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Generation of highly efficient self-pumped phase conjugation femtosecond pulse using photorefractive BaTiO3:Co crystal
1 September 2001
Optics Communications 196 (2001) 281±284
www.elsevier.com/locate/optcom
Generation of highly ecient self-pumped phase conjugation femtosecond pulse using photorefractive BaTiO3:Co crystal Go Urushibata, Yusuke Tamaki 1, Minoru Obara * Department of Electronics and Electrical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received 28 February 2001; accepted 11 June 2001
Abstract The generation of highly ecient self-pumped phase conjugate (SPPC) pulse of 180 fs, 810 nm Ti:sapphire laser at 76 MHz in a photorefractive BaTiO3 :Co crystal is demonstrated. Brewster prisms are inserted in front of the photorefractive crystal to make wide bandwidth of the femtosecond pulse spatially separated in order to remove an in¯uence of erasure caused by beam fanning and to avoid spectrum narrowing. A maximum re¯ectivity is improved from 1.5% without prisms to 6.6% with prisms. The spectral bandwidth of the phase conjugate pulse is also improved from 2.7 to 3.8 nm. The spectral bandwidth of the phase conjugate pulse with prisms measured is approaching 5.1 nm of the incident laser pulse, whereas the phase conjugate pulse without prisms is narrower. Since the refractive index grating that contributes to the generation of the SPPC pulse is only the transmission grating, therefore, the rise-time behavior of the SPPC pulse is smoother than that of continuous wave. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Photorefractive eect; Phase conjugation; Barium titanate; Femtosecond laser
The generation of the self-pumped phase conjugation (SPPC) with photorefractive crystal has been much studied since its ®rst discovery by Feinberg [1]. SPPC is easy to be used for versatile applications because it requires no external beam or optics. The pump beam is self-generated by beam fanning and internal re¯ection of the crystal surface. So far, continuous wave (CW) [2±4], nanosecond [5] and picosecond [6] laser is used for * Corresponding author. Tel.: +81-45-563-1141; fax: +81-45566-1529. E-mail addresses: [email protected] (Y. Tamaki), [email protected] (M. Obara). 1 Present address: Cyber Laser Inc., 2-45 Aomi, TIME24 Bldg. 4F N-5, Kotoku, Tokyo 135-8073, Japan.
the generation of SPPC. Recently, the generation of SPPC with femtosecond pulses in BaTiO3 crystal has been studied [7,8]. The SPPC with femtosecond pulses will primarily be used for the compensation of wavefront distortion of the output coming from the femtosecond laser oscillator. In addition, there would potentially be versatile applications in ultrafast photorefractive optics. However, re¯ectivity of the femtosecond phase conjugate (PC) pulse is rather low compared to CW, nanosecond or picosecond PC pulses. Yau et al. [7] demonstrated the SPPC with 126 fs pulse at 800 nm in a undoped BaTiO3 crystal. They measured a maximum re¯ectivity of 5%. Yang [8] reported SPPC with 90 fs pulse at 450 nm in a undoped BaTiO3 crystal, both placed in air and in
0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 7 2 - 4
282
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
Fig. 1. Schematic of new generation setup for the SPPC pulse. I: isolator, k=2: half-wave plate, P: prism, CL: convex lens.
immersion oil. He measured a maximum re¯ectivity of 1.7% in air, and 2.4% in immersion oil. On the other hand, with SPPC of CW, the values of more than 50% in re¯ection are achieved. This drawback is due to the broad bandwidth of the femtosecond pulse. Because of the broad spectral bandwidth, refractive index grating has a low visibility. In addition, because the SPPC can be considered as a diraction of a volume hologram, the Bragg selectivity has to be taken into account. When the grating of SPPC is the transmission grating, wavelength permission width 2 Dk1=2 can be written as [9] 2 Dk1=2
4jK nK cos h; p
1
where j is the coupling constant, K is the grating spacing, n is the refraction index, and h is the half angle between the two beams. In the case of the BaTiO3 crystal and the transmission grating, 2 Dk1=2 is 1 nm. Taking advantage of the strong Bragg selectivity of the volume holograms, photorefractive crystals are used for applications such as spectral narrowing of a diode laser [10] and highcapacity holographic data storage [11], but this selectivity is not suitable for femtosecond pulse [12]. Because the broad spectral bandwidth of the femtosecond pulse is beyond the Bragg selectivity, it reduces the spectral bandwidth of the diracted signal (phase conjugate wave).
In this letter, we will report on a new scheme for the generation of the SPPC with femtosecond laser. The main idea is to broaden the wavelength components in a direction orthogonal to the beam propagation by use of prisms. Brewster prisms are used here in order to suppress the loss of the beam power due to Fresnel re¯ection. The experimental setup is shown in Fig. 1. A mode-locked Ti:sapphire laser with a pulse duration of 180 fs at 810 nm is used. The spectrum has a full width at half maximum (FWHM) of 5.1 nm and the repetition rate is 76 MHz. The average power of the incident beam in front of the photorefractive crystal is 40 mW. The crystal is 0°-cut BaTiO3 :Co (4:2 mm 4:4 mm 5:1 mm, c-axis is along 5.1 mm). The incident beam is sent through the two BK7 Brewster prisms to make angular dispersion. After passing through the prisms the beam is focused by a convex lens (f 100 mm) and adjusted not to exceed the height of the crystal (4.4 mm). Lateral width of the beam just in front of the crystal surface is 1 mm. Two half-wave plates are inserted in order to enter the beam P-polarized before the prisms and extraordinary-polarized before the crystal. PC re¯ectivity and spectral width are measured both when the prisms are inserted and not inserted. Fig. 2 shows the re¯ectivity of the PC pulse in steady state. In both of the experiment, a maximum re¯ectivity is obtained with an incident angle around 70°. Two-photon absorption is not
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
Fig. 2. Re¯ectivity of the SPPC pulse vs. the incident angle. ( ) With prisms, ( ) without prisms.
observed in the crystal. Neither second harmonic generation is observed in the crystal. Although the rise time of the SPPC becomes longer when the prisms are inserted because of the broadening of the incident beam area, a maximum re¯ectivity measured is improved from 1.5% to 6.6% (4.3
283
times gain). Fig. 3 shows the spectral bandwidth. The FWHM of the spectrum is broadened by the insertion of the prisms from 2.7 to 3.8 nm. Although the spectral width is not completely the same as that of the incident pulse, the spectral bandwidth of the PC pulse with the prisms is much more similar to that of the incidental pulse than that of the PC pulse when the prisms are not inserted. A slight frequency shift indicates that the generation process of the SPPC is not due to the stimulated photorefractive backscattering (SPBS) but the four-wave mixing (FWM) process [10,13]. We also measured the rise-time behavior of the SPPC pulse compared to the case of CW signal. Fig. 4(a) shows the rise time of the SPPC pulse, and Fig. 4(b) shows the rise time when the signal beam is CW at the same wavelength as the femtosecond pulse. In Fig. 4(a), the SPPC rises to the steady state more smoothly rather than the case of CW (Fig. 4(b)). The result can be considered as follows: In the case of CW, the signal beam, fanning beam, and internal re¯ection beam intersect simultaneously, then the transmission grating, the re¯ection grating, and two 2K gratings are generated. After the competition of the gratings, one grating remains and the SPPC reaches its steady state. On the other hand, in the case of
Fig. 3. Spectral bandwidth of the incident and SPPC pulse. ( ) Incident pulse, ( ) SPPC pulse with prisms, () SPPC pulse without prisms.
284
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
1.5% to 6.6%. The FWHM of the spectrum is also increased from 2.7 to 3.8 nm. Although the idea in this paper is dierent, however, Ansari et al. have demonstrated that the reduced ®eld of view that is due to walk-o experienced while performing o-axis photorefractive holography with broadband, low-coherence radiation can be overcome by use of prisms to introduce an appropriate group delay across the writing beams [14].
References
Fig. 4. Rise time of the SPPC pulse and CW. (a) SPPC of CW beam, and (b) SPPC pulse.
femtosecond pulse, spatial length of the pulse in the crystal is on the order of 10 lm so that only the transmission grating contributes to the generation of the SPPC pulse. In summary, we have demonstrated the generation of highly ecient SPPC femtosecond pulse in a novel con®guration with BaTiO3 :Co crystal. In this con®guration, an in¯uence of erasure caused by beam fanning is decreased, and the spectrum narrowing of the PC pulse is avoided. Higher re¯ectivity is obtained by use of two prisms: A maximum re¯ectivity is increased by 4.3 times from
[1] J. Feinberg, Opt. Lett. 7 (1982) 486. [2] Y. Zhu, D. Wang, X. Wu, H. Yang, D. Zhang, Y. Jiang, P. Ye, Opt. Commun. 93 (1992) 333. [3] H.Y. Zhang, X.H. He, Y.H. Shih, Opt. Commun. 88 (1992) 424. [4] P.H. Beckwith, W.R. Christian, Opt. Lett. 14 (1989) 642. [5] L. Mager, C. Laquarnoy, P. Pauliat, M.H. Garrett, D. Rytz, G. Roosen, Opt. Lett. 19 (1994) 1508. [6] A.L. Smirl, K. Bohnert, G.C. Valley, R.A. Mullen, T.F. Boggess, J. Opt. Soc. Am. B 6 (1989) 606. [7] H.F. Yau, P.J. Wang, E.Y. Pan, J. Chen, Opt. Lett. 21 (1996) 1168. [8] C. Yang, J. Opt. Soc. Am. B 16 (1999) 871. [9] P. Yeh, Introduction to Photorefractive Nonlinear Optics, Wiley, New York, 1993. [10] A. Shiratori, M. Obara, Appl. Phys. Lett. 69 (1996) 1515. [11] J. Heanue, M. Bashaw, L. Hesselink, Opt. Quant. Electron. 25 (1993) 611. [12] Y. Ding, D.D. Nolte, Z. Zheng, A. Kanan, A.M. Weiner, G.A. Brost, J. Opt. Soc. Am. B 15 (1998) 2763. [13] R. Guo, Z. Ling, Opt. Commun. 92 (1992) 99. [14] Z. Ansari, Y. Gu, M. Tziraki, R. Jones, P.M.W. French, D.D. Nolte, M.R. Melloch, Opt. Lett. 26 (2001) 334.
Optics Communications 196 (2001) 281±284
www.elsevier.com/locate/optcom
Generation of highly ecient self-pumped phase conjugation femtosecond pulse using photorefractive BaTiO3:Co crystal Go Urushibata, Yusuke Tamaki 1, Minoru Obara * Department of Electronics and Electrical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received 28 February 2001; accepted 11 June 2001
Abstract The generation of highly ecient self-pumped phase conjugate (SPPC) pulse of 180 fs, 810 nm Ti:sapphire laser at 76 MHz in a photorefractive BaTiO3 :Co crystal is demonstrated. Brewster prisms are inserted in front of the photorefractive crystal to make wide bandwidth of the femtosecond pulse spatially separated in order to remove an in¯uence of erasure caused by beam fanning and to avoid spectrum narrowing. A maximum re¯ectivity is improved from 1.5% without prisms to 6.6% with prisms. The spectral bandwidth of the phase conjugate pulse is also improved from 2.7 to 3.8 nm. The spectral bandwidth of the phase conjugate pulse with prisms measured is approaching 5.1 nm of the incident laser pulse, whereas the phase conjugate pulse without prisms is narrower. Since the refractive index grating that contributes to the generation of the SPPC pulse is only the transmission grating, therefore, the rise-time behavior of the SPPC pulse is smoother than that of continuous wave. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Photorefractive eect; Phase conjugation; Barium titanate; Femtosecond laser
The generation of the self-pumped phase conjugation (SPPC) with photorefractive crystal has been much studied since its ®rst discovery by Feinberg [1]. SPPC is easy to be used for versatile applications because it requires no external beam or optics. The pump beam is self-generated by beam fanning and internal re¯ection of the crystal surface. So far, continuous wave (CW) [2±4], nanosecond [5] and picosecond [6] laser is used for * Corresponding author. Tel.: +81-45-563-1141; fax: +81-45566-1529. E-mail addresses: [email protected] (Y. Tamaki), [email protected] (M. Obara). 1 Present address: Cyber Laser Inc., 2-45 Aomi, TIME24 Bldg. 4F N-5, Kotoku, Tokyo 135-8073, Japan.
the generation of SPPC. Recently, the generation of SPPC with femtosecond pulses in BaTiO3 crystal has been studied [7,8]. The SPPC with femtosecond pulses will primarily be used for the compensation of wavefront distortion of the output coming from the femtosecond laser oscillator. In addition, there would potentially be versatile applications in ultrafast photorefractive optics. However, re¯ectivity of the femtosecond phase conjugate (PC) pulse is rather low compared to CW, nanosecond or picosecond PC pulses. Yau et al. [7] demonstrated the SPPC with 126 fs pulse at 800 nm in a undoped BaTiO3 crystal. They measured a maximum re¯ectivity of 5%. Yang [8] reported SPPC with 90 fs pulse at 450 nm in a undoped BaTiO3 crystal, both placed in air and in
0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 7 2 - 4
282
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
Fig. 1. Schematic of new generation setup for the SPPC pulse. I: isolator, k=2: half-wave plate, P: prism, CL: convex lens.
immersion oil. He measured a maximum re¯ectivity of 1.7% in air, and 2.4% in immersion oil. On the other hand, with SPPC of CW, the values of more than 50% in re¯ection are achieved. This drawback is due to the broad bandwidth of the femtosecond pulse. Because of the broad spectral bandwidth, refractive index grating has a low visibility. In addition, because the SPPC can be considered as a diraction of a volume hologram, the Bragg selectivity has to be taken into account. When the grating of SPPC is the transmission grating, wavelength permission width 2 Dk1=2 can be written as [9] 2 Dk1=2
4jK nK cos h; p
1
where j is the coupling constant, K is the grating spacing, n is the refraction index, and h is the half angle between the two beams. In the case of the BaTiO3 crystal and the transmission grating, 2 Dk1=2 is 1 nm. Taking advantage of the strong Bragg selectivity of the volume holograms, photorefractive crystals are used for applications such as spectral narrowing of a diode laser [10] and highcapacity holographic data storage [11], but this selectivity is not suitable for femtosecond pulse [12]. Because the broad spectral bandwidth of the femtosecond pulse is beyond the Bragg selectivity, it reduces the spectral bandwidth of the diracted signal (phase conjugate wave).
In this letter, we will report on a new scheme for the generation of the SPPC with femtosecond laser. The main idea is to broaden the wavelength components in a direction orthogonal to the beam propagation by use of prisms. Brewster prisms are used here in order to suppress the loss of the beam power due to Fresnel re¯ection. The experimental setup is shown in Fig. 1. A mode-locked Ti:sapphire laser with a pulse duration of 180 fs at 810 nm is used. The spectrum has a full width at half maximum (FWHM) of 5.1 nm and the repetition rate is 76 MHz. The average power of the incident beam in front of the photorefractive crystal is 40 mW. The crystal is 0°-cut BaTiO3 :Co (4:2 mm 4:4 mm 5:1 mm, c-axis is along 5.1 mm). The incident beam is sent through the two BK7 Brewster prisms to make angular dispersion. After passing through the prisms the beam is focused by a convex lens (f 100 mm) and adjusted not to exceed the height of the crystal (4.4 mm). Lateral width of the beam just in front of the crystal surface is 1 mm. Two half-wave plates are inserted in order to enter the beam P-polarized before the prisms and extraordinary-polarized before the crystal. PC re¯ectivity and spectral width are measured both when the prisms are inserted and not inserted. Fig. 2 shows the re¯ectivity of the PC pulse in steady state. In both of the experiment, a maximum re¯ectivity is obtained with an incident angle around 70°. Two-photon absorption is not
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
Fig. 2. Re¯ectivity of the SPPC pulse vs. the incident angle. ( ) With prisms, ( ) without prisms.
observed in the crystal. Neither second harmonic generation is observed in the crystal. Although the rise time of the SPPC becomes longer when the prisms are inserted because of the broadening of the incident beam area, a maximum re¯ectivity measured is improved from 1.5% to 6.6% (4.3
283
times gain). Fig. 3 shows the spectral bandwidth. The FWHM of the spectrum is broadened by the insertion of the prisms from 2.7 to 3.8 nm. Although the spectral width is not completely the same as that of the incident pulse, the spectral bandwidth of the PC pulse with the prisms is much more similar to that of the incidental pulse than that of the PC pulse when the prisms are not inserted. A slight frequency shift indicates that the generation process of the SPPC is not due to the stimulated photorefractive backscattering (SPBS) but the four-wave mixing (FWM) process [10,13]. We also measured the rise-time behavior of the SPPC pulse compared to the case of CW signal. Fig. 4(a) shows the rise time of the SPPC pulse, and Fig. 4(b) shows the rise time when the signal beam is CW at the same wavelength as the femtosecond pulse. In Fig. 4(a), the SPPC rises to the steady state more smoothly rather than the case of CW (Fig. 4(b)). The result can be considered as follows: In the case of CW, the signal beam, fanning beam, and internal re¯ection beam intersect simultaneously, then the transmission grating, the re¯ection grating, and two 2K gratings are generated. After the competition of the gratings, one grating remains and the SPPC reaches its steady state. On the other hand, in the case of
Fig. 3. Spectral bandwidth of the incident and SPPC pulse. ( ) Incident pulse, ( ) SPPC pulse with prisms, () SPPC pulse without prisms.
284
G. Urushibata et al. / Optics Communications 196 (2001) 281±284
1.5% to 6.6%. The FWHM of the spectrum is also increased from 2.7 to 3.8 nm. Although the idea in this paper is dierent, however, Ansari et al. have demonstrated that the reduced ®eld of view that is due to walk-o experienced while performing o-axis photorefractive holography with broadband, low-coherence radiation can be overcome by use of prisms to introduce an appropriate group delay across the writing beams [14].
References
Fig. 4. Rise time of the SPPC pulse and CW. (a) SPPC of CW beam, and (b) SPPC pulse.
femtosecond pulse, spatial length of the pulse in the crystal is on the order of 10 lm so that only the transmission grating contributes to the generation of the SPPC pulse. In summary, we have demonstrated the generation of highly ecient SPPC femtosecond pulse in a novel con®guration with BaTiO3 :Co crystal. In this con®guration, an in¯uence of erasure caused by beam fanning is decreased, and the spectrum narrowing of the PC pulse is avoided. Higher re¯ectivity is obtained by use of two prisms: A maximum re¯ectivity is increased by 4.3 times from
[1] J. Feinberg, Opt. Lett. 7 (1982) 486. [2] Y. Zhu, D. Wang, X. Wu, H. Yang, D. Zhang, Y. Jiang, P. Ye, Opt. Commun. 93 (1992) 333. [3] H.Y. Zhang, X.H. He, Y.H. Shih, Opt. Commun. 88 (1992) 424. [4] P.H. Beckwith, W.R. Christian, Opt. Lett. 14 (1989) 642. [5] L. Mager, C. Laquarnoy, P. Pauliat, M.H. Garrett, D. Rytz, G. Roosen, Opt. Lett. 19 (1994) 1508. [6] A.L. Smirl, K. Bohnert, G.C. Valley, R.A. Mullen, T.F. Boggess, J. Opt. Soc. Am. B 6 (1989) 606. [7] H.F. Yau, P.J. Wang, E.Y. Pan, J. Chen, Opt. Lett. 21 (1996) 1168. [8] C. Yang, J. Opt. Soc. Am. B 16 (1999) 871. [9] P. Yeh, Introduction to Photorefractive Nonlinear Optics, Wiley, New York, 1993. [10] A. Shiratori, M. Obara, Appl. Phys. Lett. 69 (1996) 1515. [11] J. Heanue, M. Bashaw, L. Hesselink, Opt. Quant. Electron. 25 (1993) 611. [12] Y. Ding, D.D. Nolte, Z. Zheng, A. Kanan, A.M. Weiner, G.A. Brost, J. Opt. Soc. Am. B 15 (1998) 2763. [13] R. Guo, Z. Ling, Opt. Commun. 92 (1992) 99. [14] Z. Ansari, Y. Gu, M. Tziraki, R. Jones, P.M.W. French, D.D. Nolte, M.R. Melloch, Opt. Lett. 26 (2001) 334.