Lipi Mohanty

Fiber Bragg grating microphone system

A fiber Bragg grating microphone has been developed and tested in the audible frequency range. The fiber Bragg grating is longitudinally attached to a membrane that vibrates in response to acoustic vibrations and is prestrained. The frequency and amplitude of the sound are encoded in the wavelength shift. The microphone can also pick up voices dynamically from a distance of a few meters. The simplicity of principle and structure of this fiber grating sensor provides scope for commercialization as an optical microphone.

The use of an embedded chirped fibre Bragg grating sensor to monitor disbond initiation and growth in adhesively bonded composite/metal single lap joints

Chirped fibre Bragg grating (CFBG) sensors embedded within glass fibre reinforced plastic (GFRP) adherends have been used to monitor disbond initiation and growth in an adhesively bonded GFRP/aluminium alloy single lap joint. The elevated temperature curing of the adhesive used in the manufacture of the joint leads to thermal strains being generated within the GFRP and aluminium adherends. Disbond initiation and growth between the adherends during fatigue cycling causes a relaxation of the residual thermal strains within the composite adherend and perturbations (peaks or dips) in the reflection spectra from the CFBG sensor. These perturbations can be used, when the joint is unloaded, to monitor both the initiation of a disbond in the joint and to monitor the growth of the disbond with fatigue cycling.

Pressure mapping at orthopaedic joint interfaces with fiber Bragg gratings

We present the concept of a fiber-optic sensor that can be used for pressure mapping at the prosthetic knee joint, in vitro and in vivo. An embedded array of fiber Bragg gratings is used to measure the load on the tibial spacer. The sensor gives the magnitude and the location of the applied load. The effect of material properties on the sensitivity of each subgrating is presented. The wavelength-shift maps show the malalignment of implants and demonstrate the potential of this sensor for use during total knee arthroplasty.

High relative humidity sensing using gelatin-coated long period grating

A long-period grating (LPG) coated with gelatin was developed as a high relative humidity (RH) sensor. The resonance dip or coupling intensity of the LPG spectrum varies with humidity while the resonance wavelength remains constant. The principle of operation of the sensor is based on the effect of an external medium, with higher refractive index than that of silica or cladding, on the LPG spectrum. Experimental investigations on the sensor yield a sensitivity of 1.2dB/%RH with an accuracy of ±0.25%RH, and a resolution of ±0.00833%RH. The LPG RH sensor also offers repeatability, hysteresis and stability errors of less than ±0.877%RH, ±0.203%RH and ±0.04%RH respectively. In addition to the characterization of the LPG RH sensor, further studies were conducted to determine the effect of grating periodicities on the sensitivity. Results show that higher-order cladding modes from smaller grating periods enable the sensor to achieve higher sensitivity to humidity. This method is proposed to be more cost effective as compared to more complex spectroscopic methods based on wavelength detection. This sensor can also help to solve problems in measuring high humidity with existing relative humidity measurement systems.

Pressure sensing with embedded chirped fiber grating

Abstract In this paper, we present a new design and development of embedded chirped fiber grating for sensing local pressure in the range 0–30 N along the length of the grating. The sensor can also detect the location of the force. The embedding design is supported with experimental results, which show that the change in reflected intensity varies linearly with the applied pressure. The reflected wavelength where the change is observed varies linearly with the point of application of the force.

Simultaneous measurement of load and position with an embedded chirped sampled fibre grating

In this paper we present the use of a chirped sampled grating for distributed sensing of pressure over a small area (e.g. a few centimetres). The grating is embedded in fibre-reinforced composite. It is found that the shift in the central wavelength of each sub-grating in the chirped sampled grating increases monotonically with an increase in the applied force. The central wavelength of the sub-grating gives the location of the applied pressure. When a surface comes into contact with the sensor, the distribution of the pressure determines the shift in central wavelength of the various sub-gratings. The sub-grating that experiences the maximum pressure will show maximum wavelength shift whereas adjacent sub-gratings will show less shift. This method gives the location and magnitude of the load even when the load is not directly applied to any of the sub-gratings. The sensor design comprises of a sampled chirped grating embedded in unidirectional fibre-reinforced composite prepreg. The prepreg enhances the mechanical strength and the unidirectional embedding reduces birefringence. The number of layers in the prepreg stack varies the sensitivity.

Sampled chirped fiber gratings as distributed pressure sensors

Fiber gratings have been studied for their applications in sensing and communications. Many sensing applications of the uniform fiber Bragg grating, chirped fiber grating and long period grating have been studied, proposed and commercialized. Sampled chirped gratings have been studied for multichannel dispersion compensation in DWDM systems. In this paper, we show that the sampled chirped fiber grating can be used as a distributed pressure sensor. The chirp provides ease of manufacture of many gratings. The sampling results in many small, uniform grating-like structures. This fact can be used to simulate a distributed sensor over the length of the sampled chirped grating. When a surface comes into contact with the sensor, the distribution of the pressure determines the shift in central wavelength of the various sub-gratings. The sub-grating that experiences the maximum pressure will show maximum wavelength shift whereas adjacent sub-gratings will show less shift. This can also give the location of the pressure. The sensor design comprises of a sampled chirped grating embedded in unidirectional fiber-reinforced composite prepreg. The prepreg enhances the mechanical strength and the unidirectional embedding reduces birefringence. The number of layers in the prepreg stack varies the sensitivity. Such distributed pressure sensors can be applied in robotics, ergonomics, and in the biomedical field.