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Corneal damage e®ects induced by infrared optical
parametric oscillator radiation at 3743 nm
Luguang Jiao
*
, Chao Wang, Kaizeng Zhang,
Jiarui Wang and Zaifu Yang
†
Beijing Institute of Radiation Medicine
Beijing 100850, P. R. China
*
jiaoluguang001@163.com
†
yangzf@bmi.ac.cn
Received 30 August 2020
Accepted 23 November 2020
Published 21 December 2020
The main aim of this paper is to investigate the corneal damage e®ects induced by mid-infrared
optical parametric oscillator (OPO) radiation. Experiments were performed to determine the
corneal damage thresholds of New Zealand white rabbit at the wavelength of 3743 nm for ex-
posure durations of 0.1 s, 1.0 s and 10.0 s. Through slit-lamp biomicroscope and histopathology,
corneal injury characteristics were revealed. The damage thresholds were 3.73 J/cm
2
, 7.91 J/cm
2
and 31.1 J/cm
2
, respectively, for exposure durations of 0.1 s, 1.0 s and 10.0 s. The damage data
was correlated by an empirical equation: Radiant exposure at the threshold ¼9:72 exposure
duration,0:46 where the units of radiant exposure and exposure duration were J/cm
2
and second.
At near-threshold level, corneal injuries at 1 h post-exposure mainly involved the epithelium, and
the epithelium damages repaired at 24-h post-exposure. There are su±cient safety margins be-
tween the damage thresholds and the maximum permitted exposures from current international
laser safety standard IEC 60825-1.
Keywords: Corneal damage threshold; optical parametric oscillator radiation; laser safety
standard.
1. Introduction
Infrared lasers in the wavelength range of 3–5m
have been increasingly applied in diverse ¯elds such
as environmental monitoring,
1
precise spectrum
analysis
2
and infrared countermeasures.
3
Optical
parametric oscillator (OPO) technology is receiving
much attention in recent years because it is a
practical approach to generate lasers operating in
this wavelength range, comparing to chemical deu-
terium °uoride lasers developed in 1970s.
4–9
With
the breakthrough on nonlinear crystals and ¯ber
lasers, technology in ¯ber laser pumped OPO de-
veloped rapidly and the output power increased
*,†
Corresponding authors.
This is an Open Access article. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC-BY) License. Further
distribution of this work is permitted, provided the original work is properly cited.
OPEN ACCESS
Journal of Innovative Optical Health Sciences
2150004 (9 pages)
#
.
cThe Author(s)
DOI: 10.1142/S1793545821500048
2150004-1
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continually.
10–14
In international public reports, the
maximum power of continual-wave infrared OPO
laser has been increased above 30 W, with the
wavelength tuning range of 3.2–3.9 m.
15
Because of the importance of vision and in-
creasing applications of OPO sources, attention has
to be paid to the potential ocular injuries induced
by OPO sources. In the wavelength range above
1.4 m, ocular damages mainly occur at the cornea
because radiation is signi¯cantly absorbed in the
cornea and aqueous humor.
16
Vision can be de-
creased signi¯cantly and permanently due to severe
corneal injuries.
17
For the protection of cornea, a
large amount of studies have been conducted to
determine the corneal injury thresholds induced by
CO
2
laser at the wavelength of 10.6 m,
18–31
thuli-
um laser at 2.0 m
32–34
and erbium laser at
1.54 m.
35–41
Corneal damage data from these
studies promoted the developments of laser safety
guidelines and standards.
16,42,43
However, research
on the damage e®ects induced by infrared OPO
radiation in the wavelength range of 3–5mis
lacking, thus it is necessary to perform experimental
research on the corneal injury e®ects induced by
mid-infrared OPO sources and examine whether the
maximum permissible exposures (MPEs) speci¯ed
in the current international safety standards
are appropriate for evaluating the hazard of mid-
infrared OPO sources. With above considerations, we
performed experiments to determine the rabbit in-
vivo corneal damage thresholds induced by an OPO
source operating at 3743nm for exposure durations of
0.1 s, 1.0 s and 10.0 s, revealed the corneal injury
characteristics through slit-lamp microscope and his-
topathology, and compared the determined damage
values with corresponding MPEs in the laser safety
standards.
42,43
The obtained results may provide
references for the re¯nement of laser safety standard
and the clinical treatments ofaccidental laser-induced
corneal damages.
2. Materials and Methods
2.1. Experimental set-up for corneal
exposures by OPO source
The experimental set-up is shown in Fig. 1. The
¯ber laser pumped MgO: PPLN OPO was provided
by National University of Defense Technology,
Changsha, China. The output of the OPO source
included three kinds of laser radiation, with the
wavelengths of 1070 nm (pump laser), 1498 nm
(signal laser) and 3743 nm (idler laser). Two mir-
rors, M1 and M2, were employed to separate
the idler laser with the pump and signal lasers.
The mirrors had a high re°ectivity (R>99%)in
the wavelength ranges of 1.0–1.1 m and 1.4–
1.7 m, and a high transmittance (T>95%) in the
wavelength range of 2.5–4.1 m with the incidence
angle of 45. A laser spectrum analyzer (721B,
Bristol Instruments Inc., NY, America) was used to
measure the spectral component of the radiation
after the mirror M2. As shown in Fig. 2, only a
single line existed with the central wavelength of
3743 nm and FWHM of 8 nm. The maximum power
of the idler laser was about 8.3 W, and the relative
power °uctuation was within 5:0%. A ¯xed por-
tion of the idler laser was re°ected to the laser
power meter 1# (3A, Ophir, Israel) by a GaF
2
beam
splitter with the wedge angle of 8. Thus, the sta-
bility of the laser power during laser exposures
could be monitored. Another laser power meter 2#
(30A, Ophir, Jerusalem, Israel) was positioned to
measure the power of the laser incident on the
rabbit cornea. Through the adjustments of the
driving current of the OPO source and adding
attenuators including GaF
2
plates and K9 plates,
Fig. 1. Schematic drawing of the exposure setup for the de-
termination of rabbit corneal damage thresholds induced by an
infrared OPO (OPO: Fiber laser pumped MgO: PPLN OPO.
M1: Mirror having a high re°ectivity (R>99%) in the wave-
length ranges of 1.0–1.1 m and 1.4–1.7 m and simultaneously
having a high transmittance (T>95%) in the wavelength
range of 2.5–4.1 m. M2: Mirror having same coatings with the
mirror M1. BD1: Beam dump for collecting the pump and
signal lasers. BD2: Beam dump for collecting the residual pump
and signal lasers. W: GaF
2
beam splitter with the wedge angle
of 8. BD3: Beam dump for collecting the idler laser radiation
re°ected from the ¯rst surface of the wedge beam splitter. PM1:
Laser power meter 1#. L: GaF
2
plane-convex lens with the
focal length of 500 mm. S: Electronically-controlled mechanical
shutter. M3: GaF
2
plate. LP1: Low-power 655 nm laser pointer
1#. AS: Attenuators including GaF
2
plates and K9 plates. LP2:
Low-power 655 nm laser pointer 2#. PM2: Laser power
meter 2#).
L. Jiao et al.
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the laser power incident on the animal cornea could
be changed freely. The exposure duration was con-
trolled by an electronically-controlled mechanical
shutter. The selected exposure durations were 0.1 s,
1.0 s and 10.0 s. Two low-power 655-nm laser poin-
ters, crossed at the center of the laser spot, facili-
tated the targeting of the invisible infrared laser
radiation. A GaF
2
plane-convex lens, with focal
length of 500 mm, was employed to change the spot
size on the animal cornea. The distance between the
lens and the rabbit cornea surface was kept con-
stant as 28.3 cm. At the corneal surface, the laser
irradiance was nearly Gaussian-distributed. Using
the knife-edge method,
44
the 1/e beam diameters in
the horizontal and vertical directions were deter-
mined as about 1.61 mm and 1.50 mm, respectively.
2.2. Animal subjects
New Zealand white rabbits were selected. The total
number was 13 with weight of 2.5–3.2 kg. The pro-
tocols and handling of the animals had been ap-
proved by the ethics review board of Academy of
Military Medical Science, Beijing, China. All ani-
mals were procured and maintained in the Center
for Laboratory Animal Medicine and Care, Acade-
my of Military Medical Sciences, Beijing, China and
used in accordance with the institutional guidelines
of the Animal Care and Use Committee; and the
ARVO Resolution on the Use of Animals in Re-
search. A slit-lamp microscope (Topcon, Tokyo,
Japan) and a fundus camera (Topcon, Tokyo,
Japan) were employed to examine the animal eyes.
Only the subjects with clear refractive media and
healthy fundus were included. Before laser expo-
sures, subjects were anesthetized with an intra-
muscular injection of a mixture of ketamine
hydrochloride (20 mg/kg) and xylazine (5 mg/kg).
Full pupil dilation was performed with two drops of
proparacaine hydrochloride 0.5%, phenylephrine
hydrochloride 2.5% and tropicamide 1% at a 5-min
interval, which facilitated the following observations
of corneal injuries. The anesthetized subjects were
positioned with the aid of the two laser pointers.
Corneal drying was prevented by periodic applica-
tions of physiological saline solution at room tem-
perature and by manual blinking of the lids. Irrigation
was stopped about 30 s prior to laser exposures and
the excess °uid was blotted at the limbus.
2.3. Damage determination and
experimental procedures
The criterion for the determination of minimal ep-
ithelial damage is the presence of a super¯cial,
barely visible, gray-white spot that develops within
1 h after exposure.
39
Corneas were assessed with a
slit-lamp microscopy (Topcon, Tokyo, Japan). We
re¯tted the slit-lamp microscopy by adding an
eyepiece adaptor and a Huawei P20 cell phone, thus
the corneal damage images could be captured.
Two illumination methods, including broad-beam
di®use illumination and slit-beam illumination,
were employed to observe the corneal lesions.
In the experiments, we found the damage
threshold was well de¯ned. The overlap between
exposures that produced minimal lesions and those
that did not was rare. Thus, the probit analysis was
not employed to determine the damage threshold,
as the statistical procedures would require using
more animals than necessary.
39
Speci¯cally, we ir-
radiated the rabbit cornea with step-changed inci-
dent power levels. The bracket between the power
level (PH) producing a minimal lesion and that (PL)
producing no damage was narrowed until only
about a 10% di®erence existed between the PHand
PL. The damage threshold Pth then was 1deter-
mined as ðPHþPLÞ=2. As an example, Table 1
shows the damage results for the exposure duration
of 1.0 s. The peak radiant exposure Hshown in the
Fig. 2. The spectrum after M2, showing that the radiation
incident on the rabbit cornea contained a single line with the
central wavelength of 3743 nm.
Corneal damage e®ects induced by infrared OPO radiation
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table was de¯ned by
H¼4Pt=ðabÞ;ð1Þ
where Pwas the laser power incident at the corneal
surface aand bwere the 1/e spot diameters in the
horizontal and vertical directions, respectively, and
twas the exposure duration. It was found that at
the power level of 0.158 W the exposure sites were
slightly damaged and no damage could be found at
the power level of 0.141 W, thus the damage
threshold was estimated at 0.150 W. The injury
thresholds for exposure durations of 0.1 s and 10.0 s
could be determined using the same method. By
employing the determined spot sizes, the damage
threshold Hthreshold was determined by
Hthreshold ¼4Pthresholdt=ðabÞ;ð2Þ
where Pthreshold was the laser power at the damage
threshold.
Additionally, we performed histopathologic stud-
ies. Some rabbits were euthanized at 6 h and 24 h post-
exposure. After the euthanasia, eyeballs were taken
and ¯xed in Davidson solution for 30 min and then
corneas were cut o® and ¯xed in Davidson solution for
2.5 h. The next steps were to dehydrate the corneas
with ethanol, embedded with para±n, serially sec-
tioned and the sections stained with hematoxylin and
eosin (H&E). The histological images were captured
by a microscope (BX43F, Olympus, Japan).
3. Results
Table 2shows the determined damage thresholds
for the exposure durations of 0.1 s, 1.0 s and 10.0 s.
The MPEs in the IEC-60825 standard
43
and safety
factors of Hthreshold/MPE were also included.
Figure 3shows the rabbit corneal lesions at 1-h
post-exposure for the exposure duration of 1.0 s
and incident power of 0.158 W (1.05 times the
damage threshold). At the slightly-above threshold
level, barely visible gray-white lesions could be ob-
Table 1. The damage results induced by infrared 3743 nm laser at 1 h post exposure for the exposure duration of 1.0 s.
Laser power incident
at the corneal surface P(W)
Peak radiant
exposure H(J/cm2)H=Hthreshold
Involved
eye number
Number of damage
lesions/Number of exposures
0.296 15.6 1.97 2 2/2
0.221 11.7 1.47 1 3/3
0.193 10.2 1.29 1 6/6
0.172 9.1 1.15 1 6/6
0.158 8.3 1.05 1 5/6
0.141 7.4 0.94 1 0/6
0.124 6.5 0.83 1 0/6
Table 2. The damage thresholds and MPEs for exposure durations of 0.1 s, 1.0 s and 10.0 s.
Exposure
duration (s)
Damage threshold expressed
in power incident on the
cornea Pthreshold (W)
Damage threshold expressed
in peak radiant exposure
Hthreshold (J/cm2) MPE (J/cm2)
Safety factor
of Hthreshold/MPE
0.1 0.708 3.73 0.31 12.0
1.0 0.150 7.91 0.56 14.1
10.0 0.059 31.1 1.00 31.1
Fig. 3. Corneal damages with the exposure duration of 1.0 s
and the incident power of 0.158 W (1.05 times the damage
threshold). The arrows in the images indicated the lesions. (a)
Lesions at 1-h post-exposure with broad-beam di®use illumi-
nation and (b) Lesions at 1-h post-exposure with slit-beam
illumination.
L. Jiao et al.
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served for the exposure sites under slit-lamp mi-
croscopy with di®use illumination (Fig. 3(a)). With
slit-beam illumination, super¯cial re°ective white
straps could be seen, indicating that the threshold
damage mainly involves the epithelium layer
(Fig. 3(b)). The meaning of \white strap" is
explained as follows. The normal cornea is trans-
parent to incident light, but if damaged, the inci-
dent light would be signi¯cantly scattered by the
damaged tissue which looks like \white strap"
under slit-lamp microscopy. At 24-h post-exposure,
these lesions could not be found, indicating that the
damaged epithelium repaired. At 1.97 times of
damage threshold, apparent and opaque lesion on
corneal surface with circular symmetry could be
found under broad-beam di®use illumination. Sur-
face distortion could also be found for the lesion and
the lesion edge was distinct from surrounding nor-
mal tissue (Fig. 4(a)). With slit-beam illumination,
highly re°ective white strap with a thickness less
than the cornea thickness was observed, showing
that the damage involves the epithelium and part of
the stroma (Fig. 4(b)). At 24-h post-exposure, the
lesion became blurred and the edge was no longer
distinct from surrounding tissue (Fig. 4(d)). Histo-
logical section at 6-h post-exposure showed that the
epithelium layer disappeared and the number of
cell nuclei in the partial stroma decreased obviously
due to laser irradiation (Fig. 4(c)). At 24-h post-
exposure, the epithelium repair could be found
(Fig. 4(f)).
4. Discussion
Previous corneal damage studies employed rabbit
and rhesus monkey as the animal model. It is found
that no signi¯cant di®erence for damage threshold
values exists between the rabbit and the rhesus.
45
Considering the current trends toward the reduc-
tion, re¯nement and replacement philosophy in
animal research, use of nonhuman primates should
be avoided and the rabbit model was selected to
investigate the corneal damage e®ects induced by
mid-infrared OPO radiation.
41
For infrared laser
radiation, the corneal injury is governed by thermal
mechanism above about 50 s.
29,39
In the thermal-
mechanism regime, the damage threshold variation
with exposure duration could be correlated by a
Fig. 4. Corneal damages with the exposure duration of 1.0 s and the incident power of 0.296 W (1.97 times the damage threshold).
(a) Lesion at 1-h post-exposure with broad-beam di®use illumination. (b) Lesion at 1-h post-exposure with slit-beam illumination.
(c) Histological section at 6-h post-exposure. (d) Lesion at 24-h post-exposure with broad-beam di®use illumination. (e) Lesion at
24-h post-exposure with slit-beam illumination and (f) Histological section at 24-h post-exposure.
Corneal damage e®ects induced by infrared OPO radiation
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power function law:
Hthreshold ¼Atk;ð3Þ
where Aand kwere ¯tted constant values.
For the wavelengths of 1.54 m, 2.02 m and
10.6 m,
29,32,34,39
the values of kwere in the range of
0.53–0.72 for exposure durations of about 0.1 s to
10 s, as shown in Fig. 5. For OPO radiation at
3743 nm, we selected the exposure durations of 0.1 s,
1.0 s and 10.0 s to reveal the dependence. The
damage threshold for exposure duration less than
0.1 s could not be determined due to the limitation
of the mechanical shutter. Spot size is another sig-
ni¯cant in°uence factor for the determination of
damage data.
46,47
As shown in Fig. 5, the damage
thresholds for larger beam diameters are consis-
tently lower than the data for smaller beam spots
for speci¯c laser wavelength. Generally, corneal spot
diameters of larger than about 1 mm are usually
selected. With relatively large beam diameters, the
damage thresholds can be determined under con-
ditions near one-dimensional (1D) heat transfer in
cornea. Thus damage thresholds approach the
lowest values with the increase of corneal beam spot
size. Considering above analysis, the 1/e corneal
spot size of about 1.5 mm was selected in our
experiments. Figure 6shows the corneal damage
thresholds and MPE values at the wavelength of
3743 nm. The damage values follow the empirical
power-law function
Hthreshold ¼9:72t0:46;ð4Þ
where the units of Hthreshold and tare J/cm
2
and
second. Obviously, su±cient safety margins exist
between the damage data and corresponding MPEs.
However, for a safety assessment of corneal expo-
sure for wavelength above 1400 nm, the IEC-60825
laser safety standard de¯nes an averaging aperture
where the diameter depends on exposure duration.
43
For exposure duration less than 0.35 s, the diameter
equals 1 mm and for exposure duration between
0.35 s and 10 s, the diameter increases with a 1.5t3=8
dependence. The main rationale for the increasing
averaging aperture for the determination of the
corneal exposure level is assumed eye movements
that result in a decrease of the e®ective relevant
exposure level. In our experiments, the beam di-
ameter for exposure duration of 10.0 s was less than
3.5 mm. Therefore, to compare the experimentally
determined damage threshold with the MPE value,
the exposure value was scaled to an e®ective radiant
exposure Heffective approximately by
Heffective ¼Hthreshold D2
L=D2
f;ð5Þ
where Hthreshold is the determined damage thresh-
olds in J/cm
2
,DLis the measured laser beam 1/e
diameter in cm and Dfis the averaging aperture.
48
The Heffective was calculated as 5.71 J/cm
2
for 10.0 s
Fig. 5. Previous corneal damage thresholds.
29,32,34,39
The
parameters behind the symbols denoted the laser wavelength
and the corneal 1/e spot diameter. The threshold data followed
an empirical power-laser relationship Hthreshold ¼Atk, where
Hthreshold and twas the radiant exposure at the threshold and
the exposure duration; Aand kwas ¯tted constant values.
Power-law ¯tting curves in the ¯gure showed that the values of
kranged from about 0.53–0.72.
Fig. 6. Corneal damage thresholds at the wavelength of
3743 nm and MPE curve for the wavelength range of 2600–
10
6
nm. The ¯tting curve followed the empirical power-law re-
lationship Hthreshold ¼9:72t0:46, where Hthreshold and twas the
radiant exposure at the threshold and the exposure duration.
L. Jiao et al.
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and the safety factor would be reduced from 31.1 to
5.71. It was worth noting that above assessment
was only qualitative because the e®ect of eye rela-
tive movements on radiant exposure is not well
de¯ned in the safety standard.
48
Additionally,
Dunsky et al. determined corneal damage thresh-
olds for hydrogen deuterium °uoride chemical lasers
in the Rhesus model.
49
At the wavelength of
3698 nm, the damage threshold was 4.61 J/cm
2
for
the exposure duration of 0.125 s. And at the wave-
length of 3731 nm, the damage value was 7.09 J/cm
2
for the exposure duration of 0.5 s. The two data
were also included in Fig. 6. No signi¯cant di®er-
ence exist between the damage data determined in
this report and previous data, which also shows that
rabbit is the most suitable animal model for corneal
damage research.
5. Conclusion
Corneal injuries induced by an OPO source at the
wavelength of 3743 nm were performed in the New
Zealand white rabbit model. The corneal damage
thresholds for exposure durations of 0.1 s, 1.0 s and
10.0 s were 3.73 J/cm
2
, 7.91 J/cm
2
and 31.1 J/cm
2
,
respectively. The damage values followed an em-
pirical power-law equation. At threshold level, cor-
neal damages at 1-h post-exposure mainly involved
the epithelium. At 24-h post-exposure, the epithe-
lium damages repaired. There were enough safety
margins between the damage thresholds and corre-
sponding MPEs, indicating that the MPEs in the
laser safety standard are su±cient to protect the
cornea at the wavelength of 3743 nm.
Con°ict of Interest
The authors declare no con°ict of interest.
Acknowledgment
This study was supported by the National Natural
Science Foundation of China (NSFC) (61575221).
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