Fibre Bragg grating based spectral encoder/ decoder for lightwave CDMA zyxwvuts A. zyxwvutsrqpo Grunnet-Jepsen, A.E. Johnson, E.S. Maniloff, T.W. Mossberg, M.J. Munroe and J.N. Sweetser An all-fibre technique for the coherent spectral encoding and decoding of optical pulses is proposed. The spectral coding of optical waveforms is achieved using pairs of oppositely chirped and suitably coded fibre gratings. The application potential to lightwave code-division-multiple-access (CDMA) communications is demonstrated. Lightwave code-division-multiple-access (CDMA) provides multi- access communications in optical networks by introducing chan- nel-specific code sequences that can be used for channel discrimination. Multiple users communicate over a common trans- port fibre by encoding optical bits with channel-specific codes. At the receivers, matched decoders distinguish the different channels using cross-correlationand thresholding operations. Of the numer- ous optical implementations of CDMA that have been proposed, variations have included encoding and decoding in the time and frequency domains and the use of coherent or incoherent signal processing [l zyxwvutsrqpo - 51. One of the most widely investigated coherent signal-processing techniques [ 1 - 31 employs the spectral encoding of short pulses using the free-space 4F grating-pair configuration shown schematically in Fig. la. The 4F device consists essentially of two lenses, two uniform diffraction gratings and a spatial filter. The lenses are separated from the spatial filter by their focal lengths, zyxwvutsrqponm F. The first grating-lens pair provides a mapping of the spectral content onto the position of the spatial fdter. The second lens-grating pair recombines the filtered spectral components into a single beam. The transmission properties of the spatial fdter determine the spectral coding function of the device. Code detec- tion is effected with a similar 4F grating-pair device at the receiver end that produces autocorrelation spikes on the receipt of cor- rectly coded signals. Despite the many promising features of this approach, application in the communications industry is hindered by the physical limitations of the 4F grating-pair devices. In prac- tice, due to spectral resolution limits, 4F grating-pair devices work best with subpicosecond input pulses and produce autocorrelation signatures of similar duration. Consequently, CDMA based on 4F grating pair devices requires nonlinear means for detection and thresholding of correlation signals, and is subject to severe trans- mission impairments due to the ultra-wide bandwidth involved. It follows that CDMA based on 4F grating-pair devices has poor power sensitivity and limited transport range. Furthermore, the use of large free-space bulk-optics [l, 21 is not compatible with the robust compact packaging requirements of the telecommunica- tions industry. It should be noted that interesting techniques have recently been suggested for implementing integrated optical devices offering functionality similar to that offered by the 4F grating spatial filter grating zyxwvutsrqp a chirped 1 FBG chirped B phasFegcGoded zyxwvutsr 3 8- -$, ,+ m- input bit expanded bit encoded bit zyxwvutsrqp b LE& Fig. 1 Schematic diagram of spectral encoder using free-space diffrac- tion gratings (DGs). and all-fibre construction based on pair of step- chirped fibre Bragg gratings (FBGs) a DGs b FBGs grating-pair devices [3]. In this Letter, we present an all-fibre spec- tral-encoding approach based on temporal wavelength dispersion and recombination using fibre Bragg gratings (FBGs). Fig. lb shows a schematic diagram of the proposed all-fibre spectral phase encoder. The device consists of a pair of step- chvped FBGs arranged in series and is based on the concept of spectral dispersion in the time domain as opposed to the spatial Fourier domain. By step-chirped gratings, we mean gratings com- posed of spatially adjacent subgratings each of constant but con- stantly incremented spatial period. When an input bit is incident on the first chirped grating, the wavelengths are dispersed in time and the reflected pulse is temporally expanded. When this expanded bit is reflected from a second FBG having an opposite dispersion slope, the wavelength components are resynchronised and the original pulse is reconstituted. However, if the second grating contains phase-shifts along its length, these phase-shifts are transferred to the reflected signal and the output pulse repre- sents a spectral-phase-encoded bit. Similarly, a spectral amplitude code can be impressed by varying the reflectivity of wavelength- selective grating subsections. To ensure that each subgrating controls the backward diffrac- tion of an isolated spectral band, the overall grating length should satisfy where N is the number of subgratings, n is the effective refractive index, & is the carrier wavelength in air, and zyx a is the optical bandwidth of the input bit. The contiguous subgratings have Bragg wavelengths that change by AA SA 2 - z N In the present demonstration, each grating consisted of N = 8 subgratings of length 2.4mm, arrayed contiguously for a total fibre grating length of 19.2mm. All subgratings are created in a single exposure through a patterned phase-mask. The subgratings in the first FBG have Bragg wavelengths that increase in OSnm steps from 1540.5nm (input signals see the 1540.5nm grating first). The subgratings of the second FBG are arranged in the reverse order and possess spatial phase shifts introduced during the grat- ing fabrication. Each subgrating has a reflectivity of 60%. Fig. 2a shows an autocorrelation of an incident -Ips pulse and an expanded bit measured after reflection from the first FBG. The expanded bit was measured using a fast photodiode (12ps impulse response) and a 5OGHz sampling oscilloscope. The temporal width corresponds well with the expected 192ps (T~,,~ = 2nL/c). Fig. 2b shows the measured temporal waveform after reflection from the second chirped and phase-coded FBG (shown together with the input pulse). 0.14, . , , . , I zyxwvutsrqponm 0.04, , , , , . tirne,ps time,ps a b E @ Fig. 2 Example of measured waveforms after first and second circulator of all-fibre spectral encoder shown together with -Ips input bit a Expanded bit of width -190 ps b Encoded bit of width -30 ps Fig. 3a shows schematically the construction of a lightwave ccde-division multiaccess communication system using the pro- posed spectral encoders where each encoder has a complementary matched decoder. In the present demonstration, the encoder under investigation contains gratings G1 and G2. The matched receiver contains gratings G3 and G4, which are in essence identical to those of the encoder but connected in reverse. In other words, grating pair G1 and G2 are connected to the circulator with fibre ends that are the opposite to those of G3 and G4 in order to per- form the time-reversed function of G1 and G2. After transmission of the encoded bit to the correctly matched decoder, the spectral 1096 ELECTRONICS LETTERS 24th June zyxw I999 Vol. 35 No. 13 Authorized licensed use limited to: Johns Hopkins University. Downloaded on August 31, 2009 at 17:32 from IEEE Xplore. Restrictions apply.