Phase Sensitive Gain

Phase-sensitive amplification of an optical signal by an SOA
in a nonlinear Sagnac interferometer

Yongzhang Leng, Christopher J.K. Richardson , Julius Goldhar
Presented at CLEO 08


  1. To explore the phase-sensitive amplification of optical signals using semiconductor optical amplifiers (SOA).
  2. To demonstrate optical switching with a weak signal and strong probe pulses for optical signal processing.
  3. An application of this device would be phase regeneration of DPSK signals [1.2]

Balancing a nonlinear interferometer
With pulses injected only from the left (signal is off) the minimum output is achieved from our loop mirror
(containing a non-linear element) when there is a symmetrical arrival of the pulses in the SOA (Point A in the figure below.
Point C corresponds to alternate arrival of the 2 counter propagating pulses at the SOA). (Our nonlinear device is a 1.2 mm long Alcatel 1901A SOA).

Phase sensitive amplification
A strong pump signal is injected into the Sagnac interferometer from
the left side and a weak signal is injected form the right. A piezoelectric
transducer is used to periodically stretch the fiber at a frequency of 100Hz which modulates the phase of the pump by several cycles.

The relative optical phase between the signal and the pump can be directly determined by monitoring the output from port D2. Typical output from the phase monitor and Sagnac interferometer is shown below.

The output of the interferometer shows strong dependence on the phase as seen from the plot above. Symmetrical phase sensitive gain is observed when the interferometer is set to points A or C.


  • Demonstrated the phase-sensitive amplification of optical pulses utilizing an SOA inside a Sagnac interferometer with a weak control signal and strong probe pulses.
  • Achieved an aspect ratio of 3 for the phase-sensitive gain. 
  • Obtained qualitative agreements between the experimental result and a simple numerical model of SOA. 

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