2.1.1

Deep-UV Raman Laser

As mentioned above the scientific results demonstrated with deep-UV excitation show great detail and a good signal to noise ratio (SNR) compared to other excitation wavelength in the VIS or NIR wavelength range, mainly for two reasons:

Within MIRAS II a very compact hollow cathode, quasi-continues-wave (quasi–cw) laser was selected for the BB, after an extensive investigation and trade-off. The quasi-cw operation is a compromise between pure pulsed operation (laser damage of the sample and potentially the fiber optics) and cw-operation (large power consumption and active cooling required). These compact quasi-cw lasers allow operation without cooling over a large operational temperature range and the power consumption is compatible with an instrument for planetary exploration [4]. Raman measurements are reported with these lasers [5]. Very advantageous for these Raman measurements is the extremely narrow line width of < 3 GHz (0.0005 nm) and the wide operational temperature range (-200°C – + 100 °C) of these hollow cathode lasers. On the other hand, strong plasma lines within the wavelength region of interest have to be filtered out of the excitation path. A drawback of these multimode lasers is the relatively low beam quality, with an M² above 10. This is a mayor design driver for selecting the appropriate fiber to deliver this laser light to the sample and for the optical design of the Raman optical head. Improvement of the M² is required for a dedicated custom designed laser to relax the fiber coupling conditions.

2.1.2

Fiber Coupling

Due to the multimode excitation laser only multimode fibers can be used to deliver the light to the optical head and back to the spectrometer. The laser beam quality and beam diameter are design drivers for the numerical aperture (NA) and core diameter of the fiber. On the one hand a multimode fiber with a large core requires a more complex optical system in the optical head to achieve a small focus spot (imaging of the excitation fiber end on the sample). On the other hand large core multimode fiber is less critical in fiber coupling alignment tolerances and also less critical with regard to fiber bending (assuming that the full mode-field of the fibers is excited). An underfilled fiber would result in strong intensity variations when the fiber is moved during operation.

Due to the deep-UV excitation only special UV-enhanced fibers can be applied. Normal glass fibers would suffer from laser damage inside the fiber, which would cause irreversible attenuation increase. A number of fibers (UV-enhanced, solarisation resistant, different core diameters and NA) from different fiber manufacturers were tested within the project to identify the best possible coupling fiber, which ideally allows stable guidance of all of the laser light with a small core diameter and a low, constant attenuation. Additionally the compatibility of these fibers with regard to the space environmental conditions is investigated.

Fig. 3.

Attenuation of a deep-UV enhanced all silica fiber (figure by FiberTech GmbH).

The typical attenuation of these UV-enhanced fibers is quite high compared to standard optical fibers like they are used in telecom industry. As the optical power budget of a Raman system is quite demanding and any losses within the optical train have to be minimized, the fiber length is a critical system parameter. At a wavelength of e.g. 250 nm the attenuation has reached about 0,5 dB/m, which means, that the cable length for a Raman system at these wavelength regime is restricted to a few meters.

2.1.3

Fiber Coupled Spectrometer

As mentioned above the spectral range of interest, which represents a Raman spectral range between 500cm^-1 and 4000cm^-1 covers just 40nm in the deep- UV. With the scientific goal to resolve spectral features down to 5cm^-1, a spectral resolution of 0,03 nm needs to be achieved. For the MIRAS II instrument BB a Hadamard transform spectrometer [6] was selected, which allows to achieve such a high resolution at a high optical throughput in an extremely small overall volume.

Fig. 4.

MIRAS II spectrometer with commercial CCD detector (left) and schematic view of the folded optical path (right).

An alternative spectrometer concept which was evaluated in a former project [2], an acousto optical tunable filter (AOTF) spectrometer, was not further investigated, as such a scanning system is not the preferred approach with a pulsed excitation source. Currently the BB instrument uses a commercial, back-illuminated CCD detector with deep-UV antireflection coating. With regard to the quasi-cw pulsed operation of the laser, experiments with an intensified, UV sensitive CCD (ICCD) in gated operation mode (triggered by the laser pulses) were performed. These experiments indicate, that the ICCD is the preferred detector solution for the quasi-cw excitation laser, as signal intensities of the Raman signature are extremely low and a high number of laser pulses are required to accumulate enough photons for a significant spectral signature.

Fig. 5.

Spectral data obtained with the deep-UV Hadamard spectral sensor (plasma lines of a tested deep-UV laser), demonstrating a resolution better than 0.1 nm.

2.1.4

Raman Optical Head

The Raman optical head has to fulfill the following functions and requirements (see also Fig. 6).

Fig. 6.

Schematic of the Raman optical head (filters not shown).

The excitation light coming out of the excitation fiber needs to be collimated, fiber-Raman signature and laser plasma lines, have to be filtered. The pure excitation light has to be focused on the sample onto a relatively small spot by the front objective (optics 2). The same lens system then has to collect the Raman emission spectrum with a high efficiency, which means the collection NA should be as high as possible. As the excitation path (optics 1-2) mainly has to be optimized for effective focusing of the excitation wavelength, the reception path (optics 2-3) has to be achromatic for the Raman spectral range of about 50nm, to allow effective fiber coupling of the full spectrum. The beam-folding within the excitation path is also used to remove the plasma lines and the fiber Raman signature out of the excitation spectrum. As indicated in Fig. 6 two deep- UV edge filters with a very steep cut off are used for the beamfolding. The Raman spectrum can pass the edge filter on the way back. The beamsplitter and an additional filter (not shown) block the direct laser light with an optical density (OD) of 8. The remaining pure Raman spectrum needs to be focused on the core of the reception fiber.

The detailed optical design of the optics 1-3 is driven by several requirements and restrictions like small spot size, high reception NA to collect as much Raman light as possible, core diameter and NA of the excitation and reception fibers. Due to the operation wavelength range in the deep-UV the number of glasses to work with, is drastically limited to a few UV transparent materials. Additionally, only air-spaced designs for the optical systems 1-3 can be considered, as kitted optical systems might be permanently damaged by the deep UV laser light. All surfaces have to be coated for best UV transmission.

The overall optical design of the MIRAS II Raman optical head achieved a spot size of 6 μm, 2 mm working distance and a collection NA of 0,5. Especially the working distance has to be improved in future designs to allow access to samples with a rough topography. The spot size could be further reduced, however just by increasing the focal length and diameter of the optics, thus increasing volume and weight of the head. The opto-mechanical design of the head was optimized for these optical parameters to reach a volume of 24x20x70 mm³ (including beam-folding and edge filters, not including the fiber connectors) and a weight of just 100g.