Spectrum-sliced Incoherent Light 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.

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.

Simulation on semiconductor optical amplifier intensity noise reduction for future spectrum-sliced optical networks

Spectrum-slicing techniques employing incoherent light are an economic, practical and therefore attractive solution for future all-optical networks, especially for wavelength-division multiplexing (WDM) transmission systems in local area networks (LAN). However, spectrum-sliced methods exhibit a large excess intensity noise factor that limits the performance of the system. In this paper, we investigate noise suppression of spectrum-sliced incoherent light using a saturated semiconductor optical amplifier (SOA). The system incorporating the noise reducing SOA is modeled and simulated using OptSim software, and the results are compared to practical schemes from the literature. Performance comparisons are made with two different broadband sources test-beds. The characteristics of the SOA gain saturation are also presented. In both cases, it is found that a high degree of intensity noise is suppressed by the use of the non-linear gain saturation characteristics of the SOA so as to achieve better system performance. The position of a modulator in the system is also investigated in order to greatly reduce the excess intensity noise.

1.25-Gb/s Operation at 50-GHz Channel Spacing Based on Intensity Noise Suppression of Wavelength-Locked Fabry–PÉrot Laser Diode

We demonstrate 1.25-Gb/s operation at 50-GHz channel spacing and 10-km single-mode fiber, which is based on a wavelength-locked Fabry-Perot laser diode (F-P LD) and an intensity noise suppression by a gain saturation of F-P LD in front of the receiver. The wavelength-locked F-P LD employs the spectrum-sliced incoherent light with narrow bandwidth of 35 GHz. The intensity noise of wavelength-locked F-P LD is suppressed about 6 dB by the F-P LD in the front of the receiver. As a result, the bit-error-rate curves exhibit the error-free performances over the detuning conditions to cover a whole mode spacing period of F-P LD.

RSOA-Based Wavelength-Reuse Gigabit WDM-PON

This article presents the RSOA-based re-modulation schemes for practical application to the WDM-PON link. Emphasis is put on the three methods for reducing the residual downstream signal in the upstream transmission; the simplest one is to use the RSOA gain-saturation property, the second uses selective filtering of the adiabatic-chirped DFB-LD output spectrum, and the last uses a dynamic RSOA gain control by varying its driving current according to the incident binary signal. The reflection sensitivity in the proposed re-modulation link is also qualitatively presented in the cases of using two different types of seed light: coherent light from DFB-LD array and spectrum-sliced incoherent ASE light. We show experimental results of an error-free bidirectional gigabit transmission over 20 km at -30 dB reflection with the spectrum-sliced ASE seed light.

Gain‐saturated SOA‐based optical power booster for spectrum‐sliced incoherent light

AbstractAn optical power booster based on a gain‐saturated semiconductor optical amplifier with an injected holding light is experimentally demonstrated for an optical access network using spectrum‐sliced incoherent light (SSIL). The results show that the proposed booster can provide not only a signal gain of 10.8 dB but also a Q‐factor of 8.9 for a 2.5 Gb/s SSIL signal, transmitted over 10 km of single‐mode fiber. The corresponding bit‐error rate is lowered to 6 × 10−11. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3038–3040, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22966

Optical Packet Header Processing Using Time-to-Wavelength Mapping in Semiconductor Optical Amplifiers

A new approach for optical packet header processing based on a time-to-wavelength converter is proposed. The processing is composed of transferring, through an all-optical and-gate, the header data bits into the successive single optical pulses at different wavelengths. The and-gate is performed by the semiconductor optical amplifier (SOA) and makes use of either cross-polarization modulation or four-wave mixing (FWM). Both implementations are compared. In order to improve the scalability of the design and to overcome the polarization sensitivity of the FWM solution, we propose the use of a spectrum-sliced incoherent light in generating the wavelength bits. The FWM process in this specific case is studied. The 10-Gb/s demonstrations of the various elements are presented.

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.

Super-dense WDM transmission of spectrum-sliced incoherent light for wide-area access network

This paper presents a detailed study on superdense wavelength-division multiplexing (WDM) transmission of spectrum-sliced incoherent light used for a wide-area access network. The transmission performance possible with this approach is evaluated numerically with regard to the following items: the transmission limit due to the chromatic dispersion, optimized signal-to-noise ratio design for the optical spectral width taking into account the adjacent-channel crosstalk, and the nonlinear effect generated by four-wave mixing at around the zero-dispersion wavelength. Superdense WDM transmission is also performed experimentally using 25-GHz channel-spaced, 15-GHz wide, up-to 156-Mb/s signals through up to 120 km of dispersion-shifted fiber.

Transmission of 4 x 2.5-Gb/s spectrum-sliced incoherent light channels over 240 km of dispersion-shifted fiber with 200-GHz channel spacing

We demonstrate a transmission of 4/spl times/2.5-Gb/s spectrum-sliced incoherent wavelength-division-multiplexed channels over 240 km of dispersion-shifted fiber. The channel bandwidth and the channel spacing are kept small using the intrachannel four-wave mixing at the receiver. The error-floor level is close to 1/spl times/10/sup -11/ and the dispersion penalty is within 1 dB at 1/spl times/10/sup -9/ bit-error rate. Eye opening simulation shows that incoherent channels are less sensitive to the fiber four-wave mixing than coherent channels.

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

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.

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

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.

Signal-to-noise ratio measurement of a 2.5-Gb/s spectrum-sliced incoherent light channel

We have measured the signal-to-noise ratio (SNR) of a 2.5-Gb/s spectrum-sliced incoherent light channel in the large beat-noise limit. SNR changes strongly in time depending on the receiver electrical filter and the extinction ratio. Except for some minor differences, the SNR behaved quite similar to the theory. In order to obtain a high SNR, the nonreturn-to-zero format should be used rather than the return-to-zero format and the receiver sampling time should be chosen more precisely than that of conventional receivers.

Reduction of excess intensity noise in spectrum-sliced incoherent light for WDM applications

We investigate wideband reduction of excess intensity noise in incoherent light for application to spectrum-sliced WDM systems. The noise reduction scheme is based on optoelectronic feedforward compensation. We derive expressions for the probability density function of noise-reduced incoherent light and present measurements that are in good agreement with theory. We evaluate the significant levels of improvement obtainable in the capacity of spectrum-sliced WDM channels. For example, to obtain a signal-to-noise ratio of 50, a noise-reduced channel requires six times less optical bandwidth than a spectral slice without noise reduction.