J. Kenton White

High-dynamic-range linear analog data links (1-20 GHz) using room temperature DFB laser diodes

We report the analysis and application of uncooled, directly-modulated high-speed DFB lasers with emphasis on their analogue transmission performance. Fibre-optic links employing such lasers are shown to meet the most stringent requirements of analogue systems at both high carrier frequencies and high temperatures. Spurious-free dynamic ranges (SFDR) exceeding 100dB×Hz2/3 and 90dB×Hz2/3 and input third-order intercept points (IIP3) above 20dBm and 18dBm are reported for carrier frequencies up to 20GHz at 25°C and up to 10GHz at 85°C, respectively. The error-vector magnitude (EVM) for a 256-QAM modulated signal transmitted over 15km of SMF remains below 1.9% for carrier frequencies of both 2GHz and 5GHz for all measured temperatures. The link performance is assessed by using 3GPP W-CDMA, IEEE 802.11a and IEEE 802.11b signals. In all cases the EVM remains within the standard specification, for fibre-optic link lengths of up to 10km and laser operating temperatures of up to 70°C. Finally, an IEEE 802.11b WLAN demonstrator is presented, allowing antenna remoting over up to 1000m of 62.5/125μm MMF.

Temperature dependence of threshold current: a better criterion than T0?

Experimental measurements of threshold current density as a function of temperature have been analyzed in terms of the characteristic temperature, T0, and temperature gradient (Delta) TJth equals (delta) Jth/(delta) T, for a number of semiconductor laser device structures. These include AlInGaAs/InP, InGaAsP/InP, and AlGaAs/GaAs. A theoretical model is used to investigate the possible loss mechanisms in laser diodes that cause the superlinear increase of threshold current with temperature. The characteristic temperature T0 is found to vary with temperature and device length, thus making it somewhat misleading when quoted without qualification. A different approach based on plotting ln((Delta) TJth) vs. ln(Jth) shows a linear relationship that is dependent on device structure only, allowing the use of a new figure of merit for the temperature performance of semiconductor lasers.

High-power ultra-fast single- and multi-mode quantum dot lasers with superior beam profile

Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids. We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz. GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2. Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers. Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained. Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.

High-performance directly modulated lasers: device physics

Directly modulated lasers (DMLs) have two high performance applications: 1310 nm 10 Gb/s uncooled and 1550 2.5 Gs/s extended reach. Two key elements are gain coupled gratings and buried heterostructures. Gain coupled gratings simultaneously increase the DMLs intrinsic relaxation oscillation frequency and damping, while the buried heterostructure reduces thermal chirp and parasitic capacitance. Large relaxation oscillation frequencies and reduced parasitic capacitance allow 85°C operation; large damping and reduced thermal chirp enable extended reach.

Scanning Voltage Microscopy

Scanning voltage microscopy and scanning differential resistance microscopy analyses on diode lasers are presented: the direct observation of the current blocking breakdown in a buried heterostructure laser; the effect of current spreading inside actively biased ridge waveguide lasers; origins of anomalously high series resistance encountered in ridge lasers; parasitic series resistance power dissipation; and electron overbarrier leakage. Applications to emerging fields of nanotechnology and nanoelectronics are suggested. Specific sources of artifacts (such as circuit time constants and photocurrent) are described and evaluated for optimal performance.

Simulations of statistical parameter distributions in distributed-feedback lasers using a transmission-line laser model

A transmission-line laser model has been used for simulating distributed-feedback (DFB) lasers. Statistical distributions of laser parameters like threshold current, slope efficiency, front-to-back power ratio, or side-mode-suppression ratio (SMSR) are generated by randomly varying the lasers’ facet phases. Model parameters were adjusted by comparing simulated and experimental distributions for a continuous wave (CW) index-coupled laser and a 2.5Gb/s gain-coupled directly-modulated (DM) DFB laser. For the index-coupled DFB laser, agreement with experimental data is excellent but the front-to-back power ratio, which has a larger spread than measured experimentally. For the gain-coupled DFB laser, distributions are in excellent agreement with experimental data, but the SMSR is calculated to have a median about 5dB larger than measurement. Distributions of dispersion penalties after propagation in an optical fiber are also generated for various drive conditions and design parameters. It is shown that a grating with an index coupling larger than 4.0 and a gain coupling of around 0.05 gives the highest 2dB dispersion penalty yields for a reach of 450km. There is nevertheless a compromise between high dispersion penalty yields and CW single-mode yields when using large index coupling coefficients with only a small amount of gain coupling.

Physics of output power limitations in long-wavelength laser diodes

We analyze the high-temperature continuous-wave performance of 1.3 micron AlGaInAs/InP laser diodes grown by digital alloy molecular beam epitaxy. Commercial laser software is utilized that self-consistently combines quantum well bandstructure and gain calculations with two-dimensional simulations of carrier transport, wave guiding, and heat flow. Excellent agreement between simulation and measurements is obtained by careful adjustment of material parameters in the model. Joule heating is shown to be the main heat source; quantum well recombination heat is almost compensated for by Thomson cooling. Auger recombination is the main carrier loss mechanism at lower injection current. Vertical electron escape into the p-doped InP cladding dominates at higher current and it causes the thermal power roll-off. Self-heating and optical gain reduction are the triggering mechanisms behind the leakage escalation.

Superlattice physics of digitally grown epitaxial InAlGaAs layers

The material physics of digitally grown InAlGaAs quaternary alloy systems are investigated using Molecular Beam Epitaxy (MBE) grown layers. With MBE, arbitrary epitaxial alloy compositions can be achieved, without changing the group III elemental constituents flux rates, by simple sequential shuttering of the relevant fluxes. Monolayer fluctuations create inhomogeneities that lead to a broadening of the photoluminescence (PL) spectra. Multiple PL peaks are also seen in select alloy compositions.