Sputtered Ultrafast Optics: Dispersion compensation mirror set for non-linear microscopy

IdestaQE has unique expertise in designing complex thin-film mirrors. For example, the mirrors used in our Octavius product line feature more than 100 layers per mirror. As these mirrors are used in matched pairs, the overall mirror pair optimization problem has more than 200 variables. A newly-developed proprietary merit function for the mirror performance, as well as algorithms which can run in highly parallelized fashion on massive computer grids, allow idestaQE to bring thin film design to the next level.

To ensure the highest quality coatings, we work with the best optic shops and coating companies in the industry. All of our coatings are deposited by a high-energy-sputter-coating technique. Sputter coatings not only allow for the highest precision and purity during the coating process itself, but also produce extremely robust and shift-free coatings. An in-house white-light interferometer and reflectometer are employed for rigorous quality control on optics before they ship to customers.

Idesta-QE developed a pair of Dispersion-Compensating Mirrors that correct for the phase distortions that occur when ultrashort pulses travel through an optical system. Since femtosecond pulses are comprised from many different wavelengths, pulse broadening, as a result of dispersion will occur when the laser light passes through a dielectric medium (e.g., glass in the optical system). This pulse broadening is attributed to the wavelength dependence of the refractive index of the optical components through which the light travels. Shorter wavelengths are associated with higher indices of refraction than longer wavelengths, causing them to travel slower than longer wavelengths. The pulse dispersion caused by the wavelength-dependent nature of the refractive index can be corrected using Idesta’s dispersion-compensating mirror set. These mirrors are specifically designed so that longer wavelengths experience larger group velocity delay than shorter wavelengths, thereby negating the pulse broadening caused by the optical elements within the imaging system.

Features

Dispersion vs. Wavelength for DCMP175
Dispersion vs wavelength for DCMP175		 Reflectivity vs. Wavelength for DCMP175
reflectivity vs wavelength for DCMP175

Improved Multiphoton Image Contrast Using a Dispersion-Compensating Mirror Set

The two-photon images of a mouse kidney shown below demonstrate the benefits of using the Dispersion-Compensating Mirror Set manufactured by Thorlabs’ strategic partner Idesta-QE for increasing image quality. Figure 1 shows an image of a mouse kidney specimen that was taken without the use of the Dispersion-Compensating Mirror Set, whereas Fig. 2 shows the same image acquired after adding the mirror pair into the experimental setup. In the mouse kidney specimen (Molecular Probes®, Invitrogen Corp.), the glomeruli and convoluted tubules are labeled with Alexa Fluor 488 (green) and cell nuclei are labeled with DAPI (blue). These pseudo-color images were obtained using Thorlabs’ in-house developmental multiphoton microscroscope equipped with a 40X Olympus objective (NA = 0.75). Two-photon excitation was provided by Idesta-QE’s Octavius-1G, a Ti:Sapphire oscillator that provides a repetition rate of 1 GHz and ultra-short (<6 fs) pulses. The group delay dispersion (GDD) attributed to the optical elements in the microscope is ~4200 fs2. GDD was compensated by adding the Dispersion-Compensating Mirror Set into the beam path prior to the imaging system entrance. An intensity analysis of the images shown in Figures 1 and 2 indicates that the pulse compression provided by the mirror pair increases the signal to noise by a factor of ~38 (~16 dB), thereby providing a higher quality image of the mouse kidney.


Uncompressed pulse
Compressed pulse

Application Note: Multiphoton Imaging

Multiphoton laser scanning microscopy (MPLSM), also commonly referred to as two-photon excitation microscopy or nonlinear laser scanning microscopy, is currently recognized as the premier method for obtaining high-resolution, three-dimensional images of thick biological specimens. In addition to two-photon excitation fluorescence, MPLSM benefits from the added flexibility of being able to generate signals via second harmonic generation (SHG) and three-photon excitation. The nonlinear nature of two-photon excitation confines excitation to the focal plane of the objective. This confinement produces inherent optical sectioning and eliminates fluorescence outside of the focal plane. During two-photon laser scanning microscopy, the sample is illuminated with light whose energy is half of what is necessary to excite the fluorophore being used. Therefore, in order to excite the fluorophore, two photons must be absorbed simultaneously. Consequently, MPLSM requires the use of mode-locked femtosecond pulsed lasers. The nonlinear interaction between the light and the medium necessitates the use of high intensity light sources. Therefore, to prevent damage to the specimen, it is important to have a short laser pulse present at the sample location; such a pulse can be provided using the dispersion-compensating mirror set featured here.

Advantages of Multiphoton Imaging

For more information about the two-photon microscope used in these measurements please contact Enrique Chang under echang@thorlabs.com from our strategic partner company Thorlabs.