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Scanning Fabry-Perot Interferometers
1.5 GHz FSR, 3.0 - 4.4 µm
10 GHz FSR, 820 - 1275 nm
1.5 GHz FSR, 290 - 355 nm & 520 - 545 nm
Schematic Representation of a Confocal Fabry-Perot Interferometer
Each Fabry-Perot interferometer features a thermally stable Invar® cavity.
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SA200-18C with Included Photodiode Removed
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Thorlabs' Scanning Fabry-Perot Interferometers are spectrum analyzers that are ideal for examining fine spectral characteristics of CW lasers. Interferometers are available with a Free Spectral Range (FSR) of 1.5 GHz or 10 GHz. The resolution, which varies with the FSR and the finesse, ranges from <1 MHz to 67 MHz. For information on these quantities and their applications in Fabry-Perot interferometry, see the Fabry-Perot Tutorial tab.
The confocal Fabry-Perot cavity transmits only very specific frequencies. These transmission frequencies are tuned by adjusting the length of the cavity using piezoelectric transducers, as shown in the diagram to the right. The transmitted light intensity is measured using a photodiode, amplified by the transimpedence amplifier in the SA201 controller (or equivalent amplifier), and then displayed or recorded by an oscilloscope or data acquisition card. Each Fabry-Perot interferometer has a cable ending in a BNC connector for controlling the piezo.
The mirrors in the SA200-18C and SA210-18C are made of IR-grade fused silica (Infrasil®), the mirrors in the SA200-30C are made of yttrium aluminum garnet (YAG), and the mirrors on all other models are made of UV fused silica. Each Fabry-Perot interferometer also features an internal housing made of thermally stable invar to eliminate misalignment due to temperature fluctuations.
The SA200 and SA210 models feature SM1 (1.035"-40) and SM05 (0.535"-40) threading, respectively, on the back of the instrument for detector mounting. Except for the SA200-30C, each Fabry-Perot scanning interferometer comes with a photodiode detector included, as well as an SMA-to-BNC cable to connect the detector to an amplifier. This photodiode can be removed for alignment purposes or for replacement with another detector.
For the SA30-52, which has a finesse of >1500, additional steps are necessary to ensure proper alignment of the interferometer. The system will need to be fine-tuned while observing a transmission mode in order to suppress higher order modes. For more details, see the Alignment Guide tab.
Reflectance per surface for all of our Fabry-Perot interferometers are plotted here. See the Graphs tab for more information. Custom mirror coatings for wavelengths from the UV to the mid-IR
(200 nm to 4700 nm) are available upon request. If the coatings represented in the graphs below do not suit the needs of your application, please contact Tech Support.
The plots below show the reflectance of the mirrors in our Fabry-Perot interferometers. "Mirror Finesse" refers to the contribution to the total finesse value due to the reflectance of the mirrors. In a system with near-perfect alignment, the finesse of the Fabry-Perot cavity will be limited by the reflectance of the mirrors, and the finesse values will approach those shown on the plots below. For more information on finesse, please see the Fabry-Perot Tutorial tab.
Please remember that the actual reflectance 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 Fabry-Perot Tutorial tab for more information.
Custom mirror coatings for wavelengths from the UV to the mid-IR (200 nm to 4700 nm) are available upon request. If the coatings represented in the graphs below do not suit the needs of your application, please contact Tech Support.
Interferometer with Finesse ≥1500
Interferometers with Finesse >150 or >200
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Click Here for Raw Data
Measured Reflectance Data and Calculated Mirror Finesse
This data has been smoothed in the wavelength region from 2.65 - 2.7 µm due to measurement inaccuracies introduced by absorption by water vapor in the air. For details on operation from 2.6 - 2.8 µm, see the -18C/30C Detectors tab.
Figure 2: Simple Diagram Showing Laser Focus Inside FP Cavity
Figure 1: Ray Trace of an Off-Center Input Beam
Our Scanning Fabry-Perot (FP) 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 in Figure 1, 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 4d-h4/d3, where d is the distance between 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. 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 SA30, 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, which is marked by a groove in the outer housing of the instrument (See Figures 3 and 4).
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Figure 4: SA210 Interferometer
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Figure 3: SA200 Interferometer
Coupling a Free-Space Beam into a Scanning Fabry-Perot Interferometer
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Figure 5: SA210 FP Interferometer Free-Space System
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Figure 6: SA200 FP Interferometer Free-Space System
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Figure 7: Flipper Mirror in the Up Position (left) Intercepts Beam and Directs it to Interferometer Setup. Flipper Mirror in the Down Position (right) Allows Beam to Downstream Optics.
Figure 5 above depicts a setup to integrate an SA210 FP Interferometer into a free-space system. Figure 6 shows a similar setup for the SA200. The easiest method for integration is through the use of a flipper mirror and kinematic interferometer mount. The advantage of the flipper mirror is that it allows normal system operation with the ability to measure the source spectrum when necessary, without repositioning optics and equipment. During normal operation, the mirror can simply be flipped out of the way of the beam, allowing the beam unobstructed propagation downstream. When necessary, the mirror can be flipped upright to intercept the beam and pass it down to the interferometer (see Figure 7). The system shown in Figure 5 is a single mirror system; here, the flipper mirror intercepts the beam, which is then directed toward the focusing lens and the interferometer in a kinematic mount. This minimizes space and components required, useful in a compact setup.
The SA210 FP Interferometer is shown mounted in a KC1 (the KS1 would work equally as well) kinematic mount; the SA200 FP Interferometer is shown mounted in a KC2 (the KS2 would work equally as well) kinematic mount. Figures 5 and 6 show a mounted lens threaded into the LMR1 lens mount (an unmounted lens will work just as well). The specific optics (mirror and lens) will depend on the wavelength of the system.
Measure the height of the beam from the table surface; in general, it is good practice to start with your optics centered at the height of your beam. Install the flipper mirror assembly (mirror mount with mirror, flip mount, post, and post holder) with the mirror in the up position at a 45° angle to the beam. 90° bounces make the initial alignment and walking the beam for fine-alignment much easier. The tapped holes of the optical table make an excellent guide for the initial setup.
With the flipper mirror installed and correctly deflecting the beam by 90°, secure the flipper mirror assembly to the table with a 1/4"-20 (M6) screw. Then mount the interferometer so that the beam enters the center of the aperture; the iris may be used to guide the alignment. While not necessary, if the vertical centerline of interferometer is set at the height of the beam, the initial setup should line up nicely and produce enough of a signal to simply use the kinematic mount of the flipper mirror and the kinematic interferometer mount to guide the beam into its optimal alignment.
Turn on the Fabry-Perot controller box, and start scanning the length of the cavity (set the amplitude at >10 V to ensure that more than 1 peak is displayed) since light will only be transmitted when the cavity length is resonant with the wavelength of the light beam. Connect the detector output and the trigger or ramp signal to an oscilloscope. If no signal is detected at this point, it might be necessary to remove the detector from the back of the Fabry-Perot cavity in order to coarsely align the cavity. The iris located in the back of the interferometer can also be used to guide the alignment. However, this may be unnecessary if care is taken in the initial placement of the optics. Use the kinematic mount holding the interferometer and the flipper mirror to walk the beam until the Fabry-Perot cavity is correctly aligned.
Insert the lens (according to the table above) in the path at the specified distance so that the beam waist is centered in the Fabry-Perot cavity (marked by a groove on the FP housing, see Figures 3 and 4). Adjust the height and position of the lens to center the beam on the entrance aperture. The mirror and interferometer mount can be used to tweak the signal back into its optimal levels.
Here, the higher order transverse modes are still visible. This setup requires further tweaking to optimize alignment.
Here, the higher order modes are suppressed. This setup is properly aligned.
This section describes the electrical connections between a Fabry-Perot (FP) interferometer, an SA201 control box, and an oscilloscope. The SA201 provides a voltage ramp to the piezoelectric transducer inside the FP cavity, which controls the cavity length. The oscilloscope is used to view the output from the scanning FP and the controller.
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Recommended Setup for SA200 Series Fabry-Perot Interferometers (Except SA200-30C; See Diagram to the Right)
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Recommended Setup for SA200-30C Fabry-Perot Interferometer
SA30, SA200, & SA210 Series Scanning Fabry-Perot Interferometers
Piezo (Ramp In) - BNC Male
150 V Maximum
Photodiode Out - SMA Female
SA201 Control Box for Scanning Fabry-Perot Interferometers
Trigger Output BNC Female
This trigger output signal may be used to externally trigger the oscilloscope. The trigger is capable of driving 50 Ω terminated cables, as well as Hi Z loads such as oscilloscopes. The trigger will provide an edge on the beginning and middle of the scanning ramp.
Output BNC Female
The output BNC is used to drive the SA200 scanning piezos from 1 to 45 V. The output is capable of driving 0.6 μF piezo loads at a ramp rate of 1 ms over the full voltage range. The output current is internally limited to prevent damage to the output drive.
PD Amplifier Input BNC
This input BNC is used to interface the photodetector, provided with the SA200 scanning heads, to the amplifier circuit. The photodiode amplifier is configured to operate with the Thorlabs supplied photo detectors; however, it is possible to operate user supplied photodetectors. To do so, the BNC center contact must be connected to the photo detector cathode and the BNC shell must be connected to the photodiode anode (unbiased operation). If a biased detector is to be used, the BNC shell must be connected to the bias ground and the bias voltage must be negative for the circuit to operate properly.
PD Amplifier Output BNC
This BNC is the amplifier output and may be connected directly to an oscilloscope to view the cavity spectrum. The amplifier gain will be set using the front panel 'DETECTOR' control knob. The amplifier output includes a 50 Ω series resistor to minimize noise when operating with a 50 Ω coax cable. For best results, a 50 Ω load resistor is recommended at the oscilloscope. Note, the amplifier gain will be halved with a 50 Ω load connected.
Calibrating Time Scale and Finesse
For a Fabry-Perot cavity, the finesse is a measure of the interferometer's ability to resolve closely spaced spectral features. The total finesse of an interferometer is defined as the ratio of the Free Spectral Range (FSR) to the Full Width at Half Maximum (FWHM) of the resonant peak, where Δ is the FWHM.
The equation for the total finesse is given by
The FSR and the FWHM can be in either units of meters or Hz (be sure to use consistent units); an example of the FSR and linewidth are shown in Figures 1 and 2, respectively. For more detailed information on the finesse, please see the Fabry-Perot Tutorial tab.
Calibrating the Time Scale
Figure 1: An FSR Plot, Using a 1550 nm DFB Laser (PRO8000 Series). Using the model SA200-12B, 1.5 GHz interferometer, this plot is used to calibrate the time-base of the oscilloscope. Knowing that the FSR of the interferometer is 1.5 GHz, the calibration factor is found by setting 1.5 GHz = 3.2 ms between the two peaks.
Figures 1 - 3 show example oscilloscope traces for the time scale calibration of our Fabry-Perot interferometers. For some of the more advanced oscilloscopes, as shown by the trace in Figure 3, linewidth and period analysis is automatic. These will typically display the information somewhere on the screen (in this case, it’s displayed at the bottom). For other scopes, the user may have to calibrate the time scale themselves. Figures 1 and 2 show this process for manual calibration using the SA200-12B 1.5 GHz interferometer (for many digital oscilloscopes, cursers can be used to mark the position of peaks on screen). Figure 1 shows the full FSR of the interferometer, with the two peaks separated by 1.5 GHz. By measuring the time between the peaks (3.2 ms in this example), the proper calibration can be calculated; 468.8 MHz/ms for our example. Once the time scale calibration is known, we can zoom in on one of the peaks to measure the FWHM in time (shown in Figure 2). The measured FWHM in this example is 10 µs (0.010 ms) and yields a linewidth of 4.7 MHz. From here the finesse of the system can be calculated along with any other necessary measurement.
It may be beneficial to express either the FSR or the linewidth in terms of wavelength. The conversion is given by:
Here, δλ is the FSR or linewidth in space, δν is the FSR or linewidth in frequency, λ is the wavelength of the laser, and c is the speed of light. For example, in Figure 2 we have a FSR of 1.5 GHz for a 1550 nm laser. Converting to wavelength, we find that the FSR is 0.0121 nm. Likewise, the linewidth from Figure 3 is 5.1 MHz, which yields a linewidth of 0.000038 nm.
Figure 2: This plot shows a close-up of the actual signal of the laser, which results from the convolution of the laser linewidth and finesse of the cavity; with the oscilloscope timebase calibrated from Figure 1, at 468.8 MHz/ms, we determine the FWHM for the interferometer to be 0.010 ms x 468.8 MHz/ms for a FWHM of 4.7 MHz. This provides a lower limit for the finesse of 320.
Figure 3: This plot shows an FSR plot on a scope that automatically analyzes pulse width and period. The linear yellow line shows the voltage ramp, the blue line shows the FSR trace.
Scanning Fabry-Perot Interferometers
The core of Thorlabs' scanning Fabry-Perot interferometers is an optical cavity formed by two, nearly identical, spherical mirrors separated by their common radius of curvature as shown in Figure 1. This configuration is known as a mode-degenerate, confocal cavity design, which is generally referred to as a confocal Fabry-Perot. The cavity is mode degenerate because the frequency of certain axial and transverse cavity modes are the same (degenerate). This degeneracy greatly simplifies the alignment of the instrument by eliminating the need to carefully mode match the input to the cavity. The confocal design offers several benefits over flat-plate interferometers such as easier alignment since the confocal interferometer is fairly insensitive to angular alignment. Additionally, the confocal design offers a unique property that at constant finesse as resolving power increases, so does the etendue (etendue is defined as the radiation from a source within a solid angle, Ω, subtended by an aperture with area A). By contrast, in a flat-plate interferometer as resolving power increases (at constant finesse) the etendue decreases; meaning that an increase in light intensity decreases resolution and vice-versa.
The inner concave surface of each mirror has a highly reflective coating, while the outer convex surface has a broadband antireflection coating. The curvature on the outer surface, which matches that of the inner surface, eliminates lensing effects. In a confocal system, the mirror spacing matches the curvature of radius (r) as depicted in Figure 1. To illustrate the operation of this confocal cavity, it is useful to follow a ray as it enters the cavity off-axis and travels one round trip through the cavity. As seen in Figure 2, the beam enters the cavity at a height H. A portion of the ray follows the path numbered 1, 2, 3, and 4; it is then reflected onto path 1 again. The dotted lines exterior to the cavity in Figure 2 represent the portion of the ray that is transmitted through the cavity mirrors when the cavity is resonant with the input (i.e., when the round trip distance equals mλ, where m is an integer, and λ is the wavelength of the input radiation). The approximate optical path length (L) of one round trip through the cavity can be expressed as
L = 4r
Resonance and Free Spectral Range (FSR)
In order to achieve a maximum in resonance from a Fabry-Perot cavity, the complete round-trip phase delay must be a multiple of 2π. For a plano-plano Fabry-Perot cavity with a round trip distance of 2r, this condition is satisfied when the frequency is mc/2r, were m is any integer, c is the speed of light in air, and r is the mirror separation. Therefore, the separation, or Free Spectral Range (FSR) between two transmission peaks is c/2r.
For a confocal Fabry-Perot cavity, we must take into account that the modes of the cavity are Gaussian. By taking into account the phase shift of a Gaussian mode in the confocal cavity, it can be shown  that resonance frequencies of the transverse modes either overlap or fall exactly halfway between the longitudinal mode resonances. Therefore, the FSR for a confocal cavity the free spectral range is c/4nd.
Total Corrected Round Trip Distance
Because the actual optical path length of the confocal Fabry-Perot cavity is dependent on H, the resonance condition will vary across the input beam. This variation across the input aperture is a critical practical consideration when using a confocal Fabry-Perot Interferometer.
In order to develop an equation that relates the resolution of the interferometer to H, the geometrical optical path length with a correction for spherical aberration must be considered. Doing so with the approximation that 0 < H << r, yields an optical path length of
L = 4r - H4/r3
As the input beam diameter increases, the second term in Eq. (2) becomes significant in comparison to λ.
Finesse and Resolution of a Confocal Cavity
For a Fabry-Perot cavity, the finesse is a measure of the interferometer's ability to resolve closely spaced spectral features. The minimum resolvable frequency increment of an interferometer is based on the Rayleigh Criterion, which stipulates that for two closely spaced lines of equal intensity to be resolved, the sum of the two individual lines at the midway point can at most be equal to the intensity of one of the original lines (see Figure 3).
The total finesse of an interferometer is defined as the ratio of the FSR to the FWHM of the resonant peak, where Δ is the FWHM. As can be seen in Figure 3, two lines separated by Δ are just resolvable according to the Rayleigh criterion. Therefore, Δ quantifies the resolution of the system.
The equation for the total finesse is given by
Ft = FSR/Δ
Note that during the manufacturing of the interferometers, Ft is maximized in order to adjust the cavity length to the confocal condition by maximizing its value. This method provides a very precise means for setting the required length of the cavity to better than λ.
The FSR and the FWHM of a representative lineshape are shown in Figures 4 and 5, respectively. An Ft of 294 is measured using a DFB laser with a linewidth that cannot be considered infinitely small in comparison to the resolution of the cavity. Therefore, the true Ft is about 320, assuming a 2 MHz laser linewidth.
A measured finesse has a number of contributing factors: the mirror reflectivity finesse FR, the mirror surface quality finesse Fq, and the finesse due to the illumination conditions (beam alignment and diameter) of the mirrors Fi. Therefore, the total inverse of the finesse of a system can be written as
1/Ft = [(1/FR)2 + (1/Fq)2 + (1/Fi)2]1/2
where, for mirrors with a reflectivity close to 1, the effective mirror reflectivity finesse is given by
FR = pi√/(1-R)
Here, R is the mirror reflectivity.
While the definition for the reflectivity finesse is ambiguous, Eq. (5) is presented as an effective finesse that is defined by Eq. (3) when the other contributing factors are negligible. For these interferometers, the reflectivity finesse dominates when operating with proper illumination .
Using Eq. (5), the reflective coatings in the interferometers have been designed so that the minimum FR is better than 1.5 times the minimum specified finesse across their entire operating wavelength range for each model (see the table on the product page). This fixes the first term of Eq. (4).
The second term in Eq. (4) involves Fq, which accounts for mirror irregularities that cause a symmetric broadening of the lineshape. The effect of these irregularities is a random position-dependent path length difference that blurs the lineshape. The manufacturing process that is used to produce the cavity mirrors ensures that the contribution from Fq is negligible in comparison to our specified total finesse for each model.
The final term in Eq. (4), which deals with the illumination finesse Fi, will reduce the resolution as the beam diameter is increased or as the input beam is offset. When the finesse is limited by the Fi term, the measured lineshape will appear asymmetric. The asymmetry is due to the path length difference between an on-axis beam and an off-axis beam, resulting in different mirror spacings to satisfy the maximum transmission criteria. The approximate decrease in path length for a beam at a distance H off axis is given by the second term in Eq. (2).
To quantify the effects of the variable path length on Fi, consider an ideal monochromatic input, a delta function in wavelength with unit amplitude, entering the Fabry-Perot cavity coaxial to the optic axis and having a beam radius a. The light entering the interferometer at H = +e, where e is infinitesimally small but not zero, will negligibly contribute to a deviation in the transmitted spectrum. Light entering the cavity at H = +a will cause a shift in the transmitted output spectrum, since the optical path length of the cavity will be less by an approximate distance of a4/4r3. Assuming the input beam has a uniform intensity distribution, the transmitted spectrum will appear uniform in intensity and broader due to the shifts in the optical path length. As a result, the wavelength input delta function will produce an output peak with a FWHM of H4/4r3.
Assuming that only Fi contributes significantly to the total finesse, then Eq. (3) can be used to calculate Fi for the idealized input beam:
Fi = FSR/FWHM
Substituting λ/4 for the FSR, and (H4/4r3) for FWHM, yields
Fi = (λ/4)/(H4/4r3)
The λ/4 substitution for the FSR is understood by considering that the cavity expands by λ/4 to change from one longitudinal mode to the next. For an input beam with a real spectral distribution, the effect of the shift will be a continuous series of shifted lineshapes.
It should be noted that the shift is always in one direction, leading to a broadened or assymmetric lineshape due to the over-sized or misaligned beam.
Now, using Eq. (4), the total finesse, which includes significant contributions from both FR and Fi can be found (Note: Fq is still considered to have a negligible effect on Ft):
Ft = [ (1/FR)2 + (1/Fi)2 ]-1/2
Replacing Fi and FR yields:
Ft(H, R) = [ ((1-R2)/piR)2 + (H4/λr3)2]-1/2
Eq. (8) is used to provide an estimate (albeit an overestimate) of effects of beam diameter effects on the total finesse of a Fabry-Perot Interferometer. Several assumptions lead to the overestimation of finesse. One is that the diameter of the beam is the same as the diameter of the mirror, in practice the diameter of the beam is typically significantly smaller than that of the mirror (this also helps to reduce spherical aberration) . Another assumption is that the light is focused down to an infinitesimally small waist size, even for monochromatic light the minimum waist size is limited by diffraction, and in multimode applications the waist size can be quite large at the focus. Figure 6 provides a plot of Eq. 8 for the two cavity designs offered (r = 50 mm and r = 7.5 mm). The traces in the plot were made with the assumption that the reflectivity finesse is equal to 300, which is the typical value obtained for mirrors used in our interferometers.
Spectral Resolving Power and Etendue
The spectral resolving power of an interferometer is a metric to quantify the spectral resolution of an interferometer, and is an extention of the Rayleigh criterion. The spectral resolving power, SR, is defined as:
SR = v/Δv = λ/Δλ
In Equation (9), v is the frequency of light and λ is its wavelength. It can be shown that for a confocal Fabry-Perot interferometer, the SR is given by:
SR = 4rF/λ
In Equation (10), F is the finesse of the interferometer, r is the radius of curvature of the mirrors, and λ is the wavelength. However, to achieve this maximum instrumental profile while the interferometer is in scanning mode, the aperture of the detector would need to be infinitesimally small; as the aperture is opened wide enough, the spectral resolving power begins to decrease. The spectral resolving power must be balanced with the etendue of the interferometer. The etendue (U) is the metric for the net light-gathering power of the interferometer. When the light source is a laser beam, the etendue provides a measure of the alignment tolerance between the interferometer and the laser beam. The etendue is defined as the product of the maximum allowed solid angle divergance (Ω) and the maximum allowed aperture area (A). For the confocal system the etendue is given by:
U = pi2λd/F
In Equation (11), F is the finesse of the interferometer, λ is the wavelength, and d is the mirror spacing. For proper use of the interferometer the spectral resolving power and etendue need to be balanced such that enough light is allowed to enter the system without significantly reducing the resolution of the interferometer. The accepted compromise for this balance is to increase the mirror aperture until the the spectral resolving power is decreased by 70% (0.7*SR) . Under this condition the "ideal" etendue becomes π2λr/F, where r is the mirror's radius.
The SA30-52 is a high finesse version of our 1.5 GHz FSR Fabry-Perot interferometers. The resolution of this interferometer is 1.0 MHz. The mirrors are designed for use in the 488 - 545 nm range (see the Graphs tab above).
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PDAVJ5 Detector Installed
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PDA10PT Detector Installed
The SA200 series of Fabry-Perot interferometers have a free spectral range of 1.5 GHz. With a minimum finesse of 200, the resolution of these interferometers is 7.5 MHz. Seven optical coatings are available with operating ranges from 290 to 4400 nm, including one dual-wavelength coating (SA200-2B). For details on these wavelength ranges, see the Graphs tab above.
For the SA200-18C, the included photodiode detector is sensitive to wavelengths from
The SA200-30C has a coating designed for 3.0 - 4.4 µm and does not include a detector. We recommend using the PDAVJ5 HgCdTe Amplified Photodetector, which is sensitive to wavelengths in the 2.7 - 5.0 µm range. The PDAVJ5 and the SA200-30C, which have external and internal SM1-threading respectively, can be directly mounted together without using additional threaded adapters. Please note that the PDAVJ5 includes a transimpedance amplifier, so the detector should not be connected to the transimpedance amplifier of the SA201 controller when operating the interferometer. Additionally, due to saturation effects of the diode inside the PDAVJ5 detector, the optical power entering the SA200-30C should be kept below 200 µW in order to avoid saturation.
The SA210 series of Fabry-Perot interferometers have a free spectral range of 10 GHz. With a minimum finesse of 150, the resolution of these interferometers is 67 MHz. Five wavelength ranges are available from 350 nm to 2600 nm. For details on these wavelength ranges, see the Graphs tab above.
For the SA210-18C, the included photodiode detector is sensitive to wavelengths from 1.8 - 2.6 µm. Since the reflectance of the mirrors inside this device remains over 99.0% in the 2.6 - 2.8 µm range, an alternative detector can be installed to use the instrument over an extended wavelength range. For this purpose, we recommend Thorlabs' PDA10PT detector, which can be attached to the Fabry-Perot interferometer using an SM1T2 lens tube coupler.
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, which features a power supply with a 100, 115, or 230 VAC switch-selectable input, 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 midpoint 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. A calibrated zoom capability provides a 1X, 2X, 5X, 10X, 20X, 50X, or 100X increase in the length of the ramp signal, thus allowing an extremely wide range of scan times.
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 10 kV/A, 100 kV/A, or 1000 kV/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.
The SA201 is shipped with a 120 VAC power supply line cord for use in the US, while the SA201-EC is shipped with a 230 VAC power supply line cord for use in Europe.