David Fehrenbacher

All-passive phase locking of a compact Er:fiber laser system

A passively phase-locked laser source based on compact femtosecond Er:fiber technology is introduced. The carrier-envelope offset frequency is set to zero via difference frequency generation between a soliton at a wavelength of 2 μm and a dispersive wave at 860 nm generated in the same highly nonlinear fiber. This process results in a broadband output centered at 1.55 μm. Subsequently, the 40 MHz pulse train seeds a second Er:fiber amplifier, which boosts the pulse energy up to 8 nJ at a duration of 125 fs. Excellent phase stability is demonstrated via f-to-2f spectral interferometry.

Femtosecond coherent seeding of a broadband Tm:fiber amplifier by an Er:fiber system.

We generate broadband pulses covering the Yb: and Tm:silica amplification ranges with a passively phase-locked front end based on Er:fiber technology. Full spectral coherence of the octave-spanning output from highly nonlinear germanosilicate bulk fibers is demonstrated. Seeding of a high-power Tm:fiber generates pulses with a clean spectral shape and a bandwidth of 50 nm at a center wavelength of 1.95 μm, pulse energy of 250 nJ, and repetition rate of 10 MHz.

Analysis of the carrier-envelope phase noise of passively phase-locked Er:Fiber frequency combs up to the Nyquist frequency

We study the carrier-envelope phase noise of an Er:fiber frequency comb which is passively phase-locked at the full repetition rate of 100 MHz. A novel characterization method determines an out-of-loop phase jitter of only 250 mrad when integrated over 12 orders of magnitude: from 50 μHz up to the Nyquist frequency.

Characterization of carrier-envelope phase noise of passively phase-locked fiber-based frequency combs up to the nyquist frequency

Applications of optical frequency combs in high precision metrology [1] require low-noise stabilization of its carrier-envelope offset (CEO) frequency. This task is commonly achieved via active feedback. Fully passive elimination of the CEO frequency based on difference frequency generation (DFG) between two octave-separated comb sections followed by amplification of the DFG signal in the EDFA has been demonstrated recently in an all-fiber design [2]. In this work, we develop a novel broadband characterization method of carrier-envelope phase (CEP) noise and apply it to study the passively phase-locked 100 MHz Er:fiber comb [3].

Passively phase-locked Er:Fiber frequency comb

Modern precision metrology ever more strongly relies on the availability of optical frequency combs. We review our approach to passive stabilization of the carrier envelope phase (CEP) of a pulse train at full repetition rate via difference frequency generation. Using this approach, we demonstrate a inherently offset-free Er:fiber comb directly locked onto an optical reference (85Rb) together with a possibility of an actively narrowed sub-500 mHz linewidth. This performance highlights an attractive potential of robust all-fiber comb systems toward applications in ultrahigh-precision metrology.

High-power Yb- and Tm-doped fiber amplifiers seeded by a femtosecond Er:Fiber system

Summary form only given. The generation of ultrafast optical pulses that are used for many applications is increasingly based on Er:fiber lasers. The inherent advantages of this technology are compactness, stability, and turn-key operation. Tm- and Yb-doped fiber systems are promising candidates for reaching microjoule pulse energies [1,2,3].We present a setup exploiting the well-established Er:fiber technology that provides a femtosecond pulse train suitable for coherent seeding of both Yb: and Tm:amplifiers. A seed source generates pulses with a wavelength centered at 1550 nm. Four parallel amplification branches each provide 8 nJ pulse energy at a repetition rate of 40 MHz. This source implements also a passive phase-locking scheme [4]. The pulses are compressed in a silicon prism pair and then coupled into a highly nonlinear fiber (HNF). This scheme allows us to generate tailor-cut spectra with components spanning from 800 to 2300 nm that can be finely tuned by material insertion in the prism sequence [5]. The solitonic part of the spectrum is centered at 1970 nm and optimized to cover the entire gain bandwidth of Tm:silica. The dispersive part of a second HNF is designed to fit the gain maximum of Yb-doped fibers at a wavelength of 1030 nm. The benefit of Er:fiber seeding has been proven experimentally by confirming the full coherence of the spectral components generated in the HNFs [6].MHz and the transform limit for the pulse duration is 110 fs. The Er:fiber system acts as seed source for the parallel Yb: and Tm:fiber amplifiers. We reduce the repetition rate to 10 MHz via electro-optic modulators. To amplify the spectrum delivered by the HNF at a center wavelength of 1030 nm, we stretch the pulses with a grating pair to a temporal duration of 340 ps. The amplification occurs in a first Yb:fiber preamplifier pumped at a wavelength of 976 nm. The energy is then boosted in a 1.5 m long Yb-doped photonic crystal fiber (PCF) amplifier stage. The PCF double cladding structure enables pumping with high-power multimode laser diodes. At a pump power of 40 W, we measured pulse energies of 2.2 μJ. The spectrum after amplification has a bandwidth of 12 nm (FWHM) centered at 1032 nm (see Fig. 1.a). Recompression of these pulses leads to a pulse duration of 185 fs (see SHG FROG characterization in Fig. 1.b). The seed for the Tm:amplifier is stretched in 30 m of single-mode fiber directly spliced to the HNF. In a monolithic and truly single-mode Tm:fiber amplifier pumped at 796 nm, we demonstrate pulse energies of 250 nJ at a repetition rate of 10 MHz [6]. Fig. 1.c shows the pulse spectrum after amplification. It is centered at a wavelength of 1950 nm with a bandwidth of 50 nm. Both amplifiers are operating in a linear regime and there are no signs of a significant influence of nonlinear effects. The combination of high-energy, sub 200-fs pump pulses together with passive carrier-envelope phase stability of ultrabroadband seed pulses for parametric amplification paves the way towards extremely nonlinear optics and attosecond technology at unprecedented repetition rates and stability.

High-power Yb: And Tm:fiber amplifiers seeded by a femtosecond passively phase-stable Er:System

Synchronous high-power Yb: and Tm:amplifiers both coherently seeded by the same broadband passively phase stable Er:fiber system are demonstrated. Microjoule-level pulse energy and sub-200-fs operation at a repetition rate of 10 MHz are obtained.

Tm:fiber amplifier coherently seeded by femtosecond Er:Fiber technology

Broadband seeding of a femtosecond Tm:fiber amplifier based on passively phase-locked Er:fiber technology is demonstrated. Excellent coherence properties of the seed are observed experimentally and analyzed theoretically.

All passively phase-locked Er:fiber laser system

The generation of phase-locked pulses with a well-defined electric field is essential for applications like high harmonic pulse generation [1] or precision metrology [2]. Active and passive locking schemes have been widely demonstrated utilizing Ti:sapphire technology. However, active stabilization requires cumbersome feedback loops and a locking of fCEO = 0 is very challenging at the full repetition rate of the laser.