Method and system for testing a tunable chromatic dispersion, dispersion slope, and polarization mode dispersion compensator utilizing a virtually imaged phased array

[0002] This application is a Continuation-in-part of co-pending U.S. patent application Ser. No. 09/574,421, entitled “Tunable Chromatic Dispersion, Dispersion Slope, and Polarization Mode dispersion Compensator Utilizing a Virtually Imaged Phased Arry,” filed on May 19, 2000.

[0003] The present invention relates to chromatic dispersion, dispersion slope, and polarization mode dispersion compensation, and more particularly to dispersion compensation accumulated in a wavelength division multiplexed optical fiber transmission line.

[0004] Fiber optic networks are becoming increasingly popular for data transmission because of their high speed and high capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.

[0005] In this specification, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”

[0006] One common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the channels comprising an optical signal travel through an optic fiber at different speeds, e.g., longer wavelengths travel faster than shorter wavelengths. This is a particular problem that becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion. The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel.

[0007] For dense wavelength division multiplexer (DWDM) systems or for WDM or DWDM systems with a wide wavelength spacing between the shortest and longest wavelength channels, the common approach is to allow chromatic dispersion to accumulate within spans of fiber and to compensate for dispersion at the ends of spans through the use of in-line dispersion compensator apparatuses.

[0008] A second common and well-known problem in the transmission of optical signals is polarization mode dispersion (PMD). PMD is the phenomenon by which differently polarized components, or sub-signals, comprising an optical signal propagate with different speeds or, alternatively, propagate along differing-length optical paths. This duality of speeds or paths can also cause unacceptable broadening of the digital pulses comprising a signal that increases in severity with increasing transmission speed. The maximum acceptable PMD-induced optical path length difference is the cumulative result of all PMD effects in all the optical elements through which a signal propagates, including fiber and non-fiber optical components. Although the PMD broadening of optical fiber increases as the square root of fiber length, the PMD broadening caused by birefringent components is linearly related to the cumulative optical path difference of all such components.

[0009] The chromatic dispersion characteristics of optical fibers are not constant but depend upon wavelength, as illustrated in FIG. 1, which presents graphs of Group Velocity Dispersion, D, against wavelength, for typical examples of three commonly used fiber types. In FIG. 1, the quantity D (ps-km⁻¹-nm⁻¹) is defined by the relationship of Eq. 1 $\begin{array}{cc}D=\frac{d}{d\lambda }\left(\frac{1}{v_g}\right)=\frac{1}{L}\left(\frac{d{\tau }_g}{d\lambda }\right)& \left(1\right)\end{array}$

[0010] in which λ is the channel wavelength (nm), ν_gis the group velocity (km/ps), τ_gis the group delay time (ps), and L is the fiber length (km). If ν_gdecreases with increasing wavelength (i.e., longer or “red” wavelengths travel slower than relatively shorter or “blue” wavelengths) then D is positive, otherwise D is negative. Because all three fiber types illustrated in FIG. 1 are deployed in telecommunications systems, the requirements for dispersion compensators vary widely. Furthermore, because of the existence of non-zero dispersion slope, S, a constant level of dispersion compensation does not accurately negate the dispersion of all channels. This inaccuracy can become a significant problem for high-speed data propagation, long span distances, and/or wide distances between the shortest and longest wavelength channels.

[0011] Conventional apparatuses for dispersion compensation include dispersion compensation fiber, chirped fiber Bragg gratings coupled to optical circulators, and conventional diffraction gratings disposed as sequential pairs.

[0012] A dispersion compensation fiber, which is used in-line within a fiber communications system, has a special cross-section index profile so as to provide chromatic dispersion that is opposite to that of ordinary fiber within the system. The summation of the two opposite types of dispersion negates the chromatic dispersion of the system. However, dispersion compensation fiber is expensive to manufacture, has a relatively large optical attenuation, and must be relatively long to sufficiently compensate for chromatic dispersion.

[0013] A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. A chirped fiber Bragg grating of this sort is generally coupled to a fiber communications system through an optical circulator. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. However, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system. Furthermore, fiber Bragg gratings generally do not compensate polarization mode dispersion.

[0014] A conventional diffraction grating has the property of outputting different wavelengths at different angles. By using a pair of gratings in a coupled spatial arrangement, this property can be used to compensate chromatic dispersion in a fiber communications system. In such a spatial grating pair arrangement, lights of different wavelengths are diffracted from a first grating at different angles. These lights are then input to a second grating that diffracts them a second time so as to set their pathways parallel to one another. Because the different lights travel with different angles between the two gratings, certain wavelength components are made to travel longer distances than other wavelength components. Chromatic dispersion is produced in the spatial grating pair arrangement because the wavelength components that travel the longer distances incur time delays relative to those that travel the shorter distances. This grating-produced chromatic dispersion can be made to be opposite to that of the fiber communications system, thereby compensating the chromatic dispersion within the system. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees/nm. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, the two gratings of a spatial grating pair would have to be separated by a very large distance, thereby making such a spatial grating pair arrangement impractical.

[0015] Accordingly, there exists a need for an improved chromatic dispersion, dispersion slope, and polarization mode dispersion (PMD) compensator. The improved compensator should produce an adjustable chromatic dispersion and be readily adapted to provide either positive or negative chromatic dispersion, which can provide non-uniform dispersion compensation so as to compensate for fiber dispersion slope, and can also compensate for polarization mode dispersion. The present invention addresses such a need.

[0016] The present invention provides a dispersion compensator which utilizes a Virtually Imaged Phased Array (VIPA), gratings, and birefringent wedges to moderate chromatic dispersion, dispersion slope and polarization mode dispersion, and a method and system for testing such a dispersion compensator. The dispersion compensator in accordance with the present invention propagates the composite optical signal in a forward direction; separates the wavelengths in the band of wavelengths in each of the plurality of channels, where each of the wavelengths in the band is spatially distinguishable from the other wavelengths in the band; spatially separates each band of wavelengths in the plurality of channels; spatially separates each wavelength of each separated band of wavelengths into a plurality of polarized rays; and reflects the plurality of polarized rays toward a return direction, where dispersion is added to the reflected plurality of polarized rays such that the unwanted chromatic dispersion, dispersion slope, and PMD are compensated. The dispersion compensator provides simultaneous tunable compensation of these various dispersions utilizing a single apparatus. A system which utilizes the compensator is thus cost effective to manufacture. Systems may be provides which determines the operating and performance parameters for the compensator, and measures the spectrum outputted by the compensator.

[0017] FIG. 1 is a graph of the typical dispersion characteristics of three common commercially available optical fiber types, plotted against wavelength.

[0018] FIG. 2 illustrates a Virtually Imaged Phased Array (VIPA) utilized in the preferred embodiments of dispersion compensator in accordance with the present invention.

[0019] FIG. 3 illustrates in more detail the light path through and operation of the VIPA of FIG. 2.

[0020] FIG. 4 is a diagram illustrating an apparatus which uses a VIPA and a light returning device to produce chromatic dispersion.

[0021] FIG. 5 is a more detailed diagram illustrating the operation of the apparatus in FIG. 4.

[0022] FIGS. 6aand 6bare diagrams illustrating side views of an apparatus which uses a VIPA together with a curved reflecting apparatus so as to provide, respectively, negative and positive chromatic dispersion to light.

[0023] FIGS. 7aand 7billustrate a top-view and side-view, respectively, of a first preferred embodiment of a dispersion compensator in accordance with the present invention.

[0024] FIG. 7cis a perspective view showing the locations of the focused wavelengths of the various channels upon the mirrors of the first preferred embodiment of the compensator in accordance with the present invention.

[0025] FIGS. 8a-8bfate a top-view and side-view, respectively, of a second preferred embodiment of a dispersion compensator in accordance with the present invention.

[0026] FIGS. 9aand 9billustrate a top-view and side-view, respectively, of a third preferred embodiment of a dispersion compensator in accordance with the present invention.

[0027] FIGS. 10aand 10billustrate a top-view and side-view, respectively, of a fourth preferred embodiment of a dispersion compensator in accordance with the present invention.

[0028] FIG. 11 illustrates a preferred embodiment of a system which utilizes a dispersion compensator in accordance with the present invention.

[0029] FIG. 12 illustrates a method and system for measuring operating and performance parameter of a dispersion compensator in accordance with the present invention.

[0030] FIG. 13 illustrates a method and system for measuring the pass bands of a dispersion compensator in accordance with the present invention.

[0031] FIG. 14 shows an example graph of the chromatic dispersion versus wavelength of a 100 GHz chromatic dispersion compensator in accordance with the present invention.

[0032] The present invention provides an improved chromatic dispersion, dispersion slope, and polarization mode dispersion (PMD) compensator. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

[0033] The present invention provides a dispersion compensator which comprises a Virtually Imaged Phased Array (VIPA) optically coupled on a first side to an optical communications system, and optically coupled on a second side to one or more transmission-type diffraction gratings and a mirror and one or more birefringent wedges. The VIPA assists in producing chromatic dispersion compensation, the diffraction grating assists in producing dispersion slope compensation, and the birefringent wedge assists in compensation for PMD.

[0034] FIG. 2 illustrates a VIPA utilized in the preferred embodiments of the dispersion and dispersion compensator in accordance with the present invention. The VIPA 76 is disclosed in U.S. Pat. No. 5,930,045, incorporated herein by reference. The VIPA 76 is preferably made of a thin plate of glass. An input light 77 is focused into a line 78 with a lens 80, such as a semi-cylindrical lens, so that input light 77 travels into VIPA 76. Line 78 is herein after referred to as “focal line”. Input light 77 radially propagates from focal line 78 to be received inside VIPA 76. The VIPA 76 then outputs a luminous flux 82 of collimated light, where the output angle of luminous flux 82 varies as the wavelength of input light 77 changes. For example, when input light 77 is at a wavelength λ_i, VIPA 76 outputs a luminous flux 82aat wavelength λ₁in a specific direction. When input light 77 is at a wavelength λ₂, VIPA 76 outputs a luminous flux 82bat wavelength λ₂in a different direction. Therefore, VIPA 76 produces luminous fluxes 82aand 82bthat are spatially distinguishable from each other.

[0035] FIG. 3 illustrates in more detail the VIPA 76 and light paths therein and therethrough. The VIPA 76 includes a plate 320 made of, for example, glass, and having reflecting films 322 and 324 thereon. Reflecting film 322 preferably has a reflectance of approximately 95% or higher, but less than 100%. Reflecting film 324 preferably has a reflectance of approximately 100%. A radiation window 326l is formed on plate 320 and preferably has a reflectance of approximately 0% reflectance.

[0036] Input light 77 is focused into focal line 78 by lens 80 through radiation window 326, to subsequently undergo multiple reflection between reflecting films 322 and 324. Focal line 78 is preferably on the surface of plate 320 to which reflecting film 322 is applied. Thus, focal line 78 is essentially line focused onto reflecting film 322 through radiation window 326. The width of focal line 78 can be referred to as the “beam waist” of input light 77 as focused by lens 80. Thus, the VIPA 76 focuses the beam waist of input light 77 onto the far surface (that is, the surface having reflecting film 322 thereon) of plate 320. By focusing the beam waist on the far surface of plate 320, the VIPA 76 reduces the possibility of overlap between (i) the area of radiation window 326 on the surface of plate 320 covered by input light 77 as it travels through radiation window 326 and (ii) the area on reflecting film 324 covered by input light 77 when input light 77 is reflected for the first time by reflecting film 324. It is desirable to reduce such overlap to ensure proper operation of the VIPA 76.

[0037] In FIG. 3, an optical axis 332 of input light 77 has a small tilt angle θ with respect to a line 340 perpendicular to the plane of plate 320. Upon the first reflection off of reflecting film 322, 5% of the light passes through reflecting film 322 and diverges after the beam waist, and 95% of the light is reflected towards reflecting film 324. After being reflected by reflecting film 324 for the first time, the light again hits reflecting film 322 but is displaced by an amount d. Then, 5% of the light passes through reflecting film 322. In a similar manner, the light is split into many paths with a constant separation d. The beam shape in each path forms so that the light diverges from virtual images 334 of the beam waist. Virtual images 334 are located with constant spacing 2t along a line 340 that is normal to plate 320, where t is the thickness of plate 320. The positions of the beam waists in virtual images 334 are self-aligned, and there is no need to adjust individual positions. The lights diverging from virtual images 334 interfere with one other and form collimated light 336 which propagates in a direction that changes in accordance with the wavelength of input light 77.

[0038] The spacing of light paths is d=2t sin θ, and the difference in the path lengths between adjacent beams is 2t cos θ. The angular dispersion of the VIPA 76 is proportional to the ratio of these two numbers, which is cot θ. As a result, a VIPA 76 produces a significantly large angular dispersion.

[0039] Reflecting surfaces 322 and 324 are in parallel with each other and spaced by the thickness t of plate 320 and are typically reflecting films deposited on plate 320. As previously described, reflecting surface 324 has a reflectance of approximately 100%, except in radiation window 326, and reflecting surface 322 has a reflectance of approximately 95% or higher. Therefore, reflecting surface 322 has a transmittance of approximately 5% or less so that approximately 5% or less of light incident on reflecting surface 322 will be transmitted therethrough and approximately 95% or more of the light will be reflected. The reflectances of reflecting surfaces 322 and 324 can easily be changed in accordance with the specific VIPA application. However, generally, reflecting surface 322 should have a reflectance which is less than 100% so that a portion of incident light can be transmitted therethrough. This reflectance need not be constant along the reflecting film 322.

[0040] The reflecting surface 324 has radiation window 326 thereon. Radiation window 326 allows light to pass therethrough, and preferably has no reflectance, or a very low reflectance. Radiation window 326 receives input light 77 to allow input light 77 to be received between, and reflected between, reflecting surfaces 322 and 324.

[0041] A VIPA 76 has strengthening conditions which are characteristics of the design of the VIPA 76. The strengthening conditions increase the interference of the output lights so that a luminous flux is formed. The strengthening conditions of the VIPA are represented by the following equation:

[0042] 2tcos Φ=mλ

[0043] in which Φ indicates the propagation direction of the resulting luminous flux as measured from a line perpendicular to the surface of reflecting surfaces 322 and 324, λ indicates the wavelength of the input light, t indicates the distance between the reflecting surfaces 322 and 324, and m indicates an integer. Therefore, if t is constant and m is assigned a specific value, then the propagation direction Φ of the luminous flux formed for input light having wavelength λ can be determined.

[0044] More specifically, input light 77 is radially dispersed from focal line 78 through a specific angle. Therefore, input light having the same wavelength will be traveling in many 20 different directions from focal line 78, to be reflected between reflecting surfaces 322 and 324. The strengthening conditions of the VIPA 76 cause light traveling in a specific direction to be strengthened through interference of the output lights to form a luminous flux having a direction corresponding to the wavelength of the input light. Light traveling in a different direction than the specific direction required by the strengthening condition is weakened by the interference of the output lights.

[0045] FIG. 4 illustrates an example prior-art apparatus that uses a VIPA as an angular dispersive component to produce chromatic dispersion. A description of this prior-art apparatus will assist in understanding the functioning of the compensator 700 in accordance with the present invention. As illustrated in FIG. 4, a light is output from a fiber 446, collimated by a collimating lens 448 and line-focused into VIPA 440 through radiation window 447 by a cylindrical lens 450. The VIPA 440 then produces a collimated light 451 which is focused by a focusing lens 452 onto a mirror 454. Mirror 454 can be a mirror portion 456 formed on a substrate 458. Mirror 454 reflects the light back through focusing lens 452 into VIPA 440. The light then undergoes multiple reflections in VIPA 440 and is output from radiation window 447. The light output from radiation window 447 travels through cylindrical lens 450 and collimating lens 448 and is received by fiber 446.

[0046] Therefore, light is output from VIPA 440 and reflected by mirror 454 back into VIPA 440. The light reflected by mirror 454 travels through the path which is nearly exactly opposite in direction to the path through which it originally traveled. As described in greater detail herein following, different wavelength components in the light are focused onto different positions on mirror 454, and are reflected back to VIPA 440. As a result, different wavelength components travel different distances, to thereby produce chromatic dispersion.

[0047] FIG. 5 illustrates in more detail the example prior-art apparatus illustrated in FIG. 4. Assume a light having various wavelength components is received by VIPA 440. The VIPA 440 will cause the formation of virtual images 560 of beam waist 562, where each virtual image 560 “emits” light. Focusing lens 452 focuses the different wavelength components in a collimated light from VIPA 440 at different points on mirror 454. More specifically, a longer wavelength 564 focuses at point 572, a center wavelength 566 focuses at point 570, and a shorter wavelength 568 focuses at point 574. Then, longer wavelength 564 returns to a virtual image 560 that is closer to beam waist 562, as compared to center wavelength 566. Shorter wavelength 568 returns to a virtual image 560 that is farther from beam waist 562, as compared to center wavelength 566. Thus, the arrangement provides for normal dispersion.

[0048] Mirror 454 is designed to reflect only light in a specific interference order, and light in any other interference order should be focused out of mirror 454. More specifically, as previously described, a VIPA 440 will output a collimated light. This collimated light will travel in a direction such that the optical path length difference between subsequent virtual images contributing to the collimated light is mλ, where m is an integer. The m^thorder of interference is defined as an output light corresponding to m. Each order comprises a plurality of wavelength components and the wavelength components of one order are repeated in any other order. However, collimated lights at the same wavelength for different interference orders generally travel in different directions and are therefore focused at different positions. Thus, the mirror 454 can be made to reflect only light from a single interference order back into VIPA 440.

[0049] A wavelength division multiplexed light usually includes many channels, wherein each channel has a center wavelength and the center wavelengths are usually spaced apart by a constant frequency spacing. If the thickness t between first and second surfaces 442 and 444 of VIPA 440 is set at a specific value, the arrangement will be able to simultaneously compensate for dispersion in each channel. The thickness t which permits such simultaneous dispersion compensation is such that all of the wavelength components corresponding to the center wavelengths have the same output angle from VIPA 440 and thus the same focusing position on mirror 454. This is possible when the thickness t is set so that, for each channel, the round-trip optical length through VIPA 440 traveled by the wavelength component corresponding to the center wavelength is a multiple of the center wavelength of each channel, that is, t is such that the quantity 2 nt cos θ is an integer multiple of the center wavelength of each channel. This amount of thickness t will hereafter be referred to as the “WDM matching free spectral range thickness”, or “WDM matching FSR thickness”.

[0050] Therefore, in FIG. 5, with the thickness t set to the WDM matching FSR thickness, VIPA 440 and focusing lens 452 will cause (a) the wavelength component corresponding to the center wavelength of each channel to be focused at point 570 on mirror 454, (b) the wavelength component corresponding to the longer wavelength component of each channel to be focused at point 572 on mirror 454, and (c) the wavelength component corresponding to the shorter wavelength component of each channel to be focused at point 574 on mirror 454. Therefore, VIPA 440 can be used to compensate for chromatic dispersion in all channels of a wavelength division multiplexed light. However, this prior-art VIPA-based dispersion-compensating apparatus does not compensate for dispersion slope or PMD.

[0051] FIGS. 6aand 6billustrate additional example prior-art apparatuses which use a VIPA to provide various values of chromatic dispersion to light. In FIGS. 6aand 6b, there are illustrated the travel directions of a longer wavelength 564, a center wavelength 566 and a shorter wavelength 568 of light emitted by a virtual image 560 of beam waist 562. The mirror 654 and the mirror 655 are located at or near the focal point of focusing lens 452. In FIG. 6a, mirror 654 is a convex mirror. With a convex mirror, the beam shift is magnified relative to that produced by a flat mirror. Therefore, a large chromatic dispersion can be obtained with a short lens focal length and a small amount of space. In FIG. 6b, mirror 655 is a concave mirror. With a concave mirror, the sign of the dispersion is inverted relative to that produced by a flat mirror.

[0052] With either a flat mirror 454 (FIG. 5) or a convex mirror 654 (FIG. 6a), the light of longer (“red”) wavelengths of an optical signal travels a shorter round trip distance through the apparatus then does the light of shorter (“blue”) wavelengths of said signal. Thus, negative chromatic dispersion is introduced into the signal. This form of apparatus is useful for compensating accumulated positive chromatic dispersion in an optical signal. With a concave mirror 655 (FIG. 6b), the light of “red” wavelengths of an optical signal travels a greater distance through the apparatus then does the light of “blue” wavelengths of said signal and, thus, positive chromatic dispersion is introduced into the signal. This latter form of apparatus is useful for compensating accumulated negative chromatic dispersion in an optical signal.

[0053] FIGS. 7aand 7billustrate a top-view and side-view, respectively, of a first preferred embodiment of a dispersion compensator in accordance with the present invention. The compensator 700 comprises a fiber 702, a collimator lens 703, a cylindrical lens 704, a VIPA 706 of which the thickness is equal to the WDM matching FSR thickness, a diffraction grating 710, a birefringent wedge 720, a focusing lens 712, and a first and second mirror 714a-714b. A wavelength-division multiplexed composite signal 701 is output from fiber 702, is collimated by collimator lens 703 and is then brought to a line focus at the beam waist 705 of VIPA 706 by the cylindrical lens 704. The channels 707 and 708 are two representative channels of the composite optical signal 701. As discussed previously, the VIPA 706 spatially disperses the wavelengths comprising each one of the channels of composite signal 701, such that rays of each wavelength emanate from the VIPA 706 along ray paths which are parallel to one another but of a different direction than rays of any other wavelength comprising each channel. Upon passing through and exiting the VIPA 706, the wavelengths comprising each of these channels are separated and dispersed within a vertical dispersion plane. For instance, the wavelengths of first and second channels, 707 and 708, respectively, are separated into relatively longer wavelengths 707aand 708aand relatively shorter wavelengths, 707band 708b, respectively, together with respective continua of intervening wavelengths. Because the thickness of the VIPA 706 is equal to the WDM matching FSR thickness, then, immediately upon exiting the VIPA 706, the path of wavelength 707 a overlaps that of wavelength 708aand the path of wavelength 707boverlaps that of wavelength 708b.

[0054] After exiting the VIPA 706, the wavelengths comprising each channel are separated and dispersed from one another within a horizontal dispersion plane by the transmission grating 710. The paths of the various signals are spatially dispersed from one another according to their respective wavelengths. The dispersion plane of transmission grating 710 is not parallel to that of the VIPA 706, however. Instead, these two dispersion planes are perpendicular to one another. Thus, as shown in FIG. 7a, the dispersion plane of transmission grating 710 is horizontal and, upon emerging from this grating, the wavelengths comprising the first channel 707 are output along a different horizontal direction from those of the second channel 708. The path of the relatively longer wavelength 707aof the first channel 707 is separated within a horizontal plane from that of the relatively longer wavelength 708aof the second channel 708. In similar fashion, the relatively shorter wavelengths 707b, 708bof each channel are spatially dispersed within a horizontal plane. These twice dispersed wavelengths of the plurality of channels then pass through the birefringent wedge 720 which spatially separates each such wavelength into a first and second rays of mutually orthogonal polarizations. The less-deflected horizontally polarized rays comprise the rays 707ah, 708ah, 707bhand 708bhand the more-greatly-deflected vertically polarized rays comprise the rays 707bv, 708bv, 707avand 708av. Both the horizontally polarized and the vertically polarized rays pass through and are focused by lens 712, which focuses them onto upper mirror 714aand lower mirror 714b, respectively.

[0055] FIG. 7cprovides a perspective view of the focused wavelengths of the various channels of composite optical signal 701 upon the mirrors 714aand 714bof the first preferred embodiment of the compensator in accordance with the present invention. Preferably, each of these mirrors is shaped as a curved conical mirror. Chromatic dispersion is adjusted by simultaneous movement of both mirrors 714a-714balong the adjustment direction 726 and PMD dispersion is adjusted by movement of one of the mirrors 714a-714balong the adjustment direction 728.

[0056] The mirrors 714a-714breflect the wavelengths of the channels comprising the composite optical signal 701 back through the various components of the compensator 700 so as to be recombined into channels at birefringent wedge 720 and into a dispersion compensated composite optical signal at transmission grating 710 and VIPA 706. Compensatory dispersion is introduced into the return signal by virtue of the fact that different wavelengths “return” to different virtual images of the beam waist 705 within VIPA 706 as previously described. The degree of chromatic dispersion introduced into any channel is determined by the curvature of the mirrors at the locations where the channel is reflected. The degree of introduced chromatic dispersion slope is determined by the change in curvature of the mirrors along adjustment direction 726. The degree of PMD introduced into any channel is determined by the difference in reflection angle between the horizontally polarized rays as reflected off mirror 714aand the vertically polarized rays as reflected off mirror 714b. This difference is controlled by motion of one of the mirrors 714a-714balong the adjustment direction 728.

[0057] FIGS. 8a-8billustrate a top-view and side-view, respectively, of a second preferred embodiment of a dispersion compensator in accordance with the present invention. The compensator 800 is similar in construction and operation to the compensator 700 except that the single birefringent wedge 720 and the single focusing lens 712 comprising compensator 700 are respectively replaced by the two birefringent wedges 820a-820band the two focusing lenses 812a-812bin the compensator 800. One of ordinary skill in the art will recognize that the two birefringent wedges 820a-820bmay be replaced by a single birefringent plate. The configuration of elements in the compensator 800 permits each lens mirror assembly 824a-824bto be adjusted independently of the other along adjustment direction 830. This adjustment along the adjustment direction 830 provides an additional means of compensating PMD by causing a difference between the physical path lengths of horizontally polarized rays 807ah, 807bh, 808ah, and 808bhand of vertically polarized rays 807av, 80bv, 808avand 808bv. PMD compensation may also be varied in the compensator 800 by adjusting one or more of the mirrors 714a-714baccording to adjustment direction 728.

[0058] FIGS. 9a-9billustrate a top view and side view, respectively, of a third preferred embodiment of a dispersion compensator in accordance with the present invention. Like the compensator 700, the compensator 900 simultaneously introduces dispersion into optical channels so as to compensate for chromatic dispersion, chromatic dispersion slope and PMD. However, the compensator 900 differs from the compensator 700 through the substitution of two parallel transmission gratings 910a-910bfor the single transmission grating 710 and the optional addition of a beam condenser 916 comprising two lenses 917a-917b.

[0059] The parallel transmission gratings 910a-910bof compensator 900 cause the dispersed wavelengths of all channels to propagate parallel to one another in top-view projection after emerging from the grating pair. The wavelengths 907aand 907brepresent relatively longer and shorter wavelengths of a first such channel; the wavelengths 908aand 908brepresent relatively longer and shorter wavelengths of a second such channel. Because of the parallelism of the gratings 910a-910b, the two mirrors 914a-914bdo not necessarily require the curved conical shape of the mirrors 714a-714b. In the third preferred embodiment, the mirrors 914a-914bcomprise simple conical shapes, although numerous other shapes are possible. Optionally, the separations between the channels are then condensed along a horizontal dimension perpendicular to the main axis by the two cylindrical lenses 917a-917bcomprising beam condenser 916. Thus, the degree of chromatic dispersion slope compensation may be controlled by adjustment of the beam condenser 916. The horizontally polarized rays 907ah, 907bh, 908ah, 908bhand the vertically polarized rays 907av, 907bv, 908av, 908bvof two representative channels are then focused by lens 912 onto first mirror 914aand second mirror 914b, respectively. In other aspects, the operation of the compensator 900 is similar to that of the compensator 700.

[0060] FIGS. 10a-10billustrate a top view and side view, respectively, of a fourth preferred embodiment of a dispersion compensator in accordance with the present invention. The compensator 1000 is similar in construction and operation to the compensator 900 except that the single birefringent wedge 920 and the single focusing lens 912 comprising compensator 900 are respectively replaced by the two birefringent wedges 1020a-1020band the two focusing lenses 1012a-1012bin the compensator 1000. The configuration of elements in the compensator 1000 permits each lens/mirror assembly 1024a-1024bto be adjusted independently of the other along adjustment direction 1032. This adjustment along the adjustment direction 1032 provides an additional means of compensating PMD by causing a difference between the physical path lengths of horizontally polarized rays 1007ah, 1007bh, 1008ah, and 1008bhand of vertically polarized rays 1007av, 1007bv, 1008avand 1008bv. As in the compensator 900, PMD compensation may also be varied in the compensator 1000byadjusting one or more of the mirrors 1014a-1014baccording to adjustment direction 928.

[0061] FIG. 11 illustrates a preferred embodiment of a system which utilizes the dispersion compensator in accordance with the present invention. The system 1100 comprises an input fiber optic line 1102, an optical tap 1111, a dispersion analyzer 1108, a compensator controller 1110, a dispersion compensator 1112, a polarization controller 1114, an optical circulator 1106 and an output fiber optic line 1104. The input line 1102 and output line 1104 are optically coupled to the port 1151 and to the port 1153 of circulator 1106, respectively. The system 1100 further comprises a fiber optic tap line 1105 optically coupling the optical tap 1111 to the dispersion analyzer 1108 and a fiber optic line 1103 optically coupling the dispersion compensator 1112 to port 1152 of the circulator 1106. The system further comprises first 1107 and second 1109 electronic signal or control lines respectively connected between the dispersion analyzer 1108 and the controller 1110 and between the controller 1110 and the dispersion compensator 1112. The system further comprises a third electronic signal or control line 1113 connected between the dispersion analyzer 1108 and the polarization controller 1114. The polarization controller 1114 may be one of several well-known types, such as a looped fiber device, an optical wave plate device, or an electronic liquid crystal device. The polarization controller 1114 serves to convert or rotate the polarization state of incoming uncompensated signal light into a polarization state compatible with the birefringent elements of the compensator 1112 so as to provide optimal PMD compensation. The fiber optic line 1103 preferably comprises a polarization-preserving fiber.

[0062] An uncompensated optical signal or composite optical signal 1101uis input to the system 1100 via the input fiber optic line 1102. The uncompensated signal 1101ucomprises unwanted chromatic dispersion and PMD that is to be compensated by the system 1100. The uncompensated optical signal or composite signal 1101upasses through the polarization controller 1114 to the port 1151 of the optical circulator 1106. The optical circulator 1106 directs signal 1101uto its port 1152, from which it is immediately output to the fiber optic line 1103 and input to the dispersion compensator 1112. The dispersion compensator 1112 comprises one of the dispersion compensator embodiments in accordance with the present invention.

[0063] As described previously herein, the dispersion compensator 1112 provides compensatory chromatic dispersion, dispersion slope, and PMD to the uncompensated optical signal or composite optical signal 1101uso as to output the compensated signal or composite optical signal 1101c.The compensated signal 1101cis output along the optical fiber line 1103 in the opposite direction from the input signal 1101u.The compensated signal is then input to optical circulator 1106 through its port 1152. By the well-known operation of optical circulators, the compensated signal 1101cis directed to the port 1153 of optical circulator 1106, from which it is immediately output to the output fiber optic line 1104. A small portion 1101sof the compensated output signal 1101cis split off from signal 1101cby the optical tap 1111 and diverted to the dispersion analyzer via the fiber optic tap line 1105.

[0064] The dispersion compensator 1112 is controlled by electronic signal 1118 output from controller 1110 along electronic line 1109. The controller 1110 generates control signals in response to an electronic signal or signals 1116 produced by dispersion analyzer 1108 and sent to the controller 1110 along electronic line 1107. The dispersion analyzer may comprise separate known components to analyze chromatic dispersion and PMD, such as an optical spectrum analyzer and an ellipsometer, respectively. The electronic signal(s) 1116 contains information measured by the dispersion analyzer 1108 and pertaining to the magnitude and sign of chromatic dispersion and PMD carried by the sample signal 1101s. By logical extension, these quantities also relate to the signal 1101u. In response to these measurements, the dispersion analyzer 1108 outputs a first electronic signal 1116 to controller 1110 along electronic line 1107 and, optionally, outputs a second electronic signal 1120 to the polarization controller along electronic line 1113.

[0065] The amount of compensatory dispersion provided by dispersion compensator 1112 is controlled by the electronic signal 1118 output from the controller 1110 in response to the dispersion characteristics measured by dispersion analyzer 1108. Adjusting one or more of the various optical components along its respective adjustment direction, as described previously herein, causes variation in the magnitude and sign of the compensatory dispersion. It may be necessary to separate and or rotate the polarization components of the uncompensated signal 1110u, prior to input to the dispersion compensator 1112. The polarization controller 1114 performs these polarization separation and rotation functions.

[0066] FIGS. 12-13 illustrate performance characterization of a dispersion compensator in accordance with the present invention. This performance characterization applies to any of the preferred embodiments described above. FIG. 12 illustrates a method and system for measuring operating and performance parameters of a dispersion compensator in accordance with the present invention. Such parameters are well known and comprise insertion loss (IL), wavelength-dependent polarization dependent loss (WDPDL) and polarization-dependent wavelength shift (PDWS). In the system 1200 (FIG. 12), the compensator 1206 to be tested receives light of known wavelengths from a tunable laser source (TLS) 1202. The TLS 1202 varies or sweeps the wavelength delivered to the system 1200 between known limits in a controlled way as a periodic function of time. Prior to entering the compensator 1206, the light emitted by the TLS passes through a polarization controller 1204 that is optically coupled to both the TLS 1202 and the input of the compensator 1206. The light output from the polarization controller 1204 is delivered to an optical input of the compensator 1206. The optical output of the compensator is analyzed by an optical power measurement unit 1208 optically coupled to the compensator 1206. Preferably, the optical power measurement unit 1208 comprises an optical sensor (OS) 1208a(such as a photo-diode detector of photomultiplier tube), an electronic storage, analysis and display device 1208b(such as a lightwave multimeter), and an electronic link 1208cbetween the OS 1208aand the electronic storage, analysis and display device 1208b.

[0067] The polarization controller 1204 is disposed such that light of a controlled polarization state may be delivered to the compensator 1206 that is being tested. Typically, the controlled polarization varies in a known way with time. Such polarization controllers are well-known in the art and typically comprise a quarter-waveplate, a half-waveplate and a polarizer disposed in the light path in front of the two waveplates. By rotating the polarizer and the waveplates of the polarization controller 1204 or by removing them from the light path completely, either unpolarized light or light of a known polarization state may be delivered to the compensator 1206.

[0068] By utilizing the polarization controller 1204 to vary the light polarization delivered to the compensator 1206 between or during wavelength sweeps of the TLS 1202, the WDPDL and PDWS for the compensator may be determined. Four different polarization states forming a rectangular system on the Poincaré sphere must be generated so as to calculate WDPDL by the well-known Mueller method. The polarization controller 1204 sequentially transmits such polarization states to the compensator 1206. The intensity of polarized light transmitted through the compensator 1206 is determined from the output of the OS 1208aat each wavelength of each individual sweep of the TLS 1202. This intensity data is then transmitted, in electronic form, to the electronic storage, analysis and display device 1208bvia the electronic link 1208c. The performance parameters of the compensator 1206 are then calculated from the results of the intensity measurements during these sweeps. The calculations and/or data storage and/or data display are performed by the electronic storage, analysis and display device 1208b.

[0069] FIG. 13 illustrates a method and system for measuring the pass bands of a dispersion compensator in accordance with the present invention. The compensator in accordance with the present invention produces a periodic spectrum of pass bands which must be measured to assure that they match or align, in wavelength, with a conventional transmission channel standard such as that proposed by the International Telecommunication Union (ITU). In the pass band test system 1300 (FIG. 13), a broad-band light source (BBLS) 1302 is optically coupled to the input of a compensator 1306 to be tested and an optical spectrum analyzer 1308 is optically coupled to the output of the compensator 1306. The BBLS 1302, which may be an LED, an erbium-doped fiber, an incandescent bulb or the like, provides “white” light—that is, light comprising a continuum of wavelengths within a certain test range—to the compensator 1306. The resulting spectrum of light intensity as a function of wavelength that is output by compensator 1306 comprises the set of pass bands of the compensator 1306. This spectrum is measured by the OSA 1308 optically coupled to the compensator 1306.

[0070] FIG. 14 shows an example graph of the chromatic dispersion versus wavelength of a 100 GHz chromatic dispersion compensator in accordance with the present invention. The dotted line 1402 shows the experimentally measured dispersion values of the compensator over wavelengths encompassing the well-known C-band. The solid line 1404 shows the theoretical chromatic dispersion of the that would be produced by 80 km of single mode fibers and would require compensation. The slope of the solid line 1404 is the chromatic dispersion slope; the opposite slope of the dotted line 1402 is the compensatory dispersion slope provided by a chromatic dispersion and dispersion slope compensator in accordance with the present invention. The graph in FIG. 14 thus shows that the compensator in accordance with the present invention can compensate for 90% of the dispersion slope of the single mode fiber. This 90% dispersion compensation ratio is consistent with optical system requirements.

[0071] Typical operating parameters of the compensator in accordance with the present invention have also been measured utilizing the system 1200 (FIG. 12) and the system 1300 (FIG. 13). The insertion loss is less than 11 dB for the compensator whose chromatic dispersion is illustrated in FIG. 14. The 1 dB pass band for the compensator is greater than 0.25 nm. The insertion loss variation for all the 100 GHz WDM channels in the C-band is within ±0.15 dB. These values are suitable for utilization in an optical communications system.

[0072] A dispersion compensator which utilizes a Virtually Imaged Phased Array (VIPA), gratings, and birefringent wedges to moderate chromatic dispersion, dispersion slope and polarization mode dispersion, and a method and system for testing such a dispersion compensator, has been disclosed. The dispersion compensator in accordance with the present invention provides simultaneous tunable compensation of these various dispersions utilizing a single apparatus. A system which utilizes the compensator is thus cost effective to manufacture. Systems may be provides which determines the operating and performance parameters for the compensator, and measures the spectrum outputted by the compensator.

[0073] Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.