Benson C. Lam

Observation of chaotic instability in the active mode locking of a semiconductor laser

We examine experimentally the consequence of frequency detuning an actively mode-locked external-cavity semiconductor laser from resonance. We observe a transition of the laser system from a periodic oscillation to a nonperiodic state with broadened spectral tones. By estimating the fractal dimension of the corresponding phase-space attractors, we show the presence of low-dimensional chaos. The route to chaos is a well-defined regime of three-frequency quasi-periodicity preceded by a two-frequency quasi-periodicity.

Chaotic instabilities in modulated external-cavity semiconductor lasers

Chaotic stability of external cavity semiconductor laser under modulation is examined both theoretically and experimentally. When the modulation frequency is detuned below the cavity resonant frequency, the simulations show a two-frequency to three-frequency route to chaos, as depicted by the power spectrum and time series of the laser emission at different stages of detuning. This agrees with experimental observations of 1.3 micrometer wavelength distributed feedback (DFB) laser and ridge waveguide (RW) InGaAsP laser. The phase-space attractor of both DFB and RW lasers have well-defined structures at broadband chaotic state, which signifies the presence of dynamical determinism in this state.

Design and fabrication of InGaAsP/InP waveguide modulators for microwave applications

InGaAs/InP multiple quantum well electroabsorption modulators grown by MOCVD are designed and fabricated for 1.5 micrometers wavelength operation. For rib loaded waveguide modulators fabricated on n-InP substrate with a 3 micrometers rib width and a 2 micrometers intrinsic waveguide layer, a capacitance of 0.2 to 0.3 pF and a reverse breakdown voltage > 20 V are obtained. The extinction ratio of the modulators is more than 14 dB and the 3 dB optical bandwidth is 18 GHz. The modulators RF efficiency and optical insertion loss still need to be improved. For modulators made on semi-insulating InP substrate, a capacitance in the range 0.1 to 0.2 pF is measured.

Quasi-periodic route to chaos in an actively mode-locked external-cavity semiconductor laser

Laser mode-locking is an effective modulation technique which can be used in fiberoptic link applications. In a semiconductor laser, active mode-locking can be achieved through the direct modulation of the laser at the cavity resonant frequency. If the modulation frequency is detuned from this condition, large optical intensity fluctuations can occur, which can cause a significant degradation in the noise figure of the laser, and consequently in the noise figure of the overall link. In this paper, we investigate the consequence of frequency detuning an actively mode-locked external-cavity semiconductor laser. We experimentally demonstrate the existence of chaos in a mode-locked laser upon frequency detuning. To our knowledge, chaotic instability in an actively mode-locked semiconductor laser has not been reported.<<ETX>>

Hybrid optical transmitter for microwave communication systems

We report results of an improved hybrid optical transmitter suitable for use in microwave communication systems. Based on a double up-conversion technique, a 1.3 micrometers wavelength semiconductor diode laser is actively mode-locked, and its output is externally modulated by a lithium niobate Mach-Zehnder amplitude modulator to generate a transmitter output from 19 GHz to 21 GHz. Following a brief review of recent table top system measurements and motivation, we show a second generation engineering scheme for packaging the mode-locked source components. The initial performance results of this source are -34.2 dBm(e) mode-locked output power at 13.1 GHz, with an input rf power of 14.5 dBm(e); RIN is measured at -106.5 dBc(e)/Hz.

High-speed fiber optic link by external modulation of mode-locked lasers

A 1.3-micron wavelength optical transmitter system with a 2-GHz bandwidth centered at 20 GHz has been implemented by actively mode-locking a semiconductor laser diode and then modulating the signal externally. The performance of a link using this transmitter over a 2-km fiber is evaluated. A hybrid package is developed for a mode-locked laser submodule operating at 15 GHz.

Hybrid optical transmitter for microwave communication

We demonstrate a 1.3 micron wavelength optical transmitter system with a 20 GHz RF center frequency and 2 GHz bandwidth. We accomplish this by actively modelocking a semiconductor diode laser at 14 GHz; the modelocked signal is then externally modulated between 5 GHz and 7 GHz, using a lithium niobate based Mach-Zehnder modulator. We show a hybrid prototype package for the modelocked laser source.

Analysis and measurement of the external modulation of modelocked laser diodes (relative noise performance)

The measurement of the relative intensity noise (RIN) of a ridge waveguide laser and a distributed feedback laser under CW, external cavity, direct modulation, and modelocking conditions is presented. The purpose is to determine the relative noise performance of modelocked laser diodes. The results indicate that the RIN of modelocked lasers are comparable to CW lasers but lower than both external cavity lasers (optimized for modelocking but without the applied RF) and directly modulated lasers; the difference can be as much as 5 optical dB. The microwave carriers produced optically by the direct modulation and modelocking of laser diodes are also compared. The comparison determines that modelocked lasers produce less noisy and more RF power efficient microwave carriers. However, no difference in microwave linewidth is detected within the limit of the resolution bandwidth of the detection system.

Performance optimization of externally modulated mode-locked laser diode

The performance of the high speed InGaAsP ridge waveguide lasers and the GaAlAs single-mode lasers operating as a micromixer have been investigated by using a circuit model derived from the iterative method and device rate equations. The laser mixer has many advantages over the conventional microwave mixer, including: high conversion gain, wide dynamic range, low noise figure and high local oscillator/intermediate frequency (LO/IF) isolation. The analytic results also indicate that the heterodyne microwave fiber optic link using a laser mixer provides the advantages of system simplicity, cost-effectiveness and improved system signal-to-noise ratio (SNR) than the conventional microwave fiber optic links. System SNR of two fiber optic links were measured: a 12 GHz link with 70 MHz IF has a SNR of better than 120 dB/Hz and a 26.5 GHz up-converted link has a SNR of 100 dB/Hz, without laser diode RF impedance matching. Other system applications of the laser mixer are also presented.