Fig. 1. Structure of DD-OOFDM system

Fig. 2. Image encryption and decryption framework

Fig. 3. Image double chaotic encryption and decryption model

Fig. 4. Chaos insect-population model

Fig. 5. Image of chaotic sequence values with different initial values

Fig. 6. Flowchart of double chaotic sequence encryption

Fig. 7. Electric field signal at the photodiode

Fig. 8. Schematic of the integration area

Fig. 9. KK receiver structure

Fig. 10. Block diagram of optical direct detection system based on chaotic encryption and Kramers-Kronig receiver

Fig. 11. Image and pixel histogram before and after double chaotic encryption. (a) Source image; (b) source image histogram; (c) scrambled image; (d) scrambled image histogram; (e) chaotic encryption image; (f) chaotic encryption image histogram

Fig. 12. Processed results before and after the introduction of KK receiver

Fig. 13. Chaotic decrypted images without and with KK receiver