ECEN 4606 Lab 8 Spectroscopy SUMMARY: ROBLEM 1: Pedrotti 3 12-10. In this lab, you will design, build and test an optical spectrum analyzer and use it for both absorption and emission spectroscopy. The purpose of this device is to provide a real- be time measurement of the optical power versus vacuum wavelength. This can employed to examine colors absorbed or emitted from a sample. It is thus an extremely common tool in chemistry where it is used to identifyy the amounts, states and constituents of chemical compounds. PRELAB: HOMEWORK PR ECEN 4606, UNDERGRADUATE OPTICS LAB Lab 8: Spectrosc copy Figure 1. Layout of basic gratingg spectrometer. HOMEWORK PROBLEM 2: This problem is designed to help you understand the limits of the grating spectrometer. You have a spectrometer with the layout shown in Figure 1 with F 1 = 50 mm, F 2 = 100 mm, = 1 m, L = 25 mm. Version 1.3, 8/20/12 McLeod and Gopinath 1
Part I. Ia) Using Matlab (or your computer tool of choice), plot the intensity versus x' at the camera plane of a spectrometer for 500 nm light (assume lens 2 and the camera have been positioned such that this wavelength falls on the center of the camera, i.e. this wavelength defines x'=0) and d is sufficiently small that there is no slit limit to the resolution. Ib) Superimpose on the same plot, intensity versus x' for the closest wavelength to 500 nm that is just resolvable. Part II. IIa) Calculate the slit width, d, which would illuminate the grating with the first lobe of a sinc measuring L between the first nulls. IIb) If the slit width d were doubled from the value you calculated, the diffraction from the slit would change, modifying L. Comment on the wavelength resolution of this new system in comparison to the first (Hint - You may find it helpful to make plots as in part 1. The plots are not required for full points.). This illustrates the impact of the slit width on spectrometer resolution. Extra Credit [2 points]. IIc) Plot intensity vs. x for modified slit width of part IIb for the same wavelengths as in part I. Extra Credit [2 points]. IId) Derive expression for spot size on the grating in terms of F 1,, and the numerical aperture of lens F 1. What happens to the resolution of the system as the spot size on the grating decreases? DESIGN PROBLEM: Design a grating-based absorption spectrometer that operates over the visible spectrum with the maximum resolution possible given the equipment available. The system should be designed such that it could operate at video rates if needed. In your design, address specifically the pixel, slit and grating resolution limits. Document your specifications (upper and lower wavelengths, wavelength resolution) and, component choices. Make a clear drawing of the design. Hint: The design specs, as stated, are not possible (this happens in the real world!), so something has to be relaxed look at lens focal lengths and it should be clear which spec you have to give up. Please complete the following calculations and draw a figure of your spectrometer design, labeling components and distances. Assume that your spectral spread is 300 nm (400-700 nm). 1. Calculate grating resolution. 2. Calculate resolution of system with the camera, taking into account maximum number of pixels in camera. 3. Choose lens focal lengths f1 and f2 and calculate maximum slit width that allows one to fully utilize pixels on camera. Version 1.3, 8/20/12 McLeod and Gopinath 2
4. Calculate the wavelength resolution of the system using the slit width calculated in part 3 and compare it with the resolutions calculated in parts 1 and 2. What element in the system limits the resolution? Extra credit [ 1 point]. Additional questions: 1. How could you extend the frequency range of your instrument given common optics lab equipment if you were not constrained to operate at video rates? What are the limits to the wavelength range you could possibly achieve? 2. Although this design in unconstrained on size, comment in your prelab design what you would expect to be difficult if asked to shrink the entire device to handheld size. 3. Note how to modify your design to do emission spectroscopy. TECHNICAL RESOURCES: TEXTBOOK: Chapter 12 LECTURE NOTES: Lecture 8, Spectroscopy. EQUIPMENT AVAILABLE: A broad-band lamp with a fiber-bundle A spatially-filtered, JDS Uniphase 1103P-3020 Helium Neon laser. The laser wavelength is 633 nm. We will use this for alignment. A small butane torch and/or a compact fluorescent bulb in a desk lamp Variable width slit Shim-stock measured in thousandths of one inch for roughly setting the slit width. Diffraction grating with 1200 periods / mm of total size 2 x 2. Lenses: focal lengths of 50, 100 and 250 mm A monochrome, digitizing USB CMOS camera. Specifications and Matlab routines to read and plot lines from the image files are on the references page A translation stage with at least 1 of translation. A PC for viewing the images in real time and saving data files. Bring a USB stick to take the data away with you. Color filters glass containing dyes that absorb or transmit specific ranges of wavelengths. Thin-film filters with known, narrow passband wavelengths. LAB PROCEDURE: STEP 1: SET UP AND ALIGN THE ABSORPTION SPECTROMETER Temporarily use the HeNe laser as a light source to align and focus the spectrometer. You will later switch to the white light source, so use mirrors and table layout intelligently. The HeNe is very narrow-band so you can form a sharp image of the slit on the camera without it being spread by diffraction off of the grating. Version 1.3, 8/20/12 McLeod and Gopinath 3
Set up the slit and carefully use shim-stock to adjust it to approximately the desired width (you will adjust later using the camera as a measurement tool). Illuminate the slit with the raw output of the HeNe. Place lens 1 after the slit and collimate the light (think about aberrations and the lens orientation). Note you are collimating in the plane of the table what is happening out of the plane? Use this to position the grating precisely one focal length behind lens 1. The analysis presented in class was paraxial and did not worry too much about the angle of the grating relative to the incoming beam. This is an important parameter, but is not critical for the level of understanding we want in this class. Note that you increase wavelength resolution proportionally to the amount of the grating surface you illuminate and you can influence this through the grating angle. Start with the grating normal to the beam, then tilt the grating as necessary to separate the spectrum from lens 1 so that you can easily place lens 2 and the rest of your optics. THE GRATING SURFACE IS EXTREMLY DELICATE. DO NOT TOUCH. Find the zero order (reflection) off of the grating and the +1 and -1 orders. Make very sure you have identified the zero order - it will be symmetric to the incident light around the grating normal. Then tilt the grating to direct either the + or -1 order into the path where you will place the rest of the optics train. If you use the zero order, the grating will have no function and your spectrometer will not work. Insert lens two (again, think about lens orientation) and the CMOS camera. Place the CMOS camera on a translation stage moving perpendicular to the beam path so you can adjust its location later without changing the focus condition. Focus the system by forming a sharp image of the slit on the camera. Save an image of the slit, pull it into ImageJ and verify that the slit is filling the right number of pixels. Continue until this is as perfect as you can get it. You may need to adjust the slit width to match your design. Tilt the grating to move the image across the full camera range, confirming that that you have the best possible performance across the range. Rotate the grating to put the HeNe near the proper edge of the camera so that the visible spectrum should fill the camera. Lock all the lenses down you are done aligning the spectrometer. Select a low F/# (high NA) lens and focus the white light source onto the slit in place of the HeNe. This is your condenser lens and its job is to deliver as much of the lamp light as possible onto the slit. Adjust the lamp and positions to maximize transmission through the slit. Examine the light after lens 1. Note in your lab book how this compares to what you observed using the laser light. This step is equivalent to spatial filtering the HeNe laser, but the broad bandwidth and particularly the low brightness of the lamp make the process quite different. If stray light from the source is contaminating the rest of the experiment, make some black paper tubes or baffles to contain it. Record the total efficiency you achieve at this particular slit width. In your lab book: 1. Record your setup, notes on procedure and observations about the two illumination conditions. Version 1.3, 8/20/12 McLeod and Gopinath 4
STEP 2: CALIBRATE THE ABSORPTION SPECTROMETER There are two axes that must be calibrated to make quantitative measurements: 1) the relationship of pixel location to wavelength and 2) the relationship of digitized intensity to absorption. We will calibrate the spectrometer in that order. We want the camera to be capturing the visible spectrum. Use the dielectric band-pass filters to adjust the camera location to achieve this. We don t have filters at exactly the edges of the visible range, so you will have to do some extrapolation. Once you are satisfied, do not move the camera or any other spectrometer components from this point forward. Adjust the camera gain and integration time so you are neither saturated nor in the noise-floor across the spectrum. To ideally use the camera dynamic range, you should have the maximum intensity almost at saturation, but not quite. Capture an image this is your baseline and will be used to calibrate your absorption data, later. Until the emission section (step 5), do not adjust your gain or integration time OR if you need to, record how you are changing them (e.g. increasing gain by a factor of two) and use this in your post-processing. In your lab book: The goal of this step is to create quantitative plots of the intensity transmission efficiency (NOT CAMERA VALUE) of these filters (Iout/Iin) vs. wavelength (NOT PIXEL). Use the center bandwidths of the dielectric filters to turn pixel number into wavelength. Use the spectrum without an object to calibrate your transmission. Estimate the bandwidth of the thin-film filters and compare this to specs if available. Plot your baseline spectrum. Is it completely uniform? Why or why not? What form of lamp do you think is inside the source (e.g. fluorescent, incandescent ) based on the appearance of the spectrum? We will use these calibrated spectrometers in lab 9 to characterize LEDs and lasers. Since there are multiple labs groups, you may have to quickly recalibrate at the beginning of lab 9. Think about this when you create a program to translate raw data to the calibrated plot (make it easy to put in new calibration data). STEP 3: USE THE SPECTROMETER Building and calibrating the spectrometer can take the whole lab period. Do a good job, but try to get it done in time to do at least one of the items below. If you have time, do more for extra credit. Thin film filters: o One at a time, insert at least two dielectric filters and capture the spectrum. If the filter has a strongly reflecting side, this should be placed towards the light source to minimize absorption and the associated thermal shift of the filter. These will be used to calibrate your wavelength axis be sure to note both the filter center wavelength and the bandpass. o Rotate one of the filters in increments of ~15 degrees and use the backreflection to estimate the incidence angle (or mount the filter on a rotation Version 1.3, 8/20/12 McLeod and Gopinath 5
stage if you wish). Measure the spectrum at each angle up to ~45 degrees. This will somewhat change the filter performance but will strongly shift the filter wavelength towards shorter wavelengths (always). The shift is described by λ = λ o [1-(n o /n e ) 2 sin 2 θ] 1/2 where 0 is the normal-incident peak wavelength, n o is the index of air and n e is the effective index of the filter. Find and plot the peak shift and estimate n e. What does this exercise tell you about using such filters in collimated vs. non-collimated light? Color filters o One at a time, insert two different absorptive color filters. The analysis will work best if these filters are different (one red, one blue, say). Then insert both simultaneously and capture the transmitted spectrum. Note in your lab book the appearance of the two filters (e.g. room lights appear blue when seen through the filter ). o Rotate one of the color filters and capture the spectrum. Compare this to the normal-incidence spectrum and note how it differs from the dielectric thin-film device. o In your lab book: How do the color filters differ from the dielectrics? Explain the appearance of the color filters in terms of their spectra (which wavelengths are absorbed and which are transmitted). Compare the transmission spectrum of the two color filters used simultaneously to the spectrum you would expect from their individual spectra. Is this consistent with an additive or subtractive color process? Polarization color o Insert crossed polarizers with an optic mount between them. Place a quartz plate between the polarizers and rotate it to 45 degrees. The quartz is acting as a wave plate, although not specifically a half or quarter-wave because these are changing across your spectrum. Measure the spectrum, then add a second plate at the same orientation and repeat. Emission spectroscopy o Remove the white light source but leave the condenser lens. Light sources will go where the fiber-optic was mounted. You may need to occasionally re-insert the fiber-bundle and use a thin-film filter to re-calibrate your wavelength. Another approach is to carefully record the camera position from the micrometer on the translation stage and use this to calibrate camera translations. o Fluorescent light: You have already looked at an incandescent lamp. Now insert the compact fluorescent and examine the emission spectrum. You will probably have to adjust camera gain and integration time. You may also have to shield the camera from stray light. The spectrum should be a set of narrow emission lines. If you were to combine two sources, do you expect that their spectra would add or subtract? Compare to the case with the filters, previously. If you can get light from the regular fluorescent bulb into your setup, compare to the compact fluorescent spectrum. Version 1.3, 8/20/12 McLeod and Gopinath 6
Grading Expectations Lab Report 8: Spectroscopy (100 total points) Name Name and group members. Abstract (10 points). Introduction (10 points) Methods (35 points) 1. SET UP AND ALIGN THE ABSORPTION SPECTROMETER 10 pts a. Figure of setup, 4 pts b. Description (overview of design theory), 6 pts 2. CALIBRATE THE ABSORPTION SPECTROMETER, 10 pts a. Description of method, 10 pts 3. USE THE SPECTROMETER 15 pts a. Dielectric thin-film vs. absorptive color filters, 15 pts i. Description of measurements of thin film and absorptive color filters, 15 pts b. polarization and color 15 pts i. Figure and explanation, 10 pts ii. Description, 5 pts c. Emission spectroscopy, 15 pts a. Description, 15 pts Results and Analysis (35 points) 1. SET UP AND ALIGN THE ABSORPTION SPECTROMETER 17 pts a. Description of how spectrometer was aligned, 9 points b. Could you have aligned the spectrometer with white light?, 8 pts 2. CALIBRATE THE ABSORPTION SPECTROMETER, 8 pts Plot baseline spectrum, 5 pts Comment on uniformity and lamp inside source, 3 pts 3. USE THE SPECTROMETER 10 pts a. Examine dielectric thin-film vs. absorptive color filters 10 pts i. Thin film filters 1. Plots of quantitative intensity distribution (% transmission versus wavelength), 3 pts 2. Estimate bandwidth, 2 pts ii. Color filters 1. Plots of quantitative intensity distribution, 2 pts 2. Comparison with thin film filters, 2 pts 3. Comments on appearance of color filters in terms of spectra, 1 pts b. Polarization and color, 10 pts i. Figures of spectra, 5 pts Version 1.3, 8/20/12 McLeod and Gopinath 7
ii. Modify Jones matrix code to sweep wavelength and plot spectra that is qualitatively similar to what is observed in lab, 5 pts c. Step 5: Emission spectroscopy, 10 pts i. Figure(s), 5 pts ii. Explanation of observations, 5 pts e. Conclusion (10 points) Summary of lab report f. References Include any references that you used. Version 1.3, 8/20/12 McLeod and Gopinath 8