Abstract

The sequence bands of the CO₂ laser, which can be generated by placing a hot CO₂ cell inside a CO₂ laser cavity, provide a considerable increased number of laser lines with sufficient power to pump FIR lasers and for molecular spectroscopy. Shy and Yen have been able to lock a sequence-band laser to the line center using the optogalvanic Lamb dip with a low pressure N₂-CO₂ dc discharge.¹ In this paper, we report two new line-center frequency stabilization methods for the sequence-band CO₂ laser. For the first method, which we call rf optogalvanic Lamb dip, the dc discharge is replaced by an rf discharge. This method has certain advantages over the dc optogalvanic Lamb dip: lower pressure, lower operation voltage, no need of water cooling, and better SNR. The second method, saturated 4.3-µm fluorescence, utilizes the fact that the 4.3-µm fluorescence originating from the upper level of the sequence bands does not have the photon trapping effect. We use a longitudinal cell to collect the fluorescence. The cell is a Pyrex tube with inside coated with Al. By heating part of the cell to ~200°C, we were able to observe the saturation dip in the fluorescence signal and use its derivative signal to stabilize the laser. The stability is >100 kHz for both methods. Finally, we measured the beat frequency between a sequence-band laser stabilized by rf optogalvanic Lamb dip and a frequency stabilized regular CO₂ laser, and nineteen sequence transition frequencies have been measured to an accuracy of 300 kHz.

© 1991 Optical Society of America