Mode-locked diode laser with resonant ring amplifier

We present a ring semiconductor amplifier system which is seeded by ultrashort pulses for additive amplification. An external cavity diode laser configuration is built to generate the ultrashort pulses based on a hybrid modelocking scheme. A monolithic multi-segment diode laser is utilized as a light source in the operating oscillator. It has the advantage that the gain and absorber are integrated on one chip. The oscillator operates at a fundamental repetition-rate of 206MHz and can be driven on various harmonics of this frequency. The generated pulses are injected into a tapered amplifier (TA) which consists of a ridge waveguide section (RWS) for coupling and a tapered section (TS) for amplification. The amplified pulses are coupled back after amplification towards the TAs RWS forming a ring resonator setup. By matching the cavity lengths of the oscillator and ring resonator, we can obtain additively amplified pulses. The emission spectrum of the chosen TA is centered around 850nm which is in the wavelength range of the oscillator. The spectrum of the additively amplified pulses is observed for different pumping parameters of the TA using an optical spectrum analyzer. Additionally, we characterized the system for the best seeding parameters by monitoring the output signal with an autocorrelator. We figured out that the best performance is achieved when the amplifier is seeded by pulses at the second harmonic of 412 MHz. When blocking the seeding pulses the amplifier operates in continuous wave (CW) regime. By comparing the obtained spectra for CW and additively amplified pulses, we conclude that the system operates with a CW background also in pulsed operation. However, from the comparison of the spectra, we estimate that the amplified pulsed power is about 120mW for a seed power of 1:1mW. Thus, the ring amplifier provides a significantly higher amplification than a single pass amplifier. In future work the CW background has to be suppressed, e.g. by synchronous modulation of the current into the amplifiers ridge waveguide section.