Millimeter wave molecular sensor

A millimeter wave molecular sensor system is provided. The system includes a physics cell configured to contain a sample, a directional coupler configured to receive input millimeter waves, partition the input millimeter waves into a pump signal and a probe signal for transfer to the physics cell, a receiver configured to receive millimeter waves exiting the physics cell, a Faraday rotator coupled between a pump transmitter and the physics cell, and a coupling iris coupled between the Faraday rotator and the physics cell, configured to pass millimeter waves having a first polarization into the physics cell. The Faraday rotator includes a Faraday material, and an electronic device configured to apply a magnetic field to the Faraday material parallel to a propagation direction of the millimeter waves such that when the electronic device is activated, the Faraday material rotates a polarization of the millimeter waves passing through the Faraday rotator.

TECHNICAL BACKGROUND

Millimeter wave (mmW) spectroscopy is commonly used for the detection and identification of various compounds in a wide variety of applications. Typical mmW spectroscopes are large, expensive machines, sensitive to noise and background fluctuations. Many applications require fast and precise measurements. Also, since the machines are expensive, time spent using the spectroscope is at a premium, and any wasted time is costly.

OVERVIEW

In an implementation, a millimeter wave molecular sensor system is provided. The millimeter wave molecular sensor system includes a physics cell configured to contain a sample, and a directional coupler configured to receive input millimeter waves, partition the input millimeter waves into a pump signal and a probe signal, each signal comprising a portion of power of the input millimeter waves.

The millimeter wave molecular sensor system also includes a pump transmitter coupled with the directional coupler configured to transfer the pump signal to the physics cell, a probe transmitter coupled with the directional coupler configured to transfer the probe signal to the physics cell, and a receiver coupled with the physics cell configured to receive millimeter waves exiting the physics cell after passing through the sample.

The millimeter wave molecular sensor system further includes a Faraday rotator coupled between the pump transmitter and the physics cell configured to pass millimeter waves from the pump transmitter to the physics cell, and a coupling iris coupled between the Faraday rotator and the physics cell, configured to pass millimeter waves having a first polarization into the physics cell.

The Faraday rotator includes a Faraday material, and an electronic device configured to apply a magnetic field to the Faraday material parallel to a propagation direction of the millimeter waves passing through the Faraday material such that when the electronic device is activated, the Faraday material rotates a polarization of the millimeter waves passing through the Faraday rotator.

In another implementation, a method for operating a millimeter wave molecular sensor system is provided. The method includes the following steps: a. receiving an input millimeter wave having a first polarization swept over a range of frequencies, and b. partitioning the input millimeter wave into a pump signal and a probe signal, each signal comprising a portion of power of the input millimeter wave.

The method also includes the following steps: c. transferring the probe signal into a physics cell through a probe transmitter, and d. transferring the pump signal having the first polarization through a pump transmitter, and through a Faraday rotator, the Faraday rotator configured to transform the pump signal having the first polarization into a pump signal having a second polarization when activated.

The method further includes the following step: e. transferring an output of the Faraday rotator through a coupling iris to the physics cell. The coupling iris is configured to pass pump signals having the first polarization and to block pump signals having the second polarization.

The method also includes the following steps: f. receiving millimeter waves exiting the physics cell at a receiver configured to receive millimeter waves exiting the physics cell after passing through the sample, and g. transferring an output of the receiver to an electronic recording device, configured to receive and record millimeter wave amplitude data from the receiver as the pump signal and probe signal sweep through the range of frequencies.

The method further includes the following steps: h. generating first millimeter wave amplitude data by performing steps a-g while the Faraday rotator is inactive, and i. generating second millimeter wave amplitude data by performing steps a-g while the Faraday rotator is active.

DETAILED DESCRIPTION

FIG. 1illustrates a conventional system100for a millimeter wave (mmW) gas sensor that includes a series of three one-meter-long tubes101with a set of reflectors103pumped with a large physical vacuum and supporting electronics102. The volume of the system may be on the order of 0.5-2 cubic meters. Most current mmW spectroscopy systems rely on the utilization of external pumps, valves, and large vacuum sealed chambers.

Some mmW spectroscopy systems are compact, such as the compact millimeter wave system described in U.S. Patent Application Publication Number 2019/0346814 A1, titled “COMPACT MILLIMETER WAVE SYSTEM”, filed on Apr. 30, 2019, which is hereby incorporated by reference in its entirety. In various embodiments, the present invention is capable of application in both standard and compact spectroscopy systems.

FIG. 2illustrates an example system200of a conventional millimeter wave spectroscopy system including external electronic devices. The system includes gas inlet201connected to mass flow controller202that is monitored with pressure gauges203. The gas is pumped into physics cell204, and pumped out with a second pumping device216and gas outlet217. A millimeter wave electromagnetic signal is transmitted into physics cell204.

According to an example embodiment, a frequency of the electromagnetic signal ranges from 60 GHz to 300 GHz. The electromagnetic signal is received by receiver206and synced into input207of lock in amplifier212to frequency source211and absorption across the frequency range is generated. The output signal of the receiver is monitored with a chart214or other electronic data recording device. In some example embodiments, this will tend to just be a diode detector configured to convert the output signal to a low frequency readout for comparison to the input. In example configurations, it is not plotted for monitoring, but rather tracked by the feedback and control loop (lock in amplifier). The output signal of the lock in amplifier is monitored with another chart213or other electronic recording device.

Lock In detection is a way to increase the signal to noise ratio (SNR). Essentially the lock in amplifier is a very good bandpass filter that detects signals only around certain frequency. In this example system the signal coming out of the synthesizer is frequency modulated (FM). This signal interacts with the molecules and then received on the receiver. The lock in amplifier detects only signals at the modulation frequency, and thus, the SNR is increased compared to the signal in chart214.

This example device includes a transceiver electrically coupled to the first205and second206antennas and configured to inject a transmit signal into physics cell204through the first antenna, the signal interrogates the molecules in the cavity generating absorption dips at the quantum transition frequencies of the gases in the cavity, the signal is detected in the second antenna. By scanning a frequency band of interest, it is possible to detect the presence of different gases in the cavity by identifying the quantum absorption frequencies.

Synthesizer210receives input from frequency generator209and frequency modulator211and outputs a signal to the physics cell. In one example embodiment, signal generator209initially sweeps the transmission output frequency through a band known to include the quantum transitions of the gases in physics cell204(e.g., transitioning upward from an initial frequency below the suspected quantum transition frequency, or initially transitioning downward from an initial frequency above the suspected quantum transition frequency, or other suitable sweeping technique or approach).

The transceiver monitors the received energy via an input coupled with (e.g. electrically connected to) a second conductive coupling structure in order to identify the transmission frequency associated with peak absorption by the gas in the physics cell204(e.g., minimal reception at the receiver). Once the quantum absorption frequency is identified, a loop filter moves the source signal generator transmission frequency close to that absorption frequency (e.g., 183.31 GHz), and modulates the signal at a very low frequency to regulate operation around the null or minima in the transmission efficiency representing the ratio of the received energy to the transmitted energy.

The loop filter provides negative feedback in a closed loop operation to maintain the signal generator operating at a transmit (TX) frequency corresponding to the quantum frequency of the cavity dipolar molecule gas and dynamically adjusts a frequency of the transmit signal based on the error signal. The transceiver circuit in certain implementations is implemented on or in an integrated circuit (not shown), to which the physics cell204is electrically coupled for transmission of the TX signal via output205and for receipt of the RX signal via input206.

The transceiver is operable when powered for providing an alternating electrical output signal TX to the first conductive coupling structure for coupling an electromagnetic field to the interior of physics cell204, as well as for receiving the alternating electrical input signal RX from a second conductive coupling structure representing the electromagnetic field received from physics cell204. The transceiver circuit is operable for selectively adjusting the frequency of the electrical output signal TX in order to reduce the electrical input signal receive (RX) by interrogation to operate clock generator209at a frequency which substantially maximizes the molecular absorption through rotational state transitions, and for providing a reference clock signal REF_CLK at the frequency of the TX output signal.

FIG. 3illustrates an example system300of a conventional pump/probe millimeter wave spectroscopy cell302. In this example embodiment physics cell302contains a low-pressure dipolar gas, pump transmitter304and probe transmitter308are configured to transfer millimeter waves to physics cell302, and receiver306is configured to receive millimeter waves exiting physics cell302after passing through the sample.

Directional coupler310is configured for use as a power splitter. Directional coupler310receives power as input millimeter waves through the input TX312, and directs most of the power to pump transmitter304while the remainder goes to probe transmitter308. This ratio may be set based upon an input (not shown) to directional coupler310.

Since millimeter wave sensor system300is measuring very small changes in millimeter waves during operation, it is very sensitive to noise and any fluctuations in the background. Thus, during conventional operation, millimeter wave sensor system300is operated once with an empty physics cell302, and then with a physics cell302containing the sample under investigation. The outputs of these two operations are then subtracted to obtain the desired spectroscopic data. Other systems utilize two physics cells302, one empty, and one containing the sample under investigation, allowing them to perform the two measurements quickly without having to evacuate and fill a single physics cell302.

FIG. 4illustrates an example implementation of a millimeter wave molecular sensor system400. In this example embodiment physics cell402contains a low-pressure dipolar gas. Directional coupler410is configured for use as a power splitter. Directional coupler410receives power through the input TX430in the form of input millimeter waves swept over a range of frequencies, and directs most of the power to pump transmitter404as a pump signal, while the remainder goes to probe transmitter408as a probe signal. This power ratio may be set based upon an input (not shown) to directional coupler410.

Pump transmitter404is configured to transfer the pump signal having a first polarization swept over a range of frequencies to physics cell402through a Faraday rotator and coupling iris416, and probe transmitter408is configured to transfer the probe signal to physics cell402. These millimeter waves (photons) are detected (received) by receiver406. The detected signal from receiver406is recorded422and processed424. Probe transmitter408is coupled with physics cell402through coupling link418.

Outputs from receiver406are transferred to electronic recording device422, which is configured to receive and record millimeter wave amplitude data from receiver406as the millimeter waves sweep through the range of frequencies. Processing system424receives the recorded millimeter wave amplitude data from electronic recording device422and processes the millimeter wave amplitude data to produce molecular spectroscopy data, identify molecules within the sample, and to provide a user with the identity of the molecules within the sample.

In this example embodiment, a Faraday rotator and coupling iris416have been placed between pump transmitter404and physics cell402. The Faraday rotator comprises Faraday material412, and an electronic device414configured to apply a magnetic field to Faraday material412. In this example embodiment, electronic device414is a coil substantially surrounding Faraday material412, and is configured to apply a magnetic field to Faraday material in a vector parallel to a propagation direction of the millimeter waves passing through the Faraday material412when activated.

When coil414is activated, the Faraday material412rotates a polarization of the millimeter waves passing through the Faraday rotator412from pump transmitter404to physics cell402. Dimensions of Faraday material412are selected to provide a desired rotation of the millimeter waves. In an example implementation Faraday material412is designed to rotate the millimeter waves transferred by pump transmitter404by at least five degrees when coil414is activated. Operation of the Faraday rotator is illustrated inFIG. 5and described in detail below.

In some example embodiments of the present invention, the Faraday rotator itself is a separate physics cell, and the Faraday material is a gas at low pressure. In various embodiments, the gas is a paramagnetic gas such as NO, O2, ClO2, or a diamagnetic gas such as NH3. In other example embodiments a single gas is trapped in both the Faraday rotator and the physics cell.

Coil414is controlled by signal generator420. In an example embodiment, signal generator420applies a square wave to coil414, activating and inactivating coil414in a series of cycles. This operation is illustrated inFIG. 6and described below.

Coupling iris416is located between the Faraday rotator and physics cell402, and is configured to only pass millimeter waves having a defined polarization into physics cell402. (Basically, coupling iris416is a polarizing filter.) In various embodiments, coupling iris exhibits varying degrees of variation in the polarization of the millimeter waves that it allows to pass. For example, a high-quality coupling iris may allow only millimeter waves within less than one degree of the desired polarization to pass, while a low-quality coupling iris may allow for wider variation of the millimeter waves.

In operation, when coil414is inactive, millimeter waves from pump404having a first polarization pass through Faraday material412without any change in polarization. In configurations where coupling iris416is oriented to allow millimeter waves having the first polarization to pass through to physics cell401, when coil414is inactive, the millimeter waves having the first polarization pass from pump transmitter404into physics cell402. When coil414is active, Faraday material412rotates the millimeter waves having a first polarization from pump transmitter404into millimeter waves having a second polarization different from the first polarization. In some example embodiments, the second polarization is different from the first polarization by at least five degrees.

In the example implementation described above, Faraday material412is designed to rotate the millimeter waves transferred from pump transmitter404by at least five degrees when coil414is activated, and coupling iris416blocks the millimeter waves having the second polarization from entering physics cell402. Simply, Faraday rotator and coupling iris are configured to act as a switch to either block or allow millimeter waves from pump transmitter404to enter physics cell402.

Other example embodiments may be configured such that coupling iris416transmits the millimeter waves when the Faraday rotator is activated, and blocks the millimeter waves when the Faraday rotator is inactive, all within the scope of the present invention.

This configuration allows millimeter wave molecular sensor system400to quickly make two separate measurements using a single physics cell402. One measurement is made with coil414inactive and coupling iris416allowing the millimeter waves from pump transmitter404to pass into physics cell402, and a second measurement is made with coil414active and coupling iris416blocking the millimeter waves from pump transmitter404. The first measurement provides a doppler free signal as illustrated inFIG. 7A, and the second measurement provides an absorption signal as illustrated inFIG. 7B.

In an example implementation, in operation, electronic recording device422records first millimeter wave amplitude data from receiver406as the millimeter waves sweep through the range of frequencies while coil414is inactive, and records second millimeter wave amplitude data from receiver406as the millimeter waves sweep through the range of frequencies while coil414is active. Processing system424produces molecular spectroscopy data by subtracting the second millimeter wave amplitude data from the first millimeter wave amplitude data.

In another example implementation, signal generator420applies a repeating electronic signal to coil414such that coil414alternates between being activated and inactivated on a repeating cycle. The millimeter waves sweep through the range of frequencies each time coil414is activated and inactivated, and processing system424produces molecular spectroscopy data during each cycle.

Processing system424averages the molecular spectroscopy data produced during a plurality of cycles to reduce noise in the final data and to provide final spectroscopy data for identification of molecules within the sample in physics cell402. In some example implementations, processing system424identifies the molecules within the sample in physics cell402, and provides a user with their identities.

FIG. 5illustrates an example implementation of a Faraday rotator, as illustrated inFIG. 4. In this example, Faraday material500is defined by its Verdet constant506and length502of the path linearly polarized light510travels through the material. The plane508of linearly polarized light is rotated when a magnetic field504is applied in parallel to the propagation direction of the light. The angle of rotation514is calculated by:
β=VBd
where β is the angle of rotation in radians, B is the magnetic flux density in the direction of propagation in teslas, and d is the length of the path in meters where the light and magnetic field interact. V is the Verdet constant for the Faraday material500. It is an empirical proportionality constant (in units of radians per tesla per meter) and varies with wavelength and temperature. It has been tabulated for a variety of materials. Note that due to relative path length and reasonably achievable field strength, it is difficult to rotate the millimeter waves by 90 degrees.

When a magnetic field having a flux density of B is applied to Faraday material500having a thickness of d and a Verdet constant of V, the linearly polarized light510is rotated by an angle of rotation β as shown on plane512. By selecting a Faraday material500with a particular Verdet constant, a desired thickness502of the material500may be calculated from the above equation.

FIG. 6illustrates an example operation of a signal generator420, as illustrated inFIG. 4. In this example implementation, signal generator420is configured to apply a square wave to coil414activating and inactivating the Faraday rotator and acting as a switch between pump transmitter404and physics cell402. Proceeding along the time axis610, when the output of signal generator420is high B≠0 and coil414is activated, millimeter waves from pump transmitter404are prevented from reaching physics cell402. The resulting signal received by receiver406is called the absorption signal620.

When the output of signal generator420is low B=0 and coil414is inactive, millimeter waves from pump transmitter404are allowed to reach physics cell402. The resulting signal received by receiver406is called the doppler free signal630.

FIG. 7Aillustrates an example output of an example implementation of a millimeter wave molecular sensor system when the Faraday rotator is inactive. This example illustrates an example output from receiver406when the output of signal generator420is low, coil414is inactive, and pump transmitter404is transmitting millimeter waves through physics cell402. The resulting signal is called the doppler free signal700.

FIG. 7Billustrates an example output of an example implementation of a millimeter wave molecular sensor system when the Faraday rotator is active. This example illustrates an example output from receiver406when the output of signal generator420is high, coil414is active, and pump transmitter404is isolated from physics cell402. The resulting signal is called the absorption signal710.

FIG. 7Cillustrates the results obtained by subtracting the data ofFIG. 7Bfrom the data ofFIG. 7A. In operation, electronic recording device422records first millimeter wave amplitude data from receiver406as pump transmitter402sweeps through the range of frequencies while coil414is inactive (B=0) (the doppler free signal700,FIG. 7A), and records second millimeter wave amplitude data from receiver406as pump402sweeps through the range of frequencies while coil414is active (B≠0) (the absorption signal710,FIG. 7B). Processing system424produces molecular spectroscopy data by subtracting the second millimeter wave amplitude data from the first millimeter wave amplitude data. This resulting molecular spectroscopy data720is illustrated here.

Note that while the vertical axes illustrated inFIGS. 7A-7Cshow absorption in mV, any arbitrary units may be used within the scope of the present invention.

FIG. 8is a flow chart illustrating an example method for operating a millimeter wave molecular sensor system. In this example embodiment, directional coupler410receives a millimeter wave430having a first polarization swept over a range of frequencies, (operation800).

Directional coupler410partitions millimeter wave430into a pump signal and a probe signal, each signal comprising a portion of power of millimeter wave430, (operation802). Probe transmitter408transfers the probe signal into physics cell402through coupling link418, (operation804).

The pump signal having the first polarization is transferred through pump transmitter404, and through a Faraday rotator, (operation806). The Faraday rotator is configured to transform the pump signal having the first polarization into a pump signal having a second polarization when activated.

An output of the Faraday rotator is transferred through coupling iris416into physics cell402, (operation808). Coupling iris416is configured to pass pump signals having the first polarization and to block pump signals having the second polarization.

Receiver406receives millimeter waves exiting physics cell402after passing through the sample, (operation810).

Receiver406transfers an output to electronic recording device422, configured to receive and record millimeter wave amplitude data from receiver406as the input millimeter waves sweep through the range of frequencies, (operation812).

First millimeter wave amplitude data are obtained by performing operations800-812while the Faraday rotator is inactive, (operation814). Second millimeter wave amplitude data is obtained by performing operations800-812while the Faraday rotator is active, (operation816). Processing system424produces molecular spectroscopy data by subtracting the second millimeter wave amplitude data from the first millimeter wave amplitude data.