Abstract
The recent demonstrations of ultrafast mid-infrared fiber lasers emitting sub-picosecond pulses at 2.8 μm have created an exciting potential for a range of applications including mid-infrared frequency combs and materials processing. So far, this new class of laser has been based on the I11/24-I13/24 transition in erbium-doped fluoride fibers, which lies directly in a region of high water vapor absorption. This absorption has limited the achievable bandwidth, pulse duration, and peak power and poses a serious problem for transmission in free space. In this Letter, we present an ultrafast mid-infrared fiber laser that overcomes these limitations by using holmium as the gain medium. Holmium allows the central emission wavelength to shift to nearly 2.9 μm and avoid the strong water vapor lines. This laser, which represents the longest wavelength mode-locked fiber laser, emits 7.6 nJ pulses at 180 fs duration, with a record peak power of 37 kW. At this power level, the laser surpasses many commercial free-space OPA systems and becomes attractive for laser surgery of human tissue, for industrial materials modification, and for driving broadband coherent supercontinuum in the mid-infrared.
Highlights
The recent creation of ultrashort-pulse fiber lasers operating in the 2.8 μm range [1,2,3] demonstrated that mode locking at wavelengths beyond the transmission window of silica fiber can be achieved with sub-picosecond performance
We show that by using a holmium-doped ZrF4-BrF2-LaF3-AlF3-NaF (ZBLAN) fiber, which has an approximately 100 nm red-shifted emission wavelength compared to erbium-doped ZBLAN, we can generate 180 fs pulses, via nonlinear polarization rotation (NPR) mode locking
The holmium laser emission at 2876 nm is beyond the main water vapor lines, thereby mitigating the deleterious effects of intracavity water vapor absorption
Summary
The recent creation of ultrashort-pulse fiber lasers operating in the 2.8 μm range [1,2,3] demonstrated that mode locking at wavelengths beyond the transmission window of silica fiber can be achieved with sub-picosecond performance. These strong water vapor absorption lines can limit the ultimate pulse bandwidth [1] or inhibit self-starting of the mode-locked laser [3]. We show that by using a holmium-doped ZrF4-BrF2-LaF3-AlF3-NaF (ZBLAN) fiber, which has an approximately 100 nm red-shifted emission wavelength compared to erbium-doped ZBLAN, we can generate 180 fs pulses, via nonlinear polarization rotation (NPR) mode locking.
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