Apparatus Designed and Built by Physicists who have taught in the Advanced Undergraduate Lab
Diode Laser Spectroscopy
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Observe Doppler-Free Spectroscopy of Rubidium Gas (Saturated Absorption)
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Michelson Interferometer Used to Calibrate Laser Sweep
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Observe Resonant Faraday Rotation in Rb Vapor
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Measure Temperature Dependence of Absorption and Dispersion Coefficients of Rb Vapor
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Lock Laser to Rubidium Hyperfine Transition
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Study Zeeman Splitting in Rb Spectrum at Two Wavelengths
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Study Stabilized Diode Laser Characteristics
Introduction
TeachSpin's Diode Laser Spectroscopy Lab is an affordable, student-friendly tunable laser system designed for exploring a wide range of atomic and optical physics phenomena in the undergraduate laboratory. Developed in collaboration with Professor Kenneth Libbrecht of Caltech, the instrument features a wavelength-tunable diode laser, a temperature-regulated rubidium cell, and comes with all the associated optics hardware for performing a variety of spectroscopy experiments including a student favorite - saturated absorption.In a series of well-defined yet challenging experiments, students explore the energy states of both isotopes of rubidium (85Rb and 87Rb), the Zeeman splitting of the 5P3/2 excited states in an applied magnetic field, the relationship between resonant atomic absorption and refractive index in rubidium vapor, resonant Faraday rotation, and the Clausius-Claperyon relationship, as well as the operation and characteristics of stabilized diode lasers and interferometric methods of calibrating the frequency sweep of a laser.In addition, TeachSpin and Libbrecht working under an NSF grant developed a Fabry-Perot cavity designed to build on the capabilities of this instrument. This cavity, which can also be used with other commercial or home-built lasers, is fully compatible with the existing apparatus and significantly expanded the already wide range of experiments possible with TeachSpin's Diode Laser Spectroscopy Lab.
For Professor Kenneth Libbrecht's advanced lab students laser spectroscopy, and the opportunity to observe saturated absorption, had always been a favorite experiment. As the waiting list to use the equipment grew longer, he realized he was going to need more than one extra apparatus to meet demand. Pleased with the TeachSpin instruments already in the lab, Ken offered to collaborate with TeachSpin to develop this Diode Laser Spectroscopy system. Our Student/Faculty manual will be, in great part, an expanded version of the "tried and true" materials he has long used
In addition to developing advanced laboratory explorations of atomic energy states, Professor Libbrecht has worked on a broad variety of topics in physics. At different times, he has ventured into helioseismology, laser-cooled atoms, the search for gravitational waves, and the physics of crystal growth. While chairing the Caltech physics department, teaching, and advising TeachSpin, he manages to find time to pursue his research on gravitational waves.
Perhaps it was the snows of his native North Dakota that motivated Ken to become an authority on the crystal growth of snowflakes. His web site devoted to this topic, snowcrystals.com is well worth visiting. Ken describes the physics of these unique ice structures in a beautiful coffee table book The Snowflake: Winter's Secret Beauty, which, last winter, was given front-page billing in the Science Times section of The New York Times.
The Snowflake graces our coffee table here at TeachSpin and we were delighted to be able to send complimentary copies as a special bonus with the first twenty Diode Laser Spectroscopy systems sold. We hope they have enjoyed the beautiful clarity of Ken's prose whether he is talking about delicate, one-of-a-kind snowflakes or the impressively reproducible data of our laser spectroscopy experiments.
Instrument
The centerpiece of the apparatus is the grating stabilized 780 nm diode laser. Honed to research quality by our senior scientist, George Herold, the laser is both temperature and current regulated. Optical feedback from a grating retro-reflects laser light to create an external cavity that stabilizes the laser to run at a controllable wave length. A piezo stack, mounted in the grating support,allows the grating angle to be modulated by an applied voltage. Using the internal ramp generator to modulate both the laser current and the piezo stack, students readily achieve laser frequency sweeps of as much as 10 GHz.
The outstanding quality of this stabilized diode laser makes it useful in the research laboratory. To serve this need, the laser head and controller can be purchased separately. Individual photodiode detectors are also available. The detector unit consists of a photodiode with a low-noise preamplifier and comes with complete mounting hardware.
TeachSpin's Laser Diode Spectroscopy Lab is shown here configured for simultaneous saturated absorption spectroscopy and interferometric determination of the frequency sweep of the laser. A block diagram is shown below.
Reflecting TeachSpin's instructional philosophy the controller, shown below, is divided into modules which the student must interconnect. Each module can be interrogated independently, encouraging exploration of the effects of changing a wide variety of parameters. Within the modules, students turn dials to control the laser current, the laser temperature, the cell temperature, the piezo stack, and the ramp generator as well as the gain and summing of the detector signals. A separate power supply is used to control the magnetic field created by the high homogeneity Helmholtz coils.
Controller for TeachSpin's Diode Laser Spectroscopy Lab
Two Optical System Versions can be purchased and partial systems are also available.
Diode Laser Spectroscopy (DLS1-A), the full version of the apparatus, supplies all of the optics in the photograph and block diagram shown above. In addition to the Laser Head, Complete Electronics, and 3 Diode Lasers (tested), this system includes: 3 Photodiode Detectors with preamplifiers, IR Viewing Card, CCD Camera, TV Monitor, the Absorption Cell Assembly (Rb Cell, Helmholtz Coils, Cell Heater, Rotating Stand), 2 pairs of Safety Goggles, Special Tools, a Black Anodized Aluminum Optical Bread Board 24" x 48" and an Instructor/Student Manual written by Professor Kenneth Libbrecht of Caltech. The Optics, for which all mounts and bases are supplied, include 5 Mirrors, a High Power Neutral Density Filter, 2 Neutral Density Filter sets with a holder, 0.5 degree Optical Wedge, 1.0 degree Optical Wedge, 50/50 Beam Splitter, Fixed Beam Splitter, 2 Rotating Linear Polarizers, and 2 Rotating 1/4-Wave Plates.
Basic Experiments Only is a simplified version which does not include the optics needed for simultaneous interferometry or for the magnetic field experiments. Items deleted from the complete optics package are 1 Photodiode Detector, 3 Mirrors, the 1.0 degree Optical Wedge, the Fixed Beam Splitter, 2 Rotating Linear Polarizers and 2 Rotating 1/4-Wave Plates. For schools that purchase the basic system this set of deleted optics items will be available as an upgrade.
Either the complete or basic system can be purchased without the optical breadboard.
Partial Systems are also available which will be sufficient for use in the research lab.
Tunable Diode Laser and Controller includes the Laser Diode Head, 3 Diode lasers (tested), the electronic Controller (with cell temperature controller and detector electronics included, and 2 pair of Mandatory Savety Goggles.
Photodiode Detectors, as noted earlier, are also available individually.
The Absorption Cell Assembly can also be purchased individually and includes the Rubidium Cell, Cell Heater, Thermocouple, a Rotatable Cell Mount and the 10 mT Helmholtz Coils.
Experiments
Characteristics of the Tunable Diode Laser
Students can begin these labs by studying the laser itself. Without the grating feedback, they can examine both the threshold current for lasing and the wavelength as a function of laser temperature. The wavelength can be measured with either a "homemade" spectrometer or any commercial spectrometer already on hand. Students can also observe mode hopping in the diode laser.
With the grating feedback in place, students can observe the laser's wavelength stability, the frequency sweep (using both grating angle and current modulation), and the sweep interruptions due to mode hopping. It is also possible to change the external cavity length and measure its effect.
The simplest form of optical spectroscopy, is shown below in Figure 1.
Figure 1: Block Diagram for Transmission Spectroscopy
The frequency swept laser light is passed through a cell containing rubidium vapor and the transmitted light is detected. Data for natural rubidium is shown in the oscilloscope capture below.
Figure 2: Transmitted Light vs. Laser Frequency
Figure 3: Rubidium Atomic Energy Level Diagrams
The broad absorption peaks observed by this method correspond to transitions between the ground S1/2 and the exited P3/2
states of both isotopes of rubidium.
These transitions are designated"a" and "b" on the energy level diagrams in Figure 3.
This "simple" laser technique easily resolves the ground-state hyperfine splitting of both isotopes but Doppler broadening
obscures the far smaller hyperfine splitting of the exited state.
A CCD camera, connected to a TV monitor, is used to observe the infrared fluorescence which occurs when the laser is
properly tuned to these hyperfine transitions. Note that the laser sweep is broad enough to cover both lines of both isotopes.
Saturated Absorption Spectroscopy
Now the fun begins. The students rearrange the apparatus so that the single laser beam is split into two collinear beams, a
probe (weak) and a pump (strong) which are sent through the rubidium cell in opposite directions. The block diagram in Figure 4, below, shows one possible experimental configuration.
Figure 4: Block Diagram for Transmission Spectroscopy
As the laser sweeps through an actual transition frequency, however, both pump and probe beams interact with the atoms having zero longitudinal velocity. Because the much stronger pump beam "saturates" the transitions, the intensity of the probe beam light reaching Detector 1 increases, producing the narrow features shown in Figure 5.
Figure 5: Transmitted Light vs. Laser Frequency:
(Narrow Features Indicate 5P3/2 Hyperfine Structure)
Reflection at the second interface of the initial beam splitter produces the reference beam which does not overlap the pump beam as it passes through the rubidium vapor on its way to Detector 2. The electronics of the controller allow us to subtract this reference beam from the probe beam, removing the doppler background and leaving only the narrow features. Figure 6 shows this result dramatically for the ground state F=2 transition (transition "a") of 85 Rb. With line widths of about 10 MHz, these features represent a resolution (Df/f) of about one part in forty million!
Given the DF = 0, ±1 selection rules, the energy level diagram of 85Rb in Figure 3 suggests that there should be on three,
not six, transitions from F = 2. The six peaks seen in Figure 6 include an additional three crossover transition peaks.
These additional peaks, at frequencies exactly halfway between pairs of "actual" transition frequencies, arise from atoms
moving at non-zero velocities such that the pump is in resonance with one transition and the probe is in resonance with the other transition.
Figure 6: Features with Doppler Backgroud Removed
Using the Interferometer to Calibrate the Sweep
In order to make quantitative measurements of the hyperfine splittings and compare these measurements to the handbook date, it is essential to calibrate the frequency sweep of the laser. This is accomplished with an unequal-arm interferometer shown the the upper section of the block diagram of Figure 4.
Beam splitter 2 diverts a small portion of the laser light into the interferometer assembly. The interferometer beam splitter divides the light, sending it to mirrors 1 and 2 along the long and short arms of the Michelson interferometer. Returning beams recombine and interfere at the beam splitter. Because of the unequal arm lengths, the frequency sweep of the laser generates a series of fringes in time, due to alternate constructive and destructive interference. Figure 7 shows a typical interference signal seen by Detector 3 along with the Doppler broadened transmission data.
Straight forward measurement of path lengths from the splitter to the mirrors can be used to calibrate the frequency sweep in Hz/fringe.
If DL = L1- L2 is the is the difference between the one-way lengths of the two arms, then the optical frequency
difference, df, between two successive maxima at the interferometer can be calculated as: df = c/2 DL.
For our optical arrangement, with a path length difference of 0.35 meters, the sweep calibration is 0.429 GHz/fringe. This gives a 6.65 GHz frequency difference between the cursor marked features of Figure 7. The accepted value, shown on the energy diagram in Figure 3, is 6.835 Hz.
But this is by no means the end of the story. This unit can measure Zeeman splittings of the excited states, Faraday rotation in rubidium vapor, and the refractive index of rubidium as well as the Clausius-Claperyon relationship in rubidium.
Figure 7: Transmission Data with Interferometer Fringes
Additional Resources
Specifications
Grating Feedback External Cavity Diode Laser
Sanyo DL7140-201 Laser
l = 784 nm
Maximum Power: 70 mW
Single mode index guided structure
Holographic Grating:
Spacing: 1800 lines/mm
Piezo Stack
Displacement: 3.0 mm @ 100 V
Maximum Frequency Response: 3kHz (mounted)
Thermo-electric Cooler
Maximum cooling power: 10 Watts at 2A
Temperature Range: 0 - 60oC
Temperature Stability: better than 0.01oC
External Cavity Length and Laser Polarization
Variable
Scan Range Without Mode Hop
PZT alone: approx. 5 GHz
Simultaneous Current and PZT Scan: > 10 GHz
Absorption Cell Assembly
Rubidium Cell Dimensions: 1" diameter x 1" length
Cell Heater
Thermocouple
Rotatable Cell Mount
Helmholtz Coils 10 mT@ 3 A
Electronics
Current Control
Current: 0 - 100 mA
Maximum modulation frequency: 1MHz
Current Noise: < 50 nA rms
Laser Temperature Controller
Back panel set point potentiometer and analog input for temperature sweeps
Cell Temperature Control
Temperature regulated oven: room temperature - 100oC
Ramp Generator
Frequency range: 1 mHz - 10 kHz
Output amplitude: 0 - 10 V p-p
Piezo Controller
Modulation input 0 - 100 V
Detector Control
Two detector inputs for signal subtraction and laser locking
Laser has been locked to SAS feature for more than one hour
Photodiode Detectors
PIN Photodiode: 0.25" diameter
I-V pre Amp: 333W to 10 MW "gain" in 1, 3, 10 steps