Abstract:
One embodiment of the present invention is an optical delay line that comprises a plurality of optical elements in optical communication with each other, wherein: (a) at least one of the plurality of optical elements is capable of spatially dispersing a spectrum of an optical signal to provide a spatially dispersed optical signal; (b) at least one of the plurality of optical elements is adjustable to affect one or more of a phase delay and a group delay of an optical signal; and (c) at least one of the plurality of optical elements compensates for polarization introduced into the optical signal by others of the optical elements.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention pertains to optical delay lines. In particular, one embodiment of the present invention relates to a grating-based, phase control optical delay line used, for example and without limitation, in Optical Coherence Tomography (“OCT”) and Optical Coherence Domain Reflectometry (“OCDR”). 
     BACKGROUND OF THE INVENTION 
     A low coherence, optical interferometer has been used in various apparatuses to study scattering media. FIG. 4 shows a block diagram of an Optical Coherence Tomography (“OCT”) system. As shown in FIG. 4, OCT system  50  includes an interferometer with reference path  5 , sample path  10 , splitter/combiner  15  (one embodiment of splitter/combiner  15  comprises a 50/50 beamsplitter, or a 3 dB coupler if the interferometer is embodied using optical fibers), low coherence radiation source  20 , detector  25 , and processor  30 . Scanning optical delay line  35  (ODL  35 ) is located at an end of reference path  5  of the interferometer. As further shown in FIG. 4, sample path  10  includes probe module  40  to direct radiation to sample  45 , and to collect radiation scattered from sample  45 . As is still further shown in FIG. 4, detector  25  combines a sample beam reflected from sample  45 , and a reference beam reflected by scanning ODL line  35 . Then, whenever an optical path length mismatch between sample path  10  and reference path  5  is less than a coherence length of low coherence radiation source  20 , interference between the sample beam and the reference beam occurs. As is well known in the art, if the optical path length of the reference beam is known when detector  25  senses the interference signal, the optical path length of the sample beam can be measured within the accuracy of the coherence length of the low coherence radiation source. 
     Several designs of an optical delay line for use in the reference path have been disclosed in the art. As described in an article entitled “Optical Coherence Tomography” by Huang et al. in  Science,  Vol. 254, 1991, pp. 1178-1181, a mirror is used to reflect the reference beam back to the detector. In accordance with this article, depth information from the sample medium is acquired by varying the optical path length of the reference path by moving the mirror using a stepper motor. U.S. Pat. No. 5,321,501 (Swanson et al.) discloses a change to the design of Huang et al. in which the mirror is replaced by a retroreflector to improve optical alignment stability, and the stepper motor is replaced by a galvanometer to increase the scan speed to a degree where tomographical images of living tissue became feasible. 
     U.S. Pat. No. 6,111,645 (Tearney et al.) discloses a change to the design of Swanson et al. in which the moving retroreflector is replaced by a grating-based, phase control, optical delay line. U.S. Pat. No. 6,111,645 is incorporated by reference herein. This design change further increases the scanning speed over that disclosed in Swanson et al., and also enables independent control of the phase and group delay of produced by the reference path. However, the grating-based, phase control, optical delay line disclosed by Tearney et al. has been limited thusfar to use in a laboratory environment. 
     In light of the above, there is a need for an optical delay line that can provide high scanning rates, and that is suitable for use in optical interferometers to provide tomographic images of living tissue. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention advantageously satisfy the above-identified need in the art. In particular, one embodiment of the present invention is an optical delay line (“ODL”) that is suitable for use in optical interferometers to provide tomographic images of living tissue. Specifically, a first embodiment of the present invention is an optical delay line that comprises a plurality of optical elements in optical communication with each other, wherein: (a) at least one of the plurality of optical elements is capable of spatially dispersing a spectrum of an optical signal to provide a spatially dispersed optical signal; (b) at least one of the plurality of optical elements is adjustable to affect one or more of a phase delay and a group delay of an optical signal; and (c) at least one of the plurality of optical elements compensates for polarization introduced into the optical signal by others of the optical elements. In addition, a second embodiment of the present invention is an optical delay line that comprises: (a) a collimator lens system; (b) a grating disposed to receive radiation output from the collimator lens system; (c) a collector lens system disposed to receive at least a portion of radiation diffracted by the grating; (d) a rotatable mirror disposed substantially at a back focal plane of the collector lens system; and (e) a reflector disposed to reflect at least a portion of radiation diffracted by the grating; wherein the collimator lens system, the reflector, and an output end of an optical fiber are affixed in a unit, which unit is movable by a translation mechanism. 
     Another embodiment of the present invention is an optical interferometric imaging system to be used, for example and without limitation, in a clinical setting. In particular, one embodiment of the present invention is an optical interferometric imaging system for imaging a sample that comprises: (a) an optical source capable of producing an optical signal having an optical spectrum; (b) an interferometer in communication with the optical source; (c) a detector in optical communication with the interferometer; and (d) an optical delay line in optical communication with the interferometer that comprises a plurality of optical elements in optical communication with each other, wherein: (i) at least one of the plurality of optical elements is capable of spatially dispersing a spectrum of the optical signal to provide a spatially dispersed optical signal, (ii) at least one of the plurality of optical elements is adjustable to affect one or more of a phase delay and a group delay of the optical signal, and (iii) at least one of the plurality of optical elements compensates for polarization introduced into the optical signal by others of the optical elements. 
    
    
     BRIEF DESCRIPTION OF THE FIGURE 
     FIG. 1 shows, in schematic form, a grating-based, phase control, optical delay line disclosed in U.S. Pat. No. 6,111,645 (G. J. Tearney et al.); 
     FIG. 2 shows, in schematic form, a grating-based, phase control, optical delay line fabricated in accordance with a first embodiment of the present invention; 
     FIG. 3 shows, in schematic form, a grating-based, phase control, optical delay line fabricated in accordance with a second embodiment of the present invention; and 
     FIG. 4 shows a block diagram of an optical coherence tomography (“OCT”) system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows, in schematic form, grating-based, phase control, optical delay line  90  that is one embodiment of an optical delay line (“ODL”) disclosed in U.S. Pat. No. 6,111,645 (the &#39;645 patent); see FIG. 13 of the &#39;645 patent. As shown in FIG. 1, radiation output from single mode optical fiber  100  is collimated by lens system  110  (although lens system  110  is shown as a single lens, those of ordinary skill in the art readily appreciate that lens system  110  may comprise a multiplicity of lenses), and is directed to impinge upon grating  120  (grating  120  has a predetermined grating groove density) at a grating input angle (for example, an angle Θ with respect to a normal to grating  120 ). Grating  120  disperses the beam of radiation that impinges thereon into different spectral frequency or wavelength components that are collected, and focused by lens system  130  (although lens system  130  is shown as a single lens, those of ordinary skill in the art readily appreciate that lens system  130  may comprise a multiplicity of lenses) onto mirror  140 . The optical axis of lens system  130  is disposed at a grating output angle (for example, an angle Θ with respect to a normal to grating  120 ), and mirror  140  is disposed substantially at the back focal plane of lens  130 . 
     As further shown in FIG. 1, mirror  140  directs reflected radiation back through lens system  130  to grating  120 , and from there, to double-pass mirror  150 . The radiation output from grating  120  is substantially orthogonal to mirror  150 , and as a result, it is reflected all the way back through ODL  90  into single mode optical fiber  100 . 
     As described in the &#39;645 patent, the Fourier transform of the spectrally dispersed beam of radiation is present at mirror  140 . When the surface of mirror  140  is disposed at an angle (for example, an angle γ with respect to a normal to the optical axis of lens  130 ), a phase ramp, or linear phase shift, is applied to the spectrum across mirror  140 . Then, an inverse Fourier transform of the spectrum is achieved as the beam propagates back through lens system  130  towards grating  120 . Since the inverse Fourier transform of a linear phase shift is a time delay, a temporal group delay of the beam is produced. 
     As further disclosed in the &#39;645 patent, alteration of one or more of: (a) the grating groove density, the grating input angleΘ i , and the grating output angle Θ d  of grating  120 ; or (b) the angle at which mirror  140  is disposed (see above) produces a change in the optical group delay and phase delay, see FIG. 14 of the &#39;645 patent. 
     Thus, if the angle at which mirror  140  is disposed, as shown in FIG. 1, is rapidly scanned, a time dependent, optical group delay line is produced. Then, for example, ODL  90  can be inserted into the reference path of a low coherence optical interferometer (such as that disclosed in the &#39;645 patent and described in the Background of the Invention in conjunction with FIG. 4) to perform high speed optical coherence tomography (“OCT”). Since the phase delay and group delay are adjustable, the modulation frequency which is produced in interferometric imaging techniques can be controlled, thereby simplifying detection electronics. This is especially important for detection scenarios which involve direct electronic digitization (A/D conversion) of a detected optical interference signal. 
     Referring to FIG. 1, mirror  140  is rotated along a direction indicated by arrows  160  and  170 . Mirror  140  is rotated by a rotation mechanism (not shown). The rotation mechanism can be fabricated in accordance with any one of a number of rotation mechanisms that are well known to those of ordinary skill in the art such as, for example and without limitation, a galvanometer rotation mechanism (for example, the mirror is placed on a shaft of a galvanometer), a resonant scanner, a rotating polygon mirror (see FIG. 8 of the &#39;645 patent), and a piezoelectric mirror scanner. 
     In addition to the above (where the angle of mirror  140  produces a change in group delay), by offsetting the center of rotation of mirror  140  with respect to the center wavelength of the spectrum of the beam of radiation, the phase control device can be used to adjust the phase delay and the group delay independently. Moreover, if the center wavelength of the spectrum of the beam of radiation output from grating  120  and lens system  130  is incident on the center of rotation of mirror  140 , group delay is produced without changing the phase delay. Then, in an OCT, no modulation frequency is applied to the local oscillator, even though a scanning linear group delay is produced. Thus, the interferometric signal consists of the envelope of the autocorrelation function without any modulation. This can be useful for OCT imaging systems that perform homodyne detection. If an independent phase modulation is applied to the local oscillator, the system would be capable of scanning at different speeds without changing the center frequency of the band pass filter before demodulation. A phase diversity homodyne detection system would be useful for OCT in this instance. 
     Furthermore, by translating mirror  140  so that the center wavelength is offset from the axis of rotation, an arbitrary modulation frequency can be applied to the local oscillator. This enables complete control over the center frequency of the local oscillator. The modulation frequency (i.e., phase delay) may be varied by simply translating mirror  140  perpendicular to the optical axis of the beam. The range of center modulation frequencies that may be achieved is only limited by spectral vignetting due to the finite size of mirror  140 . 
     In accordance with the above-described approximation, the group-phase delay independence of the ODL is an advantage whenever mirror  140  has an axis of rotation that intersects the mirror surface. When the mirror surface is separated from the axis of rotation, however, the group-delay and phase-delay properties are more complex. To an approximation, the group-delay is linear in angle but not in phase delay. 
     For example, while the change in group delay produced by a polygon scanning mirror is linear, the change in phase is quadratic as a function of time. Since the modulation frequency shifts linearly over the scan, a polygon scanning mirror cannot be used in conjunction with a demodulation method that incorporates a fixed band pass filter. The varying modulation frequency can be overcome, however, by using an alternative demodulation scheme, such as adaptive frequency mixing detection, where the frequency at which the demodulation is performed is varied to track the variation in the modulation frequency. Alternative phase control configurations, such as scanning the grating angle of incidence or the grating ruling density, also produce a nonlinear phase delay. As a result, these methods can only be used in conjunction with an adaptive frequency mixing demodulation scheme for OCT imaging applications. 
     In addition to enabling high speed group delay scanning, another advantage of the above described ODL for use in conjunction with OCT is the capability to compensate dispersion mismatch between the reference path and the sample path. This occurs as follows: whenever the distance L between lens system  130  and grating  120  is not one focal length, an additional wavelength dependent phase delay is added to a pulse, creating positive dispersion for L&lt;focal length and negative dispersion for L&gt;focal length. This property of the ODL enables compensation of a dispersion imbalance between the reference path and sample path in the OCT system by simply changing the lens-grating separation. 
     Despite the advantages discussed above, we have uncovered several problems with ODL  90  disclosed in U.S. Pat. No. 6,111,645. A first problem with ODL  90  arises because radiation is reflected four (4) times from grating  120 . This problem arises because gratings typically have different reflectivities for S and P polarization states. For example, for a case where an S polarization state has a reflectivity of 67%, and where a P polarization state has a reflectivity of 47%, the relative optical efficiency of the P and S states, i.e., (P/S), for ODL  90  would be (0.47/0.67) 4 =0.24. 
     This raises two issues. The first issue arises from the fact that a source of radiation may be coupled to ODL  90  through fiber optics. This is an issue because stress in fiber optics causes them to act as waveplates that modify the polarization state of radiation entering ODL  90 . As a result, the optical efficiency of ODL  90  will change. For example, since stress in an optical fiber typically varies with temperature or movement of the optical fiber, power returning from ODL  90  will vary. Such a variation will make optimization of a detector system included in an optical interferometer in which ODL  90  is used difficult. The second issue arises because optimal detection of radiation reflected from a sample path in an optical interferometer requires that a polarization state in a reference path and a sample path be matched. For example, if there is no matching, a loss of P polarized radiation from the reference path will decrease detection of P polarized radiation from the sample path. Thus, if equal amounts of P and S polarized radiation are output from a 3 dB coupler (see the Background of the Invention), this effectively drops the sample path detection efficiency of the optical interferometer by 50%. 
     A second problem with ODL  90  arises because the optical path length of the reference path needs to be varied to match it to the optical path length of the sample path. This can be accomplished (referring to FIG. 1) by moving either collimator lens system  110  or mirror  150  in an axial direction. However, this is not practical because the slightest misalignment of either of these components causes significant transmission loss in ODL  90 . 
     To eliminate the above-identified polarization dependent effects of ODL  90  disclosed in U.S. Pat. No. 6,111,645, and in accordance with one aspect of the present invention, a polarization dependent optical component is added to ODL  90  that substantially eliminates the polarization dependent effects of grating  120 . Advantageously, this will enable a reference path (for example, reference path  5  in the low coherence optical interferometer shown in FIG. 4) to have approximately equal optical efficiency for S and P polarized states. One embodiment of such an optical component is an optical window that is placed in a reference path beam, at angle, to produce polarization dependent transmission. 
     FIG. 2 shows, in schematic form, grating-based, phase control, optical delay line  190  (“ODL  190 ”) that is the same as ODL  90  shown in FIG. 1 with the addition of window  125 . Window  125  may be fabricated from any transparent material such as a piece of glass, that can alter polarization. Further, as one of ordinary skill in the art can readily appreciate, window  125  may be disposed at various angles with respect to the beam of radiation passing therethrough to vary the polarization alteration effect. For example, the polarization transmission efficiency may vary as a function of angle differently for the S and P polarization states. Still further, in a preferred embodiment, front and back surfaces of window  125  may be slightly off from parallel to ensure that multiple reflections therefrom do not couple back into ODL  190 . 
     Although FIG. 2 shows the use of a single piece of material to embody window  125 , it is within the scope of the present invention that multiple windows may be used, for example, in series, to achieve sufficient discrimination between the two polarization states to be effective in substantially eliminating the polarization effect caused by grating  120 . In a preferred embodiment, the angle of all windows used to provide window  125  may be disposed substantially at Brewster&#39;s angle to minimize loss in the P polarization state. We have demonstrated elimination of polarization dependent effects of ODL  90  by placing a microscope slide at near Brewster&#39;s angle such that the beam of radiation passes through it four (4) times (intersecting both the beam going to rotatable scanning mirror  140  and the offset beam returning therefrom). 
     As is well known, it is often necessary to attenuate power in the reference path of a low coherence optical interferometer to optimize sensitivity of the OCT system. In accordance with this aspect of the present invention, attenuation of the beam of radiation can be achieved simultaneously with polarization compensation if window  125  includes a neutral density optical attenuator, such neutral density optical attenuators are available from a wide variety of commercial sources. 
     Although the above-described embodiment of the present invention indicates that window  125  is placed so that a beam of radiation in ODL  190  passes therethrough four (4) times, the present invention is not so limited. In fact, embodiments of the present invention include embodiments where window  125  is distributed throughout beam paths of ODL  190  so that: (a) in some embodiments, a beam passes through window  125  two (2) times; (b) in some embodiments, a beam passes through one portion of a distributed embodiment of window  125  four (4) times, and another portion of a distributed embodiments of window  125  two (2) times; and (c) so forth. 
     As was described above, another problem with ODL  90  is a difficulty in maintaining alignment while adjusting the optical path length to compensate for differences in optical path length between a reference path and a sample path of a low coherence optical interferometer. This problem is solved in accordance with a second aspect of the present invention. FIG. 3 shows, in schematic form, grating-based, phase control, ODL  290  that is fabricated in accordance with the second aspect of the present invention. As shown in FIG. 3, ODL  290  is the same as ODL  190  shown in FIG. 2 with the addition that the tip of fiber  100 , collimator lens system  110 , and mirror  150  are mounted in fixture  185  so that they are moved as a unit in the axial direction to adjust path length. Fixture  185  can be moved by a translation mechanism, which translation mechanism can be fabricated using any translation mechanism (not shown) that is well known to those of ordinary skill in the art such as a stepper motor and a screw, a linear motor, and the like. This arrangement solves the alignment problem because of the following. Grating  120 , lens system  130 , and mirror  140 , as a group, act as a retro-reflector. Thus, rays in the beam of radiation that leave collimator lens system  110  and hit mirror  150  are always parallel in a well aligned ODL  290 . Hence, if mirror  150  is kept orthogonal to the beam of radiation leaving collimator lens  110 , alignment of ODL  290  will be relatively insensitive to changes in the position or angle of this beam. 
     The inventive optical delay line can be incorporated into Low Coherence Interferometry (LCI), Optical Coherence Tomography (OCT), or other interferometric based optical ranging and imaging techniques. For example, the inventive optical delay line is used in OCT systems to enable high speed reference path optical path length scanning using heterodyne or homodyne detection. Such high speed OCT imaging can be used in a variety of applications (e.g., in vivo medical imaging in human tissue). Medical applications of the inventive optical delay line in OCT imaging include but are not limited to in vivo medical diagnostic imaging of the vascular system; gastrointestinal tract; urinary tract; respiratory tract; nervous system; embryonic tissue; OB/GYN tissue; and any other internal human organ systems. Other medical applications include a rapid scanning OCT system for performing guiding surgical intervention. The inventive optical delay line may be also used in OCT imaging for non-medical applications including imaging in biological specimens, materials, composite materials, semiconductors, semiconductor devices and packages, and other applications requiring high speed imaging. 
     Those skilled in the art will recognize that the foregoing description has been presented for the sake of illustration and description only. As such, it is not intended to be exhaustive or to limit the invention to the precise form disclosed. For example, embodiments of the present invention also include all of the grating-based, single or double-pass geometry apparatus disclosed in the &#39;645 patent such as, without limitation: (a) the grating-based, phase control, optical delay line shown in a single pass configuration and a double-pass configuration, respectively, in FIGS. 1A and 1B of the &#39;645 patent; (b) the grating-based, phase control, optical delay line using an acousto-optic modulator (“AOM”) and a reflection grating to scan an input beam shown in FIG. 2 of the &#39;645 patent (in FIG. 2, the AOM scans an incident angle Θ, and in another embodiment, a telescope is used between the AOM and the grating); (c) the grating-based, phase control, optical delay line using an acousto-optic modulator and a diffraction grating to scan an input beam shown in FIG. 3 of the &#39;645 patent; (d) the grating-based, phase control, optical delay line using a scanning mirror to change the grating input angle shown FIG. 4 of the &#39;645 patent (as described in the &#39;645 patent, a telescope is disposed between the scanning mirror and the grating to prevent beam walk-off at the grating); (e) the grating-based, phase control, optical delay line using a steerable grating shown in FIG. 5 of the &#39;645 patent; (f) the grating-based, phase control, optical delay line using a radially scanned, circular, holographic, optical element shown in FIG. 6 of the &#39;645 patent; (g) the mirror configuration for pulse shaping shown in FIG. 11 of the &#39;645 patent; (h) the grating-based, phase control, optical delay line in a double-pass configuration shown in FIG. 13 of the &#39;645 patent; (i) the grating-based, phase control, optical delay line with elements that can be modified to change the scanning group delay shown in FIG. 14 of the &#39;645 patent; (j) the grating-based, phase control, optical delay line using a scanning grating shown in FIG. 16 of the &#39;645 patent; (k) the scanning, optical delay line apparatus using a rotating, circular, holographic, optical element to produce a scanning group delay shown in FIG. 17 of the &#39;645 patent; (l) the scanning optical delay line apparatus using an acousto-optic modulator and a diffraction grating shown in FIG. 18 of the &#39;645 patent; (m) the grating-based, phase control, optical delay line using a scanning mirror with its axis of rotation offset from the optical axis shown in FIG. 20 of the &#39;645 patent; and (n) the grating-based, phase control, optical delay line using a polygon mirror with its axis of rotation offset from the optical axis shown in FIG. 21 of the &#39;645 patent.