Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/25—Arrangements specific to fibre transmission
  • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
  • H04B10/2572—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to forms of polarisation-dependent distortion other than PMD
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/29—Repeaters
  • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
  • H04B10/293—Signal power control
  • H04B10/2933—Signal power control considering the whole optical path
  • H04B10/2935—Signal power control considering the whole optical path with a cascade of amplifiers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0201—Add-and-drop multiplexing
  • H04J14/0202—Arrangements therefor
  • H04J14/0204—Broadcast and select arrangements, e.g. with an optical splitter at the input before adding or dropping
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0201—Add-and-drop multiplexing
  • H04J14/0202—Arrangements therefor
  • H04J14/0205—Select and combine arrangements, e.g. with an optical combiner at the output after adding or dropping
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0201—Add-and-drop multiplexing
  • H04J14/0202—Arrangements therefor
  • H04J14/0213—Groups of channels or wave bands arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/06—Polarisation multiplex systems

Definitions

• the field of this invention is long fiber optic transmission lines with very high bandwidth, long span between repeaters, and low cost.
• these OC48 channels are created by using laser diodes operating at around 1.55 micron wavelength and carefully tuned to be about 3 x 10 10 Hertz between channels if there are 100 channels or about 10 11 Hertz if there are 32 channels. These laser diodes are usually directly modulated with the signal and then the channels are multiplexed onto the long line fiber. After about 50 km of travel the signals are amplified about lOdb by an Erbium doped fiber amplifier pumped by laser diodes operating at 0.98 or 1.48-micron wavelength.
• This device de-multiplexes (separates) the optical frequency channels, detects each channel separately, decides whether an incoming bit is "one” or “zero” and then uses this bit stream signal to modulate a new laser diode. Then the repeater multiplexes all these new signals onto a fiber going out of the repeater. Usually the fiber for this long line has several unused fibers in the same cable.
• the objects of this invention are to provide much greater total signal carrying capacity for the system and to do so at lower cost per bit and to increase the reliability of the system.
• the invention removes the repeaters entirely or at least increases the distance between repeaters.
• the prior art laser diodes which typically provide the optical beams signal carrying, are removed and replaced by an unmodulated bank of spectral lines which are generated at some central location and distributed to several transmission stations.
• these unmodulated lines are modulated with signals. If the incoming signals to be transmitted are at a bit rate higher than the system is designed for, they are demultiplexed in time down to the design bit rate. Then the signals are optically multiplexed onto the long line fiber. If there are no repeaters then the bit rate is designed to go the full length. If there are repeaters then the bit rate is designed to go the distance between repeaters.
• the signals are demultiplexed and detected but with a filtering system that allows much closer channel spacing than in use now.
• odd numbered channels are sent out in one polarization state and even numbered channels in an orthogonal polarization state this makes demultiplexing easier and also decreases crosstalk due to dielectric non-linearity
• an optical frequency channel contains an unmodulated heterodyne line in addition to the signal.
• the broad channel carries several signals in sub channels as well as the heterodyne line.
• the modulation rate of each signal is lowered to the point where no repeaters are needed for that line length.
• the bit rate per channel is decreased then more channels are needed to carry the high total bit rate. This means closer channel spacing.
• optical filters are provided which can separate close channel spacing.
• heterodyne detection is used as a part of the filter system.
• heterodyne lines are provided at the receive end.
• a pump beam is used, which pump beam is obtained not directly from a laser diode, but indirectly by first using a neodymium doped fiber laser operating around 1.47 microns.
• FIG. 1 is an illustration of a system architecture embodying the preferred embodiment of the invention
• Fig. 2 is an illustration of broad optical channels containing signal subchannels
• Fig. 3 is an illustration of upper and lower side bands generated by RF modulation
• Fig. 4 is an illustration of a double pass 2 arm interferometer
• Fig. 5 illustrates a filter curve using the filter of Fig. 4 with a single pass
• Fig. 6 is an illustration of a channel drop
• Fig. 7 illustrates a channel drop and a channel add
• Fig. 8 illustrates a two arm fiber interferometer
• Fig. 8a and 8b illustrate the filtered output
• Fig. 9 illustrates a polarization rectifier using macro optical components and one using only one fiber
• Fig. 10 illustrates a multiple stage polarization seperator
• Fig. 11 illustrates multiplexed signals on a fiber
• Fig. 12 illustrates signal channels with heterodyne lines
• Fig. 13 illustrates polarized signal channels with heterodyne lines
• Fig. 14 illustrates a signal amplified by a Brillouin pump
• Fig. 15 illustrates a frequency shifted spectral line
• Fig. 16 illustrates a frequency shifter
• Fig. 17 illustrates a two-arm interferometer to separate signals
• Fig. 18 illustrates a figure curve
• Fig. 19 illustrates multiple filtering for channel separation using 2-arm fiber interferometer
• Fig. 20 illustrates the modulation shape of a digital signal after filtering out harmonics
• Fig. 21 illustrates a signal with Brillouin gain
• Fig. 22 illustrates the separated channels
• Fig. 23 is an illustration of an Erbium laser pumped by a Neodymium doped fiber laser
• Fig. 24 is an illustration of the spectrum at the receive end.
• polarization rectifier polarization separator
• wavelength shifter bank for generating unmodulated source lines
• multi-pass Planar Fabry Perot bank for generating unmodulated source lines
• heterodyne detector bank for generating unmodulated source lines
• double-pass 2 arm interferometer and Erbium amplifier pumped by a
• Neodymium laser is Neodymium laser.
• all traffic presented to the long line system is de-multiplexed in time down to a bit rate that can be carried to the far end of the system or to an intermediate receive station without repeaters.
• bit rate For a distance of 5000 kilometers that bit rate is about 6 x 10 8 or "OC12". If the traffic to be carried is already that low or lower it is left as is.
• the fiber used for this long line system is preferably "dispersion un-shifted" fiber or more generally, fiber with high dispersion in the 1.5 micron window.
• a bank of unmodulated source lines is provided at the transmitting end of the system.
• OC12 signal is modulated onto each of these source lines. They are spaced around 2.4 x 10 9 hertz.
• each line is orthogonally polarized relative to its two nearest neighbors at the point where the signals are introduced onto the long line. This polarization orthogonality reduces cross talk due to non-linearity.
• the modulation signal is of a shape that minimizes harmonics of the basic binary bit signal or the electronic signal is filtered to remove these harmonics before modulating the signal onto the unmodulated optical source line.
• Optical amplifiers are placed around 50 kilometers apart. These are Erbium doped fiber amplifiers. They are preferably pumped by Neodymium doped fiber lasers operating around 1.45 to 1.47 microns. The Erbium amplifiers are gain flattened. At the transmission end the total signal load is maintained relatively constant by adding "drone" signals as needed to fill unused channels. When channels are "dropped” along the path of the long line this is accomplished by flat tapping a fraction of the power of all channels off the long line.
• the unmodulated line bank is preferably carried in a separate fiber along side the signal carrying fiber. When a signal is to be added one of these bank lines is modulated and "flat tapped” onto the signal carrying fiber on a channel which is not in use. Preferably the assignment of channels is semi-permanent and not done “on the fly” depending on the traffic load.
• the optical frequency channels are de-multiplexed preferably with 2-arm fiber interferometers.
• the first such interferometer separates alternate channels onto 2-arms and so on. If necessary the first split can be repeated to improve the rejection of nearby channels.
• individual signals are not multiplexed in time.
• Signals are grouped together in sub - channels with each group being provided with an un-modulated heterodyne line spaced apart from the group by an amount such that the lowest beat frequency between the heterodyne line and a signal is greater than any beat frequency between signals in the group.
• the signals were preferably around 6 x 10 8 bits per second.
• the group width can be up to around 5 x 10 9 bits per second - but no individual signal is above 6 x 10 8 .
• the spacing between groups is preferably 3 times the group width. That is, these are spaced 4 times group width center to center. Again alternate groups are preferably polarized orthogonal to each other.
• the groups are separated by 2 arm fiber interferometers as before.
• the channels are preferably separated by polarization.
• Brillouin amplification or de- amplification of heterodyne lines may be used.
• individual signals are demultiplexed in the electronic realm by beat frequency.
• a bank of unmodulated spectral lines, to be modulated spaced about 5 x 10 9 Hertz on center are generated and transmitted to a Head End 10.
• bit rate of incoming signals to be transmitted is detected and if the bit rate is too high it is demultiplexed in time , see Fig 1 at 12.
• the signals are demultiplexed in time until the bit rate for each signal is low enough so that it can go to its receive station without needing repeaters.
• the signals are optically multiplexed at 14.
• a long line is shown as 16 with amplifiers ' 18 and receiving stations 20. In many cases the different signals will go different distances on the long-line and be dropped.
• the signals which go the full length of the long line may be carried at a lower bit rate than those going only one quarter of the length before being dropped off. In some embodiments all signals are broken down to the same low bit rate.
• Electronics well known in the art routes each signal to the proper optical frequency channel so that it will be dropped at its proper drop off point. That is, signals are addressed to the proper receive station by the optical frequency channels to which they are assigned.
• each signal is modulated onto one of these RF frequencies electronically and the 60 modulated RF frequencies drive an optical modulator (not shown).
• the upper side bands lie in the sub-channels slots already mentioned.
• the lower side bands are filtered off-line by an optical filter, such as a multi-pass planar mirror Fabry-Perot. This gives a rejection of 40db at the near edge of the lower side band and 20dB for the source line.
• the signals at the edges of the desired upper side band are down by a factor of 4 so a flattening filter may be used.
• Another filter is a simple 2-arm fiber interferometer. A filter based on this makes 2 passes through the same filter, see Fig. 4. This gives a filter curve as shown in Fig. 5.
• the electronics can be controlled so the signals at the edge are up in power to compensate the filter shape.
• the power is controlled throughout the sub-channel to get a flat curve after filtering. To get even better refection of the lower side band use 2 double pass filters in series.
• the traffic on the fiber is amplified by an erbium amplifier 30 (pumped by a neodymium doped fiber laser) after the power has dropped about lOdB which is a distance of about 50 km.
• the gain of the amplifier is flattened by means known to the art.
• a "non-filter" tap 36 hereinafter referred to as a
• a second fiber travels the same route as the signals- carrying fiber but only carries an unmodulated source lines, see Fig. 7.
• Station X taps off a fraction of the power of the source signal at 36 and puts signals onto the signal carrying fiber in the same way as described before for the originating transmission station. These are tapped onto the long line with a partial tap.
• the source bank line which was used to generate the channel is filtered out from the bank line fiber and introduced onto one branch of the fiber carrying the signals - after the signals have been filtered by a multi-pass planar mirror Fabry-Perot or a 2 arm fiber interferometer or both, see Fig. 8.
• the filtered output is taken and the power divided onto 2 legs. Then a heterodyne line x is added to one leg. Each leg is detected and A is subtracted from B.
• B contains the beat frequencies between the heterodyne line X and the channel or sub-channels containing individual signals. B also contains low power beat signals between the sub-channels to the left and right of X and the sub channels of X, and also between pairs of channels that have not been completely filtered out. A is subtracted from B to eliminate spurious beats.
• any two lines must be of the same polarization to beat fully, if they are of orthogonal polarization state they don't beat at all. So to get a reliable beat signal the heterodyne line must be in the same polarization state as the signals. To accomplish this, the signals are passed through a polarization rectifier. There are two kinds of rectifiers, as shown in Fig. 9.
• the simplest kind takes a signal traveling in a fiber and transfers one linear polarization state to another fiber by lateral coupling or by macro optics.
• the second phase adjust can be tuned so that one initial polarization, whatever it may be will exit C and the orthogonal state will exit D. This allows more signals on one fiber to be multiplexed, see Fig. 11.
• the signals are passed through the full polarization rectifier and the two signals are separated and processed.
• the separation will be clean at least over about 10 10 Hertz but the spectrum may need to be filtered so that only a few adjacent channels are processed with one polarization rectifier.
• Brillouin de-amplifiers are used to remove the two adjacent heterodyne lines - or even 4 nearby lines. Brillouin amplification is known to the art. Brillouin de-amplifiers simply place the Brillouin "pump" on the low frequency side of the spectral region to be de-amplified.
• the adjacent broad channels are orthogonally polarized.
• a full polarization rectifier is used to separate the 2 polarizations, see Fig. 13. It is obvious that a Fabry Perot filter will "shade" the edges of the Sub-channels but the electronics after detection can take care of that problem provided the sub-channels are not so wide that they are individually shaded more than about 10% from edge to edge.
• the procedure set forth above is repeated, there is a split into two paths and the heterodyne line is added in one path only. The detected results are subtracted. But when the heterodyne lines are transmitted along with the signals it is already added. So in this case, the heterodyne line is subtracted from one arm using a
• Orthogonal polarization states interact very little in causing cross talk due to nonlinear dielectric constant so it is useful to use orthogonal polarization for alternate channels even when the polarization is not used for de-multiplexing.
• 2-arm interferometer filtering is used to separate channels so they can be properly filtered.
• the modulation shape of a digital signal is as shown in Fig. 20.
• the 3 rd harmonic of the basic sine wave and all higher Fourier components are filtered out. This can be done electronically before modulation or optically after modulation.
• the necessary electronic filtering is known to the art.
• This smoothed out shape has two advantages. It gives less trouble with dispersion and it reduces the bandwidth in Hertz necessary to carry a certain bit rate. This comes close to tripling the amount of bits that can be carried in a fiber.
• the signal shape distortion as the signal travels long distances is caused by the fact that the high Fourier components travel at a different speed from the lower components. So the harmonics which convert the sin wave into a square wave get out of phase and have the opposite affect. Leaving out the harmonics in the first place increases the distance between necessary repeaters at a given bit rate.
• the detection process becomes a little more prone to error.
• the electronics in the detector contains logic that decides whether a given time slot is a 1 or a 0 on the basis of that slot but also on the basis of the preceding and following time slots.
• each signal is put on a different wavelength.
• One 3 x 10 7 Hertz bandwidth channel is allotted to each signal and the channels are placed side by side or nearby. This allows 100,000 channels in the C-band gain region of an Erbium amplifier or about 300,000 in the L-Band.
• Signals of 1.5 x 10 7 Hertz bandwidth are used which is about 3 x 10 7 bits per second for filtered digital for ordinary video.
• a Brillouin pump whose optical frequency is proper to amplify that signal is used, see Fig. 21.
• About 10 6 gain (60dB) is needed to adequately separate the signal.
• At 2.5 dB per mW km, 4 mW and 6 km of fiber is needed to get 60dB of gain, preferably differential gain.
• Flat attenuation is put in one or more times to keep the absolute gain down.
• Xi has heterodyne beats ranging from ⁇ to 2 ⁇ . There are no inter signal beats in this range. So the only conflict is with other heterodyne beats.
• y 0 to yi, y 2 and y-i are Brillouin de-amplified to very low levels. Brillouin de- amplified x-i and x- 2 are also.
• Xi is Brillouin amplified by around 20db.
• the bands are filtered by any of several possible means — centering the pass band on Xi.
• the Xi heterodyne is off center and will be partially filtered out but it has been amplified enough so it doesn't hurt.
• Erbium doped fiber amplifiers are commonly pumped with laser diodes with output of either 0.98 or 1.48 microns. These are laser diodes which put out a lot of power but it is difficult to get this power into a single mode fiber. It is very desirable to deplete the ground state of the Erbium in order to improve the photon statistics of amplification so it is advantageous to pump the Erbium doped fiber from both ends. Neodymium doped fiber lasers and amplifiers are well known but not widely used.
• Neodymium doped crystalline YAG worked in this region but they failed to find a good glass composition for a fiber laser at 1.3. Most glasses worked well in the 1.35 to 1.42 micron region but no one was interested. Some glasses - for example germania doped silica, worked out to about 1.47 microns but - who cared.
• a neodymium doped germania doped silica fiber laser operating at around 1.45 to 1.47 microns is used to pump an Erbium doped fiber amplifier.
• Figure 23 shows the circuitry.
• the neodymium-doped fiber is side pumped. This is well known to the art.
• the big advantage of neodymium is that side pumping converts a lot of power in a large area from a laser diode pump (at around 0.8 microns) into a large amount of power in a small area single mode fiber.
• Transferring the neodymium output to the Erbium is very similar to the present problem of getting laser diode light in.
• the Neodymium laser we have the advantage that the whole circuit can be part of the laser cavity.
• the 2 arm interferometers transfers the neodymium laser beam to the Erbium doped fiber and removes it from that fiber and returns it to the neodymium laser. This has the advantage that the Erbium is pumped from both ends and that 1.46 power not absorbed is returned and not discarded.
• the filters Fi and F 2 are tuned to around 1.45 to 1.47 depending on which pump wavelength is to be used. These filters prevent the Neodymium from lasing at 1.06 microns or 1.40 where it has a much higher gain.
• the choice of exact pump wavelength depends on the application.
• the Erbium gain is higher at 1.47 than at 1.45 but the photon statistics of amplification are worse. Although the gain per unit power is low in the 1.45 to 1.47 micron region, the conversion of pump power to signal power is not much worse, so when handling large bandwidth, high power signal loads there is not much power penalty for pumping at the shorter wavelength.
• An advantage of the side pumped neodymium is that several independent laser diodes can be used and so if one dies the pump doesn't quit.
• a preferred embodiment of this invention for use in the United States at the present time would use 0.6 x 10 bits per second in each channel. Some channels would be delivered to the long line at OC48 (2.4 x 10 9 ). These would be demultiplexed in time to 4 optical frequency channels at OC12 (0.6 x 10 9 ). Odd number channels would be put on the long line in plane polarization on the X axis. Even numbered in y polarization, the channels would be spaced about 1.2 x 10 9 Hertz on center, so the center to center between two x channels would be 2.4 x 10 9 Hertz. In this embodiment, only 1000 channels are used adding up to a total width of 1.2 x 10 Hertz. This narrow total bandwidth is used to make gain flattening easier.
• the fiber should be dispersion unshifted. At 1.5 microns the dispersion limits the inter action length to about: Vi x lO 22 f ⁇ f 2
• the interaction length would be about 2000 kms.
• the "4 wave mixing" gain is about 1 dB per watt per kilometer at around 10 9 spread. So for the closest degenerate set of 4 the gain would be about:
• Each set of 4 channels delivered from one incoming OC48 can again be multiplexed in time to OC48 if that is necessary for the customer.
• optical frequency channels or sub-channels can carry bit rates of 2 x 10 8 bits per second or less and in some cases channels can carry individual signals not time multiplexed, such as digital TN at 2 x 10 7 bits per second.
• these narrow channels are de-multiplexed primarily by Brillouin amplification.
• the pump is broadened enough to accommodate the signal.
• the pump beam is pulsed with alternate pulses being plane x and plane y polarized. This makes amplification insensitive to signal polarization.
• the station Since the total actual signal power is beyond its control the station adds drone channels. When the total signal load drops below a certain level the station adds non- signal power in the drone channels to keep the total load on the amplifier constant. Obviously this drone traffic should be spectrally broadened enough to prevent Brillouin build up.

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• 2001
  • 2001-09-20 EP EP01971256A patent/EP1410533A1/de not_active Withdrawn
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  • 2001-09-20 WO PCT/US2001/029492 patent/WO2002025840A1/en not_active Application Discontinuation

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