Wideband tunable microwave signal generation in a silicon-based optoelectronic oscillator
Phuong T.Do, Carlos Alonso-Ramos, Xavier Le Roux, Isabelle Ledoux, Bernard Journet, Eric Cassan
WWideband tunable microwave signal generation in asilicon-based optoelectronic oscillator
Phuong T.Do , Carlos Alonso-Ramos , Xavier Le Roux , Isabelle Ledoux , BernardJournet , and Eric Cassan LPQM (CNRS UMR-8537, ´Ecole normale sup ´erieure Paris-Saclay, Universit ´e Paris-Saclay, 61 avenue du Pr ´esidentWilson, 94235 Cachan Cedex, France C2N (CNRS UMR-9001), Univ. Paris-Sud, Universit ´e Paris-Saclay, 10 Boulevard Thomas Gobert, 91120Palaiseau, France * [email protected] ABSTRACT
Si photonics has an immense potential for the development of compact and low-loss opto-electronic oscillators (OEO), withapplications in radar and wireless communications. However, current Si OEO have shown a limited performance. Si OEO relyingon direct conversion of intensity modulated signals into the microwave domain yield a limited tunability. Wider tunability has beenshown by indirect phase-modulation to intensity-modulation conversion, requiring precise control of the phase-modulation. Here,we propose a new approach enabling Si OEOs with wide tunability and direct intensity-modulation to microwave conversion.The microwave signal is created by the beating between an optical source and single sideband modulation signal, selected byan add-drop ring resonator working as an optical bandpass filter. The tunability is achieved by changing the wavelength spacingbetween the optical source and resonance peak of the resonator. Based on this concept, we experimentally demonstratemicrowave signal generation between 6 GHz and 18 GHz, the widest range for a Si-based OEO. Moreover, preliminary resultsindicate that the proposed Si OEO provides precise refractive index monitoring, with a sensitivity of 94350 GHz/RIU and apotential limit of detection of only 10 RIU, opening a new route for the implementation of high-performance Si photonicsensors.
Introduction
The generation of broadband and low noise microwave and millimeter wave signals is important for many applications, includingamong others, radars, wireless communications, optical signal processing, warfare systems, and modern instrumentation .Among the different approaches to generate microwave and millimeter signals, the optoelectronic oscillator (OEO) is aparticularly interesting solution due to its capability to provide, by direct synthesis, spectrally pure and wideband tunablesignals . A classical OEO has a fundamentally multi-mode behavior , with mode spacing associated with the km-long opticalfiber delay lines used inside the closed-loop system. To select the desired oscillation mode, a microwave filter with high qualityfactor (Q RF ) is typically included inside the closed path . To achieve variable frequency generation, this microwave filter needsto be tunable. However, a microwave filter with high Q RF and wide frequency tunability is practically hard to realize, especiallyfor high operation frequencies . In contrast, microwave photonic (MWP) are a promising alternative solution to overcome thislimitation, allowing reconfigurable microwave signal generation in OEO with a wide tuning range . In addition, the progressof integrated microwave photonics (IMWP) provides now a solid framework for the full integration of an OEO. Several effortsin this direction have been demonstrated recently . M. Merklein et al., in demonstrated ultrawide frequency tunablesignals up to 40 GHz by using OEO based on stimulated Brillouin scattering (SBS). However, the system explored thereinis complicated as it requires harnessing light-sound interactions on chip, based on non-standard chalcogenide materials andthe use of two lasers. In , an integrated optoelectronic oscillator based on InP was investigated, but the reported frequencytunability range was limited to only 20 MHz. On the other hand, the silicon on insulator (SOI) technology has been identifiedas a promising solution to implement ultra-compact and low-cost OEO, which could be fabricated using already existing largevolume fabrication facilities. The unique potential of Si to integrate photonic and electronic functionalities within a single chip,together with the availability of high-performance key building blocks, e.g. all-Si modulators and Ge on Si photodetectors , make Si an ideal candidate for the development of high-performance OEOs. However, the scarce demonstrations ofSi-based OEOs showed a limited performance in terms of tunability. Direct conversion of intensity-modulated signals into themicrowave domain has been shown based on quadratic detection of two successive transmission lines in the drop-port of thering . The microwave frequency is determined by the free-spectral-range (FSR) of the ring, limiting its tunability. In addition,1icrowave signal generation requires few-millimeters long ring resonators, which are difficult to implement. Microwavegeneration has also been demonstrated in Si-based OEO, implementing indirect phase-modulation to intensity-modulationconversion, relying on a notch filtering provided by a micro-disk operating in an all-pass configuration. This approach requiresprecise control of the phase modulation, and provided a limited tunability range between 3 and 6.8 GHz . Here, we proposea new approach for the implementation of Si OEO that allows for wideband tunability in the microwave signal generation,based on a direct intensity-modulation to microwave conversion. As schematically shown in Fig.1a, the laser source is split intwo paths. One path comprises an intensity modulator and an add-drop ring resonator (RR). The other path goes directly tothe photodetector. The oscillation signal is created by the direct translation of the intensity modulation into the microwavedomain, provided by the beating between the optical source (direct path) and one of the sideband lobes generated by theintensity modulator (path with intensity modulator and RR). This sideband lobe is selected by one transmission line of thesilicon add-drop RR, that serves as optical bandpass filter. The frequency of the generated microwave signal is determinedby the wavelength separation between the laser source and the resonance of the RR. By using only one of the transmissionlines of the RR, we substantially relax the requirements on the free-spectral-range of the ring, while providing flexible tuning.We experimentally show that by tuning the wavelength of the source, the microwave frequency generated by the OEO canbe tuned between 5.9 GHz and 18.2 GHz. This is, to the best of our knowledge, the widest tunability range reported for aSi-based OEO. A phase noise near -110 dBc/Hz at the offset frequency of 1 MHz, comparable with state-of-the-art photonicOEO , is measured for different oscillation frequencies along the 12 GHz tuning range. Concurrently, the proposed OEOperforms a precise translation of the laser-to-RR wavelength separation into the microwave domain, where it can be preciselymeasured. Then, if the laser wavelength is fixed, monitoring of the microwave frequency shifts provides accurate informationof the variations in the resonance wavelength of the RR, which can be related to variations in the refractive index. This way, byexploiting the improved spectral resolution in the microwave domain, the proposed OEO can also serve as a high-performancerefractive index sensor. Preliminary experimental results show a sensitivity of 94350 GHz/RIU, i.e. a 40-fold improvementcompared to previously reported microwave-photonic silicon refractive index sensors . Based on the measured phase noise,we estimated a remarkably low achievable limit of detection (LOD) of only 10 RIU. These results illustrate the potential ofthis approach for the implementation of high-performance Si sensors, e.g. for lab-on-a-chip biosensing applications . Results
Principle of operationFigure 1. a Schematic of the proposed OEO structure and b Principle of operation of the proposed tunable OEO. In a , IM:Intensity modulator, RR: Ring resonator and PD: Photo-detector. In b , the red curve corresponds to the optical carrier (or lasersource frequency); the orange curve illustrates the sideband lobes of the modulated signal, the blue curve indicates the opticaltransfer function of the RR and the green one represents the generated RF frequency f RF . n the proposed tunable OEO configuration, shown in Fig.1, the optical signal coming from the laser light source (frequencyf ) is separated into two arms. One is connected directly to the photodetector (PD), while the other feeds an intensity modulator(IM) followed by a silicon ring resonator (RR) in add-drop configuration. In this scheme, the input signal of the PD alwayscomprises a part of the un-modulated laser light beam. At the initial stage, the modulator output signal grows, just seeded bywhite noise existing inside the loop. If one modulation output signal can go through the optical transfer function of the resonatorat frequency f R , this signal can then be combined with the optical carrier (f ) at either its left or right sides to generate a beatingof frequency f b at the input of the PD. If the distance between the optical carrier (f ) and the signal at f R falls within the workingrange of the loop, the generated beating signal can be converted as a RF frequency f RF (f RF =f b = | f -f R | ) at the output of thePD. At the second round-trip of the loop, the generated RF signal is sent back to the modulator. At this stage, only one singlesideband modulation signal can match the RR resonance peak at frequency f R (see Fig.1b). The RR now serves as an opticalbandpass filter, selecting only one sideband lobe of the modulated signal. The signal goes to the PD at the second-round trip ofthe loop, creating again an RF signal with frequency f RF . After this point, the loop oscillates with an oscillation frequency atf RF .The main idea behind this approach is to control the frequency of the microwave signal by the wavelength spacing betweenthe laser source and the resonance wavelength of the resonator. Since this spacing can be changed either by sweeping thewavelength of the laser or by shifting the resonance peak of the RR, this approach yields a simple tunability mechanism. Demonstration of the proposed tunable optoelectronic oscillator
To demonstrate the proposed operation principle, we used an integrated Si add-drop RR and external intensity modulator,photodetector and microwave circuitry (see Fig. 2). Note that all external building blocks have already been demonstratedin the silicon technology. Thus, monolithic integration of the complete OEO is technologically feasible. Nevertheless, theproposed scheme serves as a demonstrator of the principle, while providing a simple and flexible implementation, as differentSi ring resonators can be tested using the same global circuit.
Figure 2.
Experimental setup employed for the demonstration of the proposed tunable OEO. EDFA: Erbium doped amplifier,PC: Polarization controller, OSA: Optical spectrum analyzer, G: RF amplifier and ESA: Electric spectrum analyzer.The Q in the add-drop ring resonator is one of the key parameters determining the performance of the proposed OEO.Higher Q yields better selectivity of the optical filter, that will determine the purity and stability of the microwave signalgenerated. The ring resonator was implemented on a standard SOI technology with a 220 nm thick Si thin film on top of a3 µ m buried oxide layer. We optimized the ring to operate in transverse-magnetic (TM) polarization, thereby minimizing thedetrimental effect of sidewall roughness in propagation loss.A 450 nm wide strip waveguide was chosen to ensure single-mode operation near 1.54 µ m wavelength, with a resonatorlength L of 1 mm. In the design of the RR, adiabatic bends were considered in order to reduce losses coming from the modemismatch at the transition between straight and circular bend waveguides. A series of devices with different combinations ofcoupling lengths / coupling gaps were fabricated (see Methods) with the purpose to maximize the RR optical quality factor.Figure 3a shows an electron microscope image of the add-drop ring resonator. Details of the fiber-chip grating couplers andadiabatic bends are presented in Figs 3b and 3c, respectively. Figure 3d shows the measured transmission spectra (see Methods)of both through and drop ports of the RR with 300 nm coupling gap and 4.5 µ m coupling length, respectively. An FSR l of640 pm was obtained accordingly, corresponding to FSR f re ⇡
77 GHz, with a RR optical quality factor Q opt near 8.1x10 (obtained by fitting the resonance peaks through a Lorentzian function). igure 3. a Scanning electron microscope of the fabricated RR, detail of b Bended waveguide and c Grating coupler andtextbfd Optical transmission of the silicon RR (coupling gap: 300 nm, coupling length: 4.5 µ m).Figure 2 shows the experimental setup used to demonstrate the proposed OEO approach. We used a 90/10 optical splitter toseparate the light source coming from a CW tunable laser (Yenista TUNIS-T100S), in which 90% of the optical power was sentto an Erbium doped amplifier (EDFA) followed by the intensity modulator, the silicon RR and a second EDFA. After that, a50/50 optical combiner was used to collect the signal from the output of a second EDFA and the optical power source signal(see Fig.2). In the experimental setup, polarization controllers (PC) were used in the upper arm of the splitter in order to matchthe polarization of the laser source and the signal going out from a second EDFA. At the output of the optical combiner, onearm was connected to an optical spectrum analyzer (OSA) in order to monitor the laser or resonance wavelength, while theother arm was connected to the PD. The final setup included an RF amplifier, a 90:10 RF coupler and an electrical spectrumanalyzer (ESA). Figure 4. a ) Oscillation spectrum of the generated signal based on our proposed approach, b the zoom-in viewed and c thephase noise characteristic of the created signal. uring the experiments, the resonance frequency of the RR f R was first monitored using an OSA. Then, by placing the laserwavelength (frequency f ) close to an identified resonance peak, the beating between them was created. Figure 4a illustrates theelectrical spectrum of the generated microwave signal within a frequency span of 13.5 GHz and with a resolution bandwidth of200 kHz, showing an oscillation frequency at 5.9 GHz. In addition, higher-order harmonic peaks at 11.8 GHz and 17.7 GHzwere also observed, caused by the nonlinearity in the OEO loop . The zoomed-in view of the 5.9 GHz signal with a frequencyspan of 6 MHz and a resolution bandwidth of 2.2 kHz is shown in Fig. 4b, demonstrating a high signal to noise ratio of 60 dB.To evaluate the stability and the quality of the generated signal, its phase noise was measured by the automatic setup of anelectrical RF analyzer (Agilent E4446A), working in the “phase noise” mode. The related result is shown in Fig. 4c, indicatinga noise level of -115 dBc/Hz at 1 MHz offset frequency from the carrier. This result is comparable with the phase noise recentlyreported in photonic OEO implemented in silicon . Figure 5. a Oscillation frequency generated with different laser wavelengths, b Plot of the oscillation frequency depending onthe beating frequency. f b = | f -f R | , c Phase noise characteristic for differences generated signals and d Observed phase noiselevel at 1 MHz offset frequency from carrier.In order to demonstrate the wide tunability of the proposed approach, we swept the laser wavelength while keeping theresonance peak unchanged. To do so, the RR sample was placed on a Peltier module to keep a constant temperature, therebypreventing resonant wavelength shifts produced by temperature changes. The RR resonance wavelength at 1541.25 nm wasfirst observed from the OSA. Then, the laser wavelength was scanned between 1540.10 nm and 1540.20 nm. Figure 5a plotsthe fundamental tone of the oscillation spectrum obtained by changing the laser wavelength. These experimental resultsdemonstrate an unprecedentedly wide frequency tunability for a Si-based OEO, ranging from 5.9 GHz to 18.2 GHz. Note thatthe tuning range is limited here by the bandwidth of the microwave amplifier used inside the loop.From the corresponding frequency of the laser and resonance wavelength, we calculated the beating frequency, i.e. f b = | f -f R | .The evolution of the oscillation signal (f osc ) as a function of the beating frequency is shown in Fig.5b. The oscillation frequencyclearly follows the beating frequency, showing a nearly perfect linear evolution with the modification of the laser frequency eparation from the RR resonance frequency (regression coefficient ⇡ . The OEO as refractive index sensor
Since the proposed OEO configuration had an oscillation frequency dependent on the refractive index environment of the RRwaveguides, we have tested its characteristics for application in measuring optical index variations . A simple approachto implement this index change was adopted by changing the sample temperature with the Peltier module, thus changing thetemperature of the ring resonator, shifting its resonance wavelength. We measured this wavelength shift by wavelength scanning(see Fig. 6a) and extracted the index variation. At the same time, we monitored the variations in the oscillator frequency (seeFig. 6b). Then, as shown in Fig.6c, we could plot the oscillation frequency shift as a function of the refractive index change.
Figure 6. a Resonance wavelength, b Oscillation frequency simultaneously measured when temperature applied to the Peltiermodule increased and c Calculation 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 thedesired temperature point in the Peltier module (in the range from 25 to 30 with 1 step size). The RR was first characterizedin the optical domain. Right after the optical characterization, the closed loop OEO including the RR was measured. In thecarried-out closed-loop experiment, a distributed feedback (DFB) diode laser (model 1905 LMI) operated at around 1.54 µ mwavelength was used in order to provide a highly stable light source. The plot of the collected signals is shown in Figs 6a and b. At first glance, a fairly strong microwave frequency change can be observed between 11 GHz to 18.5 GHz. The resonancewavelength shifts towards longer wavelengths with increasing temperature. This result is in agreement with previous theoreticaland experimental analyses made for SOI ring resonators . Concurrently, the generated oscillation signal frequency alsoincreases.The variation in the optical index from the collected optical spectra was estimated as n e f f = n e f f . l / l . From avectorial optical mode solver, the waveguide refractive index n e f f was calculated. Considering the first detected resonancewavelength and its related oscillation frequency as the reference point, the change in refractive index was deduced accordingly,while the oscillation frequency shift from its reference value was also estimated. The deduced variation of the oscillation signalas a function of refractive index change is plotted in Fig.6c. By using a linear fitting procedure, a slope of 94350 GHz/RIU wasobtained. This value is 40 times better than previously reported microwave-photonic silicon refractive index sensors using aSOI resonator device to detect frequency changes produced by cladding refractive index change . In terms of limit of detection(LOD), as illustrated in the previous section, our proposed approach exhibited a stable phase noise level near -110 dBc/Hzat 1 MHz offset frequency from the carrier, allowing a system resolution of 1 MHz. From this phase noise value, a limit ofdetection LOD as low as 10 RIU can be estimated.
Discussion
In summary, we have proposed and experimentally demonstrated a new approach for the implementation of widely tunableSi OEO. Previously reported Si OEO relied on direct conversion of intensity modulation to the microwave domain, withlimited tunability, or indirect phase-modulation to intensity modulation conversion. Here, we show a direct conversion schemeproviding wide tunability. In the proposed scheme, the microwave signal is created by the beating between a laser lightsource and a single 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 between the source and the ring resonance, providingsimple tunability by sweeping the laser wavelength. Capitalizing on this concept, we demonstrate microwave signal generationbetween 5.9 GHz and 18.2 GHz, only limited here in the bandwidth of the employed RF amplifier. This is the widest microwavegeneration span reported for a Si-based OEO. Additionally, a low phase noise level of -110 dBc/Hz at 1 MHz offset frequencyis achieved for all microwave frequencies, illustrating the potential of the approach for the generation of stable high oscillationfrequency signals. Furthermore, we extended this approach for refractive index sensing application, harnessing high spectralresolution in the microwave domain. We have measured a sensitivity of 94350 GHz/RIU, 40 times better than state-of-the-art Sicounterparts microwave photonic silicon refractive index sensor and have estimated a potential limit of detection as low as10 RIU for an interrogation speed of 1 MHz. We believe that the approach proposed here will expedite the developmentof a new generation of high-performance Si OEO with an immense potential for a plethora of applications, including, radar,wireless communications, optical signal processing, warfare systems and lab-on-a-chip biosensing.
Methods
Device fabrication and experimental characterization
The patterns were lithographically defined in a 100 nm ZEP-520A photoresist by using e-beam lithography. After lithography,the patterns were transferred using ICP etching with SF and C F gases. Following the waveguide fabrication, a 2 µ m thickPMMA layer was deposited over the chip surface for protection.For the optical characterization of the ring resonators a tunable laser was coupled to the input waveguide through an inputgrating coupler with a properly adjusted coupling angle and extracted the same way from an output grating. The gratingcouplers were optimized for TM polarization, yielding a fiber to fiber optical transmission of -10.5 dB at 1540 nm wavelength.A polarization controller (PC) was used to set a proper polarization at the input of the grating. Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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This work is included in the MORSE project supported by the LaSIPS (Paris-Saclay University). The sample fabricationwas performed at the Plateforme de Micro-Nano-Technologie/C2N, which was partially funded by the "Conseil Général del’Essonne". This work was also partly supported by the French RENATECH network. uthor contributions statement
P.T.D. and C.A.R. proposed the concept. P.T.D., E.C. and B.J. designed the devices and performed the simulations. X.L.R.and P.T.D. fabricated the devices. P.T.D., C.A.R., E.C. and B.J. performed the experimental characterizations. P.T.D., C.A.R.,X.L.R., I.L., B.J. and E.C. discussed the results and wrote the manuscript. .
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