Mode-locked (ML) lasers have found many important applications in nonlinear optics, biomedical imaging, and precision metrology. They generate ultrashort optical pulses on pico- and femtosecond timescales. These are among the shortest events in nature.
Special training and experience have traditionally been required to build a ML fiber laser, but this may no longer be the case. Building a ML fiber laser using carbon nanotubes (CNTs) is becoming so easy that an undergraduate student can build one from scratch in a few hours with minimal guidance and without any prior experience.
A saturable absorber (SA) that is inserted into the laser cavity is responsible for mode locking, thereby allowing creation of ultrashort optical pulses. An SA can be any device that transmits more at higher intensities. In ultrafast fiber lasers, two types of SAs are typically used, including semiconductor SA mirrors (SESAMs) and artificial SAs based on nonlinear polarization evolution (NPE).
Carbon-based nanoscale materials have many uses. One surprising (and perhaps unexpected) application is in facilitating generation of ultrashort optical pulses. Because of their unique physical properties, CNTs have ultrafast (<1ps) recovery lifetimes1 (a desirable feature for SAs), which can be used to phase-lock laser cavity modes. This new SA material has significant advantages as compared to traditional SESAMs or NPE, including fabrication simplicity, low cost, and broadband operation.
Figure 1. (top left) Schematic of our fiber-taper-based carbon-nanotube (FTCNT) saturable absorber (SA). SWCNT: Single-walled CNT. (bottom left) Packaged FTCNT SA. (right) Hand-held mode-locked fiber laser with FTCNT SA.
CNTs can be incorporated into a laser cavity in several ways. The simplest is by depositing a thin film of CNTs onto one of the cavity mirrors.2 This thin-film configuration, however, suffers from a low damage threshold, which is also a disadvantage for SESAMs. Therefore, we have developed a new type of arrangement where the interaction with the CNTs is done through the evanescent field of a thin fiber taper.3 This configuration increases the SA's damage threshold significantly, since the absorption is now distributed along the fiber taper (a few centimeters in length) instead of being concentrated in a very thin layer (<1μm). In addition, the device is now compatible with fiber format, and adding the SA into the cavity is as simple as adding a fused fiber coupler.
We have successfully built ML fiber lasers operating at different wavelengths using this new type of SA. Figure 1 shows a packaged laser operating at the telecommunications wavelength near 1550nm. We spliced a fiber-taper-based CNT (FTCNT) SA into the cavity, so that no alignment was needed. We thus constructed a hand-held, low-cost ultrafast fiber laser. We recently also demonstrated a hand-held fiber-laser system that delivered few-cycle optical pulses.4 Its heart is a ML laser oscillator with a FTCNT SA. It can be used in high-resolution optical coherence tomography, frequency metrology, or seeding high-energy optical parametric amplifiers.
Development of laser sources that operate near 2μm wavelength is urgently needed for nonlinear optics, medicine, and sensing. We successfully built one of the first ML thulium-doped fiber lasers using a FTCNT SA that provided subpicosecond pulses near 2μm.5 We also recently developed a widely tunable ML laser in this wavelength region.6 These results exemplify the great potential of FTCNT SAs, since they can be used for ML fiber lasers in various wavelength regions.
We next need to address the long-term stability of CNT SAs. FTCNT SAs exhibit great long-term stability when they are used in lasers that operate near 1550 or 2000nm. We have built a number of laser units that have been working in different laboratories for several years. Some have worked for more than 1000 hours. We observed degradation (which appeared as a reduction in absorption under laser action) in FTCNT SAs operating around 1000nm. Therefore, the stability of these SAs seems to depend on the operating wavelength. We will need to conduct more research to gain a better understanding of this effect.
CNTs are bringing robust, affordable fiber lasers to the market. It will not be surprising to see these devices at the heart of every ultrafast fiber laser in the near future.
University of Arizona
1. Y. C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y. P. Zhao, T. M. Lu, G. C. Wang, X. C. Zhang, Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55μm, Appl. Phys. Lett. 81, pp. 975-977, 2002.