Wavelength-multiplexed pumping with 478- and 520-nm indium gallium nitride laser diodes for Ti:sapphire laser

High-efficiency 940- and 969-nm brightness-maintaining wavelength-multiplexed LD-pumped 240-W thin-rod Yb:YAG amplifier

A high-efficiency ultrafast laser amplifier based on thin-rod Yb:YAG was demonstrated, featuring a 940- and 969-nm brightness-maintaining wavelength-multiplexed laser diode (LD)-pumping method. Two high-brightness LDs (940 nm and 969 nm) were spectrally combined into one beam spatially with a dichroic mirror, thus enabling twice pump power while maintaining high brightness. A maximum signal power of 240 W was obtained at a repetition rate of 1 MHz, with a power extraction efficiency (PEE) of ∼51%. To the best of the authors' knowledge, this is the first report of >50% efficiency as well as the highest average power operating at the fundamental mode for thin-rod Yb:YAG amplifiers. The beam quality factors (M²) of the amplified signal were measured to be 1.72 and 1.12 for the horizontal and vertical directions, respectively. A preliminary pulse compression was conducted at a signal power of 80 W with a chirped volume Bragg grating (CVBG) compressor. The compressed pulse duration was 744 fs with an average power of 66.5 W, corresponding to a compression efficiency of 83.1%.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Spectrally combined three-diode-pumped compact femtosecond Ti:sapphire laser exceeding 1 W mode-locked power

The achievement of a high average power exceeding 1 W remains a major challenge for direct-diode pumped and mode-locked femtosecond Ti:sapphire lasers. Herein, we demonstrate high-power soliton-like pulses from a direct spectrally combined three-diode-pumped and semiconductor saturable absorber mirror (SESAM)-based mode-locked Ti:sapphire laser. Its mode-locked output power of up to 1 W was obtained in correspondence with a 68.8 MHz repetition rate and 55 fs pulse duration; thus, the pulse energy and peak power are 14.5 nJ and 264 kW, respectively. To the best of our knowledge, this is the highest reported output power and pulse energy from a Ti:sapphire laser with three spectrally combined pump diodes (471 nm, 491 nm, and 525 nm) and a simple beam expander. For efficient pumping, the combined pump beam, directed into the lens (f = 60 mm), which comprised three aspheric lenses along the fast axis and a shared cylindrical beam telescope (8× magnification) along the slow axis, resulting in a circular-focused beam in the Ti:sapphire crystal. The beam waist was measured to be 39 μm ×38 μm along the slow and fast axes.

Influence of pump beam shaping and noise on performance of a direct diode-pumped ultrafast Ti:sapphire laser

One of the major factors that limits the widespread use of ultrafast titanium sapphire (Ti:S) lasers in life science is its expensive and complex pump source. Broad area diode laser (BA-DL) based pump sources have high potential to solve this problem, since they are compact, inexpensive and very efficient. However, their non-diffraction limited beam profile makes it challenging to achieve high powers in Kerr-lens mode-locked (KLM) operation of Ti:S lasers. In this work, we show that the ideal way to beam shape two spectrally combined BA-DLs with different beam qualities is to aim for a compromise between matching to the cavity mode diameter and the cavity mode Rayleigh range. We furthermore conclude that the relative intensity noise (RIN) of the BA-DL pumped Ti:S laser, another important parameter for imaging applications, is sufficiently low for a wide range of life science applications. However, for applications that are highly sensitive to noise, new laser diode designs are likely necessary to reduce inherent noise originating within the laser diode.

Depth resolved label-free multimodal optical imaging platform to study morpho-molecular composition of tissue

Multimodal imaging platforms offer a vast array of tissue information in a single image acquisition by combining complementary imaging techniques. By merging different systems, better tissue characterization can be achieved than is possible by the constituent imaging modalities alone. The combination of optical coherence tomography (OCT) with non-linear optical imaging (NLOI) techniques such as two-photon excited fluorescence (TPEF), second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) provides access to detailed information of tissue structure and molecular composition in a fast, label-free and non-invasive manner. We introduce a multimodal label-free approach for morpho-molecular imaging and spectroscopy and validate the system in mouse skin demonstrating the potential of the system for colocalized acquisition of OCT and NLOI signals.

Alexandrite microchip lasers

We report on a continuous-wave alexandrite (Cr³⁺:BeAl₂O₄) microchip lasers operating at 680.4 nm and 749.5 nm laser wavelengths. Microchip resonators were realized by dielectric mirrors directly deposited on the alexandrite crystal surfaces. InGaN laser diode providing up to 3.5 W of the output power at ∼445 nm wavelength was used as a pump source. More than 210 mW and 570 mW of the laser radiation have been extracted from the microchip laser systems at 680.4 nm and 749.5 nm wavelengths, respectively. The corresponding slope efficiencies related to absorbed pump power were 15 % and 39 %.

Ultrafast diode-pumped Ti:sapphire laser with broad tunability

We report a broadly wavelength-tunable femtosecond diode-pumped Ti:sapphire laser, passively mode-locked using both semiconductor saturable absorber mirror (SESAM) and Kerr-lens mode-locking (KLM) techniques. Using two pump laser diodes (operating at 450 nm), an average output power as high as 433 mW is generated during mode-locking with the SESAM. A tunability range of 37 nm (788-825 nm) was achieved with the shortest pulse duration of 62 fs at 812 nm. In the KLM regime, an average output power as high as 382 mW, pulses as short as 54 fs, and a tunability of 120 nm (755-875 nm) are demonstrated.

Direct diode-pumped Kerr Lens 13 fs Ti:sapphire ultrafast oscillator using a single blue laser diode

We demonstrate a direct diode-pumped Kerr Lens Modelocked Ti:sapphire laser producing 13 fs pulses with 1.85 nJ energy at 78 MHz (145 mW) using a single laser diode pump. We also present a similar laser using three spectrally combined diodes, generating >300 mW output power with >50 nm bandwidth. We discuss the use of far-from TEM₀₀ pump laser sources, and their effect on the Kerr lens modelocking process.