We report an approach to generation of optical frequency combs in the spectral range 2 – 20 µm. The 2-cycle, multi-Watt laser at the repetition rate 80 MHz is based on a polycrystalline Cr:ZnS. The bandwidth of the super-octave ultrafast Cr:ZnS laser source at the central wavelength 2.4 µm is extended to the long-wave IR range (5 – 20 µm) via optical rectification in non-oxide nonlinear materials: GaSe, ZGP. The key advantages of Cr:ZnS frequency comb technology is high efficiency of optical-to-optical conversion from low-cost cw EDFL light to fs MIR pulses, and ultra-low timing jitter. These advantages, in turn, has allowed us to implement shoe-box-sized, light-weight, frequency combs that open new avenues in imaging, sensing, and spectroscopy. Our preliminary evaluations confirm the applicability of the developed sources for dual-comb spectroscopy.
KEYWORDS: Frequency combs, Long wavelength infrared, Spectroscopy, Molecular spectroscopy, Mode locking, Femtosecond phenomena, Mid-IR, Optical amplifiers, High power lasers, Synchrotrons
Femtosecond mid-IR and long wave IR lasers provide tremendous opportunities for imaging and sensing because they combine high spatial coherence of the laser beams (crucial for remote sensing), high temporal coherence of mode-locked oscillators (enabling dual comb spectroscopy in important molecular fingerprint region), and few-optical-cycle pulses (enabling IR sensing with high dynamic range via electric field sampling with low cost room temperature near-IR photodetectors). The development of such a sources (especially compact and field-deployable sources) is a challenging task that relies on the most advanced bulk laser technologies, new gain and nonlinear materials for a wide range of laser wavelengths. We present a viable route to the generation of power and energy scalable few-cycle pulse trains in the IR and describe the application of the developed sources for dual comb spectroscopy. The laser architecture is based on a combination of laser and nonlinear interactions in polycrystalline Cr:ZnS media that enables simultaneous amplification of ultrashort pulses, nonlinear pulse compression to 2-optical-cycle, and nonlinear broadening of pulses’ spectrum to an optical octave. Importantly, all of the necessary optical signals for stabilization of the frequency comb with the large lever arm are generated directly inside polycrystalline Cr:ZnS. This, in turn, has allowed us to implement robust and reliable shoe-box sized middle-IR frequency combs with ultra-low timing jitter of the pulse trains, broad instantaneous spectra, and Watt-level average power.
We report a technique for generation of ultra-broadband coherent femtosecond continua in the infrared. The laser architecture is based on the Cr:ZnS–GaSe and Cr:ZnS–ZGP tandem arrangements that enable simultaneous amplification of ultrashort middle IR pulses and augmentation of pulses’ spectrum via a chain of intrapulse three-wave mixings. The first part of the tandems is based on a single-pass polycrystalline Cr:ZnS amplifier, which is optically pumped by off-the-shelf continuous wave Er-doped fiber laser and outputs 2-cycle pulses with multi-Watt average power at 80 MHz repetition rate, at the central wavelength 2.5 μm. The second stage of the tandems comprises a GaSe or ZGP crystals configured for intrapulse difference frequency generation. The Cr:ZnS–GaSe tandem has allowed us to achieve multi-octave 2–20 μm continuum with 2 W power in the range 2–3 μm and power in excess of 20 mW in the important range 3–20 μm. On the other hand, Cr:ZnS–ZGP tandem features long-wave infrared (6–12 μm) output pulses with record braking sub-Watt power level. Last but not least, Cr:ZnS–GaSe and Cr:ZnS–ZGP IR sources have small footprints and are easily convertible to the optical frequency combs with low carrier-to-envelope timing jitter.
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