Non-linear optical crystals

Non-linear optical (NLO) crystals provide an enormously flexible solution for generating new wavelengths from existing, off-the-shelf laser sources. The optical wavelength spectrum is utilized by a large and continually expanding variety of applications, from UV sterilization, quantum networking & computing, visible imaging, telecommunications, environmental sensing, through to terahertz spectroscopy as well as many others. For all of these applications, the light source is a critical component providing the required illumination wavelength, power, linewidth and other key spectral properties.

Although there are a wide variety of commercially available laser sources covering the extended optical spectrum it is still not always possible to find a direct or cost-effective light source for all
applications. It is in the cases where a practical, direct source is not available that wavelength conversion using highly efficient, non-linear optical crystals provides a powerful solution.

WHICH APPLICATIONS?

“PPLN crystals are used in a diverse range of applications from space and defence technologies through to quantum computing. Through our research and the work conducted by our customers we are constantly discovering new applications for our solutions.”
Prof. Corin Gawith, CTO, Covesion

Principle of wavelength conversion

The principle of wavelength conversion using non-linear optical effects has existed for decades and with the developments in laser technology, combined with the discovery of high-quality crystals with large optical non-linearities, the corresponding gains in conversion efficiency have enabled the practical use of NLO crystals in both research and commercial environments. ^1,2 One of the most important developments has been the adoption of materials which can be domain engineered enabling quasi-phase matching (QPM) to be used as the method by which the relative phase between the interacting waves is maintained.³

In comparison to more conventional birefringent phase matching (BPM) used in homogeneous materials, micro-structured QPM materials offer the benefits of simple co-linear optical alignment, non critical angular walk-off, access to the largest non-linear coefficients, and a highly flexible design space.

NLO crystals provide a practical solution for the generation of wavelengths that are not readily accessible via direct laser sources.

Choice of NLO crystal

The choice of NLO crystal for a particular application is driven by the required wavelength, available pump sources and NLO conversion efficiency. Secondary considerations include the required output power, linewidth, operating temperature etc. When considering different crystal materials, lithium niobate (LiNbO3) is a particularly attractive option since it has a very high non-linear coefficient.⁴

Lithium niobate is a ferroelectric material in which the domain structure can be inverted by application of an electric field. By applying a spatially patterned electric field, so called periodic poling, a periodic reversal in the in-built polarization can be produced within the crystal. This then enables QPM to be used to access the highest (d33) non-linear coefficient. Doping with 5% MgO significantly increases the optical and photorefractive resistance of the crystal while preserving its high non-linear coefficient. With a higher damage threshold MgO:PPLN becomes suitable for higher power applications.

With its high non-linear coefficient, ability to be periodically poled and broad optical transmission, MgO:PPLN becomes a highly flexible solution for the generation wavelengths from the blue (<400nm) through the mid-IR and beyond (THz). The required wavelength is obtained by tailoring the PPLN crystal design to access the most appropriate non-linear process; SHG, SFG, DFG, etc.

Non-linear optical processes

Second harmonic generation (SHG), or frequency doubling, is the most commonly used second order non-linear process. In SHG, two input pump photons with the same wavelength λ_P are combined through a nonlinear process to generate a third photon at λ_SHG, where, λ_SHG = λ_P/2 (or in terms of frequency f_SHG = 2f_P).

MgO:PPLN SHG crystals can be fabricated with QPM grating periods suitable for a wide range of commercially available pump laser wavelengths from 976 nm to 2100 nm, allowing generation of frequency doubled light between 488nm and 1050nm.

Sum frequency generation (SFG) combines two input photons at λ_P and λ_S to generate an output photon at λ_SFG , where λ_SFG = (1/ λ_P + 1/ λ_S)-1 (or in terms of frequency f_SHG = f_P + f_S).

By combining readily available fixed (e.g. 1550nm) and tunable (e.g. 780/810nm) pump laser sources MgO:PPLN SFG crystals can provide tunable output light between 500-700nm.

Difference frequency generation (DFG) occurs when two input photons at λ_P and λ_S are incident on the crystal, the presence of the lower frequency signal photon, λ_S, stimulates the pump photon, λ_P, to emit a signal photon λ_S and idler photon at λ_i , where λ_i = (1/ λ_P – 1/ λ_S)^-1 (or in terms of frequency f_i = f_P – f_S). In this process, two signal photons and one idler photon exit the crystal resulting in an amplified signal field. This is known as optical parametric amplification (OPA). Furthermore, by placing the nonlinear crystal within an optical resonator, also known as an optical parametric oscillator (OPO), the efficiency can be significantly enhanced.

MgO:PPLN DFG crystals can be designed to work with common fixed and tunable pump wavelengths (e.g 1064/1550/775nm) to cover a broad, continuous output tuning range from the near-IR to beyond 4.5μm in the mid-IR.

MgO:PPLN QPM grating design can be further extended to access third-order processes such as third harmonic generation (THG). Although 3^rd order efficiency is significantly lower than 2^nd order the generation of useful levels of UV light has been demonstrated by 3rd order SFG (1064nm + 532nm ->355nm) in MgO:PPLN. ⁵

Real world applications

MgO;PPLN can be readily manufactured into a variety of forms from bulk crystal to waveguide providing both wide application range as well as enhanced conversion efficiency. Wavelength conversion chips, either using bulk crystal or waveguide forms, can then easily be packaged with fiber-coupled input and output – for enhanced ease of use. The combination of a fiber-coupled package together with a high precision temperature controller provides a plug and play wavelength conversion solution.

Real-world examples showing the benefit of wavelength engineering using PPLN crystals include;

780nm generation from 1560nm source (SHG).

Magneto optical trapping (MOT) of Rb atoms in applications utilizing cold atom interferometry such as gravimetric sensing and atomic clocks.⁶ In this application COTS telecoms lasers at 1560nm can be efficiently frequency doubled to 780nm, with conversion efficiencies of up to 70% demonstrated for waveguide solutions.⁷ The combination of off-the-shelf 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.

Bi-directional conversion of 422nm <-> 1550nm (SFG/DFG).

Quantum networking to facilitate 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. This thereby providing a crucial component for the construction of large-scale quantum networks.⁸

To conclude

In conclusion NLO crystals provide a practical solution for the generation of a wide range of wavelengths that are not readily accessible via direct laser sources. The use of highly efficient materials that can be micro-structured to enable QPM, such as MgO:PPLN, provide a highly flexible product eco-system. As a leading supplier of PPLN-based wavelength conversion products Covesion is able to offer advice on customer specific solutions, as well as technical support in their set-up, use and optimization. With an extensive portfolio of COTS products, as well as custom design capabilities, Covesion is well placed to support the widest range of wavelength conversion applications.

References

1. Maker et al, Phys. Rev. Lett., 8(1):21–23, 1962
2. Hum et al, C. R. Physique 8 (2007) 180–198
3. Armstrong et al, Phys. Rev., 127(6):1918–1939, 1962
4. M. Houe et al, J. Phys. D Appl. Phys., 28:1747–1763, 1995
5. Hsu et al, Proc. SPIE 1126412 (2 March 2020)
6. Devani et al, CEAS Space Journal volume 12, 539–549 (2020)
7. Berry et al, OSA Continuum, Vol. 2, No. 12, 15 December 2019, 3456
8. Wright et al, Phys Rev Appl 10, 044012 (2018)

Quantum Timing and Sensing

There is a near term, high priority need for next generation timing and sensing solutions for important applications including:

• Autonomonous navigation and inertial sensing (for use in GPS denied environments)
• Gravimetric and magnetic sensing (including earth orbit environmental monitoring and land-based site surveying)

The next generation technology providing these solutions utilizes quantum effects, with a key enabler being the magneto-optical trap based upon the Rubidium atom (Rb-MOT). The Rb-MOT enables “Cold atoms” to be used as ultra-precise atomic clocks and ultra-sensitive sensors for measuring acceleration.¹

It is predicted that demand across these and other applications will drive a large increase in the quantum sensing and timing markets. Quantum sensors have a current market size of $260M and are expected to grow to $565M by 2027 (CAGR 16.8%).²

These sensing and timing applications require the quantum technology to be transferred ‘out of the lab’ and to be in a robust form capable of being deployed and operated in remote and harsh environments (land, sea, air or space based).

Wavelength Conversion

Periodically poled lithium niobate (PPLN) is a non-linear optical crystal that can be used to change the wavelength of lasers. For quantum applications, PPLN enables off-the-shelf lasers to be converted to atom- or ion-specific wavelengths that are otherwise difficult to obtain. For Rb atom traps, PPLN enables industry-standard 1560nm telecoms lasers to be converted to the 780nm wavelength needed for Rb cooling. This approach is particularly appealing for operation in harsh environments, such as space, as telecoms lasers are reliable, robust, and rated for thousands of hours of operation.

PPLN waveguides offer the further benefit of very high conversion efficiencies of up to 70%³ and can operate up to the Watt level, enabling rapid cycling of Rb sensing measurements.

UK Investment

Current global investment in quantum technologies is estimated at more than $20bn per year. Within this the UK has committed to investing £1bn over 10 years. This includes Innovate UK (IUK) funded R&D programs investigating the potential for ruggedization of quantum technologies including the enabling systems and components. Covesion has been an active participant in a number of these projects;

• QT Assemble – an underpinning program to develop the UK supply chain for quantum-enabling optical products and systems.
• CASPA – Cold Atom Space Payload. The aim of CASPA was to build a system capable of cold atom trapping Rb atoms autonomously in the space environment. CASPA was the first step to verify the basic concept and gain heritage on the subsystems and overall design of a basic cold atom demonstrator. Covesion supplied the PPLN waveguide chip for integration into the optical subsystem.
• SNORQL – Space-certified Nonlinear Optics for Rugged Quantum Lasers. The aim of SNORQL was to demonstrate Covesion fiber-coupled, PPLN waveguides in a Rb-MOT and perform trials to assess package performance in simulated environmental conditions (thermal, vibration, shock, radiation) for pre-space qualification.

“Current global investment in quantum technologies is estimated at more than $20bn per year. Within this the UK has committed to investing £1bn over 10 years.”

PPLN waveguide performance

Three key criteria need to be met in order to demonstrate that PPLN waveguides are a viable solution for wavelength conversion in harsh environments;

• The waveguide must deliver the required power and conversion efficiency needed by the quantum technology (Rb-MOT)
• Fiber coupled packaging must be available in order to move the technology out of the lab and provide plug and play system integration
• The waveguide package must demonstrate long term reliable operation and be able to withstand the environmental conditions it will be exposed to (thermal, vibration, shock, radiation).

Covesion has taken our standard, off-the-shelf, component waveguide package and tested it against these criteria. It should be highlighted that this fibercoupled module was NOT designed to withstand harsh environments and that this work was therefore undertaken to assess its performance and inform the development needed for ruggedization.

“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.

Lifetime and efficiency testing

The Covesion waveguide module has demonstrated high efficiency wavelength conversion for more than 1000 hours of operation. With an overall second harmonic generation (SHG) conversion efficiency of up to 50% the module delivers the Watt level output at 780nm needed in order to enable rapid cycling of Rb-MOT sensing measurements.

A key target of the SNORQL project was to deliver 1W SHG output for minimal pump power as a primary requirement for space-based gravimetric sensing.

Environmental testing

Environmental testing (thermal, vibration, shock, radiation) has been performed to MIL standards (MIL-STD-883K) to assess the robustness of the waveguide module and the need for further ruggedization.

Overall the module performs well despite not being specifically designed for rugged operation. A summary of the results of the testing are shown in the table, the results are split into 4 package properties; mechanical – refers to the module casing; electrical – refers to the internal electrical connections; optical path – refers to the optical beam path from fiber input to fiber output; waveguide chip – refers to the PPLN waveguide chip. For each property a tick signifies that the package has passed the specific environmental test and a ‘D’ signifies that further work is needed and a development path has been identified.

It should be noted that for all tests the PPLN waveguide chip itself passed with no evidence of damage (breakage, cracking etc) and performed SHG with the same efficiency pre- and post-test. This demonstrates that the underpinning PPLN material technology provides a robust solution to operation in harsh environments.

Both the mechanical and electrical properties of the package also survived all tests, showing that the weak point of the package (unsurprisingly) is the optical path. The optical path was degraded in a number of different ways depending on the test exposure. The fiber-pigtails suffered both thermal and radiation damage however this is easily solvable through the use high temperature and radiation hard optical fiber. Optical coupling of the fiber pigtails on the input and output suffered damage during vibration and shock testing. Improvements to the waveguide and fiber support structure are therefore required. A low risk path to achieving these has been identified via engineering re-design and the use of optimized bonding materials.

Summary

Covesion’s approach has been to leverage its internal investment through participation in IUK funded collaborations with UK and European partners in the quantum technology community. This has enabled us to develop a route to market for rugged wavelength conversion modules meeting the demands of applications within harsh and hostile environments. These modules are needed to enable the exploitation of key quantum applications including; next generation atomic clocks, ultra-sensitive accelerometers and gravitometers.

An extensive test and development program has been undertaken to extend the use of PPLN, fiber-coupled waveguide modules to harsh environments including space. Environmental testing has shown that with modest further development our existing waveguide package is suitable for ruggedization and a low risk path has been identified to develop rugged modules suitable for operation in harsh environments including space certification.

“Covesion is keen to partner with organisations who have an interest in further developing and exploiting this technology for use in harsh environments.” Corin Gawith, CTO, Covesion

Acknowledgements

Covesion acknowledge the support of Innovate UK, the UK’s national innovation agency.

References

1. M. Odstrcil, et al., “Nonlinear ptychographic coherent diffractive imaging,” Optics Express, pp. 20245-20252, 2016.
2. Hsiang-Yu Lo, et. al, “All-solid-state continuous-wave laser systems for ionization, cooling and quantum state manipulation of beryllium ions, “Applied Physics B, vol 114, pp. 17-25, 2014.
3. Diviya Devani, et al., “Gravity sensing: cold atom trap onboard a 6U CubeSat,” CEAS Space Journal, vol. 12, p. 539–549, 2020.
4. 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.
5. 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.