Spectrum-sliced Incoherent Light Source Research Articles

Mitigation of scintillation in FSO communication using a temporally and spatially incoherent spectrum-sliced light source.

We propose and demonstrate a temporally and spatially incoherent spectrum-sliced light source for scintillation mitigation in free-space optical (FSO) communications. For the temporal incoherence, we utilize the spectrum-sliced incoherent light (SSIL) modulated by gain-saturated reflective semiconductor optical amplifiers for data modulation and the suppression of excess intensity noise inherent in a narrowband incoherent light source. We then employ offset launching from a single-mode fiber to a multi-mode fiber to add the spatial incoherence to the SSIL source. The experimental demonstration shows that this temporally and spatially incoherent light is effective in mitigating scintillation. For example, this light source reduces the transmitter power required to achieve a bit-error ratio of 10-3 by ∼6 dB in comparison with the laser source. This is an ∼3 dB larger reduction in the transmitter power than the reduction achieved by a temporally incoherent light source.

Mitigation of Scintillation in FSOC Using RSOA-Based Spectrum-Sliced Incoherent Light

We propose and experimentally demonstrate the mitigation of adverse effects of scintillation using a spectrum-sliced incoherent light source based on gain-saturated reflective semiconductor optical amplifiers (RSOAs). This incoherent light source, originally developed for the cost-effective realization of wavelength-division-multiplexed passive optical networks, employs RSOAs for both data modulation and the suppression of large excess intensity noise inherent in incoherent light sources. We show through experiment that this incoherent light source is effective in suppressing the power fluctuations induced by scintillation when compared to a coherent light source, and thus improves the receiver sensitivity of free-space optical communication systems under weak atmospheric turbulence.

10-GB/S bidirectional WDM-PON transmission using spectrum-sliced ASE light sources

We report on a new WDM-PON architecture with the capacity of 10 Gb/s data rate per channel, bidirectional, using low-cost spectrum-sliced incoherent light sources. The system allows the uplink and downlink transmission of a total of 32 channels over 25 km of optical fiber. The experimental results demonstrate bit error rates (BER) below 1.2 × 10−3 for both up- and down-link transmission. With the BER performance, error-free transmission characteristics can be achieved with forward error correction. Several advanced schemes are adopted to realize the high-capacity optical access network. A large effective area fiber is used to reduce the effects of chromatic dispersion for the spectrum-sliced signals. A two-stage cascaded semiconductor optical amplifier configuration is used to suppress the excess intensity noise caused by the incoherent amplified spontaneous emission light source. Remote pumping architecture is applied to compensate for the optical loss. Pretilt process and gain-flattened grating are used to generate uniform channel power with about 3 dB spectrum flatness for uplink and downlink bands.

Optimum Linewidth of Spectrum-Sliced Incoherent Light Source Using a Gain-Saturated Semiconductor Optical Amplifier

We investigate the optimum linewidth of the spectrum-sliced incoherent light (SSIL) source using a gain-saturated semiconductor optical amplifier (SOA) for the maximum capacity and longest transmission distance. For this purpose, we carry out experimental and simulation studies on the transmission performance of a 10-Gb/s on–off keying signal generated by using the SSIL source over a wide range of the SSIL linewidth. We find out that there are two windows of the linewidth for the high-speed operation of the SSIL source: ultra-narrow (i.e., linewidth $\ll$ receiver bandwidth) and very wide (i.e., linewidth $\gg$ receiver bandwidth). However, when the linewidth of the SSIL source is very wide, the 10-Gb/s signal generated by using this SSIL suffers severely from fiber chromatic dispersion and optical filtering. The simulation results are confirmed by experimental data measured by using an ultranarrow fiber Fabry–Perot filter (bandwidth = 700 MHz) and a bandwidth-tunable optical filter (bandwidth = 20 ∼ 53 GHz). Thus, we can conclude that the optimum linewidth of SSIL for capacity and transmission distance is ultranarrow. We also present a couple of drawbacks of the ultranarrow SSIL source, compared to the conventional wide-linewidth SSIL one, such as a large spectrum-slicing loss, a large SOA input power required for the suppression of excess intensity noise inherent in the incoherent light source, and the susceptibility to in-band crosstalk.

5-Gb/s upstream transmission using an RSOA seeded by ultra-narrow spectrum-sliced incoherent light

We successfully demonstrate 5.35-Gb/s upstream transmission over 20-km standard single-mode fiber using an ultra-narrow spectrum-sliced incoherent light source and a reflective semiconductor optical amplifier (RSOA) without any dispersion compensation. The offset optical filtering and transmitter de-emphasis are exploited to reduce the excess intensity noise of the light source and to overcome the bandwidth limitation of the 1.4-GHz-bandwidth RSOA, respectively. Thanks to these two techniques and narrow linewidth of the source, we achieve the uncorrected bit-error rate of <10−3 after the transmission. The acceptable range of RSOA injection power is also investigated through experiment.

Ultranarrow Spectrum-Sliced Incoherent Light Source for 10-Gb/s WDM PON

We report on the use of an ultranarrow ( ~0.01 nm) spectrum-sliced incoherent light source for transmission of 10-Gb/s nonreturn-to-zero signals over 20-km dispersion-uncompensated standard single-mode fiber and 0.2-nm-bandwidth optical band-pass filter. A wideband amplified spontaneous emission is first generated using an erbium-doped fiber amplifier and then spectrum-sliced by an ultranarrow fiber Fabry-Perot filter (3-dB bandwidth: ~0.006 nm). The spectrum-sliced light is intensity-smoothed by using a gain-saturated reflective semiconductor optical amplifier and then modulated at 10.7 or 12.5 Gb/s, assuming forward error correction (FEC) with 7% or 25% overheads, respectively. Thanks to the narrow linewidth of the source, we are able to retain the intensity smoothing after the transmission, achieving uncorrected bit-error ratios better than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> at 10.7 and 12.5 Gb/s, respectively. We discuss the applicability of the proposed light source to wavelength-division-multiplexed passive optical networks and the choice of FEC codes for the proposed scheme.

Effects of intraband crosstalk on incoherent light using SOA-based noise suppression technique

We investigate the effects of intraband crosstalk on the intensity noise suppression of spectrum-sliced incoherent light sources using semiconductor optical amplifiers (SOAs). Both Q-factor and electrical spectrum measurements clearly show that intraband crosstalk deteriorates the SOA-based noise suppression by breaking the correlation between optical frequency components of light.

Impact of dispersion, PMD, and PDL on the performance of spectrum-sliced incoherent light sources using gain-saturated semiconductor optical amplifiers

The effects of chromatic dispersion (CD), polarization-mode dispersion (PMD), and polarization-dependent loss (PDL) on the intensity noise suppression of spectrum-sliced incoherent light sources achieved by using gain-saturated (GS) semiconductor optical amplifiers (SOAs) are investigated. Passing the spectrum-sliced incoherent light through SOAs, the excess intensity noise (EIN) originating from beating of spontaneous emission against itself can be greatly reduced. However, since the noise suppression is achieved by an elaborate balancing between numerous frequency/polarization components of light, thus, forming a high correlation between them, it is vulnerable to frequency/polarization-dependent optical phenomena. Through Q-factor and bit error rate (BER) measurements, this paper shows that CD, PMD, or PDL deteriorates the SOA-based noise suppression technique by breaking the correlation. Spectral analysis is also performed to investigate the frequency dependency of these effects. It is shown that CD and PMD negate the noise suppression giving rise to intensity noise from high frequencies, whereas there is no frequency dependence for PDL effects. Therefore, CD-, PMD-, or PDL-induced penalties for incoherent light sources using the SOA-based noise suppression technique are considerably greater than those produced by pulse broadening or distortion alone.

Signal-to-noise ratio of spectrum-sliced incoherent light sources including optical modulation effects

We have derived the beat-noise-limited signal-to-noise ratio (SNR) of spectrum-sliced incoherent light sources, including optical modulation effects. SNR changes significantly in time depending on the received optical pulse shape and the receiver electrical filter. Thus, the optimum decision point at the optical receiver may be much different from the peak signal point and the sampled SNR may be different from the continuous-wave SNR. To obtain high SNR, small rise and fall times are required and the non-return-to-zero modulation format is recommended.