The Opto-electronic Oscillator (Oeo) Review and Recent Progress
Wideband tunable microwave signal generation in a silicon-micro-ring-based optoelectronic oscillator
Phuong T. Do
1Université Paris-Saclay, ENS Paris-Saclay, CNRS, Lumin, 4, artery des Sciences, 91190 Gif-sur-Yvette, France
Carlos Alonso-Ramos
iiUniversité Paris-Saclay, CNRS, Middle de Nanosciences et de Nanotechnologies, ten Boulevard Thomas Gobert, 91120 Palaiseau, France
Xavier Le Roux
iiUniversité Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 10 Boulevard Thomas Gobert, 91120 Palaiseau, France
Isabelle Ledoux
1Université Paris-Saclay, ENS Paris-Saclay, CNRS, Lumin, 4, avenue des Sciences, 91190 Gif-sur-Yvette, France
Bernard Journet
aneUniversité Paris-Saclay, ENS Paris-Saclay, CNRS, Lumin, four, artery des Sciences, 91190 Gif-sur-Yvette, France
Eric Cassan
2Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, ten Boulevard Thomas Gobert, 91120 Palaiseau, France
Received 2019 Mar vii; Accustomed 2020 Mar 26.
- Information Availability Statement
-
The data that back up the findings of this report are bachelor from the corresponding writer upon reasonable request.
Abstract
Si photonics has an immense potential for the development of compact and low-loss opto-electronic oscillators (OEO), with applications in radar and wireless communications. All the same, current Si OEO take shown a express performance. Si OEO relying on directly conversion of intensity modulated signals into the microwave domain yield a limited tunability. Wider tunability has been shown by indirect stage-modulation to intensity-modulation conversion. However, the reported tuning range is lower than 4 GHz. Here, nosotros advise a new arroyo enabling Si OEOs with broad tunability and direct intensity-modulation to microwave conversion. The microwave point is created by the chirapsia between an optical source and single sideband modulation signal, selected by an add-drop ring resonator working every bit an optical bandpass filter. The tunability is achieved by changing the wavelength spacing between the optical source and a resonance tiptop of the resonator. Based on this concept, we experimentally demonstrate microwave signal generation betwixt 6 GHz and 18 GHz, the widest range for a Si-micro-ring-based OEO. Moreover, preliminary results indicate that the proposed Si OEO provides precise refractive index monitoring, with a sensitivity of 94350 GHz/RIU and a potential limit of detection of only ten−8 RIU, opening a new route for the implementation of high-functioning Si photonic sensors.
Discipline terms: Nanoscience and engineering, Optics and photonics
Introduction
The generation of broadband and low noise microwave and millimeter moving ridge signals is important for many applications, including among others, radars 1,two wireless communications 3 , optical signal processing four , warfare systems 5,half-dozen and mod instrumentation 7 . The optoelectronic oscillator (OEO) is a particularly interesting solution to generate microwave and millimeter signals due to its adequacy to provide direct synthesis of spectrally pure and wideband tunable signals 8,9 . A classical OEO has a fundamentally multi-mode behavior x , with mode spacing associated with the km-long optical cobweb delay lines used inside the closed-loop organization. To select the desired oscillation mode, a microwave filter with high quality gene (Q RF ) is typically included within the closed path ten,11 . To reach variable frequency generation, this microwave filter needs to exist tunable. Withal, the realization of microwave filter with loftier Q RF and wide frequency tunability is practically hard to realize, especially for high operation frequencies 12 . In dissimilarity, microwave photonics (MWP) is a promising alternative solution to overcome this limitation, allowing reconfigurable microwave signal generation in OEO with a wide tuning range 11–13 . In addition, the progress of integrated microwave photonics (IMWP) fourteen provides at present a solid framework for the full OEO integration. Several approaches have been recently demonstrated 15–17 . M. Merklein et al., in 15 reported the generation of ultrawide frequency tunable signals up to xl GHz past using OEO based on stimulated Brillouin scattering (SBS). However, this arroyo requires harnessing lite-sound interactions on flake, based on not-standard chalcogenide materials and the utilise of ii lasers. In sixteen , an integrated optoelectronic oscillator based on InP was investigated, but the frequency tunability was limited to only 20 MHz. Remarkable low phase dissonance of −110 dBc/Hz and −130 dBc/Hz at 10 kHz and 10 MHz offset frequency has been demonstrated for OEO based on nonlinear frequency comb generation in silicon nitride micro-resonators 18 . However, the frequency of the microwave betoken is determined by the free-spectral range (FSR) of the ring. In one case the band is fabricated, the FSR can merely exist slightly shifted, substantially limiting the tunability of this approach. On the other mitt, the silicon on insulator (SOI) applied science has been identified as a promising solution to implement ultra-compact and depression-price OEO, which could exist fabricated using already existing large book fabrication facilities. The unique potential of Si to integrate photonic and electronic functionalities inside a single chip, together with the availability of high-performance fundamental edifice blocks, eastward.k. all-Si modulators 19,20 and Ge on Si photodetectors 21,22 and even silicon RF power amplifiers 23 , make Si an ideal candidate for the development of high-performance OEOs. For example, remarkable low phase dissonance has been recently demonstrated for an OEO that implements all electronic and photonic building blocks with silicon engineering, expect for the laser 24 . Even if the reported tuning range of this OEO is of simply a few MHz, these results illustrate the great potential of the silicon technology for the implementation of high-performance OEOs.
However, the scarce demonstrations of Si-based OEOs showed a limited functioning in terms of tunability. Direct conversion of intensity-modulated signals into the microwave domain has been shown based on quadratic detection of two successive manual lines in the drop-port of the ring 25 . The microwave frequency is determined by the FSR of the ring, limiting its tunability. In addition, microwave signal generation requires few-millimeters long ring resonators, which are difficult to implement. Microwave generation has also been demonstrated in Si-micro-resonator-based OEO, implementing indirect phase-modulation to intensity-modulation conversion. In this instance, the notch filtering is performed by a micro-disk in operating in an all-pass configuration. This approach provided a limited tunability range between 3 and 6.eight GHz 17 . Here, we propose a new approach for the implementation of Si-micro-ring OEO that enables wideband tunability in the microwave betoken generation, exploiting direct intensity-modulation to microwave conversion. As schematically shown in Fig.1a, the laser source is split in two paths. One path comprises an intensity modulator and an add-drop ring resonator (RR). The other path goes directly to the photodetector. The oscillation indicate is created by the straight translation of the intensity modulation into the microwave domain, provided by the beating between the optical source (direct path) and one of the sideband lobes generated by the intensity modulator (path with intensity modulator and RR). This sideband lobe is selected by one manual line of the silicon add together-drop RR, that serves as optical bandpass filter. The frequency of the generated microwave signal is adamant by the wavelength separation between the laser source and the resonance of the RR. By using but one of the transmission lines of the RR, we substantially relax the requirements on the free-spectral-range of the band, while providing flexible tuning. Nosotros experimentally evidence that past tuning the wavelength of the source, the microwave frequency generated past the OEO can be tuned betwixt 5.9 GHz and 18.2 GHz. This is, to the best of our knowledge, the widest tunability range reported for a Si-micro-ring-based OEO. A phase noise well-nigh −110 dBc/Hz at the offset frequency of i MHz, comparable with land-of-the-art photonic tunable OEO 16,17 , is measured for dissimilar oscillation frequencies forth the 12 GHz tuning range. Meantime, the proposed OEO performs a precise translation of the light amplification by stimulated emission of radiation-to-RR wavelength separation into the microwave domain, where it can be precisely measured. And then, if the laser wavelength is stock-still, monitoring of the microwave frequency shifts provides authentic data of the variations in the resonance wavelength of the RR, which can be related to variations in the refractive index. This way, by exploiting the improved spectral resolution in the microwave domain, the proposed OEO can besides serve every bit a high-performance refractive alphabetize sensor. Preliminary experimental results prove a sensitivity of 94350 GHz/RIU, i.e. a 40-fold improvement compared to previously reported microwave-photonic silicon refractive alphabetize sensors 26 . We estimated the achievable limit of detection (LOD) from the phase noise measurements, obtaining a remarkably low value of x−8 RIU. These results illustrate the potential of this approach for the implementation of high-performance Si sensors, e.g. for lab-on-a-flake biosensing applications 27 .
(a) Schematic of the proposed OEO structure and (b) Principle of functioning of the proposed tunable OEO. In (a), IM: Intensity modulator, RR: Ring resonator and PD: Photo-detector. In (b), the ruby curve corresponds to the optical carrier (or laser source frequency); the orangish bend illustrates the sideband lobes of the modulated bespeak, the blue curve indicates the optical transfer part of the RR and the light-green 1 represents the generated RF frequency f RF .
Results
Principle of operation
In the proposed tunable OEO configuration, shown in Fig.1, the optical signal coming from the light amplification by stimulated emission of radiation light source (frequency f0) is separated into ii artillery. I is connected directly to the photodetector (PD), while the other feeds an intensity modulator (IM) followed past a silicon ring resonator (RR) in add-drop configuration. In this scheme, the input signal of the PD always comprises a part of the un-modulated laser light beam. At the initial stage, the modulator output point grows seeded by the white dissonance existing inside the loop. If one modulated point (f R ) can go through the optical transfer office of the resonator, it can then exist combined with the optical carrier (f0) in the PD generating a beating of frequency f b . If the distance betwixt the optical carrier (f0) and the signal at f R falls within the working range of the loop, the generated beating point tin can be converted to the RF domain f RF (f RF = f b = |f0 − f R |). At the second round-trip of the loop, the generated RF signal is sent back to the modulator. At this phase, only one single side-band modulation signal can friction match the RR resonance superlative at frequency f R (come across Fig.1b). The RR now serves equally an optical bandpass filter, selecting merely one sideband lobe of the modulated signal. The signal goes to the PD at the second-round trip of the loop, creating again an RF signal with frequency f RF . After this point, the loop oscillates with an oscillation frequency of f RF .
The main idea behind this approach is to control the frequency of the microwave signal by the wavelength spacing betwixt the laser source and the resonance wavelength of the resonator. Since this spacing tin can exist changed either past sweeping the wavelength of the laser or by shifting the resonance acme of the RR, this approach yields a simple tunability mechanism. The light amplification by stimulated emission of radiation wavelength can be swept using a tunable laser. By monitoring the microwave signal generated, it is possible to choose the proper wavelength value yielding the desired microwave frequency. In a similar style, the resonance peak of the RR can be tuned, for case exploiting the thermo-optic upshot to change the effective alphabetize of the waveguide, thereby shifting the resonance wavelength 28 .
Demonstration of the proposed tunable optoelectronic oscillator
Even if micro-disk and micro-sphere resonators could yield higher quality factors, for the implementation of the optical bandpass filter in our OEO, we have chosen a micro-band resonator. Micro-spheres are hard to integrate on chip together with other photonic components. On the other hand, high-Q micro-disk typically rely on large bending radii that result in highly multimode behavior, which could distort the response of the OEO, e.yard. by mode hopping due to the existence of several resonances inside the working range of the OEO. Conversely, micro-ring resonators can be made single-mode, regardless of the radius, just past ensuring the waveguide is single mode.
To demonstrate the proposed operation principle, we used an integrated Si add-driblet RR and external intensity modulator, photodetector and microwave circuitry (see Fig.two). Information technology should be pointed out that all external building blocks take already been demonstrated in the silicon technology. Thus, monolithic integration of the complete OEO is technologically feasible. Nevertheless, the proposed scheme serves as a demonstrator of the principle, while providing a unproblematic and flexible implementation, equally unlike Si ring resonators can be tested using the same global excursion.
Experimental setup employed for the demonstration of the proposed tunable OEO. EDFA: Erbium doped amplifier, PC: Polarization controller, OSA: Optical spectrum analyzer, Thousand: RF amplifier and ESA: Electric spectrum analyzer.
The Q in the add-drop band resonator is ane of the key parameters determining the operation of the proposed OEO. Higher Q yields better selectivity of the optical filter, that will determine the purity and stability of the microwave betoken generated. The ring resonator was implemented on a standard SOI applied science with a 220 nm thick Si thin movie on top of a 3 μyard buried oxide layer. Nosotros optimized the band to operate in transverse-magnetic (TM) polarization, thereby minimizing the detrimental consequence of sidewall roughness in propagation loss.
Figure3a shows an electron microscope image of the add-driblet ring resonator. Light is injected into the resonator through the input port and collected from the through or drop ports. A detailed view of the fiber-bit grating couplers used to inject and excerpt the light is shown in Fig.3c. A 450 nm wide strip waveguide was chosen to ensure single mode operation near 1.54 μm wavelength, with a resonator length 50 of 1 mm. In the RR, adiabatic bends 29 were implemented to reduce losses coming from the manner mismatch at the transition between directly and circular bend waveguides. Spline curve was fabricated with 20 μgrand radius and angular spline coverage angle at 45° (come across Fig.3b). Effigy3d shows the measured transmission spectra (see Methods) of both through and drop ports of the RR with 300 nm coupling gap and 4.five μm coupling length. An optical FSR λ of 640 pm was obtained, corresponding to a microwave free-spectral range of FSR fre ≈ 77 GHz. The optical quality gene of the ring resonator was Q opt ~ viii.ane × ten4 (obtained past Lorentzian plumbing fixtures of the resonance peaks).
Scanning electron microscope images of: (a) consummate add-driblet RR, (b) adiabatically bended waveguide and (c) fiber-chip grating coupler. (d) Optical transmission of the silicon RR (coupling gap of 300 nm and coupling length of 4.five μgrand).
Effigy2 shows the experimental setup used to demonstrate the proposed OEO approach. We used a 90/10 optical splitter to separate the light source coming from a CW tunable laser (Yenista TUNIS-T100S). 90% of the optical power was sent to an Erbium doped amplifier (EDFA) followed by the intensity modulator, the silicon RR and a 2nd EDFA. After that, a 50/l optical combiner was used to collect the point from the output of a 2d EDFA and the optical power source betoken (see Fig.2). Polarization controllers (PC) were used in the upper arm of the splitter to friction match the polarization of the laser source and the indicate at the output of the 2nd EDFA. At the output of the optical combiner, one arm was continued to an optical spectrum analyzer (OSA) to monitor the laser or resonance wavelength, while the other arm was connected to the PD. The terminal setup included an RF amplifier, a ninety:10 RF coupler and an electrical spectrum analyzer (ESA).
During the experiments, the resonance frequency of the RR f R was kickoff monitored using an OSA. And so, the oscillation frequency is generated by placing the laser wavelength (frequency f0) close to a resonance peak of the RR. Figure4a illustrates the electrical spectrum of the generated microwave signal within a frequency span of 13.5 GHz and with a resolution bandwidth of 200 kHz, showing an oscillation frequency at five.9 GHz. In improver, college-order harmonic peaks at xi.8 GHz and 17.vii GHz were also observed, caused by the nonlinearity in the OEO loop 17 . The zoomed-in view of the v.9 GHz signal with a frequency bridge of 6 MHz and a resolution bandwidth of 2.two kHz is shown in Fig.4b. The microwave point exhibits a high signal to racket ratio of sixty dB with the linewidth around 120 kHz. To evaluate the stability and the quality of the generated signal, we measured its phase noise with an electrical RF analyzer (Agilent E4446A). The measured phase racket, shown in Fig.4c. Information technology is worth mentioning that the relatively high and flat phase noise level close to the carrier frequency, upwardly to 100 KHz, is produced by wavelength fluctuations of our laser 25 . The effect of light amplification by stimulated emission of radiation fluctuations is negligible for higher frequencies. There, our OEO yields the measured stage racket of −115 dBc/Hz at 1 MHz offset frequency from the carrier. This result is comparable with the phase dissonance recently reported in photonic OEO implemented in silicon 17 .
(a) Oscillation spectrum of the generated betoken based on our proposed approach, (b) The zoom-in viewed and (c) The phase noise feature of the created signal.
In gild to demonstrate the wide tunability of the proposed approach, we swept the light amplification by stimulated emission of radiation wavelength while keeping the resonance pinnacle unchanged. To do so, the Si bit was placed on a Peltier module to stabilize the temperature, thereby preventing resonant wavelength shifts produced by thermal drifts. The resonance wavelength of the RR at 1540.25 nm was first measured with the OSA. Figure5a plots the fundamental tone of the oscillation spectrum obtained by sweeping the light amplification by stimulated emission of radiation wavelength betwixt 1540.10 nm and 1540.xx nm. These experimental results demonstrate an unprecedentedly wide frequency tunability for a Si-micro-resonator-based OEO, ranging from 5.9 GHz to eighteen.2 GHz. The tuning range is express here by the bandwidth of the microwave amplifier used inside the loop. Forth the 12 GHz spectrum, a power fluctuation in the 1-~3 dB range has been observed. This fluctuation could be attributed to ii main effects: i) mechanical variations of the system that change the fiber-chip coupling loss, ii) fluctuations of the supply current applied to the EDFA, as the pump electric current variation changes of the overall loop gain 11 . Nevertheless, this variation range is slightly smaller than what was reported in 17 . Note that, the minimum frequency tuning step of the OEO is adamant by the precision in controlling the wavelength of the laser and the temperature of the ring resonator. For example, working at a stock-still temperature, the minimum tuning step of the OEO is ~125 MHz, determined past the 1 pm resolution of the laser source we are currently using. Higher resolution could exist achieved with a laser source enabling narrower wavelength steps.
(a) Oscillation frequency generated with different laser wavelengths, (b) Plot of the oscillation frequency depending on the beating frequency. f b = |f0 − f R |, (c) Phase racket characteristic for differences generated signals and (d) Observed phase noise level at 1 MHz starting time frequency from carrier.
From the frequency of the laser and resonance wavelength, we calculated the chirapsia frequency, i.east. f b = |f0 − f R |. The evolution of the oscillation signal (f osc ) as a function of the beating frequency is shown in Fig.5b. The oscillation frequency clearly follows the beating frequency, showing a near perfect linear evolution with the modification of the laser frequency separation from the RR resonance frequency (regression coefficient ≈ 0.9997). In this case the RR resonance frequency is smaller than the laser one, which explains why the oscillation frequency increases with decreasing laser frequency.
We have measured the phase noise of the proposed tunable OEO, for all the generated signals (see Fig.5c). As has been explained, the phase noise at frequency offsets below 100 kHz is governed by the random wavelength fluctuations of the light amplification by stimulated emission of radiation, resulting in unstable signals that are difficult to compare. For frequency offsets beyond 100 kHz, the phase noise is governed by the bodily response of the OEO, exhibiting a phase noise variation below 7 dB in all the frequency span between v.9 GHz and eighteen.ii GHz. Effigy5d represents the noise level at 1 MHz offset frequency from the carrier, obtained from the measurements in Fig.5c. The stage racket of the generated microwave signals remains close to −110 dBc/Hz at the offset frequency of 1 MHz, which emphasizes a key reward of such an OEO to have a constant phase noise level with the increase in oscillation frequency xxx .
The OEO as refractive index sensor
The oscillation frequency of the proposed OEO is determined past the relative distance between the laser and the ring resonance. The band resonance depends on the refractive index of the waveguide and its environment. Thus, past fixing the light amplification by stimulated emission of radiation wavelength and monitoring the shifts in the microwave oscillation frequency, it is possible to precisely estimate the variations in the optical alphabetize 31,32 . This sensing mechanism is schematically described in Fig.6a. At the initial phase, the laser and the ring resonance are separated past a given distance, generating a microwave signal oscillation with frequency . Any variation in the refractive alphabetize of the waveguide or its environment shifts the ring resonance, producing a proportional shift in the frequency of the microwave oscillation, from to . As a uncomplicated ways to implement this index change, we changed the sample temperature with a Peltier module. The temperature variation of the ring resonator shifts its resonance wavelength. We measured this wavelength shift in the optical domain (encounter Fig.6b) and extracted the index variation. At the same time, we monitored the variations in the oscillator frequency (see Fig.6c). Then, every bit shown in Fig.6d, we could plot the oscillation frequency shift as a office of the refractive index change, obtaining a linear relation.
(a) Principle of operation of the sensing system based on the proposed OEO configuration. f0: Optical carrier (or laser source frequency), f n1 and f n2: resonances wavelength associated with cladding index n1 and n2,respectively, f RF1 and f RF2: generated oscillation frequency associated with cladding alphabetize n1 and ntwo, respectively, Δf = |f RFone − f RFii|. (b) Resonance wavelength, (c) Oscillation frequency simultaneously collected for each setting temperature indicate in the Peltier module and (d) Adding of the oscillation change depending on refractive index variation.
In order to get a stable local temperature over the RR sample region, experiments only started 5 minutes after setting the desired temperature point in the Peltier module (in the range from 25 °C to xxx °C with 1 °C step size). The RR was beginning characterized in the optical domain. Correct after the optical characterization, we characterized the frequency generated past the OEO. We used a highly stable distributed feedback (DFB) diode laser (model 1905 LMI) with wavelength nearly one.54μm. Optical and microwave spectra, measured while scanning the temperature with the Peltier module between 25 °C and xxx °C, are shown in Fig.6b,c, respectively. The resonance wavelength shifts towards longer wavelengths with increasing temperature, resulting in a microwave frequency increase from 11 GHz to 18.5 GHz. These results are in proficient understanding with previous theoretical and experimental studies fabricated for SOI ring resonators 28,31 . For case, in 31 the RF spectra is shown for OEO frequency changing between 10 GHz and 18 GHz. Notwithstanding, no report of phase racket is performed in 31 , hindering the evaluation of this OEO as a microwave photonic sensor.
The variation in the optical index was estimated from the measured optical spectra every bit Δn eff = n eff .Δλ/λ 33 . We calculated the waveguide refractive alphabetize n eff with a vectorial mode solver. We took the first detected resonance wavelength and oscillation frequency as the reference betoken. And so we estimated the refractive alphabetize change from the wavelength resonance shift. The oscillation frequency shift as a function of the estimated refractive alphabetize modify is plotted in Fig.6c. We obtain a gradient of 94350 GHz/RIU (linear fitting). This value is twoscore times better than previously reported microwave-photonic silicon refractive alphabetize sensors using a SOI resonator device to notice frequency changes produced by cladding refractive index change 26 . Note that both, our work and that reported in 26 realize on integrated Si ring resonator and external components. On the other hand, the sensor presented in 26 implements a noise reduction algorithm based on stage-simply filtering and uses two etching steps for the fabrication of the Si micro-resonator, one total-compose for the interconnection waveguides and i shallow etch for the disk resonator. Nosotros do not employ any mail-processing filtering technique and fabricate our device with a single compose step. Our arroyo exhibits a stable phase noise level about −110 dBc/Hz at 1 MHz offset frequency from the carrier, allowing a system resolution of ane MHz. From this stage racket value, we estimate a limit of detection (LOD) every bit low equally 10−8 RIU.
Word
In summary, nosotros have proposed and experimentally demonstrated a new approach for the implementation of widely tunable Si-micro-resonator-based OEO. Previously reported Si OEO relied on direct conversion of intensity modulation to the microwave domain, with limited tunability, or indirect phase-modulation to intensity modulation conversion. Here, we show a direct conversion scheme providing wide tunability. In the proposed scheme, the microwave signal is created by the beating between a laser light source and a unmarried sideband modulation signal selected by an add-drop ring resonator working as an optical bandpass filter. The microwave frequency is determined by the wavelength separation betwixt the source and the band resonance, providing simple tunability by sweeping the light amplification by stimulated emission of radiation wavelength. Capitalizing on this concept, nosotros demonstrate microwave signal generation between 5.9 GHz and 18.2 GHz, only limited here is the bandwidth of the employed RF amplifier. This is the widest microwave generation span reported for a Si-based OEO. Additionally, a depression phase dissonance level close −110 dBc/Hz at 1 MHz offset frequency is achieved for all microwave frequencies, illustrating the potential of the approach for the generation of stable high oscillation frequency signals. As mentioned higher up, the poor phase noise levels at frequency offset close to the carrier mainly arise from the laser wavelength fluctuations. Introducing a DFB light amplification by stimulated emission of radiation or feedback control loop could lower these effects 16 . Indeed, by using a laser with a meliorate stability, a ~40 dB of noise level reduction at 10 kHz start frequency was previously observed 34 . Moreover, the OEO phase racket could finer exist improved by increasing the quality factor of the micro-ring resonator. This could be achieved by reducing the waveguide propagation loss past thermal oxidation of the Si waveguides to reduce roughness and thus the propagation loss level 35 . Based on experimental label of propagation loss in state-of-the-art silicon waveguides, nosotros could target more a ten-fold increase of the resonator Q. And so, following the Leeson'southward model 36 , nosotros could expect phase racket reduction of the OEO exceeding ten dB. Thus, the phase noise of the proposed system could reach a level of less than −100 dBc/Hz at ten kHz start from carrier, which level is compatible with real applications 37 . Further minimization in phase racket holding of the loop could be accomplished with a total integration of the OEO organization on a unmarried chip. Furthermore, we extended this approach for refractive alphabetize sensing application, harnessing high spectral resolution in the microwave domain. We accept measured a sensitivity of 94350 GHz/RIU, 40 times better than country-of-the-fine art Si counterparts microwave photonic silicon refractive alphabetize sensor 26 and take estimated a potential limit of detection as depression as 10−eight RIU for an interrogation speed of i MHz. We believe that the approach proposed here will expedite the evolution of a new generation of high-performance Si OEO with an immense potential for a plethora of applications, including, radar, wireless communications, optical indicate processing, warfare systems and lab-on-a-scrap biosensing.
Methods
Device fabrication and experimental characterization
Fabrication started from a SOI wafer with a 220 nm thick Si thin film on top of a three μm buried oxide layer. The patterns were lithographically defined in a 100 nm ZEP-520A photoresist by using e-beam lithography. Afterward lithography, the patterns were transferred using ICP etching with SFsix and CivF8 gases. Following the waveguide fabrication, a 2 μm thick PMMA layer was deposited over the chip surface for protection. No additional post processing was done.
For the optical characterization of the ring resonators, a tunable light amplification by stimulated emission of radiation was coupled to the input waveguide through an input grating coupler with a properly adjusted coupling angle and extracted the same way from an output grating. The grating couplers were optimized for TM polarization, yielding a fiber to cobweb optical manual of −10.five dB at 1540 nm wavelength. A polarization controller (PC) was used to set a proper polarization at the input of the grating.
Acknowledgements
This work is included in the MORSE projection supported by the LaSIPS (Paris-Saclay University). The sample fabrication was performed at the Plateforme de Micro-Nano-Technologie/C2N, which was partially funded past the "Conseil Général de fifty'Essonne". This work was also partly supported by the French RENATECH network. Information presented in this manuscript are available in 34 .
Author contributions
P.T.D. and C.A.R. proposed the concept. P.T.D., Eastward.C. and B.J. designed the devices and performed the simulations. X.L.R. and P.T.D. made the devices. P.T.D., C.A.R., East.C. and B.J. performed the experimental characterizations. P.T.D., C.A.R., Ten.L.R., I.L., B.J. and Due east.C. discussed the results and wrote the manuscript.
Information availability
The data that support the findings of this study are bachelor from the corresponding author upon reasonable request.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher'due south note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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