PHY 5341 LAB 2: DETECTION OF LIGHT Date of experiment: 02/06/2020 Lab # 002 Seonghyeon Yun ha6324 Lab 2 PHY 5341 1 I. Objective The purpose of this experiment is the following: 1) To introduce to the student several methods of quantitatively measuring the power of laser light from a Helium-Neon (HeNe) laser 2) To introduce the concept of optical density 3) To introduce to the student the concept of AC signal detection with a lock-in amplifier 4) To show how the light from a laser source can be directed by introducing various optical components into its pathlength. 5) To demonstrate the non-linear response of a photodiode and the concept of saturation. II. Introduction Various types of light sources are used extensively in today’s research environments. However, the most sophisticated light source is of little use to any researcher or scientist if it cannot be measured or manipulated to suit the user’s needs. In this experiment, a Helium-Neon laser will be used as the light source. You will be measuring the power of your laser using a commercial digital power meter. You will utilize neutral density filters to attenuate the beam (lessen the power). You will then utilize a fast silicon photo-diode with a lock-in amplifier to measure the power of a “chopped” laser beam. Using the neutral density filters, you will study the response of this photo-diode as a function of the incident laser power. shared chopper Lock-in amplifier Neutral-density filter wheel reference HeNe laser mirror signal 50:50 glass beam splitter mirror Si photodiode location of laser power meter for ND filter measurement (IIIA) Lab 2 two locations of power meter for beam splitter calibration (IIIB) PHY 5341 2 III. Procedure The apparatus will be set up similar to that shown in Figure 1.1 in order to perform the following experiments. Your optical setup consists of a Helium-neon laser operating at 633 nm, a neutral density filter wheel consisting of 5 ND filters and one open window, a shared “chopping wheel” that looks like a fan blade, two mirrors to redirect the laser, a power meter and a silicon photo-diode. Data sheets have been provided for each student to record the data. Part IIIA: Calibrate the Neutral Density Filters A neutral density filter is a glass window that only transmits a fraction of the light incident upon it. If the power of the laser incident upon the ND filter is given by P0 and the power transmitted through the ND filter is given by P, then the optical density (OD) of the ND filter is given by: πΌ0 ππ· = log10 ( ) πΌ You will calculate the OD of the 5 filters given to you by measuring the incident power and the transmitted power. Note that the chopper wheel is NOT spinning in part IIIA. In fact, the wheel must be positioned so that both lasers pass through the wheel unhindered. Also note that the ND filter wheel needs to rotate two “clicks” for a new filter to be positioned in the beam. If you rotate it only one “click” the beam is blocked: useful for measuring the background light! 1. “Zero” out the background light with your power meter by pressing the “ο” button on your power meter. 2. Adjust your power meter until you reach the screen that has “Average Value” and “Std. Deviation” measurements. Please consult the user manual before asking your TA for help: https://www.thorlabs.com/_sd.cfm?fileName=17654-D02.pdf&partNumber=PM100D 3. Measure the power of the laser beam with no filter in place using the optical power meter and then insert one of the filters into the beam. Record these average powers and their standard deviations on “Datasheet: IIIA.” 4. Repeat step 3 three times for each filter. 5. Calculate the OD for each set of measurements using the equation given above. Calculate the standard deviation of your OD measurements via: 2 2 ππ ππ √ πΏπ = ( β πΏπ₯) + β― + ( β πΏπ§) ππ₯ ππ§ Lab 2 PHY 5341 π€βπππ π = π(π₯, … , π§) 3 Part IIIB: Calibrate the Beam splitter A beam splitter is a coated glass window that only reflects a fraction of the light incident upon it and transmits the rest (ignoring absorption in the glass). You should have a “fifty-fifty” or 50:50 beam splitter which reflects about 50% of the light and transmits about 50% of the light. 1. Insert the beam splitter into your laser beam in such a way that the incident and reflected beams are at roughly right angles to each other. 2. Measure the power of the laser beam transmitted through the beam splitter and the power reflected off the face, using the same method for measuring the average and standard deviation as before. Repeat each measurement three times. 3. Calculate the percentages that are reflected versus transmitted (with appropriate uncertainties). For the remainder of the experiment as long as the beam splitter is not jostled, this ratio can be assumed to remain constant. Part IIIC: Measuring photodiode voltage using a lock-in amplifier. With the chopper running, let the laser be incident on the photodiode. The signal from the photodiode should go to the lock-in amplifier. The reference signal from the chopper should go to the reference channel of the lock-in. 1. Look for signal. Adjust sensitivity so it is not overloading. This is done by selecting the “range” on the lock-in. If there is too much power, the red “OVLD” (for overload) light will come on. 2. Check if the room lights affect the measured voltage. Confirm with your classmates and TA that it is okay to turn on and off the room lights before doing so. Part IIID: Determine the consistency between the power meter and the lock-in. Repeat part IIIA using the lock-in instead of the power meter. This will give you a readout in voltage. (Hint: Before you start, make sure that the range of light intensities you will be measuring are all within the range of readable voltages on the lock-in.) Instead of getting the standard deviation of the voltages for each measurement, as you did with the power meter, take the standard deviation of the three voltage measurements with the lock-in as your error. Record your results on the appropriate table below. Use your measurements from the power meter in IIIA and assume these are results perfect. Plot the measured voltage from the lock-in versus the power measured by the power meter. Determine the linear range at which a consistent change in power to the lock-in yields a consistent increase in measured voltage. What is the power upon the lock-in that results in saturation of its detector and a lack of linearity of the voltage? Ensure the power to lock-in’s detector is well within the linear range by attenuating the beam, if necessary, and repeat IIIB using the lock-in instead of the power meter. How does your results compare to IIIB? IV. Summary of Results 1. Determine the optical density of your five ND filters. Report the result and how it was measured. Write down after the table 2. Determine the ratio in which the beam splitter splits the light from the laser. State explicitly the percentage of light in each direction and how it was measured. Which measurement method is more reliable? The ration in which the beam splitter splits the light from the laser is reflected(57%) and transmitted(43%) at the digital power meter measurement, reflected(56%) and transmitted(44%) at the photo diode and lock in amplifier. So we can easily confirm that this beam splitter is not Lab 2 PHY 5341 4 50:50 splitter. State right after the table. Using a photo diode and Lock in amplifier is much more reliable because the % of uncertainty is much less than using a digital power meter 3. Develop a calibration curve for the photo diode response (millivolts) as a function of incident laser power (microwatts). [Use a log-log graph] V. Questions to Consider in your Conclusion 1. Is this photodiode signal sensitive to room light? Should it be? Try turning the lights on and off It is very sensitive to room light. This is because the voltage measured by photodiode depends on (the photons number)/(Area). So if light is turned off, The Area where photon is entering will be smaller than light on. This explain our result correct. In lights off, we can get much bigger value. 2. Below what power level is the photo diode linear? Below the power level 2 the photo diode is linear. We can easily confirm that filter’s power level is lower than 2 blue graph is linear. This means that this photo diode can accurately measure only below filter’s power level2. 3. How has using the chopper helped in this experiment? If light enters the photodiod continuously, recombination continues to occur in the photodiodiode, which in turn results in a lack of electrons. However, the chopper allows the alternation of recombination and generation to continuously measure the amount of photon per unit area between the chopper cycles over time. Lab 2 PHY 5341 5 Data Sheet: IIIA Power Transmitted Power δPower Average OD πΏππ· 0 606.6πW 598.9 πW 594.9 πW 162.9 πW 175.9 πW 166.2 πW 57.30 πW 68.54 πW 57.32 πW 9.334 πW 10.50 πW 10.38 πW 4.753 πW 4.647 πW 4.723 πW 3.781 πW 3.709 πW 3.575 πW 19.66 πW 18.73 πW 18.52 πW 5.016 πW 4.860 πW 5.307 πW 714.1 πW 1.940 πW 1.990 πW 74.05 πW 389.1nW 391.8 πW 43.71 πW 63.30 πW 58.29 πW 101.3 πW 100.5 πW 118.5 πW 0 0 0.552 0.022 0.992 0.020 1.775 0.023 2.105 0.0088 2.211 0.0214 1 2 3 4 5 πΌ0 ππ· = log10 ( ) πΌ Optical density can measure by this equation. πΌ0 = < π‘βπ πππ€ππ ππππ π’πππ π€ππ‘βππ’π‘ ππππ‘ππ > = 600.1ππ πΏπ = √( 2 2 ππ ππ β πΏπ₯) + β― + ( β πΏπ§) ππ₯ ππ§ πΏππ· = √( Lab 2 π€βπππ π = π(π₯, … , π§) 2 2 πππ· ππ·−< ππ· > β πΏπππ€ππ) + β― = √( β πΏπππ€ππ) + β― ππππ€ππ πππ€ππ−< πππ€ππ > PHY 5341 6 IIIB Power Reflected Power Transmitted πππ£π πΏπππ£π 343.3 πW 327.8 πW 383.4 πW 262.3 πW 254.2 πW 278.9 πW 389.3 πW 5.993 πW 14.21 πW 1.463 πW 5.774 πW 9.455 πW Average % of beam 57.01% % uncertainty 42.99% 2.10% 1.95% Average Power reflected = (343.3+327.8+383.4)/3 = 351.50 πW Average Power transmitted = (262.3+254.2+278.9)/3 = 264.97 πW Average % of beam reflected = (Average Power reflected)/(Total Average Power) * 100 = (351.5)/(616.47) = 57.01% Average % of beam transmitted = (Average Power transmitted)/(Total Average Power) * 100 = (264.97)/(616.47) = 42.99% % of uncertainty = πΏπππ£π /πππ£π *100 Data Sheets: IIIC Lights on (mV) Lights off (mV) Data Sheets: IIID Filter 0 1 Lab 2 29.32mV 56.37mV Measured Voltage STD 34.09mV 33.72mV 34.40mV 25.31mV 0.278 Expected power 600.1 πW 0.156 PHY 5341 7 2 3 4 5 Lab 2 25.15mV 24.93mV 17.76mV 18.18mV 18.35mV 7.56mV 7.36mV 7.17mV 1.56mV 1.52mV 1.52mV 0.18mV 0.18mV 0.20mV 168.3 πW 0.248 61.03 πW 0.16 10.07 πW 0.019 4.708 πW 0.0094 3.688 πW PHY 5341 8 Measured Voltage STD Expected power Reflected from beam splitter 34.85mV 0.09 351.5 34.88mV 34.66mV Transmitted 27.05mV 0.18 265.1 through beam 27.47mV splitter 27.40mV Average % of beam reflected = (Average Power reflected)/(Total Average Power) * 100 Average % of beam transmitted = (Average Power transmitted)/(Total Average Power) * 100 The Power is proportional to measured voltage. Average of measured voltage of reflected = 34.80mV Average of measured voltage of transmitted = 27.31mV Average % of beam reflected = (Average measured voltage reflected)/(Total measured voltage)*100 = 34.80/62.11*100 = 56% Average % of beam transmitted = (Average measured voltage tramsmitted)/(Total measured voltage)*100 = 27.3/62.11*100 = 44% % of uncertainty = πΏπππ£π /πππ£π *100 % of uncertainty of reflected from beam splitter = 0.09/34.80*100=0.259 % % of uncertainty of transmitted from beam splitter = 0.18/27.31*100=0.659 % Lab 2 PHY 5341 9
