29-06-2012, 05:51 PM
Optical frequency combs
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INTRODUCTION
Optical frequency combs can be applied to several attractive applications,
for example, wideband multi-wavelength continuous-wave (cw) sources for
wavelength division multiplexing systems, ultra short optical pulse generator for
time division multiplexing systems and optical frequency reference. For these
purposes, high coherence, high stability, low noise, high efficiency, low cost and
simplicity are key issue. In addition, it is important to generate comb signals with
good spectral flatness, namely, each spectral component should have save
intensity. Conventional optical comb sources are based on mode-locking
techniques. Mode-locked semiconductor lasers1 and Erbium doped fiber lasers2
have been developed. However, such lasers have drawbacks in stability because a
long-length cavity is influenced by the conditions of the environment.
Furthermore, the comb spacing is almost fixed, because that is decided by the
cavity length. On the other hand, optical comb sources based on optical
modulators are good candidates for a flexible and stable source3,4. These sources
have no cavity configuration: thus, they can operate stably. Here we use Dual
Drive Mach-Zehnder modulator (MZM) to produce broad band comb signal. The
comb generator was based on an optical modulation technique using a Mach-
Zehnder modulator (MZM). The benefits of this system are high stability, low
jitter, and independent control of the comb spacing and the center wavelength.
FLAT COMB GENERATION
PRINCIPLE OF OPERATION
The optical frequency comb generator consisted of a tunable
semiconductor laser diode (TLD) and an LiNbO3 dual-drive MZM having halfwave
voltage of 5.4 V. A CW light was generated from the TLD, with an intensity
of 5.8 dBm. The CW light was introduced into the modulator through a
polarisation controller to maximise modulation efficiency. The MZM was dualdriven
with sinusoidal signals with different amplitudes (RF-a, RF-b). The RF-a
sinusoidal signal at a frequency of 10 GHz was generated from a synthesiser,
divided into half with a hybrid coupler, each half being amplified with a microwave
amplifier, and then fed to each modulation electrode of the modulator. The
intensity of RF-a injected into the electrode was attenuated a little by giving loss
to the feeder line connected with the electrode. The input intensities of RF-a and
RF-b were 35.9 and 36.4 dBm, respectively. The phase difference between RF-a
and RF-b was aligned to be zero by using a mechanically tunable delay line that
was placed in the feeder cable for RF-a. The spectra obtained from the frequency
comb generator were measured with an optical spectrum analyser.
EDFA
An Erbium Doped Fiber Amplifier (EDFA) consists of a few meters optical
fiber doped with a few parts per million of the rare earth element erbium. The
optical signal is injected into this fiber, as well as the light from a special 'pump'
laser which is designed to excite the erbium ions.