1. Introduction and types of optical transceivers

As an important part of optical communication transmission, the optical transceiver module is composed of an optoelectronic device, functional circuit, and optical interface. The optoelectronic device includes two parts: transmitting and receiving. The main function is to realize photoelectric/electrical optical conversion, including optical power control and modulation transmission.

optical transceivers
optical transceivers

There are many types of optical transceivers, we can classify them from different aspects:

  1. Classified by rate: ＜1G/1.25G/2.5G/4.25G/10G/25G/40G/100G/200G/400G
  2. Classified by wavelength: 850nm/1310nm/1490nm/1550nm/CWDM/DWDM
  3. Classified by mode: Single-mode fiber (yellow), multimode fiber (orange)
  4. Classified by the number of inserted fibers: Single Fiber (BIDI), dual fiber, parallel
  5. Classified by usability: Hot-plug (GBIC, SFP, XFP, XENPAK) and not-hot plug (1*9, SFF)
  6. Classified by package: 1*9, GBIC, SFF, SFP, XFP, SFP+, SFP28, QSFP+, QSFP28, CFP, CFP2, X2, XENPARK, QSFP-DD

Generally, the packaging form of optical modules is a more important consideration when selecting. It should be noted that branded optical module manufacturers have independently classified and coded their optical modules, for example, 10G SFP SR module in HPE(Aruba) has the product code as J9150D which in H3C named as SFP-XG-SX-MM850-A.

The common packaging types (form-factors) of optical transceivers:

  • GBIC(Gigabit Interface Converter) module
    • This is the first optical module with standardized packaging interface. It adopts SC interface, which can be hot-pluggable and can convert Gigabit electrical signals into optical signals. It occupied the mainstream market in the 1990s.
  • SFP (Small-Factor Pluggable) module
    • This is MINI-GBIC with LC interfaces, the highest number rate up to 155M/622M/1.25G/2.125G/4.25G/8G/10G, usually connected with the LC jumper. The SFP optical module includes Baizhao Gigabit SFP, SFP, BIDI SFP, CWDM SFP and DWDM SFP, each with a light module after compatibility test strictly, ensure product reliability, stability, fully compatible with various brands of switches and other equipment.
  • SFP+ (Enhanced Small-Factor Pluggable) module transceiver
    • The SFP+ is an updated version that supports higher speeds up to 10Gbps, commonly used in the short distance. SFP+ retains the basic electro-optic and photoelectric conversion functions, which reduces the signal control functions of SerDes, CDR (Clock and Data Recover, EDC, MAC (Media Access Control) in the original XFP design, thus simplifying the design of 10G optical modules. 
  • SFP28 ( Small Form-Factor Pluggable 28)
    • The SFP28 optical module is based on SFP+ packaging mode, with a transmission rate of 25Gbps. It is suitable for a single 25Ge access port. It has low power consumption and high port density, which can save network deployment cost. It is widely used in 25G Ethernet and 100G Ethernet.
  • QSFP (Quad Small Form-Factor Pluggable)
    • This optical module is a four channel small pluggable optical module. It has four independent full duplex transceiver channels. The single channel SFP is replaced by multi-channel parallel high-density optical module, and the size of the QSFP is only 30% larger than that of standard SFP module. This 4-channel pluggable interface has a transmission rate of 4x10Gbps.
  • QSFP+(Enhanced Quad Small Form-Factor Pluggable)
    • It is a version of 4x10G QSFP transceivers which applied WDM technology with MPO or dual LC connectors.
  • QSFP28 ( Quad Small Form-Factor Pluggable 28)
    • The QSFP28 is an upgraded version of QSFP+. The QSFP28 interface is very flexible as it can achieve 100G in other ways. Using a breakout connection, QSFP28 transceivers can use just 2 channels at 50G each or they can even achieve a single channel of 100G depending on the transceiver type being used. 100G QSFP28 transceivers are available in several different standards including SR4, LR4, PSM4, CWDM4 and 100G DWDM transceiver, etc.
  • QSFP-DD (Quad Small Form Factor Pluggable Double-density)
    • This optical transceiver is compliant with IEEE802.3bs and QSFP-DD MSA standards. “Double-density” means that the module supports double the number of high-speed electrical interfaces compared to the standard QSFP28 module. With NRZ modulation technology, data rates of up to 25G x 8(channels) can be achieved for 200G network transmission. Meanwhile, with PAM4 modulation technology, data rates can reach 50G x 4(channels), enabling 400G network transmission for high-performance computing data centers and cloud networks. 

2. The compatibility of optical transceivers

There are three types of optical modules in the current market: the first kind is the original optical module, which is a switch, router and other network equipment manufacturers owned brand and authorized OEM modules, such as Huawei, H3C, Cisco, HPE. The second is the second-hand optical module, which is simply used, not a new product, and then sold. The third type is the compatible optical module, which is also known as the third-party optical module. The process and raw materials that are used by the compatible modules are no different from the original optical module, and sometimes even better than the latter, and the cost of the third-party optical module is much lower than the brand optical module.

The branded modules will set a special code so that customers can only use the brand’s supporting equipment, while means that HPE optical transceivers cannot be used with Cisco network devices, and vice versa. While compatible modules crack the code settings inside and can be compatible with a variety of devices. Branded modules are relatively high priced and of good quality, while compatible optical modules are cheap and cost-effective. So, how can we ensure the compatibility of the third-party transceivers?

Due to the high compatibility requirements of branded network equipment, it is very important for third-party vendors to ensure that third-party optical modules are compatible with branded network equipment. The SFF-8472 standard is a key factor to determine whether the quality and compatibility of third-party optical modules are passing. It was developed by the SFF committee to define the Data Diagnostic Monitoring Interface (DDMI) for optical modules, which can help us understand the operating parameters of optical modules in real-time. 

switch

In addition, we should pay attention to the following three points: check whether the optical module matches the port of the network equipment; ensure that the wavelength of the optical module used at both ends matches; and the optical module at both ends must use the correct network patch cable or fiber optic patch cable when making connections.

3. What are the application scenarios of optical transceivers?

The optical modules have a wide range of applications. In short, optical modules will be used where optical fibers are used.

Optical module using equipment and its optical module type:

  1. Optical transmitter&receiver: video optical transmitter&receiver generally adopts 1 * 9 single-mode optical module, and some high-definition optical transmitter&receiver also adopts SFP optical module.
  2. Optical fiber transceiver: 1 * 9 optical module is adopted. (a few also use SFP after optical plugging)
  3. Switch: SFP optical module, XFP optical module, etc. Transmission rates range from 155M to 100G.
  4. Optical fiber network card: 1 * 9 optical module, SFP optical module, SFP + optical module, etc.
  5. Optical fiber high-speed ball machine: SFP optical module is adopted.
  6. Base station: SFP and XFP optical modules are adopted.
  7. Repeater: SFP optical module is adopted.

Currently, the most popular optical transceivers in the market are the high-speed optical transceiver, such as 25G/40G/100G transceivers. The application scenarios of them are mainly Internet data center networks, metropolitan area network optical transmission networks, and telecommunication networks represented by 5G bearer networks.

Data flow continues to grow, and the trend of large-scale and flat data centers promotes the development of optical transceivers in two aspects: (A) The increasing demand for transmission rate; (B) The increasing demand for quantity.

The large-scale trend of data centers has led to an increase in transmission distance. The transmission distance of multimode optical fiber is limited by the increase in signal rate, and it is expected to be gradually replaced by single-mode optical fiber. The cost of an optical fiber link consists of two parts: an optical transceiver and an optical fiber, and there are different applicable solutions for different distances.