THORLABS SA210 Series User manual

SA210 Series
Fabry Perot Interferometer
User Guide

Fabry-Perot Interferometer
Table of Contents
Chapter 1Warning Symbol Definitions ................................................................................. 1
Chapter 2Safety ...................................................................................................................... 2
Chapter 3Description ............................................................................................................. 3
3.1.Confocal Cavity Configuration.......................................................................................3
Chapter 4Fabry-Perot Interferometry ................................................................................... 4
4.1.Free Spectral Range........................................................................................................4
4.2.Finesse.............................................................................................................................5
4.3.Spectrum Analyzer Controller and Other Accessories ...............................................5
Chapter 5Setup ....................................................................................................................... 6
5.1.Recommended Set-up ....................................................................................................6
Chapter 6Alignment ............................................................................................................... 7
Chapter 7Specifications ........................................................................................................ 8
Chapter 8Regulatory .............................................................................................................. 9
8.1.Waste Treatment is Your Own Responsibility..............................................................9
8.2.Ecological Background ..................................................................................................9
Chapter 9Thorlabs Worldwide Contacts ............................................................................ 10

Fabry-Perot Interferometer Chapter 1: Warning Symbol Definitions
Rev D, November 30, 2017 Page 1
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your device.
Symbol Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or Chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution: Risk of Electric Shock
Caution: Hot Surface
Caution: Risk of Danger
Warning: Laser Radiation
Caution: Spinning Blades May Cause Harm

Fabry-Perot Interferometer Chapter 2: Safety
Page 2 19502-D02
Chapter 2 Safety
All statements regarding safety of operation and technical data in this instruction manual will only apply when the
unit is operated correctly. Please read the following warnings and cautions carefully before operating the device.
WARNING
Take appropriate measures for eye safety.
Never look directly into the beam, not even at the output of the SA210.
SHOCK WARNING
Never touch the output of the SA210 PZT driving unit.

Fabry-Perot Interferometer Chapter 3: Description
Rev D, November 30, 2017 Page 3
Chapter 3 Description
The SA210 is a high finesse Spectrum Analyzer used to examine the fine structures of the spectral characteristics
of CW lasers. The spectrum analyzer consists of a confocal cavity that contains two high reflectivity mirrors; by
varying the mirror separation with a piezoelectric transducer the cavity acts as a very narrow band-pass filter.
Knowing the free spectral range of the SA210 allows the time-base of an oscilloscope to be calibrated to facilitate
quantitative measurements of a laser line shape.
3.1. Confocal Cavity Configuration
Mirrors shown below are AR coated on the outer surfaces and HR coated on the inner surfaces.
FSR=c/4d
Available Coating Ranges
Item # Wavelength Range
SA210-3B 350 - 535 nm
SA210-5B 535 - 820 nm
SA210-8B 820 - 1275 nm
SA210-12B 1275 - 2000 nm
SA210-18C 1800 - 2600 nm

Fabry-Perot Interferometer Chapter 4: Fabry-Perot Interferometry
Page 4 19502-D02
Chapter 4 Fabry-Perot Interferometry
4.1. Free Spectral Range
To scan the spectra of the laser beam entering the Scanning Fabry–Perot interferometer small displacement is
applied to one of the cavity mirror mounted on piezoelectric transducers. This operation is done by fine tuning the
ramp voltage applied to the Piezoelectric elements using the controller SA201. When the mirror spacing becomes
equal to an integral number of half the wavelength of the laser, constructive interferences occur. That spectral
response of the signal can be visualized with a scope. A series of periodical peaks appear on the screen of the
scope. The distance between consecutive peaks is called the free spectral range (FSR) of the instrument.
From a users perspective a confocal cavity has a FSR that is given by c/4d instead of c/2d (where c is the speed
of light and d is the cavity length) as would be the case for a plano-plano cavity; the factor of 2 in the denominator
can be understood by inspecting the ray trace shown below in Figure 1. Note that a ray entering the cavity at a
height ‘h’ parallel to the optical axis of the cavity makes a triangular figure eight pattern as it traverses the cavity.
From this pattern it is clear that the ray makes four reflections from the cavity mirrors instead of the two that would
result in a plano-plano cavity. Hence the total round-trip path through the cavity is given as 4d instead of 2d.
Figure 1 This figure shows a simplified ray-trace for a ray entering the cavity at height ‘h’. The curvature of the
mirrors ‘R’ and the separation being set precisely to ‘R’ ensures that the input ray is imaged back onto itself after
traveling a distance of approximately 4R.
Additionally, in this configuration if a paraxial ray is traced through the system as shown in Figure 1, it is apparent
that in the confocal configuration each mirror serves to image the other mirror back onto itself so that a ray
entering the cavity will, after four traverses of the cavity, fall back onto itself, (note that the focal length of a
spherical mirror is R/2). This imaging of the beam back onto itself greatly simplifies the alignment of the cavity;
just align your input to within a few tenths of a millimeter of the center of the mirror set and restrict your input
angles to less than a few degrees. The SA200 series interferometer has two iris diaphragms that simplify this
alignment requirement.

Fabry-Perot Interferometer Chapter 4: Fabry-Perot Interferometry
Rev D, November 30, 2017 Page 5
4.2. Finesse
The finesse of the Scanning Fabry-Perot interferometer is a quantity which characterizes the ability of the
interferometer to resolve closely spaced spectral features, it defines the resolution of the instrument. For an
infinitely narrow input spectrum, the finesse determines the width of the measured spectrum.
High finesse means high resolution capability, high finesse is obtained by increasing the reflectivity of the cavity
mirrors. However, high reflective mirrors reduce the transmission of the interferometer.
Figure 2 When two equal Gaussian lineshapes just meet
the Taylor criteria for being resolvable, they
are separated by their common FWHM (∆)
as shown in the plot.
4.3. Spectrum Analyzer Controller and Other Accessories
The SA201 controller generates a voltage ramp, which is used to scan the separation between the two
cavity mirrors. A photodiode is used to monitor transmission of the cavity. Using the output sync signal
from the controller, an oscilloscope can be used to display the spectrum of the input laser. The controller
provides adjustment of the ramp voltage (1 to 30 V) and scan-time (1 ms to 5 s) to allow the user to
choose the scan range and speed. An offset control is provided to allow the spectrum displayed on the
oscilloscope to be shifted right or left, zoom capability provides up to 100X increase in spectral resolution.
Thorlabs’ KM100, Ø2" kinematic mount can be used to mount the SA210 Scanning Fabry Perot
Interferometer.

Fabry-Perot Interferometer Chapter 5: Setup
Page 6 19502-D02
Chapter 5 Setup
Knowing the free spectral range (FSR) of the SA210 allows the time-base of an oscilloscope to be calibrated to
facilitate quantitative measurements of laser line shape. With a resolution of 67 MHz, the fine structure resulting
from multiple longitudinal modes of a laser line can be resolved. Note: A saw tooth wave (0 - 20 V) would provide
approximately 2 FSR.
The SA210 should be mounted, so that it can be easily adjusted. It is recommended that Thorlabs 1-inch
Kinematic Mount KM100 be used to mount the interferometer at the 1-inch diameter flange.
The apparent beam size should be approximately 1 mm. It is recommended that a fold mirror be used to direct the
beam into the Fabry-Perot interferometer. A lens with focal length of 100 mm can be used, with the focus set
roughly at the center of the housing, approximately 25 mm in from the flange.
The maximum voltage on the piezo (ramp in) is not to exceed 150 V.
If the detector is connected directly to the scope, a 5 kΩterminator is needed. Offset adjustment (SA201) is used
to center the output on the scope.
5.1. Recommended Set-up
In a typical application the SA210 Interferometer is used in conjunction with a signal generator and an
oscilloscope, as shown below. A signal generator (Thorlabs SA201 Fabry-Perot Controller is used for
generating the required scan signals for obtaining the data in this document) that can produce either a
triangle or saw-tooth wave with an adjustable frequency (5 to 50 Hz), an adjustable amplitude (15 to 40 V),
and an adjustable offset. The signal generator is used to repetitively scan the length of the cavity by λ/4 in
order to sweep through one FSR of the interferometer. An oscilloscope is typically used to view the
spectrum and make quantified measurements of spectral features.
Figure 3 This figure shows a schematic diagram of a typical setup that is used to measure the spectrum of a laser
source. Please note that for this device to be useful the linewidth of the source must be less than the FSR of the
interferometer.

Fabry-Perot Interferometer Chapter 6: Alignment
Rev D, November 30, 2017 Page 7
Chapter 6 Alignment
To set up the SA210 Series Fabry-Perot, first mount the unit into a tip/tilt mirror mount (item # KM100). Attach all
of the connections according the drawing on page 6. Next, remove the detector from the back of the unit and
mount it in a separate mount; this will aid in the alignment. Close the input iris to its minimum aperture and center
the beam on the iris opening. Leave the back iris completely open and start to scan the unit. Use the mirror
mount's tip/tilt adjustment until the beam is centered through the body of the SA210.
Adjust the oscilloscope gain to maximize sensitivity, position the detector close to the rear opening, and slowly
close the back iris while adjusting mirror mount to keep the beam in the center of the device. Once the beam is
centered, attach the detector to the main body once more. The interferometer will then be ready for
measurements.