All-fiber passively Q-switched fiber lar with a Sm-doped fiber saturable a bsorber
Yi Lu and Xijia Gu*
Department of Electrical and Computer Engineering, Ryerson University, 350 Victoria St., Toronto, Ontario M2K
2H7, Canada
*son.ca
Abstract: We demonstrate an all-fiber passively Q-switched Yb-doped
lar using a piece of Sm-doped fiber as a saturable absorber. The lar 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 lar produced 1064 nm puls with 28
μJ pul energy and a 200 ns pul width at a repetition rate of 100 kHz.
The pul energy and peak power are an order of magnitude higher than
what previous reported which was also in all-fiber configuration. Effects of
lar parameters, such as Sm-fiber length, pump power and duration on
lar performance were prented and discusd. Stable Q-switched puls
were obtained at the repetition rate varying from 10 kHz to 100 kHz, which
makes this lar suitable for different applications.
©2013 Optical Society of America
OCIS codes: (140.3510) Lars, fiber; (140.3540) Lars, Q-switched; (060.2310) Fiber optics. References and links
1. X. Tian, M. Tang, X. Cheng, P. P. Shum, Y. Gong, and C. Lin, “High-energy wave-breaking-free pul from all-
fiber mode-locked lar system,” Opt. Express 17(9), 7222–7227 (2009).
2. F. Wang, A. G. Rozhin, V. Scardaci, Z. Sun, F. Hennrich, I. H. White, W. I. Milne, and A. C. Ferrari, “Wide
band-tuneable, nanotube mode-locked, fibre lar,” Nat. Photonics 3(12), 738–742 (2008).
3. A. A. Fotiadi, A. S. Kurkov, and I. M. Razdobreev, “All-fiber passively Q-switched Ytterbium lar,” in IEEE,
Proceedings of CLEO-Europe, 515, Munich, Germany, 12–17 June (2005)
4. T.-Y. Tsai, Y.-C. Fang, Z.-C. Lee, and H.-X. Tsao, “All-fiber passively Q-switched erbium lar using mismatch
九龙公园of mode field areas and a saturable-amplifier pump switch,” Opt. Lett. 34(19), 2891–2893 (2009).
5. T.-Y. Tsai, Y.-C. Fang, H.-M. Huang, H.-X. Tsao, and S.-T. Lin, “Saturable absorber Q- and gain-switched all-
Yb3+ all-fiber lar at 976 and 1064 nm,” Opt. Express 18(23), 23523–23528 (2010).
6. D. B. S. Soh, S. E. Bisson, B. D. Patterson, and S. W. Moore, “High-power all-fiber passively Q-switched lar
using a doped fiber as a saturable absorber: numerical simulations,” Opt. Lett. 36(13), 2536–2538 (2011).
7. S. W. Moore, D. B. S. Soh, S. E. Bisson, B. D. Patterson, and W. L. Hsu, “400 µJ 79 ns amplified puls from a
Q-switched fiber lar using an Yb3+-doped fiber saturable absorber,” Opt. Express 20(21), 23778–23789 (2012).
8. C. E. Preda, G. Ravet, and P. Mégret, “Experimental demonstration of a passive all-fiber Q-switched erbium-
and samarium-doped lar,” Opt. Lett. 37(4), 629–631 (2012).
1. Introduction
In many fiber lar applications such as cond harmonic generation and lar material processing,
puld fiber lars are preferred becau of their high peak power and pul energy at moderate average power. High peak power can be achieved typically by actively Q-switching the lar 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 lar. Passive Q-switching has attracted significant attention for its potential to offer a compact and low cost alternative. Semiconductor saturable absorber mirrors (SESAM) [1] and carbon nanotubes saturable absorbers have been ud for Q-switching and achieved up to 1 kW peak power output. However both methods offer limited pul energy of less than 10 μJ due to potential optical damage to the saturable absorbers at high energy level [2].
Rearch on more robust all-fiber designs has achieved some progress so far. A passively Q-switched Yb-doped fiber lar, with a Sm-doped fiber as a saturable absorber (SA), was
#180495 - $15.00 USD Received 26 Nov 2012; revid 22 Dec 2012; accepted 23 Dec 2012; published 17 Jan 2013 (C) 2013 OSA28 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 1997
demonstrated that generated 19 μJ, 650 ns puls [3]. However the pul to pul stability was poor. Tsai et al. reported an Er-doped fiber lar using a piece of unpumped Er-doped fiber as an SA whic
h produced 8 μJ, 80 ns puls [4]. The same group lately successfully demonstrated an all-fiber Q-switched lar with mode-field-area mismatching Yb-doped gain fiber and SA fiber which generated 2.8 μJ 280 ns puls [5]. Their rearch clearly demonstrated the feasibility of achieving stable Q-switched puls in all-fiber configuration. However the lars, so far, produced only relatively low pul energy and average power, far from the requirements for lar material processing.
In 2011, Soh et al. propod an all-fiber configuration for a passively Q-switched lar, in which, both gain fiber and SA fiber were Yb-doped, however, with a large core size difference [6]. Their simulation showed that 0.5 mJ pul energy and 14 ns pul width could be realized. Very recently, the same group successfully verified this configuration and reported Q-switched oscillator with 40 μJ and 79 ns puls at 1026 nm [7]. However, the lar ud bulk lens to couple the pump lar into both gain fiber and SA fiber and dichroic mirrors to lect wavelengths.
We report here a new configuration for a Q-switching Yb-doped cladding pumped fiber lar 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 puls with a pul width of 200 ns and pul energy of 28 μJ, which is approximate 10 times higher than what was reported in [5].
2. Experiments
The fiber lar is monolithic in design with all connections spliced, as shown in Fig. 1. The pump light from two Oclaro 25 W, 975 nm lar diodes is coupled into the large-mode area (LMA) gain fiber through a (1 + 2) x1 power combiner with a 0.5 dB inrtion 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 lar 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 who core/cladding diameters match the gain fiber.
Fig. 1. Optical diagram of the all-fiber Q-switched fiber lar.
Within the lar cavity, a piece of Sm-doped single-mode fiber with a core diameter of 6.3 μm and an NA of 0.14, was inrted. The absorption of the saturable absorber fiber at 1064 nm is estimated to be 8 dB/m. The Sm-doped SA was enclod 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 ud for reducing the up-level lifetime. However we did find about 15% increa in pul
#180495 - $15.00 USD Received 26 Nov 2012; revid 22 Dec 2012; accepted 23 Dec 2012; published 17 Jan 2013 (C) 2013 OSA28 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 1998
energy after enclo 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 u 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 puld modes and its duty cycle can be adjusted when operating in puld mode. However, its repetition rate for puld 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 pul train and lar emission spectrum, the lar 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 lar 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 puls in both amplitude and time. When the lar was pumped at the repetition rate of 100 kHz, Q-switched pul train such as shown in Fig. 2(a), was obtained. The peak amplitude variation was about 4.1% in standard deviation and the pul duration varied at 1.2%. Similar pul amplitude stability and low pul jitter were obrved at other repetition rates. The single Q-switched pul 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 pul was also reported in [8], in which, the Sm-doped fiber is also ud as SA for an Er-doped gain fiber t
o achieve Q-switching. The pul shape was interpreted as the result of Sm’s low saturable loss than its non-saturable loss.
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Fig. 2. (a) Oscilloscope trace of the Q-switching puls at 100 kHz rate; (b) a single pul with 30 μJ energy and a 200 ns (FWHM) pul width.
When the lar was Q-switched at 100 kHz, the average output power of 2.8 W was obtained that gave the energy per pul of 28 μJ, more than 15 times the energy of SESAM or carbon nanotube Q-switched lar pul and 10 times higher than what reported in [5]. The peak power of 140 W was obtained at the pul width of 200 ns. At lower repetition rates, such as 10 kHz and 60 kHz, stable puls were also obtained, as shown in Fig. 3.
#180495 - $15.00 USD Received 26 Nov 2012; revid 22 Dec 2012; accepted 23 Dec 2012; published 17 Jan 2013
(C) 2013 OSA 28 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 1999
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我们之间的爱情Fig. 3. Oscilloscope traces of the Q-switching puls at repetition rate of 10 kHz (a), and 60 kHz (b).
When the lar was operated at 100 kHz, the duty cycle of the pump was kept at ~32%, as shown in Fig. 4. This indicates that the lar had the potential to go to a higher repetition rate, and thus, further increa its average output power. One important feature of this lar is its adjustable repetition rate by changing the repetition rate of the pump lar. A lar with a variable pul rate but same pul shape will make it versatile for various applications.
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Fig. 5. A Q-switching pul train with relaxation oscillation puls pumped at 100 kHz and above 35% duty-cycle than normal operation condition.
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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 ud in Fig. 4. After Q-switched pul
#180495 - $15.00 USD Received 26 Nov 2012; revid 22 Dec 2012; accepted 23 Dec 2012; published 17 Jan 2013
(C) 2013 OSA 28 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 2000
the gain fiber continues to accumulate energy which led the generation of the relaxation puls with less energy even after the pump is turned off. When that happened, the Q-switched puls become unstable; their amplitudes fluctuate and pul jitter increas. In our experiment, the pump was turned off before the ont of the after-puls. 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-puls. When the lar was CW pumped, Q-switched puls and relaxation puls were mixed that made whole pul train chaotic.
The lar emission spectrum, at 1063.7 nm with a 0.175 nm bandwidth, is plotted in Fig. 6, which sh
ows an excellent optical signal to noi ratio of ~70 dB. There is a small peak at 1100 nm, caud by the 1100 nm cavity for Sm-doped SA. The 1100 nm peak is more than 60 dB lower than the main lar emission and should not be detrimental to the applications.
L a s e r P o w e r (d B m )
combiner.
L a s e r P o w e r (W )
Average Pump Power (W)
Fig. 7. Average lar power vs. average pump power operated at 100 kHz.
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 lected to avoid the occurrence of after-puls: it was incread firstly until a weak after-pul was obrved, and then decread with a small decrement step until the after-pul was flattened. This procedure guarantees a stable pul train at the highest pul energy without an
#180495 - $15.00 USD Received 26 Nov 2012; revid 22 Dec 2012; accepted 23 Dec 2012; published 17 Jan 2013
(C) 2013 OSA 28 January 2013 / Vol. 21, No. 2 / OPTICS EXPRESS 2001
