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PPLN Material Properties

Key properties of PPLN materials

 

The key properties of nonlinear optical materials can be categorized as follows: (1) transmittance, (2) refractive index, (3) nonlinear light intensity and phase matching scheme, and (4) damage threshold. Transmittance represents the ability of light to pass through a medium. The refractive index of a material is its ability to refract light; it is closely related to the electromagnetic properties of the medium. Nonlinear light intensity and phase matching scheme are the main factors affecting the mixing efficiency of the selected nonlinear optical material. The damage threshold characterizes the maximum input/output power that the device can handle without causing laser damage to the material surface or interior. Table 1 summarizes the transmittance and other key characteristics of nonlinear materials.

Table 1: Key Properties of Materials

 

PPLN material light transmittance

 

Figure 1 shows the wavelength transmittance curves of uncoated LiNbO3 (CLN). The transmittance between the UV and MIR transparency ranges is not 100%, which can be corrected by applying an appropriate AR coating. However, the steep slope near the UV and MIR regions is due to material absorption. Additionally, there is a transmission attenuation near 2870 nm due to OH- absorption. Figure 2 shows the transmittance to wavelength ratio in the UV and MIR regions for uncoated CLT, 7% MgCLT, and 5% MgCLN materials. Compared to MgCLN, MgCLT has a larger transmittance range (especially in the near-UV range), therefore, when the applied wavelength exceeds the transmittance range of MgCLN, the MgCLT material family is preferred. Besides the transparency range, several other factors need to be considered when selecting a suitable nonlinear optical material (such as nonlinear intensity and the feasibility of achieving effective phase matching).

Figure 1: Wavelength transmittance curve of CLN

          

Figure 2. Transmission spectra of 1 mm thick CLT, 7% MgCLT, and 5% MgCLN:

(a) UV region of ordinary wave, (b) UV region of special wave, (c) MIR region of ordinary wave, (d) MIR region of special wave.

 

Although transmittance typically begins to decrease at wavelengths <400nm and >4000nm, CLN remains suitable for these ranges, albeit with higher absorption losses.

 

PPLN material refractive index

 

The temperature-dependent refractive index is described by the following Sellmeier equation.

The temperature-related parameter ƒ is

 

 The Sellmeier coefficients are as follows:

This characteristic describes the dispersion characteristics (the relationship between refractive index and wavelength) and birefringence characteristics (polarization dependence), thus determining the phase matching conditions, phase matching bandwidth, etc.

 

 

Nonlinear intensity

 

The intensity of a medium's response to an electric field is characteristic of second-order nonlinear optics. We can define a [6×3] two-dimensional nonlinear medium tensor array, commonly known as the Kleinman d-tensor, to describe the nonlinear intensity:

In practice, due to the symmetry of crystal structures, many tensor components can be reduced to zero, depending on the type of crystal. For example, the d-tensor of LiNbO3 (which is a three-dimensional crystal) can be expressed as...

The nonlinear coefficients are d22 = 2.59 pm/V, d31 = 4.85 pm/V, and d33 = 25.3 pm/V. The nonlinear mixing efficiency is expressed as the effective nonlinear coefficient deff, in pm/V. For a QPM structure polarized along the z-axis, deff will decrease to a²/π, i.e., deff = (2/π)d33.

 

Damage threshold

Damage threshold (LIDT) is a crucial parameter characterizing the resistance of a laser-irradiated medium to laser damage. High concentrations of laser energy can cause localized deformation or even complete damage to the medium's interior or surface. The maximum laser power a medium can withstand per unit area is called its LIDT.

Factors influencing LIDT can be summarized as follows:

(1) Closely related to the medium's inherent properties, including its composition, optical homogeneity, optical absorption, and thermal properties;

(2) Related to the properties of the irradiating laser, including laser frequency, laser pulse width, and laser beam spot;

(3) Related to the processing technology of optical components, such as processing marks left on the optical components, coating methods, purity of the film material, and even the cleanliness of the experimental operating space.

From a practical application perspective, LIDT will depend on the operating wavelength, peak/average power, spatial focusing conditions, pulse width, pulse energy, surface quality, and the design of integrated packaging.