B. K. Garside

Characterization of single-mode optical fiber filters

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.

Sensitivity limits of a tunable diode laser spectrometer, with application to the detection of NO 2 at the 100-ppt level

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.

High sensitivity pollution detection employing tunable diode lasers

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.

High sensitivity point monitoring of atmospheric gases employing tunable diode lasers

A laser absorption spectrometer is described which employs a wavelength tunable diode laser in conjunction with a multipass White cell. The instrument is sensitive to absorption coefficients as low as 3 x 10(-8) m(-1) and is designed for use as a portable pollution monitor. The performance of the instrument is illustrated by describing the measurement of three gases commonly found in the atmosphere: SO(2) and N(2)O with sensitivities of 1 ppb and NH(3) with a sensitivity of better than 0.1 ppb. The linearity of response and calibration of the instrument are also discussed in some detail.

Low-loss GeO(2) optical waveguide fabrication using low deposition rate rf sputtering.

Low-loss GeO(2) thin-film optical waveguides have been prepared using rf reactive sputtering with a GeO(2) target, and the propagation properties of the waveguides prepared over a wide range of fabrication conditions have been investigated. We have found that the waveguide attenuation dramatically decreased when a very low deposition rate of rf reactive sputtering in an argon-oxygen atmosphere was used in conjunction with appropriate annealing. In particular, 3800-A thick GeO(2) thin-film optical waveguides have been prepared with propagation losses <0.7 dB/cm for the TE(0) mode at a wavelength of 0.63 microm. The average refractive index of the GeO(2) films was measured to be 1.6059 at lambda = 5461 A by an ellipsometer technique in good agreement with measurement on bulk materials. Propagation losses have also been measured at different wave-lengths, which shows that GeO(2) thin-film optical waveguides could be used over a very wide wavelength range from the visible to the near infrared.

Resonance absorption measurements of NO with a line-tunable CO laser: spectroscopic data for pollution monitoring

Experimental coincidences between CO laser lines and seventeen NO lines have been observed. Three important parameters, the separation between each laser line and the nearest NO line (Deltanu), the pressure broadening parameter (gammaL degrees ), and the line strength ((Supsilon,J)), have been determined for each laser wavelength using the best match between experimental absorption data and theoretical calculations. These parameters are compared, where possible, with previously published results. The measured data are important for pollution monitoring systems employing tunable lasers in a direct absorption approach.

Line‐tunable oscillation of a cw NH3 laser from 10.7 to 13.3 μm

A cw CO2 laser operating on the R(30) 9‐μm transition is down shifted by 180 MHz in a pair of acousto‐optic modulators, and used to optically pump sR(5,0) transition of NH3 at line center. Vibrational inversion is created in the ν2 mode, and cw oscillation is observed on 20 different NH3 lines, spanning the region between 10.7 and 13.3 μm. Single line output powers as high as 760 mW were measured in a waveguide cavity. The operation of this laser is explained using a simple thermalization model, and several potential applications are discussed.

High sensitivity point monitoring of ozone, and high resolution spectroscopy of the ν 3 band of ozone using a tunable semiconductor diode laser

Accurate measurements of the wavenumbers of more than 100 lines in the nu(3) band of ozone have been made using a tunable semiconductor diode laser and a temperature-stabilized germanium etalon. The accuracy for the line centers is estimated to be within 3-10 MHz and compares well with the accuracy attained using more complicated heterodyne techniques. The relevance of these results to ozone monitoring instruments based on CO(2) lasers is discussed. The tunability of the diode laser is also exploited to monitor ambient concentrations of ozone in the presence of interfering molecular species. A detection sensitivity of ~0.5 ppb of ozone is demonstrated.

Laser mode locking using saturable absorbers

A theoretical model for mode locking induced by saturable absorbers is presented, employing an unidirectional ring laser cavity. Both the gain and loss media are assumed to be homogeneously broadened two‐level systems, on‐resonance with the radiation field. The loss of the cavity is treated as discrete and is supposed to occur at an equivalent mirror. Laser coupled equations are then derived, employing the density matrix formalism together with Maxwells equations, using the rotating wave and slowly varying envelope approximations. Time‐invariant solutions of the coupled equations are deduced exactly and mode‐locking regions are predicted by a perturbation method. It is found that a suitable absorber can produce mode locking if the laser parameters are appropriately chosen. Generation of the steady‐state pulses (SSP) by the finite‐difference method shows that relatively short intense pulses can be obtained from a mode‐locked laser. In addition, the theoretical model is extended to account for the possibil...

cw optically pumped 12‐μm NH3 laser

A 30‐W cw CO2 laser operating on the R(30) 9‐μm transition is used to pump a ring laser cavity containing NH3. Emission at 12.08 μm is observed with cw output power of 180 mW. A Raman process is shown to be responsible for the gain at 12.08 μm.