In this paper, we propose and demonstrate a novel room temperature multiwavelength erbium-doped fiber laser (EDFL) scheme with the wavelength spacing less than homogeneous broadening linewidth based on the inter-channel four-wave mixing (FWM). A stable CW MW-EDFL is firstly implemented based on a length of high nonlinear photonic crystal fiber (HNL-PCF) and sampled-fiber Bragg grating (SFBG). We further extend the idea of FWM induced self-stable operation of MW-EDFL to the multiwavelength mode-locking fiber ring laser to generate ultra-short, ultra-fast, multiwavelength and synchronized pulse sources. 2 and 4 wavelengths anchored on ITU-T standards with 100 GHz channel spacing are successfully mode-locked at 10-GHz simultaneously, without gain competition.

A novel method for light coupling between single mode fibers and small-core photonic crystal fibers is demonstrated. The method is based on growing photopolymer micro-tips directly on the end face of single mode fibers. The advantages of this micro-tip fabrication method are its simplicity, controllability, reproducibility and being inexpensive. The shape and the size of the tips can be controlled, by adjusting the laser power, the exposure time and the oxygen diffusion concentration for polymerization, to match its mode field to the small-core photonic crystal fibers. A photopolymer micro-tip integrated on the end face of a single mode fiber is used to reduce the mode field diameter and increase the numerical aperture of the light beam coming out from the single mode fiber, so that there is a better match to the small mode field diameter and the large numerical aperture of small-core photonic crystal fibers. A 5 dB improvement in coupling efficiency between a single mode fiber and a commercial small-core, highly nonlinear photonic crystal fiber is experimentally demonstrated. This compact and efficient butt-coupling method is particularly suitable for photonic crystal fiber gas sensor applications where holes in the photonic crystal fiber need to be kept open at the joint for easier access to the evanescent field, This method also may be suitable for the connecting a single mode fiber to a hollow-core photonic crystal fiber with the central hole filled with a liquid sample for liquid sensor applications.

We report on the increase in frequency response of a semi-insulating GaAs Four Point Photo-Conductance (FPPC) remote vibration monitoring array. This simple, rugged device works by monitoring the lateral motion of an illumination pattern and has been demonstrated for remote, non-contact vibration sensing applications in the kilohertz regime at a distance over five meters. The FPPC sensor requires only a line-of-sight path to the object being probed (although later models are expected to work also with multimode fiber optics feed). Due to its construction and application to remote sensing, the FPPC device enables measuring vibrating bodies enclosed in a high temperature or pressure environment without adverse effects. This paper reports techniques for increasing the frequency response in these sensors. Experimental results will be given on sensors that have an approximate -3 db rolloff frequency of 10 kHz to 40 kHz. The performance improvements were achieved by replacing the opaque AuGe electrical contacts with transparent Indium Tin Oxide (ITO) contacts. These 80 nm thick films were deposited by RF sputtering at 3 mTorr Argon back pressure. Further frequency response improvements were obtained by increasing the array conductivity with background illumination from a red LED. These improvements are attributed to the reduction in lateral trapped space charge distribution across the active areas. The FPPC devices exhibited rise times of 1.5 microseconds and effective fall times of 100 microseconds. The pulse response exhibited tails with an underdamped oscillatory behavior; the damping increased with increasing background illumination.

In fiber Bragg grating (FBG) sensors, detecting the Bragg wavelength accurately could be difficult due to a low signal-to-noise ratio (SNR) in the FBG spectrum. Two common sources of noise are the general random noise from the broadband sources and the interferometric noise caused by the residual reflections in the sensor system. Conventional filtering techniques could be quite effective in removing random Gaussian-white noise, but not so for the interferometric noise, which is very structured. On the other hand, parameter estimation techniques such as nonlinear least squares can be used to identify the parameters in the interferometric noise and remove it accordingly. However, since the parameter estimation problem is nonlinear, the larger the number of parameters, the higher the chance that the algorithm will get trapped into a local minimum and fail to identify the correct parameters. In this paper, it is proposed to combine the nonlinear least squares method with a Kalman smoother. Hence, the number of parameters to be estimated by the nonlinear least squares algorithm will be greatly reduced. To do this, a continuous-time linear time-varying state-space model is derived for the FBG spectrum and then the model is discretized so that the Kalman smoother can be applied. An interesting point to note is that this model is linear time-varying instead of nonlinear, thus not requiring an extended Kalman filter. Computer simulations are provided in the paper to demonstrate the effectiveness of the proposed method, followed by applications to real experimental data. Improvements in the accuracy of Bragg wavelength detection are observed.

We report on the use of the temperature-tuned optical parametric oscillator for trace gas detection. A synchronization trace gas detection system was designed and demonstrated, in which the measuring errors caused by the instability of the OPO could be greatly reduced. The trace gas detection system was based on a periodically poled MgO-doped LiNbO3 optical parametric oscillator (OPO) which was pumped by a diode-pumped passively Q-switched Nd:GdVO4 laser. The OPO could produce wavelength-tunable signal output through changing the crystal temperature and the grating periods. The usefulness of the trace gas detection system for spectroscopy was demonstrated by directly measuring the photon absorption spectrum of the methane and acetylene gas cells.

The transmission spectrum of a photonic bandgap fiber filled with low index material is investigated. A simple analytical model is developed to predict the position and bandwidth of the band gap in the wavelength domain with respect to the refractive index. The wavelength of the band gap has a blue shift and the bandwidth of the band gap becomes narrow with the increasing of the refractive index of the filled material. The degree of shifting of the band gap increases with the reduction of air-filling fraction of the photonic bandgap fiber.

Support vector machine (SVM), is proposed to enhance the measurement accuracy of a temperature-tuning optical parametric oscillator (OPO) gas sensing system. The experimental results demonstrate that the minimum detecting concentration after the use of SVM decreases by more than 8 times.

An original guided-wave optical pressure sensor, which has a semi-closed space with a small hole under a diaphragm, can be used even under high quasi-static pressure without sacrificing sensitivity, unlike conventional pressure sensors. Moreover, the sensor possesses characteristics of a high-pass filter, so that it responds to only the high-frequency components of pressure change. The cutoff frequency of the high-pass filter of the sensor property is a key factor in designing the sensor, and is acquired from the step response of the sensor. In a step-like change in ambient pressure, a pressure difference is induced on the diaphragm for a short while because the small hole restricts fluid flow between the semi-closed space and the surroundings. The reciprocal of the duration of the induced pressure difference corresponds to the cutoff frequency. In this study, the step response in relation to the cross-sectional area of the small hole was examined experimentally, and the measured durations were compared with the theoretical ones. In the experiment, the duration was approximately inversely proportional to the area of the small hole as theoretically predicted although the measured durations are larger by a factor of thousands than the calculated ones.

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in July 1996. Upon investigation the National Transportation Safety Board (NTSB) determined that the probable cause of the accident was an explosion of the center wing tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank. The Federal Aviation Administration (FAA) has focused research to support two primary methods of fuel tank protection -- ground-based and on-board -- both involving fuel tank inerting. Ground-based fuel tank inerting involves some combination of fuel scrubbing and ullage washing with Nitrogen Enriched Air (NEA) while the airplane is on the ground (applicable to all or most operating transport airplanes). On-board fuel tank inerting involves ullage washing with OBIGGS (on-board inert gas generating system), a system that generates NEA during aircraft operations. An OBIGGS generally encompasses an air separation module (ASM) to generate NEA, a compressor, storage tanks, and a distribution system. Essential to the utilization of OBIGGS is an oxygen sensor that can operate inside the aircraft's ullage and assess the effectiveness of the inerting systems. OBIGGS can function economically by precisely knowing when to start and when to stop. Toward achieving these goals, InnoSense LLC is developing an all-optical fuel tank ullage sensor (FTUS) prototype for detecting oxygen in the ullage of an aircraft fuel tank in flight conditions. Data would be presented to show response time and wide dynamic range of the sensor in simulated flight conditions and fuel tank environment.

This paper presents an optical device for uninterrupted non-contact temperature measurements and detection of overheated bearing of train. The pair of proposed optical devices is suitable to be mounted along the same axis as wheel axle on both railway sides. The optimum position for optical devices mounting is 0.5m above the rail tracks level. The proposed method is based on remote infrared measurement of lid surface of axle bearing. The device consists of three parts: optical part, infrared detector and processing unit. This paper presents characteristics of used lenses and optical filter. The output signal waveforms depend on train speeds. Proposed optical device are shown to have a vital role in: keeping railroad operations safe, preventive maintenance and avoiding of damaging of axle wheel bearing in railway.

In this paper, we introduce a novel optical fiber, referred to as Fresnel fiber. This fiber has equal area rings and is fabricated using two materials only. Transmission properties of Fresnel fibers, including axial propagation constant, effective area, and dispersion, are investigated. Design of these fibers for large or small effective area and prescribed dispersion characteristics, required for sensing or communication applications, are also addressed.

An optical fibre based exhaust gas sensor has been developed from low-cost mid-infrared components which is capable of detecting carbon dioxide (CO₂) emissions from both diesel and petrol engines. The optical fibre sensor is not cross sensitive to other gaseous species in the exhaust such as water vapour (H₂O), carbon monoxide (CO), oxides of nitrogen (NO_x) or oxides of sulphur (SO_x). Initial tests of this sensor on a modern diesel engine are outlined in this paper.

This paper describes the successful test of a multi-point fiber optic hydrogen sensor system during the static firing of an Evolved Expandable Launch Vehicle (EELV)/Delta IV common booster core (CBC) rocket engine at NASA's Stennis Flight Center. The system consisted of microsensors (optrodes) using a hydrogen gas sensitive indicator incorporated onto an optically transparent porous substrate. The modular optoelectronics and multiplexing network system was designed and assembled utilizing a multi-channel opto-electronic sensor readout unit that monitored the hydrogen and temperature response of the individual optrodes in real-time and communicated this information via a serial communication port to a remote laptop computer. The sensor packaging for hydrogen consisted of two optrodes -- one doped with an indicator sensitive to hydrogen, and the other doped with an indicator sensitive to temperature. The multi-channel hydrogen sensor system is fully reversible. It has demonstrated a dynamic response to hydrogen gas in the range of 0% to 4% with 0.1% resolution and a response time of less than or equal to 15 seconds. The sensor package was attached to a custom fiber optic ribbon cable, which was then connected to a fiber optic trunk communications cable (standard telecommunications-grade fiber) that connected to the optoelectronics module. Each board in the expandable module included light sources, photo-detectors, and associated electronics required for detecting hydrogen and temperature. The presentation would discuss the sensor design and performance data under field deployment conditions.

In this paper, we present the differential strain sensitivity characteristics of core and cladding modes in weakly TFBG. Both experiment and analysis results are presented, and they are well matched. The results show that there are three different strain sensitivity regions for cladding mode resonances: the short wavelength region, the ghost mode region and the nearly linear sensitivity change region between them. By monitoring the cladding modes with different strain sensitivities, and noting that the different cladding modes have similar temperature sensitivities, weakly TFBG are attractive candidates for more accurate temperature-independent strain sensors.

The possibility of combining load history with ultrasound signal information offers, potentially, an extremely powerful method for determining the structural health of a structure. Here we describe how suitably configured fibre Bragg grating arrays can be used to both monitor strain fields and to detect the magnitude and direction of ultrasound waves. The directional responses of fibre Bragg gratings to both static and ultrasonic strain fields have been investigated and the results used to enable the FBGs to be configured into rosettes. These can be interrogated to give both the magnitude and direction of either the principle static strain or an incident ultrasound Lamb wave. Measurement of the static strains enables us to perform strain mapping and hence determine load history, whilst ultrasound detection allows us to monitor a structure for defects such as cracks. If Lamb waves are generated by, for instance, PZTs bonded to the structure, comparing signals from the rosette with data previously obtained for the healthy structure allows possible defects to be identified and located. Location is carried out by calculating the intersection point of ultrasound direction from 2 or more rosettes. The geometry of the rosette configuration is described, together with the parameters affecting efficient strain transfer and the optimisation of FBG interrogation techniques.

We are proposing a novel technique to implement low cost motion and position sensing via micro-photonic devices based on digital diffractive optics. Standard optical encoders based on conventional shadowing effect as well as more complex grating based optical encoders are now becoming commodities in the automation industry. However, they lack many of the requirements for demanding applications as in the automotive industry. The severe automotive requirements are not only linked to high performance (resolution), but also to harsh environmental issues like fast temperature drifts, vibrations, shocks, accelerations, dust, humidity, to drastic cost issues, to small size/footprint, and finally to reliability (very long MTBF or constant monitoring of the wear-out for fault tolerance). Our diffractive optics based motion and position sensors are best suited for such special environments, while providing same or higher accuracy and resolution than standard encoders without the need for slow and/or unreliable electronic interpolation methods as it is done in the vast majority of encoders on the market today. We are proposing a novel method to design, implement and replicate in mass such motion and position sensors, which make them ideal solutions for large volume - low cost - high performance - high reliability applications. As an application example, we are applying our technology to the "Steer By Wire" (SBW) technology. Our SBW implementation requires 3 different sensors, two angular encoders (hybrid and absolute) and one torque sensor.

The ability to monitor the health of an aircraft engine is desirable in a diagnostic test cell. The work described in this paper discusses testing efforts to develop a laser-induced breakdown spectroscopy (LIBS) system for monitoring the exhaust gas from a turbine engine. The goal of this effort is the detection of metallic particles within the exhaust stream of the turbine engine. The particulate matter may be derived from wear of engine components including blades, bearings, and casings. As described in the paper, a magnesium chloride (MgCl) solution was injected into the exhaust nozzle of a turbine engine, and the LIBS plasma spark was formed within the exhaust stream exiting the nozzle. The LIBS system demonstrated the ability to detect Mg within the exhaust stream during the seeded flow. The LIBS system also detected the presence other particulate matter during the testing.

The reduction of harmful environmental pollutants which can have adverse effects on human health and the development of sensors capable of monitoring low concentrations of these pollutants is a major source of concern for many researchers today. This paper describes a multipass absorption cell used to detect and monitor the presence of several gases in the ultra violet and visible regions. An integrating sphere with a highly reflective internal coating (over 99%) was adapted in order to input and output various gases. Sulphur dioxide was detected in the ultra violet region, ozone in the visible and nitrogen dioxide in both the ultra violet and visible. This paper will report the generation of effective optical path lengths of up to 70 cm using a 5 cm diameter integrating sphere. This results in an optical sensor capable of detecting sulphur dioxide concentrations as low as 10 ppm, nitrogen dioxide concentrations as low as 4 ppm and ozone levels of the order of 500 ppm.

A fibre-optic sensor for the monitoring of hazardous exhaust gases based on absorption in the ultra-violet region is described. The loss of light through a gas cell across the UV/VIS spectrum was utilised to determine the level of absorption for three of the exhaust gases present in an exhaust. The measured absorption specific to each of these test gases Nitrogen Dioxide (NO₂), Sulphur Dioxide (SO₂) and Nitric Oxide (NO) was used in a variation of the Beer-Lambert law to determine the absorption line intensities for each of the gases. Theoretical absorption line intensities for each of the exhaust gases compared favorably with our measured results. A LabVIEW program was created and utilised to interrogate the highest absorbing wavelength for each of the gases and absorption recorded at these specific absorbing wavelength were then input along with our measured absorption line intensities into the Beer-Lambert law to determine the concentrations of each of the gases present in the test cell. In this manner the concentrations were calculated immediately and then output to the user eliminating the need for processing the data after testing. A lower detection level of 1ppm for both NO₂ and SO₂ and in the order of 26ppm for NO was achieved.

Thermopile pyrometer modules are the state of the art for contactless temperature measurements in automotive applications. In such an application, the thermopile has to operate precisely in a challenging thermal environment. While the compensation of the steady state ambient temperature is a well known technique when using thermopiles for temperature measurments, transient thermal effects are still an issue. The change of the ambient temperature as well as temperature flow through the sensor can lead to substantial errors due to unwanted thermal gradients within the device. In the thermopile chip they lead to an error signal since the measurement principle is based on quantifying thermal gradients of the chip that result from the detected IR-radiation. Thermal gradients in the cap and between the cap and the thermopile chip will lead to an exchange of heat radiation between the thermopile chip and the cap, which also leads to measurement errors. Different methods were developed that separately or in combination allow for a significant improvement of the accuracy and signal stability. The methods are based on the reduction of thermal gradients within the thermopile chip and the entire sensor device (isothermal, high thermal mass cap), reduction of radiation exchange between the sensor chip and the housing (low emissive inner cap surface) and prediction and software compensation of the error signal.

The fiber optic gyroscope (FOG) is a single axis rotation sensor which is currently employed in many advanced navigation systems. A major contribution to the cost of an FOG is the price of components such as the polarizer, phase modulator and associated detection electronics. As a lower cost realization of the device is of great importance for its wide deployment in many applications, the possibility of rotation rate measurement with moderate accuracy using a simplified FOG configuration is a very interesting issue. A low-cost simplified implementation of the open loop FOG was carried out to investigate its performance in the absence of a polarizer and a phase modulator and observe the extent to which it can usefully detect the rotation rate in the presence of polarization and phase fading. This paper reports on the realization of the simplified FOG configuration and discusses the association of polarization and phase effects to the measurement errors incurred. The results indicate that the error due to the absence of the polarizer and phase modulator can be of the range of only few hundredths of the rotation rate. This is explained by noting that the phase changes in the path affect both perpendicular polarizations approximately similarly leading to Φ_x being almost equal to Φ_y and hence the polarizer importance appears when using a phase modulator which affects each polarization differently. Possible practical uses of such a simplified gyroscope configuration are suggested for low accuracy automobile guidance applications.