A method and system for generation of optical pulses of light

Abstract

A laser system for the generation of ultrashort optical pulses of light including an oscillator emitting low power and negatively chirped optical pulses with a spectral bandwidth W1, a dispersive connecting segment to maintain the sign of the chirp of the pulses of the oscillator, an optical amplifier for amplifying the optical light pulses and a negative group velocity dispersion segment for compensating phase contributions of the whole propagation process. During the propagation from the output of the oscillator to the end of the optical amplifier, the chirp of the light pulses will change once from negative to positive chirp. After a final compression stage ultrashort optical pulses can be generated.

Claims

1. A laser system comprising: an oscillator (20) producing a plurality of negatively chirped optical pulses having a first spectral width W1; an amplifier for receiving the plurality of optical pulses and amplifying the plurality of optical pulses to produce at an output of the amplifier an optical light pulse having a second spectral width W2; and a positive group velocity dispersion connecting segment connected directly between the oscillator and an input of the amplifier, wherein the connecting segment is adapted to maintain the sign of the chirp of the plurality of optical pulses; and wherein the sign of the chirp is changed between the oscillator and the output of the amplifier.

2. The laser system of claim 1, wherein the second spectra width W2 at the output of the amplifier is greater than the first spectral width WI of the oscillator.

3. The laser system of claim 1, wherein the connecting segment has a positive group velocity dispersion (β.sub.2>0) without changing the sign of the chirp.

4. The laser system of claim 1, wherein the connecting segment is adapted to change the amount of the chirp.

5. The laser system of claim 1, further comprising a second segment of positive group velocity dispersion segment (β.sub.2) connected to an output of the amplifier.

6. The laser system of claim 1, further comprising a negative group velocity dispersion segment (β.sub.2) connected to an output of the amplifier and being adapted to compensate the phase contributions of linear and nonlinear effects that have occurred during propagation through the connecting segment and the optical amplifier including the phase of the optical pulse.

7. The laser system of claim 1, wherein the amplifier is one of a fiber amplifier, a rod-type amplifier, a slab amplifier, thin disk amplifier or a solid-state amplifier.

8. (canceled)

9. laser system of claim 1, wherein the connecting segment is a combination of positive group velocity dispersion (β.sub.2) and negative group velocity dispersion segments (β.sub.2) without changing the sign of the chi.sub.rp.

10. (canceled)

11. The laser system of claim 1, further comprising at least one optical isolator located after the oscillator.

12. The laser system of claim 11, wherein the optical isolator is one of a free space or fiber coupled.

13. The laser system of claim 1, further including at least one of a pre-amplifier or an attenuator within the connecting segment.

14. The laser system of claim 13, wherein the preamplifier is one of a fiber-based amplifier, a semiconductor optical amplifier or a solid-state amplifier.

15. The laser system of claim 1, further including at least an optical pulse picker to adjust the temporal separation between the optical pulses.

16. The laser system of claim 1, wherein the oscillator comprises at least a segment of positive group velocity dispersion (β.sub.2) and a segment of negative group velocity dispersion (β.sub.2).

17. The laser system of claim 1, wherein the laser active medium of at least one of the oscillator or the amplifier (50) is selected from the group of dopants comprising Yb, Nd, Tm, or Er.

18. The laser system (10) of claim 1, wherein the oscillator (20) comprises a linear cavity with a saturable absorber at one end and a grating compressor at the other end.

19. The laser system of claim 1, wherein the amplifier is pumped by at least one of a single mode diode laser or multi-mode laser.

20. The laser system of claim 1, wherein the oscillator and amplifier are one of a single clad fiber or a double clad fiber or a combination of these.

21. The laser system of claim 1, wherein the amplifier comprises a chirp-free point at which, in use, the chirp of the optical pulses is substantially reduced to zero, wherein at the output of the optical amplifier, the optical pulses are positively chirped.

22. The laser system of claim 8, wherein at an output of the laser system a third spectral width is generated, in use, by shifting the chirp-free point to the end of the amplifier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 shows a schematic general diagram of a first aspect of the invention

[0042] FIG. 2 shows a schematic diagram a further aspect of the invention including a second segment with positive group velocity dispersion after the amplifier.

[0043] FIG. 3 shows a third aspect of the invention containing at least an optical isolator.

[0044] FIG. 4 shows a fourth aspect of the invention including at least an optical isolator and an attenuator.

[0045] FIG. 5 shows a fifth aspect of the invention including at least an optical isolator, an attenuator or an optical preamplifier.

[0046] FIG. 6 shows a sixth aspect of the invention including at least an optical isolator, an attenuator or an optical preamplifier and a pulse picker.

[0047] FIG. 7 shows schematic diagram of the other aspect of the laser with an additional negative dispersion segment within the connecting segment.

[0048] FIG. 8 shows schematic diagram of the pulse evolution inside a Stretched Pulse Laser system.

[0049] FIG. 9 shows a schematic embodiment of the Stretched Pulse oscillator configured in a linear cavity.

[0050] FIG. 10 shows a schematic embodiment of the Stretched Pulse oscillator configured in a ring cavity.

[0051] FIG. 11 shows a schematic embodiment of the Stretched Pulse oscillator configured in a sigma arm cavity.

[0052] FIG. 12 shows a schematic embodiment of the Stretched Pulse oscillator configured in a linear cavity with a dispersion compensation by using a chirped fiber Bragg grating.

[0053] FIG. 13 shows a schematic embodiment of the Stretched Pulse oscillator configured in a linear cavity with a fiber-based dispersion compensation by using photonic crystal fibers or hollow core fibers.

[0054] FIG. 14 shows a time compressed optical pulse after the compressor of the system having a pulse duration below 50 fs.

[0055] FIG. 15 shows the corresponding optical spectrum of the optical pulses at the output of the laser system having a spectral bandwidth W2.

[0056] FIG. 16 shows a typical optical spectrum of the optical pulses at the output of an oscillator having a spectral bandwidth W1.

[0057] FIG. 17 shows the corresponding optical spectrum of the optical pulses at the output of the laser system having a spectral bandwidth W3.

DETAILED DESCRIPTION OF THE INVENTION

[0058] The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiments of the invention.

[0059] FIG. 1 shows an aspect of a laser system 10 for generating ultrabroadband optical pulses of light is based on a stretched pulse oscillator 20 emitting a plurality of negatively chirped optical pulses 30. An input 41 of a positive group velocity dispersion connecting segment 40 is connected to the output 22 of the stretched pulse oscillator 20. An optical amplifier 50 having positive group velocity dispersion is connected to an output 42 of the connecting segment 40 and amplifies the optical pulses 30′ and is followed by a negative group velocity dispersion segment 60 (also called compressor). The negative group velocity dispersion segment 60 is used to compensate the phase contributions of linear and nonlinear effects that have occurred during propagation through the connecting segment 40 and the optical amplifier 50 including the phase of output pulses 30 of the oscillator 20. The sign of the chirp between the oscillator 20 and the output of the optical amplifier 50 only changes once through the optical path. In some aspects of the invention, the laser system 10 does not include the negative dispersion segment 60. In another aspect, the negative group velocity dispersion segment 60 could be replaced or complemented by an optical filter to reduce the width of the output spectrum and thus decrease the amount of amplitude noise in the output spectrum by removing light from the edges of the spectrum.

[0060] In a first aspect of the invention, the stretched pulse oscillator 20 emits the negatively chirped optical pulses as the plurality of optical pulses 30. In this first aspect, the connecting segment 40 does not change the sign of the chirp of the oscillator 20 unlike the connecting segments 40 known in the art. The input 41 of the connecting segment 40 is connected to the output 22 of the stretched pulse oscillator 20 by a splice of optical fibers or a free space coupling. The output of the connecting segment 42 is connected to the amplifier 50.

[0061] At the input 41 of the connecting segment 40, the optical pulses 30 have a larger negative chirp compared to the optical pulses 30.sup.I at the output 42 of the connecting segment 40. In other words, the negative chirp of the optical pulses 30.sup.I and therefore the pulse duration of the optical pulses 30.sup.I at the output 42 is reduced during propagating in a positive group velocity dispersion fiber which forms the connecting segment 40. Depending on the optical power of the optical pulses 30, nonlinear effects can occur in the connecting segment 40, which lead to a reduced spectral bandwidth of the optical pulses 30.sup.I at the output 42 of the connecting segment 40.

[0062] It will be appreciated that it may be necessary to implement a short connecting segment 40′ with positive group velocity dispersion after the amplifier 50, shown in FIG. 2. For example, there is a need to remove the pump light (from the optical pump) from the amplifier 50 and this is done by using a cladding light stripper to remove residual pump light from the amplifier 50. In this case the positive chirp of the optical pulse 30.sup.II slightly increases at the output of the short connecting segment 40′.

[0063] At least one optical isolator 43 or 43′ can be implemented in the connecting segment(s) 40 or 40′ after the stretched pulse oscillator 20 and is shown schematically in FIG. 3. The position of the optical isolator 43 or 43′ could be at the input 41 or 41′ of the connecting segment 40 (40′), within the connecting segment 40 (40′), or at the output 42 (42′) of the connecting segment 40 (40′). In this case the optical pulse 30.sup.III will have a slightly changed chirp due to the presence of the optical isolator 43.sup.I in the optical path.

[0064] After the (first) connecting segment 40, the negative chirped optical pulses 30.sup.I propagate into the optical amplifier 50 which is used to increase the power level of the optical pulses 30.sup.I. Due to the nonlinear effects of the afore-mentioned SPM inside the optical amplifier 50 the chirp of the optical pulses 30.sup.I is substantially reduced to zero at a “chirp-free” point 52 within the optical amplifier 50, as shown in FIGS. 1-6, as the optical amplifier 50 has a positive group velocity dispersion. At the output 54 of the optical amplifier 50, the optical pulses 30.sup.II are positively chirped and the chirp can be compensated by a negative group velocity dispersion segment 60 connected to the output 54 of the optical amplifier 50 to a near Fourier limited pulse duration 30.sup.IV. In FIG. 14 an example of a measured FROG trace of the compressed optical light pulse is displayed. The corresponding optical spectrum is show in FIG. 15. The negative group velocity dispersion segment 60 can be, but are not limited to, a grating compressor, a prism compressor, a GRISM compressor, chirped mirrors, or a hollow core fiber segment. For an ideal compression, a pulse shaper can also be integrated.

[0065] The dispersion of the negative group velocity dispersion segment 60 is estimated to be smaller than three times the sum of the group velocity dispersion of the connecting segment 40 and the amplifier 50, i.e. (3*(|b.sub.40+b.sub.50|)>|b.sub.60|) in which b.sub.40 represents the group velocity dispersion of the connecting segment, b.sub.50 represents the group velocity dispersion of the optical amplifier 50 and b.sub.60 represents the group velocity dispersion of the negative group velocity dispersion segment 60. The value of the dispersion is not, however limiting of the invention.

[0066] In a further aspect of the laser pulse system 10, the power level inside the connecting segment 40 or 40′ can be adjusted by at least an attenuator 44 (or 44′) (shown in FIGS. 4-6) or a preamplifier 45 (shown in FIGS. 5 and 6), or a combination of both (as in FIGS. 5 and 6) for controlling the non-linear effects in the connecting segment 40 and the amplifier 50. The position of the attenuators 44 or 44′ or the preamplifier 45 could be at the input 41/41′ of the connecting segments 40 and 40′, within the connecting segments 40 and 40′, or at the output 42 or 42′ of the connecting segments 40 or 40′. A higher power output can be achieved by increasing the diameter of the mode field in the optical fiber.

[0067] The preamplifier could be one of a fiber-based preamplifier, a (fiber-coupled) semiconductor optical amplifier or a solid-state amplifier

[0068] An optical isolator 43 can be used after the preamplifier 45 (as shown in FIGS. 5 and 6).

[0069] By combining a negative chirp of the optical pulses 30 and a positive group velocity dispersion in the connecting segment 40, the spectral bandwidth of the optical pulses 30.sup.I at the output 42 will be reduced, thus creating a “nonlinear band pass filter” at a “chirp free point” 52 inside the optical amplifier 50, as shown in FIGS. 1-6. It would be possible to couple the optical pulse 30 out near to this point to provide an optical pulse 30 with a very narrow spectrum W3 by reducing the length of the amplifier 50.

[0070] A sixth aspect of the laser system 10 is shown in FIG. 6 and enables the integration of an optical pulse picker 46 to reduce the repetition rate of the stretched pulse oscillator 20 to increase the pulse energy after the optical amplifier 50. A second pulse picker 46′ can be added in front of the compressor 60 within the connecting segment 40′ to generate a pulse on demand functionality.

[0071] A seventh aspect of the laser system 10 is shown in FIG. 7. An additional part of negative dispersion 40a is added to the positive dispersion 40b of the connecting segment 40 while changing only the amount of the chirp without changing the sign. This allows for an adaption of the position of the chirp free point 52 inside the amplifier 50. The optical pulse 30.sup.V at the output 42 of the connecting segment 40 will have a larger negative chirp compared to the optical pulse 30 coming from the oscillator 20 and arriving at the input 41 of the connecting segment. The position of the chirp free point 52 will shift towards the end of the amplifier 50. This mechanism can be used to adapt the chirp free point 52. The chirp of the optical pulse 30.sup.VI will be positive after the amplifier 50 and slightly increased by propagating through the optional parts 43′, 44′ and 46′ and their dispersion leading to an optical pulse 30.sup.VII.

[0072] If the chirp free point 52 is shifted to the end of the amplifier 50, an optical pulse 30 V.sup.III with a narrow spectrum W3 can be generated.

[0073] The stretched pulse oscillator 20, the connecting segment 40 and the optical amplifier 50 are connected with fiber splices. Nevertheless, transitions between any of the stretched pulse oscillator 20, the connecting segment(s) 40 and 40′ and the optical amplifier 50 can also be implemented by free space coupling. Therefore, free space isolators 43 and 43′ pulse pickers 46 and 46′, attenuators 44 and 44′ or preamplifiers 45 can also be used.

[0074] The ultrabroadband generation of the optical pulses is based on the interaction of linear effects and nonlinear effects within the amplifier and so the maximum energy, or the spectral bandwidth, can be controlled by choosing different mode field diameters during the propagation.

[0075] The optical amplifier 50 can be made in a non-limiting example of a fiber amplifier doped with Ytterbium. It is thought that the optical amplifier 50 can be adapted to all lasing materials, such as but not limited to, Nd, Tm, Er, Er—Yb.

[0076] The principle is not limited to fiber laser technology and in different aspects, the principle can also be adapted to solid state amplifiers, including for example thin disk amplifiers, slab amplifiers, crystal-based amplifiers, rod amplifiers or other types. For a more general approach the negative chirped optical pulses 30 from the stretched pulse oscillator 20 or a soliton oscillator have to propagate through a medium of positive group velocity dispersion segment, which is not limited to optical fibers, but can also be waveguides (included those implemented as micro-optics on a wafer) or materials with positive group velocity dispersion.

[0077] The use of a positive group velocity dispersion for the connecting segment 40 requires the production of the negatively chirped optical pulses 30 in the stretched pulse oscillator 20. This is illustrated in FIG. 8 and can be achieved by using two different dispersion segments inside an oscillator cavity. The oscillator cavity comprises a positive group velocity dispersion segment 21 and a negative group velocity dispersion segment 23. The overall net group velocity dispersion (GVD) has to be less than 0.1 ps.sup.2 (β.sub.2net<0.1 ps.sup.2). In this case the optical pulses 30 undergo a change of the chirp sign within the two dispersion segments, i.e., a negative dispersion segment 21 and a positive dispersion segment 23. FIG. 7 shows the evolution of the chirp (normalized y-axis in arbitrary units (arb.unit)) of an optical light pulse inside a stretched pulse cavity during propagation (x-axis in meter). Furthermore, the corresponding group velocity dispersion segments (normalized y-axis in arbitrary unit (arb. unit)) are displayed. During the propagation, the pulse has to be free of chirp twice per round trip, as can be seen in FIG. 7. According to FIG. 7 the chirp free points are located at the ends of the linear cavity. Depending in the overall net dispersion, the position of the chirp free pulses can be changed.

[0078] Starting from a chirp free point 31 within the positive group velocity dispersion segment 21 a positive chirp is generated by propagating through the positive dispersion segment 21 forming one part of the stretched pulse oscillator 20. This positive chirp will be reduced within the negative group velocity dispersion segment 23 forming a second part of the oscillator cavity 20, leading to a chirp free optical pulse at a position 31′ within the segment 23, and changing the sign of the chirp afterwards. This negative chirp increases up to the end of the negative group velocity dispersion segment 23. Finally, the negative chirp will be reduced by entering the positive group velocity dispersion segment 21 and after one roundtrip will reach the chirp free starting point 31 again. For the apparatus of this document the optical output pulses of the oscillator 30 have to have a negative chirp at the output coupler 27 of the oscillator 20.

[0079] One example of the stretched pulse oscillator 20 is shown in FIG. 9. The stretched pulse oscillator 20 comprises an optical pump 28 for generating the pump light for the laser active fiber segment 20′. The optical pump 28 is coupled to the cavity by using a pump coupler 29. The negative dispersion segment 23 is implemented by a grating compressor at one end of a linear cavity.

[0080] The optical fiber part forms the positive dispersion segment 21. The stretched pulse oscillator 20 is mode-locked by using a saturable absorber mirror 24 at the other end of the linear optical cavity. An output coupler 27 is placed behind the negative dispersion segment 23 (in propagation direction).

[0081] Other different aspects of the laser optical cavity of the stretched pulse oscillator 20 are shown in FIGS. 10 and 11. The optical cavity can also be implemented as a ring cavity when using an optical isolator 25 and a saturable absorber 24′ or as a sigma cavity when using a circulator 26 to implement a saturable absorber mirror 24 in the optical cavity.

[0082] To further reduce free space parts inside the linear cavity oscillator the negative dispersion segment 23 can be implemented using a chirped fiber Bragg grating 23′ (FIG. 12) or a negative dispersion photonic crystal fiber combined with a fiber-based mirror (23″) can be used (FIG. 13) instead of a grating compressor.

[0083] In one aspect all of the optical fibers used are polarization maintaining fibers to achieve an environmental stable system. In general, however, the laser pulse system of this disclosure is not limited to polarization maintaining fibers. Non-polarization maintaining fibers could also be used. Furthermore, the laser pulse system 10 is not limited to single clad fibers. In addition, other types of fibers as double clad fibers can be used. Depending on the type of fibers single or multimode laser diodes can be used for pumping. Mode-locking can also be achieved by using any kind of saturable absorber (24′) or virtual saturable absorber as for example nonlinear pulse evolution.

[0084] As already noted above, the laser pulse system 10 of this document can be implemented in a bulk solid, as long as there is a provision for management of the dispersion of the optical pulse. This will require at least one positive dispersion element and one negative dispersion segment.

REFERENCE NUMERALS

[0085] 10 Laser system
20 Stretched pulse oscillator
20′ Laser active fiber segment
21 Positive dispersion segment

22 Output

[0086] 23 Negative dispersion segment
24 Absorber mirror
25 Optical isolator

26 Circulator

[0087] 27 Output coupler
28 Optical pump
29 Pump coupler
30 Optical pulse
31 Chirp-free point
40 Connecting segment
40a negative dispersion (β2<0) segment within the connecting segment
40b positive dispersion (β2>0) segment within the connecting segment

41 Input

42 Output

43 Isolator

44 Attenuator

45 Preamplifier

[0088] 46 Optical pulse picker
50 Optical amplifier
52 Chirp-free point

54 Output

[0089] 60 Negative dispersion segment