Patent ID: 8681825

Claim:
A method for quasi-synchronous tuning of a grating external-cavity laser to achieve quasi-synchronous tuning of frequency selections, wherein the laser comprises a semiconductor laser diode, a grating, and a mirror, the method comprising: determining a synchronous tuning point P 0 (x 0 ,y 0 ), the synchronous tuning point having a position such that when the grating or the mirror rotates about the synchronous tuning point P 0 , a round trip phase difference of a laser beam, emitted by the semiconductor laser diode, within a resonance cavity of the grating external-cavity laser does not change; determining a quasi-synchronous tuning range by: making a coordinate transformation from an x-y plane to an X-Y plane: for a tuning by rotating the grating, { Y = [ ( x - x 0 ) ⁢ sin ⁢ Δθ 2 + ( y - y 0 ) ⁢ cos ⁢ Δθ 2 ] X = [ ( x - x 0 ) ⁢ cos ⁢ Δθ 2 - ( y - y 0 ) ⁢ sin ⁢ Δθ 2 ] ( 8 ) for a tuning by rotating the mirror, { Y = ( x - x 0 ) ⁢ sin ⁢ ⁢ Δθ + ( y - y 0 ) ⁢ cos ⁢ ⁢ Δθ X = ( x - x 0 ) ⁢ cos ⁢ ⁢ Δθ - ( y - y 0 ) ⁢ sin ⁢ ⁢ Δθ ( 9 ) determining an optical length l(α) and a boundary condition at which mode hopping occurs: for the tuning by rotating the grating, l ⁡ ( α ) = SS ⁡ ( α ) SS ⁡ ( 0 ) [ l 0 - 2 ⁢ cos ⁢ Δθ 2 ⁢ SS ⁡ ( 0 ) SS ⁡ ( α ) ⁡ [ Y ⁢ ⁢ sin ⁢ ⁢ α + X ⁡ ( 1 - cos ⁢ ⁢ α ) ] ] ( 17 ) 2 ⁢ cos ⁢ Δθ 2 ⁢  Y ⁢ ⁢ sin ⁢ ⁢ α + X ⁡ ( 1 - cos ⁢ ⁢ α )  SS ⁡ ( α ) ≤ λ ⁡ ( 0 ) 4 ⁢ 1 SS ⁡ ( 0 ) ( 18 ) where, SS (α)=sin(θ i −α)+sin(θ d −α) (19), for the tuning by rotating the mirror, l ⁡ ( α ) = SS ⁡ ( α ) SS ⁡ ( 0 ) ⁡ [ l 0 - SS ⁡ ( 0 ) SS ⁡ ( α ) ⁡ [ Y ⁢ ⁢ sin ⁢ ⁢ α + X ⁡ ( 1 - cos ⁢ ⁢ α ) ] ] ( 15 )  Y ⁢ ⁢ sin ⁢ ⁢ α + X ⁡ ( 1 - cos ⁢ ⁢ α )  SS ⁡ ( α ) ≤ λ ⁡ ( 0 ) 4 ⁢ 1 SS ⁡ ( 0 ) ( 16 ) where, SS (α)=sin θ i +sin(θ d −α) (20); wherein the quasi-synchronous tuning range comprises a region in the X-Y plane comprising points whose corresponding frequency tuning range being not less than Δv, where: Δ ⁢ ⁢ v = c λ ⁡ ( 0 ) ⁢ ( l 0 1 ⁢ ( a + ) - l 0 1 ⁢ ( α - ) ) ( 14 ) where c is the light speed in vacuum, λ(0) is the central wavelength of laser light, l 0 is the original cavity length of the laser, α ± are the maximal mode-hopping-free tuning angles allowed in positive and negative directions respectively, Δθ=θ i −θ d , θ i is an incidence angle of the laser beam on the grating, and θ d is a diffraction angle of the laser beam on the grating; selecting one point in the quasi-synchronous tuning range as a quasi-synchronous tuning point P q ; and rotating the grating or the mirror around the quasi-synchronous tuning point, wherein a distance from the quasi-synchronous tuning point to a plane on which a diffraction surface of the grating lies or to a plane on which a reflection surface of the mirror lies does not change during the rotation about the quasi-synchronous tuning point.