Photonic generation of millimeter-wave using a silicon microdisk

Optics Communications 343 (2015) 115–120
Contents lists available at ScienceDirect
Optics Communications
journal homepage: www.elsevier.com/locate/optcom
Photonic generation of millimeter-wave using a silicon microdisk
resonator
Li Liu, Ting Yang, Shasha Liao, Jianji Dong n
Wuhan National Laboratory for Optoelectronics & School of Optical and Electrical Information, Huazhong University of Science and Technology, Wuhan
430074, China
art ic l e i nf o
a b s t r a c t
Article history:
Received 17 November 2014
Received in revised form
7 January 2015
Accepted 8 January 2015
Available online 9 January 2015
A simple photonic approach to generating millimeter-wave based on a high-Q silicon microdisk resonator is proposed and demonstrated. The MDR is designed with periodical dual passbands at the drop
port so as to filter out different pairs of optical carriers from an optical frequency comb. By beating the
two optical frequency components, several millimeter-wave signals have been obtained. A proof-ofconcept experiment illustrates millimeter-wave generation of 277 GHz, 306 GHz and 335 GHz with
harmonic distortion suppression ratio over 25 dB.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Millimeter-wave generation
Microdisk resonator (MDR)
Integrated optics
1. Introduction
Traditional microwave and millimeter-wave architectures
usually make the whole systems very bulky and costly. With
competitive advantages of light weight, inherent low-loss, high
bandwidth and immunity to electromagnetic interference (EMI),
photonics has been rapidly penetrating into the radar systems for
the generation and processing of microwave and millimeter-wave
signals [1–5]. To date, many schemes have been proposed for
microwave or millimeter-wave generation, such as using stimulated Brillouin scattering [6,7], harmonic frequency locking [8],
dual-transmission-band fiber Bragg grating filter [9] and other
methods [10–13]. Especially, very narrow-band MMW was obtained by stimulated Brillouin scattering due to its intrinsic narrow-band characteristics. The basic principle is the heterodyne
techniques by beating two optical carriers in a square-law photodetector (PD). Compared to these fiber optic systems, siliconbased waveguides can offer distinct advantages of increased stability and reliability, compactness, capability of integration with
electronics [14]. In recent years, some on-chip millimeter-wave
generators have been presented, indicating good characteristics
but the design and fabrication of the devices are a little complex
[15] or generated millimeter-wave is not tunable [16]. In this letter,
by utilizing a microdisk resonator (MDR) on a single silicon-oninsulator (SOI) chip, a compact scheme of tunable millimeter-wave
generation is proposed and experimentally demonstrated.
n
Corresponding author.
E-mail address: jjdong@mail.hust.edu.cn (J. Dong).
http://dx.doi.org/10.1016/j.optcom.2015.01.024
0030-4018/& 2015 Elsevier B.V. All rights reserved.
Different from most previous approaches, the key components
only need a high-quality (high-Q) and low-loss MDR which could
be integrated and reduce the complexity and power consumption
in the millimeter-wave system.
2. Operation principle
Our proposed scheme for the millimeter-wave generation is
based on the frequency extraction from an optical frequency comb
(OFC). The key element is the optical filter with a high-Q MDR of
periodical dual radial modes. If we align the target frequencies of
the OFC at the suitable resonant region of the MDR, two optical
frequencies could be extracted and then beat in the PD. Thus we
could achieve a millimeter-wave. Moreover, as the effective indices
of the two resonant modes are different, their free spectral ranges
(FSRs) turn to be different resulting in frequency intervals with
arithmetic progression. Hence, diverse millimeter-waves could be
achieved by selecting different frequency intervals.
It should be noted that the MDR is known as a multimode
structure. Fortunately, the number of whispering gallery modes
(WGMs) in an MDR can be reduced by optimizing the dimensions
of coupled waveguides [17], increasing inner walls to the MDR [18]
or decreasing the size of the MDR [19]. In order to optimize the
MDR of only two intrinsic resonant modes and highest Q factor, we
use a finite-element mode solver (COMSOL Multiphysics) to design
the structure of MDR and bus waveguide (WG). To reduce the
transmission loss of the WG, we design a ridge WG. After optimization, the WG width of the bus WG takes about 500 nm.
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Fig. 1. Simulated results using COMSOL software: (a) effective indices of different modes of MDR and the straight waveguide (WG) and (b) calculated Q factor of different
gaps between the straight waveguide and the through-port and drop-port of the MDR.
Meanwhile, the silicon shallow and deep layers have thicknesses
of 100 nm and 240 nm, respectively. Fig. 1(a) illustrates the effective mode indices for different radial modes of MDR with a radius
of 10 μm. Obviously, the 2nd-order radial mode has an effective
index that is closer to the straight WG. Hence, this 2nd-order
mode has a better phase matching with the waveguide mode,
thereby resulting in a stronger coupling and lower Q factor. Fig. 1
(b) calculates the Q factors of MDR at different gap parameters
(gaps between the bus WG and the through-port and drop-port of
the MDR). The Q factors become larger as the coupling gaps
increase. Fig. 1(b) also shows the mode distributions of the cross
section of the ridge MDR on the oxide substrate. It is clear to see
that the major powers of the radial modes are confined at the
center of the waveguides.
We then fabricate several double side-coupled MDRs on the
corresponding commercial SOI wafers to pursue the highest Q
factor. Fig. 2(a) shows the scanning electron microscope (SEM)
image of a fabricated MDR with radius of 10 μm. The thickness of
top silicon layer of SOI wafers is 340 nm and the thickness of
buried oxide layer is 2 μm. The device pattern was transferred to
Fig. 2. SEM images of the silicon MDR (a) and the grating coupler (b).
L. Liu et al. / Optics Communications 343 (2015) 115–120
117
Fig. 3. Measured spectrum of the high-Q MDR.
photoresist (ZEP520A) by E-beam lithography (Vistec EBPG5000 þ
ES). Then the upper silicon layer was etched downward for 240 nm
to form a ridge waveguide through inductively coupled plasma
(ICP) etching (Oxford Instruments Plasmalab System100). As designed above, the bus waveguide has a width of 500 nm. Considering the extinction ratio (ER) and Q factor of the MDR together,
the best gaps of the through-port and drop-port are both designed
to be 300 nm. We employ the vertical grating coupler to couple
the optical signal from fiber to silicon waveguide, and the zoom-in
grating coupler is shown in Fig. 2(b). The grating couplers have a
period of 630 nm, and the duty cycle is 56%.The coupling loss of
the grating couplers are measured to be 8 dB for a single side.
We inject transverse electric (TE) polarized light into the device
and measure the transmission spectrum using a swept-wavelength test setup. As shown in Fig. 3, only two low-order WGMs
are effectively triggered. There are two periodic bandpass filters
with different FSRs (FSR1 ¼10.52 nm and FSR2 ¼10.76 nm) at the
drop port which results in the three wavelength intervals to be
2.22 nm, 2.46 nm and 2.7 nm, respectively. And the maximum ER
is 23 dB. Fig. 3(b) shows the two zoom-in resonances with the Q
factors of 6.2 104 and 4.4 104, respectively. In addition, without
considering the coupling loss, the insertion loss of the MDR is only
1.8 dB.
3. Experimental results and discussion
A proof-of-concept experiment for photonic generation of
millimeter-wave has been performed as shown in Fig. 4. The setup
is mainly composed of an OFC generator and a high-Q MDR. The
output power of the tunable laser source (TLS) is 10 dBm. The sinusoidal radio frequency (RF) signal of 10 GHz repetition rate is
applied on both a Mach–Zehnder modulator (MZM) and a phase
modulator (PM). In order to achieve a satisfied source, the direct
current (DC) bias voltage of the MZM is tuned to below the
quadrature point so as to cut the pulse. An erbium-doped fiber
amplifier (EDFA1) is used to compensate the power attenuation. It
should be noted that the driving RF signal applied on the MZM and
PM is synchronized by an optical tunable delay line (OTDL).
Owning advantages of bias-drift-free operation and low insertion
loss, the PM is used to generate large chirp and produce multisidebands. The following SMF is used to compensate the chirp of
the PM. Subsequently, a high-power EDFA (HP-EDFA) and highly
nonlinear fiber (HNLF) with zero-dispersion wavelength of
1600 nm are used to provide a large pump power and self-phase
modulation (SPM), respectively. Hence, as shown in Fig. 5,
broadband OFC could be generated [20].
A polarization controller (PC3) is required since the silicon
waveguide can be operated only in transverse electrical (TE) mode.
Fig. 4. Experimental setup of the proposed scheme. TLS: tunable laser source, PC: polarization controller, MZM: Mach–Zehnder modulator, PM: phase modulator, EDFA:
erbium doped fiber amplifier, OTDL: optical tunable delay line, SMF: single mode fiber, HP-EDFA: high-power EDFA, VOA: variable optical attenuator, HNLF: highly nonlinear
fiber, MDR: microdisk resonator, and OSO: optical sampling oscilloscope.
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Fig. 5. The generated broadband OFC.
Then the OFC is sent into the MDR by the vertical grating coupler
which has a 3-dB coupling bandwidth of about 30 nm in the C
band. As the MDR has two major resonant modes with high-Q
factor, by optimizing the central wavelength of the OFC in one
resonant period of the MDR, two optical frequencies could be well
filtered out and amplified by the backward EDFA2. In this case, the
millimeter-wave can be generated and the corresponding output
waveforms can be temporally recorded by an ultra-high optical
sampling oscilloscope (OSO) with 1 ps temporal resolution. When
the TLS operates at other proper wavelengths, new millimeterwaves with different frequencies could be obtained.
When we adjust the wavelength of the TLS at 1541.244 nm which
is the center of first resonant period of the MDR, the output spectrum of the filtered optical frequencies is shown in Fig. 6(a). The
wavelength interval is 2.22 nm, namely frequency interval of
277 GHz. The corresponding output temporal waveform is shown in
Fig. 6(b), which is similar to a sinusoidal function with a period of
3.6 ps. If we change the wavelength of the TLS locating at the centers
of other resonant periods, different pairs of optical frequencies could
be selected to generate new millimeter-waves. As shown in Fig. 6(c
and e), the wavelength intervals are 2.45 nm and 2.68 nm, namely
Fig. 6. Experimental measurements of the filtered optical spectra (a), (c), (e) and output temporal waveforms (b), (d) and (f), respectively.
L. Liu et al. / Optics Communications 343 (2015) 115–120
119
Fig. 7. Calculated RF spectra whose frequencies are 306 GHz (a) and 335 GHz (b), respectively.
frequency intervals of 306 GHz and 335 GHz, respectively. Correspondingly, Fig. 6(d and f) present the temporal waveforms with
periods of 3.2 ps and 3.0 ps, respectively. Hence, it is clear that the
optical spectra in Fig. 6(a), (c) and (e) perfectly match with the
output temporal waveforms in Fig. 6(b), (d) and (f), respectively.
Unfortunately, we could not measure the output RF spectra,
which is far beyond the bandwidth of commercial electrical
spectrum analyzers. Therefore we use the fast Fourier transform
(FFT) to calculate the electrical spectra [21]. We choose the storage
data of Fig. 6(d and f) as an example. Fig. 7(a and b) present two
electrical spectra of the millimeter-waves whose peaks are
306 GHz and 335 GHz, respectively. The harmonic distortion suppression ratios are both larger than 25 dB. Hence, the generated
MMW is relatively pure.
The tunability of this scheme could be improved by two
methods. First, we could fabricate a MDR with a larger radius to
achieve smaller free spectrum ranges (FSRs). Hence, more wavelength intervals could be selected to generate lots of MMWs with
different tones. Second, due to the thermo-optic (TO) effect in silicon resonators, their transmission spectra could be shifted [22–
24]. We could use a micro-heater or electrode to change the index
of the MDR which could control the red-shifts of the two MGMs.
The red-shifts are different due to the different group indices of
the two WGMs, as shown in Eq. (1). Hence, to a certain extent, the
initial wavelength intervals could be tuned to generate continuously tunable millimeter-waves.
The resonance shift Δλ owing to TO effect can be expressed as
follows [22]:
Δλ ≈
λ0
ΔnTO
ng
(1)
where λ0 is the resonance wavelength, ng is the group index, and
nTO is the silicon refractive index changes induced by TO effect.
Comparing with other papers, there are several advantages of
our scheme. First, the key component is the silicon microdisk resonator (MDR) of periodical dual passbands which is compact,
low-loss and CMOS-compatible. Second, due to the high quality
value of the MDR, it is competent to select two comb lines to
generate well MMWs. Third, this structure has a fine tunable
ability for MMW generation, such as using thermo-optic (TO)
effect.
4. Conclusions
We have proposed a simple and integrated photonic-assisted
approach to millimeter-wave generation based on a high-Q and
low-loss silicon MDR. Experimental demonstrations were performed to generate different frequencies, such as 306 GHz and
335 GHz. Our scheme is low-cost and small-size, which can be
potentially used to generate tunable millimeter-waves by using a
micro-heater or electrode in the future.
Acknowledgment
This work was partially supported by the National Basic Research Program of China (Grant no. 2011CB301704), the Program
for New Century Excellent Talents in Ministry of Education of
China (Grant no. NCET-11-0168), a Foundation for the Author of
National Excellent Doctoral Dissertation of the People's Republic of
China (Grant no. 201139), the National Natural Science Foundation
of China (Grant nos. 60901006 and 11174096), and the Fundamental Research Funds for the Central Universities, HUST:
2014YQ015. The authors would like to thank Prof. Jinsong Xia and
Dr. Qingzhong Huang in the Center of Micro-Fabrication and
Characterization (CMFC) of WNLO for the assistance in device
fabrication.
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