Showing papers on "Optical microcavity published in 1981"
3 citations
TL;DR: In this article, the authors reported a transition loss as low as 0.15 dB and the average loss per bend is (0.3! C 0.5 ) dB, for a transition whose separation length as well as height is 40 mm.
Abstract: Offset waveguides connected by a transition form an integral part of optical circuits such as interferometric modulators and analog-to-digital converters where directional changes in channel waveguides often occur. Also offset waveguides are needed to connect high density integrated optical circuits to a group of optical fibers. The ends of these optical waveguides, displaced both horizontally and vertically, are usually connected by a transition. These transitions could be a straight section’ or made up of a number of smaller sections.2 On the other hand, the transition can be a smooth curve made up of two curved bends having a constant radius of c~rva ture .~ The reported loss of 1.5 dB, for a transition whose separation length as well as height is 40 mm, is still rather high. In this paper, we report transition losses as low as 0.3 ! C 0.3 dB. The average loss per bend is (0.15 dB and is the lowest so far reported. The shape of the transition is a smooth S-shaped curve,4 represented by a raised cosine function resulting in lower tilt radiation loss. However, the masks were produced by the electron beam exposure system (EBES) where the pattern was written by moving the e-beam in discrete amounts in both the x and y direction but not at arbitrary angles to these directions. The resolution was limited to a quarter micron. Thus instead of a smooth raised cosine curve connecting the offset waveguides, we obtain a staircase structure as illustrated in Fig. 1. The staircase step is 0.25 pm, and the offset height His 100 pm, which represents very nearly the value of the 0.d. of a single-mode fiber. This then will be the minimum separation required between offset waveguides when connecting a closely packed set of fibers to an integrated optical circuit. Several sets of curves were written in the mask, and each set consists of three S-shaped raised cosine transitions, connecting straight waveguides on either side. Each curved section was widely separated (400 pm) to avoid coupling of radiated power from the bends in the case of sharp transitions. The offset length L ranged from 0.1 to 8 mm. For reference purposes, straight sections were written in the mask after every set of waveguides with transitions. Using standard photolithographic techniques, the waveguide patterns were replicated onto LiNb03 substrates. All our experiments were carried out with Z-cut Y-propagating Ti-diffused strip waveguides of 3and 5-pm widths. We report the results of the 3-pm waveguides, fabricated by depositing 300-A thick Ti strips and diffusing them at 98OoC for 4 h. The transition losses were arrived at by subtracting the throughput of the offset waveguides from that of the straight waveguides. The absorption losses in the additional linear length of the curved section can always be neglected for all values of L . Radiation loss through the transition for the TE mode as a function of L was measured. Our results indicate that the loss is primarily due to radiation losses at the bends, since the Ti diffusion seems to eliminate the sharp edges and the loss contribution associated with the quasi-periodic staircase structure. Thus we find a minimum useful transition length rather than an optimum length as previously expected. To achieve an average radiation loss of 0.3 dB through the transition, the separation length must be at least 3 mm in our case. (13 min)