F. Tan

Transistor laser with simultaneous electrical and optical output at 20 and 40 Gb/s data rate modulation

A single quantum well transistor laser (cavity length L = 300 μm) has been designed and fabricated that operates with threshold ITH = 18 mA at 15 °C and 14 mA at 0 °C. Due to the “fast” base recombination lifetime (τB < 29 ps), the transistor laser demonstrates reduced photon-carrier resonance amplitude (<4 dB) over its entire bias range and a modulation bandwidth f-3dB = 9.8 GHz at 15 °C for IB/ITH = 3.3 and 17 GHz at 0 °C for IB/ITH = 6.4. Under the same bias conditions, simultaneous electrical and optical “open-eye” signal operations are demonstrated at 20 and 40 Gb/s data rate modulation.

850 nm Oxide-VCSEL With Low Relative Intensity Noise and 40 Gb/s Error Free Data Transmission

We have designed and fabricated a high speed 850 nm oxide-confined vertical cavity surface emitting laser with an oxide aperture dimension of ~ 4 μm and a threshold current ITH=0.53 mA at room temperature (20 °C). It demonstrates a modulation bandwidth of 21.2 GHz, and achieves a laser relative intensity noise reaching standard quantum limit 2hν/P0=-154.3 dB/Hz at high bias I/ITH=10. Furthermore, error-free data transmission at 40 Gb/s is obtained at I=6.5 mA which corresponds to an energy/data efficiency of 431 fJ/bit.

Relative intensity noise of a quantum well transistor laser

A quantum well transistor laser with a base cavity length L = 300 μm has been designed, fabricated, and operated at threshold ITH = 25 mA (0 °C). As a consequence of the inherent advantage of the picosecond base recombination lifetime, the transistor laser is able to achieve nearly a quantum shot-noise limited laser relative intensity noise (RIN) with a peak amplitude of −151 dB/Hz at frequency 8.6 GHz. Compared with a diode laser (a charge storage device) at the same output power, the transistor laser (a charge flow device) has a better than 28 dB (number dependent on the laser device design) peak RIN advantage.

The effect of microcavity laser recombination lifetime on microwave bandwidth and eye-diagram signal integrity

Vertical microcavity surface-emitting lasers employing quantum wells and small aperture buried-oxide current and field confinement are demonstrated with wider mode spacing and faster spontaneous carrier recombination (enhanced Purcell factor), lower threshold current, larger side mode suppression ratio (SMSR), and higher photon density and temperature insensitivity. The result is a microcavity laser that achieves higher microwave modulation bandwidth (f−3dB = 15.8 GHz) at ultra-low power consumption (1.5 mW) with a slope for the modulation current efficiency factor (MCEF) = 17.47 GHz/mA−1/2, as well as a better quality eye diagram in high-speed data transmission. The microwave behavior model for the microcavity laser is used to estimate the enhanced recombination and reduced lifetime.

Energy efficient microcavity lasers with 20 and 40 Gb/s data transmission

Microcavity lasers (μCLs), reduced-size (≲3 μm aperture) vertical cavity surface-emitting lasers (VCSELs) defined by the buried-oxide process for current and field confinement (thus wide mode spacing), are demonstrated with low threshold current, sharp turn-on L-I characteristics, and wide bandwidth operation. Due to the enhanced spontaneous recombination rate at reduced mode and improved photon density, μCLs exhibit lower charge-field resonance peaks at a modulation bandwidth f−3 dB=18.7 GHz, thus permitting open-“eye” operation at 20 and 40 Gb/s data rates (I≲3 mA). The energy efficiency for 20 Gb/s data transmission is measured to be 4.84 Gb/s/mW, which is eight times better than 7 μm aperture VCSELs.

Voltage and Current Modulation at 20 Gb/s of a Transistor Laser at Room Temperature

Data are presented showing open-eye 20-Gb/s transmission for a quantum-well transistor laser operating at room temperature (25°C). The fast spontaneous recombination lifetime (~ 30 ps) in the base region results in a resonance-free frequency response allowing demonstration of 20-Gb/s transmission with an I/ITH=3. It is shown that higher temperature hastens the transition to the first excited state and improves bandwidth and eye-opening at low bias levels (I/ITH=2). In addition, room temperature 20-Gb/s transmission through voltage modulation of a transistor laser via intracavity photon-assisted tunneling in the base-collector junction is reported.

Transistor laser optical and electrical linearity enhancement with collector current feedback

The three-port quantum well (QW) transistor laser (TL) is shown to provide a unique solution for generating ultra-linear electro-optical signals. With a simple collector current feedback loop, the 3rd order intermodulation distortion in the electrical and optical output signals of the transistor laser can be suppressed by as much 18.2 dB and 8.4 dB, respectively. These results show that the TL can be used for direct electro-optical feedback linearization, because of the base QW carrier-photon interaction, without incurring signal losses at multiple stages of auxiliary external electro-optical conversion circuitry.

Selective oxidization cavity confinement for low threshold vertical cavity transistor laser

Data are presented for a low threshold n-p-n vertical cavity transistor laser (VCTL) with improved cavity confinement by trench opening and selective oxidation. The oxide-confined VCTL with a 6.5 × 7.5 μm2 oxide aperture demonstrates a threshold base current of 1.6 mA and an optical power of 150 μW at IB = 3 mA operating at −80 °C due to the mismatch between the quantum well emission peak and the resonant cavity optical mode. The VCTL operation switching from spontaneous to coherent stimulated emission is clearly observed in optical output power L-VCE characteristics. The collector output IC–VCE characteristics demonstrate the VCTL can lase in transistors forward-active mode with a collector current gain β = 0.48.

The effect of ground and first excited state transitions on transistor laser relative intensity noise

We report the results of relative intensity noise (RIN) measurement on the ground and first excited state transitions of a single quantum-well (QW) transistor laser (TL). Because of higher differential gain and faster recombination lifetime on the first excited state transition, a lower laser RIN is measured as compared with ground state laser operation. The minority carrier density in the base of QWTL extracted from the laser RIN shows a carrier density of 2.6–3.5 × 1016 cm−3, a more than 40× reduction from that of a conventional diode laser.

The effect of mode spacing on the speed of quantum-well microcavity lasers

Oxide-confined quantum-well microcavity vertical-cavity surface-emitting lasers (VCSELs) of three-diameters (aperture size dA∼2, 2.5, and 3.5 μm) have been fabricated that operate as nearly single-mode lasers at ultralow thresholds ITH=0.15, 0.16, and 0.20 mA. Relative spectral intensities are measured at a set higher bias current I=0.8 mA for the three VCSEL sizes to determine the dependence on mode spacing between the fundamental and second order modes, which at increasing diameter are Δλ∼2.2, 1.6, and 1.0 nm. By studying the side-mode suppression ratio and the optical microwave frequency response of the microcavity VCSELs throughout a spread-out group of modes, we are able to resolve the dependence of signal amplitude and time response on the difference in mode spacing, Δλ, higher speed response occurring at larger Δλ.