Spectral and Spatial Filtering Using Waveguide Grating Mirror

Resonant reflection of light by a waveguide grating provides simultaneous spectral and spatial filtering of the reflected beam. Narrowband transmission filter based on a plasmon resonance shows high out-of-band suppression in a wide spectral range. Dielectric waveguide grating mirror improves spatial coherence of a semiconductor laser. OCIS codes: (050.1950) Diffraction gratings; (130.2790) Guided waves; (140.3410) Laser resonators; (240.6680) Surface plasmons; (120.2440) Filters. Introduction Excitation and re-emission of guided modes in a planar waveguide grating structure is known to result in sharp features in wavelength and angular reflectance spectra. The resonant reflection was predicted in 1965 [1] and experimentally demonstrated in 1985 [2, 3]. By physical nature, the resonant reflection from a waveguide grating is similar to the Wood anomalies of light diffraction at metallic gratings associated with excitation and re-emission of surface plasmons. Due to much lower absorption in dielectric structures, the quality factor of the resonant waveguide mode excitation can be very high. To the best of our knowledge the highest reported finesse is 1.5⋅10 [4]. Potential applications include spectral filtering [5-8], optical switches [4], optical sensors [9], polarization control in lasers [10], and spatial filtering of lateral modes in large volume lasers [11]. Special cases of normal incidence [12] and two-dimensional gratings [13] have been considered. In this talk we summarize recent achievements in study of the resonant reflection of light by waveguide gratings and report our latest results on lateral mode control in lasers using the waveguide grating mirror. Narrowband filter using plasmon resonance Theory predicts 100% maximal reflection for a lossless waveguide grating and infinite size incident beam. Out-of-resonance reflection can be to a certain degree suppressed by applying antireflectance coatings. Thus, narrowband filters with low insertion losses and high out-of-band suppression can be fabricated using the resonant reflection. The resonant wavelength λ in such a filter depends on the incident angle θ through the phase matching condition of waveguide excitation: * ) sin( n = ± Λ θ λ , (1) where Λ is the grating period and * n is the modal index. In practice, however, it is hard to provide low out-of-band reflectance in a wide spectral range (e.g., communication systems require about 20-30dB rejection over Sthrough U-bands covering the spectral range from 1440nm to 1675nm). We have found that using of a transmission resonance associated with long-range plasmons supported by a periodically corrugated thin silver film with symmetric claddings provides narrowband filtering suitable for application in optical communication systems employing coarse WDM [14]. OSA TOPS Vol. 75, Diffractive Optics and Micro-Optics Robert Magnusson, ed. ©2002 Optical Society of America 286 In general, a thin film with symmetric claddings supports two plasmon modes: symmetric and asymmetric. The asymmetric mode is weakly localized so it experiences lower losses. Consequently, in the reflection and transmission spectra one can see two resonances (Fig. 1). The film and the grating, however can be engineered in such a way that the two resonances overlap forming a single second order resonance (Fig. 2). In such a filter, the out-off-band rejection is strong due to absorption in a metal film, and the resonant transmission is high due to low optical losses for an asymmetric plasmon mode. In addition, the second order line shape provides better side-band suppression compared to the Lorentzian spectrum typical for a single resonance device. For example, in the spectra shown above the full width at –20dB level is only 4 times greater than the –3dB width. In the case of a Lorentzian shape this ratio is equal to 10 99 ≈ . 145