RU2488929C2 - Method of synchronising line of laser diodes - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: resonance lattice waveguide mirror (3) is placed on a line of laser diodes (1) with a collimating cylindrical lens at an angle to the exit end of a line of laser diodes (1) with a diffraction grating on one or more media boundary surfaces of the resonance lattice waveguide mirror (3) which is in form of a corrugation. The inclination angle, parameters of the resonance lattice waveguide mirror (3) and the diffraction grating are selected such that when radiation of the line of laser diodes (1) with diffraction order of +1 or -1 falls on the resonance lattice waveguide mirror (3), two modes are excited therein, which propagate in opposite directions, upon interaction with the diffraction grating of the resonance lattice waveguide mirror (3) of which they are emitted into the media adjoining said resonance lattice waveguide mirror (3). The resonance lattice waveguide mirror (3) has antireflection properties which prevent spurious generation on Fresnel reflection and provide output of radiation in form of -1 or +1 diffraction order of the radiation of the line of laser diodes (1).

EFFECT: enabling generation of output radiation in form of one diffraction order from a line of laser diodes with divergence defined by its full aperture.

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Description

The invention relates to the field of laser technology, in particular to a method for synchronizing a line of laser diodes.

The basic principle of synchronization is to ensure the transverse spatial coherence of feedback radiation for all emitters of a line of laser diodes.

A known method of synchronizing a line of laser diodes using a resonant grating waveguide mirror. (RU, 2008140133 A, 04/20/2010). In the known method, the radiation pattern of the output radiation contains two lobes, for the formation of a single-petal radiation pattern of the output radiation of a synchronized line of laser diodes, an additional corrective phase diffraction grating is used.

The inventive method allows to exclude this additional element and to provide output radiation in the form of a single beam of diffraction quality.

The technical result to which the invention is directed is to synchronize all emitters of a line of laser diodes, and adjacent diodes emit in antiphase, and generate output radiation in the form of one +1 or -1 diffraction order from a line of laser diodes with a divergence determined by its full aperture.

The technical result is achieved by the fact that the method of synchronizing the line of laser diodes, in which the radiation of neighboring laser diodes is in antiphase, consists in placing a resonant grating waveguide mirror at an angle to the output end of the line of laser diodes with a collimating cylindrical lens a diffraction grating at one or several media interfaces of a resonant grating waveguide mirror made in the form of a corrugation, where the angle of inclination, the parameters the resonant grating waveguide mirror and the diffraction grating are selected so that in the resonance grating waveguide mirror, when the radiation diffraction order is +1 or -1, two modes of the laser diode array are excited, propagating in opposite directions, when interacting with the diffraction grating of the resonant grating waveguide mirrors they are radiated into the medium adjacent to the mentioned resonant grating waveguide mirror, while the resonant grating waveguide th mirror has antireflective properties that prevent parasitic lasing at Fresnel reflection and provides a radiation output as -1 or +1 diffraction order of an array of laser diodes.

The implementation of the method of synchronizing the line of laser diodes is as follows.

Directly behind the collimating cylindrical lens (2) installed in front of the enlightened output end of the laser diode line (1), a resonant grating waveguide mirror (3) (hereinafter referred to as the resonant mirror) is placed, made in the form of a corrugated planar waveguide having resonant reflection at the generation wavelength line of laser diodes (1) when light is incident perpendicular to its surface (see Figure 1). The radiation of a synchronized line of laser diodes (1), in which adjacent diodes emit in antiphase, is similar to radiation from the corresponding diffraction grating. The resonance mirror (3) is installed at an angle to the output end of the line of laser diodes (1), which provides resonant reflection of only one +1 or -1 diffraction order. In this case, another -1 or +1 diffraction order, for which the resonance mirror (3) is transparent, forms the output beam of the line of laser diodes (1) (see Figure 2). This beam has a diffraction divergence of radiation, determined by the full aperture of the line of laser diodes (1), and can be considered as a fundamental mode of output radiation.

The resonance mirror (3) is a dielectric coating in the form of alternating layers of high and low refractive indices deposited on the corrugated surface of a single substrate. The dielectric coating and the corrugation parameters were calculated so that, on the one hand, the width of the angular reflection resonance was much smaller than the angular divergence of the radiation of a single diode in the p-n junction plane, and on the other hand, exceeded the diffraction divergence of radiation determined by the full aperture of the laser diode array (1). In addition, the coating must have an antireflection property for radiation with angles of incidence or wavelength outside the limits of resonance.

The principle of operation of the resonance mirror (3) is as follows. The parameters of the waveguide and the diffraction grating (grating period Λ) are selected so that when a plane wave with a wavelength λ is incident, two waveguide modes propagating in opposite directions are excited to it on the surface of the waveguide. When propagating in a waveguide, these modes interact with corrugated media interfaces and are emitted into the media adjacent to the waveguide. The distance between the modes and the lattice L _rad is determined by its strength (corrugation depth σ) and in practical devices can reach several millimeters. This distance corresponds to the minimum size of the transverse coherence of a light beam reflected by a resonant mirror (3) (see, for example, “Reflection of a limited light beam on a waveguide of limited dimensions.” Quantum Electronics, 1997, v.24, p.457, D6).

Figure 3 shows, as an example, the resonance dependence of the reflection coefficient of a plane wave with a wavelength of λ = 930 nm on the angle of incidence (the vector of the electric field of the wave lies in the plane of incidence - TM polarization). In this example, the resonance mirror (3) is an antireflective two-layer dielectric coating in the form of a tantalum pentoxide film 178.2 nm thick (refractive index 2.1931) and a silicon dioxide film 102.9 nm thick (refractive index 1.4827), sequentially deposited on the corrugated surface of a fused silica substrate with a corrugation period Λ = 580 nm and its depth σ = 48 nm, respectively.

An additional advantage of the resonance mirror (3) is its wavelength selectivity. In FIG. Figure 4 shows the spectral dependence of the reflection coefficient of a resonance mirror (3).

The synchronization mechanism of the line of laser diodes (1) is illustrated in FIG. 2. The minimum divergence of the radiation of an individual diode in the line of laser diodes (1) is estimated as λ / d, where d is the full aperture of the diode, and for a value of d equal to 4 μm, at a wavelength of λ = 930 nm it is equal to 29 angular degrees.

In the case when the radiation of individual diodes is not synchronized, the radiation of the line of laser diodes (1) has a divergence of at least the divergence of an individual diode, which in turn significantly exceeds the angular width of the resonance of 0.25 degrees in the above example, the resonance mirror (3). This leads to a low effective reflection coefficient of the resonance mirror (3) and suppresses the individual generation of laser diodes.

In the case when the radiation of the line of laser diodes (1) is synchronized, and adjacent diodes emit in antiphase, then the radiation of the line of laser diodes (1) incident on the resonant mirror (3) is radiation of +1 or -1 order of diffraction of a plane wave transmitted through the corresponding phase diffraction grating (wave amplitude distribution is shown in the upper part of Fig. 2). The angular divergence of +/- 1 of the diffraction order can be estimated as A / D, where D is the full aperture of the line of laser diodes (1). For a value of D equal to 0.2 mm at a wavelength of λ = 930 nm, this divergence is estimated at 0.27 angular degrees, which is comparable to the angular width of the resonance in the above example, the resonance mirror (3). The power fraction in the corresponding diffraction order can be estimated as 0.4 of the total radiation power of the line of laser diodes (1). This diffraction order is reflected by the resonance mirror (3) with the above parameters with an efficiency of more than 75%. It diffracts back to the resonator on the order of 0.12 = 0.4 0.75 0.4 from the total radiation of the line of laser diodes (1), provided that it matches perfectly with the collimating cylindrical lens (2). Thus, feedback efficiency is expected to be near optimal.

In preliminary experiments with a ruler of 25 laser diodes located at a distance of 8 μm from each other, a collimating antireflective cylindrical lens (2) with a focal length of 290 μm, and a resonance mirror (3) with the parameters given above, 0.3 W of output radiation of a line of laser diodes (1) with divergence in the plane of the line, determined by its full aperture at a pump current two times the threshold value.

Claims (1)

A method of synchronizing a line of laser diodes, in which the radiation of adjacent laser diodes is in antiphase, which consists in placing a resonant grating waveguide mirror at an angle to the output end of the line of laser diodes with a diffraction grating on one or more surfaces on a line of laser diodes with a collimating cylindrical lens media section of a resonant grating waveguide mirror made in the form of a corrugation, where the angle of inclination, the parameters of the resonant grating waveguide mirror and the diffraction grating is selected so that in the resonance grating waveguide mirror when the radiation diffraction order is +1 or -1, two modes are excited that propagate in opposite directions, when they interact with the diffraction grating of the resonant grating waveguide, they are emitted into adjacent to the aforementioned resonant grating waveguide mirror of the medium, while the resonant grating waveguide mirror has antireflection properties excluding spurious generation by Fresnel reflection, and providing a radiation output in the form of -1 or +1 order of diffraction of radiation of a line of laser diodes.

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