# MNO - Nonlinear Spectroscopy and Magneto-Optics

## Non-Linear Laser Spectroscopy and Magneto Optical Description (MNO)

1. Note that there is NO eating or drinking in the 111-Lab anywhere, except in rooms 282 & 286 LeConte on the bench with the BLUE stripe around it. Thank You the Staff.

This is an experiment on nonlinear laser spectroscopy and magneto-optics at the 111 Advanced Undergraduate Laboratory at Berkeley. The experiment consists of three parts. In the first part, students learn to operate a diode laser system and characterize its performance using a Fabry-Perot spectrum analyzer. In the second part, Doppler-broadened laser-induced fluorescence and Doppler-free saturated absorption spectra of the rubidium D2 line (780 nm) are recorded and analyzed. Finally, in the third part of the experiment, the near-resonant magneto-optical rotation is investigated. Nonlinear light-atom interaction leads to spectacular manifestations of the resonant Faraday effect - polarization plane rotation in a magnetic field applied along the direction of light propagation radically different from the linear case. In particular, narrow (~30 Hz) effective line widths are observed in this experiment corresponding to a rotation enhancement by some seven orders of magnitude compared to the linear Faraday rotation.

1. Pre-requisites: OPT, Physics 137AB (137B may be taken concurrently)
2. Days Alloted for the Experiment: 9
3. Consecutive days: Yes

All pages in this lab. Note To print Full Lab Write-up click on each link below and print separately

III. Tuning the Laser

Reprints and reading materials can be found on the Physics 111 Library Site

This lab will be graded 30% on theory, 50% on technique, and 20% on analysis. For more information, see the Advanced Lab Syllabus.

Acknowledgment and Disclaimer. This material is based upon work supported by the National Science Foundation under Grant No. 9750873. "Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF)."

Equipco Rentals of Concord, California donated two Newport Corporation, Research Series Optical Tables with Pneumatic Vibration Isolation mounts and table stands to our Physics Department. Equipco provides rentals, sales, and service of scientific instruments. Visit the Equipco Environmental Equipment Rental Website for more information about their products or call direct at 1-888-234-5678 if you have a specific need. They have a wide selection of instruments available for donation to UC Berkeley.

Laser Restrictions. The laser in this experiment beginning in Fall 2010 is a prototype being used in the development of an improved instructional experiment for the Physics 111 lab. The laser will not be distributed to another user. Installation and use of the laser in the experiment will be under the supervision of the U.C. Berkeley Laser Safety Officer to assure that personnel are properly trained and are not exposed to dangerous levels of radiation. This laser will be held by the U.C. Berkeley Physics Department for testing purposes, after which it will be destroyed.

D. Budker, D. Orlando, V. Yashchuk

# Before the 1st Day of Lab

Complete the following before your experiment's scheduled start date:

1. View the two videos Introduction, Part-1 video and the Introduction, Part-2 video.
2. Before using the apparatus in this experiment, you must complete training in the safe use of lasers detailed on the Laser Safety Training page. This includes readings, watching a video, taking a quiz, and filling out a form.
3. Complete the MNO Pre Lab and Evaluation sheets. Print,  fill it out and turn in your answers with the sheet. The Pre-Lab must be printed separately. Discuss the experiment and pre-lab questions with any faculty member or GSI and get it signed off by that faculty member or GSI. Turn in the signed pre-lab sheet with your lab report.
4. View the optics tutorial, a review of the principles of optics, Fundamentals of Optics Tutorial and the Optical Instruments Video, Energy Levels (part-1) Video and Energy Levels (part-2) Video.
5. Last day of the experiment please fill out the Experiment Evaluation

1. Technical description and instruction manual of the extended cavity diode laser: ECDL-7840R
2. Fabry-Perot Spectrum Analyzers: "Technical Memorandum on Fabry-Perot Interferometry". Burleigh Instruments. Use this article to get some definitions and formulas. Most intermediate optics texts have a detailed analysis of how the fringes are formed, and a derivation of the transmission function
3. Fabry-Perot Interferometer Instructions. "Fabry-Perot Interferometer Instructions". A guide to the use and settings of the FP Interferometer used in this experiment.
4. D. W. Preston, "Doppler-free saturated absorption spectroscopy: Laser spectroscopy", Am. J. Phys. 64(11), (1996). p. 1432-1436. (A fairly good introductory article (none better around).) Searchable Page
5. Saturation Spectroscopy: D. W. Preston and C. E. Wieman, "Doppler-free saturated absorption: laser spectroscopy". An advanced laboratory write-up (09/1994; unpublished), p. 1-32. A more detailed version of the above.
6. Diode Lasers: C. E. Wieman and L. Hollberg, "Using Diode Lasers for Atomic Physics", Rev. Sci. Instrum. 62 (1), January 1991; p. 1-21. (A good article if you want to know how diode lasers work and how they are used.)
7. D. Budker, D. J. Orlando, and V. Yashchuk, "Nonlinear Laser Spectroscopy and Magneto-Optics", Am. J. Phys. 67, 584 (1999). (Covers the physics in the latter part of the lab.)
8. Michael Lang. "External Cavity Designs Satisfy Stringent Demands"; Laser Focus World; June 1996, Pg. 1432-1436;
9. T. J. Sumner, J. M. Pendlebury, and K. F. Smith "Conventional Magnetic Shielding"; J. Physics D. Applied Physics 20 (1987) Pg. 1095-1101
10. L. M. Barkov, D. A. Melik-Pashayev and M. S. Zolotorev "Non-Linear Faraday Rotation in Samarium Vapor"; Optics Communications Vol. 70, No. 6, 15 April 1989
11. Dmirty Budker, Valeriy Yashchiek, and Max Zolotorev "Resonant Magneto-Optical Rotation: New Twists in an Old Plot"; Nuclear Science Division Dec 1997; LBNL-41149, UC-000 Preprint
12. University of Florida, Department of Physics, Advanced Physics Lab, Saturated Absorption Spectroscopy
13. Information about the Lock-In Amplifier Used in this lab Lock-in Amp,Lock-ins

Other reprints and reference materials can be found on the Physics 111 Library Site

You should keep a laboratory notebook. The notebook should contain a detailed record of everything that was done and how/why it was done, as well as all of the data and analysis, also with plenty of how/why entries. This will aid you when you write your report.

## Objectives

• Learn what real experimental physics is about
• Learn the synergy between experimental and theoretical work
• Learn to use pieces of equipment that are commonly used in research
• Learn how measurements are performed, analyzed, and interpreted.
• Learn how to present your work and results
• Learn problem solving strategies
• Learn how to manage and organize your time

# Note on the intent and scope of this laboratory manual

This manual is intended to provide a general guidance for the lab. It is not designed to explain all the physics necessary for understanding this experiment, nor is it a "cook-book" telling you exactly which buttons to press, etc. It is up to the student to become familiar with the necessary material in the reprints, and to figure out the exact procedure necessary for completion of the assignments in the lab. Talk with an instructor often, but not until you have given the physics and procedures some thought first. We are here to instruct and help, but not to do your thinking for you.

## General

1. W. Demtroder. Laser Spectroscopy. Springer, 1996.
2. Optics Tutorial

# Note on Safety (please do not ignore)

There are two major sources of danger in this experiment: laser radiation (780 nm, 45 mW continuous wave) and moderately high voltages present on some of the equipment (250 VDC at the scanning confocal Fabry-Perot (F-P) spectrum analyzer). The onset of permanent damage to the eye is at />5 mW. Any potential difference greater than 30 volts is potentially shocking. Please observe the following simple rules that will help you stay out of harm's way:

1. When the laser is on, always wear the protective safety eyewear provided. The laser goggles available for this experiment absorb the infrared (IR) radiation from the laser and transmit visible light. Therefore, they will not hinder general visibility. While you are working on beam alignment you can use the blue night-vision viewer converting IR into visible light to see the laser beam without any danger. Please refer to Appendix IV about optics.
2. A general rule for laser operators is to remove watches and jewelry because laser beams can get accidentally reflected in an uncontrolled fashion from watches or rings when a hand crosses the laser beam path.
3. Make sure the curtains are closed when using the laser and before leaving.
4. Do not look directly into the laser beam under any circumstances.
5. This is the most important rule: always use common sense and think before doing anything.

# Introduction

The laser spectroscopy has been useful in a variety of applications throughout the years. It has been used in medicine, chemistry, material science, environmental research and many other fields. Combustion processes, atmospheric monitoring, water and vegetation surveillance, and medical diagnostics are but a few applications of the broader spectrum.

The "Nonlinear Spectroscopy and Magneto-optics" experiment is a good introduction to laser spectroscopy. The experiment itself will consist of three parts whose set up is fairly simple. In the first part you will learn a little about the diode laser and its operation. As you get familiar with the laser's operation, you will test the laser's performance using the Fabry-Perot spectrum analyzer. In the second part of the experiment you will record and analyze Doppler-broadened laser induced fluorescence and Doppler-free saturated absorption spectra of the rubidium. In the third part of the experiment we will investigate the near-resonant magneto-optical rotation. To get familiar with these concepts it is strongly recommended that you read the "Nonlinear Laser Spectroscopy and Magneto-Optics" article written by Dmitry Budker, Donald Orlando, and Valery Yashchuk. This article covers everything you need to know to complete this experiment. It is provided in the reprints for your convenience.

# Apparatus

If you can't find something, feel the equipment is faulty or needs to be aligned, or have any other issues of the sort, talk to Don.

*DO NOT TOUCH THE FOLLOWING KNOBS ON THE LASER CONTROLLER*

1. SWEEP KNOB
2. SCAN KNOB
3. OFFSET KNOB

In this lab we are going to use an ECDL-7840R Extended Cavity Diode Laser. This is a tunable, narrow line width (100 kHz) laser that has the capability of changing the wavelength range internally. Please be very gentle with this unit and do not turn any knobs unless instructed to do so. This laser system is very expensive! Do not touch the sweep or scan knobs on the laser unless absolutely necessary and never adjust the temperature without instructor assistance.

• Note: There are overhead lights to help you see the equipment--the switch for the lights is located on the top rack near the door. At the end of each lab day, be sure to power off all equipment, turn off the overhead lights, and close the curtains on all sides to keep the station clean.

### ​ECDL-7840R Laser and Electronic Control Unit (ECU)

The ECDL-7840R Laser Input is connected to ECDL-7840R Electronic Control Unit (ECU) Laser Output. The ECU allows you to control the lasers' current input, wavelength, and temperature.The ECU is connected to the laser head via a gray cable. The 1:10 divider allows you to "zoom in" on your spectrum for finer detail. The 1:10 divider is a small brown box that should be located on the desk with the computer (put back when not in use) and has a "$\div$10" label.

The ECDL-7840R (Laser) changes wave length in three ways. First by adjusting the distance of an internal cavity in the laser head with a piezo electric device (PZT). Second, by varying the applied current to the diode itself and third by controlling the temperature of the diode. The temperature of the diode should not need to be adjusted, if you think you need to change the temperature talk to Don Orlando, though almost certainly temperature is not the problem. The ECU front panel is broken up into three sections, current, thermo, and PZT controls. The sweep knob (in the current section) sets the allowed current range when being scanned from some set center current. To set the center current the level knob is used. Note: the LCD current indicator does not keep up with the sweep rate so it is only useful to display the center current, this means it is possible to sweep above the current limit without the indicator displaying this, please monitor the current limit LED instead. The PZT control section is similar to the current section. The scan knob changes the range that the PZT is scanned over and the offset changes the center wavelength the PZT scans. On the back of the ECU there is an EXT input (labeled PZT input) that is used to connect the ECU to the computer or signal generator. Once connected, the computer/signal generator can adjust the current/PZT. There is an INT/EXT switch that switches from the external source to an internal 30 Hz triangle wave. This switch should be set to EXT for the entire experiment.

### Optical Setup

The optical set up for this experiment is fairly simple. The laser beam coming from the ECDL-7840R LASER passes through a Diaphragm Iris and is split into several beams so that we can set up all three parts of the experiment using one beam at all times. The splitting of the beam is done using the beam splitters. The beam splitters in our lab are just pieces of microscopic cover glass attached to the adjustable mirror mounts. The optical set up for this experiment is shown in the Figure 1. As you read this section, refer to both the diagram in Figure 1 and also the actual optical set up on the table. Keep in mind that as you follow the actual setup, you must be careful not to touch anything. Even slightest movement of the mirrors, beam splitters and other parts will result in misalignment of the experiment not to mention that you will leave fingerprints all over the mirrors and beam splitters. If that happens, you will create more tedious work for yourself and others trying to align the components.

The first beam splitter (BS-1 as labeled in Figure 1) is used to split the incoming laser beam into two beams. One of the beams will be used for the Fabry-Perot spectrum analyzer. The other beam will be split by BS-2 and BS-3 to be used in Laser induced fluorescence and Doppler-free saturation spectroscopy. Finally, the laser beam coming out of BS-3 will be split again by BS-4 for the magneto-optical part of the experiment. The optical setup of each individual part of the experiment will be discussed in greater detail in the procedure section. Roughly speaking, the laser beam coming out of the BS-1 for the Fabry-Perot part of the experiment passes through the Diaphragm Iris and goes into the Fabry-Perot spectrum analyzer. The output of the spectrum analyzer is connected to the photodiode sensor (PD-4). The laser beam coming from BS-2 for the laser-induced fluorescence and Doppler-free saturation spectroscopy part of the experiment passes through the optical chopper and then through the center of a Rubidium cell. A second beam coming from BS-3 is reflected by a mirror, M1 as shown in Figure 1. The reflected beam also passes through the rubidium cell but in the opposite direction. This beam is then reflected by another mirror, M2, and is sent to a Photodiode sensor, PD-3. Another photodiode sensor, PD-5 is placed next to a rubidium cell as shown in Figure 1. Finally, in the magneto-optical part of the experiment, the laser beam coming from BS-4 is passed through two polarizers. It is then passed through another rubidium cell that is placed inside magnetic coils (the cell with coils is shielded). The laser beam exits the rubidium cell and enters the polarimeter (PBS) that splits the beam into two. The two beams enter two photodiode sensors, PD-1 and PD-2. On the side of the shielded cell with coils is located another photodiode, PD-6.

Check Point: Follow the path of the laser and explain the various components of the optical setup. Where is the magneto-optical part of the experiment, the rubidium cell that is placed inside shielded magnetic coils?

Figure 1: Diagram of the optical set-up

### Photodiode circuit

In this lab, we use several silicon photodiodes to measure the intensity of laser beams and of atomic fluorescence light. The simple circuitry that we use (the photodiode bias and load resistor box) is shown in Figure A2. Here, the photodiode operates in the photoconductive mode in which a reverse bias is applied to the diode and no current is flowing through the circuit in the absence of light (neglecting the dark current, which actually becomes important in small-signal applications). In the absence of current, there is also no output voltage across the load resistor. When a photon strikes the photodiode, it creates an electron-hole pair in the conductivity band in the carrier-depleted zone of the photodiode's pn-junction. The quantum efficiency of this process may reach nearly 100 %, meaning that there is one pair produced per incident photon. The charges then flow through the load resistor upon the action of the bias voltage, thus producing output voltage.

Fig. A2.

Schematic of the photodiode bias and load resistor box

PhotoDiode-10DP data Sheet

All six photodiode sensors are connected to photodiode load resistors. The PD-5 used in the Diode Scan section of the lab is connected to a 1MΩ PD Load Resistor. The output of that load resistor is then connected to AI 2 input of the NI BNC 2120 Breakout Box (located behind the computer screen, you should be familiar with this DAQ box from 111a). The BNC 2120 Breakout Box is used to input BNC cable data to the DAQ card installed in the computer. See the diagram of the electrical connections in Figure 2 and list of connections in Table 1. Figure 2 shows the connections for the Diode Scan part of the experiment and B-Field scan, although all connections are not used for both parts.

Referring to Figure 2, you can see that the PD-4 is connected to the 100KΩ Load Resistor whose output is connected to AI 0 of the Breakout Box. The PD-3 is also connected to 1MΩ Load Resistor whose output will be connected at some point of the experiment to SR830 DSP Lock-In amplifier (do not connect the PD Load resistor to the Lock-in yet). Lock-in amplifiers are used to detect and measure very small AC signals. Accurate measurements can be made even when the small signal is obscured by noise sources. Lock in amplifiers use a technique known as phase-sensitive detection to single out the component of the signal at a specific reference frequency and phase. Noise frequencies other than the reference frequency are rejected and do not affect the measurement. In our experiment we will connect Channel 1 Output of the Lock-in to the AI 1 Input of the Breakout Box via Cable 2. (The switch for the lock-in amplifier is in the back of the unit). The Ref In of the Lock-in is connected to the SR 540 Chopper Controller's "f"/f diff Input. The SR 540 Chopper Controller controls the optical chopper that chops the laser beam coming from BS-2. The PD-2 is connected to the 100kΩ Load resistor whose output goes to AI 4 of the Breakout Box. The PD-1 is also connected to the 100kΩ Load Resistor whose output is connected to AI 3 of the Breakout Box.

In the Diode Scan section of the lab, the AO 0 of the Breakout box should be directly connected to the Input of the ECDL-7840R. When doing the B-Field Scan, the AO 1 should be connected to the magnetic coils in either of the two ways. One way is to connected it through a 2.5 kΩ Load Resistor (Little Blue Box). Second way ( Highly recommended ) is to connect it to Input 1 of the COIL DRVER via Cable 3. The COIL DRIVER is located on the rack above the magnetic coils (shielded metal chamber). The COIL DRIVER OUTPUT 1 should be connected to the magnetic coils. The apparatus is already setup for the second way, and you shouldn't need to change it. The Monitor 1 input of the Coil Driver should be monitored by a DMM. The 2.5 kΩ resistor is there so that you can also "zoom-in" on your spectrum for better detail. A chart of the connections from the DAQ is below in Table 1. The cable numbers are on a label on each cable.

Table 1: List of connections involving the DAQ

Figure 2: Block Diagram of the electrical connections for the Diode Scan part.