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Coherent laser radar system using optical homodyne detection Mark Baker1 , Tom Stace1,2 1 2 School of Mathematics and Physics, The University of Queensland, St Lucia, QLD, Australia ARC Centre of Excellence for Engineered Quantum Systems (EQuS), The University of Queensland, St Lucia, QLD, Australia Coherent laser radar (LIDAR) is a powerful remote sensing technique that has found utility in accurate distance measurement, velocimetry, and vibrometry sensing [1]. Coherent laser radar has been demonstrated in CW operation, as well as pulsed amplitude and frequency operation. Distance can be measured by modulating the beam output, and measuring the round trip delay in the return signal, to high accuracy. Additionally, the return light scattered from the target also carries information about the target velocity, having undergone a Doppler frequency shift. Here we demonstrate a prototype pulsed coherent laser radar at 1.55 µum wavelengths, using off the shelf fiber telecom components, with 2ns width pulses at 100 MHz repetition rate. In this optical homodyne scheme, the laser output is split into two, with one arm acting as local oscillator reference signal, and the other arm the output which is amplitude modulated by an electro-optical modulator (EOM). The return signal is collected with the same optical setup, and optically mixed on a photoreceiver with the reference signal. The resulting pulse train and beat envelope yields simultaneous distance and velocity information. We present details of our apparatus and preliminary performance, and outline future applications including guidance systems for autonomous vehicles [2]. Figure 1: Pulsed coherent lidar, using amplitude modulation of 2 ns pulses with 50 MHz repetition rate. The CW local oscillator is mixed with the return signal, resulting in optical beat envelope modulated at the Doppler beat frequency (2 MHz), and is a measure of the target velocity. The delay between the output and return pulse train can be used to determine the target distance. References [1] C.J. Karlsson, F.A.A. Oldsson, D. Letalick, M. Harris, Appl. Opt. 39, (21), 3716-3726 (2000). [2] X. Mao, D.Inoue, H. Matsubara, M. Kagami,IEEE ITS Transactions, 14(2), 599 (2013). High bandwidth on-chip capacitive tuning of microtoroid resonators Christiaan Bekker1, Christopher Baker1, David McAuslan1, Eoin Sheridan1 and Warwick Bowen1 1School of Mathematics and Physics, The University of Queensland, Brisbane, QLD 4072 Australia Cavity Opto-electromechanical Systems (COEMS) create an interface between an optomechanical and an electromechanical system by coupling both to a common mechanical element. Since the first demonstration of a COEMS using a whispering-gallery-mode (WGM) resonator in 2010 [1,2], many new geometries for these devices have emerged, including photonic crystals [3,4,5], membrane-in-the-middle systems [6] and nanomembranes [7]. Unique facets of COEMS can be used in many applications, for example through utilising the electrical tunability of the optical resonance frequency [3], the ability to transduce electrical (microwave) signals into the optical domain [6,7,8] and the potential for microwave-to-optical quantum state transfer [5]. The latter is an enabling step for long-range networks with superconducting quantum circuits. In a recent paper [9], we demonstrate the design, fabrication and characterization of a silica microtoroid-based integrated COEMS. Whereas the electromechanical component of earlier WGM designs was derived from a sharp electrode suspended above the device, we directly fabricate two circular electrodes onto the present device to couple more efficiently with the mechanical motion of the resonator. These allow for rapid capacitive tuning of the WGM resonances while maintaining ultrahigh quality factors. Harmonic driving of the mechanics is achieved up to the range of tens of MHz, while static tuning of the optical resonance is accomplished by applying a DC bias across the electrodes, in turn inducing a capacitive force radially inward on the COEMS (see Fig 1(a)). The resonant frequency of the optical mode can be shifted by up to 20 linewidths for the largest DC bias on the electrodes, and can be accomplished orders of magnitude faster than the current state-of-the-art tuning mechanism for silica. This mechanism [10], which relies on heat to change the resonator size and refractive index, also requires much more power to be spent in achieving and maintaining the tuning. With a tuning range greater than the optical linewidth, it is possible to scan a laser of fixed wavelength from being onresonance to completely out of the optical mode. This allows for applications in switching and routing for photonic circuits. Further, efficient radio to optical frequency conversion is enabled by the ability to electrically drive the mechanical motion of the COEMS. We further present preliminary results for an improved COEMS geometry which significantly improves upon the performance of the current design. References [1] K. Lee, T. McRae, G. Harris, J. Knittel and W. P. Bowen, Phys. Rev. Let. 104, 123604 (2010). [2] T. McRae, K. Lee, G. Harris, J. Knittel and W. P. Bowen, Phys. Rev. A 82, 023825 (2010). [3] A. Pitanti, J. Fink, A. Safavi-Naeini, J. Hill, C. Lei, A. Tredicucci, and O. Painter, Opt. Express 23, 3196-3208 (2015). [4] M. Winger, T. Blasius, T. Mayer Alegre, A. SafaviNaeini, S. Meenehan, J. Cohen, S. Stobbe and O. Painter, Opt. Express 19, 24905-24921 (2011). [5] J. Bochmann, A. Vainsencher, D. Awschalom and A. Cleland, Nat. Phys. 9, 712-716 (2013). [6] R. Andrews, R. Peterson, T. Purdy, K. Cicak, R. Simmonds, C. Regal and K. Lehnert, Nat. Phys. 10, 321326 (2014). [7] T. Bagci, A. Simonsen, S. Schmid, L. Villanueva, E. Zeuthen, J. Appel, J. Taylor, A. Sørensen, K. Usami, A. Schliesser and E. Polzik, Nature 507, 81-85 (2014). [8] L. Tian, Ann. Phys. 527, 1-14 (2015). [9] C. Baker, C. Bekker, D. McAuslan, E. Sheridan, W. P. Bowen, arXiv:1605.07281 [physics.optics] (2016). Preprint at http://arxiv.org/abs/1605.07281 [10] K. Heylman and R. Goldsmith, Appl. Phys. Lett. 103, 211116 (2013). Figure 1: a) A schematic of the effect of a capacitive force on a slotted microtoroidal WGM resonator. Colour denotes displacement in the top panel (red), and intensity of the electric field between the electrodes in the bottom panel (blue). b) Microscope image of the fabricated microtoroidal COEMS. FEA and FFT modelling of harmonics from fibre Bragg gratings Nithila Dediyagala1, Greg W. Baxter1, Fotios Sidiroglou1 and Stephen F. Collins1 1 College of Engineering and Science, Victoria University, PO Box 14428, Melbourne, VIC 8001, Australia 1) Introduction: A fibre Bragg grating (FBG) is a permanent periodic index of refraction modulation along the core of an optical fibre. They are produced by UV exposure through a phase mask, or an interference technique. The phenomenon of a permanent refractive index change is called photosensitivity; its discovery and the advent of FBGs has revolutionized the telecommunication industry and optical fibre sensors fields [1]. A uniform periodic refractive index modulation arises from the interference of the diffracted 1 orders of a phase mask, giving a grating period of , which is half of the phase mask period pm. However, if other phase mask orders are present for FBGs fabrication, the grating structure becomes complex. Phase masks are designed to produce uniform FBGs patterns by maximizing the 1 diffraction orders and suppressing other orders. Typically, the 1 orders have around 35% of the total transmitted power, with the remaining power distributed among other orders; e.g. 3% zeroth order. As the diffraction of a given order produces a different periodicity, actual FBG patterns are not purely uniform as they result from the superposition of several diffraction orders. According to the literature, various studies have been conducted experimentally and numerically to investigate the contribution of other orders except first orders. These results confirm that the existence of other orders contributes to a complex refractive index structure in the plane of the UV writing beam, as shown in Fig. 1. Fig. 1. (a) Image of fibre orientation parallel to the writing beam and (b) Schematic diagram showing the interleaving planes belongs to index modulation of pm and pm/2 [2]. Here FBG harmonics, including their periodicities and efficiencies, resulting from the complex FBG obtained via the phase mask method in a single mode fibre (SMF28) is modelled, using (finite-element) COMSOL Multiphysics software and FFT analysis, to gain greater understanding of their behavior. 2) Results: To determine the various harmonic components existing along the fibre core and their positions using intensity distribution, a 1 mm FBG length was considered for fabrication using phase mask. For the fabrication process it is assumed that the fibre core was placed 72.5 µm away from the phase mask. For analysis purposes, intensity distribution produced in a 8.2𝜇m ×1000 µm region (Fig. 3) was considered in a fibre core which has unit pixel size of 0.0076 µm. To obtain the harmonic components, and grating periods associated with the intensity distribution, fast Fourier transforms (FFT) were performed by extracting line profiles of intensity distributions along the fibre core x, where y at -71 µm, -71.75 and -73.35µm in Fig. 2. Fig. 2 Simulated intensity spectrum along the fiber core using phase-mask method Periods in x , FFT Harmonic analysis (±0.0038 m µm) FFT (%) strength at (µm) -71 -71.75 -73.35 1 1.0630 9.86 3.33 0.03 2 0.5318 0.35 33.62 33.86 3 0.3546 1.05 0.26 1.04 4 0.2658 18.93 4.84 4.37 5 0.2128 1.53 0.10 0.54 6 0.1773 0.31 0.14 0.25 7 0.1520 0.42 0.11 0.15 Table 1. Results of spectral components of SMF28 evaluated grating periods using FFT analysis, their harmonics and diffraction efficiency of harmonics for different line scans 3) Discussion: According to the performed FFT analysis, the most dominant grating period is clearly half of the phase mask period which is produced by ±1 orders. However, for line scan at -71 µm, the 0th and ±3rd order become more dominant while ±1 and ±4 gave small contribution to the diffracted pattern. At -71.75 µm, although ±1 order became more dominant there is a significant contribution from the 0th and ±3th orders. Similarly at -73.35 µm ±3 order gives significant contribution to the intensity spectrum while ±1 order becomes more dominant. Therefore FFT analysis confirms the existence of other harmonics other than ±1 order when the fibre core is placed 72.5 µm away from the phasemask in FBG writing process. Therefore investigating of existence of other harmonics and their strength will help to understand the behavior of FBG which will be useful in future applications. Hence the more work has been conducted to investigate the behavior of the FBG spectrum at different wavelength. References [1] Othonos, A., Fibre Bragg Gratings, 1999. [2] Rollinson, C. M. et al., JOSA A 29, 1259, 2012. Nonequilibrium flows of superfluid Bose gases Matthew J. Davis1 1 School of Mathematics and Physics, The University of Queensland, St Lucia, QLD 4072, Australia The classical field approach to finite temperature Bose-Einstein condensates has proven to be useful for quantitatively modelling a variety of phenomena in quantum gases [1]. Examples include the equilibrium properties of low-dimensional systems, such as the momentum distribution of a trapped one-dimensional Bose gas [2], and the characteristics of the Berezinskii-Kosterlitz-Thouless transition in a trapped twodimensional Bose gas [3]. However, the method is also suited to simulating nonequilibrium experiments, such as spontaneous vortex and soliton formation in the growth of a BoseEinstein condensate [4, 5], or the generation of excitations from quenches of the interaction strength [6]. In this work we realise novel nonequilibrium steady states of Bose gases flowing between reservoirs with different thermodynamic parameters. An increasing temperature difference leads to a greater relative flow velocity between the normal and superfluid components, eventually leading to counterflow turbulence. We also describe how to use reservoir engineering as a method to develop atomtronic devices with ultracold atoms, as well as considering application to polariton-exciton superfluids. References [1] P. B. Blakie, A. S. Bradley, M. J. Davis, R. J. Ballagh, C. W. Gardiner, Advances in Physics, 57, 363 (2008). [2] M. J. Davis, P. B. Blakie, A. H. van Amerongen, N. J. van Druten, and K. V. Kheruntsyan, Physical Review A 85, 031604(R) (2012). [3] C. J. Foster, P. B. Blakie, and M. J. Davis, Vortex pairing in two-dimensional Bose gases, Physical Review A 81, 023623 (2010). [4] C. N. Weiler, T. W. Neely, D. R. Scherer, A. S. Bradley, M. J. Davis, and B. P. Anderson, Nature 455, 948 (2008). [5] G. Lamporesi, S. Donadello, S. Serafini, F. Dalfovo, and G. Ferrari, Nature Physics 9, 656 (2013). [6] C.-L. Hung, V. Gurarie, C. Chin, Science 341, 1213 (2013). Guiding and Scattering in Random Fibre Lasers Wan Zakiah Wan Ismail1,2,3,, Charlotte Hurot1,2,4 and Judith Dawes1,2 1 MQ Photonics, Department of Physics and Astronomy,, Macquarie University, Sydney, NSW 2109 Australia Centre of Excellence CUDOS, 3 Islamic Science University of Malaysia, Nilai 71800, Negeri Sembilan, Malaysia 4 Ecole Centrale de Lyon, Lyon, France 2 ARC 1) Introduction: Random lasers [1] offer potential for biomedical sensing [2] for example by amplifying the subtle effects of particle aggregation. With the requirement for high sensitivity and very small sample volumes, a liquidfilled hollow core random fibre laser appears a promising approach for this application [2]. 2) Experiments: The laser incorporated a solution of Rhodamine 6G dye in ethylene glycol (1 mM) containing either alumina or gold nanoparticles, and infiltrated by capillary action into the hollow core of a micro-structured optical fibre. To selectively fill the 7.5 µm or 20 µm diameter hollow core, the outer cladding holes were collapsed using a fusion splicer [3]. The laser was sidepumped by a frequency-doubled Q-switched Nd:YAG laser (532 nm, 10 Hz repetition rate, 5 ns pulsewidth) focussed by a cylindrical lens, to irradiate a 5 mm-length of dyeinfiltrated fibre. The random laser emission was collected by a lens from the fibre end. Laser parameters including threshold, emission intensity, and emission spectral width were measured and scattering length was estimated from the nanoparticle concentration and Mie theory. 3) Results: The laser emission arises due to a balance between the effects of scattering and wave-guiding to introduce optical feedback [4]. The threshold is a minimum for lasers with long mean scattering lengths, in which the light is guided, rather than scattered. The threshold is also a minimum for lasers with very short scattering lengths where scattering dominates and the light amplification path is long. Notably the threshold for a random fibre laser is lower than for the equivalent laser with no scatterers (see Figure 1a). The threshold is considerably lower than for the equivalent bulk random laser (pumping a similar solution in a cuvette for example). Figure 1a) shows the improvement in the laser performance when scatterers are added to the solution. The laser emission intensity is plotted with (purple triangles) and without (green squares) added alumina scatterers (50 nm nanoparticles). Figure 1b) shows the emission intensity of the random fibre lasers containing alumina nanoparticles at a fixed pump energy level plotted against the mean scattering length (with 50 nm diameter alumina nanoparticles.) The emission increases with longer scattering lengths as optical feedback is dominated by wave-guiding rather than scattering. The introduction of metallic nanoparticles facilitates plasmonic enhancement of the optical gain but also introduces absorption loss [5]. A simple Matlab model which incorporates a Monte Carlo approach with angle-dependent scattering and waveguiding predicts low thresholds for very short and very long mean scattering lengths, consistent with experimental observations. 4) Conclusions: The low threshold of the random fibre lasers is attributed to effective optical feedback arising from a combination of scattering and wave-guiding in the hollow core micro-structured optical fibre. The small sample volumes required for the random fibre lasers offer promise for biomedical sensing applications. Emission intensity at waveplate 8 The combined effects of wave-guiding and scattering in a random fibre laser containing metallic or dielectric nanoparticles are investigated. A Rhodamine dye solution with alumina or gold nanoparticles is introduced into the hollow core of a micro-structured optical fibre. The lasing threshold is considerably reduced in comparison with the bulk solution. 70000 60000 50000 40000 30000 20000 10000 0 0 1000 2000 3000 4000 5000 6000 Scattering length (µm) Figure 1a) Rhodamine dye fibre laser emission plotted against pump energy with (purple triangles) and without (green squares) added alumina nanoparticles. Figure 1b) Rhodamine dye random fibre laser emission intensity for a fixed pump energy of 0.048 mJ, plotted against mean scattering length. References [1] H Cao, Waves in Random Media, 2003, 13, R1-R39. [2] WZW Ismail, et al., Opt. Expr, 2016, 24, A85-A91. [3] L Xiao, et al., Opt. Expr. 2005, 13, 9014-9022 [4] Z Hu et al., Phys Rev Lett, 2012, 109, 253901 [5] WZW Ismail, et al., Laser Physics 2015, 25, 085001 Water temperature measurement using blue excitation and two-channel Raman spectrometer Andréa Ribeiro1, Christopher Artlett1,2, Helen Pask1 1Department of Physics and Astronomy, Macquarie University, Sydney, Australia 2Lastek PTY Ltd. Raman spectroscopy (RS) is a technique with potential to solve several problems in oceanographic remote sensing, including being able to provide reliable data about subsurface water properties (e.g. temperature, salinity). Several studies established a relationship between water Raman spectra and temperature that could be used in remote sensing equipment [1,2]. Recently, experiments conducted using RS and a 532nm (green) excitation laser found an accuracy of ±0.1°C for pure water temperature measurements [2]. However, when this technique was applied to natural waters, it was found that fluorescence from Chlorophyll at 680nm overlapped the Raman signal and adversely affected the accuracy with which temperature could be determined. Aiming to avoid the impact of fluorescence, we conducted experiments to retrieve temperature-dependent polarised Raman spectra of Milli-Q water with a pulsed (2 ns duration) 473nm (blue) laser as excitation source. The use of blue light can also be a benefit for in situ measurements, as blue light has near optimal penetration in the water column. Figure 1: Experimental setup for collection of polarized temperature-dependent Raman spectra of Milli-Q water. Our experiment setup is shown in Figure 1. A Milli-Q water sample (S) was placed inside a temperature-controlled cuvette holder and its temperature was varied from 20°C to 40°C, stepping every 2°C. The Raman signal scattered by the sample passed through a dichroic mirror (DM) and through a Long Pass filter (LP) to eliminate Rayleigh scattered photons. The Raman signal was then split into two components – perpendicular and parallel – to the polarisation of the excitation laser using a Polarising Beam Splitter Cube (PBSC). Each component was detected by a Photomultiplier (PMT): PMT1 retrieved signal from the parallel component with a 590nm Band Pass filter (F1), and PMT2 from the perpendicular component with a 545nm Band Pass filter (F2). The signal from each PMT was collected by a multi-channel oscilloscope, with averaging over 532 pulses. The band pass filters F1 and F2 were used to select parts of the spectra positioned before and after the isosbestic point and were choses on the basis of polarised Raman spectra presented in [2]. Higher signal intensities were found for the perpendicular component (see Fig 2.A) than the perpendicular (see Fig. 2.B); this is due to the tetrahedral structure of water molecule and its vibrational modes [3]. Figure 2: Parallel (A) and Perpendicular (B) polarised Raman pulses for Milli-Q water (temperatures of 20°C to 40°C). In the liquid state, water molecules bond in clusters and the number of bonds is dependent on temperature; as temperature rises these bonds break and change the molecules vibrational frequency [4]. This behaviour was detected in both components (Fig. 2). As was also found in [5], there is a linear relationship between decreasing temperature and the depolarisation ratio, i.e., the ratio between the intensities in the perpendicular and parallel components. Our results are shown in Figure 3, where the average change in ratio over this temperature was close to 1% and the R2= 0.8858. Figure 3: Depolarisation ratio as a function of temperature. To our knowledge, this is the first time that RS has been applied to determine temperature using the depolarisation ratio and blue excitation at 473nm. Most significantly, this has been achieved using a two-channel Raman spectrometer and without retrieving the full Raman spectra. Our ongoing work will be focused on natural water samples and on combining with LIDAR methods for depth-resolved measurements. References [1] Walrafen et al.,The journal of Chemical Physics, 1985 [2] Artlett & Pask, Optics express 23 (25), 2015 [3] Tominaga et al., Fluid phase equilibria 144 (1-2), 1998 [4] Carey et al., The journal of Chemical Physics 12, 1998 [5] Leonard et al., Applied Optics 18(11) Nonlinear Optical Response with Dependence on Optical Vortex Mode Robert Donaldson1 and Esa Jaatinen1 1 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, The Queensland University of Technology, Brisbane, QLD 4000, Australia Where r, z are the radial and longitudinal coordinates, w0 is the beam radius at the beam waist, w(z) the beam radius at position z and I0 the fundamental intensity. Here we discuss how modifying this profile to have a vortex intensity distribution of order n gives a theoretically higher nonlinear response than that of the T EM00 , for a given constant peak intensity value. ∞ 2 r 2 m r 2 m n dr r e− w 2 w 0 (4) Scaling the power P0 such that each vortex mode n has a peak intensity of Id shows that the nonlinear response power should increase with vortex order n for all nonlinear optical response orders m. Here we will report on how much enhancement occurs and with future work we will show the experimental results of these predictions. Z (A) Intensity Distributions for Peak Intensity Values = Id 1 vortex order n: 0.8 0.6 In 1) Fundamentals: Significant optical nonlinear response in a material can be excited if the incident light intensity is sufficiently intense, achievable typical with a laser source. Typically this source has a T EM00 Gaussian intensity profile given by: 2 2 w0 −2 r e w(z)2 (1) I(r, z) = I0 w(z) 2 π 2m n 2m κ P0 m Pm (n) = m 2 m m π w Γ (n + 1) 0.4 0.2 0 1 2 3 4 5 0 -3 -2 -1 0 r/w0 1 2 3 (B) The relative nonlinear output increases with nonlinear order "m" and with vortex order "n" relative nonlinear response The modification of the optical and material properties of material systems is reliant on high light intensity, and is usually only possible by utilising laser light [1]. Typical laser systems produce a beam with an intensity characterised by a Gaussian distribution in cross section. Thus the bulk of research performed into investigating intensity dependent nonlinear optical effects utilizes or assumes a T EM00 fundamental Gaussian mode (Eq.1) light intensity distribution. However, this particular distribution may not be optimal for inducing a specific nonlinear effect or may lead to other unwanted outcomes such as sample damage. Here we investigate the nonlinear optical response produced when the incident light has a vortex intensity distribution rather than the standard T EM00 Gaussian distribution. 15 10 5 0 0 nonlinear order m: 1 2 3 4 5 6 1 2 3 4 5 vortex order n 2) Beam Intensity Profile Modification: Equation.(1) can be modified to include vortex mode of order n, such that n = 0 returns the initial T EM00 profile[2]. An increase in nonlinear response is theoretically predicted when the peak intensity of Figure 1: (A) Intensity profiles of vortex beams of order n = each vortex mode is ≤ Id , where Id is a material dependant 0, 1, 2, 3, 4, 5, with a peak intensity equal to the material damage threshold Id . (B) The relative response for different optical damage threshold: nonlinear response orders m based on incident beams of vortex n order n. 2 2 n 2r 2e r e− w 2 (2) In (r, z, n) = Id n w References 3) Nonlinear Enhancement Outcomes: The overall nonlinear response depends on the cross- [1] R. W. Boyd, Nonlinear Optics, 3rd. ed., (Academic Press, sectional area of the exposed material and the sample thickSan Diego). ness. The power applied to the material within this area is the intensity I0 . Depending on the material a different nonlinear [2] 1. N. R. Heckenberg, R. McDuff, C. P. Smith, H. Rubinsztein-Dunlop, and M. J. Wegener, Opt. Quantum response Pm of order m may result. Therefore for frequency Electron. 24, 951 (1992). mixing nonlinear responses such as Second Harmonic Generation (SHG) (m = 2), the power of the output harmonic is given by Eq. 3: Z Pm = κ I0 m dA (3) σ Where κ is a constant. If we let I0 = In (Eq. 2) and substitute into Eq. 3 we can retrieve an expression for the nonlinear response Pm in terms of the vortex order n: