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Hallbjorner, The significance of radiation efficiencies when using S-parameters to calculate the received signal correlation from two antennas, IEEE Antennas Wirel Propag Lett 4 (2005), 97–99. VC 2013 Wiley Periodicals, Inc. LOW-POWER PHOTONIC CONTROL OF A MICROWAVE RING RESONATOR USING BULK ILLUMINATION Mohammad Ali Shirazi-Hosseinidokht and Mani Hossein-Zadeh Center for High Technology Materials, 1313 Goddard, SE, Albuquerque, NM 87106; Corresponding author: mhz@chtm.unm.edu Received 22 October 2012 ABSTRACT: We demonstrate the feasibility of bulk illumination technique for low-power photonic control of RF resonance. Using this technique, the transmitted RF power through a microstripline-ring filter on a junction-less silicon substrate is changed by 11 dB with less than 2 mW of interacting optical power. VC 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1594–1599, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27606 Key words: light-controlled RF devices; RF ring resonator; RF-photonics 1. INTRODUCTION Photonic control of RF signal propagation is an important topic that continues to be an active area of research and development in microwave photonics [1–14]. Photonic control has many advantages over conventional electrical control. High degree of electrical isolation between the control signal and the microwave circuit, immunity to parasitic electromagnetic radiation, high power handling, overall weight reduction, high-speed control, and timing precision are among the most important benefits of photonic control. In particular, electrical isolation between the control signal and the microwave structure is crucial for design and fabrication of reconfigurable antennas [15–18] where the radiation pattern and efficiency are affected by the presence of control devices and circuits in the vicinity of antenna pattern [15]. A large variety of techniques, devices, and materials have been explored for designing photonically controlled switches, phase shifters, and attenuators. In almost all these approaches, photonic carrier generation in a semiconductor controls the am- plitude and the phase of the RF signal propagating on microstrip or coplanar transmission lines. Free carrier generation in biased and unbiased junctions as well as junction-less regions have been used to control the RF field in discontinuities [8, 11], stubs [9, 13], resonators [6], and terminations [4, 5, 9]. Except few cases where the photosensitive element is added to a transmis- sion line fabricated on a low-loss RF substrate [1, 15, 18], in most proposed structures the RF circuit is fabricated on the pho- tosensitive semiconductor substrate in order to reduce the com- plexity of the fabrication process and keep the device mono- lithic. Although compound semiconductors have also been used as the structural materials in these devices [3, 10], implementa- tion of photonically controlled RF devices on silicon substrates is more attractive for monolithic integration of microwave and mm-wave devices using well-developed fabrication processes. Independent of the material and the device structure, in all these approaches and devices a laser wavelength between 600 and 900 nm is used to maximize optical absorption and carrier generation. As a result, the optically affected region has been confined at the surface (due to small optical penetration depth at these wavelengths). In contrast to the previous work, here we explore the potential application of bulk illumination at a longer wavelength (1064 nm) combined with high-Q RF resonance to maximize the RF-optical field overlap. Moreover, we use a side- coupled RF ring resonator to confine the RF field and enhance the interaction of RF field and photogenerated carriers. Note that although previously certain planar resonant structures were used for photonic RF control, the confinement of free carrier on the surface has limited the interaction of the resonant field and the free carriers. In these cases, the presence of free carriers has been mainly modifying the electrical properties at the bounda- ries of the resonator (effectively tailoring the conductor size). This article describes a photonically controlled RF ring reso- nator suitable for controlling a variety of silicon-based micro- wave integrated circuits. By optically controlling the density of free carriers in the regions of the substrate with large resonant RF field, we have demonstrated up to 8 dB of transmission loss variation with only 1.9 mW of interacting optical power from a commercial fiber pigtailed laser diode operating at 1064 nm. To our knowledge, this is the largest optical sensitivity of RF trans- mission ever reported for a passive junction-less photonically controlled microwave device. 2. PHOTONICALLY CONTROLLED RF RING RESONATOR 2.1. Bulk Versus Surface Illumination As photonic free carrier generation controls the RF propagation in most optically controlled components, strong optical absorp- tion is the main criteria for choosing the photoconductive mate- rial and the corresponding wavelength. On the other hand to reduce the fabrication cost and complexity, preferably only one type of material is used in the device structure. As a result, low- loss optical waveguides cannot be easily integrated with the de- vice to deliver light directly to the sensitive region, and almost all devices are controlled by top illumination to avoid absorption before reaching the sensitive region. Silicon is one of the most common substrates used in phonically controlled RF devices (mainly because of compatibility with IC fabrication and low fabrication cost). Figure 1(a) shows the absorption depth (d, the depth at which the light intensity drops to 36% of its value at the interface) plotted against wavelength for silicon substrate. Below 900 lm, d is less than 30 lm, and all the photogenerated carriers are effectively confined at the surface (interface between air and silicon). That is why so far the optical control has been mainly achieved by tailoring the conductive structure on top of the semiconductor substrate using laser beam. Here, we examine an alternative approach by choosing a wavelength with an absorption depth larger than the substrate 1594 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop thickness to enable bulk illumination (photogeneration) and, therefore, enhanced interaction between the RF field and the photogenerated carriers. In addition, we use an RF ring resona- tor that confines and passively amplifies the RF-field in a rela- tively small region. Bulk illumination of the volume with maxi- mum electric field intensity results in a large RF-optical (and, therefore, RF-free carrier) overlap integral. In our experiment, we use the 1064 nm wavelength with an absorption depth of about 1 mm that is two times the thickness of the substrate. So the photocarriers are generated almost uniformly distributed across the wafer. 2.2. Experimental Setup The structure consists of RF ring resonator side coupled to a microstripline. RF ring resonator has been extensively investi- gated in the context of planar antennas and frequency-selective surfaces [19–21]. The ring resonator and the transmission line are fabricated on a 500-lm silicon substrate (100 orientation) with a resistivity of about 2000 X cm. A thin layer (�2 lm) of copper was coated on both sides of the silicon wafer using RF sputtering (50 nm of chromium was used between copper and silicon to improve the attachment). Next, we patterned the top copper layer using photolithography and wet etching. Figure 1(b) shows the ring resonator side coupled to the microstripline. The ring resonator has a diameter of 5.3 mm and a width of 0.43 mm. The microstripline is a 50-X line (width � 0.43 mm), and two SMA launchers were used to couple RF power into and out of the microstripline [Fig. 1(b)]. When a ring resonator is side coupled to microstripline, the degeneracy between frequencies of the even and odd resonant modes will be removed because of the asymmetric coupling [19, 20]. As a result, two dips appear near each resonance in the transmission spectrum of the microstripline. We have used a finite element microwave modeling software (CST) to calculate the frequencies and the field distribution for the first two modes of the ring reso- nator. Figure 2 shows the simulated transmission spectrum (S21) as well as the electric field distribution for the fundamental and the second-harmonic mode of the ring resonator. Below 15 GHz, four modes are excited: the odd fundamental mode (M1- o), even fundamental mode (M1-e), odd second-harmonic mode (M2-o), and even second-harmonic mode (M2-e). Using the simulation results, we have identified three differ- ent locations corresponding to the minimum field-strength for each one of the RF modes for optical illumination [see Fig. 3(a)]. A small circular aperture on the copper ring is used to expose the substrate to laser at each one of these locations. The diameter of aperture is 0.2 mm (about 50% of the ring width) to minimize its effect on the RF resonance [Fig. 3(b)]. We fabricated three samples with identical rings and coupling gaps but with different aperture positions (so each ring has only one aperture). As shown in Figure 3(c), the 1064-nm laser light from a fiber pigtailed laser diode is fed to a fiber pigtailed collimator and is vertically coupled to the substrate through the aperture. Figure 1 (a) Absorption depth of the silicon plotted against wavelength. (b) A photograph of the ring coupled to a microstripline on a silicon sub- strate. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] Figure 2 Simulated transmission spectrum of the microstripline coupled to the ring resonator. (a) Near the fundamental resonance. (b) Near the sec- ond-harmonic resonance. The arrows show the even (fe) and odd (fo) modes for each resonance. The insets show the electric field magnitude on a plane located in the middle of the silicon substrate (250-lm depth). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1595 Figure 4 (a) The laser is illuminating Location 3 (the minimum field intensity for even second-harmonic mode). (b) The laser is illuminating Position 1 (the minimum field intensity for even fundamental mode). (c) The laser is illuminating Position 2 (the minimum field intensity for odd fundamental mode) Figure 3 (a) The location of the aperture on three different ring resonators. (b) Close-up photograph of a ring with an aperture at Position 3. (c) Sche- matic diagram of the collimator vertically coupling laser light to the silicon substrate. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] 1596 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 DOI 10.1002/mop 2.3. Measurement and Results Figure 4 shows the S21 measurement results near fundamental and second-harmonic resonance in the absence of the laser illu- mination and with three different levels of incident optical power (0.43, 1.23, and 2.71 mW). The incident optical powers are estimated based on the optical output power from the colli- mator and the aperture area. The resonant frequencies of the odd and even fundamental modes as well as the even second-har- monic modes are in good agreement with valued from the S21 measurement. According to the simulation, the M2-o is weakly coupled to the microstripline (S21 � �4 dB) and that is why it is hidden in the parasitic background transmission in the meas- ured spectrum. In Figure 4(a), the laser is illuminating Location 3 where the E-field intensity for the M2-e is minimum (according to simulation). As this location is near the maximum of the M1- e, this mode is suppressed by photocarrier generated loss. The resonant dip of the M1-o and the M2-e are also reduced; this is due to three main reasons: the resonant electric field of these modes is not completely zero near in the illumination region, the real location of the minimum field intensity might be slightly different from the simulation, and some carriers will diffuse to high-field region. In Figure 4(b), the laser is illuminating Location 1 where electric field intensity for M1-e is minimum. As expected, the M1-e is minimally affected, whereas M1-o is effectively disappeared and M2-e is severely attenuated (Position 1 has a significant overlap with the field intensity of M2-e). In Figure 4(c), the laser is illuminating Position 2 where the maximum of M2-e is located. In this case, we observe the maximum attenuation for M2-e mode. These results show that using bulk illumination one can selec- tively control the loss factor of different modes. Due to the absence of the M2-o, near 14 GHz the illumination only con- trols the magnitude of the even second-harmonic with mini- mal frequency shift. So effectively near the second-harmonic resonance, the device functions as a single frequency optical RF switch. Figure 5 shows the RF quality factor (unloaded), frequency and the transmitted RF power (S21) at fRF ¼ f2,e � 13,890 MHz (resonant frequency of M2-e) plotted against the interacting optical power. Note that at a single wavelength (1064 nm), optical reflection from the silicon–air interface can be easily canceled by depositing two layers of dielectric on top of the silicon with almost no effect on the RF proper- ties. Therefore, to estimate the ultimate performance of the device, here we have considered the optical power inside the silicon (or the interacting optical power) instead of the inci- dent optical power. The unloaded RF quality factor in each case has been estimated using the S21 spectrum based on the 3-dB linewidth measured from the bottom of each transmis- sion dip [22]. As expected, the quality factor of the modes degrades due to loss generated by free carriers. The frequency does not change when aperture numbers two and three are illuminated, and it changes only by 1% when aperture number one is illuminated. Note that in almost all resonant cases studied previously [4–6], the optically induced attenuation is accompanied with significant frequency shift. Here, the frequency change is minimal, because the photocarriers do not increase the RF-length as opposed to previous resonant structures where the photogenerated carriers at the surface change the conducting surface (therefore the resona- tor size) and the resonant frequency. Decoupling the frequency shift and attenuation is very important, because in narrow-band Figure 5 Measured values of RF quality factor (a), resonant frequency (b), and S21 at resonance (c) plotted against optical power in the substrate (interacting optical power). These parameters are only measured for M2-e (f2,e � 13,890 MHz) and V1–V3 correspond to illumination at Positions 1–3. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] TABLE 1 Comparison Between the Performance of the Proposed Devices and Previously Shown Optically Controlled RF Devices Reference Wavelength (nm) dB/(W/mm2) dB/mW Geometry Frequency (GHz) Material This work 1064 173 5.5 Ring 13.7 Silicon [4] 513 1.9 X Stub 6 Silicon [5] 513 19 X Stub 4 Silicon [10] X X 4.1 OVC 14 III–V [8] 870 X 1 Interdigital 6 Silicon [11] 820 X 0.8 CPW-gap 6 Silicon [12] Xe arc lamp 85.7 X Waveguide 50 Silicon [13] 850 X 0.35 Stub 7 Silicon [18] 808 X 0.09 Si-gap 8.4 Silicon [9] <1000 X 1.15 Stub 6.9 Silicon [14] 808 X 0.08 CPW-gap 6.9 Silicon DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 7, July 2013 1597 resonant systems (such as filters) attenuation at certain frequen- cies is desired. Table 1 summarizes the some of the results from previous works on photonically controlled RF circuits. Independent of the material and the specific design, all devices use optical illumina- tion to control the RF field through photocarrier generation. Clearly in terms of maximum RF transmission change per 1 mW of optical power, our device outperforms the previous devi- ces. All these devices function based on optical surface effects (k < 900 lm) and some of them use complex structures, junc- tions (biased and unbiased), and compound semiconductor mate- rial. The device presented in this work is completely made on a uniform silicon substrate and does not use any bias voltage or p–n junction, yet it has the largest optical sensitivity. The enhanced sensitivity is a result of large overlap between the photocarrier density and the oscillating RF field in the silicon substrate. 3. DISCUSSION The device presented here has many applications in optical switching of microwave power and the design of optically reconfigurable RF circuits and antennas. The same structure can also function at higher frequencies by reducing the ring resonator diameter or using higher harmonics of the same ring. We expect better optical sensitivity in smaller rings due to the larger ratio between the optically attenuated RF field and the total resonant RF filed. Moreover, by coupling more rings, several frequencies can be switched simultaneously resulting in more flexible and versatile RF transmission spec- trum. Due to low power consumption, one laser can feed sev- eral resonators using low-cost power splitters and collimators. Figure 6(a) shows three identical coupled ring resonators com- prising a three-pole RF band-stop filter (three stop bands shown in the figure). As shown (gray trace), laser illumination of all three rings can eliminate the second stopband without significantly changing the other stopbands. By properly illumi- nating different locations on each ring, one can tailor the transmission spectrum of the filter without any physical con- tact or using any electronic/mechanical element. Figure 6(b) shows a design where ring resonators with different diameters are side coupled to a single transmission line. In this case, a series of optically controlled interleaved stopbands (with dif- ferent mode spacing) can provide more control over the desired region (dashed box). In this work, the ring resonator is side coupled to the microstripline resulting in bad-stop operation. These configu- rations and methods can be extended to photonically con- trolled band-pass filters using capacitive gap coupled (as opposed to side coupled) ring resonators. Optically controlled band-pass RF filters with relatively low insertion loss can be designed by implementing optimized capacitive coupling methods between rings [23]. Although for the sake of simplic- ity in this proof-of-concept demonstration, we used a colli- mated beam for illuminating the su