Inhomogeneous broadening to modify the gain of an optical...

Details Inhomogeneous broadening to modify the gain of an optical... Inhomogeneous broadening to modify the gain of an optical...

1999-02-05

2002-03-12

Tarcza, Thomas H. (Department: 3662)

Optical: systems and elements

Optical amplifier

Correction of deleterious effects

C359S341300

Reexamination Certificate

active

06356385

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical amplifiers, and more specifically, to an apparatus and method of amplifying optical signals at different wavelengths such that the optical signals experience substantially equal gain.
2. Description of the Related Art
Commercially available erbium-doped fiber amplifiers (EDFAs) currently have gain over a large optical bandwidth (up to about 50 nm in silica-based fibers). Over this bandwidth, the gain may depend strongly on the wavelength of the input signal. For many applications, especially long-haul fiber communications, however, it is highly desirable to operate with wavelength-independent gain. To take advantage of the enormous fiber bandwidth, signals with different wavelengths falling within the gain bandwidth of the EDFA are carried simultaneously on the same fiber bus. If these signals experience different gains, they will have different powers at the output of the bus. This imbalance becomes more acute as the signals pass through each successive EDFA, and can be significant for very long haul distances. For example, at the output end of a transoceanic bus involving dozens of EDFAs, signals experiencing a lower gain per EDFA might carry tens of dB lower power than signals experiencing higher gain. For digital systems, the difference in signal power levels must not exceed 7 dB, or the lower power signals will be too noisy to be useful. Flattening the gain of the EDFAs would eliminate this problem and produce amplifiers that can support a considerable optical bandwidth and thus a higher data rate. Because the projected world demand for EDFAs is extremely large, developing methods to flatten the gain of amplifiers while retaining high power efficiency has been and continues to be very important.
Several methods have been developed over the past few years to produce EDFAs with as flat a gain over as broad a spectral region as possible. A first method is to adjust the parameters of both the fiber (erbium concentration, index profile, nature and concentration of the core codopants) and the pump (power and wavelength). This method can produce gains that are relatively flat (±1-2 dB), but only over a spectral region having a spectral width on the order of 10 nm, which is too limited for most applications.
Another method is to replace each EDFA by a combination of two concatenated fiber amplifiers, in which the two amplifiers have different respective gain dependencies on signal wavelength. These dependencies are designed to compensate each other and produce a fiber amplifier combination having gain that is nearly wavelength independent over a wide spectral region. (See, for example, M. Yamada, M. Shimizu, Y. Ohishi, M. Horigushi, S. Sudo, and A. Shimizu, “Flattening the Gain Spectrum of an Erbium-Doped Fibre Amplifier by Connecting an Er
3+
-Doped SiO
2
—Al
2
O
3
Fibre and an Er
3+
-doped Multicomponent Fibre,”
Electron. Lett
., vol. 30, no. 21, pp. 1762-1765, October 1994.) This has been accomplished by using fibers having different hosts (e.g., a fluoride and a silica fiber) and with an EDFA combined with a Raman fiber amplifier.
A third gain equalization method is to add a filter at the signal output end of the Er-doped fiber, in which the filter introduces loss at those portions of the spectrum exhibiting higher gain. This approach has been demonstrated using filters made from a standard blazed fiber grating. (See, for example, R. Kashyap et al., “Wideband Gain Flattened Erbium Fibre Amplifier Using a Photosensitive Fibre Blazed Grating,”
Electron. Lett
., vol. 29, pp. 154-156, 1993.) This approach has also been demonstrated using filters from long-period fiber gratings. (See, for example, A. M. Vengsarkar et al., “Long-Period Fiber-Grating-Based Gain Equalizers,”
Opt. Lett
., vol. 21, pp. 336-338, March 1996.)
A fourth method is gain clamping. With this approach, the EDFA is placed in an optical resonator where it is forced to lase. In a laser cavity above threshold, at a given laser wavelength, the round-trip gain is equal to the round-trip loss, irrespective of the pump power. (See, for example, Y. Zhao, J. Bryce, and R. Minasian, “Gain Clamped Erbium-doped Fiber Amplifiers—Modeling and Experiment,”
IEEE J. of Selected Topics in Quant. Electron
., vol. 3, no. 4, pp. 1008-1011, August 1997.)
In the gain clamping experiment of Zhao et al., the resonator was made of two fiber gratings that exhibit high reflectivity only over a very narrow bandwidth around a particular wavelength &lgr;
0
(and little reflectivity at other wavelengths within the gain spectrum of the erbium-doped fiber), so that lasing took place only at this wavelength &lgr;
0
. The selection of &lgr;
0
greatly affects the spectral shape of the EDFA gain. By selecting the proper laser wavelength &lgr;
0
(1508 nm in their experiment), the gain spectrum can be relatively flat over a fairly broad region. Furthermore, the gain at &lgr;
0
is clamped to the value of the cavity loss at this wavelength for any pump power above threshold. If the gain is homogeneously broadened, the gain at other wavelengths also remains independent of pump power (assuming the pump power is above threshold).
Another way to flatten the gain of a gain-clamped EDFA is to rely on the inhomogeneous broadening of the laser ions. Although reference is made herein to “laser ions,” the discussion can be applied to any particle that produces lasing via stimulated emission, such as ions, atoms, and molecules. In a laser medium that is purely homogeneously broadened, all the ions exhibit the same absorption and emission spectra. When such a material is pumped below laser threshold, the round-trip gain is lower than the laser resonator round-trip loss at all frequencies across the laser gain spectrum, as illustrated in
FIG. 1A
, where it was assumed without loss of generality that the round-trip loss is frequency-independent across the gain spectral region. When pumped just above threshold, it begins to oscillate at the wavelength &lgr;
1
, that satisfies the condition gain=loss (see FIG.
1
B). As the pump power is increased further (FIG.
1
C), the condition gain=loss continues to be satisfied at &lgr;
1
, i.e., the gain at &lgr;
1
remains constant. This can be understood from a physical point of view as follows. When the pump power is increased, the population inversion increases, which produces more intense laser emission. While circulating through the fiber, this larger laser signal depletes the population inversion via stimulated emission just enough so that the gain remains equal to the loss. Further, since the broadening is homogeneous, all ions contribute equally to the gain at &lgr;
1
, and therefore, the gain spectrum does not change. As a corollary, the laser wavelength (&lgr;
1
) and the laser linewidth also remain the same (see FIG.
1
C), i.e., they are independent of pump power. This is the basis for the gain stabilization method mentioned earlier.
In a laser medium that is strongly inhomogeneously broadened, on the other hand, not all ions exhibit the same absorption and emission spectra. One reason for this behavior is that not all physical sites where the laser ions reside are identical. For example, a laser ion can reside next to a silicon ion, an oxygen ion, or an aluminum ion in the case of an aluminum-doped silica-based host. Laser ions residing at identical sites (e.g., all the laser ions next to a Si ion) will exhibit the same absorption and emission spectra, i.e., they will behave homogeneously with respect to each other. On the other hand, laser ions residing at different sites, e.g., one residing next to a Si ion and another laser ion residing next to an Al ion, will exhibit different absorption and emission spectra, i.e., they will behave inhomogeneously with respect to each other. In the case of inhomogeneous broadening, the laser medium can thus be thought of as a collection of subsets of laser ions. Ions within a given subset behave homogeneously, while ions in different

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