The microphotonic RF receiver concept
The ability to develop an integrated microphotonic receiver for mm-wave has only recently become a possibility with our introduction of active high-Q microphotonic resonators. Fig. 1 illustrates the basic receiver architecture.
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| Fig.1 (a) Schematic showing basic microphotonic receiver blocks used to form an integrated front-end receiver for mm-wave RF. (b) Diagram illustrating the basic RF-to-optical conversion concept. Continuous wave (cw) lasing light power is efficiently coupled into the microphotonic resonator. The antenna feeds RF power into a high-Q cavity or lumped-element LCR circuit such that there is a large overlap between RF and photonic field intensity in the dielectric resonator. Modulated light is coupled out of the resonator and fed to optical filters which perform signal processing. | |
A
key innovation in receiver design is the use of direct RF-to-optical conversion
via the electro-optic response of high-Q lithium niobate microphotonic
resonators. The use of high-Q resonators makes the receiver very sensitive.
At the same time the receiver is power-efficient (less than 0.04 W) because
both optical and RF signals can be simultaneously brought into resonance.
Power efficiency is further enhanced because the photonic signals can be
processed in a low-loss, ultra-fast photonic signal-processing environment.
In addition, the receiver is small in size (less than 40 mm3 including
the receiver electronics) and so can be directly integrated into antenna
structures. The direct conversion from RF to optical signals also provides
electro-optic isolation in the receiver and so minimizes cross-talk and
electromagnetic interference.
The efficient RF-to-optical conversion needed for mm-wave receivers exploits novel intimate integration of high-Q RF and microphotonic components. These components achieve their high-Q due to total internal reflection of whispering gallery photon resonances at the curved air-dielectric interface of the structure (see Fig. 2). The confinement of photon field to a region near the dielectric interface and the long photon lifetime of the resonance enhances RF-photon interaction within the material. By careful electromagnetic design it is possible to optimize the overlap between the RF-field and the electro-optic response of the resonator, thereby ensuring power-efficient RF-to-optical conversion in a small volume.
Initial experiments at 7.6 GHz
Our receiver uses high-Q RF and microphotonic electro-optic resonators
that are operated in simultaneous resonance. Central to this architecture
is a microphotonic optical modulator. This component directly converts
the received RF carrier frequency and associated side-bands to an optical
carrier frequency by interaction of optical and RF electric fields via
the electro-optic effect.
As part of our initial experimental verification of the basic receiver concept, we have performed many of our experiments at 7.6 GHz. A key experiment is verification of the basic microphotonic modulator. Our initial approach to develop a microphotonic optical modulator uses a z-cut LiNbO3 disk-shaped resonator with optically-polished curved side-walls. While other electro-optic materials could be used, such as more exotic ferro-electrics or electro-optic polymers, we chose LiNbO3 because of its well characterized material properties, availability, and the availability of a mature processing technology.
Standard evanescent prism-coupling is used to couple laser light into and out of a resonant TE-polarized high-Q optical whispering-gallery mode (WGM) which exists at the periphery of the disk. At present we use diamond prisms for evanescent coupling element. In the future we intend to explore less expensive material from which to form the evanescent coupling element. A metal electrode structure fed by a RF signal is designed to overlap and be in simultaneous resonance with the optical field. The resonator's high optical Q is used to increase the effective interaction length of photons with an applied RF electric-field. When combined with a simultaneously resonant RF structure designed to provide voltage gain between the electrodes, a highly sensitive receiver at microwave frequencies is, in principle, achievable.
The free spectral range (FSR) of the optical resonator and the spatial pattern of the metal-electrode structure determine the center modulation frequency of the optical carrier. The frequency of the RF carrier fRF should be an integral multiple m of the optical FSR such that f0 = 1 / tdisk = nopt2pR / c where tdisk is the optical round-trip time of the disk and R is the disk radius. For a z-cut LiNbO3 with R = 2.92 mm a value of f0 = 7.56 GHz results and this indeed what is measured.
The inset to Fig. 3(a) shows the RF and optical configuration. Fig.
3(b) is a photograph of the experimental arrangement. Prisms are used to
couple laser light of approximate wavelength l
= 1.55
mm into and out of the WGM optical mode
of the microphotonic resonator. Input optical wavelength from a laser with
a spectral line width less than 0.5 MHz is tuned to a resonant wavelength
of the optical resonator. A RF electric field propagating on a 50 W
metal microstrip line evanescently side-couples to a metal electrode resonator
on the LiNbO3 disk. The fundamental resonant frequency of the
electrode resonator is tuned to match the optical FSR of 7.56 GHz as indicated
by the dip in the reflected RF excitation response shown in Fig. 3(a).
The measured voltage gain provided by this resonator is greater than 4.
We anticipate that improved resonator designs should be capable of achieving
voltage gains in excess of 100.
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Fig. 3 (a) Measured reflected RF power S11
demonstrating maximum coupling of energy to the microstrip resonator at
7.56 GHz. An absorption Q = 144 is measured. The geometry of the side-coupled
microstrip resonator approach is shown in the inset. (b) Photograph of
the experimental arrangement shown in (a).
To explore the modulation response of the device, experiments were performed
to measure detected optical signal as a function of the applied RF frequency,
fRF. As shown in Fig. 4(a), the resulting optical modulation
is centered at 7.56 GHz with a -3 dB bandwidth Df
= 80 MHz. This measured bandwidth coincides with independent results of
measuring optical Q in the microphotonic resonator.
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Fig. 4 (a) Detected output optical power versus input RF frequency for a R = 2.92 mm LiNbO3 disk modulator. The light exiting the disk is optically modulated at 7.56 GHz, with a bandwidth of 80 MHz. (b) Detected output optical power as a function of optical frequency is shown. Both the optical carrier at 194 THz (1.55 mm) and 7.56 GHz optical sidebands are shown. The measured optical line width is limited by the Fabry-Perot filter resolution which is f-3dB = 900 MHz.
RF modulation of the optical carrier is observed directly by passing light exiting the microphotonic resonator through a Fabry Perot interferometer with an optical resolution of f-3dB = 900 MHz. As indicated in Fig 4(b), the optical carrier at 194,000 GHz (l = 1.55 mm) is centered between two optical side-bands, each separated by 7.56 GHz (Dl = 60.5 pm) from the optical carrier. Input light has an optical bandwidth of less than 0.5 MHz which is limited by our ability to stabilize the light output of the single-mode laser source.
Fig. 5 Detected optical modulation as a function of RF power launched onto the RF microstrip resonator. Optical power of 1.0 corresponds to the measured optical power at the maximum of the optical resonance with no modulation.
Fig. 5 shows the modulated optical power at 7.6 GHz for a fixed optical wavelength, where 1.0 equals 100 % optical modulation. Small signal modulation shows a linear increase with input RF voltage. At larger voltages, nearly 100 % modulation is achieved. These voltages, which are found to be similar to values of Vp found in commercial LiNbO3 Mach Zehnder modulators, are very encouraging as we have made no attempt to optimize electrode structure. Minimum sensitivity in our initial experiments is found to be 90 mV or 160 mW. A dramatic increase in RF sensitivity may be achieved by placing the metal electrodes closer to the optical WGM, improving spatial overlap of the RF field with the optical mode, and increasing the Q of the RF resonator.
The important technical contribution of our initial experiments is that
we have successfully demonstrated a proof-of-principle that this type of
modulator is technically feasible and could be used as part of a new microphotonic
mm-wave receiver.
Finding out more
You can read more on microphotonic RF receivers in the following papers: