Midinfrared Interband Cascade Laser for Free Space Optical Communication

A free space optical (FSO) link utilizing midinfrared (mid-IR) interband cascade lasers has been demonstrated in the 3- to 5- $mu{hbox {m}}$ atmospheric transmission window with data rates up to 70 Mb/s and bit-error rate (BER) less than $10^{-8}$ . The performance of the mid-IR FSO link has been compared with the performance of a near-IR link under various fog conditions using an indoor communication testbed. These experiments demonstrated the lower attenuation and scintillation advantages of a mid-IR FSO link.      

High-speed operation of interband cascade lasers

High-speed interband cascade lasers have been fabricated and the first experimental evidence that these devices can be directly modulated at a frequency of 3.2 GHz and above is reported.     

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.     

Active mode locking of broadband quantum cascade lasers

Active mode locking in broadband quantum cascade (QC) lasers with a repetition rate of about 14.3 GHz has been achieved through the modulation of the laser bias current. At low driving currents, the active mode locking in broadband QC lasers resembles the active mode locking in single-wavelength QC lasers, while at high driving currents, the mode locking properties are governed by the broad spectral gain of these lasers. At high bias currents, the active modulation excites Fabry-Perot modes across the entire gain spectrum from 6.7 to 7.4 /spl mu/m, with clear evidence of mode locking. The spectral width of the optical gain in the broadband QC lasers exceeds 2 THz and indicates the potential for generating subpicosecond pulses.     

High temperature operation of long-wavelength infrared superlattice detector with supressed dark current

Superlattice (SL) photodetectors operating with a cutoff wavelength of lambda = 10 mum are demonstrated to respond up to a temperature of T = 140 K with a nonunity gain enabling a high responsivity of Rfr = 25 A/W. The improvement in detector performance is achieved by the optimisation of SL heterodiode design in which suppression of dark current is realised without significant impact on the photocurrent.     

Superlattice and Quantum Dot Unipolar Barrier Infrared Detectors

We report device performance and provide theoretical analysis of a modified long-wavelength complementary barrier infrared detector structure that incorporates a double tunnel junction contact designed for simpler material growth while retaining the robustness for processing. We also provide analysis of a mid-wavelength quantum dot barrier infrared detector and explain its spectral shape and turn-on characteristics.     

Evaluation of Thermal Crosstalk in Quantum Cascade Laser Arrays

The continuous-wave operation, thermal crosstalk measurements, and thermal modeling of a quantum cascade (QC) laser array are presented. Measurements and thermal modeling predict that the thermal crosstalk between array elements can be minimized by improving the heat management when the spacing of neighboring laser elements is greater than 500 mum. The output power of the demonstrated QC laser array significantly exceeds the output power of the individual lasers in the array. This shows the potential of the QC arrays as a candidate for the realization of high-power optical sources in mid-infrared (3-20 mum) spectral range     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.