Measurement of Raman gain efficiency

The International Electrotechnical Commission in 2003 published the first version of a guidance document for measuring Raman gain efficiency on all types of singlemode optical fibers. This guidance document is technical report IEC TR 62324 titled “Single-Mode Optical fibers-Raman gain efficiency measurement using continuous wave method-Guidance.”

Developed in IEC SC86A/WG1, this document was motivated by numerous publications that have shown the transmission capacity and reach in long-haul systems can be boosted considerably by using distributed Raman amplification in the transmission fiber. Systems using distributed Raman amplification and in some cases also discrete Raman amplification in dispersion compensating modules are now being implemented in the field. That calls for transmission fiber designs optimized with regard to Raman amplification as well as thorough characterization using standardized measurements. This guidance document contains a complete description of measurement setup and procedure.

As described in the guidance document, the measured Raman gain efficiency depends on:

• Raman pump wavelength

• Signal wavelength

• Effective area of the fiber

• Fiber material composition

• Polarization of signal and pump

That means Raman gain efficiency will vary slightly from fiber to fiber of the same type and that several fibers need to be measured to achieve a representative distribution. To compare Raman gain efficiency results on different fiber types, it is important to use identical Raman pump wavelengths and signal wavelengths. For different types of nonzero dispersion fibers (NZDFs), large differences in Raman gain efficiency are observed because different commercial products have different nominal effective areas. For example, some NZDFs with moderate effective area have more than 20% higher Raman gain efficiency than NZDFs with high effective area.