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Integrated optical Bragg grating with electrically controllable spectral transfer function A.V. CHAMRAI, A.S. KOZLOV, I.V. ILICHEV, M.P. PETROV Laboratory of Quantum Electronics Ioffe Physico-Technical Institute 26 Polytekhnicheskaya, St. Petersburg, 194021 RUSSIA Abstract: - A novel versatile integrated optical device based on an electrically controlled Bragg grating in the lithium niobate waveguide has been designed and fabricated. Fast electrooptic control of the device spectral transfer function has been demonstrated. Key-Words: - Optical telecommunications, Optical Networks, Integrated optical circuits, Electrooptic devices, Gratings. Introduction Principle of operation It is well known that a Bragg grating works as a reflection (stop-band or notch) optical filter with very high wavelength selectivity. The grating of 5 mm in length can provide FWHM about 0.1 nm at 1550 nm. If a Bragg grating is formed in an electrooptical material, the central reflection wavelength can be tuned by applying an external electric field that changes the average refractive index (electrically controlled diffraction) [1]. However, since the electrooptical parameters of available materials (in particular LiNbO3) are very limited the wavelength range of tuning is rather narrow. Transmission, a.u. 80 60 40 20 0 1553.6 1553.8 1554 1554.2 Wavelength detuning, nm 1554.4 1553.6 1553.8 1554 1554.2 Wavelength detuning, nm 1554.4 a. 100 80 Transmission, a.u. We have developed a novel integrated optical device based on new original technique of the electrooptical control of the spectral transfer function of Bragg gratings in LiNbO3 single-mode channel waveguides. The device could provide new functional capacity of the optical components for wavelength control in WDM systems. Potentially a high wavelength selectivity (0.1 0.01 nm), fast electrooptic control (up to 20 GHz), relatively low controlling voltage together with the integrated optical implementation, compatibility with other components in modern optical networks, and possibility of mass production make this device very promising as a key building block of various optical systems for control of narrow-band spectral channels. For instance, it could be used for building wavelength selective electrically controlled optical attenuators for optical power equalizers, electrooptic modulators (allowing modulation of one specific wavelength channel without affecting the other channels), Add/Drop multiplexers, and other wavelength switches. 100 60 40 20 0 b. Fig. 1. The spectral transfer functions of the homogeneous Bragg grating (a) and the same grating with phase shift at the midpoint (b). The parameters of the grating are as follows: n0 = 2.2., n1 = 10-4, l = 6 mm, 0 = 1554 nm. We suggest to exploit a new and more flexible technique for the control of the spectral transfer function of Bragg gratings. The unique spectral properties of the gratings with phase-, spacing- and average refractive index discontinuities [2] are used. From the theory [3, 4] it follows that if the reflection Bragg grating gets a phase shift equal to in the midpoint of the grating the spectral transfer function is changed from stop-band to pass-band mode (Fig.1). Similar results can be obtained in the case of average refractive index discontinuity. For instance, If the average refractive index n0 of the homogeneous grating transforms into n0 + n/2 for the first half of the grating and n0 - n/2 for the second half (where n = /(2l) is the refractive index shift, l is the grating length, and is the Bragg wavelength) the spectral transfer function is also modified from reflection to transmission mode. The goal of our work was to show experimentally that such methods of the control of the spectral transfer function can be implemented in integrated optical devices. In the device presented in this report an average refractive index discontinuity in the Bragg grating was produced and electrically controlled by choosing a proper spatial distribution of the applied external electric field. In this case the electrooptical nature of reconfiguration provides very high speed of control, and simple fixed gratings in LiNbO3 waveguides that are very convenient for practical application can be used. Device fabrication At the first stage a channel single mode waveguide in the LiNbO3 substrate was fabricated by the standard technique of thermal indiffusion of titanium stripes [5]. The orientation of the waveguide (light propagation direction) was along the optical C axis of the LiNbO3 crystal. The fabricated waveguides had excellent optical quality and low losses (< 1dB). Then thermal indiffusion of a Cu layer was performed to dope the LiNbO3 substrate and provide photosensitivity for Bragg grating recording. At the next stage, Cu electrodes were deposited on the substrate surface by magnetron sputtering. The electrodes of specially designed configuration provided a phase discontinuity by application of the external electric field with a specified spatial distribution in the geometry of the transverse electrooptic effect. Two single-mode optical fibers were connected to the front and back edges of the waveguide and provided input and output of the optical signal. Then the device was packed in the protective coverage. The layout of the device is shown in Fig. 2. Output SMF Input SMF Electrodes Wires Bragg grating in optical waveguide Fig. 2. Device layout. Finally a Bragg grating has to be produced in the sample. Several techniques of the Bragg grating formation have been developed. One of the ways is a holographic recording. We used a conventional scheme for recording transmitted holograms in LiNbO3 crystals with active stabilisation. A Nd-YAG laser (532 nm) was used. The grating spacing was determined by the angle of incidence of the recording beams. An angle of 49.5 deg gives a grating spacing of about 349.8 nm, which corresponds to the Bragg wavelength of about 1554 nm for the reflection geometry of readout. The excellent Bragg grating with the diffraction efficiency up to 90 % have been recorded in the waveguide. Experimental results Fig. 3a shows the spectral transfer function of a simple homogeneous holographic Bragg grating in a LiNbO3 photorefractive waveguide. The light intensity transmitted through the waveguide versus wavelength was measured using the sweeping mode of operation of a tunable semiconductor laser (PRO 800). The diffraction efficiency higher than 90% and FWHM of about 0.1 nm have been achieved. The device allows simple tuning of the central wavelength of the Bragg grating transfer function without changing its shape by application of a spatially uniform electric field. The wavelength shift of about 0.1 nm has been obtained for the 75 kV/cm of applied electric field. However, fast electrooptical tuning even in this narrow spectral range could be very interesting for wavelength locking and laser stabilization. The selected electrode configuration allows formation of the average refractive index discontinuity, which is equivalent to the phase shift at the midpoint of the grating. Theoretical analysis shows that in this case the transmission at the central wavelength of the grating transfer function should be equal to 100 %. Our experiments proved the validity of the theory. Fig. 3b shows the device spectral characteristic when an external electric field of the same magnitude (75 kV/cm) but opposite polarity was applied to different halves of the grating. The maximum of transmittance is observed at the middle of the grating stop band. So the electrooptical switching from the stop band mode to the pass band mode has been experimentally demonstrated. This mode of operation is very interesting for building optical Add/Drop multiplexers, wavelength selective electrically controlled optical attenuators for optical power equalizers, and electrooptic modulators (allowing modulation of one specific wavelength channel without affecting the other channels). Transmission, % 100 80 60 40 E=75 E=0 20 1549.5 a. 1550 Wavelength, nm kV cm 1550.5 Transmission, % 100 80 E=75 kV cm 60 40 E=0 20 0 b. 1554,0 1554,5 Wavelength, nm Fig. 3. Experimental demonstration of the electric control of the transfer function of a Bragg grating in the LiNbO3 waveguide. a tuning of the central wavelength. b - switching from stop-band mode to pass-band mode. Summary A novel versatile integrated optical device based on an electrically controlled holographic Bragg grating in the lithium niobate waveguide has been designed and fabricated. New original technique for the control of spectral transfer function of Bragg grating has been used. A high wavelength selectivity (~0.1 nm) and fast electrooptic control of the device spectral transfer function has been experimentally demonstrated. This device is very promising as a key building block of wavelength selective electrically controlled optical attenuators for optical power equalisers, electrooptic modulators (allowing modulation of one specific wavelength channel without affecting the others channels), and Add/Drop multiplexers. References: [1]. V.M. Petrov, C. Denz, A.V. Shamray, M.P. Petrov, T. Tschudi, “Electrically controlled volume LiNbO3 holograms for wavelength demultiplexing system,” Opt. Materials 18, 191-194 (2001). [2]. G.P. Agrawal, S. Radic, “Phase-shift fiber Bragg gratings and their application for wavelength demultiplexing,” IEEE Photon. Technol. Lett. 6, 995997 (1994). [3]. V.M. Petrov, C. Karaboue, J. Petter, T. Tschudi, V.V. Bryksin, M.P. Petrov, Appl. Phys. B 76, 41-44 (2003). [4]. V.M. Petrov, S. Lichtenberg, J. Petter, T. Tschudi, A.V. Chamrai, M.P. Petrov, “A dynamic wavelength Bragg-filter with an on-line controllable transfer function,” in Advances in Photorefractive Materials, Effects and Devices, Vol. 87 of OSA TOPS, pp. 564570. [5]. J. Hukriede, D. Kip, E. Krätzig, “Investigation of titanium- and copper indiffused channel waveguides in lithium niobate and their application as holographic filters for infrared light,” J. Opt. A: Pure Appl. Opt. 2, 484-487 (2000).