There are many methods to attain the high pulse energy, including to increase the pump power, to reduce the pulse repetition rate as the cavity length increase, and to change the output coupling ratio, etc. In our experiment, the maximum pumping power is about 561mW, and the pulse repetition rate is 500 kHz as we have put 400 m long SMF inside the cavity. We adjust the output coupling ratio to obtain the highest pulse energy. When the pumping power is above a certain threshold, a relatively stable continuous-wave mode-locking (CW-ML) state without pulse breaking can be achieved by properly rotating the paddles of the polarization controllers. Depending on the adjustments of the polarization controllers, either a multi-pulse operation (harmonic mode-locking) or single-pulse regime can be started. Our task is to find the highest output power within the single-pulse operation as much as possible.
Fig. 3.2 shows the RF spectrum of the mode-locked pulse train. Due to the long cavity length, the pulse repetition rate is as low as 500kHz. The corresponding time trace of ML pulse trains measured by the real-time
oscilloscope is shown in Fig.3.3. It can be seen that the time interval of pulse separation is about 2μs. The mode-locked pulse train is relatively stable as also can be seen from the time trace data.
Fig. 3.2 RF spectrum of the pulse train
3.2-1 High pulse energy
The measured average output powers (left side) and the estimated pulse energy (right side) versus the pump power is illustrated in Fig. 3.4. Using a 90/10 output coupler, the maximum pulse energy is estimated to be about 22.4nJ at the 570mW pumping level with the 500 kHz repetition rate and 11.2mW average output power. It should still be possible to further increase the pulse energy by optimizing the cavity design, so we keep trying different coupling ratios.
Fig. 3.4 Pulse energy and output power versus pump power with 90/10output coupler
As illustrated in Fig. 3.5, the corresponding pulse energy is increased to be about 63.2 nJ using the 70/30 output coupler because the maximum output power is increased to be 31.6mW. The threshold pump power for mode-locking is about 225mW. The pulse energy and average output power still grows linearly with the pump power, indicating that even higher pulse energies can be
Fig. 3.5 Pulse energy and output power versus pump power with 70/30output coupler
After increasing the output coupling ratio to be 50%, the output light intensity is equal to that storage inside the laser cavity. The pulse energy and average output power versus the pumping power are illustrated as shown in Fig.
3.6(b). The threshold pump power for mode-locking is raised to 320mW and the maximum output pulse energy is over 100 nJ. After proper rotating the PCs, we can observe the mode-locked pulses with relatively wide optical spectrum bandwidth as illustrated in Fig. 3.6(a). The center wavelength is about 1585 nm and the 3dB bandwidth is 50 nm. A broad optical spectrum bandwidth indicates that ultra-short pulses can be generated through proper external compression mechanism. After properly rotating the angles of the polarization controllers, the relative narrower spectral bandwidth about 10nm can be produced as shown in Fig. 3.7(a). Fig. 3.7(b) shows that the highest output power and pulse energy are 63 mW and 126 nJ, respectively. This indicates that the narrower spectral bandwidth case can reach higher pulse energy.
Fig. 3.6(a) Optical spectrum with broaden bandwidth (b)corresponding pulse energy and output power by increasing the pump power (with 50/50 coupler)
Fig. 3.7(a) Optical spectrum with narrow bandwidth (b)corresponding pulse energy and output power versus pump power (with 50/50 coupler)
Fig. 3.8(a) Optical spectrum with broaden bandwidth (b)corresponding pulse energy and output power by increasing the pump power (with 30/70 coupler)
Fig. 3.9(a) Optical spectrum with narrow bandwidth (b)corresponding pulse energy and output power versus pump power (with 30/70 coupler)
In order to further increase the pulse energy, we use a 30/70 output coupler in which 30% of the light is feedback into the cavity and 70% of the light is output. The reduction of pumping power in the cavity makes it more difficult to produce the mode-locking state. The measured pulse energies versus the pump powers are shown in Fig. 3.8 and Fig. 3.9. As we can see, the pulse energy cannot exceed 106nJ whether the optical spectrum bandwidth of ML pulses are broaden (40 nm) or narrow (15 nm). Figure 3.10 shows the maximum pulse energy at the 560 mW pump power using different output coupling ratios. The 50/50 coupling ratio maintains a stable mode-locking state without pulse breaking and attains the highest pulse energy about 126 nJ.
Fig. 3.10 Output coupling ratio versus pulse energy
3.2-2 Square pulse
By carefully adjusting the polarization controllers, a stable ML pulses with square shape are also observed. Fig. 3.11 shows the expanded single pulse trains measured from the high revolution oscilloscope at different pump powers with 50/50 output coupler. Unlike the previous results, the pulse maintains a stable square-like shape without pulse breaking. The 3 dB duration of square pulse increases with the pump strength, while the highest amplitude of the pulse almost remains constant as the pump power variation. Fig. 3.12 indicates the corresponding optical spectra of ML pulses with square shape at lower (solid) and higher (dash) pump powers. The center wavelength is located about 1569 nm and the 3dB bandwidth maintains a constant at different pump power.
Fig. 3.11 Expanded single pulse traces under different pump power (With50/50 coupler)
Fig. 3.12 Optical spectra of the square pulses
Fig. 3.13 Pulse energy and output power versus pump power
Fig. 3.14 Pulse energy and output power versus pump power (With 50/50 coupler)
Under the square pulse condition, the threshold pump power for stable mode-locked pulses generation is about 200 mW as shown in Fig.3.13. The maximum pulse energy and output power is 100 nJ and 50 mW under the 560 mW pumping power level. Fig. 3.14 indicates that the square pulse-width versus the pumping power grows linearly and the maximum 3 dB duration of square pulse is 4.6 ns, which is limited by the pump power injected into the cavity.
Based on the measured pulse parameters, we estimate that the peak power of the square pulse output was about 22 W at different pump powers. The square shape pulse can also be experimentally obtained by using a 70/30 output coupler.
Through proper adjusting the polarization state by the PCs and keeping the pumping at the 560 mW, the short pulse-width about 2.4 ns and highest pulse energy about 100 nJ can be experimentally obtained to achieved the highest peak power about 44 W as shown in Fig. 3.15.
Fig. 3.15 Pulse energy and output power versus pump power (With 30/70 coupler)
Fig. 3.16 Expanded single pulse traces under different pump power
The expanded single pulse trains generated from our laser with a 30/70 coupler are shown in Fig. 3.16. Like the previous results, the nanosecond square pulses can still be maintained as the pump power varies.
It is interesting to find that the center wavelength can be fine-tuned through PCs at the same pump power. The center wavelength can be effectively tuned from 1565 nm to 1600 nm by properly rotating the PCs as shown in Fig. 3.17. At the short wavelength region, the 3dB bandwidth is about 10 nm. The obtained spectrum bandwidth will become broadened at the longer wavelengths. However, the relatively narrow bandwidth appears again when the center wavelength approaches 1600 nm. Table 3.1 shows the different center wavelength versus the bandwidth.
Fig. 3.17 Tenability of the center wavelength Center
wavelength (nm)
1564 1567 1568 1580 1598
Bandwidth (nm) 5 6 11 35 9
Table. 3.1 Different center wavelength versus bandwidth
In our fiber laser, there are many different operation states that can be obtained with different polarization controller settings and intra-cavity peak powers. The operation state with broader bandwidth can more easily break to result in multiple pulse operation or harmonic mode locking [3.1]. The laser output power as a function of increasing pump power with different laser operation regimes is shown in Fig. 3.18. The laser operates at a stable fundamental mode-locking state as pump power above 200mW. The optical spectrum and the time trace of pulse trains are illustrated in Fig. 3.19 and Fig.
3.20. It can be seen that the center wavelength is about 1580 nm and the 3dB spectrum bandwidth is 38 nm. The time interval of pulse separation is about 2μs, corresponding to 500kHz repetition rate. The pulses maintained stable operation until 400mW pump power, and the obtained pulse energies are between 35 to 75 nJ.
Fig. 3.18 Pulse energy and output power versus pump power with different laser operation regimes indicated
Fig. 3.19 Optical spectrum in mode-locking state
Fig. 3.20 Time trace of the pulse train in mode-locking state
Fig. 3.21 Unstable Optical spectrum in pulse splitting state
Fig. 3.22 Time trace of the pulse train in pulse splitting state
Fig. 3.23 Time trace of the uncontrolled splitting pulses
Fig. 3.24 Time trace of the pulse train in harmonic mode-locking state
As the pump power is above 400mW, the mode-locked pulses will break up into multiple pulses and the optical spectrum becomes noisy. Fig. 3.21 and Fig.
3.22 show the obtained spectrum and pulse train in the pulse splitting state. This is because the maximum sustained energy of single pulse is limited and the pulses will break as the pumping power increase [3.2]. The splitting pulses are uncontrolled as shown in Fig. 3.23 until the pumping power is increased about 520mW. Harmonic mode-locking state is observed as illustrated in Fig. 3.24 and the repetition rate is thus increased to 1MHz. Fig. 3.25 shows the optical spectrum in harmonic mode-locking state.
Fig. 3.25 Optical spectrum in harmonic mode-locking state
3.2-3 Chirped pulse
From section 3.2 we demonstrate a novel passive mode-locked all-fiber Er-doped fiber laser with the highest pulse energy over 100 nJ. In this section, we want to characterize the chirp properties of our laser outputs for further compressing the pulse-width by an external pulse compression section to obtain the ultra-short pulses with higher peak power. In the beginning, we operate the laser to be with a relatively flat optical output spectrum. Fig. 3.24 and Fig 3.25 show the original optical spectrum and time trace of single pulse. The center wavelength is about 1570 nm and the 3dB spectrum bandwidth is 30 nm. At 411 mW pumping power, the pulse-width measured from high-resolution digital storage oscilloscope is about 350 ps. In section 2.3 we know that the parameter α and δ can be acquired by the pulse-width and the 3dB spectrum bandwidth.
To determine the nonlinear chirp parameter γ, we use a tunable optical filter to select one strip of the optical spectrum from the ML pulses and measure the corresponding pulse-width. In the beginning, we filter out more than half of optical spectrum range and keep only the center part. The center wavelength of optical spectrum is 1569 nm and the spectrum bandwidth is 3 nm as shown in Fig. 3.26. Next, we keep the filtering bandwidth about 3nm and shift the center wavelength from the short edge to the long edge every 9 nm as shown in Fig.3.28 and Fig. 3.30. The filtered pulse shapes measured from the high revolution digital oscilloscope are shown in Fig. 3.27, Fig.3.29 and Fig.3.31. We use these measured pulse-width and spectral bandwidth for nonlinear chirp estimation.
Fig. 3.26 Optical spectrum before filtering
Fig. 3.27 Time trace of single pulse
Fig. 3.28 Optical spectrum before (up) and after (down) filtering.
(middle remain)
Fig. 3.29 Time trace of single pulse (middle remain)
Fig. 3.30 Optical spectrum before (up) and after (down) filtering.
(left edge remain)
Fig. 3.31 Time trace of single pulse (left edge remain)
Fig. 3.32 Optical spectrum before (up) and after (down) filtering.
(right edge remain)
Fig. 3.33 Time trace of single pulse (right edge remain)
After filtering, the measured pulse-width change apparently at different center wavelengths. It is due to the reason that the chirp coefficient of the pulse varies at different center wavelengths, and thus the pulse-width will change at different filtered center wavelengths [3.3]. Fig. 3.32 shows the filtered pulse-width versus different center wavelengths. The pulse-width increases almost linearly as we change the center wavelength from shorter to longer edges and the measured shortest pulse-width is about 150 ps.
Fig. 3.34 Wavelength versus the pulse-width after filtering
Finally, through the filter-wavelength-dependent filtered pulse-width, the third order chirp parameter γcan be estimated. The resulting parameters are shown in Table 3.2
α 5.00458×10-27
(s2) δ 1.48699×10-23
(s2)
γ -7.52141×10-37 (s3)
Table. 3.2 Estimated values of parameters
The characteristics of our laser outputs are now experimentally confirmed.
In principle, we can use them to estimate the proper length of different characteristic fiber to further compress the output pulses.
Reference
[3.1] A. Komarov, H. Leblond, and F. Sanchez, “Passive harmonic mode-locking in a fiber laser with nonlinear polarization rotation,” Opt.
Communications 267, 162-169, 2006.
[3.2] L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen,
“Ultrashort-pulse fiber ring lasers,” Appl. Phys. B, 65, 277–294, 1997.
[3.3] G. P. Agrawal, "Nonlinear Fiber Optics," fourth edition, Academic Press, San Diego, 2001.
Chapter 4 Conclusions
We have demonstrated a passive mode-locked all-fiber Er-doped fiber laser operated with large net round trip anomalous dispersion and large nonlinearity in a long fiber cavity. About 400 m single mode fibers have been added inside the laser cavity to generate stable wave-breaking-free pulses with 500 kHz repetition rate. At 561 mW pump power, the highest output power about 63 mW and highest pulse energy about 126 nJ have been experimtally demonstrated.
Furthermore, a relatively broad and flat optical spectrum of 50 nm has been experimentally obtained with a 50/50 output coupler. This fiber laser configuration may be further applied to other applications that require a large pulse energy or peak power directly from the mode-locked fiber laser.
Nanosecond square profile pulses can also be generated in our laser system by proper adjustment of the PCs. At this state, the ML pulses can be maintained at relatively higher pump power without pulse breaking. It demonstrates that high energy pulses can not only generate in the net normal dispersion or all normal dispersion regions, but also in the large anomalous dispersion region.
The pulse-width of square-shape pulses will become broadened as the pump power increases. It may be the reason for the observed pulse-breaking-free large pulse energy generation.
Finally, the characteristic parameters of the laser output pulses are experimentally estimated. It should help to design the external pulse compression setup for further reducing the pulse width. This will be one of our future research directions.