Giving to CREOL CREOL, The College of Optics & Photonics

Seminar: "Phase-locking of arrays of weakly coupled semiconductor lasers" by Yehuda Braiman

Wednesday, October 24, 2018 12:00 PM to 1:00 PM
CREOL Room 103


Yehuda Braiman
Computational Sciences and Engineering Division
Oak Ridge National Laboratory
Department of Mechanical, Aerospace, and Biomedical Engineering
The University of Tennessee

Abstract:

Semiconductor laser diodes are employed for a wide variety of applications. Such lasers can emit light in wide range of wavelengths, exhibit very high electro-optical efficiency, are compact, and are low cost. However, a single diode’s emission power is in the range of Watts or lower. Consequently, beam combining of many diodes is required in order to provide high emission radiance from an array. The master oscillator power amplifier (MOPA) designs have been shown to allow for almost perfect semiconductor diode phase locking for arrays containing hundreds of diodes. For external cavity designs, while excellent beam quality from single mode and broad-area diode arrays has been demonstrated, the scalability to very large arrays and stacked-arrays still remains an open matter. In the case of perfect global coupling (also referred to as all-to-all or mean-field coupling) it is understood that large systems (including semiconductor lasers) with appropriately chosen parameters and sufficiently low disorder will synchronize. However, perfect global coupling can be hard (or impossible) to achieve experimentally for large diode arrays because it requires that the coupling strengths between any two diodes will not depend on the distance between the diodes. It is important to elucidate whether phase locking of large semiconductor diode arrays is possible and, if possible, what type of external cavity designs allow for phase synchrony.
A large body of experimental work has been reported for semiconductor laser arrays subject to external cavity feedback. The motivation of those experiments was to achieve phase locking (phase synchrony) of the array and subsequently achieve as close as possible to diffraction-limited emission from the array. A variety of cavity designs have been studied including the V-shape cavity, self-Fourier cavity, and external Talbot cavity. Though some of these experiments were performed using high power broad-area semiconductor diodes, a single transverse mode emission was achieved by inserting a grating into the cavity. In these experiments commercial quality diodes were employed; consequently, the heterogeneity range of the laser diodes was large. Still, an almost perfect diffraction limited beam was reported indicating that largely heterogeneous and noisy diode arrays can robustly and spontaneously phase synchronize provided the coupling network geometry is appropriately chosen. One could wonder (a) what causes these arrays to synchronize despite their heterogeneity (though it is clear that in-phase solution for such array is not possible), and (b) whether and how the degree and characteristics of phase synchronization depend on the size of the array.
We have studied the conditions for achieving almost perfect phase locking in large arrays of semiconductor diodes. We have shown that decayed non-local coupling of diode lasers can provide the necessary conditions for robust phase synchronization of an entire diode laser array, independent on the number of diodes in the array. This phase-locked state may be robust to noise, frequency and phase disorder and can be realized under periodic (fixed-intensity limit cycle) continuous-wave and chaotic behavior of lasers. When diodes are coupled via the decayed non-local coupling layout, the dominant transverse mode of the laser array has a uniform phase across the lasers and can be stable. Spatial mode selection in laser arrays has been widely studied as a plausible mechanism of passive phasing of laser arrays. Modal analysis has also been applied to single resonators as well as compound-resonators. In order to study the stability of spatial modes, we applied a modified version of Master Stability Function (MSF) theory to an array of weakly coupled semiconductor lasers described by the Lang-Kobayashi equations. Employing an extended version of the Master Stability Function (MSF) formalism, we showed that an almost perfect (but not in-phase) phase synchronous state (fixed point, periodic limit cycle, and chaotic dynamics) described by non-
constant eigenvectors can be realized. We also showed that one can generate non-synchronous transverse modes that lead to chaotic anti-phase synchronization. To the best of our knowledge, chaotic anti-phase synchronization has not yet been demonstrated in large arrays of coupled lasers and/or nonlinear oscillators. This result is an example of linear mode selection in a highly nonlinear system.

Biography:

Dr. Braiman is a Distinguished R&D Scientist at Computational Sciences and Engineering Division, Oak Ridge National Laboratory and a Joint Faculty Professor at the Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, TN. Dr. Braiman’s co-authored papers in the fields of nonlinear dynamical systems and chaos, control of chaos, synchronization and beam combining of high power laser arrays, friction and control of friction at the nanoscale, nanoscale fracture propagation in metallic glasses, arrays of superconducting Josephson junctions, cryogenic memory cell design, and accelerator physics. Dr. Braiman also co-authored patents and patent applications in the topics of coherent beam combining of diode laser arrays, friction control at nanoscale, and cryogenic memory design. Some of the Dr. Braiman research accomplishments may be attributed to: (1) discovery of chaos in Josephson junctions; (2) devised a method to control chaos (article cited more than 400 times); (3) suggested that spatiotemporal chaos in nonlinear dynamical systems can be eliminated by disordering the parameters of the array elements (Article highlighted on the cover page of Nature and in Nature News & Views); (4) proposed design for ultrafast and energy efficient cryogenic memory cell (article published in Superconductor Science & Technology and was selected for the Superconductor Science and Technology 2016 Highlight Collection); and (5) derivation of the conditions for achieving almost perfect and scalable phase locking in large arrays of semiconductor diode lasers.

For additional information:

Bahaa Saleh
besaleh@creol.ucf.edu

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