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EML in CWDM SFP+: High performance light source

Date: 2019-09-11 14:46 From: C-light Onclick:
SFP module is basically the same as GBIC in function. Therefore, it is also called mini GBIC by some switch manufacturers. It realizes photoelectric and electro-optical conversion.

It is with independent transmitting and receiving amplifier circuit, which integrates sending and receiving functions and meets the telecommunication transmission standard of optoelectronic system.
Advantages of SFP modules are usually as following :
1. Live Hot Plug-in: SFP uses the pin definition of golden finger, which makes the order of power-on of different functional feet different. It ensures that SFP module can be hot-plugged when the socket is charged, and facilitates the maintenance of the system disk.
2. Digital Diagnosis Function DDM: It can monitor and report some related working parameters of the module in real time, and can monitor its working state.
3. Low cost: optical transceiver Raw materials are saved compared with previously separated optical receiving and transmitting modules. Therefore, the optical transceiver module is the best solution to realize low-cost bidirectional transmission and optical interconnection.
4. Miniaturization: SFP module size is not only smaller than a single optical transmitter module or optical receiver module, but also smaller than other optical transceiver modules in LC package, and more intensive installation on the system disk. APC (Power Compensation) of the Transmitter Module, ATC (temperature compensation), drive, slow start protection and other functions as well as pre-amplification, limiting amplification and signal alarm of receiving module functions is accomplished by IC in the module.
5. High reliability: IC is used in the components and isolated to ensure the reliability of the circuit. At the same time, To Coaxial encapsulation of the shell ensures the service life of the core of the photoelectric device. In the manufacturing process, laser welding technology is used to improve the reliability. The transmitter and receiver parts of the module are completely independent, which reduces the series winding between them, and makes the module itself less interference and better performance.
Previous article: SFP+ vs SFP

Brief introduction of SFP structure
Modules are classified by hardware as TOSA component, ROSA component and PCBA board.

Transmitting component T0SA
The optical emission module is the main component of the optical emission module, in which the light source laser diode is the core, and the LD chip and the monitoring photodiode (MD plus other components encapsulated in the TO coaxial constitute the optical emission module TOSA).

Laser Wavelength (nm) Characteristics
LED 850/1310 For about 100M
FP 1310/1550 It can be used for medium-distance high-speed transmission
DFB 1310~1550
The cost is high and it can provide high power for long distance transmission.
VCSEL 850/1310 Low production cost, short reaches transmission
EML 1550 Very High cost for high-speed long-distance transmission
Previous article: DFB in SFP+ CWDM reflects the most complex manufacturing process of semiconductor products
Electroabsorption modulated laser (EML)
Electroabsorption modulated laser (EML) is an integrated device of electroabsorption modulator (EAM) and DFB laser (LD).
Electroabsorption modulator
An Electroabsorption Modulator (EAM) is an optical signal modulation device fabricated by using an exciton absorption effect in a semiconductor. Because of its fast response and low power consumption, it is widely used in modulation and coding of signals in high-speed fiber-optic communication.
In recent years, many laboratories around the world have studied the use of EAM strong nonlinearity for the realization of all-optical information processing such as wavelength conversion, clock extraction, and fast all-optical logic gates.
The basic structure of EAM is a PIN structure in which the N-zone portion is an alternately grown multi-layer structure corresponding to an optical antireflective film stack. The refractive index and thickness of each layer are designed according to the central wavelength according to the optical anti-reflection stack, and the lattice matching between the substrate and the substrate is realized by the strain superlattice structure. The I region is partially a multiple quantum well (MQW) structure. It is a uniquely absorbing material that is artificially produced using the quantum-limited Stark effect. The main performance is that the absorption edge is steep and the thermal stability is good. When a suitable reverse electric field is applied, the exciton absorption peak will obviously move toward the long wave direction, and the absorption spectrum can be reversibly reduced after the external electric field is cancelled. This material is realized by designing the composition and thickness of the well and the barrier of the multiple quantum well structure and the number of cycles, which is commonly referred to as "energy band engineering." The means by which it is implemented is to compensate for strain superlattice growth. Usually, the epitaxial layer constituting the bulk heterojunction must maintain lattice matching with the substrate material, and lattice matching requires the epitaxial layer to have a specific composition, and the growth away from the specific component forms lattice defects in the epitaxial layer. After the appearance of the quantum well structure, the compressive strain quantum well structure can effectively improve the performance of the III-V semiconductor laser, such as lower threshold and higher output power, higher modulation rate and better temperature stability. Within a certain strain range, the strain force can maintain no lattice defects in the epitaxial layer, which provides a new degree of freedom for selecting the composition of the material to obtain better device performance. However, there is a critical limit to the cumulative thickness of the strained quantum well multilayer strain. Later, strain compensation technology was developed, in which both the compressive strain layer and the tensile strain layer were grown during the growth of the strained quantum well to reduce the net strain to increase the total thickness of the MQW and improve the stability of the structure.
EML is a high performance light source for optical communication, which integrates small volume and low wavelength chirp by using quantum confinement Stark effect (QCSE) and DFB laser with internal grating coupling to determine wavelength. Universal ideal light source for carrier. There are many applications in access network, such as optical transmitter for optical fiber transmission in metropolitan area network and local area network, electric-optical signal conversion and long-distance transmission in phased array radar base station, etc. Compared with direct modulation DFB lasers, EML has better transmission characteristics and effects than DFB lasers, especially in high frequency modulation or long distance transmission.
EML products are also divided into chip products, component products and module products. Module products include component products and component products contain chip products. So the key core of EML is EML chip, which is the core of the electroabsorption modulation laser.
C-light EML CWDM SFP+ specification: 

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