Highly efficient (>94%) contradirectional coupling into and out of a photonic crystal waveguide is measured using a fiber taper probe as both a source and a collector

Barclay, Paul E.

Borselli, Matthew

Painter, Oskar

Srinivasan, Kartik

English

Caltech Authors - Main

Efficient input and output coupling between planar photonic 
crystal waveguides and fiber tapers 
Paul E. Barclay, Kdrtik Srinivasan, Matthew Borselli, and Oskar Painter 
Departnienr of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA. 
phone: (626) 395-6269, Jax: (626) 795-7258, e-inuil: pbarclay @caltech.edu 
Abstract: Highly efficient (> 94%) contradirectional coupling into and out of a photonic crystal waveg- 
uide is measured using a fiber taper probe as both a source and a collector. 
@ 2003 Optical Society of America 
OCIS codes: (060.1 81 0) Couplers, switches, and multiplexers; (1  30.3 120) Integrated optics devices 
A requirement for llie practical demonstration of photonic crystal (PC) devices is the ability to efficiently couple light 
into and out of them. Fiber tapers [l], used as optical probes, allow highly efficient evanescent coupling [2] between 
photonic crystal waveguidcs (PCWGs) and fiber optics [3]. In addition to its high efficiency, two attractive features of 
this coupling scheme are its mode selective nature and wafer-scale geometry, which we have exploited to measure the 
spatial and dispersive properties of PCWG modes [41. Here we study the fiber-PCWG-fiber coupling efficiency of this 
technique by measuring light which is coupled contradirectionally from a fiber taper into a ECWG and recollected by 
the backwards propagating fiber taper mode [5 ] ,  and show that the coupling efficiency is > 94%. We then discuss the 
implementation of an efficient PCWG-PC cavity probe. 
Fig. 1. (a, Schematic ot the coupliug scheme. (h) Field profile of the IE-I  mode shidled here. (c )  SEM of the PCWG and 
termnatioii. (d) nlustratlon of the expenineiital apparatus and the contradirectional coupling process. 
In order to directly measure the efficiency of this coupling scheme (shown in Fig. l(a)), we designed a PCWG 
that allows light which is coupled into it to be recollected by the backward propagating fiber taper mode. The PCWG 
was formed by a graded defect in a compressed square lattice of air holes in a silicon membrane; a typical fabricated 
PCWG is shown in Fig. l(b), and the mode studied here (labeled TE-1) is shown in Fig. l(c). A highly reflective 
waveguide termination (labeled by r l )  was realized by removing the lattice compression, resulting in a partial bandgap 
as seen by the E - 1  mode. On the opposite end of the PCWG, a poor reflector ( ~ 2 )  was realized by gradually removing 
the defect defining the ECWG. As illustrated by Fig. l(d), by measuring the signals T transmitted through the fiber 
taper and R coupled into the backward propagating fiber mode, the coupling efficiency I K I  of this technique can be 
characterized. 
Figure 2(a) shows measured T and R as a function of wavelength (h) when a fiber taper (- 2 pm diameter) is 
positioned 200 nm above the ECWG. At h = 1600 nm the fundamental fiber taper mode and 'IE-1 PCWG mode are 
phase matched, resulting in a resonant decrease in T and increase in R over a - 20 nm bandwidth. Figure 2(b) shows 
the on-resonance values of T and R as a function of taper height, as well as theoretical fits. Assuming that multiple 
reflections within the PCWG can be neglected, the maximum measured reflected signal, R,, = 0.88, can be used to 
place a lower bound on the coupling efficiency. This value is a measure of light which is coupled into the PCWG, 
reflected at r l ,  and coupled back into the fiber taper; it suggests a coupling efficiency I K I  * 2 =/[?-I I > 0.94. A 
more detailed analysis in Ref. [51 of the dependence of T and R on taper height confirms this intuitive calculation. 
These results promise to allow efficient probing of future integrated PC devices. Of particular interest is efficient 
loading of high-Q PC cavities. Figure 2(c) shows a schematic illustrating a scheme for integrating the PCWG used 
Barclay. et al., Efficient input and output coupling ... IQEC/2004 Page 2 
L 
Waveguide Cavity 
E 
I- 
O 
i 
1565 Wavelength(nm) 1625 0.5 1 1.5 
Taper height (urn) 
Fig. 2. (a) Transnussion (lop) and reflection (bottom) ofthe fiber taper as function of wavzlength for a taper height of 200 nm. 
(b) Measured taper transmission minimum, retlection maximum, and off resonant transmission as a runclion of t a p  height. 
In (a-b) Data is normalized to the taper transnussion in al~enke of the PCWG. (c) Schematic of scheme for integrating the 
PCWG studied here with a suitable high-Q cavity. 
here with a recently demonstrated 161 high-Q (40 000) cavity mode, which is expected to couple preferentially to the 
’ E - 1  PCWG mode. By replacing the mirror r1 in the experiments presented above with the cavity studied Ref. L61, an 
efficienlJber taper-PCWG-PC cavity probe can be realized. 
References 
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Efficient input and output coupling between planar photonic crystal waveguides and fiber tapers

8xperimental demonstration of evanescent coupling from optical fibre tapers to photonic crystal waveguides,”

Efficient input and output optical fiber coupling to a photonic crystal waveguide:’ Subnutted to Opt. Lett., available online at

Optical-fiber based nieasurenient of an ultra-small volume high-Q photonic crystal microcavity,” Submitted to PRL avdlable online at http://arXiv.org/quant-ph/O309

Phase-matched excitation of whisperinpgallery-itlode resonances by a fiber taper.”

Probing the dispersive and spatial properties of planar photonic crystal waveguide modes via highly efficient coupling from o p t i d fiber tapers,” Submitted to Appl. Phys. Lett., available online at http://arXiv.org/quantphl0308070

Probing the dispersive and spatial properties of planar photonic crystal waveguide modes via highly efficient coupling from optid fiber tapers,”

Single Mode Grating Coupling Between Thin-Film and Fiber Optical Waveguides,”

https://authors.library.caltech.edu/24910/1/BARcleo04.pdf

Efficient input and output coupling between planar photonic crystal waveguides and fiber tapers

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