‘Laser’ evolved from the abbreviation of LASER (Light Amplification by Stimulated Emission of Radiation). Laser is a kind of coherent light, with the characteristics of the same frequency, the same vibration direction and the same phase (or the phase difference remains constant). Optical communication system, to a large extent, depends on high quality laser light source.

Laser is the heart of an optical module, and its cost accounts for about 50% of the total cost of an optical module. This article mainly introduces the laser in an optical module. What are the types of lasers in optical modules? VCSEL, FP and DFB are all independent devices. If you want to transmit the light signal, it is driven with or without electric current, which is called Direct Modulated Laser (DML). EML is an integrated device. There are two modules on a chip, one is a laser (L), and the other is an EAM modulator, and these two formed together is called EML.

VCSEL

The full name of VCSEL is Vertical Cavity Surface Emitting Laser. It is a semiconductor laser of surface emitting type and a vertical cavity structure.

VCSEL can provide high quality laser beam and higher coupling efficiency. Compared with FP laser and DFB laser, VCSEL is easier to manufacture, making it possible to produce low-cost VCSEL based transceivers. VCSEL is mainly used for short-range optical modules.  

FP

FP (Fabry-Perot) is a semiconductor light-emitting device of edge emitting type and horizontal cavity structure, which emits multi-longitudinal mode coherent light.

FP laser is mainly used for short-range transmission, such as transmission distance within 20 km. FP is divided into two kinds of wavelengths, 1310nm and 1550nm. It features simple structure, production technology and low cost.

DFB

DFB (Distributed Feedback Laser) is based on the FP Laser, the Bragg grating is integrated into the active layer inside the Laser (that is, in the gain medium), and the mode selection structure is formed in the resonant cavity to achieve single-mode work. It belongs to the edge emission type and uses the horizontal cavity structure.

DFB generally uses two wavelengths of 1310nm and 1550nm, which can be divided into cooled and uncooled, and is mainly used for medium and long-range transmission.

Summary

The selection and use of optical modules varies according to different scenarios. In the selection of laser type and modulation type, some of the most important factors are transmission rate, transmission distance, and different wavelengths. For high speed 100G optical module, VCSEL laser is used for transmissions of a few tens of meters, DFB laser is used for 500 meters to 10 kilometers, and EML laser is used for 40 kilometers. In a 10G optical module, the wavelength of 1310nm belongs to the zero dispersion zone. The DML laser can be used when the transmission distance is 20 km. EML laser is used for the wavelength of 1550nm.