Photonic integrated circuits (PICs) produced by large-scale integration (LSI) on Si platforms have been intensively researched. Since thermal diffusion from the LSI logic layer is a serious obstacle to realizing a Si-based optical integrated circuit, we have proposed and realized athermal wavelength filters using Si slot waveguides embedded with benzocyclobutene (BCB). First, the athermal conditions were theoretically investigated by controlling the waveguide and gap width of the slot waveguides. In order to introduce the calculated waveguide structures to wavelength filters, the propagation losses and bending losses of the Si slot waveguides were evaluated. The propagation losses were measured to be 5.6 and 5.3 dB/cm for slot waveguide widths of 500 and 700 nm, respectively. Finally, athermal wavelength filters, a ring resonator, and a Mach-Zhender interferometer (MZI) with a slot waveguide width of 700 nm were designed and fabricated. Further, a temperature coefficient of -0.9 pm/K for the operating wavelength was achieved with the athermal MZI.

A negative thermal expansion ceramic substrate and an athermal fiber Bragg grating component with the substrate were subjected to reliability tests. We confirmed that the component has adequate durability for use as optical filters in the WDM system, under test conditions of damp heat, low temperature, mechanical shock and vibration. (50 words)

Several packaging methods for athermalization of Fiber Bragg Grating (FBG), which is equipped with negative expansion substrates, have been proposed. However, those methods have some deficiency resulted from the substrates such as complex structure or poor thermal expansion characteristics. In order to provide a suitable substrate for the athermalization of FBG, the authors have developed a Negative Expansion Ceramic Substrate (NECS) which has simple structure and suitable thermal expansion characteristics. NECS consists of polycrystalline β-quartz solid solution (Li2O-Al2O3-nSiO2, n>2), and has thermal expansion coefficient of about -65 to -85 10-7/C, which is sufficient large enough for total compensation of the Bragg wavelength shift. No difference in the thermal expansion was observed between the specimen as prepared and the one on which an epoxy adhesive was applied. NECS is produced by means of a sintering method, which enables flexible design of the chemical composition. It was found that the hysteresis in thermal expansion of the NECS depends upon the chemical composition and crystalline structure. We decreased thermal expansion hysteresis by controlling the SiO2 ratio in the composition and the crystal grain size. We confirmed that the temperature dependence of the FBG mounted on the NECS with an epoxy adhesive was decreased to -2.3 10-3 nm/C from 10.0 10-3 nm/C, in good agreement with the calculated value of -2.6 10-3 nm/C. The hysteresis in Bragg wavelength shift was less than 0.03 nm, that is sufficiently small enough for practical use. It was confirmed that NECS has suitable thermal expansion characteristics for the athermalization of FBG.

We have reviewed recent progress on arrayed waveguide gratings for DWDM applications. AWGs can be used to realize not only mux/demux filters with various channel spacings, but also highly integrated optical components.

The temperature dependence of central wavelength of optical filters is a serious problem for the dense WDM systems. This dependence is owing to the temperature dependence of optical path-length of the waveguide. In this study, we realized a temperature independent silica-based optical filter at 1. 55 µm wavelength using an athermal waveguide, in which optical pathlength is independent of temperature. First, we designed a silica-based athermal waveguide, and next we designed and fabricated a ring resonator using the athermal waveguide. As a result, we successfully decreased the temperature dependence of central wavelength to less than 4 10 -4 nm/K, which is 3% and 0. 3% of corresponding values of conventional silica-based and semiconductor waveguide filters, respectively.

The temperature dependence of the central wavelength of narrow-band filters is a serious problem for the dense WDM systems. In this study, we realized a temperature independent narrow-band filter at 1.3 µm wavelength. First, we designed an athermal waveguide in which optical path length is independent of temperature by using a finite element method. Using this athermal waveguide, we designed and fabricated a ring resonator. As a result, we successfully decreased the temperature coefficient of central wavelength to 710-4 nm/K, which is 7% of conventional SiO2 waveguide filters and 0.7% of conventional semiconductor waveguide filters.