Objective Chaos synchronisation key distribution driven by a common noise light has higher security because the synchronisation coefficient between the driven signal and the response signal is low, and the eavesdropper cannot restore the complete drive signal and extract the key information from the drive signal, which is essential. Distributed feedback (DFB) laser, a vertical cavity surface-emitting laser and Fabry-Perot laser chaos key distribution systems driven by noise light have all been reported. To realise the chaos key distribution in the above-mentioned scheme, the synchronisation of the chaos and external modulator must modulate the phase polarisation or intensity state of chaos, which increases the complexity of the system, Meanwhile, it is not conducive to the construction of the key distribution system. The distributed Bragg reflector (DBR) laser is a semiconductor laser with a tunable wavelength. The laser output wavelength can be changed by adjusting the current loaded in the DBR region, and then the wavelength input in the chaotic secret distribution system of the DBR laser can be realised. Therefore, it is significance to study the chaotic dynamic characteristics of noise light injected into DBR lasers. In this paper, the chaotic dynamic characteristics of the DBR laser under the injection of noise light are experimentally studied. We found that when the noise light has a large frequency detuning with the main mode, the DBR laser exhibits a completely different chaotic state and its energy is mainly concentrated in the low-frequency range. This paper provides a foundation for high-speed key distribution technology using the synchronisation of noise-light injection common-drive DBR lasers.

Methods The noise light generated by the super-luminescent diode (SLD) is filtered, amplified and refiltered by a tunable filter (TF₁), erbium-doped fibre amplifier (EDFA₁) and TF₂ before the injection of noise light in DBR laser. The optical path uses a variable optical attenuator and polarisation controller to adjust the optical power and polarisation of the injected light. The chaotic signal of the DBR laser is amplified and filtered by EDFA₂ and TF₃, respectively. It is divided into three paths of detection by two optical couplers. In the experiment, the DBR laser gain region current is set to 57.6 mA (1.33 I_th), the DBR region current is 12.1 mA, the free-running centre wavelength of the laser output is 1549.2540 nm (frequency v_DBR) and the optical power (P_DBR) is 1.85 mw. Furthermore, the bias current of SLD is 350.0 mA, while the total power of the emitted light is 12.42 mW. Both TF₁ and TF₂ have a 3-dB filter linewidth of 6 GHz and the same centre wavelength with λ_SLD (frequency v_SLD). Additionally, the filter line width and centre wavelength of the tunable filter TF₃ are adjusted according to the state of the chaotic laser emitted by the DBR laser to ensure that the entire chaotic signal can be filtered out and the interference of the detection signal on the injected noise light (Fig. 1).

Results and Discussions The optical spectra; electrical spectra and temporal waveforms of the injected light show the injected light is a noise signal (Fig. 2). We fix noise-light injection strength κ_j=PSLD/PDBR=0.32, where P_SLD is the noise-light power entering the DBR laser. When Δv=v_DBR-v_SLD is 0 GHz, that is, under the main mode noise-light injection, the laser output is like those produced by light injection into ordinary DFB lasers. Further increase of Δv when the centre wavelength of the injected noise light is in the valley between the two modes of the DBR laser, their corresponding Δv are 27.5 GHz and 81.5 GHz, respectively. At this time, the chaotic electrical spectra are inverted “check” shape, the relaxation oscillation peak are obvious, and their 80% energy bandwidth are 1.36 GHz and 1.29 GHz. The temporal waveforms show an obvious chaotic oscillation state, but the amplitude fluctuation is small. When the centre wavelength of the injected noise light is at the peak of the DBR optical spectrum side mode, the Δv values are 47.5 GHz and 104.5 GHz. The chaotic laser electrical spectra are flat in the low-frequency band, and the oscillation peaks are almost disappeared, and their 80% energy bandwidth are 1.10 GHz and 1.07 GHz (Fig. 3). Furthermore, when the noise light is at the main mode, the chaotic bandwidth decreases as the injection strength decreases. When noise light is at the side mode, the chaotic bandwidth decreases as the injection strength increases (Fig. 4). Moreover, the correlation dimension of chaos reflects the complexity of the system. When the noise light is injected into the main mode, the correlation dimension of the chaos generated by the DBR laser is 6.17±2.02; and when the side mode is injected, it is 1.86±0.36. Meanwhile, when the noise light is in the main mode, the injection strength has little effect on the fluctuation of the correlation dimension, which shows that the chaotic complexity is higher, and the fluctuation comes from chaos itself and has little relation with external noise (Fig. 5). In key distribution system driven by noise light, the driven signal will inevitably be introduced into the detection end. This process will reduce the synchronisation coefficient of the legitimate user, and the phenomenon is not conducive to the chaos key distribution. In the case of side mode injection, the filter can avoid the introduction of noise light. Additionally, the chaos correlation dimension and the fluctuation of the dimension are small. It is easy to obtain a higher synchronisation coefficient, which is conducive to chaos synchronisation key distribution.

Conclusions Facing the application requirements of chaos key distribution, this paper experimentally studies the chaotic dynamics characteristics of noise light injected into DBR lasers. The study found that when noise light is in the main mode injection interval, the DBR laser shows a chaotic state similar to that of the light-injected DFB laser. When the noise light is in the side mode injection interval, the DBR laser shows a different chaotic state. It is mainly concentrated in the low-frequency band, and the relaxation oscillation peak is not obvious. We further studied the bandwidth and correlation dimension of chaotic lasers under different frequency detuning and injection strength and found that underside mode injection the correlation dimension is 1.86±0.36, the noise component introduced in the detection process is low, which is more conducive to the construction of chaos key distribution system.