Li-Peng Sun

Ultrasensitive refractive-index sensors based on rectangular silica microfibers

We demonstrate an ultrasensitive refractive-index (RI) sensor utilizing the polarimetric interference of a rectangular silica microfiber. The measured sensitivity is as high as 18,987 nm/RIU (refractive-index unit) around the RI of 1.33, which is 1 order of magnitude higher than that of the previously reported microfiber devices. Theoretical analysis reveals that such high sensitivity not only is originated from the RI-induced birefringence variation but also relies on the unique birefringence dispersion property for the rectangular microfiber. We predict that the sensitivity can be enhanced significantly when the group birefringence approaches zero.

193nm excimer laser inscribed Bragg gratings in microfibers for refractive index sensing

We demonstrate the inscription of fiber Bragg gratings by 193 nm ArF excimer laser in microfibers drawn from the standard single mode telecommunication fiber. Fiber Bragg gratings are directly inscribed in a series of microfibers with diameter ranged from tens of μm to 3.3 μm without hydrogen loading or other treatment to photosensitize the microfibers. Four reflection peaks are observed where three correspond to high order mode resonances. The resonance wavelength depends on the fiber diameter and it sharply blueshifts as the diameter is decreased below 10 μm. The gratings are characterized for their response to ambient refractive index. The higher order mode resonance exhibits higher sensitivity to refractive index.

Miniature highly-birefringent microfiber loop with extremely-high refractive index sensitivity.

A miniature polarimetric interferometer with the twist of a highly-birefringent microfiber is demonstrated. Good transmission spectral characteristics, which are co-governed by the birefringence and the twist degree of the microfiber, are investigated. The structure exhibits extremely-high sensitivity of around 24,373 nm per refractive-index unit and excellent temperature stability of better than 0.005 nm/°C. Featured with compactness, reconfigurability, stability, robustness, and compatibility with other fiberized components, our device has potential in tunable filtering, sensing, multi-wavelength lasing, and etc.

High-efficiency ultraviolet inscription of Bragg gratings in microfibers

In this paper, we demonstrate high-efficiency inscription of Bragg gratings in microfiber. Strong Bragg gratings are easily inscribed using a 193-nm excimer laser and phase mask in microfiber drawn from the 62.5/125- μm standard multimode fiber (MMF) with no additional photosensitization treatment. The enlarged photosensitive core offered by the MMF provides a sufficient overlap between the fundamental mode and the induced refractive index modulation region, which significantly enhances the grating inscription efficiency. The grating inscription in the microfiber is even more efficient than that in the original MMF because the multibeam interference-induced fringe fuzziness in the normal size fiber does not occur as the fiber diameter down to the scale that is comparable with the Talbot length of the ultraviolet (UV)-writing light. The proposed method is beneficial for sensing and device applications of microfiber Bragg grating (mFBGs) because of the ease of fabrication, high coupling strength, and large wavelength separation between individual resonant dips.

Microfiber Mach-Zehnder interferometer based on long period grating for sensing applications.

A Mach-Zehnder interferometer (MZI) composed by a pair of long period gratings (LPGs) fabricated in silica microfiber for sensing applications is demonstrated. Each LPG is fabricated with a pulsed CO2 laser by creating six periodical deformations along fiber length with only one scanning cycle. The length of the MZI can reach as short as 8.84 mm when the diameter of the microfiber is 9.5 μm. Compared with the ones fabricated in single-mode fibers, the present MZI is much shorter owing to the large effective-index difference between the fundamental and higher order modes. The microfiber MZI exhibits a sensitivity to surrounding refractive index (RI) of 2225 nm per refractive index unit and the temperature sensitivity of only 11.7 pm/°C. Theoretical analysis suggests that the performances of the MZI sensor can be improved by using thinner microfibers with a diameter down to 3.5 μm: The sensitivity can be greatly enhanced due to the stronger evanescent-field interaction and reduced dispersion factor; the transmission dips become narrower which benefits high-resolution measurement; the thinner fiber also allows further reduction in device length. The present device has great potential in biochemical and medical sensing due to the advantages including easy fabrication, excellent compactness and high sensitivity.

Bending effect on modal interference in a fiber taper and sensitivity enhancement for refractive index measurement.

We demonstrate the bending effect of microfiber on interference fringes in a compact taper-based modal interferometer and sensitivity for refractive index (RI) measurement. For the bend curvature ranging from 0 to 0.283 mm(-1), the measured RI sensitivity distinctively increases from 342.5 nm/RIU (refractive-index unit) to 1192.7 nm/RIU around RI = 1.333 and from 3847.1 nm/RIU to 11006.0 nm/RIU around RI = 1.430, respectively. Theoretical analysis reveals that such enhancement is determined by the dispersion property of the intermodal index rather than other parameters, such as the variation of the straightforward evanescent field. The magnitude of sensitivity varies as a function of the microfiber bend curvature. Approaching a critical curvature (the intermodal-index dispersion factor approaches zero), the sensitivity is significantly enhanced, exhibiting great potential in RI sensing areas.

High-sensitivity DNA biosensor based on optical fiber taper interferometer coated with conjugated polymer tentacle

A sensitive bio-probe to in situ detect unlabeled single-stranded DNA targets based on optical microfiber taper interferometer coated by a high ordered pore arrays conjugated polymer has been presented. The polymer coating serves as tentacles to catch single-stranded DNA molecules by π-π conjugated interaction and varies the surface refractive index of the optical microfiber. The microfiber taper interferometer translates the refractive index information into wavelength shift of the interference fringe. The sensor exhibits DNA concentration sensitivity of 2.393 nm/log M and the lowest detection ability of 10(-10) M or even lower.

Bragg gratings in rectangular microfiber for temperature independent refractive index sensing

We present fiber Bragg grating fabricated in rectangular microfiber for temperature-independent refractive index (RI) measurement. The grating has two Bragg peaks due to the high geometrical birefringence of the rectangular microfiber. The two peaks present different RI responses because the modes along the orthogonal polarizations have different energy fractions in terms of evanescent field outside the silica microfiber and hence the light/liquid interaction strength are different. In contrast, they exhibit identical temperature sensitivities because most mode energy is confined in the microfiber and the thermal-optic effect of silica dominantly determines the temperature response. As a result, temperature-independent RI sensing can be realized by monitoring the wavelength separation between the two peaks.

Temperature-Insensitive Humidity Sensor Based on a Silica Fiber Taper Interferometer

A relative humidity (RH) sensor based on a silica fiber taper interferometer is proposed and demonstrated. The interferometer is formed by simply tapering a double cladded fiber down to 3.8 μm in diameter. The two transition regions excite energy transfers between the fundamental mode and higher-order mode and act as beam splitter and combiner, respectively. Interferometric fringes can therefore be observed in the transmission spectrum due to the phase difference between the two modes over the uniform region. RH measurement is carried out by monitoring the spectral shift of the fringes induced by the evanescent-field interaction. The sensor presents a sensitivity to surrounding RH of 97.76 pm/%RH and a temperature sensitivity as low as 4.74 pm/°C. It can respond to RH change within 188 ms. Compared with previous reports, the present RH sensor does not need extra functional coating and exhibits advantages including fast response, temperature insensitivity, and low manufacture cost.

Temperature-Compensated Refractive-Index Sensing Using a Single Bragg Grating in an Abrupt Fiber Taper

In this paper, temperature-compensated refractive-index (RI) sensing is realized by use of a single Bragg grating inscribed in a silica microfiber. The microfiber is tapered from a single-mode fiber with two short transition regions. The mode evolution and intermodal couplings are analyzed based on the coupled local-mode theory. Due to the sharp variation of transverse geometry along the fiber length, couplings between copropagating local modes are excited, and the energy of light can be distributed into different modes by the transition regions. The grating couples the incident light to phase-matched backward fundamental and higher-order modes at the individual wavelengths. The coupled higher-order modes can be partially retrieved as a result of the abrupt taper. As a result, several reflection peaks can be observed in the reflection spectrum. The peaks present different responses to surrounding RI and identical temperature sensitivity. Therefore, the temperature cross-sensitivity can be removed by measuring the wavelength separations. Compared with previous reports, the proposed method is simpler and more practical.