Cooperative emission due to spontaneous build-up and rapid decay of macroscopic polarization in a strongly inverted gain medium is drastically different from the lasing dynamics. The medium polarization and not the cavity field drives the emission from a device in semiclassical Maxwell-Bloch picture. Yet as pioneered by Dicke, the decay of the highest-energy state in a quantum ensemble of two-level systems proceeds through a ladder of the highest-symmetry partially deexcited states, each of which is formalistically an entangled state in mathematical sense. In this paper we summarize our experimental and theoretical studies on the two fundamental aspects of superradiance in multi-section tandem cavity laser heterostructures: (i) How can the superradiance be reached in semiconductor quantum wells albeit the ultrafast dephasing of individual microscopic e-h dipoles? (ii) Could the ensemble non-classicality be transferred to the emitted optical field and what could be the resulting photon state?
Mode-locked lasers, and in particular solid-state femtosecond lasers, are oscillators with a unique physics capable to exhibit extremely low-phase noise of the emitted pulse train. Here we report ultra-low phase noise microwave generation with a self-referenced, fully-stabilized mode-locked femtosecond laser. The system involves a 395-MHz repetition rate 1560nm laser which is self-referenced and whose repetition rate is locked to a cavity-stabilized continuous-wave laser. The selfreferencing is achieved with a f-3f nonlinear interferometer realized in a silicon nitride highly-nonlinear waveguide. Prior to optical-to-electrical conversion for X-band microwave generation, the 395-MHz repetition rate of the mode-locked laser pulse train is multiplied in a fiber interleaver to 3.16 GHz. A high-power handling photodiode converts the optical pulse train to an electrical frequency comb with 3.16 GHz frequency spacing. Finally, the 9.5-GHz harmonics is bandpassfiltered and phase noise measurements have shown a record-low phase noise floor of -175 dBc/Hz at 1-MHz offset frequency.
We report a novel ultra-short light pulse emitters utilizing transient charge carrier behaviour in a multiple wide-quantumwell (WQW) heterostructure. The optical waveguide is implemented as a tandem-cavity laser diode with electro absorber section in the middle, surrounded by two end-firing gain sections. The ultrashort pulse production is achieved by employing the gain region with three wide GaAsP tensile strained quantum wells separated by GaInP barriers in an unintentionally doped active region of the p-i-n laser diode structure. At large negative absorber bias, lasing emission spiking starts with an unusually long delay of 7 μs. By applying the current pulses of duration smaller than 7 μs it is possible to quench entirely the lasing emission. With selection of the parameters of the electrical pump pulse and the absorber voltage it is possible to obtain ultra-short light pulse regime. This optical pulse appears at the end of the electrical pump pulse, as a single optical pulse on top of wide pedestal, due to amplified spontaneous emission. The duration of the pulse is 1.2 ps and pulse energy is 80 pJ. We attribute this behaviour to quantum confined Stark effect. Removal of the external bias field, enabling stronger overlap of carriers yields a sudden increase in the radiative recombination rate and optical gain enabling SR emission. We provide a detailed report on the pulse width and optical spectral behaviour as well as on possible nonclassical correlation in the emitted light state seen from comparison to CW lasing regime.
European Space Agency (ESA) considers Mode-Locked Semi-Conductor Lasers as a promising technology for precision metrology systems in space such as High Accuracy Absolute Long Distance Measurement. We report our progress towards challenging ESA requirements: picosecond pulse duration, pulse energy 200 pJ, Pulse Repetition Frequency (PRF) 1-3 GHz, PRF stability < 5·10-9 at 1 second and PRF tunability 20 MHz. The laser should have small power consumption, be compact and robust against launch vibrations. We have reported in the past two such mode-locked (ML) laser diodes, each reaching only 90 pJ pulse energies: (i) very long (13.5mm) monolithic tapered laser and (ii) inverse bow-tie external cavity (EC) laser. The subject of the present communication is a novel passively mode-locked monolithic tapered laser achieving 201 pJ pulses. Large optical cavity with 2QWs heterostructure provides a low internal loss (~1 cm-1) together with high quantum efficiency (< 90 %) and low series resistance. To reach high energy output pulses, the tapered gain section gets a low (< 0.1 %) reflectivity dielectric coating. For passive mode-locking at fundamental cavity frequency, the saturable electroabsorber section is located at the back side of the gain chip with a high reflectivity coating (< 95 %). The monolithic cavity is made 13.5mm long by introducing an intermediate section for PRF tuning around 3 GHz. We reached passive ML at 2.9 GHz PRF with pulse energy of 201 pJ, compressed pulse width of 2.6 ps and electric power consumption of 8.2 W. PRF can be continuously tuned by 9.8 MHz. Active current modulation for hybrid ML resulted in PRF relative stability at 9.16·10-10 level on 1s intervals, while with a phase lock loop (PLL) acting on the DC gain section current we reached PRF stability of 1.15·10-10 on 1 s measurement interval.
Imaging with non-classical photons allows to bypass the Rayleigh resolution limit and classical shot-noise level. One step towards imaging demonstration with large photon numbers is the separation of non-classical photon states from the classical photons, thus increasing dynamic range and signal to background contrast on the detector. We demonstrate the feasibility of such separation by an échelle grating at high diffraction orders. In our demonstration, a PPKTP crystal generates entangled photon pairs in type-0 SPDC. The crystal is cw pumped and produces non-collinear degenerated photon pairs at 810nm. The classical light states are produced by a VCSEL at nearly same wavelength. After diffraction on echelle grating, the spatial far-field patterns and the photon arrival times are recorded by a novel 32×32 SPAD array sensor with 160 ps timing resolution. It allows real-time monitoring of the first- and second order correlation patterns. Within the observation window, we detected correlated biphoton arrivals in the four diffraction orders corresponding to their de Broglie wavelength, which is a half of the classical wavelength. Respectively a half of these diffraction orders is prohibited for classical photons. Placing a slit mask in these orders allows us to transmit only non-classical photon state and block the classical ones. We report on a series of experiments elucidating spatial and temporal correlations at the output of such quantum –classical photon discriminator. Those results could be used for the separation of biphotons from classical photons at the same wavelength in high-intensity light sources.
We report on multi-section inverse bow-tie laser producing mode-locked pulses of 90 pJ energy and 6.5 ps width (895 fs after compression) at 1.3 GHz pulse repetition frequency (PRF) and consuming 2.9 W of electric power. The laser operates in an 80 mm long external cavity. By translation of the output coupling mirror, the PRF was continuously tuned over 37 MHz range without additional adjustments. Active stabilization with a phase lock loop actuating on the driving current has allowed us to reach the PRF relative stability at a 2·10-10 level on 10 s intervals, as required by the European Space Agency (ESA) for inter-satellite long distance measurements.
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