence, that is, in the plane of the paper. Since this relationship remains unchanged during laser oscillation, the output monitor value is stable.

As described above, the present invention uses a part of the laser output for measurement using an output monitor installed outside the laser oscillator, compares the measurement result with a control signal, and performs negative feedback control of the laser excitation power source based on the amount of deviation. This is a laser oscillation device that uses a method to control and stabilize the output.
It has the following characteristics.

First, even a laser without particular frequency stabilization can maintain high precision in output monitoring, resulting in high control precision.

Second, the present invention avoids instability in control due to the coherence of laser light, allowing highly accurate control.

Thirdly, the present invention avoids fluctuations in monitor output due to fluctuations in the plane of polarization, allowing highly accurate control.

Ru.

Fourthly, since this method minimizes the amount of energy required for output monitor measurement, there is no reduction in the overall left energy effect.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of a conventional laser oscillation device that monitors the output from a total reflection mirror, Figure 2 is a diagram showing the transmission spectral characteristics of a low transmittance dielectric multilayer interference filter that monitors minute output, and Figure 3 4 is a schematic configuration diagram of a laser oscillation device according to an embodiment of the present invention, and FIG. 4 is a diagram of the same beam splitter.
Fig. 5a is a cross-sectional view showing a specific structural example of the beam splitter, Fig. 5 is a diagram showing changes in beam intensity in two-beam interference, Fig. 6 is a prismatic beam FIG. 7 is a diagram showing the optical path in the splitter, FIG. 7 is a diagram showing reflection characteristics for different planes of polarization, and FIG. 8 is a schematic diagram showing another embodiment of the present invention. 1... Laser tube, 2... Output coupling mirror, 3... Total reflection mirror for output monitoring, 4... Laser output beam,
5... Laser beam in resonator, 6-Monitor output beam, 7... Output monitor detector, 8... Output monitor detector output, 11... Laser tube, 12... Output coupling mirror 13 ... Total reflection mirror, 14 ... Laser tube optical window,
14... Blinister window, 15... Beam splitter, 16... Output monitor detector, 17... Laser excitation power source, 18... Laser beam in resonator, 19...
・Laser output beam, 20... Beam splitter, injection beam, 21... Output monitor beam, 22...
Output monitor detector output, 23...control signal, 24
...excitation power supply output, 25...beam splitter board,
26... Antireflection film, 27... Second surface reflected beam.

Claims (1)

[Claims] 1. A laser active region, a laser resonator surrounding the region,
The resonator includes a beam splitter that uses Fresnel reflection from a transparent body, a monitor output detector that measures the reflected light from the beam splitter, and a laser excitation means, which is provided outside the resonator. A laser oscillation device characterized in that the output and the control signal are compared and the deviation amount is negatively fed back to the laser excitation means. 2. The laser oscillation device according to claim 1, wherein the beam splitter is made of one of NaCl, KCl, and KBr. 3. The laser oscillation device according to claim 1, wherein an antireflection film is provided on the surface of the beam splitter opposite to the laser resonator. 4. A patent claim characterized in that the beam splitter is a parallel flat plate, and the plate thickness T satisfies the relationship where n is the refractive index of the beam splitter material φ0 and the inner diameter of the laser tube θi satisfies the following relationships: the angle of incidence on the beam splitter range 1
Laser oscillation device described in section. 5. The beam splitter has a prismatic shape, and a first button is provided on the laser resonator side surface and the opposite surface of the beam splitter. 2. The laser oscillation device according to claim 1, wherein the second reflected beam is configured to be separated on a light-receiving surface of the detector. 6. The laser oscillation device according to claim 1, wherein the laser output beam of the laser oscillation device is linearly polarized in a specific direction. 7. The laser oscillation device is characterized in that the polarization plane of the laser output beam is fixed and there is no button, and the angle of incidence of the laser beam on the laser resonator side surface of the beam splitter is 10 degrees or less. A laser oscillation device according to claim 1. 8. The laser oscillation device according to claim 1, further comprising means for shielding a reflected beam from a surface of the beam splitter opposite to the laser resonator.