Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Towards high-power mid-infrared emission from a fibre laser

In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

Aspects of the subject disclosure may include, for example, receiving, by a feed point of a dielectric antenna, electromagnetic waves from a dielectric core coupled to the feed point without an electrical return path, where at least a portion of the dielectric antenna comprises a conductive surface, directing, by the feed point, the electromagnetic waves to a proximal portion of the dielectric antenna, and radiating, via an aperture of the dielectric antenna, a wireless signal responsive to the electromagnetic waves being received at the aperture. Other embodiments are disclosed.

Method and apparatus for coupling an antenna to a device

We propose a bidirectional wavelength-division-multiplexed passive optical network by employing gain-saturated reflective semiconductor optical amplifiers (RSOAs) for wavelength-independent optical network terminals. The fabricated RSOA module has the input saturation power less than -13 dBm and its saturated gain higher than 13 dB within the C-band for arbitrary polarization states. The upstream signals are generated by remodulating the downstream signals whose modulation amplitude is squeezed through gain-saturated RSOAs. We present experimental results on bidirectional transmission with 1.25 Gb/s for upstream and 2.5 Gb/s for downstream data rates over 20-km transmission distance.

Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers

A distributed antenna and backhaul system provide network connectivity for a small cell deployment. Rather than building new structures, and installing additional fiber and cable, embodiments described herein disclose using high-bandwidth, millimeter-wave communications and existing power line infrastructure. Above ground backhaul connections via power lines and line-of-sight millimeter-wave band signals as well as underground backhaul connections via buried electrical conduits can provide connectivity to the distributed base stations. An overhead millimeter-wave system can also be used to provide backhaul connectivity. Modules can be placed onto existing infrastructure, such as streetlights and utility poles, and the modules can contain base stations and antennas to transmit the millimeter-waves to and from other modules.

Backhaul link for distributed antenna system

Photodarkening is recognized as a potentially important limiting factor on the lifetime and reliability of many Yb-doped fiber lasers and amplifiers. In particular, a photodarkening process attributed to the formation of photoinduced structural transformations can induce excess loss in the doped glass core of the fiber, resulting in reduced output power efficiency. Yet, quantifiable measurement techniques of this phenomenon have been scarce in the literature to date. Here we present a fast, simple and repeatable method to measure and compare the photodarkening rate caused by the formation of photoinduced structural transformations from Yb-doped single-mode fibers. The method relies on quantifying observations of transmission changes at visible wavelengths as an indicative measure of photodarkening at the signal wavelengths. Preliminary measurement results are presented supporting the utility of the technique for benchmarking the photodarkening behavior of different Yb-doped fibers.

Measuring photodarkening from single-mode ytterbium doped silica fibers.

Optical fibers are provided for modal discrimination which include a central core and a cladding disposed about the central core. The central core has a non-circular and non- elliptical cross-section, and it is rotated about a central axis of the optical fiber along the length of the optical fiber at a selected pitch resulting in the capability of a fundamental mode beam output for large core sizes. An optical system includes a seed optical source configured to provide a seed beam and an optical amplifier configured to receive and amplify the seed beam. The optical amplifier also includes an active optical fiber having a large mode area non-circular and non-elliptical core rotated about a central axis of said active optical fiber to provide modal discrimination and fundamental mode output.

Spun non-circular and non-elliptical fibers and apparatuses utilizing the same

Yb-doped fibers are widely used in laser applications requiring high average output powers and high-peak-power pulse amplification. Photodarkening (PD) is recognized as one limiting factor in these fibers when pumped with high-intensity radiation. We describe an approach for performing quantitative PD studies of fibers, and we present measurements of the rate of PD in Yb-doped single-mode fibers with varying inversion levels. The method is applicable to large-mode-area fibers. We observed a seventh-order dependence of the PD rate on the excited-state Yb concentration for two different fibers; this result implies that PD of a Yb-doped fiber source fabricated using a particular fiber will be strongly dependent on the configuration of the device.

Photodarkening rate in Yb-doped silica fibers.

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. Keywords: photodarkening, photoionization, rare-earth doped fibers, ytterbium, double cladding

Photodarkening in ytterbium-doped silica fibers

Yb-doped fibers are widely used in applications requiring high average output powers and high power pulse
amplification. Photodarkening is one limiting factor in these fibers. In this paper, characterization of photodarkening in
large-mode-area (LMA) fibers is presented building upon our previous work, which indicated that meaningful
comparison of photodarkening properties from different fibers can be made as long as care is taken to equalize the
excited state Yb concentration between samples. We have developed a methodology that allows rapid and reproducible
photodarkening measurements to be performed and that enables quantitative comparison of the photodarkening
propensity among fibers with different compositions and under different operating conditions. We have shown that this
measurement technique can be used effectively for LMA fibers by employing cladding pumping rather than the more
standard core pumping. Finally, we observe a seventh-order dependence of the initial photodarkening rate on the excited-state
Yb population for two different Yb-doped fibers; this result implies that photodarkening of a Yb-doped fiber source
fabricated using a particular fiber will be strongly dependent on the device configuration.

Joona Koponen

Papers

Measuring photodarkening from single-mode ytterbium doped silica fibers.

Spun non-circular and non-elliptical fibers and apparatuses utilizing the same

Photodarkening rate in Yb-doped silica fibers.

Photodarkening in ytterbium-doped silica fibers

Photodarkening Measurements in Large-Mode-Area Fibers