Optical Waveform Synthesis Research Articles

In-line synthesis of multi-octave phase-stable infrared light.

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO3 and LiGaS2, resulting in the simultaneous coverage of the 800-cm-1-to-3000-cm-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.

Optical Waveform Synthesis and Its Applications

AbstractThe quest for ever‐shorter optical pulses has been ongoing for over half a century. Although few‐cycle pulses have been generated for nearly 40 years, pulse lengths below the single‐cycle limit have remained an elusive goal for a long time. For this purpose, optical waveform synthesizers, generating high‐energy, high‐average‐power pulses via coherent combination of multiple pulses covering different spectral regions, have been recently developed. They allow unprecedented control over the generated optical waveforms, spanning an extremely broad spectral range from ultraviolet to infrared. Such control allows for steering strong‐field interactions with increased degrees of freedom. When driving high‐harmonic generation, tailored waveforms can produce bright attosecond pulse trains and even isolated attosecond pulses with tunable spectra up to the soft X‐ray range. In this paper recent progress on parametric and hollow‐core fiber waveform synthesizers is discussed. Newly developed seeding schemes; absolute, relative, and spectral phase measurement; and control techniques suitable for synthesizers are described. The progress on serial and parallel waveform synthesis based on Ti:sapphire and Ytterbium laser systems and their latest applications in high‐harmonic generation in gaseous and solid media, attosecond science, and laser wakefield acceleration is discussed.

Sub-terahertz-repetition-rate frequency comb generated by filter-induced instabilities in passive driven fiber resonators

Ultrahigh-repetition-rate frequency comb generation exhibits great potential in applications of optical waveform synthesis, direct comb spectroscopy, and high capacity telecommunications. Here we present the theoretical investigations of a filter-induced instability mechanism in passive driven fiber resonators with a wide range of cavity dispersion regimes. In this novel concept of modulation instability, coherent frequency combs are demonstrated numerically with rates up to sub-terahertz level. Floquet stability analysis based on the Ikeda map is utilized to understand the physical origin of the filter-induced instability. Comparison with the well-known Benjamin–Feir instability and parametric instability is performed, revealing the intrinsic distinction in the family of modulation instabilities. Our investigations might benefit the development of ultrahigh-repetition-rate frequency comb generation, providing an alternative method for the microresonators.

Recent progress in integrated electro-optic frequency comb generation

Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement, including spectroscopy, metrology, precision distance measurement, sensing, as well as optical and microwave waveform synthesis, signal processing, and communications. Several techniques exist to generate optical frequency combs, such as mode-locked lasers, Kerr micro-resonators, and electro-optic modulation. Important characteristics of optical frequency combs include the number of comb lines, their spacing, spectral shape and/or flatness, and intensity noise. While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation, the latter provides increased flexibility in tuning the comb spacing. For some applications in optical communications and microwave photonics, a high degree of integration may be more desirable over a very large number of comb lines. In this paper, we review recent progress on integrated electro-optic frequency comb generators, including those based on indium phosphide, lithium niobate, and silicon photonics.

Fully tailorable integrated-optic resonators based on chirped waveguide Moiré gratings

Integrated-optic cavity resonators, such as Fabry–Perot microcavities and microrings, are key building blocks of photonics integrated circuits and are used extensively in applications such as optical communications and microwave photonics. For a single, conventional, optical-cavity resonator, resonance peaks appear periodically in frequency and have Lorentzian shapes in nature, which generally cannot be broken. Here, we report on fully tailorable, integrated-optic resonators that allow for independent control of individual resonance or spectral peaks as regards their presence, linewidths and extinction ratios, resonant wavelengths, and shapes and bandwidths. The response shapes can be set to be Lorentzian, Gaussian-like, or square. The resonators are based on chirped waveguide Moiré gratings developed on a silicon-on-insulator platform. We also demonstrate that they can be implemented on compact Archimedean spiral shapes to have sizes comparable to microring and microdisk resonators, with no spectral degradation. The unprecedented spectral flexibility of these resonators makes them attractive for a variety of fields and will enable new avenues for exploration in relevant areas such as optical waveform synthesis and microwave photonics.

Optical pulse waveform generator based on spectral shaping and frequency-to-time-mapping

Based on the spectral shaping and frequency-to-time-mapping (FTTM), a novel optical pulse waveform synthesis approach is proposed. By inscribed grating into half-tapered fiber or full-tapered fiber, spectrum with triangular shape or spectrum with distinctive non-uniform resonance peaks are obtained. After FTTM by a linear dispersion element, different temporal pulse waveforms, such as triangular pulses or linearly frequency chirp pulses are generated. In contrast to the conventional pulse synthesizing method, the generated temporal output pulses in average are the stretched counterpart of the shaped spectrum, which facilitates the reconfigurable adjustment by designing grating with different reflection spectrum.

Fully stabilized multi-TW optical waveform synthesizer: Toward gigawatt isolated attosecond pulses

A stable 50-mJ three-channel optical waveform synthesizer is demonstrated and used to reproducibly generate a high-order harmonic supercontinuum in the soft x-ray region. This synthesizer is composed of pump pulses from a 10-Hz repetition-rate Ti:sapphire pump laser and signal and idler pulses from an infrared two-stage optical parametric amplifier driven by this pump laser. With full active stabilization of all relative time delays, relative phases, and the carrier-envelope phase, a shot-to-shot stable intense continuum harmonic spectrum is obtained around 60 eV with pulse energy above 0.24 μJ. The peak power of the soft x-ray continuum is evaluated to be beyond 1 GW with a 170-as transform limit duration. We found a characteristic delay dependence of the multicycle waveform synthesizer and established its control scheme. Compared with the one-color case, we experimentally observe an enhancement of the cutoff spectrum intensity by one to two orders of magnitude using three-color waveform synthesis.

40-µJ passively CEP-stable seed source for ytterbium-based high-energy optical waveform synthesizers.

We demonstrate experimentally for the first time a ~40-µJ two-octave-wide passively carrier-envelope phase (CEP)-stable parametric front-end for seeding an ytterbium (Yb)-pump-based, few-optical-cycle, high-energy optical parametric waveform synthesizer. The system includes a CEP-stable white-light continuum and two-channel optical parametric chirped pulse amplifiers (OPCPAs) in the near- and mid-infrared spectral regions spanning altogether a two-octave-wide spectrum driven by a regenerative amplifier. The output pulses are compressed and fully characterized to demonstrate the well-behaved spectral phase of this seed source.

Toward Waveform Nonlinear Optics Using Multimillijoule Sub-Cycle Waveform Synthesizers

Waveform nonlinear optics aims to study and control the nonlinear interactions of matter with extremely short optical waveforms custom-tailored within a single cycle of light. Different technological routes to generate such multimillijoule sub-optical-cycle waveforms are currently pursued, opening up unprecedented opportunities in attoscience and strong-field physics. Here, we discuss the experimental schemes, introduce the technological challenges, and present our experimental results on high-energy sub-cycle optical waveform synthesis based on (1) parametric amplification and (2) induced-phase modulation in a two-color-driven gas-filled hollow-core fiber compressor. More specifically, for (1), we demonstrate a carrier-envelope-phase (CEP)-stable, multimillijoule three-channel parametric waveform synthesizer generating a $>$ 2-octave-wide spectrum (0.52–2.4 ${\boldsymbol \mu}$ m). After two amplification stages, the combined 125- ${\boldsymbol \mu}$ J output supports 1.9-fs FWHM waveforms; energy scaling to $>$ 2 mJ is achieved after three amplification stages. FROG pulse characterization of all three second-stage outputs demonstrates the feasibility to recompress all three channels simultaneously close to the Fourier limit and shows the flexibility of our intricate dispersion management scheme for different experimental situations. For (2), we generate CEP-stable 1.7-mJ waveforms covering 365–930 nm (measured at 1% of the peak intensity) obtained from induced-phase modulation in a two-color-driven gas-filled hollow-core fiber. Using custom-designed double-chirped mirrors and a UV spatial light modulator will permit compression close to the 0.9-fs FWHM transform limit. These novel sources will become versatile tools for controlling strong-field interactions in matter and for attosecond pump–probe spectroscopy using VIS/IR and XUV/soft-X-ray pulses.

A Dual-Wavelength Rational Harmonic Mode-Locked Fiber Ring Laser With Different Repetition Frequencies

A dual-wavelength rational harmonic mode-locked fiber ring laser with different repetition frequencies is theoretically analyzed and experimentally demonstrated. In this scheme, two wavelength channels are inserted into the laser loop to form two resonant cavities. By tuning the length of each channel, required rational harmonic mode-locking conditions can be achieved, and dual-frequency mode-locked signals are simultaneously generated at different wavelengths. In addition, the supermode suppression ratio is further improved due to the mode competition caused by this dual-cavity configuration. In the experimental demonstration, when the fourth-order (or fifth-order) harmonic mode-locked signal is generated in Cavity 1, the first-, second-, and third-order (or fourth-order) harmonic mode-locked signals, respectively, can be generated in Cavity 2 by tuning its length. The measured radio frequency spectra and waveforms show that the signals have high quality. Compared with a conventional rational harmonic mode-locked laser (RHMLL), more than 10 dB supermode suppression ratio improvement is obtained. Since this system can flexibly implement multiwavelength and multifrequency signal generation without adding much cost, it may have wide applications in optical communication system, photonic microwave signal generation, and optical waveform synthesizers.

Two-octave-spanning dispersion-controlled precision optics for sub-optical-cycle waveform synthesizers

Two-octave-spanning precision dispersive mirror systems are demonstrated, providing a new enabling technology for pulse-energy and bandwidth scaling of sub-cycle optical waveform synthesizers. We propose and characterize new dispersion management schemes with advanced dielectric coating designs. Based on an analytic dual adiabatic matching structure, we implement a chirped dichroic mirror, to efficiently optimize the beam combining from different spectra, and a double-chirped mirror pair, to avoid unwanted nonlinearity during beam propagation, with custom-tailored dispersion and reflectivity over more than two octaves of bandwidth ranging from 0.49 to 2.3 μm, supporting 1.9-fs-short sub-optical-cycle pulses. The multilayer coating structures can also be applied to the design of chirped-fiber Bragg gratings and general optical filters. The proposed designs and schemes will benefit ultrabroadband applications requiring precise dispersion management, especially enabling the generation of intense sub-optical-cycle light transients.

Genetic optimization of attosecond-pulse generation in light-field synthesizers

We demonstrate control over attosecond pulse generation and shaping by numerically optimizing the synthesis of few-cycle to sub-cycle driver waveforms. The optical waveform synthesis takes place in an ultrabroad spectral band covering the ultraviolet-infrared domain. These optimized driver waves are used for ultrashort single and double attosecond pulse production (with tunable separation) revealing the potentials of the light wave synthesizer device demonstrated by Wirth et al. [Science 334, 195 (2011)]. The results are also analyzed with respect to attosecond pulse propagation phenomena.

Compact optical function generator

We demonstrate the use of a nonlinear photonic crystal to generate a harmonic comb and an ultrabroad-band acousto-optic modulator for the field amplitudes and phases of the comb to succeed in synthesizing femtosecond and subfemtosecond optical field waveforms. Nonsinusoidal fields of various shapes are synthesized and verified using shaper-assisted linear cross-correlation. The compact all-solid-state system could lead to the realization of a portable arbitrary optical waveform synthesizer that is analogous in many aspects to an RF function generator.

Optical waveform synthesizer and its application to high-harmonic generation

Over the last decade, the control of atomic-scale electronic motion by optical fields strong enough to mitigate the atomic Coulomb potential has broken tremendous new ground with the advent of phase-controlled high-energy few-cycle pulse sources. Further investigation and control of these physical processes, including high-harmonic generation, ask for the capability of waveform shaping on sub-cycle time scales, which requires a fully phase-controlled multiple-octave-spanning spectrum. In this paper, we present a light source that enables sub-cycle waveform shaping with a two-octave-spanning spectrum and 15 µJ pulse energy based on coherent synthesis of pulses with different spectra, or wavelength multiplexing. The synthesized pulse has its shortest high-field transient lasting only 0.8 cycles (amplitude FWHM) of the centroid frequency. The benefit of the approach lies in its modular design and scalability in both bandwidth and pulse energy. Full phase control allows for the synthesis of any optical waveform supported by the amplified spectrum. A numerical study shows the uniqueness of the light source for direct isolated soft-x-ray pulse generation based on high-harmonic generation, greatly reducing and eventually even eliminating the need for gating techniques or spectral filtering. The demonstrated system is the prototype of a class of novel optical tools for attosecond control of strong-field physics experiments.

Advanced Ultrafast Technologies Based on Optical Frequency Combs

This paper presents recent results in the development of novel ultrafast technologies based on the generation and application of stabilized optical frequency combs. By using novel active resonant cavity injection locking techniques, filtering, modulation and detection can be performed directly on individual components of the frequency comb enabling new approaches to optical waveform synthesis, waveform detection and matched filtering, with effective signal processing bandwidths in excess of 1 THz.

Stimulated Raman amplification and high-order Raman sideband generation in a polymer waveguide on a printed circuit

The ultrafast Raman response of C-H vibrations in polymer waveguides on a printed circuit is shown to enable a high-gain amplification and high-order stimulated Raman transformation of ultrashort laser pulses. The pump and Stokes fields coupled by the C-H vibration mode with a vibration cycle of 11 fs give rise to multiple Raman sidebands, suggesting the way toward ultrafast optical data processing and few-cycle optical waveform synthesis on a printed-circuit polymer waveguide platform.

Information processing with longitudinal spectral decomposition of ultrafast pulses

We describe what we believe to be novel methods for waveform synthesis and detection relying on longitudinal spectral decomposition of subpicosecond optical pulses. Optical processing is performed in both all-fiber and mixed fiber-free-space systems. Demonstrated applications include ultrafast optical waveform synthesis, microwave spectrum analysis, and high-speed electrical arbitrary waveform generation. The techniques have the potential for time-bandwidth products of > or =10(4) due to exclusive reliance on time-domain processing. We introduce the principles of operation and subsequently support these with results from our experimental systems. Both theory and experiments suggest third-order dispersion as the principle limitation to large time-bandwidth products. Chirped-fiber Bragg gratings offer a route to increasing the number of resolvable spots for use in high-speed signal processing applications.

Dispersion control over the ultraviolet-visible-near-infrared spectral range with HfO_2/SiO_2-chirped dielectric multilayers

We report the first realization, to the best of our knowledge, of a chirped multilayer dielectric mirror providing dispersion control over the spectral range of 300-900 nm and the first use of hafnium oxide in a chirped mirror. The technology opens the door to the reliable and reproducible generation of monocycle laser pulses in the blue-violet spectral range, will benefit the development of optical waveform and frequency-comb synthesizers over the ultraviolet-visible-near-infrared spectral range, and permits the development of ultrabroadband-chirped multilayers for high-power applications.

Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb

The control of the broadband frequency comb emitted from a mode-locked femtosecond laser has permitted a wide range of scientific and technological advances--ranging from the counting of optical cycles for next-generation atomic clocks to measurements of phase-sensitive high-field processes. A unique advantage of the stabilized frequency comb is that it provides, in a single laser beam, about a million optical modes with very narrow linewidths and absolute frequency positions known to better than one part in 10(15) (ref. 5). One important application of this vast array of highly coherent optical fields is precision spectroscopy, in which a large number of modes can be used to map internal atomic energy structure and dynamics. However, an efficient means of simultaneously identifying, addressing and measuring the amplitude or relative phase of individual modes has not existed. Here we use a high-resolution disperser to separate the individual modes of a stabilized frequency comb into a two-dimensional array in the image plane of the spectrometer. We illustrate the power of this technique for high-resolution spectral fingerprinting of molecular iodine vapour, acquiring in a few milliseconds absorption images covering over 6 THz of bandwidth with high frequency resolution. Our technique for direct and parallel accessing of stabilized frequency comb modes could find application in high-bandwidth spread-spectrum communications with increased security, high-resolution coherent quantum control, and arbitrary optical waveform synthesis with control at the optical radian level.

Erratum to: Few-cycle optical waveform synthesis

Erratum to: Few-cycle optical waveform synthesis