E.M. Koontz

Two-photon absorption in semiconductor saturable absorber mirrors

The nonlinear reflectivity of semiconductor saturable absorber mirrors has been investigated as a function of incident energy fluence. The presence of two-photon absorption in commonly used structures was confirmed via time-resolved differential reflectivity measurements. Theoretical calculations predict that two-photon absorption will expand the continuous-wave mode-locking stability regime against Q-switched mode-locking, yet may simultaneously induce multiple pulses in a laser cavity.

Erbium-ytterbium waveguide laser mode-locked with a semiconductor saturable absorber mirror

Picosecond pulses are produced using a semiconductor saturable absorber mirror in a laser based on an Er-Yb codoped planar waveguide amplifier. Continuous-wave mode-locking (CWML) with 9.8-ps pulses is obtained at repetition rates up to 100 MHz. With intracavity spectral filtering, saturable pulsewidths of 1 ps are achieved, and tunable picosecond pulses are obtained from 1534 to 1553 nm. Absorber characterization suggests that two-photon absorption within the saturable absorber mirror influences the CWML stability.

Stabilization of an active harmonically mode-locked fiber laser using two-photon absorption

Two-photon absorption provided by a semiconductor mirror structure is shown to reduce amplitude fluctuations significantly in a harmonically mode-locked fiber ring laser. Pulse dropouts are eliminated in a laser that produces picosecond pulses at a repetition rate of 2 GHz.

High fluence ultrafast dynamics of semiconductor saturable absorber mirrors

The ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors are investigated using reflective pump–probe measurements. At high fluence, ultrafast induced absorption begins to dominate over absorption bleaching. Above the InGaAs quantum well band gap, the differential reflectivity shows a ∼1 ps transient due to nonequilibrium carrier dynamics. Below band gap, the signal is dominated by a strong two-photon absorption component followed by induced absorption that decays with a time constant of ∼5 ps; these components are attributed to nonlinear absorption and subsequent carrier diffusion in the InP layer.

Recovery dynamics in proton-bombarded semiconductor saturable absorber mirrors

Reduction of device response time, resulting from the proton bombardment of InGaAs/InP-based semiconductor saturable absorbers, was studied experimentally using an ultrafast degenerate, cross- polarized pump-probe technique. Proton bombardment is shown to reduce device response times to ∼1 ps at low optical excitation densities. Under high excitation, the device dynamics are dominated by induced absorption. The extended recovery of highly excited carriers appears to be less sensitive to defects created by bombardment. Mode locking was demonstrated with the proton-bombarded samples in an erbium-doped fiber laser.

Ultrafast hot-carrier dynamics in semiconductor saturable absorber mirrors

Femtosecond pump-probe experiments have been used to study the ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors. The relative contributions of absorption bleaching and induced absorption are investigated by varying the excitation fluence over more than four orders of magnitude, well beyond complete absorption saturation. Enhanced free carrier absorption due to highly excited carriers with an extended relaxation time of 2.8 ps dominates the differential reflectivity at ultrahigh fluences and has been studied via a two-color pump-probe measurement.

Suppression of instabilities and pulsewidth limitation by two-photon absorption in mode-locked lasers

Summary form only given. The importance of mode-locking lasers using semiconductor saturable absorbers necessitates a more thorough investigation of the saturable absorber nonlinearities. Saturation measurements of a resonantly-coated InGaAs/InP semiconductor saturable absorber mirror reveal the presence of two-photon absorption at energy densities obtainable in mode-locked fiber and waveguide lasers. Observations of the dynamics of an Er/Yb waveguide laser mode-locked by this absorber indicate that two-photon absorption is responsible for both restricting the minimum pulsewidth and suppressing Q-switched-mode-locking.

Ultrafast recovery times in implanted semiconductor saturable absorber mirrors at 1.5 /spl mu/m

Summary form only given. Recovery times as short as /spl sim/1 ps are obtained with proton bombardment of InGaAs/InP-based saturable absorbers without significantly changing the non-saturable logs. Under high excitation, recovery is found to be limited by slower intraband relaxation rates.

Picosecond pulses from an Er/Yb waveguide laser passively mode-locked with a semiconductor saturable absorber mirror

Summary form only given. Semiconductor saturable absorber mirrors have been used to passively mode-lock a variety of solid state and fiber lasers. Fiber lasers typically operate at low fundamental repetition rates because of Er-doping density limits in fiber. A mode-locked fiber laser based on an Er/Yb codoped waveguide amplifier has been demonstrated, although with a P-APM mode-locking mechanism that is not scalable to short cavity lengths. Here, a semiconductor saturable absorber mirror is used for the first time to produce pulses as short as 1 ps at repetition rates of 25-100 MHz from a mode-locked Er/Yb waveguide laser in a linear cavity configuration that is scalable to very high repetition rates.