There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Recent developments in chip-based nonlinear photonics offer the tantalizing prospect of realizing many applications that can use optical frequency comb devices that have form factors smaller than 1 cm3 and that require less than 1 W of power. A key feature that enables such technology is the tight confinement of light due to the high refractive index contrast between the core and the cladding. This simultaneously produces high optical nonlinearities and allows for dispersion engineering to realize and phase match parametric nonlinear processes with laser-pointer powers across large spectral bandwidths. In this Review, we summarize the developments, applications and underlying physics of optical frequency comb generation in photonic-chip waveguides via supercontinuum generation and in microresonators via Kerr-comb generation that enable comb technology from the near-ultraviolet to the mid-infrared regime. This Review discusses the developments and applications of on-chip optical frequency comb generation based on two concepts—supercontinuum generation in photonic-chip waveguides and Kerr-comb generation in microresonators.

Photonic-chip-based frequency combs

Zakljucno, potrebno je i nadalje svakodnevno raditi na prevenciji demencije, osiguravati resurse i osmisljavati strategije borbe protiv demencije, jer ce broj osoba s demencijom i u Hrvatskoj u buducnosti biti veci. No, s obzirom da se u svijetu ulažu znatna sredstva u podrucju istraživanja demencije, za ocekivati je pozitivnih pomaka, kako na podrucju terapije, rane dijagnostike, tako i na podrucju rehabilitacije osoba s demencijom. Stoga - vrijeme je na nasoj strani!

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Dementia: a public health priority

Integration of the whole mode-locked laser onto a single piece of semiconductor offers a number of advantages, including total elimination of optical alignment processes, improved mechanical stability, and the generation of short optical pulses at much higher repetition frequencies. Semiconductor laser processing technologies were used to implement the colliding-pulse mode-locking (CPM) scheme, which is known to effectively shorten the pulses and increase stability, on a miniature monolithic semiconductor cavity. The principles of and recent progress in monolithic CPM quantum-well lasers are reviewed. >

Monolithic colliding-pulse mode-locked quantum well lasers

Lasers have both ubiquitous applications and roles as model systems in which non-equilibrium and cooperative phenomena can be elucidated1 The introduction of novel concepts in laser operation thus has potential to lead to both new applications and fundamental insights2 Spintronics3, in which both the spin and the charge of the electron are used, has led to the development of spin-lasers, in which charge-carrier spin and photon spin are exploited Here we show experimentally that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 gigahertz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers Surprisingly, this ultrafast operation of the resultant spin-laser relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both of which are commonly viewed as detrimental in spintronics3 and conventional lasers4 Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication Room-temperature modulation frequencies exceeding 200 GHz are demonstrated in birefringent semiconductor spin-lasers by coupling the spin of the charge carriers to the light polarization

Ultrafast spin-lasers.

We report the achievement of colliding pulse mode-locked (CPM) regimes on a novel on-chip mode locked laser diode (OCCP-MLLD). The advantage of the resonator structure that we present is that the end-mirrors are defined through multimode interference reflectors (MIRs), which provide precise control of the cavity length avoiding the need for cleaved facets. This simplifies positioning the saturable absorber at the center of the resonator to achieve the colliding pulse mode-locked regime and double the repetition rate, reaching the millimeter wave frequency range. An additional advantage is that the pulsed output is delivered within the Photonic Integrated Circuit chip for further processing (i.e. modulation). We demonstrate a colliding pulse passive mode locked regime with pulse widths below a picosecond (Δτ = 0.64 ps), timing jitter σT = 75 fs and amplitude noise NAM = 0.012 dBc. The samples were fabricated in a generic InP foundry service through multi-project wafer (MPW) runs.

/pdf/on-chip-colliding-pulse-mode-locked-laser-diode-occp-mlld-13ubtvl6ly.pdf

On-Chip Colliding Pulse Mode-locked laser diode (OCCP-MLLD) using multimode interference reflectors

This Letter presents a photonics-based millimeter wave and terahertz frequency synthesizer using a monolithic InP photonic integrated circuit composed of a mode-locked laser (MLL) and two pulse interleaver stages to multiply the repetition rate frequency. The MLL is a multiple colliding pulse MLL producing an 80 GHz repetition rate pulse train. Through two consecutive monolithic pulse interleaver structures, each doubling the repetition rate, we demonstrate the achievement of 160 and 320 GHz. The fabrication was done on a multi-project wafer run of a generic InP photonic technology platform.

/pdf/mode-locked-laser-with-pulse-interleavers-in-a-monolithic-uyq17sv7x7.pdf

Mode-locked laser with pulse interleavers in a monolithic photonic integrated circuit for millimeter wave and terahertz carrier generation.

We report for the first time to the best of our knowledge, an on-chip multiple colliding pulse mode-locked semiconductor laser source. The device structure is fully integrated, replacing cleaved facet mirrors by using multimode interference reflectors; this allows us to precisely control the location of the saturable absorbers within the cavity length, which is critical to achieve the multiple colliding regime. In this paper, we succeeded to achieve this regime generating a repetition rate at four times the fundamental round-trip frequency, demonstrating a repetition rate within the millimeter wave frequency range, at 100 GHz using a 25 GHz resonator cavity length. We also demonstrate the advantage of having the signal on-chip including a boost semiconductor optical amplifier in order to increase the output optical power. This novel structure was fabricated on a generic InP photonic integrated technology through a multi-project wafer run.

On-Chip Multiple Colliding Pulse Mode-Locked Semiconductor Laser

This paper reviews the current state of the art of photonic-enabled generation of radio-frequency signals with frequencies within the millimeter wave range (30 to 300 GHz) and above using photonic-integrated circuits (RF-PICs). One of the most important applications to date is the generation of carrier wave frequencies for ultrabroadband wireless communications systems, with data rates up to 100 Gb/s. Among the different photonic signal generation techniques that are available, we focus on the approaches for which photonic integrated solutions have been explored. Optical heterodyning is first presented, based on achieving the integration of a dual wavelength sources. The second approach is through onchip integrated mode locked lasers, with excellent performance in terms of frequency stable, low phase-noise narrow linewidth sources. We review the different laser structures that have been reported, to support the advantages of the new structures that we propose.

Photonic Integrated Circuits for Radio-Frequency Signal Generation

A record low-repetition rate from an on-chip monolithic InP extended cavity colliding-pulse mode-locked laser is experimentally reported. The device, fabricated in generic InP-based active-passive integration technology, makes use of integrated mirrors to enable its use as a building block within a photonic integrated circuit. This structure allows us to generate an electrical frequency comb with mode spacing of 1 GHz, determined by the 40.5 mm long resonator. Passive and hybrid mode-locking regime conditions are experimentally demonstrated. In the passive regime, an electrical beat tone at the fundamental repetition rate with an electrical linewidth (LW) of 398 kHz and a signal-to-noise ratio (SNR) >30  dB is measured. In the hybrid regime, the optical comb is locked to a continuous wave signal source, improving the LW of the generated signal and the SNR>40  dB.

Carlos Gordon

Papers

On-Chip Colliding Pulse Mode-locked laser diode (OCCP-MLLD) using multimode interference reflectors

Mode-locked laser with pulse interleavers in a monolithic photonic integrated circuit for millimeter wave and terahertz carrier generation.

On-Chip Multiple Colliding Pulse Mode-Locked Semiconductor Laser

Photonic Integrated Circuits for Radio-Frequency Signal Generation

1 GHz InP on-chip monolithic extended cavity colliding-pulse mode-locked laser