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 nonlinear 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 Menlo Systems' 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.

The highly reflective coating for the 700 - 1000 nm range is deposited on the surface using ion-beam sputtering (IBS) technology. During the automated IBS process, the materials are directly sputtered onto the substrate with a high degree of accuracy. This is a highly repeatable and controllable technique, resulting in dense, durable, high-damage-threshold thin films.

Mounting Option
The DCMP175 Dispersion-Compensating Mirrors can be mounted in KM100C Kinematic Cylindrical Lens Mount as shown in the photograph to the right. The mount accepts any cylindrical or rectangular optic up to 65 mm tall. Unlike most guillotine mounts, the KM100C has no mounting components that obstruct the optical axis. The kinematic tip/tilt adjustments are driven by two 1/4"-80 actuators.

The mount can be attached to any of Thorlabs' Ø1/2" TR Series posts, which feature an #8-32 (M4) tapped hole.Alternatively, as shown in the figure to the right, the KM100C mount can also be attached to an RS1.5P Ø1" Pedestal Pillar Post, which has a height of 1.5", and secured to the breadboard using a CF125 Clamping Fork.

Multiphoton Imaging With and Without a Dispersion Compensating Mirror Pair

Multiphoton microscopy is recognized as the premier method for obtaining high-resolution, three-dimensional images from within thick biological samples. Compared to confocal microscopy, the confinement of signal to the focal plane of the objective in multiphoton imaging allows for more efficient fluorescence detection and a reduction in photodamage. Furthermore, the use of near infrared excitation allows for improved imaging depth due to a reduction in scattering.

Since multiphoton microscopy requires simultaneous absorption of two or more photons by the fluorophore, mode-locked pulsed lasers with high repetition rates and ultra short pulses are employed. These lasers provide the high peak powers necessary to generate sufficient multiphoton absorption in the focal
plane while preventing damage to the sample by keeping the average power absorbed low.

However, the standard optical components found in most imaging systems tend to broaden ultra short laser pulses to the point where the quality of the image will be affected. This pulse broadening occurs because of the wavelength depedence of the refractive index of 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 dispersion of the laser pulse is corrected using dielectric mirrors with specialized coatings that are specifically designed so that longer wavelengths experience a larger group delay than shorter wavelengths, thereby negating pulse broadening caused by other optical elements within the imaging system. The two-photon images of a mouse kidney shown below demonstrate the benefits of using the Dispersion-Compensating Mirror Set for increasing image quality.

The figure to the right shows an images of a mouse kidney specimen that were taken prior to and after inserting the Dispersion-Compensating Mirror Set into the In the specimen (Molecular Probes®, Invitrogen Corp.), the glomeruli and convoluted tubules were labeled with Alexa Fluor 488 (green) while the cell nuclei were labeled with DAPI (blue). Simultaneous excitation of both fluorophores wasaccomplished using Menlo Systems' Octavius- 1G, a Ti:Sapphire oscillator that produces ultra-short (<6 fs) pulses with a 1 GHz repetition rate. Two-photon fluorescence images were obtained with Thorlabs' in-house multiphoton microscope equipped with a 40X objective (Olympus, NA = 0.50).experimental setup.

Figure 1. Pseudocolored multiphoton images of a mouse kidney obtained without (frame a) and with (frame b) the use of Dispersion-Compensating Mirror Pairs. The glomeruli and convoluted tubules are stained with Alexa Fluor 488 (green) while the nuclei are stained with DAPI (blue). By introducing the mirror pair into the setup, the signal to noise was increased by a factor of approximately 38, thereby providing a higher quality image of the mouse kidney.