We successfully demonstrated a low-loss, flat-passband, and athermal arrayed-waveguide grating (AWG) multi/demultiplexer with a Mach-Zehnder interferometer (MZI) as an input router. Resin-filled trenches were formed in the longer arm of the MZI as well as the slab in the AWG to compensate for the temperature dependence. A 32-channel athermal multi/demultiplexer was fabricated using silica-based planar lightwave circuit (PLC) technology. A small temperature-dependent wavelength shift of 0.02 nm was obtained over the temperature range of-5 to 65oC with low-loss (3.3-3.7 dB) and flat-passband spectra.
©2007 Optical Society of America
The exploding demand for communication service has led to the push for greater optical transmission capacity. In order to meet this demand, dense wavelength division multiplexing (DWDM) systems have been extended from long-haul transmission systems to metropolitan and access area networks. In such systems, many nodes are incorporated in ring or mesh configuration. Accordingly, multi/demultiplexers in each node must have a flat and wide spectral response to allow the concatenation of many multi/demultiplexers. Various techniques have been proposed to flatten the passband of arrayed-waveguide gratings (AWGs) [1–14], which have been widely used as multi/demultiplexers in DWDM systems. To flatten the passband of AWGs with low intrinsic loss, a technique using a combination of two synchronized routers [4,7–14] is a promising approach. Low-loss and flat-passband characteristics have been achieved with a Mach-Zehnder interferometer (MZI) for the input of an AWG  in a compact chip. Temperature independent characteristics are also desirable in order to reduce the running costs of the systems. Due mainly to refractive index changes in silica materials, central wavelengths of conventional silica-based AWGs are temperaturedependent. Hence, athermalization techniques have been proposed in which resin-filled trenches are formed in a waveguide array [15,16] or a slab [17,18].
We have reported an athermal AWG whose passband was flattened by using Y-branch waveguides at the input-to-slab interface . However, this passband-flattening technique suffers from intrinsic loss due to the image mismatch from input mode to output mode. For this research, we demonstrated an athermal AWG multi/demultiplexer using an MZI as an input router to achieve low-loss, flat-passband, and temperature insensitive response.
2.1 Optical circuit structure
The optical circuit of the flat-passband athermal multi/demultiplexer is shown in Fig. 1. It consists of a two-input AWG and an MZI, which is connected to the first slab of the AWG, as an input router synchronized with the AWG . To obtain a flat and low-loss demultiplexing function, the free spectral range of the MZI was set to the same value as the channel spacing of the AWG.
We found that, in a multiple-input AWG, insertion loss can be reduced if the input waveguides just before the first slab have narrower core and gap widths  because the coupling efficiency between the fundamental mode of the output waveguide and the superposition of the fundamental modes of two input waveguides at each interface to the slab increases for narrower core and gap widths. Thus, to reduce the insertion loss, we introduced a coupler that consists of two tapers and narrow waveguides at the edge of the first slab. In each taper, the core width gradually decreases from 5.2 µm (quasi-single mode condition  for 1.0%-Δ waveguides) to 1.8 µm as the core approaches the slab. To keep sufficient gap width between the two tapers, which results from limitations in the fabrication process such as etching resolution, each taper has an asymmetrical shape with respect to the central axis of the straight waveguides.
2.2 Athermalization of AWG part
To athermalize the AWG part, wedge-shaped trenches were formed in the first slab and filled with silicone resin to compensate for the temperature dependence of the optical path-length difference between adjacent waveguides in the waveguide array [17,18]. The temperature dependence of the refractive index of the resin, dnr/dT, has a negative value, where nr is the refractive index of the resin and T is the temperature. To reduce diffraction loss, trenches were divided and arranged using the appropriate interval [16,17]. Lateral diffraction loss for this structure can be eliminated because the light is freely propagated in the lateral direction in the slab region.
The sum of the opening angles of the trenches, θ, should be optimized so that the temperature dependence of ΔS=Si-Si-1 compensates for the temperature dependence of the optical path-length difference in the array, where Si is the optical path length in the first slab from the input side to the edge of the i-th waveguide of the array . The ΔS is given by
where ns is the effective refractive index of the slab, D is the interval of the waveguide array at the edge of the slab, Ls is the focal length of the slab and Lt is the distance between the input side of the first slab and the center of the trenches. The temperature-independent performance is achieved when the sum of the optical path-length difference in the first slab, ΔS, and the optical path-length difference between adjacent waveguides in the array, naΔLAWG, becomes temperature-independent:
where na is the effective refractive index of the waveguide and ΔLAWG is the geometrical length difference between adjacent waveguides in the array. By substituting Eq. (1) into Eq. (2), the optimal opening angle θ is derived as
where α is the thermal expansion coefficient of the substrate.
2.3 Athermalization of MZI part
In a cascaded interferometer structure like the one proposed, generally, the temperature dependence of optical path-length difference should be compensated among all paths in the interferometers. In this circuit, an input MZI has two arms with different path lengths. Hence, the temperature dependence of optical path-length difference between two arms in the MZI should be also compensated. Thus, resin-filled trenches were also inserted in the longer arm of the MZI as well as the slab in the AWG. Like Eq. (2), the total optical path length of the trenches, Str, in athermal conditions is given by
where ΔLMZI is the geometrical length difference between the longer and shorter arms.
To reduce the lateral diffraction loss at the trenches inserted in the MZI arm, lateral spot size around the trenches were increased by widening the core around the trenches from 5.2 µm to 10 µm. To reduce reflected light at the trenches due to the difference in refractive index between the silica waveguide (~1.46) and the silicone resin (~1.40), the walls of the trenches were tilted in the lateral direction of the waveguides.
3. Chip fabrication
We fabricated a 32-channel athermal chip with an MZI as an input router by using silicabased planar lightwave circuit (PLC) technology. The design parameters are listed in Table 1, and the layout of a 32-channel multi/demultiplexer with 100-GHz channel spacing is shown in Fig. 2. The channel spacing of the AWG was 100 GHz. The relative index difference between core and cladding, Δ, was 1.0%, slightly higher than that of conventional Δ of 0.8%. Consequently, the chip size was 76 x 27 mm, which allowed us to arrange two chips on a 4-inch wafer.
The chip fabrication process was as follows. First, Ge-doped SiO2 core was formed on a silica substrate and then etched by using photolithography and reactive ion etching (RIE) process. The splitting ratio of the coupler with the tapering-down structure was more sensitive to the variation in core width than that of conventional couplers. Hence, the deviation in core width at the narrow waveguides of the coupler was carefully controlled to less than+/-0.1µm. The process was followed by over-cladding formation by chemical vapor deposition (CVD). Then vertical trenches (18 trenches in the first slab and 6 trenches in the MZI arm) were formed by photolithography and RIE process again. After dicing, the trenches were filled with silicone resin, which was hardened by applying heat. The phase of the MZI was then adjusted by controlling the refractive index of a part of the MZI arm by ultra-violet exposure.
4. Results and discussion
The spectral responses for the 16th output port of the fabricated chips, with and without resinfilled trenches, are plotted in Fig. 3. These chips were fabricated using the same photomask. There was no significant difference in spectra between chips with and without trenches. The minimum insertion loss of the chip with trenches was 3.3 dB, and the one without trenches was 2.8 dB. Thus, the excess loss due to the trenches (including AWG and MZI parts) is estimated to around 0.5 dB. The temperature dependence of the spectral response for the chip with trenches is also plotted in Fig. 3(a). The temperature-dependent wavelength shift was 0.02 nm over the temperature range of-5 to 65oC. This value is comparable to conventional athermal AWGs . Note that the spectra were little changed as the temperature change. It indicates that both the AWG and MZI parts were successfully athermalized.
The chromatic dispersion for the chip with trenches is plotted in Fig. 4. The chromatic dispersion was -1.8 to -6.1 ps/nm within 1-dB bandwidth. Although the device was theoretically linear phase, the fabricated one had small but non-zero chromatic dispersion. This was because the power splitting ratio of the couplers in the MZI was not strictly 50:50.
The spectral responses for all 32 output ports of the chip with trenches are plotted in Fig. 5. The 1-dB bandwidth was 56–57% of the channel spacing for all 32 channels. It indicates that sufficiently flat passband was obtained by using a combination of an AWG and MZI. The minimum insertion loss was 3.3 dB near the central port and 3.7 dB for the marginal ports. These values are much lower than those for a flat-passband athermal AWG using Y-branches we previously reported . The crosstalk was -32 dB, comparable to the results of a conventional 0.8%-Δ AWG . The polarization-dependent wavelength shift was 0.02–0.03 nm. The polarization dependence will be improved by carefully adjusting the birefringence of the waveguides in the waveguide array and MZI arms by controlling the core widths [21,22].
We demonstrated a low-loss, flat-passband, and athermal AWG multi/demultiplexer using an MZI as an input router. Resin-filled trenches were formed in the longer arm of the MZI as well as the first slab in the AWG to compensate for the temperature dependence of optical path-length difference among all paths in the interferometers. We fabricated a 32-channel athermal chip using silica-based PLC technology. The small temperature-dependent wavelength shift (0.02 nm) over the temperature range of -5 to 65°C was achieved with lowloss (3.3–3.7 dB) and flat-passband spectra. These results indicate that the proposed multi/demultiplexer will be useful for WDM system applications.
References and links
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