Dong-Yo Jheng

Low-loss propagation in Cr 4+ :YAG double-clad crystal fiber fabricated by sapphire tube assisted CDLHPG technique

Cr4+:YAG double-clad crystal fiber with an uniform 10-microm core was fabricated by using a sapphire tube as a heat capacitor to stabilize the power fluctuation of the CO2 laser in the co-drawing laser-heated pedestal growth system. The uniformity of the fiber core showed a factor of 3 improvement compared to that without the use of sapphire tube. The variation of the core diameter is within the +/-1.35-degree adiabatic criterion and has a autocorrelation length of 1.7 mm. The measured propagation loss is only 0.02 dB/cm. The sapphire tube also reduces the vertical temperature gradient during the crystal fiber growth process so the 10-microm crystal core exhibits a smooth perimeter. The sapphire tube assisted system can be applied to the growth of many other optical crystal materials.

Efficient and low-threshold Cr 4+ :YAGdouble-clad crystal fiber laser

A significant advancement of cw lasing in Cr4+:Y3Al5O12 (Cr4+:YAG) double-clad crystal fiber grown by the codrawing laser-heated pedestal growth technique was demonstrated at RT. The optical-to-optical slope efficiency of 33.9% is the highest, to the best of our knowledge, among all Cr4+:YAG lasers, whether they are in bulk or fiber forms. The low-threshold lasing of 78.2 mW and high efficiency are in good agreement with the simulation. The keys to the high laser efficiency are twofold: one is the improved Cr4+ emission cross section and fluorescence lifetime due to release of the strain on the distorted Cr4+ tetrahedron, which also mitigates photobleaching in Cr4+:YAG; the other is the improved core uniformity at long fiber lengths. In addition, because of the low threshold, the impact of excited state absorption of the pump light is significantly reduced. The effects of crystal-orientation, self-selected, and pump-dependent linear polarization states were also addressed.

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.

Diode-Laser-Pumped Glass-Clad Ti:Sapphire Crystal-Fiber-Based Broadband Light Source

Glass-clad crystal fibers were grown using the codrawing laser-heated pedestal growth method. Both single-clad and double-clad structures were attempted. Using a 446-nm laser diode as the pump, 2.45 mW of collimated broadband emission was generated with a 3-dB bandwidth of 180 nm. As much as 7.7 mW of output power with a 3-dB bandwidth of 182 nm was generated using a 532-nm solid-state pump laser. The broadband light source is advantageous to use in optical coherence tomography with an axial resolution of 1.5 in air and a low image pixel crosstalk.

Cladding YAG crystal fibers with high-index glasses for reducing the number of guided modes

Yttrium aluminium garnet (YAG) crystal fibers with a core diameter of 40 μm were cladded by high index glasses using the co-drawing laser-heated pedestal growth method. Due to the extremely large cooling rates in the fabrication processes, unexpected and phenomenally large index drops of 0.018 and at least 0.02 were found from the as-grown capillary and the YAG crystal fiber cladding compared with bulk N-SF57’s, respectively. The high-index glass cladding is effective in reducing the number of guided modes, and the intensity profiles of the crystal fiber show there are only four guided modes at 532 nm.

Intracavity and resonant Raman crystal fiber laser

We demonstrated an efficient, compact, and continuous-wave Raman crystal fiber laser (RCFL) using an intracavity and resonant χ(3) approach. The gain and nonlinear medium was Cr4+:Y3Al5O12 double-clad crystal fiber grown using the codrawing laser-heated pedestal growth technique. The RCFL threshold was only 50 mW, and the slope efficiency reached 14.3% above a pump power of 350 mW. The result is in good agreement with theory, which indicates a near-100% quantum efficiency of resonant stimulated Raman scattering.

High-Luminance White-Light Point Source Using Ce,Sm : YAG Double-Clad Crystal Fiber

Ce and Sm ions codoped yttrium aluminum garnet (Ce,Sm : YAG) double-clad crystal fibers (DCFs) were successfully fabricated using the codrawing laser-heated pedestal growth method. At a pump wavelength of 446 nm, white-light point source was produced from a 10--core DCF. The amplified spontaneous emissions and the residual pump resulted in a directional high brightness point source. A luminance of 4.5 × 109 cd/m2 was generated. The luminous flux of the point white light was 6.2 lm. When coupled to a 200-core multimode fiber, the output power and coupling efficiency were 4.8 mW and 36.8%, respectively. Point white-light sources are useful for biomedical endoscopic application and long-range directive illumination. The generated broadband yellow emission can also be served as the light source for optical coherence tomography system. Using the 91-nm yellow emission, an axial resolution of 1.5-optical coherence tomography can be achieved for ultrahigh-resolution ocular and industrial applications.

Direct Side Pumping of Double-Clad Fiber Laser by Laser Diode Array Through the Use of Subwavelength Grating Coupler

An electron-beam-fabricated subwavelength grating coupler for direct side coupling of light emission from a high-power laser diode array (LDA) was studied theoretically and implemented experimentally. A gold-embedded silica-based design for grating coupler was employed to minimize the thermal expansion caused by the accumulated heat from light absorption by metal part of the grating coupler. In addition, with the consideration of the backward diffraction loss and the groove wall nonverticality caused by fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. According to the experimental results, the grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave-operated LDA into the inner clad of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, an LDA side-pumped ytterbium-doped DCF laser by using the grating coupler was demonstrated. By fine tuning the slow-axis collimation lens array, the laser-pumping scheme can easily be switched between bidirectional pumping and unidirectional pumping. Compared with the unidirectionally pumped fiber laser of the same gain fiber length, the laser slope efficiency of the bidirectionally pumped fiber laser was increased by 18% because of a better gain distribution over the fiber length. Finally, the signal output power of 10 W with a slope efficiency of 61% was achieved for the bidirectional side-pumped fiber laser.

Broadband and high-brightness light source: glass-clad Ti:sapphire crystal fiber.

High-brightness near-infrared broadband amplified spontaneous emission (ASE) was generated by glass-clad Ti:sapphire crystal fibers, which were developed using the co-drawing laser-heated pedestal growth method. As much as 29.2 mW of ASE power was generated using 520 nm laser diodes as the excitation source on an a-cut, 18 μm core-diameter Ti:sapphire crystal fiber (CF). The 3 dB bandwidth was 163.8 nm, and the radiance was 53.94  W·mm(-2) sr(-1). The propagation loss of the glass-clad sapphire CF measured using the cutback method was 0.017  cm(-1) at 780 nm. For single-mode applications, more than 100 μW of power was coupled into a SM600 single-mode fiber.

Broadly Tunable and Low-Threshold Cr 4+ :YAG Crystal Fiber Laser

We demonstrate a Cr4+:YAG crystal fiber laser tunable from 1353 to 1509 nm using a birefringent filter and with a threshold pump power of 70 mW operating at room temperature. It was found that the coupling loss between the crystal fiber and the collimator due to lens aberration is critical to the fiber laser performance and can be reduced by choosing an aspheric lens with a short focal length. The crystal fiber gain was optimized by aligning the pump polarization to the crystal axis with the highest gain. The advantages of the crystal fiber compared to the bulk crystals include reduced pump threshold due to light confinement and better thermal dissipation due to high surface area-to-volume ratio. The temperature rise of the crystal fiber with a 400-mW incident peak pump power with a duty cycle of 5% was only 1.7 °C using passive cooling. This low-threshold crystal fiber laser is suitable for the application as a tunable seed laser for the power amplifier chain.