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We hence foresee that the BWOPO will play a critical role in gas sensing for environmental monitoring.
4.1 Applications of BWOPOs in gas sensing
Fig. 6. Internal conversion efficiency and energy of the parametric waves as a function of the pump energy. The roll-off in conversion efficiency is due to generation of BSPS [ 36 ].
In telecommunications, precise wavelength control is necessary for dense wavelength division multiplexing where the BWOPO can be used as a reference laser [ 37 ]. Also, coherent communication relies on precise phase and frequency control of the optical signal which requires a reference laser [ 38 ]. The insensitivity to environmental disturbance would make the BWOPO ideal as a local oscillator laser in these systems.
In quantum technologies, ultra-stable light sources are indispensable for manipulating qubits in quantum computing, generating entangled photons for secure quantum communication, and powering ultra-precise optical atomic clocks. Spontaneous parametric down-conversion( SPDC) in nonlinear crystals is the workhorse of quantum optics because of its reliable ability to generate entangled photons and squeezed states of light. The automatic separation of the down-converted photons in the case of a BWOPO provides a pathway for generating high-purity and narrowband heralded single photons [ 39 – 41 ]. It can pave the way for efficient biphoton sources but also serve as simple and robust generators of squeezed states, hence providing a novel component for applications in optical quantum networks, quantum sensing, and related fields. The unique phase-matching conditions of BWOPOs also allow for efficient and coherent generation of quantum states of light, contributing to advancements in fields like quantum cryptography and quantum metrology.
The BWOPO can also find biomedical applications including improved imaging resolution in optical coherence tomography( OCT), enhancing sensitivity in Raman spectroscopy, and targeting specific wavelengths in photodynamic therapy. The ability to generate coherent radiation across a wide range of wavelengths with exceptional spectral purity making them ideal for precision analysis of atomic and molecular transitions. This has applications in remote sensing where BWOPOs will be valuable for probing molecular structures with high spatial and spectral resolution, particularly in the near to mid-infrared region where many molecules have strong absorption features.
Mid-IR lidar systems directly benefit from the exceptional spectral properties of BWOPOs, allowing precise measurements of atmospheric components. The narrowband, tunable output of PPKTP BWOPOs is ideal for enabling single or multi-species detection with high resolution. In addition to CO 2 monitoring, BWOPOs hold promise for detecting other trace gases, such as methane, ozone, and nitrous oxide, which are pivotal in understanding anthropogenic impacts on the environment.
An important feature of the BWOPO for gas sensing is the possibility to choose wavelength band throughout the mid-IR region by addressing absorption lines with low or high absorption cross sections depending on measurement distance. This enables high detection sensitivity even in low-concentration scenarios. Additionally, the resilience to damage of PPKTP [ 42, 43 ], the high energy efficiency and the small footprint makes the BWOPO well suited for spaceborne platforms. These characteristics ensure reliable performance in harsh environments, such as outer space or high-altitude operations, where traditional systems may falter. The compact and portable design will also be important in deployment for real-time gas monitoring in industrial, environmental, or medical settings, and the stability and robustness of BWOPOs make them well-suited for continuous and long-term CO 2 monitoring under varying environmental conditions.
4.1.1 CO 2 monitoring
A first demonstration of BWOPO gas sensing was done by Vågberg et al. who exploited the backward wave’ s narrow linewidth and stability to accurately detect ambient CO 2 at the strong absorption lines at 2.7 lm [ 44 ]. This verified the BWOPO capability and the potential for atmospheric studies and climate monitoring. The simple experimental configuration seen in Figure 2 was used. The pump laser was a multi-longitudinal mode, Q-switched micro-chip laser with a wavelength centered around 1030 nm. The backward wave generated at 2712 nm at room temperature, but the output could be tuned by 3 nm by changing the temperature of the PPKTP crystal, see Figure 7. This corresponds toatuningrateof�1.77 GHz / K. The linewidth was as narrow as 43 pm, or 1.75 GHz, with a temporal stability of 65 MHz, and it was obtained without any active means of stabilization. The beam was launched a distance of 2.9 m in the laboratory and the air transmission was measured while the wavelength was scanned, see red graph in Figure 7. At2.7lm there is a partial overlap between CO 2 and H 2 O absorption lines, while other trace gases could be neglected. By using the HITRAN data base a concentration of 410 ppm of CO 2 and 17.5 % relative humidity could be deduced. The simulated absorption curve for these concentrations representing a best fit and can be seen as thebluecurveinFigure 7.
These findings underscore the potential of BWOPOs as a simple and robust platform for future differential