WO2000042465A1

WO2000042465A1 - Optical isolator using stimulated brillouin scattering phase conjugation mirrors and its application to an optical amplifier system

Info

Publication number: WO2000042465A1
Application number: PCT/KR2000/000028
Authority: WO
Inventors: Hong-Jin Kong
Original assignee: Kong Hong Jin
Priority date: 1999-01-16
Filing date: 2000-01-17
Publication date: 2000-07-20
Also published as: KR100318520B1; JP2002535696A; CN1178086C; JP4405684B2; KR20000050976A; CN1310806A

Abstract

An optical isolator with perfect rejection ratio and its application to an optical amplifier system. Stimulated Brillouin scattering phase conjugation mirrors are symmetrically disposed to form a cross-type optical amplifier system, which makes the optical system insensitive to its optical alignment. If the optical amplifier system is used for laser amplification, the output power of the laser amplifier system can be increased as desired with same repetition rate.

Description

OPTICAL ISOLATOR USING STIMULATED BRILLOUIN

SCATTERING PHASE CONJUGATION MIRRORS AND

ITS APPLICATION TO AN OPTICAL AMPLIFIER SYSTEM

TECHNICAL FIELD

The present invention relates to an optical device, and more particularly, to an optical isolator using stimulated Brillouin scattering phase conjugation mirrors. The present invention also relates to the use of such optical isolators in an optical amplifier system. BACKGROUND ART

When a device is used with a laser, the beam reflected from the device may disturb a laser if the beam returns to the laser. Especially, for a pulse type laser, the reflected beam is amplified unexpectedly and it may damage optical elements of the laser. Therefore, an optical isolator is used to prevent damages. Examples of prior optical isolators are a passive optical isolator using a 1/4 wavelength plate(Quarter Wave plate; hereinafter referred to as "QW") or a Faraday Rotator(hereinafter referred to as "FR") and an active optical isolator using Pockels Cell(hereinafter referred to as "PC"). However, the isolation effect of these optical isolators is not perfect due to imperfect rejection ratio of polarizer to other orthogonal polarization components. For example, the rejection ratio of nonlinear crystal polarizer is around 1000 and that of multi-layered polarizer is around 100. That is, the rejection ratio of prior optical isolators is between 100 and 1000, which may induce amplification of a reflected beam in a laser or if the reflected beam is combined with an amplifier in the next stage, it may cause a serious damage on the laser. The operation examples of prior optical isolators are described herein in conjunction with the accompanying drawings and their problems are discussed.

FIG. 1 A is a block diagram of a prior passive optical isolator. The passive optical isolator 100 is comprised of a polarization beam splitter (hereinafter referred to as "PBS") 110, and a polarization converter 120. The PBS 110 is a polarizing separator which passes horizontally polarized beam but reflects vertically polarized beam. The polarization converter 120 is either a QW which delays a phase by a 1/4 l wavelength or a FR which rotates a polarization plane by 45°. That is, each time the beam passes QW, a linearly polarized beam is converted to a circularly polarized beam, or vice versa. Therefore, double passes of a beam through the QW can result in a polarization state which is orthogonal to a previous polarization state. In a similar way, double passes of a beam through the FR can give the same result as above. To understand the operation of the passive optical isolator 100, let's assume that a horizontally polarized pulse input beam 105 passes through PBS 110 as shown in FIG. 1A. When a beam from the PBS 110 passes through a polarization converter 120, it is converted to a circularly polarized beam or a linearly polarized beam with a polarization plane rotated by 45°. Thereafter, when the beam is reflected and passes again through the polarization converter 120, it is converted to a vertically polarized beam which is orthogonal to a horizontally polarized beam, then it is emitted as a reflected beam 130 from the PBS 110. This will result in optical isolation effect.

FIG. IB is a block diagram of a prior active optical isolator. The active optical isolator 150 is comprised of two PBS 160 and 165, and a PC 170. The first PBS 160 passes a horizontally polarized beam, and the second PBS 165 passes a vertically polarized beam. When a high voltage is applied to the PC 170 for a predetermined time interval, the polarization direction can be rotated perpendicularly only during the time interval. To see the operation of the active optical isolator 150, let's assume that a horizontally polarized pulse input beam 145 passes through the first PBS 160 as shown in FIG. IB.

When the horizontally polarized beam from the first PBS 160 passes through the PC 170, if a high voltage is applied to the PC 170 for a time interval corresponding to the pulse-width of the passing beam, the horizontally polarized beam is converted to a vertically polarized beam as it passes through the PC 170. Therefore, the vertically polarized beam passes through the second PBS 165 with no change. Thereafter, when the vertically polarized beam returns after reflected from somewhere, the vertically polarized beam is reflected instead of being passed from the first PBS 160 because there is no high voltage applied to the PC 170. This will result in an optical isolation effect, too.

However, there are usually defects in an optical isolator component such as a PBS, PC, QW or FR. For example, the separation ratio of vertically polarized beam and horizontally polarized beam in a PBS is only around tens or hundreds. The polarization rotation of a PC, QW or FR is not perfect either, and the rejection ratio is not perfect overall. The leakage of a reflected beam into an input end through an imperfect optical isolator results in an amplification coupling between neighboring amplifiers, which causes damages in optical devices or an unstable oscillation in the output mode of laser oscillator.

Therefore, Jackel et al. suggested an optical isolator employing a Stimulated Brillouin Scattering Phase Conjugation Mirror (hereinafter referred to as "SBS- PCM"). However, even in this structure, the reflectance of the SBS-PCM is less than 100 %, and the amplifier can not completely cut off the beam reflected back to a laser oscillator. So, an optical isolator with a perfect rejection ratio is required to make a high power laser amplifier.

The inventor of the present invention has presented a solid state laser with high repetition rate and high power in U.S. Pat. No. 5,832,020. The repetition rate in a solid state laser depends on cooling speed, and in the prior art technologies the diameter of a laser rod used for an amplifier system should be increased according to the increase of desired laser output, which decreases repetition rate. The technology disclosed in U.S. Pat. No. 5,832,020 solved the problem, but even under such laser structure an imperfect wavelength plate or a PBS do not cut off returned beam completely, and the optical system can be damaged by amplified beam.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide an optical isolator which can prevent damages on an optical system by a perfect beam isolation effect.

Another object of the present invention is to provide an optical amplifier system which can increase output without harming optical systems such as a laser.

The optical isolator of the present invention to achieve the object is similar to the prior art technology in that it employs a polarization beam splitter and a polarization converter, but can be characterized by a stimulated Brillouin scattering phase conjugation mirror to reflect beam passed through a polarization converter. For a polarization conversion, a 1/4 wavelength plate which delays a phase of beam by 1/4 wavelength or a Faraday Rotator which rotates polarization plane by 45 ° can be used.

The optical amplifier system of the present invention to achieve another object is comprised of at least two optical amplifier stages, and each optical amplifier stage includes the optical isolator of the present invention. That is, each amplifier stage is provided with an optical isolator, and a beam amplifier which reflects amplified beam from an optical isolator but the polarization is converted and the converted beam is combined with a beam which passes through a polarization beam splitter that is included in the optical isolator by the reflection at the polarization beam splitter. The amplifier stages are arranged in a chain type so that beam from the previous amplifier stage is introduced into the polarization beam splitter included in the next amplifier stage. A stimulated Brillouin scattering phase conjugation mirror can be used to reflect a beam from the optical isolator and either a 1/4 wavelength plate or a Faraday rotator can be used to convert a polarization of the beam. If the optical amplifier system is configured to introduce a beam from a laser oscillator into a polarization beam splitter included in the first of the optical amplifier stages, a high power beam can be obtained. The optical amplifier system preferably further comprises beam size expansion means disposed between adjacent optical amplifier stages in order to expand the size of a beam. The optical amplifier system can be a rod-type or a slab-type depending upon the types of the laser. In addition, at least one of the optical isolator and the optical amplifier can be constructed in an array type, and in this case, a wedge-type beam splitter is added in an optical path to transmit beams to each of the array.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A is a block diagram of a prior passive optical isolator;

FIG. IB is a block diagram of a prior active optical isolator;

FIG. 2 is a graph of reflectance of a stimulated Brillouin scattering phase conjugation mirror versus input pulse energy; FIG. 3 A and FIG. 3B are block diagrams of optical isolators according to the embodiments of the present invention;

FIG. 4 is a block diagram of an optical amplifier system according to another embodiment of the present invention;

FIG. 5 shows an application example of the optical amplifier system of the present invention to a rod-type laser;

FIG. 6 shows another application example of the optical amplifier system of the present invention to a slab-type laser;

FIG. 7A to FIG. 7C are drawings to explain alignment sensitivity of an optical system; and

FIG. 8 A to FIG. 8C show amplifier stages of optical amplifier systems of the present invention which comprises compensation means for compensating for the thermal induced birefringence of the laser rod.

BEST MODE FORCARRYING OUTTHE INVENTION

When a pulse-type laser beam is incident on a SBS-PCM used for both an optical isolator and an optical amplifier system of the present invention, the reflectance versus input energy is non-linear as shown on the graph of FIG. 2. In FIG. 2, the critical value where the non-linear reflectance of the SBS-PCM increase abruptly from zero is defined as I_th, and the energy of incident beam with 90% of reflectance is defined as I₀₉.

FIG. 3 A and FIG. 3B are block diagrams of optical isolators according to the embodiments of the present invention.

Referring to FIG. 3 A, an optical isolator 300 is comprised of a PBS 310, a QW 320 and a SBS-PCM 340. When a vertically polarized beam 305 from a laser oscillator (not shown) having an energy higher than I_th of FIG. 2 is incident on the PBS 310, then it is reflected by the PBS 310 and is incident upon the SBS-PCM 340 after passing through the QW 320. At this time, if the energy of beam incident upon the SBS-PCM 340 is higher than I_th, the beam is reflected because the reflectance of the SBS-PCM 340 is not zero. Especially, when a beam with energy higher than I₀₉ is incident upon the SBS-PCM 340, the reflectance of the SBS-PCM 340 is above 90% and it has a reflectance of conventional mirrors. Generally, the reflected beam by a SBS-PCM is a phase conjugation wave and has a phase conjugation relation with an incident beam. It means that the phase of beam is compensated when an optical system distorts it. Additionally, the phase conjugation wave has a characteristic to trace back the incident path when reflected.

Furthermore, since the reflectance of a SBS-PCM is non-linear as shown in FIG. 2, it shows a spatial frequency filtering effect. That is, the spatial frequency component with weak beam intensity has a low reflectance by a SBS-PCM, and the spatial frequency component with strong beam intensity has a high reflectance by a SBS-PCM, therefore, only the spatial frequency component with strong beam intensity is amplified and presents a spatial filtering effect. Therefore, the beam quality reflected by a SBS-PCM is superior to that of a conventional mirror.

When the beam reflected by the SBS-PCM 340 passes the QW 320 again, it is converted to a horizontally polarized beam which is orthogonal to the incident beam, and it passes through the PBS 310. On the other hand, when the output beam from the PBS 310 is reflected back by the next optical system, most of the reflected beam I_back is reflected by the PBS 310. The beam I_leak which passes through the PBS 310 is again incident on the SBS-PCM 340, but this beam is very weak and the intensity is less than a reflectance critical value I_th of the SBS-PCM. It means no beam is reflected back to the laser oscillator.

As shown in FIG. 3B, when a beam 345 passed through a PBS 360 is horizontally polarized, the beam returning back through a QW 370 and a SBS-PCM 390 is reflected by the PBS 360, and it gives the same result as shown in FIG. 3A. FIG. 4 is a block diagram of an optical amplifier system according to another embodiment of the present invention and a SBS-PCM optical isolator is applied on a general multi-stage amplifier system. Referring to FIG. 4, the optical amplifier system is comprised of a plurality of optical amplifier stages 400, 450, ....

The function of the optical amplifier system will be described with the first optical amplifier stage 400. When a vertically polarized laser pulse beam 395 is incident on a PBSl 412 which is included in the first optical isolator 410, it is reflected by the PBSl 412 and is converted to a circularly polarized beam when it passes through a QW1 414. When the circularly polarized beam passes through the QW1 414 after reflected by a SBS-PCM 416, it is converted to a horizontally polarized beam and is able to pass trough the PBSl 412. When the beam is amplified by passing through a multi-pass amplifier 1 422 included in the first SBS optical amplifier 420, passes through a QW2 424, is reflected by a SBS-PCM2 426, and passes through the QW2 424 again, then it is converted to a vertically polarized beam. The QW2 424 can be substituted by a Faraday Rotator which rotates the polarization plane by 45°. Then, it is amplified again by passing through the multi-pass amplifierl 422, is reflected by the PBSl 412, and it is incident on a PBS2 462 included in the second optical isolator 460 of the second amplifier stage 450. The second amplifier stage 450 has the same components with the first amplifier stage 400, and is comprised of the second optical isolator 460, and the second SBS optical amplifier 470. On the whole, each of the optical amplifier stages 400, 450, ... has the same structure, and they are arranged in a chain type so that beam from the previous amplifier stage is introduced into the PBS included in the next amplifier stage. The configuration of an optical amplifier system in this way provides an enhanced amplification of a laser beam and removes damages on a laser by the reflected beam.

FIG. 5 shows an application example of the optical amplifier system of the present invention to a rod-type laser. Referring to FIG. 5, a beam 505 from a laser oscillator 500 is amplified by the first SBS optical amplifier 530 when it passes through the first amplifier stage 510, and the returning beam is cut off by the first optical isolator 520. The difference of each amplifier stage between that of the optical amplifier system of FIG. 4 is that there is a lens in front of a SBS-PCM to control the focus of a beam. Another difference is that the second SBS optical amplifier 560 in the second amplifier stage 540 is a 2x2 array type, and wedge type beam splitters 562 are used to transmit beams to each array. In the third amplifier stage 570, the third SBS optical amplifier 590 is a 4x4 array type and the third optical isolator 580 is a 2x2 array type. Wedge type beam splitters 582 and 592 are also used to transmit beams to each array. Furthermore, there are beam expanders 535, 565, ... between the optical amplifier stages 510, 540, 570, ... to control the size of beams. By arranging the laser amplifier system in this way, amplifier stages can be added as desired, and the output energy can be enhanced without damaging the optical system while maintaining the same repetition rate.

FIG. 6 shows another application example of the optical amplifier system of the present invention to a slab-type laser. Compared to the optical amplifier system of FIG. 5, it is different in that the second SBS optical amplifier 660 is a 2x1 array type, the third SBS optical amplifier 690 is a 4x1 array type, and the third optical isolator 680 is a 2x1 array type to be suited for a slab-type laser.

The optical system of the present invention has an advantage that it is insensitive to the alignment, which is described in FIG. 7A to FIG. 7C showing several optical systems. Identical elements have been given the same reference numeral in the FIG. 7A to FIG. 7C.

FIG. 7A shows the alignment sensitivity of the optical amplifier system of the present invention using two symmetrically disposed SBS-PCMs. In this configuration, even though a beam from a laser oscillator 700 is incident on a PBS 710 which is deviated by Δθ from the alignment, the position and direction of output beam from the symmetrically positioned SBS-PCMs 720 aligns with the direction of the incident beam.

FIG. 7B shows the alignment sensitivity of the optical system using only one SBS-PCM. In this case, if the PBS 710 is deviated by Δθ from the alignment, the output beam deviates by Δψ (= 2ΔΘ) from the alignment. FIG. 7C shows the alignment sensitivity of the optical system using conventional mirrors 722 at both ends. In this case, the output beam deviates by δ from the alignment.

On the other hand, in the optical amplifier system of the present invention, thermal induced birefringence of a laser rod can be problematic. The birefringence effect can be compensated by constructing optical amplifier stages as shown in FIG. 8A to FIG. 8C.

FIG. 8A shows a 2-pass optical amplifier stage, which includes two amplifier means 820 and 822. Between them, there is a 90° polarization rotator 830.

On both ends of the amplifier stage 800, there are 45° Faraday rotators or 1/4 wavelength plates 832. Beam reflected by a SBS-PCM 810 is re-incident on the one end of the amplifier stage 800.

FIG. 8B shows a 4-pass optical amplifier stsage. The amplifier stage 850 includes an amplifier means 870, and there are a PBS 890 and a 45° Faraday rotator or a 1/4 wavelength plate 832 on both ends of the amplifier stage 800. The split beam by the PBS 890 is reflected by a mirror 892 and an SBS-PCM 860, respectively, and the beam from the amplifier means 870 passes through a PC 882.

FIG. 8C shows an optical amplifier stage installed with a birefringent nonlinear crystal therein. It allows part of a beam reflected by a SBS-PCM 896 to pass through a 90° polarization rotator 834, which is then allowed to pass through a birefringent nonlinear crystal 894 with the remaining part. Thereafter, the beam passes through amplifier means 872 and a 45° polarization rotator or a 1/4 wavelength plate 832, consecutively.

The optical amplifier system of the present invention may further comprise means for collimating beams by a phase locking of beams reflected at the stimulated Brillouin scattering phase conjugation mirrors included in the optical amplifier system. The collimating means may utilize either a self-generated back seeding method or an acousto-optic induced phase locking method.

INDUSTRIAL APPLICABILITY

When the optical isolator of the present invention is used, damages on an optical system by reflected beam can be completely prevented. Also, according to the optical amplifier system of the present invention, output energy can be increased while maintaining the same repetition rate. It is also convenient to handle because the optical system is insensitive to the alignment.

Claims

WHAT IS CLAIMED IS :

1. An optical isolator comprising: a polarization beam splitter which reflects or passes an incident beam depending upon the polarization of the incident beam; polarization conversion means for converting the polarization of beam which is reflected or passed at said polarization beam splitter perpendicular to the polarization of the beam before it passed through said polarization conversion means after the double passes of the beam through said polarization conversion means; and a stimulated Brillouin scattering phase conjugation mirror for reflecting the beam passed through said polarization conversion means.

2. The optical isolator of claim 1, wherein said polarization conversion means is a 1/4 wavelength plate which delays the phase of beam by 1/4 wavelength, or a Faraday Rotator which rotates the polarization plane by 45°.

3. An optical amplifier system having at least two optical amplifier stages, each amplifier stage comprising: a polarization beam splitter which reflects part of beam and passes rest of the beam depending upon the polarization of the incident beam; a stimulated Brillouin scattering optical isolator, including a first polarization conversion means for converting the polarization of beam which is reflected at said polarization beam splitter perpendicular to the polarization of the beam before it passed through first said polarization conversion means after the double passes of the beam through said first polarization conversion means, and a first stimulated Brillouin scattering phase conjugation mirror for reflecting the beam which is passed through said first polarization conversion means; and a stimulated Brillouin scattering optical amplifier, including amplifying means for amplifying the beam which is passed through said polarization beam splitter during the double passes of the beam through said amplifying means, a second polarization conversion means for converting the polarization of the amplified beam perpendicular to the polarization of the beam before it passed through said second polarization conversion means after the double passes of the beam through said second polarization conversion means, and a second stimulated Brillouin scattering phase conjugation mirror for reflecting the beam which is passed through said second polarization conversion means; wherein said amplifier stages are arranged in a chain type so that beam from the previous amplifier stage is introduced into the polarization beam splitter included in the next amplifier stage.

4. The optical amplifier system of claim 3, wherein each of said first and second polarization conversion means is a 1/4 wavelength plate which delays the phase of beam by 1/4 wavelength, or a Faraday Rotator which rotates the polarization plane by 45°.

5. The optical amplifier system of claim 3, wherein a beam from a laser oscillator is incident on a polarization beam splitter included in the first of said optical amplifier stages.

6. The optical amplifier system of claim 5, further comprising beam size expansion means disposed between adjacent optical amplifier stages in order to expand the size of a beam.

7. The optical amplifier system of claim 5, wherein said stimulated Brillouin scattering optical amplifier is either a rod-type or a slab-type

8. The optical amplifier system of claim 5, wherein at least one of said stimulated Brillouin scattering optical isolator and said stimulated Brillouin scattering optical amplifier is constructed in an array type, and a wedge-type beam splitter is added in an optical path to transmit beams to each of said array.

9. The optical amplifier system of claim 7, further comprising compensation means for compensating for the thermal induced birefringence of said amplifying means.

10. The optical amplifier system of claim 5, further comprising means for collimating beams by a phase locking of beams reflected at said stimulated Brillouin scattering phase conjugation mirrors.

11. The optical amplifier system of claim 10, wherein said collimating means utilizes either a self-generated back seeding method or an acousto-optic induced phase locking method.