Rare earth doped fluoride glass fibers have been shown to be excellent laser materials. Besides their unique properties as fiber lasers, they offer more metastable levels as compared to silica, thus improving performance, adding new wavelengths and allowing easier upconversion lasing. Room temperature cw three-level, self-terminating, upcon- version,... lasers were demonstrated. Energy transfer and cross-relaxations devices with high efficiency and low threshold have been reported. Effort is now brought on diode-pumped and tunable fluoride fiber lasers.
This review paper gives an exhaustive bibliography in the field of fluoride fiber lasers and related topics, and details late new results obtained in the field.
Erbium-doped fiber amplifiers (EDFA’s) operate in the 1.5?m wavelength telecommunications window and have achieved high gain, high output power and near ideal noise performance. The feasibility of using semiconductor laser diodes to pump EDFA’s insures that EDFA’s are practical devices which will have great impact on optical communications as power boosters, optical repeaters and optical preamplifiers.
Ultrafast all-optical, fibre optic switches are of great interest as all-optical signal processing elements in telecommunication systems and as passive mode-lockers for all-fibre mode-lock lasers. We describe the operation and characteristics of two such switches, the conventional and amplifying Sagnac switch, and describe the use of these components in a recently developed, passive, self-starting, mode-locked erbium doped fibre laser capable of the generation of solitons with durations as short as 320 femtoseconds.
The characteristics of Er-doped superfluorescent fiber sources are simulated for pump wavelengths in the 980 nm and 1.48 ?m pump absorption bands. Because absorption near 980 nm occurs to a short-lived pump state, while absorption near 1.48 ?m occurs to the long-lived upper-laser state, sources pumped in these wavelength regions have different characteristics. Both pump bands are found to have optimal pump wavelengths for stability and power (976 nm and 1.475 ?m). While both pump bands are efficient, the 1.48 ?m pump band has a lower threshold pump power level and a higher slope efficiency. On the other hand, the 980 nm pump band produces broader spectra and permits the use of shorter fiber lengths. These and other source characteristics are discussed in detail.
The lasing characteristics of a neodymium-doped silica fiber laser mode-locked with a linear external cavity with vibrating mirror are measured. The investigation of the dependence of the duration of the laser pulses on the mismatch between the lengths of laser resonator and external cavity shows that mode-locking is impossible if both cavities are exactly matched. The shortest pulses have a duration less than 32 ps. A simple analytical and numerical study gives insight in the physical mechanism underlying this interesting mode-locking technique.
Excited state absorption (ESA) spectra were measured for the 4I11/2? 4F7/2 transition at 980 nm in Er-doped fluoride and fluorophosphate glass, by varying the pump wavelength from a tunable Ti:sapphire laser and monitoring the relative strength of the green upconverted fluorescence. The ESA cross section spectra are the same order of magnitude in strength as the 980 nm ground state absorption, but shifted to shorter wavelength by 4-6 nm. The 980 nm ESA may limit the efficiency of high power erbium-doped fiber amplifiers, and provides a possible pump mechanism for IR upconversion pumped fiber lasers operating in the visible.
Excited state lifetimes, ground state absorption, excited state absorption, and stimulated emission cross sections govern the performance of luminescent sources and amplifiers. The role of host glass in determining the shape and magnitude of these quantities is outlined and useful relationships among cross sections are presented. Using the theory of McCumber it is possible to obtain the shape of a cross section spectrum from a measurement of the inverse process and to set limits on the noise figure of resonantly pumped amplifiers. A simple analysis can be used together with easily obtained information to identify glass compositions most promising as hosts for Nd3+ amplifiers at 1300 nm.
Resonance-fluorescence-line-narrowing studies have been performed on a representative group of Er3+ -doped glasses for excitation in both the 1530 and 980-nm bands at 4.2, 77, 200, and 300 K. Fluorozirconate, fluorophosphate, phosphate, silicate, Ge/P-doped and A1/P-doped silica preforms, and Al/P-doped and Ge-doped fibers were examined. At 77 K the homogeneous linewidths were found to be relatively insensitive to composition. In particular, they vary from 1.4 to 1.8 cm-1 for the different hosts using excitation at 1531 nm. At 300 K weak resonant features were observed in most samples, consistent with a small inhomogeneous contribution to a predominantly homogeneous fluorescence emission.
An all-optical feedback loop is demonstrated which effectively suppresses slow, signal-induced gain
fluctuations in Er-doped fiber amplifiers (EDFA's). Gain control is achieved by operating the
amplifier in a ring laser configuration. Such a feedback loop which can be fabricated out of all-fiber
passive components could be easily implemented in any system using EDFA's.
Filters and isolators placed within the EDFA's can be used to increase the gain , and decrease the noise figure. By use of an accurate model the placement of the filters and isolators are optimized. The optimization are performed for situations with pump lasers emittiing at 0.980?m and 1.480?m and signal wavelengths at either the emission cross section peak near 1.530?m or at the silica loss minimum around 1.550?m.
Simple, experimentally-verified expressions are presented for the signal gain, pump absorption and output noise spectrum of erbium-doped fiber amplifiers having arbitrary numbers of signals and pumps of arbitrary power levels and propagation directions.
This paper reports the experimental results of a Er-doped amplifier using a self-made intercavity frequency doubling Q-switched(5-10KHz) Nd:YAG laser as the pump source, a 1.536?m LD as the signal source. A gain of 30dB was obtained for only 24.4mW of pump power.
Rare earth-doped fluorozirconate fibres show potential as optical amplifiers from the visible to the mid IR. This paper reviews the current status of their application in telecommunications, concentrating on Tm and Er-dopants for the 1st window and Nd and Pr-dopants for the 2nd window. A 1300 nm system operating at 2.5 Gbits/s using a Pr-doped fibre amplifier is described.
Optical properties of rare-earth ions in glasses based on barium, indium, gallium, zinc, yttrium/lutetium, and thorium fluorides are presented. Multiphonon relaxations are determined and are found to be significantly lower than in ZBLAN fluoride glasses. Also, spectroscopic properties of the 1.3?m emission of neodymium (III) and praseodymium (III) ions in fluoride glasses are studied. Excited state
absorption from the 4F3/2 emitting level of neodymium (III) as well as radiative and non-radiative properties of the 1G4 level of praseodymium (III) are compared in Ba-In-Ga and ZBLAN glasses.
We report a thulium-doped fluorozirconate fiber amplifier pumped at 785nm, with gain being observed between 800nm and 830nm. A small signal gain of 23dB was measured at 806nm. Modeling of this device suggests that a diode- pumped amplifier with 30dB gain and a 3dB bandwidth of 10nm should be feasible when using an optimized fiber. Tunable and Q-switched laser oscillation has also been demonstrated in this wavelength region.
The performances of diode-pumped 2.7?m fiber laser have been studied. From the population inversion estimated for a fluorozircoaluminate (AZF) glass with different ErF3 doping, a higher gain was expected for a higher ErF3 doping. The laser performances were improved by adjusting fiber length. Maximum output power of 18.5mW and slope efficiency of 7% were obtained by using 10mol% ErF3 doped, 25mm long AZF fiber in the 800nm pumping. A cw lasing was achieved by diode-pumping at 0. 98pm and the effect of Yb3+ sensitization was discussed to improve the performance.
We have demonstrated the feasibility of transoceanic fiber amplifier systems using an experimental technique that measures bit error rates in a circulating loop. We have achieved a transmission distance of 9,000 km at 5 Gb/s and 21,000 km at 2.4 Gb/s using a non-retum-to-zero pseudo-random data pattern. The bit rate distance product of the 2.4 Gb/s result was 51 Tb-km/sec.
In the light of achievable component performance, we discuss from both a theoretical and an experimental point of view the applications of erbium-doped fiber amplifiers (EDFA) in long distance repeaterless digital transmission and analog cable TV distribution. In digital repeaterless systems, EDFA postamplifiers, preamplifiers and remotely pumped amplifiers are considered. In analog TV distribution networks, EDFA postamplifiers and in-line amplifiers are evaluated.
Design of a distributed Erbium doped fiber amplifier for a long distance transmission line is investigated, using a very accurate model. The design is evaluated for a bidirectional pumping scheme, taking nonlinearities into account. Bit error rates based on amplified spontaneous emission are calculated. For distances between each pumping station equal to 100km, bit error rates lower than 10-10 at 4GBit/s is found.
Erbium doped fiber amplifiers in eight different system configurations are investigated, using a very accurate model. The in-line placement of the amplifier is shown to be optimum. A distributed amplifier is compared to the remote pumping scheme showing no system improvement. By the use of a booster, an inline remote pumped amplifier and a preamplifier up to 180 km of extra transmission fiber can be added.
We have investigated the signal and noise power evolution along cascaded erbium-doped fiber amplifier systems, in various configurations, with and without optical in-line filters. We showed what the key parameters are and what are the conditions to obtain a given signal to noise ratio performance.
The quantum conversion efficiency (QCE) and noise figure of erbium-doped fiber power amplifiers have been investigated experimentally. Under 980 nm co-directional pumping, efficiencies up to 89% were achieved. Very good agreement of the measured QCE, gain and noise figure with those predicted by a numerical model using only measured input parameters was obtained. The model was utilized to analyze the influence of fiber waveguide design, erbium confinement, signal wavelength and the signal input level on the QCE and noise figure for power amplifiers pumped at 980 nm or 1480 nm. Increasing the NA from 0.15 to 0.25 was found to increase the QCE by up to 52%, and decreasing the confinement factor from 1.0 to 0.5 produced up to an 18% increase of QCE. The noise figure is predicted to be up to 3 dB higher for pumping at 1480 nm than for pumping at 980 nm, and up to 2 dB higher for a signal wavelength of 1532 nm than for one at 1555 nm. As the signal input level increases, the QCE saturates and the noise figure begins to increase. This leads to a pump-power dependent optimum input signal level if both QCE and noise figure are a consideration.?
We describe 2?m wide ridge waveguide lasers emitting at a wavelength of 980nm using a strained single quantum well epitaxial structure. These devices exhibit high coupling efficiency into single mode fiber. We have coupled up to 60mW using an industry standard 14-in dual-in-line package. Such devices are ideal for pumping Er-doped Fiber Amplifiers.
The dependence of the output saturation power on length is determined for an aluminosilicate erbium-doped fiber amplifier. For low gain amplifiers the output saturation power is found to decrease with increasing length. But for amplifiers with small signal gains above about 20 dB the output saturation power increases with length. The self-saturation by amplified spontaneous emission of such high gain amplifiers is responsible for the observation of increased saturation output powers.
The erbium doped silica fibre amplifier has become almost universal in the 1.53-1.57?m band, due to its high gain and power handling, good linearity, and lack of crosstalk. In this paper, we report results on amplification at wavelengths as long as 1.62?m in the erbium/silica system, greatly extending the existing gain window.
The performance and efficiency of erbium-doped fibers pumped in the 800-nm band has been analyzed using a quantitative amplifier model. Both a silica and a fluorophosphate host were investigated. The analysis showed that under optimized conditions the fluorophosphate is superior to the silica with higher gain and up to 60 % higher quantum conversion efficiency. Only with respect to noise figure is the fluorophosphate poorer, because of the shorter lifetime of the metastable state.
This paper describes the latest results on laser oscillation and amplifier operation with Erbium(Er3+)-and Neodymium(Nd3+)-doped fluoride glass fiber. Er3+-doped fluoride fiber amplifiers have a 20 dB gain in the 35 nm wavelength range around 1.55?m. Also, up-conversion lasing is observed in highly Er3+-doped fiber, which produces many kinds of emission. Nd3+-doped fiber amplifiers show a 10 dB gain and a high saturation signal output around 1.33 ?m. The gain limit is discussed in comparison with that of Praseodymium(Pr3+)-doped fiber.
The steady state behaviour of erbium-doped fibre laser amplifiers and oscillators has been modelled operating at 1.5?m and at 2.7?m. The model has been used to optimise a multimode 2.7?m CW laser using erbium doped fluoride fibre, end pumped by a 793nm laser diode. The laser has been constructed and performed as predicted, giving a threshold of 200mW and a slope efficiency of around 0.5%.
Using an accurate and comprehensive computer model, we study the pump wavelength dependence of amplifier performance in the 800 nm band in erbium-doped fiber amplifiers. A comparison is made for both forward and bidirectional pumping over a wide range of pump powers. It is shown that 800-810 nm pumping is more efficient if bidirectional pumping is used, while 810-820 nm pumping is more efficient if only forward pumping is carried out. As we show, this is due to the effects of excited state absorption (ESA), in the 800 nm pump band. In alumino-phosphate-silicate (APS) erbium-doped fiber amplifiers the detrimental effects of ESA can be reduced significantly by using bidirectional pumping. Further, if a “Band Optimum Length” (BOL) is selected, a near optimum small signal gain (SSG) can be constant over a broad pump wavelength region and this is due to the combined effects of ESA and ground state absorption (GSA). The results show that single-longitudinal-mode or multi-longitudinal-mode, high power, reliable and inexpensive GaAlAs laser diodes can be used as pumping sources for EDFAs having SSG > 40 dB, noise figures of < 4 dB and output signal powers approaching 100 mW and these could be used as line repeaters, preamplifiers and power amplifiers in different lightwave systems.
We also analyze the performance that can be expected in fluorophosphate glass erbium-doped fiber amplifiers (EDFAs), with 800 nm band pumping. Due to the smaller excited state absorption, the fluorophosphate host glass shows about a 4-6 dB advantage in small signal gain (SSG) over alumino-silicate EDFAs with bidirectional pumping, and nearly double the output signal power for power amplifiers under similar conditions.
With theoretical modeling, we optimize the fiber design for distributed erbium-doped fiber amplifiers which maintain a constant signal level across 10 - 100 km lengths. A constant signal level is maintained by fully inverting an amplifier with very low doping concentrations in the 10 - 50 ppb range. Included in the analysis are the pump wavelength, pumping configuration, erbium doping concentration, noise, temperature effects, macrobending loss and chromatic dispersion. We found that the optimum design would consist of a V value of 1.775 with a corresponding doping concentration of approximately 22 ppb.