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Described in detail is a laser induced fluorescence system which has been successfully interfaced with two aircraft sampling platforms (i.e., Sabreliner jet and an L-188C Electra). This system, which has been under development for four years, presently consists of the following major components: (1) a Nd–Yag laser driven oscillator–amplifier dye laser; (2) a sampling manifold with associated fluorescence detection optics; (3) an OH calibration chamber; (4) a laser beam steering assembly; and (5) sampling electronics and data processing hardware. During the last three years, this system has been flown some 50 000 air miles making tropospheric OH radical measurements over the latitude range of 70 °N to 57 °S. OH concentrations measured during these flights have ranged from 30 parts-per-quadrillion (3.7×105 molecules/cm3) at altitudes of 6 km to 0.8 parts-per-trillion (2.0×107 molecules/cm3) at 0.5 km. Computations have been completed which indicate that the existing aircraft system with modest modifications should also be capable of detecting natural tropospheric levels of NO, SO2, CH2O, NO2, HNO2, NO3, H2O2, and CS2 by using both conventional laser-induced fluorescence methodology and multiphoton techniques.

Kerr nonlinearities can cause severe distortions in fiber transmission systems. The penalties introduced by these nonlinearities rise as the transmission capacity is pushed to its limits. Ken nonlinearities can in principle be cancelled using a device with negative n2 or by mid-span optical phase conjugation (OPC) at alternate repeater sites 1, 2. The latter has been used only in limited instances because of a fundamental issue: OPC can compensate for Kerr nonlinearities only in the absence of dispersion slope and when the OPC device is followed by a fiber with negative absorption 3. The last requirement is obviously unphysical, and therefore the only choice was to reduce absorption by using short fiber spans. A more severe limitation is met when long dispersive fiber spans are used with relatively higher bit rates and when one attempts to cancel cross phase modulation (XPM). Under these conditions, dispersion effects combined with a lack of symmetric power distribution with respect to the OPC device makes the distortion introduced by Kerr-nonlinearities irreversible. that compensates both for Kerr nonlinearities and fiber dispersion. The technique is based on a compact LiNb03 phase conjugator combined with Raman amplification. The demonstration is z Z performed up to at a data rate of 100 Gb/s, which In this paper we demonstrate a novel scheme P is the highest reported data rate for any transmis-~ LINb03 OPC sion experiment with OPC. We show cancella-F, tion of all Kerr-nonlinearities, four-wave mixing, self-phase modulation (SPM) and cross-phase D,a,n, (XPM), in long fiber 'pans Fig. 1: Our Ke,-cancellation scheme: Raman counter-pumping in the second fiber span provides aOur technique uses counterpropagating Raman decreasing absorption needed for canceling any nonpumping in all fiber spans or at alternate fiber linear phase using phase conjugation. spans in conjunction with OPC. This creates a good approximation to a symmetric power distribution profile that allows taking full advantage of the time reversal property of OPC. Fig. 1 shows this arrangement; this unit can then be repeated many times in order to reach the total desired transmission length. The nonlinear phase accumulated over the absorption length (shaded area) in the first fiber span will be compensated by the nonlinear phase accumulated over a comparable length towards the end of the second span (where the bits in the WDM channels achieve their original relative timing too).We first test the proposed scheme by canceling FWM in Tmewave fiber. The experimental setup is the same as shown in Fig. 1. We use two 75 km long fiber spans and with zero dispersion wavelengths (Lo) chosen such that the channels and their phase conjugated counterparts always overlap the region of zero .

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