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We report on a tunable noise-like pulse (NLP) generation in a mode-locked Tm fiber laser with a SESAM. A tuning range of 1895-1942 nm, while keeping the spectral bandwidth of 10-19 nm under NLP mode-locked operation, was obtained by a tunable filter based on chromatic dispersion of telescope lenses. At the center wavelength of 1928 nm, the maximum output power of 195 mW with the spectral bandwidth of 18.9 nm was obtained. The repetition rate was 20.5 MHz and the corresponding pulse energy was 9.5 nJ. To our knowledge, this is the first report of a tunable NLP mode-locked laser based on chromatic dispersion of a lens system.
© 2016 Optical Society of America
1. Introduction
Over the last decade, two micron fiber lasers have attracted huge attention due to their scalability of mode filed area, mitigation of nonlinear processes and their applications such as LIDAR [1], pump source for a mid IR light source, polymer processing [2], and silicon processing [3]. Pulsed operation is especially attractive in terms of its high peak power, short pulse duration and/or broad spectrum bandwidth. Thulium (Tm) doped fiber is one of the most attractive gain medium to obtain short pulses at 2 μm wavelength range due to its broad and smooth fluorescence spectrum. In standard soliton mode-locked fiber lasers, the pulse energy Ep and the pulse duration Δt are governed by a soliton equation, Ep λβAeff /Δt, where β is dispersion parameter, and Aeff is effective mode field area. The equation limits the available pulse energy from soliton mode locked fiber lasers generally to below ~10s pJ level [4] in combination with finite gain bandwidth of gain fiber and nonlinear process such as stimulated Raman scattering and self-phase modulation. At the wavelength range of 2 μm, β and Aeff of silica fiber are about 2-3 times larger than that around 1.55 μm wavelength that could allow an order of magnitude larger pulse energy for a soliton mode-locked fiber laser. Dispersion-managed soliton (DMS) and all-normal dispersion (ANDi) mode-locked fiber lasers [5] have extended the available pulse energy with a short pulse duration. Pulse energy as high as 4.9 nJ with sub ps pulse duration [6] and pulse energy up to 7.6 nJ with 130 fs pulse duration after dechirping were achieved by DMS (hybrid self similariton) mode-locked laser at 2 μm wavelength range [7]. Further power scaling of DMS laser and development of ANDi mode-locked laser in 2 μm wavelength range are, however, are big challenges due to a large anomalous dispersion of silica glass at the wavelength range. Besides them, noise-like pulse (NLP) mode-locked laser provides another way to access a high power moderate short pulses. In NLP mode-locked operation, a bunch of short pulses composes ps to ns envelope by a soliton fission effect and positive feedback [8]. The NLP mode-locked operation enables very high effective pulse energy with a penalty of low coherence and longer pulse duration (typically above ps envelope) than other mode-locked operations. NLP mode-locked laser is useful for some specific applications such as material processing and super continuum generation where a high coherence and sub ps short pulse duration are not always necessary [9]. Recently, a pulse energy as high as 0.25 μJ has been obtained by a nonlinear polarization rotation NLP mode-locked fiber laser at 2 μm spectral region [10].
For some applications, a tunable short pulse light source is desirable, because it allows us to select the wavelength for different targets or aims. Tunable lasers have been investigated in various wavelength ranges with various types of wavelength selective elements, such as grating, Lyot filters, Fabry-Perot Tunable Filter, and Acousto-Optic tunable filter [11–14]. Most of them have a high wavelength selectivity (high resolution) that is favorable for a narrow linewidth continuous wave (CW) operation. For a tunable mode-locked operation, wavelength tunable element with appropriate bandwidth, however, is required to sustain a broad spectral bandwidth of a short pulse duration. The availability of commercial filters with a broad bandwidth, high transmission, and high peak power handling, especially at the developing wavelength ranges such as the 2-3 μm, is strongly limited. Only a limited number of tunable mode-locked fiber lasers have been reported at 2 μm so far [15–17].
In this paper, we report on a tunable NLP generation in a mode-locked Tm fiber laser with a SESAM. The tuning range from 1895 nm to 1942 nm while keeping the spectral bandwidths of 10-19 nm for NLP mode-locked operation was obtained by a simple tunable filter based on chromatic dispersion of telescope lenses and slit-like effect of a fiber core. This simple method requires neither expensive component, fragile fiber part (such as taper), nor temperature control. The maximum output power was 195 mW with the spectral bandwidth of 18.9 nm at the center wavelength of 1928 nm. The repetition rate was 20.5 MHz and the corresponding pulse energy was 9.5 nJ. To our knowledge, this is the first report of a tunable NLP mode-locked fiber laser based on chromatic dispersion of a lens system.
2. Experimental setup
A schematic picture of the tunable NLP mode-locked Tm fiber laser setup is shown in Fig. 1. All the fibers used in the cavity work in single transverse mode at 2 μm region, and no polarization control element was used. As a modulation element, we used a free space semiconductor saturable absorber mirror (SESAM, BATOP GmbH). SESAM possess relatively high damage threshold than other saturable absorbers such as CNT and the free space arrangement allows us easy control of the fluence onto the SESAM that is favorable for a high power pulsed laser operation. As a gain medium, we used 1.5 m Tm-doped single clad silica fiber (core diameter is 10 μm, NA is 0.13, CTm is 0.2 wt.%, fabricated in Uni. Southampton). The Tm fiber was pumped by our home-built Er:Yb fiber MOPA system (1.55μm, 4.8 W maximum output power) through a fused type WDM. A dielectric mirror coated passive fiber of the reflectivity of ~60% was fusion spliced to the signal port of the WDM and served as an output coupler. At the end of the Tm doped fiber, a SMF28 fiber with APC (Angled Physical Contact) connector was spliced to avoid unfavorable feedback from the facet. The light from the APC fiber end was focused onto a high reflection (HR) mirror or SESAM by f = 18.75 mm (H-LAK54) and f = 30 mm (N-BK7) telescope lenses. The telescope lenses used in the cavity have large chromatic dispersion. In combination with the slit like effect of the fiber core, the telescope lenses works as a bandpass filter whose bandwidth could be broad enough to sustain 100 fs order pulse mode-locked operation. The center wavelength of the filter shifts by moving the position of the mirror along optical axis and therefore it works as a tunable bandpass filter. This tuning method is very simple with less components and can be adapted for any other wavelength region within the material transparency range of the lens. Recently, a tunable CW Pr doped ZBLAN fiber laser at 600 nm wavelength range with a tunable bandwidth of 20 nm based on a very similar tuning method has been reported [18]. The filter bandwidth depends on the combination of the lenses and fiber core diameter. The calculated filter property for different lens set with SMF28 fiber are shown in Fig. 2(a) and 2(b). The calculation is based on a shift of the focusing length due to chromatic dispersion and other aberration effects were not taken into account. The used lenses for the calculation are listed in Table 1. The telescope lenses used in the experiment (f = 18.75 mm (H-LAK54) and f = 30 mm (N-BK7)) show the calculated FWHM of ~35 nm which would be broad enough to sustain short pulsed operation. The filtering property can be optimized by selecting the proper lens combination (Table 1).
Fig. 1 schematic picture of tunable NLP mode-locked Tm fiber laser. The SESAM has modulation depth of 20%, non-saturable loss of 16%, relaxation time of 10 ps, and saturation fluence of 50 μJ/cm2. The Tm fiber has a core diameter of 10 μm, NA of 0.13 and Tm doping level of 0.2 wt.%. The fiber, telescope lenses and the end mirror at the right side also work as a tunable bandpass filter.
Fig. 2 (a) Calculated filter property at specific mirror position for the lenses of focal length of 15 mm (circles), 30 mm (squares) and 60 mm (triangles) and (b) for the lenses made of N-BK7 (circles) and CaF2 (squares).
Table 1. Calculated FWHM of the filter for different lens combinations.
3. Result & discussion
At first, we optimized the SESAM position (bandpass filter) for the highest output power. The power property and the spectrum at the maximum output power are shown in Fig. 3(a) and 3(b), respectively. Stable mode-locked operation self-started at the output power of 176 mW (at low pump power level the laser showed Q-switching instability). The maximum output power was 195 mW at the incident pump power of 3.1 W and the slope efficiency was ~7%. The spectral bandwidth of 18.9 nm at the center wavelength of 1928 nm was obtained. The dips in the spectra are water vapor absorption in the air. The Fourier transform (FT) limited pulse duration of the measured spectrum is ~280 fs. The pulse trains are shown in Fig. 4(a)-4(c). The repetition rate was 20.5 MHz equal to the cavity length and the corresponding pulse energy was 9.5 nJ. The measured pulse duration by a 7 GHz oscilloscope (resolution of ~140 ps) was 250 ps (Fig. 4(c)). The autocorrelation traces are shown in Fig. 5(a) and 5(b), which showed double scale structure constructed with the narrow sub-ps peak (~533 fs) and broad sub-ns pedestal (>200 ps, longer than the available measurement time window of our autocorrelation system) components. The narrow sub-ps peak is much broader FT pulse duration of the measured spectrum so that it is not a coherent spike.
Fig. 4 Temporal profile of mode-locked operation measured by 7 GHz oscilloscope. (a) 10 μs time window (b) 200 ns time window (c) 2 ns time window.
Above experimental results showed common characteristic properties of NLP in mode-locked laser [19, 20]. First, the estimated pulse energy of 9.5 nJ is much higher than the soliton mode-locked laser. Such a high pulse energy operation could be available in a DMS or ANDi mode-locked laser, but our setup was all anomalous dispersion cavity which does not work in such regions. Second, the measured spectra did not show characteristic structure for a multi pulsing and Kelly sideband even with such a high pulse energy level. Third, the autocorrelation trace showed the double scale structure. Those results strongly indicate our laser works in NLP mode-locking region. In our experiment, the laser always showed NLP property and standard single pulse soliton or dissipative soliton mode-locking were not observed.
Next, tunable NLP mode-locked laser operation was demonstrated by moving the position of the SESAM along optical axis with the same cavity. To investigate the influence of the SESAM on the tunable property, we also demonstrated tunable CW operation with a HR mirror instead of the SESAM. Figure 6(a) and 6(b) show the spectra of CW and mode-locked operations at each HR mirror or SESAM position, respectively. The CW operation showed multi-peak spectra and the tunable range of 1893-1950 nm with the maximum output power of 213 mW. The mode-locked operation showed the tunable range of 1895-1942 nm (peak wavelength) with 10-19 nm spectral bandwidths with the maximum average power of 195 mW. Outside the tunable range, we observed a parasitic lasing around ~1920 nm. The measured output power and center wavelength as a function of the relative position of the HR mirror and SESAM are shown in Fig. 7(a) and 7(b), respectively. In either operations, the output power kept over 150 mW whole tuning range and the spectra show similar linear red shift by moving the mirror away from the lens. The calculated center wavelength of the tunable filter as a function of relative mirror positon are also shown in Fig. 7. The experimental (squares) and calculation (triangles) results are in good agreement that proves the tunability came from the chromaticdispersion of the lenses. In Fig. 7(b), a deviation between experimental and calculated results can be seen at the longer wavelength edge, which could be explained that in the NLP mode-locked operation, the spectral bandwidths were broad and its longer edge reaches to the limitation of tunable range at first(Fig. 6(b)). Therefore it has narrower (center wavelength) tuning range. The main limitation factor of the tunable range of our current setup would be the WDM (2000nm ± 20nm in spec sheet), cut off of the high reflection coating of the SESAM and HR mirror (high reflectance above ~1890 nm), and spherical aberration of the telescope lenses. By further optimization of the components, much broader tuning range would be available. The maximum spectral bandwidth of 18.9 nm in our current NLP laser also could be restricted by the bandwidth of our current bandpass filter of 35 nm (Fig. 2) so that tunable NLP mode-locked laser operation with much broader spectral bandwidth could be obtained by using a lens set of longer focusing length or of lower chromatic dispersion (see Table1).
Fig. 6 Tunable spectral property as a function of the position of the mirror. (a) CW operation with the HR mirror. (b) NLP mode-locked operation with the SESAM. (b). The insets indicate the relative positions of the mirror.
Fig. 7 Average power and center wavelength as a function of the relative position of the mirror (a) CW operation with HR mirror. (b) NLP Mode-locked operation with SESAM. The circles and solid curve are measured output power and fitting. The squares and doted curve are measured peak wavelength and fitting. The triangles and dashed line is calculated peak wavelength of the tunable filter and fitting.
4. Conclusions
In conclusion, we have developed a tunable NLP mode-locked Tm fiber laser with a SESAM at the wavelength range of two micron. The tuning range was from 1895 nm to 1942 nm while keeping spectral bandwidths of 10-19 nm in NLP mode-locked operation. This was realized by a tunable filter based on chromatic dispersion of lenses and the slit-like effect of fiber core. The maximum output power was 195 mW with the spectral bandwidth of 18.9 nm at the center wavelength of 1928 nm. The repetition rate was 20.5 MHz and the corresponding pulse energy was 9.5 nJ. To our knowledge, this is the first report of the tunable NLP generation in a mode-locked Tm fiber laser based on chromatic dispersion of a lens system. This simple tuning scheme would be useful at other wavelength ranges, especially where the availability of tunable filters is limited. The filter property of the scheme can be tuned by telescope lens arrangement such as multiple lenses in series and/or material of the lenses.
Funding
JSPS KAKENHI Grant-in-Aid for Young Scientists (B) (16K17526); The Amada Foundation; and Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
Acknowledgments
The authors thank Prof. Andy Clarkson in University of Southampton for comments and providing the Tm doped fiber. The authors also thank Dr. Henrik Tünnermann and Dr. Peter Shardlow for private discussion.
References and links
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