Applications of interferometer are countless both in the research and commercial world. Laser sources offer precise
measurements of relative path difference between two interfering beams. An exciting example is LIGO (laser
Interferometer for Gravitational Observatory), which is aiming to resolve length change as small as 10-19 m over a 4 km
length for detection of gravitational waves. However, laser is a disadvantage for microscopic imaging and surface
topography applications usually required in semiconductor industry. A different approach for microscopy is to use white
light in place of laser. White light due to its limited temporal coherence offers a multitude of benefits for imaging
applications. An immediate benefit from white light is the sharp localisation of interference fringe that makes the 3D
topography construction or OCT (Optical Coherence Topography) realisable using a Scanning White Light
Interferometer (SWLI) imager. In Mirau Mode, SWLI performs high resolution imaging; whereas in Michelson mode
Fourier Transform Spectroscopy (FTS) is realised. SWLI can easily be modified into PUPS (Pupil Plane SWLI) for
Ellipsometry. Superimposing Michelson Interferometer known as VISAR (Velocity Interferometer System for Any
reflector) can form interference fringes even in presence of wide angle light scattered from a moving illuminated object.
This paper describes work undertaken at Nanometrics (UK) on simulation of SWLI fringes including high Numerical
Aperture (NA) applications, thin film characterisation, OCT generation and Zemax modelling of compact dispersion-free
vibration-immune Fourier-Transformed spectrometer. VISAR as a modified Mach-Zehnder Interferometer is also
discussed based on the work at Rutherford-Appleton laboratory (UK).
The effects of optical feedback in edge-emitting semiconductor lasers and vertical cavity surface emitting lasers (VCSELs) have attracted considerable attention because the characteristics of both kind of lasers are very sensitive to the effects of reflected light. The phenomena arising in laser diodes subject to optical feedback can be classified according to operating five regimes conventionally identified as regimes I-V. Optical feedback also affects the noise properties of semiconductor lasers. There have been a number of investigations of the noise properties of semiconductor lasers and VCSELs subject to optical feedback. However, such work has been concerned with only some of the above-mentioned regimes of operation. A thorough experimental investigation of the effect of optical feedback in RIN of edge-emitting semiconductor lasers has yet to appear in the literature. In this paper, we present a detailed study of the effect of optical feedback on the average RIN of edge-emitting semiconductor lasers at different bias currents. An experimental study has been performed of the relative intensity noise (RIN) of a semiconductor laser in optical feedback regimes I to V. At low bias current, a low RIN is observed with low feedback feedback ratio, the RIN increased in the coherence collapse regime, then decreased again in regime V. The RIN in regime V is lower than that of the solitary laser. It is observed experimentally that an increase of the RIN occurs for intermediate levels of feedback. Such an increase in RIN appears to be related to enhanced intensity fluctuations. We will consider theoretical approaches to explaining the observed RIN characteristics.
Vertical cavity surface emitting lasers (VCSEL) exhibit self-pulsations in the polarization-resolved light output at certain bias currents, where the dominant polarization direction suffers a transition from one direction to its orthogonal direction. This pulsation in the polarization resolved output could be controlled by an external optical injection in a master-slave configuration with VCSEL used as slave laser. Experimentally it has been demonstrated that the stabilization linear polarization directions preferred in the solitary VCSEL. The exact frequency of the optical injection to effect stabilization is dependent on the injection power and polarization.
A nearly single mode vertical cavity surface emitting laser (VCSEL) subject to optical injection have been investigated experimentally. It is found that regions of polarization-resolved chaotic behavior exist for both positive and negative detuning from the stable injection locking regime. Outside the chaos regimes, several nonlinear dynamical phenomena including frequency pushing, nearly degenerate four-wave mixing, injection locking, limit cycle and period doubling, were also observed.
The relative intensity noise (RIN) of a semiconductor laser subject to optical feedback ahs been experimentally studied. At low bias current, a low RIN is observed with low feedback ration, the RIN increased in the coherence collapse regime (regime IV) and decreased in regime V. The RIN in regime V is lower than that of the solitary laser. The measurements are found to be in good qualitative and quantitative agreement with theoretical predictions. For higher bias current, a higher feedback ratio is needed for the semiconductor laser to transit from regime IV to V.
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