We report on the carrier-envelope phase (CEP) stabilization of a Yb-doped fiber amplifier system delivering 30 μJ pulses at 100 kHz repetition rate. A single-shot, every-shot measurement of the CEP stability based on a simple f-2f inter-ferometer is performed, yielding a CEP standard deviation of 320 mrad rms over 1 s. Long-term stability is also assessed, with 380 mrad measured over 1 h. This level of performance is allowed by a hybrid architecture, including a passively CEP-stabilized front-end based on difference frequency generation and an active CEP stabilization loop for the fiber amplifier system, acting on a telecom-grade integrated LiNbO3 phase modulator. Together with recent demonstrations of temporal compression down to the few-cycle regime, the presented results demonstrate the relevance of the Yb-doped high repetition rate laser for attoscience. Carrier-envelope phase (CEP) stabilized few-cycle amplified laser systems are one of the keys enabling attoscience [1,2]. Since the first demonstration of isolated attosecond pulse generation [3], applications requiring a precise control of the electric field have considerably expanded. To date, titanium-doped sapphire (Ti:Sa) amplifier systems have mostly provided the required short and energetic pulses at repetition rates ranging from 50 Hz to 10 kHz [4,5]. A large number of applications would benefit from further repetition rate scaling to ≥100 kHz, reducing acquisition times and improving signal-to-noise ratio. This is particularly true for low yield experiments involving coincidence detection [6]. At repetition rates above 10 kHz, high-power Ti:Sa amplifiers become complicated and costly, and do not provide the long-term stability and robustness required by today's science. A proposed and demonstrated alternative is using CEP-stable Ti:Sa to seed optical parametric chirped pulse amplifiers (OPCPAs) [7,8]. Of particular interest, OPCPAs can be configured to work at different wavelengths opening new opportunities. However, OPCPAs are limited by the low conversion efficiency from the pump to the useable beam (∼15%) before spatial and temporal distortions occur. Very powerful hence, costly, pump sources have to be employed to generate only a few tens of watts [9]. In the past years, laser sources based on Ytterbium gain material have shown stunning performances with >kilowatt average power and >millijoule energy per pulses [10,11]. Furthermore, efficient nonlinear compression of these laser sources down to the few-cycle regime has been demonstrated [12,13] and used to drive high-photon flux extreme ultraviolet (XUV) sources [14,15] through high-harmonic generation (HHG). However, robust CEP stabilization is still missing. This is due to a number of reasons such as noisy fiber oscillator characteristics [16], a large stretching and compression ratio, and enhanced intensity to CEP noise transfer in different nonlinear stages [17]. To date, CEP stabilization of Ytterbium-based amplified systems have been realized in a linear amplification regime at the microjoule level with a regenerative Yb:KGW system at 1 MHz seeded by a solid-state Yb:KGW oscillator [18] and with a fiber amplifier at 80 MHz seeded by a CEP-stable Ti:Sa oscil-lator [19]. Both systems revealed CEP fluctuations comparable to Ti:Sa and OPCPA sources, but are not directly scalable in average power for the former one [18] and in pulse energy for the latter one [19]. A CEP-stable Tm-doped linear fiber CPA architecture at 2 μm has also been demonstrated recently [20]. In this Letter, we report, to the best of our knowledge, on the first high-energy CEP-stabilized Yb-doped fiber chirped-pulse amplifier (FCPA) system, including a multi-pass cell (MPC) nonlinear compression stage [21]. It delivers 30 μJ 96 fs pulses at 100 kHz and relies on the following several key elements: • A passively CEP-stable front-end at a central wavelength of 1030 nm. • A FCPA system, including preamplifiers, a rod-type fiber power amplifier, and a large stretching/compression ratio. • An active CEP feedback loop, including an in-line in-focus f-to-2f interferometer and an integrated electro-optic phase modulator (PM) as an actuator. The CEP stability is characterized in detail both at the full 100 kHz bandwidth over 1 s and at 10 kHz over 1 h, revealing Letter Vol. 44, No. 16 /