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A high-power transverse-mode-switchable all-fiber picosecond laser in a master-oscillator power-amplifier (MOPA) configuration is demonstrated. The master oscillator is a gain-switched laser diode delivering picosecond pulses with 25 MHz repetition rate at the wavelength of 1.06 μm. After multi-stage amplification in ytterbium-doped fibers, the average output power is scaled to 117 W. A mechanical long-period grating is employed as a fiber mode convertor to achieve controllable conversion from the fundamental (LP01) to the second-order (LP11) mode. Efficient mode conversion is demonstrated and the output characteristics for both modes are investigated. It is shown that LP01 and LP11 modes have nearly identical optical-to-optical conversion efficiency during amplification, but the nonlinear spectral degradation is significantly alleviated for LP11 mode operation. Owing to the compact all-fiber architecture, this high-power transverse-mode-switchable fiber laser is reliable during long-term operation and thus promising for many practical applications, e.g. high-resolution laser micro-processing.
© 2016 Optical Society of America
1. Introduction
High-power short-pulse lasers are highly demanded in a broad spectrum of applications. Based on the development of high-power fiber components and the fusion-splicing technique, all-fiber-integrated lasers without utilizing free-space optics become a promising scheme due to their compactness and reliability. To date, the average power of pulsed all-fiber lasers in the picosecond regime has reached hundreds of watts [1–5]. These advancements can be attributed primarily to the consistently improved fabrication technics of large-mode-area double-clad fibers (LMA-DCFs) and the widely used master-oscillator power-amplifier (MOPA) configuration.
Fiber lasers operating in controllable spatial transverse modes can offer great flexibility and are capable of meeting the different requirements of diverse applications. Although the fundamental mode is frequently expected, specific higher-order modes (HOMs) with unique spatial intensity, phase and polarization distributions are also desirable in many applications. For example, high-power fiber lasers delivering doughnut-shape vortex or vector beams, which are variations of LP11 mode, hold great potential for laser micro-processing [6] and micro-machining [7]. Another attractive characteristic of HOMs is the relatively large effective mode area which is beneficial for further power-scaling [8].
The LMA-DCFs for high-power laser application commonly have a large core diameter of above 15 μm [1–5]. As a consequence, they naturally support more than one transverse mode in the fiber core. It is usually convenient to obtain a pure fundamental mode from such few-mode fibers by introducing mode-selective loss or gain to favor the fundamental mode and suppress the unwanted HOMs [9,10]. In contrast, to eliminate the fundamental mode and obtain a single HOM is challenging. For this purpose, specially designed mode selection elements (few-mode FBGs, polarization components, or spatial light modulators) were employed to excite the desired HOMs and even to achieve fast transverse-mode switching in fiber laser oscillators [11–17]. However, their applicability and power-scaling capability is limited either by the bulky configuration [11,12,15,17] or by a fragile offset splicing spot with low mode conversion efficiency [13,14,18].
As an alternative method, individual HOM excitation can be implemented through mode conversion from the fundamental mode by employing a mode convertor [19]. It was demonstrated that a low-power HOM seed laser can be efficiently amplified in a few-mode fiber amplifier [20,21], which implies that the power-scaling of HOM fiber lasers in a MOPA configuration to hundred-watt level is theoretically feasible. However, due to the absence of a universal and reliable mode conversion approach, such fiber lasers with high-average-power HOM output have rarely been reported. Recently, an up to 25 W picosecond vortex laser based on a stressed LMA few-mode fiber amplifier and its frequency doubling have been demonstrated [22,23]. However, the mode conversion efficiency based on off-axis injection is relatively low (25%~30% as reported) and the bulky free-space coupling between the solid-state master-oscillator and fiber amplifier may cause instability issues during long-time operation under practical environments. To further scale up the power and enhance the stability of such a few-mode fiber MOPA, a high-efficiency all-fiber mode convertor should be utilized.
In this paper, we experimentally demonstrate an all-fiber-integrated picosecond MOPA laser system with over 100 W average output power. This high-power fiber MOPA incorporates an all-fiber mode convertor for efficient mode conversion. The output can be switchable between the fundamental LP01 mode and the second-order LP11 mode by applying an appropriate pressure to a mechanic long-period grating (LPG). The mode conversion efficiency is dramatically improved in comparison with the previously used off-axis free-space coupling. The LMA few-mode fiber amplifier exhibits over 60% optical-to-optical conversion efficiency for both LP01 and LP11 modes. To the best of our knowledge, this is the first experimental demonstration of a high-power all-fiber laser with switchable transverse mode output based on conventional LMA-DCFs.
2. Experimental setup
A schematic of the high-power all-fiber MOPA is shown in Fig. 1. The seed laser is a monolithic linearly-polarized picosecond fiber laser in a MOPA configuration. The master-oscillator is a fiber-pigtailed gain-switched laser diode with a repetition rate of 25 MHz. The pulse width is about 200 ps and the central wavelength is 1059.3 nm. Inside the seed laser, a two-stage polarization-maintaining ytterbium-doped fiber amplifier (YDFA) chain is used to pre-amplify the milli-watt signal to an average power of about 1 W. The pigtail fiber of this seed laser is a 10/125 DCF which is a single-mode fiber at the operating wavelength. The seed laser is succeeded by a mode field adapter and then a 25/250 LMA cladding-pumped YDFA which is used as a pre-amplifier to provide sufficient signal power for the final stage. In this pre-amplifier, fundamental mode operation is ensured by coiling the gain fiber to a specific diameter to introduce an additional bend induced loss to HOMs [5]. The fundamental mode output from this pre-amplifier can be converted to the second-order mode through an all-fiber mode convertor. The final-stage power amplifier is another LMA cladding-pumped YDFA. The core diameter and numerical aperture (NA) of the LMA fibers are 25 μm and 0.06, respectively. This corresponding to a normalized V parameter of 4.45 at the operating wavelength; therefore, both LP01 and LP11 modes are well supported in such few-mode fibers. The power amplifier is pumped with six temperature-stabilized high-power 975 nm LDs. The gain fiber is loosely coiled and placed on a water-cooled heat sink. A pump dumping section is employed to strip off the residual pump light and the fiber output end is angle-cleaved to avoid harmful back-reflection. All of the fiber components are connected by fusion-splicing and there is no complicated and sometimes problematic free-space coupling in the whole system.
Fig. 1 Schematic of the transverse-mode-switchable all-fiber MOPA. MFA, mode field adapter. LD, laser diode. LMA-YDF, large-mode-area ytterbium doped fiber. DCF, double-clad fiber.
3. Results
3.1 All-fiber Mode Convertor
The key component for transverse mode switching is the high-efficiency all-fiber mode convertor, which is schematically illustrated in Fig. 2. Its working principle is based on the mode coupling induced by periodical stress introduced to the fiber core. The LPG is composed of periodically arranged ridges on the top plate, while on the bottom it is a plane plate. Adjustable pressure can be applied to the top plate to squeeze the fiber placed underneath. If the period of mechanical stress imposed to the fiber matches the beat length between two specific modes, the light transmitting through this grating will be coupled from one mode to the other and vice versa. Such mechanical LPGs are commonly used in conjunction with standard single mode fibers for spectral filtering [24]. They have also been used as a mode conversion device for few-mode fibers [25] and recently employed for mode-multiplexed fiber communication [26–28]. However, all of them are designed for single-clad fibers and low-power applications. Here we propose that, for a few-mode LMA-DCF, it is also feasible and convenient to fabricate such an all-fiber mode convertor with high conversion efficiency.
Fig. 2 Schematic of the fiber mode convertor for a large-mode-area double-clad fiber (LMA-DCF). LPG, long-period grating.
According to the definition of modal beat length between LP01 and LP11 modes, i.e. , we have numerically calculated the dependence of LB at 1.06 μm on two parameters: the core diameter and NA of a few-mode fiber. As shown in Fig. 3, for the most commonly used LMA-DCFs, the LP01-LP11 modal beat length is between 1 to 4 mm. Using the core diameter (25 μm) and NA (0.06) of the LMA-DCF employed in the experiment, we found the modal beat length between LP01 and LP11 modes to be 3 mm. Therefore, a mechanic LPG with can cause effective coupling between the two modes. Such a mechanical LPG fiber mode convertor is easy to fabricate and only introduces a very low insertion loss in comparison with other approaches. Moreover, because the mechanical pressure can be finely adjusted in flexible manners, the mode coupling process is highly controllable and nearly complete mode conversion can be achieved [25,26].
Fig. 3 Dependence of LP01-LP11 modal beat length on the numerical aperture and core diameter of large-mode-area few-mode fibers.
We fabricate a mechanical LPG based fiber mode convertor according to the above design, and test its performance by measuring the output power of the pre-amplifier before and after mode conversion from LP01 to LP11 mode. The experimental results are shown in Fig. 4(a). It can be seen that the mode convertor has over 90% efficiency at all power levels. This result also demonstrates the power-handling capability of this all-fiber mode convertor which is usually not achievable by other methods such as off-axis splicing [13,14].
Fig. 4 (a) Output power characteristics of the pre-amplifier and conversion efficiency of the mode convertor. (b) Out spectrum of the pre-amplifier at 11.5 W pump power. The inset shows the details of the spectral curve.
Figure 4(b) depicts a typical output spectrum of the pre-amplifier at moderate pump power. It is clearly shown that, at kW-level peak power, nonlinear effects have occurred and lead to the broadening of signal spectrum. The two central peaks are caused by self-phase modulation and the tiny spikes may be a feature of four-wave mixing. The nonlinear degradation of spectrum is detrimental for many applications such as spectroscopy and frequency conversion [2]. Therefore, we restrict the pump power to a moderate level (11.5 W) to ensure the spectral bandwidth acceptable.
3.2 Mode-switchable Power Amplification
We then demonstrate the power amplification of LP01 and LP11 modes in the final-stage LMA fiber amplifier. The switching between two modes is implemented by applying/releasing the pressure on the mechanical LPG. The output power characteristics and corresponding slope efficiency are summarized in Fig. 5. For both LP01 and LP11 modes, we have obtained 117 W average output power at the maximum pump power of 178 W, corresponding to an overall optical-to-optical efficiency of 66%. The slope efficiency is 62.2% and 62.5% for LP01 and LP11 modes, respectively. It is somewhat surprising that the power characteristics of LP01 and LP11 modes are almost identical, which may be partly attributed to the fact that the main amplifier is highly gain-saturated. The output beam profiles of LP01 and LP11 modes as shown in Fig. 5 are measured at a relatively low power level. However, little change has been observed during the entire process of power increasing. At the maximum pump power, the output modes still maintain high purity. The output power is stable during continuous operation over tens of minutes. The temperature of key components including the pump dump section and the fiber combiner is always blew 60 °C, implying that there is still room for further power scaling.
Fig. 5 Output power characteristics and pump-to-signal conversion efficiency of the power amplifier operating in (a) LP01 and (b) LP11 mode, respectively. The insets are the corresponding output mode profiles.
HOM instead of the fundamental mode operation can help suppress the nonlinear distortion of signal spectrum in the power amplifier. We have measured the output spectrum when operating in either LP01 or LP11 mode, respectively. As shown in Fig. 6, for LP01 mode, a large portion of the signal power has been transferred to the longer wavelengths above 1100 nm through Raman scattering in the fiber amplifier. In contrast, Raman scattering is partly suppressed when the fiber amplifier is operating in LP11 mode. Due to the relatively large mode area of LP11 mode, spectral broadening induced by self-phase modulation and four-wave mixing is also alleviated to some extent, resulting in a reduced spectral bandwidth (0.6 nm at −3 dB level and 2 nm at −10 dB level) as compared to LP01 mode (1 nm at −3 dB level and ~9 nm at −10 dB level).
Fig. 6 Out spectrum of the power amplifier at maximum pump power for LP01 (green) and LP11 (blue) modes, respectively. The inset shows the details of the spectral curves.
4. Conclusion
To conclude, we have implemented an all-fiber MOPA capable of delivering high-power picosecond pulses with switchable spatial transverse modes. For this purpose, we propose and demonstrate a simple but highly efficient mode convertor compatible with the LMA double-clad fibers widely used in high-power fiber lasers and amplifiers. This fiber mode convertor can be readily integrated into an all-fiber MOPA system. In the experiment, we have obtained 117 W average output power from this all-fiber MOPA in both LP01 and LP11 modes. We also find that the efficiency of the main fiber amplifier is almost immune to mode switching. This all-fiber-integrated system is reliable for long-time operation and further power-scaling is feasible. Generally, exploiting the various spatial transverse modes of few-mode fibers can open new avenues for enhancing the ability of fiber-based devices and systems to transport information or deliver energy. Recently, the research topic on few-mode fiber amplifiers capable of providing gain for multiple transverse modes has been renewed in the wake of the rise of space-division multiplexing (SDM) [29]. We anticipate that, as another example of the possible use of few-mode fiber amplifiers, transverse-mode-switchable all-fiber MOPAs may find its application in a wide range of fields as well.
Funding
National Natural Science Foundation of China (NSFC) (61235008); National High Technology Research and Development Program of China (2015AA021101).
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