Paul Jansz

A transmit reflect detection system for fiber Bragg grating photonic sensors

Traditional Fibre Bragg Grating (FBG) sensing systems acquire data about the measurand via the spectral response of the FBG. Edge filter methods are also used in the acquisition of data from FBGs. In edge filter systems, the spectral shift in the FBG due to the measurand is converted into an optical power change. This optical power change can then be easily measured using conventional optoelectronic devices. We demonstrate the use of a Transmit Reflect Detection System (TRDS) for Fibre Bragg Grating (FBG) sensors. The TRDS is in essence a dual edge filter detection method. In conventional edge filter detection schemes, the reflected portion of the incident spectrum is monitored to determine the change in the measurand. In the TRDS, both the transmitted and reflected portions of the input spectrum, from a narrow band light source, are utilised. The optical power of the transmitted and reflected signals are measured via two separate photoreceivers, where each generates a single edge filter signal. As the spectral response of the FBG shifts due to the measurand, the transmitted power will increase, and the reflected power will decrease, or vice versa. By differentially amplifying the transmitted and reflected components, the overall signal is increased. This results in improved sensitivity and efficiency of the photonic sensor. In this work, the FBG sensor and TRDS are used in the measuring and monitoring of temperature, force and strain. As such, results are presented for the FBG TRDS for all of the measurands.

Modelling of device structure effects on electrical crosstalk in back illuminated CMOS compatible photodiodes

Standard CMOS fabrication processes provide the means to realize the further development of back illuminated photodiode arrays for imaging systems. We have simulated crosstalk effects in a back illuminated CMOS compatible photodiode array, and compared this effect with that predicted for front illuminated arrays, using a two dimensional simulation model. It was found that the crosstalk in back illuminated arrays is generally greater than that for front illuminated arrays with identical structure, although this effect can be reduced by decreasing the thickness of the array. The n-well junction depth had little effect on the crosstalk predicted for the back illuminated case.

Simulation of optical delay lines for Optical Coherence Tomography

We present an analytical model which can be used to simulate different optical delays in time domain Optical Coherence Tomography (OCT). Its primary purpose is to compare the conventional moving optical delay line to quasi-stationary and stationary optical delay lines.

A proposed fibre optic time domain optical coherence tomography system using a micro-photonic stationary optical delay line

Conventional time domain Optical Coherence Tomography (OCT) relies on a reference Optical Delay Line (ODL). These reference ODLs require the physical movement of a mirror to scan a given depth range. This movement results in instrument degradation. We propose a new optical fibre based time domain OCT system that makes use of a micro-photonic structure as a stationary ODL. The proposed system uses an in-fibre interferometer, either a Michelson or a Mach-Zhender. The reference ODL makes use of a collimator to expand the light from the optical fibre. This is them expanded in one dimension via planar optics, that is, a cylindrical lens based telescope, using a concave and convex lens. The expanded beam is them passed through a transmissive Spatial Light Modulator (SLM), specifically a liquid crystal light valve used as an optical switch. Light is then reflected back through the system off the micro-photonic structure. The micro-photonic structure is a one dimensional array of stagged mirror steps, called a Stepped Mirror Structure (SMS). The system enables the selection of discrete optical delay lengths. The proposed ODL is capable of depth hoping and multicasting. We discuss the fabrication of the SMS, which consists of eight steps, each approximately 150 μm high. A change in notch frequency using an in-fibre Mach Zhender interferometer was used to gauge the average step height. The results gave an average step height of 146 μm.

Double boundary trench isolation effects on a stacked gradient homojunction photodiode array

The effect of the width of inter-pixel double boundary trench isolation on the response resolution of a two dimensional CMOS compatible stacked gradient homojunction photodiode array was simulated. Insulation and P-doped double boundary trench isolation were compared. Both geometries showed improved crosstalk suppression and enhanced sensitivity compared to photodiode geometries previously investigated, combined with a reduction in fabrication complexity for the insulation DBTI configuration.

Stepped mirrored structures for generating true time delays in stationary optical delay line proof-of-principle experiments for application to optical coherence tomography

Conventional time domain Optical Coherence Tomography (OCT) relies on the detection of an interference pattern generated by the interference of backscattered light from the sample and a reference Optical Delay Line (ODL). By referencing the sample interference with the scan depth of the ODL, constructive interference indicates depth in the sample of a reflecting structure. Conventional ODLs used in time domain OCT require some physical movement of a mirror to scan a given depth range. This movement results in instrument degradation. Also in some situations it is necessary to have no moving parts. Stationary ODLs (SODLs) include dual Reflective Spatial Light Modulator (SLM) systems (Type I) and single Transmissive SLM with match-arrayed-waveguide systems (Type II). In this paper, the method of fabrication and characterisation of a number of Stepped Mirrored Structures (SMS) is presented. These structures are intended for later use in proof-of-principle experiments that demonstrate Type II SODL: a six step, 2 mm step depth macro-SMS, an eight step 150 um deep micro-SMS with glue between steps, and a six step 150 um deep micro-SMS with no glue between steps. These SMS are characterized in terms of their fabrication, step alignment and step height increment precision. The degree of alignment of each step was verified using half of a bulk Michelson interferometer. Step height was gauged using a pair of vernier callipers measuring each individual step. A change in notch frequency using an in-fibre Mach-Zhender interferometer was used to gauge the average step height and the result compared to the vernier calliper results. The best aligned SMS was the micro-SMS prepared by method B with no glue between steps. It demonstrated a 95% confidence interval variation of 1% in reflected intensity, with the least variation in intensity within steps. This SMS also had the least absolute variation in step height increment: less than 8 um. Though less variation would be ideal, for producing micro-SMS for proof of principle experiments for Type II stationary ODL, of the method compared, method B, with no glue between steps, produced more reproducible step height increments and step alignment.

Effect of a polywell leometry on a CMOS photodiode array

The effect of a polywell geometry hybridized with a stacked gradient poly-homojunction architecture, on the response of a CMOs compatible photodiode array was simulated. Crosstalk and sensitivity improved compared to the polywell geometry alone, for both back and front illumination.

A micro-photonic stationary Optical Delay Line for fibre optic time domain OCT

We present results for the characterisation of a micro-photonic stationary optical delay line. The delay line is intended to generate true time delays for a fibre optic based optical coherence tomography system.

A Comparison Of Wet And Dry Etching To Fabricate A Micro-Photonic Structure For Use In OCT

In conventional time-domain Optical Coherence Tomography (OCT), a moving mirror is used as a reference optical delay line. This motion can result in instrument degradation, and in some situations it is preferable to have no moving parts. Stationary optical delay lines using a variety of methods have been proposed. Of particular interest, due to its low cost, is the use of a micro-photonic stationary optical delay line, made up of an addressable Stepped Mirror Structure (SMS) using a liquid crystal optical switch. Here the individual steps of the SMS can be selected by the liquid crystal array. For use in OCT, the discrete nature of the SMS needs to be overcome by having the step height less than the coherence length of the low coherent light source. Typical coherence lengths in current OCT systems are on the order of 10μm. Hence, micrometer size steps require the use of a relevant fabrication method. In this paper, we compare SMSs fabricated using wet and dry etching methods. Specifically, Reactive Ion Etching (RIE) using CF4/O2 and chemical bath etching, using a solution of HF, HNO3 and Acetic acid. Three inch diameter silicon wafers, 400μm thick, were etched by both methods. The RIE was used to produce a SMS with five 5μm high steps each step approximately 1 cm wide. The wet etching produced an SMS with three 15μm steps approximately 2 cm wide. The overall structures of the SMSs were compared using optical profilometry. The RIE step quality was far superior to the wet etch method due to the ability to control the anisotropy of the RIE method.

Characterizing the resolvability of real superluminescent diode sources for application to optical coherence tomography using a low coherence interferometry model.

Abstract. The axial resolution is a critical parameter in determining whether optical coherent tomography (OCT) can be used to resolve specific features in a sample image. Typically, measures of resolution have been attributed to the light source characteristics only, including the coherence length and the point spread function (PSF) width of the OCT light sources. The need to cost effectively visualize the generated PSF and OCT cross-correlated interferogram (A-scan) using many OCT light sources have led to the extrinsic evolution of the OCT simulation model presented. This research indicated that empirical resolution in vivo, as well as depending on the light source’s spectral characteristics, is also strongly dependent on the optical characteristics of the tissue, including surface reflection. This research showed that this reflection could be digitally removed from the A-scan of an epithelial model, enhancing the stratum depth resolution limit (SDRL) of the subsurface tissue. Specifically, the A-scan portion above the surface, the front surface interferogram, could be digitally subtracted, rather than deconvolved, from the subsurface part of each A-scan. This front surface interferogram subtraction resulted in considerably reduced empirical SDRLs being much closer to the superluminescent diodes’ resolution limits, compared to the untreated A-scan results.