Over the past years, pulsed laser have been an attractive tool widely employed in vast applications, such as medical, optoelectronics and materials engineering. Pulse laser as compared to continuous wave, gives many advantages such as providing higher pulse energy and peak power. In general, pulse lasers can be initiated by either active or passive techniques. Active technique requires a more complex system, which employs an electro- or acousto-optic modulator with additional optical components such as lenses, mirror, and U-bench units. Those additional devices introduced the insertion loss in the laser systems. Therefore, passive mechanism is preferable since it is more compact, provides better stability, and is relatively low in cost, without the addition of external modulator.
Passive technique exploits the saturable absorption characteristic of suitable material, which exhibits a reduction of absorption ability with the increase of light intensity inside a laser cavity. Conventionally, semiconductor saturable absorber mirrors (SESAMs) were widely used as a saturable absorber (SA) in commercial laser systems. However, the development of SESAM based laser systems are limited by their complexity in fabrication, limited operating wavelength and reasonably high cost. To cope up with this limitation, carbon-base materials (carbonnanotubes, graphene, graphite, graphene oxide) have been alternatively proposed. These materials have very short recovery time around 500 fs, higher damage threshold and wider operation spectrum, but still they suffer from complex fabrication and limited applications. After graphene, low dimensional materials were used in the fields of nanotechnology and science such as transition metal-dichalcogenides (TMDs) and Topological insulators (TIs). TMDs possess high second order receptivity, robust coupling orbit and high carrier mobility. TIs have low saturation threshold and high nonlinear coefficients. However, band gap of TIs dependent on the operational wavelength which makes the TIs fabrication complex. The third type of 2D material is black phosphorus (BPs), which is considered as a thermo-dynamically stable material due to its small band gap. Unfortunately, BP has high sensitivity to air and water. Recently, metal oxides and metal carbides are also reported as SAs for the generation of pulsed lasers. Moreover, MXene, a new type of 2D material was used as SA in fiber laser cavity as it has a high nonlinear optical property to generate ultrafast laser.
An optical ring configuration of the Q-switched EDFL using lawsone saturable absorber is made in this experiment. The SA functions to modulate the intracavity losses by modulating the intracavity losses and thus the Q factor of the laser resonator for the generation of Q-switched pulses. A 2.4 m long erbium-doped fiber (EDF) is pumped by a 980 nm single mode semiconductor laser (LD), through a 980/1550 nm wavelength division multiplexer (WDM). A 50:50 fiber- merged coupler is used to fed back 50% of the oscillating light to the cavity. The output of the laser was tapped out from the second port of the 50:50 coupler. The EDF has a numerical aperture of 0.23, core and cladding diameter of 4 µm and 125 µm respectively, and the absorption coefficient of about 25 dB/m at 980 nm. The unidirectionality of the light inside the optical resonator is assured by inserting a polarization insensitive isolator in between WDM and the SA device. The frequency of signal was observed with radio frequency spectrum analyzer (RFSA) and the time domain analyses were reported by Oscilloscope (GDS-3352). Both the RFSA and Oscilloscope were connected via a 1.2 GHz photodetector. We got the output optical spectrum by OSA, having the resolution of 0.07 nm. Besides that, optical power meter was used to measure the output power in milliwatt (mW). The total cavity length was approximately 7 m.
In this study, the power of the input pump was increased steadily and the output of the EDFL was analyzed using an oscilloscope and a spectrum analyzer. In the beginning, only a continuous wave laser was produced when no lawsone SA was used in the laser configuration. The CW laser persisted throughout the range of power available to the input pump. However, when a lawsone film SA was positioned between the coupler and isolator in the cavity, a stable and self-started Q-switched pulses replaced the CW laser starting from the threshold pump power of 26 mW until 43.3 mW. Result shows the pulse trains and single pulse plots of the Q-switched laser at pump power of 26, 33.6, and 43.3 mW respectively. They were captured via a 1.2 GHz photodetector connected to an oscilloscope.
The pulse train shown is produced at the pump power of 26 mW. It has a repetition rate of 70 kHz and a pulse width of 2.25 µs. At the pump power to 33.6 mW, the repetition rate of the pulse train increases to 75.4 kHz while its pulse width drops to 1.8 µs. At the maximum pump power of 43.3 mW, the pulse repetition rate and pulse width are 80 kHz and 1.7 µs. It was observed that the pulse train remained smooth with little noise or fluctuation for more than 48 h continuously. This observation proves the stability of the Q-switched laser. The experiment was repeated several times, and each time the lawsone SA was able to produce the same Q-switched laser output without a noticeable difference in quality. However, when the pump power went past 45 mW up to the maximum value of 300 mW, the Q-switched pulses became unstable. After reducing the pump power back to 43.3 mW, the stable Q-switched pulse trains reemerged. This shows that the SA was not damaged and its damage threshold is more than 300 mW.
In this paper, we have successfully prepared a lawsone film and used as a saturable absorber for generating a passively Q-switched EDFL. The stable range of Q-switched operation is observed from 26 to 43.3 mW pump power, at the center wavelength of 1564 nm and the repetition rate of 70–80 kHz. The highest pulse energy and shortest pulse width are obtained to be 53.7 nJ and 1.7 µs, respectively. The lawsone material results show a promising operation of passively Q-switched EDFL cavity (at 1.5-micro region).
Author: Prof. Dr. Moh. Yasin, M.Si.
Detailed information from this research can be seen on our article at:
https://www.sciencedirect.com/science/article/pii/S1068520021000869
Rawan S.M. Soboh, Ahmed H.H. Al-Masoodi, Fuad. N.A. Erman, Ab. H.H. Al-Masoodi, B. Nizamani, H. Arof, M. Yasin, S.W. Harun., Lawsone dye material as potential saturable absorber for Q-switched erbium doped fiber laser.
https://doi.org/10.1016/j.yofte.2021.102537
