Light Sources for CARS Microscopy: Continuous Evolution

Past advances in CARS microscopy have been possible due to the constant improvement in ultrafast laser sources. This development continues today, to push CARS technology to the next level, and thus fulfill the demands of emerging applications in cellular biology and biomedical science.

PAST:

Two tightly synchronized Mode-locked Ti:Sapphire lasers

PROS:

• High spectral resolution (1-3 cm^-1 for 5 ps pulse)
• Broad Tunability (500-3500cm^-1)

ROOM FOR IMPROVEMENT:

• Timing jitter limits the ultimate performance
• Complex: two laser sources
• The system has short operation wavelengths in the IR (690-1080 nm). Longer operation wavelengths are desired to increase the penetration depth and reduce photodamage.

PRESENT:

Intracavity-doubled synchronously-pumped Optical Parametric Oscillator (OPO)

PROS:

• Complete elimination of pulse jitter
• High spectral resolution (1-2 cm^-1)
• Broad tunability (500-3500 cm^-1).
• Longer operation wavelengths than previous systems (Stokes: 1064 nm; Pump: 780-920 nm)

PROS:

• Single source for both Pump and Stokes
  Colinear Pump and Stokes beams directly from the OPO. No need for optics to combine the two beams.
• Raman shift selection controlled by the temperature of a single crystal (500-3800 cm^-1)
• No pulse jitter
• High spectral resolution (1-2 cm^-1)
• Wavelengths available over hundreds of nanometers (740-1450 nm)
• Long operating wavelengths minimize sample photodamage and increase penetration depth

PROS:

• Significantly lower cost than solid state system
• No need of optical table or water cooling
• No misalignment
• No pulse jitter
• Lower repetition rate

FUTURE:

We are working on the future of CARS imaging...Surprises are on the way!