Microstructured fibers have been investigated extensively over most of the last decade.1,2 They have found numerous applications in areas such as nonlinear optics and power delivery. However, for practical use in environments outside the laboratory, such waveguides must be suitably protected in a robust sheath and, ideally, terminated with conventional connectors. Upon termination, microstructured fibers must be sealed to prevent ingress of moisture or debris, which would otherwise compromise their light-guiding properties and structural integrity.
Currently, after cleaving, photonic-crystal fibers (PCFs) are sealed by heating their end faces. This causes the air holes to collapse, and forms a hermetic seal. It can then be further cleaved and/or polished. The collapse length (the region between the point at which the holes have collapsed and the end face) is solid and coreless. Consequently, the field emerging from the fiber diverges by an amount that depends on the collapse length and the operational wavelength. A butt-coupled connection between two such terminated fibers will result in a coupling loss that is directly proportional to the collapse length. One can minimize the collapse length by polishing the end face. However, in practice, this is nontrivial because of uncertainties in the collapse length prior to polishing. In addition, for smaller core diameters, the collapse length needs to be further minimized (similarly as for an increase in operational wavelength). Therefore, we are seeking a method to terminate PCFs so that an efficient butt-couple connection can be effected. We have investigated the use of self-imaging in short lengths of multimode fiber (MMF), butt-coupled to PCF end faces, to assess the feasibility of realizing sealed, robust PCF patch cords. In the latter, the coupling mechanism is effectively transparent in relation to its influence on the propagating field.
Multimode-interference effects in square and circular geometries have been studied extensively for many years. They have found many practical applications.3,4 Essentially, any electric field injected into a multimode waveguide will excite a characteristic set of modes that combine in phase and intensity to exactly reproduce the input field at discrete, calculable points along the fiber. We have exploited this self-imaging phenomenon. Two PCFs, each spliced to the correct length of MMF, should demonstrate very-low-loss transmission efficiency when butt-coupled in a similar manner to conventional fiber connectors. We laser cleaved a commercially available MMF to a length of 6.86mm, i.e., the length at which self-imaging would occur for the particular MMF used. We then butt-coupled a length of commercially available PCF to one of the MMF's end faces and recorded the output profile. Self-imaging occurred as expected (see Figure 1).
Figure 1. (top) Output field from a photonic-crystal fiber (PCF) used as input to a multimode fiber (MMF). (bottom) Output field from the MMF, showing re-imaging of the input field.
The concept of such a connector effectively requires propagation through two self-imaging lengths of MMF. Therefore, we set up a length of MMF equal to twice that of the self-imaging range and, indeed, we found that self-imaging occurred. We located other lengths of MMF at the output face of the PCF and recorded the resulting profiles. We found that flat-topped and doughnut-shaped profiles result for distances not equal to the self-imaging length, confirming the critical nature of MMF length. Finally, we set up a PCF-MMF-PCF chain to replicate the proposed PCF connector (see Figure 2). We measured an overall coupling loss of 1.2dB (76% transmission efficiency), including Fresnel losses at the fiber interfaces. This is the first time that the output from a microstructured fiber has been re-imaged in a waveguide using the multimode-interference effect.
Figure 2. Microscope photograph of our PCF-MMF-PCF chain. The tape holding the MMF in place is visible across the fiber's top.
Adoption of PCFs in nonlaboratory environments requires robust and efficient termination and connectorization. Using commercially available MMF, we have shown that multimode-interference effects can be used to achieve this. Currently, this represents a proof of principle. Our next challenge will be to splice MMF to PCF within the requisite alignment tolerances, and then terminate the composite fiber end in a conventional connector. This is the goal of our ongoing research.
Craig Stacey is a senior scientist. He received his PhD in high-power optical-fiber amplifiers from Heriot-Watt University (Edinburgh, UK) in 2006. Previously, he worked at the UK's Defence Evaluation and Research Agency/QinetiQ. Craig has extensive experience in fiber-laser technology, fiber-optic devices, and laser-directed energy.
Department of Physics
Imperial College London
Roy Clarke, David Wesley Charlton
Optics and Lasers Technology
BAE Systems Advanced Technology Centre