Going the distance

With ECOC on the horizon, Tom Eddershaw reports on the latest advances of laser technology for optical communications

In a report published in June 2014, BCC Research predicted the VCSEL market to grow from $501.0 million in 2013 to nearly $2.1 billion in 2018. At this year’s ECOC exhibition and conference – which will take place in Cannes at the Palais des Festivals from 22-24 September – visitors will be able to see the latest VCSEL technology for the optical communications market, all turning data into pulses of light to be transmitted along optical fibres.

VCSELs are now dominating the market for shorter range communications – under 500m – and the laser technology is extending its reach in both market share and transmissible distance. On 25 February 2014, the Optical Society of America announced that IBM researchers had transmitted data at a rate of 64 gigabits per second (Gb/s) over a cable 57m long using a VCSEL which beat the previous record by around 14 per cent.

The distance information can travel before needing repeating, and the speed of data transmission is reliant on these light sources and both attributes are improving constantly. Erin Byrne, TE Connectivity’s director of fibre optics engineering, said: ‘Laser suppliers are always trying to increase the distances served by their products. Particularly due to the growth in large data centres, the demand for longer reach VCSELs is increasing. Longer reach products are the result of R&D investment by companies like TE in response to customers’ unmet needs.’

These customers also require ever faster data transmission. Byrne continued: ‘Overall bandwidth demand for communications and computing is also driving the need to increase the rate of transmitted data. TE’s Coolbit product technology platform was introduced to address the need for VCSEL-based products at 25Gb/s data rates. Products made using Coolbit technology aggregate these 25Gb streams into multifibre transceivers and active optical cables that transport up to 400Gb/s of data at distances up to 100m. Each increase in data rate is accompanied by an ecosystem development of signal-generating integrated circuits, backplane and I/O interconnects, and standards development.’

Multimode fibre which works best with 850nm wavelength light is where Byrne said VCSELs are most heavily favoured. However she said: ‘Since attenuation is higher in multimode fibre, the optical signal degrades faster with distance, thereby shortening the distances achievable but it has more to do with the fibre than the laser.’

Transmitting this data over a certain distance can prove problematic due to the weakening of the signal. This attenuation is inherent to the glass fibres used, as Bill Diamond, vice president of product management for Oclaro’s optical network solutions department, explained. ‘There are three typical wavelengths that are commonly used: 850nm which the VCSELs commonly produce and 1,310nm and 1,550nm for longer distance communications.

‘The reason the three different wavelengths are important is because glass doesn’t pass all wavelengths of light equally. Some wavelengths are transmitted by the glass more efficiently than others; 1,310 and 1,550nm are two points in the spectrum that pass through more easily, resulting in less loss of light intensity or brightness, even after travelling a long distance down the glass. The 850nm also transmits very efficiently, but not as effectively as the other two wavelengths.’

Errors can also come into play if the light pulses used to transmit the data interfere with each other and Byrne explained that this could be caused by a phenomenon known as chromatic dispersion. This is the spreading out of light over distance, as light with a different wavelength travels through the fibre at different speeds. If the light pulse spreads out enough, and the pulse behind it is spreading as well, the second pulses can catch up with the first and they can interfere with one another. This makes it very hard to distinguish one pulse from the other and can produce erroneous data.

The varying wavelengths of multimode light cause more errors to be produced over distance and mean the distances covered by multimode fibre tend to be lower. The alternative is to spread out the pulses which would reduce the amount of cross over, however this would slow the flow of data. While chromatic dispersion is more prevalent when using multimode fibre, in data centres or enterprise systems where they are commonly used, this is rarely problematic as the distance the pulses have to travel is much less.

Dr Periklis Petropoulos, Professor at the Optoelectronics Research Centre (ORC), University of Southampton, pointed out that increasing power is not necessarily beneficial to the distance. ‘In the context of communications, the availability of high power sources is not an issue. In fact, transmitting data at high powers is not preferred, for the reason that the transmission channel needs to be viewed as nonlinear and its behaviour dependent on the power of the optical field, especially when long lengths are involved.’ He explained that rather than opting for high power transmitters, it is preferred to maintain as low optical powers along the transmission link as possible which is maintained by amplifying the signal more frequently.

He commented: ‘Again, however, the situation is likely to change with the wider adoption of sophisticated digital signal processing techniques, which might prove capable of mitigating the effects of optical nonlinearities. Also, the development of novel fibre technologies exhibiting much lower nonlinearities might change the perspective in this area, for example, photonic bandgap fibres guiding light in an air region.’

In it for the long haul

When considering long-haul communications, Petropoulos stated, for a long time people have been using semiconductor DFBs, which give reasonable amounts of power, and a relatively narrow linewidth. Then several lasers emitting at slightly different wavelengths would be combined in the same fibre to create a wavelength-division multiplexed (WDM) system.
Oclaro’s Diamond explained that it was the longer, 1,550nm wavelength glass used for WDM, and said: ‘Wavelength division multiplexing involves emitting light at slightly varying wavelengths, by one or two nanometres, which allows for a much greater data traffic flow. Each slightly different colour can be passed down the fibre at the same time without interfering with the other.’

Petropoulos said:‘The WDM channels have historically been spaced by 100GHz (more recently this spacing has been reduced to 50 or even 25GHz). In this sense, the channel spacing is much greater than the bandwidth of the demultiplexing filters and the detection bandwidth, so no interference of the channels will be detected at the receiver.’

He continued: ‘The situation changes with the re-emergence of coherent communications. In this case, where the system relies on the use of a local oscillator at the receiver, lasers that exhibit a much narrower linewidth are required.

‘Coherent communications allow one to transmit more information per transmitted optical pulse. This is because information can be encoded, not only on the binary state of the intensity of the pulse – as typically happens in on-off keying communications – but also on the phase of the optical carrier. Therefore, a pulse no longer represents a single bit, but multiple bits at the same time.’

Not going it alone

‘The technology has got better and better over the time,’ observed Diamond. In terms of the laser’s evolution, the typical route of the technology shrinking is not seen here. Diamond pointed out that it is occasionally the case that the chips actually get bigger. However, he said: ‘You wouldn’t recognise any significant difference until you look at the chip under a microscope.’

‘When you are talking about optical communications, you have a laser on a chip; the chip has to go into a system. Within this box you need a power supply, a fibre to take the light away and many other components. It’s one thing to have a laser capable of turning off and on 25 billion times a second, but you also need electrical circuits that can deliver the information at these speeds.’

‘In data communications you see a lot of transceivers which take data in and give light out in one direction, and take light in and give data out on the other,’ said Diamond: ‘Manufacturers and researchers are constantly trying to make these smaller, provide faster data transmission, and make them more energy efficient. The more energy efficient they are, the more you can place on a circuit board, and the better port density you have. This in turn means you can have a lot of data traffic capability in a small space.’

Diamond remarked that the overall size of the transceiver is not that dependent on the size of the laser chips, but the other components within the device are constantly shrinking.

He concluded: ‘This is very important for data centres or telephone exchanges; you have more processing power and more information going through. You need to handle that with physically smaller technology that is going to use up less power and generate less heat. More speed, lower power consumption, lower cost; these are the things that are changing year on year.’