Importance & Definition of Optical Modulation Index (OMI)
The primary factors in the performance of optical communications systems are Optical Power and Optical Modulation Index. The OMI must be set properly to achieve optimum Carrier-to-Noise Ratio (CNR) while avoiding unacceptable large non-linear distortions and threshold clipping effects. Static clipping behavior increases the lasers non-linear distortions and imposes an upper limit on its channel capacity.
The mathematical calculation of CNR for a multi-channel carrier system is a statistical calculation comprised of several variables: OMI (m), video noise BW (B), receiver photocurrent (I), receiver noise equivalent current (N), and laser Relative Intensity Noise (RIN): CNR=(m*I)2/[2B(2qi+N+(RIN*I2)]
As you can see, OMI (m) is a key variable along with optical power in the CNR equation. Therefore, in order to optimize the system, it is vital to get as much optical power from a laser as the result of setting OMI to the optimum level. The FOS1000A from M2 Optics is the only instrument available in the market with the ability to measure/monitor OMI (while taking all variables into account), making it an extremely valuable tool.
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