A 50-year-old idea is finally flourishing into compact eye-safe LIDARs. The mirrorless optical parametric oscillator (MOPO) can generate eye-safe, near- and mid-infrared coherent light like conventional parametric oscillators, without the alignment issues that arise from complex optical cavities. A MOPO can be as simple as a pump laser and a periodic nonlinear crystal. The biggest obstacle to building a MOPO is the extremely fine pitch in the periodic structure of the nonlinear crystal. The mirrorless parametric interaction occurs only when the orientation of the crystal changes with a period shorter than the pump laser wavelength. Typical laser wavelengths at around 1 micrometer set tight requirements for fabricating the crystal. Now, researchers at the Royal Institute of Technology in Stockholm, Sweden, have excelled in making high-quality crystals with sub-micrometer periodicity. Moreover, their method allows scaling up of the total length of the crystal, which decreases the threshold for parametric interaction. These advances have made it possible to pump the MOPO with ever longer laser pulses and to raise the curtain for inexpensive and simple sources of eye-safe pulses to be used, for example, in environmental monitoring.
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