Overview of the Development of Fiber Optic Transceivers

Introduction to Fiber Optic Transceivers

Fiber optic transceiver, also called optical module, is used to realize the conversion between electrical and optical signals. It is the core device for connecting communication equipment with optical fibers. The optical module is usually composed of Transmitter Optical Subassembly (TOSA, containing a laser LD Chip), Receiver Optical Subassembly (ROSA, containing a photodetector PD Chip), a driving circuit, and an optical and electrical interface. Its schematic is shown in Figure 1.

Figure 1 Internal structure diagram of optical module

The lasers inside the optical module can be divided into vertical cavity surface emitting lasers (VCSEL), Fabry-Perot lasers (FP), distributed feedback lasers (DFB), electro-absorption modulated lasers (EML) and narrowband tunable lasers, etc.; Photodetectors can be classified into PIN junction diodes (PIN) and avalanche photodiodes (APD). Different types of lasers and photodetectors have differences in performance and cost, so we can choose different chip solutions according to their specific specifications.

Let’s take the 25G gray optical module as an example to introduce the basic functional block diagram of the optical module.

Figure 2 Basic functional block diagram of the optical module

At the sending end, the electrical signal at a certain rate is processed by the driver chip to drive the laser (LD) to emit a modulated optical signal at a corresponding rate, and the optical signal with stable power is output through the optical power automatic control circuit. At the receiving end, after entering the module, the optical signal at a certain rate is converted into an electrical signal by a photodetector (PD), and then output an electrical signal of a corresponding rate after passing through a preamplifier.

In addition to lasers and detectors, the optical module also contains some electrical chips, the specific functions can be seen in the figure below.

Figure 3 Introduction to the functions of the internal components of the optical module

Classification of optical modules

 

Optical modules can be classified in many ways, such as according to packaging method, rate, transmission distance, modulation format, whether to support wavelength division multiplexing (WDM) applications, optical interface working mode, working temperature range, etc. The specific classification is shown in the following table.

Table 1 Classification of optical modules

Classification of Optical Modules
By form factor	SFP,SFP+,SFP28,QSFP28,CFP2,QSFP-DD,OSFP,etc.
By transmission distance	2.5Gb/s,10Gb/s,25Gb/s,40Gb/s,50Gb/s,100Gb/s,200Gb/s,400Gb/s,800Gb/s,etc.
By Modulation mode	NRZ,PAM4,DP-QPSK/n-QAM,etc.
Whether wavelength division multiplexing(WDM) is supported	gray light module(WDM not supported),color optical module (WDM supported)
By working module of ptical interface	Duplex,BiDi
By operating temperature range	Commercial grade (0~70℃), industrial grade (-40~85℃), etc

The evolution of the packaging method can most intuitively reflect the evolution trend of high speed and miniaturization of optical modules. From SFP packaging to QSFP, QSFP-DD packaging, the rate of optical modules has increased from 1Gbps to 800Gbps, and the improvement of packaging technology has enabled optical modules to adapt to higher rate requirements with little change in volume.

Figure 4 Evolution of optical module packaging

With the advent of 5G, optical module manufacturers have launched colored optical modules to support higher bandwidth requirements. The colored optical module adopts WDM technology, which combines optical signals of different wavelengths on one optical fiber for transmission, which greatly improves the signal transmission bandwidth of the link. At present, the colored module is mainly divided into CWDM, LWDM, MWDM, and DWDM.

Table 2 Comparison of colored optical modules

Main types of color optical modules	Channel interval	Scheme	Cost	Major commercial area
CWDM	20nm	uncooled DML+PIN	low	China
LWDM	800GHz	cooled DML+PIN	medium	Japan and South Korea
DWDM	100GHz	cooled EML+APD	high	North America, Japan, and South Korea
MWDM	CWDM wavelength+-3.5nm	cooled DML+PIN	medium	China

Its wavelength distribution is as follows:

Figure 5: wavelength distribution of colored optical module

Main application fields of optical modules

Optical modules are mainly used in the field of telecommunications and data centers. In the field of telecommunications, it is mainly used in wireless fronthaul, middle and backhaul, backbone transmission, FTTX, etc.; In the field of data centers, it is often used in the internal data transmission of the main data centers and the interconnection among data centers.

 Application of Fiber Optic Transceivers in the Telecom Field

Taking the 5G bearer network as an example, it is generally divided into the metro access layer, the metro aggregation layer, the metro core layer/provincial trunk line to realize the front-haul and mid-haul functions of 5G services. The devices at each layer are interconnected mainly through optical modules, and their typical application scenarios and requirements analysis are shown in Table 3.

Table 3: Analysis of 5G bearer optical module application scenarios and demand

Network layering	Metro access layer		Metro convergence layer	Metro core layer/trunk line
	5G front-haul	5G mid-and-back-haul	5G mid-and-back-haul+DCI	5G mid-and-back-haul+DCI
Transmission distance	<10/20km	<40km	<40km-80km	<40km-80km/hundreds km
Networking topology	Star type is dominant, with ring network auxiliary	Ring network dominates, with a small number of chain or star links	Ring network or dual uplink link	Ring network or dual uplink link
Customer interface rate	eCPRI:25Gb/s, CPRI:N*10/25Gb/s	Early 5G:10/25Gb/s Scale business：N*25/50Gb/s	Early 5G:10/25Gb/s Scale business：N*25/50/100Gb/s	Early 5G: 25/50/100Gb/s Scale business：N*100/400Gb/s
Line interface rate	10/25/100Gb/s gray light or N*25/50Gb/s WDM color light	25/50/100Gb/s gray light or N*25/50Gb/s WDM color light	100/200Gb/s gray light or N*100Gb/s WDM color light	200/400Gb/s gray light or N*100/200/400Gb/s WDM color light

Typical requirements for optical modules in 5G front-haul application scenarios are as follows:

(1) Within the industrial temperature range and satisfying high reliability requirements: Considering the AAU full outdoor application environment, the fronthaul optical module needs to be within the industrial temperature range of -40°C~+85°C and meet the dustproof requirements.
(2) Low cost: The total demand for 5G optical modules is expected to exceed that of 4G, especially the demand for front-haul optical modules may be in the order of tens of millions. Low cost is one of the main demands of the industry for optical modules.

Table 4 Current situation of 5G fronthaul optical modules

Rate	Form factor	Transmission distance	Operating wavelength	Modulation format	Optical chip
25Gb/s (eCPRI/CPRI)	SFP28	70-100m	850nm	NRZ	VCSEL+PIN
	SFP28	300m	1310nm	NRZ	FP/DFB+PIN
	SFP28	10km	1310nm	NRZ	DFB+PIN
	SFP28 BIDI	10km/15km/20km	1270/1330nm	NRZ/PAM4	DFB+PIN(or APD)
	SFP28	10km	CWDM	NRZ	uncooled DFB+PIN
	SFP28	15km	LWDM	NRZ	cooled DFB+PIN
	SFP28	10km	MWDM	NRZ	cooled DFB+PIN
	SFP28	10km	DWDM	NRZ	cooled EML+APD
	SFP28 Tunable	10km/15km	DWDM	NRZ	cooled EML+APD

5G medium and backhaul covers the access layer, aggregation layer and core layer of the metropolitan area. The required optical module technology is not much different from that used in the existing transmission network and data centers. The access layer will mainly use 25Gb/s, 50Gb/ s, 100Gb/s and other gray light or colored light modules, the convergence layer and above will mostly use 100Gb/s, 200Gb/s, 400Gb/s and other DWDM colored light modules.

Application of optical modules in data centers

A data center is a huge data center group composed of multiple computer rooms. For the normal use of Internet services, it is necessary to coordinatate operation of data centers. A large amount of information between data centers converges at the same time, which creates the demand for data center interconnection network, and optical fiber communication is a necessary solution to realize network interconnection; Different from traditional telecom access network transmission equipment, data center interconnection needs to achieve larger and more intensive transmission of information, which requires switching equipment to have higher speed, lower power consumption, and more miniaturized performance. Fiber optic transceiver is a core factor that determines whether these performances can be realized.

The connection types of optical modules in data center can be divided into three types: internal information transmission in data centers, interconnection among data centers, and information transmission from data centers to users. At present, the internal communication of data centers accounts for the vast majority of data center communications. The great development of data center construction has promoted the development of high-speed optical modules, and the application prospects of high-speed optical modules are getting better and better.

Figure 6 Diagram inside the data center

Optical module industry chain

The optical module industry chain consists of upstream optoelectronic chip suppliers, midstream optical module suppliers, and downstream telecommunications, network equipment vendors, internet service providers and Internet manufacturers.

Figure 7 Industry chain of optical modules

There are many suppliers of upstream optoelectronic chips, but high-end optical chips and electrical chips have high technical barriers and high R&D costs. The optical module industry is located in the middle of the industry chain, which belongs to the packaging link with relatively low technical barriers. It is under the pressure of the top and bottom of the chain, with relatively weak bargaining power and fierce competition within the industry. After years of development, Chinese companies have occupied half of the global optical module market by virtue of their advantages in labor costs, market size, and support from equipment manufacturers and operators.

Development trend of Fiber Optic Transceivers

High speed is the inevitable trend of optical modules. With the evolution of optical modules to high speeds such as 400G, 800G and even 1.6T, the market has increasingly high requirements for low power consumption, miniaturization, and low cost. There will be a technical bottleneck for traditional optical module technology. Due to its high integration and low power consumption, silicon photonics integration technology will break through this bottleneck and usher in a technological breakthrough. At present, more and more manufacturers from home and abroad are investing in the research and development of silicon photonics integrated modules. Some high-speed optical modules using silicon photonics integration technology have been mass-produced and put into use in data centers.