High-gain optical parametric amplification with a continuous-wave pump using a domain-engineered thin-film lithium niobate waveguide

The escalating capacity limitations of conventional near-infrared telecommunication bands have spurred urgent investigations into wide-band optical communication systems spanning from the near-infrared to mid-infrared regimes. This has motivated the development of optical components combining broadband bandwidth with high-speed operation. The state-of-the-art modulators face challenges in achieving broad operational bandwidth due to waveguide dispersion and velocity mismatch. Here we demonstrate a thin-film lithium niobate (TFLN) electro-optic (EO) modulator with an unprecedented 800-nm operational bandwidth, covering the full O-U telecom bands and extending into the 2-μm regime. The TFLN modulator exhibits >67 GHz EO bandwidth across O-U bands (~100 GHz at O-/S-/C-/L-bands) and >50 GHz at 2-μm band. It enables single-lane exceeding 240 Gbps PAM-4 transmission across O-U bands and a record 170 Gbps PAM-4 transmission at 2-μm band. This breakthrough establishes TFLN as a compelling platform for multispectral photonics, bridging conventional telecom infrastructure with emerging 2-μm technologies for next-generation optical communications.

Summary form only given. The coherent synthesis of custom-tailored, intense, sub-cycle optical waveforms is promising for attosecond science and strong-field physics, e.g., for precision control of strong-field interactions in atoms, molecules and solids, for the generation of intense isolated attosecond pulses, and for attosecond pump-probe spectroscopy.Recently, coherent pulse synthesis based on supercontinuum generation in a hollow-core fiber compressor allowed the generation of sub-cycle -300-μJ optical pulses. However, in this case ionization losses in the gas medium prevent further scaling to the mJ level. In contrast, parametric synthesizers do not face an energy scaling limit, and allow for spectral extension into the particularly appealing MIR region. In previous works we demonstrated coherent pulse synthesis of two optical parametric chirped-pulse amplifiers and of two optical parametric amplifiers (OPAs) on the few-μJ level [3]. Here we present the ongoing development of a novel 3-channel parametric synthesizer for generating a 2-octave-wide spectrum, which can easily be upscaled to the mJ-level. We start from a cryogenically cooled Ti:sapphire chirped-pulse amplifier (150 fs, 22 mJ, 0.8 μm, 1 kHz) and generate a CEP-stable continuum (0.5-2.3 μm) [5], by white-light generation in a YAG crystal pumped by the second harmonic (1.06 μm) of the CEP-stable idler of a NIR OPA. The continuum is split with custom-designed dichroic beam splitters (which will also be used for the final beam recombination) and seeds three OPAs, a VIS noncollinear OPA (NOPA), a NIR and an IR degenerate OPA (DOPA), each composed of 2 (later 3) amplification stages, and pumped by the pulses at 0.8 μm (IR DOPA), and by its second harmonic at 0.4 μm (VIS NOPA, NIR DOPA). To synthesize a coherent ultrashort pulse from these three OPAs, the relative timing of the pulses will be tightly locked using feedback loops with balanced optical cross-correlators, that can achieve sub-cycle synchronization with s0-as RMS timing jitter [2,3]. Figure 1(a) shows the measured output spectra and the energies from the second amplification stages operating in parallel. The transform-limited (TL) pulse duration from the synthesis of these spectra is 1.9 fs FWHM, corresponding to 0.7 optical cycles at 785 nm center wavelength. Our current work aims to scale up the energies (by a third amplification stage) to -0.5 mJ for the VIS NOPA/NIR DOPA and -2 mJ for the IR DOPA using the 18.5 mJ of pump remaining after the first two amplification stages. State-of-the-art double-chirped mirror pairs (at present in fabrication) will allow for the final pulse compression with ultralow residual ripple in the resulting total group-delay dispersion over the full bandwidth (0.52-2.3μm). Temporal characterization of the synthesized two-octave-spanning optical waveforms will be performed by two-dimensional spectral shearing interferometry (2DSI). We foresee that our 3-channel synthesizer, when further scaled in energy to the mJ level, will become a versatile tool for controlling strong-field interactions in atoms, molecules and solids and for attosecond pumpprobe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions.

Optical source generation has attracted significant attention recently, especially in fiber optical communications. Today there is a growing a demand for optical source generation beyond conventional telecommunication wavelength bands. However, high quality and versatile optical source is generally not available over those wavelength bands due to the lack of efficient gain medium. Thanks to fiber optical parametric amplifier (FOPA), which is based on the third order nonlinear susceptibility of optical fibers, offers ultrafast response, wide-gain bandwidth, high gain and large frequency detune from the pump, serves as a promising candidate for signal amplification over those wavelength bands. By using the corresponding fiber optical parametric oscillator (FOPO) configuration, widely tunable source from continuous-wave (CW) to sub-picosecond pulses can be potentially generated to serve different applications from communication to biomedical imaging.\n\nIn this thesis, we first demonstrate an all-fiber widely-tunable picosecond FOPO using highly-nonlinear fiber (HNLF). The tuning range is as wide as 250 nm, which is higher than previous picosecond FOPOs reported in the 1550-nm region. Second, time-dispersion tuning of the FOPO is investigated with fixed pump wavelength. It is a relatively simple and economic approach, and there will be no filter induced cavity loss. We then describe using FOPO to generated nearly-transform limited sub-picosecond pulses with a 60-nm tuning range. Another FOPO with a tuning range of 440-nm with dispersion-shifted fiber (DSF) as the gain medium is proposed and demonstrated. Compared with FOPOs demonstrated using HNLF as the gain medium, the use of DSF offers two key advantages: a wider tuning range and a narrower linewidth.\n\nIn addition to picosecond FOPO, CW FOPO is also of great interest in fiber optical communications and biomedical imaging. We also demonstrate an all-fiber CW single-longitudinal-mode (SLM) FOPO with tuning range covers the S and L bands. SLM oscillation with a side-mode suppression ratio greater than 43 dB is achieved, which has been extended to 1-μm region under stable operation.\n\nApart from static tuning, dynamic wavelength tuning of the FOPO is also discussed in this thesis with a cumulative speed exceeds 4,000,000 nm/s, which is higher than previous work reported in wavelength-swept FOPOs. The high-speed swept source would be useful in biomedical imaging and sensing applications.\n\nThe amplification of the sub-picosecond pulses of the FOPO output is also investigated, for the first time to our knowledge, by using a fiber optical parametric chirped pulse amplifier(FOPCPA).The totally fiber-integrated nature of the whole system allows complete self-alignment and further integration to other fiber-based systems.\n\nAll these research effort will show the versatility of FOPO techniques for generating wide range of optical sources for varies applications. These schemes may be useful in generating CW and short pulse for potential optical communication and biomedical imaging in non-conventional wavelength bands.

A novel superstructure fiber Bragg grating (SFBG) based comb gain equalizer for fiber optical parametric amplifier (OPA) is proposed. This gain equalizer is characterized by its comb reflectivity spectrum. It can have two functions at least, one is flattening the gain in multi ITU-T regulated channels, the other is it can be used as multi-channel isolation filters taking full advantage of its comb reflectivity spectrum. In addition it can be dispersionless or have dispersion compensation function. A one-bump fiber OPA, 30dB bandwidth >15nm, is designed with one of its peak gains (41dB) located at the center of work wavelengths in a 16-channel 100GHz spaced WDM optical transmission system according to corresponding Chinese criterion (1548.51nm~ 1560.61nm). The object reflectivity spectra of gain equalizer are calculated in two parts independently. Inner channel part for every channel is designed according to the gain spectrum of fiber OPA. As a comb filter, its inter-channel part reflectivities are the smaller the better. But inter-channel part reflectivities can not be simply zeroed, for which would result in the rising and descending edges of reflectivity spectrum being too sharp to be performed by any filter. To split the difference, inter-channel part reflectivities are calculated with Gaussian hypergeometric function (-0.5dB bandwidth 0.1nm, -45dB bandwidth 0.3nm) to obtain slower rising and descending edges than the simply zeroed case, which is very necessary and important to be sure the object reflectivity spectrum not only has sharp enough edges to take the shape of comb but also is physically realizable. Adopting channel-by-channel windowing method to apodize the object reflectivity spectrum, a superstructure fiber Bragg grating is designed with lay-peeling inverse scattering technique. At last, according to the coupling coefficient calculated, reflection spectrum and group time delay are analyzed based on the method of transformation matrix. Calculation results show that this SFBG satisfies all requirements. Further calculation results show that, using this SFBG as comb gain equalizer, the gain of fiber OPA is flattened in every channel to within ±0.4dB among 16 channels, with channel isolation high up to -35dB. It needs to point out that this method can also be used to design gain equalizer for other optical amplifiers.

The spontaneous fluorescence background in optical parametric amplifiers is generally attributed to the zero-point fluctuations of the electromagnetic field. These are amplified in parallel to the seed light and lead to an uncompressible superfluorescence background that deteriorates the contrast in optical parametric chirped pulse amplifiers (OPCPA). The absolute level of the underlying parametric fluorescence has not been reported so far. Comparing the fluorescence to low level cw seed light and quantitatively monitoring the output of a noncollinear optical parametric amplifier for both sources, the level is now determined. In a situation of 50 nm visible output bandwidth and low Gaussian spatial modes about 58 photons are found in the signal direction within the femtosecond time window of the amplifier. The superfluorescence level is observed to be proportional to the pump area for constant signal amplification. The implications for the background in high power OPCPA are discussed.

We report on theoretical and experimental investigation of the signal and superfluorescence amplification in a 7 TW optical parametric chirped pulse amplifier (OPCPA) system. We demonstrate that the compressed pulse contrast ratio can be improved by using larger seed pulse energies and by careful adjustment of the pump intensity in each amplification stage. Comparison of the measured amplified signal and superfluorescence energies yielded good agreement between calculations and experiments.

Optical parametric chirped pulse amplification has apparent advantages over the current schemes for high energy ultrashort pulse amplification. High gain in a single pas amplification, small B-integral, low heat deposition, high contrast ratio and, especially the extremely broad gain bandwidth with large-size crystals available bring people new hope for over multi-PW level at which the existing Nd:glass systems suffered difficulties. In this paper we present simulation and experimental studies for a high gain optical parametric chirped pulse amplification system which may be used as a preamplifier to replace the current complicated regenerative system or multi-pas Ti:sapphire amplifiers. Investigations on the amplification pump and chirped broadband pulse as a seed, respectively. Analysis and discussions are also given.

I will discuss progress on the development of ultra-low loss thin film lithium niobate (TFLN) photonic platform and its application in classical and quantum information processing. Examples of devices to be presented include low power consumption and high bandwidth electro-optic (EO) modulators and EO frequency comb sources, as well as their integration with high power lasers, of interest for applications in optical communications and microwave photonics. TFLN is also promising for spectral and temporal control of non-classical light. In particular, I will discuss the development of quantum transducers and photonics needed for realization of multiplexed quantum repeaters.

Photonics on thin-film lithium niobate (TFLN) has emerged as one of the most pursued disciplines within integrated optics. Ultracompact and low-loss optical waveguides and related devices on this modern material platform have rejuvenated the traditional and commercial applications of lithium niobate for optical modulators based on the electro-optic effect, as well as optical wavelength converters based on second-order nonlinear effects, e.g., second-harmonic, sum-, and difference-frequency generations. TFLN has also created vast opportunities for applications and integrated solutions for optical parametric amplification and oscillation, cascaded nonlinear effects, such as low-harmonic generation; third-order nonlinear effects, such as supercontinuum generation; optical frequency comb generation and stabilization; and nonclassical nonlinear effects, such as spontaneous parametric downconversion for quantum optics. Recent progress in nonlinear integrated photonics on TFLN for all these applications, their current trends, and future opportunities and challenges are reviewed.

In this work, we present a free-space transmissive light amplitude modulator based on thin-film lithium niobate on an insulator platform with an indium tin oxide meta-grating. The design leverages guided mode resonances induced by the transparent conductive oxide layer, enabling efficient electrooptical modulation in the near-infrared region. By integrating transparent conductive oxide both as electrical contact and as the resonating structure, the device eliminates the need for complex alignment during fabrication and minimizes optical losses associated with metallic contacts. We experimentally demonstrate that the device achieves a fundamental mode resonance at 968.5 nm with a quality factor of 440. The electrooptical tuning efficiency is thoroughly investigated across different modes using measurements and simulations. A notable resonance shift of 0.38 nm is observed for the fundamental mode under a ±10 V bias, while a maximum modulation amplitude of 4.6% is achieved with a higher-order mode. Furthermore, the device exploits incident angle tuning as an additional degree of freedom, effectively splitting and sensitively shifting resonances. The new resonances can provide electrooptic tunability. These results highlight the potential of this compact and scalable design for applications in spatial light modulation, optical communications, and tunable optics.

We have investigated the possibility of using optical parametric amplifiers (OPA) as optical parametric chirped pulse amplifiers (OPCPA), in which energy from a narrow bandwidth pump source is efficiently transferred to a chirped signal pulse prior to its recompression (1).

We have investigated the generation of new undesirable frequencies in a two-pump optical parametric amplifier, which are generated by the four-wave mixing of a signal or idler with one of the strong pump waves. The powers of the new frequencies impose upper bounds on the device gain and output power if cross talk is to be avoided. In the parametric amplifier we investigated, however, we found that, for 7.5 dB of gain and −3.5 dBm of output power, the ratio of power in the new frequencies and the signal is less than −30 dB. This demonstrates that cross talk is acceptable for low gain and output power applications of these devices such as frequency conversion. We present simple scaling relationships for the power in the new undesirable frequencies that can be used to help design optical parametric amplifiers.

At signal critical wavelength degeneracy (CWD), super-broad parametric gain bandwidths (SBGBs) can be obtained in nonlinear crystals. A 280 nm SBGB, for ∼60 small signal peak gain, has been calculated considering a collinear optical parametric chirped pulse amplification (OPCPA) at CWD in a 40 mm long, 58.3% partially deuterated KDP (P-DKDP) crystal, pumped by a frequency doubled nanosecond Nd:glass laser at 1 GW/cm2 pump intensity. This gain bandwidth in the 1 µm spectral range is significantly broader compared to the 150 nm gain bandwidth, calculated under similar conditions, for the broad-band non-collinear (NC) OPCPA in DKDP crystals in the 900 nm spectral range. It can support the amplification of significantly shorter femtosecond pulses, giving rise to a higher peak power at the same amplified pulse energy. The spectral profile of the gain bandwidth can be shaped by slightly adjusting the deuteration ratio (DR) of the P-DKDP crystals. A small-angle non-collinear OPCPA near the CWD can be used for the spatial separation of the signal and idler beams with the same polarization, keeping at the same time an ultra-broad gain bandwidth. A SBGB of 255 nm FWHM has been calculated for a non-collinear OPCPA stage with a 40 mm long P-DKDP crystal, 10.3 mrad signal-pump wavevectors internal angle, 57.8% DR, pumped by a few-ns 527 nm wavelength laser at 1 GW/cm2 pump intensity. The half-meter size P-DKDP crystals, pumped by frequency doubled few-ns multi-kJ Nd:glass lasers, are able to amplify super broad-band nanosecond stretched pulses up to the kJ energy level. They might be appropriate amplifying media for the development of single-beam 100-PW class femtosecond laser systems based on OPCPA at CWD.

In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.

Supercontinuum generation (SCG) with spectral coverage across the full visible and ultraviolet (UV) ranges is crucial for numerous quantum computing and atomic systems. Here, such ultrabroad‐bandwidth SCG is demonstrated in low‐loss thin‐film lithium niobate (TFLN) nanophotonic waveguides fabricated by photolithography‐assisted chemo‐mechanical etching (PLACE) technique, without additionally introducing complex periodic poling. The MgO‐doped waveguides are designed to exhibit anomalous dispersion in the telecom band by tailoring the waveguide thickness, simultaneously enabling dispersive wave emergence, second harmonic, and third harmonic generation to broaden the spectrum. Thanks to the utilization of the strong χ (2) and χ (3) nonlinear processes in the fabricated low‐loss (≈3 dB m −1 ) waveguide, the SCG with a −80‐dB spectral bandwidth as large as 2.7 octaves spanning from 330 to 2250 nm is observed by pumping the waveguide with a 1550‐nm femtosecond pulsed laser with 0.687 nJ, agreeing well with numerical simulation. Meanwhile, the SCG sustains its stable spectral envelope for a long period of time thanks to the mitigation of the photorefractive effect of lithium niobate by doping of MgO. And theoretical simulation is carried out to verify the coherence of the fundamental and second harmonic waves.