Compensating for chromatic dispersion in optical fibers

Abstract

An optical chromatic dispersion compensator (60) betters optical communication system performance. The dispersion compensator (60) includes a collimating means (61) that receives a spatially diverging beam of light from an end of an optical fiber (30). The collimating means (61) converts the spatially diverging beam into a mainly collimated beam that is emitted therefrom. An optical phaser (62) receives the mainly collimated beam from the collimating means (61) through an entrance window (63), and angularly disperses the beam in a banded pattern that is emitted from the optical phaser (61). A light-returning means (66) receives the angularly dispersed light and reflects it back through the optical phaser (62) to exit the optical phaser near the entrance window (63) thereof.

Description

or
4h²(n²−sin ²φ)=m²λ²  (2)
where

• • n is the index of refraction of material forming the optical phaser 62
  • θ is the angle of incidence on the surface 65 of light reflecting internally inside the optical phaser 62
  • φ is the angle of refraction of light exiting the optical phaser 62 through the surface 65
  • h is the thickness of the optical phaser 62
  • m is the order of interference
    The angular dispersion capability of the optical phaser 62, set forth in the relationship (3) below, can be derived from equation (2).

$\begin{array}{cc}\frac{\delta \phi }{\delta \lambda }\approx \frac{n^2-{\sin }^2\phi }{\lambda \sin \phi \cos \phi }& \left(3\right)\end{array}$
The optical phaser 62 produces a large angular dispersion of light exiting through the surface 65 if φ is near critical angle. A large angular dispersion may also be realized if φ approaches normal to the surface 65 of the optical phaser 62. The latter orientation for light emitted from the surface 65 corresponds to the orientation of light emitted from the parallel plate 58 of a VIPA.

Although both the optical phaser 62 and VIPA have similar angular dispersion capabilities, their diffraction patterns differ significantly. As illustrated schematically in FIG. 6A, the beam waist inside the parallel plate 58 of the VIPA must be very small to simultaneously reduce both the angle φ and loss of optical energy. Consequently, for a given wavelength of light λ the narrow beam waist within the parallel plate 58 of the VIPA produces a large angular divergence of refracted beams. In other words, the energy of light diffracted by the VIPA is distributed into multiple orders. Due to the different diffraction properties of the beams of different order, as stated previously for the VIPA only one of the diffraction orders may be used for dispersion compensation. Consequently, the VIPA is an inherently high-loss device. Alternatively, the beam width inside the optical phaser 62 is similar to the thickness h of the optical phaser 62. This wide beam width within the optical phaser 62 causes optical energy of light refracted at the surface 65 to be mainly concentrated in a single order for any beam of light at a particular wavelength as illustrated schematically in FIG. 6B.

To compensate chromatic dispersion in an optical communication system containing multiple WDM channels, it is preferable to design the beam incidence angle inside the optical phaser 62, θ, in accordance with the following equation (4).

$\begin{array}{cc}\cos \theta =\frac{c}{2h\Delta \mathrm{fn}}& \left(4\right)\end{array}$
where

• • c is the speed of light
  • Δf is the frequency separation between adjacent WDM channels.
    Note that n, the index of refraction of the optical phaser 62, is wavelength dependent. The incidence angle θ therefore varies with wavelength. In particular, for light of each WDM channel λ_i, there exists a specific incidence angle θ_i. Angular spreading of the light beam inside the optical phaser 62 is enabled by the angular dispersion produced by the prism 77 or bulk diffraction grating 77a of the collimating means 61. If the incidence angle θ is near the angle of total internal reflection, as preferred for the current embodiment, the optical phaser 62 not only produces large angular dispersion at a particular wavelength as shown by relationship (3), the optical phaser 62 also amplifies angular dispersion of the collimating means 61. Amplification of the angular dispersion provides a means for reducing dispersion ripple.

To reduce loss of light entering the optical phaser 62 from the collimating means 61 and to also produce preferably only one order for any beam of light at a particular wavelength in the diffraction pattern of the beam exiting the surface 65 of the optical phaser 62, or perhaps a few orders, the angular dispersion produced by the collimating means 61, i.e. the collimation of the beam emitted by the collimating means 61, preferably has a beam waist w_o in the plane of the plate that is perpendicular to the parallel surfaces 64, 65 in accordance with relationship (5) below.
w_o≈h sin θ  (5)
where

• • h is the thickness of the optical phaser 62
  • θ is the angle of incidence on the surface 65 of light reflecting internally inside the optical phaser 62
    Collimating the beam of light emitted from the collimating means 61 in accordance with relationship (5) above ensures that more than fifty-percent (50%) of the energy in the mainly collimated beam of light impinging upon the entrance window 63 diffracts into fewer than three (3) diffraction orders for any beam of light at a particular wavelength in the angularly dispersed light emitted from the optical phaser in the banded pattern.

Several alternative embodiments for the optical phaser 62 are illustrated in FIGS. 7A through 7F. In those various alternative embodiments of the optical phaser 62, the entrance window 63 may be formed either by a beveled surface as illustrated in FIG. 4, or by a prism 82 that projects out of one of the parallel surfaces 64, 65 as illustrated in FIGS. 7D through 7F. Light entering the entrance window 63 of the prism 82 reflects internally within the prism 82 before impinging for a first time on one of the parallel parallel surfaces 64 or 65. As illustrated for the various alternative embodiments, the reflective surface 64 may either be coated with a high-reflectivity film or be partially transparent. If the surface 64 is partially transparent, the optical phaser 62 exhibits greater optical loss. However, for such configurations of the optical phaser 62 light leaking from the surface 64 may be used for performance monitoring. It should be noted that if the reflectivities of the parallel surfaces 64, 65 were made polarization independent by special optical coatings, polarization control produced by the collimating means 61 for light impinging upon the entrance window 63 of the optical phaser 62 is unnecessary.

As described previously, the preferred embodiment of the light-returning means 66 includes the light-focusing element 67 and a curved mirror 68 placed near the focal plane of the light-focusing element 67. The light-focusing element 67 may be a lens as indicated in FIG. 4. Alternatively as illustrated in FIG. 8A, a concave mirror may also be used for the light-focusing element 67 in a folded configuration of the light-returning means 66. The light-focusing element 67 is preferably located along the direction of the diffracted beam emitted from the surface 65 of the optical phaser 62 at a distance, as illustrated in FIG. 8B, which is approximately one focal length, i.e. f, of the light-focusing element 67 away from the surface 65.

In the preferred embodiment of the light-returning means 66, the light beams emerging from the surface 65 of the optical phaser 62 are collected by the light-focusing element 67 for projection onto the curved mirror 68 that is located near the focal plane of the light-focusing element 67. Reflected back by the curved mirror 68, the beams reverse their trajectory through the light-returning means 66, the optical phaser 62 to exit therefrom through the entrance window 63, and proceed through the collimator 71 of the collimating means 61. Preferably, as illustrated in the plan view of FIG. 9A, light returning through the collimator 71 can be spatially separated from light entering therethrough by slightly tilting the light-focusing element 67 perpendicular to a plane of symmetry of the dispersion compensator 60. Alternatively, as indicated in FIG. 9B light returning collinearly through the collimator 71 can also be separated from light entering the collimator 71 by a circulator 86. While FIG. 9B illustrates the circulator 86 as being located between the optical fiber 30 and the collimating means 61, alternatively the circulator 86 can be inserted between the collimating means 61 and the optical phaser 62.

The chromatic dispersion, β, produced by the dispersion compensator 60 follows a relationship (6) set forth below.

$\begin{array}{cc}\beta \approx -\frac{2{\left(n^2-1\right)}^2{f}^2}{c\lambda R{\cos }^2\phi }& \left(6\right)\end{array}$
where

• • R is the radius of curvature of the curved mirror 68.
    Note that R is defined as positive for a convex mirror and negative for a concave mirror. For fixed diffraction angle φ and fixed focal length f, relationship (6) indicates that the chromatic dispersion generated by the dispersion compensator 60 is directly proportional to curvature of the curved mirror 68. In particular, by adjusting the curvature of the curved mirror 68, it is always possible to completely cancel chromatic dispersion of a particular optical transmission system for a specified wavelength of light traveling therethrough.

Furthermore, the small angular dispersion introduced by the prism 77 or the bulk diffraction grating 77a of the collimating means 61 produces a banded pattern that angularly disperses beams of light of differing wavelengths emerging from the surface 65 of the optical phaser 62. That is, the optical phaser 62 diffracts WDM channels having differing wavelengths of light at slightly different angles. Furthermore, light having a particular frequency within each specific band of the banded pattern is angularly displaced from light at other frequencies within that same band. Moreover, the banded pattern of angularly dispersed light generated by the optical phaser 62 exhibits a rate of angular change with respect to a center frequency within a particular band that differs from the rate of angular change with respect to center frequencies of other bands. Consequently, as indicated schematically in FIG. 10 the light-focusing element 67 projects this banded pattern for light of each WDM channel to a distinct location on the curved mirror 68 located at the focal plane of the light-focusing element 67.

Projection of the banded pattern by the light-focusing element 67 to distinct locations on the curved mirror 68 may be exploited advantageously if the curved mirror 68 has a curvature which varies across the focal plane of the light-focusing element 67. Employing a curved mirror 68 having an appropriately varying curvature permits the dispersion compensator 60 to concurrently compensate for chromatic dispersion for all WDM channels propagating through the optical fiber 30. FIG. 11 displays preferred shapes for the curved mirror 68 of exemplary embodiments of the dispersion compensator 60 that fully compensate GVD and dispersion slope for various different types of commercially available optical fibers 30.

In one exemplary embodiment of the dispersion compensator 60, the optical phaser 62 is made from a plate of silicon that is approximately 1 mm thick. In accordance with the various embodiments for the optical phaser 62 depicted in FIGS. 7A-7F, the entrance window 63 is formed on an outer surface of the plate. The reflective surface 64 has a gold coating, and the refractive surface 65 is polished. The beam incidence angle θ inside the optical phaser 62 is approximately sixteen degrees (16°), and the focal length of the light-focusing element 67 is approximately 100 mm.

The dispersion compensator 60 of the present invention provides several advantages and distinctions of over existing dispersion compensation devices.

First, the dispersion compensator 60 enables independent control both of GVD and of dispersion slope. Specifically, for any optical fiber 30 of a specified length, its GVD can be compensated by an appropriate curvature of the folding curved mirror 68, and its dispersion slope can be compensated by appropriate curvature variations of the same folding curved mirror 68.

Second, the nearly collimated beam inside the optical phaser 62 concentrates energy of the diffracted light into a few diffraction orders for any beam of light at a particular wavelength, resulting in broad pass-band width and minimum throughput loss. For example, the dispersion compensator 60 of the present invention exhibits a 0.5 dB bandwidth greater than 40 GHz for a WDM system having immediately adjacent channels spaced 100 GHz apart.

Third, according to relationship (6), the GVD and dispersion slope produced by the dispersion compensator 60 change linearly with curvature of the folding curved mirror 68. Therefore, the shape of the curved mirror 68 that provides full GVD and dispersion slope compensation for an optical system is uniquely determined by the type of optical fiber 30, and changes linearly with the length of the optical fiber 30. Accordingly, in FIG. 11 the vertical axis associate with the graphic depiction of various curvatures for different curved mirrors 68 is normalized to the length of the various different optical fibers 30.

Fourth, the dispersion compensator 60 can be designed with minimum dispersion slope, for example, by setting the radius of the folding curved mirror 68 in accordance with the following relationship (7).
R cos ²φ≈cons tan t  (7)
The dispersion compensator 60 equipped with a curved mirror 68 in accordance with relationship (7) can be useful for compensating chromatic dispersion in optical communication systems where:

• • 1. the wavelength of the light is unstable, such as that from an uncooled laser; or
  • 2. the spectrum of the light is broad, such as that from a directly modulated laser.

Finally, the dispersion compensator 60 introduces little dispersion ripple into light propagating through an optical communication system. Therefore, the dispersion compensator 60 can be used for terminal chromatic dispersion compensation, as well as for inline chromatic dispersion compensation of long-haul fiber optical systems. For inline chromatic dispersion compensation of long-haul fiber optical systems a number of dispersion compensators 60 are installed at spaced apart locations along the optical fiber 30 inline with the optical fiber 30. For example, FIG. 12 displays a eye-diagram results from a simulation of a 10 Gbps fiber optical transmission system containing 4000 km of fiber compensated by dispersion compensators 60 of the present invention spaced at 80 km apart along the optical fiber 30. As is apparent to those skilled in the art, the eye-diagram of FIG. 12 exhibits little degradation due to chromatic dispersion.

Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. For example, the embodiments of the dispersion compensator 60 described above preferably include a prism 77 or bulk diffraction grating 77a to provide angular dispersion of light emitted from the collimating means 61 that impinges upon the entrance window 63. However, it is not intended for the dispersion compensator 60 as encompassed in the following claims necessarily include such an angular dispersion element. As stated previously, in applications of the dispersion compensator 60 in which dispersion slope compensation is not critical, any other type of mode coupler that produces a nearly collimated beam of light with efficient optical coupling between the optical fiber 30 and the optical phaser 62 may be employed as the collimating means 61. Such a coupler may simply be a standard optical collimator.

As described above, the entrance window 63 of the optical phaser 62 is preferably coated with an antireflective film. However, it is not intended that the dispersion compensator 60 as encompassed in the following claims necessarily have such a coating. The only requirement is that the entrance window 63 of the optical phaser 62 must simply be partially transparent at the wavelength of light impinging thereon.

In the above embodiments of the present invention, the birefringent plate 73 and the half-wave plates 76, 78 linearly polarize the beam of light received from the optical fiber 30 before suitably polarized beams impinge on the prism 77 and the optical phaser 62 to become angularly dispersed thereby. However, it is not intended that the dispersion compensator 60 encompassed by the following claims be limited to using these specific polarization components. Instead, the dispersion compensator 60 simply requires that an appropriately polarized beam of light impinge on the entrance window 63 of the optical phaser 62.

Further, as described above the parallel surfaces 64, 65 of the optical phaser 62 may be coated with films such that the corresponding reflectivities are insensitive to beam polarizations. If such coatings are applied to the parallel surfaces 64, 65, then polarization of the beam of light impinging upon the entrance window 63 need not be controlled, and the polarization control components, e.g. the birefringent plate 73 and the half-wave plates 76, 78, may be eliminated from the collimating means 61. Analogously, to increase optical efficiency an antireflective coating may be advantageously applied to the light-focusing element 67 to reduce loss of light passing through a lens. For optical efficiency it is also advantageous if the curved mirror 68 have a highly reflective coating.

As described above, the preferred spacing between the surface 65 of the optical phaser 62 and the light-focusing element 67 equals the focal length, f, of the light-focusing element 67. However, the dispersion compensator 60 encompassed by the following claims is not limited to that specific geometry. Instead, the distance between the focusing element 66 and the surface 64 of the phaser 61 may be set to any value. As described in greater detail below, that distance may, in fact, even be adjustable.

In general, the chromatic dispersion produced by the apparatus of the present invention is related to its geometry by the following relationship (8).

$\begin{array}{cc}\beta \propto \left(f-u+\frac{f^2}{R}\right)& \left(8\right)\end{array}$
where

• • u is the distance from the surface 65 of the optical phaser 62 to the light-focusing element 67 along the optical axis of the light-focusing element 67
  • f the focal length of the light-focusing element 67
  • R the radius of curvature of the folding curved mirror 68.
    A tunable dispersion compensator 60 in accordance with the present invention can be implemented by adjusting u, or R or both u and R. For the adjustment of u, a preferred embodiment of the dispersion compensator 60 is to place the light-returning means 66 on a translation stage, as indicated in FIG. 4 by an arrow 69. Alternatively, the curved mirror 68 can be made with adjustable curvature.

There exist numerous different ways which may be employed to make the curvature of the curved mirror 68 adjustable. One way is to apply elastic bending forces to the curved mirror 68 in the direction indicated in FIG. 4. by arrows 70 Such bending forces may be generated mechanically such as by push screws. Alternatively, the forces may also be generated electrostatically or electromagnetically such as by a micro electro-mechanic system. The curvature of the curved mirror 68 may also be adjusted thermally if the mirror is formed from a bi-metallic material. Optimal mirror shapes may be achieved by forming the curved mirror 68 to have varying stiffness, or by applying bending forces at multiple locations on the curved mirror 68, or by a combination of both techniques. Translating a curved mirror 68 having uneven curvatures that is located near the focal plane of the light-focusing element 67 transverse to the optical axis thereof, i.e. translating along the focal plane of the light-focusing element 67, also adjusts the curvature of the curved mirror 68. The curvature of the curved mirror 68 may also be adjusted by replacing a curved mirror 68 having a particular shape with another one having a different shape.

Consequently, without departing from the spirit and scope of the invention, various alterations, modifications, and/or alternative applications of the invention will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the invention.

Claims

1. An optical chromatic dispersion compensator adapted for bettering performance of an optical communication system comprising:

a collimating means for receiving a spatially diverging beam of light which contains a plurality of frequencies as may be emitted from an end of an optical fiber included in an optical communication system, the collimating means also converting the received spatially diverging beam of light into a mainly collimated beam of light that is emitted from the collimating means;

an optical phaser which provides an entrance window for receiving the mainly collimated beam of light from the collimating means and for angularly dispersing the received beam of light in a banded pattern that is emitted from the optical phaser, whereby the received beam of light becomes separated into bands so that light having a particular frequency within a specific band is angularly displaced from light at other frequencies within that same band; and

a light-returning means which receives the angularly dispersed light having the banded pattern that is emitted from the optical phaser, and for reflecting that light back through the optical phaser to exit the optical phaser near the entrance window thereof.

2. The compensator of claim 1 wherein the mainly collimated beam of light emitted from the collimating means has a divergence which ensures that more than fifty-percent (50%) of energy in the mainly collimated beam of light impinging upon the entrance window diffracts into fewer than three (3) diffraction orders for any beam of light at a particular wavelength in the angularly dispersed light emitted from the optical phaser in the banded pattern.

3. The compensator of claim 1 wherein light enters the optical phaser through the entrance window at near normal incidence.

4. The compensator of claim 1 wherein the entrance window of the optical phaser is at least partially transparent to light impinging thereon.

5. The compensator of claim 1 wherein the light-returning means includes a light-focusing means and a mirror disposed near a focal plane of the light-focusing means, the light-focusing means collecting the angularly dispersed light having the banded pattern emitted from the optical phaser for projection onto the mirror, the mirror reflecting light impinging thereon back towards the light-focusing means.

6. The compensator of claim 5 wherein the light-focusing means projects to a distinct location on the mirror each band in the banded pattern of angularly dispersed light generated by the optical phaser.

7. The compensator of claim 5 wherein a distance between the light-focusing means and the optical phaser is adjustable.

8. The compensator of claim 5 wherein the mirror is curved.

9. The compensator of claim 8 wherein curvature of the mirror is adjustable.

10. The compensator of claim 9 wherein curvature of the mirror is adjusted by bending the mirror.

11. The compensator of claim 10 wherein force for bending the mirror is selected from a group consisting of mechanical, electrical, magnetic and thermal.

12. The compensator of claim 9 wherein the mirror has multiple curvatures, and curvature of the mirror is adjusted by translating the mirror.

13. The compensator of claim 9 wherein the mirror is replaceable, and curvature of the mirror is adjusted by replacing the mirror with another mirror having a different curvature.

14. The compensator of claim 1 wherein the optical phaser is made from a plate of material having two parallel surfaces between which light after entering the optical phaser through the entrance window reflects, and with the entrance window being formed on an outer surface of the plate.

15. The compensator of claim 14 wherein the entrance window is formed by a beveled edge of the plate.

16. The compensator of claim 14 wherein the entrance window is formed by a prism which projects out of one of the two parallel surface surfaces of the optical phaser, and light entering the optical phaser through the entrance window undergoes internal reflection within the prism before impinging upon one of the two parallel surface surfaces.

17. The compensator of claim 14 wherein one of the two parallel surface surfaces of the optical phaser is partially transparent to allow a portion of light impinging thereon to exit the optical phaser.

18. The compensator of claim 17 wherein light emitted from the optical phaser through the partially transparent surface detracts at an angle which exceeds forty-five degrees (45°) from a normal thereto.

19. The compensator of claim 1 wherein the optical phaser is made from a material having an index of refraction which is greater than the index of refraction of medium surrounding the optical phaser.

20. A chromatic dispersion compensation method that is adapted for bettering performance of an optical communication system comprising the steps of:

collimating into a mainly collimated beam of light a spatially diverging beam of light which contains a plurality of frequencies as may be emitted from an end of an optical fiber included in an optical communication system;

impinging the mainly collimated beam of light onto an entrance window of an optical phaser for angularly dispersing the mainly collimated beam of light into a banded pattern emitted from the optical phaser whereby the mainly collimated beam of light becomes separated into bands so that light having a particular frequency within a specific band is angularly displaced from light at other frequencies within that same band; and

reflecting the angularly dispersed light back through the optical phaser to exit the optical phaser near an entrance window thereof.

21. The method of claim 20 wherein the mainly collimated beam of light has a divergence which ensures that more than fifty-percent (50%) of energy in the mainly collimated beam of light impinging upon the entrance window diffracts into fewer than three (3) diffraction orders for any beam of light at a particular wavelength in the angularly dispersed light emitted from the optical phaser in the banded pattern.

22. The method of claim 20 wherein a light-returning means for reflecting the angularly dispersed light back through the optical phaser includes a light-focusing means and a mirror disposed near a focal plane of the light-focusing means, the method further comprising the steps of:

the light-focusing means collecting the angularly dispersed light having the banded pattern emitted from the optical phaser for projection onto the mirror; and

the mirror reflecting light impinging thereon back towards the light-focusing means.

23. The method of claim 22 wherein the light-focusing means projects to a distinct location on the mirror each band in the banded pattern of angularly dispersed light generated by the optical phaser.

24. The method of claim 22 further comprising the step of adjusting a distance which separates the light-focusing means from the optical phaser.

25. The method of claim 22 further comprising a step of adjusting a curvature of the mirror.

26. The method of claim 25 wherein curvature of the mirror is adjusted by bending the mirror.

27. The method of claim 26 wherein force for bending the mirror is selected from a group consisting of mechanical, electrical, magnetic and thermal.

28. The method of claim 25 wherein the mirror has multiple curvatures, and curvature of the mirror is adjusted by translating the mirror.

29. The method of claim 25 wherein the mirror is replaceable, and curvature of the mirror is adjusted by replacing the mirror with another mirror having a different curvature.

30. The method of claim 20 wherein light is emitted from the optical phaser through a partially transparent surface thereof, the emitted light being defracted at an angle which exceeds forty-five degrees (45°) from a normal to the partially transparent surface.

31. A chromatic dispersion compensation method that is adapted for bettering performance of an optical communication system comprising the steps of:

collimating into a mainly collimated beam of light a spatially diverging beam of light which contains a plurality of frequencies as may be emitted from an end of an optical fiber included in an optical communication system;

impinging the mainly collimated beam of light onto an entrance window of an optical phaser (62), the optical phaser (62) having an entrance window (63) for receiving a beam of light, and having opposed parallel surfaces (64, 65) one of which is highly reflective and one of which is at least partially transmissive, and wherein the optical phaser (62) is arranged such that the angle of incidence of the beam's impingement upon the partially transmissive diffractive surface (65) is slightly less than the angle of total internal reflection;

angularly dispersing in the optical phaser the mainly collimated beam of light into a banded pattern emitted from the optical phaser whereby the mainly collimated beam of light becomes separated into bands so that light having a particular frequency within a specific band is angularly displaced from light at other frequencies within that same band; and

reflecting the angularly dispersed light back through the optical phaser to exit the optical phaser near an entrance window thereof.

32. The method of claim 31 wherein the mainly collimated beam of light has a divergence which ensures that more than fifty-percent (50%) of energy in the mainly collimated beam of light impinging upon the entrance window diffracts into fewer than three (3) diffraction orders for any beam of light at a particular wavelength in the angularly dispersed light emitted from the optical phaser in the banded pattern.

33. The method of claim 31 wherein a light-returning means for reflecting the angularly dispersed light back through the optical phaser includes a light-focusing means and a mirror disposed near a focal plane of the light-focusing means, the method further comprising the steps of:

the light-focusing means collecting the angularly dispersed light having the banded pattern emitted from the optical phaser for projection onto the mirror; and

the mirror reflecting light impinging thereon back towards the light-focusing means.

34. The method of claim 33 wherein the light-focusing means projects to a distinct location on the mirror each band in the banded pattern of angularly dispersed light generated by the optical phaser.

35. The method of claim 33 further comprising the step of adjusting a distance which separates the light-focusing means from the optical phaser.

36. The method of claim 33 further comprising a step of adjusting a curvature of the mirror.

37. The method of claim 36 wherein curvature of the mirror is adjusted by bending the mirror.

38. The method of claim 37 wherein force for bending the mirror is selected from a group consisting of mechanical, electrical, magnetic and thermal.

39. The method of claim 36 wherein the mirror has multiple curvatures, and curvature of the mirror is adjusted by translating the mirror.

40. The method of claim 36 wherein the mirror is replaceable, and curvature of the mirror is adjusted by replacing the mirror with another mirror having a different curvature.

41. The method of claim 31 wherein light is emitted from the optical phaser through a partially transmissive surface thereof, the emitted light being defracted at an angle which exceeds forty-five degrees (45°) from a normal to the partially transmissive surface.

42. An optical chromatic dispersion compensator adapted for bettering performance of an optical communication system comprising:

an optical phaser (62) having an entrance window (63) for receiving a beam of light, and having opposed parallel surfaces (64,65) one of which is highly reflective and one of which is at least partially transmissive for angularly dispersing the received beam of light in a banded pattern that is emitted from the optical phaser, through the partially transmissive diffractive surface (65), whereby the received beam of light becomes separated into bands so that light having a particular frequency within a specific band is angularly displaced from light at other frequencies within the same band; and

a light-returning means (66) which receives the angularly dispersed light having the banded pattern that is emitted from the partially transmissive diffractive surface (65) of the optical phaser, and for reflecting that light back through the optical phaser to exit the optical phaser near the entrance window thereof;

43. The compensator of claim 42 wherein the mainly collimated beam of light emitted from the collimating means (61) has a divergence which ensures that more than fifty-percent (50%) of energy in the mainly collimated beam of light impinging upon the entrance window diffracts into fewer than three (3) diffraction orders for any beam of light at a particular wavelength in the angularly dispersed light emitted from the optical phaser (62) in the banded pattern.

44. The compensator of claim 42 wherein light enters the optical phaser (62) through the entrance window at near normal incidence.

45. The compensator of claim 42 wherein the entrance window of the optical phaser (62) is at least partially transparent to light impinging thereon.

46. The compensator of claim 42 wherein the light-returning means (66) includes a light-focusing means (67) and a mirror (68) disposed near a focal plane of the light-focusing means (67) collecting the angularly dispersed light having the banded pattern emitted from the optical phaser for projection onto the mirror, the mirror (68) reflecting light impinging thereon back towards the light-focusing means.

47. The compensator of claim 46 wherein the light-focusing means (67) projects to a distinct location on the mirror (68) each band in the banded pattern of angularly dispersed light generated by the optical phaser (62).

48. The compensator of claim 46 wherein a distance between the light-focusing means (67) and the optical phaser (62) is adjustable.

49. The compensator of claim 46 wherein the mirror (68) is curved.

50. The compensator of claim 49 wherein curvature of the mirror (68) is adjustable.

51. The compensator of claim 50 wherein curvature of the mirror is adjusted by bending the mirror.

52. The compensator of claim 51 wherein force for bending the mirror is selected from a group consisting of mechanical, electrical, magnetic and thermal.

53. The compensator of claim 50 wherein the mirror (68) has multiple curvatures, and curvature of the mirror is adjusted by translating the mirror.

54. The compensator of claim 50 wherein the mirror (68) is replaceable, and curvature of the mirror is adjusted by replacing the mirror with another mirror having a different curvature.

55. The compensator of claim 42 wherein the optical phaser (62) is made from a plate of material having two parallel surfaces (64, 65) between which light after entering the optical phaser through the entrance window reflects, and with the entrance window being formed on an outer surface of the plate.

56. The compensator of claim 55 wherein the entrance window is formed by a bevelled edge of the plate.

57. The compensator of claim 55 wherein the entrance window is formed by a prism which projects out of one of the two parallel surfaces (64, 65) of the optical phaser, and light entering the optical phaser through the entrance window undergoes internal reflection within the prism before impinging upon one of the two parallel surfaces.

58. The compensator of claim 42 wherein light emitted from the optical phaser (62) through the partially transparent surface (65) diffracts at an angle which exceeds forty-five degrees (45°) from a normal thereto.

59. The compensator of claim 42 wherein the optical phaser (62) is made from a material having an index of refraction which is greater than the index of refraction of medium surrounding the optical phaser (62).