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In this paper, we present the improvement of optical properties of lx8 arrayed waveguide grating (A WG) de multiplexer by adding Transmission Star Couplers (TSC) into Free Propagation Region 1 (FPR1) which has refractive index 1.455. It is found that the appropriate value of refractive index of TSC area is 1.460. Using this method, the insertion loss of A WG decreased from 1.79 dB to 1.58 dB while the crosstalk increased from -23.94 dB to -22.49 dB. To maintain the problem of the increased crosstalk, the traditional output guide structure is replaced by a taper structure which has start width 11 /lm and end width 6 /lm. Then, the insertion loss is reduced from 1.58 dB to 1.09 dB, and the crosstalk is reduced from -22.49 dB to -24.94 dB, respectively. Therefore, by using all design concepts, the appropriate structure of proposed A WG can be achieved. The size of the proposed A WG is the half of that of the traditional A WG, and the insertion loss of the proposed A WG is lower than that of the traditional A WG.

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The chirped grating is useful in distributed feedback (DFB) lasers as it removes threshold degeneracy, enhances wavelength tuning range', and reduces spatial hole burning2. Moreover, it lowers input power necessary for optical bistability in DFB laser amplifiers and enlarges operation wavelength range as welP4. This optical bistability provides means of all optical flip-flop and wavelength conversion5, and therefore very important for optical networking.For the purpose of the optical bistability the linearly chirped grating as illustrated in Fig. 1 is advantageous. However, making such gratings is difficult if the holographic exposure is used. Even with the electron beam exposure, patterning linear c h i r p i n g is not straightforward as the grating pitch has to be changed typically by several ppm every grating line. One method of controlling the grating pitch beyond the maximum positioning resolution of electron beam writers is to incorporate ability to change the field size. Gratings with slightly different pitches for WDM laser arrays have been successfully fabricated by this method6. However, linearly chirped gratings require even finer controllability. In this paper, we have therefore introduced the weighted exposure technique7 into the field size modulation, thus acquiring pitch controllability sufficient to write linear clurped gratings. Figure 2 depicts the idea; the field size A+1 is one digit larger size than A. The first line on the lefthand-side is written using the field size A. Then the next line on the right is formed by the combination of the fields A and A+1 with larger dose for A. The one after the next is also the combination but larger dose for A+1. The fourth one is delineated by using A+l only. In this t CRESTEC Corporation tt University of Rochester ttt also with CREST, Japan Science and Technology Corporation way, one may effectively achieve higher pitch resolution than what the filed size modulation can actually give.We then fabricated linearly chirped gratings on InP substrates utilizing an electron beam writer with the field size modulation capability (CRESTEC CABL2000). Scanning electron micrographs in Fig. 3 demonstrate one example of the gratings thus fabricated along with a uniform pitch (235 nm) grating as a reference. Half pitch and full pitch diffeences appear at Lines 308 and 437, respectively. This quadratic phase shift is the evidence of linear chirp and agrees well with our design. Such continuous linear change of grating pitch is only achievable by the combination of the field size modulation and the weighted dose exposure.Next we fabricated a gain-coupled DFB laser having the linearly-chirped absorptive grating. As can be seen in Fig. 4, the chirped grating DFB laser showed a below-threshold spectrum peculiar to the linear chirping (compare with that of the uniform grating laser). Finally, we conducted an optical transmission experiment across the DFB lasers biased below threshold (DFB SOA). As was predicted, the optical bistable switching occurred at lower input optical ...