TUNABLE LASER AND METHOD TO TUNE A WAVELENGTH OF A LIGHT EMITTED BY THE LASER

Abstract

The present invention relates to a method to tune a wavelength of a coherent light signal emitted by a tunable laser, the tunable laser comprising: a cavity, the cavity including: a gain medium, an optical tunable filter, a first and a second mirrors, one of which is partially reflective, wherein the optical tunable filter includes: a first and a second electrodes, a liquid crystal, the method comprising: applying a voltage difference between the first and second electrodes to apply an electric field to the liquid crystal; wherein applying a voltage difference includes: applying the voltage difference for at least a driving time interval lasting less than 1 μs; and varying the voltage difference applied between the first and second electrodes within the driving time interval so that a maximum applied voltage difference is reached and said maximum applied voltage is above 0.1 kV.

Claims

1-20. (canceled)

21. A method to tune a wavelength of a coherent light signal emitted by a tunable laser, the tunable laser comprising: a cavity, the cavity including: a gain medium, an optical tunable filter, a first and a second mirrors, one of which is partially reflective, wherein the optical tunable filter includes: a first and a second electrodes, a liquid crystal, the method comprising: applying a voltage difference between the first and second electrodes to apply an electric field to the liquid crystal; wherein applying a voltage difference includes: applying the voltage difference for at least a driving time interval lasting less than 1 μs; and varying the voltage difference applied between the first and second electrodes within the driving time interval so that a maximum applied voltage difference is reached and said maximum applied voltage is above 0.1 kV.

22. A tunable laser comprising: a cavity, the cavity including: a gain medium, an optical tunable filter, a first and a second mirrors, one of which is partially reflective, Wherein the optical tunable filter includes: a first and a second electrodes, a liquid crystal, the liquid crystal being subject to an electric field created by the first and second electrodes, and a voltage generator (160) to apply a voltage difference between the first and second electrodes, said voltage generator being programmed for: applying the voltage difference for at least a driving time interval lasting less than 1 μs; and varying the voltage difference applied between the first and second electrodes within the driving time interval so that a maximum applied voltage difference is reached and said maximum applied voltage is above 0.1 kV.

23. Method or tunable laser according to claim 21, wherein applying a voltage difference includes applying the voltage difference for at least a driving time interval lasting between 1 ns and 1 μs.

24. Method or tunable laser according to claim 21 wherein the liquid crystal comprises a nematic liquid crystal.

25. Method or tunable laser according to claim 21, wherein the liquid crystal or the nematic liquid crystal is doped with polar additive.

26. Method or tunable laser according to claim 21, wherein varying the voltage difference applied to the first and second electrodes within a driving time interval includes varying the voltage difference between a minimum and a maximum.

27. Method or tunable laser according to claim 26, wherein varying the voltage difference applied to the first and second electrodes within a driving time interval includes varying the voltage difference linearly with time.

28. Method or tunable laser according to claim 26, wherein varying the voltage difference applied to the first and second electrodes within a driving time interval includes: varying the voltage difference between a minimum value to a maximum value; and varying the voltage difference between the maximum value to the minimum value afterwards.

29. Method or tunable laser according to claim 26, wherein varying the voltage applied to the first and second electrodes within a driving time interval includes: varying the voltage difference between a minimum value to a maximum value; interrupting the voltage difference application.

30. Method or tunable laser according to claim 21, wherein the applied voltage difference has a repetition rate comprised between 100 kHz and 100 Mhz.

31. Method or tunable laser according to claim 21, wherein the optical tunable filter (150) further comprises a first and a second high reflectivity dielectric layer sandwiching the liquid crystal.

32. Method or tunable laser according to claim 31, wherein the first and/or second high reflectivity dielectric layer defines a reflectivity, the reflectivity being above or equal to 95%.

33. Method or tunable laser according to claim 21, wherein the liquid crystal defines a liquid crystal thickness, and wherein the distance between the first and second electrodes is comprised between 10 micron and 200 micron.

34. Method or tunable laser according to claim 25, wherein the polar addictive has a negative dielectric anisotropy.

35. Method or tunable laser according to claim 21, wherein the first and the second electrodes are at least partially transparent to a radiation having a given wavelength resonating in the cavity.

36. Method or tunable laser according to claim 21, wherein: the first mirror is partially reflective and the second mirror is substantially totally reflective; the first mirror defines a mirror free spectral range and the optical tunable filter defines an filter free spectral range; and wherein the mirror free spectral range is of the same order of magnitude of the filter free spectral range.

37. Method or tunable laser according to claim 36, wherein the filter free spectral range and/or the mirror free spectral range is >20 nm.

38. An Optical Coherence Tomography (OCT) system including: the tunable laser according to claim 22, emitting a coherent light signal; an optical interferometer for dividing the coherent light signal into a reference arm leading to a reference reflector and a sample arm leading to a sample; an optical element to selectively direct a sample light signal exiting the sample arm to a specific portion of the sample, so that for each selection in the optical element a different specific portion of the sample is illuminated; an optical detector for detecting an interference signal generated by a combination of reference and sample returning signals from the reference arm and from the sample arm, reflected by the reference reflector and the sample, respectively.

39. The OCT system according to claim 38, further including: a processing unit, said processing unit being programmed for, for the same selection in the optical element illuminating a specific portion of the sample: i. defining a sweeping time interval ΔT; ii. changing the coherent light signal leading to the sample light signal illuminating the specific portion of the sample from a minimum wavelength to a maximum wavelength and in the same sweeping modifying the wavelength of the coherent light signal so that it reaches the same value between the minimum wavelength to the maximum wavelength at least twice during the sweeping; iii. elaborating the detected interference signal for obtaining an OCT image of the specific portion of the sample.

40. An optical coherence tomography analysis method, comprising: providing an Optical Coherence Tomography system according to claim 38, wherein the method further comprises: sweeping the tunable laser for a time interval ΔT, so that a wavelength of the coherent light signal leading to the sample light signal illuminating the specific portion of the sample changes from a minimum wavelength to a maximum wavelength, wherein said sweeping includes: applying a voltage difference between the first and second electrodes to apply an electric field to the liquid crystal; wherein applying a voltage difference includes: applying the voltage difference for at least a driving time interval lasting less than 1 μs; and varying the voltage difference applied between the first and second electrodes within the driving time interval so that a maximum applied voltage difference is reached and said maximum applied voltage is above 0.1 kV; detecting the interference signal generated by the sweeping; elaborating the detected interference signal generated by the sweeping to obtain an OCT image of the specific portion of the sample.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0209] The present invention will be better understood with non-limiting reference to the appended drawings, where:

[0210] FIG. 1 represents a behavior of the variation of the wavelength (λ) over time (t) in a light source according to the prior art;

[0211] FIG. 2 is a schematic representation of a SS-OCT system according to the invention;

[0212] FIG. 3A is a detail of the system of FIG. 2;

[0213] FIG. 3B is a detail in enlarged view of FIG. 3A;

[0214] FIG. 4 represents as a solid line a first embodiment of a behavior of the variation of the wavelength (Δλ), expressed in nanometers, over time (t) in a light source of system of FIGS. 2 and 3A-B according to the present invention, the shown dotted line represents the signal of FIG. 1;

[0215] FIG. 5A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the prior art;

[0216] FIG. 5B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 4 is used to illuminate the same portion of the same sample of FIG. 5A according to the invention;

[0217] FIG. 5C represents the superposition of FIGS. 5A and 5B;

[0218] FIG. 6 represents a second embodiment of a behavior of the variation of the wavelength (Δλ), expressed in nanometers, over time (t) in a light source of system of FIGS. 2 and 3A-B according to the present invention, the shown dotted line represents the signal of FIG. 1 according to prior art;

[0219] FIG. 7A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the prior art;

[0220] FIG. 7B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 6 is used to illuminate the same portion of the same sample of FIG. 7A according to the invention;

[0221] FIG. 7C represents the superposition of FIGS. 7A and 7B;

[0222] FIG. 8A represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 1 is used to illuminate a portion of a sample according to the invention, where two reflections are present;

[0223] FIG. 8B represents the amplitude (A) in arbitrary units of the resulting interference signal over time (t), expressed in microseconds, when the signal of FIG. 6 is used to illuminate the same portion of the same sample of FIG. 8A according to the invention;

[0224] FIG. 8C represents the superposition of FIGS. 8A and 8B;

[0225] FIG. 9A shows the amplitude (A) in arbitrary units of the fast Fourier transform (FFT) over frequency (f) in arbitrary units for the interference signal of FIG. 8A;

[0226] FIG. 9B shows the amplitude (A) in arbitrary units of the fast Fourier transform (FFT) over frequency (f) in arbitrary units for the interference signal of FIG. 8B; and

[0227] FIG. 9C shows the superposition of FIGS. 9A and 9B.

DESCRIPTION OF PREFERRED DETAILED EMBODIMENTS OF THE INVENTION

[0228] In FIG. 2, an optical coherence tomography scanner 100 for SS-OCT is illustrated. The scanner is used to illuminate a sample 110, a typical sample being tissues at the back of the human eye.

[0229] The scanner 100 includes a spatially coherent source of light, 101. This source is preferably a Swept laser Source.

[0230] Further, the scanner includes an interferometer 105, for example including two arms called reference and sample arms, 103, 104 realized with optical fibers.

[0231] Light from source 101, i.e. a coherent light signal, is routed to illuminate the sample 110 via the sample arm 104 of the interferometer 105. Further, the light from source 101 illuminates a reference reflector 106 via the reference arm 103.

[0232] The scanner 100 further includes an optical element 107 positioned between the end of the sample arm 104 and the sample 110. The optical element is able to scan light exiting the arm 104 on the sample 110, so that the beam of light (dashed line 108) sweeps over the area or volume to be imaged. This area or volume of the sample which is imaged at a given time by the optical element is called selected portion of the sample 110.

[0233] The direction of light propagation of the light towards the sample outputted from the sample arm defines a Z direction or depth. A plane perpendicular to it, where the sample lies at least partially, defines a (X, Y) plane.

[0234] Light scattered from the sample 110 is collected, typically into the same sample arm 104 used to route the light for illumination of the selected portion of the sample 110.

[0235] Reference light derived from the same source 101 travels a separate path, involving reference arm 103. The light outputted by the reference arm 103 is reflected by a reflector 108. A reflected light from the reflector is thus travelling backwards in the reference arm 103.

[0236] These two “returning” sample and reference lights back-propagating in the sample and reference arms 103, 104 are collected. Collected sample returning light is combined with collected reference returning light, typically in a fiber coupler 111, to form interference light which is routed to a detector 120, such as a photodiode. The output from the detector 120 is supplied to a processor 130. The results can be stored in the processor.

[0237] The interference causes the intensity of the interfered light to vary across the spectrum. For any scattering point in the sample, there will be a certain difference in the path length between light from the source and reflected from that point, and light from the source traveling the reference path. The interfered light has an intensity that is relatively high or low depending on whether the path length difference is an even or odd number of half-wavelengths, as these path length differences result in constructive or destructive interference respectively. Thus the intensity of the interfered light varies with wavelength in a way that reveals the path length difference; greater path length difference results in faster variation between constructive and destructive interference across the spectrum.

[0238] The Fourier transform of the interference spectrum reveals the profile of scattering intensities at different path lengths, and therefore scattering as a function of depth in the sample.

[0239] The profile of scattering as a function of depth is called an axial scan (A-scan). A set of A-Scans measured at neighbouring locations (various selected portions) in the sample produces a cross-sectional image (tomogram) of the sample.

[0240] The range of wavelengths at which the interference is recorded determines the resolution with which one can deter mine the depth of the scattering centers, and thus the axial resolution of the tomogram.

[0241] A more detailed view of the laser source 101 used in the scanner 100 according to the invention is depicted in FIG. 3A. The laser source, in order to tune the wavelength of the emitted signal, uses a liquid crystal 150 based etalon with a Free Spectral Range of 25 nm and a frequency response of around 10 MHz.

[0242] The laser source 101 includes a cavity 141 delimited by a first and a second mirror. The first mirror 142 is a highly reflective mirror, while the second mirror 143 is a partially transparent mirror having a mirror FSR and has the function of output coupler. The output of the etalon 150 is indicated with 146 in the figure.

[0243] The cavity further includes a gain medium or gain chip 144, pumped in a known way, and a collimating lens 145 to focus the light on the etalon 150. Etalon 150 is connected to a voltage generator 160.

[0244] The processor 130 connected to the laser changes the etalon driving voltage via the voltage generator 160 so that, during an A-scan, the wavelength of the coherent light signal emitted from the laser 101 changes according to the invention.

[0245] In FIG. 3B, a more detailed view of the etalon 150 is shown in an enlarged view.

[0246] The etalon 150 includes a liquid crystal element 151. The liquid crystal element may include any of: CCN-47, MLC-20180, HNG715600-100 available from Nematel GmbH (Germany), Merck (USA), Jiangsu Hecheng Display technology (china), respectively.

[0247] The liquid crystal element 151 is preferably doped with a polar addictive, preferably 2, 3-dicyano-4-pentyloxyphenyl 4′-pentyloxybenzoate (DPP), CAS 67042-21-1 available from UAB Tikslioji Sinteze, Lithuania.

[0248] More information about the used liquid crystal material can be found in “Enhanced nanosecond electro-optic effect in isotropic and nematic phases of dielectrically negative nematics doped by strongly polar additive”, published in Journal of Molecular Physics, December 2017, written by Bingxian Li et al.

[0249] Two opposite sides of the LC element 151 are coated with a high reflectivity dielectric multilayer 152 and the resulting structure is sandwiched between two electrodes 153 attached to the voltage generator 160. The reflectivity of the multilayer is of about 95%.

[0250] Two glass slabs 154 then closes the etalon 150.

[0251] The voltage generator applies a suitable voltage to the electrodes 153 so that the refractive index of the LC 151 changes. A linear voltage variation implies a linear change in the wavelength of the output 146. Voltage difference values applied to the electrodes vary between 0 and few kV.

[0252] In FIG. 4, a first preferred embodiment of the sweeping for an A scan which last ΔT is shown, the sweeping duration ΔT is divided is sub intervals of equal duration Δt.

[0253] In this embodiment, as visible in the figure, in each of these sub intervals of duration Δt, the wavelength of the coherent light output 146 is increased linearly and monotonously for a duration Δt.sub.A. Further, in the same sub sweeping interval, the wavelength is decreased linearly and monotonously for a duration Δt.sub.B where preferably Δt.sub.B<<Δt.sub.A. The resulting wavelength behaviour of the wavelength of the coherent light signal 146 over t is a periodic function in time with period Δt=Δt.sub.A+Δt.sub.B. The wavelength defines substantially, if Δt.sub.B<<Δt.sub.A, a slightly “deformed” sawtooth function of time as represented in FIG. 4. The sawtooth scan can be made or with a very fast reset of the tuneable filter 150 if the electro-optical material is enough fast or using a beam splitter for dividing a the light source in two or more portions and an optical delay line(s) to combine said portions in a sawtooth profile line and an optical switch with the laser working in a symmetric way as in the preferred embodiment.

[0254] Therefore, the voltage difference is applied to the electrodes for a time Δt before being switched off and starting again to be applied for another Δt. Within each Δt, the applied voltage difference varies linearly between 0 kV and 1 kV (at the maximum before being decreased again).

[0255] In FIG. 4, the prior art tuning of the wavelength is also shown (linear dashed curve equivalent to FIG. 1), where the wavelength linearly increase for the whole duration of the sweeping ΔT.

[0256] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 4 is swept over the selected portion of the sample is depicted in FIGS. 5A and 5B, in the prior art result in FIG. 5A and the present invention case in FIG. 5B. Further, in FIG. 5C a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0257] In FIG. 5A, prior art case, the interference signal is a sinusoid.

[0258] In FIG. 5B, the interference signal shows a sinusoid and some “noise portions”. It is possible to see from FIG. 5B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. This portions are thus preferably discarded. These regions correspond to the portions Δt.sub.B of the sub sweeping intervals. However, it can also be seen that in the remaining part of the curve (i.e. outside the noise discarded portions) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to a much smaller maximum and repeating this change several times. This can be clearly seen in FIG. 5C where the two signals correspond perfectly outside the “noise” portions.

[0259] It can be shown that, if Δt.sub.B is reduced to a minimum, the resulting portions to be discarded can be reduced as well. The smaller Δt.sub.B is, the smaller the part of the resulting interference signal that needs to be not considered becomes (e.g. the discarded portions become smaller).

[0260] In FIG. 6, a second preferred embodiment of the sweeping for an A scan which last ΔT is shown, the sweeping duration ΔT is divided in sub intervals of equal duration.

[0261] In each of these sub intervals of duration Δt, the wavelength is varied linearly and monotonously for the whole duration Δt. However, the variation is alternatively either increasing or decreasing. In a first sub sweeping interval, the wavelength is for example increased linearly and monotonously and in the following sub sweeping interval the wavelength is decreased linearly and monotonously. The slope of the linear curve is the same albeit opposite. In other words, if in the i-th sub-interval the slope of the segment defined by the function wavelength (t) is m, the slope of the curve in the (i+1)th sub-interval is −m.

[0262] This behaviour of the signal is obtained increasing with a certain speed the voltage applied to the electrodes 153, reaching a maximum, and then decreasing the voltage till the minimum at the same speed of the increase.

[0263] The voltage difference is applied to the electrodes for a time 2Δt before being switched off and starting again to be applied for another 2Δt. Within each Δt, the applied voltage difference varies linearly between 0 kV and 1 kV and then from 1 kV to 0 kV.

[0264] In FIG. 6, the prior art tuning of the wavelength is also shown (linear dashed curve equivalent to FIG. 1), where the wavelength linearly increase for the whole duration of the sweeping ΔT.

[0265] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 6 is swept over the selected portion of the sample is depicted in FIGS. 7A and 7B. The prior art results are shown in FIG. 7A and the present invention case in FIG. 7B. Further, in FIG. 7C a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0266] In FIG. 7A, prior art case, the interference signal is a sinusoid.

[0267] In FIG. 7B, the interference signal shows a sinusoid and some “noise portions”. It is possible to see from FIG. 7B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. These regions correspond to the boundary between one sub sweeping interval and the next sub sweeping interval. They also correspond to the point in which the wavelength changes behavior, from increasing to decreasing. However, it can also be seen that in the remaining part of the curve (i.e. outside the discarded portions) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to maximum and from the maximum to the same minimum, repeating this change several times. This can be clearly seen in FIG. 7C where the two signals correspond perfectly outside the “noise” regions.

[0268] FIG. 8A-8C show the simulations results using the second embodiment sweeping signal of FIG. 6, however in this case two reflections separated by 10 m are present in the sample.

[0269] A numerical simulation of the signal from the OCT detector 120 of the interference signal obtained in case the signals (prior art and invention) of FIG. 6 is swept over the selected portion of the sample is depicted in FIGS. 8A and 8B. The prior art result are shown in FIG. 8A and the present invention case in FIG. 8B.

[0270] Further, in FIG. 8C a superposition of the two signals is made (dashed line=prior art, solid curve=present invention).

[0271] In FIG. 8A, prior art case, the interference signal is a superposition of two sinusoids having different frequency. Each frequency represents a different reflection on the sample.

[0272] In FIG. 8B, the interference signal shows also two sinusoids superimposed, and some “noise portions”. It is possible to see from FIG. 8B that the interference signal in the invention presents a plurality of regions where the signal cannot be used. These regions correspond to the boundary between one sub sweeping interval and the next sub sweeping interval. They also correspond to the point in which the wavelength changes behavior, from increasing to decreasing. However, it can also be seen that in the remaining part of the curve (i.e. outside the noise discarded portions) the signal is in perfect agreement with the prior art signal, i.e. there is substantially no difference in varying the wavelength continuously from a minimum to a “high” maximum and varying the wavelength from a minimum to maximum and from the maximum to the same minimum, repeating this change several times. This can be clearly seen in FIG. 8C where the two signals correspond perfectly outside the “noise” regions.

[0273] FIG. 9A-9C show the fast Fourier transform (FFT) for this interference signal of FIG. 8A-8C (respectively) where the two reflections can be clearly distinguished. In the two cases of prior art and present invention. It is possible to see that the two spectral behaviors are very similar with only a small added noise for the present invention case.

Examples

[0274] The laser can emit light at 1550 nm using InP based gain chip. The emission wavelength change by tuning the intra cavity tunable filter at different transmission wavelength by varying the voltage applied to the electro-optical material (in our case a thin liquid Chrystal film inside a Fabry-Perot cavity). The output of the laser is coupled at the input of an interferometer (a 2×2 in fiber coupler). At the other input arm is coupled a fast photodiode (bandwidth around 1 GHz) connected with a signal processor. At the end of one of the output arms is fixed the reference mirror and at the other output arm the scanning element based on a collimating lens and a scanning mirror. The length of the two output arms is preferably balanced for optimum interferometer work.

[0275] The sweeping time is set to be equal to 1 μs and it is divided in N=4 sub sweeping interval, each of 250 ns.

[0276] What is called “prior art” signal is substantially the sweeping of FIG. 1, obtained maintaining the laser source sweeping for 1 us covering 100 nm.

[0277] The signal as depicted in FIG. 6 is obtained sweeping the laser for 250 ns increasing the output wavelength of 25 nm than inverting the sweep for other 250 nm returning at the initial wavelength and then we repeat the previous two sweeps a second time. During this 1 μs (4×250 ns) the optical element of the OCT remain fixed on the same measurement point.

[0278] The signal of FIG. 4 is obtained sweeping the output wavelength linearly for 225 ns at a slightly higher speed covering 25 nm than reset in 25 ns and repeat four time this cycle (see FIG. 4). As in the previous example during this 1 μs (4×250 ns) the optical element of the OCT remain fixed on the same measurement point.

[0279] The electrical signal from the photodiode is then amplified and sampled (in the example 10 sample per ns). The resulting 10000 samples are then Fourier transformed using a Cooley-Tukey Fast Fourier Transform (FFT) algorithm.

[0280] The structure of the tunable optical filter or etalon 150 is the following:

[0281] Glass 154: each glass slab has a thickness smaller than 1 mm, preferably smaller than 0.75 mm; [0282] ITO 153: each electrode has a thickness smaller than 100 nm, preferably smaller than 50 nm; [0283] High reflectivity dielectric multilayer 152: it has a thickness comprised in the range between 0.5 μm and 5 μm as a function of desired reflectivity; [0284] Liquid crystal 151: it has a thickness smaller than 100 μm, preferably smaller than 50 μm, more preferably smaller than 30 μm (the free spectral range is affected by the thickness choice, which is in turn affected by liquid crystal characteristics as refractive index, induce birefringency, and externally applied electro-magnetic field);