Dr. Byoung-Uk Sohn Profile

Dr. Byoung-Uk Sohn

at Singapore Univ of Technology and Design

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Publications (4)

Proceedings Article | 23 September 2020 Presentation + Paper

Advances in CMOS nonlinear optics: amplification, solitons, and optical waveform manipulation

D. T. H. Tan, D. K. Ng, J. W. Choi, E. Sahin, P. Xing, B.-U. Sohn, H. Gao, Y. Cao, A. Gupta, G. F. Chen

Proceedings Volume 11461, 1146125 (2020) https://doi.org/10.1117/12.2567332

KEYWORDS: Waveguides, Solitons, Nonlinear optics, Refractive index, Dispersion, Photonic crystals, Four wave mixing, Bragg gratings

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Proceedings Article | 23 May 2018 Presentation

Ju Won Choi, Byoung-Uk Sohn, George F. R. Chen, Doris K. T. Ng, Dawn T. H. Tan

Proceedings Volume 10684, 106840T (2018) https://doi.org/10.1117/12.2306492

KEYWORDS: Waveguides, Wavelength division multiplexing, Optical transfer functions, Single photon, CMOS technology, Optical amplifiers, Linear filtering, Quantum computing, Quantum communications, Electronics

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Correlated single photons provide a means to drive applications such as quantum computing and quantum communications. Correlated single photons can be generated via parametric down conversion in second–order nonlinear media or spontaneous four–wave mixing in third–order nonlinear media. In particular, complementary metal–oxide–semiconductor (CMOS) technology allows for seamless integration with electronics, providing the potential for a completely on-chip solution for quantum information processing. Ultra–silicon–rich nitride platform is a backend CMOS compatible platform, that has already been used to obtain high gain optical parametric amplification, wideband supercontinuum and enhanced nonlinearity in photonic crystal waveguides due to its large nonlinearity. In this work, we demonstrate correlated photon pair generation based on spontaneous four–wave mixing using ultra-silicon-rich nitride waveguides for the application in CMOS–based optical quantum technologies. A CW pump at a wavelength of 1555.747nm amplified using an EDFA is filtered through five wavelength division multiplexers (WDM) with a bandwidth of 1.2nm, providing 175dB suppression of EDFA induced pump sideband noise. The filtered quasi–TE pump, adjusted using a fiber polarization controller, is coupled into an ultra–silicon–rich nitride waveguide using a lensed fiber. A SiO2 cladded waveguide with a width of 550nm and height of 300nm possesses a high nonlinear parameter of 530W^-1/m with anomalous dispersion necessary for spontaneous four-wave mixing. The waveguide output is coupled into a lensed fiber and 7 cascaded WDMs are used to provide 245dB of residual pump filtration. The pump–suppressed output is spectrally separated into signal/idler part using WDMs. We refer to lower (higher) frequency photon as the signal (idler). The spontaneously generated signal and idler photons are filtered using cascaded tunable band pass filters (OTF) centered at 1571.24nm and cascaded WDMs centered at 1540.56nm, respectively. The bandwidth of the tunable OTF and WDM is 0.5nm and 1.2nm, therefore the correlated signal/idler photons are observed within the bandwidth window of 0.5nm induced by the OTF. The signal and idler photons are measured using InGaAs/InP avalanche photodetectors. The time correlation between signal and idler photons is obtained using a time interval analyzer with a detection efficiency of 20% and dead time of 15μs.The time bin is set to 81ps and the photon collection time is 240s. The coincidence peak is located ~11ns in the time–bin histogram due to the optical-path difference between the tunable OTF and WDM at respective signal and idler sides. The experimental raw coincidence counts (Hz), calculated by subtracting the accidental rate from the coincidence peak, show a quadratic increase with respect to coupled pump power. At the maximum coupled power of 5mW, the raw coincidence count is ~1Hz. We achieve a raw coincidence–to–accidental ratio (CAR) of up to 3. Therefore, we succeeded to observe correlated photon pair generation based on spontaneous four–wave mixing using the ultra–silicon–rich-nitride waveguide as a CMOS compatible platform, for future applications in quantum technologies.

Proceedings Article | 23 May 2018 Presentation

Nonlinear optical properties of germanium at mid-infrared wavelengths (Conference Presentation)

Byoung-Uk Sohn, Corentin Monmeyran, Lionel Kimerling, Anuradha Agarwal , Dawn Tan

Proceedings Volume 10684, 1068403 (2018) https://doi.org/10.1117/12.2306265

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Group IV photonic materials such as Si and Ge have been used for monolithic applications including on-chip sensors and optical devices in the mid-IR spectral range. The optical properties of germanium have not been reported experimentally to the same extent as silicon in spite of expected merits over silicon. Germanium is expected to possess advantages as large nonlinear optical coefficients, a broad transparency beyond 10um, and large carrier mobility. In this paper, we report nonlinear refractive indices and multi-photon absorption coefficients over the wavelength range between 2um-5um, measured using closed and open z-scan measurements. Samples are scanned through the focal point of a plano-convex spherical CaF2 lens, where an intensity variation with respect to its spatial location exists. The transmitted laser power is measured by a power meter at different sample positions relative to the focal region. Closed Z-scan measurements utilize an aperture in front of the detector whereas open Z-scan measurements do not. Closed Z-scan measurements are typically used for the quantification of nonlinear refractive indices while open Z-scan measurements are used for the characterization of nonlinear absorption coefficients. The closed Z-scan measurement values are then divided by the open Z-scan measurement values to remove the effects of nonlinear absorption before the nonlinear refractive index is measured by fitting [B.-U. Sohn et al., APL 111(2017)] Ultrashort pulses with a temporal width of 150fs at a1kHz repetition rate are used for the measurements. The mid-infrared optical pulses originate from an optical parametric amplifier and difference frequency pumped by a Ti:sapphire regenerative amplifier. The low repetition rate of the pulses and ultrashort temporal pulse width which is much shorter than the thermal diffusion time scale of 40µs in Ge, is ideal to mitigate effects of heat phonons on the nonlinear effects under study. The nonlinear refractive index of Ge is characterized to possess the highest value, 9.1*10-5cm2/GW at a wavelength of 3um, corresponding to the two photon absorption edge. This result is supported by Kramers-Kronig relation between two photon absorption and nonlinear refractive index n2. The value of n2 is observed to vary between 4*10-5cm2/GW to 5*10-5 cm2/GW within the 3.5-5µm wavelength range. Considering Ge’s bandgap of 0.66eV, two photon absorption and three photon absorption occur in Ge at wavelengths between 2-3.6µm and 3.6-5.5µm respectively. The two photon absorption coefficient has the largest value, 25.6cm/GW at 2.2µm and possesses a relatively constant value with average of 0.71cm3/GW2 between 4-5.5µm. The four photon absorption coefficient is measured to be 0.007cm5/GW3 at 6µm. We further investigate the nonlinear figure of merit (FOM), which is proportional to n2 and inversely related to multi photon absorption coefficient. A large FOM is achieved in wavelengths where n2 is large and multi-photon absorption effects are weak. The FOM has a high value of 0.08 between 2.5 - 3 µm making germanium an efficient material for applications in nonlinear optical devices.

Proceedings Article | 23 May 2018 Presentation

Broadband four-wave mixing and optical parametric gain of broadband incoherent light using ultra-silicon-rich nitride waveguides (Conference Presentation)

Ju Won Choi, Byoung-Uk Sohn, George F. R. Chen, Doris K. T. Ng, Dawn T. Tan

Proceedings Volume 10684, 106840X (2018) https://doi.org/10.1117/12.2306075

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Four–wave mixing (FWM) serves as the physical basis for various nonlinear phenomena including wavelength conversion, parametric amplification, and frequency combs. FWM on a chip has been implemented using CMOS platforms, chalcogenide glasses and III–V materials. On-chip, waveguide based stimulated FWM techniques have been mostly demonstrated using a coherent pump and coherent signal to focus on broadband spectral tuning for operation in high–speed and multi–channel wavelength division multiplexing network. Though FWM using incoherent light has the potential to provide large optical conversion efficiency, such demonstrations remain largely confined to fiber–experiments and involved narrow–band signals/idlers. Furthermore, the FWM based on a pulsed laser and a broadband incoherent source has yet to be implemented. In this work, we demonstrate integrated ultra–silicon–rich nitride parametric converters that perform wavelength conversion of a broadband incoherent source with a bandwidth of ~100nm at the -20dB level. A 500fs pulsed pump is combined with an incoherent superluminescent diode (SLD) as the signal and parametric gains between 12dB – 27dB is demonstrated as well as cascaded FWM. A 500fs pulsed laser centered at 1.555μm and an incoherent SLD with a 20dB bandwidth spanning from 1.6 – 1.7μm are used as the pump and signal respectively. The pump and signal are combined with a wavelength division multiplexer and coupled into an ultra–silicon–rich nitride waveguide with 10mm length, 700nm width and 400nm height. The waveguide is designed to have a larger nonlinear parameter of 330W^-1/m while possessing anomalous dispersion of -0.92ps^2/m, necessary for phase matched parametric conversion. At a coupled peak power of 4.6W, an idler spanning from 1.43 – 1.52μm at the -20dB level is generated. At a maximum input signal power of 0.71mW, a second idler appears at the blue side of the first generated idler because of cascaded FWM induced between pump of 1.555μm and the first idler peak of 1.48μm. At a coupled peak power of 2.8W, an idler spanning from 1.46 to 1.52μm is generated. The experimental idler bandwidth agrees well with the calculation based on degenerate FWM phase–matching condition. The broadened idler powers are calculated by integrating the energy of each signal and idler with respect to wavelength to obtain optical conversion efficiencies. The integrated idler power is 3.4dBm and 13.4dBm, corresponding to idler parametric gain of 12dB and 18dB respectively at a coupled peak power of 2.8 and 4.6W, respectively. The application of the SLD signal to a supercontinuum that is generated at a coupled peak power of 26W spectrally spanning 1.1 – 1.7μm is observed to generate an idler power of 14dBm within the wavelength range of 1.18 – 1.42μm as well as an idler conversion efficiency/gain of 27dB. Therefore, we achieved broadband wavelength conversion based on stimulated FWM using a pulsed pump and broadband incoherent signal that facilitate the spectrum spanning from 100nm, sufficient to cover parts of the E– and S–bands an representing large conversion efficiency and parametric gains of 12dB – 27dB.

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