Abstract:
A method and apparatus for implementing a new type of colorless Mach-Zehnder-interferometer (MZI)-based tunable dispersion compensator (TDC) that has only three MZI stages (two in a reflective version) and two adjustable couplers which are responsive to one control voltage, making it compact, low power, and simple to fabricate, test, and operate.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to optical dispersion compensators and, more particularly, to a method and apparatus for implementing a colorless Mach-Zehnder-interferometer-based tunable dispersion compensator. 
   BACKGROUND OF THE INVENTION 
   Optical signal dispersion compensators can correct for chromatic dispersion in optical fiber and are especially useful for bit rates 10 Gb/s and higher. Furthermore, it is advantageous for the dispersion compensator to have an adjustable amount of dispersion, facilitating system installation. It is also advantageous if the tunable dispersion compensator (TDC) is colorless, i.e., one device can compensate many channels simultaneously or be selectable to compensate any channel in the system. 
   Previously proposed colorless TDCs include ring resonators [1] , the virtually imaged phased array (VIPA) [2] , cascaded Mach-Zehnder interferometers (MZIs) [3,4,5] , temperature-tuned etalons [6] , waveguide grating routers (WGRs) with thermal lenses [7] , and bulk gratings with deformable mirrors [0] . The bracketed references [1]  refer to publications listed in the attached Reference list. The cascaded MZI approach is particularly promising since it exhibits low loss, can be made with standard silica waveguides, and can be compact. However, previous MZI-based TDCs required 8 stages and 17 control voltages in one case [3]  and 6 stages with 13 control voltages in two others [4, 5] . This large number of stages and control voltages is expensive and power-consuming to fabricate and operate, especially when compensating 10 Gb/s signals. Because fabrication accuracy cannot guarantee the relative phases of such long path-length differences, every stage of every device must be individually characterized. Also, a large number of stages often results in a high optical loss and a large form factor. Additionally, the more the stages, the more difficult it is to achieve polarization independence. 
   What is desired is a simplified MZI-based TDCs having reduced number of stages and control voltages. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, I disclose a method and apparatus for implementing a new type of colorless Mach-Zehnder-interferometer (MZI)-based tunable dispersion compensator (TDC) that has only three MZI stages (two in a reflective version) and two adjustable couplers which are responsive to one control voltage, making it compact, low power, and simple to fabricate, test, and operate. Such an MZI-based TDC with a 25-GHz-free-spectral-range version can compensate ˜±2100 ps/nm for 10 Gb/s signals. Having a free-spectral range equal to the system channel spacing divided by an integer makes it possible for the TDC to compensate many channels simultaneously. 
   More particularly, one embodiment of my tunable chromatic optical signal dispersion compensator comprises 
   three cascaded Mach-Zehnder interferometers, MZIs, a first MZI including a fixed 50/50 coupler for receiving an input optical signal, a second MZI including a first adjustable coupler that is shared with the first MZI and a second adjustable coupler that is shared a third MZI, and the third MZI including a fixed 50/50 coupler for outputting a dispersion-adjusted output optical signal,
 
wherein the path-length difference between the two arms in the second MZI is twice that of the first MZI, and the path-length difference between the two arms in the first MZI is equal to that of the third MZI and
 
wherein said first and second shared adjustable couplers are adjusted with equal coupling ratios using a single control signal to provide adjustable dispersion compensation to the output signal.
 
   In a reflective embodiment, my tunable chromatic optical signal dispersion compensator comprises 
   a first MZI including a fixed 50/50 coupler for receiving an input optical signal at a first port and an adjustable coupler, that is shared with a second reflective MZI, the path-length difference between the two arms in the second MZI is equal to that of the first MZI and
 
wherein the adjustable coupler is responsive to a control signal for controlling the amount of signal dispersion added by said compensator to the input optical signal to form the output optical signal.
 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully appreciated by consideration of the following Detailed Description, which should be read in light of the accompanying drawing in which: 
       FIG. 1  illustrates, in accordance with the present invention, a tunable dispersion compensator (TDC) that has only three stages and one control voltage. 
       FIG. 2  illustrates the TDC of  FIG. 1  where the adjustable couplers are each implemented using an MZI-based adjustable coupler. 
       FIG. 3  illustrates the electrical layout for using a single control signal, C 1 , to control the two MZI-based adjustable couplers of FIG.  2 . 
       FIG. 4  illustrates, in accordance with the present invention, a reflective design of a tunable dispersion compensator (TDC) that uses only one control voltage. 
       FIGS. 5A and 5B  illustratively show the transmissivity and group-delay characteristics of my TDC at three different settings of the adjustable coupler(s). 
       FIGS. 6A and 6B  show the use of my TDC in illustrative optical transmission systems. 
       FIGS. 7   a  and  7 B show my TDC arranged together with an Erbium amplifier. 
   

   In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g.,  101  is first located in FIG.  1 ). 
   DETAILED DESCRIPTION 
   With reference to  FIG. 1  there is shown, in accordance with the present invention, an illustrative diagram of my tunable dispersion compensator (TDC) that has only three stages and uses one control voltage. The three stages  103 ,  105 , and  107  are implemented using Mach-Zehnder-interferometers (MZIs). The first and second MZIs  103 ,  105  share an adjustable coupler  104  and the second and third MZIs  105 ,  107  share an adjustable coupler  106 . The two adjustable couplers  104 ,  106  are always set equally. The first and third MZI have path-length differences ΔL, and the center MZI has a path-length difference of 2ΔL (plus any phase offset from the couplers). 
   The TDC operates as follows. An input optical signal at port  101  is split equally to the two arms of the first MZI  103  by the y-branch coupler  102 . In the first MZI  103 , one arm is longer, by ΔL, than the other arm so that when the optical signals are recombined in the first adjustable coupler  104 , the amount of light sent to each of the two arms of the second MZI  105  depends on the wavelength. The first adjustable coupler  104  in response to a control signal C 1  controls the sign and amount of dispersion introduced to the signals outputted from the coupler  104  to the arms of the second MZI  105 . Similarly, the second adjustable coupler  106  in response to a control signal C 1  controls the sign and amount of dispersion introduced to the signals received from the arms of the second MZI  105  and outputted from the coupler  106  to the arms of the third MZI  107 . If positive dispersion is desired, a predetermined control signal C 1  to adjustable couplers  104 ,  106  is used to enable the longer wavelengths to predominantly travel the longer arms of the second MZI  105  and third MZI  107 , respectively. The third MZI  107  then performs a function similar to the first MZI in that the wavelengths on its arms are recombined in the final y-branch coupler  108  and are sent to the output port  109 . 
   Note that when the TDC device is set for zero dispersion, the two adjustable couplers  104 ,  106  are 100/0 (i.e., the couplers perform a simple cross-connect function—an input to the upper left-hand port of the adjustable coupler goes to the lower right-hand output port of the adjustable coupler and vice versa). In such a zero-dispersion case, the optical signals through the TDC traverse equal path lengths. While only the differential arm lengths are shown in  FIG. 1 , in MZIs  103  and  107 , the actual arm lengths are L+ΔL and L and in MZI the actual arm lengths are L+2ΔL and L. Thus, the signal path from one output port of y-branch coupler  102  to the output port  109  of y-branch coupler  108  follows a path of length L+ΔL through MZI  103 , L through MZI  105 , and L+ΔL through MZI  107 , giving a total length of 3L+2ΔL; and the other path consists of L, L+2ΔL, and L, also giving a total length of 3L+2ΔL. Thus for the zero dispersion setting, the TDC device acts simply as a waveguide of length 3L+2ΔL and so introduces no significant chromatic dispersion. 
   In the above description ΔL determines the free spectral range (FSR) of the TDC. The FSR is equal to
 
 FSR=C   0   /ΔL·n   g 
 
   Where 
   C 0  is 300 km/s (vacuum speed of light) 
   n g  is the group refractive index of the MZI waveguides. 
   In one illustrative design, for an optical signal data rate of 10 Gb/s, the FSR would be about 25-GHz. Such an MZI-based TDC with a 25-GHz-free-spectral-range version can compensate ˜±2100 ps/nm for 10 Gb/s signals. In a multi-wavelength channel system, having a FSR equal to the system wavelength channel spacing divided by an integer makes it possible for the TDC to compensate many channels simultaneously. Thus, my TDC is colorless, i.e., it can compensate many channels simultaneously or be selectable to compensate any channel in a multi-wavelength channel system 
   In a well-known manner, MZIs  103 ,  105 ,  107  may be implemented together as a planar optical integrated circuit or may be implemented using discrete optical elements mounted on a substrate. 
   The dispersion of TDC can be tuned positive or negative by adjusting couplers  104  and  106  toward 50/50 using a control signal C 1 . As will be discussed with reference to  FIG. 3 , by selecting a control signal C 1  that is higher or lower that the zero dispersion control signal C 1  setting, TDC can be set to a positive or negative dispersion level. The design is similar to the birefringent crystal design of Ref. [9], except that the device of [9] was not tunable, using only a fixed 50/50 coupling ratio. Advantageously, my TDC design is colorless, i.e., it can compensate many channels simultaneously or be selectable to compensate any channel in the system. 
   Note that while the adjustable couplers  104  and  106  are controlled by a common control signal C 1 , if desirable separate control signals may be used. Separate controls could be useful, for example, if the couplers have unequal characteristics due to fabrication non-uniformities. 
     FIG. 2  illustrates, in accordance with the present invention, a TDC of  FIG. 1  where the adjustable couplers  104  and  106  are implemented using two MZI-based adjustable couplers. As shown, the adjustable couplers  104 ,  106  are implemented using small MZIs with controllable phase shifters. Each MZI includes a 50/50 fixed evanescent coupler  201 , upper phase shifter  202 , lower phase shifter  203 , and 50/50 fixed evanescent coupler  204 . Driving both the lower phase shifters  203  of both MZIs with the same control signal C 1  at a first level pushes the dispersion in one direction, and driving both upper phase shifters  202  at a second level pushes the dispersion in the other direction. Depending on the orientations of the main MZIs, there may be a small path-length difference between the two arms in the adjustable coupler MZI. 
   If the phase shifters  202 ,  203  are thermooptic heaters, then a convenient electrical layout that requires only one control signal C 1  is shown in FIG.  3 . The control signal C 1  voltage is varied between the levels V 1  and V 2 , where V 2  is greater than V 1 . When control voltage C 1  is at a predetermined zero dispersion level Vz between V 1  and V 2 , then the same current flows through both the upper and lower phase shifters establishing zero dispersion and, hence, adjustable couplers  202 ,  203  perform a simple cross-connect function as discussed previously. When control signal C 1  is at level V 1  then no current flows through the upper phase shifters  202  and current flows through the lower phase shifters  203  establishing the maximum amount of a dispersion of a first polarity. When the desired dispersion level is somewhere between zero dispersion level Vz and the maximum first polarity dispersion level V 1 , then control signal C 1  is suitably adjusted to a voltage level between V 1  and Vz. At control signal C 1  levels between V 1  and Vz, the upper  202  and lower  203  phase shifters are operated in a push-pull arrangement. That is, for example, in the upper phase shifter  202  current is increasing while in the lower phase shifter current is decreasing. 
   When control signal C 1  is at level V 2  then no current flows through the lower phase shifters  202  and current flows through the upper phase shifters  203  establishing the maximum amount of a dispersion of a second polarity. When the desired dispersion level is somewhere between zero dispersion level Vz and the maximum second polarity dispersion level V 2 , then control signal C 1  is suitably adjusted to a voltage level between Vz and V 2 . This push-pull operation of the upper  202  and lower  203  phase shifters results in a low worst-case thermooptic power consumption and roughly constant power dissipation for all tuning settings [10].    
   With reference to  FIG. 4  there is shown, in accordance with the present invention, a reflective design of a tunable dispersion compensator (TDC) that also uses only one control voltage. Since the TDC arrangement of  FIG. 1  is symmetric, as shown in  FIG. 4  it can be implemented using a simpler reflective design, at the expense of requiring a circulator. In the reflective design of  FIG. 4 , MZI  403  performs the function of the first  103  and third  107  MZIs of FIG.  1  and reflective MZI  405  performs the function of MZI  105  of FIG.  1 . 
   An input optical signal at port  400  passes through circulator  401  and is split equally to the two arms of the MZI  403  by the y-branch coupler  412 . In the MZI  403 , one arm is longer, by ΔL, than the other arm so that when the optical signals are recombined in the first adjustable coupler  404 , the amount of light sent to each of the two arms of the reflective MZI  405  depends on the wavelength. The adjustable coupler  404  operates in response to a control signal C 1  that controls both the sign and amount of dispersion introduced to the signals outputted from the coupler  404  to the arms  407 ,  408  of the reflective MZI  405  and also establishes the same sign and amount of dispersion introduced to the signals outputted from the coupler  404  to the arms of MZI  403 . Note that the reflective MZI  405  has a reflective facet  406  for reflecting signals received from the two arms  407  and  408  back to these arms. Since the signal traverses twice through arms  407 ,  408 , both left-to-right and then right-to-left, the length of arm  407  is need only be ΔL longer than arm  408 . The reflected signals then traverse MZI  403  in the right-to-left direction (to act like MZI  107  of  FIG. 1 ) and are combined in y-branch coupler  402  (which acts like y-branch coupler  108  of FIG.  1 ). The output signal from y-branch coupler  402  then passes through circulator  401  to output port  409 . Reflective TDC of  FIG. 4 , using control signal C 1 , can control the sign and amount of dispersion introduced to the signal outputted from output port  409  in the same manner that is achieved by TDC of FIG.  1 . 
   Note that one can create an adjustable coupler by other methods than as shown in FIG.  2 . For example, instead of two 50/50 evanescent couplers  201  and  204  one can use two 50/50 multi-section evanescent couplers. Multi-section evanescent couplers can give a more accurate 50/50 splitting ratio in the face of wavelength, polarization, and fabrication changes. Another possibility is to use multimode interference couplers. 
   Likewise, couplers  102  and  108  could be other 50/50 couplers than y-branch couplers. For instance, they could be multimode interference couplers. 
     FIG. 5A  shows the simulated transmissivity and  FIG. 5B  shows chromatic dispersion (group delay characteristic) through my TDC at three different settings (0, +π/2, −π/2) of the adjustable couplers (s) of  FIGS. 1 and 4 . In  FIGS. 5A and 5B , the free-spectral range is 25 GHz, at the limits and center of the tuning range. The wavelength is 1550 nm. The marked phases denote the phase difference between the MZI arms in the tunable couplers of FIG.  2 . The loss is theoretically zero and does not increase at the channel center as the dispersion is tuned away from zero. At maximum dispersion, there is a transmissivity ripple of 1.25 dB peak-to-peak; the dispersion reaches ±2500 ps/nm. The bandwidth is not very wide, though: the transmitter frequency error must be less than ˜±2.5 GHz (±20 pm). This is achievable for wavelength-locked transmitters. Practically, for 10 Gb/s signals in this case the dispersion is limited to ˜±2100 ps/nm 
     FIGS. 6A and 6B  show the use of my TDC in illustrative optical transmission systems.  FIG. 6A  shows a pre-transmission dispersion compensation system where the first location  600  includes an optical transmitter unit  601 , a TDC  602  used for pre-transmission dispersion compensation, an optical amplifier  603 , and a wavelength multiplexer  604 , if needed. The output signal is sent over the optical facility  610  to a second location  620  that includes a wavelength demultiplexer  621  (if needed), an amplifier  623 , and an optical receiver unit  622 . Since the illustrative optical transmission systems is bi-directional, the first location also includes a demultiplexer  621  (if needed), an amplifier  623 , and an optical receiver unit  622  connected over optical facility  630  to the second location  620  which includes an optical transmitter unit  601 , a TDC  602  used for pre-transmission dispersion compensation, an optical amplifier  603 , and a multiplexer  604  (if needed). Note that the optical transmitter unit  601  and the optical receiver unit  622  are typically packaged together as a transponder unit  640 . 
     FIG. 6B  shows a post-transmission dispersion compensation system where the first location  600  includes an optical transmitter unit  601 , an optical amplifier  603 , and a wavelength multiplexer  604  (if needed). The output signal is sent over the optical facility  610  to a second location  620  that includes a wavelength demultiplexer  621  (if needed), an amplifier  623 , a TDC  602  for post-transmission dispersion compensation, an optical filter  605  [e.g., an amplified spontaneous emission (ASE) filter], and an optical receiver unit  622 . Since the illustrative optical transmission systems is bi-directional, the first location also includes a demultiplexer  621  (if needed), an amplifier  623 , a TDC  602 , an optical filter  605 , and an optical receiver unit  622  connected over optical facility  630  to the second location  620  which includes an optical transmitter unit  601 , an optical amplifier  603 , and a multiplexer  604  (if needed). The order of the TDC  602  and ASE filter  605  could be reversed without affecting system performance. 
   Note that for a system having a standard single mode fiber (SSMF) optical facility  610  less than about 80 Km, no dispersion compensation is typically needed. For a SSMF optical facility  610  in the range of about 80-135 Km the pre-transmission dispersion compensation system of  FIG. 6A  is preferable. For a SSMF optical facility  610  in the range of about 135-160 Km the pre-transmission dispersion compensation system of  FIG. 6B  is preferable. 
   In the system arrangements of  FIGS. 6A and 6B , it should be noted that TDC  602  can be integrated together with one or more of the optical components, such as optical transmitter  601 , optical amplifier  603 , optical filter  605 , wavelength multiplexer  604 , wavelength demultiplexer  621 , and optical receiver  622 . For example, the TDC could be monolithically integrated in InGaAsP with a laser and an optical modulator to form an optical transmitter with built-in dispersion precompensation. 
     FIG. 7A  shows on illustrative design of my TDC arranged together with an Erbium amplifier. In this arrangement, the TDC  700  is arranged in a polarization diversity scheme, in order to make the TDC function polarization independent even if the TDC device itself is polarization dependent, in which polarization-maintaining fibers (PMFs)  702  and  703  are spliced to a circulator/polarization splitter (CPS)  701  of the type described in Ref. [11]. In operation, an input optical signal  700  received by the circulator is split in the polarization splitter and coupled via PMF  702  to TDC  700 . The dispersion compensated optical signal from TDC  700  is coupled via PMF  703  to polarization splitter and the circulator to Erbium amplifier  710 . The circulator/polarization splitter (CPS)  701  eliminates the need for an input signal isolator  711  in Erbium amplifier  710 . Thus, the Erbium amplifier  710  need only include the Erbium fiber output isolator  713  and either forward pump and coupler  714  or back pump and coupler  714 . It should be noted that since the TDC of  FIG. 1  has only three stages, it can relatively simply be made polarization independent on its own and therefore does not need the polarization diversity scheme using PMFs  702  and  703  and circulator/polarization splitter (CPS)  701 . 
     FIG. 7B  shows a polarization independent reflective TDC  751  of  FIG. 4  arranged together with Erbium amplifier  710 . A circulator  750  is used to couple the input optical signal  700  to TDC  751  and to couple the dispersion compensated optical signal to Erbium amplifier  710 . 
   With reference to  FIG. 1 , I illustratively describe the initial setup of an exemplary prototype TDC that was made and tested. The TDC was temperature controlled with a thermoelectric cooler. Because the path-length differences in MZIs  103 ,  105 , and  107  are so large, after fabrication the relative phase in each MZI stage was random. Thus the arms are permanently trimmed using hyper-heating [12 ]. The procedure is as follows: with no power applied, the adjustable couplers  104 ,  106  are set for 100/0 (i.e., the couplers look like waveguide crossings in FIG.  1 ), and the transmissivity spectrum is flat. Then the left coupler  104  is adjusted to be 0/100, causing the transmissivity spectrum to have a full sinusoidal ripple. The position of a valley is marked. Then the left coupler  104  is restored to 0/100, and the right coupler  106  adjusted to 0/100. The path-length differences in the two outermost MZIs  103 ,  107  are correct when the ripples from the two cases are wavelength-aligned. If they are not, one of the outer MZIs&#39; arms is hyperheated to make them aligned. Then, with both couplers  104 ,  106  at 100/0, the center MZI  105  arms are hyperheated in order to maximize the transmissivity. After trimming, the fiber-to-fiber loss of the TDC apparatus, including the CPS, is 4.0 dB. 
   Various modifications of this invention will occur to those skilled in the art. Nevertheless all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed. 
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