# Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation
###### Abstract  
High-frequency signal oscillators are devices needed for a variety of scientific disciplines. One of their fundamental requirements is low phase noise in the micro- and millimeter wave ranges. The opto-electronic oscillator (OEO) is a good candidate for this, as it is capable of generating a signal with very low phase noise in the micro- and millimeter wave ranges. The OEO consists of an optical resonator with electrical feedback components. The optical components form a delay line, which has the advantage that the phase noise is independent of the oscillator's frequency. Furthermore, by using a long delay line, the phase noise characteristics of the oscillator are improved. This makes it possible to widen the range of possible OEO applications. In this paper we have reviewed the state of the art for OEOs and micro- and millimeter wave signal generation as well as new developments for OEOs and the use of OEOs in a variety of applications. In addition, a possible implementation of a centralized OEO signal distribution as a local oscillator for a 5G radio access network (RAN) is demonstrated. opto-electronic oscillator; phase noise; microwave signal; millimeter wave signal; 5G radio access network; long-term stability; multimode operation; wideband OEO; integrated OEO +
Footnote †: journal: _Electronics_  
resonator [35; 36], or an electro-absorption modulated laser [37] that decrease the size of the OEO. In 2017, a fully integrated OEO was reported in the literature by [PERSON] et al. [38; 39]. In addition, a theoretical and experimental study of the characteristics of an injection-locked OEO was presented and published in several journals [40; 41; 42; 43; 44]. Recently, a W-band OEO was introduced by [PERSON] et al. [45]. The OEO provided a phase noise characteristic of \(-101\)\(\mathrm{d}\mathrm{Bc}/\mathrm{Hz}\) at a 10 kHz offset from the 94.5 GHz carrier. On the other hand, an opto-electronic parametric oscillator [46] was reported in 2020. There are some more recent developments in the use of OEOs in various applications. One example of this is terahertz (THz) photonic signal generation using an OEO [47; 48; 49]. Another possible application of an OEO is to use it as a local oscillator (LO) in the central office of a 5G radio access network (RAN) [50; 51; 52]. The single-loop OEO can be combined with an optical fiber path selector to measure the free spectral range (FSR) and side-mode suppression ratio (SMSR) of the OEO for different lengths of the optical delay line [53]. There are other applications of OEOs such as an acoustic sensor [54], low-power radio frequency (RF) signal detection [55], phase-locked loops [56; 57; 58], parity time-symmetric OEO [59; 60], silicon micro-ring-based OEO [61] and linear frequency-modulated waveform generation [62], etc. Long-term stability and side modes (multimode operation) are the main challenges affecting the stabilization of an OEO. The OEO uses an optical fiber that is mainly affected by the temperature variations in the environment. This leads to fluctuations in the oscillation frequency over time, which is referred to as the frequency drift (in other words, long-term stability). On the other hand, electrical bandpass filters have bandwidth limitations in the micro- and millimeter wave ranges. Electric bandpass filters are used in the oscillator loop to destroy the side modes in the RF spectrum and determine the main mode of the oscillation. Due to the bandwidth limitation of the filter, the side modes are not completely filtered out, and they can therefore be seen in the RF spectrum. The ratio between the fundamental mode and the spurious side modes is called SMSR. The short-term stability (i.e., phase noise) is mainly based on the length of the delay line of the OEO. The OEO can use a long delay line to achieve the lowest possible phase noise. However, using a long delay line boosts the power of the side modes because the FSR becomes lower, and the side modes are more difficult to filter out due to the bandwidth limitation of the electrical filter.