Traveling-wave Laser Amplifier Research Articles

Effects of Spectral Broadening and Cross Relaxation on the Gain Saturation Characteristics of Quantum Dot Laser Amplifiers

Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-µm InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.

Photon point process for traveling-wave laser amplifiers

The authors determine the evolution of the photon point process of a light beam as it passes through a traveling-wave laser amplifier (TWA). In particular, when coherent light is presented to the input of the amplifier, the output photon statistics are characterized by a marked-Poisson (MP) point process, which has a noncentral-negative-binomial (NNB) output photon-number distribution (PND). Using this distribution we calculate the probability of error (PE) for an ON-OFF keying (OOK) direct-detection photon-counting communication system, and show that the results differ somewhat from those obtained when the Gaussian-PND approximation is used. It is shown that receiver performance is optimized by filtering the amplifier output. Analysis of the point process is of interest because it permits the time response of the amplifier to be determined; this, in turn, allows the effects of intersymbol interference to be calculated. >

Optical switching in the twin-guide travelling-wave laser amplifier

Detailed calculations have been made of the optical switching behaviour in twin-guide travelling-wave laser amplifiers driven with equal current in each guide. For typical device parameters, appropriate to 1.55 μm InGaAsP devices, the analysis indicates that the switching occurs at about one quarter of the saturated power, which in practice means at about 1 mW for an unsaturated gain of 26 dB. The factors influencing this switching power are discussed and some suggestions offered as to ways of reducing the value.

Real-time in situ monitoring of antireflection coatings for semiconductor laser amplifiers by ellipsometry

Very strict requirements need to be met for producing a high-quality single-layer antireflection coating on a traveling-wave laser amplifier facet. In order to obtain a facet reflectivity of 10/sup -4/ or less, the index of refraction and the layer thickness of single-layer coatings have to be controlled to better than 0.03 and 30 AA, respectively. An innovative approach to highly controlled antireflection layer deposition based on in situ real-time ellipsometry is presented. Index control within +or-0.01 and a facet reflectivity on the order of 10/sup -4/ are reproducibly obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evolution of the statistical properties of photons passed through a traveling-wave laser amplifier

The authors determine the evolution of the photon statistics of a light beam as it passes through a traveling-wave laser amplifier, modeled as a birth-death immigration (BDI) medium. The relationship between the input and output probability distributions and probability generating functions with given (but possibly varying) birth, death, and immigration rates for arbitrary input statistics is obtained. The case of constant birth, death, and immigration rates is considered in particular detail. The photon statistics at the output of a general BDI traveling-wave amplifier are always broader than those at the input, and they can take many forms. The most general solution can be applied when the input distribution to the amplifier takes the form of a negative-binomial transform.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase noise in traveling-wave inverted-population-type optical amplifiers

The Heisenberg equations of motion are used to calculate the phase noise of inverted-population-type traveling-wave laser amplifiers generally and semiconductor amplifiers specifically. Agreement with previously reported results in the limit of perfect inversion is demonstrated.

Transmission-line laser modelling of semiconductor laser amplified optical communications systems

Previously, the transmission-line laser model (TLLM) has been used to simulate the optical noise characteristics of semiconductor lasers and semiconductor laser amplifiers in the time-domain. The paper shows that, with the addition of a suitable photodiode model, the base-band electrical noise characteristics of amplified optical systems can also be simulated with the TLLM. A simple theory of the baseband noise properties of a detected optical signal is presented and photodiode model for the TLLM is developed. The baseband electrical noise spectra of travelling-wave, Fabry–Perot and DFB laser amplifiers are numerically simulated, and the travelling-wave laser amplifier spectrum is in excellent agreement with simple theory. This new approach to amplified optical communications system modelling allows the effects of different optical and electrical filters on the signal and the noise to be studied simultaneously over a wide bandwidth. As an example, a 10 Gbit/s system is simulated, and the effects of the optical filter on the signal and the noise are investigated.

Pitchfork bifurcation and all-optical flip-flop operation with a coupled traveling-wave amplifier system

Mirrorless optical bistability arising through pitchfork bifurcation is predicted in a coupled anisotropic traveling-wave laser amplifier system. It is shown theoretically that an all-optical flip-flop operation can be attained by utilizing the complementary bistable characteristics of counterpropagating beams.

Limits on amplification of picosecond pulses by using semiconductor laser traveling-wave amplifiers

The limiting factors in picosecond pulse amplification by semiconductor traveling-wave laser amplifiers are investigated, including the effects of the spontaneous emission. For the amplifier with a negligible amount of spontaneous emission, amplification is limited by the energy saturation for low-repetition-rate pulses, while the power saturation is the limiting factor for high-repetition-rate pulses. The spontaneous emission decreases both the signal gain of the amplifier and the signal-to-noise ratio considerably. The effective gain recovery time and the effective saturation energy are strong functions of the input pulse energy, and the amount of the spontaneous emission. >

Low noise characteristics of a GaAs-AlGaAs multiple-quantum-well semiconductor laser amplifier

Low noise characteristics are experimentally demonstrated in a multiple-quantum-well laser amplifier. The measured noise figure of a GaAs-AlGaAs traveling-wave laser amplifier at a 20 dB signal gain was found to be 4.6 dB. This is the smallest value reported for semiconductor laser amplifiers. A signal gain of 20 dB was obtained at a low bias current of 23.5 mA. >

Applications of traveling-wave laser amplifiers in subcarrier multiplexed lightwave systems

Describes applications of traveling-wave laser amplifiers (TWLAs) in subcarrier-multiplexed distribution systems for both frequency-modulated (FM) and amplitude-modulated vestigial-sideband (AM-VSB) video signal distribution. The characteristics of a 1300-nm facet-angled and antireflection-coated TWLA are described. Analytical carrier-to-noise ratio (CNR) characterizations of subcarrier-multiplexed systems employing TWLAs are given. Experimental results of using TWLAs in multichannel FM and AM-VSB video signal systems are described. The potential optical-reflection problems associated with the residual reflectivities of optical fiber connectors and splices while using TWLAs are briefly described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Detuning characteristics and conversion efficiency of nearly degenerate four-wave mixing in a 1.5- mu m traveling-wave semiconductor laser amplifier

The optical nonlinearity in a semiconductor laser gain medium is investigated through copropagating nearly degenerate four-wave mixing (NDFWM) in a 1.5- mu m InGaAsP traveling-wave laser amplifier (TWA). The FWM signal output powers vary symmetrically with the sign of probe detuning with respect to the pump frequency, while the pump and probe output powers vary asymmetrically. The NDFWM conversion efficiency from the probe input to the FWM signal output is a maximum of 8.3 dB around zero detuning and has positive gain in the range of +or-6 GHz. This demonstrates highly efficient nonlinear interaction due to both large optical gain and large third-order susceptibility. The NDFWM efficiency is also investigated in connection with the TWA gain saturation characteristics and is found to be a maximum for operation around the saturation intensity of the TWA. >

Stabilisation of mode-locked pulses using travelling-wave semiconductor laser amplifier

Numerical and experimental results show, for the first time, that a saturated travelling-wave laser amplifier (TWLA) can be used to amplify and limit the amplitudes of pulses from an actively mode-locked semiconductor laser while removing unwanted secondary pulses.

Theoretical analysis of nonlinear optical phenomena taking into account the beating vibration of the electron density in semiconductor lasers

Nonlinear optical phenomena in semiconductor lasers are induced by two types of mechanisms. The first mechanism is beating vibration (i.e., pulsing modulation) on spectral distributions of injected carriers (i.e., electrons and holes) due to lasing frequencies, which is observed as the spectral hole burning effect and whose relaxation is characterized with the intraband relaxation time on the order of 10−13 s. This effect occurs even when the numbers of injected carriers are constant. The second mechanism is beating vibration on the number of injected carriers, whose relaxation is characterized with the electron lifetime on the order of 10−9 s due to band-to-band transition. Master equations including these two nonlinear phenomena are obtained in this paper in a general form based on the density matrix formalism. As examples of the equations, characteristics of the saturated gain profile in a laser oscillator and the four-wave mixing effect (or generation of the phase-conjugate wave) in a traveling-wave laser amplifier are analyzed. A saturated gain profile on optical frequency in a laser oscillator contains both symmetric and asymmetric properties induced by the first and the second mechanisms, respectively, where the strengths of these properties are almost the same. While in the case of the four-wave mixing effect in a traveling-wave laser amplifier, the second mechanism is more effective than the first mechanism. These results coincide well with previous analyses by other authors which treated the above-mentioned phenomena individually.

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. >

The properties of a GaAs-GaAlAs traveling-wave laser amplifier

Abstract The reflectivity of the cavity mirrors of a GaAs-GaAlAs DH semiconductor laser are reduced to 1∗10−4 by sputtering a film Si3N4. The gain of traveling-wave laser amplifiers is measured. The highest gain is about 25 dB. The dependence of gain on the injection current is measured. The measured frequency bandwidth shows the great advantage over other types of semiconductor laser amplifiers.

A comparison between Fabry-Perot and travelling wave laser amplifiers in an 8 Gbps repeatered optical system using a time domain model

A time domain model is used to investigate the use of semiconductor laser amplifiers in repeatered communication systems. This is based on the transmission line laser model and allows modelling over a wide optical bandwidth. Simulations show that the travelling wave laser amplifier provides a high gain and large dynamic and optical bandwidths, whereas the Fabry-Perot device has a smaller gain and optical bandwidth. The filtering action of the Fabry-Perot device is shown to distort the signal in both the time and frequency domains.

Travelling-wave Brewster-angled Stripe InGaAsP Laser Amplifier at 1·3 µm

Optical amplification of a Brewster-angled stripe travelling-wave laser amplifier operating at 1·3 µm is described. An actively mode-locked external cavity semiconductor laser generating pulses of ...

Multichannel FM video transmission using travelling-wave laser amplifier in 1300 nm subcarrier multiplexed optical system

A 1300 nm travelling-wave laser amplifier has been employed to provide a power gain of 10 dB in a subcarrier multiplexed optical communication system which carried 10 broadcast quality C-band satellite FM video signals.

Homogeneous gain saturation in 1.5 μm InGaAsP traveling-wave semiconductor laser amplifiers

Spectral line broadening in semiconductor lasers is studied experimentally through the gain saturation characteristics of 1.5 μm InGaAsP traveling-wave laser amplifiers. Wide signal gain spectrum under the injection of an intense saturating signal is measured using a weak probe signal. The saturated signal gain spectrum is found to coincide exactly with the unsaturated spectrum under a less biased condition, thus verifying that the semiconductor laser gain saturates homogeneously over the entire gain spectrum. Cross-saturation characteristics between the two signal channels having identical input powers are also investigated and found to be in good agreement with theoretical calculations based on the homogeneous gain model. The degree of gain saturation is confirmed to be uniquely determined by the total output power from both channels.