T. Chau

Improved intrinsic dynamic distortions in directly modulated semiconductor lasers by optical injection locking

The effects of optical injection locking on the nonlinear distortions of directly modulated semiconductor distributed feedback (DFB) lasers are investigated experimentally. The second harmonic distortion (SHD) and third harmonic distortion (THD), as well as the third-order intermodulation distortion (IMD3), are measured as functions of modulation frequency for both the free-running and injection-locked lasers. Under strong injection locking with -8-dB injection ratio, the SHD and THD of the DFB laser have been suppressed by 15 dB from 2 to 4 GHz. Moreover, nearly 15-dB reduction in IMD3 has been observed from 1.4 to 3.0 GHz with the same injection conditions. We also found that the injection locking is not effective in reducing the low-frequency distortions.

Distributed balanced photodetectors for broad-band noise suppression

A novel velocity-matched distributed balanced photodetector with a 50-/spl Omega/ coplanar waveguide output transmission line has been experimentally demonstrated in the InP/InGaAs material system. Distributed absorption and velocity matching are employed to increase the saturation photocurrent. A common-mode rejection ratio greater than 27 dB has been achieved. The radio-frequency (RF) link experiment conducted at 4.16 GHz shows that the relative intensity noise of the laser has been suppressed by more than 24 dB and shot-noise limited performance has been achieved. Significant improvement of signal-to-noise ratio has been observed over a wide range of frequencies and phase mismatch of input RF signals.

A novel monolithic distributed traveling-wave photodetector with parallel optical feed

We report on a novel monolithic distributed traveling wave photodetector with parallel optical feed to an array of individual photodiodes using an integrated multimode interference (MMI) coupler to attain high saturation current. The parallel optical feed reduces the maximum photocurrent and photocurrent density seen by any single photodiode, thus increasing the maximum linear photocurrent of the detector. We have successfully fabricated a device with a maximum linear photocurrent of 20 mA and a responsivity of 0.13 A/W.

Generation of millimeter waves by photomixing at 1.55 /spl mu/m using InGaAs-InAlAs-InP velocity-matched distributed photodetectors

Millimeter-wave generation by photomixing at 1.55 /spl mu/m in long-wavelength velocity-matched distributed photodetector (VMDP) is experimentally demonstrated. The VMDP exhibits a dc responsivity of 0.25 A/W, a breakdown voltage of 7-9 V, and up to 15-mA dc photocurrent without damage. Millimeter-wave output powers of -21 dBm at 75 GHz and -24 dBm at 95 GHz were measured.

Demonstration of an analog fiber-optic link employing a directly modulated semiconductor laser with external light injection

We demonstrate an analog fiber-optic link employing a directly modulated semiconductor distributed-feedback (DFB) laser under optical injection locking. The results show that injection locking can significantly improve performances of analog fiber-optic systems at frequencies considerably beyond the bandwidth of free-running semiconductor lasers.

Gain and bandwidth enhancement of directly modulated analog fiber optic links using injection-locked gain-coupled DFB lasers

We report on the experimental demonstration of directly modulated analog fiber optic links with gain-coupled DFB lasers under strong optical injection locking. Successful transmission of BPSK signals up to 18 GHz has been achieved.

Long-wavelength velocity-matched distributed photodetectors

We have experimentally demonstrated an InGaAs-InAlGaAs-InP high-speed, high-power long wavelength velocity matched distributed photodetector (VMDP) using optical lithography. A 3-dB bandwidth of 18-GHz and an overall quantum effiency of 34% have been achieved.

Mode locking of external-cavity semiconductor lasers with saturable Bragg reflectors

We demonstrate the mode locking of external-cavity semiconductor lasers by using a saturable Bragg reflector as an external reflector. Output pulses of 1.9 ps were generated from the semiconductor lasers without dispersion compensation. By coupling the output to a standard single-mode filter with a length of 35 m to compensate for the linear chirp, we have achieved mode-locked pulse durations as short as 880 fs.

Suppression of intermodulation distortion in DFB lasers by external optical injection locking

We report on the experimental investigation of third-order intermodulation distortion in directly modulated distributed feedback (DFB) lasers under optical injection locking. Nearly 15 dB reduction in third-order intermodulation distortion has been observed from 1.4 to 3.0 GHz.

High speed velocity-matched distributed photodetectors

We report work on long-wavelength (1.55 /spl mu/m) velocity-matched distributed photodetectors (VMDP) for RF photonic systems and millimeter-wave photomixers. The VMDP consists of an array of photodiodes connected by an optical waveguide. The output is collected in phase by the velocity-matched 50 /spl Omega/ microwave transmission line. The VMDP offers many advantages for high frequency photodetectors with high saturation power. The distributed absorption is also desirable for increasing the damage threshold due to thermal runaway. Recently, we have experimentally demonstrated millimeter-wave generation at 95 GHz using a 1.55 /spl mu/m VMDP. We review the state of the art of high-speed photodetectors, and present recent results in VMDP development for millimeter-wave generation.