All-fiber phase actuator based on an erbium-doped fiber amplifier for coherent beam combining at 1064 nm

Active phase control in fiber amplifiers is of considerable interest for low-noise single-frequency amplifiers and for coherent beam combining. We demonstrate phase control at 1064 nm by use of an erbium-doped fiber. We investigated the phase shift by guiding the beam through an erbium-doped fiber amplifier in a Mach-Zehnder configuration and applied the results to stabilize the relative phase of two ytterbium-doped fiber amplifiers. To the best of our knowledge, this is the first demonstration of an all-fiber coherent beam combining at 1064 nm employing an erbium-doped fiber as a phase actuator.     

73-nm tuning of a double-clad Yb<Superscript>3+</Superscript>-doped fiber laser based on a hybrid array

We report on a wide wavelength tuning in a double-clad ytterbium-doped fiber laser. The laser cavity consists of an array of broadband high-reflection fiber Bragg gratings and a bulk grating as the output coupler and wavelength selection element. The proposed fiber laser configuration combines a low intracavity loss of the fiber Bragg grating mirrors with a wide wavelength tuning of the bulk gratings. We demonstrate a >70-nm wavelength tuning range, limited only by the available fiber Bragg gratings.     

Fault analysis: optical performance investigation of fiber connections with imperfect physical contact

We investigated the insertion and return losses of optical fiber connections with various air-filled gaps. Such gaps might occur when a fiber connection using physical contact unexpectedly fails, resulting in imperfect physical contact. The optical performance of these fiber connections depends on wavelength and gap size. We revealed that the measured optical performance of fiber connections with an imperfect physical contact, i.e., with an air-filled gap, might deteriorate to ~18 dB for insertion loss and 9.4 dB for return loss at worst.     

高功率、低量子亏损同带抽运掺镱光纤放大器

开展了基于同带抽运的高功率、低量子亏损的掺镱光纤放大器实验研究. 搭建了一台输出功率为21 W的1018 nm短波 长掺镱光纤激光器, 并利用其对双包层掺镱光纤进行同带抽运, 获得18.6 W的1080 nm波段激光输出, 光-光转换效率高达90.86%. 关键词： 光纤放大器 同带抽运 双包层光纤 转换效率     

High-power tandem pumped fiber amplifier with an output power of 2.9 kW

We report on a high power fiber amplifier tandem pumped by an industrial style thin-disk laser. An output power of 1 kW and a very good slope efficiency of 73% have been obtained for a state-of-the-art photonic crystal fiber originally designed for short-pulse amplification. We also compare this result to an experiment, where a power of 2.9 kW could be obtained from a step-index large-mode-area fiber with the same slope efficiency.     

Polarization-maintaining, double-clad fiber amplifier employing externally applied stress-induced birefringence

We report a new approach to obtaining linear-polarization operation of a rare-earth-doped fiber amplifier in which the gain fiber is coiled under tension to induce birefringence. We demonstrated this method by constructing an Er/Yb-doped, double-clad, single-mode fiber amplifier with an output power of 530 mW and a polarization extinction ratio of >17 dB (when seeded with linearly polarized light) at a wavelength of ~1.5 microm . The technique is achromatic, permits single- or multiple-pass operation of the amplifier, requires no additional components in the optical path, leaves the fiber ends unobstructed, and is inexpensive to implement.     

Demonstration of a microwave spectrum analyzer based on time-domain optical processing in fiber

We demonstrate a novel method for spectral analysis of microwave signals that employs time-domain processing in fiber. We use anomalous dispersion in single-mode fiber to perform a Fresnel transform followed by a matched amount of dispersion-compensating fiber to perform an inverse Fresnel transform of an ultrashort pulse. After the Fresnel-transformed waveform is modulated by the microwave signal, the waveform at the output of the dispersion-compensating fiber represents the ultrashort pulse convolved with the microwave spectrum. An experimental system for spectral analysis of microwave signals in the range 6-21 GHz is demonstrated.     

Single-mode operation of a coiled multimode fiber amplifier

We report a new approach to obtaining single-transverse-mode operation of a multimode fiber amplifier in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. We demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 microm and a numerical aperture of ~0.1 (V approximately 7.4) . When the amplifier was operated as an amplified-spontaneous-emission source, the output beam had an M(2) value of 1.09 +/- 0.09 ; when seeded at 1064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique will permit scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.     

2R-regenerative all-optical switch based on a highly nonlinear holey fiber

We report the fabrication of a highly nonlinear holey fiber made from pure silica with an effective area of just ~2.8mu;m(2) at 1550 nm. We believe this to be the smallest effective area yet measured for a holey fiber at 1550 nm. We also report the operation of a 2R regenerative optical switch based on just 3.3 m of the fiber that is shown to have 30 times the nonlinear figure of merit of previous devices based on dispersion-shifted fiber.     

CO(2) laser writing of long-period fiber gratings in optical fibers under tension

We demonstrate experimentally that the efficiency of writing a long-period fiber grating in a single-mode fiber by CO(2) laser pulses increases significantly with the axial stress applied along the fiber. We attribute the enhancement in writing efficiency to the generation of nonuniform inelastic frozen-in strains across the fiber under tension by CO(2) laser heating. Controlling the axial stress distribution along a fiber during the CO(2) laser writing process thus provides an additional degree of freedom for control of the grating characteristics.     

Emission Dependence on Length in a 1.48-mum Pumped Erbium-Doped Laser with an All-Fiber Configuration

We report a study on the main features of a 1.48-mum pumped erbium-doped fiber laser, such as threshold pump power, output power, and slope efficiency. In particular we have focused on its dependence on active fiber length. We employed an all-fiber laser configuration, which included mirrors directly coated onto the polished fiber endfaces. Launched pump power and laser output power were introduced and extracted, respectively, from the laser cavity by means of couplers. Our measure ments show the existence of an optimum erbium-doped fiber length, where threshold power reaches minimum and output power maximum.     

Thermally induced mode coupling in rare-earth doped fiber amplifiers

We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.     

Optical Measurement Techniques for Optical Fiber and Waveguide Devices

We describe three major optical characterization methods for fiber and fiber devices. A simple servo controlled scanning fiber-optic confocal microscope is proposed for determining the refractive index profile of an optical fiber. To measure the chromatic dispersion of a short length fiber a Mach-Zehnder fiber interferometer with a novel interferometric distance meter is introduced. At the end, a tomographic method is demonstrated for determining the 2-D stress profile of a fiber.     

Coherent transfer of an optical carrier over 251 km

We transfer an optical frequency over 251 km of optical fiber with a residual instability of 6x10(-19) at 100 s. This instability and the associated timing jitter are limited fundamentally by the noise on the optical fiber and the link length. We give a simple expression for calculating the achievable instability and jitter over a fiber link. Transfer of optical stability over this long distance requires a highly coherent optical source, provided here by a cw fiber laser locked to a high finesse optical cavity. A sufficient optical carrier signal is delivered to the remote fiber end by incorporating two-way, in-line erbium-doped fiber amplifiers to balance the 62 dB link loss.     

Integrated fiber Michelson interferometer based on poled hollow twin-core fiber

We propose an integrated fiber Michelson interferometer based on a poled hollow twin-core fiber. The Michelson interferometer can be used as an electro-optic modulator by thermal poling one core of the twin-core fiber and introducing second-order nonlinearity in the fiber. The proposed fiber Michelson interferometer is experimentally demonstrated under driving voltages at the frequency range of 149 to 1000 Hz. The half-wave voltage of the poled fiber is 135 V, and the effective second-order nonlinear coefficient χ2 is 1.23 pm/V.     

Simple model for frozen-in viscoelastic stress in optical fibers

We have measured residual stress in a commercial single mode optical fiber and observed its relaxation in an annealing process with a polariscopic stress measurement method. After annealing the fiber for an hour at various high temperatures in a specially made image furnace we have measured the temperature dependent relaxation of frozen-in viscoelastic stress in the fiber. We have proposed a simple physical model to explain recently observed relaxation of frozen-in viscoelastic stress in optical fibers based on the Kelvin-Voigt model of viscoelastic materials. The experimental results are explained with our model of frozen-in viscoelastic stress by introducing simplified four-step procedures for fiber drawing.     

Velocity of heat dissipative solitons in optical fibers

In the fiber fuse, a pulse of high temperature travels toward the input end of the fiber, where high-power laser light is launched into the fiber. At any point along the fiber, the soliton can be ignited. The fiber core is damaged in the process so that light cannot propagate beyond the hot spot. This phenomenon is an example of a dissipative soliton that can exist only in the presence of an external energy supply and internal loss. We analyze this phenomenon, derive an expression for the velocity of the soliton, and determine its width as functions of the physical parameters of the laser and the fiber material.     

Equalization of Erbium Gain Spectrum Using the Multiport Lattice Filter

We propose an accurate modeling technique of concatenated long-period fiber gratings. The proposed technique is then applied to the synthesis of long-period fiber gratings for the erbium gain equalization using the simulated annealing and the steepest descent minimization technique. We have obtained design parameters of the piecewise-uniform long-period fiber gratings that fit the inverted erbium gain spectrum of a commercially available erbium-doped fiber amplifier (EDFA) over the entire 1525 nm to 1570 nm range.     

All-fiber Yb:fiber frequency comb

We demonstrate an all-fiber Yb:fiber frequency comb with a nonlinear-amplifying-loop-mirror-based Yb:fiber laser oscillator. The fiber-spliced hollow-core photonic bandgap fiber was used as dispersion compensator, which was also directly spliced to a piece of tapered photonic crystal fiber for an octave-spanning spectrum. The spectrum of the compressed 107 fs laser pulses was broadened, covering 600 nm to 1300 nm in a high-nonlinearity tapered fiber for f to 2 f beating. The signal-to-noise ratio of offset frequency was measured to be 22 dB.