An increasing interest in making sensors based on fiber Bragg gratings (FBGs) written in polymer optical fibers
(POFs) has been seen recently. Mostly microstructured POFs (mPOFs) have been chosen for this purpose
because they are easier to fabricate compared, for example, to step index fibers and because they allow to tune
the guiding parameters by modifying the microstructure. Now a days the only technique used to write gratings
in such fibers is the phase mask technique with UV light illumination. Despite the good results that have been
obtained, a limited flexibility on the grating design and the very long times required for the writing of FBGs
raise some questions about the possibility of exporting POF FBGs and the sensors based on them from the
laboratory bench to the mass production market. The possibility of arbitrary design of fiber Bragg gratings
and the very short time required to write the gratings make the point-by-point grating writing technique very
interesting and would appear to be able to fill this technological gap. On the other end this technique is hardly
applicable for microstructured fibers because of the writing beam being scattered by the air-holes. We report on
the design and realization of a microstructured polymer optical fiber made of PMMA for direct writing of FBGs.
The fiber was designed specifically to avoid obstruction of the writing beam by air-holes. The realized fiber has
been used to point-by-point write a 5 mm long fourth order FBG with a Bragg wavelength of 1518 nm. The
grating was inspected under Differential Interferometric Contrast microscope and the reflection spectrum was
measured. This is, to the best of our knowledge, the first FBGs written into a mPOF with the point-by-point
technique and also the fastest ever written into a polymer optical fiber, with less than 2.5 seconds needed.
The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm
region has been well documented to date but little has been presented on the reliability of this technology. Early
experiments highlighted that photodarkening can be a significant concern when Tm-doped silica fibers are exposed to
high intensity blue light. We present a discussion of the processes responsible for the production of blue light in Tmdoped
fibers pumped at 790nm and how fiber composition influences these processes. Through optimization of fiber
composition we have demonstrated highly efficient lasers exhibiting less than 1% output power degradation per thousand
We describe and analyze the phenomenon of partial image revivals in a multi-channel directional-coupler (MCDC)
structure. Using supermodes, a MCDC is described as a composite waveguide structure and then analyzed in terms of
multi-mode interference and the principle of self-imaging. As we bring the waveguides in the array closer together the
supermodes beat to form partial images in between the complete mirror- and self-images. In this case, we have the
advantage of the increased coupling and hence shorter image lengths, but with the disadvantage that the images are
partial revivals of the input field. Nonetheless, we can take advantage of the partial image revivals that occur near
complete mirror- and self-images, as they are almost complete images themselves. To demonstrate the use of this
phenomenon we describe the simple design of a compact 1×2 wavelength division multiplexer based on a MCDC. The
design highlights a trade-off between device length and isolation ratio, where although we use the partial self-image at
1.55 μm for a more compact device, the trade-off is a reduced isolation ratio when compared to 1.3 μm where a complete
mirror-image is present.
We have theoretically investigated twin-core all-solid photonic bandgap fibers (PBGFs) for evanescent wave sensing of
refractive index within one single microfluidic analyte channel centered between the two cores. The sensor can achieve
ultrahigh sensitivity by detecting the change in transmission. We find novel features in the sensing characteristics: the
sensitivity is higher at the short wavelength edge of a bandgap than at the long wavelength edge, the effective index of
the odd supermode (nodd) is more sensitive to ambient refractive index change compared with that of the even supermode
We report on the development of a compact, all fibre laser source operating at 1 μm with a linearly polarized (extinction ratio > 20 dB) and very narrow linewidth (12 pm) output. The unique cavity design included a fibre Bragg grating high reflector and output coupler, inscribed via the point-by-point method directly into the active core. A single splice within the cavity between the fibre incorporating the high reflector and the output coupler permitted re-orientation of the stressors at an angle of 90 degrees to each other, which produced a single lasing polarisation. This simple technique removed the need for a more complicated and expensive polarization controller.
The refractive index and thermo-optic coefficient of composite polymers of polystyrene and polymethylmethacrylate were measured at 1.55μm using an optical fiber refractometer and were found to be 1.483 and 1.570, respectively, at room temperature. The refractive index and thermo-optic coefficient of the copolymers were consistent with the rule of mixtures.
Simulations and experimental results for novel refractometric-sensing platforms are presented here. The first platform is based on a
multi-mode interference coupler (MMIC) in which the top and sidewalls of the coupler are exposed to a humidity-sensing enrichment layer. Sensor operation is based on the creation of self-images of the input field into the coupler, at regular intervals along the coupler. This phenomenon is due to interference between the optical modes in MMICs. Changes in the refractive index of the sensing layer cause predictable shifts in the position of the output image, which in turn affects the amount of light coupled into the output waveguide. Sensitivity enhancement has been demonstrated by fabricating longer MMICs capturing higher-ranking self-images, which are shifted more than the first self-image. Consequently, more significant changes in the amount of light coupled to the output are observed for a given refractive index change.
The second platform demonstrated is a Multi-Channel Directional Coupler sensor (MCDC). It differs from the MMIC in that the sensing
region now consists of multiple single-mode waveguides, which are in close enough proximity to allow light to transfer between the waveguides. Sensitivity dependance on platform length has been investigated and compared with that of the MMIC.
The devices have been fabricated by the direct laser writing process on UV curable hybrid sol-gel materials. Such materials allow
implementation of planar technology enabling integration on a silicon substrate. Future applications of these platforms include chemical and bio-chemical sensing is the areas of process, environmental and bio-diagnostic monitoring.
Semiconducting conjugated polymers have recently attracted significant interest as amplifying media for solid-state lasers due to their functional photo-physical properties and simple fabrication. Distributed feedback (DFB) cavities are proving to be the most attractive for polymer lasers, since they can combine the properties of transverse optical pumping, low threshold and practical output beams. To date, in most polymer DFB lasers the feedback is provided by second order diffraction. This has the advantage of surface emission, though it also imposes extensive scattering losses. In this work, we present the use of alternative structures that attempt to reduce the threshold of polymer DFB lasers, and also achieve dual wavelength operation. The former was addressed with cavities formed by alternative symmetries of the Brillouin zone of a square lattice. Using the diagonal ΓM symmetry first order feedback was attained. The threshold energy was thus reduced by almost an order of magnitude as compared with the more commonly used ΓX symmetry of second order square gratings. Finally, we show that two lasing wavelengths may be set independently in a semiconducting polymer laser by using a doubly periodic (i.e. Moiré) DFB grating.