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Electrically controlled volume LiNbO3 holograms for wavelength demultiplexing systems
Optical Materials 18 (2001) 191±194
www.elsevier.com/locate/optmat
Electrically controlled volume LiNbO3 holograms for wavelength demultiplexing systems V.M. Petrov a,b,*, C. Denz a, A.V. Shamray b, M.P. Petrov b, T. Tschudi a a
Institute of Applied Physics, Darmstadt University of Technology, Hochschulstrasse 6, 64293 Darmstadt, Germany b A.F. Ioe Physical Technical Institute, Politechnicheskaya st., 26, 194021 St.-Petersburg, Russia
Abstract We demonstrate the equivalence of the electric ®eld and spectral multiplexing (SM) technique for holographic recording in photorefractive materials. The proposed technique of electric ®eld multiplexing (EFM) provides a spectral selectivity of recorded holographic ®lters up to a few picometers. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Spectral ®lters; Volume holograms
1. Introduction The electrically controlled volume LiNbO3 holograms are intensively investigated now because they can be used as electrically tunable ®lters. A set of dierent holograms can be recorded in a single photorefractive crystal at dierent external electric ®elds. To switch-on the proper hologram one has only to apply the proper electric ®eld. This is the technique of electric ®eld multiplexing (EFM) [1±5]. In the previous papers the optimal con®guration providing the maximal diraction eciency at a minimal controlling electric ®eld [6], the crosstalks between the electrically controlled holograms [4,7], and the phenomenon of electric ®eld shift during recording [8] were reported. In this paper we pay attention on the property of electrically controlled volume holograms as a spectral ®lter.
*
Corresponding author. Fax: +49-06151-163122. E-mail address: [email protected] (V.M. Petrov).
A volume hologram recorded by two plane waves can be considered as a spectral ®lter with the selectivity of [4,5,9] Dk Dn K ; k n T
1
where Dk and Dn are the wavelength and refractive index variations, K k=2n sin H is the grating spacing, T is the hologram thickness, n is the refractive index of the material. According to Eq. (1), one can use the method of EFM instead of the method of spectral multiplexing (SM) for creating of the spectral ®lters. 2. Experimental results 2.1. Holographic recording and restoring using a tunable semiconductor laser The experimental setup is shown in Fig. 1(a). The holograms were recorded and restored using a tunable semiconductor laser (1), k 783:066± 783:208 pm, Pout 54 mW. The strongly doped
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 1 6 5 - 3
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V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
Fig. 1. (a) Experimental setup. 1 ± laser (tunable semiconductor, or Nd±YAG CW, second harmonic), 2 ± beam forming system, 3 ± beam splitter, 4 ± mirrors, 5 ± shutter, 6 ± crystal, 7 ± photodiode. (b) Orientation of the crystal: q; ri are the recording beams, a is the angle between the optical axis ~ C and the direction of the light propagation, in our experiments a 30°; ~ P is the wave polarization, ~ E is the external electric ®eld, T is the crystal thickness. (c) The double-thickness sample. (d) The triple-thickness sample.
Fe2 sample of LiNbO3 crystal (6) and a focused laser beam (beam diameter was approximately 0.5 mm) were used. After passing through the optical collimating system (2), the light beam was splitted into two arms ri and q. The Bragg angle was approximately 88°. The crystal orientation is shown in Fig. 1(b). The sample size T along the light propagation was 9.25 mm. A pair of electrodes was deposited on the right and left lateral surfaces, the interelectrode separation being 2.0 mm. After hologram recording, beam ri was blocked by a shutter (5), and the reconstructed beam r was detected by a photodiode (7). The dependences of the diraction eciency of the reconstructed hologram on the applied electric ®eld E and on the wavelength k were measured. During the experiment, two holograms were recorded at one wavelength, but at two dierent values of the external electric ®eld: k 783:150 pm, E1 0, E2 3:8 kV/cm. (Such an electric ®eld separation between the holograms is equivalent to the doubled electric ®eld selectivity.) Fig. 2(a) shows the diraction eciency of the two recorded holograms as a function of the external electric ®eld. Fig. 2(b) shows the diraction eciency of the same recorded holograms as a function of k. It
is clearly seen, that two holograms recorded at two dierent external electric ®elds and on the equivalent recording wavelength have two maxima of diraction eciency at k 783:145 2 pm and at k 783:105 pm 2 during reconstruction. The spectral separation between the two maxima is 40 pm and the electric ®eld separation is 3.8 kV/cm. We now can conclude that these holograms, were recorded at two dierent electric ®elds and at one wavelength, are providing a high spectral selective properties and can therefore be considered as spectral ®lters. Moreover, as far as these holograms can be switched on (or o) by applying the proper electric ®eld, these holograms can be considered as electrically tunable spectral ®lters. 2.2. Holographic recording and restoring using a Nd±YAG CW laser In this part of work the same geometry of the experimental setup (except the laser and the crystal) was used. The second harmonic of a Nd±YAG laser (k 532 nm) was used for the recording. The concentration of Fe2 was less than 0.01 mol%, the sample thickness T was 8 mm.
V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
193
Fig. 2. (a) Diraction eciency of two recorded holograms versus the external electric ®eld. The holograms were recorded at the next conditions: E1;ext 0, E2;ext 3:8 kV/cm, krec 783:150 pm. The dependence was measured at the same wavelength. (b) Diraction eciency of the same holograms versus the wavelength. The dependence was measured at Eext 0.
Fig. 3. (a) Diraction eciency of one recorded hologram versus the external electric ®eld (measured data) and versus the wavelength (calculated scale) (squares: crystal thickness 8 mm, circles: double-thickness sample, triangles: triple-thickness sample). (b) Diraction eciency of four recorded holograms. The double-thickness sample (squares: experimental data, curves only for eye's help).
From the data plotted on Fig. 3(a) one can estimate the electric ®eld selectivity (EFS) of the hologram recorded in the samples with the dierent thicknesses T: 8, 16, and 24 mm. Using the method described in [7] and the data about EFS one can calculate the spectral selectivity (SS) Dk of one hologram. It is clearly seen what double- and triple-thickness samples provides two- and threetimes higher values of the spectral selectivity. To achieve the most ecient multiplexing the holograms have to be recorded at Rayleigh's criterion that means in our case the measured values of EFS. Fig. 3(b) shows the example of four electrically multiplexed holograms. The holograms were recorded at the next conditions: two ®rst holograms were recorded at Rayleigh' criterion, the third hologram was recorded with the separation on the double Rayleigh' criterion, and the fourth hologram was recorded again at the Ray-
leigh' criterion. Potentially in this gap of electric ®elds ®ve holograms could be recorded, but the position for the central hologram which could be placed between the two pairs of holograms was empty. In the region of value 2 kV/cm one can see the small maximum that can be considered as a cross talk between recorded holograms. Because these holograms were recorded in the doublethickness crystal (T 16 mm), they can be considered as the electrically tunable ®lters with the spectral selectivity of 4 pm.
Acknowledgements Financial support of the Alexander von Humboldt Foundation (Grant IV RUS 1063840) is gratefully acknowledged.
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V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
References [1] M.P. Petrov, S.I. Stepanov, A.A. Kamshilin, Ferroelectrics 21 (1978) 631. [2] M.P. Petrov, S.I. Stepanov, A.A. Kamshilin, Opt. Commun. 29 (1979) 44. [3] A. Kewitsch, M. Segev, A. Yariv, R.R. Neurgaonkar, Opt. Lett. 18 (1993) 534. [4] M.P. Petrov, A.V. Shamray, V.M. Petrov, Opt. Memory Neural Networks 7 (1998) 19. [5] M.P. Petrov, A.V. Shamray, V.M. Petrov, J.S. Mondragon, Opt. Commun. 153 (1998) 305.
[6] A.V. Shamray, M.P. Petrov, V.M. Petrov, in: OSA TOPS, vol. 27, Advances in Photorefractive Materials, Eects and Devices, Elsinore, Denmark, 1999, p. 515. [7] V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov, T. Tschudi, Appl. Phys. B 71 (2000) 43. [8] V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov, T. Tschudi, Appl. Phys. B 72 (2001) 701. [9] J.V. Alvarez-Bravo, R. Muller, L. Arizmendi, Europhys. Lett. 31 (1995) 443.
www.elsevier.com/locate/optmat
Electrically controlled volume LiNbO3 holograms for wavelength demultiplexing systems V.M. Petrov a,b,*, C. Denz a, A.V. Shamray b, M.P. Petrov b, T. Tschudi a a
Institute of Applied Physics, Darmstadt University of Technology, Hochschulstrasse 6, 64293 Darmstadt, Germany b A.F. Ioe Physical Technical Institute, Politechnicheskaya st., 26, 194021 St.-Petersburg, Russia
Abstract We demonstrate the equivalence of the electric ®eld and spectral multiplexing (SM) technique for holographic recording in photorefractive materials. The proposed technique of electric ®eld multiplexing (EFM) provides a spectral selectivity of recorded holographic ®lters up to a few picometers. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Spectral ®lters; Volume holograms
1. Introduction The electrically controlled volume LiNbO3 holograms are intensively investigated now because they can be used as electrically tunable ®lters. A set of dierent holograms can be recorded in a single photorefractive crystal at dierent external electric ®elds. To switch-on the proper hologram one has only to apply the proper electric ®eld. This is the technique of electric ®eld multiplexing (EFM) [1±5]. In the previous papers the optimal con®guration providing the maximal diraction eciency at a minimal controlling electric ®eld [6], the crosstalks between the electrically controlled holograms [4,7], and the phenomenon of electric ®eld shift during recording [8] were reported. In this paper we pay attention on the property of electrically controlled volume holograms as a spectral ®lter.
*
Corresponding author. Fax: +49-06151-163122. E-mail address: [email protected] (V.M. Petrov).
A volume hologram recorded by two plane waves can be considered as a spectral ®lter with the selectivity of [4,5,9] Dk Dn K ; k n T
1
where Dk and Dn are the wavelength and refractive index variations, K k=2n sin H is the grating spacing, T is the hologram thickness, n is the refractive index of the material. According to Eq. (1), one can use the method of EFM instead of the method of spectral multiplexing (SM) for creating of the spectral ®lters. 2. Experimental results 2.1. Holographic recording and restoring using a tunable semiconductor laser The experimental setup is shown in Fig. 1(a). The holograms were recorded and restored using a tunable semiconductor laser (1), k 783:066± 783:208 pm, Pout 54 mW. The strongly doped
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 1 ) 0 0 1 6 5 - 3
192
V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
Fig. 1. (a) Experimental setup. 1 ± laser (tunable semiconductor, or Nd±YAG CW, second harmonic), 2 ± beam forming system, 3 ± beam splitter, 4 ± mirrors, 5 ± shutter, 6 ± crystal, 7 ± photodiode. (b) Orientation of the crystal: q; ri are the recording beams, a is the angle between the optical axis ~ C and the direction of the light propagation, in our experiments a 30°; ~ P is the wave polarization, ~ E is the external electric ®eld, T is the crystal thickness. (c) The double-thickness sample. (d) The triple-thickness sample.
Fe2 sample of LiNbO3 crystal (6) and a focused laser beam (beam diameter was approximately 0.5 mm) were used. After passing through the optical collimating system (2), the light beam was splitted into two arms ri and q. The Bragg angle was approximately 88°. The crystal orientation is shown in Fig. 1(b). The sample size T along the light propagation was 9.25 mm. A pair of electrodes was deposited on the right and left lateral surfaces, the interelectrode separation being 2.0 mm. After hologram recording, beam ri was blocked by a shutter (5), and the reconstructed beam r was detected by a photodiode (7). The dependences of the diraction eciency of the reconstructed hologram on the applied electric ®eld E and on the wavelength k were measured. During the experiment, two holograms were recorded at one wavelength, but at two dierent values of the external electric ®eld: k 783:150 pm, E1 0, E2 3:8 kV/cm. (Such an electric ®eld separation between the holograms is equivalent to the doubled electric ®eld selectivity.) Fig. 2(a) shows the diraction eciency of the two recorded holograms as a function of the external electric ®eld. Fig. 2(b) shows the diraction eciency of the same recorded holograms as a function of k. It
is clearly seen, that two holograms recorded at two dierent external electric ®elds and on the equivalent recording wavelength have two maxima of diraction eciency at k 783:145 2 pm and at k 783:105 pm 2 during reconstruction. The spectral separation between the two maxima is 40 pm and the electric ®eld separation is 3.8 kV/cm. We now can conclude that these holograms, were recorded at two dierent electric ®elds and at one wavelength, are providing a high spectral selective properties and can therefore be considered as spectral ®lters. Moreover, as far as these holograms can be switched on (or o) by applying the proper electric ®eld, these holograms can be considered as electrically tunable spectral ®lters. 2.2. Holographic recording and restoring using a Nd±YAG CW laser In this part of work the same geometry of the experimental setup (except the laser and the crystal) was used. The second harmonic of a Nd±YAG laser (k 532 nm) was used for the recording. The concentration of Fe2 was less than 0.01 mol%, the sample thickness T was 8 mm.
V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
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Fig. 2. (a) Diraction eciency of two recorded holograms versus the external electric ®eld. The holograms were recorded at the next conditions: E1;ext 0, E2;ext 3:8 kV/cm, krec 783:150 pm. The dependence was measured at the same wavelength. (b) Diraction eciency of the same holograms versus the wavelength. The dependence was measured at Eext 0.
Fig. 3. (a) Diraction eciency of one recorded hologram versus the external electric ®eld (measured data) and versus the wavelength (calculated scale) (squares: crystal thickness 8 mm, circles: double-thickness sample, triangles: triple-thickness sample). (b) Diraction eciency of four recorded holograms. The double-thickness sample (squares: experimental data, curves only for eye's help).
From the data plotted on Fig. 3(a) one can estimate the electric ®eld selectivity (EFS) of the hologram recorded in the samples with the dierent thicknesses T: 8, 16, and 24 mm. Using the method described in [7] and the data about EFS one can calculate the spectral selectivity (SS) Dk of one hologram. It is clearly seen what double- and triple-thickness samples provides two- and threetimes higher values of the spectral selectivity. To achieve the most ecient multiplexing the holograms have to be recorded at Rayleigh's criterion that means in our case the measured values of EFS. Fig. 3(b) shows the example of four electrically multiplexed holograms. The holograms were recorded at the next conditions: two ®rst holograms were recorded at Rayleigh' criterion, the third hologram was recorded with the separation on the double Rayleigh' criterion, and the fourth hologram was recorded again at the Ray-
leigh' criterion. Potentially in this gap of electric ®elds ®ve holograms could be recorded, but the position for the central hologram which could be placed between the two pairs of holograms was empty. In the region of value 2 kV/cm one can see the small maximum that can be considered as a cross talk between recorded holograms. Because these holograms were recorded in the doublethickness crystal (T 16 mm), they can be considered as the electrically tunable ®lters with the spectral selectivity of 4 pm.
Acknowledgements Financial support of the Alexander von Humboldt Foundation (Grant IV RUS 1063840) is gratefully acknowledged.
194
V.M. Petrov et al. / Optical Materials 18 (2001) 191±194
References [1] M.P. Petrov, S.I. Stepanov, A.A. Kamshilin, Ferroelectrics 21 (1978) 631. [2] M.P. Petrov, S.I. Stepanov, A.A. Kamshilin, Opt. Commun. 29 (1979) 44. [3] A. Kewitsch, M. Segev, A. Yariv, R.R. Neurgaonkar, Opt. Lett. 18 (1993) 534. [4] M.P. Petrov, A.V. Shamray, V.M. Petrov, Opt. Memory Neural Networks 7 (1998) 19. [5] M.P. Petrov, A.V. Shamray, V.M. Petrov, J.S. Mondragon, Opt. Commun. 153 (1998) 305.
[6] A.V. Shamray, M.P. Petrov, V.M. Petrov, in: OSA TOPS, vol. 27, Advances in Photorefractive Materials, Eects and Devices, Elsinore, Denmark, 1999, p. 515. [7] V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov, T. Tschudi, Appl. Phys. B 71 (2000) 43. [8] V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov, T. Tschudi, Appl. Phys. B 72 (2001) 701. [9] J.V. Alvarez-Bravo, R. Muller, L. Arizmendi, Europhys. Lett. 31 (1995) 443.