Optical Properties of Fluorinated Polyimides and Their Applications ...

all PI films, the absolute values of dn TE /dT are largerthan dn TM /dT, indicating remarkable anisotropy (dn TE /dT−dn TM /dT) in the TO coefficients in the range of −9 ~ −39 ppm/K. These anisotropy is not closely relatedto the degrees of refractive index or birefringence.This phenomenon may cause significant polarizationdependence in the performance of TOswitches, but this can also be used for polarizationsensitiveoptical filters or splitters etc.3 Applications of FPIs to Optical Use [34]3.1 Thin FPI-Film Substrates for MultilayerFiltersFlexible substrates for supporting “thin film filters”is a good example of the applications of outstandingoptical transparency of FPI over the visibleand NIR regions with good heat resistance, pliability,and good processability as thin films. For opticalcommunication applications, optical filters have afunction that they are transparent for specific wavelengthsbut reflective for other wavelengths. An opticalfilter consists of a dielectric multilayer, whichperforms transmission/reflection of light, and it isformed on a supporting substrate film. It is stronglyrequired that the thickness of a filter inserted into opticalcircuits, such as an optical fiber or opticalwaveguides, should be less than 20µm in order toreduce the radiation loss accompanying with a grooveformed in the optical circuits. For conventional bulktypefilters, multilayered thin film of TiO 2 and SiO 2is formed on an inorganic glass or silica substrate bysputtering, in which the cost for making thin film andgrinding and polishing the substrate is rather high.On the other hand, since formation of FPI thin filmsis easy and economical by spin-coating [35], and theoptical transparency and heat resistance is excellent,FPI is the best material for substrates of multilayerfilters. Paying attention to the suitable properties ofan FPI 7, thin film filters were fabricated using thisPI as a substrate. Optical filters having a function of1.3 µm-transmission / 1.55 µm-reflection were producedusing inorganic glass and FPI substrates, andalmost the same properties were obtained. At present,thin PI film filters are produced and widely used inthe WDM modules for optical surveillance systemsin fiber-optics communications.3.2 Thin FPI-Film Waveplates“Thin film polyimide waveplate” is a good exampleof application using the large birefringence natureof FPIs having rigid-rod molecular structures withoutstanding optical transparency. Silica-based planarlightwave circuits (PLCs) formed on Si substratesare widely used for optical communications becauseSpinCoatingDryingSolventPeelingPoly(amic acid)SolutionSubstratePoly(amic acid)FilmCuttingThermal Curingof their high performance and mass-productivity.However, the optical components using interferenceof light frequently show apparent polarization dependenceresulting from the birefringence of opticalwaveguides, and this had been a serious problem [36].As a technique for solving this, the polarization modeexchanging method which inserts a half-waveplate atthe center of optical path of PLC was proposed. However,when using a conventional half-waveplate madeby quartz (92 µm-thick), excess insertion loss (radiationloss) is more than 5 dB due to the large thickness.In order to reduce the loss, the thickness of awaveplate should be reduced, and thus, it is necessaryto produce a waveplate using highly birefringentmaterials. Moreover, ease at insertion, outstandingoptical transparency at the operating wavelength, andhigh reliability over high temperature and high humidityare required as same as for optical filters. Andoet al. [37-40] focused on the rigid-rod molecule structureof a FPI 7. Although PI thin films can be easilyformed by spin-coating of polyamic acid solution (precursor)on a substrate followed by heating, the filmsthus produced is optically isotropic in the film plane.Hence, no in-plane birefringence (∆n // =∆n TE1 −∆n TE2 )is generated. Then, spin-coated polyamic acid solutionwas partially dried at a low temperature, andpeeled from the substrate. The film was fixed to ametal frame by two sides of the longer direction. ThePI molecules are spontaneously orientated along thefixed direction during thermal imidization at 350°C,and thus large birefringence (∆n // =0.053) was generatedin the film plane. (Fig. 7). The magnitude ofbirefringence is controllable with high precision bychanging the curing conditions, in particular the highestimidization temperature. A PI waveplate exhibitinga large ∆n (0.18), that is 20 times larger than quartz,is producible by using this technique. As a result, thethickness of a novel PI waveplate of 14.5 µm for an TE2n TE1Metal FrameFig. 7 Method of preparing polyimide waveplate usingthe spontaneous orientation behavior [37-40].224