## 2 Experimental Setup and Signal Bandwidth Estimation  
On top of the optical transport network, there is an Internet protocol (IP) network utilizing the packet forwarding engine (PFE) application-specific integrated circuit (ASIC) to forward the IP packets. It is very desirable to physically integrate the coherent transponder with the PFE, which will remove any client-side optical transponders to reduce the cost and power consumption. Figure 1 shows the architecture, which closely integrates the PFE, the digital signal processing (DSP) ASIC, and the coherent optical transponders. The client's signal is received by the DSP and converted to the polarization-division-multiplexed quadrature phase shift keying (PDM-QPSK) modulation format at 30.1 giga-baud (GBd). The coherent optical transponder is a highly integrated C form factor two analog coherent optics (CFP2-ACO) module [17]. The payload of each transponder is either 100 gigabit Ethernet (GbE) or Optical Channel Transport Unit 4 (OTU4). The IP traffic is converted into optical signal through the coherent transmitter and then transmitted through the long-haul optical communications system, which can have multiple cascading ROADMs. During the transmission, multiple optical impairments, such as chromatic dispersion (CD), polarization mode dispersion (PMD), and polarization dependent loss (PDL), can accumulate. At the receiver, the optical signal is coherently detected by beating with the local oscillator (LO). Then, the signal is converted back to the digital domain through the ADC. Most of the optical impairments are compensated by the DSP ASIC. Figure 1 depicts this detailed configuration. In addition, a network management layer oversees the whole long-haul system. The digital analog converter (DAC), the circuit trace, the connector for pluggable optics, the radio-frequency (RF) amplifier, and the electro-optical modulator form the analog interface between the DSP chip and the coherent transmitter (Tx). Together with the ADC, the photodiode, the trans-impedance amplifier, the circuit trace, and the connector in the pluggable ACO form the analog interface between the coherent receiver (Rx) and the DSP chip. The received signal bandwidth is influenced by the bandwidth of both these analog interfaces. Although this influence is relatively static, it might degrade over the lifetime of the coherent transponder. Thus, a dynamic mechanism to compensate the bandwidth narrowing effect would also improve the tolerance to the component degradation. Figure 2 shows the experimental setup to study the bandwidth narrowing effect. CD, PMD, and PDL emulators are used to emulate various impairments in the transmission system. The \(4\times 4\) non-blocking optical switches can reconfigurably load either individual impairment or combined impairments. The noise produced by the amplified spontaneous emission (ASE) is added to adjust the optical signal-to-noise ratio (OSNR). An optical filter whose center frequency and pass-band are tunable is placed in front of the coherent receiver. The tunable filter is Finisar's WaveShaper built on the liquid crystal on silicon (LCOS) technology. The filter's passband closely resembles that of a wavelength selective switch (WSS), which is widely used in modern ROADM [4]. One noticeable difference is that with multiple cascading ROADMs, there is a steep roll-off at high-frequency content of the filter's frequency response, which cannot be emulated by the WaveShaper. Despite this slight difference, one can adjust the pass-band of the tunable filter, allowing the emulation of the bandwidth narrowing effect to the coherent signal. To achieve adaptive compensation of the bandwidth narrowing effect, we firstly estimate the signal's bandwidth. The coherent DSP ASIC can provide a snapshot of the ADC raw data and store them in the random-access memory (RAM). The network management layer can extract the ADC raw data and perform fast Fourier transform (FFT) to calculate the spectrum of the modulated signal. The sample rate of the ADC is 45 GHz.