3-Axis NanoMax™ Flexure Stages

Insights into Optical Fiber

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• What factors affect the amount of light coupled into a single mode fiber?
• Is the max acceptance angle constant across the core of a multimode fiber?

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What factors affect the amount of light coupled into a single mode fiber?

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Figure 193B  Conditions which can reduce coupling efficiency into single mode fibers include anything that reduces the similarity of the incident beam to the optical properties of the fiber's guided mode. 

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Figure 193A  For maximum coupling efficiency into single mode fibers, the light should be an on-axis Gaussian beam with its waist located at the fiber's end face, and the waist diameter should equal the MFD. 

Adjusting the incident beam's angle, position, and intensity profile can improve the coupling efficiency of light into a single mode optical fiber. Assuming the fiber's end face is planar and perpendicular to the fiber's long axis, coupling efficiency is optimized for beams meeting the following criteria (Figure 193A):

• Gaussian intensity profile.
• Normal incidence on the fiber's end face.
• Beam waist in the plane of the end face.
• Beam waist centered on the fiber's core. 
• Diameter of the beam waist equal to the mode field diameter (MFD) of the fiber.  

Deviations from these ideal coupling conditions are illustrated in Figure 193B.

These beam properties follow from wave optics analysis of a single mode fiber's guided mode (Kowalevicz).

The Light Source can Limit Coupling Efficiency
Lasers emitting only the lowest-order transverse mode provide beams with near-Gaussian profiles, which can be efficiently coupled into single mode fibers.

The coupling efficiency of light from multimode lasers or broadband light sources into the guided mode of a single mode fiber will be poor, even if the light is focused on the core region of the end face. Most of the light from these sources will leak out of the fiber. 

The poor coupling efficiency is due to only a fraction of the light in these multimode sources matching the characteristics of the single mode fiber's guided mode. By spatially filtering the light from the source, the amount of light that may be coupled into the fiber's core can be estimated. At best, a single mode fiber will accept only the light in the Gaussian beam output by the filter.

The coupling efficiency of light from a multimode source into a fiber's core can be improved if a multimode fiber is used instead of a single mode fiber. 

References
Kowalevicz A and Bucholtz F, "Beam Divergence from an SMF-28 Optical Fiber (NRL/MR/5650--06-8996)." Naval Research Laboratory, 2006.

Date of Last Edit: Jan. 17, 2020

Is the max acceptance angle constant across the core of a multimode fiber?

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Figure 193C  Step-index multimode fibers have an index of refraction ( n ) that is constant across the core. Graded-index multimode fibers have an index that varies across the core. Typically the maximum index occurs at the center.

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Figure 193E  Graded-index multimode fibers have acceptance angles that vary with radius ( ρ ), since the refractive index of the core varies with radius. The largest acceptance angles typically occur near the center, and the smallest, which approach 0°, occur near the boundary with the cladding (0 < ρ₁ < ρ₂ ). Air is assumed to surround the fiber.

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Figure 193D  Step-index multimode fibers accept light incident in the core at angles ≤|θ_max | with good coupling efficiency. The maximum acceptance angle is constant across the core's radius ( ρ ). Air is assumed to surround the fiber.

It depends on the type of fiber. A step-index multimode fiber provides the same maximum acceptance angle at every position across the fiber's core. Graded-index multimode fibers, in contrast, accept rays with the largest range of incident angles only at the core's center. The maximum acceptance angle decreases with distance from the center and approaches 0° near the interface with the cladding.

Step-Index Multimode Fiber
The core of a step-index multimode fiber has a flat-top index profile, which is illustrated on the left side of Figure 193C. When light is coupled into the planar end face of the fiber, the maximum acceptance angle (θ_max ) is the same at every location across the core (Figure 193D). This is due to the constant value of refractive index across the core, since the acceptance angle depends strongly on the index of the cladding. 

Regardless of whether rays are incident near the center or edge of the core, step-index multimode fibers will accept cones of rays spanning angles ±θ_max with respect to the fiber's axis. 

Graded-Index Multimode Fibers
The core of a typical graded-index multimode fiber, shown on the right side of Figure 193C, has a refractive index that is greatest at the center of the core and decreases with radial distance ( ρ ). The equation included below the diagram in Figure 193E shows that the radial dependence of the core's refractive index results in a radial dependence of the maximum acceptance angle and numerical aperture (NA). This equation also assumes a planar end face, normal to the fiber's axis that is surrounded by air.

Cones of rays with angular ranges limited by the core's refractive index profile are illustrated Figure 193E. The cone of rays with the largest angular spread (±θ_max ) occurs on the fiber's axis (ρ = 0). The angular spread decreases as the radial distance to the axis increases. 

Step-Index or Graded Index?
A step-index multimode fiber has the potential to collect more light than a graded-index multimode fiber. This is due to the NA being constant across the step-index core, while the NA decreases with radial distance across the graded-index core.

However, the graded-index profile causes all of the guided modes to have similar propagation velocities, which reduces the modal dispersion of the light beam as it travels in the fiber.

For applications that rely on coupling as much light as possible into the multimode fiber and are less sensitive to modal dispersion, a step-index multimode fiber may be the better choice. If the reverse is true, a graded-index multimode fiber should be considered.

References
Keiser G, "Section 2.6." Optical Fiber Communications. McGraw-Hill, 1991.

Date of Last Edit: Jan. 2, 2019

PIC Demo Testing Station

Thorlabs offers a variety of components that enable construction of a photonic integrated circuit (PIC) testing station, including imaging systems, fiber optics, precision piezos, and manual stages. This page presents a demo system that drives an on-chip laser and collects and analyzes the emitted light. The laser chip employed in this example PIC testing setup is a fully integrated, heterogeneous GaAs-on-SiN platform with an array of tunable lasers, with each source capable of coarsely covering the range from 765 to 795 nm. This PIC chip was supplied by Nexus Photonics. The scheme of this PIC testing demo can be generalized and customized to meet the needs of testing various PIC chips and is not restricted to the specific circuit used here. Click on the highlighted areas in Figure 787B or the links in Table 787A to find descriptions of the subsystems that comprise the demo testing station. Tables 787C and 787D provide a comprehensive list of all parts used in the demo.

Figure 787B  PIC Testing Demo Setup

Imaging

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Figure 787E  Image of PIC chip, probes, and collection fiber acquired by the imaging system. The lensed fiber used for light collection can be seen in the lower left of the image. The scale bar shows 1 mm.

A widefield imaging system^a is necessary to facilitate alignment of the probe card and light collection (lensed fiber) systems relative to the the PIC chip. Thorlabs' SFM2 Cerna^® Mini Microscope provides a simple means to construct an imaging system^b. An LP126CU Kiralux^® Scientific Camera attached to the SFM2 Mini Microscope displays the image of the PIC chip, probe card, and collection fiber (shown in Figure 787E). Illumination from a light source like the OSL2 High-Intensity Fiber-Coupled Illuminator can be guided to the chip via the OSL2YFB gooseneck fiber bundle. The combination of an MCM301 Stepper Motor Controller and an MCMK3 USB Joystick with the PLSXY and PLSZ Motorized Translation Stages enables three-dimensional motorized control of the imaging system.

1. In this context widefield means an imaging field comparable to the size of the PIC chip.
2. An EF-Mount Adapter Ring such as the SM2CEFM can be used to connect the camera lens to the Cerna microscope.

Cerna® Mini Microscopes	Halogen Fiber Optic Illuminators	Gooseneck Fiber Bundles	EF-Mount Adapter Rings	Kiralux® Scientific Cameras	Compact Stepper Motor Controller	Joystick for Stepper Motor Controller

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Figure 787F  PIC Chip Close Up Showing Probe Card, Collection Fiber, Illumination, and Imaging Microscope

Light Collection

Figure 787F shows a close up of the light collection geometry. Light emitted from the chip is collected by an LFM1F-1 Lensed Multimode Fiber and coupled into an optical spectrum analyzer (as discussed in the previous section). If multiple facets of analysis are desired, the signal can be split using a multimode fiber optic coupler such as the TM50R2F1B. To achieve adequate signal-to-noise ratio, fine positional control of the collection fiber is necessary. Thorlabs offers many components enabling precision control of fiber optics. Thorlabs' NanoMax™ line provides precision fiber positioning, and a PY004 base allows pitch and yaw control. The NanoMax MAX311D contains piezos, which can be controlled for auto-alignment with a combination of the KNA-VIS and KPC101 K-Cube^® Controllers. The KCH301 and KCH601 K-Cube mounts are available as a USB hubs for the K-Cubes.

Lensed Multimode Fibers	Multimode Fiber Optic Couplers	NanoMax™ Fiber Flexure Stages	High-Load Pitch and Yaw Platform	K-Cube Auto-Alignment Controllers	K-Cube Piezo Controller	K-Cube USB Controller Hubs

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Figure 787G  Screenshot of Spectrum Acquired with the OSA202C

PIC Control and Readout

In order to drive the laser circuit on the chip, current is supplied to the PIC chip via the probe card. Two LDC202C Laser Diode Drivers serve as the current sources here. Emitted light collected from the chip is directed into the fiber-coupled OSA202C Optical Spectrum Analyzer, which provides analysis and display of the signal. Figure 787G shows an example screenshot of the spectrum acquired by the OSA202C.

Depending on the particular PIC chip/testing application, various beam characterization and polarization measurement instruments can be used to construct a custom analysis setup to meet specific needs. Examples include the CCT10 Compact CMOS Spectrometer for spectroscopy, the PNA1 Intensity Noise Analyzer for high frequency-axis noise analysis, and the PAX1000IR1 Polarimeter for polarization measurements.

Laser Diode Drivers	Optical Spectrum Analyzers	Compact CMOS Spectrometers	Intensity Noise Analyzer	Polarimeters

Positioning

MVS05 Vertical Translation Stage	XRV1 Vertical Translation Stage	Compact X-Y Translation Stages

PIC Chip Stage

Vertical (Z-axis) control of the PIC chip aids in focusing the widefield imaging system. A vertical drive stage such as the MVS05 Stage provides this additional degree of freedom. If more range is needed, the MVS1 Vertical Translation Stage allows up to 1" of travel.

Probe Card

The probe card serves to supply a multipoint electrical contact directly to the on-chip devices. To achieve optimal/flush contact with the chip multiple degrees of freedom are necessary. A vertical translation stage such as the XRV1 Stage provides vertical translation, and the LX20 Stage allows XY translation.

Thorlabs offers a wide selection of manual stages including linear translation, rotation, and multi-axis stages.

Optical Tables, Breadboards, and Supports	Solid Aluminum Breadboards	ScienceDesk Workstations	ScienceDesk Shelving	Cable Management

General Setup, Organization, and Cable Management

Thorlabs offers a wide selection of general components and tools for construction, organization, and management of a PIC testing station. Optical tables, breadboards, and supports provide the foundation of any testing setup. ScienceDesk™ workstations can provide a convenient means to construct a vibration isolated table with a small footprint, and shelving affords convenient options for holding tools, equipment, and instruments. Cable management products for electric and fiber optic cables are also available.

Custom Capabilities

Both the lensed fiber holder and probe card holder were custom made for this specific PIC testing station demo application. Thorlabs offers a wide variety of custom solutions and manufacturing capabilities for various applications. Contact techsales@thorlabs.com for further information.

Multi-Axis Stage Selection Guide

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Figure 83A  In the above application, a 3-Axis NanoMax flexure stage is aligned in front of a 6-axis stage at the proper 112.5 mm deck height using an AMA554 Height Adapter.

3-Axis Stages
Thorlabs offers three different 3-Axis Stage variations: NanoMax flexure stages, MicroBlock compact flexure stages, and RollerBlock long-travel stages. Each stage features a 62.5 mm nominal deck height. Our NanoMax line of 3-axis stages offers built-in closed- and open-loop piezos as well as modular drive options that include stepper motors, differential drives, or additional piezos. The MicroBlock stages are available with differential micrometer drives or fine thread thumbscrews; these drives are not removable. Finally, our RollerBlock stage drivers can be switched out for any actuator that has a Ø3/8" (9.5 mm) mounting barrel.

4- and 5-Axis Stages
Our 4- and 5-axis stages are ideal for the static positioning of waveguides or complex optical elements with respect to our 3-axis or 6-axis high-performance alignment stages. Thorlabs' 5-axis stages have nominal heights of 62.5 mm or 112.5 mm. The AMA554 Height Adapter can be used to raise the deck height of the 3-axis or 4-axis stages to 112.5 mm for compatibility with our 5-axis MicroBlock or 6-Axis NanoMax Stages.

6-Axis Stages
Thorlabs' 6-Axis NanoMax Nanopositioners are ideal for complex, multi-axis positioning and have a nominal deck height of 112.5 mm. These stages offer a common point of rotation and a patented parallel flexure design that allows all actuators to be coupled directly to the base to minimize any unwanted motion in the system. Built-in closed- and open-loop piezo options are available. A selection of modular drive options allows any axis to be manual or motorized with the option for external piezos. Our units without included actuators are also available in right- or left-handed configurations. To increase the stage height of the 3-axis stages to 112.5 mm, we recommend our AMA554 Height Adapter, shown in Figure 83A.

A complete selection and comparison of our multi-axis stages is available below.

3-Axis Stages