This photograph of the bent fiber demonstrates the hollow-core portion of the fiber that transmits the green pump light, and the dye-doped portion emits orange-colored laser light.
Water droplet on a flat As2S2 surface and its reflection. The horizontal dashed line is drawn along the water/substrate interface. The droplet surface intersects the surface at a 92-degree angle.
Schematic of motion control set-up
Dynamic surface-emitting fiber lasers
Another key aspect of the SEFL is that the gain medium need not be fixed in place with respect to the fiber. In static SEFLs, the copolymer plug bonds to the inside of the fiber walls as a result of the polymerization process. In these structures, the doped monomer solution is first inserted into the hollow core and subsequently polymerized to prevent it from spreading and wetting the inside surface.
While the polymerization process prevents disintegration of the plug, it forces a constraint on the laser’s potential—a static lasing position. Indeed, a static lasing position characterizes all lasers to date. However, with the opportunity to introduce a mobile gain medium into the fiber core, it is possible to engineer an SEFL with a dynamically tunable lasing position.
The physical parameters that need to be considered in creating a dynamic SEFL structure are the surface energies of the host solution and the inside fiber walls. The goal is to introduce a liquid into the core that does not wet the surface, such that it could be transported from one point in the fiber to another without leaving a trail. Water is an ideal host fluid due to its high surface tension and large contact angle (greater than 90°) with As2S3.
We have designed and built an SEFL that permits real-time control of the fluidic gain medium position within the fiber. Motion control is achieved from one end of the fiber by a series of electronically controlled microdispensing solenoid valves connected to a positive pressure source and a vacuum generator to allow displacement of the dye-doped water plug in both axis directions. The pump beam is coupled to the other end of the fiber, which is open to ambient pressure.
Once one has the ability to position the gain medium at any point along the fiber in real time, the entire surface area of the fiber may be used for lasing. The figure above shows three instances of laser emission from a dynamic SEFL. Note that the fiber is stationary, while the liquid plug has been positioned at three different locations within the fiber. In the figure, laser emission from the dye-doped water plug is seen reflecting from the “rLe AT MIT” logo in the background, positioned 7 cm behind the fiber.
Future applications of SEFLs
Surface-emitting fiber lasers offer unique control over the position, direction and polarization of the lasing wavefront, are inherently wavelength-scalable and can be used for the remote delivery of radial laser emission. They provide the ability to control the gain medium location, spatial extent, and concentration in a mechanically flexible fiber. Thanks to these unique characteristics, these lasers will pave the way for new and exciting applications as well as the enhancement of existing technologies.
In medical imaging, for example, the ability to perform in vivo sensing of intact organisms is of great importance. One such technique is fluorescence molecular tomography, in which the emission of near-infrared excited fluorochromes is used to tomographically reconstruct a three-dimensional organism. Another emerging technology is diffusive optical tomography, in which an object is illuminated with an array of sources while an array of detectors measures the light scattered by the object.
Dynamic radial lasing is demonstrated as the dye-doped water plug is moved along the hollow core photonic bandgap fiber. The three photos correspond to three different instances in time. The “rLe AT MIT” logo in the background and the fiber are fixed in space, while the microliter water droplet moves along the fiber. The green pump light is seen propagating along the fiber from the left and the orange surface emitting laser light is seen reflecting from the logo in the background, positioned approximately 7 cm behind the fiber.
A model of the propagation physics is then used to determine the properties of the illuminated tissue. For both techniques, using a denser array of sources can lead to a great improvement in the reconstruction resolution. The SEFL, being a flexible large-area laser that can form any shape and is effectively a large number of point sources, can enhance the capabilities of such imaging techniques.
Recently, side-emitting silica core fibers have received much attention due to potential technologies that might use large area, light-emitting fabrics. Because fibers can be shaped to arbitrary contours, side-emitting fibers may prove to be a particularly useful technology for security systems as infrared perimeters.
SEFLs are well suited for this application because they allow one to have control over the emission direction, which can increase target sensitivity in the effective security zone. In addition, the coherent radiation could be used to detect specific biological or chemical gases, which are traced by specific molecular transitions that match the laser radiation field.
Ofer Shapira, Nicholas D. Orf, Ayman F. Abouraddy, John D. Joannopoulos and Yoel Fink are with the Research Laboratory of Electronics at the Massachusetts Institute of Technology in Cambridge, Mass. Alexander Stolyarov is with the School of Engineering and Applied Sciences at Harvard University. Ken Kuriki is with Global Marketing at GE Plastics in Japan.
References and Resources
>> C. Wilmsen et al. Vertical Cavity Surface-Emitting Lasers, Cambridge University Press (1999).
>> D.A. Boas et al. “Imaging the Body with Diffuse Optical Tomography,” IEEE Sig. Proc. Mag. (Nov. 2001).
>> R. Weissleder et al. “Fluorescence molecular tomography resolves protease activity in vivo,” Nature Medicine 8, 757-60 (2002).
>> J. Spigulis. “Emitting Fibers Brighten Our World in New Ways,” Opt. Photon News 16(10), 34 (2005).
>> O. Shapira et al. “Surface-Emitting Fiber Lasers,” Opt. Express 14, 3929-35 (2006).