JPS6147683A - Laser oscillation device - Google Patents

Abstract

PURPOSE:To realize an inexpensive unit while enabling the easy control of a pulse width, by arranging a 1/4 wavelength plate in series with an electro-optical element between a double refraction element and a total reflection mirror. CONSTITUTION:An incident light L3 entering into a 1/4 wavelength plate 6 goes out as a light L4, which has been circularly polarized counterclockwise. A phase of lambda/4 is further applied thereto by an electro-optical element 3. As a result, a light L5 applied to a reflection mirror 1 has a plane of polarization rotated by 90 deg. with respect to that of the incident light L3. When a light L6 reflected by the total reflection mirror 1 enters into the electro-optical element 3 in the opposite direction, it is again circularly polarized counterclockwise. A phase difference of lambda/4 is applied thereto by the 1/4 wavelength plate 6. Accordingly, a light L7 returning to a double refraction element 4 has the same plane of polarization with the incident light L3. If the application lambda/4 voltage to the electro-optical element 3 is stopped, the returning light L7 becomes an extraordinary ray in the double refraction element 4, and it goes along an optical path P and returns as a randomly polarized light L. Thus, the resonance conditions of a resonator are satisfied, realizing high Q, and the laser is caused to oscillate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser oscillation device that performs Q-switching operation using electro-optic effects to generate short pulses.

In a YAG laser, methods for obtaining short pulses on the order of nanoseconds include a Q-switch method using an electro-optic effect, a Q-switch method using an acousto-optic effect, a Q-switch method using the Kerr effect, and a Q-switch method using an electro-optic effect or a Q-switch method using the Kerr effect. A cavity dump method using an acousto-optic effect is used. Cavity dump requires a complex optical system;
Further, in the Kerr effect, the applied voltage becomes very high, and in the Q-switch method using the acousto-optic effect, the switching start-up is slow, so the output pulse width is large, making it unsuitable for applications in, for example, ophthalmology.

For this reason, in the YAG laser oscillation device for ophthalmology,
A Q-switch method using electrical and optical effects is adopted, and an output pulse of several tens of nanoseconds can be obtained. By the way, when using this electro-optical element, the simplest method is to use it in combination with a polarizer, but the flash lamp pump causes thermal distortion in the YAG rod, and the incident light is linearly polarized. Due to the photoelastic effect, the plane of polarization undergoes rotation of spatially different magnitudes, and the desired effective area of the plane of polarization is confined near the center.

The magnitude of this effect is determined by the relationship between excitation and cooling, and when changing the frequency of use, that is, the number of emission pulses and their intervals, it may cause fluctuations in the output power for each pulse, and YAG is sometimes called a hot spot
There is a possibility that a phenomenon that leads to the destruction of the rod may occur.

Several examples have been reported as methods for eliminating the effects of photoelastic effects caused by thermal distortion, one of which is to use a birefringent plate made of calcite instead of a polarizer and apply a voltage to the electro-optic element. The optical paths of the ordinary ray and the extraordinary ray are made to match when the Q of resonance is high when . (1883) reported a method that gave a refraction that did not occur.

Here, FIG. 1 shows the laser configuration, and FIGS. 2(a) and 2(b) show the state of ray refraction when Q is low and when Q is high, respectively. In FIG. 1, l is a total reflection mirror,
Laser output is taken out from output mirror 2. Between the total reflection mirror 1 and the output mirror 2, an electro-optical element 3,
Fused silica prism 4 illustrated in FIGS. 2(a) and (b)
Birefringent element 4 combining a and a calcite prism 4b
, YAG rods 5 are arranged.

When Q is low and no voltage is applied to the electro-optical element 3,
In FIG. 2(a), the calcite prism 4 of the birefringent element 4
Of the randomly polarized light incident perpendicularly to b, the extraordinary ray L1 is refracted by an angle determined by the optical axis of the calcite prism 4b and the direction of incidence, while the ordinary ray L2 travels straight. Each of these rays L1 and L2 has a refractive index Ne, No for the extraordinary ray and ordinary ray of calcite in the fused silica prism 4a.
From the relationship between and the refractive index N of fused silica, the light ray Ll',
1 Reflection mirror 1
After reflection, the light propagates in opposite directions through different polarized light paths, passes through the fused silica prism 4a, and returns to the calcite prism 4b.
The extraordinary ray is Ll'' and the ordinary ray is L2''
As a result, the resonant condition deviates from the resonant condition, and the Q of the resonator becomes low and no oscillation occurs.

On the other hand, when a λ/4 voltage is applied to the electro-optical element 3, the second
As shown in Figure (b), the extraordinary ray L1. Since the ordinary ray L2 is rotated by 90 degrees after making one round trip through the electro-optical element 3, the reflected lights L1° and L2' on the total reflection mirror 1 will each travel backwards along the same trajectory as the incident light, which is a high Q condition. is fulfilled. In this case, since there is no change in loss depending on the plane of polarization unlike when a polarizer is inserted, the YAG rod 5 can be used effectively and the above-mentioned drawbacks are eliminated. However, when transitioning to a higher Q, the electro-optical element 3
It is necessary to apply a voltage to produce a phase difference of 4. Crystal KD''P used as electro-optical element 3
In (KD2PO4), this voltage is about 3 KV, and a device for rapidly raising this voltage in about nanoseconds would be large-scale. Further, it is necessary to vary the pulse width depending on the repetition conditions, but the equipment for this also becomes expensive.

In this way, the conventional method has a stable output Q with no polarization.
Although this is a useful method for obtaining switching pulses, it is necessary to apply a high voltage pulse with a fast rise to the electro-optical element 3 in order to perform the Q-switching operation, which has the drawback that it is difficult to achieve a practical cost.

An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a structure in which a 174-wave plate is disposed between an electro-optical element and a birefringent element.
The purpose is to provide a laser oscillation device that can be realized at low cost and whose pulse width can be easily controlled. and an electro-optical element, and is characterized in that a quarter-wave plate is arranged between the birefringent element and the total reflection mirror in series with the electro-optical element. .

The present invention will be explained in detail based on the embodiment shown in FIG. 3 and below. Note that the same reference numerals as in FIGS. 1 and 2 represent the same members or those with equivalent properties.

FIG. 3 shows a configuration diagram of a YAG laser oscillator according to the present invention, and in addition to the conventional example shown in FIG. . This one
The /4 wavelength plate 6 is made of biaxial crystal such as a mica plate,
Its thickness is such that the phase difference between the extraordinary ray and the ordinary ray is λ/4, and the extraordinary ray and ordinary ray from the birefringent element 4 are different from the extraordinary ray and ordinary ray from the quarter-wave plate 6. The optical axis is arranged so that the light enters at an angle of 45 degrees. Also, l
The /4 wavelength plate 6 may be a uniaxial crystal such as calcite, whose optical axis is the resonator axis, and whose thickness is 4n+1 (n is an integer) times λ/4. . 1
It is assumed that the input and output surfaces of the /4 wavelength plate 6 are coated with an AR coating (anti-reflection coating) in order to reduce insertion loss that causes parasitic oscillation.

In this arrangement, the Q-switching oscillation operation is performed as follows. To simplify the explanation, only the trajectory of the ordinary ray 1.2 in the birefringent element 4 will be considered. If the axes of the extraordinary ray and ordinary ray of the 1/4 wavelength plate 6 are arranged as shown in FIG.
In the case of an axial crystal, that is, when the refractive index is in the relationship Ne>No, as shown in FIG. The light L4 becomes counterclockwise circularly polarized light. Due to the electro-optical element 3 having an axial arrangement in which the phase difference to which the λ/4 voltage is applied is added with the same sign, a phase of λ/4 is further generated, and as a result, the light L5 incident on the reflection mirror l is different from the incident light L3. has a polarization plane of 90
The light is rotated by a degree.

When the light L6 reflected by the reflection mirror 1 enters the electro-optical element 3 in the opposite direction, this incident light L6 becomes counterclockwise circularly polarized light again, and the quarter-wave plate 6 further increases the phase difference by λ/4. is added, and the return light L7 to the birefringent element 4 becomes light with the same polarization plane as the incident light L3. As described above, this is refracted by the birefringent element 4 as light L8 in a direction deviating from the resonator conditions, and the resonator enters a low Q state and oscillation is stopped.

When the application of the λ/4 voltage to the electro-optical element 3 is stopped, the incident light L3 receives a λ/4 phase difference twice, and the polarization planes of the incident light L3 and the returned light L7 are different from each other due to the λ/2 phase difference. Rotated by 90 degrees, the returned light L7 becomes an extraordinary ray in the birefringent element 4,
It passes through the optical path P and returns as randomly polarized light.

That is, the resonance condition of the resonator is satisfied, the Q becomes high, and the laser oscillates. Here, the position of the quarter wavelength plate 6 is
It is clear that it may be between the electro-optical element 3 and the reflecting mirror 1.

When the 174 wavelength plate 6 is arranged in series with the electro-optic element 3 between the birefringent element 4 and the reflection mirror l, the electro-optical element 3 and the quarter-wave plate 6 function as a composite element. When the electro-optical element 3 is in the active state with a λ/4 voltage applied, the phase difference is λ/2 per round trip, and when the λ/4 voltage is not applied to the electro-optical element 3. By arranging them so that they have a phase difference of λ/4, it is possible to change the voltage polarity of Q switching applied to the electro-optical element 3 from a high side to a low side.

As explained above, the laser oscillation device according to the present invention has
By inserting a quarter-wave plate into the resonator, Q-switching is replaced from the conventional voltage application type to a voltage release type, making it easy to control the pulse width and stabilize the oscillation output. can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a conventional example, FIGS. 2(a) and (b) are diagrams explaining its operation, FIG. FIG. 4 is a diagram showing the relationship between the principal axis and the incident wavefront, and FIG. 5 is an explanatory diagram of the operation. 1 is a total reflection mirror, 2 is an output mirror, 3 is an electro-optical element, 4 is a birefringent element, 4a is a fused silica prism, 4b
is a calcite prism, 5 is a YAG rod, and 6 is a quarter wavelength plate. Figure 1 Figure 2 (CI) Figure 3 Figure 4

Claims (1)

[Claims] 1. A laser oscillation device using a Q-switch method using an electro-optic effect, which includes a birefringent element and an electro-optical element, and between the birefringent element and a total reflection mirror, A laser oscillation device characterized in that a quarter wavelength plate is arranged in series with the electro-optical element. 2. The electro-optical element and the quarter-wave plate function as a composite element, and when a voltage corresponding to λ/4 is applied to the electro-optical element, a phase difference of λ/2 is generated to the electro-optical element.
2. The laser oscillation device according to claim 1, wherein the laser oscillation device is configured to provide a phase difference of λ/4 when the voltage corresponding to .