What Is an Optical Module and Its FAQs

Transmitter

• Average transmit power

  The average transmit power refers to the optical power output by the light source at the transmit end of the optical module under normal working conditions, which can be considered as the luminous intensity. The transmit power is related to the proportion of signal 1s in the transmitted data signals. More signal 1s indicate higher optical power. When the transmitter sends pseudo-random sequence signals, the number of signal 1s is approximately equal to the number of signal 0s. In this case, the power obtained in the test is the average transmit power, in the unit of W, mW, or dBm. W and mW are linear units, and dBm is a logarithmic unit. In communications, dBm is typically used to represent optical power.

• Extinction ratio

  The extinction ratio refers to the minimum ratio of the average optical power when the laser transmits all signal 1s (emits light) against the average optical power when the laser transmits all signal 0s (does not emit light) in complete modulation mode. Based on these two parameters, the extinction ratio indicates the capability of identifying signal 0 and signal 1. As such, the extinction ratio can be considered as a measure of the laser operating efficiency. The typical minimum extinction ratio ranges from 8.2 dB to 10 dB. As shown in Figure 1-3, the laser of the optical module's transmitter converts electrical signals into optical signals based on the bit rate of the input electrical signals.

  Figure 1-3 Working principle of a laser
  
• Center wavelength of an optical signal

  The center wavelength is the wavelength measured at the midpoint of a half-amplitude line in the transmit spectrum. Lasers of different types or two lasers of the same type may have different center wavelengths due to factors such as techniques and production processes. Even the same laser may have different center wavelengths under different conditions. Generally, the manufacturers of optical components and optical modules provide the center wavelength parameter, whose value is generally a range. Currently, there are three types of center wavelengths for commonly used optical modules: 850 nm, 1310 nm, and 1550 nm.

  Why are they defined in these three bands? This is related to the fiber loss of the optical signal transmission medium. After continuous research and experiments, it is found that the fiber loss is likely to decrease as the wavelength increases. The fiber loss at the 850 nm wavelength is small, but the loss at the 900–1300 nm wavelength increases. The fiber loss starts to decrease at the 1310 nm wavelength and is the lowest at the 1550 nm wavelength, and the loss at the 1650 nm wavelength or higher tends to increase. As such, 850 nm is a short-wavelength window, and 1310 nm and 1550 nm are long-wavelength windows.

Receiver

• Overload optical power

  Overload optical power, also known as saturated optical power, refers to the maximum average input optical power that can be received by the receiver of an optical module under a certain bit error rate (BER, which is usually 10^-12). The unit is dBm.

  Note that the photodetector will have saturated photocurrent when exposed to strong light. When this occurs, the photodetector needs some time to recover. In this case, the receiver sensitivity decreases, and the received signals may be misjudged, causing bit errors. Simply speaking, if the input optical power exceeds the overload optical power, the device may be damaged. Therefore, avoid direct exposure to strong light to prevent the input optical power from exceeding the overload optical power.

• Receiver sensitivity

  The receiver sensitivity refers to the minimum average input optical power that can be received by the receiver of an optical module under a certain BER (BER = 10^-12). If the transmit optical power refers to the luminous intensity at the transmit end, the receiver sensitivity refers to the luminous intensity that can be detected by the optical module. The unit is dBm.

  Generally, a higher rate indicates poorer receiver sensitivity. That is, a higher minimum receive power means higher requirements on the receiver of an optical module.

• Receive power

  The receive power refers to the average optical power range that can be received by the receiver of an optical module under a certain BER (BER = 10^-12). The unit is dBm. The upper threshold of the receive power is the overload optical power, and the lower threshold is the maximum receiver sensitivity.

In general, when the receive power is lower than the receiver sensitivity, the signals may not be received normally because the optical power is too weak. When the receive power is greater than the overload optical power, signals may fail to be received because of bit errors.

Comprehensive Performance Counters

• Interface rate

  The interface rate is the maximum rate of electrical signals that an optical component can transmit without bit errors. The interface rates defined in Ethernet standards include 125 Mbit/s, 1.25 Gbit/s, 10.3125 Gbit/s, and 41.25 Gbit/s.

• Transmission distance

  The transmission distance of an optical module is limited by loss and dispersion. The loss of light energy is caused by absorption and dispersion of the medium and the leakage of optical signals when optical signals are transmitted over the optical fiber. This part of energy is dissipated at a certain rate as the transmission distance increases. Dispersion is generated because electromagnetic waves of different wavelengths are transmitted at different speeds in the same medium. As a result, different wavelength components of optical signals arrive at the receive end at different time points due to the accumulation of transmission distances. As a result, pulses are broadened and signal values cannot be identified.

  The dispersion-limited distance of the optical module is far greater than the loss-limited distance. Therefore, the dispersion-limited distance can be ignored. The loss-limited distance can be estimated according to the following formula: Loss-limited distance = (Transmit power – Receiver sensitivity)/Optical fiber attenuation. The attenuation of optical fibers is strongly related to their types.

For CloudEngine series switches, you can run the display interface transceiver verbose command to check the general information, manufacturing information, alarms, and diagnostic information about the optical module on a specified interface, as described in Table 1-2.

Classification by Transmission Rate

To meet various transmission rate requirements, optical modules with different rates are provided, including 400GE, 100GE, 40GE, 25GE, 10GE, GE, and FE optical modules.

Classification by Form Factor

A higher transmission rate depends on a more complex structure. Different form factors in different structures are provided for varying transmission rates. Huawei switches support optical modules of the following form factors: Small Form-factor Pluggable (SFP)/Enhanced Small Form-factor Pluggable (eSFP), SFP+, SFP28, Quad Small Form-factor Pluggable Plus (QSFP+), 120 Gb/s eXtended-capability Form Factor Pluggable (CXP), Centum Form-factor Pluggable (CFP), QSFP28, and QSFP-Double Density (QSFP-DD).

Classification by Mode

Optical fibers are classified into single-mode and multimode fibers. Therefore, optical modules are also classified into single-mode and multimode modules to support different optical fibers.

• Single-mode optical modules have a typical center wavelength of 1310 nm or 1550 nm, and are used with single-mode fibers. Single-mode fibers support a wide band and large transmission capacity, and are used for long-distance transmission.

• Multimode optical modules have a typical center wavelength of 850 nm, and are used with multimode fibers. Multimode fibers have lower transmission performance than single-mode fibers because of modal dispersion, but are more cost-effective. They are used for small-capacity, short-distance transmission.

The transmit power of a long-distance optical module is often larger than its overload power. Therefore, when using such optical modules, select optical fibers of an appropriate length to ensure that the actual receive power is smaller than the overload power. If the optical fibers connected to a long-distance optical module are too short, use an optical attenuator to prevent the remote optical module from being burnt. The optical attenuator is the attenuation value of the optical fiber per unit length, in dB/km.

Classification by Center Wavelength

The operating wavelength of an optical module is a range. To facilitate description, the center wavelength is used, in unit of nm.

To support transmission of optical signals in different optical bands, optical modules with different center wavelengths, such as 850 nm, 1310 nm, and 1550 nm, are provided.

Classification by Light Color

The biggest difference between colored optical modules and other types of optical modules lies in the center wavelength.

• Generally, the center wavelength of an optical module can be 850 nm, 1310 nm, or 1550 nm. The center wavelength of an optical module is simple, and the light is called gray light.
• A colored optical module carries light with different center wavelengths. This type of light is called colored light.

Colored optical modules are classified into two types: coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). Within the same band, DWDM modules are available in more types and use wavelength resources more efficiently than CWDM modules. DWDM and CWDM modules allow lights with different center wavelengths to be transmitted on one fiber without interfering each other. Therefore, a passive multiplexer can be used to combine the lights into one channel, which is then split into multiple channels by a demultiplexer on the remote end. This reduces the optical fibers required. DWDM and CWDM modules are used for long-distance transmission.

Category (Example)

According to the preceding classification, the following table lists the types of some common optical modules based on different features.

Main Causes for Optical Module Failures

The main causes of optical module failures are optical modules' performance deterioration due to ESD damages and optical links' unavailability incurred by optical bore contamination and damage. The possible causes of optical bore contamination and damage are as follows:

1. The optical bore is exposed to the environment and contaminated by dust.
2. The end face of the fiber connector is contaminated.
3. The end face of the fiber connector with a pigtail is improperly used. For example, the end face is scratched.
4. A poor-quality fiber connector is used.

Two main approaches are available to effectively prevent optical module failures: ESD prevention and physical protection.

ESD Prevention

ESD damage deteriorates the performance of optical components or even causes optical component failures. In addition, it is difficult to detect optical components with ESD damage. If an optical component fails due to ESD damage, this failed optical component is hard to locate within a short time.

Operation instructions

1. When you transport optical modules, ensure that they are in ESD packages, and do not take them out unless necessary or place them at random.
   Figure 1-4 Optical modules in an ESD box
   
   Figure 1-5 ESD label
   
   Figure 1-6 Optical modules in an ESD bag
   
2. Before touching an optical module, wear an ESD wrist strap or ESD gloves. Take full ESD measures when installing it.
   Figure 1-7 ESD gloves
   
   Figure 1-8 ESD wrist strap
   
3. Ensure that a device is properly grounded before you test or use it.
   Figure 1-9 Grounded device
   

   Do not take optical modules out of ESD packages or stack them at random without any ESD prevention measures.

Physical Protection

Lasers and thermoelectric coolers (TECs) inside optical modules can be easily broken or disconnected after collisions. Therefore, physical protections are required during optical module transportation and usage.

Use a dedicated cotton swab to gently rub the stain on an optical bore. A non-dedicated cotton swab may cause damage to the optical bore. If the cotton swab is used with excessive force, the metal in the cotton swab may scratch the ceramic end face.

Tests on installation and removal of an optical module are based on manual operations, so are the push force and pull force. Therefore, do not use tools to install or remove an optical module.

Operation instructions

1. Handle optical modules gently and protect them from falling.
2. When installing an optical module, push it gently by hand. When removing an optical module, unlatch it first. Never use any metal tools during installation and removal.
   Figure 1-10 Installing an optical module
   
   
3. Use a dedicated cotton swab to clean an optical bore and do not insert any metal object into it.
   Figure 1-11 Cleaning an optical bore with a cotton swab
   

Context

Theoretically, optical modules of the same connector type can be connected. In practice, pay attention to the transmit and receive power ranges and transmission distance. Table 1-6 lists the main factors that affect optical module interconnection.

Fault Symptom

Two optical interfaces on different switches are interconnected through optical fibers. The local optical interface is Down, leading to a communication failure between the two corresponding optical modules.

Possible Causes

• The optical modules used on the switches are not certified for Huawei data center switches.
• The optical modules and optical fibers do not match.
• The local optical interface is shut down.
• The transmit power of the local optical module is too low or too high.
• The receive power of the local optical module is too low or too high.
• The two optical modules do not match.

Troubleshooting Procedure

1. Check whether the optical module of the Down interface has been certified for Huawei data center switches. CloudEngine series switches must use optical modules certified for Huawei data center switches. Those that are not certified for Huawei data center switches cannot ensure service reliability, and the corresponding optical interfaces may fail to go Up.
2. Check whether optical modules and optical fibers match.
   • A single-mode optical module (typically with a center wavelength of 1310 nm or 1550 nm) must be used with single-mode optical fibers (typically yellow).
   • A multimode optical module (typically with a center wavelength of 850 nm) must be used with multimode optical fibers (typically orange).
3. Run the display interface transceiver command to check whether any alarm information has been generated for the optical module.

   <HUAWEI> display interface 10ge 1/0/1 transceiver
   
    10GE1/0/1 transceiver information:
   -------------------------------------------------------------------
    Common information:
      Transceiver Type                      :10GBASE_SR
      Connector Type                        :LC
      Wavelength (nm)                       :850
      Transfer Distance (m)                 :30(62.5um/125um OM1)
                                             80(50um/125um OM2)
                                             300(50um/125um OM3)
                                             400(50um/125um OM4)
      Digital Diagnostic Monitoring         :YES
      Vendor Name                           :HUAWEI
      Vendor Part Number                    :02318169
      Ordering Name                         :
   -------------------------------------------------------------------
    Manufacture information:
      Manu. Serial Number                   :AQG269Y
      Manufacturing Date                    :2013-10-20
      Vendor Name                           :HUAWEI
   -------------------------------------------------------------------
    Alarm information:
   -------------------------------------------------------------------

   If the LOS alarm is displayed, the local optical interface does not receive signals from the remote interface. Run the display this command in the interface view to check whether the two interfaces have been shut down. If an interface is shut down, run the undo shutdown command on the interface.

4. Run the display interface transceiver verbose command to check diagnostic information about the optical module on the local interface. Specifically, check whether the diagnostic information contains alarms about abnormal transmit or receive power.

   <HUAWEI> display interface 10ge 1/0/1 transceiver verbose
   
    10GE1/0/1 transceiver information:
   -------------------------------------------------------------------
    Common information:
      Transceiver Type                      :10GBASE_SR
      Connector Type                        :LC
      Wavelength (nm)                       :850
      Transfer Distance (m)                 :30(62.5um/125um OM1)
                                             80(50um/125um OM2)
                                             300(50um/125um OM3)
                                             400(50um/125um OM4)
      Digital Diagnostic Monitoring         :YES
      Vendor Name                           :HUAWEI
      Vendor Part Number                    :02318169
      Ordering Name                         :
   -------------------------------------------------------------------
    Manufacture information:
      Manu. Serial Number                   :AQG269Y
      Manufacturing Date                    :2013-10-20
      Vendor Name                           :HUAWEI
   -------------------------------------------------------------------
    Alarm information:
   -------------------------------------------------------------------
    Diagnostic information:
      Temperature (Celsius)                 :33.68
      Voltage (V)                           :3.29
      Bias Current (mA)                     :7.97
      Bias High Threshold (mA)              :13.20
      Bias Low Threshold (mA)               :4.00
      Current RX Power (dBm)                :-2.15
      Default RX Power High Threshold (dBm) :1.00
      Default RX Power Low Threshold (dBm)  :-11.90
      Current TX Power (dBm)                :-2.07
      Default TX Power High Threshold (dBm) :1.00
      Default TX Power Low Threshold (dBm)  :-9.30
   -------------------------------------------------------------------

   The display interface transceiver verbose command displays the current transmit and receive power values of the optical module, as well as the default maximum and minimum power values.

   • If RxPower Low is displayed, the strength of signals received by the local optical module is too low. As a result, the local interface may not go Up or discard packets after it is Up. In this case, check whether the distance between the two switches exceeds the maximum transmission distance of the remote optical module. If the distance is within the transmission distance of the remote optical module, check whether the optical module and optical fiber on the interface are damaged.
   • If RxPower High is displayed, the strength of signals received by the local optical module is too high. The possible reason is that the distance between the two switches is short but a long-distance optical module is used on the remote end. In this case, install an optical attenuator on the remote optical module to protect the local optical module.
   • If TxPower Low is displayed, the strength of signals sent from the local optical module is too low, or the optical module is faulty. This may cause low receive power on the remote optical module. Then, the remote interface may not go Up or discard packets after it is Up. In this case, contact technical support engineers.
   • If TxPower High is displayed, the strength of signals sent from the local optical module is too high. This may cause high receive power on the remote optical module. If the high receive power lasts for a long time, the remote optical module will be burnt. The local optical module may have failed and you are advised to replace it.

   To ensure normal communication between two interconnected interfaces that have optical modules installed, check for transmit and receive power alarms. Ensure that the transmit and receive power values of the two optical modules are in the normal ranges. Otherwise, traffic forwarding on the optical interfaces may be abnormal or the optical modules may be damaged.

5. If no transmit and receive power alarms are displayed on the two ends but the interface is still Down, collect detailed information and logs about the optical modules, and then replace the optical modules or optical fibers. If the interface can go Up, the original optical modules or optical fibers are faulty. If the interface is still Down, contact technical support engineers.