Optical transceiver module: How it increases speed

Optical transceiver modules have been 1G → 10G → 40G → 100G → 400G → 800G And so, speeds have increased in response to the demands of the times.
In this article, we will explain the mechanism that achieves high speed in terms that are easy for even beginners to understand.

Optical transceiver module: How it increases speed
① Increase the data rate
②1 Send a lot of data at once
3) Increase the number of lanes

Increase Data Rate

The first method is to simply increase the data rate of the signal. For an NRZ signal, which represents a digital signal with a low level for 0 and a high level for 1, this means speeding up the repetition period.
For example, a 1Gbps signal sends 1,000,000,000 0s and 1s per second, but if you send 10 times this speed, that is, 10,000,000,000 per second, it becomes 10Gbps.
In addition, integrated circuits that process signals, analog circuits that amplify signals, electrical-optical conversion sections, and optical systems must also be able to handle that data rate.
Also, and this gets a bit technical, the faster the signal is, the more bandwidth it will occupy.
The difficult part is that it is not possible to simply make things faster indefinitely.

Sending a lot of information at once

In an NRZ signal, a digital signal represents 0 as a low level and 1 as a high level, but there is a way to make this signal level division finer and achieve higher speeds.
For example, a digital signal can be divided into four levels: low level, slightly weaker level, slightly stronger level, and high level. Then, by assigning two bits (00, 01, 10, 11) to each of the four levels, it is possible to send two bits of 0 or 1 information at a time.
This method is commonly known as PAM4, and is actually used to increase the speed of optical transceiver modules.
By using PAM4, the data rate can be doubled with the same signal repetition rate as NRZ.
Again, to get a little technical, the occupied bandwidth of the signal in this case is the same for PAM4 and NRZ (the data rate doubles but the occupied bandwidth remains the same).
However, since the gap between signal levels becomes narrower, this has the disadvantage of making it more susceptible to noise and more difficult to distinguish.

Increase the number of lanes

One way to achieve higher speeds is to increase the number of lines that transmit signals, just like adding more lanes to a road.
The transmission paths that transmit high-speed signals consist of differential signal lines, each consisting of a pair of lines for a positive signal and a negative signal, and these two lines (1 pair) are called 1 lane.
For example, when sending a 10 Gbps signal over one pair of differential signal lines, it can be said that 10 Gbps is transmitted over one lane.
If four lanes are used, the transmission speed will be four times faster, at 40 Gbps.

Examples of speed improvements

Let's look at a couple of examples of how faster speeds are achieved using SFP, SFP+, and QSFP+.

Example 1: 1G SFP → 10G SFP+

​-1G SFP transmits 1Gbps​ ​NRZ signals over one lane.
-10G SFP+ transmits a 10Gbps​ ​NRZ signal in one lane.
(Decimals of the rates are omitted here.)

The speed is increased by simply increasing the data rate by a factor of 10.

Example 2: 10G SFP+ to 40G QSFP+

-10G SFP+ transmits a 10Gbps​ ​NRZ signal in one lane.
-40G SFP+ transmits 10Gbps​ ​NRZ signals over four lanes.
(Decimals of the rates are omitted here.)

The data rate per lane remains the same, but the number of lanes has been quadrupled to achieve faster speeds.

Optical transceiver module: How it increases speed

We used examples to introduce methods for increasing the speed of optical transceiver modules, such as increasing the data rate and number of lanes.
For more information, we also have an article explaining the optical transmission standards for optical transceiver modules.

100G edition: https://www.macnica.co.jp/business/semiconductor/articles/coherent/144352/

400G Edition: https://www.macnica.co.jp/business/semiconductor/articles/coherent/144904/

800G edition: https://www.macnica.co.jp/business/semiconductor/articles/coherent/145049/

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