An opto-electronic oscillator (OEO) is one of the most popular types of oscillators for generating micro- and millimeter wave signals.
High-precision signal oscillators are needed in a variety of fields such as satellite communications, optical communications, radar applications, radio-over-fiber communications, etc. [1]. In its most basic form, an oscillator consists of a resonator and a feedback component. When the Barkhausen conditions are satisfied, the oscillator starts generating the fundamental oscillation signal. An opto-electronic oscillator (OEO) is one of the most popular types of oscillators for generating micro- and millimeter wave signals [2][3]. The OEO has a number of optical components such as a laser diode [4][5], an optical fiber [6] and a photodiode [7]. The electrical components including an electrical bandpass filter and an electrical amplifier are used to complete the feedback loop. The laser of the OEO can be modulated directly, or it can use external modulation with an electro-optic modulator such as a Mach Zehnder modulator (MZM) [8] or an electro-absorption modulator [9]. A typical externally modulated OEO is shown in Figure 1.
Figure 1. Single-loop opto-electronic oscillator (OEO) with electrical components.
Currently, −163 dBc/Hz at a 6 kHz offset from the carrier for an operating frequency of 10 GHz [10] is the lowest phase noise achieved so far. Different types of configurations for the OEO have already been presented in the literature. The dual-loop and multi-loop configurations [11][12][13][14], coupled OEO [15][16][17][18], injection-locked OEO [19][20][21][22], OEO with quality multiplier [23], and OEO with feedback loop [24] are some of the well-known configurations. Moreover, optical solutions are possible by adding components such as optical filters [25][26][27] and optical amplifiers [28][29] or by adjusting the optical link to achieve an optical gain [30]. These are already used to improve the stabilization of the OEO. Since an OEO consisting of such bulky components is very large, some methods to reduce the size of the oscillator device have already been reported. There are several solutions such as using a whispering-gallery-mode resonator (WGMR) [31][32][33][34], a ring 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 J. Tang 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 G.K.M. Hasanuzzaman et al. [45]. The OEO provided a phase noise characteristic of −101 dBc/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. For instance, the use of a 1-km fiber has an FSR of 200 kHz, while a 15 km fiber has an FSR of 13.4 kHz. The relationship between SMSR and phase noise performance of the OEO at different optical lengths is shown in Figure 2.
Figure 2. Comparison of phase noise and side-mode suppression ratio (SMSR) performance of OEO with 1 km and 15 km delay line length [1]. Reprinted with permission from ref. [1]. Copyright 2015 IEEE.
As can be seen from the experimental results in Figure 2, there is a tradeoff between the short-term stability and multimode operation of the OEO. The 1 km OEO has about a 30 dB improvement performance in the SMSR, but the 15 km OEO has a significant improvement in phase noise, which is about 20 dB at 1 kHz and 10 kHz offsets from the carrier.
2. Current Progress of the Common Topologies of the OEO
In this section, the paper focuses on recent advances in the development of OEOs and the main challenges that they face: multimode operation, as well as short-term and long-term stability. In the first subsection, the paper focuses on multimode operation and short-term stability, with long-term stability following this subsection.
2.3. General Overview
In this section of the paper, we would like to compare the performance of different configurations of the OEO in multimode operation: short-term stability (i.e., phase noise) and long-term stability. In the first table, different configurations of the OEO are described by comparing the SMSR and the phase noise.
For Table 1, OEOs with the same or similar frequency were selected (except for the OEO with a high quality opto-electronic filter) to allow a more accurate and scientific comparison of the phase noise in different solutions. However, in theory, the OEO has a stable phase-noise characteristic that is independent of the operating frequency [1], so higher frequencies can be used for the comparison. In the SMSR comparison, the optical delay line length and the bandwidth of the electrical and/or optical filter are more important for the comparison. When considering the phase noise, the injection-locked OEO achieves the best performance among the other solutions. On the other hand, the cascaded micro-wave photonic filter solution achieved a better result in terms of the SMSR.
Configuration | Optical Delay Line Length | Central Frequency | SMSR | Phase Noise (@10 kHz offset from the carrier) |
---|---|---|---|---|
Dual-loop OEO [64] | 7-core fiber (105 m) |
From 3.5 GHz to 17.1 GHz | 61 dB | −100 dBc/Hz |
Injection-locked OEO [68] | Single-mode fibers (1 km and 0.7 km) |
10 GHz | N/A | −130 dBc/Hz |
Coupled OEO [72] | Erbium-doped fiber (4 m) |
10 GHz | 72.5 dB | −123.6 dBc/Hz |
OEO with high-quality opto-electronic filter [73] | Dispersion-shifted fiber (3 km) | 29.99 GHz | 83 dB | −113 dBc/Hz |
Cascading microwave photonic filter [49] | Single-mode fibers (2 km and 0.2 km) |
17.33 GHz | 125 dB | −103 dBc/Hz |
Narrowband microwave laser with dual-loop OEO [74] | Single-mode fibers (2.5 km and 3 km) |
From 1.85 GHz to 10.24 GHz | 55 dB | −116 dBc/Hz |
In Table 2, different solutions are compared to evaluate the performance of the OEO in terms of long-term stability and phase noise.
Configuration | Optical Delay Line Length | Central Frequency | Long-term Stability | Phase Noise (@10 kHz offset from the carrier) |
---|---|---|---|---|
Temperature stabilization [75] | N/A | 10 GHz | 0.1 ppm/K | −143 dBc/Hz |
OEO with feedback-control loop [76] | 15 km | 3 GHz | 0.05 ppm/K | < −130 dBc/Hz |
Optical delay stabilization system [77] | 3 km | 3 GHz | 0.02 ppm/K | −123 dBc/Hz |
OEO with PLL [78] | 500 m (Dispersion deduced fiber) |
3 GHz | 6.98 × 10−14 (average time of 1000 s) |
< −100 dBc/Hz |
The optical delay line system showed good performance in terms of long-term and short-term stability. The phase-noise performance could be improved by using a longer optical fiber. An OEO with a PLL does not have good phase noise performance because a short delay line is used. However, a classic solution such as temperature stabilization has good phase noise performance and short-term stability.
This entry is adapted from the peer-reviewed paper 10.3390/electronics10070857