Amplified spontaneous emission (ASE) noise is one of the main noise sources in optical amplifiers. ASE is initiated by spontaneous decay of electrons, creating photons which could travel in any direction. In modern optical amplified systems, after propagating multiple spans, accumulated ASE noise is not negligible compared to wavelength division multiplexed (WDM) signals. For both constant power and constant gain amplifier control, if ASE power is not considered in the total target output power at each network node, a system will take penalty due to lower than expected channel launch power since significant ASE will share the total output power of the amplifier with the WDM signals. With state-of-the-art amplifier control schemes, ASE compensation has been implemented; however, this ASE compensation assumes ASE is evenly distributed across each channel. This assumption was valid for traditional optical networks which include fixed optical add-drop multiplexers.
In a normal amplified system, noise across channel is either evenly distributed or filtered down by channelized devices. However, the addition of reconfigurable optical add-drop multiplexers (ROADMs) alters this assumption. ROADMs are a form of optical add-drop multiplexer that adds the ability to remotely and dynamically switch traffic from a WDM system at the wavelength layer. This allows individual wavelengths carrying data channels to be added and dropped from a fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.
Referring to FIG. 1, an exemplary embodiment of a micro-electromechanical system (MEMS)-based wavelength-selective switch (WSS) 10 is illustrated. An input fiber including multiple wavelengths λ1, λ2, . . . λn of optical signals is input into a de-multiplexer 11, such as a diffraction grating. The de-multiplexer 11 separates each wavelength from the common input, and optionally a variable optical attenuator (VOA) 14 can be included following the de-multiplexer 11. VOAs 14 are configured to provide variable attenuation to the wavelength, and the VOAs 14 can be remotely and dynamically set to a range of values. The WSS 10 includes a MEMS mirror 12 for each of the wavelengths λ1, λ2, . . . λn. The MEMS mirror 12 is a micro-mirror that deflects the optical signal to an appropriate output port 13. Advantageously, the WSS 10 is fully reconfigurable for adding, dropping, and expressing through optical signals. Since there is a MEMS mirror 12 for each of the optical signals, any signal can be dropped to any of the output ports 13. Additionally, multiple wavelengths including all wavelengths can be dropped to a single port 13, such as an express port.
Referring to FIGS. 2a and 2b, WSSs 15,16 are configured to direct each wavelength from a common input port to any one of multiple (e.g., “N”) output ports. To indicate device fan out, these devices are often classified as “1×N” devices, with a “1×9” WSS meaning a 10 port device, with 1 common input and 9 output ports. For example, the WSS 15 is a 1×9 WSS with a common input, eight drop ports, and one express port. The WSS 15 can be utilized at a node where up to eight optical signals need to be dropped, with the remaining optical signals pass through as express signals. Alternatively, the WSS 16 utilizes the same hardware configured for multiple express ports, such as where a node has multiple degree interconnection. Advantageously, the WSS 15,16 provides nodal flexibility to add, drop, and express optical signals with the same MEMS-based hardware.
Disadvantageously, MEMS-based WSSs have side-lobes for each channel. The side-lobes are a passband distortion caused by channel attenuation or filter cascading. Currently the designs of ROADM are either based on MEMS tilting mirror array or liquid-crystal modulators (LCM). Note, the passband sag at the edges are possible in both technologies. Furthermore, in practice, both MEMS and LCM devices may require that a finite gap exist between adjacent pixels. The edge effects are known to cause attenuation dependent passband side-lobe.
Generally, the side-lobes are located out of signal bandwidth, leading to more noise pass through in the WSS than a traditional system (without WSS), which has an evenly distributed ASE across channel. In this case, even with current ASE migration compensation scheme, the total ASE power can still be under-estimated. When multiple ROADMs are present in a network, the cascading effect will lead the side-lobes to grow along the spans, and the accumulated noise power can increase much faster than traditional flat-noise system. The severe under-estimating accumulated ASE power will cause unexpected signal power drop and lead to OSNR penalty in the system. The OSNR penalty introduced by WSS side-lobes is mainly caused by launching lower than expected signal power into the system along the amplifier chain, due to larger than expected ASE and under-estimated amplifier target power, i.e. side-lobes utilize amplifier power designated for the signal bandwidth.
Further, if a WSS is used to realize a dynamic gain equalization (DGE) function in networks, large dynamic range of attenuation on every channel through WSS is needed. Meanwhile, the size of side-lobes of a WSS is dependent on the attenuation of the WSS. Normally, the higher of the attenuation of the WSS, the bigger the side-lobes, and the more unexpected ASE can accumulate along the system.
Referring to FIG. 3, graph 19 illustrates a study showing a network of up to 16 spans with a ROADM at every network node (noise figure=9, span loss=20 dB). With a typical WSS side-lobe, when WSS attenuation is set at 0 dB (no side-lobe), 4 dB (typical) and 8 dB (worst case), and assuming all the channels have same attenuation, optical signal-to-noise ratio (OSNR) migration in the network has been calculated. In a typical WSS scenario (i.e., WSS attenuation=4 dB) more than ½ dB OSNR penalty has been observed after propagation through 16 ROADMs; and for the highest attenuation (i.e., 8 dB), more than 4 dB OSNR penalty can be incurred by the system. This study was performed assuming each WSS is working at typical conditions. Alternatively, if the WSSs are working at the worst-case condition in their specification, the system has to take even more OSNR penalty due to the side-lobes. Accordingly, side-lobes due to WSS ROADMs significantly impair network performance, and current systems and methods are inadequate for compensation. Thus, systems and methods are needed to compensate for the OSNR penalty caused by the side-lobe of MEMS-based ROADMs.