Laser and photodetector coupled by planar waveguides

1. An optical apparatus, comprising

a substrate;

a first planar optical waveguide formed on the substrate comprising a first transmission core and cladding;

a second planar optical waveguide formed on the substrate comprising a second transmission core and cladding;

a third planar optical waveguide formed on the substrate comprising a third transmission core and cladding;

a laser positioned so as to launch at least a portion of its optical output to propagate along the first transmission core toward its proximal end;

a photodetector positioned so as to receive at least a portion of an optical signal propagating along the second transmission core away from its proximal end;

a first tap core segment connecting the first and third transmission cores at their respective proximal ends; and

a second tap core segment connected to the proximal end of the second transmission core,

wherein:

the first and second tap core segments are positioned in a substantially parallel spaced-apart arrangement, so that a first fraction of the laser optical output propagating along the first transmission core toward its proximal end is transferred to propagate along the second transmission core away from its proximal end by optical transverse coupling between the first and second tap core segments;

a second fraction of the laser optical output propagating along the first transmission core toward its proximal end is transferred to propagate along the third transmission core away from its proximal end by optical propagation along the first tap core segment;

a volume between the tap core segments is substantially filled with cladding so that the second transmission core and the second tap core segment are spatially disjoint from the first tap core segment and the first and third transmission cores; and

the second tap core segment and a proximal portion of the second transmission core form an obtuse angle with respect to one another, and respective proximal portions of the second and third transmission cores form an acute angle with respect to one another.

2. The apparatus of claim 1 , wherein the tap core segments are substantially modal-index-matched.

3. The apparatus of claim 2 , wherein the tap core segments comprise substantially the same material and have substantially the same cross-sectional size and shape as one another.

4. The apparatus of claim 1 , wherein the tap core segments are arranged at substantially the same height relative to the substrate, and are spaced apart laterally by less than about 2 μm between their respective mode axes.

5. The apparatus of claim 1 , wherein acute angle is greater than about 5° and less than about 10°.

6. The apparatus of claim 1 , wherein the tap core segments are each between about 2 μm long and about 40 μm long.

7. The apparatus of claim 1 , wherein:

the cladding comprises silica or doped silica;

the proximal portions of the three transmission cores and the two tap core segments each comprise silicon nitride, are each about 75 nm thick and about 1 μm wide, and are arranged at substantially the same height relative to the substrate;

the tap core segments are each about 7 μm long;

the tap core segments are spaced apart by about 1.6 μm between their respective mode axes; and

the acute angle is about 8°.

8. The apparatus of claim 1 , wherein:

the cladding comprises silica or doped silica;

the proximal portions of the three transmission cores and the two tap core segments each comprise silicon nitride, are each about 70 nm thick and about 1.2 μm wide, and are arranged at substantially the same height relative to the substrate;

the tap core segments are each about 12 μm long;

the tap core segments are spaced apart by about 2.2 μm between their respective mode axes; and

the acute angle is about 8°.

9. The apparatus of claim 1 , wherein:

the three transmission cores and the two tap core segments are each less than

about 250 nm thick and each comprise silicon nitride or silicon oxynitride; and the cladding comprises silica or doped silica.

10. The apparatus of claim 9 , wherein the three transmission cores and the two tap core segments are each less than about 100 nm thick.

11. The apparatus of claim 9 , wherein:

the three transmission cores and the two tap core segments are arranged at substantially the same height relative to the substrate; and

the three transmission cores are each less than about 2 μm wide.

12. The apparatus of claim 1 , wherein:

a ratio of an electronic signal produced by the photodetector in response to the first fraction of the laser optical output optical power to a delivered optical power of the second fraction of the laser optical output defines a tracking ratio; and

the laser, photodetector, the first, second, and third planar optical waveguides, and the first and second tap core segments are arranged so that the tracking ratio remains between about 0.7 times a nominal tracking ratio and about 1.3 times the nominal tracking ratio over a selected operating wavelength range for the laser.

13. The apparatus of claim 12 , wherein the laser, photodetector, the first, second, and third planar optical waveguides, and the first and second tap core segments are arranged so that the tracking ratio remains between about 0.8 times a nominal tracking ratio and about 1.2 times the nominal tracking ratio over a selected operating wavelength range for the laser.

14. The apparatus of claim 12 , wherein the laser, photodetector, the first, second, and third planar optical waveguides, and the first and second tap core segments are arranged so that the tracking ratio remains between about 0.9 times a nominal tracking ratio and about 1.1 times the nominal tracking ratio over a selected operating wavelength range for the laser.

15. The apparatus of claim 12 , wherein a wavelength dependence of a ratio of the optical power of the first fraction of the laser optical output to the optical power of the second fraction of the laser optical output is selected so as to at least partly compensate for at least one other wavelength-dependent property of the apparatus, thereby yielding a desired wavelength dependence of the tracking ratio.

16. The apparatus of claim 1 , wherein:

a ratio of a sum of optical powers of the first and second fractions of the laser optical output to optical power of the laser optical output propagating along the first transmission core defines an optical throughput; and

the first, second, and third planar optical waveguides and the first and second tap core segments are arranged on the substrate so that the optical throughput is greater than about 0.9 over a selected operating wavelength range for the laser.

17. The apparatus of claim 16 , wherein the first, second, and third planar optical waveguides and the first and second tap core segments are arranged on the substrate so that the optical throughput is greater than about 0.95 over a selected operating wavelength range for the laser.

18. The apparatus of claim 1 , wherein the photodetector is arranged to generate a laser monitor electronic signal in response to that portion of the second fraction of the optical output of the laser received by the photodetector.

19. The apparatus of claim 18 , further comprising a feedback circuit for controlling optical output power of the laser in response to the laser monitor electronic signal.

20. The apparatus of claim 1 , wherein:

distal segments of the first and second transmission cores are larger in cross-sectional area than proximal segments thereof; and

a segment of each of the first and second transmission cores tapers in at least one transverse dimension toward the corresponding proximal end thereof.

21. The apparatus of claim 20 , wherein the tapered segments of the first and second transmission cores are tapered sufficiently gradually so as to provide substantially adiabatic optical transitions between corresponding proximal and distal segments of the first and second transmission cores.

22. The apparatus of claim 1 , further comprising another optical waveguide optically coupled with the third planar optical waveguide at a distal portion thereof, so as to receive the delivered optical power of the second fraction of the laser optical output.

23. The apparatus of claim 22 , wherein the other optical waveguide comprises an optical fiber end-coupled to a distal end of the third planar optical waveguide.