Advanced Photonics, 2020, 2 (4): 044001, Published Online: Jul. 27, 2020
Recent advances in optoelectronic oscillators 
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Fig. 1. Selected key developments in OEOs over the past 24 years. The chronological order refers to the first date when it appeared in the literature. The concept of OEO was proposed by Yao and Maleki in 1996,2,3 where a fiber-based single-loop structure was demonstrated for the generation of single-frequency microwave signals. After that, there were the demonstrations of coupled OEO as well as optical pulse generation,7 multiloop8 OEO, resonators-based5 OEO, wideband frequency tunable OEO,9 format conversion,10 injection-locked OEO,11 frequency-doubled12 OEO, optical frequency comb generation,13 chaotic RF/microwave signal generation,14 signal detection,15 measurement,16 frequency-multiplied OEO,17 sensing,18 and OEO serving as a seed source to obtain RF/microwave waveforms.19 Recently, new mode control and selection methods based on FDML20 and PT symmetry21,22 have been proposed and demonstrated, which overcome the mode building time and mode selection problems in a traditional OEO. Integrated OEOs with compact size and low power consumption have also been demonstrated in InP23 and silicon24 platforms.
2,3" target="_self" style="display: inline;">3 where a fiber-based single-loop structure was demonstrated for the generation of single-frequency microwave signals. After that, there were the demonstrations of coupled OEO as well as optical pulse generation,7" target="_self" style="display: inline;">7 multiloop8" target="_self" style="display: inline;">8 OEO, resonators-based5" target="_self" style="display: inline;">5 OEO, wideband frequency tunable OEO,9" target="_self" style="display: inline;">9 format conversion,10" target="_self" style="display: inline;">10 injection-locked OEO,11" target="_self" style="display: inline;">11 frequency-doubled12" target="_self" style="display: inline;">12 OEO, optical frequency comb generation,13" target="_self" style="display: inline;">13 chaotic RF/microwave signal generation,14" target="_self" style="display: inline;">14 signal detection,15" target="_self" style="display: inline;">15 measurement,16" target="_self" style="display: inline;">16 frequency-multiplied OEO,17" target="_self" style="display: inline;">17 sensing,18" target="_self" style="display: inline;">18 and OEO serving as a seed source to obtain RF/microwave waveforms.19" target="_self" style="display: inline;">19 Recently, new mode control and selection methods based on FDML20" target="_self" style="display: inline;">20 and PT symmetry21" target="_self" style="display: inline;">21,22" target="_self" style="display: inline;">22 have been proposed and demonstrated, which overcome the mode building time and mode selection problems in a traditional OEO. Integrated OEOs with compact size and low power consumption have also been demonstrated in InP23" target="_self" style="display: inline;">23 and silicon24" target="_self" style="display: inline;">24 platforms." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 2. Schematic diagram of a typical single-loop OEO. It has a hybrid positive feedback loop formed with an optical path and an electrical path that is capable of producing self-sustained oscillation signals. LD, laser diode; EOM, electro-optic modulator; EDFA, erbium-doped fiber amplifier; PD, photodetector; EA, electrical amplifier.
Fig. 3. An FDML OEO. (a) A schematic illustration of the FDML OEO.20 A frequency-scanning filter, rather than a statistical one as in traditional OEOs, is incorporated into the FDML OEO cavity, and the tuning period of the filter is synchronized to the cavity round-trip time to achieve FDML operation. (b) The dynamic frequency window in the FDML OEO cavity.99 The passband of the filter changes in time. E/O, electrical-to-optical conversion; O/E, optical-to-electrical conversion. Panel (a) is reproduced from Ref. 20, licensed under a Creative Commons CC BY license. Panel (b) is reproduced from Ref. 99, © 2018 The Optical Society (OSA).
20 A frequency-scanning filter, rather than a statistical one as in traditional OEOs, is incorporated into the FDML OEO cavity, and the tuning period of the filter is synchronized to the cavity round-trip time to achieve FDML operation. (b) The dynamic frequency window in the FDML OEO cavity.99" target="_self" style="display: inline;">99 The passband of the filter changes in time. E/O, electrical-to-optical conversion; O/E, optical-to-electrical conversion. Panel (a) is reproduced from Ref. 20, licensed under a Creative Commons CC BY license. Panel (b) is reproduced from Ref. 99, © 2018 The Optical Society (OSA)." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 4. Experimental setup and generated signal of the FDML OEO.20 (a) The experimental setup. An MPF based on PM-IM conversion is used as the frequency-scanning filter in the experiment. The frequency-scanning MPF consists of a frequency-scanning LD, a PM, an optical notch filter, and a PD. (b) The spectrum of the generated X band (8 to 12 GHz) LCMW. (c) The details of the spectrum with a much smaller span. (d) The temporal waveform. (e) The instantaneous frequency–time diagram. (f) The compressed pulse by autocorrelation. Figures reproduced from Ref. 20, licensed under a Creative Commons CC BY license.
20 (a) The experimental setup. An MPF based on PM-IM conversion is used as the frequency-scanning filter in the experiment. The frequency-scanning MPF consists of a frequency-scanning LD, a PM, an optical notch filter, and a PD. (b) The spectrum of the generated X band (8 to 12 GHz) LCMW. (c) The details of the spectrum with a much smaller span. (d) The temporal waveform. (e) The instantaneous frequency–time diagram. (f) The compressed pulse by autocorrelation. Figures reproduced from Ref. 20, licensed under a Creative Commons CC BY license." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 5. Dual-chirp FDML OEO.102 (a) A schematic diagram of the dual-chirp FDML OEO. A frequency-scanning dual-passband MPF based on PM-IM conversion by the use of an optical notch filter and two LDs is incorporated into the OEO cavity. (b) The temporal waveform of the generated signal. (c) Instantaneous frequency–time diagram of the generated waveform. PC, polarization controller; VOA, variable optical attenuator; OC, optical coupler; PM, phase modulator; OSA, optical spectrum analyzer; OSC, oscilloscope; ESA, electrical spectrum analyzer. Figures reproduced with permission from Ref. 102, © 2019 OSA.
102 (a) A schematic diagram of the dual-chirp FDML OEO. A frequency-scanning dual-passband MPF based on PM-IM conversion by the use of an optical notch filter and two LDs is incorporated into the OEO cavity. (b) The temporal waveform of the generated signal. (c) Instantaneous frequency–time diagram of the generated waveform. PC, polarization controller; VOA, variable optical attenuator; OC, optical coupler; PM, phase modulator; OSA, optical spectrum analyzer; OSC, oscilloscope; ESA, electrical spectrum analyzer. Figures reproduced with permission from Ref. 102, © 2019 OSA." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 6. Microwave photonic radar based on FDML OEO.107 (a) A schematic diagram of the FDML OEO-based radar and photograph of the targets. (b) A frequency–time diagram of the generated waveform of the FDML OEO. (c) The electrical spectrum of the dechirped echo of the targets after nonlinearity compensation. (d) The calculated inverse synthetic aperture radar image of the targets. ADC, analog-to-digital converter; DSP, digital signal processor. Figures reproduced from Ref. 107, © 2020 OSA.
107 (a) A schematic diagram of the FDML OEO-based radar and photograph of the targets. (b) A frequency–time diagram of the generated waveform of the FDML OEO. (c) The electrical spectrum of the dechirped echo of the targets after nonlinearity compensation. (d) The calculated inverse synthetic aperture radar image of the targets. ADC, analog-to-digital converter; DSP, digital signal processor. Figures reproduced from Ref. 107, © 2020 OSA." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 7. Microwave photonic frequency-to-time mapping based on FDML OEO.108 (a) The principle of operation of the microwave photonic frequency-to-time mapping system. (b) The measured pulse envelopes when a single-tone microwave signal with a different frequency is injected into the FDML OEO. (c) The measured results and errors. Figures reproduced from Ref. 108, © 2018 OSA.
108 (a) The principle of operation of the microwave photonic frequency-to-time mapping system. (b) The measured pulse envelopes when a single-tone microwave signal with a different frequency is injected into the FDML OEO. (c) The measured results and errors. Figures reproduced from Ref. 108, © 2018 OSA." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 8. PT symmetric phases in OEOs.21 (a) A traditional single-loop OEO oscillates without a narrowband filter. All longitudinal modes with a positive net gain will oscillate. (b) A traditional dual-loop OEO oscillates without narrowband filters. All longitudinal modes with a positive net gain will oscillate. (c) A PT symmetric OEO with two coupled loops oscillates under the PT broken phase condition. The gain and loss of each loop are balanced. By adjusting the gain, loss, and coupling efficient of the two loops, the loss can overcompensate the gain for all longitudinal modes except the one with the highest gain. As a result, a single-mode oscillation can be established at the longitudinal mode with the highest gain, while other modes will be suppressed. DPMZM, dual-polarization Mach–Zehnder modulator. Figures reproduced from Ref. 21, licensed under a Creative Commons CC BY license.
21 (a) A traditional single-loop OEO oscillates without a narrowband filter. All longitudinal modes with a positive net gain will oscillate. (b) A traditional dual-loop OEO oscillates without narrowband filters. All longitudinal modes with a positive net gain will oscillate. (c) A PT symmetric OEO with two coupled loops oscillates under the PT broken phase condition. The gain and loss of each loop are balanced. By adjusting the gain, loss, and coupling efficient of the two loops, the loss can overcompensate the gain for all longitudinal modes except the one with the highest gain. As a result, a single-mode oscillation can be established at the longitudinal mode with the highest gain, while other modes will be suppressed. DPMZM, dual-polarization Mach–Zehnder modulator. Figures reproduced from Ref. 21, licensed under a Creative Commons CC BY license." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 9. Schematic diagram and spectra of a PT symmetric OEO using a polarization multiplexed modulator.21 (a) A block diagram of the PT symmetric OEO. (b) The multimode oscillation measured with a span of 2 GHz and an RBW of 100 kHz. (c) The single-mode oscillation measured with a span of 2 GHz and an RBW of 100 kHz. PC, polarization controller; MZM, Mach–Zehnder modulator; PR, polarization rotator; PBC, polarization beam combiner; SMF, single-mode fiber; TA, tunable attenuator; TDL, tunable delay line. Figures reproduced from Ref. 21, licensed under a Creative Commons CC BY license.
21 (a) A block diagram of the PT symmetric OEO. (b) The multimode oscillation measured with a span of 2 GHz and an RBW of 100 kHz. (c) The single-mode oscillation measured with a span of 2 GHz and an RBW of 100 kHz. PC, polarization controller; MZM, Mach–Zehnder modulator; PR, polarization rotator; PBC, polarization beam combiner; SMF, single-mode fiber; TA, tunable attenuator; TDL, tunable delay line. Figures reproduced from Ref. 21, licensed under a Creative Commons CC BY license." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 10. Schematic diagram and spectra of a PT symmetric OEO using a BPD.22 (a) A schematic digram of the PT symmetric OEO. (b) The multimode oscillation measured with a span of 100 MHz and an RBW of 3 MHz. (c) The single-mode oscillation measured with a span of 100 MHz and an RBW of 3 MHz.
22 (a) A schematic digram of the PT symmetric OEO. (b) The multimode oscillation measured with a span of 100 MHz and an RBW of 3 MHz. (c) The single-mode oscillation measured with a span of 100 MHz and an RBW of 3 MHz. Σ, microwave combiner. Figures reproduced from Ref. 22, licensed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC)." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 11. Schematic diagram and spectra of a frequency tunable PT symmetric OEO using a photonic integrated MDR.111 (a) A schematic diagram of the frequency tunable PT symmetric OEO. Frequency tuning is achieved by thermally tuning the MDR. (b) The single-mode oscillation measured with a span of 1 MHz and an RBW of 3 kHz. (c) The frequency tunability of the PT-symmetric OEO with a tuning range from about 2 to 12 GHz. Cir, circulator; PBC, polarization beam combiner; ED, electrical divider. Figures reproduced with permission from Ref. 111, © 2020 IEEE.
111 (a) A schematic diagram of the frequency tunable PT symmetric OEO. Frequency tuning is achieved by thermally tuning the MDR. (b) The single-mode oscillation measured with a span of 1 MHz and an RBW of 3 kHz. (c) The frequency tunability of the PT-symmetric OEO with a tuning range from about 2 to 12 GHz. Cir, circulator; PBC, polarization beam combiner; ED, electrical divider. Figures reproduced with permission from Ref. 111, © 2020 IEEE." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 12. InP-integrated OEO. (a) A block diagram of the InP-integrated OEO.93 (b) A photograph of the InP-integrated OEO.93 (c) A photograph of the monolithically integrated photonic parts.93 (d) The electrical spectra of the generated microwave signals.115 A frequency tuning range of around 20 MHz is achieved. (e) The measured phase noise performance of the generated signal.115 DML, directly modulated laser; ODL, optical delay line; PD, photodetector. Panels (a)–(c) are reproduced with permission from Ref. 93, © 2018 IEEE. Panels (d) and (e) are reproduced from Ref. 115, © 2018 OSA.
93 (b) A photograph of the InP-integrated OEO.93" target="_self" style="display: inline;">93 (c) A photograph of the monolithically integrated photonic parts.93" target="_self" style="display: inline;">93 (d) The electrical spectra of the generated microwave signals.115" target="_self" style="display: inline;">115 A frequency tuning range of around 20 MHz is achieved. (e) The measured phase noise performance of the generated signal.115" target="_self" style="display: inline;">115 DML, directly modulated laser; ODL, optical delay line; PD, photodetector. Panels (a)–(c) are reproduced with permission from Ref. 93, © 2018 IEEE. Panels (d) and (e) are reproduced from Ref. 115, © 2018 OSA." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 13. Silicon photonic integrated OEO formed by integration of an MPF.116 (a) A perspective view of the silicon photonic integrated OEO. Three key components of the integrated OEO, including a high-speed PM, a thermally tunable high-selectivity MDR, and a high-speed PD are integrated on a silicon photonic chip. The MPF in the integrated OEO is based on PM-IM conversion using the above three key components and an external laser. (b) The electrical spectra of the generated microwave signals. (c) The measured phase noise at 10 kHz offset frequency of the generated microwave signals at different center frequencies. Figures reproduced with permission from Ref. 116, © 2018 IEEE.
116 (a) A perspective view of the silicon photonic integrated OEO. Three key components of the integrated OEO, including a high-speed PM, a thermally tunable high-selectivity MDR, and a high-speed PD are integrated on a silicon photonic chip. The MPF in the integrated OEO is based on PM-IM conversion using the above three key components and an external laser. (b) The electrical spectra of the generated microwave signals. (c) The measured phase noise at 10 kHz offset frequency of the generated microwave signals at different center frequencies. Figures reproduced with permission from Ref. 116, © 2018 IEEE." class="imgSplash img-thumbnail" style="cursor:pointer;">Fig. 14. Silicon-integrated OEO formed by hybrid-integration of a photonic chip and an electronic chip.117 (a) Schematic diagram. All photonic and electronic building blocks of an OEO are integrated, except for the laser. (b) Microphotograph of the hybrid-integrated optoelectronic feedback loop. (c) The measured spectrum of the integrated OEO when its frequency is locked to a reference RF source. (d) The measured phase noise performance of the generated microwave signal. TIA, transimpedance amplifier. Figures reproduced from Ref. 117, © 2019 OSA.
117 (a) Schematic diagram. All photonic and electronic building blocks of an OEO are integrated, except for the laser. (b) Microphotograph of the hybrid-integrated optoelectronic feedback loop. (c) The measured spectrum of the integrated OEO when its frequency is locked to a reference RF source. (d) The measured phase noise performance of the generated microwave signal. TIA, transimpedance amplifier. Figures reproduced from Ref. 117, © 2019 OSA." class="imgSplash img-thumbnail" style="cursor:pointer;">Tengfei Hao, Yanzhong Liu, Jian Tang, Qizhuang Cen, Wei Li, Ninghua Zhu, Yitang Dai, José Capmany, Jianping Yao, Ming Li. Recent advances in optoelectronic oscillators[J]. Advanced Photonics, 2020, 2(4): 044001.
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