How is 255 Tbit/s processed in optical fiber communication?
It seems like you're referring specifically to http://www.nature.com/nphoton/journal/v8/n11/full/nphoton.2014.243.html .  It can be read here: https://www.researchgate.net/publication/269099858_Ultra-high-density_spatial_division_multiplexing_with_a_few-mode_multicore_fibre .  

In this case, it's slightly more complicated than "an optical signal".  The link in question uses multiple forms of parallelism to achieve that 255 Tbps figure:


Dense wavelength division multiplexing is used to cram 50 different wavelengths into the fiber on 50 GHz intervals (~0.8 nm in the 1550 nm C band), each one carrying 1/50th of the data.  
The fiber used is a custom-made 7 core, few-mode fiber, with 3 modes per core, each with 2 polarizations, for 7*3*2 = 42 independent (more or less) channels.  It seems like the selling point of their fiber is that the isolation between the cores is quite good, so the receiver only has to equalize the crosstalk between the modes and polarizations of each core separately (7 parallel 6x6 instead of 42x42).  


Then, they used a 24.3 Gbaud 32-QAM signal format (5 bits per symbol, 24.3*5 = 121.5 Gbps) for all 42*50 channels, for an overall bandwidth of 0.1215*42*50 = 255.15 Tbps. 

Now, these guys actually cheated a little bit here: they take 50 lasers, multiplex them together, modulate that with a single IQ modulator, then decorrelate the polarizations and adjacent channels with fixed delays to emulate using independent transmitters.  So it's really just one signal at 121.5 Gbps, repeated 2100 times in parallel.  And the transmitted signal is probably just a pseudorandom binary sequence (PRBS) that's generated on the fly and not read out of memory.  Or it could be read out of fast SRAM or an array of DRAM in a high performance arbitrary waveform generator. 

On the receive side, digital signal processing is required to recover the original data by compensating for crosstalk between the modes and polarizations in each core and applying error correction.  The paper does mention a figure of 200 Tbps net, which would be the data rate before encoding for transmission (similar to how 1000BASE-X gigabit ethernet is 1 Gbps before encoding and 1.25 Gbps after, or PCIe is 2/4/7.877 Gbps before encoding and 2.5/5/8 Gbps after) but it's unclear what encoding and forward error correction scheme they are assuming.  

It also looks like they didn't build an actual receiver, but they are instead using two high speed oscilloscopes with coherent detectors to capture raw data and then doing the signal processing and equalization offline.  They also had to do a fancy time interleaved capture because they had to run coherent detection on all 3 modes from each fiber core at the same time, but they only had 2 fast oscilloscopes available.  And even that setup only allows them to receive 1 wavelength on 1 fiber core at a time--729 Gbps, and only in short bursts. 

But all of this is fine, because the paper is about the fiber and not the actual link.  

TL;DR: the 255 Tbps figure is a little bit misleading - they did not build a transceiver capable of that, but they evaluated the multicore fiber they made with 2100 copies of a 121.5 Gpbs signal and a single receiver.