OMI | What is Optical Modulation Index
This page describes optical modulation index equation. AM and FM modulations are used in optical fiber. Optical modulation index in both electrical and optical domain is described.
Modulation Index in AM/FM
The modulation index for AM is expressed as follows.
m = Vm/Vc, where Vm is modulating signal and Vc is carrier signal.
The modulation index for FM is expressed as follows.
m = Fd/Fm, where Fd is frequency deviation and Fm is modulating signal.
Refer AM FM modulation index >>.
Optical Modulation Index
The term OMI (Optical Modulation Index) is defined as amount by which instantaneous power of optical carrier varies around its average power. OMI is defined in RFoG (RF over Glass Fiber) specification for amplitude modulation.
It is expressed in units of percentage (%). OMI is 100% when peak of amplitude modulated sine wave on optical carrier modulates instantaneous power of carrier from zero power to twice of average power value. The figure-1 depicts curve between optical power vs modulating current.
OMI is expressed by following equation in the optical domain.
OMI = (Pp - Pt)/Pt,
Where,
Pp = Peak optical power output of laser
Pt = optical power value at bias current
OMI is expressed by following equation in the electrical domain.
OMI = Ip/(Ib-Ith),
Where,
Ip = peak modulating current
Ib = bias current
Ith = threshold current of laser
➨OMI should always be measured with CW carrier which has same average power as the desired one.
➨OMI value always refers to modulation index of entire RF signal.
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