AC Stark effect of optically pumped FIR lasers had becn studied theorctically and experimentally by many authors. All analysis were based on the assumption that the FIR signal intensity was much smaller than the pumping field intensity and the pumping power density was kept constant everywhere throughout the sample tube. This was not true

because there were energy exchange between the pumping and FIR signals

so the pumping power density decayed continuously along the sample tube and at the same time the FIR power density grew relevantly. So the AC Stark splitting measured at the output end of an optically pumped FIR laser(OPFIRL) systems with sample tube of different length should not be the same.According to the theoretical analysis

we concluded as follows: For an actual optical pumped FIR laser excited by pumping laser of constant power density with pumping detuning x=0

the AC Stark splitting and its spectral line width observed at the output end should not be the same for different length of sample tube. The longer the length of the sample tube

the AC Stark splitting would be smaller and the line width of the laser line would be broader. These effects would be more considerable if the pumping power density was smaller.