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Ph.D. Thesis - Benny Mikkelsen - December 1994
Optical amplifiers and their system applications.
Ph.D. thesis, Department of Electromagnetic Systems, Technical University of Denmark, Kgs. Lyngby, Denmark.
Supervisors: Kristian Stubkjr.

Benny Mikkelsen

Basic characteristics of erbium doped fiber amplifiers and semiconductor optical amplifiers and their applications in optical systems are investigated both experimentally and theoretically. More than 28 dB of fiber-to-fiber gain is reported for DCPBH-SOAs featuring window and taper sections. The measured polarisation sensitivity and gain ripple is less than 1 dB at 25 dB gain. Ridge waveguide amplifiers with angled facets are shown to have modal reflectivities of less than 10-5. Pump efficiencies and noise figures are reported for EDFAS pumped with either 980 or 1480 nm pump wavelength. With 980 nm as pump wavelength, the measured pump efficiencies are 3.7 and 2.8 dB/mW at 1533 and 1553 nm signal wavelength, respectively, while the noise figure is ∼3.5 dB at both wavelengths. Models that predict the spontaneous emission accurately are presented for both EDFAS and SOAs. The modelling work reveals, in agreement with measurements, that backward travelling ASE generally increases the noise figure of high gain amplifiers. Additionally, it is shown that the best ratio between the saturation power and the noise figure is achieved with a ∼500 μm long SOA. The moment generating function (MGF) describing the noise statistics of an. optically preamplified direct detection receiver including an arbitraryly chosen electrical filter is derived. It is shown that the saddlepoint approximation (applied to the receiver MGF) gives almost exact results, whereas the modified Chernoff bound is not very accurate at high gain values. The Gaussian approximation (GA) predicts sensitivities that are at most 0.6 dB worse than the sensitivities found by using an exact approach based on a fast Fourier transform. Significant improvements in receiver sensitivities for SOA based pre-amplifier receivers are reported at 2.5 Gbit/s (-40.1 dBm) and at 10 Gbit/s (-32.4 dBm) for amplifiers with 28 dB fiber-to-fiber gain (polarisation insensitive). Best performances are achieved with short SOAs. For EDFAS the potential of a mixed pump configuration combining the advantages of the low noise properties of 980 nm pumping and the high possible conversion efficiency of 1480 nm pumping is demonstrated. Pre-amplifier sensitivities of -43.5 dBm at 2.5 Gbit/s and -34.5 dBm at 10 Gbit/s are achieved. The dynamic range of SOAs is investigated at 2.5 and 10 Gbit/s. Experiments reveal that short SOAs have best performances and that approximately 50 SOAs can be cascaded at 2.5 Gbit/s while only 10 can be cascaded at 10 Gbit/s. Furthermore, repeaterless transmission over more than 300 km of standard fiber at a bit rate of 2.5 Gbit/s is demonstrated with refined EDFA based booster and pre-amplifiers. In the experiments the transmitter employs FM/IM conversion to achieve information bandwidth limited signals and to suppress the SBS-threshold. The multi channel capability of preamplified receivers is investigated for both SOA and EDFA based pre-amplifiers. The inherent AM-crosstalk in SOAs impose severe constrains on the number of channels that can be received simultaneously. Experiments show that the total capacity of SOA based multi channel receivers is limited to approximately 25 Gbit/s independent of the channel spacing and the bit rate per channel. Moreover, a four channel (4 × 5 Gbit/s) repeaterless transmission experiment over 204 km standard non-dispersion shifted fiber employing EDFA booster and pre-amplifiers is reported. The potential of SOAs as functional elements is exploited. An optical modulation bandwidth in excess of 20 GHz is measured. This high bandwidth is shown to be caused by a self-filtering effect. Furthermore, theory predicts that a modulation bandwidth as high as 40 GHz is feasible with SOAs. Optical wavelength conversion of 20 Gbit/s data signals by use of cross-gain modulation in SOAs is reported. Additionally, a monolithic integrated interferometric converter realised by placing two SOAs in a Michelson interferometer is used to perform wavelength conversion of 10 Gbit/s. For the first time, penalty-free up- and down-conve ??rsion of 10 Gbit/s signals is demonstrated within a wavelength range of 35 nm. The integrated converter also features converted signals with very high spectral purity enabling transmission of 10 Gbit/s signals over more than 60 km of standard non-dispersion shifted fiber. Furthermore, bit rate transparent conversion of up to 10 Gbit/s signals is demonstrated by a four section DBR-laser. Finally, the high modulation bandwidth of SOAs has been used to accomplish optical demultiplexing of 20 Gbit/s OTDM signals by use of the integrated SOAs within the Michelson interferometer. Novel demultiplexer configurations based on interferometric configurations are discussed.

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

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