Shanghai-based scientists say they have successfully tested a laser that can reach 160 metres below the sea surface — a development that would significantly boost China's naval deterrence capabilities.

Key points: For comparison, sunlight rarely reaches 200 metres below the water's surface

For comparison, sunlight rarely reaches 200 metres below the water's surface Once developed, the technology would fundamentally change submarine warfare

Once developed, the technology would fundamentally change submarine warfare The technology is part of a program to create a beam to penetrate depths of 500 metres

Sunlight, for comparison, rarely reaches below the water's surface beyond 200 metres, though it can travel to 1,000 metres deep depending on the right circumstances, according to the US National Oceanic and Atmospheric Administration (NOAA).

A team from the Shanghai Institute of Optics and Fine Mechanics reportedly tested the laser mounted on a plane flying over the South China Sea earlier this year, and the results were made public this month.

However, the test's exact location and the environmental conditions present at the time of the test were not revealed.

In order for a laser to penetrate water effectively, the power used to generate the beam must be significant.

Most sunlight is unable to pass the euphotic zone which generally extends to depths of 200 metres. ( US National Oceanic and Atmospheric Administration )

The South China Morning Post reported that the laser was made up of green and blue light beams, which are believed to be better equipped with the right wavelength to travel below the water's surface where it is harder for light to penetrate.

Song Chengtian, associate professor at the Beijing University of Technology, told reporters the power used on previous tests was stymied by the size limitations placed on the devices that housed lasers given that they had to fit onto planes, but added that the latest test may have achieved an important technological breakthrough.

Technology could fundamentally shift submarine warfare

The research team's laser depth claim would put it in range of Australia's operational submarines. ( ABC News: Jarrod Fankhauser )

To put the laser's reach in context, Australia's current submarines, known as the Collins class, are reported to have a diving depth of 180 metres below sea level, though its ultimate diving depth could be much deeper, given that it remains classified information.

The laser technology, when deployed, could fundamentally change submarine warfare, given that the Chinese military — or any other — would possess the ability to spot another submarine before it entered their country's territorial waters.

Most submarine detection methods in operation today rely on Sonar — or sound navigation ranging — where sound is utilised to show where objects are in a body of water.

Sonar can either be deployed actively or passively: the former uses sound pulses which bounce off of an object, while the latter simply listens for the sound made by vessels.

This technology becomes flawed when a water body gets busier, as it is harder for sound to identify the exact vessel someone's looking for when there are a number of objects all producing various sounds, or shapes in which to bounce sound from.

In these moments, that is where airborne, or even satellite-mounted lasers, could come in handy.

"Until now submarine warfare has been conducted using acoustic systems, where you ping sounds off of underwater objects," Manabrata Guha, a researcher in war theory at UNSW Canberra, told the ABC.

"Given the inherent technological weaknesses of acoustic systems, obviously there has been a logical trend to explore [alternatives]."

China wants to see the sea at 500m below the surface

Most operational submarines use sound to find out where other vessels may be in a body of water. ( AFP: Guang Niu )

The Post also reported that the Shanghai research team were part of China's 'Guanlan' (Sea Watcher) project, which aims to build a laser satellite that fire a beam that can reach 500 metres below the sea surface.

The program was launched in May 2018 with over 20 research institutes and universities reportedly signing up to it.

Dr Guha said a depth of 500 metres would be beyond the maximum depth thresholds for most submarines in operation today, however, there may be submarines in operation that can dive much deeper — given that most submarine tactical information remains classified.

He added that the Shanghai team's laser test simply formed another piece added to China's "anti-access" military strategy, which is designed to deny anyone from entering its territorial waters.

"China's biggest operational fear is what happens if a few American submarines were to slip into Chinese territorial waters," Dr Guha said.

"That's when the detection systems become a problem because then their acoustic [surveillance] systems would be at a disadvantage there."

He explained that laser-based systems would allow Chinese military planners to be able to "triangulate and identify an American [vessel] underwater".

Tests using similar technology by the US Department of Defence have been reported to be able to penetrate 200 metres below the water's surface, but the effectiveness and ability to detect submarines while airborne has remained a question mark.

Considering that public images of the recent laser test shows the researchers flying above cloud level, scaling up to a satellite-based system was "quite far away", according to Dr Guha.

"If you're thinking about space-based systems then the challenges mount exponentially — it's not really easy to do this," he said.

He added the challenge was in being able to keep the strength of the laser stable enough across the journey from the satellite to the desired depth of 500 metres below sea level.

Similar tests by the US military also found that lasers become less effective in busy waters, as they may be confused by sea life, clouds or murky water.

And, considering the application of submarine-spotting lasers in space could be years, if not decades away, Dr Guha noted that news of the trial's success may have served another purpose.

"If I developed — and was sure about — the capability to deter my adversary, would I publicly announce this?" he said.

"The fact that [China] has gone to the extent of issuing a photograph of [the test] suggests that they're saying, 'Hey look, don't assume we can't do this'."

The latest announcement follows a trend of reports from China rolling out and displaying new technologies that could bolster its military capabilities, some of which were seen during China's 70th anniversary National Day celebrations this week.

The Shanghai Institute of Optics and Fine Mechanics were contacted for comment but have not replied as Chinese public institutions are on holiday until October 7 for a national holiday week.