G. Pakulski

External cavity InAs∕InP quantum dot laser with a tuning range of 166nm

We have studied the tuning behavior of an external cavity laser in Littrow configuration using antireflection/high-reflection coated InAs∕InGaAsP∕InP quantum dot laser diodes as the amplifying element. Adding the coatings improves the performance of the setup, and the tunability of the external cavity laser output has been increased up to 166nm. Detailed investigations have revealed that laser diode length and width influence the magnitude of the tuning range. Furthermore, the external differential quantum efficiency is systematically increasing as the external cavity laser wavelength is decreasing. These characteristics are discussed in terms of energy levels available in the inhomogeneous broadening of the self-assembled quantum dots.

An L-band monolithic InAs/InP quantum dot mode-locked laser with femtosecond pulses

We have developed an InAs/InP quantum dot (QD) gain material using a double cap growth procedure and GaP sublayer to tune QDs into the L-band. By using it, a passive L-band mode-locked laser with pulse duration of 445 fs at the repetition rate of 46 GHz was demonstrated. The 3-dB linewidth of the RF spectrum is less than 100 KHz. The lasing threshold injection current is 24 mA with an external differential quantum efficiency of 22% and an average output power of 27 mW. The relationship between pulse duration and 3-dB spectral bandwidth as a function of injection current was investigated.

Gain and lifetime of GaInNAsSb narrow ridge waveguide laser diodes in continuous-wave operation at 1.56μm

The continuous-wave (cw) operation of GaInNAsSb lasers at 1560nm is reported. Light-current measurements were made before and after a 100h cw burn in at 20°C, during which a 3×890μm2 device with 72mA initial threshold current and 14mW maximum output power experienced a 15% drop in peak output power. These preliminary lifetime results provide insight into the reliability of GaInNAsSb active regions and reinforce the promise of this material for C-band devices. High-resolution modal gain spectra were extracted from the amplified spontaneous emission spectra acquired after the burn in, providing reliable values for the internal loss, transparency current, and differential gain.

Low noise InAs/InP quantum dot C-band monolithic multiwavelength lasers for WDM-PONs

A 50-GHz-spacing C-band multiwavelength laser (MWL) module based on an InAs/InP quantum-dot (QD) Fabry-Perot cavity has been developed for the first time. We have evaluated its system performance to show our QD-MWL potentials for WDM-PONs.

Ultra-broadband quantum-dot semiconductor optical amplifier and its applications

We have developed an ultra-broadband InAs/InGaAsP quantum-dot semiconductor optical amplifier around 1520 nm with the 3-dB bandwidth of 150 nm. The four-wave mixing process and multi-wavelength lasers have been demonstrated by using our QD-SOAs.

Pressure tuning of GaInNAs laser diodes in external cavity

High hydrostatic pressure can be used for wavelength tuning of semiconductor laser diodes in a wide spectral range. Coupling the laser with external grating leads to wavelength tuning within the gain spectrum (i.e. in a narrower range than with pressure) but allows for a narrow emission line and nearly continuous tuning (mode-hop free if anti-reflecting coating is applied). Here we demonstrate a combination of pressure and external-resonator tuning for the GaInNAs laser emitting at 1343 nm at ambient conditions. Using the specially designed liquid pressure cell working up to 20 kbar we shift the emission down to 1170 nm while the external grating (used in Littrow configuration) allows for fine tuning in the ~10 nm range (at each pressure).

Intrinsic Dynamic Properties of High-Characteristic Temperature GaInNAs Laser Diodes Operating at 1.3

The properties of a high-characteristic temperature (T0=155K) 1.3-mum GaInNAs-GaAs laser are presented with an emphasis on laser dynamic characteristics evaluated by linewidth enhancement factor and relative intensity noise. It is found that the relatively high differential gain of GaInNAs-GaAs quantum wells leads to a small linewidth enhancement factor of 2.8, indicating a small magnitude of frequency modulation with modulation current. The relative intensity noise measurements indicate a relaxation frequency of 4.7 GHz at a moderate bias current, from which the maximum intrinsic modulation bandwidth was calculated to be 9.7 GHz. The experimental determination of the low linewidth enhancement factor and high relaxation frequency reinforce the potential of dilute nitride lasers for high-speed directly modulated fiber links

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The room-temperature 1.55 &mgr;m continuous-wave (CW) operation of single-lateral mode GaInNAsSb ridge waveguide lasers grown on GaAs is reported. Detailed measurements of the light output power and spectral properties were used to assess the device characteristics as a function of applied current and temperature in both CW and pulsed operation. An exemplary, 3&mgr;×750&mgr;m, device with a 92% high-reflectivity back facet coating exhibited a record low CW threshold current of 63~mA, with a peak output power of 15~mW. High-resolution modal gain spectra were extracted from amplified spontaneous emission measurements yielding the internal loss (8.0~cm-1, transparency current (50~mA) and the wavelength dependence of the differential gain. The latter was used with careful measurements of the Fabry-Perot mode shift with injection current to determine the linewidth enhancement factor of 2.8 at the transparency current. The first measurement of intrinsic modulation frequency in 1.55 &mgr;m GaInNAsSb lasers is reported, based on the observed relative intensity noise (RIN). The RIN measurements indicate a maximum modulation frequency of 7.2~GHz, which is a promising result for future telecommunications applications.

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We have designed, fabricated and characterized self-assembled InAs/InGaAsP QD-waveguide devices around 1.55 μm. In order to obtain optimal performance, we have investigated several QD-based semiconductor optical amplifiers (SOAs) / lasers with different core geometry and doped profiles. To make the fair comparison between QD-SOA and QW-SOA, InAs/InGaAsP QW-SOAs with the same structure and the doped profiles have been designed and characterized. The experimental results indicate the QD-SOA is much better than QW-SOA in term of optical spectral bandwidth, temperature sensitivity and output power stability. The 3-dB and 10-dB bandwidths of the amplified spontaneous emission (ASE) spectra of the QD-SOA are 150 nm and 300 nm around 1520 nm. By using CW pump and probe signals we have demonstrated a non-degenerated four-wave mixing (ND-FWM) process and the experimental results indicate that the asymmetry of the FWM conversion efficiencies is eliminated by using the QD-SOA. To make use of the inhomogeneous broadening which is one of the specific properties of QD waveguide devices, we have designed and investigated the QD-based multi-wavelength semiconductor laser. A stable multi-wavelength laser output with a 93-channel multi-wavelength laser with maximum channel intensity non-uniformity of 3-dB were demonstrated on the basis of a single InAs/InGaAsP QD F-P cavity chip. All channels were ultra-stable because of the inhomogeneous gain broadening due to statistically distributed sizes and geometries of self-assembled QDs.

Properties of GaInNAsSb narrow ridge waveguide laser diodes in continuous-wave operation at 1.55um

Transparency current density (Jtr) was studied in GaInNAs ridge waveguide lasers. The devices employ Ga1-xInxNyAs1-y multiple quantum wells and were grown on GaAs substrates using solid-source molecular beam epitaxy (MBE) with an RF plasma cell. The transparency current density is sensitive to material quality: defects, traps and other sources of non-radiative recombination. It is also dependent on the rate of thermionic emission from quantum wells. Wavelength, polarization and temperature dependence of transparency carrier density of annealed material was studied. Record low transparency carrier densities of 20 and 90 A/cm2/well were observed (for TM and TE polarizations) in devices based on GaInNAs material designed for emission at 1340 nm after optimized rapid thermal annealing. This low value of Jtr confirms the excellent quality of the GaInNAs material and demonstrates that GaInNAs lasers with excellent material properties can be grown for long wavelength applications provided appropriate annealing is applied. It is believed that the low transparency current density is a unique feature of GaInNAs and is due to the band structure and band alignment of the material system.