Diffraction Grating Efficiency Essay Samples

Diffraction Grating Efficiency

...m. Procedure We used the computer simulation “PCGrate2000” to investigate the workings of diffraction gratings (this experiment looked at reflection gratings in particular). Firstly we looked at the diffraction pattern of a 500lines/mm reflection grating with 600nm incident light, which was incident at varying degrees (between 0 and 89º) to the macroscopic surface of the grating. We looked at the angles at which the various orders were reflected, and how much light intensity was directed into each order. Next we put the light incident normal to the surface of the grating, and looked at the angles of reflection for the various orders for wavelengths between 200 and 1200nm. We changed to a blazed grating, specifying the Littrow order to be –1. The profile type was changed from sine to sawtooth, and the groove frequency was changed to 260 lines/mm. We looked at the diffraction pattern for wavelengths between 200 and 700nm. After looking at the intensity of the diffracted beams, we were required to explain this behaviour, and find the maximum resolving power of the arrangement. We printed the graph of diffraction order intensity versus wavelength. We changed the groove frequency of the grating to 1000 lines/mm, and the facet angle to 11.5º, then compared the resolving power and the shape of the scan graph to the previous. Again, we repeated the scan after changing the groove frequency to 1500 lines/mm, the facet angle to 13º, and using TM(S) polarised incident light. We compared the scan graph to the previous two, and explained the anomaly seen in it. Finally, we were required to choose our own parameters for a 40 mm wide blazed grating with a resolving power of at least 20,000 using TE polarised 500nm light. We needed to have the grating in Littrow, and achieve an efficiency of at least 80%, and to do this, had to choose the blaze angle, order, and groove frequency. Once this was done, we repeated it for the orders -2 and -3. Results We found that by manipulating the grating equation, we could determine what the minimum and maximum orders seen would be (i.e. we found the range of the allowed orders for particular values of wavelength, groove frequency and incident angle). The minimum allowed order was given by mmin  -asini / (i.e. the first integer to satisfy this equation), while the maximum order was given by mmax  a(1 – sini) / . We can see then, that as the wavelength is increased, the range of m, -asini /  m  a(1 – sini) / . This was observed as we stepped through the wavelengths ranging between 200 and 700nm: with the increasing wavelength, the number of orders present decreased. With the light impinging on the blazed grating, we saw that stepping through the (increasing) wavelengths changed the order with the greatest intensity. We found that by increasing the groove frequency of the grating, we could increase the resolving power since the number of slits present in the grating increases. The resolving power could also be increased by looking at a higher order of diffraction, however by looking at high orders, we find that the intensity is not as great, so the increase in the resolving power may not be of any use unless the incident light is bright enough. Increasing the groove frequency, however, may be hard to do precisely, so looking at a high order may be necessary. When we looked at the logarithmic efficiency plot for the blazed grating with TM(S) polarisation, line frequency 1500 lines/mm, wavelength 300nm and facet angle 13, we saw that there were sudden ‘jumps’ in the efficiency of the 0th and -1th or...

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