Optical modulators are one of the important components in optical systems, which are generally based on electro-optic effects, thermo-optic effects, nonlinear effects, and elastic-optic effects[
Chinese Optics Letters, Volume. 16, Issue 4, 040605(2018)
Investigation on an all-optical intensity modulator based on an optical microfiber coupler
An all-optical intensity modulator based on an optical microfiber coupler (OMC) is presented. The modulator works at 1550 nm wavelength and is modulated directly by heating the coupling region with 980 nm pump light injected through the coupling port of the OMC. The OMC is controlled to have at least a 30 mm long coupling region with diameter smaller than 8 μm, and the uniform waist region diameter is about 3 μm. This is helpful to ensure the optical modulation function based on the light induced thermal effect in the coupling region, while pump light is injected. The modulation response is measured to show good linearity when the 980 nm pump light has a lower intensity (with power below 2.5 mW), which proves that the OMC acts as an all-optical modulator. The bandwidth of the modulator can be at 0.2–50 kHz with the average power of the intensity-modulated pump light about 2 mW, which can be further improved by optimizing the design of the coupler. The demonstrated modulator may have potential value for the application in an all-optical integration system.
Optical modulators are one of the important components in optical systems, which are generally based on electro-optic effects, thermo-optic effects, nonlinear effects, and elastic-optic effects[
The OMC is fabricated with two twisted conventional fibers in the drawing system based on the improved flame-brushing method, according to the fabrication techniques of conventional fiber couplers and OMs[
Figure 1.Schematic illustration of the composite OMC and the all-optical modulation theory of the OMC.
It has been studied that the doped optical fiber has a weak optical modulation ability due to the limited optical field area in the fiber (usually an 8–10 μm diameter in the core of a conventional single-mode fiber) and large thermal conduction volume (usually 125 μm diameter for a conventional single-mode fiber)[
Sign up for Chinese Optics Letters TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
In order to meet the optical modulation requirements, the OMC should have the coupling capability and maximally enhance the interaction between light and waveguide materials, as the coupling region of the OMC could be regarded as two OMs, which had been tightly adjacent. The study on all-optical phase modulators shows that the modulation efficiency is proportional to fiber length, and it is inverse to the fiber diameter in a certain range[
We have fabricated an OMC with the properties described above. A laser diode at 1550 nm wavelength is used as the light source of the OMC. The throughput port (Port3) and the coupling port (Port4) are monitored by two photodectors (OE-200). During the drawing process of the OMC, the output of Port3 is shown in Fig.
Figure 2.(a) Transmission intensity of the microfiber coupler Port3 during the fabrication process. (b) Input and output optical spectra of the OMC.
Figure
Figure 3.Image of the OMC in an optical microscope.
According to the coupling theory, the light power of the two output ports can be expressed as[
When the injected modulated pump light is injected with lower power, and the pump light can be expressed as
We set up an all-optical intensity modulation system to measure responses of OMC modulators driven by the intensity-modulated 980 nm light. An optical fiber generally shows absorption at a wide spectral range from ultraviolet to infrared wavelengths. The proposed modulation method exploits the light induced thermal effect and can be realized using light sources at other wavelengths as pump light. We used a light source at 980 nm just because it is widely used and easily available in labs.
The experimental configuration is illustrated in Fig.
Figure 4.Schematic of the measurement system.
Firstly, we measured the output of the 980 nm pump laser directly at the FC placed with another photodetector and an oscilloscope when a sinusoidal modulation signal with a frequency of 1 kHz and an amplitude of 160 mV was imposed on the 980 nm pump driver. We observed that the intensity of the pump light was modulated clearly, corresponding to the modulation signal. We had adjusted the modulation depth of the 980 nm pump light signal to be near 1 in the next experiments. Then, we connected the 980 nm pump laser into the system again, as shown in Fig.
Figure 5.Oscilloscope display of the output of the system (top waveform) and the modulation signal (bottom waveform): (a) 1 kHz; (b) 20 kHz.
We replaced the OMC separately with a conventional 3 dB coupler and a coupler with an 8 μm tapered waist diameter, and then we could not observe the same phenomenon. The above processes have verified that the OMC could work as a modulator driven by the intensity-modulated 980 nm pump light, and it could be an all-optical modulator based on the light induced thermal effect.
To measure the responses of the OMC modulator, we sampled the output of the oscilloscope by a computer and calculated the amplitude of modulation produced by the OMC modulator. The linear response property of the OMC modulator was measured by varying the amplitude of modulating signals imposed on the 980 nm laser driver at the frequencies of 1 and 5 kHz. The experimental results are plotted in Fig.
Figure 6.Measured (dots) and fitted linear responses of the OMC modulator at 1 and 5 kHz frequencies.
The frequency responses of the OMC modulator were measured in the system. Meanwhile, the average power of the 980 nm modulated light was constant, which had been measured to be 2 mW. In order to eliminate the effects of the length, local loss of OMC, and the effects of the intensity variation of the 980 nm modulation light, we normalized the values of the measured frequency responses with the phase modulation amplitude at a frequency of 0.2 kHz. The measured data of the frequency response curves are plotted in Fig.
Figure 7.Measured frequency response of the all-optical OMC modulator.
For further understanding of the modulation response speed, the 980 nm pump laser is injected in a square-wave form. Figure
Figure 8.Oscilloscope display of the output of the system (CH2) and the modulation signal (CH1) for square-wave modulation.
To ensure that there is enough light extinction ratio and modulation speed, which are essential for real applications, we should optimize the structure and function design of OMCs and further increase the response speed. For example, we could add metallic or polymer coatings on a tiny waist region to enhance the nonlinear and thermal-optic effects[
In conclusion, this Letter analyzes the modulation mechanisms and characteristics of thermal-optic-effect-based OMC all-optical modulators. By optimizing the design of the structure, the output intensity of 1550 nm probe light through the OMC is modulated directly by heating the coupling region with 980 nm pump light injected into the OMC. The OMC modulator shows good linear modulation responses with the intensity of the 980 nm pump light (with power below 2.5 mW), and the modulation efficiency is inversely proportional to the modulation frequency. The frequency responses of the modulator can be at 0.2–50 kHz, which may be further improved by optimizing the design of the coupler. Further works will be carried out on the modulation efficiency, the package, and the selection of the modulating light wavelength of the OMC modulator. As a passive, linear, all-optical modulator, the OMC will obtain fast development and real applications in minimized and integrated all-optical communication and sensing systems.
[17] R. Wang, D. Li, H. Wu, M. Jiang, Z. Sun, Y. Tian, J. Bai, Z. Ren. IEEE Photon. J., 99(2017).
[19] Y. Jung, G. Brambilla, D. J. Richardson. Opt Express, 17, 5273(2009).
[22] S. Wang, H. Yang, Y. Liao, X. Wang, J. Wang. IEEE Photon. J., 8, 6804209(2016).
[23] L. Sun, Y. Semenova, Q. Wu, D. Liu, J. Yuan, X. Sang, B. Yan, K. Wang, C. Yu, G. Farrell. IEEE Photon. J., 8, 6805407(2016).
[27] Y. Yu, Q. Bian, X. Zhang. Chin. J. Lasers, 45, 0606003(2018).
Get Citation
Copy Citation Text
Yang Yu, Qiang Bian, Nan Zhang, Yang Lu, Xueliang Zhang, Junbo Yang. Investigation on an all-optical intensity modulator based on an optical microfiber coupler[J]. Chinese Optics Letters, 2018, 16(4): 040605
Category: Fiber Optics and Optical Communications
Received: Dec. 9, 2017
Accepted: Feb. 9, 2018
Published Online: Jul. 12, 2018
The Author Email: Xueliang Zhang (xueliang.john@hotmail.com)