Need help understanding lasing threshold I’m currently studying laser systems and doing some experimental work with a Nd:YAG laser that has a laser diode as the pump source. I think I understand the concept of lasing threshold, but there’s an experimental result I’ve been struggling with for the past week. It would be amazing if somebody could help me out of this.
About the pump source
The laser diode that acts as the pump source has two ways of working: continuous emission and square-wave emission. This square wave operational mode is used to measure the lifetime of an atomic transition and to demonstrate spiking in the laser cavity. This is the part I’m having problems with. The laser diode has two parameters that one can control, one of which is the amount of current circulating through it, which I’ll call “injection current”. The pump power is proportional to the injection current.
Using the laser diode in continuous operation I’ve been able to determine the minimum injection current circulating through the pumping source necessary for the Nd:YAG laser to start lasing. I’ve measured the intensity emitted by the Nd:YAG for various values of the injection current and I’ve obtained the following data, from which I’ve extrapolated the value of the injection current threshold, which is around 600 mA. https://image.ibb.co/cL9ZEd/plot1.png
Here comes the problem: if I use the laser diode in the square wave mode, I’m able to get lasing activity at a much lower value of the injection current than in continuous operation. You can see the effect in this picture.
https://image.ibb.co/jmNqLJ/IMG_18062018_192411.jpg
One of the signals is the injection current and the other one is the measured intensity of the Nd:YAG laser. As you can see, after the spiking the laser signal relaxes to a constant value and only falls down to zero when the input signal is cut off. I don’t understand why this is happening since I needed to almost double the pump power in order to get lasing in continuous operation of the pump source.
Thank you in advance.
 A: This appears (as many optoelectronic oddities often do) to be an issue of heat.  When you run your laser in continuous mode, you are continuously dissipating heat in your pump laser diode.  On the other hand, in square-wave mode, with what appears to be a 50% duty cycle, there is less thermal dissipation so the average temperature of the diode during operation will be a bit lower for the same peak current.  As nicely described in this document (especially see Figure 3.4), the wavelength and output power critically depend on this temperature (as an aside, have you seen a color shift between the two operating modes?).  
Higher temperatures cause a number of things to change in your pump diode.  The cavity length increases, the refractive index changes, the carrier energy distribution spreads out, the scattering rates change, and generally you'll have more loss and less gain in the cavity.  The complex interplay of temperature dependencies ultimately determines the lasing threshold.  But essentially, by reducing the duty cycle of your lasing, you can achieve the same injection current (during the "on" state) at a lower average diode temperature, which decreases your lasing threshold.
