Development of photodarkening in two similar large-mode-area ytterbium doped fibers from different sources is compared. The excess loss induced by photodarkening is derived from transmission loss measurements of pristine and pumped or photodarkened samples. To accelerate the photodarkening process, cladding pumping is used so as to achieve high and uniform inversion through the sample. Further, intensity profiles are measured and compared in effort to detect
possible radial variations in the induced losses.
Fiber lasers offer substantial advantages compared to conventional solid-state lasers due to their high efficiency,
compact size, diffraction-limited beam quality, tunability, and facile thermal management. A number of important
applications require high peak powers and pulse energies, which has generated great interest in Yb-doped, large-modearea
(LMA) fibers. Liekki has pioneered a new manufacturing technology for rare-earth-doped fibers, Direct
Nanoparticle Deposition (DND), that is capable of producing fibers uniquely well suited to power scaling.
Conventional fiber fabrication methods are characterized by poor process accuracy and flexibility due to the large
particle sizes and relatively small number of deposition layers (2-10). In contrast, DND provides independent control of
the composition of hundreds of layers that make up the core, thereby allowing previously unattainable precision,
accuracy, and uniformity in the index and rare-earth-dopant profiles. DND allows the simultaneous use of both gasphase
and liquid precursors, providing unprecedented flexibility in the glass composition. Furthermore, DND enables
fabrication of fibers with extremely high rare-earth concentrations, which minimizes the required fiber length and
correspondingly raises the threshold power for nonlinear processes. Finally, the single-step, direct-deposition process
makes manufacturing of fibers rapid and cost-effective, even for fibers with large core diameters or sophisticated
geometries and dopant distributions. DND fibers have shown high conversion efficiency (low clustering), low
photodarkening, and high damage threshold. DND thus promises to revolutionize the use of fiber lasers in applications
previously restricted to bulk, solid-state lasers and to enable new applications of high-power lasers.
Many high power fiber laser applications require doped fibers having large mode area but still working in the single mode regime. The most common techniques to keep a large mode area fiber in the single mode regime are to reduce the core numerical aperture, to strip the high order modes by coiling the fiber, to launch only a single transverse mode, or to use photonic crystal fibers. All these methods have limits and disadvantages.
In this paper we demonstrate by simulation the effectiveness of another method to suppress the high order modes in large mode area active fibers by optimizing the rare earth dopant concentration across the core while keeping the step index structure of the core of the fiber. This method was not previously employed because the traditional doped fiber manufacturing technologies do not have the required capability to radially control the dopant concentration. However, Direct Nanoparticle Deposition (DND) can be used to manufacture large mode area fibers having any radial distribution of active element concentration and any refractive index profile. Thus, DND fibers can be designed to benefit from this high order mode suppression technique.
The simulation results presented in this paper have been obtained using Liekki Application Designer v3.1, a software simulator for fiber lasers and amplifiers.
Ytterbium-doped fibers are widely used in applications requiring short fiber amplifiers for high peak power pulse amplification. One of the key challenges posed on the performance and reliability of such amplifiers is mitigating photodarkening of the active fiber. Photodarkening manifests itself as a temporal increase in broadband absorption centered at visible wavelengths, and varies on how the active fiber has been manufactured. The tail of the photodarkening absorption extends to the 1μm region, thus in some cases seriously degrading the fiber efficiency over time. Accurate measurement methods for characterization of photodarkening must be developed in order to better understand its causes and create techniques to eliminate it so as to secure widespread commercialization of reliable Yb-doped fibers. This paper presents a simple method to characterize photodarkening in both single-mode and double clad Yb-doped fibers. A short length of fiber is pumped using high brightness source in order to achieve high and uniform inversion. The high inversion speeds up the formation of the color centers to the high degradation level, thus reducing the analyzing time from weeks to few of hours. With this method, photodarkening can be measured even from relatively short fibers by monitoring loss at visible wavelengths, where the degradation is greatest. We have analyzed the repeatability of the measurement method against the pumping conditions and fiber sample properties. The impact of photodarkening in different applications is discussed. We present the results of recent optimization of Liekki Yb1200 product family and also compare these with some other commercially available fibers.
Large-mode-area double clad fibers offer excellent efficiency and beam quality, high output power as well as lightweight, robust and reliable packaging. The addition of polarization maintaining property though use of well-know Panda-structure has further increased the interest in double clad fibers, especially in fibers doped with ytterbium (Yb). Many material processing, military and R&D applications benefit from wavelength conversion by nonlinear effects, from IR through UV, of 1064nm Q-switched pulses through polarization maintaining large-mode-area double clad Yb-fiber amplifiers. The possibility of power scaling through coherent beam combining has also been identified by the military. The design of a polarization maintaining large modea area double clad fiber for the above mentioned applications must address several key performance parameters: provide large mode area (>300μm<sup>2</sup>), high efficiency (>80% slope PCE), high average power (>100W), high birefringence (>2*10-4) and offer good beam quality (M<sup>2</sup> <1.5), short fiber length (<3m), as well as high reliability and good usability. Further optimization of the fiber design must take into consideration the impairment of the fiber by thermal loading as well as coiling of the fiber for elimination of higher order modes. This paper presents the key design considerations of such fibers for high-average-power pulsed amplifiers and provides the latest experimental techniques to verify the results. The design and results on high performance highly Yb-doped polarization maintaining large mode area fiber manufactured by the Direct Nanoparticle Deposition technology are presented and possibilities and opportunities brought by this technology are discussed.
Wavelength routing and reconfigurable cross connects are emerging concepts for optical multiwavelength telecommunication networks. They provide more efficient usage of the network resources, as individual wavelength channels can be added or dropped from the wavelength multiplex. However, problems arise with the erbium-doped fiber amplifiers (EDFAs), whose gain is dependent on the input power level. If the gain of the EDFA is not by some means controlled, transient effects will occur due to the EDFA's slow gain dynamics. In this paper a simple device for controlling the gain of the EDFA is studied. The gain- controlling scheme is based on a fast pump laser control. Part of the total input power to the amplifier is detected by the gain controlling circuitry, which then compensates for the changing gain by adjusting the pump laser power. In the study, transient effects due to changing number of channels in an EDFA are measured. The response time and the transient suppression of the gain-controlling device are verified through measurements. The effect of the amplifier gain tilt is also studied. A comparison against other proposed gain-controlling schemes is done.
The design, fabrication and properties of various types of optical waveguides on silicon substrates as well as Fabry- Perot devices accomplished mainly at the Microelectronics Center of VTT Electronics are introduced. Different waveguides, directional couplers, multimode interference couplers, switches and power splitters have been studied. For waveguide fabrication on silicon substrates principally three materials were used depending on the application: silicon oxynitride, silicon on insulator (SOI). Silicon waveguides with large Si-core and a thermo-optical switch have been fabricated by SOI technology. Silicon Fabry-Perot structures have been fabricated for wavelength scanning applications in instrumentation and to measure chirp properties of lasers used in optical communications.
We present a new survivable, reconfigurable wavelength division multiplexing (WDM) ring architecture for the metropolitan area or regional networks. It uses bi- directional WDM and space division multiplexing with simple switches for 1 + 1 protection at optical channel layer. The ring can be realized with available technology at relatively low cost. This architecture has advantages in scalability and reconfigurability. Achievable ring sizes are studied, and adaptability to varying traffic needs, utilizing multi-hop connections, is evaluated.
A multilayer capillary fiber was designed for optical sensor applications and its optical properties were evaluated by using a fluorescent layer immobilized on its inner surface. This fiber structure combines a large interaction surface (the inner wall of the capillary fiber) for binding of fluorescent indicators with evanescent wave fluorescence measurement, which may facilitate the design of pseudohomogeneous assays without separation of the bound from nonbound fluorescent indicator. The inner surface of the capillary was derivatized by aminosilanization, followed by biotinylation and addition of streptavidin. This biotin- streptavidin coating facilities subsequent immobilization of any biotinylated species (e.g. antibodies, antigens, etc.) participating in specific molecular recognition. We have evaluated some properties of this capillary fiber design by using fluorescent proteins immobilized on the inner wall of capillary by biotin-avidin-interaction. Fluorescence was excited by a HeNe-laser (fluorescent indicator APC; (lambda) <SUB>em</SUB> equals 660 nm) and by a Ar-laser (fluorescent indicator RPE, (lambda) <SUB>em</SUB> equals 578 nm), and measured with a spectrum analyzer.
The feasibility of a passive fiber optic bus that uses bidirectional integrated optics bus access couplers is studied. The number of bus nodes can be increased when an optimized asymmetric bus access coupler is used instead of a symmetric one. A modified Ag-Na ion exchange process provides the possibility of manufacturing integrated optics asymmetric couplers for the bus. The process is suitable for the processing of integrated optics components that combine deep multimode waveguides with different core sizes on a single substrate. An access coupler is designed, modeled, and processed. The coupler is optimized for seven nodes; the asymmetry of the coupler is 5 dB.
We introduce a new technique to measure the amplified spontaneous emission and signal gain of the fiber amplifier. We have constructed a measurement system that consists of a tunable signal source, a fast chopper, detectors, and a microcomputer. This amplifier measurement system can be used to characterize amplifier properties as a function of input signal wavelength and power.
The optical fiber amplifier is a device where signal light near 1.55 micrometers is amplified by stimulated emission of the exited erbium ions as the signal passes through the erbium doped fiber. The critical properties of erbium doped fiber amplifiers are the output power, gain, and noise. These amplifier parameters should be measured as a function of wavelength. For this reason we have constructed a measurement system that consists of a tunable signal source, fast chopper, detectors, and microcomputer. This amplifier measurement system can be used to characterize amplifier properties as a function of wavelength and input or output powers.
The feasibility of a passive bidirectional fiber optic bus and the packaging considerations of a bus access module have been studied. The bus uses 110/125 micrometers HCS fiber and passive integrated optic couplers for bus access. The access couplers are asymmetric and were fabricated using a Ag-Na ion exchange process. The asymmetry of the coupler was 5 dB, the launch loss to the bus was 6 dB and the tap-off loss to the node was 11 dB. With the integrated optics coupler it is possible to connect 6 nodes to the bidirectional bus. It is also possible to realize a simple, easy-to-use, and reliable bus access module for intramachine communication. The integrated optics coupler, a LED chip, and a PIN-diode chip and transceiver electronics are packaged in an electrical connector with a two-fiber optical cable pigtail. Active and passive components are butt coupled to the coupler. The 0.5 dB alignment tolerances for the fiber pigtails, the LED, and the PIN-diode chips are +/- 5 micrometers .
A dry silver ion exchange process is described, in which silver evaporation and ion exchange are performed successively in a vacuum chamber. This makes possible the reproducible fabrication of large, nearly step index multimode waveguides. Asymmetric power dividers have been fabricated with the process.
The dependence of gain in Erdoped fibre on Vvalue refractive index dip confinement of Er atoms and diffusion at the core cladding interface is presented. The absorption of the pump light and the gain of the signal light are calculated by solving the population density equations as a function of radial and axial position in the fibre. When the pump wavelength is 980 nm the optimum Vvalue at signal wavelength 1530 nm of a step index fibre increases from 1. 16 to 1. 29 with the pump power increasing from 2 mW to 8 mW. When the pump wavelength is 1490 nm the optimum Vvalue is 1. 37 without pump power dependence. With the confinement of the dopant atoms the gain can be improved as much as 68 . The confinement increases the optimum Vvalue to around 1 . 44. The gain is decreased 37 by an index dip which is 75 of the core diameter and 100 of the index difference. The diffusion of the dopants at the interface between the core and the cladding decreases the calculated gain 17 when the core radius increases 40 due to the diffusion measured at 10 index difference level.