“When using VOAs in Dense Wavelength Division Multiplexing (DWDM) optical networks, there are two critical performance parameters that must be minimized: wavelength-dependent loss (WDL) and the polarization-dependent loss (PDL). The WDL refers to the variation in attenuation loss over the specified wavelength range. The PDL refers to the variation in attenuation loss over all states of input polarization. In a VOA, WDL and PDL can vary as a function of the attenuation level. WDL and PDL are undesirable because they contribute to increasing differences in optical power between wavelength channels, which in turn increases the need for channel power equalization and increases the cost and complexity of optical networks. Various design approaches have been proposed to reduce WDL in ES-VOA, see for example U.S. Pat. No. 7,295,748.
“In a further development the ES-VOA component is packaged inside a Small Form factor Pluggable (SFP) housing. This product is referred to as SFP VOA. The SFP VOA offers several advantages compared to the stand-alone pigtailed ES-VOA component described above: (1) the SFP VOA is pluggable, the customer can gradually populate SFP VOA slots on the host system board as the system capacity is increased, (2) the SFP VOA pluggability allows for easy replacement, (3) no fiber management is required since the SFP VOA is connectorized, (4) the interface is digital and the attenuation level is set by a firmware instruction from the host board, the customer does not need to design control and drive hardware and does not need to know the specific characteristics of the ES-VOA component.
“However, compared to the stand-alone pigtailed ES-VOA, the SFP VOA suffers from higher WDL and higher PDL. The inventors have investigated the possibility that this may be caused by modal interference between the fundamental mode and co-propagating cladding modes launched in the output fiber..
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