# A Data Transmission Method with Spectral Switches via Electroabsorption
## 4 Discussion and Conclusions  
As mentioned in the Introduction, in the past a data-transmission scheme based on the EO effect to control the refractive index to manipulate the spectra was presented [3]. Here we use the electroabsorption effect to control the absorption coefficient and spectral shifts to perform the same task. The electroabsorption modulation typically operates at greater speeds and at lower voltages than electro-optic modulators, operating based on the refractive-index response to applied electric fields. The data can still be transmitted by modulating monochromatic light intensity. Nevertheless, one advantage of using polychromatic light spectral shift lies in that only the spectral shift (blue or red) needs to be judged or confirmed. This is independent of the absolute detected spectrum intensity. In the monochromatic light intensity modulation scheme, monitoring and maintaining correct intensities corresponding to bit 0 or 1 are necessary, and may be challenging. However, in the polychromatic scheme, an optical spectrum analyzer or fiber spectrometer is required to record the modulated spectrum.  
It seems that this method is similar to the conventional frequency modulation (FM) because the shifted spectral peaks (red shift or blue shift) have different wavelengths (or frequencies). However, they are essentially different as explained below. Direct FM modulation can be achieved by directly feeding the message into the input of a voltage-controlled oscillator. Thus, at any instantaneous moment there is only one frequency component (a higher or lower one representing digital cone 1 or 0, respectively) being transmitted--it is a \"frequency-changing\" scheme. In our method, no oscillator is used, only an electroabsorption material is used to control the broad-band light source spectrum. There are many wavelength components that remain in the spectrum at any instantaneous moment, as can be seen in Figure 7. The digital data are encoded through the red/blue shift of the spectra; thus, it is a \"spectral-shift\" scheme.  
This work studies in detail the effects of a semiconductor quantum-well waveguide's electroabsorption on a polychromatic light. We demonstrated that for light sources with suitable central wavelength and bandwidth, the spectrum peak shifts can be controlled by varying the magnitude of the applied voltage. This feature can be employed to perform data transmission jobs on a chip scale, which is of value because the waveguide can be fabricated and integrated with a light source on a single chip. Possible candidates for this kind of polychromatic light source include LEDs or edge emitting light emitting diodes (ELEDs). Compared with its monochromatic counterpart, such as DFB lasers, LEDs (or ELEDs) have the advantages of simple structure, easy fabrication and low cost.  
Figure 8: Illustration for the data encoding and transmission by controlling the magnitude of the applied field. The two-level data 0 and 1 are related to the spectral shifts R and B, respectively, which can be obtained by varying the magnitude of the applied voltage \(V\).  
**Author Contributions:** Conceptualization, P.H.; methodology, J.-P.C.; validation, J.-H.W.; data curation, H.-C.H.; writing--original draft preparation, H.-C.H.; writing--review and editing, P.-Y.L.; writing--review and editing, P.H.; funding acquisition, P.H. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under contract number MOST 107-2221-E-005-059-MY3.  
Not applicable.  
**Acknowledgments:** We would like to express our thanks for the support from National Chung Hsing University, and Central Taiwan Science Park 110 RB02.  
The authors declare no conflict of interest.