Hwa-Yaw Tam

Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer

A fiber-optic strain sensor is demonstrated by using a short length of polarization-maintaining photonic crystal fiber (PM-PCF) as the sensing element inserted in a Sagnac loop interferometer. Spectrum shift in response of strain with a sensitivity of 0.23pm∕μe is achieved, and the measurement range, by stretching the PM-PCF only, is up to 32me. Due to the ultralow thermal sensitivity of the PM-PCF, the proposed strain sensor is inherently insensitive to temperature, eliminating the requirement for temperature compensation.

Simultaneous strain and temperature measurement using a superstructure fiber Bragg grating

A novel and simple fiber-optic sensor based on a superstructure fiber Bragg grating (SFBG) for simultaneous strain and temperature measurement is proposed and demonstrated. The transmission spectrum of the sensor possesses several narrow-band loss peaks situated on the slope of a broad-band loss peak. By measuring the transmitted intensity and wavelength at one of the loss peaks, strain and temperature can be determined simultaneously. The accuracy of the sensor in measuring strain and temperature is estimated to be /spl plusmn/20 /spl mu//spl epsiv/ in a range from 0 to 1200 /spl mu//spl epsiv/ and /spl plusmn/1.2/spl deg/C from 20/spl deg/C to 120/spl deg/C, respectively.

Mode-division multiplexed transmission with inline few-mode fiber amplifier

We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fibers LP01 and two degenerate LP11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature

A novel and short (5 mm long) fiber grating based sensor with a fiber grating Fabry-Perot cavity (GFPC) structure was fabricated and tested for simultaneous measurement of strain and temperature. The sensor exhibits unique properties that it possesses two spectral peaks within its main reflection band and the normalized peak power difference, in addition to its peak wavelength shift, changes linearly with strain or temperature. The accuracy of this particular sensor in measuring strain and temperature are estimated to be /spl plusmn/30 /spl mu/s in a range from 0 to 3000 /spl mu/s and /spl plusmn/0.4/spl deg/C from 20/spl deg/C to 60/spl deg/C, respectively.A novel and short (5 mm long) fiber grating based sensor with a fiber grating Fabry-Perot cavity (GFPC) structure was fabricated and tested for simultaneous measurement of strain and temperature. The sensor exhibits unique properties that it possesses two spectral peaks within its main reflection band and the normalized peak power difference, in addition to its peak wavelength shift, changes linearly with strain or temperature. The accuracy of this particular sensor in measuring strain and temperature are estimated to be /spl plusmn/30 /spl mu/s in a range from 0 to 3000 /spl mu/s and /spl plusmn/0.4/spl deg/C from 20/spl deg/C to 60/spl deg/C, respectively.

Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling

A novel in-fiber structure for power-referenced refractometry with the capability to measure surrounding refractive index (SRI) as low as 1.33 is proposed and demonstrated. A short optical fiber stub containing a weakly tilted Bragg grating is spliced to another fiber with a large lateral offset. The reflection from this structure occurs in two well-defined wavelength bands, the Bragg reflected core mode and the cladding modes. The cladding modes reflect different amounts of power as the SRI changes, while the core-mode reflection from the same weakly tilted FBG remains unaffected by the SRI. The power reflected in the core mode band can be used as a reliable reference to cancel out any possible power fluctuations. The proposed refractometer with improved sensitivity for low SRI measurement together with the tip-reflection sensing feature, is a good candidate for sensing in chemical and biological applications.

High-pressure and high-temperature characteristics of a Fabry–Perot interferometer based on photonic crystal fiber

A fiber-optic Fabry-Perot interferometer was constructed by splicing a short length of photonic crystal fiber to a standard single-mode fiber. The photonic crystal fiber functions as a Fabry-Perot cavity and serves as a direct sensing probe without any additional components. Its pressure and temperature responses in the range of 0-40 MPa and 25°C-700°C were experimentally studied. The proposed sensor is easy to fabricate, potentially low-cost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications.

Tilted fiber grating accelerometer incorporating an abrupt biconical taper for cladding to core recoupling

We demonstrate a compact power-referenced fiber-optic accelerometer using a weakly tilted fiber Bragg grating (TFBG) combined with an abrupt biconical taper. The electric-arc-heating induced taper is located a short distance upstream from the TFBG and functions as a bridge to recouple the TFBG-excited lower-order cladding modes back into the fiber core. This recoupling is extremely sensitive to microbending. We avoid complex wavelength interrogation by simply monitoring power change in reflection, which we show to be proportional to acceleration. In addition, the Bragg resonance is virtually unaffected by fiber bending and can be used as a power reference to cancel out any light source fluctuations. The proposed sensing configuration provides a constant linear response (nonlinearity < 1%) over a vibration frequency range from DC to 250 Hz. The upper vibration frequency limit of measurement is determined by mechanical resonance, and can be tuned by varying the sensor length. The tip-reflection sensing feature enables the sensor head to be made small enough (20~100 mm in length and 2 mm in diameter) for embedded detection. The polymer-tube-package makes the sensor sufficiently stiff for in-field acceleration measurement.

Ultrasonic hydrophone based on distributed Bragg reflector fiber laser

We demonstrate a novel fiber-optic hydrophone that uses a dual polarization distributed Bragg reflector (DBR) fiber laser as sensing element. The operation principle is based on the modulation of the birefringence of the fiber laser by high-frequency ultrasound. By measuring the amplitude and frequency of the sidebands as well as the polarization beat frequency of the output of the fiber laser using a photodetector and a radio-frequency spectrum analyzer, the amplitude and frequency of the acoustic pressure, and temperature can be determined simultaneously. The DBR fiber laser hydrophone has a linear response to acoustic pressure and can detect acoustic frequency up to at least 40 MHz.

Temperature-independent fiber Bragg grating tilt sensor

We report a vertical-pendulum-based fiber Bragg grating tilt sensor, which can detect the magnitude as well as the direction of the inclination from the horizontal direction. The sensor is insensitive to temperature, and preliminary results show that tilt angle accuracy and resolution of better than 0.1/spl deg/ and 0.007/spl deg/ can be easily achieved.

Fiber-laser-based wavelength-division multiplexed fiber Bragg grating sensor system

A wavelength-division-multiplexing technique for interrogating 10-cascaded fiber Bragg grating sensing elements based on a ring-compounded-cavity fiber laser is reported. A feedback control technique was introduced to a tunable Fabry-Perot filter that tracks the wavelength-shift of the sensor. The sensors were demodulated using an unbalanced scanning Michelson interferometer and a sensitivity of 1.682/spl deg///spl mu//spl epsi/ has been demonstrated.