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The development of efficient, low-cost and robust high peak power pulsed lasers in the ~ 2 – 2.1 µm spectral region is required for many application areas in the mid-infrared (mid-IR) photonics sector. In particular, such laser sources can be used to efficiently access the deeper mid-IR region through optical parametric frequency conversion techniques, utilizing nonlinear crystals such as ZGP and OP-GaAs, or supercontinuum gener¬ation in highly nonlinear fibers. Such mid-IR frequency comb systems are of particular interest for laser countermeasures, remote sensing, high precision spec¬troscopy, and environmental monitoring. Compact and efficient ultrafast 2 µm lasers can also be used as seed sources for developing high energy amplifier systems operating in the 2 – 7 µm region which will benefit many applications from the areas of laser material pro-cessing, strong-field phys¬ics, as well as the development of tabletop X-ray coherent sources.
So far, the work on the development of pulsed lasers that operate in the 2-2.1 µm spectral region is rather limited and based predominantly on Ho-doped gain media which require relatively expensive and bulky Tm-laser pump sources.
Tm3+-doped cubic sesquioxides RE2O3 (RE=Lu, Sc, and Y) occupy a prominent position amongst other Tm3+-doped gain media. They possess advanta¬geous thermo-mechanical properties and spectroscopic features that make them ideal for high power lasers development in the 2 – 2.1 µm region using a low-cost laser diode pump platform around 800 nm. In particular, their thermal conductivities are in range of 13-17 W/m·K (compared to that of YAG which is 11 W/m·K) and in the case of Lu2O3 it decreases negligibly when the rare-earth-ion doping concentration is increased allowing high-power operation under direct diode pumping. In contrast to most Tm3+-doped crystalline and amorphous gain media, their broadband emission spectra extend well beyond 2 µm allowing efficient operation close to 2.1 µm reaching atmospheric transparency window. The attractive characteristics of rare-earth ion doped crystalline sesquioxides gain media have also led them to being studied extensively as ceramics. Currently, high optical quality sesquioxide ceramics can be produced by nanocrystalline and vacuum-sintering technologies. Such ceramic gain media possess stronger fracture toughness than single crystals and afford a high potential for size scalability thereby offering practical advantages in high-power laser implementations.
Here we report on our recent progress in the development of a diode-pumped Tm-doped sesquioxide class ceramic ultrafast lasers operating around 2.1 µm region. In particular, a diode-pumped femtosecond Tm:Lu2O3 laser is demonstrated generating directly transform-limited <500 fs pulses with an average output power in excess of 1 W and a peak power of >30 kW at a center wavelength of 2070 nm. Both semiconductor saturable absorber mirror and Ker-lens mode-locking techniques were investigated. The perspectives for further power scaling during ultrashort pulse generation under direct diode pumping around 800 nm and in-band pumping at 1.6 µm region will be discussed.
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