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J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026)
band-gap, they possess substantially higher second-order susceptibility compared to the oxide type nonlinear crystals employed in the first stage.
The non-resonant OPO( NRO), in which none of the waves are resonant and the signal- and idler-waves leave the cavity after just one round trip in opposite directions, can alleviate the degradation of the beam quality and conversion efficiency caused by back conversion which occurs in conventional resonant OPOs. One of the NRO cavity mirrors is highly reflective( HR) at the signal and highly transmissive( HT) at the idler wavelength, while the other cavity mirror has the opposite properties( HT signal / HR idler). In contrast to the conventional OPOs where this is just an option to reduce the oscillation threshold, the pump wave in a NRO must be retro-reflected through the cavity to propagate in both directions in order to realize the feedback for the signal and idler waves.
The idea of such a resonatorless parametric oscillator was suggested theoretically as early as 1970 as a noncollinear scheme( to avoid the use of dichroic mirrors) using LiNbO 3 [ 2 ], and realized experimentally in the same year as a collinear NRO( to utilize the higher parametric gain) using an a-HIO 3 crystal [ 3 ]. The optical components in this early realization consisted of retro-reflecting prisms and polarizers as alternatives to dichroic mirrors which was possible thanks to the type-II phase-matching chosen. With the availability of high quality, damage resistant dichroic mirrors, and having in mind that their design as long-pass filters is easier due to the absence of parasitic reflectivity bands, the generic scheme of a modern linear cavity NRO will look like Figure 1.
It shall be emphasized that the early interest in resonatorless OPOs or NROs was motivated in those years by the fact that in the absence of both suitable laser sources and nonlinear crystals, the parametric gain was insufficient for travelling-wave type optical parametric generation( OPG) without any cavity. In this sense the NRO shall be distinguished from a related idea suggested even earlier, in 1966, the so-called backward-wave or mirrorless OPO( MOPO) [ 4 ], experimentally demonstrated more than 40 years later in 2007 [ 5 ]. In the MOPO, the feedback is realized by counterpropagating waves which requires very large birefringence: thus while the original work suggested the use of a Se single crystal which exhibits huge birefringence but unfortunately also high residual absorption [ 4 ], the realization became possible through engineered quasi-phasematching( QPM) in a KTiOPO 4( KTP) crystal [ 5 ]. In contrast, the NRO is based on mirrors although no longitudinal modes exist in the cold cavity. The two concepts were compared in an early review paper in terms of threshold which was estimated to be much higher for the MOPO [ 6 ].
Unfortunately, the first publications on the NRO concept [ 2, 3 ] remained largely unknown and the same idea was“ rediscovered” 20 years later in [ 7 ] where a type-I b-BaB 2 O 4( BBO) crystal was employed in a collinear scheme. This experiment revealed that a NRO can provide similarly high slope and conversion efficiency as a singlyresonant oscillator( SRO). In the same paper [ 7 ], also an original scheme employing an intracavity quarter-wave plate was suggested for a single output degenerate NRO.
While initially the NRO was considered for nanosecond pumping by frequency doubled Nd lasers in the green due to thehigherparametricgain [ 2, 3, 7 ], another 15 years later it was realized using type-II KTP pumped at 1064 nm in a ring cavity [ 8, 9 ]. These authors preferred to use the term cross-resonant oscillator( CRO) to emphasize the fact that in the presence of a pump wave( hot cavity), certain phase relationships have to be fulfilled after one cavity round-trip. However, this conclusion appears to be related to their choice to use the same ring cavity mirrors for circulating the pump. This does not correspond to the original NRO design [ 2, 3 ] and is technically more challenging to realize both in ring and linear cavities. Nevertheless, under reasonable assumptions( undepleted plane-wave limit and continuous-wave( CW) pumping) the authors of [ 9 ] compared theoretically their CRO to a SRO in terms of threshold, built-up time, saturation, etc., and concluded that the CRO is characterized by less back conversion and stronger pump depletion than the SRO, shorter build-up time and weaker saturation, supporting this with experimental results in the nanosecond regime. Thus, while degradation of beam quality and conversion efficiency caused by back conversion will be intrinsically suppressed in an NRO because the intracavity intensity of both the signal and idler waves is reduced to a minimum, as could be expected the pump threshold will be higher compared to a conventional SRO with 100 % feedback for one of the waves and a retroreflected pump wave [ 8 ]. Nevertheless, the NRO is a very promising concept for power scaling.
Another practical advantage of the CRO or NRO compared to the SRO is the dual wavelength output, in particular when it comes to DFG in the second stage of cascade down-conversion schemes for the mid-IR spectral range [ 1 ]. This has been experimentally demonstrated in [ 8 ] using KTP in the CRO and CdSe for DFG. It is obvious that the NRO scheme( see Fig. 1) is also ideal for seeding at one of the wavelengths, including narrowband and singlefrequency seeding. In this aspect the NRO can be in fact considered also as a multi-pass parametric amplifier and this has been experimentally realized in [ 10, 11 ] asacomponent of a complex, single-frequency nanosecond system with impressive tunability from the visible to the mid-IR. This experiment in fact supports the interpretation of the NRO as a resonatorless parametric device.
Maintaining a narrowband spectrum at high average power is still, however, challenging for an NRO just as for an SRO since the parametric gain bandwidth increases with the pump power beyond the DFG limit determined by the difference of the signal and idler group velocities [ 1 ], which further broadens the output spectrum if no spectral constraining elements are employed. Broad spectral extent of the pump wave for the second stage of cascade parametric frequency down-converters not only clamps the conversion efficiency but will result in broadband output in the mid- IR. Volume Bragg Gratings( VBGs) in bulk glass acting on the signal or idler wave in the first stage OPO can be an elegant solution with relatively low insertion losses and high damage resistivity in a compact cavity configuration. In addition, Transversely Chirped VBGs( TC-VBGs) can be used not only for spectral narrowing of the OPO output