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Broadband Supercontinuum Based Measurements of High-Index

Contrast Photonic Bandgap Devices from 1 to 2 Jlm

Peter T. Rakich, Juliet T. Gopinath, Hideyuki Sotobayashi, Chee Wei Wong, Steven G. Johnson, John D. Joannopoulos, and Erich P. Ippen

Research Laboratory of Electronics Massachusetts Institute of Technology. Cambridge MA 02139

rakich@mit.edu

Abstract: We demonstrate ultra-broadband supercontinuum as a white-light source for characterizing high-index-contrast photonic circuits at telecommunications wavelengths. Supercontinuum-based measurements are used to map the band structure of one­dimensional photonic crystal microcavities over the wavelength range 1.2-2.0 J.!m.

Introduction High-index contrast photonic crystal waveguides are of increasing interest for a variety of applications such as integrated optics, quantum optics and super resolution. Conventional measurement techniques employ tunable lasers to scan the broad spectral features of photonic crystals, most notably photonic bandgaps. However, few practical tunable sources span the entire 1-2 J.!m spectral range that is critical for the characterization of broadband photonic devices at telecommunications wavelengths.

In order to meet this challenge in measurement capability, we have demonstrated a fiber-based supercontinuum source for the measurement of nano-scale photonic bandgap waveguides over a broad spectral range, spanning from 1.2 to 2.0 J.!m. Previous work has used supercontinuum centered at 800 nm to measure two-dimensional photonic-crystal slab waveguides with a system utilizing free space optics [1-2]. In contrast, we use a supercontinuum source centered at telecommunications wavelengths (1.55 J.!m) in an all-fiber measurement apparatus which has been designed to optimize coupling and collection efficiencies, reducing the need for high power continuum sources. We demonstrate the utility of this new technique through the measurement of high-index contrast (HIC) ID photonic crystal microcavities.

Measurement Apparatus A number of different nonlinear fibers were used to generate broad supercontinuum spectra enabling high fidelity measurements of photonic bandgap microcavities. The source used in this study employed 9 m of highly nonlinear Furakawa fiber (HNL-fiber) having a dispersion of -1.259 ps/nmlkm at 1550 nm, and a

nonlinear coefficient of 22 (W·kmr1. Supercontinuum (SC) was produced by coupling 150 fs pulses centered at 1540 nm from an optical parametric oscillator (OPO) into the fiber. The resulting SC light was coupled in and out of the waveguide with high-numerical-aperture lensed fibers (LF). A detailed schematic of the apparatus and a characteristic SC spectrum are shown in Fig. 2.

Due to the nonlinear nature of the supercontinuum light source, small changes in laser or fiber state can cause significant and rapid fluctuations in the output spectrum. In addition, spectral variations of 10-20 dB were typical for many of the sources studied. For this reason, it was necessary to measure a small fraction of reference power (reference) directly from the SC light source with a broadband fiber optic coupler, in order to normalize the waveguide transmission measurements. This was done by imaging the light from the waveguide (signal) and the reference, through a monochromator onto two identical photodetectors at the exit plane of the monochromator. Polarization control of the light launched into the waveguide, and polarization analysis of the light collected from the guide was performed with the use of broadband integrated polarizers (lP) and strain-type polarization controllers (PC) as seen in Fig. 1.

Device under study The device studied with this apparatus is a one-dimensional photonic-bandgap microcavity [3]. It consists of a silicon strip on top of an oxide layer, which forms a single mode waveguide, supporting only a TE-like mode. The photonic crystal is formed by etching a periodic array of holes through the waveguide. The microcavity defect is formed by increasing the hole spacing from that of the lattice constant (a) to ad, the defect length. The design investigated is a symmetric microcavity, having a defect in the middle of the photonic crystal such that it is surrounded by five holes on either side.

Transmission measurements of the photonic bandgap and microcavity resonance, taken by the method described above, are shown in Fig. 2. A 290 nm stop-band is clearly visible from approximately 1400-1690 nm, and is seen as a 10-20 dB drop in transmission over these wavelengths. In addition, a sharp microcavity resonance can be seen at 1541 nm, demonstrating efficient power coupling through the

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photonic crystal (approximately 73% transmission). For comparison, band structure calculations of this same structure were performed based on parameters extracted from scanning electron microscope (SEM) measurements. The computed band structure predicts a bandgap from 1470-1730 nm, which differs in center frequency from the measurement by 2.7%. Given that the uncertainty of such SEM measurements is 5 percent, the discrepancy between simulations and measurements is reasonable.

A measurement of the same microcavity resonance visible in Fig. 2(a) can be seen in Fig. 2(b) with 0.1 nm spectral resolution. A Lorenzian fit of the microcavity resonance yields a spectral width 3.6 nm,

demonstrating a microcavity Q of approximately 430. Deviation of the micro cavity spectral shape from Lorenzian is primarily due to Fabry-Perot oscillations which result from reflections at the end facets of the waveguide.

Conclusion In conclusion, we have demonstrated that supercontinuum based white-light sources can be utilized to perform high fidelity optical transmission measurements of waveguides and photonic crystals over broad spectral ranges in the near-IR. Direct coupling of continuum light spanning wavelengths from 1.2 to 2.00 11m into the devices facilitated the mapping of the photonic bandgaps as well as the cavity mode frequencies and quality factors with a wavelength resolution comparable to that achievable with tunable lasers.

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Wavelenqth (nm) Figure 1. (a) A schematic of measurement apparatus. A small fraction of SC light is diverted by a coupler for reference measurement (Ref) while the remainder (Signal) is passed through the waveguide. The reference and signal intensities are then simultaneously imaged through the same monochromator and onto similar photodetectors. (b) A characteristic supercontinuum spectrum generated by coupling OPO pulses into the HNL-fiber. Apparent roll-off of SC spectrum at 2 11m is due to decreasing photodetector response.

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Figure 2 (a) A transmission measurement of photonic crystal microcavity (TE polarization) which has been normalized to that of a similar straight waveguide. A photonic bandgap (1400-1690 nm) and microcavity resonance are observed. SEM of photonic crystal device is shown on inset of transmission measurement. (b) High resolution (0.1 nm) measurement of the microcavity resonance. Fabry-Perot oscillations resulting from waveguide end facets result in some distortion of microcavity resonance.

We thank Takeshi Yagi, Ryuichi Sugizaki , Jiro Hiroish and Ryo Miyabe of Furakawa for supplying the highly nonlinear fiber.

References [I] R.T. Neal, M.D.C. Charlton, G.J. Parker, C.E. Finlayson, M.C. Netti, J.J. Baumberg, App!. Phys. Lett. 83, 4598-600 (2003). [2] M.C. Netti, C.E. Finlayson, lJ. Baumberg, M. Charlton, M. Zoorob, J. Wilkinson, G. Parker, App!. Phys. Lett. 81,3927-9 (2002) [3] J.S. Foresi, P.R. Villeneuve, J. Ferrera, E.R. Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L.C. Kimerling, H.I. Smith, E.P. Ippen, Nature 390, 143-5 (1997).

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