Patent Application
20170068082
A rapid switch between different spatial light distributions is required in various areas of optics. For example, one can use such a possibility in laser screen microscopy in order to be able to switch very rapidly between different focal fields. This is especially important when one would like to influence only the light in the focus of one lens. Here, a method in which a three-dimensional illustration of optically thick tests is achievable was recently demonstrated whereby the background light is discriminated [1, 2, 6].
In this case, the basic feature is that a property only influences the fluorescence that is generated in the focus temporarily, whereby the beam should not be modulated outside the focus. Until now, this method has been based on rapid switches of the optical phase in the pupil of a lens. Until now it has been demonstrated that the phase is switched in two half pupils.
Similar to the half pupil switching depicted above, switching between the optical phases of other split beams can also be used expediently. Moreover, it is advantageously possible to use not only the phases but generally field mode switching in order to generate a temporary modulation of the beam from the focal volume, whereby the beam is not modulated outside the focal volume over time. Moreover, rapid mode switches for this purpose should be discussed here.
Besides phase switching, switching polarization is, moreover, also proposed as a further possibility in order to shift rapidly between two different field modes of the excitation spot whereby the beam coming from the focal volume is modulated. It has been recognized that a similar effect can be achieved in this case as with switching the phase. The effects of the switching particularly influence the stimulation of fluorescence in the focus.
Qualifiable solutions should lead to a modulation in the area of several MHz. With that said, they are basically suitable for advantageous use in laser scanning microscopes (LSM) to increase the depth of penetration without losses of scanning speed. Another increase of the speed ensues by parallelization using multi-spot microscopy. However, a slower modulation is also always possible and can be analogously discontinued.
Because of their advantageously high modulation frequencies, essentially only rapid switching opto-electronic elements such as, for example, AOMs and EOMs, qualify. With these solutions, a property should be switched that essentially ultimately has an effect on the field in focus and results in modulating the focus field, while the essential items that are out of focus are not significantly modulated.
The invention is characterized by the features of the independent patent claims.
Preferred additional improvements are the object of the dependent claims.
A schematic description of various advantageous embodiments of the invention follows below:
The present inventions will be described in greater detail, using examples with reference to the annexed drawings, in which
In an initial embodiment, it is assumed that the phases are generated on a passive element which introduces a polarization-dependent phase deviation in different spatial areas, preferably in a lens pupil. In order to generate a rapid temporary modulation, the polarization is manipulated over time using, for example, an EOM or another suitable element such as, for example, a nematic crystal, which responds more slowly, however, or constructions that generate a different polarization using a path segmentation and rapidly shifts this path using an AOM/AOTF.
By switching as well as rotating the polarization in
The passive phase elements displayed by way of example here consist, in each case, of a combination of a double refractive crystal, a phase plate P is represented in the form of a shared λ/2 plate and, with respect to the phase, a component that is independent of the direction of polarization that is, by way of example, made of glass. In the process, the phase plate is aligned in such a way that the rapid direction of the crystal is advantageously aligned in parallel and/or vertically or in some other optimized angle to the irradiated laser polarization.
Various modifications of the phase plate P are represented in
Glass is used here only by way of example. Amorphous quartz [Suprasil] or other non-double refractive materials can also be used.
In each case, as shown by an arrow, the orientation of the extraordinary axis of the respective λ/2 part is represented. If a polarized beam of light enters parallel to the direction of the arrow of this element, then a phase retardation of about half a wave length relative to the glass part will be generated in the λ/2 part. If, however, its polarization is vertically oriented to the direction of the arrow, then no phase retardation will be generated.
The element P is in connectivity with the EOM that causes a rapid rotation of the polarization direction PR of the incidental beam of light L, usually a linearly polarized laser beam, by corresponding excitation.
After passing through P, the beam of light proceeds further, as is also shown in the arrays below, in the direction of the microscope M via a scanning unit that is not represented here for testing, as is also familiar from the current state of the technology.
By virtue of the half-page alteration of the plate P, the beam of light undergoes the field modulation typical for FMM.
A standard technique is also part of the invention, whereby, advantageously, the formation of the phase element, by way of example by exchange or excitation of an SLM, is altered and by measuring the modulation contrast, e.g., by measurements with a switched on plate P and an outwardly tilted plate P (without FMM), whereby an optimization of the FMM signal can take place.
Within the context of the invention, even a rotation of the plate P can take place in a fixed beam of light L instead of rotating the polarization by the EOM.
In
This embodiment is especially advantageous if, in the actual optical system/appliance/microscope/scanning head, there is little room for construction.
In contrast to the state of the technology (e.g., Little et al. [1]), here an electro-magnetic insulation of the EOM can take place by applying the passive phase elements in connectivity with the fiber that is used which advantageously avoids influence from the measuring arrangement through the high frequency of electric fields emitted from the EOM. The EOM can also be conveniently accommodated separately in a laser module.
In 1D, 4 possible phase plates, P1-P4, are again shown by way of example.
In another advantageous embodiment, a solution is realized according to the invention in which the two modes are already available and are switched by means of an EOM, AOM or AOTF between these modes. This principle still differs widely from the state of technology documented up to this point, since now switching in the focus does not place by switching the optic phases but between two optic field modes. In doing so, an equivalent beginning and end state is produced, whereby, however, the transition from one configuration into the other configuration, distinguished by the respective focus field structure, takes place incoherently.
Illustration 2A shows a rapid optical mode switch based on an EOM to which a polarizing beam splitter PBS is subordinated in the light path.
For redirecting light, M1-M4 are scheduled after the PBS mirror.
A polarization direction Poll generated (reflected) by PBS is diverted via M1-M4.
There is a phase element between M2 and M3 that generates a donut mode DM of the beam distribution for a specific polarization direction (SPP spiral phase plate, [5]) that is unified again at PBS with the part going through PBS after M4.
Illustration 5B shows an example of a spiral phase plate from:
This linearly polarized laser beam L is propagated by the wave-retarding EOM, which acts like a rapidly switchable polarization rotator. In this way, the light is transmitted on its further path by the following polarizing beam splitter (PBS) (e.g., the polarized light that is parallel to the level of the plate). For the other polarization direction, the light is directed upward in the drawing and encounters an element like a spiral phase plate SPP that generates a donut mode from the Gauss mode (e.g.,[5]).
Alternatively, here too other mode switching elements such as sub-wavelength structures or DOE's (Diffractive Optical Elements) can be introduced to this beam. Accordingly, the light is again reflected to the PBS and is alternatively overlaid in a timely fashion with the beam of the second polarization. A modulation of the EOM consequently causes a modulation of the different field modes (poll 2 as well as GM) and thereby modulation of the light in the focus of the microscope lens.
For the time-dependent intensity of the light, which is emitted from the focal volume:
I(t)∝∫C(r)Ip(r,t)dr
applies.
Here, C(r) denotes the spatially varying concentration of excitable molecules. The integral stretches over an area that contains the focal volume. Ip(r,t) denotes the varying stimulation intensity over time, which in the case of a change-over between two modes is accounted for by:
Ip(r)∝G(r)(1+Cos(ωt)),+LG(r)(1+Sin(ωt))
In this case, it is important that essentially only the light in the focus undergoes this temporal modulation, but all the light outside the focus is essentially not, or, alternatively, fundamentally more weakly modulated.
By means of a lock-in-detection the temporally varying portion then lets itself be separated from the temporally constant portion. This temporally varying portion corresponds to the difference between the two different focal fields whereby the unmodulated out of focus portion is temporally constant and cancelled by the in-phase detection using, for example, a lock-in-detection.
Illustration 2B shows the intensity distribution of a Gauss mode and a donut mode (Laguerre-Gauss-Mode). The axes mark a scaled lateral spatial expansion in the focal level.
The proposed solution especially differs from the state of the technology in that here the light field is not dismantled in the pupil into its individual parts and influenced using a phase modulator in different spatial parts but that a temporally varying switch takes place between two different focal fields. In doing so, a series of advantages for its practical realization emerge. First of all, the most significant advantage consists in the attainable modulation speed.
Another advantage of the solution is its simple expandability by several focal volumes. In doing so, one would, for example, shine several Gaussian beams into the arrangement and reshape these in the corresponding optical way by means of an array transformer (e.g., a spiral phase mask array). The production of several focal volumes by itself is, for example, described in DE19904592 (see also
A schematic embodiment is shown in
Here, a multi-spot-variation for mode modulation is represented by the example of 4 specifically collimated beams L1-4 that are finally focused in the microscope M in the direction of the focus level.
The flow path of L1-4 takes place analogously to that in
For the passageway of the beams L1-4 from the EOM, this can feature an appropriately large cross-section and/or also be pre-arranged in the direction of the light of the multi-spot-production or several EOM's for L1-4 can be provided, even with different modulation frequencies or a segmented EOM with differently actuated cross-section areas.
In
Switching between two modes can also take place very rapidly by means of, for example, an acousto-optic modulator (AOM).
A PBS is again provided.
In the continuous beam path after the PBS, an AOM is arranged after a quarter wave plate QWP which performs polarization rotation in order to be able to direct the returning beam through the PBS in the direction of the additional microscope M; in the O of the AOM and initial alignment there are reflecting elements S which reflect the light back in the same direction in the AOM.
In the 0. arrangement a “donut mode generator” DMG (e.g., a spiral phase plate or “radial polarizer”) is provided in the beam path between the AOM and S.
This embodiment of the invention presents an advantageous variation of Embodiment 2a mentioned above.
In so doing, a Gaussian beam with a linear polarization is beamed in through a polarizing beam splitter and a quarter wave plate influencing the polarization in an AOM or AOTF actuated by a control unit AS (for very rapid switching an AOM; for slower, but polychromatic modulation, an AOTF can also be used).
In the AOM, a temporally varying grid can now be developed by beaming in an acoustic field in which this beam is, for example, in each case refracted in its 0. and its 1. ordering. Between these orderings, there is therefore rapid (high frequency) switching on and off. Then the beam again encounters a mode producing element on one of its paths such as, for example, a donut mode generating element DMG whereby, in addition to the donut modes, also other mode generating elements, e.g., for the production of higher Gauss-Laguerre modes, are conceivable and are subsequently reflected on an initial mirror. After that, the element is encountered a second time. The other ordering is, however, only reflected on one mirror. Both orderings then again encounter the AOM and in so doing are thereby again brought into the AOM into a beam direction, whereby they normally, though in individual cases depending on the mode producing element, again leave the AOM in the linear exit polarization. By rotating this field by means of a quarter wave plate QWP, the beam is then directed to reflect on the polarizing beam splitter cube PBS and can thereby get to the direction of the microscope M and the test.
The detection of the emitted fluorescence beam occurs with appropriate opto-electronic detectors and an optical or electronic demodulation.
Explanation of the detection technique that is used:
Lock-in techniques (LIT) are based on the phase dependent measurement of temporally modulated. signals, the basis of which forms a frequency reference. Characteristically, the test is stimulated with a certain frequency during which LI measures the signal with the reference frequency.
The lock-in system LI detects the signal with this specific reference signal depending on the reference phase situation.
In a solid relationship to the reference phase, LI amplifiers generate their own internal measuring phase (normally through a so-called phase-locked loop)
LIref=VL sin (ωLLt+θref)
In a classic LI amplifier, the signal is at first reinforced and subsequently multiplied with the LI reference: LIref by means of a phase sensitive detector (PSD) or a multiplier.
The signal is described by, for example:
S(t)=VS sin (ωst+θs)
so that at the exit of the PSD one obtains:
VPSD=VSVL sin (ωst+θs) sin (ωLt+θref)=½VSYL(cos ([ωr−•L]t+θS−θref)−cos ([ωr+wL]t+θS+θref)
Consequently, the exit of the PSD provides two temporally varying signals: one with the difference and one with the sum frequency from the LI reference phase and signal phase. This signal VIPSD is low-pass filtered whereby all high frequency signals are cancelled. Then, with a perfect low-pass, a signal finally only emerges, if (ωr−•L)=0 applies. Thereby, upon exiting one obtains:
V PSD,TP=½VSVL cos (θS−θref)
Consequently, the LI amplifier extracts the signal with the frequency ωr=ωL and itself provides an exit signal, which depends linearly on the signal amplitude with this reference frequency.
Further on, on the basis of
According to the invention, optical modulators are used in detecting demodulation or the operating mode of the detectors for demodulation.
Illustration 4A shows a microscope configuration for increasing the penetration depth/suppression of diffuse reflection whereby, in a schematic beam path of a laser L in the direction of a test P, a dichroic mirror/beam splitter D for separating illumination and the beam path detector is provided.
The detection is split up into two partial beam paths using a beam splitter BS, in which detectors d1, d2 are located:
Moreover, optical modulators m1, m2, an element Ph for phase adjustment, a filter F, a focus modulator and a subtraction operator O- are depicted.
The laser L of the fluorescence excitement is modulated with a focus modulator (FM) such as, for example, that depicted on the basis of
The fluorescence beam of the test (P) reflected by the dichroic filters (D) and now also modulated with ω is phase-sensitively detected using a 50/50 beam splitter (BS) from two detectors d1 and d2. In addition, by way of example, in each case an additional modulator m is inserted (m1, m2 in 3a) before d1 and d2, or the detector gain from d1 and d2 is modulated (3b).
The plated signals (filter F, e.g., integrator) are then subtracted from each other (operator O-).
The modulators M1, m2 can advantageously be optical modulators such as an EOM; m1 is directly modulated and m2 undergoes a phase retardation via Ph, whereby two partial signals are detected which thereby show a phase difference with respect to each other (delta phi) and are mixed and filtered with low-pass filters; an optic demodulation therefore also advantageously occurs with a discontinued relative phase, in the ideal case 90 degrees between the two detection components so that the conditions with intact focus and postponed conditions are disturbed in those of the focuses by the destructive interference in order to be able to be subtracted from each other (-operator); so the out of focus background (diffuse light) is withdrawn from the focus on each pixel of the signal.
In
The detectors d1, d2 are, for example, switched on and off with frequency co (and discontinued phase retardation).
Optical modulation in the detection beam, path:
According to the invention, a beam switch for demodulation by means of AOM is used.
Illustration 4C shows an beam path similar to what is depicted in
Here, however, there is an advantageous switchover between detectors d1 and d2 with frequency co by means of an AOD (A) similar to that depicted in
On the other hand, and as already depicted and detailed, a subtraction of the in focus and out of focus signals and low-pass filers takes place.
In so doing, so that no beam splitter is used, half of the signal intensity gets lost in each case.
Using Ph, the phase difference can be cancelled.
In
The first supplement involves the structure of the phase plate itself. This was originally described in such a way that it consists of a combination of a lambda/2 plate and a plate that operates isotropically. However, more extensively, additional combinations are also possible.
For example, this can be achieved, if, as in
The various designs of a combination of different areas represented in
It is especially advantageous, moreover, to achromatize the phase deviation of the two parts against each other.
This can be realized by using so-called “achromatic wave plates” or “achromatic retardation plates” that, for example, consist of various double refractive crystals and exhibit the same optical qualities (the same phase retardation) for a broad band wave length area and whereby then the phase spectra of the isotropic partial plate are adjusted to it while, for example, plates that consist of at least two glasses are used.
With respect to the illumination of the phase plate, it was implied that the polarization direction of the illumination is very rapidly switchable. For this an EOM was proposed above, for example, with which even this rapid rotation of the polarization direction would be made possible. At that point, the lighting to the phase plate was described in such a way that lighting can take place using an optical fiber; in which case the polarization must remain preserved. Furthermore, it has, however, been recognized that another and in various respects more advantageous solution can be achieved, if the polarized light is supplied using two fibers from the phase plate. In this case, as shown in
Another and especially advantageous solution emerges by virtue of the fact that the frequencies fac1 and fac2 permanently reside in the acousto-optical crystal and the amplitudes of the two components are temporally modulated. For the technique, this can be meaningfully demonstrated in such a way that the sum of intensity remains constant over time. A technical realization can, for example, take place by means of a DDS (Direct Digital Synthesizer).
The advantages of the represented embodiment emerge from the fact that:
1) it provides for switching an acousto-optic element that is small and shows clear advantages for handling in a microscope;
2) the fibers can be used directly for lighting from the laser module to the scanning head of an LSM.
Likewise, it is possible that, with an EOM in combination with a PBS, a rapidly switching fiber coupling is realized. In this case, a PBS is located behind the EOM which distributes the light directly into both fibers irrespective of the polarization condition (
It can, moreover, be advantageous if the polarization of the PBS has a pre-determined value. For this, a rotatable polarizer or one that is in some other way adjustable with respect to its polarization influence can be installed after the phase plate.
Having done this, even an intensity calibration can take place per the optical fibers between the two partial beams.
The measurement relating to this can be carried out in the microscope or with an external monitor diode.
For more exact calibrations between the partial beams and the adjustment of the AOM/EOM and the adjustable polarizer's pole, part of the beam can be disconnected after the PBS to a diode D.
Among other things, the fluctuations in intensity that are measured with the diode are used as a control quantity in order to set the maximum amplitudes of the frequencies fac1 and fac2.
Up to this point, in every case two fibers and frequencies were assumed. This is to be understood in the sense of an example, The technique can generally be used in switching from N light conditions with, for example, N fibers.
In what follows, the illustrations are to be described in detail.
The light of the laser light wave L is beamed in over a lens L1 at the level of an AOD (acousto-optic deflector) whose sound wave in the crystal with a frequency F is switched between different acoustic frequencies F1 and F2 which generate the alternatingly different deflection angle of the laser beam.
The alternatingly deflected laser beams La1, La2 are in each case introduced via fiber couplers Fkein into optical fibers F1, F2 and disconnected by means of a fiber uncoupler FKaus at different positions on a pole splitter PBS.
In this case, fibers F1 and F2 are arranged so that their (preferably linear) polarization is parallel to each other at the PBS and vertical to each other from above.
This is achieved by appropriate rotation of the ends of the fibers as well as the decoupling units, whereby in each case the optimal position can be adjusted behind the PBS for F1 and F2 on the basis of the signal yield from the PBS.
After the PBS, the phase plate PP is arranged as described in
Using a beam splitter ST, a monitor diode D can be disconnected that is used to adjust the signal intensities and the relationship of the two pole components by using a control unit ACE.
In this case, a rotating polarizer is advantageously provided between PP and ST that can be used for fine tuning the relationship between the two pole components and calibrating possible errors in orienting the fibers F1, F2 to each other as mentioned above.
Using ACE, detection can be synchronized using the monitor diode at the frequency of the switch by the AOD so that the individual intensity components let themselves separate from each other.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102010047352.9 | Oct 2010 | DE | national |
| 102011013613.4 | Mar 2011 | DE | national |
This application is a divisional of U.S. patent application Ser. No. 13/251,452 filed on Oct. 3, 2011, which claims the benefit of U.S. provisional patent application 61/480,707 filed Apr. 29, 2011 which claims priority to German patent application no. DE 10 2011 013 613.4 filed Mar. 8, 2011 which claims priority to German patent application no. DE 10 2010 047 352.9, filed Oct. 1, 2010, the contents of which are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13251452 | Oct 2011 | US |
| Child | 15298424 | US |
