Introduction

When using an optical spectrum analyzer (OSA) to measure light propagating in free space, and because the input to an OSA is typically via a fiber optic connection, an adaptive fixture is often required.* This fixture, more commonly referred to as a jig, enables coupling of the light into an optical fiber. This application note introduces free space light measurement jigs as a recommendation for measuring the emission spectrum of a light source that propagates in free space or the optical transmission spectrum of an optical filter. It describes four types of jigs for light sources and one type for filters.
 

Specialized Setups for Complex Optical Applications

In recent years there has been a dramatic increase in optics-related R&D and manufacturing applications across multiple industries (e.g., telecom, biomedical, environmental monitoring, gas sensing/analysis) that necessitate advanced optical spectral measurements, and specifically, the measurement of light in free space.

Given the complexities of measuring free space light, the use of a specialized jig is typically employed to ensure that data is captured accurately and with the highest precision possible.

Figure 1: The AQ6373E Optical Spectrum Analyzer from Yokogawa Test&Measurement covers the visible wavelength range of 350 nm to 1200 nm and is designed for bio-photonics applications
 

Selecting a Jig Type

When selecting the best jig type for a project, it is important to consider the target object, required coupling efficiency, and budget.

*1 The actual wavelength range is limited by the wavelength bandwidth of optical fibers and lenses
Table 1: Jig types L-1, L-2, L-3, and L-4 are used for light source measurements 
 

*1 The actual wavelength range is limited by the wavelength bandwidth of optical fibers and lenses
Table 2: Jig type T-1 is used for optical filter measurement

Jig Type L-1: General Purpose

L-1 jig types are used for general purpose light source measurement. Applicable for both divergent and collimated light, L-1 jigs are easy to use, low-cost, and enable precise optical alignment with XY-axis translation mount.

Considerations

• Device under test (DUT) must be fixed and roughly aligned by user
• Measurement wavelength range may be limited by wavelength band of the optical fiber
• Large-diameter multimode fiber recommended

Alignment Steps

• If available, use of a viewer card is recommended for more precise optical alignment
• Place the DUT optical output as close as possible to the center of the optical fiber connector end face, making certain not to touch it
• Adjust the position of the optical fiber connector end face with the XY-axis translation mount to maximize the amount of power measured

Figure 2: Jig type L-1 component configuration

Jig Type L-2: TO-CAN

L-2 jig types are used for measurements relating to TO-CAN (divergent) light sources. Simple to use and low-cost, they utilize a cage system for easy optical alignment with the XY-axis translation mount.

Considerations

• If measuring divergent light sources other than TO-CAN, an L-1 jig type can be used
• Measurement wavelength range may be limited by wavelength band of the optical fiber
• Large-diameter multimode fiber recommended

Alignment Steps

• Fix the DUT optical output into the cage plate and as close as possible to the center of the optical fiber connector end face, making certain not to touch it
• Adjust the position of the optical fiber connector end face with the XY-axis translation mount to maximize the amount of power measured

Figure 3: Jig type L-2 component configuration
 

Jig Type L-3: Low-Loss (TO-CAN)

L-3 jig types are used for measurements of divergent light with low coupling loss due to aspheric lenses or parabolic mirrors. They use a cage system plus two translation mounts for precise optical alignment.

Considerations

• Applicable to TO-CAN-specific divergent light sources only
• Measurement wavelength range may be limited by the aspheric lens and optical fiber wavelength band
• Small-diameter multimode fiber recommended

Alignment Steps

• Remove both the XY-axis translation mount and the Z-axis translation mount
• Adjust optical alignment between the parabolic mirror and the optical fiber adapter to create a collimated beam (Block A in Figure 4)
• Assemble the XY-axis translation mount, fitted with the aspheric lens and the Z-axis translation mount, with the DUT using cage assembly rods
• Adjust the collimation of the DUT output beam (Block B in Figure 4) and configure to the components from Block A
• Adjust the XY-axis translation mount to maximize the amount of power measured

Optional Steps

• Use visible laser source with single-mode output and shearing interferometer (collimation tester) for easier optical alignment
• Input the visible laser’s output through the optical fiber adapter and to the parabolic mirror within the mirror mount
• Input the parabolic mirror’s output light into the shearing interferometer
• Adjust the mirror mount and the XYZ-axes movement mount until the interference fringes become a collimated beam
• Adjust XYZ-axis so the DUT output light on the Z-axis translation mount is collimated by the aspherical lens on the XY-axis translation mount
• Check the state of collimation using an alignment disk

Figure 4: Jig type L-3 component configuration
 

Jig Type L-4: Low-Loss (Parallel Light)

L-4 jig types measure divergent light with low coupling loss due to aspheric lenses or parabolic mirrors using a cage system plus two translation mounts for precise optical alignment.

Considerations

• Applicable only for TO-CAN-specific divergent light sources
• Use Type L-1 for divergent light sources other than TO-CAN
• Measurement wavelength range may be limited by wavelength band of the optical fiber and aspheric lens
• Small-diameter multimode fiber recommended

Alignment Steps

• Remove both the XY-axis translation mount and the Z-axis translation mount
• Adjust optical alignment between the parabolic mirror and optical fiber adapter to create a collimated beam (Optical Layout Block in Figure 5)
• Assemble the XY-axis translation mount, fitted with the aspheric lens and the Z-axis translation mount, with the DUT using cage assembly rods
• Adjust the collimation of the DUT output beam and configure to the components from Optical Layout Block
• Adjust the XY-axis translation mount to maximize the amount of power measured

Optical Alignment

• Adjust the optical alignment between the parabolic mirror and the optical fiber adapter to create a collimated beam  
• Adjust the optical alignment of the DUT using the above collimated beam (can use a viewer card or alignment disk to assist)
• Adjust the XYZ-axis translation mount to maximize the power measured by the OSA
   

Figure 5: Jig type L-4 component configuration
 

Jig Type T-1: Optical Filters Transmission Characteristics

Designed for large-diameter multimode fiber and using reflective optics, T-1 jig types measure the transmission characteristics of optical filters with low insertion loss over a wide wavelength range and do not require optical alignment adjustments.

Considerations

• Applicable only to one-inch optical filters
• Measurement wavelength range may be limited by wavelength band of the optical fiber
• Large-diameter multimode fiber (> 200 μm) recommended

Figure 6. Jig type T-1 component configuration

Selecting a Light Source

A broadband light source is required to measure the transmission characteristics of optical filters. The four primary broadband light source types are amplified spontaneous emission (ASE), super luminescent diode (SLD), super-continuum (SC), and white light.

Figure 9: Wavelength bands and comparison of light sources
 

Figure 10. Comparison of light sources

Selecting Optical Fibers

For jig types L-1, L-2, L-3, and L-4, optical fiber selection is made according to the measurement wavelength and required wavelength resolution.

• Use silica optical fibers for <2300 nm
• Use fluoride optical fibers for >2300 nm
• The smaller the optical fiber core diameter, the better the OSA wavelength resolution
• The larger the core diameter, the easier it is to capture light and adjust optical alignment
• Large-core fibers are recommended unless high wavelength resolution is required
• Use multimode fibers >100 μm for coupling efficiency in jig types L-1 and L-2
• Use multimode fibers <200 μm or single-mode fibers for jig types L-3 and L-4

Figure 11: Optical fiber examples

For jig type T-1, select a suitable optical fiber according to wavelength, required dynamic range, wavelength resolution, and light source. When selecting optical fiber, it’s important to note that even when multiple optical fibers are made of the same material, they can sometimes have different wavelength characteristics.

Figure 12: Optical fiber examples
 

Alignment Tools

Viewer Card
Viewer cards visualize the irradiation point of infrared light and facilitate optical alignment. By moving the card back and forth in the direction of light propagation, it becomes possible to confirm the light is parallel via size change. Viewer card models differ depending on light source wavelength

Shearing Interferometer (Collimation Tester)
A shearing interferometer is used to determine if light is collimated (i.e., interference fringes are parallel to the reference line). A single-mode visible laser, such as a He-Ne laser, is used for collimation (Fabry-Perot lasers and broadband light sources do not produce interference fringes so single-mode fiber-coupled visible light sources can be used).

Alignment Disk
Like viewer cards, alignment disks visualize the irradiation point of infrared light to facilitate optical alignment. Additionally, they are compatible with a cage system
 

Conclusion

Depending on the source and application, there are multiple jig options and configurations available to measure the emission spectrum of light propagating in free space, as well the optical transmission spectrum of an optical filter. Whether constructing a straightforward optical arrangement or an intricate experimental system, various jig setups are available that enable customization precisely suited to your requirements. These setups encompass a spectrum of configurations, from basic spectrometer-based arrangements to sophisticated combinations that integrate optical fibers, collimators, and mirrors, to enable precise and accurate measurements.

*Please note these recommendations are applicable only to OSAs that do not have an internal connector and use a free space input (such as the AQ637x series) and are not applicable to OSAs with an internal input connector design (e.g., the AQ6380 OSA).