Experimental verification of the practicality of
photonic crystal fibre photonic lanterns
Jiro Funamoto
(309216494)
Contents
1 List of abbreviations and symbols
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2 Acknowledgements
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3 Overview
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4 Background and theory
4.1 MMFs vs SMFs for light collection in astronomy . . . .
4.1.1 Optical fibres, numerical aperture, and modes . .
4.1.2 MMFs vs SMFs . . . . . . . . . . . . . . . . . . .
4.2 The PCF photonic lantern . . . . . . . . . . . . . . . . .
4.2.1 PCF . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 The PCF photonic lantern . . . . . . . . . . . . .
4.2.3 Theory behind photonic lanterns - multi-mode to
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5 Verification of a practical PCF photonic lantern setup
5.1 The experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Experimental difficulties . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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6 Experimental results
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7 Discussion and implications
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8 Conclusion
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List of abbreviations and symbols
A/Prof.
OSA
Dr.
Prof.
MMF
SMF
FBG
IR
OH
PCF
fibre
NA
vs
2
associate professor
optical spectrum analyser
doctor
professor
multi-mode fibre
single-mode fibre
fibre Bragg grating
infrared
hydroxide
photonic crystal fibre
optical fibre
numerical aperture
versus
Acknowledgements
• A/Prof. John O’Byrne for managing this project (including coming down to the lab
every time he was free) despite his very busy schedule. Indeed, as a lady (anonymous)
in the physics building had said, John is overworked and underpaid!
• Christopher Trinh for showing me how to strip, cleave, and splice the optical fibres,
as well as how to analyse the light from a fibre using the optical spectrum analyser
(OSA).
• Dr. Sergio G. Leon-Saval for explaining some basic aspects of the photonic lantern
and helping in imaging the ends of the optical fibres.
• Prof. Joss Bland-Hawthorn for showing me where the photonic lantern fits in to the
other developments in astrophotonics.
• Prof. Dick Hunstead and Dr. Michael J. Biercuk for their time and effort in organising
these very worthwhile TSP activities.
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Overview
A photonic lantern is an optical device which acts as the transition between a multi-mode
optical fibre (MMF or ‘wide cored optical fibre’) and several (or more) single-mode optical
fibres (SMFs or ‘thin cored optical fibres’). The photonic lantern allows one to use the
benefits of MMFs (greater light collection and propagation) with SMF-only technologies
(such as fibre Bragg grating (FBG) filters), and has much application in astronomy. This is
particularly true for infrared (IR) astronomy which requires OH suppression (single-modeonly) filters to be used with MMFs (which collect the light from the telescope focal point).
A few types of photonic lanterns currently exist, and this report will focus on one particular type - namely the photonic crystal fibre (PCF) photonic lantern. The PCF photonic
lantern was one of the first low loss photonic lanterns ever fabricated (created in 2006),
and uses a PCF as an intermediary for converting a MMF input into seven SMF outputs.
Despite the relatively early fabrication of the PCF photonic lantern and subsequent analyses of performance [1], this particular device has never been experimentally verified for use
in a practical setup. This report will therefore experimentally verify the practicality of the
PCF photonic lantern in a setup most relevant for IR astronomy - using FBG filters on a
MMF input at infrared wavelengths1 .
4
Background and theory
4.1
MMFs vs SMFs for light collection in astronomy
This section will clarify the differences between MMFs and SMFs. This will act to highlight
the main reason for the invention of the photonic lantern. Firstly, however, it is necessary
to become familiar with a few parameters used to describe different types of optical fibres.
4.1.1
Optical fibres, numerical aperture, and modes
Optical fibres (or ‘fibres’ for short) are electromagnetic waveguides used to confine and
transport light from one place to another. These fibres are commonly created from high
purity silica, and consist of a core (where most of the light is confined2 ) and a cladding
(a lower refractive index material surrounding the core responsible for the aforementioned
light confinement).
Note: unless explicitly stated, all optical fibres in this report refer to those created for use
1
this is a setup that would not provide the desired output without a MMF to SMF converter such as
the photonic lantern [2]. Greater detail on the reason as to why FBGs do not work with MMFs is found
later in the report.
2
a small amount of light is present as an exponentially decaying evanescent wave in the cladding.
3
at IR wavelengths around 1550 nm (as silica has the lowest absorption and scattering losses
for wavelengths roughly around 1550 nm, and is the wavelength used in the telecommunications industry).
A typical schematic of an optical fibre can be seen in Fig.1, in which a simple geometric optics interpretation of light propagation (using total internal reflection), is illustrated.
Figure 1: A labeled schematic of an optical fibre (in this case a step index MMF - a step index fibre
is one in which the transition of refractive index between the core and the cladding is discontinuous and
‘step-like’). The simplified model of light propagation using geometric optics and total internal reflection,
as well as the acceptance cone, is illustrated. Figure obtained from [4].
Also seen in Fig.1 is an important parameter expressed as the acceptance cone, the cone
created using the maximum angle at which light can enter the optical fibre core and still
be confined within the core. This is a very crucial parameter (for astronomy) as the larger
the cone, the easier it is to couple (transfer energy from one medium to another) light into
the fibre. One equivalent measure of this acceptance angle used commonly in the literature
is the numerical aperture (NA), which is defined as the sine of half the acceptance cone
angle multiplied by the refractive index of the medium from which light is entering (i.e. in
this case air):
acceptance cone angle
N A = n sin
(1)
2
where n ≈ 1 for air. This is a direct measure of the acceptance cone scaled to suit situations in which the external medium (of refractive index n) is not air. NAs of MMFs
are typically much larger than that of a SMF, and consequently, much more precision is
needed to accurately couple light into a SMF [6].
Just like the acceptance cone, the higher the NA, the greater the ability of the optical
fibre to accept light at larger angles, and consequently the easier it is to couple light into
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the fibre.
Another important parameter used to categorise optical fibres is the number of ways (or
‘modes’) light can propagate down the fibre, categorising fibres into MMFs and SMFs3 .
Each mode is a solution to the electromagnetic wave equation solved inside the fibre (with
the appropriate boundary conditions imposed by the fibre) and therefore represent the
different ways light can propagate down the fibre4 . More ways that light can travel down
an optical fibre also signifies an increased capacity in carrying light. Therefore MMFs have
much larger light carrying capabilities per fibre than their SMF counterparts.
Note: the number of modes is dependent on the wavelength of light propagating down
the fibre (with shorter wavelengths allowing more modes to propagate down a fibre with
a given core width. Conversely a wider core allows more modes to propagate for a fixed
wavelength).
MMFs allow multi-modal light to propagate due to their large core diameter d which
is one of the critical factors for the number of possible propagation modes:
number of modes ≈
π 2 d2 (N A)2
4λ2
(2)
Note: For the approximate wavelength range of interest, 1550 nm, the core thickness of
SMFs (only one propagation mode) is less than 10 µm, whereas for MMFs the core thickness
is usually anywhere between 50 to 100 µm.
4.1.2
MMFs vs SMFs
From the above subsection, it will have become evident that MMFs are much more desirable for the collection of light in astronomy due to their greater NA (allowing light
from larger angles to enter the fibre) and because MMFs can collect much more light per
fibre (due to the wider core and the presence of multiple modes). Although the MMF
does suffer from effects such as modal dispersion (where one mode progresses down the
fibre slower than another5 resulting in unwanted phase changes between different areas of
the input light), the MMF is still the most desirable choice for use in telescope focal points.
However, this multi-modal light propagating through the MMF can not be used with many
devices invented in photonics such as the FBG (a filter composed of alternating variations
of refractive index which reflect a narrow waveband (centred on what is named the Bragg
3
a third category, ‘few-mode fibres’ (FMFs) has also been recently proposed in [3] to categorise fibres
between the MMFs and SMFs regimes.
4
These modes are often called transverse modes due to the different pattern the electric field of each
mode can make when viewed in a cross sectional plane perpendicular to the length of the fibre.
5
this can be thought of in geometric optics, by imagining two rays with different total internal reflection
angles, and therefore difference path lengths in a given length of fibre
5
wavelength) almost 100% back towards the source) which depend entirely on single-modal
light for proper functioning. For the FBG, since the Bragg wavelength depends directly on
the spacing between the alternating refractive indices, if multi-modal light is sent into a
FBG the different modes will each have a different Bragg wavelength - since different modes
travel in different paths within the fibre (as mentioned in the prior paragraph) changing
distance between alternating refractive indices. This makes FBGs almost useless for use
with MMFs.
The answer to allowing MMFs to collect light while using SMF-only technologies therefore
came in the form of the photonic lantern - a device which couples the multi-modal light
propagating down a MMF into a number of individual SMFs, available to then be fed into
FBGs and other SMF-only technologies.
4.2
The PCF photonic lantern
This section will describe the basic structure and theory of the PCF photonic lantern.
However, a brief explanation of the PCF will precede.
4.2.1
PCF
PCFs are another type of optical fibre similar in a working sense to ‘standard’ optical fibres
described in the sections above. However, the main difference is that PCFs do not have a
separate cladding material around the core. As a substitute, the core material of the PCF
extends as far as a typical cladding outer diameter, except that the core material has holes
where the cladding material should be. A cross sectional image of a PCF is shown in Fig.2.
Having the air holes effectively lowers the average refractive index of the core material in
Figure 2: A optical micrograph of a PCF. In particular this is the MMF end of the PCF photonic lantern.
From [5]
the region of the holes, and acts essentially like a cladding. PCFs has several advantages
over conventional fibres - one of which is the capability to vary the average refractive index
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by increasing or decreasing the density of holes - thereby controlling various parameters
such as the NA.
4.2.2
The PCF photonic lantern
For the first ever low loss photonic lantern (created at the University of Bath), a PCF
design was used due to the perceived ease of manufacturing of the photonic lantern. An
image of the PCF photonic lantern is given in Fig.3.
Figure 3: The PCF photonic lantern. The pencil is approximately 18 cm for scale. Adapted from [5]
The PCF photonic lantern was created by firstly manufacturing the cross section seen in
Fig.4 (which is a number of holey fibres placed inside a protective jacket of identical material).
Following this, seven SMFs are inserted into the seven open honeycomb structure spacings
(seen in Fig.4). This structure is then heated and drawn out till the cross sectional diameter is at the desired width.
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Figure 4: The first stage of the creation of the PCF photonic lantern
The photonic lantern thus takes the form as seen in Fig.3, with one side (the MMF end)
tapering down to a multi-mode PCF (MMF PCF) as seen in Fig.2, and the other side with
seven standard SMFs extruding from the large PCF jacket (Fig.3).
4.2.3
Theory behind photonic lanterns - multi-mode to single-mode
The theory behind photonic lanterns is not yet well understood. It is indeed still a slight
mystery on how the multi-modal light in the MMF can split efficiently and with low loss
into individual SMFs.
Analogies do exist, however, which can explain the transition from a MMF to SMFs and
vice versa. One such analogy is with the one dimensional Kronig-Penny model of quantum
mechanics which describes electrons in a one dimensional crystal lattice (modelled as a
regular array of potential wells). The analogy can be extended such that the wave function
trapped in each lattice of the electron is like the electric field trapped within each SMF,
and by bringing the wells closer together into a single large well, the transition of the SMFs
merging into one MMF analogy can be formed. This is still just an analogy, however, and
as such no further details on the model will be included in this report6 .
However, one important consideration of photonic lanterns has been developed from a
number of experimental results and theoretical considerations [1, 2, 5]. That is, for mini6
more information about this model can be found at [6]
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mum loss, the number of output SMFs must be equal to the number of input propagating
modes in the MMF. A lack of SMFs for propagation modes, or an abundance of SMFs for
propagation modes, (i.e. a mode number mismatch), in both cases appears to result in a
decrease of the net throughout of the device[1].
Unfortunately for the PCF photonic lantern analysed in this report, there was quite a
severe mode number mismatch caused by the core diameter of the MMF end of the lantern
being too large. Specifically, is has been calculated that 21 modes will be excited in the
MMF [1], however, as outlined previously, the PCF photonic lantern only has 7 SMF
outputs. This is expected to decrease the efficiency of the photonic lantern by at least
(1 − 7/21) = 67%.
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5.1
Verification of a practical PCF photonic lantern setup
The experimental setup
Experimental verification of the practicality of the PCF photonic lanterns simulating a
realistic situation was done using the setup illustrated in Fig.5
Figure 5: Experimental setup using the PCF photonic lanterns. The photonic lantern splits multi-modal
light input into single-modal light propagating through the seven SMFs with imprinted FBGs. The seven
SMFs are then connected to a photonic lantern in reverse. In essence, this is two photonic lanterns backto-back with FBGs in between. Light is shown to come in from the left and exit to the optical spectrum
analyser (OSA) on the right. Green lines represent MMFs, orange representing SMFs.
IR light (at 1555 nm, 50 nm bandwidth) is sent into the left PCF photonic lantern through
a MMF. The light is then split into seven SMFs and sent through individual FBG filters.
Lastly, a photonic lantern in reverse re-combines the SMFs into a single MMF heading
for the optical spectrum analyser (OSA) (an instrument that measures the intensity as a
function of wavelength) to detect whether the light has been filtered by the FBGs.
The FBGs were those manufactured to filter (i.e. have a Bragg wavelength of) around
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1555 nm. This was confirmed initially with measurements using the OSA before the setup
seen in Fig.5 was fused together. The average filtered wavelength of each of the seven
FBGs was confirmed to be in the range 1555.06 ± 0.02 nm. A graph showing the effect of
the filter at around 1555.06 nm is shown in Fig.6.
Figure 6: The notch created in the spectrum of light by the FBG. Obtained using the OSA and light of
wavelength 1550 nm with a 50 nm bandwidth fed into a single FBG. The wavy nature on both sides of the
notch are results of partial reflection of close-by wavelengths by the FBG.
If the setup of Fig.5 is successful in coupling multi-modal light (in the MMF) into a
number of single-modal light rays (into separate SMFs), then the output at the OSA from
the setup should be very similar to that of Fig.6.
Note: this experimental setup was chosen as it was a practical setup most relevant for
IR astronomy and one for which the photonic lantern was designed.
5.2
Experimental difficulties
There were several complications in the process which will most likely affect the results of
this experiment. These are as follows:
• fibre adaptors: the multi-mode PCF end of the photonic lantern did not fit into the
standard 125 µm bare fibre adaptors needed to connect the lantern to the OSA. Fusing
a standard size MMF onto the MMF end of the PCF lantern was also attempted but
failed due to the inability of the fusion splicer (fibre fusion device) to fuse a PCF
with a standard MMF (see Fig.7).
Therefore a technique called ‘butt coupling’ had to be used (which is plainly placing
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Figure 7: Showing the screen of the fusion splicer when attempting to fuse together the MMF PCF end of
the photonic lantern with a standard MMF together. The image displayed on the left of the screen shows
the X view, and the right shows the Y view of the two fibres up for fusing. On each of the views, the left
fibre is the standard MMF, and the right fibre is the MMF PCF. The screen displays areas of differing
refractive indices in different shades of grey. As can be seen, the core and cladding of the standard MMF
is clearly visible, however, that of the MMF PCF is not. The air holes of the MMF PCF obstruct the view
of the inner core. The inability to see a clear core causes an error in the fusion splicer machine and results
in an error - disallowing the two fibres to fuse.
the ends of two fibres up against each other). This is expected to severely affect the
power throughput of the device. There were two butt couples in the setup at each of
the multimode ends, and hence the power throughput is predicted to be extremely
low.
• mode number mismatch: the severe mode number mismatch of the PCF photonic
lantern in use (as mentioned in the previous section) will cause power loss in the
MMF to SMFs and the reverse transition.
• splice losses: at every interface or boundary there will always be coupling losses (losses
caused by the reflection of some light at any interface - even a splice between identical
fibres). Such losses will be present at the splices between the photonic lantern SMFs
and the FBGs.
• photonic lantern losses: from the performance analyses of the PCF photonic lantern
(in [5]) it is seen that there is an average loss on each of the photonic lanterns used
of around -2.6 dB ≈ 45% power loss. Therefore, purely two lanterns in series should
result in ≈ 70% power loss of the original input power. PCF photonic lanterns with
lower loss do exist, however, were not used for this report as they will be saved for
the time when we obtain proper fibre adaptors. The results below are therefore also
only intermediate results.
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6
Experimental results
One of several graphs obtained from the OSA is shown in Fig.8 below.
Figure 8: Intensity distribution of the light out of the photonic lantern setup as analysed by the OSA.
As can be seen in Fig.8, compared to the expected curve of Fig.6, the graph is extremely
noisy. This is due to the very low power throughput of the system (high noise level relative
to the power throughput) with almost 3 orders of magnitude less power than just using a
single FBG. (Note: the same input power was used for both cases). Such high power loss
is predicted to be caused predominantly be the butt coupling of the fibres, as well as the
inherent photonic lantern losses.
Despite the very low power throughput, the notch in the spectrum created by the FBGs
is clearly visible at the predicted wavelength (≈1555.06 nm). The width of the notch as a
function of the power above the Bragg wavelength in Fig.8 is also very similar to that of
Fig.6.
Another graph obtained from the OSA is given in Fig.9.
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Figure 9: Intensity distribution of the light out of the photonic lantern setup as analysed by the OSA. As
compared o Fig.8, this graph is skewed in the opposite direction. What was changed was the orientation of
the butt coupling fibres.
The only difference in the setup between Fig.8 and Fig.9 was the orientation of one of
the butt coupled fibres. This provides support that the skew of Fig.8 was not a result of
the PCF photonic lantern device, but was a product of the rough butt coupling of fibres.
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Discussion and implications
The above results show very high loss using the current photonic lantern setup. However,
as mentioned, this is predicted to be caused predominantly by the combination of factors
described previously in the “Experimental difficulties” section. The butt coupling of fibres
is predicted to be the main cause of the very high (3 orders of magnitude greater) loss of
power, as the inherent photonic lantern loss is only predicted to be ≈ 70%.
As also previously mentioned, the PCF photonic lanterns used were not the lowest loss
pair of lanterns available. The lowest loss PCF photonic lantern has a power loss of around
-0.32 dB [1] and will be used once the correct fibre adaptors are obtained. If two of these
lowest loss lanterns were created, the theoretical throughput of the back-to-back photonic
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lantern setup is predicted to be around 14 % which is relatively low loss. This would make
this experimental setup feasible for practical use.
Nonetheless, the intermediate results obtained show that the FBG Bragg wavelength notch
is of the same characteristics as that for a SMF input and occurs at the same Bragg wavelength of ≈ 1555.06 nm (which would not be true for a FBG used on a MMF, as multiple
Bragg wavelengths would result). This confirms that the PCF photonic lantern is successful in splitting the multi-modal light from a MMF input into separate individual SMFs.
This in itself shows the feasibility of the photonic lantern concept in allowing a MMF to
be used with SMF-only devices such as the FBG.
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Conclusion
This report has provided experimental support that the PCF photonic lantern does convert multi-modal light (from the MMF) into single-modal light (down several SMFs). By
using a PCF photonic lantern, this will allow SMF-only technologies such as the FBG to
be used with a MMF input (such as the light commonly collected from the telescopes).
In particular, the lowest loss setup is predicted to obtain a minimum of 14% power loss
through the back-to-back system, and hence will prove feasible for practical use.
Future work will verify the sources of this extreme power loss in the setup of (Fig.5) by using the correct fibre adaptors as well as the lowest PCF photonic lantern pair. The results
will then be compared to the minimum predicted loss of 14% and the true practicality of
the PCF photonic lanterns will be judged.
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References
[1] T. A. Birks, A. W. Witkowska and S. G. Leon-Saval, Multimode to Single-Mode Transitions II, University of Bath
[2] S. G. Leon-Saval, T. A. Birks, J. Bland-Hawthorn, M. Englund, Multimode fiber devices
with single-mode performance, October 1, 2005 / Vol. 30, No. 19 / OPTICS LETTERS,
2545
[3] A.J. Horton and J. Bland-Hawthorn, 2007, Coupling light into few-mode optical fibres
I: The diffraction limit
[4] Gringer, Image: Optical-fibre.svg
Available at: http://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Opticalfibre.svg/500px-Optical-fibre.svg.png
[5] J.W. OByrne, J. Bland-Hawthorn, S.G. Leon-Saval, A.W. Witkowska, T.A. Birks, Multimode fibres with single-mode performance: a low loss 7-mode converter, 2006, Optical
Society of America
[6] S.G. Leon-Saval, A. Argyros and J. Bland-Hawthorn, Photonic lanterns: a study of light
propagation in multimode to single-mode converters, 2009, Optical Society of America
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