One of the fundamental problems which exist in long haul high rate optical communications is chromatic dispersion of light pulses passing through fiber optic lines. This dispersion causes different wavelengths to travel through the optical waveguide at different speeds. An optical communications light pulse which is part of a bit stream is created by a transmission laser operating at a predetermined carrier optical frequency or wavelength (the words frequency, represented by f, and wavelength in vacuum, represented by l, will be used interchangeably, as is currently done in the art, with l related to f by the equation l=c/f, where c is the speed of light, viz. 299 792 458 m/s).
The pulse has a given intensity and duration which can be detected at a receiver located at an opposite end of the communications waveguide (typically the optic fiber) some distance away from the transmitter, perhaps a few to tens of kilometers away. The light pulse comprises spectral components covering a certain bandwidth Df, which is generally in the range 10-60 GHz in modern optical communications systems operating in the bit rate range of 2.5-10 Gb/s. In optical fibers used for communications, the group velocity associated with each spectral component varies with its optical frequency (or wavelength). This means that the lowfrequency spectral components of a light pulse do not travel at the same velocity as its high frequency components. This causes a light pulse initially injected into a fiber with duration Dt to broaden out (in others words to become "dispersed") to a duration Dt' after propagating through a certain length of optical fiber. As an example, in conventional fibers deployed over land, the dispersion is typically 17 ps/nm/km. This means that two spectral components separated by say 10 GHz at a carrier frequency of 193 000 GHz (l=1553.329 nm), corresponding therefore to a wavelength separation of 0.0804 nm, would undergo a differential delay (or dispersion) of 17 ps/nm/km times 0.0804 nm, i.e. 1.37 ps/km, or 137 ps after 100 km of fiber (the sign of the dispersion in conventional fibers is such that low frequency light travels at a lower group velocity than highfrequency light). Light pulses that may be initially 25 ps in duration in a 10 Gb/s optical communication system have spectral components that cover several tens of GHz. The time dispersion between the low- and the high-frequency spectral components of such a light pulse are therefore more than 137 ps after a 100-km stretch of fiber. Such a large amount amount of dispersion is intolerable since, the pulses being 100 ps apart, dispersion will make subsequent pulses start overlapping to a considerable degree and increase the bit error rate.
Therefore, dispersion causes a spreading of laser pulses over great distances (even in the most non-dispersive optic fibers) such that the pulse energy is spread out and lower-frequency spectral components trail behind higher-frequency components of the laser pulse.
When the bit time slot in an optical communication system is large enough, dispersion causes a small portion of the pulse energy to spill over into adjacent bit time slots. The pulse height at the receiver will be marginally lower, but still detectable as a bit, and empty bit slots (e.g. representing zeros) will contain small amounts of spill over light, which will be below the predetermined detection threshold. However, as the bit time slots are made smaller so as to increase the bit rate and increase transmission capacity, dispersion of a very short light pulse may significantly reduce the light intensity in the bit time slot and even cause enough spill over into adjacent empty bit time slots so as to cause difficulty in the detection of zeroes.
Dispersion thus produces the spreading of short optical pulses, and thus affects negatively the quality of a communication link. If the dispersion of a light pulse is great enough, the communication link utilizing the light pulses becomes unstable, and eventually, unusable.
In addition to problems in optical communications, dispersion is also a major problem in the generation of very short, high power optical laser pulses. The spreading of the pulses reduces the achieved peak power, and thus reduces the efficiency of the laser pulses.
Various attempts have been made to address the problem of dispersion in optical signals. Three basic approaches have been developed. The first approach is to pass the optical signal transmitted, which has suffered dispersion due to the optical transmission waveguide, through a length of waveguide having an opposite sign of dispersion at the communications wavelength before feeding the signal to the receiver. U.S. Pat. No. 4,969,710 to Tick et al. is an example of such a method. The second approach has been to use optical devices to separate the light pulse into its wavelength components, then to suject the separated components to different delays before recombining the components into a single dispersion compensated optic signal. U.S. Pat. No. 5,473,719 to Stone is an example of this approach. The third approach has been to use chirped in-fiber Bragg gratings to reflect each wavelength component at different points such that the reflected optical signal has been dispersion compensated. This approach is disclosed in a paper by Franqois Ouellette, titled "Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides", Optics Letters, vol. 12, pp. 847-849, October 1987. Experimental results have been reported in the paper by W. H. Loh, R. I. Laming, N. Robinson, A. Cavaciuti, F. Vaninetti, C. J. Anderson, M. N. Zervas, and M. J. Cole, titled "Dispersion compensation over distances in excess of 500 km for 10-Gb/s systems using chirped fiber gratings", IEEE Photonics Technology Letters, vol. 8, pp.944-946, July 1996.
Other known prior art attempts to resolve these problems arm disclosed in U.S. Pat. No. 5,570,439 to Ido et al.; U.S. Pat. No. 5,568,583 to Akasaka et al; U.S. Pat. No. 5,559,920 to Chraplyvy et al.; U.S. Pat. No. 5,530,778 to Rast; U.S. Pat. No. 5,504,829 to Evans et al.; U.S. Pat. No. 5,448,674 to Vengsarkar et al.
Known dispersion compensators are bulky, generally limited to a single channel wavelength and/or have significant losses. The advantage of the dispersion management device proposed here is that it could me manufactured using integrated optic techniques so that it could be very compact and low-cost. In addition the taper lends itself very naturally to tayloring the dispersion as a function of wavelength in a way to match (and compensate) that of an optical fiber. In 10 Gb/s and higher bit rate system the variation of the dispersion with wavelength (from 16 ps/nm/km at 1530 nm to 17 ps/nm/km at 1580 nm, for example, in certain optical fibers) is a factor that is now considered. Our device can offer this wavelength taylored dispersion profile and can do so over very broad wavelength bands.