Optical Modulation Index (OMI) is key variable in determining carrier power and thus Carrier-to-Noise Ratio (CNR), which is a primary factor in the performance of optical communications systems (as you will see in the equation below).
CNR = (m * I)2 / [2B(2qi + N + (RIN * I2)]
Where: OMI (m), video noise BW (B), receiver photocurrent (I), receiver noise equivalent current (N), and laser relative intensity noise (RIN)
Simply put, OMI is a measure of how much the modulation signal affects the light output, and is measured in %. OMI is used to set and verify the optimum operating point (light or current bias) that provides the best tradeoff between noise (under-modulation) and distortions (over-modulation). Thus, it is the ideal point at which a laser will provide the best performance.
Why is setting OMI so important?
With systems operators increasing the number of services being delivered, it is crucial that OMI is set properly in order to achieve peak system performance. In addition, before laser transmitters are shipped out or deployed in a system, there are a number of variables (manufacturer setup, front panel settings, etc) that can lead to inaccuracies that significantly affect performance in the field. By initially setting proper OMI and routinely checking it in the future, operators can ensure lasers are always at the optimal level, as well as spot potential issues prior to failure.
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