Compact design of a tunable high-pass filter based on one-dimensional photonic crystal waveguide

Abstract

In this work, we proposed a compact design of a tunable high-pass filter based on the one-dimensional photonic crystal waveguide. The device design is simple and cut-off wavelength can be tuned by a few geometric parameters. The spectral characteristics of the high-pass filter are calculated for the wavelength range of 1200 nm to 2000 nm. The numerical simulations suggest that the extinction ratio of >22 dB is obtained for the wavelength range lying in the photonic bandgap for the device footprint of 3.4 x 1 µm2 . We believe that our findings are useful for the realization of photonic integrated circuit filters with miniaturized footprint.

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Compact design of a tunable high-
pass filter based on one-dimensional
photonic crystal waveguide
Butt, Muhammad, Fomchenkov, Sergey
Muhammad A. Butt, Sergey A. Fomchenkov, "Compact design of a tunable
high-pass filter based on one-dimensional photonic crystal waveguide," Proc.
SPIE 11793, Optical Technologies for Telecommunications 2020, 117930C (22
June 2021); doi: 10.1117/12.2591784
Event: Eighteenth International Scientific and Technical Conference "Optical
Technologies for Communications", 2020, Samara, Russian Federation
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Compact design of a tunable high-pass filter based on one-dimensional
Photonic Crystal waveguide
Muhammad A. Butt*a, Sergey A. Fomchenkova,b
aSamara National Research University, Samara, Russia
bInstitute of RAS-Branch of the FSRC “Crystallography and Photonics” RAS, Samara, Russia
ABSTRACT
In this work, we proposed a compact design of a tunable high-pass filter based on the one-dimensional photonic crystal
waveguide. The device design is simple and cut-off wavelength can be tuned by a few geometric parameters. The
spectral characteristics of the high-pass filter are calculated for the wavelength range of 1200 nm to 2000 nm. The
numerical simulations suggest that the extinction ratio of >22 dB is obtained for the wavelength range lying in the
photonic bandgap for the device footprint of 3.4 x 1 µm2. We believe that our findings are useful for the realization of
photonic integrated circuit filters with miniaturized footprint.
Keywords: Photonic Crystal, high-pass filter, ridge waveguide, silicon photonics.
1. INTRODUCTION
Photonic integrated circuits (PICs) based on high-index contrast silicon-on-insulator (SOI) platform are perfect due to
their competence of on-chip operations such as optical interconnections and optical sensing, among others1-7. These
elements can be developed with standard complementary metal-oxide-semiconductor (CMOS) manufacturing
technology. Wavelength filtering devices are primary elements that are used in applications such as fluorescence
microscopy, switching, spectroscopy and wavelength division multiplexing8,9. One-dimensional Photonic crystals are the
straightforward structure in a photonic crystal family. Intriguingly, these crystals still hold numerous stimulating
characteristics such as modifiable dispersion and birefringence, analogous to homogeneous materials10-12. The
uncomplicated structure of one-dimensional Photonic crystals is easily integrated with the existing photonic devices
without varying fabrication process in comparison with two-dimensional or three-dimensional photonic crystals.
Accordingly, several high-performance silicon-based devices have been studied and demonstrated by making use of
those exciting properties of one-dimensional Photonic crystals13-16.
In general, One-dimensional Photonic crystal works in the following three regimes, depending on the ratio between the
period (ᴧ) of the structure and the operational free-space wavelength. i) Diffraction region- the incoming light is
scattered in different orders, ii) Bragg-reflection region- the incoming beam is reflected backward, iii) Sub-wavelength
region-the diffraction and reflection effects are concealed due to the periodicity of the structure17-19. Recently, Photonic
crystal waveguides have gathered a significant interest as they might direct to the apprehension of compact photonic
interconnection networks and finally to high-density PICs. Theoretical predictions have shown that an optimized
Photonic crystal waveguide is capable of guiding electromagnetic wave with a minimum loss, even around bends with a
radius of curvature analogous to the wavelength of guided light20. There are several applications which are demonstrated
such as modulators21, sensors22, 23, filters24, 25, polarizer26, 27 and lens28 among others. Optical band-pass/band-rejection
filters are an important element in PICs, which can suppress a desired range of wavelengths to very low levels whereas
transmitting the required wavelengths of light29, 30. Broadband tunable optical band-rejection filters based on multimode
one-dimensional Photonic crystal waveguide offering a bandwidth of 84 nm31. The device has a footprint of 40x1 µm2.
Grande et al. established a vertical directional coupler based on the coupling between a polymer waveguide and W1
Photonic crystal waveguide32. A bandpass filter is demonstrated using an apodized subwavelength grating coupler which
provides a sidelobe suppression ratio of 27 dB and 3 dB bandwidth of 8.8 nm33. However, the footprint of the device is
113x2 µm2. The spectral characteristics of these filters are good. Nevertheless, many of them failed to offer broad band-
rejection/band-pass bandwidth or compact footprint.
In this work, a simple design of a high-pass filter is proposed which is based on a one-dimensional Photonic crystal
waveguide. The filter design is simulated via COMSOL Multiphysics 5.5 based 3-dimensional-finite element method
Optical Technologies for Telecommunications 2020, edited by Vladimir A. Andreev, Anton V. Bourdine,
Vladimir A. Burdin, Oleg G. Morozov, Albert C. Sultanov, Proc. of SPIE Vol. 11793, 117930C
© 2021 SPIE · CCC code: 0277-786X/21/$21 · doi: 10.1117/12.2591784
Proc. of SPIE Vol. 11793 117930C-1
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(3D-FEM). The electromagnetic wave frequency domain (ewfd) is selected as a physics interface. The FEM subdivides
outsized elements into smaller parts known as finite elements. This can be obtained by a space discretization in the space
dimensions, which is carried out by constructing a mesh of the object. The accurate meshing is determined on the
precision of the solution and the processing power of the computer. We used a mesh size of λ/30 for the whole filter
model. This meshing provides correct simulation results based on our system processing speed. The open computational
domain is employed as it allows the electromagnetic wave to travel without back reflections. The open geometry is
allocated by employing the scattering boundary conditions at the outer edges of the simulation window. An x-oriented
plane wave is coupled at the input of the waveguide. Furthermore, parametric sweep function is employed to simulate
different geometric parameters of the filter versus the wavelength range. The filter performance is calculated in terms of
extinction ratio (ER) which is expressed in equation (1).
10 log out
in
P
ER P

, (1)
Where Pout and Pin are the output and input power, respectively. And the specific high-pass range can be tuned by varying
the period of the Photonic crystal holes.
2. FORMATTING OF MANUSCRIPT COMPONENTS
At first, the Photonic crystals were explored by Yablonovitch and John34, 35 which has attracted great interest due to its
astonishing electromagnetic behavior and probable employment in optoelectronic-related applications. Photonic crystals
have a photonic bandgap due to Bragg scattering in a periodical dielectric structure which can be compared to electronic
band gaps of semiconductors. Usually, Photonic crystals are designed to eliminate certain surplus wavelengths and can
be employed in one dimension (1D), two dimensions (2D) and three dimensions (3D) depending on the necessity. The
transmission characteristics of the Photonic crystal is exceptional which allows the development of miniaturized optical
waveguides proficient of delivering high performance. The filter design is composed of a one-dimensional photonic
crystal waveguide based on silicon-on-insulator (SOI) platform. The refractive indices of silicon (Si) and silicon dioxide
(SiO2) at room temperature are 3.478 and 1.444, respectively. The periodic array of holes are located in the middle of the
ridge waveguide as shown in figure 1. The radius of the holes is denoted as r where d is the lattice constant. The width
and height of the ridge waveguide are represented as W and H as shown in the inset of figure 1. To simplify the filter
analysis, W and H are maintained at 400 nm and 220 nm throughout the paper, respectively.
d
rSi
SiO2
Top view
Cross-section view
WH
SiO2
Figure 1. Schematic representation of a high-pass filter based on one-dimensional photonic crystal waveguide. Inset shows
the cross-section of the ridge waveguide.
3. RESULTS AND DISCUSSION
One-dimensional Photonic crystal waveguide composed of an array of holes placed in the center of a ridge waveguide
offers even smaller size and stronger grating strength compared to the two-dimensional Photonic crystal waveguides36.
Here, the spectral characteristics of the high-pass filter based on a one-dimensional photonic waveguide are studied as
shown in figure 2. In figure 2a, the transmittance is plotted for the wavelength range of 1200 nm to 2000 nm. The
influence on ER and cut-off wavelength (λcut-off) is determined by varying r and d for a constant number of the period
(N=20). It can be observed that, λcut-off experiences a redshift as r increases. Moreover, there is no prominent variation in
Proc. of SPIE Vol. 11793 117930C-2
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ER of the filter which is >24 dB. λcut-off for the high-pass filter can be tuned from 1422 nm to 1618 nm by varying r from
80 nm to 120 nm. Therefore, if a large high-pass wavelength range is desired then r should be kept as small as possible.
For instance, a high-pass wavelength range of 1422 nm to 2000 nm can be obtained for r=80 nm and d=260 nm. In
figure 2b, the effect of d on λcut-off is determined at r=100 nm and N=20. A Similar trend for λcut-off is observed as seen in
figure 2a. This is because the Bragg reflection region shifts to a higher wavelength as the lattice constant increases.
Therefore, it is suggested that λcut-off can be tuned by either varying r or d. Moreover, no change in the ER is observed. In
the end, the outcome of N on ER is shown in figure 2c while maintaining r and d at 100 nm and 300 nm, respectively.
There is no effect of N on λcut-off. However, ER deteriorates at N>10. Therefore, it is possible to design a high-pass filter
with a smaller footprint compared to other techniques. The spectral characteristics of the high-pass filter based on one-
dimensional Photonic crystal waveguide are presented in table 1.
Table 1. Spectral characteristics of high-pass filter based on the one-dimensional photonic waveguide.
Variation in d at constant N
Variation in N at constant r and d
Parameters
λcut-off
High-pass range
N
λcut-off
Footprint (µm)2
ER
r=100 nm, N=20, d=280 nm
1428
1428 nm-2000 nm
5
1620
1.9x1
16.68
r=100 nm, N=20, d=290 nm
1482
1482 nm-2000 nm
10
1550
3.4x1
22.99
r=100 nm, N=20, d=300 nm
1532
1532 nm- 2000 nm
15
1536
4.9x1
22.4
r=100 nm, N=20, d=310 nm
1578
1578 nm- 2000 nm
20
1532
6.4x1
22.49
r=100 nm, N=20, d=320 nm
1624
1624 nm-2000 nm
25
1542
7.9x1
23.13
a) b)
c)
Figure 2. Spectral characteristics of a high-pass filter, a) Extinction ratio versus radius of the holes, b) Extinction ratio versus lattice
constant, c) Extinction ratio versus the number of periods.
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The Ex-field distribution of the propagating mode is plotted in two different wavelength regions i.e., Photonic bandgap
and bandpass. The geometric parameters of the filter design used in this demonstration are W=400 nm, H=220 nm, r=100
nm, d=300 nm and N=30. For this specific filter geometry, the photonic bandgap is in the range of 1200 nm to 1536 nm,
therefore we decided to plot the Ex-field at 1512 nm. The propagating mode suffers a high reflection in the photonic
bandgap as shown in figure 3a. Beyond the photonic bandgap, the electromagnetic field at certain wavelengths forms a
standing wave comprising of forwarding and backward Bloch waves resulting in transmission resonances. The Ex-field
distribution is plotted for the first resonance order at 1536 nm where the light travels in the Photonic crystal waveguide
without any attenuation as can be seen in figure 3b.
λ=1512 nm
λ=1536 nm
r=100 nm, d=300 nm, W=400 nm, H=220 nm, N=35
a)
b)
Ex-field distribution
xy
Figure 3. Ex-field distribution of a propagating mode in, a) Photonic bandgap region, b) Bandpass region.
4. CONCLUSION
Here, we presented an attractive and simple design of a high-pass filter based on one-dimensional photonic crystal
waveguide designed for the wavelength range of 1200 nm to 2000 nm. The numerical analysis is conducted via a 3-
dimensional finite element method. The cut-off wavelength of the high-pass filter can be controlled by manipulating the
period of the structure which shifts the Photonic bandgap to high or low wavelength range. Consequently, a desired high-
pass regime can be obtained. The number of periods has a significant effect on the extinction ratio of the wavelength
range in the photonic bandgap up to a certain limit. An extinction ratio of greater than 21 dB can be obtained for a device
footprint of 3.4x1 µm2 at a cut-off wavelength of 1550 nm. This study provides a guideline for the realization of
integrated optical high-pass and low-pass filters with a small footprint.
ACKNOWLEDGEMENT
This work was financially supported by the Russian Foundation for Basic Research (Grant No. 16-29-09528_ofi_m) for
numerical calculations, by the Ministry of Science and Higher Education within the State assignment FSRC
«Crystallography and Photonics» RAS (Grant No. 007-GZ/Ch3363/26) for theoretical results.
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