Dongning N. Wang

Whispering gallery mode (WGM) resonator has many advantages such as high quality factor (Q), small mode volume, long photon life time and low power consumption, hence is widely used in optical filters, modulators, micro-lasers, frequency comb generators, nonlinear optics and high-sensitivity sensor devices. However, most of the WGM resonators are discrete optical devices, which cannot be coupled directly with incoming light radiations, and an external coupling scheme by use of tapered fiber, side-polished fiber, waveguide or prism has to be used. This creates a difficulty when applying WGM resonator in the optical fiber system. Here we propose and demonstrate an all optical fiber dual WGM resonator device, which is naturally integrated with optical fiber and being easily coupled by the light traveling in the fiber core. The device consists of an embedded cylinder inside a cylindrical air-cavity within the fiber core, fabricated by use of femtosecond laser micromachining and fast hydrofluoric acid etching technique. When the optical transmission path in the fiber core is tangent to the inner wall of the cylindrical air-cavity, the light is coupled into the cavity via the evanescent field and excites WGM. When light entering the cylindrical air-cavity is tangential to the surface of the embedded cylinder, it is coupled into the cylinder cavity, circulates along the cylinder surface, and excites WGM resonance. Thus a dual WGM operation is generated. The device is compact in size, flexible in construction, simple in fabrication and convenient in operation, and exhibits promising potential applications in photonics.

A highly sensitive optical fiber gas pressure sensor with temperature monitoring is proposed and demonstrated. It is based on a slightly tapered fiber with an inner micro-cavity forming an in-fiber Mach–Zehnder interferometer (MZI), and a micro-channel is drilled into the lateral wall of the in-fiber micro-cavity using a femtosecond laser to allow gas to flow in. Due to the dependence of the refractive index (RI) of air inside the micro-cavity on its gas pressure and the high RI sensitivity of the MZI, the device is extremely sensitive to gas pressure. To prevent fiber breakage, the MZI is housed in a silicate capillary tube with an air inlet. Multiple modes are excited by slightly tapering the inner micro-cavity, and the resonance dips in the sensor’s transmission spectrum feature different linear gas pressure and temperature responses, so a sensitivity matrix algorithm can be used to achieve simultaneous demodulation of two parameters, thus resolving the temperature crosstalk. As expected, the experimental results demonstrated the reliability of the matrix algorithm, with pressure sensitivity reaching up to ~−12.967 nm/MPa and temperature sensitivity of ~89 pm/°C. The features of robust mechanical strength and high air pressure sensitivity with temperature monitoring imply that the proposed sensor has good practical and application prospects.

Whispering gallery mode (WGM) resonators are usually discrete optical devices, which have integration difficulties with an optical fiber system. Here we present a new, to the best of our knowledge, type of optical fiber whispering gallery mode resonator based on a cylindrical cavity, which is located in the multimode fiber core and fabricated by femtosecond laser micromachining together with fast hydrofluoric acid etching techniques. When light traveling in the fiber core is tangent to the cylindrical cavity wall, it is coupled into the cavity and circulates along the cavity wall to excite WGM resonance before being coupled out to the same tangential path and continuing propagation in the fiber core. The device is fully integrated into the optical fiber, simple in fabrication, convenient in operation, low in cost, and has a good quality factor (Q) of 1.06 × 10⁴. The device enriches the family of WGM resonators and is expected to have promising applications in photonics.

A reflective whispering gallery mode microsphere resonator is presented. The device consists of a single-mode fiber, a thick-wall capillary tube, and a microsphere. The capillary tube and single-mode fiber are firstly conically fused together using a fusion splicer, followed by tapering the capillary tube, then cut at the end of tapered region of capillary tube, and finally, the microsphere is inserted into the tapered region of capillary tube by use of a fiber taper and a small amount of glue. The device is alignment free, has compact size, good robustness, and stable operation.

A whispering gallery mode microsphere resonator based on a capillary is presented, which consists of a tapered capillary with thick wall and an inserted microsphere, fusion spliced using conical fusing mode with single mode fiber at both of its ends. The device is featured with alignment-free coupling of light traveling in the capillary wall into the microsphere, high robustness due to the thick capillary wall, simple in fabrication, low in cost and good operation stability, which make it exhibit broad application prospects in the field of photonics.

We demonstrated a new optical fiber modal interferometer (MI) for airflow sensing; the novelty of the proposed structure is that an MI is fabricated based on a piece of HAF, which makes the sensitive MI itself also a hotwire. The interferometer is made by applying arc-discharge tapering and then flame tapering on a 10 mm length high attenuation fiber (HAF, 2 dB/cm) with both ends spliced to a normal single mode fiber. When the diameter of the fiber in the processing region is reduced to about 2 μm, the near-infrared dispersion turning point (DTP) can be observed in the interferometer’s transmission spectrum. Due to the absorption of the HAF, the interferometer will have a large temperature increase under the action of a pump laser. At the same time, the spectrum of the interferometer with a DTP is very sensitive to the change in ambient temperature. Since airflow will significantly affect the temperature around the fiber, this thermosensitive interferometer with an integrated heat source is suitable for airflow sensing. Such an airflow sensor sample with a 31.2 mm length was made and pumped by a 980 nm laser with power up to 200 mW. In the comparative experiment with an electrical anemometer, this sensor exhibits a very high air-flow sensitivity of −2.69 nm/(m/s) at a flowrate of about 1.0 m/s. The sensitivity can be further improved by enlarging the waist length, increasing the pump power, etc. The optical anemometer with an extremely high sensitivity and a compact size has the potential to measure a low flowrate in constrained microfluidic channels.

An optical fiber Mach-Zehnder interferometer based on two cascaded single-mode fiber tapers with an open cavity is proposed and demonstrated for refractive index sensing. The device is fabricated by firstly employing heating and pulling method to form two cascaded fiber tapers, followed by using femtosecond laser to inscribe a group of vertical line structure in the fiber cladding between the two separated tapers, and finally creating an open cavity with the help of hydrofluoric acid corrosion. The experimental results obtained show that the refractive index sensitivity achieved is ~1107.14 nm/RIU (refractive index unit) in the refractive index range of 1.338 - 1.368, which is more than 40 times larger than that of the Mach-Zehnder interferometer based on pure two tapers. This low-cost and highly sensitive optical fiber refractive index sensor has high potential in chemical or biochemical sensing applications.

A high-sensitivity and fast-response relative humidity (RH) sensor based on a fiber-tip interferometer coated with agarose gel is proposed and demonstrated. To fabricate the interferometer, focusing the femtosecond laser on the fiber core near the end face of a well cleaved single-mode fiber (SMF) for less than 0.1 second to create a tiny refractive index-modulated dot is enough, then a thin film of hydrophilic material (agarose gel) is deposited on the end facet of the SMF. Owing to a high RI sensitivity of the interferometer and a humidity-induced refractive index (RI) change in the agarose gel, the fiber-tip sensor is qualified for RH sensing. The experimental results show that the sensor is extremely sensitive to high RH, with a maximum sensitivity of −2.04 dB/%RH and 0.673 nm/%RH at 90%RH. Furthermore, it exhibits low hysteresis as demonstrated by a breath monitoring of about 60 ms ultrafast response time and 160 ms recovery time. Additionally, it also shows a good long-term stability with a small standard deviation. The prominent advantages of high sensitivity, fast response, easy fabrication and compact size (total length of about 50 μm) imply that the proposed sensor has many potential applications for static or dynamic RH sensing.