A FIBER ARRANGEMENT, AN OPTICAL SYSTEM AND A METHOD OF OPERATING AN OPTICAL SYSTEM

Abstract

A fiber arrangement, in particular for providing broadband light, preferably in the ultraviolet frequency range, comprises: a hollow core optical fiber (11), and at least one housing (15) having a chamber (27) in which a piezo device (25) is arranged, the piezo device (25) having a variable length and the chamber (27) being in fluid communication with a hollow core (13) of the fiber (11) such that a change of length of the piezo device (25) causes a change of the size of a volume (23) which is provided by the hollow core (13) and the chamber (27) for a fluid, such as a gas or a mixture of gases.

Claims

1. A fiber arrangement for providing broadband light, comprising: a hollow core optical fiber, and at least one housing having a chamber in which a piezo device is arranged, the piezo device having a variable length and the chamber being in fluid communication with a hollow core of the fiber such that a change of length of the piezo device causes a change of the size of a volume which is provided by the hollow core and the chamber for a fluid, such as a gas or a mixture of gases.

2. The fiber arrangement of claim 1, wherein the housing comprises a through-hole having an opening at a first axial end of the housing and an opening at a second axial end of the housing, and wherein the fiber is inserted in the through hole.

3. The fiber arrangement of claim 2, wherein the housing is a ferrule which is attached to the fiber, and wherein the ferrule comprises a tube-like body which provides the through-hole in which the fiber is inserted.

4. The fiber arrangement in accordance claim 1, wherein (i) the housing is a ferrule, and wherein the ferrule is attached to an end of the fiber; or (ii) wherein at least two housings are provided, the at least two housings being ferrules, with one housing of the at least two housings being attached to one end of the fiber and the other housing of the at least two housings being attached to the other end of the fiber.

5. The fiber arrangement in accordance with claim 2, wherein an optical window closes the opening of the through-hole on the first axial end of the respective housing and a fiber end is inserted in the second axial end of the through-hole of the housing.

6. The fiber arrangement in accordance with claim 1, wherein the chamber has an annular form and is arranged in a circumferential direction around a through-hole of the housing.

7. The fiber arrangement in accordance with claim 1, wherein the chamber comprises a center axis which is arranged in parallel or at an angle to a center axis of a through-hole of the housing.

8. The fiber arrangement in accordance with claim 2, wherein the ferrule is a fiber optic ferrule configured to hold and align the fiber, and wherein the piezo device is arranged in the ferrule such that the volume of the fiber is controlled via the piezo device and via the fiber optic ferrule.

9. fiber arrangement in accordance with claim 1, wherein a filling, the filling comprising at least one of a gas tight filling and a filling of a deformable material, is arranged between an outer surface of the piezo device and an inner surface of the chamber.

10. The fiber arrangement in accordance with claim 1, wherein in a maximally extended position, the piezo device extends over the complete length of the chamber in which the piezo device is arranged.

11. The fiber arrangement in accordance with claim 1, wherein one or more fluid channels extend in a radial direction through the housing to fluidly connect the chamber with the a through-hole provided in the housing.

12. The fiber arrangement in accordance with claim 11, wherein a number of fluid channels are spaced evenly around the circumference of the through-hole, or wherein in a circumferential direction around the through-hole four fluid channels are arranged offset by 90 degrees.

13. The fiber arrangement in accordance with claim 1, wherein the fiber arrangement is gas-tight.

14. The fiber arrangement in accordance with claim 1, wherein the fiber is an anti-resonant hollow core fiber or a hollow core photonic bandgap fiber.

15. An optical system for providing broadband light, the optical system comprising: a fiber arrangement, the fiber arrangement comprising: (i) a hollow core optical fiber, and (ii) at least one housing having a chamber in which a piezo device is arranged, the piezo device having a variable length and the chamber being in fluid communication with a hollow core of the fiber such that a change of length of the piezo device causes a change of the size of a volume which is provided by the hollow core and the chamber for a fluid, such as a gas or a mixture of gases; a pulsed laser source for providing laser pulses to the fiber arrangement; and at least one piezo driver for driving the piezo device arranged in at least one housing of the fiber arrangement.

16. The optical system in accordance with claim 15, wherein at least one of: (i) the at least one piezo driver is configured to drive the piezo device in dependence on a pulse repetition rate of the laser pulses, and (ii) wherein by changing the length of the at least one piezo device the change in density or pressure of the fluid is more than 50%.

17. The optical system in accordance with claim 15, wherein the fiber arrangement comprises at least two housings, the at least two housings being ferrules, arranged at a distance from each other along the fiber, wherein the at least one piezo driver is configured to drive the piezo devices of the at least two housings to generate a desired pressure distribution in the hollow core of the fiber, to either generate a homogeneous pressure distribution or a pressure gradient in the hollow core of the fiber.

18. The optical system in accordance with claim 15, wherein the fiber arrangement comprises at least one housing, the at least one housing being a ferrule, at each fiber end.

19. A method of generating broadband light in the UV frequency range, with an optical system, the optical system comprising (a) a fiber arrangement, the fiber arrangement comprising: (i) a hollow core optical fiber, and (ii) at least one housing having a chamber in which a piezo device is arranged, the piezo device having a variable length and the chamber being in fluid communication with a hollow core of the fiber such that a change of length of the piezo device causes a change of the size of a volume which is provided by the hollow core and the chamber for a fluid, such as a gas or a mixture of gases; (b) a pulsed laser source for providing laser pulses to the fiber arrangement; and (c) at least one piezo driver for driving the piezo device arranged in at least one housing of the fiber arrangement, the method comprising: inputting laser pulses into the fiber arrangement to generate broadband pulses, which are output by the fiber arrangement, while inputting the laser pulses, controlling the piezo device arranged in the at least one housing to generate the broadband pulses with a desired frequency spectrum or to generate the broadband pulses such that different pulses comprise different frequency spectra.

Description

[0053] Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which:

[0054] FIG. 1 shows schematically a cross-sectional view of a fiber arrangement known from prior art.

[0055] FIG. 2 shows schematically a cross-sectional view of an embodiment of a fiber arrangement in accordance with the present invention.

[0056] FIG. 3 shows schematically also the fiber arrangement of FIG. 2 in a cross-sectional view.

[0057] FIG. 4 shows schematically a further cross-sectional view of the fiber arrangement of FIG. 2.

[0058] FIG. 5 shows schematically a cross-sectional view of a further embodiment of a fiber arrangement in accordance with the present invention.

[0059] FIG. 6 shows schematically a further embodiment of a fiber arrangement in a cross-sectional view.

[0060] FIG. 7 shows schematically a cross-sectional view of a further embodiment of a fiber arrangement in accordance with the present invention.

[0061] FIG. 8 shows schematically an optical system in accordance with the present invention.

[0062] The fiber arrangement shown in FIG. 1 comprises an optical fiber 11 which has a hollow core 13. The core 13 runs along the fiber's length, where it forms the center portion of the fiber 11. The core 13 is surrounded by at least one cladding of the fiber.

[0063] As the core 13 is hollow, it can be filled with a fluid, for example a gas or a mixture of gases.

[0064] The fiber arrangement 11 comprises a housing 15, which is in the described examples a ferrule. The ferrule 15 has a tube-like body 17 that provides a through-hole for insertion of the fiber 11. The through-hole of the tube-like body 17 is open at one axial end. An end of the fiber 11 is inserted into the open end of the through-hole of the tube-like body 17 as shown in FIG. 1. The other axial end of the through-hole in the ferrule 15 is closed with an optical window 19, which can serve as an entrance window for laser pulses that are input into the optical fiber 11.

[0065] The housing can also be another element than a ferrule, for example, it can be a material block that provides a chamber for a piezo device and a through-hole for insertion of the fiber 11 (not shown).

[0066] A light cone 21 of a focused laser pulse is shown in FIG. 1. The fiber end of the fiber 11 is distal from the optical window 19. Thus, also within the length of the ferrule 15, a laser pulse can be focused down to a cross-sectional spot size which is smaller than the diameter of the fiber core 13. Thereby, ablation of material of the cladding of the fiber 11 can be avoided. A typical material of a cladding is silica. The ablation thresholds for silica surfaces are typically on the order of 3 J/mc.sup.2 for femtosecond pulses. An exemplary value of a diameter of the fiber core 13 is 27 ?m.

[0067] A corresponding ferrule 15 can be arranged at the opposite end of the fiber 11 (not shown), and the optical window 19 of this ferrule can serve as an exit window for broadband light pulses generated in the fiber 11 from the input laser pulses. Both ferrules, i. e. the ferrule 15 at the first fiber end and the other ferrule at the second fiber end, can close the fiber ends in a gas-tight way.

[0068] The volume 23 which is provided in the interior of the tube-like body 17 of the ferrule 15 and in the hollow core 13 can be filled with a fluid, such as a gas or a mixture of gases. The fluid can interact with the input laser pulses and, for example, form broadband or even supercontinuum light pulses due to nonlinear processes occurring during the interaction between the input light pulses and the fluid.

[0069] FIGS. 2 to 4 show an embodiment of a fiber arrangement in accordance with the present invention. The fiber arrangement comprises a piezo device 25 which is arranged in the ferrule 15. More specifically, the piezo device 25 is arranged in a chamber 27 of the ferrule 15. The chamber 27 is in fluid communication with the interior of the tube-like body 17 and the core 13 of the hollow fiber 11.

[0070] In the embodiment of FIGS. 2 to 4, one or more fluid channels 29 extend in a radial direction through the tube-like body 17 from the chamber 27 to the interior of the tube-like body 17. Thus, the volume which is available for the fluid is extended by the volume of the chamber 27.

[0071] As shown in FIG. 4, in the described embodiment, four fluid channels 29 are arranged offset by 90? with respect to the circumferential direction C and extend in a radial direction through the tube-like body 17 to connect fluidly the chamber 27 and the interior of the tube-like body 17 and the hollow core 13.

[0072] As illustrated with regard to FIGS. 2 and 3, the length of the piezo device 25 can be changed. A change of the length of the piezo device 25 causes a change of the size of the volume which is available for the fluid. Consequently, as is for example clear from the ideal gas law, the pressure of the fluid in the volume 23 can be changed in dependence on the length of the piezo device 25.

[0073] In the embodiment of FIGS. 2 to 4, the chamber 27 has an annular form and it is arranged in a housing section 31 which extends in the circumferential direction C around the tube-like body 17 of the ferrule 15. The housing section 31 and the tube-like body 17 preferably form a single piece. Furthermore, preferably, the chamber 27 is dimensioned such that the piezo device 25 is appropriately received in the chamber 27. Therefore, the piezo device 25 can fill out the chamber 27. In particular, as shown in FIG. 3, when the piezo device 25 has reached its maximally extended position, the piezo device 25 extends over the complete length of the chamber 27.

[0074] The piezo device can for example have an outer diameter of 8 mm, an inner diameter of 4 mm and an axial length of 36 mm. The axial length can be changed by 25 ?m with a modulation up to a resonant frequency of 40 kHz. Such an actuator can modulate the volume with up to 0.9 mm.sup.3.

[0075] In some embodiments, two or more of such piezo devices 25 can be arranged in series behind each other and arranged in the same chamber 27, so that even a larger volume modulation can be achieved.

[0076] The embodiment of a fiber arrangement shown in FIG. 5 differs from the previously described embodiment of FIGS. 2 to 4 in that the piezo device 25 comprises a piezo element 33, which has an annular form, and an end plate 35 which is fixed to an end of the piezo element 33 as shown in FIG. 5. The end plate 35 faces the volume 23 which is available for the fluid in the chamber 27. The end plate 35 has an annular form. The cross-sectional size of the end plate 35 corresponds in substance to the cross-sectional size of the chamber 27, when seen in a radial direction. The piezo element 33 can be controlled, in particular by use of a piezo controller which is connected to the piezo element 33 (not shown in FIG. 5), such that the axial length of the piezo element 33 can be extended or contracted. The piezo element 33 and the end plate 35 can act as a piston that can push the fluid out of the chamber 27 when the piezo element 35 is extended or it can pull the fluid into the chamber 27 when the piezo element 35 is contracted.

[0077] A filling 37 is arranged between outer surfaces 39 of the piezo element 33 and inner surfaces 41 of the housing section 31. In the embodiment of FIG. 5, the inner surfaces 41 are surfaces of guiding rings 43 which support the piezo element 33. The guiding rings 43 can be concentrically arranged at the radial inside and at the radial outside of the piezo element 33 to hold the piezo element 33 and the filling 37 in place and support a change in length of the piezo element 33.

[0078] The filling 37 can also extend between a radially outer surface of the end plate 35 and the opposing inner surface of the housing section 31. The filling 37 can be made of a deformable material. The filling 37 can seal the chamber 27 against the space which is taken up by the piezo device 25 in the housing section 31. In particular, the filling 37 can form a seal in the region between the radially outer surface of the end plate 35 and the inner surface of the housing section 31.

[0079] Referring again to the embodiment of FIGS. 2 to 4, it can also comprise a filling 37 between the radially outer end inner surfaces of the piezo device 25 and the opposing surfaces of the housing section 31 and of the tube-like body 17. The filling can act as a gas-tight seal and compensate a decrease in width of the piezo device caused by an extension of the length of the piezo device 25.

[0080] In the fiber arrangement of FIG. 6, the piezo device 25 is arranged in a chamber 27 which is provided by a housing section 31 that is arranged on the outer side of the tube-like body 17. A center axis B of the housing 31 extends at an angle of 90? with regard to the center axis A of the tube-like body 17 of the ferrule 15 and the hollow fiber 11. The housing section 31 can have any shape, for example cylindrical, quadratic or rectangular.

[0081] Between the inner surface of the housing section 31 and the piezo device 25, a filling 37 is arranged. The filling 37 is made of a flexible material and it can also act as a sealing. The volume 23 can extend into the chamber 27 in such a way that the material of the tube-like body 17 which is covered by the housing section 31 is completely removed. In other words, the fluid channel through the tube-like body 17 is formed by a rather large sidewall hole in the tube-like body 17 which has a cross-sectional area that corresponds in substance to the cross-sectional area of the chamber 27.

[0082] The fiber arrangement shown in FIG. 7 is based on the design of the embodiment of FIGS. 2 to 4. However, the ferrule 15 of the fiber arrangement of FIG. 7 does not include an optical window 19. Instead, both ends of the tube-like body 17 are open ends and the fiber 11 protrudes through the tube-like body 17. In other words, the ferrule 15 is arranged on an intermediate portion of the fiber 11, and the fluid channels 29 run in a radial direction through the tube-like body 17 and the fiber 11 to connect fluidly the chamber 27 and the hollow core 13 of the fiber 11.

[0083] Depending on the length of the fiber 11, more than one ferrule 15 can be arranged on the fiber 11 (not shown). For example, the ferrules 15 can be arranged at regular intervals on the fiber 11.

[0084] FIG. 8 shows schematically an optical system for providing broadband light, in particular supercontinuum light, which comprises a pulsed laser source 45 for providing laser pulses to a fiber arrangement 47, and a piezo driver 49.

[0085] The fiber arrangement 47 includes an optical fiber 11 with a hollow core 13 as described before. A ferrule 15a is arranged at a first end of the fiber 11, and a further ferrule 15b is arranged on a second end of the fiber 11 as shown in FIG. 8. The ferrules 15a and 15b can in particular correspond to the ferrules as described previously with regard to FIGS. 2 to 6. Optionally, the fiber arrangement can further comprise ferrule 15c which is arranged on an intermediate portion of the fiber 11. The ferrule 15c can in particular correspond to the ferrule 15 as described with regard to FIG. 7.

[0086] The piezo driver 19 is connected to the piezo devices 25 (see FIGS. 2 to 7) in the ferrules 15a, 15b and 15c and configured to drive individually each piezo device 25.

[0087] In operation, for example, the piezo driver 49 can drive the piezo devices 25 in phase. Then, all piezo devices 25 reach simultaneously their maximally extended length and their minimally extended length. In another example, the piezo devices 25 are being operated out of phase. Different piezo devices 25 therefore reach their maximally extended length at different times.

[0088] Moreover, in some embodiments, each piezo device 25 can be controlled so that its length changes over time according to a defined profile. This is in particular possible, since the piezo devices 25 can be controlled individually.

[0089] In some embodiments, the piezo devices 25 are controlled such that a desired homogeneous pressure distribution is obtained along the length of the hollow core 13 of the fiber 11.

[0090] In other embodiments, the piezo device 25 are controlled such that a desired pressure gradient is obtained along the length of the hollow core 13 of the fiber 11.

[0091] In one embodiment the length change of the piezo devices can be modulated at a steady frequency over a period of time. In one such embodiment a pulsed laser is focused into the hollow fiber and the pulses of this laser arrives at a second steady frequency in the same interval of time. In one such embodiment, the frequency of the modulation of the piezo is matched to the frequency of the laser pulses or to a integer fraction e.g. ?, ?, ?. . . 1/n of the frequency of the laser pulses. In one such embodiment, one can additionally control the delay or phase shift between the frequency of the laser pulses and the frequency of the piezo modulation.

[0092] In some embodiments, the entire sealed volume, which is basically formed by the fiber core 13 and the volume 23 and the volume of the chamber 27, can be varied by a factor of approximately 3 due to a change of the length of the piezo device 25. This in turn will vary the pressure and density of the fluid with a factor of approximately 3.

[0093] In some embodiments, the change in density or pressure of the fluid is more than 50%, 60%, 70%, 80% or 90%.

[0094] In some embodiments the change in density or pressure of the fluid is more than 10%, such as more than 20%, such as more than 25%, such as more than 30%, such as more than 40%, such as more than 50%, such as more than 75%, such as changed by a factor or 1:2, such as changed by a factor or 1:3 such as changed by a factor or 1:4 such as changed by a factor or 1:5 such as changed by a factor or 1:7.5 such as changed by a factor or 1:10.

LIST OF REFERENCE SIGNS

[0095] 11 optical fiber [0096] 13 hollow core [0097] ferrule, housing [0098] 15a ferrule [0099] 15b ferrule [0100] 15c ferrule [0101] 17 tube-like body [0102] 19 optical window [0103] 21 light cone [0104] 23 volume [0105] piezo device [0106] 27 chamber [0107] 29 fluid channel [0108] 31 housing section [0109] 33 piezo element [0110] end plate [0111] 37 filling [0112] 39 outer surface [0113] 41 inner surface [0114] 43 guiding ring [0115] pulsed laser source [0116] 47 fiber arrangement [0117] 49 piezo driver [0118] A center axis [0119] B center axis [0120] C circumferential direction