We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W in the region
between 2.7 and 4.2 μm. Two configurations have been studied in order to improve the frequency stability and
the linewidth of the OPO emission. First, we stabilized the signal frequency to a high-finesse Fabry-Perot cavity.
Then, we locked both pump and signal frequency to the frequency comb generated by a NIR fs mode-locked
fibre laser, linked to the caesium primary standard. With this last configuration we carried out saturation
spectroscopy of several transitions belonging to the ν1 rovibrational band of CH3I, resolving their electronic
quadrupole hyperfine structure, and determining the absolute frequency of the hyperfine components with a
50-kHz-uncertainty. An upper limit for the idler linewidth has been estimated as 200 kHz FWHM.
We developed several prototypes of ceramic Nd:YAG slab lasers pumped by different laser diode stacks. Compactness,
efficiency and ruggedness have been the principal design drivers. Nevertheless beam quality and insensitivity to diodes
temperature have also been pursued. An edge-pumped device with less than 12% power variation over a 20°C
temperature range has been demonstrated.
Fiber Bragg Gratings (FBG) sensors are a very promising solution for strain and/or temperature monitoring in hostile or hazardous environments. In particular, their typical immunity to EMI and the absence of electrical signals and cables, encourage the use of FBG sensors in aerospace structure. Moreover, FBG sensors can be embedded in composite materials, allowing the fabrication of the so-called smart-materials. In this paper we experimentally demonstrate that a Fabry-Perot cavity, integrated in a low-loss all-silicon rib waveguide, and realized by standard dry etching technique, is suitable for FBG monitoring. The reflected signal for the sensor passes through the cavity which is tuned by means of thermo-optic effect. The optical circuit ends with a photodetector that, for each tuning step, produces a photocurrent proportional to the convolution integral between the FBG and the FP spectral response. Because the finesse of a silicon FP cavity in air is not so high (about 2.5), it is advantageous an extended tuning over a wavelength range longer than the cavity free spectral range, that is convolving the FBG response with more than one FP transmission peak. The photodetector output signal, once acquired, is elaborated using standard FFT algorithm and pass-band filtered, in order to extract the main harmonic. After a final I-FFT step, a fitting procedure returns the FBG reflection peak position. The experimental accuracy, using as reference the peak wavelength measure made with a commercial high-performance Optical Spectrun Analizer, is in the order of few tenths of picometers.
Recent progress in telecom-based optoelectronic devices and low power laser sources has provided new powerful tools for in-situ monitoring of volcanic areas in real-time and for long periods, with unprecedented accuracy and resolution.
Such techniques represent a great improvement if compared to the traditionally used "in-situ" sampling techniques such as gas chromatography and mass spectrometry which result often impractical or dangerous, and do not allow temporally continuous monitoring
Optical communication diode lasers, which emit visible and IR radiation with mW power, operating on single mode at room temperature, are ideal sources to develop high sensitivity gas analysers based on absorption spectroscopy techniques. Their possible use in conjunction with fiber components is particularly advantageous in the perspective of developing networks for simultaneous monitoring of many geochemical and geophysical parameters.
We report on the field tests of a novel Portable Diode Laser Spectrometer (PDLS), operating at a wavelength of 1.99μm and based on direct absorption spectroscopy, for high precision, accurate and real-time, simultaneous measurements of CO2 and H2O concentrations.
We used an open-path configuration, positioning the detection bench, equipped with a a Herriott cell especially designed for measurements in hazardous environment, at different sites of the fumarolic areas. With such a configuration we can take advantage of the long optical path-length allowed by the multiple-pass cell (20.3m).
The measured CO2 and H2O concentrations vs. time are shown.
This work was supported by GNV-INGV, in the framework of the project "Development of spectroscopic integrated system for remote and continuous monitoring of volcanic gases".
Optical fiber sensors are the ideal system to monitor "smart structures" and on-site/real time stress measurements: they can be in fact easily embedded or attached to the structures under test and are not affected by electro- magnetic noise. In particular a signal from a Fiber Bragg grating sensor (FBG) may be processed such that its information remains immune to optical power fluctuations. Different interrogation methods can be used for reading out Bragg wavelength shifts.
In this paper we propose a very simple interferometric method for interrogating FBG sensors, based on bi-polished silicon sample acting like an etalon tuneable filter (ETF).
The Bragg wavelength shift can be evaluated by analyzing the spectral response of signal reflected by the FBG sensor and filtered by the ETF that is continuously and rapidly tuned. Tuning was obtained by rotating the ETF. Variation in the strain at the FBG causes a phase shift in the analyzed signal. The overall spectral signal, collected with time, consists in an interferometric figure which finesse and fringe contrast depending on the geometrical sizes and facets reflectivity of the silicon sample. The fringe pattern, expressed by the Airy's formula, depends on the wavelength l of the incident radiation and on the angle of incidence. The phase of fringe pattern can be retrieved by a standard FFT method giving quantitative measurements of the quasi-static strain variation sensed by the FBG. In this way, the method allows a valuable visualization of the time-evolution of the incremental strain applied to the FBG.
Principle of functioning of this method is described and first results obtained employing such configuration, are reported.
In this paper a new method for reading out Bragg wavelength shifts experienced by fiber Bragg gratings is described. The system is based on a bi-polished silicon sample acting like a Fabry-Perot filter. The spectral response of the Silicon Fabry-Perot filter allows to convert the Bragg wavelength shift into a variation of the light intensity, which can be read by a photodiode. The method efficacy is proved monitoring dynamic strain characteristics of a simple structure. So, the vibration mode of an aluminium cantilever has been sensed by means a FBG sensor attached on the surface of the cantilever itself. In proposed demodulation method the sensitivity and accuracy depend on the spectral band width of the filter. The filter can be designed according to the range of the amplitude vibrations, assuring linear response of system, as function of the thickness of the silicon sample. Moreover, thank to great tuning capability of the Silicon Fabry-Perot filter, it is possible to place Bragg grating spectral response on the central portion of the linear region of the FP response.
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