Kent Bertilsson

Soliton stability in optical fibers with polarization-mode dispersion

This letter presents a detailed investigation of how solitons broaden in the presence of polarization-mode dispersion. For a given amount of polarization-mode dispersion and pulsewidth there exists an optimal choice of group-velocity dispersion, for which solitons are most resistant to the effect of polarization-mode dispersion. We also optimize the soliton power enhancement factor and find that solitons can propagate roughly twice the distance compared to linear pulses in the absence of group-velocity dispersion.

Soliton interaction penalty reduction by receiver filtering

We present the first detailed numerical study of the impact of receiver low-pass filtering in soliton interaction-limited communication systems. We propagate 40-Gb/s pseudorandom bit sequences (PRBSs) of 2/sup 8/-1 solitons up to one soliton collision distance (z/sub c/) and optimize the filter bandwidth to minimize the interaction induced eye opening penalty. The distance at 0.5-dB penalty is increased over 40% through filtering. The penalty is essentially pulsewidth independent when considering distances normalized by z/sub c/. We also show that PRBSs of at least 2/sup 8/-1 solitons are required to accurately simulate random data.

Characterization of an InGaAsP semiconductor laser amplifier as a multifunctional device

Multifunctional properties of an InGaAsP semiconductor laser amplifier have been evaluated. A bit error rate of 10/sup -9/ at 100 Mb/s was obtained using the amplifier as a detector at a received optical power of -27 dBm with simultaneous cavity gain of 16 dB. The bandwidth of the amplifier detector was 300 MHz and the maximum responsivity was 30 V/W. The amplifier had a maximum gain of 29 dB and a very large optical on/off ratio of 50 dB. When the amplifier was used as a switch the cavity gain was 19 dB and the extinction ratio was 22 dB. >

Noise figure of erbium doped fiber amplifiers in the saturated regime

We have made an experimental and theoretical study of the noise figure of an erbium doped fiber amplifier in the saturated regime. The saturated amplified spontaneous emission at the signal wavelength was measured using a very accurate pulsed source technique. We have quantified the noise figure dependence on compression, in excellent agreement with theory, to be less than 1.5 dB for a gain compression as high as 15 dB when the small signal gain was 26 dB, and the 1.48 /spl mu/m pump power was 27 mW.<<ETX>>

Modeling of noise in erbium-doped fiber amplifiers in the saturated regime

We have made a theoretical study of the noise figure of erbium-doped fiber amplifiers in the saturated regime. The noise figures of amplifiers subjected to specific gain and gain compression requirements were calculated for various amplifier lengths. The resulting noise figures together with the required pump and input signal powers map out all possible solutions given constraints on gain, compression, pump power, output signal power, and noise figure. In some cases, requirements on the output signal power prohibit any solutions. A way to solve this problem is the introduction of a post-amplifier loss. For this configuration, two possible solutions exist, which collapse into one solution at a certain critical loss, under which there exist no solutions. When the impact of amplified spontaneous emission is neglected in the model, only one solution is obtained, and the critical loss is much smaller than when the amplified spontaneous emission is included in the model. We conclude that amplified spontaneous emission generally has to be taken into account, even when the gain is as low as 10 dB, to accurately predict the noise performance of erbium-doped fiber amplifiers. >

Impact of spectral inverter fiber length on four-wave mixing efficiency and signal distortion

We compare the performance of spectral inversion through four-wave mixing in a long (13.5 km) and a short (1.5 km) dispersion-shifted optical fiber, respectively. While the effectiveness of the long fiber is limited by nonlinear phenomena such as stimulated Brillouin scattering and cross-phase modulation, the effectiveness of the short fiber is limited only by the available power, since the critical power levels where the deleterious nonlinear phenomena occur increase when the fiber length decreases. With the requirement of negligible spectral deformation of the inverted signal, the shorter fiber gives 60% higher conversion efficiency and four times more converted optical power. Measured with 12-nm wavelength conversion from the signal to the inverted replica. >

Numerical study of moderate distance high bit-rate alternating-amplitude soliton systems

We present a numerical investigation of alternating-amplitude soliton systems. We propagate 100 Gb/s, pseudorandom bit sequences of 2/sup 5/-1 to 2/sup 7/-1 solitons through fibers of different lengths and calculate the corresponding eye opening penalty at the receiver. The influence of different amplitude ratios, amplifier spacings, pulse widths, and dispersion slopes as well as of the soliton self-frequency shift are studied. We also study the effect of compression of the alternative-amplitude solitons with the larger amplitudes to preserve their soliton character and the impact of the relative initial phase between the alternating-amplitude solitons. When the amplifier spacing is 10 km the system length can be at least 400 km with alternating-amplitude solitons compared to only 200 km in the case of equal amplitude solitons with similar penalties. Our simulations show-that third-order dispersion and the soliton self-frequency shift limit the maximum allowable amplitude ratio.

Impact of spectral-inverter fiber length on FWM efficiency and signal distortion

Midspan spectral inversion (MSSI) using four-wave mixing (FWM) in dispersion-shifted fibers (DSF) has become an attractive technique for compensating for fiber dispersion.1 It is, however, important to investigate methods for increasing the conversion efficiency and the output power.2 We have studied the influence of the length of the DSF on the FWM efficiency, as well as on self-phase modulation (SPM), cross-phase modulation (XPM), and stimulated Brillouin scattering (SBS).

Soliton Transmission at High Bit Rates through Distributed Erbium-doped Fibres

In ultra-long transmission systems, the combined effects of fibre-dispersion and optical Kerr effect, can be used to transmit soliton pulses well beyond the dispersion limit [1]. To overcome fiber loss in an all-optical soliton link, both lumped and distributed amplification have been used. Excursions in the signal power reduce the stability of the soliton. To sustain stable soliton propagation in a system with lumped amplifiers, the spacing between the amplifiers, Za, has to be much shorter than the soliton period, Z0, i.e. Za/Z0 > 1 [3], when using distributed amplification. Distributed amplification may be realized through the use of the Raman effect [4] or distributed erbium-doped fibres (d-EDFs) [5]. Here we present an experimental characterization of the transmission of solitons over 90 km using d-EDFs. We characterize the performance of the d-EDF according to the relation Za/Z0 and the magnitude of the signal excursion along the d-EDF, and verify experimentally that stable soliton transmission also is achievable when Za/Z0 > 1.