Parametric devices are flexible and convenient sources of widely-tunable coherent radiation, encompassing all time-scales from the femtosecond pulse to the continuous-wave. In these, a coherent beam of electromagnetic radiation referred to as the pump wave is used to stimulate a non-linear process in a non-linear (optical) material, resulting in the division of the energy/power in the coherent pump into two generated waves, typically referred to as the signal and idler waves. The signal wave is usually defined as the wave providing the useful output from the device, although that is not invariably the case. In the present application, the signal wave has the longer wavelength of the two generated waves.
Parametric devices can operate in a variety of configurations including amplifiers, oscillators and generators. In a parametric amplifier an intense coherent pump wave is made to interact with the non-linear material to produce amplification at the signal and idler wavelengths. A parametric oscillator uses a parametric amplifier inside a cavity resonant at one or both of the signal and idler waves. In this case, the signal/idler waves are either self-starting from noise/parametric fluorescence or the cavity is injection seeded by a suitable source operating at the signal/idler wavelength.
Oscillators that are resonant at only one of the signal and idler wavelengths are referred to as being singly-resonant, whilst those that are resonant at both are referred to as doubly-resonant oscillator. As is well established in the literature the doubly-resonant oscillator has the advantage of a significantly lower oscillation threshold in terms of the pump power/energy required to bring the oscillator into oscillation compared to the singly-resonant oscillator. However, the doubly-resonant oscillator has serious disadvantages in terms of the attainment of smooth and continuous tuning of the signal/idler waves.
Parametric generators generate signal/idler waves by the interaction of an intense pump wave with a non-linear material to parametrically produce these two other waves. No cavity is provided for these down-converted waves since the parametric gain in this case is sufficiently high as to allow adequate transfer of energy/power to these waves with only non-resonant single (or multiple) passing of the signal and/or idler waves through the non-linear material. Again in this case the signal and/or idler waves are self-starting from noise/parametric fluorescence or the generator is injection seeded by a suitable source operating at the signal and/or idler wavelengths.
There is considerable interest in extending the spectral coverage of parametric devices. This is because they are often used as sources of coherent electromagnetic radiation in spectral regions either not covered by any other sources, or where a single parametric-wave source is capable of replacing a number of sources that would otherwise be needed in order to provide the spectral coverage required. However, a serious limitation encountered in attempting to extend the spectral coverage of parametric generation to new regimes of the electromagnetic spectrum is the detrimental effect of absorption within the non-linear material of one or more of the three waves involved in the non-linear interaction. Indeed, the spectral coverage attainable with a particular parametric generation scheme is often determined by the onset of such absorption rather than by the non-linear or phase-matching characteristics of the non-linear material. Hence, it follows that elimination of such a restriction would result in improved spectral coverage attainable through the parametric generation process.
One solution to the problem of absorption is to employ a configuration of interacting waves such that the wave subject to excessive absorption exits the non-linear material as rapidly as possible after its generation. This wave is usually, but not invariably, the signal wave. One method for doing this is based on using non-collinear phase matching in such a way as to cause the wave subject to absorption to rapidly walk out from the non-linear material in a direction that is substantially lateral to the propagation direction of the pump wave. This is illustrated in FIG. 1, which shows the geometry of the interacting pump 1, idler 2 and signal 3 waves in a non-linear material 4. FIG. 2 shows the phase-matching process through a so-called k-vector diagram where kp, ki, ks are the wave vectors of the pump, idler and signal respectively within the non-linear material, angle θ is the angle subtended by the pump 1 and idler 2 waves and angle φ the angle subtended by pump wave 1 and signal wave 3. By altering the angle θ between the pump 1 and idler 2 waves, the signal wave can be rapidly tuned over a wide range.
To maintain the necessary non-linear interaction between the pump wave 1 and the idler wave 2 of FIGS. 1 and 2, they must be of sufficient radial (transverse) extent to maintain overlap between them throughout the length of the non-linear material. The parametric gain scales with the radial extent of these beams. As a consequence of the limitation placed on the interaction between the three waves due to the lateral walk-off of the signal wave, the radial extent of the beams needs to be of the order of the absorption length of the signal wave in the non-linear medium in order to optimise the gain experienced by the idler wave 2.
Examples of the technique of FIGS. 1 and 2 are described in the articles “Efficient, tunable optical emission from LiNbO3 without a resonator”, by Yarborough et al, Applied Physics Letters 15(3), pages 102-4 (1969); “Coherent tunable THz-wave generation from LiNbO3 with monolithic grating coupler”, by Kawase et al, Applied Physics Letters 68(18), pages 2483-2485 (1996); and “Terahertz wave parametric source” by Kawase et al, Journal of Physics D: Applied Physics 35(3), pages R1-14 (2002), the contents of which are incorporated herein by reference.
A problem with the arrangement of FIGS. 1 and 2 is that because of the reduced interaction between the three waves, the oscillation threshold is increased compared to conventional devices where the waves are all collinear. This has the undesirable consequences of limiting the applicability of the technique to materials exhibiting high non-linear coefficients, as well as requiring pump waves of high energy/power, and so demanding the undesirable use of high-energy/power lasers. This latter requirement prevents the development of compact and portable versions of these devices, so limiting their utility.
An alternative approach to that illustrated in FIGS. 1 and 2 is to arrange for the pump 1 and idler 2 waves to propagate collinearly through the non-linear material 4 while still maintaining the substantially lateral propagation of the signal wave 3. This condition of operation is effected by the use of a slant-stripe-type periodically-poled crystal as the non-linear material. FIG. 3 illustrates this hybrid collinear/non-collinear phase-matching process. FIG. 4 illustrates the phase-matching process through a k-vector diagram, in which K is the grating vector that describes the slant-stripe, periodically-poled non-linear crystal. Examples of this technique are described in co-pending international patent application PCT/GB2005/002912, the contents of which are incorporated herein by reference.
In this case, the presence of the additional vector K allows the pump 1 and idler 2 waves to propagate collinearly within the non-linear crystal 4 while the signal wave exits substantially laterally as required. Indeed it is apparent that it can be so arranged that the signal wave 3 propagates orthogonal to the collinear pump 1 and idler 2 waves. Having the pump 1 and idler 2 waves collinear means that common elements can be used such as, but not restricted to, mirrors for the guidance or resonance of these waves. This can simplify otherwise complicated arrangements. In addition, the common-path approach associated with the collinear propagation of the pump 1 and idler 2 waves confers the advantage of enhanced geometrical/mechanical stability.
Whilst the arrangement illustrated by FIGS. 3 and 4 confers some technical advantages, because it requires the fixing of the propagation direction of the idler wave 2 to be collinear with the propagation direction of the pump wave 1, the ability to attain wide and continuous tuning of the parametric process through the use of angle tuning is lost.
For the purpose of minimising the external pump power required to reach oscillation threshold, an approach adopted in the prior art is to place the optical parametric oscillator within the cavity of the laser used to generate the pump wave and in such a way that all three waves are collinear within the non-linear crystal, an arrangement generally referred to as an intracavity optical parametric oscillator. Because the non-linear medium experiences a pump wave with the intensity associated with the internal radiation field of the pump laser, which is generally substantially greater than the external radiation field available under optimum output coupling from the same pump laser, the requirements on the energy and power of the pump laser are significantly relaxed, leading to more compact devices. Examples of this are described in U.S. Pat. No. 3,628,186; U.S. Pat. No. 5,117,126; GB 2,252,840 A; U.S. Pat. No. 5,195,104; U.S. Pat. No. 5,181,211; U.S. Pat. No. 5,291,503; WO 94/24735; and EP 0 644 636 A2, the contents of which are incorporated herein by reference. However, none of the systems described in these allow for the rapid exit of the required signal wave from the non-linear material, simultaneously with wide and continuous tuning.