J. Eur. Opt. Society-Rapid Publ. 21, 9( 2025) 89
Figure 9. Relationship of the speed of sound to the Brillouin frequency for a grating period of d = 3lm and d = 568 nm. If on the other hand the improved speed of sound resolution is not required, the smaller fringe spacing has the benefit of requiring shorter time signals to achieve the same resolution in the speed of sound. In the case of a hydrogel cube embedded in water, which have a difference in the speed of sound of 17 m / s( 1.1 %), the frequency difference between the two materials in the crossed beam setup would be 7 MHz and in the counter propagating setup would result in 29 MHz( Fig. 9 on the right). Measurement of such a difference would require a 140 ns long signal in the crossed beams setup, compared to only 35 ns in the counter propagating setup. This would allow for a higher spatial resolution, because the time signal shortens with decreasing focus size. For a given speed of sound resolution, the counter propagating setup thus enables a higher spatial resolution. But the approach with the counter propagating pump is harder to align than the setup depicted in Figure 1, where the probe and pump beam automatically fulfill the Bragg condition if focused correctly at the grating.
When modifying the pump beam profile for a better spectral resolution and SNR, it is crucial to bear in mind the impact on the spatial resolution. Altering the initial pump profile by expanding it along the x-axis using a cylindrical lens leads to enhancements in spectral resolution and spatial resolution along the y-axis. However, it also results in a notable decrease in spatial resolution along the x-axis. This decline is mostly influenced by the crosstalk of the different media and strongly sample dependent. The transmission efficiency of the interface and the attenuation of the material affect the amount of crosstalk. In our case this leads to a spatial resolution of 180 lm in x-direction.
The final challenge to enable fast imaging is to find an alternative to stage scanning, which limits the frame rate at the moment. The hurdle is, that during scanning of the probe beam the Bragg condition needs to be fulfilled. Thus, the beams incident to the grating should just shift laterally orthogonal to the grating, which can be realized with a 2D galvo scanning system.
7 Conclusion
In this paper, we investigated the influences of the excitation parameters of ISBS on the SNR and how this effects the temporal and spatial resolution. High temporal resolution is important to achieve high-throughput measurements.
The pulse energy and average power are the most significant excitation parameters for increasing the signal strength. Using longer pulses length lowers the peak power of the pulse, which increases the maximum allowed pulse energy. Through the introduction of an line shaped pump profile in the x-direction the spectral resolution got improved by a factor of 4 with probe beam size of 40 40 lm 2. The probe beam size is currently limited by the focal length of the used optics, but can be improved further to 14 14 lm 2 [ 29 ]. By also spreading the pulse energy over a larger area, higher pulse energies are possible, which positively influence the SNR. The here presented temporal resolution of 20 ls / pixel of ISBS is two orders of magnitude higher compared to SpBM and one order of magnitude higher than the previous best temporal resolution reported for ISBS [ 29 ]. In combination with a detector array, line measurements of 100 pixel would result in an effective measurement rate of 200 ns / pixel, which would enable real-time imaging.
Acknowledgments
The authors would like to thank Anna Taubenberger( Biotechnology Center, TU Dresden) for providing hydrogel samples.
Funding Deutsche Forschungsgemeinschaft( Cz55 / 44-1).
Conflicts of interest The authors declare no conflicts of interest.
Data availability statement
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Author contribution statement All authors made significant contributions to this work.
References
1 Kennedy BF, Wijesinghe P, Sampson DD, The emergence of optical elastography in biomedicine, Nat. Photonics 11, 4( 2017). https:// doi. org / 10.1038 / nphoton. 2017.6.