Patent Application: US-201213457297-A

Abstract:
an improved interferometer measurement system is presented . in a preferred embodiment , a chirped fiber bragg grating is used as a reference surface in a fizeau interferometer arrangement for optical coherence tomography imaging of the eye . the grating creates a virtual reference surface near the sample and allows for a relatively short reference arm while maintaining close to zero delay interference conditions .

Description:
the present invention uses a chirped bragg grating to create a reference ‘ surface ’ in a fizeau interferometer configuration as diagrammed in fig1 . while the embodiment in the figure is directed towards an ophthalmic optical coherence tomography ( oct ) imaging system , the invention described herein could be applied to any type of oct system . the oct device should be interpreted broadly to include forms of white light interferometry allowing a relatively static reference arm , including but not limited to spectral domain oct ( sd - oct , se - fd - oct ), and optical frequency domain imaging (‘ swept - source ’ oct , ss - oct , te - fd - oct ). the device should be construed to contain all necessary computers , drive electronics , processors , display commonly understood to be necessary to achieve the diagnostic or guidance function provided by the device . referring to fig1 , light is provided by source 101 . the source may be either a broad bandwidth source for se - fd - oct , or a swept source for te - fd - oct . in the case of te - fd - oct , the source must have sufficient coherence length to produce observable interference at the distance between the proximal end of the grating and the most distant portion of the sample to be observed . the light is split by a beam splitter 102 with one portion directed to the optical path leading to the sample , and the remaining portion optionally directed to a source monitor detector 103 for monitoring the source . the fiber beamsplitter shown in the preferred embodiment represents only an efficient means to do this with state of the art components . optical circulators or free space optics could be used to perform a similar function . in the illustrated embodiment , the optical path leading to the sample includes an optical fiber 104 . a chirped bragg grating 110 is formed in fiber 104 near the distal end thereof . the chirp of the grating is structured such that portions of the long wavelengths of the source are reflected after traversing a short distance in the fiber , while portions of the shorter wavelengths of the source are reflected after traversing a longer distance in the fiber . in this way the chirp is structured similarly to a classical pulse stretcher , with an important difference that the phase ramp is intended to be approximately linear . the chirped grating introduces a phase ramp across the spectrum that is much greater than would be achieved if the light were reflected by a simple reflector at the maximum extent of the grating . this reduces the rate at which the phase changes over the spectrum , thereby reducing the demand for very high density or , high speed detection electronics given a relatively large delay between the sample and reference . while the reflections are actually occurring within the fiber , the phase change in the reference light is equivalent to the change that would occur if the light were reflected at a ‘ virtual reference plane ’ placed near the sample . the effective length of the chirped grating is proportional to the length one would like the virtual reference plane 105 projected beyond the end of the grating depending on the type of imaging or sample under investigation . referring to fig2 , suppose the short wavelength reflecting end of the grating 202 is located at position r s ( near the end of the fiber 104 closest to the sample ), the long wavelength reflecting end 203 of the grating is located at distance r l from the short wavelength reflecting end ( closer to the source ), and the virtual reference plane 105 is located at an optical path length , s , beyond the short wavelength reflecting end . ( the ends of the gratings are actually the points at which the reflected wavelengths appear to come , and are approximated by the ends of the physical grating .) the end - to - end phase difference accumulated in the sample arm across the spectrum ( δφ ts ) from short wavelength ( λ s ) to long wavelength ( λ l ) is defined as : similarly , the end - to - end phase difference accumulated in the reference arm from short wavelength ( λ s ) to long wavelength ( λ l ) is defined as : by defining the short wavelength reflecting portion of the grating at position 0 , the phase acquired by λ s will be zero , resulting in : we wish to achieve the same end to end phase difference , δφ t , in both the sample and the reference arms : solving for r l in the equation for δφ tr yields : substituting in the equation for the phase difference in the sample arm for δφ t since all phase differences are equal : the negative value of r l indicates that the long wavelength reflecting end of the grating should be further away from the virtual reference plane than the short wavelength reflecting end . if the grating is designed to impose a linear phase ramp , it will achieve the correct phase difference ( i . e . the same one as in the sample arm ) for any pair λ 1 , λ 2 within the wavelength band between λ l and λ s . for a typical system λ l = 885 nm , λ s = 795 nm , the path length of the bragg grating is about 11 % of the path length of the sample arm beyond the grating . supposing the fiber has an index of about 1 . 4 and the sample arm has a path length about 100 mm , the fiber grating length would be about 8 mm . note that if a longer grating is desired , such as to increase the reflectivity or for other manufacturing purposes , the distance to the sample should be increased , and may be achieved by moving the grating down the fiber towards the source . in additional embodiments of the invention , some flexibility in the position of the reference plane relative to the sample may be achieved by various methods , as might be desired for example to compensate for eyes of variable length or to image at different depth locations within a sample . one simple method may include a mechanism to adjust the distance between the bragg grating and the beam delivery optics 106 . other methods may include changing properties of the bragg grating . the effective periodicity of the grating may be changed by changing the physical spacing of the index variations within the grating and / or by changing the index of refraction of the medium in which the grating is inscribed . for example a fiber bragg grating may be stretched , changing both its physical length and introducing stress modulation of optical index . this can be achieved by stretching the fiber section with the grating coiled around a piezo - electric material . also piezo - electric coatings around the fiber have been demonstrated to modulate the grating structure . the index of the medium in which a bragg grating in a silicon planar waveguide is written may be modified by adjusting the injection current in the region . changing the temperature of a bragg grating material has also been demonstrated to tune a grating . note that while the phase ramp is intended to be primarily linear , arbitrary phase dispersion can be added if desired by proper design of the bragg grating as is known by those skilled in the art . such phase dispersion may be added to compensate for dispersion in the sample arm , or because there are advantages to having higher order dispersion mismatch between the two arms . the method described herein provides a means to create a reference reflection which has the advantages of the common path configuration including very little opportunity for fiber in the difference path to create polarization variability . additionally , the reference can be precisely controlled for dispersion mismatch . the reference can be made very compact , which is advantageous particularly for surgical or diagnostic devices which must be handheld , or placed into small spaces such as an artery or the inside of a product to be inspected . because the entire reference arm is inscribed into a piece of optical fiber , or integrated waveguide structure , this implementation also has the potential to reduce cost . the resulting interference between the light reflected from the sample and the reference light reflected from the chirped bragg grating is combined at beamsplitter 102 and directed to detector 107 . the detection may be a simple detector in the case of te - fd - oct or may include a spectrometer array in the case of se - fd - oct . in the case of se - fd - oct , the optical resolution of the spectrometer must be sufficient to achieve a coherence length to produce observable interference at the distance between the proximal end of the grating and the most distant portion of the sample to be observed . to overcome the limitation of the fizeau interferometer , that balanced detection using the out of phase return arm of the interferometer is impossible , light sampled from the source ( for example using the source monitor detector 103 ), which does not contain the interference information from the interferometer , but does contain the noise information from the source , may be used in balanced detection . the results can be stored in the processor 108 or displayed on display 109 . although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein , those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings . the structure connecting the source , detector and interferometer should be interpreted broadly to consist of a means to direct light from a source to a fizeau interferometer , and back to a detection means . the fizeau interferometer consists of reference reflection provided by a bragg grating , preferably embodied in a fiber bragg grating ; and a sample reflection such as from a biological tissue or material to be inspected . the scanning and focusing means may be required in some applications where it is desired to produce an image from the white light interferogram . in these cases , means to produce a scanned beam are well understood by those skilled in the art and the variety of which cannot be described here . u . s . pat . no . 5 , 718 , 738 kohnke et al “ method for making continuously chirped fiber bragg gratings ” u . s . patent publication no . 2010 / 0091265 franz et al “ fiber optic sensor head and interferometric measuring system ” international publication no . wo 2007 / 002969 barton et al “ optical coherence tomography probe device ” a . l . oldenburg et al “ fast - 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