Optical waveguides – Having nonlinear property
Reexamination Certificate
2001-08-13
2004-11-09
Cherry, Euncha P. (Department: 2872)
Optical waveguides
Having nonlinear property
Reexamination Certificate
active
06816656
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a technology for pulse shaping, treatment of non-linearity and monitoring in optical communication networks, preferably in optical fiber links. The present invention is a Continuation-In-Part to a U.S. patent application Ser. No. 09/780,572, filed Feb. 12, 2001.
BACKGROUND OF THE INVENTION
Three basic physical factors, that are known as limiting the achievable bit-rate in optical communication links, are chromatic dispersion, power losses and non-linearity. It is well known that power losses can be compensated by all-optical Erbium-doped or Raman amplifiers periodically installed into a long fiber link. Dispersion can also be compensated by means of periodically inserted relatively short elements with the opposite sign and large absolute value of the dispersion, which makes it possible to have the average dispersion nearly equal to zero. As such dispersion-compensating elements, a specially fabricated fiber, or very short pieces of a fiber with the Bragg grating written on it, may be used.
Nonlinearity, which manifests itself as a nonlinear phase shift accumulated by a light signal while being transmitted via an optical fiber, is generated by the so-called Kerr effect in glass. Owing to this effect, the refraction coefficient of the optical material changes with the intensity of the optical signal according to the following formula:
n=n
0
+K|E|
2
, (1)
where K is the Kerr coefficient.
WO 00/49458-A1 describes a method and an apparatus for compensating optical non-linearity in optical devices and transmission systems. Two second order interactions are cascaded in phase-mismatched second harmonic generation to accumulate a non-linear phase shift of a fundamental wave. The non-linear phase shift can be set to provide a desired amount of non-linearity compensation. Compensation takes place in a compensating medium having a negative effective non-linear refractive index at the design operating conditions of the compensating medium. Compensators incorporating these principles may be incorporated as passive or active components in optical transmitters, repeaters or receivers. Active components may be tuned by varying the operating condition of the compensating medium, for example by controlling temperature or applied stress. Embodiments of the invention use the compensator as pre- or post-compensators in an optical amplifier, to eliminate or reduce self-phase modulation in the optical amplifier that occurs as a result of the Kerr effect.
C. Pare et al. in their paper “Split compensation of dispersion and self-phase modulation in optical communication systems” (Optics Letters, 1 Apr. 1996, Vol 21, No. 7, p. 459-461, Opt. Soc. of America) discuss an idea of alternating the sign of the non-linearity along with the sign of the local dispersion by using a (generally, unspecified) medium exhibiting simultaneously a negative Kerr coefficient and specially tailored dispersion. The authors briefly mention that available non-linear media with a negative Kerr coefficient may be semiconductor wave-guides or media utilizing the cascading mechanism. The authors further point out that, though these materials are only available in the form of short samples with the size ~1 cm, the non-linearity of the media might be strong enough to compensate for kilometers of low fiber non-linearity, using pre-amplification if necessary.
It is necessary to note that their estimate was too optimistic: in fact, the semiconductor wave-guides are not acceptable at all, due to the strong two-photon absorption in them; as for the SHG materials, a realistic estimate shows that, in order to compensate the non-linear phase shift accumulated in a typical span of the fiber ~50 km long, the necessary optical path in the second-harmonic-generating material must be no less than ~5 m.
According to one possible way of the full signal restoration discussed in the paper, the dispersion compensation and negative Kerr effects must occur simultaneously, using, for example, a grating structure created on a non-linear wave-guide with a negative Kerr coefficient. Another possible way proposed in the article was to split the compensation process, i.e., the dispersion compensation can be applied first and then, in the next step, the Kerr-induced non-linear effects would be cancelled.
The SHG media known in the art can be represented, inter alia, by nonlinear optical crystals capable of producing higher harmonics of an optical signal from its fundamental harmonic. Such crystals, for example potassium titanyl phosphate (KTP), potassium dihydrogen phosphate (KDP), barium borate optical crystals (BBO) and the like have found their use in various types of laser generators. Examples of such systems can be found in JP 08201862 A2, U.S. Pat. No. 6,047,011, and others.
Notwithstanding the possible degree of the compensation of the dispersion and nonlinearity, they cannot be completely neglected, as they alter the shape of pulses on which the standard non-return-to-zero (NRZ) format of the data transmission in fiber-optic links is based. Ideally, a pulse representing a “one” bit of data must have a rectangular shape. In reality, the nonlinearity and dispersion convert it into a smoothed signal which is usually close to a Gaussian. The deviation of the data-carrying pulses from the ideal rectangles gives rise to problems produced by overlapping of their extended “tails” belonging to adjacent pulses. The tail overlapping of such tails may give rise to the appearance of parasitic maxima between the “one”-bits, which poses an additional factor limiting the achievable bit-rate, known as inter-symbol interference (ISI). While a partial solution to this problem may be provided by the above-mentioned dispersion compensation, only strong reshaping of the Gaussian pulses (i.e., periodic restoration of the desired near-rectangular form) would provide for a complete solution of the ISI problem.
T. Zhang and M. Yonemura, in the paper “Pulse Shaping of Ultrashort Laser Pulses with Nonlinear Optical Crystals” in Jpn.J.Appl.Phys., Vol. 38 (1999), pp.6351-6358, describe a technique which uses a time-delay optical crystal and a Type-II KDP optical crystal for pulse shaping of a set of two ultrashort pulses carried by the fundamental harmonic. In order to achieve pulse shaping, the interacting pulses must first satisfy the condition that the group velocity of the second-harmonic wave is close to the average group velocity of the two fundamental-harmonic pulses. If this condition is met, pulse shaping is possible by correctly selecting the fundamental intensity, intensity balance, delay time and crystal thickness.
Neither of the above-mentioned references propose a practical method/device for pulse shaping and compensation of non-linearity in fiber-optic links having various lengths, values of the fiber etc.
Further, there is a known technique for monitoring of optical pulse transmission by splitting the pulse signal and obtaining information on the transmission parameters from a minor split out portion of the signal.
OBJECT OF THE INVENTION
It is the objective of the invention to provide a method, a device and a system for pulse shaping, control of non-linearity and/or monitoring in telecommunication fiber links.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, the above object can be achieved by providing a method for handling an optical pulse signal, the handling including at least one of operations for: pulse shaping, treatment of nonlinearity and monitoring, the method comprising steps:
providing a signal handling device capable of performing a cascaded second harmonic generation (SHG) with respect to a particular fundamental harmonic (FH),
selecting an optical path length in said signal handling device, suitable for performing at least one of said operations with respect to an incoming optical pulse signal carried by a wavelength defined by said particular fundamental harmonic (FH),
conveying the incoming optical pulse signal carried by said wavelen
Gutin Michael
Mahlab Uri
Malomed Boris
Browdy and Neimark , P.L.L.C.
Cherry Euncha P.
ECI Telecom Ltd.
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