Ultra‐Low Threshold Titanium‐Doped Sapphire Whispering‐Gallery Laser

Abstract

Titanium doped sapphire (Ti:sapphire) is a laser gain material with broad gain bandwidth benefiting from the material stability of sapphire. These favorable characteristics of Ti:sapphire have given rise to femtosecond lasers and optical frequency combs. Shaping a single Ti:sapphire crystal into a millimeter sized high quality (Q) whispering gallery mode resonator (Q ≈ 108) reduces the lasing threshold to 14.2 mW and increases the laser slope efficiency to 34%. The observed lasing can be both multi‐mode and single‐mode. This is the first demonstration of a Ti:sapphire whispering‐gallery laser. Furthermore, a novel method of evaluating the gain in Ti:sapphire in the near infrared region is demonstrated by introducing a probe laser with a central wavelength of 795 nm. This method results in decreasing linewidth of the modes excited with the probe laser, consequently increasing their Q. These findings open avenues for the usage of whispering gallery mode resonators as cavities for the implementation of compact Ti:sapphire lasers. Moreover, Ti:sapphire whispering‐gallery laser can also be utilized as an amplifier inside its gain bandwidth by implementing a pump–probe configuration.

Wafer-Scale Fabrication of a Titanium-Sapphire Laser Substrate by Thermal Diffusion

“…28 Significant efforts have been made on miniaturizing Ti:Sa lasers, addressing scalability challenges posed by factors like the Ti 3+ ion's short fluorescence lifetime, low doping concentrations, and residual absorption during crystal growth. Innovations, including thermal diffusion for channel waveguides, 29 pulsed laser written waveguide lasers, 30 and ridge waveguide lasers, 31 as well as crystal microfibers 32,33 and microcavities employing whispering gallery modes, 34 have significantly improved modal confinement and drastically reduced the lasing threshold compared to traditional bulk systems. However, due to the lack of high-index contrast structures, the optical modes produced by these methods are still large, and the lasing threshold remains above tens of milliwatts to hundreds of milliwatts.…”

Optically pumped Milliwatt Whispering-Gallery microcavity laser

“…These lasers leverage the inherent advantages of semiconductors, including well-established manufacturing processes and high refractive indices, which have been widely acknowledged as the primary avenue for developing on-chip laser sources for photonic integrated circuits. On the other hand, solid-state WGM lasers, as an alternative lasing strategy, have garnered continuous and intense interest 25 – 27 since the first observation of optical WGM in 1961 28 . Their unique three/four-level lasing system has found broad applications not only in on-chip light sources but also in non-Hermitian optics 29 , 30 , optical communication 31 , biosensors 32 , 33 , and more.…”

“…The most desired approach is integrating semiconductor lasers onto the same photonic platform as the rare-earth-doped solid-state laser, allowing for electric pumping and compactness at the system level 34 . However, due to the challenges involved in heterogeneous integration on the semiconductor platform, the standard pumping method is to couple the pumping laser from an off-chip light source into the waveguide 15 , 16 , fiber taper 17 , 18 , 26 , or prism 27 , and then into the WGM, which result in a low efficiency of energy injection into the microcavity. Therefore, there is a strong motivation to develop a novel coupling technique that enables direct injection of the pumping laser into the on-chip WGM.…”

Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators

“…Such systems have been proposed and demonstrated in a large variety of material platforms [5,6]. For example, ultra-low threshold lasing of a titanium doped sapphire whispering gallery laser has been shown recently [7].…”