Watt-class semiconductor optical amplifiers based on ridge-waveguide structures
Fig. 1a: Photograph of RW amplifier, 1b: Calculated modal reflectivity vs. tilt angle for different ridge widths
Fig. 2: Dependence of CW output power (amplifier) on input power. Inset: optical spectra (dB) at different input powers (mW)
Fig. 3a: Pulse power in dependence on time for different amplitudes of current pulses injected into the PA, 3b: Mapping of optical spectrum
A number of applications, such as non-linear frequency conversion and coherent optical communication, require compact laser sources which deliver high-power, spectrally stable and diffraction-limited radiation. The combination of semiconductor-based master oscillator (MO) and power amplifier (PA) devices in a MOPA system meets these requirements. The MO realized by a distributed-feedback (DFB) or distributed Bragg reflector (DBR) laser is operated in continuous wave (CW) mode and emits a low-power but spectrally stable and diffraction-limited beam. After subsequent injection into the PA, the beam is amplified keeping its spectral and spatial properties. Either a tapered or a ridge waveguide (RW) amplifier could be used as PA. At lower power levels an RW amplifier is more efficient, needs very low input power and delivers a beam which can be easily collimated and focused – compared to the very astigmatic beam of a tapered amplifier. Now, research at FBH has successfully demonstrated a very strong increase in power level of such RW amplifiers. The key issue of development is an epitaxial structure design which allows efficient amplification over distances of 4 mm and longer and minimizes the facet reflectivity to avoid self-lasing.
A top view of the developed RW amplifier is illustrated in Fig. 1a. The length of the PA, which is soldered p-side up on a CuW submount, is 6 mm. The ridge with 4 µm width is tilted with respect to the axis of the cavity. Fig. 1b shows the calculated power reflectivity of the fundamental mode for different tilt angles and ridge widths. It can be seen that for the range of tilt angles investigated the reflectivity drops the stronger, the larger the ridge width is. For a tilt angle of only 3° and a ridge width of 4 µm a reflectivity as small as 10-4 can be reached for an as-cleaved facet. Further reduction of reflectivity is obtained by an additional anti-reflection coating. Thus, self-lasing is effectively suppressed, despite low tilt angles. A low tilt angle, in turn, allows easy collimation of the amplifier output beam.
To demonstrate the performance of the RW amplifier, an inhouse-developed 4 mm long DBR laser is used as MO. Fig. 2 shows the dependence of the CW output power of the amplifier on the input power. The RW amplifier is driven at a current of IPA = 1.5 W. At an injection of PMO = 0.2 mW an abrupt increase of the output power PPA of the PA is observed. A further increase of the seed power causes only a small increase of PPA, which approaches a maximum value of PPA = 1 W limited by the current of IPA = 1.5 A injected into the amplifier.
Fig. 3a shows the power that can be achieved when short (5 ns long) current pulses are injected into the amplifier. A peak power of the generated optical pulses of PPA = 4 W is obtained for an amplitude of the current pulses of IPA = 8 A. Compared to directly pulsed diode lasers no relaxation oscillations occur. Additionally, the peak wavelengths of the emitted pulses are independent of the current injected into the amplifier as Fig. 3b reveals. As a result, the spectrum remains very stable. Thus, the combination of a CW operated master oscillator and a CW or pulsed driven power amplifier yields spectrally stable as well as diffraction limited radiation which can be utilized for a large variety of applications.
Publication
A. Klehr, H. Wenzel, O. Brox, S. Schwertfeger, R. Staske, and G. Erbert, "Dynamics of a gain-switched distributed feedback ridge waveguide laser in nanoseconds time scale under very high current injection conditions", Opt. Express, vol. 21, no. 3, pp. 2777-2786 (2013).
FBH research: 09.01.2015