Patent Application
20240295581
The present disclosure relates generally to sensor systems, and specifically to an optical accelerometer system.
Many types of instruments have been developed for measuring acceleration. One such example is a force-balanced accelerometer. For example, in a pendulous electrostatic force-balanced accelerometer, electrostatic forcing in a closed loop system is employed to position and obtain an output from a pendulous inertial mass or proof mass. The electrostatic forcing system may employ a capacitive pickoff electrode on each side of a pendulous member that has been etched from a silicon substrate. A control pulse can be employed to sequentially apply a constant amount of charge to each electrode. A variable force can be applied to the inertial mass by varying the amount of time (e.g., duty cycle) the charge is left on a respective plate. The amount of time the charge is left on a respective plate is based on the displacement of the inertial mass relative to a null position. However, electrostatic force-balanced accelerometers can be subject to a number of deleterious phenomena, such as accelerometer bias uncertainty which can be a major source of error in inertial measurement and/or navigation systems.
One example includes an optical accelerometer system. The system includes an optical cavity system comprising a variable optical cavity that propagates a first optical beam and a fixed optical cavity that propagates a second optical beam. The fixed optical cavity can have a fixed cavity length and the variable optical cavity can have a cavity length that changes in response to an external acceleration. The optical cavity system can also include optics to provide a beat optical beam that is a combination of the first and second optical beams. The system also includes a detection system to monitor the beat optical beam to generate a beat voltage that is indicative of the frequency of the beat optical beam. The detection system can compare the beat voltage with a stable frequency reference voltage to determine a magnitude of the external acceleration.
Another example includes a method for measuring an external acceleration. The method includes generating a first optical beam and a second optical beam. The method also includes providing the first optical beam in a variable optical cavity having a cavity length that changes in response to the external acceleration, and providing the second optical beam in a fixed optical cavity having a fixed cavity length. The method also includes combining the first and second optical beams to generate a beat optical beam, and providing the beat optical beam to a photodetector to generate a beat voltage that is indicative of the frequency of the beat optical beam. The method further includes comparing the beat voltage with a stable frequency reference voltage to determine a magnitude of the external acceleration.
Another example includes an optical accelerometer system. The system includes an integrated optical cavity chip. The chip includes a variable optical cavity waveguide comprising a first gain medium, a first reflective Bragg grating, and a spring-mounted reflector configured to provide a variable cavity length for a first optical beam in response to an external acceleration. The chip also includes a fixed optical cavity waveguide comprising a second gain medium, a second reflective Bragg grating, and a reflector that is fixed to a housing of the integrated optical cavity chip to provide a fixed cavity length for a second optical beam. The chip also includes a waveguide combiner portion configured to optically combine the variable optical cavity waveguide and the fixed optical cavity waveguide to generate a beat optical beam that is a combination of the first and second optical beams. The chip further includes a photodetector configured to monitor the beat optical beam to generate a beat voltage that is indicative of the frequency of the beat optical beam. The system further includes a detection system configured to compare the beat voltage with a stable frequency reference voltage to determine a magnitude of the external acceleration.
The present disclosure relates generally to sensor systems, and specifically to an optical accelerometer system. The optical accelerometer system can be implemented in any of a variety of applications that may require accurate determination of an external acceleration acting upon a moving platform (e.g., an aerospace craft). As an example, the optical accelerometer system can be implemented in an inertial navigation system (INS) of a moving platform.
The optical accelerometer system can include an optical cavity system that includes a variable optical cavity and a fixed optical cavity. A first optical beam can propagate in the variable optical cavity having a variable cavity length, and a second optical beam can propagate in the fixed optical cavity having a fixed cavity length. As an example, the optical cavity system can include an optical source that is configured to generate the first and second optical beams. The optical source can correspond to at least one laser, such as a laser diode. For example, a single laser diode can generate a multimode optical beam that can be filtered to provide two optical modes corresponding to the first and second optical beams, respectively. As an example, the fixed optical cavity can include a reflecting surface that is mounted on a housing of the optical cavity system, and the variable optical cavity can include a spring-mounted reflective surface that is mounted to the housing of the optical cavity system. Therefore, the spring-mounted reflective surface can move in response to the external acceleration, thereby changing the frequency of the first optical beam relative to the frequency of the second optical beam.
As described herein, the term “spring-mounted” with reference to a reflective surface or reflector refers to any of a variety of ways to mount the reflective surface or reflector in a manner that allows the reflective surface or reflector to move relative to the housing of the optical cavity system based on a spring constant. The spring-mounted reflective surface is demonstrated herein as including spring mounts on opposite sides of the spring-mounted reflective surface. However, the spring-mounted reflective surface can include other spring-mounting arrangements, such as a spring cantilever or any of a variety of other spring-mounting techniques.
The optical cavity system can include optics configured to combine the first and second optical beams to generate a beat optical beam. The combination of the first and second optical beams to provide the beat optical beam can result in the beat optical beam having a frequency that is a difference of the frequencies first and second optical beams, and thus significantly less than the frequency of the first and second optical beams. For example, the beat frequency can have a frequency in the radio frequency (RF) spectrum (e.g., MHz to GHz). The beat optical beam can be provided to a photodetector, such that the frequency of the beat optical beam can be converted to a beat voltage signal. The beat voltage signal can be mixed with a stable frequency reference voltage generated by a frequency (e.g., a crystal oscillator) to generate a voltage difference signal. The voltage difference signal can be provided to an acceleration circuit that compares the voltage difference signal with a reference voltage to generate an output signal that corresponds to the magnitude of the external acceleration.
The optical accelerometer system can thus operate to determine the external acceleration with optical precision based on the use of optical beams, but with simple electronic components based on the conversion of the beat optical beam to a manageable RF frequency signal. As described in greater detail herein, the optical accelerometer system can also implement additional features for more accurate control, such as force-rebalance feedback, laser stabilization, and optical generation of the stable frequency reference voltage. Furthermore, the optical accelerometer system can be implemented in a very compact form-factor, as described in greater detail herein, which can also provide greater optical stability. Accordingly, the optical accelerometer system can be a significantly smaller and more efficient accelerometer than typical force-rebalance accelerometers or other typical optical accelerometers.
The optical accelerometer system 100 includes an optical cavity system 102, and a detection system 104. In the example of
The first and second optical beams OPT1 and OPT2 are provided to a set of optics 108 of the optical cavity system 102. As an example, the optics 108 can include collimating lenses and/or filters (e.g., an interference filter) to provide the first optical beam OPT1 in a variable optical cavity 110, and to provide the second optical beam OPT2 in a fixed optical cavity 112. As an example, the variable optical cavity 110 and the fixed optical cavity 112 can each be formed between a respective reflector and the optical source(s) 106, such as a lasing medium of a laser diode. As an example, the fixed optical cavity 112 can include a reflecting surface that is mounted on a housing of the optical cavity system 102, thereby exhibiting a fixed cavity length of the fixed optical cavity 112. However, as described herein, the variable optical cavity 110 has a variable cavity length that changes in response to the external acceleration. As an example, the variable optical cavity 110 can include a spring-mounted reflective surface that is mounted to the housing of the optical cavity system 102. Therefore, the spring-mounted reflective surface can move in response to the external acceleration, thereby changing the frequency of the first optical beam OPT1 relative to the frequency of the second optical beam OPT2.
As an example, the optics 108 can be configured to combine the first and second optical beams OPT1 and OPT2 to generate a beat optical beam OPTBT. The combination of the first and second optical beams OPT1 and OPT2 to generate the beat optical beam OPTBT can result in the beat optical beam OPTBT having a frequency that is a difference of the frequencies of the first and second optical beams OPT1 and OPT2. Therefore, the frequency of the beat optical beam OPTBT can be significantly less than the frequency of the first and second optical beams OPT1 and OPT2, such as in the radio frequency (RF) spectrum (e.g., MHz to GHz). In the example of
The optical cavity system 202 includes a variable optical cavity 218 that is defined by a spring-mounted reflector 220 and the laser diode 206. The optical cavity system 202 also includes a fixed optical cavity 222 that is defined by a fixed reflector 224 and the laser diode 206. Both the spring-mounted reflector 220 and the fixed reflector 224 are coupled to an optical cavity housing 226. The example of
The fixed reflector 224 therefore defines a fixed cavity length of the fixed optical cavity 222, such that the fixed optical cavity 222 can be substantially insensitive to mechanical shock and/or acceleration acting upon the optical accelerometer system 200. However, while the spring-mounted reflector 220 is coupled to the optical cavity housing 226, flexures on the spring-mounted reflector 220 define a variable cavity length of the variable optical cavity 218. Therefore, the spring constant of the flexures of the spring-mounted reflector 220 result in the variable optical cavity 218 having a cavity length that changes in response to a vector component of an external acceleration acting upon the optical accelerometer system 200, with the vector component being parallel with a propagation direction of the first and second optical beams 214 and 216.
In the example, the optical cavity system 202 includes a reflector (e.g., beam splitter) 228 arranged between the collimating lens 210 and the interference filter 212. The reflector 228 can thus reflect the multimode beam 208 to generate a beat optical beam OPTBT that is output from the optical cavity system 202. The beat optical beam OPTBT is thus a combination of the optical modes of the first and second optical beams 214 and 216, and therefore has a frequency that is a difference of the frequencies of the first and second optical beams 214 and 216. Therefore, the frequency of the beat optical beam OPTBT can be significantly less than the frequency of the first and second optical beams 214 and 216, such as in the radio frequency (RF) spectrum (e.g., MHz to GHz).
In the example of
The beat voltage signal VBT is demonstrated as being provided to a mixer 232. The detection system 204 also includes a local oscillator 234 that is configured to generate a stable frequency reference voltage VFRQ. The mixer 232 mixes the beat voltage signal VBT with the stable frequency reference voltage VFRQ to generate a voltage difference signal VDIFF. The voltage difference signal VDIFF can have an amplitude that is proportional to the frequency difference between the beat voltage signal VBT and the stable reference frequency voltage VFRQ.
The voltage difference signal VDIFF is provided to an acceleration circuit 236, along with a reference voltage VREF. As an example, the reference voltage VREF can have an amplitude that corresponds to the constant frequency of the second optical beam 216. Therefore, the acceleration circuit 236 can compare the voltage difference signal VDIFF with the reference voltage VREF to determine an error magnitude that is proportional to the difference between the frequencies of the first and second optical beams 214 and 216. Therefore, the acceleration circuit 236 can be configured to provide the error as an output signal ACCEL, such that the output signal ACCEL can be indicative of a rebalance force that can be dependent on the spring constant of the flexures of the spring-mounted reflector 220. Therefore, the output signal ACCEL corresponds to the magnitude of the external acceleration in the vector direction parallel with the first and second optical beams 214 and 216.
As another example, the detection system 204 can be configured to convert the frequency of the beat voltage signal VBT to a DC voltage having an amplitude that is proportional to the frequency of the beat voltage signal VBT. In this example, the mixer 232 and the local oscillator 234 can be omitted from the detection system 204. Therefore, the acceleration circuit 236 can compare the converted DC voltage directly with the reference voltage VREF to generate the output signal ACCEL. Similar to as described above, the output signal ACCEL can thus be indicative of the rebalance force, and thus the magnitude of the external acceleration.
As described above, the optical accelerometer system 200 can include additional features to provide for a more stable and accurate measurement of the external acceleration acting upon the optical accelerometer system 200.
The force-rebalance system 300 also includes a first set of electrodes 304 that are positioned proximal with and above the spring-mounted reflector 220, and a second set of electrodes 306 that are positioned proximal with and below the spring-mounted reflector 220. The force-rebalance system 300 further includes a force-rebalance servo controller 308 that is configured to receive the output signal ACCEL. As described above, the output signal ACCEL can be indicative of a force associated with the external acceleration, and can thus correspond to a rebalance force that can counteract the external acceleration in an equal and opposite manner.
In the example of
The laser stabilization system 402 includes a vapor cell 404 and an optical detection system 406. As an example, the vapor cell 404 can be filled with an alkali metal vapor (e.g., rubidium or cesium). In the example of
Photons of the second optical beam OPT2 are thus absorbed by a population of the alkali metal vapor based on the frequency of the second optical beam OPT2, such as based on any of a variety of Doppler-broadened absorption spectroscopy techniques. The amount of absorption of the photons of the beat optical beam OPTBT by the alkali metal vapor is measured by the optical detection system 406, such as based on monitoring the intensity of the second optical beam OPT2 exiting the vapor cell 404. Therefore, the offset of the frequency of the second optical beam OPT2 from a resonant frequency of the alkali metal vapor can be determined in a highly accurate manner, thereby providing an accurate determination of the frequency of the second optical beam OPT2.
In the example of
The integrated optical cavity chip 500 includes a variable optical cavity waveguide 502 that includes a first gain medium (“GAIN MEDIUM 1”) 504 and a first reflective Bragg grating 506, a first fixed optical cavity waveguide 508 that includes a second gain medium (“GAIN MEDIUM 2”) 510 and a second reflective Bragg grating 512, and a second fixed optical cavity waveguide 514 that includes a third gain medium (“GAIN MEDIUM 3”) 516 and a third reflective Bragg grating 518. The gain media 504, 510, and 516 each correspond to a respective one of the optical sources 106 in the example of
In the example of
The integrated optical cavity chip 500 also includes a first waveguide combiner portion 526 and a second waveguide combiner portion 528. The waveguide combiner portions 526 and 528 and the microlenses 524 can collectively correspond to the optics 108 in the example of
The second waveguide combiner portion 528 is configured to optically combine the second and third optical beams OPT2 and OPT3 to generate a reference frequency optical beam OPTFRQ. Because of the difference in cavity lengths of the first and second fixed optical cavity waveguides 508 and 514, the reference frequency optical beam OPTFRQ can have a beat frequency, similar to the beat optical beam OPTBT, that is a difference in the frequency of the second and third optical beams OPT2 and OPT3. Because of the cavity lengths of the first and second fixed optical cavity waveguides 508 and 514 are fixed, the frequencies of the second and third optical beams OPT2 and OPT3 can be very stable, even with frequency drift resulting from environmental considerations (e.g., temperature) based on the integration of the first and second fixed optical cavity waveguides 508 and 514 in the integrated optical cavity chip 500. Therefore, the beat frequency of the reference frequency optical beam OPTFRQ can likewise be very stable despite environmental considerations. The integrated optical cavity chip 500 also includes a second photodetector 532 configured to monitor the reference frequency optical beam OPTFRQ to generate the stable frequency reference voltage VFRQ. The stable frequency reference voltage VFRQ can thus be provided to the detection system 104, such as to replace the local oscillator 234 that is configured to generate a stable frequency reference voltage VFRQ in the example of
In view of the foregoing structural and functional features described above, methodologies in various aspects of the description will be better appreciated with reference to
What have been described above are examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements. As used herein, the term “includes” means includes but not limited to, and the term “including” means including but not limited to. The term “based on” means based at least in part on.
This application claims priority from U.S. Patent Application Ser. No. 63/449,340, filed 2 Mar. 2023, which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63449340 | Mar 2023 | US |
