Tim Whitley

Measurement and analysis of pump-dependent refractive index and dispersion effects in erbium-doped fiber amplifiers

In a doped fiber, at wavelengths close to an absorption, the refractive index, and hence the dispersion, is expected to be a strong function of wavelength, as described by the Kramers-Kronig relationship. Furthermore, this spectral variation itself will be a function of pumping. This paper describes an accurate and sensitive experimental determination of the pump dependence of the refractive index in an erbium-doped fiber amplifier. A Mach-Zehnder interferometric measurement is described where only one arm is comprised of doped fiber. Particular attention is paid to accounting for the significant difference in power levels in the two arms and the effects of polarization and incoherent light. The measured refractive-index change matches well with that theoretically predicted. The pumped and unpumped dispersion of the amplifier are calculated. While the extra dispersion is of similar magnitude to that of silica fiber, the impact on long telecommunications systems is likely to be negligible. However, this extra dispersion may be important in shorter systems such as soliton lasers. Given the good agreement between theory and experiment, the variation of dispersion with fractional inversion is calculated from absorption and gain measurements. The presence of codopants is known to alter the absorption and emission spectra; the effect on the dispersion is calculated, and a near linear dependence on germanium concentration is observed.

Quarter-watt output at 1.3 mu m from a praseodymium-doped fluoride fiber amplifier pumped with a diode-pumped Nd:YLF laser

The characteristics of a praseodymium-doped fluoride fiber amplifier (PDFFA) pumped using a high-power diode-pumped Nd:YLF laser operating at 1.047 mu m are described. High small signal gains (29.5 dB) and the highest saturated output yet reported for PDFFAs (250 mW) have been achieved using this commercial diode-pumped source. The spectral characteristics have also been examined in both the large and small signal regimes. It is concluded that there is no significant penalty associated with pumping PDFFAs in the long wavelength wing of their pump band, even though this pump wavelength is almost 40 nm away from the peak of the absorption band.<<ETX>>

High Output Power from an Efficient Praseodymium Doped Fluoride Fibre Amplifier1

A praseodymium doped fluoride fiber amplifier (PDFFA) exhibiting efficient operation in both the large- and the small-signal regimes is described. The amplifier, based on a high NA fluoride fiber, exhibited a maximum small-signal gain of 29 dB and a low-pump small-signal. efficiency of 0.13 dB/mW. In the saturated regime a maximum output power of 212 mW was achieved. In this format, the signal-out versus pump-in characteristic, exhibited a slope efficiency of 30%, representing the most efficient conversion from pump to signal yet reported. Detailed spectral characterization reveals small-signal gain in excess of 20 dB over a wavelength range of almost 50 nm and in excess of 100 mW of saturated output available over a 30-nm wavelength range.<<ETX>>

The effect of pump and signal field overlap with the ion distribution on the efficiency of an Er3+-doped fibre amplifier

Abstract We report the use of a far-field scanning system to measure the pump and signal fields within erbium doped fibre amplifiers. Measurements were carried out on fibres with both uniform dopant throughout the core and dopant confined to the central region of the core. We have demonstrated that the improvement in amplifier performance associated with ion confinement is consistent with the change in overlap between pump and signal fields and the radial ion distribution. It is shown that most of the improvement available through the process of confinement can be obtained through restricting the dopant to the inner 40% of the pump mode field radius. The effects of radial ion confinement on the small-signal gain performance of amplifiers exhibiting pump excited states absorption are discussed and compared to the effects on amplifiers free from pump ESA. We conclude that the process of confinement is only of benefit in amplifiers free from the parasitic effects of pump excited state absorption.

Analytic Expression for Gain in an Idealised 4-level Doped Fibre Amplifier 1

The last year has seen the emergence of praseodymium doped fluoride fibre amplifiers (PDFFA’s) as a means of providing optical amplification within the important second telecommunications window around 1.3µm [1], These devices have already demonstrated many of the attractive characteristics of the ubiquitous erbium doped fibre amplifier (EDFA) and are currently the subject of intense international research. Unlike EDFA’s, the gain mechanism within the praseodymium doped fluoride fibre amplifier is 4-level in nature and therefore must be modelled in a slightly different way to its erbium counterpart[2,3].

Quarter Watt Output at 1.3µm From a Praseodymium Doped Fluoride Fibre Amplifier Pumped with a Diode-Pumped Nd:YLF Laser

The fact that most terrestrial optical fibre communications systems installed around the world today operate at 1.3µm means that a practical fibre amplifier operating in this window is of key strategic importance. The primary contender to provide amplification within this window is the praseodymium doped fluoride fibre amplifier(PDFFA). To date high gains (>30dB) and high saturated output powers ( >200mW)[1] have been achieved but have required the use of large- frame ion lasers as primary optical pump sources. The main difficulty in using semiconductor laser diodes as a pump source is the very high level of pump power needed in this comparatively low efficiency system. One approach that has proved successful in erbium doped fibre power amplifiers is to use a solid state crystal laser to convert the large, but spatially incoherent output from 800nm semiconductor laser diode bars into a TEM00 output which can then be efficiently coupled into the doped fibre[2].

Rare-earth-doped fluoride fibers for optical amplification (Invited Paper)

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.

Twenty-Four Channel FM-TV Transmission Over 200km Using Three Cascaded Praseodymium-Doped Fluoride Fiber Amplifiers

We report the investigation of a twenty-four channel FM-SCM TV transmission system incorporating 200km of conventional fibre, cascaded PDFFAs and a directly modulated DFB-LD transmitter.

Recent progress in high-gain, high-output-power praseodymium-doped fluoride fiber amplifiers for the 1.3-μm spectral region

This paper describes recent progress at BTLabs in high gain high output power praseodymium doped fluoride fiber amplifiers around 1.3 micrometers . Firstly, results using Ti:sapphire pumping will be described; using a high numerical aperture fiber, a net gain of 15 dB was achieved for 200 mW pump power, with a maximum small signal net gain of 29.5 dB. When operated as a power amplifier > 200 mW output power was obtained, with a 30% slope efficiency. This represents the first achievement of efficient operation in both small and large signal regimes. In a separate experiment, a diode-pumped Nd:YLF laser was used to pump a medium numerical aperture fiber. 29.5 dB small signal gain was again achieved, with a saturated output of 250 mW when operated as a power amplifier. We believe that this represents the highest amplified output from such an amplifier, irrespective of pump power. Additional details of the spectral performance of the two amplifiers will be given, for both small signal and large signal operation.