Broad Bandwidth Laser and Nonlinear Optical Light Sources for OCT
Optical coherence tomography (OCT) achieves very high axial image resolutions independent of focusing conditions, because the axial and transverse resolution are determined independently by different physical mechanisms. This implies that axial OCT resolu
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Optical coherence tomography (OCT) achieves very high axial image resolutions independent of focusing conditions, because the axial and transverse resolution are determined independently by different physical mechanisms. This implies that axial OCT resolution can be enhanced using broad bandwidth, low coherence length light sources. The light source not only determines axial OCT resolution via its bandwidth and central emission wavelength, but also determines the penetration in the sample (biological tissue), the contrast of the tomogram and OCT transverse resolution. A minimum output power with low amplitude noise is also necessary to enable high sensitivity and high speed – real time – OCT imaging. Furthermore, ultrabroad bandwidth light sources emitting at different wavelength regions might also enable a potential extension of OCT, e.g., spectroscopic OCT. Hence, it is obvious that the light source is the key technological parameter for an OCT system and proper choice is imperative [1]. Several main criteria have to be considered when choosing a light source for OCT imaging. A light source and its usability for OCT can be characterized by: • • • • • •
Center wavelength Bandwidth, spectral shape Power Noise Single transverse mode Stability
In principle, thermal light sources are capable of achieving high axial resolution because of their large spectral bandwidth, but their use for clinical OCT applications is limited by the low power contained in a single spatial mode, which is necessary for high sensitivity, high speed in vivo clinical OCT imaging. As stated previously, the depth resolution of OCT is defined as being equal to the coherence length of the light source.
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A. Unterhuber et al.
Ti:sapphire laser are excellent light sources for ultrahigh resolution (UHR) OCT because of the extraordinary large gain bandwidth and high optical output power. With advanced mirror technology, dispersion control and adapted cavity design, optical bandwidth of up to 300 nm at full width of half maximum (FWHM) centered at about 800 nm could be achieved resulting in submicrometer axial resolution OCT in tissue. Broad bandwidth Cr3+ :LiSGaF lasers are cost-effective, directly diode pumped alternative light sources for OCT in the 800 nm wavelength region. Recent efforts also focused on developing broad bandwidth light sources in the 1,300 nm wavelength range that would permit OCT micrometer-scale resolution along with millimeter range penetration depth. A laser spectrum covering the 1,230–1,580 nm wavelength region with an optical bandwidth of 250 nm (FWHM) was generated directly out of an all-solid state Cr:forsterite laser. Cr4+ :YAG lasers have the ability to produce sub-20-fs pulses enabling broad optical bandwidth laser emission in the wavelength range from 1,300 to 1,600 nm. These lasers operate at room temperature, do not require a vacuum, and have larger gain bandwidths than Er-doped fiber lasers. Originating in Ti:sapphire laser’s nonlinear media, microstructured fibers (MF) are investigated and explored in terms of nonli
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