###### Abstract  
Quasi-coherent optical receivers have recently emerged targeting access networks, offering improved sensitivity and reach over direct-detection schemes at the expense of a higher receiver bandwidth. Higher levels of system integration together with sufficiently wideband front-end blocks, and in particular high-speed linear transimpedance amplifiers (TIAs), are currently demanded to reduce cost and scale up receiver data rates. In this article, we report on the design and testing of a linear TIA enabling high-speed quasi-coherent receivers. A shunt-feedback loaded common-base topology is adopted, with gain control provided by a subsequent Gilbert cell stage. The circuit was fabricated in a commercial 130 nm SiGe BiCMOS technology and has a bandpass characteristic with a 3 dB bandwidth in the range of 5-50 GHz. A differential transimpedance gain of 68 dB\(\Omega\) was measured, with 896 \(mV_{pp}\) of maximum differential output swing at the 1 dB compression point. System experiments in a quasi-coherent receiver demonstrate an optical receiver sensitivity of \(-30.5\) dBm (BER = \(1\times 10^{-3}\)) at 10 Gbps, and \(-26\) dBm (BER = \(1\times 10^{-3}\)) at 25 Gbps. The proposed TIA represents an enabling component towards highly integrated quasi-coherent receivers. transimpedance amplifier; optical receivers; high-speed circuit design; SiGe BiCMOS 2021102349 12249 12249 12249 12249 12249 12249 12249 12249 12249 12249 12249 12249 1249 simplified coherent receiver is reported achieving \(-37.6\) dBm sensitivity (BER = 1 \(\times\) 10\({}^{-3}\)) at 10 Gbps. A similar approach is followed in [2], reaching \(-35\) dBm sensitivity at 25 Gbps. In both cases, however, the receiver complexity is still relatively high, requiring 3 \(\times\) 3 optical couplers and three photodiodes, and resorting to offline digital signal processing (DSP). Recently, quasi-coherent receivers have gained increased attention, offering an attractive balance between performance, cost, and receiver complexity [8]. In a quasi-coherent scheme, a local oscillator (LO) laser is used to boost receiver sensitivity, with the additional advantage of filterless wavelength division multiplexing (WDM) operation by tuning of the LO wavelength. Compensation for fiber chromatic dispersion at the receiver is also a possibility, since phase information is preserved. A block diagram representation of a quasi-coherent receiver with polarization diversity is shown in Figure 1. The received optical signal is combined with an LO laser using an optical coupler. A polarizing beam splitter (PBS) divides the signal into orthogonal polarization components to be processed by independent receiver branches. Mixing of the received signal and LO is achieved in the photodetectors, with the resulting photocurrent expressed as [9]:  
\[I_{p}(t)=RP_{s}(t)+RP_{LO}+2R\sqrt{P_{LO}P_{s}(t)}cos(\omega_{IF}t+\phi_{IF}) \tag{1}\]  
The direct-detection term \(RP_{s}(t)\) and a DC component \(RP_{LO}\) appear in the photocurrent expression, where \(R\) represents the photodetector responsivity, \(P_{s}(t)\) is the power of the received signal, and \(P_{LO}\) the LO power. These two terms are not required for the receiver operation and can thus be filtered out. The third term represents a bandpass intermediate frequency (IF) component, being the one processed in a quasi-coherent receiver. It contains the amplitude information of the received signal boosted by the LO laser, as well as phase information contained in \(\omega_{IF}=\omega_{s}-\omega_{LO}\) and \(\phi_{IF}=\phi_{s}-\phi_{LO}\) (with \(\omega_{s}\),\(\omega_{LO}\) and \(\phi_{s}\), \(\phi_{LO}\) representing, respectively, wavelength and phase for received signal and LO). For intensity-modulated transmissions, the IF takes the form of an amplitude modulated (AM) signal with carrier frequency at \(\omega_{IF}\) and two sidebands containing the transmitted data.