This application example will simulate a quarter-wave-shifted index-coupled Distributed Feedback (DFB) laser with the parameters given in Reference , and will plot its multi-mode spectrum vs current and spectral gain width. It will also plot the LI curve obtained from the simulation for the laser and compare it with that obtained in Reference .
The minimum product version for this example is 2020a.
In contrary to the conventional laser design with discrete mirrors at the ends of the optical cavity, like Fabry-Perot laser, the DFB laser uses grating as the wavelength selective element. The figure below shows the DFB laser circuit in the example file dfbLaser.icp.
The grating is combined in the Traveling Wave Laser Model (TWLM) and it is enabled for the DFB design.
Open and run script file spectrum.lsf. Here laser gain FWHM is set to be 1000 nm (flat gain) and bias current is 28mA. The power spectrum measured by the Optical Spectrum Analyzer (OSA_1) for the DFB laser is plotted below and corresponds well to the result in the reference:
In the script, increase the bias current to 63mA and run the script again. As shown in the plot generated and in a good agreement with the reference, the second mode has gained strength due to spatial hole burning at high bias current and side mode suppression ratio, SMSR, is decreased.
For a more realistic gain shape, decrease the FWHM to 10nm in the script. As shown in the plot, the side mode is weakened and SMSR is improved. SMSR in this case is measured to be ~35dB that is in agreement to the 38dB SMSR reported in reference .
NOTE: This example shows the importance of realistic spectral gain profile for accurate side mode suppression ratio simulation. To accurately model a gain medium, check our MQW gain solver and MQW edge emitting laser example.
The sweep object "sweep" defined in the example file sweeps the driving current of the DFB laser and records the laser output power as the result. Run the sweep and then run the script file plotLI.lsf; the comparison of the LI results obtained from the simulation and from the text file LIcurveDataRefFig2a.txt will be plotted. The text file LIcurveDataRefFig2a.txt stores the results obtained from Reference . Following is the comparison.
Side mode suppression ratio (SMSR) vs. current
The calculation of SMSR as a function of input current can be automatized by using Lumerical's script language in addition to the sweep object that was used to sweep over current and save power and spectrum results. Script file SMSR_vs_current.lsf contains the post-processing steps to automatically generate SMSR vs. current plot after the sweep is done. This file uses a function for spectrum filtering from file filtered_spectrum.lsf, which should be placed in the same folder.
The steps to generate SMSR as a function of input current are the following:
Obtain spectra for different input currents from the sweep object.
Filter spectra with a Gaussian profile with user specified standard deviation to remove noise.
Find peaks in spectra and subtract two most dominant magnitudes to get SMSR in dBm.
The result for the original sweep with 10nm gain FWHM is given in the figure below.
If we set the gain quality factor in TWLM to correspond to 25 nm gain FWHM, rerun the sweep, and recalculate SMSR vs. current, then we can see that SMSR reduces with input current due to effects such as spatial hole burning of the dominant mode that causes gain saturation of the dominant mode and the reduction of the threshold gain difference between the dominant mode and a side mode:
 Arthur J. Lowery, Adrian Keating, and Casper N. Murtonen , “Modeling the Static and Dynamic Behavior of Quarter-Wave-Shifted DFB Lasers”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 28, NO. 9. SEPTEMBER 1992