Hsiao-Hua Wu

Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber

We propose an operation switchable ring-cavity erbium-doped fiber laser (EDFL) via intra-cavity polarization control. By using a semiconductor saturable absorber mirror in the EDFL cavity, stable Q-switching, Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic mode-locking pulses can be manipulated simply by detuning a polarization controller while keeping the pump power at the same level. All EDFL operation states can be obtained under the polarization angles detuning within 180 degrees. Continuous-wave mode-locking of EDFL with 800-fs pulsewidth repeated at 4 MHz has been obtained, for which the output pulse energy is 0.5 nJ and the peak power is 625 W. Interaction between solitons and the accompanied non-soliton component will lead to either pulse splitting or 5th-order harmonic mode-locking at repetition rate of 20 MHz.

12 GHz passive harmonic mode-locking in a 1.06 μm semiconductor optical amplifier-based fiber laser with figure-eight cavity configuration

We report the generation of passively harmonic mode-locked pulses using a 1.06 μm semiconductor optical amplifier (SOA) in a figure-eight laser configuration operated in the all-normal-dispersion regime. Different orders of harmonic mode-locking can be obtained from 30 MHz to 12.02 GHz by changing the injection current of the SOA from 80 to 660 mA together with the adjustment of polarization controllers. The highest pulse repetition rate increases almost linearly with the SOA current. As SOA current is set to 660 mA, we obtain the intracavity power of 46 mW at the highest repetition rate of 12.02 GHz, corresponding to the 1202th harmonic of the fundamental mode-locking frequency. To our best knowledge, this is the lowest intracavity power to generate the highest repetition rate with a passively mode-locked laser in the all-normal-dispersion regime.

Optoelectronic phase tracking and electrooptic sampling of free-running microwave signals up to 20 GHz in a laser-diode-based system

Reports optoelectronic phase-tracking and electrooptic sampling of microwave signals from a free-running oscillator in a laser-diode-based system. A voltage-controlled oscillator (VCO) in a photoconductive harmonic mixer based phase-lock loop is utilized to gain switch a 0.8 /spl mu/m laser diode for phase-tracking of the microwave signal at frequencies up to 20 GHz. The phase and amplitude of the microwave signal in a microstrip line is also measured using another 1.3-/spl mu/m laser diode driven by the VCO. >

The detuning characteristics of rational harmonic mode-locked semiconductor optical amplifier fiber-ring laser using backward optical sinusoidal-wave injection modulation

The modulation waveform, power, and frequency detuning dynamics of a semiconductor optical amplifier fiber-ring laser (SOAFL) harmonic and rational-harmonic mode-locked by using backward injection of a large-signal sinusoidal-wave modulated Fabry-Pe/spl acute/rot laser diode (FPLD) are studied. The shortest pulsewidth of 53.3 ps at 1 GHz with spectral linewidth of 0.09 nm at modulation power of 17 dBm can be obtained, which corresponds to a time-bandwidth product of 2.8. The threshold modulation power and the maximum frequency detuning range for harmonic mode-locking of the SOAFL are 11.5 dBm and 180 kHz, respectively. The mode-locked SOAFL gradually recovers back to sinusoidal-wave modulation mode at the detuning frequency beyond /spl plusmn/400 kHz. The pulse broadening slopes for the positive- and negative-frequency detuning regions of 0.48 and 0.375 ps/kHz are determined, respectively. By enlarging the modulating power and detuning the modulation frequency of FPLD, the SOAFL can be rational-harmonic mode-locked up to 12 GHz with 33-ps pulsewidth.

Dark current and trailing-edge suppression in ultrafast photoconductive switches and terahertz spiral antennas fabricated on multienergy arsenic-ion-implanted GaAs

We report ultrafast (∼2.7ps, instrument limited) switching responses of a multienergy-implanted GaAs:As+ photoconductive switches (PCSs) with suppressed trailing edge and reduced dark current. This material is highly resistive with dark current as low as 0.94μA at a bias of 40V. The carrier mobility of the former is ∼590cm2∕Vs, resulting in a small-signal optical responsivity of ∼2mA∕W. Pumped at 100mW and biased at 80V, the multienergy-implanted GaAs:As+ PCS exhibits peak response (0.35V) comparable to the best result of single-energy-implanted ones. The improvement on photoconductive response is crucial for the generation of shorter terahertz emission pulses from spiral antennas fabricated on multienergy-implanted GaAs:As+ (0.8ps) than single-energy-implanted GaAs (1.2ps), with the central frequency blueshifted to 0.2THz (from 0.15THz) and the spectral bandwidth broadened to 0.18THz (from 0.11THz).

Formation of off-axis beams in an axially pumped solid-state laser

I show that an axially pumped solid-state laser is able to support off-axis beams in a plane-concave cavity when the cavity length is adjusted within a region close to the hemispherical resonator configuration. These beams are characterized by the fact that even if an object is placed inside the cavity on axis in front of the concave mirror they can still oscillate off-axially in a V-shape path. One can further select and manipulate the desired off-axis beam by inserting a mask into the cavity. Experimental observations of cavity-length-dependent behavior of an Nd: YVO(4) laser at different pump power show that the gain-assisted self-image property in the vicinity of the hemispherical resonator configuration is responsible for the formation of these beams.

Electrooptic phase tracking of microwave signals beyond 18.5 GHz using an integrated electrooptic modulator as an optoelectronic harmonic mixer in a phase lock loop

We demonstrate electrooptic phase tracking of microwave signals beyond 18.5 GHz using a gain-switched laser diode as the optical clock. The key element of this system is a fiber-pigtailed integrated-optic Mach-Zehnder modulator (IOM). The IOM functions as an optoelectronic harmonic mixer in a phase lock loop. The conversion loss of the OEHM is 66 dB. The phase error and the single sideband phase noise density of the phase-locked 18.5 GHz signal is 5.6/spl times/10/sup /spl minus/5/ rad//spl radic/(Hz) and -54 dBc/Hz, respectively. >

Spatiotemporal dynamics of a passively mode-locked Nd:GdVO 4 laser

By examining beam patterns, temporal pulse forms, and radio-frequency spectra as functions of resonator configuration and pump power, we found that a passively mode-locked Nd:GdVO4 laser exhibited four main regimes of operation that were single-transverse-mode Q-switched mode locking (QSML), single-transverse-mode continuous-wave mode locking (CWML), multiple-transverse-modes QSML, and multiple-transverse-modes CWML. The effect of multiple transverse modes on CWML was giving rise to amplitude instability in the pulse train. In the regime of multiple-transverse-modes QSML, we observed the phenomenon of spatiotemporal dynamics that spatial patterns vary with the time.

High-repetition-rate passive harmonic mode-locking in an SOA-based fiber laser with normal dispersion

Harmonic mode-locking is operated in an 18-m figure-8 laser cavity having SOA gain medium at 1060 nm under normal dispersion. We can tune the repetition rate from 0.336 to 1.75 GHz by adjusting intracavity polarization at 100 mA pump.

Stable QML of SAM based picosecond Nd:GdVO 4 lasers by gain modulation

Gain modulation was applied to semiconductor saturable absorber mirror based picosecond Nd:GdVO4 lasers for achieving stable Q-switched mode locking and enhancing its peak power to 8 times higher than that operated at continuous-wave mode locking.