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Optics Letters

Pump-induced dispersion of erbium-doped fiber measured by Fourier-transform spectroscopy

Robert K. Hickernell, K. Takada, Makoto Yamada, Makoto Shimizu, M. Horiguchi

Published Jan 1, 1993• 25 citations
Publisher

Abstract

We report the measurement of group index and dispersion in an erbium-doped fiber amplifier by Fourier transformation of low-coherence interferograms. In a germania-codoped fiber whose background dispersion was - 14 ps/(km nm) we measured resonant gain-induced changes as high as 9 and -12 ps/(km nm) near 1.536 microm. The interferometric measurements agree with calculations based on a Kramers-Kronig transformation of absorption and emission spectra.

DOI
10.1364/ol.18.000019

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Pump-induced refractive index modulation and dispersions in Er/sup 3+/-doped fibers

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A novel measurement system provides determination of pump induced phase shifts in erbium doped fibers with an accuracy of /spl sim//spl pi//20. Using this system, a systematical analysis of the pump induced modulation of the refractive index and dispersions for a signal at 1550 nm and a pump at 980 nm is reported. The analysis contains measurements of pump induced refractive index changes as function of wavelength, pump power, and doping concentration. A model taking account of the contribution to the refractive index changes from optical transitions between /sup 4/I/sub 15/2/ states and /sup 4/I/sub 13/2/ states in Er/sup 3+/ yields good agreement to experimental results apart from a wavelength independent offset. The offset is interpreted to originate from high energetic optical transitions. The results show that for a large refractive index modulation, a short and highly doped fiber should be used with limited amplified spontaneous emission effect. In optical communication systems comprising erbium doped fiber amplifiers, a tradeoff between dispersion and amplification must be made.

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Evaluation of chromatic dispersion in erbium-doped fibers

D.S. Gasper, Paul F. Wysocki, William A. Reed +1 more

Transoceanic fiber optic systems that use erbium doped fiber amplifiers(EDFAs) require careful tailoring of chromatic dispersion along the length of the span. One of the parameters of interest to system designers is the additional chromatic dispersion introduced by the erbium ions in the EDFA. The effect of resonant chromatic dispersion due to erbium is also evident in the performance of short length fiber devices such as soliton lasers. Several methods have been used in the past to measure erbium-induced chromatic dispersion. Specifically, Fourier transform spectroscopy and the use of complex atomic susceptibility have been successfully demonstrated. In this paper, we evaluate the complex atomic susceptibility of these fibers with different dopant levels and derive the total dispersion (waveguide and material) of these fibers as a function of wavelength in the 1550 nm range.>

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Study of the complex atomic susceptibility of erbium-doped fiber amplifiers

E. Desurvire

A study of the complex atomic susceptibility of erbium-doped fiber amplifiers operating near the 1.53- mu m transition (/sup 4/I/sub 13/2/-/sup 4/I/sub 15/2/) developed from a semiclassical theoretical description is presented. Expressions for the emission cross section sigma /sub e/( lambda ) and absorption cross section sigma /sub a/( lambda ) of erbium:glass as a quasi-three-level laser system with Stark-split sublevel manifolds are derived. The results are used to calculate the cross sections of aluminosilicate erbium-doped fibers at low temperatures (T=4.2, 77 K) and at room temperature. It is demonstrated that an expression for the gain coefficient of the fiber amplifier can be derived from the expression of the susceptibility. Using Kramers-Kronig relations, an expression for the refractive index change in the fiber is derived. The theoretical spectral gain profile of the fiber amplifier and concurrent refractive index changes are analyzed for different pumping regimes.>

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Excited-state absorption and gain in erbium-fiber amplifiers between 1.05 and 1.35 mu m

Mark C. Farries

Measurements of the transmission of an erbium-doped fiber amplifier in the wavelength range 1.05-1.35 mu m are reported. Excited-state absorption at 1.14 mu m and gain at 1.24 mu m are observed when the amplifier is pumped at 528 nm. There is good agreement between the measured excited-state absorption and gain and that predicted from the energy bands. An excited state absorption between 1.5 and 1.75 mu m is predicted from absorption measurements.>

IEEE Photonics Technology Letters•Jul 1, 1991•3 citations
We report the measurement of group index and dispersion in an erbium-doped fiber amplifier by Fourier transformation of low-coherence interferograms. In a germania-codoped fiber whose background dispersion was - 14 ps/(km nm) we measured resonant gain-induced changes as high as 9 and -12 ps/(km nm) near 1.536 microm. The interferometric measurements agree with calculations based on a Kramers-Kronig transformation of absorption and emission spectra.
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.
This paper reports the measurement and analysis of signal power-induced changes in the refractive index spectrum of an aluminosilicate erbium-doped fiber amplifier (EDFA). For a constant 980 nm pump power of 11 mW, a 1553 nm signal power of 4.6 mW caused a peak-to-trough change in refractive index of (3.8/spl plusmn/0.4)/spl times/10/sup -8/ across the EDFA gain spectrum. A Kramers-Kronig transform of the signal-induced change in absorption coefficient agreed well with the measured change in refractive index. This result conflicts with the behavior of the system predicted by a homogeneously broadened two-level model and suggests limitations in the representation of the Er/sup 3+/:glass system by a discrete set of homogeneously broadened transitions.
A novel measurement system provides determination of pump induced phase shifts in erbium doped fibers with an accuracy of /spl sim//spl pi//20. Using this system, a systematical analysis of the pump induced modulation of the refractive index and dispersions for a signal at 1550 nm and a pump at 980 nm is reported. The analysis contains measurements of pump induced refractive index changes as function of wavelength, pump power, and doping concentration. A model taking account of the contribution to the refractive index changes from optical transitions between /sup 4/I/sub 15/2/ states and /sup 4/I/sub 13/2/ states in Er/sup 3+/ yields good agreement to experimental results apart from a wavelength independent offset. The offset is interpreted to originate from high energetic optical transitions. The results show that for a large refractive index modulation, a short and highly doped fiber should be used with limited amplified spontaneous emission effect. In optical communication systems comprising erbium doped fiber amplifiers, a tradeoff between dispersion and amplification must be made.
Transoceanic fiber optic systems that use erbium doped fiber amplifiers(EDFAs) require careful tailoring of chromatic dispersion along the length of the span. One of the parameters of interest to system designers is the additional chromatic dispersion introduced by the erbium ions in the EDFA. The effect of resonant chromatic dispersion due to erbium is also evident in the performance of short length fiber devices such as soliton lasers. Several methods have been used in the past to measure erbium-induced chromatic dispersion. Specifically, Fourier transform spectroscopy and the use of complex atomic susceptibility have been successfully demonstrated. In this paper, we evaluate the complex atomic susceptibility of these fibers with different dopant levels and derive the total dispersion (waveguide and material) of these fibers as a function of wavelength in the 1550 nm range.>
A study of the complex atomic susceptibility of erbium-doped fiber amplifiers operating near the 1.53- mu m transition (/sup 4/I/sub 13/2/-/sup 4/I/sub 15/2/) developed from a semiclassical theoretical description is presented. Expressions for the emission cross section sigma /sub e/( lambda ) and absorption cross section sigma /sub a/( lambda ) of erbium:glass as a quasi-three-level laser system with Stark-split sublevel manifolds are derived. The results are used to calculate the cross sections of aluminosilicate erbium-doped fibers at low temperatures (T=4.2, 77 K) and at room temperature. It is demonstrated that an expression for the gain coefficient of the fiber amplifier can be derived from the expression of the susceptibility. Using Kramers-Kronig relations, an expression for the refractive index change in the fiber is derived. The theoretical spectral gain profile of the fiber amplifier and concurrent refractive index changes are analyzed for different pumping regimes.>