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Technology  GaN lasers


peak power of 2.25 W, a duration of 0.35 ps, and a repetition rate of 16.8 GHz that is set by the cavity roundtrip time. Applying a tapered waveguide and a longer cavity, researchers at the University of Dundee and Alcatel-Thales III-V Lab have demonstrated a peak power of 15 W, and a pulse duration of 0.8-1 ps at a repetition rate of 10 GHz. Self-mode locking does have the downside of a relatively high timing jitter for the generated optical pulses, but this can be addressed by applying a periodic modulation to the absorber at the pulse repetition rate. Do this and the laser operates in the active mode-locking regime.


A vastly different form of laser output is also possible, which is referred to as the Dicke superradiance regime – it involves spontaneous emission of a solitary coherent optical pulse. Prior to the emission of this pulse, the active region must be densely populated with a non- equilibrium of electron-hole pairs. Creating this condition in a GaAs laser enabled researchers at Lebedev Physical Institute to produce pulses with a peak power of a few hundred watts and a width of just 180 fs, operating in a pulse-on-demand mode.


Bridging the gap


It is tempting to think it would be easy task to transfer the technology used to create ultra-short pulses in GaAs-based and InP-based lasers to blue and violet GaN lasers. But that’s not the case. One must realize that although the requirements for making red and infrared femto-second monolithic semiconductor lasers have been known for more than 20 years, practical realisation of such devices is not that advanced – industrial developers of such sources are, in general, still refining these sources in their labs. The reality is that there are very few commercial monolithic mode- locked laser products on the market – we know of only one industrial company offering such a product in reasonable quantities.


One major distinction between the nitrides and their more traditional III-V cousins is a significant difference in their set of intrinsic characteristics. For example, nitrides have a higher effective hole and electron mass. This not only limits the available output power in a GaN laser, but also restricts its pulse width to 30-50 ps (assuming that a conventional design is used and the device is operated in the gain switching regime). However, the large effective hole mass might offer advantages for (quasi-) continuously pumped femtosecond mode- locked GaN lasers, thanks to a high density of hole states. The reality is that this work is in its infancy, and there is a whole spectrum of unanswered questions concerning ultra-fast carrier dynamics in InGaN alloys.


Figure 3.(a) Q-switching performance measured by researchers at Fraunhofer IAF revealed generation of 18ps pulses of 0.6 W as seen on a spectrally resolved streak camera trace.(b) Pulse train detected by researchers from Lebedev Physical Institute on single-shot streak camera,obtained when multiple section InGaN/GaN laser was driven under self-starting passive modelocking conditions.Pulse width is 7 ps, repetition rate is 32 GHz.(c) Features of superradiant emission (top) vs Q-switching (bottom) detected in the laboratories of CSEM using ultrafast detector and sampling scope.As opposed to Q-switching, narrow-width optical pulses with a high jitter in the superradiance regime disappear after a low pass filter at the bandpass of detector has been applied in data processing


Another distinguishing feature of the nitrides is that they have a wurtzite crystalline symmetry, which is markedly different from the zinc blende symmetry associated with conventional III-V materials (see Figure 2). T


he wurtzite crystal structure is responsible for an internal spontaneous and piezoelectric polarization field, which pulls apart the electrons and holes in nitride quantum wells, reducing carrier overlap and quashing radiative efficiency. If the well is too wide it can eliminate optical gain in this active region and thereby prevent lasing.


To realize mode-locking and superradiance in any class of semiconductor laser, the device must contain a suitable saturable absorber. Fortunately, it is relatively easy to form this in a monolithic-cavity with multiple contacts. In conventional III-V devices, tweaking the reverse bias voltage of the absorber section provides control of saturable absorption and output pulse parameters via the quantum confined Stark effect (or Franz-Keldysh effect in bulk active layers). In this class of laser, increasing the negative bias produces a red-


To realize mode-locking and superradiance in any class of semiconductor laser, this device must contain a suitable saturable absorber. Fortunately, it is relatively easy to form this in a monolithic-cavity with multiple contacts


January/February 2012 www.compoundsemiconductor.net 29


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