The greatest peak power of 1.86 kW (25 ps pulse duration) was demonstrated by Tang et al. Passive mode-locking has been achieved using several different saturable absorber materials, producing picosecond scale pulses with peak powers around the 1 kW level. Īt the time of writing, the majority of ultrafast mid-infrared fiber laser research has focused on the 3 µm band. We have previously used the frequency shifted feedback technique to achieve tunable pulsed operation, producing pulses as short as 53 ps with energies up to 1.38 nJ over a 215 nm wavelength range. A pulse duration of 3.8 ps was estimated from spectral broadening assuming a transform limited temporal Gaussian shape. used a black phosphorous saturable absorber to mode-lock with a pulse energy of 1.38 nJ at 28.91 MHz.
Previous demonstrations of mode-locked fiber lasers at 3.5 µm have produced picosecond scale pulses. Ultrafast fiber systems could therefore see broader implementation and use in potential new applications.
These include greater average output power, improved beam quality, robustness, and affordability. Fiber based sources offer several potential advantages over conventional methods of producing ultrafast pulses in the mid-infrared wavelength region using optical parametric oscillators and optical parametric amplifiers. Prior to this work, sub-picosecond pulses at this wavelength have been achieved only by using nonlinear techniques to frequency shift shorter wavelength lasers. This makes them an interesting source for polymer processing, molecular spectroscopy, and breath analysis. Lasers operating around 3.5 µm are of great interest because they directly excite and probe C-H and N-O vibrational bonds. The pulse energy is 3.2 nJ, corresponding to a peak power of 5.5 kW. The laser uses dual-wavelength pumping and nonlinear polarization rotation to produce 3.5 µm wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. We report, to the best of our knowledge, the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 µm. Hopefully this novel design will open new opportunities for a diverse range of optical mid-IR applications. The pulse energy was 3.2 nJ, corresponding to a peak power of 5.5 kW. In contrast to the well-known way of direct pumping of the upper laser level, the authors have used a double-wavelength pumping scheme to decrease population build-up in a metastable "virtual ground state." This design enabled an increase in efficiency of laser emission almost three times and to produce 3.5 μm wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. The design is based on laser diode pumped erbium-doped zirconium fluoride fiber and non-linear polarization rotation technique. The authors of this publication in Optics Letters have designed the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 μm. Limited availability of gain materials in the mid-IR region for passively mode-locked laser sources, and high cost of optical parametric oscillators and amplifiers used for traditional mid-IR laser sources, push the investigators to apply new technologies and innovations in laser design. Ultrafast mid-infrared (mid-IR) laser sources have drawn attention due to irreplaceable applications in molecular spectroscopy, mid-IR non-linear conversion processes and bio-diagnostics.
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