Terahertz Source Employs Intracavity Optical Parametric Oscillator

Richard Gaughan

Terahertz radiation — electromagnetic waves in the spectral region between the far-infrared and microwaves — has applications from medical imaging and the detection of illicit drugs to security screening. But terahertz-generation schemes typically suffer from poor energy conversion and low out-put powers. A new method involving an intracavity optical parametric oscillator promises to overcome this.

Terahertz radiation is useful for a variety of applications, including the detection of illicit drugs, because it is transmitted through many materials that are opaque to optical and radio wavelengths (top). A new setup generates terahertz radiation using an MgO:LiNbO₃ nonlinear crystal in an Nd:YAG laser cavity, offering a lower threshold and higher output powers than other techniques. Courtesy of Tom J. Edwards.

The approach, developed by physicists at the University of St. Andrews in the UK and at Macquarie University in North Ryde, Australia, employs an MgO:LiNbO₃ nonlinear crystal in the cavity of an Nd:YAG laser. Besides those that form the laser cavity, another pair of mirrors acts as a resonant cavity for the idler wave generated within the crystal. The mirrors are mounted on a common, rotatable platform.

Tom J. Edwards of St. Andrews explained that there is one combination of signal refractive index, wavelength and direction at a specific idler direction that complements one idler refractive index and wavelength. Through phase-matching, the pump and idler directions define the wavelength and direction of the generated terahertz signal. The geometry forces the terahertz waves on a path about 60° to the pump beam, out the side face of the crystal. Silicon prisms bonded to the crystal face allow efficient output coupling of the terahertz beam.

Lower threshold

When the Q-switched laser was end-pumped with 20 W of 808-nm radiation in a 500-µs pulse from a quasi-CW laser diode from Lissotschenko Mikrooptik GmbH of Dortmund, Germany, the optical parametric oscillator threshold was reached. That was with a 45-ns pulse with 0.7 mJ of intracavity energy — an improvement over previous setups, in which 18 mJ of pump energy was required to exceed threshold.

With the pump operated at twice the threshold value, the down-conversion efficiency of the source in experimental tests was 50 percent, with half of the energy going into the signal and idler beams. For an angular change over the range of 1° to 3°, the output wavelength varied from 100 to 250 µm, a frequency range of 1.2 to 3.0 THz. Using the source, the scientists generated 5 nJ of terahertz radiation in a pulse less than 8 ns in length. They measured the output pulse energy using a composite silicon bolometer from QMC Instruments Ltd. of Cardiff, UK.

The initial results are encouraging, but improvements are under way. Better cooling should increase the pulse repetition rate from 15 to 400 Hz, improved coatings should enhance output coupling through the prism faces, and raising the optical damage threshold on the face of the nonlinear crystal should enable a fivefold or better increase in pump power.

These and other refinements are expected to increase the average power from the current system’s 75 nW to the range of 10 µW.

Optics Express, Feb. 20, 2006, pp. 15821589.