EP4154055A1 - A device and a method for speckle-free laser illumination - Google Patents

A device and a method for speckle-free laser illumination

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Publication number
EP4154055A1
EP4154055A1 EP20764808.0A EP20764808A EP4154055A1 EP 4154055 A1 EP4154055 A1 EP 4154055A1 EP 20764808 A EP20764808 A EP 20764808A EP 4154055 A1 EP4154055 A1 EP 4154055A1
Authority
EP
European Patent Office
Prior art keywords
laser
light
fiber
illumination
laser beam
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP20764808.0A
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German (de)
French (fr)
Inventor
Vid AGREZ
Rok Petkovsek
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Univerza v Ljubljani
Original Assignee
Univerza v Ljubljani
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Univerza v Ljubljani filed Critical Univerza v Ljubljani
Publication of EP4154055A1 publication Critical patent/EP4154055A1/en
Pending legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/48—Laser speckle optics
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
    • G02B27/0938—Using specific optical elements
    • G02B27/0994—Fibers, light pipes

Definitions

  • the invention belongs to the field of laser devices, more precisely to the field of lasers used for illumination.
  • the invention relates to a device and method for generating speckle-free homogenous illumination with laser light.
  • laser-based illumination systems are desirable.
  • the advantage of laser light is its high brightness and capability of fast modulation.
  • illumination with laser light also has problems due to its high coherency, which causes an interference pattern called laser speckle on the detector.
  • Speckle causes non-homogeneous light field, can hide the observed features and can even saturate the detector.
  • the non-homogeneous image on the sensor occurs in transmissive and reflective illumination techniques.
  • speckle patterns can be objective or subjective.
  • the objective pattern is formed directly in the detection plane while the subjective speckle is formed after being imaged with the imaging system on to the detector. In most cases, the speckle pattern makes it impossible to recognize the colour or structural pattern on the object under the inspection.
  • the homogeneity of the illuminated area on the detector can be described by the speckle contrast C or, in other words, the average value of the deviation of the intensity in each part of the image from the overall average.
  • Speckle contrast is dependent on the wavelength, angle and polarization diversity of the used illumination sources as shown by Manni and Goodman (2012; doi: 10.1364/OE.20.011288). More diverse light source producing lower speckle contrast and in consequence more homogenous illumination field.
  • the increase of the number of diverse and with each other uncoherent sources is in practice hard to achieve due to complexity and cost. Especially in the case of laser sources capable of producing short laser pulses at different wavelengths.
  • the technical problem solved by the present invention is the design of a device and a method for homogenization of the illumination field, wherein the device is primarily intended for use with lasers generating pulses on the time scales under 1 ps. Namely, most solutions for homogeneous laser illumination fail on such time scales, because modulators or averagers of the interference pattern are not capable of such fast modulation (at frequencies exceeding 1 kHz). Further, solutions based on spontaneously emitted light and broad spectrum also encounter difficulties on such time scales.
  • the problem of homogenous laser illumination can be solved in various ways.
  • the existing solutions and inventions can be divided on two approaches.
  • the first approach is development of the laser source with high brightness and inherent low coherency, while the second approach is based on the design of the apparatus for lowering or averaging the speckle contrast produced by the coherent laser source.
  • mechanical variants of the homogenization of the light field include solutions like deformable mirrors that change the light distribution over time as shown by Tran et al. (2014; doi: 10.1364/OE.22.011152).
  • patent application US 3633999 discloses a method for improving the homogeneity of the light field by laser beam splitting.
  • the beam splitting is achieved by the use of several beam splitters and mirrors, which delay each beam for a coherent distance in a free space arrangement.
  • the mutually delayed beams from the same laser source then illuminate the object and form several interference patterns that average each other to some extent.
  • the benefit of the approach is spatial averaging in comparison to the time averaging and thus supporting short exposure times.
  • An improved method of using a laser beam splitter array in a free-standing optical layout is presented in patent application WO 2000043822.
  • the primary laser pulse is duplicated by passing through each successive module of the invention. In the module, the laser pulse is split in way that, one replica is delayed and obtains additional spatial aberration. Finally, all copies of the pulses are combined on the illumination surface and their speckle patterns are averaged in to the homogeneous illumination field.
  • Document US 5233460 discloses a device for homogenizing the illumination field by using a series of coherent laser pulses.
  • the solution is implemented with free space optics splitting the laser pulses by the polarization and delaying each pulse replica at one polarization to the replica transmitted at orthogonal polarization forming a train of uncorrelated pulses.
  • patent US 7463352 B2 discloses a laser sample inspection device that includes use of an integration sphere, a set of optical fibers of different lengths, and a diffraction grating. This solution is based on mixing multiple beams, which are then further emitted as multiple beams, which differs from the present invention.
  • a disadvantage of the device according to US 7463352 is lower brightness as the power is distributed on a larger surface and emitted from several sources.
  • the essence of the device and method for providing speckle-free homogeneous laser light according to the invention is that, in a fully fiber-integrated setting, the number of mutually non-coherent laser beams is increased in a manner that does not distort the laser pulse shape.
  • the invention is thus a compact device, which uses the principle of decoupling laser light from a common channel to two or more channels and back to the common channel resulting in decreased coherency consequently allowing homogenous illumination without speckles.
  • the multiplier being coupled to the light source or the optional light combiner, wherein the said multipliers comprise means for separating laser beam into two or more channels and means for combining the separated channel into one common channel,
  • the light source may be any suitable source, for example a multimode laser diode or single mode or near single mode laser source coupled to the fiber in the form of laser diode, mode locked laser, Q-switched laser, gain switched laser.
  • the sources used can be of the different types, working in pulsed mode or continuous mode, or in case of several sources one may be operating in pulsed mode and the other in continuous mode.
  • the essential part of the device is the laser beam multiplier, which ensures that the received light along light guiding channel is mixed, delayed and forwarded in to the next light guiding channel. Mixing is achieved by separation and combination of light beams inside the multiplier.
  • - laser beam multiplier units are connected in series each multiplying the output from the previous one potentiating the number of the pulse replicas at the illumination system output;
  • a photodiode connected in parallel to the laser light source for direct monitoring of the laser light shaping in time.
  • the invention is based on the relation that connects the number of mutually uncoherent and non-correlated laser sources Mwith the reduction of speckle contrast:
  • the multiplication factor N is the number of the fiber sub-channels used in one laser beam multiplication unit. It is also possible to exchange one laser source with two to nineteen sources all combined in to the common channel. If these laser sources are not correlated between themselves, the speckle contrast falls in a way:
  • the multimode laser diodes are the preferable but not exclusive choice, as they can provide high power pulses with the duration of several tens of nanosecond that can be shaped and modulated in time without additional modulators. Used in the application of high-speed photography this illumination system can compete directly with the high-speed camera setups for fast phenomena photography.
  • This example of the embodiment of the invention further includes a dedicated electronic driver that can modulate the electric current from 1 ns to 1 ps.
  • the multimode laser diodes can be exchanged with any laser source known to the expert in the field ranging from the Q-switched to mode locked lasers. By using laser sources producing pulses several picoseconds long or shorter with the presented method one can realize ultra-fast illumination system for fast phenomena photography.
  • the invention can be realized with different types of fibers ranging from a multimode to single mode, having different dopants known to the expert in the field, have an arbitrary cross section shape or structure like microstructure and photonic crystal fibers.
  • Multimode optical fibers are preferred because they can withstand high powers for all kinds of illumination applications.
  • the multimode optical fiber is used with the numerical aperture changing from 0.46 in the common channel to the 0.22 in the sub channels of the beam multiplying unit. The expert in the filed knows that the numerical aperture can be changed without affecting the invention’s function.
  • the mutually different length of the fiber in the beam multiplying unit can be exchanged in the variation of the embodiment with fibers of same length, but mutually different numerical apertures and fiber diameters selected in such ways that the dispersion of the pulse replica is similar to the coherence length difference in the same type of fiber.
  • the laser beam multiplier unit may comprise multimode all fiber beam splitters/combiners in the laser beam multiplier unit or single mode multiplexers and 50/50 beam splitters.
  • the principle of operation is the same as disclosed in the present disclosure.
  • the single mode multiplier consists of input fiber, single mode fiber beam splitter, splitting laser beam from single to at least two fibers and the single mode fiber beam combiner combining light from the at least two input ports to the common single fiber.
  • the single mode laser beam splitter can be exchanged with the 50/50 coupler where on the input side only one port is used. In both cases, the light guided in the single/common channel is split in to the two channels by using the single mode fiber beam splitter.
  • the so split laser beams are mutually delayed by the coherence length and then recombined in to the single/common channel.
  • the preferable number of the two channel multipliers is eight to strike a balance between compactness, cost and low contrast.
  • the single mode multiplier can be redesigned so that each channel is further split in to the two channels mutually delayed by the coherence length and split again. When the preferred pulse replicas of eight are achieved, these replicas are combined back in to the common channel.
  • the recombination can be achieved by using the same count of single mode beam combiners arranged in the inverse tree structure that was used in the laser beam splitting.
  • An alternative option in order to decrease the component count is the use of the reflective element such as fiber Bragg grating at the end of each divided channel and reflect pulse replicas back in to the splitting tree to be recombined in to the single channel.
  • the reflective element such as fiber Bragg grating at the end of each divided channel and reflect pulse replicas back in to the splitting tree to be recombined in to the single channel.
  • the beam splitter can be also used in the arrangement where the number of the split channels is greater than three and the output fiber arranged in such a way that one fiber output is emitting light in the main optical axis, while other fibers are arranged at small angle offsets to the fist fiber. Together they form a fiber cluster for angle diversity of the illuminating light and a high NA illumination system output.
  • This embodiment of the invention serves to illuminate transparent objects having boundaries of variable refraction index like glass lens in a liquid medium which otherwise makes the lens region opaque in the optical setups like shadowgraphs.
  • the output of the device can be a single fiber output or can include a beam splitter to split low coherence output in to several beams.
  • the number of the spited beams is equal to the number of views that are required for illumination of the application.
  • the beam splitter is preferably of the all fiber version realized by the inverse all fiber combiner, guiding the split beams in the three fibers positioned orthogonally to one another. Combining this with industrial grade cameras, the phenomena like shockwaves in liquid can be visualized at short exposures at the synchronized times form different directions forming a 3D picture of the dynamics, which was previously costly and hard to realize with the high-speed cameras.
  • the method for providing speckle-free laser illumination with the above described device comprises the following steps: a) optional combining of laser sources into a single channel with a fiber combiner, if two or more sources are used; b) connecting the single channel to a beam splitter to decouple light from the common channel to two to nineteen fiber channels, wherein an inversely oriented beam combiner can be used for multimode fibers; c) guiding laser beam replicas produced in step b) into a multichannel interconnection comprising of two to nineteen fibers with the mutual length difference greater than the light coherence length acquiring the phase difference between each other; d) combining the multiple laser pulse replicas back into the common channel using a fiber beam combiner; e) performing and optionally repeating steps b) to d) in the laser beam multiplier unit(s); and f) leading the multiple replicas of the laser beam via the single channel to an output.
  • the method is applicable to many laser sources emitting coherent light, which is by itself unsuitable for illumination applications. These being for example high-speed imaging, machine vision in broad sense with the specialized fields of application in high-speed shock wave photography, flow monitoring, welding monitoring, etc. All these applications have in common the requirement for the high brightness highly directivity illumination source capable of high modulation bandwidths in other words short exposure times.
  • this can be achieved by combining the invention with the pulsed coherent laser source in the form of any fiber-coupled laser system on the market.
  • the benefit of the method is that it can combine two completely different types of laser system in to the single channel to address more complex illumination requirements. For example, short and long pulsed laser system at different wavelengths in single channel for imaging different fast phenomena from single side on to the camera.
  • Figure 1 Schematic view of the first exemplary embodiment of the device Figure 2
  • Figure 3 Schematic view of the second exemplary embodiment of the device
  • Figure 4 Schematic view of the third exemplary embodiment of the device
  • Figure 5 Schematic view of the fourth exemplary embodiment of the device
  • Figure 6 Schematic views of the fifth exemplary embodiment of the device using a single mode laser beam multiplier having three different architectures (panels a, b, c)
  • Figure 7 Example of illumination with multimode laser diodes in the case with (right) and without (left) use of the device according to the invention.
  • Figure 1 shows a schematic view of the first embodiment of the device for providing speckle free homogeneous laser light illumination, wherein multimode laser diodes 101 as a laser light source are used. These lasers can have continuous output or pulsed where pulsing is achieved by mode locking, Q-switching or gain switching.
  • the illumination system has a monitoring photodiode 102 connected in parallel to the laser source for monitoring the modulation of the illuminating light. Presence of the photodiode is not essential, while its position, if used, is arbitrary so it can also be connected to the illumination system along the line in the direction of the light propagating from the output to the laser source or from the laser source to the output.
  • the output light from number M of multimode laser diodes or several laser light sources of another type is combined to the common channel through the light combiner 103 and guided to the laser beam multiplier 104 producing M times N pulse replicas.
  • Laser beam multipliers are connected in the series each multiplying the output from the previous one by the factor N potentiating the number of the pulse replicas at the illumination system output 105.
  • FIG. 2 is a schematic view of the preferred laser beam multiplier 104 architecture, wherein two fiber beam combiners 202 are used.
  • the first beam combiner is used in the opposite direction and splits light from the common laser channel at the laser beam multiplier input port 201 into a multichannel arrangement 203 comprising N channels.
  • the length of the fiber in each channel differs from the rest by a coherent length depending on the laser source used.
  • Two to nineteen multichannel interconnections realized by optical fibers may be used.
  • the channels represented by the optical fibers are then coupled back into the common channel, producing N copies of the laser beam.
  • the optical fibers are not needed to be equal in diameter or light guiding properties.
  • Light form each channel is then combined by the all fiber beam combiner 202 in to the common channel, producing N copies of the laser beam at the laser beam multiplier output port 204.
  • Figure 3 shows a second embodiment of the device, wherein two consecutive laser beam multipliers are used and each individual multiplier has N channels. After passing two multipliers, N 2 copies of the laser beam are generated. This is the multiplication factor to the number of the input beams M.
  • Figure 4 shows a third possible embodiment of the device, wherein the single fiber output 105 is exchanged with the beam splitter 401 to split low coherence output in to several beams to illuminate the area from several directions with the replicas output illumination beam.
  • the number of the spited beams is equal to the number of views that are required for illumination of the application.
  • the beam splitter is of the all fiber version, guiding the splitted beams in the three fibers 402,403,404 positioned orthogonally to one another.
  • FIG. 5 Fourth exemplary embodiment of the device is shown in Figure 5, wherein the single fiber output 105 is exchanged with beam splitter 401 to split low coherence output in to several fiber channels.
  • One fiber output is emitting light in the main optical axis 501 while other fibers are arranged at small angle offsets to the fist fiber 501 forming a fiber cluster 502 for angle diversity of the illuminating light and high NA illumination system output 503.
  • the described embodiment of the invention serves to illuminate transparent objects having variable refraction index for the illuminating light.
  • FIG. 6 Fifth exemplary embodiment of the device is shown in Figure 6, wherein the single mode components in the form of singe mode splitters from single to two channels 601 and single mode fibers are used.
  • the a) is variant using the single mode laser beam multiplier constructed from two single mode splitters, each multiplier producing by a factor of 2 more laser beam replicas at the output.
  • a less compact variant is shown in b) where the same number of pulse replicas are achieved at the output as in a) but the component count required is great with the benefit of tailoring the beam delay in each channel separately.
  • the c) single mode variant uses a circulator 603 to couple input light in to the multiplication chain where it is multiplied by beam splitters 601 , reflected in each channel by reflective element being mirror or fiber Bragg grating 602, combined back to the common channel by inversely propagating trough the beam splitters 601, and coupled trough the circulator 603 to the output port.
  • Figure 7 is an example of aluminium surface illumination with multimode laser diodes.
  • the surface is illuminated directly by the multimode laser diodes. Due to coherency of the light, a speckle pattern is produced making it impossible to discern the surface features which shows in high variation of intensity profile.
  • homogenous illumination field is achieved by the device according to the invention and much more smooth intensity profile is achieved. Smaller variations are due to the surface features and imperfections.

Abstract

The invention belongs to the field of laser devices, more precisely to the field of lasers used for illumination. The invention relates to a device and method for generating homogenous illumination with laser light without speckles. The essence of the invention is that, in an all fiber design, the number of mutually non- coherent laser beams is increased in a manner that does not distort the laser pulse shape. The device uses the principle of decoupling laser light from a common channel to two or more channels and back to the common channel resulting in decreased coherency consequently allowing homogenous illumination without speckles. The device comprises: - at least one light source producing a laser beam, - at least one laser beam multiplier comprising a suitable number of optical fibers, wherein the said multipliers comprise means for separating laser beam into two or more channels and means for combining the separated channel into one common channel, - and an output for emitting the output speckle-free laser beam/light for illumination.

Description

A device and a method for speckle-free laser illumination
Field of the invention
The invention belongs to the field of laser devices, more precisely to the field of lasers used for illumination. The invention relates to a device and method for generating speckle-free homogenous illumination with laser light.
Background of the invention and the technical problem
In many applications such as laser projectors, high-speed illumination, high-speed flash, etc. laser-based illumination systems are desirable. The advantage of laser light is its high brightness and capability of fast modulation. However, illumination with laser light also has problems due to its high coherency, which causes an interference pattern called laser speckle on the detector. Speckle causes non-homogeneous light field, can hide the observed features and can even saturate the detector. The non-homogeneous image on the sensor occurs in transmissive and reflective illumination techniques. Thus formed speckle patterns can be objective or subjective. The objective pattern is formed directly in the detection plane while the subjective speckle is formed after being imaged with the imaging system on to the detector. In most cases, the speckle pattern makes it impossible to recognize the colour or structural pattern on the object under the inspection. The homogeneity of the illuminated area on the detector can be described by the speckle contrast C or, in other words, the average value of the deviation of the intensity in each part of the image from the overall average. Speckle contrast is dependent on the wavelength, angle and polarization diversity of the used illumination sources as shown by Manni and Goodman (2012; doi: 10.1364/OE.20.011288). More diverse light source producing lower speckle contrast and in consequence more homogenous illumination field. However, the increase of the number of diverse and with each other uncoherent sources is in practice hard to achieve due to complexity and cost. Especially in the case of laser sources capable of producing short laser pulses at different wavelengths.
The technical problem solved by the present invention is the design of a device and a method for homogenization of the illumination field, wherein the device is primarily intended for use with lasers generating pulses on the time scales under 1 ps. Namely, most solutions for homogeneous laser illumination fail on such time scales, because modulators or averagers of the interference pattern are not capable of such fast modulation (at frequencies exceeding 1 kHz). Further, solutions based on spontaneously emitted light and broad spectrum also encounter difficulties on such time scales.
State of the art
The problem of homogenous laser illumination can be solved in various ways. In general, the existing solutions and inventions can be divided on two approaches. The first approach is development of the laser source with high brightness and inherent low coherency, while the second approach is based on the design of the apparatus for lowering or averaging the speckle contrast produced by the coherent laser source.
The first approach is efficient for the application where low continuous power is enough. Document US 4938556 discloses a source for producing spontaneous emitted light in wide spectral range. Redding et al. (2015, doi: 10.1364/OL.40.004607) also present a solution for spontaneous emitted light with low coherence. Nixon et al. (2013; doi: 10.1364/OL.38.003858) describe a laser source for homogenous illumination field on the detector, wherein the source modulates the shape of the light filed at the output by varying the aperture inside the resonator. The present invention differs from these solutions, as it belongs to the second type of the above-mentioned possible approaches. The method disclosed in patent US 7817693 falls between the two approaches and is based on the increase of the number internal semiconductor elements in the illumination source. The emitted light beams from each element are combined through a complicated optical arrangement in to the light guide filed with liquid. The speckle free illumination is thus provided by the large diameter waveguide filled with liquid, high number of emitters and small difference in emitted wavelength between the elements.
Solutions belonging to the second approach, i.e. based on the mechanical averaging of the speckle pattern, are generally too slow for the fast illumination applications where the illumination pulse durations of several 10s of nanoseconds may be required. Kajenski et al. (1992; doi: 10.1109/50.156882) report that shaking of the fiber guiding the light generates a time changing speckle pattern on the illuminated surface. This is the averaged in time by the log exposure times of the camera. An improvement has been presented by Mehta et al. (2012; doi: 10.1364/AO.51 .001894), according to which the light is coupled in to the fiber bundle to achieve the similar result. In addition to the vibration of the optical waveguide, mechanical variants of the homogenization of the light field include solutions like deformable mirrors that change the light distribution over time as shown by Tran et al. (2014; doi: 10.1364/OE.22.011152).
Solutions to minimize speckle and ensure high homogeneous illumination field that are closer to the present invention are passive in nature. For example, patent application US 3633999 discloses a method for improving the homogeneity of the light field by laser beam splitting. The beam splitting is achieved by the use of several beam splitters and mirrors, which delay each beam for a coherent distance in a free space arrangement. The mutually delayed beams from the same laser source then illuminate the object and form several interference patterns that average each other to some extent. The benefit of the approach is spatial averaging in comparison to the time averaging and thus supporting short exposure times. An improved method of using a laser beam splitter array in a free-standing optical layout is presented in patent application WO 2000043822. The primary laser pulse is duplicated by passing through each successive module of the invention. In the module, the laser pulse is split in way that, one replica is delayed and obtains additional spatial aberration. Finally, all copies of the pulses are combined on the illumination surface and their speckle patterns are averaged in to the homogeneous illumination field.
Document US 5233460 discloses a device for homogenizing the illumination field by using a series of coherent laser pulses. The solution is implemented with free space optics splitting the laser pulses by the polarization and delaying each pulse replica at one polarization to the replica transmitted at orthogonal polarization forming a train of uncorrelated pulses.
Various passive methods of averaging speckle pattern have been described in patent US 7463352 B2, which discloses a laser sample inspection device that includes use of an integration sphere, a set of optical fibers of different lengths, and a diffraction grating. This solution is based on mixing multiple beams, which are then further emitted as multiple beams, which differs from the present invention. A disadvantage of the device according to US 7463352 is lower brightness as the power is distributed on a larger surface and emitted from several sources.
The method of improving the homogeneity of the illumination field in a compact optical fiber embodiment was described in document US 20140071406 A1 , where the dispersion of laser light guided in an optical fiber is utilized. According to this solution, one or more laser sources guided by a multimode optical fiber of a length longer than the coherent length of the guided light are used. The modes of the guided light are delayed due to the different dispersion they encounter in the multimode fiber and form separate speckle patterns that are spatially averaged, even with very short exposures of the imaging camera. The principle of delaying particular modes of the laser beam is not employed in the present invention.
All of these solutions approach the problem of homogenization of the laser light illumination field in a unique way. They are specialized for a given type of a laser source and are thus not universal. Most of them rely on the free space elements to modify and multiply the laser beam, which increases the complexity of the system and lowers the compactness and ruggedness of such device, which is essential for application in the industrial environment. Further disadvantage is the lack of adaptability to the laser source pulse width and its spectral properties. It is the aim of the present invention to overcome said disadvantages of known methods and devices for speckle-free homogenous laser illumination.
Description of the solution of the technical problem
The essence of the device and method for providing speckle-free homogeneous laser light according to the invention is that, in a fully fiber-integrated setting, the number of mutually non-coherent laser beams is increased in a manner that does not distort the laser pulse shape. The invention is thus a compact device, which uses the principle of decoupling laser light from a common channel to two or more channels and back to the common channel resulting in decreased coherency consequently allowing homogenous illumination without speckles.
The device for providing speckle-free homogenous laser illumination comprises:
- at least one light source for generating a light beam,
- optional light combiner coupled to the light sources in case two or more light sources are used,
- at least one, preferably four laser beam multipliers comprising a suitable number, preferably seven, optical fibers, the multiplier being coupled to the light source or the optional light combiner, wherein the said multipliers comprise means for separating laser beam into two or more channels and means for combining the separated channel into one common channel,
- and an output for emitting the laser light for speckle-free illumination.
The light source may be any suitable source, for example a multimode laser diode or single mode or near single mode laser source coupled to the fiber in the form of laser diode, mode locked laser, Q-switched laser, gain switched laser. The sources used can be of the different types, working in pulsed mode or continuous mode, or in case of several sources one may be operating in pulsed mode and the other in continuous mode. The essential part of the device is the laser beam multiplier, which ensures that the received light along light guiding channel is mixed, delayed and forwarded in to the next light guiding channel. Mixing is achieved by separation and combination of light beams inside the multiplier. This ensures a monolithic and simplified design of the device without any free space elements, as the light beam is coupled from a common waveguide via directly spliced fibers in to individual fibers. Even different light beams at different wavelengths and modulations may be coupled and mixed using the said multiplier.
This crucially differs from the closest known solution described in US 7463352, as the multiplier in their device does not split and then combine several light beams. Instead, the beams are forwarded from multiple to multiple channels (fiber bundles).
The preferred embodiment comprises:
- the multimode fiber combiners and splitters that are combined in to the pairs to form laser beam multiplier unit;
- optical fibers forming from two to nineteen multichannel interconnections with the mutually length difference greater than the light coherence length;
- laser beam multiplier units are connected in series each multiplying the output from the previous one potentiating the number of the pulse replicas at the illumination system output;
- laser light source in the form of one to nineteen multimode laser diodes combined in to the common channel before the emitted light encounters the laser beam splitters; and
- optionally, a photodiode connected in parallel to the laser light source for direct monitoring of the laser light shaping in time.
Nineteen multichannel interconnections are currently the highest number of fibers, which can be combined by using current state of the art fiber combiner technique. However, the device and the method can also support higher number of multichannel interconnections in case the multipliers will support twenty or more fibers. The invention is based on the relation that connects the number of mutually uncoherent and non-correlated laser sources Mwith the reduction of speckle contrast:
In the example of the invention using number of the sequential laser beam multiplying units with multiplication factor of N, one can write the drop of speckle contrast for one laser source M= 1 as:
C = N~k I2
The multiplication factor N is the number of the fiber sub-channels used in one laser beam multiplication unit. It is also possible to exchange one laser source with two to nineteen sources all combined in to the common channel. If these laser sources are not correlated between themselves, the speckle contrast falls in a way:
This formulation of the reduction of the speckle contrast and in consequence, homogenization of the illumination field is demonstrated in the example of the invention embodiment where multiple laser diodes are used as a laser source. The multimode laser diodes are the preferable but not exclusive choice, as they can provide high power pulses with the duration of several tens of nanosecond that can be shaped and modulated in time without additional modulators. Used in the application of high-speed photography this illumination system can compete directly with the high-speed camera setups for fast phenomena photography. This example of the embodiment of the invention further includes a dedicated electronic driver that can modulate the electric current from 1 ns to 1 ps. The multimode laser diodes can be exchanged with any laser source known to the expert in the field ranging from the Q-switched to mode locked lasers. By using laser sources producing pulses several picoseconds long or shorter with the presented method one can realize ultra-fast illumination system for fast phenomena photography.
It should be noted that the invention can be realized with different types of fibers ranging from a multimode to single mode, having different dopants known to the expert in the field, have an arbitrary cross section shape or structure like microstructure and photonic crystal fibers. Multimode optical fibers are preferred because they can withstand high powers for all kinds of illumination applications. In the preferred embodiment of the invention, the multimode optical fiber is used with the numerical aperture changing from 0.46 in the common channel to the 0.22 in the sub channels of the beam multiplying unit. The expert in the filed knows that the numerical aperture can be changed without affecting the invention’s function. Further, the mutually different length of the fiber in the beam multiplying unit can be exchanged in the variation of the embodiment with fibers of same length, but mutually different numerical apertures and fiber diameters selected in such ways that the dispersion of the pulse replica is similar to the coherence length difference in the same type of fiber.
The laser beam multiplier unit may comprise multimode all fiber beam splitters/combiners in the laser beam multiplier unit or single mode multiplexers and 50/50 beam splitters. The principle of operation is the same as disclosed in the present disclosure. The single mode multiplier consists of input fiber, single mode fiber beam splitter, splitting laser beam from single to at least two fibers and the single mode fiber beam combiner combining light from the at least two input ports to the common single fiber. The single mode laser beam splitter can be exchanged with the 50/50 coupler where on the input side only one port is used. In both cases, the light guided in the single/common channel is split in to the two channels by using the single mode fiber beam splitter. The so split laser beams are mutually delayed by the coherence length and then recombined in to the single/common channel. The preferable number of the two channel multipliers is eight to strike a balance between compactness, cost and low contrast. Optionally, the single mode multiplier can be redesigned so that each channel is further split in to the two channels mutually delayed by the coherence length and split again. When the preferred pulse replicas of eight are achieved, these replicas are combined back in to the common channel. The recombination can be achieved by using the same count of single mode beam combiners arranged in the inverse tree structure that was used in the laser beam splitting. An alternative option in order to decrease the component count, is the use of the reflective element such as fiber Bragg grating at the end of each divided channel and reflect pulse replicas back in to the splitting tree to be recombined in to the single channel. These way single mode fibers can be used for enhanced beam quality at the single mode fiber output. Here, the benefit of the embodiment is the lower divergence of the laser beam at the output with the applications in the far field illumination and laser vibrometry where the unwanted speckle formation usually represents noise to the detected varying signal.
The beam splitter can be also used in the arrangement where the number of the split channels is greater than three and the output fiber arranged in such a way that one fiber output is emitting light in the main optical axis, while other fibers are arranged at small angle offsets to the fist fiber. Together they form a fiber cluster for angle diversity of the illuminating light and a high NA illumination system output. This embodiment of the invention serves to illuminate transparent objects having boundaries of variable refraction index like glass lens in a liquid medium which otherwise makes the lens region opaque in the optical setups like shadowgraphs.
Further, the output of the device can be a single fiber output or can include a beam splitter to split low coherence output in to several beams. The number of the spited beams is equal to the number of views that are required for illumination of the application. The beam splitter is preferably of the all fiber version realized by the inverse all fiber combiner, guiding the split beams in the three fibers positioned orthogonally to one another. Combining this with industrial grade cameras, the phenomena like shockwaves in liquid can be visualized at short exposures at the synchronized times form different directions forming a 3D picture of the dynamics, which was previously costly and hard to realize with the high-speed cameras.
The method for providing speckle-free laser illumination with the above described device according to the invention comprises the following steps: a) optional combining of laser sources into a single channel with a fiber combiner, if two or more sources are used; b) connecting the single channel to a beam splitter to decouple light from the common channel to two to nineteen fiber channels, wherein an inversely oriented beam combiner can be used for multimode fibers; c) guiding laser beam replicas produced in step b) into a multichannel interconnection comprising of two to nineteen fibers with the mutual length difference greater than the light coherence length acquiring the phase difference between each other; d) combining the multiple laser pulse replicas back into the common channel using a fiber beam combiner; e) performing and optionally repeating steps b) to d) in the laser beam multiplier unit(s); and f) leading the multiple replicas of the laser beam via the single channel to an output.
The method is applicable to many laser sources emitting coherent light, which is by itself unsuitable for illumination applications. These being for example high-speed imaging, machine vision in broad sense with the specialized fields of application in high-speed shock wave photography, flow monitoring, welding monitoring, etc. All these applications have in common the requirement for the high brightness highly directivity illumination source capable of high modulation bandwidths in other words short exposure times. With presented method, this can be achieved by combining the invention with the pulsed coherent laser source in the form of any fiber-coupled laser system on the market. The benefit of the method is that it can combine two completely different types of laser system in to the single channel to address more complex illumination requirements. For example, short and long pulsed laser system at different wavelengths in single channel for imaging different fast phenomena from single side on to the camera.
The device and the method for providing homogeneous laser light illumination without speckles according to the invention will be described in further detail based on exemplary embodiments and figures showing:
Figure 1 Schematic view of the first exemplary embodiment of the device Figure 2 A detailed schematic view of a possible embodiment of the laser beam multiplier shown in figure 1 Figure 3 Schematic view of the second exemplary embodiment of the device Figure 4 Schematic view of the third exemplary embodiment of the device Figure 5 Schematic view of the fourth exemplary embodiment of the device Figure 6 Schematic views of the fifth exemplary embodiment of the device using a single mode laser beam multiplier having three different architectures (panels a, b, c)
Figure 7 Example of illumination with multimode laser diodes in the case with (right) and without (left) use of the device according to the invention.
Figure 1 shows a schematic view of the first embodiment of the device for providing speckle free homogeneous laser light illumination, wherein multimode laser diodes 101 as a laser light source are used. These lasers can have continuous output or pulsed where pulsing is achieved by mode locking, Q-switching or gain switching. In preferred embodiment the illumination system has a monitoring photodiode 102 connected in parallel to the laser source for monitoring the modulation of the illuminating light. Presence of the photodiode is not essential, while its position, if used, is arbitrary so it can also be connected to the illumination system along the line in the direction of the light propagating from the output to the laser source or from the laser source to the output. The output light from number M of multimode laser diodes or several laser light sources of another type is combined to the common channel through the light combiner 103 and guided to the laser beam multiplier 104 producing M times N pulse replicas. Laser beam multipliers are connected in the series each multiplying the output from the previous one by the factor N potentiating the number of the pulse replicas at the illumination system output 105.
Figure 2 is a schematic view of the preferred laser beam multiplier 104 architecture, wherein two fiber beam combiners 202 are used. The first beam combiner is used in the opposite direction and splits light from the common laser channel at the laser beam multiplier input port 201 into a multichannel arrangement 203 comprising N channels. The length of the fiber in each channel differs from the rest by a coherent length depending on the laser source used. Two to nineteen multichannel interconnections realized by optical fibers may be used. The channels represented by the optical fibers are then coupled back into the common channel, producing N copies of the laser beam. The optical fibers are not needed to be equal in diameter or light guiding properties. Light form each channel is then combined by the all fiber beam combiner 202 in to the common channel, producing N copies of the laser beam at the laser beam multiplier output port 204.
Figure 3 shows a second embodiment of the device, wherein two consecutive laser beam multipliers are used and each individual multiplier has N channels. After passing two multipliers, N2 copies of the laser beam are generated. This is the multiplication factor to the number of the input beams M.
Figure 4 shows a third possible embodiment of the device, wherein the single fiber output 105 is exchanged with the beam splitter 401 to split low coherence output in to several beams to illuminate the area from several directions with the replicas output illumination beam. The number of the spited beams is equal to the number of views that are required for illumination of the application. In the preferred embodiment of the invention the beam splitter is of the all fiber version, guiding the splitted beams in the three fibers 402,403,404 positioned orthogonally to one another.
Fourth exemplary embodiment of the device is shown in Figure 5, wherein the single fiber output 105 is exchanged with beam splitter 401 to split low coherence output in to several fiber channels. One fiber output is emitting light in the main optical axis 501 while other fibers are arranged at small angle offsets to the fist fiber 501 forming a fiber cluster 502 for angle diversity of the illuminating light and high NA illumination system output 503. The described embodiment of the invention serves to illuminate transparent objects having variable refraction index for the illuminating light.
Fifth exemplary embodiment of the device is shown in Figure 6, wherein the single mode components in the form of singe mode splitters from single to two channels 601 and single mode fibers are used. The a) is variant using the single mode laser beam multiplier constructed from two single mode splitters, each multiplier producing by a factor of 2 more laser beam replicas at the output. A less compact variant is shown in b) where the same number of pulse replicas are achieved at the output as in a) but the component count required is great with the benefit of tailoring the beam delay in each channel separately. The c) single mode variant uses a circulator 603 to couple input light in to the multiplication chain where it is multiplied by beam splitters 601 , reflected in each channel by reflective element being mirror or fiber Bragg grating 602, combined back to the common channel by inversely propagating trough the beam splitters 601, and coupled trough the circulator 603 to the output port.
Figure 7 is an example of aluminium surface illumination with multimode laser diodes. On the left, the surface is illuminated directly by the multimode laser diodes. Due to coherency of the light, a speckle pattern is produced making it impossible to discern the surface features which shows in high variation of intensity profile. On the right, homogenous illumination field is achieved by the device according to the invention and much more smooth intensity profile is achieved. Smaller variations are due to the surface features and imperfections.
Within the scope of the invention as described herein and defined in the claims, other embodiments of a device for providing speckle-free homogeneous laser light illumination that are clear to one skilled in the art of laser technology may be possible, which does not limit the essence of the invention as described herein and defined in patent claims.

Claims

Patent claims
1. A device for laser speckle-free laser illumination, characterized in that it is a fully fiber-integrated device, in which laser light from a common channel is decoupled to two or more channels and back to the common channel, wherein the number of mutually non-coherent laser beams is increased in a manner that does not significantly distort shape of the laser pulse.
2. The device according to claim 1 , characterized in that it comprises:
- at least one light source for generating a light beam,
- an optional light combiner coupled to the light sources in case of two or more light sources,
- at least one, preferably four laser beam multipliers being coupled to the light source or the optional light combiner, wherein the said multipliers comprise means for separating laser beam into two or more channels, preferably seven, and means for combining the separated channel into one common channel, the channels being optical fibers, and
- an output for emitting the laser light for speckle-free illumination.
3. The device according to claim 1 or claim 2, characterized in that is based on relation of the number of mutually uncoherent and non-correlated laser sources M with speckle contrast C:
1
C 7 wherein in case of one laser source M= 1 and sequential laser beam multiplying units kwith multiplication factor of N, which is the number of the fiber sub-channels used in one laser beam multiplication unit, the speckle contrast is:
C = N~k/2 and wherein in case of more laser sources the speckle contrast is:
4. The device according to any of the preceding claims, characterized in that the light source may be any suitable source selected from a group consisting of: a multimode laser diode; a single mode or near single mode laser source such as laser diode, mode locked laser, Q-switched laser, gain switched laser, preferably a multimode laser diode, and wherein the selected source may operate in a pulsed mode or a continuous mode, or in case of several sources one may be operating in pulsed mode and the other in continuous mode.
5. The device according to any of the preceding claims, characterized in that the optical fibers are multimode fibers or single mode fibers, any of which is optionally doped with well-known dopants, wherein the cross-section shape, numerical aperture, structure and length are arbitrary.
6. The device according to any of the preceding claims, characterized in that the optical fibers in laser beam multiplier interconnections / sub channels can be of different cross section shape, numerical aperture or length to achieve mutually different coherence length for the illumination laser beam.
7. The device according to any of the preceding claims, characterized in that the laser beam multiplier comprises at least two multimode all fiber beam splitters and combiners or at least two single mode multiplexers or at least two 50/50 beam splitters, in order to ensure that the received light along light guiding channel is mixed, delayed and forwarded in to the next light guiding channel, wherein mixing is achieved by separation and combination of light beams inside the multiplier.
8. The device according to the preceding claim, characterized in that the laser beam multiplier is a single model multiplier and comprises:
- an input single mode fiber,
- a single mode fiber beam splitter for splitting laser beam from single to at least two fibers,
- matching number of the single mode optical fibers with the mutual length difference greater than the coherence length for the illumination light that delay the splitted laser beams and - a single mode fiber beam combiner for combining light from the at least two input ports to the common single fiber; wherein the single mode laser beam splitter can be exchanged with the 50/50 coupler where on the input side only one port is used.
9. The device according to claim 7, characterized in that the laser beam multiplier is a single model multiplier and comprises a circulator to couple input light in to the multiplication chain where it is multiplied by beam splitters, reflected in each channel by reflective element being mirror or a fiber Bragg grating mutually delayed by the different length of the fibers used in each channel, combined back to the common channel by inversely propagating trough the beam splitters, and coupled trough the circulator to the output port.
10. The device according to any of the preceding claims, characterized in that it further comprises a photodiode connected in parallel to the laser light source for direct monitoring of the laser light shaping in time
11. The device according to any of the preceding claims, characterized in that the output of the device can be a single fiber output or can include a beam splitter to split low coherence output in to several beams, wherein the number of the split beams is equal to the number of views that are required for illumination of the application.
12. The device according to the preceding claim, characterized in that the output fiber is arranged in such a way that one fiber output is emitting light in the main optical axis, while other fibers are arranged at small angle offsets to the fist fiber thereby forming a fiber cluster for angle diversity of the illuminating light and a high NA illumination system output.
13. The device according to any of the preceding claims, characterized in that the device comprises: - laser light source in the form of one to nineteen multimode laser diodes combined in to the common channel before the emitted light encounters the laser beam splitters;
- four multimode fiber combiners and four splitters that are combined in to the pairs to form laser beam multiplier unit;
- optical fibers forming from two to nineteen multichannel interconnections with the mutually length difference greater than the light coherence length;
- the laser beam multiplier units are connected in series each multiplying the output from the previous one potentiating the number of the pulse replicas at the illumination system output;
- optionally, a photodiode connected in parallel to the laser light source for direct monitoring of the laser light shaping in time.
14. A method for providing speckle-free laser illumination using the device according to any of the preceding claims comprises the following steps: a) optional combining of laser sources into a single channel with a fiber combiner, if two or more sources are used; b) connecting the single channel to a beam splitter to decouple light from the common channel to two to nineteen fiber channels, wherein an inversely oriented beam combiner can be used for multimode fibers; c) guiding laser beam replicas produced in step b) into a multichannel interconnection comprising two to nineteen fibers with the mutual length difference greater than the light coherence length acquiring the phase difference between each other; d) combining the multiple laser pulse replicas back into the common channel using a fiber beam combiner; e) performing and optionally repeating steps b) to d) in the laser beam multiplier unit(s); and f) leading the multiple replicas of the laser beam via the single channel to an output.
15. Use of the device and the method according to any of the preceding claim in applications that require illumination sources capable of bright and uniform illumination with modulation of bandwidths, for example high-speed imaging, machine vision, high-speed shockwave photography, flow monitoring, and welding monitoring.
EP20764808.0A 2020-06-15 2020-06-15 A device and a method for speckle-free laser illumination Pending EP4154055A1 (en)

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