Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002

Figure 17 Optical coherence tomographic images of a calcified aortic atherosclerotic plaque taken with the 1064 nm filtered REDF and the 1300 nm SLD. Bar represents 500 m.

Yb : silica is pumped at 980 nm because low cost, high power diodes are available at this wavelength. However, the gain-narrowed bandwidth for a cladding-pumped Yb : silica REDF in the single-passed SFS configuration is approximately 15 nm, which corresponds to a coherence length of only 35 m (Fig. 18). The claddingpumped Yb : silica REDF was obtained from M. Muendel, Polaroid Corporation, MA. Like the Nd : silica REDF, a shorter coherence length is desirable, and spectral notch filtering must be used to decrease peak narrowing at 1100 nm while allowing ASE to pass unfiltered outside the bandwidth of the notch filter.

For comparison with the Bragg grating filtered Nd : silica REDF, a claddingpumped Yb : silica REDF ASE source has been constructed that uses a cascade of custom wavelength division multiplexers (WDMs) to perform the spectral filtering. The WDM-filtered Yb : silica REDF was obtained from S. Chernikov, Imperial College, London. A schematic of the WDM-filtered Yb : silica source is shown in Fig. 19. The ASE source consists of two lengths of Yb : silica fiber pumped by using three separate 980 nm diodes in a double-passed SFS configuration.

The spectrum of the WDM-filtered Yb : silica REDF source is shown in Fig. 20. The spectrum is very rectangular, which gives rise to severe artifacts in the autocorrelation. The autocorrelation function for this WDM-filtered Yb : silica REDF source is shown in Fig. 21. The 75 nm bandwidth centered at 1075 nm, produced by the Yb : silica REDF source, provides a FWHM coherence length of

Figure 21 Autocorrelation function of the WDM-filtered Yb : silica REDF ASE source. The FWHM bandwidth is 13 m.

13 m with an output SFS power of 10 mW. Like the Bragg grating filtered Nd : silica REDF, the WDM-filtered Yb : silica source contains side lobes due to the rectangular shape of the spectrum. These side lobes are similar in magnitude to those of the Bragg grating filtered Nd : silica REDF.

Images of an in vitro human breast carcinoma were acquired with the WDMfiltered Yb : silica REDF ASE source (Fig. 22). Although some tissue features can be identified, such as adipose cells near the surface of the image, side lobes present in the autocorrelation function cause blurring of the surface and the internal structure. Because of these severe artifacts, a second-generation source must be designed that uses WDM filters to not only reduce gain peaking but also shape the spectrum to be more Gaussian.

3.3.3Erbium

A very common single-mode doped fiber source used in telecommunications applications is the Er : silica REDF. The erbium fiber is pumped at 980 nm and has a

Figure 22 Optical coherence tomographic image of human breast carcinoma acquired using the WDM-filtered Yb : silica REDF. Bar represents 500 m.

three-level transition with peak emission at 1550 nm and a bandwidth of approximately 50 nm [9]. The Er : silica REDF exhibits gain narrowing, so spectral filtering would be necessary to produce a sufficient coherence length. Because the Er : silica emission is a three-level transition, the cladding-pumped geometry cannot be used owing to absorption of the signal. In addition, although scattering is lower at 1550 nm that at any of the previous wavelengths, the erbium emission band overlaps with the high water absorption peak at 1480 nm. For this reason, it is expected that OCT imaging with Er : silica REDF ASE would have a lower penetration depth than imaging with other sources discussed in this chapter.

3.3.4Praseodymium

Praseodymium-doped silica fiber has a gain-narrowed spectrum that is significantly broader than the gain-narrowed spectrum of Nd : silica REDF. A 590 nm pumped Pr : silica REDF ASE source has been reported to have a broad Gaussian spectrum with a FWHM at 25 nm [10]. With a pump power of 250 mW, the Pr : silica REDF produces 60 mW SFS at 1049 nm. The primary problem with the Pr : silica REDF is that there are no inexpensive high power single-mode diodes at 590 nm. Instead, Shi and Poulsen [10] used a dye laser to pump the Pr : silica REDF. Until an inexpensive diode source at 590 nm is available, the Pr : silica REDF is unlikely to become a clinically viable source for OCT.

3.3.5Thulium

An intriguing spectral range for OCT imaging exists at wavelengths longer than the prominent water absorption band at 1480 nm. Between 1600 and 1800 nm, water absorption decreases, and near 2:0 m it rises sharply again. Because of the inverse dependence of scattering on wavelength, the attenuation in tissue due to scattering in this wavelength range is low. Thus, sources between 1650 and 1800 nm could give rise to OCT imaging penetration depths equal to or greater than the penetration at 1300 nm.

Thulium-doped silica is an REDF with a quasi-four-level transition at 1800 nm. The maximum absorption peak is very narrow and is located at 785 nm. Figure 23 shows a plot of a single-mode Tm : silica REDF emission spectrum pumped at 785 nm in the single-passed SFS configuration. The Tm : silica REDF was obtained from L. Nelson and E. P. Ippen, Massachusetts Institute of Technology, Boston, MA. The single-mode doped fiber was 2 m long and was pumped with 500 mW of Ti : Al2O3 power at 785 nm. The SFS bandwidth is 80 nm, which gives rise to a FWHM coherence length of 18 m. The gain-narrowed spectrum is asymmetrical but more Gaussian than the spectrally filtered ASE sources. The Tm : silica REDF produced a total integrated SFS output power of 4 mW. Figure 24 shows a comparison between an image of a calcified aortic plaque acquired using the Tm : silica source (Fig. 24a) and an image of the same plaque acquired with a 1300 nm SLD (Fig. 24b). Both OCT images were taken with the same SNR, 102 dB.

As can be seen in these images, the Tm : silica REDF shows sharp delineation of boundaries in the image, such as the calcified foci within the plaque. In addition, no blurring due to autocorrelation side lobes is seen at the surface. Finally, the penetration depth in this particular image of the heavily calcified aortic plaque seems to be at least equal to the penetration depth of the 1300 nm SLD image.

One major disadvantage to Tm : silica is that it cannot be used in a claddingpumped configuration because it is a quasi-four-level system. In a three-level or quasi-four-level system, the ground-state absorption of the SFS signal extinguishes the emission over a long length of fiber. Single-mode pumps at 785 nm are commercially available but expensive, and for this reason Tm : silica REDF ASE sources may not be as desirable as other less expensive, high power REDF ASE sources.

3.4SOLID-STATE LASERS

Lasers that rely on a crystalline gain medium are one type of solid-state laser. Rare earth ions can be doped into the crystal, replacing one of the host ions at a specific lattice site. The choices of crystal host and rare earth ion allow many different possibilities of excitation and emission wavelengths. Over the last 15 years, several new laser crystals have been developed that take advantage of the vibrational interaction between the crystalline host and the dopant ion to significantly broaden the absorption and emission spectra [11]. The results have been quite impressive; these phonon-broadened sources provide greater spectral widths than any other laser sources.

The broad emission spectra of a solid-state laser can be used for OCT imaging in two ways, either as superluminescent sources [12] or through mode locking [13– 15]. Because fluorescence is emitted isotropically in space and the laser crystal does not provide the light guiding and confinement of an optical fiber, it is difficult to produce more than a few microwatts of superluminescent light from solid-state sources. Mode locking provides a much more attractive alternative. By controlling the phase relationship of many longitudinal resonator modes, short duration pulses and therefore broad spectra can be produced. In addition, mode-locked solid-state lasers can produce hundreds of milliwatts of power and typically lower relativeintensity noise.

In this section, the application of two of the solid-state lasers through mode locking to OCT imaging will be reviewed. Both of these lasers rely on passive, Kerr lens mode locking and use similar resonator designs (Fig. 25). The resonator uses four mirrors, one of which is partially reflective to provide output coupling, and dispersion-compensating prisms. The laser crystal is mounted between two concave spherical mirrors and is excited longitudinally. Kerr lens mode locking takes advantage of the inherent nonlinearity of the laser crystal itself to provide a resonator loss that is dependent upon the intensity of the light in the cavity. This provides preferential gain for pulses of short temporal duration.

3.4.1Ti : Al2O3

Mode-locked solid-state lasers have been used as high power and high resolution sources for OCT. Kerr lens mode locking (KLM) in Ti : Al2O3 oscillators (Fig. 25) has been shown to produce high average power near-infrared pulses with duration < 10 fs [16–18]. Ti : Al2O3 oscillators have been used for OCT imaging with outstanding resolution [13,15]. To maintain the high resolution of the Ti : Al2O3 source, dispersion imbalance between the interferometer arms must be precisely canceled [12]. To perform dispersion balancing, a fused silica prism pair with faces contacted and index matched to form a variable thickness window is inserted in the reference

Figure 27 OCT images of an onion acquired with the 800 nm Ti : Al2O3 laser and the 1300 nm SLD source. Bar represents 100 m.

ing to a spot size of approximately 17 m. Both of these images have dimensions of 120 vertical and 360 horizontal pixels. The image acquisition time was 2.5 s.

The KLM Ti : Al2O3 laser coupled to an optimized OCT system enables high resolution, high power imaging of biological structures. Additionally, the application of KLM to other solid-state laser materials will provide high power, short coherence length source at wavelengths with greater penetration in tissue such as 1:3 m from Cr4þ : forsterite and 1:5 m from Cr4þ : YAG.

3.4.2Cr4+ : Mg2SiO4

Although Ti : Al2O3 lasers can produce unrivaled resolution for OCT imaging, optical scattering in biological tissues near 800 nm limits imaging depth of penetration. Previous comparisons of imaging depth using 1:3 m and 800 nm light from SLDs motivated the development of other solid-state lasers capable of providing broad spectra at longer wavelengths. Like Ti : Al2O3, Cr4þ : forsterite is a phonon broadened, tunable solid-state laser material that can be used for the generation of femtosecond optical pulses. Application of Kerr lens mode locking to a Nd : YAG-pumped Cr4þ : forsterite oscillator has been achieved with pulse duration as short as 25 fs [19,20].

A Cr4þ : forsterite laser has been constructed for use as a high power, high resolution OCT source by pumping Cr4þ : forsterite with 6.0 W of 1.06 mm light