Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Towards high-power mid-infrared emission from a fibre laser

A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from ∼1.9 to 4.5 μm is generated with ∼0.7 W time-average power in wavelengths beyond 3.8 μm. The laser outputs a total average power of up to ∼2.6 W from ∼8.5 m length of ZrF4─BaF2─LaF3─AlF3─NaF (ZBLAN) fiber, with an optical conversion efficiency of ∼9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 μm is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 μm by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 μm laser diode pulses with ∼2.5 kW peak power generate a SC extending beyond 2.1 μm in ∼25 m length of standard single-mode fiber (SMF). The ∼2 μm wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to ∼270 nm SC long wavelength edge extension and ∼2.5× higher optical conversion efficiency to wavelengths beyond 3.8 μm are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrodinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN.

Supercontinuum generation from ~19 to 45 μmin ZBLAN fiber with high average power generation beyond 38 μm using a thulium-doped fiber amplifier

We briefly review the development of gain-switched rare-earth-doped fiber lasers and their applications in wavelength conversion to mid-IR, supercontinuum generation, and medicine in recent years. We illustrate the similarities between gain-switching and Q-switching techniques that will provide tools for the design and optimization of the gain-switched fiber lasers. From the nature of the gain-switched fiber lasers, benefits of this kind of lasers to 2-μm region and in-band-pumped (two-level system) laser systems are obvious. Advantages of in-band-pumped 2-μm lasers are discussed and analyzed with a simple numerical simulation in terms of Tm-doped fiber lasers. We also propose the key factors in the development of the gain-switched fiber lasers and predict the future tendency.

Development and applications of gain-switched fiber lasers [Invited]

Microstructured optical fibers (MOFs) are traditionally prepared using the stack and draw technique. In order to avoid the interfaces problems observed in chalcogenide glasses, we have developed a new casting method to prepare the chalcogenide preform. This method allows to reach optical losses around 0.4 dB/m at 1.55 µm and less than 0.05 dB/m in the mid IR. Various As(38)Se(62) chalcogenide microstructured fibers have been prepared in order to combine large non linear index of these glasses with the mode control offered by MOF structures. Small core fibers have been drawn to enhance the non linearities. In one of these, three Stokes order have been generated by Raman scattering in a suspended core MOF pumped at 1995 nm.

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Low loss microstructured chalcogenide fibers for large non linear effects at 1995 nm.

High-power mid-infrared supercontinuum sources: Current status and future perspectives

We report what we believe to be the first demonstration of stable short pulse (10 ns) generation from a gain-switched Tm-doped fiber laser in the 2 μm spectral region. A modulated 1.55 μm pump laser was used for fast gain switching to regulate the conventionally chaotic spiking in the gain-switched fiber laser. From a 6.3 μm core all-fiber oscillator, over a kilowatt of peak power was obtained at the slope efficiency of 50% and freely varying repetition rate up to 500 kHz.

Parviz Tayebati

Papers

Stable 10 ns, kilowatt peak-power pulse generation from a gain-switched Tm-doped fiber laser.