High-power sampled-grating-based master oscillator power amplifier system with 23.5 nm wavelength tuning around 970 nm.

Tunable high-power diode lasers are key components in various established and emerging applications. In this work, we present a compact hybrid master oscillator power amplifier (MOPA) laser system. The system utilizes a tunable GaAs-based sampled-grating (SG) distributed Bragg reflector (DBR) laser as the master oscillator (MO), which emits around a wavelength of 970 nm in a single longitudinal mode with a spectral width below 20 pm. The SG-DBR laser consists of two SGs, each of which can be thermally tuned with microheaters. By tuning one of the two SGs, a discrete wavelength tuning of 21.1 nm can be obtained with a Vernier mode spacing of about 2.3 nm. By tuning both SGs, 23.5 nm of quasi-continuous tuning is obtained, with a mode spacing of about 115 pm. The coupling of the beam emitted by the MO into a tapered power amplifier provides an amplified output power in the watt range having a nearly diffraction-limited beam with a propagation factor of M1/e22=1.6. The combination of high power and wide wavelength tuning in a compact system makes this light source ideal for, among other things, nonlinear frequency conversion.

Frequency doubling of a passively mode-locked monolithic distributed Bragg reflector diode laser.

In this work, frequency doubling of a passively mode-locked 3.5 mm long monolithic distributed Bragg reflector diode laser is investigated experimentally. At 1064 nm, optical pulses with a duration of 12.4 ps are generated at a repetition rate of 13 GHz and a peak power of 825 mW, resulting in an average power of 133 mW. Second-harmonic generation is carried out in a periodically poled MgO-doped LiNbO3 ridge waveguide at a normalized nonlinear conversion efficiency of 930%/W. A maximum average second-harmonic power of 40.9 mW, corresponding to a pulse energy of 3.15 pJ, is reached in the experiment at an opto-optical conversion efficiency of 30.8%. The normalized nonlinear conversion efficiency in mode-locked operation is more than 2 times larger compared to continuous-wave operation.

Dynamics of a gain-switched distributed feedback ridge waveguide laser in nanoseconds time scale under very high current injection conditions.

We present detailed experimental investigations of the temporal, spectral and spatial behavior of a gain-switched distributed feedback (DFB) laser emitting at a wavelength of 1064 nm. Gain-switching is achieved by injecting nearly rectangular shaped current pulses having a length of 50 ns and a very high amplitude up to 2.5 A. The repetition frequency is 200 kHz. The laser has a ridge waveguide (RW) for lateral waveguiding with a ridge width of 3 µm and a cavity length of 1.5 mm. Time resolved investigations show, depending on the amplitude of the current pulses, that the optical power exhibits different types of oscillatory behavior during the pulses, accompanied by changes in the lateral near field intensity profiles and optical spectra. Three different types of instabilities can be distinguished: mode beating with frequencies between 25 GHz and 30 GHz, switching between different lateral intensity profiles with a frequency of 0.4 GHz and self-sustained oscillations with a frequency of 4 GHz. The investigations are of great relevance for the utilization of gain-switched DFB-RW lasers as seed lasers for fiber laser systems and in other applications, which require a high optical power.

High-power 894 nm monolithic distributed-feedback laser.

A ridge-waveguide InGaAs/GaAsP laser, emitting up to 250 mW in a single lateral and longitudinal mode at a wavelength of 894 nm, is presented. The distributed feedback is provided by a second order grating, formed into an InGaP/GaAs/InGaP multilayer structure. Owing to the stable lasing frequency, the large side mode suppression ratio (> 40 dB) and small spectral line width (< 200 kHz) the diode laser is well suited for caesium D1 spectroscopy. This was verified by the measurement of the hyperfine structure of the D1 line.

Nonlinear dynamics of semiconductor lasers with active optical feedback.

An in-depth theoretical as well as experimental analysis of the nonlinear dynamics in semiconductor lasers with active optical feedback is presented. Use of a monolithically integrated multisection device of submillimeter total length provides access to the short-cavity regime. By introducing an amplifier section as a special feature, phase and strength of the feedback can be separately tuned. In this way, the number of modes involved in the laser action can be adjusted. We predict and observe specific dynamical scenarios. Bifurcations mediate various transitions in the device output, from single-mode steadystate to self-pulsation and between different kinds of self-pulsations, reaching eventually chaotic behavior in the multimode limit.

Excitability of a semiconductor laser by a two-mode homoclinic bifurcation.

We report on the preparation of optical excitability in a distributed feedback semiconductor laser. The device integrates a single-mode laser and a 250 microm long passive section with cleaved facet. The phase of the light fed back from the passive section is tunable by current. The theoretical analysis shows an ultimate hop between external cavity modes within every phase cycle that is associated with a two-mode homoclinic bifurcation close to which the system becomes excitable. This excitability is clearly demonstrated in the experimental response to optical injection comparing well with simulation calculations.

Self-organization in semiconductor lasers with ultrashort optical feedback.

The dynamical behavior of a single-mode laser subject to optical feedback is investigated in the limit, when the delay time is much shorter than the period of the relaxation oscillations. Use of an integrated distributed feedback device allows us to control the feedback phase. We observe two kinds of Hopf bifurcations associated with regular self-pulsations of different frequencies.