J. Eur. Opt. Society-Rapid Publ. 22, 4( 2026) 33
but also to ensure tunable operation in these conditions [ 12 ]. The transparency of such photo-thermo-refractive glass makes the choice of the signal wavelength preferable.
In our previous work we demonstrated that QPM materials, such as periodically-poled LiNbO 3( PPLN) and KTP( PPKTP) are ideally suited for use in NROs because of the high parametric gain they can provide [ 13, 14 ]. However, exactly for such QPM structures the output of a NRO is normally broadband due to the same polarizations of all three waves( Type-0 interaction), in particular close to degeneracy. We also established that the use of VBGs at the signal wavelength is simpler to implement and provides higher spectral contrast [ 13, 15 ] when compared to laser seeding [ 16 – 18 ], at least when single-frequency operation is not targeted. This is related to the continuing action of the VBG at increased power levels during the build-up process and the absence of transversal spectral narrowing dependence.
In this work we review our ongoing efforts on power scalable and tunable operation of nanosecond, narrowband PPLN NROs. Using TC-VBGs as spectral narrowing elements, tuning ranges of 1860 – 1900 nm for the signal wave and 2420 – 2486 nm for the idler wave are obtained with spectral bandwidths less than 2 nm. The total average output power of the tunable, narrowband PPLN NRO using 1-mm thick PPLN is successfully scaled up to 9.84 W at a pulse repetition rate of 30 kHz. Increasing the PPLN thickness to 3 mm enabled some energy scaling at safer pump levels in [ 15 ], however, the oscillation threshold increased and the conversion efficiency dropped. The use of both thicker( 3-mm vs. 1 mm in [ 13 ]) and longer( 50-mm vs. 25 mm in [ 15 ]) PPLN that recently became commercially available can produce simultaneously the lowest NRO threshold and the highest output( total average power of 11.35 W) and conversion efficiency( 63 %) in the narrowband regime( sub-1-nm bandwidths achieved through a fixed wavelength VBG), at minimum risk for optical damage [ 19 ]. The results in terms of output spectra are supported by numerical simulations based on a splitstep method within the plane-wave approximation that accounts for pump depletion and back-conversion. Although diffraction is not included, spatial effects are incorporated in the model, taking into account the transversal intensity distributions, in order to better reproduce the input-output power characteristics.
2 Experimental setup
The PPLN-NRO is pumped at 1064 nm by a multi-longitudinal mode( spectral linewidth ~ 0.66 nm) Nd: YVO 4 master oscillator power amplifier( MOPA) laser system( Canlas GmbH), delivering a maximum average power of 21 W with a beam quality factor of M 2 ~ 1. 15. The pulse duration varies roughly between 5 and 17 ns depending on the repetition rate that could be varied from 5 to 50 kHz. A halfwave plate and a polarizer are used to adjust the pump power whilst keeping other characteristics( pulse duration and spatial quality) constant. A Faraday isolator( FI) is employed to prevent optical feedback to the pump laser,
Figure 1. Generic scheme of a linear NRO.
and a second half-wave plate after the FI rotates the polarization to vertical for type-0( eee) phase-matching in the PPLN crystal.
Two different anti-reflection( AR) coated 5 % MgO doped PPLN slabs were employed, both supplied by HC Photonics Corp.( Taiwan). The first one had an aperture of 7.4 mm( wide) 1 mm( thick) and a length of 20 mm, and contained triple gratings of 2 mm width but we used only the QPM period of 31.78 lm. Its residual reflectivity per surface was < 0.1 % for the pump and about 0.2 % and 8 % in the respective signal and idler ranges. The second, 3 3mm 2 aperture, 50-mm long 5 % MgO doped PPLN( Product OPMIR-SD) had a QPM period of 32.25 lm. Its measured surface reflectivity was 0.2 % at the pump wavelength, 1.5 % at the signal wavelength and 3.5 % at the idler wavelength. Both samples had a parallelism specified as 3’. They could be heated in standard ovens( de facto a heater plate onto which the sample is fixed) supplied by the manufacturer with temperature control up to 200 ° C.
The pump beam was focused into the thin PPLN crystal to a 1 / e 2 spot diameter of 590 lm by a spherical lens( f = 400 mm). For the thick PPLN the pump beam was down-collimated by an achromatic beam expander( GBE2-C, Thorlabs, USA) to a diameter of 1.26 mm( vertical) and 1.15 mm( horizontal) in the position of the crystal.
The signal output coupler was 95 % reflective for the idler and 95 % transmissive for the signal. The idler outcoupler was 99 % reflective for the signal and 90 % transmissive for the idler. The dichroic mirrors coupling the pump in and out of the NRO were highly transmissive for the signal( 95 %) and idler( 91 %) and highly reflective at the pump and its second harmonic. The pump retro-reflecting mirror was highly reflective at the pump and transmissive at 532 nm, enabling the double-pass pumping of the NRO and simultaneously outcoupling parasitic green second harmonic light due to higher order QPM.
Two commercial AR-coated TC-VBGs( OptiGrate, USA) with glass aperture 25 mm wide 5.5 mm high but grating aperture 23 mm wide 5 mm high were employed in the NRO with the thin PPLN substituting the idler output coupler, each of them with a tuning range of ± 10 nm for the signal wave. Tuning is achieved by translating the TC-VBGs along their width. The TC-VBGs were 5.5 mm thick, with specified diffraction efficiency > 96 %. The reflectivity bandwidth was also certified by the supplier( ~ 1nm) but this parameter obviously depends on the beam size. The two TC-VBGs were selected in such a way in order to ensure a gapless tuning range of about 40 nm for the signal wave. The specified residual reflectivity for the signal wave was