WO2018104938A1

WO2018104938A1 - A radio-frequency (rf) system and a method thereof

Info

Publication number: WO2018104938A1
Application number: PCT/IL2017/051316
Authority: WO
Inventors: Uriel Levy; Liron STERN
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem Ltd.
Priority date: 2016-12-05
Filing date: 2017-12-05
Publication date: 2018-06-14

Abstract

The present invention describes a sensing method and a sensing system and an RF source thereof comprising two frequency locked lasers coupled to two resonance cavities, one being configured as a sensing element being sensitive to environmental perturbations to be measured, and the second being a reference resonator. Each locking scheme relies on the acquisition of a signal proportional to the difference between the laser carrier frequency and the resonator frequency. This error is fed back to the laser's frequency actuator to fully stabilize both the laser and the resonator. The frequency difference between the two resonators is measured by combining the lasers to illuminate a photodetector. The measured signal is converted from the optical domain to the RF domain. The present invention also relates to an RF source.

Description

A RADIO-FREQUENCY (RF) SYSTEM AND A METHOD THEREOF

TECHNOLOGICAL FIELD

The invention relates to optical resonators. More specifically, the invention describes a radio-frequency system and a method thereof.

BACKGROUND ART

References considered to be relevant as background to the presently disclosed subject matter are listed below:

1. T. Claes, W. Bogaerts, and P. Bienstman, "Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit," Opt. Express 18, 22747 (2010).

2. D. Kim, P. Popescu, M. Harfouche, J. Sendowski, M.-E. Dimotsantou, R. C.

Flagan, and A. Yariv, "On-chip integrated differential optical microring refractive index sensing platform based on a laminar flow scheme," Opt. Lett. 40, 4106 (2015).

3. P. Del'Haye, O. Arcizet, A. Schliesser, R. Holzwarth, and T. J. Kippenberg, "Full Stabilization of a Microresonator-Based Optical Frequency Comb," Phys. Rev. Lett. 101, 053903 (2008).

4. L. Stern, I. Goykhman, B. Desiatov, and U. Levy, "Frequency locked micro disk resonator for real time and precise monitoring of refractive index," Opt. Lett. 37, 1313-1315 (2012).

5. A. Naiman, B. Desiatov, L. Stern, N. Mazurski, J. Shappir, and U. Levy, "Ultrahigh-Q silicon resonators in a planarized local oxidation of silicon platform," Opt. Lett. 40, 1892 (2015).

6. B. Desiatov, I. Goykhman, and U. Levy, "Demonstration of submicron squarelike silicon waveguide using optimized LOCOS process," Opt. Express 18, 18592-18597 (2010).

7. J. Cardenas, C. B. Poitras, J. T. Robinson, K. Preston, L. Chen, and M. Lipson, "Low loss etchless silicon photonic waveguides," Opt. Express 17, 4752 (2009). 8. M. P. Nezhad, O. Bondarenko, M. Khajavikhan, A. Simic, and Y. Fainman, "Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks," Opt. Express 19, 18827 (2011).

9. M. S. Taubman and J. L. Hall, "Cancellation of laser dither modulation from optical frequency standards," Opt. Lett. 25, 311 (2000).

10. L. Stern, R. Zektzer, N. Mazurski, and U. Levy, "Enhanced light-vapor interactions and all optical switching in a chip scale micro-ring resonator coupled with atomic vapor," Laser Photon. Rev. (2016).

Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

BACKGROUND

Resonance cavities are excellent transducers to convert small variations in the local refractive index into measurable spectral shifts. As such, these cavities are being used extensively in a variety of disciplines ranging from e.g. bio-sensing, chemical sensing, temperature sensing and pressure gauges to atomic and molecular spectroscopy. Specifically, chip-scale microring and microdisk resonators (MRR) are widely used for these purposes owing to their miniaturize size, relative ease of design and fabrication, high quality factor and versatility in the optimization of their transfer function.

The principle of operation of such resonant sensors is based on monitoring the wavelength dependence of the resonator subject to minute variation in its surroundings (e.g. different types of atoms and molecules, gases, pressure, temperature, etc.). Traditionally, wavelength monitoring has been achieved either by comparing the spectra prior to and after the sensing event, or by monitoring the resonators' temporal intensity variations at a fixed frequency. And yet, both techniques are akin to thermal drifts and other noise sources of both the MRR and the interrogating laser, which limit the sensitivity and accuracy of such measurements, both in the long and the short terms.

GENERAL DESCRIPTION

Chip scale high precision measurements of physical quantities such as temperature, pressure, refractive index, and analytes have become common in nanophotonics and nanoplasmonics resonance cavities. Despite several important accomplishments, such optical sensors are still limited in their performance in the short and in particular in the long time regimes. Two major limitations are environmental fluctuations which are imprinted on the measured signal, and the lack of miniaturized, scalable robust and precise methods of measuring optical frequencies directly.

As described above, resonant sensors have a limited sensitivity and accuracy due to thermal drifts and other noise sources of both the resonance cavity and the interrogating laser. Thus, in order to monitor such minute perturbations to the refractive index over time, (representing for example the temporal changes in a concentration of a molecule), one needs to have both the resonance cavity and the laser fully stabilized. The level of such stabilization dictates the sensitivity limit of the system.

Considering a silicon photonic chip operating as a refractive index sensor, with a target refractive index sensitivity of 10 ^s which is beyond the current state of the art, one needs to stabilize the resonance cavity to the about 100 μΚ regime, while the laser needs to be stabilized to the MHz level. Stabilization to such values is highly challenging. In order to overcome frequency uncertainties and enable real time and precise sensing, it is desired to implement a differential sensing scheme. Indeed, such schemes have been explored using either external reference systems or by the use of a reference MRR on chip [1,2].

While the concept of a reference resonator provides a significant advance, the implementation of highly precise sensing is still limited to the quality of the local oscillator, e.g. the laser that is being used and the ability to precisely define the resonance frequency by using conventional spectroscopic measurements. Active frequency stabilization schemes can enable overcoming these bottlenecks. Frequency modulation (FM) spectroscopy, wavelength modulation (WM) and the similar Pound- Drever-Hall techniques may be used to lock the radio and optical frequencies to a desired resonance frequency and to measure the dispersive properties of resonant phenomena. Indeed, by using such schemes, the stabilization of different sources has been made possible [3]. Moreover, one can simultaneously monitor the resonator frequency variations with high precision [4]. Indeed, it should be noted that the precision of such frequency variations can substantially exceed the Q factor. For example, in an atomic clock, a frequency uncertainty (Af/f) of 10^"11 at time constant of 1 sec is achieved, being often 5 orders of magnitude smaller than the inverse Q factor of the atomic line.

The present invention overcomes these limitations and converts the frequency difference between the two resonance cavities from the optical domain to the radio frequency domain by utilizing a frequency locked loop combined with reference resonator. More specifically, the sensing system comprises two frequency locked lasers coupled to two resonance cavities, one being configured as a sensing element being sensitive to environmental perturbations to be measured, and the second being a reference resonator. Each locking scheme relies on the acquisition of a signal proportional to the difference between the laser carrier frequency and the resonator frequency. This error is fed back to the laser's frequency actuator to fully stabilize both the laser and the resonator. The frequency difference between the two resonators is measured by combining the lasers to illuminate a photodetector. The frequency difference between the two resonators is converted from the optical domain to the RF domain. In this connection, it should be understood that as the frequency difference between the resonance cavities is in the radio-frequency regime, the sensing system of the present invention has the capability to transduce minute environmental perturbations, (e.g. in the form of pressure variations, temperature variations or the presence of analytes and particles) to a radio-frequency signal. By doing so, and considering the ability to measure radio-frequencies exceptionally precisely, a conceptual breakthrough in nanophotonic based sensing is achieved.

According to a broad aspect of the present invention, there is provided a sensing system comprising: (a) two independent lasers, each laser being configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from the other laser; (b) two resonance cavities having two resonance frequencies, respectively, wherein the two resonance cavities are optically coupled to at least one optical signal from the two independent lasers, the two resonance cavities and the two independent lasers defining respectively two pairs of laser-resonance cavities; (c) a servo loop module comprising (i) a first detector associated with the two resonance cavities and being configured and operable to generate a photocurrent being indicative, for each pair of laser-resonance cavities, of an optical error signal corresponding to a difference between the laser carrier frequency and the respective resonance cavity frequency; and (ii) two lock-in circuits operable for locking the two lasers, respectively, to two different modulation frequencies, each lock- in circuit being configured and operable to receive the photocurrent, to demodulate the signal at the respective one of the different carrier frequencies, the servo loop module thereby redirecting the optical error signals back to the respective lasers, providing alignment of each of the two laser frequencies with each of the resonant frequencies of the resonance cavities, respectively; (d) a second photodetector configured and operable to receive from the two lasers combined aligned wavelength modulated optical signal formed by the signals of the two different frequencies and generate an output frequency signal having a frequency beat in radio-frequency (RF) domain, such that the frequency difference between the two resonance cavities is converted from the optical domain to the RF domain. One of the two resonance cavities is thus configured as a sensing element being environment and sensitive to environmental perturbations to be measured, and the second resonance cavity is configured as a reference resonator is screened from the environment, such that the output frequency beat signal in RF domain is indicative of a differential optical sensing signal. The environmental perturbations include (but are not limited to) different types of atoms and molecules, gases, pressures, temperatures, refractive indexes, etc.

Each laser source provides an initial light signal, which is wavelength modulated with an AC modulation signal at a given initial carrier frequency /. The laser source is operated with a DC current being constantly modulated at the frequency /and amplitude such that the wavelength of the laser scans the dedicated resonator cavity around its resonance frequency by a respective AC modulation signal. The waveform of the modulation may be chosen to be sinusoidal or non- sinusoidal such as triangular.

Each of the two resonance cavities may be micro-cavity and may comprise at least one selected microspherical cavity, microtoroidal cavity, microring-cavity, microdisk cavity, multi-layer Bragg cavity, photonic crystal defect cavity, disordered photonic crystal waveguide, Fabry-Perot cavity, photonic crystal resonator waveguide, donor-type photonic crystal cavity, acceptor-type photonic crystal cavity, a fiber ring resonance cavity, and an optical resonator operating in one or more whispering-gallery modes (WGMs).

The two resonance cavities may have at least one input port being optically coupled to a radiation from the two independent lasers and at least one output port. The two resonance cavities may be arranged in a cascaded fashion. In this case, the two resonance cavities define a common input port being optically coupled to a combined radiation from the two independent lasers and a common output port.

In some embodiments, each of the two lasers is configured to generate the wavelength modulated optical signal having a modulation depth in the order of a dimension of the resonance cavity.

In some embodiments, the sensing system is configured as a chip-scale platform. The two independent lasers are frequency locked to two resonance cavities which are situated in the vicinity of each other on the same chip. The present invention may be implemented to provide an ultra-precise optical to radio-frequency based chip scale refractive index and/or temperature sensor. The present invention provides a highly precise on-chip both short and long time sensing device with sensing precision approaching 10 ^s in effective refractive index units, and 90 μΚ in temperature. The present invention also provides single particle detection and high precision chip scale thermometry. For example, the system of the present invention may measure local temperatures and temperature gradients on a chip. This latter issue is relevant in the modern era, when the local heating of central processing units (CPUs) is becoming one of the major bottlenecks preventing the improvement of computer performance beyond the current state of the art. The system of the present invention provides a CMOS compatible platform which performs precise temperature measurements without being sensitive to electromagnetic noise. This can be implemented by inducing a temperature gradient between the two resonance cavities. By doing so, there is provided an on chip local thermometer capable of measuring temperature differences as low as 0.8mK over a course of minutes, corresponding to a measurement uncertainty of 1.2ηιΚ τ^"1/2.

In some embodiments, the sensing system further comprises an RF counter configured and operable to receive from the second photodetector an output frequency signal having a frequency beat in the RF domain and to measure the signal accordingly. The frequency beat is indicative of variations of physical quantities including at least one of temperature, pressure, refractive index, different types of atoms and molecules, gases, analytes and particles.

In some embodiments, each lock-in circuit comprises a lock- in amplifier for generating an output error signal and an integrator for integrating the output error signal from the lock-in amplifier and feeding the frequency locked independent laser with an integrated error signal. According to another broad aspect of the present invention, there is provided a sensing method comprising: generating two wavelength modulated optical signals at a different carrier frequency from each other by using two independent lasers; coupling at least one optical signal from the two independent lasers to two resonance cavities; illuminating a photodetector with the optical signal outputted by the two resonance cavities and generating a photocurrent being indicative, for each pair of laser-resonance cavities, of an optical error signal corresponding to a difference between the laser carrier frequency and the respective resonance cavity frequency; locking the two lasers respectively to the two different modulation frequencies; redirecting to the two respective lasers the optical error signals; providing alignment of each of the two laser frequencies with each of the resonant frequencies of the two resonance cavities; and generating an output frequency signal having a frequency beat in the RF domain such that the frequency difference between the two resonance cavities is converted from the optical domain to the RF domain.

In some embodiments, the method comprises combining the aligned wavelength modulated optical signals formed by the signals of the two different frequencies.

The frequency signal may comprises frequency beat being spectral shifts which may be indicative of at least one variation in the local refractive index of the resonance cavities, and temperature gradient between the resonance cavities. The sensing method may therefore measure physical quantities comprising at least one of temperature, pressure, refractive index, analytes and particles.

In some embodiments, redirecting to the respective lasers, the optical error signals provides active frequency stabilization of the two independent lasers and of the two resonance cavities.

In some embodiments, the sensing method provides a differential optical sensing signal.

According to another broad aspect of the present invention, there is also provided an RF source comprising two independent lasers, each of the two lasers being configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from the other laser; two resonance cavities being configured to define a theoretical frequency difference in the RF regime, the two resonance cavities being optically coupled to at least one optical signal from the two independent lasers, the two resonance cavities and the two independent lasers defining, respectively, two pairs of laser-resonance cavities; a memory configured and operable to tune the frequency resonance of at least one of the resonating cavity to provide a frequency difference between the two resonance cavities to be substantially identical to the theoretical frequency difference; a servo loop module comprising a first detector associated with the two resonance cavities, and being configured and operable to generate a photocurrent being indicative, for each pair of laser-resonance cavities, of an optical error signal corresponding to a difference between the laser frequency and the respective resonance cavity frequency; and two lock-in circuits operable for locking the two lasers, respectively, to two different modulation frequencies, each lock-in circuit being configured and operable to receive the photocurrent and demodulate it at the respective one of the different carrier frequencies, the servo loop module thereby redirecting the optical error signals back to the respective lasers, providing alignment of each of the two laser frequencies with each of the resonant frequencies of the resonance cavities, respectively; a second detector configured and operable to receive from the two lasers, a combined aligned wavelength modulated optical signal formed by the signals of the two different frequencies and generate an output signal having a frequency beat in radio-frequency (RF) domain.

In some embodiments, the memory is configured for generating an electric field in the vicinity of at least one of the two resonance cavities.

In some embodiments, the RF source is configured as a chip-scale platform.

In some embodiments, at least one of the two resonance cavities, the two lasers and the two lock-in circuits may be configured as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of the main functional parts of the system of according to some embodiments of the present invention;

Fig. 2a is a schematic representation of cascaded MRR's according to some embodiments of the present invention; Fig. 2b is an illustration of the spectrum of the reference MRR and the sensing MRR of Fig. 2a with and without the refractive index change;

Fig. 3a is a scanning electron micrograph (SEM) image of cascaded local oxidization of silicon (LOCOS) MRRs according to some embodiments of the present invention; Fig. 3b is a measured spectrum of the cascaded MRRs of Fig. 3a; Fig. 3c is a zoomed transmission around two adjacent dips, separated by 78.5 pm/10.03 GHz;

Fig. 4 is a schematic illustration of the sensing system according to some embodiments of the present invention;

Fig. 5a shows calibrated integrators output as a function of time of both servo- loops; Fig. 5b shows difference between the integrators output as function of time;

Figs. 6a-6b show a normalized frequency difference as a function of time for two different operation regimes; in particular Fig. 6a shows one laser locked to MRR and the second laser free running, and subsequently both lasers locked to both MRRs; Fig. 6b shows two lasers locked to both MRRs and subsequently both lasers free running;

Fig. 7 shows an overlapping Allan deviation (based on measurements in Figs. 6a-6b) of the refractive index presented for three cases: a single laser locked to an MRR with the second laser free running, two lasers free running, and the case where each of the two lasers is locked to its dedicated MRR;

Fig. 8a is schematic illustration of a near field optical microscope (NSOM) tip illuminating light on the left MRR, and thus creating a heat gradient via optical absorption in the silicon bottom layer; Fig. 8b shows a temperature difference between both MRR as inferred from the measured beat frequency as a function of time, whilst changing the optical power illuminated by the NSOM probe; and;

Fig. 9 is a schematic block diagram of the main functional parts of the RF source according to some embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is made to Fig. 1 illustrating, by way of a block diagram, the general configuration and operation of a sensing system of the present invention. The sensing system 100 comprises inter alia two frequency locked independent lasers 120 denoted as Source A and Source B. Each laser (Source A and Source B) is configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from each other ; and fi. The sensing system 100 also comprises two resonance cavities 140 A and B, each of them may have an input port (e.g. waveguide) being optically coupled to optical signals from the two independent lasers Source A and Source B respectively and each of them may have an output port (e.g. waveguide). Alternatively resonance cavities 140 A and B may be optically coupled to each other and may have a common input and output ports. Each resonance cavity (A and B) has a certain resonance frequency Ri and R2 respectively. One of the resonating cavities serves as a sensing unit, while the other plays the role of a reference, eliminating environmental and system fluctuations (temperature, laser frequency, etc.). By utilizing a servo loop locking scheme, the measured effects are translated from the optical domain to the radio frequency domain. By doing so, the system capabilities can be quantified by utilizing well-established RF technologies, such as frequency counters, spectrum analyzers, and atomic standards. One of the two resonance cavities may be thus configured as a sensing element being sensitive and exposed to environmental perturbations to be measured, and the second resonance cavity may be configured as a reference resonator screened to the environment such that the frequency signal outputted by the system of the present invention is indicative of a differential optical sensing signal. The environmental perturbations include (but are not limited to) different types of atoms and molecules, gases, pressure, temperature, refractive index etc. The novel system of the present invention enables to provide an optical heterodyne detection of physical quantities. The sensing system 100 also comprises a servo loop module 160 for providing an error signal and a servo loop redirecting the output error signal to the frequency locked independent lasers Source A and Source B. Servo loop module 160 also comprises (i) a detector 162 configured for receiving the optical signal outputted by the output port(s) of the two resonance cavities A and B and for generating a photocurrent being indicative of a difference between the laser carrier frequency/; andi and the resonance cavities frequency Ri and R2 for each pair of laser-resonance cavity (fi- Ri and /2- Ri) and (ii) two lock- in circuits 164 A and B referenced to two different modulation frequencies coi and a>2 of the two lasers Source A and Source B with carrier frequencies /; and/2. Each lock- in circuit A and B is configured and operable to receive the photocurrent from detector 162, to demodulate the signal at the different carrier frequencies /; and 72 respectively, and to provide an output error signal. The demodulated signals provide error-signals that are redirected to Sources A and B. Such error- signals, which can be viewed as the differential of the resonance cavity lineshape, provide the lasers with the needed "correction" in order to be aligned with the resonance cavity. The output error signals are fed back to the lasers 120. Each laser Source A and Source B is then modulated at a modulation frequency coi and a>2 respectively. Modulation of the locking is either performed by modulating the laser sources directly, or by the use of an external electro-optic modulator. Wavelength modulation is performed such that each of the two laser carrier frequencies ; and fi is respectively aligned with each of the resonant frequencies Ri and R2 of the resonance cavities A and B. Each laser source then generates an aligned wavelength modulated optical signal with respect to each resonance cavity. The sensing system 100 also comprises a second photodetector 180 configured and operable to receive an aligned wavelength modulated superposed optical signals at a different carrier frequency ; and fi and to generate a photocurrent having a characteristic frequency response signal having a frequency beat in the radio-frequency (RF) domain. The frequency difference between the two resonance cavities Ri and R2 is thus converted from the optical domain to the RF domain. The frequency signal comprises frequency beat(s) being spectral shifts which may be indicative of at least one of variations in the local refractive index of the resonance cavities, or temperature gradient between the resonance cavities. The sensing technique of the present invention may therefore measure physical quantities comprising at least one of temperature, pressure, refractive index, analytes and particles.

A schematic representation of a partial view of the differential sensing system according to some embodiments of the present invention is sketched in Fig. 2a, where a chip 30 consisting of two cascaded MRRs 32 and 34 coupled to a bus-waveguide 36 is represented. The sensing system of the present invention may thus be configured as a chip- scale platform. The two independent lasers (not shown) are frequency locked to two resonance cavities 32 and 34 which are situated in the vicinity of each other on the same chip 30. Although MRR are represented in the figure, the present invention is not limited to any type of resonance cavity. Bus waveguide 36 has an input port I and an output port O enabling optically coupling the two resonance cavities to the frequency locked lasers (not shown) via the input port I and to one detector (not shown ) via the output port O. Bus waveguide 36 also optically couples MRRs 32 and 34 one to each other. In this specific and non-limiting example, the first right MRR 32 serves as the reference MRR, while the second left MRR 34 is the sensing unit, which is subject to the perturbation to be measured (analytes, temperature, pressure, etc.). The refractive index variations of this sensing MRR are monitored, which manifest as a resonance frequency shift, as illustrated in Fig. 2b. The spectrum of the reference MRR is represented by curve 39 and the sensing MRR is represented by curve 38. Sensing curve 38 illustrates two adjacent resonance lines 38 originating from each of the cascaded MRRs. The sensing MRR curve 38 is illustrated with (point A) and without the refractive index change (point B). The frequency difference between these two lines of curve 38 is 10 GHz, and is assumed to be relatively constant, as both MRRs are subject to the same environment. By applying a perturbation to the sensing MRR, this frequency difference changes, as illustrated in point B in Fig. 2b. Thus, monitoring the frequency difference yields a precise and accurate method to measure small changes in any physical quantity such as refractive index, temperature or pressure. Moreover, as the ability to accurately (to orders of magnitude better than known in the art) measure radio-frequencies is readily available, are relatively cheap and miniaturized, the technique of the present invention offers a prominent advantage with respect to optical differential schemes.

Reference is made to Fig. 3a showing a scanning electron micrograph (SEM) image of two cascaded MRRs. The MRR chips were fabricated using low-loss waveguides based on the concept of local oxidization of silicon (LOCOS) [5-8]. The wave guides dimensions were 450nm width and 220nm height, whereas the MRRs radius was 30μιη. Thereafter, the transmission spectrum of the cascaded MRRs is characterized. Fig. 3b shows the measured spectrum of the cascaded MRRs. Fig. 3b is obtained by scanning one of the laser source being optically coupled to the two resonating cavities shown in Fig. 3a and receiving the transmission of the MRRs as a function of wavelength as generated by detector 162 of Fig. 1. A few distinct absorption dips separated by the free spectral range (FSR) of the MRR (corresponding to about 3nm) are observed. By closely examining the spectrum, Fig. 3c shows a zoomed transmission around two adjacent dips. It can be observed that each dip within an FSR consists of two distinct dips, with a separation of 78.5pm corresponding to 10.03GHz. Such frequency separation complies with the abovementioned requirement for an easily detectable separation in the RF regime. Finally, the resonance of each dip is about 30pm wide, corresponding to a quality factor of about 50000, with extinction of about 15dB. Reference is made to Fig. 4 illustrating a specific and non-limiting example of the sensing system 200 of the present invention having a dual locking scheme. Two lasers 120 (Source A and Source B) are coupled to two cascaded MRRs 140 such that each of the lasers is aligned to its respective MRR. More specifically, in this specific and non-limiting example, two lasers 120 labeled as Source A and Source B are combined by using a beam combiner (not shown) and are locked simultaneously to two resonance cavities 140. In this specific and non-limiting example, the resonance cavities 140 are implemented by two cascaded MRRs optically coupled to a bus waveguide having an input port I and an output port O placed on a chip. The system 200 is thus configured as an on chip sensor capable of detecting, unprecedentedly, small frequency changes which can be traced back to minute perturbations in refractive index or temperature. Both lasers 120 are wavelength modulated by two different carrier frequencies ; and fi being in the few 100Hz regime, and with a modulation depth selected to be in the order of a dimension of the resonance cavity e.g. corresponding to a fraction of the width of the resonators. The signal of both lasers 120 is coupled through the input port I of the bus waveguide and the output port O is connected via an optical coupling element (e.g. an lensed fiber) to a detector 162 (e.g. InGaAs detector). The generated photocurrent of detector 162 feeds two lock-in- amplifiers (LIA) 164, referenced to two different modulation frequencies coi and a>2 of the two lasers with carrier frequencies ; and fi. The demodulated signals provide error-signals that are redirected to the sources A and B. The error signal of each laser may be directly fed back to each laser respectively or may be received by a pair of laser frequency actuators 168 (e.g. a piezo-mounted cavity mirror) coupled to each laser Source A and Source B respectively and being configured and operable to adjust the frequency of each laser such that each of the two laser carrier frequencies /; and 72 is respectively aligned with each of the resonant frequencies Ri and R2 of the resonance cavities A and B. The laser frequency actuators 164 may be an integral part of the lasers 120 or may be separate entities. Therefore, both lasers 120 are tuned to the vicinity of the resonance dips and the two servo loops 160 are closed. In this way, the two lasers carrier frequencies/; andi are aligned with two adjacent resonant frequencies Ri and R2, each originating from a different resonance cavity. The inventors have shown that by tracking the error signals of the servo loops 160, while each error signal drifts significantly (about few GHz), the difference in error drifts is about two orders of magnitude less (about 50 MHz). Generally, error signals cannot be directly mapped to frequency deviations (due to laser frequency drifts, laser piezo hysteresis etc.). And yet, this result exemplifies the advantage of using a reference resonator to address environmental perturbations. To fully exploit the system capabilities to convert optical frequencies to the RF domain, the beat frequency between the two lasers is directly measured. First, by directly observing frequency over time, it was observed that the laser drift is significantly smaller than the drifts of the MRRs in an uncontrolled environment. Hence, MRRs are shown to be unstable frequency references over long periods of time, and thus the sensing capabilities are limited. In contrast, by analyzing the frequency beat of the locked lasers 120, stable frequency has been observed, up to a level of about 10 MHz/hour. The difference between these two frequency locked lasers 120 by beating them upon a photo detector 180, achieved a highly sensitive sensor capable of measuring a refractive index uncertainty of 1.5 · 10^~7 RIU · τ^{_1 2} (τ being the averaging time constant), with an unprecedented noise floor of 1.5 · 10^~8RIU , equivalent to temperature uncertainty of about 90 μΚ at about 200 sec. Typical drifts were measured to be 10^"7 RlU/hour enabling long time stability in the state of the art level, and even beyond.

An additional pair of integrators 166 integrate the output error signals from the two lock-in amplifiers respectively and feed the two lasers 120 with an integrated error signal and serve as a "memory" for each individual servo-loop 1 and 2 160 as implemented for example in [9] . Integrators 166 produce steadily changing output voltage for a constant input voltage. Integrators 166 take both the intensity (input voltage magnitude) and time into account, generating an output voltage representing total error signal. Therefore, the two lasers 120 receive an input changing voltage (i.e. DC current) from integrators 166 tuning the two lasers to the operating frequency. Finally, in order to monitor the frequency difference between both MRRs 140, the sources A and B are combined by using a beam combiner (not shown) to illuminate a detector 180 (e.g. fast photo detector, oscilloscope or a spectrum analyzer). The frequency beat may be monitored by using an optional RF frequency counter 182 configured and operable to receive from detector 180 a frequency signal having a frequency beat in the RF domain and to measure the frequency signal accordingly. The RF frequency counter 182 may be a digital RF frequency counter measuring the frequency of repetitive signals and measuring the elapsed time between events. The frequency counter measures the number of oscillations or pulses per second in the periodic electronic signal within a specific period of time. The frequency beat is indicative of variations of physical quantities including at least one of temperature, pressure, refractive index, different types of atoms and molecules, gases, analytes and particles.

An optional electrical switch (not shown) may be connected to the output of modulation fi and f₂ to periodically switch off the signals. If integrators 166 are used, the electrical switch is connected to the output of the integrators to switch off the signals from the integrators. It should be noted that using wavelength modulation, i.e. operating at low frequencies and relatively high modulation depths, yields a dithered beat spectrum. Obviously such spectra are difficult to interpret. By switching the modulation (and "freezing" the integrator's output) clear beats can be obtained. This switching can be avoided using either frequency modulation, where the modulation frequency has to exceed the resonators width, or dither cancelation techniques [9] .

To verify that indeed the difference in resonance frequencies between the two resonance cavities is relatively stable over time, both integrator outputs (representing the error signals) were monitored as a function of time. In this connection, reference is made to Figs. 5a-5b plotting typical error signals of both the resonance cavities. In particular, Fig. 5a shows a calibrated integrators output as a function of time of both servo-loops referenced as MRR #1 and MRR #2. One can clearly observe that both error signals follow each other, thus the lasers track each resonance cavity individually in a correlated fashion. This correlation in error signal is attributed to be mainly due to temperature fluctuations in the room. Such temperature fluctuations affect both resonance cavities almost identically, as both resonance cavities are subjected to a very similar heat environment: they are situated on the same chip, in close vicinity to each other, and are of the same dimensions and materials. Using the thermos-optic coefficient of silicon (1.8· 10^"4 RIU/K), and by calibrating the frequency modulation transfer function of the lasers, room temperature fluctuations were estimated to be about 0.1 °K, which is a typical value measured in the laboratory over a time scale of an hour. Fig. 5b shows the error signal difference between the integrators output as a function of time. It can be readily seen that the error signal difference deviations are much smaller than the deviations of each of the error signals. The deviations of the error signals are of the order of a few GHz, whereas those of the error signal difference are one and a half orders of magnitude lower. It should be noted that error signals do not represent frequency adequately, due to the fact that they may incorporate laser drifts, as well as piezoelectric (such as hysteresis and creep). Indeed, a better choice is to analyze the beat directly. This is because the beat frequency can be fully attributed to the resonance cavities frequency separation and decoupled from the laser fluctuations.

The beat frequency was obtained and analyzed by tapping about 10% of the signal emerging from each of the two lasers, combining it into a single fiber, and detecting the combined signal using a fast photodetector 180 connected to a frequency counter 182 of Fig. 4. For this purpose, a wafer scale set of cascaded MRRs was fabricated by UV lithography, and a specific set of MRRs separated by about 4GHz was selected. Once again, this frequency separation is most likely a consequence of fabrication tolerances. Reference is made to Figs. 6a-6b illustrating normalized frequency difference (Δ //) as a function of time for two different operation regimes: Fig. 6a for one laser locked to a resonance cavity, and the second laser free running, and subsequently both lasers locked to both resonance cavities. More specifically, the normalized frequency difference is plotted as a function of time, while switching between three modes of operation of the servo-loops in order to reveal the different stability characteristics of the system components. In Fig. 6a, the initial mode (denoted as t=0) presents each of the two lasers locked to its dedicated resonance cavity, followed by the mode in which one laser is free running, while the other is still locked, and finally once again both lasers are locked. As can be seen, when the lasers are locked to both resonance cavities, a relatively constant frequency difference is obtained. This is in contrast to the case where one laser is locked to a resonance cavity and the system drifts. Following, in Fig. 6b the measured normalized frequency difference is plotted for the case where initially each of the two lasers is locked to its dedicated resonance cavity, and subsequently both lasers are free running. Once again, the system seems to drift significantly when the lasers are free running, representing the relative instability of the two lasers. The magnitude of this drift is appreciably lower than that presented in Fig. 6a. Therefore, when operating in the case of a single servo loop, considering the relatively long time constants, the resonance cavity drift is most likely tracked which is dominant with respect to the laser drift. Indeed, as the resonance cavities' temperature is not stabilized in any manner, such drifts which correspond to a relative frequency drift of about 10^-5 over the course of about 40 min, are highly likely, as they correspond to a temperature drift of about O.lmK (considering the thermo optic coefficient in the order of about 10^-4/C). Next, the instability of the beat frequency representing the frequency difference instabilities between the two MRRs is analyzed. In order to comply with the widespread and conventional frequency stability analysis, an overlapping Allan deviation is applied to the measured normalized frequency presented in Figs. 6a-6b. The Allan variance is a highly common time domain measure of frequency stability. Similar to the standard variance, it is a measure of the fractional frequency fluctuations, and yet has the advantage of being convergent for most types of noise. For a discrete series of N measurements, the Allan variance can be defined as follows [9]:

where y_k is the fractional frequency of sample k, averaged on the time interval τ. Here, an overlapping Allan deviation is used, that is an implementation of the Allan deviation utilizing all possible combinations of the measured dataset.

The normalized frequency is translated to refractive index units by multiplying the normalized frequency by the effective group index of the guided mode. Reference is made to Fig. 7 showing an overlapping Allan deviation based on measurements in Figs. 6a-6b) of the refractive index presented for three cases: a single laser locked to an MRR with the second laser free running referred to as single MRR lock curve, two lasers free running referred to as free running curve and the case where illustrating the sensing system of the present invention, each of the two lasers is locked to its dedicated resonance cavity, referred to as double MRR lock curve. More specifically, in Fig. 7, the overlapping Allan deviation is plotted as a function of integration time τ for three different scenarios. The first curve, denoted as Double MRR lock, corresponds to the sensing system of the present invention. The significance of applying this approach is twofold. First, it quantifies the sensor metrics in the time domain, i.e. it reveals the instability of the sensor at different time constants. Second, the Allan deviation is a very powerful tool to discern between different noise sources by examining the slope at different time constants. For instance, as one can see, the instability of the locked system averages out at a rate of about l/Vr revealing the system of the present invention at these time constants (3-100 sec) to be white frequency noise limited. At the time constant of about 200 sec, the slope levels out, which is typical of frequency flicker noise. The sensing system reaches a floor of about 1.5· 10 ^s representing the ability to measure variations in index of refraction with unprecedented precision approaching 10 ^s at these time constants. Even if a realistic bio-sensing or gas sensing scenario is taken into account, in which the optical mode interacts with the analyte only, partially due to the limited mode confinement in the cladding, a beyond the state-of-the-art precision is still maintained. Next, the result of the free running curve and the single MRR lock curve of Fig. 7 are compared. Clearly, one can see the trend illustrated in Figs. 6a-6b revealed in this analysis. The single MRR lock curve, as well as the free running lasers curve exhibit significant instable operation for long periods of time when compared to the double MRR lock curve. Such plots exemplify the long time stability advantages of the sensing system of the present invention. A single MRR lock curve shows excellent short time stability (about 3 sec) comparable to the two resonance cavities. And yet, as lasers drifts (free running curve) and temperature drifts of the resonance cavities (single MRR lock curve) become dominant, this scheme loses its ability to measure refractive index changes precisely. It should be noted that even when the relatively deterministic linear drift is removed from the analysis (not shown here), both the free running lasers and the single MRR lock schemes still exhibit significant instability in comparison with the system of the present invention. It should be noted that although both resonance cavities are subjected to the same thermal environment, varying temperature gradients across the chip (resulting from the absence of thermal management and/or isolation of the chip, in the current demonstration) might induce slight fluctuations and drifts in the fractional frequency. Another mechanism which may induce such drifts is related to imperfections in the servo-loop, e.g. in the form of a parasitic (and drifting) input to the integrator. Indeed, a small frequency drift is observed in the order of 10^"7 RlU/hour. Generally, it should be further noted that the lasers line- width and overall performance affects servo-loop performance. Indeed, the lasers line-width is narrower than the line-width of the ring resonances. In principle, lasers with larger phase noise would require longer integration times in order to achieve the same performance.

When translating the normalized frequency uncertainty of the sensing system of the present invention in terms of temperature sensitivity (right axis in Fig. 8b below), this reveals an unprecedented ability to implement on a chip, precise thermometry. Here, a temperature difference precision of 1.2ηιΚ τ^"1/2, with a floor of -90 μΚ at 200 sec is predicted. In order to demonstrate such capabilities explicitly, a localized source of light illuminating one of the resonance cavities was provided, and thus by direct absorption a deliberate temperature gradient was created. In order to create such localized illumination, a near field optical microscope (NSOM) with a fiber coupled probe with an aperture of 300nm, was used in illumination mode. In this connection, reference is made to Fig. 8a showing a schematic illustration of a NSOM tip illuminating light on the left resonating cavity 34, and thus creating a heat gradient via optical absorption in the silicon bottom layer. As shown in Fig. 8a, the NSOM probe is set to be in contact with the surface, and is positioned at the center of one of the resonating cavities 34. Operating at a wavelength of 980nm, the light was expected to diffract into the silicon dioxide layer (having a thickness of 2μιη), and then be absorbed at the silicon substrate beneath it. Such a process generates a lateral heat gradient across the chip which can be measured precisely using the sensing system of the present invention. Fig. 8b shows a temperature difference between both resonance cavities as inferred from the measured beat frequency as a function of time, whilst changing the optical power illuminated by the NSOM probe. The inset of Fig. 8b shows temperature as a function of illuminating power. More specifically, in Fig. 8b, the temperature difference (calculated using the relation ΔΓ = n_gA f /(fa), where is the thermo-optic coefficient of silicon and n_g is the effective group index of refraction) between the two resonance cavities as a function of time, was plotted. The temperature gradient is controlled by varying the power coupled to the NSOM probe. In order to maintain a reference baseline, the laser is turned off in between each illumination sequence, and compensates for a small linear drift. As the power level decreases, the time intentionally increases, and the temperature difference is measured in order to obtain better signal to noise level. As can be seen in the inset of Fig. 8b, an optical power in the range of a few hundreds of nW was applied (calibrated separately using a photodetector) and a linear temperature offset of a few mK was obtained. For the last illumination sequences in Fig. 8b (corresponding to power levels of about 90nW and about 70nW, see dashed horizontal lines) corresponding average temperatures of 3.09mK and 2.32mK were measured, i.e. a difference of 800 μΚ between the two measurements. From the Allan deviation curve the uncertainty was estimated to be about 90 μΚ at the measurements time constant (about 300 sec). It is thus not surprising that these two temperatures can be easily differentiated. In this connection, it should be noted that there is significant effort to construct optical chip scale thermometers, with a design exploiting both photonic crystal cavities and resonance cavities. Such devices offer prominent advantages with respect to other temperature measurement techniques, as they offer high sensitivity, large temperature range and immunity to electro-magnetic disturbances. The sensing system of the present invention, not only compares favorably in its sensitivity, but also allows one to keep this high degree of precision over long periods of time, without the need to stabilize both the chip and the interrogating system. The sensing system of the present invention can be integrated with relatively cheap and compact lasers such vertical-cavity surface-emitting lasers (VCSELs).

Therefore, as detailed above, by utilizing the well-established techniques borrowed from the disciplines of frequency metrology, and by calculating the Allan deviation, it was found that the sensing system of the present invention is capable of observing extremely low perturbations in a refractive index, down to the level of about 10 ^s at 200 seconds. The above mentioned value can be used for the purpose of temperature sensing down to the 90 μΚ level. Indeed, taking advantage of the inter-band transition, a near field light probe was used as a local heat source and the capability of the sensing system of the present invention has been demonstrated for temperature sensing applications. While having an on chip thermometer with 50 μΚ temperature precision is best as compared to thermometers of state of the art in the field. The precision scales with the quality factor and the signal to noise ratio. Silicon MRRs with quality factors of about 5· 10⁶ can be achieved [5]. Thus, assuming the same signal to noise ratio as obtained in our experiments described above, one may expect temperature sensing precision below ΙμΚ and refractive index sensing in the 10^"10 regime. For implementing a temperature sensor, in contrast to a refractive index sensor, the temperature sensing MRR should be spatially separated from the reference MRR. For instance, in order to probe the variation in temperature across the chip, several MRRs can be deployed in different locations across the chip. For implementing a refractive index sensor, both MRRs may be subjected to the same cladding environment (discarding the sensing analytes). For example, a liquid-based refractive index sensor having a common flow cell above both MRRs that is able to maintain the flow of two solutions above each MRR separately is implemented as described in [2]. A second approach is to construct a gas-based sensor, where both MRRs have hollow chambers above them. Here, the reference MRR is encapsulated and the sensing MRR is exposed to the environment. Finally, as described above, the system of the present invention is fully integrated and is configured as a chip-scale sensor, including sources, detectors and servo-loops. To this end, miniaturized components such as vertically-cavity surface-emitting lasers (VCSELs), micro-processors and voltage controlled oscillators are used.

Reference is made to Fig. 9 illustrating, by way of a block diagram, the general configuration and operation of an RF source according to some embodiments of the present invention. As described above, one of the issues related to the use of photonic resonators in optical systems is the accuracy in setting the resonance wavelength. The actual resonance wavelength may deviate from the designed one due to fabrication imperfections and environmental effects (e.g. temperature change). The issue of fabrication imperfections restricts the applicability of photonic resonators primarily in on-chip configurations. For example, given two resonators which are designed for two different resonance frequencies, separated by a theoretical frequency difference in the RF regime (e.g. 100 GHz), fabrication imperfection may lead to a lower or higher separation (e.g. 95 GHz).

The present invention provides a novel chip- scale RF source in which the frequency difference between the resonating cavities is controlled by a memory configured and operable to accurately tune the frequency resonance of at least one of the resonating cavity. In this way, the typical limited accuracy of the resonance cavity is compensated by the memory. This configuration enables the resonators to be separated in frequency exactly by the desired value. In parallel, it allows both lasers to be fully stabilized, with relative frequency shift between them that perfectly corresponds to the desired frequency to be generated. .

The RF source 300 comprises inter alia two frequency locked independent lasers 120 denoted as Source A and Source B. Each laser (Source A and Source B) is configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from each other ; and fi. The RF source 300 also comprises two resonance cavities 142 A and B each of them may have an input port (e.g. waveguide) being optically coupled to optical signals from the two independent lasers Source A and Source B respectively and each of them may have an output port (e.g. waveguide). Alternatively resonance cavities 142 A and B may be optically coupled to each other and may have a common input and output ports. Each resonance cavity (A and B) is configured to have certain resonance frequency Ri and R2 respectively (e.g. by selecting an appropriate dimension). The two resonance cavities are screened to the environment. The resonating cavities are configured to have a difference between R2 and Ri in the RF range. However, as described above, given two resonators which are designed for two different resonance frequencies, (e.g. separated by 100 GHz), fabrication imperfection may lead to a lower or higher separation, (e.g. 95 GHz). Therefore to provide an accurate frequency separation (e.g. of 100 GHz) between the resonators, the technique of the present invention utilizes a memory 900 attached to at least one resonator and configured for generating electric field in the vicinity of the resonator. Memory 900 provides a frequency difference between the two resonance cavities to be substantially identical to the theoretical frequency difference. Such electric field applied on the resonator provides for varying the charge carrier's density within the resonator resulting in a free carrier plasma effect inducing variation in the refractive index of the resonator. The memory 900 is preferably configured to allow selective trapping of charged carriers to thereby provide stable electric field and eliminate the need for maintaining connection to a power source. Memory 900 may be implemented for example as described in US patent No. 2017/176780, or in the following articles: Barrios et al: "Silicon photonic real-only memory, in Journal of lightwave Technology, vol. 24, no. 7, pp. 2898-2905, July 2006; "Modeling and analysis of high-speed electro-optic modulation in high confinement silicon waveguides using metal-oxide-semiconductor configuration", J. Appl. Phys., vol. 96, no. 11, pp.6008-6015, Dec. 2004; Soref et al. "Electrooptical effects in silicon", IEEE J. Quantum Electron, vol. QE-23, no. 1, pp. 123-129, Jan. 1987. Memory 900 enables a post-production tuning of the resonating cavities by allowing fixed (certain profile) fine-tuning of resonance frequency of the resonator cavity by providing trapped charge carriers in the memory associated with the resonator, as well as dynamic variation of the resonance frequency. The configuration of the present invention allows for "electronic" trimming of the resonators by trapping charges in the memory to desirably shift the resonance frequency without the need for physical changes and/or without the need to reduce the Q-factor. Generally, each pair of resonators, e.g. 142A and 142B are configured by trapping appropriate charge in the corresponding memory, to resonate at the initial expected frequencies having a frequency difference in the RF regime. It should be noted, and as indicated above, that trimming of resonators to provide expected/theoretical resonance frequencies utilizing the technique of the present invention is simple and changeable operation. Also, at any desired stage, the resonators may be re-trimmed to be coupled at a different selected resonance frequency by varying the charge trapped in the corresponding memory. Memory 900 thus tune the separation between the two different resonance frequencies by operating one or both resonating cavities, compensating for the fabrication imperfections. Memory 900 tunes one or both resonating cavities, to resonate at the expected/theoretical frequency difference in the RF range.

The RF source 300 also comprises a servo loop module 160 for providing an error signal and a servo loop redirecting the output error signal to the frequency locked independent lasers Source A and Source B. Servo loop module 160 also comprises (i) a detector 162 configured for receiving the optical signal outputted by the output port(s) of the two resonance cavities A and B and for generating a photocurrent being indicative of a difference between the laser carrier frequency/; and/2 and the resonance cavities frequency Ri and R2 for each pair of laser-resonance cavity (fi- Ri and /2- Ri) and (ii) two lock-in circuits 164 LC A and LC B referenced to two different modulation frequencies coi and a>2 of the two lasers Source A and Source B with carrier frequencies /; and fi. Each lock- in circuit A and B is configured and operable to receive the photocurrent from detector 162, to demodulate the signal at the different laser carrier frequencies /; and fi respectively, and to provide an output error signal. The demodulated signals provide error-signals that are redirected to Sources A and B. Such error- signals, which can be viewed as the differential of the resonance cavity lineshape, provide the lasers with the needed "correction" in order to be aligned with the resonance cavity. The output error signals are fed back to the lasers 120. Each laser Source A and Source B is modulated at a modulation frequency coi and a>2 respectively. Modulation of the locking is either performed by modulating the laser sources directly, or by the use of an external electro-optic modulator. Wavelength modulation is performed such that each of the two laser carrier frequencies/; and/2 is respectively aligned with each of the resonant frequencies Ri and R2 of the resonance cavities A and B. Each laser source then generates an aligned wavelength modulated optical signal with respect to each resonance cavity. The RF source 300 comprises a detector 180 (e.g. fast photodetector, oscilloscope or a spectrum analyzer) configured and operable to receive an aligned wavelength modulated superposed optical signals at a different carrier frequency/; and i and to generate an output signal having a characteristic frequency response signal having a frequency beat in the radio-frequency (RF) domain.

Claims

CLAIMS:

1. A sensing system comprising:

(a) two independent lasers, each of said two lasers being configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from the other laser;

(b) two resonance cavities having two resonance frequencies, respectively, and the two resonance cavities being optically coupled to at least one optical signal from said two independent lasers, said two resonance cavities and said two independent lasers defining, respectively, two pairs of laser- resonance cavities;

(c) a servo loop module comprising a first detector associated with the two resonance cavities, and being configured and operable to generate a photocurrent being indicative, for each pair of laser-resonance cavities, of an optical error signal corresponding to a difference between the laser carrier frequency and the respective resonance cavity frequency; and two lock-in circuits operable for locking the two lasers, respectively, to two different modulation frequencies, each lock-in circuit being configured and operable to receive the photocurrent and demodulate it at the respective one of the different carrier frequencies, the servo loop module thereby redirecting the optical error signals back to the respective lasers, providing alignment of each of the two laser frequencies with each of the resonant frequencies of the resonance cavities, respectively;

(d) a second photodetector configured and operable to receive from said two lasers, a combined aligned wavelength modulated optical signal formed by the signals of the two different frequencies and generate an output signal having a frequency beat in radio-frequency (RF) domain, such that the frequency difference between the two resonance cavities is converted from the optical domain to the RF domain.

2. The sensing system of claim 1, wherein said two resonance cavities have at least one input port and at least one output port, said at least one input port being optically coupled to a radiation from said two independent lasers.

3. The sensing system of claim 1 or 2, wherein said two resonance cavities are arranged in a cascaded fashion, thereby defining a common input port and a common output port, said common input port being optically coupled to a combined radiation from said two independent lasers.

4. The sensing system of any one of claims 1 to 3, wherein each of the two lasers is configured to generate the wavelength modulated optical signal having a modulation depth in the order of a dimension of the resonance cavity.

5. The sensing system of any one of claims 1 to 4, wherein said two resonance cavities include at least one of the following: microring resonance cavities, microdisk resonance cavities, whispering-gallery mode resonance cavities, Bragg gratings, Fabry- Perot resonance cavities, photonic crystal resonators.

6. The sensing system of any one of claims 1 to 5, wherein one of said two resonance cavities is configured as a sensing element being exposed to environment and sensitive environmental perturbations to be measured, and the second resonance cavity is configured as a reference resonator screened from said environment, such that said output signal having a frequency beat in RF domain is indicative of a differential optical sensing signal.

7. The sensing system of any one of claims 1 to 6, further comprising an RF frequency counter configured and operable to receive from said second photodetector said output frequency signal having a frequency beat in the RF domain and to measure the signal accordingly.

8. The sensing system of any one of claims 1 to 7, configured as a chip-scale platform.

9. The sensing system of any one of claims 1 to 8, wherein each of said two lock-in circuits comprises a lock-in amplifier for generating an output error signal and an integrator for integrating said output error signals from the lock-in amplifier and feeding the frequency locked independent laser with an integrated error signal.

10. The sensing system of any one of claims 1 to 9, wherein the frequency beat is indicative of variations of physical quantities comprising at least one of temperature, pressure, refractive index, analytes and particles.

11. A sensing method comprising:

generating two wavelength modulated optical signals at a different frequency from each other by using two independent lasers; coupling at least one optical signal from said two independent lasers to two resonance cavities;

illuminating a photodetector with the optical signal outputted by the two resonance cavities and generating a photocurrent being indicative, for each pair of laser- resonance cavities, of an optical error signal corresponding to a difference between the laser carrier frequency and the respective resonance cavity frequency;

locking the two lasers respectively to the two different modulated frequencies; redirecting, to the respective lasers, the optical error signals;

providing alignment of each of the two laser frequencies with each of the resonant frequencies of the two resonance cavities respectively; and

generating an output frequency signal having a frequency beat in RF domain such that the frequency difference between the two resonance cavities is converted from the optical domain to the RF domain.

12. The sensing method of claim 11, comprising combining two wavelength modulated optical signals in one combined optical signal.

13. The sensing method of claim 11 or 12, comprising combining the aligned wavelength modulated optical signals formed by the signals of the two different frequencies.

14. The sensing method of any one of claims 11 to claim 13, wherein redirecting to the respective lasers, the optical error signals provides active frequency stabilization of the two independent lasers and of the two resonance cavities.

15. The sensing method of any one of claims 11 to claim 14, comprising providing a differential optical sensing signal.

16. The sensing method of any one of claims 11 to claim 15, comprising measuring physical quantities comprising at least one of temperature, pressure, refractive index, analytes and particles.

17. The sensing method of claim 16, wherein said frequency beat comprises spectral shifts being indicative of at least one of variations in the local refractive index of the resonance cavities, and temperature gradient between the resonance cavities.

18. An RF source comprising:

(a) two independent lasers, each of said two lasers being configured and operable to generate a wavelength modulated optical signal at a different carrier frequency from the other laser; (b) two resonance cavities being configured to define a theoretical frequency difference in the RF regime, the two resonance cavities being optically coupled to at least one optical signal from said two independent lasers, said two resonance cavities and said two independent lasers defining, respectively, two pairs of laser- resonance cavities;

(c) a memory configured and operable to tune the frequency resonance of at least one of the resonating cavity to provide a frequency difference between said two resonance cavities to be substantially identical to the theoretical frequency difference;

(d) a servo loop module comprising a first detector associated with the two resonance cavities, and being configured and operable to generate a photocurrent being indicative, for each pair of laser-resonance cavities, of an optical error signal corresponding to a difference between the laser carrier frequency and the respective resonance cavity frequency; and two lock-in circuits operable for locking the two lasers, respectively, to the two different modulation frequencies, each lock-in circuit being configured and operable to receive the photocurrent and demodulate it at the respective one of the different carrier frequencies, the servo loop module thereby redirecting the optical error signals back to the respective lasers, providing alignment of each of the two laser frequencies with each of the resonant frequencies of the resonance cavities, respectively;

(e) a second detector configured and operable to receive from said two lasers, a combined aligned wavelength modulated optical signal formed by the signals of the two different frequencies and generate an output signal having a frequency beat in radio-frequency (RF) domain.

19. The RF source of claim 18, wherein said memory is configured for generating an electric field in the vicinity of at least one of said two resonance cavities.

20. The RF source of claim 18 or claim 19, wherein said two resonance cavities have at least one input port and at least one output port, said at least one input port being optically coupled to a radiation from said two independent lasers.

21. The RF source of any one of claims 18 to 20, wherein said two resonance cavities are arranged in a cascaded fashion, thereby defining a common input port and a common output port, said common input port being optically coupled to a combined radiation from said two independent lasers.

22. The RF source of any one of claims 18 to 21, wherein each of the two lasers is configured to generate the wavelength modulated optical signal having a modulation depth in the order of a dimension of the resonance cavity.

23. The RF source of any one of claims 18 to 22, wherein said two resonance cavities include at least one of the following: microring resonance cavities, microdisk resonance cavities, whispering-gallery mode resonance cavities, Bragg gratings, Fabry - Perot resonance cavities, photonic crystal resonators.

24. The RF source of any one of claims 18 to 23, configured as a chip- scale platform.

25. The RF source of any one of claims 18 to 24, wherein each of said two lock- in circuits comprises a lock-in amplifier for generating an output error signal and an integrator for integrating said output error signals from the lock-in amplifier and feeding the frequency locked independent laser with an integrated error signal.