Verification report! How far can you fly with Coherent's high-power version 400G-ZR+?

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Base network module Coherent

Coherent (formerly II-VI) has released a 400G-ZR/ZR+ optical transceiver module. General 400G-ZR/ZR+ products are manufactured with silicon photonics, and the optical transmission power is set to -10dBm, which is low, but Coherent's 400G-ZR/ZR+ is different. The optical transmission power can be output to about 0 dBm, which is higher than normal products, so this article describes it as "high power version 400G-ZR/ZR+". Coherent's products have the advantage of being able to fly farther without using an amplifier and being able to be directly connected to ROADMs that require large optical input power.

This time, I would like to introduce an in-house experiment to see how far the Coherent "High Power Version 400G-ZR+" can fly. In addition, the contents of this article are also explained in the video, which can be viewed by filling in the simple form at the end of the article, so please take a look.

The conclusion is as follows.

・When I checked one-to-one communication using 80km bare fiber, it was possible to communicate without errors.

・When I checked WDM communication using a simple amplifier, error-free communication was possible up to 192 km with bare fiber and up to 77 km with dark fiber.

Now, let us introduce the detailed experimental results.

1. Is it possible to communicate one-to-one with 80km bare fiber?

We looped back the transmission and reception of Coherent's high-power 400G-ZR+ optical transceiver module over 80km of bare fiber to confirm error-free communication.

1.1 Measurement system

The optical signal transmitted from the high-power version 400G-ZR+ optical transceiver module returns to the receiving optical transceiver module through 80km bare fiber. The loss for this 80km bare fiber was 15.6dB.

The high-power 400G-ZR+ optical transceiver module is mounted on Coherent's evaluation board, and this evaluation board is connected to a PC with a dedicated GUI installed via a USB cable.

The GUI can control the optical signal sent from the optical transceiver module and analyze the received optical signal.

Figure 1-2 Equipment used for measurement

1.2 Measurement results

Looking at the analysis results displayed in the GUI, Pre-FEC BER = 2.07E-04, Post-FEC BER = 0, that is, error-free.

PreFEC-BER is the bit error rate before error correction. PostFEC-BER is the bit error rate after error correction, and 0 indicates no error.

400G-ZR+ uses O-FEC error correction, and the FEC limit is 2E-02, which is 0.02. Exceeding this FEC limit can lead to link instability and even loss of link due to the inability to correct errors.

As a result of this measurement, Pre-FEC BER = 2.07E-04 is 0.000207, so there is still a margin of about two digits with respect to the FEC limit of 0.02.

1.3 Summary

We were able to confirm error-free communication with one-to-one communication using 80 km of bare fiber. Furthermore, we found that there is still plenty of room to meet the FEC limit.

2. How far can it fly with WDM communication using an optical amplifier?

Earlier, I looped back the transmission and reception of Coherent's high-power 400G-ZR+ optical transceiver over 80km of bare fiber. We changed this to a WDM configuration and added an optical amplifier to verify how far it could fly.

2.1 Measurement system

The optical signal transmitted from the high power version 400G-ZR+ optical transceiver module passes through Mux for wavelength multiplexing and is amplified by Booster Amp. The gain of this amplifier is 10dB.

The Booster Amp and another Pre Amp use simple QSFP module type optical amplifiers. Therefore, the gain is smaller than that of general rack-type optical amplifiers.

The amplified optical signal passes through 80km of bare fiber and is further attenuated by an optical attenuator.

Gradually increase the attenuation of this optical attenuator to find out where the link breaks. This will give you an idea of how much you can increase the fiber loss.

Then it is amplified again with Pre-Amp. The gain of this amplifier is 17dB.

Finally, the wavelength-multiplexed signal passes through DeMux to separate it and returns to the reception of the optical transceiver module.

Figure 2-2 Appearance of actual measurement

2.2 Measurement results

Fig. 2-3 Measurement result Bit error rate before the link is broken

As the attenuation of the optical attenuator is increased, the bit error rate deteriorates. More specifically, as the OSNR degrades, the bit error rate degrades.

Figure 2-3 shows the bit error rate when the OSNR tolerance is almost at its limit and just before the link breaks. Pre FEC BER, or bit error rate before error correction, is 1.614E-02, or 0.01614.

We can say that this is very close to the FEC limit of 0.02. Post FEC BER, or bit error rate after error correction, is just barely error-free.

And the attenuation of the optical attenuator at this time was 22.8dB.

Figure 2-4 Attenuator value before the link breaks

Since the attenuation of the optical fiber was 15.6dB, it is 15.6 + 22.8, and we can see that we can keep the error free up to a total loss of 38.4dB.

Adding more loss results in a non-zero Post FEC BER, or bit error rate after error correction. In other words, bit errors occur even after error correction.

Then, the link becomes unstable and the link goes down somewhere.

Bare fiber loss is approximately 0.2 dB per kilometer, so this 38.4 dB corresponds to 192 km of bare fiber length. In the case of dark fiber, assuming a loss of 0.5dB per 1km, it can travel up to 77km.

2.3 Summary

Experiments using a simple optical amplifier revealed that bare fiber can travel up to 192 km, and dark fiber [*] can travel up to 77 km.

[*] When the loss per 1km is 0.5dB

Summary of this article

We conducted two tests to find out how far we can fly using Coherent's "High Power Version 400G-ZR+".

In the first verification, one-to-one communication using 80km bare fiber, we confirmed that communication was possible without errors. The Pre-FEC BER is also generous, so it is possible to fly longer distances.

In the second verification using an optical amplifier in a WDM configuration, it was found that bare fiber with 0.2 dB per km could be extended up to 192 km. Also, this result means that if it is a dark fiber of 0.5 dB per 1 km, it can fly up to 77 km.

Table 3-1 Summary of verification results of flight distance

Verification condition		Verification result of high power version 400G-ZR+
without optical amplifier	bare fiber	Confirmed that it can fly 80 km without problems
with optical amplifier	bare fiber	Can fly 192km without problems
with optical amplifier	dark fiber	Can fly 77km without problems

This time, we focused on ZR + and verified it. ZR+ consumes a lot of power, and you may not have equipment that can handle it. Coherent also has a ZR that can be used with lower power consumption. I will explain the performance of such ZR in the next article.

Watch the video of this article

The content of this article was introduced at a seminar in the past. You can also see the actual experiment, so if you would like to see this seminar video, please register using the form below.

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Inquiry

If you are thinking about achieving 400G transmission over tens of kilometers, please consider Coherent's high-power version ZR/ZR+. At Macnica, we can make proposals tailored to your own communication environment and desired conditions. If you are having trouble installing the optical transceiver module, or if you have any problems regarding the optical transceiver module, please contact us using the form below.