1. Introduction

Mode-locked fiber lasers have proved to be reliable, non-complex and powerful sources of ultra-short pulses, which are rapidly developed for the last two decades [1]. This resulted in numerous different mode-locking techniques used in all-fiber lasers today. Passive mode-locking can be achieved in different approaches, for example using nano-tubes [2], SESAM [3], graphene [4], topological insulators [5] nonlinear polarization rotation (NPR) [6] and Figure-8 (F8) resonator configuration [7]. Majority of all-fiber mode-locked configurations, working in net anomalous dispersion regime, have a limited pulse energy due to the necessarily of maintaining balance between dispersion and the nonlinearity arising in the resonator. This problem is connected with so called soliton area theorem [8]. High energy laser pulses are important in a wide range of applications, for example in medical equipment, high-energy physics experiments, laser material processing, laser range finding [9–13]. Direct generation of intense laser pulses typically requires using Q-switching [14], cavity dumping [15,16] or additional amplifiers [17], which are usually very complex in realization. Different approach to this problem was predicted by Chang et al. in his work published in 2008 [18].

Relying on his calculations, a passively mode-locked laser, designed with parameters enabling existence of so called Dissipative Soliton Resonances (DSR) in the cavity, will permit the soliton energy to increase virtually indefinitely, while keeping its amplitude at a nearly constant level. His theory was well modeled by Zheng et al. in 2008 [19], for a particular laser design, in which he described formation of square-shaped pulses in a Nonlinear Amplifying Loop Mirror (NALM). Based on the theoretical models, several experimental realizations of DSR mode-locked, high energy, square-shaped F8 and NPR lasers have been presented [20–23]. Nevertheless, no reports of utilizing the DSR effects in DC fiber lasers can be found in scientific papers. Among the main advantages of using DC active fibers in mode-locked lasers we can name their relatively high gain factor, single-mode operation and the possibility of using cheap, high power multimode pump sources. Moreover, DC Er:Yb active fibers have an unique attribute of dual-wavelength amplification. However, taking advantage of their high gain very often entails precise resonator design, which includes dispersion control and non-all-fiber configuration of the laser. In this paper we will present first demonstration of an all-fiber, dual-wavelength laser with a novel resonator configuration, enabling generation of record high energy, square-shaped laser pulses in the telecom band and simultaneous efficient amplification of 1064 nm seed signal using DC Er:Yb doped fiber. We found this laser to be an perfect, ultra-simple source for generating mode-locked, duration-tunable pulses in the Mid-Infrared (Mid-IR) wavelength region, via difference frequency mixing in PPLN.

2. Experimental setup

The setup of the dual-wavelength, all-fiber, mode-locked laser is depicted in Fig. 1. The DSR mode-locking at telecom wavelength is obtained by building the resonator in an ultra-simple all-fiber F8L configuration with relatively high negative net dispersion, equal to −5.355 ps². Two loops were combined using a 3 dB coupler in the center. The right loop was designed as a typical NALM [24] in which a 5-meter-long erbium and ytterbium co-doped DC active fiber was used as an amplifying medium. Pump power provided by two fiber pigtailed, 9 W multimode diode lasers was delivered to the active fiber through a standard pump-beam combiner. The additional 200-meter-long SMF28 fiber was added into this loop to ensure the DSR will be the dominant pulse shaping effect and stable mode-locking of the telecom-band laser pulses could be obtained. The left ring was designed as a directional loop, built with only two elements: a polarization controller and a circulator. Opposed to standard realizations of F8L (an isolator and a coupler [25]), in our system the unidirectional propagation of the pulses was forced by using a fiber circulator.

 

Fig. 1 Schematic of the dual-wavelength figure-8 laser. CIR – circulator, PC – polarization controller, COMB – pump-beam combiner, Er/Yb DC – erbium-ytterbium double clad fiber, SMF28 – spool of single-mode fiber, ISO – fiber isolator, WDM – wavelength division multiplexer, FC-APC – fiber connector, CL – collimating lens, FL – focusing lens, PPLN – 40 mm long periodically poled lithium niobate crystal, GF – germanium filter, MCT – mercury cadmium telluride detector.

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Appropriate arrangement of the ports ensures counter-clockwise circulation of the pulses (1-2 port), at the same time outputting all optical power traveling in the forbidden direction (2-3 port). This solution allowed generating record high energy pulses directly from the resonator without risk of damage to the isolating component. The idea of using Er:Yb DC fiber for simultaneous efficient amplification of 1.06 µm and 1.55 µm wavelengths has been studied by our Group in previous years, with promising results [26,27]. The process is possible due to the non-optimal energy transfer occurring between ytterbium and erbium ions, under high pump power conditions [28]. Part of the excited erbium ions back-transfer their energy to ytterbium ions, which are responsible for generating ASE (Amplified Spontaneous Emission) near the vicinity of 1 µm wavelength region. As showed in our previous work, this process can be easily controlled by forcing the excited Yb ions to lase in a controlled manner, e.g. via an auxiliary signal. In the presented setup we used a standard 30 mW distributed feedback (DFB) fiber pigtailed diode lasing at 1064 nm. The seed signal was introduced into the DC active fiber with a wavelength division multiplexer (WDM) coupler. Fiber isolator was spliced at the output of the seed diode to minimize the risk of damage. The amplified 1064 nm signal was out-coupled right after the active fiber section by means of a second WDM coupler. The third WDM coupler was introduced into the setup to finally join the mode-locked nanosecond pulses generated in the F8L at 1566 nm and the amplified seed signal before reaching the frequency conversion part of the experiment. Single fiber delivery system was required, as we used a single-collimator-based optical assembly. The difference frequency conversion part of our experiment relied on an off-the-shelf, 40 mm-long bulk magnesium doped periodically poled lithium niobate (MgO:PPLN) crystal, enclosed in an oven. Both beams emerging from the FC-APC connector ferule were firstly collimated and then focused in the crystal structure by a set of achromatic lenses, anti-reflection (AR) coated for both wavelengths. Generated Mid-IR radiation was collimated using a calcium fluoride (CaF₂) lens and characterized by a fast MCT detector (VIGO Systems) and a FT-IR spectrometer. Unabsorbed near-IR radiation was out-filtered using a 5 mm thick germanium window. All optical elements used in the frequency conversion part of our experiment were assembled in appropriate holders and mounted onto a Thorlabs 40 mm rod-based Cage System. Lower conversion efficiency put aside (due to non-optimal overlapping of focusing points), the single-collimator optical setup has proven to be robust and provide reproducible results throughout several experiments. Optimal quasi phase matching (QPM) conditions for 1566 nm and 1064 nm wavelengths were achieved using 30.49 µm period and crystal temperature of 95°C.

3. Experimental results

The spectrum generated by the fiber laser was analyzed with an optical power meter, optical spectrum analyzer, oscilloscope and a RF spectrum analyzer, for both wavelengths separately. Square shaped, mode-locked pulses at 1566 nm were observed at the output for pump power above 2 W, after adjusting the polarization controller paddles into optimal position. Worth noting is the fact, that after forcing the laser into mode-locked state, the polarization could be changed in a wide range without perturbation to the pulsed operation. If the paddles were nut-secured in the optimal position, self-starting of mode-locking was achieved during each ON-OFF cycle of the laser, with no alteration to parameters of generated pulses. Graph depicted in Fig. 2(a) shows average output power for both wavelengths (1064 nm and 1566 nm) in function of pump power delivered to the active fiber.

 

Fig. 2 Average output power for both wavelengths (a), and mode-locked pulses duration and pulse energy in function of pump power delivered to the active fiber (b).

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At maximum pump power the laser was capable of delivering square-shaped, mode-locked pulses with an average power of 1.7 W, which at 800 kHz repetition frequency corresponds to record pulse energy of 2.13 µJ, generated directly form the resonator without cavity dumping, Q-switching or additional amplifiers. Figure 2(b) presents achieved pulse duration and pulse energy in function of pump power, and shows good linear response. The generated pulse could be linearly tuned from 20 ns to 170 ns. The proposed dual-wavelength configuration allowed for simultaneous amplification of the low power 1064 nm seed to 1.36 W, which equals to a 16.5 dB increase in signal, originating from energy unexploited in standard DC Er:Yb doped amplifiers. This is the first demonstration of a dual-wavelength setup working in such configuration, allowing efficient amplification of the second signal required for DFG process. The registered exponential increase in amplification for this wavelength was also observed in our previous experiments [26,27]. The effect of pump power dependent increase in duration of generated square-shaped pulses has been earlier observed in F8 and NPR mode-locked lasers [20–23]. According to Mei [29], the PPC effect can be explained by investigating the NALM transmission as a function of peak pulse power. This allows to analyze the loop as a nonlinear switching device, which operation depends on the circulating power, due to the SPM effects occurring in the fibers. If the conditions in the resonator are satisfying for the mode-locking to occur, a pulse will be formed. In first milliseconds, the pulse oscillating in the cavity increases in duration until it reaches a point in which its peak power saturates the gain. From this moment, the peak power of the generated pulses is “clamped” in a region of maximum transmission of the loop. Because of this effect, if any change in losses in the cavity, or increase in gain (pump power delivered to the active fiber) will occur, the generated peak power will remain at a nearly constant level, at the same time change in pulse duration will be observed.

Under maximum pump power conditions, the amplified 1064 nm signal had a signal to noise ratio (SNR) of 33 dB -Fig. 3(a), which was comparable to the side mode suppression ratio (SMSR) of the seed diode used in the experiment (~34 dB). Investigation of the amplified output signal showed no noticeable power fluctuations, self-pulsing or spontaneous lasing on other wavelengths. The mode-locked telecom band pulses had a center wavelength of 1566 nm and 3 dB FWHM of 6.3 nm at 2 W of pump power, which increased to 7.4 nm at maximum pump power -Fig. 3(b). The presence of PPC effect pulse shaping mechanism can be clearly verified by examining the pulse shape evolution depicted in Fig. 4(a), which shows pulse shapes for several pump power conditions. While the average output power and pulse duration grows with increasing pump power (black graph in Fig. 2(a)) the registered detector signal remained nearly unchanged (around 12 mV) throughout whole pump tuning range. Graphs depicted in Fig. 4(b) clearly show, that the square shaped mode-locked 1566 nm pulses have been successfully reproduced in the Mid-IR wavelength region by mixing them with amplified continuous wave (CW) 1064 nm radiation in the PPLN crystal.

 

Fig. 3 Optical spectrum of the amplified 1064 nm seed signal under maximum pump power conditions (a), optical spectrum of 1566 nm pulses registered at four different pump power settings (b).

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Fig. 4 Pulse shapes registered for different pump conditions at 1566 nm (a) and 3322 nm (b) wavelengths (see Visualization 1 and Visualization 2). The insets present pulses registered at a 40 μs span. RF spectrum of laser pulses registered for 1566 nm (c) and 3322 nm (d) wavelengths.

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The observed difference in signal amplitude registered by the Mid-IR detector for each pulse duration originates from pump power related increase in 1064 nm signal amplification (red graph in Fig. 2(a)). Recorded SNR for RF beat-notes of the mode-locked pulses were 63 dB and 61 dB, for 1566 nm and 3322 nm wavelengths, respectively.

At maximum pump power the frequency conversion stage generated Mid-IR pulses with a FWHM of ~12 nm, centered at 3322 nm and an average output power of ~475 µW. The calculated conversion efficiency reached ~0.02%/W and was limited by non-optimal length of the PPLN crystal, in relationship to the bandwidth of the mode-locked pulses [30], and single collimator optical assembly used in this experiment. Further experiments will address the conversion efficiency issue by incorporating a 10 mm long, fiber pigtailed wave-guide PPLN module into the setup (conversion efficiency reaching 10%/W). As presented in Fig. 5(a), the spectrum of the generated Mid-IR pulses overlaps with strong absorption lines of water vapor, which will be targeted in our preliminary experiments with photo thermal gas sensing techniques in this wavelength region [31,32]. Being able to easily control the pulse duration of the generated pulse, we will be able to investigate the relationship between pulse parameters and amplitude of generated photo thermal signal.

 

Fig. 5 Optical spectra of square-shaped pulses generated in the Mid-IR via DFG nonlinear process at 17 W of pump power delivered to the active fiber. Blue graph in figure (a) represents water vapor absorption lines plotted for 10 m path length using Hitran database. Figure (b) shows average idler output power plotted against squared input power delivered to the crystal.

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4. Conclusions

The experiment served as a proof of concept for the dual-wavelength mode-locked configuration. The proposed setup is novel in several aspects. Work presented in this paper is the first published attempt to construct an dual-wavelength all-fiber DSR mode-locked F8 laser using DC active fiber. Appropriate resonator construction enabled easy pulse duration tuning in a wide 150 ns range, via PPC effect. Moreover, including a circulator as an isolating element into the directional loop of the F8 laser, enabled generating pulses with record energy of 2.13 µJ, directly from the resonator, without adding further complexity to the laser, like cavity dumping, Q-switching or additional high power amplifiers. Additionally, the unique dual-wavelength configuration of the laser supported simultaneous and efficient amplification of 1064 nm seed signal, making the proposed laser an ultra-simple, fully integrated, all-fiber source for generating tunable square-shaped Mid-IR pulses in DFG setups. Further improvements will involve building an all-PM fiber version of the laser and incorporating a fiber-pigtailed WG-PPLN crystal. The generated Mid-IR square-shaped pulses will be used in photothermal trace gas sensing experiments.