Nonlinear Optics Group - University of St Andrews

Figure 1 | . Full animated Image

Figure 1 illustrates the basic concept of the device. The nonlinear crystal (in the present case MgO:LiNbO3) is located at the point of intersection of the optical axis of the cavity (pump-wave cavity) of the laser used to pump the parametric oscillator, in this case a Nd:YAG laser generating the pump-wave, and the optical axis of the idler-wave cavity associated with the parametric oscillator itself (intersecting cavity geometry). The optical parametric oscillator is singly-resonant at the frequency of the idler-wave (very close to the frequency of the pump-wave). The optical axis of the idler-wave cavity is non-collinear with the optical axis of the pump-wave cavity (non-collinear phase match geometry). As may be seen from the wave vector diagram describing the phase matching, included in Figure 1, and the simulation diagram, changing the angle between the two cavity axes tunes the frequency of the generated THz radiation. The nonlinear crystal being located in the cavity of the pump laser means that it is subject to the high circulating intracavity field of the pump laser (intracavity geometry).

The adopted approach therefore has the following advantages. The use of a non-collinear geometry ensures that the generated THz wave (the signal wave) propagates at a large angle to the pump and idler waves, as may be seen from Figure 1, thereby rapidly exiting the nonlinear crystal; an essential requirement since this wave experiences high absorption in this crystal. The intersecting cavity geometry enables the nonlinear medium to be placed within the cavity of the pump laser where it is subject to the high circulating intracavity field. Since this field is typically over an order of magnitude greater in intensity than the external field obtainable from the pump laser under optimum output coupling conditions, the need for high-energy and hence bulky pump lasers, a particular detraction of earlier work in this field [1], is avoided and a compact, portable device results. An additional bonus is that the coupling optics between the pump laser and the parametric oscillator are eliminated. Finally, the ability to independently rotate the optical axis of the separate idler-wave cavity over a few degrees provides the wide spectral coverage (1-3 THz) associated with angle tuning. This novel intersecting cavity geometry is the subject of a current patent application.

Significant improvement compared to earlier work in the down-conversion efficiency from pump wave to extracted THz wave (signal wave) was observed. For example, a previously reported parametric device [1] employing a separate pump laser required pump pulses with energies in excess of 18mJ just to exceed oscillation threshold for the parametric process, while pump pulses with energies of 30mJ were required to generate THz pulses with energies around 200pJ. Using our intersecting cavity scheme, in which the nonlinear crystal is placed within the cavity of the pump laser itself, reduced the pump laser requirement such that a device capable of delivering output pulses of the order of only 0.7mJ was sufficient to take the parametric process above threshold (greater than tenfold reduction in pump energy requirement). When operated at twice threshold, corresponding to pumping with pulses of around 1.3mJ energy, we recently generated THz pulses with energies in excess of 20nJ (representing a eighty-fold increase in THz pulse energy over earlier work). The reduced pump energy requirement allows efficient and compact diode-lasers to be used to excite the Nd:YAG pump laser, as was the case here.

Figure 2 | Temporal behaviour of pump and idler pulses. large Image

The oscillation threshold associated with the previously described idler cavity was observed to correspond to the case when a pump pulse energy of around 0.7mJ at 1064nm could be optimally output coupled from the pump laser. This required the Nd:YAG gain medium to be excited with only 20W of diode laser power (over a rectangular pulse of duration 500µs). The associated peak intracavity intensity of the pump radiation at 1064nm was 12MWcm-2, with a pump pulse duration of around 45ns (FWHM). Note that this intensity is some ten times greater than that coupled out of the cavity under optimum output coupling conditions, indicative of the advantage of the intracavity approach. A further advantage of the intracavity approach over external pumping is that idler gain is now associated with both the forward and backward transits of the idler cavity by the idler wave

Figure 3 | THz tuning range. large Image

Figure 2 displays the temporal behaviour of the idler-wave and the pump-wave, the latter both in the absence of down-conversion (undepleted pump pulse-dashed line) and in the presence of down-conversion (depleted pump pulse-solid line), for a pump pulse energy of 1.3 mJ (corresponding to pumping at 2x threshold, and requiring only 36W of diode laser pump power), when the associated peak intracavity intensity is 25 MWcm^-2 with a pulse duration of the (undepleted) pump pulse of 45ns (FWHM) as previously. It may be seen from Figure 3 that under these operating conditions the pump pulse depletion is substantial indicating close to 50% down-conversion of the pump energy into the signal (THz) and idler waves combined. Further, the pump pulse is significantly depleted by this down-conversion process just after the pump pulse has passed through its maximum intensity. This corresponds to an optimum condition for what can be regarded as the effective “cavity dumping” of the pump field by the nonlinear down conversion process since at this point the majority of the stored energy in the Nd:YAG gain medium has been extracted, through Q-switching, into the circulating field within the pump cavity. Under the conditions above corresponding to pumping at twice threshold where the down-converted energy per pulse was estimated as 0.6 mJ, the externally measured THz pulse energy at 1.6 THz was 20 nJ, indicating a peak pulse power of in excess of 1W. The quantum efficiency associated with the generation of THz radiation at this frequency is 0.6%, so that the THz energy per pulse generated internal to the nonlinear crystal in the present case is around 4 µJ. The observation that the externally extracted energy is a factor of 200 below this level is indicative of the widely-known deleterious effects of THz wave absorption in the lithium niobate itself (quoted absorption coefficients are typically in the range 20cm^-1 to 30cm^-1 at 1.5 THz).

Figure 4 | THz OPO device. large Image

The present device was continuously tuned over the range 1.2-3.05 THz, corresponding to a wavelength range of 100-250µm. The THz wavelengths were not measured directly but were inferred from the known pump wavelength (1064nm) and the changing idler wavelengths measured with an optical spectrum analyser. Figure 3 shows the internal (to the lithium niobate crystal) and external angles between pump wave and idler wave as a function of the frequency/wavelengh of the generated THz radiation. Currently measured linewidths of the THz wave are of the order 50-80 GHz.

Figure 4 is a photograph of the bench-top version of the device. The footprint of the device can be judged from the 300mm ruler alongside the cavity. The Nd:YAG gain module and Q-switch are to the left. The swing arm on which the off-axis idler-wave cavity is mounted is clearly visible towards the centre, with the nonlinear crystal sitting directly underneath the central rotation mount. It will be realised that many of the components shown are to allow flexibility in alignment and adjustment of an experimental device, and would not be included in any prototype built for applications. It is anticipated that in such a prototype the overall dimensions of the base-plate would not exceed 400mm by 100mm.

References:

[1] K. Kawase et al., J. Phys. D, Appl. Phys., 35, R1-R14, (2002), and references therein (journal link).