K. J. Knopp

High bandwidth-efficiency resonant cavity enhanced Schottky photodiodes for 800–850 nm wavelength operation

High-speed resonant cavity enhanced Schottky photodiodes operating in 800–850 nm wavelength region are demonstrated. The devices are fabricated in the AlGaAs/GaAs material system. The Schottky contact is a semitransparent Au film which also serves as the top reflector of the Fabry–Perot cavity. The detectors exhibit a peak quantum efficiency of η=0.5 at λ=827u2009nm wavelength and a 3 dB bandwidth of more than 50 GHz resulting in a bandwidth-efficiency product of more than 25 GHz.

Optical constants of (Al0.98Ga0.02)xOy native oxides

We report the optical constants of oxidized crystalline and low-temperature-grown (LTG) Al0.98Ga0.02As films, as determined by variable angle spectroscopic ellipsometry. Data were acquired at three angles of incidence over 240–1700 nm and fitted to a Cauchy dispersion function. For oxidized crystalline material, we observe a variation in the real index of ±0.5% for layer thickness variations of ±6%. We show that upon oxidation, LTG material can expand by >25% while crystalline material contracts by <2%. Atomic force microscopy analysis indicates thickness-dependent variations in the oxide microstructure. Additionally, an optical scattering loss of 2.1×10−4%/pass is calculated based on surface roughness measurements for a thin layer of oxidized crystalline material.

Vertical-cavity surface-emitting lasers with low-ripple optical pumping windows

A general technique for numerically optimizing the optical admittances in GaAs QW vertical-cavity laser structures is used to suppress the interference ripple in the typical reflectance/transmittance spectra. This technique is applicable to any vertical-cavity device whose photonic properties at various wavelengths requires modification for specific applications. In this paper, we report the use of this optimization method to enhance the coupling of pump light into 850-nm vertical-cavity surface-emitting lasers (VCSELs). We have designed and fabricated novel lasers which contain a wideband window of low reflectance amidst the typical interference fringe spectrum. The 750-800-nm region for the low-ripple design has an average reflectance of 5%; the peak-to-peak amplitude of the ripple is 0.25%. The sensitivity of these devices to temperature variations and layer-thickness manufacturing variations is also studied. The low-ripple pump window shifts at a rate of 0.036 nm//spl deg/C, the peak-to-peak ripple of the reflectance varies less than 2%, and the pump bandwidth remains constant, over temperatures ranging from 0/spl deg/C to 100/spl deg/C. The low-ripple structure substantially reduces the temperature and wavelength variation of the pump-field overlap by creating a window of nearly constant reflectance.

Pump intensity profiling of vertical-cavity surface-emitting lasers using near-field scanning optical microscopy

We have mapped the internal pump intensity distribution of an optically pumped vertical-cavity surface-emitting laser. Spontaneous emission from quantum wells placed throughout the distributed Bragg reflectors is correlated to the pump intensity. The emission is monitored along the cleaved edge using the high spatial resolution and shallow depth of field provided by near-field scanning optical microscopy. Our results show a distinct buildup of optical intensity between the mirror stacks. Simulations performed using the transfer matrix method match well with the experimental data.

Compound semiconductor oxide antireflection coatings

We report the development of high quality, broad-bandwidth, antireflection (AR) coatings using the low index provided by wet thermally oxidized Al0.98Ga0.02As. We address the design criteria, fabrication, and characterizations of AR coatings composed of surface and buried oxide layers on GaAs. We show, using native-oxide dispersion data, that surface oxide coatings can be designed to offer a nearly zero minimum of reflectance and a reflectance of 250 nm have been experimentally demonstrated at a design wavelength of 1 micrometer. Additionally, buried oxide coatings can be designed with an AlxGa1−xAs matching layer of any composition to exactly match the admittance of any substrate with effective index between 2.5 and 3.5. We have demonstrated buried oxide coatings, also designed for 1 micrometer, having a reflectance minimum of 0.4% and a reflectance of <1% over 21 nm. The calculate...

Vertical‐cavity surface‐emitting lasers with low‐ripple optical pump bands

We have used multilayer mirror optimization methods to enhance the coupling of pump light into vertical‐cavity surface‐emitting lasers (VCSELs). With previously reported devices, pump light was coupled into VCSEL cavities through interference notches in the mirror reflectance spectrum. This approach is sensitive to temperature dependent reflectance spectrum shifts. We have created devices with a wide pump‐band window of low reflectance. We report the simulation, growth, and optically pumped lasing of such optimized low‐ripple VCSELs. Further, broadband pump windows open the possibility of spectrally‐broad optical pumps and they eliminate need for costly tunable pump lasers.

Thin-film design for enhanced stability of optically pumped vertical-cavity surface-emitting lasers (VCSELs)

We have employed a novel approach to enhance the optical pump-coupling stability of vertical-cavity surface-emitting lasers (VCSELs). These structures are composed of thin-film semiconductor multilayers and are manufactured entirely by thin-film deposition. In the past, pump light was coupled into VCSEL cavities through the short wavelength interference notches in the reflectance spectrum. However, the steep slope and narrow width of the notches inherently makes pump-coupling sensitive to device temperature. We have employed traditional thin-film optimization to the multilayer etalon to create a low ripple, wideband pump region of low reflectance while maintaining cavity-mode field overlap at the quantum wells. We demonstrate a factor of 2.5 times better stability across a 35 nm spectral range.

Spectral mapping of multimode vertical-cavity surface-emitting lasers by near-field scanning optical microscopy

We have studied the spatial and spectral characteristics of vertical-cavity surface-emitting laser emission using near- field scanning optical microscopy. We report the multi- transverse-mode characteristics of 15 micrometers diameter proton- implanted 850 nm devices used in a 2 Gbit/s multimode fiber- optic links. Spectrally resolved and integrated intensity scans over a 20 X 20 area were performed. The intensity of each resolvable transverse mode was integrated and its wavelength range false-colored at each scan position. The resulting composite image displays relative intensity and spatial distribution information for each transverse mode. Correlation with the shear force data allows mapping of the optical distributions to topographical features. Lasing filaments were observed at high drive currents. Gain competition among spatially overlapping transverse modes was observed while spatially isolated modes coexisted without competition.