Diode Laser Lab Anthony Mannino Lab Partners: Adam Keegan, Chris Dahdouh Abstract: The purpose of the following four experiments were to observe the properties of the 808 nm diode laser using such equipment as a function generator, an oscilloscope, and the Nd:YAG material, and taking note of how certain changes affect the laser. First, to familiarize ourselves with the oscilloscope, we made it a task to find the time constraint of an RC circuit with the oscilloscope. Then, we commenced our experiments concerning the diode laser itself. We first analyzed the relationship between the power of the laser and its diving current in order to find the threshold current of the laser. Then we observed how the output power was altered by changes in the temperature at which the laser operated. Our final experiment details our findings when measuring the fluorescence lifetime of the Nd:YAG material. These experiments and their results are explained in detail in the report that follows. Objective: Observe the properties of a diode laser and the YAG crystal with the use of lab equipment and familiarize ourselves with said equipment in the process. Equipment: - LEOI-50A Diode-Pumped YAG Laser an Frequency-Doubling – Enhanced Model with current driver and temperature controller Tektronix Oscilloscope RC circuit Introduction and Theory Common light emission is caused by two processes: stimulated absorption and spontaneous emission. The medium of the common light source is affected by an external source of energy in such a way that the energy is absorbed (stimulated absorption). This, in turn, “excites” the electrons so much so that they move to an upper energy state. Since the lifetime of electrons in an upper energy state is approximately a nanosecond, they are spontaneously released to a lower energy state. This essentially immediate change in energy states results in the emission of a photon with an energy of ℎ = 2 − 1 where h is Planck’s constant, is the frequency of the light emitted in Hertz, and 2 and 1 are the respective energies of the upper and lower states. This phemonenon is know as spontaneous emission. In contrast to common light emission, LASER (light amplification by stimulated emission of radiation) is caused by a process known as stimulated emission. When an atom in energy state 2 is stimulated by a photon possessing an energy of ℎ = 2 − 1 , the atom is released to the lower state 1 . This results in the emission of a photon with energy, phase, polarization, and directivity identical to that of the stimulated photon. The emission of the two identical photons results in the amplification of light. The processes is depicted in the diagram below: This stimulated photon, however, will often cause stimulated absorption rather than emission under standard conditions and thermal equilibrium unless the population of atoms in the upper state is greater than that of the lower energy state. This is known as population inversion and is necessary for the generation of laser light. The gain medium used for this experiment was the Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) material. Using thisgain medium, population inversion is achieved so that laser light can be generated. The Nd3+ ions are pumped by the 808-nm laser from the ground state (4I9/2) to the excited state (4F5/2). These ions are then transferred to the upper energy level (4F3/2). The subsequent stimulated emissions from the upper level to the lower level (4I11/2) give off photons at a wavelength of approximately 1064 nm. The following diagram thoroughly depicts this process: A crucial component for any laser is the resonator. Lasers consist of three primary components: a pumping source, a gain medium, and a resonator. The resonator is a specific arrangement of mirrors configured around the gain medium in such a way that feedback is created. The pumped laser enters the resonating cavity at a certain wavelength, 808 nm in our case, and mode. Standing waves are then produced inside the cavity at a certain mode and wavelength (1064 nm) depending on the configuration and types of mirrors used. This resultsin the emission of laser light. It is important to match the modes of the pumped laster and 1064-nm light for an efficient pumping outcome. Below is a diagram further demonstrating how the resonating cavity is set up. Part One: Finding the RC Time Constant (Τ) of a Circuit Using an Oscilloscope Procedure: In order to determine the time constant of an RC circuit, we ran an RC circuit through an oscilloscope. The oscilloscope generated a waveform depicting how the voltage varied as a function of time. Using the relationship V(t) = V0 − and the observed maximum/initial voltage, we calculated the time constant. Observations: After observing that the initial voltage (Vmax) of the circuit was approximately 19.40 Volts (±0.01 Volts), we then divided Vmax by e, which gave us VΤ = Vmax = 7.14 Volts (±0.01 Volts). As was previously defined, Τ is the RC time constant of the circuit and VΤ is the total electric potential when time (t) is equivalent to the RC time constant. Thus, after calculating VΤ , we were able to measure the time, Τ, from the point at which the electric potential of the circuit was at its maximum (t = 0.00s) to the point at which the electric potential was VΤ (t = Τ). Using the oscilloscope, we observed that Τ = 124.0 μs. Thus, every value of the circuit’s electric potential, V, dissipates by a factor of 1 every 124.0 μs. Since we know the resitance and capacitance of the circuit with which we were working, we can calculate the theoretical value of Τ and the percent error. With a resistance of 1000 Ω and a capacitance of 0.14 μF, the theoretical value of the time-constant is 140 μs. Thus, there was a percent error of 11.4%. This is most likely due to a slight inaccuracy using the oscilloscope on our part or due to the inherent error of the labeled capacitance on a capacitor, which usually is ±20% in terms of error. This would account for the small error we calculated. Having more advanced equipment might allow us to receive a value closer to the theoretical constant. Part Two: Electro-Optic Characteristics of a Diode Laser Procedure: In this experiment, our objective was to find the current threshold of the 808 nm diode laser. To do so, we observed and graphed the relationship between the driving current and the observed power of the diode laser for 24 different measurements of current. Data and Observations: Power (mW) 0.50 0.40 0.60 23.00 60.00 86.00 116.00 153.50 190.00 234.00 327.00 441.00 480.00 520.00 560.00 590.00 618.00 653.00 684.00 720.00 733.00 750.00 754.00 767.00 Power-Current Curve 900 800 700 Power (mW) Current (A) 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.40 600 500 400 300 200 100 0 0 0,5 1 1,5 2 2,5 3 Current (A) The recorded data show that the output power of a laser is defined as a function of the operating current. This information is most useful in calculating an approximate threshold current for the diode laser. At this threshold current, the diode will begin to steadily emit laser light. This is due to a steep increase in power. From our data, it is a safe assumption to say that the threshold current is 0.40 A (±0.01 A). As seen in the data table to the left, the point at which the power begins to increase is when the diode-operating current is 0.40 A. The accuracy of our observed relationship between power and current, however, seems to be questionable. The power-current curve we obtained is seemingly slightly different from other P-I curves for other diode lasers in that the maximum output power was far smaller than that reported even in the manual (2000 mW). This could be due to a number of reasons, the most likely of which is equipment malfunction. The data was taken again, yielding the following results. Power (mW) 0 0 0 25 59 76 102 133 165 199 281 305 419 454 490 517 546 573 590 604 630 643 679 412 411 Power-Current Curve 800 700 600 Power (mW) Current (A) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 500 400 300 200 100 0 0 0,5 1 1,5 2 2,5 3 Current (A) The similarity between this data set and the former is indicative of our general accuracy in taking the data (any reltive errors may be due to slight changes in set up). This may further be indicative of malfunctioning equipment. This is also supported by the fact that the laser should be transmitting approximately 2.00 Watts of power at 2.5 A, which neither of our sets show. Part Three: Absorption of Nd:YAG Material Procedure: In this experiment, we further analyzed how the laser’s output power varied in relation to other variables. In the following case, our goal was to record the relationship between the outpout power and temperature in order to analyze how the laser’s absorption spectrum changed with regards to temperature. We took measurements for the 23 temperatures below. Since the power reading fluctuated when taking each measurement, we took the average of the maximum and minimum power readings for each temperature. We then used these averages to generate a power-temperature curve. Using this power-temperature and the absorption spectrum provided in the manual, we observed how the absorption spectrum changed as a result of changes in temperature. Data and Observations: Temperature (C) 40.0 39.0 38.0 37.0 36.0 35.0 34.0 33.0 32.0 31.0 30.0 29.0 28.0 27.0 26.0 25.0 24.0 23.0 22.0 21.0 20.0 19.0 18.0 Max Power (mW) 44.0 42.0 51.0 47.0 70.0 92.0 89.5 85.0 83.0 84.0 100.0 107.0 100.0 107.0 120.0 110.0 123.0 121.0 117.0 117.0 123.0 141.0 145.0 Minimum Power (mW) 32.0 33.0 31.0 40.0 36.0 69.0 60.7 62.0 64.0 66.0 72.0 74.0 88.0 97.0 102.0 100.0 103.0 107.0 112.0 117.0 119.0 135.0 135.0 Average Power (mW) 38.0 37.5 41.0 43.5 53.0 80.5 75.1 73.5 73.5 75.0 86.0 90.5 94.0 102.0 111.0 105.0 113.0 114.0 114.5 117.0 121.0 138.0 140.0 Power-Temperature Curve Average Relative Power (mW) 160 140 120 100 80 60 40 20 0 15 20 25 30 Temperature (C) 35 40 45 Despite the above curve presenting an accurate depiction of the data we recorded, I noticed an unusually stark difference between this set of data and the data we took in a second trial for the purpose of determining the consistency and reproducibily of our results. The second set of data is represented by the following power-temperature curve. Power-Temperature Curve 300 Power (mW) 250 200 150 100 50 0 10 15 20 25 30 35 40 45 Temperature (C) The obvious inconsistency may be due to an equipment malfunction. Human error may have played some small roll, but taking this along with the results for the power-current curve into consideration is evidence enough to believe the equipment was malfunctioning in some way. However, since the second curve is less linear and more realistic, we will be referring to that when discussing the effect on the absorption coefficient. Using the above plot of the absorption spectrum for diode lasers, we can compare the graphs and analyze how the absorption coefficient is affected when temperature is altered. First, it is noteworthy to state that we are assuming the wavelength to be 808 nm at 25 degrees Celsius since that was the temperature at which we operated the laser for the last experiment. We are also assuming that the wavlength is only undergoing slight alterations (± 7.0 nm) throughout this experiment. So comparing both plots and analyzing the absorption coefficients for the appropriate wavelengths, we see, for example, at 40 degrees Celsius, the wavelength could very well be approximately 815 nm and the absorption coefficient approximately 1.0 cm-1. We can make this assumption because we know that output power and wavelength share an inverse relationship. Furthermore, from our data, we know that power decreases as temperature increases; thus, wavelength increases as temperature increases. Now our assumptions concerning the coefficients could be very inaccurate because we have no means of calculating the exact coefficient as of now. The one piece of information of which we are certain is, based upon the absorption plot, the absorption coefficient when the wavelength of the diode laser is 808 nm is approximately 12.0 cm-1. Part Four: Fluorescence Lifetime of Nd:YAG Material Procedure: For the final part of this lab, we used the oscilloscope to measure the flourescence lifetime of the Nd:YAG crystal. Since voltage and intensity are directly proportional, this process was, for the most part identical to finding the time constant of the RC circuit with the oscilloscope. We used the oscilloscope to generate voltage-time plots and find the grime constants for two cases: the laser in the presence and in the absence of the YAG. Then, by finding the difference between the two time consants, this gives us the fluorescence lifetime of the crystal itself. Data and Observations: *Note: The following pictures do not represent the actual numerical results; rather, they represent the relationships in the presence and absence of the Nd:YAG material. When measuring the fluorescence lifetime of the diode laser without the YAG crystal in place, we received the following reading: In the above case, the response time of the setup in the absence of the Nd:YAG material (τ) was measured as 100 μs (±1.00 μs). Furthermore, we now know that the intensity decreases as a function of time described by = 0 −100, where 0 = 6.08 mW/nm2. In the presence of the YAG, we observed the following reading on the oscilloscope: In this case, τ = 690 μs (±1.00 μs) and an 0 of 5.08 mW/nm2. Thus, the the function describing this behavior is = 5.08 −690 . With this information, we can now calculate our observed fluorescence lifetime of the YAG crystal itself, which is nothing by the difference of the two response times we measured. Accordingly, the fluorescence lifetime of the YAG crystal is 590 μs. Conclusion: Though our results at times seemed skewed for an unknown reason, this lab was very informational. We came away with more knowledge on how to use the equipment and how to apply the theory we learned. The equipment may need investigation as there may have been malfunctions which may have skewed our data. More measurements must be taken to improve the accuracy of our results. Despite the apparent inaccuracy of our data, however, the experiments were successful. We were able to accurately depict the relationships between certain quantites and characteristics related to how the laser functioned such as the output power, operating temperature, and driving current. For that reason, the overall experiment was a success, however, to truly determine the consistency of our methods, we would have to perform this lab again with equipment that functions in the manner it should. Only then will we know if this was truly a success.