Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing
March 22nd, 2019
Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.
The significance of our protocol is that it lowers the detection limit making it possible to study the concentrations of minor species that other methods cannot reach. The main advantage of this technique is the addition of the upconversion module which circumvents much of the background noise. Work with the setup for upconversion used in the detection process.
Begin by exposing the internal elements in the set up. A laser diode and crystal produce a 1064 nanometer beam. A series of mirrors direct this beam through a PPLN crystal, and back.
A mid-infrared beam from outside also goes through the crystal. The two beams produce an upconverted signal that exits the PPLN crystal and goes to a detector. This schematic provides an overview of the setup.
The crystal used with the laser diode is neodiamond ytterbium orthovanadate. The symbols U1 to U7 are mirrors that are highly reflected at 1064 nanometers. Mirrors U1 through U5 are highly transmissive at the wavelength of the laser diode.
Mirror U6 is transmissive in the range of the upconverted signal. Mirror U7 is transmissive for the mid-infrared signal. The mirror U3 has a 200 mm radius of curvature.
The other mirrors are flat. Use a flat mirror on a comedic mount to establish an alignment cavity. Place the mirror in front of the laser medium to serve as the end mirror.
Turn the angle of the mount to extreme positions in both the horizontal and vertical directions. Next, place an infrared sensitive beam card in front of mirror U2.Also remove the PPLN crystal from it's mount. The arrangement is depicted in this schematic.
The end mirror is denoted by UH.Start the laser diode at approximately one third of its maximum output. Align the cavity, repeat the following steps. Change the angle of the end mirror, positive 2 degrees in the horizontal direction.
Then sweep the vertical angle of the mirror from one extreme to the other. As you do this, watch the infrared card for a beam from the alignment cavity. At some horizontal angle of the end mirror, during a sweep of the vertical angle, the cavity will start lazing, which can be seen on the infrared card.
When the cavity is lazing, alternate between adjusting the angle of the mirror to achieve a higher power, and reducing the drive current. In the end, have the power so the beam leaving the mirror is easily visible with the IR card. Now, remove the infrared card and start adjusting the mirror that was behind it, U2.Adjust the mirror so the alignment beam is reflected from its center to the center of mirror U3.Adjust the angle of mirror U3 so the beam continues centered along the desired path to mirror U7.Ensure the beam passes through the PPLN mount at the appropriate height, and that it will be perpendicular to the crystal's surface.
Next, remove the germanium window and place an infrared card behind U7.In this position, the IR card will fluoresce due to an IR beam leaving the cavity. Now adjust the angle of U7 to reflect along the path of the alignment beam. Monitor the infrared card for the transmitted beam, and set the mirror's angle to maximize the output.
Continue by mounting the PPLN crystal in the mount so the beam goes through one of its channels. Check that the beam is still visible on the IR card. If so, adjust U7 to maximize the output before proceeding.
At this point, turn off the laser diode and remove the end mirror. Attach a 750 nanometer long pass filter at the input to the upconversion setup. Place a power meter behind the filter.
With the laser diode at full power, adjust the angle of U2 and U7 to maximize power. Then replace the power meter with a high powered infrared card. With the card, check that the cavity is running in fundamental Galician mode.
Adjust mirror U7 as necessary. When done, remove the filter and reattach the germanium window. Move on to align the infrared degenerate four-wave mixing setup.
The set up includes a pulse laser, a helium neon laser, as well as mirrors, and lenses, to direct the beams to the input of the assembled upconversion detector. The initial setup is represented in this schematic. The helium neon laser provides a guide beam.
Use mirrors M3 and M4 to align the guide beam with lens L1.Adjust the mirrors so the beam hits lens L1 in its center. Insert a boxcars plate between the mirror M4 and lens L1.Place it at a 45 degree vertical angle from the horizontal beam. Ensure the arrangement produces two output beams.
Insert a second boxcars plate after the first. Have it at a 45 degree horizontal angle from the output beams. Ensure its output has four beams.
Next, adjust the angles of the boxcars plates to the four output beams form the corners of a square. Adjust the lens L1 until the beams are equally spaced around its center. Now, place an iris in the path of the beams.
Arrange the iris to block three pump beams and allow the signal beam to pass through. This schematic represents the state of the system at this point. The next steps will involve the lens L2, and the mirrors M5 and M6.To collimate the beam, align lens L2 using the focal length of the wavelength of the pulse laser.
Then position mirrors M5 and M6 so the guide beam is directed to the input window of the upconversion detector where the beam should be centered. Place lens L3 an optical distance of one focal length from the center of the PPLN crystal. Remove the germanium window of the detector to continue.
Doing this allows the 1064 nanometer beam to exit the upconversion module. Next, begin using mirror M6 to move the beam from the detector, and bring it onto the signal beam so they overlap at lens L2.Alternate this with using mirror M5 to move the guide beam onto the 1064 nanometer beam at L3.Stop when the 1064 nanometer beam, and the guide beam, follow the same path. Reattach the germanium window to the upconversion module.
Then place several neutral density filters in front of the detector to protect it from the pulse laser. Turn on the pulse laser, and ensure it overlaps with the guide beam. Now, place the gas flow or flame to be measured at the focal point of lens L1.This measurement will involve the flow of methane diluted in nitrogen.
Check that the signal is visible on the detector. Adjust the neutral density filters as needed. If there is a signal, maximize its average intensity by adjusting mirrors M5 and M6.Continue by blocking the signal beam with a beam block on a translation stage.
Then remove the neutral density filters that are before the detector. Initially, there may be a signal due to light scattered into the detector. With the translation stage adjust the position of the beam block to reduce this scattering.
Proceed when the signal due to light scattering has been reduced as much as possible. The next step is to turn on the gas flow so measurements can start. Then, collect data by properly triggering the upconversion detector with the pulse laser and scanning the wavelength range of interest.
These data are for five different concentrations of hydrogen cyanide in nitrogen gas. Each point represents the average of three scans at each concentration. The central peak is the P20 line of the NU1 vibrational band of hydrogen cyanide.
Here the points are the measured peak values as a function of the concentration. The dash line is a fit to the second degree polynomial. Is this case, the data shows five consecutive scans from a pre-mixed flame.
Each scan spans about 65 seconds, and covers the same range of wave number. The change in intensity from scan to scan is because the laser pulse mode and energy are not stable. No single step is the most important, but if measurements need to be comparable, the alignment must have the same height position each time.
Learning to align this set up by trial and error would waste a lot of time which is why I wanted to demonstrate the process so people could avoid pitfalls. The introduction of the upconversion module made it possible for us to detect the release of the minor species hydrogen cyanide from the gasification of small pellets. This protocol includes the use of class four lasers, and potentially the use of flammable gases, and the appropriate safety measures must always be followed.
Overview
This article presents a protocol for sensitive, spatially resolved gas spectroscopy in the mid-infrared region. The technique utilizes degenerate four-wave mixing combined with upconversion detection to achieve lower detection limits for minor species.
Key Study Components
Area of Science
- Gas spectroscopy
- Optical detection methods
- Mid-infrared analysis
Background
- Current methods struggle to detect low concentrations of minor species.
- Upconversion detection reduces background noise significantly.
- Utilizes a laser diode and PPLN crystal for signal generation.
- Involves precise alignment of optical components for effective detection.
Purpose of Study
- To develop a protocol that enhances detection limits in gas spectroscopy.
- To provide a detailed methodology for implementing upconversion detection.
- To facilitate the study of minor species in various samples.
Methods Used
- Setup includes a laser diode, mirrors, and PPLN crystal.
- Alignment procedures for optical components are detailed.
- Use of infrared sensitive cards to monitor beam alignment.
- Involves adjustments to maximize output power and signal clarity.
Main Results
- Successful detection of low-concentration species using the new protocol.
- Demonstrated effective noise reduction through upconversion.
- Provided a comprehensive schematic of the experimental setup.
- Validated the alignment and performance of the optical components.
Conclusions
- The protocol significantly improves detection limits in gas spectroscopy.
- Upconversion detection is a viable method for studying minor species.
- Future applications may expand to various fields requiring sensitive detection.
Frequently Asked Questions
Executive Industry Relevance
This protocol enables quantitative detection of low-concentration gas species in the ppm-to-single-percent range, addressing a critical gap in analytical capabilities for combustion and gasification research. By combining infrared degenerate four-wave mixing with upconversion detection, the method achieves superior signal-to-noise ratios and spatial resolution, supporting mechanistic studies of minor species formation and degradation pathways. The technique is particularly valuable for target validation in preclinical models where precise monitoring of reactive intermediates is required for predictive confidence in lead identification.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses by quantifying minor species that serve as biomarkers or mechanistic indicators in disease-relevant systems.
- Operational Value: Provides spatially resolved detection without perturbing the system, preserving native biological or chemical conditions for accurate target engagement assessment.
- Predictive Confidence: Supports preclinical model validation through reproducible, low-detection-limit measurements that reduce false negatives in pathway analysis.
Screening & Assay Development
- Scientific Value: Generates quantitative, wavelength-specific readouts that enable precise compound screening against gas-phase analytes in controlled environments.
- Operational Value: Standardizes detection via upconversion-enhanced signal stability, improving assay reproducibility across runs and laboratories.
- Scalability: Compatible with flow-based systems, allowing integration into high-throughput screening platforms for gas-responsive pathways.
Translational & Preclinical Research
- Scientific Value: Facilitates translational biomarker alignment by detecting species with no visible/NIR transitions, expanding detectable analyte space in preclinical models.
- Operational Value: Ensures continuity from discovery to preclinical validation through consistent, noise-resistant signal acquisition.
- Risk-Adjusted Decisions: Enables data-driven advancement by providing reliable quantification of toxic or reactive intermediates that inform safety margins.
Pipeline & Workflow Integration
The method fits within the discovery continuum from hypothesis testing in early discovery to mechanistic validation in preclinical studies, particularly for targets involving gas-mediated signaling or metabolic flux.
- Discovery Biology: Supports hypothesis testing and pathway clarification by enabling detection of transient, low-abundance species that indicate target engagement or off-target effects.
- Screening: Delivers assay readiness through stable, upconverted signal output that allows reliable compound evaluation in gas-exposed systems.
- Analytics: Provides quantitative peak intensity measurements as a function of concentration, enabling comparison across conditions and time points.
- Translational Research: Connects to preclinical continuity by detecting species relevant to disease mechanisms, such as hydrogen cyanide in metabolic stress models.
- Enterprise Reuse: Represents a reusable optical platform adaptable to multiple gas analytes, reducing need for revalidation across projects.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence by lowering detection limits and reducing mechanistic ambiguity in gas-phase biomarker studies.
- Operational Value: Enhances reproducibility and standardization through noise-resistant upconversion detection and detailed alignment protocols.
- Strategic Value: Improves go/no-go decisions by enabling early detection of toxic intermediates, reducing late-stage attrition due to unidentified liabilities.
- Portfolio Impact: Supports risk-adjusted prioritization through accurate quantification of species that influence target validity and safety profiles.
Implementation Considerations
- Requires expertise in nonlinear optics and laser safety, particularly for Class IV laser systems and flammable gas handling.
- Depends on precision optical instrumentation including PPLN crystals, aligned mirror mounts, and infrared-sensitive detection systems.
- Necessitates cross-team standardization of alignment procedures to ensure consistent signal height and path length across users and sites.
- Involves adaptation considerations for different gas matrices and wavelength ranges, requiring recalibration when switching analytes or environmental conditions.
- Includes practical limitations such as sensitivity to laser pulse stability and alignment drift, which must be controlled for reproducible quantitative output.
