Experimental Demonstration of Optical Laser Oscillation with Population Inversion in Helium-­‐Neon Plasma †
Ved Chirayath† Stanford University Department of Physics, Stanford, California
Population inversion in the energy levels of Neon in a Helium-­‐Neon plasma situated in a Fabry-­‐
Perot optical resonator is observed and confirms the four-­‐level laser pumping mechanism. Amplification in the active medium, by means of a measured increase in photon flux density, is observed for the 𝟑𝒔𝟐 → 𝟐𝒑𝟒 transition in Neon resulting in lasing state at 6328 Ȧ with a continuous power output of 20 ± 3 μW. Introduction density decreases, and in the case that the levels are Lasers rely on the amplification of stimulated equally populated, transmission occurs. emission in a resonator cavity to achieve high photon In common He-­‐Ne lasers, level C. transitions to flux density, coherence and spectral purity – properties level D. (3Sà2P) (Ne), with a corresponding emitted that offer a radically new source of electromagnetic photon wavelength of 632.8 nm. Transitions from the radiation. Fundamental to the process of light 2P (B.) to 1S (A.) Ne level occur primarily as a result of amplification in an optical resonator is the population spontaneous emission and allow the pumping process inversion that occurs between energy states in the to continue. active medium. We examine how the relative population densities of energy levels in a Helium-­‐Neon D. Short-­‐lived C. Long-­‐lived level level plasma differ between lasing and non-­‐lasing states. Optical lasers can be classified according to their active mediums and number of energy levels involved in lasing. In the case of the He-­‐Ne laser, there are four energy levels utilized for lasing (Figure 1, A-­‐D) and the active medium is a mixture of Helium and Neon gas through which a high voltage electric current is passed. Upon application of high voltage electric current, the gas mixture transitions to a plasma consisting of excited and neutral Helium and Neon atoms as well as free ions and electrons. Due to the B. Short-­‐lived level electric field, the free ions and electrons gain additional kinetic energy. These particles proceed to collide with Helium and Neon atoms and excite them into higher energy states. This excitation, namely from A. to D. (Figure 1), plays the role of pumping in the four-­‐level He-­‐Ne system. However, the 2! S and2! S (He) levels are short lived (D.) and thus quickly transition to adjacent metastable Ne levels where the population accumulates as a result of metastability (C.). Figure 1 depicts a few of the possible A. Ground level consequent transitions. Of the two gases, Neon’s transition role is dominant in laser emission and its energy levels are subject to population inversion. Figure 1 -­‐ Helium and Neon four level system. Helium and Population inversion is defined as the state in Neon energy levels and associated emissions. Dashed lines represent spontaneous emission, solid lines are the lasing which there are more atoms in the second level (C.) 1
transitions and () is the A coefficient. than there are in the lower level (A.). In this scenario, light amplification can occur and lasing is possible. Experimental Methods However, in the absence of population inversion, The basic laboratory setup is depicted in Figure attenuation takes place whereby the photon flux 2. The Helium Neon discharge tube, charged with a high voltage current, is situated in a Fabry-­‐Perot type optical resonator (laser tube) and aligned to achieve a continuous lasing state. Emissions from the discharge tube are focused using an optical lens onto the HR-­‐320 monochromator entrance slit (15𝜇m ± 0.1). Output wavelength is adjusted by a sine-­‐bar type motor driver (periodic error measured as ±0.01 Ȧ per 20 Ȧ) controlled by a computer serial interface (GPIB). The monochromator has a reflective diffraction grating (58x58mm) with 2400 grooves/mm allowing for a theoretical resolving power2 of !
!"
= 𝑁𝑛 → ∆𝜆 =
0.047 Ȧ for the 6401 Ȧ line in Neon. The light exits the spectrograph through a slit of (10𝜇m ± 0.1) to the Hamamatsu R928 photomultiplier which is charged using a 900V source. The change in measurable intensity between emissions in lasing and non-­‐lasing states is on the order of 1-­‐10%3. Initial analysis with only a spectrograph and photomultiplier detector (PMT) did not provide statistically significant evidence of population inversion in the active medium. signal. The resulting lock-­‐in output is a DC signal proportional to the difference in the intensity between lasing and non-­‐lasing states, ∆I (x 5 mV). With the two signals in phase, the spectrograph scans the discharge tube emissions for a range of wavelengths associated with Neon transitions. Output from the PMT and lock-­‐in allow for direct observation of intensity, I, and the change in intensity between lasing states, ∆I, as a function of wavelength (Figures 4,5). If the reference signal and PMT input are in phase, the lock-­‐in outputs a DC signal proportional to the difference in relative intensities between lasing and non-­‐lasing states. Population inversion is measured by the relative sign of the lock-­‐in output. Results and Data Analysis The photomultiplier and lock-­‐in data were compared for a range of Ne transitions (Figures 4, 5). Evidence of population inversion is immediately evident for the lasing wavelength of 6328 Ȧ, corresponding to the 3𝑠! → 2𝑝! transition in Neon., by means of a positive value of ∆I and a percent change in intensity on the order of+20±1%. Results for five such transitions are tabulated in Table 1. Alignment of the optical resonator achieved a constant power output of 20 ± 3 µμW as measured by a laser power meter photodiode 4 cm away from the transmitting mirror. 4
Figure 2 – Laboratory setup To overcome this sensitivity barrier, a phase sensitive lock-­‐in amplifier is used in conjunction with a chopper wheel. Within the laser cavity, the chopper is set to spin at 1kHz effectively stopping the lasing process once every thousandth of a second and providing a reference signal. This reference signal is measured with a photodiode at the transmitting end of the laser cavity and fed into the lock-­‐in amplifier. The lock-­‐in is calibrated by centering the scanning spectrograph on an intense peak (6401 Ȧ in Neon). A sensitivity of 5 mV was chosen based on the value of 1% of the PMT’s voltage output while centered on a peak. On the lock-­‐in, the PMT’s output is configured to be in phase with the chopper’s reference Figure 3 – 6300 − 6335 Ȧ range showing positive values of ∆I, demonstrating population inversion for the lasing wavelength of 6328 Ȧ. The apparent shift in peaks between the lock-­‐in amplifier and PMT outputs can be seen in Figures 4 and 5. The magnitude of the shifts, 0.3 Ȧ on average, can be explained by the relatively long time constant (1 sec.) used by the lock-­‐in to bin input. If the monochromator were to scan the wavelength range slower, this systematic error could be further reduced, but would significantly increase the data acquisition time for a relatively small increase in peak synchronicity. The lowest sensitivity that could be achieved while maintaining a constant phase lock on the lock-­‐in amplifier was 5 mV. Lower sensitivities were observed to provide cleaner lock-­‐in output, but failed to maintain phase discrimination for low intensity peaks. Additional sources of errors included the periodic error in the spectrograph’s motor driver (±0.01 Ȧ/20Ȧ), which contributed to errors in wavelength over a large scanning range. The reference signal error (±0.01 Hz) is suggested to explain the observed fluctuations in intensities in lock-­‐in output on the order of ±1 mV. Effects such as current and voltage fluctuation in the discharge tube could not be accounted for, but likely influenced the population ratios5 to an insignificant degree. Statistical errors were negligible based on Lorentzian curve fits to the data. Ne transition 2𝑝! → 1𝑠! 3𝑠! → 2𝑝! 3𝑠! → 2𝑝! 3𝑠! → 2𝑝! 2𝑝! → 1𝑠! λ(Ȧ) 6677.4±0.6 6401.4±0.6 6350.7±0.6 6328.3±0.6 6096.1±0.5 ∆𝐈/𝐈 (%) -­‐10±1 +7±1 +30±1 +20±1 -­‐12±1 Table 1 – Observed Neon transitions and respective percent change in intensity from lasing and non-­‐lasing states. A negative sign indicates normal population distribution, whereas a positive sign is indicative of population inversion. Conclusions Using a chopper and lock-­‐in amplifier combination, the change in spectral intensity was observed for a total of five transitions in the He-­‐Ne four-­‐level system. The relative sign of ∆I directly indicated whether populations were inverted, able to contribute to amplification, or non inverted contributing to attenuation. Finally, a lasing state with a constant power output of 20 ± 3 μW and a wavelength of 6328±0.5 Ȧ was achieved. Population inversion in Neon’s upper and lower energy levels was observed. The phase sensitive lock-­‐in amplifier and oscillating chopper proved a novel and precise tool for confirming the difference in the relative spectral intensities of a lasing and non-­‐lasing state. Further refinements to this experiment could include a slower monochromator scanning speed to reduce the peak discrepancy between the lock-­‐in and PMT outputs as well as a more stable high voltage power source for the He-­‐Ne discharge tube. References 1
Adapted from Laser Electronics. Verdeyen. 2001. Melissinos, Experiments in Modern Physics, pp.328-­‐335 3
“Experiment 02” lab manual. Vuletic and Pam. 2009. 4
Adapted from “Experiment 02” lab manual. Vuletic and Pam. 2009. 5
nd
B. Saleh, M. Teich. Fundamentals of Photonics 2 edition. 2006. 2
Figure 4 -­‐5940 − 6100 Ȧ range. Here, the relative sign of ∆I is negative for the last two peaks confirming that the populations are not inverted.