Bragg grating reflection strength control
A tunable laser comprises a gain section and two or more reflectors. At least one of the reflectors is a Bragg grating comprising a reduced strength section comprising a base order periodic pattern of marks and spaces from at which at least some of the marks or spaces are missing. This enables the reflective strength of the grating to be controlled relative to the other reflector. The Bragg grating may be a phase change grating with individual sections being reduced in strength by line removal.
1. A tunable laser comprising a gain section and two or more reflectors, at least one of the reflectors being a Bragg grating, the grating comprising a reduced strength section comprising a periodic pattern of marks and spaces, the periodic pattern comprising a base order pattern from which at least some of the marks or spaces are missing, the periodic pattern being defined by the base order pattern modulated by a higher order envelope function that determines which marks and spaces are missing from the base order pattern, so that the reflective strength of the grating is controlled relative to the reflective strength of at least one other of the two or more reflectors.
2. The tunable laser of claim 1 , wherein n out of every m marks or spaces of the base order pattern is missing in the reduced strength section, where m is an integer greater than n, and n is one or an integer greater than one.
3. Previously Presented) The tunable laser of claim 1 , wherein the reduced strength section extends the whole length of the grating.
4. The tunable laser of claim 1 , wherein the grating is a chirped grating, and wherein the period of the base order pattern varies along the length of the grating.
5. The tunable laser of claim 1 , wherein the grating is a comb grating adapted to produce a reflection spectrum comprising a plurality of reflective peaks at discrete wavelengths.
6. The tunable laser of claim 5 , wherein the grating is a stepped pitch grating comprising a plurality of grating sections, the period of the base order pattern of each section being different, and wherein some or all of the grating sections are reduced strength sections having marks or spaces missing from their respective base order pattern.
7. The tunable laser of claim 5 , wherein the grating comprises a plurality of grating sections formed by a base order periodic pattern of marks and spaces, the grating sections being separated by phase changes of π radians, wherein some or all of the grating sections are the reduced strength sections, each reduced strength section comprising the periodic pattern of marks and spaces defined by the base order pattern from which at least some of the marks or spaces are missing.
8. The tunable laser of claim 1 , wherein at least two of the reflectors are Bragg gratings, and one or more of the gratings comprises a reduced strength section comprising a base order periodic pattern of marks and spaces from which at least some of the marks or spaces are missing to so that the relative strength of the two gratings relative to each other is controlled.
9. The tunable laser of claim 8 , wherein the reduced strength section of one of the gratings has one out of every M marks or spaces missing from its base order pattern, and the reduced strength section of the other of the gratings has one out of every L marks or spaces missing from its base order pattern, where M and L are different integers greater than one.
10. The tunable laser of claim 8 , wherein the reduced strength section of one of the gratings has one out of every M marks or spaces missing from its base order pattern, and the other grating has no marks or spaces missing from its base order pattern.
11. The tunable laser of claim 1 , wherein the reflectors comprise a front reflector and a rear reflector bounding the gain section, and wherein the Bragg grating with the reduced strength section forms at least part of the front reflector or the rear reflector.
12. The tunable laser of claim 11 , wherein the reflective strength of the reflectors is controlled so that the front reflector has a relatively low reflective strength and the rear reflector has a relatively high reflective strength.
13. The tunable laser of claim 11 , wherein the front reflector is a chirped grating and the rear reflector is a comb grating.
14. The tunable laser of claim 11 , wherein the front and rear reflectors are comb gratings.
15. The tunable laser of claim 1 , wherein the reduced strength section compensates for a non-uniform wavelength gain profile in the gain section of the laser.
16. A method of balancing the relative reflective strengths of reflectors formed by Bragg gratings in a tunable laser, wherein the gratings comprise sections formed by a base order periodic pattern of marks and spaces, the method comprising removing some of the marks or spaces from at least one of the gratings to reduce the strength of said at least one grating by forming a periodic pattern defined by the base order pattern from which some of the marks or spaces are missing, wherein the periodic pattern is defined by the base order pattern modulated by a higher order envelope function that determines which marks and spaces are missing from the base order pattern.
17. The tunable laser of claim 16 , wherein n out of every m lines is removed from at least one of the gratings, where m is an integer greater than n, and n is one or an integer greater than 1.
18. The tunable laser of claim 16 , wherein the marks and spaces are formed in a waveguide by electron beam lithography.