Moez Haque

Temperature-compensated fiber-optic 3D shape sensor based on femtosecond laser direct-written Bragg grating waveguides

Temperature-compensated 3D fiber shape sensing is demonstrated with femtosecond laser direct-written optical and Bragg grating waveguides that were distributed axially and radially inside a single coreless optical fiber. Efficient light coupling between the laser-written optical circuit elements and a standard single-mode fiber (SMF) was obtained for the first time by 3D laser writing of a 1 × 3 directional coupler to meet with the core waveguide in the fusion-spliced SMF. Simultaneous interrogation of nine Bragg gratings, distributed along three laterally offset waveguides, is presented through a single waveguide port at 1 kHz sampling rate to follow the Bragg wavelength shifts in real-time and thereby infer shape and temperature profile unambiguously along the fiber length. This distributed 3D strain and thermal sensor is freestanding, flexible, compact, lightweight and opens new directions for creating fiber cladding photonic devices for a wide range of applications from shape and thermal sensing to guidance of biomedical catheters and tools in minimally invasive surgery.

Multi-level diffractive optics for single laser exposure fabrication of telecom-band diamond-like 3-dimensional photonic crystals

We present a novel multi-level diffractive optical element for diffractive optic near-field lithography based fabrication of large-area diamond-like photonic crystal structure in a single laser exposure step. A multi-level single-surface phase element was laser fabricated on a thin polymer film by two-photon polymerization. A quarter-period phase shift was designed into the phase elements to generate a 3D periodic intensity distribution of double basis diamond-like structure. Finite difference time domain calculation of near-field diffraction patterns and associated isointensity surfaces are corroborated by definitive demonstration of a diamond-like woodpile structure formed inside thick photoresist. A large number of layers provided a strong stopband in the telecom band that matched predictions of numerical band calculation. SEM and spectral observations indicate good structural uniformity over large exposure area that promises 3D photonic crystal devices with high optical quality for a wide range of motif shapes and symmetries. Optical sensing is demonstrated by spectral shifts of the Gamma-Zeta stopband under liquid emersion.

Femtosecond laser-assisted etching of three-dimensional inverted-woodpile structures in fused silica.

Three-dimensional inverted-woodpile (WP) structures were embedded in a microchannel by femtosecond laser direct-writing of fused silica followed by chemical etching with diluted hydrofluoric acid. We show the hole size is linearly dependent on laser-scanning depth for various pulse energies, permitting the control of laser exposures to facilitate close 5 µm periodic packing of uniform microcapillary arrays. Exposure compensation for depth-dependent etching rate and optical beam aberrations yielded stable and crack-free uniform inverted-WP structures. The direct formation of the inverted-WP structure together with microchannels in an all-fused silica substrate, offers chemical stability and inertness, and biocompatibility to be exploited as new microfluidic systems for chromatography and electro-osmotic pumps.

Nanograting Bragg responses of femtosecond laser written optical waveguides in fused silica glass

Multiple Bragg nanograting stop bands are reported for the first time in single and multi-mode optical waveguides generated by femtosecond laser direct writing in bulk fused silica glass. The stop bands observed in the guided broadband light spectra originated with the orthogonal alignment of volume nanogratings co-generated with the waveguides. Rapid shifting of stop bands across the near UV and visible spectrum was sensitively controlled by laser exposure and sample scanning direction. Bragg periods anticipated from the observed stop bands concurred with the nanograting structural pitches revealed by scanning electron microscopy. The spectroscopic characterization of nanogratings along macroscopic-scale (12.5 mm long) waveguide sections constitutes a non-destructive, convenient and sensitive approach to examine long-range order and uniformity of the self-organized periodic structures that may assist to unravel the laser-glass interaction physics of nanograting formation.

Microstructuring of Polypyrrole by Maskless Direct Femtosecond Laser Ablation

Ultrafast laser micromachining was optimized for microstructuring polypyrrole as a facile new approach towards tailoring electrochemical and mechanical responses desirable for microactuator, sensors, neural probing, and nerve conduit applications. Laser perforation of high-density and high aspect ratio through-holes generated greater than 5-fold increase in surface area. The flexible machining technique offers micron-size resolution and fast prototyping capability for optimizing properties and opening new directions for polypyrrole-based devices.

Laser-written photonic crystal optofluidics for electrochromatography and spectroscopy on a chip.

Femtosecond laser processes were optimized for nonlinear interactions with various optical materials to develop a novel biophotonic lab-on-a-chip device that integrates laser-formed waveguides (WGs), microfluidic channels and photonic crystals (PCs). Such integration seeks the unique demonstration of dual PC functionalities: (1) efficient chromatographic separation and filtration of analytes through a porous PC embedded inside a microfluidic channel and (2) optofluidic spectroscopy through embedded WGs that probe PC stopband shifts as varying analyte concentrations flow and separate. The building blocks together with their integration were demonstrated, providing embedded porous PCs through which electrochromatography drove an accelerated mobile phase of analyte and an optical stopband was probed via integrated buried WGs. Together, these laboratory results underpin the promise of simultaneous chromatographic and spectroscopic capabilities in a single PC optofluidic device.

Integration of an O-band VCSEL on silicon photonics with polarization maintenance and waveguide coupling

We demonstrate the hybrid integration of an O-band vertical-cavity surface-emitting laser (VCSEL) onto a silicon photonic chip using a grating coupler that is optimized to simultaneously provide feedback to maintain the single emission polarization and efficient in-plane coupling. The grating coupler was fabricated on silicon-on-insulator using a standard silicon photonics foundry process, and integrated with a commercially available VCSEL. A transparent VCSEL submount was fabricated with femtosecond laser templating and chemical etching to simplify the passive and active alignment steps. A record-high VCSEL-to-chip coupling efficiency of -5 dB was obtained at a bias current of 2.5 mA. The slope efficiency and output power are competitive with microcavity hybrid silicon lasers. The results show the feasibility of VCSELs as low threshold current on-chip sources for silicon photonics.

Flexible fabrication of three-dimensional optical-domain photonic crystals using a combination of single-laser-exposure diffractive-optics lithography and template inversion

We demonstrate inversion of three-dimensional photonic crystal templates fabricated in a large area with diffractive-optics lithography. A custom-designed two-dimensional diffractive optical element was used to generate highly uniform, bicontinuous, three-dimensional photonic crystal templates in a single-laser exposure. Chemical vapor deposition successfully infiltrated the thick periodic polymer structure to deposit amorphous silica and thereby define an all-silica inverted photonic crystal after polymer removal, as confirmed by focused ion-beam milling and energy-dispersive x-ray spectroscopy. The diffractive-optics lithography permitted a large number of uniform layers to form that manifested in the recording of a strong -28 dB stopband in the telecom band.

Femtosecond Laser Inscription of Photonic and Optofluidic Devices in Fiber Cladding

Femtosecond laser internal microstructuring has emerged as a powerful tool for inscribing devices inside transparent materials. This chapter addresses the challenge of laser processing inside cylindrically shaped optical fibers to provide technical solutions for embedding highly functional photonic devices that efficiently interconnect with the fiber core waveguide. Chemical etching of laser modification tracks is further introduced to open microfluidic and other structures that, together with photonic devices, define a promising all-fiber platform of photonic, optofluidic, and microelectromechanical systems of broad interest to telecommunication, sensing, and biomedical applications. Aberration-free focusing of the femtosecond laser light with high numerical aperture oil-immersion lenses was developed for distortion-free writing of three-dimensional optical and optofluidic devices in arbitrary positions anywhere within the core and cladding of single-mode and coreless optical fibers. Various approaches for efficient coupling of light from core to cladding-formed waveguides are presented, building into demonstrations of more functional photonic devices including Mach-Zehnder interferometers and shape-temperature sensors based on distributed Bragg-grating waveguide circuits. Waveguide birefringence is exploited to define in-fiber polarization splitters and polarization-selective taps while laser trimming of waveguides with femtosecond laser-stressing tracks is shown to offer strong birefringence tuning up to 2 × 10−3 on which submillimeter length wave retarders were embedded in fiber. Femtosecond laser irradiation with chemical etching was further harnessed to form three-dimensional microfluidic networks, reservoirs, and micro-optical resonators with optically smooth sidewall roughness that were combined with cladding waveguides to demonstrate an in-fiber fluorescence detector and optofluidic Fabry-Perot refractive index sensor. The techniques presented in this chapter enable new directions for fabricating highly functional photonic microsystems and lab-in-fiber devices for complex laboratory-level diagnostics in a compact and flexible optical fiber platform.

Inversion of 3-dimensional polymer photonic crystal fabricated by diffractive optics laser lithography

Three-dimensional photonic crystal templates fabricated in polymer by single-step laser exposure through a 2D diffractive optical element have been inverted in silica to provide robust structures with strong photonic stopbands in the telecom band.