Introduction

Nowadays, scientists are focusing on the development of narrow linewidth, powerful and stable lasers based on compact experimental setups and small footprint components. These lasers are popular for applications such as high resolution spectroscopy, environmental science, optical clocks, fundamental research and quantum technology. Because of its high non-linear coefficient, lithium niobate (LN) is one of the most important crystals, delivering frequency conversion from IR to visible in a basic single-pass and compact configuration. Several different mechanisms, e.g. second harmonic generation, sum frequency generation, spontaneous parametric down conversion, difference frequency generation or optical parametric oscillation, can be used to produce narrow linewidth, high beam quality, stable sources of light across its wide transmission window.

MgO doped periodically poled lithium niobate (MgO:PPLN) is a non-linear optical crystal for high efficiency wavelength conversion in the 400nm-5100nm range. Adding 5% MgO to lithium niobate significantly increases the optical and photorefractive resistance of the crystal while preserving its high nonlinear coefficient. This allows more stable operation at visible wavelengths and lower temperature operation than a similar undoped crystal. MgO:PPLN can be operated at ambient  temperatures and in some cases, without temperature stabilization. With temperatures from ambient up to 200°C, MgO:PPLN offers significantly wider wavelengths operation than undoped PPLN.

Applications

Covesion offers PPLN solutions for efficient frequency conversion of lasers allowing users to reach wavelengths that cannot be achieved with conventional solid-state or diode lasers.. MgO:PPLN can be used to:

• Frequency double a 1064nm laser to 532nm, for use in iodine clocks or seabed surveying equipment¹.
• Convert 1064nm to 3um, used for gas detection or microscopy imaging techniques.
• Generate a narrow linewidth laser source for targeting specific atomic transitions for atom cooling and trapping applications.

Alternatively, PPLN has often been used to frequency double a high power tuneable 1550nm fiber source as a low cost and compact alternative to the Ti:Sapphire laser. Such a source can be used in microscopy systems for live-cell imaging, or terahertz time-domain spectroscopy where chemical fingerprints can be identified for homeland security applications.

PPLN devices are commonly used for high power mid-IR generation in an optical parametric oscillator. Tuneable mid-IR systems are used in a wide range of microscopy imaging techniques as well as spectroscopic applications for environmental imaging. With pulse energies in excess of 1mJ, these mid-IR sources are also used in the defence industry for laser countermeasures and LIDAR systems.

Terahertz generation

Terahertz (1-10THz) radiation has important applications in our daily life, such as security checks, biomedicine and quality inspection. Due to its strong nonlinearities, high photorefractive damage threshold, and small absorption coefficient in the infrared spectral region, LN is one of the best-suited materials for THz generation, which can be pumped with ultrashort femtosecond pulsed laser or CW sources. Such crystals have been used to generate THz radiation in different ways, for example, optical rectification in periodically poled crystals, as well as phase-matched operation in a terahertz parametric oscillator or in an injection seeded terahertz parametric generator.  MgO:PPLN is a promising candidate because its phase matching condition can be optimised through application specific design of the periodically poled structure. In this case, multi-cycle THz pulses are obtained from MgO:PPLN via the optical rectification effect². MgO: LN waveguides can also be used for CW THz-DFG, by realizing the phase matching in a non-collinear emission scheme within a LN surface waveguide. Waveguides have the advantage of a reduction in of the area of interacting wave fronts which minimize THz absorption losses³.

Femtosecond lasers

An Optical Frequency Comb is a broad spectrum source composed of equidistant narrow lines. Initially developed for frequency metrology, scientists also use it for spectroscopy over broad spectral bandwidths, of particular relevance to molecules. The spectrum in the visible and near infrared has good overlap with the electronic transitions of optical clocks and alkali atoms, while frequency combs in the mid-IR enable vibrational spectroscopy for molecular detection^3. Mid-IR frequency combs are typically generated by nonlinear optics, e.g. difference frequency generation (DFG) and optical parametric oscillation (OPO), providing high average power, high power per comb line and narrow comb linewidth. An efficient version of DFG is to divide the spectrum of an amplified and spectrally broadened femtosecond NIR laser into two portions, which are used as pump and signal for DFG. Researchers have demonstrated a simple and powerful method for generating broadband frequency combs across the 3-5um mid-IR atmospheric window using intrapulse DFG driven by few-cycle pulses with a MgO:PPLN crystal⁴. Synchronously- pumped OPOs using MgO:PPLN provide another efficient way to transfer fs NIR frequency combs to the mid-IR region⁵.

Bio-Photonics

Laser-based spectroscopy and microscopy is becoming an essential tool in  biochemical and medical applications. Coherent anti-Stokes Raman Spectroscopy (CARS) is a nonlinear process, not only sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy, but also employs multiple photons to address the molecular vibrations, and produces a coherent signal, by using a pump and  a probe, causing Stokes waves interact with a sample, generating an anti-Stokes wave that contains information about molecular vibrations⁶. CARS can also be combined with other nonlinear imaging techniques e.g. two-photon excitation fluorescence microscopy (TPEF) and second harmonic generation (SHG) on a single microscope system using a multi-channel output scheme. Combining these imaging methods has been done via a technique called multimodal-CARS, which can be used in a variety of applications that require structure- and chemical-specific image contrast. Researchers have used MgO:PPLN crystal to build a compact and reliable, tuneable, CW seeded synchronization-free OPA with a robust, commercial pico-second pump laser. Another MgO:PPLN frequency doubling crystal is used to generate visible spectra to excite the CARS anti-Stokes signals. This laser and OPA combination have been shown to be well suited for label-free CARS and concurrent SHG and TPEF microscopy in an epi-detection geometry⁷.

Fluorescence lifetime imaging microscopy (FLIM)is a powerful imaging technique based on the differences in the exponential decay rate of the photon emission of a fluorophore from a sample. FLIM is a useful tool for observing the localization and migration of specific molecules and proteins in cells and tissue. The fluorescence lifetime is determined by the types of fluorescent molecules and the environment surrounding the molecules, it shows little dependence on the concentration of fluorescent molecules, photobleaching, and excitation/ detection efficiency. It’s more quantitative than fluorescence intensity. Researchers have presented scan-less full-field FLIM based on one-to-one correspondence between 2D image pixels and frequency-multiplexed radio frequency signals. Dual-comb optical beats are obtained by using femtosecond laser, PPLN crystal and beam splitter. It will be very useful for rapid quantitative fluorescence image in life science⁸.

Quantum Optics

“Quantum technology (QT) is an exciting area of science which is already making a difference to our lives.”

The impact of QT will increase in the coming years and Nonlinear Optical (NLO) Crystals will have a key role to play in the commercialization of the Technology. QT is expected to have utility across multiple applications in three primary categories:

MgO:PPLN crystals can deliver frequency conversion using a number of different mechanisms (e.g. second harmonic generation, sum frequency generation, spontaneous parametric down conversion, difference frequency generation, optical parametric amplification, etc.) which makes them a very flexible solution for producing narrow linewidth, high beam quality, stable sources of light across their transmission window.

Laser cooling and trapping is the technique to cool atoms down to near absolute zero, and to confine and support these atoms in the traps. The atoms in their ground state can store quantum information and long-range interactions between highly excited Rydberg atoms are essential for successful operation of many quantum information protocols in quantum computing. The high precision and scalable technology offered by atom interferometry enables more sensitive detection of gravity features e.g. smaller size or greater depth . Many atom optics applications favour high laser power whilst maintaining a narrow linewidth and high spatial beam quality. As an example, 780nm generation from a 1560nm source (SHG) is required for Magneto optical trapping (MOT) of Rb atoms in applications utilizing cold atom interferometry such as gravimetric sensing and atomic clocks⁹. In these applications commercial off-the-shelf (COTS) telecoms lasers at 1560nm can be efficiently frequency doubled to 780nm, with conversion efficiencies of up to 70% demonstrated for waveguide solutions¹⁰. The combination of COTS pump laser components together with a frequency doubling crystal provides cost effective generation of both the 780nm power and narrow linewidth required for supporting Rb atom trapping.

“By exploiting the quantum properties of cooled and trapped Rubidium atoms, ultra-precise gravity measurements can be taken, which have many potential practical applications.” Tristan Valenzuela, Head of Quantum Sensors, STFC RAL Space.

Quantum keys are used in secure transmission of data. It enables two parties to share a random secret key known only to them, which can then be used to encrypt and decrypt messages. Bi-directional conversion of 422nm ↔ 1550nm (SFG/DFG) facilitates quantum key distribution (QKD). This application requires efficient conversion between the short wavelength, atomic transitions used for trapped-ion qubits and the telecom C-band for low loss fiber transmission. The use of specially designed PPLN crystals has demonstrated both up- and down- conversion at the single photon level between 422nm (Sr+ emission) and 1550nm, thereby providing a crucial component for the construction of large-scale quantum networks¹¹.

Environment Sensing

Environmental monitoring is of great interest because of increased awareness of environmental harm caused by air pollution, caused by human activities in industrial processes. Various international agreements call for limiting and reducing CO2 and other gaseous emissions, Mid-IR contains fundamental vibrational-rotational absorption bands of various gases, which allows remote or local gas analysis of the atmosphere using lasers with suitable wavelengths. These laser sources are widely used in atmospheric pollution monitoring and remote detection using techniques such as differential absorption lidar. Optical parametric oscillators (OPO) via MgO:PPLN crystal are used in the range of 3-5um because of its wide tunability range and narrow linewidth¹².

Laser remote sensing is a technique which is widely used in environmental analysis. High-power single frequency laser systems have become attractive because of their high spatial and temporal coherence. Researchers have used MgO:PPLN crystal to produce a high power laser system for use with Rubidium Atom Traps to generate very sensitive gravity measurements. This technology has practical applications in areas such as identifying what infrastructures are hidden under a road before starting civil engineering projects. Rubidium Atoms Traps can also be used for a number of climate monitoring activities such as measuring water tables, remote surveying and ice mass monitoring.

Conclusion

MgO:PPLN is nonlinear optical crystal for high efficiency wavelength conversion in the range of 400nm-5100nm, which allows users to produce narrow linewidth wavelengths difficult and expensive to achieve using conventional sources. Covesion Ltd. has more than 20 years of manufacturing experience delivering PPLN bulk crystals and waveguides to optical research labs and OEMs worldwide. Covesion has patented, innovative approaches to bulk crystal poling and waveguide fabrication which can meet our customers’ needs.

We work with partners worldwide to integrate our crystals and waveguides into a range of scientific instrumentation including optical sources & detectors, frequency combs, frequency convertors, gravitometers, and many more. We design and manufacture PPLN products that can be used in many spectroscopic or environmental science applications. These include narrow-linewidth laser sources for gas spectroscopy applications or picosecond and femtosecond ultrafast laser sources for specific wavelengths in fluorescence spectroscopy. Our products are already successfully integrated into a number of existing systems enabling future research and development into novel applications. Our wide range of bulk crystals and waveguides are commonly used in quantum systems where narrow linewidth lasers are needed to access specific atomic transitions. Covesion PPLN crystals can be used in a range of cold atom applications utilizing Rb, Sr, Be and Ca, as well as: entangled photon generation, sensing & detection, quantum computing and cold atom applications. We also enable users to develop innovative experimental and practical applications. Our team can discuss your requirements and advise on the right solution utilising their unrivalled capabilities for engineering PPLN. This means we can assist with many wavelengths that are applicable to a wide range of pump powers, whether your source is pulsed or CW. If we don’t have a stock item to fit your arrangement, we can custom design crystals and waveguides to meet your specific need.

References

1. Y. Liao, et al , “Reduction of Scattering Clutters in an Underwater Lidar System by Using an Optical Vortex,” IEEE Photonics Technology Letters , vol. 34, no. 17, pp. 927-930, 2022.
2. J. Hamazaki, et al., “THz Pulse Generation Emitted From Slant-Stripe-Type PPLN Via Optical Rectification Effect,” OSA Technical Digest, p. cc_6_2, 2019.
3. A. Schliesser, et al. , “Mid-infrared frequency combs,” Nature Photonics, vol. 6, pp. 440-449, 2012.
4. A. J. Lind, et al. , “Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle pulses and x(2) nonlinear optics,” Physical Review Letters , vol. 124, p. 133904, 2020.
5. M. Vainio, et al., “Fully stabilized mid-infrared frequency comb for high-precision molecular spectroscopy,” Optics Express, vol. 25, p. 4190, 2017.
6. C. Evans et al. , “Coherent anti-stokes raman scattering microscopy: chemical imaging for biology and medicine,” The Annual Review of Analytical Chemistry , vol. 1, pp. 883-909, 2008.
7. D. Xu et al. , “Widely-tunable synchronisation-free picosecond laser source for multimodal CARS, SHG, and two-photon microscopy,” Biomedical Optics Express, vol. 12, p. 1010, 2021.
8. T. Mizuno et al. , “Full-field fluorescence lifetime dual-comb microscopy using spectral mapping and frequency multiplexing of dual-comb optics beats,” Science Advances , vol. 7, 2021.
9. Diviya Devani, et al., “Gravity sensing: cold atom trap onboard a 6U CubeSat,” CEAS Space Journal, vol. 12, p. 539–549, 202
10. Sam A. Berry, et al, “Zn-indiffused diced ridge waveguides in MgO:PPLN generating 1 watt 780 nm SHG at 70% efficiency,” OSA Continuum, vol. 2, no. 12, pp. 3456-3464, 2019.
11. Thomas A. Wright, et al, “Two-Way Photonic Interface for Linking the Sr+ Transition at 422 nm to the Telecommunication,” Phys. Rev. Applied, vol. 10, p. 044012, 2018.
12. D B Kolker, et. al, “Tunable mid-infrared laser sources for trace-gas analysis,” J. Phys.: Conf. Ser., vol. 2067, 2021.
13. D. Popa et al. , “Towards Integrated Mid-Infrared Gas Sensors,” Sensors, vol. 19, p. 2076, 2019.