Fig. 1. Typical external cavity lasers^[3-5]. (a) Littrow structure; (b) Littman-Metcalf structure

Fig. 2. Ionon's external cavity tunable laser^[6-7]. (a) Schematic; (b) butterfly package device diagram

Fig. 3. Intel's tunable external cavity laser employing thermally tuned etalons^[8]

Fig. 4. NEC's external cavity tunable lasers based on etalon and liquid crystal mirror^[9]. (a) Structure schematic; (b) tuning schematic

Fig. 5. NEC's liquid crystal external cavity tunable laser^[10-11]

Fig. 6. NEC's silicon oxynitride external cavity lasers^[15-16]. (a) Structure diagram; (b) tuning spectrum

Fig. 7. External cavity tunable lasers developed by NEC with three microrings ^[17]. (a) Structure diagram; (b) tuning spectrum diagram

Fig. 8. Hybrid integrated external cavity lasers produced by Shanghai Jiao Tong University ^[18-19]. (a) Chip structure; (b) butterfly package

Fig. 9. Santur's tunable laser^[12]. (a) Chip; (b) schematic of package structure

Fig. 10. NEC's 8-channel array lasers^[22]

Fig. 11. Tuning spectra of 6 pieces of 8-channel DFB array lasers produced by NEC^[22]

Fig. 12. Cascade DFB tunable lasers from Nortel, Canada^[11]

Fig. 13. Schematic of tunable VCSEL structure^[6]

Fig. 14. Schematic of typical SG-DBR structure^[6]

Fig. 15. DS-DBR lasers^[34]. (a) Chip; (b) reflection spectra from forward and backward grating

Fig. 16. Schematic of MG-Y lasers^[38-39]

Fig. 17. Schematic of structure of three section DBR lasers^[12]

Fig. 18. NTT's monolithic integrated microring tunable lasers^[48]. (a) Chip; (b) tuning spectrum

Fig. 19. Multi-etched groove lasers^[51-52]. (a) Schematic; (b) output spectra

Fig. 20. Schematic of the tunable V-cavity laser with integrated MZI^[55]

Fig. 21. Tuning principle of the tunabe V-cavity laser^[55]

Fig. 22. Schematic of symmetrical expansion of the tunable V-cavity laser based on reflective 2×2 coupler^[55]

Fig. 23. Change of output light field with input phase difference^[55]. (a) Coupling phase difference is 90°; (b) coupling phase difference is 180°

Fig. 24. Half-wave coupled V-cavity tunable laser^[56]. (a) Chip structure; (b) tuning spectra

Fig. 25. Tunable V-cavity laser^[57]. (a) Tuning spectra; (b) tuning curves at four different temperatures

Fig. 26. Schematic and typical output spectrum of COBRA's tunable V-cavity laser^[58]

Fig. 27. O-band tunable lasers^[59]. (a) Tuning spectra; (b) direct modulation 2 Gbit/s eye diagram; (c) 6 Gbit/s eye diagram;(d) 4 Gbit/s eye diagram; (e) 8 Gbit/s eye diagram

Fig. 28. HP's O-band tunable V-cavity laser^[60]. (a) Tuning spectra; (b) 3 dB bandwidth

Fig. 29. Mid-infrared tunable V-cavity laser^[61]. (a) Typical spectrum; (b) tuning curves at different currents and temperatures;(c) 47-channel tuning spectra in the range of 53 nm

Fig. 30. 16×16 MMI^[62]. (a) Tunable V-cavity lasers; (b) SEM image

Fig. 31. 16×16 MMI coupler V-cavity laser^[62]. (a) Tuning spectra under single-temperature and single-electrode; (b) single-electrode tuning spectra combining two temperatures

Fig. 32. Tunable V-cavity laser with monolithic integrated M-Z modulator^[63]. (a) Chip; (b) 64-channel tuning spectra; (c) electro-optical response curve of integrated M-Z modulator

Fig. 33. 10 Gbit/s optical module^[65]. (a) 41 nm broadband tuning spectra; (b) typical eye diagram of direct modulation; (c) typical eye diagram of electro-absorption

Fig. 34. Bit error rate curves of tunable V-cavity lasers at different wavelengths and distances^[65]. (a) Bit error rate curves at wavelength of 1559.79 nm; (b) bit error rate curves at different wavelengths

Fig. 35. 25 Gbit/s tunable EML lasers^[65]. (a) Chip; (b) device; (c) module; (d) 32-channel spectra; (e) typical eye diagram

Fig. 36. Frequency stability and power stability of tunable V-cavity laser within 5000 h. (a) Frequency stability; (b) power stability

Fig. 37. G.698.4 network architecture^[66]

Fig. 38. Schematic of automatic wavelength pairing

Fig. 39. Distributed optical transceiver switching system. (a) Schematic; (b) demonstration system