Optical communications – Multiplex – Time division
Reexamination Certificate
2003-05-08
2004-11-16
Phan, Hanh (Department: 2633)
Optical communications
Multiplex
Time division
C398S152000, C398S065000, C398S184000, C398S182000, C398S183000, C398S161000, C398S147000, C398S079000
Reexamination Certificate
active
06819872
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical fiber communications and in particular to multiplexed communications that uses time-division 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.
Such systems typically use lengths of optical fibre or other delay means to provide a required optical path length difference between two paths such that a predetermined delay between two data streams is provided to achieve bit interleaving.
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'98 Sep. 20-24, 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'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.
Although the prior art optical circuits to some degree provide solutions for polarization time-division multiplexing, the '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.
In another aspect of the invention a bulk delay optical circuit is provided wherein to optical paths are provided wherein light propagating along the two paths is unguided. The unguided beams comprising separate bit streams are then combined to provide a single time-multiplexe
Cheng Yihao
Dunn Paul E.
Farries Mark
Finch Andrew
Munks Timothy C.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
JDS Uniphase Corporation
Phan Hanh
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