Cross-talk free and ultra-compact fiber optic sensor for simultaneous measurement of temperature and refractive index.

We propose and demonstrate a cross-talk free simultaneous measurement system for temperature and external refractive index (ERI) implemented by dual-cavity Fabry-Perot (FP) fiber interferometer. The sensing probe consists of two cascaded FP cavities formed with a short piece of multimode fiber (MMF) and a micro-air-gap made of hollow core fiber (HOF). The fabricated sensor head was ultra-compact; the total length of the sensing part was less than 600 mum. Since the reflection spectrum of the composite FP structures is given by the superposition of each cavity spectrum, the spectrum measured in the wavelength domain was analyzed in the Fourier or spatial frequency domain. The experimental results showed that temperature could be determined independently from the spatial frequency shift without being affected by the ERI, while the ERI could be also measured solely by monitoring the intensity variation in the spatial frequency spectrum. The ERI and the temperature sensitivities were approximately 16 dB/RIU for the 1.33-1.45 index range, and 8.9 nm/ degrees C at low temperature and 14.6 nm/ degrees C at high temperature, respectively. In addition, it is also demonstrated that the proposed dual-cavity FP sensor has potential for compensating any power fluctuation that might happen in the input light source.

Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer.

A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000 degrees C with a step of 50 degrees C confirm that it could be applicable as a high-temperature sensor.

Lensed photonic crystal fiber obtained by use of an arc discharge.

A lensed photonic crystal fiber (PCF) is proposed as an effective element for an optical free-space interconnector. By simultaneously forming a beam-expansion region and a focusing lens on a single piece of PCF, effective coupling between PCFs could be achieved. A long working distance of up to 1 mm with wide longitudinal and lateral tolerances was measured. The optical characteristics of the lensed PCFs and the connectors made from them are analyzed experimentally and theoretically.

Impact of interstitial air holes on a wide-bandwidth rejection filter made from a photonic crystal fiber.

We have investigated the spectral properties of a band rejection filter made with a long-period fiber grating written in photonic crystal fiber that has interstitial air holes. Experiments showed that only one mode was coupled strongly to the fundamental core mode over a 600 nm spectral range. The central wavelength of the filter could be tuned over that range without being appreciably affected by any other mode. By using the multipole method, we found that the interstitial air holes of the photonic crystal fiber played a critical role in limiting the number of modes that could strongly interact with the fundamental mode and in obtaining well-separated resonance peaks. Excellent agreement between theory and experiment was obtained.

All fiber polarimetric modulation using an electro-optic fiber with internal Pb-Sn electrodes.

All fiber electro-optic modulation using a polarimetric cell based on an electro-optic fiber with internal Pb-Sn electrodes was demonstrated. For the polarimetric cell, the electro-optic fiber with two 149 cm long internal electrodes was fabricated by injection of a molten Pb-Sn alloy into the holes of the fiber. The characteristics of the modulation were explained by the electric field induced phase retardation due to the polarization dependent electro-optic Kerr effect. The difference in the effective second order nonlinearity between TM and TE polarization directions was obtained to be 0.0035 pm/V at the applied DC voltage of 6.5 kV.

All-optical 2 x 2 switching with two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair.

We have demonstrated a new type of all-optical 2 x 2 switch by using two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair and a 3-dB fiber coupler. A 400-Hz square-wave pulse train at approximately 1549.4 nm was fully switched between the two output ports up to 200 Hz by modulated pump signals at 976 nm with a maximum pump power of approximately 35 mW, where the extinction ratio at approximately 1549.4 nm between the on and the off states was approximately 17.5 dB.

Polarization controlled multi-wavelength Er-doped fiber laser using fiber Bragg grating written in few-mode side-hole fiber with an elliptical core.

We propose and experimentally demonstrate a novel method for multi-wavelength erbium-doped fiber laser by using fiber Bragg gratings written in few-mode side-hole fiber with an elliptical core. The resonance peaks can be made to split by properly adjusting the polarization state due to the high birefringence of the fiber with the elliptical core. The number of lasing wavelengths can thus be switched using the polarization controller. This multi-wavelength laser has the advantages of low cost, novel and simple configuration.

Multi-wavelength linear-cavity tunable fiber laser using a chirped fiber Bragg grating and a few-mode fiber Bragg grating.

A novel and simple multi-wavelength linear-cavity tunable fiber laser source using a chirped fiber Bragg grating (CFBG) and a few-mode fiber Bragg grating (FMFG) is demonstrated using erbium-doped fiber (EDF) as the gain medium. In our linear-cavity configuration, the FMFG acts as full-reflecting mirror and wavelength selector while the CFBG with the 3-dB bandwidth of over 30 nm acts as a broadband partially reflecting mirror. The number of lasing wavelengths can be controlled by changing the state of polarization inside the cavity using a polarization controller. The large bandwidth of the CFBG enables continuous tuning of the lasing wavelengths by application of mechanical strain or thermal heating on the FMFG.

Fabrication of helical long-period fiber gratings by use of a CO2 laser.

We present a method of helical long-period fiber grating (H-LPFG) fabrication by use of a CO2 laser for use as an optical torque sensor. A conventional optical fiber grating has periodic vertical index changes along its fiber axis, but a H-LPFG has a screw-type index modulation. The helical index modulation is obtained with the asymmetric index change caused by a single-side laser beam exposure. The H-LPFG shows peak shifts with codirectional or contradirectional torsion to the helix. Also, the polarization-dependent loss is measured to be relatively small compared with that of a conventional long-period fiber grating.

Tunable photonic crystal fiber coupler based on a side-polishing technique.

A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range.

All-optical switching application based on optical nonlinearity of Yb(3+) doped aluminosilicate glass fiber with a long-period fiber gratings pair.

We propose a new fiber-type all-optical switching device based on the optical nonlinearity of Yb(3+) doped fiber and a long-period fiber gratings(LPG) pair. The all-optical ON-OFF switching with the continuous wave laser signal at ~1556nm in the LPG pair including the 25.5cm long Yb(3+) doped fiber was demonstrated up to ~200Hz upon pumping with the modulated square wave pulses at 976nm, where a full optical switching with the ~18dB extinction ratio was obtained at the launched pump power of ~35mW.

Multi-wavelength lasing oscillations in an erbium-doped fiber laser using few-mode fiber Bragg grating.

We propose a simple erbium-doped fiber laser configuration for obtaining multi-wavelength oscillation at room temperature, in which a few-mode fiber Bragg grating was used as the wavelength-selective component. An amplitude variation of 1.6 dB over 120 second period was obtained for three-wavelength oscillation at room temperature, which demonstrates stability of the output power. This multi-wavelength laser can be switched between dual- and triple-wavelength operations by properly adjusting polarization controller in the cavity. This multi-wavelength laser has the advantage of simple configuration, high stability, low cost and stable operation at room temperature.

Measurement method for profiling the residual stress of an optical fiber: detailed analysis of off-focusing and beam-deflection effects.

The effects of off-focusing and beam deflection on polarimetric stress measurements of optical fibers are investigated. A simple method for reducing the distortion of the phase retardation caused by unwanted beam deflections in residual stress measurement is introduced. The method is examined numerically by ray-tracing techniques and experimentally by use of hollow silica fibers into which various index-matching liquids have been inserted. An autofocusing technique is introduced. The error in stress measurement reproducibility was determined to be less than 4%. We tested the absolute error in measured stress by applying incremental external tension and determined that it is less than 0.464 MPa.

Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities.

Unambiguous simultaneous measurement of strain and temperature based on dual long-period fiber gratings by controlling their thermal and strain sensitivities is proposed and experimentally demonstrated. The difference in the wavelength peak shift and the separation with the variation of strain and temperature allows discrimination between the strain and temperature effects, respectively.

Absorption and emission properties of Tm2+ ions in germanosilicate glass fibers.

Absorption and emission properties between 350nm and 1600nm of the Tm2+ ions in optical fibers were investigated using the Tm2+-Tm3+ co-doped germanosilicate glass fibers and its fiber preform. Strong broad absorption band due to Tm2+ ions was found to appear from 350nm to ~900nm together with the absorption bands due to Tm3+ ions. Broad emission from ~600nm to ~1050nm and the other emission from ~1050nm to ~1300nm, which were not shown in the Tm3+ ions, were found upon the Ar ion laser pumping at 515nm. Both absorption and emission results confirm that the Tm2+ ions in the germanosilicate glass have the 4f-5d energy band from 350nm to ~900nm and the 4f-4f energy level at ~1115nm.

Effect of CO2 laser irradiation on the refractive-index change in optical fibers.

The effect of CO2 laser irradiation on the refractive-index change in optical fibers is investigated by measuring the interference fringe shift formed by a long-period fiber grating pair. The refractive-index decrease on CO2 laser irradiation was due to relaxation of the residual stress, which was formed in optical fibers during the drawing process, and the refractive-index decrease was found to increase linearly with the drawing force. The effect of the CO2 laser output power on residual-stress relaxation, and fiber elongation was also studied.

Measurement method for profiling the residual stress and the strain-optic coefficient of an optical fiber.

A method, believed novel, is demonstrated for determining the strain-optic coefficient profile as well as the residual-stress profile of an optical fiber by use of a modified polariscope combined with a fiber-elongation apparatus. Measurement results of the residual-stress and the strain-optic coefficient profiles for Ge-doped and Er-Ge-Al-doped optical fibers are demonstrated with this method.

Photonic crystal fiber coupler.

Fiber couplers made with photonic crystal fibers (PCF) are reported. Two types of PCF were fabricated by means of stacking a group of silica tubes around a silica rod and drawing them. The fiber couplers were made by use of the fused biconical tapered method. With a fiber that had five hexagonally stacked layers of air holes, a 33/67 coupling ratio was obtained, and with a one-layer four-hole fiber, a 48/52 coupling ratio was obtained. The fabrication processes and the characteristics of the PCFs and the PCF couplers are presented.

Spectral polarization-dependent loss of cascaded long-period fiber gratings.

We have measured the spectral polarization-dependent losses of cascaded long-period fiber gratings composed of single, double, and triple identical gratings and fabricated by exposure of a UV laser beam on one side of a fiber. The measurement was made by use of the Mueller matrix method for scanning the wavelength of the incident beam. The polarization-dependent loss was dramatically increased as the number of cascaded gratings was increased. The formation of the UV-induced geometric birefringence in the fiber grating and its relationship to the fine interference fringes formed by cascading gratings are discussed.

Complete determination of the stress tensor of a polarization-maintaining fiber by photoelastic tomography.

The use of photoelastic tomography to obtain the two-dimensional axial stress profile of a polarization-maintaining (PM) fiber with high resolution and accuracy is described. We illustrate, for what is believed to be the first time, the two-dimensional distribution of the local principal axes of the fiber's cross section, which is directly related to the fiber's PM ability. We demonstrate that the stress-induced anisotropy as well as all the stress tensor components of the fiber can be fully determined.