Carrier-envelope Offset Research Articles

Demonstration of carrier envelope offset locking with low pulse energy

We demonstrate a carrier-envelope-offset (CEO)- locked frequency comb with 230-pJ fiber coupling pulse energy by using a passively mode-locked Er-fiber amplifier laser. For the generation of an octave-bandwidth spectrum in a highly nonlinear fiber and the second harmonic in a self-referenced interferometer with the lower pulse energy, we use a tellurite photonic crystal fiber and a direct-bonded quasi-phasematched LiNbO3 ridge waveguide, respectively. Our method is feasible for locking the CEO with a lower pulse energy to obtain a low-noise and highaccuracy optical frequency comb at telecommunications wavelengths.

Diode-pumped Yb:KYW femtosecond laser frequency comb with stabilized carrier-envelope offset frequency

We describe the detection and stabilization of the carrier envelope offset (CEO) frequency of a diode-pumped Yb:KYW (ytterbium-doped potassium yttrium tungstate) femtosecond oscillator that is spectrally centered at 1033 nm. The system consists of a diode-pumped, passively mode-locked femtosecond laser that produces 290 fs pulses at a repetition rate of 160 MHz. These pulses are first amplified, spectrally broadened and temporally compressed to 80 fs, and then launched into microstructured fiber to produce an octave-spanning spectrum. An f-2f nonlinear interferometer is employed with the broadened spectrum to detect and stabilize the CEO frequency through feedback to the pump laser current. These results demonstrate that such a Yb-doped tungstate laser can provide an efficient, compact, high-repetition-rate optical frequency comb with coverage from 650–1450 nm.

Tellurite-based fibers and their applications to optical communication networks

This paper reviews tellurite-based fibers and their applications to optical communication networks. First, an investigation of tellurite-based glass and the fabrication of conventional step-index fibers and photonic crystal fibers (PCF) are described. By purifying the raw materials and employing a novel PCF fabrication process, low background losses were achieved for an Er3+-doped, an undoped tellurite-based fiber and a tellurite-based PCF. Second, the optical properties of Er3+-doped tellurite-based glass and fiber, and the gain characteristics of erbium doped tellurite fiber amplifiers (EDTFAs) were studied. A seamless, low noise and gain flattened C + L band EDTFA was realized, and an S + C + L band amplifier was constructed by combining an EDTFA and a thulium-doped fluoride fiber amplifier (TDFFA) in parallel. Third, it is confirmed that the Raman scattering characteristic of tellurite-based fiber has such a large gain coefficient and Stokes shift that it is possible achieve a wideband tellurite-based fiber Raman amplifier. To overcome several problems, a distributed/discrete hybrid tellurite- and silica-based fiber Raman amplifier (FRA) was constructed as an S + C + L band WDM repeater with a gain bandwidth of 127-nm. Fourth, Brillouin amplification and the simulated performance of slow light generation in a tellurite-based fiber were investigated. The fiber exhibits the largest time delay per unit power of 19.9 ns/mW. Finally, a carrier-envelope offset (CEO)-locked frequency comb with low fiber coupling pulse energy (230 pJ) was demonstrated by using a tellurite-based PCF. This method has the potential to lock the CEO with a lower pulse energy and thus provide a low-noise and high-accuracy optical frequency comb at telecommunication wavelengths. These tellurite-based fibers with low background loss thus offer attractive functions for applications in the optical communication field.

Bandwidth-independent linear method for detection of the carrier-envelope offset phase

We propose and demonstrate a novel linear procedure for measurement of the carrier-envelope offset (CEO) phase of femtosecond oscillators. The technique is based on a Mach-Zehnder interferometer, a ring resonator, and a spectrograph. In this scheme, interference between subsequent pulses from a pulse train may frustrate the interference between identical pulses in the Mach-Zehnder, resulting in a modification of interference contrast depending on the CEO phase. We suggest spectrally and spatially resolved interferometry for robust detection of the fringe visibility. It is shown by numerical simulations and experimentally demonstrated that the visibility of such fringes uniquely depends on the CEO phase of the pulse train. Since the method relies only on linear interactions and does not require any nonlinear conversion, it allows characterizing the CEO frequency of mode-locked oscillators with virtually arbitrarily low bandwidth and power levels.

Intense 1.5-cycle near infrared laser waveforms and their use for the generation of ultra-broadband soft-x-ray harmonic continua

We demonstrate sub-millijoule-energy, sub-4 fs-duration near-infrared laser pulses with a controlled waveform comprised of approximately 1.5 optical cycles within the full-width at half-maximum (FWHM) of their temporal intensity profile. We further demonstrate the utility of these pulses for producing high-order harmonic continua of unprecedented bandwidth at photon energies around 100 eV. Ultra-broadband coherent continua extending from 90 eV to more than 130 eV with smooth spectral intensity distributions that exhibit dramatic, never-before-observed sensitivity to the carrier-envelope offset (CEO) phase of the driver laser pulse were generated. These results suggest the feasibility of sub-100-attosecond XUV pulse generation for attosecond spectroscopy in the 100 eV range, and of a simple yet highly sensitive on-line CEO phase detector with sub-50-ms response time.

Highly stable, frequency-controlled mode-locked erbium fiber laser comb

A mode-locked Er:fiber laser-based optical frequency comb with high stability in the repetition frequency and carrier-envelope offset (CEO) frequency is realized. The CEO beat signal was detected right after the supercontinuum generation by a compact single-beam f–2f self-referencing interferometer, which does not require further delay compensation. The stabilized repetition frequency has an out-of-loop tracking stability of 1.3×10-13/\(\sqrt{\tau}\) for an integration time τ less than 1000 s, which is limited by the stability of the frequency measurement system. The stabilized CEO frequency has a residual fluctuation of 0.52 mHz measured with a 1 s gate time. This is, to our knowledge, the highest tracking stability realized for fiber laser-based optical frequency comb.

Carrier‐envelope phase‐stabilized chirped‐pulse amplification system

AbstractWe demonstrated a carrier‐envelope phase (CEP) stabilized chirped‐pulse amplifier system. This amplifier system is composed of grating based pulse‐stretcher and compressor, a regenerative amplifier and a multi‐pass amplifier. We employed a new pulse‐pick‐up method to select CEP stabilized seed pulses. This pulse selection method is different from established practice which is based on pulse train timing, but is based on CEP of seed pulse. We measured amplitude‐to‐phase noise conversion coefficient of microstructure fiber and evaluated the additional out‐of‐loop error of carrier‐envelope offset (CEO) control. We also investigated the effect of beam pointing of the measured fringe shift in self‐referencing spectral interference method. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(3): 35–42, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20334 Copyright © 2006 Wiley Periodicals, Inc.

5.1 fs pulses generated by filamentation and carrier envelope phase stability analysis

Intense 5.1 fs CEO (carrier envelope offset) phase stable pulses were generated through two-fold filamentation in a noble gas at atmospheric pressure. The preservation of the CEO phase during the filamentation process was investigated. We show that generating these short pulses using filaments is not detrimental for the CEO phase stabilization, and that the more than one octave-spanning spectrum intrinsically generated by the process is feasible, and offers certain benefits, for direct use in single shot f–2f spectral interferometry.

Optical phase noise and carrier-envelope offset noise of mode-locked lasers

The timing jitter, optical phase noise, and carrier-envelope offset (CEO) noise of passively mode-locked lasers are closely related. New key results concern analytical calculations of the quantum noise limits for optical phase noise and CEO noise. Earlier results for the optical phase noise of actively mode-locked lasers are generalized, particularly for application to passively mode-locked lasers. It is found, for example, that mode locking with slow absorbers can lead to optical linewidths far above the Schawlow–Townes limit. Furthermore, mode-locked lasers can at the same time have nearly quantum-limited timing jitter and a strong optical excess phase noise. A feedback timing stabilization via cavity length control can, depending on the situation, reduce or greatly increase the optical phase noise, while not affecting the CEO noise. Besides presenting such findings, the paper also tries to clarify some basic aspects of phase noise in mode-locked lasers.

Carrier-envelope phase Stabilized Chirped-pulse Amplification System

We demonstrated a carrier-envelope phase (CEP) stabilized chirped-pulse amplifier system. This amplifier system is composed of grating based pulse-stretcher and compressor, a regenerative amplifier and a multi-pass amplifier. We employed a new pulse-pick-up method to select CEP stabilized seed pulses. This pulse selection method is different from established practice which is based on pulse train timing, but is based on CEP of seed pulse. We measured amplitude-to-phase noise conversion coefficient of microstructure fiber and evaluated the additional out-of-loop error of carrier-envelope offset (CEO) control. We also investigated the effect of beam pointing on the measured fringe shift in self-referencing spectral interference method.

Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology

An optically integrated self-referenced frequency comb laser is demonstrated. The system consists of a passively-modelocked Er-fiber laser, a butt-coupled periodically poled lithium niobate (PPLN) waveguide phase-sensor and an electronic feedback loop for carrier-envelope-offset (CEO) phase stabilization. The fceo-beat-signal has a linewidth of 62 kHz and is detected with a S/N-ratio of 40 dB, with greatly reduced pulse energy requirements compared to bulk crystal phase-sensors. To our knowledge this is the first self-referenced frequency-comb system entirely based on guided-wave technology.

Generation of intense, carrier-envelope phase-locked few-cycle laser pulses through filamentation

Intense, well-controlled light pulses with only a few optical cycles start to play a crucial role in many fields of physics, such as attosecond science. We present an extremely simple and robust technique to generate such carrier-envelope offset (CEO) phase locked few-cycle pulses, relying on self-guiding of intense 43-fs, 0.84 mJ optical pulses during propagation in a transparent noble gas. We have demonstrated 5.7-fs, 0.38 mJ pulses with an excellent spatial beam profile and discuss the potential for much shorter pulses. Numerical simulations confirm that filamentation is the mechanism responsible for pulse shortening. The method is widely applicable and much less sensitive to experimental conditions such as beam alignment, input pulse duration or gas pressure as compared to gas-filled hollow fibers.

Requirements for the sampling source in coherent linear sampling

Complex envelope measurement using coherent linear optical sampling with mode-locked sources is investigated. It is shown that reliable measurement of the phase requires that one of the optical modes of the mode-locked laser be locked to the optical carrier of the data signal to be measured. Carrier-envelope offset (CEO) is found to have negligible effect on the measurement. Measurement errors of the intensity profile and phase depend on the pulsewidth and chirp of the sampling pulses as well as the detuning between the carrier frequencies of the data signal and the center frequency of sampling source.

Carrier-envelope phase dynamics of synchronized mode-locked lasers

The mode-combs of two lasers with synchronized repetition rate, a femtosecond Ti:sapphire- and a picosecond Nd:YVO 4-laser, are characterized by measuring the carrier-envelope offset (CEO) frequencies of the lasers using a second/third harmonic interference scheme and direct interference in the overlap region, respectively. This method enables the measurement of the CEO frequency of narrow band lasers, here a picosecond Nd:YVO 4 laser. Together with efficient nonlinear frequency conversion schemes, high power picosecond lasers may be used to extent optical frequency metrology from a Ti:sapphire standard to the UV and near IR spectral regions.

Role of the carrier-envelope offset phase of few-cycle pulses in nonperturbative resonant nonlinear optics.

We study the influence of the carrier-envelope offset phase of few-cycle pulses on nonperturbative resonant extreme nonlinear optics in a semiconductor. If the Rabi frequency becomes comparable to the light frequency, the different Rabi sidebands interfere around twice the laser center frequency, giving rise to a signal which strongly depends on the carrier-envelope offset phase. This signature should be measurable in GaAs samples.

Carrier–envelope-offset dynamics and stabilization of femtosecond pulses

We analyze and stabilize fluctuations of the relative phase between the carrier and the envelope of a mode-locked laser. Mechanisms generating fluctuations of the carrier–envelope-offset (CEO) phase are experimentally identified in lasers with and without prisms for dispersion compensation. One mechanism is amplitude-to-phase coupling via self-steepening. This mechanism translates power changes into variations of the CEO phase. A similar but much stronger effect is caused by beam-pointing variations in lasers with intracavity prisms. Both mechanisms convert power noise of the laser into phase noise and can be used to externally control or stabilize the CEO frequency by adjusting the pump power. Our measurements are well explained by a theoretical model. This investigation allowed us to obtain an unsurpassed stabilization of the CEO phase to 0.02 rad rms for a frequency range from 0.01 Hz to 10 kHz. We extend the discussion to pulse-amplification schemes and show that beam-pointing variations are also expected to have a strong influence on the CEO phase of amplified pulses. We discuss methods to reduce or avoid CEO noise by suitable design of the dispersion-compensation scheme, both in oscillators and in amplifiers.

White-light Continuum Generation for Measurements of Carrier-Envelope Phase Changes of Femtosecond Laser Pulses.

We demonstrated measurement of carrier-envelope phase (CEP) changes of amplified laser pulses by utilizing spectral interferometry between the white-light continuum and its second harmonic. The white-light continuum was generated by self-phase modulation (SPM) in a gas-filled hollow fiber. This method enables us to measure the CEP changes of low-repetition rate laser pulses on a shot-to-shot basis. The CEP-change of the amplified laser pulses exhibits random distribution that is mainly due to the CEP change fluctuation of the seed pulses from a carrier-envelope-offset (CEO) uncontrolled oscillator. We also demonstrated the CEO measurement of a mode-locked Ti:Sapphire oscillator. We utilized the self-referencing method, which measures the temporal beat frequency between the white-light continuum and its second harmonic. The white-light continuum was generated by the SPM in a micro-structure fiber. The temporal evolution of the beat signal showed CEO fluctuation of the uncontrolled oscillator.

Carrier-envelope offset phase control: A novel concept for absolute optical frequency measurement and ultrashort pulse generation

The shortest pulses periodically emitted directly from a mode-locked Ti:sapphire laser are approaching the two-optical-cycle range. In this region, the phase of the optical carrier with respect to the pulse envelope becomes important in nonlinear optical processes such as high-harmonic generation. Because there are no locking mechanisms between envelope and carrier inside a laser, their relative phase offset experiences random fluctuations. Here, we propose several novel methods to measure and to stabilize this carrier-envelope offset (CEO) phase with sub-femtosecond uncertainty. The stabilization methods are an important prerequisite for attosecond pulse generation schemes. Short and highly periodic pulses of a two-cycle laser correspond to an extremely wide frequency comb of equally spaced lines, which can be used for absolute frequency measurements. Using the proposed phase-measurement methods, it will be possible to phase-coherently link any unknown optical frequency within the comb spectrum to a primary microwave standard. Experimental studies using a sub-6-fs Ti:sapphire laser suggesting the feasibility of carrier-envelope phase control are presented.