High Speed Sub-GHz Spectrometer for Brillouin Scattering Analysis

The overall goal of this spectrometer is to measure Brion scattering signatures of tissue and biomaterials Brion scattering spectra provide non-contact non-invasive information about material properties such as the longitudinal elastic modulus. This method can help answer key questions in tissue biomechanics. For example, in corneal tissue, we can measure the corneal strength for diagnosis and treatment of keratoconus.

The main advantage of this technique is that we can perform spectral analysis with low light power, making it safe for use in vivo, which enables biological imaging. Begin with a camera and optical fiber in position on an optical bench Here, an E-M-C-C-D camera is mounted at one end of the optical path. The camera should be acquiring images and displaying them on a monitor.

Approximately 1600 millimeters in front of the camera. Have a fiber collimator at camera height. Connect the collimator to a single longitudinal mode laser.

Use laser power of less than 0.1 milliwatts and check the alignment by moving the iris along the desired optical axis. This schematic provides an overview of the setup at this point. Next, add an aromatic lens pair in front of the camera.

This matched aromatic lens pair has a focal length of 30 millimeters. Now begin to add elements for the horizontal stage. First on a horizontal translation stage, add a horizontal mask so it is imaged on the camera by the aromatic lens.

The stage should move perpendicular to the beam path. Monitor the image on the computer screen to verify that the image shows sharp edges as in this photo. Move on to prepare for placing a spherical lens, 600 millimeters in front of the horizontal mask.

First, prepare two irises each on a post and post holder to help with beam alignment, place one iris between the intended lens position and the horizontal mask. Place the other iris beyond the intended lens position. Ensure the height of the irises allows the beam to cleanly pass through both of them.

When done checking the alignment, move on to work with the post mounted spherical lens with a focal length of 200 millimeters use. Use a post holder to place the lens 600 millimeters in front of the horizontal mask between the two irises and secure the post holder. Here is a schematic of the setup after the spherical lens, S one is in position.

The next element to add is the first of the virtually imaged phase arrays or vipa S.To add the vipa have a horizontal translation stage in place on the bench. About 200 millimeters in front of the spherical lens orient the stage to move perpendicular to the beam affixed to the stage a vipa holder mounted on a post and post holder. Now get the vipa that is to be mounted.

Each vipa has an entrance slit, which must be oriented properly for this vipa. Place it in the vipa holder with the slit oriented vertically make fine adjustments to the position of the vipa mount. So the vipa entrance is exactly in the focal plane of the spherical lens.

Continue by translating the vipa out of the beam to continue the setup. Next, prepare to place the second spherical lens, 200 millimeters in front of the horizontal mask with irises on either side for alignment, note that Vipa two is displaced in the schematic to indicate it is out of the beam path. Mount the spherical lens using a post on a translation stage between irises.

Orient the translation stage to move along the optical path before proceeding. Check that the back reflection from the lens passes cleanly through the iris. Also, check that the outgoing beam passes cleanly through the iris on the opposite side of the lens.

Then move the vipa so that its entrance SL is again in the beam path. Do this by adjusting the position of the translation stage to which it is attached. As light enters the vipa, observe the vertical lines of the camera image on the computer monitor.

Move on to adjust the position of the second spherical lens while observing the lines on the monitor.Fine. Adjust the translation stage to sharpen the lines and stop when the lines come into focus. Next, work with the vipa holder.

To adjust the spectrum, use the horizontal translation degree of freedom to tune the entrance position of the beam to the lon. Observe the response of the image on the computer monitor as the entrance slit is translated. In addition, use the horizontal tilt degree of freedom to tune the input angle of the beam into the al on.

Again, monitor the image on the computer screen for feedback on the tilt adjustments. At this point, the horizontal stage of the spectrometer is complete. To set up the vertical stage, slide the vipa out of the beam using the translational stage.

Next at a position, 200 millimeters in front of the spherical lens. S one place a vertical mask mounted on a vertical translation stage to allow the mask to move out of the beam path. Make sure the vertical mask is sharply imaged on the CCD camera.

The next component will be a cylindrical lens. 600 millimeters in front of the vertical mask. Use a post holder to place a 200 millimeter focal length lens between two irises.

The second vipa will be on a vertical translation stage 200 millimeters in front of the cylindrical lens. Have the vipa holder on a vertical translation stage in place to allow the vipa to be moved out of the beam path. Mount the vipa using a post and post holder to the stage being careful to orient its entrance slit horizontally fine.

Adjust the position of the vipa to place it exactly in the focal plane of the cylindrical lens. Then move it out of the beam path. The final element is a second cylindrical lens.

Placed 200 millimeters in front of the vertical mask. For this lens, position a translational stage oriented along the optical path mount and align the second cylindrical lens on the stage. Return to the most recently placed vipa vipa one and translate it back into the beam path.

Use the vertical translation degree of freedom to tune the entrance position of the beam into the al on. Observe the effect of tuning on the camera image on the computer monitor. Continue by adjusting the vertical tilt degree of freedom to tune the input angle of the beam.

Watch the computer monitor for feedback on the effects of the adjustments. Now it is time to combine the two stages of the system. Do this by sliding VIPA two, the first one mounted into the optical path.

Adjust the vipa S while observing the horizontally and vertically spaced dots. The computer monitor stop when the dots the spectral signature of the single frequency laser are in focus. Take an image of the signature using the camera software in this software.

Right click on the image and choose the line plot option. Drag the cursor diagonally across the image to generate a line plot. Then release the cursor to view the plot that will be used to determine the finesse in this sample plot, the two required measurements are indicated.

First, measure the diagonal distance between two dots, the free spectral range. Next, measure the full width at half maximum. Divide the free spectral range by the full width at half maximum, and proceed if the ratio is greater than 30.

Back at the bench, surround the spectrometer with a box skeleton covered with blackout fabric demonstrated here with a different setup before continuing, ensure that the masks and the vipa are easily accessible. Finally, connect the fiber to a standard optical probe. In this case, a confocal microscope go here.

An uncovered setup is used to demonstrate the steps in measuring the brion shift. First, select one of the masks to be adjusted. In this case, begin closing the horizontal mask using its micrometer.

Observe the laser signature on the computer monitor. The goal is to block the laser signature seen in the corners. Use the translation stage to center the mask horizontally to help block the signature.

Continue to observe the behavior of the laser signature on the monitor. Iterate between closing and translating the mask until the signature is blocked. Then do the same with the vertical mask.

With the sample loaded, open the vertical stage mask to start optimizing the vertical stage throughput. Move on to the vertical stage vipa vipa one vary the vipa tilt position to range over input angles to the AL on scan through spectrometer orders, and stop at the order with the sharpest signal. Close the mask again until the laser signal disappears.

Repeat the optimization process with the horizontal mask and vipa before taking measurements. Here are Brion spectra measured for methanol, ethanol, and polystyrene. In these plots, the measured data are in blue.

The Lian curve fit is in red. The vipa used are five millimeters thick, giving a free spectral range of about 20 gigahertz. The integration time was 100 milliseconds.

100 measurements were taken and averaged. The Brion shifts agree with previous published results. This histogram is of the Brion shifts found in 250 sequential measurements of methanol.

The data helped determine if the spectrometer alignment is optimal. A well-aligned spectrometer with five milliwatts of light on the sample and an integration time of 100 milliseconds will have a standard deviation of about 10 megahertz. While attempting this procedure, it's important to remember to change the camera settings when laser light or scattered light is used.

Otherwise, the E-M-C-C-D camera will be saturated and degrade over time. Don't forget that working with lasers can be extremely hazardous and precautions such as never looking straight into the laser beam. Wearing safety glasses and aligning below eye level should always be taken while performing this procedure.