WO2002101895A2

WO2002101895A2 - High power, high brightness wide area laser device

Info

Publication number: WO2002101895A2
Application number: PCT/IB2002/001894
Authority: WO
Inventors: Vladimir Alexandrovitsh Sychugov
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-06-04
Filing date: 2002-05-28
Publication date: 2002-12-19
Anticipated expiration: 2003-12-04
Also published as: RU2197772C1; WO2002101895A3

Abstract

High power, high brightness laser comprising a periodically segmented wide active stripe (2), a high reflectivity rear mirror (1), a wavelength selective mirror (6) making an angle υ with the optical axis z so as to reflect into the active stripe the wave satisfying the Bragg condition in the autocollimation regime on the periodically segmented wide laser stripe for the minus first diffraction order.

Description

High power, high brightness wide area laser device

Field of the invention

The present invention relates to a high power edge emitting, broad area planar laser which provides narrow line and low divergence light output.

Background of the invention

High power semiconductor lasers are increasingly needed in a number of applications of optical techniques, for instance in optical communications for pumping erbium doped fibre amplifiers in the absorption regions of erbium ions at 970 or 1480 nm wavelengths, and for pumping Raman fibre amplifiers in the broad wavelength domain ranging from 1200 to 1650 nm. High power semiconductor lasers are also increasingly needed for pumping solid state lasers for laser machining, for instance in Nd: YAG lasers, and are even increasingly considered as the primary light source for laser machining. Another field of applications is in the generation of nonlinear effects such as in second harmonic generation, as well as in optical parametric oscillators.

In most of the applications mentioned above, not only high output power must be provided, but high brightness as well, which enables the large power emitted by a broad area laser to be spatially focused onto a restricted diffraction limited spot. Edge emitting wide stripe lasers naturally deliver light in the form of several unstable transverse modes which give rise to a beam of poor spectral and spatial coherence exhibiting broad wavelength spectrum and large angular divergence which can not be focused onto a diffraction limited spot. In particular, the effect of filamentation in the active area of semiconductor lasers leads to further degradation and instabilities of the spatial coherence of laser emission.

Several solutions have been disclosed for selecting one of the transverse modes of the wide stripe of a laser. One of the solutions, represented by documents US 5,572,542 and US 6,212,216 Bl, uses two external mirrors whose normal makes a small tilt angle with the optical axis of the wide stripe semiconductor laser, or array of semiconductor lasers. The angular tilt of the external mirrors provides a larger reflection feedback for a narrow angular spectrum corresponding to a restricted group of transverse modes of the broad laser stripe. A second type of solutions, represented by documents US 4,783, 788 and US 5,651,018, involves a mode selector which is integrated monolithically onto the semiconductor substrate. In document US 5,651,018, and in patents disclosing the same type of solutions, the mode selector is also an integrated semiconductor amplifier in the form of a flared waveguide section, the complete device representing a master oscillator power amplifier (MOP A). The solutions disclosed by the above mentioned documents provide an improvement of the spatial coherence of the emission of a wide stripe laser. The improvement is however limited. The above documents do not solve the problem of the stability of high power, high brightness laser emission. High power light generation in semiconductor materials is accompanied by several nonlinear effects of large magnitude. If the power density is large, filamentation, spatial hole burning are inherently present and affect laser emission. Transverse mode selection according to the prior art does not prevent such nonlinear effects to take place because the latter depend on the power density. They do not disclose means to adjust the emitted wavelength. There is therefore the need for a solution which selects a stable transverse mode of a wide stripe laser with high discrimination. The object of the invention is to provide a high power, high brightness, wide stripe laser with stable transverse mode operation without being unduly long.

Summary of the invention.

The above object is met with a wide stripe laser with high reflectivity rear mirror in which an index and gain grating in the wide active stripe is created by a periodical segmentation with longitudinal segments of the wide active stripe and by a wavelength selective front mirror. The periodical wide laser stripe is composed of equally spaced straight segments oriented along the wide stripe laser optical axis. In the present invention the front mirror sustains the stimulated emission in the wide active laser stripe of a wave substantially satisfying the Bragg reflection condition in the autocolimation regime on the periodically segmented wide laser stripe at a wavelength selected in the wavelength reflection domain of the front mirror.

The invention exhibits the following advantages:

• the disclosed laser exhibits high angular and wavelength selectivity since the external mirror sustains the amplification of a transverse modal field supported by the segmented wide laser stripe,

• the disclosed laser is compact since the angularly selective front mirror, placed on the laser stripe or externally close to the front facet, provides all at once high feedback reflectivity for the amplifying wave, and high transmission for the emitted wave,

• the disclosed laser allows wavelength stabilisation, therefore eases the achievement of wavelength interleaved pump laser systems,

• the disclosed semiconductor laser exhibits low vulnerability to filamentation since the gain is spatially and temporally clamped in the segmented stripe of segment width essentially equal to the smaller width of the filaments present in a non-segmented wide stripe laser made of the same semiconductor material system at high current injection,

• the disclosed semiconductor laser can be synchronously spatially multiplexed in the form of a coherent array.

Brief description of the drawings.

FIG. 1 is the top view of the first embodiment of a wide stripe laser according to the invention with integrated front mirror.

FIG. 2 is a cross-sectional view of the segmented wide laser stripe of FIG. 1 in the plane normal to the optical axis z.

FIG. 3 is another cross-sectional view of the segmented wide laser stripe of FIG. 1 in the plane normal to the optical axis z.

FIG. 4 is another cross-sectional view of the segmented wide laser stripe of FIG. 1 in the plane normal to the optical axis z.

FIG. 5 is the top view of a second embodiment of a wide stripe laser according to the invention with cleaved front mirror.

FIG. 6 is the top view of a third embodiment of a wide stripe laser according to the invention with external front mirror.

FIG. 7 is a top view of a first external mirror of the embodiment of FIG. 6.

FIG. 8 a is the top view of an integrated assembly of the mirror and of the lens of the embodiment of FIG. 6.

FIG. 8b is the cross-sectional view of the integrated mirror-lens assembly of FIG. 8a normal to the wide laser stripe and containing the optical axis. FIG. 8c is another cross-sectional view of a more integrated mirror-lens assembly of FIG. 8a normal to the wide laser stripe and containing the optical axis. FIG. 9 is the top view of a second external mirror of the embodiment of FIG. 6. FIG. 10 is the top view of a more compact third external mirror of the embodiment of FIG. 6. FIG. 1 1 is the top view of a fourth and of a fifth external mirror of the embodiment of FIG. 6. FIG. 12 is the top view of a sixth external mirror assembly of the embodiment of FIG. 6 with external mirror comprising a reflection grating and a mirror.

FIG. 13 is the top view of a seventh external mirror assembly of the embodiment of FIG. 6 comprising two symmetrical reflection gratings.

FIG. 14 is the top view of a more monolithic seventh external mirror assembly of FIG. 13. FIG. 15 is the top view of an eighth external mirror of the embodiment of FIG. 6 with external mirror comprising a cylindrical mirror and an angularly selective mirror. FIG. 16 is the top view of a fourth embodiment of the wide stripe laser in the form of an array of coupled wide stripe lasers placed side by side with external mirror according to FIG. 9. FIG. 17 is the cross-sectional view of a fifth embodiment of the wide stripe laser in a plane orthogonal to the planes of the wide stripe lasers and containing the optical axis showing a stack of coupled wide stripe lasers placed on top of each other with external mirror according to FIG. 9.

FIG 18a is the top view of a sixth embodiment of the wide stripe laser in the form of a two- dimensional array of coupled wide stripe lasers placed side by side with external mirror according to FIG. 12.

FIG. 18b is the cross-sectional view of the sixth embodiment of the wide stripe laser of FIG. 18a in a plane orthogonal to the optical axis showing a stack of coupled wide stripe laser arrays placed on top of each other.

Description of the preferred embodiments

A laser device according to the present invention is shown in FIG. 1. The wide laser stripe 2 has a length L along the optical axis z of the wide stripe laser, a width W, a rear mirror 1 of large reflection coefficient, a front edge 21, and a set 24 of active segments 23, the individual segments 23 being oriented along the optical axis z and arranged substantially periodically with period Λ along the y axis normal to the optical axis z and contained in the plane of the wide stripe 2. In this first embodiment, the front mirror 61 is a wavelength selective Bragg reflection grating of period Λβ made on top or within laser stripe 2. The k-vector of grating 61 is within the plane (y,z) of the wide stripe 2 and makes an angle θ_s with the optical z axis of the wide stripe laser so as to reflect into the wide stripe laser a wave emitted by the laser whose wave vector makes an angle θ_s with the z axis. The integrated front mirror 61 sustains the stimulated emission of the wide stripe laser in a direction θ_s corresponding to the laser feedback configuration of the 1^st order autocolimation, or Littro regime, for the active grating of period Λ which the segmented wide stripe 24 represents. The angle θ_s substantially satisfies the Littrow condition on the segmented wide stripe laser: sinθ_s = λ/(2Λ) where λ is the wavelength in the wide stripe layer 2. λ is equal to the wavelength λ₀ in vacuum divided by the effective refractive index of the lasing mode of layer 2, this mode having a field confined in the direction x perpendicular to the wide laser stripe. Consequently, the period Λβ of the Bragg reflection grating mirror 61 is Λβ = λ/2.

The wavelength λ₀ is within the gain bandwidth of the active material which the active wide stripe laser is made of, preferably close to the maximum of the gain curve. The laser output takes place under an angle θ_s relative to the optical axis z in the direction of the zeroth order of the grating which the segmented stripe 24 represents. The output beam is incident on the Bragg mirror 61 under the angle 2Θ_S. It will not be reflected since the wavelength selective mirror 61 is also angularly selective. Those familiar to the art of integrated optics will know how to engineer the refractive index distribution in layer 2 in the x direction to prevent the Bragg grating 61 from diffracting the output beam into the adjacent media.

FIG. 2 is the cross-section of the wide stripe laser in a plane orthogonal to the optical axis z. It corresponds to an optically pumped layer 2 made for instance of rare-earth doped oxide or fluoride, or of a doped polymer on a substrate 25 or of a semiconductor. The segmentation of the wide stripe 2 in the form of periodical segments 23 is made either by spatially selective doping of active species such as diffusion of erbium ions, or by optically pumping a uniformly doped layer 24 by a pump beam exhibiting bright fringes of period Λ directed along the optical axis z.

FIG. 3 is another cross-section of the wide stripe laser in a plane orthogonal to z. It corresponds to an electrically pumped layer 2 with a uniform top electrode 3 and bottom electrode 32. Layer 2 contains a periodical array 24 of active segments 23 of period Λ. The active segments 23 can be segments of electrically pumped light emitting polymer, or periodically distributed segments of semiconductor junctions embedded into buffer layers between electrodes 3 and 32.

FIG. 4 is another cross-section of the wide stripe laser in a plane orthogonal to z. The wide laser stripe 2 is uniform. The top electrode 3 is segmented with period Λ. The injection current lines flow from each electrode segment 31 to a common electrode 32 located under the active layer 2. They define an index and gain grating 24 in the active layer 2. The active layer 2 and the adjacent layers 26 can be of any usual type defining a heterostructure or double-heterostructure and comprising a quantum well or multiple quantum well. The ratio between the width of the electrodes segments 31 and space between them, as well as the distance between the set of electrodes 3 and the common electrode 32 located under the active layer 2 are adjusted so as to lead to an adequate overlap of the current lines in the active layer 2. The overlap is preferably such that the parts of the active layer 2 under the spacing between electrodes 31 receive enough injected current to be at least above threshold so as to prevent absorption losses in the laser cavity.

The so formed gain and index grating 24 prevents filamentation and related instabilities to take place as the gain is spatially and temporally clamped in the form of a regular mesh attached to the material structure. Under high power generation conditions, a uniform wide stripe semiconductor laser is known to exhibit filaments whose width may shrink from 40 to 10 micrometers as the power increases and tend to stop decreasing at an injection current equal to about 1.5 times the threshold current as reported in A.P. Bogatov et al., "Brightness and filamentation of a beam of powerful cw quantum-well Ino.₂Gao.₈As/GaAs laser", Quantum Electronics, 30(5) 401-405 (2000). The period Λ between electrodes 31 must therefore be chosen of the same order as the smaller filament width exhibited in the used material system under high power conditions.

Not illustrated is a further embodiment where both the electrode 3 and the active layer 2 are segmented, the electrode segments 31 being aligned above the active layer segments 23. FIG. 5 is the top view of a second embodiment of the device according to the invention where the front mirror 62 is the edge of the laser 2 cleaved under and angle θ_s relative to the optical axis z with a wavelength and angularly selective multilayer coating, θ_s being the angle between the optical axis z and the normal to cleaved edge 21. FIG. 6 is the top view of a third embodiment of the device according to the invention where the front mirror is the external mirror 6. The edge 21 of the front facet of the laser is coated with an antireflection dielectric multilayer 4. A cylindrical lens 5 of axis parallel to y lying in the plane of the wide laser stripe and normal to the optical axis z collimates the beam emitted by the laser edge 21. The external plane mirror 6 is placed with its normal within the plane of the wide laser stripe and making an angle θ_a with the optical axis z. The external mirror 6 is angularly selective to allow for a compact, short length laser device. It exhibits high reflection for a wave under substantially normal incidence and low reflection for a wave under incidence angle 2θ_a. The angle θ_a of the external mirror 6 relative to the z axis is essentially given by sin θ_a = λo/(2Λ) if the external medium is air. The external mirror 6 is made of a wavelength selective dielectric multilayer, the peak reflection wavelength being within the gain bandwidth of the semiconductor material. The external mirror 6 can also be a grating waveguide resonant reflector with limited reflection wavelength bandwidth. The periodically segmented laser stripe 2 is provided with a feedback mechanism and configuration which sustains spatially and spectrally coherent emission. It is often assumed, as in document H.Yang, L.J. Mawst, and D. Botez, "1.6 W continuous-wave coherent power from large-index-step (Δn=0.1) near-resonant, antiguided diode laser arrays", Applied Physics Letters, Vol. 76, March 2000, pp. 1219-1221, disclosing a periodically segmented active laser zone, that feedback is best exerted by a mirror oriented normally to the optical axis since this feedback configuration favours the emission of the fundamental mode of the wide laser stripe. However, the loss difference between first order modes is not sufficient to ensure the lasing of the sole fundamental mode. This results in a large divergence of the emitted beam with side lobes. In the present invention a much finer selective feedback mechanism is provided. It acts on a wave which satisfies the Bragg reflection condition substantially corresponding to the - 1^st order Littrow configuration for the periodically segmented wide laser stripe 2. To that end, external mirror 6 makes an angle θ_a with the laser front edge 21 essentially satisfying the condition sin θ_a = λrj/2Λ where λ₀ is the central wavelength in vacuum or air of the narrow wavelength range Δλ reflected by mirror 6. The wave reflected by external mirror 6 enters the wide laser stripe 2 through cylindrical lens 5, which can be an optical fibre, and front facet 21 where it experiences -1^st order Bragg reflection on the gain and index grating which the segmented wide stripe 2 represents. The -1^st order Littrow Bragg reflection first involves a high efficiency coupling to a transverse mode of the segmented wide stripe 2, then the propagation of the transverse mode with amplification on the way down to mirror 1 where it is reflected back with amplification on the way back to the edge 21 where it exits in the direction of mirror 6 for further round trips in the laser cavity. The index and gain grating 24 of period Λ of the segmented layer 2 in FIG. 2 and FIG. 3, or the index and gain grating 24 of period A created by the periodically segmented electrode 3 of FIG. 4, and the Bragg diffracted wave reflected by external mirror 6 give rise to electric field lobes in the segmented active stripe 2 with alternate minima and maxima. The amplified wave has the same field pattern as the Bragg reflected wave and consequently has high spatial and spectral coherence with a wave front parallel to mirror 6. The transmitted wave consequently has high spatial and spectral coherence. The relationship between the reflectivity of external mirror 6 and the reflectivity of the front facet 21 of the wide laser stripe 2 is such that the Bragg reflection feedback sustained by the external mirror 6 dominates the non-selective feedback of the front facet 21 with antireflection multilayer 4. For instance, if the front facet reflection is 10%, the reflection of external mirror 6 is preferably larger than 30%. If the reflection of the front facet 21 is made as small as 0.5% by means of antireflection multilayer 4, the external mirror reflectivity should preferably be larger than 10%. However, the reflectivity of mirror 6 can be largerand preferably 100% so as to have most output power generated by the laser transmitted to^the output port of the laser device. The output port of the device is the zeroth reflection order of the gain and index grating 24.

FIG. 7 is the top view of a first external plane mirror 6 in a plane parallel to the wide laser stripe 2 comprising a transparent substrate 601, and a wavelength selective multilayer mirror 602. Multilayer 602 provides high reflection for the incident wave normal to its surface, and low reflectivity for the exit wave with angle of incidence 2θ_a in the wavelength range of laser operation in order to enable the positioning of mirror 6 close to the cylindrical lens 5, therefore to lead to a compact, short length external cavity laser. The desired angular selectivity can be achieved by depositing a large number of substantially "λ/4" layers of low refractive index contrast and resorting to the a multilayer design tool available on the market. The side 603 of mirror 6 opposite to multilayer mirror 602 is preferably coated by an antireflection coating to ease the exit of the emitted wave.

Mirror 6 can also have the multilayer mirror 602 turned toward the outside of the laser device, the incident wave impinging first on side 603 of transparent substrate 601. This configuration allows a more integrated assembly of mirror 6 and lens 5. FIG. 8a is the top view of the lens- mirror assembly in a plane containing the incident beam and parallel to the wide laser stripe 2. FIG. 8b is the cross-section of the mirror-fibre assembly of FIG. 8a in a plane normal to the wide laser stripe 2 and containing the optical axis z. The cylindrical lens 5 forms with mirror 6 a hybrid assembly. The cylindrical lens being parallel to the front edge of the wide laser stripe, the angle θ of the multilayer mirror 602 is essentially sinθij = λo/(2n_mΛ) where n_m is the refractive index of substrate 601. FIG. 8c is another cross-section as in FIG. 8b of an even more integrated assembly of lens 5 and mirror 6 where a cylindrical diffractive lens is realized in substrate 601 by means of planar lithographic and etching technologies. For a given period A of the segmented stripe 2, the lasing wavelength λo and mirror angle θ_a are related by λ₀ = 2Λsinθ_a. This means that a rotation of the external mirror 4 around an axis parallel to x, normal to the plane of the active stripe, achieves a sweeping of the emitted wavelength λo provided the reflection bandwidth Δλ of the feedback mirror 6 is as wide as the sweeping range. The described tuning mechanism can also be used to stabilize the emitted wavelength at a prescribed value by adjusting the mirror angle θ_a.

FIG. 9 is the top view of a second external mirror 6 in a plane parallel to the wide laser stripe 2 comprising transparent substrate 601, a preferably singlemode slab waveguide 604^' with coupling grating 605 of period Λ_g whose lines are normal to the plane of the wide laser stripe 2. The normal to waveguide layer 604 makes an angle θ_a with the optical axis z. The reflection mechanism is that of abnormal reflection, (also called resonant reflection) whereby the incident beam of finite cross-section couples into waveguide 604 by means of grating 605 and is then coupled back in the reflection direction with high reflection coefficient. It is known by those familiar to the art that this resonant reflection results from the destructive interference in transparent substrate 601 between the 0^lh transmitted order of the grating 605 on the one hand, and the wave which is coupled into waveguide 604 by means of grating 605, then re-radiated out of waveguide 604 by the action of the same grating 605 on the other hand. A destructive interference in the substrate 601 implies a high reflection into the incident medium per energy conservation. For abnormal reflection to take place, two conditions are fulfilled. The first condition is the synchronism condition of waveguide mode excitation which in the case of normal incidence on mirror 6 states essentially Λ_g = λo/n_e(λ) where λo is the emission wavelength in vacuum and n_e(λo) is the wavelength dependent effective index of a propagating mode of the grating coupled slab waveguide 604. The second condition is that the product wα of the incident beam width w by the radiation coefficient α of the waveguide grating 605 is larger than 1 , preferably larger than essentially 2π if the requested value of reflection coefficient of mirror 6 is close to 100%. Waveguide excitation conditions are well known to those familiar to the art (see for instance: T. Tarnir, Integrated Optics, T. Tamir Ed., Vol. 7, 1979, p. 105, Springer- Verlag, Topics in Applied Physics). Abnormal reflection from a waveguide grating, and the conditions therefore, are also known to those familiar to the art and are abundantly described as for instance in LA. Avrutsiky, V.A. Sychugov, "Reflection of a beam of finite size from a corrugated waveguide", J. Modern Optics, Vol. 36, pp. 1527- 1539, 1989. relates to the coupling strength of grating 605; it depends on the waveguide and grating opto geometrical parameters. In the device according to the invention, the width w of the wide laser stripe 2 is rather small, for instance 200 μm. This implies that the width of the beam incident on mirror 6 as about 200 μm also since the period Λ of the segmented laser stripe 2 is of the order of 10 μm, therefore the angle θ_a is small, typically 3 degrees, at the wavelength of 970 nm. In order to give rise to a strong value for α equal to or larger than essentially 200 cm^"1, the grating must be highly efficient. This is achieved by using a high refractive index waveguide 604 on a low index substrate 601 for a high modal field confinement, for instance a Ta₂O₅ or TiO₂ layer on a glass or quartz substrate in the 400 to 1600 nm wavelength range, or a single crystal silicon layer on a quartz or pyrex substrate in the infra-red range (λo above 1100 nm). The radiation coefficient α also depends on the polarization and whether the grating corrugation is first performed in substrate 601 or after the deposition of layer 604 as illustrated in FIG. 9. In a preferred embodiment, the incident beam has its electric field polarized in the plane of the laser stripe 2, as it is known to those familiar to the art. This polarization is consequently coupled by grating 605 to the TMQ mode of waveguide 604. TM mode coupling under normal incidence is more efficient (α larger) in the case of a grating having a single undulation 605, and no undulation between waveguide 604 and substrate 601. As an example at 970 nm wavelength λo, an ion plated Ta₂O₅ film 604 of 300 nm thickness w and 2.22 refractive index on a fused quartz substrate 601 and a grating depth of 60 nm and period Λ_g = 550 nm give rise to a radiation coefficient α of essentially 200 cm^"1. The described resonant reflection provides a high spectral and angular selectivity. In particular, the transmitted exit beam does not couple to the TMo mode and crosses mirror 6 with low reflection. It also crosses mirror 6 without diffraction to non-guided orders since only the 0^th diffraction order can propagate. Very low reflection can be achieved if the grating waveguide film 604 has a thickness t such that it represents a zeroth order Fabry-Perot filter in transmission under 2θ_a incidence angle. This condition writes essentially t = 0.5λo/(n_m ²-sin² 2θ_a) " where n_m is the refractive index of substrate 601. There are two conditions to fulfil for the operation of the described abnormal mirror 6 in the device according to the invention: λo = 2Λsinθ_a as set by the Bragg condition on the segmented laser stripe, and λo = Λ_gn_e(λo) as requested for abnormal reflection under normal incidence . The method for adjusting the working point of the laser is to first find out the angle θ_a from the period A of the segmented wide laser stripe 2, and from the wavelength λ_c at the center of the laser material system gain curve. Then the period Λ_g is determined from λ_c and n_e(λ_c) in a waveguide 604 satisfying the condition for large α and low reflection coefficient for the transmitted beam. A fine adjustment of θ_a, therefore of λo, then provides the fulfilment of both conditions stated above. The above description relates to a grating waveguide having only one corrugated surface. Those familiar to the art can also consider a grating waveguide having two corrugated boundaries as well as the coupling to another polarization. For instance, the polarization emitted by the segmented wide laser stripe 2, which is in general TE as known by those familiar to the art, is coupled to the TEo mode of the waveguide by means of grating 605 having its groove lines parallel to the plane of the wide laser stripe 2. The condition for high abnormal reflection is here αw' > 1, preferably larger than essentially 2π for close to 100% reflection, where w' is the width in the x direction of the incident beam collimated by lens 5. Grating 605 can also be of another type. For instance it can be a phase grating photo- imprinted in the waveguide layer 604 made of a photosensitive Dupont resin (OmniDex HRF- 600 or HRF-700) achieving a refractive index modulation of up to 0.06; it can also be a phase grating made in a quartz substrate by means of spatially resolved Ti ion implantation; phase gratings present the advantage of exhibiting lower scattering than corrugation gratings. The waveguide 604 and grating 605 can also be combined so as to form a multilayer waveguide made of a stack of alternate low and high index layers where all interfaces are corrugated. A further embodiment of the device of the invention comprises a saturable absorber in the laser cavity so as to achieve a Q-switched regime. A saturable absorber plate can be placed between lens 5 and mirror 6. A preferred and more compact embodiment is achieved by waveguide 604 being made of a saturable absorber material as for instance a chromium doped YAG layer grown on a YAG substrate 601 by liquid phase epitaxy or a saturable absorber semiconductor layer 604 deposited on transparent semiconductor substrate 601. A third external mirror giving rise to a particularly compact laser device is shown in FIG. 10 where the lens 5 is absent and the reflective surface 602 of FIG. 7 and 604 of FIG. 9 making the angle θ_a with the edge 21 of the wide laser stripe is kept at a distance from edge 21 inferior to the Rayleigh depth of focus d = λo/2sin²(φ/2), (φ being the total angular aperture of the beam emitted by the wide laser stripe in the plane normal to the laser stripe plane and containing the optical axis; usually φ is between 40 and 50 degrees, i.e., d is essentially equal to 4λo) of the emitted beam by segmenting mirror 6 in a number of saw tooth mirror segments 608 of generic lines normal to the plane of wide laser stripe 2, the long side of each segment making an angle θ_a with edge 21. Edge 21 can have an antireflection coating 4 as well as the back side 603 of mirror 6. The number of saw tooth segments 608 is larger than substantially wθ_a/d. For instance, if θ_a = 3 degrees, w = 200 μm, and d = 4 μm, the number of segments 608 is essentially larger than 2. For the segmented mirror 6 to perform as mirror 6 of FIG. 7 and of FIG. 9, the blazed grating formed by segments 608 must be coherent, i.e., the product 2πu/λo must be an integer number of π where u is the height of the saw tooth profile. The segmented reflective layers 609 can be a wavelength and angularly selective multilayer or a grating waveguide with grating lines parallel to the wide laser stripe 2 exhibiting abnormal reflection. The condition on d to be smaller than the Rayleigh depth of focus is however not a necessary condition for the operation of the high brightness laser device; in case d is somewhat larger than the Rayleigh depth of focus, the resulting loss can be compensated for by a larger reflection coefficient of mirror 6.

A fourth external mirror is illustrated in FIG. 11 where the wavelength selective reflection is achieved by means of a reflection grating in the Littrow mount for the -1^st order. The reflection diffraction grating 610 of period Λ_s is placed at a distance from the wide stripe edge 21 where it does not intercept the emitted beam. The laser output takes place in the 0^th order of the grating of period A which the wide laser stripe 2 represents. The relationship between the reflected wavelength λ_s, the period Λ_s of grating 610 and the angle α between the surface of grating 610 and the emitted beam is λ_s = 2Λ_S sinα. Besides, the relationship between the emission angle θ_a, λ_s and the period Λ of the wide laser stripe is λ_s = 2Λ sinθ_a. The angle α can be close to the emitted beam angle θ_a so that the emitted beam illuminates a large number of periods Λ_s of grating 610. The spectral resolution can therefore be so fine that the emission of a single transverse mode of the wide laser stripe 2 is favoured. The diffraction efficiency of such grating under large incidence angle (π/2-α) can be made large by using the teaching of an article of the scientific literature (V.A. Sychugov, B.A. Usievich, K.E. Zinoviev, O. Parriaux, "Autocollimating diffraction gratings based on waveguides with leakage modes", Quantum Electronics, Vol. 30, No 12, pp. 1091- 1098, 2000). The wavelength selective mirror 6 comprises a plane mirror 611, and a dielectric layer 612 in which or on top of which the grating 610 is formed.

A fifth external mirror assembly, also shown in FIG. 11 , enhances further the wavelength selectivity by increasing the number of times the emitted beam experiences the wavelength selective reflection on mirror 6. It comprises a second mirror 618 in addition to the wavelength selective grating mirror 6. Mirror 618 can be a standard, non selective, 100% reflection mirror making an angle θ_a with the Y axis as illustrated in FIG. 11, or it can also be a highly reflective second mirror (not illustrated) of the type of mirror 6, placed symmetrically to grating mirror 6 with respect to the optical axis z, the laser output taking place along the zeroth order of the first grating mirror 6.

A sixth external mirror is illustrated in FIG. 12 where the wavelength selective reflection is achieved by the combination of a wavelength selective grating mirror 6 and a standard broadband mirror 613 in the form of a Littman-Metcalf mount. The wavelength selective mirror 6 of period Λ_s is placed at a distance from the wide stripe edge 21 where it does not intercept the zeroth order emitted beam. The front surface of grating 610 makes an angle with the -1^st order Bragg reflected beam. Grating 610 is a wavelength selective reflection grating operating according to US patent 6,219,478 Bl. It is composed of a plane mirror 611 which can be made of metal layer or of a dielectric multilayer, a dielectric layer 612 on top of mirror 61 1 , a corrugation or index grating in layer 612. The beam coming from the laser edge 21 is diffracted with high efficiency by mirror 610 in the direction of mirror 613 which is neither angularly nor wavelength selective. The reflecting surface of mirror 613 makes an angle (ε_c+α-θ_a) with the optical axis z where ε_c is the angle of the beam diffracted by grating 610 relative to the normal to grating 610. Mirror 613 reflects the beam diffracted by grating 610 back into the segmented wide stripe laser and acts as the external mirror at a vacuum wavelength λ_s satisfying the condition λ_s=Λ_s(cosα+sinε_s). The angles α and ε_c, the period Λ_s are chosen so that λ_s is within the gain bandwidth of the laser material. Furthermore, the thickness of layer 612 and the groove depth of grating 610 are chosen from US patent 6,219,478 B 1 so that the diffraction efficiency of grating 610 is large at the wavelength λ_s and so that the angular dependence of the diffraction efficiency of grating 610 is narrow enough to sustain the amplification of the wave satisfying the Bragg reflection condition for order -1 by the segmented laser wide stripe 2. The device disclosed by US patent 6,219,478 Bl allows a very large, up to 100% diffraction efficiency even under grazing incidence. The narrow beam of width W emitted by the wide laser stripe 2 is therefore made to see a large number N_s of grating periods Λ_s thanks to a small angle α; this increases the wavelength selectivity of the external mirror assembly. As an example, assuming α = θ_a = 0.025 radian, the condition of the present invention as a result of A = 20 μm, assuming a 1 μm wavelength λ_s, a number N = 10 stripes, thus a width W = 200 μm, is that the number N_s of grating lines of grating 610 illuminated by the beam is about N_s = 2NΛ²/(λ_sΛ_s) which leads to a spectral resolution of the external mirror assembly of about 7000.

A seventh external mirror assembly is illustrated in FIG. 13. If the transverse modes of the wide laser stripe 2 are spectrally very close to each other, the device of FIG. 12 may fail to resolve them spectrally. The seventh external mirror assembly comprises the grating mirror 6 of FIG. 12 and a second grating mirror 63 similar to mirror 6, and placed exactly symmetrically to mirror 6 with respect to the optical axis z. The spectral resolution of the external mirror assembly is further enhanced by the presence of the second mirror 63 and by the number of round trips within the laser cavity. Gratings 6 and 63 are identical except that grating 6 has a close to 100% diffraction efficiency whereas grating 63 has a somewhat different grating depth and exhibits lower diffraction efficiency, and thus represents the laser output. Both gratings make an angle relative to the optical axis z such that the sole 0^th and -1^st diffraction orders exist and such that the -1^st diffraction order is essentially normal to the optical axis z at the wavelength of the filtered lasing transverse mode. The two gratings can be made on two separate plates, the space between them being filled with air as shown in FIG. 13. The two gratings can also be part of a monolithic block 630 and be placed at either side of a transparent substrate 627 with an integrated cylindrical lens 5 at the side turned towards the wide laser stripe as illustrated in FIG. 14. Mirrors 6 and 63 comprise a plane mirror 611, a low index buffer layer 624, a high index layer 626, and a corrugation grating 610 between layers 624 and 626. The refractive index of substrate 627 is comprised between the index of layer 624 and of layer 626. The thickness of buffer layer 624 is such that the -1^st order diffracted by grating 610 in the direction of mirror 611 adds up constructively after reflection on mirror 611 with the -1^st order diffracted directly into substrate 627.

An eighth external mirror is illustrated in FIG. 15. The circularly or parabolically cylindrical mirror 615 is placed at a distance from the wide stripe edge 21 where the -1^st order Bragg reflected beam is separated spatially from the zeroth order emitted beam. The generic lines of mirror 615 make an angle of 90 degrees with the optical axis z and are parallel to the plane of the wide laser stripe 2. The focal line of mirror 615 is aligned on the edge 21. Between edge 21 and mirror 615 is a wavelength selective mirror 616 on a substrate of the types described by reference to FIG. 6, 7, and 9. The normal to mirror 616 makes an angle θ_a with the optical axis so as to sustain the amplification of the wave satisfying the -1^st order reflection diffraction at the segmented laser wide stripe 2.

Not illustrated is a further embodiment where the cylindrical mirror 615 forms with the wavelength selective mirror 616 a single monolithic element.

FIG. 16 represents a fourth embodiment of the device of the invention comprising an array 8 of high brightness segmented laser stripes 81 arranged side by side at the surface of the same substrate 82 in the y direction. Cylindrical lens 5 collimates the beam of all segmented laser stripes 81. The single mirror plate 9 with slab waveguide 91 supports the abnormal grating mirrors 92 of all laser stripes 81. The normal to mirror plate 9 lies in the common plane of the wide laser stripes and makes an angle θ_a with the optical axis z. The lines of gratings 92 are parallel to the direction x and are normal to the substrate surface 82. The abnormal grating mirrors are designed so as to allow a part of at least 5% of the wave field power coupled into waveguide 91 to propagate in the gratingless waveguide sections up to the nearest neighbours where it is mixed with the locally reflected field. The individual spatially and spectrally coherent wide laser stripes 81 are coupled to each other, favouring the coherent emission of all lasers 81 of the laser array 8.

FIG. 17 represents a fifth embodiment of the device of the invention comprising a stack 10 in the x direction of individual high brightness segmented wide stripe lasers 101 having parallel stripe planes each with its collimation lens 51. The single mirror plate 9 with slab waveguide 91 supports the abnormal grating mirrors 93 of all laser stripes 101. The normal to mirror plate 9 lies in a plane parallel to the planes of the wide laser stripes and makes an angle θ_a with the optical axis z. The lines of gratings 93 are parallel to the planes of the wide laser stripes 101. The abnormal grating mirrors 93 are designed so as to allow a part of at least 5% of the wave field power coupled into waveguide 91 to propagate in the gratingless sections of waveguide 91 up to the nearest neighbours where it is mixed with the locally reflected field. The individual spatially and spectrally coherent laser stripes 101 are coupled to each other, favouring the coherent emission of all lasers 101 of the laser array 10. FIG. 18 represents a sixth embodiment of the device of the invention comprising a two- dimensional array of high brightness segmented stripe lasers placed side by side along axis y, and on top of each other along axis x. FIG. 18a is the top view of array 8 of wide stripe lasers 81 with an external synchronization feedback exerted by a reflecting diffraction element 621 and a reflector 620. The diffraction element 621 comprises a grating 610, a dielectric film 612 on top of a mirror 611, and operates according to US patent 6,219,478 Bl. The -1^st order reflected from each segmented wide laser stripe 81 makes an angle ε_c with the normal to the plane of grating 610, and diffracts with high efficiency in a direction making an angle α with the plane of grating 610. As illustrated, the angle α is small so that the diffracted beams emanating from each wide stripe laser 81 propagating towards reflector 620 partially overlap on the said reflector, allowing a synchronization of the individual wide laser stripes 81 of the laser array 8. The period Λ_s of grating 610 is given by Λ_s = λ_s/(cosα+sinε_c) where the wavelength λ_s is within the gain bandwidth of the laser material. The angle η between the mirror 611 and the y axis is η=θ_a-ε_c; the angle γ between the mirror 620 and the z axis is γ=α- η. An additional, partially reflective plane mirror 110 placed in a plane normal to the optical axis z at an abscissa along z where the beams emitted under angle θ_a by the adjacent wide stripe lasers cross each other reinforces the synchronism coupling between adjacent lasers. FIG 18b is the cross-sectional view of the reflecting diffraction element 621 and of the reflector 620 in the plane orthogonal to the optical axis z. The grooves of grating 610 are parallel to the axis x. The reflector 620 comprises a number of reflecting right angle corners 622 equal to the number of laser arrays placed on top of each other in the x direction. Whereas grating 610 synchronises the wide stripe lasers 81 placed side by side along y, the set of corners 622 synchronises along x the laser arrays along y.

Claims

1. Wide stripe laser device having an optical axis and comprising: a periodical set (24) of active segments (23) of period Λ parallel to the optical axis and defining a plane substantially parallel to a plane defined by the wide stripe (2), a high reflectivity rear mirror (1), a front facet (21), a wavelength selective mirror (61), the normal to said selective mirror being in a plane parallel to the plane of the wide stripe and making an angle θ with the optical axis, this angle being substantially related with the period A and the wavelength λ of the wave emitted by the laser device in the material in which this emitted wave propagates towards the front facet (21) by the minus first (-1^st) order Bragg reflection condition on the periodical set of segments (sinθ = λ/(2Λ)).

2. Device according to claim 1 where the wavelength selective mirror is an external mirror (6).

3. Device according to claims 2 comprising a cylindrical lens (5) between the front facet (21) and the wavelength selective mirror (6).

4. Device according to claims 2 comprising a cylindrical mirror (615) between the front facet (21) and the wavelength selective mirror (616).

5. Device according to any of claims 1 to 4 wherein the edge (21) of the wide stripe laser has a low reflection coating (4).

6. Device according to any of claims 1 to 5 where the wide stripe laser is a semiconductor wide stripe laser.

7. Device according to any of claims 1 to 6 where the active wide stripe (2) is segmented.

8. Device according to any of claims 1 to 7 where the top electrode (3) is segmented.

9. Device according to any of claims 2 to 4, and of claims 5 to 8 depending on claim 2 wherein the wavelength selective external mirror (6) is a resonant waveguide grating mirror (604).

10. Device according to any of claims 2 to 4, and of claims 5 to 8 depending on claim 2 wherein the wavelength selective external mirror is a multilayer mirror (602).

11. Device according to any of claims 2, 3 and of claims 5 to 8 depending on claim 2 wherein the external mirror comprises a high efficiency wavelength selective reflection grating (6) and a mirror (613) in a Littman-Metcalf mount.

12. Device according to any of claims 2, 3 and of claims 5 to 8 depending on claim 2 wherein the external mirror comprises two high efficiency wavelength selective reflection gratings (6) and (63) in the Littman-Metcalf mount placed symmetrically with respect to the optical axis.

13. Device according to any of claims 2, 3 and of claims 5 to 8 depending on claim 2 wherein the external mirror is a high efficiency wavelength selective reflection grating (6) in a Littrow mount for the -1^st order.

14. Device according to any of claims 2, 3 and of claims 5 to 8 depending on claim 2 wherein the external mirror comprises a high efficiency wavelength selective reflection grating (6) in a Littrow mount for the -1^st order and a second mirror (618).

15. Device according to claim 4 and any of claims 5 to 8 depending on claim 2, and of claim 9, 10 and 13 wherein the wavelength selective mirror (616) is located between the wide stripe edge (21) and the cylindrical mirror (615).

16. Device according to any of claims 2 to 4, and of claims 5 to 8 depending on claim 2, and of claims 9 to 15 wherein a saturable absorber plate is placed between the wide stripe edge (21) and the external mirror (6).

17. Device according to claim 9 wherein the waveguide layer (604) of the external mirror is made of a saturable absorber material.

18. Device according to any of claims 2 to 4, of claims 5 to 8 depending on claim 2, and of claims 9 to 14 wherein the wavelength selective mirror (6) rotates around an axis normal to the plane of the wide laser stripe (2) for wavelength adjustment, stabilisation or tuning.

19. Device according to any of claims 2, 3, and 5 to 8 depending on claim 2, and of claim 9 consisting of at least two high brightness, high power segmented wide laser stripes (81) parallel to each other and oriented along the optical axis, placed side by side in a direction orthogonal to the optical axis on the same substrate (82), the individual external waveguide grating mirrors (92) being on the same plate (9) with the normal to said plate lying in a plane parallel to the plane of the wide laser stripes (81) and making an angle θ_a with the optical axis, with the waveguide (91) extending all over the plate (9), each grating mirror (92) with grating lines normal to the wide laser stripe plane achieving resonant reflection only partially and coupling part of the incident power to the two adjacent grating mirrors by means of the gratingless waveguide sections.

20. Device according to any of claims 2, 3, and 5 to 8 depending on claim 2, and of claim 9 consisting of at least two high brightness, high power segmented wide laser stripes (101) parallel to each other and oriented along the optical axis, placed on top of each other in a direction orthogonal to the wide laser stripes (101), the individual external waveguide grating mirrors (93) being on the same plate with the normal to said plate lying in a plane parallel to the plane of the wide laser stripes (101) and making an angle θ_a with the optical axis, with the waveguide (91) extending all over the plate (9), each grating mirror (93) with grating lines in a plane parallel to the wide stripe planes achieving resonant reflection only partially and coupling part of the incident power to the two adjacent grating mirrors by means of the gratingless waveguide sections.

21. Device according to any of claims 2, 3, and 5 to 8 depending on claim 2, and of claim 11 consisting of a two-dimensional array of wide stripe lasers (81) placed side by side on boards (8) in planes parallel to the plane of the wide laser stripes, and on top of each other in the direction normal to the wide laser wherein the wide stripe lasers of each board (8) are synchronously coupled by the wavelength selective grating mirror (621) and mirror (620) in the Littman-Metcalf mount, and additional mirror 110, and the boards (8) are synchronously coupled by the corner mirrors (622) of mirror (620).