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Saturated Absorption Spectroscopy Systems
(Breadboard Sold Separately)
Rubidium Vapor Cell
See the SA Spectroscopy Tab for Measurement Details
The Thorlabs SKSAS kit offers a proven set of components to construct a compact, fiber-coupled Saturated Absorption Spectroscopy (SAS) setup. It offers a method for producing a highly stable tunable laser locking reference at the peak of atomic hyperfine structure transitions. The kit also allows for the study of the hyperfine structure and Doppler broadening of atomic transitions.
The kit has been designed using stock optics and mechanics, as well as compatible custom components. This makes these kits adaptable using other Thorlabs cage system and lens tube components. For a list of the major components and subsystems included in the SAS kit, please see the Kit Contents tab.
While the vapor cell heater is included in the kit, please note that the vapor cell and temperature controller must be purchased separately. Currently, we offer rubidium and potassium reference vapor cells, which are available below. A variety of custom vapor cells are also available; please contact firstname.lastname@example.org for more details. Thorlabs' TC200 Temperature Controller is compatible with the cell heater included in each kit. We also offer pre-assembled Herriott cells for gas absorption spectroscopy applications.
Saturated Absorption Spectroscopy
Please note that the Spectroscopy Kits do not come with vapor cells, temperature controller, or a breadboard. Rubidium and Potassium vapor cells are available below. The TC200 temperature controller is compatible with the gas cell heater included with each kit.
SAS Kit Contents
Thorlabs' SAS Kits contain the following subsystems (breadboard is sold separately):
The fiber input is designed with our F220FC-780 fiber collimator and cage system components. The collimator collimates the input from an FC/PC-terminated PM fiber, which must be purchased separately. For Rubidium, we recommend using our P1-780PM-FC-5 patch cable. For users who would prefer a free-space input, please contact us at email@example.com for a quote for a kit without the fiber collimator and mounting mechanics.
Once the input is collimated, the light passes through a WPMH05M-780 half-wave plate. The wave plate allows the user to vary the relative intensity of the pump and probe beams, which is helpful during alignment of the kit.
The input prism assembly is constructed on a KM100PM kinematic mount using a specially-designed mounting platform accessory. Three prisms are then epoxied into place on the mounting platform: a MRA10-M01 right-angle prism mirror, a BS011 50:50 non-polarizing beamsplitter, and a PBS102 polarizing beamsplitter.
The polarizing beamsplitter divides the input into the pump and probe beams. The power into each beam can be adjusted using the half-wave plate (part of the fiber input assembly described above). The 50:50 non-polarizing beamsplitter picks off a portion of the input for a reference beam that measures the direct Doppler-broadened absorption spectrum.
The GCH25-75 heater assembly is provided with the kit and is designed to hold Ø9 mm, Ø19 mm, or Ø25 mm vapor cells 75 mm in length. Please note that neither vapor cells nor a temperature controller for the GCH25-75 are included with the SKSAS. Rubidium and Potassium cells are available on the bottom of this page, and other atomic vapor cells are available upon request. The TC200 temperature controller is compatible with the vapor cell heater included with each kit.
The output prism assembly is constructed similarly to the input prism assembly, using a custom mounting platform attached to a KM100PM. Three prisms are epoxied to the platform: two MRA10-M01 right-angle prism mirrors, and a PBS102 polarizing beamsplitter.
The polarizing beamsplitter reflects the pump beam to counterpropagate with the probe beam through the cell. The two right-angle mirror prisms are designed to reflect the probe and reference beams so that they may be detected using the balanced detector. A diagram showing the beam path through the system is shown in the "Pump Mirrors, Folding Mirrors, and Balanced Detector" section below.
In addition to the input and output prism assemblies, there are four PF05-03-M01 Ø1/2" gold mirrors that are mounted in KS05 mounts. Two of these mirrors are used to direct the pump beam so that it can counterpropagate with the probe beam and thus create the SAS Doppler-free spectrum. The other two mirrors are for directing the probe and reference beams onto the two photodiodes of the balanced detector. A beam path diagram is shown to the right, which details the function of these four mirrors.
The PDB210A Balanced Detector provides three output signals: two are the signals from each photodiode, and the third is the difference between these two signals. The voltages from each detector are useful when aligning the system and to investigate the features of the spectra in teaching labs, while the difference signal is useful for laser locking.
Saturated Absorption Spectroscopy
Saturated Absorption Spectroscopy Basics
When the laser has a frequency of f0, however, both counter-propagating beams can only interact with atoms that are at rest. Therefore, there will be a depletion of zero-velocity ground state atoms, which will be evidenced by dips in the Doppler-broadened absorption profile, which can be seen in the blue line in figure 1. These dips create very narrow peaks which can be used as a frequency locking point for a tunable laser.
Figure 3: Measurement signal from a balanced detector, showing the difference between the direct absorption and the Doppler-free absorption spectra.
The SAS System
A third reference beam is directed from the reflection of the 50/50 beamsplitter, through the vapor cell, and to the second port of the balanced detector. By taking the difference of the reference port (direct transmission) and the probe beam (Doppler-free transmission) signals from the balanced detector, a signal like the one in figure 3 is recorded. This signal removes the Doppler-broadening to reveal several narrow peaks. To use these signals for laser frequency stabilization, the laser is locked to a frequency corresponding to the sharp edge of a transition peak. There is a voltage V(f0) corresponding to this locking frequency. As the laser frequency drifts, the voltage of the difference signal will be different, indicated by V(f) on figure 3. An error signal, calculated by Error(f) = V(f0) = V(f), can then be used by a feedback loop to adjust the lasing frequency until Error(f) = 0. In this manner, the laser frequency can be locked to the transition.
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Borosilicate Glass Transmission
Thorlabs offers Potassium and Rubidium cells that are compatible with our Spectroscopy Kits. These reference cells are fabricated from borosilicate glass, a rugged material known to resist chipping and cracking. They are tested to ensure that the transmission through the cell exceeds 84% for light in the 350 nm to 2.2 µm range.
MSDS sheets are available and can be found by clicking on the red Docs icon () below.
Please note: These reference cells are subject to hazardous goods regulations and must be shipped separately using specifically regulated shipping methods and may require special shipping and handling charges. Next day delivery is not available. All orders will ship from our US warehouse regardless of destination and cannot be returned. Due to hazardous materials shipping regulations, we are currently unable to ship our reference cells to Brazil, China, or Uruguay.