1.

INTRODUCTION

SuperCam is one of the seven instruments selected for the Mars 2020 mission landed on Mars on February 2021. It includes a Nd:YAG laser oscillator delivering either 24 mJ of energy at 1064 nm for LIBS spectroscopy or 12 mJ at 532 nm for Raman spectroscopy [1]. The weight and footprint of this laser are similar to the ChemCam laser, which has been operating on Martian soil for now 10 years. ChemCam and SuperCam lasers have now jointly fired more than 1, 000,000 shots on Mars. They are both mounted on the masts of Curiosity and Perseverance rovers. For future missions, it could be of great interest to have the LIBS instrument directly on a helicopter to explore the Martian soil. For that, it is necessary to drastically reduce the size and weight of the instrument and therefore of the laser.

The main characteristics required for the laser are the following:

This communication describes a laser solution to fit with these characteristics. The MiniCam laser is based on a passively Q-switch Nd:YAG laser. We will present a first mechanical definition of this laser. Results on table top version of this laser will also be presented.

2.

FIRST MECHANICAL DEFINITION

The mass of SuperCam laser is 526 g, far away from the 100 g aimed for MiniCam laser. Even if we only consider the infrared part of SuperCam, its mass would still be of 335 g. Therefore, it is necessary to drastically reduce the weight of the laser at least by a factor of 3. The main weight contributors are the following:

We propose to replace the active Q-switch by a passive Q-switch with a Cr⁴⁺:YAG saturable absorber. Therefore, we only have one Cr⁴⁺:YAG crystal instead of having a Pockels cell, a wave plate and a polarizer with their associated mechanics and electronics used to generate the high voltage. Cr⁴⁺:YAG is chosen as the passive material for this study because it has already been used in space lasers [2][3][4]. It has proved long life in space environment with billion shot lifetime. It is also tolerant to both gamma and proton ionizing radiation. Finally, Cr⁴⁺:YAG also promises minimal sensitivity to thermal variation, implying that the laser could be temperature cycled with limited impact on its performance.

The sealed function achieves by itself the 100g goal for the MiniCam laser. That is why we study an unsealed laser.

We propose a MiniCam laser concept based on an unsealed passively Q-switched laser. These two points are the main changes. For the rest, we try to keep as much as possible the heritage of ChemCam and SuperCam lasers. We present a first mechanical definition of the laser (Figure 1 and Figure 2).

Figure 1 :

3D view of MiniCam laser

Figure 2:

Sectional view of MiniCam laser

The laser oscillator is based on a Nd:YAG rod longitudinally pumped by a multicolor stack, insuring pump absorption over large temperature range. This allows both the diode and the rod to be conductively cooled and requires no active cooling, neither for the laser diode nor for the laser medium to operate on large temperature range. Oscillator is linear, closed on one side by the HR coated rod side and by the output coupler on the other side. As previously mentioned, the Q-switch function is done by a Cr: YAG saturable absorber with 7 mm diameter. The output coupler is not adjustable which differs from SuperCam. In order to align the laser cavity, we add two Risley prisms between the saturable absorber and the output coupler. As the Pockels cell, the wave plate and the polarizer are removed, it allows us to shorten the cavity length to 8 cm instead of almost 13 cm in SuperCam laser. This reduces the length of the cone, which is one of the heaviest piece of the laser.

The global mechanical structure is based on the same principle as ChemCam and SuperCam lasers with a titanium body and collar that is used as a mechanical reference. The other mechanical mounts, like those for the saturable absorber or the Risley prisms, are directly or indirectly fixed on the collar. Apertures in the cone allow to adjust the prisms in order to align the laser cavity. The saturable absorber is removable. We are not sure that it is necessary but for a first mechanical definition, we prefer to keep the possibility of aligning the cavity in a relaxed mode.

The main body and the collar are a single mechanical piece made on titanium as the other mechanical mounts of the laser. Titanium is heavier than aluminum but more rigid. Nevertheless, we keep in mind the mass reduction goal and we lighten as much as possible the structure. For instance, we reduced the collar thickness.

At the end, the mass of the laser is below 100g. Laser dimensions are 122 x 38 x 50 mm. Note that stack alimentation cables are not included in this total.

3.

EXPERIMENTAL SETUP AND RESULTS

We assemble a table top version of the laser. The oscillator previously described in paragraph 2 is built using optical components from SuperCam program. Figure 3 shows the experimental setup. This version is slightly different from the mechanical design because we align the cavity directly with an output coupler instead of Risley prisms. It allows to easily align the laser cavity. We have different saturable absorbers with 62%, 35%, 30% and 20% of transmission. Their diameters are ½ inch or 7 mm diameter. These latters fit within the mechanical definition. We also have several output couplers either plan or convex. Their reflectivity varies from 20% to 60%. We have a lot of possible combinations of saturable absorber / output coupler that we can investigate. The laser operates in the nanosecond regime at 3 Hz repetition rate.

Figure 3 :

Scheme of the experimental setup of the table top laser

A measurement bench is added to the setup in order to characterize the laser beam. It includes:

3.2

Experiment on short cavity length

We reduce the cavity length to 8 cm accordingly to the mechanical definition of MiniCam laser. We first repeat the comparison of the three absorbers A, B and C with a 32 % reflectivity output coupler. It leads us to the same conclusions that those for the long cavity. We can obtain only few mJ with absorber B, so we will not consider it for the rest of the experiments. Energy above 10 mJ is obtained but higher energy leads to higher current threshold. Pulse width is around 4 ns with a jitter of few microseconds.

We focus then our attention to find the best saturable absorber / output coupler configuration. We test saturable absorbers with 35%, 30% and 20% of transmission and output couplers between 20% and 60% reflectivity. Special care needs to be done to beam profile. Indeed, depending on the cavity alignment we can extract more energy but with a higher current threshold and poorer beam quality. For example, Figure 4 shows two beam profiles in the far fields obtained with the same configuration, a 60% reflectivity output coupler and a 20% saturable absorber. We optimize either the beam profile (a) or the output energy (b). In the first case, we obtain 10.9 mJ with M² =1.1 and a threshold current of 120 A. Whereas, if we align otherwise the oscillator cavity we can have an energy as high as 15.9 mJ but with a poorer beam quality (M²x = 2.2, M²y = 1.1) and an increased current threshold of 172 A. A compromise has to be done between energy, beam profile and current threshold.

Figure 4:

Beam profile in the far field with 20% saturable absorber and 60% output coupler. Beam quality factor optimization (a) or energy optimization (b)

Finally, we choose the following configuration: 60% 4m cx output coupler and a 30% saturable absorber, as it seems the best compromise. Indeed, 12.6 mJ output energy is obtained with good beam quality (M² = 1.2) and with low current threshold (130 A). All the beam characteristics are summarized in Table 2.

Table 2:

Characteristics of the best compromise configuration

Cavity length	8 cm
Output coupler	60 % 4 m cx
Saturable absorber	30%
Energy (mJ)	12,6
Stability rms (%)	1,60 %
Threshold current (A)	130
Pulse width (ns)	4
M2	1,2
Beam profile in the far field2

4.

CONCLUSION

In this study, we presented a lighter version of SuperCam laser for LIBS only operation. We investigated through CAD simulation the potential of an unsealed passively Q-switched Nd:YAG laser. We were able to reach a laser mass of less than 100 g which fills in a 122 x 38 x 50 mm envelope. Table top experiments were conducted. Several configurations achieved more than 10 mJ energy with M² <2. At the end, we chose a 30% saturable absorber with a 60% output coupler. 12.6 mJ energy with 4 ns pulse width were obtained with a beam quality factor of 1.2 and a threshold current of 130A.