Align a Laser Beam Level to the Optical Table

Two methods for aligning a laser beam so that it propagates parallel to the surface of the optical table are demonstrated.

The first technique adjusts the pointing angle of a laser, whose tip and tilt can be adjusted. Using a ruler, the laser beam is leveled and directed along a row of tapped holes in the table.

Starting with this aligned beam, the technique for changing both the direction and the height of a beam from a fixed laser source is demonstrated. Two mirrors, which are set at different heights, direct the beam along another row of tapped holes in the table. The beam is then leveled at the height of the second mirror using two irises.

Components include a PL202 laser module, KM100 kinematic mounts, AD11NT adapter, BHM1 ruler, PF10-03-P01 mirrors, and IDA25 irises.

Date of Last Edit: Sept. 8, 2020

Avoid Screw-Length Pitfalls When Securing a Post Holder to a Table or Base

A common, unfortunate result of securing a post holder to a base or optical table is threads poking up through the bottom of the post holder. These exposed threads limit the height adjustment range offered by the post holder. Additional frustrations can result after rotating the post in the post holder, since this can unintentionally screw the post onto the exposed threads.

The solution is to keep screw length in mind when selecting a setscrew or cap screw to secure a post holder. In this video, observe consequences unfold due to threads projecting up from the bottom of the post holder, and learn techniques for overcoming this problem. The options of securing a post holder to a base or directly to the table are also compared.

Components used in this demonstration include Ø1/2" post holders, a BA2 base, Ø1/2" posts, cap screws, setscrews, and an iris.

Date of Last Edit: Sept. 24, 2020

Align a Free-Space Faraday Isolator for Operation at the Laser Wavelength

Align a Faraday isolator to ensure optimal transmission of optical power from the source, as well as effective suppression of reflections traveling back towards the source. Alignment is demonstrated using an IO-3-532-LP polarization-dependent free-space isolator with a 510 nm to 550 nm operating range, an R2T post collar, a PL201 linearly polarized and collimated 520 nm laser, a S120C silicon power sensor, and a PM400 power meter.

These optical isolators output linearly polarized light and provide best performance when the input beam is linearly polarized.

Always follow your institution's laser safety guidelines. Unlike the low-power source used in this demonstration, other laser sources may be damaged by back reflections. Many stray reflections, which can endanger colleagues and the laser, can be avoided by blocking the laser beam when it is not needed. 

Date of Last Edit: Sept. 10, 2020

Align a Linear Polarizer's Axis to be Perpendicular or Parallel to the Table

The beam paths through many optical setups are routed parallel to the optical table. When this is the case, both the plane of incidence and the p-polarization state are typically oriented parallel to the table's surface, while the s-polarization state is perpendicular. Therefore, polarizers aligned to pass p- or s- polarized light effectively have their axes aligned to be parallel or perpendicular, respectively, to the table's surface.

A procedure for optically aligning the axis of a polarizer to be perpendicular to the optical table is discussed and demonstrated using optical power readings of light transmitted through the polarizer. Then, three options for aligning the axis of a polarizer to be parallel to the table are outlined. The method of crossed polarizers is demonstrated. Tips and tricks for obtaining more precise measurements are also shared.

Components used in this demonstration include a collimated laser, a polarizing beam splitter, linear polarizers, precision rotation mounts, an optical power sensor, and a power meter. There are also special appearances by a post collar and a ruler.

Date of Last Edit: Oct. 23, 2020

Cleave a Large-Diameter Silica Fiber Using a Hand-Held Scribe

An optical-quality end face can be achieved when a large-diameter optical fiber is manually cleaved using a hand-held scribe. The procedure is demonstrated using a multimode fiber with a 400 µm diameter core.

After stripping the protective polymer buffer from the end of the fiber and securing the fiber to a flat surface, a hand-held scribe is used to score the top surface of the fiber. The scribe should create a shallow nick in the fiber's cladding, away from the fiber's core. When cleaving smaller-diameter fibers, avoid creating too deep of a nick by reducing the scribing force and sweeping motion. In some cases, it is sufficient to lightly press a stationary scribe to the fiber. Applying a longitudinal tension to the fiber, across the nicked region, cleaves the fiber.

Also demonstrated is the visual evaluation of the end face quality using an eye loupe. A good quality end face will be a flat plane, perpendicular to the fiber's long axis. The light output from the cleaved end face was also observed on a viewing screen, and tips are shared for inspecting the output light distribution for information about the quality of the end face.

Components used in this demonstration include a tool for stripping the fiber buffer, a ruby scribe, an SMA bare fiber terminator, a 10X eye loupe, fiber grippers, a fiber-coupled LED, a viewing screen, and a quick-release adjustable fiber clamp on a free-standing platform.

Date of Last Edit: Nov. 3, 2020

Measure the Insertion Loss of a Fiber Optic Component

Insertion loss measures the drop in optical power caused by the addition of a device to a fiber optic network. All sources of optical loss contribute to a device's insertion loss, including reflections, absorption and scattering due to intrinsic material properties, micro- and macrobending losses, split ratios, splice loss, and connector loss.

A single-ended insertion loss measurement is demonstrated. In this approach, a reference cable is attached to the source, and then the power at the cable's output is measured. Next, a mating sleeve is used to attach the component under test to the reference cable. The optical power at the selected output port of the component is measured. The insertion loss is calculated by first taking the ratio of this power reading to the power measured at the reference cable output, and then expressing the ratio in terms of decibels (dB).

The single-ended insertion loss measurement includes the loss from coupling light into the device, which is often mostly due to the misalignment of the fiber cores in the mating sleeve. However, this measurement does not include the same type of loss that occurs when coupling the light output from the device into the next leg of the fiber optic network. Note also that the insertion loss is wavelength dependent and will differ for each combination of device input and output ports selected for the measurement. This is due to the differences in split ratio, bend loss, absorption, scattering, reflections, and all other individual sources of attenuation along the optical path between the two ports.

Components used in this demonstration include a fiber-coupled laser source, a mating sleeve, a power sensor designed for fiber-coupled sources, a power meter, a 50:50 fiber coupler, and single mode fiber patch cables.

Date of Last Edit: Dec. 3, 2020

Create Circularly Polarized Light Using a Quarter-Wave Plate

Circularly polarized light can be generated by placing a quarter-wave plate in a linearly polarized beam, provided a couple of conditions are met. The first is that the light's wavelength falls within the wave plate's operating range. The second is that the wave plate's slow and fast axes, which are orthogonal, are oriented at 45° to the direction of the linear polarization state. When this is true, the incident light has equal-magnitude components parallel to the wave plate's two axes. The wave plate delays the component parallel to the slow axis by a quarter of the light's wavelength (/2) with respect to the component parallel to the fast axis. By creating this delay, the wave plate converts the polarization state from linear to circular.

An animation at the beginning of the demonstration illustrates the results of aligning the input linear polarization state with the wave plate's fast axis, slow axis, and angles in between. The perspective used to describe the angles and orientations is looking into the source, opposite the direction of light propagation. The procedure is then demonstrated for orienting input and output polarizers to define the reference orthogonal polarization directions, as well as provide polarization-dependent power measurements. The wave plate is placed between the two polarizers, and the effects of different orientations are explored. The quality of the circularly polarized light output by the wave plate is checked by rotating the second polarizer's transmission axis. The light's polarization is closer to circular when the power reading fluctuates less during rotation. 

Components used in this demonstration include a HeNe laser equipped with an optical isolator, a V-clamp mount, precision rotation mounts, a quarter-wave plate, linear film polarizers, a power sensor, SM1 thread adapter for the power sensor, a SM1 lens tube, and an optical power meter.

Date of Last Edit: Dec. 30, 2020

Align a Linear Polarizer 45° to the Plane of Incidence

The transmission axis of a linear polarizer can be set at a 45° angle to the plane of incidence, with the help of two additional linear polarizers. The two supporting polarizers are aligned so that one's axis is parallel and the other's is perpendicular to the plane of incidence. Orienting the transmission axis of the third polarizer at a 45° angle to the plane of incidence is done by rotating the transmission axis to make 45° angle with the axes of the polarizer pair.

This is demonstrated for the case in which the plane of incidence is parallel to the optical table. The procedure used to align the first polarizer's axis parallel to the table requires repeatedly rotating the polarizer 180° around the vertical axis. The linear polarizer assemblies have rectangular bases, and the fixed position retainer (fork) provides a fixed corner reference on the table. The fork allows the polarizer assembly to be quickly and precisely placed after flipping it around the vertical axis. Following each 180° flip, the transmission axis of the polarizer is first rotated by hand and then finely tuned using the mount's integrated micrometer (mic). After the first polarizer's axis is aligned, the second polarizer's axis is then crossed with it. When this is done, the light transmitted by the second polarizer is minimized.

The third polarizer is then placed between the other two polarizers and its transmission axis is rotated. The rotation angle affects the optical power transmitted by this set of three polarizers, and the throughput is maximized when the angle is 45° with the plane of incidence. One way to check the result, as well as determine the maximum possible power that can be obtained at the detector, is to use the cosine squared (Malus') law. It calculates the power output by a linear polarizer when the incident light is linearly polarized.

Components used in this demonstration include a collimated laser module, linear film polarizers, precision rotation mounts, a power sensor, a SM1 thread adapter for the power sensor, a SM1 lens tube, and an optical power meter.

Date of Last Edit: Feb. 8, 2021