Frequency-doubling of a cladding-pumped Er³⁺/Yb³⁺ femtosecond fiber laser system using a periodically-poled LiNbO₃

Abstract

As real-world ultra-fast optical devices proliferate, there is a growing need for highly reliable and compact sources of femtosecond pulses [1]. Currently most of these applications require moderate power sources operating around 800 nm, which is ideally compatible with frequency-doubling of femtosecond Er³⁺-fiber lasers. Previously integrated high-power fiber laser systems were developed based on chirped-pulse amplification schemes relying on chirped fiber gratings for pulse stretching and compression to minimize the nonlinearities of femtosecond fiber amplifiers [2]. The component count of such systems can be considerably reduced and the optical efficiency increased by implementing aperiodically poled lithium niobate [3] (APPLN), as APPLN allows a unique integration of chirped pulse amplification with frequency-doubling.
Here we demonstrate the first system application of a APPLN frequency-doubler in conjunction with a high-power cladding-pumped Er³⁺/Yb³⁺ fiber laser.
The experimental set-up is shown in Fig. 1. The fiber seed system is based on an environmentally stable fiber soliton laser [1] and generates bandwidth-limited 250 fsec pulses with pulse energies of 300 pJ at a repetition rate of 40 MHz at a wavelength of 1.56µm. To operate the cladding pumped power amplifier in saturation the pre-amplifier is used, which boosts the average signal power to 35 mW. Prior to amplification in the cladding-pumped power amplifier the pulses are stretched to ~ 1.7 psec in a 2.9 m length of positive dispersion fiber.
Using a coupled pump power of ~6 W at 980 nm into the power amplifier, we obtain a signal power of 600 mW. After frequency-doubling in a length of 2 cm of APPLN an average power of 180 mW is obtained at 780 nm. The frequency-doubled pulse energy is 4.5 nJ. Note that the crystal was not AR-coated and the internal SH power was ~210 mW. The internal SH conversion efficiency was 40 %.
An autocorrelation trace and the corresponding pulse spectra at the frequency-doubled wavelength are shown in Fig, 2. The pulse width is 290 fsec and assuming a gaussian pulse shape the time bandwidth product is 0.51, i.e. the pulses were within 20% of the bandwidth limit.
Since currently APPLN allows the recompression of pulses up to 15 psec in width [3], we can expect that this technology may be upscaled to producing femtosecond pulses at Watt-level powers at 780 nm.
Fig. 1: High-power frequency-doubled Er/Yb fiber laser. Fig. 2: Autocorrelation and spectrum of the pulses generated at 780 nm. The pulse width is 290 fsec and the time-bandwidth product is 0.51 assuming a Gaussian shape.

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