Abstract:
An optical coherence tomography system utilizes an optical swept laser that has improved coherence length in the swept optical signal. This is accomplished using an intra-cavity element that extracts the tunable optical signal at the optimal location within the laser&#39;s resonant cavity. Generally this location is between the intracavity tuning element and the cavity&#39;s gain element so that light coming from the tuning element is extracted. In general in lasers, the gain element adds noise and chirp and this degrades the tunable optical signal&#39;s coherence length.

Description:
BACKGROUND OF THE INVENTION 
     Optical coherence analysis relies on the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection. 
     There are several different classes of OCT, but Fourier domain OCT currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diversity detection. It has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum-encoded OCT, are not available. 
     In swept source OCT, the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive optical frequency sampling intervals and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its frequency sweep rate and tuning accuracy, along with its coherence length characteristics. 
     The swept sources for OCT systems have typically been tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. A tunable laser is constructed from a gain element, such as a semiconductor optical amplifier (SOA) that is located within a resonant laser cavity, and a tuning element such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. 
     Currently, some of the highest tuning speed/sweep rate lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. The use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass, high mechanical resonant frequency deflectable MEMS membranes that have the capacity for high speed tuning/sweep rates. 
     Another laser architecture is termed a Fourier-domain mode-locked laser (FDML). This type of laser stores light in a long length of fiber for amplification and recirculation in synchronism with the laser&#39;s tuning element. See “Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography”, R. Huber, M. Wojtkowski, and J. G. Fujimoto, 17 Apr. 2006/Vol. 14, No. 8/OPTICS EXPRESS 3225. The drawback of these devices is their complexity, however. Moreover, the ring cavity including the long storage fiber creates its own performance problems such as dispersion and instability. 
     An important metric for swept sources is coherence length. This refers to the propagation distance over which the source&#39;s optical signal maintains a specified degree of coherence. In OCT systems, longer coherence lengths enable imaging over longer depth ranges. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an OCT method and system and swept laser designs that can be used to improve coherence length of the swept optical signal. This is accomplished using an intra-cavity element that extracts the tunable optical signal at the optimal location within the laser&#39;s resonant cavity. Generally this location is between the intracavity tuning element and the cavity&#39;s gain element so that light coming from the tuning element is extracted. The present invention also concerns the simultaneous or selective generation of a tunable optical signal with different coherences lengths. 
     In general in lasers, the cavity gain element adds noise and/or distorts the spectral content of the light in the laser cavity. For example, the gain element often adds amplified spontaneous emissions (ASE) and this noise degrades the tunable optical signal&#39;s coherence length. This noise, however, is removed by the tuning element. The distortion that is added by the gain element arises from a different source. High speed swept lasers such as those often used in OCT systems exhibit a form of mode-locking, termed swept mode locking See e.g., U.S. Pat. Appl. Pub. No. US 2012/0162662 A1, which is incorporated herein by this reference. As a consequence, during operation, light within the laser cavity circulates in the form of one or more pulses, which strongly modulate the gain. Each pulse “hops” to a new optical frequency with the laser tuning, but there is also considerable chirp to the pulses. The chirp is added by the gain element. When these pulses are filtered, the pulses are longer and have smaller chirp than those that have just passed through the gain element. 
     Thus, by extracting the tunable optical signal post filtering but before amplification, the coherence length is improved. 
     In general, according to one aspect, the invention features a swept laser that generates a swept optical signal. The laser comprises a laser cavity in which the swept optical signal is generated, a tuning element for a controlling an optical frequency of the swept optical signal, a gain element for amplifying light in the laser cavity, and an optical signal extraction element located between the tuning element and the gain element for coupling the swept optical signal from the laser cavity after being filtered by the tuning element but before amplification by the gain element. 
     In embodiments, the laser cavity is a linear cavity and the signal extraction element is located downstream of the tuning element but upstream of the gain element. In one case, the signal extraction element is a beam splitter and the tuning element is a Fabry Perot tunable filter. In examples, quarter wave plates on either side of the tuning element are used to rotate the polarization of the optical signal within the laser cavity so that light transmitted through the tunable filter has a polarization that is appropriate for amplification by the gain element whereas light that is rejected by the tunable filter has a polarization that is orthogonal to the polarization at which the gain element amplifies light. The gain element can be a reflective semiconductor optical amplifier. 
     In one embodiment, a low coherence signal extraction port is provided that generates a lower coherence version of the swept optical signal. 
     In another embodiment, the laser cavity is a ring cavity. 
     In general, according to one aspect, the invention features an optical coherence tomography system comprising: an interferometer that combines a swept optical signal from a sample and from a reference path to generate an interference signal, a detection system that detects the interference signal, and a swept laser. This laser generates the swept optical signal and comprises a laser cavity, a gain element for amplifying light in the laser cavity, and an optical signal extraction element located within the laser cavity for coupling the swept optical signal from the laser cavity prior to amplification by the gain element. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIG. 1  is a schematic diagram illustrating a swept laser (linear cavity) according to the present invention; 
         FIG. 2  is a schematic diagram illustrating a linear cavity swept laser according to a second embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating a linear cavity swept laser according to a third embodiment of the present invention; 
         FIG. 4  is a schematic diagram illustrating a linear cavity swept laser according to a fourth embodiment of the present invention; 
         FIG. 5  is a schematic diagram illustrating a linear cavity swept laser according to a fifth embodiment of the present invention; 
         FIG. 6  is a schematic diagram illustrating a ring cavity swept laser according to a sixth embodiment of the present invention; 
         FIG. 7  is a schematic diagram illustrating a ring cavity swept laser according to a sixth embodiment of the present invention; and 
         FIG. 8  is a schematic diagram of an optical coherence tomography system using the inventive swept laser. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Turning now to the drawing,  FIG. 1  shows a swept laser source system  100  according to a first embodiment of the present invention. 
     In a preferred embodiment, a majority or all of the components of the swept laser  100  are installed on a common bench  105 . The bench  105  is termed a micro-optical bench and is preferably less than 10 millimeters (mm) in width and about 25 mm in length or less. This size enables the bench to be installed in a standard, or near standard-sized, butterfly or DIP (dual inline pin) hermetic package. In one implementation, the bench  105  is fabricated from aluminum nitride. A thermoelectric cooler is disposed between the bench and the package (attached/solder bonded both to the backside of the bench and inner bottom panel of the package) to control the temperature of the bench  105 . 
     As is characteristic of lasers, the swept laser  100  includes a laser (resonant) cavity  125 . Light within the cavity  125  is coupled from it via a high coherence output port defined by output lens  118 . In the illustrated example, the light or tunable signal is transmitted from the laser  100  and off of the bench  105  on an optical fiber  110 . Typically the optical fiber  110  extends through a fiber feedthrough in the hermetic package. 
     In other examples, the output port is defined by a window in the hermetic package. The tunable signal is coupled from the bench  105  and through the package as a beam, thus avoiding the use of the fiber. 
     A gain element  126  is provided in the cavity  125 . In a typical example, the gain element  126  is a semiconductor optical amplifier (SOA), which is mounted to the bench  105  via a submount. In other examples, a rare earth doped optical fiber gain element is used. Still other examples are solid-state optical gain media. The gain element  126  amplifies light within the cavity  125 . 
     In the current embodiment, the input facet  128  of the SOA chip  126  is angled relative to the axis of the cavity and anti-reflection (AR) coated. The back facet  130  is coated to be reflective to define one end of the laser cavity  125 . In the illustrated example, an edge emitting chip is used with a curved or arcuate ridge waveguide  115 . 
     The other end of the laser cavity  125  is defined by mirror  102 . Preferably, this mirror  102  also functions as a polarizing filter to remove light that is orthogonal to the gain polarization of the cavity  125 . The gain polarization of the cavity at the location of the mirror  102  is actually orthogonal to the gain polarization of the SOA  126  due to a pair of quarter waveplates  114 ,  118 . 
     The material system of the chip  126  is selected based on the desired spectral operating range. Common material systems are based on III-V semiconductor materials, including binary materials, such as GaN, GaAs, InP, GaSb, InAs, as well as ternary, quaternary, and pentenary alloys, such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs, GaInNAs, GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, InGaSb, InAsSb, and InGaAsSb. Collectively, these material systems support operating wavelengths from about 400 nanometers (nm) to 2000 nm, including longer wavelength ranges extending into multiple micrometer wavelengths. Semiconductor quantum well and quantum dot gain regions are typically used to obtain especially wide gain and spectral emission bandwidths. Currently, edge-emitting chips are used although vertical cavity surface emitting laser (VCSEL) chips are used in different implementations. 
     In one implementation, the gain element  126  amplifies light at only one polarization, the gain polarization. It provides little or no gain at the orthogonal polarization. 
     Also within the cavity  125  is a tuning element  116 , which is preferably mounted to the bench  105 . The tuning element typically has a tunable passband (in reflection or transmission) that scans over a scan band. This passband overlaps with the gain spectrum of the gain element  126 . This configuration allows optical energy within the passband to be amplified within the laser cavity  125  and thus coupled onto optical fiber  110  via the output port. 
     In some embodiments, the tuning element  116  is a micro mechanical system (MEMS) Fabry Perot tunable filter. In other examples, grating based tuning elements are used. Still other examples are acousto optic tunable filters. 
     The swept source system  100  is generally intended for high speed tuning to generate swept optical signals that repeatedly scan over the scan band(s) at rates of greater than 1 kiloHertz (kHz). In current embodiments, the laser system  100  tunes at speeds greater than 20 or 100 kHz. In very high speed embodiments, the multi-sweep rate swept source system  100  tunes at speeds greater than 200 or 500 kHz. 
     Typically, the width of the tuning or scan band is greater than 10 nanometers (nm). In the current embodiments, it is usually between 50 and 150 nm, although even wider tuning bands are contemplated in some examples. On the other hand, the bandwidth of the narrowband emission has a full width half maximum (FWHM) bandwidth of less than 20 or 10 GigaHertz (GHz), and is usually 5 GHz or less. For optical coherence tomography, this high spectral resolution implies a long coherence length and therefore enables imaging deeper into samples, for example deeper than 5 millimeters (mm). On the other hand, in lower performance applications, for example OCT imaging less than 1 mm deep into samples, broader FWHM passbands are sometimes appropriate, such as passbands of about 200 GHz or less. 
     The tuning speed can also be expressed in wavelength per unit time. In one example, for an approximately 110 nm tuning band or scanband and 100 kHz scan rate, assuming 60% duty cycle for substantially linear up-tuning, the peak sweep speed would be 110 nm*100 kHz/0.60=18,300 nm/msec=18.3 nm/μsec or faster. In another example, for an approximately 90 nm tuning range and 50 kHz scan rate, assuming a 50% duty cycle for substantially linear up-tuning, the peak sweep speed is 90 nm*50 kHz/0.50=9,000 nm/msec=9.0 nm/μsec or faster. In a smaller tuning band example having an approximately 30 nm tuning range and 2 kHz scan rate, assuming a 80% duty cycle for substantially linear tuning, the peak sweep speed would be 30 nm*2 kHz/0.80=75 nm/msec=0.075 nm/μsec, or faster. 
     Thus, in terms of scan rates, in the preferred embodiments described herein, the sweep speeds are greater than 0.05 nm/μsec and preferably greater than 5 nm/μsec. In still higher speed applications, the scan rates are higher than 10 nm/μsec. 
     According to the invention, the laser cavity  125  further comprises an optical signal extraction element  122 . The signal extraction element  122  is located downstream of the tuning element  116  but upstream of the gain element  126  and extracts light from the tuning element  116  before it is transmitted to the gain element  126 . 
     The advantage of extracting the tunable optical signal from this location within the laser cavity  125  is that the light has just been transmitted through the Fabry Perot tunable filter tuning element  116 , in fact twice in the illustrated embodiment since its last amplification. Thus, its bandwidth corresponds to the passband of the tuning element  116 . This extraction of the tunable optical signal, however, occurs prior to its amplification in the gain element  126 . Typically, a gain element adds noise in addition to amplifying light at the passband. In the case of a semiconductor optical amplifier, this noise includes amplified spontaneous emissions. 
     Moreover, as discussed previously, the gain element  126  also tends to modulate the light circulating within the cavity  125 . When operated as a swept source, with the tuning element  116  tuning over the scan band at a high rate, light circulates within the cavity as one or more pulses and the gain element chirps these pulses. The spectral broadening of the light circulating within the cavity by this chirping is counteracted by the double passing of the light through the tuning element  116  prior to extraction by the signal extraction element  122 . 
     Thus, by extracting the tunable optical signal prior to amplification, the tunable optical signal is relatively free of the noise and spectral distortion that would be added by the gain element  126 . 
     In the illustrated embodiment, the signal extraction element  122  is a beam splitter such as a partial beam splitter or a polarization beam splitter. It reflects light such as at a 90° angle. Often, it only couples approximately 1% to 10% of the light as the output tunable optical signal  160 . In illustrated embodiment, this tunable optical signal is coupled by the lens  118  into the optical fiber  110 . 
     Light that is outside the passband of the Fabry Perot tunable filter tuning element  116  is reflected by this filter. This light should not be amplified by the gain element  126  to ensure laser operation. As result, two quarter wave plates  114 ,  118  are located on either side of the Fabry Perot filter  116 . These wave plates  114 ,  118  rotate the polarization of the optical signal within the laser cavity  125  so that light transmitted through the tunable filter has a polarization that is appropriate for amplification by the gain element  126 . That is, the light is polarized parallel to the gain polarization in the case of a semiconductor optical amplifier gain element  126 . In contrast, light that is rejected by the tunable filter  116  has a polarization that is orthogonal to the polarization at which the gain element  126  amplifies light. 
     In the illustrated example, a series of lenses  112 ,  120 , and  124  are used to couple light between the various elements within the laser cavity  125  and on the bench  105 . Specifically, lens  112  couples light between the mirror  102 , through the first quarter wave plate  114  and into the tuning element  116 . Lens  120  couples light between the tuning element  116 , the second quarter wave plate  118 , and the signal extraction element  122 . Finally, lens  124  couples light between the front facet  128  of the gain element  126  and the signal extraction element  122 . 
       FIG. 2  shows a swept laser  100  according to a second embodiment of the present invention. This embodiment includes a low coherence high power extraction port defined by lens  132 . Specifically, light that is returning from the gain element  126  on a path to the tuning element  116  is also reflected by the signal extraction element  122  to provide a low coherence version  162  of the tunable optical signal. Since this light includes the noise and spectral distortion contributed by the gain element  126 , it generally has a lower coherence length than the tunable optical signal  160 , but a much higher power since it comes directly from the gain element  126 . This light is collected by lens  132  and coupled into an optical fiber  134  in one example. In other examples, it is coupled from the bench  105  as a beam. 
       FIG. 3  shows a swept laser according to a third embodiment of the present invention. This embodiment further includes a medium coherence extraction port defined by lens  164 . In general, the coherence length of the tunable signal  165  generated at this port will be lower than the tunable signal  160  but higher than tunable signal  162  since it has been filtered by one pass through the filter  116 . 
     Specifically, mirror  102 , which possibly further functions as the polarizing filter, is partially reflective/transmissive to allow the light that has been filtered by the tuning element  116  to pass through the partial mirror  102  to be collimated by lens  164  as a medium coherence version  165  of the tunable optical signal. This medium coherence version  165  of the tunable optical signal is coupled into optical fiber  166  in the illustrated embodiment. 
       FIG. 4  shows a swept laser according to a fourth embodiment of the present invention. This embodiment also includes a medium coherence extraction port. Specifically, lens  112  couples light of the cavity  125  into fiber  166 . A fiber mirror  102   f , such a grating or partially reflective fiber splice, functions as a partially reflective/transmissive mirror to define the end of the cavity  125  and also as the medium coherence extraction port. The medium coherence version  165  of the tunable optical signal being transmitted through the fiber mirror  102   f  on fiber  166 . The advantage of this embodiment is that longer laser cavities can be created that consequently have spectrally smaller longitudinal mode spacing. 
       FIG. 5  shows a swept laser  100  according to a fifth embodiment of the present invention. This embodiment also includes a reflective cavity extender  170  between lens  112  and the mirror  102 . It includes two antireflection coated facets  134 ,  136  to allow the light from the cavity  125  to be coupled into and out of the extender  170 . In the extender, light propagates in a zig-zag pattern which increases the effective optical length of the laser cavity  125 . Here again, longer laser cavities can be created that consequently have spectrally smaller longitudinal mode spacing. 
       FIG. 6  shows a swept laser  100  according to a sixth embodiment of the present invention. 
     This swept laser  100  has a ring cavity configuration. Specifically, the laser cavity  125  transmits light counterclockwise through the laser cavity  125  that is implemented on bench  105 . 
     In more detail, light is amplified in the gain element  126 . In the illustrated example, a semiconductor optical amplifier is used in which both the front facet  128  and the rear facet  130  are antireflection coated. Currently an edge-emitting chip with a linear ridge waveguide is preferred. 
     The light that exits from the gain element  126  is collected by a lens  140 . The light is then reflected by a first fold mirror  142  and a second fold mirror  144 . A third lens  146  couples the light into the tuning element  116 . The light exiting from the tuning element  116  is collected by a third lens  148 . The light is then reflected by two fold mirrors  150  and  152  to be directed back to the gain element  126 . 
     This embodiment utilizes an angle-isolated tuning element  116 . Specifically, light that is transmitted through the tuning element  116 , such as a Fabry Perot tunable filter, stays within the laser cavity  125 . Light that is outside the passband, and reflected by the tunable filter  116 , is reflected at an angle relative to the axis of the laser cavity  125  and in this way does not return back to the gain element  126  to be amplified. 
     A signal extraction element  122  is located between the tuning element  116  and the gain element  126 . It is specifically located downstream of the tuning element  116  and upstream of the gain element  126  within the ring cavity  125 . In this way, the signal extraction element  122  functions as a high coherence optical signal output port and diverts a portion of the light circulating within the optical cavity  125  as the output tunable optical signal  160 . This tunable optical signal in the illustrated example is collected by the output lens  118  and coupled into the optical fiber  110 . 
       FIG. 7  shows a swept laser  100  according to a seventh embodiment of the present invention. 
     This swept laser  100  also has a ring cavity configuration. Specifically, the laser cavity  125  transmits light counterclockwise through the laser cavity  125 , which is implemented on bench  105 . 
     In more detail, light is amplified in the gain element  126 . In the illustrated example, a semiconductor optical amplifier is used in which both the front facet  128  and the rear facet  130  are antireflection coated. 
     The light that exits from the gain element  126  is collected by a lens  140 . 
     The lens  140  collimates the light so that it is transmitted through a first polarization beam splitter  180 . The configuration of the polarization beam splitter  180  and the polarization of the light exiting from the gain element  126  is such that the light is transmitted directly through the polarization beam splitter  180 . For example, if the light from the gain element  126  is polarized in a direction that is parallel to the plane of the bench  105 , then the first polarization beam splitter is transmissive to that parallel polarization. 
     The light exiting from the first polarization beam splitter  180  is then focused by a lens  146  to be coupled into the tuning element  116 . Light exiting from the tuning element  116  is been collected by lens  148  and collimated. The light is then transmitted through a second polarization beam splitter  182 . Again, the polarization of the light and the second polarization beam splitter  182  are configured so that the light is transmitted directly through the second polarization beam splitter  182 . 
     Light exiting from the second polarization beam splitter  182  is then coupled into a beam splitter  184 . The beam splitter  184  is configured to reflect a portion of the light and allow the other portion to pass directly through the beam splitter  184 . In one example, the beam splitter will  184  reflects about 50% of the light. In other examples, it reflects 80% or more of the light. 
     The light that is reflected by the beam splitter  184  remains within the laser cavity  125 , in the illustrated embodiment. Specifically it is reflected by a first fold mirror  144 . It is then reflected by a second fold mirror  150 , followed by a subsequent fold mirror  152 . These fold mirrors complete the ring cavity. 
     The light passing through the ring cavity is then collected by lens  145  and coupled into the entrance facet  128  of the gain element  126 . 
     The light that is transmitted through the beam splitter  184  is reflected to be returned back to pass through the tuning element  116 . Its polarization, however, on this return path is rotated 90°. As a result, it is reflected by both the second polarization beam splitter  182  and the first polarization beam splitter  180 . 
     In more detail, the light that is transmitted through the first polarization beam splitter  184  is reflected by a series of fold mirrors  185 ,  186 , and  187  or other optical elements such as fiber to form a return path. On this return path through the series of mirrors, a half wave plate  188  is used to rotate the polarization of the light by 90°. 
     As a result, with this rotated polarization, the light reflected by fold mirror  187  and received by the second polarization beam splitter  182  is reflected to be collected by lens  148  and again coupled into the tuning element  116 . Its direction of propagation is counter to the predominant direction of propagation for the light in the laser cavity  125 . 
     Light exiting from the tuning element  116 , propagating in the contra propagation direction, is collimated by lens  146  and coupled into the first polarization beam splitter  180 . With its polarization, this returning light is reflected by the polarization beam splitter  180 , which also functions as the light extraction element, to be coupled to the output port. Specifically, in the illustrated embodiment, the light is collected by the lens  118  and coupled into the optical fiber  110 . 
     The advantage of this embodiment is that the light that is produced at the output port has been twice filtered by the tuning element  116  to counteract the spectral broadening from the chirp introduced by the gain element  126 , for example. 
       FIG. 8  shows an optical coherence analysis system  10  using the swept laser  100 , which has been constructed according to the principles of the present invention. 
     The swept laser  100  generates the tunable or swept optical signal on optical fiber  110  that is transmitted to interferometer  200 . The swept optical signal scans over a scan band with a narrowband emission. 
     In some embodiments, the light with other coherence lengths is provided such as on optical fibers  134  and  166  in the previously described embodiments. 
     In other cases, longer coherence length versions of the tunable signal are provided to a k-clock system  202 . In one example, the tunable signals provided on fibers  110  or  166  are used by the k-clock system  202  to generate the kclock, whereas the higher power versions of the tunable signal on fibers  134  are provided to the interferometer  200  and the sample 5. This system filters the tunable signal as produced k-clock signals that are used to trigger the sampling of the data acquisition system  255  at evenly spaced increments of the scanning of the tunable signal through the scan band. 
     A controller  290  generates a drive waveform that is supplied to a digital to analog converter  272 . This generates a tunable optical element drive signal  108  that is amplified by amplifier  274  and applied to the tuning element of  116  of the swept laser  100 . 
     The swept laser  100  is generally intended for high speed tuning to generate swept optical signals that repeatedly scan over the scan band(s) at rates of greater than 1 kiloHertz (kHz). In current embodiments, tuning element drive signal  108  that is applied to the tuning element  116  of the swept laser  100  repeatedly tunes the element  116  over the scanband at speeds greater than 20 or 100 kHz. In very high speed embodiments, the swept laser  100  tunes at speeds greater than 200 or 500 kHz. 
     Typically, the width of the tuning or scan band provided by the tuning element  116  is greater than 10 nanometers (nm). In the current embodiments, it is preferably between 50 and 150 nm, although even wider tuning bands are contemplated in some examples. 
     In the current embodiment, a Mach-Zehnder-type interferometer  200  is used to analyze the optical signals from the sample 5. The swept optical signal from the swept optical source system  100  is transmitted on fiber  110  to a 90/10 optical fiber coupler  210 . The swept optical signal is divided by the coupler  210  between a reference arm  220  and a sample arm  212  of the system. 
     The optical fiber of the reference arm  220  terminates at the fiber endface  224 . The tunable optical signal light  160 R (or alternatively  162 R or  164 R) exiting from the reference arm fiber endface  224  is collimated by a lens  226  and then reflected by a mirror  228  to return back, in some exemplary implementations. 
     The external mirror  228  has an adjustable fiber to mirror distance, in one example. This distance determines the depth range being imaged, i.e. the position in the sample 5 of the zero path length difference between the reference arm  220  and the sample arm  212 . The distance is adjusted for different sampling probes and/or imaged samples. Light returning from the reference mirror  228  is returned to a reference arm circulator  222  and directed to a  50 / 50  fiber coupler  240 . 
     The fiber on the sample arm  212  terminates at the sample arm probe  216 . The exiting swept optical signal  160 S (or alternatively  162 S or  164 S) is focused by the probe  216  onto the sample 5. Light returning from the sample 5 is returned to a sample arm circulator  214  and directed to the 50/50 fiber coupler  240 . 
     The reference arm signal and the sample arm signal are combined in the fiber coupler  240  to generate an interference signal. 
     The interference signal is detected a detection system  250 . Specifically, a balanced receiver, comprising two detectors  252 , is located at each of the outputs of the fiber coupler  240 . The electronic interference signal from the balanced receiver  252  is amplified by amplifier  254 . 
     A data acquisition system  255  of the detection system  250  is used to sample the interference signal output from the amplifier  254 . In one embodiment, the sampling is performed in response to the k-clock from the k-clock system  202 . 
     Once a complete data set has been collected of the sample 5 by spatially raster scanning the focused probe beam point over the sample, in a Cartesian geometry, x-y, fashion or a cylindrical geometry theta-z fashion, and the spectral response at each one of these points is generated from the frequency tuning of the swept laser  100 , the rendering/display system  280  performs a Fourier transform on the data in order to reconstruct the image and perform a 2D or 3D tomographic reconstruction of the sample 5. This information generated by the rendering system  280  can then be displayed on a video monitor. 
     In one application, the probe  216  is inserted into blood vessels and used to scan the inner wall of arteries and veins. In other examples, other analysis modalities are included in the probe such as intravascular ultrasound (IVUS), forward looking IVUS (FLIVUS), high-intensity focused ultrasound (HIFU), pressure sensing wires and image guided therapeutic devices. In still other applications, the probe is used to scan different portions of an eye or tooth or other structure of a patient or animal. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.