SINGLE-MODE OPTICAL FIBER HAVING NEGATIVE CHROMATIC DISPERSION

Abstract

A single-mode optical fiber that reduces the chromatic dispersion of an optical pulse due the laser chirp in an optical communication system operating in the O-band has a cable cutoff wavelength less than 1250 nm, a zero-dispersion wavelength greater than 1334 n, and a nominal mode field diameter of said fiber at 1310 nm between 8.6 and 9.5 microns.

Claims

1. A single-mode optical fiber that reduces the chromatic dispersion of an optical pulse due the laser chirp in an optical communication system operating in the O-band, wherein properties of the fiber comprise: a cable cutoff wavelength less than 1250 nm; a zero-dispersion wavelength greater than 1334 nm; a nominal mode field diameter of said fiber at 1310 nm between 8.6 and 9.5 microns.

2. A single-mode fiber according to claim 1, wherein a chromatic dispersion is negative for all transceiver operating wavelengths in the optical communications O-band.

3. A fiber according to claim 1, wherein the cutoff wavelength reduces high order modes in said single-mode fiber, so that coherent multipath interference at an optical interface is reduced.

4. A cable comprising at least one single-mode optical fiber according to claim 1.

5. A single-mode optical fiber that minimizes inter-symbol interference and coherent multipath interference penalties in the 1310 nm operating window using directly modulated semiconductor lasers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1a shows a Fabry Perot laser spectrum showing multiple longitudinal nodes

[0013] FIG. 1b shows a distributed feedback laser narrow linewidth spectrum.

[0014] FIG. 2 shows IEEE 802.3 Ethernet SMF Wavelength Grids.

[0015] FIG. 3 is a graph showing pulse delay as a function of wavelength.

[0016] FIG. 4 shows a plot of the chromatic dispersion of a typical SMF over the wavelength range of 1250 nm to 1370 nm.

[0017] FIG. 5 shows the plot of a SMF with a shifted zero dispersion wavelength.

[0018] FIG. 6 is a plot showing the relationship between multi-path interference and cutoff wavelength.

DETAILED DESCRIPTION OF THE INVENTION

[0019] An optical fiber in accordance to the present invention has a zero-dispersion wavelength shifted to a longer wavelength compared to industry Standards unshifted single-mode fiber Types IT U-G.652, and/or IT U-G.657, where the ZDW is specified to be between 1302 nm and 1322 nm. A fiber compliant with the present invention has a ZDW greater than 1334 rnm, so that essentially all transmitted operating wavelengths in the 1310 nm window undergo a negative chromatic dispersion when propagating through said optical SMF channel. A negative dispersion compensates for the chromatic dispersion due to laser chirp, thereby reducing the signal pulse-width and hence, the dispersion penalty of the channel.

[0020] In FIG. 2, we plot the spectral grids and wavelength ranges for 8 SMF laser transceiver options specified in IEEE 802.3 Ethernet Standards for data rates ranging from 25 Gb/s to 400 Gb/s. Transceivers can include 1, 4, or 8 discrete signal wavelengths. For these Ethernet specified transceivers, the maximum operating wavelength is 1337.5 nn, which is utilized in the 200GBASE-FR4 transceiver. We can calculate the required ZDW that would provide a negative dispersion for all transceivers shown in FIG. 2, as follows.

[0021] The chromatic dispersion is caused by the wavelength dependence of the optical fiber and includes two components, material dispersion given by,

dn.sub.1/dλ≠0

where n.sub.1 is the core refractive index, and profile (or waveguide) dispersion given by,

dΔ/dλ≠0

[0022] where, Δ is the ratio between the core radius and wavelength. We can compute the dispersion by numerically fitting pulse delay data as a function of wavelength as shown in FIG. 3, using the following least-mean-square-error criterion,

τ(λ)=A+Bλ.sup.2+Cλ.sup.2.

where, τ(λ) is the spectral group delay as a function of wavelength and A, B, and C are fitted parameters.

[0023] The chromatic dispersion coefficient D(λ), is defined as,

[00001]$D\left(\lambda \right)\equiv \frac{d\tau }{d\lambda }$$\mathrm{Hence},D\left(\lambda \right)=\frac{d}{d\lambda }\tau \left(\lambda \right)=2\left(B\lambda -C{\lambda }^{-3}\right)$

Using the fitted parameters B and C, we can compute the “zero-dispersion wavelength,” λ.sub.0, where, D(λ)=0.

[00002]${\lambda }_0={\left(\frac{C}{B}\right)}^{1/4}$

Solving for the parameter C in terms of λ.sub.0 we get,

C=Bλ.sub.0.sup.4

The dispersion slope, S(λ), is the first derivative of the dispersion with respect to wavelength, i.e.,

[00003]$S\left(\lambda \right)=\frac{d}{d\lambda }D\left(\lambda \right)=\frac{d}{d\lambda }\left[2\left(B\lambda -C{\lambda }^{-3}\right)\right]=2B+6C{\lambda }^{-4}$

At the zero-dispersion wavelength, the dispersion slope is represented by S.sub.0, hence,

[00004]$S_0=S\left({\lambda }_0\right)=8B\mathrm{And},B=\frac{S_0}{8}$

rewriting D(λ) in terms of λ.sub.0 and S.sub.0, we get for the dispersion:

[00005]$D\left(\lambda \right)=\frac{S_0}{4}\lambda \left(1-\frac{{\lambda }_0^4}{{\lambda }^4}\right)$

[0024] In FIG. 4, we plot the chromatic dispersion for the exemplary ITl-G652D SMF over the wavelength range of 1250 nm to 1370 nm.

[0025] For a nominal Standards compliant SMF with a specified ZDW between 1302 mu and 1322 nm, analysis shows a positive dispersion coefficient for wavelengths longer than 1310 nm, and consequently, dispersion due to laser chirp is exacerbated. By shifting the dispersion curve, as shown in FIG. 5, so that the ZDW is approximately 1340 nm, all transceiver wavelengths (illustrated by the green horizontal line) will experience a negative dispersion coefficient thereby compensating for chirp.

[0026] According to the present invention, said SMF has a ZDW greater than 1334 nm so that all optical transmission signals for a given applications such as IEEE 802.3 Ethernet, undergo a negative chromatic dispersion to compensate for laser chirp. Hence, the ZDW of said fiber for this application, where the maximum wavelength is 1337.5 nm, should be greater than 1347.5 nm with a tolerance of ±10 nm, typical of current industry standards limits for SMF.

[0027] A second optical penalty in single-mode channels containing short fiber segments, such as patch cords, is coherent multi-path interference (MPI). MPI results when an optical pulse travels to the detector via two or more optical paths. Under these conditions, the wave components arrive at the receiver detector with a relative phase shift and consequently result in destructively interfere at the receiver detector causing signal noise. Spectral loss measurements in single-mode fiber show a correspondence between MPI and fiber cutoff wavelength, where for high cutoff, the generation of higher order fiber modes (HOM) increase the channel MPI. A fiber with a specific core diameter D, transmits light in a single-mode only at the wavelengths longer than the cutoff wavelength λ.sub.c, given by,

[00006]${\lambda }_c=\frac{\pi D\sqrt{n_0^2-n_1^2}}{2.4}$

[0028] where n.sub.0 is the core refractive index, and n.sub.1 is the cladding refractive index.

[0029] In FIG. 6, we plot empirical data showing the relationship between cutoff wavelength and MPI, where for a given operating wavelength the MPI is higher for longer cutoff wavelengths. For a fiber of diameter D having a cutoff close to the operating wavelength, the MPI is within the transition region between the two extreme conditions shown in FIG. 6, where there is roughly a linear relation between cutoff wavelength and MPL, Using this data, we can relate MPI to the difference Δλ between the operating wavelength λ and cutoff λ.sub.c, where,

[00007]$\Delta \lambda =\left(\lambda -{\lambda }_c\right)$$\mathrm{and},\mathrm{MPI}\propto \frac{1}{\Delta \lambda }$

[0030] Hence, empirical data shows MPI be reduced with a shorter cutoff for O-Band Ethernet applications.