UV photonic integrated circuits for far-field structured illumination autofluorescence microscopy.

Ultra-violet (UV) light has still a limited scope in optical microscopy despite its potential advantages over visible light in terms of optical resolution and of interaction with a wide variety of biological molecules. The main challenge is to control in a robust, compact and cost-effective way UV light beams at the level of a single optical spatial mode and concomitantly to minimize the light propagation loss. To tackle this challenge, we present here photonic integrated circuits made of aluminum oxide thin layers that are compatible with both UV light and high-volume manufacturing. These photonic circuits designed at a wavelength of 360 nm enable super-resolved structured illumination microscopy with conventional wide-field microscopes and without modifying the usual protocol for handling the object to be imaged. As a biological application, we show that our UV photonic chips enable to image the autofluorescence of yeast cells and reveal features unresolved with standard wide-field microscopy.

Fiber-interface directional coupler inscribed by femtosecond laser for refractive index measurements.

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as ∼8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing.

Surface plasmon resonance refractive index sensor based on fiber-interface waveguide inscribed by femtosecond laser.

A novel surface plasmon resonance (SPR) configuration based on fiber-interface waveguide was proposed and realized by combining the technology of femtosecond laser writing waveguide with SPR effect for measuring refractive index (RI) of analyte. A U-shaped waveguide is inscribed in the coreless fiber and its bottom is very close to the fiber surface, which can produce strong evanescent field being sensitive to ambient media. When the fiber surface is coated with a layer of gold film, the strong evanescent field can excite the SPR effect on the fiber surface. Most importantly, different from some types of fiber SPR sensors with a fragile physical structure, the fiber-interface waveguide SPR sensor exhibits an excellent mechanical strength. Such a SPR sensor exhibits a high sensitivity of ∼3352 nm/RIU at the RI value of ∼1.395, which may have important practical applications in medicine, environmental monitoring, and food safety.

Femtosecond laser-inscribed fiber interface Mach-Zehnder interferometer for temperature-insensitive refractive index measurement.

A new fiber interface Mach-Zehnder interferometer has been fabricated, to the best of our knowledge, in coreless fiber by femtosecond laser-inscription for temperature-insensitive refractive index measurement. A straight waveguide was inscribed along the central axis of the coreless fiber as the reference arm, and the other curved waveguide (interface waveguide) was then inscribed bending toward the cladding interface to obtain a strong evanescent field sensitive to ambient refractive index. This fiber interface Mach-Zehnder interferometer exhibits a high refractive index (RI) sensitivity of ∼3000 nm/RIU at an RI value of 1.432. Moreover, with the significant advantages of high mechanical strength and temperature independence, such a fiber Mach-Zehnder interferometer may find many potential applications in biochemical sensing.

Bragg resonance in microfiber realized by two-photon polymerization.

A new method for microfiber Bragg gratings (µ-FBGs) fabrication by means of two-photon polymerization in photosensitive resin is reported. Such polymerized µ-FBGs were cured along with the surface of microfibers without any damage or distortion to the substrate. The laser intensity was optimized to improve the spectral properties of the polymerized gratings. The refractive index measurement was performed and the maximum sensitivity obtained is ~207 nm/RIU at the refractive index value of 1.440 with the fiber diameter being 1.7 µm. This work opens a new idea for optical structure integration and further optical functionality integration.

Optofluidic gutter oil discrimination based on a hybrid-waveguide coupler in fibre.

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of -112 743 nm RIU-1, a detection limit of 2.08 × 10-5 RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils.

Fiber surface Bragg grating waveguide for refractive index measurements.

A fiber surface Bragg grating waveguide (BGW) fabricated in the surface of single-mode fiber by direct femtosecond laser inscription is demonstrated and successfully applied for refractive index (RI) measurements. Prior to laser inscription of the fiber surface BGW, an X-coupler is first inscribed across the fiber core to couple light from the core to the fiber surface. The light transmitted in the fiber surface BGW efficiently interacts with the surrounding medium due to a strong evanescent field, and obtains an acceptable RI sensitivity approaching ∼16 nm/RIU. The novel design efficiently couples the light guided in the core with the surrounding medium using a non-destructive, single-step micromachining process, and is expected to have potential applications in fiber biochemical sensing.