818 zyxwvutsrqpo IEEE PHOTONICS TECHNOLOGY LETTERS. VOL. 2. NO. I I. NOVEMBER 1990 Polarization Mode Filter in GaAs zy - AlAs Superlattice Fabricated by Si0 Cap Disordering YASUHIRO SUZUKI, HIDETOSHI IWAMURA, AND OSAMU MIKAMI Abstract-Novel TE and TM mode filters in GaAs-AIAs superlattice are proposed and fabricated by zyxwvutsrqponm SO, cap disordering. The structure consists of mode selective channel waveguides with bending regions. Functions of mode filters are confirmed by observing near-field patterns, yielding an extinction ratio of 14 dB at 1.15 zyxwvutsrq pm wavelength. zyxwvutsrq ::'ORDER ,' Pin I. INTRODUCTION E/TM mode filters are key devices in integrated optical T circuits. zyxwvutsrqpon So far, several mode filters have been reported in various materials including glass and LiNbO, [ 11 -[3]. Mode selective channel waveguides have also been fabricated in semiconductors using a refractive index change induced by superlattice disordering. However, these semiconductor de- vices can pass only TE mode [4], [5]. Recently, both TE and TM mode selective channel wave- guides have been proposed and demonstrated in GaAs- AlAs superlattices using SiO, cap disordering [6]. This letter reports novel TE/TM mode filters consisting of bending mode selective channel waveguides. The filter was fabricated in GaAs- AlAs superlattice using SiO, cap disord- ering and its mode selectivity was confirmed. 11. DEVICE STRUCTURE AND FABRICATION The structure of TE/TM mode filters is shown in Fig. 1. These mode filters consist of bending mode selective channel waveguides fabricated by disordering superlattices [6]. The operation principle of a mode filter is based on the relation- ship among the refractive indexes of superlattices in the TE and TM modes (n::, n:r) and that of disordered superlat- tices (n,) [4], For a TM mode filter, the channel core is formed with a disordered SL, while the cladding region is formed with a nondisordered SL. The refractive index of the core is higher than that of the cladding region only for the TM mode. Therefore, this channel structure functions as a TM mode selective waveguide. This channel waveguide has a bending region, so that the TM mode light can be guided along the channel to the output, while the TE mode light, or the unwanted polarization light, is diffused into the substrate at Manuscript received July 27, 1990. The authors are with NTT Opto-Electronics Laboratories, Kanagawa IEEE Log Number 9040228. 243-01, Japan. (a) (b) Fig. 1. Schematic configurations of fabricated mode filters (a) TM mode filter. (b) TE mode filter. the bend. For a TE mode filter, the structure is just the reverse. Several processes have been used to achieve disordering of superlattices such as impurity induced disordering, ion im- plantation, and SiO, cap annealing. In this work, SiO, cap annealing was used because impurities were not introduced and free carrier absorption was expected to be small [7], [8]. Disordering was locally induced with SL under the SiO, cap, so channel waveguides are easily obtained by patterning the SiO, cap before annealing. On a (100) semi-insulating GaAs substrate, an Al,Ga, _,As zy (x = 0.43) cladding layer, and,a superlattice (SL) structure con$sting of 62 periods of 94-A-thick GaAs well layers and 56-A-thick AlAs barrier layers were ogrown by molecular beam epitaxy (MBE). Next, a 2000-A-thick layer of SiO, was deposited on an SL using plasma chemical vapor deposi- tion (P-CVD). Then, the SiO, was patterned by conventional photolithography and reactive ion etching followed by anneal- ing at 950°C for 30 s in an H, atmosphere at a heating rate of 30"C/s. This annealing partially disorders the SL struc- ture under the SiO, . The SiO, cap width in a TM mode filter is 3 pm and the SiO, capless width in a TE mode filter is 15 pm. A channel waveguide formed by disordering has a bend where the bending angle is 1" and a total length is about 5 mm . 111. CHARACTERISTICS OF MODE FILTERS First, the propagation loss was measured by a cut-back method for channel waveguides fabricated by disordering. The light wavelength for the measurement was 1.15 pm. The He-Ne laser beam was endfire-coupled into mode filters through a cleaved edge with a 20 x objective lens. The measured loss was about 5 dB/cm in both filters. This value is the same as that in slab waveguides fabricated by the same disordering method [6] and less than one-tenth of that of a 104 1 - 1 135/90/ 1 100-08 18$0 1 .OO zyxwvuts 0 1990 IEEE