New phase noise filter could enable cheaper compact lasers

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New filter chips developed at the University of Pennsylvania could enable high-quality lasers at a fraction of their current size and cost.

At present, lasers with adequately low frequency noise are bulky, expensive and an impractical choice for mass manufacturing. Penn Engineers have set out to solve this problem with a device called a 'phase noise filter' that can turn low-cost, compact lasers into those suitable for lidar and more.

Firooz Aflatouni, Skirkanich assistant Professor in Electrical and Systems Engineering have developed a way of reducing low-cost lasers’ frequency noise, achieving the same, if not better, performance as the larger, more expensive lasers.

Aflatouni and Idjadi published a study outlining the performance of their filter in Nature Photonics.

Aflatouni and Idjadi had previously published research on how a similar electro-optic system could be used to reduce noise in a low-cost laser’s frequency by forming a loop around the laser, feeding back the laser noise to itself. Now, they have shown how their new, 3mm2 filter chip can take the output of low-cost laser chips and convert it such that it has the same frequency noise as the expensive, state-of-the-art lasers that are hundreds of times bigger and significantly more expensive.

As an important advantage over their previous work, the implemented filter operates independently of the laser chip and thus can work with many different types of lasers.

Because a laser’s frequency corresponds to the number of times it toggles in a second, it is also reflected in its tone, or colour.

'The highest-quality red laser, for example, would generate a single, pure red colour at its output. That means its frequency can be represented with a thin line on the colour spectrum corresponding to that exact tone of red,' Aflatouni said. 'In practice, however, due to noise and other factors, lasers may generate multiple closely packed tones, resulting in a thicker line on the spectrum. The width of this line, also known as the laser linewidth, is therefore a way of measuring laser performance; the thinner the line, the closer it is to an ideal single-colour laser.'

Narrow linewidth lasers are essential in many applications such as communication and lidar.

'For example, in an advanced type of lidar, so-called "coherent’" lidar, the achievable range is inversely proportional to the laser linewidth; the lower the linewidth, the higher the range,' Aflatouni said.

Like their previous work, Aflatouni and Idjadi’s new filter chip uses a feedback loop system to lower the input laser’s linewidth.

'Our implemented chip measures the noise that broadens the linewidth, amplifies it and subtracts it from the laser output light in a loop, ultimately narrowing its linewidth,' Idjadi said.

Since their filters could be incorporated into existing manufacturing processes for the laser chips found in fibre-optic modems, these laser systems would also be considerably more cost-effective for mass production than their larger counterparts. This technology could enable low-cost, compact lidars and handheld diagnostic systems, and will reduce the size and expense of high-data-rate optical communications systems.


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