1. Introduction

Tunable ultrafast lasers have attracted great interests due to the wide applications, including ultrafast spectroscopy, nonlinear microscopy, chemical/biomedical diagnostics [1–4]. A wide wavelength tuning of a laser relies on a broad bandwidth of the gain medium. Rare earth doped fibers take great advantages of lasing covering a broad wavelength range. Erbium-doped fibers and ytterbium-doped fibers have an emission bandwidth of ~200 nm at 1.5 µm and ~230 nm at 1.0 µm, respectively [5, 6]. Thulium-doped fibers (TDFs) may extend even broader attainable emission range of ~600 nm from 1.6 µm to 2.2 µm [7]. A few works have been done on wavelength tunable TDFLs, both in continuous wave (CW) and Q-switched (QS) / mode-locked pulse regime. In CW regime, W. A. Clarkson demonstrated an 1860 to 2090 nm tunable TDFL using volume grating [8]. J. Li et. al extended the tunable range to the longer wavelength from 1925 nm to 2200 nm (275 nm) in a cladding-pumped TDFL [9]. The widest tunable range more than 330 nm from 1723 nm to 2061 nm has been achieved by combining core and cladding pumping [10]. Besides CW regime, nanosecond pulses ranging between 1892 nm to 2041 nm (149 nm) was demonstrated in an acousto-optic modulation based active Q-switched fiber laser [11]. The volume gratings or fiber Bragg gratings with narrow bandwidth (<1 nm), used in the reported CW/QS works, are no longer suitable, especially for the femtosecond pulse lasers. The ultrafast pulse generation requires a filter with larger bandwidth support. Furthermore, compared with CW and QS fiber lasers, mode-locked fiber lasers normally operate under a higher power pump, and the mode-locking lasers require critical dispersion and nonlinearity management. These facts cut down the range of the operating wavelength. As a result, the widest tunable range in mode-locked TDFLs were tens of nanometer, using tunable filter effect induced by nonlinear polarization evolution (NPE) [12, 13], nonlinear amplified loop mirror [14, 15], fiber taper [16], etalon [17] and multimode interference [18]. In 1995, Nelson et. al reported a sub-500 femtosecond fiber laser with a tunable range of 104 nm from 1798 nm to 1902 nm [19]. The laser consisted of bulk optics and wavelength tuning was enforced by an intra-cavity birefringent plate. In our previous work, we have demonstrated an all-fiber tunable range as wide as 136 nm from 1842 nm to 1978 nm, but the continuous tuning range is much smaller and the second resonant wavelength was not well suppressed at the boundary of the tuning range. More importantly, the laser operated at picosecond (ps) regime [20].

In this paper, a 143 nm wavelength tunable thulium-doped all-fiber laser is demonstrated. The compact NPE based mode-locked laser produces pulses with durations of between 329 fs and 392 fs over the whole tunable range from 1867 nm to 2010 nm. To our best knowledge, this is the widest tunable range obtained from a mode-locked TDFL. The broadly tunable range and ultrafast pulse generation attribute to the ultra-compact cavity design, which enables a large free spectrum range of the artificial filter and a low net dispersion.

2. Experimental setup and results

The all-fiber oscillator was mode-locked based on NPE mechanism [21]. A hybrid fiber based device with integrated 3-in-1 functions of wavelength division multiplexer (WDM), polarization sensitive isolator (PSISO) and output coupler (OC) was used in the cavity [22]. The designed configuration of the hybrid device (WDM-PSISO-OC) and the schematic setup of the experiment are illustrated as Fig. 1. Both of WDM and OC were made in the reflective scheme using a partial reflection plate and a dichroic plate. The pump source, made of a 1560 nm laser diode and an EDFA, was reflected by WDM port and launched into the cavity in the counter-clockwise direction. And the laser was tapped out with a 10% of the intra-cavity power by OC. The PSISO ensured the unidirectional operation (counter-clockwise) of the laser and applied NPE mechanism together with a squeezing polarization controller (PC). The whole cavity length was around 83 cm including 40 cm TDF, 40 cm pigtail fiber (SMF-28) and 3 cm hybrid device. The TDF has a core diameter of 4.2 µ m, a numerical aperture of 0.27 and Tm³⁺ doping concentration of 3.5 wt%. The net dispersion of the cavity is calculated to be −45440$\pm$5080 fs² where fiber dispersion is −760$\pm$85 fs²/cm of SMF-28 and −376$\pm$42 fs²/cm of TDF (‘ + ’ and ‘-’ are corresponding to shortest and longest wavelength of 1.9 µm and 2.0 µ m, respectively). The output laser was delivered through a section of ~40 cm SMF.

 

Fig. 1 The schematic setup of the compact mode-locked fiber laser.

Download Full Size | PDF

The output spectrum and pulse trains were measured with an optical spectrum analyzer (OSA, Yokogawa 6375) and a digital oscilloscope (Agilent, 2.5 GHz bandwidth) along with a photon detector (EOT, 12.5 GHz bandwidth). The oscillating frequency and stability were monitored by an RF spectrum analyzer (Rohde & Schwarz, FSU26). The pulse durations were characterized by FROG technique (Mesaphotonics) with a grid of $256\times 256$.

The stable mode-locking operation self-started when the pump power increased to 575 mW. Once mode-locking was triggered, the pump power can be lowered down to 380 mW without affecting the single-pulse mode-locking of the oscillator. When the pump power was less than 720 mW, no multi-pulse or harmonic mode-locking was observed. The typical mode-locked pulse profile and pulse train measured by the oscilloscope are shown in Fig. 2(a). The corresponding radio frequency (RF) spectra are depicted in Fig. 2(b). We measured the spectra at fundamental frequency of 247.984 MHz and the 10th order harmonic frequency of 2.47984 GHz (for jitter analysis) in a resolution bandwidth (RBW) of 10 Hz and 100 Hz, respectively. A 10 GHz span spectrum in 10 MHz RBW was also given. The signal to noise ratio (SNR) was as high as 80 dB at the fundamental frequency, which indicates the stable, single-pulse mode-locking operation.

 

Fig. 2 (a) Single pulse and pulse train of fundamental mode-locking; (b) RF spectrum, top left: fundamental frequency f₁ = 247.984 MHz (10 Hz RBW and 5 KHz span), top right: 10th order harmonic frequency f₁₀ = 2.47984 GHz (100 Hz RBW and 20 MHz span), bottom: 10 MHz RBW and 10 GHz span.

Download Full Size | PDF

The hybrid device not only enables dispersion-compensation-free feature for femtosecond pulses generation, but also broadens wavelength tunability. The wavelength tuning could be easily achieved by squeezing and rotating the PC in the cavity. The maximum tunable range of 143 nm from 1867 nm to 2010 nm of the central wavelengths was obtained. Continuous tuning over 100 nm (1890 nm to 1990 nm) was obtained just by rotating the PC without changing the squeezing strain. Figure 3(a) shows the output spectra with a spacing of ~20 nm between the neighboring frames. The spectral profile did not change during the central wavelength tuning. The spectral width slightly varied between 15.4 nm and 17.8 nm of full width at half maximum (FWHM). It supported transform limited pulse durations less than 275 fs under the assumption of sech² pulse profile. Figure 3(b) plots the zoomed-in spectrum of 1940 nm output as the center of the tunable range. All the spectra display strong Kelly sidebands, which indicates the oscillator worked in soliton regime. The significant sidebands were resulted from the phase match coupling between dispersive radiation and the soliton [23]. It well coincided with the all-anomalous cavity design. The irregular dips on the spectra (<1950 nm) are due to the absorption lines of the O-H bounds.

 

Fig. 3 (a) The series of optical spectra with a ~20 nm spacing; (b), The optical spectrum at the central wavelength of 1940 nm.

Download Full Size | PDF

The pulse width was able to directly measure by SHG-FROG without amplification due to the ultrashort pulses and high average output power (> 30 mW). The retrieved pulse widths over the whole tunable range with a sampling of ~10 nm were shown in Fig. 4(a). The measured and retrieved FROG traces at the operating wavelength of 1940 nm are imaged as Fig. 4(b). The statistic FROG error over the whole tunable range is 0.012$\pm$0.004. The statistic pulse duration is 364$\pm$17 fs (standard derivation, s.d.) and 3 dB spectral bandwidth is 16.4$\pm$0.7 nm. The shortest and longest pulses were measured as 329 fs at 1900 nm and 392 fs at 1980 nm, respectively. With pairing the measured spectral bandwidth and pulse width, the time-bandwidth product (TBP) is calculated as 0.477 ± 0.032, larger than the transform limited pulse’s TBP of 0.315. The 40 cm output delivery fiber with anomalous dispersion broadens the pulse duration away from transform limited duration.

 

Fig. 4 (a) Pulse width and spectral bandwidth at the different operating wavelength; (b) The measured and retrieved FROG traces at the central wavelength of 1940 nm.

Download Full Size | PDF

The laser typically yielded an average power of ~35 mW, the pulse energy is calculated as 141 pJ. The average power and pulse energy has a little variation at different wavelengths but within 10% difference. With subtracting the 49% contribution of Kelly sidebands by integration of spectrum, the intra-cavity soliton energy is calculated as 720 pJ (10% output ratio, 72 pJ). The theoretical soliton energy has a comparable value of 777 pJ for single mode fiber.

The oscillator would work in the mode-locking state with a long-term stability, which was verified by a stability test of 100-hour continuous operation under a temperature controlled environment (19 degrees, Class 1000 clean room). A standard deviation of 0.30 mW at the operation wavelength of 1940 nm was measured at 34.6 mW average power. For the pulse-to-pulse stability, the periodic jitter can be extracted from the measured RF spectrum at the 10th order of harmonic frequency of 2.47984 GHz (Fig. 2(b)). Following D. von de Linde’s model, the noise intensity is integrated over the frequency offset from ± 200 Hz to ± 10 MHz [24, 25]. The periodic jitter of the output pulses is calculated as 174 fs.

3. Discussion

It is not trivial to achieve a wavelength tunable range larger than 100 nm in a single TDF laser, especially for the mode-locked fiber lasers. Commercial tunable filters are rarely available at ~2 µm range, even without considering the bandwidth limitation for ultrafast pulse generation. Lyot filters, artificial filters based on birefringence, provide an attainable way for laser tuning [26]. Lyot filters normally consist of a sequence of birefringence plates/fibers and polarizers. Thus laser cavities based on NPE configuration are born with fiber-based Lyot filtering effect. Lyot filters have a periodical (sinusoidal) wavelength-dependent transmission. The free spectral range (FSR) is determined by accumulated birefringence and central wavelength ($\text{λ}$), that is expressed as Eq. (1). The transmission spectrum is formulated as Eq. (2) and band-pass bandwidth of FWHM is approximated as $\text{FSR}/2$.

In case that polarization maintaining (PM) fibers or single mode fibers with a long distance are used in the cavities, it would result in a small band-pass bandwidth and a small FSR, e.g. 8.6 nm in [27] and 10 nm in [20]. One direct consequence is losing the capacity for femtosecond pulse generation in a lack of sufficient spectral bandwidth. And this requirement gets more serious at 2000 nm compared with that at 1030 nm and 1550 nm. The other one is that multiple wavelengths may lase within the gain bandwidth, which prevents the continuous tuning over a larger wavelength range and single pulse operation.

In our experiment, the FSR of the Lyot filter, formed by the single mode fiber of 0.8 meters, was experimentally measured. When the laser operated below pump power of the mode-locking threshold, a dual-wavelength CW lasing at 1842 nm and 2006 nm indicates an FSR of 164 nm. The effective birefringence of the fiber is calculated as $2.87\times {10}^{-5}$, which approximates well with the reported experimental birefringence of $3.17\times {10}^{-5}$ and corresponding a FSR of 154 nm [14]. Figure 5(a) shows the measured spectrum (blue solid line) and the retrieved transmission spectrum of the Lyot filter (red dash line). The FSR of our fiber-based Lyot filter is more than 2 times as the earlier report using a birefringence plate [19], thus a larger continuous tunable range can be achieved. It should be noted that fused fiber devices (fused coupler/or fused WDM) are absent in our cavity. Fused fiber devices based on mode-coupling mechanism typically have a narrow operation bandwidth, which probably reduces the tunable range.

 

Fig. 5 (a) Experimental measurement of the FSR; (b) Transmission spectrum of the hybrid component and ASE of the TDF.

Download Full Size | PDF

The cutoff wavelengths of the tunable range are limited by two aspects. One is the lower transmission efficiency of the hybrid component at the short wavelength, and the other is the lower gain efficiency at the longer wavelength. Shown as blue curve in Fig. 5(b), the efficiency gradually decreases from 1860 nm and has a hard-cut at 1820 nm, but keeps a high value of >70% at the longer wavelength extending to 2100 nm. The amplified spontaneous emission (ASE) spectrum of the TDF was plotted as red dash line in Fig. 5(b). And the O-H bounds absorption lines were presented on the spectrum (1780 nm-1950 nm). The emission intensity is getting weaker above 2000 nm, which indicates a lower gain efficiency at the longer wavelength.

To further extend the tunable range, a hybrid component with a shorter cutoff wavelength (e.g. 1700 nm) intends to be fabricated, for fully utilizing the emission range of TDF. Another simpler and feasible approach is to replace the singly TDF by a thulium/holmium co-doped fiber. It can provide an efficient gain from 1850 nm to 2200 nm because of the ⁵I₈ –⁵I₇ transition of Ho³⁺ [28, 29].

4. Conclusion

We have demonstrated a tunable all-fiber mode-locked laser at 2 µm. The NPE effect is responsible for activating the mode-locking, and the fiber-based Lyot filter enables the wavelength tuning. The tunable range is as large as 143 nm (1867 nm to 2010 nm). To the best of our knowledge, it is the widest in femtosecond mode-locked TDFLs. Pulse duration is shorter than 400 fs over the whole tunable range. It is worth mentioning, if the TDF in the cavity is replaced by a THDF, the tunable range is expected to expand larger than 300 nm.