1つ目のステアリングミラーは、新しい光路上に配置された2つ目のミラーに向けてビームを反射します。2つ目のステアリングミラーは、新しい光路に沿うようにアライメントします。2つのステアリングミラーでレーザ光をアライメントする手順は、Walking the Beam(ビームの移動)として説明することがあり、その結果はFolded Beam Path(折りたたまれたビーム路)と呼ばれることがあります。動画4の例では、ビームを新しい光路にアライメントするために2つのアイリスが使用されています。新しい光路は光学テーブル面に対して平行で、タップ穴列に沿っています。
参考文献  Ping-Shine Shaw, Zhigang Li, Uwe Arp, and Keith R. Lykke, "Ultraviolet characterization of integrating spheres," Appl.Opt.46, 5119-5128 (2007).  Jan Valenta, "Photoluminescence of the integrating sphere walls, its influence on the absolute quantum yield measurements and correction methods," AIP Advances8, 102123 (2018).  Robert D. Saunders and William R. Ott, "Spectral irradiance measurements: effect of UV-produced fluorescence in integrating spheres," Appl. Opt.15, 827-828 (1976).  Ping-Shine Shaw, Uwe Arp, and Keith R. Lykke, "Measurement of the ultraviolet-induced fluorescence yield from integrating spheres," Metrologia46, S191 - S196 (2009).
Figure 1: C-mount lenses and cameras have the same flange focal distance (FFD), 17.526 mm. This ensures light through the lens focuses on the camera's sensor. Both components have 1.000"-32 threads, sometimes referred to as "C-mount threads".
Figure 2: CS-mount lenses and cameras have the same flange focal distance (FFD), 12.526 mm. This ensures light through the lens focuses on the camera's sensor. Their 1.000"-32 threads are identical to threads on C-mount components, sometimes referred to as "C-mount threads."
The C-mount and CS-mount camera system standards both include 1.000"-32 threads, but the two mount types have different flange focal distances (FFD, also known as flange focal depth, flange focal length, register, flange back distance, and flange-to-film distance). The FFD is 17.526 mm for the C-mount and 12.526 mm for the CS-mount (Figures 1 and 2, respectively).
Since their flange focal distances are different, the C-mount and CS-mount components are not directly interchangeable. However, with an adapter, it is possible to use a C-mount lens with a CS-mount camera.
Mixing and Matching C-mount and CS-mount components have identical threads, but lenses and cameras of different mount types should not be directly attached to one another. If this is done, the lens' focal plane will not coincide with the camera's sensor plane due to the difference in FFD, and the image will be blurry.
With an adapter, a C-mount lens can be used with a CS-mount camera (Figures 3 and 4). The adapter increases the separation between the lens and the camera's sensor by 5.0 mm, to ensure the lens' focal plane aligns with the camera's sensor plane.
In contrast, the shorter FFD of CS-mount lenses makes them incompatible for use with C-mount cameras (Figure 5). The lens and camera housings prevent the lens from mounting close enough to the camera sensor to provide an in-focus image, and no adapter can bring the lens closer.
It is critical to check the lens and camera parameters to determine whether the components are compatible, an adapter is required, or the components cannot be made compatible.
1.000"-32 Threads Imperial threads are properly described by their diameter and the number of threads per inch (TPI). In the case of both these mounts, the thread diameter is 1.000" and the TPI is 32. Due to the prevalence of C-mount devices, the 1.000"-32 thread is sometimes referred to as a "C-mount thread." Using this term can cause confusion, since CS-mount devices have the same threads.
Measuring Flange Focal Distance Measurements of flange focal distance are given for both lenses and cameras. In the case of lenses, the FFD is measured from the lens' flange surface (Figures 1 and 2) to its focal plane. The flange surface follows the lens' planar back face and intersects the base of the external 1.000"-32 threads. In cameras, the FFD is measured from the camera's front face to the sensor plane. When the lens is mounted on the camera without an adapter, the flange surfaces on the camera front face and lens back face are brought into contact.
Figure 5: A CS-mount lens is not directly compatible with a C-mount camera, since the light focuses before the camera's sensor. Adapters are not useful, since the solution would require shrinking the flange focal distance of the camera (blue arrow).
Figure 4: An adapter with the proper thickness moves the C-mount lens away from the CS-mount camera's sensor by an optimal amount, which is indicated by the length of the purple arrow. This allows the lens to focus light on the camera's sensor, despite the difference in FFD.
Figure 3: A C-mount lens and a CS-mount camera are not directly compatible, since their flange focal distances, indicated by the blue and yellow arrows, respectively, are different. This arrangement will result in blurry images, since the light will not focus on the camera's sensor.
Figure 7: An adapter can be used to optimally position a CS-mount lens on a camera whose flange focal distance is less than 12.526 mm. This sketch is based on a Zelux camera and its SM1A10 adapter.
All Kiralux™ and Quantalux® scientific cameras are factory set to accept C-mount lenses. When the attached C-mount adapters are removed from the passively cooled cameras, the SM1 (1.035"-40) internal threads in their flanges can be used. The Zelux scientific cameras also have SM1 internal threads in their mounting flanges, as well as the option to use a C-mount or CS-mount adapter.
The SM1 threads integrated into the camera housings are intended to facilitate the use of lens assemblies created from Thorlabs components. Adapters can also be used to convert from the camera's C-mount configurations. When designing an application-specific lens assembly or considering the use of an adapter not specifically designed for the camera, it is important to ensure that the flange focal distances (FFD) of the camera and lens match, as well as that the camera's sensor size accommodates the desired field of view (FOV).
Made for Each Other: Cameras and Their Adapters Fixed adapters are available to configure the Zelux cameras to meet C-mount and CS-mount standards (Figures 6 and 7). These adapters, as well as the adjustable C-mount adapters attached to the passively cooled Kiralux and Quantalux cameras, were designed specifically for use with their respective cameras.
While any adapter converting from SM1 to 1.000"-32 threads makes it possible to attach a C-mount or CS-mount lens to one of these cameras, not every thread adapter aligns the lens' focal plane with a specific camera's sensor plane. In some cases, no adapter can align these planes. For example, of these scientific cameras, only the Zelux can be configured for CS-mount lenses.
The position of the lens' focal plane is determined by a combination of the lens' FFD, which is measured in air, and any refractive elements between the lens and the camera's sensor. When light focused by the lens passes through a refractive element, instead of just travelling through air, the physical focal plane is shifted to longer distances by an amount that can be calculated. The adapter must add enough separation to compensate for both the camera's FFD, when it is too short, and the focal shift caused by any windows or filters inserted between the lens and sensor.
Flexiblity and Quick Fixes: Adjustable C-Mount Adapter Passively cooled Kiralux and Quantalux cameras consist of a camera with SM1 internal threads, a window or filter covering the sensor and secured by a retaining ring, and an adjustable C-mount adapter.
A benefit of the adjustable C-mount adapter is that it can tune the spacing between the lens and camera over a 1.8 mm range, when the window / filter and retaining ring are in place. Changing the spacing can compensate for different effects that otherwise misalign the camera's sensor plane and the lens' focal plane. These effects include material expansion and contraction due to temperature changes, positioning errors from tolerance stacking, and focal shifts caused by a substitute window or filter with a different thickness or refractive index.
Adjusting the camera's adapter may be necessary to obtain sharp images of objects at infinity. When an object is at infinity, the incoming rays are parallel, and location of the focus defines the FFD of the lens. Since the actual FFDs of lenses and cameras may not match their intended FFDs, the focal plane for objects at infinity may be shifted from the sensor plane, resulting in a blurry image.
If it is impossible to get a sharp image of objects at infinity, despite tuning the lens focus, try adjusting the camera's adapter. This can compensate for shifts due to tolerance and environmental effects and bring the image into focus.
Why can the FFD be smaller than the distance separating the camera's flange and sensor?
Flange focal distance (FFD) values for cameras and lenses assume only air fills the space between the lens and the camera's sensor plane. If windows and / or filters are inserted between the lens and camera sensor, it may be necessary to increase the distance separating the camera's flange and sensor planes to a value beyond the specified FFD. A span equal to the FFD may be too short, because refraction through windows and filters bends the light's path and shifts the focal plane farther away.
If making changes to the optics between the lens and camera sensor, the resulting focal plane shift should be calculated to determine whether the separation between lens and camera should be adjusted to maintain good alignment. Note that good alignment is necessary for, but cannot guarantee, an in-focus image, since new optics may introduce aberrations and other effects resulting in unacceptable image quality.
Figure 9: Refraction causes the ray's angle with the optical axis to be shallower in the medium than in air (θm vs. θo), due to the differences in refractive indices (nm vs. no ). After travelling a distance d in the medium, the ray is only hm closer to the axis. Due to this, the ray intersects the axis Δf beyond the f point.
Figure 11: Tolerance and / or temperature effects may result in the lens and camera having different FFDs. If the FFD of the lens is shorter, images of objects at infinity will be excluded from the focal range. Since the system cannot focus on them, they will be blurry.
Figure 10: When their flange focal distances (FFD) are the same, the camera's sensor plane and the lens' focal plane are perfectly aligned. Images of objects at infinity coincide with one limit of the system's focal range.
A Case of the Bends: Focal Shift Due to Refraction While travelling through a solid medium, a ray's path is straight (Figure 8). Its angle (θo ) with the optical axis is constant as it converges to the focal point (f ). Values of FFD are determined assuming this medium is air.
When an optic with plane-parallel sides and a higher refractive index (nm ) is placed in the ray's path, refraction causes the ray to bend and take a shallower angle (θm ) through the optic. This angle can be determined from Snell's law, as described in the table and illustrated in Figure 9.
While travelling through the optic, the ray approaches the optical axis at a slower rate than a ray travelling the same distance in air. After exiting the optic, the ray's angle with the axis is again θo , the same as a ray that did not pass through the optic. However, the ray exits the optic farther away from the axis than if it had never passed through it. Since the ray refracted by the optic is farther away, it crosses the axis at a point shifted Δf beyond the other ray's crossing. Increasing the optic's thickness widens the separation between the two rays, which increases Δf.
To Infinity and Beyond It is important to many applications that the camera system be capable of capturing high-quality images of objects at infinity. Rays from these objects are parallel and focused to a point closer to the lens than rays from closer objects (Figure 10). The FFDs of cameras and lenses are defined so the focal point of rays from infinitely distant objects will align with the camera's sensor plane. When a lens has an adjustable focal range, objects at infinity are in focus at one end of the range and closer objects are in focus at the other.
Different effects, including temperature changes and tolerance stacking, can result in the lens and / or camera not exactly meeting the FFD specification. When the lens' actual FFD is shorter than the camera's, the camera system can no longer obtain sharp images of objects at infinity (Figure 11). This offset can also result if an optic is removed from between the lens and camera sensor.
An approach some lenses use to compensate for this is to allow the user to vary the lens focus to points "beyond" infinity. This does not refer to a physical distance, it just allows the lens to push its focal plane farther away. Thorlabs' Kiralux™ and Quantalux® cameras include adjustable C-mount adapters to allow the spacing to be tuned as needed.
If the lens' FFD is larger than the camera's, images of objects at infinity fall within the system's focal range, but some closer objects that should be within this range will be excluded. This situation can be caused by inserting optics between the lens and camera sensor. If objects at infinity can still be imaged, this can often be acceptable.
Not Just Theory: Camera Design Example The C-mount, hermetically sealed, and TE-cooled Quantalux camera has a fixed 18.1 mm spacing between its flange surface and sensor plane. However, the FFD (f) for C-mount camera systems is 17.526 mm. The camera's need for greater spacing becomes apparent when the focal shift due to the window soldered into the hermetic cover and the glass covering the sensor are taken into account. The results recorded in the table beneath Figure 9 show that both exact and paraxial equations return a required total spacing of 18.1 mm.
The movement of Thorlabs' manual translation stages is driven by a micrometer or other adjuster, which can be replaced with a motorized actuator that has a compatible travel range and barrel diameter. Before making the substitution, it is important to fully retract the installed adjuster to protect the stage from the mechanical shock of a sudden release of spring energy.
Video Clip 1: It is important to completely retract the installed micrometer or other adjuster as the first step. If the adjuster is extended when it is released from the stage, the top plate of the stage will be propelled backwards into a hard stop. The mechanical shock may damage the stage.
Video Clip 3: After the micrometer on the XR25P stage is completely retracted, the locking cap screw on the stage's barrel clamp can be loosened with a 5/64" (2 mm) hex key and the adjuster removed. The barrel of the motorized actuator, which is the DC-servo-motor-driven Z825B in this example, is then inserted and the locking setscrew is tightened until snug.
Video Clip 2: After the adjustment screw on the MT1B stage is completely retracted, the locking cap screw on the stage's barrel clamp can be loosened with a 3/32" hex key and the adjuster removed. The barrel of the motorized actuator, which is the stepper-motor-driven ZFS13B in this example, is then inserted and the locking setscrew is tightened until snug.
Check Barrel Diameter and Travel Range Compatibility These stages secure the actuator using a barrel clamp, which makes it necessary to match the actuator's barrel diameter to the clamp's specifications. Both stages chosen for this demonstration accept actuators with a 3/8" (9.5 mm) diameter barrel.
The travel range of the actuator must not exceed that of the stage. An actuator with a larger travel range can potentially force the stage to extend beyond its limit, which may damage both the stage and the actuator's motor. An actuator with a shorter travel range will cause no mechanical harm to itself or the stage, but the stage's travel range will be reduced. The MT1B and XR25P linear translation stages included in this demonstration have travel ranges of 0.5" and 1" (13 mm and 25 mm), respectively.
Retract the Adjuster to Avoid Damaging the Stage Fully retracting the installed adjuster, before doing anything else, avoids a significant cause of damage to these stages. When the adjuster is fully retracted, the top plate is positioned as far backwards as possible to relieve the spring tension. Ideally, the tip of the adjuster would no longer be in contact with the stage.
An extended position is dangerous due to the force exerted by the stage's internal springs. The spring force keeps the top plate, or moving world, in contact with the tip of the adjuster (Clip 1). If the adjuster is extended when it is released from the stage, the spring force on the top plate will propel it backwards into a hard stop. The mechanical shock of this collision can be severe and potentially misalign the stage's components, affect the ball bearings, and introduce angular deviations to the stage's travel.
Make the Replacement With the installed adjuster completely retracted, the locking cap screw in the barrel clamp of the MT1B or XR25P stage can be loosened using a 3/32" or 5/64" (2 mm) hex key, respectively. This will release the holding force on the barrel of the adjuster, so that it can be removed (Clips 2 and 3).
Insert the barrel of the motorized actuator and tighten the locking cap screw until it is snug, but not too tight. The spring load on the top plate should not be able to push the actuator out of the barrel clamp, but the locking screw should not be so tight that it deforms the barrel, which could affect the linearity of the actuator.
Want additional Insights on translation stages? Watch the full video.
How can the strength of a material's Faraday effect be measured?
Since the Faraday effect causes the polarization state of light to rotate as it propagates through a material in the presence of a magnetic field, one approach to determining the effect's strength in a material is to input linearly polarized light, apply a strong magnetic field through the material, and observe the induced change in the orientation of the output polarization state. It is not necessary to directly measure the output polarization state to determine the change in its orientation. Instead, the output light can be analyzed by measuring the optical power transmitted through a rotating linear polarizer. The measured power oscillates with a phase dependent on the orientation of the linear polarization state incident on the rotating polarizer. This was demonstrated using a CdMgTe crystal. Measurements of light output from the crystal were used to calculate its Verdet constant, which characterizes the strength of a material's Faraday effect.
Figure 1: Faraday effect measurements can be made with the sample placed between a linearly polarized light source and a polarization-sensitive detection system. The CdMgTe crystal was approximately a third of the length of the annulus magnet's bore, and the plastic sample holder was used to position and immobilize the crystal at the center of the bore. In the detection system, the optical power sensor was placed as close as possible to the output side of the linear polarizer, which was installed in an indexed rotation mount. An advantage of this setup is that it requires minimal alignment.
Figure 3: Optical power measurements were made while rotating the detection polarizer's transmission axis in 2° increments. Data were acquired with the magnet out of (triangles) and in (squares) the setup. Malus' law (solid lines) was used to model a to fit each curve. The phase shift (Δθ ) between the two curves is 36°.
Figure 2: The crystal under test was placed in the bore of the annulus magnet (left). The 2.2 mm long crystal was positioned in the center of the 6.35 mm long bore, where the magnetic field was strongest, most uniform, and directed along the N-S axis (right).
Curve Fit to Data Acquired Without Magnet
Curve Fit to Data Acquired With Magnet
Faraday Rotation (Δθ )
Faraday Rotation The rotation of the polarization state due to the Faraday effect is called the Faraday rotation, which is directly proportional to both the magnetic field strength (B ) in the material and the physical distance (L ) the light propagates through the material in the presence of the field. The proportionality constant is called the Verdet constant (V ).
This intrinsic material parameter, which is wavelength and temperature dependent, characterizes the strength of the material's Faraday effect. When the Verdet constant is known, the Faraday rotation (Δθ ),
due to different magnetic field strengths and material lengths can be calculated. One approach to obtaining the Verdet constant is to measure the Faraday rotation for a specific material length and a known magnetic field strength.
Faraday Effect Measurement Using this approach, and the setup illustrated in Figure 1, the strength of a CdMgTe crystal's Faraday effect was measured.
The linearly polarized light source consisted of a collimated fiber-coupled laser whose 785 nm emission was transmitted through a fixed linear polarizer.
An annulus super magnet was used in order to provide a magnetic field strong enough to induce a measurable Faraday rotation. The crystal was mounted in the center of the magnet's bore, as that is where the magnetic field is the strongest (Figure 2).
The light output from the crystal was transmitted through a second linear polarizer, which was secured in an indexed rotation mount, and to a power sensor. The power sensor was positioned as close as possible to the output side of the linear polarizer.
Two measurement sets were acquired, one with, and the other without, the magnet in the setup. Each data set (Figure 3) recorded average power measurements taken at 2° increments of the second linear polarizer's transmission axis angle. The curves oscillate with the same period but are phase shifted (Δθ ) from one another.
Calculate the Verdet Constant The phase shift between the two curves plotted in Figure 3 is the Faraday rotation. It can be determined after each data curve is fit using Malus' law,
in which Io is the intensity of the incident light and is the angle of the second linear polarizer's transmission axis. The fit parameter (θ ) is a constant that was optimized individually (θbase and θmag, respectively, in the table) for each data set. The difference in the two fit parameters is the Faraday rotation (36°). Using this Faraday rotation value, the Verdet constant can be calculated,
The magnetic field strength (B ) was 5800 Gauss at the center of the magnet's bore, the crystal's length (L ) was 0.22 cm, and the Faraday rotation angle (Δθ = 36°) was multiplied by a factor of 60 to convert it to 2160 arcmin.
Content contributed by and based on work performed by Zoya Shafique. Date of Last Edit: Jan. 14, 2021
参考文献  Chris Emslie, in Specialty Optical Fibers Handbook, edited by Alexis Mendez and T. F. Morse (Elsevier, Inc., New York, 2007) pp. 243-277.  Malcolm P. Varnham et al., "Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers," J. Lightwave Technol., LT-1(2), 332-339 (1983).  Zhenyang Ding et al., "Accurate Method for Measuring the Thermal Coefficient of Group Birefringence of Polarization-Maintaining Fibers," Opt. Lett., 36(11), 2173-2175 (2011).  M. Shah Alam and Sarkar Rahat M. Anwar, "Modal Propagation Properties of Elliptical Core Optical Fibers Considering Stress-Optic Effects," World Academy of Science, Engineering and Technology, Open Science Index 44, International Journal of Electronics and Communication Engineering, 4(8), 1170 - 1175 (2010).
参考文献  Chris Emslie, in Specialty Optical Fibers Handbook, edited by Alexis Mendez and T. F. Morse (Elsevier, Inc., New York, 2007) pp. 243-277.  Edward Collett, Polarized Light in Fiber Optics (Elsevier, Inc., New York, 2007) pp. 45-53.
参考文献  Chris Emslie, in Specialty Optical Fibers Handbook, edited by Alexis Mendez and T. F. Morse (Elsevier, Inc., New York, 2007) pp. 243-277.  Malcolm P. Varnham et al., "Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers," J. Lightwave Technol., LT-1(2), 332-339 (1983).
参考文献  Chris Emslie, in Specialty Optical Fibers Handbook, edited by Alexis Mendez and T. F. Morse (Elsevier, Inc., New York, 2007) pp. 243-277.  Malcolm P. Varnham et al., "Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers," J. Lightwave Technol., LT-1(2), 332-339 (1983).  Zhenyang Ding et al., "Accurate Method for Measuring the Thermal Coefficient of Group Birefringence of Polarization-Maintaining Fibers," Opt. Lett., 36(11), 2173-2175 (2011).  M. Cavillon, P. D. Dragic, and J. Ballato, "Additivity of the coefficient of thermal expansion in silicate optical fibers," Opt. Lett, 42(18), 3650 - 3653 (2017).
参考文献  Edward Collett, Polarized Light in Fiber Optics (Elsevier, Inc., New York, 2007) pp. 45-53.  Russell A. Chipman, Wai-Sze Tiffany Lam, and Garam Young, Polarized Light and Optical Systems (CRC Press, New York, 2019) pp. 80-83.