MEASUREMENT AND SIMULATION OF THE TURN-ON DELAY TIME JITTER IN GAIN-SWITCHED SEMICONDUCTOR-LASERS

The turn-on delay time jitter of four different unbiased gain-switched laser types is determined by measuring the temporal probability distribution of the leading edge of the emitted optical pulse. One single-mode 1.5-mu-m distributed feed-back laser and three multimode Fabry-Perot lasers emitting at 750 nm and 1.3-mu-m are investigated. The jitter is found to decrease for all lasers with increasing injection current. For multimode lasers it decreases from 8 ps excited slightly above threshold down to below 2 ps at three times the threshold current. The jitter of the distributed feedback (DFB) laser is a factor of 3-5 larger than the jitter of the three multimode lasers. A new model to predict the turn-on delay time jitter is presented and explains the experiments quantitatively.

Analysis of the optical confinement factor in semiconductor lasers

We derive formulas for the optical confinement factor Gamma from Maxwell's equations for TE and TM modes in the slab waveguide. The numerical results show that the formulas yield correct mode gain for the modes propagating in the waveguide. We also compare the formulas with the standard definition of Gamma as the ratio of power flow in the active region to the total power flow. The results show that the standard definition will underestimate the difference of optical confinement factors between TE and TM modes, and will underestimate the difference of material gains necessary for polarization insensitive semiconductor laser amplifiers. It is important to use correct optical confinement factors for designing polarization insensitive semiconductor laser amplifiers. For vertical cavity surface-emitting lasers, the numerical results show that Gamma can be defined as the proportion of the product of the refractive index and the squared electric field in the active region. (C) 1996 American Institute of physics.

ON THE RATE-EQUATIONS OF SEMICONDUCTOR-LASERS FOR MEASURING SPONTANEOUS EMISSION FACTOR

The rate equations used for measuring spontaneous emission factor beta is examined through the comparison of numerical results, The results show that beta obtained by using total spontaneous emission rate R(sp) = N/tau sp is about double of that using R(sp) = BN2, The magnitude difference between the measured beta and that predicted by classical theory [8] will disappear by using more reasonable R(sp) = BN2. The results also show that the magnitude of beta may be underestimated by ignoring the nonradiative recombination rates.

Measurement of Cavity Loss and Quasi-Fermi-Level Separation for Fabry-Perot Semiconductor Lasers

A fitting process is used to measure the cavity loss and the quasi-Fermi-level separation for Fabry- Perot semiconductor lasers. From the amplified spontaneous emission (ASE) spectrum, the gain spectrum and single-pass ASE obtained by the Cassidy method are applied in the fitting process. For a 1550nm quantum well InGaAsP ridge waveguide laser, the cavity loss of about ～24cm~(-1) is obtained.

Properties and applications of injection locking in 1.55 μm quantum-dash mode-locked semiconductor lasers

Mode-locked semiconductor lasers are compact pulsed sources with ultra-narrow pulse widths and high repetition-rates. In order to use these sources in real applications, their performance needs to be optimised in several aspects, usually by external control. We experimentally investigate the behaviour of recently-developed quantum-dash mode-locked lasers (QDMLLs) emitting at 1.55 μm under external optical injection. Single-section and two-section lasers with different repetition frequencies and active-region structures are studied. Particularly, we are interested in a regime which the laser remains mode-locked and the individual modes are simultaneously phase-locked to the external laser. Injection-locked self-mode-locked lasers demonstrate tunable microwave generation at first or second harmonic of the free-running repetition frequency with sub-MHz RF linewidth. For two-section mode-locked lasers, using dual-mode optical injection (injection of two coherent CW lines), narrowing the RF linewidth close to that of the electrical source, narrowing the optical linewidths and reduction in the time-bandwidth product is achieved. Under optimised bias conditions of the slave laser, a repetition frequency tuning ratio >2% is achieved, a record for a monolithic semiconductor mode-locked laser. In addition, we demonstrate a novel all-optical stabilisation technique for mode-locked semiconductor lasers by combination of CW optical injection and optical feedback to simultaneously improve the time-bandwidth product and timing-jitter of the laser. This scheme does not need an RF source and no optical to electrical conversion is required and thus is ideal for photonic integration. Finally, an application of injection-locked mode-locked lasers is introduced in a multichannel phase-sensitive amplifier (PSA). We show that with dual-mode injection-locking, simultaneous phase-synchronisation of two channels to local pump sources is realised through one injection-locking stage. An experimental proof of concept is demonstrated for two 10 Gbps phase-encoded (DPSK) channels showing more than 7 dB phase-sensitive gain and less than 1 dB penalty of the receiver sensitivity.

Bidirectional communication using delay coupled chaotic directly modulated semiconductor lasers

Chaotic synchronization of two directly modulated semiconductor lasers with negative delayed optoelectronic feedback is investigated and this scheme is found to be useful for e±cient bidirectional communication between the lasers. A symmetric bidirec- tional coupling is identified as a suitable method for isochronal synchronization of such lasers. The optimum values of coupling and feedback strength that can provide maxi- mum quality of synchronization are identified. This method is successfully employed for encoding/decoding both analog and digital messages. The importance of a symmetric coupling is demonstrated by studying the variation of decoding efficiency with respect to asymmetric coupling.

Multi-user bidirectional communication using isochronal synchronisation of array of chaotic directly modulated semiconductor lasers

Isochronal synchronisation between the elements of an array of three mutually coupled directly modulated semiconductor lasers is utilized for the purpose of simultaneous bidirectional secure communication. Chaotic synchronisation is achieved by adding the coupling signal to the self feedback signal provided to each element of the array. A symmetric coupling is effective in inducing synchronisation between the elements of the array. This coupling scheme provides a direct link between every pair of elements thus making the method suitable for simultaneous bidirectional communication between them. Both analog and digital messages are successfully encrypted and decrypted simultaneously by each element of the array.

Performance characteristics of positive and negative delayed feedback on chaotic dynamics of directly modulated InGaAsP semiconductor lasers

The chaotic dynamics of directly modulated semiconductor lasers with delayed optoelectronic feedback is studied numerically. The effects of positive and negative delayed optoelectronic feedback in producing chaotic outputs from such lasers with nonlinear gain reduction in its optimum value range is investigated using bifurcation diagrams. The results are confirmed by calculating the Lyapunov exponents. A negative delayed optoelectronic feedback configuration is found to be more effective in inducing chaotic dynamics to such systems with nonlinear gain reduction factor in the practical value range.

Suppression of chaos through reverse period doubling in coupled directly modulated semiconductor lasers

The effect of coupling on two high frequency modulated semiconductor lasers is numerically studied. The phase diagrams and bifurcatio.n diagrams are drawn. As the coupling constant is increased the system goes from chaotic to periodic behavior through a reverse period doubling sequence. The Lyapunov exponent is calculated to characterize chaotic and periodic regions.

Numerical studies on bi-directionally coupled directly modulated semiconductor lasers

Results of a numerical study of synchronisation of two directly modulated semiconductor lasers, using bi-directional coupling, are presented. The effect of stepwise increase in the coupling strength (C) on the synchronisation of the chaotic outputs of two such lasers is studied, with the help of parameter space plots, synchronisation error plots, phase diagrams and time series outputs. Numerical results indicate that as C increases, the system achieves synchronisation as well as stability together with an increase in the output power. The stability of the synchronised states is checked by applying a perturbation to the system after it becomes synchronised and then noting the time it takes to regain synchronisation. For lower values of C the system does not regain synchronisation. But, with higher values synchronisation is regained within a very short time.

Study of Synchronisalion and Control of Chaos in Directly Modulated Semiconductor Lasers

Chaos is a subject oftopical interest and, studied in great detail in relation to its relevance in almost all branches of science, which include physical, chemical, and biological fields. Chaos in the literal sense signifies utter confusion, but the scientific community has differentiated chaos as deterministic chaos and white noise. Deterministic chaos implies the complex behaviour of systems, which are governed by deterministic laws. Behaviour of such systems often become unpredictable in the long run. This unpredictability arises from the sensitivity of the system to its initial conditions. The essential requirement for ‘sensitivity to initial condition’ is nonlinearity of the system. The only method for determining the future of such systems is numerically simulating its final state from a set ofinitial conditions. Synchronisation