Mode-locking in semiconductor lasers

Claim 2

A mode-locked semiconductor laser comprising, in combination, a semiconductor active medium characterized by the property of saturable optical absorption an optical cavity resonator including therein said medium, means for pumping said medium to generate stimulated coherent radiation in a plurality of longitudinal modes within a range of optical frequencies, said medium being disadvantageously characterized by the dispersive property that radiation at higher frequencies within said range undergoes different time delays than radiation at lower frequencies within said range, means for coupling said radiation out of said resonator, and multiple-reflection means disposed within said resonator for compensating said dispersive property so that radiation at substantially all frequencies within said range undergoes substantially the same time delay.

Claim 3

The laser of claim 1 including means for synchronously modulating the longitudinal modes of said laser.

Claim 4

The laser of claim 1 wherein said active medium includes at least one p-n junction characterized by positive dispersion whereby radiation at higher frequencies within said range undergoes longer time delays than radiation at lower frequencies within said range and said multiple-reflection means is characterized by negative dispersion whereby radiation at higher frequencies within said range undergoes shorter time delays than radiation at lower frequencies within said range.

Claim 5

The laser of claim 3 wherein said multiple-reflection means comprises at least one Gires-Tournois interferometer.

Claim 6

The laser of claim 4 including a second Gires-Tournois interferometer in spaced parallel relation with said one interferometer, said interferometers being obliquely disposed with respect to the axis of said resonator so that radiation undergoes multiple reflections between said interferometers.

Claim 7

The laser of claim 4 wherein said interferometer is characterized by a periodic time delay versus frequency characteristic including within each period substantially linear portions having both positive and negative slopes, said interferometer being adapted so that the center frequency of said laser radiation coincides with a substantially linear negative slope portion within one of said periods.

Claim 8

The laser of claim 6 wherein said interferometer comprises a first mirror of high reflectivity and a second mirror of intermediate reflectivity in spaced parallel relation with said first mirror, the power reflectivity of said second mirror being approximately 25 percent and wherein said interferometer is constructed to satisfy approximately the following relationship where ne is an equivalent index of refraction given ne n + Nu (dn/d Nu ) and Nu is the optical center frequency of said laser radiation; e is the optical path length separation said mirrors; T (e) is a tuning factor varied between about 0.5 and 1.0; L is length of said active medium; and c is the speed of light in a vacuum.

Claim 9

The laser of claim 7 wherein said medium includes a high reflectivity first end surface transverse to said junction and a low reflectivity second end surface parallel to said first surface, said first end surface and said first mirror of said interferometer forming said cavity resonator, and said second end surface forming said second mirror of said interferometer.

Claim 10

The laser of claim 8 wherein said first mirror is planar and including between said first and second mirrors a lens to match the wavefront of radiation emitted from said junction to said planar first mirror.

Claim 11

The laser of claim 8 wherein said first mirror is concave, the curvature thereof substantially matching the wavefront of radiation emitted from said junction.

Claim 12

The laser of claim 10 wherein said concave mirror is spherical and of radius equal to the separation between said first mirror and said second mirror.

Claim 13

The laser of claim 3 wherein said multiple reflection means comprises a first diffraction grating disposed at an oblique angle with respect to the axis of said resonator, a second diffraction grating, substantially identical to said first diffraction grating, disposed in spaced parallel relation with said first diffraction grating so that said radiation undergoes multiple reflections between said gratings.

Claim 14

The laser of claim 12 wherein said diffraction gratings are constructed to satisfy approximately the following relationship where dne/d Nu is the dispersion of said active medium, Nu is the optical center frequency of said radiation, lambda is the wavelength corresponding to Nu , c is the speed of light in a vacuum, b is the slant spacing of the gratings, L is the length of said laser medium, ne is given by ne n + Nu (dn/d Nu ) where n is the index of refraction of said medium and T( lambda ) is given by where gamma is the angle of incidence between the radiation and the normal to the gratings and d is the ruling spacing of the gratings.

Claim 15

The laser of claim 3 wherein said medium includes first and second planar, parallel p-n junctions in spaced relation to one another in the transverse dimension of said medium and a pair of oppositely facing, parallel end surfaces normal to said junctions thereby defining said resonator, said junctions extending along only a portion of the longitudinal dimension of said medium, said first junction extending from one of said end surfaces and said second junction extending from the other of said end surfaces, and said multiple reflection means comprises an acoustic diffraction grating established in said medium obliquely to said junction planes so that radiation emitted in the plane of either of said junctions is diffracted to propagate in the plane of the other of said junctions.

Claim 16

THe laser of claim 14 in combination with optical absorption means disposed to intercept at least radiation emitted into an n-type region of said medium from one of said junctions.

Claim 17

The laser of claim 15 wherein said absorption means comprises an absorbing dopant disposed between the end of said one junction and the end surface spaced therefrom.

Claim 18

The laser of claim 15 wherein said absorption means is provided by a low reflectivity region on the end surface of said medium spaced from an interior end of said one junction.

Claim 19

The laser of claim 15 wherein said multiple reflection means comprises an acoustic transducer crystal contiguous with a major longitudinal surface of said medium, aid crystal having its crystallographic angle cut so that in response to an electric field applied thereacross said acoustic grating is established obliquely to said junctions.

Claim 20

The laser of claim 15 wherein said multiple reflection means comprises an acoustic transducer contiguous with a major longitudinal surface of said medium, said major surface being inclined at an angle with respect to said junctions so that in response to an electric field applied across said transducer said acoustic grating is established normal to said major surface and obliquely to said junctions.

Claim 21

The laser of claim 3 wherein said laser includes first and second parallel end surfaces transverse to said junction thereby forming said resonator, and including on one major surface thereof a stripe electrical contact in a plane parallel to said junction, said contact comprising first and second parallel segments extending from opposite ones of said end surfaces parallel to the axis of said resonator and along only a portion of the length of said resonator, said first and second segments being separated by a distance b, and a third segment extending transverse to said axis thereby forming first and second junctures with said first and second segments, respectively, said third segment forming an angle 2 theta with each of said first and second segments, means for establishing at each of said junctures an acoustic grating at an angle theta to each of said first and second segments, and optical absorption means disposed to intercept radiation emitted into the region of said medium between each of said junctures and the end surface opposite thereto.

Claim 22

The laser of claim 20 wherein said acoustic means comprises first and second acoustic transducers contiguous with opposite, parallel longitudinal side surfaces of said medium, said transducers being disposed so that in response to an electric field applied thereto each respectively establishes an acoustic grating at each of said junctures.

Claim 23

The laser of claim 20 wherein said laser is constructed to satisfy the following relationships cos theta Nu sc/2ncsl and where ne is an equivalent index of refraction given by ne n + Nu l (dn/d Nu l), n is the index of refraction of said medium, Nu l is the optical center frequency of said laser radiation, Nu s is the acoustic frequency, c is the speed of light in a vacuum, cs is the speed of sound in said medium, and L is the length of said medium.