Institute of

Technical Physics

Stuttgart

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The Institute of Technical Physics works in

selected fields of optics and photonics. In

particular it is engaged in the investigation

and development of new laser sources and

the application of laser radiation in coher-

ently coupled and adaptive optical systems.

The research work and projects are here

essentially directed towards the mainte-

nance of the defense capability and security

of the Federal Republic of Germany. The

main activities can be divided into three

branches:

- Development and investigation of a new

generation of solid-state lasers with high

brightness, i.e. high output power with

good beam quality. The work includes

both diode pumped solid-state lasers and

also fiber lasers, and concentrates on the

one hand on fixed frequency and variable

frequency lasers of high average output

power and on the other hand on the co-

herent coupling of fiber lasers, i.e. fiber

Modeling of the propagation of ultra-short

laser pulses. The graphic shows the multi

filamentation of a femtosecond laser beam.

laser arrays. While with the fixed frequen-

cy lasers the focus is on the implementation

of new design concepts with high bright-

ness (e.g. thin disk lasers), the tunable fre-

quency laser research is devoted to pulsed

high energy sources in the mid infrared.

Here the main area of application is in the

investigation of directed optical counter-

measures for the protection of aircrafts.

In addition the work also serves as a basis

for many LIDAR applications in the field of

environmental measurement technology.

Due to the physically limitation of output

power of single fiber lasers, the coupling

of a large number of individual emitters

provides the only possible way towards an

ongoing power scaling. Preliminary experi-

ments demonstrate the potential for effi-

cient and compact systems with high inte-

gration capability in the near future.

- In the investigation and development of

high energy lasers the Chemical Oxygen

Iodine Laser (COIL) assumes a special role.

With its excellent scaling properties, high

efficiency, and good beam quality, it of-

fers unprecedented advantages for high

power, high brightness applications. It is

therefore regarded as a candidate for a

laser-based air defense system. Its 1.3 µm

wavelength provides excellent transmis-

sion through the atmosphere and optimal

laser-effects on relevant targets. At

present the Institute operates the most

powerful laser of this kind in Europe.

- Active/adaptive optical systems represent

a seminal area of application for coherent

radiation. Particular importance is attri-

buted to the development and provision

of an active high-resolution imaging sys-

tem. The objective is to restore to a large

extent the quality of an image that has

been degraded by dispersive media. Here

a wide area of application is being ad-

dressed for both military and civilian tasks.

Finally an automatic pointing and tracking

method is also being investigated and

developed with an accuracy not previ-

ously achievable, using adaptive optical

methods.

The use of optical components is in many

cases limited by the damage or destruc-

3

tion they receive as a result of radiation.

For this reason the Institute operates a

laser damage laboratory to investigate

and characterize damage thresholds and

the related processes. Here the lasers

that are used have pulse lengths from

nanoseconds down to femtoseconds.

The ultra-short pulse lasers are opening

up completely new fields of laser inter-

action and propagation.

The aim of the research work is to secure

a technological base for the solution of

problems in the fields cited. Most of the

work is integrated into European or trans-

atlantic collaborative projects, ensuring

good international networking. The appli-

cation of the results in products and pro-

cesses with partners from science and in-

dustry takes place in a systematic manner

by means of technology transfer projects

that are reflected in the third-party fund-

ing received by the Institute. Particular

emphasis should be given to the Institute’s

successful participation in the European

Security Research Programme with regard

to the protection of civil aircraft against

terrorist attacks.

Highlights from the current research ac-

tivities are described in more detail in the

following accounts.

Solid-state lasers and

non-linear optics

Optical Parametric Generator with a periodi-

cally poled Lithium Niobate crystal (PPLN)

covering the mid infrared spectral range. The

bright colors result from higher order harmonics

of the nonlinear process. The IR radiation is

rendered visible using a thermal foil (center

screen).The most important research themes

concerning solid-state lasers and fiber

lasers are concentrating on the one

hand on fixed frequency and variable

frequency lasers of high average out-

put power, and on the other hand on

the coherent coupling of fiber lasers,

i.e. fiber laser arrays. In the case of

fixed frequency lasers the objective is

to investigate new concepts (e.g. thin

disk laser) with high output power and

good beam quality. With the variable

frequency lasers the mid-infrared range,

in particular between 3 and 5 µm, is

being opened up to high output pow-

ers with the aid of optical parametric

processes. Applications in environment-

al measurement technology also require

high frequency stability. On account of

the inherent limitation of the output

power of individual fiber lasers a scal-

ing up of output power can only be

achieved by coupling of a large num-

ber of individual emitters. To this end

various methods of phase control and

coupling are being investigated.

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Directed optical countermeasures

The increasing threat to aircrafts from

ground-to-air missiles (e.g. ManPADS, Man

Portable Air Defense Systems) demands

innovative measures to improve survivabili-

ty and to provide protection. This is all the

more true since the threat from the future

generation of IR seeker heads will not for

the most part be fought off by traditional

countermeasures such as flares. In particular

the “Directed Optical Countermeasure” in

the mid-infrared spectral range or DIRCM

(Directed Infrared Countermeasure) is be-

coming ever more important.

With the directed optical countermeasure

the heat-seeking head of a homing missile

is disrupted either reversibly (causing glare)

or irreversibly (causing damage) by means

of incident laser radiation that is spectrally

matched.

Besides a compact integrability and high

efficiency, the generation of high average

output power in a wide frequency range

is an essential requirement.

Since no direct laser sources of sufficient

output power in the mid-infrared are avail-

able up to the present time, the optical

parametric oscillator (OPO) or generator

(OPG) is the method of choice for the in-

frared region between 3 µm and 5 µm.

Here the laser radiation of fixed freuency

solid-state lasers (pump lasers), radiating

at a wavelength of 1 µm or 2 µm, is con-

verted into two new variable frequency

laser waves by means of non-linear pro-

cesses in optical materials (crystals). This

generates the so-called signal and idler ra-

diation. The idler radiation covers the rele-

vant wavelength region in the mid-infrared.

Alternative OPO and OPG concepts are

being evaluated in the laboratory, and are

being optimized and assessed with regard

to laser output power, beam quality, emis-

sion wavelength and efficiency. This work

includes intensive research into the non-

linear crystals that are available and their

optical characteristics; in particular an

exact determination of damage limits is

indispensable, on account of the high peak

pulse output powers that occur.

Another important factor is the availabil-

ity of the fixed frequency pump lasers,

which cannot be obtained commercially

with the required properties. These pump

lasers are being specified in collaboration

with laser design and development com-

panies, and usually custom-built.

Since the DIRCM system is primarily envis-

aged for use on airborne platforms, in ad-

dition to the actual generation of the laser

radiation its propagation through the flow

field generated by the aircraft is also of

particular importance. For this reason the

degradation of the beam quality during

propagation through an appropriately

simulated flow is being investigated in the

laboratory. Thus a total system evaluation

of the DIRCM system is made possible for

the first time.

As a supporting activity an extended com-

puter simulation has been developed that

models the structure and propagation of

the radiation field as well as the resultant

beam quality, so that a better design of ex-

periments can be anticipated.

Modeling of the propagation of the pump

(above) and idler radiation (below) of an op-

tical parametric oscillator.

5

High Energy LasersChemical Oxygen Iodine Laser.

The High Energy Lasers/COIL branch

concentrates on the investigation and

development of a Chemical Oxygen

Iodine Laser (COIL). Due to its excel-

lent scaling properties, high efficiency,

and short wavelength at 1.315 µm, it

offers unprecedented advantages for

high power, high brightness applica-

tions. Therefore, it is currently con-

sidered to be one of the most promis-

ing candidates for a laser-based air de-

fense system. The Institute operates an

unique large facility in order to vali-

date various power scaling concepts

for future military implementation. In

addition, the potential of the chemical

oxygen iodine laser is being evaluated

in field trials.

For the interdisciplinary COIL projects

a 10 kW COIL laser facility, a 1 kW COIL

test bed to accommodate innovative

components, as well as a separate test

bench to validate oxygen generators

of the 10 kW class are operated at the

Lampoldshausen test site. An open air

transmission range is under construc-

tion.

The COIL research and development

activities are focussed on optimizing

total efficiency and increasing the

brightness of the laser radiation. There-

fore, compact resonator architectures

are being developed that allow the ex-

traction of high laser output power at

excellent beam quality of the emitted

radiation. In addition the transmission

properties of the laser beam are being

tested at different atmospheric condi-

6

tions, as well as the effect of COIL

radiation on particular materials and

complex structures.

Further research is focussed on the in-

vestigation of efficient methods for the

generation of excited oxygen, which

is the energy source in the laser process

and on the development of sophisti-

cated solutions for new types of pres-

sure recovery systems. The technology

for the preparation and storage of the

“laser propellant” BHP is also being

continuously developed further.

Resonators for high brightness laser

radiation

Laser radiation of high brightness requires

high laser output power and high beam

quality at the same time. The brightness

qualifies the effectiveness of the laser beam.

A high value allows the laser output power

to be focused on a minimal area, even at

a large distance from the source of the

beam.

In addition to the achievement of high

brightness the development of resonators

requires attention to be paid to mechan-

ical stability and compactness of the reso-

nator structure. Only with this combina-

tion the system is fully operational, includ-

ing deployment under the more difficult

operating conditions outside the labora-

tory (field conditions).

Because of its low amplification coefficient

and the rectangular geometry of the ac-

tive medium of a cross-flow laser, the

chemical oxygen iodine laser imposes spe-

cific requirements on the resonator that

cannot be fulfilled with conventional reso-

nator geometries. For the COIL either hy-

brid resonators that couple a stable with

an unstable resonator part or totally un-

stable resonators with off-axis geometries

are to be preferred. At the same time the

requirement for ruggedness of the total

system leads to simple resonator concepts

with a small number of optical components.

These must work reliably, while the adjust-

ment procedure for such a system must

be quick, simple and effective.

The design of the resonators is carried out

using numerical methods. The boundary

conditions for the calculation models are

based on comprehensive investigations

concerning the amplification and power

extraction characteristics of the chemical

oxygen iodine laser. In addition to the ac-

tual resonator design, this close intermesh-

ing of theory and experiment allows re-

liable statements to be made concerning

absolutely vital manufacturing tolerances

for the mirror geometries and resonator

adjustment. Only the overall result enables

a decision concerning the technical imple-

mentation of a particular type of resona-

tor. This procedure provides a quick and

cost-effective instrument for the analysis

and evaluation of new resonator concepts.

Following the theoretical studies, promis-

ing systems are tested on the 10 kW fa-

cility. Among the hybrid systems it has al-

ready been possible to achieve a near-

diffraction-limited laser radiation within

the NBHR concept (NBHR: Negative-Branch

Hybrid Resonator).

Further investigations concentrate on a

modified unstable resonator (MNBUR: Mod-

ified Negative-Branch Unstable Resonator),

developed in-house and patented, and

also on folded resonator design concepts.

The results from the individual investiga-

tions can reliably be scaled up to COIL sys-

tems in the higher output power classes.

With higher amplification in the system en-

hanced beam qualities are achieved with

the resonators under discussion. The bright-

ness increases overproportionately with

the output power class.

Hybrid resonator and measured intensity pro-

file of the resulting laser beam in the far field.

7

Active Optical

Systems

The active optical systems (AOS) busi-

ness sector is engaged in the develop-

ment of adaptive optical methods as a

basis for active high-resolution imaging

systems and highly accurate automatic

pointing and tracking systems. Here

the primary concern is with turbulence

compensation by the monitoring and

control of the phase fronts of laser

beams.

New possibilities for laser propagation

and application ensue from the use of

ultrashort pulse lasers. Because of the

extremely high peak intensities non-

linear propagation phenomena occur

that result in laser beam filamentation

and the generation of white light.

Laser induced damage

Work is focused on the vulnerability and

protection of optronic components from

laser radiation. To carry out these investi-

gations the AOS business sector operates

a test facility for the analysis of laserin-

duced damage in the infrared, visible and

ultraviolet spectral regions. Existing exper-

tise in the study of radiation damage has

been expanded to include the testing of

optics under vacuum conditions. The mo-

tivation for this work is the use of laser sys-

tems in space planned by European com-

panies and the European Space Agency

ESA. The long-term operation of laser sys-

tems under vacuum conditions with mission

durations of several years places extreme

requirements on the precision and reliabil-

ity of the optics used.

The verification and validation of the optics

in the periods leading up to missions is cor-

respondingly complex and time-consuming.

The objectives of these missions include in

particular surveys of planetary surfaces

and determination of the concentration

of atmospheric gases and their circulation

characteristics.

The long-term operation of laser systems

under vacuum conditions contains risks and

poses questions that can only be answered

in an interdisciplinary manner. The optics

used - and in particular their coatings -

must be designed for the particular laser

energy density to be applied. Under vac-

uum certain optics display a degradation

(the so-called air-vacuum effect). More-

over, long-term exposure leads to a form

of optical fatigue which leads to a reduced

laser damage threshold.

One important factor is the contamination

of the optics. Under the vacuum condi-

tions prevailing in the satellite outgasing of

the volatile components of adhesive com-

pounds and plastics is unavoidable. When

exposed to laser radiation these substances,

which are present at extremely low partial

presures, can decompose and accrete on

the optics in a chemically modified form.

The layers that are thus created can mass-

ively increase the surface absorption and

in the course of time can lead to partial or

total functional failure.

The verification and validation of the op-

tics requires on one hand the sensitive

detection of deposits on optical surfaces,

Modular UHV chamber for long-term tests of

laser optics.

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and on the other hand the detection of

optical damage occurring during irradi-

ation by a laser in a vacuum chamber. For

this purpose it was necessary to develop

new methods of detection or to improve

existing methods. For the sensitive detec-

tion of the smallest amounts of deposits

the fluorescence imaging measurement

method is used. It has been shown that

deposits created originally from aliphatic

or aromatic hydrocarbons with thickness

layers of only a few nanometers could be

detected with this method on-line and in

situ. The transient pressure sensor is a new

patented method with which the removal

of even the smallest amounts of material

from the surface of the optics can be de-

tected. The standard method for the detec-

tion of optical damage is the scattered

light monitor, with which an increase of

the scattered light from the test piece is

assessed as an indicator of damage.

Recently, a better understanding of the

processes which are responsible for the

degradation of laser optics under long term

operation in vacuum has been achieved. It

has been shown that porous optical layers

exhibit severe degradation when being ex-

posed to vacuum. In the case of compact

layers no reduction in damage resistance

caused by a vacuum has been detected.

In the contamination tests adhesives that

emit aromatic hydro-carbons are proving

to be particularly susceptible to the for-

mation of deposits. The speed of growth

of the deposits could be reduced by the

addition of water or oxygen at a low par-

tial pressure. Questions still to be answered

concern in particular the interaction be-

tween laser radiation and organic sub-

stances and the selective formation of

deposits on the surface irradiated by the

laser.

Nomarski interference contrast micrographs of

irradiated optics: delamination of an antire-

flection coating at 355 nm in vacuum (above);

carbon deposit on laser optics as a result of

laser-induced contamination at 1064 nm (below).

9

View into the combustion chamber of the Laser

Lightcraft. The spherical plasma can clearly be

seen.

Studies and Concepts

The Studies and Concepts group con-

ducts research on the following topics:

Experimental and theoretical investi-

gation of specific application areas in

fields in which basic laser research has

already been carried out; conceptual

design studies for future laser systems

and their application; system technol-

ogy aspects and inter-system issues re-

lated to the implementation of laser

devices. Those efforts supplement the

other activities of the Institute.

In addition to providing an intensive

scientific and technical consultancy

service to government authorities and

military users, current tasks include the

Laser Lightcraft experimental study.

Laser propulsion

The “Laser Lightcraft” is an innovative

propulsion concept for very small satellites.

The pulsed laser propulsion opens up a cost-

effective alternative for the transfer of

small payloads (1 to 10 kg) to high altitudes

and low Earth orbits.

The transmission of the propulsion energy

for a space vehicle from the ground into

space with the aid of a laser beam pro-

vides the possibility of a significant improve-

ment in the economics of space flight. The

transportation of an energy source becomes

unnecessary, and the typical restrictions of

a propellant based on chemical energy fall

away. As a result the payload proportion

can be substantially increased, leading to

a dramatic reduction in transport costs.

The principle of the laser propulsion is

based on a repetitively pulsed propulsion

mechanism. A combustion chamber in the

form of a parabolic mirror concentrates

the incoming laser radiation energy at its

focus. As a result a high-density plasma is

formed from the propellant, e.g. air, and

this rapidly expands. A high pressure, high

temperature spherical shockwave is created

that sets the gas (propellant) in motion.

When the wave hits the combustion cham-

ber wall the compressed gas propels the

missile forward, until the gas pressure falls

to ambient pressure. Additional thrust is

generated by the gas flowing out of the

combustion chamber (through conserva-

tion of momentum).

The pulsed, electron beam sustained,

multi-spectral laser developed at the Insti-

tute of Technical Physics is used as the

laser source; it features high pulse energies

with excellent pulse reproducibility. With

CO2 as the laser gas it has demonstrated

pulse energies of more than 450 J at a

wavelength of 10.6 µm and a repetition

rate of up to 100 Hz. The use of stable

and unstable resonators geometries allows

for different beam parameters and beam

divergences.

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Launch of the Lightcraft in the laboratory

(long-term exposure).

The work on the Laser Lightcraft includes

the demonstration of a “Lightcraft” launch

in the laboratory, as well as single pulse

experiments on a pendulum test rig to de-

termine the impulse coupling coefficient

at atmospheric pressure and also at re-

duced pressure (encountered at high alti-

tude) through to vacuum conditions. Addi-

tional propellant can increase performance

during a flight through the atmosphere. It

is essential for flights at high altitudes and

in the vacuum of space. First experiments

with Delrin, a plastic that burns without

generating soot, have been undertaken.

On the basis of the experimental results it

is possible to derive the requirements for a

Laser Lightcraft system in each of the re-

gions cited above. There is a requirement

for a pulsed laser source with an average

output power of 1 MW per kilogram of

launch mass. The repetition rate should lie

between 10 and 100 Hz. Since the laser

radiation must exhibit high transmission

through the atmosphere the choice of

wavelength and thus of the type of laser

is severely restricted. Sufficient beam quali-

ty for the transmission of the energy over

large distances is essential and directly

linked with the design of the beam guid-

ance system. An adaptive telescope of

appropriate size (several meters) with an

active tracker and turbulence compensa-

tion ensures the reliable transmission of

the laser energy into the combustion

chamber of the missile up to a distance of

1000 km through the atmosphere. The

missile itself should exhibit an aerodynamic

shape to minimize air resistance in the at-

mosphere. A compressor in the front part

of the structure can improve the fullness

of the combustion chamber. The surfaces

serving to focus the laser beam (on the in-

ner side of the combustion chamber) should

be designed to be highly reflecting to mini-

mize losses (from heat input). Furthermore

a flight and attitude control is required to

stabilize the missile on the laser beam

(“beam riding”) as well as to incline the

Lightcraft relative to the laser beam e.g.

when injecting into an earth orbit.

Future work will concentrate on experi-

ments with solid propellants and studies

to analyze optimal flight path parameters.

From this it will then be possible to derive

a complete flight system including flight

and attitude control.

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Contacts for

more information

German Aerospace Center (DLR)

Institute of Technical Physics

Pfaffenwaldring 38-40

70569 Stuttgart /Germany

Telephone: +49 (0)711 6862-773

Telefax: +49 (0)711 6862-788

E-mail: itp@dlr.de

http://www.dlr.de/TP

Institute Director:

PD Dr. Adolf Giesen

Telephone: +49 (0)711 6862-302

E-mail: Adolf.Giesen@dlr.de

Public Relations:

Dr. Hans-Albert Eckel

Telephone: +49 (0)711 6862-714

E-mail: Hans-Albert.Eckel@dlr.de

Acquisition & Support:

Dr. Wolfram Wittwer

Telephone: +49 (0)711 6862-774

E-mail: Wolfram.Wittwer@dlr.de

Scientists responsible:

Solid-state Lasers and Non-linear Optics

PD Dr. Adolf Giesen

Telephone: +49 (0)711 6862-302

E-mail: Adolf.Giesen@dlr.de

High Energy Lasers

Dr. Jürgen Handke

Telephone: +49 (0)6298 28-230

E-mail: Juergen.Handke@dlr.de

Active Optical Systems

Dipl.-Phys. ETH Wolfgang Riede

Telephone: +49 (0)711 6862-515

E-mail: Wolfgang.Riede@dlr.de

Studies and Concepts

Dr. Hans-Albert Eckel

Telephone: +49 (0)711 6862-714

E-mail: Hans-Albert.Eckel@dlr.de

DLR at a glance

Institute of Technical Physics

Pfaffenwaldring 38-40

70569 Stuttgart /Germany

Telephone: +49 (0)711 6862-773

Telefax: +49 (0)711 6862-788

www.DLR.deInsti

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DLR is Germany´s national research center for aeronautics and space.

Its extensive research and development work in Aeronautics, Space,

Transportation and Energy is integrated into national and interna-

tional cooperative ventures. As Germany´s space agency, DLR has

been given responsibility for the forward planning and the implemen-

tation of the German space program by the German federal govern-

ment as well as for the international representation of German in-

terests. Furthermore, Germany’s largest project-management agency

is also part of DLR.

Approximately 5,300 people are employed in DLR´s 28 institutes and

facilities at eight locations in Germany: Koeln-Porz (headquarters),

Berlin-Adlershof, Bonn-Oberkassel, Braunschweig, Goettingen,

Lampoldshausen, Oberpfaffenhofen, and Stuttgart. DLR also oper-

ates offices in Brussels, Paris, and Washington, D.C.

DLR's mission comprises the exploration of the Earth and the Solar

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ally-compatible technologies, and for promoting mobility, communi-

cation, and security. DLR's research portfolio ranges from basic re-

search to innovative applications and products of tomorrow. In that

way DLR contributes the scientific and technical know-how that it

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