Photonic-crystal distributed-feedback and distributed bragg-reflector lasers
View Patent ↗A photonic-crystal distributed-feedback laser includes a laser cavity with a waveguide structure that has a cavity length L c and is bounded by two mirrors; an active region for producing optical gain upon receiving optical pumping or an input voltage; at least one layer having a periodic two-dimensional grating with modulation of a modal refractive index, the grating being defined on a rectangular lattice with a first period along a first axis of the grating and a second period along a second perpendicular axis of the grating, and wherein the grating produces three diffraction processes having coupling coefficients κ 1 ′, κ 2 ′, κ 3 ′; and a lateral gain area contained within a second area patterned with the grating that has substantially a shape of a gain stripe with a width W, with the gain stripe tilted at a first tilt angle relative to the two mirrors. The rectangular lattice of the grating is tilted at a second tilt angle substantially the same as the first tilt angle with respect to the gain stripe, and the ratio of the first and second grating periods is equal to the tangent of the first tilt angle, with the first tilt angle being between about 16° and about 23°. The hexagonal lattice does not need to be tilted with respect to the two mirrors. The laser's output emerges along the normal to a facet irrespective of the operating laser wavelength, facilitating coupling the laser light into a fiber or other optical system while avoiding beam steering. The two-dimensional nature of the feedback in the laser provides for varying the wavelength through angle tuning. Wavelength tuning by changing the propagation direction (propagation angle) permits a straightforward selection of different wavelengths from photonic crystal devices monolithically fabricated on a single wafer. The fabrication procedure is straightforward since no ridges need to be defined. The single-mode spectral purity of the rectangular-lattice PCDFB is robust, owing to the near absence of side modes, and exhibits good beam quality.
1. A photonic-crystal distributed-feedback laser with an output power, a beam quality, a wavelength spectrum, and a product of a modal loss and a linewidth enhancement factor, comprising:
a laser cavity with a waveguide structure that has a cavity length L c and is bounded by two mirrors;
an active region for producing optical gain upon receiving optical pumping or an input voltage;
at least one layer including a periodic two-dimensional grating with modulation of a modal refractive index, said grating being defined on a rectangular lattice with a first period along a first axis of said grating and a second period along a second perpendicular axis of said grating;
said grating produces three diffraction processes having coupling coefficients κ 1 ′, κ 2 ′, κ 3 ′;
a lateral gain area contained within a second area patterned with said grating that has substantially a shape of a gain stripe with a width W, with said gain stripe tilted at a first tilt angle relative to the two mirrors; and
wherein said rectangular lattice of said grating is tilted at a second tilt angle substantially the same as the first tilt angle with respect to said gain stripe and the ratio of said first and second grating periods is equal to the tangent of said first tilt angle, with said first tilt angle being between about 16° and about 23°.
2. A photonic-crystal distributed-feedback laser as in claim 1 , with said cavity length at least 1.5 mm.
3. A photonic-crystal distributed-feedback laser as in claim 2 , wherein said product of the modal loss and linewidth enhancement factor is no larger than about 40 cm −1 .
4. A photonic-crystal distributed-feedback laser as in claim 3 , wherein said stripe width W is between about 350 μm and about 450 μm and wherein said output power into a single spectral mode is optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 2 cm −1 ≦|κ 1 ′|≦10 cm −1 ; 0.5≦|κ 1 ′|L c ≦1.5; 30 cm −1 ≦|κ 2 ′|≦60 cm −1 ; and 1.3≦|κ 2 ′|W≦2.1.
5. A photonic-crystal distributed-feedback laser as in claim 3 , wherein said output power and said beam quality are both optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width W to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 2 cm −1 ≦|κ 1 ′|≦10 cm −1 ; 0.5≦|κ 1 ′|L c ≦1.5; and 0.5≦|κ 2 ′|W>1.5.
6. A photonic-crystal distributed-feedback laser as in claim 2 , wherein said product of the modal loss and linewidth enhancement factor is between about 40 cm −1 and about 100 cm −1 .
7. A photonic-crystal distributed-feedback laser as in claim 6 , wherein said stripe width W is between about 150 μm and about 350 μm and wherein said output power into a single spectral mode is optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 4 cm −1≦|κ 1 ′|≦15 cm −1 ; 1.0≦|κ 1 ′|L c ≦2.0; 60 cm −1 ≦|κ 2 ′|≦150 cm −1 ; and 1.5≦|κ 2 ′|W≦2.5.
8. A photonic-crystal distributed-feedback laser as in claim 6 , wherein said output power and said beam quality are both optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width W to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 4 cm −1 ≦|κ 1 ′|≦10 cm −1 ; 1≦|κ 1 ′|L c ≦1.5; and 0.5≦|κ 2 ′|W≦1.5.
9. A photonic-crystal distributed-feedback laser as in claim 2 , wherein said product of modal loss and linewidth enhancement factor is larger than about 100 cm −1 .
10. A photonic-crystal distributed-feedback laser as in claim 9 , wherein said stripe width W is no larger than about 150 μm and wherein said output power into a single spectral mode is optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 8 cm −1 ≦|κ 1 ′|≦15 cm −1 ; 1.5≦|κ 1 ′|L c ≦2.5; 150 cm −1 ≦|κ 2 ′|≦250 cm −1 ; and 2.0≦|κ 2 ′|W≦2.5.
11. A photonic-crystal distributed-feedback laser as in claim 9 , wherein said output power and said beam quality are both optimized by requiring said coupling coefficients, said cavity length L c , and said stripe width W to substantially satisfy the relations: |κ 3 ′|≈|κ 1 ′|; 1.5≦|κ 1 ′|L c ≦2; and 1.5≦|κ 2 ′|W≦2.0.