Download Coherent laser radar system using optical homodyne detection

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: