One of the goals of the Neural Engineering System Design (NESD) program in the United States and of similar programs around the world is to develop an interface able to read from one million neurons in parallel. This is well beyond the capabilities of traditional multi-electrode arrays (MEAs), which are inherently limited in both spatial resolution and number of channels, due to issues with power dissipation and wiring.1, 2 To overcome these roadblocks our group has proposed a novel optrode array that measures electrical activity and uses light for both signal transduction and transmission, thus decoupling the bio-potentials from the signal acquisition circuitry.3 The technology relies on the sensitivity of a particular class of liquid crystals (LCs) to small electric fields and is analogous to a LC display, where the intensity of each pixel (optrode, in our case) is controlled by the electrical activity of the biological tissue. Here, we present the first use of such a transduction mechanism to record from cardiac tissue and investigate stimulus artifact suppression in rabbit sciatic nerve. Our results pave the way to the development of high-density high-channel-count optrode arrays for electrophysiology studies and brain-machine interfaces.
We propose the feasible design of a device that can offer a unique method for controlling the terahertz spectrum allocation, exploiting existing photonics-based technologies. We successfully designed a multiplexer and demultiplexer using arrayed waveguide grating, where the device can multiplex and demultiplex four frequency channels (8.64 GHz bandwidth) in the same operating range defined by the IEEE 802.15.3d standard with at least 10 dB separation. This study reveals that proposed solution play a major role in maturing the next generation wireless communication (beyond 100 GHz) networks.
We present a compact design for a 1064 nm Q-Switched waveguide laser based on a liquid crystal transducer. Directly integrating the input-coupling mirror on the chip and utilising a Grin lens to also integrate the modulator optics enables a miniaturised setup. The preliminary experimental results have demonstrated that the Q-switched laser pulses with a pulse width of 45 ns and average output power of 4.5 mW can be achieved with a pump power of 350 mW, when an electrical signal with a repetition rate of 5 kHz, a peak-to-peak voltage of 30 V and a duration of 4 µs is applied. This work was supported by the Office of Naval Research Global (N62909-18-1-2147).
We report on the latest development of our photonics-based brain-machine interface. This work done in collaboration between UNSW and Macquarie University – and supported by the US Office of Naval Research – directly addresses the long-term DARPA challenge of producing implantable chips with 1 million neural connections. To the best of our knowledge, no technology has demonstrated the potential so far to scale up to such a massive number of channels.
This paper presents an investigation into a novel electro-optic device for bi-directional brain-machine interface (BMI) by using both a chiral smectic C* liquid crystal to sense neuronal signals and the photovoltaic effect to stimulate neuronal tissues. By leveraging both the optical and electrical domains, this new electro-optic device can achieve high density of channel count and we have so far demonstrated up to 323 such channels. We focus here on tissue stimulation by adding a photovoltaic PN junction into the LC sensing structure described elsewhere to achieve a full bi-directional neuronal interface.
Nerve conduction and activity is a marker of disease and wellness and provides insight into the complex way the nervous system encodes information. We propose an electro-optical detection system and show the recordings from an electrically stimulated in-vitro nerve preparation. The system converts the action potential at the probing position to light intensity before any amplification and detection. Thence the light signal is detected by a photodetector. The new detection system has the ability of isolating the probing point and the amplification circuits, which reduces the electrical interference from the circuit. Moreover, the sampled signal transmitted via optical fibres rather than cables or wires makes it more robust to environmental noise. From the experiment, we demonstrated that the electro-optical detection system is able to detect and amplify the nerve response. By analysing the data, we can distinguish the response from the stimulus artifact and calculate CAP (compound action potential) propagation speed.
Multielectrode arrays are a powerful tool for recording biopotentials, however they are limited by issues related to wiring complexity and channel-count. We present a novel concept for a liquid crystal-based optical electrode (optrode) that does not require the electrical circuitry associated with reading and amplifying each channel, thus providing superior spatial resolution and signal-to-noise ratio. Through computational modeling, we show that it is possible to accurately image biopotentials by coupling them to the electrodes of a LC cell and measuring their re ectance under parallel polarisers.
We describe a fibre optic hydrophone array system that could be used for underwater acoustic surveillance applications
e.g. military, counter terrorist and customs authorities in protecting ports and harbors, offshore production facilities or
coastal approaches as well as various marine applications. In this paper we propose a new approach to underwater sonar
systems using voltage-controlled Liquid Crystals (LC) and simple multiplexing method. The proposed method permits
measurements of sound under water at multiple points along an optical fibre using low cost components (LC cells),
standard single mode fibre, without complex interferometric measurement techniques, electronics or demodulation
Liquid crystal (LC) cells can be used in conjunction with optical fibres to develop cheap and efficient sensors, such as
voltage sensors or hydrophones. In this paper we apply an effective tensor model to describe reflection from gold-coated
deformed-helix ferroelectric liquid crystal (DHFLC) cells. We show that, depending on the polarisation of the incident
light, it is possible to obtain a linear electro-optical response to the voltage applied to the cell. Theoretical results are
compared with experimental results yielding accurate agreement.
In this paper we propose a new approach to fibre optic voltage sensors via voltage-controlled Liquid Crystals (LC),
which would allow direct measurement of up to 400 kV/m electric fields at multiple points. In addition, a novel
polarization independent fibre optic sensor configuration is presented that exhibits a linear electro-optic (EO) response
to variations of the electrical field under test.
Recent advances in the production of high-purity synthetic diamonds have made diamond an accessible host material for
applications in present and future optoelectronic and photonic devices. We have developed a scalable process for
fabricating photonic devices in diamond using reactive ion etching (RIE) and photolithography as well as using ion
implantation to provide vertical confinement. Applying this we have demonstrated a few-moded waveguide with a large
cross section for easier coupling to optical fibre. We present our work towards in-plane coupling to diamond waveguides
and consequently characterisation of these waveguides. We also examine the application of diamond waveguides to other
photonic applications for achieving light confinement in a subwavelength cavity site using a slot-waveguide design. Such
cavities may be used to enhance photon-emission properties of a built-in diamond colour centre and to achieve strong
light-matter interactions on the single-quantum level necessary for quantum information technology. Using single
cavities as building block, we also show that these structures can be suitably coupled to form one-dimensional coupled-resonator
Diamond has a range of extraordinary properties and the recent ability to produce high quality synthetic diamond has
paved the way for the fabrication of practical diamond devices. This paper details the recent progress in the fabrication of
waveguide structures in diamond which are desirable as the basis for quantum key distribution (QKD), quantum computing and high-power, high speed microwave chips. The diamond ridge waveguide structures are produced by photolithography and reactive ion etching (RIE) with some additional processing with a focused ion beam (FIB). The processes currently used are discussed along with experimental results. Future fabrication goals and potential methods for achieving these goals are also presented.
We propose a novel 6-port planar waveguide coupler device for adding and/or dropping tow different wavelengths from a WDM channel using a single grating. By writing a blazed Bragg grating into a few-moded core at a slight angle to the waveguide propagation direction, power can be coupled from the fundamental mode into higher-order, backward-propagating modes and vice-versa. Each such mode is channelled into a particular output port using an adiabatic splitter. Modeling results indicate that more than 99.99 percent of the power in the fundamental mode can be coupled into the higher-order modes at their respective Bragg wavelengths.
The unification of transverse electric (TE) and transverse magnetic (TM) beam propagation algorithms is made possible through a transformation which converts the wave equation for TM fields in planar waveguides into a form identical to the corresponding TE wave equation. The transformation can be applied to any smoothly varying waveguide. This transformation can be made independently of any paraxial or other approximations. Thus, any TE propagation algorithm can also be applied immediately to TM fields without additional approximations. This includes the classical fast Fourier transform beam propagator, which has not previously been applied successfully to TM propagation. We also specifically develop a Finite Difference Beam Propagation Method that applies to both TE and TM propagation in 1D (planar) geometry. Previous implementations for the TM case involve an approximation that in certain circumstances leads to severe errors (including the totally unphysical occurrence of field amplification). This is the first TM propagator which exactly conserves power. We also investigate the role of the reference background wavenumber (or index) and clarify its role as it is dynamically adapted. The algorithms proposed are easily adaptable to wide-angle beam propagators and to modern transparent boundary conditions. The extension of these ideas to rapidly varying structures (such as Bragg gratings) is also briefly discussed.
The cutoff wavelengths of the higher-order supermodes of symmetric square-core and rectangular-core step-profile couplers are determined in order to interpret the spectral response of a practical single-mode buried-channel waveguide coupler. Our results suggest the possibility of simple diagnostic tests for such devices and highlight the design constraints imposed by second order supermodes. We also compare the behavior of these couplers with that of circular-core couplers to illustrate how the geometry of each core affects the coupler cutoff characteristics.
We show how the concepts central to object oriented programming--inheritance, polymorphism and encapsulation-- can be used efficiently to implement extensible and reusable code that can be readily applied to integrated optics modeling. In particular, we will show how object oriented programming can lead to elegant and adaptable simulation code that can be easily tailored for specific needs. This approach leads to small and elegant code that can--in certain instances--be preferable to fully mouse driven commercial programs. Examples are given. We have developed a collection of modern modeling techniques using object oriented programming aimed at integrated optics. This readily available code library encompasses beam propagation methods, mode solvers, gratings and coupled mode analysis. Being based on a general programming language, namely C++, it is possible to use the library to automate tedious calculations and/or optimization problems. The library can also be used to rapidly develop custom made software for rapid prototyping and/or to apply to specific research problems. Moreover, due to its object oriented nature, the library pieces fit together naturally and versatile simulation code can be written.
Fused taper single-mode fiber couplers are readily fabricated with specific spectral properties, including 3 dB splitting in the 1310 and 1550 nm windows, and 1310/1550 nm wavelength division multiplexing (WDM) and demultiplexing. The specific functionality of the coupler is achieved through precise control over the heating and drawing processes. A possible alternative approach to obtaining the required functionality is to use UV- trimming or post-tuning. The spectral response of single-mode fused taper fiber couplers can be shifted by exposing the entire coupler to intense UV light. Furthermore this occurs without any prior hydrogenation, and there is also no discernible increase in excess loss in the exposed couplers. Results will be presented showing the effect of varying both the fluence and the wavelength of the UV-source on the spectral shift of these and other types of couplers. Wavelength shifts of over 100 nm are possible with the 1310/1550 nm WDM couplers, using sufficient fluence. A simple slab model of the complete tapered coupler confirms that the major contribution to the wavelength shift due to the change in the core index originates mainly in the down- and up-taper regions and, to a lesser extent, in the central waist region. This model explains the greater sensitivity of the spectral shift of the more-tapered WDM couplers to UV-irradiation compared to the shift in the 3 dB couplers.
Existing optical fiber and fiber-device fabrication techniques have been complemented recently by the development of plasma enhanced chemical vapor deposition (PECVD) processes for the fabrication of buried channel waveguides and associated devices. These processes rely on new forms of plasma reactors and diagnostic systems, which allow in-situ control of optical parameters such as refractive index, and are also being complemented by the direct writing of waveguides into photosensitive PECVD materials. Both the plasma and direct-write processes allow the fabrication of optical devices which are not readily feasible in fiber technology. The low-temperature PECVD process reported here offers the potential to integrate photonic devices with semiconductor sources and detectors to realize a compact, hybrid photonic-optoelectronic chip, complete with fiber pig-tailing. Because of their compactness and potential low cost, these types of photonic chips are attractive components for future high-capacity optical telecommunications and other networks now being planned as part of the information superhighway.
We demonstrate all optically (UV) written waveguide grating structures in germanosilicate trilayers grown by conventional PECVD, as well as novel results with a new hollow cathode PECVD growth technique which is capable of producing films having either positive or negative UV photosensitivity.