Comparison of the characteristics of the most relevant FBG based refractometers
The in situ and real time measurement of a variety of chemical and biological parameters is important in diversified environments ranging from industrial processes, medicine to environmental applications. In this context, the demand for novel sensing platforms capable of multiplexing, real time and remote operation in electromagnetic or chemically hazardous environments has increased significantly in recent years.
The combination of fiber optic technology with optical sensing mechanisms has many benefits that make it a promising alternative to standard technologies. Immunity to electromagnetic interferences, small size, and capability for in-situ, real-time, remote, and distributed sensing are some of the most appealing characteristics that motivate a growing scientific community.
Biochemical sensing typically requires that optical signal interacts with the external media, either directly with a given analyte or through an auxiliary membrane, which contains an indicator dye. Some of the most appealing techniques regarding sensitivity and specificity rely on the use of colorimetric or fluorescent indicator dyes. Although some of the intrinsic problems of indicator based sensor like, leaching, photobleaching and temperature dependence have reported solutions, some limitations restrict further developments. A variety of excitation sources, detectors and filters are needed to deal with the large variety of spectral characteristics of dye based sensors. Moreover, these wavelength ranges demand for the use of special optical fibers and optoelectronics, severely limiting its compatibility with the standard telecom optical fiber technology.
In this context, label free optical sensing based on the measurement of refractive index (RI) represents an interesting solution. Such approaches do not interfere with the analyte properties and require, instead, the design of sensitive layers that experience a refractive index change in its presence. This can be achieved by using biomolecules with a natural affinity to the target, or chemical species having analyte specific ligands. The combination of such membranes with refractive index sensors can therefore provide attractive solutions for biochemical sensing.
The aim of this chapter is to expose the basic principles of evanescent field based fiber optic refractometers, suitable to biosensing field and capable to remote and real time operation. Initially, the principles of the technology are described. Thereafter, recent progress in the area is presented where several fiber optic devices will be detailed, ranging from the popular fiber Bragg gratings, the well known long period gratings, a variety of modal interferometers including tapers, mismatched fiber sections and also multimode interference based structures. Emphasis will be given to the description of the sensing structures and its sensing mechanism, advantages and disadvantages and wherever possible, the sensing performance of each sensing device will be compared in terms of sensitivity and detection limit.
2. Fiber optic refractometers: Principle
Optical fiber consists of a core and a cladding with different refractive indices. The refractive index of the core
Since light is totally reflected inside the core, no electromagnetic field is propagating in to the cladding. Nevertheless, the electromagnetic field actually penetrates a short distance into the lower refractive index medium, propagating parallel to the interface core-cladding and decaying exponentially with the distance from the interface (See figure 1). The physical explanation for this phenomenon is that when applying Maxwell equations to the interface between two dielectrics, the tangential components of both the electric and magnetic fields must be continuous across the interface, this is, the field in the less dense medium cannot abruptly become zero at the interface and a small portion of light penetrates into the reflecting medium. This boundary condition can only be satisfied if the electromagnetic field crosses the interface, creating the so-called evanescent wave . The penetration depth
The majority of the fiber refractometers are based on evanescent field interactions. However, fibers were originally designed for optical communications. A typical single mode optical fiber has a core diameter between 8 and 10.5 µm, a cladding diameter of 125 µm and light propagates confined in the core. Therefore, the penetration depth is far smaller than the cladding thickness and there is almost no interaction between the optical signal and the external medium. Strategies must be devised in order to provide interaction with the surrounding medium. Typically, the evanescent field can be exposed by removing partially or totally the cladding of the optical fiber. This can be done by chemical etching, tapering or side polishing techniques. In alternative, it is possible to use specific tools capable to transfer energy from the fundamental core mode to cladding modes. Fiber gratings are an example of these devices. In such a cases the optical radiation can interact with the external environment due to the evanescent field formed at the cladding/external medium interface. In this case, the penetration depth is given by:
Fiber optic biochemical sensors based on evanescent field configurations rely on the use of sensing layers deposited on the sensitive surface that experience a refractive index change in presence of an analyte. This can be achieved by using biomolecules with a natural affinity to the target, or chemical species having analyte specific ligands. When exposed to an analyte, a chemical/biochemical interaction takes place within this layer or on its surface. In this case, only a portion of the optical radiation which comes out of the sensor (evanescent field) is modulated, depending on the thickness of the interaction region.
Biological sensing is based on the specific binding between biorecognition molecules (antibodies, oligonucleotides, aptamers or phages) immobilized on the sensor surface and the targeted biological species, which causes a change in the effective thickness or density of the surface of fiber and consequently a change on the optical signal. Figure 2 conceptually shows an example of label free fiber optic biosensor. A functional coating is used to support and enhance the attachment of the bioreceptor molecules, which bind the analyte .
In the following sections the most relevant fiber refractometric platforms based on evanescent field interactions and capable for label free biochemical sensing will be presented, including their measurement principle and some examples of most important works presented till now.
3. Fiber Bragg gratings
Fiber Bragg grating (FBG) sensors have generated great interest in recent years because of their many industrial and environmental applications. FBGs are simple, versatile, and small intrinsic sensing elements that can be written in optical fibers and which consequently have all the advantages normally attributed to fiber sensors. In addition, due to the fact that typically the measurand information is encoded in the resonant wavelength of the structure, which is an absolute parameter, these devices are inherently self-referenced. Moreover there are several intrinsic advantages associated with FBG technology such as reflection operation mode, narrowband spectral response and their compatibility with standard telecom technology, therefore can be easily multiplexed, which is particularly important in the context of remote, multi-point and multi-parameter sensing . Based on diffraction mechanism, they consist on the periodic perturbation of the core of the optical fiber (typically half-wavelength) that is induced by exposing the fiber to an interference pattern of UV light or femtosecond radiation. They are characterized by the periodicity
The full width at the half maximum (FWHM) of the resonant peak of the Bragg grating is typically a few hundred picometers. It depends on the physical length of the grating, which is usually few millimeters. Figure 3 illustrates the principle of operation of an FBG. When a broadband optical signal reaches the grating, a narrow spectral fraction is reflected and the remaining is transmitted. The peak wavelength of the reflected signal is defined by the Bragg resonance wavelength.
FBG sensors have been widely used for strain and temperature measurement . Bragg gratings works mainly with radiation confined to the fiber core, this way strategies have to be devised in order for the radiation to interact with the external medium. Typically, FBG based refractometers rely on the evanescent field of the core modes under fiber etching conditions, which enables interaction with the surrounding medium.
The first demonstration of an FBG as a refractometer was done in 1997 by Asseh
Regarding sensitivity enhancement and temperature compensation, in 2001 Schroeder
A simpler solution for thermal compensation was published by Iadicicco
In the past few years, microfibers have attracted increasing interest due to their intrinsic advantages such as large evanescent field, small effective mode field diameter and low-loss interconnection to single mode fibers. Microfibers can be produced by the use of standard flame brushing technique. Bragg gratings written in microfiber have been also explored for refractive index sensing. In 2010 Fang
Etched FBG, side polished FBG or microfiber Bragg gratings are interesting devices that exploit the influence of the surrounding refractive index (by the evanescent field interaction) on the effective index of the core mode, and consequently on the Bragg wavelength (
A different approach to develop fiber optic refractometers based on FBG technology was proposed in 2001 by Laffont
The TFBGs used in the experiment of Laffont
TFBGs are a suitable option for refractometric sensing in terms of performance and robustness of the fiber structure. However, a TFBG couples the core mode to a number of cladding modes in a large wavelength bandwidth, which renders difficult the signal readout and multiplexing. In addition, the fact that the measurement must be made in transmission, requiring access to the sensor from both sides, can represent a difficulty in some applications. Recently a few authors have been exploring the possibility to excite the cladding modes of standard FBG by transferring power from the fundamental core mode to the cladding modes in the upstream of the FBG. Thereby, the FBG will couple back the light to the fundamental core mode. This arrangement enables the possibility to read the cladding mode of the Bragg grating in the reflected spectrum
In 2010 Han
Owing to reflective nature of this devices a few FBG based Fabry-Perot cavities were presented for refractive index measurement. In 2005 Liang
|Etched FBG||Spectral Shift
|Polished FBG||Spectral Shift||2001||Near 1.45||300 nm/RIU||10-6(*)|||
|Microfiber FBG||Spectral Shift
|Spectral Shift||2007||Near 1.32||11.2 nm/RIU||10-4|||
|LPG/FBG||Spectral Shift||2010||Near 1.45||2.32 nm/RIU||10-4(*)|||
|MMF/FBG||Spectral Shift||2010||1.40-1.44||7.33 nm/RIU||10-4(*)|||
|FP-FBG||Spectral Shift||2005||Near 1.33||71.4 nm/RIU||1.4×10-5|
Several FBG based refractometers have been described rely on the measurement of the refractive index changes for the measurement of sucrose, salt, ethylene glycol, Isopropyl Alcohol among others [5-7]. Using functional layers just few works were presented. The first demonstration of the concept of biosensor based on FBG, was done by Chryssis
4. Long period fiber gratings
A Long period grating (LPG) is one of the most popular fiber optic refractive index sensor and it has been widely used for chemical and biological sensing. Like FBG, LPG is also a diffraction structure, where the refractive index of the fiber core is modulated, with a period between 100µm to 1000µm that is induced in the optical fiber using different techniques: UV laser irradiation, CO2 laser irradiation, electric-arc discharge, mechanical processes and periodic etching . This periodic perturbation satisfies the phase matching condition between the fundamental core mode and a forward propagating cladding mode of an optical fiber. Thereby, in an LPG, the core mode couples into the cladding modes of the fiber, resulting in several attenuation bands centered at discrete wavelengths in the transmitted spectrum, where each attenuation band corresponds to the coupling to a different cladding mode. The spectral width of the resonant dip varies from few nanometers up to tens of nanometers depending on the physical length of the grating.
LPGs are intrinsically sensitive to external refractive index exhibiting changes in the position of the resonance wavelength. The resonant wavelength of light coupling into a particular cladding mode is given by the phase matching condition :
Where Λ is the grating period, and are the effective indexes of the core and
The sensitivity of an LPG is then typically defined as a shift of the resonance wavelength induced by a measurand. The sensitivity characteristic of a bare LPG to surrounding refractive index changes has an increasing (in modulus) non-linear monotone trend. The result is that the maximum sensitivity is achieved when the external index is close to the cladding index while for lower refractive indices (around 1.33) the LPG is scarcely sensitive. Figure 7 shows the behavior of resonance wavelength and its optical power to refractive index changes. The behavior changes when a thin layer of sub-wavelength thickness (few hundreds of nanometers) and with higher refractive index than the cladding is deposited thereon. The use of high refractive index (HRI) overlays in fiber optic sensors refractometers based on evanescent wave was explored initially by Schroeder
The HRI overlay draws the optical field towards the external medium extending its evanescent wave. As a result there is an increased sensitivity of the device to the surrounding RI. Due to the refractive-reflective regime at the cladding-overlay interface, the cladding modes in a HRI coated LPG are bounded within the structure comprising the core, the cladding and the overlay. This means that a relevant part of the optical power carried by the cladding modes is radiated within the overlay. The field enhancement in the overlay depends strongly on the overlay features (thickness and refractive index) and the SRI. For a fixed overlay thickness and refractive index, by increasing the SRI, the transition from cladding to overlay modes occurs: the lowest order cladding mode (cladding mode with highest effective refractive index) becomes guided into the overlay. At the same time, the higher order modes move to recover the previous effective indices distribution. This is reflected through the phase matching condition in the shift of each attenuation band toward the next lower one . Resulting from this modal transition that the attenuation bands can exhibit a sensitivity of thousands of nanometers per refractive index unit.
High order cladding modes that strongly penetrate the external medium, on the other hand, offer higher sensitivity, and obviously these are the most desirable for sensing purposes. An increase in the order of the coupled cladding mode is obtained by decreasing the grating period . Pilla
LPGs show great sensitivity to the surrounding RI, but also at the same time to temperature. In the other hand, the measurement of the refractive index is strongly dependent on the temperature due to the thermo-optic coefficient. Thus, measurement and compensation of this parameter is an important issue for this kind of platforms. A number of techniques have been proposed in order to get rid of the temperature cross-sensitivity mainly based on the use of a second grating sensitive only to temperature [35, 36].
LPG based interferometers have shown higher resolution to refractive index measurement compared to the use of a single LPG. The advantage of using those structures lies on their interferometric nature and its principle of operation, where the coupled core and cladding modes from one LPG combine again at a second matched LPG to form interference fringes. The core and cladding paths constitute the arms of an all fiber Mach–Zehnder interferometer (see figure 8) . In 2002, Allsop
More recently, in 2010 Mosquera
Long period gratings are the most popular fiber optic sensor for label free sensing, since in 1996 Bhatia
LPGs coated by functional layers have been successfully exploited for chemical sensing. Gu
LPG has been widely used for biochemical sensing; on this case a biomolecule with affinity to a target can be used as functional coating. The earliest demonstration of biomolecule detection using this structure was done by DeLisa
LPGs applied for label free detection of specific bacteria using physically adsorbed bacteriophages were presented for the first time by Smietana
Lately, an enzyme coated LPG was used for glucose detection by Deep et al . The authors demonstrated the successful immobilization of glucose oxidase on to the 3-aminopropyl-triethoxysilane (APTES) silanized LPG fibers for the development of a new glucose sensing technique.
5. Modal interferometers
Fiber modal interferometers have recently concentrated the focus of research because of their potential sensing capabilities and in some cases the reduced cost and simplicity of fabrication. In the previous section an LPG based modal interferometer was introduced. The LPGs were used as mechanism to couple light from core to cladding and subsequently from cladding to core. There are different mechanisms through which the high order modes could be selectively excited, by tapering a single mode optical fiber, through a core diameter mismatching structure (larger or thinner) or by a simple misaligned splice. Other kind of devices relies on multimode interference, in such a cases a small section of multimode fiber is properly inserted between single-mode fibers. The aim of this section is to describe the sensing mechanism of this kind of devices and to address the most relevant contributions for chemical and biosensing field.
5.1. Tapered single-mode fiber
Tapering a single mode fiber involves reducing the cladding diameter along with the core and it is made by heating a section of the fiber and pulling on both ends of the fiber in the opposite directions, either under a constant speed, force or tension. The heat source can be a gas burner flame, a focused CO2 laser beam or an electric arc formed between a pair of electrodes. When the optical fiber is tapered, the core–cladding interface is redefined in such a way that the light propagation inside the core penetrates to the cladding and it is confined by the external medium.
A fiber taper consists of three contiguous parts: one taper waist segment with small and uniform diameter, and two conical transition regions with gradually changed diameter. Depending on the pulling conditions it is possible to fabricate tapers with different shapes and properties. Fiber tapers may be divided into two distinct categories: adiabatic and non-adiabatic. An adiabatic fiber taper is characterized by a very smooth change in the profile (small taper angle) in order to ensure a smooth mode conversion without significant losses in the transmitted signal. In this case, the main portion of the radiation remains in the fundamental mode (
On the other hand, non-adiabatic fiber tapers (abrupt taper angle) can be done in such a way that coupling occurs primarily between the fundamental mode of the un-pulled fiber and the first two modes of the taper waveguide (
Fiber refractometer based in non-adiabatic tapers has been proposed recently as platform for label free sensing. Zibaii
5.2. Core mismatch
Abrupt Tapered devices show high sensitivity to refractive index measurements. However, after the tapering, due to reduced fiber diameter, the structure becomes very fragile and special handling is needed. Recently, a different approach based on core mismatched sections have been investigated. In this case, mismatched sections are proposed as valid alternatives as mode-coupling mechanisms to transfer optical power between core and cladding modes in optical fiber. The idea is to couple and recouple the fundamental mode and high order cladding modes through two mismatched sections. It can be done by using a misaligned splice or a short section of a special fiber.
A core offset splice based refractometer was presented by Tian
The core-offset technique presents difficulties to control the amount of light power splitting. In alternative, Pang
The idea of fiber a core diameter mismatch (CDM) based interferometer for refractive index sensing has been reported by Rong
5.3. Multimode interference
Modal interference involving more than two modes has also been studied, resulting into a spectral transfer function that is no co-sinusoidal but instead show sharp peaks at specific wavelengths. It is common to refer this approach as multimode interference (MMI). MMI in optical fiber devices is usually obtained by splicing a MMF section between two single mode fibers, thus forming a SMF-MMF-SMF (SMS) fiber configuration. Based on multimodal interference and the self-imaging or re-imaging effect, the SMS structure acts as an optical band filter that has been widely explored for optical communication and sensing applications.
The SMS fiber concept relies on the fact that when the light field coming from the input SMF enters the MMF, exciting several high order modes, generating a periodic interference pattern along the MMF section. Depending on the wavelength and geometrical length, the light into de MMF can interfere constructively or destructively resulting, at the end, in a device with different spectral characteristics. Therefore the length of the MMF determines the spectral features of the MMI device. Depending where the interference pattern is ‘intersected’, constructive or destructive interference results, at different wavelengths yielding the transmission of resonant peaks or resonant losses respectively. The transmitted spectral power distribution is, therefore, highly sensitive to the optical path length of the MMF section. It is important to refer that in MMI devices based on standard MMF, the optical signal does not access the external medium. Therefore, they are insensitive to the SRI. MMI based refractometers usually relies on etched cladding MMF, tapered MMF or coreless multimode fibers (CMF). Figure 13 shows conceptually a SMS device based in a CMF, where constructive interference is present resulting in a resonant peak in the transmitted spectrum.
MMI fiber devices are very attractive due to their high potential for refractive index sensing. In 2006, Jung
Good sensitivity, ease to fabricate and possibility to build robust devices are some of the advantages of SMS structures for label-free sensing. However, these structures produce a broad optical band spectrum, resulting in a small Q factor and thus poor resolution in the measurement of spectral shift. Concerning improvements in the interrogation schema, Lan
A different approach for multimodal interference devices was presented by Xia
In this chapter a review of evanescent field based refractometric platforms for label free sensing was given. Several aspects regarding the implementation of label free biochemical sensors using standard optoelectronics were address. Different structures were described, including fiber gratings, modal interferometers and multimodal devices. Emphasis was given to the description of fiber optic device and their sensing mechanism, advantages and limitations and the sensing performance of each sensing technology was evaluated. Table 2 summarizes the main features of the refractometric configurations.
||Well developed technology
||Well developed technology||Low sensitivity
Low temperature cross-sensitivity
Optical fiber gratings, including fiber Bragg gratings and long-period fiber gratings, have also been explored for refractive index sensing. They consist in a periodic modulation of the refractive index of the core of the fiber, where the LPG’s period is much longer (hundreds of microns) than the FBG’s period (typically a half-wavelength). This structural difference results in devices with fundamentally different properties. FBGs work mainly with radiation confined to the fiber core, in this way strategies have to be devised in order for the radiation to interact with the external medium. Typically, FBG based refractometers rely on the evanescent field of the core mode under fiber etching conditions. FBG based configurations are more attractive for the purpose of multipoint sensing due to their very narrow spectral response. Nevertheless, the etching process introduces fragility in the fiber sensor. Tilted FBGs do not require etching therefore maintain the fiber integrity. Although, a TFBG couples the core mode to a number of cladding modes in a large wavelength bandwidth, it renders difficulty for signal readout and multiplexing. The refractive index sensitivity to these devices (FBG and TFBG) in the biological range is quite low which means that these devices are not very promising for field of biosensing.
Long period gratings (LPG), on the other hand, provide evanescent interaction by exciting cladding modes, and are therefore intrinsically sensitive to external refractive index changes. They maintain fiber integrity and probably represent the most popular device for label free sensing. They present high sensitivity to refractive index measurement, which can be increased and tuned by using HRI overlays. The HRI overlay draws the optical field towards the external medium extending its evanescent wave. As a result there is an increased sensitivity of the device to the SRI. The field enhancement in the overlay depends strongly on the overlay thickness and refractive index. This technique allows the coupling of the optical design and sensitivity optimization of the device, together with the functionalization. The careful design by means the proper choice of the grating period, the overlay RI and a very controlled deposition method, together with the integration on the HRI of sensitive materials or biological active agents, provide a powerful platform for advanced optical label free biochemical sensing. However, LPGs are also highly sensitive to temperature, they need an extra mechanism to compensate temperature changes.
LPG interferometers based on Michelson or Mach-Zehnder layouts or even Fabry-Perot intracavity were also demonstrated showing high sensitivity when compared with single bare LPG, and great potential for the biosensing applications. Nevertheless, the device length (few tens of centimeters) can be a constraint for some applications. Fiber tapers, due its highly reduced cladding diameter have an enhanced evanescent interaction and have long been explored for refractive index measurements by monitoring the transmitted optical power. In spite of high sensitivity and very compact size (few millimeters), however, these structures are very fragile and special packaging is needed.
On the other hand, new configurations using special fibers provide new sensing opportunities. Modal interferometers based on core diameter mismatch, by using thin core fibers or multimode fiber used as cladding coupling mechanism have shown good sensitivity, ease of fabrication and potential low cost. Nevertheless, these configurations are difficult to reproduce and to control the mode excitation and the amount of power transferred.
Multimode interference based refractometers are also interesting solutions that rely on the concept of re-imaging effects of MMI patterns present in multimode waveguides. In these devices, the transmitted spectral power distribution is highly sensitive to the optical path length of the multimode fiber and its SRI. Usually based on singlemode-multimode-singlemode structures, they can be easily fabricated and applied in different situations. However, these configurations are also difficult to reproduce and present very broad spectral resonance making for instance multiplexing a very difficult task. The table 2 shows the most relevant evanescent field based fiber refractometers.
Overall, evanescent field fiber refractometers are very attractive due to their immunity to electromagnetic interferences, small size, and capability for in-situ, real-time, remote, and distributed sensing. Most of the applications, however, focus on the measurement of parameters such as the concentration of ethylene glycol, sucrose, salt, ethanol, among others. Nevertheless, this approach is not the most reliable due to the possible interference of other species present in the solution, which are different from the analyte of interest. Thus, the use of sensitive materials containing biomolecules with a natural affinity to the target, or chemical species having analyte specific ligands, has increased, mainly based on LPGs. Several works were reported regarding antibody-antigen interaction and also DNA hybridization. Regarding chemical application several sensing probes were presented to measure pH, Ethanol vapor, ammonia.
||Normalized Optical Power||-||2x10-5|||
AcknowledgmentsThis work was partially supported by COMPETE program and FCT by funding project n.º FCOMP-01-0124-FEDER-019439 (Refª. FCT PTDC/AGR-ALI/117341/2010). Carlos Gouveia would like to acknowledge the financial support of FCT (SFRH/ BD/ 63758/ 2009)
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