Non-critical Phase-matching Wavelength Research Articles

Widely Tunable Angular Non-Critical Phase-Matching Wavelengths from 0.72 to 1.42 μm Based on RE1xRE21−xCOB Mixed Crystals

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.

Growth and non-critical phase-matching frequency conversion properties of LaxY1−xCOB crystals

A new series of nonlinear optical crystals LaxY1−xCa4O(BO3)3 (LaxY1−xCOB) with multiple components were grown by using the traditional Czochralski pulling method. X-ray powder diffraction and single-crystal X-ray diffraction experimental results proved that all these crystals formed in the LaxY1−xCOB solid solution state with a monoclinic structure of space group Cm. The optical birefringence was found to be adjustable by changing the compositional parameter x for the LaxY1−xCOB crystals. Taking advantage of the optical parametric oscillator laser, the non-critical phase-matching (NCPM) second-harmonic-generation (SHG) and third-harmonic-generation (THG) wavelengths along the Y-axis were determined at ambient temperature. In addition, the relationships between and among the optical birefringence, NCPM wavelength and crystal structure were discussed, and the NCPM SHG and THG of the 1064 nm laser were further achieved from the La0.120Y0.880COB and La0.140Y0.860COB crystals by adjusting the temperature, respectively. Using a 1064 nm Nd:YAG laser as the fundamental light source, the angular acceptance, temperature acceptance, and optical conversion efficiencies were determined and found to be on the order of 107.6 mrad, 18.3 °C and 48.8% for SHG of the La0.120Y0.880COB crystal, respectively, and 126.8 mrad, 68.6 mrad, 10.6 °C and 32.4% for THG of the La0.140Y0.860COB crystal, respectively. It is demonstrated that LaxY1−xCOB is a promising NLO crystal for NCPM frequency conversion applications in the visible and ultraviolet spectral regions.

Phase matching characteristics of deuterated ammonium dihydrogen phosphate crystals

Refractive indices for crystals ammonium dihydrogen phosphate (ADP), 30% deuterated ADP (DADP), 50% DADP, and 70% DADP are measured from 253 to 1529 nm with 5 × 10-6 accuracy. Numerical fits to modified double-pole Sellmeier equation are made. Second-harmonic generation, third-harmonic generation phase matching (PM) angles, and noncritical PM (NCPM) wavelengths are calculated using the Sellmeier parameters. The deuterated crystals show smaller PM angles than pure crystal. Fourth-harmonic generation process can be realized by DADP in smaller deuterium content than deuterated potassium dihydrogen phosphate (DKDP). The measured NCPM wavelengths are consistent with the calculated value. PM characteristics are compared between DADP and DKDP.

Blue light production by type-I non-critical phase matching second-harmonic generation in La(Ca1−xSrx)4O(BO3)3single crystals

The optical birefringence of La(Ca1−xSrx)4O(BO3)3 nonlinear optical crystals can be controlled by changing the compositional parameter x. Therefore, it is possible to achieve non-critical phase matching (NCPM) for type-I second-harmonic generation (SHG) of specific wavelengths in the range of 990 to at least 946 nm. La(Ca1−xSrx)4O(BO3)3 single crystals (with x = 0.0, 0.1, 0.3 and 0.4 starting compositions) of large size and good optical quality were grown by the Czochralski method. X-ray diffraction measurements have been carried out to characterize the structural changes with compositional parameters x. The NCPM wavelengths along the Z-axis were experimentally determined for each grown crystal. According to our assumptions, the obtained results demonstrate that by an appropriate choice of the compositional parameter x in La(Ca1−xSrx)4O(BO3)3 crystals, we can obtain blue light in the range of 495–473 nm through type-I NCPM SHG of the corresponding fundamental wavelengths, which are very attractive for practical applications.

Mechanical phase matching of birefringent non-linear crystals.

Second-order nonlinear processes such as second harmonic generation or parametric amplification have found numerous applications in the scientific and industrial world, from micromachining to petawatt laser facilities. These nonlinear interactions are mostly carried out in birefringent crystals because of their low cost and the possibility to operate at high powers Phase-matching configurations in birefringent crystals are determined by their refractive indexes. Here, we show that an important mechanical stress can be used to significantly change the phase-matching properties of a birefringent crystal. As an example, we demonstrate the shift of second harmonic non-critical phase matching wavelength of LiB3O5 (LBO) crystal at room temperature from 1200 nm to 1120 nm by applying compressive forces up to 100 MPa. We believe that this mechanical phase matching can be used as an additional degree of freedom to optimize nonlinear optical frequency mixing geometries.

Bulk crystal growth of congruent MgO-doped LiNbO 3 crystal with stoichiometric structure and its second-harmonic-generation properties

A new LiNbO 3 bulk crystal has been grown by doping with MgO (cs-MgO:LN; Li 2O:Nb 2O 5:MgO=45.30:50.00:4.70, (Li 0.906Mg 0.047V Li 0.047)NbO 3), which successfully has the congruent point coinciding with the stoichiometric point. Its second-harmonic-generation (SHG) properties were evaluated. It was found that cs-MgO:LN has a much more homogeneous composition leading to uniform in-plane distribution of the non-critical phase-matching wavelength than the conventional LiNbO 3 crystals such as congruent LiNbO 3 (c-LN), stoichiometric LiNbO 3 (s-LN), and MgO-doped congruent LiNbO 3 (5MgO:LN). This homogeneity arose from the observation that none of the solute components including ionic species were segregated at the interface during growth. The SHG conversion efficiency of cs-MgO:LN is comparable to those of s-LN and 5MgO:LN.

Growth and type-I noncritical phase-matching second-harmonic-generation of Gd 1−xR xCa 4O(BO 3) 3 (R 3+[dbnd]Sc 3+ or Lu 3+) crystals

Single crystals of Gd 1− x R x Ca 4O(BO 3) 3 (R 3+ Sc 3+ or Lu 3+) with large size and good quality have been grown by Czochralski method. The chemical compositions of the grown crystals were determined and X-ray diffraction measurements have been carried out to characterize the structural changes with compositional parameter x. The solubility limits of R 3+ ions in the Gd 4Ca 4O(BO 3) 3 crystals were also determined. Type-I noncritical phase-matching wavelengths for second-harmonic generation in Gd 1− x R x Ca 4O(BO 3) 3-grown crystals were measured and compared with theoretical predictions.

Non-critical phase-matched second harmonic generation in GdxY1-xCOB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut GdxY1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd0.28Y0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser.

Non-critical phase matching of Gd xY 1− xCa 4O(BO 3) 3(Gd xY 1− xCOB) crystal

By using an optical parametric amplifier (OPA), we have measured the non-critical phase-matching (NCPM) wavelengths for frequency-doubling (including type I and type II) in different Gd x Y 1− x COB crystals. From the Sellmeier equations of YCOB and GdCOB, we calculated the ranges of NCPM wavelengths in Gd x Y 1− x COB, and compared them with the data obtained from our experiment. When the output power of a mode-locked Nd/YAG laser is 110 mW (λ=1.064 μm), a conversion efficiency of 37% is obtained for the type II NCPM frequency-doubling in Gd 0.2Y 0.8COB crystal. Using a KTP crystal as frequency-doubler, we have achieved NCPM third harmonic generation (THG) in Gd 0.22Y 0.78COB crystal. When the output power of Nd/YAG laser is 94 mW and the power of the second-harmonic wave is 38 mW, THG conversion efficiency can reach 21%.

Influence of [K]/[Li] and [Li]/[Nb] ratios in melts on the TSSG growth and SHG characteristics of potassium lithium niobate crystals

KLN crystals were grown along the 〈110〉 axis by the TSSG method from different melts with 30–32mol% K2O, 23–27mol% Li2O and 43–46mol% Nb2O5. Lattice constants were measured by the single crystal XRD analysis. Curie temperatures were determined with an LCR meter HP 4284A at 1kHz. The blue SHG characteristics of the KLN crystals were investigated by using a 3900S Ti: sapphire CW tunable laser with the wavelength range of 675–1100nm. It is found that the non-critical phase matching (NCPM) wavelength at room temperature decreases significantly from 974 to 827.4nm with the increase of [K]/[Li] and [Li]/[Nb] ratios in the melts. On the basis of this survey, the melt compositions suitable for the TSSG growth of high quality KLN crystals with high lithium content are outlined, as indicated by the large c-axis lattice constant, high Curie temperature and short NCPM wavelength.

RESEARCH ON THE NON-CRITICAL PHASE-MATCHING WAVELENGTH OF GdxY1-xCa4O(BO3)3 CRYSTAL

Using the Sellmeier equations of GdCOB and YCOB, we determined the wavelength range in which non-critical phase matching in GdxY1-xCOB can be realized. Using a 704-5 type optical parametric amplifier, we measured the non-critical phase-matching wavelengths in several GdxY1-xCOB crystals with different compositional parameter x.

High-efficiency intracavity frequency doubling of a styryl-9 dye laser with KNbO_3 crystals

Efficient (1.1%) frequency doubling of a low power cw (single frequency) dye laser at 850−870-nm wavelengths in KNbO3 has been demonstrated. The fundamental power on the crystal was enhanced by (i) use of a passive resonator and (ii) by mounting the crystal within the dye laser resonator. The room temperature noncritical phase-matching wavelength for the nonlinear optical coefficient d32 = (20.5 ± 0.3) pm/V has been determined to be λpm = (859.7 ± 0.3) nm for the KNbO3 crystals used in these experiments.