For the first time, the angular non-critical phase-matching (A-NCPM) second-harmonic-generation (SHG) characteristics of a family of monoclinic oxoborate crystals, RECaO(BO) (RECOB, RE = Tm, Y, Gd, Sm, Nd and La), were comprehensively investigated. For all of the realizable A-NCPM SHG styles, the feature parameters including PM wavelength, angular, wavelength and temperature acceptance bandwidths, have been derived from the theory and verified by the experiments. We discovered that the closer the ion radius between RE and Ca, the smaller the birefringence, and the better the A-NCPM SHG properties. As a result, for the Type-I SHG on Y-axis which has the largest effective nonlinear optical coefficient (d) among the three realizable A-NCPM styles, NdCOB crystal presents the longest PM wavelength (927 nm), the largest angular acceptance bandwidth (Δθ⋅l = 84.3 mrad·cm, Δϕ⋅l = 58.8 mrad·cm), and the broadest wavelength acceptance bandwidth (8.7 nm). This discovery will contribute to the design of new NCPM materials, at the same time the parameter formula will be helpful for the theoretical prediction of NCPM performance.

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The angular non-critical phase-matching (A-NCPM) second-harmonic-generation (SHG) properties of RE1xRE21−xCOB (RE1, RE2 = Y, Gd, La, Tm, Sm, and Nd) type mixed crystals including NCPM wavelength and conversion efficiency were detailedly investigated. Theoretical calculations manifest that the A-NCPM SHG scope of these crystals is 0.72~1.42 µm, and in experiments, the A-NCPM SHG waveband of 0.72~1.25 µm has been realized, by changing the ratio of the rare-earth elements RE1 and RE2 in RE1xRE21−xCOB crystals. Comparing to the temperature-dependent A-NCPM SHG of 0.95~1.34 µm in LiB3O5 (LBO) crystal, the composition-dependent A-NCPM SHG of 0.72~0.95 µm in RE1xRE21−xCOB type crystals is unique and has special significance for the frequency conversion of Ti:Sapphire lasers. Relationships between the birefringence and radius of rare-earth ion RE3+ in RE1xRE21−xCOB mixed crystals were discussed. Aiming for the A-NCPM SHG of 0.72~1.42 µm, we supply a clear, completed, and optimized solution on how to select the compositions of RE1xRE21−xCOB mixed crystals. Under focusing light beam conditions, high efficient A-NCPM SHG for both OPO and Ti:sapphire lasers were realized experimentally by using long Y- and Z-cut RE1xRE21−xCOB crystal samples.