  Fiber optic sensing is becoming an important means to physically secure
today's network infrastructure. However, a network-wide deployment of the
monitors will require cost reduction of the interrogator system, which can only
be achieved by highly integrated system components. In this contribution, we
report on the use of an in-house designed single-chip coherent transceiver for
acoustic fiber sensing. The transceiver on the basis of silicon photonics
contains a high-speed dual-polarization IQ-modulator as well as a coherent
receiver with balanced photodiodes and trans-impedance amplifiers, as defined
by the OIF integrated coherent transmit-receive optical sub assembly (IC-TROSA)
implementation agreement. The laser, used for transmission and as local
oscillator, is provided external to the photonic integrated circuit and can be
chosen according to the line-width requirements of the sensing system. The
acoustic sensing demonstration is using a correlation-based optical time domain
reflectometry with coherent detection. This method is able to detect, besides
the amplitude information, the phase of the back-scattered signal, which has a
significantly higher sensitivity to environmental effects on the fiber, like
temperature and strain. As a proof of concept, sensing of an acoustic signal
after a fiber span of 20 km is demonstrated by evaluating the obtained phase
information, providing information on external dynamic events up to a frequency
of 1.75 kHz.
