Patent Publication Number: US-2023160938-A1

Title: Measurement system and method for analyzing rf signals

Description:
TECHNICAL FIELD 
     The present disclosure relates to signal analysis in microwave communication, and in particular to a measurement system for analyzing radio frequency (RF) signals and a corresponding method. 
     BACKGROUND ART 
     Atom-based quantum effects may be used in developing quantum sensors for various physical quantities, as is known in the art. 
     For example, in highly excited energy states wherein an outermost electron is substantially farther away from a nucleus as in a ground energy state, atomic vapor may exhibit sensitive responses to external electromagnetic RF fields ranging from several to hundreds of gigahertz (GHz). This coincides with a microwave frequency range between 300 MHz and 300 GHz, respectively. 
     For spectrum analysis of RF signals, it is crucial that the underlying quantum sensor provides a fine-tunable and frequency-dependent control of sensitivity and dynamic range. 
     Conventionally, attenuators typically attenuate all frequency components, whereas filter banks can attenuate certain frequency ranges but have to be programmed in accordance with previously measured data. 
     SUMMARY 
     In view of the above-mentioned drawbacks and limitations, the present disclosure aims to improve an RF spectrum analysis of the background art. An objective is to provide systems and methods offering the above-mentioned fine-tunable and frequency-dependent control of sensitivity and dynamic range. 
     The objectives are achieved by the embodiments as defined by the appended independent claims. Preferred embodiments are set forth in the dependent claims and in the following description and drawings. 
     A first aspect of the present disclosure relates to a measurement system for analyzing RF signals, comprising: an optically transparent enclosure comprising an optically pumpable medium being exposed to an electromagnetic field of the RF signal to be analyzed; an optical pump for penetrating the medium with intensity-modulated light, wherein the intensity defines an absorption sensitivity of the medium; a field generator for generating an electric and/or magnetic field within the enclosure, wherein a strength of the generated field defines an absorption frequency of the medium; a controller for controlling the absorption sensitivity of the medium in dependence of the absorption frequency of the medium; and a detector for detecting an optical property of the penetrating light passing through the medium. 
     The optical pump may comprise one of: a directly modulated laser, and an externally modulated laser. The field generator may be configured to generate a homogeneous field within the enclosure. The controller may be configured to sweep the absorption frequency of the medium over a frequency range of the RF signal to be analyzed. The detector may comprise a photodiode. 
     The externally modulated laser may comprise an acousto-optic modulator. 
     The optical pump may comprise a spatially resolving light modulator. The field generator may be configured to generate a spatially resolved inhomogeneous field within the enclosure. The controller may be configured to spatially resolve the absorption sensitivity of the medium in accordance with the spatially resolved absorption frequency of the medium. The detector may comprise a camera. 
     The spatial resolution may be present in a lateral direction relative to a propagation direction of the light. 
     The spatially resolving light modulator may comprise a transmissive light valve. 
     The medium may comprise at least one constituent of: atoms, molecules or ions in a gas, and point defects in a solid. 
     The optical pump may comprise a coherent light source being resonant to at least one energy transition of the at least one constituent to an initial energy state. 
     The initial energy state may comprise a Rydberg state. 
     The gas may comprise an alkali metal. 
     The gas may comprise Rubidium. 
     The solid may comprise a diamond. 
     The optical pump may be configured to penetrate the medium in a continuous wave, CW, mode. The detector may be configured to continually detect the optical property of the penetrating light passing through the medium. 
     The optical property may comprise one of: an intensity, and a photon count. 
     A second aspect of the present disclosure relates to a method for analyzing RF signals, comprising generating an electric and/or magnetic field in an optically transparent enclosure comprising an optically pumpable medium being exposed to an electromagnetic field of the RF signal to be analyzed, wherein a strength of the generated field defines an absorption frequency of the medium; penetrating the medium with intensity-modulated light, wherein the intensity defines an absorption sensitivity of the medium; controlling the absorption sensitivity of the medium in dependence of the absorption frequency of the medium; and detecting an optical property of the penetrating light passing through the medium. 
     Advantageous Effects 
     The present disclosure provides systems and methods for spectrum analysis of RF signals based on an optically pumpable medium in an optically transparent enclosure, such as an atomic vapor cell, for example. 
     The constituents of the medium, which denote N-level quantum systems (N≥2), are optically pumped to an initial energy state. The initial energy state may optionally be only marginally short of the vacuum/ionic state, wherein an outermost electron is substantially farther away from the nucleus as in a ground state. Owing to their extraordinarily large dimension with respect to regular particles, the constituents of the medium are particularly sensitive to exposure to external electric and/or magnetic fields. 
     When exposing the constituents of the medium to a—static—magnetic and/or electric field generated within the enclosure, this resulting in splitting of the initial energy state to a plurality of initial energy states due to Zeeman or Stark splitting. 
     Zeeman splitting may refer to splitting of a spectral line into several components in the presence of a static magnetic field. Stark splitting, the electric-field analogue of Zeeman splitting, may refer to splitting of a spectral line into several components in the presence of a static electric field. 
     The constituents of the medium may easily be taken to even higher energy levels by absorption when exposed to external electro-magnetic RF fields. Such a (near) field may be induced around an electrical transmission line guiding an RF signal to be analyzed through the enclosure, radiated by an antenna, or simply be around (i.e., an ambient field). 
     An intensity of the light penetrating the medium (and thus pumping its constituents) defines a population of the initial energy state, and hence an absorption sensitivity of the medium. More specifically, a low laser intensity enables detection of low-power RF signals, whereas a high laser intensity reduces the sensitivity but increases a dynamic range. The key parameter here is the pumping rate. A low light intensity leads to a low pumping rate, which requires less RF power to cause a significant population change while the optical pump  12  is in operation. 
     A strength of the generated static electric and/or magnetic field determines an absorption frequency (or a plurality thereof) of the medium. 
     The intensity of the light and the strength of the field may be associated in that the absorption sensitivity of the medium depends on the absorption frequency of the medium. A spectral analysis of RF signals may thus enjoy fine-tunable and frequency-dependent control of sensitivity and dynamic range. That is, a better fine-tuning and less sensitivity loss are achieved. 
     The technical effects and advantages described above in relation with the measurement system for analyzing RF signals equally apply to the method for analyzing RF signals having corresponding features. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above-described aspects and implementations will now be explained with reference to the accompanying drawings, in which the same or similar reference numerals designate the same or similar elements. 
       The features of these aspects and implementations may be combined with each other unless specifically stated otherwise. 
       The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to those skilled in the art. 
         FIG.  1    illustrates a measurement system for analyzing RF signals in accordance with an implementation of the present disclosure; 
         FIG.  2    illustrates a measurement system for analyzing RF signals in accordance with a further implementation of the present disclosure; and 
         FIG.  3    illustrates a flow chart of a method for analyzing RF signals in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTIONS OF DRAWINGS 
       FIG.  1    illustrates a measurement system  1 A for analyzing RF signals in accordance with an implementation of the present disclosure. 
     The measurement system  1 A, like the measurement system  1 B explained below, comprises an optically transparent enclosure  11  (such as an atomic vapor cell, a.k.a. gas cell) comprising an optically pumpable medium that is exposed to an electromagnetic field of the RF signal to be analyzed. The medium may comprise at least one constituent of: atoms, molecules or ions in a gas, and point defects in a solid. The gas may comprise an alkali metal, in particular Rubidium (Rb), whereas the solid may comprise a diamond having nitrogen-vacancy (NV) point defects. 
     The measurement systems  1 A,  1 B further comprise an optical pump  12  for penetrating the medium with intensity-modulated light. Optical pumping may refer to a process of stimulating quantum systems having at least two energy levels/states, such as the aforementioned constituents of the medium, through resonant absorption of photons of adequate frequency. The intensity of the light defines an absorption sensitivity of the medium. In particular, the optical pump  12  may comprise a coherent light source  121  (such as a laser) being resonant to at least one energy transition of the at least one constituent to an initial energy state. The initial energy state may comprise a Rydberg state. Optionally, the measurement systems  1 A,  1 B may comprise further coherent light sources besides the optical pump  12 . 
     The measurement systems  1 A,  1 B further comprise a field generator  13  for generating an electric and/or magnetic field within the enclosure  11 . For example, the field generator  13  may include an electromagnet. A strength of the generated field defines an absorption frequency (or a number of absorption frequencies) f RF  of the medium. 
     The measurement systems  1 A,  1 B further comprise a controller  14  for controlling the absorption sensitivity of the medium in dependence of the absorption frequency f RF  of the medium. That is to say, the controller  14  is configured to sweep the absorption frequency f RF  of the medium (via the strength of the electric and/or magnetic field generated by the field generator  13 ) and concurrently to control the absorption sensitivity of the medium (via the intensity of the light of the optical pump  12 ). 
     The measurement systems  1 A,  1 B may further comprise an electrical transmission line  15  for guiding the RF signal to be analyzed through the enclosure  11 , thereby inducing the electromagnetic field of the RF signal to be analyzed. The transmission line  15  may comprise an electroconductive waveguide, such as a metallic wire. Alternatively, the measurement systems  1 A,  1 B may further comprise an antenna (not shown) radiating the electromagnetic field of the RF signal to be analyzed into the enclosure  11 . 
     The measurement systems  1 A,  1 B further comprise a detector  16  for detecting an optical property of the penetrating light passing through the medium. Actually, the detection may capture emitted light as well: When the particles of the medium absorb the light of the optical pump  12 , they also emit photons to all directions, which could be collected by a lens, for instance, and fed to the detector  16  as well. 
     The optical property may comprise one of: an intensity, and a photon count. Forming a difference (or ratio) of the respective optical property of the penetrating light before and after passing through the medium may yield a measure of absorption by the medium. The optical property may further comprise a phase change of the light upon passage through the medium. 
     The optical pump  12  may be configured to penetrate the medium in a continuous wave, CW, mode, and the detector  16  may be configured to continually detect the optical property of the penetrating light passing through the medium. In other words, the optical pump  12  and the RF signal to be analysed are preferably applied during the entire measurement. 
     The specific measurement system  1 A depicted in  FIG.  1    implements a sweep-based spectral analyzer. 
     More specifically, the field generator  13  may be configured to generate a homogeneous field within the enclosure  11 , preferably directed in a lateral direction L relative to a propagation direction P of the light, which lacks a field gradient and thus a spatial resolution in the lateral direction L. Therefore, the detector  16  may comprise a photodiode  161  for punctiform detection. The generated homogeneous field results in a specific absorption frequency (or a number of specific absorption frequencies) f RF  of the medium. 
     The optical pump  12  may comprise one of: a directly modulated laser (not shown), and an externally modulated laser  121 ,  122 . Rydberg excitation usually requires a plurality of lasers, but in that case, it is sufficient to modulate one laser of the plurality. 
     A laser may refer to a device configured to emit light based on stimulated emission of electromagnetic radiation. For example, the externally modulated laser  121 ,  122  may comprise an acousto-optic modulator  122 . An acousto-optic modulator may refer to a device configured to use the so-called acousto-optic effect (i.e., sound waves) to influence, inter alia, an intensity of incident light. 
     As the name suggests, the controller  14  of the sweep-based spectral analyzer may be configured to sweep the absorption frequency f RF  of the medium over a frequency range of the RF signal to be analyzed, by varying a strength of the generated homogeneous field. As mentioned above, the absorption sensitivity of the medium may be controlled in dependence of the absorption frequency f RF  of the medium, by adapting the intensity of the light penetrating the medium. 
       FIG.  1    shows an exemplary intensity profile I(f RF ) in dependence of the absorption frequency f RF  of the medium, corresponding to a frequency-specific absorption sensitivity of the medium. The intensity profile I(f RF ) may be provided as user input to the controller  14 . 
       FIG.  2    illustrates a measurement system  1 B for analyzing RF signals in accordance with a further implementation of the present disclosure. 
     As mentioned above, the same or similar reference numerals as in  FIG.  1    designate the same or similar elements. 
     The specific measurement system  1 B depicted in  FIG.  2    implements a field gradient-based spectral analyzer. 
     As such, the field generator  13  may be configured to generate a spatially resolved inhomogeneous field within the enclosure  11 , wherein the generated field exposes a field gradient. For example, the field generator  13  may comprise an electromagnet whose magnetic field coils generate counter-directed fields. The field gradient ensures availability of constituents of the medium in a ground state for each frequency in a desired frequency range, such as an RF/microwave frequency range. The spatial resolution may be present in a lateral direction L relative to a propagation direction P of the light. Therefore, the detector  16  may comprise a spatially resolving camera  162 , and the optical pump  12  may comprise a spatially resolving light modulator  123 , such as a transmissive light valve. A transmissive light valve may refer to a device configured to attenuate components of light passing through the device in dependence of their lateral position relative to a propagation direction of the light (i.e., spatially resolved). The inhomogeneous field generated by the field generator  13  results in a spatially resolved absorption frequency f RF  of the medium (i.e., a dependency of the absorption frequency f RF  of a—preferably lateral—position within the enclosure  11 . 
     Like its name suggests, the controller  14  of the field gradient-based spectral analyzer may be configured to spatially resolve the absorption sensitivity of the medium, by adapting the intensity of the light penetrating the medium in accordance with the spatially resolved absorption frequency f RF  of the medium. 
     The intensity profile I(f RF ) introduced in  FIG.  1    may be re-used as the user input to the controller  14 . 
       FIG.  3    illustrates a flow chart of a method  2  for analyzing RF signals in accordance with the present disclosure. 
     The method  2  comprises a step of generating  21  an electric and/or magnetic field in an optically transparent enclosure  11  comprising an optically pumpable medium being exposed to an electromagnetic field of the RF signal to be analyzed, wherein a strength of the generated field defines an absorption frequency f RF  of the medium. 
     The method  2  further comprises a step of penetrating  22  the medium with intensity-modulated light, wherein the intensity defines an absorption sensitivity of the medium. 
     The method  2  further comprises a step of controlling  23  the absorption sensitivity of the medium in dependence of the absorption frequency f RF  of the medium. 
     The method  2  further comprises a step of detecting  24  an optical property of the penetrating light passing through the medium.