Researchers at the University of Tokyo and at Alnair Laboratories Corp. of Kawaguchi, Japan, have demonstrated what they believe is the first 1300-nm laser mode-locked with carbon nanotubes. The accomplishment is an extension of their earlier work in which they mode-locked 1550-nm lasers with the nanotubes. Figure 1. The carbon nanotubes acted as a saturable absorber and mode-locked the ring laser. An isolator within the praseodymium-doped amplifier ensured unidirectional oscillation around the ring.Carbon nanotubes act as a saturable absorber in the host laser resonator, and their performance is comparable to that of other saturable absorbers, such as semiconductor saturable absorber mirrors and bleachable dyes. However, carbon nanotubes offer the advantages of ultrafast recovery time (~500 fs), high optical damage threshold, chemical stability and inexpensive manufacturing.They resemble semiconductor saturable absorber mirrors in that their saturable absorber characteristics derive from their energy bandgaps. The bandgap energy of a single-walled carbon nanotube depends on the tube diameter, so fabrication of an effective saturable absorber requires that the nanotubes in the sample have approximately the same diameter. The researchers used a proprietary purification method to ensure that the nanotubes’ diameters were within an allowed tolerance. They then sprayed the nanotubes onto a quartz substrate, creating a coating less than 1 µm thick.Figure 2. The output pulse train had a repetition frequency of 3.18 MHz. The pulse-to-pulse amplitude variation is an artifact introduced by the sampling scope.They placed the substrate with the nanotubes inside the laser resonator between a pair of focusing lenses (Figure 1) and oriented the plane of the nanotubes at an oblique angle to avoid intracavity etalon effects that can prevent mode-locking. They minimized intracavity dispersion by balancing positive-dispersion fibers with negative-dispersion ones.The laser produced a train of mode-locked pulses at a 3.18-MHz repetition rate (Figure 2). The spectrum was centered at 1294 nm and had a bandwidth of 0.30 nm, full width half maximum. With slightly less than 1 W of total pump power, the laser produced a mode-locked output of 1.67 mW. Figure 3. The amplified spontaneous emission spectrum of the praseodymium-doped amplifier was centered at ∼1299 nm and showed gain ripple peaks separated by about 0.55 nm.The researchers observed the spectrum of amplified spontaneous emission from the praseodymium-doped fiber amplifier and discovered a series of gain ripples, possibly due to etalon effects stemming from stray reflections within the amplifier. The ripples, spaced by ~0.55 nm, had an amplitude of ~0.1 dB (Figure 3).They also observed that, although the amplified spontaneous emission was centered at ∼1299 nm, wavelength selectivity within the resonator forced the laser to oscillate at 1294 nm. By adjusting the intracavity polarization controller, they were able to take advantage of these effects to obtain simultaneous mode-locking at two amplified spontaneous emission peaks. The resulting spectrum indicated two output wavelengths separated by 1.1 nm. The two pulse trains overlapped, generating slightly greater power than the single wavelength.