High Speed Diode Lasers

The following sections are included:

• INTRODUTION

• CONTENTS

In this paper we summarize the most important recent advances and results in high speed diode lasers. These advances have primarily come about as a result of physical understanding of the properties that affect the dynamic performance of these lasers. A great deal of progress has been made in understanding the active region properties of such devices, including the electron and hole transport dynamics as well as the effect of active region doping and strain. At the very high frequencies characteristic of the highest speed lasers reported to date the microwave signal propagation becomes an important issue that can limit the laser bandwidth. The distribution of signals along the laser length due to these effects are discussed, analyzed, and measured, and conclusions about bandwidth and device operation are drawn from the analysis. All of these different issues are summarized in this paper from both a theoretical and experimental perspective. The laser structures that address and overcome the problems caused by such factors are presented along with the best results obtained to date.

The high speed direct modulation of semiconductor lasers has made tremendous progress since the early works of Suemune et al. that proposed for the first time the potential for semiconductor lasers to be directly modulated to frequencies approaching 60–90 GHz. Technical achievements as well as an improved understanding of the role of carrier dynamics in the modulation response have made much of these advances possible. This paper discusses the various methods that have been used to improve the direct modulation bandwidth over the last decade. An understanding of the theoretical and technical concepts that have led to the development of millimeter-wave directly modulated lasers is critical in understanding how these lasers can be further optimized for efficient operation in a number of optical communication links.

This paper describes the essential elements for creating a practical wide bandwidth directly modulated laser source. This includes considerations of the intrinsic limitations of the laser structure, due to the resonant frequency and damping of the laser output, together with carrier transport issues to allow carriers in the device active region to be efficiently modulated at high speeds. The use of a P-doped compressively strained multiple-quantum well active region to provide high intrinsic speed and remove transport limitations is described, together with record setting results of 25 GHz modulation bandwidth for a 1.55 μm Fabry–Perot laser and 26 GHz bandwidth for a 1.55 μm DFB laser. The challenges of providing high bandwidth electrical connections to the laser on a suitable submount, together with fiber attachment and microwave packaging are discussed. Results of fully packaged 1.55 μm DFB lasers with 25 GHz modulation bandwidth are shown. Digital modulation of the packaged 1.55 μm DFB including impedance matching is described, and the transient wavelength chirp is presented. This low chirp is reduced further using an optical filter, to provide a 10 GBit/s source that can transmit error free over 38.5 km of standard optical fiber.

The high speed performance of partly gain-coupled (GC) DFB lasers consisting of periodically etched strained-layer quantum wells (QW's) is reviewed with comparisons to the equivalent index-coupled (IC) DFB lasers with the same active layers. It is shown that the GC DFB laser has a – 3 dB modulation bandwidth of 22 GHz at 10 mW with a stable single mode oscillation at the longer side of the Bragg stop-band due to in-phase gain coupling. A theoretical analysis is also presented based on the local-normal-mode transfer-matrix laser model which takes into account both the longitudinal distribution of laser parameters and carrier transport effects. The mechanism for high modulation bandwidth of the GC DFB laser is attributed to a higher differential gain due to a reduced carrier transport time which is provided by an efficient carrier injection from the longitudinal etched interface of the QW's.

The paper reviews the recent progress in technology, theory and applications of diode lasers mode locked at microwave frequencies.

Recent experiments have shown that laser diodes are capable of generating pulses as short as 160 fs. thus approaching the performance of more conventional femtosecond lasers such as Ti:sapphire lasers or dye lasers. Regular progress is also accomplished in terms of power and energy efficiency or spectral and spatial brightness with the development of novel semiconductor laser structures. In this paper, we review decisive improvements which have placed laser diodes in category of sub-picosecond lasers. A number of experimental results are used to illustrate the specific characteristics of short-pulse laser diodes as compared to other laser systems. The effects of dispersion and phase modulation are discussed on a basis of theoretical model of chirp mode locking.

In this paper we discuss the possibility of confinement factor and dual (confinement & pumping current) modulation of laser diodes. In the initial experiment, four-terminal Fabry-Pérot lasers of ridge guide type with two additional side electrodes were tested. The possibility of controlling the lateral part of the optical confinement factor by applying an external voltage to the side contacts was proved in this experiment. To analyze the dynamic behavior of the four-terminal laser under confinement factor and dual modulation, a simple model and a more accurate new model were developed, the last one based on the simultaneous solution of laser rate equations and matrix equation relating potentials and currents. The simulations carried out on the basis of the advanced model allowed us to propose an optimized four-terminal laser structure promising enhanced performance under dual modulation. It is shown that dual modulation may result in very efficient correction of output waveforms which leads to a much better opening of transmitted eye patterns at high bit rates as compared to direct modulation.