The generation of infrared picosecond pulses (.about.10.sup.-12 s pulse duration) has received increasing attention over the last several years principally for optical communications, for the investigation of multiple-photon absorption and vibrational energy transfer processes in molecules, and for laser fusion. In particular, the infrared region between 10 and 20 .mu.m is of great importance since most molecules have characteristic absorptions there. To our knowledge, the only existing picosecond source in this range is the 10 .mu.m CO.sub.2 laser. However, this laser provides coverage of only a small portion of the desired wavelengths. The present invention, in contrast, provides broadly and continuously tunable high power infrared radiation with picosecond temporal resolution between 10 and 20 .mu.m. The bandwidth and operating wavelength are completely controlled by the use of a diffraction grating and the device is simple, stable and reliable. The device utilizes two lithium niobate crystals positioned in series but separated by the grating and optically pumped by a 1.06 .mu.m mode-locked Nd:YAG laser to generate and amplify infrared radiation between 1.9 and 2.4 .mu.m. The generated radiation is subsequently converted into 10-20 .mu.m picosecond radiation by difference-frequency mixing in a cadmium selenide crystal.
Picosecond pulses have been generated in the infrared region (1.3-3.6 .mu.m) in the past using 1.06 .mu.m optically pumped lithium niobate optical parametric devices in the travelingwave mode which, with modification, is what is done by the instant invention. (See, e.g., Laubereau et al., App. Phys. Letters 25, 87 (1974)) However, several disadvantages associated with these previous devices are successfully overcome as a result of said modifications. In the conventional generation techniques, in intense, short duration light pulse at fixed frequency .nu..sub.p is sent through one (or more) nonlinear crystals (LiNbO.sub.3, for example) which act as parametric, threephoton amplifiers. Starting from spontaneous noise at frequency .nu..sub.s, a significant magnitude signal of frequency .nu..sub.s, is produced in the first crystal amplifier. The signal may be further amplified in subsequent crystals. The energy for the production of .nu..sub.s comes from the .nu..sub.p electromagnetic field. A second frequency called the idler, .nu..sub.i, is also generated in the crystal such that .nu..sub.s +.nu..sub.i =.nu..sub.p . This condition is simply a consequence of energy conservation. The specific .nu..sub.i,.nu..sub.s set which will be amplified is determined by the "phase matching" conditions, k.sub.p =k.sub.s +k.sub.i. k.sub.p, k.sub.s and k.sub.i are the wave vectors, defined as k.sub.p =2.pi.n.sub.p .nu..sub.p, k.sub.s =2.pi.n.sub.s .nu..sub.s and k.sub.i =2.pi.n.sub.i .nu..sub.i, of the pump, signal and idler, respectively. n.sub.p, n.sub.s, and n.sub.i are the corresponding indices of refraction. The high intensity requirement of .nu..sub.p is necessary to produce a high amplification within a path length of a few centimeters. Single pulses have the advantage of allowing the application of such high pump intensities without crystal damage.
The first disadvantage that the instant invention overcomes is that of unusably broad spectral bandwidths. The bandwidth in previous devices was primarly determined by the phase-matching conditions in the non-linear crystal. Spectrally narrow pulses are normally generated only when the device is tuned far from the degenerate point (.nu..sub.s .about..nu..sub.i), thereby severely restricting the useful operating range. With our invention, the bandwidth is determined by the character of the diffraction grating and its distance from the second lithium niobate crystal. An advantage of being able to operate near the degenerate point is that in the subsequent mixing of the signal and idler frequencies in the cadmium selenide crystal we can generate radiation continuous from 10 to 20 .mu.m as the difference, .vertline..nu..sub.s -.nu..sub.i .vertline.. Previous to this invention there was no such picosecond infrared source available.
A second disadvantage of the usual parametric traveling-wave scheme is that non-collinear components can experience significant amplification resulting in a highly divergent beam. Although this can be improved considerably (&lt;3 mrad beam divergence) by positioning a second crystal where the pump beam overlaps the signal beam and amplifies only collinear components, the beam still may be spectrally broad. Furthermore, when two or more crystals are used, the frequency tuning process is tedious and difficult because the crystals must be simultaneously and precisely rotated to maintain the aforementioned phase-matching condition, and the entire apparatus becomes unstable to mechanical and thermal fluctuations.
Our invention (See Campillo et al., Opt. letters 4, 325 (1979), and Opt. Letters 4, 357 (1979)) successfully overcomes these difficulties by first generating a broadband (1.9-2.4 .mu.m) picosecond continuum in a LiNbO.sub.3 crystal and then isolating and injecting a specific spectral-spatial component into a second broadband parametric amplifier by means of a diffraction grating. The grating additionally provides the required narrow spectral bandwidth. Thus the instant invention provides a high power, broadly and continuously tunable picosecond infrared source which is also simple, stable and reliable.