Objective A picosecond pulse at 780 nm has played an important role in practical applications and scientific researches. A Ti: sapphire laser is a famous light source at this wavelength range. However, due to its bulk design and complex architecture, the Ti: sapphire laser is sensitive to its environment. Compared with Ti: sapphire lasers, fiber lasers have the advantages of compact structure, high beam quality, and high stability. An alternative way to generate a picosecond pulse at 780 nm is to utilize frequency doubling of the pulse output from an Er-doped fiber laser. In this study, we experimentally demonstrated a frequency doubling efficiency as high as 55.7% based on an all-polarization-maintaining Er-doped fiber laser. Moreover, this scheme favors the advantages such as compact structure and good stability, and can be further applied in many fields such as ultrafast spectroscopy and semiconductor testing.
Methods In this study, a passively mode-locked fiber oscillator and a cascaded fiber amplifier, including a single mode fiber pre-amplifier and a double-cladding fiber amplifier, were used to produce a picosecond pulse at 1550 nm. The fiber oscillator was mode-locked by a semiconductor saturable absorber mirror (SESAM). A segment of Er-Yb co-doped double-cladding fiber with a 12 μm fiber core was used in the main amplifier to provide a laser gain and suppress nonlinear effects at a watt-level average power. The frequency conversion was achieved in a periodically poled lithium niobate (PPLN) crystal with 10 mm×3 mm×1 mm size and a 19.34 μm poling period. The fundamental-wave was focused on the PPLN crystal by a lens with a 19 mm focal length. A temperature-controlled oven was used to achieve the thermal phase matching condition for frequency doubling. A collimating lens and two dichroic mirrors were used for splitting the two beams at 775 nm and 1550 nm.
Results and Discussions The average power from the oscillator was 30 mW at a 100-MHz repetition rate. Subsequently, the pulsed laser was amplified to 1.3 W by the cascaded Er-doped fiber amplifiers. The amplified pulse had a spectral bandwidth of 1.01 nm centered at 1549.6 nm, and a pulse duration of 11.6 ps [Fig. 2(a) and Fig. 2(b)]. The polarization extinction ratio of the pulse was measured to be 13 dB. In order to avoid optical damage on the end facet of the crystal, the maximum incident power of the fundamental-wave was controlled below 1.1-W average power. The optimum temperature of this PPLN crystal was 34 ℃. In our experiment, the second harmonic average power increases monotonically with the fundamental-wave power. With an incident average power of 1.1 W, the second harmonic laser achieves a power as high as 613 mW, yielding a conversion efficiency of 55.7% [Fig. 3(a)]. The spectral bandwidth and pulse duration of the frequency doubled pulse are 0.68 nm and 11.4 ps, respectively [Fig. 2(c) and Fig. 2(d)]. Benefiting from the all-polarization-maintaining fiber architecture, the frequency doubled pulse exhibits an excellent long-term stability. As shown in Fig. 3(c), the average power instability is as low as 0.6% within 3 h.
Conclusions A picosecond laser architecture for 780-nm spectral range was demonstrated by using an all-polarization-maintaining Er-doped fiber laser and a PPLN frequency doubling crystal. As high as 613-mW average power and 55.7% conversion efficiency were achieved. The proposed laser scheme has the characteristics of compact structure and high stability, which is a good candidate to replace Ti:sapphire lasers in some circumstances.
