DISTRIBUTED FEEDBACK LASER

Abstract

A distributed feedback laser, including: an output end including an active region including a grating including a λ/4 phase-shift region; and a non-output end including a reflecting region including a grating with uniform period. The length of the active region is smaller than or equal to 200 μm. The end facet of the output end of the laser is coated with an anti-reflection film.

Claims

1. A distributed feedback laser, comprising: an output end comprising an active region, the active region comprising a first grating, and the first grating comprising a λ/4 phase-shift region; and a non-output end comprising a reflecting region, the reflecting region comprising a second grating having a uniform period; wherein a length of the active region is smaller than or equal to 200 μm; and an end facet of the output end of the laser is coated with an anti-reflection film.

2. The laser of claim 1, wherein the second grating is a Bragg reflective grating having a period A calculated according to the following equation: $\Lambda =\frac{m.Math..Math.\lambda }{2.Math..Math.n_{\mathrm{eff}}}$ wherein, m represents a series number of the second grating; λ represents a Bragg wavelength corresponding to the second grating, at which the second grating is capable of producing a maximum reflection; and n.sub.eff is an effective refractive index of a waveguide.

3. The laser of claim 2, wherein the series number of the second grating is m=1.

4. The laser of claim 1, wherein a waveguide of the reflecting region and a waveguide of the active region adopt same core layer structures; and a waveguide core layer of the reflecting region adopts active quantum well materials.

5. The laser of claim 1, wherein a length of the reflecting region is regulated by self-definition according to a required reflectivity; the length of the reflecting region and a reflectivity of the reflecting region are in positive correlation; and a maximum reflectivity of the reflecting region exceeds 80%.

6. The laser of claim 2, wherein a length of the reflecting region is regulated by self-definition according to a required reflectivity; the length of the reflecting region and a reflectivity of the reflecting region are in positive correlation; and a maximum reflectivity of the reflecting region exceeds 80%.

7. The laser of claim 1, wherein a coupling coefficient of the second grating of the reflecting region is regulated by self-definition according to a required reflectivity; the coupling coefficient of the reflecting region and a reflectivity of the reflecting region are in positive correlation; and the coupling coefficient is regulated to make the reflectivity of the reflecting region exceed 80%.

8. The laser of claim 2, wherein a coupling coefficient of the second grating of the reflecting region is regulated by self-definition according to a required reflectivity; the coupling coefficient of the reflecting region and a reflectivity of the reflecting region are in positive correlation; and the coupling coefficient is regulated to make the reflectivity of the reflecting region exceed 80%.

9. The laser of claim 1, wherein the period of the second grating of the reflecting region is different from that of the first grating of the active region.

10. The laser of claim 2, wherein the period of the second grating of the reflecting region is different from that of the first grating of the active region.

11. The laser of claim 1, wherein a non-output end facet of the reflecting region adopts a window section, or a horizontally inclined end facet, or a coated anti-reflection film, or a combination thereof.

12. The laser of claim 2, wherein a non-output end facet of the reflecting region adopts a window section, or a horizontally inclined end face, or a coated anti-reflection film, or a combination thereof.

13. The laser of claim 1, wherein a reflectivity of the anti-reflection film of the end facet of the output end is smaller than 1%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is described hereinbelow with reference to the accompanying drawings, in which:

[0020] FIG. 1 is a structure diagram of a distributed feedback laser in accordance with one embodiment of the invention;

[0021] FIG. 2 is reflective spectra of a reflecting region with different lengths acquired from simulation in accordance with one embodiment of the invention;

[0022] FIG. 3A is a chart illustrating the effective index difference between the active region and the reflecting region, versus the threshold gain in accordance with one embodiment of the invention; and

[0023] FIG. 3B is a chart illustrating the effective index difference between the active region and the reflecting region, versus the threshold gain difference between a dominant mode and a side mode in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] For further illustrating the invention, experiments detailing a distributed feedback laser are described below. It should be noted that the following examples are intended to describe and not to limit the invention.

[0025] A distributed feedback laser having a short cavity length is illustrated in FIG. 1. The distributed feedback laser comprises: an active region 1, also called an optical gain region, and a reflecting region 2. The active region 1 comprises, from the top down: an electric contact layer 3, a waveguide upper cladding layer 4, a grating layer 5, an upper optical confinement layer 6, a Multi-quantum well layer 7, a lower optical confinement layer 8, and a waveguide lower cladding layer 9. The reflecting region 2, from the top down, comprises: a waveguide upper cladding layer 4, a grating layer 5, an upper optical confinement layer 6, a Multi-quantum well layer 7, a lower optical confinement layer 8, and a waveguide lower cladding layer 9. The grating layer of the active region comprises a λ/4 phase-shift region. The gratings at two sides of the phase-shift region are uniform gratings having the same period. The introduction of the λ/4 phase-shift region enables a Bragg wavelength to be a resonant wavelength of the laser cavity so as to be lasing wavelength of the laser. Therefore, the lasing wavelength of the laser can be controlled by accurately controlling the Bragg wavelength of the grating. Besides, as the grating is only able to provide the most effective reflection at the Bragg wavelength, the laser possesses good single-mode characteristics.

[0026] A section of uniformly distributed Bragg grating is introduced to a non-output end of the laser to form the reflecting region. The additional feedback of the reflecting region is able to reduce a threshold gain of the laser, therefore realizing a short cavity length. In addition, the feedback of the reflecting region increases an output power from the output end of the laser, so that a slop efficiency of the laser is improved.

[0027] A period A of the Bragg reflective grating is calculated according to the following equation:

[00002]$\Lambda =\frac{m.Math..Math.\lambda }{2.Math..Math.n_{\mathrm{eff}}}$

in which, m represents a series number of the grating, λ represents a Bragg wavelength corresponding to the grating where the grating is capable of producing a maximum reflection, and n.sub.eff is an effective refractive index of a waveguide.

[0028] The feedback of the reflecting region is regulated via the following two means:

[0029] 1. regulating a length of the section of the grating; under a certain coupling coefficient, the longer the length is, the larger the equivalent reflectivity of the reflecting region is.

[0030] 2. regulating the coupling coefficient of the section of the grating; under a certain length of the grating, the larger the coupling coefficient is, the larger the equivalent reflectivity of the reflecting region is.

[0031] Theoretically, the two means are able to increase the reflectivity of the reflecting region to approaching 1. However, the waveguide exists with intrinsic loss, which makes the reflectivity smaller than 1. The loss of the waveguide is simulated and designed to be a typical value of 20 cm.sup.−1 to acquire a relation between the wavelength and the equivalent reflectivity as shown in FIG. 2. It is known from the chart that the reflectivity at the Bragg wavelength exceeds 85%. As the grating is processed by high accuracy fabrication means, such as adopting the electron beam lithography technology, the reflection phase provided by the grating of the reflecting region can be accurately controlled, thus the feedback provided by the reflecting region and the feedback provided by the grating of the optical gain region are able to keep the same phase, ensuring that the laser is able to lase at a maximum feedback wavelength, which realizes the following effects: 1) the threshold of the laser can keep at a relatively low level, which is important when the cavity length reduces; 2) a lasing wavelength of the laser can be accurately controlled, through controlling the Bragg wavelength of the grating; and 3) the single-mode yield of the laser is high, as the lasing wavelength is always the wavelength that gets the highest feedback, which is the Bragg wavelength of the grating, and in contrast, the feedback of other wavelengths is much weaker, therefore, their threshold gain is much higher. The conventional cleaved facet coated with a high reflective film can easily provide a reflection exceeding 90%, which can reduce the threshold of the laser. However, as the position of the cleaved facet cannot be controlled accurate enough, it cannot be ensured to provide the reflection with the same phase as the grating of the optical gain region, resulting in inaccurate control of the lasing wavelength of the laser and serious problem of single-mode yield.

[0032] The reflecting region and the optical gain region of the distributed feedback laser in this invention comprise the same waveguide structure, and the waveguide core layers are both active layers 7. The difference is that the reflecting region does not have the metal electrode 3, current is not injected and gain cannot be acquired, thus only functioning in increasing the reflectivity of an end facet. No additional etching and regrowth technology are required by the process, which simplifies the fabrication process.

[0033] When current is not injected into the active region and the reflecting region of the distributed feedback laser, their effective refractive indexes should keep the same. When current is injected to the active region, the effective refractive index of the active region varies from that of the reflecting region. When the gratings of both the reflecting region and the active region adopts the same period, the Bragg wavelengths of the gratings of the two regions are slightly different from each other, resulting in reduction of the effective reflection of the reflecting region. In practice, the period of the grating of the reflecting region can be properly regulated to compensate this portion of difference to make the Bragg wavelengths of the gratings of the two portions keep the same. However, even when such compensation measurement is not carried out, it is anticipated that the variation of the effective refractive index caused by current injection may only lead to very small influence. Simulation is made as follows: gain is produced in the active region when current is injected, and such gain is clamped to the threshold gain after lasing of the laser, and in general condition that the threshold gain approaches 40 cm.sup.−1. The reflecting region is optically pumped into transparency by the emission from the laser itself. In such a state, the quantum wells of the waveguide core of the reflecting region neither produce gain nor produce absorption, and a net gain of the waveguide in such condition remains −20 cm.sup.−1, therefore a difference between the net gains of the reflecting region and the gain region is approximately 60 cm.sup.−1. This portion of gain difference will produce a corresponding difference of effective refractive index. They are connected through the linewidth enhancement factor. As the high-speed directly modulated laser generally adopts an InGaAlAs quantum well material, the linewidth enhancement factor often keeps at between 1 and 2, which means that the difference of the effective refractive indexes between the reflecting region and the gain region is smaller than 0.005. When a length of the active gain region is simulated to be 150 μm and a length of the reflecting region is simulated to be 75 μm, relation between the variation of the effective refractive index and the threshold gain and relation between the effective refractive index and the threshold gain difference between a dominant mode and a side mode are charted, as shown in FIG. 3. It is known that when the variation of the effective refractive index n is controlled within 0.005, the threshold gain always keeps below 40 cm.sup.−1, and the threshold gain difference between the dominant mode and the side mode is always larger than 5 cm.sup.−1. Generally, the distributed feedback laser having the threshold gain difference between the dominant mode and the side mode being larger than 5 cm.sup.−1 is able to achieve good single mode characteristic. It is indicated from simulations that even when the optical gain region and the reflecting region adopt the same grating period, the threshold gain and the side-mode suppression ratio of the laser will not be greatly deteriorated.

[0034] To reduce the influence of the reflection of the cleaved facet, the non-output facet of the reflecting region of the laser adopts a window section or a horizontally inclined cleave facet, or is coated with an anti-reflection film, so that the final reflection of the cleavage plane does not affect the performance of the laser.

[0035] An output end of the distributed feedback laser is coated with the anti-reflection film, and a reflectivity of the anti-reflection film is optionally smaller than 1%.

[0036] To further increase the feedback of the resonant cavity of the laser, a section of grating that is the same as the reflecting region is added to the output end of the laser. The grating is able to provide additional reflection, and functions in reducing a lasing threshold of the laser in general, which enables the laser to work in condition of reducing the length of the active region. However, this part of grating also produces additional loss and therefore reduces the output efficiency of the laser. Compared to the grating of the reflecting region, the grating added to the output end should not be too long.

[0037] Unless otherwise indicated, the numerical ranges involved in the invention include the end values. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.