Picosecond pulses with 40 W peak power from a tapered laser diode for THz application
Fig. 1: Schematic diagram of the tapered diode laser.
Fig. 2: (a) Pulse width and (b) peak power as functions of TP current under different bias conditions of the saturable absorber of a tapered diode laser with double quantum well.
Fig. 3: Measurement results of the tapered diode laser from Fig. 2 emitting 2.6 ps pulses with 40 W peak power. (a) Measured pulse autocorrelation curve and its sech2-fit, (b) optical spectrum, (c) RF spectrum with 30 GHz range and (d) zoomed-in spectrum around 6.3 GHz.
Fig. 4: Tapered laser soldered on a conduction cooled package (CCP).
Terahertz time-domain spectroscopy with its non-destructive testing method is a powerful technique for material characterization. An optical pumping source providing ultrashort pulses with high peak power is the essential component to generate THz pulses in such systems. Thus, a diode laser with its compactness and cost-efficiency could be ideally suited. However, two main drawbacks of optical pulse generation from diode laser have to be overcome: insufficient pulse power and relatively long pulse duration.
To address these issues, FBH scientists have realized a monolithic double quantum well laser featuring a tapered (TP) gain section. The diode laser is 6 mm long and consists of a gain-guided TP gain section and an index-guided ridge waveguide (RW) part. The RW part is further subdivided into three sections, where the one adjacent to the TP section is operated as an RW gain section and the other two as saturable absorbers. By applying proper pumping currents (Itp and Irw) on the gain sections and a reverse bias voltage (U) on the absorber sections, the laser operates passively mode-locked and generates ultrashort optical pulses.
The shortest pulses measure 2.4 ps in pulse duration (full width at half maximum). For slightly longer pulses with 2.6 ps width, the pulse peak power is as high as 40 W. To the best of our knowledge, this is the highest peak power achieved from a monolithic mode-locked quantum well diode laser. The corresponding central emitting wavelength is 832 nm with a 1 nm 3-dB optical bandwidth. The pulse repetition rate is 6.3 GHz. The generated pulses are not transform-limited, which means they can be further compressed to a shorter pulse width with more intense peak power and can be used to drive THz antennas.
This work is funded by the Deutsche Forschungsgemeinschaft (DFG) under project reference number KN 1333/1-3.
Publication
S. Wohlfeil, H. Christopher, J. Fricke, H. Wenzel, A. Knigge, and G. Tränkle "Picosecond pulses with 40 W peak power from a passively mode-locked tapered quantum well laser", Electronics Letters, Vol. 59, No. 4, doi: 10.1049/ell2.12736 (2023).