Research

Phosphorus quantum dot saturable absorbers for mode-locked fiber lasers
 

    Ultrafast fiber laser sources have become an essential tool facilitating a wide range of scientific and industrial applications. In fiber lasers, ultrashort pulses can be generated by passive mode-locking. This requires the aid of a nonlinear component called saturable absorber (SA) which acts as a passive optical switch in a laser cavity. The current dominant saturable absorber is semiconducting saturable absorber mirror (SESAM), which has its own drawbacks, such as narrow operating bandwidth, complex fabrication and packaging issues. These limitations are driving research into the exploration of alternative materials for SA applications; of particular interests are two-dimensional (2D) layered nanomaterials with high optical nonlinear susceptibility, ultrafast carrier dynamics and broadband working wavelength. Among these 2D materials, black phosphorus (BP) has received particular interest with a direct bandgap varying from 0.3 eV in bulk to 2 eV in monolayer and thus offers potential in bridging the gap between zero-bandgap graphene and large-bandgap semiconducting transition metal dichalcogenides (s-TMDs).

In our experiment, the BPQDs are firstly prepared by ultrasound-assisted liquid phase exfoliation. The BPQD dispersion is then deposited on the microfiber by using optical deposition method. The BPQD-based SA is realized by the nonlinear interaction of processed nanomaterial with the evanescent field of light in a microfiber. The microscope images of the microfiber BPQD SA device are shown in Fig 1 (a). The upward image shows the microfiber coated with BPQDs at a magnification of 500 times; the downward image and the inset, at a magnification of 500 and 1000 fold respectively, present the evanescent field of light in the microfiber device after injecting a 650 nm He-Ne laser source. The nonlinear optical absorption of the integrated BPQD-SA is shown in Fig 1 (b). The saturable average power and normalized modulation depth of the device are 1.69 mW and 8.1 %. Thus, the BPQD-SA shows strong saturable absorption property illustrating potential to be used for short pulse generation.

Apart from the 2D layered structure, ultrasmall quantum dot (QD), another form of nanomaterials, exhibiting unique properties owing to the quantum confinement and edge effects, has been reported to possess prospective homogenous size and sizeable bandgap; and thus it offers new opportunities for photonic applications. Here, we fabricate the ultrasmall BP quantum dot (BPQD) based SA as an ultrafast mode-locker for short pulse generation. We demonstrate the self-starting mode-locked pulses generated from an erbium-doped (Er-doped) fiber laser to underscore its applicability as a broadband SA material.

 
 
 

Fig 1. (a)Photograph of the microfiber deposited with PQDs; (b)saturable absorption property of the PQD-SA device

 

We developed an erbium (Er)-doped fiber laser consisting of single-mode all-fiber integrated components for an alignment-free and compact system, as shown in Fig 2. The fiber amplifier consists of a length of 0.7 m single-mode Er-doped fiber (EDF) pumped by a 980 nm pump laser diode. In addition to the fiber amplifier, the cavity includes a polarization-independent optical isolator (PI-ISO) to ensure unidirectional propagation, 10% fiber output coupler to deliver the signal for both spectral and temporal diagnostics, and polarization controllers (PC) to enable a thorough and continuous adjustment of the net cavity birefringence. The cavity total length is ~37.8 m and the laser operated in the average-soliton regime.

 
 
 

Fig 2. The schematic of ultrafast Er-doped fiber laser

 
 
 

Fig 3. Mode-locking performance: (a)pulse train; (b)optical spectrum; (c)autocorrelation; (d)radio frequency spectra

 

Self-starting mode-locking is observed at the fundamental repetition frequency of the cavity of 5.47 MHz [Fig 3 (a)], with 24.7 pJ single pulse energy. The spectrum is centered at 1561.7 nm, with a full width at half maximum (FWHM) of 3 nm [Fig 3 (b)]. The corresponding pulse duration is 882 fs, shown in Fig 3 (c). The time-bandwidth product is calculated to be 0.325, close to the Fourier transform limit of a sech2 pulse. The fundamental frequency shows a high signal to background extinction ratio of ~67 dB, indicating low-amplitude fluctuations, and stable mode-locking operation performance. The inset of Fig 3 (d) shows high cavity harmonics, recorded on a span of 150 MHz, without any noticeable sign of Q-switching instabilities, implying good pulse-train stability. To further evaluate the operating stability of the microfiber-based BPQD SA device and mode-locking performance of the fiber laser, we recorded the optical spectra of the laser every 20 mins over 2 hours, presented in Fig 4 (a). No evident variation of both central wavelength and spectral bandwidth could be observed [Fig 4 (b)], suggesting the stability of mode-locking operation performance.

 
 
 

Fig 4. (a)Measured optical spectra at 20 mins interval; (b)the drift of the central wavelengths and the 3 dB spectral widths

 

We experimentally found that BPQD dispersion in NMP solvent possesses a reasonably good stability for half a year without degradation. The laser continuously emits high-quality pulses, indicating that this nanomaterial could be a promising SA candidate for ultrafast optics.

 
 

Meng Zhang, associate professor, school of electronics and information engineering, Beihang University, E-mail: mengzhang10@buaa.edu.cn

 
 

Reference

Du J, Zhang M, Guo Z, et al. Phosphorene quantum dot saturable absorbers for ultrafast fiber lasers.[J]. Scientific Reports, 2017, 7:42357.