# Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing
## 2 Comb-Based Superchannels
### Superchannel Transmitter Requirements
#### 2.1.1 OSNR Requirements  
For comb-based superchannels, the key assumption is that the underlying comb source can provide the required line power to reach sufficient OSNR at the transmitter side. This can be analyzed by comparing a comb-based transmitter to a transmitter using a laser array, as illustrated in Figure 3. In the case of a laser array, each laser is directly connected to the modulators, and assuming equal line power is therefore natural. This is the classical system and will be our benchmark. A multi-wavelength comb source cannot, generally, beat the OSNR of a free-running laser since practical power limitationswill always degrade the maximum line power. The required input power to the modulator to reach a certain transmitter side OSNR is:  
\[P_{\text{Mod}}=\text{OSNR}_{\text{TX}}+\text{NF}_{\text{EDFA}}+\text{IL}_{\text{ Mod}}-58\,\text{dBm}, \tag{2}\]  
where \(\text{OSNR}_{\text{TX}}\) is the desired transmitter side OSNR, \(\text{NF}_{\text{EDFA}}\) the noise figure of the post-mux EDFA in Figure 3 and \(\text{IL}_{\text{Mod}}\) the overall loss of the modulation stage. The \(-58\) dBm is the vacuum fluctuation-limited, dual-polarization noise power (The value \(-58\) dBm can be calculated from \(h\
u_{0}\Delta\
u_{0}\), where \(h\) is the Planck constant, \(\
u_{0}\) is the frequency of the carrier and \(\Delta\
u\) is the noise bandwidth, corresponding to \(0.1\,\text{nm}\). More details can be found in e.g., [79]). When free-running lasers are used, \(P_{\text{Mod}}\) is the required output power needed. In the case of comb-based transmission, the losses of the comb generation stage need to be taken into account to calculate the output power of the comb seed laser, \(P_{\text{Comb}}\). Assuming a comb generator without amplification, the input power to the comb generation stage is then:  
\[P_{\text{Comb}}=P_{\text{Mod}}+10\cdot\log_{10}(N)+\text{IL}_{\text{Comb}}+P _{\text{Line Diff.}}, \tag{3}\]  
where \(10\cdot\log_{10}(N)\) represents the number of lines among which the power is divided, \(\text{IL}_{\text{Comb}}\) the insertion loss of the comb regeneration stage and \(P_{\text{Line Diff. }}\) the difference between average and minimum line power. As a simple comparison point, assume that the post-mux EDFA has a noise figure of \(5\,\text{dB}\) and the required OSNR is \(30\,\text{dB}\). The loss for modulating the high spectral-efficiency signal including the multiplexer to combine all signals prior to amplification into the span is around \(15\,\text{dB}\). The required output power for each laser in a laser array is then \(-8\,\text{dBm}\). Assuming that a comb is used to generate these lines, the required power grows fast with the number of lines. Considering the case of a 50-line loss-free comb source and a perfect flat envelope without excess lines (\(N=50,\text{IL}_{\text{Comb}}=0,P_{\text{Line Diff. }}=0\)), only the power division between the lines needs to be considered. The input power to the comb then has to be \(9\,\text{dBm}\), which is clearly reachable using a non-amplified pump line. In practice, however, combs are neither loss-free, nor spectrally flat or without any excess lines. Considering the case of a 50-line EO-comb with the spectral flatness \(P_{\text{Line Diff. }}\) of about \(6\,\text{dB}\) and total insertion loss of \(12\,\text{dB}\)[67], the required input power is \(27\,\text{dBm}\). While this is easily reachable using high-power amplifiers, one would ideally like to avoid such components and have the laser directly connected to the comb source. Depending on the technology used, these values can vary, but they still show the challenge in designing comb-based transmitters for multiple lines and high OSNRs. Moreover, depending on the comb-technology used, tuning the number of lines, as well  
Figure 3: Schematic of (**a**) a comb-based transmitter and (**b**) a transmitter using a laser array. Mod. : Modulator. as the flattening can be very difficult.