All-fiberized amorphous carbon nitride (a-CNx) based passive Q-switcher

This research presents the first-ever demonstration of a passively Q-switched fiber laser in the 1.5 μm wavelength region using amorphous carbon nitride (a-CNx) as a saturable absorber (SA) in an all-fiber configuration. The a-CNx thin film was prepared using plasma-enhanced chemical vapor deposition. The laser operation showed remarkable stability, achieving a repetition rate of 57.99 kHz and a pulse duration of 1.36 μs at maximum input pump power. The peak power and pulse energy reached 100.56 mW and 136.77 nJ, respectively. The utilization of a-CNx as an SA offers potential benefits such as a simpler and more flexible cavity design, making it an attractive candidate for various applications, including material processing and medical laser equipment. This work contributes to the advancement of fiber lasers and expands the possibilities for utilizing carbon nitride-based SAs in practical laser systems.


Introduction
Extensive researches have been done on different materials as potential saturable absorber (SA).The wide range of applications ranging from basic researches to material processing, offerable by lasers in the 1.5 μm operating region has made it highly sought after these past few years.As a gain medium, Erbium doped fiber (EDF) possesses high gain bandwidth with operating wavelength of around 1550 nm, particularly appealing in optical communication.This is due to the minimum attenuation given by the fiber allowing for more stable transmission of information signals.The works reported on this particular wavelength are substantial with researches achieving Q-switching and mode-locking operations [1][2][3] using various materials as SAs.When compared, Q-switching has longer pulse duration (μs) and lower repetition rates in the kHz range, however, with less complicated procedures making it more easily achievable than that of mode-locking.
Q-switching can be achieved by either the active or passive approach with the passive approach being preferable as it provides a simpler and compact laser design as well as easier to operate with lower cost.Unlike active Q-switching, passive Q-switching uses SA instead of any external control element to control the intracavity losses [4].Continuing the effort for simpler and more compact lasers, the use of semiconductor saturable absorber mirror (SESAM) is becoming less dominant for its inapt downside of intricate and expensive fabrication process.Real SAs has been the go-to alternatives of such traditional ones, offering distinct properties contributing towards various degrees of achievements of fiber lasers.The characteristics of SAs vary from one material to another depending on their own physical and optical attributes.
Carbon based SAs has been the talk of the town in the latest years for their outstanding optical and physical properties.With Graphene at the centre stage, other carbon based SAs such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have also been highly researched.Another addition to the carbon family is carbon nitride.While being widely researched on its exceptional chemical properties due to its tri-s-triazine ring structure as well as its polymerization ability [5] making it known for its application in bioimaging, electrocatalysis, and photoreactivity [5][6][7], carbon nitride's potential optical properties have not been given the same attention and less focus is geared toward carbon nitride as a potential saturable absorber.
A few research works has been done on the material including the one from Fan et al. [8] where the first graphitic carbon nitride (g-CN) based Q-switched laser near the 3.0 μm wavelength region was demonstrated.The research uses bulk g-CN powder through ultrasonic pulverisation to produce g-CN flakes with thickness of around 7 to 15 layers.A passively Q-switched laser is also reported at 2.95 μm producing a pulse duration of 420 ns with repetition rate of 93 kHz [9] using a g-CN SA nanosheets which is fabricated through liquid phase exfoliation method.The application of the SA in the visible wavelength is first reported in 2018 where it is found that the saturable absorption is stronger for the particular SA [10].The amount of reported mode-locked fiber laser based on g-C 3N4 SA is also present.A thulium-doped fiber laser incorporating iron-doped g-CN nanosheets fabricated through thermal condensation of iron precursors was demonstrated yielding pulse energy of 6.3 nJ with a repetition rate of 15.3 MHz [11].The following year, a tunable multi-wavelength mode-locked fiber was reported for the first time operating at 11.3 MHz with corresponding pulse duration of 1.9 ns using a g-CN nanosheets based SA [12].All of the aforementioned laser generations were generated through a solidstate laser setup while no research has yet to be reported based on a carbon nitride SA compatible with an all-fiber laser configuration.Excellent optical properties have also been reported on amorphous carbon nitride (a-CNx) including changeable bandgap, and high transparency and photoconductivity [13,14], but no related research works have yet been done in the field of photonics.With the few works reported on g-CN, all of which was based on solid-state laser, none was ever reported on a Q-switched operation in 1.5 μm region.This research will introduce, for the first time, the use of a-CNx as SA in an all-fiber based Q-switched laser regime.

a-CNx Thin Film Preparation
A homogeneous a-CNx thin film was prepared using a home-built plasma enhanced chemical vapour deposition (PECVD) technique with aided of 13.56 MHz radio frequency (RF) as shown in Figure 1.The a-CNx thin film was deposited on a crystalline quartz substrate.Prior to actual deposition process, the substrate was exposed to a hydrogen plasma treatment for 10 minutes at 50 sccm H2 flow rate and 50 W RF power.This was done to minimise the contamination of the sample thus enhance the adhesive of deposited CN layer onto the substrate.For the a-CNx deposition process, two carrier gases were used which is from the dissociation of acetylene (C2H2) at 20 sccm and nitrogen (N2) at 50 sccm flow rate.The deposition of a-CNx thin film was performed for 30 minutes at RF power of 70 W and pressure at

a-CNx Thin Film Characterization
The Raman characteristics of the fabricated a-CNx SA was investigated by using Raman spectroscopy.Figure 2 shows the D-band, G-band, and 2D-band profiles of the SA.The bands act as a defining 'fingerprint' to differentiate one material to another.The D-band and G-band centered at around 1361 cm -1 and 1560 cm -1 , respectively.The distinct recorded peaks indicated that the fabricated a-CNx based SA agrees with graphitic trait [15].Apart from that, Field Emission Scanning Electron Microscopy (FESEM) was used to characterize the surface morphology of the a-CNx SA as shown in Figure 3.The modulation depth of the a-CNx was obtained through a twin-balanced measurement.Figure 4 shows the twin-balanced detector setup for the nonlinear response measurement with the mode-locked seed of ˂150 fs of pulse width and 100 Mhz repetition rate.The mode-locked seed was sourced from Elmo Femtosecond Erbium Laser (MenloSystems) with average power up to 15 mW.The laser source was amplified through an Erbium-doped fiber amplifier (EDFA) (KEOPSYS).Via the twin-balanced detector, 50% of the light was channelled towards a bare fiber ferrule while the other 50% was directed at the fiber ferrule with attached aCNx film.The nonlinear absorption measurement in Figure 5 shows that the modulation depth of the SA was measured at 13.5% with a saturation intensity of 32.1 μW/cm 2 .The nonsaturable loss was also measured at approximately 86%.The modulation depth recorded is significantly higher than reported works on the same material [8,9,11] indicating better light modulation efficiency.

Experimental Setup
The experimental setup of the Q-switched EDFL with the a-CNx film as SA is illustrated in Figure 6.A 3 m long Erbium-doped fiber (EDF)(IsoGain I-25(980/125) was used as the gain medium.The EDF was forward-pumped by a 980 nm laser diode via a 980/1550 nm wavelength division multiplexer (WDM).The SA was placed in the cavity by sandwiching it in between two ferrule which are connected with a connector.An optical isolator was also spliced in the cavity to ensure unidirectional propagation of the light.The output of the laser was observed and obtained through an optical spectrum analyser (OSA) (Yokogawa AQ6370C), an optical power meter (OPM), a radio frequency spectrum analyser (RFSA), and an oscilloscope (Keysight Infiniivision DSOX3102T) which were connected through a 3 dB optical coupler.

Modulation depth
= ~ 13.5 % Figure 6.Experimental setup of the proposed Q-switched EDFL based on a-CNx SA

Experimental Results and Discussions
The Q-switching lasing of the laser can be observed as the input pump power is tuned over 10.58 mW.
As compared to other carbon based SAs, the minimum pump power required to induce a Q-switching operation is tremendously lower than that reported in [8][9] as well as when using other carbon based SAs [16][17][18][19].The Q-switching operation is kept stable as the tunable pump power is increased steadily over 90 mW.However, the Q-switching ceased operation as the pump power is increased over 92.12 mW.The Q-switching operation resumed when the pump power is reduced below 92.12 mW indicating that the SA is not damaged due to high heat during laser operation.This also shows the reversibility of the laser output depending on the pump power tunability and only limited by the SA's absorption and saturation capabilities.The central operating wavelength of the laser is as shown in Figure 7.The laser operated at 1567.23 nm with a peak power of -10.3 dBm at maximum input pump power.The 3 dB spectral bandwidth of the spectrum was around 0.6 nm.   9 shows the Q-switched laser's pulse train with constant spacing and its corresponding single pulse envelope, respectively, at 92.12 mW.The repetition rate of a Q-switched laser increases with the increasing input pump power and this normally also resulted in the simultaneous decrement of the pulse duration.At maximum input pump power, the repetition rate is recorded at 57.99 mW with corresponding pulse duration of 1.36 μs.The repetition rate of 57.99 mW is higher, it not comparable to that of graphene oxide and carbon nanotubes based SAs [16][17][18][19].11 shows how repetition rate and pulse width, as well as the instantaneous peak power and pulse energy, respectively, are related to the input pump power.Figure 10 contained two trends describing the relationship between the repetition rate and the pulse duration with the pump power.The scatter line graph clearly shows the steady-like increment of the repetition rate with the increasing pump power.However, the same trend is not applied to the pulse duration.The pulse duration is observed to decrease with the increasing pump power which is to be expected of a Q-switched laser.

μs
Tuning the input pump power from 10.58 mW to 92.12 mW resulted in the increase of the repetition rate from 18.34 kHz to 57.99 kHz while the pulse duration is decreased from 5.23 μs to 1.36 μs.The sharp drop in the pulse duration as the pump power is tuned from 10.58 mW to 19.64 mW is due to the threshold intensity of the SA being reached [20].The shortest pulse duration recorded in the experiment is lower than the ones reported in works related to carbon based SAs such as graphene and carbon nanotubes [16][17][18][19].
By increasing from threshold to maximum input pump power, the instantaneous peak power and pulse energy also increases almost monotonically.The highest peak power recorded is 100.56 mW which is the highest ever reported when using carbon nitride as the SA.A high pulse energy is also recorded at 136.77 nJ.When compared to the Q-switching works on carbon based SAs [16][17][18][19], this work yielded a higher peak power and/or pulse energy with a relatively low pulse duration, in the same wavelength region.The first beat node at the repetition rate of 57.99 kHz is approximately 48 dB, as shown in Figure 12, indicating that the pulse produced is stable with low amplitude noise fluctuations.The comparison is tabulated in Table 1 for easier reference.Q-switched operations based on carbon nitride in 1, 2, and 3 μm region are also tabulated in Table 2 for comparison purposes.

Conclusion
A successful demonstration of a passively Q-switched fiber laser in 1.5 μm region incorporating an a-CNx SA is reported for the first time in an all-fiberized laser configuration.The highly stable operation of the laser yielded a maximum repetition rate and minimum pulse duration of 57.99 kHz and 1.36 μs, respectively.From there, a 100.56 mW and 136.77nJ of peak power and pulse energy, respectively, is measured at maximum input pump power of 92.12 mW.The advantage of a simpler and more flexible cavity design for the carbon nitride based SA with excellent yield can be applied to various field of applications ranging from material processing to aesthetic and medical laser equipment.

Figure 1 .
Figure 1.Schematic diagram of RF-PECVD and the fabricated a-CNx thin film

Figure 2 .Figure 3 .
Figure 2. Raman profile of the a-CNx thin film

Figure 4 .
Figure 4. Twin-balanced detector setupAs the input of the attenuator was gradually reduced, the OPM readings for both with and without a-CN x SA were recorded.A transmission fitting equation as shown in Equation 1 below was used for curve fitting.Where αs is the modulation depth, I is the input intensity, Isat is the saturation current and αns is the non-saturable absorption.

Figure 5 .
Figure 5. Modulation depth of a-CNx thin film

Figure 7 .
Figure 7. Optical spectrum at maximum input pump power of 92.12 mW

Figure 8
Figure8and Figure9shows the Q-switched laser's pulse train with constant spacing and its corresponding single pulse envelope, respectively, at 92.12 mW.The repetition rate of a Q-switched laser increases with the increasing input pump power and this normally also resulted in the simultaneous decrement of the pulse duration.At maximum input pump power, the repetition rate is recorded at 57.99 mW with corresponding pulse duration of 1.36 μs.The repetition rate of 57.99 mW is higher, it not comparable to that of graphene oxide and carbon nanotubes based SAs[16][17][18][19].

Figure 8 .
Figure 8. Oscilloscope trace at maximum pump power of 92.12 mW

Figure 9 .
Figure 9. Single pulse envelop of the pulse train at 92.12 mW Figure 10 and Figure11shows how repetition rate and pulse width, as well as the instantaneous peak power and pulse energy, respectively, are related to the input pump power.Figure10contained two trends describing the relationship between the repetition rate and the pulse duration with the pump power.The scatter line graph clearly shows the steady-like increment of the repetition rate with the increasing pump power.However, the same trend is not applied to the pulse duration.The pulse duration is observed to decrease with the increasing pump power which is to be expected of a Q-switched laser.

Figure 10 .Figure 11 .Figure 12 .
Figure 10.Repetition rate and pulse width in a function of pump power

Table 1 .
Laser performance comparison on carbon based SAs.

Table 2 .
Laser performance comparison on carbon nitride based SAs