IEEE TRANSACTIONS ON EDUCATION, VOL. 45, NO. 3, AUGUST 2002 227

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

Abstract—The authors have developed a senior-level under-graduate laboratory course with an integrated theme of lasersand optics to teach experimental methods and reinforce funda-mental physics concepts. This laboratory includes six experimentsconcerning modulators, laser kinetics, waveguiding, CO2 laseroperation, ultrashort pulse characterization, and nonlinear optics.The authors discuss the philosophy and structure of this course.

Index Terms—Instruction, lasers, modulators, optics, secondharmonic generation.

I. INTRODUCTION

T HE SENIOR undergraduate laboratory course servesas the capstone experimental course for undergraduate

physics students. In the past, students were asked to do anumber of experiments involving several disciplines, such asnuclear, plasma, laser, and solid-state physics. In the last fewyears, however, this course has been restructured to createa more integrated curriculum using lasers and optics as thecentral theme.

The authors developed this laser laboratory course to accom-plish several goals. First, they selected experiments in lasers andoptics because of student and Air Force interests in this field.The presence of lasers and optics in military and civilian appli-cations is widespread and continues to grow. The authors alsowanted this course to tie in concepts from other physics courses,such as quantum mechanics, electricity and magnetism, statis-tical mechanics, and solid-state physics. Laboratory experiencewith lasers and optics reinforces ideas learned in these coursesand allows for spiral learning of important concepts.

The authors want the course to help students develop criticalthinking and independent thought. They do not use a text-book. Usually the students have not had any previous coursesinvolving lasers. The authors give general guidelines only onthe experiments and allow the students to have freedom in de-termining the outcome. The work required of the students andthe subsequent grading system, discussed below, help developgood oral and written presentation skills that are useful laterin life. Overall, this course bridges the gap between textbooklearning and independent learning required for research.

Manuscript received August 30, 1999; revised January 19, 2002. This workwas supported by the Air Force Office of Scientific Research, the U.S. AirForce 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 ofPhysics, U.S. Air Force Academy, CO 80840 USA.

W. R. White was with the Laser and Optics Research Center, Departmentof Physics, U.S. Air Force Academy, CO 80840 USA. He is now with theAerospace Corporation, Colorado Springs, CO 80910 USA.

B. V. Zhdanov was with the Laser and Optics Research Center, Departmentof Physics, U.S. Air Force Academy, CO 80840 USA. He is now with DirectedEnergy Solutions, Inc., Colorado Springs, CO 80921 USA.

Publisher Item Identifier S 0018-9359(02)05057-4.

II. COURSEOUTLINE

Every student must complete six selected experiments inlasers and optics: 1) optical modulators, 2) optical waveguiding,3) Nd : YAG laser kinetics, 4) COlaser characterization, 5)ultrashort laser pulse measurements, and 6) optical harmonicsgeneration.1 These experiments were designed so that con-cepts learned in one experiment will be used in subsequentexperiments.

Before each lab, the students receive a page of learning ob-jectives, which includes references. At the beginning of eachlaboratory, the students also receive a list of prelab questionsthat they must answer and turn in by the second lesson of thatparticular lab. During the first lesson of each lab, the studentsare given an overview of the apparatus and some backgroundon the theory behind each experiment. The students are able tocomplete the experiment in about ten classroom hours.

III. EXPERIMENTS

Students are required to complete six experiments in thesemester.

A. Optical Modulators

In this experiment, the students study the principles of modu-lation of light [1]. Modulators have important technological ap-plications in fiber optics and optical communications. The setupis shown in Fig. 1. In the first part of the experiment, the elec-trooptical modulator is studied. Applying a voltage to the crystalresults in changing of the indexes of refraction and, hence, thepolarization of the transmitted light. From this part of the lab,the students develop a good understanding of polarization andhow birefringent materials can change the optical polarization.From the data, they calculate the value of the electrooptical co-efficient for LiNbO . They are expected to compare their resultswith accepted published values.

The acoustooptical modulator (AOM) is then studied. Asound wave, inside the AOM quartz crystal, is created bya piezoelectric transducer resulting in a volume diffractiongrating. The students measure the Bragg diffraction angle andits dependence on the acoustic wave frequency. They alsomeasure diffraction efficiency dependence on the acousticwave power. The students understand how sound waves in amaterial can cause diffraction similar to a grating. Using theexperimental data, they calculate the speed of sound in quartz.

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

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228 IEEE TRANSACTIONS ON EDUCATION, VOL. 45, NO. 3, AUGUST 2002

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

B. Optical Waveguiding

This experiment examines the principles of optical waveg-uiding [2]. Light can be guided in a medium surrounded bymaterials of lower indexes of refraction. This is the principleused in fiber optics. The apparatus is shown in Fig. 2. The ex-periment consists of two parts: the physics of waveguiding andwriting a simple LabView2 program to control the experiment.The index of refraction of the thin film is several percent largerthan that of the substrate because of the Ge doping. Light willbe waveguiding only if the proper boundary conditions are sat-isfied, which depend on the wavelength, polarization, and inci-dent angle. The students first manually rotate the stage holdingthe waveguide and observe that at about six or seven angles (forour particular film), the light is waveguided down the thin film.They then spend several lessons developing a LabView pro-gram, which will automatically rotate the stage and measure thetransmitted light. The program is used to take data for severaldifferent wavelengths and for both laser polarizations.

The students learn that boundary conditions are important inthe propagation of an electromagnetic wave in a one-dimen-sional waveguide. They develop an appreciation for the pro-gramming necessary to control an experiment. Analysis of thedata allows them to determine the index of refraction, its disper-sion, and the thickness of the doped film.

C. Nd : YAG Laser Radiation Kinetics

The purpose of this experiment is to acquaint students witha solid-state Nd : YAG laser3 and measurement of various laserparameters [3]. The setup is shown in Fig. 3. Continuous-wave(CW) and pulsed -switched regimes of laser operation are in-vestigated in this experiment. First, measurements of the laserpower are made as a function of the arc lamp current. For the

-switched regime, the pulse width and the power as a functionof the repetition rate are also measured. They use a knife-edgemounted on a translation stage to measure the beam profile bothclose and far away from the laser. Students can compare thesedata 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.com3Quantronix, East Setauket, NY, http://www.quantron.com/

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

laser relaxation oscillations and measure the frequency of theseoscillations and the dependence on pump power. Finally, the stu-dents observe production of second harmonic light in a LiNbOcrystal for both the CW and pulsed regimes.

Students learn about the concept of the threshold of the laseroperation and the relationship between gain and loss. They areasked to compare the average and peak powers for CW andpulsed regimes. The overall electrical to laser power efficiencyis calculated. From the decrease in peak-pulsed laser power as afunction of repetition rate, the energy storage time of Nd : YAGis also calculated. The beam profile is calculated from the knifeedge experiments, and the beam divergence is determined. Thestudents are asked to compare their results to a simple model ofdiffraction. They learn about transverse laser modes. They alsoobserve a large increase in green light produced by second har-monic generation, when -modulation is switched on as com-pared to CW operation. The conversion of infrared into greenlight serves as an introduction to nonlinear optics that is utilizedin the last two experiments.

D. CO Laser Characterization

In this experiment, the students learn how a COlaser oper-ates [4]. The apparatus is shown in Fig. 4. The students are askedto measure the output power as a function of discharge current,various gas mixtures, and total pressure. The laser typically op-erates in the TEMradial doughnut mode, and the students areasked to observe this pattern and identify it. The output power ismeasured using output couplers with transmissions between 50

KNIZE et al.: UNDERGRADUATE LASER PHYSICS LABORATORY 229

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

and 95%. Finally, the output wavelength is measured with thespectrum analyzer.

To understand how the laser operates, the students need tolearn about the quantum mechanics of the different vibrationaland rotational modes of the COmolecule. They learn the im-portance of the three gases, CO, N , and He, in optimizingthe output power. The students are introduced to different trans-verse laser cavity modes. From the measurements with differentoutput couplers, they determine the optimal output mirror andcan calculate the gain and loss coefficients and the saturationintensity for the laser [5]. The output wavelength can be used todetermine which vibrational and rotational modes of the COmolecule is lasing.

E. Ultrashort Light Pulses Generation and MeasurementTechnique

The mode-locked Nd : YAG laser and the optical measure-ment of ultrashort light pulses are studied in this experiment[6]. The laser used in this experiment4 can produce pulses withwidth of about 150 ps. First, the students investigate the fouroperation regimes of this laser: continuous wave,-switched,active mode locking, and simultaneous-switched and mode-locking ones. The students measure the average power, the pulseduration, and the peak power for the different regimes. An im-portant part of this experiment is the optical correlator measure-ment of the time duration of short light pulses. The correlatoris designed similar to a Michelson interferometer in which oneof the two beams has a variable time delay with respect to theother beam (see Fig. 5). The two beams are mixed in the non-linear KTP crystal, and the intensity of the second harmonic isproportional to the product of the instantaneous intensities ofthe overlapping input beams. A computer and LabView-basedsoftware are used for controlling the optical correlator and forstoring and analyzing the experimental data.

4Quantronix Model 416 laser.

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

Fig. 5. Schematic diagram of the autocorrelator for the ultrashort pulsesgeneration and measurement experiment. The input beam is split by the BS intwo beams (A and B). The A beam is reflected back by mirror M3. The B beampasses through the variable optical delay: retroreflecting prism (RR) mountedon the travel translation stage (TTS) with step motor driven by computer. Thesetwo beams are focused by lens (L) into nonlinear crystal (NC). The frequencydoubled light (after filter F) is measured by the photodetector (PD1) and thereference signal by photodetector (PD2).

The observed green light power as a function of opticaldelay distance is described by the correlation function of theintensities of the two beams. The students use the computer todetermine which function, i.e., Gaussian, Lorenztian, rectan-gular, etc., best describes the data. They determine the widthof 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 differenceoptical frequencies to create light with different wavelengths.The setup is shown in Fig. 6. A -switched pulsed Nd : YAGlaser5 is used in this experiment. Students use three potassiumdihydrogen phosphate (KDP) crystals of 0.5, 2, and 4 cm

5Quanta-Ray DCR-3, Spectra-Physics, Mountain View, CA,http://www.spectra-physics.com

230 IEEE TRANSACTIONS ON EDUCATION, VOL. 45, NO. 3, AUGUST 2002

Fig. 6. Schematic diagram of the optical harmonics generation experiment.Nonlinear crystals for second (SHG) and third (THG) harmonics generationcan be installed into the rotation stages (RS1 and RS2 correspondingly) andtuned to the phase-matching directions (RS1 is computer controlled, andRS2 is manually controlled). BSs are used to direct a part of correspondingfundamental and harmonics radiation to the photodetectors (PD1, PD2, orPD3). The photodetectors are absolutely calibrated using the absolute powermeter. Filters (F) are used to separate corresponding harmonics.

lengths and a 2-cm LiNbOcrystal for studying the SHGeffect. The 4-cm-long KDP crystal is used to obtain the thirdharmonic by sum frequency generation of the fundamentaland second harmonic.

The students study the phase-matching conditions for SHG,where the fundamental and the second harmonic waves travelinside the crystal at the same velocity. They measure the secondharmonic power as a function of the angle between the beamand the optical axis. Then they study the influence of the crystaltype, its length, and its input power on the efficiency of SHG.They mix the second harmonic with the fundamental to gen-erate the third harmonic in another KDP crystal that can also berotated to achieve phase matching. They measure the third har-monic power as a function of fundamental input power.

From the data, the students learn that the second harmonicpower is dependent on the square of the input power. They alsolearn that at the phase-matching angle, the power varies quadrat-ically with the length. Comparison of results allows determi-nation of the relative second-order nonlinear susceptibility ofLiNbO as compared to KDP. The absolute value of the secondharmonic coefficients can also be estimated for these materials.Finally, they determine that third harmonic generation is less ef-ficient and depends on the third power of the input intensity.

IV. GRADED WORK

Every student is required to present two oral presentations,three written reports, and one poster presentation. The authorstry to have each student in the group do a different type of pre-sentation for each experiment, if possible. Students are able tolearn from each other through the oral presentations. After thesecond written report, the paper is given to another student forpeer review. The student is graded on the quality of the peer re-view report. The students are then required to incorporate theinstructor’s and peer’s comments into a revised paper. Postersare presented at the end of the semester to the course instruc-tors and to other physics faculty. The requirements for written,oral, and poster presentations develop professional skills that areuseful in future academic and professional environments.

V. LASER SAFETY

The use of lasers in an undergraduate laboratory always hasthe associated risk of laser damage to the eye [8]. Therefore,laser safety is stressed frequently during the semester. At thebeginning of the course, a short lecture on laser safety and theuse of protective eyewear is presented.

VI. CONCLUSION

The students have generally enjoyed this course. Even thoughthey may have had little prior education in lasers, they havebeen able to research and learn the basic concepts required forunderstanding the experiments. The authors have been ableto incorporate concepts from their other theoretical coursesinto the experiments, and the connection between the variousexperiments has been extremely beneficial. Students acceptedincreased 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 aprism 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 dis-tributed 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 labo-ratory,” 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 aResearch Physicist. He was an Assistant Professor at the University of SouthernCalifornia from 1988 to 1996. In 1996, he joined the U.S. Air Force Academyas an Associate Professor of physics. In 2000, he became a Professor. His areasof interest include lasers, nonlinear optics, and atomic physics.

W. Roc White received the B.S. degree in physics and mathematics from theU.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-PattersonAFB, OH, in 1984 and 1989, respectively.

He was an Assistant Professor of physics at the USAF Academy from 1990 to1993, then an Associate Professor. In 1998, he became a full Professor of physicsand was Deputy Department Head. His area of interest is in lasers and nonlinearoptics. Currently, he is with Aerospace Corporation, Colorado Springs, CO.

Boris V. Zhdanov received the Master degree in physics and the Ph.D. degreefrom Moscow State University, Moscow, Russia, in 1970 and 1975, respectively.

He became an Assistant Professor at Moscow State University in 1979 and anAssociate Professor in 1988. He was a Visiting Professor at the U.S. Air ForceAcademy, Colorado Springs, CO, from 1998 to 2000. His area of interest is inlaser physics and nonlinear optics.