## 1 Introduction  
Orange lasers with wavelengths near \(\sim\)600 nm play an important role in a number of scientific applications such as photodynamic therapy [1], flow cytometry for excitation of fluorescent probes of 595 nm excitation wavelength [2], spectroscopy [3], astronomy [4], optogenetics and neuroscience [5], and laser projection displays [6]. So far, however, there have been no available conventional diode lasers in this spectral region due to a lack of suitable direct band gap materials [7]. Therefore, compact, stable, and efficient all-solid-state laser sources generated by nonlinear effects have gained popularity over the past thirty years with a view of overcoming this limitation. The abilities of sum frequency generation (SFG) to efficiently mix two fundamental wavelengths to generate a third wavelength is a potential technique to attain such new wavelengths with periodically poled ferroelectric crystals [8]. On the other hand, the quasi phase-matching (QPM) technique using periodically poled lithium niobate (PPLN) is quite attractive for implementing broadband SFG owing to its wide optical transparency window (350-5000 nm), a large nonlinear coefficient (\(d_{33}\)), a flexible design, and a high conversion efficiency [9]. However, reaching high efficiencies in broadband wavelength conversion remains a challenge. Nevertheless, in order to increase the input wavelength acceptance range and achieve the high efficiency of a QPM process, complementary properties of uniform poled and step-chirped crystals in a cascaded configuration can be exploited as proposed herein. The pump and signal lasers are focused into the first crystal and the generated sum frequency light co-propagates in the second crystal in which the pump and the signal are re-focused. In both crystals, SFG occurs separately, increasing the total output power and, consequently, improving the overall conversion efficiency. Previous studies demonstrating the generation of continuous wave (CW) coherent light using cascaded crystals have been reported. For instance, a two-crystal cascade to generate 491 nm and 429 nm light with conversion efficiencies of 2.4%/W and 3.1%/W respectively in KNbO\({}_{3}\) crystals was demonstrated [10]. Up to 13 W of green light at 532 nm with 5%/W conversion efficiency based on two-crystal cascade PPLT crystals has also been reported [11]. Using two PPLN crystals, 780 nm second harmonic generation has been achieved with 5.6%/W conversion efficiency. Recently, 5.5 W SFG of green light was demonstrated in a two-crystal cascade of PPLN and PPLT crystals with a 50% conversion efficiency of the total fundamental power [12]. In the orange spectral region, a tunable orange laser from 601 nm to 604 nm range was successfully generated by tuning the temperature of quasi-periodically poled lithium tantalate (LT) superlattice from 150 to 195 \({}^{\circ}\)C, yielding a maximum output of 310 mW [13]. Up to 23 mW of narrow-line 593 nm light via intracavity SFG of a CW dual-wavelength Nd:YVO\({}_{4}\) laser has also been demonstrated using two volume Bragg gratings in a three-mirror cavity configuration with a temperature tunability of \(\sim\)5 pm/\({}^{\circ}\)C [14]. Recently, also, more than 1 W all-solid state optical parametric oscillator (OPO) in the spectral range from 605 to 616 nm based on MgO:PPLN with different poling sections was reported [15]. In this scheme, tuning was achieved by varying the crystal position, temperature and etalon tilt angle. Whilst various schemes of generating visible light in the 600 nm spectral range have been extensively studied, no work has been reported on high efficiency, tunable wavelength conversion by cascaded nonlinear crystals in this range. Here, we report the generation of all-solid state, efficient, tunable orange light by single-pass sum frequency mixing of \(\sim\)1545.7 nm and \(\sim\)975.2 nm laser sources in two cascaded MgO:PPLN bulk crystals.