Simulation of Broad Spectral Bandwidth Emitters at 1060 nm for Optical Coherence Tomography

The simulation of broad spectral bandwidth light sources (semiconductor optical amplifiers (SOA) and superluminescent diodes (SLD)) for application in ophthalmic optical coherence tomography is reported. The device requirements and origin of key device parameters are outlined, and a range of single and double InGaAs/GaAs quantum well (QW) active elements are simulated with a view to application in different OCT embodiments. We confirm that utilising higher order optical transitions is beneficial for single QW SOAs, but may introduce deleterious spectral modulation in SLDs. We show how an addition QW may be introduced to eliminate this spectral modulation, but that this results in a reduction of the gain spectrum width. We go on to explore double QW structures where the roles of the two QWs are reversed, with the narrow QW providing long wavelength emission and gain. We show how this modification in the density of states results in a significant increase in gain-spectrum width for a given current. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Quantum well and dot self-aligned stripe lasers utilizing an InGaP optoelectronic confinement layer

We demonstrate and study a novel process for fabrication of GaAs-based self-aligned lasers based upon a single over-growth. A lattice-matched n-doped InGaP layer is utilized for both electrical and optical confinements. Single-lateral-mode emission is demonstrated initially from an In0.17Ga0.83 As double quantum well laser emitting similar to 980 nm. We then apply the fabrication technique to a quantum dot laser emitting similar to 1300 nm. Furthermore, we analyze the breakdown mechanism in our devices and discuss the limitations of index guiding in our structures

Sensitivity Advantage of QCL Tunable-Laser Mid-Infrared Spectroscopy over FTIR Spectroscopy

Interest in mid-infrared spectroscopy instrumentation beyond classical FTIR using a thermal light source has increased dramatically in recent years. Synchrotron, supercontinuum, and external-cavity quantum cascade laser light sources are emerging as viable alternatives to the traditional thermal black-body emitter (Globar), especially for remote interrogation of samples ("stand-off" detection) and for hyperspectral imaging at diffraction-limited spatial resolution ("microspectroscopy"). It is thus timely to rigorously consider the relative merits of these different light sources for such applications. We study the theoretical maximum achievable signal-to-noise ratio (SNR) of FTIR using synchrotron or supercontinuum light vs. that of a tunable quantum cascade laser, by reinterpreting an important result that is well known in near-infrared optical coherence tomography imaging. We rigorously show that mid-infrared spectra can be acquired up to 1000 times faster - using the same detected light intensity, the same detector noise level, and without loss of SNR - using the tunable quantum cascade laser as compared with the FTIR approach using synchrotron or supercontinuum light. We experimentally demonstrate the effect using a novel, rapidly tunable quantum cascade laser that acquires spectra at rates of up to 400 per second. We also estimate the maximum potential spectral acquisition rate of our prototype system to be 100,000 per second

Strain balancing of MOVPE InAs/GaAs quantum dots using GaAs0.8P0.2

MOVPE growth of stacked InAs/ GaAs QDs with and without GaAs 0.8 P 0.2 strain balancing layers has been studied. The GaAsP layers reduce the accumulated strain whilst maintaining the electrical characteristics. This should enable closer stacking of QD layers leading to higher gain and improved laser performance

Strain Balancing of Metal-Organic Vapour Phase Epitaxy InAs/GaAs Quantum Dot Lasers

Incorporation of a GaAs0.8P0.2 layer allows strain balancing to be achieved in self-assembled InAs/GaAs quantum dots (QDs) grown by metal organic vapor phase epitaxy. Tuneable wavelength and high density are obtained through growth parameter optimization, with emission at 1.27 μm and QD layer density 3 × 10 10 cm-2. Strain balancing allows close vertical stacking (30 nm) of the QD layers, giving the potential for increased optical gain. Modeling and device characterization indicates minimal degradation in the optical and electrical characteristics unless the phosphorus percentage is increased above 20%. Laser structures are fabricated with a layer separation of 30 nm, demonstrating low temperature lasing with a threshold current density of 100 A/cm2 at 130 K without any facet coating

Incorporating Structural Analysis in a Quantum Dot Monte-Carlo Model

We simulate the shape of the density of states (DoS) of the quantum dot (QD) ensemble based upon size information provided by high angle annular dark field scanning transmission electron microscopy (HAADF STEM). We discuss how the capability to determined the QD DoS from micro-structural data allows a MonteCarlo model to be developed to accurately describe the QD gain and spontaneous emission spectra. The QD DoS shape is then studied, with recommendations made via the effect of removing, and enhancing this size inhomogeneity on various QD based devices is explored

O-band excited state quantum dot bilayer lasers

Bilayer InAs/GaAs quantum dot(QD) lasers operating in the excited state at wavelengths that span the O-band are demonstrated. The higher saturated gain and lower scattering time of the excited states of the ensemble of QDs offers the opportunity for fast direct-modulation lasers. We predict an increase in K-factor limited modulation bandwidth from QD lasers operating in the excited state due to a reduction in carrier transport and scattering times whilst maintaining high peak modal gain

Quantum dot superluminescent diodes for optical coherence tomography: device engineering

We present a 18 m W fiber-coupled single-mode super-luminescent 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 mu m, 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

The mid-infrared swept laser:Life beyond OCT?

Near-infrared external cavity lasers with high tuning rates ("swept lasers") have come to dominate the field of near-infrared low-coherence imaging of biological tissues. Compared with time-domain OCT, swept-source OCT a) replaces slow mechanical scanning of a bulky reference mirror with much faster tuning of a laser cavity filter element and b) provides a ×N (N being the number of axial pixels per A-scan) speed advantage with no loss of SNR. We will argue that this striking speed advantage has not yet been fully exploited within biophotonics but will next make its effects felt in the mid-infrared. This transformation is likely to be driven by recent advances in external cavity quantum cascade lasers, which are the mid-IR counterpart to the OCT swept-source. These mid-IR sources are rapidly emerging in the area of infrared spectroscopy. By noting a direct analogy between time-domain OCT and Fourier Transform Infrared (FTIR) spectroscopy we show analytically and via simulations that the mid-IR swept laser can acquire an infrared spectrum ×N (N being the number of spectral data points) faster than an FTIR instrument, using identical detected flux levels and identical receiver noise. A prototype external cavity mid-IR swept laser is demonstrated, offering a comparatively low sweep rate of 400 Hz over 60 cm-1 with 2 cm-1 linewidth, but which provides evidence that sweep rates of over a 100 kHz should be readily achievable simply by speeding up the cavity tuning element. Translating the knowledge and experience gained in near-IR OCT into mid-IR source development may result in sources offering significant benefits in certain spectroscopic applications. © 2015 SPIE

Room Temperature Tuneable THz Generation Based on 2nd Order Non-linear Optical Effects in GaAs/AlGaAs Multi-quantum Well Excitons

Summary form only given. We report the generation of tuneable THz radiation (0.75-3 THz) through second order nonlinear effects in the excitation of excitons in GaAs/AlAs multi-quantum wells (MQWs), using readily available continuous wave (CW) laser diodes at room temperature. A MQW GaAs/AlAs sample was designed to have excitonic resonances at a wavelength accessible by commercially available lasers (850nm, 260mW), and have ElHHl-ElLHl splitting of 9.1meV. The sample was grown by MBE with the MQW region containing 30 repeats of 11.9nm GaAs separated by 7.1nm AlAs barriers. These preliminary measurements removed the substrate and cap layer. The sample was then capillary bonded to a diamond heat spreader [1]. Two collimated lasers were used to excite the excitonic resonances. Both lasers were normally incident to the sample surface. Figure 1(a) shows THz power obtained in collinear and crossed polarisations of the lasers with one laser resonant with the HH exciton, and the other laser tuned across the excitonic bands with both lasers operating at 260mW. A clear signal is observed in the case of collinear excitation which scales with the density of states of the excitons. Power dependence measurements confirm this is a second order non-linear effect. Using a simple interferometer, and fitting the measured power with the expected transmission of a Fabry-Perot etalon, frequency measurements indicate the ability to tune the THz radiation from 0.75-3THz. See Figs 1(b-d). For excitation at the peaks of HH and LH, a conversion efficiency of 1.2×10-5 was obtained. This was achieved without the use of plasmonic effects, nor any kind of an antenna, nor an applied E-field to the structure. This offers the opportunity for the creation of compact, low cost, tuneable room temperature THz source