There has been a recent surge in interest for optical satellite communication (SatCom) utilizing lasers. It is clear to see why, as optical SatCom is capable of higher speed, lighter weight, higher directionality, and higher efficiency versus their radio-based counterparts. Research into optical SatCom has focused on devices operating in the short-wave infrared (SWIR), which is due to the maturity and commercial availability of such component’s thanks to significant development in terrestrial telecommunications networks. However, SWIR performs poorly in fog and heavy weather, prompting investigations into longer mid-wave and long-wave infrared bands for optical communication instead due to reduced atmospheric losses. This paper provides a comprehensive review of laser transmitters, detectors, and the science behind selecting longer wavelengths for optical SatCom to boost optical SatCom between ground stations and low earth orbit satellite constellations being deployed.

In this paper, a novel approach to modelling intracavity second harmonic generation (SHG) in periodically poled MgO-doped lithium niobate (PPLN) is presented and verified against experimental results. This approach involves combining the coupled nonlinear wave equations with a rate equation model for a diode-pumped solid-state Nd:YVO₄ laser, taking into account both the depletion of the fundamental wave due to the energy conversion from the fundamental wave to the SHG wave and the reduction of the fundamental wave within a laser cavity due to the loss as a result of the SHG nonlinear process. It was shown that the theoretical simulation matched the experimental results well, while also providing physical insight into the importance of the fundamental wave depletion in the intracavity SHG nonlinear processes. The resulting model is computationally simple and has the potential to generalize to the other nonlinear processes such as three-wave mixing and optical parametric oscillation.

Mid-IR light detection based on intracavity upconversion using a compact structure has been studied experimentally and theoretically. The mid-IR detector consists of a 47.5 mm MgO doped periodically poled lithium niobate crystal placed in a resonant cavity of a 1064 nm diode-pumped Nd:YVO₄ laser to enhance efficiency. The generated 1064 nm light is mixed with a mid-infrared source emitting at 3469 nm using an intracavity dichroic mirror. This produces short wave infrared 814.2 nm light via sum frequency generation (SFG). The upconverted light overlaps with the high responsivity for commercial off-the-shelf silicon photodetectors, enabling high speed and high sensitivity detection, surpassing direct mid-infrared detection. The lowest power detected was 150 nW, and the theoretical noise equivalent power for state-of-the-art Si detectors is 1.7⋅fW/Hz. The free-running cavity requires no active stabilization, and the total packaged prototype size is 3.75 × 3.0 × 8.0 cm, which is relatively compact. An experimental power conversion efficiency of up to 36.0% is observed, which agrees well with theoretical simulations. A systematic theoretical study is performed to investigate the potential for further device optimization.

We report a diode-pumped solid-state (DPSS) laser used for intracavity pump-enhanced difference frequency generation (DFG) to create a 3.5-micron laser. Using a 50 mm-long periodically poled lithium niobate (PPLN) crystal inside the cavity of an Nd:YVO₄ solid-state laser at 1064 nm with 4.5 W pump power at 808 nm, and a 310 mW C-band signal at 1529 nm, up to 31 mW of mid-infrared output power at 3499 nm is obtained. The cavity requires no active stabilization and/or locking, and the entire cavity is <8 cm in length. The obtained output power corresponds to a black-box efficiency of 2.20%W⁻¹, which is the highest value reported to date for continuous-wave DFG based on a bulk nonlinear optical crystal with no active stabilization. Potential future applications in free-space optical communication are also discussed.

In this paper, we report a theoretical systematic study of continuous wave intracavity backwards optical parametric oscillators based on periodically poled lithium niobate (PPLN) for mid-infrared (mid-IR) light generation. We study the effects of varying different cavity parameters including nonlinear crystal length, cavity size, pump laser diode spot size, output coupler radius, and cavity loss values on the output power and threshold of the proposed mid-IR laser. The effects of different physical phenomena are included in the model including pump depletion due to the nonlinear conversion process, the thermal lens effect, and mode overlap between the beams in the nonlinear crystal. We show that high output powers in the mid infrared (>500 mW at 3.2 μm) can be achieved with proper cavity design and that a laser threshold with a PPLN as short as 2 cm can be reached.

This study presents a compact intracavity difference-frequency generation (DFG) source designed for mid-infrared (mid-IR) applications. By optimizing overlap between the pump and signal beams, we obtained a black-box efficiency (${\eta }_{BB}$) of 3.4% W⁻¹ and a maximum DFG output power of 63 mW at a wavelength of 3437 nm for a 50 mm long periodically poled lithium niobate (PPLN) crystal. The mid-IR laser was systematically studied for various PPLN lengths (29 mm, 40 mm, and 50 mm), demonstrating extensive tunability across a range of mid-IR wavelengths. Stability assessments over short-term (2 min) and long-term (2 h) periods were also investigated, showing stability values of 0.91% and 1.08%, respectively. These advancements position our DFG laser as a promising candidate for potential applications in greenhouse gas detection and satellite communication, addressing the growing demand for reliable and efficient mid-IR sources.