We demonstrate a novel method of mid-infrared (mid-IR) supercontinuum (SC) generation with the use of a 2 µm gain-switched self-mode-locked thulium-doped fiber laser. SC radiation ranging from ~1.9 to 3.8 µm wavelength, generated in a single-mode ZBLAN fiber with a zero-dispersion wavelength (ZDW) shifted to ~1.9 µm, is reported. An average output power of 0.74 W with 0.27 W at wavelengths longer than 2.4 µm was measured. It is, to the best of our knowledge, the first report on such an approach to generate a mid-IR SC in optical fibers.
©2013 Optical Society of America
Numerous methods have been demonstrated on SC generation using different laser systems and nonlinear optical media. Optical fibers seem to be the best candidates for SC generation providing easy confinement of a pump laser beam into a small area core and a significant media length for nonlinear interaction. The most available and thus most commonly used silica fibers have been successfully employed for watt-level continua generation form ultraviolet [4,5] to ~2.7 µm wavelength [6,7]. Further spectral broadening in this nonlinear medium is ineffective due to intrinsic absorption losses beyond 2.4 µm in silica glass . Therefore, recent efforts have been directed to the use of soft glass fibers composed from tellurite [9,10], chalcogenide [11,12] and fluoride [1,13] glass. Tellurite and fluoride glasses are transparent to about 5 µm [9,13], while chalcogenide glass can support propagation of light even above 7 µm wavelength . However, at present only the drawing technology of heavy metal fluoride fibers, especially ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers seems to be sufficiently mature to be useful for SC generation with an output power above 10 W .
Mode-locked lasers, Q-switched mode-locked (QML) lasers, ns-scale pulsed fiber Master Oscillator Power Amplifiers (MOPAs) and optical parametric oscillators have been used as pump sources for SC generators based on ZBLAN fibers [e.g 1,13,16–18.]. Qin at al . presented an ultra broadband low power (<20 mW) SC light in the range from UV to 6.28 µm generated out of 2-cm long ZBLAN fiber pumped by fs pulses (~50 MW peak-power) at 1540 nm. Mid-IR SC generation (wavelength range from ~1.9 to 3.6 µm) in a ZBLAN fiber pumped by actively QML fiber laser with an output power of 1.08 W was also reported . Pumping a ZBLAN fiber by a tunable optical parametric amplifier system, providing 110 fs pulses at 2 µm, Agger at al. achieved SC spread from 0.7 to 4.3 µm . An alternative approach to generate SC in a tandem of silica and fluoride fibers was proposed by Xia at al . They pumped a nonlinear fiber by ~1 ns 1.55 µm pulses of distributed feedback (DFB) laser amplified in a cascade of fiber amplifiers. The SC was initiated by the breakup of ns pulses into a train of short femtosecond sub-pulses through a modulation instability and then, as a result of Raman scattering, red shifted reaching the long wavelength cut-off of 4.5 µm. A unique feature of this solution is the possibility of scaling up the output average power while keeping the spectrum shape and width relatively constant. A very attractive solution towards flat and high power density SC generation in silica fibers was proposed by Larsen et. al , where a diode-pumped gain-switched ytterbium fiber laser, delivering ~200 ns pulses, was used as a pump source. This concept could be also adopted for pumping of soft glass fibers.
In this paper we report an octave spanning mid-IR SC generated in a 20-m long, customized, single-mode ZBLAN fiber pumped directly into an anomalous region of its dispersion characteristic. A novel, gain-switched mode-locked (GSML) fiber laser and amplifier system generating an average output power of up to 2.3 W at a repetition rate of 26 kHz was used as a pump source. It delivered gain-switched pulses with an envelope width of ~50 ns with simultaneous self-mode-locked sub-pulses spaced by the laser cavity round-trip time. The mode-locked pulses were characterized by 200-300 ps duration and the maximum peak-power of up to 27.8 kW. An average SC output power as high as 0.74 W in a wavelength range of ~(1.9-3.8) µm is demonstrated.
2. SC experimental setup
The SC source consists of a 1.55 µm fiber MOPA, a GSML thulium-doped fiber laser (TDFL) and a thulium-doped fiber amplifier (TDFA). The principal scheme of this configuration is shown in Fig. 1 .
To obtain a stable pulse train, fast gain-switching and resonant pumping are required. In-band pumping ensures a rapid population inversion that can be depleted by a single short gain-switched pulse. Then the cross-relaxation and excited state absorption processes can be reduced leading to generation of stable, short 2-µm pulses, as was shown in . Furthermore, in gain-switched TDFLs an output pulse duration is determined by an active medium gain and the round-trip time, which means that the length of active fiber directly affects output time characteristics of a laser. In such a situation, only core-pumping can provide a suitable gain while keeping a resonator length short and thus supports generation of short (tens of ns) pulses . For this reason a ~20-cm long, single-mode double-clad, Tm-doped (~2 wt.%), silica fiber (TDF) was used as an active medium of the GSML laser. It had a 10/130 µm core/clad diameter and a corresponding numerical aperture NA of 0.15/0.46. The active fiber was core-pumped by a 1.55 µm fiber MOPA system, delivering pulses of 20-700 ns duration at the frequency independently changeable in the range from 26 to 320 kHz. The MOPA system consisting of a DFB laser seed followed by a cascade of erbium- and erbium:ytterbium-doped fiber amplifiers (EDFA and EYDFA) was capable to deliver pulses with energy of several tens of µJ and average power of up to 3.5 W. About 90% of pump power was absorbed by the active dopant. The output from the MOPA system was directly spliced to the pigtail of a 3-cm long high reflector (HR) fiber Bragg grating (FBG). The laser cavity was formed by a HR FBG having a reflectivity of >99% at 1994.5 nm, and an output coupler (OC) FGB with a reflectivity of 90% at 1994.6 nm and a 3dB reflection bandwidth of 1.5 nm. The FBGs were cleaved and fusion spliced very close to the active fiber to keep the resonator short. In the next step, the TDFL output was optically isolated and spliced to an input of TDFA that was built with the use of ~2.5-m long TDF, characterized by the same parameters as mentioned above. The amplifier was cladding-pumped by two fiber-pigtailed 4.75 W 790-nm pump laser diodes with 105 µm core diameter and 0.22 NA via a (2x1) + 1 pump combiner with a signal feedthrough. Finally, the 2 µm amplified pulse train was launched into a ZBLAN fiber with the use of a telescope allowing for coupling efficiency of ~60%. The 20-m long ZBLAN fiber (manufactured by Le Verre Fluore) had a core/clad diameter of 7/125 µm and a NA of 0.23. The ZDW and cut-off wavelength of this fiber were 1.9 µm and 2.04 µm, respectively. Both ends of the fluoride fiber as well as the output fiber end of the TDFA were polished at an angle of 8° to avoid any back reflections.
The benefit of using the GSML Tm3+-doped fiber laser as a pump source is the lower cost and reduced complexity of the system, compared to e.g. Q-switched or QML lasers. Besides, comparing with CW pumping scheme, the increased peak power of self-mode-locked pulses, amplified in a chain of fiber amplifiers, can eliminate the critical dependence on the ZDW, as it was reported in .
SC spectrum from 1.2 µm to 2.4 µm wavelength was measured by using an optical spectrum analyzer (OSA) whereas the longer wavelengths were measured with a grating monochromator and a thermoelectrically cooled HgCdTe detector. A sampling digital oscilloscope with 6 GHz bandwidth and two detectors with rise/fall time of <35 ps were used to measure time characteristics.
3. Experimental results and discussion
The gain-switched TDFL was pumped by ~100 ns 1.55 µm pulses with the energy of up to 80 µJ at the pulse repetition frequency (PRF) ranging from 26 to 40 kHz. After reaching the lasing threshold, the 2 µm output pulses of long duration appeared and with the increase in pump power both the pulse build-up time and pulse width shortened. Further increase in pump energy lead to self-started mode-locking operation with a full modulation depth, which is shown in Fig. 2 .
The envelope width of the gain-switched pulse was ~50 ns whereas the duration of sub-pulses was 200-300 ps (190 MHz spectral space), the time of which is much shorter than the laser cavity round-trip time (5.3 ns). The mode-locking depth as well as the exact form of generated pulse train was strictly dependent on the pumping conditions (PRF, pump pulse width and shape, pump pulse energy). We believe that the origin of self-starting mode-locking in our GSML fiber laser is a self-phase modulation effect and an interplay of mode beats. This phenomenon is still being examined .
In the next step the self-mode-locked pulses from the gain-switched fiber laser (presented in Fig. 2) were boosted in the TDFA. Figure 3 shows the evolution of output average power at 2 µm for the PRF of 26 kHz and 40 kHz as a function of TDFA pump power. For 26 kHz (Fig. 3(a)) the average power of 2.3 W, with corresponding 88 µJ energy in 50 ns duration gain-switched pulse was measured. Increasing the PRF to 40 kHz results in average power scaling up to 2.6 W with corresponding pulse energy of 65 µJ (Fig. 3(b)). The amplifier operated with a 26.5% and 31% slope efficiency for 26 kHz and 40 kHz, respectively.
Figure 4 presents pulse energy and peak power for selected mode-locked pulses in a 50-ns gain-switched envelope for the PRF of 26 kHz. The inset presents a typical SML pulses within an envelope of a gain-switched pulse. During the experiment we did not carry out autocorrelation measurements and the pulse structure was recorded with the use of a sampling oscilloscope and a fast photodiode and then analyzed in OriginPro software. The timing of the sub-pulses was slightly unstable, owing to amplitude fluctuations of 1.55 µm pump pulses. Furthermore, we also noticed that careful adjustment of the pump power and the pump pulse time characteristic led to the improvement of output pulse structure stability. This relation is still being analyzed .
As can be seen in the inset in Fig. 4, the width of the first and last sub-pulses was longer than those in the center of the gain-switched envelope. The highest peaks were characterized by the shortest duration and thus the highest peak-power. The generated pulse train consisted of ~15-20 sub-pulses. The maximum energy of the gain-switched pulse (calculated by dividing the average output power by the PRF) was 88 µJ whereas the energies of three highest mode-locked peaks (marked in the inset as 4, 5 and 6) were 8.3 µJ, 13.7 µJ and 15.7 µJ (with corresponding peak power of 22.2 kW, 24.7 kW and 27.8 kW), respectively.
Finally, the amplified pulse train was launched into a ZBLAN fiber with ~60% coupling efficiency. As was mentioned earlier, the fluoride fiber had the ZDW at 1.9 µm in order to be pumped in anomalous dispersion region but relatively close to the ZDW, which facilitates spectral broadening towards mid-IR . The output SC power as a function of launched pump power, for both PRFs applied, is shown in Fig. 5 . The output power increases linearly with the increase of pump power and was only limited by the available pump power. For instance, for 26 kHz increasing the incident pump power to 1.23 W results in the total SC power of 0.74 W, out of which 0.27 W is in the wavelength range beyond 2.4 µm. Over 0.12 W was detected for λ > 3 µm. When operating at 40 kHz, the output power was as high as 0.9 W, where 240 mW and 86 mW were measured for λ > 2.4 µm and 3 µm, respectively. The slope efficiency η of generated SC in all spectral SC band was 62.8% and 60% for the PRF of 40 kHz and 26 kHz, respectively. Even though higher PRFs provided higher generation efficiency of total output power, the generation efficiency of longer wavelength radiation was higher for lower PRF, being a consequence of higher peak power of pump pulses.
The measured SC output spectrum, corrected for the spectral responsivity of the HgCdTe detector and the monochromator gratings response, is presented in Fig. 6 .
The spectrum of SC, measured for the PRF of 26 kHz, spreads from ~1.9 to 3.8 µm with an octave spanning within −45 dB level with respect to the peak at the 2 µm pump wavelength. A modulation instability effect leading to the creation of solitons and then soliton self-frequency shift are mostly responsible for spectrum extension, which is typical for pumping a nonlinear medium in the anomalous group-velocity dispersion (GVD) region [1,18,23]. Additionally, the red-shifted solitons were broadened by self-phase modulation and cross-phase modulation, making the SC spectrum smooth and flat. As can be seen in Fig. 6 the 10 dB flatness of spectral intensity was maintained in the wavelength interval from ~2100 to 3600 nm (span of 1500 nm). Another characteristic feature of the spectrum is that it is mainly broadened towards longer wavelength with reference to the pump wavelength, in contrary to spectra obtained in laser systems utilizing 1.55 µm pulses and pumping in the normal GVD of ZBLAN fibers [e.g 1,13,15,18.]. Further spectral broadening towards mid-IR was difficult due to the rapidly increasing fiber attenuation >100 dB/km at ~4 µm and >1000 dB at 4.5 µm (inset in Fig. 6), caused that red wavelengths generated in the fluoride fiber were highly attenuated preventing long wavelengths cut-off extension. The length of the ZBLAN fiber used in the experiment was not optimized. It will be the subject of further investigation taking into account the pump pulse characteristics of our laser source. As was shown in [e.g 13.], by proper selection of fluoride fiber length with regard to pump pulse peak power the inherent material losses of the fiber can be overcome leading to spectrum extension even beyond 6 µm.
In conclusion, we have demonstrated for the first time, to the best of our knowledge, an octave spanning (~1.9-3.8 µm) mid-IR supercontinuum generation in a single-mode ZBLAN fiber, pumped by self-mode-locked pulses from a gain-switched Tm3+-doped fiber laser. The pump laser delivered ~50 ns gain-switched pulses with simultaneously mode-locked sub-pulses with 200-300 ps duration and a maximum peak-power of up to 27.8 kW. The maximum SC output average power for the PRF of 26 kHz was 0.74 W and the power beyond 2.4 µm and 3 µm were measured to be 0.270 W and 0.125 W, respectively. Both an output power and spectrum width can be further scaled up.
This work was supported by the Polish National Science Centre under Project No. 724/N-MIFL/2010/0.
References and links
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