Spectral efficiency is also a nifty metric because, given some bandwidth and signal-to-noise ratio, there is an upper bound to which you can compare a result [0].
Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.
Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.
[0] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...