Optical coherence tomography technique

Abstract

An optical coherence tomography device comprises a light generator, a dispersive medium, an optical coupler and a detector. The light generator is adapted to generate a series of input pulses of coherent light, each input pulse having an input pulse width. The dispersive medium has an input that is optically coupled to the light generator and an output for output pulses. The dispersive medium is adapted to stretch the input pulse width to an output pulse width of each of the output pulses by chromatic dispersion. The optical coupler is adapted to couple the output pulses into a reference arm and a sample arm. The optical coupler is further adapted to superimpose light returning from the reference arm and the sample arm. The detector is adapted to detect an intensity of interference of the superimposed light with a temporal resolution of a fraction of the output pulse width.

Claims

1. A device for optical coherence tomography, or OCT, the device comprising: a light generator adapted to: generate a series of input pulses of coherent light, each input pulse of the series having an input pulse; width and an input spectral range; and change an axial resolution of the device independently of an axial scanning depth of the device by changing the input spectral range; a single dispersive medium having an input optically coupled to the light generator and an output for output pulses, the dispersive medium being adapted to stretch the input pulse width to an output pulse width of each of the output pulses by chromatic dispersion, the dispersive medium comprising a plurality of taps at different positions along the length of the dispersive medium to provide different dispersion; an optical coupler adapted to couple the output pulses from the output into a reference arm and into a sample arm, and to superimpose light returning from the reference arm and from the sample arm; an optical switch adapted to change the axial scanning depth independently of the axial resolution by selectively coupling the light generator to one of the taps as the input or by selectively coupling one of the taps as the output to the optical coupler; and a detector adapted to detect an intensity of interference of the superimposed light with a temporal resolution of a fraction of the output pulse width.

2. The device of claim 1, wherein each input pulse in the series has an at least essentially time-independent input center wavelength.

3. The device of claim 1, wherein each output pulse has at least one of a time-dependent instantaneous output peak wavelength and a time-dependent instantaneous output spectral range.

4. The device of claim 3, wherein the input spectral range is multiple times broader than the instantaneous output spectral range.

5. The device of claim 1, wherein the dispersive medium includes an optical fiber.

6. The device of claim 1, wherein a path of light propagation in the dispersive medium from the input to the output is longer than 1 km.

7. The device of claim 1, wherein a dispersion parameter of the dispersive medium is greater than 10000 ps/(km.Math.nm).

8. The device of claim 7, further comprising a field generator adapted to generate an external field acting on the medium, wherein the dispersion parameter of the medium is controlled or controllable by the external field.

9. The device of claim 1, wherein the detector is further adapted to sample the intensity for a plurality of consecutive fractions corresponding to one output pulse width.

10. The device of claim 9, wherein the plurality of sampled fractions is at least 500.

11. The device of claim 1, wherein the fraction is shorter than 200 ns.

12. The device of claim 1, wherein the detector includes at least one of a photodiode and a balanced detector.

13. The device of claim 12, wherein the detector further includes a gate unit connected to the photodiode and adapted to read the intensity for each of the fractions.

14. The device of claim 1, wherein the light generator includes a pulsed titanium-sapphire laser or a pulsed supercontinuum light source.

15. The device of claim 9, wherein the light generator generates the series of input pulses at a repetition rate and the detector initiates the sampling at the repetition rate.

16. The device of claim 1, wherein the light generator includes a continuous wave light source and a shutter operatively arranged between the continuous wave light source and the input of the dispersive medium.

17. The device of claim 1, wherein the optical coupler includes at least one of a beam splitter, an optical fiber coupler, a circulator and a 1-by-2-coupler.

18. A method of performing optical coherence tomography, or OCT, the method comprising: generating by a light generator a series of input pulses of coherent light, each input pulse of the series having an input pulse width and an input spectral range; stretching the input pulse width of each of the input pulses to an output pulse width of output pulses by means of chromatic dispersion in a single dispersive medium, the dispersive medium comprising a plurality of taps at different positions along the length of the dispersive medium to provide different dispersion; changing an axial resolution for the OCT independently of an axial scanning depth for the OCT by changing the input spectral range; changing the axial scanning depth independently of the axial resolution by selectively coupling the light generator to one of the taps as input or by selectively coupling one of the taps as output to an optical coupler; coupling by the optical coupler the output pulses into a reference arm and into a sample arm, and superimposing light returning from the reference arm and from the sample arm; and detecting an intensity of interference of the superimposed light with a temporal resolution of a fraction of the output pulse width.

19. The method of claim 18, further comprising: generating an external field acting on the medium, wherein the dispersion parameter of the medium is controlled or controllable by the external field.

20. The method of claim 18, further comprising: sampling the intensity for a plurality of consecutive fractions corresponding to one output pulse width.

21. The method of claim 20, further comprising: generating a series of input pulses at a repetition rate; and initiating the sampling at the repetition rate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, advantages and technical effects of the disclosure will become apparent in below detailed description of exemplary embodiments with reference to the accompanying drawings, in which:

(2) FIG. 1 schematically illustrates a first embodiment of a device for optical coherence tomography including a dispersive medium;

(3) FIG. 2 schematically illustrates a second embodiment of a device for optical coherence tomography including a dispersive medium;

(4) FIG. 3 schematically illustrates a third embodiment of a device for optical coherence tomography including a dispersive medium;

(5) FIG. 4 shows a schematic diagram of a first input power distribution at an input of the dispersive medium of FIG. 1, 2 or 3;

(6) FIG. 5 shows a schematic diagram of a first output power distribution at an output of the dispersive medium of FIG. 1, 2 or 3 as a result of the first input power distribution;

(7) FIG. 6 shows a schematic diagram of a second input power distribution at an output of the dispersive medium of FIG. 1, 2 or 3;

(8) FIG. 7 shows a schematic diagram of a second output power distribution at an output of the dispersive medium of FIG. 1, 2 or 3 as a result of the second input power distribution; and

(9) FIG. 8 shows a measurement diagram of a dispersion parameter of the dispersive medium of FIG. 1, 2 or 3.

DETAILED DESCRIPTION

(10) Modern Optical Coherence Tomography (OCT) is dominated by so-called Fourier Domain OCT (FD-OCT), which achieves a better Signal-to-Noise Ratio (SNR) compared to classical Time Domain OCT (TD-OCT). Furthermore, by avoiding a mechanical z-scan (i.e., a reference arm which optical length is mechanically changed), significantly higher scanning rates, e.g., f.sub.R>100 kHz, are realizable. The FD-OCT is implemented as Spectral Domain OCT (SD-OCT) or Swept Source OCT (SS-OCT).

(11) A modern SD-OCT system is described in the article Extended in vivo interior eye-segment imaging with full-range complex spectral domain optical coherence tomography by J. Jungwirth et al. in the Journal of Biomedical Optics, page set 050501, Vol. 14 (5), 2009. An intrinsic limitation of a scanning depth and a way of doubling the limited scanning depth by means of a so-called full-range complex (FRC) technique is discussed.

(12) An SS-OCT system is described with reference to FIG. 2 in the article High-speed optical frequency-domain imaging by S. H. Yun et al. in Optics Express, Vol. 11, No. 22, pages 2953-2963. Aside the scanning rate, a bandwidth emitted by a light source of an OCT device and a instantaneous linewidth of the emission bandwidth significantly influence a performance of the OCT device, including an axial resolution z.sub.min and an axial scanning depth z.sub.max, as detailed below.

(13) The axial resolution z.sub.min (in both SD-OCT and SS-OCT) is determined by the bandwidth of the light source according to:

(14) $z_{\min} = \frac{2 \ln 2}{} \frac{_{0}^{2}}{n},$
wherein n denotes an index of refraction of a sample, such as tissue. For example, n=1.37 for a cornea of an eye. The symbol .sub.0 denotes a center wavelength in the bandwidth defined by the Full Width at Half Maximum (FWHM) bandwidth of the light source.

(15) A limitation of the axial scanning depth z.sub.max is set, in SD-OCT and SS-OCT, by the spectral resolution of detection or the instantaneous linewidth of the swept laser, respectively, according to:

(16) $z_{\max} = \frac{\ln 2}{n} \frac{_{0}^{2}}{},$
wherein .sub.0 and n denote the center wavelength and the index of refraction, respectively. In the case of SD-OCT using a spectrometer with a linear detector array, the spectral resolution is limited by a pixel size of the linear detector array, on which a transversely decomposed spectrum of light is radiated.

(17) Physical principles underlying FD-OCT thus relate limits of the scanning depth z.sub.max and the axial resolution z.sub.min according to:

(18) $z_{\max} = \frac{}{2 .Math.} z_{\min} .$

(19) Consequently, a requirement for an SD-OCT system may aim at detecting the full spectral width (i.e., aiming at a large ) and, at the same time, may aim at a high spectral resolution (i.e., at a small ). However, the spectral resolution and the bandwidth covered by the detector are not independent but related by a number of pixels in the linear detector array. If the spectral resolution is improved by increasing the transversal spread of the spectral decomposition that is radiated on the linear detector array, the bandwidth covered by the linear detector array may be reduced. Manufacturable pixel size and the number of pixels in the linear detector array of spectrometers, as well as apertures of achromatic lenses and diffraction gratings, determine technical limitations to independently improving the axial resolution z.sub.min and the scanning depth z.sub.max.

(20) The technical limitations thus limit the performance of SD-OCT systems and may exclude certain applications. By way of example, modern SD-OCT systems may achieve a rather high axial resolution z.sub.min, but with a relatively low scanning depth z.sub.max, as illustrated by below numerical example.

(21) Using a Ti-sapphire laser (TiS laser) as the light source with central wavelength .sub.0=800 nm and a rather advanced bandwidth =200 nm in conjunction with an advanced pixel number of N=4096 pixels, a good axial resolution z.sub.min=1.0 m and a scanning depth z.sub.max=2.1 mm in tissue with n=1.37 is possible. In this example, the limited number of pixels entails a limited spectral resolution
=/N=200 nm/4096 px=0.05 nm/px,
which in turn limits the scanning depth z.sub.max. The high axial resolution thus obviates a large scanning depth, and vice versa.

(22) In the case of SS-OCT, a laser light source is rapidly tuned through the entire emittable bandwidth , resulting in an instantaneous linewidth with which the laser light source oscillates at a time t of the tuning. The instantaneous bandwidth is, however, limited by a quality factor (or Q factor) of a cavity of the laser light source and by a settling time of the oscillation. Moreover, the bandwidth of tunable light sources required for SS-OCT is typically below 120 nm, which is why SS-OCT often achieves lower axial resolutions z.sub.min at a slightly wider scanning depth z.sub.max compared to SD-OCT. Furthermore, currently no laser light sources for SS-OCT with sufficient tunable bandwidth are available in a spectral range covering .sub.0=800 nm, which significantly reduces the axial resolution, since z.sub.min is proportional to the square of .sub.0. Using very advanced swept laser light sources, a SS-OCT system may achieve the performance of below numerical example.

(23) Assuming the swept laser light sources has a center wavelength .sub.0=1060 nm, a total bandwidth =120 nm and provides an instantaneous line width =0.06 nm, results in an axial resolution z.sub.min=3.0 m (for cornea tissue) or z.sub.min=4.1 m (in air) and a scanning depth z.sub.max=3.0 mm (for cornea tissue) or z.sub.max=4.1 mm (in air).

(24) As has become apparent from above principle considerations for FD-OCT and corresponding numerical examples, a high axial resolution z.sub.min and a wide scanning depth z.sub.max cannot be simultaneously realized.

(25) FIG. 1 schematically illustrates an arrangement of optical components and their mutual coupling according to a first embodiment of a device 100a for optical coherence tomography. The device 100a comprises a light generator 110, a dispersive medium 120 having an input 122 and an output 124, an optical coupler 130, and a detector 140. The light generator 110 is optically coupled to the input 122. The light generator 110 and the dispersive medium 120 form a light source 150 of the device 100a.

(26) The optical coupler 130 receives the light that is output by the light source 150 and equally splits the light by means of a semi-transparent mirror 132 into a reference arm 160 terminated by a mirror 162 and into a sample arm 170. Light propagating towards the coupler 130 in the reference arm 160 and in the sample arm 170 is superimposed by the coupler 130 in a detection arm 180.

(27) The detector 140 comprises a photodiode 142, a gate unit 144, a buffer 146, a data acquisition unit 143, a processing unit 145, a storage unit 147 and a display 148. The photodiode 142 is arranged on the detection arm 180. The photodiode 142 has a response time below 50 ps, preferably on the order of 35 ps or 40 ps, or therebetween. A temporal resolution may also depend on dead time of the photodiode of approximately 100 ps, which can be reduced or avoided by consecutively using more than one photodiode 142 or more than one detector 140. The gate unit 144 is electrically connected to the photodiode 142. The gate unit 144 sequences an intensity signal of the photodiode 142 in temporal fractions t.sub.gate. The buffer 146 temporarily stores samples of the sequenced intensity signal. Each sample represents one fraction in the sequence of fractions. The buffer 146 stores the intensity of each fraction in association with a consecutive number of the fraction, a detection time, or a time-dependent wavelength (which is detailed with reference to FIGS. 5 and 7). The data acquisition unit 143 is an interface via which the processing unit 145 retrieves the data of those fractions corresponding to one of the output pulses 302, 304, 306.

(28) The processing unit is adapted to read the intensity samples of one sequence from the buffer 146 via the data acquisition unit 143 and to perform a Fourier transformation thereof. A result of the Fourier transform is permanently stored in the storage unit 147 and/or displayed to a user at the display 148.

(29) The sample arm 170 comprises an xy-scanner 172 and a scanner lens 174. The xy-scanner 172 includes a pair of pivotable mirrors 176 and 178 that deflect the light in the sample arm 170 (propagating either from the coupler 130 to the lens 174 or in the other direction) in a first transversal direction and a second transversal direction perpendicular to the first transversal direction, respectively. The scanning lens 174 forms an approximately Gaussian beam spot, which beam waist is focused inside a sample 190, such as the cornea or retina of an eye.

(30) FIG. 2 schematically illustrates a second embodiment of a device 100b for optical coherence tomography. Corresponding reference signs relate to components and features as described in the context of the embodiment 100a. The device 100b differs in that the optical coupler 130 includes a 2-by-2 fused-fiber coupler. A first interface pair of fused fibers is optically coupled to the output 124 of the dispersive medium 120 and to the detector 148, respectively. A second interface pair of the fused fibers is optically coupled to the xy-scanner 172 and the reference arm 160, respectively.

(31) FIG. 3 schematically illustrates a third embodiment of a device 100c for optical coherence tomography. The device 100c includes components and features denoted by corresponding reference signs as described above with reference to the FIG. 1 or 2. The device 100c differs in that the optical coupler 130 includes a circulator 134 and a 1-by-2 coupler 136. The circulator 134 has three ports and is adapted to transmit power entering any port to a next port in a circulation direction indicated by an arrow 138. A first port of the circulator 134 is optically coupled to the output 124 of the dispersive medium 120. A second port (that follows the first port in the direction of circulation) of the circulator 134 is optically coupled to the single port of the 1-by-2 coupler 136. A third port (that follows the second port in the direction of circulation) of the circulator 134 defines the detection arm 180. The 1-by-2 coupler 136 outputs light from the second port of the circulator 134 into both the reference arm 160 and the sample arm 170. Light returning from the reference arm 160 and/or the sample arm 170 is combined into the single port of the 1-by-2 coupler and thus enters the second port of the circulator 134. One or both of the reference arm 160 and the sample arm 170 optionally includes a polarization controller.

(32) The light generator 110 is a broadband TiS laser or a pulsed supercontinuum source (SC source). The light generator 110 generates pulses with a center wavelength .sub.0=800 nm, 1050 nm or 1300 nm at a repetition rate f.sub.R=1/T.sub.R of the pulses. A spectral-temporal power distribution S.sub.IN for (a short portion of) a series of pulses 202, 204 and 206 at times T.sub.1, T.sub.2, and T.sub.3 is schematically illustrated in a diagram 200a of FIG. 4. Time is shown on the horizontal axis and wavelength on the vertical axis of the diagram 200a. The spectral-temporal power distribution is schematically illustrated as a spectral-temporal density that is a function of both time and wavelength. FIG. 6 schematically illustrates in a diagram 200b a variant of the spectral-temporal power distribution S.sub.IN at the input 122. Extreme wavelengths in the wide spectral range are provided by the light generator 110 over essentially the full pulse width .sub.0. Closed lines schematically indicate lines of equal power density.

(33) In a variant of each of the devices 100a, 100b and 100c, the light generator 110 includes a broadband Continuous Wave (CW) light source, such as a super luminescent diode (SLD) or an Amplified Spontaneous Emission (ASE). The CW light source has a high intensity or luminescence. The CW light source provides a broadband spectrum corresponding to the input spectral range . The CW light source is optically chopped by means of a fast shutter. The shutter operates at a frequency of approximately f.sub.R=1 MHz. The chopped light is input to the dispersive medium 120. The diagram 200b in FIG. 6 may schematically illustrate (e.g., more realistically than the diagram 200a in FIG. 4) the spectral-temporal power distribution S.sub.IN provided by the shutter.

(34) Each pulse 202, 204 or 206 in the series of pulses is essentially identical as to its distribution of power in time and frequency or wavelength. The TIS laser pulses have an input spectral range on the order of 200 nm. An exemplary TIS laser is described in the article Compact, low-cost Ti:Al.sub.2O.sub.3 laser for in vivo ultra high-resolution optical coherence tomography by A. Unterhuber et al., Optics Letters, Vol. 28, No. 11, p. 905-907, 2003. An input pulse width .sub.0 is a pulse duration defined as the time of power above a 1/e.sup.2-level with respect to a power peak. (An alternative definition uses a 3 dB level, i.e. the FWHM in time.) The input pulse width .sub.0 is in the range of 10 fs to 10 ns, preferably 1 ps to 1 ns or 2 ns. The input spectral range is defined as the FWHM bandwidth, i.e. at a 3 dB level of the spectrum. An alternative definition may use a level of 10 dB (i.e. the spectral range is defined at a 10%-level) for complex spectra, or rarely at a 1/e.sup.2-level. The input spectral range defines an effective swept spectrum by means of the dispersive medium 120, as described below with reference to FIGS. 5 and 7.

(35) The broadband input pulse 202, 204 or 206 is stretched in time as the pulse passes through the highly dispersive medium 120. In the embodiments 100a, 100b and 100c shown in the FIGS. 1, 2 and 3, respectively, the dispersive medium 120 is an optical fiber. The input pulse is subject to a linear dispersion of group velocity. The alternative of a non-linear group velocity dispersion is discussed below with reference to FIG. 8. As a result, spectral components of the input pulse are differently delayed or temporally dispersed with respect to each other. The delay is a function of the wavelength such that, in the case of positive dispersion, long wavelengths propagate faster in the dispersive medium 120 resulting in output pulses 302, 304, and 306, which spectral-temporal power distribution S.sub.OUT is schematically illustrated by diagram 300a in FIG. 5. A spectral-temporal power distribution S.sub.OUT at the output 124 of the dispersive medium 120 resulting from the input pulses according to the diagram 200b of FIG. 6 is schematically illustrated by diagram 300b in FIG. 7. An original spectrum 308 of the input pulses 202, 204 and 206 is essentially unchanged when averaged over a time scale longer than an output pulse with .sub.p of each of the output pulses 302, 304 and 306. More specifically, non-linear effects such as a parametric gain, a second-harmonic generation, a dispersion of arbitrary orders, a self-phase modulation and a four-wave mixing are absent or negligible in the dispersive medium 120. The dispersive medium 120 is a linear medium. Specifically, the center wavelength .sub.0 is conserved.

(36) On the significantly shorter time scale of the fractions t.sub.gate temporarily resolved by detector 140, the spectral separation of different wavelengths gives rise to an up-chirped output pulse, which wavelength (t) is a function of time, as is indicated in each of the diagrams 300a and 300b for the times t.sub.1 and t.sub.2. An instantaneous output spectrum 310, exemplarily shown for the instant t.sub.1 in FIGS. 5 and 7, is a narrow line with an instantaneous spectral range out of the full input spectral range .

(37) The dispersion of the medium 120 is a chromatic dispersion or a group-velocity dispersion (GVD). The dispersion is (at least partially) described by a dispersion parameter D. Specifically designed fibers are available with large dispersion parameters for almost any given spectral range of interest, particularly for 600 nm to 1000 nm. Large Mode Area fibers (LMA fibers) are Photonic Chrystal Fibers (PCF) provide a GVD of the different spectral components with the dispersion parameter |D|>500 ps/(nm.Math.km).

(38) FIG. 8 shows a diagram 400 of the dispersion for a polarization maintaining Large Mode Area UV fiber. The diagram 400 is a measurement result of Prof. P. Hartmann, Westschsische Hochschule. The LMA UV fiber has a diameter of 125 m, a first Mode Field Diameter MFDx=2.6 m and a second Mode Field Diameter MFDy=4.3 m. A delay per length, /L, is shown with reference sign 402 and the dispersion parameter D is shown with reference sign 404. Corresponding results on the dispersion parameter of a PCF are provided in the journal Optics Express, Vol. 12, No. 2, 2004 on p. 301 in FIG. 1(a). Furthermore, an advanced variant of the each of embodiments of the devices 100a, 100b and 100c uses a so-called concentric-core fiber with a dispersion parameter D on the order of 13200 ps/(nm.Math.km), as reported in the journal Laser Focus World, July 2011, p. 9.

(39) The input pulse width .sub.0 of the input pulse 202, 204 or 206 is strongly stretched, i.e. prolonged in time, to the output pulse width .sub.p of the output pulse 302. The input pulse width .sub.0 is on the order of 1 fs to 1 ns. The output pulse width .sub.p is on the order of 100 ns to 10 s. The dispersion of the medium 120 relates the output pulse width .sub.p to the input pulse with .sub.0 according to:
.sub.p=.sub.0+|D|.Math.L.Math.,
with the spectral dispersion parameter D of the dispersive medium 120 (in ps/(nm.Math.km)), a length L of a path of light propagation (in km, e.g. the fiber length), and the input spectral range (in nm). Preferred lengths L include 1 km, 10 km or any length L in between, as detailed in below numerical example. A power of the light generator 110 is chosen such that the (peak) power of the output pulses 302, 304, 306 (e.g., at the output 124) is at least 1 mW, 5 mW, 10 mW, 20 mW, 50 mW or a power between 5 to 50 mW. The power may take into account an attenuation in the dispersive medium 120.

(40) The broadband input spectrum S.sub.IN of the input pulses 202, 204 and 206 with the broad input spectral range and the short input pulse width .sub.0 is thus largely stretched in time to the output pulse with .sub.p without changing the time-average spectral range at the output 124. In the case of positive dispersion shown in each of the FIGS. 5 and 7, red spectral components are in the beginning of the stretched output pulse 302, 304 or 306. Blue spectral components follow in a temporal tail of the output pulse 302, 304 or 306. The different spectral components thus reach the detector 140 at different times, which sequentially detects the spectral components in the fractions t.sub.gate subdividing the output pulse width .sub.p.

(41) The temporal spread, i.e., the output pulse width p, is selected, depending on an application, by changing the dispersion parameter D. The dispersion parameter D is changed by selectively switching to the dispersive medium 120 out of a plurality of different dispersive media. In order to avoid moving components in the optical arrangement of the device 100a, 100b or 100c, a variant of the embodiment changes the dispersion parameter D by applying an external electric or magnetic field acting on a dispersive medium 120, wherein the dispersion parameter D of the medium 120 is sensitive to the external field. Alternatively or in addition, the length L of the dispersive medium 120 is changed. In an advanced variant of the embodiment of the device 100a, 100b or 100c, the dispersive medium 120 includes a plurality of taps 123 along the length L. Each of the plurality of taps allows coupling light into or out of the dispersive medium 120 at a position of the tap along the length L. At least one of the input 122 and the output 124 can be selected along the length L of the dispersive medium 120. An optical switch 121 automatically uses one of the taps 123 as the input 122 or as the output 124 depending on the application.

(42) At a sufficient temporal dilatation of the output pulse 302, 304 or 306 (i.e., by a is sufficiently large T.sub.p) and for a sufficiently fast detector 140 (i.e., for a sufficiently short t.sub.gate), only the sharp instantaneous spectral range (i.e., the instantaneous spectral linewidth) is detected within the temporal fraction t.sub.gate defined by the gate unit 144. The instantaneous output spectral range (i.e., the instantaneous spectral linewidth) thus defines the coherence length, which is twice the axial resolution according to:

(43) $l_{c} = 2 .Math. z_{\max} = \frac{2 .Math. \ln 2}{n .Math.} .Math. \frac{_{0}^{2}}{} .$

(44) Here it is assumed that the depth at which the OCT signal drops to 6 dB (corresponding to 20.Math.log(A), wherein A is the amplitude of the signal) defines the axial resolution. In other words, the axial resolution z.sub.max is half of the coherence length. It is pointed out that, even for a given temporal resolution t.sub.gate of the detector 140, the spectral resolution (i.e., the instantaneous spectral range ) can be improved by increasing the output, pulse width .sub.p, which is caused by the dispersive medium 120, independently of the spectral range of the light generator 110 and/or to decrease the gate time resolution t.sub.gate.

(45) As a result of the independence, the axial resolution z.sub.min (which is proportional to 1/) and the axial scanning depth z.sub.max (which is proportional to 1/, and thus proportional to .sub.p/(.Math.t.sub.gate)) can be chosen independent of each other. In other words, the OCT technique, which may also be referred to as a Pulse-Stretched Swept Source OCT (PSSS-OCT), allows to almost freely choose the axial resolution z.sub.min and the axial scanning depth z.sub.max, e.g., depending on the application. The spectral resolution is no longer limited by a pixel size of a detector array in a spectrometer (as opposed to SD-OCT) and is no longer determined by an instantaneous bandwidth of a tunable or swept light source (as opposed to SS-OCT). A large bandwidth (which is provided by the light source 110) and a small spectral resolution (which is caused by the dispersive medium 120) are not mutually exclusive using the PSSS-OCT.

(46) Parameters for an exemplary implementation of each of the devices 100a, 100b and 100c are summarized. The TiS laser used as the light generator has a center wavelength .sub.0=800 nm and a bandwidth =200 nm for an input pulse width .sub.0=1 ps to 1 ns. The dispersive medium 120 is a fiber of L=2 km length arranged as a coil. The fiber has a dispersion parameter D=13200 ps/(nm.Math.km). The detector 140 has a temporal resolution for sampling the fractions t.sub.gate100 ps, which clocking is supported by all components 142 to 148 of the detector 140. The output pulse width .sub.p is thus (at least, since the contribution of the first term, .sub.0, is neglected):
.sub.p=|D|.Math.L.Math.=5.3 s.

(47) For a sampling interval t.sub.gate=100 ps, a plurality of N fractions is sampled per pulse:

(48) $N = \frac{5.3 s}{100 ps} = 5.3 .Math. 10^{4} .$

(49) The instantaneous output spectral range , which is the spectral resolution detected by the detector 140 detecting the intensity signal of the photodiode 142 in the temporal fraction t.sub.gate, is thus:

(50) $= \frac{}{N} = 0.004 nm .$

(51) The exemplary implementation of the device 100a, 100b or 100c can thus achieve an axial resolution z.sub.min=1.0 m and an axial scanning depth z.sub.max=26 mm. The embodiments of the devices 100a, 100b and 100c thus achieves the high axial resolution (comparable to SD-OCT) in combination with an axial scanning depth that (for sufficient signal strength, i.e., spectral power) is long enough to scan the entire length of an eye.

(52) The scanning depth z.sub.max is freely adjustable, even without changing the optical arrangement of the device 100a, 100b or 100c, by changing the sampling interval of the fractions t.sub.gate.

(53) Alternatively or in addition, particularly suitable for the case of a very short input pulse width .sub.0, the temporal spread of the input pulse yielding the output pulse of output pulse width .sub.p may be described by the relation:

(54) $_{p} =_{0} \sqrt{1 + {(4 .Math. \ln (2) .Math. \frac{D_{2}}{_{0}^{2}})}^{2}} 4 .Math. \ln (2) .Math. \frac{D_{2}}{_{0}},$
wherein D.sub.2=.sub.2.Math.L is the Group Delay Dispersion (e.g., the Group Velocity Dispersion related to the specific length L of the dispersive medium). The symbol .sub.2 denotes the Group Velocity Dispersion:

(55) $_{2} = - \frac{D_{} .Math._{0}^{2}}{2 .Math. c} (in units of {fs}^{2} / m),$
wherein D.sub. denotes the dispersion parameter (also referred to as Group Delay Dispersion Parameter) in units of ps/(km.Math.nm), an example of which is shown in FIG. 8.

(56) In a set of numerical examples, the dispersion parameter is D.sub.=13200 ps/(km.Math.nm) and light is generated at a center wavelength .sub.0=800 nm, which yields .sub.2=4481781 fs.sup.2/m, such that for a length L=2 km the output pulse .sub.p is approximately 1.65 s; for a length L=10 km the output pulse .sub.p is approximately 8.25 s; or for a length L=20 km the output pulse .sub.p is approximately 16.5 s.

(57) By further increasing the input spectral range of the light generated by the TiS laser 110, the axial resolution z.sub.min is further improved, even without a negative effect on the scanning depth z.sub.max.

(58) As has become apparent from above description of embodiments of a device for optical coherence tomography, some embodiments allow overcoming limitations or mutual interdependencies of at least one of the axial resolution z.sub.min and a scanning depth z.sub.max. An output pulse width .sub.p can exceed 1 s. The detector can resolve temporal fractions t.sub.gate shorter than 100 ps. A dispersion can be linear with respect to a wavenumber or frequency of spectral components in an input pulse, which allows sampling the fractions t.sub.gate of an intensity signal uniformly in time for direct Fourier transformation.

(59) It will be apparent that various changes may be made in the form, construction and arrangement of above exemplary embodiments without departing from the scope of the invention or without sacrificing all of its advantages. Because the invention can be varied in many ways, it will be recognized that the invention should be limited only by the scope of the following claims.

Optical coherence tomography technique

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PHYSICS

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A61B3/10

HUMAN NECESSITIES

Abstract

Claims

Description