Correspondence 1741 3s 3P U 16.6eV Ground State Fig. 1-Energy levels of neon (simplified).Fig. 2-Tracings of odloscope photographs showing laser cross modulation. (Time wale 20 lrsec per division.) (a) Light signal from pumping lamp. (h) Cross modulation ( uenching) of red-laser output. (c) Switch-W 07 red laser by high-intenslty flash. (d) Enhancement of IR-laser output.placed in one of the 2P states, it may decay to other levels in the 1s series which have shorter decay times than the 1 S g metastable.In the CiV gaseous He-Ne laser, both the 0.633 ("red") and 1.15 p ("IR") transitions end on the neon 2 P 4 state. Since this laser operates with a rather small level population difference (-lO*/crnJ), its power output may be decreased by increasing the population of the lower state through optical absorption of the neon metastables. We enclosed a Xenon flashtube (GE FT-91) and a laser gaseous discharge (approx. 20 cm long) in an optical cavity. The laser output was monitored by a photocell shielded from stray radiation of the flash lamp. T o avoid optically pumping the helium 1s and 3 s metastable states the flash lamp was en-closed in a filter, which absorbs in the near infrared (XlO.8 p ) . In Fig. 2 (a) and (b) it will be observed that the red-laser cross modulation is a reasonably faithful replica of the flash signal providing one operates at suitably low light excitation (about 13J flash input) or suitably high laser power. A similar quenching of the laser power was also observed on the IR laser signal. Furthermore, when the laser is oscillating weakly, it may be pulsed off for a considerable interval, and the time for the signal to recover progressively lengthens as the laser output is reduced (Fig. 2(c)). Fig. 2(d) shows that the output of the IR laser may be enhanced by placing a violet filter Kodak #36 over the flash tube; the o p tical excitation of the neon metastables then proceeds favorably to the 3P levels rather than the 2P. The 3P levels will spontaneously radiate to the 2 s levels, which include the upper level of the IR laser. Thus the flash-lamp radiation causes the IR laser to operate in a cycle closed by the opticalpumping source. This effect was less pronounced compared with the quenching action because of the low transmission (one to five per cent) of this filter in the spectral region of interest (0.33 ~-0 . 3 5 P ) . The increase of output was a maximum of 10 per cent above the CiV output. B. PARISER
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