Tiny Tip Couples Diode Laser to Single-Mode Fiber
The inefficient coupling of a diode laser's power into a single-mode fiber has been a major stumbling block in modern fiber optic telecommunications systems. These coupling losses are one of the motivations for the development of fiber lasers as telecommunications transmitters because fiber-to-fiber can be less lossy than semiconductor-to-fiber coupling.
When a diode laser is coupled to a fiber, a microlens is usually placed between the two to maximize the coupling efficiency, and many approaches to design and fabrication of these microlenses have been explored. As an alternative, researchers at Centre National de la Recherche Scientifique in Troyes, France, have demonstrated that a tiny tip, grown on the end of a single-mode fiber, provides one of the highest coupling efficiencies yet reported between a diode laser and a single-mode optical fiber.
Figure 1. In (left), researchers grew a tiny polymer tip on the cleaved facet of a 125-µm-diameter single-mode telecom fiber. In (right), an enlargement shows the tip's radius of curvature to be ~3 µm.
They grew the tip by photopolymerization of a photosensitive liquid deposited on one facet of the fiber. A few microwatts of 542-nm light launched into the other end of the fiber polymerized the volume of the liquid immediately adjacent to the fiber's core (Figures 1a and 1b).
They found that they could control the radius of curvature of the tip by varying the exposure time: They obtained radii of curvature ranging from less than 1 µm to approximately 11 µm by varying the exposure time from less than a second to 20 s. For longer exposure times, the entire drop deposited on the fiber's facet became polymerized, and the radius of curvature approached 100 µm.
Figure 2. By forcing the four-lobed LP21 mode to propagate in the fiber, the scientists grew a four-pronged tip on the end of the fiber.
The infrared single-mode fiber supported multiple modes at the green photopolymerization wavelength, but by mechanically stressing the fiber, the researchers were able to select a single propagation mode. Under certain conditions, for example, they could polymerize the liquid with light in the fiber's linearly polarized LP21 mode, creating a four-pronged tip (Figure 2).
Of greater interest for coupling diode-laser output into a single-mode fiber, however, was a single tip similar to the one in Figure 1. They positioned the tipped fiber in front of the laser, a commercial multiple-quantum-well InGaAsP diode that produced 9.5 mW at 1310 nm (Figure 3), and experimented with tip radii of curvature and with distances between the tip and the laser. The researchers obtained the best results when they placed a tip with a 2.4-µm radius approximately 4 µm from the laser.
Figure 3. Positioning the fiber with its tiny tip in front of the laser, the researchers coupled as much as 70 percent of the diode laser's output into the single-mode optical fiber.
Under these conditions, 70 percent of the light emitted from the diode was coupled into the fiber, a value they believe to be comparable to the best results of any other coupling technique. They hypothesize that the 1.5-dB loss was primarily a result of geometrical mode mismatch between the laser and the tip and between the tip and the fiber.
To model the experimental results, they used a two-dimensional, finite-element method and inserted the physical parameters of their experiment. The model predicted that optimal coupling between the laser and the fiber would occur when they were separated by 4 µm, consistent with the experimental findings.
MORE FROM PHOTONICS MEDIA