Semiconductor Laser Amplifier Research Articles

Pulse energy gain saturation in subpico- and picosecond pulse amplification by a traveling-wave semiconductor laser amplifier

Optical pulses with durations ranging from 0.49 to 21 ps are amplified by a traveling-wave semiconductor laser amplifier. The pulse energy gain is determined by pulse energy only. The dependence of pulse energy gain on output pulse energy does not change in the pulse duration range. The saturation characteristics are successfully explained by a four-level system model. >

Lightwave systems with optical amplifiers

Fiber-optic communication systems using semiconductor laser amplifiers are investigated theoretically and experimentally. The noise and bit-error-rate characteristics of lightwave systems with optical amplifiers are calculated and the dependence of system performance on amplifier characteristics such as optical bandwidth, noise figure, gain, etc., is shown. Experimental results for both a 4-Gb/s optical preamplifier as well as coherent and direct detection systems with four inline amplifiers are presented. >

Crosstalk and its power penalty in multichannel transmission due to gain saturation in a semiconductor laser amplifier

A theoretical study of channel crosstalk due to gain saturation in a laser amplifier in multichannel transmission is discussed. When the amplifier is gain-saturated, the gain of any channel is influenced by the light intensity of the other channels, which causes channel crosstalk in intensity-modulated multiplexing transmission. On-off-keying modulation systems with more than two channels are discussed. Considering the probability distribution, the power penalty due to crosstalk is presented for practical system design. Calculated examples reveal that the power penalty is dependent on both the level of gain saturation and the number of multiplexed channels. >

Wave-optical effects in semiconductor laser amplifiers

A wave-optical model is used to investigate the power scaling of semiconductor laser amplifiers. The configurations investigated are single-pass amplifiers, double-pass amplifiers, phase-conjugate double-pass amplifiers, regenerative amplifiers, and injection-locked lasers. We are interested in a comparison of conventional double-heterostructure and quantum-well gain media, the factors affecting optical power extraction, and conditions leading to filamentation in the laser beam.

Tunable optical bistability of bichromatic fields interacting with four-level systems

A simple four-level model is used to describe two-band systems such as dyes and semiconductors. The system is irradiated by a bichromatic input in an optical cavity and both fields exhibit optical multistability. The calculations take into account the possibility of noncoherent (electrical) pumping of the system, thus generalizing the model to include semiconductor laser amplifiers. Since both fields are allowed to resonate in the cavity, a low-intensity control beam and/or the noncoherent pumping intensity can 'tune' and adapt the response of the second input. Some applications of this tunability are outlined. >

Amplification and compression of weak picosecond optical pulses by using semiconductor-laser amplifiers

A novel pulse-compression scheme is proposed and demonstrated for compressing weak picosecond pulses having energies as small as 0.1 pJ. A semiconductor-laser amplifier is shown to impose a nearlylinear frequency chirp on the pulse as a result of self-phase modulation occurring because of gain-saturation-induced index changes. The amplified chirped pulse can be compressed by passing it through a dispersive delay line. The theory shows that, depending on the operating conditions, the pulse width can be reduced by a factor of ~2-4 and the peak power can be enhanced by a factor of ~10-100. The preliminary experimental results are in agreement with theory.

Coherent detection of 565 Mbit/s DPSK data using semiconductor laser amplifier as phase modulator

Coherent optical detection and demodulation of DPSK data, impressed using a semiconductor laser amplifier as the phase modulator, has been demonstrated. When the amplifier was modulated by a pseudorandom bit sequence of length 27−1 at 565 Mbit/s, the received power was −39.1 dBm for a biterror ratio of 10−9.

Subcarrier multiplexed transmission using wavelength division multiplexing and optical amplifier

Measured bit-error-rate performance is reported for a wavelength-division-multiplexed subcarrier-multiplexed transmission experiment. Six digital channels were carried over two wavelengths in the 1.5µm wavelength region and amplified using a semiconductor laser amplifier which gave a 12dB increase in tolerable path loss with no noticeable penalty.

Direct optical phase modulation in semiconductor laser amplifier

Efficient optical phase modulation in a nearly travelling-wave semiconductor laser amplifier, via modulation of the amplifier injection current, is proposed and demonstrated. Using a 500 mu m-long GaInAsP buried heterostructure amplifier and a 1530 nm optical carrier, phase deviations of the order of 0.1 rad/mA are obtained for modulation frequencies in the range 100 MHz to 2 GHz. >

High-speed and high-gain optical amplifying photodetection in a semiconductor laser amplifier

A high-speed and high-gain photodetection was experimentally demonstrated in a distributed feedback semiconductor laser amplifier by detecting its forward voltage changes induced by the injection of weak intensity modulated light. Physical phenomenon involved in the photodetection stems from the reduction of quasi-Fermi level separation through the depletion of carrier density by optical amplification. A bandwidth as wide as 500 MHz was measured together with a narrowband spectral selectivity for the amplifier biased just below the lasing threshold. The responsivity and the noise equivalent power were found to be comparable with those of the conventional detection scheme using a similar optical preamplifier and a Ge avalanche photodetector.

Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52- mu m mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarisation-insensitive, near-travelling-wave semiconductor laser amplifiers at 1.5 [micro sign

A polarisation-insensitive, near-travelling-wave semiconductor laser amplifier (TWSLA) is presented. Polarisation sensitivity of less than 1 dB is demonstrated. We believe this to be the lowest reported polarisation sensitivity for a TWSLA. In addition, the saturated output power is in excess of 5.2 dBm.

Gain and noise characteristics of a 1.5 μm near-travelling-wave semiconductor laser amplifier

The spectral dependence of the gain and noise characteristics of a gas source molecular beam epitaxy (GSMBE)-grown BH-GaInAsP near-travelling-wave semiconductor laser amplifier are presented, from which an indication of nonresonant internal losses is given. An intrinsic noise figure of 6 dB at an internal gain of 28 dB is obtained.

Influence of extinction ratio on performance of optical receivers incorporating laser preamplifiers

We report the performance degradation of optical receivers incorporating semiconductor laser amplifiers (SLAs) caused by the nonzero extinction ratio of the input optical signal. The resulting sensitivity penalty dependencies on bit rate, SLA gain and facet reflectivity are also investigated. The results clearly show that the new receiver can be more affected by a finite extinction ratio compared to a conventional pin or APD receiver. Using a bandpass optical filter to reduce amplifier spontaneous emission noise will increase the extinction ratio penalty.

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>

546 km, 140 Mbit/s FSK coherent transmission experiment through 10 cascaded semiconductor laser amplifiers

The effect of the spontaneous emission noise on coherent systems in a multistage semiconductor laser amplifier repeater system is experimentally examined. By appropriate adjustment of the input power level to each amplifier, 546 km FSK transmission at 140 Mbit/s using 10 cascaded amplifiers is demonstrated without power penalty.

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.

Modelling spectral effects of dynamic saturation in semiconductor laser amplifiers using the transmission-line laser model

Previously, the transmission-line laser model (TLLM) was shown to be a good model for ultra-short pulse propagation in semicondcutor laser amplifiers under heavy saturation. However, the spectral shift associated with dynamic saturation has not yet been considered. The paper shows that the TLLM correctly predicts frequency shifts caused by index changes during amplification.

Semiconductor laser amplifier optimization: an analytical and experimental study

Long-wavelength semiconductor laser amplifiers are investigated with respect to spectral gain properties such as peak gain wavelength shift and width of gain curve, employing different structural parameters such as thickness of the active layer and amplifier length. The model takes into account Auger recombination, thermal effects, and spontaneous emission. It is shown that there exists an optimum thickness of the active layer with respect to current density for a given gain and that increased length of the amplifier allows higher gains and reduced variation of peak gain wavelength with respect to variation of peak gain at the expense of increased saturation by amplified spontaneous emission and increased excess noise. An experimental verification of the theoretical model is reported. >

Pulse compression mechanisms in semiconductor laser amplifiers

Semiconductor laser amplifiers can exhibit either temporal pulse compression or broadening. A transmission-line laser model was used to isolate the mechanisms responsible and to investigate the conditions required for this form of pulse distortion. Results show that gain saturation is the dominant temporal compression mechanism in travelling wave laser amplifiers, and that filtering may account for some compression in Fabry-Perot laser amplifiers. Compression is highly dependent on the input pulse shape, being zero for Gaussian-like pulses. This means that it is unlikely to be useful for shortening pulses produced by highly modulated injection lasers. However, compression can occur with pulses with a slowly decaying trailing edge, but only over a limited range of input powers. Pulse broadening is also caused by gain saturation and is again dependent on the input pulse shape, being zero for square-edged pulses. For Gaussian pulses, the broadening is negligible for input pulse energies below the amplifier's saturation energy. This suggests that TWLAs may be used to boost the power of pulses from mode-locked semiconductor lasers. These results are in excellent agreement with other worker's experimental results. >