Plot the intensity of normal laser light emerging from an optical fiber and you will see a familiar bell curve with greatest intensity in the center, tapering to zero at the edges. But for more than 35 years, it has been possible to coax light to spin down a fiber, producing a vortex mode with unique features, including an annular intensity profile and orbital angular momentum (L).
Optical vortices, such as the Laguerre-Gaussian mode (an eigenmode of the paraxial propagation electromagnetic equation), have applications in areas such as optical tweezers,1,2 superresolution microscopes,3,4 and laser ablation.5,6 To date, scientists have proposed several mode-conversion techniques from a Gaussian to the vortex mode using holographic spatial-light modulators or passive optical fibers. However, these approaches waste significant beam power, and their conversion efficiencies and output powers are mostly limited.
We have produced high-power, pulsed optical-vortex generation from a diode-pumped solid-state master laser oscillator in combination with an active fiber, i.e., a stressed, ytterbium (Yb)-doped, large-mode-area fiber power amplifier.7 The basic concept behind our method (see Figure 1) features a collimated master laser output with a Gaussian profile that is off-axially injected into a large-mode-area fiber. This enables in-phase coupling of two orthogonal LP11 modes at high efficiency. These modes are 90 or −90° out of phase with each other at the end of the fiber by providing appropriate stress to the fiber at 45° to the vertical direction. The fiber amplifier will then generate an annular vortex mode.
Figure 1. In our concept of mode conversion in the fiber amplifier, a Gaussian input (top) is converted to an annular output (bottom) via the stressed optical fiber.
The experimental setup (see Figure 2) features a master laser, a continuous-wave, mode-locked, neodymium-doped vanadate laser of our own design. Its output has a lasing frequency of 1064.4nm, a pulse width of 4.5ps, a pulse-repetition frequency of 150MHz, and an output power of 300mW. It uses a polarization-maintained, large-mode-area, Yb3+-doped double-clad, 3m-long fiber with a core diameter of 30μm, a core numerical aperture (NA) of 0.06, a cladding diameter of 400μm, and a cladding NA of 0.46. A 30W, 400μm-diameter, fiber-coupled, 975nm laser diode pumps the fiber amplifer. Output is measured as it emerges from a dichroic mirror.
Figure 2. Experimental setup of the picosecond vortex fiber laser. LD: Laser diode. DM: Dichroic mirror. CW: Continuous wave. Yb: Ytterbium. NA: Numerical aperture.
Figure 3 shows the experimental vortex output power as a function of pump-diode power. The amplified vortex output exhibits a pulse duration of 4.5ps, and its average power measures 8.5W at the maximum pump level, corresponding to a peak power of 12.5kW.
Figure 3. Vortex output power as a function of pump power.
We also investigated interferometric fringes formed by interference between the laser output beam and a spherical reference wavefront. A single spiral in the fringes is evidence that the annular output has a phase singularity with an orbital angular momentum of L = 1. Selective control of the rotational direction of the spiral was also possible by varying the stress applied to the fiber amplifier (see Figure 4).
Figure 4. Interferometric fringes formed by the amplified vortex and a spherical reference wavefront. The vortex outputs exhibit clockwise (left) and counterclockwise (right) single arms, respectively.
Our arrangement exhibits nonlinear effects, such as self-phase modulation, that limit power scaling of the system. These effects also induce pulse broadening of the amplified output. With this system, the B integral (the product of the pulse intensity, the nonlinear refractive index of the fiber, and the fiber length) is estimated at 1.02 × 10−3. Thus, further power scaling will be possible by employing a more powerful pump diode. We have already obtained >20W picosecond vortex-pulse generation using a similar setup.
This laser system requires no phase elements for generating high-power vortex outputs, and it will also be easily extended to produce high-energy nanosecond and high-power continuous-wave outputs. One direction for further research is frequency extension of optical vortex into the UV region. Another direction is pulse shortening toward the femtosecond regime.
Chiba University (CU)
Takashige Omatsu received BS and PhD degrees in applied physics from the University of Tokyo (Japan) in 1983 and 1992, respectively, for research on frequency extension of metal-vapor lasers. In 1992, he became a research associate at CU, where he worked on development of diode-pumped solid-state lasers and optical-phase conjugation. In 2007, he became a professor. He is currently working on high-power solid-state and fiber lasers, as well as on singular optics.
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