Pulsed photothermal interferometry (PTI) gas sensor with hollow-core photonic bandgap fibre (HC-PBF) is demonstrated with a Sagnac interferometer-based phase detection system. Under the condition of constant peak pump power, the optimal pulse duration is found to be > 1:2 <i>μs</i> for detecting low-concentration of trace gases in nitrogen, limited by thermal conduction of gases within the hollow-core. Preliminary experiments with a 0.62-mlong HC-PBF gas cell, low peak power ( ~ 20:2<i>mW</i>) and a boxcar averager with 10k average times demonstrated a detection limit of 3:3 <i>p:p:m</i> acetylene. Detection limit down to ppb or lower is expected with high peak power pump pulses.
We present all-fiber resonating Fabry-Perot gas cells made with a piece of hollow-core photonic bandgap fiber (HCPBF) sandwiched by two single mode fibers with mirrored ends. A HC-PBF cavity made of 6.75-cm-long HC-1550-06 fiber achieved a cavity finesse of 128, corresponding to an effective optical path length of 5.5 m. Such HC-PBF cavities can be used as absorption cells for high sensitivity gas detection with fast response. Preliminary experiment with a 9.4-cm-long resonating gas cell with a finesse of 68 demonstrated a detection limit better than 7.5 p.p.m. acetylene.
We exploit photothermal effect in gas-filled hollow-core photonic bandgap fibers, and demonstrate remarkably sensitive all-fiber (acetylene) gas sensors with noise equivalent concentration of 1-3 parts-per-billion and an unprecedented dynamic range of nearly six orders of magnitude. These results are two to three orders of magnitude better than previous direct absorption-based optical fiber gas sensors. The realization of photothermal spectroscopy in fiber-optic format will allow a new class of sensors with ultra-sensitivity and selectivity, compact size, remote and multiplexed multi-point detection capability.
We demonstrate an all-optical-fiber photoacoustic (PA) spectrometric gas sensor with a graphene nano-mechanical resonator as the acoustic detector. The acoustic detection is performed by a miniature ferrule-top nano-mechanical resonator with a ∼100-nm-thick, 2.5-mm-diameter multilayer graphene diaphragm. Experimental investigation showed that the performance of the PA gas sensor can be significantly enhanced by operating at the resonance of the grapheme diaphragm where a lower detection limit of 153 parts-per-billion (ppb) acetylene is achieved. The all-fiber PA sensor which is immune to electromagnetic interference and safe in explosive environments is ideally suited for real-world remote, space-limited applications and for multipoint detection in a multiplexed fiber optic sensor network.
The effects of modal interference (MI) on the performance of hollow-core photonic bandgap fiber (HC-PBF) gas sensors are investigated. By optimizing mode launch, applying wavelength modulation with proper modulation parameters as well as appropriate digital signal processing, an estimated lower detection limit of <1 ppmv acetylene is achieved with 13-m long HC-PBF. The impacts of drilling side-hole on the MI and response time are also studied. With a 62-cm long sensing HC-PBF drilled with multiple side-holes, an acetylene sensor with a lower detection limit of 11 ppmv and a recovery time of 2 minute is demonstrated.
Compact ferrule-top nano-mechanical resonators with all-fiber optical interrogation are demonstrated. The resonators comprise of a suspended multi-layer graphene film supported by a ceramic ferrule with a bore diameter of 125 μm. The mechanical resonance of the graphene film is excited and detected via a single optical fiber cable, and experimental test shows that the resonant frequency and quality factor of the resonators are in the range of 170-520 kHz and 58.4-250, respectively. The integration of graphene resonator with optical fiber transmission and interrogation would allow the development of practical fiber-optic sensors for force, mass and pressure measurements.
Phase sensitivity of the fundamental mode of hollow-core photonic bandgap fiber to gas pressure applied internally to its core is investigated. The measured phase sensitivity for a 95-cm-long fiber is 9.92 rad/kPa, over two orders of magnitude higher than that to external pressure. The large phase sensitivity is attributed mainly to the pressure-induced refractive index change of air inside the fiber core. Such an effect may be exploited for high sensitivity pressure sensing and biochemical and environmental process analysis involving pressure variations.
We report a novel type of highly birefringent suspended core (SC) photonic microcells made by selectively inflating four air-columns in a solid core photonic crystal fiber. The wavelength-scale SC has two-fold rotational symmetry and exhibits high phase and group birefringence of ~3 x10<sup>-3</sup> and ~5 x10<sup>-3</sup> respectively at 1550 nm. By incorporating such microcells into Sagnac fiber loop interferometers, we demonstrated refractive index, temperature and gas pressure sensors with good performances.
We report the fabrication of in-line fiber-optic photonic microcells by post-processing commercial photonic crystal fibers. With such microcells, novel photonic devices such as in-fiber amplifiers, grating filters, and accelerometers are created.
The acoustic pressure sensitivities of hollow-core photonic bandgap fibers (HC-PBFs) with different thicknesses of silica outer-cladding and polymer jacket were experimentally investigated. Experiment with a HC-PBF with 7 μm-thick silica outer cladding and 100 μm-thick Parylene C jacket demonstrated a pressure sensitivity 10 dB higher than the commercial HC-1550-02 fiber and 25 dB higher than a standard single mode fiber. The significant enhancement in sensitivity would simplify the design of fiber hydrophones and increase the number of sensors that could be multiplexed in a single fiber.
The spectrophone performance for QEPAS is numerically investigated by using a finite element method. The effect of
varying system parameters such as the excitation frequency, relative position between the acoustic resonant tubes and the
quartz tuning fork, and the dimensions of resonant tubes are examined A pair of rigid tubes, each with a length of 5.1
mm and an inner diameter of 0.2 mm, positioned 0.6 μm down from the opening and 20 μm away from the edge of
tuning fork is suggested for optimal spectrophone performance.
A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm. The
graphene also acts as a light reflector which, in conjunction with reflection at the fiber end/air interface, forms a low
fineness Fabry-Perot cavity. The graphene-based micro-cavity sensor demonstrated a pressure sensitivity of ~34.8
nm/kPa with a diaphragm diameter of ~ 25 μm, nearly two orders of magnitude higher than previously reported sensors
with similar diaphragm diameters.
Quartz-enhanced photoacoustic spectroscopy with a near infrared distributed feedback diode laser at 1.53 μm is
demonstrated for acetylene detection at atmospheric pressure and room temperature. The P(9) absorption line in the
ν<sub>1</sub>+ν<sub>3</sub> band of C<sub>2</sub>H<sub>2</sub> is selected for light absorption and photoacoustic pressure wave excitation. A pair of resonant tubes
with optimal dimensions is used in combination with a quartz tuning fork for photoacoustic signal enhancement. The
wavelength of diode laser is modulated at half of the resonant frequency of tuning fork for second harmonic signal
detection. The effect of residual amplitude modulation is theoretically analyzed and compared with the experimental
results. A noise-limited minimum detectable concentration (1σ) of 2 part-per-million (ppm) is achieved with a 7-mW
laser power and a 1-s lock-in time constant, corresponding to a normalized noise equivalent absorption coefficient of
5.4×10<sup>-8</sup> cm<sup>-1</sup> W/√Hz.
This paper reports some of our recent work on in-line devices based on air-silica microstructrue optical fibers.
These devices are fabricated by use of a CO<sub>2</sub> laser/a femtosecond infrared laser and include strong long period
gratings in index-guiding fibers and air-core photonic bandgap fibers, in-fiber polarizers, polarimeters, and modal
interferometers. Applications of such devices for strain, temperature, directional bend, twist, and gas sensing are
This paper reports the development of a fiber-optic gas detection system capable of detecting three types of
dissolved fault gases in oil-filled power transformers or equipment. The system is based on absorption spectroscopy and
the target gases include acetylene (C<sub>2</sub>H<sub>2</sub>), methane (CH<sub>4</sub>) and ethylene (C2H<sub>4</sub>). Low-cost multi-pass sensor heads using
fiber coupled micro-optic cells are employed for which the interaction length is up to 4m. Also, reference gas cells made
of photonic bandgap (PBG) fiber are implemented. The minimum detectable gas concentrations for methane, acetylene
and ethylene are 5ppm, 2ppm and 50ppm respectively.
This paper reports recent development and application of optical fiber gas sensors using absorption spectroscopy,
including open-path gas sensors using fiber coupled micro-optic cells and photonic bandgap (PBG) fibers. A fiber-optic
sensor system capable of detecting dissolved fault gases in oil-insulated equipment in power industry is presented. The
gases include methane (CH<sub>4</sub>), acetylene (C<sub>2</sub>H<sub>2</sub>) and ethylene (C<sub>2</sub>H<sub>4</sub>). In addition, the development of gas sensor using
PBG fiber will be reported.
Sol-gel entrapment technique is proposed for glucose oxidase immobilization in long period grating glucose sensor. The
glucose oxidase is encapsulated in transparent sol-gel matrix to detect the presence of D-glucose molecules. A sensitivity
of 39.8mM/nm was achieved for the fabricated glucose biosensors.
Long period gratings in hollow-core photonic-bandgap fibers were fabricated by use of a pulsed CO<sub>2</sub> laser. The resonant
wavelengths of these gratings are sensitive to strain but insensitive to temperature, bend and external refractive index.
A series of low-contract photonic band-gap (PBG) fibers were fabricated by filling the holes of a commercial air-silica
hollow-core PBG fiber with different refractive index liquids. The PBGs and the transmission characteristics of these
fibers were investigated theoretically and experimentally. An increase in the refractive index of liquid filling the holes
causes blue-shift of the PBG and a narrow down of the PBG width, which may be exploited for sensitive refractive index
This paper reports the development of a wavelength detection system that can be used to detect the Bragg wavelength of fiber Bragg grating sensors. The system can interrogate up to 100 sensors with wavelength detection resolution on the order of picometers. The results of medium-term test will be reported. Possible applications will be discussed.
Photonic crystal fibers (PCFs) have special wave-guiding properties that cannot be achieved in conventional optical fibers. The properties of PCFs can be controlled via the geometry of their microstructured cladding. This opens up new opportunities for numerous applications in the areas of light transmission, nonlinear optics, fiber-optic components and sensors. In this paper, we overview the applications of PCF in photonic sensing.
This paper reports the results of our recent investigation on the noise limited performance in heterodyne interferometric demodulation systems for fiber Bragg grating strain sensors. Theoretical and simulation results are presented and compared with experimental results.
The cut-off wavelengths of the two linearly polarized states in PCF can be designed by varying the diameters and pitch of the holes. Simulations show that single polarization operation over a range of up to 100 nm can be realized around 1.3 μm and 1.55 μm bands.
We report a fiber Bragg grating sensor for the measurement of static and dynamic measurands (e.g., strain and temperature). Initial experiments demonstrated detection resolutions of ~2.6me and ~7ne/(Hz)1/2 for static and dynamic strain measurements respectively.
A multi-point gas sensor network based on a frequency-modulated continuous wave technique and wavelength modulation spectroscopy has been demonstrated for the detection of acetylene gas. A minimum detectable concentration of 6.75ppmrn is obtained with a three-sensor system. The crosstalk between the sensors is estimated about -22 dB.
The cost per sensing point may be reduced by networking a number of gas sensors that shares the same tunable laser and/or the same signal processing electronics. In this paper we report on the use of a frequency modulated continuous wave (FMCW) technique for addressing the remote optical fiber gas sensor arrays. The sensor network is of a ladder topology and is interrogated by a tunable external-cavity semiconductor lasers. The system performance in terms of detection sensitivity and crosstalk between sensors is investigated. By using appropriate wavelength modulation/scanning coupled with low pass filtering, the coherent interferometric noise can be reduced greatly. Computer simulation shows that an array of 20 acetylene (C<SUB>2</SUB>H<SUB>2</SUB>) gas sensors with 2000 ppm (2.5 cm gas cell, or 50 ppm.m) detection sensitivity for each sensor may be realized. A two-sensor acetylene gas detection system is experimentally demonstrated with detection sensitivity of 165 ppm/(root)Hz (2.5 cm gas cell or 4 ppm.m/(root) Hz) and crosstalk of -25 dB.
Quantitative measurement of gas concentration based on wavelength modulation spectroscopy and second-harmonic detection is demonstrated by applying low-frequency wavelength modulation to an external-cavity tunable diode laser. The tunable laser operating at 1.53 micrometers region is used to detect acetylene (C<SUB>2</SUB>H<SUB>2</SUB>) with laser absorption spectroscopy. A time-division multiplexed three- sensor system with a forward-coupled ladder topology have been implemented and experimentally tested. The sensor system uses single path 25 mm absorption cells and have demonstrated sensitivity of 81 ppm. The crosstalk between the sensors was found to be -30 dB. Power budget analysis shows that a sensor network consisting of 37 sensors could be realized with the same multiplexing topology.
An external-cavity tunable diode laser operating at near-IR region is used to detect acetylene with laser absorption spectroscopy. Quantitative measurement of gas concentration is obtained by applying low-frequency wavelength modulation to the tunable laser and by phase-sensitive-detection of the fundamental and second-harmonic signals using a lock-in amplifier. A single-sensor and a time-division multiplexed two-sensor system have been implemented and experimentally tested. The sensor systems use single path 25 mm absorption cells and have demonstrated sensitivity of 75 ppm for the single sensor system and 182 ppm for the multiplexed two- sensor system.
Interferometric noise in fiber optic grating sensors is investigated. Interference between signal wave and reflected waves causes signal fluctuation in the output which limits the wavelength detection accuracy of the sensing system. The measurement error limited by interferometric noise are calculated for both reflective type and transmission type sensors.