We report on the demonstration of a passively Q-switched 1.55 µm fiber laser utilizing a colloidal PbS quantum dot (QD) thin film as a saturable absorber. Colloidal PbS QD films have several features that are advantageous in passively Q-switched fiber laser operation, including a large operation wavelength range, cost-effectiveness, and a low saturable absorption intensity. We conducted thorough material and optical studies to verify the advantages of PbS QDs in Q-switched laser operation and successfully generated 801 nJ pulses with a 24.2 kHz repetition rate. To the best of our knowledge, the developed Q-switched fiber laser is the first based on colloidal PbS QDs.
© 2016 Optical Society of America
Q-switched fiber laser sources in the 1.55 μm eye-safe region are of interest for many applications, such as environmental sensing, eye-safe LIDAR, and medical surgery. In recent years, the output powers of continuous-wave (cw) and pulsed Er3+-doped fiber (EDF) lasers and amplifiers have increased to the hundred-watt and kilowatt levels, respectively [1,2]. There are also increasing demands for compact Q-switched 1.55 μm fiber laser sources that are driven by applications in fields such as material processing, sensing, medical treatment, and so on [3,4]. Several types of actively and passively Q-switched fiber lasers have been reported. In all-fiber actively Q-switched fiber laser systems, extensive and complex fiber pigtailed acousto-optic or electric optic devices are generally required. Although stable pulses with high pulse energies can be generated in such fiber laser systems, the use of high voltages and radio-frequency trigger sources makes these systems expensive and complex . Unlike actively Q-switched laser systems, passively Q-switched fiber laser systems could be made much simpler and more cost-effective by adopting saturable absorbers (SAs). Several types of materials have been fabricated for use as SAs thus far, such as semiconductor SA mirrors (SESAMs) , carbon nanotubes (CNTs) , graphene [8,9] and transition metal-doped crystals . Although SESAM-based SAs have been widely used and commercialized, they are expensive and their operation wavelength ranges are relatively small. On the other hand, most CNT- and graphene-based thin-film SAs are cost-effective and easy to assemble. However, the absorption bands in CNT devices are mainly controlled by the CNT diameters as well as chirality , and the bandwidths are relatively narrow compared with those of graphene-based SAs. Although the absorption of graphene is wavelength-independent, the absorption at 1550 nm is insufficient to provide a reasonable modulation depth. In addition, passively Q-switched lasers using transition metal-doped crystals require free-space optical components, making the systems more complex. Therefore, a novel SA material that has a large wavelength operation range, is cost-effective, and has a low saturable absorption intensity would be beneficial in this field.
PbS quantum dots (QDs) are nanocrystals made of a semiconductor with an 18 nm exciton Bohr radius and a bulk bandgap of about 410 meV. Because of the relatively large Bohr radius and small energy bandgap, the absorption in the 1–2.5 μm wavelength range can be easily achieved and adjusted by varying the diameters of the QDs between 2 nm and 7 nm . Recently, PbS and PbSe QD-doped glasses have been used as SAs in Q-switched fiber lasers . Compared with PbS QD-doped glass, a colloidal PbS QD device would be simpler to fabricate and could be much more cost-effective. Therefore, we experimentally investigated the use of a cost-effective colloidal QD device as an SA in the near-infrared wavelength range for a compact, all-fiber, Q-switched EDF laser. In general, methods of producing colloidal PbS QDs can be divided into two categories, wet-chemical and organometallic synthesis methods, depending on the nature of the solvent. The wet-chemical process uses low-temperature polar solvents such as water or methanol, while the organometallic synthesis method uses high-temperature, non-polar solvents such as trioctylphosphine oxide. Within each category, there are numerous variations in the exact precursors, solvents, and special additives, as well as in the required temperatures and pressures . Compared with the organometallic synthesis process, the wet-chemical process does not require high temperature and pressure. Moreover, this method is relatively low-cost and less toxic. In this study, colloidal semiconductor nanocrystals were synthesized from precursor compounds dissolved in solutions, much like in traditional wet-chemical processes. The absorption wavelength could be adjusted to match the Er3+ radiation wavelength, making the material suitable for use as an SA in EDF lasers.
2. Colloidal PbS thin film fabrication
The PbS core-type QDs employed in this research were purchased from Sigma-Aldrich (St. Louis, MO, USA). They were oleic-acid-coated with the first excitonic peak at 1524 nm and were dissolved in toluene to a concentration of 10 mg/ml. Referring to Moreels et al. , who studied the relationship between the energy levels and diameters of QDs, the average diameter of the PbS QDs was estimated to be 6.8 nm. To prepare the QD–polymer composite film, a QD-polymer mixture was obtained as follows. First, 1 mL of a colloidal QD solution was completely dissolved in a solution of the polymer xylene at 4°C. A One-Touch vortexer mixer was used to distribute the QDs uniformly throughout the polymer. The QD–polymer layer was dip-coated onto a slide, which was then spun at 3200 rpm for 1 min. The plated film was stored at room temperature until it had completely dried and was then put into an oven and annealed at 35°C for 1 h. After the film was cooled back to room temperature, it could easily be removed from the slide. Such QD-based films could tolerate high temperatures for hours. The completed film had a QD concentration of 2.1 × 10−2 mol/L and a thickness of 30 μm. The absorbance spectrum of the film was measured by a UV-visible spectrometer and is shown in Fig. 1. The absorption covers the 1400-1600 nm wavelength range, with a peak at 1524 nm, making such films usable in Er3+-doped fiber lasers.
To characterize the saturable absorption of the aforementioned PbS QD thin film, we measured its absorption as a function of the incident peak power. For this measurement, we used a single-mode mode-locked 1565 nm all-fiber EDF laser as the signal source. The home-made soliton mode-locked laser had the maximum pulse energy of 3.2 nJ with the pulse width of 644 fs. The laser intensity was attenuated through fiber-based directional couplers with different transmission coefficients. The intensity was first coarsely adjusted by switching the directional couplers; then, it was fine-tuned by changing the launched pump power. The laser pulse intensity was carefully calibrated by measuring the pulse energy and pulse width through the reference arm of the directional coupler. The PbS QD thin film samples were directly sandwiched between two SMF28 fiber ferrule connectors, and the transmittance was determined by measuring the ratio of the transmitted laser energies with and without the QD thin film. The mode-locked laser pulses were of sub-picosecond duration, which was less than the picosecond-level excited-state lifetime of the QDs in the thin film, thereby avoiding the reabsorption effect. As indicated by the relationship between the measured transmittance and the incident intensity that is shown in Fig. 2, the modulation depth of the QD SA is about 6%.Eq. (2), Isat is a function of the ground-state absorption cross-section σgs and the radiative lifetime t, and σgs can be determined based on the small measured signal absorption at 1565 nm:Eq. (2), σgs for the PbS QD thin film at 1565 nm was determined to be 1.21 × 10−18 m2, and the upper-state lifetime of the PbS QD thin film was found to be 2.34 ps.
3. Experimental setup
The experimental setup of the 1.55 µm Q-switched EDF laser based on the PbS QD thin film is shown in Fig. 3. The EDF was a 10-μm-diameter single-mode fiber with a fundamental fiber mode (LP01) output beam. The total cavity length was about 10 m. The beam from a 140 mW, 976 nm pump laser was passed through a wavelength-division multiplexing (WDM) system and was then launched into the EDF to provide the laser gain. A ring cavity design was adopted, including a polarization-independent isolator (ISO), an output coupler, and a polarization controller (PC). A 50% directional coupler (DC) served as the output coupler. The 30 μm PbS QD thin film was sandwiched between two fiber ferrules and was installed in the laser ring cavity. Then, 10% of the output light was sent to the oscilloscope, while the other 90% of the light was measured by the power meter.
4. Results and discussion
In the experiment, the EDF laser began CW lasing at a pump power of 30.8 mW and then transitioned to Q-switching operation at 36.4 mW. The Q-switching operation was optimized by fine-tuning the PC. Once the Q-switching pulses were optimized, no further tuning was required during the power scaling. Figure 4 depicts the oscilloscope trace of the Q-switched laser pulse trains under different pump powers. As shown, as the pump power increases from 60.8 mW to 115.4 mW, the repetition rate of the stable pulse train increases, verifying that passive Q-switching operation was successfully achieved. Once the pump power exceeded 128.7 mW, the Q-switched pulses became unstable. To assess whether this instability was caused by thermal damage to the QD-based SA, we gradually decreased the pump power back to 0 mW, then increased the power to 36.4 mW, and again observed stable Q-switching operation, indicating that the QD-based SA had not been damaged. Therefore, the instability could only have originated from the relatively large thermal loading, which was caused by the high circulator intensity.
As a general rule, the repetition rate of a passively Q-switched laser is proportional to the pumping ratio, (Ppump/Pth)-1, where Ppump is the pump power and Pth is the Q-switching threshold pump power, while the pulse duration is inversely proportional to this pumping ratio . As shown in Fig. 5(a), we recorded the pulse repetition rate and duration as functions of the pump power. As the pump power increases from 36.4 mW to 128.7 mW, the pulse repetition rate increases from 4.4 kHz to 24.2 kHz. As expected, the pulse duration responds inversely to the pumping rate, decreasing from 18.3 μs to 3.3 μs. The pulse duration could be further reduced by shortening the laser cavity lifetime or improving the modulation depth of the QD thin film. Figure 5(b) shows a single pulse profile, which has a full-width at half maximum of 3.3 μs under a maximum pump power of 128.7 mW. The generated Q-switching pulses are stable and uniform during the several-hour operation, which confirms that QD thin films have good mechanical properties for multiple practical applications.
The measured average output power and the calculated corresponding single-pulse energy are shown in Fig. 6. The average output power almost linearly increases with the input pump power up to a maximum of 19.4 mW at a pump power of 128.7 mW. The corresponding efficiency, as given by the slope, is 20.2%. Such high efficiency combined with the long-term stability verifies the advantageous optical and mechanical properties of the PbS QD thin film. Figure 6 also shows that the measured pulse energy, which initially increases linearly, tends to become saturated after a pump power of 104.3 mW. The maximum pulse energy of 801 nJ is much higher than those of Q-switched fiber lasers using CNTs  and graphene [9,16–18] as passive Q-switching devices. Further improvement could be achieved by optimizing the fabrication parameters of the PbS QD thin film-based SA.
The output optical spectrum is shown in Fig. 7. It has a center wavelength at 1562.7 nm and a 3 dB bandwidth of 0.63 nm. The laser output wavelength was selected through the gain competition, without requiring any special wavelength-dependent device.
We experimentally demonstrated a passively Q-switched all-fiber laser at 1562.5 nm by using a PbS QD thin film SA. To the best of our knowledge, this is the first study in which a colloidal PbS QD thin film has been used as the SA in a Q-switched fiber laser. The PbS QD thin film was fabricated by employing a wet-chemical process. Important material characteristics of the film were reported, such as its small signal absorption and its saturable absorption properties. This PbS QD-based SA exhibited a modulation depth of 6.4% and a maximum transmission of 82.1%. The saturable Q-switching operation was successfully achieved with an efficiency as high as 20.2% and a maximum average power of 19.4 mW. Furthermore, 3.3 μs pulses with a repetition rate of 24.2 kHz and a pulse energy of 801 nJ were obtained under a pump power of 128.7 mW. All of the results verify that the presented colloidal PbS QD thin film has optical and mechanical properties that make it advantageous for SA applications in cost-effective Q-switched EDF lasers.
This research is based upon work supported by the Ministry of Science and Technology (MOST) of Taiwan under the award number NSC-101-2218-E-027-003-MY3, and the Ministry of Economic Affairs (MOEA) of Taiwan under the award number 104-EC-17-A-07-S3-011.
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