Narrow linewidth tunable diode lasers are an important tool for spectroscopic instrumentation. Conventional external cavity diode lasers are designed as laboratory instrument and do not allow hand-held operation for portable instruments. A new miniaturized type of tunable external cavity tunable diode laser will be presented. The presentation will focus on requirements on the assembly technology of micro-optic components as well as on the physical properties of such devices. Examples for the realization of this new technology will be given in the NIR for Alkaline Spectroscopy as well as in the MIR at 1908nm.
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.
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 briefly review one procedure for calculating the mirror loss spectra of devices containing such features. We then go on to describe a method for synthesising onedimensional slot patterns. This technique allows the lasers emission wavelength to be specified to a high degree of accuracy.
We describe a new approach to achieving high brightness emission from laser diodes using a compound waveguide containing an unusual high order mode. This mode is predominantly single lobed and unusually localized in the low index region of the waveguide. To examine the utility of this mode we calculated the single-mode fiber butt coupling efficiency for an AlGaAs/GaAs compound waveguide of 61 micrometer aperture width, emitting at a wavelength of (lambda) equals 980 nm. Our results show a 2.3 times enhancement of the fiber coupling efficiency when compared with a simple broad area waveguide. Furthermore the coupling efficiency is still almost a factor of two greater than the coupling efficiency of the emission from a single mode ridge waveguide.
We present experimental and theoretical investigations of the temperature dependence of self-pulsation in CD laser diodes. We use a rate equation model to predict the device dynamic behavior over a large temperature range and identify the role of carrier diffusion. We show experimentally and by calculating that the temperature dependence of the threshold current is driven by the carrier diffusion--particularly at low temperature. We experimentally show that for several temperatures the self-pulsation variation with respect to normalized bias current is highly linear. These results call into question whether pulsations in CD laser structures are undamped relaxation oscillations. Our results also suggest that the highly temperature dependent carrier diffusion does not play a first order role in CD laser diode self- pulsation.
COST 240 is a pan-European action collaborating on the investigation of techniques for modeling and measuring photonic components. This action has concentrated on inter- laboratory comparison of measurement and modeling techniques using round-robin measurement of sample devices. The present paper reviews the work performed within this action on measurement of and parameter extraction from single frequency semiconductor laser diodes. Specifically, those measurements that have been made in order to estimate laser parameters include; Relative Intensity Noise, modulation response, emission linewidth, several static characteristics and amplified spontaneous emission below threshold. Some of the parameters that can be estimated from these measurements include; threshold current, external efficiency, diode resistance, internal loss, characteristic temperature, differential gain, gain compression parameter, facet reflectivities, facet phases, index and gain coupling coefficients, and group refractive index. Following a review of the typical measurements performed on circulated lasers within the COST 240 Action by participating laboratories, a brief description will be presented of the physical models adopted to extract the laser diode parameters. Examples will be presented and conclusions given as to the suitability of certain techniques for the extraction of diode parameters for single frequency lasers.
We give an overview of near IR laser diode based methods for gas sensing discussing both the devices enabling this technology and the techniques utilized. Of particular importance for this technology is development of single frequency laser diodes for laser based gas sensing. We discuss and present results on laser diode based gas sensing using sources such as distributed feedback laser diodes, vertical cavity laser diodes and other novel single frequency devices. Detection limits for particular gases of interest, e.g. methane, oxygen and hydrogen sulphide are discussed.
The operation of an efficient cw thulium-doped fiber laser emitting at wavelength, lambda equals 2.31 micrometer is reported. The operation of the fiber laser is optimized with a view to producing a small and efficient laser source for optical absorption based gas sensing. A 2 mW output power combined with a high slope efficiency make this fiber laser a useful source for sensing spectroscopic based hydrocarbon gases which absorb at lambda equals 2.3 micrometer. A lower limit of detection of 1000 ppm meters for methane is routinely demonstrated using an unoptimized laser system. The tunability of the fiber laser has also been investigated in order to target specific hydrocarbon absorption features.
Spectral measurements of the v1 + v2 + v3 combination absorption band of H2S were made over a 10-nm wavelength range around 1.57 m with two tunable distributed feedback (DFB) laser diodes. Targeting the strongest observed H2S absorption feature, a detection limit at atmospheric pressure of 50 ppm/m is indicated from the measured detectivity at higher concentration. We also probed the 2v1 + 2v2 + v3 overtone and combination absorption band of CO2 and investigated the dependence of absorption linewidth on pressure and assessed its implications for expected low-detection limits. These results demonstrate the utility of DFB laser diodes for industrial applications in the area of toxic and trace gas monitoring.
For many years it has been assumed that nonradiative recombination plays a dominant role in determining the high temperature performance of long wavelength laser diodes. We show that this view is inconsistent with the measured temperature dependence of spontaneous emission from light emitting diodes. We conclude that net gain primarily determines the temperature sensitivity of threshold in long wavelength semiconductor lasers.