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Saturated Absorption Spectroscopy Kit


Saturated Absorption Spectroscopy Systems

  • Ideal for Lasers Locking or Teaching Labs
  • Rubidium or Potassium Vapor Cells Available Separately


  • Saturated Absorption Spectroscopy for Locking to Atomic Transitions
  • Rubidium or Custom Vapor Cell Options
  • PM Fiber-Coupled Setup
  • Ideal for Laser Locking or Teaching Labs

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 lock for tunable lasers 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 reference vapor cells, which are available below. A variety of custom vapor cells are also available. Thorlabs' TC200 Temperature Controller is compatible with the cell heater.

Saturated Absorption Spectroscopy
Saturated Absorption Spectroscopy (SAS) systems provide a means to create a highly sensitive lock tied directly to an atomic transition. When an atom absorbs (or emits) a photon, the absorption (or emission) frequency is Doppler shifted. To create a more narrow laser lock, Doppler Broadening is eliminated by use of the well-known saturated absorption technique that resolves the hyperfine structure of atomic transitions. For a detailed tutorial on SAS, please see the SA Spectroscopy tab.

Custom Options and Assembly Services
Our spectroscopy kits are adaptable to most tunable lasers with user-supplied feedback mechanisms. While our standard kit is designed to accept PM fiber-coupled sources, we can also offer kits for free-space input. We can also offer assembly services upon request.


Required Input Power ~500 µW
Input Fiber Termination* FC/PC
Detector Bandwidth 1 MHz
Detector Output Range ±10 V
Reference Cell Temperature (Max.) 50 °C

* Alternate fiber inputs are available. Please contact us.


Spectroscopy Kit-Compatible Vapor Cells

Thorlabs offers Potassium and Rubidium cells that are compatible with our Spectroscopy Kits. These reference cells are fabricated from Pyrex, a rugged material known to resist chipping and cracking, and incorporate flat windows made from optical-quality Schott Borofloat glass. They are tested to ensure that the transmission through the cell exceeds 84% for light in the 350 nm to 2.4 µm range. The content of each reference cell is also tested by measuring the absorption spectrum of a well known transition using a tunable diode laser.


SAS Kit Contents

Thorlabs' SAS Kits contain the following subsystems:

  • Fiber Input and Half-Wave Plate
  • Input Prism Assembly
  • Vapor Cell and Heater
  • Output Prism Assembly
  • Pump Mirrors, Folding Mirrors, and Balanced Detector


Fiber Input and Half-Wave Plate

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.

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.


Input Prism Assembly

The input prism assembly is constructed on a KM100P kinematic mount using a specially-designed mounting platform accessory. Three prims 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.


Vapor Cell Heater Assembly

The GCH25-75 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. Thorlabs offers the TC200 temperature controller which is compatible with the vapor cell heater.


Output Prism Assembly

The output prism assembly is constructed similarly to the input prism assembly, using a custom mounting platform attached to a KM100P. 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.


Pump Mirrors, Folding Mirrors, and Balanced Detector

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.


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