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We design and fabricate a silicon photonic crystal L3 resonator for chip-scale analog signal transmission. The lattice constant (a) is 420 nm, and the radius of holes (r) is 126 nm. The three holes adjacent to the cavity are laterally shifted by 0.175a, 0.025a and 0.175a, respectively. We experimentally evaluate the performance of silicon photonic crystal L3 resonator for chip-scale analog signal transmission. The spurious free dynamic ranges (SFDRs) of the second-order harmonic distortion (SHD) and the third-order harmonic distortion (THD), which are important factors to assess the analog link performance, are measured for the chip-scale analog signal transmission through the fabricated silicon photonic crystal L3 resonator. The SHD SFDR and THD SFDR are measured to be ~34.6 dB and ~52.2 dB even with the input optical carrier sitting at the dip resonance wavelength of the fabricated silicon photonic crystal L3 resonator. The influences of the optical carrier wavelength and input optical power on the SHD SFDR and THD SFDR are studied in the experiment. The impacts of geometric parameters of the cavity structure (lattice constant, radius of holes, shift of the hole) on the analog signal transmission are also analyzed, showing favorable analog link performance with relatively large fabrication tolerance to design parameters.
© 2015 Optical Society of America
1. Introduction
Silicon photonics is considered to be a promising technology to address ever increasing challenges of future chip-scale optical interconnections and optical data processing owing to the availability of a mature silicon technology, low power consumption, compactness and potential for complementary metal-oxide-semiconductor (CMOS) compatibility which enables low-cost mass production [1,2]. Silicon material has many potential properties for photonic applications in the spectral region extending from near- to mid-infrared [3]. Those properties of transparent to infrared light with wavelengths above approximately 1100 nm, high-index-contrast for waveguiding, large thermal conductivity, hardness, low density and high optical damage threshold make silicon-based integrated systems to be a great potential platform for chip-scale optical communications and signal processing functions [4–6].
While analog system applications represent a smaller volume market than that for digital data transmission in optical communications, there are significant deployments of analog electro-optics systems for some applications, such as broadband wireless access networks, sensor works, and radio over optical transmission systems [7–9]. In analog links, it is important to maintain the linearity of transmitted analog signals. Recently, a high-contrast-gratings-based hollow-core-waveguide which features low nonlinearity, low loss and high thermal stability [10] and a tunable bandpass filter which can change the bandwidth and center wavelength of filter [11] were reported to show the linearity of analog transmission in various systems. Considering the increasing interest of silicon photonics with aforementioned advantages, one laudable goal would be to realize chip-scale analog signal transmission using silicon photonic devices.
Photonic crystal cavities are promising candidates for building blocks of future photonic integrated circuits [12]. Considerable research efforts have been devoted to the optimized photonic crystal device structures with high quality (Q) factor and small mode volume (V), which might facilitate efficient chip-scale optical signal processing operations [13–16]. The enhanced nonlinear optical responses of these devices with high Q factor, small mode volume, and resultant high Purcell factor and strong radiation-matter interaction have seen wide interesting applications, such as optical switching [17–20], optical logic [20,21], optical memories [22,23], optical diode [24], wavelength conversions [25], and Raman shift [26]. For instance, one of the photonic crystal cavities known as photonic crystal L3 resonator with high Q factor and small mode volume has been proposed and demonstrated in optical signal processing applications with impressive operation performance [12,14,15,17,21,24,27]. Beyond those optical signal processing functions, one would also expect to see the potential use of photonic crystal cavities (e.g. photonic crystal L3 resonator) in analog links. So far there have been limited research efforts on chip-scale analog signal transmission through photonic crystal L3 resonator.
In this paper, we design and fabricate a silicon photonic crystal L3 resonator. Compared to previous photonic crystal L3 resonators used for optical signal processing, the fabricated silicon photonic crystal L3 resonator with proper transmission spectrum enabled by fine control of three air holes adjacent to the cavity is applied to chip-scale analog links in the telecom C band. We experimentally evaluate the performance of silicon photonic crystal L3 resonator for analog signal transmission. The spurious free dynamic range (SFDR) of the second-order harmonic distortion (SHD) and the third-order harmonic distortion (THD) are characterized to assess the chip-scale analog link performance. In addition, we study the dependence of SHD SFDR and THD SFDR on the optical carrier wavelength and input optical power. Moreover, we analyze the impacts of geometric parameters (lattice constant, radius of holes, shift of the hole) on the analog link performance.
2. Experimental setup
The experimental setup for analog signal transmission through the proposed photonic crystal L3 resonator is shown in Fig. 1. At the transmitter side, the output of external cavity laser (ECL) is injected into a Mach-Zehnder modulator (MZM). The light source is modulated by a 5-GHz radio frequency (RF) in the MZM modulator and then amplified by an erbium-doped fiber amplifier (EDFA). A variable optical attenuator (VOA) is employed to control the optical input power of the photonic crystal L3 resonator. The polarization controller (PC) is used to adjust the polarization state of the analog signal to be aligned with the optimized polarization of the vertical grating coupler. Assisted by the vertical grating couplers at input and output ports, the analog signal is coupled from optical fiber to photonic crystal L3 resonator and vice versa. At the receiver side, after transmitting through the photonic crystal L3 resonator, the analog signal is amplified by EDFA, attenuated by VOA, sent to a photo-detector (PD), and measured by an electrical spectrum analyzer (ESA).
Fig. 1 Experimental setup for analog signal transmission through a silicon photonic crystal L3 resonator. ECL: external cavity laser; MZM: Mach-Zehnder modulator; EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; PC: polarization controller; PD: photo-detector; ESA: electrical spectrum analyzer.
3. Silicon photonic crystal L3 resonator
Figure 2 shows the designed and fabricated silicon photonic crystal L3 resonator. The photonic crystal L3 resonator consists of a photonic crystal membrane with a line of three holes missing. It is fabricated on a silicon-on-insulator (SOI) wafer (220-nm-thick silicon on 3000-nm-thick silica). The calculated mode profile is shown in Fig. 2(b). The measured scanning electron microscope (SEM) image of the fabricated photonic crystal L3 resonator is shown in Fig. 2(c). To fabricate the silicon photonic crystal L3 resonator, electron beam lithography (Vistec EBPG 5000 plus) and inductively coupled plasma (ICP) etching are used to define and form patterns on an SOI wafer. The cavity is processed by diluted hydrofluoric acid solution to strengthen optical confinement in the normal direction and increase the symmetry of the structure. In the optimized design of the silicon photonic crystal L3 resonator, the lattice constant (a) is 420 nm, the radius of air holes (r) is 126 nm, and the three air holes adjacent to the cavity are laterally shifted by 0.175a, 0.025a and 0.175a, respectively. The resonant wavelength of the silicon photonic crystal L3 resonator is within the telecom C band. Vertical light coupling is employed to couple light from fiber to chip and from chip to fiber by use of vertical grating couplers. Shown in Fig. 2(d) is the measured SEM image of the grating coupler for vertical coupling. The period of vertical grating coupler is 630 nm, and the duty cycle is 50%. The total fiber-chip-fiber insertion loss of vertical coupling is assessed to be ~13.6 dB. The 3-dB bandwidth of the vertical grating coupler is measured to be ~55 nm.
Fig. 2 (a) Proposed structure of silicon photonic crystal L3 resonator. (b) Calculated mode profile for the Ey field component of mode distribution. (c) SEM image of the fabricated photonic crystal L3 resonator. (d) SEM image of the grating coupler.
4. Experimental results and discussions
Figure 3(a) shows the measured typical transmission spectrum of the fabricated silicon photonic crystal L3 resonator with a resonance wavelength of ~1552.99 nm. The extinction ratio and 3-dB bandwidth of the fabricated silicon photonic crystal L3 resonator are ~18.4 dB and 0.19 nm, respectively. Figure 3(b) shows the measured typical RF spectrum of RF carrier at 5 GHz, SHD at 10 GHz and THD at 15 GHz after transmission through the fabricated silicon photonic crystal L3 resonator.
Fig. 3 (a) Measured transmission spectrum of the fabricated silicon photonic crystal L3 resonator with a resonance wavelength of ~1552.99 nm. (b) Measured RF spectrum of RF carrier, SHD and THD after transmission through the fabricated silicon photonic crystal L3 resonator.
Figures 4(a) and 4(b) show the measured output power of the RF carrier and distortions (SHD, THD) as a function of the RF input power at input optical carrier wavelength of 1552.83 nm and 1552.99 nm, respectively. To evaluate the performance of analog signal transmission, SFDR which is an important common judgment criterion to assess the linearity level of an analog link is taken in to consideration. It is defined by the RF input power range at the left and right boundaries of which the fundamental RF carrier power and SHD/THD power are equal to the noise floor [28,29]. A higher SFDR system generally means a more linear signal transmission. SFDRs can be achieved by measuring the intercepting points of output power curves (RF carrier, SHD, THD) and the noise floor. As shown in Fig. 4(a), for input optical carrier wavelength of 1552.83 nm, the SHD SFDR (~29.8 dB) and THD SFDR (~50.0 dB) degrade slightly. As shown in Fig. 4(b), for input optical wavelength of 1552.99 nm (i.e. resonance wavelength), slight degradations of SHD SFDR (~34.6 dB) and THD SFDR (~52.2 dB) are observed. Actually, when the optical carrier is tuned at the resonance wavelength, the modulated optical sidebands after MZM (RF modulation), which are related to the RF carrier and distortions (SHD, THD), are offset from the dip resonance wavelength. As a result, it might not be the worst case for analog signal transmission with the optical carrier sitting at the resonance wavelength of the photonic crystal L3 resonator.
Fig. 4 Measured output power of RF carrier and distortions (SHD, THD) as a function of RF input power at different optical carrier wavelengths of (a) 1552.83 nm and (b) 1552.99 nm.
To further study the analog signal transmission performance in the fabricated silicon photonic crystal L3 resonator, we also measure the SHD SFDR and THD SFDR as functions of input optical carrier wavelength and optical input power. Figure 5(a) shows the dependence of SHD SFDR and THD SFDR on the input optical carrier wavelength. Figure 5(b) shows the dependence of SHD SFDR and THD SFDR on the optical input power. The obtained results shown in Figs. 3–5 indicate favorable analog link performance using the designed and fabricated silicon photonic crystal L3 resonator.
Fig. 5 Measured SHD SFDR and THD SFDR as functions of (a) optical carrier wavelength and (b) optical input power.
5. Simulations of analog signal transmissions in silicon photonic crystal L3 resonator
For the proposed analog signal transmissions in silicon photonic crystal L3 resonator, the geometric parameters of the cavity structure such as lattice constant, radius of holes, and shift of the hole might affect the analog signal transmission performance. In order to characterize the analog link performance, we further simulate the transmission spectrum and SHD/THD SFDR under different design parameters. A three-dimensional finite difference time domain (3D-FDTD) simulation is performed to study the silicon photonic crystal L3 resonator.
Figure 6 depicts simulated resonant wavelength and Q factor (i.e. transmission spectrum) as functions of the lattice constant, radius of holes, and shift of the hole. As shown in Fig. 6(a), the radius of air holes is 0.30a and the resonant wavelength increases linearly with the lattice constant. When the lattice constant is 420 nm, the simulated Q factor reaches its maximum. As shown in Fig. 6(b), the lattice constant is 420 nm and the resonant wavelength decreases with the increase of the radius of air holes. When the radius of air holes is 0.30a, a maximum Q factor is achieved. As shown in Fig. 6(c), the lattice constant is 420 nm and the radius of air holes is 0.30a. The resonant wavelength increases with the shift of the air hole adjacent to the cavity. When the shift of the air hole is 0.175a, the Q factor finds its maximum. The simulated Q factor with three-holes-shifting is also shown in Fig. 6(c), which is larger than that of the silicon photonic crystal L3 resonator with one-hole-shifting. The mode volume of the designed silicon photonic crystal L3 resonator keeps almost unchanged as varying the lattice constant, radius of air holes, and shift of the air hole.
Fig. 6 Simulated resonant wavelength and Q factor of the designed silicon photonic crystal L3 resonator as functions of (a) lattice constant, (b) radius of holes, and (c) shift of the hole adjacent to the cavity. The diamond in (c) represents the Q factor of silicon photonic crystal L3 resonator with three-holes-shifting.
Figure 7 plots simulated SHD SFDR and THD SFDR as functions of the lattice constant, radius of holes, and shift of the hole corresponding to Fig. 6. By comparing Figs. 7(a)–7(c) with Figs. 6(a)–6(c), one can see similar dependence of Q factor and SHD/THD SFDR on the design parameters (lattice constant, radius of holes, shift of the hole). Based on the simulation results shown in Figs. 6 and 7, a lattice constant of 420 nm and a radius of holes of 0.30a are used in the design of the silicon photonic crystal L3 resonator. The positions of the three air holes adjacent to the cavity are optimized with the lateral shift of 0.175a, 0.025a and 0.175a, respectively. From Fig. 7, one can see favorable performance of analog signal transmissions in the designed silicon photonic crystal L3 resonator with relatively large fabrication tolerance to geometric parameters.
Fig. 7 Simulated SHD SFDR and THD SFDR of analog signal transmissions in the designed silicon photonic crystal L3 resonator as functions of (a) lattice constant, (b) radius of holes, and (c) shift of the hole adjacent to the cavity.
The designed and fabricated silicon photonic crystal L3 resonator with proper transmission spectrum features favorable analog signal transmission performance in the telecom C band. Remarkably, with future improvement, when a sharp notch appears in the transmission spectrum of the photonic crystal cavity and a relatively large radio frequency is employed, e.g. the radio frequency for analog modulation is larger than the 3-dB bandwidth of the sharp notch transmission spectrum, the performance of chip-scale analog links based on photonic crystal cavity could be affected by the optical carrier wavelength, optical input power, and geometric parameters (lattice constant, radius of holes, shift of the hole). Similar theoretical analyses are still available to comprehensively evaluate the chip-scale analog signal transmission performance.
5. Conclusion
In summary, we design and fabricate a silicon photonic crystal L3 resonator for analog signal transmission. The SHD SFDR and THD SFDR are adopted to evaluate the analog link performance. Using the fabricated silicon photonic crystal L3 resonator, we demonstrate the analog signal transmission and assess the operation performance. The influences of input optical carrier wavelength and input optical power on the analog signal transmission are also studied in the experiment. Furthermore, the dependences of transmission spectrum and analog link performance on geometric parameters (lattice constant, radius of holes, shift of the hole) are theoretically analyzed. The obtained results show favorable analog signal transmission performance using silicon photonic crystal L3 resonator. The demonstrated chip-scale analog signal transmission in silicon photonic crystal L3 resonator, combined with chip-scale digital signal transmission and chip-scale optical signal processing, might find wide interesting applications of chip-scale optical communications, i.e. possible integration of complete optical communication systems on a monolithic chip.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) under grant 61222502, the Program for New Century Excellent Talents in University (NCET-11-0182), the Wuhan Science and Technology Plan Project under grant 2014070404010201, the Fundamental Research Funds for the Central Universities (HUST) under grants 2012YQ008 and 2013ZZGH003, and the seed project of Wuhan National Laboratory for Optoelectronics (WNLO). The authors thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in the manufacturing process of silicon microring resonator and the facility support of the Center for Nanoscale Characterization and Devices of WNLO. The authors would also like to thank Yun Long and Qi Yang for their valuable technical supports and helpful discussions.
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