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This paper compares the performance of two polymeric substance, polyvinyl alcohol (PVA) and polyethylene oxide (PEO) as a host in graphene saturable absorber (SA) fabrication. By applying a low-cost solution casting method, graphene flakes which was prepared using the electro-chemical exfoliation technique will be mixed with PEO or PVA solvent as a host. Single concentration (12.04 wt.%) of graphene-PVA and graphene-PEO has been prepared for comparison. At 1.5 µm wavelength region, 2.8 meters erbium doped fiber (EDF) was applied as a gain medium in ring laser cavity setup. Generally, our measurement confirmed stable Q-switched pulses generation (>30 dB) for both SAs at 1.5 µm region. Evidently, the generated pulse by graphene-PVA at 1531.1 nm produced shorter pulse width of 1.85 µs as compared to 18.1 µs by graphene-PEO at maximum permissible pump power. Although smaller pulse energy was calculated for graphene-PVA, the ability of this type of SA to withstand higher pump-power proved its superiority as compared to its counterpart. Notably, this is the first ever effort that assess the performance of PVA and PEO host in graphene-based saturable absorption, particularly at 1.5 µm region.

In this study, a novel and simple optical fiber sensor is proposed, simulated, and experimentally demonstrated for simultaneous measurement of high refractive index (RI) and temperature. The sensor consists of a section of single-mode-silica rod-single-mode (SSRS) fiber structure cascaded to fiber Bragg grating (FBG) (SSRS-FBG). The simultaneous measurement of high RI and temperature was realized in this study by monitoring the output power level and wavelength shift of the SSRS-FBG single dip transmission spectrum corresponding to different surrounding high RI and ambient temperature values. The experimental results showed that the cascaded sensor had a sensitivity of 108.07 dBm/RIU for high RI ranging at 1.45-1.531, while the temperature sensitivity was 9.31 pm/°C from 35 °C to 85 °C. This sensor is most suitable for high RI applications, such as for monitoring oil quality stored in terminal and power transformers.

An all‐fiber Mach‐Zehnder interferometer based on symmetrical offset coreless silica fiber (CSF) is proposed and demonstrated for temperature sensing. The sensor is constructed by a symmetrical offset splicing of a short CSF section between two CSF sections. Thus, two distinct optical paths are created inside and outside of the offset section resembling a Mach‐Zehnder interferometer. For temperature sensing, the offset gap is filled with refractive index liquid with high thermo‐optic coefficient of 3.34 × 10−4. Measurement is established by monitoring wavelength shift resulting from interference of modes between the two distinct optical paths. A very high temperature sensitivity of 3220 pm/°C has been achieved with a short sensor length of 1.5 mm. Due to its compact size and high sensitivity, the proposed structure would be an attractive option for low temperature measurement such as for human body temperature monitoring.

An all-fiber Michelson interferometer based on double-clad fiber (DCF) is proposed for temperature and refractive index (RI) sensing. The sensor is formed by continuous splicing between single mode fiber, multimode fiber, and DCF. Temperature measurement is achieved by monitoring the dip wavelength shift of the interferometer spectra resulting from thermo-optic effect on core and first cladding of DCF. RI measurement is established from the power change in spectra due to the Fresnel's reflection at the DCF tip. Experimental result on 18.5 mm sensor manifests that temperature and RI responses are distinguishable. For instance, power level at 1540.5 nm exhibits quadratic shift to RI changes while unaffected to temperature changes. Dip wavelengths of sensor spectra demonstrate linear shift to temperature changes while unaffected by RI changes. The proposed sensor is cost effective as sensor fabrication only requires conventional splicing. Sensor capability to distinguish temperature and RI parameters makes it practical for broad range applications including for biomedical and food industries.

In this paper, graphene oxide (GO) was doped into SU-8 polymer to examine the functionalities of polymer waveguide material via dopant inclusion. In material preparation, step by step processes starting from cyclopentanone mixing will be explained. It is shown that doped material's refractive index can be tailored by having different weight percentage of GO. Finally, a single mode ridge waveguide loss of GO doped SU-8 has been characterized to be 1.9 dB/cm at 1550 nm wavelength.

This paper explains about the performance of graphene nanopowder (GNP) based saturable absorber (SA) at 1.5-micron region which is prepared by dissolution in polyvinyl alcohol (PVA) polymer. Two different GNP flakes thickness (AO2-8 nm and AO4-60 nm) are tested. By applying a solution casting method, three weight ratio of GNP to PVA (12.04, 8.03 and 3.11 wt.%) have been prepared and fabricated as a composite thin film. To characterize for the SA performance, 4 mm 2 area of GNP-PVA thin film is embedded in a 14 meters long ring cavity with 3 meters Erbium doped fiber (EDF) as a gain medium. Our characterization results show that the GNP-PVA thin film act as a Q-switcher which produce stable laser pulses for 12.04 wt.% with maximum repetition rate of 39.22 kHz and shortest pulse width of 11.79 µs. Meanwhile, unstable Q-switched pulses of 8.03 wt.% and 3.11 wt.% have been observed with recorded signal to noise ratio (SNR) of only 21 dB and 17 dB, respectively. The threshold pumping power for Q-switched lasing to emerge is recorded as low as 30 mW. Apparently, it shows that GNP concentration and flakes thickness in fabricated SA composite plays vital role in the performance of generated Q-switch laser, particularly at 1.5 µm region. Full Text: PDF References T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P.H. Tan, A.G. Rozhin, A.C. Ferrari, "Nanotube–Polymer Composites for Ultrafast Photonics", Adv. Mater. 21, 3874 (2009). CrossRef Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, "Atomic-Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers", Adv. Funct. Mater. 19, 3077 (2009). CrossRef Z. Luo, M. Zhou, J. Weng, G. Huang, H. Xu, C. Ye, Z. Cai, Opt. Lett. 35(21), 3709 (2010). CrossRef D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, A.C. Ferrari, "Graphene Q-switched, tunable fiber laser", Appl. Phys. Lett. 98, 3106 (2011). CrossRef Y.M. Chang, H. Kim, J.H. Lee, Y. Song, "Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers", Appl. Phys. Lett. 97, 211102 (2010). CrossRef M. Jiang, Z. Ren, Y. Zhang, B. Lu, R. Zhang, J. Guo, Y. Zhou, J. Bai, "Passive Q-Switching with Graphene Saturable Absorber in Nd:YAG Operating at 1064nm", Mater. Sci. Forum 694, 700 (2011). CrossRef N. Hussin, M.H. Ibrahim, F. Ahmad, H. Yahaya, S.W. Harun, "Graphene Nanoplatelets (GnP)-PVA Based Passive Saturable Absorber", Telkomnika 15(2), 814 (2017). CrossRef F.C. Mat, M. Yasin, A.A. Latiff, S.W. Harun, Photonics Letters of Poland 9, 100 (2017). CrossRef E.K. Ng, K.Y. Lau, H.K. Lee, N.M. Yusoff, A.R. Sarmani, M.F. Omar, M.A. Mahdi, "L-band femtosecond fiber laser based on a reduced graphene oxide polymer composite saturable absorber", Opt. Mater. Express 11, 59 (2021). CrossRef N.H.M. Apandi, S.N.F. Zuikafly, N. Kasim, M.A. Mohamed, S.W. Harun, F. Ahmad, "Observation of dark and bright pulses in q-switched erbium doped fiber laser using graphene nano-platelets as saturable absorber", Bull. Electr. Eng. Inform. 8, 1358 (2019). CrossRef N.U.H.H.B. Zalkepali, N.A. Awang, Y.R. Yuzaile, Z. Zakaria, A.A. Latif and F. Ahmad, "Graphene Nanoplatelets as Saturable Absorber for Mode-locked Fiber Laser Generation", J. Adv. Res. Dyn. Control Syst. 12(2), 602 (2020). CrossRef X. Zhu and S. Chen, "Autoencoder-Based Transceiver Design for OWC Systems in Log-Normal Fading Channel", IEEE Photonics J. 11, 7105109 (2019). CrossRef

An in-line all-fiber Mach–Zehnder interferometer (MZI) based on the combination of etched single mode fiber (ESMF) and double cladding fiber (DCF) is proposed for refractive index and discrete liquid level sensing. In the proposed structure, two sensing points are devised using two ESMF section of length. The two sections are separated by a refractive index insensitive region formed by DCF section. The sensor may be deployed to measure refractive index in single- or dual-point configuration. The experimental work revealed maximum refractive index (RI) sensitivity at − 37.2 nm/RIU for 1.305–1.335 RI range, with 1 mm compact sensor size. Additionally, the sensor performed as a two-point liquid level sensor with wavelength shift ranging between 0.84 nm and 1.44 nm. The proposed sensor displayed great potential to be deployed in liquid chemical storage to monitor liquid quality and level.

Optical fiber based on the polymer coating is one of the best options for water detection sensors in liquid insulation. The water diffusion can be simulated through the finite-element method (FEM) as it provides the finite element analysis to determine the performance of the moisture sensor based on optical fibers. In this work, a commercial COMSOL Multiphysics software was used for the FEM simulation of the coated optical fiber based moisture sensor. The response of the sensor was estimated for different thickness to select the coating thickness before the hardware design to opt for the best sensitivity. The electric field propagation used the 2D model and was estimated by using the boundary mode analysis and frequency domain study. Transport dilute species uses 3D model species to study the diffusion of water into the polymer by varying the thickness and concentration of water (mol/m3). The light propagation was simulated based on electromagnetic fields and water diffusion using Fick's law. The modelling of the sensor was done, and the response time required for the sensor was estimated along with the effect of the electric field with varying thickness of PVAc coating.

Partial discharge (PD) detection in high voltage equipment is a vital solution for the identification of insulation degradation. It is a regular strategy of detecting the PD by acoustic emission using conventional methods like piezoelectric transducers (PZT) but this method has drawbacks such as low sensitivity and false alarms due to electromagnetic interference (EMI). In order to avoid these kinds of drawbacks, a thin polymer film based Single-mode fiber (SMF) – multimode fiber (MMF) – single-mode fiber (SMS) was fabricated and studied. The thin polymer film SMS sensor was tested with two types of light source; Broadband source (BBS) and Tunable laser source (TLS). The experiment has been carried out with air ultrasonic transmitter and high voltage source for creating PD to detect the acoustic signal generated. The signal to noise ratio (SNR) of both sensors was observed and measured. The PD signal detection was carried out in air medium and the phase-resolved partial discharge patterns were recorded and the performance was compared with the conventional measuring instrument. The acoustic signals have been detected by the sensor generated from the transmitter and surface discharge using BBS and TLS. Identification of the best light source for PD detection has been explained.

Bending and rotation sensing based on high coherence interferometry in seven-core fiber (SCF) is proposed here for the first time. In the suggested scheme, distinct optical paths are formed from the selected cores, thus creating a very compact in-fiber Michelson interferometer. The interferometer consists of an in-line taper structure fabricated on the SCF for a light splitting/coupling element, and a Faraday rotating mirror used as a reflective element. Different strains are exerted on the reference and sensing cores because of the dissimilar distance from the neutral axis. The phase difference between these two interferometer arms resulting from the variations in strain is resolved using a fringe counting technique. The proposed sensor is revealed to be extremely sensitive for curvature and rotation measurement, with sensitivities of 629 fringe/m−1 and 6.2 fringe/°, respectively. The sensor is capable of detecting the smallest curvature of 0.0032 m−1 or radius of 312 m. The sensor also responds linearly to rotation, with a sensitivity of 6.2 fringe/°. This response is highly repeatable; linear; insensitive to slow-varying parameters, such as temperature; and promising for the detection of direction. The proposed sensor is particularly attractive for robotics, wearable medical devices, and structural health-monitoring applications.

An intensity-modulated temperature sensor based on fiber Mach-Zehnder interferometer (MZI) is proposed and demonstrated. The sensor is constructed using single mode fiber (SMF) and multimode fiber (MMF) in SMF-MMF-SMF-MMF-SMF sequence. The sensing system is power-interrogated, relies on an optical bandpass filtering of the sensor output at a predetermined wavelength which converts any change in sensor spectra into power change. This sensing scheme is very cost effective, as the temperature can be directly quantified from photodetector voltage measured at the filter output. Furthermore, sensor fabrication only involves conventional fusion splicing process between standard SMF and MMF. Experimental result shows that a sensor with length 20 mm produces linear response of -0.48 mV/°C at 1532 nm passband wavelength, good long-term stability with negligible voltage drift and fluctuation, and wide measurement range within 30-180°C. The proposed scheme potentially can be applied in fiber interferometer sensors for measurement of other physical parameters such as strain, refractive index and vibration.

This paper presents the study on the impact of the nonlinear phase shift on the performance of optical transmission systems and its suppression using digital backpropagation (DBP) technique. The proposed DBP employed digital optical phase conjugation (DOPC) at the receiver and a virtual fiber, thereby having the combined benefits of DBP and optical phase conjugation. Optical signal propagation was achieved by solving the nonlinear Schrödinger equation (NLSE) using the split-step Fourier method (SSFM). Implementation of DBP was conducted on 100-Gbps, DP-16QAM systems and it has demonstrated its efficacy in suppressing nonlinear phase shift, by improving the minimum bit error rate (BER) from 1 × 10−2 to 6.1 × 10−5.

This work presents simulation of light propagation inside tapered structure-plastic optical fibre (POF) subjected to various surrounding temperatures and refractive index (RI). The tapered structure served as Mach-Zehnder interferometer (MZI) which split light into core modes and clad modes that recombined at the end of the taper. Simulation was done by using Beam Propagation Method (BPM) solved using Finite Difference Method (FDM). The light launched in to the MZI was varied from 611nm to 655nm. In order to give temperature effect, the temperature was varied from 30°C to 80°C with increment of 10°C. The results showed that the free spectral range (FSR) of the MZI are shifted as temperature increased. The sensitivity for the peak and dip of the FSR are -0.0158 nm/°C and -0.0153 nm/°C, respectively. For RI sensitivity investigation, the surrounding RI was varied from 1.333 to 1.349. The sensitivity of peak FSR and dip FSR to RI change are -98.638 nm/RIU and 105.4 nm/RIU. Therefore, the MZI can be used to measure RI and temperature simultaneously.

Partial discharges (PD) occur in high voltage equipment are considered as major problem that required to be prevented to avoid any breakdown, damage and loss from happening. In addition, this pre-breakdown phenomenon can be detected using appropriate sensors such as acoustic emission sensor. In this study, acoustic emission sensor was chosen as the method to detect the PD in high voltage insulation. Low-cost piezoelectric transducer (PZT) with pre-amplifier and optical fiber sensor (OFS) were used as a sensing device to detect PD but OFS was expected to highlight its usage towards PD detection. Development of optical fiber sensor (OFS) based on Single Mode-Multi Mode-Single Mode (SMS) structure which implementing a Multimode Interference effects (MMI) was focused on. PD measurement test was conducted according to IEC 60270 and IEC TS 62478 standards. A coated SMS based OFS was fabricated and then was used in PD measurement test. Furthermore, after implementing the pre-amplified PZT sensor and coated OFS in PD measurement test, results were compared with a measuring impedance by analyzing the Phase Resolved Partial Discharge (PRPD) patterns. From the results, all sensors can detect PD as accurately as the MI but the usage of pre-amplifier on PZT has shown a great deal of advantage in removing noises and amplifying small signal, but the SMS coated OFS has shown similar ability in detecting the PD.

peer-reviewed An improved and compact gas sensor using mid-infrared spectroscopy for highly responsive Carbon Dioxide (CO2) detection is presented. The sensing principle is based on open-path direct absorption spectroscopy in the mid-infrared region. The improved reflective structure of optical gas sensor consists of low cost and compact components. Several gas cell configuration which includes SISO, 2-MISO, 4-MISO and 8-MISO were simulated using ZEMAX®12 using non-sequential ray tracing technique to get the optimum radius of the reflective curved surface. Sensitivity analysis has shown that the optimised structure of 4-MISO yields the highest sensitivity and power efficiency of -0.2895%-1 and 12.298% respectively. The developed gas sensing system using the optimised gas cell has shown the capability of accurately detecting CO2 concentration between 1.5% and 5.8% with no cross-sensitivity with other gases present in the gas cell. The newly developed CO2 sensor exhibits high responsivity with the recorded rise time and fall time of less than 1 second respectively.

A novel low cost and simple temperature sensor based on multimode interference (MMI) is formed by a successive singlemode fiber (SMF)-bent multimode fiber (MMF)-SMF structure. Due to the small curvature radius of bent MMF section mimicking a balloon shape, MMI effect from the core is spreading into coating. The high temperature sensitivity is mainly due to the large thermo-optical coefficient of the existing acrylate coating of the MMF fiber. A temperature sensitivity of up to −2060 pm/°C and −25.1 nW/°C for wavelength- and intensity-based interrogation, respectively, have been achieved in the range between 27 °C and 31 °C. This sensor is most suitable in high sensitivity and low-temperature application such as the measurement of human body temperature.

Multiple Bragg gratings are inscribed in a photosensitive single-mode polymer fiber using phase mask method and also by applying strain to the fiber. Due to the large strain tuning range of the polymer fiber, multiple Bragg gratings are inscribed with a large peak wavelength separation between adjacent peaks. The effect of hysteresis of the polymer fiber is also considered and its influence on the fabrication of the grating is studied. From this paper, it is found that a fiber elongation up to 0.9% of the total fiber length can be used to inscribe multiple Bragg gratings and give a quality grating array. Fiber elongation ${>}{0.9\%}$ of the total length affects the grating reflectivity and bandwidth due to the creep and hysteresis developed in the polymer fiber from the large strain. Within the 0.9% elongation limit, multiple gratings can be inscribed with a maximum peak wavelength separation of 12 nm between adjacent fiber gratings. The fabricated single-mode polymer fiber Bragg grating array can be used in applications, where temperature and humidity compensations are required, while measuring the strain or as multipoint sensors.

This paper reports a simple sealed-void photonic crystal fiber (PCF) interferometric humidity sensor that operates in reflection mode and is presented here for the first time. The sensor was fabricated by collapsing the holes of the PCF at both ends to form an interferometer, which excites the core and cladding modes. As a result, the reflection spectrum of the sensor exhibits a sinusoidal interference peak that shifts when the adsorption and desorption of H2O molecules happen along the surface of the PCF. This 1.3 cm long PCF interferometer sensor exhibits a sensitivity of 20.3 and 61.6 pm/%RH in the range of 60%RH-80%RH and 80%RH-95%RH, respectively. The sealed-void configuration sensor has a fast response time of ~ 750 ms and also protects the holes of the PCF from contamination. In addition, the all-silica nature of the sensing element has made the sensor insensitive to temperature. The proposed sensor has potential in medical applications for monitoring breathing conditions.

A novel simple fiber sensor to sense liquid level is presented. The operation principle is based on the relative Fresnel reflective intensity. The sensor consists of a fiber splitter with the configuration of one input to multiple fiber outputs, i.e. 1×4, 1×8 and 1×12 arrangements that act as a discrete liquid level. A broadband source (BBS) is used as the light source supply. The total reflected power intensity is measured using a power meter. Experimental results show that the power intensity decreases as the level of liquid is increased. The sensor has a simple configuration, low cost, and it can be customized for a wide height measurement range spanning from a few centimeters up to a hundred meters.

In this paper, a low-cost fiber optic displacement sensor (FODS) using a bundle of plastic optical fiber (POF) as a probe is developed and presented. The sensor consists of a high power light emitting diode (LED) as light source, a probe with multiple receiving plastic optical fiber and a photodiode detector. The sensor is characterized at millimeter distance and the sensor output is analyzed from 0 mm to 13 mm displacement. The sensitivity of the sensor is found to be 5.38 mV/mm over 2.6 mm sensing range. The sensor is very useful for close distance target since it is highly sensitive at the front slope. The low development cost, high degree of sensitivity and simplicity of the design make it suitable for wide range of industrial applications.

We propose a novel non-hygroscopic humidity sensor based on a photonic bandgap fiber (PBF) and a digital wavelength modulation spectroscopy (WMS). PBF is employed as the sensing element due to the large overlap of transmitted light with humid air inside the core of the PBF. High sensitivity detection is achieved by using WMS–this shifts the detection bandwidth to a higher frequency. We use the ratio between second harmonic $(2f)$ and the first harmonic $(f)$ of the WMS signals at the water vapor absorption peak around 1368.59 nm for detection of relative humidity. In addition, a PBF fiber reference cell is introduced to lock the DFB laser source at 1368.59 nm. With the WMS and reference cell, various disturbances, such as laser power fluctuations, attenuation variations of the sensing element and fiber can be suppressed. We experimentally implement the scheme with a PBF sensing head of 5 cm and a PBF reference cell of 7 cm. High humidity resolution of ${\sim}{0.2\%}$ Relative humidity (RH) over the range from 0 to 95% RH is achieved.

We proposed a novel relative humidity (RH) sensor based on the air guided photonic crystal fiber (AGPCF) using the direct absorption spectroscopic method in this paper. The wavelength scanning around the water vapor absorption peak around 1368.59 nm was realized by injecting the saw-tooth modulated current to a distributed-feedback laser diode. A reference signal was used as a zero absorption baseline and to help reduce the interference from the distributed-feedback laser source and probed region. The humidity level was determined by the normalized voltage difference between the reference signal and sensor signal at the peak of water vapor absorption. We demonstrated that a length of 5-cm AGPCF with a fixed small air gap between the single mode fiber (SMF) and hollow core fiber as an opening achieved a humidity detection resolution of around 0.2% RH over the range 0 to 90% RH which did not require the use of any hygroscopic coating material.

Polymer micro-fibers with inscribed Bragg gratings are reported in this Letter. Starting with a single-mode polymer optical fiber and implementing a two-stage tapering process, a 16 μm diameter micro-fiber is fabricated and a Bragg grating is inscribed in it that exhibits a peak reflected wavelength circa 1530 nm. The growth dynamics of the polymer micro-fiber Bragg grating are also observed and analyzed. A maximum reflectivity of 5% is obtained after an exposure time of 3 min to a 50 mW power He-Cd laser of 325 nm wavelength. The temperature and strain characterization results of the micro-fiber Bragg grating with different diameters are also presented. Such polymer micro-fiber Bragg gratings can be used as sensors for high-sensitivity measurements in a number of application areas.

We propose a novel relative humidity (RH) sensor based on hollow core fiber using direct absorption spectroscopic method in this paper. The wavelength scanning around water vapor absorption peak around 1368.59 nm is realized by injecting saw-tooth modulated current to a DFB laser diode. A reference signal is used as a zero absorption baseline and to help reduce the interference from DFB laser source and probed region. The humidity level is determined by the normalized voltage difference between reference signal and sensor signal at the peak of water vapor absorption. We demonstrate that a length of 5 cm hollow core fiber with a fixed small air gap between SMF and hollow core fiber as an opening achieves a humidity detection resolution of around 0.02%RH over the range 0 to 50%RH which does not require the use of any hygroscopic material.

We propose a novel ring laser for non-hygroscopic coating relative humidity (RH) fiber sensor by means of infrared absorption spectroscopy. A semiconductor optical amplifier (SOA)-based fiber ring laser is used in this scheme. No tunable optical filter is required for the ring laser scheme as wavelength scanning is introduced in the ring using a distributed feedback (DFB) laser. An air-guided photonic crystal fiber (AGPCF) is included in the ring cavity that acts as a sensing head. The detection of gas humidity inside the air holes of AGPCF is determined by DFB wavelength scanning around 1368.59 nm water vapor absorption peak with SOA as a gain medium in the ring. We have experimentally implemented the wavelength scanning of SOA-based fiber ring laser scheme with an AGPCF sensing head of 5 cm and a small gap between single mode fiber and AGPCF to allow air diffusion in and out of the air holes inside the AGPCF. The sensitivity of the sensor is increased from 2.47 to 10.93 mV/1% RH over the range from 0 to 90% RH when the non-lasing mode (single-pass absorption spectroscopy) of the sensor is changed into the lasing mode (multi-pass absorption spectroscopy).

We propose a novel relative humidity (RH) sensor based on a hollow core-photonic bandgap fiber (HC-PBF) using a wavelength scan and a reference technique in this paper. The wavelength scan of water vapor absorption peak around 1368.59 nm is realized by injecting sawtooth modulated current to a DFB laser diode. The humidity level is determined by the voltage difference (ΔV) between the reference signal and absorption signal at the peak of water vapor absorption. The reference signal is used not only as a zero absorption baseline but also to reduce the influence of intensity noise from DFB laser source. We have experimentally implemented the wavelength scanning modulation scheme with a HC-PBF sensing head of 5 cm and a small gap between SMF and HC-PBF to allow air diffusion into air holes of HC-PBF. High humidity sensitivity of 3.02 mV/1% RH over the range from 0 to 90% RH is achieved without the use of any coating material for humidity sensing.

peer-reviewed A compact and low-cost gas sensor using absorption spectroscopy for Carbon Dioxide (CO2) measurement is presented. The sensing principle is based on open-path direct absorption spectroscopy in the mid-infrared range. The improved reflective structure of optical gas sensor consists of low cost and compact components, such as filament emitter, multispectral pyroelectric detector, Calcium Fluoride (CaF2) window and aluminium curve reflective surface. In the previous investigation, the optimized gas cell structure was simulated using ZEMAX®12 software prior to fabrication for measuring CO2 gas concentration. The developed gas sensing system using the optimized gas cell structure has shown the capability of accurately detecting CO2 concentration. The sensor utilizes a CaF2 narrow bandpass (NBP) filter for detection of CO2 gas with no cross-sensitivity with other gases present in the gas cell. The repeatability of sensor’s response of detecting CO2 was tested with response times were calculated as being less than 1 second.