Ph.D. Defense: Dmitriy Churin

    Monday, June 29, 2015 - 10:00am
    Meinel 647

    The development of high average- and peak-power ultrashort pulsed fiber lasers is important for many critical research, industrial and defense applications. However, the performance of mode-locked fiber oscillators still lags behind that of solid-state counterparts such as Kerr-lens mode-locked titanium:sapphire lasers. Despite the drawbacks in cost, size and required maintenance, titanium:sapphire remains the workhorse of ultrafast science. One of the remaining challenges for fiber lasers to overcome is their limited set of accessible wavelengths. Unfortunately, readily available ytterbium, erbium and thulium fiber lasers can produce coherent radiation only near 1, 1.55 and 2 micrometers, respectively. There remain a significant number of wavelength regions that fiber lasers cannot address. In this thesis, novel fiber lasers producing ultrashort pulses at wavelengths not currently accessible with established active rare-earth-doped fibers are investigated. The main approach is to use various nonlinear optical effects to generate new laser wavelengths.

    First, a watt-level synchronously pumped Raman fiber oscillator generating tens of nanojoules femtosecond pulses is demonstrated. Stimulated Raman scattering in a passive fiber within an oscillator cavity allows formation of Raman pulses that are spectrally redshifted with respect to the pump pulses. World records in output pulse energy and conversion efficiency have been achieved with this femtosecond Raman fiber laser design. Churin's group have also demonstrated a high-power, widely tunable all-fiber optical parametric oscillator based on four-wave mixing in a passive fiber. The FOPO is synchronously pumped with an Yb3+-doped mode-locked fiber laser working at approximately 1,040 nanometers. The FOPO produces ultrashort pulses tunable from 760 to 1,560 nanometers. Record pulse energy of 5.7 nanojoules is generated at the output of the FOPO. Depending on the configuration of the FOPO, the duration of produced pulses varies between 170 femtoseconds and 3 picoseconds. This new laser source has similar performance to standard titanium:sapphire femtosecond lasers so it can potentially replace the latter in many applications.

    Ultrashort optical pulses in the mid-infrared and long-infrared range (2-20 micrometers) have many important applications in gas sensing, counter-measures, etc. The realization of the ultrashort pulses in the mid-infrared and long-infrared wavelengths requires the use of free-space nonlinear crystals. An efficient mid-infrared source based on difference frequency generation in an AgGaS2 crystal using femtosecond erbium/thulium pump fiber laser has been proposed and demonstrated. The photon conversion efficiency of the pump wave (1.55 micrometers) to idler wave (9.2 micrometers) has been measured to be 16 percent, which is today a record for conversion of near-infrared light radiation from fiber lasers to 9 micrometers spectral range. Potentially the photon conversion efficiency can be increased up to 60 percent by using pump pulses having higher peak power.

    Finally, generation of supercontinuum light in the mid-infrared spectral range is also demonstrated. It is well known that SC produced in standard optical fibers is limited to approximately 6 micrometers by material absorption. The liquid core optical fiber platform has been proposed to address this matter. Several highly nonlinear liquids have minimal absorption in the mid-infrared wavelength range, which potentially allows us to create broadband SC light in this spectral region. SC generation up to 2.4 micrometers in an integrated hollow core optical fiber filled with CS2 has been demonstrated. Further development of the liquid core optical fiber platform should allow generation of the SC covering wavelengths beyond 6 micrometers.

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