Green External-Cavity Diode Lasers for Space-Borne Raman Spectroscopy

Introduction

An important component in the exploration of planetary bodies, such as Mars, relies heavily on in-situ mineralogical analysis to uncover the geological history and also investigate potential habitability and organic content. Raman spectroscopy has emerged as a cornerstone technique in planetary in-situ exploration as it enables the identification of minerals and organic compounds [1]. It had its debut in planetary research with two Raman instruments on NASA’s Perseverance rover [2,3] which landed in Jezero crater, Mars, in February 2021 and has been collecting in-situ Raman data from the Martian surface since then. Raman spectroscopy will also be used in upcoming planetary missions, such as JAXA’s Martian Moons eXploration (MMX) mission with the RAman spectrometer for MMX (RAX) [4, 5] and ESA’s ExoMars rover with the RLS (Raman Laser Spectrometer) [1], both operating at 532 nm. In this study we present external-cavity diode lasers (ECDLs) as a promising alternative to frequency-doubled Nd:YAG lasers for space-borne Raman Spectroscopy.

Background

External-Cavity diode lasers (ECDLs) address the growing demand for compact, tunable, and energy-efficient laser sources in planetary Raman spectroscopy, where in-situ mineralogical analysis requires high spectral resolution. Due to their compact design, high electrical-to-optical conversion efficiency, direct electrical modulation capability, and low étendue [6], diode lasers are especially attractive for space-borne instruments. Recent advances in semiconductor technology now enable green-emitting diodes (520–530 nm), offering a flexible, semiconductor-based alternative to traditional frequency-doubled Nd:YAG lasers at 532 nm. By embedding these diodes in an external cavity, we narrow their broad gain spectrum to longitudinal single-mode operation with linewidths of 1cm⁻¹, meeting the requirements for high-precision Raman measurements. The tunability of ECDLs further enables shifted-excitation Raman difference spectroscopy (SERDS), which suppresses fluorescence by capturing spectra at slightly shifted wavelengths (~1 nm), thereby enhancing the signal-to-noise ratio for weak Raman signals in fluorescent backgrounds [7, 8].

Methodology

In this work, we present two external-cavity setups for green diodes, specifically designed for in-situ Raman measurements under planetary conditions:

• A Littrow configuration, known for its simplicity and reliability (see Figure 1).
• A filter-based configuration, combining a narrow bandpass filter with a cat’s-eye reflector.

By embedding the diodes in an external cavity, we narrow their inherently broad emission to achieve longitudinal single-mode operation with linewidths of 1 cm⁻¹ (Littrow setup), meeting the requirements for high-resolution Raman spectroscopy.

For the Littrow configuration, we systematically evaluated eight laser diodes based on emission wavelength, optical output power, and tuning range. The filter-based setup was tested with two diodes to assess their tuning range and maximum single-mode output power. The Littrow configuration was selected as the primary setup due to its established reliability and simplicity, making it ideal for initial testing and characterization of the diodes and ECDL system.

Figure 1: Sketch of the Littrow configuration, where the grating is mounted on a piezo actuator for fine adjustment of the cavity length [9].

Results  

Our results with the Littrow setup demonstrate wide tuning ranges across multiple diodes, with three diodes achieving the target wavelength of 532 nm which is demonstrated in Fig. 2. This positions them as direct, semiconductor-based alternatives to frequency-doubled Nd:YAG lasers for instruments like RAX [9]. The filter-based setup exhibits different tuning behavior compared to the Littrow configuration. Furthermore, its maximum single-mode output power is lower, indicating limited suitability for our application. Additionally, initial radiation hardness tests confirm the operational robustness of the diodes, marking a critical step toward flight qualification.

Figure 2: Tuning range of eight different diodes in the Littrow setup. Three of them achieving the target wavelength of 532nm.

Conclusion

This study bridges the gap between laboratory characterization and practical space applications, demonstrating that green ECDLs are a viable alternative to Nd:YAG lasers for next-generation planetary instruments. Key findings show that the Littrow setup is better suited for such applications than the filter-based configuration. The compact size, efficiency, and tunability of ECDLs make them a compelling alternative to traditional solid-state lasers, while their compatibility with 532 nm ensures seamless integration into existing mission instruments. Additionally, their ability to perform SERDS further enhances their utility, enabling high-sensitivity Raman measurements even in the presence of strong fluorescence.

References:

[1]: Rull et al., 2017
[2]: Bhartia et al., 2021
[3]: Lopez-Reyes et al., 2025
[4]: Hagelschuer et al., 2022
[5]: Schröder et al., this conference
[6]: Angel et al., 1995
[7]: Zhao et al., 2002
[8]: Böttger et al., 2017
[9]: Lukaszewski et al., in revision