Fiber Laser Components Technology Readiness Overview

NASA Electronic Parts and Packaging Program

Melanie Ott, Sigma Research and Engineering,

<Melanie.ott@gsfc.nasa.gov>, 301-286-0127

Parts, Packaging and Assembly Technologies Office Code 562

NASA Goddard Space Flight Center, Greenbelt MD 20771

### Abstract

Presented here is a summary of information concerning the usage of fiber
lasers in a space flight environment. The scientific needs of NASA are
addressed in reference to laser systems and applications and how fiber
lasers fit those needs are summarized. The technology is explained
briefly with focus on harsh environment reliability of the components
that are used in typical fiber laser systems towards the goal of
assessing the technology for space flight missions.

**1. Introduction**

Some of the more revolutionizing commercial technologies whose further
development was spawned during the expansion and success of the
telecommunications market is the fiber laser, fiber amplifier and its
associated components. Fiber laser technology is an emerging technology
primarily comprised of commercial off the shelf components with fusion
spliced optical fiber links between components. The technology utilizes
rare earth doped optical fiber for amplification purposes with pumping
provided by compact diode lasers. Fiber lasers can provide broadband or
narrow linewidth light depending on the system components utilized in
the configuration. A wide variety of configurations exist that cater to
different applications. However, the main components used in contriving
many of these configurations are common among these various systems. A
full understanding of the component structures of typical fiber laser
constructions will allow investigation of these systems from a
components level perspective. Through further investigation of these
components for degradation modes as a result of exposure to harsh
environments, a knowledge base for establishing performance and enabling
the technology for space flight missions will be achieved.

# 1.1 Future and Current Space Flight Needs

The fiber laser is an advanced newly available technology and there is
no space flight certification testing requirements available for
establishing which instruments and components will be suitable for harsh
environments. The NASA Office of the Earth Science Enterprise Code Y
conducted an independent laser assessment following the difficulties
with Vegetation Canopy Lidar and determined that there was a great lack
of characterization and space certification knowledge for lasers in
space flight environments. The Earth Science laser assessment report
also requested that in depth laboratory testing of lasers under space
like conditions be extensively performed. In addition to in depth
testing, other recommendations as a result of this study included:

• NASA should consider identification and intensive development of
critical fundamental technology elements applicable to multiple
missions,

• NASA needs to develop guidelines that define how basic laser
technology development is carried out among centers and private vendors,

• A technology alliance should be formed for the development of
space-based lasers. As a result of this study the Integrated NASA Lidar
Systems Strategy Team (INLSST) was formed with multi-center
participation to provide guidance with enabling Lidar technology for
better risk assessment during proposal process. \[1,2\].

![](media/image1.jpeg){width="6.0in" height="4.5in"}

Figure 1: Earth Science Lidar and Laser Needs

> Due to the large interest in utilizing compact lasers in space flight
> applications there are current leveraging opportunities including work
> being performed by the Integrated NASA Lidar Systems Strategy Team
> (INLSST). Currently, the NASA GSFC segment of this team is developing
> a platform laser technology in order the meet many of the demands
> shown in Figure 1. ESTO has funded a variety of laser based programs
> that can provide insight to some of the component issues. In
> particular the following projects are currently underway and funded by
> ESTO:

-   Laser Sounder Technology for Atmospheric CO2 Measurements from
    Space, GSFC.

-   A compact Highly-Efficient, and Rugged All Solid-State UV Source
    Based on Fiber lasers for UV-DIAL, JHU, Code 562 GSFC.

-   High Efficiency, Double-Pulsed, High Beam Quality, Nd Laser for
    Global Ozone, Fibertek, Inc.

-   Advanced Optical Heterodyne Receiver Development for Coherent
    Doppler Wind Lidar, JPL

-   Efficient, Compact, Conduction Cooled Laser Diode-Pumped YB: YAG
    laser for Atmospheric Composition and Ozone Measurements., Standford
    University.

-   High Efficiency Remote Sensing Laser Technology, Aerospace
    Corporation.

-   Water Vapor DIAL transmitter at .94 Micrometers, LaRC.

-   Efficient, Conductively-Cooled 2-Micron Laser Transmitter for
    Multiple Lidar Applications, LaRC.

All of the above mentioned projects provide leveraging opportunities and
can also benefit from studies performed on any laser components, many of
which are present in fiber lasers as well. The John's Hopkins Applied
Physics Laboratories are currently conducting component level studies
for the INLSST laser platform as well as components for fiber lasers for
remote optical communications applications. The Naval Research
Laboratories Optical Sciences Group has developed a state-of-the-art
fiber laser for a variety of applications including high power, narrow
linewidth, remote sensing usage.

**1.2 Applications of Fiber Laser Technology**

In many ways the available fiber laser systems perform better than
typical space flight lasers currently under development with high pulse
energies, high repetition rates, narrow pulsewidths, better wall plug
efficiencies (20% or higher), diffraction limited output that is
transverse single mode. In addition, the added advantage of smaller size
and lighter weight due to the lack of requirement for an optical bench
for alignment, makes them more attractive for such missions that require
compact sizes as in deep space missions. Fiber lasers can be used in
either continuous wave with external modulation for communications or in
a pulsed capacity for lidar, resolving objects from space or for probing
through dense medium for communications purposes.

There is a great need for fiber lasers for space flight usage for remote
sensing and in optical communications applications. As stated
previously, fiber laser instrumentation is an advanced technology
recently available for applications related to space flight. For remote
sensing applications the fiber laser provides: an ultra stable compact
source for laser vibrometry, a multi-wavelength Q-switched source for
LADAR-based imaging, a dual-wavelength optical transmitter for
Differential Absorption Lidar (DIAL) for atmospheric characterization,
an ultraviolet optical transmitter for fluorescence Lidar for remote
detection of biological agents and a space based pulsed source for
precision altimetry. For optical communications the fiber optic laser
can fit into several configurations including: an optical transmission
system for highly parallel optical data links for high speed computing,
an optical transmission system for wavelength division multiplexed (WDM)
and/or time division multiplexed communications, a WDM transmitter for
very large throughput free space communications links, and as part of an
all optical clock recovery circuit for high speed optical receivers. In
particular, the Laser Communications Demonstration Experiment (LCDE)
scheduled for ISS will demonstrate a high data rate free space
communications link between a ground station and the space station. It
is proposed to utilize fiber amplifier technology as part of the high
speed transmitter.

**2. Status of Available Technology**

The fiber laser market has shifted from focus on the telecommunications
industry in the 1990's to focusing on needs of the medical industry in
the early 2000's to a current focus on military and space lidar and
communications as well as industrial manufacturing. There is currently
more opportunity for space flight instrument development with less
dependence on the telecommunications market wavelengths now that the
focus has changed. With greater focus on alternative markets there will
be more emphasis on providing stability, narrow line widths, short
pulses with high repetition rates to accommodate these market. Higher
power developments mean higher power at doubled and tripled wavelengths
to accommodate more science applications for all enterprises.

One great advantage of fiber lasers is that they are comprised of
commercial off the shelf components and therefore provide easily
manufacturable platform technologies that can be utilized in a variety
of applications. Laser wavelengths can be tailored to a variety of
purposes if enough power output is achieved in the platform laser. A
variety of fiber lasers exist to cater to different applications.
Advances in providing higher powers without damaging optical fiber have
emerged to accomplish this as well as methods to provide stability.

There are a variety of vendors who provide fiber laser components making
many of the common components readily available. Since fiber lasers are
simply comprised of commercial components linked together with fiber
optics and some small bulk optic pieces, construction and
manufacturability is non complicated. For commercial procurements, IPG
and IMRA America seem to be among the more prominent manufacturers of
state of the art fiber lasers currently available. Other manufacturers
such as Spectra Physics, Synchronous Inc, Southwold Enterprises, and
Optigain Inc. are providers for fiber amplifiers that can be used in
fiber laser systems. The next generation of fiber laser technology is
being developed by non commercial research groups such as one at the
Naval Research Laboratory. By groups such as this, many of the road
blocks that kept fiber lasers from achieving higher powers are overcome,
in prototype form and close to being fully manufacturable.

**2.1 Key Components**

In order for this technology to become space flight ready there is a
need to comprise characterization methods and achieve an understanding
of the failure and degradation modes of optical fiber lasers from a
component perspective. Towards that goal several available designs are
being studied to identify the most common components used.

2.1.1 Amplifier Laser Fiber:

Most rare earth doped fiber lasers and amplifiers consist of either
Erbium or Erbium/Ytterbium fiber with fewer consisting of Ytterbium.
Erbium based fiber lasers are more closely related to typical fiber
communications wavelengths in that they emit near around 1550 nm while
Ytterbium type fiber lasers emit near 1060 nm making them more closely
related to Lidar applications or free space communications where
diffraction limited beams are necessary. For applications that require
wavelengths in the UV range either type of fiber laser can be utilized
by frequency conversion using nonlinear crystals.

2.1.2 General Components:

For applications that require shorter wavelengths (visible or UV), the
larger the output power of the amplification stage to the nonlinear
crystals the more likely crystals can be used that not require
additional thermal control for frequency conversion to those
wavelengths. Fiber lasers can also include fiber bragg gratings for
control of narrow linewidths. Other passive optical components are used
in a variety of designs. In many designs, single mode doped laser fiber
is used with double cladding to protect the fiber from optical damage
but also to allow coupling from multimode pump sources. The ability to
couple multimode sources allows for higher pump powers. In more recent
state of the art designs under development, multimode rare earth doped
optical fiber is being utilized to overcome problems with creating
enough gain in the amplification stage in addition to the advantages of
coupling greater power.

2.1.3 Pumping methods

There are several methods of coupling pump laser diodes to the
amplification fiber and a wide variety of pump diodes available at
select wavelengths. In many cases wavelength division multiplexing
couplers are used for pumping laser diodes into the amplification fiber.
Less common are side pumping methods in which part of the fiber is
etched in a manner than allows for pump light to enter the fiber through
the cladding or in such way to allow for the ends of the amplification
fiber to be available to the other components in the system. The pump
diode wavelengths are chosen based on which rare earth doped fiber is
selected for the amplification stage. The idea is to pump with the
highest power, laser diode available for maximum efficiency to the
output power. Pump diodes range from simple laser diode modules to bar
diodes and arrays. Pump laser diode wavelengths are typically near or in
close range to 808 nm, 980 nm, 1064 nm, and 1480 nm.

**2.2 Reliability Concerns for a Space Flight Environment**

Under current study are the failure and degradation modes of fiber laser
components to further the flight readiness for current and future fiber
laser designs. Most systems are designed to function in the thermal
range of 0ºC to 70ºC because they are based on commercially available
components. How much of a harsh environment a system can withstand will
always depend on the most vulnerable component or components present in
the system. Since this study is currently underway, full reliability
assessments that are not currently available, will be available in
future reports.

2.2.1 Vibration and Vacuum Considerations:

It is expected that fiber lasers will be very robust against vibration
since most of the components are linked together with fiber fusion
splices. However, different coupling methods require some bulk optic
parts and additional study of the packaging methods for the bulk optical
component alignment will be necessary. In most cases there are very few
parts that are coupled with bulk optics and are usually limited to the
initial pump coupling if a side pumped method is used and the coupling
to and from the nonlinear crystals if wavelength conversion is
necessary. To assure stability in a vacuum environment additional study
is require for materials analysis on key components.

2.2.2 Thermal Considerations:

Thermal transients in a vacuum environment may effect the performance
and stability of components such as bragg gratings, and the wavelengths
of the absorption and emission band of the amplifier fiber, for example.
For gratings the effects can be limited by thermally annealing the
components during manufacturing. The changes in the spectrum of the
amplifier fiber can be characterized to see how much of an effect will
be transferred to the system performance. The double clad fiber has an
external polymer cladding over a silica material causing a CTE mismatch
and this too will be affected during thermal transients. In spite of the
fact that thermal transients may have an effect on the components, it is
expected that these effects will produce minor performance changes.

2.2.3 Radiation Considerations:

In general, when Erbium fiber laser amplifiers were tested as a system
in a radiation environment along with other passive components, the
results showed that the majority of the damage was a result of color
center formation in the Erbium fiber. The pump power as well as the
amplified signal were absorbed.\[6\] In other studies, Erbium and
Erbium/Ytterbium fiber were tested in a radiation environment but far
less data exists on Ytterbium fiber. In several cases of study, models
that are typically used for telecommunications optical fiber were
applied to extrapolate the high dose rate exposure data for the rare
earth doped results to a lower dose space flight environment. It was
concluded that when this modeling was applied that the extrapolation
results over estimated the expected absorption based losses and that
other models were necessary. Some of this has been documented.\[7-8\]
Another assumption applied to the testing and modeling of radiation
effects on rare earth doped fiber is that as with telecommunications
optical fiber, gamma radiation should provide a worse case scenario than
proton exposure. It may be the case that this assumption is also
incorrect and further study is necessary.\[8\]. It is however, true of
rare earth doped fiber as it is with telecommunications grade fiber,
that the dopants do have an enormous effect on the radiation induced
performance. There are currently a wide variety of available optical
fiber choices for usage in fiber amplifiers. Due to the vast number of
choices available, it will be necessary to study the effects of the most
common types of fiber used since the dopants used among these different
products can also contain wide variations. Different Ytterbium fiber
products in particular were shown to behave differently in a radiation
environment based on the different dopants contained in each.\[9\]

Double clad rare earth fiber, which makes the injection of higher pump
powers possible, have outer claddings comprised of fluorine doped
polymers with low optical index. Polymers are typically more sensitive
to radiation in general than silica optical materials and fluorine doped
fiber is typically more sensitive to radiation effects than fiber not
containing fluorine. The fact that polymers are used for this outer
cladding will also make them more prone to embrittlement as a result of
radiation exposure.

Pump laser sources will have to be characterized for proton induced
displacement damage but in general are considered quite robust against
typical environments. More research is necessary to assess the other
components of the fiber laser system and will be available in future
reports.

## 3. Conclusion

At present there are no qualified fiber lasers for usage as a platform
technology for space flight missions. The studies currently underway
will enable space flight usage of fiber laser technology within the next
five years. Advanced designs currently underway will also enable more
reliable performance in that they do address thermal and vibration
issues as a matter of component and packaging selection.

In future reports, more information will be provided on what type of
fiber lasers are currently available based on key parameters such as
wavelengths, linewidths, power output for continuous wave and energy
outputs for pulsed systems, pulse repetition rates and other critical
specifications. The designs and components associated with these
different systems will be outlined as well.

**4. References**

1.  Upendra N. Singh, *Lidar Technology Development Strategy and Plans*,
    presentation, NASA LaRC, October 25, 2001.

2.  G.R. Asrar, S.L. Venneri, J. F. Creedon, A. V. Diaz, W. S.
    Heaps, U. N. Singh, *Integrated NASA Lidar Systems Strategy Team
    Report*, presentation to Dan Goldin, June 18, 2001.

3.  M. Kavaya, U.N. Singh, F. Amzajerdian, "A New NASA Technology
    Program for Risk Reduction of Space-Based Lidar Missions,"
    presentation to the Working Group on Space Based Lidar Winds, Jan 02
    Meeting.

4.  J. Abate, J. Simpson, D.L. Brownlow, D. Digiovanni, S. Kannan, M.
    Luvalle, P. Lemaire, A. Stentz, " Reliability Concerns for Double
    Clad Fiber Lasers for Space Based Laser Communications," IEEE
    Military Communications Conference Proceedings, MILCOM 97 Vol.2,
    pages 936-942.

5.  J. Koplow, D.A.V.Kilner, L. Goldberg, "Single-mode operation of a
    coiled multimode fiber amplifier," OSA Optics Letters, Vol. 25, No.
    7, April 1, 2000, pages 442-444.

6.  E.W. Taylor et al. "Gamma-ray induced effects in Erbium doped fiber
    optic amplifiers," SPIE Vol. 3440, Photonics for Space Environments
    VI, July 1998, pages 16-23.

7.  G.M. Williams, B.Wright, W.D.Mack, E.J.Friebele, "Projecting the
    Performance of Erbium-Doped Fiber Devices in a Space Radiation
    Environment," SPIE Vol. 3848, Optical Fiber Reliability and Testing,
    1999, pages 271-280.

8.  G.M. Williams, E.J. Friebele, "Space Radiation Effects on
    Erbium-Doped Fiber Devices: Sources, Amplifiers and Passive
    Measurements," IEEE Transactions on Nuclear Science Vol. 45, No.3,
    June 1998, pages 1531-1536.

9.  H.Henschel, O. Kohn, H.U.Schmidt, "Radiation-induced loss of Rare
    Earth doped silica fibres," IEEE Fourth European Conference on
    Radiation and Its Effects on Components and Systems, RADECS, 1997,
    pages 439-444.