Optical sensor arrangement including different scan times through a frequency interval for first and second light sources

Abstract

An optical sensor arrangement for measuring an observable including at least one light source for generating a first light component of a first frequency including a first mode and a second light component of a second frequency including a second mode orthogonal to the first mode, an optical resonator having differing optical lengths for the first and second modes, at least one of the optical lengths being variable depending on the observable and a dependence of the respective optical length being different for the first and second modes, and a detector unit coupled to the optical resonator for coupling out the two light components and being configured for detecting a frequency difference between a resonance frequency of the optical resonator for the first mode and a resonance frequency of the optical resonator for the second mode.

Claims

1. An optical sensor arrangement for measuring an observable comprising: at least one light source for generating a first light component of a first frequency comprising a first mode and a second light component of a second frequency comprising a second mode orthogonal to said first mode; an optical resonator having differing optical lengths for the first and second modes within a frequency interval including the first and second frequencies, at least one of the optical lengths being variable depending on the observable and a dependence of the respective optical length on the observable being different for said first and second modes, wherein the at least one light source is optically coupled to the optical resonator for feeding the first and second light components into the optical resonator, and the at least one light source comprises a first light source for generating the first light component and a second light source for generating the second light component, the first and second light sources being coupled to the optical resonator and being independently tunable of each other; a detector unit being coupled to the optical resonator for coupling out the light components and being configured for detecting a frequency difference between a resonance frequency of the optical resonator for the first mode and a resonance frequency of the optical resonator for the second mode; a mode converter arrangement situated between the optical resonator and the detector unit, the mode converter arrangement configured to rotate the first and/or second mode such that the modes comprise components in a matching direction; and a control unit for controlling the at least one light source and/or the optical resonator, the control unit configured such that a scan time through the entire frequency interval is different for the first and second light sources, and the scan time of the first light source is at least five times the scan time of the second light source.

2. The optical sensor arrangement of claim 1 wherein the control unit preferably tunes the at least one light source through said frequency interval.

3. The optical sensor arrangement of claim 1 wherein the mode converter arrangement is configured to rotate the first and/or second modes such that both modes comprise components in a matching transversal direction.

4. The optical sensor arrangement of claim 3 wherein the mode converter arrangement comprises a polarization rotator and a polarization splitter.

5. The optical sensor arrangement of claim 1 wherein the first and second modes are different polarizations of light or orthogonal TEM modes of light.

6. The optical sensor arrangement of claim 1 wherein the optical resonator is at least partially covered with an active layer of covering material for selectively adsorbing a group of substances comprising a substance to be detected and wherein the covering material is configured such that at least one of the resonance frequencies is shifted when the substance contacts the active layer.

7. The optical sensor arrangement of claim 1 wherein the detector unit comprises a photo detector and/or an electronic spectrometer and/or wherein the detector unit is configured for determining said frequency difference as a beat frequency of a superposition of the first light component and the second light component.

8. The optical sensor arrangement of claim 1 wherein the optical resonator is a ring resonator or a Fabry-Perot resonator.

9. The optical sensor arrangement of claim 1 wherein the at least one light source is coupled to the optical resonator through at least one optical path comprising an optical waveguide.

10. The optical sensor arrangement of claim 1 wherein a coupler is situated before or behind the optical resonator.

11. A method for measuring an observable by means of an optical sensor arrangement, the method comprising: coupling light of a first light source of a first frequency comprising a first mode into an optical resonator; coupling light of a second light source of a second frequency comprising a second mode into the optical resonator, wherein the second mode is orthogonal to the first mode and the optical resonator having differing optical lengths for the first and second modes within a frequency interval including the first and second frequencies, at least one of the optical lengths being variable depending on the observable and a dependence of the respective optical length on the observable being different for the first and second modes; changing the first and second frequencies such that a scan time through an entire frequency interval is different for the first and second light sources, and the scan time of the first light source is at least five times the scan time of the second light source; detecting a frequency difference between the resonance frequency of the first and second mode; and coupling the light of the first and/or second frequency to a mode converter arrangement situated behind the optical resonator, the mode converter arrangement configured to rotate the first and/or second mode such that the modes comprise components in a matching direction.

12. The method of claim 11 wherein the light of the first and the light of the second frequency is coupled to the optical resonator simultaneously.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The embodiments of the disclosure are subsequently explained with reference to FIGS. 1-4.

(2) FIGS. 1a and 1b are diagrams illustrating an optical sensor arrangement and the respective intensity spectrogram due to the different optical lengths of an optical resonator, according to some embodiments of the disclosure;

(3) FIGS. 2a through 2c are diagrams illustrating an optical sensor arrangement and the detection of a frequency difference by superposing the first and second light components after their having passed through the optical resonator by analyzing the beat of the superposed light components, according to some embodiments of the disclosure;

(4) FIG. 3 is a diagram illustrating an optical sensor arrangement, according to some embodiments of the disclosure; and

(5) FIG. 4 is a diagram illustrating an optical sensor arrangement on a silicon substrate, according to some embodiments of the disclosure.

DETAILED DESCRIPTION

(6) FIG. 1a is a diagram illustrating an optical sensor arrangement 1 according to a first aspect of the disclosure. The optical sensor arrangement 1 comprises a tunable laser 2 for generating a first light component comprising a first mode and a second light component comprising a second mode. In some embodiments, both components are generated simultaneously. The first light component comprising the first mode relates to the horizontal transversal polarization of the electromagnetic wave, while the second light component is vertically polarized and thus defines the second mode. Both the first light component and the second light component are of the same frequency because the tunable laser emits the light of said frequency. Consequently, the first and second light components are coupled due to their generation within a single laser.

(7) The tunable laser 2 is coupled to an optical wire 3 that may be embodied by a rib made of silicon on a silicon substrate. It may be buffered through a SiO.sub.x layer. Various other materials may be used for producing such optical wires. The optical wire 3 transmits the first and second light component generated by tunable laser 2 and runs by an optical ring resonator 4 that is a closed silicon ring on a silicon substrate. As the distance between the optical wire 3 and the optical ring resonator 4 is chosen such that the light components transmitted by the optical wire 3 may couple through their evanescent fields with the optical ring resonator, the first and second light components are transmitted into the optical ring resonator.

(8) Optical ring resonator 4 has an optical length defined by its physical length and the effective refractive optical index of the material of which the resonator is made. In the present case, the optical ring resonator 4 has different optical lengths for the first and second modes within a frequency interval through which the tunable laser 2 may be tuned. Hence, as the optical lengths of the optical resonator differ for the first and second modes, different resonant frequencies are detected in the optical resonator. Only resonant frequencies may pass through the optical ring resonator and may re-couple into optical wire 5, which transmits the light of a resonant frequency into detector unit 6.

(9) In addition to having differing optical lengths for the first and second modes within said frequency interval, optical resonator 4 is covered with an active layer of covering material for selectively adsorbing a group of substances comprising a substance to be detected. When a substance contacts the active layer, the optical length of the resonator for the first and/or second mode is shifted. In the example, both the optical lengths for the first and for the second mode are shifted.

(10) For illustration of this principle, the reader is referred to FIG. 1b, wherein frequency F is marked on the x-axis and intensity A on the y-axis. The intensity A is proportional to the squared value of the amplitude of the emitted first and second light components.

(11) The tunable laser 2 may be tuned within the frequency interval F. Within the interval, resonance frequency f.sub.1 refers to the resonance of the optical ring resonator when no substance is in contact with the active layer, while f.sub.2 is the resonance frequency for the second mode of the optical ring resonator when no substance contacts the active layer. The difference f.sub.12 is also shown.

(12) When the tunable laser 2 is tuned throughout the frequency interval F, the first light component is coupled into optical wire 5 when the frequency is f.sub.1. In a similar fashion, the second light component is coupled into the optical wire 5 when the tunable laser is tuned to frequency f.sub.2.

(13) In the presence of a substance contacting the active layer, the optical lengths for the optical resonator are shifted from f.sub.1 to f.sub.1 and from f.sub.2 to f.sub.2, respectively. It can be easily seen that the difference f.sub.12 is different from f.sub.12.

(14) A higher-mode resonance frequency f.sub.3 of the optical lengths is also shown in FIG. 1b in order to illustrate that resonance frequencies are not scanned by the tunable laser when they are outside the chosen frequency interval.

(15) Detector unit 6 is equipped with, for example, photo detectors or electronic spectrometers to measure the light intensity throughout the frequency interval. The detector unit, for example comprising photo detectors, measures an intensity of 0 when the frequency of the tunable laser is outside a narrow gap between the resonance frequencies f.sub.1, f.sub.2, f.sub.1, and f.sub.2. When the intensity curves as shown in FIG. 1b are found, one can easily indentify from the difference between the resonance frequencies detected by the detector unit whether the substance is present on the active layer of the optical resonator 4 or not. It should be noted that even though FIG. 1b displays both resonance frequency differences (with and without the chosen substance present on the active layer, such as a particular molecule or DNA snippet), usually only one of the two shown differences will be measurable under working conditions. When the optical circuit of the optical sensor arrangement 1 is produced and one is certain that no substance has contacted the active layer, the difference in the resonance frequencies f.sub.1 and f.sub.2 should be determined.

(16) FIG. 2a is a diagram illustrating an optical sensor arrangement 10, according to some embodiments. Optical sensor arrangement 10 comprises a first tunable laser 11 and a second tunable laser 12, which are coupled through optical wires 13 and 14 and are transmitted to an optical coupler 15, which leads to a superposing of the first and second light components each emitted by the first and second tunable lasers 11 and 12, respectively, through an overlap of the evanescent fields of the emitted light waves. The superposed wave is then transmitted through an optical wire 16 and coupled into an optical ring resonator 17 that is comparable to the optical ring resonator 4.

(17) In contrast to the arrangement shown in FIG. 1a, the arrangement of FIG. 2a comprises two tunable lasers, so that the first and second light components emitted at a first and second frequency may have different frequencies at the same time. Hence, in contrast to the arrangement shown in FIG. 1a, it is possible to scan through several resonances at the same time. Referring back to FIG. 1b, it is thus possible to scan resonance frequency f.sub.1 of the first mode emitted by the first tunable laser 11 at the same as the second resonance frequency f.sub.2 of the second mode emitted by the second tunable laser 12. Similarly, when a substance is contacting the active layer of the optical resonator 17 (not shown), the detectable resonance frequencies f.sub.1 and f.sub.2 may also be detected at the same time.

(18) FIG. 2b is a diagram illustrating a scanning scheme, according to some embodiments of the disclosure. The x-axis shows the scan time. Within a scan time interval of T, tunable laser 11, emitting a first light component at a first frequency f.sub.m1, scans through the frequency interval F six times. During the scan time T the tunable laser 12, emitting a second light component at the second frequency f.sub.m2, scans through the frequency interval only once. This scheme may be used in such a manner that every combination of frequencies f.sub.11, f.sub.12 is scanned, which makes it possible to scan resonance frequencies f.sub.1 and f.sub.2 as shown in FIG. 1b at the same time.

(19) In the following, it is assumed that the first light component frequency f.sub.m1 is tuned to the resonance frequency f.sub.1 and the frequency of the second light component f.sub.m2 is tuned to the resonance frequency f.sub.2. As both these frequencies are resonant within the optical resonator 17, they are both emitted into the optical wire 18. They are transferred to a polarization splitter 19 that splits the horizontal and vertical polarizations. Assuming that the horizontal polarization light component is fed into the upper optical wire comprising the polarization rotator 20, for example in the form of a lambda platelet, and that the horizontal polarization light component is fed to/into the lower optical wire 21, the polarization rotator 20 rotates the vertical polarization to a horizontal polarization and ultimately both light components have the same polarization and may be superposed in the optical coupler 22.

(20) Due to the coupling of light of different frequencies of the same polarization, the superposed wave is made of two different frequency components. The first frequency component is the sum of the frequencies f.sub.1 and f.sub.2. The second frequency component is the difference between frequencies f.sub.1 and f.sub.2 and results in a modulation of the amplitude of the superposed wave.

(21) FIG. 2c is a diagram illustrating the modulation, according to some embodiments of the disclosure. The superposed wave 30 shows a modulation 31 of the amplitude, generally known as the beat of the superposed wave. The envelope has a frequency of f.sub.1-f.sub.2. The frequency of the beat may thus allow a user to determine whether a substance is present due to difference between f.sub.12 and f.sub.12. The frequency of the beat is much slower than the frequency of the optical wave and the photo detectors 24 and 25 of the detector unit 23 may measure the amplitude modulation and transfer the amplitude modulation to an electronic spectrometer, as the difference in frequencies of the optical waves is slow enough to be detected by electronic components. Hence, via means such as electronic spectrometers, it is possible to detect small frequency differences of resonances indicative of whether a substance, i.e., an observable, is located on the active layer of an optical resonator. The arrangement of FIG. 2a is thereby able to detect a shift in optical lengths or resonance frequencies of the optical resonators in the region of less than 0.1 picometers, in the optical spectral interval around 1.5 micrometers. Hence, the arrangement of FIG. 2a is suited for providing a very sensitive sensor for measuring whether a substance is present or not.

(22) FIG. 3 is a diagram illustrating an optical sensor arrangement, according to some embodiments of the disclosure. Optical sensor arrangement 40 comprises first and second tunable lasers 41 and 42 coupling to optical wires 43 and 44, respectively. Both lasers 41 and 42 emit transversally polarized light, a second light component emitted by the second laser 42 being polarized orthogonal to a first light component emitted by the first laser 41. Both light components or modes are coupled into the ring resonator 45, light emitted by the first laser 41 propagating clockwise and light emitted by the second laser 42 propagating counter-clockwise. From the ring resonator 45, the two modes are transmitted to the optical wire arrangement 47, which comprises a first branch 48 of an optical wire for the light component emitted by the second laser 42 and a second branch 49 for the light component emitted by the first laser 41. The second branch 49 comprises a polarization rotator 50 that may shift the polarization of the light emitted by the first laser 41 to be parallel to the polarization of the light of the second laser 42. Once the polarization of the first light component has been switched to the other polarization, both components are superposed in coupler 51 and transferred to detector unit 52 comprising photodiodes 53 and 54, both of which are coupled to a digital amplifier and an electronic spectrometer. In a similar fashion as the arrangement of FIG. 2a, the arrangement of FIG. 3 may be used to detect minuscule resonance shifts that are due to the presence of an observable on an active layer of the ring resonator 45 suited for being contacted by biomolecules or fluids or other substances or observables to be detected. This is due to the fact that the ring resonator 45 has at least slightly different optical lengths for the two modes, the optical length for the mode emitted by the first laser 41 depending more sensitive on the respective observable than the optical length for the mode emitted by the second laser 42.

(23) FIG. 4 is a diagram illustrating an optical sensor arrangement 60, according to some embodiments of the disclosure. Optical sensor arrangement 60 is situated on a silicon substrate 61. In contrast to the arrangements shown in FIGS. 2a and 3, the optical arrangement 60 comprises a master laser 62 coupled to two single-mode slave lasers 63 and 64 that are different in their emission wavelength and synchronized via injection locking, i.e., the phase noise is synchronized between the two slave lasers. By using this tunable source with synchronized phase noise, the width of the frequency difference may be reduced to be in the Hertz region, and thus a high resolution for detecting resonance shifts or the presence of an observable on an optical resonator is given.

(24) Despite the different laser arrangements, optical sensor arrangement 60 is similar to the arrangement of FIG. 2a. In particular, light from the tunable lasers is coupled via optical wires 65 and 66 into an optical coupler 67, then to an optical wire 68 and into the optical ring resonator 69 comprising an active layer, then into the optical wire 70, the different polarizations emitted by the first and second laser 63 and 64 being separated in the polarization splitter 71, the polarization of one of the components being rotated in the polarization rotator 72, and the two remaining waves being superposed in coupler 73. Detector unit 74 comprises photodiodes 75 and 76, both of which are connected to a digital amplifier 77.

(25) In a variation of the optical sensor arrangement 60, both lasers 63 and 64 may be configured such that they emit different TEM modes. One of the lasers 63 emits a TEM mode 01 while the other emits a TEM mode 11. Both lasers are tuned via a control unit 78, which may also receive input from the components of the detector unit 74. As the different TEM modes are orthogonal to each other and the optical ring resonator 69 may be configured to have different resonance frequencies, i.e., optical lengths, for different transversal electric modes, different resonant frequencies may be transmitted by the optical resonator and are then fed into optical wire 70, into mode splitter 71, one of the branches comprising a mode converter instead of the polarization rotator 72. An example for such a mode converter can be found in R. L. Eisenhart, A Novel Wideband TM 01 to TM 11 Mode Converter, IEEE MTT-S Intl., Jun. 7-12, 1989, vol. 1, pages 249-252. Once the TM 01 mode is converted to a TM 11 mode, they may be coupled and superposed in the optical coupler 73.