Developing Continuous-Wave Laser Systems for Raman Spectroscopy: Overcoming Significant Challenges

In 2018, the flight model of the Raman Laser Spectrometer (RLS) instrument for the ExoMars mission was delivered [1]. Among the critical parts of the RLS instrument was its laser unit, an INTA-led development and one of the most demanding units to comply with the technical and scientific performances required by the mission [2]. To ensure stable and robust operation in the Mars environment, the laser unit had to meet strict optical, thermal and mechanical criteria. This required an extremely precise alignment and integration process with very tight tolerances [3],[4].

The achievement of this milestone and the experience gained at RLS led to INTA's collaboration in JAXA's Martian Moons eXploration (MMX) project. DLR, INTA/University of Valladolid and the University of Tokyo collaborated to create the RAX instrument (Raman Spectrometer for MMX), which is part of this mission. The MMX rover, tasked with investigating the Martian moon Phobos, will carry the RAX instrument. The laser flight model created specifically for RAX was delivered and successfully completed all qualification phases required for its space deployment in 2022 [5].

These consecutive advancements in two consecutive planetary missions provided INTA with a solid technological foundation and an unmatched practical understanding of the capabilities and limitations of space-qualified laser systems. During the development of these projects, the difficulties of using solid-state lasers (DPSS) with second-harmonic generators (SHG) to achieve continuous green beam became evident. Although these systems offer very high beam quality and spectral efficiency, they present significant drawbacks in terms of mechanical integration and optical alignment, particularly when it comes to minimizing dimensions and mass while maintaining stable optical performance in space environments.

INTA launched ProtoRaman, a strategic research initiative that includes a line to investigate and validate alternative laser architectures, both continuous and pulsed, for space applications. Focusing on continuous emission lasers, and in order to overcome the drawbacks encountered in RLS and RAX, we are designing and evaluating the feasibility of working with a new generation of green lasers for proximity Raman spectroscopy that are reliable, compact and easy to integrate. The development of continuous wave (CW) green lasers based on visible emission gallium nitride (GaN) diode lasers [6], together with external cavity structures, is one of the main objectives of this INTA-funded project, which covers multiple research areas. By using visible-emitting diodes instead of nonlinear frequency-conversion elements, the chosen configuration aims to significantly simplify the laser head. However, this strategy entails the creation of external cavity feedback mechanisms that enable spectral narrowing, wavelength stabilization, and moderate power enhancement [7] to meet the demanding specifications of planetary proximity Raman spectroscopy, which include narrow linewidths, low optical noise, and spectral stability.

Finding enough optical power in the 515–525 nm range while preserving the beam characteristics necessary for efficient Raman excitation is one of the major challenges facing this strategy. Furthermore, since edge-emitting GaN diodes lack intrinsic mode selection, not only appropriate cavity geometries and optical coatings are needed, but also highly precise optical alignment and thermal control are required to maintain spectral purity and linewidth control. Furthermore, the overall system must be designed to withstand environmental stresses such as vibration, thermal cycling, and extended operation in low-pressure or vacuum environments. This work focuses on proof-of-concept implementations following theoretical modeling and basic design phases. These have revealed significant limitations related to the alignment sensitivity of the external cavity design, as well as the effects of mechanical tolerances and thermal drift on spectral linewidth and power stability. An extensive testing campaign is currently underway to evaluate the limitation ranges and the margins achieved by optical performance. These impacts will then be assessed in representative environments.

 

Keywords: Laser, Raman Spectroscopy, RLS, MMX, Planetary Exploration

 

References

 

[1] Rull, F. et al., “The Raman Laser Spectrometer for the ExoMars Rover Mission to Mars”., Astrobiology, 17 (6-7): 627-654. (2017)

[2] Rull, F. et al., "The Raman Laser Spectrometer for the ExoMars Mission: Overview and Expected Performance," Spectrochimica Acta Part A, 2020.

[3] Ribes-Pleguezuelo, P. et al. “Assembly processes comparison for a miniaturized laser used for the Exomaras European Space Agency mission” Optical Engineering 55 (11), 116107 (2016)

[4] Pérez-Canora, C. et al., "Development and Integration of the RLS Laser Unit for ExoMars," Proceedings of the European Planetary Science Congress, 2018.

[5] Bibring, J.-P. et al., “The RAX Raman Spectrometer for MMX: Scientific Goals and System Design,” International Journal of Astrobiology, 2022.

[6] Nakamura, S. et al., "Visible-Light Emitting GaN Laser Diodes: Recent Developments and Future Perspectives," Nature Photonics (2013)

[7] M. Chi et al. “Green high-power tunable external-cavity GaN diode laser at 515nm”. Optics Letters 41 (18), 4154-4157 (2016)