M. Calligaro

Phase-resolved measurements of stimulated emission in a laser

Lasers are usually described by their output frequency and intensity. However, laser operation is an inherently nonlinear process. Knowledge about the dynamic behaviour of lasers is thus of great importance for detailed understanding of laser operation and for improvement in performance for applications. Of particular interest is the time domain within the coherence time of the optical transition. This time is determined by the oscillation period of the laser radiation and thus is very short. Rigorous quantum mechanical models predict interesting effects like quantum beats, lasing without inversion, and photon echo processes. As these models are based on quantum coherence and interference, knowledge of the phase within the optical cycle is of particular interest. Laser radiation has so far been measured using intensity detectors, which are sensitive to the square of the electric field. Therefore information about the sign and phase of the laser radiation is lost. Here we use an electro-optic detection scheme to measure the amplitude and phase of stimulated radiation, and correlate this radiation directly with an input probing pulse. We have applied this technique to semiconductor quantum cascade lasers, which are coherent sources operating at frequencies between the optical (>100 THz) and electronic (<0.5 THz) ranges. In addition to the phase information, we can also determine the spectral gain, the bias dependence of this gain, and obtain an insight into the evolution of the laser field.

InP based lasers and optical amplifiers with wire-/dot-like active regions

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 µm. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelling. (Some figures in this article are in colour only in the electronic version)

Gain and noise saturation of wide-band InAs-InP quantum dash optical amplifiers: model and experiments

We present a theoretical model for gain and noise saturation in quantum dash (QDash) semiconductor optical amplifiers. The model is based on the density matrix formalism and addresses static saturation spectra. The calculations are confirmed by a series of experiments which highlight the unique properties of these amplifiers. We demonstrate a high gain, a wide bandwidth, and high saturation power. The saturation spectrum is shown to be asymmetric, emphasizing saturation at short wavelength. The asymmetry stems from the high energy tail of the density of state function in those quantum wire (QWire) like gain media as well as from the interactions with the wetting layer.

Improved CW operation of GaAs-based QC lasers: T/sub max/= 150 K

We present a substantial improvement in the CW performance of GaAs-based quantum cascade lasers with operation up to 150 K. This has been achieved through suitable changes in device processing of a well-characterized laser. The technology optimizes the current injection in the laser by reducing the size of the active stripe whilst maintaining a strong coupling of the optical mode to preserve low current densities. The reduction of total dissipated power is critical for these lasers to operate CW. At 77 K, the maximum CW optical power is 80 mW, threshold current is 470 mA, slope efficiency is 141 mW/A, and lasing wavelength /spl lambda//spl sim/10.3 /spl mu/m.

High-power room temperature emission quantum cascade lasers at /spl lambda/=9 /spl mu/m

We present two different techniques for processing InP-based /spl lambda/=9 /spl mu/m quantum cascade lasers which improve the thermal dissipation in the device. The first process is based on hydrogen implantation creating an insulating layer to inject current selectively in one part of the active region. The second process uses a thick electroplated gold layer on the laser ridge to efficiently remove the heat produced in the active region. Each process is designed to improve heat evacuation leading to higher performances of the lasers and will be compared to a standard ridge structure from the same wafer. We give evidence that the process of proton implantation, efficient in GaAs based structures, is not directly applicable to InP based devices and we present a detailed analysis of the thermal properties of devices with an electroplated gold thick layer. With these lasers, an average power of 174 mW at a duty cycle of 40% has been measured at 10/spl deg/C.

Longitudinal spatial hole burning in terahertz quantum cascade lasers

The propagation of a quasi-single-cycle terahertz pulse through an active terahertz quantum cascade laser is studied. The terahertz pulse is found to be diffracted on the optically induced longitudinal modulation of the refractive index of the laser active region. This modulation is a result of longitudinal spatial hole burning due to the formation of a standing wave in the laser cavity.

Gain, index variation, and linewidth-enhancement factor in 980-nm quantum-well and quantum-dot lasers

An experimental comparative study of the gain, index variation, and linewidth enhancement factor in 980-nm quantum-well (QW) and quantum-dot (QD) lasers structures, designed for high power applications, is presented. The gain spectra of the QW lasers at high injection level revealed three different transition energies, with a low linewidth enhancement factor (/spl sim/1.2) for E2HH2 transitions. Similar values for the linewidth enhancement factor, ranging between 2.5 and 4.5, were found for QW and QD devices, when comparing at similar values of the peak gain. This result is attributed to the contribution of excited state transitions in the measured QD lasers.

High brightness GaInAs/(Al)GaAs quantum-dot tapered lasers at 980 nm with high wavelength stability

High brightness (2 W with M2=3.4) is demonstrated at 980 nm using a gain-guided tapered GaInAs/(Al)GaAs quantum-dot laser. A remarkable low temperature shift (0.09 nm/K) of the emission wavelength is observed. Moreover, at 20 °C, the emission wavelength is quasiconstant as a function of the injected current.

High-power and high-brightness laser diode structures using Al-free active region

High bit-rate, WDM, networks are reliant on Er or Er/Yb doped fiber amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed several 980 nm laser diode structures at 980 nm, using an Aluminum free active region and standard AR/HR coatings on the facets. Our laser show low optical losses, low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of small angle index guided tapered laser diodes (emissive width of 3 mm) with an optical output power of 20W at 33A under CW operation (25°C). The far field of the slow axis has a Gaussian single lobed shape, with a FWHM of 3.5° at maximum power, which is two times less than obtained with multimode broad area lasers. With such a device, we expect to couple 10W into a 100μm diameter fiber. We also demonstrate a large aperature gain-guided tapered laser with an output power of 2.4W and a calculated M21/c2 = 3, the M21/c2 factor being calculated with the method based on measurements of the fields profiles widths at 1/c2.

High-power and high-brightness laser diode structures at 980 nm using Al-free materials

High bit rate, WDM, networks use intensively Er or Er/Yb doped fibre amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed different structures of laser diodes at 980nm, using Aluminium free materials. Our laser structure shows low optical losses together with a low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of broad area laser diodes (emissive width of 2.6mm) with an optical output power of 19W at 25A under CW operation. We have also developed a mini-bar of small angle index guided tapered laser diodes (W=2.6mm). We demonstrate 17W at 27.6A under CW operation at 20 degree(s)C. Slow axis far field has a Gaussian single mode shape, with a FWHM of 3.3 degree(s) (at 15A, 11W), which is two times less than obtained on multimode broad area lasers. With such a device, we expect to couple 10W into a 100micrometers diameter fiber. We also demonstrate, on a large aperture gain-guided tapered laser, an output power of 1.3W with an M2 of 3.3