In-line Optical Amplifiers Research Articles

Compensation for pulse waveform distortion in ultra-long distance optical communication systems by using midway optical phase conjugator

In long-distance optical communication systems using in-line optical amplifiers, the waveform distortion is induced by both the Kerr effect and the group-velocity dispersion of optical fibers. In this letter, we show that the optical phase conjugator placed at the midway of the transmission line can compensate for the Kerr effect as well as the group-velocity dispersion, preventing the waveform distortion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transmission limitations due to self-phase modulation in optical PSK heterodyne detection systems employing chromatic dispersion equalization

Even if the amplitude of a phase-modulated optical signal is constant before transmission, amplitude modulation is caused by fiber chromatic dispersion. As a result, self-phase modulation (SPM) is induced. In optical heterodyne detection, SPM cannot be compensated for by the delay equalizer (electrical domain) used to compensate fiber chromatic dispersion. However, the transmission distance limitation of multi-repeatered coherent transmission systems has not been investigated in the presence of SPM. This paper theoretically and experimentally investigates the transmission distance achievable with a phase-shift-keying (PSK) heterodyne detection system employing in-line optical amplifiers and delay equalization. The calculated results show that equalization is effective when /spl gamma/P/sub 0//2B/sup 2//spl verbar//spl beta//sub 2//spl verbar/ >

Unregenerated optical transmission at 10 Gbit/s via 204 km of standard singlemode fibre using a directly modulated laser diode

Using the new method of dispersion supported transmission, 10 Gbit/s signals from a directly modulated DFB laser at 1.53 μm wavelength are transmitted on standard singlemode fibre via 151 km without inline amplification. Using one inline optical amplifier, the record length of 204 km is bridged.

Dynamic range of an optical amplifier cascade in an ASK system with significant phase noise

The authors present detailed results concerning the dynamic range of an amplitude-shift keyed (ASK) heterodyne optical communication system where the transmitted signal is impaired by significant amounts of laser phase noise. Theoretical results are evaluated for a system with up to ten in-line optical amplifiers and a significant influence of the laser phase noise is found for the maximum allowable transmission span and the dynamic range. With ten amplifiers, a linewidth of 11% of the bit rate causes the span to be reduced by 12.2 dB and the dynamic range to be reduced by 5.7 dB. The results have been confirmed using two amplifiers in a field trial experiment at a bit rate of 636 Mb/s with an IF linewidth of 70 MHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evolution of field spectrum due to fiber-nonlinearity-induced phase noise in in-line optical amplifier systems

The spectral evolution of light propagating in in-line optical amplifier systems is investigated. Phase noise, which is caused by amplified spontaneous emission through the nonlinear Kerr effect, does not result in simple linewidth broadening but changes the lineshape of the Lorentzian carrier wave. Tails of the field spectrum are raised over a wide frequency range. It is experimentally confirmed that the phase noise is proportional to the third power of the number of amplifiers. Intensity-modulation direct-detection systems, in addition to coherent systems, are affected by the phase noise when a large number of amplifiers are cascaded. >

A noise-loading method for evaluating in-line amplifier systems

The authors propose a noise-loading method for evaluating inline optical amplifier systems. This method takes the linear accumulation of amplified spontaneous emission into account. Comparison against the measured performance of an experimental transmission system confirms that the method accurately simulates inline amplifier systems. The method can be used to evaluate transmitter-receiver pairs to be used in such systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Change of field spectrum of signal light due to fibre nonlinearities and chromatic dispersion in long-haul coherent systems using in-line optical amplifiers

The change of a field spectrum of signal light in long-haul coherent optical fibre communication systems using in-line amplifiers is measured in the normal and anomalous dispersion regions of a fibre. Results show that the interplay of nonlinear Kerr effect and fibre chromatic dispersion cause a fatal change in the field spectrum in the anomalous dispersion region.

Theory of signal degradation in semiconductor laser amplifiers in direct detection systems

Theoretical studies are carried out on long haul direct detection optical fiber communication systems, with inline optical semiconductor amplifier repeaters. Calculations are made of the noise, eye diagrams, and bit-error-rate characteristics of lightwave systems with optical amplifiers. Indications are given of the effect of amplifier characteristics such as spontaneous noise and signal distortion due to gain saturation on the system performance. The nonlinear process within semiconductor laser amplifiers leads generally to pulse amplitude-temporal distortions due to gain saturation. This theoretical study demonstrates that the system penalty caused by these nonlinear effects appears progressively as the optical input power at each amplifier is increased. For example, nonregenerated fiber transmission using traveling wave semiconductor laser amplifiers was simulated, and results obtained at 0.5 and 2.5 Gb/s are presented. In order to improve the system performance, the influence of structure and bulk dimensions of the amplifier cavity is also considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimum use of cascade coupled in-line amplifiers for soliton communication systems

Optimum distances between in-line optical amplifiers in soliton communication systems are presented, and an upper limitation of the amplifier gain is typically shown to be less than 10 dB. The use of higher-order solitons is proposed, and for N=1.4 solitons the noise level is 18 dB under the signal level after 500 km. It will also be shown that the use of N=2.0 solitons will reduce the necessary number of amplifiers for medium transmission distances of about 100 km.

Heterodyne video distribution systems sharing transmitter and local oscillator lasers

The authors present theoretical and experimental results for coherent subcarrier multiplexed (SCM) systems using a novel architecture that shares both the transmitter and local oscillator (LO) laser among multiple optoelectronic receivers. The ability to share both lasers significantly reduces the cost and complexity compared to a multichannel coherent frequency division multiplexed (FDM) system. Experimental results confirm that the system performance can be greatly enhanced by inserting an inline optical amplifier so that many receivers can share one transmitter and LO laser. For example, with an amplifier chip gain of 24 dB, increasing the optical power at the input to the amplifier by 3 dB from -27.6 to -24.5 dBm, the number of receivers can be increased from 2 to 32. >

Long-haul coherent optical fiber communication systems using optical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated. >

Power penalties due to multiple Rayleigh backscattering in coherent transmission systems using in-line optical amplifiers

The power penalties due to double Rayleigh backscattering are calculated for coherent lightwave systems with optical in-line amplifiers. It is shown that coherent systems are less sensitive to Rayleigh noise than high-speed direct-detection systems. For PSK heterodyne systems, the maximum gain of in-line amplifiers that can be achieved without optical isolation is limited to about 25 dB. >

279 km, 591 Mb/s direct detection transmission experiment using four in-line semiconductor optical amplifiers

The authors realized a direct detection intensity-modulation system experiment with four cascaded in-line semiconductor optical amplifiers. Using optical isolators between the amplifiers, they transmitted a NRZ 2/sup 23/-1 message over a distance of 279 km at a bit rate of 591.2 Mb/s. A comparison of this result with parallel experiments without optical isolators indicates that for a realistic implementation of long distance systems with semiconductor optical amplifiers, interamplifier optical isolators will be needed unless the residual facet reflectivity of the optical amplifiers is brought to a very low value ( >

Optimal Location of In-Line Optical Amplifier in a DPSK Heterodyne Link

Optimal Location of In-Line Optical Amplifier in a DPSK Heterodyne Link

Optical time domain reflectometry on optical amplifier systems

The use of optical time-domain reflectometry (OTDR) on long-span fiber transmission systems containing in-line optical amplifiers is discussed. Having identified the specific requirements for OTDR equipment, measurements were carried out on systems of up to 300 km in length, containing three semiconductor laser amplifiers. The results demonstrate that OTDR can be used not only for fault location on fiber links several hundred kilometers in length but also as an alternative to standard system supervisory techniques, thus providing the potential for minimizing the hardware in the optical amplifier stages of future long-span fiber optic transmission systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Direct detection transmission experiments at 565 Mbit/s using cascaded inline optical amplifiers

We report experimental investigations, at 565 Mbit/s, of an inline cascade of five semiconductor laser amplifiers with a total fibre to fibre gain of ~52 dB. Stable operation is achieved even in the absence of intra-amplifier optical filters and isolators, thus ensuring maximum optical system bandwidth and transparency.

Calculation of bit error rate in DPSK system with interfering echoes from optical amplifier chain

The bit error rate degradation due to echoes caused by cascaded nearly travelling wave amplifiers in an optical DPSK-heterodyne transmission system is calculated. Using these results, an estimate is derived for the maximal allowable facet reflectivity in dependence of the number of in-line optical amplifiers.

400 Mbit/s, 372 km coherent transmission experiment using in-line optical amplifiers

We report on an optical transmission experiment using four in-line optical amplifiers. With a net gain of 58,000 (47.7 dB)from the amplifiers, we were able to increase the longest non-regenerated transmission distance to 372 km. The system penalty associated with the amplifiers was only l.5 dB.

Cascaded inline semiconductor laser amplifiers in a coherent optical fibre transmission system

A coherent optical transmission system with two broadband inline optical amplifiers was investigated. The main cause of system degradation was the generation of an interfering signal owing to backward scattered waves in the amplifier chain. The perturbation was eliminated by an isolator or sufficient attenuation between both amplifiers. It is possible to convert the effective 30 dB gain of both amplifiers fully into an increase of the transmission span.