Europium-doped optical fibers used as radiation sensors
In recent years, polymer optical fibers (POFs) have generated significant interest because they are easy to handle, flexible, and have the potential to incorporate a large variety of molecules with special optical properties. These include laser-active organic dyes and rare earth complexes.1–4 Poly-methyl-methacrylate (PMMA) is a typical polymer used to fabricate such fibers. Recently, it was shown that an europium chelate showed excellent solubility in an organic polymer host and a high fluorescence intensity at 613nm, close to the low-loss window of PMMA-based POFs located around 650nm.3,4 This is desirable because, if a fiber can propagate light in its low-loss window region, losses are minimized and propagation is more efficient.
Photonic crystal optical fibers can also be used to build highly sensitive chemical and biochemical optical sensors due to their long optical paths:5 their periodic array structure creates microscopic air holes along the entire fiber length, allowing low-loss guidance of light throughout the hollow core. However, the evanescent field of the guided light must overlap with the substance to be detected if sensing is to take place. This is difficult to achieve with the usual geometry of photonic crystal fibers: even for gases, which would have to be pumped into the fiber. A more attractive solution would be to use optical fiber that is open to the air.
Recently, we have demonstrated that this is possible by drawing a star cross-sectional PMMA optical fiber. The fiber was simply extruded through a star-shaped die. The rationale for using a star shape was that light would be confined in the core region with a higher average refractive index: like a photonic crystal fiber, but with the evanescent field capable of leaking into the open air. This, we felt, would increase the interaction with ions impregnated in the canal fiber for the whole fiber length. To test the potential of our fiber for use as an evanescent field sensor, we doped it superficially with anthracene and the europium chelate Eu(DBM)3phen.
We observed both anthracene and europium ion luminescence after lateral excitation of one fiber end with nanosecond laser pulses at 355nm. The luminescence, collected at the other end, was coupled to a multimode fiber and analyzed with an optical spectrum analyzer. Figure 1 shows the star cross-sectional PMMA fiber obtained and the blue luminescence clearly propagating along the fiber.
Figure 2 shows the luminescence spectra recorded for the bare PMMA fiber (green dotted line), the anthracene-doped PMMA fiber (dashed blue line), and the Eu3+-doped PMMA fiber (red solid line). All spectra were obtained by pumping the sample at 355nm with a Nd:YAG laser.
The green dotted line shows the typical luminescence of a PMMA fiber around 430nm with the pump laser line at 355nm, and the 532nm-peak corresponding to the 1064nm second harmonic. The blue dashed line shows a typical antracene emission profile with bands around 404, 428, and 453nm. However, the 355nm-peak is not present, meaning that the laser light was completely absorbed along the fiber. Finally, the red solid spectral trace shows both 355 and 532nm laser lines, together with the PMMA luminescence features and the Eu3+ characteristic emission from the 5D0 electronic level.
In conclusion, we have developed a novel PMMA-based star cross-sectional optical fiber that is open to the air. We incorporated Eu(DBM)3phen and antracene on the fiber surface and observed their characteristic luminescence profiles with 355nm excitation. Since their luminescence properties may be considerably modified by other neighboring molecules, these new types of doped POFs have significant potential for chemical and biochemical sensing.
The authors acknowledge financial support from CNPq (Brazilian National Council for Research) and CePOF-Unicamp (Center for Optics and Photonics), supported by FAPESP (Sao Paulo State Foundation for Research).
