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Ph.D. Thesis - Michael Vaa - November 1997
Optical time division multiplexing for high capacity photonic networks.
Ph.D. thesis, Department of Electromagnetic Systems, Technical University of Denmark, Kgs. Lyngby, Denmark.
17 September 1997.
Supervisors: Kristian Stubkjr, Benny Mikkelsen, Kaj B. Jakobsen.

Michael Vaa

Photonic technologies for applications in high capacity optical time division multiplexed systems and networks are addressed, including picosecond optical pulse generation, optical signal processing for switching and demultiplexirig applications, dispersion accommodation and preamplified high bit-rate receivers. The pulse generation techniques comprise mode locked external cavity semiconductor lasers, gain switched DFB lasers combined with fibre optic linear pulse compression and mode locked erbium doped fibre ring lasers. Pulse widths of the modelocked semiconductor source, operating at 5 GHz repetition rate, ranges from 20-25 ps, limited by the optical modulation bandwidth of the laser. The pulse tail extinction ratio allows for penalty free multiplexing up to 20 Gbit/s and the minimum timing jitter (rms. standard deviation) is 1.5 ps. The amplitude and phase noise characteristics are critically dependent on detuning of the cavity resonance frequency relative to the modulation frequency. Stable operation is achieved within a detuning range of a ∼5 MHz. Gain switched MQW DFB lasers combined with linear pulse compression produce pulses with a pulse width of 6,5 ps (FWHM) at 10 GHz repetition rate. The pulse tail extinction ratio (25 ps from the peak) is better than 30 dB. Hence, the power penalty associated with interferometric noise in a 40 Gbit/s OTDM transmitter employing the gain switched source is negligible. The timing jitter is better than 1 ps. The fine properties of the gain switched source is attributed to a highly linear chirp stretching into the low power regions of the pulse trailing edge combined with an initially high on-off ratio of the non-compressed pulse. Up to 80 Gbit/s signal generation is achieved with 2-3 dB penalty due to inter-channel cross talk. The gain switched laser source is simple to operate and extremely stable. A wavelength tuneable mode locked erbium doped fibre ring laser with all polarisation maintaining fibre is constructed and tested as an OTDM source up to 40 Gbit/s. Generated pulses at 5 GHz repetition rate show FWHM pulse widths down to ∼12 ps limited by the filter bandwidth of the laser. The timing jitter is less than 1.1 ps and the residual super modes of the harmonically mode locked laser is suppressed more than 30 dB (optical). The pulse shape is almost ideal Gaussian giving a well defined relation between the pulse tail extinction ratio and the pulse width. For stabilised operating conditions the laser runs with BER> l0⁻¹0 at 40 Gbit/s for up to lh. System requirements to both demultiplexing and add/drop multiplexing in an OTDM system context is discussed. For 1:4 and 1:8 demultiplexing with 10 Gbit/s as the base data rate, the suppression ratio of the discarded channels relative to the demultiplexed signal should exceed 13 and 17 dB, respectively, for a receiver penalty less than 1 dB. For add/drop multiplexing a suppression ratio for the drop channel of ∼21 dB is required for a 1 dB penalty at 40 Gbit/s. All-optical demultiplexers and add/drop multiplexers based on monolithic integrated Michelson and Mach-Zehnder interferometers incorporating semiconductor optical amplifiers as phase shifting elements have been investigated both theoretically and experimentally. By applying a differential control scheme using two optical control signals bit-rate flexible demultiplexing and add/drop multiplexing is realised. Penalty free 40 to 10 Gbit/s demultiplexing is achieved with the SOA-MZI while the demultiplexed signal of the SOA-MI suffers from penalty of 4.5 dB, which is attributed to the all active device implementation. The ultrahigh speed capability is demonstrated in a 80 Gbit/s demultiplexing experiment using the SOA-MZI. The power penalty of 4 dB is in this case mainly attributed to interchannel crosstalk of the transmitter. High quality 40 Gbit/s add/drop multiplexing with a small signal- to-noise degradation of the add channel due to interferometric crosstalk is demonstrated. For add-drop multiplexers to operate with high suppression ratios <22 dB for ??the drop channel, the relative power level of the optical control signals should be controlled within 0.5 dB. Optical demultiplexing from 40 to 10 Gbit/s by a high speed (< 20 GHz bandwidth) multiple quantum well electroabsorption modulator is also demonstrated as well with a power penalty less than 1 dB. Dispersion accommodation by mid-span spectral inversion employing FWM in semiconductor optical amplifiers is investigated. SOA's with different cavity lengths are characterised with respect to FWM conversion efficiency and optical signal-to-noise ratio of the conjugated signal. It is found that long amplifiers are preferred for optimised FWM performance. An OSNR of 18 dB measured in 0.5 nm bandwidth is achieved in 1200 mum long amplifiers. All-optical standard fibre OTDM network (3 nodes) experiments at 20 and 40 Gbit/s have been demonstrated using cascaded high performance phase conjugators. BER assessment for transmission over 200 km of concatenated standard fibre links, including two OPC's, show small (>2 dB) or even negative power penalties. The improvement in receiver sensitivity is caused by slightly different pulse width and shape at the input and output of the transmission link. The results indicate, that the high OSNR of the conjugate signal achieved in long amplifiers (L<1 mm) is suitable for achieving high performance transmission systems using cascaded MSSI. Highly sensitive SOA-based preamplifiers operating at 10 and 20 Gbit/s are investigated, including both chip-SOA preamplifiers and a hybrid integrated module incorporating a SOA, an optical filter and a PIN-FET front-end. A preamplifier receiver sensitivity of -24.5 dBm at 20 Gbit/s RZ for both a chip-SOA and the hybrid module is obtained. At 10 Gbit/s RZ the receiver sensitivity of the module is -32 dBm. The receiver sensitivity is only 3.5 dB worse than for preamplifier consisting of a 980 nm pumped EDFA. A refined SOA-chip results in a record fibre sensitivity of -33.4 dBm at 10 Gbit/s NRZ. The results indicate fine prospects for realising cost-effective and high performance receivers for high bit-rate systems in general, and in particular for OTDM systems based on the RZ signal format.

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Record created 2009-02-19, last modified 2009-02-19

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