In this article we show the workflow to calculate Relative Intensity Noise, RIN, for Directly Modulated Lasers, DMLs. This method also applies to TWLM elements. RIN refers to the fluctuations in the output power of laser which is defined as the intensity noise of the laser normalized to the average power. This normalization makes RIN independent of the laser power. RIN can be expressed through the output power measured from the photo detector as below:
Let’s take following circuit as an example. The DML is pumped with an impulse function. The laser output is fed to a photodetector and output signal is calculated.
RIN is an operating characteristic of the laser in the steady-state. Hence, first we should find the threshold current. In the attached project file [[LaserTestBenchImpulse_DM_laser.icp]] under optimizations and Sweeps, run “lisweep” which sweeps output power vs range of input currents. From the resulted L-I curve, the threshold current can be obtained. You can run [[curveLI.lsf]] to find the threshold current by finding the second derivative of L-I curve. Running the script, threshold current is calculated to be Ith=0.106A.
Now that we know the limits to operate the laser in the steady-state, we need to set up a sweep to sweep over the currents above the threshold and record the power density spectrum and the mean output power.
Note: It is important to set the correct time interval for the PWM_1 element for calculating mean output power. This is to make sure that for all the current points in the sweep, the laser is in the steady state for the whole time interval. This can be done by checking the steady state time for different current points using OSC_2 element results and setting "start time" and "stop time" in the PWM_1 element properties.
Normalizing the output power by the mean power, RIN (dB/Hz) is obtained. You can run [[rinSweep.lsf]] to do that automatically. In this script the spectrum is also smoothed with a low pass filter in terms of convolution with a Gaussian function. The raw RIN spectrum and smoothed spectrum are as below: