aladin txa qualification and validation...• plh was the last item to undergo qualification tests,...

ALADIN TxA – Qualification and Validation

Aeolus Cal-Val Meeting

Frascati, 10 February 2015

ALADIN TxA Qualification and Validation Contents

1. ALADIN TxA qualification overview

2. Laser Diode Stack Qualification

3. Optics Coatings Qualification

4. Laser Induced Contamination

5. Zero-gravity Tests

6. EMC

7. Initial/final Tests

8. Vibration Tests

9. Thermal Vacuum Tests

10. Burn-In Tests

11. Next steps

12. Summary & Conclusions

2 © Copyright Selex ES. All rights reserved

ALADIN TxA Qualification and Validation

ALADIN TxA is an arrangement of

• The Power Laser Head (PLH), the actual generator of UV photons,

• The Reference Laser Head (RLH), a IR frequency stabilized seed laser to

determine the wavelength of the UV

photons, and

• The Transmitter Laser Electronics (TLE), a complex electronics box, plus

• Harnesses connecting these units, including a single-mode optical fiber to

get the output from RLH to PLH

Introduction

3

• RLH and TLE have been qualified on unit-level. Although challenging themselves, they

have good design heritage and qualification activities were successfully completed.

• PLH was the last item to undergo qualification tests, and as it is a completely new unit

without any heritage the risk is highest.

This presentation concentrates on the issues concerning PLH qualification, including the

special issues concerning high energy Laser Diode Stacks and Optical Coatings.

© Copyright Selex ES. All rights reserved


Laser Diode Stacks are used to pump

the Nd:YAG crystals of the Master

Oscillator (2 Stacks) and the Amplifiers

(8 Stacks each). Thus a total of 18

Laser Diode Stacks are in each laser

head.

Each Laser Diode Stack comprises 12

laser diode bars with 70 emitters each,

giving 15120 emitters per PLH.

Each laser diode bar is rated for 100

W output power, but the stacks have

been de-rated to 840 W output power

(over 200µs pulses at 50 Hz).

At the beginning of the TxA project,

laser diode stacks had just advanced

sufficiently to make a 3 year lifetime

possible.

Laser Diode Stack Qualification (1)

4

Qualifying the Laser Diode Stacks was more difficult than anticipated, but finally

succeeded.



Laser Diode Stack Qualification (2)

After initial testing of various different commercial pump diodes, a manufacturer was selected (Quantel Laser Diodes, France) and Flight Model manufacturing initiated.

Extended burn-in was performed to select best diode stacks for flight; lifetime testing has been performed over 16400 hrs.

The degradation slope is compatible with the 39 month lifetime if energy is readjusted in flight.


ALADIN TxA Qualification and Validation Optical Coatings Qualification

6

Laser Induced Damage (LID) of coatings is a well

known problem in all high-energy lasers.

At the beginning of the TxA project, the standard of

Coating Qualifications was based on S-on-1 tests (as

defined by ISO-11254-2 from 2001 – see next page),

with S (number of shots) less than 1000.

During the project the number of shots had to be

increased to 10000 to assess residual degradation

slopes.

All LID testing needed to be performed in vacuum, as

many coating show a much lower damage threshold in

vacuum than in air.


All ALADIN high power laser optics have been screened using the standard S-on-1

according test ISO Standard 21 254-2. DLR (Stuttgart) have performed >200 tests

in the same facility

• Test consists of a grid of test sites

• 150-200 sites per test; d>x3 beam radius

• The shot number, N, where damage occurs

for a given fluence, on a given site is

recorded

For a given number of shots, N, we can construct a

probability of damage versus fluence. We then accumulate

this data to give the characteristic damage curve.


Optical Coatings Qualification – S on 1 procedure


LIDT Extrapolation to EOL

For ALADIN high laser power optics test the screening goes up to 10kshots. This has to be

extrapolated to the EOL i.e. 5Gshots! The method described below was agreed with

independent laser experts.

• Take the DLR (Stuttgart) data set and apply temporal scaling factor: 𝑆. 𝐹.=𝜏𝑃𝐿𝐻

𝜏𝐷𝐿𝑅

0.35

• Apply power fitting function 𝐴. 𝑥𝐵to the data for N≥100

• Extrapolate this curve to EOL and apply a safety factor of x2

Requirement

EOL extrapolation with

power law, pulse scaling

and safety factor

EOL extrapolation with

power law, pulse scaling

DLR raw data from 10k-

on-1 test

Power law extrapolation of DLR

data for N>100



Laser raster scanning

After the damage events which occurred during the endurance test of the flight laser in 2011,

the coating technology was changed (to eliminate damage precursors), the fluence in the UV

section was lowered by a factor of x2, and a new laser raster scan screening method was

introduced for the flight optics.

Area covered by S-on-1 test (1-3%)

Damage precursor density from

HR mirror from the 2011

endurance test obtained from

ToF-SIMS chemical maps of the

damage precursors (

Results for the first flight laser

o First flight laser has now been

delivered to Airbus-DS in

Toulouse after successfully

executing 240Mshots at full

energy.

o The laser has been used to test

the rest of the emission path

optics and as of today has

undergone 450Mshots and is still

in good health.


UV beam @ THG

Inspection after 450 Mshots: no

damages have been identified


Laser Induced Contamination (LIC)

UV Energy decay in early vacuum test, indicated in bursts (600 pulses). IR energy remained constant.

Operation in low pressures of Oxygen (0.2 mbar) is sufficient to keep UV optics clean and maintain UV output.

Outgassing of organic

materials is known to generate

absorbing layers on coatings,

in particular in vacuum

operation and with UV

illumination.

Careful selection of materials

and stringent cleanliness

control are essential

prerequisites for a long laser

lifetime, but not sufficient.

Addition of low levels (20-60

Pa) of pure Oxygen is

necessary to remove

darkening layers from UV

optics.

Validation of the Oxygen

cleaning has been successfully

performed in the various Burn-

In Tests.



ALADIN TxA Qualification and Validation Sequence of TxA-level qualification tests

12

Example for a test flow of the TxA

Qualification (in this case for FM-B)

Key meetings are indicated:

• TRR = Test Readiness Review

• PTR = Post Test Review

• TRB = Test Result Review

TxA is operated in Initial Test and

Final Test in air,

Thermo Vacuum Test is a 9 day

sequence operating at various

thermal environments expected in

flight, including a large temperature

variation from -20°C to +35°C (laser

off) to show insensitivity to thermal

variations

Burn-In Test is a 5 week continuous

operation in the vacuum chamber

(O2@55Pa) to investigate longer

term effects



To assess the effect of zero-gravity on the alignment of the PLH, the laser is operated in

two different orientation (thus generating ±1g – 0g is difficult to obtain on-ground).

This test had been performed on FM-A using a special test mount, but on FM-B it has

been performed during the final steps of the integration of the PLH.

Zero-Gravity Test

13

Results show a small motion of the Master Oscillator (MO) beam position, which is well

within the tolerances for the amplifiers. © Copyright Selex ES. All rights reserved


EMC (conducted and radiated) tests have been performed at RLH and TLE level (active

units)

Radiated EMC test @ TXA level has confirmed the results obtained at RLH and TLE

level

EMC at TXA level



Initial and final tests have been performed in air. The measurements are carried out

simultaneously on UV (main beam) and on IR (two test points at MO and amplifier

outputs)

Initial /final test setup

15

UV

Line of Sight, Near Field, Far Field, Energy, Fluence

Pulse Duration, Wavelength, Polarization

IR (amplifier output)

Near field, Far field, Energy, Pulse Duration,

Frequency stability

IR (Master oscillator output)

Near field, Energy, Pulse Duration



The vibration test is performed on a shaker simulating the maximum mechanical

loads acting on the PLH. Test is performed in all three axes, requiring re-mounting

between the tests. Laser is operated before and after the test sequence.

Also tested before and after vibration test is the leak rate of the laser housing

(critical parameter for the low-pressure Oxygen system).

No variation of optical parameters has been observed.

Mechanical Qualification Tests

16

PLH on the one-axis

shaker with extra

accelerometers mounted

to monitor the response to

the input vibrations



PLH in the vacuum chamber:

PLH itself on the baseplate (right),

PLH with thermal hardware (right below), and

OGSE in front of vacuum chamber (below)

Thermal Vacuum/ Burn in Test – Set-up



Thermal Cycles (scale -25°C to 60°C). Internal and external temperatures over the full

9 day period, starting 29 May 2014

Thermal Vacuum/ Burn in test

18

Thermal Vacuum Test, example of FM-B: the temperature of the Base Plate is varied

between +35°C and -5°C, while the temperatures within the PLH (characterized by

LOB=Lower Optical Bench and MOB=Master Oscillator Bench) show changes below

±0.25°C. NOP between +50°C and -22°C


Internal PLH

temperatures

Interface

temperatures


UV-Energy for complete Initial Test / FM-B TV / Burn-In Test from 21 May to 3 July 2014 (6

weeks). Test phases and some major events are marked; ST=Sensitivity Test, EA = Energy Adjustment

Dark red= internal UV energy monitor (PD74), bright red=external energy meter (E.OGSE)

TV - Burn-In Test results

19

Six weeks of TxA operation (FM-B), the Burn-In Test starts on 3 June 2014 after the TV

test. An initial reduction of UV-energy was compensated on 10 June (EA01) with an

adjustment of Amplifier timing. Apparent energy reduction measured by external energy

meter results from darkening of support optics outside the vacuum chamber


TxA FMB Performance Parameter INITIAL FINAL UNITS

UV Pulse Energy (OGSE) 112 107 mJ

Peak Fluence 0.92 0.95 J/cm2

UV beam spot size (horizontal) 6204 6150 µm

UV beam spot size (vertical) 4697 4550 µm

UV beam divergence (EE86%) 674 635 µrad

UV Pulse Duration 21.7 20.4 ns

UV beam spot position (horizontal wrt aperture centre) -107 -276 µm

UV beam spot position (vertical wrt aperture centre) 133 -59 µm

Pointing (horizontal wrt LOS bench OCR) -1236 -1458 µrad

Pointing (horizontal wrt LOS bench OCR) 756 793 µrad

ALADIN TxA Qualification and Validation Verification campaign results – TxA FMA and FMB


TxA FMA Performance Parameter INITIAL FINAL UNITS

UV Pulse Energy (OGSE) 113 111 mJ

Peak Fluence 1.21 1.22 J/cm2

UV beam spot size (horizontal) 6097 6116 µm

UV beam spot size (vertical) 4211 4161 µm

UV beam divergence (EE86%) 684 674 µrad

UV Pulse Duration 20.4 19.6 ns

UV beam spot position (horizontal wrt aperture centre) 102 177 µm

UV beam spot position (vertical wrt aperture centre) 84 89 µm



Note: a) FMB test campaign includes the burn-in test

b) A delta acceptance test is running after the

diasporameter replacement

Comparison between initial and final tests (Main results for PLH FMA and PLH FMB)

Note: FMA Burn-in test was performed before the

qualification campaign

Next steps

Finalize the delta acceptance on TXA FMB (within March)

Activities on Flight Spare:

• Acceptance test: different scenarios have been proposed to reduce this phase

• Lifetime (six month) test: important to verify the long term stability of the PLH

Next steps

21



FMC Standard Reduced Reduced &

Reversed

Acceptance test August 2015 July 2015 Feb 2016

Lifetime test March 2016 Feb 2016 Dec 2015


UV energy (PD74) from FM-B BIT (red) and FM-A BIT II (brown) and FM-A BIT I (orange)

in the range from 80 to 120 mJ

Comparison of Burn-In Tests

22

Comparison of the UV energy from three Burn-In Tests (in Mega-shots: 140 Mshots

is 32 days) shows similar initial energy drop of FM-A and FM-B, resulting from

small evolution of thermal conduction between amplifiers and cold plate, changing

the IR beam divergence and consequently the tripling efficiency to UV.

Residual long term energy reduction results from laser diode stack degradation.

Daily cycles result from residual temperature variation of the laser cold plate.


FM-A BIT I

FM-A BIT II

FM-B


FMA

Coefficient Value

a (J) 0.1014

b (J) 8.85*e-3

c (day-1) 0.307

σageing (J/day) -8.60*e-5

Energy decrease over long term: long test results

23

FMB

Coefficient Value

a (J) 0.1012

b (J) 9*e-3

c (day-1) 0.19

σageing (J/day) -6.49*e-5

𝑃𝐸 = 𝑎 + 𝑏 ∙ 𝑒−𝑐𝑡 +𝜎𝑎𝑔𝑒𝑖𝑛𝑔 ∙ t

The UV energy has the same trend on FMA and FMB

There are 2 major contributors:

• Exponential fast thermal interface settling

• Slow linear diode degradation


FM-B FM-A BIT II

The TxA tests have shown the need for in-orbit control of the UV energy, due to

• Changes of the thermal conductivity of the interfaces,

• Aging of the laser diode stacks,

• Other, yet unknown sources.

Energy adjustments have been carried out during the Burn-In Tests of FM-A and

FM-B, to work at constant performance, based on analysis of the available

telemetry (5 optical energies, 14 different temperatures);

This approach and final in-flight operation procedures need to be validated for TxA

operation over more than 6 weeks to note the effects of laser diode degradation

(and for the other sources);

The six-month test planned for the third flight model FM-C was not designed to

verify the recovery approach.

Energy Control over long lifetimes

24



ALADIN TxA Qualification and Validation Proposal for lifetime test improvement

25

The test is a loop of following 3 phases:

1. Sensitivity test: laser operating parameters (settings) are varied by a small delta and the effect on performance is recorded. The dependency of performance on

settings is called sensitivity matrix

2. Un-perturbed operation of about 7 days: all along this phase, all parameters are recorded and trends are computed

3. Recovery: correction of operating parameters is determined starting from:

I. sensitivity data (phase 1) and

II. trend analysis (phase 2)

and applied to restore original laser performance

Test sequence matches in-orbit procedure sequence


Summary and Conclusions

26


A complex system as the TxA

caused unexpected issues but step

by step they have been removed

from the system.

Qualification testing of FM-A and

FM-B has been successfully

completed.

Optical, mechanical and thermal

testing demonstrates good design

margins of the TxA.

The long duration test of FM-C will

demonstrate the stability of the

laser.

The ALADIN TxA team


aladin txa qualification and validation...• plh was the last item to undergo qualification tests,...

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