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We demonstrated a widely tunable 1-μm optical vortex laser formed from a 0.532-μm optical vortex pumpedoptical parametric oscillator with a singly-resonant cavity configuration employing cascaded non-critical phase-matching LiB3O5 crystals. With this system, the topological charge of the pump beam can be selectively transferred to the signal or idler output, and a vortex output in the wavelength range of 850–990nmor 1130-1300nmcould be obtained.A maximum signal vortex output energy of 0.9 mJ was achieved, corresponding to an optical efficiency of 10%.
© 2015 Optical Society of America
1. Introduction
Optical vortex lasers, exhibiting a helical wavefront characterized by an azimuthal phase, exp(ilϕ) (wherelis an integer termed the topological charge), with a doughnut-shaped spatial form and an orbital angular momentum (OAM) of lħ [1–4], have potential applications in a variety of fields,includingoptical manipulation [5–8], space-division multiplexing telecommunications [9,10],materials processing [11,12],super-resolution microscopy [13–15], quantum information [16], and nonlinear spectroscopy [17]. In particular, optical vortex lasers enable the twisting of materials to create chiral structureson the nanoscale [18–20]. Such chiral nanostructures, which are difficult to fabricate even by utilizing advanced chemical techniques, will potentially open new avenues in chiral materials science, such as selective identification of the circular dichroism of molecules and chemical composites.
However, the lasing frequency of the optical vortices generated by conventional spiral phase plates typically used in previous studies,is restricted to set specified frequencies [21, 22]. Tunableoptical vortex sources with wavelengthversatility, allowing the absorption band of target materials to be matched, are greatly desired for the above-mentioned applications.
Recently,we have developed a tunable 2-μmvortex laser formed from a 1.064-μm optical vortex pumped KTiOPO4 (KTP) optical parametric oscillator(OPO)with a stable cavity configuration [23,24]. We have demonstrated a widely tunable mid-infrared (6.3–12 μm) vortex laser based on the 2-μm vortex laser in combination with a difference-frequency generator formed from a nonlinear ZnGeP2 (ZGP) crystal [25]. This is the first reported moderate pulse energy, tunable near-infrared optical laser that can be applied as a base source in materials processing.
In this paper, we report on a widely tunable vortex laser based on a singly-resonant OPO formed from a non-critical phase-matching LiB3O5 (NCPM-LBO) crystal. With this system, a millijoule-level vortex pulse was obtained within a wavelength region of 850–990 nm and 1130–1300nm.
2. Singly resonant cavity configuration
Conservation of orbital angular momentum (OAM) in nonlinear frequency up-conversion processes, such as second harmonic generation [26,27] and sum frequency generation [28], has been well established. However, for the conservation of OAM in an optical parametric down-conversion process, it remains an open question as to how the orbital angular momentum of the pump beam is divided between the signal and idler outputs.
In our previous studies of 2-μm vortex lasers based on a 1-μm optical vortex pumped OPO with a plane parallel cavity configuration [29], the cavity acted as a doubly-resonant cavity for both signal and idler outputs, thereby generating a fractional vortex, formed of coherently coupled Gaussian and vortex outputs. In a 1-μm optical vortex pumped OPO with a stable cavity configuration, the large walk-off effects of the KTP crystal prevent vortex mode operation of the idler output (a low frequency output with extraordinary polarization). Hence a singly-resonant cavity for the signal output (a high-frequency output with ordinary polarization) is used to encourage vortex mode operation of the signal output.The resulting tuning bandwidth (range) was measured to be ~200 nm (1953–2158 nm).
A type-I, non-critical phase-matching LiB3O5 (NCPM-LBO) crystal, a conventional nonlinear crystal for 1-μm OPO with less walk-off effects, produces signal and idler outputs with the same polarization. Thus, the NCPM-LBO OPO, pumped by a first order optical vortex with a topological charge lof 1, easily establishes a doubly-resonant cavity for the signal and idler outputs and encourages the signal and idler outputs to lase in a mixed mode (incoherent coupling between Gaussian and first-order vortex modes) by utilizing a stable cavity configuration.In fact, an OPO with a linear cavity configuration [see Fig. 1(a)] permitted double resonance for the signal and idler outputs, and forced the laser to operate in mixed mode, evidenced by an intensity profile with a shallow dip and a pair of Y-shaped fringes with a low modulation depth arising from incoherently spatial overlap between Gaussian and first-order vortex (with a topological charge of 1) modes.
Fig. 1 (a)Vortex pumped NCPM-LBO parametric oscillator with a linear cavity configuration. (b) Self-referenced interferometry employing a transmission grating.
Figure 2 shows the experimental spatial forms and wavefronts of the pump,signal and idler outputs. The wavefronts of the pump, signal and idler outputs were observed by laterally sheared interferometry [Fig. 1(b)]using a transmission grating with a low spatial frequency (10 lines/ mm), in whichthe positiveand negative firstorder diffracted beams of the signal (or idler) output were selectively collected by a spatial filer and a lens on a CCD camera to form a self-referencedinterferogram.
Fig. 2 (a) Spatial form and (b) self-referenced fringes of the pump beam.(c) Spatial form and (d) self-referenced fringes of the signal (950 nm) output and (e) spatial form and (f) self-referenced fringes of the idler (1209 nm) output from a vortex pumped NCPM-LBO parametric oscillator with a linear cavity configuration.
To establish vortex-mode operation in the NCPM-LBO OPO, it is necessary to design a singly-resonant cavity for the signal or idler output, in which the nonlinear interaction between the signal (or idler) and pump electric fields forces the oscillation of the signal (or idler) output. The nonlinear gain can then be determined by the spatial overlap efficiency η between the signal (or idler) and pump electric fields, given by formula (1):
whereEs and Ep are the electric fields of the signal (or idler) and pump beams, and ls and lp are the topological charges of the signal (or idler) and pump beams, respectively. This relationship indicates that an OPO with a singly-resonant cavity configuration for the signal (or idler) output allows the signal (or idler) output to lase at the vortex mode with the same topological charge as that of the pump beam. The resulting idler (or signal) output is forced to lase at a Gaussian mode. To encourage the vortex mode oscillation in the NCPM-LBO OPO, a singly-resonant cavity configuration utilizing an internal folding mirror is proposed.3. Experiments
Figures 3(a) and 3(b) show a schematic diagram of an OPO with an internal folding mirror.
Fig. 3 Experimental setup for a 1-μm vortex pumped singly-resonant NCPM-LBO OPO showing singly-resonant cavities for the (a) signal and (b) idler outputs.
A frequency-doubledQ-switched Nd:YAG laser (pulse duration 25 ns,PRF 50 Hz;wavelength 0.532 μm, maximum pulse energy 9 mJ) was used as a pump laser. Its output was converted into a first-order optical vortex with a topological charge,l, of 1by utilizing a continuous spiral phase plate(RPC Photonics, VPP-1c).
The first-order optical vortex beam was collimated to a ϕ750 μm spot size and was incident on cascaded LBO crystals. The singly-resonant cavity for the signal output was formed from a flat input mirror with high transmission (HT) for 532 nm and high reflectivity (HR) for 980 nm, a folding mirror with HR for <980 nm (signal) and HT for >1180 nm (idler), and an 80% reflective output coupler for 980 nm [Fig. 3(a)]. The cavity for the idler output was also formed from a flat input mirror and a 60% reflective output coupler for 1180 nm [Fig. 3(b)]. The spatial forms and self-interference fringes of the signal and idler outputs were then observed by a conventional digital CCD camera.The length of both cavities was ~200 mm.
Cascaded NCPM-LBO crystals (θ = 90°, φ = 0°) with dimensions of 30 × 3 × 3 mm3 were employed to increase the parametric gain and narrow the lasing spectrum bandwidth of the signal and idler outputs. The crystals were mounted on an oven to control and maintain the crystal temperature. With this system, the wavelengths of the signal and idler outputs could be tuned by changing the crystal temperature. The lasing wavelength bandwidth of the signal and idler outputs was measured to be<1.5 nm, even near the degenerate condition.
3. Results and discussions
As shown Fig. 4, a tunable vortex output with a topological charge lof 1 was obtained in a wavelength range of 850–990 nm and 1130–1300nm by controlling the temperature of the LBO crystals. Anasymmetric transfer of the topological charge from the pump beam to the signal (idler) output was established. A maximum signal (idler) vortex pulse energy of 0.9 mJ (0.2 mJ) was achieved at a pump energy of 9 mJ, corresponding to an optical efficiency of 10% (2%). Figure 5 shows the power scaling of the signal output at a wavelength of 970 nm. The signal and idler outputs exhibited a mixed-mode spatial form within a wavelength region of 990–1130nm,arising from their double resonance. A narrowband folding mirror with high reflectance for the signal output and high transmittance for the idler output will allow us to fill the wavelength gap of the vortex mode generation seen in the wavelength regionof 990–1130nm. In a LBO-OPO without cascaded LBO crystals, the tunability of the vortex output was limited to a wavelength region of 930–1250 nm. The wavelength gap (980–1170 nm) for vortex mode generation was also relatively wide owing to the low parametric gain. The maximum signal (idler) vortex pulse energy was limited to 0.39 mJ (0.08 mJ), even at the maximum pumping level, corresponding to an optical efficiency of 4.5% (1%).
Fig. 5 Power scaling of signal and idler outputs in OPOs with cascaded LBO and without cascaded LBO.
Figure 6 shows the spatial forms and self-referenced interference fringes of the signal and idler outputs.An OPO with a cavity configuration for signal output forced the signal output to lase in vortex mode [Fig. 6(a)]. The wavefront of the signal output shows a pair of Y-shaped fringes [Fig. 6(c)], indicating that the topological charge of the pump beam was selectively transferred to the signal. In fact, the idler output exhibited a Gaussian spatial form [Fig. 6(b)] without any phase singularities. Also, notice that the handedness of the signal output is identical with that of the pump beam[see Figs. 2 (a) and (b)].For an OPO with a cavity configuration for idler output, the idler output was permitted to lase in vortex mode [Fig. 6(f)]with the same handedness as that of the pump beam[Fig. 6 (h)]and, resulting in the production of a Gaussian signal output[Fig. 6(e)] and [Fig. 6(g)].
Fig. 6 (a) Spatial form and (c) self-referenced fringes of the signal output, and (b) spatial form and (d) self-referenced fringes of the idler output generated from an NCPM-LBO OPO with a singly-resonant cavity configuration for the signal output. (e) Spatial form and (g) self-referenced fringes of the signal output, and (f) spatial form and (h) self-referenced fringes of the idler output generated from an NCPM-LBO OPO with a singly-resonant cavity configuration for the idler output.
These results indicate that the singly-resonant cavity configuration in an NCPM-LBO optical parametric oscillator enables us to selectively transfer the topological charge of the pump beam to the signal or idler output.
4. Conclusion
We have successfully demonstrated awidelytunable 1-μm optical vortex laser formed of a 0.532-μm vortex pumped non-critical phase matching LiB3O5 optical parametric oscillator with a singly-resonant cavity configuration. Asymmetrictopological charge transfer from the pump beam to the signal or idler output occurred, resulting in a tuning range of 850–990 nm and 1130–1300 nm. The maximum signal vortex output energy of 0.9 mJ was obtained at a pump energy of 9 mJ.
This system will be extremely useful in various fields, e.g., as a pump source for 3–5-μm tunable optical vortex generation, and it can be extended to generate mid-infrared to terahertz vortex output by utilizing a nonlinear crystal ZnGeP2 or LiNbO3 [30] for difference frequency generation.
Acknowledgements
The authors acknowledge support from a Grant-in-Aid for Scientific Research (No. 24360022) from the Japan Society for the Promotion of Science.
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