Birefringence or Etalon Effect Used for Mode Selection in C315M?
So, how does the temperature of the KTP (KTP TEC) and overall cavity (RES TEC) control mode selection and single mode operation? Without some form of mode selection, the C315M laser would almost certainly operate with multiple longitudinal modes because the cavity is long and the gain medium is not at all the way at one end aginst the mirror. It would not be single requency, stability would be poor, and amplitude noise in the output would be high due to mode competition enhanced by the non-linear behavior of the KTP (the “green noise problem”). Most of what follows applies to the C415M (and probably to the C215M) as well. The Coherent 315M cavity has an effective optical length of about 55 mm resulting in a cavity mode spacing of only about 0.01 nm (2.7 GHz). Since this is much less than the 0.5 nm (140 GHz) gain bandwidth of Nd:YAG, many modes would fit under the gain curve and oscillate simultaneously. The KTP crystal is 5 mm in length. The only other optical element between the mirr
