Single sideband modulation in Brillouin optical correlation domain analysis

Highly resolved distributed monitoring of strain and temperature can be easily achieved by means of fiber-optic Brillouin-based sensors. The majority of available Brillouin systems relies on Brillouin Optical Time Domain Analysis (BOTDA) techniques in which pulsed pump and/or signal waves are exploited, allowing a spatial resolution fundamentally restricted to about 1 meter [1]. Brillouin Optical Correlation-Domain Analysis (BOCdA) schemes have also been proposed where pump and probe are instead subjected to synchronous random phase modulation which guarantees an efficient Brillouin amplification confined only in a narrow section (even less than 1 cm) of the fiber under test (FUT), corresponding to the correlation peak [2, 3]. BOCDA architectures typically employ a standard push-pull Mach-Zehnder modulator (MZM) to shift the probe frequency in the Brillouin amplification range by means of the dual sideband (DSB) amplitude modulation of the probe wave. Narrow optical passband filtering, performed for example by using a temperature-controlled fiber Bragg grating, is also required in order to select only the probe sideband experiencing the Stokes Brillouin amplification [1-3]. In this paper we present a BOCDA solution, exploiting the capabilities of a nested I/Q MZM, usually employed for fiber optical communications with complex formats modulation, not only to obtain the required frequency detuning, but also to generate a single sideband (SSB) probe modulation [4] with a significant attenuation of the anti-Stokes probe sideband and a complete suppression of the optical carrier. This approach enables to effectively remove the gain-loss balance of the two sidebands, allowing a precise observation of the Stokes Brillouin gain spectrum (BGS) in order to derive the Brillouin Frequency Shift (BFS) and thus the temperature and strain information, without the need of any demanding optical filtering.