The SA200 and SA210 Series of Scanning Fabry-Perot (FP) Interferometers are high-finesse spectrum analyzers that are frequently used to examine the fine structure of the spectral characteristics of CW lasers. The confocal FP cavity acts as a very narrow bandpass filter. The transmission wavelength of the cavity is tuned by adjusting the length of the cavity using piezoelectric transducers, which are driven using the SA201 controller or an equivalent function generator. The transmitted light intensity is measured using a photodiode, amplified by the transimpedence amplifier in the SA201 (or equivalent amplifier) and then displayed / recorded by an oscilloscope or data acquisition card. For more information on confocal Fabry-Perot cavities, please see our online tutorial.
Alignment The confocal design of the FP Interferometer cavity is relatively insensitive to the alignment of the input beam. As a result, the optical axis of the FP Interferometer can be aligned with sufficient accuracy to the input beam by mounting the interferometer on a standard kinematic mirror mount (see the Alignment Guide tab for more details).
Schematic Representation of a Confocal Fabry Perot Interferometer
1) Controller (BNC) to Piezo (Attached) Cable, Non-removable Part of FP Interferometer 2) Photodiode (SMA) to Controller (BNC) Cable, Included with FP Interferometer
3) Amplified Photodiode Output (BNC) to Oscilloscope Cable, Not Included 4) Trigger Output of Controller (BNC) to Oscilloscope Cable, Not Included
5) Optional Connection that Allows the User to Monitor the Signal used To Drive the Piezoelectric Transducers
The SA201 controller generates the sawtooth or triangle wave voltage required to repetitively scan the length of the cavity by λ/4 (or more) in order to sweep through one FSR (or more) of the interferometer. The SA201 controller also houses a transimpedance amplifier that can be used to amplify the output of the photodiode detector in the FP Interferometer. The intensity of the used to measure the intensity of the light transmitted through the confocal FP cavity. The controller also provides a trigger signal to the oscilloscope, which allows the oscilloscope to easily trigger at the beginning or the middle of the scan. The time axis of the oscilloscope can be precisely by calibrated measuring the time elapsed between the same spectral feature by separated one FSR (see Figures 4 and 5 in the online tutorial for details).
Theoretical Mirror Reflectivity and Mirror Finesse
The Excel file contains the data used to make the plot above. Please remember that the actual reflectivity of the mirror will vary slightly from coating run to coating run within the specified region and can vary significantly from coating run to coating run outside of the specified region. The total cavity finesse depends on additional factors. Please see the online tutorial for more information.
The SA200 and SA210 series of Scanning Fabry-Perot Interferometers have confocal FP cavities. Since the transverse modes of a confocal cavity are degenerate, the cavity is fairly insensitive to the alignment of the input beam. As seen in the ray trace to the right, even an off axis input beam that is not parallel to the optical axis of the FP cavity will make one round trip through the cavity with an approximate path length of 4r-H4/r3 where r is the radius of curvature of the mirrors and H is the distance that the input beam is from the optical axis when the beam enters the cavity. As long as the second term in the path length expression is much less than the wavelength of the light, then the off axis input beam will be degenerate with the on axis input beam (see the online tutorial for more information). The second term in the path length expression also limits the diameter of the input beam. In practice, the cavity can be aligned by mounting the confocal FP interferometer in a standard kinematic mirror mount (KS2 for SA200 and KS1 for SA210) which is then placed in a free space beam after a fold mirror. While the cavity is being scanned, iteratively adjust the position of the mirror and FP interferometer until the cavity is aligned with the input beam. After the cavity is aligned to the beam, a lens should be placed in the beam so that a beam waist with the specified diameter is formed in the center of the cavity.
SA200 Series The diameter of the collimated free space beam before the lens should be ~4 mm and a 250 mm focal length lens should be positioned ~220 mm in front of the flange on the interferometer. This will create the 600 µm diameter beam waist at the center of the confocal cavity.
SA210 Series The diameter of the collimated free space beam before the lens should be ~1 mm and a 100 mm focal length lens should be positioned ~75 mm in front of the flange on the interferometer. This will create the 150 µm diameter beam waist at the center of the confocal cavity.
For examples of how to integrate an SA200 or SA210 series interferometer into Thorlabs' cage system, see below.
Example Setup that Facilitates the Coupling of a Free Space Beam into an SA210 Series Fabry-Perot Interferometer Using Cage System Components
Below is an explanation of the parts used at each numbered stage, a description of how the system was aligned, and a table of the products used in the setup pictured above.
Stage 1: A BE02M-A is used to reduce a beam of light to the required collimated beam diameter of 1 mm, as specified in the manual (the magnification factor of the beam expander/reducer is application specific). The beam expander/reducer is held in the 30 mm cage system using a CP06 cage plate. Notes: The beam expander/reducer should be adjusted so that it outputs a collimated beam prior to inserting it into the cage system. Alternatively, the mirror can be removed from the first KCB1 mount to allow a sufficiently long beam path to ensure that the output of the beam expander/reducer is collimated.
Stage 2: A BB1-E02 is held in a KCB1 kinematic cage mount. This is the first of two steering mirrors used to control the propagation direction of the beam of light being coupled into the SA200 series Fabry-Perot Interferometer. The KCB1 is supported by a TR series post and PH4E pedestal post holder.
Stage 3: A BB1-E02 is held in a KCB1 kinematic cage mount. This is the second of two steering mirrors. The KCB1 is supported by a TR series post and PH4E pedestal post holder.
Stage4: A CT1 30 mm cage translation stage is used to hold the mounted AC254-100-A1-ML achromatic lens. The center of the lens should be positioned approximately 250 mm from the front plate of the KC1 mount with the z-axis micrometer set in the middle of its adjustment range. The CT1 can be replaced by the combination of a CP02 cage plate and an SM1V05 adjustable length lens tube.
Stage 5: A KC1 is a 30 mm kinematic cage mount that is being used to mount the SA210 series Fabry-Perot interferometer. The KC1 is being supported by a TR series post and an PH4E pedestal post holder.
Alignment Guide:
For a complete alignment procedure, please see the operations manual.
After the beam expander was adjusted so that it produced a collimated 1 mm diameter beam of light when inserted into the free space laser beam used for testing this setup, the entire cage system was assembled as shown in the picture. The assembly was then situated on the optical table and adjusted so that the free space beam, which was already propagating parallel to the table, passed through the center of the CPA1 alignment guide when it was placed before and after the stage 1 components. At this point the pedestal post holders were locked down to the optical table using the CF175 clamps. The CPA1 alignment guide was then positioned immediately before stage 3 and the stage 2 kinematic mirror was used to steer the beam to the alignment mark. The CPA1 was then positioned immediately before the stage 5 components and the stage 3 kinematic mirror was used to steer the beam to the alignment mark. Turn on the Fabry-Perot controller box and start scanning the length of the cavity since light will only be transmitted when the cavity length is resonant with the wavelength of the light beam. At this point it might be necessary to remove the detector from the back of the Fabry-Perot cavity in order coarsely align the cavity, however this was an unnecessary step when this setup was tested. Iteratively adjust the adjustment knobs on the mounts in stages 3, 4, and 5 until the Fabry-Perot cavity is correctly aligned.
SA210 Series Fabry-Perot Alignment Setup Parts List with Links
a The correct choice for a beam expander/reducer is dependent on the properties of the beam of light that needs to be coupled into the Fabry-Perot cavity. The beam after the beam expander/reducer should be collimated and approximately 4 mm in diameter. Be sure to choose a beam expander with the correct AR coating. b In addition to the visible (400-700 nm) achromatic lens listed in the table Thorlabs also sells achromatic lenses suitable for the 650-1050 nm and 1050-1620 nm spectral ranges. However, these lenses are not mounted and as a result an SM1L03 should be purchased to mount an unmounted achromatic lens. c In addition to the visible (400-750 nm) spectral range broadband dielectric mirror in the table Thorlabs sells Broadband Dielectric Mirrors suitable for the 750-1100 nm and 1280-1600 nm spectral ranges. Alternatively, a protected metallic mirror made from silver or aluminum could also be used. d Optional 30 mm cage system alignment guide. This guide precisely locates the optical axis of a 30 mm cage system. This greatly simplifies the alignment of the Fabry-Perot Interferometer the setup pictured above is used.
Example Setup that Facilitates the Coupling of a Fiber Coupled Source into an SA210 Series Fabry-Perot Interferometer Using Cage System Components
When using a fiber coupled light source, stages 1 and 2 of the free space setup shown at the top of the page can be replaced with a cage mounted fiber collimation system. The diameter of the beam output from the fiber collimator needs to be approximately 1 mm (not larger) in diameter. One possible fiber collimation solution would be to use a CFC-5 series adjustable collimator. The collimator could be incorporated into the 30 mm cage system using an HCFN adapter and a CP06 cage plate.
Example Setup that Facilitates the Coupling of a Free Space Beam into an SA200 Series Fabry-Perot Interferometer Using Cage System Components
Below is an explanation of the parts used at each numbered stage, a description of how the system was aligned, and a table of the products used in the setup pictured above.
Stage 1: A BE02M-A is used to reduce a beam of light to the required collimated beam diameter of 4 mm, as specified in the manual (the magnification factor of the beam expander/reducer is application specific). The beam expander/reducer is held in the 30 mm cage system using a CP06 cage plate. Notes: The beam expander/reducer should be adjusted so that it outputs a collimated beam prior to inserting it into the cage system. Alternatively, the mirror can be removed from the first KCB1 mount to allow a sufficiently long beam path to ensure that the output of the beam expander/reducer is collimated.
Stage 2: A BB1-E02 is held in a KCB1 kinematic cage mount. This is the first of two steering mirrors used to control the propagation direction of the beam of light being coupled into the SA200 series Fabry-Perot Interferometer. The KCB1 is supported by a TR series post and PH4E pedestal post holder.
Stage 3: A BB1-E02 is held in a KCB1 kinematic cage mount. This is the second of two steering mirrors. The KCB1 is supported by a TR series post and PH4E pedestal post holder.
Stage4: A CP02 holds the SM1V10 adjustable length lens tube in which the AC254-250-A1-ML achromatic lens is mounted. The center of the lens was positioned approximately 22 cm from the front plate of the KC2 mount with half of the threads on the adjustable length lens tube threaded into the CP02 cage plate. An LCP02 is used to convert the cage system from the 30 mm format to the 60 mm format.
Stage 5: A KC2 is a 60 mm kinematic cage mount that is being used to mount the SA200 series Fabry-Perot interferometer. The KC2 is being supported by a TR series post and an PH4E pedestal post holder.
Alignment Guide:
For a complete alignment procedure, please see the operations manual.
After the beam expander was adjusted so that it produced a collimated 4 mm diameter beam of light when inserted into the free space laser beam used for testing this setup, the entire cage system was assembled as shown in the picture. The assembly was then situated on the optical table and adjusted so that the free space beam, which was already propagating parallel to the table, passed through the center of the CPA1 alignment guide when it was placed before and after the stage 1 components. At this point the pedestal post holders were locked down to the optical table using the CF175 clamps. The CPA1 alignment guide was then positioned immediately before stage 3 and the stage 2 kinematic mirror was used to steer the beam to the alignment mark. The CPA1 was then removed and the LCPA1 was positioned immediately before the stage 5 components and the stage 3 kinematic mirror was used to steer the beam to the alignment mark. Turn on the Fabry-Perot controller box and start scanning the length of the cavity since light will only be transmitted when the cavity length is resonant with the wavelength of the light beam. At this point it might be necessary to remove the detector from the back of the Fabry-Perot cavity in order coarsely align the cavity, however this was an unnecessary step when this setup was tested. Iteratively adjust the adjustment knobs on the mounts in stages 3, 4, and 5 until the Fabry-Perot cavity is correctly aligned.
SA200 Series Fabry-Perot Alignment Setup Parts List with Links
a The correct choice for a beam expander/reducer is dependent on the properties of the beam of light that needs to be coupled into the Fabry-Perot cavity. The beam after the beam expander/reducer should be collimated and approximately 4 mm in diameter. Be sure to choose a beam expander with the correct AR coating. b In addition to the visible (400-700 nm) achromatic lens listed in the table Thorlabs also sells achromatic lenses suitable for the 650-1050 nm and 1050-1620 nm spectral ranges. However, these lenses are not mounted and as a result an SM1L03 should be purchased to mount an unmounted achromatic lens. c In addition to the visible (400-750 nm) spectral range broadband dielectric mirror in the table Thorlabs sells Broadband Dielectric Mirrors suitable for the 750-1100 nm and 1280-1600 nm spectral ranges. Alternatively, a protected metallic mirror made from silver or aluminum could also be used. d Optional 30 mm cage system alignment guide. This guide precisely locates the optical axis of a 30 mm cage system. This greatly simplifies the alignment of the Fabry-Perot Interferometer the setup pictured above is used. e Optional 60 mm cage system alignment guide. This guide precisely locates the optical axis of a 60 mm cage system. This greatly simplifies the alignment of the Fabry-Perot Interferometer when the setup pictured above is used.
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Posted Comments:
Poster: Thorlabs
Posted Date: 2010-08-16 16:05:08.0
Response from Javier at Thorlabs to Lars Sandström: we do not have test data for the damage threshold of these Fabry-Perot interferemometers. However, the limiting factor is the power handling capability of the detector. As a guideline, it is recommended to limit the input to below 100 mW/cm^2.
Poster: lsandstrom
Posted Date: 2010-08-16 12:40:47.0
What is the maximum allowed input power for the SA210-5B to avoid dammage to the device?
Poster: Adam
Posted Date: 2010-04-28 10:00:10.0
A response from Adam at Thorlabs to jdonnelly: We recommend using as much force as possible. It is possible for these to get stuck and become impossible to remove. If that is the case, we can take it back for repair.
Poster: jdonnelly
Posted Date: 2010-04-27 20:49:21.0
It is not clear to me how to remove the detector unit, as specified in your alignment directions. The threaded piece at the back looks like it should turn but it doesnt, at least by hand. We are reluctant to use more force unless you so recommend.
Poster: Tyler
Posted Date: 2009-11-30 18:10:59.0
A response from Tyler at Thorlabs: In addition to a plot of the mirrors reflectance, an Excel file has been added to the Graphs tab that contains the data used to make the plot.
Poster: Tyler
Posted Date: 2009-11-20 08:54:33.0
A response from Tyler at Thorlabs: Thank you for suggesting the need to have the mirror performance data on this webpage. I added the Graphs tab to the presentation with this data on it. Please let us know if you need any additional information.
Poster: klee
Posted Date: 2009-11-19 10:55:27.0
A response from Ken at Thorlabs: We can send you the coating curves if you can provide your email address.
Poster:
Posted Date: 2009-11-18 20:54:27.0
would be useful to have the coating curves
Poster: Greg
Posted Date: 2009-03-12 10:09:53.0
A response from Greg at Thorlabs to dergachev: Thank you for your interest in our SA200 series of Fabry-Perot Interferometers. We should be able to make a custom SA200 that fits your needs. A member of our Technical Support team has e-mailed you to discuss this possibility.
Poster: dergachev
Posted Date: 2009-03-11 14:13:08.0
Do you have a SA-200 version suitable for operation at 2 um?
Please advise.
Thanks.
Alex Dergachev
Poster: Tyler
Posted Date: 2009-02-12 09:45:01.0
A response from Tyler to melsscal:For the SA200-14A, which has a wavelength range from 1450 nm to 1625 nm, the following parts are required to replicate the setup shown in the Alignment Guide tab. PH4E (Qty. 2), TR4 (Qty. 2), ER6 (Qty. 8), ER4 (Qty. 8), LCP02 (Qty. 1), CP02 (Qty. 1), SM1V10 (Qty. 1), AC254-250-C (Qty. 1), SM1L03 (Qty. 1), KCB1 (Qty. 1), BB1-E04 (Qty. 1), CP02FP (Qty. 1), and PAF-X-18-PC-C (Qty. 1). The optional alignment guides, which I strongly recommend, are the LCPA1 and CPA1 (1 Each). There are definitely other ways to do this so I recommend talking to a member of our tech support department. I will email you this list, which can then be turned into a quote if you desire. Thank you for using this feedback forum to post your question, as I think that this information may be useful to other customers.
Poster: melsscal
Posted Date: 2009-02-12 05:52:05.0
Can you please confirm the parts list for the fiber coupled setup with SA200-14A.
Poster: jwk
Posted Date: 2008-06-17 11:10:31.0
Hello, I used your FP interferometer well. Could you tell me whether you can supply the FP for 1800-2100 nm? I look forward to your reply.
Best regards,
Ji Won
Poster: Tyler
Posted Date: 2008-03-19 08:40:39.0
Response from Tyler at Thorlabs to melsscal: Thank you for your interest in our product. The output connector on the photodiode in the Fabry-Perot Interferometer is SMA. Included with the interferometer is a cable with an SMA connector on one end and a BNC connector on the other end. This information will soon be part of our product presentation. Thank you once again for taking the time to help us make our product presentation better.
Poster: melsscal
Posted Date: 2008-03-19 06:24:39.0
How we will get the output of this etalon?
Poster: kbuffington
Posted Date: 2007-10-18 11:45:57.0
Consider adding the following:
If launching a free space beam into the interferometer, the beam size should be approximately 4mm. For optimum performance, the beam should then be focused to a waist with less than 600um diameter using a lens with approximately 250mm focal length. The center of the Fabry-Perot cavity should be located near the beam waist. To achieve this, use an FC/PC collimator (Thorlabs item# F260FC-*, page 1010 Thorlabs catalog, Vol 19) to collimate the beam from your fiber coupled laser. Then a lens with 250mm focal length should be inserted into the collimated beam to produce the desired spot size. We recommend the Thorlabs item# LB1056-C found on page 708 in the Thorlabs catalog, Vol 19.
* The input aperture of the FP cavity is larger than the Max Beam Diameter. However, if the beam exceeds this specification, the resolution of the instrument will decrease. ** The mirror substrates for the SA200-18B are IR-Grade Fused Silica (Infrasil)
* The input aperture of the FP cavity is larger than the Max Beam Diameter. However, if the beam exceeds this specification, the resolution of the instrument will decrease. ** The mirror substrates for the SA210-18B are IR-Grade Fused Silica (Infrasil)
Adjustable DC Offset of Scan Voltage (Center Signal on Scan Midpoint)
Adjustable (0.01 - 10 s) Scan Time
Triangle or Sawtooth Scan Voltage
Transimpedance Gain Amplifier for Photodiode Output
The SA201 is specifically designed to control Thorlabs' Fabry-Perot interferometers by generating a highly stable, low-noise voltage ramp. This ramp signal is used to scan the separation between the two cavity mirrors. The controller provides adjustment of the ramp voltage and scan time, allowing the user to choose the scan range and speed while an offset control is provided to allow the spectrum displayed on the oscilloscope to be shifted right or left. The output trigger allows the user to externally trigger an oscilloscope on either the beginning or midpoint of the ramp waveform. The ability to trigger the oscilloscope from the mid-point makes zooming in on a line shape more convenient; just place the spectral component of interest on the center of the screen and increase the timebase of the scope. There is no need to use the offset to re-center the signal; the scope expands about the point of interest.
Another convenient feature of the controller is a calibrated zoom capability that provides a 1X, 2X, 5X, 10X, 20X, 50X and 100X increase in the length of the ramp signal, thus allowing an extremely wide range of scan times. The output TTL level trigger allows the user to externally trigger an oscilloscope on either the beginning or midpoint of the ramp waveform.
The SA201 also includes a high precision photodetector amplifier circuit used to monitor the transmission of the cavity. The amplifier provides an adjustable transimpedance gain of 10k, 100k, and 1MV/A when driving a high impedance load, such as an oscilloscope. Using the output sync signal from the controller, an oscilloscope can be used to display the spectrum of the input laser. The detector circuitry incorporates a blanking circuit, which disables the photodiode response during the falling edge of the sawtooth waveform.
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SA200-3B
- 350-535 nm, Extended Range Scanning Fabry-Perot, 1.5 GHz FSR
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