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.
