Systems and methods disclosed herein include an imaging system that may include a laser diode source; an objective lens positioned to direct a focus tracking beam from the light source onto a location in a sample container and to receive the focus tracking beam reflected from the sample; and an image sensor that may include a plurality of pixel locations to receive focus tracking beam that is reflected off of the location in the sample container, where the reflected focus tracking beam may create a spot on the image sensor. Some examples may further include a laser diode light source that may be operated at a power level that is above a power level for operation at an Amplified Spontaneous Emission (ASE) mode, but below a power level for single mode operation.

A semiconductor laser source including a Mach-Zehnder interferometer, this interferometer including first and second arms. Each of the arms is divided into a plurality of consecutive sections, the effective index of each section located immediately after a preceding section being different from the effective index of this preceding section. The lengths of the various sections meet the following condition:

[00001]$\underset{n=1}{\overset{N_2}{.Math.}}.Math.L_{2,n}.Math.{\mathrm{neff}}_{2,n}-\underset{n=1}{\overset{N_1}{.Math.}}.Math.L_{1,n}.Math.{\mathrm{neff}}_{1,n}=k_f.Math.{}_{\mathrm{Si}}$

where: k.sub.f is a preset integer number higher than or equal to 1, N.sub.1 and N.sub.2 are the numbers of sections in the first and second arms, respectively, L.sub.1,n and L.sub.2,n are the lengths of the nth sections of the first and second arms, respectively, neff.sub.1,n and neff.sub.2,n are the effective indices of the nth sections of the first and second arms, respectively. The first and second arms each comprise a gain-generating section.

A method for determining operating conditions of a particle detector that includes a multimode Vertical Cavity Surface Emitting Laser (VCSEL) includes providing an electrical drive current to the multimode VCSEL such that a laser beam is emitted by the multimode VCSEL and varying the electrical drive current within a predefined range of electrical drive currents. The method further includes determining, as a function of the electrical drive current, an intensity signal of an optical wave within a laser cavity of the multimode VCSEL, determining, as a function of the electrical drive current, a noise measure of the intensity signal, determining a range of electrical drive currents for which the noise measure is below a predefined threshold noise measure value, and determining operating conditions of the particle detector by choosing an electrical drive current for particle detection out of the determined low noise range of electrical drive currents.

A semiconductor laser device with external resonator with stable longitudinal mode regardless of variation of drive current is disclosed. The device includes: a semiconductor light-emitting element having a pair of end faces with a light emitting section disposed therebetween, and an external resonator configured to oscillate light emitted from the semiconductor light-emitting element, the external resonator being formed by a resonator mirror disposed outside the semiconductor light-emitting element and one of the pair of end faces that is farther from the resonator mirror, wherein, as the semiconductor light-emitting element, a semiconductor light-emitting element having a structure which does not oscillate light emitted therefrom by itself is used. The device further includes a wavelength control element disposed in the optical path within the external resonator and configured to select a wavelength range of the light, and a driver circuit configured to perform fast modulation drive of the semiconductor light-emitting element.

An example laser has a rear reflector, a front facet spaced from the rear reflector, and a laser cavity defined between the rear reflector and the front facet. The laser comprises a Bragg grating located in the laser cavity, where a length of the Bragg grating (L.sub.g) is in a range from 40% to 60% of a distance from the rear reflector to front of the Bragg grating, and a grating strength (Kappa*L.sub.g) is in a range from 0.6 to 1.5.

A method for determining operating conditions of a particle detector that includes a multimode Vertical Cavity Surface Emitting Laser (VCSEL) includes providing an electrical drive current to the multimode VCSEL such that a laser beam is emitted by the multimode VCSEL and varying the electrical drive current within a predefined range of electrical drive currents. The method further includes determining, as a function of the electrical drive current, an intensity signal of an optical wave within a laser cavity of the multimode VCSEL, determining, as a function of the electrical drive current, a noise measure of the intensity signal, determining a range of electrical drive currents for which the noise measure is below a predefined threshold noise measure value, and determining operating conditions of the particle detector by choosing an electrical drive current for particle detection out of the determined low noise range of electrical drive currents.

Methods, devices and systems for improving single-frequency operation of diode lasers are described. One such method includes ramping up an operational current of a diode laser for a first predetermined number of steps, and measuring an associated current value indicative of optical power within the laser diode for each of the first predetermined number of steps. Next, operational current of the diode laser is ramped down for a second predetermined number of steps, and an associated current value indicative of optical power within the laser diode is measured for each of the second predetermined number of steps. Using the measured data current values at which a mode hop or a multimode operation is likely to occur are identified, and a contiguous range of operating currents that is devoid of identified likely mode hops or multimode regions of operation is determined as the operating current range of the diode laser.

A semiconductor laser device is provided with a semiconductor layer including an active layer and a plurality of cladding layers sandwiching the active layer. The active layer includes a stripe-shaped active region, a pair of first refractive index regions and a pair of second refractive index regions sandwiching the active layer and the pair of first refractive index regions. When ? is the laser oscillation wavelength, n.sub.a is the effective refractive index of the active region, n.sub.c is the effective refractive index of the first refractive index regions, n.sub.t is the effective refractive index of the second refractive index regions, w is the width of the active region, and m is a positive integer, the semiconductor laser device satisfies n.sub.a>n.sub.t>n.sub.c, and the conditions of equations (5), (8) and (9).

A semiconductor laser diode includes a semiconductor body having an emitter region; and a first connection element that electrically contacts the semiconductor body in the emitter region, wherein the semiconductor body is in contact with the first connection element in the emitter region, and at least in places in the emitter region, the semiconductor body has a structuring that enlarges a contact area between the semiconductor body and the first connection element.

In an embodiment, the gain-guided semiconductor laser includes a semiconductor layer sequence and electrical contact pads. The semiconductor layer sequence includes an active zone for radiation generation, a waveguide layer, and a cladding layer. The semiconductor layer sequence further includes a current diaphragm layer which is electrically conductive along a resonator axis (R) in a central region and electrically insulating in adjoining edge regions. Transverse to the resonator axis (R), the central region includes a width of at least 10 ?m and the edge regions includes at least a minimum width. The minimum width is 3 ?m or more. Seen in plan view, the semiconductor layer sequence as well as at least one of the contact pads on the semiconductor layer sequence are continuous components extending in the central region as well as on both sides at least up to the minimum width in the direction transverse to the resonator axis (R) adjoining the central region and beyond the central region.