Today,let's talk about what are the main factors limiting the transmission distance of optical modules?In fact,the transmission distance of optical modules is mainly limited by loss and dispersion.
The loss is caused by the loss of light energy due to the absorption and scattering of the medium as well as the leakage when the light is transmitted in the optical fiber,and this energy is dissipated at a certain rate with the increase of the transmission distance.The loss limit can be estimated based on the formula:loss-limited distance=(transmit optical power-receive sensitivity)/fiber attenuation.The higher the loss,the shorter the transmission distance of the optical module,and vice versa.
Dispersion is mainly due to the different wavelengths of electromagnetic waves in the same medium propagation speed,resulting in different wavelength components of the optical signal due to the accumulation of transmission distance and reach the receiving end at different times,resulting in pulse spreading,and then can not distinguish the signal value.www.fs-transceivers.com
The optical module is a connection module that plays the role of photoelectric conversion,in which the transmitter converts the electrical signal into an optical signal,and then converts the optical signal into an electrical signal at the receiver after transmission through optical fiber.Optical module consists of optoelectronic devices,functional circuits and optical interfaces,etc.The optoelectronic devices include two parts:transmitting and receiving.
The transmitting part:input a certain code rate of electrical signals through the internal driver chip processing to drive the semiconductor laser(LD)or light-emitting diode(LED)to emit the corresponding rate of modulated optical signals,with its internal optical power automatic control circuit(APC),so that the output optical signal power to maintain stability.
Receiving part:the optical signal of a certain code rate is input to the module and then converted into an electrical signal by the light detecting diode,and the electrical signal of the corresponding code rate is output after the preamplifier.The following is a measure of the performance of the optical module what are the indicators?
First,the transmitter
1,the average transmit power(Average transmit power)
Average transmit power is the light power output from the light source at the transmitting end of the optical module under normal operating conditions,which can be understood as the intensity of light.Transmitted optical power and the proportion of"1"in the data signal sent is related,the more"1",the greater the optical power.When the transmitter sends a pseudo-random sequence of signals,"1"and"0"roughly half of each,when the test power is the average transmit optical power,the unit is dBm.
2,extinction ratio(Extinction ratio)
Extinction ratio refers to the full modulation conditions of the laser in the launch of the full"1"code when the average optical power and the full"0"code when the average optical power ratio of the launch of the minimum value,the unit is dB.
When converting an electrical signal to an optical signal,the laser in the transmitting section of the optical module is converted to an optical signal according to the code rate of the input electrical signal.
The average optical power of the full"1"code means the average power of the laser light,the average optical power of the full"0"code means the average power of the laser does not light,the extinction ratio that characterizes the difference between 0,1 signal,so the extinction ratio can be seen as a Laser operating efficiency measurement.The typical minimum value of extinction ratio ranges from 8.2dB to 10dB.
3,the central wavelength of the optical signal
In the emission spectrum,the wavelength corresponding to the midpoint of the line segment connecting 50%of the maximum amplitude value.Different kinds of lasers or two lasers of the same kind may have differences in the central wavelength due to process and production,and even the same laser may have different central wavelengths under different conditions.
Generally,manufacturers of optical devices and optical modules,provide users with a parameter,the central wavelength(e.g.850nm),which will generally be a range.There are three main types of center wavelengths of Optical Transceiver commonly used today:850nm band,1310nm band,and 1550nm band.
Second,the receiving end
1,overload optical power
Overload optical power,also known as saturation optical power,refers to the optical module in a certain BER conditions,the maximum input average optical power received by the components at the receiving end,the unit is dBm.
It should be noted that the optical detector will appear in the strong light irradiation photocurrent saturation phenomenon,when this phenomenon,the detector needs a certain amount of time to recover,at this time the reception sensitivity decreases,the received signal may be misjudged and cause the phenomenon of false code.
Simply put,the input optical power exceeds the overload optical power,may cause damage to the equipment,in the use of operation should try to avoid strong light exposure to prevent the overload optical power.
2、Receiving sensitivity
Receiving sensitivity refers to the optical module in a certain BER conditions,the minimum average input optical power that can be received by the receiver component.If the transmit optical power refers to the light intensity of the transmitter,then the receive sensitivity refers to the light intensity that can be detected by the optical module.The unit is dBm.
In general,the higher the rate the worse the reception sensitivity,that is,the greater the minimum received optical power,the higher the requirements for the optical module receiving end device.
3、Receive optical power
Receive optical power refers to the Optic Transceiver in a certain BER conditions,the average optical power range received by the receiver component,the unit is dBm.receive optical power for the upper limit of the overload optical power,the lower limit for the maximum value of the receive sensitivity.
Collectively,it means that when the received optical power is less than the received sensitivity,the signal may not be received properly,because the optical power is too weak.When the received optical power is greater than the overload optical power,may also not be able to receive the signal normally,because there is the phenomenon of error code.
The performance indicators affecting the optical module are mainly the average transmit optical power,extinction ratio,optical signal center wavelength,overload optical power,receive sensitivity,receive optical power,by detecting whether these values are within the normal range of values can determine the performance of the optical module.
