FAQs

Please refer to our frequently asked questions which answer common queries raised when using PPLN technologies both in research and industrial applications. If you can’t find what you need, please contact our experienced customer support team who will be happy to provide guidance and advice with regards to your particular requirements.

What conversion efficiency can I expect from a bulk MgO:PPLN crystal?

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.

Interaction Efficiency Pump Source Output power Crystal
SHG@532 nm 1.5 %-2 %/W/cm 10 W CW 1064 nm ~2.5 W 532 nm MSHG1064-1.0-20
SHG@780 nm 0.3 %/W/cm 30 W CW 1560 nm 11 W 780 nm MSHG1550-1.0-40
SHG@775 nm 0.6 %/W/cm 10 W CW 1550 nm ~1 W 775 nm MSHG1550-1.0-20
SFG @626 nm 2.5-3.5%/W/cm 8.5 W CW 1050 nm + 8.5 W CW 1550 nm ~7 W 626 nm MSFG626-0.5-40
DFG @ 3.35 µm ~16 % Pump: 1 ns, 26 W, 25 MHz, 1063 nm Signal: 0.85 ns, 12.7 W, 25 MHz, 1435-1570 nm ~6.2 W 3350 nm MOPO1-1.0-40
SHG @489 nm ~34 % 1.54 W pulsed  976 nm ~0.52 W 489 nm MSHG976-0.5-10
SHG@976 nm ~75  % 35ps, 3.2 W, 1 MHz 1952 nm 2.4 W 976 nm MSHG2100-0.5-20
SHG@775 nm ~30-50 % 100 fs, 100-200 mW average power, 100 MHz rep. rate 1550 nm ~40-80 mW 775 nm MSHG1550-0.5-1
OPG @ ~3 µm 30% signal 66% idler Pump: 1030 nm, 400 fs, 43 MHz, 8 W, Signal: 1500-1650 nm, 5 mW CW, <0.2 nm bandwidth 30 % signal 66 % idler 2750-3150 nm MOPO1-0.5-10

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What is the temperature acceptance bandwidth, pump acceptance bandwidth, and expected walk-off of my crystal?

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 the in following table.  Walk-off time is the group velocity mismatch multiplied by the crystal length.

Interaction Period Temperature Length/mm Temperature acceptance/°C Pump acceptance/nm Walk-off /ps
SHG@1550nm 19.10 um ~101°C 0.3 240 39 0.09
0.5 176 24 0.15
1 83 12 0.3
10 7.9 1.2 3
20 3.9 0.6 6
40 2.0 0.3 12
Interaction Period Temperature Length/mm Temperature acceptance/°C Pump acceptance/nm Walk-off /ps
SHG@1064nm 6.96 um ~33 °C 1 25 2 0.8
10 2.5 0.2 8
20 1.3 0.1 16
40 0.6 0.05 32

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What is the relationship between focusing (Rayleigh range) and 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.

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What should I check if there is no SHG signal?

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.

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Which laser polarisation do I need?

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.

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How do I find the optimum crystal operating temperature?

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.

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What is the damage threshold of PPLN?

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.

Regime

Peak Intensity/ Energy Density/ Power

Damage?

Note

CW

500 kW/cm2

N

1064 nm SHG

10 W

CW

200 kW/cm2

N

532 nm, 2.2 W

CW

50 W

N

1550 nm SHG

50 W

ns

2 J/cm2

>2 mJ pulse energy

Y

1064 nm SHG

10-20 ns, 21 Hz, ~30 µm spot size 

ps

1.8 MW/cm2

Y

530 nm OPO

20 ps, 230 MHz, 500 mW

ps

7 GW/cm2

N

1030 nm pump OPG

6 ps, 14 W aver. power, 100 kHz

fs

8 GW/cm2

N

1550 nm SHG

150 fs, 80 MHz, ~4 W average power

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What is the aperture size of a Covesion MgO:PPLN waveguide chip? What are the expected Mode Field Diameter (MFD) and Numerical Aperture (NA) of the chip? What FWHM can I expect?

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)

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How many modes does a Covesion MgO:PPLN waveguide support? Is it single mode at 780nm? What happens if I launch 780nm for Spontaneous Parametric Downconversion (SPDC)?

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

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What is the insertion and propagation loss of a Covesion PPLN waveguide chip?

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)

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What is the expected conversion efficiency of a Covesion MgO:PPLN waveguide chip when used for SHG of a 1560nm fs laser?

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.

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What is the lead time of products?

For all the stocked items, shipping is 1 week after receipt of order/ prepayment. For custom items, our typical lead time is 12 weeks after receipt of order, depending on volume, complexity and AR coating requirements.

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How long is the product warranty?

One-year warranty for oven, temperature controller and mount adaptor.

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