Síntesis de los problemas a resolver u oportunidades a aprovechar… 23

We studied the influence of the PRF on the SC source developed. Figure 4.37 repre- sents the pump laser average power (after the isolator), as a function of the pump diodes power. An efficiency of 13-23 % is measured, and a threshold of 0.5-1 W. This average power corresponds to the power coupled into the tapered PCF. It can be observed that the pump laser average power depends slightly on the PRF but maintains similar values for PRFs higher than 100 kHz. Figures 4.38 and 4.39 show

FIGURE4.37: Pump laser average power for different output power of the pump diodes and different PRFs.

FIGURE4.38: SC average power for different output power of the pump diodes and different PRFs.

respectively the SC average power and the PCF transmission at different PRFs and powers. The PCF transmission is calculated as the ratio between the SC average power and the pump laser average power. The PCF transmission increases with the PRF. A transmission of 25-40 % corresponds to a coupling efficiency of typi- cally 60-75 %. It can be observed that the PCF transmission decreases fast with power for a PRF of 25 kHz. This behaviour is not observed for higher PRFs. The measurements were first performed at 25 kHz, therefore, this abrupt decrease in transmission at 25 kHz is due to thermal drift of components until stabilisation. Figure 4.40 represents the SC spectra for different PRFs at maximum power (240- 720 mW depending on the PRF, corresponding to an output power of the pump diodes of∼ 7.85 W). The SCG is narrower when the PRF increases.

FIGURE 4.39: Taper 10.7-5 PCF transmission for different output power of the pump diodes and different PRFs.

FIGURE4.40: SC spectra for different PRFs at maximum power.

The PRF-dependent changes observed in Fig. 4.39 and 4.40 are linked to the changes in the pump laser peak power. The peak power is expressed as:

Ppeak =

Pav

∆T · P RF, (4.1)

where, Pavis the average power and ∆T the pulse duration. Table 4.6 presents di-

verse parameters measured for different PRFs at maximum power. The larger the peak power, the broader the SC spectrum, according to section 3.1.4 (chapter 3). Therefore, the lower the PRF, the broader the spectrum, confirmed by Fig. 4.40. The broader the spectrum, the higher the losses in the PCF due to nonlinear effects. This explains why the PCF transmission increases with the PRF. A peak power of ∼ 10 kW is sufficient to achieve a blue edge of 475 nm (similar value as seen in Table 4.1, section 4.1.2). This is effectively observed for PRFs of 25 kHz and 50 kHz.

PRF [kHz] 25 50 100 250 500 1 000 5 000 Pump average power [W] 1.02 1.3 1.64 1.72 1.74 1.78 1.86 Pump pulse duration** [ns] 2.3 2 1.65 1.56 1.53 1.5 1.48 Pump peak power* [kW] 17.7 13 9.9 4.4 2.3 1.2 0.25 SC average power [mW] 240 335 460 550 607 660 717 Experimental blue edge [nm] 475 475 525 600 700 800 1000

TABLE4.6: Pump laser and SC parameters at maximum power for different PRFs. *: calculated. **: estimated from Fig. 4.24, with 65 % transmission through the

isolator.

As explained in section 4.3, the main spectral bands of interest for sPA, PAM and MPAM cover the visible range and are included in the 500-840 nm band (VARIA). The 500-600 nm band is especially important for haemoglobin imaging and oxy- gen saturation measurements. Therefore, by looking at the spectra presented in Fig. 4.40, a PRF higher than 50 kHz is not suitable for visible PAM and MPAM. OCT can employ several spectral bands, in our case we used 850 nm or 1300 nm as the central wavelengths. By observing Fig. 4.40, a PRF up to at least 500 kHz is usable. Using a PRF of 625 kHz, for instance, instead of 25 kHz, reduces the OCT A- scan noise by a factor of 5 (√25 =√625/25), according to section 2.3.7 (chapter 2). This SC source shows promising results for multimodal sPA-PAM-MPAM-OCT.

FIGURE4.41: SC spectra at 100 kHz and 250 kHz before and after power enhance- ment.

As expressed previously, at higher PRFs, the pump peak power is too low to generate a broad SC spectrum. The power limitation of our source is mainly driven by the MOPA limitations in peak power and average power. It is possible to in- crease the pump laser power for higher PRFs, to obtain a higher peak power that improves the SCG. We focused our efforts on PRFs below 250 kHz. However, the

study can be extended to higher PRFs, but we were unsure on how much power the MOPA can handle. Figure 4.41 presents the SC spectra at 100 kHz and 250 kHz for two different pump laser powers: same as presented in Fig. 4.40, and enhanced. Table 4.7 presents the different parameters before and after power enhancement. A blue shift of the spectrum of approximately 50 nm is observed as a consequence of increasing the peak power coupled into the PCF. For a PRF of 100 kHz, after power enhancement, the peak power is larger than 10 kW. Consequently, the blue edge of the SC spectrum reaches 475 nm, value similar to that achieved by using a PRF of 25 kHz or 50 kHz. Figure 4.42 shows the SC spectrum for PRFs lower than 250 kHz.

PRF [kHz] 100 250

Power enhancement? No Yes No Yes Pump average power [W] 1.64 2.1 1.72 2.87 Pump pulse duration** [ns] 1.65 1.42 1.56 1.2 Pump peak power* [kW] 10.9 14.8 4.6 9.6 SC average power [mW] 460 536 550 836 Experimental blue edge [nm] 525 475 600 550

TABLE 4.7: Pump laser and SC parameters at 100 kHz and 250 kHz before and after power enhancement. *: calculated. **: estimated from Fig. 4.24, with 65 %

transmission through the isolator.

FIGURE4.42: SC spectra for different PRFs, after power enhancement for PRFs of 100 kHz and 250 kHz.

For PRFs lower than 100 kHz, light is generated at wavelengths shorter than 600 nm. A pulse duration of 1-10 ns with tens of kHz of PRF is achieved. Light is generated at around 850 nm and 1300 nm for a PRF of up to 500 kHz-1 MHz. There- fore, according to sections 2.2.5 and 2.3.6 (chapter 2), this SC source is suitable for sPA, PAM, MPAM and OCT in terms of spectral covering and PRF.