Daniel A. Tauber

Large and small signal dynamics of vertical cavity surface emitting lasers

Continuous wave relative intensity noise (RIN) measurements and pulsed electrical gain switching measurements were performed on vertical cavity surface emitting lasers in order to investigate the high speed potential of these devices. A high resonance frequency–square root of power coefficient of 7.6 GHz/√mW was obtained from the RIN measurements. The gain switched pulses exhibited very fast relaxation oscillations with a maximum observed relaxation oscillation frequency of 71 GHz.

HIGH SPEED SEMICONDUCTOR LASERS

The strategy and methods to design high speed semiconductor lasers are reviewed here. The formalism for the analysis of intensity modulation, frequency modulation and intensity noise in quantum well lasers is first derived. Using this formalism the process of optimizing the laser structure for high speed operation is presented. In addition to the conventional factors such as the differential gain, photon density, photon lifetime and device parasitics, we also review the critical effects of carrier transport and microwave signal transmission on the dynamic characteristics and design of high speed semiconductor lasers.

The microstrip laser

We present a novel semiconductor laser structure, the microstrip laser, where the epitaxial laser layers sit directly above a thick gold layer, instead of on a conventional semiconductor substrate. This design provides advantages for both high frequency and high-power performance compared to conventional ridge waveguide structures. Results indicative of an improved structure are presented, including a factor of 3 reduction in the thermal resistance of the microstrip laser compared to a conventional laser.

Dynamic responses of widely tunable sampled grating DBR lasers

The modulation bandwidth, dynamic mode suppression ratio, and wavelength chirp of directly modulated sampled grating DBR lasers have been measured. Although the tuning range can be up to an order of magnitude larger than in simple DBR lasers, the chirp is about the same or better over a wide range of operation parameters. The modulation bandwidth was in excess of 4 GHz and the dynamic MSR remained larger than 40 dB as long as the current did not swing below threshold. The linewidth enhancement factor was extracted from the measured chirp parameters and ranged from three to eight for different lasing wavelengths of the tunable lasers. The dispersion of the linewidth enhancement factor is consistent with published theoretical predictions.

Dynamics of Wide Bandwidth Semiconductor Lasers

In this paper we summarize the most important recent advances and results in high speed diode lasers. These advances have primarily come about as a result of physical understanding of the properties that affect the dynamic performance of these lasers. A great deal of progress has been made in understanding the active region properties of such devices, including the electron and hole transport dynamics as well as the effect of active region doping and strain. At the very high frequencies characteristic of the highest speed lasers reported to date the microwave signal propagation becomes an important issue that can limit the laser bandwidth. The distribution of signals along the laser length due to these effects are discussed, analyzed, and measured, and conclusions about bandwidth and device operation are drawn from the analysis. All of these different issues are summarized in this paper from both a theoretical and experimental perspective. The laser structures that address and overcome the problems caused by such factors are presented along with the best results obtained to date.

Distributed microwave effects in high speed semiconductor lasers

In this paper we analyze and experimentally show that high speed semiconductor lasers approximately 300 /spl mu/m long and operating at frequencies above 25 GHz should be treated as distributed electrical elements. The analysis and experiments indicate that the microwave propagation is lossy, slow wave and dispersive, and that these distributed effects have important implications for the use of directly modulated lasers in high speed optical links.<<ETX>>

70-GHz relaxation oscillation in a vertical cavity surface emitting laser

Summary form only given. Very-high-frequency relaxation oscillations were measured in an electrically pumped vertical cavity surface emitting laser (VCSEL). The measurements were performed over a range of drive currents, and a 71-GHz relaxation oscillation was observed when current injection was maximum. The lasers used for the measurements consisted of an active region of 3 In/sub 0.2/Ga/sub 0.8/As quantum wells. CW (continuous-wave) measurements showed a very high resonance frequency-square root of power coefficient of 7 GHz/ square root mW. The maximum resonance frequency was limited to 7 GHz by heating. The larger signal pulse responses also showed a linear dependence of relaxation oscillation frequency on square root of current above threshold, with a maximum of 71 GHz for a peak drive voltage on the order of 15 V. >

Direct modulation of widely tunable sampled-grating DBR lasers

The linewidth, chirp and mode-suppression-ratio (MSR) of directly modulated sampled grating DBR (SGDBR) lasers has been measured. Although the tuning range can be up to an order of magnitude larger than in simple DBR lasers (i.e. 60 nm vs 6 nm), we find that the linewidth, chirp and MSR are about the same or better (i.e. (Delta) (nu) < 5 MHz, (alpha) equals 3-8, and MSR > 40 dB) over a wide range of operating parameters. The modulation bandwidth was in excess of 4 GHz, and the dynamic MSR remained > 40 dB as long as the current did not swing below threshold.

The Microstrip Laser

Semiconductor lasers that operate at high power and high frequency are important components for optical communication networks and systems. High power operation requires efficient heat remova1 from the active region of the device and high frequency operation requires careful consideration of electrode structure to minimize the deleterious effects of electrical parasitics and poor microwave signal propagation [ 1-31. Efficient heat remova1 has often been achieved by mounting the laser ridge side down onto a diamond heat sink. The highest frequency lasers have utilized a coplanar waveguide electrode geometry with thick metallization to minimize the electrical problems [3]. The microstrip laser, a schematic of which is shown in Figure 1, is inherently an excellent thermal and microwave structure because of the thick gold layer beneath the lower cladding. The schematic drawing is for a polyimide ridge waveguide laser. The improved thermal properties result from the high thermal conductivity of the gold layer which acts as a heat spreader. The improved microwave properties result from the high electrical conductivity of the gold layer, which minimizes slow wave effects and microwave signal attenuation at high frequency [1-2].