LASER

Abstract

An example laser has a rear reflector, a front facet spaced from the rear reflector, and a laser cavity defined between the rear reflector and the front facet. The laser comprises a Bragg grating located in the laser cavity, where a length of the Bragg grating (L.sub.g) is in a range from 40% to 60% of a distance from the rear reflector to front of the Bragg grating, and a grating strength (Kappa*L.sub.g) is in a range from 0.6 to 1.5.

Claims

1. A laser having a rear reflector, a front facet spaced from the rear reflector and a laser cavity defined between the rear reflector and the front facet, the laser comprising a Bragg grating located in the laser cavity, wherein a length of the Bragg grating (L.sub.g) is in a range from 40% to 60% of a distance from the rear reflector to front of the Bragg grating and wherein a grating strength (Kappa*L.sub.g) is in a range from 0.6 to 1.5.

2. The laser of claim 1, wherein the laser is a distributed feedback laser.

3. The laser of claim 1, wherein the Bragg grating is elongated along a length of the laser cavity.

4. The laser of claim 1, wherein the length of the Bragg grating (L.sub.g) is in a range from 45% to 55% of the distance from the rear reflector to the front of the Bragg grating.

5. The laser of claim 1, wherein the grating strength is in a range from 0.8 to 1.3.

6. The laser of claim 1, wherein the laser is configured to function, in operation, in Fabry-Perot mode.

7. The laser of claim 6, wherein the laser is configured such that if material defining the laser cavity is cut to form a new front facet not more than 100 nm closer to the rear reflector than the said front facet, the new front facet having a same reflectivity as the said front facet, the laser, in operation, functions in the Fabry-Perot mode.

8. The laser of claim 1, wherein the front facet is a cleaved facet.

9. The laser of claim 1, wherein the front facet is coated with an anti-reflection coating.

10. The laser of claim 1, wherein the front facet of the laser is optically coupled to a lens.

11. The laser of claim 1, wherein the rear reflector is planar, and wherein the said distance from the rear reflector to the front of the Bragg grating is measured in a direction perpendicular to the rear reflector.

12. The laser of claim 1, wherein the laser cavity comprises a first semiconductor layer of a first doping type, a second semiconductor layer of a second doping type opposite to the first doping type, and an active region located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the rear reflector and the front facet.

13. The laser of claim 12, wherein the Bragg grating is located between the first and second semiconductor layers.

14. The laser of claim 1, wherein the laser cavity comprises an amplifier.

15. The laser of claim 1, wherein the laser cavity comprises a modulator.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0028] The present invention will now be described by way of example with reference to the accompanying drawings.

[0029] In the drawings:

[0030] FIG. 1 shows a laser with a Bragg grating positioned adjacent to the front facet of the laser cavity.

[0031] FIG. 2 illustrates a laser with a Bragg grating spaced from the front facet of the laser cavity.

[0032] FIG. 3 illustrates a laser coupled to an optical fibre.

DETAILED DESCRIPTION OF THE INVENTION

[0033] As illustrated in FIG. 1, one form of laser comprises a semiconductor block which has a front face or facet 1, a rear face or facet 2 opposite to the front face or facet and a laser cavity formed therebetween. The total length of the laser cavity is Lt. A high reflection (HR) coating 3 is applied to the rear facet and an anti-reflection (AR) coating 4 is applied to the front facet. The back facet with the HR coating acts as a rear reflector.

[0034] In the example shown in FIG. 1, the laser cavity comprises an active layer 5 interposed between layers of p- and n-type semiconductor material, shown at 6 and 7 respectively. A Bragg grating 8 is positioned adjacent to the front facet between the active layer 5 and the p-type semiconductor layer 6. The grating may alternatively be positioned between the active layer and the n-type semiconductor layer 7. The Bragg grating is integral with the cavity of the laser. The Bragg grating has a length L.sub.g. The Bragg grating is elongated along the length of the cavity. The elongation of length of the grating is orthogonal to the rear facet. Light exits the laser cavity at the front facet, shown at 9.

[0035] It is preferable that the front and rear facets are aligned parallel to one another. Preferably the rear facet is orthogonal to the length of the cavity and/or to the Bragg grating. Preferably the front facet is orthogonal to the length of the cavity and/or to the Bragg grating.

[0036] In this example, the semiconductor layers are made from InP. However, other semiconductor materials, such as GaAs, may be used. The material forming the cavity may be selectively doped in the region of the p- and n-type layers 6, 7. The Bragg grating 8 may be positioned between different semiconductor layers to those shown in the example of FIG. 1.

[0037] For the laser of FIG. 1, the length of the grating (L.sub.g), shown at 8, is between 40% to 60% of the total laser cavity length, L.sub.t. Preferably, L.sub.g is in the range from 45% to 55% of the total laser cavity length, L.sub.t. The grating coupling strength, K*L.sub.g, (where K represents the coupling coefficient, kappa) is between 0.7 and 1.4. Preferably, K*L.sub.g is between 0.8 and 1.3. Values of L.sub.g and grating strength within these narrower ranges can be expected to result in better performance compared to the broader ranges specified above.

[0038] This configuration results in a laser that is a hybrid between a Distributed Feedback (DFB) laser and a Fabry Perot (FP) laser.

[0039] Modelling has shown that for a laser with the above characteristics, the random facet location relative to the grating phase as a result of the cleaving process does not have a great impact on the optical mode profile along the laser cavity. Hence, the laser can be expected to have a better yield than conventional DFB lasers, and to be more insensitive to external optical reflection.

[0040] For such a laser, the single mode laser wavelength is selected from the FP modes by the partial grating in the section of the laser cavity between the rear HR facet and the grating. The FP mode is formed by the grating also acting as a reflector together with the HR coated rear facet. Optionally there may be a second grating located at or near the rear facet which may contribute to the laser operating in FP mode.

[0041] For such a laser, the lasing mode profile along the cavity and the yield of the laser is not affected by the random phase of the cleaved facets. Additionally, the front/back output power ratio remains consistent, with a low spread of around 6 to 15, in comparison with standard DFB lasers.

[0042] The laser configuration described above also reduces the spatial hole burning effect, which can also cause low yield. Where cleaved facets result in a random phase of the reflected waveforms, there will be an uneven distribution of optical modes along the cavity. This can result in uneven depletion of charge carriers. At some positions, there will be a strong optical mode inside the cavity and charge carriers are depleted quickly. At other positions, there will be a higher density of charge carriers where the mode is weaker. By using a laser with the configuration as described above, the mode is more evenly distributed along the cavity.

[0043] As described above, preferably the front facet is coated in an AR coating. By using an AR coating on the front face, the value of K*L.sub.g can be between 0.7 and 1.4. If the facet is more reflective, K*L.sub.g is preferred to be closer to 1 in order to operate in FP mode.

[0044] The laser is configured to operate in Fabry Perot mode regardless of any small variations in the position of the front face as a result of the cleaving process. The position of the front face may vary by 100 nm, 50 nm, or 20 nm as a result of the cleaving process. The optical mode profile along the cavity is not affected by the random phase of the front face as a result of the cleaving process.

[0045] Instead of the front grating being positioned adjacent to the front face of the laser cavity, the grating may be spaced from the front face, towards the rear face, as shown in FIG. 2. The grating may be spaced from the front face by greater than the pitch of the grating, or more than 2, 3, 4 or more times the pitch of the grating. In this example, the front section of the cavity between the grating and the front face can act as an optical amplifier. The length of the grating L.sub.g is between 40% to 60% of the total laser cavity length, L.sub.t. The grating coupling strength, K*L.sub.g, is between 0.7 and 1.4.

[0046] As shown in FIG. 3, the laser may be coupled to an optical fibre 10 by a coupling lens 11.

[0047] The Bragg grating may be fabricated by electron beam lithography. This allows the accuracy of the grating spacing to be controlled very accurately. The pitch of the grating may be approximately 300 nm, 200 nm, or 50 nm.

[0048] The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. The layer comprising the grating may be fabricated from a p-doped or n-doped semiconductor material.

[0049] The laser structure may be integrated with another optically functional structure, for example an electroabsorption modulator, a Mach-Zehnder modulator, or an amplifier.

[0050] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.