Fiber Bragg grating

A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to ...

We have applied the diffractive optics and digital holography technologies to the design and fabrication of fiber Bragg gratings (FBG). We studied the phase mask near field diffraction using the numerical simulation. Our new understanding on the phase mask has critical importance for fabrication of complex FBGs, such as the sampled grating DBR laser, multi-channel dispersion compensator, and phase-shift band-pass filters. In the design of the FBGs we introduced the phase-only sampling multi-channel FBGs using the Dammann ...

... b Experimental Mechanics Laboratory, Structural Engineering Research Centre, Council of Scientific and Industrial Research (CSIR), Chennai, India. Received 20 August 2009; revised 28 October 2009; accepted 28 October 2009. Available online 10 November 2009. Abstract. ...

A fiber Brag grating sensor interrogator has been developed which is capable of gathering vectors of information from individual fiber Bragg gratings by capturing the full optical spectrum 3 kHz. Using a field programmable gate array with high speed digital-to-analog converters and analog-to-digital components, plus a kilohertz rate MEMS optical filter, the optical spectrum can be scanned at rates in excess of 10 million nanometers per second, allowing sensor sampling rates of many kilohertz while maintaining the necessary resolution to understand sensor changes. The autonomous system design performs all necessary detection and processing of multiple sensors and allows spectral measurements to be exported as fast as Ethernet, USB, or RS232 devices can receive it through a memory mapped interface. The high speed - full spectrum - fiber Bragg grating sensor interrogator enables advanced interrogation of dynamic strain and temperature gradients along the length of a sensor, as well as the use of each sensor for multiple stimuli, such as in temperature compensation. Two examples are described, showing interrogation of rapid laser heating in an optical fiber, as well as complex strain effects in a beam that had an engineered defect.

In this paper, we discuss the realization of an optical microphone array using fiber Bragg gratings as sensing elements. The wavelength shift induced by acoustic waves perturbing the sensing Bragg grating is transduced into an intensity modulation. The interrogation unit is based on a fixed-wavelength laser source and - as receiver - a photodetector with proper amplification; the system has been implemented using devices for standard optical communications, achieving a low-cost interrogator. One of the advantages of the proposed approach is that no voltage-to-strain calibration is required for tracking dynamic shifts. The optical sensor is complemented by signal processing tools, including a data-dependent frequency estimator and adaptive filters, in order to improve the frequency-domain analysis and mitigate the effects of disturbances. Feasibility and performances of the optical system have been tested measuring the output of a loudspeaker. With this configuration, the sensor is capable of correctly detecting sounds up to 3 kHz, with a frequency response that exhibits a top sensitivity within the range 200-500 Hz; single-frequency input sounds inducing an axial strain higher than ~10nɛ are correctly detected. The repeatability range is ~0.1%. The sensor has also been applied for the detection of pulsed stimuli generated from a metronome.

The Erbium doped fiber laser (EDFL) has demonstrated to be the ideal source for optical communications due to its operating wavelength at 1550 nm. Such wavelength matches with the low-loss region of silica optical fiber. This fact has caused that the EDFL has become very important in the telecomm industry. This is particularly important for Dense Wavelength Division Multiplexing (DWDM) which demands the use of single emission sources with different emission wavelengths. In the long run, this increases the capacity of transmission of information without the necessity to increase the infrastructure, which makes tunable laser sources an important component in DWDM applications. Many techniques for tuning have been demonstrated in the state of the art and we can mention, for example, the ones using birefringence plates, bulk gratings, polarization modified elements, fiber Bragg gratings, and very recently the use of multimode interference (MMI) effects. The MMI consists in the reproduction of single images at periodic intervals along the propagation direction of a multimode optical fiber, taking into account that these single images come from a single mode fiber optic. Here, a compact, tunable, erbium-doped fiber laser is experimentally demonstrated. The mechanism for tuning is based on the multimode interference self-imagining effect, which results in a tunable range of 12 nm and optical powers of 1mW within the region of 1549.78-1561.79nm.

A method based on the quantum-behaved particle swarm optimization algorithm is presented to design a bandpass filter of the fibre Bragg gratings. In contrast to the other optimization algorithms such as the genetic algorithm and particle swarm optimization algorithm, this method is simpler and easier to implement. To demonstrate the effectiveness of the QPSO algorithm, we consider a bandpass filter. With the parameters the half the bandwidth of the filter 0.05 nm, the Bragg wavelength 1550 nm, the grating length with 2cm is divided into 40 uniform sections and its index modulation is what should be optimized and whole feasible solution space is searched for the index modulation. After the index modulation profile is known for all the sections, the transfer matrix method is used to verify the final optimal index modulation by calculating the reflection spectrum. The results show the group delay is less than 12ps in band and the calculated dispersion is relatively flat inside the passband. It is further found that the reflective spectrum has sidelobes around -30dB and the worst in-band dispersion value is less than 200ps/nm . In addition, for this design, it takes approximately several minutes to find the acceptable index modulation values with a notebook computer.

A novel combined fiber Bragg grating (FBG) and interferometric based sensor is proposed and demonstrated. The sensor is based on two overlapped Michelson interferometers working at different wavelengths in a Sagnac loop and two FBGs used as wavelength selective mirrors. The advantage of the system is that it combines the benefit of point measurement with FBG and the high sensitivity of long gauge interferometric sensor.

The study and development of Structural Health Monitoring (SHM) systems for aerospace applications is one of the best challenges for the research in the field of fiber optic (FO) sensors. The harsh environments in which these aerospace structures have to work are the major limit for the employment of standard fiber optic sensors for the thermo-mechanical monitoring processes. Thermal loads which act on these structures do not allow using standard fiber optic sensors used for classic avionics application. In fact, many aerospace structures can be exposed to temperatures up to 1000°C, higher than the operation temperature of the standard fiber optic sensors. In this paper a new fiber optic system for structural analysis of ultra high temperature ceramic (UHTC) materials is proposed. A tunable laser source is used to easily measure the spectral response of different fiber optic sensors. Moreover the employment of an in-fiber optical circulator and TLC 1x4 optical switch, allows to perform a multi-sensor interrogation, to analyse many physical parameters, such as: temperature, strain, pressure, etc.. In particular the monitoring system has been used to test high temperature resistant Fiber Bragg Grating sensors. The first tests at high temperature, up to 600°C, have shown a good response in terms of: sensitivity, resolution, repeatability and dynamic range of the measurement. At last, the flexibility of the electro-optical system developed for the interrogation of the fiber optic sensors, allows the extension of the instruments to mechanical stress analysis, using custom fiber optic strain sensors currently under development.