# A DFB-SOA Based Optical Vector Network Analyzer for Characterization of Bandpass Optical Devices
## 1 Introduction  
Optical Vector Network Analyzers (OVNAs) are indispensable tools for the precise measurement and comprehensive characterization of optical devices. Utilizing an OVNA, we can acquire both phase and magnitude responses, which facilitate the design and enhancement of optical links. Traditional OVNAs are fundamentally implemented through the use of interferometers or modulation-based phase shifts [1, 2, 3]. Nevertheless, the inherent lack of stability and the comparatively coarse tuning resolution of the wavelength-scanning laser constraints the precision of measurements to an extent that it falls short of the requirements for comprehensive spectrum characterization of high-Q optical components [4, 5, 6, 7]. To address the issue at hand, researchers have introduced an OVNA that utilizes optical single sideband (OSSB) modulation. This technique effectively converts the transmission spectrum of the ODUT from the optical domain to the electrical domain, as detailed in references [8, 9, 10, 11, 12, 13, 14, 15]. Correlated with the benefits of the extraordinary high-frequency tuning capabilities of the electrical vector network analyzer (EVNA), the OVNA utilizing OSSB modulation can attain measurement resolutions in the range of several kHz or even less. Typically, to quantify the transmission response of an ODUT, it is essential that the optical carrier possesses a robust energy level capable of beating with the sidebands, which encapsulate the alterations inherent to the ODUT's transmission response. Nevertheless, the OVNA employing OSSB modulation is ill-suited for bandpass optical components, as the optical carrier frequency migrates beyond the transmission bandwidth of the ODUT, resulting in substantial attenuation. In order to achieve spectral characterization of ODUTs featuring bandpass responses, several methods have been proposed recently. An OVNA capable of quantifying both passband and stopband characteristics is presented [16]. The optical carrier is preserved and unaltered, while the sideband is separately allocated to delineate the passband of the ODUT, through the bifurcation of the carrier and sidebandonto two distinct channels. Due to the differential phase shifts experienced by the carrier and sidebands across channels, this OVNA exhibits heightened sensitivity to environmental fluctuations. Consequently, the acquisition of reliable phase response using this approach is challenging. An OVNA design that enhances measurement precision for bandpass response evaluation is established, as referenced in [17]. By employing four distinct measurement stages, an enhanced resolution and a minimized error in the measurements are achieved. Nonetheless, the incorporated Brillouin-assisted optical carrier processor results in the OVNA being both exorbitantly priced and intricate. In our previous work, we introduced an OVNA that employs a distributed-feedback semiconductor optical amplifier (DFB-SOA) [18]. The DFB-SOA is a DFB laser operating below the lasing threshold, which could exhibit dual functionality as both an optical filter and an optical amplifier concurrently [19; 20]. The optical-to-electrical conversion of the ODUT's spectrum is effectuated through the implementation of our proposed OVNA. Initially, the optical carrier undergoes modulation via an EVNA utilizing a phase modulator prior to its injection into the ODUT. Subsequently, the lower sideband traverses the optical spectrum that is intended for measurement in the ODUT, concurrent with a marked reduction in the optical carrier and the accompanying undesired sidebands. In the DFB-SOA, the primary optical carrier experiences significant amplification, whereas no appreciable gain is observed for the sidebands. Eventually, the optical carrier with enhanced power beats with the scanning sideband within the photodetector, resulting in the generation of a microwave signal. This signal coherently transports the transmission response of the ODUT, which is subsequently received and spectrally characterized by an EVNA.