All-reflective multiphoton microscope

All-reflective freeform microscope objective for ultra-broadband microscopy

Conventional refractive microscope objective lenses have limited applicability to a range of imaging modalities due to the dispersive nature of their optical elements. Designing a conventional refractive microscope objective that provides well-corrected imaging over a broad spectral range can be challenging, if not impossible. In contrast, reflective optics are inherently achromatic, so a system composed entirely of reflective elements is free from chromatic aberrations and, as a result, can image over an ultra-wide spectral range with perfect color correction. This study explores the design space of unobscured high numerical aperture, all-reflective microscope objectives. In particular, using freeform optical elements we obviate the need for a center obscuration, rendering the objective's modulation transfer function comparable to that of refractive lens systems of similar numerical aperture. We detail the design process of the reflective objective, from determining the design specifications to the system optimization and sensitivity analysis. The outcome is an all-reflective freeform microscope objective lens with a 0.65 numerical aperture that provides diffraction-limited imaging and is compatible with the geometric constraints of commercial microscope systems.

Reflective multi-immersion microscope objectives inspired by the Schmidt telescope

Imaging large, cleared samples requires microscope objectives that combine a large field of view (FOV) with a long working distance (WD) and a high numerical aperture (NA). Ideally, such objectives should be compatible with a wide range of immersion media, which is challenging to achieve with conventional lens-based objective designs. Here we introduce the multi-immersion 'Schmidt objective' consisting of a spherical mirror and an aspherical correction plate as a solution to this problem. We demonstrate that a multi-photon variant of the Schmidt objective is compatible with all homogeneous immersion media and achieves an NA of 1.08 at a refractive index of 1.56, 1.1-mm FOV and 11-mm WD. We highlight its versatility by imaging cleared samples in various media ranging from air and water to benzyl alcohol/benzyl benzoate, dibenzyl ether and ethyl cinnamate and by imaging of neuronal activity in larval zebrafish in vivo. In principle, the concept can be extended to any imaging modality, including wide-field, confocal and light-sheet microscopy.

Chromatically Corrected Multicolor Multiphoton Microscopy

Simultaneous imaging of multiple labels in tissues is key to studying complex biological processes. Although strategies for color multiphoton excitation have been established, chromatic aberration remains a major problem when multiple excitation wavelengths are used in a scanning microscope. Chromatic aberration introduces a spatial shift between the foci of beams of different wavelengths that varies across the field of view, severely degrading the performance of color imaging. In this work, we propose an adaptive correction strategy that solves this problem in two-beam microscopy techniques. Axial chromatic aberration is corrected by a refractive phase mask that introduces pure defocus into one beam, while lateral chromatic aberration is corrected by a piezoelectric mirror that dynamically compensates for lateral shifts during scanning. We show that this light-efficient approach allows seamless chromatic correction over the entire field of view of different multiphoton objectives without compromising spatial and temporal resolution and that the effective area for beam-mixing processes can be increased by more than 1 order of magnitude. We illustrate this approach with simultaneous three-color, two-photon imaging of developing zebrafish embryos and fixed Brainbow mouse brain slices over large areas. These results establish a robust and efficient method for chromatically corrected multiphoton imaging.

Harmonic mode-locked noise-like pulses under a Q-switched envelope in an erbium doped all-fiber ring laser

We report for the first time, to the best of our knowledge, harmonic mode-locked noise-like pulses under a Q-switched envelope in an all-fiber erbium doped ring laser cavity, mode locked using the nonlinear polarization rotation (NPR) technique. For a cavity with a fundamental repetition rate of 1.33 MHz, stable mode-locked noise-like pulses, with few nanoseconds durations, single pulse energies around 30-40 nJ, and Q-switched repetition rates up to 31 kHz, were produced and characterized from the fundamental to the eighth harmonic. The formation and evolution of Q-switched harmonic mode-locked noise-like pulses were preceded by the formation of Q-switched mode-locked noise-like pulse bunches. This is also the first report on noise-like bunches under a Q-switched envelope, to our knowledge. Our studies also provide further insights into the interplay of different physical mechanisms involved in the production of such ultrashort pulses. Such sources should prove especially useful for efficient supercontinuum generation and laser micromachining.

Soliton microcomb based spectral domain optical coherence tomography

Spectral domain optical coherence tomography (OCT) is a widely employed, minimally invasive bio-medical imaging technique, which requires a broadband light source, typically implemented by super-luminescent diodes. Recent advances in soliton based photonic integrated frequency combs (soliton microcombs) have enabled the development of low-noise, broadband chipscale frequency comb sources, whose potential for OCT imaging has not yet been unexplored. Here, we explore the use of dissipative Kerr soliton microcombs in spectral domain OCT and show that, by using photonic chipscale Si₃N₄ resonators in conjunction with 1300 nm pump lasers, spectral bandwidths exceeding those of commercial OCT sources are possible. We characterized the exceptional noise properties of our source (in comparison to conventional OCT sources) and demonstrate that the soliton states in microresonators exhibit a residual intensity noise floor at high offset frequencies that is ca. 3 dB lower than a traditional OCT source at identical power, and can exhibit significantly lower noise performance for powers at the milli-Watt level. Moreover, we demonstrate that classical amplitude noise of all soliton comb teeth are correlated, i.e., common mode, in contrast to superluminescent diodes or incoherent microcomb states, which opens a new avenue to improve imaging speed and performance beyond the thermal noise limit.

Superluminescent diodes, that provide a broadband spectrum are typically used in spectral domain coherence tomography. Here, the authors use chipscale silicon nitride resonators to generate soliton microcombs with a lower noise flor that could substitute the diode sources.

Characterization of multiphoton microscopes by the nonlinear knife-edge technique

Imaging submicron fluorescent microspheres are the standard method for measuring resolution in multiphoton microscopy. However, when using high-energy pulsed lasers, photobleaching and heating of the solution medium may deteriorate the images, resulting in an inaccurate resolution measurement. Moreover, due to the weak higher-order response of fluorescent microspheres, measuring three-photon resolution using three-photon fluorescence (3PEF) and third-harmonic generation (THG) signals is more difficult. In this report, we demonstrate a methodology for complete characterization of multiphoton microscopes based on second- and third-harmonic generation signals from the sharp edge of GaAs wafers. This simple methodology, which we call the nonlinear knife-edge technique, provides fast and consistent lateral and axial resolution measurement with negligible photobleaching effect on semiconductor wafers. In addition, this technique provides information on the field curvature of the imaging system, and perhaps other distortions of the imaging system, adding greater capability compared to existing techniques.

Single-cavity dual-wavelength all-fiber femtosecond laser for multimodal multiphoton microscopy

A single-cavity dual-wavelength all-fiber femtosecond laser is designed to generate 1030 nm wavelength for high resolution multiphoton imaging and 1700 nm wavelength for long penetration depth imaging. Considering two-photon and three-photon microscopy (2PM and 3PM), the proposed laser provides the single-photon wavelength equivalent to 343 nm, 515 nm, 566 nm and 850 nm, that can be employed to excite a wide variety of intrinsic fluorophores, dyes, and fluorescent proteins. Generating two excitation wavelengths from a single laser reduces the footprint and cost significantly compared to having two separate lasers. Furthermore, an all-reflective microscope is designed to eliminate the chromatic aberration while employing two excitation wavelengths. The compact all-fiber alignment-free laser design makes the overall size of the microscope appropriate for clinical applications.

Improved nondegenerate multiphoton microscopy and axial registration with a reflective objective

Conventional, degenerate multiphoton microscopy (D-MPM) requires the use of a high-numerical-aperture (NA) objective. Nondegenerate MPM (ND-MPM) imposes the additional demand for precise spatiotemporal overlap of two distinct excitation sources. We demonstrate that the axial focal shift introduced by refractive objective chromatic aberration hinders the spatial requirement of ND-MPM, whereas the use of a reflective objective overcomes this challenge and allows for improved ND excitation efficiency in spite of a lower NA. Moreover, we demonstrate that reflective objective focusing eliminates the axial misregistration of volumetric stacks in traditional D-MPM experiments when multiple excitation wavelengths are used.

Diagnosing pituitary adenoma in unstained sections based on multiphoton microscopy

PURPOSE

If we can find a new method that can achieve rapid diagnosis of adenoma during operation, it will help surgeon shorten the operation time and enhance the treatment efficacy. This study discusses the feasibility of multiphoton microscopy (MPM) in diagnosing pituitary adenoma.

METHOD

MPM, based on two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) is performed for the diagnosis of pituitary adenoma in unstained sections.

RESULTS

Our results show that MPM can reveal the variation of reticulin fiber by SHG signals of collagen, combined with the measurement of area of acinus, thickness of collagen fiber and collagen percentage. MPM can further reflect the change of meshwork in normal pituitary and hyperplasia quantitatively. And the characteristics of typical growth patterns of pituitary adenoma are demonstrated by the overlay of SHG and TPEF images. What's more, we can identify the boundary of normal pituitary, hyperplasia and adenoma from MPM images. And the experiment also results verify the feasibility of this method in frozen sections.

CONCLUSION

These results indicated that MPM can make a diagnosis of pituitary adenoma by the morphological changes without routine pathological processing including hematoxylin-eosin (H&E) staining and other special staining. Therefore, this technique is expected to help diagnosis of pituitary adenoma during operation.

Application of a reflective microscope objective for multiphoton microscopy

Reflective objectives (ROs) mitigate chromatic aberration across a broad wavelength range. Yet, a systematic performance characterisation of ROs has not been done. In this paper, we compare the performance of a 0.5 numerical-aperture (NA) reflective objective (RO) with a 0.55 NA standard glass objective (SO), using two-photon fluorescence (TPF) and second-harmonic generation (SHG). For experiments spanning ∼1 octave in the visible and NIR wavelengths, the SO leads to defocusing errors of 25-40% for TPF images of subdiffraction fluorescent beads and 10-12% for SHG images of collagen fibres. The corresponding error for the RO is ∼4% for both imaging modalities. This work emphasises the potential utility of ROs for multimodal multiphoton microscopy applications.