Standard He-Ne lasers consist of a gas discharge tube were the mirrors, forming the resonator, are fixed at both ends. In this way the light is amplified within the tube to several Watts of power. A small portion of this power is coupled out of the laser by a mirror having a transmission of a few percent. So, the output power of a standard He- Ne laser is in the range of Milliwatts. A setup with an external laser allows to use the resonator- internal power for measurements. This is often used for particle measurements. Any particle flowing to the strong laser beam generates a signal that can be analyzed or counted. This is used for instance to measure the purity of clean rooms.
Extra cavity Helium-Neon laser for particle measurement
