Realisation of photonic Hilbert transformer with a simple planar Bragg grating

Photonic Hilbert transformers (PHTs) are desirable for the direct processing of optical signals at high speeds and operation bandwidths, allowing optical networks to outperform current electronic technologies. We practically demonstrate a photonic Hilbert transformer in planar geometry; utilising a pi-phase shift planar Bragg grating with proper apodization profile. The device is fabricated by direct UV grating writing technology in silica-on-silicon [1]. The PHT has a pi-phase shift at the zero point of the frequency response, whereas the amplitude remains constant. The pi-phase shift in PHT is simply induced by placing a pi-phase shift in the refractive index modulation. The constant amplitude is achieved by precise apodization of the grating coupling strength, while the apodization profile is given by [2]. With our current direct UV writing technology, the proposed grating can be fabricated in a much higher accuracy then the conventional fibre Bragg grating manufacturing technique. We will present our latest work on more complex apodized gratings to obtain the ideal realisable frequency and temporal responses for PHTs

Power-efficiency enhanced thermally tunable Bragg grating for silica-on-silicon photonics

A thermally tunable Bragg grating device has been fabricated in a silica-on-silicon integrated optical chip, incorporating a suspended microbeam improving power efficiency. A waveguide and Bragg grating are defined through the middle of the microbeam via direct ultraviolet writing. A tuning range of 0.4 nm (50 GHz) is demonstrated at the telecommunication wavelength of 1550 nm. Power consumption during wavelength tuning is measured at 45 pm/mW, which is a factor of 90 better than reported values for similar bulk thermally tuned silica-on-silicon planar devices. The response time to a step change in heating is longer by a similar factor, as expected for a highly power-efficient device. The fabrication procedure involves a deep micromilling process, as well as wet etching and metal deposition. With this response, the device would be suitable for trimming applications and wherever low modulation frequencies are acceptable. A four-point-probe-based temperature measurement was also done to ascertain the temperature reached during tuning and found an average volume temperature of 48 °C, corresponding to 0.4 nm of tuning. The role of stress-induced buckling in device fabrication is included

Direct grating writing: single-step Bragg grating and waveguide fabrication for telecommunications and sensing applications

Direct Grating Writing (DGW) has been developed over the past decade as a means of rapidly prototyping waveguides with integrated Bragg grating structures in silica-on-silicon substrates [1]. The technique allows complicated waveguide structures and Bragg grating arrays to be fabricated and characterised in house

Integrated all-optical SSB modulator using photonic Hilbert transformer with planar Bragg gratings

Photonic Hilbert transformers (PHT) offer potential for a wide range of applications in telecommunication, information processing and signal analysis [1]. They provide operational bandwidths and speeds far beyond current electronic technologies. PHTs have been realized in discrete free space components and recently in fiber using multi-tapped sampled fiber Bragg gratings (FBGs), and single phase-shifted FBGs with appropriate apodisation profile [2]. Recently we have presented preliminary spectra from a fabricated PHT device in a planar waveguide [3]. This device employs a single pi phase shift and an apodised grating profile. In this work we use the freedom of the planar format to fabricate a single-side-band suppression (SSBS) module combining regular and phase shifted gratings on a single chip

Integrated planar Bragg grating devices for advanced optical communication systems

This thesis presents theoretical and experimental studies of planar Bragg grating devices for all-optical signal processing. Bragg gratings offer a route to realise many systems for all-optical signal processing including photonic Hilbert transformers. The fabrication of Bragg gratings in a planar format allows monolithic integration with traditional planar components and micro-heaters to realise devices with desired performance.Photonic Hilbert transformers offer potential for a wide range of applications such as single-sideband modulation, and also provide operational bandwidths and speeds far beyond current electronic technologies. A series of experimental demonstrations of photonic Hilbert transformers based on apodised planar Bragg gratings with phase-shifts are presented. Devices implementing fractional order Hilbert transform are also investigated. Grating structures are synthesised to achieve improved spectral quality including the demonstration of devices with Terahertz bandwidths.A direct UV grating writing technique based on phase-controlled interferometry is proposed and demonstrated to fabricate arbitrary Bragg gratings in a silica-on-silicon platform. Electro-optic phase modulation of one beam in the interferometer is used to manipulate the fringe pattern and control the parameters of Bragg gratings and waveguides. Along with the unique micron-order writing spot, modulation linearity and translation consistency ensures the desired grating apodisation profile is achieved. Furthermore, the significant improvement in fringe control results in larger grating index contrast, enabling Terahertz bandwidth devices.Finally, the system utilising the phase controlled UV writing technique is applied to the inscription of fibre Bragg gratings. Various kinds of fibres are experimentally tested in the work. The small writing spot shows potential and significant capability to inscribe arbitrary Bragg gratings into fibre structures

Phase controlled integrated interferometric single-sideband filter based on planar Bragg gratings implementing photonic Hilbert transform

The first monolithically integrated all-optical single-sideband filter based on photonic Hilbert transform and planar Bragg gratings is proposed and experimentally demonstrated. Single-sideband suppression of 12 dB at 6 GHz and sideband switching are achieved via thermal tuning. An X-coupler, photonic Hilbert transformer, flat top reflector and a micro heater are incorporated in a single silicon-on-silica substrate. The device can be thermally tuned by the micro heater on top of the channel waveguide. The device is fabricated using a combination of direct UV grating writing technology and photolithography

Recent Advances in Light-Induced Thermoelastic Spectroscopy for Gas Sensing: A Review

Light-induced thermoelastic spectroscopy (LITES) is a promising optical approach for gas sensing, which uses a quartz tuning fork (QTF) as a photothermal detector, instead of a commercial photodetector. Since the QTF has the advantages of low cost, small size, high resonance frequency, high-quality factor (Q-factor), and a wide spectral response range, and the LITES sensor has received extensive attention and obtained great development. This review paper summarizes and discusses the advances of the QTF-based, state-of-the-art LITES gas sensing technique in recent years and presents the development prospects of LITES sensor in the future

Integrated tunable all-optical filters based on push-pull fractional order photonics Hilbert transformers

We proposed and demonstrated a tunable all-optical filter based on the integration of X coupler, grating based fractional Hilbert transformers and optical phase push-pull elements, realizing controllable flattop notch filters and microwave photonic SSB filters

Mid-Infrared Hollow-Core Fiber Based Flexible Longitudinal Photoacoustic Resonator for Photoacoustic Spectroscopy Gas Sensing

Photoacoustic spectroscopy (PAS) has received extensive attention in optical gas sensing due to the advantages of high sensitivity, gas selectivity, and online detection. Here, a mid-infrared hollow-core fiber (HCF) based flexible longitudinal photoacoustic resonator for PAS-based gas sensing is proposed and theoretically demonstrated. A mid-infrared anti-resonant HCF is designed to innovatively replace the traditional metallic acoustic resonator and obtain a flexible photoacoustic cell in PAS. Optical transmission characteristics of the HCF are analyzed and discussed, achieving single mode operation with below 1 dB/m confinement loss between 3 and 8 μm and covering strong absorptions of some hydrocarbons and carbon oxides. With varied bending radii from 10 mm to 200 mm, the optical mode could be maintained in the hollow core. Based on the photoacoustic effect, generated acoustic mode distributions in the HCF-based flexible photoacoustic resonator are analyzed and compared. Results show that the PAS-based sensor has a stable and converged acoustic profile at the resonant frequency of around 16,787 Hz and a favorable linear response to light source power and gas concentration. The proposed novel photoacoustic resonator using HCF presents bring potential for advanced flexible PAS-based gas detection