Frequency Doubling Femtosecond Fibre Lasers

Frequency Doubling Femtosecond Fibre Lasers

One of the most common applications of our crystals is for the generation of femtosecond pulses at around 780nm which can also be used as a low cost and compact alternative to the Ti:Sapphire laser. This is typically achieved by frequency doubling a high power 1560nm Er 3+ fibre source using a 1mm long MgO:PPLN crystal (MSHG1550-0.5-xx) at room temperature (no PPLN crystal oven needed). Such a source can be used in microscopy systems for live-cell imaging, or terahertz time-domain spectroscopy where chemical fingerprints can be identified for homeland security applications.

The crystal length is an important factor when choosing a crystal due to the acceptance bandwidth of the device. The crystal needs to be long enough to achieve good temporal overlap of the pulses, and yet short enough to accommodate the bandwidth. The MSHG1550-0.5-xx is available in lengths as short as 0.3mm for <30-100 fs, but typically for 100-200fs pulse durations, a 1mm long crystal is recommended.

For frequency doubling femtosecond pulses, if the pump bandwidth is significantly wider than the acceptance bandwidth, it is still possible to achieve high conversion efficiency. The pump frequencies outside of the acceptance bandwidth can still contribute to the conversion efficiency via sum frequency generation, essentially squeezing the broadband pump into a relatively narrower-band SHG pulse [1].

Using a 1mm crystal length and 5-10μm focussed spot sizes (1/e2 radius), customers have reported efficiencies of 40-60% for ~100fs, 100MHz and 100-200mW average powers. Due to the very wide temperature acceptance bandwidth, <1mm long crystals can be used at room temperature, and with no temperature controller, for SHG at 1550 or 1560nm.

Huang et al. have reported a Multiphoton Microscopy System based on MgO:PPLN with an SHG conversion efficency of 40% under the following conditions [4]:

    • MgO:PPLN crystal: 1mm long, 0.5mm thick (MSHG1550-0.5-1)
    • Aspheric lens, f = 7.5 mm
    • Pump: 1.58um, 250fs, 67MHz, 200mW
    • SHG output: 786nm, 150fs, 80mW

The table below shows the available lengths for the MSHG1550-0.5-xx and the recommended lengths based on input pulse duration and pump acceptance bandwidth. The temperature acceptance bandwidth of the crystal length indicates that crystal lengths less than 1mm do not need to be temperature stabilised in a PPLN crystal oven. The maximum SHG bandwidth is the maximum bandwidth expected from the crystal due to its length.

Length (mm)
Pump acceptance bandwidth (nm) Temperature acceptance bandwidth (C) Maximum SHG bandwidth (nm) Input pulse duration
0.3 40 265 20 <30 – 100 fs [2]
0.5 24 170 12 50 – 100 fs
1 12 90 6.0 100 – 200 fs
3 4.0 30 2.0 200 – 500 fs
5 2.4 20 1.2 0.5 – 2 ps
10 1.2 10 0.6 1 – 3 ps [3]


References

[1] K. Moutzouris et al., Optics letters, vol. 31, no. 8, pp. 1148–50, (2006)
[2] C. W. Freudiger et al., Nature Photonics 8, 153–159 (2014)
[3] C. Peuntinger et al., Phys. Rev. Lett. 113, 060502
[4] L. Huang et al., Biomed. Opt. Express, vol. 7, no. 5, p. 1948, (2016)