Jun-long Kou

Miniaturized fiber taper reflective interferometer for high temperature measurement

We present an ultra-small all-silica high temperature sensor based on a reflective Fabry-Perot modal interferometer (FPMI). Our FPMI is made of a micro-cavity (approximately 4.4 microm) directly fabricated into a fiber taper probe less than 10 mum in diameter. Its sensing head is a miniaturized single mode-multimode fiber configuration without splicing. The sensing mechanism of FPMI is the interference among reflected fundamental mode and excited high-order modes at the end-faces. Its temperature sensitivity is approximately 20 pm/degrees C near the wavelength of 1550 nm. This kind of sensor can work in harsh environments with ultra-large temperature gradient, but takes up little space because of its unique geometry and small size.

Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe

We experimentally demonstrate an all-silica first-order fiber Bragg grating (FBG) for high temperature sensing by focused ion beam (FIB) machining in a fiber probe tapered to a point. This 61-period FBG is compact (~36.6 μm long and ~6.5 μm in diameter) with 200-nm-deep shallow grooves. We have tested the sensor from room temperature to around 500 °C and it shows a temperature sensitivity of nearly 20 pm/°C near the resonant wavelength of 1550 nm. This kind of sensor takes up little space because of its unique geometry and small size and may be integrated in devices that work in harsh environment or for detecting small objects.

Microfiber-probe-based ultrasmall interferometric sensor

We report an ultrasmall microfiber-probe-based reflective interferometer for highly sensitive liquid refractive index measurement. It has a 3.5 microm micronotch cavity fabricated by focused ion beam micromachining. A sensitivity of 110 nm/RIU (refractive index unit) in liquid is achieved with over 20 dB extinction ratio. Theoretical analysis shows this kind of device is a hybrid of Fabry-Perot and modal interferometers. In comparison with normal fiber interferometers, this probe sensor is very compact, stable, and cheap, offering great potentials for detecting inside sub-wavelength bubbles, droplets, or biocells.

Microfiber-based Bragg gratings for sensing applications: a review

Microfiber-based Bragg gratings (MFBGs) are an emerging concept in ultra-small optical fiber sensors. They have attracted great attention among researchers in the fiber sensing area because of their large evanescent field and compactness. In this review, the basic techniques for the fabrication of MFBGs are introduced first. Then, the sensing properties and applications of MFBGs are discussed, including measurement of refractive index (RI), temperature, and strain/force. Finally a summary of selected MFBG sensing elements from previous literature are tabulated.

An Optical Fiber Tip Micrograting Thermometer

An ~ 12-μm-long Bragg grating was engraved in an ~ 5-μm-diameter optical fiber tip by focused ion beam (FIB) milling. An ~ 10-dB extinction was achieved at 1570 nm with only 11 indentations. The grating was used for temperature sensing, and it exhibited a temperature sensitivity of ~ 22 pm°C.

Miniature tapered photonic crystal fiber interferometer with enhanced sensitivity by acid microdroplets etching

We fabricate a miniature tapered photonic crystal fiber (PCF) interferometer with enhanced sensitivity by acid microdroplets etching. This method is very simple and cost effective, avoiding elongating the PCF, moving and refixing the device during etching, and measuring. The refractive index sensing properties with different PCF diameters are investigated both theoretically and experimentally. The tapering velocity can be controlled by the microdroplet size and position. The sensitivity greatly increases (five times, 750 nm/RIU) and the size decreases after slightly tapering the PCF. The device keeps low temperature dependence before and after tapering. More uniformly and thinly tapered PCFs can be realized with higher sensitivity (∼100 times) by optimizing the etching process.

Ultra-highly sensitive surface-corrugated microfiber Bragg grating force sensor

We experimentally demonstrate a microfiber Bragg grating force sensor with ultra-high sensitivity. The fiber Bragg grating (FBG) is inscribed in a microfiber tapered from standard non-photosensitive single-mode fiber by focused ion beam machining method, and has a compact size (∼112 μm in length). Small diameter increases the force sensitivity of such grating when acting as a force-sensing element under tensile loads. We have demonstrated force sensitivity as high as ∼3146 nm/N around the resonant wavelength of 1538 nm, which is three orders of magnitude larger than FBGs in untapered fibers.

Platform for enhanced light–graphene interaction length and miniaturizing fiber stereo devices

Sufficient light–matter interactions are important for waveguide-coupled graphene optoelectronic devices. Subwavelength-diameter microfibers (MFs) with a strong evanescent field are attractive for graphene integration in fiber optics system, which can be realized by covering or wrapping a graphene sheet on a straight and thin MF. However, it is challenging to handle such a thin MF and graphene for sufficient length and strength of interaction. Using an MF-based lab-on-a-rod technique, we present a platform for ultralong light–graphene interaction and design graphene-integrated helical MF devices. Using this approach, we experimentally demonstrate polarization manipulation by wrapping an MF on a rod pretreated with a graphene sheet. The device can operate as not only a broadband polarizer but also a high-Q single-polarization resonator by tuning the geometry of MF coils. By specializing the rod surface and coil geometry, we believe the platform could contribute to advancing the research for more graphene–MF-integrated devices including modulators and photodetectors.

Highly Birefringent Slot-Microfiber

We propose a possible approach to realize a highly birefringent slot microfiber by postprocessing a circular microfiber (CMF). The shape can be fabricated with very high repeatability, reliability, and accuracy. Light field can be enhanced and confined in the nanometer-wide low index slot, and the birefringence achieves as high as 4 × 10-2. The slot-microfiber has the advantages both of a microfiber and a slot waveguide. It is perfect for miniature fiberized polarization manipulation devices and its unique geometry can also greatly enhance the refractive index sensitivity (nearly ten times higher than that of CMFs) for evanescent-field-based gas sensors.

Miniaturized Metal-Dielectric-Hybrid Fiber Tip Grating for Refractive Index Sensing

We fabricate a miniaturized metal-dielectric-hybrid fiber tip grating (FTG) using a focused ion beam (FIB) method with high accuracy for refractive index sensing applications. It is the smallest fiber grating by now (nearly 10 μm in length located at the fiber tip of 3 μm in radius) and it has strong surface corrugations with notches periodically. The grating shows refractive index sensitive [125 nm/refractive index unit (RIU)] and insensitive (nearly zero) properties for reflection channels of different resonant modes by metal-dielectric cladding structure. It can be used as a multichannel sensor for simultaneous refractive index and temperature/pressure measurements. Taking advantage of its flexible design, tiny size, unique modal and spectral characteristics, the metal-dielectric-hybrid FTG has great potential in fast-response detection of ultrasmall objects.