lab result
Characterization of Light Sources using a Grating Based Spectrometer
Performed: 16 March 2018 and 23 March 2018
Submitted: 27 April 2018
Abstract
Using standard benchtop components we have built an optical spectrometer capable of analyzing the spectral patterns for a variety of light sources. We have tested the spectrometers with several different standard dispersive elements and were able to obtain wavelength values within 5% of the equipment specification values. Finally, we used a non-standard dispersive element, i.e. a holographic Colgate toothpaste box, and were able to obtain the grating spacing of the diffractive cardboard paper to within an order of magnitude of industry reference values.
1. Introduction
Optical spectroscopy is often used to determine the composition of a material based on the light emitted from the material source. When this light is passed through a device called a spectrometer the emitted light from the source will separate into distinct lines, the relative position of the lines can be used as a sort of fingerprint as different materials will have characteristic spectral patterns based on the energy levels of their atomic constituents. In this experiment we will be building a benchtop spectrometer using standard optical components; we will then use this spectrometer to characterize several different light sources.
A spectrometer separates a light beam into components based on wavelength, the separation can happen in multiple ways--such as through a diffraction grating, through a prism, or through some other type of dispersive element. The dispersive elements being used for our investigations are a standard prism, two blazed gratings--one with 600 lines/mm, the other with 1200 lines/mm--and a Colgate toothpaste box.
A blazed grating is a grating with an asymmetric orientation, similar to a sawtooth pattern. Incident light onto the gratings excites dipoles whose far-field radiation results in a tighter beam for the diffracted orders of light, as
seen on the observing screen. A diagram of the orientation of elements is shown in Figure 1.1.
Fig 1.1: Diagram showing the orientation of incident light and relevant distances used in the grating equation.
y
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x
Blazed Grating
1st Order Diffracted Light
Viewing screen
Collimated Beam
2. Materials & Methods
The equipment used in this experiment was a L405P20 – 405nm, 20mW Laser Diode[1], a TCLDM9 – TE – Cooled Mount[2], a TLD001 – T-Cube Laser Diode Controller[3], a TTC001 – T-Cube TEC Controller[4], a collimated laser diode module 532nm/0.9mW round beam[5], SLS201 - Stabilized Fiber-Coupled Light Source w/ Universal Power Adapter, 300 - 2600 nm, 1/4"-20 Tap[6], a LEDWE-15 - Epoxy Encased White LED, Qty. of 5, 7.5° Half Viewing Angle[7], a VA100/M - Adjustable Mechanical Slit, M4 Tap, Metric Micrometer[8], two ACL2520U-B - Aspheric Condenser Lens, Ø25 mm, f=20.1 mm, NA=0.60, ARC: 650-1050 nm[9], a LB1471-A - N-BK7 Bi-Convex Lens, Ø1", f = 50.0 mm, ARC: 350-700nm[10], a LB1676-B - N-BK7 Bi-Convex Lens, Ø1", f = 100.0 mm, ARC: 650-1050 nm[11], a GR25-1205 - Ruled Reflective Diffraction Grating, 1200/mm, 500 nm Blaze, 25 x 25 x 6 mm[12], a GR25-0605 - Ruled Reflective Diffraction Grating, 600/mm, 500 nm Blaze, 25 x 25 x 6 mm[13], a Uniphase Model 155 HeNe laser and a Colgate toothpaste box.
The first task of the experiment involved building a grating-based spectrometer. The setup for the spectrometer can be seen below in Fig 2.1.
Fig 2.1: Experimental setup of the grating based spectrometer (Reproduced from Thorlabs.com[17]). Image shows the complete beam path of the light source through the setup. Parts used are listed in the box located in the upper left corner.
A light source was directed towards two aspheric condenser lenses, which are used to collimate the light source. The light is then directed towards a focusing lens that centers the light on a variable slit. The slit width is changed to obtain an optimal image and control the resolution of the spectrometer. The further the slit is opened, the brighter/more intense the image becomes, and the tighter it is closed, the sharper the image becomes[14]. The light is then guided towards the imaging lens, which focuses the beam on the diffraction grating. The diffraction grating splits and diffracts light into several beams travelling in different directions[15]. These beams of light were observed on the viewing screen. Either one wavelength of light or multiple wavelengths could be observed depending on the light source chosen.
The next part of the experiment involved measuring the wavelength of three different laser sources using the spectrometer setup discussed above. The three laser sources used were a violet laser diode with a known wavelength of 405nm, a green laser diode with a wavelength of 532nm and a HeNe laser with a wavelength of 632nm. The lasers were coupled to a fiber optic cable and directed through the spectrometer setup seen in Fig 2.1. Diffraction gratings with 600 lines/mm and 1200 lines/mm were used in this part of the experiment. The first order diffraction maximum was used in the measurements for the wavelength.
The third part of the experiment was to measure the wavelengths of light that make up a particular LED source using two different diffraction gratings. A diffraction grating can separate polychromatic light into its components without using any of the properties of refraction[16]. The LED was directed through the spectrometer setup seen in Fig 2.1. Diffraction gratings with 600 lines/mm and 1200 lines/mm were used in this part of the experiment. The first order diffraction maximum was used in the measurements for the wavelength. This LED light source was comprised of three separate wavelengths of light, red, green and blue; therefore, measurements for each wavelength were taken for this part of the experiment.
The fourth part of the experiment involved measuring the wavelengths of light from a broadband light source using two different diffraction gratings. The broadband light source was coupled to a fiber optic cable and directed through the spectrometer setup seen in Fig 2.1. Diffraction gratings with 600 lines/mm and 1200 lines/mm were used in this part of the experiment. The first order diffraction maximum was used in the measurements for the wavelength. The wavelengths of light seen from this light source were red, green and blue; therefore, measurements for each wavelength were taken for this part of the experiment.
In the last part of the experiment the number of gratings on a Colgate toothpaste box with a holographic coating were measured. For this part of the experiment the setup is exactly the same except a Colgate toothpaste box replaced the diffraction grating. The green laser diode with a wavelength of 532nm was used for this part of the experiment. A known wavelength needed to be used in order to determine the number of gratings on the box.
3. Results
Table 1. Measurements of the wavelengths of several light sources using a home-made spectrometer with a blazed grating of 600 grooves/mm.
Table 2. Measurements of the wavelengths of several light sources using a home-made spectrometer with a blazed grating of 1200 grooves/mm.
4. Discussion
Our experiment went as expected with no major events that would have induced significant errors. The first part of the experiment agrees with our expectation that the wavelength is proportional to the angle over the order of diffraction, lambda = d*sin(theta)/m. In general, the measured value of any scientific experiment only agrees with the real value within experiment error. For example, using the 600 lines/mm grating, the wavelengths of the blue, red and green were found to be respectively 391.07 nm, 618.98 nm, and 513.91 nm. The accepted values for these colors are 405 nm, 632 nm, and 532 nm. The percentage error between our measurements and the actual values were 3.44%, 2.06%, and 3.4% respectively. However, these errors seems to diminish when we replaced the 600 lines/mm grating with 1200 lines/mm. The percentage errors went from 3.44%, 206%,and 3.4% to 1.29%, 0.32%, and 0.13% for the blue, red and green respectively. This proves that the higher the grating, the more accurate the measurement. Since the discrepancy between our measurements and the real values overlap the percentage error, we can state that our measurement agree with the thors lab..
In the second part of the experiment in which we break down the LED light into spectral components, our measurements still follow the proportionality between the wavelength and the order of diffraction. However, the errors in this part of the experiment are slightly higher than the first part. Our measured values for the blue, red, and green were 411.13 nm, 689.67, and 518.4 nm respectively using the 600 lines/mm grating and for the 1200 lines/mm, we found 372.68 nm, 616.62nm, and 520.58 nm respectively for the blue,red and green colors. The deviation for the blue color was the highest in our experiment because most blue colors contain a mixture of other colors. Also the challenge arising from an accurate measuring of the blue light is that it overlaps other neighboring colors, making it harder to get its exact position with respect to the first order.
The last part of the experiment supports our hypothesis that for a given material, we can measure the grating density. To do this we return to the diffraction grating equation and solve for the inverse of the grating spacing or 1/a; this will give us the grating density. Using a colgate toothpaste box as our grating and the first order m=1 for a diffracted 530 nm light source, we found the groove density of the grating on the Colgate box to be 973 lines/mm.
5. References
[1]https://www.thorlabs.com/thorproduct.cfm?partnumber=L405P20
[2]https://www.thorlabs.com/thorproduct.cfm?partnumber=TCLDM9
[3]https://www.thorlabs.com/thorproduct.cfm?partnumber=TLD001
[4]https://www.thorlabs.com/thorproduct.cfm?partnumber=TTC001
[5]https://www.thorlabs.com/thorproduct.cfm?partnumber=CPS532-C2
[6]https://www.thorlabs.com/thorproduct.cfm?partnumber=SLS201
[7]https://www.thorlabs.com/thorproduct.cfm?partnumber=LEDWE-15
[8]https://www.thorlabs.com/thorproduct.cfm?partnumber=VA100/M
[9]https://www.thorlabs.com/thorproduct.cfm?partnumber=ACL2520U-B
[10]https://www.thorlabs.com/thorproduct.cfm?partnumber=LB1471-A
[11]https://www.thorlabs.com/thorproduct.cfm?partnumber=LB1676-B
[12]https://www.thorlabs.com/thorproduct.cfm?partnumber=GR25-1205
[13]https://www.thorlabs.com/thorproduct.cfm?partnumber=GR25-0605
[14] https://www.thorlabs.com/drawings/bd4fc59b2afcd5bc-864C164A-C451-B72A-33EA85807525713A/EDU-SPEB1-EnglishManual.pdf
[15] https://en.wikipedia.org/wiki/Diffraction_grating
[16] http://web.cerritos.edu/cmera/SitePages/Ph102L/labs/Ph102labs/Ph102/diffgrat.ht