Optical signal interleaver and deinterleaver devices with chromatic dispersion compensation

A dispersion compensated optical signal interleaver having GTI based interferometer for interleaving and deinterleaving sets of odd and even channels and having an additional GTI interferometer for compensating the chromatic dispersion arising from the first interferometer.

DETAILED DESCRIPTION FIG. 3 shows a schematic diagram of a deinterleaver 10 in accordance with the present invention. The deinterleaver 10 receives an input signal at input port 12 . The input signal contains a set of even channels and a set of odd channels, the channel spacing being fixed at, for example, 50 GHz between alternating even and odd channels. Thus, in this example, the spacing between even channels is 100 GHz, and the spacing between odd channels is 100 GHz. A collimator 14 passes the input signal to a translator 16 . The translator 16 includes a walk-off crystal 18 and a half-wave plate 20 . The input signal passes through the walk-off crystal 18 and emerges from the walk-off crystal 18 as two separate beams, a vertically polarized beam and a horizontally polarized beam. The horizontally polarized beam passes through a half-wave plate 20 and emerges from the half-wave plate 20 as a vertically polarized beam. Thus the input is translated by the translator 16 into a vertically polarized input signal consisting of two parallel beams, each vertically polarized. The translator 16 outputs this vertically polarized input signal to a polarization beam splitter 22 . The translator 16 can be operated in the reverse direction to remove polarization from an output signal. The translator described above adds and removes vertical polarization. A translator for adding and removing horizontal polarization is similar but has the half-wave plate intercepting the other beam. The term translator as used in this application is intended to include any device that performs the functions just described. The polarization beam splitter 22 reflects the vertically polarized input signal to a quarter wave plate 24 . The polarized input signal passes through the quarter-wave plate 24 , the polarization of the input signal being transformed to a circular polarization by the quarter-wave plate 24 . The circularly polarized input signal then goes to a dispersion compensating interferometer 26 . The dispersion compensating interferometer 26 is a Gires-Tournois interferometer (GTI) as shown in FIG. 2 . The dispersion compensating interferometer 26 outputs a dispersion compensated circularly polarized input signal to the quarter-wave plate 24 . The signal passes through the quarter-wave plate 24 , is transformed to a horizontally polarized signal by the quarter wave plate 24 , and emerges from the quarter-wave plate 24 as a dispersion compensated horizontally polarized input signal. The horizontally polarized input signal is transmitted through the polarization beam splitter 22 , then through another polarization beam splitter 28 , then through a 22.5° cut half-wave plate 30 which rotates the polarization through forty five degrees positive, and then through a garnet 32 which rotates the polarization through forty five degrees negative. The input signal as it emerges from the garnet 32 remains a horizontally polarized input signal. The horizontally polarized input signal then enters the interferometer section 34 of the interleaver 10 . The interferometer section 34 includes a polarization beam splitter 36 and a the signal processing interferometer which in this embodiment of the invention is a nonlinear interferometer NLI 38 . The nonlinear interferometer 38 in this embodiment of the invention is a modified GTI interferometer as shown in FIG.4 . The horizontally polarized input signal is transmitted through the polarization beam splitter 36 to the nonlinear interferometer 38 . The nonlinear interferometer 38 outputs a signal to the polarization beam splitter 36 which transmits a horizontally polarized signal containing the set of odd channels (horizontally polarized odd channel signal) and reflects a vertically polarized signal containing the set of even channels (vertically polarized even channel signal). The horizontally polarized odd channel signal passes from the polarization beam splitter 36 out of the interferometer section 34 and through the garnet 32 which rotates the polarization forty five degrees, through the 22.5° degree cut half-wave plate 30 which rotates the polarization another forty five degrees and outputs a vertically polarized odd channel signal to the polarization beam splitter 28 where it is reflected to translator 40 . The translator 40 receives the vertically polarized odd channel signal, removes the polarization and outputs the odd channel signal. Translator 40 includes a half-wave plate 42 and a walk-off crystal 44 . A part of the vertically polarized odd channel signal enters the walk-off crystal 44 directly and a part passes through the half-wave plate 42 before entering the walk-off crystal 44 , the latter part becoming horizontally polarized. The vertically polarized part and the horizontally polarized part are recombined in the walk-off crystal 44 and outputted from translator 40 as the odd channel output signal which then passes through the collimator 46 to the output port 48 . Referring back to the vertically polarized even channel signal that was reflected by the polarization beam splitter 36 , that signal goes to translator 50 . The translator 50 includes a half-wave plate 52 and a walk-off crystal 54 and removes polarization from the polarized even channel signal just as translator 40 does for the odd channel signal. The depolarized even channel signal passes from the translator 50 through a collimator 56 to output port 58 . FIG. 4 shows a schematic cross-section of a dispersion compensating interferometer 60 that is suitable for use as a dispersion compensator in the present invention. The dispersion compensator 60 is a Gires-Tournois interferometer having a first partially reflective mirror 62 spaced apart from and parallel to a highly reflective mirror 64 , with a cavity 66 between the two mirrors. The partially reflective mirror 62 provides a single input/output port 68 to allow light to enter and leave the cavity 66 . The optional spacers 70 are made of ultra-low expansion material. The amplitude response of the Gires-Tournois interferometer is flat (i.e. independent of wavelength), the phase response is &phgr;(&lgr;)&equals;2 tan −1 &lsqb;&lsqb;(1&plus;{square root} R )/(1−{square root} R )&rsqb;tan(2&pgr; d /&lgr;)&rsqb;,  (2) the group delay is &tgr;(&lgr;)&equals;0.01&lgr; 2 /6&pgr;&lsqb; d&phgr; (&lgr;)/ d &lgr;&rsqb; (ps),  (3) and the dispersion is D (&lgr;)&equals;10 −3 d &tgr;(&lgr;)/ d &lgr; (ps/nm).  (4) In the above equations &lgr; is wavelength, R is the power reflectivity of the partially reflective mirror, and d is the length of the cavity 66 from the partially reflective mirror 62 to the highly reflective mirror 64 . The reflectivity of the highly reflective mirror is preferably about 100%. FIG. 5 shows a graph of group delay calculated for a Gires-Tournois interferometer dispersion compensator, having a cavity length d&equals;3 mm and reflectivity R&equals;0.28% for the partially reflective mirror. FIG. 6 shows, in cross section, a nonlinear interferometer suitable for use as the nonlinear interferometer 36 in FIG. 1 . The nonlinear interferometer 100 is a modified GTI interferometer having a first partially reflective mirror 62 , spaced apart from and parallel to a highly reflective mirror 64 with a cavity 66 between the two mirrors. The partially reflective mirror 62 provides a single input/output port 68 to allow light to enter and leave the cavity 66 . The partially reflective mirror has a reflectivity preferably of approximately 18.5%. The highly reflective mirror preferably has reflectivity of approximately 100%. A quarter-wave plate 70 is located in the cavity 66 and introduces a 180° round trip phase change between an o beam and an e beam of the signal inside the cavity 66 . The external &lgr;/8 plate 72 introduces a round trip phase change of &pgr;/2 between the o beam and the e beam. FIG. 7 shows the relationship between polarization direction and the c axis of 22.5° cut half-wave plate as used in the first and second embodiments of the invention. FIG. 8 shows a graph of group delay for a 50 GHz dispersion compensated interleaver and deinterleaver according to the first embodiment of the invention shown in FIG. 3 FIG. 9 shows a graph of dispersion for a 50 GHz dispersion compensated interleaver and deinterleaver according to the first embodiment shown in FIG. 3 . FIGS. 5, 8 and 9 are the results of calculations wherein the parameters were GTI cavity length&equals;3 mm, and R&equals;0.28%. From FIG. 9 the dispersion is only &plus;/−4.7 ps/nm in bandwidth of &plus;/−0.8 nm (&plus;/−10 GHz). In order to compensate dispersion, the peak of the group delay of the compensator must be aligned with the bottom of the group delay of the interleaver. FIG. 10 shows an interleaver in accordance with the present invention. All of the elements in FIG. 10 are the same locations in FIG. 10 as in FIG. 3 except that the garnet 32 and the 22.5° cut half-wave plate 30 have changed places. In this case the odd channel input signal is changed from vertical polarization to horizontal polarization on the way to the NLI and the horizontally polarized output signal from the NLI to the dispersion compensating interferometer is not changed. FIG. 11 shows the relevant portion of a deinterleaver that is the same as that shown in FIG. 3 except for the garnet 32 which in FIG. 11 is oriented as indicated by the facet in a specific direction, with the “A” face facing the polarization interferometer 36 . FIG. 12 shows an interleaver that has the same elements as the deinterleaver of FIG. Except for the garnet 32 as indicated by the facet 33 .which has the “A” face facing the quarter wave plate 30 FIG. 13 shows a schematic of a second dispersion compensated deinterleaver in accordance with the present invention. Numeral identifiers used in FIG. 3 are used again in FIG. 13 to identify like elements. All of the parts shown below the dashed line AA in FIG. 13 have like numbered counterparts in FIG. 3 . Therefore, the description of the deinterleaver 110 can conveniently begin at the point where the horizontally polarized input signal leaves the garnet 32 and enters the interferometer section 112 . The interferometer section 112 includes a 50/50 beam splitter 114 optically coupled to the garnet 32 , optically coupled to a reflecting mirror 116 and optically coupled to an interferometer 118 . The beam splitter 114 splits the horizontally polarized input signal into a portion which is reflected by the mirror 116 back to the beam splitter 114 , and a portion which is reflected by the interferometer 118 back to the beam splitter 114 . The combination of the Gires-Tournois interferometer 118 with the beam splitter 114 and the mirror 116 , is a Gires-Tournois-Michelson interferometer (GTMI). The Gires-Tournois interferometer 118 is shown, in cross-section in FIG. 4 . The signals reflected from the mirror 116 and from the Gires-Tournois interferometer 118 go back to the beam splitter 114 and interfere within the beam splitter 114 so that the beam splitter 144 outputs a horizontally polarized signal containing the set of odd channels (horizontally polarized odd channel signal) to the garnet 32 and outputs a horizontally polarized signal containing the set of even channels (horizontally polarized even channel signal) to the translator 120 . The horizontally polarized odd channel signal passes through the garnet 32 which rotates the polarization through forty five degrees. The signal then passes through the 22.5° cut half-wave plate 30 which rotates the polarization through an additional forty five degrees so that the signal becomes a vertically polarized odd channel signal. As described with regard to FIG. 3 this signal is reflected in polarization beam splitter 28 and is depolarized in translator 40 which sends the odd channel output signal via collimator 46 to output port 48 . The horizontally polarized even channel signal goes from the beam splitter 114 to translator 120 . The translator 120 includes a half-wave plate 122 and a walk-off crystal 124 . Part of the horizontally polarized even channel signal enters the walk-off crystal 124 directly and part passes through the half-wave plate 122 , where it becomes vertically polarized, before entering the walk-off crystal 124 . Both parts of the signal are recombined in the walk-off crystal 124 to form a depolarized signal that is the even channel output signal which passes through the collimator 126 to input/output port 128 . The dispersion of a 50 GHz dispersion compensated GTMI based interleaver is calculated to be &plus;/−10 ps/nm as compared to &plus;/−50 ps/nm calculated for a GTMI interleaver without dispersion compensation. The above described embodiments of the invention are to be regarded as illustrative of the invention are not intended to be construed as limiting. Accordingly the scope of the invention should be determined by the following claims and their legal equivalents.