The Effect of Moisture and Temperature on Optical 

Coatings Used in Eye-safer Lasers

by Jeffrey O. White, Aaron Z. Chan, and Carl E. Mungan

ARL-TN-0408 September 2010

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Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TN-0408 September 2010

The Effect of Moisture and Temperature on Optical 

Coatings Used in Eye-safer Lasers

Jeffrey O. White, Aaron Z. Chan, and Carl E. Mungan

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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4. TITLE AND SUBTITLE

The Effect of Moisture and Temperature on Optical Coatings Used in Eye-safer 

Lasers

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Jeffrey O. White, Aaron Z. Chan, and Carl E. Mungan

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U.S. Army Research Laboratory

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Erbium (Er):yttrium aluminum garnet (YAG) lasers are currently under development at the U.S. Army Research Laboratory 

(ARL) as eye-safer alternatives to neodymium (Nd):YAG for countering rockets, artillery, and mortar (CRAM). Anomalous 

increases in laser output have been observed during the 30–60 min period after turn-on. A hypothesis was that one or more of 

the three optical coatings (anti-reflection, dichroic, and partially reflecting) in the laser cavity were absorbing or desorbing 

moisture. On the basis of the baking-out measurements described here, we attribute the increase in laser power to moisture 

being driven out of the dichroic coating, increasing its reflectivity at the laser wavelength. We have also measured the effect of 

operating temperature on a dichroic coating that has already been baked-out.

15. SUBJECT TERMS

Moisture, optical coatings, lasers

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19a. NAME OF RESPONSIBLE PERSON

Jeffrey O. White

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(301) 394-0069

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iii

Contents

List of Figures iv

1. Introduction/Background 1

2. Experiment/Calculations 2

3. Results and Discussion 2

4. Conclusions 8

5. References 9

List of Symbols, Abbreviations, and Acronyms 10

Distribution List 11
iv

List of Figures

Figure 1. Diode-pumped solid-state rod laser.................................................................................1

Figure 2. (Left to right) Partial reflector, AR coating, and dichroic coating. .................................1

Figure 3. Transmission spectra for AR-coated sample after repeated heating at 100 °C. ..............3

Figure 4. Transmission of AR-coated sample vs. bake time. .........................................................3

Figure 5. Transmission spectra of PR-coated sample after heating at 100 °C................................4

Figure 6. Transmission of PR-coated sample vs. bake time...........................................................4

Figure 7. Transmission spectra of HR-coated sample after baking at 100 °C................................5

Figure 8. Transmission of HR-coated sample vs. bake time. .........................................................5

Figure 9. Temperature-controlled oven for use inside the spectrophotometer. .............................7

Figure 10. Transmission spectra of HR-coated sample for different temperatures. .......................7

List of Tables

Table 1. Index of refraction of the PR substrate, the witness samples, and the laser rod...............2
1

1. Introduction/Background

Erbium (Er):yttrium aluminum garnet (YAG) Er:YAG lasers are currently under development at 

the U.S. Army Research Laboratory (ARL) as eye-safer alternatives to neodymium (Nd):YAG 

for countering rockets, artillery, and mortar (CRAM). Anomalous increases in laser output have 

been observed during the 30–60 min period after turn-on. A hypothesis was that one or more of 

the optical coatings in the laser cavity were absorbing or desorbing moisture. 

The three basic coatings used in these lasers are (1) high reflecting (HR), (2) anti-reflecting

(AR), and (3) partially reflecting (PR) at λL = 1645 nm (figure 1). The partial reflector is used as 

the output coupler (OC). Longitudinal diode pumping at λp = 1532 nm implies a low-quantumdefect (7%), but necessitates a dichroic HR coating that has a high transmission at P and a high 

reflection at L. The HR coating can be either on the rod face or on a separate mirror substrate.

Figure 1. Diode-pumped solid-state rod laser.

All three are lossless dielectric multilayer coatings; therefore, we can assume that the reflection 

plus the transmission equal one R + T = 1 (figure 2). The PR is deposited on BK7 glass with the 

back side anti-reflection-coated for 1645 nm (1). The AR and HR coatings are deposited on 

witness samples of SF6 glass, which has an index of refraction close to that of YAG (table 1) (2, 

3). The back sides are polished, but not AR coated, so multiple reflections are present. 

Figure 2. (Left to right) Partial reflector, AR coating, and dichroic coating.
2

Table 1. Index of refraction of the PR substrate, the witness samples, and the laser rod.

BK7 SF6 YAG

1532 nm 1.50088 1.76441 1.80725

1645 nm 1.49944 1.76272 1.80565

2. Experiment/Calculations

The three samples were immersed in water overnight to furnish a starting point that could be 

reproduced easily. It is not known whether the moisture absorbed in this case is greater or less 

than the amount absorbed over a period of weeks or months at ambient relative humidity 

(~40%). The experimental sequence was to bake the samples, cool them to room temperature, 

measure the transmission, and repeat. We assume that the baking process was cumulative, i.e.,

changes in moisture content occurred only during the baking time, because the cool-down was 

comparatively rapid, and spectral scanning was done at ~25 °C. The samples were baked in air 

on a hotplate at a temperature (100 °C) recommended by a vendor, high enough to produce a 

change in a reasonable time, but far lower than what would damage the coating (1). After 

baking, the samples were cooled for a period of 5–10 min. 

The optical transmission was measured at 0° incidence with a spectrophotometer (4). Although 

the only wavelengths of immediate interest are 1532 and 1645 nm, spectra were acquired from 

355–1700 nm in case features correlated to water content turned up at other wavelengths (none 

did). We do not expect multiple reflections from the sample faces to produce interference 

fringes in the spectra, because the sample faces are not intentionally parallel and the 

spectrophotometer does not have sufficient resolution.

3. Results and Discussion

The transmission of the AR-coated sample increased by ~1% during 4 h of baking out; after 

which no further changes were observed (figure 3). The transmission of the sample is less than 

that of the AR coating alone because of the ~7.6% reflection expected from the air/SF6 interface 

on the back side. The fine-scale structure in the spectra is due to noise.
3

1500 1550 1600 1650 1700

91.5

92.0

92.5

93.0

93.5 Transmission (%)

Wavelength (nm)

Before Heating After 60 min

120

180

240

Figure 3. Transmission spectra for AR-coated sample after repeated heating at 100 °C.

On a shorter timescale than that shown in figure 3, the transmission shows an initial drop, which 

may be spurious (figure 4). The 1532-nm curve in figure 4 is an interpolation between the data 

taken at 1530 and 1535 nm.

0 50 100 150 200 250

91.0

91.5

92.0

92.5

93.0 Transmission (%)

Bake Time (min)

1532

1645

Figure 4. Transmission of AR-coated sample vs. bake time.

The PR sample showed a decrease in T (increase in R) over the course of several hours of baking

(figure 5). At 1645, the reflectivity increased from 76.4% to 77.2% (figure 6). The coating run 

for this substrate was specified to yield a 70% reflector at 1645 nm when ordered. The 

reflectivity was measured to be 72% by the vendor shortly after deposition, and six months later 
4

measured to be ~80% at ARL. The mirror was returned to the vendor, where baking at 100 °C 

overnight restored the ~70% reflectivity (1)

*

. Currently, with an identical mirror (except for the 

substrate radius of curvature), we are seeing an increase in reflectivity during bakeout, for 

reasons that are unknown.

1500 1550 1600 1650 1700

22

24

26

28 Transmission (%)

Wavelength (nm)

Before Heating After 60 min

120

180

240

300

Figure 5. Transmission spectra of PR-coated sample after heating at 100 °C.

0 50 100 150 200 250 300

23

24

25

26 Transmission (%)

Bake Time (min)

1532

1645

Figure 6. Transmission of PR-coated sample vs. bake time.

 

*This observation originally lead us to suspect moisture desorption as the cause for the slow increases in Er:YAG output 

power after turn-on.
5

The HR spectrum shifted 32 nm toward the shorter wavelengths during the baking (figure 7). 

The decrease in transmission at 1532 nm is unfortunate for the Er:YAG laser output power, but 

the increase in reflectivity (from 95% to >99.5%) at 1645 nm is beneficial (figure 8). 

1500 1550 1600 1650 1700

0

20

40

60

80

100 Transmission (%)

Wavelength (nm)

Before Heating After 15 min

30

60

90

120

180

240

Figure 7. Transmission spectra of HR-coated sample after baking at 100 °C.

0 50 100 150 200 250 300 350

75

80

85

90

95

1532

1645

Time (min) Transmission at 1532 nm (%)

0

1

2

3

4

5 Transmission at 1645 nm (%)

Figure 8. Transmission of HR-coated sample vs. bake time.

We believe that moisture desorption from the HR coating on the Er:YAG rod is the most likely 

explanation for the gradual increase in laser output power. The effect of moisture on the AR 

coating was negligible. Although the PR coating was effected by moisture, it not subject to 
6

significant heating during laser operation because the PR is located several centimeters from the 

rod, and it does not absorb at 1532 or 1645 nm. The Er:YAG rod does absorb the pump light at 

1532 nm. To reduce heating the HR coating and the O-ring seal to the point of damage, the rod 

has a 10-mm undoped cap on the end where the pump is incident. The undoped YAG can still 

conduct some heat from the doped portion out to the coating. Monitoring the temperature of the 

coating, or the rod, is difficult because the thermal conductivity is low compared to a metal, and 

a thermocouple placed close enough to the region of interest would shadow the sample, absorb 

light by itself, etc. 

The middle two thirds of the 60-mm rod are cooled with water at 15 °C to dissipate the 

30–300 W of heat that is generated when in operation. When not being pumped, the cooling 

water could induce some condensation on the coated ends of the rod. In view of the care taken to 

cool the rod, it is ironic that heating turns out to have the beneficial effect of removing moisture 

from the HR coating. In the future, steps will probably be taken to keep the coatings in a dry 

atmosphere when not in use.

The effect of moisture on optical coatings has been studied in the past, but not a great deal has 

been published. The following paragraph is an excerpt from reference 5:

“In many coatings voids in the microstructure are large enough to allow water 

which condenses from the atmosphere to enter and spread laterally through the

layers (6–8). This process may take place over a long period of time and is an 

important cause of temporal instability of thin-film properties. The most 

prominent optical effect is a shift of the spectral profile of the coating toward 

longer wavelengths (9–12) due to the increase in effective index of the wet 

material.”

Our observations on the HR coating are consistent with the red shift due to moisture absorption 

and a blue shift due to moisture removal.

Because the dichroic HR coating may be inadvertently heated during use, despite care taken to 

keep it cool, we thought it prudent to see how operating temperature affects coating performance. 

An oven was built and placed inside the spectrophotometer so that transmission spectra could be 

recorded while a sample is hot (figure 9). 
7

Figure 9. Temperature-controlled oven for use inside 

the spectrophotometer.

The HR sample was baked for several hours before the measurement began. The results show a 

±5-nm movement in the edge, only a slight change in transmission at 1645 nm, and close to zero 

change at 1532 nm (figure 10).

1500 1550 1600 1650 1700

0

20

40

60

80

100 Transmission (%)

Wavelength (nm)

25C

50C

75C

100C

Figure 10. Transmission spectra of HR-coated sample for different temperatures.
8

4. Conclusions

The results of this study indicate that moisture desorption from the HR dichroic coating on the 

Er:YAG rod is the most likely explanation for the gradual increase in laser output power. 

Moisture induces a 30-nm red shift of the transition wavelength in the HR dichroic coating. 

Coatings by other manufacturers may be less porous and less susceptible to moisture absorption. 

Another solution would be to keep the laser heads in a dry atmosphere when not in use. 

Temperatures in the range 25–100 °C had a less-pronounced effect on the transmission spectra.
9

5. References

1. Lattice Electro Optics Web page, 1324 E. Valencia Dr., Fullerton, CA 92831, 

http://www.latticeoptics.com (accessed 2010).

2. TwinStar Optics and Coatings Web page, 6741 Commerce Ave., Port Richey, FL 34668.

http://www.twinstaroptics.com (accessed 2010).

3. Refractive Index database Web site, 2010. www.refractiveindex.info (accessed 2010).

4. Perkin Elmer Web page, 940 Winter St., Waltham, MA 02451, model Lambda 950.

http://www.perkinelmer.com (accessed 2010).

5. Gee, J. R.; Hodgkinson, I. J.; Macleod, H. A. Moisture-dependent Anisotropic Effects in 

Optical Coatings. Applied Optics 1985, 24, 3188–3192.

6. Pulker, H. K. Recent Investigations of Dielectric Thin Films for Applications in Optics. 

Optik 1971, 32, 496.

7. Macleod, H. A.; Richmond, D. Moisture Penetration Patterns in Thin Films. Thin Solid 

Films 1976, 37, 163.

8. Gibson, D. R.; Lissberger, P. H. Optical Properties of Narrow-Band Spectral Filter Coatings 

Related to Layer Structure and Preparation. Appl. Opt. 1983, 22, 269.

9. Hall, Jr., J. F.; Ferguson, W.F.C. Dispersion of Zinc Sulfide and Magnesium Fluoride Films 

in the Visible Spectrum. J. Opt. Soc. Am. 1955, 45, 74.

10. Gonella, J. R.; Robrieux, B. Transformation of Thin Calcium Fluoride Films. Opt. Acta

1962, 9, 101.

11. Shklyarevskii, I. N.; Umerov, R. I.; Lupashko, E. A.; Khramtsova, V. I. Effect of Absorbed 

Moisture on the Position of the Passband of Narrow-Band Interference Filters. Opt. 

Spectrosc. 1973, 35, 206.

12. Koppelmann, G. Irregularities in Evaporated Interference Films and Their Connection with 

Light Scattering. Optik 1960, 17, 416.
10

List of Symbols, Abbreviations, and Acronyms

AR anti-reflecting

ARL U.S. Army Research Laboratory 

CRAM countering rockets, artillery, and mortar

Er erbium 

HR high reflecting

Nd neodymium 

OC output coupler

PR partially reflecting

YAG yttrium aluminum garnet
11

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