This paper reports on the experimental characterization by means of optical frequency-domain reflectometry of a White-type multipass gas cell used for trace gas spectroscopy. The fractional Lambertian reflections inevitably arising from the three high reflectivity mirrors of this multipass cell is precisely detected due to the high sensitivity of the reflectometer. Each bounce of light on the mirror surface generates backscattered light, which returns to the sensing system. Then, using the measured distribution of multiple back-reflections as a function of distance the position of the 3mm-thick CaF2 entrance window is clearly identified, thanks to the spatial resolution of 731μm. In addition, the physical distance between mirrors at both sides of the cavity is accurately assessed to be 40.72cm, delivering the exact optical path length of light inside the multipass cell of 30.9853m, which is an important parameter for improving the accuracy of the computation to retrieve the gas concentration from the measured light absorption spectrum.
The most cost-effective solution for modulating data onto an optical carrier is via direct modulation of a semiconductor
laser. Unfortunately, this approach suffers from high chirp. The chirp can be reduced by reducing the on/off modulation
contrast ratio (i.e. by keeping the signaling laser well above threshold when generating both logical ‘0’ and ‘1’ bits), but
the low contrast ratio itself compromises performance. Other techniques can better suppress chirp, e,g., based on selfinjection
or optical injection locking of the directly-modulated laser (slave) to another laser (master) that emits CW light.
However, this technique although very efficient at eliminating chirp, also requires the slave laser be operated well above
threshold. We show however that the issue of the limited on/off modulation contrast can be addressed in this instance by
subtraction of the carrier using a component of the master beam and an interferometric arrangement.
Discrete Mode Laser Diodes (DMLDs) present an economic approach with a focus on high volume manufacturability of
single mode lasers using a single step fabrication process. We report on a DMLD designed for operation in the 1550 nm
window with high Side Mode Suppression Ratio (SMSR) over a wide temperature tuning range of -20 °C < T < 95 °C.
Direct modulation rates as high as 10 Gbit/s are demonstrated at both 1550 nm and 1310 nm. Transmission experiments
were also carried out over single mode fibre at both wavelengths. Using dispersion pre-compensation transmission from
0 to 60 km is demonstrated at 1550 nm with a maximum power penalty measured at 60 km of 3.6 dB.
KEYWORDS: Semiconductor lasers, Modulation, Receivers, Digital signal processing, Temperature metrology, Heterodyning, Phase shift keying, Waveguides, Modulators, Signal detection
We report on discrete mode laser diodes designed for narrow linewidth emission and demonstrate a linewidth as low as
96 kHz. A discrete mode laser diode with a minimum linewidth of 189 kHz was also characterised in a coherent
transmission setup using quadrature phase shift keying modulation. Similar performance to an external cavity laser is
demonstrated at baud rates as low as 2.5 Gbaud. The effect of increased linewidth on transmission performance is also
investigated using lasers with linewidths up to 1.5 MHz.
The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive
index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index
of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the
transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these
perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the
unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be
manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced
below that of the other cavity modes. Here we review the latest results obtained from devices containing such features.
These results clearly demonstrate that these devices exceed the specifications required for a number of FTTH and
Datacomms applications, such as GEPON, LX4 and CWDM. As well as this we will also present initial results on the
linewidth of these devices.
Widely tuneable laser diodes operating in the 1520 ≤ λ ≤ 1570 nm are characterised and compared for use as sources for tuneable laser diode gas absorption spectroscopy. Three gases hydrogen cyanide, ammonia and acetylene with overlapping absorption features within the 50 nm tuning range of the devices were targeted employing wavelength modulation spectroscopy with second harmonic detection.
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