P.E. Barnsley’s research while affiliated with British Telecom and other places

Advanced optoelectronic components based on semiconductor laser amplifier technology are showing enormous potential for a range of complex network functions. In this paper some current achievements in wavelength conversion, optical space and time switching and clock extraction are outlined. Extensions of the current technology offer a wide range of solutions to network problems.

Bit-error-ratio (BER) measurements have been demonstrated with all-optical clock recovery from a 3.2 Gbit/s NRZ sequence using two-contact semiconductor devices. A nonlinear optical amplifier generated the missing clock component in the NRZ modulation spectrum which then locked the output of a selfpulsating laser. BER measurements showed that the resulting clock signal was extremely pure and stable giving a 0.2 dB improvement in receiver sensitivity when compared to measurements using a clock from the transmitter. This type of optical clock recovery technique could have a major impact on future gigabit per second telecommunication systems.

It is shown theoretically that the widths of the hysteresis loops in absorptive bistable laser diodes are strongly dependent on the spontaneous emission coefficient. As a consequence, a correction is required to the method previously presented for determining the differential gain coefficient from experimental L-I plots. An analytical approximation for this correction is presented.

For the first time, all-optical clock extraction from 2.5 Gbit/s NRZ data is demonstrated using a two contact InGaAsP semiconductor selfpulsating laser diode (SP-LD) for what the authors believe to be the first time. The saturable absorber region of the device was doped with Zn ions to reduce the carrier lifetime such that strong selfpulsations at frequencies up to 4 GHz were obtained. Injection of a 10 mu W optical data signal at a wavelength approximately 15 nm lower than the lasing wavelength was sufficient to synchronise the selfpulsations to the incoming NRZ data stream. Similar effects were seen for RZ formated data. Such all-optical clock extraction techniques will find application in future multigigabit per second optical networks and for OEIC applications.

The authors demonstrate successful 5 Gb/s operation of an all-optical clock extraction circuit in a 20 Gb/s optical time division multiplexing OTDM system. An optical clock signal was obtained by synchronizing a self-pulsating 1.56 mu m laser diode to a 5 GB/s RZ data signal, demultiplexed from a 4*5 Gb/s OTDM transmission system. Synchronization was achieved for pseudo-random-bit sequences from 2/sup 7/-1 to 2/sup 31/-1. At 2/sup 7/-1 full system bit-error-ratio (BER) measurement on one of the 5 GB/s channels using the extracted optical clock for synchronization were made with no observed penalty over transmitter clock measurements. Such clock extraction techniques may be of great importance in a wide variety of telecommunications applications.< >

A wavelength converter based on a semiconductor nonlinear optical amplifier (NLOA) has been used to switch 155 MB/s data from 1.3 mu m into a 1.55 mu m WDM wavelength routed network. The data were converted to 1.56 mu m, one of the WDM system wavelengths, and transmitted over 63 km of step-index fiber. Bit-error-ratio measurements on the full system showed a 2 dB 10/sup -9/ receiver sensitivity penalty, demonstrating the usefulness of NLOAs as wavelength switches for future multiwavelength optical networks.< >

All-optical clock extraction from 5 Gb/s RZ data is demonstrated using a two-contact InGaAsP semiconductor self-pulsating laser diode. The saturable absorber region of the device was doped with zinc ions to reduce the carrier lifetime in this region such that strong self pulsations at frequencies up to 5.2 GHz were obtained. Injection of a 10- mu W optical data signal at a wavelength close to one of the Fabry-Perot wavelengths was sufficient to synchronize the self pulsations to the incoming data stream. The measured 3-dB RF electrical bandwidth of the synchronized optical clock signal was 10 Hz, approximately the same as that of the electrical transmitter clock. Initial bit-error-rate measurements for 2/sup 7/-1 5 Gb/s data result in a 0.5-dB receiver sensitivity penalty when using the optical clock signal compared to the transmitter clock.< >

Analytical expressions are obtained for the output power and bias current at switch on and switch off for absorptive bistable semiconductor laser diodes. Manipulation of these expressions yielded a simple analytical expression for the product of the carrier lifetime tau ₂ and the differential gain sigma ₂ in the absorber region in terms of the hysteresis width and the switch-off output power of the measured L-I characteristics. Experiments on a number of absorptive bistable devices indicated a mean value of 4.1*10¹⁶ cm² for sigma ₂.

A novel approach to clock extraction for non-return-to-zero (NRZ) formatted data is demonstrated using two contact semiconductor nonlinear optical amplifiers (NLOAs). The NLOA amplifies the transmitted data (fiber-fiber approximately 5 dB) and due to the absorptive nonlinearity, induced by bleaching a region of saturable absorption within the NLOA, an electrical clock component is generated at the absorber electrical contact. Narrow bandpass electrical filtering of the absorber contact electrical RF spectrum is all that is required in order to extract a stable clock signal. Bit-error-rate measurements at 155.6 Mb/s showed no receiver sensitivity penalty when using the NLOA extracted clock compared to the transmitter clock. Using quantum well material strong clock component generation has been achieved at modulation rates from < 100 Mb/s up to 5 Gb/s.< >