Kuang-Yao Huang

Cr(4+):YAG double-clad crystal fiber laser.

We report what we believe to be the first demonstration of a room-temperature, continuous-wave Cr(4+):Y(3)Al(5)O(12) (Cr(4+):YAG) double-clad crystal fiber laser grown by the codrawing laser heated pedestal growth method. The threshold is below 100 mW, which is a factor of 4 lower than previously reported Cr(4+)-doped lasers. A slope efficiency of 6.9% was obtained, and is in good agreement with the numerical simulation. In additional to small core diameter, the low-threshold lasing is made possible by the low propagation loss of 0.08 dB/cm and the high crystallinity of the core.

Ce3+:YAG double-clad crystal-fiber-based optical coherence tomography on fish cornea.

A Ce(3+):YAG double-clad crystal fiber (DCF) visible emission was used as the light source for optical coherence tomography (OCT). The visible emission was produced from a 10 microm core DCF pumped by a diode laser. The broadband emission and short central wavelength of this light source enabled the realization of 1.5 microm axial resolution in air. The relatively clean spectrum reduced the side lobe of its point-spread function, and therefore facilitated the generation of a high-quality image with less crosstalk between adjacent image pixels. As a demonstration, an Aplocheilus lineatus goldfish was experimented on to map out the stroma of its cornea. This visible-light-based OCT can be utilized for industrial inspection as well as ocular applications.

Preform fabrication and fiber drawing of 300 nm broadband Cr-doped fibers

The fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform is presented. The Cr-doped YAG preform was fabricated by a rod-in-tube method. By employing a negative pressure control in drawing-tower technique on the YAG preform, the Cr-doped fibers with a better core circularity and uniformity, and good interface between core and cladding were fabricated. The amplified spontaneous emission spectrum showed a broadband emission of 1.2 to 1.6 mum with the output power density about a few nW/nm. The results indicate that this new Cr-doped fiber may be used as a broadband fiber amplifier to cover the bandwidths in the whole 1.3-1.6 mum range of low-loss and lowdispersion windows of silica fibers.

Yb3+:YAG silica fiber laser.

We demonstrate a compact, room-temperature, cw Yb(3+):Y(3)Al(5)O(12) silica (Yb(3+):YAG silica) fiber laser grown by the codrawing laser-heated pedestal growth technique. A slope efficiency of 76.3% was achieved from a 7 mm Yb(3+):YAG silica fiber, corresponding to an extracted power of nearly 1 W/cm. A laser side-mode suppression ratio of 70 dB was obtained with a 3 dB linewidth of 0.15 nm. Additionally, the propagation loss and emission cross section were determined by analyzing the lasing thresholds and slope efficiencies.

400-nm-Bandwidth Emission From a Cr-Doped Glass Fiber

Cr4+-doped glass fiber by a laser-heated pedestal growth method is reported. By analyzing the absorption spectra, the transitions of Cr3+ and Cr4+ in the fibers are identified. With appropriate pumping wavelength and divalent doping concentration, both Cr3+ and Cr4+ emission bands can be excited simultaneously, and become comparable in their fluorescent intensities. As a result, more than 400-nm-width emission peaked at 1144 nm was generated at room temperature. As much as 12muW of amplified spontaneous emission was obtained by pumping with a 900-nm Ti : sapphire laser

Broadband Chromium-Doped Fiber Amplifiers for Next-Generation Optical Communication Systems

We report the first experimental breakthrough of a net gain of optical signals in a broadband chromium-doped fiber amplifier (CDFA) for next-generation optical communication systems. Current fiber amplifiers, including commercial erbium-doped fiber amplifier, cover only a relatively small portion of the entire transmission bandwidths (1300-1600 nm) of the low-loss windows of silica fibers. The newly developed CDFAs have opened up the possibility of utilizing the 300-nm entire spectrum of the low-loss fibers to further increase the transmission capacity. In this paper, we present the experimental demonstration of a net gain of 1.2 dB employing gain enhancement technique. With the help of an optical-fiber system examination for the CDFA, a 40-Gb/s error-floor free data transmission was successfully demonstrated by realizing the high-speed transmission of signal with gain through the chromium-doped fibers (CDFs). Further gain improvement in the CDFAs employing few-mode or single-mode CDFs will be presented and discussed.

Composition dependence of the microspectroscopy of Cr ions in double-clad Cr:YAG crystal fiber

We have demonstrated the use of microspectroscopy for measuring the Cr3+ and Cr4+ fluorescence spectra in double-clad Cr:YAG crystal fiber. The emission spectra of Cr3+ and Cr4+ are detected from core and inner cladding. The Cr3+ spectrum in the inner cladding shows a broad-band emission from 650to950nm, while the emission of Cr4+ occurs in the range of 1.15–1.55μm with a peak around 1.22μm. The characteristic of Cr ion at high-field sites shows a narrow-band emission (E2→A24 for Cr3+; E1→A23 for Cr4+), whereas that at low-field sites shows a broad-band emission (T24→A24 for Cr3+; T23→A23 for Cr4+). The emission intensity ratio of high-field sites to low-field sites in the inner cladding with different compositions has been investigated. It varies from 20% to 29% for Cr3+ and from 7.1% to 11.3% for Cr4+ when the concentration of SiO2 increases from 26.9to43.0wt%.

Study on the core/cladding interface in Cr:YAG double-clad crystal fibers grown by the codrawing laser-heated pedestal growth method

This study reports, for the first time, atomic-scale high-resolution transmission electron microscope (HRTEM) microstructure studies on the interface between the crystal core and polycrystalline inner cladding and strain relaxation mechanism of high-quality Cr:YAG double-clad crystal fibers grown by a codrawing laser-heated pedestal growth method. HRTEM analysis indicates that the core has high crystallinity and a sharp core/inner-cladding interface, exhibiting coherent planes with a preferred orientation relationship to the γ-Al2O3 nanocrystals in the inner cladding. The slightly distorted γ-Al2O3 lattice facilitates the release of residual strain, and eliminates misfit dislocation at the interface. Lattice strain analysis and image processing reveal a ∼18.2 nm layer near the interface for strain relaxation.

Enhancement of Cr4+ Concentration in Y3Al5O12 Crystal Fiber by Pregrowth Perimeter Deposition

Chromium ions tend to diffuse outward during the laser-heated pedestal growth (LHPG) of Y3Al5O12 (YAG) crystal fiber, in which the average Cr4+ ion concentration decreases significantly after each diameter-reduction step. The Cr4+ ions are replenished using an electron gun to deposit Cr2O3 and divalent-ion oxide (CaO or MgO) on the source rod circumference before LHPG. It was observed that Ca2+ has better efficiency to diffuse into the source rod more efficiently than Mg2+ generating fewer defects and stacking faults. By CaO deposition and post growth annealing at 1350 °C under an oxygen environment, a 110% increase in Cr4+ concentration was obtained. The achieved Cr4+ concentration and the ratio of Cr4+ to total Cr were 1.76×1018 cm-3 and 5.5, respectively. The Ca,Cr:YAG crystal fiber can thus be used as an ultra broadband laser material and amplified spontaneous emitter.

Bidirectionally pumped Cr 4+ :YAG crystal fiber light source for optical coherence tomography

A broadband Cr4+:YAG double-clad crystal fiber light source with a collimated output power of 3.15mW was demonstrated by using a bidirectional pump scheme. The bidirectional pump scheme increased the pumping efficiency and reduced the thermal problem along the fiber. Good agreement on the output power of the broadband fluorescence was achieved between experiment and theory, which showed a peak net gain coefficient of 0.03cm−1 at the center wavelength near 1380nm. The 3dB bandwidth of 222nm with a Gaussian-like spectrum makes it eminently suitable for broadband interferometric technique to have low image cross talk. The calculated interference signal shows a 3dB width of 3.62μm with a cross talk smaller than −24dB for adjacent axial image pixels.