Ultrafast Saturable Absorber Research Articles

Ultra-fast quantum-well saturable absorber devices and their application to all-optical regeneration of telecommunication optical signals

We review recent advances in the investigation of vertical cavity saturable absorber devices and their use for the all-optical regeneration of telecommunication signals in ultra-long-haul transmission systems. Such devices are polarization-insensitive and operate as fully passive nonlinear optical elements. Two approaches for obtaining fast recovery absorber materials are described, relying upon ion irradiation or upon iron doping. The vertical micro-cavity devices are designed so as to optimize the switching contrast and the operating power. Their functional behaviour as extinction ratio amplifiers has been characterized and their optimal operating conditions have been determined. The potential application of these devices to all-optical regeneration has been investigated through numerical simulations and fully demonstrated in several long-distance transmission loop experiments, with results obtained at 10, 20 and 40 Gbit/s, showing significant improvements in system haul or operational margins. A four-channel fibered module has also been fabricated, as a perspective towards the development of wavelength division multiplexing (WDM) saturable absorber modules.

Numerical model of the dynamic absorption variation in QW-EAM for ultrafast all-optical signal processing

Noise reduction techniques, such as all-optical 3R regeneration, can be realised using ultrafast saturable absorbers, both for the clock recovery block and for the reshaping block. Electroabsorption modulators (EAM) can be used as saturable absorbers when they are traversed by a strong pump optical pulse, which can reduce the absorption experienced by a second probe signal that propagates along the device together with the pump: this feature is called cross-absorption modulation (XAM). The authors present a novel numerical model to evaluate the dynamic behaviour of a quantum well EAM absorption, that can be applied in all-optical signal processing schemes for ultrafast transmission systems. The effects of both electric field screening and exciton saturation are taken into account. Numerical results agree with the experimental device behaviour reported in the literature.

Ultimate performances of an all-optical sampler based on ultrafast saturable absorbers created by heavy ion irradiation

We analyze the ultimate performances of an all-optical sampler based on ultrafast bulk GaAs saturable absorbers created by ion irradiation. The ion irradiation technique provides a unique way to adapt the sampler resolution to the optical pulse width since the absorber relaxation time can be adjusted over more than three decades with the irradiation dose. Subpicosecond time resolutions with pulse contrast of /spl sim/100 are achievable by using high irradiation doses. In standard conditions, the sampler noise performances are limited by the detector noise and sampling pulse amplitude fluctuations. Picosecond laser diode pulses with an average power of less than one microwatt are already measurable. The ultimate limit is imposed by the absorber luminescence shot noise.

Comparison of light- and heavy-ion-irradiated quantum-wells for use as ultrafast saturable absorbers

We have compared light- and heavy-ion irradiation of InGaAs/InAlAs multiple-quantum wells for ultrafast saturable absorption applications. Under heavy-ion impacts, defect clusters were produced, as observed via transmission electronic microscopy. By contrast, in proton-irradiated samples, only point defects were formed. Nonlinear absorption measurements were performed with excitonic resonance pumping. The relaxation time of absorption saturation (minimum value 2 ps) did not depend on the irradiating ion, and was practically independent of the pulse repetition rate (up to 10 GHz) and optical excitation fluence (0.1 mJ/cm2). We conclude that irradiating multiple-quantum wells with light ions is as effective as using heavy ions, when fabricating ultrafast saturable absorber devices operating at high bit rate and near bandedge wavelength.

Low-temperature-grown surface-reflection all-optical switch (LOTOS)

The low-temperature-grown surface-reflection all-optical switch, or LOTOS, is an ultrafast saturable absorber made with Be-doped strained InGaAs/InAlAs MQWs. It's operation principle is the carrier-induced change in the excitonic absorption of the MQWs. Its ultrafast operation (250 fs) is a result of the MQW region being grown at a low temperature (200 °C) and doped with Be. Large optical nonlinearity is obtained by compressively straining the quantum wells and using a reflection geometry combined with a low-reflectivity DBR: a so-called asymmetric Fabry–Perot etalon with an extremely low finesse. All of these things enable ultrafast all-optical switching operation with a high on/off ratio, a wide range of operating wavelengths, and polarization insensitivity. A spin-polarization scheme results in on/off ratios more than 20 dB better than those obtained with the conventional scheme, and handling the two orthogonal components of the signal pulse at two different points makes the switch operation polarization insensitive. Tests confirm that the LOTOS offers an ultrafast demultiplexer capable of simultaneous multi-output operation future high-speed optical communications systems.

Picosecond semiconductor lasers with a multisection saturable absorber, fabricated by heavy ion implantation

A method of implanting high-energy heavy ions across the emitter layer of a semiconductor laser has been developed to create distributed regions of ultrafast saturable absorber integrated into the cavity of a quantum-well laser. This method was used to fabricate picosecond laser diodes having high average powers and to demonstrate colliding-pulse mode-locking of a diode laser with a multisection saturable absorber.

Q-switching in single-heterojunction lasers and generation of ultrahigh-power picosecond optical pulses

Q-switching has been obtained for single-heterojunction lasers. The ultrafast saturable absorber in the laser cavity is created by implantation of high-energy heavy ions. The peak power generated by the lasers in a 150 μm stripe width is 380 W with a pulse length of 40 ps.

Integrated ultrafast saturable absorber

An ultrafast saturable absorber was demonstrated experimentally in AlGaAs, operated with a photon energy below half the band gap, near 1555 nm. Both the saturation intensity and the linear transmission can be independently designed in the structure. The device is based on spatial soliton emission from a tapered channel waveguide with a corresponding increase in transmission of 50%. The experimental data compare favorably with numerically simulated results.