Download Analysis of optical damage mechanisms in hollow core

Analysis of optical damage mechanisms in hollow core
waveguides delivering nanosecond pulses from a Qswitched Nd:YAG laser.
J P Parry, T J Stephens, J D Shephard, J D C Jones and D P Hand
Heriot-Watt University, Riccarton, Edinburgh, UK
[email protected]
Abstract. Fibre optic delivery of high peak power pulses can offer advantages to a
wide variety of applications including laser micromachining, medical and metrological
applications giving increased manoeuvrability of the beam and simplified access to
enclosed volumes. Delivery of high peak powers through fibre optics is however
challenging. With conventional step-index fibres large core diameters (~1 mm) are
required for pulse energies in the 10’s of mJ range (ns pulse length). Such fibres are
not suitable for applications involving tight focusing, as the delivered beam-quality is
low. Hollow core waveguides consisting of a glass capillary tube with an internal
reflective coating (typically silver with a dielectric) offer a potential solution. Such
waveguides are capable of guiding pulse energies of 10’s of mJ’s and the delivered
beam has an improved beam-quality compared with step index fibres of similar core
diameter. In previous work we have demonstrated the use of such waveguides to
deliver Q-switched pulses at 532nm for high speed gas flow measurements within a
total-internal-combustion engine.
The work presented here demonstrates the
capability of these fibres to deliver high power Q-switched pulses at the fundamental
(1064nm), second (532nm) and third (355nm) harmonics of an Nd:YAG laser, both in
terms of peak power and beam-quality delivered. The primary limitation of these
waveguides in terms of peak power delivery is the occurrence of bend-induced optical
damage to the reflective coating. The damage mechanism and the influential factors
are analysed, in particular the dependence upon the number of guided modes, core
diameter, coating thicknesses and input polarisation alignment.
1. Introduction
Fibre optic delivery of laser light is desirable for many processes requiring the application of
high peak-power laser pulses. Conventional step-index fibres are limited in such applications
as they are prone to damage within the core either at the end face or as a result of the
nonlinear process of self focusing. Large core fibres or fibre bundles can support greater
pulse energies but at the expense of delivered beam quality which is an issue for many
applications. Hollow waveguides [1] offer advantages for the delivery of laser pulses over
short distances. These consist of a glass capillary tube with an internal metallic reflective
layer. Additionally, a dielectric layer is often included to both protect the metal coating and
enhance reflection at a specific wavelength [1]. Waveguides were supplied by Tohoku
University (Japan).
Interest in this type of waveguide is increasing for many applications, for example for
tissue ablation in medicine [2] and spark formation [3] (useful as a source of ignition). In
particular our group is interested in fibre delivery for the fluid flow measurement technique of
Particle Image Velocimetry (PIV) [4]. PIV requires the delivery of laser pulses focused into a
thin and extended light sheet of high intensity to image tracer particles in the flow. The
typical light source is a Nd:YAG laser operating at 532 nm due to the spectral response of
the cameras used. Previously PIV measurements have been made inside a test combustion
engine using hollow waveguides to deliver a light sheet [4].
Hollow waveguides are also available for the delivery of UV wavelengths. These may
benefit applications such as laser induced fluorescence for fuel film measurements in
internal-combustion engines [5] or temperature measurements in gas turbines [6].
These waveguides are capable of supporting very high pulse energies when straight.
However, significant losses occur in a bent waveguide and the supportable pulse energy is
reduced imposing significant restrictions for practical use. This paper is concerned with
developing a better experimental understanding of the limitations of these waveguides with
respect to the mechanisms and characteristics of optical damage and the influential factors.
2. Beam Quality
The focusability (beam quality) of the beam delivered is important for PIV and many other
applications where a small focused spot is required. Beam quality can be quantified by the
M2 value [7] which compares the dimensions of the real focussed spot to a diffraction-limited
focussed spot at the same wavelength. Due to the relatively large core size of these
waveguides, light is easily coupled into high order modes so the beam quality of light
launched into the guide is not maintained.
In order to simulate use in a real application, M2 values were measured for the
waveguides with a 180° bend of 300 mm radius. A short focal length lens was used to
overfill the input and to encourage guidance of as many modes as possible. An imaging lens
was placed at the output to create a magnified beam waist, in order that the waist diameter
and Rayleigh range could be readily measured, and hence the M2 value could be calculated.
The numerical aperture (NA) of each waveguide was calculated from its measured M2.
This value is not well-defined for such waveguides as guiding is by Fresnel reflection (rather
than by total internal reflection), but gives an indication of what cone angle will be accepted
or delivered, dependent on the range of modes that may be guided without excessive loss.
2.1. Measured beam quality
Only one waveguide was available to test at 1064 nm with an internal diameter of 1 mm.
Initially at 532 nm three different waveguide diameters were tested: 0.54 mm, 0.7 mm and
1.0 mm. Waveguides with an improved reflective coating were later tested at this
wavelength for two core diameters, 320 m and 540 m. The coating was improved in the
sense that the surface finish of the metal layer was smoother than in other examples,
providing improved reflection efficiency. This was achieved by depositing the coating in a
thinner layer. Tests were conducted at 355 nm on waveguides with core diameters of
540 m and 1 mm. These waveguides were coated with a reflective layer of aluminium due
to the high reflectivity of this metal at short wavelengths. No dielectric coating was included.
Measured M2 values and the calculated NAs are shown in table 1.
By comparing waveguides at the same wavelength it is clear that the delivered beam
quality is dependent on the internal diameter of the guide. Smaller diameter waveguides
deliver a lower M2 (higher beam quality). This is in agreement with waveguide theory [8,9],
which states that the attenuation of high order modes is greater in smaller diameter
waveguides leading to preferential guiding of low order modes.
Comparison of waveguides of the same diameter but operating at different wavelengths is
less intuitive as modal attenuation is dependent on several factors that vary differently with
wavelength. Waveguide theory [8,9] states that attenuation increases with wavelength.
However this does not take account of the effects of surface roughness, the effect of bend
induced losses and the interdependence of these additional loss mechanisms [10]. Looking
at the results for 1 mm diameter waveguides the 532 nm waveguide can be seen to deliver
light with the greatest M2 value. The 1064 nm waveguide gives a low M2 value as fewer
modes are supported at longer wavelengths. The 355 nm waveguide should support the
greatest number of modes giving the largest M2. However large reflection losses filter these
out giving an M2 that is intermediate between guides tested at longer wavelengths.
1. Measured M2 and calculated Numerical Aperture.
Waveguide diameter/mm
1.0 (1064nm)
1.0 (532nm)
0.7 (532nm)
0.54 (532nm)
0.54 (Smooth coating, 532nm)
0.32 (Smooth coating, 532nm)
1.0 (355nm)
0.54 (355nm)
Delivered M2
54
370
176
121
109
53
132
22
NA
0.037
0.125
0.084
0.078
0.068
0.056
0.030
0.0092
3. Damage Threshold
At high pulse energies air breakdown may occur at the focal point of the input lens. This can
result in damage to the tip of the waveguide. Air breakdown can be avoided by placing the
waveguide tip in a vacuum or, more conveniently, in an inert gas such as helium [1]. For the
work reported in this paper, immersion in a Helium atmosphere was achieved by mounting
the end of the waveguide in a gas cell at just above atmospheric pressure, and allowing the
gas to flow through the hollow core. The arrangement used for damage tests is shown in
figure 1.
1. Launch arrangement used for damage tests.
A 150 mm focal length lens was used for input coupling in all cases (unless otherwise
stated). For initial experiments on the 1064 nm waveguide a 100 mm focal length lens was
chosen to match the measured NA. However this was found to produce a plasma in the
helium atmosphere at pulse energies of approximately 40 mJ. A 150 mm lens gave a larger
diameter beam waist and avoided problems with plasma formation. In all damage tests
(unless otherwise stated) the waveguide was held with a 180° bend of radius 300 mm.
Damage was observed before any measurable change in output occurred and was
characterised by bright points that were visible on the outside of the bends in the waveguide
as the pulse energy was stepped up. These points were found to correspond to gaps in the
reflective layer that were visible after tests.
3.1. Measured damage thresholds
One waveguide was tested at 1064 nm. The polarisation state of the laser used was linear
and aligned horizontally (parallel to the plane of the bend in the waveguide). Three
diameters of waveguide were tested at 532 nm. At this wavelength the polarisation state of
the laser was linear and vertical (perpendicular to the plane of the bend in the waveguide).
Two waveguide diameters were tested with an improved/smoother coating and two
diameters were tested at 355nm. The available laser did not produce sufficient pulse energy
at 355 nm to damage the 1 mm diameter waveguide. Results are shown in table 2.
2. Measured damage thresholds (*no damage observed).
Waveguide diameter, mm
E in, mJ
E out, mJ
Efficiency, %
1.0 (1064nm)
1.0 (532nm)
0.7 (532nm)
0.54 (532nm)
0.54 (Smooth coating, 532nm)
0.32 (Smooth coating, 532nm)
1.0 (355nm)
0.54 (355nm)
105
66.3
41.7
36.0
16.42
3.27
84
10.29
60.6
31.6
18.2
14.1
8.7
1.68
4.71*
0.29
57.7
47.7
43.6
39.2
53
51.4
5.6
2.8
The limiting damage mechanism in these waveguides manifests as ablation of the
reflective coating on the outside of bends. Penetration of light into a metal is small, around
1/20th of a wavelength so the predominant energy transfer mechanism is thermal [11]. The
heat wave penetration depths for silver and aluminium produced by an 8 ns laser pulse are
1.05 m and 0.803 m respectively based on reference [12]. The 532 nm waveguides tested
here had a coating thickness in the region of 100-120 nm (measured using transmission of
visible light), which is substantially less than the heat wave penetration depth. In this regime
the damage threshold of the coating can be expected to vary linearly with coating thickness
[11]. Comparisons of the damage threshold of waveguides at 532 nm of 0.54 mm diameter
with the standard coating (14.1 mJ delivered) and smooth coating (8.7 mJ delivered) confirm
this; to achieve a smoother coating the metal had to be deposited in a thinner layer of
approximately 40-70m, about half the thickness of the standard coating. This also helps to
explain the higher damage threshold of the 1064 nm waveguide (60.6 mJ delivered)
compared to the 532 nm waveguide of the same diameter (31.6 mJ delivered). For guiding
at a shorter wavelength the impact of surface roughness is more significant so the surface of
the 532 nm guide was made smoother [1], again at the expense of surface thickness.
However in this case the higher photon energy at 532 nm and the slightly lower reflectance
of silver at that wavelength may have an additional impact.
Comparing results for waveguides of different diameter, but with the same coating type
and operating wavelength, it is found that waveguides of smaller diameter damage at lower
energies. This is expected as the area of the reflective surface scales with diameter, so for a
given pulse energy the energy density at the surface will be greater in a smaller guide. In
addition high order modes are attenuated (absorbed) more strongly in small diameter
waveguides so overall losses tend to be greater.
The UV waveguides exhibit high bend losses so it is not surprising that damage occurs to
the 0.54 m diameter example at a low delivered pulse energy. The 1 mm diameter
waveguide proved very robust, being able to support the maximum pulse energy available
from the laser (84 mJ) without showing signs of damage, although with considerable bend
losses. Some damage to the gas cell window was observed after a number of pulses at the
highest energy illustrating the difficulties of delivering Q-switched pulses at UV wavelengths.
4. Dependence on number of guided modes.
The primary damage mechanism in these waveguides is believed to be a result of bendinduced mode coupling. If this is true then the damage threshold should show a dependence
on the number of guided modes. This was tested experimentally using different focal length
lenses to couple light into a waveguide. For a given beam diameter a short focal length lens
will focus light with a greater NA and therefore couple light into a greater number of modes.
Figure 2 shows three images of the output from a waveguide taken using a single lens to
image the core onto a CCD camera. Profiles are shown for input lenses of 88 mm, 125 mm
and 250 mm focal length. The speckled interference pattern gives an indication of the
number of modes supported. The pattern becomes progressively less coarse and of a
higher spatial frequency as the NA is increased indicating that more modes are supported.
A 540 m waveguide with a smooth coating was used for these experiments. Initially
transmission measurements were made for each lens through a straight waveguide (table 3).
As the input NA is increased the transmission efficiency of the waveguide reduces. This is
consistent with a large NA exciting a greater number of lossy high order modes. Low order
modes excited by a small NA are guided most efficiently.
For damage tests the number of waveguides available was limited. A single waveguide
was cut into sections of approximately 15cm held with a bend that was consistent throughout
the experiments. As damage occurs initially at the start of the first bend this should not affect
results. Here the polarisation state of incident light was parallel to the plane of the bend.
Damage threshold results are shown in table 3. In all cases damage was observed as
ablation of the reflective coating on the outside of the bend. The damage threshold appears
to increase with increasing NA. Although launching with a low NA lens leads to more
efficient guiding of light in fewer modes it also leads to a lower damage threshold.
Increasing NA
2. Hollow waveguide output for input lens of focal length: (a) 88mm, (b) 125mm, (c) 250mm.
3. Transmission efficiency through 540m waveguide for different input lens.
Lens, mm
Input NA
Efficiency (straight) %
E in, mJ
E out, mJ
88
0.045
59.4
11.5
5.88
125
0.032
72.4
8.81
4.11
250
0.016
90.9
4.18
2.86
It is thought that damage occurs at bends as light couples from low to higher order modes.
When a small number of low order modes propagate bend-induced coupling causes a large
amount of energy to couple between relatively few modes. For a greater number of
propagating modes the energy exchange in mode coupling is spread out giving a lower
average energy exchange between modes and reducing the occurrence of damage.
5. Dependence on Polarisation Alignment
Theory suggests that the polarisation alignment to bends in the waveguide has an impact on
transmission efficiency [8,9]. To assess the effect on damage thresholds tests were
conducted on a 0.54mm diameter waveguide at 532nm for two input polarisation states
(varied using a half-waveplate) aligned parallel and perpendicular to the plane of the bend.
Results are shown in table 4.
The effect of input beam polarisation on both bend loss and damage threshold is
significant. For a polarisation alignment parallel to the plane of the bend the bend loss is
greater and the damage threshold is much lower than for the case with a perpendicular
polarisation state. Theory [8,9] suggests that polarisation alignment influences mode
coupling at bends. Light polarised parallel to the bend couples into a set of modes with
higher losses. It is also useful to consider how polarised light is reflected from metallic
surfaces: in general light that is polarised parallel to the plane of incidence is reflected less
efficiently or rather it is absorbed more strongly. In a waveguide if light is polarised parallel to
the plane of the bend then light inside the waveguide will be incident on the outer surface of
the bend such that its polarisation state is parallel to the plane of incidence. This will result in
a greater absorption on the outer surface of the bend (where damage tends to occur first)
than for the case where light is polarised perpendicular to the plane of the bend and
therefore perpendicular to the plane of incidence with the outer surface.
4. Damage threshold for different polarisation alignment.
Polarisation alignment to bend
Parallel
Perpendicular
E in, mJ
5.68
12.14
E out, mJ
2.2
7.82
Efficiency, %
38.7
64.4
6. Conclusions
The work presented here demonstrates that the use of hollow waveguides is limited primarily
by bend losses and bend-induced damage to the reflective surface. Damage appears to
arise from the bend-induced coupling of light from low order to higher order modes and is
dependent on the number of modes initially propagating. A small number of guided modes
corresponds to an increased likelihood of damage. This is also the situation where both
losses and delivered beam quality are best so a compromise must be made. Transmission
efficiency and bend losses may be improved by improving the smoothness of the reflective
coating. However this is currently achieved at the expense of making the coating thinner
which leads to a reduction in damage threshold. Significant improvements in damage
threshold may be achieved by ensuring the polarisation state is aligned perpendicular to the
plane of any bend. For bends in multiple planes a circularly polarised input beam is best.
References
[1] Y. Matsuura, et al. Appl. Opt. 41(3), 442-5 (2002)
[2] S. Sato, et al. Opt. Lett. 25(1), 49-51 (2000)
[3] A.P. Yalin,et al. Opt. lett. 30(16), 2083-2085 (2005)
[4] T.J. Stephens, et al. Meas. Sci. Technol. 16 (5), 1119-1125 (2005)
[5] M.C. Jermy, et al. 12th Intl. Symposium on Appl. of Laser Techniques to Fluid
Mechanics: Lisbon 2004.
[6] J.P. Feist, et al. In Optical Methods for Data Processing in Heat and Fluid Flow, CC
Greated, J. Buick, J.M. eds., London: IMechE. (2002)
[7] International Standard Organisation EN ISO 11146:2000 (and EN ISO 11146:1999)
(2000)
[8] E.A.J. Marcatili and R.A. Schmeltzer. AT&T Tech. J. 43, 1783-1809 (1964)
[9] M. Miyagi and S. Kawakami. J. Lightwave technol. LT-2, 2, 116-126 (1984)
[10] O.B. Danilov, et al. J. Opt. Soc. Am. B, 7(9), 1785-1790 (1990)
[11] E. Matthias, et al. Thin Solid Films, 254, 139-146 (1995)
[12] A.M. Prokhorov, et al. In Laser heating of metals. IOP Publishing Ltd, pp. 42-43 (1990)