# Extended L-Band InAs/InP Quantum-Dash Laser in Millimeter-Wave Applications
## 1 Introduction  
The fast paced revolution and proliferation of mobile technology have spurred the fifth-generation (5G) wireless (WL) technology to meet the bandwidth-intensified demand and provide ubiquitous connectivity. Besides, this technology has a vision of attaining ultra-low latency of 1 ms and offers data rates of up to 10 Gbits/s [1]. To meet these requirements, a new millimeter-wave (MMW) spectrum band from \(\sim\)30 to 300 GHz has been identified to address the communication sector's demands by offering multi-gigabit-capacity networks [2], which are considered as the cornerstone to support the fronthaul segment in 5G networks [2]. More specifically, the 20-30 GHz frequency band has been selected by the 5G public private partnership (PPP) to be used for mobile networks above 6 GHz [3] for short-reach WL propagation distances, owing to the high atmospheric attenuation of these frequencies and the exploitation of smaller cell sizes (pico-cells). Moreover, end-users prefer the high flexibility and mobility offered by WL connections. As such, to bridge this gap, the most viable method for the transmission of MMW is through optical fiber cables, i.e., radio-over-fiber (RoF) technology. This can be implemented by seamless integration with the existing fiber-to-the-\(\times\) (FTTX) infrastructure without the need for a significant overhaul. This yields low capital expenditure (CAPEX) [4] while offering customers security, protocol- and bitrate transparency, as well as scalability and upgradability. Hence, a significant reduction in future optical networks' operating expenditure (OPEX) is also expected [5]. Likewise, in the future, on the road towards complete convergence,bidirectional RoF with wavelength-division-multiplexed passive optical networks (WDM-PON) is a promising next-generation access network. This architecture could potentially increase channel capacity and flexibility and reduce latency [6; 7] by operating beyond the classical C-band, i.e., mid- and far-L-band wavelengths, as this spectrum window still exhibits the least fiber attenuation, which is paramount in long-reach access systems [8]. Furthermore, the use of fiber cables may not be feasible under several practical scenarios, such as impairments in terrains due to obstructions, rivers, etc. As such, the integration of highly resilient flexible heterogeneous access network technologies, such as RoF WL [9] and RoF-over-free-space-optics (FSO) WL technologies [10], has been perceived for the next generation of optical networks. In general, an MMW transmission system's performance strongly depends on the generated MMW beat tones' noise and linewidth characteristic [11] and the spectral purity and coherence of employed optical sources [12]. Thus, ultra-narrow optical linewidth and low-phase-noise optical sources are fundamental. In the literature, the heterodyne generation of MMW signals is the most popular method of signal generation, which is demonstrated by using various optical sources viz dual-wavelength lasers (DWLs) [12], mode-locked laser (MLL)-based optical frequency combs (OFCs) [13], and gain-switched distributed-feedback (DFB) lasers [14]. Besides, additional assisting methods have also been deployed in tandem to improve the wavelength modes' spectral purity. This includes phase locking [15] and optical external injection locking [16]. Moreover, self-assembled InAs/InP quantum-dash (QD)-nanostructure-based multiwavelength laser diodes [17] have recently been garnering attention as a potential source for MMW applications. Thanks to their inherent superior performance characteristics compared to quantum-well and bulk-active-region counterparts and the potential to cover emission wavelengths from S- to U-bands, they have emerged as promising MMW photonic sources for the next-generation extended C- and L-band networks [12]. In the literature, to the best of the author's knowledge, demonstrations of QD laser diodes (QD-LDs) in MMW applications are only limited to the C-band region.