Optical node including three-stage optical amplifier with automatic gain and level control

The present invention relates to the field of optical networks, and particularly to constructing an optical node in ultra long haul backbone networks.

Next generation of backbone networks may interconnect optical nodes by using dense wavelength division multiplexer (DWDM), ultra long haul regeneration free transmission, optical cross connect (OXC), or optical add drop multiplexer (OADM) technologies. The drive is less for the sheer capacity boost but more for the fundamental shift in replacing the voice-centric, hard-to-scale, and slow-to-provision digital network with a data-centric, scalable, and easy-to-provision optical network.

An optical node typically performs the following basic functions. The first function is optical 2R (regenerate and reshape), including high-power and low-noise optical amplifications and dispersion management. The second function is DWDM maintenances, including gain equalization and dispersion slope control. The third function is 100% wavelength processing, including 100% wavelength manipulation such as OADM or OXC. OXC must have all optical bypassing to ensure the full transparency of the optical layer. The fourth function is wavelength monitoring, which comprises monitoring of channel wavelength, optical signal-to-noise ratio (OSNR), and all other optical intelligence. Power consumption and channel density are also significant [figure-of-merit] figures-of-merit. A shortcoming in a conventional optical node is the lack of capabilities to deal with high-power and low-noise optical amplifications, dispersion slope control, gain equalization, OADM, and OXC all the same time.

Accordingly, it is desirable to have an optical node that has the functional power, features, and performance for operation in ultra long haul networks.

The invention discloses an optical node for operation in an ultra long haul backbone network that provides DWDM optical transmission and wavelength networking functionalities. The optical node is designed with capabilities for amplification, dispersion compensation, and add/drop functionalities. In one embodiment, three erbium-doped fiber amplifiers (EDFA) are cascaded using low nonlinearity and low loss dispersion compensating module (DCM).

Advantageously, the optical node in the present invention produces a more efficient power consumption and channel density. The present invention also advantageously does not rely on, but is complementary to, other ultra long haul technologies, e.g. Raman amplification, EDFA band splitting, and return-to-zero (RZ) coding.

Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

FIG. 1 is an architectural diagram illustrating an optical node configuration for ultra long haul backbone networks in accordance with the present invention.

FIG. 2 is an architectural diagram illustrating a recirculating loop for verifying the integrated long haul processor in accordance with the present invention.

FIG. 3 is a flow diagram illustrating the process for operating an optical node in an ultra long haul network in accordance with the present invention.

FIG. 4A is a graphical diagram illustrating a measurement of the optical signal-to-noise ratio for six channels at 3000 km in accordance with the present invention; FIG. 4B is a graphical diagram illustrating an eye diagram of six channels at 3000 km in accordance with the present invention.

FIG. 1 is an architectural diagram illustrating an optical node configuration 10 for ultra long haul backbone networks. The optical node 10 includes three stages of optical amplifier gain block: a G 1 amplifier 12 , a G 2 amplifier 15 , and a G 3 amplifier 17 . Apart from the optical amplification blocks there are control loops that are required to construct an optical node. Automatic gain control/automatic level control AGC/ALC blocks are control loops to ensure that a node is operating properly. Each G 1 and G 2 has its own ALC. Three loops are defined in the optical node 10 . First, automatic gain control/automatic level control (AGC/ALC) 19 a provision takes the input and the output levels and uses the error to control the total gain and output level of the whole optical node by adjusting the third stage G 3 17 via pump control input 19 c . The other two loops are local loops. ALC 13 controls the output level of the first stage G 1 12 via an input 13 a . In one embodiment of the invention, the first stage G 1 12 is an optical fiber amplifier OFA) such as an erbium-doped fiber amplifier (EDFA), and input 13 a is a pump control input used to control the power output of the laser pump in the OFA. ALC 16 controls the output level of the second stage G 2 15 via a pump control input 16 a . In one embodiment of the invention, the second stage G 2 15 is an optical fiber amplifier (OFA) such as an erbium-doped fiber amplifier (EDFA), and input 16 a is a pump control input used to control the power output of the laser pump in the OFA.

An OADM/OXC has an access of 15 dB loss budget between G 2 15 and G 3 17 . A dispersion compensation module (DCM) and a dynamic gain equalizer (DGE) 14 is an element between G 1 12 and G 2 15 that operates with the following features features: tunable DCM, dispersion slope compensation (DSC), dynamic gain equalization (DGE), optical performance monitoring (OPM), and low nonlinearity. Preferably, the nonlinearity and insertion loss of the passive elements (DCM, DSC, DGE, OPM, and OADM/OXC) are very low, for the optical node architecture to operate effectively. Low nonlinearity implies that very high optical output power from G 1 12 can be launched into DCM portion 14 , without invoking optical nonlinearities that may cause unwanted signal distortion. This, in turn, allows the first stage G 1 12 to impose very high gain on an incoming optical signal via pin 11 a . In tandem amplification such as this three-stage chain, high-gain first stage in G 1 12 ensures that the overall noise of the chain is dominated by first stages G 1 12 . To characterize this in another way, once the G 1 12 operates at very high gain with a low-noise figure, the overall noise figure of the G 1 /G 2 /G 3 chain is similar to that of the G 1 12 , with little noise degradation resulting from signal propagation through the G 2 15 and the G 3 17 . Fiber-based DCM reaches nonlinearities at about 0 dBm power per channel. One may not put a conventional DCF in between the G 1 12 and the G 2 15 without suffering severe degradation in the overall noise figure, because the G 1 12 can operate only at moderate gain. If a DCM product is not fiber based, it can tolerate very high optical power without invoking nonlinearities. For the same reason, insertion losses of all the passive components should not be large compared to available gains from the G 1 12 and the G 2 15 . A suitable DCM has loss <10 dB and 100% 32 channel OADM with loss for express channels 5 dB. A suitable commercial product for implementing a DCM is a PowerShaper™, made by Avanex Corporation™.

The optical node 10 , with OADM/OXC access and built-in DCM/DSC/DGE/OPM, is designed to offer all the functionalities required by an ultra long haul backbone network, instead of focusing only on transport functionalities, as is the case for most conventional optical line amplifiers. The optical node 10 expands current two-stage line amplifier to three-stage, with functional characteristics of having low nonlinearity and low loss DCM. The resultant extra 15 dB access for OADM/OXC is necessary for wavelength networking, and it costs less than the 15 dB gain available from distributed Raman amplification, which has a newer deployment stage than EDFA technology, and has intrinsically poorer energy conversion efficiency than EDFA technology.

FIG. 2 is an architectural diagram illustrating a recirculating loop 20 for verifying the integrated long haul processor. The recirculating loop 20 has three optical nodes, a first node 21 , a second node 22 , and a third node 23 . The third node 23 has both a DCM and +15 dB 26 , while the first node 21 and the second node 22 do not have a DCM, but a 15 dB loss adjustment 24 in the first node 21 and a 15 dB adjustment 25 in the second node 22 . The 15 dB represents the equivalent of inserting an add/drop multiplexer. A loss equivalent of 15 dB is inserted in placed of an actual add/drop multiplexer. A suitable DCM is a PowerShaper™, which has −5000 ps/nm dispersion at every ITU grid with 50 GHz channel spacing, made by Avanex™. Six channels spaced 50 GHz apart are combined and modulated with 2 31 −1 PRBS data at 2.5 Bg/s using a LiNbO 3 modulator. The composite DWDM signal is boosted launched into the 3-span recirculating loop. The signal is tapped out of the loop, demultiplexed and detected using eye analyzer.

The recirculating loop 20 contains three spans 27 , 28 , and 29 of 100 km Allwave™ fiber. The first node 21 is placed between the first span 27 and the second span 28 , while the second node 22 is placed between the second span 28 and the third span 29 . A suitable commercial product for implementing the first node 21 and the second node 22 is PowerExpress™, a two-stage optical line amplifier, made by Avanex Corporation™. A 15 dB variable optical attenuator 30 is inserted between the first stage 21 and the second stage 22 , which is equivalent the loss of 32 channel 100% OADM. A three-stage optical amplifier in the third node 23 is inserted between the third span 29 and a loop switch 34 . Variable optical attenuators (VOA) 31 , 32 , and 33 are used to adjust launching power into a fiber for optimizing overall system performance.

Input signals are feed into a transmitter 35 and 36 for transmitting the input signals to a mux 37 . In one embodiment, the mux 37 has 6 channels of inputs. A modulator 38 modules with electric signals. A load switch 39 is constructed in a reverse manner from the loop switch 34 . Initially, the load switch 39 opens, and the loop switch 34 closes, so the input signal is passed into the loop. A 3 dB coupler 40 couples between the load switch 39 , the recirculating loop, and a 3 dB coupler 41 , which further couples to a demux 44 folowed by a receiver 42 and a receiver 43 and an optical spectrum analyzer (OSA) 45 . The signal continues to pass through the load switch 39 for τ amount of time, representing the time duration for the light to travel 300 km. By the time that the first signal reaches 300 km, the loop switch 34 opens, and the load switch 39 closes, having already circulated one full loop. For example, at time 0 , the load switch 39 opens, and the loop switch 34 closes. At time τ, or the amount of time for a signal to travel 300 km; the loop switch 34 opens, and the load switch 39 closes. In this illustration, a signal runs through the recirculating loop 20 ten times, or 3000 km in total distance. At the end of 3000 km, it is determined whether the signal strength still recovers.

FIG. 3 is a flow diagram illustrating the process 50 for operating an optical node in an ultra long haul network. The G 1 amplifier 12 receives 51 an input optical signal and amplifies 52 the input optical signal with a G 1 gain. The ALC 13 controls 53 the optical level of the input optical signal. The DCM/DGE 14 compensates 54 the dispersion of the optical signal from the G 1 12 amplifier, and equalizes 55 the gain G 1 . The G 2 15 amplifies 56 the optical signal received from the DCM/DGE 14 with an amplification gain of G 2 . The ALC 16 couples to the G 2 amplifier 14 for controlling 58 the level of the optical signal generated form the G 2 amplifier 15 . The process 60 then drops or adds to the optical signal from the G 2 amplifier 15 . At a third stage, or last stage in this embodiment, the G 3 amplifier 17 amplifies 59 the optical signal after add/drop operation has been performed. An optical performance monitor 18 monitors 60 the optical performance. The AGC/ALC 19 adjusts 61 control gain/level between the G 3 amplifier 17 and the G 1 amplifier 12 . Finally, the process 60 generates 62 an amplified output optical signal.

FIG. 4A shows the measured better than 18 dB/0.1 nm optical signal-to-noise ratio (OSNR) of all 6 channels at 3000 km. The 50 GHz spaced humps in the noise floor are due to the cyclic bandpassing characteristics of a PowerShaper™.

FIG. 4B shows the eye diagram of these channels at 3000 km. The input power into each span is about 1 dBm/channel, achieved by using fixed attenuator at the output of each PowerExpress™. An 80-channel system would need 19 dBm total output power from PowerExpress™ output EDFA, which is available form commercial DWDM EDFAs. This indicates that 80 wavelengths can be processed fully using only three commercial DWDM EDFAs.

The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. Although the optical node described in the present invention is in the context of an ultra long haul network, the optical node is applicable to other networks including along haul network, a metropolitan network, and an access network. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practices without departing form the scope of the invention as set forth in the appended claims.