Compact Wavelength Demultiplexers and Spectrometers for Integrated Photonics Enabled by Dispersion Engineering in Photonic Crystals
Compact wavelength demultiplexing devices are of great interest for several integrated photonic applications including optical information processing, optical communications and networking, as well as integrated optical sensing (e.g., lab-on-a-chip) systems. The compactness of these demultiplexing devices and their compatibility
with integrated photonic and electronic platforms are among their main advantages over other wavelength demultiplexing solutions. These advantages are the result of the wide range of controllable dispersive properties in a very compact structure enabled by using photonic crystals. Such dispersive properties are not available in conventional structures that usually rely on bulk or grating-based materials.
For efficient implementation of wavelength demultiplexing devices, we use photonic crystals as the core material. Recently, photonic crystals have been proposed to control optical properties of the material through fabricating subwavelength periodic features. It has been shown that photonic crystals can have unique dispersive properties
that are not available in naturally occurring optical materials. In particular, when properly designed, photonic crystals can angularly separate optical beams of different wavelengths in considerably different directions – an effect called the superprism effect in an analogy with the way conventional prisms work. The superprism effect in photonic crystals is the primary physical effect used for the separation of wavelength channels of an optical signal (i.e.,
wavelength demultiplexing) as shown in Fig. 1(a).
Figure
1. The three
dispersive properties of photonic crystals used for wavelength
demultiplexing are schematically demonstrated: (a) the superprism
effect, (b) the negative diffraction, and (c) negative refraction.
Ideally, to spatially separate different wavelengths in a compact fashion, a large angular difference between the directions of propagation of different wavelengths is desired. This would translate into a strong superprism effect for the photonic crystal of choice. However, rapid variation of the angle with wavelength in the photonic crystal band
structure naturally occurs along with strong divergence of the optical beams. As a result, the beams corresponding to different wavelengths will have a large divergence angle in these structures, and thus, undergo large broadening during propagation. The complete spatial separation of optical beams at different wavelengths under such a large divergence (as performed in the former implementation of superprism-based photonic crystal demultiplexers) requires large propagation lengths, which takes away the advantage of compactness in these structures especially for high-resolution applications.1,2
A compact wavelength demultiplexing device can be realized if the effect of the diffractive broadening is avoided in the wavelength demultiplexing device. Photonic crystals have been shown to have the possibility of diffraction effects negative to that of ordinary bulk materials –an effect called negative diffraction,3,4 shown in Fig. 1(b). Propagation in a proper length of any material with inverse diffraction effects can cancel out the diffractive broadening (or beam divergence) and result in small spot sizes at the output plane of the wavelength demultiplexing device.5-7 Another
important need of the wavelength demultiplexers is the separation of stray signals (e.g., unwanted wavelengths, unwanted polarizations, etc.) at the output of the demultiplexer. This feature can also be added to the photonic crystal demultiplexers by combining the superprism and the negative diffraction effects with another dispersive property of the photonic crystal, i.e. negative refraction, for the desired signal, as shown in Fig. 1(c).
We have shown the possibility of engineering the band structure of photonic Figcrystals through geometrical optimization of the photonic crystal lattice to achieve structures that simultaneously have a strong superprism effect, negative diffraction, and negative refraction.7 Such photonic crystals, therefore, provide the perfect optical material for a compact wavelength demultiplexing device. Figure 2(a) shows the overall device schematic with the three dispersive properties working together for a compact and efficient design. A scanning electron microscopy (SEM) image of a fabricated device on a silicon-on-insulator wafer is shown in Fig. 2(b). Figure 3(a) shows the performance of a 70µm×100µm photonic crystal structure designed as a 4-channel wavelength demultiplexer.7 This compact structure is capable of separating 4 wavelength channels with 8 nm channel spacing and better than 6.5 dB crosstalk isolation with considerably smaller size compared to all other proposed implementation of photonic crystal demultiplexers with similar performance measures.
Figure
2. (a) Schematic of a compact wavelength demultiplexing device formed by simultaneous
presence of three dispersive properties of photonic crystals shown in
Figure 1. (b) SEM image of a fabricated device on a SOI wafer. The
output of the photonic crystal device is sampled by an array of ridge
waveguides.
Figure
3. Spectral response
of the 4-channel wavelength demultiplexer shown in Figure 2(b) is
demonstrated. The wavelength spacing is 8 nm with better than 6.5 dB
crosstalk reduction.
References
1. T. Baba and T. Matsumoto, “Resolution of photonic crystal
superprism,” Appl. Phys. Lett. 81, 2325-2327 (2002).
2. B. Momeni and A. Adibi, “Optimization of photonic crystal
demultiplexers based on the superprism effect,” Appl. Phys. B 77,
555-560 (2003).
3. M. Qiu, L. Thylén, M. Swillo, and B. Jaskorzynska, “Wave
propagation through a photonic crystal in a negative phase
refractive-index region,” IEEE J. Sel. Topics Quantum Electron. 9,
106-110 (2003).
4. B. Momeni and A. Adibi, “An approximate effective index model
for efficient analysis and control of beam propagation effects in
photonic crystals,” J. Lightwave Technol. 23, 1522-1532
(2005).
5. J. Witzens, T. Baehr-Jones, and A. Scherer, “Hybrid superprism
with low insertion losses and suppressed cross-talk,” Phys. Rev. E
71, 026604 (2005).
6.
T. Matsumoto, S. Fujita, and T. Baba, “Wavelength demultiplexer
consisting of photonic crystal superprism and superlens,” Opt.
Express 13, 10768-10776 (2005).
7.
B. Momeni, J. Huang, M. Soltani, M. Askari, S. Mohammadi, A. Adibi,
and M. Rakhshandehroo, “Compact wavelength demultiplexing using
focusing negative index photonic crystal superprisms,” Opt. Express
14, 2413-2422 (2006).