Abstract
Coherent in-phase quadrature (IQ) transponders are ubiquitous in the long-haul and the metro optical networks. During the transmission, the coherent signal experiences a bandwidth narrowing effect after passing through multiple reconfigurable optical add-drop multiplexers (ROADMs). The coherent signal also experiences a bandwidth narrowing effect when electrical or optical components of the coherent IQ transponder experience aging. A dynamic method to compensate the bandwidth narrowing effect is thus required. In the coherent optical receiver, signal bandwidth is estimated from the raw analog-to-digital converter (ADC) outputs. By adaptively adjusting the tap coefficients of the finite impulse response (FIR) filter, simple post-ADC FIR filters can increase the resiliency of the coherent signal to the bandwidth narrowing effect. The influence of chromatic dispersion, polarization mode dispersion, and polarization dependent loss are studied comprehensively. Furthermore, the bandwidth information of the transmitted analog signal is fed back to the coherent optical transmitter for signal optimization, and the transmitter-side FIR filter thus changes accordingly.
Highlights
Coherent line-cards using the polarization-division-multiplexed quadrature amplitude modulation (PDM-QAM) have been the de-facto standards for the metro and the long-haul optical fiber communications systems [1]
When a modulated signal passes through multiple reconfigurable optical add-drop multiplexers (ROADMs), the bandwidth of the propagating signal narrows down due to two effects
The tolerance of the coherent receiver to the bandwidth narrowing effect is improved by 5 GHz, which is equivalent to the bandwidth degradation of the signal passing through >10 ROADMs [16]
Summary
Coherent line-cards using the polarization-division-multiplexed quadrature amplitude modulation (PDM-QAM) have been the de-facto standards for the metro and the long-haul optical fiber communications systems [1]. In [15], the data stream from the transmitter passes through a digital filter and generates the duobinary data pattern, which is more tolerant to the bandwidth narrowing effect. We adaptively adjust the FIR filter in the coherent receiver to provide compensation according to the received signal bandwidth. The tolerance of the coherent receiver to the bandwidth narrowing effect is improved by 5 GHz, which is equivalent to the bandwidth degradation of the signal passing through >10 ROADMs [16]. The system tolerance to the bandwidth narrowing effect through the cascaded ROADMs is further improved This scheme is relatively simple to implement, and it is complementary to the other methods discussed above [10,11,12,13,14,15]
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