There are now commercial laser systems that cover a huge range of the electromagnetic spectrum. From the development of efficient quantum cascade lasers that cover most of the mid-infrared region to excimer lasers that provide a range of different UV wavelengths, advances in laser technologies mean that most applications can now make use of an appropriate laser source.
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However, while there are a myriad of laser technologies and options available, it is not always easy to find a technology with completely suitable pulse characteristics for all applications. For example, for many spectroscopy applications, it can be advantageous to have a tunable wavelength source as not all samples absorb or reflect in the same wavelength range. Many solid-state diode laser sources, for example only have single frequency emission, and so need to be coupled to additional optical components like a master oscillator power amplifier to provide any degree of tuneability.
In order to provide a wider spectra range, many devices contain several laser sources that can be switched between as required. The need for multiple excitation sources can become problematic for optical layouts and the compactness of a device, so finding alternative ways to tune the wavelength such as temperature control of the laser source can be desirable.
Miniaturization of laser devices and optical components has been an important part of realizing the advantages offered by photonic technologies. Now, developments in semiconductor laser technology have made it possible to integrate the Pockels effect into a waveguide-based laser design.
Pockels effect is the change in the birefringence of a material when an electrical field is applied. In pulsed laser systems, Pockels effect is exploited in what are called Pockels cells, voltage-controlled devices that can be used to either block or propagate the light depending on the voltage settings.
By finding materials that can tolerate high pulse powers, Pockels cells have become an integral part of many pulsed laser systems. As the driving frequency of the cell and so the switching rate of when the laser is blocked or transmitted can be modulated at very high frequencies, Pockels cells can be used to create a sequence of laser pulses, rather than continuous wave irradiation.
Many applications benefit from pulsed laser sources rather than continuous wave as pulsed lasers tend to have higher peak intensities. Non-linear optical techniques that rely on high peak intensities to drive the desired non-linear effect benefit from temporally short, high intensity pulses at the highest possible repetition rate. For non-linear microscopy applications, repetition rates as high as megahertz can be desirable.
The ability to create Pockels effect functionality with a switching speed of up to 50 MHz into a small semiconductor laser is an important step for addressing current issues with fast tuning and reconfigurability. While integrated solid-state laser systems are finding many applications in areas such as machine vision for autonomous vehicles and for information transfer, it is still challenging to replicate the functionality of many of the optical components for controlling laser pulse shapes and characteristics that are found in more standard benchtop lasers.
Pockels cells assemblies for standard Nd:YAG lasers and other types are typically quite bulky as they need large drivers to generate and switch the high electric fields applied to the materials used to create the Pockels cells. There are also some limitations to the switching speeds that are typically achievable due to thermal effects in the dielectric materials used – typically at around the megahertz level.
For applications such as LIDAR, extremely high (>MHz) repetition rates would be desirable to maximize data transfer rates and minimize the number of downtimes between data collection events. For autonomous vehicles, achieving very high data collection rates is an essential safety feature, as the vehicle needs to be able to constantly refresh the information it has on its surroundings in order to make informed decisions about how to react to external events in the environment or how to steer and navigate.
Atomic and molecular physics experiments and sensing applications also benefit similarly from a high repetition rate laser source. Detection of gas phase molecules requires sufficient sensitivity so that low number densities can be recognized and one way to achieve this is to use high repetition rates so a large number of sampling events can be performed in a relatively short acquisition time.
The successful miniaturization of components to achieve switching at 50 MHz and also the very wide tuneability of 2.0 EHz/s with a narrowband laser with a fundamental linewidth 11.3 kHz has been essential for creating a multi-color integrated laser.
The rapid tunability of the laser and the very large tuning bandwidth available of the hybrid integrated III-V/Lithium Niobate structure can provide a chip-sized source that is highly versatile. The developments were achieved with the creation of a Pockels effect being used to electro-optically tune a phase shifter section to allow fine control of the final laser pulse characteristics and is an important step in realizing new capabilities for miniature semiconductor lasers.
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References and Further Reading
Botez, D., KIrch, J., Boyle, C., Oresick, K., Sigler, C., Kim, H., Knipfer, B., Ryu, J., III, D. L., Earles, T., Mawst, L. J., & Flores, Y. V. (2018). Quantum cascade lasers. Optical Materials Express, 8(5), 1378–1398. https://doi.org/10.1364/OME.8.001378
Beggs, S., Short, J., Rengifo-pardo, M., & Ehrlich, A. (2015). Applications of the Excimer Laser : A Review. Dermatologic Surgery, 1201–1211. https://doi.org/10.1097/DSS.0000000000000485
Traub, T., Anstett, G., Goeritz, G., & L'huillier, J. (2014, May). 2.6 um to 12 um tunable ZGP parametric master oscillator power amplifier. In Nonlinear Optics and Its Applications VIII; and Quantum Optics III (Vol. 9136, pp. 170-175). SPIE. https://doi.org/10.1117/12.2052288
Conti, C., Botteon, A., Bertasa, M., Colombo, C., Realini, M., & Sali, D. (2016). Portable Sequentially Shifted Excitation Raman spectroscopy as an innovative tool for in situ chemical interrogation of painted surfaces. Analyst, 141, 4599–4607. https://doi.org/10.1039/c6an00753h
Li, M., Chang, L., Wu, L., Staffa, J., Ling, J., Javid, U. A., Xue, S., He, Y., Lopez-rios, R., Morin, T. J., Wang, H., Shen, B., Zeng, S., Zhu, L., Vahala, K. J., Bowers, J. E., & Lin, Q. (2022). Integrated Pockels laser. Nature Communications, 13, 5344. https://doi.org/10.1038/s41467-022-33101-6
Squier, J., & Muller, M. (2016). High resolution nonlinear microscopy : A review of sources and methods for achieving optimal imaging. Review of Scientific Instruments, 72(7), 2855–2867. https://doi.org/10.1063/1.1379598
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