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In this study, a hollow optical waveguide with omni-directional reflectors in silicon-based materials was design, fabricated and characterized. By using dry etching technique, plasma-enhanced chemical vapor deposition for Si/SiO2 thin films and covering another wafer with omni-directional reflector together, the waveguides can be formed with an air core of 1.2mm×1.3mm. A uniform propagation loss of the waveguide to be around 1.7dB/cm for C+L band was found for the TE and TM modes. Polarization-independent hollow optical waveguides were obtained with the hollow waveguide structure.
©2004 Optical Society of America
1. Introduction
Photonic crystal structures efficiently control light flow [1–2]. One-dimensional (1-D) photonic crystals (PC), also known as Bragg gratings, i.e., dielectric stack of alternating layers of different refractive indices, have been developed for many applications such as filters. Dielectric mirrors have the advantages over uncoated metals of lower absorption, higher reflectivity and higher mechanical robustness. Recent studies have demonstrated both theoretically and experimentally that highly reflective mirrors which is angle and polarization insensitive can be achieved by sufficiently enlarging the index difference between the high and low-index materials. The resulting structure is known as the omni-directional reflector (ODR) [3–5].
Traditional waveguides are formed from highly transparent materials with index confinement. Such waveguide structures suffer the fundamental limitations associated with light propagation through solids, such as dispersion, absorption, scattering and nonlinear effects. Hollow waveguides may minimize the dependence of light transmission on the optical properties of the waveguide materials. Furthermore, since the hollow waveguide core consists only of air, the temperature dependence of the refractive index can be eradicated. This characteristic can help improve stability over traditional integrated optic devices which are highly sensitive to temperature, such as arrayed waveguide gratings. By using highly reflective walls in a hollow core, the light can be constrained to form a waveguide. For example, hollow metallic waveguides that are very efficient in the millimeter wavelength range can be fabricated using this concept. However, the metallic materials may suffer strong absorption loss in high-frequency electro-magnetic waves. This behavior restricts the metallic waveguides to for low-frequencies applications. An alternative approach is to apply dielectric coating on the hollow waveguide core. [6,7] Antiresonant reflecting optical waveguides (ARROWs) [6] and hollow waveguides with semiconductor multilayer mirror [7] have been developed. The waveguide loss of ARROWs depends strongly on the wavelength making them useful in sensing. [6] Hollow waveguides with GaAs/AlAs mult ilayer mirror should be merged with the phase-control layer to lower the polarization dependence of the propagation loss. The structure with 30-GaAs/AlAs pair multilayers achieved a low polarization-dependent propagation loss of 0.2dB/cm. [7] Since the core size of the hollow waveguide was around 20μm, the multimode behavior indicates that the waveguides are unsuitable for high-speed fiber optics communication systems. Due to the low index contrast between GaAs and AlAs (∆n=0.46), large multilayer pair numbers are necessary for low propagation loss of around 0.35dB/cm and 0.53dB/cm for the TE and TM modes, respectively. Theoretical investigations have revealed that the Bragg grating can be merged into the core of the semiconductor-based hollow waveguides for ultrafast optic applications. [8]
Omni-guide fibers were first demonstrated by using the similar hollow waveguides concept with multilayer coating to form ODR [9]. Furthermore, ODR has also been realized in semiconductor materials. [10–11] Recently, we have proposed a novel air waveguide structure with omni-directional reflectors. [12] However, e-beam photolithography should be used in the fabrication process. Furthermore, the single-mode semiconductor hollow waveguides formed by ODR (SHOW-ODR) are not known to have been experimentally demonstrated. In this study, a rectangular SHOW-ODR structure was designed, fabricated and characterized. Low polarization-dependent and low propagation loss were found to be attainable.
2. Design and fabrication of SHOW-ODR
Figure 1 presents the schematic framework of the hollow waveguide with ODR. In the structure, six multilayer pairs of Si/SiO2 were coated on a Si substrate. The refractive indices of Si and SiO2 were 3.48 and 1.48, respectively. The thickness of the Si and SiO2 layers was one quarter of the wavelength of 1.55µm in the materials to be 0.111µm and 0.258µm, respectively. The bandgap of the omni-directional reflector was calculated to be 1190–1720nm. The mode pattern of structure was calculated using the two-dimensional (2-D) finite-difference time -domain (FDTD) algorithm. The single mode is obtainable as the width and the height of the air core are 1.2µm and 1.3 µm, respectively as demonstrated in Fig. 2(a). Using the 2-D radial beam propagation method, the imaginary part (k) of the effective index of the SHOW-ODR with a circular air core is approximately calculated to be around 7.6×10-6. The loss coefficient is obtained to be around 0.6cm-1 by 4πk/l where λ is the wavelength, 1.55µm. The corresponding propagation loss is 2.6dB/cm.
To manufacture the SHOW-ODR, a groove was etched by inductive coupled plasma with photolithography on (100) silicon wafer. The width of the groove varied from 3.5–5.5µm for different waveguide designs. The etching depth of the groove was 1.2µm. After the dry etching process, plasma enhanced chemical vapor deposition (PECVD) technique was applied to deposit six pairs of Si/SiO2 (0.111/0.258µm) on the sample. Since the thin film deposition by PECVD is quasi-isotropic, the thickness of the thin films on side wall of the groove can be nearly identical to that on the bottom of the groove. Thermal annealing was performed at 200°C for 20sec and 850°C for 45 sec under nitrogen atmosphere. The top of the sample was covered by another identical ODR to form the SHOW-ODR. The ends of the waveguides were treated by mechanical polishing.
Fig. 2. Mode pattern of SHOW-ODR as (a) with well-bonded the ODRs and (b) with a 1µm gap between the ODRs.
3. Characterization and discussion
Figure 3 illustrates the cross-section image of SHOW-ODR taken by scanning electron microscope. The height and the width of the air core are 1.2µm and 1.3µm, respectively. A 1μm air-gap was found between the two ODRs owing to the rough surface of the PECVD-coated thin film. The mode pattern was re-calculated using the FDTD algorithm as shown in Fig. 2(b). A small leakage was noted on the single mode pattern possibly lead to higher propagation loss. Furthermore, the thickness of the thin films was not uniform for the bottom and the sidewall of the groove, thus narrowing the ODR bandgap. The minimum bandgap of the ODRs with the non-uniform thin films is estimated to be 1480–1640nm for the TE and TM modes. This bandwidth implied that the SHOW-ODR is still within appropriate to be applied for optical fiber communication.
A broadband light source with a bandwidth from 1520nm to 1620nm was directly coupled into the SHOW-ODR with an objective lens. A polarizer was applied to control the input light polarization. Figure 4 illustrates the output mode patterns measured by an infrared CCD. The horizontal and vertical mode size is around 1.3µm and 1.4µm, respectively, by comparing it with that of a single mode fiber of 9µm diameter. The output power of the SHOW-ODR was measured by an optical spectrum analyzer. The propagation loss spectra were measured by end-fire coupling of an optical fiber to the SHOW-ODR of different lengths (2.9cm, 1.9cm and 0.9cm). For each wavelength, the propagation loss was obtained by calculating the slope of the relation between the output power and the sample length. Fig. 5 illustrates the propagation loss spectra which are about 1.7±0.3 and 1.7±0.1dB/cm for the TE and TM modes, respectively. The propagation loss obtained by the experiment and by the simulation is in the same order of magnitude. The polarization dependent loss was low owing to the quasi-symmetrical geometry of the air core mode of the SHOW-ODR as illustrated in Fig. 2.
The propagation loss of the conventional waveguides for integrated optics is usually below 1dB/cm. Although, the propagation loss of SHOW-ODR approaches that for integrated optic applications, the light leakage may cause heavy crosstalk in the devices. According to the FDTD simulation, the air gap leakage can be greatly reduced when the air gap is less than 0.5µm and the mode pattern is similar to that in a traditional rib waveguide. Therefore, decreasing the surface roughness of the deposited thin films can decrease SHOW-ODR’s propagation loss for the practical applications.
Fig. 4. (a) Mode pattern of a conventional single mode fiber with a core diameter of 9µm.(b) Mode pattern of SHOW -ODR. The horizontal and vertical mode size is around 1.3µm and 1.4µm, respectively.
4. Conclusion
This work has demonstrated experimentally a hollow waveguide with ODR, achieving a propagation loss of around 1.7dB/cm. Polarization independent loss is uniform for a wide C+L bandwidth. The loss due to the air gap between the ODRs can be ameliorated by depositing the thin films using low-pressure chemical vapor deposition method to reduce the surface roughness of the ODR and the wafer bonding technique. Study of bent SHOW-ODR and power splitter using multimode interference are currently in progress.
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