We review the present status of the rapidly developing field of semiconductor laser diodes based on self-organized quantum dots (QDs). Several milestones have been achieved since the first realization of such a device in 1994: above room-temperature cw operation, the lowest threshold current density of any semiconductor laser diode, high temperature stability, an extended wavelength range on GaAs substrate and high power operation. After a brief introduction we discuss the tremendous advances in epitaxial growth, device performance, and theoretical understanding of QD lasers. Three applications of large commercial interest are discussed in detail: 1300 nm QD lasers on GaAs substrate, QD surface emitting lasers, and high power QD lasers. Finally, we give an outlook on future developments.

The present status of quantum dot lasers

By analyzing the propagating behavior of the supermodes in a coupled-waveguide system, we have derived a universal criterion for designing adiabatic mode transformers. The criterion relates e, the fraction of power scattered into the unwanted mode, to waveguide design parameters and gives the shortest possible length of an adiabatic mode transformer, which is approximately 2/πe1/2 times the distance of maximal power transfer between the waveguides. The results from numerical calculations based on a transfer-matrix formalism support this theory very well.

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Adiabaticity criterion and the shortest adiabatic mode transformer in a coupled-waveguide system

An apparatus for monitoring and controlling the wavelength of laser radiation includes at least one optical filter for receiving laser radiation and for transmitting and reflecting first and second filtered beams, respectively. Alternatively, the first and second beams may be transmitted by separate filters (32, 34). The beams are filtered according to respective first and second spectral filter functions that cross at least one crossing wavelength. A beam comparison element (20) compares the first and second filtered beams (36,38) and produces an error signal (22) representative of the deviation of the wavelength of the laser radiation from a set-point wavelength. The beam comparison element (20) can include first and second optical detectors (40, 42) and error signal (22) by taking a ratio or the difference of the signals detected by the detectors. Varying the angle of incidence of the laser radiation upon at least one optical filter varies the spectral filter function of that filter for selecting or varying the operating wavelength of the laser (12). A laser wavelength controller (24) can receive the error signal (22) for stabilizing or tuning the wavelength of the laser radiation. Several embodiments of the invention are disclosed.

Method and apparatus for monitoring and control of laser emission wavelength

We present the first optical study of 2-D photonic crystals (PCs) deeply etched in an InP/GaInAsP step-index waveguide. Following the same internal light source approach proposed by Labilloy et al. (1997,1999) for the investigation of GaAs-based 2-D PCs, transmission measurements through simple PC slabs and 1-D Fabry-Perot (FP) cavities between PC mirrors were performed. Details are given on the experimental setup which has been implemented with respect to the original scheme and adapted to InP-based systems working at 1.5-/spl mu/m. 2-D plane-wave expansion and finite difference time-domain (FDTD) methods are used to fit the experimental data. Out-of-plane losses were evaluated according to a recently introduced phenomenological model. In spite of the complex hole morphology in the measured samples, preliminary results are presented which indicate the possibility of separating different loss contributions from finite etch depth and hole shape. As for 1-D cavities, both FDTD and classical theory for planar resonators are applied in order to deduce the optical properties of the PC mirrors. The origin of an anomalously high transmission observed inside the stopgap is discussed and arguments are given to demonstrate the need for further modeling efforts when working in the bandgap regime.

Optical study of two-dimensional InP-based photonic crystals by internal light source technique

Reference LOEQ-CONF-2001-009View record in Web of Science Record created on 2007-08-31, modified on 2017-05-12

We present an efficient method for an almost arbitrary variation of the complex coupling coefficient of distributed feedback lasers by using superstructure gratings, As shown by detailed transfer matrix model calculations the coupling coefficient can be reduced locally according to the duty cycle of a sampled grating. The additional supermodes caused by the superstructure can be strongly suppressed by choosing a small superperiod so that the supermodes are shifted apart from the gain curve. This novel approach for the realization of nonuniform coupling coefficients is applied to analyze experimentally the influence of the complex coupling strength on the performance of loss coupled InGaAs-InGaAlAs-InP-DFB lasers. >

Variation of coupling coefficients by sampled gratings in complex coupled distributed-feedback lasers

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

We have realized laser diodes with integrated spot-size transformer to achieve a high-coupling efficiency to a standard single-mode fiber (SMF) without microoptical elements. A coupling loss of about 1 dB has been achieved with tolerances of 12 /spl mu/m in both vertical and horizontal direction. We have fabricated both distributed-feedback (DFB) and Fabry-Perot (FP)-type lasers. In the case of the FP lasers the longitudinal single-mode emission was stabilized by means of an external fiber Bragg-grating which was directly butt-coupled to the laser. We have experimentally and theoretically investigated the optical and electrical properties of the devices using a transmission line model.

Laser diodes with integrated spot-size transformer as low-cost optical transmitter elements for telecommunications

The characteristics of loss-coupled distributed feedback (DFB) semiconductor laser arrays are investigated both theoretically and experimentally. Using simulations based on a transfer matrix method, the strong influence of the residual facet reflectivity on the singlemode yield and the statistical fluctuation of the emission wavelength for as-cleaved and AR/HR coated loss-coupled DFB lasers is pointed out and compared to purely index-coupled /spl lambda//4 phase-shifted devices. Experimental results and the fabrication techniques are given for loss-coupled 1.55 /spl mu/m InGaAs/InGaAlAs/InP DFB laser arrays with four channels and integrated striped thin-film heaters, which were successfully used for fine tuning the channel spacings.

E. Kuphal

Papers

Variation of coupling coefficients by sampled gratings in complex coupled distributed-feedback lasers

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

Laser diodes with integrated spot-size transformer as low-cost optical transmitter elements for telecommunications

Properties of loss-coupled distributed feedback laser arrays for wavelength division multiplexing systems

Very high compositional homogeneity of 1.55 μm strain-compensated InGaAsP MQW structures by MOVPE under N2 atmosphere