All Optical Wavelength Conversion and
Parametric Amplification in Ti:PPLN
Channel Waveguides for
Telecommunication Applications
Dem Department Physik der
Universität Paderborn
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
vorgelegte
Dissertation
von
Rahman Nouroozi
1. Gutachter: Prof. Dr. Wolfgang Sohler
2. Gutachter: Prof. Dr. Christine Silberhorn
Tag der Einreichung: 19.10.2010
dedicated to
Nasrin
&
Sana
Abstract
Efficient ultra-fast integrated all-optical wavelength converters and parametric
amplifiers transparent to the polarization, phase, and modulation-level and -format
are investigated. The devices take advantage of the optical nonlinearity of Ti:PPLN
waveguides exploiting difference frequency generation (DFG). In a DFG, the signal
(ls) is mixed with a pump (lp) to generate a wavelength shifted idler (1/li = 1/lp –
1/ls). The mode-selective excitation of the pump (shorter wavelength) is difficult in
a directly pumped DFG. However, by internal generation of the pump via cascaded
second harmonic generation and DFG (cSHG/DFG) or sum frequency generation
and DFG (cSFG/DFG), the phasematched pump (SH or SF) mode can be excited
selectively. Therefore, efficient generation of the pump in Ti:PPLN channel guides is
investigated using different approaches.
In the waveguide resonators, first a resonance of the fundamental wave alone is
considered. It is shown that the maximum power enhancement of the fundamental
wave, and therefore the maximum SHG efficiency, can be achieved with low loss
matched resonators. By this way, SHG efficiency of ~ 10300%/W (10.3 %/mW) has
been achieved in a 65 mm long waveguide resonator. Its operation for cSHG/DFG
requires narrowband reflector for fundamental wave only. Thus, the SH (pump)
wave resonator is investigated. The SH-wave resonator enhances the intracavity SH
power only. Based on this scheme, an improvement of ~ 10 dB for cSHG/DFG-
based wavelength conversion efficiency has been achieved with 50 mW of coupled
fundamental power in a 30 mm long Ti:PPLN. However, operation was limited to
relatively small fundamental power levels (< 50 mW) due to the onset of photo-
refractive instabilities destroying the cavity stabilization.
The cSHG/DFG efficiency can be considerably improved by using a double-pass
configuration in which all the interacting waves were reflected by a broadband
dielectric mirror deposited on the one endface of the waveguide. However, due to the
wavelength dependent phase change by the dielectric folding mirror phase
compensation is required to maintain an optimum power transfer. Three different
approaches are investigated and up to 9 dB improvement of the wavelength conver-
sion efficiency in comparison with the single-pass configuration is achieved.
Polarization-insensitive wavelength conversion is based on a polarization main-
taining fiber loop configuration. Since both polarization components can be
converted in a contra-directional single-pass waveguide, differential group delay
(DGD) equalization between them is automatically provided. With such polarization
diversity scheme an error-free polarization insensitive conversion of 320 Gb/s
differential quaternary phase shift keying (DQPSK) data with signal pulses of 1.4 ps
width has been achieved using the packaged and pigtailed cSHG/DFG-based
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