Evaluating damage in optical elements using an amplified spontaneous emission beam

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_High Power Laser Science and Engineering_, (2015), Vol. 3, e30, 6 pages.
© The Author(s) 2015. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2015.31

# Evaluating damage in optical elements using an amplified spontaneous emission beam

Qiong Zhou 1 _,_ 2, Jiangfeng Wang 1, Yajing Guo 1, Dean Liu 1, and Jianqiang Zhu 1

1 _National_ _Laboratory_ _on_ _High_ _Power_ _Laser_ _and_ _Physics,_ _Shanghai_ _Institute_ _of_ _Optics_ _and_ _Fine_ _Mechanics,_ _Chinese_ _Academy_ _of_
_Sciences, Shanghai 201800, China_

2 _University of Chinese Academy of Science, Beijing 100049, China_

_(Received 11 September 2015; revised 21 October 2015; accepted 2 November 2015)_

Abstract
A method to evaluate damage in optical elements with the near field of an amplified spontaneous emission (ASE) beam
has been developed. Local peak intensities are generally distributed randomly in the near field of a laser beam. The
partial coherence of the ASE source results in a very smooth beam profile. The coherence time of ASE is much less
than the pulse width. Small-scale intensity modulations can be smoothed out rapidly within the time of a pulse width.
In the experiments, ASE is generated from a multifunctional high-performance Nd:glass system, with a pulse duration
of 3 ns, a spectral width (full width at half maximum, FWHM) of 1 nm and an adjustable energy range from 1 to 10 J.
The damage thresholds of samples induced by ASE are two to three times higher than those induced by a laser with the
same size of test spot. Furthermore, the ASE beam has great potential for the detection of defects over a large area and
the conditioning of optical elements.

Keywords: amplified spontaneous emission; damage threshold test; partially coherence; uniform intensity distribution

1. Introduction

Accurately predicting the lifetime of optics in high-power fusion lasers is very important in managing laser facilities [1–3] .
The defects randomly distributed in optical elements and
the light spot which irradiates the sample surface together
determine the damage threshold [4] . As is well known, a
large-area spot and a smooth intensity distribution can reveal
more defects, which are the major factors for an accurate
damage measurement. The far field of the test spot is
usually used because of its smooth Airy pattern, in which
a majority of the energy is concentrated in the main lobe.
But the beam size is very small. When a 1-to-1 test is
used, there is a clear dependence on the beam size, and
not all the defects are revealed [5] . If a raster-scan test is
used, the beam shape - Gaussian or flat top - needs to
be considered, because of beam overlap. Also scanning
times are very long with small size beams [5] . Hence, a
test beam in the near field would seem to be a better
choice, because a large size spot can be acquired. A laser
beam is usually used as the irradiation source in the laserinduced damage threshold (LIDT) test. But, during laser
propagation through the laser system, the near-field intensity

Correspondence to: J. Zhu, No. 390, Qinghe Rd., Jiading, Shanghai
201800, China. Email: jqzhu@siom.ac.cn

distribution is strongly influenced by random modulation
of both the phase and amplitude. Local peaks of intensity
result, randomly distributed in the near field. Combined
with the defects of the sample, nonuniformity of the laser
source makes the damage distribution more complicated to
determine. If the intensity distribution on the sample surface
was uniform, then damage would be most likely to occur
in the areas where defects in the samples were located.
Thus, the damage distribution would correspond to that of
the defects. Compared to a laser source, a light source
with a uniform intensity distribution should be more suitable
to test damage thresholds and derive directly the damage
characteristics induced by defects in samples.

Amplified spontaneous emission (ASE) is one typical kind
of partially coherent light (PCL) [6, 7] . The coherence time
of ASE is far shorter than the pulse duration due to temporal incoherence. There is less spatial modulation during
ASE beam propagation to the sample surface, leading to a
reduction in intensity fluctuations in the irradiated spot on
the sample. Therefore, ASE could be a preferable irradiation
source in the damage threshold test. Furthermore, if the
output energy of ASE from the experimental installation
were sufficiently high, a test spot with a large size could be
acquired and the test time could also be shortened.

In this letter, a method using an ASE-induced damage
threshold test is proposed and validated. The ASE beam is

2 _Q. Zhou et al._

Figure 1. Multifunctional high-performance Nd:glass system based on a four-pass amplifier.

Figure 2. Schematic of a damage threshold testing facility using the ASE beam and the laser beam respectively.

generated from a multifunctional high-performance Nd:glass
system based on a four-pass amplifier (Figure 1) with a
certain spectral width and a smooth intensity distribution.
The energy of the ASE can reach 10 J or more. The intensity
distribution of the ASE and the laser are compared. Three
kinds of samples are tested with this method, including TiO2
high-reflection optical film, fused silica glass and K9 glass.
Their ASE-induced damage threshold (AIDT) is clearly two
to three times higher than when tested using a laser beam
with the same size of test spot, leading to an accurate
evaluation of the damage threshold.

2. Experiments

_2.1._ _Experimental setup and method_

The AIDT testing facility was built as shown in Figure 2.
The broadband fluorescent source was coupled into multistage amplifier chains consisting of an Nd:glass regenerative
amplifier, a beam shaping system and a four-pass amplifier.
The spectral width of the ASE light was 4 nm (full width
at half maximum, FWHM) because of gain narrowing in the
Nd:glass rod. On passing through a narrow band filter, the

final output spectral width was 1 nm (FWHM), as shown
in Figure 3. An output energy of 10 J with a beam size of
18×18 mm could be obtained, while the energy injected into
the four-pass amplifier was about 1 mJ. An optical shutter
shaped the ASE light with an opening time of 3 ns (Figure 4).
In the measurement setup, the ASE light was focused on the
surface of the samples by a focusing lens with a diameter of
50 mm and a focal length of 1580 mm. Samples, which were
fixed on a three-dimensional translation stage, included TiO2
high-reflection film, fused silica glass and K9 glass. The
optical distance between the sample and the focusing lens
was fixed, leading to a square spot with a size of 0 _._ 914 ×
0 _._ 914 (mm). One shot was delivered every 15 min, with an
output energy level that could be varied from 1 J to more than
10 J by adjusting the power supply voltage. Therefore, the
total fluence irradiated on the sample surface could be varied
during the experiment. The spatial profile was recorded by
a camera-based spot profiler and the temporal profile was
recorded by a photocathode coupled to an oscilloscope.

According to the standard ISO-11254 for the determination of damage thresholds of optical surfaces, 1-on-1
measurement was adopted. One shot of ASE was radiated
on each testing site of the sample surface. Then the light

_Evaluating damage in optical elements using an amplified spontaneous emission beam_ 3

Table 1. Comparison of damage test results in samples.

Sample LIDT (J _/_ cm 2 ) AIDT (J _/_ cm 2 )

TiO2 4 12.5
SiO2 14 30.5
K9 10.5 27.5

Figure 3. Spectrum profile of the ASE beam.

Figure 4. Temporal profile of the ASE beam.

spot was moved to the next testing site, no matter whether
the previous site had been damaged or not. The energy
density was gradually increased within the damage energy
density range, and the damage morphologies at different
energy densities were detected.

In order to assess any improvements in using the AIDT
test method, a testing facility using LIDT was also built, as
shown in Figure 2. The laser beam outputted from the main
oscillator had a wavelength of 1053 nm, a pulse duration of
3 ns and a spectral width (FWHM) of 10 −3 nm.

_2.2._ _Experimental results_

The output ASE and laser beam from this four-pass amplifier
have flat-top profiles as shown. Because of the complete
coherence of the laser light, interference, diffraction and
nonlinear effects arise, which cause small-scale intensity

perturbations. Thus, coherent fringes (marked by red box)
and diffraction rings with a high contrast ratio are clearly
observed in the intensity distribution of laser spot shown in
Figures 5(b) and 5(h). The near-field fill factor of the laser
beam is 52% and the modulation is 1.9. However, the ASE
is PCL with a larger beam divergence angle and a broader
spectral width of 4 nm (FWHM) after gain narrowing. The
coherence time of the ASE is 3 ps, which is much less than
the pulse width of 3 ns. Therefore, small-scale intensity
modulations can be smoothed out rapidly within the time of a
pulse width. There are no distinguishable coherence fringes
observed in the intensity distribution of the ASE in Figures
5(a) and 5(g). In addition, diffraction patterns are found in
the intensity distributions both of the ASE and laser spots,
caused by the finite aperture of the optics. But, according to
the fringe contrast of these diffraction patterns, the variations
of the laser intensity distribution are apparently smaller in
scale spatially than those of the ASE intensity distribution.
This is due to PCL smoothing out the modulation to a certain
extent. The near-field fill factor of the ASE is 60% and the
modulation is 1.6.

The damage threshold is determined by the ratio of the
damage area to the light spot area. Based on statistical
analysis of the test results, the energy density which results in
a ratio of zero is taken as the damage threshold of a sample.
A curve relating the damage ratio to the energy density is
nonlinearly fitted. The damage threshold of three kinds of
samples measured using the ASE beam and the laser beam
are shown in Figure 6 and Table 1.

The experimental results show that the AIDT is two to
three times higher than the LIDT. The two main factors
leading to this are the random defect distribution of the
sample and random nonuniformities in the near field. During
laser propagation, phase front distortion of the near field is
caused by various sources of noise modulation, leading to
large random intensity nonuniformities in the near field. For
a fixed defect density in the samples, damage may occur
first in the defect area due to nonuniformity of the test
light spot. Thus, the damage threshold tested by a laser
beam is much lower than the typical damage threshold of
the samples. Therefore, under the joint action of defects
and beam nonuniformity, which are randomly distributed,
inaccuracy occurs in the evaluation of the laser damage
resistance of the optical elements. However, nonuniformity
of the intensity distribution can be suppressed by using PCL,
such as ASE. The coherence time of ASE is far less than
the pulse duration, due to spatial and temporal incoherence.

4 _Q. Zhou et al._

(a) (b)

(e) (f)

(g) (h)

Figure 5. Two-dimensional intensity distribution of the ASE spot (a) and the laser spot (b); one-dimensional intensity distribution of the ASE spot (c), (e);
and the laser spot (d), (f); enlarged view of the ASE spot (g) and the laser spot (h).

_Evaluating damage in optical elements using an amplified spontaneous emission beam_ 5

Figure 6. Damage threshold nonlinearly fitted using ASE and a laser as the test source, respectively.

6 _Q. Zhou et al._

Hence, the diffraction patterns of each frequency component
overlap and the nonuniformity is quickly smoothed out to a
certain extent during the duration of pulse. Thus, there is
less noise modulation in the near field of the ASE. Using
ASE as the damage threshold test source, nonuniformity of
the irradiation intensity caused by interference, diffraction
and other nonlinear coherent effects can be neglected and
the defects of elements can be studied in isolation. Thus, the
AIDT is generally higher than the LIDT. In our experiment,
the size of the test spot may not be large enough, due to
small size samples. For large size optical components, we
can test with large size spots of ASE. More defects can thus
be revealed, leading to an accurate evaluation of the damage
threshold.

3. Conclusions

Compared with the intensity distribution of a laser, the near
field of ASE is very smooth, with less spatial modulation in
the near field, and fewer hot spots in the far field. Thus, the

effect of beam nonuniformity can be ignored in carrying out
the damage threshold test. The experimental results show
that the AIDT is clearly higher than the LIDT, leading to a
more precise evaluation of the damage threshold.

References

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(1996).
3. J. Hue, F. Y. Genin, S. M. Maricle, and M. R. Kozlowski, Proc.
SPIE 2966, 451 (1997).
4. A. F. Stewart and A. H. Guenther, Appl. Opt. 23, 3741 (1984).
5. L. Lamaign`ere, Laser-induced damage density measurements
of optical materials, in _Advanced_ _Topics_ _in_ _Measurements_
(InTech, 2012).
6. H. Nakano, T. Kanabe, K. Yagi, K. Tsubakimoto, M.
Nakatsuka, and S. Nakai, Opt. Commun. 78, 123 (1990).
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