Dusun Hwang

High temperature fiber sensor with high sensitivity based on core diameter mismatch

We report a simple fiber sensor for measurement of high temperature with high sensitivity. The sensing head is a multimode-single mode-multimode (MM-SM-MM) fiber configuration formed by splicing a section of uncoated single mode fiber (SMF) with two short sections of multimode fibers (MMF) whose core is composed of pure silica. Because of the mode-field mismatch at the splicing points of the SMF with 2 sections of MMFs, as well as index matching between the core of the MMF and the cladding of the SMF, optical power from the lead-in fiber can be partly coupled to the cladding modes of the SMF through the MMF. The cladding modes of the SMF then re-coupled to the lead-out fiber, in the same fashion. Due to the effective index difference between the core and cladding modes, an interference pattern in the transmission spectrum of the proposed device was obtained. The interference pattern was found to shift to the longer wavelength region with respect to temperature variation. The temperature sensor can measure temperature stably up to more than 900 degrees C with sensitivity of 0.088 nm/ degrees C.

Novel auto-correction method in a fiber-optic distributed-temperature sensor using reflected anti-Stokes Raman scattering.

A novel method for auto-correction of fiber optic distributed temperature sensor using anti-Stokes Raman back-scattering and its reflected signal is presented. This method processes two parts of measured signal. One part is the normal back scattered anti-Stokes signal and the other part is the reflected signal which eliminate not only the effect of local losses due to the micro-bending or damages on fiber but also the differential attenuation. Because the beams of the same wavelength are used to cancel out the local variance in transmission medium there is no differential attenuation inherently. The auto correction concept was verified by the bending experiment on different bending points.

Simultaneous measurement of temperature and strain using a Lyot fiber filter incorporated with a fiber Bragg grating in a linear configuration

We report a fiber sensor capable of simultaneously measuring temperature and strain. A fiber Bragg grating (FBG) was incorporated in a Lyot fiber filter (LFF) by fusion splicing an FBG and a section of high birefringence fiber (PM fiber), which is an elliptical core side-hole fiber, and then placing them between two polarizers. Measured in the transmission mode, the fringe resulting from the LFF and the resonance wavelength dip of the FBG have different responses when a variation of temperature or strain is applied. The proposed device can therefore measure both strain and temperature simultaneously.

Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters.

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a pi phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to 10(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.

Intensity-based optical fiber strain sensor using long-period fiber gratings and a core mode blocker

We report an intensity-based fiber strain sensor using a pair of long-period fiber gratings (LPFGs) and an in-line core mode blocker. By inserting a core mode blocker fabricated by the arc discharge method between two LPFGs, a band-pass filter can be formed. We used an LED and an optical power meter to measure the transmittance near the resonance wavelength of the LPFGs and we obtained a relation between the axial strain applied on one of the LPFGs and the transmitted power. The measurement results indicate that the sensitivity of the power meter output voltage to the applied strain is 6.37 pV ???1.

Restoration of Reflection Spectra in a Serial FBG Sensor Array of a WDM/TDM Measurement System

A restoration method for reflection spectra in a serial FBG sensor array with spectral shadowing is proposed and experimentally demonstrated in a WDM/TDM combined multiplexing system. The SNR of each FBG sensor is formulated and analyzed as a function of the number and reflectivities of serial FBG sensors. The maximum number of FBG sensors in a single fiber line can be determined by the approximate formula. In the test using two FBG sensors, the restored reflection spectrum of second FBG sensor is shown to be very well matched with the original reflection spectrum. Using the proposed restoration method, the maximum peak detection error in a strain experiment is suppressed drastically by almost seven-fold, from 0.074 nm to 0.011 nm.

Gain-clamped discrete Raman amplifier with suppressed low-frequency relative intensity noise pump-to-signal transfer

We demonstrate an optically gain-clamped discrete Raman amplifier with a suppressed low-frequency relative intensity noise transfer from pump sources to an input amplified signal, which is accomplished by employing a cascaded second-order Raman fiber resonator as an optical amplification. The input signal dynamic range for a 3-dB gain compression is controllable without affecting the clamping level. In addition, we find that the Raman net gain is unsusceptible to fiber parameters such as the gain and attenuation coefficients at the pump, first-, and second-order Stokes lines. These characteristics fit our proposed gain-clamped discrete Raman amplifier for practical deployment although it works at the expense of the efficiency.

Stabilization of the output power from a Raman fiber laser by generated-amplified spontaneous emission

In this work, an optical approach is theoretically presented for suppressing the low-frequency relative intensity noise transfer from the pump source to the Raman fiber laser output. This is accomplished by generating a sufficiently high-amplified spontaneous emission intensity at one Stokes shift in the wavelength from the lasing line to stabilize the latter at the cost of lasing efficiency. In addition, the clamped laser output can be easily controlled in a large dynamic range. These special properties suit the proposed Raman fiber laser to usage in Raman fiber amplifiers as the pump source.

Analytic Formulation of Transmission Light Intensity of Hole Blockers in Intensity-based Polymer Optical Fiber Sensors

Intensity-based optical fiber sensors are devised using a blocker which is located between two polymer optical fibers(POFs), one fiber is light-in and the other is light-out. This blocker is moved by an external displacement. Therefore, finding a general formulation of the relation between this displacement and transmission light intensity of various blockers is important to help develop intensity-based optical fiber sensors. In this paper, we consider blockers with arbitrary shapes from circular holes to inclined angled blockers. The transmission light intensities of such blockers should be determined by this generalized equation. In order to verify this equation, the calculated intensities of the blockers are compared with the values acquired from experiment. In the comparison, it is shown that the analytic equation can give the exact values of the transmitted light intensities for the assorted blockers. The range of the displacement measurement is also shown to be about 6 times of the radius of the hole in the case of a 9 degree inclined angle blocker.

Ultra-narrow dual-channel filter based on sampled fiber Bragg gratings

We have proposed a new kind of ultra-narrow dual-bandpass fiber Bragg gratings. Based on the proposed grating, an ultra-narrow dual-channel filter is designed. Our filter has the same bandwidth and the even strength, as well as the ultra-narrow bandwidth as small as <1 pm.