Supercontinuum light source comprising tapered microstructured optical fiber

Abstract

The invention relates to a supercontinuum light source comprising a microstructured optical fiber and a pump light source. The microstructured optical fiber comprises a core and a cladding region surrounding the core, as well as a first fiber length section, a second fiber length section and an intermediate fiber length section between said first and second fiber length sections. The first fiber length section comprises a core with a first characteristic core diameter. The second fiber length section comprises a core with a second characteristic core diameter, smaller than said first characteristic core diameter, where said second characteristic core diameter is substantially constant along said second fiber length section. The intermediate length section of the optical fiber comprises a core which is tapered from said first characteristic core diameter to said second characteristic core diameter over a tapered length.

Claims

1. A microstructured optical fiber for generating supercontinuum light upon feeding of light having a first wavelength .sub.1, the optical fiber having a length and a longitudinal axis along its length and comprising a core region for guiding light along the length of said optical fiber, and a first cladding region surrounding said core region, said first cladding region comprising a plurality of microstructure elements, wherein the plurality of microstructure elements are arranged in a plurality of layers within the first cladding region; said optical fiber, along its length, comprises a first fiber length section, a second fiber length section, and an intermediate fiber length section between said first and second fiber length sections; said first fiber length section has a core region with a first characteristic core diameter W.sub.1 in a cross-section through the microstructured optical fiber perpendicularly to the longitudinal axis; said second fiber length section has a core region with a second characteristic core diameter W.sub.2 in a cross-section through the microstructured optical fiber perpendicularly to the longitudinal axis, where said second characteristic core diameter W.sub.2 is smaller than said first characteristic core diameter W.sub.1; said intermediate length section of the optical fiber comprises a core region which is tapered from said first characteristic core diameter W.sub.1 to said second characteristic core diameter W.sub.2 over a tapered length L.sub.i; said first fiber length section has normal dispersion at said first wavelength .sub.i and a zero dispersion at a wavelength ZDW.sub.1 where ZDW.sub.1>about .sub.1+20 nm; and said second fiber length section has zero dispersion at a second wavelength ZDW.sub.2, where said second fiber length section has anomalous dispersion at wavelengths above said second wavelength ZDW.sub.2.

2. The optical fiber according to claim 1, wherein said first wavelength .sub.1 is up to about 1100 nm.

3. The optical fiber according to claim 1, wherein said first characteristic core diameter is larger than about 8 m.

4. The optical fiber according to claim 1, wherein said second characteristic core diameter is larger than about 3 m.

5. The optical fiber according to claim 1, wherein said inner cladding comprises a microstructure, said microstructure comprising a plurality of microstructure elements having a microstructure element diameter, d.sub.f, said microstructure elements being arranged at a pitch , said pitch being a measure of a spacing between the microstructure elements, wherein said microstructure is at least partially maintained along the first and second fiber length sections of the optical fiber, wherein the relative size (d.sub.f/) of the microstructure elements is larger in the second fiber length section of the optical fiber than in the first fiber length section of the optical fiber, said relative size being the ratio between the diameter (d.sub.f) of the microstructure elements and the pitch .

6. The optical fiber according to claim 5, wherein the relative size (d.sub.f/) of the microstructure elements is chosen so that the first fiber length section is a single mode fiber at least at the first wavelength .sub.1.

7. The optical fiber according to claim 1, said optical fiber further comprising a second cladding surrounding said first cladding, wherein: said core region is adapted to guide an optical signal at said first wavelength .sub.1, said core region having an effective refractive index n.sub.core, and where said core region comprises a material doped with at least one rare earth element; said first cladding being arranged for guiding light at a third wavelength .sub.3, said first cladding having an effective refractive index n.sub.first-clad; said second cladding having an effective refractive index n.sub.second-clad; and n.sub.core>n.sub.first-clad>n.sub.second-clad and .sub.1>.sub.3.

8. The optical fiber according to claim 1, wherein the first characteristic core diameter is larger than about 7 m.

9. The optical fiber according to claim 1, wherein ZDW.sub.2 is up to about .sub.1+50 nm.

10. A supercontinuum light source comprising: a microstructured optical fiber for generating supercontinuum light at a wavelength .sub.1, said microstructured optical fiber comprising a core region that is capable of guiding light along a longitudinal axis of said optical fiber, and a first cladding region surrounding said core region, said first cladding region comprising a plurality of microstructure elements; and a pump light source arranged to feed light into said core region at an input end of said optical fiber, said light having, a first wavelength, .sub.1, wherein: the plurality of microstructure elements are arranged in a plurality of layers within the first cladding region; said optical fiber has a length and a longitudinal axis along its length, wherein said optical fiber comprises a first fiber length section, a second fiber length section as well as an intermediate fiber length section between the first and second fiber length sections; said first fiber length section has a core region with a first characteristic core diameter W.sub.1; said second fiber length section has a core region with a second characteristic core diameter W.sub.2 in a cross-section through the microstructured optical fiber perpendicularly to the longitudinal axis, where said second characteristic core diameter W2 is smaller than said first characteristic core diameter W.sub.1; said intermediate fiber length section of the optical fiber comprises a core region which is tapered from said first characteristic core diameter W.sub.1 to said second characteristic core diameter W.sub.2 over a tapered length L.sub.i; said first fiber length section has normal dispersion at a first wavelength .sub.1 and a zero dispersion at a wavelength ZDW.sub.1 where ZDW.sub.1>about .sub.1+20 nm; and said second fiber length section has zero dispersion at a second wavelength ZDW.sub.2, where said second fiber length section has anomalous dispersion at wavelengths above said second wavelength ZDW.sub.2.

11. The supercontinuum light source according to claim 10, wherein the pump light source has a pulse duration t, wherein the pulse duration is more than about 1 ps.

12. A supercontinuum light source according to claim 10, wherein an output pulse from said light source has a pulse length of less than about 20 ns.

13. A supercontinuum light source according to claim 10, said microstructured optical fiber further comprising a second cladding surrounding said first cladding, wherein: said core region is adapted to guide an optical signal at said first wavelength .sub.1, said core region having an effective refractive index n.sub.core, and where said core region comprises a material doped with at least one rare earth element; a second pump light source is arranged for feeding light at a second wavelength .sub.2; said first cladding being arranged for guiding light at said second wavelength .sub.2, said first cladding having an effective refractive index n.sub.first-clad; said second cladding having an effective refractive index n.sub.second-clad; and n.sub.core>n.sub.first-clad>n.sub.second-clad and .sub.1>.sub.2.

14. The supercontinuum light source according to claim 10, wherein the first characteristic core diameter is larger than about 7 m.

15. The supercontinuum light source according to claim 10, wherein ZDW.sub.2 is up to about .sub.1+50 nm.

16. A method of using a supercontinuum light source according to claim 10, the method comprising delivering supercontinuum light from the supercontinuum light source and using the light delivered by the supercontinuum light source for at least one of the following applications: photoacoustic measurements, multi-spectral imaging, LIDAR (Light Detection and Ranging), STED (Stimulated Emission Depletion).

17. A photoacoustic imaging system comprising a supercontinuum light source according to claim 10, a detector for detecting ultrasonic emission waves, and an image processor for forming an image of the detected ultrasonic waves.

18. The photoacoustic imaging system of claim 17 further comprising an optical coherence tomography (OCT) imaging system, wherein the photoacoustic imaging system and the OCT imaging system use said supercontinuum light source as a common light source.

19. A method of performing photoacoustic imaging of a biological tissue the method comprising providing a photoacoustic imaging system as claimed in claim 17 and delivering supercontinuum laser pulses from said supercontinuum light source to said biological tissue and collecting ultrasonic emission waves from said biological tissue by said detector and forming the image using said image processor.

Description

BRIEF DESCRIPTION OF DRAWINGS

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

(2) FIG. 1 is a schematic drawing of a cross-section of a microstructured fiber, along the longitudinal axis;

(3) FIG. 2 shows a picture of a cross-section of a microstructured fiber, perpendicular to the longitudinal axis;

(4) FIG. 3 shows part of a cross-section of a microstructured fiber, perpendicular to the longitudinal axis;

(5) FIG. 4a shows a schematic drawing of a microstructured optical fiber according to the invention;

(6) FIGS. 4b and 4c show cross-sections of a microstructured optical fiber, perpendicular to the longitudinal axis, at a first and second fiber length section, respectively;

(7) FIG. 5 is a cross-section of a microstructured optical fiber according to the invention, along the longitudinal axis;

(8) FIG. 6 is a schematic drawing of a supercontinuum light source comprising a microstructured optical fiber and a pump light source;

(9) FIG. 7 is a graph showing dispersion curves for two optical fibers as a function of wavelength;

(10) FIG. 8 is a supercontinuum spectrum from a supercontinuum light source of the invention, and

(11) FIG. 9 is an illustration of a pump light source suitable for a super-continuum light source and comprising a system for active feedback.

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

(13) 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.

(14) 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.

(15) 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.

(16) With reference to FIG. 1 is described a schematic drawing of a cross-section of a microstructured fiber, along the longitudinal axis. The cross-section is an enlarged view of a very short length of microstructured fiber 1. The microstructured fiber 1 has a core region 2 and a cladding region 3 surrounding the core region 2.

(17) The microstructured optical fiber 1 has a length consisting of a length 7 of the microstructured fiber as well as a length 8 of an end cap, as well as an outer diameter D. The microstructures of the microstructured fiber 1 are holes. These holes have been collapsed as shown in the figure in the area of fiber length 7 adjacent to the end cap 8. One way of carrying out such a collapse is by heating of the optical fiber 1.

(18) In this prior art embodiment, the microstructured fiber is a non-tapered fiber. The characteristic diameter of the core is of the order of magnitude some m, typically 3.5 or 5 m; the extension of the end cap length 8 in the longitudinal axis of the fiber is of a magnitude of 100 m, e.g. 200 m, whilst the microstructured fiber length is e.g. several meters, for example 10 meters.

(19) Light has been fed into the fiber; the reference number 4 indicates light confined within the core region of the fiber 1, reference number 5 indicates light spreading from the core region of the length 7 of the fiber into the end cap 8, whilst reference number 6 indicates light exiting from the end cap 8 of the optical fiber 1. It may be seen that the light spreads out in the region of the collapsed microstructures and in the end cap 8 compared to the length 7.

(20) FIG. 2 shows a picture of a cross-section of a microstructured fiber 1, perpendicular to the longitudinal axis. The fiber is a microstructured fiber comprising a core region 2 and a cladding region 3, the cladding region surrounding the core region. The cladding region comprises low-index cladding features 9, here including features in the form of holes or voids extending in the longitudinal direction of the fiber, and an inner cladding background or base material. The core region 2 comprises a refractive index profile such that the core region comprises material with a refractive index n.sub.core 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. 3.

(21) FIG. 3 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. 2. In FIG. 3 is shown the core area or core region 2 as defined as the area of a circle inscribed by the microstructure elements of the cladding arranged to immediately surround the core 2. 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, and the microstructure elements are arranged at a pitch , where the pitch is a measure of the spacing between the microstructure elements. As shown in FIG. 3 the pitch is indicated as the distance between the centers of two adjacent microstructure elements.

(22) FIG. 4a shows a schematic drawing of a microstructured optical fiber 10 according to the invention, and FIGS. 4b and 4c 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 .sub.1 preferably up to about 1100 nm, such as from about 900 nm to about 1100 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.

(23) The optical fiber 10, along its length, comprises a first fiber length section 12, a second fiber length section 14 as well as an intermediate fiber length section 13 between the first and second fiber length sections 12, 14.

(24) In one embodiment the total length of the optical fiber 10 is such as less than about 50 m, such as less than about 30 m, such as less than about 20 m.

(25) FIG. 4b shows a cross-section of the microstructured fiber, perpendicular to the longitudinal axis, at the first length section 12. It is indicated that the first fiber length section 12 has a core region with a first characteristic core diameter W.sub.1 in a cross-section through the microstructured optical fiber perpendicularly to the longitudinal axis. The first characteristic core diameter W.sub.1 is larger than about 7 m. The second fiber length section 14 has a core region with a second characteristic core diameter W.sub.2 in a cross-section through the microstructured optical fiber perpendicularly to the longitudinal axis, where the second characteristic core diameter W.sub.2 is smaller than the first characteristic core diameter W.sub.1. The microstructure elements of the microstructured optical fiber at the first fiber length section 12 are arranged at a pitch .sub.1 and have a microstructure element diameter d.sub.1, whilst the microstructure elements of the microstructured optical fiber at the second fiber length section 14 are arranged at a pitch .sub.2 and have a microstructure element diameter d.sub.2, as indicated in FIGS. 4b and 4c.

(26) The intermediate fiber length section 13 of the optical fiber 10 comprises a core region which is tapered from the first characteristic core diameter W.sub.1 to the second characteristic core diameter W.sub.2 over a tapered length L.sub.i.

(27) The first fiber length section 12 has normal dispersion at the first wavelength .sub.1 and the second fiber length section has zero dispersion at a second wavelength ZDW.sub.2, where ZDW.sub.2 is up to about .sub.1+50 nm. The second fiber length section has anomalous dispersion at wavelengths above the second wavelength ZDW.sub.2.

(28) FIG. 5 shows a cross-section of the tapered microstructured optical fiber 10 along the longitudinal axis. From FIG. 5 it may be seen that the structure of the microstructured optical fiber 10 comprising a core region 2 and a cladding region 3 is maintained throughout the first fiber length section 12, the tapering in the intermediate fiber length section 13 and the second fiber length section 14. As explained in relation to FIGS. 4a-4c, the size of the core region as well as the pitch and the microstructure diameter differ in the different fiber length sections 12, 13, 14 of the microstructured optical fiber, but the number of microstructures is kept, except at the end cap region 8 of the microstructured optical fiber 10. The vertical hatched lines in FIG. 6 indicates the transition between the first fiber length section 12 and the intermediate fiber length section 13 and between the second fiber length section 14 and the intermediate fiber length section 13.

(29) Referring again to FIGS. 4a-4c, it may be seen from FIG. 5a 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 microstructured optical fiber from the first length section 12 to the second fiber length section 14. In the first fiber length section 12, the first characteristic core diameter W.sub.1 is substantially constant, and in the second fiber length section 14, the second characteristic core diameter W.sub.2 is substantially constant. Moreover, the microstructure element pitch .sub.1 and microstructure element diameter d.sub.1 are substantially constant in the first fiber length section 12; the microstructure element pitch .sub.2 and microstructure element diameter d.sub.2 are substantially constant in the second fiber length section 14, whilst the microstructure pitch and microstructure element diameter differ along at least a part of the intermediate fiber length section 13 of the microstructured optical fiber 10.

(30) FIG. 5 shows a cross-section through a microstructured tapered fiber 10 according to an embodiment of the invention, along the longitudinal axis of the microstructured optical fiber. The microstructured optical fiber comprises an end cap 8, a first fiber length section 12, an intermediate fiber length section 13 and a second fiber length section 14. The vertical dashed lines indicate the transition between the end cap 8 and the first fiber length section, between the first fiber length section 12 and the intermediate fiber length section 13 and the transition between the intermediate fiber length section 13 and the second fiber length section 14, respectively.

(31) FIG. 5 shows that the microstructured optical fiber 10 has a core region 2 and a cladding region 3. The cladding region comprises low-index cladding features 9, for example features in the form of holes or voids extending in the longitudinal direction of the microstructured optical fiber, and an inner cladding background or base material in which the cladding features are embedded. FIG. 5 shows that the core region 2 and thus the characteristic core diameter is larger in the first fiber length section 12 than in the second fiber length section 14, whilst the characteristic core diameter changes along the length of the intermediate fiber length section 13. Moreover, FIG. 5 shows that the number of low-index cladding features 9 is unchanged in the first fiber length section 12, the intermediate fiber length section 13 and the second fiber length section 14. However, the cladding region 3 is larger in the first fiber length section 12 than in the second fiber length section 14, whilst the size of the cladding region changes along the length of the intermediate length section 13.

(32) As an example only, the characteristic core diameter at the first fiber length section is 10 m and the characteristic core diameter at the second fiber length section is 5 m.

(33) It is an insight of the inventors that fiber damage due to high optical peak powers and/or pulse energies often takes place at the transition between the end cap and the first fiber length section 12, and not at the end cap input of the microstructured optical fiber. Therefore, by providing a fiber with enlarged input dimensions in the form of a fiber with a first fiber length section having enlarged core diameter compared to the second fiber length section, it is achieved that the microstructured optical fiber is able to receive more peak power and/or pulse energy without being damaged. It is believed that this is due to the effect that the light fed into the first fiber length section is distributed over the large core in the first fiber length section.

(34) FIG. 6 is a schematic drawing of a supercontinuum light source 100 comprising a microstructured optical fiber 10 and a pump light source 20. 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 fiber length section 12 of the optical fiber. The light fed into the end cap 8 of the optical fiber 10 continues to the intermediate fiber length section 13 and the second fiber length section 14. Due to the large size of the core of the microstructured optical fiber in the first fiber length section 12, a large amount of light may be fed into the microstructured optical fiber 10 without damaging it. The light is confined to the core region, and as the core region of the microstructured optical fiber is reduced throughout the intermediate fiber length section, the intensity of the confined light increases. However, due to the relatively long intermediate fiber length section 13, the transition of the light intensity from the first fiber length section 12 to the second fiber length section 14 takes place adiabatically.

(35) FIG. 7 is a graph showing dispersion curves for two optical commercially available fibers, LMA-10 and SC-5.0-1040 (NKT Photonics A/S), as a function of wavelength . The dispersion curve for the optical fiber LMA-10 is shown by a solid curve, whilst the dispersion curve for the optical fiber SC-5.0-1040 is shown by a dashed curve. FIG. 8 shows that the zero dispersion wavelength, ZDW.sub.LMA, for the optical fiber LMA-10 is about 1180 nm, whilst the zero dispersion wavelength, ZDW.sub.SC, for the optical fiber SC-5.0-1040 is about 1040 nm. As the skilled person knows, the area below the horizontal line indicating zero dispersion corresponds to normal dispersion, whilst the area above the horizontal line indicating zero dispersion corresponds to anomalous dispersion. From this it can be seen that the first fiber length section in an embodiment of the microstructured fiber of the invention for example may have a cross-sectional structure corresponding to the optical fiber LMA-10, such that ZDW.sub.1=ZDW.sub.LMA, and the second fiber length section may have a cross-sectional structure corresponding to the optical fiber SC-5.0-1040, such that ZDW.sub.2=ZDW.sub.SC.

(36) FIG. 8 is a supercontinuum spectrum from a supercontinuum light source 100 of the invention. The supercontinuum light source 100 comprises a pump light source 20 and a microstructured optical fiber for generating supercontinuum (SC) light (microstructured SC fiber 10).

(37) The pump laser 20 applied was a passive q-switched laser, comprising a Nd:YAG crystal combined with a Cr:YAG. The backside of the Nd:YAG was coated with HR coating at 1064 nm and a semi-transparent mirror with a 60% reflection@1064 was placed in front of the Cr:YAG. The cavity was pumped using an 808 nm diode pump. The resulting laser cavity emitted pulsed light at 1064 nm with a repetition rate of app. 18-20 kHz and a pulse width of 1.2 ns. Measured output power was 600 mW, but with a portion of this light arising from non-absorbed pump laser light (residual 808 nm light).

(38) The pump laser was attempted coupled into the commercially available fiber SC-5.0-1040 from NKT Photonics. The fiber has pitch =3.3 m, and relative hole size d/A=0.52 giving core size 5 m. It was observed that the laser damaged the fiber in such a way that no light could be transmitted and that the damage seemed to occur instantaneously. This is a typical failure mechanism when the pulse energy is too high.

(39) It is known from prior tests that the SC-5.0-1040 can withstand 1.2 ns laser pulses at 1064 nm having pulse peak power in the range of 10-12 kW, and that such pulses generate a broad supercontinuum from 0.5 to 2.4 m in the fiber. This fiber is used for NKT Photonics commercial product SuperK Compact.

(40) The pump laser 20 was applied to an example of the microstructured optical fiber of the invention comprising an intermediate tapered fiber length section in order to prove that this fiber could sustain the pulse peak power which a normal non-linear fiber cannot. The microstructured optical fiber used for the experiment had the following characteristics: d/=0.52 in the first fiber length section, the second fiber length section as well as in the intermediate fiber length section; Characteristic core diameter in first fiber length section: 10 m; Characteristic core diameter in second fiber length section: 5 m; Length of first fiber length section: app. 1 m (0.1 m); Length of taper, viz. intermediate fiber length section: app. 1 m (0.1 m); Total length of the microstructured optical fiber: app. 15 m (0.5 m).

(41) Both ends of the microstructured optical fiber were cleaved and collapsed with a 200 m25 m collapse length, giving end caps.

(42) The input end of the microstructured optical fiber viz. the input end of the first fiber length section, was positioned in front of the pump laser, and a pair of lenses was used to couple the light from the pump laser into the fiber. The lens pair consisted of a collimating lens having a 100 mm focal length and a focusing lens having a focal length of 3.5 mm. The two lenses were separated by a distance of app. 30 mm. The lenses were aligned in the x- and y-axis until seed light was coupled into the core of the microstructured optical fiber. Focus on the fiber was achieved by moving the microstructured optical fiber end towards the focusing lens until maximum power was measured using a thermal detector from Ophir Optronics (Ophir power head model#3A).

(43) The alignment procedure continued until maximum power out of the microstructured optical fiber was measured. This was achieved around 70 mW output power.

(44) Once a supercontinuum was achieved through the tapered microstructured optical fiber, the result was recorded using an Optical Spectrum Analyser (OSA) from ANDO, model #6315B. A spectrum was recorded from 400 to 1750 nm with a 5 nm resolution. The spectrum was recorded through an integrating sphere (Ocean Optics) with a multimode fiber having an 1 mm fiber core.

(45) The coupling from the seed laser into the tapered microstructured optical fiber operated at maximum achieved output power (app. 70 mW) for app. 10 minutes in order to verify that the microstructured optical fiber core would not burn due to the increased pulse peak power. In contrast when the laser was fed directly into the prior art fiber SC-5.0-1040, the core of the optical fiber was destroyed within a few seconds once optimal power has been achieved due to the high peak power and pulse energy of the pump source. A significant portion of the measured 600 mW input power is believed to arise from non-absorbed 808 nm pump light which can be seen in the spectrum. To achieve the best injection efficiency into single-mode fiber, the direction, position, size and divergence of the beam from the pump light source are advantageously all optimized. In the present experiment, is believed that the lens pair was not optimally aligned with respect to the core of the microstructured optical fiber resulting in a significant portion of light being lost instead of reaching the microstructured optical fiber. Thus, it is believed that considerable more output power is achievable, in the order of magnitude 200-300 mW.

(46) The experiment demonstrates that the first fiber length section having an enlarged size compared to the second fiber length section and compared to the standard non-linear fiber is capable of coupling the light into the second fiber length section which is here a single mode part of the microstructured optical fiber, thus creating a supercontinuum. Thus, light in the first fiber length section and/or in the intermediate fiber length section does not exit the microstructured optical fiber at end of these the respective first and intermediate fiber length sections, but is being coupled into the core of the single mode structure in the second fiber length section of the microstructured optical fiber.

(47) The experiment also demonstrates that the expanded input in the form of the first fiber length section having a large first characteristic core diameter is able to sustain a greater pulse peak power than the prior art non-linear supercontinuum fibers. This is seen in that the relatively high peak power provided by the pump laser tends to damage the input end facet of a prior art standard supercontinuum, non-linear fiber but did not damage the tapered microstructured fiber of an embodiment of the invention.

(48) Thus, the experiment shows that the microstructured optical fiber according to the invention is useful for relatively high energy pulse power compared to prior art fibers used for SC generation.

(49) In an embodiment of the invention, the pump laser 200 is a q-switched laser comprising an active feedback to control the emission frequency (repetition rate) of the pump laser, and/or to reduce the timing jitter between the pump laser pulses.

(50) Passive Q-switched pump lasers comprising an saturable absorber (e.g. a Cr:YAG Crystal) and a laser diode to pump the saturable absorber (e.g. a CW laser diode with a peak wavelength at 808 nm) are well known in the art. Here pulses are emitted from the pump laser whenever the saturable absorber becomes transparent (bleaches) thus changing the transmission through the crystal from a low transmission value to a high transmission value. This is e.g. described in the W. Koechner's book Solid-State Laser Engineering on pp. 522-523 (Springer, Sixth revised and Updated Edition, 2010, ISBN-13: 978-14419-2117-8). The repetition rate of the emitted pulse train is random in nature due to a number of limitations in the seed laser cavity (noise, fluctuation in input power, temperature fluctuation, etc.).

(51) In an aspect the inventors have found that a pump light source comprising a system for active feedback as shown in FIG. 9 in principle can be applied as pump source for any kind of supercontinuum fiber, such as non-tapered microstructured supercontinuum, in a supercontinuum light source. In an embodiment the pump light source comprises a q-switched laser comprising an active feedback system for active feedback as described below.

(52) The pump light source generally provides a very stable emission frequency (repetition rate) of the pump laser and/or a reduced timing jitter between the pump laser pulses.

(53) In an embodiment the control is obtained by modulating the output from the laser diode. This might e.g. be obtained by detecting whenever a pulse is emitted from the pump laser 200. The feedback signal is used to switch the laser diode from ON (emitting light) to OFF (no emission).

(54) FIG. 9 is a diagram of an embodiment of the pump light source comprising a system for active feedback. As mentioned this pump light source may advantageously be used as a part of the supercontinuum light source of the invention but it may in principle be applied as a light source in any optical systems.

(55) In the embodiment illustrated in FIG. 9, the pulse detection is obtained by placing a photodiode so that it receives part of the emitted pulses from the pump light source. The photodiode should be sensitive to light emitted from the pump laser (1064 nm), but not sensitive to light emitted from the laser diode (808 nm) as light from the laser diode may give rise to noise in the detection. This is obtained by either careful selection of photodiode type or by placing an appropriate filter in front of the detector.

(56) When the laser diode 30 is turned ON it will start to pump energy into the pump laser cavity 31. No light is emitted from the pump laser until the saturation state of the saturable absorber is changed allowing light from the seed laser cavity to emit a pulse. When the pulse is emitted from the pump laser it will be detected by the photodiode 32. The signal from the photodiode is sent to a control unit 33, which sends a signal to the laser diode to shut it OFF thus preventing further light to be pumped into the laser cavity.

(57) With the laser diode turned OFF no light is pumped into the pump laser cavity, and consequently no further seed laser pulses are emitted. The laser diode may be turned to ON again whenever a new pulse is requested by the supercontinuum laser system. This request may either be at a constant frequency or by a user trigger input. The maximum obtainable repetition rate is achieved when the requested frequency exceeds the pump capacity of the pump source laser. This happens when a new pulse is requested before the previous pulse is yet emitted from the pump laser. The laser diode will then operate continuously providing the maximal repetition rate possible by the pump laser.

(58) During the laser diode OFF state the laser may be kept on a threshold level where no light is emitted from the laser. Keeping the laser diode at threshold will enable a faster rise time of the laser diode unit and thus a faster response of the pump laser.

(59) The use of active feedback enables full control of the pump laser repetition rate up to the level where the laser diodes operates continuously, which typically occurs at a pump laser repetition rate of 40-50 kHz. Furthermore a much more stable pulse-to-pulse signal will be obtained as the pulse-to-pulse jitter is only dependent on the amount of energy (and thus time) the laser diode requires in order to provide sufficient energy for the pump laser to emit a pulse. The timing jitter between pulse emissions from the pump laser is reduced to a few hundred ns (100-300 ns) where a normal passive q-switched pump laser may have a pulse-to-pulse jitter in the range of a few ms (2-10 ms). As the pump laser only acquires the amount of energy sufficient in order to emit a single pulse at a time, the emitted pulse itself is expected to be less sensitive to pulse jitter such as pulse width jitter and pulse amplitude jitter.

(60) In one embodiment of the invention the pump laser has a variable repletion rate.

(61) In one embodiment of the invention the pump laser pulses are externally triggered. This could e.g. by used in connection with a measurement method having a finite sampling time, where it is advantageous to have the same number of pulses within each of the sampling intervals. One example is hyperspectral imaging with a given shutter time.

(62) In one embodiment of the invention the pump laser timing jitter is such as less than about 1 ms, such as less than about 500 ns, such as less than about 300 ns, such as less than about 200 ns.

(63) The supercontinuum light source of the invention has been found to be highly suitably for performing photoacoustic imaging due to its ability to generate supercontinuum pulses with very high pulse energy.

(64) The invention also comprises a photoacoustic imaging system comprising a supercontinuum light source as described above, a detector for detecting ultrasonic emission waves and an image processor for forming an image of the detected ultrasonic waves. The detector can be any that is suitable for ultrasonic emission waves, preferably with a high sensitivity e.g. such as ultrasonic emission waves used in prior art photoacoustic imaging systems.

(65) The processor for forming an image of the detected ultrasonic waves can for example be in prior art photoacoustic imaging system.

(66) Photoacoustic imaging is well known and is based on the photoacoustic effect. In photoacoustic imaging, non-ionizing laser pulses are delivered into biological tissues. Some of the delivered energy will be absorbed and converted into heat, leading to transient thermoelastic expansion and thus wideband (e.g. MHz) ultrasonic emission. The generated ultrasonic waves are then detected by ultrasonic transducers to form images. It is known that optical absorption is closely associated with physiological properties, such as hemoglobin concentration and oxygen saturation. As a result, the magnitude of the ultrasonic emission (i.e. photoacoustic signal), which is proportional to the local energy deposition, reveals physiologically specific optical absorption contrast. 2D or 3D images of the targeted areas can then be formed.

(67) The invention further comprises a method of performing photoacoustic imaging of a biological tissue the method comprising providing a photoacoustic imaging system as described above and delivering supercontinuum laser pulses from said supercontinuum light source to said biological tissue and collecting ultrasonic emission waves from said biological tissue by said detector and forming the image using said image processor.

(68) Due to the very high power and pulse energy of the supercontinuum light source of the invention, the supercontinuum light source has further been found to be very suitable for use in multimodal imaging, e.g. combining photoacoustic and optical coherence tomography (OCT) imaging. The invention therefore also relates to a multimodal photoacoustic and optical coherence tomography (OCT) image acquisition system comprising photoacoustic imaging system as described above combined with an OCT imaging system, wherein the photoacoustic imaging system and the OCT imaging system using said supercontinuum light source as a common light source, said multimodal photoacoustic and optical coherence tomography (OCT) image acquisition system preferably further comprises a detector for collecting reflected light and an image processor for forming an image of the detected reflected light.

(69) Optical coherence tomography is an established medical imaging technique. It is widely used, for example, to obtain high-resolution images of the anterior segment of the eye and the retina. OCT is advantageous for delivering high resolution because it is based on light, rather than sound or radio frequency.

(70) An optical beam is directed at the tissue to be analyzed, and a small portion of this light that reflects from sub-surface features is collected. Most of the light is not reflected but, rather, scatters off at large angles. OCT uses a technique called interferometry to record the optical path length of received photons allowing rejection of most photons that scatter multiple times before detection. Thus OCT can build up clear 3D images of thick samples by rejecting background signal while collecting light directly reflected from surfaces of interest. The invention is in particular suited for spectral domain OCT.

(71) The multimodal photoacoustic and optical coherence tomography (OCT) image acquisition system preferably comprises a filter for selecting a spectral portion of the light beams to be applied in the OCT. Due to the very high power the multimodal photoacoustic and optical coherence tomography (OCT) image acquisition system advantageously comprises an intensity filter for reducing the intensity applied in OCT in dependence on the subject to be analyzed. Where OCT is applied for analysis of sensitive tissue, such as eye tissue, it is desired that the intensity is kept relatively low in order not to damage the tissue.

(72) 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.

(73) 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.

(74) 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.