Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-12-24
2002-04-23
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06377378
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to methods and systems for suppressing polarization hole burning in rare-earth doped fiber amplifiers. More particularly, the present invention relates to methods and systems for suppressing polarization hole burning using acousto-optic modulation to vary a state of polarization of an input signal.
Long distance optical communication systems have been known to suffer from various polarization dependent effects that may cause a signal-to-noise ratio of the system to lessen. Polarization hole burning (PHB) is one of the polarization dependent phenomena that can severely impair the performance of erbium-doped fiber amplifiers (EDFAs) located in optical fiber communication systems. PHB occurs when a strong, polarized optical signal is launched into an EDFA and causes anisotropic saturation of the amplifier. This effect, which is related to the population inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same polarization as the saturating signal. Thus, PHB causes a signal having a state of polarization (SOP) orthogonal to the saturating signal to have a gain greater than that of the saturating signal.
In a chain of saturated EDFAs, amplified spontaneous emission (ASE) noise can accumulate faster in the polarization orthogonal to a saturating information signal than along the polarization parallel to the signal. ASE orthogonal to a saturating signal will accumulate at each amplifier stage of the transmission line. The build-up of orthogonal ASE reduces the signal-to-noise ratio (SNR) of the optical transmission system, thus causing possible errors in the received data stream. Accordingly, it is desirable to reduce the effects of PHB in amplified systems in order to maintain a system with good SNR characteristics.
Operating EDFAs in gain compression helps to cause the undesired PHB effect. The degree of gain compression Cp indicates the difference of gain of the amplifier in its operative condition of propagation of a signal with low optical power (i.e., a non-saturating signal experiencing maximum gain, called “Go”) with respect to the value experienced by the optical signal in the power level condition at which it is operating (G). An amplifier's operating gain in decibels can be measured with a saturating signal of input power Si as the following:
G=So−Si,
(1)
where So is the saturated output power. Accordingly, the amount of gain compression equals the following:
Cp=Go−G.
(2)
The gain in the orthogonal polarization, on the other hand, can be measured using a probe signal with an input polarization orthogonal to the saturating signal as the following:
Po−Pi=G+&Dgr;G,
(3)
Pi and Po being the input and output power of the probe signal. In equation (3), &Dgr;G corresponds to the PHB value.
Moreover, the amount of PHB increases as the amplifier goes deeper into gain compression.
FIG. 1
is a graph of experimental measurements showing the relationship between the amount of gain compression and the amount of PHB in an EDFA. As shown in this graph, the amount of PHB is only about 0.08 dB for a single EDFA that operates with 3 dB of gain compression. However, as the gain compression increases, so does the PHB. When the EDFA operates in a saturated condition with Cp equal to about 9-10 dB, the PHB is more significant and quantifiable at around 0.2 dB per EDFA.
Furthermore, the amount of PHB in an EDFA depends on the degree of polarization (DOP) of the saturating signal passing through the amplifier.
FIG. 2
is a graph of experimental results on an EDFA operating at 10 dB of gain compression. As can be seen from this graph of
FIG. 2
, as the degree of polarization of the saturating signal diminishes from 100%, the variation of gain induced by PHB also diminishes. This fact illustrates that the deleterious effects from PHB may be lessened by varying the state of polarization. PHB can be reduced by scrambling the SOP of the transmitted optical signal at a rate that is much higher than 1/t
s
, where t
s
is the anisotropic saturation time. Because an EDFA takes about 0.5 msec to reach a gain stable condition after variation of a signal's SOP, the signal's SOP should be scrambled at about 10 kHz or more in order to overcome the PHB phenomenon.
The literature has proposed several arrangements for mitigating PHB effects in optical communication systems. EP 615,356 and U.S. Pat. No. 5,491,576 disclose a technique for reducing nonlinear signal degradation by simultaneously launching two optical signals of different wavelengths, comparable power levels, and substantially orthogonal relative polarizations into the same transmission path. The resulting overall transmitted signal is therefore essentially unpolarized, and the impact of detrimental polarization dependent effects within the transmission system are reportedly minimized. The combined signal is modulated by a polarization independent optical modulator so that both wavelength components of the combined signal carry the same data, or each wavelength path is separately modulated prior to their combination. Similar disclosure of a system that launches two signals of different wavelengths can be found in Bergano et al., “Polarization Hole-Burning in Erbium-Doped Fiber-Amplifier Transmission Systems,” ECOC '94, pp. 621-628.
U.S. Pat. No. 5,107,358 describes a method and apparatus for transmitting information and detecting it after propagation through a waveguide by means of a coherent optical detector. In particular,
FIG. 3
shows a transmitter comprising an optical source generating a single carrier signal which is fed to a modulator. An optical splitter generates two versions of the modulated signal. The first version is fed to a first polarization controller, while the second version is fed via a frequency shifting circuit to a second polarization controller. The polarization of this signal is adjusted by the second controller to be orthogonal to the polarization of the signal from the first controller. The orthogonally polarized signals are then combined by a polarization selective coupler for transmission.
It should be understood that in all the examples described in the '358 patent, the two optical carrier frequencies will typically be separated by two to three times the bit rate in Hertz. Applicants have observed that by superposing an optical signal with a version of the same having orthogonal polarization and being shifted in frequency by two to three times the bit rate, an optical signal with a bandwidth of the same magnitude (two to three times the bit rate) is obtained. The bandwidth of the filters to be used at the receiver must be equal to or greater than the signal bandwidth. Due to this large filter bandwidth, the noise at the receiver, in the case of a long distance amplified optical telecommunication system, would be too high to allow a good signal reception, particularly for a bit rate greater than 1 Gbit/s.
It is also known from, for example, U.S. Pat. No. 5,327,511 and Heismann et al., “Electro-optic polarization scramblers for optically amplified long-haul transmission systems,” ECOC '94, pp. 629-632, to generate a carrier signal having a single wavelength, modulate the carrier signal with data, and then send the modulated carrier signal through a polarization modulator or scrambler to help alleviate the effects of polarization hole burning. These documents disclose the use of a lithium niobate-based electro-optic modulator with a single path for passing the carrier wavelength and modulating its polarization at, for example, modulation frequencies of 40 kHz and 10.66 GHz. These polarization modulators or scramblers create highly randomized polarization states for the signal. Such devices affect the output polarization according to a control signal and use relatively high levels of power.
From Electronics Letters, Vol. 30, No. 18, p. 1500-1501, Sep. 1, 1994 an acousto optical Ti:LiNbO
3
device is known whose
Bentivoglio Ravasio Filippo
Frascolla Massimo
Ottolenghi Paolo
Schmid Steffen
Finnegan, Henderson, Farabow, Garrett & Dunnner, L.L.P.
Pascal Leslie
Pirelli Cavi e Sistemi S.p.A.
Singh Dalzid
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