1064 nm wavelength stabilized hybrid ns-MOPA diode laser system for high peak power and low spectral width

Abstract

Summary form only given. High-power diode lasers capable of generating spectrally stable and nearly diffraction-limited optical pulses in the nanosecond range can be used in a variety of applications including free-space communications, metrology, material processing and frequency doubling. Especially nanosecond laser systems emitting more than 10 W optical peak power are of potential interest for differential absorption light detection and ranging of atmospheric species (DIAL). Due to the pressure broadening of the absorption lines a spectral line width below 0.01 nm together with excellent wavelength stability is necessary. MOPA systems where a distributed Bragg reflector (DBR), a distributed feedback (DFB), or an external cavity diode laser (ECDL) acting as master oscillator (MO) and a tapered amplifier as power amplifier (PA) have promising characteristics. They combine the narrow spectral line width and the wavelength stability of the MOs with high output power of the PA.In this work a hybrid ns-MOPA system comprising a 1064 nm DFB laser operated in CW mode as a master oscillator (MO) and a multi-section power amplifier for pulse generation and amplification will be presented (see Fig. 1). The power amplifier is based on an InGaAs triple quantum well embedded in asymmetric AlGaAs based super large optical cavity with a narrow vertical divergence of about 15° (FWHM). The total length is 6 mm. The facets of the amplifier were both anti-reflection coated with a reflectivity R <; 5x10-4. The device was mounted p-side up on a C-mount and operated at a heat sink temperature of 25°C. The amplifier consists of three ridge-waveguide (RW) sections and one tapered section. The three RW sections are in total 2 mm long and have a ridge with a width of 5 μm. One RW section is driven pulsed to modulate the transparency [1] with a pulse width Pulse <; 4 ns and gate optical pulses from the CW input signal. The other two RW sections have a length of 500 μm each and were operated near transparency to avoid unwanted feedback effects. The tapered section is also excited with current pulses to decrease the ASE between the generated optical pulses. Current pulses up to 20 A with a width of 6 ns and a repetition frequency of 800 kHz are applied. Fig. 2 shows the temporal shape of the generated optical pulses at a delay time of 3 ns between amplifier pulse and gate pulse at 13.5 A pulse peak current. The optical pulse has a pulse width of 3 ns and a pronounced plateau with a power amplitude of 12.5 W. The rise and fall times are 0.4 ns and 0.7 ns, respectively. ASE is only generated at the beginning and the end of the amplifier pulse and is <; 5%. Dependent on the length of the pulses optical peak powers up to 16 W were observed. In best cases the ASE power amounts are less than 1 % in comparison to the laser power. The dependence of the pulse shape and the ASE on the delay time Delay between the pulses injected into the RW and tapered sections and the physics behind them will be discussed in detail at the conference. The peak wavelength of = 1063.075 nm and the measured spectral line width (FWHM) of 10 pm given by the resolution limit of the spectrometer coincide for the DFB laser and the MOPA. The DFB laser has a side mode suppression ration (SMSR) of 52 dB whereas the SMSR of the MOPA is deteriorated to 42 dB indicating the ASE contribution. In Fig. 3 a spectral map in the amplifier current range between 3 A and 15 A is shown. A stable wavelength in the pulse peak power range up to 16 W is reached. In comparison to monolithic lasers no variation of the emission wavelength and linewidth of the laser emission occur with increasing pulse power, which allows the application of the system for the measurements of absorption lines of molecular species under atmospheric conditions. No saturation of the output power was observed, so a further increase of the output power seems to be possible with current pulses of higher amplitude.