A novel passively Q-switched all-fiber laser using a single mode-multimode-single mode fiber device as the saturable absorber based on the Kerr effect of multimode interference is reported. Stable Q-switched operation of an Er3+/Yb3+ co-doped fiber laser at 1559.5 nm was obtained at a pump power range of 190-510 mW with the repetition rate varying from 14.1 kHz to 35.2 kHz and the pulse duration ranging from 5.69 μs to 3.86 μs. A maximum pulse energy of 0.8 μJ at an average output power of 27.6 mW was achieved. This demonstrates a new modulation mechanism for realizing Q-switched all-fiber laser sources.
© 2015 Optical Society of America
Q-switched fiber lasers capable of producing microsecond or nanosecond pulses have attracted extensive attention for a variety of applications including material processing, laser surgery, remote sensing, optical communications, and defense. Various Q-switched fiber lasers have been demonstrated with active modulation devices (acousto-optic modulators and electro-optic modulators) [1, 2 ], passive modulation devices (various saturable absorbers) [3–10 ], gain-switching , and other techniques [12–15 ]. Passive Q-switching techniques possess the advantages of compactness, simplicity, low-cost and an all-fiber format and have been studied extensively with different saturable absorption materials, such as semiconductor saturable absorption mirrors (SESAMs) , carbon nanotubes (CNTs) [4, 5 ], graphene [6, 7 ], transition metal-doped crystals , rare-earth-doped fibers , and MoS2 . However, the operating wavelengths of SESAMs, CNTs, transition-metal-doped crystals, and rare-earth-doped fibers are usually limited by their absorption bands. Graphene has been proven to be a remarkable saturable absorber with an ultra-broad operation wavelength range owing to its unique bandgap. Besides 1 µm, 1.55 µm, and 2 µm wavelength ranges, graphene Q-switched fiber lasers at 1.2 µm and 3 µm have also been demonstrated recently [6, 7 ]. Nevertheless, all these saturable absorbers need complicated fabrication processes for the materials and devices and have relatively low damage threshold. Utilizing the nonlinear effects in passive optical fibers is an attractive alternative approach to achieve passively Q-switched operation of a fiber laser because of its intrinsically high power damage threshold, all-fiber format, and wavelength independence. Due to the short relaxation oscillation pulse of stimulated Brillouin scattering (SBS), several passively Q-switched all-fiber lasers induced by the strong feedback of SBS have been demonstrated [13–15 ]. This Q-switching technique can be extensively used to obtain pulsed fiber lasers at arbitrary wavelengths because SBS is independent of laser wavelength. However, long lengths of single-mode optical fiber are required for low-threshold SBS and Q-switched operation of the fiber laser is not stable because of the inherent chaotic property of SBS and the influence of stimulated Raman scattering [13–15 ].
In this paper, we propose and demonstrate a new nonlinear effect induced modulation mechanism that can be used to achieve Q-switched operation of all-fiber lasers. It is based on the Kerr-effect of multimode interference (MMI) in a single mode-multimode-single mode (SMS) fiber device. An Er3+/Yb3+ co-doped fiber laser operating at 1559.5 nm has been successfully Q-switched by incorporating a 4.9-cm multimode fiber segment with a core diameter of 50 µm into the fiber ring laser.
2. Kerr effect of multimode interference
MMI is the interference of the excited modes in a multimode waveguide when single-mode light is coupled into a multimode waveguide. The field at any position inside the multimode waveguide is a superposition of the fields of the excited modes. Self-imaging of the input field can be obtained at certain positions where the excited modes are in phase, i.e., the accumulated phase difference between any two excited modes is an integer multiple of 2π. The self-imaging effect of MMI in multimode fibers (MMFs) was first observed by Allison  and has been experimentally demonstrated by several groups in recent years for various applications such as wavelength tunable lasers [17, 18 ], all-fiber bandpass filter , wavelength tunable fiber lens , mode-field adapters [21, 22 ] and fiber-optic sensors [23–25 ]. In addition to these passive MMI devices, active MMI devices have also attracted much attention for the development of high power fiber lasers [26, 27 ]. In 2008, X. Zhu et al. proposed an alternative approach to achieve single-transverse mode laser output with a piece of active multimode fiber by utilizing the self-imaging effect of MMI [26, 27 ], and also demonstrated that the self-imaging effect of MMI can be employed to achieve high power fiber lasers and amplifiers using short-length multimode gain fibers . Apart from the self-imaging effect of MMI, the same workers have demonstrated that the MMI effects in MMFs can be used to generate a Bessel-like beam with long propagation invariant distance and to transform a Gaussian input beam into other frequently desired beams including top-hat, donut-shaped, taper-shaped, and Bessel-like beams [29–32 ]. Nevertheless, all these efforts are focused on the linear effects of MMI. In 2013, Nazemosadat and Mafi numerically studied nonlinear MMI in a short graded-index MMF and proposed to use the SMS fiber device in nonlinear switching, optical signal processing, or as a saturable absorber to achieve a mode-locked fiber laser . In this paper, we describe and analyze the Kerr-effect of MMI in step-index MMF and demonstrate that nonlinear MMI is a reliable modulation mechanism that can be used to achieve Q-switching in fiber lasers.
The theoretical description of the Kerr effect of MMI in MMF can be addressed similarly to the description of the self-imaging effect in . When the light from an input single-mode fiber (SMF) is coupled into the MMF, a number of MMF modes are excited, i.e .
The field EMM(r,ϕ,z) along the MMF can be expressed as
Self-imaging occurs at some certain positions inside MMF, when the following condition is satisfied for all N modes
When the length of the MMF segment L is chosen to allow self-imaging to occur at a certain wavelength, (3) can be rewritten as follows
Because of the Kerr effect, the refractive index of the optical fiber depends on the light intensity and thus (4) can be expressed as below
Therefore, the wavelength of self-imaging at a high laser intensity can be described as below
According to (5) and (6), the nonlinear change of the refractive index originating from the optical Kerr effect leads to a change of the self-imaging wavelength in the MMF. In another word, the transmission spectrum of the SMS fiber device will change due to the Kerr effect of MMI. As a consequence, the transmission of the SMS fiber device at a certain wavelength exhibits a performance similar to a saturable absorber, i.e., the transmission increases at high laser power. Therefore, pulsed laser operation can be achieved at a wavelength where the SMS fiber device performs like a saturable absorber. In our work, this wavelength can be selected by using a tunable band-pass filter in a fiber ring cavity.
3. Experiments and results
The configuration of the passively Q-switched all-fiber ring laser is illustrated in Fig. 1 . A piece of 5-m Er3+/Yb3+ co-doped double-cladding fiber (Nufern, SM-EYDF-6/125-HE) was used as the gain medium. A 980 nm multimode pump laser was coupled into the gain fiber through a (2 + 1) × 1 fiber combiner. An isolator was used in the ring cavity to force the laser oscillation to propagate in the clockwise direction. The SMS fiber device used in the experiment was fabricated by splicing two SMF-28 fibers to a 4.9-cm multimode fiber with a core diameter of 50 µm (Thorlabs AFS50/125Y). The transmission spectrum of this SMS fiber device was measured with an amplified spontaneous emission source (AFR, OS-SM-1550-50-FA) and is shown in Fig. 2 . A transmission peak of 72% can be found at 1560.2 nm. It is worth noting that the 4.9-cm length of multimode fiber was selected to ensure a spectral overlap between the transmission peak of the SMS fiber device and the gain peak of the free-running Er3+/Yb3+ co-doped fiber ring laser. Due to the short self-imaging length , the length of MMF is difficult to control in practice. So in the experiment, many samples were fabricated and the device with better transmission peak was selected to use in this demonstration. In order to achieve stable Q-switched operation, a tunable band-pass filter (Jun Laser, TOF-1550-SM-L-10-NE) was used to force the fiber ring laser to operate at a wavelength (i.e., 1559.5 nm in this experiment), where strong saturable absorption occurs. Due to the Kerr effect of MMI, the transmission of the SMS fiber device at 1559.5 nm increases with increased laser power density, performing like a saturable absorber. A 70/30 coupler was used to outcouple 70% of the laser power from the ring laser cavity.
The Er3+/Yb3+ co-doped fiber laser started to lase at a pump power of 132 mW but the pulses were not stable. Stable Q-switched pulses were obtained at a pump power of 190 mW and maintained until the pump power increased to 510 mW. As the pump power further increased, the pulse trains were diminished gradually and continuous-wave output was obtained at a pump power of 532 mW. The output power of the fiber laser in CW and Q-switched operation as a function of the pump was measured by a power meter (OPHIR, 12A-V1-ROHS) and is shown in Fig. 3(a) . A maximum pulsed output power of 27.6 mW was obtained at a pump power of 510 mW. Compared with the laser without the SMS fiber device, which is in CW operation, the Q-switched output power is a bit smaller resulting from the transmission loss of SMS device. The spectrum of the Q-switched Er3+/Yb3+ co-doped fiber laser was measured by an optical spectrum analyzer (Yokogawa, AQ6375) and is depicted in Fig. 3(b). This laser has a center wavelength at 1559.5 nm and a 3-dB bandwidth of 0.07 nm. Both are defined by the tunable band-pass filter.
Stable Q-switched laser pulses were detected by a high-speed InGaAs biased photodetector (Thorlabs, DET01CFC/M) and recorded with an oscilloscope (Tektronix, DPO2024B). The typical pulse trains of the Q-switched fiber laser at pump powers of 190 mW, 374 mW, and 510 mW were recorded and are shown in Figs. 4(a)-4(c) , respectively. The pulse envelopes of the Q-switched laser at three pump powers are plotted and shown in Fig. 4(d). Clearly, the pulse duration decreases with the increased pump power. The repetition rate and the pulse duration of the Q-switched laser were measured at different pump powers and are shown in Fig. 5(a) . The repetition rate increased from 14.1 kHz to 35.2 kHz as the pump power increased from 190 mW to 510 mW. The pulse duration, however, decreased with increased pump power and a minimum pulse duration of 3.86 µs was obtained. The corresponding pulse energy and peak power of the Q-switched pulses were calculated and are shown in Fig. 5(b). Both pulse energy and peak power increased monotonously with the increased pump power. The maximum pulse energy was 0.78 µJ and the maximum peak power was 203.2 mW. For pump power in the range from 190 mW to 510 mW, no noticeable pulse jitter was observed.
Fig. 6 shows the radio-frequency (RF) spectrum of the Q-switched Er3+/Yb3+ co-doped fiber laser measured with a signal analyzer (Agilent, PXA N9030A) at a pump power of 510 mW. The inset shows the RF spectrum over a frequency range of 500 kHz. The fundamental peak is at 35.2 kHz which corresponds to the repetition rate shown in Fig. 4(c). Note that the signal-to-noise ratio (SNR) is over 85 dB, which indicates a very good stability of this laser during the measurement time.
In conclusion, we proposed a novel modulation mechanism based on the Kerr effect of MMI fiber device and experimentally demonstrated this new concept by Q-switching an Er3+/Yb3+ co-doped fiber laser using an SMS fiber device. Stable Q-switched pulses were obtained for pump powers from 190 to 510 mW. At a pump power of 510 mW, stable Q-switched pulses with a repetition rate of 35.2 kHz, pulse energy of 0.78 μJ, and pulse duration of 3.86 μs were obtained. This novel modulation mechanism can be used to obtain Q-switched all-fiber lasers at any wavelengths, where the transmission of the SMS fiber device increases with the increased laser power density.
This work was supported by NSFC (No.61335013, 61275102), the National High Technology Research and Development Program (“863” Program, No.2014AA041901), and Doctoral Fund of Ministry of Education (No.20130032110051).
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