Research in organic solid and polymer-based optoelectronic devices is driven by the physical flexibility, low cost, processability, and almost infinite spectrum of functional possibilities offered by organic materials. Many active components currently used in fiber-optic communication system reply on the use of inorganic semiconductors and mechanical means, which makes each of the components bulky, rigid and independent. Photonic on-chip integration is extremely difficult with current components and materials. Thus, there is a need for a new and emerging class of organic materials for use in a range of important optoelectronic components, which enables improvement and revolution in device performance, device miniaturization and on-chip integration.
Optical fiber fluorescence sensors are widely used for chemical sensing. Intrinsic fluorescence sensors such as evanescent-wave sensors can be used for surface sensing while extrinsic sensors are used for bulk sensing. In intrinsic/ extrinsic fluorescence sensors, typically the side-wall/end-face of a single/multi-mode fiber is used. In this paper we introduce a novel intrinsic optical fiber fluorescence sensor which uses the side-wall of a large-core multi-mode fiber to collect the fluorescence from the bulk of samples. The method is to use a corrugated long period fiber grating, fabricated by a CO2 laser, to couple radiating modes to the core modes. Using the side-wall of a fiber helps to fabricate a multitarget or a distributed sensor. We also report the sensing of lead ions with concentration as low as few ppb in water using a novel fluorescence turn-on polymer as a chemical probe.
This paper is devoted to examining the ability of a coaxial fiber-optic sensor (FOS) in detecting weak fluorescent light
and weak fluorescence “turn-on” in the presence of trace heavy metal ion Pb2+. The captured fluorescent signal is
detected by the Ocean Optics QE65000 spectrometer. The stock solutions include Pb2+ acetate in water (0.01 M) and a
small molecule probe in water. The preliminary experiment shows that this FOS offers the Pb2+ detection limit (DL) of
1.26×10-4 mg/mL. The advantages, limitations and further improvements of this coaxial FOS are discussed in
comparison with the bench-top instruments in terms of the abilities of signal light capture and stray excitation light
suppression.
Based on the lab-on-a-fiber (LOF) concept we proposed before, we further optimize its architecture while preserving its
capability in fluorescent signal collection and excitation stray light rejection. This LOF device is a short fiber taper with
a TNT sensory film overlay at one end of a 400 μm core fiber which is approximately 50 mm long. The optimized LOF
also lowers the system cost, eases the fiber replacement and maintenance, which are enabled by a reusable 3-leg
bifurcated fiber bundle with SMA connectors to connect LOF, an excitation light source and a spectrometer. This LOF
device occupies only a Φ0.4 mm × 1 mm space.
In this article a novel advanced fiber-optic fluorescent sensor is demonstrated. The sensor is based on collection of the
fluorescence from the sidewall of the multimode optical fiber which is partly de-cladded and covered by the sample
under the test (SUT). The most part of the fluorescent intensity is carried by the leaky rays which are inaccessible in
traditional evanescent-wave fluorescence fiber sensors. In the proposed structure, some part of a refracting power is
collected in the de-cladded segment and used to excite the lower order lossless modes in cladded part by a sidewall mode
mixer. In addition to the higher level of fluorescence collection, the architecture allows us to multiplex several different
channels along one fiber, since we use only a small segment of the normal (not tapered) sidewall for each channel.A
highly efficient fluorescence turn-on molecular probe is applied to this advanced fiber-optic structure, for sensitive and
selective detection of Cu+2 in water. The fluorescence turn-on molecular probe is a mixture of a fluorophore polymer
P1and M1 as a Cu receptor and a fluorescent quencher. The P1 is used as an indicator which generates the fluorescence
centered at the wavelength of 650 nm and then, with a proper amount of M1 solution, the fluorescence is quenched up to
53% of its maximum intensity. The P1-M1 pair is broken by absorption of Cu with the M1 and the fluorescence is
released again. This turn-on effect is used for detection of Cu with a low detection limit of 0.02689 g/ml.
Identification of trace-vapor TNT explosive emanating from mines or IEDs is in urgent demand due to today's mounting
terrorist threat. Amplifying fluorescent polymer (AFP) based on quenching offers an opportunity for progress in this
regard. Maximizing the performance of this material and meeting the requirements of real-life situations, however, pose
challenges to the sensing platform design. We address these challenges by establishing a multimode fiber-optic sensing
platform with a millimeter-scale sensing element and our alternative TNT sensory polymer to AFP. Theoretical
calculation is given, demonstrating the highly efficient use of the excitation power, which allows for the sensing head to
be as miniature as 0.8×0.8×1.6 mm. The exposure of our sensor to the TNT vapor gives a fast response, at the level of
30% power quenching occurring within 10 seconds. Another major advantage of our solution is that the mechanism of
stray excitation light is eliminated naturally due to the design of the platform architecture.
By depositing an amplifying fluorescent polymer (AFP) directly onto the core side wall of an optical fiber near the fiber
tip, a functional fiber-optic sensing platform is created at a scale of a mere 0.8×0.8×1.6 mm3, including the second fiber
tip for excitation light delivery. The device integrates several functional optical components, a chemical sensory film and
the necessary laboratory procedures on a minute scale. Here the Lab-on-a-Fiber (LOF) platform is conceptually
introduced and proven to be a high-performance and low-cost approach to detection of trace vapors of TNT explosives.
The low-cost potential is achieved by straightforward system construction and simple procedures for the AFP film
deposition. The high performance is achieved by a dramatic increase of fluorescence emission signal collection, virtually
complete suppression of excitation stray light and the fast response to the presence of TNT vapor, which is illustrated by
30% of quenching percentage occurring within 10 seconds.
There is an increasing demand for the efficient collection of fluorescent light emitted from a very thin layer of polymer
material such as the specifically synthesized polymer material used in TNT explosives detection. However, the
enormous transparency of this thin film poses a severe challenge for any light detection system based on a traditional
approach. For a simple two-fiber architecture with one fiber for excitation light delivery and another for emission
collection, we report that by launching the excitation light to one corner edge of a planar glass substrate covered with a
thin layer of polymer film, we are able to simultaneously dramatically enhance the collectable fluorescent signal level
and reduce the level of stray excitation light. The proposed sensing architecture opens up an efficient way of light
coupling and collecting for fluorescent-related chemical and biological sample assay.
A three-fiber probe is fabricated for the measurement of fluorescent light emitted from a flat and tilted membrane cast on a glass substrate. With two identical photonic crystal fibers (PCFs) as the receiving fibers, we demonstrate that a segment of pure glass rod formed at the front end of a large-core PCF has an enhanced capability of collecting fluorescent light. The percentage of this increase depends on the distance between the membrane and the fiber probe. An initial theoretical investigation reveals that a longer fused segment can shift the light reception cone towards the symmetric axis of the illuminating fiber where the maximum energy locates. Higher emission is expected in this area and would be able to reach this shifted reception cone. This area is a dead zone for the PCF without or with a shorter fused segment. The possible applications of this fused glass segment with a proper length are highlighted.
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