Thermo-optic effect in a material is defined as the change in the refractive index as a function of temperature change (dn/dT)[1]. The thermo-optic characteristics of dielectric optical materials such as silica and optical polymer are an important consideration in their application to waveguide devices.
The temperature dependence of lightwave devices is a serious and outstanding problem in the dense wavelength division multiplexing (DWDM) system[2]. This feature is mostly due to the temperature dependence of the optical path length. To eliminate this undesirable feature, the athermal waveguide (hybrid waveguide structure consisting of silica core and polymer overcladding) using the dn/dT characteristics of materials have been investigated[3]. In this case, the exact control of dn/dT in optical materials is important in application of athermal waveguide because the required value of dn/dT depends on the design of optical devices. Also, a waveguide material with a high dn/dT allows easy modulation of factors such as the optical path, the phase and the intensity. Thus, it can be used in fabrication of active optical devices[4, 5]. Therefore, the exact controllability of the value in dn/dT over a wide range is attractive when attempting to design and produce an effective thermo-optic waveguide device.
Recently, a sol-gel derived inorganic-organic hybrid material composed of silica and an organic material is receiving attention as a promising candidate for a waveguide material that might be used to replace silica and polymer[6]. It will be interested to measure dn/dT of hybrid materials because the silica have positive dn/dT while organic materials have negative values. However, there have been no reports on the dn/dT measurement of the hybrid material systems. In this study, I proposed new easy dn/dT measurement method of films using a prism coupler and measured dn/dT of inorganic-organic hybrid materials films. And the manipulation of structure in hybrid materials is proposed as a method for controlling dn/dT. In order to apply hybrid materials to thermo-optic devices, the athermal AWG (Arrayed Waveguide Grating) device were fabricated and characterized. In addition, the thermal stability and optical propagation loss of hybrid materials for athermal AWG were measured.
The dn/dT of inorganic-organic hybrid materials films which are composed of silica and organic are negative and the range of value can be controlled between the order of -10^{-4}/ ℃ and -10^{-5}/ ℃. In order to investigate dn/dT variation depending on hybrid materials structure, their dn/dT were measured depending on organic/inorganic ratio, modified organic species, degree of organic cross-linking, degree of inorganic condensation and formation of heterometallic oxide. The dn/dT of hybrid materials negatively increases with growing their organic contents and increasing organic size. And their dn/dT decrease with growing their inorganic contents, formation of heterometallic oxide, degree of organic cross-linking and inorganic condensation. Therefore, dn/dT of inorganic-organic hybrid materials can be easily and widely controlled by changing the hybrid structure.
According to Prod' homme theory that explains the thermo-optic effect, it can be thought that variation in dn/dT of inorganic-organic hybrid materials is mainly affected by the variation in thermal expansion.
In order to apply inorganic-organic hybrid materials to athermal AWG, the hybrid material that satisfies athermal condition $(n=1.4440, dn/dT= -1＼sim-1.3 ＼times 10^{-4} at 1550nm)$ was investigated. Thus, the hybrid material system containing fluorine and zirconium oxide can exactly satisfy the athermal condition.
The optical propagation loss of hybrid materials for athermal AWG was measured. As the zirconium oxide content increases, the optical propagation loss at 1310nm, 1550nm grows. However, the values of optical loss decrease when the fluorine concentration increases in hybrid materials.
In the thermal stability test, there is no abrupt weight loss of inorganic-organic hybrid materials up to ~400 ℃ and the 5% weight loss occurred between 370 and 390 ℃. Thus, the hybrid materials are thermally stable as compared to general optical polymer.
Finally, the athermal AWG with overcladding layer of inorganic-organic hybrid material was characterized. The temperature dependence is suppressed in comparison with all silica AWG and represents good athermal behavior.
