We have analyzed experimentally and theoretically the modal properties of a semiconductor ring laser and the
wavelength jumps that occur in connection with directional switching above threshold. A transfer matrix analysis
allow us to explain the transfer function measurements when amplified spontaneous emission in the cavity is
accounted for. Moreover the transfer matrix analysis permits to determine the threshold condition for the laser
modes, which split in two branches due to the symmetry breaking imposed by the output coupler and output
waveguides. The wavelength jumps displayed by the device above threshold are interpreted with the frequency
splitting and threshold difference between these two branches of solutions, together with the redshift of the
material gain.
The optical spectrum of monolithic Semiconductor Ring Lasers (SRLs) is measured simultaneously for both lasing
directions with a grating-based OSA, in the regimes of bidirectional and unidirectional operation. In the unidirectional
operation regime the SMSR is larger than 25 dB, and the directional extinction ratio (i.e., the ratio of the power emitted
in the two opposite directions) is larger than 20 dB. The influence of the current injected in the active output waveguides
that act as SOAs is outlined. In the unidirectional regime the linewidth of the SRL is measured by an heterodyne
technique, revealing linewidth values around 2 MHz.
Monolithic Semiconductor Ring Lasers (SRLs) are promising devices for all-optical memory and all-optical switching
applications, as they can operate in a directional bistable regime where only one directional mode (clockwise or anti-clockwise)
is active at one time. The unidirectional bistable regime can be naturally associated to a binary logic, and the
SRL represents an elementary digital memory cell that can be written all-optically, realising the function of an all-optical
flip-flop. In fact, the direction of operation can be switched by injecting an external optical signal pulse into the SRL
through one of the 4 input/output ports.
Directional switching of the SRL-based all-optical flip-flop has been demonstrated by injecting optical pulses with 5 ps
duration into one of the four input/output ports. The required switching energy is around 100 fJ, and the swiching time is
between 100 and 200 ps. The same function has been demonstrated by injecting 400 ps pulses as optical trigger.
The semiconductor ring laser (SRL) is attracting more and more interest as a potential all-optical logic device. Whilst
previous operations used electrical modulation to induce switching, for all-optical applications such as all optical
switching, regeneration, and optical memory it is necessary to switch using an external optical signal. When operated as
a monostable way at 110 mA (just above the threshold of 80 mA) where the device operates in the bidirectional regime,
SRL should also be dynamically forced to work in clockwise (CW) and anticlockwise (CCW) directions depending on
the external injection direction. In this paper the response characteristics of SRL to external optical injection which fed
into SRL by CCW direction are investigated. Both output directions have highly nonlinear relationship with injection
signal power and their responses are highly digital. This operation is also simulated in both directions and the agreement
with experiment is very good apart from the injection power scale. This confirms that the SRL power is constant above a
certain injection power level in both on and off directions, which can be further verified by future devices with 2
couplers.
Theoretical investigation and device measurements are reported to demonstrate the strict fabrication requirements of small diameter shallow etched semiconductor ring lasers. A very accurate control over the dry etching depth is crucial to both minimise the bending losses and achieve very precise control of the coupling ratio in directional couplers. A reactive ion etching process was developed on Aluminium-quaternary wafer structures, showing selectivity greater than 30 between the AlInAs core layer and the InP upper cladding. The process proved very effective in providing a complete and controllable etching of directional couplers with 500nm wide gaps. Assessment on the effect of the bending losses and on the minimum ring radius was performed through characterisation of half ring lasers. A minimum current threshold of 34mA is reported on 150μm ring radius devices emitting at 1300nm.
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