Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki UNIWERSYTET GDAŃSKI

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1 Instytut Fizyki Doświadczalnej Wydział Matematyki, Fizyki i Informatyki UNIWERSYTET GDAŃSKI

2 I. Background theory. 1. The temporal and spatial coherence of light. 2. Interaction of electromagnetic waves with a medium: a) absorption of radiation; b) spontaneous emission; c) stimulated emission; d) dipole radiation and the probability of absorption, spontaneous and forced emission; e) Einstein coefficients; f) lifetimes of excited states; g) spectral line profiles; Lorentz and Gaussian profiles. 3. Population inversion of atomic states. 4. Formation of energy bands in crystals. 5. Doped ions as luminescent centres in crystals. 6. Types of solid-state lasers. 7. Construction and operation of a triply ionised laser rare earth Nd: YAG laser. 8. Pumping of solid-state lasers. 9. Semiconductor lasers: a) radiation transitions in semiconductors; b) conductivity of semiconductors; c) p n interface in semiconductor lasers (homo and heterojunctions); d) radiation amplification in semiconductor lasers. 10. Absorption of radiation in Nd:YAG crystals. 11. The basic properties lasers and lasing: a) lasing threshold; b) types of cavities and their influence on resonance conditions; c) modal structure of laser radiation; d) performance curve of a laser; e) laser output power. II. Experimental tasks. 1. Familiarise yourself with the experimental setup shown in Pictures Measure the output power of a semiconductor laser as a function of current through the junction following steps II.3. II.12. Wear safety goggles when working with lasers. The continuous output power of the Nd:YAG laser exceeds 1mW and the laser light is very harmful to your eyes!!! Instytut Fizyki Doświadczalnej 1.

3 Experiment 23 : Studying the properties of solid-state lasers 3. Using scheme 1 in Figure 2, assemble the following elements on the optical bench: semiconductor laser (the left side of optical bench), a collimator (f = 6 mm) and a photodiode obscured by the visualisation screen (on the right end of the bench). Picture 1 Experimental setup to study the properties of a Nd:YAG laser: 1 laser diode power supply; 2 semiconducting laser (λ = 808,4 nm); 3 collimator (ƒ = 6 mm); 4 lens (ƒ = 50 mm); 5 Nd: YAG crystal; 6 KTP crystal; 7 mirror; 8 filter holder; 9 optical bench; 10 Si photodiode; 11 universal multimeter; 12 oscilloscope; 13 set of accessories (IR converter card; filters: BG39, RG1000, ND filter with optical density 1,1; photodiode cap). 4. Turn on the power to each component in your assembled setup. Check that the semiconductor laser 2, Picture 1 is connected to the power supply (connector 1 in Picture 4). Turn the temperature and current control knobs (5 and 6 in Picture 3) all the way to the left. Turn on the main power supply with its switch 6, Picture 4 (on the back of the housing). Turn key 1, Picture 3 to the ON position. An illuminated red LED on the laser means that you can adjust the temperature and current settings for the semiconductor laser junction. Figure 2. Various configurations of the Nd:YAG laser setup: 1 arrangement for measuring the semiconductor laser pump power; 2 focusing lens settings; 3 Nd:YAG crystal settings; 4 system to measure the absorption of the Nd: YAG crystal; 5 optical setup of the Nd:YAG laser; 6 optical setup of the Nd:YAG laser with frequency doubler. Instytut Fizyki Doświadczalnej 2.

4 Experiment 23 : Studying the properties of solid-state lasers 5. Set switches 2 and 3, Picture 3 to stabilised mode (Stab.) and continuous operation OFF respectively. Picture 3. View of the front panels of the LDC-01 laser power supply and DSO1002A oscilloscope: 1 laser power switch; 2 current stabiliser switch; 3 continuous/pulsed mode switch; 4 laser pulse frequency dial; 5, 6 p-n junction current and temperature controls. 6. Set the maximum current to 560 ma by turning dial 5, Picture 3 to the right. Picture 4. View of the rear panel of the laser power supply: 1 laser head power connector; 2 BNC photodiode input cable; 3 photodiode input gain dial; 4 BNC photodiode output signal; 5 BNC sync output; 6 main switch. Instytut Fizyki Doświadczalnej 3.

5 7. Set the collimator so as to obtain a parallel beam of light (1-2 mm from the laser). 8. Adjust the laser beam so that it shines on the centre of the visualisation screen. 9. Connect the photodiode to the preamplifier input 2, Picture 4. Set the gain dial 3, Picture 4 to position x Remove the screen in front of the photodiode and insert a grey filter with optical density at least 1 D. 11. Connect the amplifier output 4, Picture 4 with the universal multimeter 11, Picture 1 set to measure DC voltage. 12. Measure the laser output power P for two temperature 5 ⁰C and 45 ⁰C as a function of current I junction flowing through the semiconductor junction from 0 ma to 560 ma in steps of 25 ma. 13. Plot a graph of P = ƒ(i) for both temperatures T = 5 ⁰C and T = 45 ⁰C. Determine the threshold for stimulated emission generated by the semiconductor laser. 14. Measure the semiconductor laser output power as a function of temperature following steps II.15. II Use dial 5, Picture 3 to set a maximum current of 560 ma. 16. Set the gain dial 3, Picture 4 to position x Use dial 6, Picture 3 to adjust the temperature from 5 ⁰C to 45 ⁰C in steps of 2 ⁰C, each time noting the reading on the multimeter 11, Picture Turn the key 1, Picture 3 to the OFF position. 19. Use the plots in Figure 5 in Appendix A to convert voltage to power (taking into account the gain and attenuation filters). 20. Plot a graph of P = ƒ(t), I = const. 21. Measure the absorption of the neodymium-doped YAG crystal following steps II.22. to II Place the visualising screen in front of photodiode. 23. Place the focussing lens 4, Picture 1 behind the collimator on the optical bench. Slide it towards the collimator to the correct position such that the visualisation screen is illuminated as in scheme 2 in Figure Insert the Nd:YAG crystal 5, Picture 1 in the optical path, removing the visualisation screen the system should look like scheme 3, Figure Turn key 1, Picture 3 to the ON position, set the maximum current to 560 ma. 26. By gently moving the crystal, place it such that the multimeter indicates the maximum. 27. Set the gain dial 3, Picture 4 to position x Adjust the temperature from 5 ⁰C to 45 ⁰C in steps of 2 ⁰C while noting the voltage on the meter. 29. Plot a graph of A = ƒ(t), I = const. Determine the area of greatest absorption by the Nd 3+ ions. 30. Determine the Nd:YAG crystal luminescence decay profile following steps II.31 II Set the current to 560 ma by turning dial 5, Picture 3 to the right. 32. Use dial 6, Picture 3 to select the temperature for which the Nd:YAG crystal absorption is the highest. 33. Set switch 3, Picture 3 to the INT position. Instytut Fizyki Doświadczalnej 4.

6 34. Assemble the system according to scheme 4, Figure 2 by inserting the filter holder (8, Picture 1) with the FG 1000 filter and push the photodiode close to the filter. 35. Connect the photodiode signal to oscilloscope channel 1; Connect the sync output 5, Picture 4 to channel Save the observed waveforms to external USB memory as described in Appendix B. 37. Measure the Nd:YAG laser output power as a function of semiconductor laser power following steps II.38. II Set the current to 560 ma by turning dial 5, Picture 3 to the right. 39. Set switch 3, Picture 3 to the OFF position. 40. Assemble the system according to scheme 5 in Figure 2, inserting a filter with optical density at least 1 D just before the photodiode followed by filter RG 1000 (in holder 8, Picture 1) and the mirror (7, Picture 1) approximately 100 mm from the Nd:YAG crystal. 41. Connect the photodiode to the preamplifier input 2, Picture 4 and set the gain switch 3 to position x Change the position and orientation of mirrors 5 and 7 Picture 1 such that the signal on the meter is a maximum. 43. Use the graph in Figure 6 To plot the output power P YAG of the Nd:YAG laser as a function of power P of the semiconductor laser P YAG = ƒ(p), T const. 44. Measure the output power P SHG of the SHG laser (with frequency doubler) by inserting the frequency doubling KTP crystal (6, Picture 1) between the resonating mirrors as in scheme 6 in Figure Adjust the position and orientation of the KTP crystal in the cavity such that the signal on the meter is a maximum. 46. Use the graph in Figure 7 to plot P SHG = ƒ(p), T const. 47. Interpret the results from steps 13, 29, 30, 43, 46. III. Apparatus. 1. LDC-01 power supply for the semiconducting laser. 2. LDS 1200 semiconducting laser. 3. Collimator with focal length 6 mm. 4. Lens with focal length 50 mm. 5. Nd:YAG crystal with AR-coating mirror in XY bracket. 6. KTP crystal generating second harmonics mounted in XY bracket. 7. Mirror mounted in XY bracket. 8. Filter holders. 9. Optical bench. 10. Detector semiconductor diode with visualisation screen. 11. Universal multimeter. 12. Dual-channel oscilloscope model DSO 1002 A ( Agilent Technology) 13. Set of accessories: IR converter screen, bandpass filters 532 nm (BG39) and 1064 nm (RG 1000), ND filter with optical density 1,1D, visualisation screen mounted on photodiode. Instytut Fizyki Doświadczalnej 5.

7 IV. Literature. 1. N.W. Ashcroft, N.D. Mermin Solid State Physics, Saunders College, Philadelphia K. Shimoda Introduction to Laser Physics, Springer, W.W. Chow, S.W. Koch, M. Sargent Semiconductor Laser Physics, Springer, Berlin W. Demtröder Laser Spectroscopy. Basic Concepts and Instrumentation, Springer, H. Abramczyk Introduction to Laser Spectroscopy, Elsevier Science, Amsterdam W. Demtröder Atoms, Molecules and Photons: an Introduction to Atomic-, Molecular- and Quantum-Physics, Springer, Berlin Instytut Fizyki Doświadczalnej 6.

8 Appendix A Dependence of the laser power on the photodiode signal. Laser power as a function of voltage at the Si photodiode 300 λ=808,4 nm Y = -3, ,56*X P (mw) U (V) Figure 5. Scaled plot of semiconductor laser with wavelength λ = 808,4 nm output power as a function of photodiode voltage. Laser power as a function of voltage at the Si photodiode 100 λ=1064 nm Y = -1, ,73*X 80 P (mw) U (V) Figure 6. Scaled plot of Nd:YAG laser with wavelength λ = 1064 nm output power as a function of photodiode voltage. Instytut Fizyki Doświadczalnej 7.

9 Laser power as a function of voltage at the Si photodiode 0,4 λ=535 nm Y = -0, ,68*X 0,3 P (mw) 0,2 0,1 0,010 0,015 0,020 U (V) Figure 7. Scaled plot of SHG laser with wavelength λ=535 nm output power as a function of photodiode voltage. Instytut Fizyki Doświadczalnej 8.

10 Experiment 23 : Studying the properties of solid-state lasers Appendix B Reading and saving signals with the DSO 1002 A oscilloscope (Agilent Technology) The oscilloscope has internal non-volatile memory and a USB input (1 in Picture 8) which allows you to connect it to external storage or a printer. You can save/read data to/from one of the ten internal memory slots or to external storage with the following steps: 1. Saving to external media requires plugging the external memory into the USB port (1 in Picture 8). Picture 8. Oscilloscope front panel: 1 USB slot; 2 save/read data button. 2. Press Save/Recall (2 in Picture 8) on the oscilloscope front panel. 3. Select the internal data format Waveform or ASCII (CSV) (1 in Picture 9) by pressing Storage or turning the dial. Picture 9. Write/read menu on the oscilloscope front panel: 1 data type selection button; 2 memory manager button. Instytut Fizyki Doświadczalnej 9.

11 To write or read data to or from internal memory: a. Select Internal. b. Press Location in the menu Internal. c. Press Location or turn the dial, to select an internal memory storage location. N indicates that the slot is empty while S indicates that a waveform is stored in this slot. Press Save or Load. To write or read data to or from external memory: a. Select External. b. Use the Disk Manager to choose a folder in which to save the file 2, Picture 9. c. Select New File from the External menu, enter the filename and choose Save. To read data, select Load (it will read files with the extension.wfm). d. Press Location or turn the dial to select an external memory storage location. N indicates that the slot is empty while S indicates that a waveform is stored in this slot. e. Press Save or Load. Instytut Fizyki Doświadczalnej 10.