Long wavelength generation in optical fiber

Abstract

The invention relates to a supercontinuum source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light. The supercontinuum comprises infrared wavelengths generated in the nonlinear fiber from the pump light. The nonlinear fiber has a dispersion profile comprisinga zero dispersion wavelength,a positive peak value at a peak wavelength longer than the zero dispersion wavelength,a minimum value of dispersion at a minimum wavelength longer than the peak wavelength. The pump light is arranged to comprise substantial energy at one or more preferred pump wavelengths which are 10 nm longer than the zero dispersion wavelength or more, The invention also relates to a supercontinuum pump source comprising a nonlinear fiber having a core comprising a fluoride glass and having a core diameter smaller than 7 ?m, where the fiber has a numerical aperture of more than 0.26.

Claims

1. A supercontinuum light source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light wherein said pump light and nonlinear fiber are arranged so that a supercontinuum comprising infrared wavelengths is generated in said nonlinear fiber from said pump light, said nonlinear fiber having a dispersion profile, the dispersion profile comprising: a. a shortest zero dispersion wavelength ZDW equal to or longer than 1600 nm, b. a positive peak value at a peak wavelength longer than said zero dispersion wavelength ZDW, and c. a minimum value of dispersion D.sub.min at a minimum wavelength longer than 2 ?m, wherein said nonlinear fiber is a step-index fiber and is transparent at a wavelength longer than 3 ?m, wherein said core of said nonlinear fiber comprises fluoride glass comprising InF.sub.3, and wherein said pump light is arranged to comprise energy at one or more preferred pump wavelengths which are longer than said zero dispersion wavelength ZDW by 150 nm or more, whereby said dispersion profile causes a soliton red-shift that occurs when said supercontinuum is generated in said nonlinear fiber to be most accelerated at wavelengths at which Raman based soliton energy loss and an increasing mode field diameter cause the soliton red-shift to slow down, thereby overcoming said soliton red-shift slow down and enabling the soliton red-shift to continue to longer infrared wavelengths.

2. A supercontinuum light source according to claim 1, wherein said preferred pump wavelengths are shorter than said peak wavelength by 400 nm or less.

3. A supercontinuum light source according to claim 1, wherein said preferred pump wavelengths are longer than 1.6 microns.

4. A supercontinuum light source according to claim 1, wherein a significant part of the energy in said pump light entering the nonlinear fiber is at wavelengths 200 nm or longer than said zero dispersion wavelength ZDW, the significant part being 5% or more of the pump energy.

5. A supercontinuum light source according to claim 1, wherein said peak wavelength is longer than 1.5 ?m and shorter than 24 ?m.

6. A supercontinuum light source according to claim 1, wherein said peak wavelength is longer than said zero dispersion wavelength ZDW by 100 nm or more.

7. A supercontinuum light source according to claim 1, wherein said minimum wavelength is longer than 3 ?m.

8. A supercontinuum light source according to claim 1, wherein said nonlinear fiber is arranged so that for a range of wavelengths the dispersion is lower than the dispersion at said preferred pump wavelengths and wherein the dispersion in at least part of said range of wavelengths is at least 5 ps/(nm km) lower than the dispersion at the peak wavelength.

9. A supercontinuum light source according to claim 1, wherein the dispersion is non-negative at said minimum wavelength.

10. A supercontinuum light source according to claim 1, wherein said core of said nonlinear fiber further comprises a ZBLAN glass.

11. A supercontinuum light source according to claim 1, wherein said pump light and nonlinear fiber are arranged so that said supercontinuum at some pump powers extends to wavelengths longer than said minimum wavelength.

12. A supercontinuum light source according to claim 1, wherein a majority of the output energy in the generated supercontinuum is emitted at wavelengths longer than 1800 nm.

13. A supercontinuum light source according to claim 1, in which the core of the nonlinear fiber has a numerical aperture of more than 0.3.

14. A supercontinuum light source according to claim 1, wherein the pump light source comprises a pulse picker and/or frequency multiplier arranged to adjust the repetition rate of the supercontinuum light source.

15. A supercontinuum light source according to claim 1 that is configured to generate a low noise supercontinuum, where a relative intensity noise (RIN) in any part of the spectrum is less than ?110 dB/Hz.

16. A supercontinuum light source according to claim 1, wherein said preferred pump wavelengths are shorter than said peak wavelength by 300 nm or less.

17. A supercontinuum light source according to claim 1, wherein said preferred pump wavelengths are shorter than said peak wavelength by 200 nm or less.

18. A supercontinuum light source according to claim 1, wherein core of the nonlinear fiber has a core diameter larger than 4 ?m and a numerical aperture of more than 0.25.

19. A supercontinuum light source according to claim 1, wherein the nonlinear fiber has a length of at least 1 m.

20. A supercontinuum light source according to claim 1, wherein said pump light and said nonlinear fiber are arranged so that the generated supercontinuum has wavelengths longer than 3 ?m.

21. A supercontinuum light source according to claim 1, wherein said pump light source has a peak power of less than 10 MW.

22. A supercontinuum light source according to claim 1, wherein said pump light source is a pulsed laser emitting pulses with a duration longer than 1 ps.

23. A supercontinuum light source according to claim 1, wherein said core comprises InF.sub.3 in a concentration of less than 55 mol %, but more than 1 mol %.

24. A supercontinuum light source according to claim 1, wherein said core of said nonlinear fiber has a core diameter smaller than 7 ?m and a numerical aperture of more than 0.26.

25. A supercontinuum light source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light, where said pump light and nonlinear fiber are arranged so that a supercontinuum comprising infrared wavelengths is generated in said nonlinear fiber from said pump light, said nonlinear fiber being a step-index fiber and having a dispersion profile, wherein said core comprises fluoride glass comprising InF.sub.3 and has a core diameter smaller than 7 ?m and a numerical aperture of more than 0.26, and wherein the dispersion profile comprises: a. a zero dispersion wavelength ZDW equal to or longer than 1600 nm, b. a positive peak value at a peak wavelength longer than said zero dispersion wavelength ZDW, and c. a minimum value of dispersion D.sub.min at a minimum wavelength longer than 2 ?m, wherein said nonlinear fiber is transparent at a wavelength longer than 3 ?m, and wherein said pump light is arranged to comprise energy at one or more preferred pump wavelengths which are longer than said zero dispersion wavelength ZDW by 150 nm or more, whereby said dispersion profile causes a soliton red-shift that occurs when said supercontinuum is generated in said nonlinear fiber to be most accelerated at wavelengths at which Raman based soliton energy loss and an increasing mode field diameter cause the soliton red-shift to slow down, thereby overcoming said soliton red-shift slow down and enabling the soliton red-shift to continue to longer infrared wavelengths.

26. A supercontinuum light source of claim 25, wherein said core diameter is smaller than 6.8 ?m.

27. The supercontinuum light source of claim 25, wherein said numerical aperture is more than 0.275.

28. A supercontinuum light source according to claim 25, wherein said non-linear fiber core further comprises ZBLAN glass.

29. A supercontinuum light source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light where said pump light and nonlinear fiber are arranged so that a supercontinuum comprising infrared wavelengths is generated in said nonlinear fiber from said pump light, said nonlinear fiber having a dispersion profile, wherein said core comprises fluoride glass comprising InF.sub.3 and comprises PbF.sub.2 in a concentration of less than 30 mol %, but more than 1 mol %, and wherein said nonlinear fiber is a step-index fiber and the dispersion profile comprises: a. a zero dispersion wavelength ZDW equal to or longer than 1600 nm, and b. a positive peak value at a peak wavelength longer than said zero dispersion wavelength ZDW, and c. a minimum value of dispersion D.sub.min at a minimum wavelength longer than 2 ?m, wherein said nonlinear fiber is transparent at a wavelength longer than 3 ?m, and wherein said pump light is arranged to comprise energy at one or more preferred pump wavelengths which are longer than said zero dispersion wavelength ZDW by 150 nm or more, whereby said dispersion profile causes a soliton red-shift that occurs when said supercontinuum is generated in said nonlinear fiber to be most accelerated at wavelengths at which Raman based soliton energy loss and an increasing mode field diameter cause the soliton red-shift to slow down, thereby overcoming said soliton red-shift slow down and enabling the soliton red-shift to continue to longer infrared wavelengths.

30. A supercontinuum light source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light wherein said pump light and nonlinear fiber are arranged so that a supercontinuum comprising infrared wavelengths is generated in said nonlinear fiber from said pump light, said nonlinear fiber having a dispersion profile, the dispersion profile comprising: a. a zero dispersion wavelength ZDW equal to or longer than 1600 nm, b. only one zero dispersion wavelength ZDW shorter than 2 ?m, c. a positive peak value at a peak wavelength longer than said zero dispersion wavelength ZDW, and d. a minimum value of dispersion D.sub.min at a minimum wavelength longer than 2 ?m, wherein said nonlinear fiber is a step-index fiber and is transparent at a wavelength longer than 3 ?m, wherein said core of said nonlinear fiber comprises fluoride glass comprising InF.sub.3, and wherein said pump light is arranged to comprise energy at one or more preferred pump wavelengths which are longer than said zero dispersion wavelength ZDW by 150 nm or more, whereby said dispersion profile causes a soliton red-shift that occurs when said supercontinuum is generated in said nonlinear fiber to be most accelerated at wavelengths at which Raman based soliton energy loss and an increasing mode field diameter cause the soliton red-shift to slow down, thereby overcoming said soliton red-shift slow down and enabling the soliton red-shift to continue to longer infrared wavelengths.

31. A supercontinuum light source comprising a pump light source arranged to emit pump light and a nonlinear fiber having a core arranged to receive the pump light wherein said pump light and nonlinear fiber are arranged so that a supercontinuum comprising infrared wavelengths is generated in said nonlinear fiber from said pump light, said nonlinear fiber having a dispersion profile, the dispersion profile comprising: a. a zero dispersion wavelength ZDW equal to or longer than 1600 nm, b. a positive peak value at a peak wavelength longer than said zero dispersion wavelength ZDW, and c. a minimum value of dispersion D.sub.min at a minimum wavelength longer than said peak wavelength, wherein the dispersion is non-negative at said minimum wavelength, and wherein said nonlinear fiber is a step-index fiber and is transparent at a wavelength longer than 3 ?m, wherein said core of said nonlinear fiber comprises fluoride glass comprising InF.sub.3, and wherein said pump light is arranged to comprise energy at one or more preferred pump wavelengths which are longer than said zero dispersion wavelength ZDW by 150 nm or more, whereby said dispersion profile causes a soliton red-shift that occurs when said supercontinuum is generated in said nonlinear fiber to be most accelerated at wavelengths at which Raman based soliton energy loss and an increasing mode field diameter cause the soliton red-shift to slow down, thereby overcoming said soliton red-shift slow down and enabling the soliton red-shift to continue to longer infrared wavelengths.

Description

BRIEF DESCRIPTION OF DRAWINGS

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

(2) FIGS. 1a and 1b how the calculated dispersion and mode field diameter, respectively, as a function of wavelength for two fibers.

(3) FIGS. 2a to 2d show calculated characteristics of solitons as they propagate through a nonlinear fiber according to one embodiment of the invention.

(4) FIGS. 3a and 3b show the exemplary calculated wavelength that a soliton may reach depending on pump power and pump wavelength.

(5) FIG. 4 shows the calculated dispersion curves of a series of fibers according to embodiments of the fiber.

(6) FIGS. 5a and 5b show, for two pump powers, the calculated wavelength that a soliton may reach depending on pump wavelength in fibers according to embodiments of the invention.

(7) FIG. 6 illustrates the calculated dispersion profile for a fiber with three zero dispersion wavelengths.

(8) FIG. 7 shows exemplary calculated numerical aperture and core diameter of step index fibers according to the invention.

(9) FIG. 8 shows a diagram of an exemplary pump system of an embodiment of the invention.

(10) FIGS. 9a and 9b show a calculated dispersion profile and an example of spectra generated in one embodiment of the invention, respectively.

(11) FIGS. 10a and 10b show a calculated dispersion profile and an example of spectra generated in an embodiment of the invention, respectively.

(12) FIGS. 11a and 11b show a calculated dispersion profile and an example of spectra generated in an embodiment of the invention, respectively.

(13) FIGS. 12a and 12b show a calculated dispersion profile and an example of spectra generated in one embodiment of the invention, respectively.

(14) FIG. 13 shows an example of the relative location of the ZDW, Peak and Minimum.

(15) FIGS. 14a and 14b show an exemplary calculated final soliton wavelength as a function of wavelength with a fixed pump power and as a function of pump power at a fixed pump wavelength, respectively.

(16) FIG. 15 shows an example of a termination housing which includes a cavity filled with dry air.

(17) FIG. 16 shows an example of a termination housing in which an end cap is used to protect the facet of a nonlinear fiber

(18) FIG. 17 shows an example of a splice housing in which a nonlinear fiber is aligned with a connecting fiber and sealed off from surrounding atmosphere by the splice housing.

DETAILED DESCRIPTION OF THE DRAWINGS

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

(20) 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. 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.

(21) FIGS. 1a and 1b show the calculated dispersion and mode field diameter, respectively, as a function of wavelength for two fibers. In FIG. 1: (a) the dispersion curve of a fiber with a core diameter of 5.7 ?m and a numerical aperture (NA) of 0.30 which represents one embodiment of the invention (solid) is shown. For comparison is illustrated the dispersion curve of a positive gradient fiber with a core diameter of 7 ?m and a NA of 0.30 (dotted). In FIG. 1(b) is shown the effective mode field diameter curves of 5.7 ?m, NA 0.30 fiber (solid) and the 7 ?m, NA 0.30 fiber (dotted). These curves are based on a fiber with a cladding made from a ZBLAN material whose refractive index n(?) is given by the following Sellmeier equation:

(22) $\begin{array}{cc}n\left(?\right)=\sqrt{1+\frac{f_1{?}^2}{{?}^2-{?}_1^2}+\frac{f_2{?}^2}{{?}^2-{?}_2^2}}& \mathrm{Eq}.4\end{array}$
where f.sub.1=1.22514, f.sub.2=1.52898, ?.sub.1=0.08969 and ?.sub.2=21.3825. The refractive index of the core material is raised with a wavelength independent value sufficient to give the numerical apertures (NA) noted for the fibers mentioned. These NA were determined according to the standard equation for a fiber NA=((n.sub.core).sup.2?(n.sub.clad).sup.2).sup.1/2 where n.sub.clad and n.sub.core are the refractive index of the cladding and core respectively. It should be noted that the composition of the fiber material and the resulting Sellmeier constants can dramatically vary the position of the peak wavelength, the minimum wavelength and the fiber geometry necessary to produce them.

(23) FIGS. 2a to 2d show calculated characteristics of solitons as they propagate through a nonlinear fiber according to one embodiment of the invention. In FIG. 2 we show the result of a moment method calculation of the characteristics of a soliton as it propagates through fibers with the dispersion profiles and MFDs shown in FIG. 1. The solid line illustrates the fiber with a core diameter of 5.7 ?m and a NA of 0.30 which is one embodiment of the invention. The dotted line illustrates a positive gradient fiber with a core diameter of 7 ?m and an NA of 0.30 for comparison. FIG. 2a shows a comparison of the soliton wavelength along the fiber in the two fibers pumped at 1900 nm. FIG. 2a shows the centre wavelength of the soliton as it propagates through the fiber. FIG. 2b shows the soliton energy as a comparison of the soliton energy along the fiber in the two fibers pumped at 1900 nm. FIG. 2c shows a comparison of the soliton pulse length along the fiber in the two fibers pumped at 1900 nm. FIG. 2d shows the soliton peak power along the fibers in the two fibers pumped at 1900 nm. For both fibers it was assumed that the solitons were generated in the Modulation instability breakup of a pump with a peak power of 10 kW and a wavelength of 1900 nm and that the breakup occurred instantaneously at 0 m. Any effect of soliton collisions was ignored.

(24) FIGS. 3a and 3b show the exemplary calculated wavelength that a soliton may reach depending on pump power and pump wavelength. FIGS. 3a and 3b show the result of moment method calculations of the centre wavelength of the soliton at the output of a 10 m fiber as a function of pump wavelength for the 13 different pump peak powers 100 W, 250 W, 500 W, 1 kW, 2.5 kW, 5 kW, 10 kW, 25 kW, 50 kW, 100 kW, 250 kW, 500 kW and 1 MW respectively starting with the lowest powers at the bottom and the highest values at the top. FIG. 3a shows the result for a ZBLAN fiber with a core size of 5.7 ?m and NA of 0.30, which represents one embodiment of the invention, and FIG. 3b shows the result for a positive gradient ZBLAN fiber with NA of 0.30 and a core diameter of 7 ?m.

(25) FIG. 4 shows the calculated dispersion curves of a series of fibers according to embodiments of the fiber. In FIG. 4 a series of dispersion curves showing dispersion for nonlinear fibers according to the invention with 7 different numerical apertures (NAs) are shown.

(26) FIGS. 5a and 5b show, for two pump powers, the calculated wavelength that a soliton may reach depending on pump wavelength in fibers according to embodiments of the invention. In FIG. 5a is shown the result of moment method calculations of the centre wavelength of the soliton at the output of a 10 m fiber as a function of pump wavelength for fibers with the 7 different dispersion profiles shown in FIG. 4. In FIG. 5a we assume a pump with an initial peak power of 10 kW and in FIG. 5b we assume a pump with an initial peak power of 50 kW.

(27) FIG. 6 illustrates the calculated dispersion profile for a fiber according to one embodiment of the invention, with three zero dispersion wavelengths. In FIG. 6 the dispersion curve for one embodiment in which the dispersion at the minimum is negative (normal) is shown. Arrows mark the location of the three zero dispersion wavelengths the ZDW, ZDW-2 and ZDW-3. In this embodiment the solitons stop at a wavelength shorter than ZDW-2 and generate dispersive waves above it. In some of these embodiments the region of normal dispersion around the minimum will be narrow so that dispersive waves will also be generated at wavelengths longer than ZDW-3 thus generating light considerably above the minimum.

(28) FIG. 7 shows exemplary calculated numerical aperture and core diameter of step index fibers according to the invention. In FIG. 7 the numerical aperture and core diameter of step index fibers which will provide a dispersion profile relevant to this invention is shown for a material with a material dispersion described by the Sellmeier constants f.sub.1=1.22514, f.sub.2=1.52898, ?.sub.1=0.08969 and ?.sub.2=21.3825.

(29) The dashed curve (?) shows the maximum core diameter and minimum numerical aperture which will constitute a fiber with a peak and minimum in the dispersion curve.

(30) Fibers with numerical apertures smaller than and core diameters above this curve will provide positive gradient fibers whose dispersion curves cannot be the to have a peak or minimum.

(31) The dotted curve (B) represents the minimum numerical aperture and minimum core diameter which will provide fibers which have anomalous dispersion at the peak of the dispersion curve. If the dispersion at the peak of the dispersion curve is not anomalous the pump will not break up and create solitons and thus there will be no red-shift of solitons. Fibers with numerical aperture less than this curve and core sizes below this curve will thus not yield dispersion curves which are advantageous for long wavelength supercontinuum generation according to this invention. The solid curve (C) represents numerical apertures and core diameters which yield fibers with dispersion profiles according to an embodiment of the invention in which the dispersion at the minimum of the dispersion curve remains positive (anomalous) but is close to zero. It should be noted that the core diameters and numerical apertures related to these curves depend on the material as they represent a balance between the material dispersion and the waveguide dispersion. Similar curves can be calculated for other materials and such curves may be used to design embodiments of the invention.

(32) FIG. 8 shows a diagram of an exemplary pump system 1 of an embodiment of the invention. FIG. 8 shows a setup used in some embodiments of the invention where a red-shifted modelocked erbium laser 3 is used to deliver of short pulses at a wavelength of approx. 1.9 ?m with an average power of approx 30 mW and a repetition rate of 40 MHz. These are then amplified in a thulium amplifier 5 to deliver an amplifier output 7 of 2.7 W, 40 MHz, 1 ps pulses at 1.9 ?m which are then coupled into a ZBLAN fiber 8 with a numerical aperture of 0.27 and a core diameter of 6.2-6.4 ?m. The emitted supercontinuum 2 has more than 0.5 W output power, 40 MHz repetition rate and covers a spectrum of at least 1.5-4.2 ?m. The light source is controlled through the use of laser drivers and other electronics 9.

(33) FIGS. 9a and 9b show a calculated dispersion profile and an example of spectra generated in one embodiment of the invention, respectively. In FIG. 9b the spectra generated in one embodiment is shown. The curves are labeled according to the pump average power and have not been corrected for instrument sensitivity. In FIG. 9a is shown the approximate dispersion profile of the fiber in which the spectrum was generated. The fiber dispersion curve has a dispersion at the minimum which is only a little less than the value at the peak and the fiber is pumped with picosecond pulses of up to about 50 kW peak power at a wavelength of approx 1900 nm. In one embodiment this type of dispersion profile generates a spectrum which increases gradually with pump power but which do not reach above the minimum of the minimum of the dispersion profile.

(34) FIGS. 10a and 10b show a calculated dispersion profile and an example of spectra generated in an embodiment of the invention, respectively. In FIG. 10b the spectra generated in an embodiment is shown; the curves are labeled according to the pump average power and have not been corrected for instrument sensitivity. FIG. 10a show the approximate dispersion profile of the fiber in which the spectrum was generated. The fiber dispersion curve shows a dispersion at the minimum which is nearly zero, and the fiber is pumped with picosecond pulses of up to about 50 kW peak power at a wavelength of approx 1900 nm. In one embodiment this type of dispersion profile generates a spectrum which increases gradually up to just below the dispersion minimum after which it increases to above the minimum with a dip in the spectrum being created at the wavelengths corresponding to the minimum of the dispersion.

(35) FIGS. 11a and 11b show a calculated dispersion profile and an example of spectra generated in an embodiment of the invention, respectively. In FIG. 11b the spectra generated in an embodiment is shown. The curves are labeled according to the pump average power and have not been corrected for instrument sensitivity. FIG. 11a shows the approximate dispersion profile of the fiber in which the spectrum was generated. The fiber dispersion curve shows a dispersion at the minimum which is slightly below zero and the fiber is pumped with picosecond pulses of up to about 50 kW peak power at a wavelength of approx 1900 nm. In one embodiment this type of dispersion profile generates a spectrum which increases gradually up to just below ZDW-2. The solitons in the spectrum just below the ZDW-2 can then achieve spectral overlap with their phase matched wavelengths above ZDW-2 and generate a localized dispersive wave in the normal dispersive region surrounding the minimum of the dispersion curve. In one embodiment phase matching and spectral overlap above ZDW-3 generate a peak at wavelengths longer than ZDW-3 when sufficient pump power is applied.

(36) FIGS. 12a and 12b show a calculated dispersion profile and an example of spectra generated in one embodiment of the invention, respectively. In FIG. 12b the spectra generated in one embodiment is shown. The curves are labeled according to the pump average power and have not been corrected for instrument sensitivity. FIG. 12a shows the approximate dispersion profile of the fiber in which the spectrum was generated. The fiber dispersion curve shows a dispersion minimum in the middle of a wide wavelength interval of normal dispersion and the fiber is pumped with picosecond pulses of up to about 50 kW peak power at a wavelength of approx 1900 nm. In one embodiment this type of dispersion profile generates a spectrum which increases gradually up to just below ZDW-2 and then solitons generate a dispersive wave above the second ZDW. However since the normal dispersive region is wide there is not efficient spectral overlap with phase matched wavelengths above ZDW-3 and thus there may be no light generated above ZDW-3.

(37) In FIG. 13 an example of the relative location of the ZDW, the peak and the minimum is shown. As noted above, there may in principle be more than one local maximum following (i.e. as the wavelength is increased) the ZDW, and/or there may be more than one local minimum following the peak. In one embodiment the peak is the first local maximum following the ZDW and/or in one embodiment the minimum is the first local minimum following the peak. In one embodiment the wavelength referred to as the zero dispersion wavelength (ZDW) in this text is the shortest wavelength where the dispersion crosses zero or is substantially zero as discussed above. In one embodiment it is the second, third, four, fifth or higher shortest wavelength where the dispersion crosses zero or is substantially zero. If the dispersion profile crosses zero again at longer wavelengths after the ZDW applied in the invention these crossing wavelengths will be referred to as ZDW-2 and ZDW-3 as can be seen in FIG. 6.

(38) FIGS. 14a and 14b show an exemplary calculated final soliton wavelength as a function of wavelength with a fixed pump power and as a function of pump power at a fixed pump wavelength, respectively. In FIG. 14a the soliton wavelength reached at the end of 10 m fiber pumped at a fixed pump peak power of 10 kW is shown as a function of pump wavelength. An interval of rapid increase in wavelength referred to as the wavelength threshold has been indicated. In FIG. 14b the soliton wavelength reached at the end of 10 m fiber pumped at a fixed wavelength of 1900 nm is shown as a function of pump peak power an interval of rapid increase in wavelength which will be called the power threshold has been indicated. The two FIGS. 14a and 14b show the result for the same two fibers that were treated in FIG. 1, FIG. 2 and FIG. 3, and here the solid line also represents a fiber according to one embodiment of the invention while the dotted line is represents a positive gradient fiber. The wavelength and power thresholds which are seen for the fiber according to one embodiment of the invention occur when the initial pulse break-up generates solitons that can reach a wavelength where their red-shift are accelerated by the low dispersion near the minimum of the dispersion curve.

(39) Various embodiments of the invention may be used in a wide range of applications including, but not limited to infrared counter-measures, chemical sensing, non-contact or remote sensing of firearms, weapons or drugs, industrial chemical sensing, such as in advanced semiconductor process control, combustion monitoring, chemical plant process control, petrochemical production or control of refining processes, bio-medical imaging and/or ablation, in an optical coherence tomography configuration for semiconductor wafer imaging or defect location, or in free space or fiber based telecommunications. In all these applications the spectra that can be generated using the invention may be of benefit to the application.

(40) In an embodiment the supercontinuum was generated in a nonlinear fiber made from ZBLAN glass and pumped with 2 W average power from a 40 MHz pulse train of pulses with a full width half maximum (FWHM) length of approx 1 ps. Delivered from a setup similar to the one shown in FIG. 8. This means that the peak power of the pump was approx. 50 kW. This pump light was coupled into a ZBLAN fiber with a numerical aperture of approx 0.27 and a core size of 6-6.9 ?m with a coupling efficiency of approx. 70%. The core size numerical aperture and material dispersion of the ZBLAN fiber was estimated and used to calculate the approximate dispersion curves shown in FIG. 9-12. FIG. 9-12 also show spectra generated in different ZBLAN fiber using the setup described here and shown in FIG. 8.

(41) In another embodiment pulses with a envelope pulse length of 3 ns, a repetition rate of 30 kHz, an average power of 200 mW (pump peak power of approx 2.2 kW) and a wide spectrum stretching from 1.9 to 2.4 ?m was coupled into a similar ZBLAN fiber with an efficiency of approx. 60% and the resulting spectrum spanned from 1.5 ?m to 4.1 ?m. This spectrum had approx the same spectral width as when the same fiber was pumped with the 1 ps system described above which may confirm that when fibers with the optimized dispersion profile are pumped with sufficient peak power, additional peak power does not increase the spectral width much.

(42) FIG. 15 shows an example of a termination enclosure 10 in which includes a cavity filled with dry air. FIG. 15 displays a diagram of one embodiment of a termination enclosure 10 in which a nonlinear fiber 11 with a coating 12 is fixed in a transparent ferrule 13. The transparent ferrule 13 forms part of the termination enclosure 10 and is inserted into one side of a termination housing 14. At the other side of the termination housing 14, a transparent window or lens 16 allows the light to pass out of the termination housing 14 and thus out of the termination enclosure 10. The termination housing 14, the ferrule 13 and the window or lens 16 provides a closed cavity 15 which is filled with dry air or dry inert gas. The interfaces between fiber 11 and ferrule 13, between the ferrule 13 and the rest of the termination housing 14 and between the lens or window 16 and the rest of the termination housing 14 are all airtight, preventing humidity from the surrounding air from entering the cavity 15.

(43) FIG. 16 shows an example of a termination enclosure 10 in which an end cap 17 is used to protect the facet of a nonlinear fiber 11. In FIG. 16 is shown an embodiment of a termination enclosure 10 in which a nonlinear fiber 11 with a coating 12 is fixed in a transparent ferrule 13. The transparent ferrule 13 together with a transparent end cap 17 form part of the termination enclosure 10. The interfaces between the fiber 11 and the ferrule 13 as well as between the ferrule 13 and the end cap 17 are airtight, preventing humidity from the surrounding air from entering the cavity between the fiber 11 and the ferrule 13. The end cap 17 is transparent in order to allow the light from the fiber 11 to pass through the end cap 17 and thus out of the termination enclosure 10. In one embodiment the airtightness is ensured by bonding the ferrule 13 and/or the end cap 17 directly to the fiber 11. In one embodiment, the end cap 17 has a shape and/or refractive index distribution that allows it to function as a lens for the output light.

(44) FIG. 17 shows an example of a termination enclosure 10 wherein a nonlinear fiber 11 is aligned with a connecting fiber 18 and is sealed off from the surrounding atmosphere by a splice housing 20. FIG. 17 displays a diagram of one embodiment of a splice housing 20 in which the nonlinear fiber 11 with a coating 12 is fixed in a transparent ferrule 13 which forms part of the termination enclosure 10. The fiber 11 is input into one side of the splice housing 20 and another, connecting fiber 18 with its coating 19 is input into the splice housing 20 from the other side, so that the end facets of the two fibers 11 and 18 meet at a point 21 inside the ferrule 13, inside the splice housing 20. In one embodiment, a material fills the void between the ferrule and the fibers. This material may protect the fiber against the surrounding atmosphere, and may mechanically support the fiber so that the alignment between the fiber facets and/or the cores of the fibers is maintained at the meeting point 21. In one embodiment, the splice housing 20 also supports the nonlinear fiber 11 and the connecting fiber 18 to assist in maintaining the alignment between their cores.

(45) In one embodiment, the connecting fiber 18 has a different glass melting point than the nonlinear fiber 11.

(46) In one embodiment, at least part of the material of the splice housing is transparent in at least part of the optical spectrum where at least one of the nonlinear fiber and the connecting fiber are also transparent.

(47) 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 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.

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

(49) 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.