Thomas F. Carruthers

10-GHz, 1.3-ps erbium fiber laser employing soliton pulse shortening

An actively mode-locked single-polarization erbium fiber laser modulated at 10 GHz utilizes intracavity soliton formation to produce 1.3-ps pulses, well below the Kuizenga-Siegman limit, without passive mode locking. The observed degree of pulse shortening agrees with the predictions of recently developed soliton laser models. The pulse dropout ratio was measured to be less than 10(-12), and the rms amplitude and phase jitter are less than 1.1% and 0.16 ps, respectively.

Dispersion management in a harmonically mode-locked fiber soliton laser

Harmonically mode-locked Er-fiber soliton lasers have become a reliable source of high-repetition-rate picosecond pulses in high-speed communications and photonic analog-to-digital conversion systems because of their low-noise, dropout-free operation. We have fabricated such a laser with a strongly dispersion-managed cavity and modeled its operation, and we have found that dispersion management significantly extends the power range over which uninterrupted single-pulse production is attained and dramatically decreases the effects of amplified spontaneous emission on the phase noise of the laser.

Theoretical and experimental study of harmonically modelocked fiber lasers for optical communication systems

We study theoretically and experimentally actively modelocked fiber lasers that are used in high repetition rate optical communication systems. Using an innovative numerical technique and a reduced model, we have found that the laser can operate in four different operating regimes when the laser intensity was changed; three of the regimes were experimentally observed in a laser with a sigma configuration. An excellent quantitative agreement between the theoretical and the experimental results was obtained. The use of dispersion management in the sigma laser was found to significantly improve the laser performance.

Picosecond optical mixing in fast photodetectors

We report a simple optical correlation technique which is capable of measuring the intrinsic response speeds of fast photosensitive electronic devices. The appeal of the method is that neither high‐speed electrical connections to a device nor cross‐correlation measurements between two devices are required. An example is given of measurements of the response speed of a GaAs Schottky barrier photodiode. Under appropriate illumination conditions the detector is found to be sufficiently fast to be capable of replacing optical second harmonic generation techniques for monitoring the quality of a beam of 1.5 ps pulses.

Pulse Compression using a Tapered Microstructure Optical Fiber

We calculate the pulse compression in a tapered microstructure optical fiber with four layers of holes. We show that the primary limitation on pulse compression is the loss due to mode leakage. As a fibers diameter decreases due to the tapering, so does the air-hole diameter, and at a sufficiently small diameter the guided mode loss becomes unacceptably high. For the four-layer geometry we considered, a compression factor of 10 can be achieved by a pulse with an initial FWHM duration of 3 ps in a tapered fiber that is 28 m long. We find that there is little difference in the pulse compression between a linear taper profile and a Gaussian taper profile. More layers of air-holes allows the pitch to decrease considerably before losses become unacceptable, but only a moderate increase in the degree of pulse compression is obtained.

Photonic serial-parallel conversion of high-speed OTDM data

We demonstrate full demultiplexing of a single 100-Gb/s optical time-division multiplexed data stream into eight parallel 12.5-Gb/s streams. This is performed using a photonic serial-to-parallel converter based on lithium niobate intensity modulators, and requires no nonlinear optical elements. Error-free operation is obtained for each of the eight output channels.

4/spl times/10 GHz mode-locked multiple-wavelength fiber laser

We demonstrate a four-wavelength 10-GHz mode-locked erbium-doped fiber laser, with wavelength spacings of 3.5-5 nm. Nearly transform-limited Gaussian pulses of 11- to 14-ps duration are measured and all wavelengths are synchronous.

Passive laser mode locking with an antiresonant nonlinear mirror.

A nonlinear mirror that is based on an antiresonant ring incorporating a nonlinear element sensitive to the polarization state of the circulating light has been developed. No phase matching of the mirror to the laser cavity is necessary, so the mirror does not need active length stabilization. Spontaneous mode locking occurs when the mirror is incorporated into a cw Nd:YAG laser. Pulses of 11-psec duration are produced at 1064 nm; two output beams at 532 nm are also generated. An analysis indicates that the primary mechanism responsible for passive mode locking is self-phase modulation in the nonlinear element.

Integration of low-temperature GaAs on Si substrates

GaAs was grown on Si substrates by molecular‐beam epitaxy at low substrate temperatures. Femtosecond time‐resolved reflectivity investigations revealed a significant reduction of carrier lifetime in GaAs epilayers from 14 ps to <0.5 ps as the growth temperature was reduced from 400 to 150 °C. Photoconductors were fabricated on the epilayers, and electro‐optic sampling was used to confirm the electrical response times of <1 ps. The responsivity is comparable to that for low‐substrate‐temperature GaAs grown on GaAs substrates, with a photogenerated carrier mobility of ∼100–300 cm2/V s. The low growth temperatures allow ultrafast GaAs‐based photodetector incorporation into Si‐based integrated circuits for novel optoelectronic applications.

Visible-wavelength amplified spontaneous emission in a neodymium-doped optical fiber pumped at 1064 nm

A neodymium‐doped single‐mode fiber emits light at 496 and 633 nm, as well as at several near‐infrared wavelengths, when pumped by a mode‐locked, Q‐switched Nd:YAG laser operating at 1064 nm. A threshold peak pumping power of approximately 50 kW is required for the process; peak emitted powers of several watts were observed. The blue line is more than 15 nm wide at high pumping levels and possesses large temporal pulse dispersion across its spectrum. Amplified spontaneous emission via a three‐photon pumping process is proposed to account for the phenomenon.