Yu-Kuan Lu

A novel scheme of lensed fiber employing a quadrangular-pyramid-shaped fiber endface for coupling between high-power laser diodes and single-mode fibers

This paper proposes a new scheme for the lensed fiber employing a quadrangular-pyramid-shaped fiber endface (QPSFE) for coupling between the high-power 980-nm laser diodes and the single-mode fibers (SMFs). The QPSFE was fabricated by grinding and polishing techniques and then heating in a fusing splicer to form an elliptical microlens endface. A coupling efficiency of 83% has been demonstrated. Based on the far-field profile measurements, the higher coupling efficiency of the QPSFE lensed fiber was attributed to the better matching of both the elliptical Gaussian field distribution and the aspect ratio between the laser source and the fiber lens, The advantage of the QPSFE structure was to be able to control two axial curvatures, and, therefore, this novel QPSFE can form any different aspect ratios of elliptical microlenses to match the far field of the high-power diode lasers as well as the commercial diode lasers used in lightwave communications.

Asymmetric elliptic-cone-shaped microlens for efficient coupling to high-power laser diodes

A new scheme of asymmetric elliptic-cone-shaped microlens (AECSM) employing a single-step fabrication technique for efficient coupling between the high-power 980-nm laser diodes and the single-mode fibers is proposed. The AECSMs are fabricated by asymmetrically shaping the fiber during the single-step grinding process and elliptically lensing the fiber tip during the fusing process. A maximum coupling efficiency of 85% and a high-average coupling efficiency of 71% have been demonstrated for a 980-nm laser diode with a high aspect ratio of 5. In comparison with the previous works on asymmetric fiber microlenses fabricated by the multi-step processes with complicated fabrication, the advantages of the AECSM structure for achieving high coupling are a single-step fabrication, a reproducible process, and a high-yield output. Therefore, this AECSM can form different aspect ratios of asymmetric elliptical microlenses to match the far field of the high-power diode lasers that is suitable for use in commercial high-power pump laser modules.

Broadband emission from Cr-doped fibers fabricated by drawing tower.

We report on the first fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform. Both Cr3+ and Cr4+ ions coexist in the Cr-doped fiber, and the amplified spontaneous emission (ASE) spectrum shows a broadband emission of 1.2 to 1.55 mum which can not be realized by using currently available fiber amplifiers. This indicates that the new Cr-doped fibers have the potential for being used as a broadband fiber amplifier to cover the bandwidth of the entire 1.3-1.6 mum range which exhibit 300 nm usable spectral bands.

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.

Development of Broadband Single-Mode Cr-Doped Silica Fibers

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A single-mode characteristic of SMCDSF was observed when the propagation wavelengths were longer than 1310 nm. The transmission loss was about 8 dB/m at 1550 nm. The successful fabrication of SMCDSFs may facilitate the possibility for utilizing the SMCDSFs as a new generation broadband fiber amplifier to cover the bandwidths in the whole 1300- to 1600-nm range of low-loss windows of silica fibers.

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.

New Scheme of Double-Variable-Curvature Microlens for Efficient Coupling High-Power Lasers to Single-Mode Fibers

A new scheme of a double-variable-curvature microlens (DVCM) employing a single-step grinding technique with fully automated fabrication for efficient coupling between high-power 980-nm laser diodes and single-mode fibers is proposed and demonstrated. The grinded fiber endface exhibited a double-variable curvature in the major axis, and was clearly observed for both major and minor axes after slight fusion polish. The perfect aspherical shape of the DVCM results in less grinding offset of 0.2 μm and better curvature radii control that leads to high average coupling efficiency of 83%. This clearly indicates that the performance of the DVCM can achieve high average coupling efficiency better than any other grinding techniques to form asymmetric microlenses. The versatility in double-variable-curvature design, fully automated fabrication, and the excellent performance of aspherical microlens makes the proposed DVCM extremely attractive for use in many lightwave interconnection applications.

A new scheme of fiber end-face fabrication employing a variable torque technique

A new scheme of optical fibers with different types of end-faces by applying different types of periodical variable torques to control the polishing pressure is proposed and fabricated. Two types of mechanisms, mechanical torque and electrical torque controls, are developed to generate different types of periodical variable torques. The mechanical torque control type is mainly designed for fabricating an elliptical-cone-shaped fiber end-face that can be used in efficient coupling between a laser diode and fiber with a coupling efficiency up to 83% (Lu et al 2007 Opt. Express 15 1434). The electrical torque control type is even more versatile for fabricating optical fibers or micro-probes with polygon pyramid end-faces that may have other potential applications, such as various kinds of micro-indenters. Both types of fiber end-faces can deliver satisfactory results with single-step fabrication, a high fabrication yield and sufficient optical coupling efficiency.

Additional wavelength shift of peak gain due to inhomogeneous distributions of carriers inside semiconductor lasers

According to the traveling wave rate equations, the carrier density inside a semiconductor laser is position dependent. This will introduce an additional wavelength shift for the radiation that experiences the largest gain. Analysis has led to the establishment of the relationship between the shift and the mean square deviation of the carrier density from its mean. Calculations show that under certain circumstances the shift may approach the order of 10/sup -1/ nm.<<ETX>>

Fabrication and characteristics of ultra-broadband singlemode Cr-doped fibers

The fabrication of ultra-broadband singlemode Cr-doped fibers (SMCDFs) using fiber drawing-tower method is demonstrated for the first time. The success in fabrication of SMCDFs may open the possibility for utilizing SMCDFs as ultra-broadband fiber amplifiers.