Traveling-wave Semiconductor Optical Amplifier Research Articles

A novel all-optical switch: the wavelength recognizing switch

A new class of an all-optical switch-the wavelength recognizing switch is proposed and experimentally demonstrated. The device uses a control signal to sense the wavelength of the input packet and taps a portion of the data packet to the appropriate output port. The device is based on noncollinear four-wave mixing in a broad-area traveling-wave semiconductor optical amplifier. Measured switching efficiency is +8.2 dB with -28.8 dB of crosstalk. The recognition bandwidth is as narrow as 0.03 /spl Aring/ and the 3-dB switching bandwidth is 42 nm.

Noncollinear four-wave mixing in a broad area semiconductor optical amplifier

A high-efficiency noncollinear four-wave mixing in a broad area traveling-wave semiconductor optical amplifier is demonstrated. A key feature of the demonstrated configuration is the ability to isolate generated sidebands from the inputs. A wave-mixing efficiency of +8.2 dB with 22.6 dB of sideband isolation at 6° separation between the two pump beams was obtained. The dependence of the efficiency on pump detuning, separation angle, and probe wavelength was also investigated. Use of a high quality traveling-wave amplifier enabled us to achieve ripple-free broadband operation over 37 nm. The demonstrated configuration is a key to practical implementations of four-wave mixing based devices.

Dynamic beam propagation method for flared semiconductor power amplifiers

In this paper, we present the results of a dynamic two-dimensional time-domain beam propagation method (two spatial dimensions plus time) for simulating travelling wave semiconductor optical amplifiers with nonuniform transverse section in pulsed regime. Using the effective refractive index method, the paraxial approximation and a moving timeframe, we reduce the scalar wave equation to a nonlinear parabolic partial differential equation for the slow complex envelope of the electric field. Diffraction, refractive index dispersion up to the second order, gain saturation and gain dispersion are included in the model. We numerically solve the equation through a locally one-dimensional (LOD) method employing both finite-difference techniques and FFT-based pseudospectral methods.

Very high efficiency four-wave mixing in a single semiconductor traveling-wave amplifier

We report on high efficiency frequency-conversion obtained by four-wave mixing in a single traveling-wave semiconductor optical amplifier. Efficiency in excess of 0 dB is demonstrated for frequency conversion up to 2 THz. Measurements of the signal to the amplified spontaneous emission background ratio are also presented.

Four-wave mixing efficiency in traveling wave semiconductor optical amplifiers at high saturation

We report an experimental investigation of the conversion efficiency of a frequency converter based on four-wave mixing in semiconductor optical amplifiers. We find that the maximum of conversion efficiency has a nonlinear dependence on the unsaturated gain of the device. The observed features are explained by the presence of the Auger and bimolecular recombination.

Generation of transform-limited optical pulses using a combination of a gain-switched diode laser and a semiconductor optical amplifier

We introduce a novel method for generating nearly transform-limited picosecond optical pulses at multi-gigahertz repetition rates. Frequency-chirped optical pulses are first produced using a gain-switched semiconductor diode laser, and they are subsequently amplified by a semiconductor optical amplifier. The red frequency chirp in the gain-switched pulses is compensated for almost completely as a result of gain-saturation-induced self-phase modulation occurring in the amplification process. Experimentally, we have successfully generated nearly transform-limited 28-ps optical pulses at a 5-GHz repetition rate using a distributed feedback diode laser and a traveling wave semiconductor optical amplifier. >

Analysis of wavelength-switched high-density WDM networks employing wavelength conversion by four-wave-mixing in semiconductor optical amplifiers

In this paper transmission through an optical transparent network layer in which very high capacity data streams are processed without optical to electrical conversion is considered. Highly-dense WDM is assumed. The optical layer is superimposed to an electrical network layer in which the high capacity streams are processed at lower digital transmission hierarchies. Photonic switching is realized inside the optical layer by means of wavelength translators based on four-wave-mixing in traveling wave semiconductor optical amplifiers. The transmission performance of the network is theoretically analyzed exploiting an accurate model of the wavelength converter. ASE and phase noise accumulation along the signal path through the network, linear crosstalk and nonlinear effects during fiber propagation are taken into account. Due to the adopted method to perform wavelength conversion, chirping/dispersion induced penalties result to be greatly reduced and homo-wavelength crosstalk inside the network nodes is almost eliminated. This allows very good transmission performances to be attained even for very high capacity networks, as demonstrated by the reported theoretical results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Travelling wave semiconductor laser amplifier detector noise characteristics

The authors derive expressions for the small signal noise properties of the electrode current of a travelling-wave semiconductor optical amplifier when used as a detector. The analysis takes into account spatial variations in the amplifier carrier density and fields. Numerical simulations show that the dominant mechanisms giving rise to the detected current noise are spontaneous beat noise occurring within the amplifier gain medium, and the thermal noise of the load resistor and postamplifier. As the spontaneous emission and amplified signal are generated by the same shared gain medium, beat noises will contain not only beating within longitudinal modes but also beating between distinct modes. In previous analyses this &apos;extra&apos; noise was neglected which gave rise to errors between experiment and theory, The analysis also shows that there is an optimum amplifier gain which gives the best sensitivity. The bit error rate characteristics show good agreement with previously reported experimental results. Typically, for a 20 dB amplifier with 250 Mbit/s NRZ input data, sensitivities of the order of -27 dBm for a bit error rate of 10-9 can be achieved.

Fibre transmission of 10 Gbit/s signals followingwavelength conversionusing a travelling-wave semiconductor optical amplifier

The wavelength of an intensity-modulated signal at 10 Gbit/s was translated from 1546 to 1531 nm using cross gain-compression in a semiconductor optical amplifier. The shifted signal was transmitted with less than 1 dB penalty over a 121 km span of dispersion-shifted fibre with two in-line amplifiers. A dispersion penalty of nearly 2 dB is measured for transmission of 6 Gbit/s data over 20 km conventional fibre.

Practical method for modelling the nonlinear behaviour off a travelling wave semiconductor optical amplifier

The authors present a simple model of a travelling wave semiconductor optical amplifier with an appreciable degree of saturation. The model uses a particular way of linearising the total carrier recombination R(N) to find useful expressions for the saturation parameter and/or the carrier lifetime. By combining these expressions and the gain measurements realised, it is possible to establish the dependence between the mentioned parameters and the input optical power.

Distortion of analogue intensity modulated signals in semiconductor optical amplifiers

Distortion of analogue intensity modulated signals amplified by near travelling wave semiconductor optical amplifiers, which is observed even for input optical powers significantly lower than saturation, is shown to be strongly influenced by residual facet reflectivity. Harmonic distortion levels have been observed to vary with wavelength by more than 12 dB.

Simulation of 12.5 Gb/s lightwave optical time-division multiplexer using semiconductor optical amplifiers as external modulators

Using computer simulation techniques, a nonlinear model is developed for the use of traveling-wave semiconductor optical amplifiers as external modulators for multigigabit per second optical time-division multiplexed (OTDM) lightwave systems. The simulation results indicate that the modulation performance is sensitive to the precise time delay between the incident optical pulse and the electrical injection current pulse required to drive the traveling-wave semiconductor optical amplifier (TWSOA) external modulator. It is found that satisfactory modulation performance is obtained when the peak of the input optical pulse p/sub in/(t) coincides with the trailing edge of the injection current pulse I(t). The simulation results then indicate that, under this condition, adequate modulation performance at 2.5 Gb/s per channel, over a practically useful range of input signals, is possible provided that -10 dB >

Observation of highly nondegenerate four-wave mixing in 1.5 mu m traveling-wave semiconductor optical amplifiers and estimation of nonlinear gain coefficient

Nondegenerate four-wave mixing is measured in the 1.5 mu m traveling-wave semiconductor optical amplifier medium as a function of the pump-probe detuning frequency ranging from a few GHz to 400 GHz. It is found that two different sources are responsible for the four-wave mixing: the carrier density modulation and the nonlinear gain effect. The latter is clearly observed when the detuning frequency increases above 100 GHz. The nonlinear gain coefficient epsilon , which induces a gain grating through the pump-probe beating, is estimated to be 1.75*10/sup -23/ m/sup 3/. The change in the real refractive index associated with the nonlinear gain effect, which generates an index grating, is negligibly small. The relaxation time involved in the nonlinear gain effect is found to be less than 0.3 ps. These results support the role of the spectral hole burning rather than the carrier heating in the nonlinear gain effect.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Harmonic and phase distortion of analogue amplitude-modulated signals in bulk near travelling-wave semiconductor optical amplifiers

A theoretical model has been formulated using rate equations to analyse the response of a semiconductor optical amplifier to a single analogue amplitude-modulated input signal. The results are found to agree very closely with experimental results obtained from a bulk buried-heterostructure device, and show that signal distortion is significant even for input powers substantially below saturation. The level of distortion is strongly affected by both the saturation power of the device, and the Fabry-Perot resonance arising from residual facet reflectivity. On the basis of good agreement with experimental data, the theoretical model is used to assess a novel method of bias current feedback that reduces the effect of carrier density modulation by the modu lation of the input signal, and greatly improves the harmonic performance of the device. Using this feedback method, the calculated value of signal phase distortion has been reduced by a factor of up to 40, and the harmonic distortion by up to 11 dB.

Carrier-density modulation effects in a traveling-wave semiconductor optical amplifier: communications theory analysis and experiment

A communications theory equivalent model suitable for analyzing carrier-density modulation effects within a traveling-wave semiconductor optical amplifier is derived by linearizing the amplifier rate equation. This model is used to analyze dual-channel amplification, and it is shown to give results which are in good agreement with those given by nonlinear polarization theory and experimental measurements. The effect of four-wave mixing on the amplification of many optical channels for worst-case operating conditions is studied, and it is shown that the individual channels experience different gains. It is also shown that if the individual channel gains are to be confined to lie within a +or-1-dB band when operating the amplifier at high values of unsaturated power gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement and analysis of phase noise generated from semiconductor optical amplifiers

The phase noise generated from traveling-wave semiconductor optical amplifiers is measured. It is found that the phase noise is strongly correlated with the intensity noise and has bandwidth of 600 MHz. Phase-noise formulas are derived for semiconductor optical amplifiers to explain such phase-noise characteristics. It is shown that the phase noise is induced by the carrier density fluctuation associated with the intensity noise and the spontaneous carrier recombination. For the coherent heterodyne DPSK system, the phase noise, instead of the intensity noise, is found to be the limiting factor on the number of repeater amplifiers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of chirping-induced waveform distortion on the performance of direct detection receivers using traveling-wave semiconductor optical preamplifiers

The influence of chirping-induced waveform distortion on the performance of multigigabit-per-second traveling-wave semiconductor optical amplifier (TWSOA)/p-i-n direct detection receivers is evaluated. The results are based on a novel method of evaluating the probability of error in the presence of the signal-spontaneous and spontaneous-spontaneous beat noise components. Laser chirping causes the dependence of the receiver sensitivity on the fiber dispersion coefficient*length product DL to be different for TWSOA/p-i-n and avalanche photodiode (APD) receivers. Compared to the APD receiver, the sensitivity of the TWSOA/p-i-n receiver degrades less quickly. So for cases of practical interest, the TWSOA/p-i-n receiver is more tolerant of chirping-induced waveform distortion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sensitivity improvement of a 2 Gb/s quadrature phase shift keyed subcarrier multiplexed system by use of an optical preamplifier

A traveling wave semiconductor optical amplifier is used as a receiver preamplifier in a quadrature-phase-shift-keyed subcarrier multiplexed system. The intensity-modulated signal from a 1509-m distributed feedback laser bears two 1-Gb/s data signals on a 5-GHz subcarrier. The receiver preamplifier yields a 9.2-dB sensitivity improvement to -29.2 dBm. The system noise sources are measured. Theoretical and measured bit error rates versus received power are presented. >

Communications theory analysis of multichannel carrier density intermodulation distortion effects in a semiconductor optical amplifier

It is shown that, for worst-case conditions, four-wave mixing effects in a traveling-wave semiconductor optical amplifier can result in the individual carriers experiencing different gains depending on channel separation, multiplex size, and channel position. It is also shown that the highest-frequency carrier experiences the lowest gain and that the position of the minimum gain never exceeds Delta omega tau /sub e/>1. For worst-case multichannel system design, it is anticipated that four-wave mixing effects will be negligible if normalized channel separations >or=10 Delta omega tau /sub e/ (of order >or=3 GHz) are used, and it is envisaged that, for practical gigabit/second frequency multiplexed coherent systems, this will be automatically satisfied.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance degradations of multigigabit-per-second NRZ/RZ lightwave systems due to gain saturation in traveling-wave semiconductor optical amplifiers

A nonlinear model for a travelling-wave semiconductor optical amplifier has been used to determine eye closure degradations for 2.4 and 10 Gb/s NRZ/RZ lightwave systems due to gain saturation effects in the optical amplifier. At 10 Gb/s, with a carrier lifetime of 300 ps, the results indicate that the penalty is less than 1 dB for both NRZ and RZ systems provided that the ratio of the input power (P/sub in/) to the saturation output power (P/sub sat/) is less than -17 dB. The NRZ system penalty is slightly larger than the RZ penalty when P/sub in//P/sub sat/ is larger than -17 dB. For example, with P/sub in//P/sub sat/=-10 dB, the NRZ system penalty is about 2.8 dB versus 2 dB for the RZ system. The system penalty at 2.4 Gb/s is slightly less than that at 10 Gb/s. At P/sub in//P/sub sat/=-10 dB, the NRZ system penalty is about 2.5 dB versus 1.5 dB for RZ. >