Gerard Dang

Quantum Dashes on InP Substrate for Broadband Emitter Applications

We report on the development of InAs/InGaAlAs quantum-dash-in-well structure on InP substrate for wideband emitter applications. A spectral width as broad as 58 meV observed from both photoluminescence and surface photovoltage spectroscopy on the sample indicating the formation of highly inhomogeneous InAs-dash structure that results from the quasi-continuous interband transition. The two-section superluminescent diodes (SLDs), with integrated photon absorber slab as lasing suppression section, fabricated on the InAs dash-in-well structure exhibits the close-to-Gaussian emission with a bandwidth (full-width at half-maximum) of up to 140 nm at ~ 1.6 mum peak wavelength. The SLD produces a low spectrum ripple of 0.3 dB and an integrated power of ~ 2 mW measured at 20degC under 8 kA/cm2. The oxide stripe laser exhibits wide lasing wavelength coverage of up to 76 nm at ~ 1.64 mum center wavelength and an output optical power of ~ 400 mW from simultaneous multiple confined states lasing at room temperature. This rule changing broadband lasing signature, different from the conventional interband diode laser, is achieved from the quasi-continuous interband transition formed by the inhomogeneous quantum-dash nanostructure.

Low thermal resistance high-speed top-emitting 980-nm VCSELs

Increasing copper plated heatsink radii from 0 to 4 mum greater than the mesa in vertical-cavity surface-emitting lasers (VCSELs) reduced the measured thermal resistance for a range of device sizes to values 50% lower than previously reported over a range of device sizes. For a 9-mum diameter oxide aperture, the larger heatsink increases output power and bandwidth by 131% and 40%, respectively. The lasers exhibit a 3-dB modulation frequency bandwidth up to 9.8 GHz at 10.5 kA/cm2. The functional dependence of thermal resistance on oxide aperture diameter indicates the importance of lateral heat flow to mesa sidewalls

InGaAs/GaAs Quantum-Dot Superluminescent Diode for Optical Sensor and Imaging

We report on the design and fabrication of a novel wideband superluminescent diode (SLD) based on InGaAs/GaAs quantum-dot structure. In this device, we monolithically integrate a photon absorber section to suppress lasing action and optical feedback oscillation. The fabricated SLDs produce a close-to-Gaussian shaped spectrum centered at 1210 nm with a bandwidth of 135 nm. Spectral ripple as low as 0.3 dB has been measured

Defect Annealing of InAs–InAlGaAs Quantum-Dash-in-Asymmetric-Well Laser

We report the improvement of ~1.62-mum wavelength InAs-InAlGaAs quantum-dash-in-asymmetric-well laser performance using rapid thermal annealing. After the postgrowth annealing at 700 degC for 2 min, the internal quantum efficiency is increased from 90% to 93%, and the linewidth of the laser spectrum and the threshold current density is significantly reduced

Quantum Dash Intermixing

We investigate the intermixing effect in InAs/InAlGaAs quantum-dash-in-well structures grown on InP substrate. Both impurity-free vacancy disordering (IFVD) via dielectric cap annealing, and impurity-induced disordering (IID) using nitrogen ion-implantation techniques have been employed to spatially control the group-III intermixing in the quantum-dash (Qdash) system. Differential bandgap shifts of up to 80 nm and 112 nm have been observed from the IFVD and IID processes, respectively. Compared to the control (nonintermixed) lasers, the light-current characteristics for the 125 nm wavelength shifted Qdash lasers are not significantly changed, suggesting that the quality of the intermixed material is well preserved. The intermixed lasers exhibit a narrower linewidth as compared to the as-grown laser due to the improved dash homogeneity. The integrity of the material is retained after intermixing, suggesting the potential application for the planar integration of multiple active/passive Qdash-based devices on a single InP chip.

Quantum grid infrared spectrometer

We have designed and characterized an infrared spectrometer, which uses a linear array of quantum grid infrared photodetectors (QGIPs) as its spectral sensing elements. Each QGIP element shares the same detector material but has a different grid geometry. The detector material, which is based on a binary superlattice design, provides an 8–14 μm broadband absorption medium for the spectrometer. The geometry of the grid, which is the light coupling structure under normal incidence, selects individual absorption wavelength for each element. Using a linear array of QGIPs of different geometries, multiple wavelengths can be detected simultaneously, and the array thus forms a spectrometer. Multicolor infrared imaging can then be achieved by integrating such QGIPs in unit cells of a two-dimensional array.

Research progress on a focal plane array ladar system using chirped amplitude modulation

The Army Research Laboratory is researching a focal plane array (FPA) ladar architecture that is applicable for smart munitions, reconnaissance, face recognition, robotic navigation, etc.. Here we report on progress and test results attained over the past year related to the construction of a 32x32 pixel FPA ladar laboratory breadboard. The near-term objective of this effort is to evaluate and demonstrate an FPA ladar using chirped amplitude modulation; knowledge gained will then be used to build a field testable version with a larger array format. The ladar architecture achieves ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated (chirped amplitude modulation). The diodes output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of optoelectronic mixing, metal-semiconductor-metal detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal resulting from the mixing process whose frequency is proportional to the target range. This IF signal is continuously sampled over a period of the rf modulation. Following this, a signal processor calculates the discrete fast Fourier transform over the IF waveform in each pixel to establish the ranges and amplitudes of all scatterers.

A 32x32 pixel focal plane array ladar system using chirped amplitude modulation

The Army Research Laboratory is researching system architectures and components required to build a 32x32 pixel scannerless ladar breadboard. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (RF) subcarrier that is linearly frequency modulated (i.e. chirped amplitude modulation). The backscattered light is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. Pixel read-out is achieved using code division multiple access techniques as opposed to the usual time-multiplexed techniques to attain high effective frame rates. The raw data is captured with analog-to-digital converters and fed into a PC to demux the pixel data, compute the target ranges, and display the imagery. Last year we demonstrated system proof-of-principle for the first time and displayed an image of a scene collected in the lab that was somewhat corrupted by pixel-to-pixel cross-talk. This year we report on system modifications that reduced pixel-to-pixel cross-talk and new hardware and display codes that enable near real-time stereo display of imagery on the ladars control computer. The results of imaging tests in the laboratory will also be presented.

Demonstration of a Slow-Light High-Contrast Metastructure Cage Waveguide

We have developed a new type of Si-based 3D cage-like high-contrast metastructure waveguide with both “slow-light” and low-loss properties, which has applications in providing a long time-delay line or a high Q cavity in chip-scale optoelectronic integrated circuits (OEIC). Traditional semiconductor optical waveguides always have high loss when used in a high dispersion (slow-light) region. A preliminary computational model has predicted that there is a slow-light and low propagation loss region within cage-like hollow-core waveguide formed by 4 high-contrast-gratings walls/claddings. Using our new processing technique, we fabricated several such waveguides on a Si wafer with different core sizes/shapes and different HCGs for 1550 operation wavelength. We have conducted experimental waveguide delay test measurements using a short optical pulse which indicate that the group velocity of these metastructure waveguides are in the range of 20- 30% of the speed of the light. Using a waveguide “cut-back” method, we have experimentally determined the propagation loss of these waveguides are in the range of 2-5dB/cm. We are also developing this type of high-contrast metastructure hollow-core waveguide for different operating wavelength/frequency such as THz for different applications.

Effects of Intermixing on Gain and Alpha Factors of Quantum-Dash Lasers

Gain and alpha factors were measured on InAs-InAlGaAs quantum-dash lasers with their heterostructures intermixed by either a dielectric-capping or ion-implantation technique. The laser intermixed by the dielectric-capping technique exhibits a blue shift as much as 93 nm without degrading the laser quality. In comparison, the laser intermixed by the ion-implantation technique has a larger shift but lower differential gain and higher alpha factor. The result implies that quantum-dash lasers of different wavelengths can be effectively integrated on the same chip by the dielectric-capping intermixing technique.