Continuously-Tunable Dispersionless 44-ns All Optical Delay Element Using a Two-Pump PPLN, DCF, and a Dispersion Compensator Y. Wang (1), C. Yu(1), L. Yan(1), A. E. Willner(1), R. Roussev(2), C. Langrock(2), M. M. Fejer(2) 1.: Dept. of Electrical Engineering, University of Southern California, Los Angeles, CA 90089-2565 Tel: 213-740-1488, Fax: 213-740-8729, E-mail: yawang@usc.edu 2 : Dept. of Physics, Stanford University Abstract We demonstrate an optically controlled, continuously-tunable dispersionless optical delay element using a two-pump PPLN waveguide, dispersion compensated fiber and a dispersion compensator. A Continuous optical delay up to 44-ns is shown for 10-Gb/s NRZ applications. Introduction Optical delay lines have many potential uses in optical systems, including: (i) buffers for efficient contention resolution in a packet-switched network, (ii) synchronization of multiple bit streams through a high-speed optical switch, and (iii) various optical signal processing techniques for equalization. Such optical delays should optimally be continuously tunable over a wide range of delay times. For example, delays that can vary over a few tens of nanoseconds (i.e., the length of a 10-Gbit/s ATM data packet) have utility as packet buffers. Discretely tunable optical delay lines have been demonstrated by switching pulses out of recirculating loops [1] or different lengths of fiber [2]. Alternatively, continuously tunable delays have been demonstrated: (i) using fiber parametric amplifiers and a dispersive fiber for a 1.2-ns delay [3] (ii) using slow light techniques for a 1.5-ns delay at a bit-rate of 20 Gb/s [4], (iii) using resonant optical filters [5]. Moreover, many techniques tend to introduce some chirp or dispersion on the optical data stream as it passes through the network element, something that should probably be avoided for optimal system performance. In this paper, we demonstrate a continuously-tunable dispersionless 44-ns optical delay element using a two-pump periodically-poled lithium-niobate waveguide (PPLN), dispersion compensating fiber (DCF), and a fiber-Bragg-grating (FBG)-based dispersion compensator. The PPLN waveguide is used as a wideband rapidly tunable wavelength converter [6], and is used as the delay selection element. The large dispersion value of the DCF results in a time delay as a function of input wavelength. While intra-band dispersion from the DCF can result in signal degradation, the FBG serves as a compensator before exiting the module. A continuous optical delay up to 44 ns is demonstrated on a 10-Gb/s NRZ system. The wavelength of the output signal is the same as the input signal and this technique is not limited by the speed or modulation format of the signal. Concept and experimental setup Fig. 1(a) explains the concept of this technique. Dispersion will generate a wavelength dependent delay along with the intra channel dispersion. Assume we have an input signal around 1546.7 nm and a wide band wavelength converter converts the input signal to a calculated wavelength according to the desired delay. With 1900 ps/nm dispersion and 25 nm tuning bandwidth (limited by the EDFA gain bandwidth), we achieve an approximate 44 ns tuning range. Wavelength Converter 1 Wavelength Converter 2 Signal in λ in λ in Dispersion Compensator Relative Delay 1540 nm 1565 nm Slope = 1900 ps/nm (generated by DCF) 25 nm λ C Converted signal Dispersion Module λ c Signal out λ c λ in 45 ns Wavelength Converter 1 Wavelength Converter 2 Signal in λ in λ in Dispersion Compensator Relative Delay 1540 nm 1565 nm Slope = 1900 ps/nm (generated by DCF) 25 nm λ C Converted signal Dispersion Module λ c Signal out λ c λ in 45 ns Fig. 1 (a) Concept Signal in PPLN Calculated wavelength 1900 ps/nm LD 2 Fixed Dispersion Compensation High power EDFA LD 1 High power EDFA Signal Out PPLN Signal in PPLN Calculated wavelength 1900 ps/nm LD 2 Fixed Dispersion Compensation High power EDFA LD 1 High power EDFA Signal Out PPLN Fig. 1(b) Experimental setup Our experimental setup is shown in Fig. 1(b). The input signal is a 1546.7 nm 10-Gb/s NRZ signal and is used as the first pump of the 2-pump-PPLN waveguide. LD1 is used as the second pump which should be located equidistant to the center wavelength of the PPLN waveguide. LD2 is used as the dummy/tuning signal which is also equidistant to the center wavelength with respect to the desired output. The output signal wavelength is set by tuning the dummy signal (LD2) wavelength. These three are coupled together, amplified and sent into the PPLN waveguide with center wavelength 1551.3nm. The converted signal is then filtered out and passed through a spool of DCF with dispersion around 1900 ps/nm. After that, the delayed, converted signal is