A highly angle tolerant spectral filter has been implemented taking advantage of three-stage serially concatenated resonators in dielectric films, each of which involves a high-index cavity in a-Si, sandwiched with a pair of SiO2 films. For the constituent etalons, the free spectral range is individually adjusted by differentiating the thickness of the cavity, so that a primary infrared pass-band could be attained to present enhanced roll-off characteristics in conjunction with an appropriate bandwidth. The a-Si cavities relating to the three etalons are selected to be 117, 234, and 468-nm thick, while the SiO2 layer is uniformly 150-nm thick. The filter is actually created on a silica glass substrate, by alternately depositing SiO2 and a-Si films. The observed center wavelength, bandwidth, and peak transmission efficiency are about 900 nm, 120 nm, and over 90%, respectively, for normal incidence. In response to an angle change amounting to 60°, the relative wavelength shift and the variation in peak transmission become barely 0.03 and 8%, respectively. Finally, a detecting cell is constructed by integrating the prepared filter with a photodiode, thus rendering a 3-dB angular bandwidth of 90°. By adequately arranging three detecting cells in a fixture, a compact, portable optical receiver could then be constructed. For incoming collimated light at λ = 905 nm, the infrared receiver exhibits an extended 3-dB angular acceptance as large as 160°.
© 2013 OSA
Recently, free-space optics technology has played a pivotal role in numerous fields, such as visible light communications, multiple integrated laser engagement systems, and last-mile wireless interconnects [1–5]. A receiver used for such optical wireless applications is required to preferably secure a large angular acceptance, thereby facilitating its alignment with the corresponding transmitter. Spectral filters, which are used to select optical signals of concern, are mostly attached to the receiver, while guaranteeing a wide field-of-view [6,7]. The filter helps eliminate undesirable background noise caused by ambient sunlight or lighting equipment, so that the occasional malfunction of the receiver could be effectively prevented . To embody such filters, various schemes of multi-layered structures in low-index materials such as oxides have been actively attempted, in view of their simple structure and low cost. However, their performance has been reported to deteriorate drastically depending on the angle of incident light [9–15]. Other devices that rely on complicated photonic crystal structures have been suggested as well, yet their transmission was susceptible to pronounced angle dependence in addition to polarization sensitivity [16–18].
In this paper, we endeavored to develop a highly angle tolerant filter with boosted transmission efficiency, which operates in the near infrared (IR) band centered at λ = ~900 nm, making the best use of three-stage concatenated etalon-type resonators. Each of the constituent resonators entails a high-index cavity made of a-Si (n = 3.8), embedded in a pair of low-index thin films made of SiO2 (n = 1.45). Their free spectral ranges (FSRs) are individually tailored by differentiating the thickness of the cavity in such a way that the main pass-band centered at a resonant wavelength, which is shared by all of the resonators simultaneously, features a sharp roll-off characteristic occupying an appropriately broad spectral width and suffering from no significant side lobes. The proposed device has been designed via rigorous coupled-wave analysis (RCWA), and fabricated by interleaving SiO2 films with a-Si films on a glass substrate. It is primarily evaluated in terms of the angle tolerance and polarization sensitivity. In an effort to validate its immediate applications to optical wireless communications, an optical receiver, permitting a large field-of-view, is successfully demonstrated capitalizing on the proposed angle tolerant filter.
2. Proposed angle tolerant filter resorting to three-stage dielectric etalons
The configuration of the proposed bandpass filter is schematically illustrated in Fig. 1 , where incoming light with an angle of incidence θi is filtered out in the near-IR band centered at ~900 nm. Here, the incident light is supposed to assume either transverse electric (TE) or transverse magnetic (TM) polarization, depending on the direction of its electric field. On a silica glass substrate, triple a-Si films of unequal thicknesses are interleaved with SiO2 films of a constant thickness. The proposed device may be treated as a multi-cavity Fabry-Perot etalon filter [19, 20]. Individual resonators work as an etalon of SiO2/a-Si/SiO2 configuration, where the high-index a-Si layer in the middle acts as a cavity, while the SiO2/a-Si boundary formed at the top and bottom serves as a partially reflecting mirror, contributing a modest reflectance of ~0.16. As shown in Fig. 1, Resonator I is chosen to have a cavity of a fundamental thickness d1 = d, while Resonators II and III adopt a cavity of thicknesses d2 = 2d and d3 = 4d, respectively. Assuming Resonator I yields an FSR equivalent to Δν, Resonators II and III accordingly provide FSRs of Δν/2 and Δν/4, respectively. As a consequence, the spectral filter, based on triple serially cascaded etalons, is anticipated to exhibit periodic transmission peaks, set apart by an FSR of Δν.
Figure 2 shows the behavior of incident light in a dielectric etalon incorporating an a-Si cavity, which gives the periodic transmission in Eq. (1). Here, R denotes the Fresnel reflection induced reflectance available from the partially reflecting SiO2/a-Si boundary. The thickness and refractive index of the cavity are expressed as d and n2, respectively. θ2 is the propagation angle inside of the cavity, λ is the wavelength, and φ is the phase shift in the reflection coefficient associated with the dielectric boundary. θ2 is known to be conspicuously dependent upon the cavity index. The rate of fractional change in the resonant wavelength with respect to the angle is expressed as Eq. (2), where the incidence angle and resonant wavelength are denoted as θi and λc, respectively, implying that the relative wavelength shift (Δλc/λc) resulting from the angle change diminishes with the refractive index of the cavity [21, 22]. Therefore, to substantially raise the angle tolerance of the etalons, we decided to introduce a-Si film as a high-index cavity:
The proposed filter is designed with the assistance of an RCWA-based simulation tool, GSolver while the complicated dispersion for the a-Si cavity, SiO2 layer, and glass substrate is thoroughly taken into account . The bandwidth needs to be moderately broad enough to mitigate the issues pertaining to the possible shift in the filter’s pass-band or drift in the source wavelength. In order to make one of multiple resonant wavelengths that originate from the three resonators coincide with the targeted center at ~900 nm, as shown in Fig. 1, Resonators I, II, and III are chosen to have cavities with thicknesses of d1 = d = 117 nm, d2 = 234 nm, and d3 = 468 nm, respectively. The oxide layers enclosing the a-Si cavity are uniformly h = 150 nm thick. Figure 3 presents the theoretical transfer curves for the filter device and the constituent etalons. An efficient pass-band located at ~900 nm is obtained, yielding satisfactorily sharp roll-off and an adequate bandwidth of ~130 nm, as compared to individual etalon-type resonators. It is especially noteworthy that secondary peaks in the vicinity of the primary peak at 900-nm wavelength are discovered to be substantially suppressed.
Figure 4(a) shows the calculated optical response for unpolarized light, as the angle θi varies from 0 to 60° in steps of 10°. For normal incidence, the primary resonance in the IR band is centered at λ = ~900 nm with a peak transmission of ~95%. On the whole, the relative wavelength shift and the fluctuation in maximum transmission are estimated at roughly 0.03 and 3%, respectively. The bandwidth remains constant at ~130 nm throughout the entire range of the angle. Next, we aimed to investigate the influence of light polarization upon the device performance in terms of the angle. Figure 4(b) shows the transfer characteristics when the angle of incidence varies from θi = 0 to 60° in steps of 10°. The device discloses no remarkable polarization sensitivity. As plotted in Fig. 5 , the reflection and absorption responsible for the overall transmission of the filter are then calculated, signifying that the low transmission below the IR band is largely attributed to the relatively considerable absorption incurred by the a-Si cavity .
3. Device design and experimental results
Our filter was formed on a glass substrate, as shown in Fig. 1, by alternately depositing three different a-Si films with approximate thicknesses of d1 = 117 nm, d2 = 234 nm and d3 = 468 nm, and a 150-nm thick SiO2 film, via plasma-enhanced chemical vapor deposition. As displayed in Fig. 6 , the SEM image confirms that the device has been manufactured with high fidelity to its design.
In order to evaluate the performance, a collimated light beam from a halogen lamp (Model LS-1, Ocean Optics) is shone upon the prepared filter mounted on a motorized rotation stage, while the output is monitored by a spectrophotometer (Model USB-4000-VIS-NIR, Ocean Optics). Figure 7(a) shows the obtained transfer characteristics for unpolarized light, when the angle of incidence varies from θi = 0 to 60° in a step of 10°. For normal incidence, the center wavelength and peak transmission were initially calculated as about 900 nm and 90%, respectively. The center wavelength shifted from 900 nm down to 890 nm, as a result of the angle change from 0 to 60°. The spectral bandwidth was preserved at an almost fixed value of ~120 nm throughout the entire range of the angle. According to Fig. 7(b), it was proven both theoretically and experimentally that the relative wavelength shift and the variation in peak transmission becomes as small as 0.03 and about 8%, respectively, in response to the angle ranging up to 60°. Figure 8(a) shows the measured optical responses for the angle θi spanning 60°, for both TE and TM polarizations. As plotted in Fig. 8(b), the peak transmission and relative wavelength shift with the angle were specifically explored for the TE and TM polarizations, which seems to be in good correlation with the calculated results. As predicted, for the angle amounting to 60°, the transfer characteristics revealed no outstanding polarization dependency.
Lastly, an IR optical receiver, which can be potentially deployed for various optical wireless applications, was cost effectively constructed tapping into the proposed thin-film filter. As illustrated in Fig. 9(a) , the IR receiver is composed of three detecting cells, which are installed in a fixture with the shape of a triangular prism, making angles of 40° with each other. A unit cell includes a single IR filter linked to a Si PIN photodiode (PD), when a long-pass filter (Model FGL-850, Thorlabs), with a cutoff wavelength of 850 nm, is placed atop the filter so as to prohibit unwanted visible transmission. The receiver has dimensions of 24 (W) x 21.6 (L) x 14 (H) mm3, with each detecting cell occupying a footprint of 5 (W) x 5 (L) mm2. By use of a transmitter supplying a TE-polarized collimated beam at λ = 905 nm, the optical output power available from the detecting cell was scrutinized as a function of the angle. As indicated in Fig. 9(b), the captured optical power declines progressively with the angle, thus leading to a 3-dB angular acceptance of 45°. The total output of the IR receiver, available from the three combined detecting cells, is similarly recorded, hinting at the fact that an angular 3-dB bandwidth equivalent to 80° could be achieved, translating into a full-angle field-of-view as large as 160°.
In summary, an IR spectral filter was demonstrated drawing upon three-stage concatenated dielectric resonators, based on high-index a-Si cavities of uneven thicknesses. The main pass-band of interest was determined to be centered at the resonant wavelength, which is shared with all of the constituent etalons. The device provided an extended angular acceptance in conjunction with enhanced transmission and slight polarization sensitivity, rendering a well-defined roll-off characteristic allowing for no significant secondary peaks. To verify its practical feasibility, a miniature IR receiver was embodied by arranging three detecting cells in a fixture, comprising the proposed spectral filter linked with a PD chip. We succeeded in acquiring an enlarged field-of-view beyond 160° in full angle.
This work was supported by a National Research Foundation of Korea grant funded by the Korean government (MEST) (No. 2012-0009226 and 2012-0004922), a research grant from Kwangwoon University in 2013, and the Australian Research Council Future Fellowship (FT110100853, Dr. Duk-Yong Choi).
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