# Cavity Wavelength on Erbium-Doped Fiber Ring Laser Depending on Fabry-Perot Etalon Steering Angle
## 2 Experimental Setup  
Figure 1 shows the fiber optic ring laser architecture of this work. There are two options, both of which are suitable for different laser wavelength tuning methods. The main difference between these two configurations is that the laser output is from the opposite direction of the 3 dB coupler. In the left-hand side configuration, the output port beam from the EDFA initially traverses a 3 dB coupler before proceeding through a collimator, which facilitates the transmission of light out from the optical fiber to the FC-LP. Before entering the FC-LP, the beam undergoes a single polarization fashion through a polarizer. Upon passing through the FC-LP, the collimator guides the light back into the optical fiber, where the signal is subsequently amplified by the EDFA. It is important to highlight that the custom-fabricated FC-LP exhibits a reflectivity rate of up to 90%. Consequently, the light entering the EDFA's input port is approximately 30 dB weaker than the output light but can successfully pass through the collimator to reflect its light from the outside surface of the FC-LP cavity. This light is then reflected along its initial path, returning to the EDFA for optical signal amplification and fusion with the output at the EDFA's output port. However, it should be noted that the final laser output does not originate from the beam traveling through the FC-LP, as the EDFA input port beam's transmission through the FC-LP is minimal. Although the output beam from the EDFA's output port propagates through the FC-LP as the optical signal is continuously circulated and amplified within the ring structure, the ultimate laser output is derived from the reflection of the beam outside the FC-LP cavity. Accordingly, the reflection spectrum is directed to an optical spectral analyzer via a 3 dB coupler positioned in the opposite direction of the EDFA's output port to present the spectrum. On the other hand, in the configuration presented on the right, the output beam from the EDFA initially traverses a collimator before being filtered by a  
polarizer to achieve a single polarization. Subsequently, the beam is directed to the LC-FP. The beam passing through the LC-FP is then directed to the optical fiber via the collimator, where half of the energy is directed to the optical spectral analyzer, while the remaining half is amplified by the EDFA. The beam reflected by the LC-FP from the output port of the EDFA will be ignored, as the built-in optical isolator of the EDFA allows unidirectional routing. Additionally, the input port beam of the EDFA is also reflected by the LC-FP through the collimator; half of the energy is routed to the optical spectral analyzer, and half of the energy is returned to the EDFA for optical amplification. The resultant laser output is obtained following transmission through the LC-FP, as the output port of the 3 dB coupler aligns in the same direction as the output port of the EDFA.  
The homemade LC-FP structure comprises two glass sheets made of coated indium tin oxide (ITO), which are supported by vertically positioned spacers, thereby establishing a cavity with a gap of 40 \(\upmu\)m for the accommodation of the liquid crystal. The reflectivity of the dielectric coating is approximately 90% within the wavelength range of 1520 to 1620 nm. By connecting two electrodes to the ITO layers and applying an external voltage, the orientation of the liquid crystal molecules is altered in response to changes in the electric field. This alteration induces a change in the birefringence of the liquid crystal, which subsequently modifies the optical path difference. Incidentally, the 3 dB coupler, polarizer, and collimator are commonly available components in the marketplace.