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Example data taken with the Chromatis™ dispersion measurement system to aid in optimizing an optical coating. The targeted GDD (black dashed line) for this 1030 nm mirror is -1200 fs² over the 1020 to 1050 nm wavelength range (blue shaded area). The first two runs show significant error in the GDD.

2013 Category of Test, Measurement, and Metrology

Features

• White Light Interferometer for Measuring Dispersion
• Broadband Emission from Stabilized Halogen Light Source
• Accepts Ø1/2" and Ø1" Optics
• Standard Measurement Modes with Fixtures and Reference Optics Included
  • 0° Reflection
  • 0° - 70° Transmission
  • 5° - 70° Angled Reflection
• Mirror Pair Reflection Fixture Available (Contact Applications@thorlabs.com)
• Measures S and P Polarization Simultaneously
• Includes HeNe Reference Laser for Wavelength Self-Calibration
• Includes Red and Green Diode Lasers for Spatial Alignment
• Two User-Replaceable Detectors Available
• Electronics Contained in Rack-Mountable 2U Enclosure (3.5" Panel Height)
• Operated by Included Windows Laptop with Pre-Installed Software

Applications

• Confirming Coating Design Specifications
• Inspecting Optics
• Metrology for Quality Control

Thorlabs' Chromatis™ Dispersion Measurement System characterizes the group delay dispersion (GDD) properties of reflective and transmissive optical components that are used with femtosecond pulsed lasers. Ultrafast pulses broaden as they propagate through an optical system; short pulse widths can be recovered by knowing the GDD induced by each optic and compensating for the distortions appropriately. For highly accurate and reliable results, the Chromatis™ uses time-domain white light interferometry (WLI) to measure the GDD. This dispersion measurement system includes software that guides the user through beam alignment (fringe optimization), automatically finds the zero time delay position (maximum fringe contrast), and rapidly measures the dispersion.

The Chromatis dispersion measurement system features a fiber-coupled stabilized halogen light source with an effective wavelength range from 500 to 2100 nm. This broadband source allows for characterization of ultrafast optics designed for use with Ti:Sapphire, Ytterbium fiber, and Erbium fiber lasers. Since the interferogram is recorded in the time domain, measurements are carried out with a single photodiode detector, rather than a spectrometer, and a built-in HeNe reference laser provides calibration for the wavelength axis. A Si photodiode for wavelengths from 500 - 1100 nm is included with the system; however, an InGaAs detector can easily be switched in for measurements in the 1000 - 1650 nm wavelength range. For more information about the InGaAs detector add on, please contact Applications@thorlabs.com.

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The user-friendly software includes the measurement procedures enabled by the system. The Zero-Degree, Transmision, and Angle Measurement modes are standard, while the Mirror Pair Measurement is available as an add on; contact Applications@thorlabs.com for details.

Typically, low dispersion optics require only hundreds of femtoseconds to capture an entire interference pattern, so it is often helpful to restrict the wavelength range of the incident light with optical filters to reduce the required scan range and overall noise. The Chromatis™ dispersion measurement system features six slots for optical filters, and bandpass filters covering the 500 - 700 nm, 600 - 1000 nm, and 950 - 1100 nm wavelength ranges are included for use with the Si photodiode. Since filtering reduces the overall intensity of the scan and, therefore, the interference contrast, the photodiode has variable gain settings to compensate for this attenuation.

Measurement Modes
As a white light interferometer, the Chromatis™ dispersion measurement system consists of a broadband, low dispersion beamsplitter and two delay arms. The arm with the test mirror, which is mounted in a precision fixture, has a fixed length, and the other, containing the low-dispersion reference optic, has variable length to introduce the time delay. Two precision fixtures are included to accommodate the three standard measurement modes, as well as reference optics for verifying the system operation. Note that one fixture is used for both zero degree reflection and transmission measurements, while another is used for angled reflection measurements. More details regarding the measurement modes and reference optics are available on the Specs tab. A mirror pair precision fixture for mirror pair reflection measurements is also available separately; for more information on this fixture or to test optics of shapes and sizes other than Ø1/2" and Ø1", please contact Applications@thorlabs.com.

To eliminate the need for additional tools, all mounts for user-provided optics include a tightening thumbscrew. Please note that optics mounted without thumbscrews should not be removed or replaced. Precision fixtures are locked into place or removed via a locking wheel; when not in use, the fixtures should be mounted on the baseplate in the dedicated storage area.

Software Interface
The software included with the Chromatis™ dispersion measurement system is designed to carry users of any experience level through the entire measurement process. While the user has to perform the initial optical alignment, the software automatically finds the zero delay position, as well as records the interference pattern and determines the dispersion.

Step-by-step procedures with detailed on-screen instructions are provided for measurements on either reference or test optics. For example, to simplify the visual alignment of the interferometer, the user is shown which knobs should be adjusted, as well as animations that show fringes resulting from misalignment, near-alignment, and alignment. In the case of an advanced user, the step-by-step instructions can be switched off.

Please note that the Chromatis™ is factory calibrated to remove any residual dispersion from fixed system optics; the software is also precalibrated for the Si and InGaAs detectors.