This paper investigates the principle and performance comparison of DML lasers and EML lasers.
Let's start with the terminology, DML is short for Directly Modulated Laser. As the name implies, DML controls the intensity of the laser output by changing the laser's injection current. The intensity magnitude of the laser corresponds to signals 0 and 1 and is shown schematically as follows (image from Ref. 1):
DML lasers use current changes to modulate the signal, but changes in the injected current result in changes in carrier concentration, which in turn cause changes in the refractive index of the material. In contrast, DFB lasers use gratings for mode selection, and a change in refractive index brings about a change in wavelength. The change in wavelength causes a chirp effect, and the fiber itself has dispersion, which can lead to pulse spreading and signal distortion. Therefore, DML lasers are not suitable for long-distance transmission.
DML lasers operate with a signal 0 corresponding to a current greater than the laser's threshold current. If the current corresponding to 0 is below the threshold, each current injection takes a certain amount of time to reach the particle number reversal, which is in the order of nanoseconds. Taking the signal of 10G as an example, the time of each pulse is 100ps, and the switching delay in nanosecond scale obviously cannot meet the requirement of high-speed modulation. In practice, the injection current is always greater than the threshold value, as shown in the following figure (image from Ref. 1):
The frequency bandwidth of DML is mainly related to parameters such as relaxation oscillation frequency, damping factor and RC time, and the expression is as follows:
The bandwidth can be increased by reducing the area of the active area and increasing the differential gain (dg/dn).
There are two abbreviations for the term EML. One is External Modulated Laser, as opposed to Direct Modulated Laser, where the injection current of the laser does not change and the laser outputs continuous light and the light intensity is adjusted by an external modulator, as shown in the diagram below; the other is Electro-absorption Moduled Laser, which is an electrically absorption modulated laser. The other one is the Electro-absorption Moduled Laser. InP's Electro-absorption Moduled Laser can be integrated with the laser on a single chip, which is usually referred to as an Electro-absorption Moduled Laser (EML). In addition to the electric absorption modulator, another commonly used modulator is the Mach-Zehnder modulator based on the electro-optical effect.
(image from Ref. 1)
The electric absorption modulated laser is mainly based on the quantum confinement Stark effect. In a quantum well structure, when no electric field is applied, the photon energy is smaller than the band gap and the light field is not absorbed through the material; when an external field is applied, the energy level structure is tilted, the equivalent band gap is reduced, and the incident light is absorbed by the material. By changing the intensity of the external field, the intensity of the output optical field can be modulated.
(image from http://photonics.intec.ugent.be/research/topics.asp?ID=175)
The frequency bandwidth of EML is mainly determined by the RC time of the electrical absorption modulation region. Since an EML laser requires additional current input to the modulator, its power consumption is increased compared to a DML laser.
The following table gives a comparison of the performance of these two lasers:
In terms of performance, EML outperforms DML in all aspects (including chirp effect, extinction ratio, eye diagram, jitter, transmission distance, etc.). The advantage of DML is its small size, low cost and low power consumption. Based on this, DML is more suitable for data center applications, while EML is suitable for telecom grade applications.
The above is a brief investigation of DML and EML, and we hope to deepen our understanding of them and know what to do in the subsequent product development process.
If there are any mistakes and indiscipline in the article, please do not hesitate to point out!
1、 Christophe Peucheret， Direct and External Modulation of Light
2、 Tsuyoshi Yamamoto，High-Speed Directly Modulated Lasers