K. Kennedy

λ∼3.1 μm room temperature InGaAs/AlAsSb/InP quantum cascade lasers

Strain compensated In0.67Ga0.33As/AlAs0.8Sb0.2/InP quantum cascade lasers emitting at wavelengths near 3.1 μm at room temperature have been demonstrated. The lasers operate in pulsed mode with threshold current density of 3.6 kA/cm2 at 80 K and 19.2 kA/cm2 at 295 K. The peak optical power for an as-cleaved 3 mm long and 10 μm wide ridge device exceeds 1 W per facet at 80 K and is around 8 mW at 295 K. The observed laser performance suggests that room temperature operation for these lasers remains possible beyond the predicted threshold for Γ-L intervalley scattering of electrons in the upper laser levels.

Quantum Dot Superluminescent Diodes for Optical Coherence Tomography: Device Engineering

We present a 18 mW fiber-coupled single-mode superluminescent diode with 85 nm bandwidth for application in optical coherence tomography (OCT). First, we describe the effect of quantum dot (QD) growth temperature on optical spectrum and gain, highlighting the need for the optimization of epitaxy for broadband applications. Then, by incorporating this improved material into a multicontact device, we show how bandwidth and power can be controlled. We then go on to show how the spectral shape influences the autocorrelation function, which exhibits a coherence length of <;11 μm, and relative noise is found to be 10 dB lower than that of a thermal source. Finally, we apply the optimum device to OCT of in vivo skin and show the improvement that can be made with higher power, wider bandwidth, and lower noise, respectively.

High peak power λ∼3.3 and 3.5 μm InGaAs/AlAs(Sb) quantum cascade lasers operating up to 400 K

We demonstrate λ∼3.5 μm and λ∼3.3 μm strain compensated In0.7Ga0.3As/AlAs(Sb)/InP quantum cascade lasers operating in pulse regime at temperatures up to at least 400 K. Peak optical power exceeding 3.5 W at 300 K has been achieved at both wavelengths for 10 μm wide 4 mm long lasers with high reflectivity coated back facets. Threshold current densities of 2.5 kA/cm2 and 3.5 kA/cm2 have been observed at 300 K for the devices emitting at λ∼3.5 μm and λ∼3.3 μm, respectively.

Quantum Dot Superluminescent Diodes for Optical Coherence Tomography: Skin Imaging

We present a high-power (18 mW continuous wave exiting a single-mode fiber and 35 mW exiting the facet), broadband (85 nm full-width at half-maximum) quantum dot-based superluminescent diode, and apply it to a time-domain optical coherence tomography (OCT) setup. First, we test its performance with increasing optical feedback. Then we demonstrate its imaging properties on tissue-engineered (TE) skin and in vivo skin. OCT allows the tracking of epidermal development in TE skin, while the higher power source allows better sensitivity and depth penetration for imaging of in vivo skin layers.

Quantum dot selective area intermixing for broadband light sources

We report a comparison of different capping materials on the intermixing of modulation p-doped InAs/In(Ga)As quantum dots (QD). QD materials with different caps are shown to exhibit significant difference in their optical properties during the annealing process. The selective area intermixing technique is demonstrated to laterally integrate two and three different QD light emitting devices with a single electrical contact. A spectral bandwidth of 240nm centered at 1188nm is achieved in a device with two sections. By calculating the point spread function for the obtained emission spectra, and applying the Rayleigh criteria for resolution, an axial resolution of 3.5μm is deduced. A three section device realizes a spectral bandwidth of 310nm centered at 1145nm. This corresponds to an axial resolution of 2.4μm. Such a small predicted axial resolution is highly desirable in optical coherence tomography system and other coherence-based systems applications.

Optical and thermal characteristics of narrow-ridge quantum-cascade lasers

Quantum-cascade lasers operating at λ≈3.9μm at room temperature with narrow w≈5μm ridge widths are described. The lateral confinement due to the narrow ridge is similar to the vertical confinement and the resulting beam is circular in cross section with a single TM00 spatial mode. The beam divergence is 46° both parallel and perpendicular to the surface. The beam quality factor along the slow axis is about M2=1.6. The narrow ridges also increase the relative lateral heat dissipation from the active region, resulting in a thermal conductance per unit area of about Gth=380WK−1cm−2 for a 3mm long laser. Maximum average power is obtained with duty cycles between 10% and 30%; in spite of the very narrow ridge, the total average power with thermoelectric cooling exceeds 60mW with a peak power of 460mW. The circularly symmetric beam with very good beam quality suggests essentially zero astigmatism and indicates that these narrow-ridge quantum-cascade lasers are well suited for applications in midinfrared spectro...

InP-Based Midinfrared Quantum Cascade Lasers for Wavelengths Below 4 μm

We review the recent development of high-performance short-wavelength (λ ~ 3.05-3.8 μm) strain-compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers (QCLs). The lasers are demonstrated in which wavelengths as low as 3.05 μm are obtained at temperatures up to 295 K. We also verify that strain-compensated In0.7Ga0.3As/AlAs(Sb) QCLs with AlAs barriers in the active region operate with much better performance compared with the lasers having identical design but with AlAsSb barriers throughout the whole core region. λ ~ 3.3-3.7 μm laser emission is observed at temperatures up to at least 400 K and up to 20 W of output optical power at 285 K for the QCLs with various core region designs. Room temperature distributed feedback InGaAs/AlAs(Sb) QCLs with buried third-order gratings have been also developed, displaying single-mode operation in the wavelength range of 3.358-3.380 μm for temperatures between 270 and 360 K.

High performance InP-based quantum cascade distributed feedback lasers with deeply etched lateral gratings

We report device results and techniques for fabricating gratings for InP-based quantum cascade (DFB) lasers, grown by MOVPE. Deeply etched lateral gratings are achieved by the development of a novel two-stage ICP etch process

Fabrication of v-groove gratings in InP by inductively coupled plasma etching with SiCl4/Ar

Inductively coupled plasma (ICP) etching of v-grooves in InP is demonstrated. We use SiCl4/Ar at a temperature of 25 °C and a broadband photoresist mask. Optimized v-groove etch parameters are discussed, in addition to the effect of various etch parameters on the etch rate and profile. This process utilizes a small amount of mask erosion, and careful control of the chemical and bombardment enhanced etch processes to give a self-limiting v-groove etch profile, which is insensitive to crystallographic directions of the InP.

Design, growth, fabrication, and characterization of InAs∕GaAs 1.3μm quantum dot broadband superluminescent light emitting diode

In this paper we discuss a technique for broadening the emission and gain spectra of 1.3μm quantum dot superluminescent light emitting diodes (SLEDs). By incorporating different amounts of indium in different wells of a multi-dot-in-well stack we are able to tailor the emission and gain spectra of the devices. This technique allows us to overlap the ground state of one dot-in-well (DWELL) with the excited state of another to achieve broader and flatter emission spectra compared to a SLED design comprising DWELL layers of constant indium composition. Due to the low internal loss of these structures, this broadening is achieved without a significant reduction in the output power of the devices.