Patent Application
20230400677
This application claims benefit to European Patent Application No. EP 22178228.7, filed on Jun. 9, 2022, which is hereby incorporated by reference herein.
Embodiments of the present invention relate to a light source unit for an imaging device. Embodiments of the present invention further relate to a method for generating laser light.
Typical laser light sources generate laser light that comprises a very narrow wavelength band centered around a single wavelength. Broadband laser light sources on the other hand generate laser light that comprises a wide spectrum of wavelengths often called continuum. One common way to generate a wide spectrum is to use micro-structured glass fibers, for example photonic crystal fibers (PCF) in combination with a pulsed laser.
Broadband laser light sources are used as versatile excitation light sources in fluorescence microscopy. Typically, an acousto optic tunable filter is used to select laser light comprising wavelengths of one or more spectral bands from the wide spectrum. The selected laser light is then directed onto a sample in order to excite fluorophores located in the sample. The optimal excitation wavelength for a specific fluorescent dye or protein can easily be selected from the wide spectrum of the broadband laser light by adjusting the acousto optical tunable filter. This results in a high excitation efficiency that allows high-quality microscopy images to be obtained without causing stress to the sample, for example by overexciting the sample.
However, for maximum flexibility, light sources used in fluorescence microscopy need to be able to excite a number of different fluorescent dyes and fluorescent proteins with different excitation spectra. Therefore, broadband laser light sources intended for use as excitation sources in fluorescence microscopy should be capable of emitting excitation light over the entire visible spectrum. However, it is very complex, and therefore expensive to generate such a broad spectrum from a single laser light source. Especially when generating laser light with short wavelengths, the micro-structured glass fibers typically used in broadband laser light sources suffer greatly from the increased stress, resulting in a short lifetime. Thus, broadband laser light sources often have no, or insufficient, light output in the short-wavelength region of the visible spectrum, i.e. the blue spectral range around 400 nm.
Embodiments of the present invention provide a light source unit for an imaging device. The light source unit includes a beam extraction unit configured to receive broadband laser light, to direct at least a part of the broadband laser light having a first wavelength into an amplifier beam path, and to direct a residual laser light into a first illumination beam path. The light source unit further includes an optical amplifier unit arranged in the amplifier beam path and configured to generate amplified laser light having the first wavelength by amplifying the part of the broadband laser light, and a frequency changing unit arranged in the amplifier beam path and configured to generate laser light having a second wavelength from the amplified laser light having the first wavelength.
Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:
Embodiments of the present invention provide a light source unit for an imaging device and a method that provide a cost-effective means for generating laser light having a broad spectrum including the short-wavelength region of the visible spectrum.
The light source unit for an imaging device according embodiments of the present invention includes a beam extraction unit configured to receive broadband laser light, to direct at least a part of the broadband laser light having a first wavelength into an amplifier beam path, and to direct a residual laser light into a first illumination beam path. The light source unit also includes an optical amplifier unit that is arranged in the amplifier beam path and configured to amplify the laser light having the first wavelength. The light source unit further includes a frequency changing unit that is arranged in the amplifier beam path and configured to generate laser light having a second wavelength from the amplified laser light having the first wavelength.
The laser light having the first wavelength is extracted from the broadband laser light and directed into amplifier beam path that is separate from the illumination beam path. In the amplifier beam path, the laser light having the first wavelength is amplified. The amplified laser light is then frequency shifted from the first wavelength to the second wavelength. This arrangement can be used to generate laser light having the second wavelength which lies outside of the spectrum of the broadband laser light. Thereby, the proposed light source unit provides means to extend the spectrum of the broadband laser light, in particular into the short-wavelength region of the visible spectrum, i.e. the blue part of the spectrum around 400 nm.
Since only one source of broad band laser light is used and no additional laser light sources are needed, the proposed light source unit is therefore very cost-effective. The proposed light source unit can also be made very compact for the same reason. The short-wavelength laser light is generated outside the source of the broadband laser light. Thus, the short-wavelength laser light cannot damage the micro-structured fibers typically used in such sources, thereby extending the lifetime of the source of the broad band laser light. Additionally, if the light source for the broadband laser light is pulsed, the laser light with the second wavelength is also synchronized with the residual laser light. This is particularly important for fluorescence imaging applications based on a fluorescence lifetime, and allows the light source unit to be used for further applications, for example pulsed interleaved excitation microscopy.
In an embodiment, the frequency changing unit is configured to direct the laser light having the second wavelength into a second illumination beam path. In this embodiment, the light source unit includes two distinct illumination beam paths. The residual laser light and laser light having the second wavelength can be used independently of each other making the light source unit even more versatile.
In another embodiment, the light source unit includes a beam unification unit configured to couple the laser light having the second wavelength into the first illumination beam path. In this embodiment, the light source unit includes only one illumination beam path. Both the residual laser light and laser light having the second wavelength are combined into a single beam of phase-synchronous laser light. The resulting single beam of excitation light has a broad spectrum and can be used to excite a variety of different fluorescent dyes and/or fluorescent proteins.
In another embodiment, the light source unit includes at least one acousto optical tunable filter arranged in at least one of the first illumination beam path and the second illumination beam path. For example, the acousto optical tunable filter may be arranged in the single illumination beam path following the beam unification unit. In such an embodiment, the acousto optical tunable filter may be used to select a specific excitation wavelength or wavelength band from the extended spectrum provided by the light source unit. Thereby, the light source unit provides a very flexible source of excitation light that can excite a large number of different fluorophores. Such an excitation light source can be used to great benefit in fluorescence microscopy, in particular in scanning and/or confocal microscopy, for example. It is also possible to arrange one acousto optical tunable filter in each of the two illumination beam paths. The additional acousto optical tunable filter in the second illumination beam path can be used to select whether the laser light having the second wavelength is switched on or not, or to adjust the intensity of the laser light having the second wavelength. Alternatively, the optical amplifier unit can be controlled such that no laser light having the second wavelength is generated.
In another embodiment, the beam extraction unit is configured to direct at least a part of the broadband laser light having a first polarization into the amplifier beam path, and to direct laser light having a second polarization into the first illumination beam path. This might be accomplished e.g. by a polarizing beam splitter.
Alternatively, or additionally, acousto optical tunable filters filter out light of a certain polarization, i.e. either s-polarization or p-polarization. Therefore, when an acousto optical tunable filter is used to select a specific excitation wavelength or wavelength band, light having the polarization that is filtered by the acousto optical tunable filter cannot be utilized, for example as excitation light. This otherwise unused laser light is extracted from the broadband laser light and used to generate the laser light having the second wavelength, thereby not wasting broadband laser light and making the light source unit more efficient. In order to combine the laser light having the second wavelength and the residual laser light back together, the polarization of the laser light having the second wavelength needs to be changed, for example by a waveplate or a prism or a simple mirror-arrangement.
In another embodiment, the beam extraction unit includes an edge filter, in particular a short pass filter. An edge filter either blocks light having a wavelength above a center wavelength or below the center wavelength. In this embodiment, the beam extraction unit acts like an edge filter, in particular like a short pass filter, in the sense that the beam extraction unit directs most of the broadband laser light having a wavelength above the center wavelength into the amplifier beam path. The residual laser light, i.e. most of the broadband laser light having a wavelength below the center wavelength, is directed into the first illumination beam path. The edge filter may in particular be realized by a beam splitting element, for example a dichroic beam splitter. Alternatively, a dedicated filter element may be arranged between the beam splitting element and the optical amplifier unit. Both a dichroic beam splitter and dedicated filter element acting as the edge filter are less expensive than for example acousto optic tunable filter, thereby making the beam extraction unit more cost effective.
In another embodiment, the beam extraction unit includes a band pass having a center wavelength equal to the first wavelength. A band pass is a filter that blocks all wavelengths of light except a wavelength band around a center wavelength. In this embodiment, the beam extraction unit acts like a band pass filter in the sense that the beam extraction unit directs most of the broadband laser light having a wavelength in the wavelength band around the center wavelength into the amplifier beam path. The band pass may in particular be realized by a beam splitting element, for example a dichroic beam splitter. Alternatively, a dedicated filter element may be arranged between the beam splitting element and the optical amplifier unit. Like in the previously described embodiment, a dichroic beam splitter and dedicated filter element acting as the band pass filter are less expensive than for example acousto optic tunable filter, thereby making the beam extraction unit more cost effective.
In another embodiment, the frequency changing unit is configured to perform a parametric process in order to generate the laser light having the second wavelength from the amplified light having the first wavelength. The parametric processes uses a non-linear optical effect in order to achieve the wavelength shift from the first wavelength to the second wavelength. In particular, the parametric process is second harmonic generation or higher harmonic generation. During second harmonic generation the frequency of the amplified light is doubled, i.e. the wavelength of the amplified light is halved. During higher harmonic generation the frequency of the amplified light is changed by an integer factor. For example, during third harmonic generation, the frequency is tripled, during fourth harmonic generation, the frequency is quadrupled, and so on. By way of the parametric process, shorter wavelength light is generated from the light that has been extracted from the broadband laser light. Thereby, the spectrum of the broadband laser light is extended towards the short-wavelength region making the light source unit even more versatile.
In another embodiment, the frequency changing unit includes a first collimating lens, a nonlinear optical crystal, and a second collimating lens arranged in that order in the amplifier beam path. The nonlinear optical crystal is preferably a periodically poled lithium niobate crystal or a Barium borate crystal. However, other types of crystals may be used as the nonlinear optical crystal. Nonlinear optical crystals are passive optical elements, i.e. they require at most a simple control circuit comprising a Peltier-element for temperature control in order to function. The frequency changing unit therefore has a particularly simple design making the light source unit easier to manufacture.
In another embodiment, the second wavelength has a value between 350 nm and 450 nm, in particular between 380 nm and 420 nm. In this embodiment, the second wavelength is in the blue, i.e. short-wavelength, region of the visible spectrum. Laser light in this wavelength region is used in many fluorescence imaging applications, for example biological experiments that use DAPI as a fluorophore for staining cell nuclei. Since many broadband laser light sources do not provide light in the wavelength region and the broadband laser light sources that do are expensive, the light source unit according to the present embodiment fills an important gap, providing a cost-effective broadband laser light source that can be used in a wide range of applications.
In another embodiment, the first wavelength has a value between 700 nm and 900 nm, in particular between 760 nm and 840 nm. Many broadband laser light sources provide laser light in the red and near infrared region. This light can be extracted, amplified, and then frequency doubled into light in the range between 350 nm and 450 nm, in particular between 380 nm and 420 nm. As has been described above, this wavelength region is important for many applications in fluorescence imaging.
In another embodiment, the light source unit includes a supercontinuum light source configured to generate the broadband laser light, and to direct the broadband laser light into the beam extraction unit. Supercontinuum lasers are laser light sources that typically use a micro-structured glass fiber to generate broadband laser light from a pulsed laser light source. Typically, the pulsed laser light source has a pulse length in the femto- to picosecond range and emits light having wavelengths in the near infrared. The broadband laser light generated by supercontinuum lasers is both longer- and shorter-wavelength than the wavelength of the original pulsed laser light which is also called seed laser light. Since supercontinuum light sources use a single pulsed seed laser light source they are compact compared to arrangements that use multiple different laser light sources in order to generate broadband laser light or light having multiple single wavelengths. Thus, by using a supercontinuum light source the light source unit can also be made very compact.
In another embodiment, the light source unit includes an optical amplifier control unit. The supercontinuum light source is pulsed. The optical amplifier control unit is configured to set parameters of the optical amplifier unit in accordance with a pulse rate of the supercontinuum light source. When the amplification of the extracted light having the first wavelength is done in accordance with the pulse rate of the supercontinuum light source, the extracted light can be amplified more efficiently. Thereby, less of the extracted light is wasted and more of the extracted light can be put to use, for example to excite fluorophores. This makes the light source unit more efficient.
In another embodiment, the light source unit includes an optical amplifier control unit. The optical amplifier control unit is configured to set parameters of the optical amplifier unit in accordance with the power of the supercontinuum light source. In this embodiment, the amplification of the extracted light having the first wavelength is done in accordance with the power of the supercontinuum light source, the extracted light can be amplified more efficiently. Thereby, less of the extracted light is wasted and more of the extracted light can be put to use, for example to excite fluorophores. Further, damage to the optical amplifier unit can be prevented. This makes the light source unit more efficient.
The inventions also relates to an imaging device comprising light source unit described above. Preferably, the imaging device is a microscope, in particular a confocal microscope and/or a scanning microscope. The imaging device has the same advantages as the light source unit described above.
The invention further relates to a method for generating laser light. The method includes the following steps: Receiving broadband laser light. Directing at least a part of the broadband laser light having a first wavelength into an amplifier beam path. Directing a residual laser light into a first illumination beam path. Amplifying the light having the first wavelength. Generating light having a second wavelength from the amplified light having the first wavelength.
The method has the same advantages as the light source unit described above and can be supplemented using the features of the dependent claims directed at the light source unit.
The light source unit 100 includes a broadband laser light source 102 exemplary formed as a supercontinuum light source that is configured to generate broadband laser light 104. The broadband laser light 104 includes laser light having a wide range of wavelengths in the visible spectrum and the near infrared and might extend into the infrared. An exemplary spectrum of the broadband laser light 104 is described below with reference to
The extracted part of the broadband laser is directed into an optical amplifier unit 112 arranged in the amplifier beam path 108. The optical amplifier unit 112 is adapted to amplify laser light having the first wavelength in order to generate amplified laser light. An exemplary spectrum of the amplified laser light is described below with reference to
In the present embodiment, the laser light generated by the frequency changing unit 114 is then directed into a beam unification unit 116 arranged at an intersection of the first illumination beam path 110 and the amplifier beam path 108. The beam unification unit 116 combines the residual light with the laser light generated by the frequency changing unit 114 into combined laser light by coupling the laser light generated by the frequency changing unit 114 back into the illumination beam path 110.
An acousto optical tunable filter 118 is arranged in the illumination beam path 110 following the beam unification unit 116. The acousto optical tunable filter 118 can be used to filter one or more spectral bands from the combined laser light, for example in order to generate excitation light that can be directed onto a sample 808 (c.f.
The light source unit 200 according to
The broadband laser light 104 is typically unpolarized, i.e. all polarizations are present in the broadband laser light 104. The first acousto optical tunable filter unit 118 filters out all laser light having either s-polarization or p-polarization. This means that approximately half of the broadband laser light 104 cannot be utilized by the light source unit 100 according to
According to the present embodiment, the beam extraction unit 302 includes a beam splitting element 304 that is configured to direct laser light having a first polarization into the amplifier beam path 108, and to direct laser light having a second polarization into the first illumination beam path 110. The beam extraction unit 302 further includes a filter element 306 that is arranged in the amplifier beam path 108 between the beam splitting element 304 and the optical amplifier unit 112. The filter element 306 is optional and may be a band pass configured to filter all wavelengths but a narrow band around the first wavelength. Alternatively, the filter element 306 may also be an edge filter configured to filter laser light of all wavelengths below a certain wavelength. The filter element 306 is used to filter out wavelengths except the first wavelength in order to adapt the extracted part of the broadband laser to the amplification profile of the optical amplifier unit 112.
In the present embodiment, the beam unification unit 308 might include a polarization changing element, for example a waveplate, that is configured to change the polarization of the laser light generated by the frequency changing unit 114 to the second polarization, i.e. polarization of the residual laser light. This ensures that the laser light generated by the frequency changing unit 114 is not filtered out by the first acousto optical tunable filter 118, and can therefore be utilized by the light source unit 100.
The light source unit 400 according to
The abscissa 502 of the diagram 500 denotes wavelength in nm. The ordinate 504 of the diagram 500 denotes intensity. The exemplary spectrum of the broadband laser light 104 shown in
The abscissa 602 of the diagram 600 denotes wavelength in nm. The ordinate 604 of the diagram 600 denotes intensity. The exemplary spectrum of the amplified laser light, i.e. the laser light amplified by the optical amplifier unit 112, includes a narrow band 606 around the first wavelength which is exemplary chosen to be 785 nm in the present embodiment.
The abscissa 702 of the diagram 700 denotes wavelength in nm. The ordinate 704 of the diagram 700 denotes intensity. The exemplary spectrum of the laser light generated by the frequency changing unit 114 includes two narrow bands 706, 708. A first band 706 shown to the right in
As can be seen in the diagram 700 shown in
The first wavelength and the second wavelength in the embodiment described above with reference to
The imaging device 800 is exemplary formed as a microscope. More specifically, the imaging device 800 is formed as a fluorescence microscope configured to image the sample 808 by means of fluorescence imaging.
The imaging device 800 includes one of the light source units 100, 200, 300, 400 described above with reference to
An optical detection system 806 of the imaging device 800 is configured to generate images of the sample 808 based on fluorescence light emitted by the excited fluorophores. The optical detection system 806 according to the present embodiment includes an objective 810 directed at the sample 808, and a detector element 812. The objective 810 receives the fluorescence light emitted by the excited fluorophores and directs the fluorescence light into a detection beam path 814. A beam splitter 816 is located at an intersection of the illumination beam path 804 and the detection beam path 814, which are exemplary shown to be perpendicular to each other in the present embodiment. The beam splitter 816 is configured such that the excitation light is directed into the sample 808 via the objective 810. The beam splitter 816 is further configured such that the fluorescence light received by the objective 810 is directed towards the detector element 812.
The imaging system further includes a control unit 818 that is connected to the optical detection system 806 and the light source unit 802. The control unit 818 is configured to control the optical detection system 806 and the light source unit 802, for example based on a user input. The control unit 818 according to the present embodiment includes an optical amplifier control unit 820 configured to control the optical amplifier unit 112 of the light source unit 802.
Although the imaging device 800 has been exemplary described as a microscope, the imaging device 800 may be any other imaging device. In particular, the imaging device 800 may be any imaging device configured for fluorescence imaging.
Identical or similarly acting elements are designated with the same reference signs in all Figures. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. Individual features of the embodiments and all combinations of individual features of the embodiments among each other as well as in combination with individual features or feature groups of the preceding description and/or claims are considered disclosed.
Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus.
While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22178228.7 | Jun 2022 | EP | regional |
