Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-09-04
2003-04-15
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06549702
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and systems for generating a broadband spectral continuum, methods of making the systems and pulse-generating systems utilizing same.
BACKGROUND ART
The bandwidth demands projected for the near future will require multi-Tb/s TDM/WDM systems that are based on numerous high bit-rate channels. A supercontinuum (SC) generated in optical fiber is a convenient source for such systems because it provides a very broad bandwidth (>200 nm) that can be sliced, as required, into short pulses at individual WDM channels as illustrated in FIG.
1
. The pulse trains in each channel have the repetition rate of the source laser and, when the spectrum is flat and of uniform phase, pulse widths that are transforms of the spectral filter function. These features make the continuum source an attractive alternative to numerous discrete laser diodes, particularly for high bit-rate OTDM systems, since a single short pulse source provides chirp-free, ultra-short pulses simultaneously for multiple wavelength channels. Also, the SC source requires the relatively simpler stabilization of passive filters rather than of the operating wavelengths of multiple laser diodes. Finally, since the SC has a continuous high power spectral density outside the erbium gain band compared with thermal sources or superluminescent LEDs, it is also useful for characterizing passive components and amplifiers in new spectral regions.
Work in the 1980's first explored continuum generation in fibers. Working in the anomalous group-velocity dispersion (GVD) regime of fibers, several groups generated short pulses with extremely broad bandwidth, which they attribute to stimulated Raman scattering. Using 100-psec pulses from a Nd:YAG laser, Gouveia-Neto et al. obtained a spectrum between 1.32 and 1.54 &mgr;m.
By frequency doubling a 2.79 &mgr;m YSGG:Cr
3+
:Er
3+
laser, Vodop'yanov et al. generated 100-200 fsec pulses between 1.5 and 1.7 &mgr;m.
On the other hand, Beaud et al. witnessed pulse breakup when 0.83-psec pulses from a 1.37 &mgr;m dye laser were passed through a fiber.
Blow et al. tried to reconcile the difference between broad bandwidth and pulse breakup by theorizing that the frequency shift can be suppressed by Raman gain.
Islam et al. developed a model of the femtosecond distributed soliton spectrum (FDSS) that does not rely on stimulated Raman scattering at its peak (~440 cm
−1
below the pump frequency) but did explain all three of the above experiments. In particular, Islam et al. generated pulses with duration larger than ~100-fsec between 1.55 and 1.85 &mgr;m in a fiber pumped by a color center laser. The experiments and computer simulations showed that in the anomalous GVD regime, the narrow pulses evolve from multi-soliton collisions initiated by modulational instability and soliton self-frequency shift effects. These experiments were conducted in fiber lengths of 100 to 500 m of single mode, polarization-maintaining fiber. Cross-correlation measurements suggest that there is little or no correlation between spectral components of the FDSS that are separated by more than the 100-fsec pulse bandwidth. The experiments support a model of the FDSS as an ensemble average over fundamental solitons that have frequency shifted by different amounts.
More recently, Morioka et al. studied 1 Tb/s (100 Gb/s ×10 channels) TDM/WDM transmission using a single SC WDM source. In particular, and as illustrated in
FIG. 2
, their SC is generated in 3 km of fiber, and their SC source has a bandwidth >200 nm. Also, they used dispersion decreasing fiber with the third order dispersion flattened. Their pulses are compressed using adiabatic soliton compression (ASC) and spectral shaping is achieved through normal GVD propagation.
The most striking feature of the Morioka et al. SC is that it can generate short pulses <0.3 ps over the continuous spectral range, and that multi-wavelength, transform-limited short pulses can easily be selected by filtering with passive optical filters as illustrated in FIG.
1
. The optical frequency stability of the filtered channels was quite high (~1 GHz/C), determined by that of the filtering devices. Morioka demonstrated 100 Gb/s×10 channel optical signal generation and error-free transmission of all the 100 Gb/s×10 channels over 40 km of DS fiber using the low-noise SC WDM source with a newly developed array-waveguide grating demultiplexer and multiplexer. ASC derives from a fundamental N=1 soliton's tendency to decrease its pulse width to maintain a constant area in response to gradually decreasing dispersion or increasing energy with propagation. When resulting from a dispersion-decreasing fiber the amount of compression depends on the ratio of the initial to final dispersions. In addition, by using a fiber in which the 3rd order dispersion is flattened near the center wavelength and symmetrically concave about it, the spectrum broadens symmetrically. Also, in fiber with flattened 3rd order dispersion, stimulated Raman scattering is the dominant higher-order mechanism that shapes the continuum. Therefore, for continuum generation based on ASC in long fibers, optimization generally requires:
operation over the fiber length in both the normal and anomalous dispersion regimes;
specially designed dispersion fiber; and
suppression of the 3rd order dispersion.
A group at the University of Michigan has further optimized the SC generation in long fibers by using dispersion decreasing (DD) fibers. Using DD fiber can enhance the SC generation process. For example, using 3.3 km of DD fiber with 24.3W peak input power, one can obtain 100 nm SC that is flat over more than 20 nm and twice as broad and more uniform spectrum than dispersion-increasing or constant dispersion fiber. The DD fiber generates a broader and smoother spectrum than the other fibers because the changing zero dispersion wavelength enhances the generation of new frequencies, as self-phase modulation effects are more efficient near the zero dispersion wavelength.
Following the University of Michigan group's work on dispersion tailored fibers for continuum generation, a number of groups have studied optimization of the continuum in kilometer lengths of specialty dispersion fibers. K. Mori et al. show that the SC spectrum can be optimized by using DD fiber in which the dispersion is a convex function of frequency with two zero-dispersion wavelengths.
Okuno et al. show experimentally the generation of 280 nm wide continuum by using a kilometer length of dispersion-flattened and decreasing fiber. As illustrated in
FIG. 5
, pulses are compressed by adiabatic soliton compression. Spectral shaping is accomplished through normal GVD propagation.
In contrast, Sotobayashi and Kitayama demonstrate 325 nm wide continuum by using a two state set-up: the first stage for pulse compression and the second stage for continuum generation. As illustrated in
FIG. 4
, pulses are compressed by soliton-effect compression and spectrum shaping is accomplished through normal GVD propagation. The pulses are first compressed in a 4 km length of fiber with anomalous dispersion through the higher-order soliton compression effect. Then, continuum is generated in a 2 km length of dispersion-flattened fiber that has a constant normal dispersion throughout the fiber length.
In a similar fashion, Takushima et al. generate over 140 nm-wide supercontinuum from a normal dispersion fiber by using a mode-locked semiconductor laser source. In their two-stage set-up, the pulses are first compressed through the adiabatic soliton compression technique in a 10.2 km length of DD fiber as illustrated in FIG.
3
. An EDFA is used to boost the signal after the compression, and the output is then fed into a 1.7 km length of dispersion-flattened fiber with normal dispersion to generate the continuum through normal GVD propagation.
All of the experiments illustrated in
FIGS. 2-5
however require some type of non-commercial, specialty fiber.
Although continuum sources have bee
Islam Mohammed N.
Kim Jae-youn
Nowak George A.
Brooks & Kushman P.C.
The Regents of the University of Michigan
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