Forward Voltage drop of laser line module
This module almost certainly contains the laser controller as well as the diode.  From the little information you give it implies that the operating voltage is between 2.6 and 5V.  The actual laser diode itself may have about 1.8V drop if it is red more if green less if infra red.

When asking questions like this you should give the part number and any other information you have, picture, data sheet etc - think of how you are asking others to answer a question with very little context. You should always try searching for a data sheet before enquiring here.  Just Google it!

Edit - Now you have given us the part number I can see some of the description and it includes the Laser driver - so the external voltage is not that of the laser itself:

Digikey has a data sheet of the 650-27 (not 28) with more information.  The voltage range is between 2.6 and 5V.