Abstract:
A chromatic dispersion compensator comprises a beam delay element, such as one or more Gires-Tournois etalon (GTEs); a beam director, such as a polarizing beam splitter (PBS), a prism polarizer, a dielectric polarizer or a crystal polarizer; and a polarization changer, such as one or more quarter-wave plates. The beam director directs an inbound optical beam based on its polarization toward the beam delay element whereat a first unit of group delay is induced. The optical beam traverses the beam delay element and enters a polarization changer whereat the optical beam obtains a new polarization. The optical beam traverses the polarization changer and re-enters the beam director whereupon a path change is induced on the optical beam based on its new polarization and the optical beam is redirected toward the beam delay element whereat a second unit of group delay is induced. The compensator is arranged to advantageously perform the referenced technique contemporaneously on two constituent optical beams (having different polarizations) of an inbound optical beam and eventually re-combine the two constituent optical beams into an outbound optical beam. The beam delay element may include one or more GTEs. Inducement of path changes and direction of the optical beam to the beam delay element may be assisted by one or more ninety degree mirrors. The referenced technique may be performed on an optical beam more than twice, such that a third, fourth, fifth, etc. delay is induced on the optical beam.

Description:
CROSS-REFERENCE OF RELATED APPLICATION(S) 
     This application claims the benefit of U.S. provisional application No. 60/390,915, filed on Jun. 24, 2002, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The wavelength dependence of the group velocity of light propagating in an optical fiber leads to pulse distortion. Wavelength-dependent pulse distortion is commonly called group chromatic dispersion (CD) and can lead to high bit error rates or even signal loss if left uncorrected. The problem becomes particularly severe at high data rates and over long transmission distances. In the conventional approach to CD correction, a pulse is detected after a short transmission distance, reshaped in the electronic domain, and retransmitted. For Dense Wave Division Multiplexing (DWDM) systems, reshaping in the electronic domain is too costly. 
     It is known that CD can be corrected in the optical domain using a dispersion compensator which provides compensating dispersion. For example, a Gires-Tournois etalon (GTE) can provide periodic compensating dispersion at frequencies aligned to the International Telecommunications Union (ITU) transmission channel grids. 
     A GTE consists of a partially reflecting mirror at the input face and a totally reflecting mirror at the rear. These two mirrors are parallel and separated by a distance in between. As a result of the cavity between the mirrors, the wavelength-dependent phase shift a beam experiences upon interaction with a GTE can be written 
             ϕ   =     2   ⁢           ⁢       tan     -   1       ⁡     (         1   +     R         1   -     R         ⁢     tan   ⁡     (       ω   c     ⁢   n   ⁢           ⁢   d     )         )                 (   1   )             
 
where R is the mirror reflectivity of the front mirror, n is the index of refraction of the cavity medium and d is the space between the mirrors. Notice that the phase shift depends on the frequency of light ω.
 
     The compensating group delay can be obtained by taking a derivative of the phase shift with respect to the frequency ω. This leads to 
                 τ   =         ⅆ   ϕ       ⅆ   ω       =       σ     1   +       (       σ   2     -   1     )     ⁢       sin   2     ⁡     (     ω   ⁢           ⁢   n   ⁢           ⁢     d   /   c       )             ⁢     τ   0           ⁢     
     ⁢   where     ⁢                   (   2   )                       ⁢       σ   =       1   +     R         1   -     R           ⁢     
     ⁢   and             (   3   )                       ⁢       τ   0     =           ⅆ               ⅆ   ω       ⁢     (     2   ⁢     ω   c     ⁢   n   ⁢           ⁢   d     )       =       2   ⁢   d       v   g                   (   4   )             
 
Here, τ 0  is the round trip flight time inside the cavity; v g  is the group velocity of light inside the cavity medium. For a vacuum cavity, v g  is c, for a non-vacuum cavity, v g  is c/n.
 
     The compensating CD (in units of ps/nm) is defined as the derivative of the group delay Γ with respect to wavelength λ, that is 
             CD   =         ⅆ   τ       ⅆ   λ       =       -       2   ⁢           ⁢   π   ⁢           ⁢   c       λ   2         ⁢       ⅆ   τ       ⅆ   ω                   (   5   )             
 
     The amount of compensating dispersion provided by a single GTE interaction has generally been inadequate for long distance broadband applications. Multiple GTE arrangements—for example, dual GTEs separated by a zig-zag beam path—have achieved a higher amount of compensating dispersion. However, known multiple GTE arrangements have experienced problems of beam walk-off due to the oblique incidence of the beam&#39;s arrival at the GTEs. As well, multiple GTE interactions are needed in order to increase the bandwidth of the periodic CD function required for CD compensation. 
     In summary, to adequately correct CD in long distance, broadband applications, multiple GTE interactions are needed. However, to avoid beam walk-off, the beam must arrive at substantially normal incidence for each of the multiple GTE interactions. 
     SUMMARY OF THE INVENTION 
     The present invention, in a basic feature, provides a chromatic dispersion compensator comprising a beam delay element, such as one or more GTEs; a beam director, such as a polarizing beam splitter (PBS), a prism polarizer, a dielectric polarizer or a crystal polarizer; and a polarization changer, such as one or more quarter-wave plates. The beam director directs an inbound optical beam based on its polarization toward the beam delay element whereat a first unit of group delay is induced. The optical beam traverses the beam delay element and enters a polarization changer whereat the optical beam obtains a new polarization. The optical beam traverses the polarization changer and re-enters the beam director whereupon a path change is induced on the optical beam based on its new polarization and the optical beam is redirected toward the beam delay element whereat a second unit of group delay is induced. The compensator is arranged to advantageously perform the referenced technique contemporaneously on two constituent optical beams (having different polarizations) of an inbound optical beam and eventually recombine the two constituent optical beams into an outbound optical beam. The beam delay element may include one or more GTEs. Inducement of path changes and direction of the optical beam to the beam delay element may be assisted by one or more prismatic mirrors. Naturally, the referenced technique may be performed on an optical beam more than twice, such that a third, fourth, fifth, etc. delay is induced on the optical beam. 
     The present invention, in another feature, provides a method for chromatic dispersion compensation which comprises directing based on a first polarization an optical beam to a delay element, inducing a first unit of group delay on the optical beam at the delay element, changing the polarization of the optical beam from the first polarization to a second polarization, inducing a path change on the optical beam based on the second polarization, redirecting the optical beam to the delay element and inducing a second unit of group delay on the optical beam at the delay element. 
     These and other features of the invention will be better understood by reference to the detailed description of the preferred embodiment, taken in conjunction with the drawings which are briefly described below. Of course, the invention is defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a chromatic dispersion compensator having one PBS, one quarter-wave plate, one GTE and three ninety degree mirrors, in accordance with a first embodiment of the invention. 
         FIG. 2  shows a chromatic dispersion compensator having one PBS, one quarter-wave plate, two GTEs and three ninety degree mirrors, in accordance with a second embodiment of the invention. 
         FIG. 3  shows a chromatic dispersion compensator having one PBS, two quarter-wave plates, two GTEs and a plurality of elevator prisms, in accordance with a third embodiment of the invention. 
         FIG. 4  shows a crystal polarizer. 
         FIG. 5  shows a chromatic dispersion compensator having one crystal polarizer, two quarter-wave plates, three GTEs, one ninety degree mirror and one PBS, in accordance with a fourth embodiment of the invention. 
         FIG. 6  shows a chromatic dispersion compensator having one PBS, two quarter-wave plates, two GTEs and two ninety degree mirrors, in accordance with a fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 , a chromatic dispersion compensator  10  in accordance with a first embodiment of the invention is shown. Compensator  10  includes a PBS  110 , a first ninety degree mirror  120 , a quarter-wave plate  130 , a GTE  140 , a second ninety degree mirror  150  and a third ninety degree mirror  160 . 
     PBS  110  is made from two right angle glass prisms joined at the hypotenuse. The hypotenuse face of one prism has a dielectric coating so as to make PBS  110  reactive to the polarization of light. That Is, light is either transmitted or reflected at the hypotenuse of PBS  110  depending on its polarization. 
     First ninety degree mirror  120  is a right angle glass prism whose hypotenuse is fully reflective. 
     Quarter-wave plate  130  is a birefringent crystal which converts linearly polarized light into circularly polarized light and vice versa. When quarter-wave plate  130  is double-passed, it acts as a half-wave plate and rotates the plane of polarization of light. 
     GTE  140  has a first mirror which is partially reflective, a second mirror which is fully reflective and a cavity in between. The spacing between the mirrors (i.e. the thickness of the cavity) is generally a function of the channel spacing of a DWDM system in which compensator  10  is operative. Light arriving from PBS  110  or prismatic mirror  120  enters and exits GTE  140  through the partially reflective mirror. GTE  140  subjects different wavelength components of the light to variable delay in accordance with its resonant properties. That is, the partial reflectivity of the first mirror causes certain wavelength components to be restrained in the cavity between the first mirror and the second mirror longer than others. GTE  140  thereby imposes a group delay on the wavelength components of the light which, when implemented over multiple instances, i.e. multiple bounces, can correct CD previously induced on the light&#39;s pulses by a high speed, long haul, DWDM transmission system. 
     Second ninety degree mirror  150  is a right angle glass prism whose shortest two legs are fully reflective. 
     Third ninety degree mirror  160  is a right angle glass prism whose shortest two legs are fully reflective. 
     In operation, an input optical beam  100 , which is unpolarized, is incident into PBS  110 . PBS  110  splits beam  100  into two polarized beams A 1 , B 1 . Polarized beams A 1 , B 1  are directed (with the help of mirror  120  in the case of beam B 1 ) toward GTE  140  at normal incidence via quarter-wave plate  130 . GTE  140  contributes a first unit of group delay on polarized beams A 1 , B 1 . Upon reflecting from GTE  140  and passing through quarter-wave plate  130  a second time on the return trip, the polarization plane of beams A 1 , B 1  is rotated. Thus, when the beams A 1 , B 1  re-intersect at PBS  110 , they are recombined into an unpolarized beam and directed to mirror  150 . This completes the first cycle. 
     Prismatic mirror  150  redirects the unpolarized beam toward PBS  110 , beginning a second cycle in which GTE  140  contributes a second unit of group delay on polarized beams A 2 , B 2 . Upon reflecting from GTE  140  and double passing through quarter-wave plate  130 , the polarization plane of beams A 2 , B 2  is once again rotated. Thus, when the beams A 2 , B 2  re-intersect at PBS  110 , they are recombined into an unpolarized beam and directed to mirror  160 . This completes the second cycle. 
     Mirror  160  redirects the unpolarized beam toward PBS  110 , beginning a third cycle in which GTE  140  contributes a third unit of group delay on polarized beams A 3 , B 3 . Upon reflecting from GTE  140  and double passing through quarter-wave plate  130 , the polarization plane of beams A 2 , B 2  is once again rotated. Thus, when the beams A 3 , B 3  re-intersect at PBS  110 , they are recombined into an unpolarized beam and directed to mirror  150 . This completes the third cycle. 
     Mirror  150  redirects the unpolarized beam toward PBS  110 , beginning a fourth and final cycle in which GTE  140  contributes a fourth unit of group delay on polarized beams A 4 , B 4 . Upon reflecting from GTE  140  and double passing through quarter-wave plate  130 , the polarization plane of beams A 4 , B 4  is once again rotated. Thus, when the beams A 4 , B 4  re-intersect at PBS  110 , they are recombined into an unpolarized output optical beam  190 , which exits compensator  10 . 
     All told, compensator  10  contributes four units of group delay over four cycles. That is, four interactions with GTE  140  are made by the constituent components of input optical beam  100 , all at normal incidence. In general, any number of such interactions can be designed into this geometry. 
     In  FIG. 2 , a chromatic dispersion compensator  20  in accordance with a second embodiment of the invention is shown. Compensator  20  includes a PBS  210 , a first ninety degree mirror  220 , a quarter-wave plate  230 , a first GTE  240 , a second GTE  245 , a second ninety degree mirror  250  and a third ninety degree mirror  260 . Elements  210 ,  220 ,  230 ,  250  and  260  are similar in composition and operation to their counterparts  110 ,  120 ,  130 ,  250  and  260  in  FIG. 1 . However, use of two GTEs  240 ,  245  having different resonant properties allows for polarization mode dispersion (PMD) in which the group delays induced on the beams may be made polarization-dependent. Use of two GTEs  240 ,  245  also permits adjustments to ensure normal incidence of beams into GTEs  240 ,  245 , even if one or more of prismatic mirrors  220 ,  250 ,  260  are imperfect. Finally, use of two GTEs  240 ,  245  enables CD correction of pulses transmitted on broader channels. 
     In operation, an input optical beam  200 , which is unpolarized, is incident into PBS  210 . PBS  210  splits beam  200  into two polarized beams C 1 , D 1 . Polarized beams C 1 , D 1  are directed (with the help of mirror  220  in the case of D 1 ) toward GTEs  240 ,  245 , respectively, at normal incidence via quarter-wave plate  230 . GTEs  240 ,  245  contribute a first unit of group delay to polarized beams C 1 , D 1 , respectively. Recall that the group delay induced by GTE  240  may have different wavelength-dependence than the group delay induced by GTE  245  owing to configurably different resonant properties of GTEs  240 ,  245 . Upon reflecting from GTEs  240 ,  245 , respectively, and again passing through quarter-wave plate  230 , the polarization plane of beams C 1 , D 1  is rotated. Thus, when beams C 1 , D 1  re-intersect at PBS  210 , they are recombined into an unpolarized beam and directed to mirror  250 . This completes the first cycle. 
     Mirror  250  redirects the unpolarized beam toward PBS  210 , beginning the second cycle in which GTEs  240 ,  245  contribute a second unit of group delay on polarized beams C 2 , D 2 , respectively. All told, compensator  20  contributes four units of group delay over four cycles. That is, four interactions with GTEs  240 ,  245  are made by the constituent components of input optical beam  200  before an unpolarized output optical beam  290  exits compensator  20 . Moreover, the constituent portion of input optical beam  200  which had a first polarization is subjected to four interactions with GTE  240 , while the constituent portion of inbound beam  200  which had a second polarization is subjected to four bounces off GTE  245 , enabling PMD if desired by configuring GTE  240  and GTE  245  with different resonant properties. In general, any number of such interactions can be designed into this geometry. 
     In  FIG. 3 , a chromatic dispersion compensator  30  in accordance with a third embodiment of the invention is shown. Compensator  30  has a PBS  310 , two quarter-wave plates  320 ,  350 , two GTEs  330 ,  360  and multiple elevator prisms  340 . The principle of operation is generally the same as in  FIGS. 1 and 2  except in compensator  30  the beam migrates from ground level to higher levels with the assistance of elevator prisms  340 . Elevator prisms  340  are right angle glass prisms whose shortest two legs are fully reflective and which are disposed to cause an input optical beam to project onto a higher plane upon reflection. 
     In operation, an input optical beam  300 , which is unpolarized, is incident into PBS  310  (identified as beam stage  1  in  FIG. 3 ). PBS  310  splits beam  300  into two polarized beams. The two polarized beams are directed toward GTEs  330 ,  360 , respectively, at normal incidence via quarter-wave plates  320 ,  350 , respectively. GTEs  330 ,  360  contribute a first unit of group delay on the polarized beams, respectively. Upon reflecting from GTEs  330 ,  360  and passing through quarter-wave plates  320 ,  350  a second time on the return trip, the polarization plane of the beams is rotated. Thus, when the beams re-intersect at PBS  310 , they are recombined into an unpolarized beam and directed to an elevator prism (beam stage  2  in  FIG. 3 ). This elevator prism has been omitted from  FIG. 3  for clarity. This completes the first cycle. 
     The elevator prism elevates and redirects, the unpolarized beam toward PBS  310  (beam stage  3  in  FIG. 3 ), beginning a second cycle in which GTEs  330 ,  360  contribute a second unit of group delay on the respective polarized beams, Upon reflecting from GTEs  330 ,  360  and completing another double-pass through quarter-wave plates  320 ,  350 , the beams re-intersect at PBS  310  and are recombined into an unpolarized beam and directed to elevator prism  340  (beam stage  4  in  FIG. 3 ). This completes the second cycle. 
     All told, the beam completes beam stages  5 ,  6 ,  7 , . . .  11  in which compensator  30  contributes six units of group delay on the polarized beams, respectively, over six cycles. That is, six interactions with GTEs  330 ,  360  are made by the constituent components of input optical beam  300 , all at normal incidence, before output optical beam  370 , which is unpolarized, exits compensator  30  (beam stage  12  in  FIG. 3 ). 
     In  FIG. 4 , a crystal polarizer  40  is shown. Crystal polarizer  40  includes a birefringent crystal  410  which is reactive to the polarization of light to create spatial separation, without altering direction. That is, light is either transmitted on the plane of entry or “walks over” and is transmitted on a different plane depending on its polarization. In the case of  FIG. 4 , ordinary beam “o” having a first polarization is transmitted as output optical beam  430  on the plane of entry while extraordinary beam “e” having a second polarization walks over and is transmitted as output optical beam  420  on a lower plane than the plane of entry. Both output optical beams  420 ,  430  continue in the direction of entry. 
     In  FIG. 5 , a chromatic dispersion compensator  50  in accordance with a fourth embodiment of the invention is shown. Compensator  50  has a crystal polarizer  520 , two quarter-wave plates  510 ,  530 , three GTEs  540 ,  550 ,  560 , a ninety degree mirror  570  and a PBS  580 . 
     In operation, an input optical beam  500 , which is unpolarized, is incident into crystal polarizer  520 . Crystal polarizer  520  splits beam  500  into two polarized beams E 1  (ordinary beam “o”) and F 1  (extraordinary beam “e”) in the general manner discussed above in connection with  FIG. 4 . That is, E 1  is transmitted on the plane of entry while F 1  walks down and is transmitted on a lower plane than the plane of entry. Polarized beams E 1 , F 1  are directed toward GTE  540  at normal incidence via quarter-wave plate  530 . GTE  540  contributes a first unit of group delay on polarized beams E 1 , F 1 . Upon reflecting from GTE  540  and passing through quarter-wave plate  530  a second time on the return trip, the polarization plane of beams E 1 , F 1  is rotated. This completes the first cycle. 
     When beams E 1 , F 1  reenter crystal polarizer  520  (transitioning to beams E 2 , F 2 , respectively), E 2  walks up for transmission on a higher plane than the plane of entry while F 2  is transmitted on the plane of entry. Polarized beams E 2 , F 2  are directed toward GTEs  560 ,  550 , respectively, at normal incidence via quarter wave plate  510 . GTEs  560 ,  550  contribute a second unit of group delay to polarized beams E 2 , F 2 , respectively. Upon reflecting from GTEs  560 ,  550  and passing through quarter-wave plate  510  a second time on the return trip, the polarization plane of beams E 2 , F 2  is rotated. This completes the second cycle. 
     In similar fashion, compensator  50  contributes eight additional units of group delay on polarized beams E 3  . . . E 10 , F 3  . . . F 10 , respectively, over eight additional cycles. In all, a total of ten bounces off GTEs  540 ,  550 ,  560  are made on the constituent portions of input optical beam  500 , all at normal incidence. Then, polarized beams E 11  and F 11  are directed to PBS  580  (with the help of mirror  570  in the case of beam E 11 ). At PBS  580 , beams E 11 , F 11  re-intersect and are recombined into output optical beam  590  which is unpolarized and which exits compensator  50 . 
     In  FIG. 6 , a chromatic dispersion compensator  60  in accordance with a fifth embodiment of the invention is shown. Compensator  60  has a PBS  610 , two quarter-wave plates  620 ,  640 , two GTEs  630 ,  650  and two ninety degree mirrors  660 ,  670 . 
     In operation, an input optical beam  600 , which is unpolarized, is incident into PBS  610 . PBS  610  splits beam  600  into two polarized beams G 1 , H 1 . Polarized beams G 1 , H 1  are directed toward GTEs  630 ,  650 , respectively, at normal incidence via quarter-wave plates  620 ,  640 , respectively. GTEs  630 ,  650  contribute a first unit of group delay on polarized beams G 1 , H 1 . Upon reflecting from GTEs  630 ,  650  and passing through quarter-wave plates  620 ,  640  a second time on the return trip, the polarization plane of beams G 1 , H 1  is rotated. Thus, when the beams G 1 , H 1  re-intersect at PBS  610 , they are recombined into an unpolarized beam and directed to mirror  660 . This completes the first cycle. 
     Mirror  660  redirects the unpolarized beam toward PBS  610 , beginning a second cycle in which GTEs  630 ,  650  contribute a second unit of group delay on polarized beams G 2 , H 2 , respectively. All told, compensator  60  contributes four units of group delay over four cycles. That is, four bounces off GTEs  630 ,  650  are made by the constituent components of input optical beam  600 , all at normal incidence, before output optical beam  680 , which is unpolarized, exits compensator  60 . 
     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. The present invention is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.