Chaotan Sima

Ultra-wide detuning planar Bragg grating fabrication technique based on direct UV grating writing with electro-optic phase modulation.

A direct UV grating writing technique based on phase-controlled interferometry is proposed and demonstrated in a silica-on-silicon platform, with a wider wavelength detuning range than any previously reported UV writing technology. Electro-optic phase modulation of one beam in the interferometer is used to manipulate the fringe pattern and thus control the parameters of the Bragg gratings and waveguides. Various grating structures with refractive index apodization, phase shifts and index contrasts of up to 0.8 × 10(-3) have been demonstrated. The method offers significant time/energy efficiency as well as simplified optical layout and fabrication process. We have shown Bragg gratings can be made from 1200 nm to 1900 nm exclusively under software control and the maximum peak grating reflectivity only decreases by 3 dBover a 250 nm (~32 THz) bandwidth.

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

The monolithically integrated all-optical single-sideband (SSB) filter based on photonic Hilbert transform and planar Bragg gratings is proposed and experimentally demonstrated. An SSB 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.

Terahertz bandwidth photonic Hilbert transformers based on synthesized planar Bragg grating fabrication.

Terahertz bandwidth photonic Hilbert transformers are proposed and experimentally demonstrated. The integrated device is fabricated via a direct UV grating writing technique in a silica-on-silicon platform. The photonic Hilbert transformer operates at bandwidths of up to 2 THz (~16 nm) in the telecom band, a 10-fold greater bandwidth than any previously reported experimental approaches. Achieving this performance requires detailed knowledge of the system transfer function of the direct UV grating writing technique; this allows improved linearity and yields terahertz bandwidth Bragg gratings with improved spectral quality. By incorporating a flat-top reflector and Hilbert grating with a waveguide coupler, an ultrawideband all-optical single-sideband filter is demonstrated.

Demonstration of simultaneous mode conversion and demultiplexing for mode and wavelength division multiplexing systems based on tilted few-mode fiber Bragg gratings.

We experimentally demonstrate mode conversion by exploiting optical reflection of tilted few-mode fiber Bragg grating (FM-FBG). Mode conversions from LP(01) mode to higher symmetric and asymmetric modes are achieved, and more than 99.5% conversion efficiency from LP(01) to LP(11) mode is obtained using a 1.6°-tilted FM-FBG. Influences of the weakly tilted FM-FBG parameters on the property of mode conversion is analyzed and discussed. A simultaneous mode conversion and demultiplexing scheme for 4-mode × 3-wavelength multiplexing transmission is proposed and the modal crosstalk is analyzed based on the transmission spectra of the tilted FM-FBGs. The proposed approach shows potential applications in mode and wavelength division multiplexing communication systems.

Temperature-insensitive fiber twist sensor based on elliptical-core few-mode fiber

A highly sensitive fiber optics twist sensor is proposed and experimentally demonstrated using elliptical-core few-mode fiber (e-FMF). With the help of a fixed-phase plate for conversion from LP11 to LP01 mode, the twist angle can be linearly mapped to the spatial rotation of LP11 mode profile and consequently discriminated by monitoring the optical power variation at the standard single-mode fiber (SSMF) output. When the optical power at the e-FMF output is 10 mW, the twist angle within the range of ±39° can be successfully detected with a sensitivity of more than 20 μW/°. Meanwhile, the twist direction can be identified simultaneously. In particular, when the twist angle ranges from -12.5° to 20.1°, the sensitivity is higher than 100 μW/°. Both temperature-insensitive operation from 20°C to 150°C and e-FMF length insensitive operation are experimentally verified.

Direct UV-written planar Bragg grating sensors

Integrated photonics is a proven platform for physical and chemical sensing. It offers miniaturised solutions that are suited for use in extreme environments, including strong EM-fields, EM-pulses and contact with flammable materials, often far exceeding electronic sensors in this regard. This review looks into direct UV-written planar Bragg grating technology and its application to integrated photonic sensors. The platform has been demonstrated widely for measurement of physical properties such as temperature, pressure and strain. In addition, by using an evanescent interaction, refractive index can be measured allowing for chemical and biochemical detection. Further to this, the platform has recently been utilised in quantum information processing, where quantum gate operations and single photon detection has been shown.

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 π-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.

Simple planar Bragg grating devices for photonic Hilbert transformer

In this work the theoretical design and experimental realisation of a photonic Hilbert transformer is investigated. Planar Bragg gratings are proposed as a route to realise photonic Hilbert transformers in terms of a finite bandwidth and temporal impulse response. With proper grating apodisation and π-phase shift, the Bragg grating will display the spectral features expected from a Hilbert transformer. The unique features of Direct UV grating writing technology are used to fabricate the proposed device in silica-on-silicon

Ultra-short FBG based distributed sensing using shifted optical Gaussian filters and microwave-network analysis

Ultrashort fiber Bragg gratings (US-FBGs) have significant potential as weak grating sensors for distributed sensing, but the exploitation have been limited by their inherent broad spectra that are undesirable for most traditional wavelength measurements. To address this, we have recently introduced a new interrogation concept using shifted optical Gaussian filters (SOGF) which is well suitable for US-FBG measurements. Here, we apply it to demonstrate, for the first time, an US-FBG-based self-referencing distributed optical sensing technique, with the advantages of adjustable sensitivity and range, high-speed and wide-range (potentially >14000 με) intensity-based detection, and resistance to disturbance by nonuniform parameter distribution. The entire system is essentially based on a microwave network, which incorporates the SOGF with a fiber delay-line between the two arms. Differential detections of the cascaded US-FBGs are performed individually in the network time-domain response which can be obtained by analyzing its complex frequency response. Experimental results are presented and discussed using eight cascaded US-FBGs. A comprehensive numerical analysis is also conducted to assess the system performance, which shows that the use of US-FBGs instead of conventional weak FBGs could significantly improve the power budget and capacity of the distributed sensing system while maintaining the crosstalk level and intensity decay rate, providing a promising route for future sensing applications.

Experimental Investigation of Large Time–Bandwidth Product Photonic Hilbert Transformer Based on Compact Bragg Grating

We propose and experimentally demonstrate an integrated photonic Hilbert transformer (PHT) with substantial time bandwidth product (TBP). Due to the current substrate material and fabrication technique, PHT devices find it challenging to achieve ultrawide operative bandwidth while maintaining the minimum bandwidth level presented as the narrow central notch in frequency response. This implies that the TBP value is restricted. Here, we investigate the synthesized refractive index profile in compact Bragg grating design for theoretical simulation and the linearity-enhanced direct UV grating writing fabrication for experimental implementation. The fabricated device could process microwave photonic signals as Hilbert transformation between 50 GHz and 3 THz, with the TBP above 60, which is the largest experimental data to the best of our knowledge. This technique allows the PHT with improved spectral performance to be suitable for analog all-optical signal processing.