Frequently Asked Questions

The conversion efficiency that can be achieved with a bulk MgO:PPLN crystal depends on pump source power and pulse width and the length of the crystal. The following example data has been collated from our customers and represents operation in a single pass configuration without a cavity.

The temperature acceptance bandwidth is defined as the range at FWHM (Full Width at Half Maximum) of SHG intensity. The temperature dependence of conversion efficiency is inversely proportional to the crystal length and follows a sinc2 function, which defines the crystal temperature acceptance bandwidth. Typical values are given in the table below. Similarly the crystal Pump Acceptance Bandwidth FWHM (in nm) is inversely proportional to the crystal length. Typical values are given in the following table.  Walk-off time is the group velocity mismatch multiplied by the crystal length.

For Second Harmonic Generation (SHG) with CW lasers, a theoretical result from Boyd and Kleinman shows that optimum efficiency can be achieved when the ratio of the crystal length to the confocal parameter is 2.84, where the confocal parameter is twice the Rayleigh range. This is also true for Sum Frequency Generation (SFG) where the two pump beams should both be adjusted to have the same Rayleigh range.Reference: Boyd, G. D., and D. A. Kleinman. “Parametric interaction of focused Gaussian light beams.” Journal of Applied Physics 39 (1968): 3597.

For Difference Frequency Generation (DFG) and Optical Parametric Oscillators (OPOs), optimum efficiency requires a confocal focussing condition where the Rayleigh range is half the length of the crystal. These focussing conditions also apply to pulsed lasers, but due to the high peak powers the spot size requirements are less sensitive. The user should be aware of the crystal damage threshold (see section 6 below) and not focus the beam too tightly as this may cause damage.

In general, a good rule of thumb is that the spot size should be chosen such that the Rayleigh range is half the length of the crystal. The spot size can then be reduced in small increments until the maximum efficiency is obtained.

Should you achieve no output signal, the first thing to check is that you are focussing into the PPLN crystal and not the protective cover glass on top of the crystal. In that case you should see a more diffuse transmitted TEM00, as the cover glass has no polished apertures.

The second common thing to check is that the polarisation of the pump laser is correctly aligned to the crystal. For most applications, the laser polarisation should be linear and aligned parallel to the thickness (z-axis) of the PPLN crystal. If the linear polarisation is rotated by 90 (to be parallel with the y-axis and the long aperture edge of the crystal) then no nonlinear interaction will be observed for our standard Type-0 crystals

The highest nonlinear coefficient in lithium niobate is d33 = 25 pm/V, which corresponds to parametric interactions that are parallel to the z-axis (Type-0 phase matching). In this regime, all interactive waves must be linear e-polarized parallel to the z-axis of the crystal in order to achieve the highest conversion efficiency. Note that in periodically poled magnesium-doped lithium niobate (MgO:PPLN) the effective nonlinear coefficient, deff is typically 14 pm/V.
Covesion’s standard PPLN crystals are designed for Type-0 conversion. Please contact us to discuss custom designs for Type-I or Type-II interactions.

The optimum operating temperature can be determined by heating the crystal to 20°C higher than the calculated temperature and then allowing the crystal to cool whilst monitoring the output power at the generated wavelength.

The damage threshold of PPLN is dependent on wavelength, intensity and pulse energy. Below is a table with customer feedback regarding crystal power handling and damage threshold in various operating regimes.

The table below shows a collection of data from Covesion and from customers showing the power handling or damage thresholds under various regimes. We are continuously working together with our customers to increase the amount of information available on crystal damage thresholds. If you would like to contribute to this, please email sales@covesion.com

Our MgO:PPLN waveguides for 1560nm SHG have an aperture size of approximately 12 µm x 12 µm (width x height). The measured MFD of the 1560nm pump mode is 10.0 µm x 8.8 µm (NA = 0.094 x 0.113). For the phase-matched 780nm output the MFD is measured to be 9.9 x 8.3 (NA = 0.092 x 0.085). Please refer to the paper below for more details.
The FWHM of a 40mm long waveguide chip is 0.28 nm.
Reference: Lewis G. Carpenter, Sam A. Berry, Alan C. Gray, James C. Gates, Peter G. R. Smith, and Corin B. E. Gawith, “CW demonstration of SHG spectral narrowing in a PPLN waveguide generating 2.5 W at 780 nm,” Opt. Express 28, 21382-21390 (2020)

Our waveguide is single mode at the pump (1560 nm). When SHG light is produced at 780 nm it will be produced in the fundamental spatial mode. When running these waveguides for SPDC, if the 780 nm pump is injected into the fundamental 780 nm mode, a fundamental 1560 nm mode will be obtained. However, care must be taken to ensure mode matching the fundamental 780 nm via selective launching, as the waveguide will be multimode at this wavelength.

Our PPLN waveguides have been measured to have a total insertion loss of -1.2 dB at 1560nm, and -1.3 dB at 780nm. Propagation losses of ~0.12 dB/cm at 1560nm and 0.58 dB/cm at 780 nm are calculated as described in the following reference: Reference: Lewis G. Carpenter, Sam A. Berry, Alan C. Gray, James C. Gates, Peter G. R. Smith, and Corin B. E. Gawith, “CW demonstration of SHG spectral narrowing in a PPLN waveguide generating 2.5 W at 780 nm,” Opt. Express 28, 21382-21390 (2020).

Based on customer feedback, our waveguides have achieved 45% conversion efficiency when frequency doubling a 1560nm fs laser source. Pump parameters were: 200 fs pulse duration, 975 MHz rep rate, 275 mW average power, 1.28 kW peak power.