Widely tunable narrowband mid-infrared coherent sources, realized using optical parametric oscillators, play an essential role in spectroscopic investigations. A part of mid-infrared spectral region is a “fingerprint range” of solid-state materials, therefore, narrow linewidth is a particularly important feature. The most suitable linewidth of radiation to satisfy the required resolution for spectroscopy of solids is 2‒6 cm-1. The biggest challenge for the developer of the laser source is meeting customers’ needs and providing numerous parameters simultaneously from a single device: broad spectral range, high spectral resolution, fast wavelength tuning, high repetition rate, stable beam direction, nearly diffraction-limited divergence, etc. All this should be provided throughout the entire operational spectral range. These features are relevant for many applications, especially for Scanning Near-field Optical Microscopy (SNOM). This presentation will describe the architecture and applications of EKSPLA's broadly tunable commercial ns and ps laser sources, from 2 to 18 μm based on OP-GaAs fan-type gratings and other mid-infrared OPO nonlinear crystals. The advantages and limitations of the crystals in different narrowband OPO setups will be presented.
Broadband Sum Frequency Generation (BB-SFG) spectrometer is a complete solution for femtosecond vibrational spectroscopy designed and manufactured by EKSPLA. System produces optically coupled femtosecond broadband midinfrared pulse covering the spectral range of the molecular "fingerprint" region and narrowband visible pulse which are directed to the sample to produce sum frequency signal. Ultrashort pulses of high intensity allow to get better signal to noise ratio using lower pulse energy thus reducing the possibility of sample modification. It is especially important for aqueous and biological samples. Single measurement can cover up to 800 cm-1 bandwidth which shortens the measurement time and lets obtaining spectrum of the same sample state at the beginning and at the end of measurement, which can be different in case of scanning. The design of BB-SFG helps to overcome the main shortcoming of common broadband SFG spectrometers: a complex and energy inefficient narrowband visible pulse channel formation. We use a single all-fiber mode-locked oscillator to generate ultrashort pulses for both broadband femtosecond mid-IR and narrowband picosecond visible channels. Up to 10 μJ energy tunable broadband (>300 cm-1) pulses in 2.5-10 μm spectral range are mixed at the sample with less than 2 cm-1 bandwidth visible (532 nm) picosecond pulses in order to produce SFG signal at kHz repetition rates. Real-time spectral scanning technique allows even broader simultaneous spectral acquisition.
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