WO2007079750A1

WO2007079750A1 - A super continuum source comprising a photonic crystal fibre, a system, a method and use

Info

Publication number: WO2007079750A1
Application number: PCT/DK2007/000011
Authority: WO
Inventors: Morten Østergaard PEDERSEN; Carsten L. Thomsen; Thomas Feuchter
Original assignee: Koheras A/S
Priority date: 2006-01-11
Filing date: 2007-01-09
Publication date: 2007-07-19

Abstract

The invention relates to the field of light sources, more specifically to a light source generating a super continuum spectrum. The invention further relates to a system for, use of and a method of generating a super continuum spectrum. An object of the present invention is to provide a super continuum light source. The invention is embodied by a super continuum source comprising: a) an amplifier section comprising a length of a large-mode area (LMA) micro-structured, double clad amplifying fibre comprising a core region, inner and outer cladding regions wherein the outer cladding region comprises a ring of air holes, and wherein a numerical aperture of the inner cladding region being larger than 0.4, and b) a coupling section, and c) a non-linear section comprising a length of a small-mode field diameter non-linear fibre. The relatively low non-linearity in the amplifier section together with the relatively highly non-linear fibre of the non-linear section enables the generation of a super continuum covering the UV and the visible wavelength range using pico-second pulses. The invention may e.g. be used in applications ranging from spectroscopy to ultra-short-pulse generation, including optical radar and ranging (LIDAR)1 spectroscopy, optical computing, and reaction rate studies.

Description

A SUPER CONTINUUM SOURCE COMPRISING A PHOTONIC CRYSTAL FIBRE, A SYSTEM, A METHOD AND USE

TECHNICAL FIELD

The invention relates to the field of light sources, more specifically to a light source generating a super continuum spectrum.

The invention furthermore relates to a system for, use of and a method of generating a super continuum spectrum.

BACKGROUND ART

Super continuum (SC) generation is a nonlinear phenomenon characterized by dramatic spectral broadening of intense light pulses passing through a nonlinear media. SC generation occurs in various media and finds use in numerous applications ranging from spectroscopy to ultra-short-pulse generation, including optical radar and ranging (LIDAR), spectroscopy, optical computing, and reaction rate studies.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a super continuum light source.

It is a further object of the invention to provide a super continuum light source capable of converting a relatively low power pulse-train of picosecond length pulses to a super continuum with a relatively high spectral power density.

The objects of the invention are achieved by the invention and embodiments thereof as described in the accompanying claims and as described in the following. In one embodiment the super continuum light source is capable of converting low power of typically ≤ 100 mW average pulse-train of picosecond length pulses to a super continuum with a relatively high spectral power density.

An object of the invention is, as defined in claim 1 , achieved by a super continuum source comprising: a) an amplifier section comprising a length of a large-mode area (LMA) micro-structured, double clad amplifying fibre comprising a core region, inner cladding region, and an outer cladding region, wherein the outer cladding region comprises a plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core,, and wherein a numerical aperture of the inner cladding region being larger than 0.4, and b) a coupling section, and c) a non-linear section comprising a length of a small-mode field diameter non-linear fibre.

The relatively low non-linearity in the amplifier section together with the relatively highly non-linear fibre of the non-linear section enables the generation of a super continuum using pico-second pulses. In particular, a super continuum covering the UV and the visible wavelength range (can be provided.

The UV- and visible wavelength range may be taken as the range from 300 nm to 700 nm.

In the present context, the term 'a super continuum (spectral range)' is taken to mean a spectrum spanning a range of wavelengths larger than 100 nm, such as larger than 200 nm, such as larger than 300 nm, such as larger than 500 nm, such as larger than 800 nm, such as larger than 1100 nm. Preferably the range of super continuum optical radiation is included in the range from 200 to 2200 nm, such as the range from 400 nm to 1600 nm, such as the range from 450 nm to 600 nm, such as in the range from 800 nm to 1000 nm, such as in the range from 1200 nm to 1400 nm, such as in the range from 1500 nm to 1600 nm, such as in the range from 480 nm to 1750 nm, such as in the range from 480 nm to 800 nm, such as in the range from 480 nm to 700 nm..

In an embodiment, the power density of the super continuum source is larger than 1 mW/nm over the range of the super continuum, such as larger than 2 mW/nm, such as larger than 3 mW/nm, such as larger than 4 mW/nm, such as larger than 5 mW/nm.

An LMA micro-structured, double clad fibre comprises a core region, an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region and comprising a plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core. A fiber with such an arrangement of air holes is also referred to as an air clad. In one embodiment the plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core is in the form of at least one 'ring' of relatively large voids extending along the longitudinal direction of the fibre. The LMA micro-structured, double clad fibre may further comprise longitudinally extending micro- structured elements in the inner cladding and/or in the core regions. The micro-structured elements may be periodically or non-periodically distributed when viewed in a given transversal cross section of the fibre. Air-clad fibres are e.g. described in WO-03/019257 or US-5,907,652.

In an embodiment, the core region of 'a large-mode area' double clad fibre is taken to correspond to a mode-field diameter larger than 15 μm, such as larger than 20 μm, such as larger than 25 μm.

In an embodiment, the diameter of the inner cladding region (located within the air-clad) is larger than 100 μm, such as larger than 150 μm, such as larger than 200 μm, such as larger than 300 μm, such as larger than 400 μm.

The numerical aperture is preferably larger than 0.4. In an embodiment, the numerical aperture is larger than 0.45, such as larger than 0.5, such as larger than 0.55, such as larger than 0.6. The term 'a small mode-field diameter' should be seen in relation to the mode-field of the LMA fibre.

In an embodiment, 'a small mode-field diameter' is taken to be a mode-field diameter of about or smaller than 10 μm, such as smaller than 5 μm, such as smaller than 4 μm, such as smaller than 3 μm.

The large-mode area (LMA) micro-structured, double clad amplifying fibre comprising an air-clad may in general be of any kind providing a numerical aperture of the area limited by the air-clad. In general, the amplifying fibre may contain any optically active material including a rare earth ion, such as Er and/or Yb or the like. Fiber amplifiers based on rare earth ions are e.g. discussed in [Digonnet] (Michel J. F. Digonnet, ed., "Rare-Earth-Doped Fiber Lasers and Amplifiers", 2^nd edition, 2001, Marcel Dekker, Inc., New York- Basel). As an example, an Yb-doped LMA-25 fibre from Crystal Fibre A/S (of Birkeroed, Denmark) may be used. Air-clad fibres are e.g. described in WO- 03/019257.

In one embodiment, the coupling section comprises a length of a tapered micro-structured fibre capable of converting the mode-field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode- field diameter of the non-linear fibre.

In one embodiment, the coupling section comprises a length of a tapered standard fibre capable of converting the mode-field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode-field diameter of the non-linear fibre.

In one embodiment, the coupling section comprises a free-space single mode coupling unit converting the mode-field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode-field diameter of the non-linear fibre.

In one embodiment, the amplifying LMA fibre of the amplifying section and a fibre of the coupling section are spliced together in a fusion splice. In one embodiment, a fibre of the coupling section and the non-linear fibre of the non-linear section are spliced together in a fusion splice.

In one embodiment, the LMA micro-structured, double clad amplifying fibre is polarization maintaining. This has the advantage that it may provide an input to the non-linear section which improves the non-linear response.

The non-linear fibre may be any optical fibre in which the polarization P of responds nonlinearly to the electric field E of the light.

In one embodiment, the non-linear fibre of the non-linear section is polarization maintaining. This has the advantage that the non-linear response may be improved.

In one embodiment, the non-linear fibre of the non-linear section is a micro- structured fibre. This has the advantage of improving the design flexibility of the size of the non-linearity of non-linear section.

In one embodiment, the three sections a), b) and c) of claim 1 are fusion spliced together to form a single fibre unit.

In one embodiment, the amplifier section comprises two or more lengths of amplifying LMA fibres with increasing mode-field diameter such that the population inversion inside the LMA fibre amplifier may be minimized in the length of the amplifier with the lower pulse peak power.

In one embodiment, the super continuum source further comprises at least one seed source optically coupled to the amplifying section. Alternatively, the seed source may be a separate unit.

The invention further relates to a system for generating a super continuum comprising a super continuum source according to the invention as described above, in the following description and in the claims, and a seed source optically coupled to the amplifying section of the super continuum source. In one embodiment, the seed source provides pico-second pulses to the amplifying section of the super continuum source. In one embodiment, the pico-second pulses have a duration of from 0.5 ps to 100 ps, such as between 1 ps and 20 ps, such as around 10 ps.

In one embodiment, the low power pulse-train of the seed source has an average power smaller than 500 mW, such as smaller than 250 mW, such as smaller than 100 mW, such as smaller than 80 mW, such as smaller than 50 mW, such as smaller than 20 mW, such as smaller than 10 mW.

In one embodiment, the system further comprises one or more pump sources for the amplifier section of the super continuum source.

The invention further relates to a method of producing a super continuum source comprising the steps of a) an amplifier section comprising a length of a large-mode area micro- structured, double clad amplifying fibre comprising a core region, an inner cladding region, and an outer cladding region, wherein the outer cladding region comprises a plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core, and wherein a numerical aperture of the inner cladding region being larger than 0.4, and b) providing a coupling section, c) providing a non-linear section comprising a length of a relatively small- mode field diameter, non-linear fibre; and d) providing optical coupling between the amplifier section and the coupling section and between the coupling section and the non-linear section.

The method has the same advantages as the corresponding super continuum source as described herein and may be combined with the corresponding features of the super continuum source.

The invention further relates to the use of a super continuum source or of a system for generating a super continuum according to the invention as described herein. In a particular embodiment, the use is for optical coherence tomography, laser precision spectroscopy, or fluorescent microscopy, such as confocal microscopy for which the provision of high brightness emission in the visible part of the spectrum (as provided by a supper continuum source or a system according to the invention) is advantageous.

Optical coherence tomography is e.g. discussed by I. Hartl, X. D. Li, C. Chudoba, R.KGhanta, T. H. Ko, and J.G.Fujimoto in "Ultrahigh-resolution optical coher-ence tomography using continuum generation in a air-silica microstructure optical fiber", Opt. Lett. 26, 608 (2001). Laser precision spectroscopy is e.g. discussed by R. Holzwarth, T. Udem, T.W.Hansch, J. C. Knight, W. J. Wadsworth, and P. S. Russell in Optical frequency synthesizer for precision spectroscopy", Phys. Rev. Lett. 85, 2264 (2000). Fluorescent microscopy is e.g. discussed by C. Dunsby, P.M.P. Lanigan, J. McGinty, D. S. Elson.J. Requejo-lsidro, I. Munro, N. Galletly, F. McCann, B. Treanor, B. Onfelt, D.M. Davis, M.A.A. Neil and P.M.W. French in "An electronically tunable ultrafast laser source applied to fluorescence imaging and fluorescence lifetime imaging microscopy", J. Phys. D. 37, 3296 (2004).

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

FIG. 1 shows a system comprising a super continuum source according to the invention, FIG. 2 shows the amplification of a 5 ps pulse from 100 mW to 5 W in a conventional double-clad fibre amplifier (solid line) and in a double-clad photonic crystal fibre amplifier (dashed line), respectively, and

FIG. 3 shows a super continuum spectrum obtained using a double-clad photonic crystal fibre amplifier and a photonic-crystal, highly non-linear fibre.

The figures are schematic and simplified for clarity, and they just show details which are essential to the understanding of the invention, while other details are left out.

FIG. 1 shows a system comprising a super continuum source according to the invention. The super continuum source comprises three parts:

• An amplifier section (termed 'LMA amplifier' in FIG. 1). • A coupling section (termed 'coupling' in FIG. 1) for coupling light between the amplifier section and a

• A non-linear section (termed 'HNLF' in FIG. 1).

The system further comprises a seed source (Termed 'Seed' in Fig. 1) for generating an optical pulse train. The seed source may e.g. be a semiconductor diode pumped solid state laser generating optical pulses with a duration of 8-10 ps, a repetition rate of 80 MHz and an average optical power of 100 mW at a wavelength around 1060 nm. The seed source in FIG. 1 additionally comprises pump source(s) for the LMA amplifier section.

The optical coupling between the seed source (and the LMA-pump source(s)) and the amplifier section may comprise a fusion splice (if appropriate) or one or more optical components (lenses, mirrors, etc.).

In an embodiment, the amplifier section comprises a large-mode area (LMA) photonic crystal double clad fibre amplifier, e.g. an Yb-doped LMA-25 fibre from Crystal Fibre A/S pumped (forward or backward) around 915 nm.

In an embodiment, the coupling section comprises

• a tapered section converting the amplified output of the amplifier section (e.g. an LMA fibre) to a smaller mode-field diameter matching that of the non-linear fibre section, and • a free space single mode (SM) coupling

In an embodiment, the non-linear section comprises a small mode-field diameter, highly non-linear photonic crystal fibre.

In an embodiment, the amplifier fibre of the amplifier section and the nonlinear fibre of the non-linear fibre section have the following parameters.

The above is only one example of values of relevant parameters of a super continuum source according to the invention. The skilled person will be able to adapt the above parameters to other situations, depending on the relevant power levels, wavelength-ranges, coupling requirements, etc. Photonic crystal fibres for use as amplifier fibres as well as non-linear fibres in a super continuum source according to the invention may e.g. be purchased from Crystal Fibre A/S (of Birkeroed, Denmark). The manufacture, properties and applications of photonic crystal fibres are e.g. discussed in Bjarklev et al. (Bjarklev, Broeng, and Bjarklev in "Photonic crystal fibres", Kluwer Academic Press, 2003).

The photonic crystal fibres enable rare-earth doped fibres with mode-field diameters significantly larger than what can be achieved using conventional index-guiding fibres and still maintain a strictly single-mode beam quality. The LMA core structure may be contained within a double-clad structure comprising a ring of air holes resulting in a numerical aperture significantly larger than the largest obtainable values using polymer clad double-clad fibres. These two properties are highly advantageous for the amplification of short, high intensity pulses. In the amplifier section non-linear processes, in particular self-phase modulation (SPM) and four-wave mixing (FWM) may introduce a strong degradation of the input pulse shape. The effect of SPM is to introduce a chirp and spectral broadening of the pulse and FWM may further increase the spectral width and chirp of the pulse. This broadening leads to a significant reduction of the efficiency of super continuum generation in the subsequent section of the system. The maximum phase shift Φ introduced by SPM is given by the ratio of the amplifier length L to the non-linear length L_NL of the fibre, i.e.

after inserting the definition of the non-linear length. Since the non-linear index n₂ of glass is almost constant, where ω₀ is the angular frequency, c is the speed of light in vacuum and A_eff is the effective mode field area of the core, the maximum SPM induced phase shift Φ scales with the fibre parameters as ώ A ∞ ^L

and to minimize pulse distortion, it is advantageous to maximize the effective mode-field area and minimize the length of the amplifying fibre. The first may be brought about by the LMA structure forming the core of the photonic crystal fibre. The second, minimizing the length, may be obtained through the large numerical aperture of the double-clad structure. For a given optical pump diode(s), the brightness thereof (emitting aperture size times emission solid angle) may preferably be equal to or smaller than the double-clad structure diameter times the acceptance angle (determined by the numerical aperture (NA) of the fibre). By increasing the numerical aperture, it is possible to lower the diameter of the double-clad structure and still accept the same brightness from the pump diodes. Lowering the diameter will lead to an improved overlap between the pump light and the doped core, thus a shorter absorption length and a shorter overall length of the amplifier. The highest numerical aperture values obtained with conventional polymer clad fibres are approximately 0.45 whereas air-clad double-clad fibres can have a numerical aperture of 0.6-0.62 (technologically determined). Comparing a typical conventional fibre with a photonic crystal fibre as described here is shown below

Double-clad photonic Conventional double- crystal fibre clad fibre

The maximum nonlinear phase shift is almost 5 times smaller for the double- clad photonic crystal fibre amplifier compared to the conventional double- clad fibre amplifier.

FIG. 2 shows the amplification of a 5 ps pulse from 100 mW to 5 W in a conventional double-clad fibre amplifier (solid line) and in a double-clad photonic crystal fibre amplifier (dashed line), respectively.

FIG. 2 shows that the pulse spectrum is much better in the case of double- clad photonic crystal fibre amplifier compared to conventional double-clad fibre amplifier. The difference is that the non-linear effect is suppressed in the case of the double-clad photonic crystal fibre compared to conventional double-clad fibre, minimizing SPM and Four-wave mixing.

In an embodiment, the amplifier section comprises a combination of two double cladding LMA amplifier fibres. Two double clad LMA amplifier fibres with different mode field diameter are spliced together in order to reduce photo darkening and save cost. The smallest-mode double cladding LMA amplifier is used as preamplifier and the largest-mode double cladding LMA amplifier as booster amplifier. The splicing allows residual pump from the booster amplifier to pump the preamplifier thereby saving the cost of one pump/signal multiplexer.

The benefits of the combination of two or more double cladding LMA amplifier fibres include the following:

1. The splicing allows the combination of a preamplifier and booster amplifier without any significantly cost increase. In many cases the seed source need to be pre-amplified before the final booster amplification in order to reduce ASE from the booster amplifier and reduce photo darkening effects in the booster amplifier. The splicing allows residual pump from the booster amplifier to be used as pump for the preamplifier thereby significantly reduces the cost of the amplification step, since the cost of one pump/signal multiplexer can be eliminated.

2. The preamplifier increases the seed power before the booster amplifier thereby decreasing the photo darkening effects in the booster amplifier.

The coupling section provides optical coupling between the amplifier section (e.g. comprising a (LMA) photonic crystal double clad fibre) and the nonlinear section (e.g. comprising a highly non-linear photonic crystal fibre).

Several different methods can be employed to couple the light from the amplifier fibre into the highly non-linear photonic crystal fibre. In the following we describe two desired methods:

1. Taper section: To launch the amplified pulses into the non-linear fibre, a tapered fibre can be used with a mode field diameter in one end matching the LMA fibre and gradually tapering down to match the mode field diameter of the non-linear fibre. This can be done either by thermal processing of a conventional fibre or by suitable drawing of a fibre while modifying the draw conditions during the draw process.

2. Free space single mode (SM) coupling: To launch the amplified pulses into the non-linear fibre using an optical coupling unit consisting of two or more lenses.

In an embodiment, the amplified pulses are launched into a photonic-crystal, highly non-linear fibre with dispersion properties adjusted for optimum phase match and generation of super continuum light through a variety of nonlinear processes. A benefit of a relatively low chirp and spectral broadening of the amplifier section is to ensure a relatively high efficiency and low noise (the noise in the super continuum generation has been shown to depend on the chirp of the pulses). The advantages of a photonic-crystal, highly nonlinear fibre are (1) the relatively high confinement of the light by the glass-air structure resulting in a high non-linear coefficient thus yielding a high efficiency for the non-linear processes (2) the ability to tailor the dispersion properties to guarantee that phase matching conditions are met at desired wavelength(s) resulting in high efficiency for four-wave mixing. FIG. 3 shows a super continuum spectrum obtained using a double-clad photonic crystal fibre amplifier and a photonic-crystal highly non-linear fibre.

The relatively low non-linearity in the amplifier section together with the photonic-crystal, highly non-linear fibre enables efficient generation of a visible super continuum using high power pico-second pulses.

Utilizing the setup described above it is possible to obtain a visible super continuum source with power density exceeding 4 mW/nm in the visible spectral range (cf. FIG. 3), which significantly exceeds what have been achieved with conventional double-clad fibre amplifiers.

Some preferred 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.

Claims

1. A super continuum source comprising: a) an amplifier section comprising a length of a large-mode area (LMA) micro-structured, double clad amplifying fibre comprising a core region, an inner cladding region, and an outer cladding region, wherein the outer cladding region comprises a plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core, and wherein a numerical aperture of the inner cladding region being larger than 0.4, and b) a coupling section, and c) a non-linear section comprising a length of a small-mode field diameter non-linear fibre.

2. A super continuum source according to claim 1 , wherein the coupling section comprises a length of a tapered micro-structured fibre converting the mode-field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode-field diameter of the non-linear fibre.

3. A super continuum source according to claim 1, wherein the coupling section comprises a length of a tapered standard fibre converting the mode- field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode-field diameter of the non-linear fibre.

4. A super continuum source according to claim 1 , wherein the coupling section comprises a free-space single mode coupling unit converting the mode-field of the amplified output of the LMA fibre to a smaller mode-field diameter matching the mode-field diameter of the non-linear fibre.

5. A super continuum source according to any one of the preceding claims, wherein the LMA fibre of the amplifying section and the coupling section are spliced together in a fusion splice.

6. A super continuum source according to any one of the preceding claims, wherein the coupling section and the non-linear fibre of the non- linear section are spliced together in a fusion splice.

7. A super continuum source according to any one of the preceding claims, wherein the LMA fibre is polarization maintaining.

8. A super continuum source according to any one of the preceding claims, wherein the non-linear fibre of the non-linear section is polarization maintaining.

9. A super continuum source according to any one of the preceding claims, wherein the non-linear fibre of the non-linear section is a micro- structured fibre.

10. A super continuum source according to any one of the preceding claims, wherein at least three sections comprising a) the amplifier section, b) the coupling section and c) the non-linear section are fusion spliced together to form a single fibre unit.

11. A super continuum source according to any one of the preceding claims, wherein said amplifier section comprises two or more lengths of amplifying LMA fibres with increasing mode-field diameter, preferably such that the population inversion inside said amplifier is minimized in the length of the amplifier with the lowest pulse peak power.

12. A system for generating a super continuum comprising a super continuum source according to any one of the preceding claims and a seed source optically coupled to the amplifying section of the super continuum source.

13. A system according to claim 12 wherein the seed source is arranged to provide pico-second pulses to the amplifying section of the super continuum source.

14. A system according to any one of the claims 12 and 13 further comprising at least one pump source for the amplifier section of the super continuum source.

15. A system according to any one of the claims 13 and 14 wherein the pico-second pulses have a duration of from 0.5 ps to 100 ps, such as between 1 ps and 20 ps, such as around 10 ps.

16. A method of producing a super continuum source comprising the steps of a) providing an amplifier section comprising a length of a large-mode area micro-structured, double clad amplifying fibre comprising a core region, and an inner cladding region and an outer cladding region wherein the outer cladding region comprises a plurality of air holes elongating in the length of the LMA fibre and distributed in a ring around the core, and wherein a numerical aperture of the inner cladding region being larger than 0.4, and b) providing a coupling section, c) providing a non-linear section comprising a length of a small-mode field diameter, non-linear fibre; and d) providing optical coupling between the amplifier section and the coupling section and between the coupling section and the non-linear section.

17. Use of a super continuum source according to any one of claims 1-11 or of a system for generating a super continuum according to any one of claims 12-15.

18. Use according to claim 17 for optical coherence tomography, laser precision spectroscopy, or fluorescent microscopy, such as confocal microscopy.