Mial E. Warren

Comprehensive numerical modeling of vertical-cavity surface-emitting lasers

We present a comprehensive numerical model for vertical-cavity surface-emitting lasers that includes all major processes affecting cw operation of axisymmetric devices. In particular, our model includes a description of the 2-D transport of electrons and holes through the cladding layers to the quantum well(s), diffusion and recombination of these carriers within the wells, the 2-D transport of heat throughout the device, and a multilateral-mode effective index optical model. The optical gain acquired by photons traversing the quantum wells is computed including the effects of strained band structure and quantum confinement. We employ our model to predict the behavior of higher-order lateral modes in proton-implanted devices and to provide an understanding of index-guiding in devices fabricated using selective oxidation.

Coherent beams from high efficiency two‐dimensional surface‐emitting semiconductor laser arrays

We have fabricated and operated large two‐dimensional (2D) arrays of phase‐locked surface‐emitting semiconductor lasers. The arrays were fabricated by reactive ion beam etching of epitaxial Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs‐AlGaAs quarter‐wave mirrors. Different arrays corresponding to different pixel size (2–5 μm) and spacing (1–2 μm) were produced to investigate evanescent coupling between pixels. The arrays were photopumped so that the array size could be conveniently varied from 1×1, 2×2,... up to 20×20. Except for the 1×1 which emits a circular pattern, all arrays exhibit a well‐defined four‐lobed far‐field pattern in agreement with our theoretical analysis of the optical modes which predicts domination by the 2D out‐of‐phase eigenmode. As a consequence this pattern can be understood with simple Fraunhofer diffraction theory. The angular spread of the lobes, determined by the periodicity of the array elements, is 10° for the array with element size/spacing...

On-axis far-field emission from two-dimensional phase-locked vertical cavity surface-emitting laser arrays with an integrated phase-corrector

We have fabricated large, two‐dimensional (2D) arrays of optically pumped, phase‐locked vertical cavity surface‐emitting lasers that emit more than 50% of their light in a central on‐axis lobe. The emission of the arrays was modified from the usual four‐lobed far‐field of 2D coupled arrays by incorporation of a binary phase‐shift mask on the surface of the array. The array consists of Fabry–Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs/AlGaAs quarterwave mirrors with a multiple order AlGaAs phase‐delay layer on the top mirror stack. The phase‐shift layer was etched away on alternating elements of the array. The resulting on‐axis emission had an angular width of 2° for an array of approximately 100 elements.

Single‐frequency continuous‐wave operation of ring resonator diode lasers

We report continuous‐wave room‐temperature operation of a semiconductor ring resonator diode laser with a single Y‐junction outcoupling waveguide. This device with a 150 μm radius and 8‐μm‐wide etched‐rib waveguide has a threshold current of 72 mA and emits up to 1 mW of single‐frequency output. The side‐mode‐rejection ratio of this laser exceeds 22 dBm over a ≳50 mA current range corresponding to a stable operating mode of the laser. Studies of the emission behavior in this and other similar ring lasers suggest that the ring is lasing in ‘‘whispering gallery’’ modes, and also that the Y junction is affecting mode selection in the ring.

Polarization-sensitive subwavelength antireflection surfaces on a semiconductor for 975 nm.

We present the results of subwavelength antireflection surfaces etched into GaAs for use at 975 nm. These surfaces comprise linear gratings with periods less than the wavelength of light in GaAs. The structure appears as a homogeneous birefringent film. For one of the two polarizations, the film is directly analogous to the well-known quarter-wavelength antireflection coating. For the other polarization there is little effect on the surface reflectivity.

Amplitude and phase beam characterization using a two-dimensional wavefront sensor

We have developed a two-dimensional Shack-Hartman wavefront sensor that uses binary optic lenslet arrays to directly measure the wavefront slope (phase gradient) and amplitude of the laser beam. This sensor uses an array of lenslets that dissects the beam into a number of samples. The focal spot location of each of these lenslets (measured by a CCD camera) is related to the incoming wavefront slope over the lenslet. By integrating these measurements over the laser aperture, the wavefront or phase distribution can be determined. Since the power focused by each lenslet is also easily determined, this allows a complete measurement of the intensity and phase distribution of the laser beam. Furthermore, all the information is obtained in a single measurement. Knowing the complete scalar field of the beam allows the detailed prediction of the actual beams characteristics along its propagation path. In particular, the space-beamwidth product, M2, can be obtained in a single measurement. The intensity and phase information can be used in concert with information about other elements in the optical train to predict the beam size, shape, phase and other characteristics anywhere in the optical train. We present preliminary measurements of an Ar+ laser beam and associated M2 calculations.

High-efficiency subwavelength diffractive optical element in GaAs for 975 nm

We have fabricated subwavelength diffractive optical elements with binary phase profiles for operation at 975 nm. The individual surface-relief features of the elements are smaller than the wavelength of light in the material. By modulating the size and spacing of the features we form artificial, gradient, effective index-of-refraction surfaces. The blazed transmission gratings were designed with rigorous coupled-wave analysis and fabricated by direct-write electron-beam lithography and reactive ion-beam etching in GaAs. The gratings have minimum features 63 nm wide. Transmission measurements show 85% diffraction efficiency into the first order.

Optical Bloch waves in a semiconductor photonic lattice

We have observed multiple optical Bloch waves in a semiconductor photonic lattice. This photonic lattice comprises epitaxial quarter‐wave periodic layers surrounding a periodic quantum‐well region. After growth, the layers are structured laterally into periodic square unit cells by reactive‐ion‐beam etching. When photoexcited, the lattice emits a complex angular distribution of photons that reflects its periodic structure. Scattered light is distributed according to the Laue conditions in analogy with x‐ray diffraction from a bulk crystal. Optical Bloch waves photostimulated in the lattice are analogous to electron Bloch waves in an atomic lattice. These optical Bloch waves exhibit long‐range translational symmetry and local symmetry due to the shape of the unit cell. Interestingly, the far‐field pattern of stimulated emission gives a direct mapping of the allowed Bloch wave vectors in the Brillouin zone. The mapping exhibits a wave‐vector gap at the Bragg condition and may be associated with a photonic e...

Subwavelength, binary lenses at infrared wavelengths

We describe the nanofabrication of subwavelength, binary lenses in GaAs for operation in the infrared. Subwavelength surface relief structures create an artificial material with an effective index of refraction determined by the fill factor of the binary pattern and can be designed to yield high-efficiency diffractive optical elements. In this work, we designed and fabricated a circular-aperture, off-axis lens with a deflection angle of 20°, focal length of 110 μm, and diameter of 80 μm, for operation at 975 nm. The off-axis lens design has a theoretical efficiency of 92% and the fabricated lens exhibits a diffraction efficiency into the first order of 72% and 59% of the transmitted power for TE and TM polarization, respectively. A significant advantage of these subwavelength structures is that fabrication requires only a single-lithography-and-etch-step process, in this case, electron-beam lithography and reactive-ion-beam etching.

Damage from proton irradiation of vertical-cavity surface-emitting lasers

Damage resulting from irradiating oxide-confined vertical-cavity surface-emitting lasers became significant (threshold shift /spl ap/20%, peak power degradation /spl ap/20%) at fluence levels approaching 1/spl times/10/sup 13/ protons/cm/sup 2/. The threshold current shifted to higher values, and the peak light output power decreased. Forward-current annealing led to partial recovery of the performance of two of the three lasers for which annealing was attempted. Recent results on proton-implanted devices are summarized in a table.