Transceiver wavelength and transmission distance are important parameters, they are related to how long the signal can be transmitted. As we all know, different wavelengths of transceivers have different reach distances, so what is the relationship between wavelength and reach distance? Is wavelength a factor that affects the reach distance? Refer this article to find the answer to these questions.
Figure 1, Etern Optoelectronics FLS100C211S_100G QSFP28 LR4
Working Wavelength
Checking out the working wavelength and optical fiber mode of a transceiver is one of the common ways to estimate how long a transceiver can reach. If the transceiver works at a wavelength near 850nm (880nm) or 910nm (940nm), then the module is a multi-mode fiber(MMF) optical transceiver, and if the working wavelength is 1310nm or 1550nm, it is a single-mode fiber(SMF) transceiver. Generally, the maximum transmission distance(generally no more than 500 m) supported by a multi-mode transceiver is much shorter than that of a single-mode transceiver.
Working Optical Fiber Type
The light transmitting in fiber will produce dispersion and loss during transmission, the dispersion and loss are the main factors affecting the transmission distance of transceivers.
Dispersion: In general, single-mode transmission does not produce inter-mode dispersion, while multimode transmission supports multiple transmission modes, light will be refracted several times, which will produce inter-mode dispersion. The greater the dispersion, the shorter the transmission distance of the transceiver.
Insertion Loss: Different wavelengths of light transmission loss, from large to small 850nm> 1310nm> 1550nm, the smaller the loss, the longer the transmission distance of the transceiver. Generally, when the single-mode fiber is regular G.652D, which is most mainstream using fiber type in the world, and the loss per kilometer at the 1310nm working wavelength is about 0.35db, and at the 1550nm is about 0.22db.
Figure 2, Wavelength Vs Loss in Optical Fiber
Protocol in Industry Standard
Take a example with 100G QSFP28 LR4 10km, the link power budgets is defined in IEEE802.3ba-2010.
Power Budget, 8.5dB.
Insertion Loss, 6.3dB. Which is considered the wavelength loss in fiber and designed points loss in real fiber link,
Figure 3, Power Budget Definition of 100G QSFP28 LR4 in IEEE802.3ba Standard
Calculation of the Reach Distance
Before you get more details on how to measure the reach distance, it is necessary to have a basic understanding of transmitting optical power. The transmitting optical power can be regarded as the intensity of the light, in watt(W) or milliwatt(mW), or dBm( W or mW is a linear unit and dBm is a logarithmic unit), which is usually measured by optical power meters. dBm is the most frequently-used unit to represent optical power. The equation can be expressed as
P(dBm)=10Log(P/1mW)
Optical communication systems use a BER value to specify the performance requirement for a particular transmission link application. Defining the receiver sensitivity of a transceiver is actually measuring an optical power budget in dBm incident on the photodetector. This is a pretty useful method in low rate optical module systems for dispersion and chirping are not the main factors to transmission link length. The measuring method is as follows:
Power budget = Min(TXPx) – Max(Rxsens)
This formula here, TxPx is the Average Launch Power per Lane of Transmitter, Rxsens is the Average Receiver Sensitivity per Lane of Receiver.
Why there is a minimum power for transmitter and a minimum sensitivity for receiver? The performance of transceiver is often not consistent with each other throughout the production and is changing over the life cycle. Therefore, the worst performance needs to be considered when calculating the average power. This is different from academic research pursuing the approval of authority; products are measured by their usability and general applicability.
Figure 4, Optical Performance of Etern Optoelectronics’s100G QSFP28 LR4
Based on the power budget, the maximum supported transmission distance can be measured by the optical fiber loss in bands of 1310nm and 1550nm. In general, the average loss of the O-band and C-band is measured by 0.35 dBm and 0.25 dBm respectively. For example, 100G QSFP28 LR4 working wavelength is 1310nm, its transiting power is -4.3~4.5dBm, and its receiver sensitivity is -8.6dBm.
Power Budget=Min(TXPx) – Max(Rxsens)=-4.3- (-8.6)=4.3dB
Here the power budget is 4.3dB in worst situation, so it supports a transmission distance of about 10km, corresponding to the 100GBASE-LR4 standard according to the above calculation method. Please know that in Figure 4, the receiver sensitivity is measured with a PRBS 231-1 test pattern @25.78125 Gb/s, BER≤1E-12.
The real link situation must be token the factors like system margin, optical fiber connector, loss, and dispersion of optical fiber connector into account. Therefore, this calculated result is just the theoretical maximum distance, but its actual result is a little bit less than that. The good news is that, the real per kilometer fiber loss is much better than the design value via advanced fiber processing, and during Etern factory's shipment testing, we always follow more stringent performance standards to determine whether the products can be shipped.
For more information, visit Etern Optoelectronics website: https://www.szetern.com, or email to: sales@szetern.com