Pump laser development

To maximise the capabilities of the tapered PCFs studied in section 4.1, the pump laser should ideally have a peak power of at least 30-35 kW. Figure 4.7 shows a sketch of the SC configuration, that consists of a seed, several stages of amplifica- tion, followed by a isolator and finally, this pump laser is coupled into the tapered PCF. The seed has a power of few tens of µW, therefore more than∼ 50 dB of ampli- fication is needed. To achieve such an amplification, multiple amplification stages are required (preamplifier and booster).

FIGURE4.7: Supercontinuum source configuration with an all-fibre based pump laser and a tapered PCF. ISO: isolator, WDM: wavelength division multiplexer,

Yb: Ytterbium.

Seed

The QDLaser seed was used to build the MOPA. The pulse duration and the PRF can be selected with the supplier software with good repeatability, that makes such a seed ideal to build a flexible pump laser. For PAM, the excitation laser requires a pulse duration of typically 1-10 ns with a PRF in the kHz range, according to section 2.2.5 (chapter 2). The seed software gives the possibility to choose between

FIGURE4.8: QDLaser seed spectrum at a PRF of 100 kHz with a pulse duration of 0.9 ns. The inset displays the spectrum around 1062 nm.

FIGURE4.9: Left: measured pulse durations for different PRFs. Right: temporal pulses at 25 kHz. Horizontal scale: 2 ns/div.

discrete pulse durations. The DMD consists of a diode and a semiconductor opti- cal amplifier. The current applied to the DMD was optimised to limit the amplified spontaneous emission (ASE) and maximise the power. A seed average power of ∼ 10-20 µW is measured. Figure 4.8 presents the seed spectrum taken with the OSA used previously (Ando AQ6315, Yokogawa). The seed laser spectral peak consists of two sub-peaks which may come from the reflection profile of the reflection grat- ing used in the DMD. The seed ASE is characterized by its typical broad spectrum, and mainly originates from the semiconductor optical amplifier.

Four different pulse durations were used to optimise the MOPA: 0.9 ns, 2 ns, 4.6 ns and 7.2 ns and six different PRFs: 12 kHz, 25 kHz, 50 kHz, 100 kHz, 1 MHz and 10 MHz. Figure 4.9 shows the measured seed pulse duration and examples of

temporal pulses for a PRF of 25 kHz. The measurements are perfomed with a fast photodiode (DET08CFC, Thorlabs) and a PicoScope (Pico Technology, 9000 Series). The pulse is composed of a short gain-switched pulse (∼ 100 ps) and a main pulse (ns). The pulse duration is almost insensitive to the PRF, where a variation of 2 to 7 % is observed. However, for a pulse duration of 0.9 ns (QDLaser value), a pulse of∼ 100 ps only is measured for PRFs higher than 1 MHz. This short pulse corre- sponds to the DMD gain-switched pulse alone.

Preamplifier

The preamplifier consists of a double stage ytterbium-doped fibre amplifier (YDFA) using a single pump diode (∼ 400 mW). The power ratio distribution between the two stages as well as the Yb-doped fibres lengths were optimised to maximise the preamplifier amplification gain and limit the ASE, for different pulse durations and PRFs. 30 % of the pump diode power is used for the first stage and 70 % for the sec- ond. Both Yb-doped fibres (PLMA-YDF-10/125, Nufern) are 2 m long. Figure 4.10 shows the average power at the preamplifier output for different pulse durations and PRFs.

Figure 4.11 represents the spectrum after preamplification for different PRFs with a pulse duration of 7.2 ns. A broadband ASE spectrum extending from 1000 nm to 1140 nm is observed, corresponding to the Ytterbium emission [160] from the second stage of preamplification. Bandpass filters with 4 nm full width at half maximum are used before and after the first stage of preamplification to limit the amplification of the seed ASE. However, a high level of ASE is still observed, with a bandwidth of∼ 10 nm centred at 1064 nm, which corresponds to the filters total

FIGURE4.10: Average power after preamplification of the QDLaser seed for different pulse durations and PRFs.

FIGURE4.11: Spectrum after preamplification of the QDLaser seed for different PRFs with a pulse duration of 7.2 ns. The inset displays the spectrum around

1062 nm.

FIGURE4.12: Ratio between the maximum of the preamplifier spectrum and its ASE level, taken at 1064 nm, for different pulse durations and PRFs.

bandwidth. At higher PRFs the seed presents a larger average power and more pulses are available, thus less ASE is generated. This is observed in Fig. 4.12 which represents the difference, in dB, between the pump laser spectrum amplitude at 1062.6 nm (maximum) and the ASE level at 1064 nm. Increasing the pulse duration results in a slightly larger average power as well, so lower ASE is as well observed. High ASE is a major issue when building high power MOPA.

Booster with NKT Photonics Yb-doped fibre

The booster (final stage of amplification) consists of two multimode high power pump diodes with a double clad Yb-doped fibre. A first version of the MOPA

used an Yb-doped fibre drawn by NKT Photonics. The maximum average power reached by the MOPA is presented in Fig. 4.13 for different pulse durations and PRFs. A power of ∼ 22 W was measured. After preamplification, the average power depends on the PRF, according to Fig. 4.10. After the booster, the aver- age power depends only slightly on the pulse duration and the PRF, with a vari- ation of± 3 %. Figure 4.14 shows the dependency of the average power with the pump diode power for a pulse duration of 2 ns and different PRFs. As observed in Fig. 4.13, the MOPA average power is barely dependent on the PRF. A threshold of ∼1 W of pump diodes power and an efficiency of more than 75 % are measured.

FIGURE4.13: Maximum average power at the MOPA output for different pulse durations and PRFs.

FIGURE4.14: Average power at the MOPA output with a pulse duration of 2 ns for different PRFs and output powers of the pump diodes.

SC test

The MOPA output beam is used to pump the SC-5.0-1040 PCF and generate a SC. The final version of the source is planned to use a tapered PCF, but as a first step we used the standard SC-5.0-1040 PCF. A pulse duration of 2 ns, a PRF of 100 kHz and and an input peak power of∼10 kW (2 W average power) were used. Figure 4.15 shows the spectrum generated. A total SC power of 350 mW is measured. It can be observed that little visible light is generated. We conjecture that a lot of ASE is contained in the pump laser. This correlates with the observation of high ASE in the preamplification stage already (Fig. 4.11 and 4.12). ASE is continuous and therefore has low peak power. Therefore, ASE cannot induce nonlinear effects and is simply transmitted and attenuated through the fibre. The effective peak power used for the SCG is reduced, leading to too little spectral broadening. The MOPA developed above is not suitable to generate a SC with high energy densities in the visible. Further developments are required to generate a pump laser with lower ASE and higher peak power.

FIGURE4.15: SC spectrum using the MOPA with a pulse duration of 2 ns and a PRF of 100 kHz.

Booster with Nufern Yb-doped fibre

The double clad Yb-doped fibre from NKT Photonics exhibits high gain amplifica- tion, however specific splices at the fibre edges are required. In case of damage in the middle of the fibre, no resplicing of the fibre with itself is possible due to its de- sign. Unfortunately, the gain fibre as well as the splices in the booster stage could not resist the heat generated by such a high power (∼ 20 W output). Therefore, we decided to use a fibre that can easily be respliced in case of damage (PLMA-YDF- 15/130-VIII, Nufern).

FIGURE4.16: Average power at the MOPA output with a PRF of 25 kHz for dif- ferent pulse durations.

Figure 4.16 presents the average power of the MOPA at 25 kHz for different pulse durations and different output power of the pump diodes. The same thresh- old than with the previous configuration is measured: ∼ 1 W but the efficiency is reduced by half, ∼ 35 %. We observed, as well as with the NKT Photonics Yb- doped fibre, that the MOPA average power is barely dependent on the PRF. There- fore, only measurements corresponding to a PRF of 25 kHz are presented here. The MOPA average power is independent of the pulse duration.

FIGURE 4.17: Temporal pulses at the MOPA output with a PRF of 25 kHz for different pulse durations and average powers. Horizontal scale: 1 ns/div.

The MOPA temporal pulses for different pulse durations and average powers are presented in Fig. 4.17. For an average power of 0.05 W, just above the amplifier threshold, the temporal pulse shape is similar to that of the seed (before any am- plification, as shown in Fig. 4.9). The gain-switched pulse (∼ 100 ps) initiating the

pulse has double the amplitude of the nanosecond section of the pulse. However, while increasing the MOPA power, it can be observed that the nanosecond pulse is barely amplified in comparison to the gain-switched pulse. Therefore, a pulse du- ration of maximum 1-2 ns is achieved, while the QDLaser seed offered more than 7 ns pulse duration.

FIGURE4.18: Spectrum at the MOPA output with a PRF of 25 kHz for different pulse durations and two different average powers. The inset displays the spec-

trum around 1062 nm for a MOPA power of 0.05 W.

FIGURE4.19: Spectrum at the MOPA output with a PRF of 25 kHz, a pulse dura- tion of 7.2 ns and five different average powers. The inset displays the spectrum

around 1062 nm.

Figure 4.18 shows the MOPA spectrum for different pulse durations, and with a MOPA power of 0.05 W, just above threshold, and of 4.4 W. At a given MOPA power, the pulse duration has little influence on the spectrum. Figure 4.19 presents the MOPA spectra for five different average powers, for a fixed PRF of 25 kHz and fixed pulse duration of 7.2 ns. It can be observed that by increasing the power, a

FIGURE 4.20: Estimation of the maximum peak power achieved at the MOPA output for different PRFs and pulse durations.

stronger Raman generation is obtained without increasing significantly the MOPA power at ∼ 1062 nm. Several orders of Raman Stokes are generated. The side bands, observed at 1045 nm and 1080 nm, may be due to modulation instabilities of the pump laser [149].

As expressed previously, a peak power of ∼ 30-35 kW is required to maximise the potential of the tapered PCF for SCG. Figure 4.20 represents the maximum peak power achievable as a function of the PRF for different pulse durations. The pulse durations are assumed to be 0.9 ns, 2 ns, 4.6 ns and 7.2 ns for the calculation. The grey area corresponds to the domain where the MOPA can be used to generate op- timum SC (> 30 kW of peak power). A PRF of maximum 200 kHz is usable to generate an optimum SC. These results seem promising. For instance, the SuperK COMPACT (NKT Photonics), which is currently the SC source that offers the largest energy with nanosecond pulse duration, has a maximum PRF of only 20 kHz. We coupled this MOPA using more than 10 kW of peak power to the SC-5.0-1040 PCF. The spectral broadening of the SC obtained was poor with a blue edge of∼ 600 nm. According to section 3.1.2 of chapter 3, a blue edge of∼ 478 nm is expected. These results are similar to those shown in Fig. 4.15. Therefore changing the Yb-doped gain fibre of the booster did not help reducing the ASE of the pump laser and too little peak power can be used for the SCG.

The pump laser, made of the QDLaser seed, a preamplifier and a booster, is not suitable to generate SC because too much ASE is generated, resulting in too little SCG. The strong ASE is initially generated by the QDLaser seed, then amplified.