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All Optical Stabilizations of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Based on Asymmetric Dual-Loop Optical Feedback Configurations

All Optical Stabilizations of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Based... hv photonics Communication All Optical Stabilizations of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Based on Asymmetric Dual-Loop Optical Feedback Configurations 1 2 3 , Tahani A. Alrebdi , Mamoon Asghar and Haroon Asghar * Department of Physics, College of Science, Princess Nourah Bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia; taalrebdi@pnu.edu.sa Department of Physics, University of Gujrat, Hafiz Hayat Campus, Gujrat 50700, Pakistan; mamoonasghar319@gmail.com National Centre for Physics, Quaid-i-Azam University Campus, Islamabad 45320, Pakistan * Correspondence: haroon.asghar@ncp.edu.pk Abstract: We report feedback-induced frequency oscillations using a power-split-ratio through asymmetric dual-loop optical feedback (Loop I: ~2.2 km and Loop II: ~20 m) subject to a self- mode-locked two-section QDash laser emitting at 1550 nm and operating at 21 GHz repetition rate. To assess the suppression of frequency resonances, three chosen combinations of feedback power (Loop I: 27.27 dB and Loop II: 19.74 dB, Loop I: 22 dB and Loop II: 22 dB, and Loop I: 19.74 dB and Loop II: 27.27 dB) through asymmetric dual-loop optical feedback have been studied. Based on the chosen coupling strength, an optimum feedback ratio that yields better side-mode suppression has been identified. Our results demonstrate that side-mode suppression can be achieved by the fine adjustment of coupling power through either cavity of dual-loop feedback configurations. Furthermore, we have further demonstrated that frequency fluctuations from the RF spectra can be filtered by carefully selecting the delay phase of the second cavity. Our experimental findings suggest that semiconductor mode-locked lasers based on dual-loop feedback configurations Citation: Alrebdi, T.A.; Asghar, M.; can be used to develop noise oscillations free from integrated photonic oscillators for potential Asghar, H. All Optical Stabilizations applications in telecommunications, multiplexing, and frequency-comb generation. of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Keywords: semiconductor lasers; optical feedback; mode-locked lasers; power-split ratio; frequency- Based on Asymmetric Dual-Loop fluctuations Optical Feedback Configurations. Photonics 2022, 9, 376. https:// doi.org/10.3390/photonics9060376 1. Introduction Received: 21 April 2022 Accepted: 23 May 2022 Mode-locked lasers (SMLLs) are potential candidates for key applications in many Published: 26 May 2022 fields of optical sampling [1], frequency comb generation [2–4], data center networks [5], clock recovery [6,7], telecommunications [8–15], and spectroscopy [16]. Some ideal features Publisher’s Note: MDPI stays neutral of SMLLs include compactness, low fabrication costs, low threshold current, low amplified with regard to jurisdictional claims in spontaneous emission fast carrier dynamics, and an inhomogeneous wide spectrum [17]. published maps and institutional affil- iations. To enhance the improvement and for better applications of SMLLs in telecommunications, high timing stability is paramount. The timing stability of SMLLs can be improved using the optical fiber cavity [18], which reduces the RF linewidth and corresponding integrated timing jitter of the optical pulse train. The optical fiber loop in the feedback cavity can be Copyright: © 2022 by the authors. considered as the energy storage component, with Q-factor being determined by the length Licensee MDPI, Basel, Switzerland. of the resonator [19]. For single-loop optical feedback, there is an increase in the system This article is an open access article memory, which directly corresponds with the reduction in the timing jitter [20–26]. With distributed under the terms and improved timing jitter, single-loop optical feedback produces additional cavity sidebands conditions of the Creative Commons around the main frequency in the power spectrum [20–27]. Recently, we have proposed Attribution (CC BY) license (https:// the asymmetric dual-loop optical feedback scheme with equal feedback ratio through creativecommons.org/licenses/by/ each loop, to suppress the first sideband around the fundamental frequency; however, the 4.0/). Photonics 2022, 9, 376. https://doi.org/10.3390/photonics9060376 https://www.mdpi.com/journal/photonics Photonics 2022, 9, x FOR PEER REVIEW 2 of 9 equal feedback ratio through each loop, to suppress the first sideband around the funda- Photonics 2022, 9, 376 2 of 9 mental frequency; however, the second appears unsuppressed (modal overlap) [22]. To eliminate these cavity sidebands and modal overlap, we reported how to optimize bal- anced asymmetric dual-loop optical feedback by varying the second feedback cavity second appears unsuppressed (modal overlap) [22]. To eliminate these cavity sidebands length [23]. and modal overlap, we reported how to optimize balanced asymmetric dual-loop optical In this paper, we report the effect of unequal coupling power through asymmetric feedback by varying the second feedback cavity length [23]. dual-loop optical feedback on the adverse dynamical effects induced due to longer feed- In this paper, we report the effect of unequal coupling power through asymmetric back cavities. We identify the optimum conditions for coupling power and the second dual-loop optical feedback on the adverse dynamical effects induced due to longer feedback feedback loop length, which yields much better suppression in external cavity side modes. cavities. We identify the optimum conditions for coupling power and the second feedback These findings reveal that optimized asymmetric dual-loop feedback is a robust and po- loop length, which yields much better suppression in external cavity side modes. These tential source for overcoming the disadvantages of frequency fluctuations in mode-locked findings reveal that optimized asymmetric dual-loop feedback is a robust and potential QDash lasers, which limits the applications of semiconductor MLLs. source for overcoming the disadvantages of frequency fluctuations in mode-locked QDash lasers, which limits the applications of semiconductor MLLs. 2. Experimental Setup The device under test is an InAs/InP SML QDash laser, and details about the device 2. Experimental Setup are given in [24]. The schematic of the experimental setup is depicted in Figure 1. The The device under test is an InAs/InP SML QDash laser, and details about the device are emission from the device under test (QDash MLL) was collected using lensed fiber and given in [24]. The schematic of the experimental setup is depicted in Figure 1. The emission then fed into an optical circulator through Port 2. Port 3 collects light, and then the semi- from the device under test (QDash MLL) was collected using lensed fiber and then fed into conductor optical amplifier amplifies the optical signal. The amplified light was then an optical circulator through Port 2. Port 3 collects light, and then the semiconductor optical equally divided into two parts through a 3-dB coupler. One part of optical light is used to amplifier amplifies the optical signal. The amplified light was then equally divided into analyze the optical spectra and the other part for electrical spectra. The other part of the two parts through a 3-dB coupler. One part of optical light is used to analyze the optical light was fed into the experimental arrangements. Each feedback loop consisted of a var- spectra and the other part for electrical spectra. The other part of the light was fed into the iable optical attenuator, a polarization controller, and an optical delay line. Loop I and experimental arrangements. Each feedback loop consisted of a variable optical attenuator, Loop II consisted of fiber spool of length ~2.2 km and ~20 m, respectively. The optical a polarization controller, and an optical delay line. Loop I and Loop II consisted of fiber strength in each feedback loop was fixed through an optical attenuator. The overall feed- spool of length ~2.2 km and ~20 m, respectively. The optical strength in each feedback loop back ratio was fixed to be −22 dB and was again injected into the gain section of the laser was fixed through an optical attenuator. The overall feedback ratio was fixed to be 22 dB from Port 1 of an optical circulator. and was again injected into the gain section of the laser from Port 1 of an optical circulator. Figure 1. Experimental arrangement for asymmetric dual-loop optical feedback scheme. Acronyms: Figure 1. Experimental arrangement for asymmetric dual-loop optical feedback scheme. Acronyms: SOA, Semiconductor Optical Amplifier; ODL, Optical Delay Line; OC, Optical Circulator; PC, Po- SOA, Semiconductor Optical Amplifier; ODL, Optical Delay Line; OC, Optical Circulator; PC, larization Controller; OSA, Optical Spectrum Analyzer; VOA, Variable Optical Attenuator; ESA, Polarization Controller; OSA, Optical Spectrum Analyzer; VOA, Variable Optical Attenuator; ESA, Electrical Spectrum Analyzer; PM, Power Meter; QDash MLL, Quantum-Dash Mode-Locked Laser. Electrical Spectrum Analyzer; PM, Power Meter; QDash MLL, Quantum-Dash Mode-Locked Laser. 3. Results and Discussions In the present work, an asymmetric dual-loop optical feedback configuration has been adopted to prevent unwanted spurious sidebands from appearing due to the length of the feedback cavity. For that purpose, two approaches have been proposed: an asymmetric dual-loop feedback with various couplings of power-split-ratio via each optical feedback loop and the effect of the second feedback loop length on the cavity side modes. Photonics 2022, 9, 376 3 of 9 3.1. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on RF Linewidth and Integrated Timing Jitter In this section, we have investigated the effect of power split-ratio through an asym- metric dual-loop optical feedback scheme on the integrated timing jitter and RF linewidth of an SML QDash laser. A ~2.2 km fiber span was used in a single-loop optical feed- back scheme, and the cavity length was varied by using an optical delay line, which was optimized in steps of 1.67 ps from 0 to 84 ps. Under integer resonance, external cavity side modes with a frequency spacing of ~95 kHz appear in the RF spectrum, and they are depicted in Figure 2a. The frequency fluctuations that appear in the RF spectra limit the practical applications of QDash MLLs. To eliminate these resonance frequencies, an additional cavity loop with a delay time of more than ~100 smaller than the period of the frequency oscillations of the first loop was demonstrated. For this dual-loop scheme, the polarization controllers (PC-I and PC-II) and optical delay lines (ODL-I and ODL-II) attached with both loops were fine-tuned. The light signal was further split in different percentages into both cavities by using optical-attenuators (Att-I and Att-II). Three different Photonics 2022, 9, x FOR PEER REVIEW 4 of 9 combinations of feedback strength fed through each feedback loop of asymmetric dual-loop feedback scheme are listed in Table 1. Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz for for (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = −27.27 dB, (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = 27.27 dB, Loop Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. II = 19.74 dB; (c) Loop I = 22 dB, Loop II = 22 dB; (d) Loop I = 19.74 dB, Loop II = 27.27 dB. The experimentally measured phase noise traces of asymmetric dual-loop versus fre- Table 1. Three different combinations of feedback strength through each feedback loop with resulting quency-offset from the mode-locked frequency are shown in Figure 3. overall coupling strength of 22 dB into the gain section of QDash laser. Loop I Loop II Feedback Ration into Gain Section 27.27 dB 19.74 dB 22 dB 22 dB 22 dB 22 dB 19.74 dB 27.27 dB 22 dB Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from 10 kHz to 100 MHz. Photonics 2022, 9, x FOR PEER REVIEW 4 of 9 Photonics 2022, 9, 376 4 of 9 First, a 27.27 dB feedback strength via Loop I and a 19.74 dB via Loop II was fixed using variable optical attenuators. It should be noted that ODL-I was retuned such that Loop I modes precisely overlap with that of Loop II. Under such a situation, strong side- band elimination occurs, and all feedback-induced frequency fluctuations disappeared, as shown in Figure 2b. Our results demonstrate that the RF linewidth narrows down to 14 kHz from a 100 kHz free-running case when both feedback loops were integer resonant. The spectra were measured under a frequency span of 1 MHz, with a video-bandwidth (VBW) of 100 Hz and resolution-bandwidth (RBW) of 1 kHz. Similarly, when a balanced feedback strength (Loop I: 22 dB and Loop II: 22 dB) was passed via each external feedback cavity, then RF linewidth to as low as 20 kHz was noticed. The measured RF spectra is shown in Figure 2c under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). Furthermore, Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz asymmetric-dual-loop feedback was further implemented such that a 19.74 dB feedback for (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = −27.27 dB, strength was fed through Loop I, and 27.27 dB was fed via Loop II. Under this chosen Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. combination of feedback ratios, the RF linewidth reduces to 72 kHz. The measured RF spectra is shown in Figure 2d under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). The experimentally measured phase noise traces of asymmetric dual-loop versus fre- The experimentally measured phase noise traces of asymmetric dual-loop versus quency-offset from the mode-locked frequency are shown in Figure 3. frequency-offset from the mode-locked frequency are shown in Figure 3. Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from 10 kHz to 100 MHz. 10 kHz to 100 MHz. When each feedback loop was fully resonant, the timing jitter reduces to 2.5 ps from 3.9 ps (integrated from 10 kHz to 100 MHz) for a dual-loop feedback configuration with feedback ratio (Loop I: 19.74 dB and Loop II: 27.27 dB), 1.4 ps for dual-loop feedback configuration with coupling ratio (Loop I: 22 dB and Loop II: 22 dB), and 0.9 ps for dual-loop feedback with feedback ratio (Loop I: 27.27 dB and Loop II: 19.74 dB). The calculated RF linewidth is directly proportional to the root-mean-square (RMS) timing jitter [28]. Hence, the integrated timing jitter under feedback strength (Loop I: 19.74 dB and Loop II: 27.27 dB) was higher, as RF linewidth with such combinations of feedback ratio was 72 kHz. Similarly, the integrated timing jitter under feedback strength (Loop I: 27.27 dB and Loop II: 19.74 dB) was lower, because RF linewidth with this coupling strength was 14 kHz. A comparison of the RF linewidth and integrated timing-jitter under the stable reso- nance condition as functions of three combinations of feedback ratios is shown in Figure 4. Photonics 2022, 9, x FOR PEER REVIEW 5 of 9 When each feedback loop was fully resonant, the timing jitter reduces to 2.5 ps from 3.9 ps (integrated from 10 kHz to 100 MHz) for a dual-loop feedback configuration with feedback ratio (Loop I: −19.74 dB and Loop II: −27.27 dB), 1.4 ps for dual-loop feedback configuration with coupling ratio (Loop I: −22 dB and Loop II: −22 dB), and 0.9 ps for dual- loop feedback with feedback ratio (Loop I: −27.27 dB and Loop II: −19.74 dB). The calculated RF linewidth is directly proportional to the root-mean-square (RMS) timing jitter [28]. Hence, the integrated timing jitter under feedback strength (Loop I: −19.74 dB and Loop II: −27.27 dB) was higher, as RF linewidth with such combinations of feedback ratio was 72 kHz. Similarly, the integrated timing jitter under feedback strength (Loop I: −27.27 dB and Loop II: −19.74 dB) was lower, because RF linewidth with this cou- pling strength was 14 kHz. Photonics 2022, 9, 376 5 of 9 A comparison of the RF linewidth and integrated timing-jitter under the stable reso- nance condition as functions of three combinations of feedback ratios is shown in Figure 4. These results indicate that optimum stabilization (lowest timing jitter) in an SML QDash These results indicate that optimum stabilization (lowest timing jitter) in an SML QDash laser was achieved for the asymmetric dual-loop with a feedback ratio of −27.27 dB laser was achieved for the asymmetric dual-loop with a feedback ratio of 27.27 dB through through Loop I and −19.74 dB through Loop II. On the other hand, better side-mode sup- Loop I and 19.74 dB through Loop II. On the other hand, better side-mode suppression pression was achieved for the dual-loop with a feedback strength of −19.74 dB from Loop was achieved for the dual-loop with a feedback strength of 19.74 dB from Loop I and I and −27.27 dB via Loop II. 27.27 dB via Loop II. Figure 4. A comparison of RF linewidth and integrated timing jitter for asymmetric dual-loop feed- Figure 4. A comparison of RF linewidth and integrated timing jitter for asymmetric dual-loop back scheme. feedback scheme. 3.2. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on 3.2. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on Suppres Suppression sion of of Freque Frequency ncy Flu Fluctuations ctuations In the following, we discuss the suppression of feedback-induced frequency fluctu- In the following, we discuss the suppression of feedback-induced frequency fluctua- ations at various combinations of feedback ratios. The measured RF spectra for single tions at various combinations of feedback ratios. The measured RF spectra for single loop loop feedback with a fiber spool of 2.2 km is shown in Figure 5a under frequency spans feedback with a fiber spool of 2.2 km is shown in Figure 5a under frequency spans of 10 of 10 MHz, RBW 10 kHz, and VBW 1 kHz. However, for dual-loop optical feedback MHz, RBW 10 kHz, and VBW 1 kHz. However, for dual-loop optical feedback configura- configurations, the feedback strength in Loop I was fixed to 27.27 dB and that in Loop tions, the feedback strength in Loop I was fixed to −27.27 dB and that in Loop II to −19.74 II to 19.74 dB. The measured RF spectra for single loop feedback with a fiber spool of dB. The measured RF spectra for single loop feedback with a fiber spool of 2.2 km is shown 2.2 km is shown in Figure 5b under frequency spans of 10 MHz, RBW 10 kHz, and VBW 1 in Figure 5b under frequency spans of 10 MHz, RBW 10 kHz, and VBW 1 kHz. It was kHz. It was observed from measured spectra that modal overlap appears at a particular observed from measured spectra that modal overlap appears at a particular frequency frequency offset, corresponding to the length of the second feedback loop. These results offset, corresponding to the length of the second feedback loop. These results indicate that indicate that this chosen combination of feedback strength (Loop I: 27.27 dB; Loop II: 19.74 dB) is not suitable for better suppression of feedback-induced cavity sidebands. However, for a balanced feedback ratio (Loop I: 22 dB; Loop II: 22 dB) on a larger frequency span (10 MHz), weak side modes were observed. The RF spectra were measured under span 10 MHz (RBW 10 kHz and VBW 1 kHz) and are shown in Figure 5c. The mea- sured experimental results show that cavity sidebands cannot effectively be suppressed by considering a balanced feedback ratio through an asymmetric dual-loop feedback scheme. In order to acquire stable and flat RF spectra, we demonstrated an unbalanced-asymmetric dual-loop feedback scheme for better external cavity sideband suppression, which yields fluctuation-free RF-spectra compared to single and dual-loop feedback schemes. With feedback ratio Loop I: 27.27 dB and Loop II: 19.74 dB, fine-tuning of both external feedback cavities was carried out such that precise coincidence of the modes of Loop I with a mode of Loop II occurs. When the optical delay lines connected to each feedback loop are fully resonant, a strong side-mode suppression was noticed. A flat RF spectra can be seen Photonics 2022, 9, x FOR PEER REVIEW 6 of 9 this chosen combination of feedback strength (Loop I: −27.27 dB; Loop II: −19.74 dB) is not suitable for better suppression of feedback-induced cavity sidebands. However, for a bal- anced feedback ratio (Loop I: −22 dB; Loop II: −22 dB) on a larger frequency span (10 MHz), weak side modes were observed. The RF spectra were measured under span 10 MHz (RBW 10 kHz and VBW 1 kHz) and are shown in Figure 5c. The measured experimental results show that cavity sidebands cannot effectively be suppressed by considering a bal- anced feedback ratio through an asymmetric dual-loop feedback scheme. In order to ac- quire stable and flat RF spectra, we demonstrated an unbalanced-asymmetric dual-loop feedback scheme for better external cavity sideband suppression, which yields fluctua- tion-free RF-spectra compared to single and dual-loop feedback schemes. With feedback ratio Loop I: −27.27 dB and Loop II: −19.74 dB, fine-tuning of both external feedback cavi- ties was carried out such that precise coincidence of the modes of Loop I with a mode of Loop II occurs. When the optical delay lines connected to each feedback loop are fully Photonics 2022, 9, 376 6 of 9 resonant, a strong side-mode suppression was noticed. A flat RF spectra can be seen in Figure 5d under a span of 10 MHz (RBW 10 kHz and VBW 1 kHz). In this feedback con- figurations, the RF linewidth is 5× higher than the dual-loop configuration with feedback in Figure 5d under a span of 10 MHz (RBW 10 kHz and VBW 1 kHz). In this feedback con- ratio (Loop I: −27.27 dB and Loop II: −19.74 dB), but the side modes are eliminated. These figurations, the RF linewidth is 5 higher than the dual-loop configuration with feedback results agree well with our recently published data [23]. These finding further suggests ratio (Loop I: 27.27 dB and Loop II: 19.74 dB), but the side modes are eliminated. These that better suppression in external cavity sidebands can be achieved by precisely control- results agree well with our recently published data [23]. These finding further suggests that ling the percentage of feedback ratio through either external feedback loop. Furthermore, better suppression in external cavity sidebands can be achieved by precisely controlling the resulting setup can be implemented for applications where less noise and a stable RF the percentage of feedback ratio through either external feedback loop. Furthermore, the spectrum are desired, as in frequency-comb-generation. Most recently, it was theoretically resulting setup can be implemented for applications where less noise and a stable RF predicted that dual-loop optoelectronic oscillators could be optimized by controlling the spectrum are desired, as in frequency-comb-generation. Most recently, it was theoretically phase delay and power split ratio [29], which agrees with our experimental measure- predicted that dual-loop optoelectronic oscillators could be optimized by controlling the ments. phase delay and power split ratio [29], which agrees with our experimental measurements. Figure 5. Measured RF linewidth under a frequency span of 10 MHz, RBW 10 kHz, and VBW 1 kHz Figure 5. Measured RF linewidth under a frequency span of 10 MHz, RBW 10 kHz, and VBW 1 kHz for (a) single loop feedback and dual-loop feedback at feedback strength (b) Loop I = −27.27 dB, for (a) single loop feedback and dual-loop feedback at feedback strength (b) Loop I = 27.27 dB, Loop Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. II = 19.74 dB; (c) Loop I = 22 dB, Loop II = 22 dB; (d) Loop I = 19.74 dB, Loop II = 27.27 dB. 3.3. Effect of the Length of Second Cavity on Suppression of Frequency Resonances In this section, we further studied the effect of the length of the second feedback cavity on the suppression of laser-induced frequency resonances. Similar to the above experimental arrangement, a fiber length of ~2.2 km was fixed in Loop I, and a ~220 m fiber length was used in the second loop, with equal feedback strength through each external feedback loop. The signals of a few gigahertz repetition rate were generated with a mode spacing of 95 kHz away from the main mode-locked frequency, as shown in Figure 6. Upon fine tuning of both external feedback loops, the asymmetric dual-loop configuration suggested here (Loop I = ~2.2 km and Loop II = ~220 m) is a promising approach that leads towards significant suppression in external cavity sidebands closer to the main peak. Furthermore, when ODL-I, attached for the first feedback loop, was tuned to 24 ps and ODL II, connected to the second feedback cavity, was varied to 13 ps, the modes of Loop I overlap with the modes of Loop II. Consequently, a maximum of 30 dB sideband compression in the first frequency harmonic occurs. The measured RF spectrum is shown in Figure 5 using Photonics 2022, 9, x FOR PEER REVIEW 7 of 9 3.3. Effect of the Length of Second Cavity on Suppression of Frequency Resonances In this section, we further studied the effect of the length of the second feedback cav- ity on the suppression of laser-induced frequency resonances. Similar to the above exper- imental arrangement, a fiber length of ~2.2 km was fixed in Loop I, and a ~220 m fiber length was used in the second loop, with equal feedback strength through each external feedback loop. The signals of a few gigahertz repetition rate were generated with a mode spacing of 95 kHz away from the main mode-locked frequency, as shown in Figure 6. Upon fine tuning of both external feedback loops, the asymmetric dual-loop configuration suggested here (Loop I = ~2.2 km and Loop II = ~220 m) is a promising approach that leads towards significant suppression in external cavity sidebands closer to the main peak. Fur- thermore, when ODL-I, attached for the first feedback loop, was tuned to 24 ps and ODL Photonics 2022, 9, 376 7 of 9 II, connected to the second feedback cavity, was varied to 13 ps, the modes of Loop I over- lap with the modes of Loop II. Consequently, a maximum of 30 dB sideband compression in the first frequency harmonic occurs. The measured RF spectrum is shown in Figure 5 single-loop feedback with a fiber length of ~2.2 km (black line), ~220 m (red line), and using single-loop feedback with a fiber length of ~2.2 km (black line), ~220 m (red line), dual-loop (blue line) feedback. and dual-loop (blue line) feedback. Figure 6. Experimentally measured RF-spectra using single-loop feedback with lengths of ~2.2 km Figure 6. Experimentally measured RF-spectra using single-loop feedback with lengths of ~2.2 km (black line) and ~220 m (red line), and asymmetric dual loops having lengths of ~2.2 km for Loop I (black line) and ~220 m (red line), and asymmetric dual loops having lengths of ~2.2 km for Loop I and ~220 m for Loop II under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). and ~220 m for Loop II under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). The asymmetric dual-loop feedback scheme demonstrated here is a better technique The asymmetric dual-loop feedback scheme demonstrated here is a better technique that effectively suppresses unwanted noise-induced oscillations and yields side-band free that effectively suppresses unwanted noise-induced oscillations and yields side-band free RF spectra when an equal percentags of feedback ratio was used. Furthermore, this be- RF spectra when an equal percentags of feedback ratio was used. Furthermore, this behavior ha shows vior shows tha that bettertsuppr better suppressi ession in cavity on in ca sidebands vity sideb can ands ca be obtained n be obby tain varying ed by vthe aryisecond ng the second feedback loop length using ODL. feedback loop length using ODL. 4. Conclusions 4. Conclusions In the present work, we experimentally demonstrate how to suppress the feedback- In the present work, we experimentally demonstrate how to suppress the feedback- induced frequency fluctuations from conventional single and dual-loop feedback schemes, induced frequency fluctuations from conventional single and dual-loop feedback with feedback ratio controlled for short as well as long optical cavities. The device under schemes, with feedback ratio controlled for short as well as long optical cavities. The de- test was a two-section InAs/InP QDash MLLs operating at 21 GHz and emitting at 1550 nm. vice under test was a two-section InAs/InP QDash MLLs operating at 21 GHz and emitting These results reveal that dual-loop feedback with precise alignment of the loop lengths and fine-tuning of feedback ratio through external feedback cavities effectively suppresses external cavity sidebands. The proposed asymmetric dual-loop feedback configuration makes semiconductor mode-locked lasers promising for the development of compact and cost-effective optoelectronic oscillators with low timing jitter. The resulting setup using this method is also integrable in a hybrid integrated optics, compact fiber loops and stable OEOs. Author Contributions: Conceptualization, H.A.; methodology, H.A.; software, T.A.A., M.A. and H.A.; validation, T.A.A., M.A. and H.A.; formal analysis, T.A.A., M.A. and H.A.; investigation, T.A.A., M.A. and H.A.; resources, H.A.; data curation, H.A.; writing—original draft preparation, T.A.A. and H.A; writing—review and editing, T.A.A., M.A. and H.A.; visualization, H.A.; supervision, H.A.; project administration, T.A.A. and H.A.; funding acquisition, T.A.A. and H.A. All authors have read and agreed to the published version of the manuscript. Photonics 2022, 9, 376 8 of 9 Funding: Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R71), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: T.A extend their sincere appreciation to Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R71), Princess Nourah bint Abdul- rahman University, Riyadh, Saudi Arabia. Conflicts of Interest: The authors declare no conflict of interest. References 1. Bajek, D.; Cataluna, M.A. Fast optical sampling by electronic repetition-rate tuning using a single mode-locked laser diode. Opt. Express 2021, 29, 6890–6902. [CrossRef] [PubMed] 2. Merghem, K.; Calò, C.; Rosales, R.; Lafosse, X.; Aubin, G.; Martinez, A.; Lelarge, F.; Ramdane, A. Stability of optical frequency comb generated with InAs/InP quantum-dash-based passive mode-locked lasers. IEEE J. Quantum Electron. 2014, 50, 275–280. [CrossRef] 3. Panapakkam, V.; Anthur, A.; Vujicic, V.; Zhou, R.; Gaimard, Q.; Merghem, K.; Aubin, G.; Lelarge, F.; Viktorov, E.A.; Barry, L.; et al. 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Performance optimization of an optically combined dual-loop optoelectronic oscillator based on optical interference analysis. Opt. Eng. 2017, 56, 066111. [CrossRef] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

All Optical Stabilizations of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Based on Asymmetric Dual-Loop Optical Feedback Configurations

Alrebdi, Tahani A.; Asghar, Mamoon; Asghar, Haroon
Photonics , Volume 9 (6) – May 26, 2022
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10.3390/photonics9060376
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hv photonics Communication All Optical Stabilizations of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Based on Asymmetric Dual-Loop Optical Feedback Configurations 1 2 3 , Tahani A. Alrebdi , Mamoon Asghar and Haroon Asghar * Department of Physics, College of Science, Princess Nourah Bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia; taalrebdi@pnu.edu.sa Department of Physics, University of Gujrat, Hafiz Hayat Campus, Gujrat 50700, Pakistan; mamoonasghar319@gmail.com National Centre for Physics, Quaid-i-Azam University Campus, Islamabad 45320, Pakistan * Correspondence: haroon.asghar@ncp.edu.pk Abstract: We report feedback-induced frequency oscillations using a power-split-ratio through asymmetric dual-loop optical feedback (Loop I: ~2.2 km and Loop II: ~20 m) subject to a self- mode-locked two-section QDash laser emitting at 1550 nm and operating at 21 GHz repetition rate. To assess the suppression of frequency resonances, three chosen combinations of feedback power (Loop I: 27.27 dB and Loop II: 19.74 dB, Loop I: 22 dB and Loop II: 22 dB, and Loop I: 19.74 dB and Loop II: 27.27 dB) through asymmetric dual-loop optical feedback have been studied. Based on the chosen coupling strength, an optimum feedback ratio that yields better side-mode suppression has been identified. Our results demonstrate that side-mode suppression can be achieved by the fine adjustment of coupling power through either cavity of dual-loop feedback configurations. Furthermore, we have further demonstrated that frequency fluctuations from the RF spectra can be filtered by carefully selecting the delay phase of the second cavity. Our experimental findings suggest that semiconductor mode-locked lasers based on dual-loop feedback configurations Citation: Alrebdi, T.A.; Asghar, M.; can be used to develop noise oscillations free from integrated photonic oscillators for potential Asghar, H. All Optical Stabilizations applications in telecommunications, multiplexing, and frequency-comb generation. of Nano-Structure-Based QDash Semiconductor Mode-Locked Lasers Keywords: semiconductor lasers; optical feedback; mode-locked lasers; power-split ratio; frequency- Based on Asymmetric Dual-Loop fluctuations Optical Feedback Configurations. Photonics 2022, 9, 376. https:// doi.org/10.3390/photonics9060376 1. Introduction Received: 21 April 2022 Accepted: 23 May 2022 Mode-locked lasers (SMLLs) are potential candidates for key applications in many Published: 26 May 2022 fields of optical sampling [1], frequency comb generation [2–4], data center networks [5], clock recovery [6,7], telecommunications [8–15], and spectroscopy [16]. Some ideal features Publisher’s Note: MDPI stays neutral of SMLLs include compactness, low fabrication costs, low threshold current, low amplified with regard to jurisdictional claims in spontaneous emission fast carrier dynamics, and an inhomogeneous wide spectrum [17]. published maps and institutional affil- iations. To enhance the improvement and for better applications of SMLLs in telecommunications, high timing stability is paramount. The timing stability of SMLLs can be improved using the optical fiber cavity [18], which reduces the RF linewidth and corresponding integrated timing jitter of the optical pulse train. The optical fiber loop in the feedback cavity can be Copyright: © 2022 by the authors. considered as the energy storage component, with Q-factor being determined by the length Licensee MDPI, Basel, Switzerland. of the resonator [19]. For single-loop optical feedback, there is an increase in the system This article is an open access article memory, which directly corresponds with the reduction in the timing jitter [20–26]. With distributed under the terms and improved timing jitter, single-loop optical feedback produces additional cavity sidebands conditions of the Creative Commons around the main frequency in the power spectrum [20–27]. Recently, we have proposed Attribution (CC BY) license (https:// the asymmetric dual-loop optical feedback scheme with equal feedback ratio through creativecommons.org/licenses/by/ each loop, to suppress the first sideband around the fundamental frequency; however, the 4.0/). Photonics 2022, 9, 376. https://doi.org/10.3390/photonics9060376 https://www.mdpi.com/journal/photonics Photonics 2022, 9, x FOR PEER REVIEW 2 of 9 equal feedback ratio through each loop, to suppress the first sideband around the funda- Photonics 2022, 9, 376 2 of 9 mental frequency; however, the second appears unsuppressed (modal overlap) [22]. To eliminate these cavity sidebands and modal overlap, we reported how to optimize bal- anced asymmetric dual-loop optical feedback by varying the second feedback cavity second appears unsuppressed (modal overlap) [22]. To eliminate these cavity sidebands length [23]. and modal overlap, we reported how to optimize balanced asymmetric dual-loop optical In this paper, we report the effect of unequal coupling power through asymmetric feedback by varying the second feedback cavity length [23]. dual-loop optical feedback on the adverse dynamical effects induced due to longer feed- In this paper, we report the effect of unequal coupling power through asymmetric back cavities. We identify the optimum conditions for coupling power and the second dual-loop optical feedback on the adverse dynamical effects induced due to longer feedback feedback loop length, which yields much better suppression in external cavity side modes. cavities. We identify the optimum conditions for coupling power and the second feedback These findings reveal that optimized asymmetric dual-loop feedback is a robust and po- loop length, which yields much better suppression in external cavity side modes. These tential source for overcoming the disadvantages of frequency fluctuations in mode-locked findings reveal that optimized asymmetric dual-loop feedback is a robust and potential QDash lasers, which limits the applications of semiconductor MLLs. source for overcoming the disadvantages of frequency fluctuations in mode-locked QDash lasers, which limits the applications of semiconductor MLLs. 2. Experimental Setup The device under test is an InAs/InP SML QDash laser, and details about the device 2. Experimental Setup are given in [24]. The schematic of the experimental setup is depicted in Figure 1. The The device under test is an InAs/InP SML QDash laser, and details about the device are emission from the device under test (QDash MLL) was collected using lensed fiber and given in [24]. The schematic of the experimental setup is depicted in Figure 1. The emission then fed into an optical circulator through Port 2. Port 3 collects light, and then the semi- from the device under test (QDash MLL) was collected using lensed fiber and then fed into conductor optical amplifier amplifies the optical signal. The amplified light was then an optical circulator through Port 2. Port 3 collects light, and then the semiconductor optical equally divided into two parts through a 3-dB coupler. One part of optical light is used to amplifier amplifies the optical signal. The amplified light was then equally divided into analyze the optical spectra and the other part for electrical spectra. The other part of the two parts through a 3-dB coupler. One part of optical light is used to analyze the optical light was fed into the experimental arrangements. Each feedback loop consisted of a var- spectra and the other part for electrical spectra. The other part of the light was fed into the iable optical attenuator, a polarization controller, and an optical delay line. Loop I and experimental arrangements. Each feedback loop consisted of a variable optical attenuator, Loop II consisted of fiber spool of length ~2.2 km and ~20 m, respectively. The optical a polarization controller, and an optical delay line. Loop I and Loop II consisted of fiber strength in each feedback loop was fixed through an optical attenuator. The overall feed- spool of length ~2.2 km and ~20 m, respectively. The optical strength in each feedback loop back ratio was fixed to be −22 dB and was again injected into the gain section of the laser was fixed through an optical attenuator. The overall feedback ratio was fixed to be 22 dB from Port 1 of an optical circulator. and was again injected into the gain section of the laser from Port 1 of an optical circulator. Figure 1. Experimental arrangement for asymmetric dual-loop optical feedback scheme. Acronyms: Figure 1. Experimental arrangement for asymmetric dual-loop optical feedback scheme. Acronyms: SOA, Semiconductor Optical Amplifier; ODL, Optical Delay Line; OC, Optical Circulator; PC, Po- SOA, Semiconductor Optical Amplifier; ODL, Optical Delay Line; OC, Optical Circulator; PC, larization Controller; OSA, Optical Spectrum Analyzer; VOA, Variable Optical Attenuator; ESA, Polarization Controller; OSA, Optical Spectrum Analyzer; VOA, Variable Optical Attenuator; ESA, Electrical Spectrum Analyzer; PM, Power Meter; QDash MLL, Quantum-Dash Mode-Locked Laser. Electrical Spectrum Analyzer; PM, Power Meter; QDash MLL, Quantum-Dash Mode-Locked Laser. 3. Results and Discussions In the present work, an asymmetric dual-loop optical feedback configuration has been adopted to prevent unwanted spurious sidebands from appearing due to the length of the feedback cavity. For that purpose, two approaches have been proposed: an asymmetric dual-loop feedback with various couplings of power-split-ratio via each optical feedback loop and the effect of the second feedback loop length on the cavity side modes. Photonics 2022, 9, 376 3 of 9 3.1. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on RF Linewidth and Integrated Timing Jitter In this section, we have investigated the effect of power split-ratio through an asym- metric dual-loop optical feedback scheme on the integrated timing jitter and RF linewidth of an SML QDash laser. A ~2.2 km fiber span was used in a single-loop optical feed- back scheme, and the cavity length was varied by using an optical delay line, which was optimized in steps of 1.67 ps from 0 to 84 ps. Under integer resonance, external cavity side modes with a frequency spacing of ~95 kHz appear in the RF spectrum, and they are depicted in Figure 2a. The frequency fluctuations that appear in the RF spectra limit the practical applications of QDash MLLs. To eliminate these resonance frequencies, an additional cavity loop with a delay time of more than ~100 smaller than the period of the frequency oscillations of the first loop was demonstrated. For this dual-loop scheme, the polarization controllers (PC-I and PC-II) and optical delay lines (ODL-I and ODL-II) attached with both loops were fine-tuned. The light signal was further split in different percentages into both cavities by using optical-attenuators (Att-I and Att-II). Three different Photonics 2022, 9, x FOR PEER REVIEW 4 of 9 combinations of feedback strength fed through each feedback loop of asymmetric dual-loop feedback scheme are listed in Table 1. Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz for for (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = −27.27 dB, (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = 27.27 dB, Loop Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. II = 19.74 dB; (c) Loop I = 22 dB, Loop II = 22 dB; (d) Loop I = 19.74 dB, Loop II = 27.27 dB. The experimentally measured phase noise traces of asymmetric dual-loop versus fre- Table 1. Three different combinations of feedback strength through each feedback loop with resulting quency-offset from the mode-locked frequency are shown in Figure 3. overall coupling strength of 22 dB into the gain section of QDash laser. Loop I Loop II Feedback Ration into Gain Section 27.27 dB 19.74 dB 22 dB 22 dB 22 dB 22 dB 19.74 dB 27.27 dB 22 dB Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from 10 kHz to 100 MHz. Photonics 2022, 9, x FOR PEER REVIEW 4 of 9 Photonics 2022, 9, 376 4 of 9 First, a 27.27 dB feedback strength via Loop I and a 19.74 dB via Loop II was fixed using variable optical attenuators. It should be noted that ODL-I was retuned such that Loop I modes precisely overlap with that of Loop II. Under such a situation, strong side- band elimination occurs, and all feedback-induced frequency fluctuations disappeared, as shown in Figure 2b. Our results demonstrate that the RF linewidth narrows down to 14 kHz from a 100 kHz free-running case when both feedback loops were integer resonant. The spectra were measured under a frequency span of 1 MHz, with a video-bandwidth (VBW) of 100 Hz and resolution-bandwidth (RBW) of 1 kHz. Similarly, when a balanced feedback strength (Loop I: 22 dB and Loop II: 22 dB) was passed via each external feedback cavity, then RF linewidth to as low as 20 kHz was noticed. The measured RF spectra is shown in Figure 2c under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). Furthermore, Figure 2. Measured RF linewidth under a frequency span of 1 MHz, RBW 1 kHz, and VBW 100 Hz asymmetric-dual-loop feedback was further implemented such that a 19.74 dB feedback for (a) single loop feedback and dual-loop feedback at feedback strengths of (b) Loop I = −27.27 dB, strength was fed through Loop I, and 27.27 dB was fed via Loop II. Under this chosen Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. combination of feedback ratios, the RF linewidth reduces to 72 kHz. The measured RF spectra is shown in Figure 2d under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). The experimentally measured phase noise traces of asymmetric dual-loop versus fre- The experimentally measured phase noise traces of asymmetric dual-loop versus quency-offset from the mode-locked frequency are shown in Figure 3. frequency-offset from the mode-locked frequency are shown in Figure 3. Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from Figure 3. SSB phase-noise trace of asymmetric dual-loop optical feedback scheme integrated from 10 kHz to 100 MHz. 10 kHz to 100 MHz. When each feedback loop was fully resonant, the timing jitter reduces to 2.5 ps from 3.9 ps (integrated from 10 kHz to 100 MHz) for a dual-loop feedback configuration with feedback ratio (Loop I: 19.74 dB and Loop II: 27.27 dB), 1.4 ps for dual-loop feedback configuration with coupling ratio (Loop I: 22 dB and Loop II: 22 dB), and 0.9 ps for dual-loop feedback with feedback ratio (Loop I: 27.27 dB and Loop II: 19.74 dB). The calculated RF linewidth is directly proportional to the root-mean-square (RMS) timing jitter [28]. Hence, the integrated timing jitter under feedback strength (Loop I: 19.74 dB and Loop II: 27.27 dB) was higher, as RF linewidth with such combinations of feedback ratio was 72 kHz. Similarly, the integrated timing jitter under feedback strength (Loop I: 27.27 dB and Loop II: 19.74 dB) was lower, because RF linewidth with this coupling strength was 14 kHz. A comparison of the RF linewidth and integrated timing-jitter under the stable reso- nance condition as functions of three combinations of feedback ratios is shown in Figure 4. Photonics 2022, 9, x FOR PEER REVIEW 5 of 9 When each feedback loop was fully resonant, the timing jitter reduces to 2.5 ps from 3.9 ps (integrated from 10 kHz to 100 MHz) for a dual-loop feedback configuration with feedback ratio (Loop I: −19.74 dB and Loop II: −27.27 dB), 1.4 ps for dual-loop feedback configuration with coupling ratio (Loop I: −22 dB and Loop II: −22 dB), and 0.9 ps for dual- loop feedback with feedback ratio (Loop I: −27.27 dB and Loop II: −19.74 dB). The calculated RF linewidth is directly proportional to the root-mean-square (RMS) timing jitter [28]. Hence, the integrated timing jitter under feedback strength (Loop I: −19.74 dB and Loop II: −27.27 dB) was higher, as RF linewidth with such combinations of feedback ratio was 72 kHz. Similarly, the integrated timing jitter under feedback strength (Loop I: −27.27 dB and Loop II: −19.74 dB) was lower, because RF linewidth with this cou- pling strength was 14 kHz. Photonics 2022, 9, 376 5 of 9 A comparison of the RF linewidth and integrated timing-jitter under the stable reso- nance condition as functions of three combinations of feedback ratios is shown in Figure 4. These results indicate that optimum stabilization (lowest timing jitter) in an SML QDash These results indicate that optimum stabilization (lowest timing jitter) in an SML QDash laser was achieved for the asymmetric dual-loop with a feedback ratio of −27.27 dB laser was achieved for the asymmetric dual-loop with a feedback ratio of 27.27 dB through through Loop I and −19.74 dB through Loop II. On the other hand, better side-mode sup- Loop I and 19.74 dB through Loop II. On the other hand, better side-mode suppression pression was achieved for the dual-loop with a feedback strength of −19.74 dB from Loop was achieved for the dual-loop with a feedback strength of 19.74 dB from Loop I and I and −27.27 dB via Loop II. 27.27 dB via Loop II. Figure 4. A comparison of RF linewidth and integrated timing jitter for asymmetric dual-loop feed- Figure 4. A comparison of RF linewidth and integrated timing jitter for asymmetric dual-loop back scheme. feedback scheme. 3.2. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on 3.2. Effect of Power-Split-Ratio Controlled Asymmetric Dual-Loop Feedback Scheme on Suppres Suppression sion of of Freque Frequency ncy Flu Fluctuations ctuations In the following, we discuss the suppression of feedback-induced frequency fluctu- In the following, we discuss the suppression of feedback-induced frequency fluctua- ations at various combinations of feedback ratios. The measured RF spectra for single tions at various combinations of feedback ratios. The measured RF spectra for single loop loop feedback with a fiber spool of 2.2 km is shown in Figure 5a under frequency spans feedback with a fiber spool of 2.2 km is shown in Figure 5a under frequency spans of 10 of 10 MHz, RBW 10 kHz, and VBW 1 kHz. However, for dual-loop optical feedback MHz, RBW 10 kHz, and VBW 1 kHz. However, for dual-loop optical feedback configura- configurations, the feedback strength in Loop I was fixed to 27.27 dB and that in Loop tions, the feedback strength in Loop I was fixed to −27.27 dB and that in Loop II to −19.74 II to 19.74 dB. The measured RF spectra for single loop feedback with a fiber spool of dB. The measured RF spectra for single loop feedback with a fiber spool of 2.2 km is shown 2.2 km is shown in Figure 5b under frequency spans of 10 MHz, RBW 10 kHz, and VBW 1 in Figure 5b under frequency spans of 10 MHz, RBW 10 kHz, and VBW 1 kHz. It was kHz. It was observed from measured spectra that modal overlap appears at a particular observed from measured spectra that modal overlap appears at a particular frequency frequency offset, corresponding to the length of the second feedback loop. These results offset, corresponding to the length of the second feedback loop. These results indicate that indicate that this chosen combination of feedback strength (Loop I: 27.27 dB; Loop II: 19.74 dB) is not suitable for better suppression of feedback-induced cavity sidebands. However, for a balanced feedback ratio (Loop I: 22 dB; Loop II: 22 dB) on a larger frequency span (10 MHz), weak side modes were observed. The RF spectra were measured under span 10 MHz (RBW 10 kHz and VBW 1 kHz) and are shown in Figure 5c. The mea- sured experimental results show that cavity sidebands cannot effectively be suppressed by considering a balanced feedback ratio through an asymmetric dual-loop feedback scheme. In order to acquire stable and flat RF spectra, we demonstrated an unbalanced-asymmetric dual-loop feedback scheme for better external cavity sideband suppression, which yields fluctuation-free RF-spectra compared to single and dual-loop feedback schemes. With feedback ratio Loop I: 27.27 dB and Loop II: 19.74 dB, fine-tuning of both external feedback cavities was carried out such that precise coincidence of the modes of Loop I with a mode of Loop II occurs. When the optical delay lines connected to each feedback loop are fully resonant, a strong side-mode suppression was noticed. A flat RF spectra can be seen Photonics 2022, 9, x FOR PEER REVIEW 6 of 9 this chosen combination of feedback strength (Loop I: −27.27 dB; Loop II: −19.74 dB) is not suitable for better suppression of feedback-induced cavity sidebands. However, for a bal- anced feedback ratio (Loop I: −22 dB; Loop II: −22 dB) on a larger frequency span (10 MHz), weak side modes were observed. The RF spectra were measured under span 10 MHz (RBW 10 kHz and VBW 1 kHz) and are shown in Figure 5c. The measured experimental results show that cavity sidebands cannot effectively be suppressed by considering a bal- anced feedback ratio through an asymmetric dual-loop feedback scheme. In order to ac- quire stable and flat RF spectra, we demonstrated an unbalanced-asymmetric dual-loop feedback scheme for better external cavity sideband suppression, which yields fluctua- tion-free RF-spectra compared to single and dual-loop feedback schemes. With feedback ratio Loop I: −27.27 dB and Loop II: −19.74 dB, fine-tuning of both external feedback cavi- ties was carried out such that precise coincidence of the modes of Loop I with a mode of Loop II occurs. When the optical delay lines connected to each feedback loop are fully Photonics 2022, 9, 376 6 of 9 resonant, a strong side-mode suppression was noticed. A flat RF spectra can be seen in Figure 5d under a span of 10 MHz (RBW 10 kHz and VBW 1 kHz). In this feedback con- figurations, the RF linewidth is 5× higher than the dual-loop configuration with feedback in Figure 5d under a span of 10 MHz (RBW 10 kHz and VBW 1 kHz). In this feedback con- ratio (Loop I: −27.27 dB and Loop II: −19.74 dB), but the side modes are eliminated. These figurations, the RF linewidth is 5 higher than the dual-loop configuration with feedback results agree well with our recently published data [23]. These finding further suggests ratio (Loop I: 27.27 dB and Loop II: 19.74 dB), but the side modes are eliminated. These that better suppression in external cavity sidebands can be achieved by precisely control- results agree well with our recently published data [23]. These finding further suggests that ling the percentage of feedback ratio through either external feedback loop. Furthermore, better suppression in external cavity sidebands can be achieved by precisely controlling the resulting setup can be implemented for applications where less noise and a stable RF the percentage of feedback ratio through either external feedback loop. Furthermore, the spectrum are desired, as in frequency-comb-generation. Most recently, it was theoretically resulting setup can be implemented for applications where less noise and a stable RF predicted that dual-loop optoelectronic oscillators could be optimized by controlling the spectrum are desired, as in frequency-comb-generation. Most recently, it was theoretically phase delay and power split ratio [29], which agrees with our experimental measure- predicted that dual-loop optoelectronic oscillators could be optimized by controlling the ments. phase delay and power split ratio [29], which agrees with our experimental measurements. Figure 5. Measured RF linewidth under a frequency span of 10 MHz, RBW 10 kHz, and VBW 1 kHz Figure 5. Measured RF linewidth under a frequency span of 10 MHz, RBW 10 kHz, and VBW 1 kHz for (a) single loop feedback and dual-loop feedback at feedback strength (b) Loop I = −27.27 dB, for (a) single loop feedback and dual-loop feedback at feedback strength (b) Loop I = 27.27 dB, Loop Loop II = −19.74 dB; (c) Loop I = −22 dB, Loop II = −22 dB; (d) Loop I = −19.74 dB, Loop II = −27.27 dB. II = 19.74 dB; (c) Loop I = 22 dB, Loop II = 22 dB; (d) Loop I = 19.74 dB, Loop II = 27.27 dB. 3.3. Effect of the Length of Second Cavity on Suppression of Frequency Resonances In this section, we further studied the effect of the length of the second feedback cavity on the suppression of laser-induced frequency resonances. Similar to the above experimental arrangement, a fiber length of ~2.2 km was fixed in Loop I, and a ~220 m fiber length was used in the second loop, with equal feedback strength through each external feedback loop. The signals of a few gigahertz repetition rate were generated with a mode spacing of 95 kHz away from the main mode-locked frequency, as shown in Figure 6. Upon fine tuning of both external feedback loops, the asymmetric dual-loop configuration suggested here (Loop I = ~2.2 km and Loop II = ~220 m) is a promising approach that leads towards significant suppression in external cavity sidebands closer to the main peak. Furthermore, when ODL-I, attached for the first feedback loop, was tuned to 24 ps and ODL II, connected to the second feedback cavity, was varied to 13 ps, the modes of Loop I overlap with the modes of Loop II. Consequently, a maximum of 30 dB sideband compression in the first frequency harmonic occurs. The measured RF spectrum is shown in Figure 5 using Photonics 2022, 9, x FOR PEER REVIEW 7 of 9 3.3. Effect of the Length of Second Cavity on Suppression of Frequency Resonances In this section, we further studied the effect of the length of the second feedback cav- ity on the suppression of laser-induced frequency resonances. Similar to the above exper- imental arrangement, a fiber length of ~2.2 km was fixed in Loop I, and a ~220 m fiber length was used in the second loop, with equal feedback strength through each external feedback loop. The signals of a few gigahertz repetition rate were generated with a mode spacing of 95 kHz away from the main mode-locked frequency, as shown in Figure 6. Upon fine tuning of both external feedback loops, the asymmetric dual-loop configuration suggested here (Loop I = ~2.2 km and Loop II = ~220 m) is a promising approach that leads towards significant suppression in external cavity sidebands closer to the main peak. Fur- thermore, when ODL-I, attached for the first feedback loop, was tuned to 24 ps and ODL Photonics 2022, 9, 376 7 of 9 II, connected to the second feedback cavity, was varied to 13 ps, the modes of Loop I over- lap with the modes of Loop II. Consequently, a maximum of 30 dB sideband compression in the first frequency harmonic occurs. The measured RF spectrum is shown in Figure 5 single-loop feedback with a fiber length of ~2.2 km (black line), ~220 m (red line), and using single-loop feedback with a fiber length of ~2.2 km (black line), ~220 m (red line), dual-loop (blue line) feedback. and dual-loop (blue line) feedback. Figure 6. Experimentally measured RF-spectra using single-loop feedback with lengths of ~2.2 km Figure 6. Experimentally measured RF-spectra using single-loop feedback with lengths of ~2.2 km (black line) and ~220 m (red line), and asymmetric dual loops having lengths of ~2.2 km for Loop I (black line) and ~220 m (red line), and asymmetric dual loops having lengths of ~2.2 km for Loop I and ~220 m for Loop II under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). and ~220 m for Loop II under a span of 1 MHz (RBW 1 kHz and VBW 100 Hz). The asymmetric dual-loop feedback scheme demonstrated here is a better technique The asymmetric dual-loop feedback scheme demonstrated here is a better technique that effectively suppresses unwanted noise-induced oscillations and yields side-band free that effectively suppresses unwanted noise-induced oscillations and yields side-band free RF spectra when an equal percentags of feedback ratio was used. Furthermore, this be- RF spectra when an equal percentags of feedback ratio was used. Furthermore, this behavior ha shows vior shows tha that bettertsuppr better suppressi ession in cavity on in ca sidebands vity sideb can ands ca be obtained n be obby tain varying ed by vthe aryisecond ng the second feedback loop length using ODL. feedback loop length using ODL. 4. Conclusions 4. Conclusions In the present work, we experimentally demonstrate how to suppress the feedback- In the present work, we experimentally demonstrate how to suppress the feedback- induced frequency fluctuations from conventional single and dual-loop feedback schemes, induced frequency fluctuations from conventional single and dual-loop feedback with feedback ratio controlled for short as well as long optical cavities. The device under schemes, with feedback ratio controlled for short as well as long optical cavities. The de- test was a two-section InAs/InP QDash MLLs operating at 21 GHz and emitting at 1550 nm. vice under test was a two-section InAs/InP QDash MLLs operating at 21 GHz and emitting These results reveal that dual-loop feedback with precise alignment of the loop lengths and fine-tuning of feedback ratio through external feedback cavities effectively suppresses external cavity sidebands. The proposed asymmetric dual-loop feedback configuration makes semiconductor mode-locked lasers promising for the development of compact and cost-effective optoelectronic oscillators with low timing jitter. The resulting setup using this method is also integrable in a hybrid integrated optics, compact fiber loops and stable OEOs. Author Contributions: Conceptualization, H.A.; methodology, H.A.; software, T.A.A., M.A. and H.A.; validation, T.A.A., M.A. and H.A.; formal analysis, T.A.A., M.A. and H.A.; investigation, T.A.A., M.A. and H.A.; resources, H.A.; data curation, H.A.; writing—original draft preparation, T.A.A. and H.A; writing—review and editing, T.A.A., M.A. and H.A.; visualization, H.A.; supervision, H.A.; project administration, T.A.A. and H.A.; funding acquisition, T.A.A. and H.A. All authors have read and agreed to the published version of the manuscript. Photonics 2022, 9, 376 8 of 9 Funding: Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2022R71), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. 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Journal

Photonics – Multidisciplinary Digital Publishing Institute

Published: May 26, 2022

Keywords: semiconductor lasers; optical feedback; mode-locked lasers; power-split ratio; frequency-fluctuations

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