Direct Modulated Laser diode (DML) can be used to modulate optical power. In DML, the laser output power is adjusted by changing the pump current in the laser gain medium. The pump current is controlled by an electrical drive signal. This type of direct detection (DD) system typically uses On-Off Keying (OOK). In other words, the pump current of the DML is changed through binary signals.
After optical fiber transmission and photodetection, the receiver performs clock recovery and sets appropriate thresholds to determine whether the received symbol is logic 0 or 1.
In general, adaptive digital filtering is not used; if there is intersymbol interference, it will not be mitigated before symbol decision.
Additionally, for short-distance transmission, forward error correction (FEC) coding is typically not used.
The advantage of DML lies in its modulation process being implemented within the laser source, eliminating the need for optical coupling between the laser and modulator.
This method can minimize manufacturing and packaging costs.
DML-based DD systems are suitable for 10 Gb/s systems.
Research and laboratory demonstrations show that DML can achieve bandwidths exceeding 50 GHz and can modulate data at speeds over 100 Gb/s.
However, the application of DML-based DD systems is mainly limited by the limited extinction ratio of DML and issues such as laser chirping, which directly alters the laser frequency.
For DD systems, a popular external modulator is the electrically absorbed modulator (EAM).
EAMs are typically manufactured in the form of semiconductor waveguides and equipped with electrodes to apply an electric field perpendicular to the direction of light beam propagation.
The applied electric field alters the optical absorption spectrum of the semiconductor (thus changing the modulator‘s output power), a property known as the Franz-Keldysh effect.
EAMs can operate at low drive voltages (approximately 2 V) and provide bandwidth exceeding 100 GHz.
They can be integrated with distributed feedback lasers on the same chip, and this combination of laser and EAM is called an electrically absorbed modulator laser (EML).
Although EMLs have better extinction ratios than DMLs, they still produce modulation chirping.
To avoid this, Mach-Zehnder modulators (MZMs)can be used.
MZMs are another type of external modulator that achieves intensity modulation by combining two phase modulators in a Mach-Zehnder interferometer configuration.
Using differential drive or push-pull configurations, MZMs can achieve chirp-free modulation.
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