We present evaluation results of the 940nm 400mW transverse single-mode laser diodes (LDs) with real reflective index self-aligned (RISA) structure based on graded index separate confinement hetero structures (GRIN-SCH) for a three-dimensional (3D) depth sensor. The AlGaAs/InGaAs laser diodes that are adopted with RISA structure have many advantages over conventional complex refractive index guided lasers, what include low operating current, high temperature operation and stable fundamental transverse-mode operation up to high power levels.
Simultaneously, the RISA process is easy to control the waveguide channel width and does not require stable oxide mask for the regrowth of aluminum alloys, so it is possible to manufacture high output power and high reliability laser diodes.
At the optical power 400mW under the continuous-wave (CW) operation, Gaussian narrow far-field patterns (FFP) are measured with the full-width at half-maximum vertical divergence angle of 23°. A threshold current (Ith) of 33mA, slope efficiency (SE) of 0.81mW/mA and operating current (Iop) of 503mA are obtained at room temperature. Also, we could achieve catastrophic optical damage (COD) of 657mW and long-term reliability of 60°C with TO-56 package.
High-power femtosecond (fs) lasers in the visible wavelength regime have numerous applications in areas including micro-machining, medical eye surgery, communication, spectroscopy, etc. To generate this laser beam, frequency conversion, especially second-harmonic generation (SHG), of near-IR lasers using nonlinear optical crystals is known to be the most standard technique. However, the use of a long-length crystal, which is preferred to achieve high SHG conversion efficiency for long-pulse or cw lasers, cannot be applied to the fs laser with broad linewidth due to the tight phase matching condition and the exacerbated walk-off effect. Thus the conditions of the nonlinear optical crystal should be optimized to achieve efficient SHG generation and hence, the high power visible fs laser pulses. There are many reports for the efficient SHG of the fs lasers but not many reports about influence of the crystal length on the SHG process, such as the pulse width and the linewidth and the conversion efficiency.
Here, we report efficient SHG of femtosecond Yb lasers at 1 um by optimizing the conditions of nonlinear optical crystals. The SHG pulse and the conversion efficiency were numerically calculated to find the optimized conditions of the nonlinear optical crystals for the high power fs laser pulses with different pulse widths. Preliminary experiments were conducted using a Type I LBO crystal and the femtosecond Yb laser at 1 um, which was in good agreement with the theoretical results. The theoretical and the experimental results for LBO and BBO crystals will be reported in detail.
We present a methodology for the optimization of sampling schemes in diffuse optical tomography (DOT). The proposed method exploits singular value decomposition (SVD) of the sensitivity matrix, or weight matrix, in DOT. Two mathematical metrics are introduced to assess and determine the optimum source–detector measurement configuration in terms of data correlation and image space resolution. The key idea of the work is to weight each data measurement, or rows in the sensitivity matrix, and similarly to weight each unknown image basis, or columns in the sensitivity matrix, according to their contribution to the rank of the sensitivity matrix, respectively. The proposed metrics offer a perspective on the data sampling and provide an efficient way of optimizing the sampling schemes in DOT. We evaluated various acquisition geometries often used in DOT by use of the proposed metrics. By iteratively selecting an optimal sparse set of data measurements, we showed that one can design a DOT scanning protocol that provides essentially the same image quality at a much reduced sampling.
The optimization of experimental design prior to deployment, not only for cost effective solution but also for computationally efficient image reconstruction has taken up for this study. We implemented the iterative method also known as effective independence (EFI) method for optimization of source/detector pair configuration. The notion behind for adaptive selection of minimally correlated measurements was to evaluate the information content passed by each measurement for estimation of unknown parameter. The EFI method actually ranks measurements according to their contribution to the linear independence of unknown parameter basis. Typically, to improve the solvability of ill conditioned system, regularization parameter is added, which may affect the source/detector selection configuration. We show that the source/detector pairs selected by EFI method were least prone to vary with sub optimal regularization value. Moreover, through series of simulation studies we also confirmed that sparse source/detector pair measurements selected by EFI method offered similar results in comparison with the dense measurement configuration for unknown parameters qualitatively as well as quantitatively. Additionally, EFI method also allow us to incorporate the prior knowledge, extracted in multimodality imaging cases, to design source/detector configuration sensitive to specific region of interest. The source/detector ranking method was further analyzed to derive the automatic cut off number for iterative scheme.
We investigated a novel design concept of index-guided tapered LDs with linearly effective-refractive-index variation to make a quality beam in 808 nm for intermediate power LDs between a few decades of mW to ~ W. In this concept, the tapered width at each position in the propagation direction varies linearly depending on change in effective-refractive-index not geometry. We use GaAsP/InGaP/InGaAlP quantum well LD structure of 808 nm and standard LD fabrication processes to test. To design a detail structure, we use the effective-refractive-index method and transfer matrix method. The tapered ridge LD with linear effective-refractive-index variation shows more stable in beam quality but needs more study to optimize the structure.