Abstract:
A switchable free-spectral-range mode selector is used to change the free spectral range of a free-space delay-line interferometer. The mode selector consists of a rotatable device with at least one transparent plate selected to produce the desired change in the free spectral range of the delay-line interferometer. The device may be rotated in and out of the free-space optical path of on of the interferometer arms. If used as a DPSK demodulator, the device enables operation at multiple predetermined free spectral ranges. In the preferred embodiment, the demodulator includes a 50/50 beam-splitter cube combined with two cavities. The mode selector consists of a plurality of different transparent slabs attached to a rotatable shaft so that any one of the slabs or none may be inserted in the appropriate optical path to obtain the desired FSR mode of operation.

Description:
RELATED APPLICATIONS 
     This application is a divisional application of Ser. No. 12/541,894, filed Aug. 14, 2009, which was based on U.S. Provisional Application Ser. No. 61/094,966, filed Sep. 7, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to differential phase-shift keying (DPSK) in telecommunication. More specifically, it relates to DPSK devices for multiple free-spectral-range operation. 
     2. Description of the Prior Art 
     Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases is used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal (such a system is termed coherent). 
     Alternatively, instead of using bit patterns to set the phase of the wave, the patterns are used to set changes in the phase of the signal. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (i.e., it is a non-coherent scheme). 
     In telecommunication technology, differential phase-shift keying utilizes a decoding method in order to convert the phase-keyed signal to an intensity-keyed signal at the receiving end. The decoding method can be achieved by comparing the phase of two sequential bits. In principle, it splits the input signal beam into two channels with a small delay before recombining them. After the recombination, the beams from the two channels interfere constructively and destructively. The interference intensity is measured and becomes the intensity-keyed signal. To achieve this, one channel has an optical path longer than the other by a distance equivalent to the photon flight time of one bit. For instance, in a 40 Gbit-per-second system, one bit is equal to 25 ps and light travels 7.5 mm in that period. Thus, in this example, the optical path difference (OPD) between the two channels would be set at 7.5 mm. 
     The Mach-Zehnder type interferometer with a desired OPD between the two channels has been used for decoding purposes. Because of the properties of optical interference, a change in OPD can greatly affect interference intensity. Moreover, the optical path in each arm is much longer than its difference. Therefore, a sophisticated temperature control is required to maintain the optical path in each arm in order to assure that the change in the OPD is much less than a small fraction of one wavelength, e.g., about 10 nm. This is difficult and expensive to achieve, especially for an interferometer with a long optical path. 
       FIG. 1  shows the typical configuration of a Michelson interferometer  10 . The core of such device consists of a beam-splitter cube  12  and two mirrors  14 , 16  positioned so as to reflect the transmission and reflection beams produced by the beam splitter. Two cavity structures are shown in the figures because commonly used in the art, but it is understood that the mirrors  14 , 16  are the only relevant optical elements to the Michelson configuration and the rest of the structure can take different forms. The input signal I impinges on the beam-splitting surface  18  of the beam-splitter cube  12  and produces two beams (a reflected beam R and a transmitted beam T), each carrying 50% of the total power. After both beams R,T are reflected by the mirrors  14 , 16  along their respective optical paths, they return to the beam-splitting surface  18  and are split again, thereby producing two pairs of beams propagating in different directions. The two beams in each pair interfere, both constructively and destructively, to produce two output beams O 1  and O 2  that are 180 degrees out of phase. 
     As mentioned, the mirrors  14 , 16  are illustrated as components of cavity structures formed by combining two pieces of fused silica glass plates ( 20 , 22  and  24 , 26 ) and a number of spacers ( 28  and  30 ) interposed between them. Therefore, the cavity length is defined by the length of the spacers in the cavity. (Two spacers are illustrated for each cavity, but different numbers are often used in the art to meet other design requirements). The spacers are typically made of a Zerodur® or a ULE® (ultra low expansion) glass, both substances with a coefficient of thermal expansion (CTE) less than 0.05 ppm (i.e., very close to zero), in order to make the free spectral range of the device practically insensitive to temperature variations. The rest of the parts in the interferometer are made of fused silica, which has CTE of 0.5 ppm. The joints between the spacers, the fused silica and the beam splitter are implemented by optical contact technology. In essence, the parts are super-polished, contacted at room temperature, and then thermally treated for several hours in order to obtain a permanent bond. Mirror coatings are used in conventional manner to form the mirrors  14 , 16  on the glass plates  22 , 26  to reflect the impinging beams at the distal end of each cavity. 
     Assuming that the anti-reflection (AR) coated surfaces  32 , 34  of the glass plates  20 , 24  at the proximal end of each cavity are equidistant from the beam-splitting surface  18 , the lengths L 1  and L 2  of the cavities determine the optical path difference (OPD) between the two arms of the interferometer  10 . As is well understood in the art, the difference in the optical lengths of the two arms (which under the conditions described is the same as the difference between L 1  and L 2 ) determines the free spectral range (FSR) of the device. The relationship between the FSR of the device and the cavity lengths is given by 
                     F   ⁢           ⁢   S   ⁢           ⁢   R     =       C     2   ⁢     (       L   1     -     L   2       )         =     -     C     2   ⁢     (       L   2     -     L   1       )                     (   1   )               
where C is the speed of light. For example, an OPD of 3,000 μm will produce an FSR of 100-GHz.
 
     For optical communication DPSK demodulator applications, the relative lengths of the cavities are also selected so as to provide a time delay (τ DLI ) between the two beams R and T returning to the beam-splitting surface  18  that is close to the time interval (τ 0 ) between two adjacent bits. (If τ DLI &lt;τ 0 , the interferometer can be used as a PDPSK—partial DPSK—demodulator.)  FIG. 2  shows the spectra of two outputs O 1  and O 2  of a DPSK demodulator with FSR=50 GHz. 
     Copending U.S. application Ser. No. 11/360,959 and Ser. No. 11/485,653 describe various embodiments of novel Michelson-type interferometers used as DPSK demodulators. These demodulators provide a significant improvement over the prior art, but the operation of each device is limited to a single free spectral range. The present invention provides a solution for expanding the use of any free-space delay line interferometer (DLI) to multiple free-spectral-range applications. 
     SUMMARY OF THE INVENTION 
     This invention is based on the idea of using a switchable mode selector to change the free spectral range of a free-space delay-line interferometer. The mode selector consists preferably of a rotatable device with at least one transparent plate with a predetermined index of refraction and a corresponding thickness selected to produce a desired change in the free spectral range of the delay-line interferometer. Through an actuator, the device may be rotated in and out of the free-space optical path of the interferometer&#39;s arm (the transmission or the reflection arm) selected to determine the FSR of the interferometer. Thus, if used as a DPSK demodulator, the device of the invention enables operation at multiple predetermined free spectral ranges, each corresponding to a different transparent plate in the mode selector or to none. 
     In the preferred embodiment, the demodulator of the invention includes a 50/50 beam-splitter cube combined with two cavities that define the device&#39;s delay and free spectral range. The mode selector consists of a plurality of different transparent slabs, each judiciously designed to provide the desired change in the FSR of the demodulator, attached to a rotatable shaft so that any one of the slabs or none may be inserted in the appropriate optical path to obtained the desired FSR mode of operation. 
     Two additional components are preferably used to achieve thermal stability. A temperature-tuned phase shifter is placed in the reflection or the transmission arm to tune the phase of the DLI. A micro-heater is incorporated into the tuner to vary its temperature as necessary to phase tune the DLI. An ambient temperature compensator, preferably a plate made of the same material as the phase shifter, is used in the other arm of the interferometer to compensate for the thermal drift in the phase tuner introduced by the environment. 
     Various other advantages will become clear from the description of the invention in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments, and particularly pointed out in the claims. However, such drawings and descriptions disclose only some of the various ways in which the invention may be practiced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically a conventional Michelson interferometer in free-space configuration. 
         FIG. 2  illustrates the spectra of the two outputs, O 1  (top) and O 2  (bottom), of the interferometer of  FIG. 1  with cavity lengths selected so as to produce an FSR of 50 GHz. 
         FIGS. 3A and 3B  illustrate the two modes of operation of a dual-FSR DPSK demodulator according to the invention. 
         FIG. 4  illustrates a triple-FSR demodulator with a mode selector comprising two slab pieces with different thicknesses attached to a motor shaft. 
         FIG. 5  is a front view of the mode selector of  FIG. 4 . 
         FIG. 6  illustrates a mode selector for a quadruple-FSR DPSK demodulator. 
         FIG. 7  illustrates a demodulator with a temperature-tuned phase shifter and a corresponding ambient temperature compensator. 
         FIG. 8  shows schematically the structure of a temperature-tuned phase shifter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described throughout with reference to a Michelson interferometer. However, it is equally applicable to any free-space delay line interferometer. Therefore, the invention is intended to be so construed and it should not be limited to any particular form of interferometer configuration. 
     Referring to the figures, wherein like reference numerals and symbols are used throughout to refer to the same components,  FIGS. 3A and 3B  illustrate schematically a multiple free-spectral-range DPSK demodulator  40  according to the invention. The device, described in Michelson configuration for illustration, is the same as the Michelson interferometer of  FIG. 1 , but modified to allow the introduction of a transparent slab  42  in one of the cavities to change the OPD so as to obtain a predetermined alternative free spectral range for the demodulator. In the example, the cavity in the transmission arm of the interferometer is illustrated with an open end that permits the introduction of the slab  42  in the optical path of the transmission beam T. Because the transmission cavity is illustrated as longer than the reflection cavity, the addition of a transparent slab in the longer cavity (L 1 ) will increase the OPD and reduce the FSR of the demodulator. Alternatively, the addition of a transparent slab in the shorter cavity (L 2 ) would decrease the OPD and correspondingly increase the FSR. 
     In such dual-FSR DPSK demodulator, the transparent slab  42  can be positioned in ( FIG. 3B ) or out ( FIG. 3A ) of the optical path of the arm of the interferometer using a rotatable mode selector (not shown in the figure). As mentioned, with the slab  42  inside the path, the path length difference between the two interference beams is increased; therefore, the FSR is decreased. Thus, by switching the slab  42  in and out of the cavity, it is possible to switch the FSR of the DLI between two settings of interest. The motion of the mode selector can be controlled in conventional manner with a solenoid, a motor or other electro-mechanical device. 
       FIG. 4  illustrates a triple-FSR demodulator, where two slab pieces  46 , 48  with different thicknesses are attached to the shaft  50  of a motor  52  to obtain a three-way mode selector  54 . The thickness of each slab piece  46 , 48  corresponds to a specific FSR for the DLI. Thus, simply by rotating the motor to place a particular slab in the optical path of the transmission arm cavity, the required FSR may be set. This design obviously affords three possible modes of operation, including one with no slab in the cavity.  FIG. 5  illustrates the mode selector  54  in front view. 
       FIG. 6  illustrates a mode selector  60  for a quadruple-FSR demodulator. Three slabs  46 , 48 , 62  with three different thicknesses permit operation at three corresponding FSRs by successive 90-degree rotations of the mode selector. In addition, a fourth mode of operation is provided when no slab is present in the cavity. 
     Thus, by judiciously selecting the material of the slabs in the mode selector of the invention (i.e., with the appropriate optical and thermal properties), a multiple-FSR DLI may be implemented. For example, if a transparent slab ( 46 ,  48  or  62 ) consists of an athermal optical parallel plate with thickness d S , the change in the delay produced by the slab will be equal to (n S −1)d S , where n S  is the refractive index of the material. 
     However, even with athermal mode selector slabs, it is advisable to provide tunability to the free-space DPSK demodulator (i.e., enable tuning by shifting the central wavelength of the device to track the grid of the signal received from the network). This may be accomplished by introducing a temperature-tuned phase shifter  70  in either arm of the demodulator and a corresponding ambient temperature compensator  72  in the other arm.  FIG. 7 , for simplicity, illustrates such devices in the two arms of the conventional free-space DPSK demodulator of  FIG. 1 . The transmission arm of the DLI and its cavity are again used for illustration of the placement of the phase shifter, but it is understood that the reflection arm could be used in the same way. A micro-heater is incorporated into the phase shifter  70  to vary its temperature as necessary to tune the DLI. The ambient temperature compensator  72 , preferably a plate made of the same material as the phase shifter  70 , is used to compensate for the thermal drift in the phase shifter introduced by the environment. 
       FIG. 8  shows the typical structure of a temperature-tuned phase shifter  70 . It consists of a thermally sensitivity optical plate  74  that is placed in the optical path of the beam of interest. A micro-heater  76  is used to control the temperature of the plate, thereby affecting the optical path length therethrough. The phase shifter is commonly attached to the housing  78  of the device, which also provides a heat sink that facilitates control of temperature fluctuations. The heater  76  may be coupled to either one or both the phase shifter  70  and the compensator  72  (in the latter case to increase the tuning range). 
     The thickness of the compensator and the phase tuner are determined in conventional manner. If the transparent slabs of the FSR mode selector are made of an athermal material, the thickness of the compensator  72  should be the same as that of the phase shifter  70  to compensate for its intrinsic thermal drift (i.e., the amount of delay drift due to changes in ambient temperature). If the mode selector slabs are not athermal, then the thickness of the compensator  72  should be different from that of the phase tuner, and it should be selected as required to also compensate for the intrinsic thermal drift of each slab. Obviously, a compensator  72  can only compensate for one mode selector slab. Therefore, for each additional slab in the mode selector, a corresponding additional compensator with the appropriate thickness is required. This can be implemented by having a set of compensators mounted on a compensator selector (similar to the FSR mode selector) for alternatively introducing the appropriate compensator in the second arm of the interferometer when a given FSR slab is changed (or removed) in the first arm. 
     If this latter approach to thermal compensation is adopted, for each slab of the FSR mode selector, the thickness of the corresponding compensator would be selected so as to satisfy the relation
 
Delay=( n   P −1) d   P +( n   S −1) d   S −( n   C −1) d   C,   (2)
 
where Delay is the delay required to produce the desired FSR; and n P , n S  and n C  are the refractive indices and d P , d S  and d C  are the thicknesses of the phase-shifter, FSR slab and compensator, respectively. The same result can also be obtained by appropriately selecting different materials for the phase shifter and the compensator. As one skilled in the art would readily recognize, in such case the values of d S  and d C  would be selected according to their optical thermal coefficients (g and h, respectively) such that (d S )(g)=(d C )(h), which would render their combination athermal. Therefore, by choosing two materials with the right combinations of thickness and optical thermal coefficients, the delay-line interferometer can have the required time delay and also be athermal. Finally, it should be noted that it would also be possible to use a single compensator  72  for all FSR modes of operation if a micro-heater were coupled to the compensator  72  as well as to the phase shifter  70 .
 
     Thus, a simple device and mode of operation have been disclosed that enable the use of a free-space DPSK demodulator at alternative free spectral ranges. By providing a mode selector in an arm of the demodulator with multiple transparent plates designed to produce outputs with different predetermined FSRs, the same demodulator can be used for alternative modes of operation simply by removing or changing the plate in the optical path. Thermal compensation is preferably also provided with a temperature-tuned phase shifter in one arm of the DLI and a corresponding ambient temperature compensator in the other arm. 
     While the invention has been shown and described in what are believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention. For example, as mentioned, a multiple-FSR DPSK demodulator according to the invention could be implemented with any free-space interferometer configuration, such as illustrated in in copending U.S. application Ser. Nos. 12/498,419, 12/509,474 and 12/538,124. Similarly, a combination of transparent plates could be used with two mode selectors, one for each arm of the interferometer. Therefore, the invention is not to be limited to the details disclosed herein, but is to be accorded the full scope of the claims so as to embrace any and all equivalent apparatus and methods.