By combining a thin-film of carbon nanotubes (CNTs) with a highly reflective silver (Ag) mirror, researchers in Finland have created a saturable absorber with a bandwidth of 1 µm. The result means that a single saturable absorber mirror can be deployed at all of today's common fibre laser wavelengths: 1, 1.5 and 2 µm. (Optics Express 17 2358)

"This is the broadest operation range reported for a single CNT absorber," Samuli Kivistö, a researcher at Tampere University of Technology, told "Our fabrication method is very simple and does not involve polymers. From commercial point of view, these properties make our absorber very interesting."

In the last five years, there have been various reports of CNT-based absorbers modelocking lasers emitting at 1, 1.55 and 1.9 µm. However, these absorbers have all been optimised to operate at a particular wavelength. This new work goes further by creating a universal absorber with a significantly wider bandwidth.

To fabricate their absorber, Kivistö and colleagues developed a one-step dry-transfer contact press method. This essentially involves depositing a dense mesh of single-walled CNTs onto a film approximately 80 nm thick, and then pressing and adhering this film on to the surface of a highly reflective and broadband Ag mirror.

"Our CNT-films can be utilized as-deposited and no purification or dispersion steps are required," commented Kivistö. "Standard wet deposition methods can require several time-consuming stages such as purification, dispersion and filtering. Our approach is simple and inexpensive. The thickness of the film and its absorption characteristics can be well controlled. We also have the potential to scale to higher powers, due to the absence of polymers, and into the mid-infrared."

Kivistö explains that it is the diameter distribution of the CNTs that determines the absorption characteristics. "In our absorber, the tube diameters ranged from 1.2 to 1.8 nm allowing the broadband operation," he said.

The team has already demonstrated modelocked sub-picosecond operation of Yb-, Er- and Tm:Ho-doped fibre lasers emitting at 1.05, 1.56 and 1.99 µm, respectively, using the same CNT-based absorber.

"The broadband absorption of the CNTs and the broadband reflection of the Ag mirror enabled us to build compact and robust linear cavities," said Kivistö. "Our modelocked lasers had average output powers in the range of few tens of milliwatts and corresponding peak powers in the range of 1 kW. No aging effects or degradation of the saturable absorber were observed."

The researchers are now trying to extend the operation range of the absorber towards mid-infrared wavelengths. "We may also develop broadband CNT absorbers to modelock disc lasers," said Kivistö. "Making ultrashort pulsed laser systems for material processing and medical applications could also be considered."

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