Method

Saturable Absorber (SA): SAs are used to passively mode-lock lasers. They differ from actively mode-locked lasers in that the intrinsic properties of a material are used to mode-lock – this allows for shorter pulses and SAs can sometimes even start the mode-locking process themselves! Carbon nanotubes (CNT) have received much attention recently because of their advantageous optical properties. CNTs attenuate low optical intensities and allow high optical intensities to pass. CNTs have been incorporated into laser design in many creative ways to mode-lock lasers. We use CNTs in a polymer (PMMA) composite. Then we try and draw the CNT/PMMA solution into the core of a photonic bandgap fiber (PBG) and use this in a fiber laser for an all fiber passively mode-locked laser. Though all fiber mode-locked lasers exist, we hope this SA will prove more robust – as other CNT SAs of this sort (e.g. fiber ferrule) have low power threshold.

Tapering & Cleaving Fibers: In order to get the CNT/PMMA solution only in the center opening of the PBG fiber, we need to first taper the fiber. Once the fiber is tapered, it is then cleaved where the outside holes have collapsed and the center hole is all that is left exposed…

Vacuum: The tapered end of the PBG is placed into the CNT/PMMA solution. The other end is cleaved and placed into a vacuum chamber. Presumably the solution can then be drawn into the center hole of the fiber. All attempts to view the motion of fluid through the center of the fiber in real time have proven inconclusive. Also, viewing under a microscope after this process yields no visible sign of solution in the fiber…

Vacuum

Testing: Once we have the PBG with (we suspect) solution, we put the fiber into a pre-existing laser cavity. The throughput is first measured and if it looks good then the spectrum can be analyzed to see if the laser is mode-locked or not. So far, the laser hasn’t mode-locked…

This is the gist:

The real setup: