Temperature stability of static and dynamic properties of 1.55 µm quantum dot lasers.

Static and dynamic properties of InP-based 1.55 µm quantum dot (QD) lasers were investigated. Due to the reduced size inhomogeneity and a high dot density of the newest generation of 1.55 µm QD gain materials, ridge waveguide lasers (RWG) exhibit improved temperature stability and record-high modulation characteristics. Detailed results are shown for the temperature dependence of static properties including threshold current, voltage-current characteristics, external differential efficiency and emission wavelength. Similarly, small and large signal modulations were found to have only minor dependences on temperature. Moreover, we show the impact of the active region design and the cavity length on the temperature stability. Measurements were performed in pulsed and continuous wave operation. High characteristic temperatures for the threshold current were obtained with T0 values of 144 K (15 - 60 °C), 101 K (60 - 110 °C) and 70 K up to 180 °C for a 900-µm-long RWG laser comprising 8 QD layers. The slope efficiency in these lasers is nearly independent of temperature showing a T1 value of more than 900 K up to 110 °C. Due to the high modal gain, lasers with a cavity length of 340 µm reached new record modulation bandwidths of 17.5 GHz at 20 °C and 9 GHz at 80 °C, respectively. These lasers were modulated at 26 GBit/s in the non-return to zero format at 80 °C and at 25 GBaud using a four-level pulse amplitude format at 21 °C.

Static and dynamic characteristics of an InAs/InP quantum-dot optical amplifier operating at high temperatures.

We report on high quality InAs/InP quantum dot optical amplifiers for the 1550 nm wavelength range operating over a wide temperature range of 25 to 100 °C. A temperature dependent shift of the peak gain wavelength at a rate of 0.78 nm/K is observed. Consequently, two possible modes of operation are performed for a systematic device characterization over the entire temperature range. In the first mode, the signal wavelength is tuned to always match the peak gain wavelength while in the second mode, the signal wavelength is kept constant as the gain spectrum shifts with the temperature. Static characteristics, such as gain spectra and saturation levels, as well as dynamical properties, are presented. Distortion-less amplification of a single 28 Gbit/s signal and cross-talk free amplification of two channels, detuned by 2 nm, were demonstrated over the entire temperature range.

Optical control of capacitance in a metal-insulator-semiconductor diode with embedded metal nanoparticles.

This paper describes a metal-insulator-semiconductor (MIS) capacitor with flat capacitance voltage characteristics and a small quadratic voltage capacitance coefficient. The device characteristics resemble a metal-insulator-metal diode except that here the capacitance depends on illumination and exhibits a strong frequency dispersion. The device incorporates Fe nanoparticles (NPs), mixed with SrF2, which are embedded in an insulator stack of SiO2 and HfO2. Positively charged Fe ions induce dipole type traps with an electronic polarization that is enhanced by photogenerated carriers injected from the substrate and/or by inter nanoparticle exchange of carriers. The obtained characteristics are compared with those of five other MIS structures: two based on Fe NPs, one with and the other without SrF2 sublayers. Additionally, devices contain Co NPs embedded in SrF2 sublayers, and finally, two structures have no NPs, with one based on a stack of SiO2 and HfO2 and the other which also includes SrF2. Only structures containing Fe NPs, which are incorporated into SrF2, yield a voltage independent capacitance, the level of which can be changed by illumination. These properties are essential in radio frequency/analog mixed signal applications.

Optical efficiency and gain dynamics of modelocked semiconductor disk lasers.

Compact optically pumped passively modelocked semiconductor disk lasers (SDLs) based on active quantum wells (QWs) such as vertical external-cavity surface-emitting lasers (VECSELs) or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) are wavelength-versatile sources that offer a unique combination of gigahertz pulse repetition rates and short pulse durations. In this paper, we present record-short pulses of 184 fs from a gigahertz MIXSEL emitting at a center wavelength of 1048 nm. This result comes at the expense of low optical-to-optical pump efficiency (<1%) and average output power limited to 115 mW. We experimentally observe that shorter pulses significantly reduce the macroscopic gain saturation fluence and develop a QW model based on rate equations to reproduce the gain saturation behavior and quantitatively explain the VECSEL and MIXSEL modelocking performances. We identify spectral hole burning as the main cause of the reduced gain at shorter pulse durations, which in combination with the short lifetime of the excited carriers strongly reduces the optical pump efficiency. Our better understanding will help to address these limitations in future ultrafast SDL designs.

Phase sensitive parametric fiber amplifier for the 2 µm wavelength range.

A widely tunable phase sensitive parametric fiber amplifier employing a three fiber stages configuration and operating in the 2 µm wavelength region is demonstrated. Phase sensitive gain levels of 30 dB and a gain variation of 20 dB were measured for a pulsed pump by determining the conversion efficiency near 2 µm when a signal at 1.281 µm was applied. The amplifier operates in the wavelength range of 1952 nm to 2098 nm, with its bandwidth being around 0.1 nm. The bandwidth can be controlled by the fiber lengths and their dispersion properties.

Narrowband optical parametric amplification measurements in Ga0.5In0.5P photonic crystal waveguides.

We report the first demonstration of narrowband parametric amplification in a chip scale semiconductor waveguide. A dispersion engineered, Ga0.5In0.5P photonic crystal waveguide with a dispersion function that exhibits two zero crossings was used with a pulsed pump placed in the normal dispersion regime while a tunable probe was scanned on either side of the pump. A peak conversion efficiency of -10 dB was obtained with a peak pump power of only 650 mW. The narrowband nature of the gain spectrum was clearly demonstrated.

Coherent control in a semiconductor optical amplifier operating at room temperature.

Quantum decoherence times in semiconductors are extremely short, particularly at room temperature where the quantum phase is completely erased in a fraction of a picosecond. However, they are still of finite duration during which the quantum phase is well defined and can be tailored. Recently, we demonstrated that quantum coherent phenomena can be easily accessed by examining the phase and amplitude of an optical pulse following propagation along a room temperature semiconductor optical amplifier. Taking the form of Rabi oscillations, these recent observations enabled to decipher the time evolution of the ensemble states. Here we demonstrate the Ramsey analogous experiment known as coherent control. Remarkably, coherent control occurs even under room temperature conditions and enables to directly resolve the dephasing times. These results may open a new way for the realization of room temperature semiconductor-based ultra-high speed quantum processors with all the advantages of upscaling and low-cost manufacturing.

Dual-pump parametric amplification in dispersion engineered photonic crystal waveguides.

This paper describes a numerical simulation of narrow band parametric amplification in dispersion engineered photonic crystal waveguides. The waveguides we analyze exhibit group velocity dispersion functions which cross zero twice thereby enabling many interesting pumping schemes. We analyze the case of two pulsed pumps each placed near one of the zero dispersion wavelengths. These configurations are compared to conventional single pump schemes. The two pumps may induce phase matching conditions in the same spectral location enabling to control the gain spectrum. This is used to study the gain and fidelity of 40 G bps NRZ data signals.

Parametric gain in dispersion engineered photonic crystal waveguides.

We present a numerical simulation of parametric gain properties in GaInP PhC dispersion engineered waveguides in which the group velocity dispersion crosses zero twice and where the pump and the signal are 100 ps pulses. The simulations use the M-SSFT algorithm which incorporates dispersive nonlinear coefficients and losses. We concentrate on narrow band parametric gain which occurs for pump wavelengths in the normal group velocity dispersion regime. The effects of structural details, of pump wavelength and of losses are carefully analyzed.

Nonlinear pulse propagation in a quantum dot laser.

We investigate the nonlinear propagation of an ultra-short, 150 fs, optical pulse along the waveguide of a quantum dot (QD) laser operating above threshold. We demonstrate that among the various nonlinear processes experienced by the propagating pulse, four-wave mixing (FWM) between the pulse and the two oscillating counter-propagating cw fields of the laser is the dominant one. FWM has two important consequences. One is the creation of a spectral hole located in the vicinity of the cw oscillating frequency. The width of the spectral hole is determined by an effective carrier and gain relaxation time. The second is a modification of the shape of the trailing edge of the pulse. The wave mixing involves first and second order processes which result in a complicated interaction among several fields inside the cavity, some of which are cw while the others are time varying, all propagating in both directions. The nonlinear pulse propagation is analyzed using two complementary theoretical approaches. One is a semi-analytical model which considers only the wave mixing interaction between six field components, three of which propagate in each direction (two cw fields and four time-varying signals). This model predicts the deformation of the tail of the output signal by a secondary idler wave, produced in a cascaded FWM process, which co-propagates with the original injected pulse. The second approach is a finite-difference time-domain simulation, which considers also additional nonlinear effects, such as gain saturation and self-phase modulation. The theoretical results are confirmed by a series of experiments in which the time dependent amplitude and phase of the pulse after propagation are measured using the cross-frequency-resolved optical gating technique.

Narrowband phase sensitive fiber parametric amplifier.

We describe a widely tunable phase sensitive fiber amplifier, based on narrowband parametric amplification in dispersion shifted fiber. Using a fiber with a zero dispersion wavelength of 1549 nm and a pump wavelength in the range of 1549 nm to 1532 nm, we obtained phase sensitive amplification between 1344 nm and 1781 nm, for an overall wavelength range of 437 nm. The amplifier threshold power is 7 W, and the maximum gain is 50 dB at a pump peak power of 25 W. The variance in gain due to phase sensitivity was measured to be up to 15 dB.

Narrowband optical parametric gain in slow mode engineered GaInP photonic crystal waveguides.

We predict narrowband parametric amplification in dispersion-tailored photonic crystal waveguides made of gallium indium phosphide. We use a full-vectorial model including the dispersive nature both of the nonlinear response and of the propagation losses. An analytical formula for the gain is also derived.

Kerr-induced all-optical switching in a GaInP photonic crystal Fabry-Perot resonator.

We describe nonlinear properties of a GaInP photonic crystal Fabry-Perot resonator containing integrated reflectors. The device exhibits an extremely large static nonlinearity due to a thermal effect. Dynamical measurements were used to discriminate between the thermal and Kerr contributions to the nonlinearity. The high frequency nonlinear response is strictly due to the Kerr effect and the efficiency is similar to that obtained in self-phase modulation and four wave mixing experiments. The waveguide dispersion and the wavelength dependent integrated reflectors yield a series of transmission peaks with varying widths which determine the maximum speed at which the device can operate. Switching and wavelength conversion experiments with 92ps and 30ps wide pulses were demonstrated using pulse energies of a few pJ. The switching process is Kerr dominated with the fundamental response being essentially instantaneous so that the obtainable switching speed is strictly determined by the resonator structure.

Efficient parametric interactions in a low loss GaInP photonic crystal waveguide.

We describe time domain characterizations of dynamic four-wave mixing in a low loss modified W1 GaInP photonic crystal waveguide. Using 32 ps wide pump pulses with peak powers of up to 1.1 W we achieved a very large conversion efficiency of -6.8 dB as well as a 1.3 dB parametric gain experienced by a weak CW probe signal. Time domain simulations confirm quantitatively all the measured results.

Time domain switching/demultiplexing using four wave mixing in GaInP photonic crystal waveguides.

We describe dynamical four wave mixing (FWM) functionalities of an GaInP photonic crystal waveguide. A W1 waveguide was used to wavelength convert 100 ps pulses and for sampling a 10.56 Gbit/s data stream so as to time demultiplex it into 16 or 32 channels. In all cases, the extracted pulses at the idler wavelength are undistorted and have a high signal to noise ratio proving the high efficiency and the versatility of the FWM process in the GaInP PhC waveguides we used.

Fiber parametric oscillator for the 2 µm wavelength range based on narrowband optical parametric amplification.

We describe a widely tunable synchronously pumped coherent source based on the process of narrowband parametric amplification in a dispersion-shifted fiber. Using an experimental fiber with a zero-dispersion wavelength of 1590 nm and pump wavelengths of 1530 to 1570 nm yields oscillations at 1970 to 2140 nm-the longest reported wavelength for a fiber parametric oscillator. The long-wavelength oscillations are accompanied by simultaneous short-wavelength oscillations at 1200 to 1290 nm. The parametric gain is coupled to stimulated Raman scattering. For parametric oscillations close to the Raman gain peak, the two gain processes must be discriminated from each other. We devised two configurations that achieve this discrimination: one is based on the exploitation of the difference in group delay between the wavelengths where Raman and parametric gain peak, and the other uses intracavity polarization tuning.

Highly efficient four wave mixing in GaInP photonic crystal waveguides.

We report highly efficient four wave mixing in a GaInP photonic crystal waveguide. Owing to its large bandgap, the ultrafast Kerr nonlinearity of GaInP is not diminished by two photon absorption and related carrier effects for photons in the 1550 nm range. A four-wave-mixing efficiency of -49 dB was demonstrated for cw pump and probe signals in the milliwatt range, while for pulsed pumps with a peak power of 25 mW the conversion efficiency increased to -36 dB. Measured conversion efficiency dependencies on pump probe detuning and on pump power are in excellent agreement with a simple analytical model from which the nonlinear parameter gamma is extracted. Gamma scales approximately with the square of the slow down factor and varies from 800 W(-1) m(-1) at a pump wavelength lambda(p)=1532 nm to 2900 W(-1) m(-1) at lambda(p)=1550 nm. These values are consistent with those obtained from self phase modulation experiments in similar devices.

Resonance enhanced large third order nonlinear optical response in slow light GaInP photonic-crystal waveguides.

We report a large nonlinear response in a 1.3mm long GaInP photonic crystal waveguide. The wide band gap of GaInP (1.9 eV) ensures that no two photon absorption takes place for photons at 1.55mm improving the nonlinear performance. The nonlinearity is enhanced by a resonance effect due to the waveguide end facet reflectivities as well as by the low group velocity exhibited by the waveguide. A low CW input pump power of approximately 2mW causes a very large change in the nonlinear refractive index coefficient which manifests itself in a large, approximately p /3 phase shift in the Fabry Perot fringes. The extracted effective nonlinear coefficient g varies from 3.4 x 105W-1m-1 at short wavelengths to 2.2 x 106W-1m-1 near the band edge. These values are several orders of magnitude larger than those obtained in reported nonlinear experiments which exploit the Kerr effect. We postulate therefore that the observed nonlinearity is due to a hybrid phenomenon which combines the Kerr effect and an index change which is induced by local heating that results from the residual linear absorption. The efficient nonlinear phase shift was also exploited in a fast dynamic experiment where we demonstrated wavelength conversion with 100ps wide pulses proving the potential for switching functionalities at multi GHz rates. The index change required for this switching experiment can not be obtained, at the power levels used here, with a g value of a few thousands W-1m-1 which is a typical Kerr coefficient in similar waveguides. Hence, we conclude that the hybrid nonlinearity is sufficiently fast to enable switching with a time scale of at least 100ps.

Near field imaging of a GaAs photonic crystal cavity and waveguide using a metal coated fiber tip.

We report optical near field characterization of a GaAs photonic crystal waveguide which is side coupled to a nano cavity. We observe the effect of the metal coated probe on the resonance wavelength and the intensity distribution. The measurements fit well to finite difference time domain simulations.

Direct observation of the coherent spectral hole in the noise spectrum of a saturated InAs/InP quantum dash amplifier operating near 1550 nm.

We demonstrate a direct observation of the coherent noise spectral hole in a saturated quantum dash amplifier. Its width 500-600 GHz is determined by the response time and is responsible for high speed regeneration properties.