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The rapidly growing field of label-free biosensors demands an accurate and portable yet cheap technology. Inspired by the success of surface plasmon resonance biosensors it is investigated whether the unique light guiding properties of photonic crystal fibers (PCFs) can be made useful in this application. The presented work focuses on long-period gratings in PCFs (PCFLPGs) as these will be shown to posses the required sensitivity. Strong interaction between the sample and probing light is obtain by infiltrating the sample into the holes of the PCF. The PCF-LPG sensor is studied both experimentally and numerically. Experimentally, a setup for CO2-laser inscribed LPGs has been constructed. The setup produces LPGs with unprecedented quality and throughput. Numerically, the simulation of PCFLPGs is a demanding task and requires accurate mathematical methods such as the finite element method (FEM). The FEM is very general and can also give estimates to the attenuation constants of the lossy cladding modes as well heat transfer simulations of the rapid, intense heating and cooling during the CO2-laser inscription. As sensors PCF-LPGs are shown to detect layers of biomolecules ∼ 0.25 nm thick on average while having a refractive index sensitivity of ∼ 10−5. The PCF-LPG has a vanishing temperature cross sensitivity of ∼ 6 pm/◦C. The sensitivities are shown to be highly dependent on the PCF design, and theoretically it is shown that enhancements of these values of two orders of magnitude is realistically possible. Correct expressions for the sensitivity of PCF-LPGs for refractive index sensing, biosensing, and temperature sensing are derived and presented for the first time. The sensitivity characteristics of LPG-PCFs promises for successful label-free biosensors. Det hurtigt voksende felt labelfri biosensorer efterspørger en nøjagtig ogbærbar, men alligevel billig teknologi. Inspireret af overfladeplasmon biosensorerssucces bliver det undersøgt hvorvidt de særlige lyslederegenskaber ifotoniske krystalfibre (FKF) kan gøres brugbare til denne anvendelse. Detforeliggende studium fokuserer p°a langperiodiske gitre i FKF (FKF-LPG),idet disse vises at have den fornødne følsomhed. En stærk vekselvirkningmellem prøve og lys opn°aes ved at infiltrere prøven ind i hullerne p°a denFKF.FKF-LPG studeres b°ade eksperimentelt og numerisk. Eksperimentelt, erder blevet bygget en opstilling med en kuldioxidlaser til indprægning af LPGFKF.Opstillingen kan producere FKF-LPG’er med en hidtil uset kvaliteog antal. Numerisk er modelleringen af et FKF-LPG en krævende opgaveand kræver nøjagtige matematiske metoder s°asom finite element metoden(FEM). FEM er meget generel og kan ogs°a give svar p°a dæmpningskonstantenaf kappetilstande ligesom simulere varmeledningen under den hurtige,intense opvarmning og afkøling af FKF’en under indprægningen med kuldioxidlaseren.Som sensor vises FKF-LPG at være i stand til at m°ale et lag af biomolekyle∼ 0.25 nm tykt i gennemsnit med en brydningsindeks følsomhed p°a ∼10−5. FKF-LPG har en ubetydlig temperatur følsomhed p°a ∼ 6 pm/◦C Følsomhederne vises at være generelt meget afhængige af FKF’ens udformning,og teoretisk vises det, at forbedringer af de viste værdier medto størrelsesordener er realistisk muligt. Korrekte udtryk for følsomhedenfor brydningsindeksm°aling, biosensorer og temperaturm°alinger bliver udledt og præsenteret for første gang. Følsomhedskarakteristika for FKF-LPG forudsiger, at FKF-LPG kan blive succesfulde som labelfri biosensorer.