Optically-Generated Bessel Beam Ultrasound (OBUS)

• OBUS is an optoacoustic technique where short laser pulses generate ultrasound that is shaped into elongated, narrow Bessel‐like beams.
• It combines optical absorption with axicon and annular beam synthesis to extend depth-of-focus while maintaining high lateral resolution.
• Experimental implementations demonstrate high peak pressures and improved transcranial performance compared to conventional Gaussian beams.

Searching arXiv for recent and foundational papers on optically-generated Bessel beam ultrasound and closely related Bessel-beam optoacoustics. Optically-generated Bessel Beam Ultrasound (OBUS) denotes an optoacoustic strategy in which short laser pulses are converted into ultrasound by an absorbing emitter or by optical absorption in tissue, while the resulting excitation or detection geometry is engineered to produce Bessel-like, elongated, weakly diffracting acoustic fields rather than a conventional Gaussian-like focal spot. In current arXiv literature, the term is explicit in a miniaturized transcranial neuromodulation device that uses a conical optoacoustic emitter to form a column-shaped acoustic field (Li et al., 8 Jul 2025), while closely related work in photoacoustic microscopy and simulation establishes the underlying matched Bessel-beam illumination, extended depth-of-focus, and annular-source design logic that OBUS exploits (Ali et al., 2021, Song et al., 2020). More general nondiffracting acoustic beam theory further situates OBUS within the broader class of Bessel-beam and Frozen-Wave synthesis methods based on annular apertures, axicons, and superpositions of Bessel modes (Prego et al., 2012, Jiménez et al., 2014).

1. Definition and conceptual scope

In optoacoustics, short laser pulses are absorbed and converted into broadband ultrasound. The initial pressure $p_0$ generated in the absorbing layer is given by

$p_0 = \Gamma \,\mu_a\, F$

where $\Gamma$ is the Grüneisen parameter, $\mu_a$ is the optical absorption coefficient, and $F$ is the optical fluence (Li et al., 8 Jul 2025). OBUS uses this optoacoustic conversion together with Bessel-beam or axicon-type field shaping so that the generated ultrasound is not confined to a single diffraction-limited focal plane, but instead forms an elongated axial region with a narrow lateral core (Li et al., 8 Jul 2025, Ali et al., 2021).

The OBUS concept therefore spans two closely connected implementations. In one implementation, a purpose-built optoacoustic emitter directly generates a Bessel-like ultrasound beam, as in the miniaturized, fiber-driven transcranial stimulation device reported in 2025 (Li et al., 8 Jul 2025). In the other, an optical Bessel beam creates an elongated optoacoustic source distribution in the sample, and an acoustic axicon detector provides a matched elongated sensitivity field; this produces an elongated optoacoustic focus in the optical-resolution regime (Ali et al., 2021). This suggests that OBUS is best understood not as a single hardware archetype, but as a family of co-designed optical–acoustic systems in which the effective point-spread function is long in $z$ and narrow in $x,y$.

A recurrent distinction in the literature is between an ideal Bessel beam and a practical Bessel-like beam. The ideal zeroth-order acoustic Bessel beam has radial profile

$p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$

with a central core and concentric side lobes, together with extended depth of field and self-healing (Li et al., 8 Jul 2025). Finite apertures, finite annular structures, and real materials instead generate Bessel-like beams that approximate this behavior over a finite interval and typically exhibit truncation, multiple foci, or side-lobe artifacts (Jiménez et al., 2014, Prego et al., 2012).

2. Physical basis: optoacoustic generation and Bessel-beam shaping

OBUS relies on the optoacoustic effect, in which absorbed optical energy produces a rapid temperature rise, thermoelastic expansion, and a broadband ultrasound pulse (Li et al., 8 Jul 2025). In solids, the Grüneisen parameter is reported as proportional to the bulk modulus $K$,

$\Gamma \propto \frac{\beta K}{\rho C_v \kappa},$

with

$p_0 = \Gamma \,\mu_a\, F$0

so emitter stiffness can influence optoacoustic efficiency for a given absorber and fluence (Li et al., 8 Jul 2025). In the miniaturized OBUS device, the absorbing composite is a candle-soot–PDMS emitter, and the PDMS base:curing agent ratio was tuned between 2:1 and 10:1; experimentally, an 8:1 ratio yielded the highest peak-to-peak pressure (Li et al., 8 Jul 2025).

The Bessel aspect of OBUS arises from conical or annular geometry. In the miniaturized device, a conical optoacoustic emitting surface acts as an acoustic axicon: the superposition of conical waves along the axis generates a Bessel-like beam (Li et al., 8 Jul 2025). The depth of focus is linked to geometry through the empirical design relation

$p_0 = \Gamma \,\mu_a\, F$1

where $p_0 = \Gamma \,\mu_a\, F$2 is the radius of the conical surface and $p_0 = \Gamma \,\mu_a\, F$3 is the conical half-angle (Li et al., 8 Jul 2025). In matched Bessel-beam optoacoustic microscopy, an optical axicon lens forms a Bessel beam whose central lobe has approximately $p_0 = \Gamma \,\mu_a\, F$4 FWHM over an 8 mm propagation range, while an acoustic axicon transducer produces a thin cylindrical sensitivity field or “pencil beam” (Ali et al., 2021).

Related simulation work describes Bessel-beam generation using the ring slit method. Because the Fourier transform of a ring is a zero-order Bessel function, placing a ring slit on the back focal plane of a lens can convert incident light into a Bessel beam (Song et al., 2020). The field near the front focal plane is described as a Fourier–Bessel-type integral,

$p_0 = \Gamma \,\mu_a\, F$5

with $p_0 = \Gamma \,\mu_a\, F$6 the annular pupil function (Song et al., 2020). This provides an optical analogue of annular-source synthesis and clarifies how ring geometry controls depth of field.

3. Beam synthesis architectures

Three architectures dominate the literature relevant to OBUS: conical optoacoustic emitters, matched optical-and-acoustic axicon systems, and annular-superposition methods.

The explicit OBUS device is a miniaturized, fiber-driven optoacoustic source. Its outer diameter is 2.33 mm and its weight is 2.1 mg, or 167.6 mg including a 3D-printed adapter (Li et al., 8 Jul 2025). The emitter is a conical candle-soot–PDMS structure with radius $p_0 = \Gamma \,\mu_a\, F$7 mm and conical half-angle $p_0 = \Gamma \,\mu_a\, F$8, illuminated by a 400 $p_0 = \Gamma \,\mu_a\, F$9m core multimode fiber using 1064 nm, 2.2 ns laser pulses (Li et al., 8 Jul 2025). The optical delivery system is arranged to ensure uniform illumination over the conical surface (Li et al., 8 Jul 2025).

The matched elongated-focus optoacoustic microscopy system uses interchangeable optical illumination and acoustic detection units (Ali et al., 2021). Gaussian illumination is produced by a 10$\Gamma$0 Plan Achromat objective with NA = 0.25, while Bessel illumination is produced by replacing the objective with an axicon lens (Ali et al., 2021). The spherical transducer has an 8 mm LiNbO$\Gamma$1 element, a spherical acoustic lens with 6 mm curvature radius, and a center frequency of 60.5 MHz; the axicon transducer has a 9.6 mm LiNbO$\Gamma$2 element, a conical acoustic lens with apex angle 114.4°, and a center frequency of 61 MHz (Ali et al., 2021). The four tested configurations were Gaussian–Spherical, Gaussian–Axicon, Bessel–Spherical, and Bessel–Axicon (Ali et al., 2021).

Annular-source methods provide the general wave-synthesis framework behind both systems. In Frozen-Wave theory, a monochromatic acoustic field is synthesized as a finite superposition of equal-frequency Bessel beams,

$\Gamma$3

with

$\Gamma$4

and longitudinal wavenumbers chosen as

$\Gamma$5

to realize arbitrary axial envelopes within $\Gamma$6 (Prego et al., 2012). Separately, an axisymmetric grating of concentric rigid tori produces Bessel-like acoustic beams through the grating relation

$\Gamma$7

and the focal-distance law

$\Gamma$8

showing how radial periodicity, wavelength, and ring radius determine the elongated focus (Jiménez et al., 2014).

4. Quantitative beam characteristics and performance

The explicit OBUS device reported in 2025 produces a column-shaped field with lateral resolution 152 $\Gamma$9m and axial resolution 1.93 mm (Li et al., 8 Jul 2025). Under 1064 nm, 2.2 ns, 61 $\mu_a$0J/cm$\mu_a$1, 1 kHz operation in water, the measured peak-to-peak pressure at 2 mm from the OBUS surface was 4.1 MPa, with center frequency 10.6 MHz and $\mu_a$2 dB bandwidth 5–30 MHz, approximately 250% relative bandwidth (Li et al., 8 Jul 2025). Simulations performed with $\mu_a$3 mm showed that varying cone angle changes the lateral and axial resolutions and the peak position: for $\mu_a$4, the simulated lateral resolution was 0.33 mm, axial resolution 4.49 mm, and peak position 2.06 mm (Li et al., 8 Jul 2025).

In elongated-focus optoacoustic microscopy, the optical Bessel beam central lobe had approximately $\mu_a$5 FWHM over an axial range of 8 mm, while the Gaussian beam had a measured optical DOF of 108 $\mu_a$6m (Ali et al., 2021). The full optoacoustic depth-of-focus, defined as the axial range at which the normalized MIP is above 50% of maximum, was 75 $\mu_a$7m for Gaussian–Spherical, 75 $\mu_a$8m for Gaussian–Axicon, 662 $\mu_a$9m for Bessel–Spherical, and 1275 $F$0m for Bessel–Axicon (Ali et al., 2021). This was reported as a 17-fold extension over traditional configurations (Ali et al., 2021). Lateral resolution from a 7 $F$1m carbon fiber phantom was approximately 7 $F$2m at best focus for all configurations, and for Bessel–Axicon the FWHM remained approximately 7 $F$3m over almost the entire field of view, degrading to 9–10 $F$4m only near the far end where SNR was low and the carbon fiber bent (Ali et al., 2021).

The simulation platform for Bessel-beam photoacoustic microscopy emphasizes the trade-off between ring width and depth of field (Song et al., 2020). With outer ring radius 5 mm, slit widths of 150 $F$5m and 400 $F$6m, focal length 40 mm, and wavelength 582 nm, the lateral FWHM in the focal plane was approximately 1.2 $F$7m for the 400 $F$8m slit, and the qualitative result was that as slit width increases, the DoF decreases (Song et al., 2020). The same work reported that a true vessel width of approximately 2 $F$9m was measured as approximately 9.5 $z$0m in the reconstructed image because the side lobe of Bessel beams deteriorated the resolution (Song et al., 2020).

The following table organizes the principal quantitative configurations reported across the most directly relevant studies.

System	Key beam metrics	Reported outcome
Miniaturized OBUS (Li et al., 8 Jul 2025)	152 $z$1m lateral resolution; 1.93 mm axial resolution; 4.1 MPa peak-to-peak; 10.6 MHz center frequency	Column-shaped field for volumetric transcranial stimulation
Bessel–Axicon optoacoustic microscopy (Ali et al., 2021)	1275 $z$2m DOF; approximately 7 $z$3m lateral resolution; optical Bessel central lobe approximately $z$4m over 8 mm	17-fold DOF extension over Gaussian–Spherical
Ring-slit Bessel-beam simulation (Song et al., 2020)	approximately 1.2 $z$5m lateral FWHM; wider slit gives shorter DoF	Large volumetric image by point scanning

These results show a consistent theme: Bessel or Bessel-like shaping mainly expands usable axial extent while keeping a relatively narrow lateral core, although the achievable resolution and frequency regime vary greatly between microscopy-scale and neuromodulation-scale systems (Li et al., 8 Jul 2025, Ali et al., 2021, Song et al., 2020).

5. Transcranial stimulation, volumetric control, and biological validation

The most explicit biomedical application of OBUS is volumetric transcranial neuromodulation in rodents (Li et al., 8 Jul 2025). The reported device was designed to address trade-offs between miniaturization versus volumetric control and spatial resolution versus transcranial capability (Li et al., 8 Jul 2025). Immunofluorescence imaging of mouse brain slices confirmed the ability to stimulate cells at a depth of 2.2 mm (Li et al., 8 Jul 2025). In vivo c-Fos experiments further reported significant difference in Pearson’s coefficient between stimulated and control regions up to 2.2 mm depth, and when the 0.6 mm gel gap was added, the most activated region corresponded to approximately 2.8 mm from the OBUS device (Li et al., 8 Jul 2025).

Electrophysiological recordings in mice reported increased LFP amplitude and higher-frequency power in the 10–50 Hz band during OBUS stimulation, with significant increases at approximately 47 Hz during 3.7 and 4.1 MPa stimulation (Li et al., 8 Jul 2025). fMRI in rats showed localized activation directly underneath the OBUS device, and the BOLD signal amplitude was described as comparable to positive-control electrical hind paw stimulation (Li et al., 8 Jul 2025). The reported stimulation paradigms used 1 kHz pulse repetition frequency and broadband optoacoustic pulses, with pressures ranging from 2.8 to 5 MPa peak-to-peak depending on the experiment (Li et al., 8 Jul 2025).

Transcranial comparison with a conventional Gaussian beam was performed in simulation using the same 10 MHz center frequency and 250% bandwidth, and the same nominal focus depth of 4.8 mm (Li et al., 8 Jul 2025). Peak intensity transmission efficiency after skull versus before skull was 18.7% for OBUS and 11.0% for Gaussian ultrasound (Li et al., 8 Jul 2025). The Gaussian beam exhibited severe axial broadening after the skull, with axial FWHM increasing from 1.05 mm to 5.95 mm, whereas OBUS changed from 6.48 mm pre-skull to 2.63 mm post-skull (Li et al., 8 Jul 2025). A plausible implication is that Bessel-like columnar fields can preserve a more interpretable volume of tissue activation under skull-induced aberration than tightly focused Gaussian beams in the same frequency regime.

The optoacoustic microscopy literature offers a related but distinct notion of volumetric control. Imaging a tilted mouse ear with the matched Bessel–Axicon configuration revealed vasculature over an imaging depth exceeding 4.2 mm with optical resolution and afforded a 6-fold increase in imaging volume over the same scanning duration compared to Gaussian illumination (Ali et al., 2021). This is not neuromodulation, but it demonstrates the same systems-level advantage: an elongated excitation–detection overlap reduces the need for repeated axial refocusing.

6. Theoretical frameworks and design equations

Bessel-beam OBUS is supported by several complementary theoretical descriptions.

For optical generation in photoacoustics, the standard initial-pressure relation

$z$6

is implicit in simulation workflows and explicitly given in the miniaturized OBUS device (Li et al., 8 Jul 2025, Song et al., 2020). In the Bessel-beam simulation platform, the optical fluence corresponds to the Bessel intensity distribution, and the absorber distribution corresponds to a vascular phantom, so the initial pressure effectively follows the product of Bessel illumination and absorber geometry (Song et al., 2020).

For matched optical–acoustic imaging, the effective optoacoustic point-spread function is described as the product of optical fluence and acoustic sensitivity,

$z$7

so a Bessel beam illumination profile matched to an acoustic axicon pencil beam yields a depth-invariant lateral PSF over approximately 1.3 mm measured DOF (Ali et al., 2021). This is the central co-design principle behind elongated-focus optoacoustic microscopy.

For general acoustic Bessel-beam synthesis, Frozen-Wave theory provides a formal route to arbitrary longitudinal intensity shaping (Prego et al., 2012). A desired axial envelope $z$8 over $z$9 is synthesized by choosing

$x,y$0

computing

$x,y$1

and setting the coefficients

$x,y$2

The resulting field can realize static longitudinal envelopes, including step-like, concave, multi-peak, and exponentially rising patterns (Prego et al., 2012). This suggests that future OBUS systems could move beyond a single zeroth-order Bessel-like column toward more general axially programmed acoustic fields if coherent optoacoustic synthesis of multiple components becomes practical.

Axisymmetric gratings provide a simpler passive design law. With ring periodicity $x,y$3, diffraction order $x,y$4, wavelength $x,y$5, and ring radius $x,y$6, the focal segment is determined by

$x,y$7

(Jiménez et al., 2014). This formulation makes explicit how aperture extent and acoustic wavelength set the start and end of the elongated focal line. It is directly relevant to annular optical absorption patterns or photoacoustic ring sources that would emulate an axisymmetric grating in OBUS.

7. Limitations, trade-offs, and open directions

A persistent limitation of Bessel and Bessel-like beams is side-lobe energy. In the ring-slit simulation platform, side lobes blurred a vessel of approximately 2 $x,y$8m width to approximately 9.5 $x,y$9m (Song et al., 2020). In elongated-focus microscopy, side lobes of the optical Bessel beam generated secondary optoacoustic signals that degraded lateral resolution, motivating blind deconvolution using the measured Bessel beam profile as an initial PSF estimate (Ali et al., 2021). After deconvolution, vessel width FWHM improved from 132 $p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$0m to 75 $p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$1m and background decreased, with SNR improving by 2.1 dB (Ali et al., 2021). Side-lobe suppression is therefore not ancillary but central to practical OBUS design.

Another trade-off is between depth of focus, aperture, and localization. In ring-slit Bessel generation, increasing slit width decreased DoF (Song et al., 2020). In Frozen-Wave synthesis, increasing the number of modes $p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$2 improves axial-envelope fidelity but rapidly increases the minimum required aperture radius and the complexity of the annular source (Prego et al., 2012). In the miniaturized OBUS device, reducing diameter from 12.2 mm to 2.33 mm was reported to leave maximum intensity nearly unchanged while mainly shortening DOF, a behavior contrasted with conventional optoacoustic focused pads (Li et al., 8 Jul 2025). This suggests that miniaturization is unusually compatible with Bessel-like operation, but only within a design space where column length remains sufficient for the target anatomy.

Frequency scaling remains application-dependent. The transcranial OBUS device operates around 10.6 MHz with 5–30 MHz bandwidth and achieves 152 $p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$3m lateral resolution (Li et al., 8 Jul 2025), whereas the microscopy systems operate in the 50–61 MHz regime to achieve approximately 7 $p(r,z) \propto J_0(k_r r)\, e^{i k_z z}$4m lateral resolution (Ali et al., 2021, Song et al., 2020). This suggests that translation to larger animals or humans would likely require lower-frequency Bessel designs, with corresponding increases in core width and changes in skull transmission behavior. That implication is explicitly anticipated in the OBUS study, which notes that scaling to larger animals and humans may need lower-frequency Bessel designs (Li et al., 8 Jul 2025).

Safety and thermal loading are also active constraints. For the reported OBUS neuromodulation regime, the maximum in vivo peak-to-peak pressure was limited to 5 MPa, with MI approximately 0.93 at 5 MPa, below the FDA diagnostic ultrasound limit of 1.9 (Li et al., 8 Jul 2025). Water-tank thermal measurements gave a surface rise of 1.2–2.2 K and approximately 0.5 K at 0.5 mm depth, with less than 1 K at depths relevant to the volume of tissue activation (Li et al., 8 Jul 2025). These are encouraging values, but they remain specific to the tested geometry, duty cycles, and rodent-scale exposures.

The current literature therefore presents OBUS as a convergence of optoacoustic materials engineering, axicon and annular beam synthesis, and volumetric field design. Its defining technical proposition is that optical generation can be coupled to Bessel-like acoustic propagation to produce a narrow but elongated activation or imaging column, with performance benefits in depth coverage, tolerance to axial misalignment, and, in some regimes, transcranial robustness (Li et al., 8 Jul 2025, Ali et al., 2021). The broader Bessel-beam and Frozen-Wave literature indicates that more elaborate axial shaping is mathematically available (Prego et al., 2012, Jiménez et al., 2014). A plausible implication is that future OBUS systems will increasingly be judged not only by peak pressure or focal width, but by how precisely they can program the longitudinal envelope, side-lobe distribution, and tissue-specific volume of interaction.