Nam-Heon Kim

Near-diffraction-limited coherent emission from large aperture antiguided vertical-cavity surface-emitting laser arrays

We demonstrate that in-phase mode operation with a near-diffraction-limited beam can be realized in large aperture (up to 100 elements) antiguided vertical-cavity surface-emitting laser (VCSEL) arrays. A selective etching process with two-step metalorganic chemical vapor deposition is used for fabrication of the antiguided VCSEL array structures. Modal discrimination is enhanced by intentionally choosing a GaAs cap thickness so as to introduce suitable loss to array interelement regions. Far field patterns indicate in-phase mode operation from both triangular and rectangular geometry antiguided VCSEL arrays, which is in good agreement with theory.

Modeling, fabrication, and characterization of large aperture two-dimensional antiguided vertical-cavity surface-emitting laser arrays

We have investigated the modal behavior of two-dimensional (up to 400 elements) active-photonic-lattice-based antiguided vertical-cavity surface-emitting laser (VCSEL) arrays by both modeling and device characterization. A two-dimensional (2-D) model based on the effective index method has been constructed to analyze 2-D resonance and calculate array mode frequencies in rectangular geometry arrays. A more comprehensive three-dimensional bi-directional beam propagation code has also been developed to theoretically describe 2-D antiguided arrays with the VCSEL structure in the primary wave propagation direction. Gain spatial hole burning (GSHB) effects above laser threshold are applied to find conditions favorable for in-phase mode lasing and high intermodal discrimination. Three rectangular geometry array structures based on different interelement loss mechanisms have been designed and fabricated. Both far-field and spectral characterization were conducted on the devices to make detailed comparison with theoretical results. We found that introducing higher loss within the interelement region can allow the in-phase mode to exhibit the lowest threshold gain for a wide range of interelement widths around the in-phase resonance condition. A detailed spectral study of 5/spl times/5 arrays with the highest interelement loss design has demonstrated suppression of competing guided array modes and higher order leaky array modes at drive currents up to 10 times threshold.

Temperature sensitivity of InGaAs quantum-dot lasers grown by MOCVD

Temperature-dependent cavity length studies have been performed on multiple stack strain compensated InGaAs quantum-dot (QD) active region broad stripe laser structures grown by metal-organic chemical vapor deposition. The characteristic temperature coefficients of the threshold current density (T/sub 0/) and external differential quantum efficiency (T/sub 1/) were calculated from variable temperature measurements. The correlation of the T/sub 0/,T/sub 1/ values and the extracted values of the characteristic temperature coefficients of the transparency current density, material gain, injection efficiency, and internal loss (T/sub tr/,T/sub g0/,T/sub /spl eta/inj/,T/sub /spl alpha/i/) from the temperature-dependent study is discussed. The T/sub 1/ values are higher than 400 K for five-stack QD laser structures, comparable values to conventional quantum-well (QW) laser structures. T/sub 0/ values are lower than 100 K. Extracted material gain parameters are found to increase with increasing temperature for the three-stack QD structure, and are nearly temperature independent for the five-stack structure, different to that observed in InGaAs QW lasers.

Antiresonant reflecting optical waveguide-type vertical-cavity surface emitting lasers: comparison of full-vector finite-difference time-domain and 3-D bidirectional beam propagation methods

The cold-cavity modal characteristics of an antiresonant optical waveguide-type cylindrical vertical-cavity surface-emitting laser (VCSEL) are investigated through numerical simulations using a three-dimensional (3-D) bidirectional beam propagation method (BD-BPM) and a full-vector axisymmetric finite-difference time-domain (FDTD) method. Good agreement between the BPM- and FDTD-computed radial mode profiles as well as the mode-dependent radiation losses is obtained. The results of this paper establish the accuracy of the BD-BPM technique for simulating this class of devices and confirm effective-index method predictions that antiresonance conditions for cylindrical geometry devices (i.e., VCSELs) differ from those of planar geometry devices (i.e., edge emitters).

Leaky defect modes of two-dimensional VCSEL arrays

Two-dimensional photonic lattices with a low-index defect have been studied. Simulations demonstrate that this type of structure has potential for realizing single-spatial mode operation from a relatively large emitting aperture, making it ideal for fiber coupling applications. A 2-D finite difference model is used to calculate the radiation loss of the modes in various low-index defect based two-dimensional photonic lattices. The simulation results are also compared to the results from a comprehensive the 3-D bi-directional beam propagation model. These calculations have been used to guide mask design and the experimental realization of the defect VCSEL devices.

Modeling and observations of modal structure in antiguided VCSEL array

Modal behavior of a 2-D (square lattice geometry) antiguided vertical cavity surface emitting laser (VCSEL) array was studied by 3-D bi-directional beam propagation method. Above threshold operation of leaky modes was simulated using multiple iterations. Besides, a method based on functions of Krylov’s subspace, was developed to find a number of array optical modes in a VCSEL array with gain and index distributions established by the oscillating mode. In calculations, both Fourier and space variable descriptions of beam propagation were combined. The FFT technique was used for calculations of the Fourier image and the original. Conditions are found for favorable lasing of the in-phase mode providing high laser beam quality. Experimentally realized 5x5 laser array was studied numerically. The 2-D antiguided array results from shifting the cavity resonance between the element and inter-element regions and is fabricated by selective chemical etching and two-step metalorganic chemical vapor deposition (MOCVD) growth. In-phase and out-of-phase array mode operation is observed from top-emitting rectangular arrays as large as 400 elements, depending on the inter-element width, in good agreement with theory.

Radiative Efficiency of MOCVD grown QD lasers

The optical spectral gain characteristics and overall radiative efficiency of MOCVD grown InGaAs quantum dot lasers have been evaluated. Single-pass, multi-segmented amplified spontaneous emission measurements are used to obtain the gain, absorption, and spontaneous emission spectra in real units. Integration of the calibrated spontaneous emission spectra then allows for determining the overall radiative efficiency, which gives important insights into the role which nonradiative recombination plays in the active region under study. We use single pass, multi-segmented edge-emitting in which electrically isolated segments allow to vary the length of a pumped region. In this study we used 8 section devices (the size of a segment is 50x300 μm) with only the first 5 segments used for varying the pump length. The remaining unpumped segments and scribed back facet minimize round trip feedback. Measured gain spectra for different pump currents allow for extraction of the peak gain vs. current density, which is fitted to a logarithmic dependence and directly compared to conventional cavity length analysis, (CLA). The extracted spontaneous emission spectrum is calibrated and integrated over all frequencies and modes to obtain total spontaneous radiation current density and radiative efficiency, ηr. We find ηr values of approximately 17% at RT for 5 stack QD active regions. By contrast, high performance InGaAs QW lasers exhibit ηr ~50% at RT.

Modal characteristics of large-aperture ARROW VCSELs

Antiresonant reflecting optical waveguide (ARROW) vertical cavity surface emitting lasers (VCSELs) are designed for high power single mode operation. Scalar wave and finite-difference time domain (FDTD) studies indicate large modal discrimination in favor of the fundamental mode for large aperture (8 um) ARROW VCSELs. The modal discrimination mechanisms are identified through the total modal loss and quality factor calculations including polarization dependence. A novel design is presented, utilizing metal absorption loss to help suppress higher-order modes.