We use soft lithography replica molding to fabricate unclad polystyrene (PS) and clad SU-8 microring resonator filters. The PS resonator has an intrinsic quality factor of 1.0/spl times/10/sup 4/ at /spl lambda/=1.55 /spl mu/m, while that of the SU-8 resonator is 7100. The extinction ratios of the PS and SU-8 microring filters are -12 and -20 dB, respectively, with net insertion losses of 6.7 and 9.9 dB. The good quality factors and high extinction ratios of the microring resonator filters show the practicality of soft-lithography replica molding for the fabrication of integrated optical devices

Huang, Yanyi

Paloczi, George T.

Poon, Joyce K. S.

Yariv, Amnon

English

Caltech Authors - Main

2496 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004
Soft Lithography Replica Molding of Critically
Coupled Polymer Microring Resonators
Joyce K. S. Poon, Student Member, IEEE, Yanyi Huang, George T. Paloczi, Student Member, IEEE, and
Amnon Yariv, Life Fellow, IEEE
Abstract—We use soft lithography replica molding to fabricate
unclad polystyrene (PS) and clad SU-8 microring resonator filters.
The PS resonator has an intrinsic quality factor of 1 0 104 at
= 1 55 m, while that of the SU-8 resonator is 7100. The ex-
tinction ratios of the PS and SU-8 microring filters are 12 and
20 dB, respectively, with net insertion losses of 6.7 and 9.9 dB.
The good quality factors and high extinction ratios of the microring
resonator filters show the practicality of soft-lithography replica
molding for the fabrication of integrated optical devices.
Index Terms—Optical device fabrication, optical polymers, op-
tical resonators.
I. INTRODUCTION
AS FABRICATION technologies mature, microring res-onators have garnered significant interest in recent years
since they may open new avenues toward compact integrated
optical devices such as filters and delay elements [1]–[4]. Poly-
mers provide an attractive material platform for the microring
resonators, and more generally integrated optics, since their
material and production costs may be substantially lower than
other material systems. A fabrication technique that is impor-
tant for polymeric integrated optical circuits is replica molding.
In replica molding, a mold of the original master device is sub-
sequently used to cast nearly exact copies of the master. This
technique is powerful because it allows for high-throughput
replication of the master device, and unlike other wet-etch or
chemical processes, it does not chemically or physically alter
any dopant molecules that may be in the polymer.
Soft lithography replica molding is a particular technique
where the mold is flexible [5]. We have previously shown that
very high fidelity between the master and molded devices can
be achieved using soft lithography molding [6]. Moreover,
we have recently used this technique to fabricate a high per-
formance Mach–Zehnder modulator in a polymer doped with
electrooptic chromophores [7]. Recently, Martin et al. have
used this technique to fabricate polymeric toroid resonators
with factors of [8]. In this work, we further address the
important issues of loss and device performance in polymeric
integrated optics by demonstrating, using soft lithography
Manuscript received May 20, 2004; revised July 8, 2004. This work was
supported by the National Science Foundation (NSF), Defense Advanced Re-
search Projects Agency (DARPA), and Hughes Research Laboratories (HRL).
The work of J. Poon was supported by an NSERC-Julie Payette scholarship.
The authors are with the Departments of Electrical Engineering and Applied
Physics, California Institute of Technology, Pasadena, CA 91125 USA (e-mail:
poon@caltech.edu).
Digital Object Identifier 10.1109/LPT.2004.835610
Fig. 1. Microring resonator coupled to a waveguide. The field amplitudes are
denoted by a; b; c, and d.
replica molding, high- polymeric microring resonator notch
filters in two material systems with excellent extinction ratios.
II. THEORY
The microring resonator notch filter consists of a single res-
onator coupled to a waveguide, as shown in Fig. 1. Using the
notation in Fig. 1, the coupling between the microring and the
waveguide can be described by the matrix equation [3], [9], [10]
(1)
where is the dimensionless length-integrated coupling coeffi-
cient and . As the field propagates in the ring,
it accumulates a phase shift and may be attenuated, so
, where is the field attenuation constant,
is the ring radius, and is the propagation constant in the ring.
Hence, the transfer function of the filter is described by [11],
[12]
(2)
An important parameter that describes the losses in resonators
is the quality or factor. The loaded quality factor of a
resonator is given by [10], [13]
(3)
where is the intrinsic factor and is the external
factor from the resonator-waveguide coupling. In general, since
and in (2) are interchangeable, and cannot be
uniquely determined from the spectral response of the device.
However, when the special condition is satisfied, or al-
ternatively, when the internal power loss is equal to the coupling
. This condition is known as
critical coupling [12]. At critical coupling, the transfer function
1041-1135/04$20.00 © 2004 IEEE
POON et al.: SOFT LITHOGRAPHY REPLICA MOLDING OF CRITICALLY COUPLED POLYMER MICRORING RESONATORS 2497
Fig. 2. Fabrication process for the microring resonators. The leftmost column
shows the fabrication of the soft PDMS mold. The center column shows the
fabrication of the PS microring resonators. The rightmost column shows the
fabrication of the SU-8 microring resonators.
completely vanishes at the resonance frequencies of the ring res-
onator. Therefore, not only do critically coupled ring resonators
allow us to unambiguously determine the intrinsic losses of a
resonator through a single measurement of the spectral response
of the geometry in Fig. 1, they also enable optical notch filters
with high extinction ratios.
III. FABRICATION
With the aim of demonstrating the applicability of soft lithog-
raphy replica molding to integrated optical devices, we fabri-
cate critically coupled microring resonators. We fabricate both
unclad and clad devices in polystyrene (PS, ) and
SU-8 (Microchem, ), respectively. Fig. 2 shows the
schematic flowchart of the fabrication process. The fabrication
of the mold is described in greater detail in [6] and [14]. Briefly,
we create a poly(di-methylsiloxane) (PDMS) (General Electric)
mold from the master devices which are defined using elec-
tron-beam writing of SU-8. The waveguides in the master de-
vices have a height of 1.5 m and a width of 1.9 m. The res-
onator-waveguide gap size is about 350 nm. The PDMS molds
are highly robust, and the same molds are used repeatedly in
this experiment for the different materials without any notice-
able physical difference in the molded device characteristics.
We begin the fabrication of the molded devices by spinning
a 3- m-thick layer of OG-125 (Epotek, ), an ultra-
violet (UV) curable epoxy, as the under-cladding on a silicon
wafer. The chip is then cured with UV light and baked at 80 C
for 2 min. For the unclad microrings, we deposit 10 L of PS so-
lution (4 wt% in toluene) on the chip and press the mold against
the chip with a force of 25 N for about 20 min. The toluene evap-
orates through the PDMS mold during this time. Even though
our molding process leaves behind a thin ( 200 nm) residue
film over the chip, this film should not be detrimental on the
loss of the devices if it is sufficiently thin [14]. After the mold
Fig. 3. SEM image of the resonator-waveguide coupling region in PS. Smooth
side-walls are achieved using the soft lithography fabrication process. Inset: An
optical micrograph of the microring resonator filter.
is lifted away, the chip is baked at 80 C for 3–5 min. Finally,
the chip is cleaved to separate the devices. The deviation in the
radius of the molded and master devices is about 2%. The radius
of the master is 200 m, and the radius of the molded resonator
is about 204 m. The deviation can be reduced by decreasing
the force applied to the mold.
Fig. 3 shows a scanning electron microscope (SEM) image
of the PS microring resonator. The slight unevenness at the base
of the waveguides is replicated from the master device and is
caused by the electron-beam lithography fabrication process.
The fabrication process of the clad SU-8 resonator filters is
similar to that of the PS microrings. The only differences are that
the SU-8 must be UV-cured to harden and an over-cladding is
applied. After the SU-8 is UV-cured, the chip is baked at 80 C
for about 3 min. An extra 3- m layer of OG-125 is subsequently
applied as the over-cladding. After the second application of
OG-125, the chip is UV-cured again and finally baked at 80 C
for 3–5 min.
IV. MEASUREMENT
We measure the spectral response of the fabricated devices
by coupling light from a tunable laser to the waveguide via a
tapered fiber and collecting the transmitted light through a lens.
Fig. 4 shows the transmission spectra for the PS and SU-8 mi-
croring resonator filters for transverse electric (TE) polarized
light. The PS and SU-8 devices have maximum extinction ratios
of 12 and 20 dB, respectively, illustrating that the critical
coupling condition is essentially satisfied. The PS resonators
have a free spectral range (FSR) of 1.15 nm and a 3-dB band-
width of 0.3 nm, and hence, a finesse of 3.8. The SU-8 micror-
ings have an FSR of 1.2 nm, a 3-dB bandwidth of 0.436 nm,
and a finesse of 2.75. By taking the ratio between the reso-
nance wavelength and the full-width at half-maximum linewidth
of the resonance, factors are found to be approximately
5200 and 3555 for the PS and SU-8 microrings, respectively.
These quality factors imply that the resonators have values
of and . These intrinsic factors are compa-
rable to the highest reported values to date at the optical wave-
length of 1.55 m for integrated polymeric microring resonators
fabricated with reactive ion etching ( – in [4]).
2498 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004
Fig. 4. Transmission spectra for TE polarized light of (a) the air-clad PS
microring resonator and (b) the OG-125-clad SU-8 microring resonator.
We also fit the experimental data with the theoretical
response described by (2) to find excellent agreement be-
tween them. The fit parameters for the PS microring are
, and the group index
is 1.624. The fit parameters for the SU-8 microrings are
and . Extrapolating the factors
from the attenuation in the microring resonator, we find that
for the PS microring and for
the SU-8 microring. These values imply a distributed loss of
about 25 dB/cm in the resonator. The factors are in excellent
agreement with the direct measurements of the resonance
linewidths.
For the microring radius and refractive index contrast in this
experiment, we expect that the resonator loss will be dominated
by the side-wall scattering ( 15–20 dB/cm) and material losses
( 5 dB/cm) rather than the bend loss [4], [15], [16]. There-
fore, resonators with larger FSRs and similar factors can be
achieved by reducing the radius.
To determine the total insertion loss of the devices, we mea-
sure the difference between the transmitted powers with and
without the microring resonator devices in our experimental
setup. The total off-resonance insertion losses are found to be
6.7 and 9.9 dB for the PS and SU-8 microring resonator fil-
ters, respectively. These values indicate fairly low losses at the
cleaved end facets and good mode-matching between the fiber
taper and the waveguide. The coupling efficiency can be im-
proved by designing a suitable mode-converter between the fiber
and our device.
V. CONCLUSION
We have used soft lithography replica molding to fabricate
critically coupled high- microring resonator notch filters at
the telecommunications wavelength of 1.55 m using PS and
SU-8 with OG-125 as the cladding material. We expect that the
insertion losses of the devices can be further reduced by im-
proving the input–output coupling, reducing the side-wall scat-
tering, and reducing the material losses of the optical polymers.
This demonstration of low-loss, high-extinction integrated op-
tical microring resonator filters illustrates the potential of the
soft lithography replica molding technique for integrated optical
circuits.
ACKNOWLEDGMENT
The authors thank W. Green and Dr. J. Scheuer for helpful
discussions.
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Soft lithography replica molding of critically coupled polymer microring resonators

Compact microring notch filters,”

Critical coupling and its control in optical waveguide-ring resonator systems,”

Microring resonator channel dropping filter,”

Optical Filter Design and Analysis: A Signal Processing Approach.

Polymer micro-ring filters and modulators,”

Propagation loss in a single-mode GaAs-AlGaAs microring resonators: Measurement and model,”

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Replica molded electrooptic polymer

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Soft lithography molding of polymer integrated optical devices:

Soft lithography replication of polymeric microring optical resonators,”

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Waves and Fields in Optoelectronics. Englewood Cliffs,

https://authors.library.caltech.edu/2007/1/POOieeeptl04b.pdf

Soft lithography replica molding of critically coupled polymer microring resonators

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