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 λP are combined through a nonlinear process to generate a third photon at λSHG, where, λSHG = λP/2 (or in terms of frequency fSHG = 2fP).
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 λP and λS to generate an output photon at λSFG , where λSFG = (1/ λP + 1/ λS)-1 (or in terms of frequency fSHG = fP + fS).
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 λP and λS are incident on the crystal, the presence of the lower frequency signal photon, λS, stimulates the pump photon, λP, to emit a signal photon λS and idler photon at λi , where λi = (1/ λP – 1/ λS)-1 (or in terms of frequency fi = fP – fS). 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 3rd order efficiency is significantly lower than 2nd 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
- Maker et al, Phys. Rev. Lett., 8(1):21–23, 1962
- Hum et al, C. R. Physique 8 (2007) 180–198
- Armstrong et al, Phys. Rev., 127(6):1918–1939, 1962
- M. Houe et al, J. Phys. D Appl. Phys., 28:1747–1763, 1995
- Hsu et al, Proc. SPIE 1126412 (2 March 2020)
- Devani et al, CEAS Space Journal volume 12, 539–549 (2020)
- Berry et al, OSA Continuum, Vol. 2, No. 12, 15 December 2019, 3456
- Wright et al, Phys Rev Appl 10, 044012 (2018)
