Full stabilization and characterization of an optical frequency comb from a diode-pumped solid-state laser with GHz repetition rate

206 MHz fully stabilized all-PM dispersion-managed figure-9 fiber laser comb

High-repetition-rate optical frequency combs are useful for precision spectroscopy because of their high power per comb mode, but conventional high-repetition-rate lasers do not have a broad enough spectrum. In this study, a fully stabilized polarization-maintaining figure-9 mode-locked fiber laser with a high repetition rate of 206 MHz and a broad spectrum was demonstrated by employing simultaneous control of cavity dispersion and length. The laser exhibited a 3 dB spectral bandwidth of 88 nm and a compressed pulse width of 66 fs. Additionally, fCEO and frep phase locking were implemented, resulting in low (0.21 rad) in-loop carrier-envelope-offset frequency phase noise. To the best of our knowledge, this is the widest spectrum bandwidth and shortest pulse duration directly obtained from an all-PM figure-9 fiber laser oscillator to date. The combination of high repetition rate and broad spectral range makes this system very useful for a wide range of applications, especially in the field of precision spectroscopy.

Powerful 1-µm 1-GHz optical frequency comb

A self-referenced optical frequency comb is presented based on Kerr-lens mode-locking of ytterbium-doped CALGO. The robust source delivers 3.5 W average power in 44 fs-long pulses at 1 GHz repetition rate. The residual root-mean-square timing jitter of the emitted pulse-train is 146 fs and the residual integrated phase noise of the carrier-envelope offset frequency is 107 mrad, both in a span from 1 Hz to 10 MHz. After stabilization, 2.7 W average power remains for direct application. This work represents the first multi-mode pumped Kerr-lens mode-locked optical frequency comb at gigahertz-level repetition rate.

Low-noise Yb:CALGO optical frequency comb

We report on a compact optical frequency comb, operating in the wavelength range from 670 to 1500 nm, based on diode-pumped low-noise femtosecond Yb:CALGO amplified laser system. Both the carrier-envelope offset and repetition rate are phase-locked to reference synthesizers. A full characterization of the frequency comb, in terms of frequency stability, phase noise analysis, and optical beating against a single-frequency non-planar ring oscillator Nd:YAG laser, is presented, showing the excellent properties of the Yb:CALGO comb.

Ultra-low-noise carrier-envelope phase stabilization of a Kerr-lens mode-locked Yb:CYA laser frequency comb with a feed-forward method

We demonstrate ultra-low-noise carrier-envelope phase stabilization of a Kerr-lens mode-locked Yb:CYA laser frequency comb, which is the first time, to the best of our knowledge, that the feed-forward method has been applied to 1 μm all-solid-state lasers. We obtain a signal-to-noise ratio of more than 38 dB at a 100 kHz resolution bandwidth for carrier-envelope phase offset beat signals in two standard in-loop and out-of-loop f-2f interferometers. The residual integrated phase noise amounts to 79.3 mrad from 1 Hz to 1 MHz, corresponding to a timing jitter of 44 as. We also investigate long-term performances of the CEP stabilization in the feed-forward scheme, phase-locked feedback systems and the combining locking techniques in terms of sub-hertz frequency-resolved phase noise and Allan deviation in 1000 s. The results indicate that, although the feed-forward CEP stabilization method dominates in the range of 1 Hz to 1 MHz Fourier frequency, feedback methods with a time integration effect are superior in sub-hertz Fourier frequency phase noise suppression.

Characterization of a carrier-envelope-offset-stabilized blue- and green-diode-pumped Ti:sapphire frequency comb

Diode-pumping of Ti:sapphire provides a low-cost route to high-quality frequency-comb sources, exploiting the potential of direct diode modulation for wideband control of the carrier-envelope-offset frequency. We present here an f_REP- and f_CEO-locked, directly diode-pumped Ti:sapphire frequency comb, producing 66-fs pulses at 800 nm and employing f-to-2f interferometry and current modulation of a 462-nm blue laser diode to achieve a stabilization bandwidth extending to ∼70  kHz. Characterizations of the f_REP and f_CEO phase noise are compared to relative intensity noise spectra of the pump diodes to provide insights into how the diode design and performance transfer into the comb stability, suggesting a lower contribution to f_REP and f_CEO noise from the blue laser diode than from the green diode.

Carrier-envelope offset frequency stabilization of a thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime

We demonstrate the carrier-envelope offset (CEO) frequency stabilization of a Kerr lens mode-locked Yb:Lu₂O₃ thin-disk laser oscillator operating in the strongly self-phase modulation (SPM) broadened regime. This novel approach allows overcoming the intrinsic gain bandwidth limit and is suited to support frequency combs from sub-100-fs pulse trains with very high output power. In this work, strong intra-oscillator SPM in the Kerr medium enables the optical spectrum of the oscillating pulse to exceed the bandwidth of the gain material Yb:Lu₂O₃ by a factor of two. This results in the direct generation of 50-fs pulses without the need for external pulse compression. The oscillator delivers an average power of 4.4 W at a repetition rate of 61 MHz. We investigated the cavity dynamics in this regime by characterizing the transfer function of the laser output power for pump power modulation, both in continuous-wave and mode-locked operations. The cavity dynamics in mode-locked operation limit the CEO modulation bandwidth to ~10 kHz. This value is sufficient to achieve a tight phase-lock of the CEO beat via active feedback to the pump current and yields a residual in-loop integrated CEO phase noise of 197 mrad integrated from 1 Hz to 1 MHz.

Ultra-low noise microwave generation with a free-running optical frequency comb transfer oscillator

We present ultra-low noise microwave synthesis by optical to radio-frequency (RF) division realized with a free-running or RF-locked optical frequency comb (OFC) acting as a transfer oscillator. The method does not require any optical lock of the OFC and circumvents the need for a high-bandwidth actuator. Instead, the OFC phase noise is electrically removed from a beat-note signal with an optical reference, leading to a broadband noise division. The phase noise of the ∼15  GHz RF signal generated in this proof-of-principle demonstration is limited by a shot-noise level below -150  dBc/Hz at high Fourier frequencies and by a measurement noise floor of -60  dBc/Hz at 1 Hz offset frequency when performing 1,100 cross-correlations. The method is attractive for high-repetition-rate OFCs that lead to a lower shot-noise, but are generally more difficult to tightly lock. It may also simplify the noise evaluation by enabling the generation of two or more distinct ultra-low noise RF signals from different optical references using a single OFC and their direct comparison to assess their individual noise.

Low-noise 750 MHz spaced ytterbium fiber frequency combs

We demonstrate two low-noise 750 MHz ytterbium fiber frequency combs that are independently stabilized to a continuous-wave laser. A bulk electro-optic modulator and a single-stack piezo-electric transducer are employed as fast actuators for stabilizing the respective cavity length to heterodyne beat notes. Both combs exhibit in-loop fractional frequency instabilities of ∼10^-18 at 1 s. To the best of our knowledge, this is the first demonstration of tightly phase-locked (<1  rad root mean square phase noise integrated from 0.1 Hz to 10 MHz) fiber frequency combs with 750 MHz fundamental repetition rate.

High-repetition-rate ultrafast fiber lasers

Multi-gigahertz fundamental repetition rate, tunable repetition rate and wavelength, ultrafast fiber lasers at wavelengths of 1.0, 1.5, and 2.0 µm are experimentally demonstrated and summarized. At the wavelength of 1.0 µm, the laser wavelength is tuned in the range of 1040.1-1042.9 nm and the repetition rate is shifted by 226 kHz in a 3-cm-long all-fiber laser by controlling the temperature of the resonator. Compared with a previous work where the maximum average power was 0.8 mW, the power in this study is significantly improved to 57 mW under a launched pump power of 213 mW, thus achieving an optical-to-optical efficiency of 27%. For comparison, a similar temperature-tuning technique is implemented in a Tm³⁺-doped ultrafast oscillator but, as expected, it results in a broader tunable range of 14.1 nm (1974.1-1988.2 nm) in wavelength as compared with the value of 1.8 nm for the wavelength of 1.0 µm. The repetition rate in the process is shifted by 294 kHz. For the high-frequency range from 100 kHz to 10 MHz, the value of integrated timing jitter gradually increases with an increase in temperature. Finally, to the best of our knowledge, for the first time, a new method for tuning wavelength and repetition rate is proposed and demonstrated for a femtosecond fiber laser at the wavelength of 1.5 µm. Through fine rotation of the alignment angle between the Er/Yb:glass fiber and a semiconductor saturable absorption mirror, the peak wavelength can be tuned in the range of 1591.4-1586.1 nm and the repetition rate is shifted by 60 kHz.

Octave-spanning optical frequency comb based on a laser-diode pumped Kerr-lens mode-locked Yb:KYW laser for optical frequency measurement

We developed an optical frequency comb based on a Yb:KYW laser. Soft-aperture Kerr-lens mode-locking at the cavity transverse-mode degeneration enabled us to generate 360 mW from a 750 mW pump laser diode. This resulted in spectral broadening over one octave using just a photonic crystal fiber. We achieved a free-running linewidth of 15 kHz in the carrier-envelope offset frequency by optimizing the cavity group delay dispersion, crystal position, and pump laser power, which led to a residual phase noise of 0.51 rad during phase-locking. We measured the frequency drift of a cavity-stabilized laser for a clock transition in Yb171⁺.

6.5 GHz Q-switched mode-locked waveguide lasers based on two-dimensional materials as saturable absorbers

Two-dimensional (2D) materials have generated great interest in the past few years opening up a new dimension in the development of optoelectronics and photonics. In this paper, we demonstrate 6.5 GHz fundamentally Q-switched mode-locked lasers with high performances in the femtosecond laser-written waveguide platform by applying graphene, MoS₂ and Bi₂Se₃ as saturable absorbers (SAs). The minimum mode-locked pulse duration was measured to be as short as 26 ps in the case of Bi₂Se₃ SA. The maximum slope efficiency reached 53% in the case of MoS₂ SA. This is the first demonstration of Q-switched mode-locked waveguide lasers based on MoS₂ and Bi₂Se₃ in the waveguide platform. These high-performance Q-switched mode-locked waveguide lasers based on 2D materials pave the way for practical applications of compact ultrafast photonics.

Composite filtering effect in a SESAM mode-locked fiber laser with a 3.2-GHz fundamental repetition rate: switchable states from single soliton to pulse bunch

States that are switchable from single soliton to pulse bunch in a compact semiconductor saturable absorber mirror (SESAM) mode-locked fiber laser with a fundamental repetition rate of 3.2 GHz are experimentally investigated and further studied via simulations. A composite filtering effect comprising an intracavity low-finesse Fabry-Perot (FP) filter, an artificial optical low-pass filter, and a gain filter implements the state switching to pulse bunch. A numerical model is proposed to clarify the mechanism underlying the switching. It reveals that, for pulse interval ∆T > τA (relaxation time of the SESAM) in a pulse bunch, the laser operates in pulse-bound build up. In an inverse mechanism the state returns to single soliton, in which the ∆T is obtained from the free spectral range Ωc of the intracavity FP filter by mechanically controlling the distance between the SESAM and gain fiber. This pulse bunch regime of operation ought to be amenable to a quasi-steady-state treatment. It represents an alternative emergence trait in the temporal domain between a main soliton with strong sidelobes in both sides and a bound soliton pair with weak sub-sidelobes. Another profile of the pulse bunch state is that the side peak amplitude in the autocorrelation trace is more than 50%, which is distinct and larger than that in the conventional bound state regime in fiber lasers. The optical spectra, radio frequency spectra, and frequency chirp are further analyzed. These numerical results agree well with the experimental ones within the variation range of the crucial values of Ωc and enable the explicit understanding of such behavior in SESAM mode-locked high-repetition-rate fiber lasers.

Carrier-envelope offset stabilization of a GHz repetition rate femtosecond laser using opto-optical modulation of a SESAM

We demonstrate, to the best of our knowledge, the first carrier-envelope offset (CEO) frequency stabilization of a GHz femtosecond laser based on opto-optical modulation (OOM) of a semiconductor saturable absorber mirror (SESAM). The 1.05-GHz laser is based on a Yb:CALGO gain crystal and emits sub-100-fs pulses with 2.1-W average power at a center wavelength of 1055 nm. The SESAM plays two key roles: it starts and stabilizes the mode-locking operation and is simultaneously used as an actuator to control the CEO frequency. This second functionality is implemented by pumping the SESAM with a continuous-wave 980-nm laser diode in order to slightly modify its nonlinear reflectivity. We use the standard f-to-2f method for detection of the CEO frequency, which is stabilized by applying a feedback signal to the current of the SESAM pump diode. We compare the SESAM-OOM stabilization with the traditional method of gain modulation via control of the pump power of the Yb:CALGO gain crystal. While the bandwidth for gain modulation is intrinsically limited to ∼250  kHz by the laser cavity dynamics, we show that the OOM provides a feedback bandwidth above 500 kHz. Hence, we were able to obtain a residual integrated phase noise of 430 mrad for the stabilized CEO beat, which represents an improvement of more than 30% compared to gain modulation stabilization.