Second Harmonic Generation
PPLN can be used in a single pass configuration for SHG with the pump focussed at the centre of the crystal length. For optimum efficiency, aim for the Boyd-Kleinman focussing condition. This is where the spot size is such that the ratio of the crystal length to the confocal parameter is 2.84.
The optimum conversion efficiency that can be achieved for an SHG interaction also depends on several factors such as:
- CW or pulsed pump source
- Input power: at high power, you can reach gain saturation
- Pump/SHG wavelength: At low gain, conversion efficiency is higher for interactions involving higher energy photons (short wavelength).
1064nm → 532nm
For low gain CW the typical conversion efficiency is 2%/Wcm. For example, for 1.5W at 1064nm and a 40mm long MgO:PPLN crystal, the expected 532nm output is 180mW. At higher powers, Covesion has achieved 1.5%/Wcm with a 10W source, generating 3W at 532nm from a 20mm long crystal.
In CW systems, conversion efficiencies in excess of 50% have been demonstrated in an intracavity arrangement[1]. For nanosecond sources (~10KHz, ~50uJ), efficiencies of 50% can typically be achieved.
1550nm → 775nm
Frequency doubling of Erbium doped fiber lasers is also common, for example for 775nm or 780nm generation. For a CW source, you can typically achieve 0.6%/Wcm for low gain. At high powers an efficiency of 0.3%/Wcm has been demonstrated for generating 11W at 780nm in a 40mm long crystal with 30W pump power[2].
For a nanosecond source, up to 80% conversion efficiency has been demonstrated in a single pass pulsed system[3]. For femtosecond sources, using a 1mm crystal length, customers have reported efficiencies of 40-60% for ~100fs, 100MHz and several hundred mW average powers. Due to the very wide temperature acceptance bandwidth, our MSHG1550-0.5-1 crystal can be used at room temperature, and with no temperature controller, for SHG at 1550 or 1560nm.
Difference Frequency Generation
PPLN is often used in a DFG setup for mid-IR generation, either with a tuneable Ti:S laser and 1550nm laser, or a 1064nm source and tuneable ~1550nm laser. Optimum efficiency requires confocal focussing of both pump beams, i.e. ratio of the crystal length to the confocal parameter is 1. For CW systems, efficiencies of 0.3-0.4mW/W2cm can be achieved.
Optical Parametric Oscillator
One of the most common uses of PPLN is in an Optical Parametric Oscillator (OPO). A schematic of an OPO is shown above. The common arrangement uses a 1064nm pump laser and can produce signal and idler beams at any wavelength longer than the pump laser wavelength. The exact wavelengths are determined by two factors: energy conservation and phase matching. Energy conservation dictates that the sum of the energy of a signal photon and an idler photon must equal the energy of a pump photon. Therefore an infinite number of generated photon combinations are possible. However, the combination that will be efficiently produced is the one for which the periodicity of the poling in the lithium niobate creates a quasi-phase matched condition. The combination of wavelengths that is quasi-phase matched, and hence referred to as the operation wavelength, is altered by changing the PPLN temperature or by using PPLN with a different poling period. Nd:YAG pumped OPOs based on PPLN can efficiently produce tunable light at wavelengths between 1.3 and 5μm and can even produce light at longer wavelengths but with lower efficiency. The PPLN OPO can produce output powers of several watts and can be pumped with pulsed or CW pump lasers.
The minimum oscillation threshold can be achieved under confocal focussing conditions for the pump and resonating signal or idler, i.e. ratio of the crystal length to the confocal parameter is 1. The typical pump threshold for a singly resonant CW OPO is around 1-2W.
Sum Frequency Generation
To achieve efficient SFG, you ideally want the two pump beams to be confocally focussed into the PPLN (i.e. ratio of the crystal length to the confocal parameter is 1) and for both beams to be roughly equal in power.
SFG in PPLN is often used for laser cooling of atoms or ions where very precise control over the frequencies is required. For generation of 626nm light from 1051nm and 1551nm, efficiencies of 3.5-2.5%/Wcm have been achieved. Here, the efficiency η, is defined by[4, 5]:
Where P is the power at each wavelength, and l is the crystal length. An efficiency of 44% has been demonstrated for the generation of 7.2W of 626nm light from 1051nm (8.5W) and 1551nm (8.3W) [4].
A similar conversion efficiency of 3.2%/Wcm has also been reported for 589nm generation from 1064nm and 1319nm[6].
References
1. M.Zhou et al., Laser Physics, vol. 20, no. 7, pp. 1568-1571 (2010)
2. S. S. Sané et al., Optics Express, vol. 20, no. 8, pp. 8915–9, (2012)
3. D. Taverner et al., Optics Letters, vol. 23, no. 3 pp. 162-164 (1998)
4. H.-Y. Lo et al., Applied Physics B, doi:10.1007/s00340-013-5605-0, (2013)
5. A. C. Wilson et al., Applied Physics B, vol. 105, no. 4, pp. 741–748, (2011)
6. J. Yue et al., Optics letters, vol. 34, no. 7, pp. 1093–5, (2009)
