2.1

Oscillator

The oscillator provides the high beam quality and short pulse length needed for LIBS analysis. It is based on a Nd:YAG rod longitudinally pumped by a multicolor diodes stack. The broad emission spectrum of the diodes shift with temperature but always match some absorption peak of the lasing material. This insures pump absorption over large temperature range without the need of any temperature stabilization system for the pumping diodes. Both the diode and the rod are conductively cooled and can operate on a large temperature range (-40 °C to 40 °C for qualification).

The oscillator is actively Q-Switched with a RTP Pockels cell to produce nanosecond pulses. Oscillator resonator is linear, closed on one side by the HR coated rod side and by the output coupler on the other side. Reflectivity of the output coupler is 40 %. A polarizer, wave-plate and Pockels cell are associated to Q-switch the cavity. The oscillator provides output energy of about 30 mJ at 1064 nm with a pulse duration < 5 ns and a M2 < 2.

2.2

Second harmonic generator

The Second Harmonic Generator (SHG) is a KTP (type II) crystal, placed at the output of the oscillator. It generates the 532 nm beam used in Raman mode. A RTP electro-optical switch (same Pockels cell as for the oscillator), between the oscillator and SHG, allows the operation mode selection (LIBS or Raman) by switching between linear and circular polarization. In the absence of a high voltage, an infrared beam at 1064 nm is emitted. When the high voltage is applied to the RTP switch, beam is converted to 532 nm for Raman analysis. An SHG conversion efficiency of about 50% is obtained corresponding to about 15 mJ available for Raman spectroscopy.

2.3

SuperCam evolutions: Slab amplifiers

Following the development and manufacturing of the SuperCam model, we studied the possibility to increase further the energy of the laser for both LIBS and Raman applications [2].We added one or two identical slab amplifiers (dimensions 6 x 6 x 22 mm) to increase the energy at the oscillator output. Each amplifier is transversally pumped by one 1800 W diode stack identical to the one used for the oscillator. These slab amplifier geometry is similar to the one from ChemCam amplifier with the following modifications:

Thanks to improved thermal properties of Nd:YAG compared to Nd:KGW, a thousand shot sequence at 10 Hz can be performed compared to the 100 shots/3 Hz shooting sequence of ChemCam.

3.1

SuperCam test platform

We assembled a tabletop version of SuperCam as a flexible platform for low TRL studies (TRL 3-4). We can flexibly add various modules as depicted in figure (1). The oscillator described in paragraph 2.1 is built using optical components from the SuperCam program. As the mechanical mount are off the shelf components, and the system is opened without any covers, it is not representative in term of thermal behavior of the SuperCam Flight model. For convenience, the diode stack is cooled and temperature stabilized at 20°C by a Peltier module. For the purpose of this study, we kept a constant output energy of 20 mJ at 1064 nm for the oscillator. We choose to have this constant energy level to preserve a low M2 parameters for the oscillator (M2<1.2). Subsequent amplification in slabs allows us to recover higher energy at the fundamental wavelength before frequency conversion.

Figure 1.

SuperCam test platform on breadboard for harmonic conversion at 355 nm and 266 nm.

The repetition rate can be set at 3 Hz or 10 Hz. Following the oscillator, we choose the laser configuration by adding the following modules:

We used dichroic mirrors to separate third or fourth harmonic, 1064 nm and 532 nm beams are transmitted while harmonic beam is reflected.

3.2

Third harmonic generation module We selected the following non-linear crystal:

Both crystals are mounted on mechanical stages allowing phase matching by rotation of the crystals.

3.3

Fourth harmonic generation module

We selected the following non-linear crystal:

BBO crystal for FHG was used in program like the MOMA instruments for ExoMars mission.

3.4

Measurement set-up

The measurement set-up includes:

We took special care to remove fundamental wavelength and second harmonic generated signals using additional two additional dichroic mirrors and a Pelin Broca prism for spatial and temporal measurements.

4.1

SuperCam test platform at 1064 nm

The SuperCam laser assembled on a breadboard allows us to test different configurations to increase the energy of the oscillator. We measured energy, pulse duration and beam profile (near field, far field and M2 factor) for each configurations at the fundamental wavelength of the emission of the laser. We summarized results in table 1. All measurements were performed at 3 Hz except for the 2 slabs and beam expander configuration (3 Hz and 10 Hz).

Table 1.

SuperCam performance at 1064 in four optical configurations.

The spatial profile in the near field and far field is preserved when the beam is amplified in the different configurations.

Figure 2.

Beam profile at 1064 nm in the near field (left image) and far field (right image) measured at 3 Hz for an energy of 115mJ (2 amplifiers with a beam expander).

The platform can deliver energy from 20 mJ (oscillator only) to 115 mJ (two amplifiers including beam expander). Pulse width (FWHM) is 6 ns for all configurations.

4.2

Third Harmonic Generation

Third harmonic generation for each set-up is optimized by angle tuning of SHG and THG crystals. The highest level in energy of the third harmonic is not obtained for the highest level of second harmonic. Usually, a conversion efficiency of 40 % for the second harmonic is optimal to maximize THG energy output. We have summarized in table the maximum energy achievable for each configuration.

Table 2.

SuperCam performance at 355 nm for 4 different optical configurations.

We can deliver with the various platform tested from 5.8 mJ at 355 nm (oscillator only) to 37.4 mJ at a repetition rate of 3 Hz. Energy stability is better than 2.5% RMS except for the most energetic configuration (3.8% RMS for 2 slabs and beam expander). M2 parameter is lower than 2 on both axis for all configurations. FWHM pulse duration is reduced to 5 ns due to temporal filtering in the harmonic generation process.

Figure 3.

Beam profile at 355 nm in the near field (left image) and far field (right image) measured at 3 Hz for an energy of 37 mJ (2 amplifiers with beam expander).

For the configuration using two slab amplifiers and a beam expander (maximum energy in the fundamental), there is a decrease of converted energy by 10% when running the laser at 10 Hz rather than 3 Hz. We attribute this decrease in harmonic conversion efficiency to increased thermal lensing and aberration within the slab amplifiers.

4.3

Fourth Harmonic Generation

Fourth harmonic generation for each set-up is optimized by angle tuning of SHG and FHG crystals. The highest level in energy of the fourth harmonic is obtained for the highest level of second harmonic. We have summarized in table the maximum energy achievable for each configuration.

Table 3.

SuperCam performance at 355 nm for 4 different optical configurations.

Output energy at 266 nm is 3.1 mJ for the oscillator and up to 23.4 mJ for two slabs with the beam expander set-up. Energy stability (12 % RMS) and conversion efficiency are low in the oscillator configuration because of insufficient peak power incident on the BBO crystal. Adding one or two slabs drastically improves energy stability (better than 2.5 % RMS) and conversion efficiency (up to 19 %) thanks to higher peak power. M2 is lower than 3.4 on both axis in all configurations. Temporal width (FWHM) is measured to be 6ns. We expected shorter duration due to the nonlinear process but are limited by the detector spectral response in the UV.

Figure 4.

Beam profile at 355 nm in the near field (left image) and far field (right image) measured at 3 Hz for an energy of 37 mJ (2 amplifiers with beam expander).

As for THG, the conversion efficiency drops by 10% when running at a higher repetition rate. We noticed for all configuration the high sensitivity of angular phase matching of the fourth harmonic generator. In the case of the highest energy version including the beam expander, it is also critical to adjust its divergence precisely to be within the narrow acceptance of the BBO crystal. These constraints could be relaxed by using a shorter BBO crystal (larger acceptance angle) at the cost of a lower harmonic conversion efficiency.

4.4

Discussion and model design

From our measurements, we wrote preliminary specifications for a UV laser operating at 355 nm or 266 nm. The designed lasers would be single wavelength, without the capacity to switch between fundamental and UV. Repetition rate is chosen to be 10 Hz to have a fixed thermal load and hence constant beam divergence during the burst sequence. Mechanical design is herited from SuperCam. Thermal dissipation is purely conductive in the laser (no active cooling using Peltier module). Non-linear crystal oven are temperature controlled using heaters as for SuperCam KTP crystal. Additional driving electronics for Q-switch and diodes adds 600 g to the laser mass.

In table 4, we have summarized performances of both laser versions. Thermal effects experienced during burst shooting were taken into account (thermal dissipation, interplay between diode spectrum and Nd:YAG absorption spectrum) to estimate preliminary performances of the lasers. Different configurations with lower repetition rate and/or lower energy are possible, leading to smaller footprint for the laser.

Table 4.

Preliminary specifications for SuperCam type UV model at 355 nm and 266 nm operating at 10 Hz.