Parametric Wavelength Conversion

==Parametric Wavelength Conversion Technologies==
The first type of wavelength conversion relies on parametric wavelength conversion based on difference frequency generation in semiconductor waveguides.  This method offers the following unique parameters not available in other techniques.
#Strictly transparent
#Multichannel conversion with extremely low crosstalk
#Wide conversion bandwidth
#Polarization insensitive conversion efficiency, polarization diversified operation
#Quantum noise limited operation

[imageright|Schematic of an optical frequency mixer based on difference-frequency-generation|{UP}Projects%2fWavelengthConversion%2fWavelengthConversion.gif]
[imageright|Measured conversion efficiency tuning curve for two conversion processes, TE conversion to TM and TM conversion to TE.|{UP}Projects/WavelengthConversion/Result2.gif]
 [imageright|wavelength conversion mapping from one frequency (w1) to the other (w1’) and multichannel conversion mapping from two input channels (w1 and w2) to two output channels (w1’ and w2’)|{UP}Projects/WavelengthConversion/DFGb.gif]
[imageright|Simultaneous wavelength conversion of eight input wavelengths(1546, 1548, 1550, 1552, 1554, 1556, 1558, 1560 nm) to a set of eight converted wavelengths (1538, 1536, 1534, 1532, 1530, 1528, 1526, 1524 nm).measured (a) without a cross polarizer|{UP}Projects/WavelengthConversion/result3.gif]
[imageright|Simultaneous wavelength conversion of eight input wavelengths(1546, 1548, 1550, 1552, 1554, 1556, 1558, 1560 nm) to a set of eight converted wavelengths (1538, 1536, 1534, 1532, 1530, 1528, 1526, 1524 nm).measured (a) without a cross polarizer, and (b) with a cross-polarizer showing filtering of input wavelengths but allowing collection of converted wavelengths.|{UP}Projects/WavelengthConversion/result4.jpg]
[imageright|Schematic of (a) an optical frequency mixer based on difference-frequency-generation and (b) wavelength conversion mapping from one frequency (w1) to the other (w1’) and multichannel conversion mapping from two input channels (w1 and w2) to two output channels (w1’ and w2’)|{UP}Projects/WavelengthConversion/DFGc.gif]
Wavelength conversion is an effective solution to resolving packet contentions without requiring path deflection or packet buffering which may skew the packet sequence and may add latency.  Wavelength conversion, on the other hand, resolves the contention by transmitting at an alternate wavelength through the same route, resulting in the identical latency and packet sequence.  There are numerous wavelength conversion technologies available, and their characteristics and performance significantly influence the architecture and the performance of the WIXC.

In the proposed architecture of Next Generation Networks, it is important that both the data payload and the header are switched together in the WIXC.  This requires amplitude transparency in the wavelength converter since both digital information in the baseband and analog information in the subcarrier must be converted with high fidelity.  It is also desired that multi-channel wavelength conversion can be utilized to minimize the number of wavelength converters and complexity in the WIXC.  In the proposed work, we will investigate parametric wavelength conversion by difference frequency generation (DFG) in AlGaAs waveguides.  The DFG wavelength conversion in AlGaAs waveguides offer a number of unique properties not available in other wavelength conversion techniques.  These properties are particularly well suited for Next Generation Networks, and are summarized above.  We discuss this wavelength converter in more detail below.




Fig. II.1 illustrates such a DFG wavelength converter.  For example, a parametric mixing of an input signal at a wavelength l1= 1.53 m and a pump wave at a wavelength lp= 0.77 mm results in a generated wave at a wavelength of l1'= 1.55 mm, as is dictated by the frequency mixing relations, w1' = wp – w1 , or equivalently, 1/l1' = 1/lp – 1/l1.  The parametric wavelength converter is also capable of converting more than one wavelength at a time.  For instance, the same wavelength converter pumped by the same pump wave converts another input signal at l2= 1.55 mm to an output signal at l2'= 1.53 mm (1/l2' = 1/lp – 1/l2).  The two example conversion processes (l1= 1.53 mm to l1'= 1.55 mm, and l2= 1.55 mm to l2'= 1.53 mm ) can occur simultaneously, which allows an interchange of information between the two wavelengths.  Such multiple wavelength conversion processes can take place among any number of wavelengths within the optical bandwidth of the device.

While the DFG wavelength conversion has significant potentials, the conversion efficiency can be low due to weak nonlinear interaction and phase-matching requirements.  We propose to exploit large nonlinear optical susceptibilities (eg. c(2)GaAs=180 pm/V) in AlGaAs materials and to achieve quasi-phasematching (QPM) in a waveguide with patterned crystal orientation.  Due to dispersion in the waveguide, the phase velocities of the interacting waves are not matched for efficient nonlinear interactions unless a special arrangement such as QPM is made.  The power flow from one wave to the other is determined by the relative phase between the waves, and the differing phase velocities between the interacting waves lead to an alternation of the power flow.  The alternation of the sign of power flow results in repetitive growth and decay of the generated idler power along the length of interaction.  The distance over which the relative phase changes by p is called the coherence length.  The proposed quasi-phasematching involves repeated modulation of the sign of the nonlinear susceptibility after each propagation through a coherence length.  In crystal optics, reversal of the sign of the nonlinear susceptibility can be achieved by inversion of the crystallographic orientations.  In this proposed work, we will fabricate a waveguide with periodical patterning of semiconductor crystal orientation for quasi-phasematched nonlinear optical interaction.  The following sections discuss fabrication methods of such a waveguide.

===Wafer-bonded periodically-domain-inverted waveguide===
[imageright|Schematic of a quasi-phasematched waveguide wavelength converter with N input wavelengths.  Active gain section can be integrated in this waveguide to provide high intensity pump within the waveguide.|{UP}Projects/WavelengthConversion/QPMWAVEGUIDE.gif]
The design and fabrication of such a periodically-domain-reversed waveguide consists of those of a template and those of a waveguide. Fig. II.4 (a), (b), (c) illustrates the fabrication procedure involving wafer-bonding and epitaxial regrowth. The template is prepared by bonding two [001] orientation epitaxially grown wafers such that the [110] crystal directions of the two wafers are parallel to each other.  Fig. II.4(a) shows a schematic of the two wafers.  The wafer bonding process will achieve an intimate atomic rearrangement at the interface.  After the bonding, one of the GaAs substrates and a 1 mm Al0.8Ga0.2As sacrificial layer will be selectively etched away.  Subsequent patterning of a grating and selective wet-etching steps will reveal, alternately, a GaAs surface from the bottom substrate and a GaAs epitaxial layer from the removed top substrate.  The grating lines extend in the [110] direction and the crystal domains alternate.  The nonlinear AlGaAs waveguide to be epitaxially grown on the patterned template consists of a planarization layer, a 2 mm lower cladding layer, a 1 mm core layer, and a 2 mm upper  cladding layer.  A subsequent patterning of a waveguide stripe and Al0.6Ga0.4As regrowth will realize a buried hetero waveguide. Using this technique, we have recently demonstrated the first semiconductor DFG wavelength converter.  Fig. II. 5 shows (a) an experimentally measured spectrum for simultaneous conversion of two input channels at 1528 nm and 1536 nm and converted waves at 1548 nm and 1556 nm, and (b) measured conversion efficiency tuning curve for two conversion processes, TE conversion to TM and TM conversion to TE.  For arbitrarily polarized input waves, the converted waves also followed the two curves in Fig. II.5 (b), indicating that the conversion efficiency is polarization independent.  The measured conversion efficiency was -17 dB for 65 mW pump power, which was far below the theoretically calculated -4 dB for 100 mW pump power assuming 2 dB/cm loss at interacting waves [10].  This difference was due to imperfect planarization of waveguide corrugations during the fabrication.  The optical loss coefficient is proportional to the square of the corrugation amplitude, and as small as 25 nm corrugations in the fabricated waveguide is responsible for the 45 dB/cm loss measured at the pump wavelength, 771 nm.  In the proposed effort, we will use an atomic planarization method to realize a low loss, highly efficient parametric wavelength converter.  Reducing the corrugation amplitude to below 5 nm will bring the corresponding scattering loss to below 2 dB/cm, which is where the theoretical -4dB conversion efficiency was calculated.  The atomic planarization scheme will adopt mass flow dynamics and we anticipate much less than 1 nm corrugations in the waveguide.
[imageright|Experimentally measured spectrum for simultaneous conversion of two input channels at 1528 nm and 1536 nm and converted waves at 1548 nm and 1556 nm |{UP}Projects/WavelengthConversion/Result1.gif]
[imageright|An illustration of fabrication procedure for a waveguide with patterned crystal orientation.  (a) two wafers just before the wafer bonding process, (b) after the bonding, a grating is lithographically patterned, GaAs and InGaP layers are selectively etched to reveal a GaAs surface from the bottom substrate.  (c) subsequent growth of a superlattice layer for planarization and a waveguide consisting of a lower Al0.6Ga0.4As cladding layer, an Al0.5Ga0.5As core layer, and an Al0.6Ga0.4As layer completes the growth.  The layer thicknesses shown in the figure are for the design utilized for telecommunications wavelength converters at 1.5 microns, and the layers will be thicker for mid-infrared applications.  Shown in (d) is a SEM photograph of a fabricated waveguide crossection stained to enhance contrast of the domain boundaries.|{UP}Projects/WavelengthConversion/QPMWAVEGUIDE2.gif]
Fig II.6 shows an eight-channel conversion experiment measured in a similar setup using a single polarization input (a) without the cross polarizer allowing both input signals and converted signals to be collected, and (b) with the cross polarizer filtering out the input signals allowing only the converted signals.
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