Optical vortex switch based on multiplexed volume gratings with high diffraction efficiency.

Systems of controllable orbital angular momentum (OAM) require more compact, higher conversion efficiency and more tolerable wavelength or polarization. We introduce an optical vortex switch based on a multiplexed volume grating (MVG). The MVG recorded in a piece of photo-thermo-refractive (PTR) glass exhibits high diffraction efficiency (DE, also known as conversion efficiency in transporting), sensitive angular selectivity, and polarization-insensitivity. The effects of the incident divergence angle and polarization on the DE and the far-field diffraction profiles are demonstrated and investigated. It turns out that the divergence angle of the probe beam can greatly affect the DE. The fluctuation of the DE caused by polarization variation is less than 1.59%. This switch can be potentially applied in vortex tweezers, optical communication, and high power systems.

Fabrication of high-precision reflective volume Bragg gratings.

Reflecting Bragg gratings (RBGs) recorded in photo-thermo-refractive (PTR) glasses have been widely used in narrowing and stabilization of the laser emission spectrum. As the center wavelength of RBGs determines the final output wavelength of lasers, it is necessary to carefully control the center wavelength of RBGs during the fabrication process. In this paper, the fabrication process of high-precision RBGs was investigated. We developed a two-step method and demonstrated both theoretically and experimentally that it is effective and can be used to guide the fabrication process of high-precision RBGs. The experimental results show that the center wavelength of the fabricated RBG deviates from the target center wavelength within ±10 pm.

Design and fabrication of multiplexed volume Bragg gratings as angle amplifiers in high power beam scanning system.

In this study, the realization of multiplexed volume Bragg gratings (VBGs) working as angle amplifiers in high power beam scanning system is theoretically and experimentally investigated. The design of the multiplexed VBG for the working wavelength of 1064 nm is described. We propose a cascaded multiplexed VBGs scheme that consists of 12 grating channels. Three 4-channel multiplexed VBGs were fabricated inside photo-thermo-refractive (PTR) glasses by multiple exposures and subsequent heat treatment. The test results show that this angle amplifier can achieve discrete angle deflection ranging from -45° to + 45°. The relative diffraction efficiency of all the grating channels is more than 80% and is almost polarization independent.

Method for precise evaluation of refractive index modulation amplitude inside the volume Bragg grating recorded in photo-thermo-refractive glass.

In this paper, we present a method for achieving precise evaluation of amplitude of refractive index modulation (RIM) inside the volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glasses. The Gaussian divergence characteristics of the incident beam is theoretically considered when calculating the angular selectivity of VBG, and the profiles of experimental angular selectivity curves are utilized to determine the value of RIM with one step. The effectiveness of our proposed method is experimentally verified. This method is applicable even if the full width at half maximum (FWHM) of VBG's angular selectivity curve has the same order of magnitude as or is less than the beam divergence.

Optical-power tunable dual-wavelength laser around 1 µm in Nd3+/Yb3+ co-doped phosphate glass fiber.

This work reports the generation of an optical-power tunable 1 µm dual-wavelength fiber laser in Nd3+/Yb3+ co-doped phosphate glass fiber under 970 nm and 808 nm laser diode pumping. A bidirectional free-space pump laser setup was adopted. An optical-power tunable dual-wavelength laser can be easily realized by the control of 808 nm pump power magnitude. Based on a series of laser tests under different pump conditions and theoretical calculation of the Nd3+→Yb3+ energy transfer efficiency, the control mechanism was found to be the stimulated emission modulation of Nd3+ on Yb3+ lasing. This property is helpful to enhance the knowledge of the Nd3+/Yb3+ co-doping system.

5 W output power from a double-clad hybrid fiber with Yb-doped phosphate core and silicate cladding.

For the first time, to the best of our knowledge, we report on the realization of a laser from a Yb-doped phosphate core/silicate cladding double-clad hybrid fiber. 5 W output power was extracted with 14.6% slope efficiency and a laser spectrum of a 1027 nm central wavelength from a 20 cm long single-mode fiber with a ∼10 µm core diameter in a 20%-4% laser cavity. The laser efficiency can be significantly enhanced by correspondingly adjusting and optimizing the laser oscillator.

Influence of Stark splitting levels on the lasing performance of Yb(3+) in phosphate and fluorophosphate glasses.

Lasing properties have been investigated for Yb(3+) doped glasses with similar emission cross sections (σ(emi)) and lifetime while possessing different Stark levels. Narrow Stark splitting of Yb(3+)-phosphate glass is responsible for severe heat generation, narrow emission band and much smaller σ(emi) at lasing wavelength, making Yb(3+)-phosphate glass unsuccessful to achieve laser output, whereas 1.166W cw laser was obtained in Yb(3+)-fluorophosphate (FP) glass with broader Stark splitting. Analysis on laser system levels reveals that under room temperature, Yb(3+) laser is quasi-3.13-level in phosphate glass and quasi-3.36-level in FP glass. These demonstrations suggest that unless the Stark splitting is enlarged, conventional Yb(3+)-phosphate glass is not a good gain medium for bulk Yb(3+)-laser.

Phosphate ytterbium-doped single-mode all-solid photonic crystal fiber with output power of 13.8†W.

Single-mode ytterbium-doped phosphate all-solid photonic crystal fiber (AS-PCF) with 13.8 W output power and 32% slope efficiency was reported. By altering the diameter of the rods around the doped core and thus breaking the symmetry of the fiber, a polarization-maintaining AS-PCF with degree of polarization of >85% was also achieved, for the first time to knowledge, in a phosphate PCF.

Yb/Er co-doped phosphate all-solid single-mode photonic crystal fiber.

An all-solid Yb(3+)/Er(3+) co-doped single-mode phosphate photonic crystal fiber (PCF) with Watt-level output power and 20 µm core diameter is demonstrated for the first time. A PCF whose refractivity of the active core is lower than that of the background glass is suggested and theoretically confirmed to be in single-mode operation at 40 µm core diameter.

Intense blue up-conversion luminescence in Tm3+/Yb3+ codoped oxyfluoride glass-ceramics containing beta-PbF2 nanocrystals.

Up-conversion luminescence properties of a Tm3+/Yb3+ codoped oxyfluoride glass-ceramics under 980 nm excitation are investigated. Intense blue emission centered at 476 nm, corresponding to 1G4-->3H6 transitions of Tm3+ was simultaneously observed in the transparent oxyfluoride glass ceramics at room temperature. The intensity of the blue up-conversion luminescence in a 1 mol% YbF3-containing glass-ceramic was found to be about 40 times stronger than that in the precursor oxyfluoride glass. The reason for the intense Tm3+ up-conversion luminescence in the oxyfluoride glass-ceramics is discussed. The dependence of up-conversion intensities on excitation power and possible up-conversion mechanism are also evaluated.