Small-signal Modulation Bandwidth Research Articles

High-speed modulation of 850-nm intracavity contacted shallow implant-apertured vertical-cavity surface-emitting lasers

GaAs-AlGaAs quantum-well (850 nn) vertical-cavity surface-emitting lasers, with lateral current injection and shallow implanted apertures, show small signal modulation bandwidths of at least 11 GHz and large signal data rates of at least 10 Gb/s. The devices achieved a maximum output power of 2.1 mW, with a threshold current and voltage of 1 mA and 1.71 V, respectively. The shallow implantation step provides photolithographically precise aperture formation (using O/sup +/ ions), for efficient lateral current injection into the quantum-well active region of the laser, from intracavity contacts. The device aperture was 7 μm in diameter, and the opening in the annular top contact was 13 μm in diameter. The optical spectrum showed several transverse modes.

Monolithically integrated low-power phototransceivers for optoelectronic parallel sensing and processing applications

A low-power GaAs-based monolithically integrated phototransceiver, consisting of a high-gain heterojunction phototransistor (WPT) and a microcavity light-emitting diode (MCLED) or a low-threshold vertical-cavity surface-emitting laser (VCSEL), is demonstrated. The HPT and MCLED/VCSEL are grown by molecular-beam epitaxy in a single step. The phototransistor exhibits a responsivity of 60 A/W at an input power of 1 /spl mu/W. The input and output wavelengths are 850 and 980 nm, respectively. The MCLED-based phototransceiver exhibits an optical gain of 7 dB and power dissipation of 400 /spl mu/W for an input power of 1.5 /spl mu/W. The small signal modulation bandwidth is 80 MHz. On the other hand, the VCSEL-based phototransceiver exhibits an optical gain of 10 dB and power dissipation of 760 /spl mu/W for an input power of 2.5 /spl mu/W.

InP-based cylindrical microcavity light-emitting diodes

We have investigated the properties of InP-based microcavity light-emitting diodes (/spl lambda/=1.6 /spl mu/m). Our objective was mainly to study the effects of lateral confinement of optical modes, which was achieved by the wet oxidation of double In/sub 0.52/Al/sub 0.48/As layers. The smallest devices had a cavity radius of 0.5 /spl mu/m, which becomes comparable to /spl lambda//n, where n is the effective refractive index of the photon emitting heterostructure. Two types of devices were tested: the first without any mirrors in the vertical direction, and the second with a combination of MgF/ZnSe DBR (top) and silver (bottom) to produce a low Q/spl sim/35-45. The latter type of devices exhibited higher output power and narrower spectral linewidth; otherwise, the characteristics were very similar The output slope efficiency monotonically decreases with reduction of lateral cavity size up to /spl sim/2-/spl mu/m in diameter and then is enhanced again for smaller cavity sizes. The slope efficiency of the smallest device (aperture diameter 1 /spl mu/m) is almost equal to that measured for the largest devices. The maximum output power measured from the devices is 30 /spl mu/W. The far-field pattern of devices with aperture radii ranging from 1.5 to 20 /spl mu/m shows an angular width (FWHM) of 50/spl deg/. On the other hand, devices with smaller aperture (radius /spl sim/0.5 /spl mu/m) exhibit an angular width of 20/spl deg/. The measured small-signal modulation bandwidth increases from /spl sim/0.45 GHz for the larger devices to 0.8 GHz for the smallest devices. Our results indicate that microcavity effects can be observed with only lateral photon confinement, making device fabrication requirements less stringent compared to surface-emitting lasers.

Monolithically integrated low-power phototransceiver incorporating microcavity LEDs and multiquantum-well phototransistors

A low power GaAs-based monolithically integrated phototransceiver, consisting of a high gain phototransistor and a microcavity light-emitting diode, is demonstrated. The input and output wavelengths are 0.85 and 0.98 /spl mu/m, respectively. The phototransceiver exhibits an optical gain of 7 dB and power dissipation of 400 /spl mu/W for an input power of 1.5 /spl mu/W. The small signal modulation bandwidth is 80 MHz.

Light-emitting diode emitting at 650 nm with 200-MHz small-signal modulation bandwidth

State-of-the-art modulation bandwidths are presented for multiquantum well resonant cavity light emitting diodes (RCLED's) emitting at 650 nm. 84-/spl mu/m size epoxy coated RCLED's have a 1.4-mW (CW) output power and a small signal modulation bandwidth of 200 MHz at 40 mA bias. 150-/spl mu/m diameter devices yield 3.25-mW and 150-MHz bandwidth at 70-mA bias. An open eye-diagram at 622 Mb/s achieved for the 84-/spl mu/m device makes it very attractive for SONET OC-12 data communication links.

Highly uniform operation of high-performance 1.3-/spl mu/m AlGaInAs-InP monolithic laser arrays

We describe the fabrication of monolithically integrated 1/spl times/12 arrays of 1.3-/spl mu/m strain-compensated multiquantum-well AlGaInAs-InP ridge lasers. The laser array shows highly uniform characteristics in threshold current, slope efficiency, and lasing wavelength with a standard deviation of 0.08 and 0.27 mA, 0.012 and 0.007 W/A, and 0.59 and 0.57 nm, respectively, at 20/spl deg/C and 100/spl deg/C. Besides, each laser on the array exhibits a low threshold current of 8 mA at 20/spl deg/C and 21 mA at 100/spl deg/C, a characteristic temperature of 92 K, and a slope efficiency drop of 0.7 db between 20/spl deg/C and 80/spl deg/C. A low thermal crosstalk of less than -4 dB can be obtained from one diode as the injected current of other elements is increased to 70 mA. Also, each laser on the array has a negligible degradation after a 24-hr burn-in test at 80 mA and 100/spl deg/C. An expected lifetime of more than 20 years is estimated for the lasers when operating at 10 mW and 85/spl deg/C. The lasers have a small-signal modulation bandwidth of about 9 GHz at 25/spl deg/C and a low relative intensity noise of -155 dB/Hz without an isolator at 2.5 GHz. It can transmit a 2.5-GHz signal to 50 km through standard single-mode fiber and to 308 m through multimode fiber, with a clear eye opening in OC-48 data-rate tests.

Quantum well and quantum dot lasers: From strained-layer and self-organized epitaxy to high-performance devices

Strained heterostructures are now widely used to realize high-performance lasers. Highly mismatched epitaxy also produces defect-free quantum dots via an island growth mode. The characteristics of high-speed strained quantum well and quantum dot lasers are described. It is seen that substantial improvements in small-signal modulation bandwidth are obtained in both 1 μm (48 GHz) and 1.55 μm (26 GHz) by tunneling electrons directly into the lasing sub-band. In quantum dots the small-signal modulation bandwidth is limited by electron-hole scattering to ∼7 GHz at room temperature and 23 GHz at 80 K. The properties of these devices are described.

In(Ga)As/GaAs self-organized quantum dot lasers: DC and small-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of /spl sim/20 nm and height of /spl sim/6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T/sub 0//spl les/670 K. Typical threshold currents of 200-/spl mu/m long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of /spl sim/30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of /spl sim/8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as /spl sim/7/spl times/10/sup -14/ cm/sup 2/ at room temperature.

Highly efficient oxide-confined VCSEL arrays for parallel optical interconnects

We have designed and fabricated and VCSEL arrays at 850 and 980 nm operation wavelength, respectively, which are designed for maximum single-mode output power and high-frequency applications. GaAs VCSELs in the array show record high single-mode CW powers up to 4.8 mW. Individual devices of the InGaAs VCSEL array exhibit small-signal modulation bandwidths exceeding 10 GHz.

Modulation and free-space link characteristics of monolithically integrated vertical-cavity lasers and photodetectors with microlenses

We present temperature, modulation, and free-space link characteristics of monolithically integrated vertical-cavity lasers (VCLs) and resonant photodetectors. The devices have been integrated using a novel structure that makes it possible to fabricate devices with through-the-substrate emission and detection. Taking advantage of the substrate emitting/detecting architecture, we monolithically integrate microlenses on the substrate side of the devices and flip-chip bond arrays without via processes or substrate removal. Low-threshold high-efficiency VCLs exhibit maximum small-signal modulation bandwidths, which are limited by parasitics, of /spl sim/9.5 GHz at 20/spl deg/C and /spl sim/8.4 GHz at 70/spl deg/C. The VCLs have the lowest reported bias currents required to reach bandwidths of up to /spl sim/8 GHz. A free-space optical link is demonstrated with flip-chip-bonded arrays of microlensed, monolithically integrated VCLs and detectors. The link is found to be tolerant to temperature differences of /spl plusmn/75/spl deg/C between the VCL and detector, with error free (BER<10/sup -12/) data transmission demonstrated in each case.

15-GHz modulation bandwidth, ultralow-chirp 1.55-μm directly modulated hybrid distributed Bragg reflector (HDBR) laser source

A hybrid source has been realized, integrating a fast Fabry-Perot laser and a fiber grating. The device has shown very good performances in the 1530-1570-nm range, obtaining 16 mA of threshold current at 20/spl deg/C, 1.6-mW fiber optical power and 48 dB of sidemode suppression ratio at 50 mA bias current. The cavity length was designed to achieve a good tradeoff between chirp reduction and increasing speed. The device has shown for the first time, to our knowledge, more than 15 GHz of small-signal modulation bandwidth, and 10-Gb/s modulation capability. Moreover, a penalty-free transmission experiment at 2.5 Gb/s over 100 km of standard fiber has confirmed the very low wavelength chirp of the device. These previous characteristics together with an extremely low temperature dependence (<0.02 nm//spl deg/C) make the hybrid distributed Bragg reflector (HDBR) particularly suitable for dense wavelength-division-multiplexing systems.

QUANTUM DOT SEMICONDUCTOR LASERS

The formation of coherently strained islands during the growth of strained heterostructures has been exploited to form an array of quantum dots. The shape and size of the islands vary with growth parameters, but exhibit the electronic properties of zero-dimensional systems. One or multiple, vertically coupled, layers of such quantum dots can form the gain region of a separately confined heterostructure (SCH) laser. The properties of such InGaAs/GaAs self-organized quantum dot lasers are described here. The lasers exhibit temperature independent operation up to 100 K and beyond. Typical threshold currents of 200 μm long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5–7.5 GHz at 300 K and increased to &gt;20 GHz at 80 K. We believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as ~ 7× 10-14 cm2 at room temperature. Wavelength switching is demonstrated in these devices and preliminary results on long-wavelengths intersubband quantum dot light emitters are also presented.

The application of flip-chip bonding interconnection technique on the module assembly of 10 Gbps laser diode

Flip chip bonding technique using Pb/In solder bumps was applied to packaging of a 10 Gbps laser diode (LD) submodule for high speed optical communication systems. The effect of the flip-chip bonding interconnection technique instead of conventional wire bonding was investigated for high speed broad band devices. The broad band performance of 10 Gbps LD submodule was simulated using SPICE S/W and compared with experimental results. In this simulation, the 10 Gbps LD was modeled in a parallel RC circuit. The values of R and C used for the equivalent circuit were 5ω and 1 pF, respectively. The LD was placed in series with a 18ω thin film resistor to prevent the impedance mismatch between the LD and a 25ω transmission line. The dependence of parasitic parameters on the small signal modulation bandwidth and the scattering parameters of the LD submodule was investigated and analyzed up to 20 GHz. A small signal modulation bandwidth of 14 GHz at 10 mA dc bias current and the clean modulation response up to 20 GHz were found for the flip-chip bonded submodule. The bandwidth of flip-chip bonded 10 Gbps LD submodule is wider than that of the wire-bonded LD submodule by a difference of 3.8 GHz.

High-speed modulation of low-temperature optimisedvertical-cavity lasers over 77–300 K temperature range

The authors report high speed studies of packaged, low-temperature optimised single quantum well vertical-cavity laser diodes. The small-signal modulation bandwidths in the range 8.2-9.3 GHz are demonstrated for a 5.4 /spl mu/m diameter laser with very low drive currents over the 77-300 K temperature range.

Quantum capture times at room temperature in high-speed In/sub 0.4/Ga/sub 0.6/As-GaAs self-organized quantum-dot lasers

The quantum capture times in high-speed single-mode self-organized quantum-dot (QD) lasers with I/sub th/=15-30 mA, and small-signal modulation bandwidth f/sub -3 dB/=4.5 GHz, have been estimated from high frequency electrical impedance measurements. The effective carrier capture times, determined from this relatively simple measurement technique, vary in the range of 20-40 ps, depending on bias current, and are in excellent agreement with theoretical predictions. The results suggest that carrier capture, not damping, may prove to be the limiting factor in the modulation bandwidths of QD lasers.

Small-signal modulation and differential gain of single-mode self-organized In0.4Ga0.6As/GaAs quantum dot lasers

We report small-signal modulation bandwidth and differential gain measurements of a single-layer self-organized In0.4Ga0.6As/GaAs quantum dot laser grown by molecular beam epitaxy. The 3 dB bandwidth of single-mode ridge waveguide lasers was measured to be 7.5 GHz at 100 mA under pulsed measurements, demonstrating the possibility of high speed operation of these devices. The differential gain was measured to be 1.7×10−14 cm2.

24-GHz modulation bandwidth and passive alignment of flip-chip mounted DFB laser diodes

High-frequency lasers have been flip-chip mounted on silicon motherboards. Small-signal modulation bandwidths around 24 GHz were obtained. It was shown that the bandwidth was not limited by extrinsic parasitics associated with the mounting scheme. Lasers were passively aligned to single mode fibers in V-grooves with the self-aligning solder bump technique. By passive alignment 50% of maximum coupling efficiency obtained by active alignment was achieved.

Top surface-emitting vertical-cavity laser diodes for 10-Gb/s data transmission

Intensity modulated proton-implanted top surface-emitting vertical-cavity InGaAs QW lasers (VCSELs) with a small-signal modulation bandwidth of 12 GHz butt-coupled to multimode fibers are investigated as light source for optical interconnection. At 10-Gb/s pseudorandom data rates the bit-error rate (BER) remains under 10/sup -11/ after transmission over 500 m of graded index multimode fiber. Optimum transmission behavior is achieved for linearly polarized nearly single-mode laser operation with a side-mode suppression of better than 25 dB under modulation. Spectral characterization indicates that linearly polarized single-mode light output is essential for good BER performance.

Semiconductor MOPA with monolithically integrated5 GHzelectroabsorption modulator

A high-power monolithically-integrated master oscillator/electroabsorption modulator/power amplifier semiconductor laser is demonstrated. The device emits &gt;1 W output beam power. Extinction ratios of –15 dB are obtained with a 2.9 V reverse bias voltage on the electroabsorption modulator. The 3 dB small signal modulation bandwidth is as high as 5 GHz. High-power diffraction-limited semiconductor laser sources with high-speed modulation capability are very desirable for applications such as intersatellite space telecommunications.

High-performance 1.3 μm AlGaInAs/InP strained quantum well lasers grown by organometallic chemical vapor deposition

In this paper, we show that by using the AlGaInAs/InP instead of the GaInAsP/InP materials system, 1.3 μm lasers with excellent high temperature performance can be fabricated, and report on the optimization of the growth conditions. Compressive strained five-quantum-well AlGaInAs/InP lasers showed only a 0.3 dB change in differential quantum efficiency for a temperature change from 25 to 100°C and a large small-signal modulation bandwidth of 8.6 GHz even at 85°C. Tensile-strained three-quantum-well lasers exhibited a 0.63 dB change in differential quantum efficiency for a temperature change from 25 to 100°C. At a heat sink temperature of 25°C the maximum 3 dB modulation bandwidth, limited by heating, was 19.6 GHz for compressive-strained lasers and 17 GHz for tensile-strained lasers. In spite of the Al-containing active layer, no catastrophic optical damage was observed at room temperature up to the highest powers obtained, 218 mW for the compressive and 103 mW for the tensile strained lasers. Preliminary life tests indicated that these lasers are at least as reliable as conventional GaInAsP/InP lasers, with the mean-time-to-failure being 110 years at 85°C. These data indicate that AlGaInAs/InP lasers are attractive for uncooled and low-cost applications, such as fiber-in-the-loop (FITL).