Characterization Procedure of the Flight Laser Modules for the

ExoMars Raman Laser Spectrometer

Marina Benito-Parejo

, Pablo Rodríguez-Pérez

, Ángel Marín

, José A. Rodríguez-Prieto

Rosario Canchal

, Andoni Moral

and Fernando Rull

Ingeniería de Sistemas para la Defensa de España ISDEFE, Calle Beatriz de Bobadilla 3, 28040 Madrid, Spain

Instituto Nacional de Técnica Aeroespacial INTA, Ctra Ajalvir Km4, 28850 Torrejón de Ardoz, Spain

Universidad de Valladolid UVA, Parque Tecnológico Boecillo, 47151, Valladolid, Spain

Keywords: ExoMars, Raman Laser Spectrometer, Raman Laser Module, Raman Laser Flight Models.

Abstract: Several space missions have been sent to the surface of Mars carrying scientific instruments to study the

environment. However, only one of these missions included a laser: the ChemCam instrument on-board

NASA’s Curiosity mission. In 2020, two missions will be launched to study the Martian surface and search

for signs of life: NASA’s Mars mission carrying SuperCam instrument that will perform Raman spectroscopy

and LIBS technique; and ESA’s mission, ExoMars, with a Raman Laser Spectrometer (RLS) as part of the

rover’s payload that will study the Martian surface. One of the critical points of the RLS instrument is the

laser, due to the strict technological requirements that must be met to satisfy scientific and environmental

requirements. This paper summarizes the electro-optical characterization campaigns that passed the Flight

Model (FM) and the Flight model Spare (FS).

1 INTRODUCTION

The surface of Mars has already been studied for

some decades, since the arrival of the first rover in

1997, the NASA’s Pathfinder mission rover,

Sojourner, being the first probe to touch down on the

Martian surface. It was also a NASA mission that

took the following rovers to Mars, this time two twin

vehicles with the same instruments: Spirit and

Opportunity. They landed in opposite regions of the

planet in January 2004 with the aim of finding

evidence of water on Mars. In 2011, NASA gave up

contact with Spirit, and with Opportunity in 2018

(Mann, 2019).

NASA’s most recent rover to touch down on Mars

is Curiosity, which landed in 2012 and is the only

rover still active (Mars Exploration Program, 2019).

It is the largest vehicle on the Martian surface and its

main objective is to determine if Mars ever had what

all life needs: durable water and appropriate chemical

ingredients. This rover is still operative and could last

longer than the previous ones because it is powered

by a nuclear battery instead of solar panels. It was in

https://orcid.org/0000-0001-6192-2197

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this mission that the first specific scientific laser for

Mars exploration was sent, in the ChemCam

instrument on-board Curiosity (Mann, 2019).

The ChemCam instrument used the laser-induced

breakdown spectroscopy (LIBS) technique, with a

pulsed laser. This laser, about 600g weight, uses three

Neodymium-doped Potassium-Gadolinium

Tungstate (Nd:KGW) crystals and emits pulses at

1067 nm. It is used to vaporize rock surfaces and

soils, creating a plasma of their component gases. The

generated plasma is characterized by a spectrometer,

included also in ChemCam (NASA Science, 2019).

Although in 2018 Curiosity discovered organic

materials (Mann, 2019), the LIBS approach does not

allow detecting low presence of organic molecules

(Ciminelli, Del’Olio & Armensie, 2016).

The ChemCam successor, SuperCam instrument

included in next NASA Mars mission, to be launched

in 2020, will not only use the LIBS technique but also

Raman spectroscopy in a range up to 12m away from

the rover. The objective of this mission is to look for

signs of past or present life, and to see if humans

could one day explore Mars (NASA, 2019). The

Benito-Parejo, M., Rodríguez-Pérez, P., Marín, Á., Rodríguez-Prieto, J., Canchal, R., Moral, A. and Rull, F.

Characterization Procedure of the Flight Laser Modules for the ExoMars Raman Laser Spectrometer.

DOI: 10.5220/0008966701070113

In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 107-113

ISBN: 978-989-758-401-5; ISSN: 2184-4364

107

SuperCam laser uses a Neodymium-doped Yttrium

Aluminum Garnet (Nd:YAG) crystal and emits one

laser beam at 1064nm and another one at 532nm

(Wiens, Maurice & Perez, 2017). LIBS spectroscopy

uses the 1064 nm beam while the 532 nm beam is

used for Raman spectroscopy. Mars2020 rover will

also include an ultraviolet laser, about 400g weight,

to perform proximity Raman with the Scanning

Habitable Environments with Raman &

Luminescence for Organics & Chemicals instrument,

SHERLOC, (Beegle et al, 2015).

The next European Martian mission, an

astrobiology program by ESA and the Russian space

agency Roscosmos, ExoMars mission will be also

launched in 2020. In contrast with previous missions,

ExoMars will be the first mission capable of moving

around the Martian surface while studying the

composition of materials at depth thanks to a drill that

will extract samples up to 2m below the surface

(ESA, 2019). The scientific objectives of the

ExoMars mission are: searching for possible

biosignatures of past Martian life, characterizing the

water and geochemical distribution as a function of

depth in the shallow surface to better understand the

evolution and habitability of Mars, and achieving

incremental steps for future return samples missions

(ESA Scientific Exploration, 2019).

ExoMars mission will carry, in its analytical

laboratory (ALD), two instruments with laser devices

(Vago et al., 2017):

 The Raman Laser Spectrometer (RLS)

 The Mars Organic Molecule Analyser (MOMA)

The goal of MOMA is to seek signs of past life on

Mars by analysing a wide range of organic

compounds in the collected soil samples, studying

large molecules, inorganic minerals and volatile

organic molecules using a UV laser. From a

Neodymium/chromium-doped YAG (Nd:Cr:YAG)

crystal, its 1064 nm beam is converted to its

frequency-quadrupled of 266 nm as output beam

(Goesmann, et al. 2017).

The other laser on-board ExoMars rover belongs

to the RLS instrument, which seeks to search

biosignatures and biomarkers on Mars using Raman

Spectroscopy (European Space Agency, 2013). This

very useful technique is used to identify mineral

phases produced by water-related processes. In

addition, it will help identify organic compounds and

search for microbial life by identifying the mineral

products and indicators of biologic activities. RLS

will provide geological and mineralogical context

information with a non-destructive technique that will

allow a cooperative working with the other

instruments in ALD, and cross-correlate scientific

data (Rull, et al. 2017).

2 RAMAN LASER

SPECTROMETER

DESCRIPTION

RLS instrument consist of three main units:

 Spectrometer Unit (SPU)

 Internal Optical Head (iOH)

 Instrument Control and Excitation Unit (ICEU)

The Raman Laser Module (RLM) is located in the

ICEU, and its excitation signal is carried by means of

optical harness (OH#1) to the iOH to illuminate the

sample. Another fibre (OH#2) collects the Raman

signal to the SPU so it can be processed. These three

units, in addition to being connected by optical fibres,

are also interconnected by electrical harness (EH) that

distributes signal transmission and power supply

between them. Some of the main technical

characteristics of the instrument are the following

(Moral, et al. 2018):

 2.4 kg of mass

 Work performance in thermal environments

between -40ºC and 0ºC, and non-operational

survival between -60ºC and +50ºC

 Power consumption between 20W and 30W,

depending on the temperature range and

operational mode.

Figure 1: ExoMars Rover ALD with the RLS units layout.

The RLS laser module is one of the most critical

parts of the Raman instrument and for its design,

scientific requirements and functionalities had to be

taken into account. Thus, as top-level scientific

requirements for the instrument, the following

parameters were taken (Rull, et al. 2011):

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

108

 Spectral range of 150 to 3800 cm

-1

, so that all

important spectral bands can be detected in

Martian exploration.

 Spectral resolution of at least 6 cm

-1

(up to 2000

-1

wave numbers) or 8 cm

-1

(for wave numbers

higher than 2000 cm

-1

), so that the instrument is

able to differentiate between two Raman bands.

 Maximum power on the sample of 0.3 – 0.6

kW/cm

, depending on the sample: to obtain the

highest possible Raman signal without burning

the sample.

 Spot size of 50 µm.

2.1 Laser Module Optical

Characteristics

According to these requirements, the Raman Laser

Module (RLM) was designed and assembled with

(Moral et al. 2018):

 Wavelength of 532 nm ± 0.5nm. This is the most

suitable for planetary Raman laser spectroscopy

because it not only stimulates Raman resonance

signals in biomolecules but also has a great

performance for geology (Rull & Martínez-Frías,

2006).

 Optical output power from 20 mW to 30 mW.

High enough to obtain an adequate signal-to-noise

ratio (SNR) in the Raman signal but without

burning or altering the sample.

 Redundant laser design. Two lasers channels are

included in the unit, as a risk mitigation strategy.

 Main peak stability of ±10pm. The stability of the

main peak wavelength is vital to avoid Raman

signals disturbed by variations in the wavelength

of the emitted signal.

 Main peak linewidth of 29pm, in order to achieve

the spectral resolution at instrument level.

 Side mode suppression ratio below -20dB.

Figure 2: Flight Model of Raman Laser Module.

Both, the spectral behaviour and the level of

optical output power are the most important points,

and keeping them stable implies a very precise

temperature control. Thus, thermal control is one of

the critical parameters that must be taken over RLM.

The device has two thermal sensors embedded in the

module, capable of independently measuring the

temperature inside the housing of the laser module.

These thermal sensors, together with an external

thermo-electric module (TEM) and the associated

electronics and software allow a precise control of

thermal conditions inside the RLM, necessary to

achieve the required spectral stability. The range of

working temperatures, or setpoints, admitted by the

pumping diodes is 15ºC to 45ºC. However, the

operational temperature of the laser module itself was

confined between 20ºC and 30ºC, i.e. close to the

room temperature used during the optical components

alignment procedure. Therefore, electro-optical

characterization focused on that temperature interval.

In order to find the exact working points for each

laser contained inside the RLM, LD1 and LD2, it is

necessary to know the optical behaviour of each one

in a temperature range around 25ºC, as well as in a

feeding current range between current threshold and

the maximum current provided by the flight laser

driver, that are 0.5A and 1.5A, respectively. It is not

only important to characterize the behaviour of the

main peak at 532 nm: the pumping laser diode (PLD)

wavelength is also useful for monitoring purposes.

Even though, by design, the externally available PLD

output power is very low, its spectral contribution

could still be distinguished with a high enough SNR.

The PLD emission wavelength, close to 808 nm, is

directly related to the device temperature. Monitoring

the PLD wavelength while changing the temperature

and feeding current setpoints, for LD1 and for LD2,

will facilitate us to check the thermal operation of the

RLM, and later correlate these measurements with

those obtained after the RLM integration into the

ICEU.

These critical parameters were the main ones to

cope with in the characterization campaigns of the

flight models. Specific tests were designed to fulfil

these functional characteristics and the requirements

imposed on the RLS instrument.

3 FLIGHT MODEL OPTICAL

TEST CAMPAIGN

The RLM Flight Model (FM) characterization

campaign consisted on a series of tests designed to

characterize and evaluate the behaviour of the

Characterization Procedure of the Flight Laser Modules for the ExoMars Raman Laser Spectrometer

109

selected device, aimed to select the working points

that met the requirements. In particular, the strategy

of the Raman laser flight model characterization

campaign comprised two main steps:

 Gross output power and spectral performance

assessment over a wide temperature and current

range. This first step is performed to estimate one

or several potential setpoints within each laser

unit.

 Fine output power and spectral performance

assessment in the proximity of the candidate

setpoints in order to confirm its feasibility and

check the performance stability at the selected

setpoint.

3.1 Characterization Results

The goal of this global search is to pick up a starting

temperature and current setpoint for a detailed

assessment of laser performances. For that, a test was

carried out by sweeping in temperature (from 22.5ºC

to 27.5ºC) and currents (from 0.5A to 1.5A), and

recording in each step both output power and

spectrum using a beamsplitter (BS) fibre. As an

example, optical output power values and spectrum

are represented in Figure 3 and Figure 4, respectively.

Figure 3: Optical output power (mW) 2D map from RLM

FM LD1.

From these figures, we can identify the potential

setpoints, those temperature-current areas where the

optical power value is between 20 - 30 mW and the

secondary peak is below -20dB (Figure 5).

The spectral range recorded is 400 nm to 1100 nm,

so Nd:YAG peak at 1064 nm and PLD wavelength at

808 nm can be assessed. PLD wavelength variations

due to temperature and current in the test carried out

are relevant (Figure 6), and therefore PLD peak

information is also reported during our automated

tests (Figure 7).

Figure 4: Spectral performance close to 532nm in

normalized power density (dB/nm) from RLM FM LD1.

Figure 5: Spectrum centred in 532 nm from RLM FM LD1,

in the selected setpoint.

Figure 6: Spectral performance close to PLD wavelength in

normalized power density (dB/nm) from RLM FM LD1.

This test was repeated in the same temperature

and current ranges, but recording only optical power,

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

110

in order to obtain LIV (power, current and voltage)

curves.

Figure 7: PLD peak wavelength (nm) 2D map from RLM

FM LD1.

3.2 Stability Results

After pre-selecting the potential setpoints, a fine

characterization is made around them and stability

measurements are taken. For that, each laser is

switched on at its setpoint and optical output power

(Figure 8) and spectrum (Figure 9) are recorded.

4 FLIGHT SPARE OPTICAL

TEST CAMPAIGN

For the Flight Spare model (FS), the campaign

followed the same strategy as in the FM, with the aim

of finding the working setpoints that met the spectral

requirements as well as the optical power and

stability.

Figure 8: Optical output power stability measurements

RLM FM LD1.

Figure 9: Spectral performance stability close to main peak

from RLM FM LD1, in normalized power density (dB/nm).

4.1 Characterization Results

An extended characterization RLM FS was

performed sweeping in temperatures (from 22.5ºC to

27.5ºC) and feeding currents (from 0.5A to 1.5A).

Optical power was recorded in every temperature-

current setpoint, as well as feedback photodiode

values. After that, this test was repeated saving both

optical power and spectrum by means of a beam-

splitter.

An example of spectrum performance from RLM

FS LD2 characterization can be seen in Figure 10.

These values, combined with the ones obtained in

optical power performances (Figure 11), give us the

key to select an appropriate setpoint.

Figure 10: Spectral performance close to 532nm in

normalized power density (dB/nm) from RLM FS LD2.

0 5 10 15 20 25

OpticalPower(mW)

Time

(

min

)

Characterization Procedure of the Flight Laser Modules for the ExoMars Raman Laser Spectrometer

111

Figure 11: Optical output power (mW) 2D map from RLM

FS LD2, estimated from beamsplitter.

Figure 12: PLD peak wavelength (nm) 2D map from RLM

FS LD2. Blank data correspond to low SNR spectral signal

at PLD emission wavelength.

The characterization test was repeated, but this

time from 20ºC to 30ºC to explore a wider

temperature range. Figure 12 represents PLD peak

wavelength from the recorded spectra. White areas

are due to improper Gaussian fitting in the calculation

of PLD peak position due to a low SNR. Expressions

relating the PLD peak wavelength with the

temperature and feeding current are calculated from

the represented values.

4.2 Stability Results

In the same way as in the FM characterization

campaign, once a preliminary setpoint was selected,

the repeatability of the performance of both LDs was

checked, as well as stability behaviour (Figure 13).

Figure 13: Spectral performance stability close to main

peak from RLM FS LD2, in normalized power density

(dB/nm).

5 CONCLUSIONS

The characterization campaigns of the Raman

instrument’s laser flight models comprised a series of

tests dedicated to identify the working points where

the laser complied with the required performances.

For this purpose, an optical and spectral

characterization was performed, making a sweep in

temperature and current to identify the areas in which

each laser could give the necessary optical power and

an adequate spectral behaviour. In addition, once the

setpoints had been selected, additional measurements

were performed to assess the spectral and power

performance stability over the foreseen Raman

measurement time.

With these tests, it was possible to obtain a

detailed record of the behaviour of flight models in a

wide range of temperatures and currents, as well as to

determine the appropriate working setpoints to meet

the requirements for both lasers in each Raman Laser

Module.

ACKNOWLEDGEMENTS

This work has been funded by the Spanish MINECO

(Ministerio de Economía y Competitividad) through

ESP2014-56138-C3-1-R project.

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