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Photorefractive phase conjugator for Nd:YAG laser system
Nuclear Instruments and Methods in Physics Research A 455 (2000) 244}246
Photorefractive phase conjugator for Nd:YAG laser system Kazuyoku Tei*, Yoshito Niwa, Fumiaki Matsuoka, Masaaki Kato, Yoichiro Maruyama, Takashi Arisawa Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizucho, Sorakugun, Kyoto 619-0215, Japan
Abstract An external loop phase conjugate mirror with a 453-cut rhodium-doped BaTiO is constructed and characterized. The external loop is composed of an image-relay telescope, a 903-image rotator and a phase modulator. An Nd:YAG laser with the phase conjugate mirror produces a pulse energy of 330 mJ at 100 Hz with a beam quality of 1.2;DL. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.65; 42.65.H; 42.60 Keywords: Nonlinear optics; Phase conjugation; Lasers optical systems; Rh:BaTiO
Recently, solid-state phase conjugators in laser systems have received much attention [1}3]. The obstacle in the stimulated Brillouin scattering (SBS) in solid-state media is the existence of power threshold to generate phase conjugate wave. As the incident beam quality deteriorates, the threshold power of SBS increases and approaches the damage threshold. Therefore, the dynamic range of solidstate SBS is limited to a narrow range. Photorefractive (PR) e!ect has the advantage of wide dynamic range because the e!ect has no power threshold. Recently, Brignon et al. have demonstrated an Nd:YAG master oscillator power ampli"er (MOPA) system [4], that generates an average power of a few watts with a cat-type [5] self-
* Corresponding author. Tel.: #81-774-71-3371; fax: #81774-71-3316. E-mail address: [email protected] (K. Tei).
pumped phase conjugator. However, the cat-type phase conjugator has a long response time of several minutes. Moreover, the incident beam size should be much smaller than the crystal size because the light loop for wave mixing is formed in the crystal. Consequently, the cat-type phase conjugator is not suitable for MOPA systems that generate high-energy laser pulses. We have developed a PR self-pumped phase conjugator [6] based on ring geometry in place of the cat-type one having such disadvantages. In the phase conjugator, the re#ectivity of phase conjugate wave is not a!ected by the incident beam quality. Moreover, the phase conjugator is superior to the cat-type geometry in respect of the response time, the re#ectivity and the allowance to the pulse energy. In this paper, we report on a high-power (an average power of tens of watts and a pulse energy of hundreds of milli-joules) MOPA system with a ring self-pumped phase conjugator.
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 4 0 - 3
K. Tei et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 244}246
Fig. 1. Arrangement of phase conjugate mirror.
Fig. 1 shows the arrangement of the phase conjugator [6]. A 453-cut rhodium-doped (3200 ppm) BaTiO crystal (Deltronic Crystal Industries, Inc.) is used as the phase conjugator. This crystal is cut along the axes [1 0 0], [0 1 1] and [0 1 1 ], and its dimensions are 10.5, 9.0 and 2.3 mm. The beam entrance and exit faces, (0 1 1 ) and (0 1 1), are antire#ection coated. Firstly, to compose this external loop arrangement, an image relay telescope is used. The crystal location coincides with the conjugate points of the telescope. This consideration brings about fast response time and great ability of phase compensation in the phase conjugator. Secondly, a Dove prism in the external loop rotates the beam cross-section by 903 in order to suppress the generation of higher-order transverse modes in the direction perpendicular to the beam intersection plane. The wave plates preserve the horizontal linear polarization of beams on the crystal. Thirdly, the beams in the external plane are phasemodulated by a mirror attached to a piezo transducer. The phase modulation of feedback beams suppresses the formation of re#ection gratings in the crystal. These three considerations bring about a high re#ectivity ('60%) and a high-power stability ((1% for tens of minutes) while the phase conjugator preserves the ability of phase compensation for highly distorted beams (M'100). Fig. 2 shows an LD-pumped Nd:YAG MOPA system. The system consists of a master oscillator, two power ampli"ers and the phase conjugator.
245
Fig. 2. LD-pumped Nd:YAG MOPA system with the developed phase conjugate mirror. MO: master oscillator; DP: double Pockels cell; FI: Faraday isolator; P: polarizers; FR: Faraday rotator; j/2: half-wave plate; AMP1 and AMP2: LDpumped zig-zag slab Nd:YAG ampli"ers; P1, P2 and P3: pinholes; PCM: phase conjugate mirror.
The master oscillator runs with a linewidth of 30 MHz at a repetition rate of 100 Hz. The initial beam quality of the seed beam from the oscillator was examined by the spot size focused by a planoconvex lens that has a focal length of about 1 m. The seed beam has a Gaussian pro"le, and was focused to a spot size of 1.2 times di!raction limit (1.2;DL). Two LD-pumped zig-zag-slab ampli"ers have an identical composition [7]. The dimensions of the seed beam incident on the ampli"ers are 10 mm (vertical) ; 4.5 mm (horizontal). The two ampli"ers are connected optically by using two image-relay telescopes. The beam path between the second ampli"er and the phase conjugator is also connected with an image-relay telescope. Three telescopes have pinholes (P1, P2 and P3) for the prevention of parasitic oscillations. The diameters of P1, P2 and P3 are 1, 2 and 3 mm, respectively. The seed beam propagates each ampli"er twice until it reaches the phase conjugator, and then the pulse energy approaches about 15 mJ. After the re#ection from the phase conjugator the seed beam is reampli"ed by these ampli"ers. The output pulse energy of MOPA is 360 mJ at a repetition rate of 100 Hz. The energy extraction e$ciency of MOPA calculated in the same manner as Brignon et al. [5], becomes 50%. This is higher than the e$ciency achieved in Ref. [5].
HIGH-QUALITY LASER BEAM
246
K. Tei et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 244}246
Fig. 3. Far-"eld pro"les of (a) the output beam from a system with the phase conjugate mirror and (b) the output beam from a system with an ordinary mirror.
Fig. 3 shows the far-"eld pattern of MOPA output beam at the focus of the lens used for focusing the oscillator beam. The pattern has side lobes because the near-"eld pattern has a #at-top shape peculiar to the saturation ampli"cation. The radius of the "rst zero is 1.2 times di!raction limit (Fig. 3(a)). The output beam quality is the same as that of the seed beam except for the intensity pro"les. This means that the phase conjugator corrects the phase distortion of the laser beam induced in the system perfectly. When the MOPA system is rearranged with an ordinary mirror, a remarkable deterioration of the beam quality occurs (Fig. 3(b)). We show that the developed photorefractive phase conjugator can be used in a high-power or high-energy Nd:YAG MOPA system. The arrangement o!ers for high-power laser systems a high
ability of phase correction using the photorefractive e!ect.
References [1] A. Brignon, J.-P. Huignard, M.H. Garrett, I. Mnushkina, Opt. Lett. 22 (1997) 215. [2] H.J. Eichler, B. Liu, A. Haase, O. Mehl, A. Dehn, Proc. SPIE 3263 (1998) 20. [3] M.J. Damzen, R.P.M. Green, K.S. Syed, Opt. Lett. 20 (1995) 1704. [4] J. Feinberg, Opt. Lett. 7 (1982) 486. [5] A. Brignon, J.-P. Huignard, M.H. Garrett, I. Mnushkina, Opt. Lett. 22 (1997) 442. [6] K. Tei, Y. Niwa, M. Kato, Y. Maruyama, T. Arisawa, Jpn. J. Appl. Phys. 38 (1999), to be published. [7] K. Tei, M. Kato, F. Matsuoka, Y. Niwa, Y. Maruyama, T. Matoba, T. Arisawa, Appl. Opt. 38 (1999) 4548.
Photorefractive phase conjugator for Nd:YAG laser system Kazuyoku Tei*, Yoshito Niwa, Fumiaki Matsuoka, Masaaki Kato, Yoichiro Maruyama, Takashi Arisawa Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizucho, Sorakugun, Kyoto 619-0215, Japan
Abstract An external loop phase conjugate mirror with a 453-cut rhodium-doped BaTiO is constructed and characterized. The external loop is composed of an image-relay telescope, a 903-image rotator and a phase modulator. An Nd:YAG laser with the phase conjugate mirror produces a pulse energy of 330 mJ at 100 Hz with a beam quality of 1.2;DL. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.65; 42.65.H; 42.60 Keywords: Nonlinear optics; Phase conjugation; Lasers optical systems; Rh:BaTiO
Recently, solid-state phase conjugators in laser systems have received much attention [1}3]. The obstacle in the stimulated Brillouin scattering (SBS) in solid-state media is the existence of power threshold to generate phase conjugate wave. As the incident beam quality deteriorates, the threshold power of SBS increases and approaches the damage threshold. Therefore, the dynamic range of solidstate SBS is limited to a narrow range. Photorefractive (PR) e!ect has the advantage of wide dynamic range because the e!ect has no power threshold. Recently, Brignon et al. have demonstrated an Nd:YAG master oscillator power ampli"er (MOPA) system [4], that generates an average power of a few watts with a cat-type [5] self-
* Corresponding author. Tel.: #81-774-71-3371; fax: #81774-71-3316. E-mail address: [email protected] (K. Tei).
pumped phase conjugator. However, the cat-type phase conjugator has a long response time of several minutes. Moreover, the incident beam size should be much smaller than the crystal size because the light loop for wave mixing is formed in the crystal. Consequently, the cat-type phase conjugator is not suitable for MOPA systems that generate high-energy laser pulses. We have developed a PR self-pumped phase conjugator [6] based on ring geometry in place of the cat-type one having such disadvantages. In the phase conjugator, the re#ectivity of phase conjugate wave is not a!ected by the incident beam quality. Moreover, the phase conjugator is superior to the cat-type geometry in respect of the response time, the re#ectivity and the allowance to the pulse energy. In this paper, we report on a high-power (an average power of tens of watts and a pulse energy of hundreds of milli-joules) MOPA system with a ring self-pumped phase conjugator.
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 4 0 - 3
K. Tei et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 244}246
Fig. 1. Arrangement of phase conjugate mirror.
Fig. 1 shows the arrangement of the phase conjugator [6]. A 453-cut rhodium-doped (3200 ppm) BaTiO crystal (Deltronic Crystal Industries, Inc.) is used as the phase conjugator. This crystal is cut along the axes [1 0 0], [0 1 1] and [0 1 1 ], and its dimensions are 10.5, 9.0 and 2.3 mm. The beam entrance and exit faces, (0 1 1 ) and (0 1 1), are antire#ection coated. Firstly, to compose this external loop arrangement, an image relay telescope is used. The crystal location coincides with the conjugate points of the telescope. This consideration brings about fast response time and great ability of phase compensation in the phase conjugator. Secondly, a Dove prism in the external loop rotates the beam cross-section by 903 in order to suppress the generation of higher-order transverse modes in the direction perpendicular to the beam intersection plane. The wave plates preserve the horizontal linear polarization of beams on the crystal. Thirdly, the beams in the external plane are phasemodulated by a mirror attached to a piezo transducer. The phase modulation of feedback beams suppresses the formation of re#ection gratings in the crystal. These three considerations bring about a high re#ectivity ('60%) and a high-power stability ((1% for tens of minutes) while the phase conjugator preserves the ability of phase compensation for highly distorted beams (M'100). Fig. 2 shows an LD-pumped Nd:YAG MOPA system. The system consists of a master oscillator, two power ampli"ers and the phase conjugator.
245
Fig. 2. LD-pumped Nd:YAG MOPA system with the developed phase conjugate mirror. MO: master oscillator; DP: double Pockels cell; FI: Faraday isolator; P: polarizers; FR: Faraday rotator; j/2: half-wave plate; AMP1 and AMP2: LDpumped zig-zag slab Nd:YAG ampli"ers; P1, P2 and P3: pinholes; PCM: phase conjugate mirror.
The master oscillator runs with a linewidth of 30 MHz at a repetition rate of 100 Hz. The initial beam quality of the seed beam from the oscillator was examined by the spot size focused by a planoconvex lens that has a focal length of about 1 m. The seed beam has a Gaussian pro"le, and was focused to a spot size of 1.2 times di!raction limit (1.2;DL). Two LD-pumped zig-zag-slab ampli"ers have an identical composition [7]. The dimensions of the seed beam incident on the ampli"ers are 10 mm (vertical) ; 4.5 mm (horizontal). The two ampli"ers are connected optically by using two image-relay telescopes. The beam path between the second ampli"er and the phase conjugator is also connected with an image-relay telescope. Three telescopes have pinholes (P1, P2 and P3) for the prevention of parasitic oscillations. The diameters of P1, P2 and P3 are 1, 2 and 3 mm, respectively. The seed beam propagates each ampli"er twice until it reaches the phase conjugator, and then the pulse energy approaches about 15 mJ. After the re#ection from the phase conjugator the seed beam is reampli"ed by these ampli"ers. The output pulse energy of MOPA is 360 mJ at a repetition rate of 100 Hz. The energy extraction e$ciency of MOPA calculated in the same manner as Brignon et al. [5], becomes 50%. This is higher than the e$ciency achieved in Ref. [5].
HIGH-QUALITY LASER BEAM
246
K. Tei et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 244}246
Fig. 3. Far-"eld pro"les of (a) the output beam from a system with the phase conjugate mirror and (b) the output beam from a system with an ordinary mirror.
Fig. 3 shows the far-"eld pattern of MOPA output beam at the focus of the lens used for focusing the oscillator beam. The pattern has side lobes because the near-"eld pattern has a #at-top shape peculiar to the saturation ampli"cation. The radius of the "rst zero is 1.2 times di!raction limit (Fig. 3(a)). The output beam quality is the same as that of the seed beam except for the intensity pro"les. This means that the phase conjugator corrects the phase distortion of the laser beam induced in the system perfectly. When the MOPA system is rearranged with an ordinary mirror, a remarkable deterioration of the beam quality occurs (Fig. 3(b)). We show that the developed photorefractive phase conjugator can be used in a high-power or high-energy Nd:YAG MOPA system. The arrangement o!ers for high-power laser systems a high
ability of phase correction using the photorefractive e!ect.
References [1] A. Brignon, J.-P. Huignard, M.H. Garrett, I. Mnushkina, Opt. Lett. 22 (1997) 215. [2] H.J. Eichler, B. Liu, A. Haase, O. Mehl, A. Dehn, Proc. SPIE 3263 (1998) 20. [3] M.J. Damzen, R.P.M. Green, K.S. Syed, Opt. Lett. 20 (1995) 1704. [4] J. Feinberg, Opt. Lett. 7 (1982) 486. [5] A. Brignon, J.-P. Huignard, M.H. Garrett, I. Mnushkina, Opt. Lett. 22 (1997) 442. [6] K. Tei, Y. Niwa, M. Kato, Y. Maruyama, T. Arisawa, Jpn. J. Appl. Phys. 38 (1999), to be published. [7] K. Tei, M. Kato, F. Matsuoka, Y. Niwa, Y. Maruyama, T. Matoba, T. Arisawa, Appl. Opt. 38 (1999) 4548.