Pawel Strzebonski

Pawel Strzebonski

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Pawel Strzebonski joined professor Choquette’s research group in 2016 after finishing his B.S. degree in electrical engineering at UIUC. He has worked on various projects involving semiconductor lasers and photonics while working on his M.S. degree (electrical and computer engineering, 2018) and Ph.D. degree (electrical and computer engineering), ranging from the theory and modeling of lasers and photonic devices, to the nano-/micro-fabrication and characterization of diode lasers.

Research

Coherently-Coupled Photonic Crystal VCSEL Arrays (2020-2021)

Coherently-coupled photonic crystal VCSEL arrays have been a focus area of the PDRG for many years. These devices have been shown to have numerous novel properties and behaviors. Perhaps most promisingly, when the lasers in an array are optically coupled their modulation bandwidth increases and noise decreases, making these devices a candidate for the next generation of optical communications laser transmitters. However, tuning multiple lasers into the coherently-coupled operating regime has been a challenge.

Pawel has worked on developing automated device measurement and characterization techniques for VCSEL arrays. This includes laboratory instrument automation for measuring large amount of data, as well as developing automated data analysis methods to search through these datasets and extract information about array coherence. Many of the techniques developed are detailed in Pawel’s 2021 Photonics West manuscript, and the machine learning analysis of current-power measurements was refined in his 2021 CLEO work.

Pawel has also worked on developing methods for analyzing the links between the array structure, modes, and performance. This includes developing waveguide models, such as the 2D complex index models discussed in his 2022 JSTQE work.

Photonic Crystal Surface Emitting Lasers (2019-2021)

Photonic crystal surface emitting lasers, commonly referred to as PCSELs, are a type of semiconductor diode laser that has an in-plane optical resonance (as do edge-emitting lasers) but uses photonic crystals to out-couple the light to surface-normal emission (VCSELs also have surface-normal emission). PCSELs have gathered interest due to their ability to scale single-mode lasing to high powers and large emission areas producing very high quality beams. Pawel has explored the theory and modeling aspects of these structures, some of which are discussed in his 2021 Directed Energy S&T Symposium manuscript.

15xx nm InP Mode-Engineered Edge-Emitting Lasers (2017-2021)

The PDRG, along with industry partners, worked on a project to develop high brightness 15xx nm semiconductor diode lasers for laser pumping applications under JTO-MRI award No. 17-MRI-0619. As part of this program, Pawel explored lateral index structuring for edge-emitting lasers. His hypothesis was that lateral index structuring in edge-emitting lasers can be used to control and engineer the lateral modes, improving the laser beam. Dielectric waveguide mode-solving (using eigensolving of the finite difference formulation of the Helmholtz waveguide equation) allows the transverse modal fields to be calculated, and electromagnetic propagation software allows the beams produced by these transverse modes to be analyzed. This analysis is described in Pawel’s M.S. thesis and his 2018 ISLC and 2019 IPC conference papers.

The waveguide simulations were used to layout designs for InP edge-emitting lasers with surface-etched lateral ridges. These designs were then fabricated in cooperation with industry partners. Initial characterization of device performance was described in Pawel’s 2020 IPC conference paper/presentation “Surface-Etched Laterally Structured Semiconductor Laser Diodes for Mode Engineering”.

Theses & Dissertations

M.S. Thesis (2018)

Abstract

Semiconductor laser diodes are used in many applications, and their performance is often strongly influenced by the optical mode properties. In this work, dielectric waveguides and their guided modes have been simulated and analyzed as relevant to semiconductor diode lasers. The transverse refractive index structure of waveguides determines the modal field profiles and modal properties. By engineering the transverse index structure we show that it is possible to increase modal discrimination and impart modal selection. We further show that we can engineer the modal field profile itself for the purpose of engineering the properties of the laser beam, such as far-field brightness. Therefore waveguide engineering using index structuring may be beneficial to enhancing semiconductor laser performance.

M.S. Thesis

Abstract

The performance of a semiconductor laser for many applications is often largely determined by how well the laser beam profile and optical spectrum are matched to the application’s requirements. The beam profile and spectrum, in turn, are primarily determined by the lasing optical mode(s) and any subsequent mode/beam conversion as the mode out-couples from the laser cavity. We discuss optical mode and beam engineering theory and some of the many methods for engineering diode laser modes and beams. We expand a time-varying confinement factor analysis of diode laser dynamics with photon-photon resonance (PPR) effects to multi-cavity systems, such as vertical cavity surface emitting laser (VCSEL) arrays. We compare this analysis to the previous coupling coefficient analysis of diode laser dynamics as applied to VCSEL arrays. We propose a two-dimension complex refractive index waveguide model for 2x1 photonic crystal VCSEL array that enables the analysis of array geometry on the complex coupling coefficient and the effects of symmetric and asymmetric index suppression tuning on the array optical modes and beams. We calculate the small-single modulation response using our multi-cavity time-varying confinement factor analysis to determine that lower mode suppression enables stronger PPR effects, as does increased array asymmetry. We use guided mode expansion to model photonic crystal structures to evaluate the effects of photonic crystal geometry and epitaxial layer thicknesses on the modal resonances, losses, and asymmetry in vertical emissions. Experimental analysis of optical power measurements of VCSELs arrays shows imaginary coupling coefficient decreasing as photonic crystal lattice period is increased, and a combination of optical power and beam profile measurements show array supermode transitions associated with a dip in imaginary coupling coefficient as current injection to the two cavities is increased. We propose a Fourier method of analyzing laser array beam profiles that enables a more resilient analysis of coherence and beam-steering in VCSEL arrays. The theory and models presented in this work link the VCSEL array geometry and current injection to the optical modes and the PPR modulation enhancement, guiding future work on the optimization of array design and current biasing. The experimental analysis supports our models, and the associated computational methods will enable automated coherence tuning of VCSEL arrays.

Ph.D. Dissertation

Final Examination Slides (PDF)

Publications

Conferences Attended