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OFC/NFOEC Technical Digest © 2013 OSA
Photonic Processing Using Integrated Optical Filters
C.K. Madsen*, Q. Chen, J. Kim and Y. Zhou
Solid-State Electronics, Photonics and Nano-Engineering Laboratory, Department of Electrical and Computer Engineering,
Texas A&M University, College Station, TX, 77843-3128*
Abstract: We will discuss photonic processing tasks that are enabled or assisted by integrated optical filters.
A novel platform incorporating a highly nonlinear glass on an electro-optic substrate will be reviewed,
namely arsenic-trisulfide (As2S3)-on-LiNbO3 waveguides.
©2013 Optical Society of America
OCIS codes: (230.5750) Resonators; (130.3730) Lithium Niobate; (230.7370) Waveguides
Optical filters play a critical role in fiber-optic communication systems as well as for signal processing in radiofrequency (RF) photonics and sensing applications. While a strong emphasis has been placed on silicon photonics
and many advanced integrated optical filters realized, other material systems offer unique capabilities. In this paper,
we’ll review work on a novel platform that combines a highly nonlinear glass on an electro-optic waveguide layer,
namely arsenic-trisulfide (As2S3)-on-LiNbO3 waveguides.
Ring resonators are a fundamental building block for achieving sharp features in the frequency response with
only a few stages. The optical waveguide modes must be highly confined to provide low-loss, small-bend-radii
rings. Ring resonators with large free-spectral ranges (FSRs) and high quality factors have been demonstrated on
numerous material platforms, including silica [1], III-Vs [2], silicon-on-insulator [3] and As2S3 [4].
The higher refractive index of As2S3 allows vertical waveguide integration with LiNbO3 waveguides, which
provide electro-optic modulation and high coupling efficiency to standard singlemode optical fibers [5],as well as
the realization of ring resonators. Important properties of As2S3 include its transparency from the visible to the far
infrared (8µm) range and nonlinearity. We will review three photonic processing applications using integrated
optical filters realized with this hybrid platform
Nonlinear Frequency Modulated Waveform Generator
Two characteristics of great importance in laser detection and ranging (LADAR) systems are long range
performance and fine range resolution. Waveforms that achieve these characteristics are desired, and many different
waveforms have been investigated including linear frequency modulation (LFM) chirped waveforms, pseudorandom phase modulated waveforms, poly-phase (P4) waveforms, and others. Sidelobe reduction can be
accomplished by creating a non-linear frequency modulated (NLFM) chirp, which can also avoid carrier-to-noise
losses. We have reported a novel method for optically generating an NLFM waveform that approximates a tanhfunction using a ring resonator with a modulator in the feedback path and a standard phase modulator as shown in
the inset of Fig. 1 [6]. A simulation of the signal and its frequency chirp are shown in the graph. Through
simulation, the maximum sidelobe level of the autocorrelation of an NLFM waveform generated by a series of
tunable integrated optical ring resonators is shown to be -20 to -30 dB or lower.
A key step in realizing the capabilities of this hybrid device platform is the demonstration of electro-optic
tuning, which allows for on-chip reconfigurable optical devices and high-speed modulators. While the As2S3
waveguide is a rib waveguide that is external to the substrate, through different design parameters the mode
confinement in the As2S3 waveguide can be controlled. By adjusting the As2S3 waveguide width and thickness,
along with the silicon dioxide (SiO2) cladding/buffer layer thickness, a portion of the optical mode can be kept in the
electro-optically tunable LiNbO3 substrate. An electro-optically tunable, vertically integrated As2S3 Mach-Zehnder
interferometer (MZI) side coupled to a Ti-diffused waveguide on LiNbO3 substrate was recently demonstrated [7],
as shown in Fig. 2. Asymmetric coplanar strip electrodes were placed along the As2S3 path and show strong electrooptic tuning capability, shifting the FSR of the interferometer. Work on tuning ring resonators has also been
recently completed [8].
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
Fig. 1. Simulation of time-varying signal amplitude and instantaneous
frequency showing tanh-like chirp.
OFC/NFOEC Technical Digest © 2013 OSA
Fig. 2. Wavelength response for MZI as DC voltage is applied to
electrodes. (inset shows device schematic)
Linearized Frequency Discriminator
While MZIs represent a baseline device for frequency discriminators, ring-assisted MZIs can enhance the frequency
response linearity [9]. Nonlinearity in the frequency response produces unwanted intermodulation distortion (IMD).
In particular, suppression of 3rd order IMD (IMD3) is important because the generated frequencies are close to the
signal frequency, which makes it difficult to be filtered. A schematic of a ring-assisted discriminator is shown in the
inset of Fig. 3. An MZI has a ring resonator on one arm and a phase shifter on the other arm. Unlike most ringassisted MZI filters, for which bandwidth is determined by the MZI’s FSR and the ring assists with its unique phase
response to improve MZI’s performance, the discriminator’s bandwidth is solely determined by the ring resonator
while the FSR of the MZI provides a constant phase shift between the arms as long as the MZI’s FSR is much wider
than the ring’s FSR. Since this device does not require a precise control to match both the ring and MZI’s FSR,
fabrication tolerances of an MZI-assisted ring resonator are more relaxed than that of a ring-assisted MZI filters.
Simulation results show that linearity of the discriminator is dependent on zero magnitude of the ring resonator,
which is determined by the coupling efficiency between the ring and bus waveguide. Simulations are shown for an
89% power coupling ratio along and the subsequent enhanced linearity. Fabrication and experimental results have
recently been obtained and will be published shortly.
Linear Fit
Normalized Frequency
Normalized Frequency
Fig. 3. Simulation result for the discriminator (a) magnitude reponse and (b) its deviation from a linear fit. An ideal ring resonator (no roundtrip
loss) is assumed. The zero magnitude of the ring is |z|=3, which corresponds to 89% of coupling between ring-bus waveguide.
OFC/NFOEC Technical Digest © 2013 OSA
Nonlinear Optical Processing
For a nonlinear ring resonator device, all-optical switching can be achieved by tuning the refractive index inside the
ring waveguide via self or cross-phase modulation using substantially lower optical powers than required for straight
waveguides or MZI-based devices. Ultra-high Q, long path ring resonators have been demonstrated using As2S3on-LiNbO3 waveguides [10]. In these results, a ring resonator with 1.2 dB/cm propagation loss and a 1.7-cm
round-trip length was realized, resulting in an FSR of 7 GHz. For a nonlinear index change of 0.005, the magnitude
response is shifted substantially as shown in Fig. 4. The waveguide materials are scalable to the mid-infrared
region, where low loss waveguides have already been demonstrated [11]. We have designed dispersion-engineered
waveguides for wavelength converters based on four wave mixing (FWM) that allow efficient conversion between
the near- and mid-infrared regions. Simulation results are shown for phase-match efficiency in Fig. 5 for a pump of
2.05Pm and signal and idler wavelengths of 1.55 and 3.03 Pm [12].
FWM phase-matching efficiency
Wavelength (nm)
Fig. 4. Simulation of all-optical tunable ring magnitude response based
on fabrication parameters.
w=1.4Pm h=1.7Pm with 0.18Pm MgF2
w=1.5Pm h=1.685Pm
pump off
pump on
Magnitude response (dB)
Fig. 5. FWM phase-matching efficiency as a function
of signal wavelength
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