The latest ultra-high extinction ratio electro-optic modulator

The latest ultra-high extinction ratio electro-optic modulator

On-chip electro-optical modulators (silicon-based, triquinoid, thin film lithium niobate, etc.) have the advantages of compactness, high speed and low power consumption, but there are still great challenges to achieve dynamic intensity modulation with ultra-high extinction ratio. Recently, researchers at a joint research Center for Fiber Optic Sensing at a Chinese university have made a major breakthrough in the field of ultra-high extinction ratio electro-optical modulators on silicon substrates. Based on the high order optical filter structure, the on-chip silicon electro-optic modulator with extinction ratio of up to 68 dB is realized for the first time. The size and power consumption are two orders of magnitude smaller than that of traditional AOM modulator, and the application feasibility of the device is verified in the laboratory DAS system.

Figure 1 Schematic diagram of test device for ultra high extinction ratio electro-optic modulator

The silicon-based electro-optical modulator based on the coupled microring filter structure is similar to the classical electrical filter. The electro-optic modulator achieves flat bandpass filtering and high out-of-band rejection ratio (>60 dB) through the series coupling of four silicon-based microring resonators. With the help of a Pin-type electro-optical phase shifter in each microring, the transmittance spectrum of the modulator can be significantly changed at a low applied voltage (<1.5 V). The high out of band rejection ratio combined with the steep filter roll-down characteristic enables the intensity of the input light near the resonant wavelength to be modulated with a very large contrast, which is very conducive to the production of ultra-high extinction ratio light pulses.

To verify the modulation capability of the electro-optic modulator, the team first demonstrated the variation of the transmittance of the device with the DC voltage at the operating wavelength. It can be seen that after 1 V, the transmittance drops sharply over 60 dB. Due to the limitation of conventional oscilloscope observation methods, the research team adopts the self-heterodyne interference measurement method, and uses the large dynamic range of the spectrometer to characterize the ultra-high dynamic extinction ratio of the modulator during pulse modulation. The experimental results show that the output light pulse of the modulator has an extinction ratio of up to 68 dB, and the extinction ratio of more than 65 dB near several resonant wavelength positions. After detailed calculation, the actual RF drive voltage loaded to the electrode is about 1 V, and the modulation power consumption is only 3.6 mW, which is two orders of magnitude smaller than the conventional AOM modulator power consumption.

The application of Silicon based electro-optic modulator in DAS system can be applied to a direct detection DAS system by packaging the on-chip modulator. Different from the general local-signal heterodyne interferometry, the demodulation mode of non-balanced Michelson interferometry is adopted in this system, so that the optical frequency shift effect of the modulator is not required. The phase changes caused by sinusoidal vibration signals are successfully restored by demodulation of Rayleigh scattered signals of 3 channels using conventional IQ demodulation algorithm. The results show that the SNR is about 56 dB. The distribution of power spectral density along the entire length of the sensor fiber in the range of signal frequency ±100 Hz is further investigated. Besides the prominent signal at the vibration position and frequency, it is observed that there are certain power spectral density responses at other spatial locations. The crosstalk noise in the range of ±10 Hz and outside the vibration position is averaged along the length of the fiber, and the average SNR in space is not less than 33 dB.

Figure 2

a Schematic diagram of optical fiber distributed acoustic sensing system.

b Demodulated signal power spectral density.

c, d vibration frequencies near the power spectral density distribution along the sensing fiber.

This study is the first to achieve an electro-optical modulator on silicon with an ultra-high extinction ratio (68 dB), and successfully applied to DAS systems, and the effect of using commercial AOM modulator is very close, and the size and power consumption are two orders of magnitude smaller than the latter, which is expected to play a key role in the next generation of miniaturized, low-power distributed fiber sensing systems. In addition, the CMOS large-scale manufacturing process and on-chip integration capability of silicon-based optoelectronic devices can greatly promote the development of a new generation of low-cost, multi-device monolithic integrated modules based on-chip distributed fiber sensing systems.