Advantages of photoacoustic detection with a silicon cantilever microphone are demonstrated in the THz range. In our method, earlier membrane microphone is replaced with robust silicon cantilever microphone, which can tolerate high intensity of the input radiation in contrast to vulnerable membrane. The responsivity of the photoacoustic sensor was confirmed to be constant over almost six orders of magnitude of input power, which is not easy to achieve with any other detector of THz radiation. Another favorable feature of the photoacoustic sensor is its uniform spatial responsivity over areas of several millimeters in size. Finally, we measured nearly constant spectral responsivity of the photoacoustic sensor for the wavelength range of 0.3 μm to 200 μm.
The atmospheric window at 3 to 5 μm is one of the most important spectral regions for molecular spectroscopy. This region accommodates strong fundamental vibrational spectra of several interesting molecules, including species relevant for air quality monitoring, medical diagnostics, and fundamental research. These applications require excellent spectroscopic sensitivity and selectivity. For example, atmospheric research often needs precise quantification of trace gas fractions of down to the parts-per-trillion level (10-12), with the capability of resolving individual spectral features of different molecular compounds. This sets stringent requirements for the light source of the spectrometer in terms of output power, noise, and linewidth. In addition, the wavelength tuning range of the light source needs to be large, preferably over the entire atmospheric window, in order to enable measurements of molecular fingerprints of several compounds. Continuous-wave optical parametric oscillators (CW-OPOs) are one of the few light sources that have the potential of combining all these favorable characteristics. This contribution summarizes our progress in the development of CW-OPOs, with an emphasis on precise frequency control methods for high-resolution molecular spectroscopy. Examples of new applications enabled by the advanced CW-OPO technologies will be presented. These examples include a demonstration of world-record detection sensitivity in trace gas analysis, as well as the first characterization of infrared spectrum of radioactive methane 14CH4.
We report a mid-infrared frequency comb generator, which produces up to 3 W of output power. The comb mode spacing is 208 MHz, the spectral bandwidth is ~300 GHz, and the center wavelength is tunable between 3 and 3.4 μm. The comb generation is based on intracavity difference frequency mixing between near-infrared pump and signal beams of a continuous-wave-pumped optical parametric oscillator. The signal beam, which is resonant in the cavity, acquires a comb structure through cascading quadratic nonlinearities in a periodically poled lithium niobate crystal. This comb structure is transferred to the spectrum of the mid-infrared idler beam via the difference frequency mixing process.
We have developed and characterized a new iodine spectrometer for absolute frequency measurements of I2 transitions at the 633-nm region. This region is important especially in length metrology. To obtain good frequency accuracy, the various sources of frequency shifts in the used saturation spectroscopy configuration have been minimized by optimizing the laser beams for spatial and spectral quality. This is done by injection locking a microlens-coupled diode laser with nearly Gaussian output beam to a stable external-cavity diode laser. The use of the injection-locking scheme also reduces the detrimental effects of optical feedback, as well as residual amplitude modulation related to laser frequency modulation via injection current.
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