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We developed an octave-band tunable optical vortex laser based on a 532 nm optical vortex pumped optical parametric oscillator with a simple linear-cavity configuration by employing cascaded non-critical phase-matching LiB3O5 crystals. The optical vortex output was tunable from 735 to 1903 nm. For a pump energy of 9 mJ, an optical vortex pulse energy of 0.24–2.36 mJ was obtained, corresponding to an optical-optical efficiency of 0.3-26%.
© 2016 Optical Society of America
1. Introduction
The orbital angular momentum, which is characterized by an azimuthal phase term, exp(ilϕ), where l is an integer called the topological charge and ϕ is the azimuthal angle, is carried by helical light, i.e., optical vortices [1–4]. Optical vortices have attracted much attention in a variety of research areas, e.g., optical tweezers and manipulations [5–7], quantum optics [8–11], large-capacity optical telecommunication [12–14], nonlinear spectroscopies [15, 16], microscopies with high spatial resolution beyond the diffraction limit [17–19], and materials processing [20–25]. Additionally, in recent years it has been discovered that the orbital angular momentum of optical vortices can twist materials, such as metal, silicon, and azopolymer, to create chiral nanostructures [26–29].
A wide wavelength tunability of optical vortex sources is highly desirable for the aforementioned applications, because this would enable matching of the lasing frequency to the absorption bands of the materials being studied. Lavery et al. demonstrated an orbital angular momentum-carrying white-light beam by rotating a phase element [30]. However, the conventional phase modulation elements typically used to generate optical vortices, such as spiral phase plates and spatial light modulators, are designed for certain wavelengths, so that the frequency of the optical vortices is more or less fixed without any mechanical motion. Nonlinear frequency-conversion techniques, including second-harmonic generation [31–33], sum-frequency generation [34, 35], optical parametric generation [36, 37], and stimulated Raman scattering [38, 39], are promising techniques to expand the range of available frequencies for optical vortex sources.
In fact, we have successfully demonstrated a tunable (1953–2158 nm) optical vortex laser constructed from a 1-μm vortex pumped optical parametric oscillator (OPO) by employing cascaded KTiOPO4 (KTP) crystals [40]. Most recently, we also demonstrated a widely tunable near-infrared (NIR) optical vortex source constructed from a 532 nm optical vortex pumped singly resonant OPO with a folding-cavity configuration by employing cascaded noncritical phase-matched LiB3O5 (LBO) crystals [41]. This system, in which the topological charge of the pump beam is selectively transferred to the signal (idler) beam, allows us to generate a vortex output of 850–990 nm (1130–1300 nm). However, their tuning ranges are limited by the angle tuning of KTP, large walk-off effects, and insertion loss of the folding mirror.
In this paper, we report on the first demonstration of an octave-band tunable optical vortex laser constructed from an optical vortex-pumped OPO with a simple linear cavity configuration. The optical vortex pulse energy of this system is over 0.2 mJ from 735 to 1903 nm, although there is a wavelength gap of 990–1130 nm in the vortex mode generation.
2. Experimental setup
The tunable optical vortex laser system is illustrated in Fig. 1(a). A conventional frequency-doubled Q-switched Nd:YAG laser with a wavelength of 532 nm (pulse duration, 25 ns; repetition rate, 50 Hz; maximum pulse energy, 9 mJ; spatial form, nearly Gaussian) was used as a pump source, and its output was converted into a first-order optical vortex with l = 1 by utilizing a spiral phase plate (RPC Photonics, VPP-1c). The collimated first-order optical vortex beam (spot diameter, 1.1 mm) was delivered toward on OPO consisting of two cascaded noncritical phase-matched LiB3O5 (NCPM-LBO) crystals (θ = 90°, φ = 0°, 30 × 3 × 3 mm3) mounted on an oven, yielding high-gain and narrowband parametric emission. The wavelengths of the signal and idler were tuned by controlling the temperature of the LBO crystals.
Fig. 1 Experimental setup of the 532 nm first-order optical vortex pumped LBO OPO with a simple linear cavity configuration. (b) Self-referenced interferometry employing a transmission grating.
A singly resonant cavity for the signal beam (<1064 nm) consisted of a flat input mirror with high transmission (HT) at 532 nm and high reflectivity (HR) at 800 nm, as well as an 80% reflective output coupler (OC) for 800 nm with a radius of curvature of 500 mm. The OC was mounted on a one-dimensional translation stage, so as to allow the cavity length to be varied. The signal and idler (>1064 nm) beams were separated by a dichroic mirror (HR at <990 nm, HT at >1180 nm), and their transverse beam profiles were measured by a NIR InGaAs camera.
To assign the topological charge of the signal and idler, a self-referenced interferometric measurement was also carried out using a transmission grating with low spatial frequency (10/mm). As shown in Fig. 1(b), the plus and minus first-order diffracted beams were selectively filtered, and delivered by a lens onto the camera, thereby forming a self-referenced interferogram.
3. Results and Discussion
Figures 2(a) and 2(b), respectively, show the measured spatial mode and wavefront of the pump vortex beam with l = 1 for the NCPM-LBO OPO.
Fig. 2 (a,c,e) Transverse beam profile and (b,d,f) self-referenced fringes of the pump, signal (900 nm), and idler (1300 nm) outputs, respectively, for a compact cavity configuration (~215 mm). (g) The temporal evolution of the pump, signal and idler outputs.
A stable compact cavity (effective cavity length, ~215 mm) for the signal beam enabled lasing of the signal in a vortex mode with an annular intensity profile due to a phase singularity [Fig. 2(c)]. The signal beam carried an orbital angular momentum of l = 1, as evidenced by a pair of upward and downward Y-shaped fringes [Fig. 2(d)]. The idler beam had a Gaussian spatial form without any phase singularity [Figs. 2(e) and 2(f)]. These results indicate that the topological charge of the pump beam is selectively transferred to the signal beam, as discussed in our previous publication [40]. The wavelengths of the signal and idler were measured to be 900 nm and 1300 nm, respectively. The signal output exhibited a smooth pulse without any pre- and post-lasing [Fig. 2(g)], indicating that the signal output was selectively lased in the present cavity. In contrast, the idler output showed a similar temporal decay to that of the pump pulse.
Upon increasing the cavity length (effective cavity length within 300–330 mm), the signal and idler beams transformed into a mixed mode, as evidenced by an intensity profile with a shallow central dip arising from an incoherent spatial overlap between the Gaussian and first-order optical vortex modes [Figs. 3(e)–3(h)]. A further extension of the cavity prevented the generation of a signal beam with the vortex mode, and thus it exhibited a Gaussian spatial mode without any phase singularities [Figs. 3(i) and 3(j)]. Instead, the idler beam exhibited the vortex mode owing to an asymmetric topological charge transfer from the pump beam. In fact, the idler beam exhibited a first-order phase singularity, as evidenced by a pair of upward and downward Y-shaped fringes [Figs. 3(k) and 3(l)] that appeared when the effective cavity length was ~435 mm. When using a different OC (80% reflectivity for the 960 nm signal output, <10% reflectivity for the 1190 nm idler output, a radius of a curvature of 1000mm), the signal and idler outputs also exhibited a similar spatial transformation by increasing the cavity length [Figs. 4(a)-(f)].
Fig. 3 (a,c) Transverse beam profile and (b,d) self-referenced fringes of the signal (900 nm) and idler (1300 nm) beams, respectively, for a compact cavity configuration (~215 mm). (e,g) Transverse beam profile and (f,h) wavefronts of the signal and idler beams, respectively, for a cavity length of 315 mm. (i,k) Transverse beam profile and (j,l) self-referenced fringes of the signal and idler beams, respectively, for an extended cavity configuration (~435 mm).
Fig. 4 (a,b) Transverse beam profile of the signal (960 nm) and idler (1190nm) outputs for a compact cavity configuration (~215 mm). (c,d) Transverse beam profile of the signal (960 nm) and idler (1190nm) outputs for a cavity length of 315 mm. (e,f) Transverse beam profile of the signal and idler beams, respectively, for an extended cavity configuration (~435 mm).
Such transformations of the signal and idler beams can be understood through the Fresnel number, F, which characterizes the stability of the higher-order mode,
where is the aperture size (0.55 mm) of the cavity (e.g., radius of the pump beam), is the wavelength of the signal beam, and L is the effective cavity length, respectively. The value of F is inversely proportional to the effective cavity length, and the experimental F is estimated to be 0.86–1.76 over an effective cavity length range of 435–215 mm (see Fig. 5).Fig. 5 Estimated Fresnel number of the LBO OPO as a function of effective cavity length. Insets show the transverse beam profile of signal (900 nm) and idler (1300 nm) beams. The compact (extended) cavity forces the signal (idler) beam into the vortex mode.
Thus, a stable compact cavity with a larger F allows the signal beam to lase at a higher-order mode, such as a vortex mode, resulting in the transfer of the topological charge of the pump beam to the signal beam. Under these conditions, the idler beam exhibits a Gaussian spatial form without any phase singularities.
In contrast, an extended cavity (effective cavity length of~435 mm) with a low F (<1) prevents the signal beam to lase at the higher-order mode (vortex mode) owing to a large diffraction loss for the higher-order mode. In fact, the signal and idler beams exhibited a Gaussian and annular transverse beam profile, indicating that the topological charge of the pump beam was transferred to the idler beam. Thus, the topological charge can be selectively transferred from the pump beam to either the signal or idler simply by shortening or extending the linear cavity.
Figures 6(a) and 6(b) show the power scaling of a 900 nm signal and 1300 nm idler outputs measured for the compact and extended cavity configurations, respectively. In the case of the compact cavity, a maximum signal vortex output energy of 1.75 mJ was measured, corresponding to a slope efficiency of 26.6%. When the cavity was extended, a maximum idler vortex pulse energy of 1.03 mJ was obtained, corresponding to a slope efficiency of 16%.
Fig. 6 Power scaling of the signal and idler from the NCPM-LBO optical cavity with (a) compact and (b) extended cavity configurations.
In addition, the optical vortex output could be tuned from 735 to 990 nm (signal) and 1130 to 1903 nm (idler) by controlling the temperature of the LBO crystals, as shown in Fig. 7. An optical vortex pulse energy of 0.24–2.36 mJ was then obtained at a pump energy of 9 mJ, corresponding to an optical-optical efficiency of 0.3–26%. The cavity exhibited a coupling loss for the signal output, defined as –lnROCRinput (where ROC and Rinput are the measured reflectivities of OC and input mirror), of 0.25-2.5 in the wavelength region of 770-980 nm, as shown in Fig. 8. Nevertheless, the cavity enables selectively the lasing of the signal output in the vortex or Gaussian mode merely by tuning the cavity length. These results well support that the Fresnel’s number rather than the parametric gain and cavity coupling loss, mainly contributes to managing the topological charge transfer in the present cavity.
Note that the vortex output was unavailable from 990 to 1130 nm (wavelength gap) due to a double resonance (1020 nm and 1110 nm) [Fig. 9(a)] as well as the signal and idler, both of which exhibited a mixed-mode spatial form [Fig. 9(b)] within this wavelength region, as mentioned in our previous publication [41].
Fig. 9 Spatial form of the (a) signal (a) and (b) idler beams in the region of 990–1110 nm, where a double resonance occurs in the cavity. The double resonance occurs at 1020 nm and 1110 nm.
4. Conclusion
We have successfully demonstrated, for the first time, an octave-band tunable optical vortex laser with a milli-joule level pulse energy (0.24–2.36 mJ), constructed from a 532 nm vortex pumped NCPM-LBO OPO with a simple linear cavity configuration. Asymmetric topological charge transfer took place from the pump beam to the signal or idler beams, resulting in tuning ranges of 735–990 nm and 1130–1903 nm, respectively. In future work, this system will be extended so as to generate a mid-infrared (2–8 μm) tunable optical vortex output by utilizing difference frequency generation [42]. Moreover, the wavelength gap (990–1130 nm) for the vortex output caused by a double resonance of the signal and idler will be “filled in” by utilizing an output coupler with a narrow-band HR coating.
Acknowledgments
We 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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