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Abstract
A passively mode-locked Yb fiber laser using PbSe colloidal quantum dots (CQDs) as saturable absorber (SA) is experimentally demonstrated. An all-fiber experimental scheme was designed to understand the SA property of PbSe CQDs. The non-saturable loss, modulation depth, and saturable intensity of SA measured were 23%, 7%, and 12 MW/cm2, respectively. The PbSe CQDs were sandwiched in a fiber connector, which was further inserted into the Yb fiber laser for mode-locking. As the pump power up to 110 mW, the self-starting mode-locking pulses were observed. Under the pump power of 285 mW, a maximum average laser power with fundamental mode-locking operation was obtained to be 21.3 mW. In this situation, the pulse full width at half maximum (FWHM), pulse repetition rate, and spectral FWHM were measured to be 70 ps, 8.3 MHz, and 4.5 nm, respectively.
© 2017 Optical Society of America
1. Introduction
Owing to their short pulse duration and high peak power, the ultrashort pulses have been applied in a variety of essential regions [1–5], such as environmental monitoring, material processing, optical nonlinear generation, and free-space optical communication. The ultrashort pulses are usually obtained using mode-locking technique. Due to the advantages of fiber lasers [6–8], including excellent beam quality, high optical-optical conversion efficiency, and convenient thermal management, the mode-locked fiber lasers have become the ideal source for generating ultrashort pulses. To date, the mode-locking techniques usually used in the fiber lasers, have covered nonlinear polarization rotation (NPR), nonlinear amplifying loop mirror (NALM), and saturable absorber (SA). Among them, SA-based technique, in which only a small SA material is inserted into a fiber laser, is known as the most promising approach to obtain mode-locked pulses [9,10].
Semiconductor saturable absorber mirror (SESAM) has been developed as a SA material in fiber lasers over the past decades [9]. Today, most commercial mode-locked fiber lasers employ SESAM as SA material to achieve picosecond or femtosecond pulses. However, the shortcomings regarding SESAM impede the further development of the commercial mode-locked fiber lasers. Firstly, SESAM is a bulk material, as a result, a special coupling technique has to be used to realize the optical couple between fiber and bulk solid, which leads to the extra loss and weak environment stability. Secondly, the cost is paid close attention in the development of commercial mode-locked fiber lasers. That SESAM is a high-cost SA material owing to its preparation method, which directly increases the cost of the current commercial mode-locked fiber lasers. Furthermore, the absorption-wavelength tunable range of SESAM is relatively narrow, which limits its application.
Colloidal Quantum dots (CQDs) have the much lower preparation cost and the much larger absorption-wavelength tunable range than that of SESAM [11,12], and is known as a novel promising SA material. The QDs are a quasi-zero-nanocrystalline material. Due to their small size, carriers are limited in nano-scale space, which makes QDs exhibit several particular optical properties [13–16]. For instance, QDs exhibit the much stronger three-order nonlinear effect than that of bulk material with same components, and the absorption-wavelengths are capable of being tuned by changing the size of QDs conveniently. QDs are always prepared through epitaxial or solution approach, and the obtained QDs are called as epitaxial QDs and CQDs respectively. Compared with epitaxial QDs, CQDs have the evident advantages. Firstly, CQDs obtained merely from a chemical solution, have the much lower process price. Otherwise, CQDs extracted from solution, could be doped into different matrix, including ultraviolet (UV) gel [17,18]. Consider these facts, CQDs were employed as SA in our experiment.
In this paper, the commercial lead selenide (PbSe) CQDs were employed as SA in an Yb-doped fiber laser to achieve passively mode-locking. The mode-locking operation self-started with a mode-locking threshold of 110 mW. The maximum average laser power with fundamental mode-locking operation was obtained to be 21.3 mW, corresponding to the peak power of 37 W at the pulse repetition rate of 8.3 MHz and the pulse FWHM of 70 ps.
2. Experimental setup
The commercial PbSe CQDs which were initially dissolved in toluene, were adopted as SA in our experiment. The average diameter and the size dispersion of PbSe CQDs was measured to be 3.4 nm and 8% respectively using transmission electron microscope (TEM), while the first excitonic peak measured using a spectrophotometer, was 1060 nm with an uncertainty of ± 5 nm.
To build an all-fiber laser cavity configuration, a fiber connector containing PbSe CQDs was prepared to form a sandwich structure. Firstly, the toluene was evaporated completely to achieve pure PbSe CQDs. Then, the PbSe CQDs and the UV gel were mixed together and the PbSe CQDs were dispersed uniformly using an ultrasonic oscillator. Finally, this mixture was transferred from the beaker to the fiber end-facet of the fiber connector by using a micropipettor, and was then cured by a UV lamp. The cured PbSe CQDs doped UV gel on the fiber end-facet had a doping concentration of 8 ± 0.2 mg/mL and an estimated thickness of 70 ± 5 µm, which was obtained in the light of micropipettor and the end-face area of the fiber patch cord.
In order to understand the SA property of the PbSe CQDs doped UV gel, a simple experiment was designed as illustrated in Fig. 1. A commercial 1064 nm peak-power-tunable pulsed fiber laser with a pulse FWHM of 500 ps was employed as the light source. The energy from this source was splitted via a 1 × 2 fiber coupler with a coupling ratio of 50/50. The output powers from Port1 and Port2 were measured by a power meter from Thorlabs.
The schematic diagram of the mode-locked fiber laser constructed in our experiment, is illustrated in Fig. 2. A ring resonator was adopted because of usually the higher power stability than that of the linear configuration. The pump energy from a continuous wave (CW) operated 974 nm laser diode (LD) with a single mode fiber (SMF) pigtail and a maximum power of 800 mW, was coupled into this ring resonator through a 980/1064 nm wavelength division multiplexer (WDM). The output fiber of WDM was spliced to a 1.5-m-long Yb-doped fiber with a core diameter of 6 µm, numerical aperture (NA) of 0.12, and peak core absorption of 300 dB/m at 976 nm. A polarization-insensitive (PI) fiber-coupled isolator was employed to ensure optical single direction transmission. The stable mode-locked laser was emitted from the output fiber of the 1 × 2 fiber coupler with a coupling ratio of 20/80 at the wavelength of 1064 nm, after the SA sandwiched fiber connector was spliced between WDM and coupler.
3. Results and discussion
The SA property of PbSe CQDs doped UV gel measured was shown in Fig. 3. According to the two-level SA model of T = exp[-ΔT/(1 + I/Isat)]-Ans (T is the power-dependent transmittance, ΔT is the modulation depth, I is the incident intensity, Isat is the saturation intensity, and Ans is the non-saturable loss) [19,20], the Ans, ΔT, and Isat were estimated to be ~23%, ~7%, and ~12 MW/cm2, respectively. The uncertainty for Isat was ± 3 MW/cm2. However, it is difficult to decide who is the main contributor for these SA parameters only from Fig. 3, because the PbSe CQDs UV gel was a mixture of PbSe CQDs and UV gel. To this end, we also characterized UV gel using the identical experimental scheme as illustrated in Fig. 1. Of course, here the pure UV gel was inserted as the alternative sandwiched material instead of PbSe CQDs doped UV gel. The experimental result measured in the intensity region from 0 MW/cm2 to 100 MW/cm2, was that the transmittance of UV gel was around 90% with a negligible variation of 0.2%, which made us believe that PbSe CQDs were the leading contributor to the data in Fig. 3.
To further investigate and develop the PbSe CQDs based mode-locked fiber laser, a PbSe CQDs doped UV gel sandwiched in a fiber connector was inserted into the Yb fiber laser. The average laser power, spectrum, and pulses were measured using a power meter (S302C from Thorlabs with power range from 100 µm to 2 W), an optical spectrum analyzer (OSA, AQ6317B from ANDO with wavelength revolution of 0.05 nm around 1060 nm), and a digital oscilloscope (DSA71254 from Tektronix with bandwidth of 12.5 GHz and sampling rate of 50 GS/s) together with an 20-GHz-bandwidth optical detector, respectively.
Under the pump power of approximately 110 mW, the self-starting mode-locking pulses were observed from the oscilloscope. As pump power increased slowly, the maximum average laser power with fundamental mode-locking operation were measured to be 21.3 mW at the pump power of 285 mW. The laser spectrum curve at the maximum laser power is illustrated in Fig. 4. A shape-irregular spectrum was observed with a peak wavelength of 1068 nm and a spectral FWHM of 4.5 nm.
Meanwhile, the mode-locked pulses were measured as shown in Fig. 5. The pulse train at the average laser power of 21.3 mW was illustrated in Fig. 5(b), from which the pulse repetition rate is observed to be approximately 8.3 MHz, corresponding to the total cavity length of 25 m. The single pulse is also depicted in Fig. 5(a). The FWHM of the single pulse was about 70 ps, which was much broader than that of an Er fiber laser as a result of all-normal dispersion operation. To obtain a higher power laser output, we had increased the pump power beyond 285 mW, however, the fundamental mode-locking operation could never be maintained in this situation, as illustrated in Fig. 5(c). It is difficult to find out the suitable repetition rate for accurately describing this pulse train in Fig. 5(c).
Fig. 5 (a) Single pulse and (b) pulse train measured at the pump power of 285 mW. (c) Pulse train obtained while the pump power increased beyond 285 mW.
4. Conclusion
In summary, the PbSe CQDs were developed as a new SA material. The SA parameters of PbSe CQDs including non-saturable loss, modulation depth, and saturable intensity were measured from an all-fiber experimental configuration. A stable fundamental mode-locking operation with a pulse FWHM of 70 ps and a maximum average laser power of 21.3 mW was obtained from the PbSe CQDs based Yb fiber laser. The mid-infrared fiber laser using PbSe CQDs as SA would be deeply investigated in our future work.
Funding
National Natural Science Foundation of China (NSFC) (61705056, 61405050, 81601530); Natural Science Foundation of Zhejiang Province (LQ16F050002); Scientific Research Fund of Zhejiang Provincial Education Department (Y201533689).
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