In this paper, we report on an offshore field validation of a FBG based optical fiber sensor for simultaneous monitoring of hydrostatic pressure and temperature. The sensor consists of a femtosecond laser induced grating written in a Butterfly microstructured fiber. The sensor has an extremely low cross-sensitivity between temperature and pressure which makes it ideal for monitoring large transients in pressure or temperature, like is the case in wireline intervention. Pressure and temperature readings from the FBG based optical fiber sensor are compared with the readings from a battery powered electrical quartz gauge during an offshore wireline intervention job in an oil well. Good agreement was found between both measurements
In this paper, a packaged FBG based optical fiber sensor written by femtosecond laser pulses in highly birefringent micro-structured optical fiber (MS-FBG sensor) is presented and validated for simultaneous pressure and temperature monitoring. The MS-FBG sensor is capable of separating the temperature information from pressure information without the need for an additional transduction mechanism and this with a negligible pressure-temperature cross-sensitivity. However, in order to use the sensor for downhole applications, a ruggedized sensor housing is required that not only offers mechanical protection to the fiber, but also provides pressure transfer from the well fluid to the sensing element without inducing an additional pressure-temperature cross-sensitivity. In this article, the design of the sensor housing is reported as well as the lab-scale validation up to a temperature and pressure of 150 °C and 700 bar, respectively.
The applicability of fiber Bragg gratings written in highly birefringent Butterfly micro-structured optical fibers (MSFBG) for simultaneous High Pressure/High Temperature monitoring without a significant pressure/temperature cross sensitivity have recently been shown. This makes these MS-FBG sensors extremely interesting for downhole monitoring in the Oil and Gas industry. However, an important effect to be taken into account for these applications is the presence of hydrogen, as hydrogen is known to diffuse into the fiber structure and therefore might affect the wavelength responses of the sensor element. In this paper, the effect of hydrogen gas on the MS-FBG sensor readings by monitoring the wavelength changes of the MS-FBG sensor in a hydrogen rich environment have been investigated.
In this experiments, two MS-FBG sensors were placed in a hydrogen test chamber: one with its fiber end sealed for pressure sensing and the other with its fiber end kept open for referencing purposes. It could be demonstrated that both sensors show a similar wavelength shift after some time and that due to the hydrogen diffusion, the pressure in the airholes of the sealed MS-FBG sensor equalizes the hydrogen pressure in the chamber. Furthermore, it could be demonstrated that the refractive index seen by the waveguide of the fiber is also affected. Based on all these observations, the influence of the hydrogen on the temperature and pressure measurement performance of the MS-FBG sensor is estimated, and a mitigation scheme that partially compensates for this influence is discussed.
In this paper, we demonstrate that femtosecond laser pulse written fiber Bragg gratings (FBGs) fabricated in specialty highly birefringent micro-structured optical fiber (MSF) can be used for high pressure and high temperature monitoring in downhole applications. The design of the micro-structure allows encoding the pressure information into the spectral separation between the two Bragg peaks reflected by the obtained MS-FBG. We obtained a differential pressure sensitivity of 3.30 pm/bar over a pressure range from atmospheric up to 1400 bar and at temperatures between 40 °C and 290 °C. Owing to the negligible differential pressure-temperature cross-sensitivity of 6.06E-3 bar/°C, the proposed MSFBG sensor is an ideal candidate for pressure monitoring in the presence of high temperature transients.
Optical sensors based on Fiber Bragg Gratings (FBGs) are used in several applications and industries. Several inscription techniques and type of fibers can be used. However, depending on the writing process, type of fiber used and the packaging of the sensor a Polarization Dependent Frequency Shift (PDFS) can often be observed with polarized tunable laser based optical interrogators. Here we study the PDFS of the FBG peak for the different FBG types. A PDFS of <1pm up to >20pm was observed across the FBGs. To mitigate and reduce this effect we propose a polarization mitigation technique which relies on a synchronous polarization switch to reduce the effect typically by a factor greater than 4. In other scenarios the sensor itself is designed to be birefringent (Bi-FBG) to allow pressure and/or simultaneous temperature and strain measurements. Using the same polarization switch we demonstrate how we can interrogate the Bi-FBGs with high accuracy to enable high performance of such sensors to be achievable.
This paper outlines a demodulation technique developed for low-bandwidth, high sensor density fiber Bragg grating (FBG) applications. Currently there are no such demodulation techniques that can be easily scaled to large networks of sensors. The technique takes advantage of known differences in FBG spectral profiles to uniquely identify each multiplexed grating. Known grating profiles are individually cross-correlated with the measured spectrum to locate each Bragg peak. Cross-correlation was used because of its rapid processing speed. This paper covers preliminary experimental validations to identify accuracy limits, as well as investigations into a correction factor for improved accuracy.
This paper demonstrates a combined packaging and optical coupling scheme for optoelectronic devices in short distance
optical communication systems. The proposed scheme allows an ultra short optical path between the optoelectronic
component and the optical waveguide entry. This is achieved by embedding the bare die optoelectronics in the substrate
of the optical system. The positioning and alignment of the embedded dies is performed with a scalable passive
alignment process based on physical alignment studs which are manufactured with standard photolithography combined
with the use of Moiré interference patterns for precise alignment.
This paper describes the implementation of a low-cost technology platform for fluorescence-based optochemical sensors
made up of arrays of multimode waveguides and coupling structures integrated onto a flexible substrate. Such a
configuration is ideal for multi-analyte detection owing to a possibility of future integration of different dyes in each
waveguides. The presence of light sources, fluorescent sensing elements and photodetectors in a foil platform makes it a
compact optochemical sensor, which has wide-range of applications in medical, biochemical, and environmental
diagnostics. Flexible lightguides fabricated using soft-lithography based replication techniques, are used in combination
with 45° micromirror coupling structures, having a loss of 0.5dB. Fluorescent dyes are incorporated with the lightguides
enabling a detection of shift in fluorescence-peaks in contact with gases, which are read-out at the detection. Initial
measurements yielded promising results of the waveguides mixed with fluorescent dyes showing response to toluene.
We present a compact, portable and low cost generic interrogation strain sensor system using a fibre Bragg grating
configured in transmission mode with a vertical-cavity surface-emitting laser (VCSEL) light source and a GaAs
photodetector embedded in a polymer skin. The photocurrent value is read and stored by a microcontroller. In addition,
the photocurrent data is sent via Bluetooth to a computer or tablet device that can present the live data in a real time
graph. With a matched grating and VCSEL, the system is able to automatically scan and lock the VCSEL to the most
sensitive edge of the grating. Commercially available VCSEL and photodetector chips are thinned down to 20 μm and
integrated in an ultra-thin flexible optical foil using several thin film deposition steps. A dedicated micro mirror plug is
fabricated to couple the driving optoelectronics to the fibre sensors. The resulting optoelectronic package can be
embedded in a thin, planar sensing sheet and the host material for this sheet is a flexible and stretchable polymer. The
result is a fully embedded fibre sensing system - a photonic skin. Further investigations are currently being carried out to
determine the stability and robustness of the embedded optoelectronic components.
We present the design and fabrication of a complete optical interconnection scheme including the optoelectronic
package, containing driving Vertical Cavity Surface Emitting Lasers (VCSELs) and read-out photodiode (PDs), the
coupling scheme of the fiber or waveguide interconnect and the fabrication technology of the waveguide structures itself.
Both the optoelectronic package and the waveguide part are fabricated using polymer materials resulting in a low-cost,
flexible interconnection scheme.
The optoelectronic package consists of an ultra-thin (20 μm) chip embedded in a flexible polymer stack, connected
through metalized microvias using thin film deposition steps. A 45° deflecting micromirror is used to couple this
optoelectronic package to an optical fiber or an optical waveguide. The waveguiding structures can be integrated with the
coupling plug leading to a 1 step alignment process which significantly reduces the coupling losses. Flexible and
stretchable multimode polymer waveguides are also developed to end up with a fully flexible optical interconnect for
short (waveguide) or long distance (fiber) communication or for application in sensing.
This paper describes the development of a low-cost technology platform for fluorescence-based optochemical sensors.
These sensors were constructed by incorporating fluorescent sensing elements in the core of multimode waveguides or
lightguides, and have applications in medical, biochemical and environmental diagnostics. Flexible lightguides were
fabricated either with silsesquioxane-based or PDMS-type optical polymers using photolithography or soft-lithography
based replication techniques respectively. Spectral transmission characteristics were measured along with loss values
obtained by cut-back measurements for several wavelengths from visible to mid-IR. Propagation losses as low as
0.14dB/cm were measured for 50 x 50 μm2 waveguides. For coupling light in and out of the waveguides, different types
of coupling structures, e.g. 45° micromirror plugs were investigated.
Fiber Bragg gratings can be used for monitoring different parameters in a wide variety of materials and constructions.
The interrogation of fiber Bragg gratings traditionally consists of an expensive and spacious peak tracking or spectrum
analyzing unit which needs to be deployed outside the monitored structure. We present a dynamic low-cost interrogation
system for fiber Bragg gratings which can be integrated with the fiber itself, limiting the fragile optical in- and outcoupling
interfaces and providing a compact, unobtrusive driving and read-out unit. The reported system is based on an
embedded Vertical Cavity Surface Emitting Laser (VCSEL) which is tuned dynamically at 1 kHz and an embedded
photodiode. Fiber coupling is provided through a dedicated 45° micromirror yielding a 90° in-the-plane coupling and
limiting the total thickness of the fiber coupled optoelectronic package to 550 μm. The red-shift of the VCSEL
wavelength is providing a full reconstruction of the spectrum with a range of 2.5 nm. A few-mode fiber with fiber Bragg
gratings at 850 nm is used to prove the feasibility of this low-cost and ultra-compact interrogation approach.
We review the state-of-the-art in photonic crystal fiber (PCF) and microstructured polymer optical fiber (mPOF) based
mechanical sensing. We first introduce how the unique properties of PCF can benefit Bragg grating based temperature
insensitive pressure and transverse load sensing. Then we describe how the latest developments in mPOF Bragg grating
technology can enhance optical fiber pressure sensing. Finally we explain how the integration of specialty fiber sensor
technology with bio-compatible polymer based micro-technology provides great opportunities for fiber sensors in the
field of healthcare.
This paper reports on the latest trends and results on the integration of optical and opto-electronic devices and
interconnections inside flexible carrier materials. Electrical circuits on flexible substrates are a very fast growing
segment in electronics, but opto-electronics and optics should be able to follow these upcoming trends. This paper
presents the back-thinning and packaging of single opto-electronic devices resulting in highly flexible and reliable
packages. Optical waveguides and optical out-of-plane coupling structures are integrated inside the same layer stack,
resulting in complete VCSEL-to-PD links with low total optical losses and high resistance to heat cycling and moisture
exposure.
A highly flexible sensing skin with embedded polymer optical fibre Bragg gratings is characterised The response to
pressure and strain compare favourably to a similar skin instrumented with silica fibre Bragg grating sensors.
In this paper, we report on the strain and pressure testing of highly flexible skins embedded with Bragg grating sensors
recorded in either silica or polymer optical fibre. The photonic skins, with a size of 10cm x 10cm and thickness of 1mm,
were fabricated by embedding the polymer fibre or silica fibre containing Bragg gratings in Sylgard 184 from Dow
Corning. Pressure sensing was studied using a cylindrical metal post placed on an array of points across the skin. The
polymer fibre grating exhibits approximately 10 times the pressure sensitivity of the silica fibre and responds to the post
even when it is placed a few centimetres away from the sensing fibre. Although the intrinsic strain sensitivities of
gratings in the two fibre types are very similar, when embedded in the skin the polymer grating displayed a strain
sensitivity approximately 45 times greater than the silica device, which also suffered from considerable hysteresis. The
polymer grating displayed a near linear response over wavelength shifts of 9nm for 1% strain. The difference in
behaviour we attribute to the much greater Young's modulus of the silica fibre (70 GPa) compared to the polymer fibre
(3 GPa).
In traditional electrical sensing applications, multiplexing and interconnecting the different sensing elements is a major
challenge. Recently, many optical alternatives have been investigated including optical fiber sensors of which the
sensing elements consist of fiber Bragg gratings. Different sensing points can be integrated in one optical fiber solving
the interconnection problem and avoiding any electromagnetical interference (EMI). Many new sensing applications also
require flexible or stretchable sensing foils which can be attached to or wrapped around irregularly shaped objects such
as robot fingers and car bumpers or which can even be applied in biomedical applications where a sensor is fixed on a
human body. The use of these optical sensors however always implies the use of a light-source, detectors and electronic
circuitry to be coupled and integrated with these sensors. The coupling of these fibers with these light sources and
detectors is a critical packaging problem and as it is well-known the costs for packaging, especially with optoelectronic
components and fiber alignment issues are huge. The end goal of this embedded sensor is to create a flexible optical
sensor integrated with (opto)electronic modules and control circuitry. To obtain this flexibility, one can embed the
optical sensors and the driving optoelectronics in a stretchable polymer host material. In this article different embedding
techniques for optical fiber sensors are described and characterized. Initial tests based on standard manufacturing
processes such as molding and laser structuring are reported as well as a more advanced embedding technique based on
soft lithography processing.
This paper presents the optical and mechanical characterization of fully embedded optical links. The flexible optical
links consist of ultra thin opto-electronic transceivers, multimode polymer optical waveguides, optical coupling
structures and galvanic interconnections, all embedded inside a thin (145 μm) flexible foil. The embedded GaAs
transceivers (VCSELs and Photodiodes) are first mechanically thinned down to 20 μm thickness, enabling the
unobtrusive embedding inside the foil and allowing the chips to be bent with the foil due to their very low thickness. The
embedded links are tested for their flexible behavior by means of several measurements: the optical bending losses of the
flexible waveguides, the minimum bending radius before link failure and the bending endurance. The emitted optical
modes of the ultra thin VCSEL's were characterized and compared before and after thinning and before and after
embedding of the VCSEL's to determine the effect of these actions on the behavior of the VCSEL power and modes.
The optical power budget of the complete optical VCSEL-to-Photodiode VCSEL is investigated by simulations and
measurements of the different optical loss contributors. Also crosstalk behavior between two neighboring waveguides
and links is measured. A proof of principle demonstrator of an embedded optical link on a rigid substrate using standard
50 Ohm test equipment and a basic galvanic lay-out shows a clear open eye diagram at a speed of 1.2 Gb/s. Reliability of
the flexible optical link foil was demonstrated with temperature (-40 to 125 degrees Celsius) and humidity (85 rh/85 °C
for 1000 hours) fastened aging cycling with good results.
To realize a high density matrix of pressure sensors, mainly electrical approaches are reported. The proposed highdensity
optical pressure sensor is based on a matrix of 2 stacked layers of crossing multimode waveguides. When
pressure is applied on a crossing point, the distance between the waveguides from the upper and lower layer will
decrease and power is transmitted between these waveguides. The sensor consists of polymer waveguides embedded in
polydimethylsiloxane (PDMS) which is a very flexible material. Therefore, it is ideally suited to be applied on irregular
or moving surfaces especially for applications which require covering small areas with high density pressure sensors.
This paper describes the fabrication of a novel type of pressure sensor based on optical feedback in a Vertical Cavity
Surface Emitting Laser (VCSEL). The detection mechanism of the sensor is based on a displacement measurement
through self-mixing interferometry in the laser cavity. Using this technique a sensing resolution of half the laser
wavelength can be achieved. The use of unpackaged VCSELs allows the integration of the sensor in a flexible polymer
material, which enables thin and ultimately bendable optical sensing foils. Moreover, the use of unpackaged dies limits
the sensor dimensions and minimizes the distance between two sensing points. Consequently, dense matrices of highly
accurate sensing points can be fabricated. A proof of principle set-up for this new sensing mechanism has been developed and a first demonstrator has been fabricated and characterized.
This paper presents the latest results on the development of a thin flexible package of commercially available optoelectronics
with polymer multimode waveguides. The GaAs VCSELs and Photodiodes are thinned down to 20 μm
thickness, resulting in packages which can be bended to a bending radius of 2 mm with high reliability. With these
actives, also waveguides and out-of-plane coupling structures are embedded inside the foil. Flexible Polyimide micromirrors
were fabricated, characterized and embedded inside the foil. An embedded VCSEL to Photodiode optical
waveguide link was demonstrated at a speed of 1.2 Gbs with open eye diagram. Temperature (-40 to 125 degrees
Celsius) and humidity (85 rh/85 °C for 1000 hours) reliability was tested with good results. The total thickness of the
completed foil containing actives, waveguides and coupling elements is only 145 μm.
We present a concept for an artificial optical skin, a flexible foil in which a novel type of optical force sensing elements
is integrated. The principle relies on the change in coupling between two arrays of crossing polymer waveguides
separated by a thin layer of soft silicone. When the exerted pressure is increasing, the distance between the waveguides
decreases and consequently power is transmitted from one to another. A process flow to produce a proof of principle
demonstrator with arrays of TruemodeTM waveguides embedded in silicone is described. In a second approach also the
waveguides are fabricated in silicone using an embossing technique.
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