DML 과 EML 차이점
Directly Modulated Laser (DML)
DMLs generally use distributed feedback structure with a diffraction grating in
the waveguide for stable operation for direct modulation, and so this laser is
also called a “DFB”
(Distributed-Feedback laser diode). The modulation speed and transmission
distance strongly depends on the spectral line-width of the laser. A narrower
line-width is required for higher modulation speed (data rate) and longer
distance. Compared to a Fabry-Perot laser, the spectral line-width of a DFB is
about 1/10th, and so a DFB structure is more suitable as a high data rate DML.
In a DML, data is placed on the optical beam by modulating the
Injection Current, which is an input on/off electrical signal and is directly
applied to the laser diode chip to output a modulated optical signal. A
DML is a single chip and provides a simple electrical circuit configuration for
operation, so it may fit to a compact design and low power application.
Direct modulation changes the laser properties like its refractive index
leading to a large chromatic dispersion. Performance of a DML degrades over longer reaches (>10km)
due to larger chromatic dispersions, lower frequency response, and a relatively
low extinction ratio when compared to EMLs.
Electro-absorption Modulated Laser
An EML is a laser diode integrated with an electro-absorption modulator (EAM)
in a single chip. The laser diode section, normally with the same device
structure as DML operates under a continuous wave (CW) condition, and input
voltage on/off signals are applied to the EAM section to generate optical
output signals. The laser properties themselves are not changed by the
process of modulation, as they are in a DML. EMLs are advantageous in applications
with higher speeds and longer distance transmission, compared to a DML, because
of its smaller chromatic dispersion. An EML is
mainly used for higher speeds (>25Gbps), and longer reaches (10-40km) in
Compared to a DML, an EML has smaller chromatic dispersion with
a stable wavelength under high-speed operation, because the injection current
(input signal) to the laser section is not modulated and therefore does not
change. The frequency response of an EML depends on a capacitance in the
EAM section which enables high operating speed, even greater than 40GHz.
Extinction in an EML is caused by absorption as the coefficient changes with
the modulated voltage applied to the EAM section, and the extinction ratio
becomes higher with a large voltage input (on/off electrical signal).