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The fabrication of narrowband highly reflecting filters in single-mode step-index fibers was reported recently by Hill et al. [Appl. Phys. Lett. 32, 647 (1978)]. The underlying effect on which these filters are based is a photoinduced refractive-index change in the GeO(2) used as a core dopant in SiO(2) fibers. A study is reported aimed at the characterization of such optical fiber filters. A theoretical model is developed, and relevant fiber parameters are determined through intercomparison with experiment. In this way, both the magnitude of the photoinduced index change and its dependence on the writing power coupled into the fiber are determined.

A laser absorption spectrometer is described which uses a tunable diode laser and a 1-m multipass White cell to detect NO(2) in air with a sensitivity of better than 100 ppt. The modulation techniques employed to achieve this sensitivity are described in detail, and the noise mechanisms, which currently limit the detectable absorption coefficients to greater, similar 10(-7) m(-1), are examined.

A laser absorption spectrometer is described which employs a wavelength-tunable Pb(1-x) Sn(x) Se diode in conjunction with a multipass White cell and which is capable of measuring SO(2) concentrations in the low ppb range. We describe in some detail the modulation techniques used in signal detection which enable us to measure absorption coefficients as low as 10(-7) m(-1). In addition, calibration of the instrumentation using small sample cells is described, and the question of interference from unwanted molecular species is discussed. The instrumentation allows the measurement, basically at the same time, of a large number of other atmospheric gases which are of significance in pollution studies. For example, the present diode operates over 1050-1150 cm(-1) and can measure SO(2), O(3), N(2)O, CO(2), H(2)O, NH(3), and PAN. The addition of a second diode to the system will allow most gases of any atmospheric importance to be monitored. In general, these gases have much stronger ir absorption bands than SO(2) and hence can be detected at concentrations much less than 1 ppb.

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