# Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)
## 2 PIC Evolution  
Photonic integrated solutions are a promising technology for the 21 st-century optical systems, under the current demand for flexibility/reconfigurability in optical communication networks [5; 10]. Differentiated examples of PIC solutions include the implementation of photonic integrated optical transforms for data compression applications in different integrated platforms, e.g., indium phosphide (InP) [11; 12], silicon nitride (SiN) [13], and new organic-inorganic hybrid materials [14]; SiN-integrated photonic dispersion compensator enabling extended reach pulse-amplitude modulation with four amplitude levels (PAM-4) transmission as a low-cost interface for data center interconnects (DCI) [15]; development of state-of-the-art sub-hertz fundamental linewidth (\(<\)1 Hz) photonic integrated Brillouin laser, narrow enough to move demanding scientific applications to the chip-scale [16; 17; 18]; and a new soft-packaging flexible platform approach for photonic integrated processors, based on the spatial light modulation operation principle [19; 20]. Moreover, new concepts of PIC-to-PIC interconnects with integrated fan-in/fan-out multicore fiber applications for optical dense networks have been  
Figure 1: Diagram of a telecommunications network infrastructure, based in [3]. reported [21]. Photonic integration appears as a dominant technology in high bandwidth optical communication systems [9], offering increased valuable solutions in several innovative fields, such as bio-photonics [22], sensing [23], and space technology [24]. Nevertheless, PIC technology is still more expensive than standard microelectronics, which can restrict its application into some niche markets [5]. PICs are the equivalent of electronic integrated circuits (EIC) in the optical domain. As an alternative to transistors and other electronic components, PICs contain optical elements, such as modulators, detectors, attenuators, multiplexers, optical amplifiers, and lasers embedded in a single chip using a waveguide (WG) architecture [19]. The photonic integration technology is going through a similar evolution path as microelectronic integration, however, with a time delay of about 20 to 30 years [25]. Historically, the invention of the transistor in 1948 [26; 27] launched microelectronic integration developments; analogously, the invention of the semiconductor laser in 1969 [28] was the breakthrough of photonic integration technology. Transistor and laser technologies were primarily implemented as discrete components. Microelectronics integration technology started with Kilby in 1959 [29] and matured with the complementary metal-oxide-semiconductor (CMOS) [30] in the 1970s. A first PIC comprising a laser integrated with a modulator was reported in 1987 [31], marking the start of PIC exponential development, typically associated with [PERSON]'s law [25]. [PERSON]'s law forecasted a double in the number of components per integrated circuit about every two years, with a new technology generation introduced approximately every three years and an expected compound annual growth rate (CAGR) under the dozens [32]. Optical communications evolution has brought the advent of improved PIC, presenting an economic and sustainable alternative to data transmission [4]. PIC technology offers compelling performance advances in terms of small weight and volume, low power consumption, high mechanical and thermal stability, and the ease for assembling a substantial number of complex systems [9]. Consequently, the current PIC-related contexts are evolving at a good rate with an overall worldwide concentration set of efforts aiming to reach a sustainable PIC ecosystem model [33; 34]. From the 2000s when the first consortia started to consolidate the concepts around photonics multi-project wafer (MPW), the way for groundbreaking photonic integration developments was paved.