A Novel Sampled Grating Multiwavelength Laser Array

As a key component of the WDM system, MLA (multiwavelength laser array) has been well developed. An MLA with sampled gratings is promising because of the simplicity in wavelength control and fabrication.In the conventional sampled grating, part of the uniform base Bragg grating should be removed. High-order reflections will appear on the reflection spectrum of the sampled grating. The reflection according to Bragg wavlength is marked as the zeroth-order mode, while the high-order reflections are marked as the first-order, second-order, and so on. The different sampling period gives the different high-order mode. For this type of device, the key concept is thus to utilize the high-order reflection, e.g., the first-order reflection of the sampled grating, for laser array operation.The duty of the sampling could be from0to1. However, the zeroth-order reflection is always larger than the first-order reflection. This means that the zeroth-order mode will have a lower gain threshold, compared with the first-order mode. By expanding the Fourier Series of the sampled grating refractive index expression, this could be verified mathematically. The coefficient of the zeroth-order mode is always larger than the coefficient of the first-order mode. Making the high-order mode lasing rather than the zeroth-order mode, is the key problem. By adjusting asymmetric sampled gratings in two sections, or by using a relatively narrow gain spectrum could solve this problem. The former one suggests that mismatch the zeroth-order mode and align the first-order mode of the two section. The other category makes the first-order mode being located within the gain spectrum of the semiconductor material, while the zeroth-order mode is outside of the gain spectrum.This thesis presents a novel multiwavelength laser array with a sampled grating formed by periodically loaded n-doped injection blocking layer on top of the uniform base grating. With an optimized design of the sampled grating as well as the laser operation conditions, all the even orders, including the zeroth-order reflection wavelength, can be effectively suppressed, leaving only the first-order reflection wavelength to lase. It should be pointed out that, unlike in the conventional sampled grating where part of the base grating will have to be removed, the whole base grating is retained in the proposed structure; hence, the side-mode suppression ratio will not be jeopardized due to the weakened grating coupling strength.A comprehensive implementation of the time domain transfer matrix method, is used to simulate and optimize the proposed sampled grating structure. In the conventional transfer matrix formulas, the space step length is usually an integer multiple of the Bragg period length. However, these formulas are not suitable for sampled gratings. Because the space step length is not always an integer multiple of the Bragg period length in sampled gratings. Transfer matrix formulas are modified in this thesis to fit for sampled gratings in time domain.As an example, an array of12lasers has been designed to cover a wavelength range from1542nm to1560nm in C-band, with an SMSR beyond60dB. Probable parameters of the proposed structure are suggested, including the material composition and the sampling period length of the12lasers.