KR0155068B1 - The laser amplification method and apparatus for four path amplification compensating thermal birefringence - Google Patents

Abstract

The present invention provides an apparatus and method for compensating for thermal birefringence by placing Faraday photons between a gain medium and a reflector of a laser amplification device to achieve efficient high power laser amplification. Since thermal birefringence is compensated for, the laser resonance does not occur even if the laser medium is reciprocated twice by the gain medium, and thus a large gain GAIN can be obtained.

Description

Laser Amplification Apparatus and Method for Four-Path Amplification by Compensating Polarization Distortion

1 is a diagram showing the configuration of a four-path laser amplifier according to the present invention.

2 shows the definition of the refractive index in a gain medium according to the angle of incidence of light.

3 shows a path through which light passes to achieve four-path amplification.

4 is a graph showing the amplification factor of a four-path amplifier using QWP and a four-path amplifier using Faraday optoelectronics.

* Explanation of symbols for main parts of the drawings

1: first polarizer 2: pockels cell

3: second polarizer 4: gain medium

5: Faraday rotator 6: First mirror

7: second mirror 8: laser

31: A curve showing the amplification rate of the first path

32: Curve indicating the amplification rate of the second path

33: A curve showing the amplification factor of the fourth path when using a Faraday photon

TECHNICAL FIELD The present invention relates to an amplifier, and more particularly, to a method for amplifying by using four paths using a Faraday rotator, using a four path, and an amplifier thereof.

X-ray generators using TW-class lasers are in the spotlight these days. In addition, high power lasers have become an indispensable device for nonlinear scientific research. To obtain high output light, the laser is oscillated by mode-locking Q-switching (MLQS) and then amplified by another amplifier. When the pulse width to be amplified is very short, the method of amplifying by extending the pulse with the optical fiber and then compressing the pulse width again using a device with a large dispersion. Amplifiers used here include a regenerate amplofier and a multi-pass amplifier. A regenerative amplifier is a method of amplifying several round trips in a medium with a small pulse width gain. In the regenerative amplifier, two total reflection mirrors form a resonator, and a Pockels cell is used to pull the amplified light out of the resonator. So the Pockels cell drive circuit must be synchronized with the MLQS. While regenerative amplifiers have the advantage of high gain and high energy, synchronization problems present several challenges in practice. In general, the Pockels cell has an operating time of about 10ns, so the total length of the amplifier is inevitable. In addition, there is a problem that it is difficult to amplify a long pulse of more than 10ns.

In view of solving the above problems, a multipath amplifier that does not require synchronization is a simple structure and a very convenient amplifier. In general, the amplification medium used in the multipath amplifier has a small signal amplification rate larger than that of the regenerative amplifier, so that a large amount of energy can be obtained in only a few round trips. The advantage of multipath amplifiers is that they do not use active devices and do not have a wavelength selection device, so they can be used for very short pulse amplification.

Recently Henry Plaessmann and poetry. Le Blanc et al. Have shown how to take advantage of multipath amplifiers to achieve high power. However, their multipath amplifiers use an angle multiplexing method that injects light into the gain medium at various angles and then extracts the amplified light. Therefore, since the laser beam is concentrated in the medium, optical damage may be caused when a large energy light is amplified. Therefore, instead of amplifying small beams at multiple angles, if a large beam can be amplified in a degenerate multipass like a reproduction amplifier, the energy amplification efficiency is not only high but also optical damage can be avoided.

V. V. N. Bolousov et al. Also propose a two-path amplifier in which the advantages of a multipath amplifier are applied in Laser and Particle Beam No. 9 (1991). As is well known, optical pumping of a laser medium with a flash lamp, or the like, generates heat, which causes the medium to be birefringent. As a result, polarization distortion occurs when light is amplified while reciprocating. However, in the case of the Belosorb, since the λ / 4 wave plate (QWP) is used, polarization distortion cannot be compensated for and the light is reciprocated only once. Proper use of Faraday optoirs instead of QWPs will compensate for polarization distortion and allow for multipath amplification.

Accordingly, it is an object of the present invention to create an efficient high power laser by placing the Faraday photonizer, which compensates for thermal polarization distortion, in an appropriate position to make the laser light pass through four paths and amplify it.

Other features and advantages, including the above-described features and advantages of the present invention, can be clearly appreciated by those skilled in the art by the following detailed description described with reference to the accompanying drawings in which like reference numerals are attached to the same elements. have.

First, referring to FIGS. 1, 2, and 3, a structure of a four-path amplifier according to the present invention and a method of compensating polarization distortion of the amplifier will be described.

1 shows one preferred embodiment of the construction of a four-path amplifier in accordance with the present invention. One preferred embodiment of the four-path amplifier according to the present invention is a second polarizer in which the polarization peripheral side is rotated 90 degrees with respect to the laser source 8, the first polarizer 1, and the first polarizer as shown in FIG. 3) a Pockels cell (2), a gain medium (4), Faraday optometries (5) and first and second mirrors (6 and 7), which rotate the polarization of the light passing through the first polarizer by 90 °. Include. In a preferred embodiment of the present invention, the gain medium is a phosphate glass containing neodymium (Nd), which has been polished at 6 ° inclined on both sides of the gain medium to prevent oscillation and then subjected to antireflection coating. In general, however, any solid gain medium can be used. The polarizers used a Glan-Thompson split polarizer, with two mirrors being planar total mirrors.

Incident light from the laser 8 is polarized by the first polarizer and only the light polarized in one direction passes. The light passing through the first polarizer is incident on the Pockels cell 2 and the light passing through the Pockels cell 2 is rotated by 90 ° to reach the second polarizer 3.

In FIG. 3, the laser light passing through the second polarizer 3 proceeds through the gain medium 4 and then rotates by 45 ° as it passes through the Faraday rotator 5. This light is reflected off the first mirror 6. Each time the light passes the Faraday photon, the polarization is rotated by 45 °. The light reflected from the first mirror 6 passes again through the Faraday optometries 5 and the back light is rotated 90 ° with respect to the light incident on the first gain medium in accordance with the polarization effect described below (Formula 2). This light is reflected out of the second polarizer at a right angle, reflected by a second mirror disposed on the path of light, and returned back to the second polarizer 3, where initially the Pockels cell 2 is present in the second polarizer. From the same path of light incident on the second polarizer to the first mirror, where it is reflected and passes back through the gain medium. After passing through the second polarizer to reach the Pockels cell (PC) (2). At this time, since the polarization of the light passing through the PC 2 does not change since the Pockels cell 2 has already stopped, the light reaching the first polarizer 1 is reflected at right angles.

The most important in this configuration is the position of the Faraday beneficiator 5.

When optically pumping neodium glass with a uniform coefficient of thermal expansion with a flash lamp, the temperature distribution is different and the refractive indices for γ and θ polarization change. When the incident polarization direction is the same as the x-axis of FIG. 2, since the polarized light incident on the x-axis and the y-axis is the same as the direction of the major axis of the index ellipsoid, the polarization direction does not change even when passing through the medium. However, the light incident on the x-axis and the position of each ø is phase shifted by γ and θ polarization with respect to the new principal axis by øγ and øβ, respectively, and cannot generally maintain an initial polarization state. By using Faraday optoi, however, it is possible to compensate for the thermal birefringence effect by rotating the polarization of light by 90 °, as it is easily amplified in the following arguments, thus degrading amplification in the medium due to energy loss due to resonance. Prevent and gain large gains (gain = E _OUT / E _IN ).

When traveling to and from Faraday Beneficiaries, the Jones Matrix of Faraday Beneficiaries

And the Jones matrix of the gain medium in the coordinate system of FIG. Therefore, the case where the linearly polarized light starting from the second polarizer reciprocates the gain medium and the Faraday optometrist is expressed by the Jones matrix as follows.

As can be seen from the above result, when the Faraday photonizer is placed between the gain medium and the reflector, the incident polarization becomes the vertical polarization after the round trip irrespective of the birefringence of the gain medium. If the Faraday photon is placed between the second polarizer and the gain medium, the output polarization component

This will not compensate for thermal birefringence. Therefore, when the gain of the medium is large, the first mirror and the second mirror form a resonator and oscillate, and thus fail to amplify. As mentioned earlier, V. yen. VN Belousov et al. See Faraday optometrists in front of the gain medium in a two-path amplifier using a phase conjugate mirror in Laser and Particle Beam No. 9 (1991). Can be. In these amplifiers, there is a loss of light by E ² x / E ² y due to thermal birefringence, so it is better to place Faraday photons behind the gain medium.

Consider a case where a QWP is placed between a gain medium and a mirror instead of a Faraday beneficiator. Under this configuration, the output light component once reciprocated is:

In this equation, since the QWP cannot compensate for the rotation of polarization caused by thermal birefringence, it can be seen that the laser oscillates between the first mirror and the second mirror when the pumping energy is large. And E ² x / E ² y is a loss of the reproduction amplifier using the QWP.

However, since the thermal birefringence is small and the condition of E ² x / E ² y × g ² ₀ <1 (g ₀ : single path gain) is not oscillated, the amplifier can be configured using QWP. Therefore, QWP cannot compensate for thermal birefringence, but when the thermal birefringence is small, it can be used for an amplifier having a larger aperture than the amplifier for which Faraday photon is used. Or, even when the medium has its own birefringence, such as polarization fiber, it can be used in a four-path amplifier because the direction of polarization does not change while light passes through the medium.

Now, the performance and the effect of the four-path amplifier for which the polarization distortion is compensated according to the present invention, the gain ratio of the pumping energy per unit volume of the four-path amplifier using the QWP wave plate and the four-path amplifier using the Faraday photonizer A description with reference to FIG. 4 is shown.

First, let's look at the case of a four-path amplifier in which QWP is placed between the amplification medium and the reflector. Amplification rates were measured for linearly polarized MLQS single pulses (1.6 μJ, 40 ps) and long pulses of 10 ns and 200 ns while increasing the pumping energy while the QWP was placed between the amplification medium and the reflector. Since the oscillation threshold is lowered not only by thermal birefringence but also when the QWP is not accurate, in order to prevent this, the QWP is set to be slightly inclined around the QWP spindle so that the QWP becomes exactly λ / 4 wavelength plate for 1053 nm. In FIG. 4, the curve 31 is the first path amplification measured in front of the first mirror, the curve 32 is the second path amplification measured in front of the second mirror, and the curve 33 is the fourth path amplification. .

However, in the experiment using QWP, the maximum amplification factor was 6 × 10 ³ times regardless of the pulse length, and the output was about 10 mJ when the incident energy was 1.6 μJ (40 psec). The gain of more than 6x10 ³ times in four-path amplification is due to laser oscillation when the amplifier gains more gains than previously considered. In addition, the oscillation threshold was lowered after a quick repeated pumping. This is evidence that the cause of the rash is a thermal effect.

Next, experiments were performed with a four-path amplifier in accordance with the configuration according to the present invention as described above, using Faraday beneficiaries instead of QWP. The amplification factor for the shunt polarized MLQS single pulse (1.6μJ, 40ps) and long pulses of 10ns and 200ns was measured while increasing the pumping energy by placing the Faraday beneficiator between the amplification medium and the reflector. As can be seen from the curve 33 of FIG. ^3, an amplification factor of 2 × 10 ³ times was obtained and no oscillation phenomenon was seen.

Therefore, the present invention can improve the amplification rate because it can be amplified four times by one amplifier, and there is no optical damage to the medium since the light is amplified in a redundant path without being connected in the medium.

Although the present invention has been described in detail with respect to a preferred four-path amplifier embodiment, the method for compensating for thermal birefringence according to the present invention can be used in a two-path amplifier or a flash lamp pumped regenerative amplifier that has been proposed so far. Accordingly, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and background of the invention as defined by the appended claims.

Claims (9)

A laser amplifying apparatus for amplifying laser light, comprising: a laser source 8 for generating the laser light, a laser light from the laser source 8 disposed on a front optical path from the laser source 8 A first polarizer for flattening the light. It is disposed on the front optical path from the first polarizer 1 and rotates the polarization direction of the laser light incident from the first polarizer 1 by 90 ° when the electric field is applied, and when the electric field is not applied, the rear optical path is rotated. Pockels cell (2) for transmitting the incident laser light as it is. A polarization direction difference of [90 ± 90 × 2n] ° (where n is a positive integer) with respect to the laser light from the first polarizer 1 disposed on the front optical path from the Pockels cell 2 A polarization main axis rotated by 90 ° relative to the polarization main axis of the first polarizer 1 so as to transmit the laser light having a polarization direction and reflect the laser light having a polarization direction difference of [90 ± 90 × (2n-1)] °. A second polarizer (3) having a gain medium (4) for amplifying the laser light which is disposed on the front optical path from the second polarizer (3) and is incident along the front and rear optical paths, and the gain medium ( 4, a laser beam incident from the Faraday beneficiator 5 for rotating the polarization direction of the laser light incident on the front and rear light paths by 45 °. A laser reflected from the first mirror 6 and the second polarizer 3 for total reflection of light And a second mirror (7) for total reflection of the second polarizer (3), the second polarizer (3), the gain medium (4), the Faraday photonizer (5), and the second The amplified laser light, which is amplified by being repeatedly reflected along the optical path defined by the first mirror 6 and the second mirror 7, and incident on the second polarizer 3 along the rear light path, When the polarization direction has a difference of [90 ± 90 × 2n] ° from the polarization direction of the laser light from the first polarizer 1, the light is transmitted through the second polarizer along a rear optical path and is emitted. Light is transmitted by passing through the Pockels cell (2) along the rear light path and is incident and reflected by the first polarizer (1), characterized in that the laser beam amplification device. 2. The laser beam amplifying apparatus according to claim 1, wherein the Faraday optograph is arranged on an optical path between the gain medium and the first mirror. 6. The laser beam amplifying apparatus according to claim 1, wherein said gain medium (4) is a solid gain medium and has an antireflective coating surface that is inclined polished at a small angle on both sides. The laser beam amplifying apparatus of claim 1, wherein the first polarizer and the second polarizer are Glan-Thompson polarizers. A method of amplifying a laser beam from a laser source (8), wherein the laser beam is incident on the first polarizer (1) to obtain a first laser beam by polarization separation, and the polarization direction of the first laser beam is 90 °. Rotating to obtain a second laser light, the second laser light is incident to the second polarizer 3 having a polarization main axis rotated by 90 ° relative to the polarization main axis of the first polarizer 1 to transmit the third laser light Obtaining light, and amplifying the third laser light by injecting the third laser light transmitted through the second polarizer 3 into the laser amplifying means 4 alternately along the front and rear optical paths, wherein the laser amplification is performed. An amplifying step in which the polarization direction is rotated by 90 ° every time the amplification of the means 4 is reciprocated once, and when the amplified laser light has a polarization direction parallel to the polarization main axis of the second polarizer 3 The amplified ray Outputting low light through the second polarizer along the rear light path and obtaining the output light by causing the emitted laser light to be incident and reflected by the first polarizer 1 along the rear light path; A laser beam amplification method characterized by the above-mentioned. 6. The method according to claim 5, wherein the step of obtaining the second laser light comprises a step of Pockels cell (2) rotating the polarization direction by 90 degrees. 6. The laser beam amplification method according to claim 5, wherein the amplifying step includes a step in which the Faraday photon beamer (5) rotates the polarization direction of the incident light by 45 degrees. 6. The laser beam amplification method according to claim 5, wherein the amplifying step includes amplifying the third laser beam through four paths through the laser amplifying means. 6. The laser beam amplification method according to claim 5, wherein the step of obtaining the output light includes a step in which the Pockels cell (2) transmits the laser light emitted through the second polarizer as it is.