of 7
High resolution on-chip optical filter
array based on double subwavelength
grating reflectors
Yu Horie,
1
Amir Arbabi,
1
Seunghoon Han,
1
,
2
and Andrei Faraon
1
,
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, 1200 E
California Blvd, Pasadena, CA 91125, USA
2
Samsung Advanced Institute of Technology, Samsung Electronics, Samsung-ro 130, Suwon-si,
Gyeonggi-do 443-803, South Korea
faraon@caltech.edu
Abstract:
An optical filter array consisting of vertical narrow-band
Fabry-P
́
erot (FP) resonators formed by two highly reflective high contrast
subwavelength grating mirrors is reported. The filters are designed to cover
a wide range of operation wavelengths (
λ
/
λ
=
5%) just by changing
the in-plane grating parameters while the device thickness is maintained
constant. Operation in the telecom band with transmission efficiencies
greater than 40% and quality factors greater than 1,000 are measured
experimentally for filters fabricated on the same substrate.
© 2015 Optical Society of America
OCIS codes:
(050.6624) Subwavelength structures; (050.2230) Fabry-Perot; (130.7408) Wave-
length filtering devices.
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#250298
Received 17 Sep 2015; revised 30 Oct 2015; accepted 2 Nov 2015; published 5 Nov 2015
(C)
2015
OSA
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Nov
2015
| Vol.
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No.
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| DOI:10.1364/OE.23.029848
| OPTICS
EXPRESS
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1. Introduction
The spectrometer is one of the most important instruments in the field of optics. Miniaturized
spectrometers have been the subject of considerable recent interest due to their applications
in a wide range of applications in sensing and various test and measurement equipment [1].
Several integrated optics approaches have been investigated, including ring resonator-based
optical circuits [2], integrated diffraction grating [3], and photonic crystal cavity arrays [4];
however, low free-space to chip optical coupling efficiency reduces the sensitivity of these
spectrometers. For free-space spectroscopy, one of the attractive designs consists of an array of
on-chip narrowband wavelength optical filters directly placed on photodetector arrays [5]. In
this design, a collimated incident light beam with a broadband spectrum of interest is filtered out
top SWG
reector
reected
bottom SWG
reector
λ
1
λ
2
λ
3
transmitted
Photodetector layer
Fig. 1. Schematic illustration of narrowband wavelength filter array composed of vertical
FP resonators realized using two-layers of SWG reflectors separated by a spacer layer.
When broadband input light is illuminated, the spectrum is filtered out by the narrowband
filters with different central wavelengths, and the optical powers detected by the underlying
photodetector pixels are used to reconstruct the original spectral information.
#250298
Received 17 Sep 2015; revised 30 Oct 2015; accepted 2 Nov 2015; published 5 Nov 2015
(C)
2015
OSA
16
Nov
2015
| Vol.
23,
No.
23
| DOI:10.1364/OE.23.029848
| OPTICS
EXPRESS
29849
1400
1450
1500
1550
1600
Wavel
ength (nm)
0.25
0.30
0.35
0.40
0.45
0.50
Duty
cycle
Reectivity
0.90
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1.00
1400
1450
1500
1550
1600
Wavel
ength (nm)
0.25
0.30
0.35
0.40
0.45
0.50
Duty
cycle
Phase (
2π)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
(a)
(b)
Fig. 2. (a) Simulated reflectivity contour map as a function of duty cycle of SWGs and
wavelength, for the
α
-Si on SiO
2
1D SWGs with 900 nm period and SU-8 polymer cladding
for the normally incident TE-polarized light (i.e. electric field parallel to the grating bars)
and (b) the corresponding reflection phase contour map.
by the narrowband wavelength filters with different central wavelengths, and the corresponding
underlying photodetectors are used to detect the optical power of the filtered spectra, allowing
to reconstruct the spectral information for the incident light. Such a narrowband optical filter
array is conventionally implemented by using FP resonators formed by two highly reflective
distributed Bragg reflectors (DBRs) with spatially varied cavity length such that each of the
resonators has a different resonance wavelength [6]. Grayscale lithographic technology enables
the realization of cavities with spatially varied thickness in a controllable way as reported in [7].
However, such a high-end and expensive lithography is not readily available and hinders the
realization of the narrowband optical filter array in mass production.
In this paper, we propose and demonstrate a method to design a set of narrowband filters that
covers a wide range of wavelengths, based on a vertical FP resonators formed by two-layers
of highly reflective subwavelength grating (SWG) reflectors [8, 9]. SWG reflectors not only
provide broadband reflection spectra comparable to DBRs thus being regarded as the thin-layer
alternatives of DBRs, but also, by changing the grating geometry, they allow for engineering the
reflection phase while keeping their reflectivity very high [10]. In contrast to the conventional
DBR-based vertical FP resonators, the resonance wavelengths of the SWG FP resonators can
be controlled by adjusting in-plane geometries of the SWG reflectors [11]. Therefore, a set
of narrowband filters can be easily fabricated using well-established top-down lithographic
processes. As schematically shown in Fig. 1, each of the narrowband optical filters is made of
two identical SWG reflectors separated by a distance on the order of a wavelength of interest, to
satisfy the symmetric FP condition. Such symmetric FP resonators are critically coupled, have
theoretical transmission of 100% at their resonance wavelength, and reflect back off-resonance
portion of the incident light.
2. Design
To investigate the operation of the narrowband filters, we first designed a SWG reflector that
operates in a wide wavelength range around the telecommunication band (
λ
=
1550 nm). The
broadband SWG reflector design typically involves a subwavelength periodic structure made of
a high-refractive index material surrounded by a low refractive index material. The subwave-
length geometry of the grating suppresses higher order diffraction for normally incident light.
#250298
Received 17 Sep 2015; revised 30 Oct 2015; accepted 2 Nov 2015; published 5 Nov 2015
(C)
2015
OSA
16
Nov
2015
| Vol.
23,
No.
23
| DOI:10.1364/OE.23.029848
| OPTICS
EXPRESS
29850