Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002
.pdf
Optical Sources |
71 |
Several manufacturers of superluminescent diodes have used multiple spectrally offset semiconductors in the attempt to synthesize broader spectra. Seemingly small distortions of the spectral distribution away from the well-defined Gaussian or hyperbolic secant shape, however, can significantly distort the OCT axial point spread function. As an example, two Gaussian spectra are combined in Fig. 2. The modified spectrum (solid line) was generated by combining a second, offset Gaussian with a narrower width with the Gaussian distribution of the previous figure. In the logarithmic plot of the spectra, this modification is barely discernible. In the logarithmic plot of the axial point spread function, however, the far-field wings are found to be substantial.
Noise superimposed onto a spectrum also has significant implications for OCT imaging. In Fig. 3 the Gaussian spectrum of the previous figures was modified by a multiplicative, random noise with a maximum amplitude of 10%. Although the noise appears small on the spectral plots, it acts to dramatically increase the apparent noise floor of the axial point spread function. In this case the noise was not periodic. In cases where the noise is oscillatory ripple, the point spread function develops discrete wings or echoes located at delay values determined by the period of the oscillation.
Figure 2 Power spectra and autocorrelations for Gaussian spectra that are modified with a second spectral peak. This is an example of the spectra that might result from combining two sources.
72 |
Bouma and Tearney |
Figure 3 Power spectra and autocorrelations demonstrating the influence of noise or ripple superimposed on a Gaussian spectrum.
A final type of nonuniform spectrum frequently encountered in OCT system development is the clipped spectrum. This can arise from an element in the interferometer that acts as a notch, high-pass, or low-pass filter. In bulk optical systems, this type of filtering can be found in dielectric beamsplitters or mirrors. The phase control rapid scanning optical delay line [7] can also act to clip the spectrum, because this device uses a diffraction grating to spread the spectrum and a galvanometer mirror to reflect the spectrally dispersed light. In an effort to optimize the repetition rate of the galvanometer, the angular momentum of the mirror should be kept as small as possible, requiring minimization of the mirror width. If the mirror is not wide enough, however, a portion of the spectrum can either miss the mirror or be aberrated at the mirror edge. The influence of spectral clipping is shown in Fig. 4. In this case, the wings of the spectrum have been deleted below the 10% level. The resulting sharp edges give rise to weakly damped periodic oscillations in the wings of the point spread function and significantly increase the far-field response.
3.1.3Amplitude Modulation
A final source characteristic that will be discussed is amplitude stability. In the most common implementation of OCT, light returning from the sample and reference interferometer paths is recombined at a photodiode and the electric current gener-
Optical Sources |
73 |
Figure 4 Power spectra and autocorrelation demonstrating the effect of spectral clipping.
ated by the photodiode is measured as a function of delay time. The idealized interferogram depicted in Fig. 5a consists of an envelope function modulated by a sinusoid. The radio-frequency spectrum is the Fourier transform of this and consists of a single-peaked, localized distribution (Fig. 5b). Prior to digitization of the interferogram, filtering electronics are used to reduce out-of-band noise. This can be done by using either bandpass filters or lock-in detection.
Frequently, optical sources generate noise in the form of amplitude modulation. In semiconductor sources, this can result from fluctuations in drive current.
Figure 5 Autocorrelation function and spectrum of ideal Gaussian spectral shape.
74 |
Bouma and Tearney |
Figure 6 The effect of amplitude modulation either in the source or induced through mechanical instability in the interferometer. Periodic amplitude modulation appears as a discrete spike in frequency space (Fourier transform from interferogram).
Similarly, in solid-state sources, including doped fibers, amplitude modulation can result from fluctuations in pump laser power. Additional sources of light source amplitude instability are longitudinal mode beating and relaxation oscillations. Mechanical perturbations of the interferometer can also give rise to modulation in the photocurrent. The example of Fig. 6a depicts an interferogram that would result from a light source with amplitude modulation of a characteristic time constant. In the frequency domain (Fig. 6b), the modulation gives rise to a discrete peak that in this case is separated from the portion of the spectrum containing ranging information. Understanding the characteristic noise of the light source is important in OCT, because in most systems the heterodyne frequency can be chosen to optimize noise filtering.
The data of Fig. 7 were generated with an OCT system using a diffraction grating based rapid-scanning optical delay line [7]. The heterodyne frequency was adjusted to be 1.0 MHz, and the signal bandwidth was 500 kHz. In this spectrum, the primary contributions to noise are 1=f noise and a peak characteristic of the semiconductor light source of this system (Fig. 7a). Because the source noise peak occurring at 300 kHz is outside the information-containing band, a passive filter network (Butterworth filter) can be used to significantly improve the signal (Fig. 7b).
Figure 7 (a) Frequency distribution of OCT detector signal showing broad signal component, 1=f noise, and discrete frequency spike arising from amplitude modulation in the source.
(b) Same signal after passive bandpass filter.
Optical Sources |
75 |
3.2SEMICONDUCTOR SOURCES
The development of OCT technology has relied significantly on the availability of advanced devices developed for the telecommunications market. Perhaps the most influential telecom contributions have been in the area of broadband light source development. The first reports of coherence domain ranging and, later, OCT imaging were based on superluminescent diodes (SLDs). The primary advantages to the semiconductor sources are compactness, reliability, environmental stability, and ease of use. Until recently, however, the low power and bandwidth of these sources limited their application to slow acquisition and modest resolution imaging. This section will review typical characteristics of SLDs and the recently developed multiple quantum well semiconductor optical amplifier sources.
3.2.1Superluminescent Diodes
As mentioned above, SLDs dominated the early work of OCT because of their simplicity and relatively low cost. Most of the early SLDs used for OCT imaging had center wavelengths near 850 nm and provided bandwidths of approximately 20 nm, supporting an OCT resolution of 15 m. The total cost for these SLDs has been on the order of $5000–10,000 including the diodes, current sources, and thermoelectric cooler controllers. Typical powers for the early SLDs were below 1 mW. The smooth Gaussian spectra of these devices provided low noise, echo-free OCT imaging, but image acquisition times were typically several minutes.
Recent advances in SLD development have enhanced the power, bandwidth, and spectral coverable of available devices. Currently, broadband SLDs are available with center wavelengths near 670, 800, 980, 1300, and 1500 nm. The web site of one of the leaders in SLD development for OCT, Superlum, Ltd (http:==www:superlum:ru=), provides complete specifications of some of the more advanced devices currently on the market.
3.2.2Multiple Quantum Well Semiconductor Optical Amplifiers
A source developed in 1997 by AFC Inc. has become the de facto standard for clinical OCT imaging systems. The primary driving market behind this source was broadband telecommunications. The 1300 nm AFC source is based on a multiple quantum well semiconductor optical amplifier and uses proprietary filtering to achieve bandwidths of 50–80 nm and fiber-coupled powers of 9–30 mW.
At the core of the AFC source is a semiconductor chip containing several quantum wells with chirped periods. The varied confinement dimensions of the wells leads to a combined broad spectrum. The semiconductor chip is configured in a butterfly package with fiber-optic input and output. To further increase the power of the amplified spontaneous emission of these devices, one output direction can be spectrally filtered to precompensate for gain narrowing incurred during a second, amplifying pass through the waveguide. One potential drawback of this source is that its emission is unpolarized. Polarization-sensitive OCT imaging, which requires a polarized source, can therefore access only half of the available power.
An example image acquired using a 1300 nm AFC source is depicted in Fig. 8. This system used a nonreciprocal fiber-optic interferometer and a fiber-optic catheter
76 |
Bouma and Tearney |
Figure 8 (A) Endoscope view showing linear scanning OCT catheter in human esophagus. (B–D) Although interand intrapatient variability of layer thickness and structure is noticed, five discrete layers are always observed in the normal esophagus. The OCT images were acquired using a semiconductor multiple quantum well optical amplifier emitting 10 mW at 1:3 m.
to image the gastrointestinal tract of human patients [6]. Typical images of the human esophagus are shown.
3.3DOPED FIBER SOURCES
The need for a high power, clinically viable low coherence source has motivated the investigation of diode-pumped rare-earth-doped single-mode fibers (REDFs). These single-mode, broad-bandwidth superfluorescent light sources are compact, relatively simple, and inexpensive. In addition to evaluating the capability of the REDFs as a high resolution, high power source for OCT, research using REDFs at different wavelengths has provided insight for the determination of appropriate wavelengths for OCT imaging in tissue.
If the excitation of the REDF is strong, the fluorescent light emitted from the fibers is amplified by stimulated emission. The pump energy is stored primarily as a population inversion, which in turn amplifies the guided fluorescence [8]. The wavelength dependence of the gain, however, leads to a narrowing of the emitted spectrum with amplification. In addition, if any backreflections are present in the fiber, either from a backscattering site within the device or from Rayleigh scattering from the fiber, lasing will commence and further reduce the spectral width [8]. For these reasons, many REDFs require spectral filtering to compensate for gain narrowing so that useful power levels can be achieved.
Figure 9 depicts several different fiber configurations used for creating REDF superfluorescent sources [9]. In order to suppress lasing, feedback from either one or both fiber ends can be eliminated by angle cleaving or polishing. The forward superfluorescent signal (SFS) configuration is the simplest to implement but reduces the
Optical Sources |
77 |
Figure 9 Rare-earth-doped single-mode fiber (REDF) pump and fiber configurations.
(a) Single-pass forward SFS; (b) double-passed forward SFS; and (c) double-passed backward SFS.
useful superfluorescent power by 50% (Fig. 9a). In the single-pass forward SFS case, both fiber ends are cleaved and the superfluorescence is detected from the fiber face opposite the pump. Many of the REDF sources presented in this chapter use the single-pass forward SFS configuration because of its simplicity and complete elimination of feedback from both fiber ends. Figures 9b and 9c depict double-pass configurations. Double-pass SFS geometries enable the use of both forwardand backward-propagating superfluorescence.
One problem with REDF superfluorescent sources is the need for a high power ( 500 mW) single-mode pump for optimal coupling into the doped single-mode core. Although some compact semiconductor diode sources produce a single-mode output with these powers, they are typically very expensive. Because of the need for an inexpensive and compact pump, a cladding-pumped geometry has been adopted to enable coupling into the fibers with less expensive multimode diodes (Fig. 10). The cladding-pumped REDF consists of an asymmetrical cladding surrounding the doped core [9]. The shape of the inner cladding can be tailored to match the geometry of the pump diode emissions. For a given dopant density, the length of the cladding-pumped fiber must be much greater than that of the single-mode doped fiber because the absorption of the pump light per unit length is much lower than for standard single-mode REDFs. For four-level systems, such as Nd : silica and Yb : silica, which exhibit little absorption in the emission band, the greater fiber length does not pose a prbolem. For three-level systems or quasi-four-level systems,
78 |
Bouma and Tearney |
Figure 10 REDF cladding-pumped geometries.
however, such as Er : silica and Tm : silica, the increased length of the fiber causes increased absorption of the superfluorescence. For this reason, rare earth ions with three-level or quasi-four-level transitions have not been used in the cladding-pumped geometry.
3.3.1Neodymium
Neodymium : silica excited at 800 nm has a strong four-level transition at 1060 nm and is therefore well suited as a dopant for the cladding-pumped fiber geometry. Initial experiments were performed using a double-clad fiber to evaluate the suitability of this source for OCT. Because optical scattering in tissue at 1060 nm is stronger than the scattering at 1300 nm, one would expect the penetration to be less. However, the absorption at 1060 is lower than the absorption at 1300 nm. One goal of the preliminary OCT studies with the Nd : silica fiber was to determine the significance of the differences in absorption and scattering in tissue.
Figure 11 depicts the fluorescence spectrum of a 10 m single-mode Nd : silica REDF pumped at 800 nm with 100 mW Ti : Al2O3 power. The Nd : silica REDF was obtained from J. Minelli, University of Southampton, UK. Although the shape of the fluorescence spectrum seems reasonable for OCT imaging without significant side lobes in the autocorrelation function, the total integrated power emitted was only 16 W. At higher pump powers the amplified spontaneous emission exhibits significant narrowing. The Nd : silica REDF pumped with 600 mW produces 3.5 mW of SFS, but the spectrum contains significant gain narrowing at the dominant emission line, 1060 nm (Fig. 12). The width of the dominant peak is now only 8 nm, corresponding to a coherence length of 62 m. If this peak narrowing can be suppressed, the wings of the SFS can be amplified and the REDF spectrum can be broadened. The next section describes the use of an in-line long-period Bragg grating to broaden the Nd : silica REDF SFS spectrum.
To decrease the gain narrowing in Nd : silica at high pump power, a Bragg grating filtered, cladding-pumped Nd : silica REDF has been developed. The Bragg grating filtered Nd : silica REDF was obtained from E. A. Swanson, Lincoln Laboratory, MA. A schematic of the system is shown in Fig. 13. This system uses 3.7 m of cladding-pumped Nd : silica fiber in a double-passed backward SFS configuration. To eliminate the spectral gain narrowing at 1060 nm, an in-line optical
Optical Sources |
79 |
Figure 11 Spectrum of single-mode Nd : silica REDF pumped with 100 mW. The integrated emitted power was 16 W.
notch filter is spliced between the REDF and the mirror. The notch filter is a longperiod Bragg grating that is written into a single-mode optical fiber (Fig. 14). The Bragg grating flattens the spectrum by diminishing the peak at 1060 nm while allowing amplified spontaneous emission (ASE) to pass unfiltered outside of the bandwidth of the notch filter (Fig. 15). The output power from the filtered REDF is 7 mW, with a multimode diode pump power of 200 mW at 810 nm. The bandwidth of the filtered spectrum is 39 nm, corresponding to a measured FWHM coherence length of 16 m.
Figure 12 Gain-narrowed superfluorescent spectrum for a double-clad Nd : silica fiber pumped with 600 mW of 800 nm power. The total integrated power is 3.5 mW.
80 |
Bouma and Tearney |
Figure 13 Schematic of the Nd : silica REDF superfluorescent source with the long-period Bragg fiber grating.
Figure 14 Long-period Bragg grating notch filter transmission spectrum.
Figure 15 Double-clad SFS spectrum of Nd : silica REDF (a) without notch filter and (b) with notch filter.