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Stable passively Q-switched and gain-switched Yb-doped all-fiber laser based on a dual-cavity with fiber Bragg gratings Dongchen Jin, Ruoyu Sun, Hongxing Shi, Jiang Liu, and Pu Wang* Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China * [email protected]

Abstract: We demonstrate a stable passively Q-switched and gain-switched Yb-doped all-fiber laser cladding-pumped by a continuous fiber-coupled 976 nm laser diode. By use of an all-fiber dual-cavity, the efficient elements of the laser mainly include the fiber Bragg gratings and rare-earth doped fiber, allowing the oscillator to be integrated in a compact size with reliable and stable output. In this scheme, an efficient laser output with 45 ns pulse width, 62 μJ pulse energy, and 1.4 kW peak power operating at 1081 nm was obtained. To the best of our knowledge, this is the minimum pulse width in this similar kind of all-fiber configuration at present. Sequential nanosecond pulses were obtained at the repetition rate of several to tens of kHz with the variation of the diode pumping power. Effects of laser parameters such as pump power, cavity length, external-cavity wavelength, and FBG reflectivity on laser performance were also presented and discussed. ©2013 Optical Society of America OCIS codes: (060.2310) Fiber optics; (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched.

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#197677 - $15.00 USD Received 16 Sep 2013; revised 10 Oct 2013; accepted 11 Oct 2013; published 23 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026027 | OPTICS EXPRESS 26027

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1. Introduction Fiber lasers have distinguished properties of good beam quality, high conversion efficiency and stable operating performance, and they have attracted worldwide attention for various applications, such as range finding, remote sensing, material processing and pumping sources for optical parametric oscillators. Q-switched fiber lasers, which normally operate in tens to hundreds of nanosecond pulsed region, become more and more competitive because of their capabilities in building up large accumulated gain and producing pulses with relatively high energy [1–6]. Passively Q-switched operation is an alternative approach to achieve a compact and rugged laser source with short-duration and high-intensity Q-switched pulses. In recent years, there have been reports of fiber lasers using different types of fiber saturable absorbers (SAs) for the pulse generation, which are different from traditional bulk SAs [7–9]. Accordingly, all-fiber laser system using rare-earth ion-doped fiber, such as Yb [10], Sm [11], Bi [12], Cr [13], and Ho [14], as gain fiber or SA element can accomplish a simple system in relatively lower cost, and gradually attract more and more attention. A. A. Fotiadi et al. reported a passively Q-switched laser using an Yb-doped gain fiber and a Sm-doped SA fiber to produce 650 ns, 19 μJ pulses. A segment of Sm-doped fiber was placed in the cavity of Yb-doped fiber laser to get the pulse generation, but the pulse to pulse stability was relatively poor [15]. In 2009, a self-Q-switching mechanism using mismatch of mode field areas in a standing-wave fiber laser was proposed and demonstrated [16]. Tsai et al. reported an Yb-doped fiber laser which was saturable absorber Q-switched at 976 nm and gain-switched at 1064 nm, using the method of mode-field-area mismatch [17]. The numerical simulation of an all-fiber passively Q-switched fiber laser was achieved by Soh et al., who used a large mode area Yb-doped fiber as gain medium and unpumped single-mode Yb-doped fiber as SA [18]. However, the typical problems associated with the passive fiber SAs, such as long pulse duration, large amplitude fluctuation and timing-jitter hindered further development of such passively Q-switched fiber laser. Recently, V. V. Dvoyrin proposed and demonstrated a pulsed fiber laser with cross-modulation of laser cavities in which he achieved pulse train with low timing jitter and high pulse energy. In his opinion, the pulse mode was realized by switching off the gain of the external cavity, and the pulse duration was mainly controlled by the external cavity [19]. Here, by use of an all-fiber dual-cavity, we demonstrate a passively Q-switched and gainswitched nanosecond Yb-doped fiber laser with relatively high stability. The dual-cavity, which includes an external cavity and an internal cavity, consists of Yb-doped fiber and fiber Bragg grating (FBG) pairs, removing the complex free-space elements to achieve a simple, reliable and stable all-fiber configuration. In this paper, all components are directly spliced, allowing the laser to be integrated in a compact size with reliable and stable output. In this design, the external cavity is Q-switched by a piece of Yb-doped single-mode SA fiber used in the internal-cavity, and the internal-cavity is gain-switched by the pulses generated in external-cavity pulses. Moreover, the gain-switched pulses would be further amplified in the propagation of the external-cavity. Therefore, sequential stable high energy pulses would be obtained at wavelength defined by the internal cavity. Effects of laser parameters such as pump power, cavity length, pump wavelength, and FBG reflectivity on laser performance are presented and discussed.


2. Experimental set-up

Fig. 1. Schematic design of the dual-cavity fiber laser.

The dual-cavity is constructed in a linear cavity configuration depicted in Fig. 1. The dualcavity, which includes an external-cavity of 1040 nm and an internal-cavity of 1081 nm, consists of Yb-doped double-clad single-mode fiber and FBG pairs (writing on the single-clad passive fiber with core diameter of 6 μm). The external cavity consists of two narrow linewidth, high reflectivity (HR≥99%) fiber Bragg gratings (FBGs) with the central wavelength of 1040 nm and a segment of Yb-doped double-clad fiber (DCF). The core/inner cladding diameters and numerical aperture of the Yb-doped fiber are 7/125 μm and 0.19/0.45, respectively. The cladding absorption coefficient of the 7/125 DCF is 5 dB/m at 975 nm, and the mode field diameter (MFD) of 7/125 fiber is calculated to be 6.0 μm at 1040 nm. Accordingly, the internal 1081nm cavity is composed of a pair of FBGs, a HR one and a low reflectivity one (R = 90%) used as the output port, and a piece of 5/130 sing-mode Yb-doped fiber with cladding absorption coefficient of 1.7 dB/m at 975 nm and NA of 0.12. The MFD of 5/130 fiber is calculated to be 7.2 μm at 1080 nm. The entire system is fiber-integrated, making it misalignment free and largely immune to mechanical vibrations. All these elements are directly spliced to ensure a truly all-fiber configuration and the integration in a compact size. In this scheme, the gain medium of the 1040 nm external-cavity is 2 m long, which is CW cladding-pumped by a 976 nm laser diode through a (2 + 1) × 1 pump combiner. And the lasing 1040 nm radiation is Q-switched by the 2 m long Yb-doped DCF of the 1081 nm internal-cavity. However, there is little output of the external radiation as a result of the high reflectivity of 1040 nm FBGs. Then the internal cavity is core-pumped by the generated 1040 nm radiation. The FBG with 90% reflectivity of 1081nm is used as the output port, and the 1081 nm pulses can be further amplified in the propagation of the external cavity. 3. Results and discussion The threshold pump power for the generation of pulses under the condition of 1040 nm external-cavity is almost 1.5 W, and the repetition rate of the pulse train is about 5 kHz. The output power as a function of pump power is plotted in Fig. 2, which is proportional to the pump power. And the maximum output power is ~1.8 W at ~4.3 W incident pump power with 60.3% slope efficiency. The experimental results have demonstrated the potential for higher energy pulses with larger core-diameter fiber and more powerful pump diodes. The slope efficiency is much higher than that in the mode-field-area mismatch method, which is caused by the low coupling loss and the amplification of the generating pulses propagating in the external cavity. The mechanism using mismatch of mode field areas makes use of high intensity of the ASE in the single-mode fiber to bleach the absorption, thereby making the SA fiber transparent, before the onset of gain depletion in the gain fiber, resulting in short pulses in the sacrifice of optical energy and efficiency.


Fig. 2. Output power versus pump power of the dual-cavity fiber laser.

Fig. 3. Optical spectrum of the dual-cavity fiber laser.

Figure 3 is the optical spectrum of all-fiber passively Q-switched and gain-switched laser. The center wavelength is 1081 nm and the full width at half maximum (FWHM) bandwidth is ~0.6 nm measured by optical spectrum analyzer (YOKOGAWA AQ6370B) with resolution of 0.02 nm. The tiny structure of the peak spectrum may be induced by the FBGs working in the high power condition. Besides, a negligible output of 1040 nm can be found from the spectrum, owing to the leakage of radiation from the FBG with high reflectivity.

Fig. 4. (a) Oscilloscope trace of the stable pulse train at 30 kHz repetition rate; (b) A single pulse with 45 ns pulse width and 62 μJ pulse energy.

Figure 4(a) is the measured oscilloscope trace over a 300 μs time scale at 30 kHz repetition rate. Stable pulses can be obtained at the repetition rate varying from 5 kHz to 30 kHz as the pump power increases. And the minimum pulse width is 45 ns with 62 μJ pulse energy, as shown in Fig. 4(b). To the best of our knowledge, this is the minimum pulse width obtained at such low pump power in this similar kind of all-fiber configuration at present. At the meantime, the Q-switched pulse duration is estimated to be about 90 ns observed by the oscilloscope. Figure 5(a) plots how the pulse width and repetition rate change as a function of pump power. The pulse width is not only related to the pump power, and it can be further


shortened by reducing the dual-cavity length including the active and passive fiber length. Furtherly, we can change the active fiber to another one with higher dopant, keeping enough absorption of the pump at a shorter cavity length, to generate shorter pulses. Moreover, the reflectivity of the two FBG pairs plays an important role in the output power and the pulse width. Higher reflectivity FBGs can accumulate more energy to bleach the SA fiber, leading to relatively low extraction of the stored energy. The relation of peak power and pulse energy versus pump power is shown in Fig. 5(b). The peak power and pulse energy are proportional to the pump power, and the maximum peak power is 1.4 kW with maximum 62 μJ pulse energy at 4.3 W incident power. If the pump power further increases, there is no obvious change in the pulse energy, and it may induced by the limit of the extractable energy of the single-mode fiber. Despite this, the pulse energy and peak power are much higher than some of the previous reports which were also in all-fiber configuration.

Fig. 5. Pulse characterics of the laser. (a) Pulse width, repetition rate versus pump power; (b) Peak power, pulse energy versus pump power.

In addition, we have checked the laser performance with this similar configuration for the external cavity with different wavelengths. The laser can operate stably when the wavelength of external cavity is 1040 nm, 1050 nm or 1060 nm, respectively. When λ = 1050 nm, pulse train can be obtained at the repetition rate varying from 6 kHz to 52 kHz with the pump power changing from 1.1 W to 6.7 W. The maximum output power is 3.2 W with 53.7% slope efficiency. The minimum pulse width is 78 ns at the repetition rate of 52 kHz. While there is still potential to improve the laser performance when λ = 1060 nm. By comparing the parameters and laser performance of the all-fiber pulsed laser, a conclusion can be made that the pulse width is related to the length of dual-cavity, wavelength of external cavity and the reflectivity of FBGs. If the shorter wavelength of external cavity is selected (λ≥976 nm), the higher core-absorption of the gain fiber in the internal cavity would help to reduce the length of dual-cavity to generate shorter pulses, owing to the shorter round-trip time. Then, based on the previous research, the reflectivity of FBGs also plays the important role in generating short pulses with high energy. If we change the reflectivity of output FBG from 80% to 90%, sequential pulses can be obtained with the minimum pulse width changing from 171 ns to 155 ns, but it might go against the extraction of the stored energy, especially in the condition of the low slope efficiency. If the external lasing wavelength and reflectivity of FBGs have been set, the cavity length should be adjusted to modify the laser performance. The further decrease of cavity length would achieve shorted pulses while the gain fiber of external cavity should be long enough to provide ASE to bleach the SA fiber in the internal cavity. Table 1. Details of the parameters and laser performance of the all-fiber pulsed laser λ, nm 1040 1050 1060

L1,nm 2 4 4.5

L2,nm 2 3.4 10.5

R, % 90 90 80

η,% 60.3 53.7 35

Pav,W 1.8 3.2 2

τ,ns 45 78 171

Rep,kHz 30 52 36

P, W 1400 794 322

Ppump,W 4.3 6.7 7.2


λ: the wavelength of external cavity; L1: the length of 7/125; L2: the length of 5/130; R: reflectivity of the OC FBG; η: the slope efficiency of the laser; P: output power; τ: pulse width; Rep: repetition rate of the pulse train; P: peak power; Ppump: pump power For a Q-switched laser, Herda et al. have described a novel mechanism of pulse shortening induced by the gain compression effect under strong pumping conditions [20]. They estimated the pulse width from Eq. (1) in the limit of large coupling ratio, where the cavity loss is much larger than the saturable absorber loss.

τ=

7.04Tr 3.52Tr = Δg q0 + A P Pthreshold − 1

(1)

With A=

2Tr

τg

⋅ log(

Esat , g

τ g P0

) ⋅ (l + q0 )

(2)

g P (3) = 0 Pthreshold l + q0 Where q0 is the saturable absorber loss and g0 is the small signal gain. Esat,g is the saturable energy and τg is the recovery time for the saturable gain. l is the cavity loss and Tr is the round-trip time. From the equation, we learn that the pulse width is proportional to the roundtrip time and inversely proportional to the pump power, which is in agreement with Fig. 5. As mentioned above, it has also been identified in experiment that the pulse width depends on the cavity length including both the active and passive fiber. If shorter lasing wavelength in the external-cavity is selected, the absorber coefficient of SA fiber core-pumped by the externalcavity will be higher, so the total cavity length can be further shortened in order to reduce the round-trip time. Meanwhile, the gain fiber and SA fiber can be shorter with higher dopant on the condition of providing enough gain absorber. The double-clad core-pumped fiber can provide relatively high optical gain in a short length, leading to a more compact cavity design for short pulse width. The inset of Fig. 6 is the typical pulse trains operating at the maximum output power at different sequential time, which shows the stability in amplitude and time domain overall. To further analyze the stability of the all-fiber pulsed laser, the timing jitter of multi-pulses is monitored, just as shown in the Fig. 6, which implies a tolerable fluctuation in time domain. Moreover, the inevitable instability of pump source may also affect the total performance of our system. In addition, the measured fluctuation in the peak-peak value is superior to 3%, indicating the stability in the amplitude. The operating stability of the laser system was also monitored over a few hours with little fluctuations in laser performance.

Fig. 6. Timing jitter measurement of the all-fiber pulsed laser. Inset is the pulse trains at different sequential time.


4. Conclusion We use a novel dual-cavity design to realize a truly all-fiber passively Q-switched and gainswitched laser with relatively high stability in detail. All elements are directly spliced, allowing the oscillator to be integrated in a compact size with reliable and stable lasing output. The laser is saturable-absorber Q-switched and gain-switched by a single-mode Yb3+doped fiber, and the generated pulses can be further amplified in the propagation of the external-cavity. In this scheme, an efficient Q-switched and gain-switched all-fiber pulsed laser with 45 ns pulse width, 62 μJ pulse energy, and 1.4 kW peak power operating at 1081 nm is obtained. To the best of our knowledge, this is the minimum pulse width in this similar kind of all-fiber configuration at present. Moreover, effects of laser parameters such as pump power, cavity length, external-cavity wavelength, and FBG reflectivity on laser performance are presented and discussed. The experimental results have demonstrated great potential for high energy pulses with large core-diameter fiber and powerful pump diode for various applications. Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC, Nos. 61235010 and 61177048), and the Beijing University of Technology, China.
