The 'optical distance' is the luminous point distance from the reference plane where light appears to be. When light passes through a material with a high refractive index, since it seems to only advance a short distance, a glass window is situated in the LD's exit port, and the optical distance from the reference plane appears longer than the luminous point distance (mechanical distance). More specifically, the luminous point is positioned 1.27mm from the reference plane, but optically seems to be 1.35mm away. In practice an optical distance of 1.35mm is important. For CAN packages without a glass window the optical distance and luminous point (mechanical) distance are the same: 1.35mm. In other words, LD manufacturers change the LD chip die bonding position based on the presence/absence of the glass window in order to ensure that the optical distance from the reference plane is the same.
Short for Moisture Sensitivity Level, MSL is a JEDEC (Joint Electron Device Engineering Council) standard established for the purpose of preventing device failure due to volumetric expansion of atmospheric moisture introduced into the resin package of semiconductor devices during reflow. The floor life of IC packages, during which the components must be mounted and reflowed after opening the moisture-resistant packaging, is broken down into 8 MSL levels.