Method and apparatus for fabrication of controlled chirp gratings
In some aspects of the present application, an apparatus for producing an interference pattern on a photosensitive portion formed on a surface of a sample is disclosed. The apparatus can include an optical system for providing interference between two coherent spherical wavefronts impinging on a thin-film photosensitive material formed on a surface of a sample, wherein a plane of the surface normal of the sample is arranged at an angle with respect to a plane defined by center propagation vectors of the two coherent spherical wavefronts; and one or more actuating elements operable to actuate one or more optical elements in the optical system, the sample, or both the one or more optical elements and the sample in one or more degrees of freedom to control a relative magnitude of a longitudinal and a transverse chirp of the interference pattern.
1. A tunable laser device comprising:
a chirped distributed feedback (DFB) grating disposed over an area of a laser structure, wherein the chirped DFB grating comprises a grating pitch having grating lines that varies across the area of the DFB grating and is characterized by both longitudinal and transverse chirp parameter; and
a pump laser disposed to provide a pump beam illumination in a sub-area with respect to the area covered by the chirped DFB grating, wherein the sub-area is arranged in a stripe geometry with a long axis of the pump stripe essentially perpendicular to each of the grating lines of the chirped DFB grating, the position of the sub-area being changed to provide for continuous tuning of a lasing emission wavelength from the optically pumped laser structure;
wherein the extent of the longitudinal chirp across the length of the sub-area stripe is comparable to or less than the bandwidth of the stop-band produced by a fixed pitch grating of comparable depth and duty cycle to the chirped DFB grating.
2. The device of claim 1 , wherein the grating lines of the chirped DFB grating are arranged at a tilt to one or more edges of the optically pumped laser structure to reduce an impact of any Fabry-Perot resonances on the lasing emission.
3. The device of claim 1 , wherein the chirped DFB grating is formed by etching into a material of a clad layer of the optically pumped laser structure, wherein the material comprises one or more of Ge, GaSb, AlGaSb, AlAsSb, AlGaAsSb, InAlAsSb, GaAlAsSb, AlSb, AlInSb, AlSbAs, or AlGaSbAs.
4. The device of claim 1 , wherein the chirped DFB grating is formed by an interferometric lithography technique.
5. The device of claim 1 , wherein the chirped DFB grating comprises a continuously-varying grating period.
6. The device of claim 1 , further comprising a discrete set of DFB gratings that each DFB grating comprises a fixed grating period and the chirp is realized in different fixed grating periods.
7. The device of claim 1 , wherein the optically pumped laser structure employs one or more type-II quantum well active regions in the GaSb/InAs such that the material optical gain is in the mid-infrared spectral region.
8. The device of claim 1 , wherein the continuous tuning is provided without lasing mode hops caused by the longitudinal chirp.