Experimental Demonstration of Optical Oscillation with in -

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 ��� → ��� 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 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. (3Sà2P) (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 plasma differ between lasing and non-lasing states. level 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 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 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 consequent transitions. Of the two gases, Neon’s A. Ground level 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 which there are more atoms in the second level (C.) represent spontaneous emission, solid lines are the lasing 1 than there are in the lower level (A.). In this scenario, transitions and () is the A coefficient. 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 signal. The resulting lock-in output is a DC signal continuous lasing state. Emissions from the discharge proportional to the difference in the intensity between tube are focused using an optical lens onto the HR-320 lasing and non-lasing states, ∆I (x 5 mV). monochromator entrance slit (15�m ± 0.1). Output With the two signals in phase, the spectrograph wavelength is adjusted by a sine-bar type motor driver scans the discharge tube emissions for a range of (periodic error measured as ±0.01 Ȧ per 20 Ȧ) wavelengths associated with Neon transitions. Output controlled by a computer serial interface (GPIB). The from the PMT and lock-in allow for direct observation of monochromator has a reflective diffraction grating intensity, I, and the change in intensity between lasing (58x58mm) with 2400 grooves/mm allowing for a states, ∆I, as a function of wavelength (Figures 4,5). If ! theoretical resolving power2 of = �� → ∆� = the reference signal and PMT input are in phase, the !" lock-in outputs a DC signal proportional to the 0.047 Ȧ for the 6401 Ȧ line in Neon. The light exits the difference in relative intensities between lasing and spectrograph through a slit of (10�m ± 0.1) to the non-lasing states. Population inversion is measured by Hamamatsu R928 photomultiplier which is charged the relative sign of the lock-in output. using a 900V source. The change in measurable intensity between emissions in lasing and non-lasing Results and Data Analysis states is on the order of 1-10%3. Initial analysis with The photomultiplier and lock-in data were only a spectrograph and photomultiplier detector (PMT) compared for a range of Ne transitions (Figures 4, 5). did not provide statistically significant evidence of Evidence of population inversion is immediately evident population inversion in the active medium. for the lasing wavelength of 6328 Ȧ, corresponding to the 3�! → 2�! 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 µW as measured by a laser power meter photodiode 4 cm away from the transmitting mirror.

Figure 2 – Laboratory setup4

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 Figure 3 – 6300 − 6335 Ȧ range showing positive values of set to spin at 1kHz effectively stopping the lasing ∆I, demonstrating population inversion for the lasing process once every thousandth of a second and wavelength of 6328 Ȧ. providing a reference signal. This reference signal is measured with a photodiode at the transmitting end of The apparent shift in peaks between the lock-in the laser cavity and fed into the lock-in amplifier. amplifier and PMT outputs can be seen in Figures 4 and The lock-in is calibrated by centering the 5. The magnitude of the shifts, 0.3 Ȧ on average, can be scanning spectrograph on an intense peak (6401 Ȧ in explained by the relatively long time constant (1 sec.) Neon). A sensitivity of 5 mV was chosen based on the used by the lock-in to bin input. If the monochromator value of 1% of the PMT’s voltage output while centered were to scan the wavelength range slower, this on a peak. On the lock-in, the PMT’s output is systematic error could be further reduced, but would configured to be in phase with the chopper’s reference significantly increase the data acquisition time for a Ne transition λ(Ȧ) ∆�/� (%) relatively small increase in peak synchronicity. 2�! → 1�! 6677.4±0.6 -10±1 The lowest sensitivity that could be achieved 3�! → 2�! 6401.4±0.6 +7±1 while maintaining a constant phase lock on the lock-in 3�! → 2�! 6350.7±0.6 +30±1 amplifier was 5 mV. Lower sensitivities were observed 3�! → 2�! 6328.3±0.6 +20±1 to provide cleaner lock-in output, but failed to maintain 2�! → 1�! 6096.1±0.5 -12±1 phase discrimination for low intensity peaks. Table 1 – Observed Neon transitions and respective percent Additional sources of errors included the change in intensity from lasing and non-lasing states. A periodic error in the spectrograph’s motor driver negative sign indicates normal population distribution, (±0.01 Ȧ/20Ȧ), which contributed to errors in whereas a positive sign is indicative of population inversion. wavelength over a large scanning range. The reference signal error (±0.01 Hz) is suggested to explain the Conclusions observed fluctuations in intensities in lock-in output on Using a chopper and lock-in amplifier the order of ±1 mV. combination, the change in spectral intensity was Effects such as current and voltage fluctuation observed for a total of five transitions in the He-Ne in the discharge tube could not be accounted for, but 5 four-level system. The relative sign of ∆I directly likely influenced the population ratios to an indicated whether populations were inverted, able to insignificant degree. contribute to amplification, or non inverted Statistical errors were negligible based on contributing to attenuation. Finally, a lasing state with a Lorentzian curve fits to the data. 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. 2 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. Figure 4 -5940 − 6100 Ȧ range. Here, the relative sign of ∆I 5 B. Saleh, M. Teich. Fundamentals of 2nd edition. is negative for the last two peaks confirming that the 2006. populations are not inverted.