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Attentions of Using Long Distance Transmission Optical Transceiver


There are many kinds of 10G SFP + optical transceivers in the market, some optical transceivers can transmit 80km, and some others can transmit 100km, even 120km. Do you know the reason why the transmission distance is different? And why the long-distance optical receives, such as 80km 100km and 120km optical transceivers, are often burned down when we use them?

As we know the transmission distance of optical transceiver is related to optical power and receiver sensitivity. However, dispersion is also an important factor which affects the transmission distance of optical transceivers.

Fiber loss α (dB/km) is one of the most important specifications of an optical fiber because it largely determines the maximum distance between a transmitter and receiver.

Therefore, users need to select appropriate optical transceivers to meet their demands according to their networking situation. The actual transmission distance depends on the output power of the optical transceiver, the transmission attenuation of the optical fiber, and the receiver sensitivity of the optical transceiver.

Transmitter optical power and receiver sensitivity are important parameters to affect transmission distance.

Optical transceiver estimated transmission distance: L (max) = (Output Power-Receiver Sensitivity) / α (dB/km)


10G SFP+ 100km Optical and Electrical Characteristics

Parameter

Symbol

Min

Typical

Max

Unit

Notes

Transmitter

Centre Wavelength

λc

1530

1550

1565

nm

 

Side-Mode Suppression Ratio

SMSR

30

-

 

dB

 

Average Output Power

Pout

0

 

+4.0

dBm

1

Extinction Ratio

ER

6.0

 

 

dB

 

Data Input Swing Differential

VIN

180

 

850

mV

2

Input Differential Impedance

ZIN

90

100

110

Ω

 

TX Disable

Disable

 

2.0

 

Vcc

V

 

Enable

 

0

 

0.8

V

 

TX Fault

Fault

 

2.0

 

Vcc

V

 

Normal

 

0

 

0.8

V

 

Receiver

Centre Wavelength

λc

1260

 

1600

nm

 

Receiver Sensitivity

 

 

 

-25

dBm

3

Receiver Overload

 

-7

 

 

dBm

3

LOS De-Assert

LOSD

 

 

-26

dBm

 

LOS Assert

LOSA

-34

 

 

dBm

 

LOS Hysteresis

 

0.5

 

4

dB

 

Data Output Swing Differential

Vout

300

 

900

mV

4

LOS

High

2.0

 

Vcc

V

 

Low

 

 

0.8

V

 

 


The main reason why the dispersion appears is that the speed of optical signals with the different wavelengths travel is different in the fiber. Then the optical signal with different wavelength reaches the receiving end at a different time due to the accumulation of the transmission distance. After that, the pulse to be widened, and then the signal values won’t be able to distinguish.

Signal dispersion is a consequence of factors such as intermodal dispersion, intramodal dispersion, polarization mode dispersion, and higher-order dispersion effects. The group velocity is the speed at which energy in a particular mode travels along with the fiber.

The dispersion coefficient is the arrival time difference between the two optical waves that have a wavelength interval of 1nm and a light wave transmission length of 1km, the unit is PS/nm km.

The relationship between dispersion and transmission rate

The influence of group velocity dispersion on the specific ratio can introduce the criterion B △ T < 1, which does not produce the overlapping of adjacent pulses

B is the bit rate, △ t is the pulse broadening caused by group velocity dispersion

The higher the transmission rate is, the smaller dispersion should be controlled to ensure the transmission of the correct signal.

                                                                                △T=DL δλ

L--- transmission distance  D---dispersion coefficient  δλ--- light source of rms-20dB spectrum width δλ-20

                                                                              δλ=δλ-20 /6.07   

The typical dispersion value of G.652 fiber is 17ps / nm · km near the wavelength of 1550nm. After solved the attenuation problem of optical fiber, the dispersion limitation turns to the major problem that determines the transmission distance.

The dispersion tolerance of 10G SFP+ is 1600ps/nm (80km) and 2400ps/nm (120km).


                                                     



Why the optical transceiver of long-distance optical transceivers are often damaged?

When the optical transceiver does not work, We usually need to check the DDM information of optical transceivers.

                                                                                 

 

First, we test the transmitter is normal, no sensitivity in the receiver test, RX monitoring display is -3.12dBm when no optical input and the module working current is too large. We assumed that is caused by the APD abnormality based on the phenomenon in the preliminary detection. After that, use the multimeter to measure the APD voltage and display the abnormality.

According to the above test and analysis results, it is determined that APD can’t work normally due to the breakdown damage of large input power.

When removing the To case and observe it with a high-power microscope, we can see the APD has been damaged by the breakdown.


                                                                                    

   

10G SFP + 80km receiver overload - 7dBm. Please make sure the input power of APD to be≤ - 6dBm when using it. Optical power is too large that will cause the APD to break down instantaneously.


Summary: Review the following safety precautions to avoid injury and prevent damage to the optical transceiver.

1. We should protect the optical interface of the optical modules and optical fiber cable to prevent dust cross-contamination; before using, wipe up the end faces of the optical fiber cable with cleaning paper; When we uninstall the optical modules, immediately replace dust protective caps on the optical modules and optical fiber cables.

2. To prevent damage, while using the optical transceiver, we should pay attention to the plug-in method and strength of the optical fiber cable. The cable should be inserted in parallel and gently to avoid products damaged by improper use.

3. We should pay attention to the output current and voltage of the instrument when using. The working voltage ranges from 3.3 ± 0.5V. If the voltage exceeds the allowed working voltage or the voltage is unstable and the instantaneous pulse current is too large, It often causes optical module damage.