The design, fabrication and measurement of a silicon-on-insulator (SOI) two-dimensional photonic crystal ring resonator are demonstrated in this study. The structure of the photonic crystal is comprised of silicon nano-rods arranged in a hexagonal lattice on an SOI wafer. The photonic crystal ring resonator allows for the simultaneous separation of light at wavelengths of 1.31 and 1.55μm. The device is fabricated by e-beam lithography. The measurement results confirm that a 1.31μm/1.55μm wavelength ring resonator filter with a nano-rod photonic crystal structure can be realized.
©2007 Optical Society of America
Interest in two-dimensional photonic crystals (PCs) has been increasing because of their potential for use in optical fiber communication system devices such as air-core waveguides, channel drop filters, power splitters, light sources, and so on [1–4]. The photonic bandgap (PBG) effect of photonic crystals provides the highly confined mode needed to control the propagation of the light. Due to their nanometer feature size, the device packing density can be increased. One important PC device is the channel drop filter which is utilized in wavelength division multiplexer (WDM) communication systems [5–7]. Research has been done to obtain high quality, channel drop filters based on the micro-ring resonator structure [8–11]. Such devices have a circulating mode in ring form which is excited by the coupling of the propagating wavelength of the main bus waveguide. The performance of the micro-ring resonator is very sensitive to the radius of the device . To overcome the challenges involved in using a micro-ring resonator, it is necessary to reduce the mode number of the operating bandwidth (i.e. by reducing of the radius of the ring). Ring resonator filters with a PC structure are a good candidate to solve the problem because the bent ring waveguide provides good optical confinement. The high-Q, high wavelength selectivity, and ultra small size of the resonator, compared to that of the minimum-useable-size strip-based rings, can be achieved by optimization of the coupling length and radius of the periodic structure of the PC ring resonator structure. Recently, we reported on a PC directional coupler formed by dielectric nano-rods  in which the air waveguides were formed by linear defects in the photonic crystal structure of the dielectric rods. The guiding material for minimizing the problems associated with absorption and material dispersion is air. The structure helps to confine a high-powered laser beam by the air-waveguide. We also proposed using omnidirectional reflectors on and below the two-dimensional PC formed dielectric rods; this can be done to obtain vertical confinement of the light propagating in the PC waveguide. To reach this goal, we have recently succeeded in realizing the formation of air waveguides by omnidirectional reflectors . We believe that air waveguides formed by dielectric rods and omni-directional reflectors can significantly reduce propagation loss.
In this present work, we investigate the formation of a PC ring resonator formed from line-defect PC waveguides and a single PC ring with hexagonally arranged rods on an SOI wafer. The device is designed to separate the wavelengths at 1.31 and 1.55μm. E-beam lithography is used to define the nano-structures of the device. The results demonstrate that a 1.31μm/1.55μm wavelength ring resonator filter with a PC structure can be realized. The compactness of the device, greater than that of conventional ring resonators , means that it can be used in integrated optics systems for such applications as optical fiber communications.
2. Design and simulation
The SOI wafer used in this study consists of a 0.8μm-thick silicon device layer and a 4μm-thick SiO2 lower cladding layer. The schematic SOI structure is shown in Fig. 1. We use the silicon ridge waveguides to launch the light into the PC structure. The refractive index of silicon is higher than that of the lower cladding layer, SiO2. The two-dimensional plane wave expansion method is used to calculate the photonic bandgap of the dielectric rods periodically arranged in hexagonal and square lattices for the polarization of the E-field parallel to the dielectric rods (TM mode). The refractive index of the silicon rods is 3.46 (for a wavelength of 1.55μm). Figure 2 shows that the photonic bandgaps for the different radius (r)/lattice constant (a) ratios are larger for the hexagonal lattice PC than for the square lattice PC. Therefore, the hexagonal dielectric rod arrangement is chosen for designing the PC ring resonators. When the radius of the rods is 0.185a, the normalized frequency of the photonic bandgap is between 0.29 and 0.47. When the refraction index of silicon at a wavelength of 1.31μm needs to be 3.5, the normalized frequency of the photonic bandgap is between 0.28 and 0.48. The result shows that the index difference between 1.31 and 1.55μm has only a slight influence on the normalized frequency of the photonic bandgap.
A larger bandgap can improve the process and the optical field confinement. For a PC device with a larger bandgap, the r/a ratio chosen is 0.185. The normalized frequencies are 0.374 and 0.442 for wavelengths of 1.55μm and 1.31μm, respectively. The lattice constant obtained is 0.58μm, and the corresponding radius of the rods is 0.108μm. A hexagonal ring is formed by removing 42 nano-rods as shown in Fig. 3. The transmission spectra of the ring resonator are investigated by using the two-dimensional finite-difference time-domain method. A pulse in the TM mode is first launched into the devices to obtain the approximate transmission spectrum of the PC ring resonator. The results are shown in Fig. 4. We can see optical power peaks around 1.31μm and 1.55μm, respectively. The free spectral range is around 20nm. The bandwidth is around 10nm and 7nm for the 1.31 and 1.55μm, respectively. A detailed study based on the approximate results obtained above is now done, by launching continuous waves with different wavelengths into the PC ring resonator. The optical field distribution is illustrated in Fig. 3. We can observe that the light at wavelengths 1.31 μm and 1.55μm is received at output ports 1 and 2, respectively. This behavior confirms that the device can be used as a wavelength division multiplexing filter. The crosstalk is defined as the ratio of the intensity for 1.31μm to 1.55μm at the output port. The crosstalk is 7.5dB and 6.5dB for the outputs of port 1(1.31μm) and port 2(1.55μm), respectively.
3. Sample preparation and characterization results
This sample is grown by plasma enhanced chemical vapor deposition (PECVD). A 4μm-thick SiO2 cladding layer is grown on the Si substrate, following by the deposition of a 0.8μm-thick Si guiding layer. E-beam lithography is used to define the patterns on the photonic crystal regions. The sample is coated by hexamelthyl-disilazane (HMDS) to increase the adhesion of the negative photoresist. Following the negative photoresist, the sample is then coated with maN-2403 before being baked at 90°C for 90sec. The pattern of the rod array is defined by an e-beam writer. Dry etching is performed using an inductively coupled plasma (ICP) etcher with a C4F8/SF6 plasma mixture to etch the Si after the e-beam lithography. Sidewall protection is achieved by C4F8 gas. The sidewall protection mechanism makes for a good sidewall profile for the nano-rods.
To obtain higher coupling efficiency between the conventional waveguides and PC waveguides than obtained in our previous results , we adopted PC tapers at the input/output ports of the PC waveguides. The etching depth of the device is 0.8μm, and the width of the ridge waveguides is 4μm. Figure 5 shows scanning electron microscopic images of the PC ring resonator. The insets to Fig. 5 show a tilted view of the samples and a top view of the nano-rods. The diameter of the nano-rods is around 0.22μm which corresponds to the design diameter. Before measurement, the Si substrate is thinned by polishing and cleaved to yield a mirror-like facet at the end of the input and output ridge waveguides.
The measurement system of the PC ring resonator is shown in Fig. 5. The wavelength of the tunable laser is 1.55μm; the 1.31μm light source is a DFB (Distributed Feedback) laser. The two light sources are identical in power (3dBm). They are coupled to a fiber using a 3dB coupler. The light is polarized in TM mode using a polarizer which is coupled to the 4μm width waveguide using a lensed fiber. The spot size of the lensed fiber is 2.5μm. The use of this type of fiber can reduce the optical loss in the measurement system. The positions of the device and the lensed fiber are controlled by piezoelectric stages. The output spectra are analyzed using an optical spectrum analyzer.
The crosstalk between two channels is an important parameter to evaluate the degree of interference between two different signals in a fiber communication system. The crosstalk in our device is determined by the ratio of the optical powers of two different optical signals (1.31μm/1.55μm). Figures 7(a) and 7(b) show the optical spectra measured at port 1 and port 2, respectively. It can be seen in Fig. 7(a) that the 1.55μm signal is below the noise level, so the crosstalk at the port 1 should be larger than 15dB. The crosstalk at port 2 is around 16dB. The optical loss of the device is around 70dB. The crosstalk of the device may be improved by tuning the refractive index or the feature size of the rods between the two PC waveguides. The former, which changes the material index locally, is difficult to achieve. The latter can be achieved by controlling the electron dose for each rod.
In our previous study , where the PC directional coupler was formed by nano-rods arranged in a square lattice, the crosstalk was only around 3 dB, and the optical loss around 80dB. We believe however that the arrangement of the nano-rods of the PC structure in a hexagonal lattice show lead to a larger bandgap than that of the square lattice, and the tapered structure should help to reduce the optical loss, at least at 10dB, and therefore improve the crosstalk problem. However, the amorphous silicon layer means high loss in the near infrared. Using single crystal silicon as guiding layers may reduce the device loss. An anti-reflective coating on the facets of the input waveguide could also reduce the Fresnel loss due to light source coupling.
In this study, the diameter of the ring resonator is 8μm, which is much compact than that of the conventional ring resonator, around 300μm . As shown in Fig. 3, the light in the PC ring resonator is highly confined. This result may not be obtained with dielectric ring resonators with such a small diameter and shows the advantage of using the PC structure for ultra compact devices.
The resonant frequency of the ring resonator is quite sensitive to the fabrication tolerance. In e-beam lithography, the tolerance is defined as ΔLe/Le, where ΔLe and Le are the dimension error and the device size defined by e-beam writer, respectively. With a larger device size (Le), the dimension error (ΔLe) will increase, due to the larger aberration of the magnetic and electrostatic lenses used to scan the electron beam. This ratio can be regarded as a constant when the scanning field is small. This ratio can also be used to distinguish the performance of the e-beam writer. The resonant frequency of the resonant cavity can be expressed as ΔL/L=-(Δv/v) , where L, ΔL, v and Δv are the cavity length, the dimension error of the cavity length, the resonant frequency, and the change of the resonant frequency, respectively. This relation shows that the resonant frequency sensitivity (Δv/v) is influenced by the ratio of the dimension error to the cavity length (ΔL/L). Therefore using an e-beam writer with a low aberration can ameliorate the deviation of the resonant frequency due to the fabrication error. This phenomenon also shows that the reduction of device size (from the conventional dielectric ring resonator to the PC ring resonator) will not aggravate the resonant frequency sensitivity to the fabrication error.
In this work we discuss the formation of a PC ring resonator with an SOI structure arranged in a hexagonal lattice. We use e-beam lithography to yield round rods. The size of the nano-rods is optimized close to that of the simulation parameters. The results show that with the device we can simultaneously separate the wavelengths at 1.31 and 1.55μm. The low crosstalk, of 15dB, makes this device suitable for optical fiber communications. The hexagonal lattice PC nano-rod structure allows for an improvement in optical loss and crosstalk problem over our previous results. The crosstalk may be ameliorated by varying the diameter of the rods between the PC waveguides using the e-beam technique.
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