Multi-wavelength light sources by narrow-band spectral filtering of multi-mode lasers have been studied for wavelength division multiplexing (WDM) applications. Long-wavelength Fabry-Perot (FP) semiconductor lasers provide many modes, whose mode spacing can be fixed with precise frequency by using a mode locking technique. However, increased intensity noise has been observed in the spectral filtering of multi-modes generated by mode-locked semiconductor lasers. This is known as mode partition noise. The noise appears as an extreme increase in the relative intensity noise (RIN) spectrum in low frequency regions. This limits transmission applications because of degradation of signal to noise ratio (SNR). In the paper, we demonstrate reduction of the mode partition noise by using a semiconductor optical amplifier (SOA). 16-dB SNR recovery is reported for spectral filtering of FP lasers.

Reduction of mode partition noise by using semiconductor optical amplifiers

In this paper, we present an experimental and numerical study of semiconductor optical amplifier (SOA)-based noise suppression and its relevance to high-channel-density spectrum-sliced wavelength-division-multiplexed systems. We show that the improvement in signal quality is accompanied by spectral distortion, which renders it susceptible to deterioration in the presence of subsequent optical filtering. This phenomenon originates from the loss of intensity correlation between spectral components of the SOA output when the signal spectrum is altered. As a consequence, a design tradeoff is introduced between intensity noise and crosstalk in high-channel-density systems. These adverse effects can be overcome by optimized SOA design, resulting in a significant improvement in signal quality.

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Noise suppression of incoherent light using a gain-saturated SOA: implications for spectrum-sliced WDM systems

We simulate transmission of a spectrum-sliced WDM channel operating at high bit rates (e.g., 622 to 2488 Mb/s). We calculate the bit error rate using the non-Gaussian statistics of thermal light sources that are commonly used in spectrum slicing and account for the effects of fiber dispersion. We evaluate the tradeoff in optical slice linewidth between signal-to-excess optical noise ratio and dispersion penalty in spectrum-sliced WDM systems, and determine the channel slicewidth that minimizes transmission penalty for a given link length and bit rate. We compare our simulations against the measured performance of a 1244 Mb/s channel over 20 km of fiber. The results in this paper provide useful information for the design of spectrum-sliced WDM networks.

Transmission performance of high bit rate spectrum-sliced WDM systems

A laboratory-simulated free-space optical link under various turbulence levels is implemented to propose and experimentally demonstrate the use of saturated optical amplifiers as a simple and efficient approach for suppression of scintillation due to atmospheric turbulence. The use of erbium-doped fiber amplifier (EDFA) or semiconductor optical amplifier (SOA) requires the received signal be coupled into a fiber. The system performance of receiver structures employing a saturated EDFA and a SOA (in saturation and conversion modes) are measured and compared to that of fiberless direct detection (DD). It is shown that in higher turbulence levels, where no data transmission can be achieved by DD, remarkable eye opening results when using saturated amplifiers

Suppression of Turbulence-Induced Scintillation in Free-Space Optical Communication Systems Using Saturated Optical Amplifiers

Theoretical and experimental results on the reduction of the intensity noise in spectrum-sliced wavelength-division-multiplexing systems using a saturated semiconductor optical amplifier (SOA) are presented. The influence of the injected current and the input power to the saturated SOA on the noise reduction and its bandwidth are studied. The optimum condition for a high noise suppression ratio and a large bandwidth is derived. For the optimum operation of the SOA, an increase of 13.5 dB in the intensity-noise-limited signal-to-noise ratio for a bitrate of 2.488 Gb/s and of 17.5 dB for a bitrate of 622 Mb/s are obtained experimentally in a single-stage SOA.

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Analysis and optimization of intensity noise reduction in spectrum-sliced WDM systems using a saturated semiconductor optical amplifier

We demonstrate 250-km nondispersion-shifted fiber transmission of directly modulated 2.5-Gb/s signals. The chirping-induced power penalty is reduced by using a spectral filter whose transmission peak is tuned to "mark" wavelength of the modulated laser output. A well-known intensity modulation with a sufficiently high extinction ratio is used to suppress the intersymbol interference induced by nonuniform frequency modulation response of semiconductor lasers. The observed results are explained theoretically by using eye closure penalty.

Transmission of directly modulated 2.5-Gb/s signals over 250-km of nondispersion-shifted fiber by using a spectral filtering method

Using a gain-saturated semiconductor optical amplifier, we have suppressed the intensity noise in spectrum-sliced incoherent light communication systems. A large increase has been achieved in the signal-to-noise ratio that is much better than previous results to our knowledge. Using this technique, we have transmitted a 0.36-nm bandwidth 2.5-Gb/s incoherent light signal over 20 km of conventional single-mode fiber without any dispersion compensation. The transmission penalty was 1.5 dB and the error floor level was less than 10/sup -10/.

Intensity noise suppression in spectrum-sliced incoherent light communication systems using a gain-saturated semiconductor optical amplifier

We introduce a new all-optical technique to transmit a spectrum-sliced incoherent channel with its optical bandwidth smaller than the conventional limit. As a demonstration, we reduce the optical bandwidth to only 0.1 nm for the 2.5-Gb/s incoherent channel transmission. Even though the signal-to-noise ratio (SNR) is poor during the transmission, sufficient SNR can be obtained through the intrachannel four-wave mixing at the receiver. With this slim bandwidth transmission technique, the maximum number of spectrum-sliced wavelength-division-multiplexed channels can be increased greatly and the dispersion penalty can be reduced simultaneously.

0.1-nm narrow bandwidth transmission of a 2.5-Gb/s spectrum-sliced incoherent light channel using an all-optical bandwidth expansion technique at the receiver

Two cascaded semiconductor optical amplifiers are used to suppress the intensity noise of a spectrum-sliced incoherent channel with a bit rate as high as 10 Gbit/s for 2.6 nm channel bandwidth. The incoherent channel is transmitted over 100 km of dispersion-shifted fibre with a negative transmission penalty caused by intra-channel four-wave mixing.

Suppression of intensity noise in 10 Gbit/s spectrum-sliced incoherent light channel using gain-saturated semiconductor optical amplifiers

Transmission of 2.5 Gbit/s spectrum-sliced fibre amplifier light source channels over 200 km of dispersion-shifted fibre (DSF) has been achieved, the highest bit-rate distance product 500 Gbit/skm using incoherent light sources reported to our knowledge. The total power penalty was as low as 0.2 dB for the 1.8 nm bandwidth channel centred at the zero-dispersion wavelength of DSF.

Jung-Hee Han

Papers

Transmission of directly modulated 2.5-Gb/s signals over 250-km of nondispersion-shifted fiber by using a spectral filtering method

Intensity noise suppression in spectrum-sliced incoherent light communication systems using a gain-saturated semiconductor optical amplifier

0.1-nm narrow bandwidth transmission of a 2.5-Gb/s spectrum-sliced incoherent light channel using an all-optical bandwidth expansion technique at the receiver

Suppression of intensity noise in 10 Gbit/s spectrum-sliced incoherent light channel using gain-saturated semiconductor optical amplifiers

2.5 Gbit/s transmission of spectrum-sliced fibre amplifier light source channels over 200 km of dispersion-shifted fibre