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An efficient dispersion compensating fiber (DCF) design is proposed based on a coaxial fiber structure, which provides dispersion compensation over a broad wavelength region matching that of an erbium-doped fiber amplifier gain spectrum. The design can provide dispersion coefficient that is nearly twice those of existing DCF designs. Due to the basic nature of the dispersion curve of this design, the total link dispersion cannot only be compensated but also can be flattened.Keywords dispersion compensating fiber, broadband dispersion com pensation, coaxial optical fiber Upgradation of the installed 1310 nm optimized optical fiber links for operation at 1550 nm is attracting considerable interest. The relatively high value of a dispersion coefficient of a 1310 nm optimized fiber operation at 1550 nm can be compensated by employing various schemes of dispersion compensation [1]. Currently, one of the most attractive options to achieve dispersion compensation seems to be the use of a large negative dispersion singlemode fiber which is more commonly known as dispersion compensating fiber [2][3][4]. Furthermore, one of the most attractive options available for increasing the information carrying capacity in a fiber network is to employ wavelength division multiplexing (WDM) technique, i.e., by simultaneously sending signals at multiple wavelengths. This concept has become more attractive because of the availability of the EDFAs, which can provide broadband gain over about 30 nm between 1530 and 1560 nm [5][6][7][8]. In order to use multiple wavelengths in the nonzero dispersion shifted fiber (NDSF), the DCF should be able to provide dispersion compensation at all the operating wavelengths simultaneously. The matched-clad DCFs have a positive dispersion slope similar to that of an NDSF operating in the 1550 nm wavelength 21

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For a sinusoidal modulation of the injection current we present optical and photocurrent spectra computed by a numerical large signal LD model, and compare the outcome to previous analytical and experimental results [2]. We also treat successfully some cases of overmodulation, where temporarily the current falls below threshold. For lower modulation frequencies and below-threshold currents the optical power becomes significantly determined by spontaneous emission; in these cases our present deterministic model fails. I IntroductionThe commercial impact of extended subscriber services with higher transmission capacity seems to be tremendous, if cost-effective means of connecting to subscribers can be employed. A very promising way is to upgrade existing copper or optical fibre cables by hybrid fibre radio networks, where wireless transmission channels employing the microwave and mm-wave frequency range 20. . .70 GHz bridge inexpensively the 'last mile' to a subscriber. The direct modulation of a laser diode (LD) has the potential to be very cost-effective, especially if harmonic upconversion by transmission over a fibre or any other suitable dispersive element is exploited 11) 12). For an optimization of this technique a model of a chirping LD is needed, which we discuss in the following. I1 Laser diode spectrumThe optical output field of a LD is represented by an analytic signal a(t) with complex amplitude A(t), phase cp(t), and total output power P,(t) leaving the resonator (time t, angular optical frequency wg = 27rf0, vacuum speed of light c , vacuum wavelength XO, frequency fo = ./A,, elementary charge e, Planck's constant h, time constant T R from finite resonator mirror reflectivities, photon lifetime r p = 3 ps, effective carrier lifetime 7 ,~ = 3ns, differential quantum efficiency vd = T~/ T R = 0.28, field confinement factor r = 0.2). Further, the normalized quantities Z(t) for the photon number N p ( t ) , Y ( t ) for the carrier number N~c ( t )inside the active volume VI< sz 0.16 x 3 x 420pm3, and n(t) for the injection current I ( t ) (threshold values indexed by S, threshold current 1s = 41 mA, bias pumprate 170 = 3 with corresponding approximate DC values y0 = 1, 2 0 = 2, 20 N Pao = 18mW) are introduced, Further parameters are the normalized gain GN with the normalized transparency carrier number & = 0.82, the normalized differential gain d G~/ t l Y = G N~ M 1 / ( 1 -yt) = 5.56, the threshold gain rGs x l / r p , the gain saturation parameter EG = 10 x 10-17cm3, its normalized version EG = 0.075, the adiabatic angular frequency shift w, = 27r x 8.0GHzI the Henry factor of amplitude-phase coupling Q = 3.5, and the factor of spontaneous emission Q = We adopt the notation X ( t ) = Xo + X l ( t ) for any quantity X ( t ) dependent on time t around a constant operating point Xo; this formulation does not necessarily imply a small signal approximation for X,(t).The quasi-stationary resonance frequency w(t) = WO + w l ( t ) of ,a single-mode field in a laser resonator with length L, effective modal r...

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