Application Cases

Experimental Title: Liquid Crystal-Filled Photonic Crystal Fiber Coupling Device and Its Sensing Characteristics

Research Focus: A nematic liquid crystal was infiltrated into one of the air holes in the innermost ring of a PCF to fabricate a PCF directional coupling structure. The response of the transmission spectrum of this structure to an external electric field, along with its corresponding electro-optic switching and modulation characteristics, were investigated in detail.

Test Equipment: High-voltage amplifier, signal generator, oscilloscope, tunable laser, photodetector, etc.

Experimental Process:

Figure 1: (a) Variation of sample transmission spectrum with increasing applied voltage, (b) Relationship between the coupling peak wavelength and the applied voltage.

A sinusoidal voltage with a fixed frequency of 1000 Hz was first applied to the conductive glass, and its RMS value was gradually increased from 0 to 130 V_rms. The change in the sample's transmission spectrum with increasing applied voltage is shown in Figure 1(a). The coupling peak at 1365 nm, labeled DipA, was monitored. It can be observed that the sample's spectrum remained largely unchanged until the applied voltage reached 60 V_rms. As the voltage increased from 60 V_rms to 80 V_rms, the wavelength of this coupling peak shifted nonlinearly towards longer wavelengths, approaching 1400 nm. This nonlinear shift is attributed to irregular slight changes in the refractive index of the liquid crystal. Liquid crystal molecules typically have a threshold voltage required to overcome the surface anchoring energy and molecular thermal motion. When the applied voltage is below this threshold, the alignment of the liquid crystal molecules within the PCF air hole either remains unchanged or begins to undergo slight irregular deflection as the voltage increases, leading to minor changes in the refractive index. This ultimately manifests in the device's spectrum, evolving from initial stability to a slight nonlinear shift.

As seen in Figure 1(a), by applying sinusoidal voltages with different RMS values, the transmittance of light at a specific wavelength in the liquid-filled PCF can be controlled. When a suitable voltage is applied and the depth of the device's coupling peak is sufficient, complete turning on and off of light can even be achieved. Simultaneously, it was found experimentally that there was no significant delay in the change of the sample's transmission spectrum when voltage was applied, enabling the proposed device to function as an all-fiber fast electro-optic switch.

Experimental Results:

The electro-optic switching characteristics of the device were experimentally investigated. A tunable laser was used to output single-wavelength laser light, which was coupled into the liquid crystal-filled PCF via a single-mode fiber. The light output from the PCF was fed through another segment of single-mode fiber into a photodetector. The output of the photodetector was connected to an oscilloscope. The photodetector received the light from the sample, converted the optical signal into an electrical signal, which was then displayed on the oscilloscope. The switching response characteristics of the device were studied by monitoring the waveform on the oscilloscope. The same signal generator and high-voltage amplifier as mentioned previously were used to apply the electrical signal to the ITO glass for electrical modulation of the PCF sample.

First, the tunable laser was set to output 1550 nm single-wavelength light. The optimal sinusoidal voltage magnitude and frequency for achieving all-fiber electro-optic switching with this device at this input wavelength were tested. Subsequently, sinusoidal signals with different RMS values at a frequency of 1 kHz were used as the carrier wave in the signal generator. A square wave signal with a period of 0.4 ms was used as the modulation wave to perform amplitude modulation on the carrier wave with 100% modulation depth. This generated an intermittent sinusoidal voltage signal output from the signal generator. After amplification by the high-voltage amplifier, this signal was applied to the ITO conductive glass, creating an intermittent spatial electric field acting upon the liquid crystal-filled PCF.

Figure 2: (a) Switching response of the device under different voltage signal RMS values and (b) under different voltage signal frequencies.

A schematic of the voltage signal applied to the conductive glass and the device's switching response under different voltage RMS values are shown in Figure 2(a). It can be seen that for input light at 1550 nm, the device achieved the best on/off ratio at an RMS voltage of 110 V_rms. The influence of electrical signal frequency on the device's switching characteristics was then investigated. With the carrier voltage signal RMS value fixed at 110 V_rms, the frequency (f) of the voltage signal was varied. Similarly, amplitude modulation with 100% depth was applied using a square wave modulation signal with a period of 0.4 ms, generating an intermittent voltage signal applied to the ITO conductive glass. The switching performance of the device was tested at different voltage signal frequencies. Figure 2(b) shows a schematic of the applied voltage signal and the device's switching response results at different frequencies. It can be observed that the switching efficiency was highest at 1 kHz. In summary, for the proposed device with input light at 1550 nm, using a voltage signal with an RMS value of 110 V_rms and a frequency of 1 kHz yielded the best switching performance.

High-Voltage Amplifier Recommendation: ATA-7015

Figure: ATA-7015 High-Voltage Amplifier Specifications and Parameters

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