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Herriott Cell for Multipass Absorption Spectroscopy
Angled Optical Ports for
Ø2" Concave Mirrors
Gas Ports with
Herriott Cell with
A Herriott cell increases the path length of a beam through the cell.
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The interface on each mounting plate is angled at the necessary beam injection angle for ease of alignment.
See the Getting Started tab above for details.
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Each mounting plate has an internal SM1 thread for compatibility with our Ø1" Lens Tubes or other
SM1-threaded components such as photodetectors.
Thorlabs' Herriott cell is a compact solution for achieving long path lengths in gas absorption spectroscopy applications. This Herriott-type multipass cell features a 10.4 m optical path length and concave mirrors with protected gold coatings designed for enhanced performance in the mid-IR. Each cell provides two gas connection ports for use in either a static pressure configuration or as a flow cell. All Herriott cells are individually vacuum and leak tested to make sure each unit meets specifications. The optical apertures on each end feature mounting plates for a variety of Thorlabs optomechanical standards for ease of alignment. To request a quote for a custom Herriott cell with other windows, mirror coatings, or gas ports, please contact Tech Support.
Herriott-Type Multipass Cells
Usually referred to as Herriott cells, these long-optical-path absorption cells are used primarily for weak-trace-gas absorption spectroscopy applications such as environmental monitoring, combustion processes, medical diagnostics, and fundamental atomic and molecular physics. Compared to other long-optical-path absorption cells, such as the White cell, the Herriott cell is more stable to small perturbations. The major advantages of the Herriott cell are that it provides a relatively long absorption path in a compact design and that it is much simpler than high-finesse optical cavities (which would typically require spatial mode matching, precise optical alignment, or resonant excitation). Please refer to the Herriott Cell Mirrors presentation for more details on the principles behind Herriott cells, and see the Applications tab above for three application examples.
Each Herriot cell is equipped with two gas connection ports. If not operated in a flow cell configuration, the second gas port could instead be used to connect a pressure gauge or other similar device to the cell. The gas ports are equipped with standard Swagelok stainless steel tube fittings (male connectors). The HC10L-M02 Cell features a fitting for a 1/4" OD tube while the HC10L/M-M02 features a fitting for a 6 mm OD tube. The tube fittings can be disconnected for access to the G1/4" parallel female thread connector in the cell housing. However, note that the leakage rate is not guaranteed if the gas fitting is replaced or adjusted by the end user. If you are unsure how to connect the cell to your existing vacuum or gas supply system, please contact Tech Support.
Angled Optical Ports
This multipass Herriot cell has an aperture on each end with a mounting plate that provides compatibility with Thorlabs' FiberPort Collimators, 30 mm Cage System, and Ø1" Lens Tubes. The surface of each mounting plate is manufactured to be perpendicular to the nominal beam path to simplify alignment. For details on how to take advantage of these mounting features, even when operating the cell with free-space optics not directly attached to the cell, please refer to the Getting Started tab above.
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This graph shows the measured reflectance of our MIR-enhanced-gold-coated mirrors near normal incidence. The shaded region in the graph denotes the range over which we guarantee the specified reflectance. Please note that the reflectance outside of this band is typical and can vary from lot to lot, especially in out-of-band regions where the reflectance is fluctuating or sloped.
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This graph shows the measured transmission of an uncoated calcium fluoride window at normal incidence.
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This graph shows simulated throughput for the HC10L(/M)-M02 Herriott cell using measured transmission values for the calcium fluoride windows and measured reflectance values for the MIR-enhanced-gold-coated mirrors. The shaded region in the graph denotes the recommended operating wavelength range. For wavelengths longer than 8 µm, the calcium fluoride cell windows must be replaced by windows made of an appropriate material, e.g. zinc selenide. For more information, please see the Troubleshooting section of the Getting Started tab above, or contact Tech Support.
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Each mounting plate has tapped holes for Ø6 mm cage rods, tapped holes for a FiberPort Collimator, and an optical aperture with internal SM1 threads.
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The mounts are compatible with 1/4"-20 (M6) cap screws and their spacing is adjustable. Each locking setscrew has a 5/64" (2.0 mm) hex.
First place the Herriott cell mounts on an optical table or breadboard. Next, place the Herriott cell on the mounts and make sure that the cell rods snap into the mounts. Adjust the mount spacing to match the hole spacing of your table or breadboard. Secure the cell using the locking setscrews in each mount, shown in the photo to the right. Then remove the end caps (replacement item # SM1CP2) from each optical port. The cell can be used in either direction, in the sense that there is no designated input or output. Note that to reduce spectroscopic background effects, the cell windows are mounted perpendicular to the common optical axis of the Herriott cell mirrors. This means they are approximately 3° from being perpendicular to the nominal optical beam path (refer to the Specs tab for the angles of incidence).
The angled mounting plates on each end of the cell provide compatibility with Thorlabs' FiberPort Collimators, 30 mm Cage System, and Ø1" Lens Tubes, as illustrated in the photo to the far right. The interface surfaces of the mounting plates are manufactured to be perpendicular to the nominal beam path, simplifying alignment even when operating the cell with free-space optics not directly attached to the cell. See the example in the Free-Space Operation section below for more details.
Each gas port is sealed with an end plug, which must be removed prior to connecting a tube or gas pipe to the port. The plug can be removed using a 9/16" (14 mm) wrench; make sure the gas port adapter is held firmly in the cell housing by using a 1/4" (19 mm) wrench while removing the Swagelok plug. To seal a gas port, leave a plug tightened onto it. Note that the leak test during manufacturing was performed with the plug installed on the port in the unengraved housing and with the port in the engraved housing connected to the vacuum and leak test system.
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Each mounting plate is angled at the necessary beam insertion angle for ease of alignment. ER10 Cage Rods with CPA1 Alignment Plates are shown.
We also recommend the use of two folding mirrors prior to the Herriott cell to position and angle the incident beam. After initial alignment, make sure that the beam is not truncated by the apertures. This can be done by carefully introducing small misalignments to the beam and monitoring the cell transmitted intensity with a large-area photodetector (making sure all transmitted light is being detected); the transmitted power should not change as the small misalignments are introduced.
Please note that truncation of the beam by any element along the path will result in a residual background signal imposed on the spectroscopic signal, reducing the system's overall performance. When operating the cell with invisible laser light, pre-aligning the system with with visible laser light (e.g. in the reverse beam direction) can simplify the alignment of the invisible beam.
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The mounting plates are also compatible with FiberPort Collimators. Note that the FiberPorts can be prealigned with a visible laser when working with infrared light.
It is possible that the spectroscopic signal observed after the multipass cell is deteriorated by fringes. Among other reasons (e.g. standing waves in optical elements before or after the cell, or standing waves between different optical components), this problem can occur when the light beam is truncated. Please ensure that:
If the performance is still insufficient, a weak fringing effect might arise from the cell windows (Item # WG50530). If this is the factor limiting overall system performance, it is possible to replace the windows with wedged windows (e.g. Item # WW50530). Each window is secured with a POLARIS-SM05RR SM05-Threaded Retaining Ring, which can be removed with an SPW603 Spanner Wrench. There is also a Viton O-ring (9.25 mm inner diameter x 1.78 mm thickness) between each window and the cell and another between each window and retaining ring.
Herriott Cell Considerations
The Herriott cell can be implemented into optical setups in a wide variety of configurations, both from an optical point of view and a vacuum system point of view. Optically, there are two fundamentally different approaches when using the cell for analytical spectroscopic applications. These approaches differ in how the spectrum is resolved in the system. One of these approaches is to couple in a tunable narrowband light source (most often a laser) into the cell and then use a photodetector on the other side of the cell for analysis. In this case, the light source is the spectrally resolving part; by tuning the frequency of the light source over a spectral feature of the target analyte and by knowing the relevant experimental parameters, quantitative spectroscopy can be performed. The other approach incorporates a broadband source (e.g. Globar, LED, superluminescent source, or a broad linewidth laser) and a spectrally resolving device at the output of the cell, for instance a Fourier-based optical spectrum analyzer, a grating, or a tunable optical filter. It is important in all cases that the system is capable of precisely selecting a target feature at a specific wavelength.
Before a gas sample can be analyzed, it must fill the Herriott cell. A number of parameters must be determined in order to find a system design that meets the requirements imposed by the spectroscopic application. The answers to the following questions will help you decide on the experimental setup:
The three sample configurations below include either a pressure gauge, a mass flow controller, or both. Additional valves may be useful for improved control over the pressure or flow rate.
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Figure 1: Flow Cell Configuration at Atmospheric Pressure
Example 1: Flow Cell Configuration at Atmospheric Pressure
Example 3: Steady State Sub-Atmospheric Pressure Experiment
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Figure 2: Flow Cell Configuration at Sub-Atmospheric Pressure
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Figure 3: Steady State Sub-Atmospheric Pressure Experiment
Damage Threshold Data for Thorlabs' Mid-Infrared Enhanced Mirrors
The specifications to the right are measured data for Thorlabs' mid-infrared enhanced gold mirror coatings. Damage threshold specifications are constant for a given coating type, regardless of the size or shape of the mirror.
Laser Induced Damage Threshold Tutorial
The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.
Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.
Continuous Wave and Long-Pulse Lasers
When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) . Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.
When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.
Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.
In order to use the specified CW damage threshold of an optic, it is necessary to know the following:
Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below.
The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).
Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):
While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.
As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.
Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism . In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.
Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately . A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):
You now have a wavelength-adjusted energy density, which you will use in the following step.
Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT . For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.
The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:
Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1998).