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DFB EML - Externally modulated DFB laser

Simulation with PICWave software

PICWave can model semiconductor lasers and their interaction with a photonic integrated circuit. This example shows a quarter-wavelength shifted distributed-feedback laser diode (DFB laser diode) simulated with PICWave. The output of the distributed-feedback laser is modulated on the right-hand side with an electro-absorption modulator.

DFB laser and modulator in PICWave

Quarter-wavelength shifted DFB laser with electro-absorption modulator
designed in PICWave

Design of the DFB laser
Simulation of the modulated DFB laser in the time-domain
Longitudinal hole burning

Design of the DFB laser

The distributed-feedback laser consists of two sections whose sinusoidal grating profiles are offset by 180 degrees. This is equivalent to introducing a quarter-wavelength optical phase shift in the middle of the DFB laser. The DFB laser has a total length of 320um. The K coupling coefficient is set to 0.008um-1 leading to a K.L product of 2.56. For a value such as this, the longitudinal mode intensity profile should peak at the location of the phase shift.

Quarter-wavelength phase shift in the DFB grating

Longitudinal profile of the DFB grating showing quarter-wavelength phase shift

The electro-absorption modulator is driven by a NRZ voltage signal. For the electro-absorption modulator, PICWave allows you to define the escape rate and the absorption (negative gain) as a function of the applied voltage.

Electro-absorption modulator
(a)                                                             (b)

Electro-absorption modulator: (a) absorption and
(b) escape rate plotted as a function of the applied voltage

Simulation of the modulated DFB laser in the time-domain

The simulation is run in the time-domain. You can see below the un-modulated output of the laser emitted on the left-hand side and the modulated output on the right-hand side. The overall shape is the same as the one emitted from the left-hand side, but the signal has been modulated by the NRZ voltage drive.

Output of the DFB laser
(a)                                                             (b)

Output of the DFB laser: (a) un-modulated and
(b) transmitted by the electro-absorption modulator driven by an NRZ signal

PICWave allows you to plot the absorption in the modulator, shown here along with the voltage drive. Plotted over a smaller time scale, it allows you to see how the voltage signal drives the absorption.

Absorption in the electro-absorption modulator
(a)                                                             (b)

Dynamics of the electro-absorption modulator : (a) absorption and
(b) voltage drive plotted in the time-domain

The spectrum of the laser output is shown below around the central wavelength. You can see that the side modes are strongly attenuated, and that the laser emits as expected at a single frequency.

Output spectrum of the DFB laser

Output spectrum of the DFB laser around the central
wavelength showing attenuated side modes

Time-evolving spectra can also be plotted.

Output spectrum of the DFB laser in the time-domain

Time-evolving spectrum of the DFB laser

Longitudinal hole burning

Plotting the mode power along the axis of the DFB laser shows a strong intensity peak at the location of the phase shift. Plotting the carrier density against z shows how the intensity peak causes longitudinal hole burning with a dip in the carrier density. This behaviour is expected given the value of the K.L product and the position of the phase shift at the centre of the DFB laser.

Longitudinal hole burning
(a)                                                             (b)

Longitudinal hole burning in the quarter-wavelength shifted DFB laser:
(a) optical power along the z-axis (blue: total, green: forward, red: backward)
and (b) carrier density along the z-axis at the centre of the laser