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November,
2002
EEEL Researchers Quantify Resolution and Accuracy of Relative Group Delay Measurements for Optical Fiber Telecommunications |
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Shellee Dyer and Joey Espejo, of the Optoelectronics Division, have quantified the effects of signal-to-noise ratio on low-coherence interferometric measurements of optical components’ relative group delay. In optical fiber telecommunications, accurate measurements of group delay and its spectral dependence are critical for high-data rate applications. Low-coherence interferometry offers a fast and flexible means to achieve these measurements. However, the Fourier transform relationship involved in the calculation of group delay often obscures the tradeoffs between measurement noise and the achievable spectral and temporal resolutions.
Shellee and Joey used a hydrogen cyanide gas cell as their test sample; it is commonly used as a wavelength reference standard, but it also exhibits a predictable group delay profile and is a potential candidate for a group delay calibration reference. Their theoretical model started with an accurate measurement of the cell’s transmission function, and then they calculated the complex refractive index using the Kramers-Kronig theorem. From the complex refractive index, they created a model of the output of the interferometer, and they used this model to investigate the effects of noise, truncation length, and quantization on the measurement of relative group delay. Using this simulation program, they predicted both the group delay and its resolution as a function of both signal-to-noise ratio and wavelength resolution. Shellee and Joey designed and constructed a low-coherence interferometer to test the theoretical results. The measured group delay profile agreed with their theoretical prediction to better than 0.3 ps at 6 pm wavelength resolution. They also demonstrated an agreement better than 0.02 ps between simulated and measured group delay resolution at this wavelength resolution.
These results illustrate the high-resolution capability of the low-coherence interferometric measurement system and the important but often misunderstood tradeoffs between wavelength resolution and group delay resolution. In order to compare the group delay resolutions of different measurement systems, they all must operate (or be made to operate) at the same wavelength resolution.
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