Optical sampling of subnanosecond light pulses

In this paper we shall describe a technique for the display of subnanosecond light pulses which is the optical analog of the technique used in electronic sampling oscilloscopes. The optical pulse waveform to be displayed is assumed to repeat itself with period T. A mode-locked laser provides a source of sampling pulses of a few picoseconds duration. The period of this laser is adjusted to be T + δT, where δT < T. The two beams are sent into a nonlinear crystal set at the phase-matching angle for optical sum frequency generation. If the signal and the probing beam traverse the crystal simultaneously, a short pulse of light at the sum frequency is generated. This pulse represents a sample of the signal beam. Its energy Ysis proportional to ∫ I1I2dt, where I1and I2are the intensities of the light signal beam and sampling beam, respectively. If the signal beam is slowly varying compared to the very brief sampling pulse, we have: Ys= constant × I1(tn) where tnis the arrival time of the nth sampling pulse at the nonlinear crystal. Because of the slightly unequal periods, the sampling pulse scans the light signal in steps of δT seconds. Displays of the sum frequency signal Yson a conventional oscilloscope therefore constitute a sampled display of the light signal. In an experiment to test the method, sampled displays of subnanosecond pulses generated by a mode-locked He-Ne laser (wavelength 0.633µ) were obtained. The source of sampling pulses, ∼4 psec in duration, was a mode-locked Nd:glass laser (wavelength 1.06µ). The two unfocussed beams were mixed in a KDP crystal and the sum frequency signal at 0.397µ was detected by a photomultiplier and displayed on an oscilloscope. The period T of the He-Ne laser pulses was 12.44 nanoseconds, and the sampling step δT was varied between 100 psec and 400 psec. Using this method the He-Ne laser pulse width was observed to vary between 700 psec and 900 psec depending on the He-Ne laser adjustment. These measurements were confirmed with a fast photodiode and an electroni- c sampling oscilloscope. The average He-Ne laser power was ∼1 mw. In the KDP crystal the sampling pulses converted red light into UV light with an efficiency of about 5%. Since with focussed beams an efficiency of better than 20% can be achieved, it appears possible to optically sample light signals with average powers much less than a milliwatt.