Microstructured optical fiber, supercontinuum light source comprising microstructured optical fiber and use of such light source

Abstract

The invention relates to a microstructured optical fiber for generating incoherent supercontinuum light upon feeding of pump light. The microstructured optical fiber has a first section and a second section. A cross-section through the second section perpendicularly to a longitudinal axis of the fiber has a second relative size of microstructure elements and preferably a second pitch that is smaller than a blue edge pitch for the second relative size of microstructure elements. The invention also relates to an incoherent supercontinuum source comprising a microstructured optical fiber according to the invention.

Claims

1. A microstructured optical fiber for generating incoherent supercontinuum light upon feeding of pump light, said microstructured optical fiber having a length and a longitudinal axis along its length and comprising, along its length, a core region that is capable of guiding light along the length of said microstructured optical fiber, and a first cladding region surrounding said core region, said first cladding region comprising a microstructure having a plurality of microstructure elements, wherein said microstructured optical fiber, along its length, comprises: a first section with a first length L.sub.1, wherein the microstructure elements of said optical fiber at least at a first cross-section through the first section perpendicularly to the longitudinal axis has a first pitch Λ.sub.1, a first microstructure diameter d.sub.1 and a first relative size d.sub.1/Λ.sub.1 of microstructure elements, wherein the microstructured optical fiber at least in said first cross-section has a first zero dispersion wavelength ZDW1.sub.1 in the range from about 920 to about 1120 nm a second section with a second length L.sub.2, wherein the microstructure elements of said optical fiber at least at a second cross-section through the second section perpendicularly to the longitudinal axis has a second pitch Λ.sub.2, a second microstructure diameter d.sub.2 and a second relative size d.sub.2/Λ.sub.2 of microstructure elements, wherein said microstructured optical fiber at least in said second cross-section through the microstructured optical fiber has a first zero dispersion wavelength ZDW2.sub.1 and a second zero dispersion wavelength ZDW2.sub.2, said second zero dispersion wavelength being about 2200 nm or shorter, wherein the sum of the first length L.sub.1 and the second length L.sub.2 is about 1 meter or larger.

2. A microstructured optical fiber according to claim 1, wherein said second pitch Λ.sub.2 is smaller than a blue edge pitch Λ.sub.blue, where said blue edge pitch Λ.sub.blue is defined as a specific pitch giving the shortest possible blue edge wavelength of the supercontinuum light for said second relative size d.sub.2/Λ.sub.2 of microstructure elements.

3. A microstructured optical fiber according to claim 1, wherein the relative size d.sub.1/Λ.sub.1 of microstructure elements in the first cross-section is about 0.75 or less.

4. A microstructured optical fiber according to claim 1, wherein said microstructured optical fiber in said second cross-section has a group velocity matched wavelength GVMW.sub.2 in the range from about 650 nm to about 800 nm.

5. A microstructured optical fiber according to claim 1, wherein the second relative size d.sub.2/Λ.sub.2 of the microstructure elements is about 0.75 or less.

6. A microstructured optical fiber according to claim 1, wherein the second pitch Λ.sub.2 is about 0.9 times the blue edge pitch Λ.sub.blue or smaller.

7. A microstructured optical fiber according to claim 1, wherein said first zero dispersion wavelength ZDW2.sub.1 of said second cross-section is less than about 1000 nm, such as less than about 900 nm.

8. A microstructured optical fiber according to claim 7, wherein the second pitch Λ.sub.2 is smaller than the first pitch Λ.sub.1.

9. A microstructured optical fiber according to claim 7, where said second pitch Λ.sub.2 is in the range from about 1.1 μm to about 1.7 μm.

10. A microstructured optical fiber according to claim 1, wherein the microstructured optical fiber further comprises a second tapered section L.sub.i2 and a third section with third length L.sub.3, wherein the microstructure elements of said optical fiber at least at a third cross-section through the third section perpendicularly to the longitudinal axis has a third pitch Λ.sub.3, a third microstructure diameter d.sub.3 and a third relative size d.sub.3/Λ.sub.3 of microstructure elements; wherein the third pitch Λ.sub.3 is larger than the second pitch Λ.sub.2.

11. A microstructured optical fiber according to claim 1, wherein a cross-section through the first tapered section perpendicularly to the longitudinal axis of the fiber comprises microstructures at a first taper pitch Λ.sub.t1, a first taper microstructure diameter d.sub.t1 and a first taper relative size d.sub.t1/Λ.sub.t1 of microstructure elements, wherein the first taper section of the microstructured optical fiber has a taper group velocity matched wavelength GVMW.sub.t1 corresponding to a second zero dispersion wavelength ZDW.sub.t1 in said cross-section, where said taper group velocity matched wavelength GVMW.sub.t1 is in the range from about 400 nm to about 500 nm for any cross-sections of the first tapered section.

12. A microstructured optical fiber according to claim 1, wherein the first section is coupled to the second section by a splicing.

13. An incoherent supercontinuum source comprising: i. a microstructured optical fiber for generating incoherent supercontinuum light upon feeding of pump light, said microstructured optical fiber having a length and a longitudinal axis along its length and comprising, along its length, a core region that is capable of guiding light along the length of the microstructured optical fiber, and a first cladding region surrounding said core region, said first cladding region comprising a microstructure having a plurality of microstructure elements, ii. a pump source with a center wavelength between about 1000 nm and about 1100 nm and a pulse length of longer than about 500 fs, wherein said microstructured optical fiber, along its length, comprises: a first section with a first length L.sub.1, wherein the microstructure elements of said optical fiber at least at a first cross-section through the first section perpendicularly to the longitudinal axis has a first pitch Λ.sub.1, a first microstructure diameter d.sub.1 and a first relative size d.sub.1/Λ.sub.1 of microstructure elements, wherein said microstructured optical fiber at least in said first cross-section has a first zero dispersion wavelength ZDW1.sub.1 in the range from about 920 to about 1120 nm; a second section with a second length L.sub.2, wherein the microstructure elements of said optical fiber at least at a second cross-section through the second section perpendicularly to the longitudinal axis has a second pitch Λ.sub.2, a second microstructure diameter d.sub.2 and a second relative size d.sub.2/Λ.sub.2 of microstructure elements, wherein said microstructured optical fiber at least in said second cross-section through the microstructured optical fiber has a first zero dispersion wavelength ZDW2.sub.1 and a second zero dispersion wavelength ZDW2.sub.2, said second zero dispersion wavelength being about 2200 nm or shorter, wherein the sum of the first length L.sub.1 and the second length L.sub.2 is about 1 meter or larger.

14. An incoherent supercontinuum source according to claim 13, wherein the second pitch Λ.sub.2 is smaller than a blue edge pitch Λ.sub.blue, wherein said blue edge pitch Λ.sub.blue is defined as a specific pitch giving the shortest possible blue edge wavelength of the supercontinuum light for said second relative size d.sub.2/Λ.sub.2 of microstructure elements.

15. An incoherent supercontinuum source according to claim 13, said microstructured optical fiber further comprising a first tapered section with length L.sub.t1, wherein the tapered section connects the first section and the second section.

16. An incoherent supercontinuum source according to claim 13, wherein the first section is coupled to the second section by a splicing.

17. The incoherent supercontinuum source according to claim 13, wherein the pump source is a seed laser arranged to provide seed pulses with a seed pulse frequency F.sub.seed, the supercontinuum source further comprising a pulse frequency multiplier (PFM) arranged to multiply the seed pulses and convert the seed pulse frequency F.sub.seed to pump pulses with a pump pulse frequency F.sub.pump, where said pump pulse frequency F.sub.pump, is larger than said seed pulse frequency F.sub.seed.

18. The incoherent supercontinuum light source according to claim 13, said pump light source comprising a mode-locked laser and at least one amplifier, said supercontinuum light source having an output being spliced onto said input end of said microstructured optical fiber.

19. An optical coherence tomography (OCT) acquisition system comprising a supercontinuum light source according to claim 13, and a detector for collecting reflected light and an image processor for analyzing the detected reflected light.

20. An optical coherence tomography (OCT) acquisition system of claim 19 wherein the acquisition system is configured to determine a wavefront aberration in a coherent signal to thereby provide a wavefront sensor.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be explained in more detail below in connection with embodiments and with reference to the drawings in which:

(2) FIG. 1 shows dispersion profiles of three fibers having different pitch, but constant hole-to-pitch ratio of d/Λ=0.52;

(3) FIG. 2 shows the blue edge wavelength as a function of pitch for group-velocity matching to a loss edge of 2300 nm or 2400 nm;

(4) FIG. 3 shows power and noise as a function of the wavelength of a supercontinuum spectrum;

(5) FIG. 4a shows a picture of a cross-section of a microstructured fiber, perpendicular to the longitudinal axis;

(6) FIG. 4b shows part of a cross-section of a microstructured fiber, perpendicular to the longitudinal axis;

(7) FIG. 5 shows the group velocity matched wavelength corresponding to the second zero dispersion wavelength as a function of pitch for four fibers with relative size of microstructure elements of 0.34, 0.52, 0.61 and 0.8;

(8) FIGS. 6a and 6b show systems for narrow band and wide band spectral noise measurement, respectively.

(9) FIG. 7a shows a schematic drawing of an embodiment of a microstructured optical fiber according to the invention;

(10) FIGS. 7b and 7c show cross-sections of a microstructured optical fiber, perpendicular to the longitudinal axis, at a first and second fiber length section, respectively;

(11) FIG. 8 shows supercontinuum spectra obtained from an embodiment of a microstructured optical fiber according to the invention, comprising a tapered section.

(12) FIGS. 9 and 10 show noise spectra as a function of wavelength for an embodiment of an optical fiber according to the invention as compared to a uniform fiber.

(13) FIG. 11 shows a graph of the average noise for amplification levels between 20% and 100% of an incoherent supercontinuum source comprising an embodiment of an optical fiber according to the invention, as well as for a prior art optical fiber.

(14) FIG. 12 shows broad band noise spectra as a function of wavelength for an embodiment of a tapered optical fiber according to the invention and from a uniform fiber;

(15) FIGS. 13a, 13b and 13c show a cascaded optical fiber according to an embodiment of the invention and cross-sections through a first and second section thereof.

(16) FIG. 14 shows graphs of the noise of a cascaded optical fiber for different lengths of the first section;

(17) FIG. 15 shows supercontinuum spectra obtained from another embodiment of a microstructured optical fiber according to the invention, comprising a cascade, and from a uniform fiber;

(18) FIG. 16 shows noise spectra as a function of wavelength for an embodiment of a cascaded optical fiber according to the invention as compared to a uniform fiber;

(19) FIG. 17 shows a noise spectrum for amplification levels between 20% and 100% of an incoherent supercontinuum source comprising an embodiment of a cascaded optical fiber according to the invention as well as for a prior art optical fiber;

(20) FIG. 18 shows supercontinuum spectra obtained by four different optical fibers, and

(21) FIG. 19 is a schematic drawing of a supercontinuum light source comprising a microstructured optical fiber and a pump light source.

(22) The figures are schematic and are simplified for clarity. Throughout the description, the same reference numerals are used for identical or corresponding parts.

(23) Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

(24) The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope.

(25) Some preferred embodiments have been described in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

(26) FIG. 4a shows a picture of a cross-section of a microstructured fiber 50, perpendicular to the longitudinal axis. The fiber is a microstructured fiber comprising a core region 52 and a cladding region 53, the cladding region surrounding the core region. The cladding region comprises an inner cladding background or base material in which microstructure elements 59 in the form of low-index cladding features are provided. The microstructure elements 59 shown are features in the form of holes or voids extending in the longitudinal direction of the fiber, and an. The core region 52 comprises a refractive index profile such that the core region comprises material with a refractive index score being different from the refractive index of a material in the inner cladding region. In order to tune various properties of the optical fiber it may be preferred to have a special refractive index profile of the core region. The region A denotes an area of the fiber to be shown enlarged in FIG. 4b.

(27) FIG. 4b shows part of a cross-section of a microstructured fiber, perpendicular to the longitudinal axis, corresponding to an enlargement of the square region denoted A in FIG. 4a. In FIG. 4b is shown the core area or core region 52 as defined as the area of a circle inscribed by the microstructure elements of the cladding arranged to immediately surround the core 52. The circle has characteristic core diameter W being the diameter of the largest circle that may be inscribed within the core without interfering with any microstructure elements or cladding features of the fiber, in a cross-section through the fiber perpendicularly to the longitudinal axis thereof. The cladding comprises a microstructure with a plurality of microstructure elements or cladding features each having a microstructure element diameter d.sub.f. The microstructure elements are arranged at a pitch Λ, where the pitch Λ is a measure of the spacing between the microstructure elements. As shown in FIG. 4b the pitch A is indicated as the distance between the centers of two adjacent microstructure elements.

(28) FIG. 5 shows the group velocity matched wavelength corresponding to the second zero dispersion as a function of pitch for four fibers with relative size d/Λ of microstructure elements of 0.34, 0.52, 0.61 and 0.8. In prior art it has been shown that a tapered fiber with very large relative hole size, viz. a very large relative size of microstructure elements, can be used to obtain supercontinuum extending down to very short wavelengths. This is exemplified by the curve of an optical fiber with relative size d/Λ of 0.8 (not a part of the present invention). The curve shows a minimum GVMW of around 360 nm for a pitch Λ=2.0 μm, which is in agreement with the prior findings of the above referenced articles by Kudlinksi, Travers and Møller. As it can be seen in FIG. 5 the blue edge pitch is the pitch where the curve has a minimum. Decreasing the relative size of microstructure elements provides a decrease in minimum GVMW. For a relative size of microstructure elements d/Λ=0.61, the minimum GVMW is 430 nm and is obtained at a pitch Λ=2.4 μm. It is noticed that for a relative size of microstructure elements d/Λ=0.61, the GVMW is below 500 nm for all pitches between 1.5 and 4.0 μm. Hence if a fiber with d/Λ=0.61 is tapered down whilst keeping the relative size of microstructure elements constant, then the GVMW will be below 500 nm for all cross sections in the taper. Thus, in an example a tapered fiber with a relative size of microstructure elements of around d/Λ=0.61 can lead to a supercontinuum having a broad wavelength peak below 500 nm, as will be further explained in relation to FIG. 18.

(29) In an embodiment of the invention an incoherent supercontinuum source having low noise from 680 nm to 920 nm is obtained. In this embodiment the second section of the fiber has a group velocity matched wavelength GVMW.sub.2 in the range from about 650 nm to 800 nm. For a relative size of microstructure elements of d/Λ=0.52, FIG. 5 shows that this requires the pitch to be Λ<1.5 μm. A tapered fiber having a constant relative size microstructure elements of d/Λ=0.52 is further described in relation to FIGS. 7 to 12. If the relative size of microstructure elements is d/Λ=0.34, the requirement on the pitch changes to be Λ<2.5 μm, corresponding to a mode field diameter of around 3.5 μm at 1064 nm. This enables low loss splicing to a standard supercontinuum fiber as e.g. SC-3.7-975 from NKT Photonics as will be further described below in relation to FIGS. 13 to 17.

(30) FIGS. 6a and 6b show systems for narrow band and wide band spectral noise measurement, respectively.

(31) The noise of an optical spectrum may be measured in a number of different ways depending on the frequency range and spectral resolution. The applications requiring low spectral noise are typically divided into two categories: narrow spectral resolution of 0.1 nm or less, which is typically required for applications such as spectral domain optical coherence tomography, and large spectral resolution of 10 nm or more for applications where the detailed spectral information is not required.

(32) The actual noise performance is quite different for the two ranges of spectral resolution, and two different methods have therefore been applied to characterize the noise, viz. a narrow band spectral noise measurement (illustrated in FIG. 6a) and a broad band spectral noise measurement (illustrated in FIG. 6b).

(33) FIG. 6a shows a system for narrow band spectral noise measurement (<0.1 nm). For the narrow band spectral noise characterization a high resolution, high speed spectrometer is used to perform the measurements. The system shown in FIG. 6a comprises a light source 1 under investigation, a single-mode coupling unit 2, and a high resolution spectrometer 3 or narrow band spectrometer. The spectrometer 3 includes a high speed line camera for fast acquisition of the spectral data (typical line rates are between 10 and 140 kHz, with 1024 to 8096 pixels per spectrum). The coupling unit 2 includes means of signal power attenuation and band pass filtering that is matched to the spectral range of the spectrometer 3 in order to avoid ghosting effects. The spectrometer 3 is adjusted to a specific integration time and line scan rate. The photons from the light source 1 are detected on the camera for the duration specified by the integration time and are subsequently read out. For a pulsed source this means that the number of pulses collected per spectrum is determined by the integration time multiplied by the pulse frequency of the source (e.g. 10 ρs×80 MHz=800 pulses), and the measured spectrum is the sum of these pulses.

(34) The spectral noise is in the context of this text defined as the relative standard deviation of the individual spectral bin (i.e. defined by the spectral resolution) over time. The standard deviation is calculated over a number of spectra—typically the spectrometer reads out a number of spectra (e.g. 500 or 1000) in a frame. As the result is a statistical value, it may be relevant to perform a number of such measurements to get a value with sufficiently high confidence.

(35) The spectral noise can be measured as a function of wavelength (which may be relevant if this is wavelength dependent) or as an average value across the spectrum.

(36) If the spectrometer noise (e.g. thermal noise, circuit noise or shot-noise) is comparable to the noise of the light source 1 at low spectrometer readings, this can have a significant impact on the measurement result, where the measured noise decreases as the power increases. In such a case it may be necessary to perform measurements at different input power levels by stepwise changing the attenuation from the light source to the spectrometer in order to ensure that the weaker parts of the spectrum are not significantly influenced by the spectrometer noise (similar to high dynamic range imaging).

(37) FIG. 6b shows a system for broad band or wide band spectral noise measurements (>10 nm).

(38) For wide band spectral noise characterization of a pulsed light source 1, the relative standard deviation of the individual pulse energy is measured in the relevant spectral width.

(39) The measurement system comprises a light source 1 under investigation, an attenuation and wide band spectral filtering unit 4, a fast optical detector 5, and a high speed digital oscilloscope 6. The electrical bandwidth of the detector and the oscilloscope must be much larger that the frequency of the light source to avoid crosstalk from one pulse to the next.

(40) For pulses with durations much shorter than the response time of the electronic detection system, the detected pulse amplitude will be proportional to the energy of the detected pulse. Hence, to evaluate the statistics of the pulse energy it is sufficient to measure the statistics of the amplitudes of the detected signals. This may be performed by using internal oscilloscope analysis functions which determine the peak amplitude of each pulse over a certain measurement ensemble (typical 1000 individual pulse traces) and calculate the average and standard deviation of this parameter.

(41) The measurement is repeated at different source power levels to determine the power dependence of the noise and using band pass filters with different center wavelengths to determine the wavelength dependence.

(42) FIG. 7a shows a schematic drawing of an embodiment of a microstructured optical fiber 10 according to the invention, and FIGS. 7b and 7c show cross-sections of the microstructured fiber, perpendicular to the longitudinal axis, at a first and second fiber length section, respectively. The microstructured optical fiber is arranged for generating supercontinuum light upon feeding of light having a first wavelength λ.sub.1 from about 900 nm to about 1100 nm, such as between 1000 nm and 1100 nm, for instance 1064 nm. The optical fiber 10 has a length and a longitudinal axis along its length and comprises a core region for guiding light along the length of the optical fiber, and a first cladding region surrounding the core region.

(43) The optical fiber 10, along its length, comprises a first fiber length section 12, a second fiber length section 14 as well as a tapered section 13 between the first and second fiber length sections 12, 14. The optical fiber 10 optionally includes an end cap 8. The extension of the end cap length 8 in the longitudinal axis of the fiber 10 is of a magnitude of 100 μm, e.g. 200 μm, whilst the total length of microstructured fiber is e.g. several meters, for example 10 meters or 50 meter.

(44) In an embodiment the sum of the first and second lengths of the microstructured fiber 10 is such as less than about 50 m, such as less than about 30 m, such as less than about 20 m, such as less than 10 m. In an embodiment, the second length L.sub.2 is larger than about 1 m, such as larger than about 3 m, such as larger than about 5 m, such as larger than about 10 m.

(45) FIG. 7b shows a cross-section of the microstructured fiber, perpendicular to the longitudinal axis, at the first length section 12. As it can be seen the microstructure elements are arranged in a hexagonal pattern in the cladding. It is indicated that the first fiber length section 12 has a core region with a first characteristic core diameter W.sub.1 and a cladding region with a first pitch Λ.sub.1, a first microstructure diameter d.sub.1 and a first relative size of microstructure elements Λ.sub.1/d.sub.1 in a cross-section through the fiber perpendicularly to the longitudinal axis. The second fiber length section 14 has a core region with a second characteristic core diameter W.sub.2 and a cladding region with a second pitch Λ.sub.2, a second microstructure diameter d.sub.2 and a second relative size of microstructure elements Λ.sub.2/d.sub.2 in a cross-section through the fiber perpendicularly to the longitudinal axis. The second pitch Λ.sub.2 is chosen so as to be smaller than the blue edge pitch Λ.sub.pitch. The first pitch Λ.sub.1 is larger than the second pitch Λ.sub.2. The first relative size of microstructure elements Λ.sub.1/d.sub.1 is about 0.75 or less, such as about 0.65 or less, such as about 0.55 or less. The second relative size of microstructure elements Λ.sub.2/d.sub.2 is equal to or less than the first relative size of microstructure elements Λ.sub.1/d.sub.1.

(46) The tapered section 13 of the optical fiber 10 comprises a core region and cladding, which is tapered from parameters of the first section to the parameters of the second section over a length L.sub.t1 of the tapered section The length L.sub.t1 of the tapered section is at least about 0.1 m, such as least about 0.2 m, such as at least about 0.5 m, such as at least about 1 m, such as at least about 1.5 m, such as at least 2 m, such as at least 5 m, such as at least 10 m.

(47) It may be seen from FIG. 7a that the tapering from the first fiber length section to the second fiber length section means a substantially monotonic decrease of the dimensions of the fiber from the first length section 12 to the second length section 14. In the first length section 12 the dimensions of the fiber, viz. the core diameter, the first pitch, the first microstructure diameter, the first relative size of microstructure elements, are substantially constant, and in the second length section 14 the second characteristic core diameter W.sub.2, the second pitch, the second microstructure diameter, and the second relative size of microstructured elements are substantially constant. In the tapering section, whilst the microstructure pitch and microstructure element diameter vary along at least a part of the intermediate length section 13 of the fiber 10. In an embodiment, the relative size of microstructure elements is be constant throughout the fiber, such that the first relative size of microstructure elements is substantially equal to the second relative size of the microstructure elements. However, in an alternative embodiment, the relative size of microstructure element varies in the tapered section, so that the first relative size of microstructure elements is different from the second relative size of the microstructure elements.

(48) FIGS. 8 to 19 shows results from measurements where a non-linear microstructured optical fiber is pumped by a pulsed laser source. The noise of the output from a uniform fiber is compared with the output from a tapered fibre and with a cascaded fiber, viz. a fiber where two different microstructured optical fibers have been spliced together.

(49) In the measurements, a pump laser source was 1064 nm and the temporal duration of the pump pulses just prior to the microstructured optical fiber was 8 ps. The bandwidth of the laser pulses was 10 nm. The output power from the pump source was adjustable with the maximum power being 10 W. The maximum output power will be referred to as 100% of the maximum power, whilst a power of e.g. 5 W will be referred to as 50% of the maximum power. The output of the pulsed laser source was spliced to the input of the microstructured optical fiber being measured.

(50) The reference fiber for the measurements was a straight, uniform section of the commercial fiber SC-3.7-975 from NKT Photonics. The pitch was 2.6 μm and the relative hole size (d/Λ) was 0.52. The length of the reference fiber was 10 m. This fiber is used in NKT Photonics product series SuperK™ EXW (currently comprising the following variants: EXW-1, EXW-4, EXW-6 and EXW-12). The EXW fiber used as a reference fiber in the context of this application is the one denoted SC-3.7-975.

(51) FIG. 8 shows supercontinuum spectra obtained from a microstructured optical fiber 10 according to the invention comprising a tapered section. The optical fiber 10 is an optical fiber as the one shown in FIG. 7a. In an alternative embodiment, the optical fiber 10 does not include an end cap.

(52) The optical fiber 10 comprises a first uniform section 12 which is followed by a tapered section 13, where the tapered section 13 is followed by a second uniform section 14. The dimensions of the second uniform section 14 are smaller than those of the first uniform section 12. The second uniform section following the tapered section is also referred to as the waist of the taper.

EXAMPLE 1

(53) As one example the dimensions of the first section includes a first length L.sub.1=1 m, a first pitch Λ.sub.1=2.6 μm and a first relative size of microstructure elements d/Λ.sub.1=0.52. In Example 1, the microstructure elements are holes, and the term “relative size of microstructure elements” is thus seen as equivalent to “relative hole size”.

(54) The tapered section 13 of the optical fiber 10 has a length L.sub.t1=5 m, and the dimensions of the second section 14 includes a second length L.sub.2=30 m, a second pitch Λ.sub.2=1.3 μm and a second relative hole size d.sub.2/Λ.sub.2=0.52. The second section has a first zero dispersion wavelength ZDW.sub.21=850 nm, a second zero dispersion wavelength ZDW.sub.22=1167 nm and a group velocity match wavelength GVM.sub.2=721 nm.

(55) FIG. 8 shows supercontinuum spectra measured from the output of the optical fiber with the above dimensions. The supercontinuum spectra are shown for powers of the pump light source of 60% and 100%. FIG. 8 shows that the spectrum extends from about 450 nm to about 1300 nm for both powers of pump light source.

(56) FIGS. 9 and 10 show narrow band noise spectra as a function of wavelength for an optical fiber having the dimensions as indicated in Example 1 (“tapered fiber”), as compared to a uniform fiber (“straight fiber”). FIG. 9 shows the noise spectrum as a function of wavelength for an optical power of the pump light source of 60%, whilst FIG. 10 shows the noise spectrum as a function of wavelength for an optical power of the pump light source of 100%. The noise spectra of FIGS. 9 and 10 are taken from the same tapered optical fiber according to an embodiment of the invention as well as from the same uniform fiber used as a reference.

(57) The narrow bandwidth noise spectra shown in FIGS. 9 and 10 were measured with a diode array spectrometer with higher resolution, as previously described. In the wavelength range from 680-920 nm, FIGS. 9 and 10 shows a reduction in noise for an embodiment of the microstructured tapered optical fiber of the invention compared to the uniform reference fiber. Only for the smallest wavelengths, the noise is higher for the tapered fiber 10 compared to the uniform reference fiber.

(58) FIG. 11 shows a graph of the average noise for amplification levels between 20% and 100% of an incoherent supercontinuum source comprising an embodiment of an optical fiber according to the invention as well as for a prior art optical fiber. In FIG. 11, a comparison of noise for tapered fiber according to Example 1 and the uniform reference fiber. In the two cases the noise is compared at identical pump power. For a given amplification level, the average noise is obtained by averaging the noise in the wavelength range from 680 to 920 nm. Thus, the graphs of FIG. 9, corresponding to the noise in the wavelength range from 680 to 920 nm result in one point in the graph of FIG. 11 for the tapered fiber and another point in the graph of FIG. 11 for the uniform reference fiber. FIG. 11 shows that the average noise in this wavelength range from 680 to 920 nm is lower for the tapered fiber according to the invention compared to the uniform reference fiber. This is observed for all power levels between 20% and 100%.

(59) Furthermore, a cut-back experiment was performed where the length of the second length was gradually reduced from 30 m. It was observed that the spectrum and noise between 680 and 920 nm were nearly independent of the length of the second section. However, when the tapered section was cut-off directly at the waist, meaning that the length of the second section is neglectable, the noise showed a small increase of a couple of percent compared to the previous level.

(60) FIG. 12 shows broad band noise spectra as a function of wavelength for an embodiment of a tapered optical fiber according to the invention and from a uniform referenced fiber.

(61) The broad band noise spectra were measured with a photo detector and band pass filters, as previously described in relation to FIG. 6b. FIG. 12 shows the comparison of the broad bandwidth noise for the uniform reference fiber and the tapered fiber of Example 1. In the two cases, the noise has been measured at identical pump power. The results are shown for a 50% power level. The results for both the tapered fiber of Example 1 and the uniform reference fiber show that the noise is rapidly decreasing from 600 to 700 nm. At wavelengths longer than 700 nm the noise reaches a more or less constant level until approximately 900 nm. The tapered fiber of Example 1 has lower noise than the uniform reference fiber in the entire range from 600 nm to 900 nm. It is noticed that in contrast to what has been described in the article “Control of pulse-to-pulse fluctuations in visible supercontinuum”, by Kudlinski et al, the tapered optical fiber of the invention, viz. of Example 1, decreases the noise in region from about 700 to 900 nm as compared to the uniform reference fiber. From around 900 to 1100 nm the noise in the taper is larger than in the uniform reference fiber, but from 1100 nm to 1300 nm the noise is again lower in the taper. Above 1300 nm there is no power in the output from the tapered fiber which is also evident from FIG. 8 of the spectrum from the tapered fiber of Example 1. Therefore, the noise cannot be measured at wavelengths above 1300 nm.

(62) FIGS. 13a, 13b and 13c show an embodiment of a cascaded optical fiber 30 according to the invention and cross-sections through a first and second section thereof, perpendicular to the longitudinal axis, at a first and second fiber length section, respectively. The microstructured optical fiber 30 is arranged for generating supercontinuum light upon feeding of light having a first wavelength Λ.sub.1 from about 900 nm to about 1100 nm, such as between 1000 nm and 1100 nm, for instance 1064 nm. The optical fiber 30 has a length and a longitudinal axis along its length and comprises a core region for guiding light along the length of the optical fiber, and a first cladding region surrounding the core region.

(63) The optical fiber 30, along its length, comprises a first section 31, a second section 32 and a splicing 33 between the first and second sections 32, 33. The optical fiber 30 may optionally include an end cap 8 (not shown in FIG. 13a).

(64) In an embodiment the sum of the first and second lengths of the microstructured fiber 10 is such as less than about 50 m, such as less than about 30 m, such as less than about 20 m, such as less than 10 m.

(65) FIG. 13b shows a cross-section of the microstructured fiber, perpendicular to the longitudinal axis, at the first section 31. It is seen that the first section 31 has a core region with a first characteristic core diameter W.sub.1 and a cladding region with a first pitch Λ.sub.1, a first microstructure diameter d.sub.1 and a first relative size of microstructure elements Λ.sub.1/d.sub.1 in a cross-section through the first section 31 perpendicularly to the longitudinal axis.

(66) FIG. 13c shows a cross-section of the microstructured fiber, perpendicular to the longitudinal axis, at the second section 32. The second section 32 has a core region with a second characteristic core diameter W.sub.2 and a cladding region with a second pitch Λ.sub.2, a second microstructure diameter d.sub.2 and a second relative size of microstructure elements Λ.sub.2/d.sub.2 in a cross-section through the fiber perpendicularly to the longitudinal axis.

(67) Throughout the first section 31 the dimensions of the fiber, viz. the core diameter, the first pitch, the first microstructure diameter, the first relative size of microstructure elements, are substantially constant, and throughout the second section 32 the second characteristic core diameter W.sub.2, the second pitch, the second microstructure diameter, and the second relative size of microstructured elements are substantially constant.

(68) In an embodiment the first relative size d.sub.1/Λ.sub.1 of the microstructure elements is larger than the second relative size of the microstructure elements d.sub.2/Λ.sub.2, such as being about 1.2 times the second relative size d.sub.2/Λ.sub.2 of the microstructure elements or larger, such as about 1.3 times the second relative size d.sub.2/Λ.sub.2 of the microstructure elements or larger, such as about 1.4 times the second relative size d.sub.2/Λ.sub.2 of the microstructure elements or larger, such as about 1.5 times the second relative size d.sub.2/Λ.sub.2 of the microstructure elements. It is advantageous that the first relative size d.sub.1/Λ.sub.1 of the microstructure elements is larger than the second relative size of the microstructure elements d.sub.2/Λ.sub.2, in that this will reduce the splicing loss between the first section and the second section of the fiber compared to a situation where a first section and a second section having equal or similar relative size of microstructure elements were spliced together.

(69) It has been shown that for a tapered fiber with a relative size d/Λ of microstructure elements of 0.52 in both the first and second section, the lowest noise was obtained when the fiber was tapered down so that the second section had a second pitch Λ.sub.2 of around 1.3 μm.

(70) A fiber with a second section having a second pitch Λ.sub.2 of around 1.3 μm has a simulated effective area of 3 μm at 1064 nm.

(71) Table 1 below shows the relative size of microstructure elements d/Λ, the pitch Λ, the calculated mode field area and the calculated mode field diameter for a uniform reference fiber SC-3.7-975, a down-tapered part of an optical fiber, viz. the second section of a tapered optical fiber according to an embodiment of the invention, as well as the second section of a cascaded fiber according to an embodiment of the invention.

(72) From Table 1 it is seen that the a second section of a tapered fiber having a relative size of microstructure elements of 0.5 being close to the relative size of microstructure elements in the uniform reference fiber, has an effective mode field area close to one third of the mode field diameter of the uniform reference fiber. The calculated minimum splice loss between such a tapered fiber and the reference fiber with Λ=2.6 μm and (d/Λ)=0.52 is 1.6 dB. This large splice loss will lead to local heating of the fiber, which decreases the power handling capability and life time of the splice.

(73) The calculated minimum splice loss between the reference fiber and the cascaded fiber having the parameters of Table 1 is 0.1 dB. Therefore, it is advantageous that the relative size of microstructure elements in the second section of an embodiment of a cascaded fiber according to the invention is smaller than the relative size of microstructure elements in the first section.

(74) TABLE-US-00001 TABLE 1 Fiber d/Λ Λ (μm) Aeff (μm.sup.2) MFD (μm) SC-3.7-975 0.52 2.6 8.5 3.3 Taper waist 0.5 1.3 3 2 Cascade 0.34 2.2 10.5 3.6

(75) In an embodiment, the second pitch Λ.sub.2 of the second section is at about 2 μm or larger, such as at about 2.3 μm or larger, such as at about 2.6 μm or larger, such as at about 3 μm or larger.

(76) In an embodiment the first length L.sub.1 is in the range from about 1 m to about 5 m or less, such as in the range from about 2 m to about 4 m.

EXAMPLE 2

(77) Example 2 is an example of an embodiment of the invention providing lowered noise. Example 2 is a cascaded fiber 30, which is a combination of a first section 31 of a uniform microstructured fiber and a second section 32 of a microstructured optical fiber spliced together at the splicing 33. Originally, the first section 31 of the cascaded fiber 30 was 10 meter of uniform fiber for supercontinuum generation. Throughout the measurements on the fiber of Example 2, the first section was shortened, as described in relation to FIG. 14, in order to compare the influence of the length of the first section on the noise. The first pitch Λ.sub.1 was 2.6 μm and first relative hole size d.sub.1/Λ.sub.1 was 0.52.

(78) The second section 32 was a microstructured optical fiber with a second pitch Λ.sub.2 of 2.2 μm, a second relative hole size d.sub.2/Λ.sub.2 of 0.36 and a length of 10 m. The second section had a second zero dispersion wavelength ZDW.sub.22=1800 nm and group velocity match wavelength GVM.sub.2=770 nm. A simulation using the Gaussian radius approximate shows that the first section has an effective mode area of 10.5 μm at 1064 nm, whereas the second section has an effective mode area of 8 μm. Using the standard formula for the coupling loss between Gaussian modes, this gives a minimum obtainable loss of 0.1 dB. In practice a splicing loss of 0.5 dB was obtained.

(79) Table 2 below indicates the parameters of the first and second section of the cascade optical fiber of Example 2.

(80) TABLE-US-00002 TABLE 2 First section Second section (~SC-3.7-975) (NL-1060-1800) d/Λ 0.5 0.36 Λ 2.45 μm 2.2 μm ZDW ~965 nm 1060 nm ZDW2 >2800 nm 1800 nm

(81) FIG. 14 shows graphs of the noise of an embodiment of a cascade optical fiber according to Example 2 for different lengths of the first section. The length of the first section of the cascaded fiber was gradually reduced from 10 m and the spectrum and noise were measured for lengths of 10 m, 6 m, 5 m, 4 m, 3 m, 2 m and finally 1 m. It was seen that for this particular choice of fibers, the minimum noise is obtained for a length L.sub.1 of the first section of about 3 m.

(82) FIG. 15 shows supercontinuum spectra obtained from the microstructured optical fiber of Example 2 and from a uniform reference fiber. The reference fiber for the measurements was again a uniform section of fiber. The pitch was 2.6 μm and the relative hole size (d/Λ) was 0.52. The length of the reference fiber was 10 m. This fiber is used in NKT Photonics product series SuperK EXW

(83) FIG. 15 shows noise spectra as a function of wavelength for an embodiment of a cascaded optical fiber of Example 2 as compared to a uniform reference fiber as indicated above. In the wavelength range from 750 to 920 nm, a reduction in noise is observed for the optical fiber of the invention, viz. in accordance with Example 2, compared to the uniform reference fiber. For wavelengths shorter than 750 nm, the noise in the output from the cascaded fiber of Example 2 is slightly higher than the uniform reference fiber. However, the average noise in the recorded wavelength interval is lower for the cascaded fiber, as it is clear from FIG. 17.

(84) FIG. 16 shows narrow bandwidth noise spectra as a function of wavelength for a cascaded optical fiber according to an embodiment of the invention as compared to a uniform fiber. The noise spectra of FIG. 16 are obtained for an amplification level of 60% for both the cascaded optical fiber according to an embodiment of the invention and for the reference fiber.

(85) FIG. 17 shows a noise spectrum for amplification levels between 20% and 100% of an incoherent supercontinuum source comprising an embodiment of a cascaded optical fiber according to the invention, as well as for a prior art optical fiber.

(86) In FIG. 17, a comparison of noise for tapered fiber according to Example 2 and the uniform reference fiber. In the two cases the noise is compared at identical pump power. For a given amplification level, the average noise is obtained by averaging the noise in the wavelength range from 680 to 920 nm. Thus, the graphs of FIG. 16, corresponding to the noise in the wavelength range from 680 to 920 nm result in one point in the graph of FIG. 16 for the cascaded fiber of Example 2 and another point in the graph of FIG. 16 for the uniform reference fiber. FIG. 17 shows that the average noise in this wavelength range from 680 to 920 nm is lower for the tapered fiber according to the invention compared to a uniform fiber for all power levels between 30% and 100%.

(87) FIG. 18 shows supercontinuum spectra obtained by four different optical fibers, two prior art optical fibers and two embodiments of optical fibers according to the invention.

(88) The solid curve (“Straight fiber”) shows the spectrum from a straight, uniform prior art fiber with a pitch of Λ=3.65 μm.

(89) The dashed curve (“Tapered for blue edge”) shows the spectrum from a fiber tapered to the blue edge, viz. a fiber tapered in order to obtain the maximum blue shift. This fiber has been tapered from a pitch Λ=3.65 μm to a pitch Λ=2.4 μm.

(90) The dotted curve (“Under tapered 1”) shows the spectrum obtained from an embodiment of a tapered fiber according to the invention. This fiber has been tapered to a pitch smaller than the blue edge pitch. This fiber has been tapered from a pitch Λ=3.65 μm to a pitch Λ=1.65 μm. Thus, the fiber corresponding to the dotted curve has been tapered to a smaller pitch than the fiber tapered down to the blue edge pitch, corresponding to the dashed curve.

(91) The dash-dotted curve (“Under tapered 2”) shows the spectrum obtained from an embodiment of another tapered fiber according to the invention. This fiber has been tapered from a pitch of Λ=3.65 μm to a pitch of Λ=1.50 μm. Thus, the fiber corresponding to the dash-dotted curve has been tapered to an even smaller pitch than the fiber corresponding to the dotted curve.

(92) From FIG. 18 it is clear that undertapering shifts the blue edge of the spectrum towards shorter wavelengths. Moreover, FIG. 19 show that the spectrum from the fiber denoted “Under tapered 2” has a relatively high power at wavelengths between about 430 nm and about 780 nm. In the range from about 430 nm to about 520 nm, the power is about 5 nW/nm or higher. The spectrum from the fiber denoted “Under tapered 1” has a quite high power, at or above 4 nW/nm, at wavelengths between about 430 nm and 475 nm.

(93) FIG. 19 is a schematic drawing of a supercontinuum light source 100 comprising a microstructured optical fiber 10 and a pump light source 20.

(94) FIG. 19 shows that the microstructured optical fiber 10 is a tapered fiber; however, the supercontinuum light source is not limited to sources with tapered fibers. Instead, a cascaded fiber could be the optical fiber of the supercontinuum light source.

(95) The pump light source 20 has an output 25 arranged to feed light into the end cap 8 of the microstructured optical fiber 10, adjacent to the first length section 12 of the optical fiber. The light fed into the end cap 8 of the optical fiber 10 continues to the intermediate length section 13 and the second length section 14. Due to the large size of the core of the fiber in the first length section 12, a large amount of light may be fed into the fiber 10 without damaging it. The light is confined to the core region, and as the core region of the fiber is reduced throughout the intermediate length section, the intensity of the confined light increases. However, due to the relatively long intermediate length section 13, the transition of the light intensity from the first length section 12 to the second length section 14 takes place adiabatically.

(96) It should be emphasized that the term “comprises/comprising” when used herein is to be interpreted as an open term, i.e. it should be taken to specify the presence of specifically stated feature(s), such as element(s), unit(s), integer(s), step(s) component(s) and combination(s) thereof, but does not preclude the presence or addition of one or more other stated features.

(97) All features of the inventions including ranges and preferred ranges can be combined in various ways within the scope of the invention, unless there are specific reasons for not to combine such features.

(98) Some embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims.