PROBE WITH OPTIMIZED FOCAL DEPTH, WORKING DISTANCE AND AXIAL LIGHT INTENSITY UNIFORMITY

Abstract

A probe with optimized focal depth, working distance and axial light intensity uniformity, including a single-mode fiber for guiding light, a first gradient index fiber for improving light propagation efficiency and regulating mode energy, a large core fiber for generating mode interference field (MIF) and regulating an mode phase difference, a second gradient index fiber and a no-core fiber for magnifying the MIF, and a third gradient index fiber for focusing.

Claims

1. A probe with optimized focal depth, working distance and axial light intensity uniformity, comprising a single-mode fiber, a first gradient index fiber, a large core fiber, a second gradient index fiber, a no-core fiber and a third gradient index fiber, the single-mode fiber, the first gradient index fiber, the large core fiber, the second gradient index fiber, the no-core fiber, and the third gradient index fiber are spliced by fusion in sequence; the second gradient index fiber magnifies a MIF (Mode Interference Field) on an interface between the large core fiber and the second gradient index fiber to an entrance pupil of the third gradient index fiber, a length of the no-core fiber satisfies the requirement that the magnified MIF fully fills the aperture of the third gradient index fiber.

2. The probe with optimized focal depth, working distance and axial light intensity uniformity according to claim 1, wherein the MIF is generated at an end of the large core fiber, the MIF is adjusted by a length of the first gradient index fiber and a length of the large core fiber; the first gradient index fiber adjusts a mode energy of the MIF, and the large core fiber adjusts a mode phase difference of the MIF.

3. The probe with optimized focal depth, working distance and axial light intensity uniformity according to claim 1, wherein the length of the first gradient index fiber is zero.

4. The probe with optimized focal depth, working distance and axial light intensity uniformity according to claim 1, wherein an outer diameter of each fiber is the same as that of a standard single-mode optical fiber.

5. The probe with optimized focal depth, working distance and axial light intensity uniformity according to claim 1, wherein a length of the third gradient index fiber used for focusing realizes a lateral resolution required by an OCT system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1(a) is a structural schematic diagram of a conventional probe; SMF, single-mode fiber, NCF, no-core fiber, GIF, gradient index fiber;

[0021] FIG. 1(b) is the probe structure of the present disclosure; SMF, single-mode fiber, GIF1, first gradient index fiber; LCF, large core fiber, GIF2, second gradient index fiber; NCF, no-core fiber; GIF3, third gradient index fiber; P1 and P2 are a pair of conjugate focal planes of GIF2;

[0022] FIG. 2 shows the simulated field intensity of the output beams in air normalized to the peak intensity in SMF for six typical cases of the designed probe.

[0023] FIG. 3(a) is the microscope image of the fabricated conventional probe; SMF, single-mode fiber, NCF, no-core fiber, GIF, gradient index fiber;

[0024] FIG. 3(b) is the microscope image of the fabricated of the present disclosure; SMF, single-mode fiber, GIF1, first gradient index fiber, LCF, large core fiber, GIF2, second gradient index fiber; NCF, no-core fiber, GIF3, third gradient index fiber;

[0025] FIG. 4 is the schematic of the probe-based swept source OCT system;

[0026] FIG. 5 (a) shows the full width half maximum (FWHM) diameter of the output beam from the conventional probe versus depth in air, WD represents working distance;

[0027] FIG. 5 (b) shows the full width half maximum (FWHM) diameter of the output beam from the probe of the present disclosure versus depth in air; WD represents working distance;

[0028] FIG. 5(c) is the cross-sectional reflectivity profiles of the resolution target at the probe's foci imaged by the conventional probe;

[0029] FIG. 5(d) is the cross-sectional reflectivity profiles of the resolution target at the probe's foci imaged by the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] The present disclosure will be described in detail below with reference to the drawings and embodiments.

[0031] As shown in FIG. 1(a), a conventional all-fiber probe includes a single-mode fiber (SMF), a no-core fiber (NCF), and a gradient index fiber (GIF). The SMF is used for light delivery, the NCF is used for beam expansion, and the GIF is used for beam focusing. In order to extend a focal depth, a series of fiber sections GIF1-LCF-GIF2 are inserted between the SMF and NCF, as shown in FIG. 1(b). The first gradient index fiber (GIF1) is used to manipulate the modes excited in the following large core fiber (LCF). Without GIF1, the incident power will be distributed among high order modes with increased insertion loss due to mismatched numerical aperture between the LCF and the SMF. Light field in the LCF can be decomposed into linearly polarized (LP) modes with different propagation constants. Due to the symmetry of the probe structure and the limited V-number of the LCF, only LP.sub.01 and LP.sub.02 modes with cut-off frequency lower than the V-number are actually excited and can steadily propagate in the LCF. The interference of LP.sub.01 mode and LP.sub.02 mode forms a mode interference field (MIF) at the end of LCF. This MI field (MIF) functions as a pupil filter for DOF extension, and is regulable by length tuning of the LCF. On the other hand, to increase the WD of the probe, the beam size of this MIF needs to be expanded. One way of MIF expansion can be realized by direct propagation of this MIF in the NCF. However, due to light diffraction in the homogeneous medium, the evolved MIF is not exactly an expanded version of the original MIF, and its performance as a pupil filter might be inferior to the expectation. The second way of MIF expansion can be done by magnified imaging. Hence, the GIF2 is utilized to relay the MIF at the LCF-GIF2 interface (labeled as P1 in FIG. 1(b)) onto the entrance pupil (labeled as P2 in FIG. 1(b)) of the GIF3 with magnification.

TABLE-US-00001 TABLE 1 Parameters of optical fibers used by the probe (wavelength is 1.3 μm) GIF1/ GIF/ SMF GIF2 LCF NCF GIF3 core diameter/μm 50 25 — 62.5 cladding diameter/μm 125 125 125 125 125 mode field diameter/μm 9.2 — refractive index 1.4607 1.4469 1.4469 1.4728 in the fiber axis cladding refractive index 1.4469 1.4434 1.4469 1.4469

[0032] It is speculated that the MIF at the end of the LCF and the way of MIF expansion might affect the output beam of the probe. In view of the excited modes allowable in the LCF, three situations are considered, including mainly LP.sub.01 mode with negligible mode interference and two mixed LP.sub.01 and LP.sub.02 modes with significant mode interference but distinct mode phase difference at the end of the LCF. These three situations of allowable modes in combination with two choices of MIF expansion result in six typical cases for the designed probe. Table 1 lists the parameters of each fibers in the probe (wavelength is 1.3 μm). Table 2 lists the fiber lengths and the characteristics of the output beam under six typical cases, where magnified MIF with the GIF2 is adopted in cases I (negligible mode interference), II and III (significant mode interference but distinct mode phase difference), while diffracted MIF without the GIF2 is applied in cases IV (negligible mode interference), V and VI (significant mode interference but distinct mode phase difference) for comparison.

TABLE-US-00002 TABLE 2 Fiber lengths and characteristics of the output beam under six typical cases of the designed probe I II III IV V VI L.sub.Gif1 (μm) 285 0 485 285 0 485 L.sub.LCF2 (μm) 820 890 1250 1315 1310 1115 L.sub.GIF2 (μm) 390 390 390 0 0 0 L.sub.NCF (μm) 300 300 300 300 300 300 L.sub.GIF3 (μm) 160 160 160 160 160 160 MBD (μm) 5.2 5.8 4.2 5.5 5.0 4.5 DOF (μm) 190 305 240 175 290 195 WD (μm) 185 187 200 122 150 142 NDOFG 1.16 1.50 2.25 0.96 1.92 1.59

[0033] The normalized depth of focus gain (NDOFG) of the output beam from the probe is expressed as:

[00001]$\mathrm{NDOFG}=\frac{0.1274\lambda .Math.\mathrm{DOF}}{n.Math.{\mathrm{FHWM}}^2},$

[0034] wherein λ is a central wavelength, n is a refractive index of a medium outside the probe, the beam diameter (FHWM) is defined as the full width at half maximum of the lateral light intensity of the beam, and a depth of focus (DOF) is defined as a depth range where the beam diameter is less than twice its minimum value. For Gaussian beams, NDOFG equal to 1.

[0035] The mode phase difference at the end of the LCF is:

[00002]$\mathrm{\Delta \varphi }={\mathrm{\Delta \varphi }}_0\left(L_{\mathrm{GIF}1}\right)+\Delta \stackrel{\_}{\beta }{L}_{\mathrm{LCF}},$

[0036] wherein Δϕ.sub.0 is an initial mode phase difference at the GIF1-LCF interface. Δβ is a difference between the propagation constant of the LP.sub.02 mode and propagation constant of LP.sub.01 mode in the LCF, L.sub.GIF1 and L.sub.LCF is a length of the GIF1 and the LCF respectively.

[0037] The lengths for GIF1 and LCF in six cases listed in Table 2 are chosen with high coupling efficiency to realize negligible two-mode interference, or significant two-mode interference with distinct mode phase difference. For cases I and IV, a quarter-pitch-length of the GIF1 is chosen to excite mainly the LP.sub.01 mode with negligible LP.sub.02 mode, while the length of the LCF with insignificant effect on the MIF is determined to realize Δϕ with π difference than that in cases II and V. The lengths of the GIF1 for cases II, III, V and VI are chosen to obtain significant two-mode interference aiming to the maximized DOF gain. However, Δϕ at the end of the LCF for cases II and V are distinct from cases III and VI, which might induce different axial intensity profile of the output beam. The lengths of the NCF are chosen so that the diffracted MIF can fully fill the aperture of the following GIF3. The lengths of the GIF2 are determined by the conjugated relation between P1 and P2. The lengths of GIF3 are chosen to yield an output beam with ˜5 μm minimal beam diameter (MBD). To demonstrate controllable manipulation of the output beam, light field simulations on six typical cases of the designed probe are conducted. Beam propagation method and angular spectrum method are adopted for segmented fibers and homogeneous medium (NCF, air), respectively. The field intensity distributions of output beams in air for six cases are shown in FIG. 2, with characteristics listed in Table 2. Significant DOF gain (≥1.5) is realized in cases II, III, V and VI where mode interference is significant. Also, destructive interference in the focus region is observed in cases II and V but not available in cases III and VI due to distinct mode phase difference. Hence, the manipulation of the mode phase difference is crucial for the uniformity of the output beam. Furthermore, both uniformly focusing and maximized DOF gain are realized in case III by the magnified MIF approach, which is thus chosen for the proposed probe. Compared to the existing probe design, the proposed probe increases the WD from 130 μm to 200 μm in air.

[0038] To fabricate the probe based on case III, each optical fiber is sequentially cut by a fiber cleaver and spliced to the end of the probe by a fiber fusion splicer. A conventional probe with the same lateral resolution was also fabricated for comparison, as shown in FIG. 3(a). In order to reduce the reflection of the probe end face, a short eight-degree angle-cleaved NCF was attached to the ends of the above two probes. FIG. 3(b) shows the microscope images of the two fabricated probes. The distal optics consisting by segmented fibers with the same diameter as that of a standard SMF make the probe robust in mechanical property and flexible for applications.

[0039] In order to compare and illustrate the advantages of the probe of the present application in OCT imaging, the above two probes were connected to the established swept source OCT system, as shown in FIG. 4. The central wavelength of the swept source is 1.3 μm and the bandwidth is 100 nm. In order to achieve two-dimensional or three-dimensional imaging, the probe is kept stationary, while the sample is placed on a two-dimensional motorized linear stage for lateral scanning. The collected interference spectrums are uniformly sampled in the wavenumber domain, and the dispersion compensation algorithm as well as the fast Fourier transform are applied to obtain OCT images.

[0040] The diameters and DOFs of the output beams are calibrated by OCT imaging on a 1951 USAF resolution test target. Elements in group 6, 7 of the test target are chosen, corresponding to line pair period from 4.4 μm to 13.9 μm. The beam diameters versus depth are plotted in FIG. 5(a) for the conventional probe and FIG. 5(b) for the proposed probe, corresponding to a DOF of 103 μm and 211 μm, respectively. The target's reflectivity profiles at the probes' foci are given in FIGS. 5(c) and (d), indicating that the minimized lateral resolutions for two probes are similar and better than 4.4 μm. Thus, it can be concluded that, compared with the conventional probe, two times of DOF gain is achieved by the proposed probe without loss of lateral resolution. Also, it can be observed from FIG. 5(a)(b) that the measured WD is 100 μm for the conventional probe and 174 μm for the proposed probe. Both WDs are slightly shortened due to the mentioned appended NCFs at the end of the probes. Furthermore, no degradation in axial resolution is observed in the proposed probe within its DOF range, and the measured axial resolutions for both probes are 11.3 μm.

[0041] A probe for OCT that utilizes fiber mode interference to simultaneously achieve focal depth extension, working distance extension, and axial light intensity uniformity optimization is provided. The diameter and the length of the distal optical component of the probe is 125 μm and 2.6 mm, respectively. Compared with the conventional probe with the same lateral resolution (better than 4.4 μm), the probe of the present application has twice the depth of focus and 1.7 times the working distance. Due to the advantages of optimized imaging quality, easy manufacturing, reliable structure and flexible application scenarios, the probe of the present application has potential application in important fields.