Optical fiber modulators

The iavention of the laser ia 1958 prompted the beginning of the story of optical fiber communications. This device was capable of produciag a high iatensity, coherent beam of light which could be modulated at a high rate (see Lasers). StiU, no transmission medium of suitable clarity was available. 

Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

In context with methane detection during offshore oil drilling, another infrared fiber optic methane sensor was reported25. The detector comprises 3 main units a microcomputer-based signal processing and control unit, a nonconducting fiber optic gas sensor, and an optical fiber cable module. The system operates at an absorption line of methane where silica fibers have very low losses. 

P. Yuan and D. R.Walt, Calculation for fluorescence modulation by absorbing species and its applications to measurements using optical fibers, Anal. Chem. 59, 2391-2394 (1987). 

A typical optical sensor consists of modules found in conventional spectroscopy, for example, source, monochromator, cuvet, detector, and so on. The big difference is that light travels between these modules through optical fibers or waveguides. As it travels, the beam of light interacts with interfaces between media of different optical densities and therefore of different refractive indices. The laws that govern its behavior at these boundaries are the most important aspects of the operation of optical sensors (Okamoto, 2000). 

Related to the plasmon resonance physics is the micromirror optical sensor for hydrogen (Butler, 1991). Like gold and silver, palladium is a free-electron gas metal in which charge groupings such as phonons or plasmons are likely to occur. As we have seen already, palladium has a natural selectivity due to its sorption of monoatomic hydrogen. In that sensor, the reflectivity of the thin Pd film mirror mounted at the end of cladded optical fiber (Fig. 9.19) is modulated by absorption of hydrogen. 

The optical source is a diode laser and fiber-emitter and fiber-detector coupling is accomplished using standard optical fiber connectors. The detector is a PIN photodiode connected to a transimpedance preamplifier and the signal is amplified and filtered using a lock-in amplifier that also tunes the modulation frequency of the laser source. The analytical signal is collected and treated by a PC. 

Because NL(t) has the same periodicity as A(t) the comb structure of the spectrum, as derived in section 3, is not affected. In an optical fiber self-phase modulation can be quite efficient even though the nonlinear coefficient in fused silica is comparatively small. This is because the fiber core carries a high intensity over an extended length. 

This simplified picture of self-phase modulation neglects dispersion, time-delayed nonlinearities and shock formation which is all known to occur in optical fibers. While no in fused silica is at least as fast as a few fs, the GVD broadens the pulses as they travel along the fiber so that the available peak power Pa is decreased. Effective self-phase modulation however takes place when the so called dispersion length is much smaller then the nonlinear length whose ratio is given by [27]... 

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