Femtosecond Pulse Compressor

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The shaded region in this plot denotes the precompensation values that the Femtosecond Pulse Compressor can provide.

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This plot shows the calculated transmission of the Femtosecond Pulse Compressor.

Thorlabs' Femtosecond Pulse Compressor helps correct for the pulse width broadening that occurs in all multiphoton microscopes. As shown by the sample images above, pulse width broadening can lead to significantly decreased image contrast in two-photon microscopy.

Multiphoton microscopes like Thorlabs' Bergamo^® II rely upon femtosecond lasers that emit near-infrared pulses with durations of roughly 100 fs. In order to be short in duration, the pulses consist of a broad range of wavelengths.

Ideally, the pulse width at the sample would be as short as the pulse emitted by the laser. However, when propagating through the optical elements in the microscope (such as fluorescence filters, dichroic mirrors, scan lenses, and objectives*), the pulse width broadens: each wavelength travels at a different velocity because the glasses' indices of refraction vary as a function of wavelength. In contrast, when propagating through air, the pulse width is conserved: all wavelengths within the pulse effectively travel together because air's index of refraction is relatively constant as a function of wavelength.

This broadening phenomenon, known as group delay dispersion (GDD), is detailed in the Group Delay tab. The femtosecond pulse compressor compensates for GDD imparted by the microscope on the laser pulse, ensuring that the pulse arriving at the sample is as short as possible, thereby maintaining high contrast.

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Dispersion Adjustment Knob and Readout

Adjustable Dispersion Compensation
The pulse compressor supports operating wavelengths from 700 to 1050 nm, making it compatible with Coherent's Chameleon™ laser family and other Ti:Sapphire lasers. The plot above and to the right shows the range of possible dispersion compensation values as a function of wavelength.

To provide optimal GDD compensation for a wide range of microscope configurations, the dispersion can be adjusted by rotating a knob on the outside of the housing, shown in the photo to the right. The knob controls the insertion of a prism into the beam. This feature helps ensure that performance is not compromised as you change filters, objectives, or the laser wavelength.

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Aperture Being Installed into Alignment Holes

As the knob is rotated, a display shows how far the prism has been inserted. Higher values correspond to more glass in the beam path and hence more positive dispersion. The side of the unit contains a female 3.5 mm audio jack, which outputs a voltage that is proportional to the prism displacement for use as a diagnostic. A plot that converts between the prism insertion and dispersion is available in the Group Delay tab.

Installation
The femtosecond pulse compressor should be placed in the beam path between the laser and the microscope. Two beam heights are supported: 4.75" (120.7 mm), for Coherent's Chameleon™ lasers, and 4.25" (108.0 mm), for other common commercial lasers. The beam height is set by a base plate that is included with the unit. To set the beam height at 4.25", remove the base plate by unscrewing three bolts compatible with a 9/64" hex key or balldriver. Three CL5 and three CL8 Table Clamps are included for securing the pulse compressor to the optical table.

When the pulse compressor is first installed, it is necessary to align the input and output beams. We provide two custom-built apertures for initial alignment; one is shown being installed in the photo to the left. The manual contains detailed alignment instructions. All internal optical elements are prealigned at Thorlabs before the unit is shipped.

* See P. E. Hänninen and S. W. Hell, Bioimaging 2, 117 (1994); M. Müller, J. Squier, and G. J. Brakenhoff, Opt. Lett. 20, 1038 (1995); G. J. Brakenhoff, M. Müller, and J. Squier, J. Microsc. 179, 253 (1995); J. B. Guild, C. Xu, and W. W. Webb, Appl. Opt. 36, 397 (1997); and M. Müller, J. Squier, R. Wolleschensky, U. Simon, and G. J. Brakenhoff, J. Microsc. 191, 141 (1997).

How the Femtosecond Pulse Compressor Improves Image Contrast

When laser pulses are sent into a multiphoton microscope without dispersion compensation, the pulse experiences positive dispersion as it propagates through the optical elements in the microscope. In a positively dispersed pulse, the longer wavelengths contained within the pulse travel ahead of the shorter wavelengths (that is, red travels ahead of blue). As a result, the laser pulse at the sample is lengthened in duration, leading to reduced image contrast.

The Femtosecond Pulse Compressor (FSPC) compensates for this effect by applying negative dispersion to the pulse before it is sent into the multiphoton microscope. In a negatively dispersed pulse, the shorter wavelengths contained within the pulse travel ahead of the longer wavelengths (that is, blue travels ahead of red). The microscope then applies positive dispersion to this pulse, and as a result, the pulse is short again at the sample, improving contrast.

Experimental Flexibility

The FSPC provides negative group delay dispersion (GDD) for 700 - 1050 nm laser pulses, adjustable by rotating the knob on the side of the unit. Because the dispersion is adjustable, the unit is easily re-optimized when an objective, fluorescence filter, or other optical element is changed in the setup. The contour plot below shows the achievable GDD values as a function of wavelength. It provides a conversion between the prism displacement (shown on the display) and the dispersion applied to the pulse. (The prism displacement magnitude is defined by a calibration procedure, which is detailed in the manual.)

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