IL107299A - Method and apparatus for generating bright light sources - Google Patents

Description

nmm ηιτη. ^v.. TIN mm pa m»s>* ipnm no>ei METHOD AND APPARATUS FOR GENERATING BRIGHT LIGHT SOURCES METHOD AND APPARATUS FOR GENERATING BRIGHT LIGHT SOURCES The present invention relates to a method and apparatus for generating bright light sources. The invention is particularly useful for generating bright laser sources for use in laser surgery and is therefore described below with respect to this application, but it will be appreciated that the invention could be advantageously used in other applications requiring bright light sources, e.g., metal welding.

In many applications, particularly laser surgery procedures, it is necessary to concentrate relatively high laser power onto a small area, e.g., for ablating or cutting tissue. This application of lasers requires that the laser source be of high brightness; that is, it must emit a high power beam of small area and of small divergence. Brightness is directly proportional to the output power and inversely proportional to the source area and also to the solid angle of the light beam. It is therefore difficult to increase the brightness of a source by increasing the output power, since this also tends to increase the source area and/or the solid angle of the light beam such that the net effect on the brightness is relatively small.

It is well known, e.g., see UK Patent Application GB 2256503A, to increase the brightness of a light source by using a polarising beam combiner, in which two differently-polarised beams are combined into a single composite beam. However, this technique is limited to doubling the brightness since it combines only two (polarised) beams.

An object of the present invention is to provide a method and apparatus for generating a beam of bright light which can be used to combine two or many more beams and thereby to increase the brightness of a light source more than two times if desired.

According to a broad aspect of the present invention, there is provided a method of generating a bright light source and applying same onto a working surface, comprising: generating a plurality of source beams of light of different wavelengths; multiplexing the plurality of source beams of light into a single composite beam; and directing the single composite beam onto the working surface.

The novel method is thus based on the technique called wavelength division multiplexing (WDM) . This technique is widely used in communications systems for multiplexing a plurality of communication channels onto a single channel at the transmitting end, and then demultiplexing the plurality of channels into their separate channels at the receiving end. The WDM technique, which is the optical equivalent of freqency division multiplexing employed in radio frequency (RF) communication networks, is used to increase the information transfer capacity of the transmission medium, e.g., an optical fibre. In the WDM technique as used in such multi-channel communication systems, each discrete data channel is modulated onto an optical carrier of a fixed wavelength before being applied to the optical transmission medium. At the optical receiver, the individual data channels are reestablished by demultiplexing the composite carrier into its individual wavelength components . In the known WDM multi-channel communication system, the transmission path between the multiplexer at the transmitting end and the demultiplexer at the receiving end is generally very long (e.g., kilometers), the input power of each channel at the transmission end is generally in the order of milliwatts, and the output power at the receiver end is generally in the order of microwatts or nanowatts .

In contrast with the WDM technique used in multichannel communication systems, the WDM technique is used in the present invention for generating a bright light source to be applied onto a working surface, such as tissue to be subjected to a surgical laser treatment. The input power of each light source is of the order of many watts or tens of watts, rather than of milliwatts; the produced composite beam is transmitted a relatively short distance (e.g., up to a few meters) such that the losses are very low or negligible; and therefore the power of the composite beam is substantially equal to the sum of the powers of the input beams, again many watts or tens of watts, as distinguished from microwatts or nanowatts in WDM communication systems. In addition, the composite beam is not demultiplexed at the receiving end.

The above WDM technique for producing a bright light source is applicable generally with respect to a wide variety of light sources, but is particularly attractive in the case of laser diodes. It is known that laser diodes are small, efficient, easy to drive and modulate, very reliable, and of relatively low cost. However, the brightness of the presently known laser diodes, even the high power laser diodes, is generally far below the brightness of gas lasers or solid state lasers. Thus, the resulting power per unit area of laser energy generated by a laser diode, even when focused on a patient's tissue, is generally insufficient for surgical laser applications.

The present invention exploits the above advantageous characteristics of laser diodes, and the further characteristic that laser diodes can be made to lase in a wide range of wavelengths. For example, the presently known high power laser diodes made of GaAlAs emit light in a range of 780 to 880 nm, and diodes made of InGaAs emit light in the range of 920 to 980 nm. The above-described WDM technique for increasing the brightness of a beam of light thus enables laser diodes to be used in applications, such as in laser surgery, requiring high power outputs.

Thus, in the preferred embodiment of the invention described below, the source beam generators are high power diode lasers of different wavelengths. In the described example, these generators include a first laser of 780 nm, a second laser of 860 nm, and a third laser of 960 nm. The power of each laser is at least 1 0 watts, preferrably about 1 5 watts, so that the power of the composite output beam is approximately 45 watts, sufficient for use in many surgical laser applications. It will be appreciated that if higher powers are desired, more than three source beams can be combined by the WDM technique in accordance with the present invention .

According to further features in the described preferred membodiment, the plurality of beams are multiplexed into the single composite beam such that both the cross-sectional area, and the solid angle, of the single composite beam is substantially the same as in each of the source beams .

There are several known methods for wavelength division multiplexing (WDM) . One method utilises a dichroic combiner, sometimes referred to as an interference filter; examples of this method are described in US Patents 4,244,045, 4,431 ,258, 4,482,994, 4,675,860, 4,630,255 and 4,701 ,012. Another method utilizes a grating, or holographic element; examples of this method are described in US Patents 4,111 ,524, 4,198,117, 4,359,259, 4,387,955, 4,784,614, 4,622,662, 4,736,360 and 4,643,519. When the number of light sources is small and the wavelength separation is large (tens of nanometers), the method using dichroic combiners, or interference filters, is generally preferred as this method generally produces higher throughput efficiency and involves lower cost.

The invention also provides apparatus for generating a bright light source and applying same onto a working surface in accordance with the above method.

Further features and advantages of the invention will be apparent from the description below.

The invention is herein described, by way of example only, with reference to the single drawing figure illustrating one form of apparatus constructed in accordance with the present invention.

The apparatus illustrated in the drawing includes three diode lasers , IL^ and L^, generating laser beams of different wavelengths, e.g., of 780, 860 and 960 nm, respectively. As the laser energy from each laser is emitted from a relatively wide area (typically about 10 mm), the output from each laser is directed to a fibre bundle 11, 12 and 13 respectively, which is tightly packed at the output end, e.g., to a circle of about 1 mm in diameter. Lenses 14, 15 and 16, respectively, collimate the light from each fibre bundle to produce three essentially collimated beams 17, 18 and 19, respectively.

The illustrated apparatus further includes wavelength division multiplexer (WDM) means in the form of plurality of wavelength combiners for combining the plurality of light beams 17, 18 and 19, respectively, into single composite beam. Thus, a first combiner 20 combines beams 17 and 18 into a composite beam 21; and a second combiner 22 combines beams 19 and 21 into a single composi In the illustrated example, combiners 20 and 22 are both dichroic elements, e.g., interference filters, are highly transparent to one wavelength band and highly reflective to the other wavelength band. Thus, dichroic combiner 20 is highly transmissive with respect to the wavelength of laser L1 and is highly reflective with respect to the wavelength of laser IL^; whereas combiner 22 is highly transmissive with respect to the wavelengths of both lasers , L2, and is highly reflective with respect to the wavelength of laser .

Each of the lasers , I^ and L^, should output at least one watt of power, preferably at least 10 watts of power in laser surgery applications of the invention. In the illustrated example, each outputs 15 watts of power, so that the power of the single composite beam 23 is approximately 45 watts. This is normally sufficient to permit the laser beam to be used for many laser surgery procedures .

The illustrated apparatus further includes a safety shutter 24 in the path of the single composite beam 23, in order to selectively block the transmission of the single composite beam to the working surface, shown at 25.

The illustrated system further includes a fourth laser which generates a laser beam of visible light and of low power, to serve as a visible aiming beam to be projected onto the working surface 25 in order to permit aiming the high powered composite beam 23. In the illustrated example, the aiming beam laser is a laser diode generating a laser beam of 635 nm wavelength and of a few milliwatts of power. This laser beam is collimated by a lens 26 to produce a collimated beam 27 which is combined by a third dichroic combiner 28 with the high power composite beam 23 from the three lasers , I^, L^. Dichroic combiner 28 would thus be highly transmissive with respect to the wavelengths of the three power lasers , and L^, but highly reflective with respect to the wavelength of the aiming laser L^. The output of the dichroic combiner 28 is thus a single composite beam 29, consisting of the outputs of the three high power lasers - L^, and the output of the aiming beam laser L^.

The output 29 of dichroic combiner 28 is then directed via a focusing lens 30 into the inlet end 31a of a guiding optical fibre 31. The single composite beam exits from the outlet end 31b of the optical fibre onto the working surface WS .

In the described apparatus, the combiners 20, 22 and 28, as well as the optics of the apparatus, are such that the product of the cross-sectional area and the solid angle of the single composite beam 29 outputted by combiner 28 is substantially the same as in each of the beams exiting from the power lasers , and via their respective bundles .

Thus, the outlet end 31b of the guiding optical fibre 30 now becomes a light source of high brightness, almost three times that of each of the power lasers , L2, L^, thereby enabling the use of laser sources, particularly laser diodes, for applications requiring high power, such as laser surgery applications.

While the dichroic combiner 28 is highly transmissive of the wavelengths of the three power lasers L1 , 1*21 L-j, in the composite beam 23, a small fraction of the energy in the composite beam is reflected by the combiner 28. This small fraction reflected by combiner 28 is used for monitoring the power produced in the composite beam 29. For this purpose, the small fraction of the laser energy reflected by combiner 28 is reflected by a mirror 32 through a focusing lens 33 onto a power detector 34 for measuring the laser energy so reflected from the combiner 28. Since the proportion of laser energy reflected by combiner 28 to the laser energy passing through the combiner is known, depending on the characteristics of the combiner, 3V the output of power detector -33 also provides a measurement of the total energy in the composite beam 29 applied, via optical fibre 31, onto the working surface 25.

It will be appreciated that whereas the illustrated system includes a dichroic combiner, other types of wavelength combiners may be used for multiplexing the laser beams, e.g., a holographic element or grating, as known in the prior art. It will also be appreciated that whereas the illustrated apparatus shows combining three light sources, namely lasers , ,Ι^, L^, together with the aiming beam laser L^, only two, or more than three, light sources could be combined, according to the requirements of a particular appication.

Many other variations, modifications and applications of the invention will be apparent. 107299/2

Claims (22)

1. CLAIMS A method of performing a medical laser application including the steps of: generating a first laser beam of a first frequency; generating a second laser beam of a second frequency; said first and second laser beams being generated by diode lasers; said first frequency and said second frequency being different frequencies that are differentiatable by frequency responsive optics but close enough in frequency to have the same effect on tissue; combining said first and second laser beams into a composite laser beam using frequency responsive optics; and shining said composite laser beam into tissue to perform said medical laser application.

2. The method according to claim 1 , further including the steps of: generating a third laser beam of a third frequency; said third frequency being different from said first frequency and from said second frequency with respect to frequency responsive optics but being close enough in frequency to said first frequency and said second frequency to have the same effect on tissue; combining said third laser beam with said first and second laser beams into said composite laser beam.

3. The method according to claim 2, wherein: said third laser beam is generated by diode lasers. 107299/2

4. The method according to claim 2 wherein said first laser beam is of a wavelength of 780 nm and of a power of at least 10 watts; said second laser beam is of a wavelength of 860 nm and of a power of at least 10 watts; and said third laser beam is of a wavelength of 960 nm and of a power of at least 10 watts.

5. The method according to claim 1 or 2, wherein at least said first laser beam, said second laser beam and said composite laser beam each has a cross-sectional area and a solid angle such that the product of the cross-sectional area and the solid angle of the composite laser beam is substantially the same as that of each of said first and second laser beams.

6. The method according to claim 1 or 2 wherein at least said first and second laser beams are combined into said composite beam by a dichroic combiner.

7. The method according to claim 1 or 2 wherein at least said first and second laser beams are combined into said composite beam by a holographic element.

8. The method according to claim 1 or 2 wherein at least said first and second laser beams are combined into said composite beam by a grating.

9. The method according to claim 1 or 2 wherein said diode lasers are at least one watt of power each. 107299/2

10. The method according to claim 1 or 2 wherein a small fraction of the composite beam is split-off and is directed to a power detector for measuring the power of said composite beam.

11. The method according to claim 1 or 2 wherein an aiming beam of visible light and of a different wavelength than said first and second laser beams is also generated and combined into said composite beam.

12. The method according to claim 11 , wherein said aiming beam is of a wavelength of 635 nm and of a power of a few milliwatts.

13. The method according to claim 1 or 2 wherein said composite beam is directed into said tissue by an optical fibre.

14. A bright light source including: a first plurality of semiconductor lasers in which eachof said first plurality of semiconductor lasers emits a beam of light of a first frequency; a second plurality of semiconductor lasers in which each of said second plurality of semiconductor lasers emits a beam of light of a second frequency; said first frequency and 107299/2 said second frequency being different frequencies that are differentiatable by frequency responsive optics but close enough in frequency to have the same effect on tissue; a first bundle of optical fibers for receiving the light from ones of said first plurality of semiconductor lasers and providing a first combined laser beam of a predetermined shape and having a cross-sectional area and a solid angle; a second bundle of optical fibers for receiving the light from ones of said second plurality of semiconductor lasers and providing a second combined laser beam of a predetermined shape and having a cross-sectional area and a solid angle; and frequency responsive optics for combining said first and second combined laser beams into a composite laser beam, said composite laser beam having substantially the same product of cross-sectional area and solid angle as each of said first and second combined beams.

15. A bright light source as defined in claim 14 wherein said first plurality of semiconductor lasers are arranged in a first monolithic array; and said second plurality of semiconductor lasers are arranged in a second monolithic array.

16. A bright light source as defined in claim 14 further including: a third plurality of semiconductor lasers in which each of 107299/2 said third plurality of semiconductor lasers emits a beam of light of a third frequency; said third frequency being different from said first frequency and from said second frequency with respect to frequency responsive optics but being close enough in frequency to said first frequency and said second frequency to have the same effect on tissue; a third bundle of optical fibers for receiving the light from ones of said third plurality of semiconductor lasers and providing a third combined laser beam of a predetermined shape and having a cross-sectional area and a solid angle.

17. The apparatus according to any one of claims 14-16, wherein said semiconductor lasers are high power diode lasers.

18. The apparatus according to any one of claims 14-16, wherein said frequency responsive optics include a dichroic combiner.

19. The apparatus according to any one of claims 14-16, wherein said frequency responsive optics include a grating.

20. The apparatus according to any one of claims 14-16, wherein said frequency responsive optics include a holographic element.

21. The apparatus according to any one of claims 14-16, further including a power detector for receiving a small fraction of the composite beam and for measuring the power of the combined beam.

22. The apparatus according to any one of claims 14-16, 107299/2 wherein each of said combined beams is at least 10 watts of power. The apparatus according to any one of claims14-16, wherein said first plurality of semiconductor lasers each generates a laser beam of between 780 nm and 880 nm inclusive; and said second plurality of semiconductor lasers each generates a laser beam of between 920 nm and 940 nm inclusive. The apparatus according to any one of claims 14-16, including a light source for generating an aiming beam of visible light and of a different wavelength than said plurality of semiconductor lasers; and a multiplexer for multiplexing said aiming beam into said combined laser beam. The apparatus according to claim 24, wherein said further light source generates an aiming beam of 635 nm P-936-IL