Lens focusing
Firstly you need to focus laser lens.
The best way to do it - use camera that can see IR light.
You need to fix laser lens holder with screws, install lens with extender.
Lens approximate position:
You need to direct laser to a surface at big distance (> 2m), and rotate lens to get image with sharp edges.
Example:
Left spot at this photo is produced by focused Lidar, right spot is produced by laser pointer (given for size comparation).
After you get sharp laser spot, you need to adjust lens holder position.
It is very helpful to use fast oscilloscope for that. Oscilloscope should be connected to J1 and J2 (for sync) connectors.
Also it is useful to use "LidarTestingUtility" to see signal level (by observing histogram width (wide is bad) and pulse width - wide is good).
Photodiode lens must be installed. Laser should be pointed to a wide surface at 1.5-2m. APD voltage should be close to breakdown. Laser voltage should be > 13V.
You need to unscrew laser lens holder a bit, and move it along PCB surface. You need to find position with maximum received amplitude.
You should get TIA overdrive at good lens holder position (see animated GIF image):
[CH1 is laser current, CH2 is TIA output]
Process of laser holder adjustment could be iterative: move lens, tighten screws a bit, move holder again.
At the end you should get fixed holder and high received amplitude.
You need to decrease received signal somehow (use darker/more far object/decrease APD voltage) to leave TIA overdrive state - see shape of the received signal at the oscilloscope. After that you need to rotate lens to get max amplitude if it is possible.