We demonstrate an all-fiber passively Q-switched Yb-doped laser using a piece of Sm-doped fiber as a saturable absorber. The laser was pumped by two 25W, 975 nm fiber coupled diodes and Q-switching was initiated when the ASE generated in the core of the gain fiber bleached the Sm-doped saturable absorber. The laser produced 1064 nm pulses with 28 μJ pulse energy and a 200 ns pulse width at a repetition rate of 100 kHz. The pulse energy and peak power are an order of magnitude higher than what previous reported which was also in all-fiber configuration. Effects of laser parameters, such as Sm-fiber length, pump power and duration on laser performance were presented and discussed. Stable Q-switched pulses were obtained at the repetition rate varying from 10 kHz to 100 kHz, which makes this laser suitable for different applications.
©2013 Optical Society of America
In many fiber laser applications such as second harmonic generation and laser material processing, pulsed fiber lasers are preferred because of their high peak power and pulse energy at moderate average power. High peak power can be achieved typically by actively Q-switching the laser with an acoustic optic modulator (AOM). However, complexity of coupling light in and out of a bulk AOM efficiently and extra electronic control of AOM make this method an expensive option for a fiber laser. Passive Q-switching has attracted significant attention for its potential to offer a compact and low cost alternative. Semiconductor saturable absorber mirrors (SESAM)  and carbon nanotubes saturable absorbers have been used for Q-switching and achieved up to 1 kW peak power output. However both methods offer limited pulse energy of less than 10 μJ due to potential optical damage to the saturable absorbers at high energy level .
Research on more robust all-fiber designs has achieved some progress so far. A passively Q-switched Yb-doped fiber laser, with a Sm-doped fiber as a saturable absorber (SA), was demonstrated that generated 19 μJ, 650 ns pulses . However the pulse to pulse stability was poor. Tsai et al. reported an Er-doped fiber laser using a piece of unpumped Er-doped fiber as an SA which produced 8 μJ, 80 ns pulses . The same group lately successfully demonstrated an all-fiber Q-switched laser with mode-field-area mismatching Yb-doped gain fiber and SA fiber which generated 2.8 μJ 280 ns pulses . Their research clearly demonstrated the feasibility of achieving stable Q-switched pulses in all-fiber configuration. However these lasers, so far, produced only relatively low pulse energy and average power, far from the requirements for laser material processing.
In 2011, Soh et al. proposed an all-fiber configuration for a passively Q-switched laser, in which, both gain fiber and SA fiber were Yb-doped, however, with a large core size difference . Their simulation showed that 0.5 mJ pulse energy and 14 ns pulse width could be realized. Very recently, the same group successfully verified this configuration and reported Q-switched oscillator with 40 μJ and 79 ns pulses at 1026 nm . However, the laser used bulk lenses to couple the pump laser into both gain fiber and SA fiber and dichroic mirrors to select wavelengths.
We report here a new configuration for a Q-switching Yb-doped cladding pumped fiber laser using a Sm-doped fiber SA. A truly all-fiber design, with all connection spliced together, can put two pump diodes, a fiber Bragg grating reflector, active fiber, saturable absorb fiber and an output coupler into a small package. The design has achieved temporal stable pulses with a pulse width of 200 ns and pulse energy of 28 μJ, which is approximate 10 times higher than what was reported in .
The fiber laser is monolithic in design with all connections spliced, as shown in Fig. 1 . The pump light from two Oclaro 25 W, 975 nm laser diodes is coupled into the large-mode area (LMA) gain fiber through a (1 + 2) x1 power combiner with a 0.5 dB insertion loss per pump port. The input and output fibers of the combiner match 10/125 μm core/cladding double cladding gain fiber. The 4.5 m long Yb-doped gain fiber (Nufern, LMA-YDF-10/130-VIII) has a cladding absorption coefficient of 3.9 dB/m at 975 nm. The laser cavity consists of a highly reflective fiber Bragg grating (FBG) of more than 99% reflectivity and 0.3 nm bandwidth at 1064 nm spliced at the input port of the combiner, the gain fiber, and an output coupler (OC) FBG of 10% reflectivity and 0.27 nm bandwidth, spliced at the other end of the gain fiber. The OC-FBG was fabricated on the fibers whose core/cladding diameters match the gain fiber.
Within the laser cavity, a piece of Sm-doped single-mode fiber with a core diameter of 6.3 μm and an NA of 0.14, was inserted. The absorption of the saturable absorber fiber at 1064 nm is estimated to be 8 dB/m. The Sm-doped SA was enclosed in a FBG cavity of 1100 nm made on HI1060 fiber. Since Sm ion has a fast relaxation time of 5 ns, the cavity was not used for reducing the up-level lifetime. However we did find about 15% increase in pulse energy after enclose the SA in 1100 nm cavity. The mechanism of this improvement is still under study.
Since the actual core diameter of the gain fiber is 11 μm, the core area ratio of the gain to SA fiber reaches 3:1. When the 975 nm pump excites the population inversion in Yb-doped gain fiber, the amplified spontaneous emission (ASE) is confined in the core and propagating to the left to bleach the SA fiber. A large area ratio will help to store more power in gain fiber before bleaching the SA to start Q-switching. The use of the single-cladding SA fiber will not lead to the loss of pump light in this configuration.
The power supply for the pump diodes can operate in both CW and pulsed modes and its duty cycle can be adjusted when operating in pulsed mode. However, its repetition rate for pulsed mode is limited to 100 kHz. The fiber of the OC-FBG was spliced to an angle-polished connector with a same 10/125 μm core/cladding diameter. To measure the Q-switched pulse train and laser emission spectrum, the laser output was imaged by a lens of 62 mm focal length onto the connector of a patch cord which was directed into either an optical spectral analyzer or a fast photodiode with 1 GHz in bandwidth.
3. Results and discussions
The laser was tested at the repetition rate varying from 10 kHz to 100 kHz. The pump current and duration were adjusted to obtain stable Q-switched pulses in both amplitude and time. When the laser was pumped at the repetition rate of 100 kHz, Q-switched pulse train such as shown in Fig. 2(a) , was obtained. The peak amplitude variation was about 4.1% in standard deviation and the pulse duration varied at 1.2%. Similar pulse amplitude stability and low pulse jitter were observed at other repetition rates. The single Q-switched pulse is shown in Fig. 2(b) which has a slightly asymmetric shape with a steep rising edge and a slower tailing edge. This quasi- symmetric pulse was also reported in , in which, the Sm-doped fiber is also used as SA for an Er-doped gain fiber to achieve Q-switching. The pulse shape was interpreted as the result of Sm’s low saturable loss than its non-saturable losses.
When the laser was Q-switched at 100 kHz, the average output power of 2.8 W was obtained that gave the energy per pulse of 28 μJ, more than 15 times the energy of SESAM or carbon nanotube Q-switched laser pulse and 10 times higher than what reported in . The peak power of 140 W was obtained at the pulse width of 200 ns. At lower repetition rates, such as 10 kHz and 60 kHz, stable pulses were also obtained, as shown in Fig. 3 .
When the laser was operated at 100 kHz, the duty cycle of the pump was kept at ~32%, as shown in Fig. 4 . This indicates that the laser had the potential to go to a higher repetition rate, and thus, further increase its average output power. One important feature of this laser is its adjustable repetition rate by changing the repetition rate of the pump laser. A laser with a variable pulse rate but same pulse shape will make it versatile for various applications.
In general, the pump duration should be kept as long as possible to store more energy in the gain medium before Q-switching. However excessive long pump time will trigger relaxation oscillation as illustrated in Fig. 5 which was obtained at the same pump amplitude of 52W, however, with a longer duty cycle than what used in Fig. 4. After Q-switched pulse the gain fiber continues to accumulate energy which led the generation of the relaxation pulses with less energy even after the pump is turned off. When that happened, the Q-switched pulses become unstable; their amplitudes fluctuate and pulse jitter increases. In our experiment, the pump was turned off before the onset of the after-pulses. For example, at the 100 kHz repetition rate, the pump time was kept at less than 3.2 μs which prevented the start of the after-pulses. When the laser was CW pumped, Q-switched pulses and relaxation pulses were mixed that made whole pulse train chaotic.
The laser emission spectrum, at 1063.7 nm with a 0.175 nm bandwidth, is plotted in Fig. 6 , which shows an excellent optical signal to noise ratio of ~70 dB. There is a small peak at 1100 nm, caused by the 1100 nm cavity for Sm-doped SA. The 1100 nm peak is more than 60 dB lower than the main laser emission and should not be detrimental to the applications.
The average pump power was measured after the combiner when pump laser operated at the pulse mode. The corresponding laser output power was measured as a function of pump power as shown in Fig. 7 . The laser efficiency is indicated by the slope of the linear fitting line and is calculated to be 28%, which is far exceeding the slope efficiency of 9% in . There is no trend of saturation at the line, indicating the potential of producing higher laser power without sacrificing efficiency. Further power scale-up can be made by increasing the maximum power of each pump diode, or place more diodes with a (1 + 6) x1 power combiner.
For a given SA length of 24.5 cm at the pump frequency of 100 kHz, we measured the pump duration as a function of pump power amplitude. The pump duration was carefully selected to avoid the occurrence of after-pulses: it was increased firstly until a weak after-pulse was observed, and then decreased with a small decrement step until the after-pulse was flattened. This procedure guarantees a stable pulse train at the highest pulse energy without an after-pulse. The pump duration decreased from 5.8 μs to 3.2 μs with the increase of pump amplitude from 21 W to 52 W, as shown in Fig. 8(a) . The corresponding laser pulse width and pulse energy were plotted in Fig. 8(b), which showed the pulse width decreases and the pulse energy increases as the pump amplitude increases. The results as such are expected; as the pump amplitude increases, the ASE in the gain fiber takes less time to build up and to bleach the SA. Moreover, since the pump light intensity was higher, once the SA was bleached, the higher population inversion in the gain fiber was depleted faster which resulted in shorter pulse width and higher pulse energy extracted in the Q-switching process. The high power intensity in the cavity improved the stimulated emission process. Therefore, an effective way to increase the peak power and pulse energy is to increase the amplitude of the pump power. The slope of the pulse energy plotted in Fig. 8(b) shows no roll-off at high pump power, indicating that higher pump power can be employed to further increase the pulse energy. However, at a very high pump level, due to limited availability of Yb-ions in the gain fiber, both the pulse energy and pulse width would eventually saturate.
In order to determine the effects of SA length on the laser performance, we adjusted the pump duration for achieving stable Q-switch at three different SA lengths of 50cm, 35cm and 24.5cm, respectively. We found that shorter SA fiber decreased the pump duration, reduced only slightly the laser pulse width and increased the pulse energy by about 15%. We considered that the threshold for ASE to bleach the SA decreases with a shorter of SA fiber, which led to the shorter pump duration required to reach the ASE threshold. Due to the fast decay and high loss of Sm-doped fiber, the shorter the SA, the less energy lost in propagating through the SA which led to the increase of output pulse energy. However, if higher pump power is available, longer SA fiber is preferred to force the gain fiber to higher population inversion and therefore produce pulses with higher energy. The optimized SA length is required for the available pump power.
We demonstrated a new configuration for a passively Q-switching Yb-doped fiber laser, which has achieved a pulse energy and peak power higher than those of other all-fiber Yb-doped Q-switched lasers previous reported. The high pulse energy of 28 μJ and a peak power of 140 W were obtained at a repetition rate of 100 kHz. The pump duration as a function of pump power amplitude was studied for the purpose of achieving stable Q-switched pulses. The all-fiber design makes the laser simple, compact, easy to maintain and low cost. The variable repetition rate makes it versatile for different applications. The laser can be used as a stand-alone unit for some material processing such as cutting and drilling, or as a seed laser for a single stage fiber amplifier to achieve over hundreds of watts of average power, mJ-level pulse energy, and multiple kW of peak power.
References and links
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