Moustafa Ahmed

Generation of vortex beam using Bragg reflector waveguide

We demonstrated the generation of vortex beams using a GaAs-based ring-shaped Bragg reflector waveguide functioning as a highly dispersive angular diffraction element for potential applications in integrated mode-multiplexing in a multimode fiber. The polarization measurement of far-field patterns shows the formation of a radially polarized vortex beam. A tunable ring-shaped far-field pattern was demonstrated by tuning the wavelength of coupled light. We also integrated multi-ring vortex beam emitters, demonstrating the possibility of the creation and multiplexing of vortex beams with different orbital momentums.

Enhancing Modulation Bandwidth of Semiconductor Lasers beyond 50 GHz by Strong Optical Feedback for Use in Millimeter-Wave Radio over Fiber Links

We present the modeling on high-speed modulation of semiconductor lasers beyond 50 GHz over a frequency band of 2 GHz by using strong optical feedback (OFB). This ultra high modulation frequency is achieved by adjusting the external-cavity resonance frequency close to the modulation frequency and optimizing strong OFB conditions to achieve stable operation. We show that this optimized strong OFB results in about 20 dB improvement in the link gain of a 55.8 GHz radio over fiber (RoF) link. We also discuss the influence of environmental variations on the simulated modulation characteristics.

Enhancing the modulation bandwidth of VCSELs to the millimeter-waveband using strong transverse slow-light feedback

We present modeling on the millimeter (mm)-wave modulation of vertical-cavity surface-emitting laser (VCSEL) with a transverse coupled cavity (TCC). We show that strong slow-light feedback can induce 300% boosting of the modulation bandwidth of the TCC-VCSEL. Also, the strong lateral feedback can induce resonance modulation over passbands centered on frequencies as high as 3.8 times the VCSEL bandwidth. The slow-light feedback is modeled by a time-delay rate equation model that takes into account the multiple round trips as well as the optical loss and phase delay in each round trip in the feedback cavity.

High Frequency Modulation of Transverse-Coupled-Cavity VCSELs for Radio Over Fiber Applications

We demonstrate a high-speed and efficient direct modulation of a novel transverse-coupled-cavity vertical cavity surface emitting laser for radio over fiber applications. A bow-tie joint connection between two oxide apertures results in a leaky traveling wave in the lateral direction between the two cavities. The measured L/I characteristics and lasing spectra demonstrate coherent coupling of the two cavities. The small signal response of the fabricated device shows a large enhancement of over 30 dB in the modulation amplitude at frequencies . The resonantly enhanced modulation response is suitable for efficient narrow-band modulation in the millimeter wave range over 25 GHz far beyond the intrinsic modulation bandwidth of the laser without optical feedback.

Compact electro-absorption modulator integrated with vertical-cavity surface-emitting laser for highly efficient millimeter-wave modulation

We demonstrate a compact electro-absorption slow-light modulator laterally-integrated with an 850u2009nm vertical-cavity surface-emitting laser (VCSEL), which enables highly efficient millimeter-wave modulation. We found a strong leaky travelling wave in the lateral direction between the two cavities via widening the waveguide width with a taper shape. The small signal response of the fabricated device shows a large enhancement of over 55u2009dB in the modulation amplitude at frequencies beyond 35 GHz; thanks to the photon-photon resonance. A large group index of over 150 in a Bragg reflector waveguide enables the resonance at millimeter wave frequencies for 25 μm long compact modulator. Based on the modeling, we expect a resonant modulation at a higher frequency of 70 GHz. The resonant modulation in a compact slow-light modulator plays a significant key role for high efficient narrow-band modulation in the millimeter wave range far beyond the intrinsic modulation bandwidth of VCSELs.

Noise performance of high-speed radio over fiber links employing vertical-cavity surface-emitting lasers

This study investigates the intensity noise in high-speed vertical-cavity surface-emitting lasers (VCSELs) and its contribution to the noise performance of radio over fiber (RoF) links. We evaluate the sinusoidal modulation of VCSELs in terms of the second-order harmonic distortion (2HD) and third-order intermodulation distortion (IMD3) in additions to the relative intensity noise (RIN). The spurious-free dynamic range of the proposed VCSEL is estimated. The noise performance of the RoF link is assessed by the noise figure. The modulation characteristics of the VCSEL and the gain and noise factor (NF) of the fiber link are compared under conventional and high-speed modulations of VCSELs. Also, we present comparison of the NF between short (300 m) and relatively long (2 km) fibers.

850 nm transverse-coupled-cavity vertical-cavity surface-emitting laser with direct modulation bandwidth of over 30 GHz

An 850-nm-band transverse-coupled-cavity vertical-cavity surface-emitting laser (VCSEL) is demonstrated for the first time, showing a 3 dB modulation bandwidth of over 30 GHz, which is the record for VCSELs. The transverse-coupled cavity is formed by making lithography-defined, battledore-shaped mesas. The coupling strength can be increased by reducing the absorption loss in a passive feedback cavity. Further increase in the coupling strength and/or mode selection via current injection into the feedback cavity shows the possibility of overclocking the device by optical equalization. Large-signal measurement shows eye-opening at 40 Gb/s. After the coupled mode is stabilized, higher bitrate modulation can be expected.

Wavelength tuning and controlled temperature dependence in vertical-cavity surface-emitting lasers with a thermally and electrostatically actuated cantilever structure

We demonstrate the wavelength tuning and the control of its temperature dependence in 850 nm GaAs vertical cavity surface emitting lasers (VCSELs) using a thermally and electrostatically actuated cantilever structure. The temperature dependence can be controlled from +0.035 to −0.14 nm/K by changing the length of a micromachined cantilever, which can be electrostatically actuated for wavelength tuning. Continuous wavelength tuning over 36 nm was obtained keeping almost constant temperature dependence. The result indicates a possibility of realizing widely-tunable VCSELs with engineered temperature dependence.

Sub-gigahertz beam switching of vertical-cavity surface-emitting laser with transverse coupled cavity

We report a high-speed electrical beam switching of vertical cavity surface emitting laser with a transverse coupled cavity. A high speed (sub-gigahertz) and large deflection angle (>30°) beam switching is demonstrated by employing the transverse mode switching. The angular switching speed of 900u2009MHz is achieved with narrow beam divergence of below 4° and extinction ratio of 8u2009dB. We also measured the near- and far-field patterns to clarify the origin of the beam switching. We present a simple one-dimensional Bragg reflector waveguide model, which well predicts the beam switching characteristic.

Study of the optical feedback-induced noise in semiconductor lasers and applications to the optic disc system

The dynamics and noise of semiconductor lasers under optical feedback (OFB) have been simulated. The study is performed as applied to an optic-disc system in which laser radiation is reflected by the disc surface and re-injected into the laser cavity. We examine the possibility of suppressing OFB-induced noise in the optic-disc system by the technique of superposition of high-frequency current. The study is based on numerical integration of the time-delay rate equations of semiconductor lasers under OFB. The laser noise is evaluated in terms of the spectral profile of relative intensity noise (RIN). It is shown that RIN is enhanced when states of chaos are generated, and attains minimum levels under continuous-wave operation just before the laser starts the route to chaos. The suppression of RIN in the low-frequency regime is achieved when the superposition-current frequency exceeds the laser resonance frequency by factors of 0.8, 1.0, and 1.1 and when the modulation depth exceeds 0.4.