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Abstract
Our first demonstration of an all-PM fiber double clad erbium-ytterbium figure-8 laser mode-locked in a dissipative soliton resonance regime is presented. The laser generated µJ-level rectangular-shaped pulses with a maximum average output power of 1 W at 994 kHz repetition rate. The proposed configuration was characterized for two values resonator lengths – 44 and 205 m (total net-dispersion −0.9274 ps2 and −4.3084 ps2, respectively) to verify the possibility of non-complex tailoring of pulse parameters. The long-term stability of the all-PM configuration and self-starting of the mode-locking was experimentally confirmed by exposing the laser to forces of –5G to 7G magnitude on a vibration generator.
© 2017 Optical Society of America
1. Introduction
The impressive number of publications focusing on both the physical principles and the experimental realizations of various configurations of Dissipative Soliton Resonance (DSR) mode-locked lasers proves, that these relatively non-complex fiber lasers gathered much attraction amongst researchers around the world. Although DSR lasers are mainly limited to directly generating pulses in the picosecond or nanoseconds regime, these sources have several key advantages that are beneficial when it comes to constructing mode-locked lasers with very high average output power. The main drawback of all-fiber, soliton mode-locked lasers, is their average output power, which originates from low gain offered by standard single-mode active fibers [1]. Unfortunately, direct implementation of double-clad (DC) active fibers in passively mode-locked all-fiber configurations is limited by the soliton area theorem [2]. In net-anomalous dispersion soliton lasers, stable pulse generation can be observed when a subtle steady-state is achieved between the cavity nonlinearity and dispersion. Soliton pulses forming in cavities constructed with DC active fibers (thus introducing high gain) quickly accumulate large values of nonlinearities, perturbing the harmony. Such lasers tend to produce pulses with harmonic repetition rates – harmonic mode-locking [3,4]. Satisfying the soliton area theorem in high-gain DC laser configurations requires using complicated and bulk-optics-based stretch and pulse re-compression stages, which are less robust than all-fiber configurations and increase the overall build cost. In 2008, Chang et al. [5] published a theoretical paper, in which they described a unique mode-locking regime – DSR. According to that prediction, a precise set of laser cavity parameter values would allow the circulating soliton pulse to accumulate indefinite values of energy, without breaking the pulsed operation, or dividing the pulse into harmonically mode-locked copies – thus circumventing the soliton area theorem limitations. The theoretical work was followed by an outburst of publications experimentally documenting the existence of such stable states in laser cavities in numerous non-complex configurations. It has been shown that DSR mode-locking is obtainable in cavities exploiting various passive mode-locking mechanisms (SESAM, nonlinear polarization rotation, figure-8, graphene), in various spectral regions and net-cavity dispersion values [6–19].
Worth mentioning is also the fact, that DSR pulses can be easily amplified in external, standard fiber amplifiers, what was experimentally verified by Zhao et al., who reported a 100 W average output power 2 µm DSR master oscillator power amplifier (MOPA) configuration [20]. Although the pulses have relatively long durations (mostly falling in the ns range), in some DSR laser configurations the chirp, although giant, can be linearly distributed over the entire pulse envelope [21]. This was demonstrated by Li et al., who presented a DSR re-compression configuration, which yielded a factor of 82, thus outputting pulses with large energy and 760 fs duration [22]. Moreover, appropriate cavity design enables tuning of the pulse parameters over a very wide range, thus permitting non-complex tailoring for a specific application [23]. Although simple and built in all-fiber configurations, most of the reported DSR lasers required precise intra-cavity polarization control and their self-starting capabilities were questionable. Additionally, based on non-PM fibers and components, the mode-locking operation was susceptible to environmental noise. Up to our best knowledge, Armas-Rivera et al. presented first all-PM fiber configuration of a DSR mode-locked laser, working in the 1 µm wavelength region [24]. Nevertheless, based on singlemode active fiber, the configuration was capable of generating pulses with an average output power of ~0.35 mW, thus not taking any advantage of the predicted “limitless” energy accumulation of DSR mode-locked pulses. The main focus of the work presented in this paper was to develop a DSR mode-locked DC erbium-ytterbium (Er:Yb) laser in a configuration permitting using only polarization maintaining (PM) fibers and components. Taking all advantages of standard DSR configurations, the all-PM version would additionally be purely self-starting and exceptionally invulnerable to external perturbations. Moreover, incorporating a DC active fiber as the gain medium should allow reaching high average output powers.
2. Experimental setup
The laser is built in an all-fiber, ultra-simple configuration, using only PM fibers and components. The schematic is depicted in Fig. 1.
Fig. 1 Schematic of the all-PM fiber, figure-8, DSR mode-locked laser. MS – mode striper, PM Er:Yb DC – 2.5 m of PM-type erbium-ytterbium co-doped double clad fiber, COMB – pump-beam combiner, 3dB – 3 dB fiber coupler, CIR – circulator,PM1550 – additional spool of singlemode PM fiber.
The resonator is built in an typical Figure-8 Laser (F8L) configuration, which consists of two independent fiber loops, connected via a 3dB coupler in the center. The left loop includes a 2.5 meter-long piece of DC active fiber (Nufern, PM-EYDF-6/125-HE, dispersion −19 ps2·km−1), which was optically pumped by a single, multimode 976 nm semiconductor diode, via a standard fiber pump-beam combiner. Unabsorbed pump light was dumped from the cladding in a mode-stripper (MS), placed at the end of the DC fiber. Having no uni-directional component, the left loop acted as a standard nonlinear amplifying loop mirror (NALM) [25]. Pulses entering the active loop are split into two copies by the 3 dB coupler and circulate in opposite directions, accumulating disproportional nonlinear phase-shift, due to the deliberately asymmetrical placement of the active fiber [25]. After a full cycle in the active loop the dissimilarly phase-shifted pulses interfere at the coupler before exiting the active loop. This effect forms an optical-power-dependent transmission characteristic at the 3 dB coupler, which favors pulsed, mode-locked operation of the F8 laser configuration. The right, passive loop, consists of merely two components – a fiber circulator and a spool of PM1550 fiber (dispersion −21 ps2·km−1). Because the fiber components used in the experiments were manufactured to have PM1550 passive fiber on the input and output ports, we can assume their dispersion parameters are similar the PM1550 fiber, when calculated per meter of length. Historically, in F8L with NALM’s, uni-directional operation of the passive loops was enforced using optical isolators [26,27]. Due to the fact that DSR mode-locked lasers are capable of generating optical pulses with energies beyond µJ-levels, constant exposure of fiber isolators to such optical intensities is not reasonable, especially if long-term stability and reliability is required. We found that with an appropriate port configuration a standard fiber circulator can act as isolator and an output coupler, simultaneously. The proposed configuration works as follows: optical pulses exiting the NALM loop are divided in the 3dB coupler, while entering the passive, right loop. Pulses traveling in the counter-clockwise direction are transmitted through the circulator, using the 1 - 2 port direction. Pulses circulating in the clockwise direction are completely out-coupled from the loop (port 2 – 3 direction), rather than being absorbed in an isolator, compared to standard NALM laser configuration. Moreover, this configuration ensures that the overall optical power circulating in the passive loop is minimized before being transmitted through the additional long spool of PM1550 fiber, thus limiting nonlinear-related spectra and pulse distortion. The spool of PM1550 fiber introduced additional dispersion that was necessary to ensure DSR would be the dominant mode-locking mechanism. Minimum anomalous cavity dispersion value permitting stable DSR mode-locking operation through the entire pump power range was found experimentally. Resonators with a total cavity length shorter than ~40 m (total net-dispersion higher than −0.835 ps2) were susceptible to randomly falling into harmonic mode-locking or noise-like pulse regime.
3. Experimental results
The performance of the all-PM fiber DSR mode-locked F8L was analyzed for two values of total resonator length – 44.4 (total net-dispersion −0.9274 ps2) and 205.4 m (total net-dispersion −4.3084 ps2), to demonstrate the possibility of non-complex tailoring of pulse parameters. Because the cavity was formed using only PM fibers and components, the laser did not require any additional procedures to obtain DSR mode-locking regime, apart of delivering adequate pump power. Average output power registered in function of pump power is depicted in Fig. 2 (measured using Thorlabs - PM100 thermal power meter).
Fig. 2 Average output power (squares) and pulse energy (dots) registered for two laser configurations (44.4m and 205.4m total cavity length) in function of pump power delivered to the active fiber. Solid lines represent linear fit. The insets in both Figs. present 50 ps autocorrelation measurements, showing no signs of a fine-pulse-structure hidden under the giant-pulse envelope.
Stable DSR mode-locking operation for both laser configurations was achieved for pump powers between 0.5 and 4.4 W. At maximum pump power the laser was capable of generating 915 mW and 1005 mW of average output power, for the 44.4 m and 205.4 m configuration, respectively. Considering negligible optical noise build-up between the mode-locked pulses, and repetition frequencies of 4.6 MHz and 0.994 MHz, this corresponds to maximum pulse energies of 0.199 µJ and 1.011 µJ, which, to our best knowledge are the highest recorded values for all-PM DSR configuration working in the 1.55 µm wavelength region.
Both output power characteristics can be linearly fitted with minimal error. Worth noting is the fact, that although built simple, using widely available fiber components, the presented all-PM-fiber DSR lasers yielded and impressive slope efficiency of 21.5% and 24.4%, for the 4.6 MHz and 0.994 MHz configurations, respectively. The insets in Figs. 2(a) and 2(b) present autocorrelation measurements performed with APE PulseCheck autocorrelator, for 50 ps scan range (maximum available). No trace of a fine-structures hidden in the giant rectangular pulse envelopes confirms, that the constructed lasers produced purely DSR mode-locked pulses. The DSR mode-locking was also verified by measuring the RF beatnote between the modes circulating in the cavity, using a RF signal analyzer (Keysight N9010A, 3.6 GHz BW) and a fast photodiode (Lab Buddy, DSC2-50S, 12 GHz BW). The measurement is depicted in Fig. 3.
Fig. 3 Beatnote registered with an RF analyzer for fundamental repetition frequency of each configuration, at maximum pump power. Insets show the beatnote for wider spans and a zoom-in on the envelope modulation, intrinsic to DSR mode-locking, which is directly connected to the pulse duration.
The spectrum measured with an RF analyzer confirms, that the DSR regime was indeed obtained for both laser cavity lengths, generating a train of evenly spaced, phase-locked pulses. The supermode suppression ration (SSR) was ~76 dB for both configurations. The insets presented in Figs. 3(a) and 3(b) show a beatnote envelope modulation, registered for wider spans, which is an intrinsic feature of the DSR mode-locked laser configurations, generating giant rectangular-shaped pulses. The modulation period is inversely proportional to the duration of pulses generated in the cavity for a given input pump power (10 MHz envelope for 95 ns pulse duration and 230 MHz for 4.3 ns pulses). Pulses exiting the cavity were also analyzed using a wide bandwidth oscilloscope (Keysight DSO90604A, 6 GHz BW) and a fast photodiode (Lab Buddy, DSC2-50S, 12 GHz BW). Pulse shapes registered for both configurations in function of pump power delivered to the active fiber are depicted in Fig. 4.
Fig. 4 Pulse shapes registered for both laser configurations in function of increasing pump power. The inset show pulses registered for acquisition times of 10 µs and 40 µs.
The high resolution oscilloscope measurements clearly confirm that both lasers generated pulses with shapes close to rectangular. Moreover, the DSR-intrinsic pump power-related increase of duration was also observed. Pulses generated from the 0.994 MHz laser configuration show minor traces of gain saturation at the leading edges, for pulses generated at high pump power conditions (above 3.5 W of pump power). This effect was also observed in our previous configurations [23]. The relation between pump power and pulse duration, and the corresponding peak powers, for both configurations are plotted in Fig. 5.
Fig. 5 Pulse duration and peak power plotted in function of input pump power, for both laser configurations
Both configurations experienced nearly linear pump-power related increase in pulse duration, which is characteristic for DSR mode-locked lasers. The pulse duration was tunable in the 0.92 ns - 4.35 ns and 16 ns – 92 ns range, for the 4.6 MHz and 0.994 MHz configuration, respectively. At maximum pump power the lasers generated pulses with peak powers equal to 42.9 W and 10.4 W, for the 4.6 MHz and 0.994 MHz configuration, respectively. The output of both laser configurations was also analyzed using an optical spectrum analyzer (Yokogawa AQ6370B), for increasing pump power conditions – Fig. 6.
Fig. 6 Optical spectrum registered for both laser configurations in function of increasing pump powers (0.8 - 4.4 W). The insets show optical spectra, plotted in linear scale, gathered for maximum pump power conditions.
The measurements depicted in Fig. 6 show no severe optical spectra distortion for increasing input pump powers. The broadening of the optical spectra, registered for the 4.6 MHz configuration was previously observed for other configurations [19,20,23], and is connected with higher peak powers of the pulses circulating in the laser cavity, thus producing more evident self-phase-modulation broadening. Higher noise floor registered for lower pump power conditions in both configurations is connected with the fact that we used the same sensitivity settings for all measurements and centered every peak at 0 dBm (to clearly demonstrate any shape changes).
4. Durability test
Because the entire laser cavity was designed based only on PM components and fibers, the laser was a purely self-starting configuration and was invulnerable to external vibrations. Moreover, the output pulse parameters were repeatable at each ON-OFF power cycle. To prove this fact, we have enclosed the 0.994 MHz laser cavity in a standard 20x10x1.5 cm3 fiber tray, which was mounted onto a vibration generator (only the multimode pump diode had to be mounted outside the tray, to enable sufficient heat dissipation). The results showing a 30 minute stability and self-starting capability are depicted in Fig. 7.
Fig. 7 Polarization (azimuth and ellipticity) and output power stability measured for 30 minutes at maximum pump power (0.994 MHz configuration) while exposing the laser to forces of ~7 G – graph b), inset shows measurement taken with a G-sensor. 1 minute close-up of the durability measurement – a). Self-starting of the DSR mode-locking presented for 6 consequent ON-OFF cycles (while being exposed to ~7G) – c). With pump switched off, the polarimeter registered random values (black and red noise between ON-cycles). Video showing the experiment provided in supplementary files (Visualization 1 and Visualization 2).
The durability test was split into two parts. Firstly, the laser was turned on for a 5 minute period with maximum pump power input, so the parameters would stabilize. After the heat-up period we began to register polarization parameters – azimuth and ellipticity (Thorlabs, PAX5710IR3-T), and output power. The first minute of the test, conducted without exposing the laser to vibrations, was to show the initial values of the output parameters (can be seen as a flat part of the G-sensor graph on the inset in Fig. 7(b). During the next 28 minutes the laser was exposed to Z-axis forces of –5G to 7G magnitude, and frequency of ~4 Hz (G-force measurement presented as an inset in Fig. 7(b) and gray background graph in Fig. 7(a). The Fig. 7(a) presents a 1 minute close-up of the Fig. 7(b) measurement. Although being constantly exposed to severe G-forces, the proposed all-fiber configuration, delivering µJ-level pulse energies, sustained the initial polarization state and average output power. Standard deviation of the parameters were 0.06 deg. and 0.03 deg. for azimuth and ellipticity, and 0.266 mW for the average output power. The second part of the measurement was performed after a 2 hour cool-down period. Next, the vibration generator was turned-on and was constantly exposing the laser to G-forces similar as in the previous experiment. To verify the self-starting capability of the constructed laser, the pump power was cycled 6 times - Fig. 7(c), showing excellent repeatability, both in the polarization and the average output power parameters. The registered increase in output power is connected only with the initial temperature-related active-fiber gain change. During the durability experiments we have not observed any difference in the duration, shape or the optical spectra of the generated pulses.
5. Conclusions
The goal of our experiments was to verify the possibility of designing and constructing a DSR mode-locked all-PM laser, capable of delivering µJ-level pulses. Optimized length of the DC active fiber yielded efficiencies reaching 24%, enabling generation of pulses with µJ-level energies, while at the same time requiring a single 5W multimode pump diode. The proposed laser configuration was tested for two different resonator lengths (44.4 m and 205.4 m) to confirm the possibility of non-complex tailoring of the generated pulse parameters (duration, repetition rate) via simply adjusting the length of the additional spool of the PM1550 fiber. Although built extremely simple, using widely available and inexpensive PM-fiber components, the proposed configurations were purely self-starting, compact and remarkably resistant to external vibrations. Following experiments will include using the constructed laser as an excitation source in photothermal spectroscopy applications in the mid-infrared wavelength region [28,29].
Funding
National Science Centre (NCN) (DEC-2014/14/M/ST7/00866); Statutory funds of the Chair of EM Field Theory, Electronic Circuits and Optoelectronics, Wroclaw University of Technology (0401/0094/16).
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