How to convert spontaneous emission into stimulated?
Well the conversion of spontaneous to stimulated emission is a part of the lasing process. My recommendation is that you you take look at the a book on lasers, my personal recommendation is : &quot;Introduction to Laser Technology, Fourth edition. C. Breck Hitz, J. Ewing, and J. Hecht&quot;, its super easy to read and it teaches you the fundamentals.
Here's an illustration from that book that might be a good explainer:

In each of the rods, you basically have a lasing effect as shown down here:

In some cases you use the same principle and skip having a cascade by adding a resonator:

References -
All illustrations are from the book &quot;Introduction to Laser Technology, Fourth edition. C. Breck Hitz, J. Ewing, and J. Hecht&quot;