LASER SYSTEM FOR COHERENTLY COMBINING MULTIPLE LASER SOURCES

Abstract

A method and system for combining two or more optical fields is disclosed. A first continuous-wave high powered output field generated by a solid-state master laser is injected into a first solid state optical amplifier to produce a single output field from the laser system that exhibits a high phase-coherence with the output field of the master laser. The power of the output field equals the sum of powers of the master laser and that generated by the first optical amplifier, while exhibiting similar beams characteristics to that produced by the output field of the master laser i.e. it exhibits low noise, in a single transverse and longitudinal mode Gaussian beam, and has a single polarisation. The laser system is highly scalable in that N optical amplifiers may be located in series with the master laser to provide a single low noise, high power output field.

Claims

1. A laser system comprising: a first laser, the first laser comprising a Ti:sapphire gain medium that generates a first continuous-wave output field having a first frequency (f.sub.1) and a first power (P.sub.1), and a first optical amplifier, the first optical amplifier comprising a Ti:sapphire gain medium that generates a second continuous-wave output field having a second frequency (f.sub.2) and a second power (P.sub.2), wherein a fraction of the first continuous-wave output field is injected into the first optical amplifier to seed the first optical amplifier resulting in the second continuous-wave output field having the same frequency (f.sub.1) as the first continuous-wave output field, and a remainder of the first continuous-wave output field is combined with the second continuous-wave output field forming a single output field for the laser system at the first frequency (f.sub.1) and a power (P.sub.out) substantially equal to the sum of the first (P.sub.1) and the second (P.sub.2) powers.

2. A laser system as claimed in claim 1 wherein the second power (P.sub.2) is in the range 0.5 P.sub.1≤P.sub.2<1.5 P.sub.1.

3. A laser system as claimed in claim 1 wherein the first power (P.sub.1) is greater than or equal to 1 Watt.

4. A laser system as claimed in claim 1 wherein the first power (P.sub.1) is greater than or equal to 4 Watts.

5. A laser system as claimed in claim 1 wherein the first optical amplifier comprises a ring cavity within an arm of which is located the Ti:sapphire gain medium.

6. A laser system as claimed in claim 1 wherein the laser system further comprises a first injection locking module located within an optical path between the first laser and the first optical amplifier wherein the injection locking module provide a means for ensuring that the mode of the first continuous-wave output field matches that of the first optical amplifier.

7. A laser system as claimed in claim 1 wherein the laser system further comprises a first optical isolator located within an optical path between the first laser and the first optical amplifier.

8. A laser system as claimed in claim 7 wherein the first optical isolator comprises one or more dichroic mirrors.

9. A laser system as claimed in claim 1 wherein the laser system further comprises a first frequency lock control loop to frequency lock the second continuous-wave output field to the frequency of the first continuous-wave output field.

10. A laser system as claimed in claim 1 wherein the laser system further comprises a second optical amplifier comprising a Ti:sapphire gain medium that generates a third continuous-wave output field having a third frequency (f.sub.3) and a third power (P.sub.3) wherein a fraction of the single output field is injected into the second optical amplifier to seed the second optical amplifier resulting in the third continuous-wave output field having the same frequency (f.sub.1) as the first continuous-wave output field; and the remainder of the single output field is combined with the second continuous-wave output field forming a single output field for the laser system at the first frequency (f.sub.1) and a power (P.sub.out) substantially equal to the summation of the first (P.sub.1), second (P.sub.2) and third (P.sub.3) powers.

11. A laser system as claimed in claim 10 wherein the laser system further comprises a second injection locking module located within an optical path between the first and second optical amplifiers wherein the second injection locking module provide a means for ensuring that the mode of the second continuous-wave output field matches that of the second optical amplifier.

12. A laser system as claimed in claim 10 wherein the laser system further comprises a second optical isolator located within an optical path between the first and second optical amplifiers.

13. A laser system as claimed in claim 12 wherein the second optical isolator comprises one or more dichroic mirrors.

14. A laser system as claimed in claim 10 wherein the laser system further comprises a second frequency lock control loop to frequency lock the third continuous-wave output field to the frequency of the second continuous-wave output field.

15. A method of combining two or more optical fields the method comprising: providing a first laser comprising a Ti:sapphire gain medium to generate a first continuous-wave output field having a frequency (f.sub.0) and a first power (P.sub.1); providing a first optical amplifier comprising a Ti:sapphire gain medium to generates a second continuous-wave output field having a second frequency (f.sub.2) and a second power (P.sub.2); injecting a fraction of the first continuous-wave output field into the first optical amplifier to seed the first optical amplifier causing the second continuous-wave output field to have the same frequency (f.sub.1) as the first continuous-wave output field; and combining the remainder of the first continuous-wave output field with the second continuous-wave output field to form a single output field for the laser system at the first frequency (f.sub.1) and a power (P.sub.out) substantially equal to the sum of the first (P.sub.1) and the second (P.sub.2) powers.

16. A method of combining two or more optical fields as claimed in claim 15 wherein the second power (P.sub.2) is in the range 0.5 P.sub.1≤P.sub.2<1.5 P.sub.1.

17. A method of combining two or more optical fields as claimed in claim 15 wherein the first power (P.sub.1) is greater than or equal to 1 Watt or greater than or equal to 4 Watts.

18. A method of combining two or more optical fields as claimed in claim 15 wherein providing a first optical amplifier comprises providing a ring cavity within an arm of which is located the Ti:sapphire gain medium.

19. A method of combining two or more optical fields as claimed in claim 15 wherein the method further comprises matching the mode of the first continuous-wave output field to that of the first optical amplifier.

20. A method of combining two or more optical fields as claimed in claim 15 wherein the method further comprises optically isolating the first solid state laser from the first optical amplifier.

21. A method of combining two or more optical fields as claimed in claim 15 wherein the method further comprises frequency locking the first optical amplifier to the first continuous-wave output field.

22. A method of combining two or more optical fields as claimed in claim 15 wherein the method further comprises providing a second optical amplifier comprising a Ti:sapphire gain medium to generates a third continuous-wave output field having a third frequency (f.sub.3) and a third power (P.sub.3); injecting a fraction of the single output field into the second optical amplifier to seed the second optical amplifier causing the third continuous-wave output field to have the same frequency (f.sub.1) as the first continuous-wave output field; and combining the remainder of the single output field with the third continuous-wave output field forming a single output field for the laser system at the first frequency (f.sub.1) and a power (P.sub.out) substantially equal to the summation of the first (P.sub.1), second (P.sub.2) and third (P.sub.3) powers.

23. A method of combining two or more optical fields as claimed in claim 22 wherein the method further comprises matching the mode of the second continuous-wave output field to that of the second optical amplifier.

24. A method of combining two or more optical fields as claimed claim 22 wherein the method further comprises optically isolating the first optical amplifier from the second optical amplifier.

25. A method of combining two or more optical fields as claimed in claim 22 wherein the method further comprises frequency locking the second optical amplifiers to the second continuous-wave output field.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0058] There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

[0059] FIG. 1 presents a schematic representation of a laser system in accordance with an embodiment of the present invention;

[0060] FIG. 2 presents a schematic representation of:

[0061] (a) an optical amplifier employed within the laser system of FIG. 1 and

[0062] (b) an alternative optical amplifier employed within the laser system of FIG. 1;

[0063] FIG. 3 presents a schematic representation of a laser system in accordance with an alternative embodiment of the present invention; and

[0064] FIG. 4 presents a schematic representation of a laser system in accordance with a further alternative embodiment of the present invention.

[0065] In the description which follows, like parts are marked throughout the specification and drawings with the same reference numerals. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of embodiments of the invention.

DETAILED DESCRIPTION

[0066] Details of the laser system will now be described with reference to FIG. 1. In particular, FIG. 1 presents a schematic representation of a laser system in accordance with a first embodiment of the present invention, as generally depicted by reference numeral 1.

[0067] The laser system 1 can be seen to comprise a Ti:sapphire laser 2 employed as the master (or seed) laser within the system 1. The master Ti:sapphire lasers 2 is optically pumped at 532 nm by a dedicated continuous wave diode-pumped solid-state (DPSS) laser source 3 to produce a continuous-wave output field 4. The applicant's proprietary SolsTiS® laser is a suitable example of a Ti:sapphire laser for use as the master laser 2 while the pump laser 3 may comprise a commercially available diode-pumped solid-state (DPSS) laser.

[0068] The pump laser 3 has the capability of providing up to ˜18 Watts of pump power to the master Ti:sapphire laser 2. In the presently described embodiment, the pump laser 3 is arranged to provide ˜18 W of pump power to the master Ti:sapphire laser 2 to provide a tuneable continuous-wave output field 4 (tuneable between 660 nm and 1060 nm) with a power of around ˜4.7 W at 880 nm. Due to the presence of a number of intracavity elements (e.g. etalons) and control electronics the generated tuneable continuous-wave output field 4 operates at a single frequency (f.sub.0), as a single transverse and longitudinal mode and exhibits low phase noise. In practice, the quality of the output field 4 can be maintained when the master Ti:sapphire laser 2 is configured to operate at a power as low as 400 milliwatts.

[0069] As can be seen from FIG. 1, the continuous-wave output field 4 is injected into a first optical amplifier 5 i.e. the continuous-wave output field 4 is employed as a seed optical field for the first optical amplifier 5 resulting in the first optical amplifier 5 being slaved to the master Ti:sapphire laser 2. A first dichroic mirror 6a is employed to redirect the seed optical field 4 into the first optical amplifier 5.

[0070] It is preferable for an injection locking module 7 to also be located within the optical path between the master Ti:sapphire laser 2 and the slave optical amplifier 5. The injection locking module can be seen to comprise beam shaping optics 8 located between two dichroic mirrors 6b and 6c. The combined effects of the beam shaping optics 8 provide a means for ensuring that the mode of the seed optical field 4 matches that of the slave optical amplifier 5.

[0071] FIG. 2(a) presents a schematic representation of the first optical amplifier 5 employed within the laser system 1. The first optical amplifier 5 comprises a ring cavity 9 within an arm of which is located a Ti:sapphire gain medium 10, the Ti:sapphire gain medium 10 being optically pumped at 532 nm by a dedicated continuous wave DPSS laser source 11 to produce a continuous-wave output field 12. The applicant's proprietary SolsTiS® amplifier is an example of a suitable optical amplifier 5 for use within the laser system 1. Unlike the master Ti:sapphire laser 2, the first optical amplifier 5 comprises no additional intracavity elements. Therefore, in the absence of the seed optical field 4 the first optical amplifier 5 is effectively free running, and thus produces a bi-directional output at 770 nm when pumped at 532 nm by the dedicated continuous wave DPSS laser source 11. In the presently described embodiment the pump laser 11 is arranged to provide ˜18 W of pump power to the Ti:sapphire laser 5.

[0072] The cavity of the first optical amplifier 5 is arranged to be resonant at the frequency (f.sub.0) of seed optical field 4, namely the 880 nm output field 4. As described above, the continuous-wave output field 4 is employed as a seed optical field for the first optical amplifier 5. This is achieved by arranging for the continuous-wave output field 4 to be incident upon the output coupler 13 of the first optical amplifier 5. As a result, a fraction the optical field 4 enters the ring cavity 9 of the first optical amplifier 5 while the remainder is reflected from the output coupler 13 of the first optical amplifier 5.

[0073] The fraction of the output field 4 that enters the ring cavity 9 acts as a “seed” for the first optical amplifier 5. This “seed” results in stimulated emission within the first optical amplifier 5 at the same frequency as the output field 4 of the master Ti:sapphire laser 2, as well as causing the first optical amplifier 5 to operate in a unidirectional manner.

[0074] The fraction of the seed optical field 4 entering the ring cavity 9, and amplified therein, is also in phase with that reflected from the output coupler 13, thus ensuring temporal phase coherence between the seed optical field 4 and the output field 12 of the first optical amplifier 5.

[0075] Spatial phase-coherence between the fraction of the optical field 4 which is reflected from the output coupler 13, and that which is amplified within the ring cavity 9, is ensured by having a high degree of mode-matching between the seed optical field 4 and the output field 12 of the optical amplifier 5. The injection locking module 7 is employed to assist with mode-matching the seed optical field 4 to the output field 12

[0076] The output field 12 of the optical amplifier 5 is therefore combined with the output field 4 of the master Ti:sapphire laser 2 such that the laser system 1 provides a single output exhibiting a single transverse and longitudinal mode Gaussian beam, that exhibits a single polarisation and a high phase-coherence with the seed optical field 4 i.e. their frequencies are in phase to <<1 cycle of the waveform. The output field 12 has a power ˜9.4 W which is effectively the sum of the powers of the output field 4 of the master Ti:sapphire laser 2 and the output field 12 of the first optical amplifier 5. Most significantly, the wavelength of the output field 12 is tuneable over a 400 nm wavelength range (around 660 nm to 1060 nm) as controlled by the operating wavelength of the master Ti:sapphire laser 2. By comparison the Nd:YAG laser systems of the prior art are only tuneable over a range of around 0.05 nm. Thus, unlike the Nd:YAG laser systems of the prior art the present laser system 1 able to be employed within optical traps where it is necessary to be able to tune the laser source to different atomic wavelengths depending on the particular atom required to be trapped e.g. Barium, Beryllium, Caesium, Magnesium, Rubidium, Strontium and Ytterbium.

[0077] Since the first optical amplifier 5 is a resonant device, and as explained above, injection locking occurs when the seed optical field 4 is in resonance with the ring cavity 9 of the optical amplifier 5, it is beneficial for the stable operation of the laser system 1 to frequency lock the resonance condition of the first optical amplifier 5 to the seed optical field 4.

[0078] In the embodiment presented in FIG. 2(a) a preferred frequency lock control loop 14 based on the Hänsch-Couillaud technique is employed to achieve the desired frequency locking. This technique was first described in Optics Communication Volume 35, Issue 3, pages 441 to 444 (1980) and exploits the fact that the Ti:sapphire gain medium 10 of the first optical amplifier 5 is Brewster cut crystal. As a result, there exists an asymmetry in the two orthogonal polarisation components of the resonance frequency. To exploit this fact, a half-wave plate 15 is employed to introduce an offset to the polarisation of the fraction of the the seed optical field 4 that enters the ring cavity 9 to introduce a component of polarisation orthogonal to the plane of incidence to the Brewster-cut Ti:sapphire gain medium 10. A component 16 of the output field 12 is then directed towards a polarisation analyser 17 employed to detect dispersion shaped resonances which can provide an error signal generated by locking electronics 18 for electronic frequency stabilisation via a piezo-mounted cavity mirror 19.

[0079] In an alternative embodiment presented in FIG. 2(b) a frequency lock control loop 20 based on the Pound-Drever-Hall technique is employed to achieve the desired frequency locking. This technique is described in Applied Physics. B, Volume 31, Pages 97 to 105 (1983). In this technique a frequency modulator 21 applies a frequency-modulation to the seed optical field 4 that is detected within a component 22 of the output field 12 by a photodiode 23. This frequency modulated signal is then demodulated by via a frequency mixer 24 to generate an error signal. Frequency locking electronics 25 then provide a correction signal for electronic frequency stabilisation via the piezo-mounted cavity mirror 19.

[0080] It should be noted that the above described frequency locking techniques of the optical amplifiers 5 do not themselves provide for phase-coherence between the seed optical field 4 and the output field 12. Instead, phase coherence is a direct result of the amplification process within the first optical amplifier 5. The frequency locking techniques merely ensures that the frequency of the resonance of the first optical amplifier 5 remains coincident with the frequency of the seed optical field 4.

[0081] In order to confirm that the output field 12 comprises the combined output of the master laser 2 and the first optical amplifier 5 a fibre launch module 26 was employed to analyse the output field 12. As can be seen from FIG. 1, a fraction of the output field 12 (˜2%) is picked-off by employing a glass substrate before being coupled into a single mode fibre 27. A coupling efficiency of >80% through single mode fibre 27 served to demonstrate that the output field 12 occupies a single transverse spatial mode. In an independent measurement of beam quality, the M.sup.2 value was found to be 23 1.1 which, again, demonstrates that the output field 12 occupies a single transverse spatial mode.

[0082] As discussed above, before injection locking has been achieved, the first optical amplifier 5, not containing any internal elements to force unidirectional operation, will operate bi-directionally, with half of its output power being directed back towards the master laser 2. However, the presence of the dichroic mirrors 6a, 6b and 6c prevent the backward going oscillation of the free running optical amplifier 5 at 770 nm from returning to the master laser 2. Preferably the optical isolation provided by the dichroic mirrors 6a, 6b and 6c is capable of providing optical isolation of <−50 dB. This method of optically isolating the master laser 2 from the first optical amplifier 5 is particularly suited for laser systems where the wavelength of the output field of the free running optical amplifier 5 is significantly different (i.e. greater than 10 nm different) from the operating wavelength the master laser 2.

[0083] It will be appreciated by the skilled reader that an alternative form of optical isolation may be located between the master laser 2 and the optical amplifier 5. By way of example, FIG. 3 presents an alternative laser system in accordance with a second embodiment of the present invention, as generally depicted by reference numeral 28. The laser system 28 presented in FIG. 3 is similar to the laser system 1 described above, and presented in FIGS. 1 and 2, with the exception that the dichroic mirrors 6 are replaced by standard mirrors 29 and an optical isolator 30 is located within the optical path between the master laser 2 and the optical amplifier 5. This embodiment does not require there to be a difference between the wavelength of the output field of the free running optical amplifier 5 and the operating wavelength the master laser 2.

[0084] The above described laser systems 1 and 28 can provide a high power output field 12 (˜9.4 W) which is greater than that which can be achieved by the independent operation of either the master laser 2 or the first optical amplifier 5. Most significantly, the wavelength of the output field 12 is tuneable over a 400 nm wavelength range (around 660 nm to 1060 nm) as controlled by the operating wavelength of the master Ti:sapphire laser 2. This is around two orders of magnitude greater that the Nd:YAG laser systems known in the prior art e.g. those disclosed within U.S. Pat. No. 5,027,360. This is achieved even though the power of output field 4 generated by the master laser 2 is at least one order of magnitude greater than those employed within the laser systems of U.S. Pat. No. 5,027,360. The applicants have found that the decreased stability of employing a higher power master laser, although not acceptable within free space communication systems and gravitational wave detectors, is an acceptable trade off for an optical trap laser system where increased tuneability of the operating wavelength of the generated output field is highly beneficial.

[0085] A further significant advantage of the laser systems 1 and 28 presented within FIGS. 1 to 3 is the fact that they are readily scalable through the inclusion of additional optical amplifiers 5. By way of example, FIG. 4 presents an alternative laser system in accordance with an embodiment of the present invention, as generally depicted by reference numeral 31.

[0086] As can be seen from FIG. 4, the continuous-wave output field 12 is now employed as a seed optical field for a second optical amplifier 5b i.e. the continuous-wave output field 12 is employed as a seed optical field for the second optical amplifier 5b resulting in the second optical amplifier 5b also being slaved to the master Ti:sapphire laser 2. A fourth dichroic mirror 6d is employed to redirect the optical field 12 into the second optical amplifier 5b.

[0087] It is again preferable for a second injection locking module 7b to also be located within the optical path between the first and second slave optical amplifiers 5 and 5b to ensure that the mode of the seed optical field 12 matches that of the second optical amplifier 5b. It is also beneficial for the stable operation of the laser system 31 to frequency lock the resonance condition of the second optical amplifier 5b to the seed optical field 12, in a similar manner to that described above.

[0088] The output field 32 of the second optical amplifier 5b is therefore combined with the output field 12 of the first optical amplifier 5b, and hence the output of the master Ti:sapphire laser 2, such that the laser system 31 provides a single output exhibiting a single transverse and longitudinal mode, single polarisation and a high phase-coherence with the first 4 and second 12 seed optical fields. Most significantly is the fact that the output field 32 has a power ˜14 W which is effectively the sum of the powers of the output field 4 of the master Ti:sapphire laser 2, the output field 12 of the first optical amplifier 5 and the output field of the second optical amplifier 5b. Significantly, the wavelength of the output field 32 is again tuneable over a 400 nm wavelength range (around 660 nm to 1060 nm) as controlled by the operating wavelength of the master Ti:sapphire laser 2.

[0089] It is envisaged that the above described laser system 1 could in fact be scaled up by to provide an output field having a power equal to the sum of the power of the output field 4 of the master laser 2 and N additional optical amplifiers 5.

[0090] A number of phase locked injection-locked laser system have also been disclosed. These systems have the advantage that they provide a single output exhibiting a single transverse and longitudinal mode, a single polarisation and a high phase-coherence with the respective seed optical fields. An advantage of the disclosed laser systems is the fact that the output field of the injection-locked laser system is highly scalable thus providing a means for increasing the power of the generated output field.

[0091] A further advantage of the disclosed laser system resides in the fact that the generated output field is tuneable over a 400 nm wavelength range (around 660 nm to 1060 nm) as controlled by the operating wavelength of the master laser 2. This is a result of the fact that Ti;Sapphire gain media are employed within the master laser 2 and the one or more optical amplifiers 5 located in series with the master laser 2. By comparison Nd:YAG laser systems of the prior art are only tuneable over a range of around 0.05 nm. Thus, unlike the Nd:YAG laser systems of the prior art the present laser system able to be employed within optical traps where it is necessary to be able to tune the laser source to different atomic wavelengths depending on the particular atom required to be trapped.

[0092] A method and system for combining two or more optical fields is disclosed. A first continuous-wave high powered output field generated by a solid-state master laser is injected into a first solid state optical amplifier to produce a single output field from the laser system that exhibits a high phase-coherence with the output field of the master laser. The power of the output field equals the sum of powers of the master laser and that generated by the first optical amplifier, while exhibiting similar beams characteristics to that produced by the output field of the master laser i.e. it exhibits low noise, in a single transverse and longitudinal mode Gaussian beam, and has a single polarisation. The laser system is highly scalable in that N optical amplifiers may be located in series with the master laser to provide a single low noise, high power output field.

[0093] Throughout the specification, unless the context demands otherwise, the term “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0094] Furthermore, reference to any prior art in the description should not be taken as an indication that the prior art forms part of the common general knowledge.

[0095] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.