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and optics to teach experimental methods and reinforce funda- mental physics concepts. This laboratory includes six experiments concerning modulators, laser ...
IEEE TRANSACTIONS ON EDUCATION, VOL. 45, NO. 3, AUGUST 2002

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Undergraduate Laser Physics Laboratory Randall J. Knize, W. Roc White, and Boris V. Zhdanov

Abstract—The authors have developed a senior-level undergraduate laboratory course with an integrated theme of lasers and optics to teach experimental methods and reinforce fundamental physics concepts. This laboratory includes six experiments concerning modulators, laser kinetics, waveguiding, CO2 laser operation, ultrashort pulse characterization, and nonlinear optics. The authors discuss the philosophy and structure of this course. Index Terms—Instruction, lasers, modulators, optics, second harmonic generation.

I. INTRODUCTION

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HE SENIOR undergraduate laboratory course serves as the capstone experimental course for undergraduate physics students. In the past, students were asked to do a number of experiments involving several disciplines, such as nuclear, plasma, laser, and solid-state physics. In the last few years, however, this course has been restructured to create a more integrated curriculum using lasers and optics as the central theme. The authors developed this laser laboratory course to accomplish several goals. First, they selected experiments in lasers and optics because of student and Air Force interests in this field. The presence of lasers and optics in military and civilian applications is widespread and continues to grow. The authors also wanted this course to tie in concepts from other physics courses, such as quantum mechanics, electricity and magnetism, statistical mechanics, and solid-state physics. Laboratory experience with lasers and optics reinforces ideas learned in these courses and allows for spiral learning of important concepts. The authors want the course to help students develop critical thinking and independent thought. They do not use a textbook. Usually the students have not had any previous courses involving lasers. The authors give general guidelines only on the experiments and allow the students to have freedom in determining the outcome. The work required of the students and the subsequent grading system, discussed below, help develop good oral and written presentation skills that are useful later in life. Overall, this course bridges the gap between textbook learning and independent learning required for research. Manuscript received August 30, 1999; revised January 19, 2002. This work was supported by the Air Force Office of Scientific Research, the U.S. Air Force Academy, the European Office of Airspace Research and Development (London), and the International Laser Center of Moscow State University. R. J. Knize is with the Laser and Optics Research Center, Department of Physics, U.S. Air Force Academy, CO 80840 USA. W. R. White was with the Laser and Optics Research Center, Department of Physics, U.S. Air Force Academy, CO 80840 USA. He is now with the Aerospace Corporation, Colorado Springs, CO 80910 USA. B. V. Zhdanov was with the Laser and Optics Research Center, Department of Physics, U.S. Air Force Academy, CO 80840 USA. He is now with Directed Energy Solutions, Inc., Colorado Springs, CO 80921 USA. Publisher Item Identifier S 0018-9359(02)05057-4.

II. COURSE OUTLINE Every student must complete six selected experiments in lasers and optics: 1) optical modulators, 2) optical waveguiding, 3) Nd : YAG laser kinetics, 4) CO laser characterization, 5) ultrashort laser pulse measurements, and 6) optical harmonics generation.1 These experiments were designed so that concepts learned in one experiment will be used in subsequent experiments. Before each lab, the students receive a page of learning objectives, which includes references. At the beginning of each laboratory, the students also receive a list of prelab questions that they must answer and turn in by the second lesson of that particular lab. During the first lesson of each lab, the students are given an overview of the apparatus and some background on the theory behind each experiment. The students are able to complete the experiment in about ten classroom hours. III. EXPERIMENTS Students are required to complete six experiments in the semester. A. Optical Modulators In this experiment, the students study the principles of modulation of light [1]. Modulators have important technological applications in fiber optics and optical communications. The setup is shown in Fig. 1. In the first part of the experiment, the electrooptical modulator is studied. Applying a voltage to the crystal results in changing of the indexes of refraction and, hence, the polarization of the transmitted light. From this part of the lab, the students develop a good understanding of polarization and how birefringent materials can change the optical polarization. From the data, they calculate the value of the electrooptical coefficient for LiNbO . They are expected to compare their results with accepted published values. The acoustooptical modulator (AOM) is then studied. A sound wave, inside the AOM quartz crystal, is created by a piezoelectric transducer resulting in a volume diffraction grating. The students measure the Bragg diffraction angle and its dependence on the acoustic wave frequency. They also measure diffraction efficiency dependence on the acoustic wave power. The students understand how sound waves in a material can cause diffraction similar to a grating. Using the experimental data, they calculate the speed of sound in quartz.

1Setups for experiments 1, 3, 5, and 6 were developed at the International Laser Center of Moscow State University, Moscow, Russia, http://www.ilc.msu.su.

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Fig. 1. Schematic diagram of the optical modulator experiment. Light from the HeNe laser is focused by lens (L1) into either the electrooptical or the acoustooptical modulator (M). Polarizer (P) is used only for the electrooptical modulator experiment. The lens (L2) is used for focusing the beam into the photodetector (PD).

B. Optical Waveguiding This experiment examines the principles of optical waveguiding [2]. Light can be guided in a medium surrounded by materials of lower indexes of refraction. This is the principle used in fiber optics. The apparatus is shown in Fig. 2. The experiment consists of two parts: the physics of waveguiding and writing a simple LabView2 program to control the experiment. The index of refraction of the thin film is several percent larger than that of the substrate because of the Ge doping. Light will be waveguiding only if the proper boundary conditions are satisfied, which depend on the wavelength, polarization, and incident angle. The students first manually rotate the stage holding the waveguide and observe that at about six or seven angles (for our particular film), the light is waveguided down the thin film. They then spend several lessons developing a LabView program, which will automatically rotate the stage and measure the transmitted light. The program is used to take data for several different wavelengths and for both laser polarizations. The students learn that boundary conditions are important in the propagation of an electromagnetic wave in a one-dimensional waveguide. They develop an appreciation for the programming necessary to control an experiment. Analysis of the data allows them to determine the index of refraction, its dispersion, and the thickness of the doped film. C. Nd : YAG Laser Radiation Kinetics The purpose of this experiment is to acquaint students with a solid-state Nd : YAG laser3 and measurement of various laser parameters [3]. The setup is shown in Fig. 3. Continuous-wave (CW) and pulsed -switched regimes of laser operation are investigated in this experiment. First, measurements of the laser power are made as a function of the arc lamp current. For the -switched regime, the pulse width and the power as a function of the repetition rate are also measured. They use a knife-edge mounted on a translation stage to measure the beam profile both close and far away from the laser. Students can compare these data to that measured by a computer-controlled beam profiler, based on a charge-coupled device camera. Then they observe 2National

Instruments Corporation, Austin, TX, http://www.natinst.com East Setauket, NY, http://www.quantron.com/

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Fig. 2. Schematic diagram of the optical waveguiding experiment. Light from a single line Ar or HeNe laser is focused into the waveguide via a prism. The waveguide consists of a quartz substrate with a Ge-doped film of several microns deposited on one side. The transmitted light is focused onto the photodetector. A Newport motion controller PMC200-P is used to rotate the stage (manually or from the computer). A computer records the detected light intensity and the rotation angle. The polarization of light can be rotated by the half wave plate to excite either transverse electric (TE) or transverse magnetic (TM) modes.

laser relaxation oscillations and measure the frequency of these oscillations and the dependence on pump power. Finally, the students observe production of second harmonic light in a LiNbO crystal for both the CW and pulsed regimes. Students learn about the concept of the threshold of the laser operation and the relationship between gain and loss. They are asked to compare the average and peak powers for CW and pulsed regimes. The overall electrical to laser power efficiency is calculated. From the decrease in peak-pulsed laser power as a function of repetition rate, the energy storage time of Nd : YAG is also calculated. The beam profile is calculated from the knife edge experiments, and the beam divergence is determined. The students are asked to compare their results to a simple model of diffraction. They learn about transverse laser modes. They also observe a large increase in green light produced by second harmonic generation, when -modulation is switched on as compared to CW operation. The conversion of infrared into green light serves as an introduction to nonlinear optics that is utilized in the last two experiments. D. CO Laser Characterization In this experiment, the students learn how a CO laser operates [4]. The apparatus is shown in Fig. 4. The students are asked to measure the output power as a function of discharge current, various gas mixtures, and total pressure. The laser typically operates in the TEM radial doughnut mode, and the students are asked to observe this pattern and identify it. The output power is measured using output couplers with transmissions between 50

KNIZE et al.: UNDERGRADUATE LASER PHYSICS LABORATORY

Fig. 3. Schematic diagram of the Nd : YAG laser radiation kinetics experiment. Part of a laser beam is directed by beamsplitter (BS) to PD and analyzed by an oscilloscope and spectrum analyzer. The other part of the beam is either measured by an absolute power meter or directed by mirror M into the nonlinear crystal (LiNbO ) for second harmonic generation. The HeNe laser is used for alignment of the Nd : YAG laser cavity.

and 95%. Finally, the output wavelength is measured with the spectrum analyzer. To understand how the laser operates, the students need to learn about the quantum mechanics of the different vibrational and rotational modes of the CO molecule. They learn the importance of the three gases, CO , N , and He, in optimizing the output power. The students are introduced to different transverse laser cavity modes. From the measurements with different output couplers, they determine the optimal output mirror and can calculate the gain and loss coefficients and the saturation intensity for the laser [5]. The output wavelength can be used to determine which vibrational and rotational modes of the CO molecule is lasing. E. Ultrashort Light Pulses Generation and Measurement Technique The mode-locked Nd : YAG laser and the optical measurement of ultrashort light pulses are studied in this experiment [6]. The laser used in this experiment4 can produce pulses with width of about 150 ps. First, the students investigate the four operation regimes of this laser: continuous wave, -switched, active mode locking, and simultaneous -switched and modelocking ones. The students measure the average power, the pulse duration, and the peak power for the different regimes. An important part of this experiment is the optical correlator measurement of the time duration of short light pulses. The correlator is designed similar to a Michelson interferometer in which one of the two beams has a variable time delay with respect to the other beam (see Fig. 5). The two beams are mixed in the nonlinear KTP crystal, and the intensity of the second harmonic is proportional to the product of the instantaneous intensities of the overlapping input beams. A computer and LabView-based software are used for controlling the optical correlator and for storing and analyzing the experimental data. 4Quantronix

Model 416 laser.

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Fig. 4. Schematic diagram of the CO laser experiment. The HeNe laser is used for aligning CO laser cavity. The gas flow control system can supply CO , N , and Ne at various pressures. The transmission of the output couplers can be changed and output measured with the power meter. The infrared spectrometer is used to measure the radiation wavelength.

Fig. 5. Schematic diagram of the autocorrelator for the ultrashort pulses generation and measurement experiment. The input beam is split by the BS in two beams (A and B). The A beam is reflected back by mirror M3. The B beam passes through the variable optical delay: retroreflecting prism (RR) mounted on the travel translation stage (TTS) with step motor driven by computer. These two beams are focused by lens (L) into nonlinear crystal (NC). The frequency doubled light (after filter F) is measured by the photodetector (PD1) and the reference signal by photodetector (PD2).

The observed green light power as a function of optical delay distance is described by the correlation function of the intensities of the two beams. The students use the computer to determine which function, i.e., Gaussian, Lorenztian, rectangular, etc., best describes the data. They determine the width of the laser pulses. The students also study the principles of -switching and mode locking. F. Optical Harmonics Generation Nonlinear optical effects such as second harmonic generation (SHG) and optical mixing are studied in this experiment [7]. Nonlinear optical crystals can be used to sum and difference optical frequencies to create light with different wavelengths. The setup is shown in Fig. 6. A -switched pulsed Nd : YAG laser5 is used in this experiment. Students use three potassium dihydrogen phosphate (KDP) crystals of 0.5, 2, and 4 cm 5Quanta-Ray DCR-3, Spectra-Physics, http://www.spectra-physics.com

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V. LASER SAFETY The use of lasers in an undergraduate laboratory always has the associated risk of laser damage to the eye [8]. Therefore, laser safety is stressed frequently during the semester. At the beginning of the course, a short lecture on laser safety and the use of protective eyewear is presented. VI. CONCLUSION

Fig. 6. Schematic diagram of the optical harmonics generation experiment. Nonlinear crystals for second (SHG) and third (THG) harmonics generation can be installed into the rotation stages (RS1 and RS2 correspondingly) and tuned to the phase-matching directions (RS1 is computer controlled, and RS2 is manually controlled). BSs are used to direct a part of corresponding fundamental and harmonics radiation to the photodetectors (PD1, PD2, or PD3). The photodetectors are absolutely calibrated using the absolute power meter. Filters (F) are used to separate corresponding harmonics.

lengths and a 2-cm LiNbO crystal for studying the SHG effect. The 4-cm-long KDP crystal is used to obtain the third harmonic by sum frequency generation of the fundamental and second harmonic. The students study the phase-matching conditions for SHG, where the fundamental and the second harmonic waves travel inside the crystal at the same velocity. They measure the second harmonic power as a function of the angle between the beam and the optical axis. Then they study the influence of the crystal type, its length, and its input power on the efficiency of SHG. They mix the second harmonic with the fundamental to generate the third harmonic in another KDP crystal that can also be rotated to achieve phase matching. They measure the third harmonic power as a function of fundamental input power. From the data, the students learn that the second harmonic power is dependent on the square of the input power. They also learn that at the phase-matching angle, the power varies quadratically with the length. Comparison of results allows determination of the relative second-order nonlinear susceptibility of LiNbO as compared to KDP. The absolute value of the second harmonic coefficients can also be estimated for these materials. Finally, they determine that third harmonic generation is less efficient and depends on the third power of the input intensity.

The students have generally enjoyed this course. Even though they may have had little prior education in lasers, they have been able to research and learn the basic concepts required for understanding the experiments. The authors have been able to incorporate concepts from their other theoretical courses into the experiments, and the connection between the various experiments has been extremely beneficial. Students accepted increased responsibility for their learning, and the report, review, and listen cycle proved to enhance comprehension considerably. REFERENCES [1] A. Yariv, Quantum Electronics, 2nd ed. New York: Wiley, 1975, pp. 327–370. [2] R. Ulrich and R. Torge, “Measurement of thin film parameters with a prism coupler,” Appl. Opt., vol. 12, p. 2901, 1973. [3] W. Silfvast, Laser Fundamentals. New York: Cambridge Univ. Press, 1996, pp. 449–455. [4] C.K.N. Patel, “High-power carbon dioxide lasers,” Sci. Amer., vol. 219, p. 22, Aug. 1968. [5] W. W. Rigrod, “Homogeneously broadened CW lasers with uniform distributed loss,” IEEE J. Quantum Electron., vol. QE-14, p. 377, 1978. [6] O. Svelto, Principles of Lasers, 4th ed. New York: Plenum, 1968, pp. 305–364. [7] P. W. Milonni and J. H. Eberly, Lasers. New York: Wiley, 1988, pp. 635–641. [8] H. Weichel, W. A. Danne, and L. S. Pedrotti, “Laser safety in the laboratory,” Amer. J. Phys., vol. 42, pp. 1006–1013, 1974.

Randall J. Knize received the B.A. degree from the University of Chicago, Chicago, IL, in 1975 and the Ph.D. degree from Harvard University, Cambridge, MA, in 1981, both in physics. Subsequently, he was with Princeton University, Princeton, NJ, until 1988 as a Research Physicist. He was an Assistant Professor at the University of Southern California from 1988 to 1996. In 1996, he joined the U.S. Air Force Academy as an Associate Professor of physics. In 2000, he became a Professor. His areas of interest include lasers, nonlinear optics, and atomic physics.

IV. GRADED WORK Every student is required to present two oral presentations, three written reports, and one poster presentation. The authors try to have each student in the group do a different type of presentation for each experiment, if possible. Students are able to learn from each other through the oral presentations. After the second written report, the paper is given to another student for peer review. The student is graded on the quality of the peer review report. The students are then required to incorporate the instructor’s and peer’s comments into a revised paper. Posters are presented at the end of the semester to the course instructors and to other physics faculty. The requirements for written, oral, and poster presentations develop professional skills that are useful in future academic and professional environments.

W. Roc White received the B.S. degree in physics and mathematics from the U.S. Air Force (USAF) Academy, Colorado Springs, CO, in 1976 and the M.S. and Ph.D. degrees from the Air Force Institute of Technology, Wright-Patterson AFB, OH, in 1984 and 1989, respectively. He was an Assistant Professor of physics at the USAF Academy from 1990 to 1993, then an Associate Professor. In 1998, he became a full Professor of physics and was Deputy Department Head. His area of interest is in lasers and nonlinear optics. Currently, he is with Aerospace Corporation, Colorado Springs, CO.

Boris V. Zhdanov received the Master degree in physics and the Ph.D. degree from Moscow State University, Moscow, Russia, in 1970 and 1975, respectively. He became an Assistant Professor at Moscow State University in 1979 and an Associate Professor in 1988. He was a Visiting Professor at the U.S. Air Force Academy, Colorado Springs, CO, from 1998 to 2000. His area of interest is in laser physics and nonlinear optics.