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Introduction to DML and EML modulation methods for optical modules

Time: 2024-03-13
         ⦁ Background
         We will talk about the modulation method when mentioning the parameters of the optical module, and the modulation method at the transmitter side is also stated in the datasheets of some optical modules, as shown in the following figure:
Figure 1 Moduletek Limited 25G ER module datasheet states that the modulation mode of the transmitter laser is EML
Figure 2 Moduletek Limited 10G ZR module datasheet states that the modulation of the transmitter laser is EML
         There are two modulation techniques for optical modules, DML and EML, which are briefly introduced in this article.
         ⦁ The basic principle of the optical module
         The basic function of the optical module is to realize the photoelectric conversion function, the following is its simple working principle diagram:
Figure 3 Working principle diagram of optical module
         The optical signal transmitted in the optical fiber is not constant, but is modulated, intensity changes in the optical signal, the following is a description of the characteristics of the DML and EML modulation technology and the difference between the applications.
         ⦁ DML modulation
         DML English full name Directly Modulation Laser, the basic principle of direct control of the current through the laser, to send out different intensities of light, the following are the modulation signal for the high level and low level is the principle of operation illustrated:
         When the modulating signal is high level, the modulating current flows through the LD diode and the laser emits light normally, as shown below:
Figure 4 DML modulation - high level
         When the modulating signal is low level, the modulating current does not flow through the LD diode and the laser does not emit light, as shown below:
Figure 5 DML Modulation - Low Level
         With direct modulation, the laser always works in an unstable state, so there are many nonlinear effects that affect the quality of the output signal.
         ⦁ EML Modulation
         EML English full name External Modulation Laser, the basic principle is a constant current through the laser, through the external modulator to change the ratio of light through the light, to get a different intensity of light, the following are the modulation signal for the high level and the low level is the principle of operation of the illustration:
         When the modulating signal is high level, the electric absorber does not absorb the light from the LD, and most of the light passes through the electric absorber, as shown below:
Figure 6 EML modulation - high level
         When the modulating signal is low level, the electric absorber absorbs most of the light, and most of the light cannot pass through the electric absorber, as shown below:
Figure 7 EML Modulation - Low Level
         Under EML modulation technique is usually used for high speed (usually 10Gbps or more) and long distance (usually more than 40KM) communication system;
         In summary, the biggest difference between DML and EML modulation technology is that the LD laser under DML modulation is in the unstable state of intensity change, while the LD laser under EML modulation is in the constant light-emitting state, and the EML is more suitable for long-distance and high-rate applications.
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