Cars microscope
To measure homodyne interference with a CARS microscope, a supercontinuum beam is used as a light source. A supercontinuum beam is generated using a nonlinear optical fiber that has normal dispersion in which the coherence between pulses is maintained. As the phases of the interference components of detected beams are the same between pulses, it is possible to integrate the interference components and thus improve the signal-noise ratio.
1. A coherent anti-Stokes Raman scattering (CARS) microscope comprising:
a pulsed laser light source configured to generate a first beam with a frequency ωP;
a normal dispersion nonlinear optical fiber configured to receive the first beam and generate a supercontinuum beam;
an optical element configured to select, from the supercontinuum beam, a second beam with a frequency ω ST that is lower than the frequency of the first beam, and to select, from the supercontinuum beam, a third beam as a reference beam with a frequency ω AS =2ω P −ω ST ;
a mechanism including one or more mirrors configured to adjust phases of the first beam and the second beam;
an optical unit configured to align the first beam and the second beam on a same axis;
an objective lens configured to focus the first beam and the second beam that are coaxial beams onto an observation sample;
a stage configured to move to scan the observation sample with the coaxial beams;
an interference optical unit configured to cause a CARS beam generated from the observation sample and the third beam to interfere with each other;
a plurality of beam splitters that are each configured to decompose a respective interference beam obtained from the interference optical unit;
a plurality of spectrometers each configured to detect a respective beam decomposed by one of the beam splitters;
a computing unit configured to process signals from the spectrometers; and
a display device configured to display an image based on the signals processed by the computing unit.
2. The CARS microscope according to claim 1 , wherein: the nonlinear optical fiber has a normal dispersion at least in a frequency region in which the supercontinuum beam is generated.
3. The CARS microscope according to claim 1 , wherein:
each respective interference beam is split in at least two beams,
a phase difference between the beams split from the interference beam is an integral multiple of 90 degrees, and
at least one phase difference is 90 degrees.
4. The CARS microscope according to claim 1 , wherein the nonlinear optical fiber has a polarization-maintaining optical fiber.
5. The CARS microscope according to claim 1 , wherein the supercontinuum beam has a bandwidth of approximately 3000 cm −1 in a high-frequency direction and a low-frequency direction about the frequency ω P of the first beam as a center.
6. The CARS microscope according to claim 1 , wherein an interference beam spectrum is corrected by the computing unit using the signals from the spectrometers which correspond to the second beam.
7. The CARS microscope according to claim 1 , further comprising:
an optical filter configured to narrow a spectral bandwidth of the first beam.