P.C. Teh

A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings

We report a range of elementary optical coding and decoding experiments employing superstructured fiber Bragg grating (SSFBG) components: first, we perform a comparative study of the relative merits of bipolar and unipolar coding: decoding schemes and show that the SSFBG approach allows high-quality unipolar and bipolar coding. A performance close to that-theoretically predicted for seven-chip, 160-Gchip/s M-sequence codes is obtained. Second, we report the fabrication and performance of 63-chip, 160-Gchip/s, bipolar Gold sequence grating pairs. These codes are at least eight times longer than those generated by any other scheme based on fiber grating technology so far reported. Last, we describe a range of transmission system experiments for both the seven- and 63-bit bipolar grating pairs. Error-free performance is obtained over transmission distances of /spl sim/25 km of standard fiber. In addition, we have demonstrated error-free performance under multiuser operation (two simultaneous users). Our results highlight the precision and flexibility of our particular grating writing process and show that SSFBG technology represents a promising technology not just for optical code division multiple access (OCDMA) but also for an extended range of other pulse-shaping optical processing applications.

Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s quarternary phase coding gratings

In this letter, we report the fabrication and application of 255-chip 320 Gchip/s quaternary phase superstructure fiber Bragg gratings (SSFBGs) for optical code generation and recognition in a four-channel wavelength-division-multiplexing (WDM)/optical code-division-multiplexing (OCDM) experiment. Individual users of the system operate with different coding schemes, repetition rates, and wavelengths. Our experiments show that a single SSFBG can be used to perform simultaneous optical decoding and wavelength channel selection.

Raman effects in a highly nonlinear holey fiber: amplification and modulation

We experimentally demonstrate that a short length of highly nonlinear holey fiber (HF) can be used for strong L(+) -band (1610-1640-nm) Raman amplification and ultrafast signal modulation. We use a pure silica HF with an effective area of just 2.85mum(2) at 1550 nm, which yields an effective nonlinearity ~15 times higher than in conventional silica dispersion-shifted fiber. Using a 75-m length of this fiber, we obtained internal Raman gains of more than 42 dB and a noise figure of ~6 dB under a forward single-pump scheme, and the Raman gain coefficient was experimentally estimated to be 7.6 chi 10(-14)m/W . Also, an 11-dB signal extinction ratio in a Raman-induced all-optical modulation experiment was achieved with the same fiber.

A holey fiber-based nonlinear thresholding device for optical CDMA receiver performance enhancement

We demonstrate the use of an optical thresholder based on a short length of holey fiber to achieve enhanced code recognition quality in a 255-chip 320-Gchip/s superstructured fiber Bragg grating-based optical code-division multiple access code:decode system. The nonlinear thresholder is based on bandpass filtering of spectrally broadened components generated by self-phase modulation and assisted by the Raman effect in an 8.7 m length of highly nonlinear holey fiber. Error free penalty free system performance is obtained with complete recovery of the original input pulse shape.

Phase encoding and decoding of short pulses at 10 Gb/s using superstructured fiber Bragg gratings

A 7-chip, 160-Gchip/s phase-shift keyed code is assigned to short pulses after reflection from a superstructured fiber Bragg grating. The code is then recognized by a second grating, which is nominally identical to the encoder grating. Such an encoding/decoding action is required in code-division multiple-access systems and packet-switched networks.

A grating-based OCDMA coding-decoding system incorporating a nonlinear optical loop mirror for improved code recognition and noise reduction

We demonstrate an elementary grating-based optical code division multiple access (OCDMA) code generation and recognition system incorporating a nonlinear optical loop mirror (NOLM) within the receiver. We show that the NOLM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code-recognition pulse. The pedestal rejection allows for an improved code recognition signal-to-noise ratio (SNR) relative to simple matched filtering alone, and reduced intra- and interchannel interference noise due to code overlap. The system benefits of using the NOLM are experimentally demonstrated under both single- and multiuser operation within a variety of seven- and 63-chip 160-Gchip/s code generation, recognition, and transmission experiments based on the use of bipolar superstructure fiber Bragg grating (SSFBG) coding-decoding pairs. Incorporation of the NOLM is shown to allow error-free penalty-free operation at data rates as high as 2.5 Gb/s under single-user operation, and to provide error-free performance with reduced power penalty in two-user experiments. The narrowed pulse recognition signature offers major advantages in terms of the further all-optical processing of decoded signals, such as code regeneration and recoding.

Reconfigurable multilevel phase-shift keying encoder-decoder for all-optical networks

We demonstrate the application of a uniform Bragg grating as a dynamically reconfigurable phase encoder-decoder for optical systems. Precise discrete phase modulation between chips is obtained simply by heating segments along the grating with fine resistive wires. Its reliability to generate and recognize various phase code sequences is demonstrated in a 16-chip 20-Gchip/s quaternary phase-shift keying coherent optical code-division multiple access experiment. The bit-error-rate response is also included to highlight its performance.

Reduction of interchannel interference noise in a two-channel grating-based OCDMA system using a nonlinear optical loop mirror

We show that a nonlinear optical switch can be used to suppress the interchannel noise generated under multiuser operation within a coherent, direct-sequence optical code-division multiple-access (OCDMA) system. By incorporating a simple nonlinear optical loop mirror (NOLM) within the receiver, we demonstrate a 3.6-dB power penalty reduction in a two-channel 1.25-Gb/s 64-chip 160-Gchip/s grating-based direct-sequence OCDMA system. Even greater improvements in system performance were obtained at a data rate of 2.5 Gb/s, where the noise due to the overlap of adjacent decoded data bits also needs to be suppressed. In both instances, the system performance under two-channel operation with nonlinear filtering was shown to be comparable to that achieved under single-channel operation using the conventional matched-filter approach.

Demonstration of a 64-chip OCDMA system using superstructured fiber gratings and time-gating detection

We investigate the benefits of using time-gating detection in an optical code-division multiple access (OCDMA) system that comprises bipolar 64-chip long superstructure fiber Bragg grating encoders and decoders. Transmission of the codes is demonstrated, and it is shown that correlation combined with time-gating detection can provide some resilience to the distorting effects of dispersion, as well as the multiple access interference.

All-optical modulation and demultiplexing systems with significant timing jitter tolerance through incorporation of pulse-shaping fiber Bragg gratings

In this letter, we demonstrate the use of a superstructured fiber Bragg grating to preshape optical pulses to obtain optimal operation of nonlinear all-optical switches. Specifically, we demonstrate the conversion of 2.5-ps soliton pulses into 20-ps rectangular pulses at the input to both fiber and semiconductor optical amplifier-based switches, and show that rectangular switching windows can be achieved thereby providing a 5-10-fold reduction in timing jitter sensitivity. Error free penalty free optical time-division-multiplexing switching was readily achieved over a /spl plusmn/7-ps timing mismatch range for the square pulse driven fiber nonlinear optical loop mirror switch versus a /spl plusmn/1-ps range for the switch driven directly with 2.5-ps laser pulses.