## 4 Discussion and Application
### Application: Fiber OPO Pumping  
In the frame of achieving efficient pulse generation in the so-called biological window around 1.7 \(\upmu\)m, we then used the homemade erbium fiber laser described above as the pump source of a fiber optical parametric oscillator (FOPO), as shown in Figure 9. In the configuration yielding optimal FOPO performances, the laser delivered pump pulses with a 35-ps duration, 4-nm bandwidth, and 13 nJ, corresponding to a peak power of 370 W. To reach such long pulse durations, the filter bandwidth was increased to lower its impact on pulse shortening inside the laser cavity. The resultant mode-locking regime exhibited longer pulses with a narrower spectra. Considering the anomalous dispersion FOPO reported here (for the idler wave), we stress that the pump pulse chirp must be high enough to allow for an overlap between the pump pulse and the idler pulse, which was long enough to get high conversion efficiencies. The chirp parameter was hence optimized by increasing the width of the pump laser intra-cavity filter, yielding pulses \(\approx\)40 times longer than their Fourier limit. A fibered isolator was used to avoid any back-reflection, which could perturb the mode-locked operation. The FOPO was then pumped through the 90% port of the input coupler (C1). In its cavity, a 7-m-long dispersion shifted fiber (DSF) was used as the parametric gain medium. It had a core diameter of 3.98 \(\upmu\)m, the nonlinear coefficient at 1560 nm was estimated to be \(\gamma\) = 6.6 W\({}^{-1}\).km\({}^{-1}\), and its zero dispersion wavelength was estimated to be 1563 \(\pm\) 5 nm. More details on the DSF can be found in Ref. [13]. Figure 8: Evolution of the pulse shape along the cavity through its kurtosis parameter. Figure 9: Experimental setup of the optical parametric oscillator [14]. After the DSF, 90% of the power was coupled out to diagnostics instruments through the second coupler and 10% was sent into the FOPO feedback loop. Synchronization of the resonating pulses' roundtrip time with the repetition rate of the mode-locked laser was achieved by adding an optical delay line (ODL) comprising a segment of single-mode fiber (SMF), a fiber circulator (CIRC), and free-space elements for fine adjustments. The total length of SMF (including the fiber components) used in the ODL was about 19 m. The cavity-averaged dispersion was then estimated to \(-0.095\) ps/nm around 1400 nm, and its losses were estimated to be about 29.5 dB. Tuning of the FOPO output wavelength could then be achieved by adjusting the two following parameters: (i) by changing the pump laser wavelength through the adjustment of its intracavity spectral filter while maintaining the same level of output energy; and (ii) via the time-dispersion-tuning technique by adjusting the FOPO cavity length, i.e., the delay in the ODL. It is worth noting that in our specific case, using highly-chirped pump pulses synchronized with the resonating idler waves at each round-trip, which are much shorter in time, provided an extra degree of freedom in wavelength selectivity as only overlapping spectral components were amplified. The combination of these two techniques allowed us to obtain a tunability of the parametric sidebands signal and idler waves ranging from 1319 to 1518 nm, and from 1617 to 1876 nm, respectively, as shown in Figure 10. A good agreement between experimental measurements of the FOPO sideband positions and the numerical computation of the phase-matched parametric sidebands as a function of the pump wavelength was obtained as also shown Figure 10. It is worth noting that the nanojoule level was only reached in an 80 nm-bandwidth with wavelengths between 1640 nm and 1720 nm while pumping between 1556 nm and 1568 nm. A typical FOPO output spectrum for a pump wavelength of 1566 nm is then shown in Figure 11. In this particular and optimal situation, the pump was synchronized with the idler wave and the position of the delay line allowed us to obtain signal and idler waves at wavelengths of 1475 nm and 1668 nm, respectively.