High-power alexandrite laser developed for space lidar

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A team at Imperial College London has demonstrated a diode-pumped alexandrite laser with powers 20 times higher than current alexandrite technology, it was announced at the Photonex conference. The researchers are developing new laser technology for the European Space Agency (ESA) that is hoped will be part of a space lidar instrument for Earth observation.

Professor Michael Damzen, who is leading the work at Imperial College London, gave the keynote address at the Optical Engineering and Design session at Photonex, which took place from 15 to 16 October in Coventry, UK.

The team have generated powers of more than 26W with the alexandrite laser and have achieved q-switching for pulsed operation. ‘Now we are operating at kilohertz pulse rates, which is an order of magnitude more than most of the lidar technologies up there [in space] at the moment,’ Damzen said in his presentation.

However, he said that many challenges remain before the instrument can qualify for the incredibly demanding environment in space.

According to ESA figures, the UK space industry is growing at a long-term average of 8.6 per cent, with the revenue generated by space currently sitting at £13 million. There is a target to increase this revenue to £40 billion by 2030, which would represent 10 per cent of the global space market.

Damzen commented: ‘Space is a complex business opportunity… Rather than a volume type of business, it is rather bespoke, but it can be quite a high value.’

Earth observation satellites use lidar for monitoring atmospheric greenhouse gases, the effects of climate change and natural disasters, and vegetation cover, among other variables. The systems fire laser pulses at the Earth’s surface or atmosphere and then analysing the back scattered signal.

The alexandrite laser that Professor Damzen’s research team are developing at Imperial College London will complement existing Nd:YAG technology, which while used in lidar system has some limitations. Alexandrite is a broadly tunable laser material − from 700-750nm in the fundamental, and in the UV and blue wavelengths for the second harmonic. This means the device can access new wavelength bands. ‘You can choose your wavelength and do new and better science,’ said Damzen. ‘In the fundamental band of alexandrite there are water vapour bands; this is very important for atmospheric modelling and weather and so on.’

Another advantage with alexandrite is that the fundamental band sits across what is known as the ‘red edge’ for vegetation; the region where there is a steep gradient in reflectivity as plant health deteriorates. This therefore makes it ideal for agriculture monitoring. ‘Vegetation lidar is becoming very important at the moment for understanding the health of plants and for many commercial aspects of agriculture,’ noted Damzen.

Although this technology seems promising for space, currently all alexandrite lasers are lamp pumped, but for space compatibility, it would need to be diode-pumped to meet the strict lifetime and efficiency requirements.

Only a handful of papers have worked on diode pumping alexandrite, and the powers produced have been quite poor, Damzen explained: ‘The highest power is just over 1W; most of these are milliwatts, way below what is required.’

Damzen’s research team, over the last four years, has been investigating different geometries for packing the laser. The 26W power levels were produced by end-pumping, which is more efficient and involves delivering the light through the small end of the laser crystal. ‘More recently we have got better diodes available that allow us to do end-pumping,’ he said.

Damzen and his team are currently focusing on raising the Technology Readiness Level (TRL) of the technology for space qualification. ‘This needs to be a very robustly engineered system; it is going to be very challenging,’ he said. ‘I think we are going to need help to do this – there are many technologies we need to bring together at a high TRL level. Once you reach that level of TRL, this is something that can be considered to go into a satellite mission.’

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Imperial College London