Progressive report 7
1. Characteristic of long pass filters
As I mentioned in my previous progressive report (Progressive report6), I updated my
IDL code to read exactly output signal of photodiode in a given wavelength and measured the output of photodiode. That measured values are also compared to direct measured values to verify IDL code and characteristics of long pass filters. Generated
Light sources are intermediate level to prevent saturation in the long wavelength.
Below table shows the setup for measurement of each filter
Filter number
2
3
4
5
Start wavelength(nm)
300
560
635
685
End wavelength(nm)
340
630
695
750
Intensity of light source
100
50
35
35
Gain selection
High
Low
Low
Low
Below 4 figures show the comparison between measurement with updated IDL code
(solid line) and direct measurement (dotted line). As we can see from all figures, measurement with updated IDL code coincides with direct measurement one. It represents IDL code can read the output signal exactly in a given wavelength.
In addition, we can see the characteristics of each long pass filters as a function of wavelengths. Cut off wavelength of each filters are 320nm (filter 2), 590nm (filter 3),
665nm (filter 4) and 715nm (filter 5), respectively.

<Figure 1, Verification of long pass filters and IDL code>
2. Verification of 2 photodiodes
I simply tested 4 channels (ch0 – ch 3) with 2 photodiodes (Ch0- Si of photodiode1, Ch1-
Ge of photodiode 1, Ch2-Si of photodiode 2, Ch3-Ge of photodiode 2) to verify their working and IDL code and saved raw data as a function of wavelength (300nm – 1800nm with 100nm increase).
I selected filter 1 which is open and grating 1 which has 1200mm/g with blaze=500nm.
You should note that only photodiode 1 can be installed on the monochromator and measure the output light from monochromator. Si of photodiode 1 looks most sensitive at around 1000nm wavelengths. It shows very similar results of company’s QE documents. Ge of photodiode 1 is also working well in the long wavelength (above
1000nm). But their values are too small since I did not select high gain. So if we use different gratings having different blaze wavelength such as blaze =1μm with high gain, we may possible to get better results from Ge.

<Figure 2, Measurement of output as a function of wavelength of photodiode 1. Left is for Si and right is for Ge>
Below figures show the measurement of photodiode 2. But it (channel 2 and 3) is just connected to bud box and slightly open the cab on the room. This is why figures of ch 2 and 3 show constant in the long wavelengths. Variations of plot on the first part of figures are caused by intentionally moved photodiode cap to see and check their working. Finally, you should note that x-axis does not mean real wavelength. Instead of it, we read all values at a same time when we read the photodiode 1.
<Figure 2, Measurement of output as a function of wavelength of photodiode 2. It is not installed on monochroamator and placed on the table instead with slightly open the cab.
Left is for Si and right is for Ge>
Based above test, I verify all channels and 2 photodiodes are working well.