Slow light amplifier and method for improving signal strength in photoacoustic tomography - Patents.com

Abstract

A cascaded slow light amplifier and a system including the cascaded slow light amplifier are described, the cascaded slow light amplifier including units, each unit including at least one host crystal optically configured to be doped with ions to provide at least one amplification zone.

Description

[Technical field]

[0001] The present invention relates generally to a method for increasing the detected signal in photoacoustic tomography. In particular, the present invention relates to how a signal can be greatly amplified while limiting the noise from amplified spontaneous emission. Additionally, the present invention relates to a method for re-coupling the signal light into the imaging medium to allow for even greater signal amplification via mode selection. [Background technology]

[0002] The ability to project light into turbid, opaque, light-scattering media, i.e., media that scatters light, is important in many settings, including biomedical applications. Optical imaging provides sensitive molecular contrast simply and non-invasively. Conventional optical microscopy relies on the transmission of light through shallow layers, on the order of tens of micrometers. Confocal microscopy extends the penetration depth to around 0.1 mm, while optical coherence tomography can reach depths of around 0.5 mm. This is approximately the limit of high-resolution optical imaging modalities that rely on the suppression of diffusely scattered light. At greater depths, diffuse light scattering has a significant effect on the interaction of the light field with the light scattering medium. The attenuation of a light ray at a particular depth is determined largely by the scattering of the light-scattering medium. This fundamental aspect means that the maximum penetration depth at which light can be detected, even at the most favorable wavelengths, is limited to around 10 cm in typical biological tissue.

[0003] Other optical analysis and imaging techniques have been developed in the diffuse scattering region, from about one millimeter up to several centimeters. One example is pulse oximetry for determining oxygen saturation in the blood. One example of optical imaging is diffuse optical tomography, which can be performed, for example, using endogenous tissue contrast, using administered fluorescent contrast agents or administered fluorescent agents. Typically, diffuse scattering limits spatial localization using these techniques to within a few millimeters to a centimeter. Additionally, the light intensity of the detected light is very low, and detection techniques are often inefficient, cumbersome, and require sophisticated and expensive equipment.

[0004] Photoacoustic tomography techniques have been devised to improve spatial localization when using light to probe turbid, opaque, light-scattering media. In photoacoustic tomography, acoustic waves are locally emitted by the slight heating of living tissue due to the absorption of a laser pulse and are detected by an ultrasound transducer. Photoacoustic tomography exploits the fact that ultrasound waves scatter several orders of magnitude less than light waves in biological tissue. In this way, the spatial localization of the source of the acoustic wave can be achieved to within about a millimeter.

[0005] Another example of using acoustics to improve the localization of optical interactions in biological tissue is the use of acoustic photon tagging, as described, for example, in "Acoustic Photon Tagging" by Friedrich Schmidt, "Analyzing the Localization of Optical Interactions in Living Tissues," Proc. By insonifying tissue and directing laser light within the tissue, only the laser light passing through the area occupied by the acoustic field will be frequency shifted by the acoustic frequency. Then, by optically detecting only this frequency-shifted light, it is possible to know that the sound field has interacted with the light, making it occupy a small, localized volume compared to the light.

[0006] However, this photoacoustic tomography technique is still limited by the amount of light that reaches and interacts with the sound field. Therefore, methods and apparatus for improving the amount of light that interacts with the focusing of ultrasound would be advantageous.

[0007] This increase in light intensity can be achieved by wavefront shaping, which allows for orders of magnitude higher intensity at the desired location, as described in Non-Patent Document 2. A similar approach to achieve this wavefront shaping is by mode competition on the roundtrip through an optical amplifier, as described in Non-Patent Document 3.

[0008] Another type of optical amplifier is the cascaded or multi-stage amplifier, where each stage uses a doped crystal to amplify the signal. These types of amplifiers are described, for example, in US Pat. No. 5,399,623, US Pat. No. 5,499,633, US Pat. No. 5,523,367 and US Pat. No. 5,523,697.

[0009] Thus, an iterative approach that amplifies only the frequency-shifted light rapidly increases the intensity of the optical field at the optoacoustic interaction point. A problem that arises from the amplification of scattered light collected from biological tissue is that the amplified spontaneous emission (ASE) cannot be discarded without discarding a large amount of the signal. Conventional countermeasures to ASE include using low amplification or the inclusion of an angle-selective stage, both of which have drawbacks in the above applications. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] China Patent Application Publication No. 110895377 [Patent Document 2] U.S. Patent No. 5,872,650 [Patent Document 3] Japan Patent Application Publication No. 2010205903 [Patent Document 4] U.S. Patent Application Publication No. 2003156605 [Patent Document 5] SUWEIDEN Patent Application Publication No. 2151298

Non-licensed documents

[0011]

Non-patent document 1

Non-patent document 2

Non-patent document 3

Non-patent document 4

[0012] Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more deficiencies, disadvantages or problems in the technology, e.g. as described above, respectively or in any combination thereof, by providing an apparatus, system or method as set out in the appended claims for amplifying slow light to improve signal strength in photoacoustic tomography. [Means for solving the problem]

[0013] A first aspect of the present invention relates to a cascaded slow light amplifier. The cascaded slow light amplifier comprises units, each unit comprising at least one host crystal optically configured to be doped with ions to provide at least one amplification zone.

[0014] The unit may then comprise at least one time filter zone.

[0015] Each zone may also be constituted by a single crystal, the unit being obtained by bonding at least two crystals.

[0016] The single crystal may then comprise at least one amplification zone and at least one time filter zone.

[0017] Also, a cascaded slow light amplifier may comprise a chain in which multiple units are coupled together.

[0018] The chain may then comprise a number of amplification zones and a number of temporal filter zones, each amplification zone being followed by one of the temporal filter zones.

[0019] Furthermore, the chains may be composed of single crystals.

[0020] Additionally, the last time filter in the chain may act as a frequency shifter.

[0021] The amplification zone may also be a narrow band amplifier.

[0022] The amplification zone may also be a two stage amplifier.

[0023] Also, the amplification zones of the chain may be pumped sequentially, with the amplification zones of the chain having an amplification level such that the amplified spontaneous emission (ASE) is lower than the input signal.

[0024] Additionally, a temporal filter zone, which may be placed between the two amplification zones, may act as a temporal buffer.

[0025] The amplification may also have strong suppression outside the gain bandwidth and has an increased slow light effect.

[0026] In a further aspect of the present invention, a system for regenerating an amplifier stage is described. The system includes a cascaded slow light amplifier according to the present invention. The system also includes a power supply and a connector connected between the power supply and the amplification zone of the cascaded slow light amplifier. A connector may be configured to provide an electric field across one or more amplification zones to regenerate an amplifier stage with the electric field.

[0027] In another aspect of the invention, a system for localizing light in a light scattering medium is described. The system may include a light source configured to transmit an optical signal having a frequency into the optical scattering medium. The system may also include an ultrasonic device configured to generate an acoustic field in the optically scattering medium and a system for regenerating the amplifier stages described herein.

[0028] Additionally, the system may include a collection light guide configured to collect the light signal that has traversed the light scattering medium. During use, the system provides an amplifier circuit in which an optical signal traverses the optical scattering medium through various optical modes, some of which interact with the acoustic field causing the modes interacting with the acoustic field to frequency shift portions of their power spectra, and the light collected by the collecting light guide is transmitted to an optical subsystem where a system for regenerating the amplifier stage amplifies the frequency-shifted modes and suppresses all other modes before converting the amplified modes back to the frequency of the optical signal before transmitting them into the optical scattering medium.

[0029] The repetition rate of the ultrasound system may then be timed relative to the time of the amplifier cycle.

[0030] The repetition rate of the ultrasonic device may also be gradually changed, thereby shifting the acoustic field deeper into the light scattering medium, so that the amplified mode follows the ultrasonic field deeper into the light scattering medium with each amplification round.

[0031] The term "slow light" refers to a propagating pulse that is effectively slowed by interaction with the medium through which it propagates. "Slow light" may have a very low group velocity of light, several orders of magnitude slower than the speed of light in a vacuum; for example, the group velocity may be thousands or millions of times smaller than the speed of light, c.

[0032] For example, "Slow Light" is c / 10 3 Smaller group velocity, e.g., c / 10 4 etc., where c(=3x10 8 m / s) is the speed of light in a vacuum. For example, in a slow light amplifier this means that the group velocity of the light pulses being amplified is less than 300 km / s, eg less than 30 km / s.

[0033] As used herein, the term "light" is not limited to light visible to the human eye, but also includes wavelengths in the ultraviolet and infrared regions.

[0034] The term "comprising" as used in this specification is to be interpreted as indicating the presence of stated features, integers, steps or components, and does not exclude the presence or addition of one or more other features, integers, steps, components or groups thereof. [Brief description of the drawings]

[0035] These and other aspects and features by which embodiments of the present invention may be practiced will become apparent and elucidated from the following detailed description of the embodiments of the invention, taken in conjunction with the drawings.

[0036] [Figure 1] 1 shows a schematic embodiment of a setup for imaging with acoustic photon tagging, including an optical amplification loop to boost the signal level. [Diagram 2] 1 shows a schematic configuration of a cascade type slow light amplifier. [Diagram 3]Absorption versus frequency after population inversion for a slow-light two-level amplifier is shown. [Figure 4] We show the absorption versus frequency of the slow light amplifier and how the Stark effect can be used to continually regenerate amplification by shifting the ion distribution. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be limited to the embodiments set forth herein; rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0038] The following invention focuses on embodiments of the present optical subsystems 1-4 that are applicable to improve the localization of light interactions in light scattering media. The invention may be applicable to measuring oxygen saturation deep in biological tissue, such as the brain, heart, female breast, muscle tissue, etc. However, the present description is not limited to this application and can be applied to many other systems in which it is useful to localize light interactions in a light-scattering medium.

[0039] The present invention generally complements an earlier application, US Pat. No. 6,293,333, entitled "System and Method for Localizing Light in a Light Scattering Medium," which is incorporated herein by reference.

[0040] In FIG. 1, system 1 is an example of a system identified in US Pat. No. 5,399,633. As in the previous application, the system 1 includes a light source 1-2 (eg, a laser) at frequency f1 and an ultrasonic device 1-10 at frequency f2 that emits an acoustic field 1-3 into the optically scattering medium 1-1. The laser can be coupled into the biological tissue via an optical subsystem 1-4, which is the focus of this invention. Light from the optical subsystem 1-4 can be directed into the light scattering medium 1-1 via an illumination waveguide 1-5. In the light scattering medium 1-1, light traverses the light scattering medium 1-1 through different light modes 1-6, some modes, e.g., mode 1-6-2, interact with the sound field 1-3, and other modes, e.g., modes 1-6-1 and 1-6-3, do not interact with the sound field 1-3. When light interacts with a sound field, part of its power spectrum is frequency-shifted to a frequency f3=f1+n*f2 (n is an integer). However, this light suffers losses due to local absorption in the sound fields 1-3. The light can then be collected in a collection waveguide 1-7, which guides the light to the optical subsystem 1-4. The optical subsystem 1-4 amplifies the frequency-shifted light and then converts it back to the frequency of the original light source 1-2 and injects most of this light back into the illumination waveguide 1-5. However, a small portion of this light can be extracted from the optical subsystem 1-4 as a monitor signal 1-8 that can be monitored by a detector 1-9.

[0041] The re-injected amplified light is inherently connected to the optical mode 1-6 traversing the acoustic field 1-3, so that the optical mode 1-6 traversing the acoustic field 1-3 is advantageous for the light that loops multiple times through the system, in order, optical subsystem 1-4, illumination waveguide 1-5, light scattering medium 1-1, collection waveguide 1-7, optical subsystem 1-4, etc.

[0042] Also, during multiple revolutions, the competition for amplification in the optical subsystem 1-4 favors mode 1-6-2, which has the least loss through the light scattering medium 1-1.

[0043] Both of these effects, i.e. the effect of only amplifying modes interacting with the acoustic field 1-3 and the effect of competition between these modes, allow a greater amount of light to reach the acoustic field 1-3 for the same light intensity injected into the light scattering medium 1-1 through the illumination waveguide 1-5. This allows for a higher intensity signal to be read out. Also, the small movements of the sound fields 1-3 between revolutions allow this high intensity to track the sound fields within biological tissue. This allows both avoiding waiting for the intensity to build up at various probe points within the light scattering medium 1-1 during image acquisition, and also allows for global imaging of a greater depth.

[0044] The optical subsystem 1-4 may include a cascaded slow light amplifier 2, an example of which is shown in FIG. The cascaded slow light amplifier 2 may comprise at least one host crystal doped with ions, or may be a number of such crystals spliced ​​or sandwiched together. Also, in this population of dopant ions, the ion distribution has been spatially and spectrally engineered to include spatially contiguous amplification zones 2-1 and filter zones 2-2. The filter zone may include a slow light filter as described in the earlier application, US Pat. No. 5,399,633.

[0045] The slow light filter may consist of at least one host crystal doped with ions that have strong absorption at the original frequencies of the light source 1-2. When the ions replace other atoms in the host crystal, they can slightly distort the crystal lattice. Although the absorption of these ions individually is strong and narrow in frequency, the entire collection of doped ions may experience different crystal fields that cause slight changes in the frequencies at which they absorb relative to each other. The large number of different ion classes gives rise to a total absorption profile of the ensemble of doped ions in the host crystal, the frequency of which is several orders of magnitude wider than any frequency shift caused by the acoustic field.

[0046] With this absorption profile, it is possible to construct different permanent spectral configurations using optical pumping techniques. This optical pumping can be performed with or without the application of electric and / or magnetic fields to split the energy levels of the ions. A side effect of this modified absorption profile can be that the group velocity of the frequency-shifted light can be reduced by several orders of magnitude. Thus, time gating allows the light shifted by the sound field to be further distinguished from the original light frequency.

[0047] Amplification zones can be created by momentarily reversing the distribution of ions. In such a state, the inverted ions amplify the field by stimulated emission. Additionally, by adjusting the timing of the population inversion relative to the propagation of the optical field through the amplifier, the noise contribution from amplified spontaneous emission (ASE) can be reduced. Furthermore, by applying an electric field, the lack of amplification at the desired wavelength can be avoided.

[0048] As described above, light having both the original optical frequency f1 and the frequency f3 shifted by the acoustic field enters collection waveguide 1-7. This light is guided to optical subsystem 1-4, which consists of cascaded slow light amplifiers. The frequency difference between f1 and f3 is 10 6 It may be on the order of Hz. In order to select only f3 for amplification, a very sharp optical transition must be used so that f1 is not simultaneously amplified. Such a sharp gain profile can be achieved in ion-doped crystals, such as Pr:YSO. While each ion doped into the crystal has a very sharp optical transition, inhomogeneities in the crystal cause the center frequencies of those transitions to shift. The total number of ions doped in the crystal is 10 9 It has very high absorption capable of covering a wide frequency span not exceeding 100 Hz. At the same time, single ions can be targeted within the ensemble and used to generate 10 4 It is possible to create configurations of the spectral region with resolution of the linewidth of ion transitions on the order of Hz.

[0049] Slow light filters can be constructed in these materials by transferring ions of the aggregates to other long-lived ground states, as described above and in US Pat. No. 5,399,633. However, ions can also be excited to excited states, which are also often long-lived, e.g., 10 -5 s~10 -2 Has s. If more than half of the population can be excited, the incident light can stimulate emission, resulting in optical amplification. In conventional optical amplifiers, this population inversion requires three or more energy levels, as ions are excited to successively higher states. Upon successive excitation via optical pumping, a steady state population configuration that is half excited and half ground state is generated by stimulated emission. Such a distribution exhibits no optical gain since there is equal amounts of absorption and amplification. However, by applying a so-called pi-pulse, the total distribution can be inverted simultaneously and instantly. Such a population inversion also inverts the absorption, as illustrated in FIG. 3, where absorption 3-1 is plotted against frequency 3-2. This negative absorption is synonymous with amplification. On the other hand, negative absorption leads to a larger absorption gradient and therefore a larger dispersion. In an amplifier stage having such a population inversion, the slow light effect becomes even greater than in a normal slow light filter. The light that is not subject to amplification, i.e., the light with frequency f1, can be time differentiated from the light that is subject to amplification, i.e., the light with frequency f3.

[0050] As mentioned herein above, a problem with optical amplifiers is amplified spontaneous emission (ASE). When the population is inverted, the spontaneous emission also stimulates emission in the amplifier. When the spontaneous emission is larger than the input signal, the amplified light after the amplification stage is dominated by the ASE noise. This limits the amount of amplification that can be applied at each stage, since increased amplification leads to increased spontaneous emission. By cascading multiple slow light amplification stages, the amplification can be kept below the limit at each stage where ASE becomes a problem. An optional slow light filter may be placed between the amplification stages to act as a post-amplification time buffer and frequency cleaner. The time buffer also allows for population inversion of the next amplification stage.

[0051] A final problem to be solved arises from the fact that amplification is proportional to the population inversion and therefore to the total population of ions. To maximize amplification, a large number of ions need to be inverted, but this is limited by the number of ions per amplification stage and frequency. The distribution can be increased by either increasing the physical length of the amplifier stage or, if the dopant levels are low, by increasing the dopant concentration. A third approach to increasing the number of ions that contribute to the amplification is by applying an electric field. When subjected to an external electric field, ions undergo the Stark effect and their transition frequencies shift. This shift causes the transition center frequency to increase or decrease, depending on each ion's Stark coefficient. First, the ions are inverted over a broad frequency region, and then an electric field is applied which maintains amplification in the center of the inversion region by continuously ramping the electric field across the amplification stage.

[0052] For example, to have continuous amplification in a frequency region such as 3-3, two strong optical fields can be applied at frequencies above and below the target frequency. This is illustrated in FIG. 4, which also plots absorption 4-1 against frequency 4-2, but where the absorption is now split into two ion distributions 4-3 with Stark coefficients of opposite signs (Non-Patent Document 4). A strong optical field 4-4 is applied on either side of the amplified optical field 4-5 to invert the ion distribution. The electric field then causes the different ion distributions 4-3 to move in opposite directions due to the Stark effect. The two distributions combine in frequency space to produce an amplification of the signal optical field 4-5. This approach makes it possible to continually provide a perfectly inverted distribution, i.e., maximum amplification, for each round trip through the amplifier. If this population inversion were not recreated, the amplification would slowly decrease with each revolution until a steady state population was reached, half excited and half ground, where the final population configuration would not change with the pi-pulse.

[0053] By applying an appropriate electric field to a slow-light filter while a light pulse is propagating within its transmission window, the frequency of this light pulse can be shifted (Non-Patent Document 5). By applying such an electric field to the last filter zone 2-3, the light frequency f3 can be returned to the original frequency f1. This resets the optical frequency state to its input value, but does not remove the mode configuration. Using an amplifier chain to implement both the regeneration and this frequency shifter described in FIG. 4 as optical subsystem 1-4 of FIG. 1 allows multiple amplification rounds through the light scattering medium 1-1 with equal amplification factors. These round trips therefore allow the construction of light transmitted through mode 1-6-2, as described above. To facilitate an electric field across the various zones of the amplifier chain, one or more crystals comprising the amplifiers can be connected to one or more power sources, for example via metal electrodes.

[0054] By timing the repetition frequency of the acoustic pulses relative to the amplifier round trip time (i.e. the time it takes for light to loop through the system from 1-4 to 1-5 to 1-1 to 1-7 to 1-4 in turn, which can be a few microseconds due to the slow-light effects of the amplifiers), the optical mode 1-6-2 interacting with the acoustic field 1-3 is amplified while all other modes are suppressed. Once this is achieved, by increasing the pulse repetition frequency, the acoustic fields 1-3 are shifted deeper into the tissue with each amplifier cycle. If this observed spatial shift is such that the overlap of the sound fields between the two amplifier round trips is large enough, the high light intensity is maintained at the location of the sound field even as it is continually lowered into the biological tissue. While this intensity is maintained, the modes that interact primarily with fields 1-3 at their deep end in one revolution will see their frequency-shifted light fraction increase in the next revolution, so that these modes become stronger in the mode competition and thus obtain stronger amplification. As a result, modes that interact with the shallow ends of fields 1-3 will lose out in subsequent laps, and the strongest modes in one lap will be diminished in the next.

[0055] In this way, the mismatch between the acoustic pulse repetition rate and the amplifier cycle allows a higher optical field to reach the acoustic field deeper in the biological tissue, effectively increasing the imaging depth.

[0056] The above is a description based on the embodiment of the present invention. However, other embodiments than those described above are equally possible within the scope of the invention. The method may be implemented using hardware or software to provide different method steps than those described above within the scope of the invention. The various features and steps of the invention may be combined in other combinations than those described. The scope of the present invention is limited only by the appended claims.

[0057] As used in this specification and claims, the indefinite articles "a" and "an" mean "at least one" unless otherwise specified. The term "and / or" as used in this specification and claims refers to either or both of the elements conjointly present in some cases and non-conjointly present in other cases.

Claims

1. In a cascade type slow light amplifier having a unit, A cascaded slow light amplifier, each of said units comprising a host crystal optically configured to be doped with ions to provide at least one amplification zone.

2. 2. A cascaded slow light amplifier according to claim 1, wherein said units comprise a time filter zone.

3. each of said zones being composed of a single crystal; A cascaded slow light amplifier according to claim 1 or claim 2, wherein the unit is obtained by combining at least two crystals.

4. 3. The cascaded slow light amplifier of claim 2, wherein a single crystal comprises said amplification zone and said temporal filter zone.

5. A cascaded slow light amplifier according to any one of claims 1 to 4, comprising a chain in which a plurality of said units are coupled together.

6. the chain comprises a plurality of the amplification zones and a plurality of the time filter zones; 6. A cascaded slow light amplifier as claimed in claim 5, wherein one of the temporal filter zones follows each of the amplification zones.

7. A cascaded slow light amplifier according to claim 5 or claim 6, wherein the chain is constructed from single crystals.

8. The cascaded slow light amplifier according to any one of claims 5 to 7, wherein the last time filter in the chain functions as a frequency shifter.

9. The cascaded slow light amplifier according to any one of claims 1 to 8, wherein the amplification zone is a narrow band amplifier.

10. The cascaded slow light amplifier according to any one of claims 1 to 8, wherein the amplification zone is a two-stage amplifier.

11. The amplification zones of the chain are pumped sequentially; A cascaded slow light amplifier according to any one of claims 5 to 10, wherein the amplification zones of the chain have an amplification level such that the amplified spontaneous emission (ASE) is lower than the input signal.

12. The cascaded slow light amplifier according to any one of claims 6 to 11, wherein the temporal filter zone, which is arranged between two amplifying zones, acts as a temporal buffer.

13. The cascaded slow light amplifier according to any one of claims 1 to 12, wherein the amplification has strong suppression outside the gain bandwidth and has an increased slow light effect.

14. 1. A system for regenerating an amplifier stage, comprising: A cascade type slow light amplifier according to any one of claims 1 to 13, a power source, and a connector connected to the power source and the amplification zone of the cascade type slow light amplifier, 11. A system for regenerating an amplifier stage, wherein the connector is configured to provide an electric field across one or more of the amplification zones to regenerate the amplifier stage with the electric field.

15. 1. A system for localizing light in a light scattering medium, comprising: a light source configured to transmit an optical signal having a frequency into an optical scattering medium, an ultrasonic device configured to generate an acoustic field in the optical scattering medium, an optical subsystem comprising the system for regenerating an amplifier stage according to claim 14, and a collecting light guide configured to collect the optical signal that has traversed the optical scattering medium, A system for localizing light in an optical scattering medium, wherein during use, the system provides an amplifier circuit in which the optical signal traverses the optical scattering medium through various optical modes, some of the modes interacting with the acoustic field causing a portion of their power spectrum to be frequency shifted, the light collected by the collecting light guide is transmitted to the optical subsystem, and a system for regenerating the amplifier stage amplifies the frequency-shifted modes and suppresses all other modes before transmitting the amplified modes back to the frequency of the optical signal into the optical scattering medium.

16. 16. The system for localizing light in a light scattering medium of claim 15, wherein a repetition rate of the ultrasonic device is timed relative to the time of the amplifier cycle.

17. shifting the acoustic field deeper into the light scattering medium by gradually changing the repetition rate of the ultrasonic device; 17. The system for localizing light in an optically scattering medium of claim 16, wherein the amplified mode follows the ultrasonic field deeper within the optically scattering medium with each amplified round.

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