I am still trying to understand the modulator in an optical free space transmitter.
I have found the following block scheme:
It is from "Direct-detection free-space laser transceiver test-bed". I am trying to make clear for me each part of this scheme.
What does 1/99 tap (read) mean?
From referenced article:
The CW master laser output is modulated using two LiNbO3 Mach-Zehnder modulators manufactured by EOSpace.
MZM are the same color and just before the 1/99 tap.
From: Segmented Mach–Zehnder Modulator With 32-nm CMOS Distributed Driver
Output light from the modulator is tapped 1% for power monitoring, while 99% passes through an optical amplifier (OA).
The 1/99 tap extracts 1% of the optical power for power monitoring allowing 99% to be output to the next stage.
I'm suspecting that it splits 1% of the power off to the bias control blocks and allows the remaining 99% of the power through.
This site has some description in a somewhat different context: https://learn.lightbrigade.com/blog/pon/fiber-optic-tap-couplers-for-fttx-systems
... tap splitters offer a unique solution for low-density installations. These products can taper the split percentages in increments ranging from as low as 1/99 percent up to the standard 1:2 (50/50) types. With a tap splitter in place, the cable near the first subscriber would have a drop cable spliced to the 1% leg of the splitter, and the other 99% of the optical power would be transmitted down the span.
As each subscriber would have different attenuation levels due to the splitter and span variations, a loss budget would need to be calculated for each subscriber between the OLT and their ONT. The maximum loss allowed per subscriber would vary as the distance from the OLT increases. This — along with splitter attenuation differences based on the split percentage — requires attention to detail when planning the system.
This first subscribers on each fiber would use the 1/99 tap splitters until the loss budget required a larger split percentage such as a 2/98 splitter. The percentages would continue to increase to maintain the optical power level at the ONT until the last splitter is installed, which would normally be a 1:2 (50/50) split.
