Transverse Interference Technique

There are many methods to measure the RIP of the preform and the fiber. However, the RIP might be changed by the heat-drawing process, but some fabrication methods, such as coextrusion, can produce optical fibers directly from raw materials, not via preforms (see Chapter 5). In addition, we are interested in the final RIP of the fiber and not in the RIP of the preform during fabrication. Thus, this section focuses on measurements of the RIP of the fiber. 

In silica MMFs, a steady-state mode power distribution had been established in many previous studies because silica MMFs were expected to be used in long-haul networks. A steady-state mode power distribution was actually established in MMFs during real use [6]. However, MMFs, especially POFs, 

In the UFL condition, a silica-based MMF is selected as the probe fiber to excite restricted small mode groups. The core size and NA ofthe silica-based MMF used were 50 pm and 0.12, respectively. Because GI POFs typically have core diameters of 120-500 pm and NAs of less than 0.3, only small groups of lower order modes 

In the OFL condition, a very short SI POF is used as the probe fiber to uniformly excite all the modes. The core diameter and NA of the SI POF used were 980 pm and 0.5, respectively, which are much larger than those of typical GI POFs. 

In addition, the mode power distribution from even a short SI POF is uniform in the entire core region. Therefore, all the modes of the Gl POF are uniformly excited [8]. 

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A transverse interferometric technique using interference microscopy is adopted to measure the RIP of the POP because of its high accuracy, high resolution, and ease of sample preparation. Figure 6.1 shows the principle of the technique, which assumes that the fiber has a rotationally symmetric structure around its axis. Here, only the principle of the technique is discussed, as the detailed mathematical explanations are given elsewhere [4, 5]. 

Figure 6.2 Interference fringes observed by transverse interferometric technique, (a) Total splitting, (b) Partial splitting.

When metals have Raman active phonons, optical pump-probe techniques can be applied to study their coherent dynamics. Hase and coworkers observed a periodic oscillation in the reflectivity of Zn and Cd due to the coherent E2g phonons (Fig. 2.17) [56]. The amplitude of the coherent phonons of Zn decreased with raising temperature, in accordance with the photo-induced quasi-particle density n.p, which is proportional to the difference in the electronic temperature before and after the photoexcitation (Fig. 2.17). The result indicated the resonant nature of the ISRS generation of coherent phonons. Under intense (mJ/cm2) photoexcitation, the coherent Eg phonons of Zn exhibited a transient frequency shift similar to that of Bi (Fig. 2.9), which can be understood as the Fano interference [57], A transient frequency shift was aslo observed for the coherent transverse optical (TO) phonon in polycrystalline Zr film, in spite of much weaker photoexcitation [58],... 

Oblique ultrasonic waves sent to a composite at frequencies that excite plate wave modes induce the leaky lamb wave phenomenon. When the leaky Lamb wave is generated, the specular reflection is distorted. When the specular reflection and the leaky Lamb wave interfere, a phase cancellation occurs, and two components are generated with a phase between them. Because each type of defect has a unique response, this technique can be used to determine material eleastic constants and to estimate the volume content of resin as well as porosity content. Detection of transverse cracking and delamination in a 24-layer unidirectional graphite-epoxy laminate has also been reported [140], and oblique incidenee back-scattering techniques give accurate fiber orientation of the first composite layers [15],... 

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