On-chip large dispersion using cladding-modulated Bragg gratings

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FM3A.4.pdf Frontiers in Optics 2017 © OSA 2017
On-chip large dispersion using cladding-modulated Bragg
gratings
Ezgi Sahin1,2,3, Kelvin J. A. Ooi1, C. E. Png2 and Dawn T. H. Tan1, *
1Photonics Devices and Systems Group, Engineering Product Development, Singapore University of Technology and Design (SUTD), 8 Somapah
Road, 487372, Singapore
2Department of Electronics and Photonics, Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR),
Fusionopolis Way, #16-16 Connexis, 138632, Singapore
3ezgi_sahin@mymail.sutd.edu.sg
*dawn_tan@sutd.edu.sg
Abstract: A cladding-modulated Bragg grating is realized for creating ultra-large dispersion.
Normal dispersion as large as -11.5 ps/nm is demonstrated on a silicon chip. Devices possess
absolute group delay dispersion × bandwidth product up to 218 ps.
OCIS codes: (130.3120) Integrated optics devices; (260.2030) Dispersion.
1. Introduction
The generation of chip-scale dispersion still falls short of magnitudes needed for advanced optical signal processes
such as dispersive Fourier transforms [1], dispersion compensation [2] and temporal lensing [3]. Photonic crystal
waveguides and ring resonators have been explored for the generation of optical dispersion. However, photonic
crystal waveguides suffer from both coupling and propagation losses. Addressing the coupling losses [4] comes at
the cost of higher device complexity and larger footprint. Whereas ring resonators’ limited bandwidth makes them
unsuitable for the management of signals with high spectral content.
We propose a design for generation of on-chip dispersion in a small footprint. Cladding-modulated Bragg
gratings consist of a silicon channel waveguide and adjacent pillars which provide the mechanism for varying the
effective index perturbation, magnitude and sign of dispersion generated [5]. Coupling coefficient is changed by
pillars’ positioning which alters dispersion and bandwidth [6]. Using spiral waveguide structures, it was
demonstrated that the dispersion scales with waveguide length. Using this method, we demonstrate dispersion as
large as -11.5 ps/nm with a |GDD| × bandwidth product of 218 ps with a footprint of 1.27 × 1.7 mm2.
2. Design and fabrication
The schematic diagram given in Fig. 1 (a) shows the cladding modulation by positioning of pillars adjacent to a
silicon waveguide. The period of the pillars increases linearly in the z-direction, where the average pitch, Λave is
between 306 nm and 315 nm to target the Bragg wavelength within the C-band. The period of the pillars as a
function of z is given by, Λ(z)= Λave+ (z/L) ×∆Λ, where ∆Λ is the device chirp. Positive chirp creates anomalous
dispersion. To eliminate the ripples in stop band, gap between pillars and waveguide is apodized using the function
Gapod(z)=Fapod×cos2(π (z/L – 0.5)) +G in straight waveguides [6].
Fabrication was done on a silicon-on-insulator (SOI) platform with a 3 µm buried oxide layer and 250 nm thick
Si layer. E-beam lithography (EBL). Scanning electron micrograph of a fabricated device is shown in Figure 1 (b).
Straight cladding-modulated Bragg gratings with Λave = 306 nm and 315 nm, G = 50 nm, 100 nm, 150 nm and 200
nm, ∆Λ = 4 nm, 7 nm, 10 nm and 13 nm are fabricated. Using these as a starting point, we designed and fabricated
spiral dispersive elements as shown in Fig. 1 (c), aiming to maximize the dispersive length to achieve high
compactness, while avoiding phase distortions of optical field induced by waveguide stitching, as well as
maintaining an adiabatic change.
3. Results
An interferometric method [7,8] using Fabry-Pérot (FP) oscillations within the stop band is used to extract the
dispersion characteristics of the devices. For straight waveguides, one input port of the device generates normal
dispersion while the other generates anomalous dispersion. For waveguides of a spiral structure, devices with only
negative chirp hence generating normal dispersion, were fabricated. A y-junction [9] was added to the design for
ease of alignment during the optical characterization process. A broadband source with a wavelength range of 1520
nm- 1610 nm was used with an in-line polarizer to maintain TE polarization. The reflected spectrum was measured
using a circulator and an optical spectrum analyzer (OSA).
Group delay dispersion (GDD) information was extracted using the oscillation periods within the stopband, and
calculated using the path length difference, L(λ)=λ2 / (Δλ ng(λ)), where ng(λ) represents the group index for the

FM3A.4.pdf Frontiers in Optics 2017 © OSA 2017
silicon waveguide (calculated to be 4.4) and Δλ represents the FP oscillations as seen in Fig. 1 (d). The group delay
was calculated by τ(λ)=2 ng L(λ)/c. A linear fit was applied to the group delay data to calculate GDD from the slope
of the linear curve as GDD is given by first derivative of group delay with respect to wavelength. Fig. 1 (e)
demonstrates this calculation step for a straight cladding modulated device, the slope of the curve giving
GDD=0.838.
Fig. 1. (a) Device schematic for dispersive element. Fapod=100 nm, L=1 mm, W=500 nm and r=100 nm. (b) SEM
micrograph of fabricated device. (c) Schematic of spiral structure used to increase device length. (d) Peak detection over
stop band to extract period of FP oscillations where Λave =306 nm, Λ=10 nm, G=50 nm, (e) linear curve fitting on group
delay calculated from (d). (f) Group delay extracted from FP oscillations from device having L=10.6 mm. (g) Scaling of
GDD with length for normal dispersion where Λave =311 nm, Λ=7 nm G=50 nm.
Characterization of the straight cladding-modulated Bragg gratings revealed that |GDD| decreases with larger
chirp. A smaller gap G generates a wider bandgap which results in group delay being distributed over a larger stop
band, therefore, as gap increases, |GDD| increases. Using the results from these straight waveguides as a starting
point, we designed spiral dispersive element as shown in Fig. 1 (c), to create very large dispersion on chip. This
spiral cladding-modulated Bragg gratings increase the waveguide length that can be achieved on the same footprint.
Using the spiral structures, we demonstrated a |GDD| × Δω product of 218 where GDD is -11.5 ps/nm for a grating
length of 10.6 mm, group delay curve of this device is given in Fig. 1(f). This large value of dispersion is achieved
over a large bandwidth of 20 nm. The relationship between device length and the achievable large GDD values are
shown in Fig. 1 (g).
Achieving large anomalous dispersion values is possible by applying increasing pitch to the pillar positions,
rather than decreasing pitch. Generating dispersion as large as -11.5 ps/nm over a bandwidth of 20 nm is
demonstrated which has potential applications for ultrafast pulse processing applications such as dispersive Fourier
transforms, dispersion compensation, optical pulse compression and high speed sampling which today still leverage
non-compact, heterogeneous integration with fiber based dispersive elements.
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