Fiber Bragg gratings (FBGs) are wavelength selective optical reflectors with excellent strain
sensitivity and small sensing footprint, which makes them suitable as diagnostic sensors for structural health
monitoring applications. In this work, we explore the narrowband wavelength selectivity of FBGs for optical
feedback in a tunable fiber ring laser. The fiber ring laser consists of an erbium doped fiber laser that is
pumped with a Raman laser (980 nm) to produce population inversion and amplified spontaneous emission
(ASE) in the C-band. The ASE light is used to illuminate a FBG sensor connected to the ring, and the
reflected light from the sensor is fed back into the laser cavity to produce stimulated emission at the
instantaneous center wavelength of the sensor. As the wavelength of the sensor shifts due mechanical or
thermal strains, the wavelength of the optical output from the ring laser shifts accordingly. By combining the
ring laser with a dynamic spectral demodulator for optical readout, the instantaneous wavelength of the ring
laser is tracked with high temporal resolution. The fiber ring laser system offers several potential advantages
in the diagnostic sensing of mechanical strains for SHM applications including, fully integrated laser and
sensor system, high source power levels at the sensor wavelength, narrow spectral line-width, coherent
spectral demodulation, and low system costs.
In this work, we present experimental results that detail the feasibility of dynamic spectral tuning of
the fiber ring laser at frequencies up to hundreds of kilohertz using a single FBG sensing element. Using
multiple sensing elements, the fiber ring laser system would allow for active monitoring of dynamic strains in
a multi-point sensor array configuration, which is particularly suitable for the localization of high frequency
mechanical strains produced by impact loading and cracking events in structures.
A photoacoustic imaging system that incorporates a commercial ultrasonic camera for real-time imaging of two-dimensional (2-D) projection planes in tissue at video rate (30 Hz) is presented. The system uses a Q-switched frequency-doubled Nd:YAG pulsed laser for photoacoustic generation. The ultrasonic camera consists of a 2-D 12×12 mm CCD chip with 120×120 piezoelectric sensing elements used for detecting the photoacoustic pressure distribution radiated from the target. An ultrasonic lens system is placed in front of the chip to collect the incoming photoacoustic waves, providing the ability for focusing and imaging at different depths. Compared with other existing photoacoustic imaging techniques, the camera-based system is attractive because it is relatively inexpensive and compact, and it can be tailored for real-time clinical imaging applications. Experimental results detailing the real-time photoacoustic imaging of rubber strings and buried absorbing targets in chicken breast tissue are presented, and the spatial resolution of the system is quantified.
Fiber Bragg gratings (FBGs) are excellent tools for monitoring mechanical and thermal strains, and
have widespread application in the structural health monitoring (SHM) of aerospace, civil, and mechanical
structures. A common approach used for interrogating FBG sensors involves the illumination of the sensor
with a broadband laser source and the narrowband reflected light reflected from the FBG is monitored with a
wavelength sensitive optical detection system. The thermal or mechanical perturbations experienced by the
FBG sensor lead to a shift in its reflectivity spectrum. In this work, an alternative interrogation scheme is
presented that uses an FBG based narrowband tunable laser source produced by incorporating the FBG into a
fiber ring laser cavity as an optical feedback element. The laser cavity consists an erbium doped fiber (EDF)
connected to the FBG at the output of the fiber ring, which allows for the generation of the required amplified
stimulated emission (ASE) in the C-band and lasing at the center wavelength of the FBG reflectivity spectrum.
With this interrogation scheme, the wavelength of the resulting narrowband laser source tracks the center
wavelength of the FBG sensor as it drifts due to quasi-static and/or dynamic mechanical and thermal strains.
In addition, the instantaneous spectral line-width of the laser source is effectively narrowed owing to the long
length of the laser cavity, which facilitates highly sensitive demodulation of dynamic shifts of the lasing
wavelength with a high coherence optical interferometer.
Recent work by our research group on the dynamic demodulation of strain-induced wavelength shifts in fiber Bragg grating (FBG) sensors show that these sensors are suitable for the detection of high frequency ultrasonic waves produced by impact loading. A FBG sensor is incorporated into an optical detection system that uses a broadband tunable laser source in the C-band, a two wave-mixing photorefractive interferometer, and a high-speed photodetector. When an ultrasonic wave interacts with the FBG sensor, the wavelength of the reflected light in the fiber is dynamically shifted due to strain-induced perturbation of the index of refraction and/or the period of the grating in the fiber. The wavelength shift is converted into an intensity change by splitting the light into signal and pump beams and interfering the beams in an InP:Fe photorefractive crystal (PRC). The resulting intensity change is measured by a photodetector. The two-wave mixing (TWM) photorefractive interferometer allows for several FBG sensors to be wavelength multiplexed in one PRC and it also actively compensates for low frequency signal drifts associated with unwanted room vibrations and temperature excursions. In this work, we present preliminary experimental results on the detection of impact signals using a low power (1 mW) TWM PRC based demodulation system. The response time of the PRC is optimized by focusing the signal and pump beams into the crystal allowing for adaptivity of the demodulation system to quasi-static strains or temperature drifts. The TWM intensity gain of the system is optimized for efficient wavelength demodulation through resonant enhancement of the space charge electric field formed in the PRC. The low power demodulation system would facilitate significant reduction in the overall cost of the system.
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