Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-02-19
2001-12-11
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C359S199200
Reexamination Certificate
active
06330383
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical dispersion compensation and optical pulse manipulation, and more specifically, to devices and systems having an optical grating capable of causing wavelength-dependent delays.
BACKGROUND
Dispersion in optical waveguides such as optical fibers causes optical waves of different wavelengths to travel at different speeds. One parameter for characterizing the dispersion is group velocity which is related to the derivative of the propagation constant of an optical wave with respect to frequency. The first-order group velocity dispersion is typically expressed as a change in light propagation time over a unit length of fiber with respect to a change in light wavelength. For many fibers used in telecommunication, the first-order group velocity dispersion is on the order of 10 ps
m/km at 1550 nm.
In many applications, an optical signal is composed of spectral components of different wavelengths. For example, a single-frequency optical carrier may be modulated in order to impose information on the carrier. Such modulation generates modulation sidebands at different frequencies from the carrier frequency. For another example, optical pulses, which are widely used in optical data processing and communication applications, contain spectral components in a certain spectral range. The dispersion effect may cause adverse effects on the signal due to the different delays on the different spectral components.
Dispersion in particular presents obstacles to increasing system data rates and transmission distances without signal repeaters in either single-channel or wavelength-division-multiplexed (“WDM”) fiber communication systems. Data transmission rates up to 10 Gbit/s or higher may be needed in order to meet the increasing demand in the marketplace. Dispersion can be accumulated over distance to induce pulse broadening or spread. Two adjacent pulses in a pulse train thus may overlap with each other at a high data rate. Such pulse overlapping can cause errors in data transmission.
One way to reduce the dispersion effect in fibers is to implement a fiber grating with linearly chirped grating periods. The resonant wavelength of the fiber grating changes with the position due to the changing grating period. Therefore, different spectral components in an optical signal are reflected back at different locations and thus have different delays. Such wavelength-dependent delays can be used to reduce the accumulated dispersion in a fiber link.
SUMMARY
The present disclosure describes a nonlinearly chirped grating having a mechanism to adjust the Bragg phase-matching conditions. The dispersion of such a nonlinearly chirped grating can be dynamically adjusted to produce a desired dispersion with desired relative delays among different spectral components in a controllable manner.
One embodiment of the nonlinearly-chirped grating includes a grating that has an effective index n
neff
(x) and the grating period &lgr;(x) are configured to produce a grating parameter n
neff
(x)&lgr;(x) as a nonlinear function of the position along the fiber optic axis. Such a grating reflects optical waves satisfying a Bragg condition of &lgr;(x)=2n
neff
(x)&lgr;(x). A single Bragg reflection band is generated where the bandwidth is determined by the chirping range of the grating parameter n
neff
(x)&lgr;(x).
A grating tuning mechanism may be implemented by using a grating control unit to control either the effective index n
neff
(x) or the grating period &lgr;(x). This allows for adjustment of the grating parameter n
neff
(x)&lgr;(x) and thus to the relative delays for signals at different wavelengths within the bandwidth of the reflection. A transducer, e.g., a piezoelectric element, may be used as the control unit to compress or stretch the overall length of the grating in order to produce a tunable dispersion profile. A magnetostrictive element may also be used to change the grating length according to an external control magnetic field. If the grating material is responsive to a spatially-varying external control field such as an electric field, an electromagnetic radiation field, or a temperature field along the grating direction, a control unit capable of producing such conditions can be used to change effective index of refraction and to produce a tunable dispersion profile.
In addition, the frequency response of a nonlinearly chirped grating may be tuned by using an acoustic wave propagating along the grating direction. The acoustic wave induces additional modulation sidebands in the frequency response of the grating. Such modulation sidebands are displaced from the baseband by a frequency spacing that is dependent on the frequency of the acoustic wave. Therefore, an adjustable dispersion can be achieved by tuning the frequency of the acoustic wave.
The present disclosure also provides a sampled nonlinearly-chirped grating for changing relative time delays of signals at different wavelengths. This sampled nonlinearly-chirped grating includes a wave-guiding element having a refractive index that varies along its optic axis according to a multiplication of a first spatial modulation and a second special modulation. The first spatial modulation is an oscillatory variation with a nonlinearly-chirped period along the optic axis. The second spatial modulation is a periodic modulation with a period different than the nonlinearly-changing period.
The first and second modulations effect first and second gratings that spatially overlap each other in the wave-guiding element along its optic axis. The first grating a nonlinearly-chirped grating. The second grating may have a grating period greater than the first grating. The first grating and second grating couple with each other and operate in combination to produce a plurality of reflection bands at different wavelengths and with a bandwidth determined by the first grating.
A nonlinearly-chirped grating can be further configured to change relative time delays of two different polarization states in an optical signal. One embodiment of such a grating comprises a wave-guiding element formed of a birefringent material that exhibit different refractive indices for the two polarization states. A nonlinearly-chirped grating is formed in the wave-guiding element along its optic axis and has a varying grating period that changes as a monotonic nonlinear function of a position. The grating operates to reflect two polarization states of an input optical signal at different locations along the optic axis to cause a delay between said two polarization states.
One aspect of the nonlinearly-chirped gratings is dispersion compensation. A nonlinear chirped grating can be disposed at a fiber link to reduce the effects of the dispersion. The dispersion produced by such a grating is actively tunable to compensate for varying dispersion in a fiber link which includes a dispersion analyzer and a feedback control. This tunability can be advantageously used in a dynamic fiber network in which communication traffic patterns may change over time. For example, a given channel may be originated at different locations in the network from time to time so that the accumulated dispersion of that given channel in a specific fiber link is a variable. Therefore, the dispersion compensation required for that fiber link needs to change accordingly. Also, the operating conditions for point-to-point transmission may also change, resulting in variations in the accumulated dispersion for signals in a fixed fiber link.
Another aspects of the nonlinearly-chirp gratings include dispersion slope compensation, polarization mode dispersion, chirp reduction in directly modulated diode lasers, and optical pulse manipulation.
These and other embodiments, aspects and advantages of the invention will become more apparent in light of the following detailed description, including the accompanying drawings and appended claims.
REFERENCES:
patent: 5450427 (1995-09-01), Fermann et al.
patent: 5499134 (1996-03-01), Galvanauskas et al.
patent: 5511083 (1996-04-
Cai Jin-Xing
Feng Kai-Ming
Khosravani Reza
Lee Sanggeon
Peng Jiangde
Connelly-Cushwa Michelle R.
Fish & Richardson P.C.
Lee John D.
University of Southern California
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