WO2002025840A1

WO2002025840A1 - Long line fiber optic communication system without repeaters or with widely spaced repeaters and with inexpensive amplifiers

Info

Publication number: WO2002025840A1
Application number: PCT/US2001/029492
Authority: WO
Inventors: John W. Hicks
Original assignee: Hicks John W
Priority date: 2000-09-21
Filing date: 2001-09-20
Publication date: 2002-03-28
Also published as: AU2001291161A1; EP1410533A1

Abstract

A long distance fiber optic transmission system for carrying signals over 2000 kilometers without repeaters or dispersion compensation between a head end (10) and a receiving station (20), with total signal carrying capacity of over 500 billion bits per second and 500 billion Hertz. A bank of unmodulated spectral lines, to be modulated spaced about 5 gigahertz on center are generated and transmitted to a head end (10). The bit rate of incoming signals to be transmitted is detected and if the bit rate is too high it is demultiplexed in time (12). The signals are demultiplexed in time until the bit rate of each signal low enough so that it can go to its receive station without needing repeaters.

Description

Title

Long line fiber optic communication system without repeaters or with widely spaced repeaters and with inexpensive amplifiers

Field of the Invention The field of this invention is long fiber optic transmission lines with very high bandwidth, long span between repeaters, and low cost.

Description of relevant art

Long line fiber optic transmission lines are known to the art. Some of theses carry 100 optical frequency channels each carrying about 2.5 x 10⁹ bits per second (known as OC-48). There is talk of introducing more than 100 channels.

In the common commercial system these OC48 channels are created by using laser diodes operating at around 1.55 micron wavelength and carefully tuned to be about 3 x 10¹⁰ Hertz between channels if there are 100 channels or about 10¹¹ 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.

After about 300 km the shape of the signal modulation is degraded to the point where the bit error rate of detection is about to be a problem. So a "repeater" is put in. 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.

Objects of this Invention

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.

Brief Summary of the Invention

To accomplish these objectives the invention removes the repeaters entirely or at least increases the distance between repeaters. At the initial transmitting station 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. At the transmission station 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. At the receive side of the repeater or at the receiving station the signals are demultiplexed and detected but with a filtering system that allows much closer channel spacing than in use now.

In one embodiment, 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

In another embodiment, an optical frequency channel contains an unmodulated heterodyne line in addition to the signal.

In still another embodiment, the broad channel carries several signals in sub channels as well as the heterodyne line. In most embodiments, if the long line is more than 300 km long the modulation rate of each signal is lowered to the point where no repeaters are needed for that line length. When the bit rate per channel is decreased then more channels are needed to carry the high total bit rate. This means closer channel spacing.

In the method of this invention, optical filters are provided which can separate close channel spacing.

In still another embodiment, heterodyne detection is used as a part of the filter system. When no heterodyne lines are provided with the signal channels in transmission then in some embodiments heterodyne lines are provided at the receive end.

In still another embodiment, it is advantageous or necessary to separate channels by polarization or to at least "rectify" the polarization of a given channel. When heterodyne lines are provided at the receive station and not transmitted with its signals then it is almost necessary to rectify the polarization of these signals.

In a preferred embodiment, for amplification, 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.

Because in most routes in the United States there are idle, unused fibers and because in this invention the repeaters are removed or reduced and the cost of amplifiers is drastically reduced, then the economic decision as to whether to increase the total traffic of a fiber in use or to simply light up another fiber - may favor the latter - until the dark fibers are used up.

Brief Description of the Drawings

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.

Detailed Description of the Preferred Embodiment

Several novel components are variously used in the practice of this invention, specifically, polarization rectifier; polarization separator; wavelength shifter; bank for generating unmodulated source lines; multi-pass Planar Fabry Perot; heterodyne detector; double-pass 2 arm interferometer and Erbium amplifier pumped by a

Neodymium laser.

In the preferred embodiment of this invention used in cases where several un- used fibers are available no attempt is made to carry the maximum possible amount of traffic per fiber but instead the intent is to carry traffic at the lowest possible system cost per bit.

In this embodiment 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. For a distance of 5000 kilometers that bit rate is about 6 x 10⁸ 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. One

OC12 signal is modulated onto each of these source lines. They are spaced around 2.4 x 10⁹ hertz.

In the best embodiment 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.

At the receive end 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.

In this preferred embodiment where unlit fiber is available, in order to min -nize cost per bit, the full spectrum of Erbium amplifiers is not used. Typically only about 1.0 x 10¹² to 2 x 10¹² Hertz is used to minimize the cost of gain flattening the Erbium amplifiers.

In an alternate preferred embodiment 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. In the previous embodiment the signals were preferably around 6 x 10⁸ bits per second. In this embodiment the group width can be up to around 5 x 10⁹ bits per second - but no individual signal is above 6 x 10⁸. 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. At the receive end the groups are separated by 2 arm fiber interferometers as before. In addition the channels are preferably separated by polarization. In addition Brillouin amplification or de- amplification of heterodyne lines may be used. Finally, individual signals are demultiplexed in the electronic realm by beat frequency.

In this embodiment where individual signals are multiplexed in optical frequency but not in time, it is possible to carry analog signals as well as digital.

Long Lines

In a preferred embodiment of this invention, a bank of unmodulated spectral lines, to be modulated spaced about 5 x 10⁹ Hertz on center are generated and transmitted to a Head End 10.

The 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.

That is, 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.

When individual bit rates are lower than the broad optical frequency channel bandwidth then two or more signals are placed in sub-channels within the broad channel. For multiplexing at 14, there are 60 RF frequencies in electronic form available each signal is modulated onto one of these RF frequencies electronically and the 60 modulated RF frequencies drive an optical modulator (not shown). This produces upper and lower side bands for each sub-channel, see Fig. 3. 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.

This has the advantage that the top is flatter than that achieved with Fabry-Perots. 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.

Amplification

Referring to Fig. 6, 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. To drop traffic at X, a "non-filter" tap 36 hereinafter referred to as a

"flat tap" removes about 10% of the power of all channels. This is done right after an amplifier -preferably. The amplifier has enough gain so that the power is proper after the tap. If needed the tapped off power passes through another amplifier 34 powered by the same pump as shown in Fig. 6.

Note that the broad channel A which is to be used at station X is not removed from the long line fiber. So a new signal cannot be introduced onto this optical frequency channel. In this invention bandwidth is so inexpensive and plentiful that it isn't worth the trouble to clean out and re-use the channel.

In the preferred embodiment 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.

This is repeated where signals are to be added until the far end receive station is reached.

To separate and receive a desired station de-multiplexing channel, 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.

However, 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.

After separating the x and y polarization the y is rotated to become x and the two are combined with a 50% coupler. By adjusting the phase before combining one can get 50% of the power in each leg A and B.

So a sample is taken from each leg and detected and compared. The result feeds back and adjusts the phase to split the power equally.

So the light leaving A will be plane polarized and half the original power. So now if the heterodyne line is also plane polarized and the same polarization as the signal then there will be a constant reliable beat between the two.

There is a theorem that if two signals start out in the same polarization state they will after transmission in a fiber end up in the same state. If they start in orthogonal states they will end in orthogonal states. There are caveats. The signals should be not too far apart in optical frequency. And nowhere should polarization states be separated into 2 paths.

This theorem may seem implausible but if it were not true then a signal consisting of a sine wave modulation of 10¹⁰ Hertz would be lost in transmission unless the upper and lower side bands remained in the same polarization state. Another stage is added to the one stage polarization rectifier, as shown in Fig. 10.

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.

At the receive end 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 10¹⁰ Hertz but the spectrum may need to be filtered so that only a few adjacent channels are processed with one polarization rectifier.

These signals can be demultiplexed at the receive station with a multipass plane mirror Fabry Perot with only 2 passes. In this less dense traffic case heterodyne lines can be carried with the signals, see Fig. 12.

But in this case it is difficult to filter out B and C well enough to get a clean signal. So in this embodiment 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.

In the preferred embodiment, the adjacent broad channels are orthogonally polarized. In this embodiment, 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.

In another embodiment, 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

Brillouin pump to de-amplify the heterodyne line.

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.

In some systems to carry narrow signal channels but not use heterodyne detection - for example, when an individual signal is to be transferred to another fiber without detecting and re-modulating a Brillouin pump is used to amplify that signal together with some coarse filtering, see Fig. 14. To carry this out a very large bank of Brillouin pump beams is needed. So we use a less dense bank is used and the nearest one is frequency shift, see Fig. 15.

To shift a line, the see Fig. 16 one modulator is driven by coswt and the other by sinwt. The phase is adjusted to give: sinwt cosft + coswt sinft out of arm A. which is: sin (f+w) t. So by adjusting w the bank line is shifted by up to 10¹⁰ Hertz. This shifter can also be applied to the bank lines used for source lines for signal transmission rather than creating 2 side bands by modulation and filtering out one of them.

So in the preferred embodiment 2-arm interferometers are used to separate channels, Fig. 17, 18.

Every fourth channel exits A and every fourth alternate exits B pretty unscathed. Channels 1,3,5 - are botched up so more splitting is required, Fig. 19. After the first split, splitting continues until there is one channel on each leg. But that channel is not at all well filtered. But filtering is not really the purpose. The purpose is to deliver one channel to each Fabry - Perot to be filtered without wasting a lot of signal power or concatenating several complex filters in series. Granted there will be about 9 two - arm interferometers in series but these are all-fiber, not fiber to macro- optics (lenses, mirrors, etc.) and back.

Thus, 2-arm interferometer filtering is used to separate channels so they can be properly filtered.

In a very similar way the channels are multiplexed at the transmit station without precise filtering. Modulation Shape

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.

As for "dispersion" limitations it is necessary to correct the prevailing notion about the affect of dispersion on signal shape. It seems to be the common belief that it is dispersion which causes square bits to lose their shape. The fact is, it is the slope of the fiber dispersion curve that causes the trouble - not the dispersion itself. Therefore there is no advantage in using dispersion-shifted fiber. There is a strong disadvantage - namely cross talk - when operating at near zero dispersion. So in the preferred embodiment of this invention "unshifted" fiber is used, which is, fortunately the most abundant fiber in the existing systems.

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.

Having smoothed out the modulation shape, the detection process becomes a little more prone to error.

However, the electronics in the detector (not shown) 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.

Preferably, each signal is put on a different wavelength. One 3 x 10⁷ 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 10⁷ Hertz bandwidth are used which is about 3 x 10⁷ bits per second for filtered digital for ordinary video.

To demultiplex the channels, a Brillouin pump whose optical frequency is proper to amplify that signal is used, see Fig. 21. About 10⁶ 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.

The channels are laid out like this, as shown in Fig. 22 with x meaning one polarization and y being orthogonal to x. Xi has heterodyne beats ranging from Δ to 2 Δ . There are no inter signal beats in this range. So the only conflict is with other heterodyne beats. y₀ to yi, y₂ 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.

To get an even cleaner signal the filtered product can be divided into 2 streams and the XI line is then Brillouin de-amplified and then the detected signals are subtracted in electronic form. 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. There was a large effort awhile back to develop such a laser for use at around 1.30 microns signal wavelength where fiber transmission has a "local" minimum and where the dispersion is zero. 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.

In this preferred embodiment 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.

With 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 F₂, 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 10⁹). These would be demultiplexed in time to 4 optical frequency channels at OC12 (0.6 x 10⁹). 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 10⁹ Hertz on center, so the center to center between two x channels would be 2.4 x 10⁹ 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 photon count per bit should be around 100 to 300 just before a lOdB, amplifier, times 2 times the number of amplifiers in the lines. Across the U.S. there would be about 100 amplifiers - so 20,000 photons per bit just before and 200,000 just after amplification. With about 5 x 10¹⁸ photons per watt sec this is: 4 x 10^"14 watts/bit x 0.6x 10⁸ bits = 2.4 x 10^"6 watts per OC12 channel. This falls to 2.4 x 10^"7 watt just before amplification.

Consider "4 wave mixing".

The x channels don't mix with y channels the x channels are 2.4 x 10⁹ Hertz apart The fiber should be dispersion unshifted. At 1.5 microns the dispersion limits the inter action length to about: Vi x lO²² fιf₂

Or

For the next combination the interaction length would be about 2000 kms. The next pair shorter, etc.. The "4 wave mixing" gain is about 1 dB per watt per kilometer at around 10⁹ spread. So for the closest degenerate set of 4 the gain would be about:

10^"6 x ldB/watt x 4000 kilometer Or

4 x lO^"3 dB. So all "4 wave mixing" should come to less than 1 % which is tolerable.

This all assumes the 100 bit case. When Erbium ground state is poorly depleted the 300 bit case applies. In one preferred embodiment no heterodyne lines are used. At the receive end the spectrum is divided into 8 broad regions, see Fig. 24. For region 1 is used a 2-leg interferometer removing 2,4,6,8 followed by another removing 3,7. Followed by another removing 5. These are all coarse interferometers easy to tune and stabilize. Then for each region the two polarizations are separated.

Again with 2-leg interferometers this is repeated, first go through either 2 interferometers or make; a double pass through one and then each leg is taken and double passed again. Then pass through 5 more single pass filters and at each branch we have one well-filtered channel.

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.

In the United States some of the optical frequency channels or sub-channels can carry bit rates of 2 x 10⁸ bits per second or less and in some cases channels can carry individual signals not time multiplexed, such as digital TN at 2 x 10⁷ bits per second.

In this embodiment these narrow channels are de-multiplexed primarily by Brillouin amplification. When the signal bandwidth is too great for a narrow Brillouin pump then the pump is broadened enough to accommodate the signal. When the signal is not rectified to plain polarization state then the pump beam is pulsed with alternate pulses being plane x and plane y polarized. This makes amplification insensitive to signal polarization.

In a preferred embodiment of this invention the problem of amplifier saturation and the need for monitoring gain and adjusting pump power is overcome by a system which maintains relatively constant total signal power.

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.

This does not mean the power is constant along the length of a long line but only that the power at any given amplifier remains reasonably constant.

The foregoing description has been limited to a specific embodiment of the invention. It will be apparent, however, that variations and modifications can be made to the invention, with the attainment of some or all of the advantages of the invention.

Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.

Having described my invention what I now claim is:

Claims

1 1. A long line fiber optic system for carrying signals over 2000 kilometers

2 without repeaters or dispersion compensation between a first transmission end and a last

3 reception end, with total signal carrying capacity of over 500 billion bits per second and

4 500 billion Hertz which comprises:

5 means for introducing at least one signal line into the transmission end;

6 means to detect the bit rate of the introduced signal;

7 means for modulating the signal onto a carrier frequency;

8 means for multiplexing the signal to form a plurality of optical frequency

9 channels carrying optical signals; 0 means to amplify the signals; 1 a partial lateral tap between the transmission end and the receiving end for a 2 signal drop or addition; and 3 means for selecting a desired signal.

1 2. The system of claim 1 wherein the means to amplify comprises an Erbium

2 doped amplifier pumped by a Neodymium doped fiber laser operating in the 1.45 to 1.48

3 micron region.

1 3. The system of claim 2 which comprises:

2 filters tuned to between around 1.45 to 1.47 to prevent the Neodymium from

3 lasing at 1.06 microns or 1.40 microns.

1 4. The system of claim 1 wherein the means for introducing a signal

2 comprises:

3 unmodulated spectral lines provided from a spectral line bank.

1 5. The system of claim 1 wherein the means for introducing comprises:

2 an unmodulated spectral line bank wherein several spectral lines are generated

3 from one laser source.

1 6. The system of claim 1 wherein the means to multiplex comprises:

2 a wavelength shifter.

1 7. The system of claim 6 wherein the wavelength shifter comprises:

2 two parallel modulators one driven by sin 2πft and the other by cos 2πft operating on the same source line with their outputs combined and their phases adjusted to send one side band out of one exit branch with optical frequency f₀ + f or f₀ -f where f₀ is the optical frequency of the source line.

8. The system of claim 6 wherein the wavelength shifter comprises: an input unmodulated line spliced into a fiber ring with a wavelength shifter in the ring and an optical amplifier in the ring to maintain power level, with one more spectral line generated by each cycle around the ring and with a fraction of the power of all the shifted lines tapped off.

9. The system of claim 1 which comprises: means for adding a heterodyne line to each optical frequency channel at either the transmit end or at the receive end with optical frequency spacing such that the beat frequency between the heterodyne line and its associated signal channel is different from any beats between signals themselves.

10. The system of claim 9 wherein the means for selecting comprises: a demultiplexing system where optical filters for coarse filtering and wherein heterodyne lines are present, said lines are filtered by either Brillouin amplification of the desired heterodyne line or Brillouin de-amplification of the neighboring line or both.

11. The system of claim 1 wherein the means for selecting comprises: a filter to select a channel; means for splitting the power of the signal in the channel equally onto two legs; means for adding a heterodyne line to one leg; and means for detecting the power on both legs and subtracting the two electrical outputs.

12. The system of claim 11 wherein the heterodyne lines accompanying the channels comprises: means for subtracting the selected heterodyne leg from one branch after splitting.

13. The system of claim 1 which comprises: means for demultiplexing in time the bit rate of the signals introduced into the transmit end such that deterioration of modulation shape over the distances that the signal travels on the fiber is obviated.

14. The system of claim 1 wherein the multiplexed optical frequency channels comprise at least first and second sets, one set of optical frequency channels is introduced onto a fiber with one polarization state and another set is introduced with a polarization state orthogonal to the first state.

15. The system of claim 14 which comprises: means for separating the two polarization sets at the receive end.

16. The system of claim 1 which comprises: means for receiving a signal in any single polarization state which may vary in time and converting that state into a constant plane polarization state said device comprising a first path split with one plane polarization on one branch and the other plane polarization on the other branch and with either branch having a dynamic path length adjuster and with one branch rotated 90 so the two branches have the same plane polarization followed by a 50 % flat tap followed by two branches with detection on each branch and electrical feed back to the path length adjuster to adjust the path length so that both branches carry the same amount of signal power and both branches carry plane polarizated light.

17. The system of claim 1 which comprises: means for receiving a signal on an optical fiber in a single but varying polarization state and converting a constant fraction of its power in an output fiber into a plane polarization signal of constant polarization.

18. The system of claim 1 which comprises: means for receiving signals on a fiber with a first set of signals in one polarization state and a second set of signals orthogonal to the first set; and means for separating the two sets onto two output branches, said devices comprising: means for receiving a signal in any single polarization state which may vary in time and converting that state into a constant plane polarization state said device comprising a first path split with one plane polarization on one branch and the other plane polarization on the other branch and with either branch having a dynamic path length adjuster and with one branch rotated 90° so the two branches have the same plane polarization followed by a 50 % flat tap followed by two branches with detection on each branch and electrical feed back to the path length adjuster to adjust the path length so that both branches carry the same amount of signal power and both branches carry plane polarizated light followed by an optical path length adjustment on one of the two final branches.

19. The system of claim 1 wherein the optical frequency channels are demultiplexed with means for detecting beat frequencies between said channels and an unmodulated heterodyne line.

20. The system of claim 1 wherein the signals which are multiplexed in optical frequency but not multiplexed in time.

21. The system of claim 1 wherein the means to amplify comprises : a multi line generator comprising a fiber ring containing an optical amplifier with a narrow laser output line fed into the ring said ring then producing muiti-Brillouin off- spring with a fraction of the power of all off-spring tapped off the ring.

22. The system of claim 21 further comprising: a filter with progressively greater transmission at lower, optical frequencies inserted in the ring to increase the number of Brillouin offspring.

23. The system of claim 1 wherein the means to amplify comprises a Brillouin amplifier with very high differential gain between the spectral region to be amplified and the remainder of the spectrum but with less net gain than differential gain of the selected spectral region.

24. The system of claim 23 which comprises: means to widen the Brillouin pump beam to broaden the Brillouin gain.

25. The system of claim 1 wherein the means to amplify comprises: an erbium doped fiber laser used to pump an erbium doped fiber amplifier.

26. The system of claim 1 wherein the signals are amplified in the L band.

27. The system of claim 1 wherein a long line fiber carries signals with a photon count per bit representing the number "one" of between 200 and 600 times the number of amplifiers in the long line, between repeaters or terminals this count being taken at the low point in power just before amplification.

28. The system of claim 1 where if a time multiplexed signal reaching the transmission end is at a bit rate too high to transmit over the necessary distance without repeaters comprising: means to demultiplex in time to a lower bit rate and transmitted over more than one optical frequency channel.

29. The system of claim 1 which comprises: means to demultiplex two or more channels in optical frequency and combines and multiplexes the channels in time at a higher bit rate at the receive end than the bit rate transmitted.

30. The system of claim 1 wherein the total signal power entering any means to amplify is held relatively constant for means of adding drone channels at transmit stations with drone power increasing as total signal power decreases.