Nonlinear transmission is investigated in a hydrogenated amorphous silicon optical fiber extending into the mid-infrared region. Low losses past the two-photon absorption edge allow for strong spectral broadening in this important wavelength regime

Badding, J.V.

Day, T.D.

Healy, N.

Peacock, A.C.

Shen, L.

Sparks, J.R.

Southampton (e-Prints Soton)

Two-Photon Absorption and Self-Phase Modulation inSilicon Optical Fibers into the Mid-Infrared RegimeL. Shen1, N. Healy1, T. D. Day2, J. R. Sparks2, J. V. Badding2, and A. C. Peacock11. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom2. Department of Chemistry and Materials Research Institute, Pennsylvania State University 16802 PA, USAacp@orc.soton.ac.ukAbstract: Nonlinear transmission is investigated in a hydrogenated amorphous silicon opticalfiber extending into the mid-infrared region. Low losses past the two-photon absorption edge allowfor strong spectral broadening in this important wavelength regime.OCIS codes: (060.2270) Fiber characterization; (060.2290) Fiber materials; (060.4370) Nonlinear optics.Nonlinear silicon photonics is a rapidly growing area of research with applications ranging from optical switching to photonpair generation. More recently, hydrogenated amorphous silicon (a-Si:H) has become an increasingly popular material fornonlinear silicon photonics due to its low transmission losses, high Kerr nonlinearity and low fabrication costs [1]. To date,this material has largely been characterized at telecoms wavelengths where the nonlinear losses are expected to be lowerthan crystalline silicon (due to the larger bandgap and hence the shorter two-photon absorption cut-off wavelength), but yetthese have still been shown to be non-negligible [1,2]. In this paper we extend our nonlinear investigations of the a-Si:H corefibers to present the first characterization of both the TPA parameter βTPA and the Kerr nonlinear index n2 over a wavelengthregion that crosses the TPA edge. The low optical losses approaching 2 µm allow for strong spectral broadening indicatingthe suitability for these fibres to find applications in the mid-infrared regime.Our silicon core fibers are fabricated using the well-established high pressure microfluidic chemical technique to deposita-Si:H material into the central hole of a silica capillary [2]. The fiber used in our characterization has a core diameter of∼ 6 µm and a linear loss starting at 2.5dB/cm for 1540nm and decreasing to 1.5dB/cm at 2000nm. The bandgap energy ofamorphous silicon is known to be around Eg ∼ 1.7eV. To characterize βTPA across the TPA edge (Eg/2) we used a Ti:sapphirepumped OPO with 200fs pulses at 80MHz, tuned from 1400nm to 2200nm. By measuring the output power as a function ofthe coupled input power, βTPA can be obtained from the slope of the inverse of the transmission given by 1/T = Pin/Pout [1].The corresponding values are plotted as a function of wavelength in Fig. 1(a) with the results showing that βTPA drops rapidlyas it approaches 1750nm, indicating a bandgap energy for our material of ∼ 1.4eV. Following this, it is useful to characterizethe nonlinear spectral broadening for wavelengths both above and below the estimated TPA edge. Figs. 1(b) and (c) show thespectral broadening due to self-phase modulation at 1540nm and 1850nm, respectively. It is clear that whilst both spectraexhibit significant broadening at high pump powers, a slightly larger bandwidth of ∼ 250nm is obtained at 1850nm comparedto ∼ 80nm at 1540nm. By fitting the experimental spectral broadening using a modified form of the Schro¨dinger equationwe have previously characterized n2 to be ∼ 1.8× 10−13 cm2/W at 1540nm, which is more than twice that of crystallinesilicon [2]. Using similar analysis we will characterize the values of n2 for our a-Si:H core material across the full wavelengthregion and determine the relative nonlinear figure of merit of these fibers in both the near and mid-infrared regions.50W(a) (b) (c)Fig. 1. (a) TPA parameter as a function of wavelength. Spectral broadening for (b) 1540nm and (c)1850nm, input peak powers given in the legend.References1. B. Kuyken, H. Ji, S. Clemmen, S. K. Selvaraja, H. Hu, M. Pu, M. Galili, P. Jeppesen, G. Morthier, S. Massar, L.K. Oxenlwe, G.Roelkens, and R. Baets, “Nonlinear properties of and nonlinear processing in hydrogenated amorphous silicon waveguides,” Opt.Express 19, B146–B153 (2011).2. P. Mehta, N. Healy, N. F. Baril, P. J. A. Sazio, J. V. Badding, and A. C. Peacock, “Nonlinear transmission properties of hydrogenatedamorphous silicon core optical fibers,” Opt. Express 16, 16826–16831 (2010).

Two-photon absorption and self-phase modulation in silicon optical fibers into the mid-infrared regime

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