We examine the quasi-distributed interrogation of fiber Bragg grating arrays (FBGA) with the method of stepped incoherent optical frequency domain reflectometry (IOFDR) combined with wavelength scanning of a tunable laser source. The technique of IOFDR provides some advantages over traditional time division multiplexing schemes, because continuous wave modulation of the light intensity instead of short and low energy pulses is used, maximizing the reflected signal received from the sensor fiber. Moreover, an electrical heterodyne demodulation by a microwave vector network analyzer provides a low-noise detection, while achieving high spatial resolutions down to the centimeter range. A quasi-distributed temperature measurement with ten FBGs of 0.5% reflectivity is demonstrated. In the experiment, a root mean square temperature error of 0.3 K and a maximum error of 1 K is observed, using a fiber-coupled power of only -12 dBm. A spatial two-point resolution of 2.14 cm is achieved, which enabled a successful interrogation of FBGAs with 20 and 30 cm spacings. The potential multiplexing capability of the given setup could reach more than 50 gratings in a 30 cm spacing at a measurement rate of 50 s/nm.

A Q-switched laser based system for broadband absorption spectroscopy in the range of 1390-1740 nm (7200-5750 cm⁻¹) has been developed and tested. In the spectrometer the 1064 nm light of a 25 kHz repetition-rate micro-chip Nd:YAG laser is directed into a photonic crystal fiber to produce a short (about 2 ns) pulse of radiation in a wide spectral range. This radiation is passed through a 25 km long dispersive single-mode fiber in order to spread the respective wavelengths over a time interval of about 140 ns at the fiber output. This fast swept-wavelength light source allows to record gas absorption spectra by temporally-resolved detection of the transmitted light power. The realized spectral resolution is about 2 cm⁻¹. Examples of spectra recorded in a cell with CO₂:CH₄:N₂ gas mixtures are presented. An algorithm employed for the evaluation of molar concentrations of different species from the spectra with non-overlapping absorption bands of mixture components is described. The uncertainties of the concentration values retrieved at different acquisition times due to the required averaging are evaluated. As an example, spectra with a signal-to-noise ratio large enough to provide species concentrations with a relative error of 5% can be obtained in real time at a millisecond time scale. Potentials and limitations of this technique are discussed.

Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.