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• Abstract

SECTION I

## INTRODUCTION

Pulsed fiber lasers are highly in demand in both scientific laboratories and industry for their high energy and peak power at moderate pump power. Their extensive applications can be found in laser radars, material processing, second or third harmonic generation [1], [2], and supercontinuum generation [3], [4]. Actively Q-switching by means of acoustic optical modulator (AOM) usually generates the expected results. However, due to the use of bulk optics, coupling loss between fiber and AOM crystal is relatively high. Since it is necessary to suppress the first few giant pulses in AOM, the control electronics is more complicated.

The passively Q-switched fiber lasers which use semiconductor saturable absorber mirrors (SESAM) [5] or saturable absorber (SA) fibers [5]– [6] [7] [8] [9] [10][11] are much preferred due to their simple structure and high reliability. Yang et al. reported a configuration using SESAM and Tm/Ho-doped double clad fiber to achieve 1.14 $\mu\hbox{J}$ 490 ns pulses [5]. However, the pulse energy was limited by the low damage threshold of SESAM in their design. Research on the more robust all-fiber designs has achieved some progress so far. The first passively Q-switched Yb-doped fiber laser in an all-fiber configuration was reported by A. Fotiadi et al. in which, Sm-doped fiber was used as a saturable absorber and 19 $\mu\hbox{J}$ 650 ns pulses were demonstrated [6]. However, the generated pulse train was not very stable. Earlier this year, we reported an all-fiber passively Q-switched Yb-doped laser using a piece of Sm-doped fiber as a saturable absorber and achieved stable 28 $\mu\hbox{J}$, 200 ns pulses [7].

When selecting a proper SA fiber for an efficient Q-switched laser, its absorption cross-section should be larger than the emission cross-section of the gain fiber at the lasing wavelength. Larger absorption cross-section leads to faster saturating SA, and hence shorter pulse width. This constrain limits the type of materials that can be used to make saturable absorber fibers. Tsai et al. proposed the use of mode-field-area (MFA) mismatching method in an Er-doped fiber laser using a piece of unpumped Er-doped fiber as an SA and successfully generated 8 $\mu\hbox{J}$ 80 ns pulses [8]. The need for large absorption cross-section in SA was overcome by using a large core gain fiber and a small core SA fiber so the photon density can be significantly enhanced in the SA fiber to initiate Q-switching. The same group lately demonstrated an all-fiber Q-switched laser with a MFA mismatching Yb-doped gain fiber and a SA fiber which produced 2.8 $\mu\hbox{J}$ 280 ns pulses [9]. Their research clearly demonstrated the feasibility of using Yb-doped fibers as a viable SA to achieve Q-switched pulses in an all-fiber configuration. However, the issue remains for the Q-switched laser to obtain high energy and shorter pulse width so that the peak power could reach kW level for material processing or other applications. In 2011, Soh et al. proposed an all-fiber configuration for passively Q-switched lasers, in which the SA fiber was enclosed in a second cavity at a longer wavelength to improve the relaxation [10]. Their simulation showed that for a high mode-field-area mismatching ratio, a short pulse duration of 14 ns could be realized. Very recently, the same group successfully demonstrated a passively Q-switched laser with 40 $\mu\hbox{J}$ and 79 ns pulses at 1026 nm [11]. However, the laser used bulk lenses to couple the pump laser into both the gain fiber and the SA fiber, and used dichroic mirrors to select wavelengths. An all-fiber Er-doped Q-switched fiber laser was reported by Kurkov et al. with a piece of Tm-doped fiber as a saturable absorber [12]. The high peak power of 3.5 kW was achieved with a pulse width of 100 ns. However, this laser has a low average power of 0.7 W and a low slope efficiency of 12%. Another Yb-doped fiber laser, which is Q-switched and gain-switched with a dual-cavity structure, was reported recently [13]. Although a peak power of 1.4 kW was achieved, its 62 $\mu\hbox{J}$ pulse energy and 1.8 W average power are on the low end for this laser to be used as a stand-alone unit for micro-machining.

We present here a passively Q-switched Yb-doped fiber laser in an all-fiber configuration. In order to significantly increase the peak power, a piece of Yb-doped fiber with a smaller core than the gain fiber is used as an SA. The laser produces temporal stable pulses with a pulse duration of 140 ns and a pulse energy of 141 $\mu\hbox{J}$, which is more than 3 times higher than what has been reported in [11]. The peak power reached $\sim$1 kW which is twice as high as that reported in [11]. The average power of 14 W at 100 kHz is also sufficiently high to make this laser a stand-alone unit for certain material processing. We believe that this laser achieves the highest pulse energy and peak power reported in MFA mismatching Yb-doped all-fiber lasers. It also retains the advantage of a variable repetition rate as reported in [7]. Such a laser is compact and versatile which can be employed as a portable device for surgical applications. For example, this laser was recently used to cut a 4 mm thick bone at a speed of 0.5 mm per second.

SECTION II

## EXPERIMENTAL SETUP

In this experiment, we used a power combiner made by ITF Lab to couple the pump light, provided by two 55 W diodes at 976 nm, into the laser cavity. The combiner has an insertion loss of 0.3 dB between signal to output port, and of 0.4 dB between pump to output port. The schematic diagram of the laser is shown in Fig. 1.

Fig. 1. The configuration of all-fiber passively Q-switched fiber laser with a smaller core Yb-DCF as a saturable absorber.

The gain fiber, a 15 $\mu\hbox{m}$ core and 130 $\mu\hbox{m}$ cladding Yb-doped double-cladding fiber, has a nominal absorption rate of 5.4 dB/m at 976 nm (Nufern, Part #: LMA-YDF-15/130-VIII) with a core numerical aperture of 0.078. The length of the gain fiber is 4.5 m which is spliced to the output port of the combiner in a backward pumped configuration. The 0.4 m long Yb-doped absorber fiber has a 7$\mu\hbox{m}$ core and 128 $\mu\hbox{m}$ cladding. The SA fiber was spliced between two fiber Bragg gratings (FBGs) inscribed at 1100 nm on a HI1060 fiber to form a cavity which reduces the recovery time after saturation, as suggested in [10]. The unabsorbed pump power is stripped between the gain fiber and the 1100 nm HR-FBG to prevent pump power entering the cladding of the SA fiber. Counter-pump configuration was used in Fig. 1 since amplified spontaneous emission (ASE) generated in gain fiber propagated to the left to bleach the SA. Co-pump may stimulate the lasing in the gain fiber due to the reflection from the output coupler FBG, which was the reason that the co-pump was not used.

The one end of the SA cavity is spliced to the gain fiber using the mode-field matching program of a fusion splicer (Ericsson FSU995FA, program p6). The estimated splice loss is 0.2 dB. It is well-know that mode-matching splicing can significantly reduce the splice loss between the fibers of different core diameters. This process is widely used in the assembly of Er-doped fiber amplifier. A highly reflective FBG of & 99% at 1064 nm was spliced to the left side of the SA fiber as a highly reflective mirror and an FBG of 4% reflectivity at 1064 nm was spliced to the input signal port of the combiner to form the main laser cavity. This arrangement also makes the main cavity reasonably short. Notice that the ratio of the core areas between gain fiber and SA fiber is 4.6. Such a high ratio in MFA shall lead to rapid saturation of the SA fiber and high extraction efficiency of the stored energy in the gain fiber. The two fiber ends are spliced to angle polished fiber connectors (FC/APC) in order to prevent reflection back to the cavity. The residual pump power in the cladding is stripped before the output fiber connector. To measure the spectrum or pulse train of the laser emission, the laser output was imaged, by a lens of 62 mm long focal length, onto an angle-polished connector of a patch cord. The other end of the patch cord can be inserted into either a photodiode (NewFocus model:1811) or an optical spectral analyzer (ANDO, model: AQ6317). The amount of light coupled into the patch cord can be reduced by inserting a neutral density filter in the path of the light.

SECTION III

## RESULTS AND DISCUSSION

The principle of Q-switching process is similar to the laser reported in [7], however with a different SA fiber. When pump power is absorbed in the gain fiber, the ASE starts to build up. Nevertheless, the high loss of the SA in the cavity prevents the lasing. When ASE reaches a sufficiently high level to bleach the SA, the sudden increase of Q-factor in the cavity initiates the Q-switch process since the higher population inversion is already established in the gain fiber. The Q-switched pulse depletes population inversion which returns the cavity to an absorbing state. A larger diameter ratio between the gain fiber and the SA fiber allows more energy stored in the gain fiber before Q-switching.

The two pump diodes were connected in series and driven by a pulsed power supply, which has an adjustable duty cycle and frequency up to 100 kHz. The laser was tested at three repetition rates, namely, 10, 60, and 100 kHz. At each repetition rate, the laser output was recorded at different current amperage varying from 5 to 14 A; which corresponded to the pump power amplitude from 53 W to 136 W. The duty cycle was adjusted for each operation condition to ensure that the relaxation oscillation pulses would not occur.

Steady pulse trains were successfully obtained in all of these conditions. At the 100 kHz repetition rate and a pump amplitude of 136 W, stable pulses with an average power of 14.1 W, and a pulse width of 140 ns were generated. Energy per pulse of 141 $\mu\hbox{J}$ was calculated from the measured average power divided by the repetition rate. The peak power of 1007 W was estimated by integrating a single pulse signal over the time and normalizing the pulse energy to 141 $\mu\hbox{J}$. The pulse train recorded by an oscilloscope is shown in Fig. 2(a) and a single pulse is shown in Fig. 2(b) with no averaging used. The pulse train was measured at 10000 sampling points per trace with a sampling rate of 125 MHz to ensure sufficient data point to track the shape of Q-switched pulses. The pulse amplitude variation was calculated to be 2.8% in a standard deviation and the pulse width varied at about 4%. Similar pulse amplitude stability was observed at lower frequencies with different pump power and pump duty cycles. The stability is comparable to the reported pulse stability in [7], indicating that we have successfully scaled up the power with a Yb-doped fiber SA instead of an Sm-doped fiber SA while retained its stability. Fig. 2(c) and (d) shows stable pulse trains at the repetition rate of 60 kHz and 10 kHz, respectively. We noticed that the longer pump duration was required, for example, 12% long at 60 kHz and 32% at 10 kHz than that at 100 kHz in order to maintain similar pulse width and pulse energy. The laser pulses remain stable at both repetition rates. Stability of a pulsed laser is an important parameter for many applications. The high stability of this laser is also an evident to show that the SA is well recovered between pulses.

Fig. 2. (a) Oscilloscope trace of the Q-switching pulses at 100 kHz rate; (b) a single pulse with 141 $\mu\hbox{J}$ energy and 140 ns (FWHM) pulse width. (c) Oscilloscope traces of the Q-switching pulses at 60 kHz, and (d) 10 kHz.

The laser spectra were measured with an optical spectrum analyzer at a resolution of 0.01 nm. When the laser was operated at a repetition rate of 100 kHz and a pump amplitude of 136 W, the laser line width of 0.16 nm at 1064 nm was observed. A signal-to-noise ratio of more than 65 dB was achieved in the measurement as shown in Fig. 3(a). Laser emission spectrum at the pump amplitude of 63 W is shown in Fig. 3(b) for comparison and a narrower line width of 0.13 nm was obtained. No significant laser emission at 1110 nm (the inner cavity resonance wavelength enclosing the SA) was observed, indicating that the undesirable emission at output was well suppressed. The output beam showed a perfect circular shape with high intensity in the center as observed with an infrared viewer when the laser was projected to a far-away screen.

Fig. 3. Emission spectrum of the laser at (a) 136 W of power amplitude (line width is 0.16 nm), and (b) 63 W of power amplitude (line width is 0.13 nm).

A broad emission peak around 1120 nm was observed and identified as the emission from stimulated Raman scattering (SRS). However, the SRS amplitude is about 70 dB below the main lasing emission. In general the threshold condition of the SRS is given by: $P_{th}^{SRS} \approx 16\pi a^{2}\Gamma ^{2}g/g_{R}$ [14], where $a$ is core radius, $\Gamma$ is the ratio of the mode field radius to the core radius, $g_{R}$ is the Raman gain coefficient, and $g$ is the laser gain per meter. Given that $a = 7. 5 \ \mu\hbox{m}$, $\Gamma = 0. 8$, $g_{R} = 0. 5 \times 10^{- 13} \ \hbox{m/W}$, and $g = 0. 44 \ \hbox{m}^{- 1}$, the peak power threshold of the SRS in this gain fiber is approximately 6.2 kW. It indicates that this laser configuration still has the potential for further power scaling up despite the onset of SRS effect.

At 100 kHz, the average pump power is calculated by multiplying pump amplitude with the modulated pump duty cycle, and is plotted against the measured average laser output power in Fig. 4(a). The slope efficiency of 51% was achieved which is more than 4 times higher than what reported in [12].

Fig. 4. (a) Output laser power versus average pump power; and (b) a comparison of pulsed pump power (dots) and output laser power (solid) at 100 kHz.

The pump duration needs to be reduced as pump amplitude increases in order to prevent the occurrence of relaxation oscillation pulses. At pump amplitude of 136 W, the duty cycle was reduced to 27% or 2.7 $\mu\hbox{s}$ at 100 kHz as shown in Fig. 4(b). The current rise time is less than 25 ns. We noticed that time jitter of the pulses will increase significantly if the duty cycle was not reduced, or the relaxation oscillation started to occur. After the pump turned off, it needs around 1 $\mu\hbox{s}$ for the gain medium to recover and be ready for the next Q-switch cycle. Therefore, this laser has a potential to be operated up to 250 kHz with stable output pulses. Notice that two pump diodes require only square-shaped current pulses in operation, therefore, the power supply for this Q-switched fiber laser is much simpler than the power supply for the AOM based pulsed laser.

We measured the pulse width and pulse energy of the laser output as a function of pump power amplitude at the repetition rate of 100 kHz and the results were plotted in Fig. 5(a). When the pump amplitude increased from 53 W to 136 W, the pulse width decreased from 288 ns to 140 ns, and the pulse energy increases from 77 $\mu\hbox{J}$ to 141 $\mu\hbox{J}$. Higher pump amplitude leads to higher gain in the gain fiber and thus it takes less time to deplete the population inversion after the on-set of Q-switching. As a result, higher pulse energy with a narrower pulse width was generated at higher pump amplitude. Neither the pulse energy curve nor the pulse duration curve shows the trend of saturation, indicating the possibility of further power scalability if more pump power become available. Fig. 5(b) shows the linearity of the laser peak power as a function of pump amplitude.

Fig. 5. (a) Laser pulse width and pulse energy versus pump power amplitude; and (b) laser peak power versus pump power amplitude.

A theoretical model was developed according to the travelling wave method [15], [16] for both the gain fiber and the SA fiber. The pump power in cladding was mostly absorbed by the gain fiber and the unused pump power was stripped before entering the SA fiber and thus spontaneous emission is omitted in SA fiber. However, since the ASE generated in the gain fiber entering the core of the SA fiber to bleach it and the secondary cavity at 1100 nm induces the emission at 1100 nm to expedite the recovery of the SA to its ground state after Q-switching, both absorption cross-section and emission cross-section in the SA are included in the simulation. Pulsed pump scheme is used in the simulation, where the pump is turned off in each Q-switching cycle when the Q-switched pulse reaches its peak.

In the simulation, we used parameters as close as those used in the experiment. Splice loss of 0.2 dB for all mode-field matching splice point and zero for other splice point were assumed. The simulated pulses in Fig. 6(b) have a good agreement with the measured pulse train obtained from the oscilloscope in Fig. 6(a). The simulated pulses have a slightly higher peak power of $\sim$1.1 kW and a higher pulse energy of 157 $\mu\hbox{J}$. The small discrepancy can be attributed to the underestimated splicing losses of the cavity.

Fig. 6. A comparison of (a) the measured pulse train and (b) the simulated pulse train.

From the simulation, the direction for laser output improvement can be learnt. For example, shortening the cavity length leads to a shorter pulse width, however the gain fiber has to be sufficiently long in order to fully absorb the pump power. Increasing pump power amplitude and Yb-concentration in gain fiber lead to higher pulse energy; further experiment is planned for power-scaling up using high power pump diodes and high concentration Yb-doped active fibers.

SECTION IV

## CONCLUSION

We demonstrated a stable Q-switched laser in an all-fiber configuration with a high pulse energy of up to 141 $\mu\hbox{J}$ and a high peak power of 1 kW. The repetition rate of the laser is adjustable, tested up to 100 kHz with a potential to increase to 250 kHz. High slope efficiency of 51% was achieved. Since the stimulated Raman scattering threshold is estimated at $\sim$6.2 kW, the output peak power can be further scaled up to 5–6 kW range if the pump diodes with higher power are available. Due to its high peak power and adequate average output power of 14 W, the laser can be used as a stand-alone module for certain material processing such as cutting thin stainless steel sheets and laser marking. Because of its all-fiber structure, the laser is currently packaged on a 12” square-shaped optic breadboard which can be further reduced. Beside a stand-alone module, the laser is also suitable for being used as a seed laser for a power amplifier. A single-stage amplifier should be able to scale its peak power up to 40 to 50 kW range with an adjustable repetition rate for producing customer-desirable average power.

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