DEVICE FOR GENERATING LASER RADIATION

Abstract

The present invention relates to a device for generating laser radiation.

A problem addressed by the present invention is that of specifying a device for generating laser radiation using a nonlinear crystal, which device has a simple construction and low optical losses.

The device according to the invention comprises an optical amplifier having an active zone, wherein the optical amplifier has a front facet and a rear facet, between which the active zone extends; and a resonator having a first resonator element and a second resonator element, between which the optical amplifier extends, wherein the first resonator element is arranged on a side of the active zone facing away from the front facet and the second resonator element is arranged on a side of the active zone facing the front facet, and wherein the second resonator element comprises a nonlinear crystal having periodic poling.

Claims

1. A device for generating laser radiation, comprising: a) an optical amplifier with an active zone, b) wherein the optical amplifier has a front facet and a rear facet, between which the active zone extends; and c) a resonator with a first resonator element and a second resonator element, between which the optical amplifier extends, wherein the first resonator element is arranged on a side of the active zone facing away from the front facet and the second resonator element is arranged on a side of the active zone facing the front facet, d) wherein the second resonator element comprises a nonlinear crystal with periodic poling, e) wherein the device is configured to only actively adjust the temperature of the nonlinear crystal and to passively adjust the temperature of the optical amplifier.

2. The device of claim 1, wherein the optical amplifier is realized in the form of an electrically pumped optical semiconductor amplifier, and wherein the active zone is designed for emitting radiation of a first wavelength.

3. The device of claim 2, wherein the ratio of the reflectivity of the crystal for the first wavelength to the reflectivity of the front facet for the first wavelength is greater than or equal to 10.

4. The device of claim 3, wherein the ratio of the reflectivity of the crystal for the first wavelength to the reflectivity of the front facet for the first wavelength is greater than or equal to 100.

5. The device of claim 2, wherein a reflectivity of the front facet for the first wavelength is smaller than 0.001.

6. The device of claim 2, wherein the nonlinear crystal is designed for converting radiation of the first wavelength into radiation of a second wavelength by means of nonlinear frequency conversion.

7. The device of claim 6, wherein the first wavelength amounts to double the second wavelength.

8. The device of claim 1, wherein no optical isolators and/or no optical filters are arranged between the front facet of the optical amplifier and an input facet of the crystal.

9. The device of claim 1, wherein the optical amplifier and the crystal are aligned relative to one another in such a way that the radiation emitted by the optical amplifier is coupled into an input facet of the crystal.

10. The device of claim 9, wherein the boundaries of periodically arranged polarity layers of the crystal extend at an angle unequal to 90° relative to the radiation coupled into the crystal.

11. The device of claim 9, wherein the boundaries of periodically arranged polarity layers of the crystal extend perpendicular to the radiation coupled into the crystal, and wherein the nonlinear crystal comprises no beam-guiding elements.

12. The device of claim 1, wherein the periodic poling is a homogenous periodic poling.

13. The device of claim 1, wherein the periodic poling extends over the entire length of the crystal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Exemplary embodiments of the invention are described below with reference to the corresponding drawings. In these drawings:

[0047] FIGS. 1a and 1b show reflection spectrums of two periodically poled ridge waveguide crystals,

[0048] FIG. 2 shows a schematic sectional representation of an inventive device for stabilizing an optical amplifier with the aid of reflections of a periodically poled crystal for frequency conversion,

[0049] FIG. 3a shows a schematic top view of a device according to a first design variation of the invention,

[0050] FIG. 3b shows a schematic side view (sectional representation) of a device according to the first design variation of the invention,

[0051] FIG. 3c shows a schematic side view (sectional representation) of a device according to another design variation of the invention, and

[0052] FIG. 4 shows a sectional representation of the semiconductor amplifier illustrated in FIGS. 3a and 3b.

DETAILED DESCRIPTION OF THE DRAWINGS

[0053] FIGS. 3a and 3b show a top view and a sectional representation of a device according to a first design variation, in which a periodically poled crystal with ridge waveguide is used as resonator mirror for a semiconductor amplifier with a ridge waveguide structure.

[0054] The periodically poled crystal 5 has the same periodic poling as the crystal, the reflection spectrum of which is illustrated in FIG. 1b), but the domains are arranged at an angle θ=92° relative to the waveguide 10. The aim is to additionally use the reflection spectrum of the crystal 5 as spectral filter and to thereby define the emission wavelength of the laser operation at 1065 nm. A variation of the temperature of the crystal and the different scaling of the Bragg resonances and the phase matching for the frequency doubling ultimately make it possible to adjust the emission wavelength of the laser in such a way that light is during passage through the crystal 5 optimally converted into frequency-doubled radiation (SHG).

[0055] The semiconductor amplifier 1 was processed by means of organometallic vapor phase epitaxy on gallium arsenide (GaAs). The amplifier 1 has a length W1=4 mm and the ridge waveguide 3 has a width W2=4 μm. A planar waveguide with a thickness W3=4.8 μm is formed in the vertical direction. The rear facet 2 is mirror-coated and therefore represents the rear resonator mirror (21 in FIG. 2). The front facet 4 has an antireflection coating and a reflectivity of less than 0.01% for the operating wavelength of 1065 nm.

[0056] The periodically poled crystal 5 consists of lithium niobate and is commercially available, for example, from HP Photonics Corp. The crystal 5 has a length W4=10 mm whereas the period of the periodic poling W5 amounts to approximately 6.6 μm. The domains of the periodic poling are arranged at an angle θ=92° relative to the waveguide. The ridge waveguide 10 has a width W6 of approximately 6 μm and a height W7=4 μm. Although the ridge waveguides 3, 10 of the amplifier 1 and the crystal 5 have slightly different dimensions, the basic modes guided therein (i.e. at approximately 1065 micrometer) largely correspond. Consequently, two aspherical lenses 8 and 9 with a focal length of 4 mm are used for optically coupling both components. These lenses are respectively positioned in such a way that the distance W8 from the front facet 4 of the amplifier 1 and the distance W9 from the input facet 6 of the crystal 5 respectively correspond to the effective focal length of the lenses 8 and 9. The distance between the two lenses 8, 9 may amount up to several meters as long as it does not reach the order of magnitude of the Rayleigh length of the laser beam to be coupled. This length usually is greater than 1 m when lenses with a focal length of 4 mm are used.

[0057] In comparison with the crystal 5, the crystal 11 has no beam-guiding element and the radiation being coupled in therefore propagates freely through the crystal. A lens 12 that differs from the lens 9 consequently is used for the coupling.

[0058] The vertical layer structure of the semiconductor amplifier 1 is illustrated in FIG. 4. The epitaxic layer sequence is applied on a GaAs substrate 14. An InGaAs triple quantum well forms the active zone 17, which is asymmetrically arranged in a waveguide. The waveguide is composed of an n-doped AlGaAs waveguide core 16 with a thickness of 4000 nm and a p-doped AlGaAs waveguide core 18 with a thickness of 800 nm. This results in a total of 4.8 μm, which is indicated as W3 in FIG. 3b. The waveguide is respectively surrounded by an n-doped outer layer 15 with a thickness of 500 nm and a p-doped outer layer 19 with a thickness of 500 nm.

[0059] The ridge waveguide 3, which is produced by means of edging, once again protrudes beyond the outer layer 19 with the height W11 of 800 nm. Electric contacting is ultimately ensured by the p-contact 20 and the n-contact 13.

[0060] Since the Bragg resonances of the crystal 5 sometimes deviate from the optimal wavelength for frequency doubling by several 10 nm, it is not always possible to use the Bragg resonances as wavelength-selective element and to simultaneously achieve optimal conditions for the frequency conversion. Nevertheless, the non-evanescent reflectivity of the crystal of at least approximately 0.01% can be used for achieving the laser threshold due to periodic poling. The front facet 4 of the amplifier has to be highly non-reflecting and have a reflectivity of 10.sup.−6 or less. In this case, the crystal 5 once again acts as front resonator mirror 22, but without wavelength-selective effect. The rear resonator mirror 21 has to be realized in the form of a wavelength-selective element in order to still define the emission wavelength. One potential design is the integration of a surface grating directly into the rear section of the ridge waveguide 3.

REFERENCE LIST

[0061] 1 Optical amplifier [0062] 2 Rear facet [0063] 3 Ridge waveguide [0064] 4 Front facet [0065] 5 Optical crystal (periodically poled) [0066] 6 Input facet [0067] 7 Output facet [0068] 8 Lens [0069] 9 Lens [0070] 10 Ridge waveguide (crystal) [0071] 11 Optical crystal (periodically poled without beam-guiding elements) [0072] 12 Lens [0073] 13 n-contact [0074] 14 Substrate [0075] 15 n-conducting outer layer [0076] 16 n-conducting core layer [0077] 17 Active zone [0078] 18 p-conducting core layer [0079] 19 p-conducting outer layer [0080] 20 p-contact [0081] 21 Rear resonator mirror (first resonator element) [0082] 22 Front resonator mirror (second resonator element)