I am running a script for same problem (source):
xset dpms force off
This turns monitor off and locks the session. The
sleep 0.5 is there to avoid the monitor turning on when you release the Ctrl+Alt keys; maybe you don't need this.
You can try other solution also from there (didn't tried this one yet).
The solution I use is:
I have added the Inhibit Applet to my top panel (this lets me toggle the sleep mode off and on)
In power management I set Put display to sleep when inactive for: 1 minute.
I use the Ctrl+Alt+L (I actually changed my shortcut to Ctrl+Alt+Spacebar) to lock the screen.
So when I am using the laptop the first thing I do is click on my Inhibit Applet in my top panel so my display does not go to sleep in 1 minute of inactivity.
When I am all done and want to lock the screen and turn off the monitor I click on the Inhibit Applet so it allows the sleep timer to work, and I hit my combination of keys to lock the screen Ctrl+Alt+L.
The Screen is locked and in 1 minute the display turns itself off.