CA2447927A1

CA2447927A1 - Depolarized optical signal transmitter

Info

Publication number: CA2447927A1
Application number: CA002447927A
Authority: CA
Inventors: Xueyan Zheng; Masoud Vaziri
Original assignee: Qtera Corporation; Xueyan Zheng; Masoud Vaziri
Current assignee: Qtera Corp
Priority date: 2002-03-22
Filing date: 2003-03-19
Publication date: 2003-10-09
Also published as: WO2003084101A1; AU2003230687A1; EP1490990A1

Abstract

A depolarized return-to-zero (DRZ) transmitter provides a DRZ output signal with improved immunity to polarization dependent gains and losses associated with optical fiber transmission systems.

Description

DEPOLARISED OPTICAL SIGNAL TRANSMITTER
Field of the Invention [0001] The present invention relates to optical communications, and in particular to providing a depolarized, return-to-zero optical transmitter.
Background of the Invention

[0002] Long-haul optical fiber transmissions systems suffer from polarization dependent loss (PDL) and polarization dependent gain (PDG) caused by the polarization sensitivity of the components used to create the transmission systems. Unfortunately, the PDL and PDG vary from component to component within a given system. To complicate matters further, the polarization state of an optical signal within the system will change randomly due to temperature variations, vibrations, polarization mode dispersion (PMD), and the like. These changes in polarization state result in changes in the received optical signal to noise ratio (OSNR) associated with the signal. As the transmission distance and number of optical components in the transmission system increase, the changes in polarization state increase, making the fluctuation of the received OSNR larger and the system design more difficult. Accordingly, there is a need to reduce the impact of PDL and PDG on long-haul transmission systems in an efficient and cost effective manner.
Summary of the Invention

[0003] A depolarized return-to-zero (DRZ) transmitter provides an output signal with improved immunity to polarization dependent gains and loses associated with optical fiber transmission systems. In select embodiments, a polarized clock signal of return-to-zero (RZ) optical pulses is split into two orthogonal optical clock signals. A first of the optical signals is transmitted along a first optical transmission path providing a first transmission delay and the second of the optical signals is transmitted along a second optical transmission path providing a second transmission delay. The first and second transmission delays of the respective transmission paths differ by an integer multiple of the period of the polarized clock signal and the pulses of WO 03/084101 ~ PCT/US03/08488 the main and orthogonal polarized signals at the end of the first and second transmission paths are orthogonally aligned. Furthermore, the delays should be larger than the coherent length of the laser source used in the transmitter, so the combined two orthogonal optical clocks have a random phase relationship and a DRZ optical clock signal is realized. The DRZ optical clock signal is then modulated by the polarization insensitive modulator (PIM) with data to provide a DRZ output signal.

[0004] In one embodiment, the first and second transmission paths are the two principle polarization axes of a polarization maintenance (PM) fiber. In this documents, we refer to those axes as the main and the orthogonal axes of the PM fiber. The RZ optical clock pulses from a polarization sensitive modulator may be coupled to the first end of the PM fiber at approximately a 45 degree angle from the main axis to effectively generate the main and orthogonal polarized clock signals on the main axis and the orthogonal axis, respectively.
In the PM fiber, the two orthogonal optical clocks have different delay. If the PM fiber is long enough and the delay will be longer than the coherent length of the laser used in the transmitter, a DRZ clock is realized. The DRZ clock is coupled into a polarization vinsensitive modulator (PIM), which encodes the data into the optical pulse stream.

[0005] In another embodiment, a polarization beam splitter is configured to split the polarized clock signal into main and orthogonal polarized clock signals; couple the main polarization clock signal to an input end of a first polarization maintenance fiber, which forms the first transmission path; and couple the orthogonal polarization clock signal to an input end of a second polarization maintenance fiber, which forms the second transmission path. A
polarization beam combiner is used to combine the output ends of the first and second polarization maintenance fibers in a manner combining the main and orthogonal polarized clock signals on the main axis and the orthogonal axis, respectively. If the delay between the two paths is longer than the coherent length of the laser used in the transmitter, a DRZ optical clock signal is realized.

[0006] In still another embodiment, a first PM coupler is used to split optical power of the polarized clock signal into two equal parts; couple one part of the polarized clock signal to a main axis of a first polarization maintenance fiber, which forms part of the first transmission path; and couple the other part of the polarized clock signal to a main axis of a second polarization maintenance fiber. The two parts are combined in a second PM
coupler. However, polarization of one of the paths is rotated by 90 degree before the second coupler. If the delay between the two paths is longer than the coherent length of the laser used in the transmitter, a DRZ optical clock signal is realized.

[0007] In yet another embodiment, the transmitter includes a first data modulator coupled to a data source and a first polarization maintenance fiber coupled between the switching means and the first modulator to form the first transmission path. The transmitter also includes a second data modulator coupled to the data source and a second polarization maintenance fiber coupled between the switching means and the second modulator to form the second transmission path. An optical polarization maintaining combiner is used to combine the output of the first and second modulators to form the DRZ optical signal on a single fiber.

[0008] In another embodiment, the transmitter includes first and second modulators for providing first and second polarized clock signals of RZ
optical pulses based on a clock signal. A coupler is provided to couple the first polarized clock signal to the main axis at a first end of a polarization maintenance fiber and couple the second polarized clock signal to the orthogonal axis at the first end of the polarization maintenance fiber to provide a DRZ clock signal. A polarization insensitive modulator is used to encode the data onto the pulse stream.

[0009] In another embodiment, the transmitter includes a clock modulator adapted to provide a polarized optical clock signal. A polarization sensitive modulator is coupled to an output of the clock modulator and adapted to modulate the clock signal with data from a data source to provide a polarized modulated signal. The polarized RZ optical signal is launched into a piece of PM fiber which main axis is 45-degree angle to polarization of the optical signal. In the PM fiber, the RZ optical signal is spitted into two orthogonal modes, which have different delays in the PM fiber. After the PM fiber, the RZ
optical signal will have two delayed orthogonal modes in each bit, to reduce the degree of polarization (DOP), and a DRZ signal is realized.

[0010] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.
Brief Description of the Drawing Figures

[0011] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

[0012] Figure 1 is a block representation of a depolarized return-to-zero transmitter according to a first embodiment of the present invention.

[0013] Figure 2 is a block representation of a depolarized return-to-zero transmitter according to a second embodiment of the present invention.

[0014] Figure 3 is a block representation of a depolarized return-to-zero transmitter according to a third embodiment of the present invention.

[0015] Figure 4 is a block representation of a depolarized return-to-zero transmitter according to a fourth embodiment of the present invention.

[0016] Figure 5 is a block representation of a depolarized return-to-zero transmitter according to a fifth embodiment of the present invention.

[0017] Figure 6 is a block representation of a depolarized return-to-zero transmitter according to a sixth embodiment of the present invention.
Detailed Description of the Preferred Embodiments

[0018] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0019] In general, the present invention provides a depolarized return-to-zero (DRZ) optical transmitter capable of providing a depolarized optical signal, which has data modulated thereon at a desired clock rate. The transmitter generates a polarized return-to-zero (RZ) clock signal, splits the RZ clock signal, and transmits one of the signals along a first path and the other signal along a second path. These paths are ultimately coupled to corresponding main and orthogonal axes of a single fiber. The resultant signals are modulated with data corresponding to the clock rate to provide a DRZ output signal. The following embodiments are illustrative of the various types of implementations of such a DRZ transmitter.

[0020] A DRZ transmitter 10, according to a first embodiment of the present invention, is illustrated in Figure 1. Initially, a polarized continuous-wave (CW) light source 12 provides polarized CW light 14 to a clock modulator 16. The clock modulator 16 receives a pulse stream forming an electrical clock signal 18 from a clock source 20 to effectively modulate the polarized CW light 14 with the electrical clock signal 18. The output of the optical clock modulator 16 is a polarized clock signal 22. For the purposes of illustration, the parallel lines associated with each clock pulse of the polarized clock signal 22 identify pulses to be transmitted via a main axis in a polarization maintenance (PM) fiber.

[0021] The clock modulator 16 is preferably a LiNb03 modulator and is used to generate RZ optical clock pulses, which are polarized. Those skilled in the art will recognize other modulators capable of generating polarized optical clock pulses. Those skilled in the art will also recognize that the CW
light 14 may be generated from various types of sources, such as DFB lasers and the like. Further, the functionality of the clock modulator 16 and the polarized CW light source 12 may be combined such that an optical clock is generated directly from a laser, such as that made possible by a gain switched laser or a fiber ring laser, which are available to those skilled in the art.

[0022] The polarized clock signal 22 is immediately modulated by data modulator 24 with electrical data 26, which is provided by a data source 28.
The output of the data modulator 24 is a polarized data signal 30. As illustrated, the data 26 used to modulate the polarized clock signal 22 begins with the pattern 1, 0, 1, 1,...; thereby resulting in a return-to-zero (RZ), polarized data signal 30 as illustrated wherein data corresponding to a logical 1 results in a pulse and data corresponding to a logical 0 results in no pulse in traditional return-to-zero fashion.. The polarized data signal 30 is coupled to a PM fiber 32 at an angle of 45 degrees from the main axis (of the PM fiber 32) via an appropriate optical connector 33. Coupling the polarized data signal 30 at an angle of 45 degrees from the main axis of the PM fiber 32 results in dividing the polarized data signal 30 into two signals, which are orthogonal to one another and travel along a main and orthogonal axis within the PM fiber 32.

[0023] Notably, the inherent nature of PM fibers causes light to travel down the main and orthogonal axes at different speeds. Accordingly, the delay between corresponding pulses in the main and orthogonal axes increase as the length of the PM fiber 32 increases and results in a depolarized data signal 34 wherein the main data pulses and the corresponding orthogonal data pulses are slightly misaligned at the end of PM fiber 32.

[0024] In this embodiment, the length of the PM fiber 32, and thus the amount of depolarization, is limited by the receiver's ability to correlate corresponding pulses between the main and orthogonal axes. With such limitations, the degree of depolarization is limited and the resulting depolarized data signal 34 typically maintains approximately 50% polarization or less.

[0025] Although the above embodiment is easy to implement, a more substantially depolarized data signal provides even greater immunity to PDL
and PDG associated with the transmission system. Preferably, the resulting depolarized signal has main and orthogonal components having a random phase relationship. With reference to Figure 2, a DRZ transmitter 10, capable of providing a depolarized signal having a random phase, is illustrated.
Separate polarized CW light sources 12A and 12B provide corresponding CW
light 14A and 14B to clock modulators 16A and 16B, respectively. Each clock modulator 16A and 16B is driven by a clock signal 13 provided by the clock source 20. Notably, the CW light signals 14A and 14B inherently have a random phase with respect to one another, given the use of separate polarized CW light sources 12A and 12B. Accordingly, the phase relationship between any optical signals derived from these CW light signals 14A and 14B
have a random phase relationship with respect to one another.

WO 03/084101 ~ PCT/US03/08488

[0026] The output of the clock modulator 16A is a polarized clock signal 22, which is provided to a polarization combiner 36 via a PM fiber 38. The output of clock modulator 16B is also a polarized clock signal 22, albeit having a phase relationship that.is random from that of the output of clock modulator 16A. The output of clock modulator 16B is delivered to a polarization rotator 40 via a PM fiber 42. The polarization rotator is essentially any optical device, connector, or coupling capable of delivering the polarized clock signal 22 from the clock modulator 16B to an orthogonal axis of a PM fiber 44, which couples to the polarization combiner 36. The orthogonal polarized clock signal 46 is combined with the (main) polarized clock signal 22 by the polarization combiner 36 to create a depolarized clock signal 50 with random phase relationship, which is delivered to the data modulator 24 via PM fiber 48. The depolarized clock signal 50 is modulated with data 26 from the data source 28 driving a fiber 52 with a depolarized signal 54. In this embodiment, the depolarized signal 54 has main and orthogonal components having a random phase relationship with respect to one another. Further, the respective pulses for the main and orthogonal pulse stream are aligned, unlike those in the first embodiment, making the transmitter in this embodiment capable of providing superior results over longer runs while maximizing the immunity to PDL and PDG.

[0027] Preferably, the data modulator 24 in this embodiment is an electrical absorption modulator, which is insensitive to the depolarized clock signal 50. Those skilled in the art will recognize other polarization insensitive modulators capable of being used with the present invention. In this second embodiment, the depolarized signal 54 has a 0% degree of polarization and provides exceptional immunity to PDL and PDG, yet requires multiple CW
light sources 12A and 12B, which will increase the cost of the DRZ transmitter 10.

[0028] Turning now to Figure 3, a DRZ transmitter 10 is illustrated that is capable of providing the depolarized signal 54 from a single CW light source 12, wherein the main and orthogonal pulse streams are aligned and have a random phase with respect to one another. Accordingly, a single polarized CW light source 12 provides CW light 14 to a clock modulator 16, which is modulated with a clock signal 18 from the clock source 20. The output of the WO 03/084101 g PCT/US03/08488 clock modulator 16 is a polarized clock signal 22, which is coupled to a PM
fiber 56 such that the polarized clock signal 22 is input to the PM fiber 56 at a 45 degree angle from the main axis by connector 33. The result is a polarized clock signal 22 appearing on the main and orthogonal axes of the PM fiber 56.
As noted, transmission along the main and orthogonal axes occurs at different speeds.

[0029] For the current example, assume the main axis facilitates a higher transmission speed than the orthogonal axis. The PM fiber 56 between the connector 33 and the input of the data modulator 24 is selected such that pulses of the polarized clock on the main axis arrives at the data modulator at the same time as a pulse of a polarized clock on the orthogonal axis.
Accordingly, the different axes simply provide a delay in the respective polarized clock signals, and the length of the PM fiber 56 is selected such that there is an integer delay period between the clock pulses on the main axis and the orthogonal axis. The resultant depolarized clock signal 50 received by the data modulator 24 is illustrated. The delay between the main and orthogonal axes should also correlate to a distance greater than the coherent length of CW light source 12. Separating corresponding pulses in the main and orthogonal axes by a distance greater than the coherent length of the PM
fiber 56 ensures a depolarized clock signal 50 having a random phase relationship. The depolarized clock signal 50 is modulated by the data signal 26 provided by data source 28 to provide the depolarized signal 54. The resultant pulse width and intensity of the depolarized signal 54 is essentially the same as that provided by the architecture of Figure 2.

[0030] Turning now to Figure 4, another embodiment of a DRZ transmitter is shown wherein a single, polarized CW light source 12 is used to generate a depolarized signal 54. As described above, the output of the clock modulator 16 provides a polarized clock signal 22. Instead of inputting the polarized clock signal 22 into a single PM fiber at a 45 degree angle from the main axis, the polarized clock signal 22 is delivered to a polarization beam splitter (PBS) 58 which provides two output signals corresponding to polarized clock signal 22, which are delivered to a polarization beam combiner (PBC) 60 after traveling across a first PM fiber 62 and a second PM fiber 64, respectively. The first output, which is the main polarized clock signal, travels along the first PM fiber 62 wherein the second output signal, which is the orthogonal polarized clock signal 66, travels along the second PM fiber 64.

[0031] The first PM fiber 62 is significantly longer than the second PM fiber 64. Both lengths of the first and second PM fibers 62 and 64 are sufficiently long wherein the pulses from the main and orthogonal polarized clock signals are aligned when combined by the PBC 60 to provide a depolarized clock signal 50. Preferably, the depolarized clock signal 50 has a random phase relationship between the main and orthogonal components given the relationship between the corresponding pulses. Further, the difference in lengths of the first and second PM fibers 62, 64 is greater than the coherent length of CW light source 12. This difference ensures the pulse of the respective streams will have a random phase relationship at PBC 60 even though they are derived from a single light source.

[0032] The depolarized clock signal 50 is received by the data modulator 24 and modulated with the data signal 26 from a data source 28 to provide the depolarized signal 54, which is for all practical purposes identical to that provided in the embodiments of Figures 2 and 3. The embodiment of Figure 4 may reduce the length of PM fiber required between the clock modulator 16 and the data modulator 24 over the embodiment of Figure 3.

[0033] Figure 5 provides another embodiment capable of providing a depolarized signal 54 from a single, polarized CW light source 12. As described above, the clock modulator 16 modulates the CW light source 14 with a clock signal 18 to provide the polarized clock signal 22. Polarized clock signal 22 is input to a PM coupler 68, which facilitates delivery of the polarized clock signal 22 to PM coupler 70 via a first PM fiber 72 along its main axis.
The polarized clock signal 22 is also sent to a connector 74 along the main axis of a second PM fiber 76.

[0034] A third PM fiber 78 connects the connector 74 to the PM coupler 70 in a manner wherein the polarized clock signal 22 is coupled to the orthogonal axis of the third PM fiber 78 by the connector 74. Thus, a polarized clock arrives at the PM coupler 70 along a main axis of the first PM fiber 72 and along the orthogonal axis of the third PM fiber 78. Both signals are combined to provide the depolarized clock signal 50. As with the latter embodiments, the lengths of the PM fibers 72, 76, and 78 are such that pulses on the main WO 03/084101 l0 PCT/US03/08488 and orthogonal axes at the output of the PM coupler 70 are aligned yet do not emanate from the same original pulse of the polarized clock signal 22 so as to provide a random phase relationship. Preferably, the length of the first PM
fiber 72 is significantly longer than the combined lengths of the second and third PM fibers 76 and 78. Preferably, difference in lengths of the PM fibers 76 and 78 is greater than the coherent length of CW light source 12. The depolarized clock signal 50 is then modulated with data 26 by the data modulator 24 to provide the depolarized signal 54 at the output 52.

[0035] Turning now to Figure 6, yet another embodiment of a DRZ
transmitter 10 is illustrated wherein the depolarized signal 54 can be generated from a single, polarized CW light source 12. Again, CW light 14 is modulated with a clock 18 by modulator 16 to provide a polarized clock signal 22. The polarized clock signal 22 is sent to a PM coupler 80 for splitting and delivering each of the split signals to a first data modulator 82 and a second data modulator 84, respectively. A first PM fiber 90 connects the PM coupler 80 to the first modulator 82, and a second PM fiber 92 couples the PM coupler 80 to the second data modulator 84. Again, the first PM fiber 90 is significantly longer than the second PM fiber 92 and both PM fibers 90, 92 have a length wherein pulses from the polarized clock signal 22 arrive at the respective first and second data modulators 82 and 84 simultaneously.
Again, the difference in the lengths of PM fibers 90 and 92 is greater than the coherent length of CW light source 12 to ensure a random phase relationship between the two pulse streams. The polarized clock signal 22' of the first PM
fiber 90 and the polarized clock signal 22" of the second PM fiber 92 are shown attenuated to reflect the splitting of power resulting from splitting the polarized clock signal 22 into two signals by the PM coupler 80.

[0036] As noted, pulses from. the polarized clock signals 22' and 22" arrive at the first data modulator 82 and the second data modulator 84 simultaneously, and are modulated by the data signal 26 from the data source 28. The output of the first data modulator 82 is provided to a PM fiber 94 and represents a polarized signal 96. The delay between the first path, PM fiber 94, and the second path, PM fiber 98 and connector 94, should be the same in order to avoid the distortions of DRZ from PBC 88. Similarly, the second data modulator 84 outputs a polarized signal to a connector 86, which effectively provides a 90 degree rotation of the polarized signal prior to being delivered to, the data modulator 84. The rotation effectively places the orthogonal polarized signal 100 on the orthogonal axis of PM fiber 98.
Accordingly, the PBC 88 receives the polarized signal 96 along the main axis of the PM fiber 94, as well as a corresponding orthogonal polarized signal 100 along the orthogonal axis of the PM fiber 98 and combines the two signals to form the depolarized signal 54 at the output 52. Accordingly, the depolarized signal 54 has a random phase with respect to its main and orthogonal components. The depolarized signal 54 provides significant immunity to PDL
and PDG caused by the polarization sensitivity of components appearing between the transmitter and the receiver in the transmission system. The nature of the depolarized signal 54 reduces the fluctuation of OSNR caused by PDL and PDG and allows for longer transmission distances.

[0037] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Claims

Claims What is claimed is:

1. A depolarized return-to-zero (DRZ) optical transmitter comprising:

a) means for providing a polarized clock signal of return-to-zero (RZ) optical pulses;

b) means for splitting the polarized clock signal into main and orthogonal polarized clock signals;

c) a first optical transmission path providing a first transmission delay for the main polarized clock signal;

d) a second optical transmission path providing a second transmission delay for the orthogonal polarized clock signal, the first and second transmission delays differing by an integer multiple of a period of the polarized clock signal, wherein pulses of the main and orthogonal polarized signals at the end of the first and second optical transmission paths are orthogonally aligned and have a random phase relationship for forming a DRZ
optical clock signal.

2. The transmitter of claim 1 wherein the first and second transmission paths are a main axis and an orthogonal axis, respectively, of a single, polarization maintenance fiber having an input end and an output end, and further comprising a polarization insensitive modulator coupled to the output end of the polarization maintenance fiber and adapted to modulate the DRZ optical clock signal with data to provide a DRZ
signal.

3. The transmitter of claim 2 wherein the means for splitting couples the RZ optical pulses into the first end of the polarization maintenance fiber at approximately a 45 degree angle from the main axis to effectively generate the main and orthogonal polarized clock signals on the main axis and the orthogonal axis, respectively.

4. The transmitter of claim 1 wherein the splitting means is a polarization beam splitter configured to:

a) split the polarized clock signal into main and orthogonal polarized clock signals;

b) couple the main polarization clock signal into an input end of a first polarization maintenance fiber, which forms the first transmission path; and c) couple the orthogonal polarization clock signal into an input end of a second polarization maintenance fiber, which forms the second transmission path, and the transmitter further comprising a polarization beam combiner coupled to output ends of the first and second polarization maintenance fibers in a manner combining the main and orthogonal polarized clock signals on the main axis and the orthogonal axis, respectively, on a third polarization maintenance fiber to provide the depolarized DRZ optical clock signal.

5. The transmitter of claim 4 further comprising a polarization insensitive modulator coupled to an output end of the third polarization maintenance fiber and adapted to modulate the DRZ optical clock signal with data to provide a DRZ signal.

6. The transmitter of claim 1 wherein the splitting means is a first coupler configured to:

a) split the polarized clock signal into the main and orthogonal polarized clock signals;

b) couple the main polarization clock signal into a main axis of a first polarization maintenance fiber, which forms at least part of the first transmission path; and c) couple the orthogonal polarization clock signal into a main axis of a second polarization maintenance fiber via a second input end, and the transmitter further comprising a third polarization maintenance fiber, and a second coupler having inputs coupled to output ends of the first and third polarization maintenance fibers, and a connector connecting an output end of the second polarization maintenance fiber and an input end of the third polarization maintenance fiber such that the second coupler provides the main and orthogonal polarized clock signals on the main axis and the orthogonal axis, respectively, on a fourth polarization maintenance fiber to provide the depolarized DRZ
optical clock signal, wherein the second and third polarization fibers form at least part of the second transmission path.

7. The transmitter of claim 6 further comprising a polarization insensitive modulator coupled to an output end of the fourth polarization maintenance fiber and adapted to modulate the DRZ optical clock signal with data to provide a DRZ signal.

8. The transmitter of claim 2 further comprising a polarization insensitive modulator adapted to modulate the DRZ optical clock signal with data to provide a DRZ signal and means for coupling the main and orthogonal polarized signals at the end of the first and second transmission paths to the input of the polarization insensitive modulator as the DRZ optical clock signal.

9. The transmitter of claim 1 further comprising:

a) a first data modulator coupled to a data source;

b) a first polarization maintenance fiber coupled between the switching means and the first modulator to form the first transmission path;

c) a second data modulator coupled to the data source; and d) a second polarization maintenance fiber coupled between the switching means and the second modulator to form the first transmission path.

10. The transmitter of claim 9 further comprising combining means for combining the outputs of the first and second modulators to form the DRZ optical signal on a single fiber.

11. The transmitter of claim 1 further comprising a polarized, continuous-wave light source and wherein the means for generating the polarized clock signal is a modulator adapted to modulate continuous-wave light with a clock signal to generate the polarized clock signal.

12. The transmitter of claim 1 wherein the difference between first transmission delay and the second transmission delay corresponds to a difference greater than a coherent length of a continuous wave light source from which the polarized clock signal is provided.

13. A depolarized return-to-zero (DRZ) optical transmitter comprising:

a) first means for providing a first polarized clock signal of return-to-zero (RZ) optical pulses based on a clock signal;

b) second means for providing a second polarized clock signal of RZ
optical pulses based on the clock signal;

c) a polarization maintenance fiber having main and orthogonal axes;

d) means for coupling the first polarized clock signal to the main axis at a first end of the polarization maintenance fiber and coupling the second polarized clock signal to the orthogonal axis at the first end of the polarization maintenance fiber to provide a DRZ clock signal; and e) a polarization insensitive modulator coupled to a second end of the polarization maintenance fiber and a data source and adapted to modulate the DRZ clock signal with data from the data source to provide a DRZ signal modulated with the data at a rate corresponding to the DRZ optical clock.

14. A depolarized return-to-zero (DRZ) optical transmitter comprising:

a) a clock modulator adapted to provide a polarized clock signal of return-to-zero (RZ) optical pulses based on a clock signal;

b) a modulator coupled to an output of the clock modulator and adapted to modulate the clock signal with data from a data source to provide a polarized modulated signal;

c) a polarization maintenance fiber having a main axis providing a first transmission path with a first transmission delay, an orthogonal axis providing a second transmission path with a second transmission delay, and a length wherein the first and second transmission delays .differ by an integer multiple of the period of the polarized modulated signal; and d) a polarization coupler for coupling the output of the modulator to a first end of the polarization maintenance fiber such that the polarized modulated signal is coupled at approximately a 45 degree angle to the main axis of the polarization maintenance fiber, wherein pulses from the main and orthogonal axes at the end of the polarization maintenance fiber are partially, orthogonally aligned to form a DRZ optical signal.

15. A method comprising:

a) generating a polarized clock signal of return-to-zero (RZ) optical pulses;

b) splitting the polarized clock signal into main and orthogonal polarized clock signals;

c) transmitting the main polarized clock signal along a first transmission path providing a first transmission delay; and d) transmitting the orthogonal polarized clock signal along a second transmission path providing a second transmission delay, the first and second transmission delays differing by an integer multiple of a period of the polarized clock signal, wherein pulses of the main and orthogonal polarized signals at the end of the first and second transmission paths are orthogonally aligned and have a random phase relationship for forming a DRZ optical clock signal.

16. The method of claim 15 further comprising modulating the DRZ optical clock signal with a data signal to provide a DRZ signal modulated with data at a rate corresponding to the DRZ optical clock.

17. The method of claim 15 wherein the difference between first transmission delay and the second transmission delay corresponds to a difference greater than a coherent length of a continuous wave light source from which the polarized clock signal is provided.

18. A system comprising:

a) means for generating a polarized clock signal of return-to-zero (RZ) optical pulses;

b) means for splitting the polarized clock signal into main and orthogonal polarized clock signals;

c) means for transmitting the main polarized clock signal along a first transmission path providing a first transmission delay;

d) means for transmitting the orthogonal polarized clock signal along a second transmission path providing a second transmission delay, the first and second transmission delays differing by an integer multiple of a period of the polarized clock signal, wherein pulses of the main and orthogonal polarized signals at the end of the first and second transmission paths are orthogonally aligned and have a random phase relationship for forming a DRZ optical clock signal.

19. The system of claim 18 further comprising modulating the DRZ optical clock signal with a data signal to provide a DRZ signal modulated with data at a rate corresponding to the DRZ optical clock.

20. The system of claim 18 wherein the difference between first transmission delay and the second transmission delay corresponds to a difference greater than a coherent length of a continuous wave light source from which the polarized clock signal is provided.

21. A method comprising:

a) generating a polarized clock signal of return-to-zero (RZ) optical pulses;

b) splitting the polarized clock signal into two polarized clock signals;

c) transmitting a first of the two polarized clock signals along a first transmission path providing a first transmission delay;

d) transmitting and rotating a second of the two polarized clock signals along a second transmission path providing a second transmission delay, the first and second optical transmission delays differing by an integer multiple of a period of the polarized clock signal, wherein pulses at the end of the first and second transmission paths are orthogonally aligned and have a random phase relationship for forming a DRZ optical clock signal.

22. The method of claim 21 further comprising modulating the DRZ optical clock signal with a data signal to provide a DRZ signal modulated with data at a rate corresponding to the DRZ optical clock.

23. The method of claim 21 wherein the difference between first transmission delay and the second transmission delay corresponds to a difference greater than a coherent length of a continuous wave light source from which the polarized clock signal is provided.

24. A depolarized return-to-zero (DRZ) optical transmitter comprising:

a) means for generating a polarized clock signal of return-to-zero (RZ) optical pulses;

b) means for splitting the polarized clock signal into two polarized clock signals;

c) means for transmitting a first of the two polarized clock signals along a first transmission path providing a first transmission delay;

d) means for transmitting and rotating a second of the two polarized clock signals along a second transmission path providing a second transmission delay, the first and second transmission delays differing by an integer multiple of a period of the polarized clock signal, wherein pulses at the end of the first and second transmission paths are orthogonally aligned and have a random phase relationship for forming a DRZ optical clock signal.

25. The transmitter of claim 24 wherein the difference between first transmission delay and the second transmission delay corresponds to a difference greater than a coherent length of a continuous wave light source from which the polarized clock signal is provided.