Chapter 4
NOLM Simulation Software
The first step before the physical realization of the experimental setup is the implementation of a simulation software. The software was necessary to understand the complete behaviour of the interferometric structure and its realization allowed us to evaluate the correct setting of a number of variables like fiber length and splitting ratio, which were to be chosen among fixed values.
4.1 National Instruments LabVIEW
The NOLM simulator was realized using the National Instruments “LabVIEW” package. This package is specifically designed to permit a quick implementation of a computer-controlled data gathering and analysis system which can be extensively customised. LabVIEW is a graphical software system for developing high-performance scientific and engineering applications. LabVIEW can acquire data and control devices via IEEE-488 (GPIB), RS-232/422 and modular (VXI or CAMAC) instruments as well as plug-in I/O boards. LabVIEW programs, called "virtual instruments" (VIs), are created using icons instead of conventional text-based code. A VI consists of a front panel and a block diagram. The front panel (with
4.2 NOLM Simulator
The software we realized simulates the behaviour of a SPM-based NOLM
Fig.4.1 Schematic illustration of SPM-based NOLM.
computing the input/output power characteristic. Input values we can set are fiber length [km], fiber attenuation [dB/km], nonlinear coefficient γ[W-1
km-1], lumped loss [dB] and coupler splitting ratio:
As seen in the previous chapter, Ein JG
[W1/2] is the NOLM input field with a generic splitting ratio ρ at the coupler. For clockwise and counterclockwise paths, the counter-propagating signals E1 and can be expressed as:
JG 2 EJG 1 i E = ρ ⋅E JG JG n E2 = 1− ρ ⋅Ein JG JG
Due to the presence of the lumped loss the field intensities are unbalanced and experience different phase shifts. The clockwise travelling signal, whose initial power is ρ·Pin,
experiences SPM and the induced phase shift is given by:
c P Lin eff
φ = γρ
where Leff (the effective fiber length) is computed as:
L 1 e Leff −α − = α
α being the fiber attenuation per unit of length. On the other side also the counterclockwise travelling signal, whose power after the lumped loss is a·(1- ρ)·Pin (a < 1 is the attenuation
introduced by the concentrated loss), experiences SPM and the induced phase shift is:
cc a(1 )P Lin eff
φ = γ − ρ
After having covered the entire loop (each one proceeding along its own way) the two signals travel again through the coupler. We assume the two signals to be polarized on the same axis; this is true for Polarization Maintaining fibers, while for Not-PM ones a polarization controller is required to be inserted into the loop in order to align the signals into the coupler. The output signal is then the result of two fields:
(1)
out in in
EG = a E exp[ jρG φ + γρ0 j P L ]eff
and E(2)out originates from the counterclockwise signal: JG
(2)
out in in
E = a (1− ρ)E exp[ jφ + γ0 j a(1− ρ)P Leff + ]
JG JG
π
φ0 = βLeff is the linear phase shift( being β the propagation constant). The presence of π is due
to the two crossed-passes through the coupler, each one responsible of a phase shift of π/2. The output signal’s power is 2.
out out