KR20000057679A - Distributed fiber amplifier for solitons - Google Patents

Abstract

PURPOSE: A distributed optical amplifier is provided to reduce the effects of soliton-soliton interaction by introducing an optimum excursion in the soliton intensity. CONSTITUTION: A distributed optical amplifier for use with soliton transmission, which reduces the effects of soliton-soliton interaction by introducing an optimum excursion in the soliton intensity. An optimum operating regime is found wherein both the pulse intensity oscillations and the soliton-soliton interactions are taken into account.

Description

Fiber dispersion amplifier for soliton {DISTRIBUTED FIBER AMPLIFIER FOR SOLITONS}

The concept of optical dispersion amplifiers was published in February 1991 by the Journal of Optical Technology Vol. 9, No. 2, "Performance of Erbium-doped Distributed Mobile Fiber Dispersion Amplifiers" by Simson et al. 75-B-I, No. 3 is described in Wada et al. "Noise Performance and Loss Compensation Properties of Erbium-doped Light Dispersing Fibers". In each publication, the need to keep soliton energy deviations small was emphasized. Refer to Shimson et al. On page 228, column 2, second paragraph, "Very high bit-rate systems will require a visible transmission line with only a small half amplitude in signal amplification." And, summarizing page 231, "Erbium." "Continuous efforts to produce low density and low loss fibers will improve the performance of these optical dispersion amplifiers." See page 75, column 2, the first paragraph of Wada et al., "Especially for soliton propagation with N = 1, it is important to keep the pulse energy deviation below about 20% to maintain pulse shape along the propagation distance. Therefore, optical signal transmission with little change is required. " Wada et al. Also discuss the benefits of using bidirectional pumping to provide the longest compensation length (see Summary, page 75, first and second paragraphs).

In contrast to these publications, the present invention presents other limitations to soliton signal changes in medium length systems, for example, in the range from 50 km to 500 km in length. In addition, in the above system, the present invention contrasts with the above publication regarding the benefits of bidirectional pumping as opposed to unidirectional pumping. The present invention provides a marginal evaluation of the interaction between the timing jitter of soliton pulses and the solitons that can cause soliton collisions in which two neighboring solitons collapse into one pulse in a medium length system. Caused by.

The present invention relates to an optical fiber dispersion amplifier devised for soliton signal transmission. In particular, the optical dispersion amplifier is most suitable for harmonizing magnetic phase modulation and linear dispersion as well as minimizing solitton interaction effects at very high bit rates at long unreproduced distances.

Soliton is a light pulse that does not change in time or in the spectral wavelength domain when propagated over an ideal optical waveguide fiber. In the case of an ideal waveguide, the waveguide is lossless and has a total dispersion called group velocity dispersion that maintains the soliton's invariance with the soliton's magnetic phase modulation. Nonlinear magnetic phase modulation is dependent on soliton intensity. Thus, solitons are invariant only when the intensity is at a level required to balance magnetic phase modulation and total waveguide dispersion. If the travel distance in the optical waveguide is not large, the soliton can convey information when the level of intensity is different from the level required for invariance.

By introducing optical amplifiers in telecommunications, soliton data transmission has become practical. In the case of local or concentrated optical amplifiers, for example, erbium doped waveguide fibers should be tens of meters or less in length, and the spacing between amplifiers should be less than the soliton period. The soliton period is given by the relationship z ₀ = 9.53 × 10 ^-5 × T ² / λ ² D, where z ₀ is the soliton period, T is the soliton pulse width in ps, and λ is the soliton wavelength in nm And D is the total variance in units of ps / nm-km. As can be seen in the form of this relationship, the soliton period is reduced by reducing the soliton pulse width. Therefore, in a system requiring a high data rate, which means that the solitons are narrow, the spacing between the optical amplifiers becomes short. At 40 Gbps, the optical amplifier spacing must be less than 10 km, so the initial and maintenance costs for the system are enormous.

A possible solution to this spacing problem is provided by an optical dispersion amplifier in which a dopant, for example erbium, is essentially dispersed throughout the waveguide fiber. By connecting the desired pump light energy to the waveguide at appropriately spaced lengths, the waveguide fibers can be made locally so that there is no change in soliton intensity.

In order to satisfy the transmission conditions of this intrinsically invariable soliton using an optical dispersion amplifier,

Pump signal attenuation at the waveguide must be taken into account;

Erbium concentration should be selected high enough along the waveguide fiber to provide lossless transmission and pump light efficiency;

The erbium concentration must be low enough along the waveguide fiber to keep the local gain low to provide a half amplitude of small soliton strength.

1 is a schematic cross-sectional view of an erbium doped waveguide fiber,

2A is a graph of soliton maximum pulse intensity versus one-way pumping distance,

2B is a graph of soliton maximum pulse intensity versus bidirectional pumping distance,

3A is a graph of soliton maximum pulse intensity versus one-way pumping distance,

FIG. 3B is a diagram showing the temporary change of two soliton pulse groups from FIG. 3A at a specific distance of the waveguide. FIG.

4A is a graph of soliton maximum pulse intensity versus one-way pumping distance,

4B is a view showing the change of two soliton pulse groups with respect to time from FIG. 4A after a 90 km movement in the waveguide.

5 is a graph of the power required for fiber transparency to input signal power.

A first feature of the present invention is an erbium-doped light dispersion amplifier that provides soliton pulse power half amplitudes in the range of about +3.0 kW to +5.2 kW to increase the distance of the electronic signal generation period. As described above, the soliton power half-amplitude is measured as a ratio with the reference soliton power, with a reference soliton power of 1 in which the magnetic phase modulation is balanced with the total dispersion (D). The system operating wavelength λ is the zero dispersion wavelength in the so-called irregular region where λ> λ ₀ . The dispersion amplifier fiber has a positive net variance (D). The total variance (D) may have a negative half amplitude at a smaller length compared to the soliton period (z ₀ ) which provides a positive net variance (D). An example of an optical waveguide fiber having a core refractive index profile that produces a positive dispersion (D) is described in US patent application Ser. No. 08 / 559,954. Pump light is coupled to the fiber amplifier to activate the erbium atoms in an energy state that provides signal power amplification. The erbium is limited to the core region of the fiber.

The embodiment of the first feature has a concentration in the range of about 20 ppb to 200 ppb depending on the division of the position where the pump light is connected to the waveguide and is erbium doped substantially uniformly along the waveguide fiber core.

The dispersion amplifier waveguide fibers are doped and pumped to provide light transmission to the fiber length. That is, the soliton power at the output of the optical dispersion amplifier is essentially the same as the soliton power at the output end of the amplifier fiber.

In a preferred embodiment of the present invention, in contrast to the aforementioned publications, the light dispersing vaporizer is pumped in only one direction. The pump light can propagate in the same or opposite direction to the soliton signal.

A second aspect of the invention is a telecommunication system associated with the novel optical dispersion amplifier. In telecommunication systems, the pumping means are spaced apart by waveguide fiber lengths where the dispersion amplifier provides amplification of the soliton signal sufficient to achieve optical transparency.

Preferred embodiments include one-way pumping in the same or different directions as the signal. In other words, the pump light can be propagated countercurrently or in parallel with the signal light in the amplifying fiber.

Depending on the type and nature of pump lasers available, the pumping means may be spaced between about 20 km and 90 km.

Typical soliton communication systems require that the soliton pulse is time-divided at least five times the soliton period. In the present invention, soliton spacing may be less than this conventional amount. Thus, in another embodiment, the time division of the soliton pulse is less than about five times the soliton pulse width. This need for time intervals is sufficient to limit soliton interactions or collapses to provide very high data rates. For example, for a 200 Gbps data rate, a soliton pulse width of 1 ps and a pulse interval not exceeding 5 ps is sufficient. This type of system with pumping means spaced about 50 km apart is believed to have a bit error rate limit of about 10 ⁻² at about 500 km system length.

The novel amplifiers employed in telecommunications provide a long distance between error-prone soliton interactions. In particular, the distance traveled by the 1ps soliton before the collapse or collision has occurred is in the range of at least about 30 km to 50 km, and may be as large as 300 km to 400 km.

At very high data rates transmitted over long unreproduced distances, special methods must be developed to obtain a practical signal-to-noise ratio, and total waveguide dispersion (D) and nonlinear effects must be considered. Using soliton as an information carrier is one of the methods. Soliton signaling is attractive because the two dispersion effects in the formation and retention of soliton pulses cancel each other out. That is, the total wavelength dispersion (D) and the pulse intensity independent nonlinear magnetic phase modulation interact with each other to form and maintain the shape of the soliton.

Since the magnitude of the magnetic phase modulation depends on the pulse intensity, a major concern in maintaining soliton form is the control of soliton intensity as the soliton travels along the waveguide fibers. This interest led to the study of optical dispersion amplifiers for use with soliton transmission. For very high data rate transmissions above about 40 Gbps, establishing waveguide transparency, i.e., ensuring that the input power is equal to the output power at a selected distance, is insufficient to maintain the soliton form. There is a need for a waveguide having local attenuation equal to zero, thereby having optical transparency at essentially all lengths.

In addition, another configuration is provided in which soliton interaction is managed in a high data rate soliton telecommunication system. The soliton traveling in the optical waveguide fiber exerts an attractive or repulsive force on the neighboring soliton, depending on its relative phase. When the distance from neighboring solitons is sufficiently small, mutual attraction can cause timing jitter in the solitons. Since the force between solitons increases exponentially as the distance decreases, at smaller intervals the soliton pairs collapse or collide in one high intensity pulse. Thus, mutual attraction or repulsion is a source of bit error, especially in telecommunication systems with closed pulse intervals such as those encountered in high data rate systems.

What is found in the present invention is that by inducing a predetermined half amplitude of soliton intensity, the susceptibility of solitons perturbed or colliding with each other can be reduced. This intensity half-amplitude can be derived using an optical dispersion amplifier having a selected dopant concentration with a pump source of selected intensity. In particular, the dopant concentration, which is erbium and pump power, expressed as ppb Er- ³ for about 1550 nm operation, is synthesized to maintain optical transparency from end to end of the dispersion amplifier fiber, while at about 3.0 kW to This produces an amplitude half amplitude of 5.2 dB. The reference power may be taken to provide a balance between total variance D (group velocity variance) and magnetic phase modulation. For systems up to about 500 km described here, this concept contrasts with the prior art in which soliton forms as information carriers are best maintained by carefully limiting the intensity half amplitude to keep the magnetic phase modulation constant.

The cross section of the optical dispersion amplifier waveguide fiber is shown in FIG. 1. The core region 4 comprises erbium or other dopant, represented by the region 6, inside the core. Region 6 is typically smaller in diameter than core 4 to increase the cross section of impact between the signal light and the dopant atoms. The coating layer 2 has a lower refractive index than at least a portion of the core region 4 for guiding light along the fiber.

A model simulation of the propagation soliton undergoing an intensity half amplitude is shown in FIG. 2A. Each curve represents the soliton maximum intensity versus travel in the dispersion amplifier waveguide. Nine solitons with 1ps width and 5ps spacing, corresponding to a 200Gbps rate, were modeled as they propagate in the waveguide. Group velocity dispersion (D) in the fiber was set to 2 ps / nm-km. The pump spacing is 20 km and the pumping is in one direction. The waveguide is optically transparent at the distance between the pumping means. The soliton traveling in a lossless waveguide without power half amplitude is represented by the curve (8). A large intensity spike at 10 km is a collision between solitons, indicating the effective operating length of the system. When a small power half amplitude is induced, as shown by curve 9, the soliton collision occurs farther along the waveguide. Subsequent power half amplitudes, represented by curve 12, do not cause soliton collisions at travel distances greater than 30 km. The noticeable periodic ripple in curve 10 is the soliton width variation due to the power half amplitude. At the highest power half amplitude of curve 10, for example, referring to intensity spike 16, the soliton begins to lose shape after traveling about 25 km. When the power half amplitude is lower than the curve 12, soliton collision occurs at about 30 km. Thus, the optimum power half amplitude is between curve 12 and curve 10, and ranges from about 3 Hz to 5.2 Hz.

As a comparative example, FIG. 2B is a graph of soliton pulses propagating in the waveguide under the same conditions as described in FIG. 2A except that pumping is bidirectional. As before, the lossless case indicated by curve 18 represents a soliton collision after about 10 km travel in the waveguide. However, the highest power half amplitude curve illustrated by curve 22 in this case does not exhibit an increased soliton configuration over a distance of 10 km. The intensity curve shows a large amplitude after traveling about 19 km. Similarly, soliton breakdown at the lowest power half amplitude was demonstrated by the intensity spike 20 after about 18 km. Comparing FIGS. 2A and 2B, it has been evident that one-way pumping provides a soliton configuration at longer distances than two-way pumping.

This result is counterintuitive because bidirectional pumping allows better control of the gain in distributed waveguide amplifier lengths. In the case of one-way pumping, the description of improved performance is as follows. The high gain near the start end of the waveguide is adiabatic, ie, slowly suppresses the soliton pulse. That is, the magnetic phase modulation, which depends on the intensity, is greater near the start end of the waveguide. The magnetic phase modulating portion is not canceled by linear (group velocity) variance D, which narrows the time width of the soliton pulse. As a result, the soliton is spaced farther apart at the start end of the distributed waveguide fiber amplifier and does not interact strongly. No collisions occur due to wide pulse intervals in time, and the timing jitter induced by the soliton interactions is smaller. Although this description is believed to be correct, the invention is not limited by this description and does not depend on its accuracy.

The second simulation is shown in FIG. 4A. The coefficients used in the model are the same as in the previous example, except that the total variance (D) is 0.67 ps / nm-km. In the case of lossless, soliton impingement occurs at about 30 km, as indicated by the intensity spike 38. The lowest power half amplitude of the soliton is represented by the curve 42. The soliton maximum intensity represents the predicted amplitude at the same distance as before. Soliton intensity is maintained up to about 80 km travel in the waveguide. The intensity spikes indicated at 85 km to 100 km are strong interactions between solitons. Intensity spike 46 in curve 42 is a collision between solitons.

Curve 40 represents the maximum pulse intensity for the high power half amplitude distance of the soliton. The amplitude due to the soliton intensity change is much larger than the curve 42 as expected. However, collisions between solitons do not occur at 100 km waveguide travel. The best balance of soliton amplitude due to increased collision free movement occurs in the power half amplitude between curves 40 and 42. This observation is most clearly seen in Fig. 4b, where the intensity of the soliton pulses with respect to time after a travel distance of 90 km in the waveguide is shown. At a power half amplitude of about 5.2 Hz, curve 48 represents the first nine solitones that maintain their time intervals with the amplitude expected to produce an appropriate signal-to-noise ratio.

Although the amplitude is small in curve 50 because the power half amplitude is only 3 dB, the strong interaction between solitons, as seen at time position 52, will change the time interval drastically. Thus, higher power half amplitudes provide error-free soliton signal transmission at longer waveguide lengths. This result is different from the prior art mentioned above.

The third embodiment shows the interaction of the soliton power half amplitude with the pump interval and the soliton pulse width. In FIG. 3A, the soliton pulse width is 2 ps and the pump interval is 50 km. The total dispersion (D) of the waveguide was set to 0.67 ps / nm-km. The intensity spike 28 shows a lossless case where collisions between solitons appear at about 120 km. The maximum pulse intensity curve for the soliton travels 26 and 24 represents a large amplitude for the case of the high power half amplitude 24. The edge shape of curve 24 begins to distort the soliton's time interval at about 180 km travel. The clear shape in the low power half amplitude curve 26 begins to induce data errors after the travel distance exceeds 250 km. Thus, for wide soliton pulses and wide pump intervals, low power half amplitudes provide long error-free transmission.

The time evolution of the soliton pulse at a selected distance along the waveguide is shown in FIG. 3B. Initial pulse intervals for low and high half amplitudes are shown as a series of soliton pulses 33 and 36. At 200 km, a timing error occurs in the pulse train 34 of 5.2 kHz power half amplitude. In contrast, in the case of the 3 kHz power half amplitude, as shown by the pulse trains 35 and 31, no error occurs until the travel distance exceeds 400 km.

The power per kilometer needed to reach waveguide transparency for the input signal power is shown in FIG. 5. Here curve 54 relates to the 49km waveguide length, curve 56 relates to 55km, curve 58 to 50km and curve 60 to 154km. The graph shows that the pump power required for the normal soliton system operating area can be obtained even when the soliton input power whose curve is steeply sloped is usually above 0 dBm. For example, a 154 km link operating at an input signal power of about 5 kW and having an Er- ³ concentration in the range of 20 to 200 ppb requires a total pump power of about 60 kW, which is possible with the present invention.

Thus, the present invention provides an optical dispersion amplifier for use with soliton transmission, which induces an optimal half amplitude in soliton intensity, thereby reducing the effect of soliton interaction. The optimum mode of operation has been found to control both the pulse intensity amplitude and the interaction between solitones.

While specific embodiments of the invention have been shown and described, the invention is limited only by the following claims.

Claims (12)

An optical waveguide fiber in a ground state and containing erbium dispersed along the length of the waveguide fiber and having a core area in contact with the surrounding coating layer, positive net dispersion (D), and a predetermined length; Optical pumping means for providing pump light to activate erbium in an energy state above the ground state; And And coupling means for injecting pump light and soliton pulses into the waveguide fiber, Magnetic phase modulation generates a half amplitude at the maximum power of each soliton in the range of about +3.0 kW to +5.2 kW for a reference soliton power with an ordinal number of 1 in a standardized unit that balances group velocity variance (D). Erbium dope optical waveguide fiber dispersion amplifier made with. The erbium-doped optical waveguide fiber dispersion amplifier according to claim 1, wherein the erbium is distributed substantially uniformly along the optical waveguide fiber, and wherein the Er ⁻³ concentration is in the range of about 20 ppb to 200 ppb. 3. The erbium dope optical waveguide fiber dispersion amplifier according to claim 2, wherein the pump light has a power level selected to provide optical transparency to the waveguide fiber length. The erbium-doped optical waveguide fiber dispersion amplifier according to claim 1, wherein the pump light propagates in one direction in the waveguide fiber. At least one erbium-doped optical waveguide fiber having a core length, positive net dispersion (D), and a predetermined length, including a core region in the bottom energy state and containing erbium dispersed along the length of the waveguide and in contact with the surrounding coating layer; ; Optical pumping means connected to said erbium-doped waveguide and having a preselected input power to activate erbium in an energy state above the ground state; And And a means for generating a plurality of solitones that are propagated in the erbium-doped waveguide, The half amplitude at the maximum power of each soliton is characterized in that it ranges from about +3.0 kW to +5.2 kW for the reference soliton power with an ordinal number of 1 in a standardized unit where the magnetic phase modulation is balanced with the group velocity variance (D). Optical waveguide telecommunication systems. 6. The optical waveguide telecommunication of claim 5, wherein the optical pumping means are spaced along the waveguide fiber length and the preselected power of the optical pumping means provides optical transparency of the waveguide length between neighboring pumping means. system. 7. An optical waveguide telecommunications system according to claim 6, wherein the light from the pumping means propagates in one direction in the waveguide fibers. 8. An optical waveguide telecommunications system according to claim 7, wherein an interval between said optical pumping means is in a range of about 20 km to 90 km. 6. The optical waveguide telecommunications system of claim 5, wherein the soliton pulses have a time interval that is about five times or less the soliton time width. 10. The optical waveguide telecommunications system of claim 9, wherein the soliton time width is about 1 ps. 11. The optical waveguide telecommunications system of claim 10, wherein the bit error rate is about 10 ^-9 or less for a pumping means spacing of 90 km. 10. The optical waveguide telecommunications system of claim 9, wherein the system length traveled by the soliton is about 30 km to 500 km before the soliton interaction results in a transmission air rate of about 10 ^-9 or less.