The phenomena occurring in semiconductor lasers due to weak external optical feedback are reviewed, including mode hopping and related mode-hopping noise, linewidth narrowing and broadening, and the transition to the coherence collapse regime. Guidelines are given for designing semiconductor lasers, both edge emitters and VCSEL's, with high endurance against external optical feedback. >

External optical feedback phenomena in semiconductor lasers

The basic equations governing noise phenomena are derived from first principles and applied to examples in optical communications. Quantum noise arises from two sources, the momentum fluctuations of electrons at optical frequencies and the uncertainty-related fluctuations of the electromagnetic field. Shot noise results from the beating of the noise sources with the signal field. In high-gain amplifiers, the spontaneous-emission noise dominates shot noise and results in a noise figure of at least 3 dB. It is shown explicitly how, at high power, both the laser field and the laser noise source become classical. The increase in noise in lasers with open cavities is not due to enhanced spontaneous emission as once thought, but to single-pass amplification. The noise fields and spontaneous currents have Gaussian distributions, while nonlasing modes have exponential photon-number distributions. Low-frequency intensity fluctuations arise from the electric current driving the laser and can be sub-Poissonian, in contrast to shot noise, which has a Poissonian distribution. The calculational tools are a wave equation for the field operator and a rate equation for the carrier-number operator, each containing spontaneous current noise sources. The correlation functions of these sources are determined by the fluctuation-dissipation theorem. [S0034-6861(96)00503-X]

Quantum noise in photonics

A detailed nonlinear model of the performance of single-frequency laser structures operating both below and above threshold is discussed. Arbitrary series combinations of uniform pitch grating, linearly-chirped pitch grating and plane guide sections can be analyzed-taking longitudinal-mode spatial hole burning into account. The material properties of each section of a device structure can be specified. The output power from each end of the laser and the emission wavelength, and the longitudinal intensity, carrier density, and relative permittivity profiles are predicted as a function of drive current above threshold for each lasing mode. The linewidth is also estimated while allowing for the nonuniform longitudinal distribution of spontaneous emission into the mode in a physical manner. The model was used to provide the data with which to trade off the numerous interacting performance parameters of a lambda /4 phase-shifted DFB (distributed-feedback) laser. The design options are then summarized. >

The design assessment of lambda /4 phase-shifted DFB laser structures

We have developed an L-band tunable distributed feedback laser array (TLA) with a new design to reduce the spectral linewidth. A wide wavelength tuning range of ~ 40 nm is obtained with a high fiber output power of 20 mW and a high side-mode suppression ratio of >50 dB in the TLA module. A narrow linewidth of less than 580 kHz is achieved over the entire tuning range. Furthermore, we investigated the causes of linewidth variation. We found that a TLA with a longer cavity is more tolerant to external feedback, which reduces the variation in linewidth.

Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array

The influence of longitudinal mode spatial hole burning (LMSHB) on the performance of distributed feedback (DFB) laser structures is examined in detail. A comprehensive model has been used to interpret the experimental results and to construct a theoretical framework that was utilized to develop more advanced device designs. An increasing side mode intensity with output power, movement of the lasing mode relative to the stopband, and curvature of the light-current characteristic at low power can all be manifestations of the influence of LMSHB on the static device performance. The dynamic behavior can also be affected, with extended wavelength chirp and amplitude patterning effects on the timescale of the effective carrier recombination time being particularly important. >

The static and dynamic characteristics of single and multiple phase-shifted DFB laser structures

To calculate the linewidth for an index-guided semiconductor laser, one usually neglects a correction factor for the spontaneous emission rate, which is introduced by the longitudinal field distribution within the laser cavity. For FabryPerot lasers with cleaved facets the correction factor is small. However, for DFB lasers this correction factor may become quite significant, yielding a linewidth enhancement for DFB laser diodes.

J. Wang

Papers

Linewidth enhancement for DFB lasers due to longitudinal field dependence in the laser cavity