What is the purpose of Electro-Optical Modulator (EOM)?

[OrangeYogi]

Hello,
I'm studying the setup of distributed Brillouin sensor (using fibre optics) and don't quite understand the purpose of EOM in the sensor. It says that
"to generate both the pump and the probe waves from a single physical light source by using an electro-optical modulator (EOM)", but since we have a reflected fibre end, why do we have to generate two waves?

Thank you.

 

[deskswirl]

Direct modulation of a laser diode increases the phase noise of a laser output. The reason is that the number of carriers injected directly determines the number of stimulated photons and vice versa. The result is that the refractive index is also directly modulated leading to the cavity (from the photons perspective) to change in effective length. Hence at the laser output you would measure increased spectral broadening than is naturally present due to spontaneous emission.

One method to avoid this is to use an external modulator where the laser is operated in a continuous-wave (CW) mode and the beam itself is modulated. In fiber optics a Mach-Zender interferometer is usually used whereby the optical signal is split into two arms and one of the arms in directly modulated (electrically with a RF signal) and then the two beams are recombined. This will result in information to be encoded onto the optical carrier waveform.

I suspect, although I'm not familiar with distributed Brillouin sensors, that they require a source with a narrow linewidth to operate effectively.