Exploring The Structure Performance Indicators And Types Of Optical Modules

Structure of optical module

The optical module is an optoelectronic device that realizes the mutual conversion of light and electricity during the transmission of optical signals.

At the transmitting end, the optical module realizes the conversion of electrical signals to optical signals. The optical signal is transmitted through optical fiber. At the receiving end, the optical module realizes the conversion of optical signals to electrical signals.

The optical module is usually composed of optical transmitting components, optical receiving components, laser chips, detector chips and other components.

At the transmitting end, the driver chip processes the original electrical signal and then drives the semiconductor laser (LD) or light emitting diode (LED) to emit a modulated optical signal.

At the receiving end, after the optical signal comes in, it is converted into an electrical signal by the optical detection diode, and the electrical signal is output after passing through the preamplifier.

Key performance indicators of optical modules!

1. Average transmitted optical power - determines the signal transmission distance. The average optical power output by the light source at the transmitting end of the optical module under normal working conditions can be understood as the intensity of light. The output optical power refers to the output optical power of the light source at the transmitting end of the optical module, and the unit is W or mW or dBm (milliwatt decibel). , where W or mW is a linear unit, and dBm is a logarithmic unit. In communication, we usually use dBm to represent optical power. The optical power is attenuated by half, reduced by 3dB, and the optical power of 0dBm corresponds to 1mW.

In optical communication systems, data transmission is achieved through optical signals. These optical signals represent 0 and 1 in binary code with different intensities. These 1 and 0 are combined to transmit a large amount of information, such as text, pictures, videos, etc. The transmitted optical power is related to the proportion of "1" in the transmitted data signal. The more "1", the greater the optical power. When the transmitter sends a pseudo-random sequence signal, "0" and "0" account for roughly half each. At this time, the power obtained by the test is the average transmitted optical power. The average transmitted optical power determines how far the signal can be transmitted. 

2. OMA optical power & extinction ratio - "1" and "0" distinction OMA (Optical Modulation Ampltude) optical power is closely related to the extinction ratio (ER) of the optical module. Both are used to describe the distinction between "1" and "0" in the optical signal, and the unit is dBm. OMA optical power refers to the difference between the optical power at high level and low level, while the extinction ratio is the ratio of the optical power corresponding to these two levels. Within a certain range, the greater the distinction between "1" and "0", the "clearer" the signal, and the better the quality of the transmitted signal.

The extinction ratio refers to the minimum value of the ratio of the average optical power of the signal to the average optical power of the empty number under full modulation conditions, indicating the ability to distinguish between 0 and 1 signals. The two factors that affect the extinction ratio in the optical module are bias current (bias) and modulation current (Mod). Let's take ER=Bias/Mod. The larger the extinction ratio, the better the optical module, but the better the optical module is if the extinction ratio meets the 802.3 standard. 

3. The center wavelength of the optical module

In optical communication systems, different optical signals can be transmitted simultaneously in the same optical fiber through wavelength division multiplexing (WDM) technology. In order to ensure that these signals do not interfere with each other and can be effectively transmitted in the optical fiber, each signal needs to have a precise center wavelength.

The center wavelength of the optical module is the center wavelength used when the optical module is working. When the optical module matches the center wavelength of the optical signal it transmits, the transmission effect is best. At the same time, different optical fibers need to be selected according to different signal center wavelengths, because different optical fiber types have different attenuation and dispersion characteristics for light of different wavelengths.

For example, single-mode optical fiber has low attenuation in the two wavelength ranges of 1310nm and 1550nm, so these two wavelength ranges are widely used in long-distance and high-speed optical communications.

4. Overload optical power (saturated optical power, optical saturation)

Also known as saturated optical power, it refers to the maximum input average optical power that the receiving end component can receive under the condition of a certain bit error rate (10^-10~18^-12) of the optical module. The unit is dBm.

It should be noted that the photodetector will experience photocurrent saturation when exposed to strong light. When this phenomenon occurs, the detector needs a certain amount of time to recover. At this time, the receiving sensitivity decreases, and the received signal may be misjudged. causing code errors. Simply put, if the input optical power exceeds this overload optical power, it may cause damage to the equipment. During operation, strong light exposure should be avoided to prevent exceeding the overload optical power.

5. Receive sensitivity

Receive sensitivity refers to the minimum average input optical power that the receiving end component can receive under a certain bit error rate condition. If the transmitted optical power refers to the light intensity at the transmitting end, then the receiving sensitivity refers to the light intensity that the optical module can detect, and the unit is dBm. The receiving sensitivity determines how weak the optical signal can be detected by the optical module. If the perception ability is weak, some weak signals may be ignored.

Generally speaking, the higher the rate, the worse the receiving sensitivity, that is, the greater the minimum received optical power, and the higher the requirements for the receiving end components of the optical module.

6. Received optical power

Received optical power refers to the average optical power range that the receiving end component can receive under a certain bit error rate condition of the optical module. The unit is dBm.

The lower the received optical power, the less likely it is to miss any small signal and possible error.

The upper limit of the received optical power is the overload optical power, and the lower limit is the maximum value of the receiving sensitivity. In general, when the received optical power is less than the receiving sensitivity, the signal may not be received normally because the optical power is too weak. When the received optical power is greater than the overload optical power, the signal may not be received normally, causing damage to the optical module.

7. Interface rate

The maximum electrical signal rate that an optical device can carry without error transmission is specified by the Ethernet standard as follows: 125Mbits, 1.25Gbits, 10.3125Gbit/s, 4125Gbit/s50Gbit/s, 100Gbit/s. The interface rate determines how much data the optical module can transmit per second.

8. Transmission distance

The transmission distance is how far the optical module can send the signal. The transmission distance of the optical module is mainly affected by loss and dispersion.

Loss is the loss of light energy caused by absorption, scattering and leakage of the medium when light is transmitted in the optical fiber. This part of energy dissipates at a certain rate as the transmission distance increases. The loss limit can be estimated according to the formula: loss-limited distance = (transmitted light power-receiving sensitivity)/fiber attenuation. The attenuation of the optical fiber is strongly related to the actual optical fiber selected.

The dispersion is mainly caused by the different speeds of electromagnetic waves of different wavelengths when propagating in the same medium, which causes the different wavelength components of the optical signal to arrive at the receiving end at different times due to the accumulation of transmission distance, resulting in pulse broadening, and then the signal value cannot be distinguished. In terms of dispersion limitation of the optical module, its limited distance is much larger than the limited distance of loss, so it can be ignored.

Types of optical modules

1. Classification by transmission rate

In order to meet the needs of various transmission rates, optical modules with different rates have been produced: 400GE optical module, 100GE optical module, 40GE optical module, 25GE optical module, 10GE optical module, GE optical module, FE optical module, etc.

2. Classification by package type

The higher the transmission rate, the more complex the structure, which leads to different packaging methods. Common package types are: SFP (1GE), SFP+ (10GE), SFP28 (25GE) QSFP+ (40GE), CFP (40GE-100GE), CFP2/CFP2-DCO (100GE-400GE), QSFP28 (100GE), QSFP-DD (400GE)

GBIC, which is Gia Bitratelnterface Converter (Gigabit Interface Converter) Before 2000, GBIC was the most popular optical module package and the most widely used Gigabit module form.

SFP, small form pluggable

SFP, full name Small Form Pluggable, is a small form pluggable optical module. Its small size is relative to GBIC package. The volume of SFP is half that of GBIC module, and more than twice the number of ports can be configured on the same panel. In terms of function, the two are not much different, and both support hot plugging. The maximum bandwidth supported by SFP is 4Gbps.

XFP is a 10-Gigabit Small Form.-factor Pluggable, a 10G SFP. XFP uses a full-speed single-channel serial module connected by an XFI (10Gb serial interface), which can replace Xenpak and its derivatives.

SFP+, like XFP, is a 10G optical module. The size of SFP+ is the same as SFP, which is more compact than XFP (reduced by about 30%) and consumes less power (reduced some signal control functions).

SFP28, an SFP with a rate of 25Gbps, was mainly developed because the price of 40G and 100G optical modules was too expensive at the time.

OSFP, Quad Small Fomm-factor Pluggable, a four-channel SFP interface. Many mature key technologies in XFP are applied to this design. According to the speed, QSFP can be divided into 4x10GQSFP+, 4x25GQSFP28, 8x25GQSFP28-DD optical modules, etc. Take QSFP28 as an example, it is suitable for 4x25GE access ports. Using QSFP28, you can upgrade directly from 25G to 100G without going through 40G, greatly simplifying the wiring difficulty and reducing costs.

QSFP-DD, established in March 2016, DD refers to "Double Density". The 4 channels of QSFP are increased by one row of channels, becoming 8 channels. It is compatible with the QSFP solution, and the original QSFP28 module can still be used, just insert another module. The number of electrical port gold fingers of QSFP-DD is twice that of OSFP28. OSFP-DD uses 25GbpSNRZ or 50GbpSPAM4 signal format for each channel. Using PAM4, it can support up to 400Gbps rate.

OSFP, OctalSmall Form Factor Pluggable, "0" represents "octal", officially launched in November 2016. It is designed to use 8 electrical channels to achieve 400GbE (8*56GbE, but the 56GbE signal is formed by a 25G DML laser under PAM4 modulation), with a size slightly larger than QSFP-DD, a higher wattage optical engine and transceiver, and slightly better heat dissipation performance.

CFP, Dense Wavelength Division Optical Communication Module

CFP, Centum gigabits Fomm Pluggable, dense wavelength division optical communication module. The transmission rate can reach 100-400Gbps. CFP is designed based on the SFP interface, with a larger size and supports 100Gbps data transmission. CFP can support a single 100G signal, one or more 40G signals. The difference between CFP, CFP2, and CFP4 lies in the volume. The volume of CFP2 is half of that of CFP, and CFP4 is one quarter of that of CFP. CFP8 is a packaging form specifically proposed for 400G, and its size is comparable to that of CFP2. Supports 25Gbps and 50Gbps channel rates, and achieves 400Gbps module rate through 16x25G or 8x50 electrical interfaces.

400G optical module

The early 400G optical module used a 16-channel 25GbSNRZ implementation method and adopted CDFP or CFP8 packaging. The advantage of this implementation method is that it can borrow the mature 25GNRZ technology on the 100G optical block. But the disadvantage is that 16 channels of signals are required for parallel transmission, and the power consumption and volume are relatively large, which is not suitable for data center applications. Later, PAM4 began to replace NRZ.

On the optical port side, 8 channels of 53GbpSPAM4 or 4 channels of 106GbpSPAM4 are mainly used to achieve 400G signal transmission, and 8 channels of 53GbpS PAM4 electrical signals are used on the electrical port side, using OSFP or OSFP-DD packaging.

In comparison, the QSFP-DD package size is smaller (similar to the QSFP28 package of the traditional 100G optical module), and is more suitable for data center applications.

The OSFP package size is slightly larger, and because it can provide more power consumption, it is more suitable for telecommunications applications.

For optical modules, if you want to achieve a rate increase, you must either increase the number of channels or increase the rate of a single channel. The traditional digital signal uses the NRZ (Non-Retumn.t0-Zer0) signal at most, that is, high and low signal levels are used to represent the 1 and 0 information of the digital logic signal to be transmitted, and each signal symbol period can transmit 1 bit of logic information. The PAM (4PulseAmplitudeModulation) signal uses 4 different signal levels for signal transmission, and each symbol period can represent 2 bits of logic information (0, 1, 2.3). Under the same channel physical bandwidth, PAM4 transmits twice the information of the NRZ signal, thereby doubling the rate.

3. Classification by mode (single mode, multi-mode)

According to mode classification, optical fibers are divided into single-mode optical fibers and multi-mode optical fibers. In order to use different types of optical fibers, single-mode optical modules and multi-mode optical modules are produced.

The center wavelengths of single-mode optical modules are generally 1310nm and 1550nm, and are used in conjunction with single-mode optical fibers.

Single-mode optical modules use a single optical fiber to transmit data and can provide high-speed, large-capacity data transmission over long distances, but require high-quality optical fiber lines and precise alignment.

The center wavelength of multi-mode optical modules is generally 850nm and is used in conjunction with multi-mode optical fibers. Multimode optical modules use multiple beams of light to transmit data. Due to dispersion defects, they are suitable for data transmission over shorter distances, such as between the same building or adjacent buildings. Its advantages are relatively simple installation and maintenance and low cost, but the transmission speed and distance are relatively limited.

As an important component of modern communication systems, the performance of optical modules directly affects the speed and reliability of the network. As data demand continues to grow, the development of optical modules is also constantly advancing to meet the needs of higher bandwidth and longer distances.