Optics and Photonics Journal, 2013, 3, 61-65
doi:10.4236/opj.2013.32B015 Published Online June 2013 (http://www.scirp.org/journal/opj)
Description of the FDML Laser with Quasi-steady State
Model of the SOA
Zhi Wang, Limei Zhang, Lanlan Liu, Zhenchao Sun, Yingfeng Liu, Fu Wang
Institute of Optical Information, School of Science, Beijing, Jiaotong University,
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing, China
Email: zhiwang@bjtu.edu.cn
Received 2013
ABSTRACT
Experiments and simulations demonstrate that an SOA-based ring cavity can operate as a tunable laser, wavelength-
swept laser or Fourier-domain-mode-locking laser according to the relation between the roundtrip frequency and the
sweeping frequency of the filter.
Keywords: Mode Locked Laser; Semiconductor Optical Amplifiers; Tunable Filter
1. Introduction
In the Fourier Domain Mode Locking laser (FD ML), which
was first presented by R. Huber [1,2], a narrowband op-
tical band pass filter is driven in resonance with the optical
roundtrip time of the laser cavity. The required resonator
length of several kilometers is realized by a long delay line
consisting of single mode fiber (SMF) and dispersion
management fibers. As each wavelength component cir-
culates in the cavity such that it is transmitted through
the filter at every pass, FDML represents a stationary
operating regime. Lasing does not have to build up re-
petitively as in conventionally wavelength swept laser
(WSL) sources, resulting in improved noise performance,
coherence length, output power and higher maximum
sweep repetition rates [1].
In spite of the numerous applications of FDML lasers
demonstrated [3-8] so far, up to now, only one model f or
the theoretical description of FDML is proposed by
Christian Jirauschek [9]. In their model, a dynamic equa-
tion is derived to identify the physical effects relevant for
FDML, and clarify the role of amplified spontaneous
emission (ASE) for self-starting and for the steady state
operation of FDML lasers. In 2012, they employed a
numerical simulation based on this model to investigate
the temporal evolution of the instantaneous power spec-
trum at different points in the laser cavity, and gained
deeper insight into the role of the physical effects gov-
erning FDML dynamics, such as gain recovery and
linewidth enhancement in the SOA, dispersion and self-
phase modulation (SPM) in the optical, and the filter
sweeping action [10].
However, there are a few defaults in Christian's model,
and a novel mechanism for SOA-based ring cavity
FDML laser (SOA-R-FDML) will be established based
on the quasi-steady state SOA [11] in this manuscript.
The improvement mainly comes from four aspects: first,
frequency dependence of the spectral gain (including the
material gain and absorption) are considered; second, the
gain properties of the SOA is simulated based on the
steady state model, which can give us the gain character-
istics for any incident frequency and power, including the
gain saturation; third, the ASE is always included in the
steady state model, accompanying with the resonance
light in the cavity and in the SOA; fourth, the FP trans-
mission function is used to accurately describe the sweep
filter.
2. Building up Laser Activity
Figure 1(a) is the basic structure of the SOA-R-FDML,
in which the SOA is the gain medium, the tunable filter
is driven by external signal, the coupler is for feedback
and output, isolators (ISO), polarization control (PC),
and dispersion management fibers are also included in
the cavity.
The cavity length of the SOA-R-FDML is about tens
of meters or kilometers, the roundtrip time of light in the
cavity is about hundreds of ns or us. The experiments
show that tens of roundtrips is necessary to build up the
laser from ASE with the amplifications by the SOA, so it
will cost about a few us or ms, which is much longer than
the gain recovery time of SOAs, which is abou t hundreds
of ps, so the SOA can be modeled as a steady-state ele-
ment.
When the injection curren t of the SOA is 200 mA, the
cavity is 20 m long, and the coupler is 70:30 (feedback :
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