JPH09199781A - Laser amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser amplifier which can be coupled easily to the laser beam fed from a main laser oscillator. SOLUTION: On the side surfaces of an Nd:YVO4 crystal 1, excitation lights are projected by laser diodes 2. To the excitation surfaces, two-wavelength coats 6 are applied respectively. An incident beam 3 is projected on the incident surface of the Nd:VYO4 crystal 1 to make its incident angle θin equal to Brewster's angle θb Representing the width of the incident surface as 2W, the length of the Nd:VYO4 crystal 1 is set to be integer times as large as 2W/tan(sin<-1> (sinθb /n)}. Therefore, the incident beam 3 is reflected by two excitation surfaces and the opposite surface to the incident surface to return again to the incident surface. An outgoing beam 4 is radiated again at an outgoing angle θout (= the incident angle θin ) from the incident surface (the incident angle θin = Brewster's angle θb = the outgoing angle θout ).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to CW (continuous oscillation).
Alternatively, the present invention relates to the technology of a laser amplifier used when the output of a pulse oscillator is small and more power is required during laser processing.

[0002]

2. Description of the Related Art Conventionally, a laser amplifier has been used as a means for further increasing the laser power of a main oscillator. A general laser amplifier is an amplifier using a laser rod, and in the case of a lamp excitation system, excitation light is incident from the entire side surface of the rod. That is, the excited volume is the entire rod. The beam diameter coupled from the master oscillator to the laser amplifier is about 60 to 70% of the rod diameter, which is about 50% of the energy accumulated in the rod.
Are extracted as an amplified beam. That is, the extraction efficiency is about 0.5. Normally, an elliptic cylinder concentrator is used and the central part of the rod is strongly excited, so when the beam from the main oscillator is coupled to the center of the rod amplifier,
Energy can be extracted a little more efficiently, and in fact, an extraction effect of about 0.6 to 0.7 can be obtained.

In order to extract the energy from the amplifier as efficiently as possible, it is better to expand the beam diameter from the main oscillator as much as possible and couple it to the amplifier. This is basically the same even in the LD excitation type amplifier. Of course, it is desirable to spread the beam from the master oscillator and make it enter the amplifier, because it delays the gain reaching the saturation density in the amplifier.

As described above, in addition to improving the extraction efficiency of the amplifier, an amplifier using a slab crystal can be considered as another means. The slab amplifier is usually designed so that in the slab-shaped active medium, the incident laser light from the main oscillator repeats multiple reflections on the side surface of the slab and passes through the excited region as much as possible, and the input excitation energy is It is intended to be used without waste. For example, IEEE J. Quantum Electro
As can be seen in n.27 (1991) pp.2327, it is devised at the time of design so that there is no dead space in the excitation region. Considering the ease of cooling the slab and the efficiency, the slab amplifier can input a larger excitation energy to a large excitation region of the active medium, and is superior to the rod-shaped amplifier described above. I can say.

Regarding the shape of the slab, US Pat. No. 5,29
As shown in No. 9,220 and IEEE J. Quantum Electron 28 (1992) pp.986, the design is such that the laser beam is propagated while cutting the end face at an acute angle and performing zigzag reflection on the slab side face. ing.

[0006]

The first problem of the above-mentioned prior art is that it is difficult to adjust the incident angle on the slab end face that determines the mode overlapping efficiency between the excitation region and the laser beam in the slab crystal. That is. If the adjustment of the incident angle is deviated, there is a problem that the overlapping efficiency becomes lower than the design value and the power cannot be taken out. Also, because amplified light cannot be taken out as designed from the center of the exit surface that faces the entrance surface, especially in the case of high-power slab amplifiers, when amplification is performed with insufficient adjustment, the output end surface is cut at an acute angle. The amplified beam may be irradiated in the vicinity of the existing edge or in the vicinity of the opposite edge. In this case, there is a problem in that the reflected light may be applied to the holder portion of the slab and may cause a great damage. In the worst case, this heat may distort and destroy the end face of the slab itself, which may lead to very expensive slab damage.

The second problem is that a sharp cut must be formed on the end face of the slab laser.
General slab laser, while totally reflecting on the side of the slab,
It is designed to create as little dead space as possible in the excitation region. Therefore, the slab end face is sharply cut, which is not only costly in manufacturing the slab, but also requires careful handling. Further, this end face portion is usually a place where it cannot be cooled, and the problem of slab crystal destruction due to thermal strain remains a bottleneck in practical application of a high power slab laser.

The third problem is the problem of scalability of the slab laser amplifier. When increasing the amplifier gain, usually the length of the slab in the longitudinal direction is increased. However, due to the limitation of the boule of the laser crystal, the length is at most about 200 mm. Since it is impossible to make it longer than this, if it is absolutely necessary to obtain more gain, the slab will be coupled to a tandem. However, unlike the case where the rod-shaped amplifier is coupled in tandem, it is very difficult to couple the slab in tandem because the laser-amplified light is not emitted straight from the end face of the slab.

There have been the above-mentioned problems in principle of the slab amplifier.

Therefore, it is an object of the present invention to provide a laser amplifier having a structure capable of easily coupling a laser beam from a main oscillator to a slab amplifier. Further, the adjustment at this time is to be kept simple and the overlap efficiency with the slab excitation region is increased. Furthermore, it is an object of the present invention to provide a structure and a structure that can be manufactured as inexpensively as possible, instead of an expensive slab as in the past.

[0011]

In order to solve the above problems, according to the present invention, in a laser amplifier that pumps a laser medium by using a pumping light source to generate a gain, first laser beams parallel to the laser medium are provided. A surface and a second surface, and between the first surface and the second surface, an excitation surface on which light from the excitation light source is incident is formed at a right angle to the first and second surfaces, The amplified laser light is made incident on the first surface at Brewster's angle, is reflected by the excitation surface, and reaches the second surface.

With the above arrangement, the entrance / exit surface of the laser medium may be polished perpendicularly to the longitudinal direction thereof, so that the entrance / exit surface is 1/2 to half that of the conventional device having an acute entrance / exit surface.
Production will be possible at a cost of 2/3.

Further, by making the incident beam incident at the Brewster angle, the amplified outgoing beam can be automatically emitted at the same Brewster angle.
The configuration is simplified.

Furthermore, it is not necessary to attach a non-reflective coating to the entrance / exit surface. If a normal laser medium such as YAG, YLF, or YVO ₄ is used, the incident angle on the side surface in the longitudinal direction of the slab for zigzag reflection also becomes larger than the critical angle, so that the total reflection coating is not necessary. As described above, it is possible to construct a structure in which not only the aforementioned polishing cost is reduced, but also the coating cost is unnecessary.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment of the present invention will be described.
A detailed description will be given with reference to the drawings. FIG. 1 is a block diagram of the first embodiment of the present invention. 1A is a front view and FIG. 1B is a side view. In this embodiment, the slab is Nd: YV.
Although the example using the O ₄ crystal 1 is given, YAG, YLF, YVO ₄ or the like may be used as the laser medium.

The excitation is an LD excitation method. Nd: YVO
₄ than at least one of the sides perpendicular to the incident surface of the crystal 1, a configuration for inputting excitation light from the laser diode (LD) 2. In this embodiment, the LD 2 is a light emitting unit 10 m.
An example using a m × 1 mm diode array (stack) is shown. Alternatively, an arc lamp, a flash lamp, or the like may be used as the excitation light source.

LD2 is Nd: Y without using a coupling optical system.
It is excited in the vicinity of the VO ₄ crystal 1. Nd: YVO ₄
The refractive index of crystal 1 for extraordinary rays (p-polarized light) is n = 2.1
Since it is 652 (λ = 1064 nm), the Brewster angle θ _b is 65.21 ° (θ _b = tan ⁻¹ n). When the incident beam 3 is incident on the incident surface (first surface) at an incident angle θ _in = Brewster angle θ _b , it is emitted into the Nd: YVO ₄ crystal 1 at 24.79 °. Therefore, the excitation light of the LD 2 is incident on the excitation surface at 65.21 °.

Since the critical angle of the p-polarized component of the Nd: YVO ₄ crystal 1 is θ _c = 27.5 °, the beam incident on the excitation surface has an incident angle of θ = 65.21 ° on this excitation surface. Even if the total reflection coating is not applied, it is totally reflected. Amplified light is totally reflected by the excitation surface, the incident surface and opposed surface (second surface), emitted as a beam 4, is emitted by the output angle θ _out = 65.21 ° is equal to the incident angle .theta.i _n ( That is, exit angle θ _out = incident angle θ _in = Brewster angle θ _b ).

In the case of Nd: YVO ₄ crystal 1, the absorption coefficient for the LD excitation light is large, the spectral width of the excitation light is narrow, and when the Nd absorption wavelength is 809 nm, the Nd concentration is 1 at. % Is as large as 31 cm ^-1 , 1
There will be about 95% absorption at mm thickness. According to the experiments by the inventors, since the commercially available LD has a spectral width of about 4 to 5 nm, the absorption of about 100% is actually about 2.0 to 2.5 mm. It is known that there is. Therefore, also in this embodiment, the excitation light has almost complete absorption of the excitation energy at a depth of 2 mm from the excitation incident surface. Therefore, the mode matching with the excitation region of the Nd: YVO ₄ crystal 1 can be made close to 100% by introducing the φ4 mm incident beam 3 from 2.3 mm from the LD excitation incident surface.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, Nd: YVO ₄ crystal 1
When the width of the incident surface of is 2 W, the crystal length is 2 W / t
An {sin ^-1 (sin θ _b / n)} was set, and a total reflection film for the wavelength of the incident light laser to be amplified was attached to the surface (second surface) facing the incident surface. The incident beam 3 is made incident from the center of the incident surface (the position W from the end of the width 2W). Further, the LD excitation light is made incident from both side surfaces (the longitudinal direction of the crystal) adjacent to the incident surface by the same method as in the first embodiment. Also in the second embodiment, the LD 2 uses an LD array having a width of 10 mm. Therefore, the length of the crystal is 10 mm and W = 2.3 mm. The incident beam 3 has a Brewster angle (θb) of 65.
By making light incident from the center of the incident surface with a width of 4.6 mm at 21 °, an emission angle (θ _out ) of 6 is obtained as the emission beam 4 from the same point where the amplified laser light is incident.
The beam is extracted at 5.21 ° in a direction completely symmetrical to the incident beam 3 with respect to the longitudinal axis of the crystal.

As described in the first embodiment, the incident beam diameter is selected to be close to 90% of 2 W, and the distance between the LD 2 and the excitation light incident surface is adjusted to match the beam diameter at the incident end surface. It is possible to obtain almost 100% mode matching with respect to light, and the energy accumulated in the slab crystal of YVO ₄ can be efficiently extracted.

In the present embodiment, the excitation light incident surface is a dual wavelength coat 6 (dichroic coat) that totally reflects the laser light (1064 nm) and is non-reflective with respect to the excitation light (809 nm). An example is shown below. The reason why the amplified laser light is totally reflected in the slab crystal is that the coating is performed because the dual wavelength coating 6 reduces the loss of the excitation light (about 13.6%). Because you can. Since the Nd: YVO ₄ crystal 1 has a large refractive index, the end face has a large reflectance, and therefore it is judged that coating is preferable. For example, a YLF crystal having a small refractive index has a reflectance of about 4%, and in this case, there is no problem even without coating. As described above, the loss of the pumping light greatly depends on the type of the slab amplifier crystal used. Therefore, the decision as to whether or not to apply a coating should be made according to the type of crystal.

Next, a third embodiment of the present invention will be described with reference to FIG. In the present embodiment, the excitation light is made incident from the surface (second surface) facing the incident surface shown in the second embodiment. This surface is totally reflected with respect to the wavelength (1064 nm) of the amplified laser light, and the pumping light wavelength (80
9 nm) is coated with a dual wavelength coat 8 which is non-reflective. Further, in the present embodiment, the excitation light is Nd: Y using a lens duct (reducer 7) for reducing the beam width to 4.6 mm with respect to the LD emission width of 10 mm.
Up to VO ₄ crystal 1 is introduced. Also in this embodiment, the birefringent coat 6 is applied to the excitation light incident surface.
According to this embodiment, the amplification factor of the laser amplifier can be increased by adding the pumping light.

Next, a fourth embodiment of the present invention is shown in FIG.
This example is the same as the Nd: YVO ₄ crystal 1 shown in the second example.
In this example, the length in the longitudinal direction of is doubled and the excitation volume is doubled. As a result, the amplification factor of the laser amplifier can be increased.

Next, a fifth embodiment of the present invention will be described with reference to FIG. In the present embodiment, the light reflected and emitted once on the excitation light incident surface shown in the first embodiment is made to enter the second-stage amplifier symmetrically arranged with respect to the axis in the crystal longitudinal direction. I have to. Similarly, by arranging the amplifiers in tandem, a highly scalable amplifier can be constructed. In the fourth example, the available crystal length is limited, and the slab excitation volume increase is limited. On the other hand, according to the present embodiment, there is an effect that a desired amplification degree can be obtained by increasing the number of amplifier stages.

Next, a sixth embodiment of the present invention is shown in FIG.
The present embodiment is an example in which the configuration of the fourth embodiment is tandemly coupled using one or two coupling mirrors (total reflection mirror 9 in the figure). This makes it possible to easily combine the double-pass amplifier configuration shown in FIG. 4 (a configuration in which the beam incident from the first surface is reflected on the second surface and is emitted again from the first surface).

Next, a seventh embodiment of the present invention will be described with reference to FIG. The present embodiment is a configuration example in which the number of stages can be further overlapped with the configuration of the sixth embodiment. Nd: Y
Two VO ₄ crystals 1 are horizontally arranged as shown in FIG.
Four of the pairs were arranged continuously in the longitudinal direction. Nd in the left column of Figure 7: In each of the YVO ₄ crystal 1, Nd: YVO ₄ pumping light by LD2 through the reducer 8 from the left end surface of the crystal 1 is irradiated, the right column of Figure 7 Nd: YVO ₄ Each of the crystals 1 is irradiated with excitation light by the LD 2 from the right end face of the Nd: YVO ₄ crystal 1 via the reducer 8. A dual wavelength coat 8 is applied to each of the end faces of the Nd: YVO ₄ crystal 1 on which the excitation light is incident. That is,
In this embodiment, the excitation light is not incident from the side surface in the longitudinal direction of the slab crystal 1, but is incident from the surface (the surface having the total reflection film 8 for folding) facing the input / output end surface to the slab amplifier. ing.

Assuming that the distance between both end faces of the slab amplifier units facing each other is d, the Nd: YVO ₄ crystal 1 is stacked in the direction orthogonal to the longitudinal direction at a distance given by d tan θ _b to make it scalable. It can be configured. The incident beam 3 is incident on the uppermost Nd: YVO ₄ crystal 1 in the right column of FIG. 7 at an incident angle θ _in (Brewster angle θ _b ), and as shown in FIG.
After passing through the d: YVO ₄ crystal 1, it is reflected by the total reflection mirror 9 and again passes through each Nd: YVO ₄ crystal 1 in a zigzag manner.
The outgoing beam 4 is output at an outgoing angle θ _out (= incident angle θ _in ).

In contrast to the fifth embodiment, which extends long in only one direction, the method of this embodiment can be used to make it more compact. With this configuration, it is possible to configure a large output amplifier in a very small size.

In the slab amplifier of the above-mentioned embodiment, the incident end face and the face opposite to each other are parallel to each other, so that parasitic oscillation may occur when a large excitation energy is increased. In such a case, it is possible to obtain a desired gain by suppressing the excitation energy per stage and stacking them in tandem.

[0031]

The first effect of the present invention is that the manufacturing cost can be reduced by using the laser amplifier according to the present invention. The reason is that polishing is easy because the slab end face is not an acute angle as in the prior art. Further, the volume of the base material for polishing can be minimized. Further, the cost is reduced because it is not necessary to apply a non-reflective coating on at least the incident surface.

The second effect is that the efficiency of extracting the energy stored in the amplifier is high. The reason is that since the shape is simple, polishing accuracy can be easily obtained, and the design value of the slab amplifier can be realized with high accuracy. As a result, the overlap efficiency between the excitation region and the incident laser beam can be as high as designed. Furthermore, by adjusting the incident angle of the incident laser beam to the incident surface to Brewster's angle and the incident position accurately to the center of the incident surface, after that, it automatically follows the designed path, and The amplified beam can be emitted at the emission position and the emission angle as designed.

The third effect is that tandem coupling is easy and the number of stages can be increased to obtain a large total gain. The reason is that it has a feature that the incident angle of the laser beam incident on the slab amplifier is the same as the outgoing angle of the amplified beam. For this reason, when connecting and coupling, the configuration becomes easy. In particular, when a double-pass amplifier configuration is adopted, simply combining the slabs of the present invention by using one or two folding reflecting mirrors allows the paired amplifiers to be stacked in multiple layers to easily form a large output amplifier. it can.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention,
(A) is a front view and (b) is a side view.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 5 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 6 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a seventh embodiment of the present invention.

[Explanation of symbols]

1 Nd: YVO ₄ crystal 2 LD (laser diode) 3 Incident beam 4 Emission beam (amplified light) 5 Total reflection coating (1064nm HR θ = 24.79 °) 6 Dual wavelength coating (1064nm HR θ = 65.21 °, 809nm AR)
θ = 0 °) 7 Reducer (Coupling optical system) 8 Dual wavelength coating (1064nm HR θ = 24.79 °, 809nm AR
θ = 0 °) 9 Total reflection mirror

Claims (5)

[Claims] 1. A laser medium is excited by using an excitation light source,
In a laser amplifier that generates a gain, a first surface and a second surface that are parallel to each other are formed in the laser medium,
A laser beam to be amplified is formed between the first surface and the second surface by forming a pumping surface on which pumping light from the pumping light source is incident, at a right angle to the first and second surfaces. A laser amplifier, which is incident on the first surface at a Brewster's angle, is reflected by the excitation surface, and reaches the second surface. 2. When the refractive index of the laser medium with respect to p-polarized light is n, the width of the first surface is 2 W, and the Brewster angle is θ _b , a distance between the first surface and the second surface is set. 2. The laser amplifier according to claim 1, wherein the length of is a multiple of 2 W / tan {sin ⁻¹ (sin θ _b / n)}. 3. A reflection surface parallel to the excitation surface is formed on the laser medium, and an incident angle si is formed on the second surface.
The laser amplifier according to claim 2, wherein a thin film coating that totally reflects at n ⁻¹ (sin θ _b / n) is applied, and the amplified laser light is incident on the center of the first surface. 4. The method according to claim 3, wherein the excitation light is not incident only from the excitation surface, but is incident from any one or two or all of the excitation surface, the reflection surface, and the second surface. The laser amplifier described. 5. A laser amplifier, wherein a plurality of laser amplifiers according to claim 1 are arranged, and an emission beam from one laser amplifier is made an incident beam of another laser amplifier. .