Programmable broadband optical field spectral shaping with megahertz resolution using a simple frequency shifting loop

Cascaded-mode interferometers: Spectral shape and linewidth engineering

Interferometers are essential tools for measuring and shaping optical fields, widely used in optical metrology, sensing, laser physics, and quantum mechanics. They superimpose waves with a mutual phase delay, modifying light intensity. A frequency-dependent phase delay enables spectral shaping for filtering, routing, wave shaping, or multiplexing. Conventional Mach-Zehnder interferometers generate sinusoidal output intensities, limiting spectral engineering capabilities. Here, we propose a framework that uses interference of multiple transverse modes within a single multimode waveguide to achieve arbitrary spectral shapes in a compact geometry. Designed corrugated gratings couple these modes, enabling energy exchange akin to a beam splitter for easy multimode handling. We theoretically and experimentally demonstrate spectra with independently tunable linewidth and free spectral range, along with distinct spectral shapes for various transverse modes. Our method applies to orthogonal modes of different orders, polarization, and angular momentum, offering potential for sensing, calibration, metrology, and computing.

Broadband microwave signal generation with programmable chirp shapes via low-speed electronics-controlled phase-modulated optical loop

Broadband microwave signals with customized chirp shapes are highly captivating in practical applications. Compared with electronic technology, photonic solutions are superior in bandwidth but suffer from flexible and rapid manipulation of chirp shape or frequency. Here, we demonstrate a concept for generating broadband microwave signals with programmable chirp shapes. Our realization is based on a recirculating phase-modulated optical loop to ultrafast manipulate the laser frequency, which breaks the limitation of the buildup time of the laser from spontaneous emission. Through heterodyne beating the frequency-agile lasers with a continuous-wave laser, microwave signals with ultrafast and programmable chirp shapes are generated. Besides, signal parameters, such as bandwidth, center frequency, and temporal duration, can be reconfigured. In the experiment, highly coherent microwave signals with various customized chirp shapes are generated, where the time resolution for programming the chirp shape is 649 ps. This flexible frequency manipulation characteristic holds promise for many applications, including LiDAR, broadband radar systems, and spectroscopy.

Silicon photonic microresonator-based high-resolution line-by-line pulse shaping

Optical pulse shaping stands as a formidable technique in ultrafast optics, radio-frequency photonics, and quantum communications. While existing systems rely on bulk optics or integrated platforms with planar waveguide sections for spatial dispersion, they face limitations in achieving finer (few- or sub-GHz) spectrum control. These methods either demand considerable space or suffer from pronounced phase errors and optical losses when assembled to achieve fine resolution. Addressing these challenges, we present a foundry-fabricated six-channel silicon photonic shaper using microresonator filter banks with inline phase control and high spectral resolution. Leveraging existing comb-based spectroscopic techniques, we devise a system to mitigate thermal crosstalk and enable the versatile use of our on-chip shaper. Our results demonstrate the shaper's ability to phase-compensate six comb lines at tunable channel spacings of 3, 4, and 5 GHz. Specifically, at a 3 GHz channel spacing, we showcase the generation of arbitrary waveforms in the time domain. This scalable design and control scheme holds promise in meeting future demands for high-precision spectral shaping capabilities.

Photonic stepped-frequency radar with 150-m unambiguous detection and centimeter range resolution

Photonic stepped-frequency radars based on optical frequency-shifting modulation have shown attractive properties such as wide bandwidth, centimeter range resolution, inherent frequency-time linearity with low spectrum spurs, and reduced system complexity. However, existing approaches typically exhibit meter- or centimeter-level radar range ambiguity, inversely proportional to the frequency step, due to the large frequency shift determined by acousto-optic or electro-optic (EO) modulators. Here, we overcome this limitation by injecting a narrowband, stepped-frequency signal into an optical frequency-shifting fiber cavity to achieve, for the first time, to our knowledge, a broadband photonic stepped-frequency radar with 150-m unambiguous detection and centimeter range resolution, surpassing the reported photonic- and electronic-based counterparts. The demonstrated approach effectively resolves the trade-off between ambiguity range and shifting frequency while maintaining the signal quality and bandwidth, bringing its practicality into reach for outdoor applications.

Design, tuning, and blackbox optimization of laser systems

Chirped pulse amplification (CPA) and subsequent nonlinear optical (NLO) systems constitute the backbone of myriad advancements in semiconductor manufacturing, communications, biology, defense, and beyond. Accurately and efficiently modeling CPA+NLO-based laser systems is challenging because of the complex coupled processes and diverse simulation frameworks. Our modular start-to-end model unlocks the potential for exciting new optimization and inverse design approaches reliant on data-driven machine learning methods, providing a means to create tailored CPA+NLO systems unattainable with current models. To demonstrate this new, to our knowledge, technical capability, we present a study on the LCLS-II photo-injector laser, representative of a high-power and spectro-temporally non-trivial CPA+NLO system.

Pulse generation with programmable positions based on a phase-modulated optical frequency-shifting loop

An approach to generating pulses with programmable positions is proposed and demonstrated based on a phase-modulated optical frequency-shifting loop (OFSL). By setting the OFSL to operate in the integer Talbot state, pulses are generated in the phase-locked positions, since the additional phase introduced by the electro-optic phase modulator (PM) in the OFSL is equal to an integer multiple of 2π in each round trip. Therefore, the pulse positions can be controlled and encoded by designing the driving waveform of the PM in a round-trip time. In the experiment, linear, round-trip, quadratic, and sinusoidal variations of pulse intervals are achieved by applying the corresponding driving waveforms to the PM. Pulse trains with coded pulse positions are also realized. In addition, the OFSL driven by waveforms with repetition rates equal to double and triple the free spectral range of the loop is also demonstrated. The proposed scheme paves a way to generate optical pulse trains with user-defined pulse positions, which can be used for such applications as compressed sensing and lidar.

Phase-sensitive distributed Rayleigh fiber sensing enabling the real-time monitoring of the refractive index with a sub-cm resolution by all-optical coherent pulse compression

We have developed a novel architecture enabling distributed acoustic sensing in a commercial single-mode fiber with a sub-cm spatial resolution and an interrogation rate of 20 kHz. More precisely, we report the capability of real-time and space-resolved monitoring of the distributed phase and of the refractive index variations along the sensing fiber. The system reported here is optimal in many aspects. While the use of broadband light waveforms enables a sub-cm spatial resolution, the waveforms are quasi CW, delaying the occurrence of non-linear effects. Coherent detection ensures direct access to the distributed phase and to the local variations of the refractive index. Moreover, an all-optical pulse compression feature enables to lower the detection bandwidth down to 10 MSa/s. Based on a bi-directional frequency shifting loop, the architecture makes use of a single CW laser, commercial telecom components, and low frequency electronics. It is expected to open new avenues in distributed acoustic sensing applications, where high spatial resolution and high interrogation rates are required.

High-resolution wide-band optical frequency comb control using stimulated Brillouin scattering

We introduce a technique to manipulate an optical frequency comb on a line-by-line basis using stimulated Brillouin scattering (SBS). The narrow-linewidth SBS process has been used to address individual lines in optical frequency combs, but previous demonstrations required a dedicated laser to modulate each comb tooth, prohibiting complete comb control. Here, we use a pair of frequency shifting fiber optic loops to generate both an optical frequency comb and a train of frequency-locked pulses that can be used to manipulate the comb via SBS. This approach enables control of the entire frequency comb using a single seed laser without active frequency locking. To demonstrate the versatility of this technique, we generate and manipulate a comb consisting of 50 lines with 200 MHz spacing. By using polarization pulling assisted SBS, we achieve a modulation depth of 30 dB. This represents a scalable approach to control large numbers of comb teeth with high resolution using standard fiber-optic components.

Laser control of a dark vibrational state of acetylene in the gas phase—Fourier transform pulse shaping constraints and effects of decoherence

We propose a methodology to tackle the laser control of a non-stationary dark ro-vibrational state of acetylene (C2H2), given realistic experimental limitations in the 7.7 μm (1300 cm−1) region. Simulations are performed using the Lindblad master equation, where the so-called Lindblad parameters are used to describe the effect of the environment in the dilute gas phase. A phenomenological representation of the parameters is used, and they are extracted from high-resolution spectroscopy line broadening data. An effective Hamiltonian is used for the description of the system down to the rotational level close to experimental accuracy. The quality of both the Hamiltonian and Lindblad parameters is assessed by a comparison of a calculated infrared spectrum with the available experimental data. A single shaped laser pulse is used to perform the control, where elements of optics and pulse shaping using masks are introduced with emphasis on experimental limitations. The optimization procedure, based on gradients, explicitly takes into account the experimental constraints. Control performances are reported for shaping masks of increasing complexity. Although modest performances are obtained, mainly due to the strong pulse shaping constraints, we gain insights into the control mechanism. This work is the first step toward the conception of a realistic experiment that will allow for population characterization and manipulation of a non-stationary vibrational “dark” state. Effects of the collisions on the laser control in the dilute gas phase, leading to decoherence in the molecular system, are clearly shown.

All-optical coherent pulse compression for dynamic laser ranging using an acousto-optic dual comb

We demonstrate a new and simple dynamic laser ranging platform based on analog all-optical coherent pulse compression of modulated optical waveforms. The technique employs a bidirectional acousto-optic frequency shifting loop, which provides a dual-comb photonic signal with an optical bandwidth in the microwave range. This architecture simply involves a CW laser, standard telecom components and low frequency electronics, both for the dual-comb generation and for the detection. As a laser ranging system, it offers a range resolution of a few millimeters, set by a dual-comb spectral bandwidth of 24 GHz, and a precision of 20 µm for an integration time of 20 ms. The system is also shown to provide dynamic measurements at scanning rates in the acoustic range, including phase-sensitive measurements and Doppler shift velocimetry. In addition, we show that the application of perfect correlation phase sequences to the transmitted waveforms allows the ambiguity range to be extended by a factor of 10 up to ∼20 m. The system generates quasi-continuous waveforms with low peak power, which makes it possible to envision long-range telemetry or reflectometry requiring highly amplified signals.

Radial Structure of OAM-Carrying Fundamental X-Waves

We investigate the spectral degree of freedom of OAM-carrying localized waves and its influence on their transverse intensity distribution. In particular, we focus our attention on exponentially decaying spectra, which are very tightly connected to fundamental X-waves; we then show how it is possible to structure their transverse intensity distribution, thus creating a radial structure similar to that of Bessel beams.

Far-field Talbot waveforms generated by acousto-optic frequency shifting loops

We report on the description of the optical fields generated by acousto-optic Frequency-Shifting Loops (FSL) in the temporal Fraunhofer domain when the loop is operated in the vicinity of integer or fractional Talbot conditions. Using self-heterodyne detection, we experimentally demonstrate the equivalence of the Talbot phases generated at fractional conditions with the Gauss perfect phase sequences, and identify deviations from the standard frequency-to-time mapping description of the far field. In particular, we show the existence of ripples in the pulse intensity, of unavoidable pulse-to-pulse interference in the pulse train, of small oscillations, of the order of hundreds of MHz, in the expected linear pulse chirp, and the capture of the phase at the pulse's trailing edge by the adjacent pulse. Using asymptotic analysis, we construct a field model that accounts for these features, which are due to corrections to the frequency-to-time mapped field created by the sharp spectral edge of the FSL spectrum, in analogy to diffraction. Practical design consequences for signal generation and processing systems based on FSL are discussed.