Fiber Lasers Q-switched by acousticoptic modulator with

In this section we attempt to enhance the loss modulation by controlling the polarization of the incident beam into the AO Q-switch. As a result of the effective phase-grating formed in the AO crystal is transverse to the incident beam, the diffraction efficiency or modulation loss is accordingly polarization-dependent. Hence it should be beneficial to produce a higher extinction rate at lower repetition rate by polarization controlling.

5.4.1 Experimental setup

The schematic of the experimental setup for the passively Q-switched Yb-doped fiber laser is shown in Fig. 5.18. The setup comprises a 3-m Yb doped fiber and an external feedback cavity. The external cavity comprises a re-imaging lens, a Fabry-Pérot etalon for controlling the lasing wavelength, an acousto-optic Q-switch mounted on a copper heat sink and was chilled actively by water, a polarization beam splitter for polarization control, and a highly reflective mirror at 1060 nm~1100 nm for feedback.

The end facets of the fiber were cut to be normal incident. One side of the cavity was a dichroic mirror and was coated high reflection at 1030 nm~1100 nm; the other side was the fiber end which is cut normally as the lasing output with ~4% reflection.

100 μs/div

100 μs/div 50 ns/div50 ns/div

Fig. 5.17. Oscilloscope traces of a train and a typical Q-switched envelope of the hybrid Q-switched laser at maximum pulse energy.

The fiber has a peak cladding absorption coefficient of 10.8 dB/m at 976 nm and a double-clad structure with a diameter of 350 µm octagonal outer cladding, diameter of 250 µm octagonal inner cladding with a numerical aperture (NA) of 0.46, and 30-µm circular core with a NA of 0.07.

The pump source was a 35-W 976-nm fiber-coupled laser diode with a core diameter of 400 µm and a NA of 0.22. A focusing lens with 25 mm focal length and 90% coupling efficiency was used to re-image the pump beam into the fiber through a dichroic mirror with high transmission (>90%) at 976 nm and high reflectivity (>99.8%) at 1066 nm. The pump spot radius was approximately 200 µm. With launching into an undoped fiber, the pump coupling efficiency was measured to be approximately 80%. The pulse temporal behavior was recorded with a digital oscilloscope (LeCroy Wavepro 7100; 10G samples/sec; 4 GHz bandwidth) and a fast InGaAs photodiode. The spectral information was measured by an optical spectrum analyzer (Advantest Q8381A) with a resolution of 0.1 nm.

5.4.2 Experimental results and discussion

Figure 5.19 shows the average power as a function of the frequency of the AO modulation at three different incident pump powers. The average power is almost constant over the operating frequency for the four pump power. From this figure we can calculate the ratio of average power in this experiment to that obtained in section

HT@976 nm

Fig. 5.18. Schematic configuration of the actively Q-switched fiber laser with polarization control.

5.1. The ratio was obtained to be 96 %, which reveals the loss introduced by the PBS is low. However, the loss still limits the range of the operating frequency, which is shorter than that obtained in section 5.1. The lower operating frequency is reduced from 53 kHz in section 5.1 to 20 kHz through the help of the polarization control with a power efficiency of 96 %. Consequently the method of polarization control by a polarization beam splitter is efficient and uncomplicated. The average power also reveals that no significant ASE loss occur at the lowest operating frequency. Fig.

5.20 shows the corresponding pulse energy of the average powers multiplying the operating frequency. Lower pulse energy is approximately observed to be 100 µJ to 120 µJ; however, we did not see there was any upper limit of pulse energy for each pump power as seen in section 5.1.

It is reasonable that at high repetition rate the gain of fiber is modulated at high repetition rate that the undeflected beam can not lase the fiber. Oppositely the polarization beam splitter weakened the undeflected beam such that we can operate at lower repetition rate frequency. The maximum pulse energy was obtained at the pump power of 24 W and the repetition rate of 20 kHz.

0 20 40 60 80 100 120 140

0 2 4 6 8 10 12 14

Average power (W)

Repetition rate (kHz)

11 W 15 W 20 W 24 W

Fig. 5.19. Average power as a function of the operating frequency for four incident pump power.

Figure 5.21 shows the pulse width as a function of pulse repetition rate. The pulse width is shortened from 250~270 µJ ns at high repetition rate to 45~60 ns at the lowest repetition rate. The results are similar to that in section 5.2 where we used saturable absorber to enhance the loss modulation. From Fig.5.20 and Fig. 5.21 the maximum pulse peak power is calculated to be around 14 kW. Fig. 5.22 shows the

0 20 40 60 80 100 120 140 Fig 5.20. Pulse energy as a function of the pulse repetition

Fig 5.21. Pulse width as a function of the pulse repetition rate.

pulse train and a single pulse at repetition rate at 20 kHz and 120 kHz under pump power of 24 W. Under careful alignment, the pulse-to-pulse stability was found to be < 5% at 20 kHz and < 15% at 120 kHz. Fig. 5.23 shows the output spectrum of the Q-switched fiber laser at 20 kHz under pump power of 24 W. The FWHM of the linewidth is about 1.68 nm and the ratio of signal to ASE is estimated 50 dB.

Fig. 5.21 (b) Oscilloscope traces of a train and a typical Q-switched envelope of Q-switched pulses at pulse repetition rate of 20 kHz.

20μs/div

20μs/div 200ns/div200ns/div

100μs/div

100μs/div 50ns/div50ns/div

Fig. 5.22. Oscilloscope traces of a train and a typical Q-switched envelope of Q-switched pulses at pulse repetition rate of (a) 120 kHz; (b) 20 kHz.

(a)

(b)

5.5 Conclusion

We have demonstrated Q-switched Yb fiber lasers by use of acousticoptic Q-switch and combination of SESA and polarization control respectively. . Firstly by use of an acousticoptic Q-switch to Q-switch a fiber laser we have obtained a pulse with energy of 250 µJ and duration of 100 ns. The laser can operate at repetition rate from 53 kHz to 200 kHz with an average power of 13 W. Secondly we combined the active Q-switch and the passive Q-switch to generate a pulsed fiber laser. The laser possesses the merits of larger loss modulation than the former experiment and smaller timing jitter at low pump power than the passively Q-switched laser depicted in section 4.2. The hybrid Q-switched fiber laser can provide a larger pulse energy up to 560 µJ with a pulse duration of 50 ns. Finally we added a polarization control into the first experiment by inserting a PBS inside the external cavity. This method is efficient and simple to enhance the modulation loss. Pulse energy of 600 µJ with pulse duration of 45 ns was obtained at repetition rate of 20 kHz. Besides, all the three experiments are narrowband with a FWHM of < 1.5 nm.

1040 1050 1060 1070 1080

0.0

1030 1040 1050 1060 1070 1080 1090 1100

-50

Fig 5.23. Spectral spectrum of the Q-switched laser at 20 kHz under the pump power of 24 W.

The left one shows the intensity in linear scale while the right is in log scale.

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