Fig. 1. (a) Diagram of gradient deuterium DKDP. A high-quality Type-II PM direction gradient deuterium DKDP crystal with dimensions of 20 mm × 20 mm × 80 mm is employed in the THG scheme. The deuterium content ranges from 50% to 60% with a change of 10% in an 80-mm-thickness crystal and maintains 55% at the center of the crystal. (b) The variation of phase mismatch in SFG as a function of crystal length. The calculations assumed a monochromatic light of frequency mixing at 1058, 529 and 352 nm, respectively. Insert: photo of gradient deuterium DKDP crystal. (c) Intensity evolution for SFG of the fundamental frequency (blue) and second (green) and third (red) harmonics. The input broadband fundamental frequency light has an intensity of 0.8 GW/cm². Assuming the total transmission loss before the KDP crystal is 20% and the efficiency of broadband SHG with KDP is 50%, the intensity broadband SHG is about 3.2×10¹² W/m² and the given narrowband intensity is comparable to the SHG.

Fig. 2. (a) Schematic setup of compact THG based on a low-coherence broadband laser. The optical axis of the SHG (THG) crystal is located in the horizontal (vertical) plane. (b) Pulse profile for SHG (green). Insert: pulse profile for the fundamental frequency (blue). (c) SHG efficiency is varied with the external angle (black dots) and its quadratic-polynomial fit (purple curve).

Fig. 3. Dependence of tripling efficiency on the rotation angle of the gradient deuterium DKDP crystal (black dots) and its quadratic-polynomial fit (purple curve).

Fig. 4. High-efficiency schematic diagram of THG through broadband and narrowband laser sources. The optical axis of the doubler (tripler) is also located in the horizontal (vertical) plane. Time synchronization of the broadband SHG pulse (green) and narrowband fundamental frequency pulse (blue) is maintained. DM, dichroic mirror; RM, reflection mirror; Syn., time synchronization. Insert: the black, blue and green solid lines represent the reference, narrowband and broadband pulses, respectively.

Fig. 5. Measured spectra of fundamental frequency (blue), SHG (green) and THG (red) in the high-efficiency THG scheme. The spectra are centered at 1058, 528.2 and 352.1 nm, respectively.

Fig. 6. Conversion efficiency as a function of the crystal angular rotation for (a) gradient deuterium DKDP crystal and (b) KDP crystal. The experimental data (quadratic-polynomial fit) are represented by black dots (purple curve). The energy of narrowband and broadband pulses remained at 100 and 50 mJ, respectively.

Fig. 7. Measured efficiency as a function of broadband intensity at 1058 nm for (a) gradient deuterium DKDP crystal and (b) KDP crystal. Black dots and purple curve represent experimental data and the quadratic-polynomial fitted curve, respectively. The energy of the narrowband pulse is fixed at 100 mJ.