Abstract:
A micro-optical delay element for a polarization time-division multiplexing scheme is disclosed wherein two light beams are provided to a polarization beam splitter/combiner (PBS/C) in the absence of optical fiber. At least one beam exiting a modulator is collimated and reaches the (PBS/C) unguided as a substantially collimated beam. In this manner the polarization state of the beam is substantially unchanged. This obviates a requirement for polarization controllers.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to optical fiber communications and in particular to multiplexed communications that uses polarization multiplexing. 
     BACKGROUND OF THE INVENTION 
     High-speed time-division-multiplexing (TDM) is a very attractive way of enhancing the spectrum efficiency of a large-capacity wavelength-division multiplexing (WDM) system. One common architecture employs two modulators having a same bit rate, wherein two separately modulated streams of data bits are combined into a high-speed single serial stream of data bits. Instead of providing a single higher-cost higher-speed modulator capable of providing modulation at a frequency of n Hz, two modulators having a frequency of n/2 Hz are provided and their outputs are time-interleaved providing a signal having a frequency of n Hz. However, one drawback to such a scheme, particularly in high-speed dense systems is that pulses from adjacent time slots spread and partially overlap one another and detection errors sometimes occur at a receiver end. 
     One remedy for this is provided by an enhanced TDM system wherein adjacent interleaved pulses are distinguishable as they are orthogonally polarized. Such a scheme is described in a paper entitled 1.04-Tbit/s SWDM Transmission Experiment Based on Alternate-Polarization 80-Gbit/s OTDM Signals, by Yutaka Miyamoto et al., published in ECOC&#39;98 20-24 September 1998 Madrid, Spain. In this paper alternate-polarization optical-TDM is described to increase the bit rate while keeping the signal spectrum from broadening. Here two modulated signals are time-division multiplexed with additional enhancement being achieved by polarization multiplexing of the two interleaved TDM streams. 
     Another system using enhanced polarization optical TDM is described and illustrated in U.S. Pat. No. 5,111,322 in the name of Bergano et al, entitled Polarization Multiplexing Device with Solitons and Method Using Same, incorporated herein by reference. In this patent, a transmission system&#39;s capacity is increased by using a combination of polarization and time-division multiplexing. More specifically, two streams of differently (preferably orthogonally) polarized solitons are interleaved (time-division-multiplexed) at a transmitter, and later separated at the receiver to recover both data streams. 
     The multiplexing of 2 channels of 2.5 Gbits/s each, into a single 5 Gbits/s channel, and the corresponding demultiplexing at the receiving end, is described in conjunction with the multiplexor of FIG. 2 in prior art U.S. Pat. No. 5,111,322. 
     In FIG. 2 the signal source for the two channels is a single, mode-locked laser  201 , producing about 35-50 ps wide soliton pulses at a 2.5 GHz rate. Its output is split into two soliton pulse streams having essentially orthogonal polarizations, in a splitter  202 , and each half separately modulated (with different information bearing signals labeled Data  1  and Data  2 ) in modulators  205  and  206 . Modulator  205  receives a first information bearing signal or data stream on line  207 , while modulator  206  receives a second data stream on line  208 . The two soliton pulse streams then recombine in a splitter  210 , but only after one of the pulse streams is delayed by one-half of the 2.5 Gbit/s bit period in an adjustable delay line  209  so that the two pulse streams are interleaved in time. 
     A few practical details concerning the apparatus of FIG. 2 are in order here. The modulators  205 ,  206  should preferably be of the LiNbO.sub.3, balanced Mach-Zehnder type, as those produce virtually no chirping of the soliton pulses, and have an adequate on-off ratio (about 20 dB). The required linear polarizations at the inputs to modulators  205 ,  206 , and for the polarization multiplexing itself, can either be maintained through the use of (linear) polarization-preserving fiber throughout the multiplexor, or through the use of polarization controllers, such as controllers  211 - 214 , both before and after modulators  205 ,  206  as shown in FIG.  2 . Polarization controllers  211 - 214  may be arranged as described in an article by H. C. Levevre, “Single-Mode Fiber Fractional Wave Devices and Polarization Controllers”, Electronics Letters, Vol. 16, p. 778, 1980. For the temporal interleaving of the two soliton pulse streams, it is necessary to make precise adjustment of the relative lengths of the two arms of the multiplexor. This can be done with adjustable delay line  209  which is shown interposed between the output of modulator  206  and polarization splitter  210 . Nevertheless, delay line  209  is not absolutely necessary. It is also possible to trim the length of one or the other arm, through one or two trials, to within a few picoseconds of the correct length so the apparatus may remain all-waveguide throughout. 
     The original soliton pulse stream output from the correctly adjusted multiplexer of FIG. 2 would appear as shown in FIG.  3 . The x and y axes represent intensities of pulses of different (orthogonal) polarizations. As an example, soliton pulses  301  and  302  have an initial polarization along the axis and a period of 400 ps. Soliton pulses  303  and  304  have an orthogonal (y direction) polarization, the same period, and are time interleaved with the first series of pulses. Information is carried in the pulse streams by virtue of the presence or absence of pulses at the expected or nominal positions on the time axis. Note that launching the soliton pulses as in FIG. 3 not only achieves the potential for combined time and polarization division demultiplexing at the receiving end, but also virtually eliminates the potential for cross-phase modulation, and hence virtually eliminates the potential for interaction during transmission, between the two channels. An alternative circuit to FIG. 2 is shown in FIG. 1, wherein two laser sources are shown, oriented to provide two orthogonally polarized beams; in all other respects, the circuit of FIG. 1 functions in a similar manner to the circuit of FIG. 2, however is absent the polarization controllers  211  and  212 . 
     The aforementioned prior art reference by Miyamoto et al. teaches the use of delay lines to time-skew the pulse trains that are to be multiplexed. For example, the paper discloses using two different lengths of polarization maintaining fibre in order to create a suitable delay. Although using different lengths of optical fibre provides a necessary delay, ensuring that this delicately balanced network is stable over a range of temperatures is not trivial. 
     Althhough the prior art optical circuits to some degree provide solutions for polarization time-division multiplexing, the &#39;322 patent for example describes a rather complex optical circuit where polarization controllers are shown to control the polarization state of the light propagating through the optical fibres. 
     In contrast, the circuit in accordance with this invention is a micro-optic circuit that does not rely on the use of polarization controllers and does not require polarization-maintaining optical fibre. 
     Furthermore, an aspect of the instant invention provides a micro-optic delay element, which utilizes the polarization difference between two data-streams to be time-multiplexed while preserving the polarization state of the two orthogonal streams. Furthermore, the instant invention provides a solution, which is considerably, more temperature-stable than using two separate waveguides and independently controlling for any temperature difference between the two waveguides. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention a circuit is provided for receiving two streams of data and for polarization time-division multiplexing the two streams of data onto a single waveguide such that one of the data streams is delayed by a time t d  from the other data stream, comprising: 
     a first modulator having an input port and an output port for modulating input light and for providing a first modulated data stream; 
     a second modulator having an input port and an output port for modulating input light and for providing a second modulated data stream; 
     a first lens for collimating light provided by the first modulator; 
     a second lens for collimating light provided by second modulator, 
     the first and second lenses each for providing a substantially collimated substantially unguided beam of light to at least another component; 
     a polarization beam splitter/combiner having two input ports at one end optically coupled to receive the substantially collimated, substantially unguided beams of light, said polarization beam splitter/combiner having a combining port at another end for combining the data streams such that one data stream delayed by a time t d  from the other data stream, 
     light traversing parallel paths from the first and second lenses respectively to the polarization beam splitter combiner being substantially unguided, so that light traversing at least one of said parallel paths will have a polarization state which is substantially unchanged. 
     In accordance with the invention, there is provided a circuit for receiving two streams of modulated data and for polarization and time-division multiplexing the two streams onto a single waveguide, comprising: 
     a polarization rotator for rotating the polarization of one of the two modulated data streams; and, 
     a birefringent crystal having at least two input ports disposed at one end to receive the two modulated data streams having orthogonal polarization states, the birefringent crystal having an output port disposed at an opposite end to receive and combine the two modulated data streams into a single time-interleaved data stream, the birefringent crystal being of suitable length for providing a path length difference between each of the at least two input ports and the output port to provide a required time delay at the output port between the two data streams. 
     In accordance with the invention, there is provided a circuit for receiving two streams of modulated data and for polarization and time-division multiplexing the two streams onto a single waveguide, comprising: 
     a light source for providing a primary signal; 
     a first and a second modulator for independently, and in parallel modulating the primary signal, the first and second modulators for providing two data streams; 
     means for operating on at least one of the two data streams relatively orthogonalizing the two data streams; and, 
     a birefringent crystal having at least two input ports at an end thereof disposed to receive the two modulated data streams having orthogonal polarization states, the birefringent crystal having an output port disposed at an opposite end thereof to receive and combine the two modulated data streams into a single time-interleaved data stream, the birefringent crystal being of suitable length for providing a path length difference between each of the at least two input ports and the output port to provide a required time delay at the output port between the two data streams. 
     In accordance with another aspect of the invention, a method of multiplexing optical signals onto an output port is provided. The method comprises the steps of: 
     providing two modulated polarized optical signals having a polarization difference between the two modulated signals of substantially 90 degrees; 
     passing one of the two modulated signals along a first path in a birefringent crystal; passing another of the two modulated signals along a second path intersecting the first path at the output port of the birefringent crystal. 
     In accordance with the invention there is provided a fibreless optical circuit for receiving two streams of modulated data and for polarization multiplexing the two data streams onto a single waveguide, comprising: 
     a modulator module for independently, and in parallel, modulating optical signals and for providing two data streams; 
     a birefringent crystal having at least two input ports at an end thereof disposed to receive the two modulated data streams having different polarization states, the birefringent crystal having an output port disposed at an opposite end thereof to receive and combine the two modulated data streams into a single multiplexed data stream, the birefringent crystal being of suitable dimensions to provide time division polarization multiplexing, the circuit being fibreless such that there is an absence of optical fibre between the modulator module and the birefringent crystal for coupling light therebetween. 
     Conveniently, if a delay is required that exceeds the delay that is provided by traversing the first and second paths of the birefringent crystal having different optical lengths, a spacer can be inserted into each of the signal paths prior to the signals reaching the birefringent crystal, wherein the spacers are of a substantially different refractive index. This method is quite suitable when optically coupling a lithium niobate modulating block with a rutile crystal, wherein no optical fibres are used except perhaps coupled to output ports. 
     In summary, the devices in accordance with this invention are small and compact and integrated. Yet still further, due to their compactness are somewhat easier to temperature control than, for example the prior art circuits shown. Yet still further, and perhaps more importantly, the optical circuit including the modulator focusing optics between the modulator and a polarization beam splitter/combiner do not require any optical fibre for coupling of light therebetween. Advantageously, by an providing a relatively unguided light path, polarization controllers or polarization maintaining fibre is not required. As well by providing block like elements coupled to one another, i.e. one or more modulator blocks coupled to rod GRIN lenses, coupled to a birefringent crystal yields a compact easy to assemble device that can be conveniently packaged. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the invention will now be described in conjunction with the drawings in which: 
     FIG. 1 is a prior art schematic diagram of a multiplexing circuit using 2 laser light sources oriented to yield orthogonal polarized light; 
     FIG. 2 illustrates a prior art schematic of a multiplexing circuit; 
     FIG. 3 illustrates the pulse field envelopes at the output of the multiplexor of FIG. 2; 
     FIG. 4 illustrates a schematic circuit block diagram of an enhanced TDM multiplexor in accordance with this invention; 
     FIG. 5 is a schematic block diagram which illustrates a portion of the circuit used for multiplexing and which illustrates the operation of that circuit portion; 
     FIG. 6 is a more detailed embodiment illustrating the circuit of FIG. 5; 
     FIG. 7 is an alternative embodiment to the circuit of FIG. 6, wherein two spacers having different refractive indices are utilized to achieve a time delay between signals traversing the two spacers; 
     FIG. 8 is a schematic block diagram of a preferred embodiment of the invention showing the modulator module coupled to a rutile crystal via a pair of substantially quarter pitch collimating GRIN lenses; and, 
     FIG. 9 is a schematic block diagram illustrating a polarization beam splitter/combiner in the form of a polarization beam splitting cube. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIG. 4, a substantially integrated micro-optic circuit is shown having a slab waveguide chip  10  having an end optically coupled with a laser  12  and having an end optically coupled with a birefringent crystal  14 . At an input end of the crystal  14 , a half waveplate  16  is provided for rotating the polarization of the light passing therethrough by 90°. 
     The slab waveguide chip is LiNbO 3  having waveguide disposed therein. The waveguides can be formed by ion implantation or alternatively by grafting polymer or other such light transmissive material into the chip. Electrical contacts  15   a ,  15   b ,  15   c  are disposed about the waveguides and in operation a voltage is applied to modulate the signal passing between the contacts. Variable attenuators are provided at the output for controlling the amplitude of the modulated signals. Although LiNbO 3  is a preferred modulator, of course other types of modulators my be used, for example electro-absorption or GaAs. Aside from the compactness and temperature stability of the circuitry shown within the waveguide  10 , the operation and interconnection of the components is substantially similar to the circuitry shown if FIG.  2 . Notwithstanding, one major difference between the circuit of the instant invention, shown in FIG.  4  and the prior art circuits, is the provision of the birefringent crystal for use as a polarization combiner and delay line for time-division polarization interleaving of pulses. One even more significant difference in this circuit and prior art circuits for time-division polarization multiplexing is the fibreless nature of the circuit from the modulator module  10  to the beam splitter/combiner, for example shown here in the form of a crystal  14 . By coupling substantially collimating lenses, for example, quarter pitch GRIN lenses to the modulator  10 , collimated beams are provided to next elements in sequence and to the crystal  14 . Since the substantially collimated beam traverses the glass spacer and half waveplate substantially unguided, its polarization state is substantially unaltered. 
     FIG. 5 illustrates a portion of the circuit shown in FIG. 4 depicting the operation of the polarization combining and multiplexing circuit. This circuit conveniently provides the added advantage of achieving a predetermined required delay. A stream of pulses spaced by 25 ps are provided at the input end of each of the GRIN lenses  50   a . Light directed through the bottom GRIN lens is rotated by 90 degrees by the half waveplate  16 . As can be seen in figure, this beam must travel a greater distance to reach GRIN lens  50   b , than the beam that follows a straight through path launched into the upper GRIN lens  50   a . This in effect skews the pulses in time that were launched simultaneously into the two GRIN lenses such that the orthogonally polarized pulses become combined and time multiplexed, as shown at the output of the GRIN lens  50   b . FIG. 6 (not drawn to scale) illustrates in more detail, dimensions of a birefringent or rutile crystal that achieves a desired time delay to provide time multiplexing of these two orthogonally polarized streams of pulses. The length of the crystal in this exemplary embodiment is 27 mm, and the with is 5 mm. Of course to some extent, the size of a crystal that is required is proportional to it cost. FIG. 7 illustrates yet another embodiment, wherein a spacer of glass  17  is inserted into the upper optical path, and a spacer of silicone  18  provides a portion of the lower optical path. By selecting light transmissive materials such as glass and silicone that have a substantially different refractive indexes in the two paths the beams must follow, delays in addition to delay provided by the birefringent crystal  14  can be enhanced and further controlled between the two. For example, in FIG. 7, the silicone spacer  18  shown, has a much higher refractive index than the glass spacer  17 ; light traveling through the silicone propagates therethrough slower than light traveling through a similar length of glass. Notwithstanding, a birefringent crystal of at least some minimum proportions is required. In the example shown, the beams propagating through the birefringent crystal  14  are collimated or near-collimated and substantially separated at the input end of the rutile. Thus, the crystal must be of dimensions that will support two beams, combine them, and provide a suitable required delay even in the instance that additional delay is provided by the silicone spacer. However, it can be seen, by comparing FIGS. 6 and 7, that the overall dimensions of the rutile, required to combine and time multplex the two pulse streams is substantially lessened in the embodiment of FIG.  7 . Nevertheless, this embodiment requires suitable antireflection coating between the GRIN lens  50   a  and the silicone spacer. 
     It is clear that either of the embodiments of FIG. 6 or FIG. 7 can be directly coupled optically to a modulator chip  10  as shown for example in FIG. 8 where the chip is illustrated as coupled to the embodiment of FIG.  7 . 
     In embodiments described heretofore, a half-wave plate is shown for rotating the polarization along one path, however, it is conceivable to provide orthogonally polarized beams of light to the modulator, obviating the requirement of a rotator.