Injection-seeded Raman fiber amplifier for photoacoustic microscopy of lipids

Photoacoustic Microscopy (PAM) is an emerging medical imaging technique that uses nanosecond laser pulses to create ultrasonic waves in tissue. PAM has advantages over traditional ultrasound or optical imaging because PAM differentiates tissue types by exploiting the optical absorption spectrum present for a specific tissue type (e.g. lipid is imaged at ~1210nm). Label-free imaging of atherosclerotic plaques and myelinated peripheral nerves are two potential applications of PAM. Conventional pulsed lasers operating at 1200 nm are very expensive. Previously, we have demonstrated a cost-effective laser using stimulated Raman Scattering (SRS) in an optical fiber to produce pulses ranging from 1000nm to 1300nm. SRS describes the inelastic scattering of light to produce lower-energy (Stokes) photons. Starting with a 1047nm pump laser, we have produced Stokes lines at 1098, 1153, 1215, and 1274nm. The third Stokes line at 1215nm is useful to image lipids with PAM. However, drawbacks of this laser are the fixed wavelength and broad spectral linewidth of the third Stokes line. To overcome these drawbacks, we introduce a second laser into the fiber to tune the wavelength and narrow the linewidth of the third Stokes peak for imaging lipids. A pump laser producing pulses at 1047nm wavelength and a seed laser with 1206nm are coupled into a 100m optical fiber. SRS of the pump produces Stokes lines at 1098 and 1153nm. SRS amplifies the seed laser to produce a Stokes line at 1206nm instead of the usual 1215nm. Furthermore, the spectral linewidth was narrowed from 30nm to 11nm. The 1206nm pulses yielded clear images of a drosophila fat body and lipid phantoms. This PAM system can produce tunable laser pulses from 1195nm to 1220nm by varying the seed laser wavelength. Therefore, we believe that this system can benefit many in vivo PAM applications for imaging lipids.