10G TOSA for 25G Transmission
Dec 06, 2025|
A
The white paper on 5G bearer optical modules mentions the concept of overclocking.
B
Let's first discuss what the eye diagram would look like if 10GTOSA were used without any processing on a 25G chip.
The typical eye diagram we see looks like this:
C
An eye diagram is actually a collection of pulses accumulated over many time intervals, like a deck of cards.
The eye diagram is structured like this: the first line in front of the main image is 101, a three-pulse segment lasting one hour. The middle line is a smaller segment with 0103 pulses. There are also segments with 111, 000, 110, 011, and so on, up to 500 or 1000 pulses – that's the eye diagram we see.
I'll just pick one out: the one with 101 and 3 pulses.
D
The amplitude of the signal is like the height of our jump; 1 is jumping up, 0 is crouching down. 101 represents the time interval from crouching to jumping and then back to crouching.The modulation capability of the laser is such that it's easy to modulate a 25 G signal using a 100 Gb/s laser. In other words, this laser has a very large bandwidth, just like a professional high jumper who jumps fast and high.
E
If my daughter were to perform this routine, it would be fine; it would be considered a normal squat jump, with average jump height and speed.
But with my own father, it wouldn't work. My dad's jumping ability is limited (like trying to do 25G work with a 10G laser). His squat jumps are clumsy and disorganized. He'll hear the command "from 0 to 1," but before he can even jump, the next command is already given, "from 1 to 0." So he barely manages to complete a 010 movement.
Therefore, using 10G TOSA to modulate a 25Gb/s signal has two main drawbacks. First, the eye diagram looks terrible.
Closed eyes are costly, resulting in a triangular eye shape and poor information transmission quality. However, it's not unusable, because as long as the receiver can recognize the signal, it's fine. It's simply a matter of identifying 0s and 1s, like a squatting and jumping motion.
If a professional judge were to assess the squatting and jumping motion, my dad's haphazard movements would definitely fail. In other words, ordinary reception wouldn't suffice.
But if my dad tries to be more attentive and jump more seriously, and my mom takes over the receiver's interpretation, then it's fine. My mom, with her rich life experience, can automatically fill in the gaps in my dad's imperfect movements.
From a technical standpoint, using a low-bandwidth laser to modulate a high-speed signal results in insufficient modulation bandwidth, high eye-closing costs, and strong inter-symbol interference. Therefore, the receiver performs equalization, which uses backend signal processing to compensate for the shortcomings in the transmitter's signal quality, aiming to distinguish between 1s and 0s.
The simplest equalization is a CTLE filter. If the high-frequency signal attenuates significantly during transmission, the receiver's filtering bandwidth remains unchanged; if the low-frequency signal attenuates less, the receiver will selectively reduce its amplitude.
10 GTOSA is used on 25G signals for two main purposes: pre-emphasis at the transmitter and, most importantly, equalization at the receiver. This allows for the identification of imperfect 0s and 1s without misinterpretation, thus eliminating bit errors.
Receiver equalization can be achieved in many ways, the simplest being CTLE, or linear equalization.
