Rami Arieli: "The Laser Adventure" Chapter 2.12 page 2

Advantages of four level lasers Compared to three level lasers:

• The lasing threshold of a four level laser is lower.
• The efficiency is higher.
• Required pumping rate is lower.
• Continuous operation is possible.

 Summary:

• In a three level laser the lower laser level is the ground state.
• In a four level laser the lower laser level is above the ground state.

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