C.-C. Chang

A femtosecond code-division multiple-access communication system test bed

This paper reports comprehensive experimental results on a femtosecond code-division multiple-access (CDMA) communication system test bed operating over optical fiber in the 1.5 /spl mu/m communication band. Our test bed integrates together several novel subsystems, including low-loss fiber-pigtailed pulse shapers for encoding-decoding, use of dispersion equalizing fibers in dispersion compensated links for femtosecond pulse transmission and also in femtosecond chirped pulse amplification (CPA) erbium doped fiber amplifiers (EDFAs), and high-contrast nonlinear fiber-optic thresholders. The individual subsystems are described, and single-user system level experimental results demonstrating the ability to transmit spectrally encoded femtosecond pulses over a 2.5-km dispersion compensated fiber link followed by decoding and high contrast nonlinear thresholding are presented.

Dispersion-free fiber transmission for femtosecond pulses by use of a dispersion-compensating fiber and a programmable pulse shaper.

We demonstrate nearly distortionless 2.5-km fiber transmission of sub-500-fs pulses, using a combination of standard single-mode fiber, dispersion-compensating fiber, and a programmable pulse shaper for simultaneous quadratic and cubic dispersion compensation. The dispersion-compensating fiber corrects the bulk of the quadratic and the cubic phases for the single-mode fiber, and the fiber-pigtailed programmable pulse shaper exactly compensates the residual dispersion terms. Together these elements permit complete recompression of pulses, which first broaden by ~400 times in the single-mode fiber.

Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5-km fiber link

We report for the first time proof-of-concept transmission experiments for femtosecond pulse code division multiple access (CDMA) operating over kilometer lengths of optical fiber in the 1.5-/spl mu/m communication band. Our CDMA link consists of a femtosecond mode-locked Er-fiber laser and two chirped pulse fiber amplifiers, a pair of low-loss fiber-pigtailed pulse shapers for encoding and decoding, a 2.5-km dispersion-compensated transmission fiber, and an ultrafast nonlinear optical receiver. Our results demonstrate high fidelity transmission of spectrally encoded femtosecond pulses over 2.5 km of fiber, followed by the use of the nonlinear CDMA receiver to discriminate between correctly and incorrectly decoded signals with 20-dB contrast.

Fiber transmission for sub-500-fs pulses using a dispersion-compensating fiber

We report transmission of /spl sim/60-fs and /spl sim/245-fs pulses, respectively, over 42-m and 2.5-km fiber links which consist of standard single-mode fibers (SMF) concatenated with dispersion-compensating fibers (DCF). The experiments using very short pulses (/spl sim/60 fs) over a short fiber length (/spl sim/42 m) demonstrate the ability to achieve simultaneous dispersion and dispersion slope compensation using this technique. Femtosecond spectral interferometry measurements of this 42-m link show that its residual dispersion slope is approximately six times lower than that of the dispersion-shifted fiber. Finally, to demonstrate that the dispersion-limited propagation distance is proportional to the cube of the pulsewidth, we transmit /spl sim/245-fs pulses over a 2.5-km SMF-DCF link and achieve comparable pulse restoration as with the shorter fiber experiments.

Effects of self-phase modulation on sub-500 fs pulse transmission over dispersion compensated fiber links

The effects of nonlinearity on sub-500 fs pulse transmission over dispersion compensated fiber links using dispersion compensating fiber technique are investigated numerically and experimentally. The pulse broadening and recompression ratio of the 2.5-km transmission link is over 300. The postcompensated and precompensated links are compared when the input pulse energy ranges from 15 to 159 pJ. At high powers, self-phase modulation (SPM) degrades the pulse recompression process and provides an upper bound on the transmitted pulse energy. The SPM effect is stronger in the postcompensated link than in the precompensated link. A dramatic spectral narrowing effect was observed in the postcompensated link. Pulse energies up to tens of pJ, consistent with high quality communication, should be possible for a sub-500 fs pulse in such dispersion compensated links.

Pulse shaping of incoherent light by use of a liquid-crystal modulator array

We demonstrate the use of a femtosecond pulse-shaping apparatus for electronically programmable phase filtering of amplified spontaneous emission from an erbium-doped fiber amplifier. Pulse shaping applied to a broadband incoherent light (noise) input results in reshaped noise, with a specially tailored electric field correlation function. Our experiments clearly reveal that phase filtering can strongly affect the coherence properties of broadband, phase-incoherent light.

Broadband fiber dispersion compensation for sub-100-fs pulses with a compression ratio of 300

We report what is, to our knowledge, the first experimental demonstration of nearly dispersion-free transmission of sub-100-fs pulses over several tens of meters of fiber. 62-fs pulses are broadened initially and recompressed by a ratio of 300 over a 42-m concatenated fiber link consisting of standard single-mode and dispersioncompensating fibers. This dispersion-compensated fiber link is estimated to have a third-order dispersion ~6 times lower than that of dispersion-shifted fiber.

Ultrafast two-photon absorption optical thresholding of spectrally coded pulses

We report studies on two-photon absorption (TPA) GaAs p-i-n waveguide photodetectors as optical thresholders for proposed ultrashort pulse optical code-division multiple-access (CDMA) systems. For either chirped optical pulses or spectrally phase coded pseudonoise bursts, the TPA photocurrent response reveals a strong pulseshape dependence and shows good agreement with theoretical predictions and results from conventional SHG measurements. The performance limits of the TPA optical thresholders set by the encoded bandwidth in the spectral encoding-decoding process are also discussed based on numerical simulations. Our results show the feasibility of applying such devices as nonlinear intensity discriminators in ultrahigh-speed optical network applications.

Dispersion compensation for ultrashort pulse transmission using two-mode fiber equalizers

We numerically simulate ultrashort pulse propagation in dispersion compensated fiber links using two-mode equalizing fibers. By choosing a proper length ratio between the conventional single-mode fiber and the compensating two-mode fiber, both the dispersion and the dispersion slope can be eliminated simultaneously. As a result, dispersion compensated propagation of sub-picosecond pulses should be possible for distances in excess of 100 km. Our analysis shows that for sub-picosecond pulses, dispersion-limited propagation distances offered by this technique may significantly exceed those possible with soliton propagation. We also evaluate nonlinearity limits due to self-phase modulation.<<ETX>>

Nonlinear detection of spectrally coded ultrashort pulse by two-photon absorption GaAs waveguide photodetectors

Our results confirm the strong intensity and pulsewidth dependence of the photocurrent from two photon absorption (TPA) waveguide photodetectors for ultrashort pulses with strong phase and amplitude modulations. We can expect the application of similar devices as the ultrafast optical thresholders for CDMA and other ultrafast network applications.