Many applications require laser light with a specific wavelength, and this has prompted decades of research into developing materials that lase at new regions of the electromagnetic spectrum. But despite great efforts, gaps still exist where the light of no laser shines. And of course, wavelength is just one feature amongst many: even if a laser at the desired wavelength existed, it may not possess other features ideal for the application. For example, several applications that currently hinge on solid-state lasers could benefit from the advantages of fiber-based devices, such as lack of sensitivity to misalignment. Unfortunately, fiber lasers only light up at a handful of spectral regions.

Besides new laser materials, an alternative route exists to access new wavelengths. Specifically, by harnessing nonlinear optical phenomena, it is possible to convert laser light from one wavelength to another. In this Optics Letters article, Yao et al. from the University of Rochester (USA) utilize nonlinear frequency conversion in a photonic crystal fiber (PCF) to realize an ultrafast, fiber-based laser source that operates at a wavelength of 1.3 μm. This region is an important spectral window for multiphoton deep tissue imaging, as the low attenuation of excitation light around 1.3 μm enables deeper tissue penetration. But because no fiber lasers exist in this spectral region, researchers have so far relied on solid-state Cr:Forsterite lasers or bulky optical parametric oscillators. Techniques relying on nonlinear “soliton self-frequency shift” in optical fibers have also been proposed, yet these suffer from low energies or the need for carefully engineered multimode fibers. Here, in contrast, Yao et al. harness an alternative nonlinear process to realise a fiber-based source outputting femtosecond pulses with nanoJoule energies around 1.3 μm.

The source examined by the Rochester-based group is made of two key sections. First, a master oscillator power amplifier system, consisting of a custom-built Ytterbium-doped fiber laser and a chirped-pulse amplification scheme, is used to create energetic pulses with 240 fs duration and a central wavelength of 1035 nm. Second, the pulses are launched into a highly nonlinear PCF, where nonlinear wavelength conversion enables up to 32.3% of the 1035 nm input to be converted to the 1.3 μm spectral region. By controlling the energy of the input pulses, the precise output wavelength can be tuned over a broad range. Compounded by the fact that the converted light maintains the femtosecond-scale duration of the input pulses, the overall source impressively enables the generation of wavelength-tuneable femtosecond pulses with energies up to 1.9 nJ in the 1.3 μm spectral region.

A key feature of the source is the particular PCF used to realise wavelength conversion. Specifically, its two closely-spaced zero-dispersion wavelengths (ZDWs) prevent the onset of a full “supercontinuum (SC) generation” that typically occurs when intense pulses are injected into highly-nonlinear fibers. Although many applications benefit from the broadband laser light created via fiber SC, the process is not ideal when efficient conversion to a particular wavelength is desired. The use of a PCF with two closely-spaced ZDWs prevents broadband SC from occurring, instead enabling the input light to be resonantly and efficiently converted to two narrowband spectral regions only. By selecting a PCF with suitable characteristics, Yao et al. ensure that one of those spectral regions is close to 1.3 μm, allowing for efficient conversion of the 1035 nm input pulses.

Besides facilitating efficient conversion to particular wavelengths, the double-ZDW fiber design also impacts beneficially on the stability of the 1.3 μm output. Specifically, for sufficiently energetic input pulses, significant fluctuations can arise when operating under typical fiber SC conditions. Here, in contrast, the authors demonstrate that the 1.3 μm output exhibits high degree of coherence, even for very large input energies. This is because, unlike modulation instability that can detrimentally impact on SC stability characteristics, the frequency conversion mechanism underpinned by the double-ZDW fiber is fully deterministic.

This interesting work illustrates that combining a Yb-doped fiber laser system with a highly nonlinear PCF with two closely-spaced ZDWs enables the coherent generation of femtosecond pulses tuneable around 1.3 μm. It opens new perspectives: once the individual components are integrated into a robust all-fiber device and the output pulse energies further increased, wide use in biomedical imaging applications can be envisioned!

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