Direct diode-laser pumping of a mode-locked Ti:sapphire laser

Watt-level tunable Ti:Sapphire laser directly pumped with green laser diodes

We demonstrate a Ti:Sapphire laser generating in excess of 1.2 W in continuous-wave operation when pumped directly with four green laser diodes eliminating the need for a complex pump laser. As a result, improvement of laser efficiency is achieved without sacrificing beam quality. Tunability within the range of 740-840 nm is attained validating the concept of a direct laser-diode pumped Ti:Sapphire laser.

Spectrally combined four-diode-pumped femtosecond Ti:sapphire laser with 16.3 nJ pulse and its application to video-rate coherent anti-Stokes Raman scattering spectro-microscopy

We demonstrate the generation of high-energy femtosecond pulses by a spectrally combined four-diode-pumped Ti:sapphire laser with semi-conductor saturable absorber mirror mode locking. To achieve energy scaling, the laser cavity was extended using a design based on the Herriott multipass cell. The laser operates at a 17.5 MHz repetition rate and generates pulses with energies as high as 16.3 nJ and 80 fs in duration. The signal-to-noise ratio at the fundamental frequency showed an extinction ratio of >50 dB relative to the carrier. This compact single laser was applied to video-rate forward and backward coherent anti-Stokes Raman scattering spectro-microscopy with a pixel dwell time of 122 ns, which is the lowest dwell time ever achieved, to the best of our knowledge.

Three-element, self-starting Kerr-lens-modelocked 1-GHz Ti:sapphire oscillator pumped by a single laser diode

We present a Kerr-lens-modelocked, three-element, diode-pumped Ti:sapphire laser producing 111-fs pulses at a repetition frequency of 1.02 GHz. Self-starting soliton-modelocked operation with an output power of 106 mW was obtained when the laser was pumped at 1.0 W with a single 527-nm laser diode. The output exhibits a relative intensity noise of 0.06% (1 Hz - 1 MHz) and locking of the repetition rate to an external reference is demonstrated with a phase error of 1.7 mrad (1 Hz-1 MHz). The simplicity of the laser makes it an attractive candidate as a module for integration into larger systems.

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.

Characterization of a carrier-envelope-offset-stabilized blue- and green-diode-pumped Ti:sapphire frequency comb

Diode-pumping of Ti:sapphire provides a low-cost route to high-quality frequency-comb sources, exploiting the potential of direct diode modulation for wideband control of the carrier-envelope-offset frequency. We present here an f_REP- and f_CEO-locked, directly diode-pumped Ti:sapphire frequency comb, producing 66-fs pulses at 800 nm and employing f-to-2f interferometry and current modulation of a 462-nm blue laser diode to achieve a stabilization bandwidth extending to ∼70  kHz. Characterizations of the f_REP and f_CEO phase noise are compared to relative intensity noise spectra of the pump diodes to provide insights into how the diode design and performance transfer into the comb stability, suggesting a lower contribution to f_REP and f_CEO noise from the blue laser diode than from the green 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.

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.

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

We experimentally reveal the pump-induced loss in a Ti:sapphire laser crystal with 451-nm indium gallium nitride (InGaN) laser diode pumping and show that 478-nm pumping can reduce such loss. The influence of the pump-induced loss at 451-nm pumping is significant even for a crystal that exhibits higher effective figure-of-merit and excellent laser performance at 520-nm pumping. We demonstrate the power scaling of a Ti:sapphire laser by combining 478- and 520-nm InGaN laser diodes and obtain CW output power of 593 mW.

Mode-coupling enhancement by pump astigmatism correction in a Ti:Sapphire femtosecond laser

To pump a solid-state femtosecond laser cavity, a beam from a CW laser is focused by a single lens into the laser crystal. To increase the output power of the laser, the overlap of the laser mode with the pump mode should be maximized. This is particularly important in the so-called mode coupling and the Kerr-lens mode locking (KLM) operation, where the change in beam waist at the position of the gain medium is exploited to enhance the mode overlap with the pump laser in the crystal. In this paper, the astigmatism in the pump beam is reduced by tilting the pump lens. A Gaussian beam is propagated through the complete focusing system-pump lens, tilted spherical mirror, and crystal cut at Brewster's angle-to show the astigmatism inside the crystal as a function of the tilt of the pump lens. A genetic algorithm is presented to optimize the mode coupling between the pump and laser beam inside the crystal by tilting the pump lens. Experimental results are presented to verify the design, showing an increase in the output power of the laser cavity of about 20%.

Laser-diode pumped glass-clad Ti:sapphire crystal fiber laser

Efficient glass-clad crystal fiber (CF) lasers were demonstrated using a Ti:sapphire crystalline core as the gain medium. With a core diameter of 18 μm, the laser diode (LD) pump source can be effectively coupled and guided throughout the crystal fiber for a low threshold and high slope efficiency laser operation. The advantage of high heat dissipation efficiency of the fiber structure can be derived from the low core temperature rising measurement (i.e., 17 K/W) with passive cooling. At an output transmittance of 23%, the lowest absorbed threshold of 118.2 mW and highest slope efficiency of 29.6% were achieved, with linear laser polarization.

Green-diode-pumped femtosecond Ti:Sapphire laser with up to 450 mW average power

We investigate power-scaling of green-diode-pumped Ti:Sapphire lasers in continuous-wave (CW) and mode-locked operation. In a first configuration with a total pump power of up to 2 W incident onto the crystal, we achieved a CW power of up to 440 mW and self-starting mode-locking with up to 200 mW average power in 68-fs pulses using semiconductor saturable absorber mirror (SESAM) as saturable absorber. In a second configuration with up to 3 W of pump power incident onto the crystal, we achieved up to 650 mW in CW operation and up to 450 mW in 58-fs pulses using Kerr-lens mode-locking (KLM). The shortest pulse duration was 39 fs, which was achieved at 350 mW average power using KLM. The mode-locked laser generates a pulse train at repetition rates around 400 MHz. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal is actively cooled, only air cooling is applied to the pump diodes using a small fan. Because of mass production for laser displays, we expect that prices for green laser diodes will become very favorable in the near future, opening the door for low-cost Ti:Sapphire lasers. This will be highly attractive for potential mass applications such as biomedical imaging and sensing.

Highly efficient and robust operation of Kerr-lens mode-locked Cr:LiSAF lasers using gain-matched output couplers

We present efficient and robust Kerr-lens mode locking (KLM) of a diode-pumped Cr:LiSAF laser using a gain-matched output coupler (GMOC). An inexpensive, battery-powered 660 nm single-spatial-mode diode was used as the pump source. GMOC enhances the effective self-amplitude modulation depth by reducing the gain-filtering effect in broadband KLM operation to provide significant improvement in efficiency and robustness. Pulsing can be initiated without careful cavity alignment and is sustained for hours. 13 fs pulses with an average power of 25 mW have been generated using only 120 mW of pump power. The corresponding pulse energy and peak power is 200 pJ and 15 kW for the 126 MHz repetition rate cavity. Optical-to-optical conversion efficiency of the system is 21%, which represents an order of magnitude improvement in reported efficiencies for such diode-pumped ultrashort-pulse KLM Cr:LiSAF lasers. The obtainable pulse width is currently limited by the dispersion bandwidth of the available optics and can be potentially reduced to below 7 fs.

CEO stabilization of a femtosecond laser using a SESAM as fast opto-optical modulator

We present a new method for intra-cavity control of the carrier-envelope offset (CEO) frequency of ultrafast lasers that combines high feedback bandwidth with low loss, low nonlinearity, and low dispersion. A semiconductor saturable-absorber mirror (SESAM) inside a modelocked laser is optically pumped with a continuous-wave (cw) laser. In this way, the SESAM acts as intra-cavity opto-optical modulator (OOM): the optical power of the cw-laser corresponds to a high-bandwidth modulation channel for CEO frequency control. We experimentally verified this method for a femtosecond Er:Yb:glass oscillator (ERGO), in which one SESAM is in parallel used for modelocking and as intra-cavity OOM for achieving a tight CEO lock. This laser can also be CEO-stabilized in the usual scheme, in which the laser pump current is modulated, i.e., the gain element acts as intra-cavity OOM. We compare the performance with gain and SESAM OOM measuring CEO transfer function, frequency noise power spectral density (PSD), and Allan deviation for integration times up to 1000 s. In the case of the gain OOM, the millisecond upper-state lifetime of the Er:Yb:glass limits the achievable CEO-control bandwidth to <10 kHz. The feedback bandwidth of the SESAM OOM was more than a factor of 10 higher than the gain OOM bandwidth and was mainly limited by the used current driver. The residual integrated phase noise (1 Hz - 100 kHz) of the ~20-MHz CEO beat was improved by more than an order of magnitude (from 720 mrad to less than 65 mrad), and the fractional frequency stability by a factor of 4 (from 1∙10<su⁻⁸ to 2.5∙10⁻⁹ at 1 s).

Vectorial analytical description of the polarized light of a high-power laser diode

A mathematical model to describe the far-field of a high-power laser diode (LD) beam is presented. The laser beam propagation is studied by the vector Rayleigh-Sommerfeld far-field diffraction integral formula The far-field distribution of the LD beam is studied in detail; the light polarized parallel and perpendicular to the junction plane are all considered. This model is employed to predict the light intensity of high-power LDs. The computed intensity distributions are in a good agreement with the corresponding measurements. This model can be easily used to analyze the propagation properties of the high-power LD beam.

Power scaling of a directly diode-laser-pumped Ti:sapphire laser

Improvements in the output power of a directly GaN diode-laser-pumped Ti:Al2O3 laser are achieved by using double-sided pumping. In continuous wave operation, an output power of 159 mW is reported. A tuning range of over 125 nm with output powers in excess of 100 mW is achieved. Pulses of 111 fs duration and an average power of 101 mW are demonstrated by mode locking the laser with a saturable Bragg reflector. Pumping with GaN diode lasers at wavelengths around 450 nm induces an additional parasitic crystal loss of about 1% per resonator roundtrip that is not observed at the conventional green pump wavelengths.

Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser

We describe a Ti:sapphire laser pumped directly with a pair of 1.2 W 445 nm laser diodes. With over 30 mW average power at 800 nm and a measured pulsewidth of 15 fs, Kerr-lens-modelocked pulses are available with dramatically decreased pump cost. We propose a simple model to explain the observed highly stable Kerr-lens modelocking in spite of the fact that both the mode-locked and continuous-wave modes are smaller than the pump mode in the crystal.

Optical trapping with "on-demand" two-photon luminescence using Cr:LiSAF laser with optically addressed saturable Bragg reflector

We demonstrate a diode-pumped Cr:LiSAF laser with controllable and reliable fast switching between its continuous-wave and mode-locked states of operation using an optically-addressed semiconductor Bragg reflector, permitting dyed microspheres to be continuously trapped and monitored using a standard microscope imaging and on-demand two-photon-excited luminescence techniques.

Femtosecond Cr:LiSAF and Cr:LiCAF lasers pumped by tapered diode lasers

We report compact, low-cost and efficient Cr:Colquiriite lasers that are pumped by high brightness tapered laser diodes. The tapered laser diodes provided 1 to 1.2 W of output power around 675 nm, at an electrical-to-optical conversion efficiency of about 30%. Using a single tapered diode laser as the pump source, we have demonstrated output powers of 500 mW and 410 mW together with slope efficiencies of 47% and 41% from continuous wave (cw) Cr:LiSAF and Cr:LiCAF lasers, respectively. In cw mode-locked operation, sub-100-fs pulse trains with average power between 200 mW and 250 mW were obtained at repetition rates around 100 MHz. Upon pumping the Cr:Colquiriite lasers with two tapered laser diodes (one from each side of the crystal), we have observed scaling of cw powers to 850 mW in Cr:LiSAF and to 650 mW in Cr:LiCAF. From the double side pumped Cr:LiCAF laser, we have also obtained ~220 fs long pulses with 5.4 nJ of pulse energy at 77 MHz repetition rate. These are the highest energy levels reported from Cr:Colquiriite so far at these repetition rates. Our findings indicate that tapered diodes in the red spectral region are likely to become the standard pump source for Cr:Colquiriite lasers in the near future. Moreover, the simplified pumping scheme might facilitate efficient commercialization of Cr:Colquiriite systems, bearing the potential to significantly boost applications of cw and femtosecond lasers in this spectral region (750-1000 nm).

Frequency-doubled DBR-tapered diode laser for direct pumping of Ti:sapphire lasers generating sub-20 fs pulses

For the first time a single-pass frequency doubled DBR-tapered diode laser suitable for pumping Ti:sapphire lasers generating ultrashort pulses is demonstrated. The maximum output powers achieved when pumping the Ti:sapphire laser are 110 mW (CW) and 82 mW (mode-locked) respectively at 1.2 W of pump power. This corresponds to a reduction in optical conversion efficiencies to 75% of the values achieved with a commercial diode pumped solid-state laser. However, the superior electro-optical efficiency of the diode laser improves the overall efficiency of the Ti:sapphire laser by a factor > 2. The optical spectrum emitted by the Ti:sapphire laser when pumped with our diode laser shows a spectral width of 112 nm (FWHM). Based on autocorrelation measurements, pulse widths of less than 20 fs can therefore be expected.