ARL-TN-0711 ● NOV 2015

US Army Research Laboratory

US Army Research Laboratory Directed Energy 

Internship Program 2014 Final Report

by Jeffrey O White

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ARL-TN-0711 ● NOV 2015

US Army Research Laboratory

US Army Research Laboratory Directed Energy 

Internship Program 2014 Final Report

by Jeffrey O White

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution is unlimited.
ii

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US Army Research Laboratory Directed Energy Internship Program 2014 Final 

Report

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Jeffrey O White

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US Army Research Laboratory

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Adelphi, MD 20783-1138

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ARL-TN-0711

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High Energy Laser Joint Technology Office

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Directed Energy Professional Society

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14. ABSTRACT

This technical note is the final report for the 2014 Directed Energy Internship program at the US Army Research Laboratory. 

Six interns were mentored by 4 government employees. The projects included 1) a comparison of traditional erbium (Er):silica

(SiO2) optical fiber to designs using Er: alumina (Al2O3) nanoparticles, 2) chirped seed amplification in a 1.6-kW ytterbium 

(Yb) fiber amplifier, 3) custom components for a 100-W Er amplifier, and 4) performance of coherent beam combining and 

power usage in fiber laser arrays with high fill-factor output. Three reports written by the interns are included as Appendices

A through C. A report on the third project was published separately as ARL-TN-0672.

15. SUBJECT TERMS

High power lasers, internship

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Jeffrey O White

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Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures v

List of Tables v

1. Introduction 1

2. Recruitment of Participants 1

3. Selection of Participants 1

4. Reporting of Student Research 2

5. Conclusion 2

6. References 3

Appendix A. A Comparison of Traditional Erbium (Er):Silica (SiO2) Optical 

Fiber to Designs Using Er:Alumina (Al2O3) Nanoparticles, by Scott 

Melis 5

A-1 Abstract 6

A-2 Acknowledgments 6

A-3 Student Bio 6

A-4 Background 6

A-5 Experiment 7

A-6 Results and Discussion 9

A-7 Conclusion 12

A-8 Related Work 12

Appendix B. Chirped Seed Amplification in a 1.6-kW Ytterbium (Yb) Fiber, 

by Zhi Yi Yang 15

B-1 Abstract 16

B-2 Introduction 16

B-3 Background 16
iv

B-4 Experiment 17

B-5 Measurement\Results 17

B-6 Conclusion 18

Appendix C. Performance of Coherent Beam Combining and Power Usage 

in Fiber Laser Arrays with a High Fill-Factor Output, by Brian Kwon, 

Jeffrey Volz, and Patrick Marple 19

C-1 Abstract 20

C-2 Acknowledgments 20

C-3 Student Bios 20

C-4 Introduction/Background 20

C-5 Experiment/Calculations 21

C-6 Results and Discussion 22

C-7 Summary and Conclusions 25

List of Symbols, Abbreviations, and Acronyms 27

Distribution List 28
v

List of Figures

Fig. A-1 A graph of the normalized fluorescence intensity of the Er-SD and 

Er-NP samples .....................................................................................10

Fig. A-2 Cross-section spectra of the Er-NP (left) and Er-SD (right) fiber 

preforms showing both stimulated emission (in blue) and absorption 

(in red). The spectra are very similar...................................................11

Fig. A-3 Near-IR upconversion fluorescence for both the Er-NP and the Er-SD 

samples.................................................................................................12

Fig. B-1 Data (points) and simulation (solid lines) showing a chirp rate of 

1×1017 Hz/s needed to reach a 1-kW output power at the SBS 

threshold (dashed line).........................................................................18

Fig. B-2 Backward power vs. forward power for various chirp rates................18

Fig. C-1 Current array (left) vs. experimental array (right) ...............................21

Fig. C-2 Diagram of experimental setup............................................................21

Fig. C-3 Experimental mask shapes for a new aperture.....................................22

Fig. C-4 Experimental results of truncated power vs. mask diameter ...............23

Fig. C-5 3D rendering of parts in Makerbot software next to printed pieces ....24

Fig. C-6 In-progress bimorph actuators next to the fiber positioner base..........25

List of Tables

Table A-1 Radiative fluorescence lifetimes of both the Er-NP and the Er-SD 

samples; their close agreement further shows the similarity of the 

samples.................................................................................................10
vi

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1

1. Introduction

The US Army Research Laboratory (ARL) operated its 11th annual Directed Energy 

(DE) Visiting Scholars program in summer 2014, funded by the High Energy 

Lasers Joint Technology Office (HEL-JTO) and coordinated by the Directed 

Energy Professional Society (DEPS). This report summarizes the activities 

including recruitment, selection among applicants, and research areas pursued. 

Three technical reports written by the participants are included as Appendices A 

through C, and a third is available as ARL-TN-0672.

1

2. Recruitment of Participants

Applicants to the summer program were recruited using several methods. A web 

page describing the program and our team’s research topics was posted on ARL’s 

Internet site and linked from the DEPS site. We also published a free ad in the 

Physics Today online classifieds, and asked the physics and electronic engineering 

(EE) departments at the University of Maryland (College Park and Baltimore 

County) and the University of Illinois to post advertisements or circulate via email 

to their students. Twenty-seven students applied; one-third had credentials of 

sufficient quality and relevance to warrant serious consideration. Two returning 

students were accepted along with 4 new students.

3. Selection of Participants

Based on their transcripts and letters of recommendation, 6 students were offered 

summer scholar positions; they all accepted.

Kevin Landsman had completed his 1st year of graduate school in the EE 

department of the University of Maryland at College Park. He was a good fit for 

Jeff White’s project on making an interlock circuit for a fiber amplifier. 

Construction of the amplifier was part of HEL-JTO contract 13-S&A-0509. He 

worked 3 days per week, spending the rest of the week studying for the exams to 

qualify for the PhD program.

Returning student, Zhi Yi Yang, had completed his Master’s degree in physics at 

Hunter College. Due to his previous experience at ARL as a DEPS Scholar in 2013, 

he was the best candidate to work with Jeff White on building a 100-W erbium (Er)

fiber amplifier using chirped seed amplification (CSA). This project was part of 

HEL-JTO contract 13-S&A-0509.
2

Scott Melis is a returning student who graduated in May 2014 from Loyola, 

Maryland University with a BS degree in physics. He was a good fit for Larry 

Merkle’s project on comparing a traditional Er:silica (SiO2) optical fiber to designs 

using Er:alumina (Al2O3) nanoparticles. He participated in ARL’s CollegeQualified Leaders (CQL) program in summer 2013. In fall 2014, he began a 

doctorate program in physics at Georgetown University where he hopes to continue 

research in materials and optics. 

Three new students were selected by Jony Liu and Leonid Beresnev to study the 

performance of coherent beam combining and power usage in fiber laser arrays 

with high fill-factor output. Brian Kwon is a graduate of The University of 

Delaware with a computer engineering degree. Jeffrey Volz is a junior attending 

The University of Delaware and majoring in computer engineering. Patrick Marple

is a junior at University of Maryland Baltimore County (UMBC) majoring in 

systems engineering. 

4. Reporting of Student Research

Scott Melis presented a poster at the Sensors and Electron Devices Directorate 

(SEDD)-Computational and Information Sciences Directorate (CISD) student 

poster session. Brian Kwon, Jeffrey Volz, and Patrick Marple presented a second 

poster. Final reports were written by Scott Melis, Zhi Yi Yang, and Kevin 

Landsman. Brian Kwon, Jeffrey Volz, and Patrick Marple wrote a joint report. The 

full text of 3 reports is included as Appendices A through C. Kevin Landsman’s 

report was turned into an ARL Technical Note: ARL-TN-0672.

1

This year, no students attended the Directed Energy Education Workshop in March 

2015, either they were unable to attend because of school or their work wasn’t 

complete by the end of the summer.

5. Conclusion

We were pleased with the quality of all 6 DE scholars this year. They were well 

worth the stipend provided by the HEL-JTO. One student, Kevin Landsman, 

applied for and was awarded a Graduate Directed Energy Fellowship in High 

Energy Lasers. We continue to believe in the importance of training the future 

Department of Defense workforce and believe that the Directed Energy Visiting 

Scholars program provides students with an excellent, much-needed, close to reallife learning opportunity. 
3

6. References

1. Landsman KA, White JO, Harrison AC. Custom Components for a 100-W 

Erbium Fiber Amplifier. Adelphi (MD): Army Research Laboratory (US);

May 2015. Report No.: ARL-TN-0672.
4

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5

Appendix A. A Comparison of Traditional Erbium (Er):Silica (SiO2)

Optical Fiber to Designs Using Er:Alumina (Al2O3)

Nanoparticles, by Scott Melis
6

A-1 Abstract

A comparison is made between traditional erbium (Er)-doped fiber (EDF) preforms 

made by solution-doping (SD), and new preforms made using Er-doped alumina 

(Al2O3) based nanoparticles (NPs) that are melted into a glass (Er-NP) in order to 

better control the separation between Er atoms in a fiber. Better separation is 

important because of laser performance degradation due to energy transfer in 

clusters of Er atoms from processes like upconversion. In the traditional method, 

Er atoms are often co-doped with aluminum (Al) atoms, but clustering still occurs. 

The new Al2O3 NP technique is believed to alleviate this by creating a cage of Al 

and oxygen (O) ions to surround the Er ion and reduce ion-ion interactions. A study 

of the absorption and stimulated emission cross sections is performed on samples 

of both materials. Overall, the Er-NP fiber preforms appear to produce similar 

cross-section spectra to the traditional counterparts. In addition, measurements of 

the upconversion-induced fluorescence of each sample have shown that the rate of 

upconversion has fallen by a significant factor. Because of this, the new Er-NP 

material shows significant potential for improving fiber lasers by allowing for 

higher Er concentrations while maintaining good efficiency.

A-2 Acknowledgments

I wish to acknowledge the mentorship and guidance of Dr Larry Merkle. In 

addition, special thanks are extended to Dr Mark Dubinskii, US Army Research 

Laboratory (ARL), for his collaboration and insight, as well as Dr Joseph Friebele, 

US Naval Research Laboratory (NRL), for his collaboration and creation of the 

EDF preforms analyzed here.

A-3 Student Bio

Scott Melis graduated from Loyola University Maryland in spring 2014 with a BS

degree in physics and minors in mathematics and engineering sciences. He has 

participated previously in ARL’s College-Qualified Leaders (CQL) program in the 

summer of 2013. In fall 2014, he began a doctorate program in physics at 

Georgetown University where he hopes to further study aspects of materials and 

optics research. 

A-4 Background

EDFs are important because of the wavelength at which they lase, which falls 

within the region of infrared (IR) wavelengths that exhibit high atmospheric 

transmission while simultaneously matching the “eye safer” region of 
7

1400–1800 nm. However, when making EDFs, the solubility of Er in traditional 

silica (SiO2)-based glass is low and the ions that successfully bond with the glass 

tend to form clusters. This is an undesirable scenario due to the ease with which Er 

ions experience upconversion when they are in close proximity. The energy gap 

between several energy levels of Er is fairly similar, which allows for an excited 

ion in a particular energy state to transfer its energy to a similarly excited ion that 

is positioned nearby. When this upconversion process occurs for a given pair of 

excited ions, one ion returns to the ground state without emitting a fluorescence 

photon. The other is excited to a much higher energy level where it can either 

fluoresce immediately and give off a photon of a shorter wavelength than desired 

or fall back to its initial excited state and fluoresce from there. Thus, either half or 

all of the energy in a pair of excited ions could be wasted.

In traditional SiO2-based Er-doped glass (Er-SD), Er is co-doped with a 

significantly higher concentration of Al ions in the fiber core using a technique 

called solution-doping. The goal is to use Al ions as buffers to separate the Er ions 

and impede energy transfer; however, the placement of the ions is a random effect. 

It does not provide a direct mechanism for controlling the atomic environment of 

the Er ions and clustering still does occur. As an idea to counteract the degrading 

effects of upconversion, Er-doped SiO2 NPs (Er-NP) are doped into the glass core 

of a fiber. This process is thought to create a cage of Al and O ions around each Er 

ion, thereby significantly reducing ion-ion energy transfer in the glass. Fiber 

preforms of this new NP-doped glass are compared in this study to traditional SD 

preforms in order to see whether they produce comparable fluorescence spectra, 

lifetimes, and cross-section spectra while also reducing upconversion in the nearIR region. If they do this, then the new NP-doping technique will improve the 

current state of EDFs.

A-5 Experiment

Experiments were carried out to compare the fluorescence spectra, fluorescence 

lifetimes, and cross-section spectra of fiber preforms that were made using both 

techniques of interest. A 0.254-mm-thick sample of Er-NP fiber preform and a 

0.474-mm-thick sample of Er-SD fiber preform were made by Dr Joseph Friebele 

of NRL. Both samples were roughly 12.3 mm in diameter with nominally 1-mmdiameter Er-doped cores. It is estimated that the Er-NP sample has roughly twice 

the concentration as the Er-SD sample.

For collecting fluorescence data on each sample, a Spectra-Physics continuous 

wave (CW) titanium sapphire (Ti:sapph) laser was used that was pumped with a 

frequency doubled Spectra-Physics neodymium-doped yttrium vanadate laser. The 
8

Ti:sapph laser was chopped at 100 Hz with a 50% duty cycle and was used to excite 

both samples at around 979 nm. The samples were excited in a Horiba Fluorolog 3 

fluorescence spectrometer that uses a laptop with Horiba’s Origin-based 

FluorEssence software to collect data. Two Thorlabs FGL1000s filters were used 

to protect the system from scattered laser light. The spectrometer also used a 

1.5-μm blazed grating and a nominally 2.6-μm liquid nitrogen-cooled indium 

gallium arsenide (InGaAs) detector whose signal was sent into a Stanford Research 

Systems SR830 lock-in amplifier before being analyzed by the Fluorolog.

For measurements of the radiative fluorescence lifetime, the Ti:sapph laser was 

again used at 979 nm. However, it was now chopped at 10 Hz with a roughly 20% 

duty cycle. The samples were not in the Fluorolog system but instead were 

positioned so that their fluorescence could be focused using a 25-mm lens directly 

onto the InGaAs detector with a Thorlabs FEL1100 and Spectrogon LP1200 filter 

for protection. The output of the detector was measured directly using a Tektronix 

TDS7104 digital phosphor oscilloscope and the data were exported to Microsoft 

Excel so that a fit to an exponential decay could be implemented for determining 

the lifetime value.

With a value for the lifetime, the fluorescence data could be converted into a 

stimulated emission cross section using the Füchtbauer–Ladenburg (F-L) 

equation.

1 The F-L equation allows for the conversion of a fluorescence intensity 

spectrum to a stimulated emission cross section given the radiative lifetime (τrad) 

and the index of refraction of the sample (n), which we assume to be approximately 

the value for fused SiO2, 1.44. The F-L equation is given by

σσssssssss.ee .

(λλ) = λλ55

88 ππ cc nn22 ττrrrrrr

II(λλ)

∫ II(λλ) λλ (A-1)

From the stimulated emission cross section, an absorption cross section can then be 

generated using the theory of reciprocity. Reciprocity insists that if the ratio of the 

partition functions of the upper and lower energy states of a transition (Zup/Zlow) 

and the zero-phonon line energy (E0) are known, then a stimulated emission cross 

section can be converted into an absorption cross section and vice versa. In a high 

temperature condition, or in a glass as is our case, the possible energies of each 

state smear together and the ratio of the partition functions of each level is simply 

the ratio of the degeneracies. For this case, where we desire fluorescence in the 

region of 1450–1750 nm, the Er ions transition from the 4I13/2 to the 4I15/2 state. This 

means Zup/Zlow is given by 14/16, where the degeneracy is equal to 2j+1. Similarly, 

the zero-phonon line energy is taken to be the energy associated with the 

 1Aull BF, Jenssen HP. IEEE J. Quantum Electronics. 1982;QE-18:925–30.
9

fluorescence peak seen at 1532 nm. Thus, E0 is 1.298 x 10–19 J. With these values 

in mind, one can apply the reciprocity equation given by2

σσ aa .

(λλ) = σσssssssss.ee .(λλ) ZZuuuu

ZZllllll

eeee �

hh cc

λλ −EE00

kk TT � (A-2)

Creating spectra for these parameters will allow for a determination of whether or 

not the Er-NP method creates a material that behaves similarly to the Er-SD 

method. What remains is to observe whether or not the upconversion is reduced. 

For this experiment, both samples are again excited in similar positions with a 

979-nm CW beam that is chopped at 100 Hz; green and near-IR wavelengths 

corresponding to transitions from higher energy states are explored using a 

Hamamatsu R2658P photomultiplier tube (PMT) with the Fluorolog spectrometer 

to scan the region. The output of the PMT is sent through the lock-in amplifier 

before analysis with the FluorEssence software due to the small signal level 

detected. The “area under the curve” of each sample’s upconversion spectrum is 

scaled by the “area under the curve” of the corresponding fluorescence from 

1450–1700 nm. This is done to hopefully correct small errors in sample positioning 

and excitation power. These scaled area values are then compared to see if the ErNP sample shows lower amounts of upconversion than the Er-SD sample.

A-6 Results and Discussion

Starting with the fluorescence spectra of each sample, it is easy to see the 

similarities between the 2 materials. First and foremost, they both fluoresce in the 

same region with a common peak at 1532 nm. As shown in Fig. A-1, both samples 

also possess the smooth nature that is to be expected of Er ions in a glass host. Small 

differences are able to be observed though. The height of the 1532-nm peak relative 

to the shoulder differs in both samples, and in the Er-NP sample, the shoulder is 

significantly more pronounced. Though different, this is acceptable and most likely 

due to the Al ions controlling which excited states of Er are most probable. Thus, 

the spectra are similar enough for all necessary laser applications.

 2Payne SA, Chase LL, Smith LK, Kway WL, Krupke WF. IEEE J. Quantum Electronics. 1992;28:2619–

30.
10

Fig. A-1 A graph of the normalized fluorescence intensity of the Er-SD and Er-NP samples

For the fluorescence lifetime measurements, the 2 samples were also found to be 

heavily similar. Table A-1, the Er-NP lifetime was slightly longer than the Er-SD 

lifetime; however, the values of τrad only differed by roughly 2%. This is not a large 

enough difference to imply that the 2 samples behave differently. Therefore, one 

can now generate the stimulated emission and absorption cross sections using these 

data and the assumptions made earlier in Eqs. A-1 and A-2. Since the lifetime 

measurements did not differ significantly and the fluorescence spectra also agreed 

fairly well, it comes as no surprise that the cross sections displayed in Fig. A-2 

maintained small structural differences of the fluorescence spectra but otherwise 

agreed well.

Table A-1 Radiative fluorescence lifetimes of both the Er-NP and the Er-SD samples; their 

close agreement further shows the similarity of the samples

Sample τrad (ms)

Er-NP 10.8

Er-SD 10.6
11

 

Fig. A-2 Cross-section spectra of the Er-NP (left) and Er-SD (right) fiber preforms showing 

both stimulated emission (in blue) and absorption (in red). The spectra are very similar.

Though the 2 processes produced similar desired fluorescence, when looking at the 

upconversion produced by each sample, there was a sharp difference. As shown in 

Fig. A-3, the near-IR upconversion was significantly lower in the Er-NP glass than 

in the Er-SD. No green upconversion that Er is famous for was able to be detected 

in either sample, though our gratings are not optimum at that wavelength. The “area 

under the curve” was calculated in Excel using the trapezoid rule and was found to 

be 4 times higher in the Er-SD case. At this time, it is not apparent whether our 

method was precise enough to truly say that there is exactly 4 times more 

upconversion in the Er-SD sample. However, there is a distinct difference in these 

values under similar excitation conditions, so it can still be said that the number of 

upconversion events was greater in the Er-SD sample. This is an excellent result 

due to the fact that the Er concentration in the Er-NP sample is estimated to be at 

least twice that of the Er-SD sample. It implies that the Er ions are indeed shielded 

by the Al and O ions in the Er-NP sample, as it exhibits less upconversion despite 

being more heavily concentrated.
12

Fig. A-3 Near-IR upconversion fluorescence for both the Er-NP and the Er-SD samples

A-7 Conclusion

Overall, it is clear that the new NP-doping approach successfully creates a material 

that behaves as well as the traditional SD method. It is also shown that the NP 

approach reduces near-IR upconversion significantly. Because it did this while 

having a higher concentration, it seems apparent that the Er-NP approach is doing 

a better job at separating the Er atoms in the glass host. If the Er-NP material were 

used for the creation of a fiber laser, there should be a good increase in output due 

to both the reduction in energy losses and the increase in fluorescence. The fiber 

could have more Er ions per unit volume for lasing than an Er-SD counterpart as 

the current preform samples appear to reduce upconversion even at twice the 

concentration. Thus, future work hopes to look into optimizing the NP approach in 

order to find the best production method and the best concentration of Er ions. In 

addition, further tests hope to look into better quantifying the upconversion process 

of both Er-NP and Er-SD materials in order to confirm the significant decrease seen 

here.

A-8 Related Work

Other projects were explored this summer, outside of the comparison between fiber 

preforms that is described in this report. They are briefly mentioned here for better 

describing the work done this summer.
13

First, a calorimetric determination of a bulk sample’s quantum efficiency was 

explored. The process attempted involved gluing a thermocouple to a sample and 

exciting the sample with a laser at a wavelength that it absorbs. The heating and 

cooling was monitored using a LabVIEW program in order to determine the heating 

rate of the sample. The incident laser power was also checked with a laser power 

meter. Then, the experiment was repeated with the same sample coated in black 

optical grease in order to determine a “calibration” factor using a scenario of 

essentially 0% quantum efficiency. If we know how much power is incident on the 

optical grease and how much heat our thermocouple detects, we can create a scale 

factor to multiply the heating rate of the uncoated sample by. Doing that should 

allow us to determine quantum efficiency using the following equation if the 

sample’s quantum defect is known.

QQQQ = 1 − �

CCCCCCCCCCCCCCCCCC PP h ee eeeeee

PP aaaa aa �

1 − QQQQQQQQQQQQQQ DDDDDDDDDD (A-3)

Unfortunately, this result has yet to confirm the well-known quantum efficiency of 

several neodymium (Nd):yttrium aluminum garnet (YAG) samples of different 

concentrations and therefore, the technique will continue to be explored by Dr Larry 

Merkle. So far, a more precise laser power meter has been obtained to better remove 

error in power measurements and an exploration of the validity of the assumption 

that the cooling behaves as a single exponential is underway.

In addition, a sample of 4H-silicon carbide (SiC) was studied to see if there was 

evidence of fluorescence. Near-IR light, which peaked at around 1000 nm and 

spanned from 850–1400 nm, was observed when the sample was excited in the 

range of 740–1000 nm. It is believed that the sample absorbs at wavelengths shorter 

than 740 nm, but due to the limits of the Ti-sapph laser used, it was impossible to 

explore much past this point. At this time, it is not known what defect or property 

caused these emissions but they were found to be horribly inefficient when 

compared to a 0.09% Nd:YAG ceramic sample. Both samples were excited at 

points where they absorbed similar amounts and yet the 4H-SiC sample fluoresced 

with roughly 1–2% the photon flux of the Nd:YAG sample. Therefore, we conclude 

that whatever this property is, it is very inefficient, and though it is interesting, it 

shows little use at this time.
14

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15

Appendix B. Chirped Seed Amplification in a 1.6-kW Ytterbium (Yb) Fiber, 

by Zhi Yi Yang
16

B-1 Abstract

As lasers are scaled to higher powers, nonlinear effects become increasingly 

prominent. This is especially pronounced in fiber lasers because of their ability to 

confine high optical power into a small area and over a long interaction length. For 

narrow linewidth lasers, the first nonlinear effect that is encountered is stimulated 

Brillouin scattering (SBS). By rapidly chirping the frequency of the seed laser, SBS

is suppressed with a more efficient use of bandwidth than by white noise phase 

modulation. In this experiment, a chirped diode laser seeds a kilowatt fiber 

amplifier and we were able to suppress SBS enough to reach 1.6 kW of output 

power while chirping at 5.2×1017 Hz/s.

B-2 Introduction

One reason fiber lasers are attractive is because of their large surface-to-volume 

ratio, which facilitates cooling. At the same time, because of a fiber laser’s small 

core size and long length, nonlinear effects occur more readily than in a rod or slab 

laser, for example. A well-known nonlinear effect that limits the power in fibers is 

SBS. The conventional method used to suppress SBS, white noise phase 

modulation, does not use the bandwidth in an efficient manner and in addition 

complicates phase sensitive applications such as coherent beam combining. Phase 

modulation also becomes a less effective means to suppress SBS in fiber as the 

delivery fiber increases in length. Chirping the frequency in a linear manner lends 

itself to phase sensitive applications because the phase varies in time in a very 

simple and deterministic way. In addition, at high chirp rates the SBS threshold 

becomes independent of the length of the delivery fiber.

B-3 Background

SBS builds from spontaneous Brillouin scattering of a laser photon from a thermal 

acoustic phonon in the fiber. This is an inelastic interaction, meaning there is an 

energy transfer that shifts the scattered photon up or down in energy by the acoustic 

frequency. Some of the spontaneously scattered photons will be guided in the 

forward or backward direction. At high enough forward laser power, the backward 

scattering process builds up to become stimulated. As the scattered photons 

propagate backward, they interact with the forward propagating beam, creating an 

interference pattern moving at the sound velocity. Through the phenomenon of 

electrostriction, the longitudinal intensity variation due to interference alters the 

density of the fiber. This causes a periodic modulation of the index that can scatter 

more of the forward propagating beam. As this cycle repeats, it will build up the 

intensity of the backward beam, which can become intense enough to damage the 
17

laser source and limit the forward laser power. The scattered photons propagating 

in the forward direction do not see this amplification due to a lack of phase 

matching between the acoustic, laser, and Stokes waves. Our laser suppresses SBS

by sweeping the laser frequency, this changes the resonant Brillouin frequency 

along the fiber for the backward propagating beam, preventing it from efficiently 

generating acoustic waves that can scatter more light. This is similar to having a 

broad linewidth laser that reduces the phase matching, thereby increasing the 

threshold for SBS. Our linear chirp occupies less bandwidth for the same threshold 

and can maintain a long effective coherence length because the very simple and 

deterministic frequency versus time relationship lends itself to coherent combining.

B-4 Experiment

Our chirped diode laser was developed jointly by Telaris and the group of Prof 

Amnon Yariv at Caltech. The vertical cavity surface emitting laser (VCSEL) at the 

heart of it was developed by Praevium, Inc. It can lase from 1000–1100 nm, 

depending on the position of an electronically controlled external mirror bonded to 

the laser. We make use of a 20-nm (5-THz) part of the spectrum that overlaps the 

gain in an ytterbium (Yb)-doped fiber. The laser diode frequency follows a triangle 

waveform in a highly linear fashion but must slow down as the chirp direction 

changes. A Mach-Zehnder interferometer is used to monitor the linearity of the 

chirp. The interferometer outputs a sinusoid with a single frequency, which 

signifies a linear chirp but during turning points this is not true. At these points, 

where the chirp rate can reach zero, SBS can build up to a point of tripping the 

laser’s interlock used to prevent any damages to the laser system. In the previous 

generation of chirped seed laser, we showed that this problem can be avoided by 

using a dual source to produce a seed that is constantly chirping. In our current 

setup, a broadband source is injected at the turning points via an electro-optic 

switch that alternates between the broadband source and the chirped diode laser. 

This setup then seeds a kilowatt fiber amplifier in a testbed at Nufern. The output 

beam is monitored by an optical power meter. A backward tap before the final stage 

is used to monitor all backward propagating power including SBS. 

B-5 Measurement\Results

From previous experiments and simulation done by Eliot Petersen, it was shown 

that a chirp of 1×1017 Hz/s would be needed to reach 1 kW of output power

(Fig. B-1). This chirp was obtained by placing the Praevium microelectromechanical system (MEMS)-VCSEL in an opto-electronic feedback loop 

developed by Telaris. 
18

Fig. B-1 Data (points) and simulation (solid lines) showing a chirp rate of 1×1017 Hz/s needed 

to reach a 1-kW output power at the SBS threshold (dashed line)

Several chirp rates were used ending with the highest chirp rate of 5.2×1017 Hz/s 

and including a 40-GHz phase modulated source for comparison. Output power 

was increased until the laser interlock tripped due to too much power in the 

backward direction. Figure B-2 shows the backward power versus the forward 

power for various chirp rates.

Fig. B-2 Backward power vs. forward power for various chirp rates

B-6 Conclusion

We were able to show that there are chirped laser sources with high enough chirp 

rates to reach a 1.6-kW output. Unfortunately there was not enough time to do a 

cut-back experiment to show that this method would have a SBS threshold that is 

independent of fiber length. This experiment will be done at a later time.
19

Appendix C. Performance of Coherent Beam Combining and Power 

Usage in Fiber Laser Arrays with a High Fill-Factor Output, 

by Brian Kwon, Jeffrey Volz, and Patrick Marple
20

C-1 Abstract

A study on the operation of a coherently combined fiber laser array in the far-field 

with respect to the near-field fill factor was performed. The experimental 

measurements for power output were performed for arrays with a variable number

of beamlets and densely packed lenses with variable size and shape. A 

representative set of data was created using a single fiber laser and a single lens 

with interchangeable masks. By measuring the energy of the legs of the Gaussian 

beam and the total energy of the beam, we were able to deduct the percentage of 

energy truncated with each shape design and size. By analyzing the trends in output 

data resulting from variations in size and shape, a much more efficient cluster of 

lenses can be designed. It was found that the truncated power could be reduced 

together with an increase of radiation in the far-field. The results allow us to design 

high energy laser (HEL) transmitter arrays with a decrease in size and complexity 

and a reduction in truncated power inside the array.

C-2 Acknowledgments

We wish to acknowledge the mentorship of Dr Leonid A Beresnev and Dr Jony 

Jiang Liu. This work is supported by the High Energy Lasers Joint Technology 

Office (HEL-JTO) student program and the US Army Research Laboratory (ARL)

summer internship program.

C-3 Student Bios

Brian Kwon is a graduate of The University of Delaware; Jeffrey Volz is a junior 

attending The University of Delaware, and Patrick Marple is a junior at the 

University of Maryland Baltimore County (UMBC). Kwon and Volz are computer 

engineering majors and Marple is an economics major. None have formal previous 

research experience. Kwon is to work for a cybersecurity company in New York 

City, Volz is working to finish his computer engineering degree, and Marple is 

working toward a systems engineering degree.

C-4 Introduction/Background

Current designs of multi-fiber laser arrays have been the latest breakthroughs in 

scalable laser technology. However, the working systems so far use standard 

circular collimating lenses, which make for a truncated area between the beams. 

The current array (Fig. C-1) uses a feedback mechanism from the truncated “tail” 

portion of the beam around the lens to assist in phase locking techniques. If a 

portion of this area was replaced by lenses using different shapes and designs, a 
21

decrease in retained system power and overall increase in throughput of the system 

could be observed. 

Fig. C-1 Current array (left) vs. experimental array (right)

Another problem presented is keeping the fiber stable so that it could be used on a 

moving vehicle. However, prototyping costs are high for small aluminum parts, 

thus a plastic 3-dimensional (3D) printer can be used for the creation of parts at a 

low cost and timely fashion. 

C-5 Experiment/Calculations

The experiment was set up using a single-fiber laser passed through a collimating 

lens (Fig. C-2). The center of the beam was then truncated using replaceable masks. 

Then, the beam of energy was focused down onto a photo detector.

Fig. C-2 Diagram of experimental setup

With this method, we can measure the percentage of power that would normally be 

truncated by standard circular lenses. The experiment was run over multiple trials 

with multiple sizes of masks to attempt to collect enough data (Fig. C-3). Multiple 

lens shapes in the array were accounted for to allow for the correct calculations to 

be made. 
22

Fig. C-3 Experimental mask shapes for a new aperture

For the 3D printing portion of the project, pieces needed to be designed in a 

computer-aid design (CAD) program and then compiled with the provided 

software. Multiple compilers or “slicing” programs and CAD programs were 

compared in order to determine which set of programs would produce the best 

results. The Makerware and openSCAD software combination was chosen due to 

the fact that it gave the precision tolerances needed. Settings such as heat, speed, 

resolution, and infill also needed exploration. The goal was to create a fully 

fabricated fiber holder piece. 

C-6 Results and Discussion

The lens masks were printed in black and determined to be easiest to cut out with 

scissors. The precision knife turned out to be imprecise when trying to cut curved 

lines, but was useful for the straight edges. The problem of finding the center of the 

mask was solved by printing a tiny cross in the center and scoring it with the knife. 

The center of the cross was then pushed onto the pin in the mask holder to 

effectively center the paper onto the lens. The scored crosses allowed for consistent 

results from our experiment. We also measured the energy before and after 

measuring the masks in order to account for power deviation during the experiment. 

After collecting and analyzing the data from the masks, we found that the truncation 

of the beam followed a predictable Gaussian pattern when using standard circular 

masks (Fig. C-4). Given this and the data from the different shaped masks, we 

should be able to make an accurate decision on the optimal lens pattern to be used

in the future.
23

Fig. C-4 Experimental results of truncated power vs. mask diameter

The 3D printing portion of the project was a great success in nearly all aspects of 

our project. The printer was easy to set up and fully functional shortly after taking 

it out of the box. Methods for effectively removing the object from the build plate 

were quickly developed. The printer software was simple to install and easy to use.

1

However, it was not very clear which printing settings were appropriate for various 

projects. It was discovered that the best way to ensure the object would be relatively 

easy to remove from the build plate would be to select the option to have the object 

print on a “raft” when compiling the build. The raft option would print a small 

platform that was easy to separate from both the build plate and the object. The 

temperature, print speed, resolution, and infill settings were more difficult to 

optimize, but after many attempts the appropriate values for each setting were 

determined. In the end, the default temperature of 230 °C worked the best; however, 

for printing at higher speeds, the temperature needed to be increased. The default 

print speed of 90 mm/s worked best for all pieces in order to ensure quality printing.

The pieces that were to be prototyped needed to be precise. Therefore, the printer’s 

highest resolution setting, 100 μm, was necessitated. The correct infill setting was 

easier to determine since it mostly affects the strength of the object. The smallest 

pieces used a 100% infill setting because the pieces needed to be sturdy. The larger 

pieces only required a 20% infill since they were larger and had slightly thicker 

walls.

 1 MakerBot Replicator 2 Desktop 3D Printer User Manual. MakerBot Industries. 2009-2013.
24

Finding a method to convert 2-dimensional (2D) blueprints into 3D models using a 

CAD program took longer than anticipated and the process could still use 

improvement. The parts were recreated using OpenSCAD, which allowed for the 

precise dimensions needed and the ability to export the file into the correct format 

to be used by the printer software.

Many problems were encountered while trying to print the experimental pieces, the 

most common of which was that the printer would stop extruding plastic while 

printing on high resolution. This problem occurred less frequently once the optimal 

print settings were determined, but when the problem occurred, further action was 

required. The best solution was to unload the filament from the print head, cut a 

fresh edge, and reload it. For a more permanent solution, a little bit of thermal paste 

was applied between the heat sink and extruder head to better facilitate cooling and 

prevent heat creep from building up above the extruder head. After applying the 

thermal paste, the printer is able to print on high settings much more reliably. Every 

50 printing hours, the printer requires maintenance, which involves cleaning the 

drive gear and lubricating the Z-axis pulley and X-axis idler pulley. To clean the 

drive gear, the cooling fan, heat sink, and extruder head were removed and cleaned.

Once the models were created, printer setting determined, and previously described 

problems overcome, the required pieces were consistently produced. Thus if more 

pieces are required for reasons of breakage or testing, they can be easily, quickly, 

and cheaply produced. Figure C-5 shows parts rendered in the software and the 

resulting printed pieces. 

Fig. C-5 3D rendering of parts in Makerbot software next to printed pieces

Along with this research, we helped with fabrication of the Bimorph actuators for 

the fiber positioners (Fig. C-6). The process of cutting out piezoelectric plates with 

a CNC machine gave us experience not only with troubleshooting a legacy 
25

Windows 95 machine, but with the complimentary subtractive process that is the 

opposite of 3D printing. A “homemade” etching process was used to remove the 

insulating layer of the piezoelectric plates in the correct areas as well, saving time 

and money in comparison to professional lithography techniques.

Fig. C-6 In-progress bimorph actuators next to the fiber positioner base

C-7 Summary and Conclusions

As a result of the gathered data, a better lens will be designed for the laser arrays. 

High energy arrays will be much easier to cool and maintain when 97% of the 

energy is being transmitted instead of the 13% absorbed by the unit. This will 

overcome one more problem with scaling these laser arrays up and allow for a lower 

cost and less complex high energy laser transmitter array.

Reflecting back, a lot was achieved from this experiment. Starting with simply 

learning the procedure for activating the laser system, we explored the 

fundamentals of Gaussian beams, became competent with the guide system 

integrated onto the worktable, and explored different methods for printing, cutting, 

and placing the masks that were used. This experiment provides a basis that can be 

reused later and applied to any shape that needs to be tested. This should allow 

further refining of lens shape and array design which could bring us toward better 

laser communication and defense.

The 3D printing provides amazing prototyping ability to the lab as a whole. These 

findings can help others prototype and keep up with this young but quickly 
26

developing technology. From here, we will work on detailed documentation to 

allow for easy use of the printer by staff. Further work could also involve 

experiments on the effectiveness of 3D printing on a 3D printer. This could 

significantly lower acquisition time and cost and allow for better pipelining of 

multi-part designs. Finally, one of the last things that could be accomplished is 

making the schematic to CAD process easier. This is probably the most challenging 

step with a new user, as hard coding the CAD models is necessary for precision. If 

a user-friendly interface could be made, we could have prewritten libraries and 

models with quick drag, drop, select, and modify options for specifying different 

tolerances and designs.
27

List of Symbols, Abbreviations, and Acronyms

2D 2-dimensional 

3D 3-dimensional 

Al aluminum

Al2O3 alumina 

ARL US Army Research Laboratory

CAD computer-aid design 

CISD Computational and Information Sciences Directorate

CQL College-Qualified Leaders 

CSA chirped seed amplification 

CW continuous wave 

DE directed energy 

DEPS Directed Energy Professional Society 

EE electronic engineering 

Er erbium 

F-L Füchtbauer–Ladenburg 

HEL-JTO High Energy Lasers Joint Technology Office 

InGaAs indium gallium arsenide 

IR infrared

MEMS microelectromechanical system 

Nd neodymium

NPs nanoparticles

NRL US Naval Research Laboratory 

O oxygen 

PMT photomultiplier tube 

SBS stimulated Brillouin scattering

SD solution-doping 
28

SEDD Sensors and Electron Device Directorate 

SiC silicon carbide

SiO2 silica 

Ti:sapph titanium sapphire 

UMBC University of Maryland Baltimore County 

VCSEL vertical cavity surface emitting laser 

YAG yttrium aluminum garnet 

Yb ytterbium 
29

1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL CIO LL

IMAL HRA MAIL & RECORDS

MGMT

1 GOVT PRINTG OFC

(PDF) A MALHOTRA

1 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL SEE E

J WHITE

RDRL CIE S

JONY JIANG LIU

1 HIGH ENERGY LASER JOINT TECHNOLOGY OFFICE (HEL-JTO)

(PDF) H ACKERMANN
30

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