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

Figure 11 Schematic diagram of version 1 of experimental apparatus for OCT measurements of optical disk, using fiber phase modulation. For direct detection measurements the reference beam is blocked and the signal beam is chopped (intensity modulated). [Adapted from Ref. 11, Fig. 10.]

coherence length is also the OCT resolution. If the spectrum were a pure Gaussian with the same width, the coherence length [from Eq. (42)] would be 24:3 m. Note that the peak-to-baseline ratio of the correlation function is limited by the broadband spectral shape (particularly its exponential-like tails). This point is discussed in more detail in Ref. 23.

This broadband source is coupled to a single-mode fiber, with a 50/50 fiber coupler serving as the interferometric beamsplitter. In one version of the apparatus, after the splitter the fiber in the reference path is wrapped around a cylindrical piezoelectric (PZ) element that provides phase modulation. The bias to the PZ element is a sawtooth, serrodyne waveform that gives a total linear 2 phase shift per cycle, thereby providing an approximately sinusoidal heterodyne signal at the modulation frequency (except for the brief retrace interval). The reference beam is collimated in free space and retroreflected from a planar reference mirror back into the collimating lens and reference arm fiber. The beam from the signal path fiber is similarly collimated and focused onto the CD-ROM using a standard CD focus objective. The signal is reflected from the CD back into the signal fiber. Spatially recombined signal and reference beams from the fiber coupler exit both ports of the coupler, with one of them leading to a photodiode detector, followed by a transimpendance amplifier. A fiber polarization controller in the signal fiber is used to align the reflected signal and reference polarizations in the detection path to maximize the heterodyne signal. The heterodyne photocurrent signal (at the modulation frequency) is bandpass filtered and detected. The CD objective lens is mounted on an x-y-z translation stage with piezoelectric fine x;y positioners on the attached low-mass lens mount. The z axis (focus) position is controlled by a computer-inter- faced stepper motor with 0:1 m resolution. A two-dimensional image of the CD is created by scanning the lens in a computer-generated rasterlike x;y pattern using the piezoelectric translator while recording the heterodyne signal. A computer data acquisition system is used to control the two-dimensional lens scan and the data A/D conversion, storage, and display. To acquire the DD output, the optical refer-

Figure 12 Characteristics of the broadband superluminescent diode used in measurements.

(a) The measured optical spectrum. (b) The measured correlation function compared with the Fourier transform of the measured frequency spectrum. [From Ref. 11, Fig. 11.]

ence path is blocked, and the beam in the free-space part of the signal path is detected at base band.

In a second version of the apparatus (shown in Fig. 13), designed to provide a higher modulation frequency and allow faster data acquisition, heterodyne detection was implemented by frequency shifting the reference beam in a fused silica acoustooptic modulator instead of phase-modulating the beam in the optical fiber. The rf drive was at 40 MHz, and double-pass propagation through the modulator imposed an 80 MHz frequency offset on the optical reference. An identical fused silica modulator device (not shown) was placed in the signal path. This device has no rf excitation applied but was used solely to provide dispersion compensation for the

Figure 13 Schematic diagram of version 2 of experimental apparatus for OCT measurements of optical disk, using acousto-optic frequency shifting of reference beam. For direct detection measurements the reference beam is blocked, the signal beam is chopped (intensity modulated), and different detection amplification is used (without the spectrum analyzer).

active modulator so as not to spread the measured coherence function and decrease the depth resolution. The heterodyne signal from the photodiode was fed into an rf spectrum analyzer with sweep frequency fixed at 80 MHz, which provided a baseband signal proportional to the heterodyne rf power from the photodiode.

14.6.2 Multilayer Sandwich

First, to demonstrate the OCT system capability and sensitivity for samples with low interface reflectivity and many layers, we performed a simple multilayer measurement. Using the first apparatus described above, a layered sample was prepared by sandwiching three layers of glycerol between four glass microscope cover slips (nominal 0.009 in. thickness). The Fresnel reflectivity at each glass/glycerol interface is about 0.001. In this instance, a weakly focused beam was used in the sample arm. Figure 14 shows the OCT heterodyne photocurrent signal versus depth and onedimensional lateral displacement for this multilayer sample, with each peak representing an interface reflection. There is excellent signal-to-noise (greater than 50 dB power) and contrast for the weak reflections. A gray-scale projection of the logarithmic signal is also shown in the lower plane of the figure. As expected, the reflections from the two exterior glass/air interfaces are stronger than those from the interior. Note that this figure shows data on a logarithmic (dB) scale, to emphasize the sensitivity to weak reflections. Later data displays of CD-ROM images will have linear scales.

14.6.3 Conventional CD-ROM

Using the first version of the apparatus, a typical display of the OCT image from a standard single-layer CD-ROM is shown in Fig. 15a, and the DD display of the same region, taken immediately after, is shown in Fig. 15b. The images are shown

Figure 14 Measured reflectivity profile of a multilayer glass/glycerol structure, scanned in depth and one lateral dimension. The bottom image is a gray-scale projection of the data using the same logarithmic gray scale as the vertical display. [Adapted from Ref. 11, Fig. 13.]

with false-color gray scales ,with signal levels indicated in the corresponding grayscale bars for each image. To compare the image contrasts more readily, we took the square of the OCT signal before creating its image, as was done mathematically in Eq. (37). The quality of the two images is very similar, as is expected for this relatively simple case of a single data layer. This demonstration [10] was the first to show the feasibility of detecting good CD-ROM signals using either OCT or DD with a low power broadband incoherent source.

14.6.4 Multilayer CD-ROMS

Prototype samples of multilayer CD-ROM media were provided for analysis by the Optical Technology Center of Imation Corporation (formerly part of the 3M Company). These prototypes had conventional CD-ROM exterior dimensions and form. The first two samples had two data layers each, one with 49 m layer separation (sample 1) and the other with 11 m separation (sample 2). Recall that the layer separation for a DVD is 30–55 m. These samples were measured with the rf acousto-optic spectral shift apparatus, enabling image scans to be acquired in 1–2 s (depending on field size and scan resolution). Another experimental feature that was implemented for the multilayer samples was automatic tracking of the reference mirror position with change in stepper motor position of the focusing lens, allowing for the effects on the optical path length of the index of refraction of the disk medium.

Representative gray-scale images for samples 1 and 2 are shown in Figs. 16 and 17, where the OCT images are shown on the left and the DD images are shown on the right. For easy comparison, the gray-scale ranges are autoscaled for each image. To study the amount of cross-talk that might not be visible by eye, we analyzed the OCT images of sample 2, with the smallest layer separation of all the samples. First, two-dimensional autocorrelations of the images from each of the layers were calcu-

Figure 15 Measured images of a single-layer CD-ROM taken using (a) OCT and (b) direct detection. The signal for the OCT image was squared, in accordance with Eq. (34), to provide a direct contrast comparison with image (b). [From Ref. 10, Fig. 4.]

lated, using a Fourier transform method. Then the cross-correlation of these two image signals was performed. All three correlation functions are shown as intensity/ gray-scale surfaces in Fig. 18. The autocorrelations each show a strong peak at zero displacement, with lower background coming from the statistical correlation of random data with themselves. Note the periodicity of the background perpendicular to the track direction and low, randomly varying amplitudes along the tracks. The cross-correlation image shows no increased level at the zero-displacement origin, indicating little interference of the signals between the two layers.

A three-layer CD-ROM with layer separations of 20 and 26 m was measured with the same method as above. The results of the OCT and DD measurements are shown in Fig. 19, where the images are shown in stacked perspective views for a

condition were smaller. Also, the expected quality of the DD signals would degrade more quickly than that of the OCT, as the layer number increases and the data layer reflectivities decrease.

14.7SUMMARY AND CONCLUSIONS

Optical coherence tomography is a potentially valuable tool for increasing the data storage capacity in optical disks. The high detection sensitivity of OCT permits a low reflectivity at each data interface, which is a prerequisite for extending high transmission of the readout beam to many layers, with minimal beam distortion. This feature combined with the increased depth resolution of OCT should allow optical readout of many closely spaced data layers with technology that is an evolution of today’s CD implementations.

Although the discussion here has used disks as an example, it also pertains to other optical media such as tape or cards. Also, the data format can take forms other than the optical phase pit we have discussed. For example, data stored as small regions of material phase change (reflectivity) would be usable as long as transmission through the data layer does not severely distort the beam. Data encoding with more than one bit of information (reflectivity level) per bit region could be accommodated by OCT. Such m-ary storage is another potentially important growth area for optical storage and matches the capabilities of OCT very well. This is because m storage symbols will produce more closely spaced signals than binary levels. The smaller signal separation will therefore be less affected by cross-talk and scattering, which should be reduced in an OCT system.

Even without using the interferometric aspect of OCT, broadband sources with direct detection reduce coherent fluctuations among cross-talk signals from different layers. These fluctuations result from interferometric addition of the fields from nonselected layers, albeit reduced by geometric defocusing. The greater the coherence of the source, the larger the interference effects become. Fluctuations in this cross-talk arise mainly from the random nature of the data patterns affecting reflections. Although these fluctuating parts of the coherently added cross-talk fields can become small, in DD there are always cross-talk power fluctuations from the individual layers. For data-free planar layers, each cross-talk power would be constant, so the cross-talk variation in the low coherence limit is caused by fluctuations from the nonselected data patterns or uncontrolled variation in layer separations on the order of optical wavelengths. Our analysis also showed that reflective interlayer cross-talk is likely to be a more severe problem than beam quality degradation on transmission through many layers.

OCT’s range discrimination allows, in principle, the elimination of all the cross-talk power fluctuations. With a coherence length sufficiently smaller than the layer separation, only the field reflected from the layer matching the reference path will be detected. In addition, the heterodyne behavior of OCT allows greater sensitivity, which ultimately can allow more layers with smaller separations.

Ultimately, the maximum number of layers may be limited by system engineering factors. As we have discussed, focusing at different layers in the medium may cause varying focal spot spherical aberration. For large ranges of focal depth, this may be alleviated with dynamic optical compensation, but at the cost of increased system complexity. OCT offers an advantage for this problem, in that it allows a

given number of layers to be stored in a smaller thickness. This reduced range of focus may eliminate the need for any spherical aberration compensation. In an OCT system, dynamic tracking of the reference mirror as well as the focal depth will be required, as well as the usual lateral signal tracking. Both reference and focus tracking problems will become more difficult with smaller layer separations. These reduced separations will also impose tighter manufacturing tolerances and possibly require the development of new multilayer disk fabrication procedures for mass production. It may also prove advantageous to incorporate formatting information that identifies each disk layer.

We have presumed the existence of multilayer media and have discussed only the detection aspect of the system. In their simplest form, such disks could be replicated in a process similar to stamping conventional CDs, with sequential layers of alternating materials (having slightly different refractive indices) being deposited and stamped by different master disks. The use of multilayer optical writing (or erasing) techniques such as those employed in single-layer phase-change or magnetooptical disks presents a much more difficult problem. The need to selectively alter data at a restricted depth in the medium would probably require a highly nonlinear absorption mechanism that will affect the medium only at the writing beam’s tightest focus or at the intersection of two different writing beams. Using two writing beams with either short pulse or low coherence could also allow better depth discrimination for the power deposition. For the present, we believe that OCT multilayer disk systems will be best suited to storage applications requiring widely distributed, very large amounts of permanently stored (read-only) data.

We have provided experimental demonstrations of CD-ROM imaging using OCT for one-, two-, and three-layer optical disks. In the course of our investigation, we have also found advantages of broadband incoherent sources for direct detection. Future evaluation of multilayer disks should show the superiority of OCT for readout of large numbers of densely spaced optical layers.

For this technology to become widely adapted, there are two aspects that must be commercially developed, the media (multilayer CD disks) and the reading instrument (multilayer CD player). Imation Corporation (3M) has fabricated multilayer CD prototypes whose data we have presented, although the disks were not designed or optimized for our technology. With this demonstration of media technology in hand, we see no barriers to fabrication of optimized disks with more layers. The OCT CD player would share most of its technology with existing players, and the addition of the interferometric components and the replacement of the laser by a superluminescent LED ought to be straightforward. Because there are no apparent technological barriers, the main factor in determining implementation will be market forces: Is there a demand for this type of high density CD storage that can justify the further development costs? Some candidate applications may suggest an answer to this: (1) archival storage of imagery (medical, satellite), (2) archival storage of data (e.g., financial records), and (3) mass market entertainment (multiple movies). The advantage of the multilayer technology would be to reduce the number of CDs required and improve the access time over the stored data. Another critical issue is whether a group or consortium of companies could agree on common standards

One concise pictorial description of this process for the two-layer DVD is given in the technical note, ‘‘Plain Talk: Dual-Layer Compact Disk,’’ 80-9550-2253-0(85.75)ii, #3M Company, June 1995.

for multilayer media and players. Such standardization was done in CD and DVD development, and it should be possible in this case once the commitment is made to the OCT data storage technology.

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