We describe the optical design of MEGARA, the future optical Integral Field Unit (IFU) and Multi-Object Spectrograph
(MOS) for the 10.4-m Gran Telescopio CANARIAS (GTC). MEGARA is being built by a Consortium of public
research institutions led by the Universidad Complutense de Madrid (UCM, Spain) that also includes INAOE (Mexico),
IAA-CSIC (Spain) and UPM (Spain).
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.
We analyze a rate equation model in the Langevin formulation for the two modes of the electric field and the
carrier density, modelling the spontaneous emission noise in a semiconductor ring laser biased in the bidirectional
regime. We analytically investigate the influence of complex backscattering coefficient when the two modes
are reinterpreted in terms of mode-intensity sum (I-Spectrum) and difference (D-spectrum). The D-spectrum
represents the energy exchange between the two counterpropagating modes and it is shaped by the noisy precursor
of a Hopf bifurcation influenced mainly by the conservative backscattering. The I-Spectrum reflects the energy
exchange between the total field and the medium and behaves similarly to the standard relative intensity noise
for single-mode semiconductor lasers. Good agreement between analytical approximation and numerical results
is found.
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.
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