The Coherent Optics Revolution: Transcending The 10G Wavelength Barrier

 The Coherent Optics Revolution: Transcending 10G Wavelengths - Part 1

The first stage of optical communication was dominated by what Mark Tinka called the simple optical detection scheme. A pulse of laser light represented a '1' and no light meant a '0' or vice versa. Cisco white papers call this 'on-off signalling'. So this approach is based on the optical power or intensity of light. The stronger a light pulse, the higher its amplitude. See the top diagram. 

The Achilles of this approach is chromatic dispersion, namely that fact different frequencies of light traverse a solid medium such as fibre glass at different speeds. Now any laser pulse is a band of frequencies. It may be narrow, but it always has non-zero width. So chromatic dispersion is inevitable (like Donald Trump continuously changing tariff rates). As the fibre path distance grows, the probable outcome is that a laser might transmit a '1 0' but the light will spread over time and the optical receiver consequently interprets it as a '1 1'. So chromatic dispersion becomes a hard stop in long haul communications whether terrestrial or undersea. 

A natural path to increase the bit transmission rate would be to chop up the optical amplitude into 4 levels and use each level to represent a binary bit pair.

So full optical power might represent '1 1', three quarters could be '1,0', half full power designates '0 1', and no light means '0 0'. But chromatic dispersion does not depend on intensity, so this scheme will fail as well. Moreover, it requires very sensitive optical receivers and drives up equipment costs. 

Fortunately, light has several dimensions. In addition to amplitude, there is phase. The phase is the position of a point on a wavelength relative to a fixed origin point. In the lower right hand diagram we see two identical wavelengths with different phases. So we can represent ordered combinations of zeros and ones by varying the phase. One simple approach that easily achieves 100G transmission is to vary the phase by 180 degrees or π radians, which means that what was the wave's peak becomes the zero point. The low signal to noise ratio permits 100G throughput over long distances. It's called Binary Phase Shift Keying or BPSK.

Of course, phase shifts do not come in discrete junks. It's a continuum of values. That's the beauty. So we can also shift phases by π/2 and since a wavelength is 2π radians, this means we have created 4 phases that can represent ordered pairs of binary bits. See the lower left hand diagram. We now have a clean way of communicating (1,1), (1,0), (1,1), and (1,0). So we effectively doubled the bit transmission rate. Naturally, this is called Quadrature Phase Key Shifting and it gets us to 200 Gbps.