Optical losses

6FDA/TFDB PMDA/TFDB = 6FDA/DMDB PMDA/DMDB 

Optical loss of these polyimides agrees well with the coloration and light absorption in the previous section. 

Relation between Optical Loss and Preparation Conditions 

The optical loss of polyimides depends on their preparation conditions. Reuter et al. have reported the optical loss of commercial fluorinated polyimides in waveguide materials containing two-C(CF3)2- groups, 6FDA/3,3 -6F or 6FDA/4,4 -6F, and estimated the optical loss to be below 0.1 dB/cm at 0.63 pm using optimized conditions. However, the loss increases to around 3 dB/cm with a 

3 dB/cm, but it increases with increasing maximum curing temperatures. 

For both 6FDA/TFDB and PMDA/TFDB, the optical losses upon curing in a nitrogen atmosphere are lower than those in an air atmosphere. The optical loss increase upon curing in an air atmosphere seems to be caused by a slight oxidative degradation that produces radicals. 

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Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

This new PA is polarized in a different direction with respect to the main optical transition, and partially overlaps with SE, causing optical losses. 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

Important parameters, characterizing fibre properties, are the attenuation a and numerical aperture NA. The attenuation (Equation 1) represents optical losses caused by absorption or scattering of the light guided through the unit fibre length. It is given by ... 

Optical fibres were originally studied and developed for the transmission of information in telecommunications and have been used for building of long-haul as well as local networks for years. Such kinds of optical fibres have to be of ultra-low optical losses, standard size, resisting to temperature changes in a large interval or to chemical influence of their surroundings, etc. In contrast to these standard widely used fibres, the special ones are... 

So the function of special optical fibres for sensing is to produce a sensitive response to changes in the fibre surroundings. Such requirements on optical hardware as durability to the analyte, transparency (i.e. minimum optical losses) in a wide spectral range and common availability should be pointed out. Related to the these requirements, the choice of the fibre material as well as of the fibre coating and fibre structure belong to fundamental tasks in the design of fibre-optic sensors. 

Close to silica fibres are silicate fibres drawn from optical glasses. Silicate fibres are typically applicable in the visible spectral region. Their optical losses in the visible region usually reach much higher values than silica fibres - at least 102dB/km. On the other hand, the refractive index can be tailored in a large interval (from 1.5 for the BK-class to 1.95 for the... 

Optical fibres composed of plastics are also transparent in the visible spectral region but optical losses reach 102 - 103 dB/km13. Their refractive index varies from 1.35 to 1.6 depending on the kind of polymer used (e.g. polymethymethacrylate PMMA -1.49). The chemical resistance is much worse than that of silica fibres and thermal stability is incomparable. On the other hand, low temperature processes of plastic fibre preparation allow us mix the starting polymer with organic dyes which enables the production of luminescent fibres suitable e.g. for fluorescence-based sensing13. 

Second-order nonlinear optics (NLO) has several applications in the field of optoelectronics.11 Several of these nonlinear processes are straightforward to experimentally demonstrate but their application in devices has been hampered by the lack of appropriate materials. Necessary requirements for second-order nonlinear optical materials include the absence of centrosymmetry, stability (thermal and mechanical), low optical loss, and large and fast nonlinearities.8... 

Optical losses in the liquid-filled ARROWs are typically measured using the cutback method. This measurement technique assumes optical transmission in the waveguide can be described according to... 

For preparative-type resolutions (chromatography or other separation techniques) it is of vital importance that the derivatives can be cleaved smoothly to avoid chemical and optical losses. In addition, it would be advantageous to recover the CDA (see Section 3.2.1.2.). The Noe lactol (Tabic 1, entry 55) fulfills such criteria almost perfectly during the course of reaction with chiral alcohols to form diastereomeric acetals220. 

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