Performance optimizer for transmission systems

Performance of a transmission system is optimized by compensating for both noise and fiber non-linearities in the transmission system. The transmission system between a transmission terminal and a reception terminal has at least two channels. A processor determines an adjustment for equalizing the predetermined characteristic for each channel, and then reduces the adjustment by a predetermined amount. A plurality of controllers, each associated with a transmitter in the transmission terminal, each receives the reduced adjustment for an associated channel and providing the reduced adjustment to an output of an associated transmitter. The determination of the adjustment may be made using measurements of received signals or may be estimated knowing the characteristics of the amplifiers in the system.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS One way of estimating bit error rate (BER), and in turn the quality Q of the system, is to degrade the system performance by moving the decision threshold. Q is impacted by the noise from the optical amplifiers, the electrical noise at the receivers, the nonlinear effects in transmission fibers, and other noise and signal beating terms at the receiver. The fiber nonlinear effects that significantly impact WDM systems are four wave mixing (FWM) and cross-phase modulation (XPM). Since both of these fiber non-linear processes scale as the fourth power of the optical power, Q can be represented as 1 Q = P 0 aP 0 + bP 0 4 ( 1 ) Where a&equals;NhvGFB e , where N is the number of amplifiers of gain G equal to the span loss L and noise figure F, h is Planck's constant, v is the wave number of the light, and B e is the receiver electrical bandwidth; P is the average output power per channel; and b is determined numerically, as set forth, for example, V. L. da Silva, et al. “Capacity Upgrade for Non-zero Dispersion-Shifted Fiber Based Systems”, National Fiber Optics Engineers Conference, September, 1999, hereby incorporated by reference in its entirety for all purposes. For noise limited systems, the “a” term dominates the Q. For fiber non-linear dominated systems, the “b” term dominates the Q. In other words, from equation (1), if the system is fiber non-linearity limited, then Q∝ 1/ P 0 (2) while if the system is OSNR limited, then Q∝{square root}{square root over (P 0 )} (3) From the proportional relationships set forth in equations (2) and (3) it is evident that the compensation for these different system features requires different approaches. A schematic of the optimizer of the present invention is shown in FIG. 1 . A transmitting terminal 10 includes a plurality of N transmitters 12 , a corresponding plurality of controllers 14 , and a multiplexer 16 . Alternatively, the controllers 14 may be integral with the transmitters, e.g., altering the drive current of the transmitter. A receiving terminal 30 includes a demultiplexer 36 and a plurality of N receivers 32 . A transmission system 20 between the transmitting terminal 10 and the receiving terminal 30 includes at least two optical amplifiers 22 and an optical fiber 24 . The optical amplifiers 22 are provided as required between fiber spans. In the particular example shown in FIG. 1 , the transmission system includes five spans of fiber and six optical amplifiers. Illustratively, the optical amplifiers are erbium doped fiber amplifiers (EDFA) or Raman amplifiers. The fibers may be non-zero dispersion shifted fiber (NZ-DSF). A telemetry link 40 is provided between the receivers 32 and controllers 14 associated with each of the transmitters 12 . The controllers 14 are any devices that can be used to selectively increase or decrease the power of the optical signal associated with the transmitter, e.g., a variable optical attenuator (VOA). The telemetry link 40 includes a processor 42 , e.g., a microprocessor, which receives an output from each receiver and supplies each controller 14 with an appropriate control signal to control the power of each channel and an appropriate signal protocol. In accordance with the present invention, this control is realized by balancing the effects of fiber non-linearities and noise as set forth below. Due to the fiber non-linearities, and as can be seen from equations (1)-(3) above, equalizing optical powers or OSNR does not necessarily optimize system performance. In a fiber non-linearity limited system, the OSNR equalization will simply invert the Q-curve with respect to the flat amplifier gain case. This can be seen in FIG. 2 , in which curve 50 is the ideal flat amplifier gain, curve 52 is the true sinusoidal gain ripple of the system, and curve 54 is the OSNR equalized output. This inversion results from the operation of the OSNR equalization, which reduces the launched powers for the strong channels and increases the launched power for the weak channels. The higher power channels are influenced the most by XPM and the lower power channels are influenced the least. The low power channels are more influenced by the ASE noise and electrical noise at the receiver. In the OSNR equalization, the XPM is not taken into account. Since XPM is dependent on the power of the signals, applying the full pre-emphasis of the OSNR equalization, the shape of the O curve will be inverted. By balancing the effects of noise and fiber non-linearities, the optimization of the present invention significantly approximates the ideal flat amplifier gain curve. The Q-curve of the optimization of the present invention is shown as curve 56 in FIG. 2 . In the particular system shown by the curve 52 of FIG. 2 , the optimization of the present invention is achieved by pre-emphasis using half the launched powers obtained from an OSNR equalization pre-emphasis algorithm, such as that noted in the techniques set forth above in the Background. As can be seen by curve 56 in FIG. 2 , the performance of the worst channels, which are limiting the overall system performance, are significantly improved. The performance of the system shown in curve 56 in FIG. 2 is optimized by the use of half the OSNR equalization evidently due to the shape of the ripple that causes the non-linearities of the system, here sinusoidal. While individual channels may display better performance for different ratios, the overall performance of the system, here within 0.2 dB of the ideal flat gain profile, is optimized by applying half the OSNR equalization. Presently, for other ripple shapes, such as a cosinusoidal ripple, half the equalization power is still optimal. Further, different systems with different contributions from noise and fiber non-linearities may require different multipliers. The present solution uses information telemetry to set the appropriate power at the transmitters. Thus, the technique of the present invention can be implemented from the initial operation of the system, with no new equipment, upgrades or adjustments are needed at intermediate points in the system. In other words, details regarding intermediate loss, gain, amplifier types, and other intermediate elements are not needed. The optimization of the present invention currently provides satisfactory performance over eight spans of conventional optical fiber. The span point at which the optimizer will need to be re-implented of course depends upon the performance of the fiber and the requirements on the system. As the system gets longer, e.g. fifteen spans, the implementation of the technique of the present invention can be broken into more than one piece. For example, as shown in FIG. 3 , the calculation for optimizing the performance may be performed at the middle of the span. Here, the elements and performance of the optimization are similar to that of FIG. 1 , with the addition of a wavelength selective switch 44 , e.g., Corning Incorporated's Dynamic Spectral Equalizer, inserted in the middle of the transmission system. This switch 44 provides the ability to operate on each wavelength separately to the processor 42 to perform optimization for the second half of the system. Multiple points of optimization may be also utilized if the ripple is too large to handle in a single optimization. The optimization points may be provided anywhere along the transmission path as desired. While a feedback configuration has been disclosed above, the optimization of the present invention may also be realized using an estimate of the optical signal to noise ratio at the end point, or any other desired point in the system. This estimate may be determined from the input power P in , the gains of the amplifiers, and the noise in the amplifiers. For a plurality N amplifiers, each having a gain G i , a loss figure L i , and a noise figure NF i associated therewith, the OSNR at a give point “b” of interest is given by: 2 OSNR b = P in &it; G 1 &it; L 1 &it; G 2 &it; L 2 &it; …G N &it; L N hvB o [ ( NF 1 &it; G 1 - 1 ) &it; L 1 &it; G 2 &it; L 2 &it; G 3 &it; &it; … &it; &it; L N - 1 &it; G N + ( NF 2 &it; G 2 - 1 ) &it; L 2 &it; G 3 &it; L 3 &it; G 4 &it; &it; … &it; &it; L N - 1 &it; G N + ( NF N &it; G N - 1 ) ] ( 4 ) where h is Planck's constant, v is the wavelength, and Bo is the bandwidth over which the noise is measured. The optical spectrum is typically divided into bins of Bo and is commonly 12.5 GHz. For a simple case, assume: G i L i &equals; 1 (i.e., amplifier gain G I fully compensates proceeding fiber span loss L I ); G 1 &equals;G 2 &equals;G 3 &equals;. . . &equals;G N &equals;G; NF 1 &equals;NF 2 &equals;NF 3 &equals;. . . &equals;NF N &equals;NF; and G i >> 1 Then equation (4) can be rewritten as: 3 OSNR B = P in NhvB o &it; NFG ( 5 ) In Equation (5), hvNFG is the spontaneous noise density of each amplifier. Since the noise figure NF and the gain G characterize the amplifier, by knowing these two values and the number of amplifiers N in the system or portion thereof being optimized, the correction can be estimated analytically. The reduction in power required to equalize the expected OSNR may then be reduced as set forth above, e.g., by multiplying by 0.5, to optimize a fiber non-linearity system. This optimization may be hardwired in the transmitter, or may be altered by a user based on any changes in the system using the processor 42 , which no longer needs to be connected to the receiver, to alter the control of the transmitters. While this technique does not allow dynamic feedback, it no longer requires detected signals and may be easily adjusted. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation.