OPTICAL COHERENCE TOMOGRAPHY SYSTEM

Abstract

Disclosed is an OCT system, in particular a mid-IR OCT system, comprising: an upconversion module configured to frequency upconvert light received or receivable by the upconversion module and which is in a wavelength range between a first wavelength and a higher second wavelength, the difference between the second wavelength and the first wavelength being at least 300 nm or larger, and the wavelength range having a center wavelength at 2.8 μm or larger, the center wavelength being defined by the average value between the first wavelength and the second wavelength.

Claims

1. An Optical Coherence Tomography (OCT) system, comprising: an upconversion module configured to frequency upconvert light received or receivable by the upconversion module and which is in a wavelength range between a first wavelength and a higher second wavelength, the difference between the second wavelength and the first wavelength being at least 300 nm or larger, and the wavelength range having a center wavelength at 2.8 μm or larger, the center wavelength being defined by the average value between the first wavelength and the second wavelength.

2. The system in accordance with claim 1, further comprising a light source, configured for providing a probe light beam, which has a spectrum that at least comprises a continuous spectral region between the first wavelength and the second wavelength.

3. The system in accordance with claim 1, wherein the wavelength range between the first wavelength and the second wavelength that can be frequency upconverted is a continuous wavelength range.

4. The system in accordance with claim 1, wherein the upconversion module is configured to employ a pump light beam having a wavelength which is smaller than the first wavelength for frequency upconverting the light in the wavelength range between the first and second wavelengths, wherein, the wavelength of the pump light beam, λ.sub.P, is in the range of 600 nm to 1.8 μm, and/or wherein the upconversion module at least partly operates by sum frequency generation using the pump light beam and light in the wavelength range, and/or wherein the upconversion module does not employ second harmonic generation (SHG) for upconverting the light.

5. The system in accordance with claim 1, wherein the difference between the first wavelength and the second wavelength is smaller than 20 μm or 15 μm or 10 μm or 5 μm or 2 μm or 1 μm.

6. The system in accordance with claim 1, further comprising an interferometer configured for receiving a probe light beam from a light source and for dividing the received probe light beam into a sample path and a reference path and for generating an interference light beam by combining probe light returning from the sample path with probe light returning from the reference path, wherein the upconversion module is configured to receive the interference light beam for generating an upconverted light beam by frequency upconversion of the interference light beam.

7. The system in accordance with claim 1, further comprising a detector configured to receive the frequency upconverted light from the upconversion module and for detecting spectral properties of the upconverted light.

8. The system in accordance with claim 1, wherein the upconversion module comprises: an upconversion element configured to enable parametric wavelength conversion, where the upconversion element comprises a quadratic nonlinear material; and/or a pump source arranged for launching a pump light beam into the upconversion element.

9. The system in accordance with any claim 4, wherein the pump light beam provided by the pump source to the upconversion element is pulsed or continuous, or wherein the upconversion module is adapted to employ a part of the probe light or of the interference light as a pump light beam in the pump upconversion module, and wherein, a continuous wave pump light beam has a spectral width of not more than 1 nm or not more than 0.5 nm.

10. The system in accordance with claim 2, wherein the probe light beam is pulsed or continuous, and wherein, pulses of a pump light beam used in the upconversion module are synchronized with pulses of the probe light beam.

11. The system in accordance with claim 1, wherein the upconversion module at least partly operates by collinear or non-collinear interaction between the interference light beam and a pump light beam.

12. The system in accordance with claim 1, wherein the probe light beam and/or the pump light beam is focused within an upconversion element of the upconversion module, or wherein the probe light beam and/or the pump light beam is not focused or unfocused or non-focused within the upconversion element, wherein, the probe light beam and/or the pump light beam travels as a collimated beam through the upconversion element.

13. The system in accordance with claim 7, wherein the detector is configured to detect light within a range of wavelengths extending from 390 nm to 2 μm.

14. The system in accordance with claim 7, wherein the detector comprises a spectrometer.

15. The system in accordance with claim 1, further comprising a long-pass filter arranged to block wavelengths in the light received from a broadband light source of the system below a defined cut-on wavelength.

16. A method for analyzing an object using an OCT system in accordance with claim 1, the method comprising: providing a probe light beam, which is a mid-IR probe light beam, dividing the probe light beam into a sample path and a reference path, where the probe light in the sample path is projected onto the object; generating an interference light beam by combining probe light returning from the sample path with probe light returning from the reference path; generating an upconverted light beam by frequency upconversion of the spectral components in the interference light beam which are in a wavelength range between a first wavelength and a higher second wavelength, the difference between the second wavelength and the first wavelength being at least 300 nm or larger, wherein the wavelength range have a center wavelength at 2.8 μm or larger, the center wavelength is defined by the average value between the first wavelength and the second wavelength, and detecting the spectral properties of the upconverted light beam.

17. The system in accordance with claim 2, wherein the system is a mid-infrared (mid-IR) OCT system and the light source is a mid-IR broadband light source.

18. The system in accordance with claim 4, wherein the wavelength of the pump light beam, λ.sub.P, is in the range of 800 nm to 1.5 μm.

19. The system in accordance with claim 13, wherein the detector is configured to detect light within a range of wavelengths selected from the group consisting of the range of 390 nm to 900 nm and the range of 900 nm to 1600 nm.

20. The system in accordance with claim 14, wherein the spectrometer is a silicon-based, Ge-based or InGaAs-based spectrometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] The above and/or additional objects, features and advantages of the present disclosure, will be further elucidated by the following illustrative and exemplary detailed description of exemplary embodiments of the present disclosure, with reference to the appended drawings, wherein:

[0101] FIG. 1 shows an OCT system.

[0102] FIG. 2 shows a flowchart for a method.

[0103] FIG. 3 shows examples of spectra.

[0104] FIG. 4 shows schematically an example of an optical setup for upconversion.

[0105] FIG. 5 shows schematically a further example of an optical setup for upconversion.

[0106] FIG. 6 shows a graph showing imaging depth as a function of spectral sampling.

DETAILED DESCRIPTION

[0107] FIG. 1 shows an overview of an embodiment of the OCT system operating with probe light having a center wavelength of 4 μm and using a NIR/VIS detector. The illustrated OCT system 100 has five modular parts: a broadband light source 101, a Michelson interferometer 102, a scanning sample translation system 103, a frequency upconversion module 104, and a silicon CMOS-based spectrometer 105. Each optical subsystem is connected via optical fiber to ease the coupling and alignment between subsystems. This is only an option. Also free-space connections between subsystems are possible.

[0108] The broadband light source 101 has a supercontinuum source based on a 1.55 μm master-oscillator power amplifier (MOPA) 106 pump laser and a zirconium fluoride fiber 107. Optionally, the fiber 107 may be a single-mode fiber, for example in the 3.5-4.5 μm region. The MOPA is for example a four-stage MOPA using an unfolded double-pass amplifier configuration based on a 1.55 μm directly modulated seed laser diode. The seed pulse duration is for example 1 ns, and the repetition rate is for example tunable between 10 kHz and 10 MHz. The seed is for example subjected to three stages of amplification in erbium-doped and erbium-ytterbium-doped silica fibers, which extend the spectrum to 2.2 μm by in-amplifier nonlinear broadening. Preferably, in order to further push the spectrum towards longer wavelengths, the erbium fiber is spliced to approximately 40 cm of 10 μm core diameter thulium-doped double-clad fiber which extends the supercontinuum spectrum to 2.7 μm. Further preferably, the thulium-doped fiber is subsequently spliced to a short piece of silica mode-field adapter fiber having a mode-field diameter of 8 μm, which provides a better match to the fluoride fiber 107. The mode adapter fiber is butt-coupled to a 6.5 μm core diameter single-mode ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber with a short length of around 1.5 m to reduce the effect of strong multi-phonon absorption beyond 4.3 μm.

[0109] The supercontinuum source 101 for example produces a continuous spectrum from 0.9-4.7 μm and is for example set to operate at 1 MHz pulse repetition rate generating 40 mW of average power above 3.5 μm. The spectral components below 3.5 μm may be blocked by a long-pass filter of the broadband light source (not illustrated in the figure for reason of simplicity), such that the probe light provided to the interferometer is a broadband spectrum with for example a bandwidth of 1.2 μm and a center wavelength at 4 μm with an average power of 20 mW being coupled to the sample arm of the interferometer.

[0110] The interferometer 102 is based on a Michelson design employing as an example a gold coated parabolic mirror collimator 108, a broadband CaF.sub.2 wedged plate beam splitter 109, a BaF.sub.2 plano-convex lens 110 in the sample arm, and a BaF.sub.2 window and flat silver mirror 111 in the reference arm. The BaF.sub.2 lens 110 may be chosen to minimize the effect of dispersion, while having a relatively short focal length of 15 mm. At 4 μm the dispersion of BaF.sub.2 is relatively low at 16.4 ps nm.sup.−1 km.sup.−1 compared to other standard lens substrates, such as CaF.sub.2 (33.0), Si (−45.8), and ZnSe (−59.9), but most importantly the dispersion slope is flat from 3.5-4.5 μm (13.6-19.1 ps nm.sup.−km.sup.−1). Even so, the residual dispersion from the 6.3 mm center thickness lens is roughly compensated by a 5 mm window and the remaining dispersion is compensated numerically. Coupling to the upconversion module 104 is performed using for example a 6 mm focal length aspheric chalcogenide lens 112 and for example a 9 μm core diameter single-mode indium fluoride patch cable 113.

[0111] Optics

[0112] The probe light beam in the sample arm of the interferometer is focused onto the analyzed object using for example a barium fluoride (BaF.sub.2) lens 110, and 2D and 3D images are acquired by moving the sample using a sample translation system 103 having motorized translation stages for moving the analyzed object in a plane perpendicular to the incoming probe light. The interfering sample and reference signals are then collected in the indium fluoride fiber 113, which may for example be a single-mode fiber, and the generated interference light beam relayed to the upconversion module 104 for frequency upconversion to the near-IR wavelength range. The upconverted light beam is then coupled to a silica fiber 114 and imaged onto the spectrometer 105 to resolve the spectrum. The silica fiber 114 is preferably a single-mode, or alternatively a multi-mode fiber.

[0113] Upconversion

[0114] The illustrated OCT system 100 operates with a center wavelength of 4 μm with more than 1 μm spectral bandwidth. Accordingly, the upconversion module is designed and optimized to upconvert the entire spectral range from 3.6-4.6 μm for fastest detection. Quasi-phase matching in a periodically poled lithium niobate (PPLN) crystal is used for the broadband upconversion, owing to its design flexibility, access to a high d.sub.eff (14 pm/V), and optical transparency up to 5 μm.

[0115] The upconversion takes place inside the PPLN crystal, where each wavelength is phase-matched at a different propagation angle. Thus non-collinear interaction among the three participating light beams is considered to phase-match over a wide spectral range. As the wavelengths of the upconverted light beam are below the pump wavelength, by choosing the pump wavelength at 1 μm, a spectrometer 105 employing conventional Si-CMOS detection can be employed for the detection of the upconverted light beam. Here, a solid state (Nd:YVO.sub.4) continuous wave (CW) linearly polarized laser operating at 1064 nm is used as the pump source. This pump source is for example driven by a broad area emitting laser diode (3 W, 880 nm). A high finesse folded solid state laser cavity may be formed by mirrors that all are HR-coated for 1064 nm and AR-coated for 700-900 nm. One mirror may act as output coupler for the upconverted light beam while other mirrors may be placed in a separate compartment to filter out the fluorescence from the laser crystal and the 1064 nm pump laser. The PPLN crystal is preferably a 20 mm long, 5% MgO-doped PPLN crystal (Covesion, preferably AR coated for 1064 nm, for example 2.8-5.0 μm on both facets). The PPLN crystal may consist of five different poling periods (Λ) ranging from 21-23 μm in steps of 0.5 μm. Each poled grating may have a 1 mm×1 mm aperture and may be separated by 0.2 mm wide regions of un-poled material. For different values of Λ, the phase-mismatch and hence the overall upconversion spectral bandwidth varies. Wider bandwidth requires larger input angles for the mid-infrared beam, which reduces the overall Quantum Efficiency (QE) as the effective interaction length is reduced. A CW intracavity power of >30 W at 1064 nm may be realized with a spot size (beam radius) of 180 μm inside the PPLN crystal. The mid-IR light (output of the fiber coupled 4 μm OCT signal), i.e. the interference light beam, is collimated and then focused into the PPLN crystal using for example a pair of CaF2 aspheric lenses (f=50 mm, AR coated for 2-5 μm). The upconverted light is collimated for example by a silica lens (f=75 mm, AR coated for 650-1050 nm). A short-pass (SP) 1000 nm and a long-pass (LP) 800 nm filter is for example inserted to block the leaked 1064 nm beam and 532 nm parasitic second harmonic light, respectively. The upconversion module may be able to upconvert all wavelengths in a relatively broad spectral range of 3.6-4.8 μm to a wavelength range of 820-870 nm, where fast and cost-efficient detectors can capture the spectral distribution of the upconverted light beam. The upconversion module may provide a fast generation of the upconverted light beam from the interference light beam to a shorter wavelength. The entire system is operated at room temperature.

[0116] Detection, Scanning and Data Processing

[0117] After the upconversion module 104 the near-IR light may be collected by a 50 μm core multimode silica fiber 114 guiding the light to a line scan spectrometer 105 (Cobra UDC, Wasatch Photonics, USA) operating for example with a maximum line rate of 45 kHz (for a bit depth of 10). The spectral range may cover wavelengths of 796 nm to 879 nm, which is for example sampled by 4096 pixels. To scan the sample, this is mounted on a double translation stage 103 (e.g., 2×ILS50CC from Newport) with for example a maximum travel speed of 100 mm/s, a travel range of 50 mm and a stepping resolution of 1 μm. The detected raw spectra are dark signal subtracted and normalized to the reference arm signal. Pixel to wavenumber translation and interferometer dispersion compensation is achieved by exploiting phase information across the pixel array retrieved for two reference interferograms showing clear interference fringes. In this way spectral resampling is performed to linearize wavenumber sampling after which a phase shift is applied for compensating the unevenly matched dispersion in the arms of the interferometer. To suppress effects stemming from the spectral envelope of the interferograms, a Hanning spectral filter is applied to the spectral region of the interferometric signals. Finally a fast Fourier transform (FFT) is applied to generate a reflectivity profile, a so-called A-scan. A compromise between signal strength and acquisition time is made that leads to an A-scan acquisition time of 3 ms. To build B-scans (2D images), the horizontal stage (X) is programmed to move continuously over a specified distance, achieving a 500 line B-scan in 1.5 seconds. 3D scans are built by stepping the vertical (Y) stage at a proportionate slower rate to assemble multiple B-scans.

[0118] As mentioned above, the sample can be scanned by mounting the sample on translation stage 103. The scanning can also be carried out by moving one or more optical elements, such as mirrors, in such a way that the light scans over the sample. In particular, a galvanometric scanning device could be employed. The galvanic scanning device could be coupled to one or two or more scanning mirrors and the device could control the one, two or more mirrors to move the light beam over the sample surface. Galvanic scanning could be fast and might help to remove artefacts from the image of the sample.

[0119] FIG. 2 shows a flowchart 220 of a method for recording OCT data, such as for determining an A scan of an analyzed object.

[0120] In step 221, the broadband probe light beam is projected onto the analyzed object and a reference element. The probe light beam is generated by passing light from a supercontinuum source as the one described above in relation to FIG. 1 through a long-pass filter. In the example of FIG. 1, the supercontinuum source provides light over a wavelength range from 0.9 μm to 4.7 μm which is wider than the bandwidth of the upconversion system that extends for example from 3.5 μm to 4.7 μm. To avoid heating of, e.g., optical components by wavelengths outside the bandwidth of the upconversion system, the supercontinuum is sent though a long-pass filter narrowing the bandwidth of the probe light to 3.5-4.7 μm. The probe light is launched from the broadband light source into an interferometer where a beam splitter divides the probe light into sample path and a reference path. The analyzed object is placed in the sample path such that the corresponding portion of the probe light is projected onto the object. The remaining portion propagates to the reference element which is located in the reference path and is reflected therefrom to interfere with probe light backscattered from the analyzed object to generate an interference light beam (step 222). The generated interference light beam covers substantially the same wavelength range as the truncated probe light spectrum (i.e. after the long-pass filter), such that the interference light beam primarily is at mid-IR wavelengths.

[0121] Detectors operating in the mid-IR range are significantly more expensive and much slower than detectors operating in the near-IR or visible wavelength range. In order to enable detection of the interference light beam using such low-cost and fast visible/near-IR detectors, the interference light beam is frequency upconverted from the wavelength range of the probe light to the near-IR and/or visible wavelength range in step 223.

[0122] The upconversion is performed by launching the interference light beam into a nonlinear crystal which is simultaneously pumped by an upconversion pump beam. The pump beam has a narrow linewidth, preferably single-frequency, to ensure that the upconversion does not cause a blurring of the spectral characteristics of the interference light beam. The pump beam and the interference beam interact through sum frequency generation such that photons of the upconversion light beam having a wavelength λ.sub.UP according to:

λ.sub.P.sup.−1+λ.sub.IR.sup.−1=λ.sub.UP.sup.−1

are generated, where λ.sub.UP is the pump wavelength and λ.sub.IR is the wavelength of a photon of the received interference light beam. The upconversion generates a compressed version of the spectrum of the interference light beam having similar spectral structures as the interference light beam and containing the same interferometric information, with the generated upconverted light beam at wavelengths below the pump wavelength. Low-cost powerful pump sources emitting light at a pump wavelength of 1064 nm are available. Using such a pump source provides that the spectral distribution of the generated upconverted light beam is at wavelengths where fast and low-cost near-infrared/visible detectors operate.

[0123] In step 224, the spectrum of the upconverted light beam is recorded using a detector operating in the near-IR and/or visible wavelength range. An upconverted light beam spectrum can be recorded for each position of the probe light beam on the analyzed object.

[0124] A so-called A-scan of the object can be determined by analysis of the recorded spectrum (optional step 225). The A-scan expresses the variations in the refractive index of the object from the surface and below, with a penetration depth determined from wavelengths of the mid-IR probe light into the object and an axial resolution which is improved by the large bandwidth if the probe light.

[0125] FIG. 3 shows examples of spectra. In FIG. 3A, the dotted line 331 shows the 0.9-4.7 μm supercontinuum generated by the MOPA pump laser and zirconium fluoride fiber described above in relation to FIG. 1 while the solid line 332 shows the truncated probe light spectrum extending between 3.5 μm and 4.7 μm defined by using a long-pass filter to block the part of the supercontinuum light below 3.5 μm. The probe light provided to the interferometer and projected onto the analyzed sample is hence the truncated supercontinuum having a bandwidth 333 of 1.2 μm, with wavelengths between λ.sub.1 and λ.sub.2, and with a center wavelength, λ.sub.C, around 4.1 μm.

[0126] From the interferometer at least a portion of this probe light is projected onto the analyzed object and probe light backscattered from the object is captured and allowed to interfere with light from the reference arm, as also illustrated in FIG. 1.

[0127] The resulting interference light beam has a spectrum 336 with a center wavelength λ.sub.C,int illustrated in FIG. 3B and carries interferometric information expressing the refractive indices of sub-surface structures of the analyzed object. The center wavelength being the average value between λ.sub.1 and λ.sub.2.

[0128] The upconversion module is configured to frequency upconvert light in the wavelength range between λ.sub.1 and λ.sub.2. Thus, the truncated supercontinuum is filtered out in such a way that at least in substance all wavelengths in the truncated supercontinuum can be upconverted by the upconversion module. The truncated supercontinuum may also be spectrally broader, but only the wavelengths between λ.sub.1 and λ.sub.2 are subject for upconversion by the upconversion module.

[0129] The upconversion of the interference light beam is driven by the pump beam of the upconversion module with a wavelength X.sub.p=1064 nm (indicated by the dotted line in FIG. 3B) and shifts the spectrum to wavelengths below λ.sub.p and simultaneously compresses the interference beam spectrum such that a single detector unit operating in the visible/near-infrared region can be used for deriving the interferometric information from the upconverted light beam spectrum 337. In the illustrated example the wavelength of the pump light beam λ.sub.p is below λ.sub.1 by a factor of more than 3, thus causing a large shift in the wavelengths of the generated upconverted beam compared to the wavelengths of the interference light beam.

[0130] The optical setup as illustrated in FIG. 4 or FIG. 5 may be employed in an OCT system 100 as shown in FIG. 1.

[0131] Now referring to FIG. 4, a pump laser 401 is used as a pump source and provides a continuously operating pump light beam 403 that has a spectral width dλ.sub.ML as illustrated in graph 405 which shows the signal strength of the pump light beam 403 as a function of wavelength. The spectral width dλ.sub.ML corresponds to the full-width half maximum (FWHM) value of the signal shown in graph 405. The pump laser 401 can for example be a Nd:YAG laser providing a pump beam with a wavelength at 1064 nm.

[0132] The pump light beam 403 is reflected from mirror 411 which is transparent for an interference light beam 407 or 409 so that the pump light beam 403 and the interference light beam 407 or 409 travel collinearly through a nonlinear medium 413 used for upconversion in upconversion module 104. The pump light beam 403 and the interference light beam 407 or 409 travel through the nonlinear medium 413 in a non-focused fashion. More specifically, the pump light beam 403 and the interference light beam 407 or 409 travel collinearly through the nonlinear medium 413 as collimated beams.

[0133] In the nonlinear medium 403, a parametric process, such as a sum-frequency generation process, can cause the generation of an up-converted light beam 407a, 409a from the respective light beam 407, 409.

[0134] A focusing lens 415 is arranged to focus the up-converted light beam 407a, 409a into an entrance of spectrometer 105. The entrance can be formed by a free space entrance window, such as a pinhole, or by a fiber front face.

[0135] A graph 417 shows signal strengths as a function of wavelength as detected by the spectrograph 105 for the up-converted light beam 407a and the up-converted light beam 409a. As an example, the light beam 407 might include a wavelength Al in the mid-infrared region. The light beam 409 might include a wavelength λ.sub.2 in the mid-infrared region (MIR). This wavelengths are up-converted to respective wavelengths λ.sub.1* and λ.sub.2* in the near-infrared region (NIR) and present in the respective beams 407a, 409a.

[0136] As shown in the graph 417, the two up-converted wavelengths λ.sub.1* and λ.sub.2* can be spectrally resolved from each other if the distance between λ.sub.1* and λ.sub.2* is larger than the spectral width dλ.sub.ML of the pump light beam 403. It is therefore advantageous to employ a cw-laser beam as pump light beam having a very small spectral width dλ.sub.ML, for example a spectral width dλ.sub.ML which is smaller than 0.5 nm.

[0137] The optical setup as shown in FIG. 5 differs from the setup of FIG. 4 in that the respective interference light beam 407, 409 is focused by use of focusing lens 419 in the nonlinear medium 413. The wavelength λ.sub.1 in the beam 407 may therefore be focused to focal point F1, whereas the wavelength λ.sub.2 in the beam 409 may be focused to focal point F2 which is different from F1.

[0138] An optical system 421 is employed to focus the respective up-converted light beam 407a, 409a into the entrance of spectrometer 105. Due to the different wavelengths λ.sub.1* and λ.sub.2 in the up-converted light beams, light beam 407a is focused to focal point F1* and light beam 409a is focused to focal point F2* which is different from F1*.

[0139] In view of the above, the setup of FIG. 4 is advantageous over the setup of FIG. 5, since in the setup of FIG. 4, focusing takes only place for coupling into the spectrometer, but the interference light beam is not focused in the nonlinear medium 413. Thus, the setup of FIG. 4 is better suited for resolving interference signals that include a larger range of frequencies.

[0140] FIG. 6 shows two signals 601 and 603 related to the imaging depth in millimeters as a function of the near-infrared spectral sampling in nanometers. The signal 601 is obtained from light at a center wavelength of 4000 nm and mixed with a pump beam at a wavelength of 1064 nm. The signal 603 is obtained from light at a center wavelength of 7000 nm and mixed with a pump beam at a wavelength of 1064 nm.

[0141] The near infrared spectral sampling corresponds to the line width (spectral width) of the pump signal. As shown, the imaging depth increases with decreasing line width. Preferably, a cw-pump beam having a line width of less than 0.5 nm is employed in order to obtain a high imaging depth.

LIST OF SELECTED REFERENCE NUMBERS

[0142] 100 system

[0143] 101 supercontinuum source

[0144] 102 interferometer

[0145] 103 translation system

[0146] 104 upconversion module

[0147] 105 spectrometer

[0148] 106 master-oscillator power amplifier

[0149] 107 zirconium fluoride fiber

[0150] 108 mirror collimator

[0151] 109 beam splitter

[0152] 110 lens focusing probe beam onto object

[0153] 111 reflective element

[0154] 112 lens collecting light for upconversion

[0155] 113 patch cable

[0156] 114 multi-mode fiber

[0157] 401 pump laser

[0158] 403 pump light beam

[0159] 405 graph

[0160] 407 light beam

[0161] 407a up-converted light beam

[0162] 409 light beam

[0163] 409a up-converted light beam

[0164] 411 mirror

[0165] 413 nonlinear medium

[0166] 415 lens

[0167] 417 graph

[0168] 419 lens

[0169] 421 optical system

[0170] 601 signal

[0171] 603 signal

[0172] F1, F1* focal point

[0173] F2, F2* focal point