tailieunhanh - Advances in Optical and Photonic Devices 2011 Part 5

Tham khảo tài liệu 'advances in optical and photonic devices 2011 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Optical Injection-Locking of VCSELs 91 4. Experiments using multimode lasers Multimode Edge Emitting Lasers EELs Optically injection-locked lasers are known to overcome many fundamental limitations of free-running systems. One of the very important improvements proposed by the employment of the optical injection-locking technique is the side-mode suppression of a multimode laser Iwashita and Nakagawa 1982 . Fig. 12 presents the superimposed optical spectra of a free-running and an injection-locked laser diode. The Fabry-Pérot modes visible in the free-running regime undergo approximately 35 dB suppression when injection-locked using a DFB laser diode. Fig. 12. The super-imposed spectra of a free running and an injection locked Fabry-Pérot EEL. Mode suppression can be observed in the injection locked spectrum. In the stable locking regime the follower laser frequency is locked to the master laser lasing frequency. The injection-locked Fabry-Pérot mode therefore becomes dominant and the unlocked modes are suppressed. Iwashita et. al demonstrated the utilization of this method for the suppression of mode-partition noise 1 . The employment of optical-injection locking for side-mode suppression in VCSELs however is not very effective. This is due to the difference in the side-mode generation mechanism between the EELs and the VCSELs. A detailed analysis of side-mode generation is presented in the following section. Single-mode operation of the follower laser however is highly desirable due to another very important reason. As presented in figure the locking-range of an injection-locked laser in the stable operation region is dependent on the injected optical power. This effective locking-range is exploitable only if the follower laser is single-mode. If the follower laser is multimode the achievable detuning frequency is limited by the Free Spectral Range FSR of the follower laser. At large detuning frequencies the master laser might come closer to an adjacent .