electronics

Review Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation

Mehmet Alp Ilgaz * and Bostjan Batagelj

Faculty of Electrical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia; [email protected] * Correspondence: [email protected]

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.

Keywords: opto-electronic oscillator; phase noise; microwave signal; millimeter wave signal; 5G



 radio access network; long-term stability; multimode operation; wideband OEO; integrated OEO

Citation: Ilgaz, M.A.; Batagelj, B. Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal 1. Introduction Generation. Electronics 2021, 10, 857. https://doi.org/10.3390/ High-precision signal oscillators are needed in a variety of fields such as satellite electronics10070857 communications, optical communications, radar applications, radio-over-fiber communica- tions, etc. [1]. In its most basic form, an oscillator consists of a resonator and a feedback Academic Editor: Geok Ing Ng 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 Received: 3 March 2021 popular types of oscillators for generating micro- and millimeter wave signals [2,3]. The Accepted: 1 April 2021 OEO has a number of optical components such as a laser diode [4,5], an optical fiber [6] Published: 3 April 2021 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 Publisher’s Note: MDPI stays neutral modulated directly, or it can use external modulation with an electro-optic modulator such with regard to jurisdictional claims in as a Mach Zehnder modulator (MZM) [8] or an electro-absorption modulator [9]. A typical published maps and institutional affil- externally modulated OEO is shown in Figure1. iations. 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 [11–14], coupled OEO [15–18], injection-locked OEO [19–22], OEO with Copyright: © 2021 by the authors. quality multiplier [23], and OEO with feedback loop [24] are some of the well-known Licensee MDPI, Basel, Switzerland. configurations. Moreover, optical solutions are possible by adding components such as This article is an open access article optical filters [25–27] and optical amplifiers [28,29] or by adjusting the optical link to distributed under the terms and achieve an optical gain [30]. These are already used to improve the stabilization of the conditions of the Creative Commons OEO. Since an OEO consisting of such bulky components is very large, some methods Attribution (CC BY) license (https:// to reduce the size of the oscillator device have already been reported. There are several creativecommons.org/licenses/by/ solutions such as using a whispering-gallery-mode resonator (WGMR) [31–34], a ring 4.0/).

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as using a whispering-gallery-mode resonator (WGMR) [31–34], a ring resonator [35,36], resonatoror an electro-absorption [35,36], or an electro-absorption modulated laser modulated [37] that decrease laser [37 the] that size decrease of the OEO. the size In 2017, of the a OEO.fully Inintegrated 2017, a fully OEO integrated was reported OEO in was the reported literature in by the J.literature Tang et al. by [38,39]. J. Tang In et al.addition, [38,39]. a Intheoretical addition, aand theoretical experimental and experimental study of the study characteristics of the characteristics of an injection-locked of an injection-locked OEO was OEOpresented was presented and published and published in several in journals several [40–44]. journals Recently, [40–44]. Recently,a W-band a OEO W-band was OEOintro- wasduced introduced by G.K.M. by Hasanuzzaman G.K.M. Hasanuzzaman et al. [45]. et The al. OEO [45]. provided The OEO a provided phase noise a phase characteris- noise characteristictic of −101 dBc/Hz of −101 at dBc/Hza 10 kHz at offset a 10 kHz from offset the 94.5 from GHz the 94.5carrier. GHz On carrier. the other On the hand, other an hand,opto-electronic an opto-electronic parametric parametric oscillator oscillator [46] was [ 46reported] was reported in 2020. in 2020.

FigureFigure 1. 1.Single-loop Single-loop opto-electronic opto-electronic oscillator oscillator (OEO) (OEO) with with electrical electrical components. components. There are some more recent developments in the use of OEOs in various applications. 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 [47–49]. One example of this is terahertz (THz) photonic signal generation using an OEO [47–49]. Another possible application of an OEO is to use it as a local oscillator (LO) in the central 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) [50–52]. The single-loop OEO can be combined office of a 5G radio access network (RAN) [50–52]. The single-loop OEO can be combined with an optical fiber path selector to measure the free spectral range (FSR) and side-mode 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]. 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 There are other applications of OEOs such as an acoustic sensor [54], low-power radio frequency (RF) signal detection [55], phase-locked loops [56–58], parity time-symmetric frequency (RF) signal detection [55], phase-locked loops [56–58], parity time-symmetric OEO [59,60], silicon micro-ring-based OEO [61] and linear frequency-modulated waveform OEO [59,60], silicon micro-ring-based OEO [61] and linear frequency-modulated wave- generation [62], etc. form generation [62], etc. Long-term stability and side modes (multimode operation) are the main challenges affectingLong-term the stabilization stability of and an OEO.side modes The OEO (multimo uses ande optical operation) fiber that are is the mainly main affected challenges by theaffecting temperature the stabilization variations of in an the OEO. environment. The OEO Thisuses leads an optical to fluctuations fiber that is in mainly the oscillation affected frequencyby the temperature over time, variations which is referred in the environmen to as the frequencyt. This leads drift (into fluctuations other words, in long-term the oscil- stability).lation frequency On the over other time, hand, which electrical is referred bandpass to as filters the frequency have bandwidth drift (in limitations other words, in thelong-term micro- and stability). millimeter On the wave other ranges. hand, Electricelectrical bandpass bandpass filters filters are have used bandwidth in the oscillator limita- looptions to in destroy the micro- the sideand modesmillimeter in the wave RF spectrumranges. Electric and determine bandpass the filters main are mode used of in the the oscillation.oscillator loop Due to to destroy the bandwidth the side limitationmodes in the of the RF filter,spectrum the side and modes determine are not the completely main mode filteredof the oscillation. out, and they Due can to thereforethe bandwidth be seen lim initation the RF of spectrum. the filter, Thethe side ratio modes between are the not fundamentalcompletely filtered mode and out, the and spurious they can side therefor modese be is calledseen in SMSR. the RF spectrum. The ratio be- tweenThe the short-term fundamental stability mode (i.e., and phase the spurious noise) is side mainly modes based is called on the SMSR. length of the delay line ofThe the short-term OEO. The OEOstability can (i.e., use aphase long noise) delay lineis mainly to achieve based the on lowestthe length possible of the phase delay noise.line of However, the OEO.using The OEO a long can delay use a line long boosts delay the line power to achieve of the the side lowest modes possible because phase the FSRnoise. becomes However, lower, using and thea long side delay modes line are b moreoosts difficultthe power to filter of the out side due modes to the bandwidthbecause the limitationFSR becomes of the lower, electrical and filter. the side For instance,modes are the more use ofdifficult a 1-km to fiber filter has out an due FSR to of the 200 band- kHz, whilewidth a limitation 15 km fiber of hasthe anelectrical FSR of filter. 13.4 kHz.For in Thestance, relationship the use of between a 1-km fiber SMSR has and an phaseFSR of noise200 kHz, performance while a 15 of km the fiber OEO has at differentan FSR of optical 13.4 kHz. lengths The isrelationship shown in Figure between2. SMSR and phase noise performance of the OEO at different optical lengths is shown in Figure 2.

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Figure 2. Comparison of phase noise and side-modeside-mode suppressionsuppression ratioratio (SMSR)(SMSR) performanceperformance of of OEO withOEO 1with km 1 and km 15and km 15 delaykm delay line line length length [1]. [1]. Reprinted Reprinted with with permission permission from from ref. ref. [1 [1].]. Copyright Copy- right2015 IEEE.2015 IEEE.

As can bebe seenseen fromfrom the the experimental experimental results result ins in Figure Figure2, there2, there is a is tradeoff a tradeoff between between the theshort-term short-term stability stability and and multimode multimode operation operation of the of OEO. the OEO. The 1 The km OEO1 km hasOEO about has about a 30 dB a 30improvement dB improvement performance performance in the SMSR, in the butSMSR, the 15but km the OEO 15 km has OEO a significant has a significant improvement im- provementin phase noise, in phase which noise, is about which 20 dB is atabout 1 kHz 20 and dB 10at kHz1 kHz offsets and from10 kHz the offsets carrier. from the carrier.In this paper, recent advances in OEO configurations are presented, using new re- sultsIn to this improve paper, the recent stability advances of short-term, in OEO long-termconfigurations and multimodeare presented, operation. using new It also re- sultshighlights to improve new areas the ofstability science, of technology short-term, and long-term engineering and where multimode the OEO operation. can be applied. It also highlights new areas of science, technology and engineering where the OEO can be ap- 2. Current Progress of the Common Topologies of the OEO plied. In this section, the paper focuses on recent advances in the development of OEOs 2.and Current the main Progress challenges of the that Common they face: Topologies multimode of the operation, OEO as well as short-term and long-term stability. In the first subsection, the paper focuses on multimode operation and In this section, the paper focuses on recent advances in the development of OEOs short-term stability, with long-term stability following this subsection. and the main challenges that they face: multimode operation, as well as short-term and long-term2.1. Progress stability. of the OEO In the toward first Lowersubsection, SMSR the and paper Phase focuses Noise on multimode operation and short-term stability, with long-term stability following this subsection. As mentioned earlier, one of the general challenges associated with OEO design is multimode operation. Since an electrical bandpass filter has bandwidth limitations, some 2.1. Progress of the OEO Toward Lower SMSR and Phase Noise optical solutions have been proposed. One of the most popular solutions is to form more thanAs one mentioned optical delay earlier, line. one These of dualthe general or multi-loop challenges OEOs associated have seen with widespread OEO design use for is multimodemore than 20operation. years to Since eliminate an electrical multimode bandpass operation filter [ 63has]. bandwidth The typical limitations, configuration some of opticala dual-loop solutions OEO have is shown been inproposed. Figure3 .One In this of the configuration, most popular one solutions loop is is used to form as a longmore thancavity, one while optical the delay other line. behaves These as dual a short or cavity.multi-loop This OEOs is achieved have se byen using widespread short and use long for morefibers than in different 20 years loops. to eliminate One of multimode the recent advancesoperation in[63]. the The dual-loop typical OEOconfiguration is the use of of a dual-loopmulticore fibersOEO is and shown the self-polarization-stabilization in Figure 3. In this configuration, technique one loop [64]. is Conventionally, used as a long cav- two ity,or more while single-mode the other behaves fibers (SMFs) as a short are used cavity. to form This a is dual- achieved or multi-loop by using configuration. short and long In fibersthis novel in different approach, loops. the combinationOne of the recent of cores advances in a multicore in the dual-loop fiber is used OEO to formis the ause short of multicoreand a long fibers cavity and [64 ].the With self-polarization-stabilization this novel approach, an SMSR technique of 61 dB [64]. was Conventionally, achieved for a twomicrowave or more OEO single-mode oscillating fibers at 7.8 (SMFs) GHz.Another are used recent to form approach a dual- isor parity multi-loop symmetry configura- of the tion.OEO In with this a novel dual-loop approach, configuration the combination [65]. The of parity cores symmetry in a multicore is achieved fiber is byused using to form two aoptical short carriersand a long with cavity different [64]. opticalWith this powers. novel Parityapproach, symmetry an SMSR provides of 61 dB additional was achieved gain for a the microwave main mode. OEO With oscillating the combination at 7.8 GHz. of parityAnother symmetry recent approach and a dual is parity loop, asymmetry 60.71 dB ofSMSR the OEO was achievedwith a dual-loop for a 10 configuration GHz central frequency. [65]. The parity symmetry is achieved by using

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Figure 3.3. Typical dual-loopdual-loop configurationconfiguration ofof anan OEO.OEO.

An injection-locked OEOOEO isis another configurationconfiguration toto improveimprove thethe SMSRSMSR performanceperformance of the OEO. It was firstfirst proposed in 2005 [[63,66].63,66]. The typical configuration configuration of an injection- lockedlocked OEOOEO isis shownshown inin FigureFigure4 4.. In In the the typical typical configuration configuration of of the the OEO, OEO, it it consists consists of of twotwo oscillatoroscillator blocks.blocks. One of themthem isis classifiedclassified asas thethe mastermaster OEOOEO andand otherother asas thethe slaveslave OEO [[63].63]. This approach isis used to suppress thethe spuriousspurious sideside modesmodes andand atat thethe samesame timetime maintain thethe qualityquality factor factor (Q-factor) (Q-factor) of of the the OEO OEO [63 [63].]. The The slave slave OEO OEO is used is used to suppress to suppress the spuriousthe spurious side side modes modes as it as employs it employs a short a shor fiber,t fiber, while while the master the master OEO OEO employs employs a long a fiberlong tofiber keep to thekeep high the Q-factor. high Q-factor. In reference In reference [67], a tunable, [67], a dual-loop,tunable, dual-loop, injection-locked injection-locked OEO was presented.OEO was presented. The OEO was The tunableOEO was from tunable 11.1 GHz from to 11.1 12.1 GHz GHz, to and 12.1 the GHz, spurious and the side spurious modes − wereside modes suppressed were belowsuppressed115 dBc/Hzbelow −115 [67 ].dBc/Hz Another [67]. approach Another was approach to form was a microwave to form a frequency divider based on the injection-locked OEO [68]. In this implementation, the microwave frequency divider based on the injection-locked OEO [68]. In this implemen- 10 GHz free-running OEO was injected with a 20 GHz microwave signal. A single-sideband tation, the 10 GHz free-running OEO was injected with a 20 GHz microwave signal. A (SSB) phase noise of −130 dBc/Hz at a 10 kHz offset was achieved. In Reference [69], an single-sideband (SSB) phase noise of −130 dBc/Hz at a 10 kHz offset was achieved. In Ref- injection-locked OEO based on stimulated Brillouin scattering (SBS) is described. This erence [69], an injection-locked OEO based on stimulated Brillouin scattering (SBS) is de- approach provided a frequency tunability up to 40 GHz with an SMSR of 60 dB and an SSB scribed. This approach provided a frequency tunability up to 40 GHz with an SMSR of 60 noise of −116 dBc/Hz at a 10 Hz offset. dB and an SSB noise of −116 dBc/Hz at a 10 Hz offset. The coupled OEO, introduced in 1997 [63,70], is another type of commonly used OEO configuration to improve SMSR. In this configuration, the OEO consists of an optical loop and an opto-electronic loop coupled via an electro-optic modulator [63]. The coupled OEO is used to improve the phase noise characteristics and the SMSR. The typical configuration of a coupled OEO is shown in Figure5. An example of a recent development in the coupled OEO was the synthesis of a 90 GHz signal using a 30 GHz coupled OEO [71]. The 90 GHz signal was obtained by biasing the MZM to the third harmonic. An SSB phase noise of −104 dBc/Hz at a 1 kHz offset from the 90 GHz carrier was achieved in this configuration [71]. On the other hand, a novel, coupled OEO with an erbium-doped fiber (EDF) has been proposed [72]. The novel configuration allows a large spatial hole burning (SHB) using the unpumped EDF to improve the SMSR and the phase noise. An SMSR of more than 72.5 dB was achieved wherein the SSB phase noise was −123.6 dBc/Hz at a 10 kHz offset from the 10 GHz carrier signal.

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FigureFigure 4. 4.Typical Typical injection-locked injection-locked OEO. OEO.

The coupled OEO, introduced in 1997 [63,70], is another type of commonly used OEO configuration to improve SMSR. In this configuration, the OEO consists of an optical loop and an opto-electronic loop coupled via an electro-optic modulator [63]. The coupled OEO is used to improve the phase noise characteristics and the SMSR. The typical configuration of a coupled OEO is shown in Figure 5. An example of a recent development in the cou- pled OEO was the synthesis of a 90 GHz signal using a 30 GHz coupled OEO [71]. The 90 GHz signal was obtained by biasing the MZM to the third harmonic. An SSB phase noise of −104 dBc/Hz at a 1 kHz offset from the 90 GHz carrier was achieved in this configura- tion [71]. On the other hand, a novel, coupled OEO with an erbium-doped fiber (EDF) has been proposed [72]. The novel configuration allows a large spatial hole burning (SHB) using the unpumped EDF to improve the SMSR and the phase noise. An SMSR of more than 72.5 dB was achieved wherein the SSB phase noise was −123.6 dBc/Hz at a 10 kHz offset from the 10 GHz carrier signal. In addition to the typical solutions listed above, there are other novel solutions to improve the phase noise and/or the SMSR characteristics of the OEO. For a millimeter- wave OEO, a high-quality opto-electronic filter was proposed, e.g., a Q-factor of 30,000 at a central frequency of 29.99 GHz [73]. This resulted in an 83 dB SMSR and a −113 dBc/Hz SSB phase noise at a 10 kHz offset. A cascaded microwave filter was presented in [49]. In this configuration, the cascaded filter configuration was implemented with a single pass- band filter having an opto-electronic filter. With this approach, an SMSR of 125 dB and an

SSB phase noise of −103 dBc/Hz for a 10 kHz offset at the 17.33 GHz carrier were achieved. Figure 5. FigureThe linewidth 5.Typical Typical configurationof configuration the laser can of of a affecta coupled coupled the OEO. OEO.phase-noise performance of the OEO. A narrow- band microcavity laser with a physical side length of 16 μm is used for the microwave 2.2. InProgress addition of the to OEO the typical Toward solutions Better Long-Term listed above, Stability there are other novel solutions to improvesignal generation the phase in noise the and/orloop of thethe SMSROEO [74]. characteristics An SSB phase of the noise OEO. of For−116 a millimeter-dBc/Hz was Long-term stability is another important characteristic of the OEO. The electrical waveachieved OEO, for a high-quality a 10 kHz offset opto-electronic from the carrier filter microwave was proposed, signal. e.g., The a Q-factor microwave of 30,000 signal at can a bandpass filter and the optical fiber are major components of the OEO that are tempera- centralbe tuned frequency between of 1.85 29.99 GHz GHz and [73 10.24]. This GHz resulted thanks in to an a 83tunable dB SMSR optical and bandpass a −113 dBc/Hz filter. ture-dependent [75]. For the frequency drift of a non-temperature-stabilized OEO operat- SSB phase noise at a 10 kHz offset. A cascaded microwave filter was presented in [49]. ing at a 10 GHz central frequency, 8 ppm/K was measured [75]. One of the useful ap- In this configuration, the cascaded filter configuration was implemented with a single passbandproaches filteris the having temperature an opto-electronic stabilization filter.of the With optical this fiber approach, and the an electrical SMSR of bandpass 125 dB filter of the OEO loop [75]. With this solution, 0.1 ppm/K was achieved. In 2016, Luka Bogataj et al. brought another approach, i.e., the OEO with a feedback control loop [24,76]. The configuration of the feedback control loop is shown in Figure 6.

Figure 6. OEO with a feedback-control loop

In the feedback control loop, the frequency discriminator controls the temperature of the laser by measuring the refractive index of the optical fiber. A proportional–integral (PI) controller is used to control the temperature of the laser. Using the feedback control

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and an SSB phase noise of −103 dBc/Hz for a 10 kHz offset at the 17.33 GHz carrier were achieved. The linewidth of the laser can affect the phase-noise performance of the OEO. A narrowband microcavity laser with a physical side length of 16 µm is used for the microwave signal generation in the loop of the OEO [74]. An SSB phase noise of −116 dBc/Hz was achieved for a 10 kHz offset from the carrier microwave signal. The

microwave signal can be tuned between 1.85 GHz and 10.24 GHz thanks to a tunable opticalFigure bandpass5. Typical configuration filter. of a coupled OEO.

2.2.2.2. Progress Progress of of the the OEO OEO toward Toward Better Better Long-Term Long-Term Stability Stability Long-termLong-term stabilitystability isis anotheranother importantimportant characteristiccharacteristic ofof thethe OEO. OEO. The The electrical electrical bandpassbandpass filter filter and and the the optical optical fiber fiber are are major majo componentsr components of the of OEOthe OEO that that aretemperature- are tempera- dependentture-dependent [75]. For [75]. the For frequency the frequency drift of drift a non-temperature-stabilized of a non-temperature-stabilized OEO operating OEO operat- at a 10ing GHz at a central 10 GHz frequency, central 8frequency, ppm/K was 8 ppm/K measured was [75 measured]. One of the [75]. useful One approaches of the useful is the ap- temperatureproaches is stabilizationthe temperature of the stabilization optical fiber of and the theoptical electrical fiber bandpassand the electrical filter of thebandpass OEO loopfilter [75 of]. the With OEO this loop solution, [75]. 0.1 With ppm/K this wassolution, achieved. 0.1 ppm/K In 2016, was Luka achieved. Bogataj etIn al. 2016, brought Luka anotherBogataj approach,et al. brought i.e., another the OEO approach, with a feedback i.e., the control OEO with loop a [feedback24,76]. The control configuration loop [24,76]. of theThe feedback configuration control of loop the feedback is shown incontrol Figure loop6. is shown in Figure 6.

FigureFigure 6. 6.OEO OEO with with a a feedback-control feedback-control loop. loop

InIn the the feedback feedback control control loop, loop, the the frequency frequency discriminator discriminator controls controls the the temperature temperature of of thethe laser laser by by measuring measuring the the refractive refractive index index of of the the optical optical fiber. fiber. A A proportional–integral proportional–integral (PI)(PI) controller controller is is used used to to control control the the temperature temperature of of the the laser. laser. Using Using the the feedback feedback control control loop, a frequency drift of 0.05 ppm/K was achieved for a single-loop OEO operating at 3 GHz. In 2017, the optical delay stabilization system (ODSS) was introduced [77] for active fiber delay stabilization at a different wavelength than used in the oscillator loop. The outstanding result of a 0.02 ppm/K frequency drift was achieved for a 3 GHz OEO. However, the frequency of the oscillator can be increased, but this does not affect the stability result because the stabilization is performed at an independent wavelength. Phase-locked loops (PLLs) are alternative solutions that are widely used in practice to improve the long-term stability of the OEO signal. In this case, the OEO signal is locked by the PLL signal [78]. The typical configuration of an OEO with a PLL is shown in Figure7. For an OEO with a PLL configuration, a stable reference signal is required to improve the long-term stability. Wen-Hung Tseng proposed another approach to improving the long-term stability that involves a fiber-delay monitoring mechanism [79]. This mechanism monitors the fiber delay using an injected probe signal. A thermal drift of 10−7 s was achieved after 4000 seconds with the monitoring mechanism. Electronics 2021, 10, x FOR PEER REVIEW 7 of 19

loop, a frequency drift of 0.05 ppm/K was achieved for a single-loop OEO operating at 3 GHz. In 2017, the optical delay stabilization system (ODSS) was introduced [77] for active fiber delay stabilization at a different wavelength than used in the oscillator loop. The outstanding result of a 0.02 ppm/K frequency drift was achieved for a 3 GHz OEO. How- ever, the frequency of the oscillator can be increased, but this does not affect the stability result because the stabilization is performed at an independent wavelength. Phase-locked loops (PLLs) are alternative solutions that are widely used in practice to improve the long-term stability of the OEO signal. In this case, the OEO signal is locked Electronics 2021, 10, 857 7 of 19 by the PLL signal [78]. The typical configuration of an OEO with a PLL is shown in Figure 7.

FigureFigure 7. 7.Typical Typical configuration configuration of anof OEOan OEO with with a PLL a PLL configuration. configuration. 2.3. General Overview For an OEO with a PLL configuration, a stable reference signal is required to improve the Inlong-term this section stability. of the paper,Wen-Hung we would Tseng like proposed to compare another the performance approach to of improving different the configurationslong-term stability of the OEOthat involves in multimode a fiber-delay operation: monitoring short-term stabilitymechanism (i.e., [79]. phase This noise) mecha- and long-term stability. In the first table, different configurations of the OEO are described nism monitors the fiber delay using an injected probe signal. A thermal drift of 10−7 s was by comparing the SMSR and the phase noise. achieved after 4000 seconds with the monitoring mechanism. For Table1, 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 comparison2.3. General of Overview the phase noise in different solutions. However, in theory, the OEO has a stable In phase-noisethis section characteristicof the paper, thatwe would is independent like to compare of the operatingthe performance frequency of different [1], soconfigurations higher frequencies of the canOEO be in used multimode for the comparison.operation: short-term In the SMSR stability comparison, (i.e., phase the noise) opticaland long-term delay line stability. length and In the bandwidthfirst table, different of the electrical configurations and/or optical of the filterOEO are are more described importantby comparing for the the comparison. SMSR and the When phase considering noise. the phase noise, the injection-locked OEO achieves For Table the 1, best OEOs performance with the same among or thesimilar other frequency solutions. were On the selected other hand,(except the for the cascadedOEO with micro-wave a high quality photonic opto-electronic filter solution achieved filter) to a betterallow resulta more in termsaccurate of the and SMSR. scientific comparison of the phase noise in different solutions. However, in theory, the OEO has a Table 1. Comparison of the SMSR and the phase noise of the current results for various advanced configurations of the OEO. stable phase-noise characteristic that is independent of the operating frequency [1], so higherOptical frequencies Delay Line can be used for the comparison. In the SMSRPhase comparison, Noise (@10 kHzthe optical Configuration Central Frequency SMSR delay Lengthline length and the bandwidth of the electrical and/or offsetoptical from filter the carrier)are more im- portant7-core for fiber the comparison. When considering the phase noise, the injection-locked OEO Dual-loop OEO [64] From 3.5 GHz to 17.1 GHz 61 dB −100 dBc/Hz achieves(105 the m) best performance among the other solutions. On the other hand, the cascaded micro-waveSingle-mode photonic fibers filter solution achieved a better result in terms of the SMSR. Injection-locked OEO [68] 10 GHz N/A −130 dBc/Hz (1 km and 0.7 km) Erbium-doped fiber Coupled OEO [72] 10 GHz 72.5 dB −123.6 dBc/Hz (4 m) OEO with high-quality Dispersion-shifted 29.99 GHz 83 dB −113 dBc/Hz opto-electronic filter [73] fiber (3 km) Cascading microwave Single-mode fibers 17.33 GHz 125 dB −103 dBc/Hz photonic filter [49] (2 km and 0.2 km) Narrowband microwave laser Single-mode fibers From 1.85 GHz to 10.24 GHz 55 dB −116 dBc/Hz with dual-loop OEO [74] (2.5 km and 3 km)

In Table2, different solutions are compared to evaluate the performance of the OEO in terms of long-term stability and phase noise. Electronics 2021, 10, 857 8 of 19

Table 2. Comparison of the different techniques to achieve long-term stability in the OEO.

Optical Delay Line Phase Noise (@10 kHz Configuration Central Frequency Long-term Stability Length offset from the carrier) Temperature stabilization [75] N/A 10 GHz 0.1 ppm/K −143 dBc/Hz OEO with feedback-control 15 km 3 GHz 0.05 ppm/K < −130 dBc/Hz loop [76] Optical delay stabilization 3 km 3 GHz 0.02 ppm/K −123 dBc/Hz system [77] 500 m 6.98 × 10−14 OEO with PLL [78] (Dispersion deduced 3 GHz < −100 dBc/Hz (average time of 1000 s) fiber)

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.

3. Recent Development of the OEO’s Application In this section we present and detail some recent advances in the application of OEOs in different fields as well as some new developments such as integrated OEOs and the wideband tunable OEO.

3.1. Wideband Tunable Frequency Generation The conventional OEO has a narrowband tunability, which is a few MHz due to the bandwidth limitation of the electrical bandpass filter. To improve the tunability of the OEO, tunable microwave resonators or microwave photonic filters have been proposed [61]. They can provide tunability from hundreds of MHz up to tens of GHz. In 2010, W. Li published the first research paper for a wideband tunable OEO [80]. In 2017, a wideband tunable frequency generator based on a dispersion compensation fiber OEO was proposed that had a tuning range of 3–42 GHz [81]. The SSB phase noise was lower than −110 dBc/Hz at a 10 kHz offset from the carrier for the whole tuning range (span of about 39 GHz). A microwave photonic filter was used for frequency tuning, while the dispersion compensa- tion fiber was used to compensate for the chromatic dispersion of the single-mode fiber. In 2019, a wideband frequency generator based on frequency division without a bandpass filter, but biasing the Mach–Zehnder Modulator (MZM) at the minimum transmission point, was introduced [82]. The wideband signal from 6 GHz to 10 GHz (or 10 GHz to 18 GHz) was obtained depending on the input signal. A phase noise improvement of 5.71 dB was achieved at the output signal [82]. A micro-ring resonator (MRR) is another possible solution to bring wideband tunability to the OEO configuration [83]. Wideband frequency generation between 0 and 20 GHz was achieved using an OEO with a MRR, wherein the SSB phase noise of −95 dBc/Hz was measured at a 10 kHz offset from the 12.23 GHz carrier frequency [83]. The configuration of the OEO with the MRR is shown in Figure8. Electronics 2021, 10, x FOR PEER REVIEW 9 of 19

proposed that had a tuning range of 3–42 GHz [81]. The SSB phase noise was lower than −110 dBc/Hz at a 10 kHz offset from the carrier for the whole tuning range (span of about 39 GHz). A microwave photonic filter was used for frequency tuning, while the dispersion compensation fiber was used to compensate for the chromatic dispersion of the single- mode fiber. In 2019, a wideband frequency generator based on frequency division without a bandpass filter, but biasing the Mach–Zehnder Modulator (MZM) at the minimum transmission point, was introduced [82]. The wideband signal from 6 GHz to 10 GHz (or 10 GHz to 18 GHz) was obtained depending on the input signal. A phase noise improve- ment of 5.71 dB was achieved at the output signal [82]. A micro-ring resonator (MRR) is another possible solution to bring wideband tunability to the OEO configuration [83]. Wideband frequency generation between 0 and 20 GHz was achieved using an OEO with

Electronics 2021, 10, 857 a MRR, wherein the SSB phase noise of −95 dBc/Hz was measured at a 10 kHz9 offset of 19 from the 12.23 GHz carrier frequency [83]. The configuration of the OEO with the MRR is shown in Figure 8.

FigureFigure 8. 8.Simple Simple configuration configuration of anof OEOan OEO with wi ath micro-ring a micro-ring resonator resonator (MRR). (MRR) 3.2. Photonic Integrated Chip 3.2. Photonic Integrated Chip The traditional OEO uses an optical fiber to form the optical delay line. This makes the OEOThe very traditional large and OEO in some uses cases, an optical bulky fiber because to form it contains the optical additional delay components line. This makes forthe the OEO improvement very large of and short-term in some and cases, long-term bulky stabilization,because it contains multimode additional operation, components etc. (anfor additionalthe improvement delay line, of additionalshort-term optical and long-t and/orerm electrical stabilization, devices, multimode etc.). To reduce operation, the etc. size(an ofadditional the OEO, delay a compact line, additi OEO wasonal proposed. optical and/or In the electrical compact OEO,devices, the etc.). optical To fiber reduce the wassize replacedof the OEO, by a a whispering-gallery-mode compact OEO was proposed. resonator In the (WGMR) compact to reduce OEO, the sizeoptical of the fiber was OEOreplaced [31–33 by]. a whispering-gallery-mode resonator (WGMR) to reduce the size of the OEO The fully integrated OEO based on an InP substrate was first presented by J. Tang et al. [31–33]. at the Microwave Photonics Conference 2017 [38,84]. All the components were assembled in a photonic The fully integrated integrated chip OEO (PIC). based An optical on an waveguide InP substrate in a spiralwas first shape presented was used by as anJ. Tang et opticalal. at the resonator. Microwave When Photonics a microwave Conference frequency 2017 was [38,84]. generated All withthe components such an integrated were assem- OEO,bled in one a ofphotonic the main integrated handicaps chip was (PIC). the length An optical of the delay waveguide line. Due in to a thespiral small shape size was of used theas an spiral-shaped optical resonator. delay line, When which a microwave was a few centimeters, frequency thewas Q-factor generated of the with OEO such was an inte- verygrated low, OEO, and thus one the of phasethe main noise handicaps characteristics was werethe length worse thanof the those delay of a line. non-integrated Due to the small Electronics 2021, 10, x FOR PEER REVIEWOEO. size of Therefore, the spiral-shaped for the integrated delay line, OEO, which an SSB wa phases a few noise centimeters, of −91 dBc/Hz the Q-factor10 atof 19 a 1-MHz of the OEO

offsetwas very from low, the 7.3 and GHz thus carrier the phase was obtained noise char [38].acteristics The integrated were OEO worse and than its tunability those of a non- andintegrated short-term OEO. performance Therefore, are for given the inintegrated Figure9. OEO, an SSB phase noise of −91 dBc/Hz at a 1-MHz offset from the 7.3 GHz carrier was obtained [38]. The integrated OEO and its tunability and short-term performance are given in Figure 9.

FigureFigure 9. The 9. Theintegrated integrated OEO. OEO. The frequency The frequency tunability tunability of the ofOEO the is OEO around is around 20 MHz, 20 MHz,with a withcen- a central tral oscillation frequency of 7.3 GHz [38]. Reprinted with permission from Reference [38]. Copy- oscillation frequency of 7.3 GHz [38]. Reprinted with permission from Reference [38]. Copyright right 2017 IEEE. 2017 IEEE. In addition to the integrated OEO on the InP substrate, an integrated OEO based on a silicon substrate was introduced to the literature by W. Zhang and J. Yao in 2018 [85]. A high-speed phase modulator, a tunable micro-disk resonator and a high-speed photode- tector were integrated on the silicon-substrate chip. The proposed silicon photonic OEO was tunable in the frequency range between 3 GHz and 8 GHz. The measured SSB phase noise was –81 dBc/Hz at a 10 kHz offset from the 4.74 GHz RF carrier signal. On the other hand, a photonic and electronic hybrid OEO was presented in 2019 [86]. In this configu- ration, the OEO consisted of photonic and electronic chips that were fabricated separately. An SSB phase noise of −103 dBc/Hz at a 100 kHz offset from the carrier was measured. As can be clearly seen from the SSB phase noise measurements of the integrated OEO [83–86], although the integrated OEOs provide the opportunity to reduce the size and enable mass production of the OEO in the chip, they unfortunately have poor short-term stability in the microwave range. However, due to their small size, the long-term stability of the OEO can be easily improved by temperature stabilization of the chip.

3.3. Optical Signal Distribution for the 5G Radio Access Network The next-generation 5G radio access network (RAN) has certain requirements to im- prove bit rate and spectral efficiency [87]. It is expected that millimeter wave bands will be used to meet these requirements [88]. The current 4G-RAN technology uses an electri- cal local oscillator (LO) in each base station to perform the frequency up-conversion and down-conversion of the data signal. Unfortunately, the phase-noise performance is de- graded when they are used for the millimeter-wave signal range. The centralized oscilla- tor’s signal distribution was reported in the literature from several sources [89–92]. For example, in [93], we proposed the use of an OEO as a LO in a 5G RAN fronthaul. The centralized oscillator signal was distributed to the base stations without the need for elec- tro-optical and opto-electronic conversion. Radio over fiber [94–96] is proposed as an ef- ficient and sophisticated solution to distribute the centralized OEO signal. The idea is shown in Figure 10. The OEO signal is distributed from the central office to the base sta- tion through an optical distribution network (ODN). The data signals and the OEO signal can be transmitted over the same fiber using a dense wavelength-division-multiplexing (DWDM) approach. This approach has the advantage of keeping the ODN infrastructure

Electronics 2021, 10, 857 10 of 19

In addition to the integrated OEO on the InP substrate, an integrated OEO based on a silicon substrate was introduced to the literature by W. Zhang and J. Yao in 2018 [85]. A high- speed phase modulator, a tunable micro-disk resonator and a high-speed photodetector were integrated on the silicon-substrate chip. The proposed silicon photonic OEO was tunable in the frequency range between 3 GHz and 8 GHz. The measured SSB phase noise was –81 dBc/Hz at a 10 kHz offset from the 4.74 GHz RF carrier signal. On the other hand, a photonic and electronic hybrid OEO was presented in 2019 [86]. In this configuration, the OEO consisted of photonic and electronic chips that were fabricated separately. An SSB phase noise of −103 dBc/Hz at a 100 kHz offset from the carrier was measured. As can be clearly seen from the SSB phase noise measurements of the integrated OEO [84–86], although the integrated OEOs provide the opportunity to reduce the size and enable mass production of the OEO in the chip, they unfortunately have poor short-term stability in the microwave range. However, due to their small size, the long-term stability of the OEO can be easily improved by temperature stabilization of the chip.

3.3. Optical Signal Distribution for the 5G Radio Access Network The next-generation 5G radio access network (RAN) has certain requirements to improve bit rate and spectral efficiency [87]. It is expected that millimeter wave bands will be used to meet these requirements [88]. The current 4G-RAN technology uses an electrical local oscillator (LO) in each base station to perform the frequency up-conversion and down-conversion of the data signal. Unfortunately, the phase-noise performance is degraded when they are used for the millimeter-wave signal range. The centralized oscillator’s signal distribution was reported in the literature from several sources [89–92]. For example, in [93], we proposed the use of an OEO as a LO in a 5G RAN fronthaul. The centralized oscillator signal was distributed to the base stations without the need for electro-optical and opto-electronic conversion. Radio over fiber [94–96] is proposed as an efficient and sophisticated solution to distribute the centralized OEO signal. The idea is Electronics 2021, 10, x FOR PEER REVIEW 11 of 19 shown in Figure 10. The OEO signal is distributed from the central office to the base station through an optical distribution network (ODN). The data signals and the OEO signal can be transmitted over the same fiber using a dense wavelength-division-multiplexing (DWDM) simple,approach. as well This as approach the base has station, the advantage because the of keepingLO is no thelonger ODN required. infrastructure Since the simple, LOs wouldas well need as the to basebe temperature station, because stabilized the LO to prov is noide longer a stable required. clock signal, Since theit is LOs more would cost- effectiveneed to be to temperatureonly develop stabilized one well-stabilized to provide aoscillator stable clock in the signal, central itis office more and cost-effective distribute itsto onlysignal develop to the onebase well-stabilized stations. In addition, oscillator the in proposal the central has office the advantage and distribute that itsthe signal total numberto the base of oscillators stations. In in addition,the whole the fronthaul proposal system has the is reduced. advantage This that would the total have number a positive of impactoscillators on inthe the energy whole savings fronthaul of the system complex is reduced. system This if the would number have of a positiveoscillators impact was re- on duced.the energy savings of the complex system if the number of oscillators was reduced.

Figure 10. UseUse of of an an OEO OEO as as a local a local oscillator oscillator (LO) (LO) for fora 5G a radi 5G radioo access access network network (RAN) (RAN) fronthaul. fronthaul. Reprinted Reprinted from Refer- from ence [93]. Reference [93]. On the other hand, the optical signal distribution of the OEO via an optical fiber pre- sents some challenges. One of the main challenges is the power penalty due to chromatic dispersion [97]. The C band is widely used in optical communications because of its ad- vantages such as a low optical loss (0.2 dB/km), but chromatic dispersion is very dominant in this band, ranging between approximately 16 and 17 ps/nm/km. Intensity-modulated links are negatively affected by the power penalty due to chromatic dispersion. Therefore, we proposed to use the tunable dispersion-compensation module (TDCM) in each base- station [93], which is shown in Figure 10. The TDCM is a device based on Bragg gratings that provides a way to compensate for the chromatic dispersion in the C band with respect to the actual fiber length. To test the performance of the optical signal distribution, we employed the experimental setup shown in Figure 11.

Figure 11. Experimental setup for dispersion penalty measurements with and without tunable dispersion-compensation module (TDCM).

For the experiment, we set up an optical delay line in combination with a vector net- work analyzer (VNA). This configuration provided the ability to apply a wideband RF signal to the optical delay line to track the full spectrum of the dispersion penalty. When the OEO was used instead of the described experimental setup, we only had the oppor- tunity to investigate a single frequency. Therefore, we could obtain limited experimental

Electronics 2021, 10, x FOR PEER REVIEW 11 of 19

simple, as well as the base station, because the LO is no longer required. Since the LOs would need to be temperature stabilized to provide a stable clock signal, it is more cost- effective to only develop one well-stabilized oscillator in the central office and distribute its signal to the base stations. In addition, the proposal has the advantage that the total number of oscillators in the whole fronthaul system is reduced. This would have a positive impact on the energy savings of the complex system if the number of oscillators was re- duced.

Figure 10. Use of an OEO as a local oscillator (LO) for a 5G radio access network (RAN) fronthaul. Reprinted from Refer- Electronicsence [93].2021, 10, 857 11 of 19

On the other hand, the optical signal distribution of the OEO via an optical fiber pre- sentsOn some the otherchallenges. hand, theOne optical of the signalmain distributionchallenges is of the the power OEO via penalty an optical due fiberto chromatic presentsdispersion some [97]. challenges. The C band One of is the widely main challenges used in optical is the power communications penalty due to because chromatic of its ad- dispersionvantages such [97]. as The a low C band optical is widelyloss (0.2 used dB/k inm), optical but chromatic communications dispersi becauseon is very of its dominant advantagesin this band, such ranging as a low between optical approximatel loss (0.2 dB/km),y 16 and but chromatic17 ps/nm/km. dispersion Intensity-modulated is very dominant in this band, ranging between approximately 16 and 17 ps/nm/km. Intensity- links are negatively affected by the power penalty due to chromatic dispersion. Therefore, modulated links are negatively affected by the power penalty due to chromatic dispersion. Therefore,we proposed we proposed to use the to tunable use the tunable dispersion-compensation dispersion-compensation module module (TDCM) (TDCM) in each in base- eachstation base-station [93], which [93], is which shown is shownin Figure in Figure 10. The 10. TDCM The TDCM is a isdevice a device based based on on Bragg Bragg gratings gratingsthat provides that provides a way ato way compensate to compensate for the for thechro chromaticmatic dispersion dispersion in in the the C C band withwith respect respectto the actual to the actual fiber fiberlength. length. To Totest test the the performance performance ofof thethe optical optical signal signal distribution, distribution, we weemployed employed the the experimental experimental setup setup shownshown in in Figure Figure 11 11..

FigureFigure 11.11. ExperimentalExperimental setup setup for for dispersion dispersion penalty penalt measurementsy measurements with with and and without without tunable tunable dispersion-compensationdispersion-compensation module module (TDCM). (TDCM).

ForFor the the experiment, experiment, we we set set up up an an optical optical delay delay line line in combinationin combination with with a vector a vector net- network analyzer (VNA). This configuration provided the ability to apply a wideband RF Electronics 2021, 10, x FOR PEER REVIEWwork analyzer (VNA). This configuration provided the ability to apply a12 wideband of 19 RF signal to the optical delay line to track the full spectrum of the dispersion penalty. When the OEOsignal was to usedthe optical instead delay of the line described to track experimental the full spectrum setup, we of only the had dispersion the opportunity penalty. When tothe investigate OEO was a used single instead frequency. of the Therefore, described we couldexperimental obtain limited setup, experimental we only had data the oppor- datarelevanttunity relevant to to investigate the to dispersion the dispersion a single penalty. penalty. frequency. The VNAThe VNA providedTherefore, provided the we RF the could signal RF signal obtain with awith rangelimited a range of aboutexperimental of about45 GHz. 45 GHz. The resultsThe results for thefor powerthe power penalty penalty with with and and without without the the TDCM TDCM are are shown shown in inFigure Figure 12 12..

FigureFigure 12. 12. Power-penaltyPower-penalty measurements measurements from from 10 MHz 10 MHz to 45 toGHz 45 over GHz a over20-km a optical 20-km opticalfiber. Re- fiber. printedReprinted from from Reference Reference [93]. [93 ].

The results show that the TDCM can be used in any base station to compensate for chromatic dispersion in the C band. In other words, to avoid the power penalty of the OEO signal distribution over the ODN, the TDCM can be used at each base station. The only challenge associated with the proposal is that the TDCM requires knowledge of the exact optical length. This challenge can be solved by using a proportional-integral-deriv- ative (PID) controller so that the TDCM can be automatically tuned by comparing the maximum RF power seen in the RF spectrum. On the other hand, there are other possible options, such as using the 1310-nm wavelength or special fibers such as a dispersion- shifted fiber (DSF). However, the use of the 1310-nm wavelength has some disadvantages such as larger optical losses compared to the C band, and the optical nonlinearities are larger in this band. In addition, DSF requires modification of the infrastructure, which might not be a cost-effective and feasible approach for a RAN.

3.4. Application for SMSR, FSR and Phase Noise Measurements As mentioned several times in the text, one of the main challenges associated with the OEO is multimode operation. There are some analytical approaches to calculating the SMSR [98,99]. These analytical approaches are challenging and require some system val- ues. We have developed an application of the OEO based on the single-loop OEO with an optical fiber path selector published in 2020 [100]. The application allows rapid measure- ments of the SMSR, FSR and phase noise of the OEO for different optical lengths. In addi- tion, various electrical bandpass filters can be tested to find the optimum one for the OEO. The application is shown in Figure 13.

Electronics 2021, 10, 857 12 of 19

The results show that the TDCM can be used in any base station to compensate for chromatic dispersion in the C band. In other words, to avoid the power penalty of the OEO signal distribution over the ODN, the TDCM can be used at each base station. The only challenge associated with the proposal is that the TDCM requires knowledge of the exact optical length. This challenge can be solved by using a proportional-integral-derivative (PID) controller so that the TDCM can be automatically tuned by comparing the maximum RF power seen in the RF spectrum. On the other hand, there are other possible options, such as using the 1310-nm wavelength or special fibers such as a dispersion-shifted fiber (DSF). However, the use of the 1310-nm wavelength has some disadvantages such as larger optical losses compared to the C band, and the optical nonlinearities are larger in this band. In addition, DSF requires modification of the infrastructure, which might not be a cost-effective and feasible approach for a RAN.

3.4. Application for SMSR, FSR and Phase Noise Measurements As mentioned several times in the text, one of the main challenges associated with the OEO is multimode operation. There are some analytical approaches to calculating the SMSR [98,99]. These analytical approaches are challenging and require some system values. We have developed an application of the OEO based on the single-loop OEO with an optical fiber path selector published in 2020 [100]. The application allows rapid measurements of the SMSR, FSR and phase noise of the OEO for different optical lengths. Electronics 2021, 10, x FOR PEER REVIEW 13 of 19 In addition, various electrical bandpass filters can be tested to find the optimum one for the OEO. The application is shown in Figure 13.

FigureFigure 13. 13. OEOOEO application application to to measure thethe phase phase noise, noise, SMSR SMSR and and free free spectral spectral range range (FSR) of (FSR) the electrical of the electrical bandpass bandpass filters filtersat different at different optical optical fiber lengths.fiber lengths. Reprinted Reprinted from Reference from Reference [100]. [100].

InIn the the configuration configuration shownshown inin Figure Figure 13 13,, a a spectrum spectrum analyzer analyzer (SA) (SA) is usedis used to verifyto verify that the Barkhausan conditions are satisfied, and a signal source analyzer (SSA) is operated that the Barkhausan conditions are satisfied, and a signal source analyzer (SSA) is oper- to measure the phase noise, FSR and SMSR. The proposed application has the advantage ated to measure the phase noise, FSR and SMSR. The proposed application has the ad- of verifying, measuring and testing the various electrical bandpass filters in different vantageoptical delayof verifying, lines to measuring find the optimum and testing one. the In this various application, electrical the bandpass optical lengths filters arein dif- ferentchanged optical without delay the lines requirement to find the to exchange optimum the one. optical In this connectors application, due to the the optical optical pathlengths areselector’s changed capability without tothe switch requirement between to different exchange fiber the spools. optical Figure connectors 14 is a due photograph to the optical of paththe proposedselector’s application capability setup,to switch while between Table3 shows different the experimentalfiber spools. resultsFigure of14 the is OEOa photo- grapharound of 9.63the proposed GHz, where application the different setup, optical while lengths Table are 3 formed.shows the experimental results of the OEO around 9.63 GHz, where the different optical lengths are formed.

Figure 14. Photograph of the proposed application of the OEO combined with the optical fiber path selector.

Table 3. FSR, SMSR and phase noise measurements of the single-loop OEO for different optical lengths [100].

Phase Noise @1 Phase Noise @10 Fiber Length FSR SMSR kHz Offset kHz Offset (km) (kHz) (dB) (dBc/Hz) (dBc/Hz) 1.25 158.5 78.1 −73.8 −120.7 2.50 79.3 71.1 −80.6 −123.3 5.00 40.2 62.5 −100.9 −123.3 7.50 27.2 39.9 −104.7 −125.0

According to the results in Table 3, the tradeoff between the phase noise and the SMSR is clearly seen when the length of the optical delay line is alternated. This applica- tion can be used to test various electrical bandpass filters in the OEO loop to find the

Electronics 2021, 10, x FOR PEER REVIEW 13 of 19

Figure 13. OEO application to measure the phase noise, SMSR and free spectral range (FSR) of the electrical bandpass filters at different optical fiber lengths. Reprinted from Reference [100].

In the configuration shown in Figure 13, a spectrum analyzer (SA) is used to verify that the Barkhausan conditions are satisfied, and a signal source analyzer (SSA) is oper- ated to measure the phase noise, FSR and SMSR. The proposed application has the ad- vantage of verifying, measuring and testing the various electrical bandpass filters in dif- ferent optical delay lines to find the optimum one. In this application, the optical lengths are changed without the requirement to exchange the optical connectors due to the optical Electronics 2021, 10, 857 path selector’s capability to switch between different fiber spools. Figure 14 is 13a photo- of 19 graph of the proposed application setup, while Table 3 shows the experimental results of the OEO around 9.63 GHz, where the different optical lengths are formed.

FigureFigure 14. 14. PhotographPhotograph of ofthe the proposed proposed application application of of the OE OEOO combinedcombined withwith the the optical optical fiber fiber path path selector. selector.

TableTable 3. 3. FSR,FSR, SMSR SMSR and and phase phase noise noise measurements measurements of of the the single-loop single-loop OEO OEO for for different different optical optical lengthslengths [100]. [100].

Phase NoisePhase @1 Noise kHz @1Phase Phase Noise Noise @10 @10 FiberFiber Length Length FSRFSR SMSR SMSR OffsetkHz Offset kHzkHz Offset Offset (km)(km) (kHz)(kHz) (dB) (dB) (dBc/Hz)(dBc/Hz) (dBc/Hz)(dBc/Hz) 1.251.25 158.5158.5 78.1 78.1 −73.8 −73.8 −120.7−120.7 2.502.50 79.379.3 71.1 71.1 −80.6 −80.6 −123.3−123.3 5.00 40.2 62.5 −100.9 −123.3 5.007.50 27.240.2 39.9 62.5 −104.7−100.9 −125.0−123.3 7.50 27.2 39.9 −104.7 −125.0 Electronics 2021, 10, x FOR PEER REVIEW 14 of 19 According to the results in Table3, the tradeoff between the phase noise and the SMSR is clearlyAccording seen whento the the results length in of Table the optical 3, the delay tradeoff line isbetween alternated. the This phase application noise and can the SMSRbe used is clearly to test variousseen when electrical the length bandpass of the filters optical in the delay OEO loopline is to alternated. find the optimum This applica- filter tionoptimumin termscan be offilter phaseused in to noiseterms test and ofvarious phase SMSR. electricalnoise Also, theand optimumbandpass SMSR. Also, optical filters the delayin optimum the line OEO can optical beloop determined to delay find linethe canwith be thedetermined use of this with application. the use In of addition, this applic withation. the proposedIn addition, application, with the the proposed phase noise appli- cation,result the of the phase OEO noise can be result obtained of the with OEO the SSA.can be For obtained instance, with in Figure the SSA.15, the For phase instance, noise in performance of an OEO with a length of 7.5 km is presented. Figure 15, the phase noise performance of an OEO with a length of 7.5 km is presented.

FigureFigure 15. 15. Phase-noisePhase-noise performance performance of of an OEO withwith aa 7.57.5 kmkm optical optical delay delay line line length. length.

As can be clearly seen in Figure 15, −104.7 dBc/Hz and −125.0 dBc/Hz were at 1 kHz and 10 kHz offset from the 10.45 GHz carrier. In this application, the phase noise of the OEO for different optical fiber lengths could be easily measured by adjusting the optical fiber path selector.

4. Conclusions The OEO is a very well-known, high-frequency oscillator for generating low-phase microwave and millimeter wave signals and is used daily in various fields of science, technology and engineering. It has already been shown that an OEO can generate a milli- meter wave signal up to the W band. The OEO has the advantage that the oscillator’s phase noise is independent of the operating frequency. However, the OEO might require stabilization for short-term, long-term and multimode operation. Moreover, the OEO can be developed as a tool/application for various scientific and engineering fields. The wideband OEO was introduced to generate high-frequency sig- nals in the microwave and millimeter wave signal ranges. This enabled the OEO to be tunable up to GHz and above. In 2017, the manufacture of an OEO on a photonic-inte- grated chip was added to the literature. The next generation of 5G RAN requires a low- phase-noise oscillator in the millimeter wave range. Thus, we propose to implement the OEO in the central station of the 5G RAN, defined as a LO for frequency up-conversion and down-conversion. This eliminates the requirement for a LO in each base station, and the clock distribution is also conducted centrally. Moreover, due to its structure, the OEO does not require opto-electronic/electro-optical signal conversion, which can be classified as another advantage. On the other hand, the simple, single-loop OEO cannot meet the requirements of the centralized OEO for 5G RAN. Thus, an injection-locked, millimeter wave OEO with temperature-stabilized components might be employed to exhibit the best performance as a centralized LO for 5G RAN. Furthermore, by combining the optical fiber path selector with the single-loop OEO, we developed an application to measure the FSR, SMSR and phase noise of the OEO for different optical fiber lengths. This application is also practical to test different electrical bandpass filters and find the optimum perfor- mance. In the future, we would like to improve the application so that it can be used for

Electronics 2021, 10, 857 14 of 19

As can be clearly seen in Figure 15, −104.7 dBc/Hz and −125.0 dBc/Hz were at 1 kHz and 10 kHz offset from the 10.45 GHz carrier. In this application, the phase noise of the OEO for different optical fiber lengths could be easily measured by adjusting the optical fiber path selector.

4. Conclusions The OEO is a very well-known, high-frequency oscillator for generating low-phase microwave and millimeter wave signals and is used daily in various fields of science, technology and engineering. It has already been shown that an OEO can generate a millimeter wave signal up to the W band. The OEO has the advantage that the oscillator’s phase noise is independent of the operating frequency. However, the OEO might require stabilization for short-term, long-term and multimode operation. Moreover, the OEO can be developed as a tool/application for various scientific and engineering fields. The wideband OEO was introduced to generate high-frequency signals in the microwave and millimeter wave signal ranges. This enabled the OEO to be tunable up to GHz and above. In 2017, the manufacture of an OEO on a photonic- integrated chip was added to the literature. The next generation of 5G RAN requires a low-phase-noise oscillator in the millimeter wave range. Thus, we propose to implement the OEO in the central station of the 5G RAN, defined as a LO for frequency up-conversion and down-conversion. This eliminates the requirement for a LO in each base station, and the clock distribution is also conducted centrally. Moreover, due to its structure, the OEO does not require opto-electronic/electro-optical signal conversion, which can be classified as another advantage. On the other hand, the simple, single-loop OEO cannot meet the requirements of the centralized OEO for 5G RAN. Thus, an injection-locked, millimeter wave OEO with temperature-stabilized components might be employed to exhibit the best performance as a centralized LO for 5G RAN. Furthermore, by combining the optical fiber path selector with the single-loop OEO, we developed an application to measure the FSR, SMSR and phase noise of the OEO for different optical fiber lengths. This application is also practical to test different electrical bandpass filters and find the optimum performance. In the future, we would like to improve the application so that it can be used for testing/verifying/measuring different configurations of OEOs, such as injection-locked, multi-loop, etc. For injection-locked or multi-loop configurations, an additional optical fiber path selector (one or more) is required to find the optimum configuration of the OEO by comparing the SMSR and the phase noise. In the current state of the art, one of the limitations is designing a stable OEO on the photonic integrated chip. In order to enable mass production of the OEO, which would make it possible to use the OEO in different scientific areas, photonic integration might be useful. However, because of its short delay line length, the photonic integrated OEO has a poor short-term stability. A future study should focus on improving the short-term stability of the integrated OEO. A high Q-factor resonator can be implemented for the photonic integrated chip. If this is achieved, the integrated OEO can easily be combined into the 5G RAN as a centralized local oscillator. On the other hand, the single- loop OEO has its own limitations for long-term stability and multi-mode operation. A variety of different configurations are proposed to reduce the challenges. Multi-loop, injection-locked or coupled OEOs have been used for many years to improve the multimode operating characteristics of the OEO. Moreover, OEOs with a feedback control loop, PLL and temperature stabilization of the optical fiber as well as an electrical bandpass filter are used to reduce the frequency drift of the oscillator signal. Another topic for future research should be the sophisticated design of wideband, tunable, integrated and ordinary OEO operating in the millimeter wave range with acceptable phase noise performance (< −120 dBc @10 kHz offset from the carrier).

Author Contributions: Conceptualization, methodology, investigation and resources M.A.I., and B.B.; formal analysis and validation B.B.; writing—original draft preparation M.A.I.; writing—review Electronics 2021, 10, 857 15 of 19

and editing M.A.I. and B.B.; supervision, project administration and funding acquisition B.B. Both authors have read and agreed to the published version of the manuscript. Funding: The work presented in this article was created within the FiWiN5G Innovative Training Network, which has received funding from the European Union’s Horizon 2020 Research and Innovation Programme 2014–2018 under the Marie Skłodowska-Curie Action grant agreement No. 642355. The authors also acknowledge the financial support from the Slovenian Research Agency (research core funding No. P2-0246). Acknowledgments: The authors would like to express their gratitude to the company InLambda BDT d.o.o. for the research equipment and devices. In addition, we would like to thank LPKF d.o.o for filter manufacturing based on laser manufacture. Conflicts of Interest: The authors declare no conflict of interest.

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