All-fiber frequency comb at 2  µm providing 1.4-cycle pulses

Abstract

We report an all-polarization-maintaining fiber optic approach to generating sub-2 cycle pulses at 2 m and a corresponding octave-spanning optical frequency comb. Our configuration leverages mature Er:fiber laser technology at 1.5 m to provide a seed pulse for a thuliumdoped fiber amplifier that outputs 330 mW average power at 100 MHz repetition rate. Following amplification, nonlinear self-compression in fiber decreases the pulse duration to 9.5 fs, or 1.4 optical cycles. Approximately 32 % of the energy sits within the pulse peak, and the polarization extinction ratio is more than 15 dB. The spectrum of the ultrashort pulse spans from 1 m to beyond 2.4 m and enables direct measurement of the carrier-envelope offset frequency using only 12 mW, or ~3.5 % of the total power. Our approach employs only commercially-available fiber components, resulting in a turnkey amplifier design that is compact, and easy to reproduce in the larger community. Moreover, the overall design and self-compression mechanism are scalable in repetition rate and power. As such, this system should be useful as a robust frequency comb source in the nearinfrared or as a pump source to generate mid-infrared frequency combs.

Single-cycle all-fiber frequency comb

“…Compared with Yb 3+ and Er 3+ ions, Al-doped silica glass can host much higher concentration of Tm 3+ ions without clustering [32,33], leading to commercially available highly-doped TDFs. Thus, we employ highly-doped Tm:fiber in the present CPA, successfully reducing the gain length by 14 times compared to our previous design [26]. Short gain fiber length greatly suppresses HOD accumulation and outputs even higher average power, leading to better pulse quality, shorter pulse duration and higher peak power.…”

“…This wavelength resides at the edge of the MIR while affording good transparency with mature, reliable and commercially available silica fiber components [21][22][23][24], leading to unique advantages in terms of amplification, dispersion engineering, and access to still longer wavelengths. Over the past few years, 2 µm lasers reaching two optical cycles by self-compression were reported [25,26], with pulses as short as 9.4 fs pulses being produced from an all-fiber frequency comb at 100 MHz repetition rate [26]. Unfortunately, accumulated higher order dispersion (HOD, β 3 and higher orders) degrades the pulse temporal quality by inducing a pedestal and satellite pulses, making it difficult to reach single-cycle realm.…”

All-fiber ultrafast amplifier at 1.9 μm based on thulium-doped normal dispersion fiber and LMA fiber compressor

“…Recently, significant progress has occurred in the development of ultrafast fiber-laser systems at with pulse durations less than 150 fs and peak powers higher than 10 kW 4 , 8 – 11 and commonly used to generate broadband coherent supercontinua. In thulium-doped fiber-laser systems, various techniques and their combinations are used to achieve such pulse characteristics, including the use of large-mode-area (LMA) active fibers 8 , nonlinear pulse compression 9 – 11 , the chirped-pulse amplification technique 4 , 10 , 11 , etc. In some works, pulses with such parameters are achieved without using an amplifier 12 , 13 .…”

The Adoption of Chalcogenide Glass Fiber as Pulse Stretcher in an All-Fiber Structured 2 μm Chirped Pulse Amplification System

“…All-fiber 2 µm ultrashort pulsed laser sources with good beam quality and high stability have attracted much attentions owed to their potential applications in such fields as optical frequency comb [1,2], multi-modal imaging [3,4], cascaded Raman laser realization [5,6], and mid-infrared supercontinuum (Mid-IR SC) generation in soft-glass fibers [7,8]. Thus far, two approaches, direct mode-locking [9][10][11] and Raman soliton self-frequency shift (SSFS) [12][13][14][15][16][17][18], are usually used to realize all-fiber 2 µm femtosecond sources.…”