1. Introduction

Q-switched lasers have received a great research interest in the last few years, as they have the potentials in several fields such as material processing, remote sensing, medicine, and telecommunications [1–4]. They can be developed by passive techniques, which have the advantages of low cost, compactness, simplicity and flexibility. Passive Q-switching can be achieved by incorporating a saturable absorber (SA) device inside the laser cavity. To date, many types of SAs have been demonstrated, particularly, semiconductor saturable absorber mirrors (SESAMs). SESAMs were among the earliest types and the most popular SAs [5]. However, they are costly and have a relatively small operation wavelength range. SESAMs dominance was suppressed by carbon nanotubes (CNTs) [6]. CNT based SA offered significant advantages in terms of fabrication and packaging. Furthermore, the operating wavelength range in CNT is much wider than that of SESAM [7]. However, CNT has its response spectral range depends on the nanotubes sizes, which restricts its practical applications [8]. Additionally, CNTs often require bandgap engineering which limits their applications [9]. Graphene has been successfully used as SA [10,11]. It has a high damage threshold and ultrafast recovery time [12,13]. But, graphene suffers from small optical absorption range and has zero optical bandgap [14,15]. Other types of SA, such as Transition Metal Dichalcogenides (TMD) and Black Phosphorous (BP) [16] have also gained many interests. They have good potential for pulse laser applications due to their unique absorption properties [17,18]. However, the process of fabricating these materials as a SA is slightly complicated; additionally, they have an insufficient purity and a low optical damage threshold.

Organic semiconductor materials (OSM) might have the potential to be utilized as a SAs, as they enjoy a broad spectral tunability and ultrafast nonlinear response, additionally, they can be fabricated simply and at low cost. [19–21]. Unlike nanomaterials, which might be dangerous on human health after long-term exposure [22], OSMs are bio-compatible, non-hazardous and environmentally-friendly, moreover, they are light-weight and enjoy mechanical flexibility [23]. They could be of great research interest in the fields biomedical laser sensing and processing [24]. OSMs have the advantages of easy control of physical properties, simple fabrication and versatile molecular design if compared with traditional inorganic materials. Hence, OSMs have been addressed in many areas, such as bi-stable memory devices [25], solar cells [26], organic thin-film transistor [27] and light-emitting diodes (OLEDs) [28]. Despite the fact that many works have demonstrated the OSMs in the linear regimes, the applications of OSMs in nonlinear optics have yet to be explored. This paper tries to demonstrate the potentials that OSM might have in pulsed fiber lasers. We used hybrid organic small molecules (HOSM) in a simple all-fiber cavity and generated Q-switching operation. As compared with above-mentioned materials, HOSM enjoys high optical damage threshold, ease and low-cost fabrication process. We believe that this is the first time that HOSM has been utilized as SA.

2. SA preparation and experimental setup

The organic small molecules materials used in this study are Tris-(8-hydroxyquinoline) aluminum (Alq₃) and N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) powders, which were purchased from Sigma Aldrich. Alq₃ and NPB powders have a CAS number, product number, purity and molecular weight of 2085-33-8, 123847-85-8; 697737-1G, 556696 ; 99.995%, 99% and 459.43 g/mol, 588.74 g/mol [29–31], respectively. These materials are embedded into the polyvinyl alcohol (PVA) film. The hybrid organic small molecules (HOSM) thin film was fabricated by solvent casting technique. Figure 1(a) illustrates and summarizes the fabrication processes of HOSM composite thin film. At first, 1g of polyvinyl alcohol (PVA) powder was dissolved into 100 ml of DI-water and then sonicated for one hour at room temperature. On the other hand, 1 ml of the distilled water was added a powder of the organic small molecule materials (5 mg of Alq₃ and 5mg of NPB). The solution was then centrifuged at 50°C for 60 minutes before adding 2 drops of acetone. The solution was stirred for about 3 hours at 30^○C after 5 ml of the prepared PVA was added to the HOSM solution. Then the mixed solution was cast into a glass petri dish and dried for 72 hours to form a thin film with 50 µm thickness.

 

Fig. 1. (a) preparation processes of HOSM PVA; (b) FTIR spectra of HOSM:PVA and PVA thin films with span between 4000 to 500 cm⁻¹, and (c) 1600 to 500 cm⁻¹

Download Full Size | PDF

Pristine PVA was also fabricated using a similar process based on solvent-casting technique. The chemical groups of both HOSM PVA and pristine PVA thin films were examined by utilizing Transform Infrared Spectroscopy (FTIR) Spectrometer. Figure 1(b) illustrates the FTIR absorbance spectra of pristine PVA and HOSM thin films, which were calculated from the transmittance mode by A = log₁₀(100/%T) where %T is the transmittance percentage. As shown in Fig. 1(b), the characteristic bands at 3000–3600, 2928, 2853 and 1730 cm⁻¹ corresponding to O–H, and CH₂, CH and C = O stretching vibration peaks of the pristine PVA, respectively. In the fingerprint region (1600-500 cm⁻¹), the vibration peaks at 1428, 1374, 1242, 946 and 604 cm⁻¹ are related to the CH₂ bending, (CH + OH) bending, CH wagging and CH₂ rocking and (OH) wagging, respectively, while at 1090, 1023 and 848 cm⁻¹ belong to C-O-C, C-O and C-C stretching vibrations for the pristine PVA [32–35], see Fig. 1(c). For the HOSM curve shown at the same Figure, two peaks significantly appeared at 1594 and 1580 cm⁻¹ are attributed to the double bond of C = C and C = N in the HOSM materials. The absorbance band at 1500-1468 cm⁻¹ belongs to C-N-C stretching and C-H bending vibrations for phenyl group of NPB material. A small peak at 1313 cm⁻¹ is related to C-N-C bond for the quinoline fragments of Alq₃ material. The peaks at 918, 550 cm⁻¹ are attributed to Al-N, Al-O stretching vibration for Alq₃. The C-H wagging vibrations at 749 and 771 cm⁻¹ are related to the quinoline group in Alq₃ and the naphthl group of NPB material. The absorbance bands at 840-791 and 690-650 cm⁻¹ belong to the C = C-H bending vibration and C-C torsion in t-phenyl of NPB [36–38].

SEM image of the SA is shown in Fig. 2(a), which indicates a uniform surface and homogeneous distribution of Alq₃ and NPB molecules along with the PVA polymer. The non-linear transmission of the SA thin film was addressed by using standard 2-arm transmission measurement. Figure 2(b) shows the HOSM PVA has a saturation intensity of 4.8 MW/cm² and modulation depth of ∼ 20.3%. The linear absorption is also checked, see Fig. 2(c). The SA has a flat linear absorption of ∼ 0.5% between 1500 to 1600 nm. The optical absorption spectrum of the prepared SA is presented in Fig. 2(d). It is clearly seen the appearance of three peaks at around 400, 335 and 317 nm are attributed to $\pi \to {\pi ^\ast }$ transition of Alq₃, NPB and PVA, respectively, which come from unsaturated bonds [39–42]. The observed optical band gaps of the SA are about 1.28 and 2.52 eV, see the T_auc plot in the inset figure. This optical band gap overlapping is to confirm the homogenous surface of the compound molecules.

 

Fig. 2. (a) SEM image, (b) nonlinear transmission, (c) linear absorption and (d) optical absorption spectrum of the fabricated SA (inset figure shows the Tauc plot); and (e) cavity setup of the fiber laser (TBF was added to tune the operating wavelength).

Download Full Size | PDF

The EDFL setup is shown in Fig. 2(e). A 2 m Erbium-doped fiber (EDF), with an absorption coefficient of 23 dB/m at 980 nm, was utilized as a gain medium. A 980 nm laser diode (LD) used to power the cavity via a wavelength division multiplexing (WDM) coupler (980/1550 nm). Additionally, an isolator (ISO) and a 90:10 optical coupler were used to prevent back-reflections and to output 10% of the signal from the cavity, respectively. The SA device was made by inserting a small piece of 1 mm² of the HOSM based SA film between two ferrule connectors. The cavity length was ∼ 5 m (the fibers used were in the setup were standard single-mode fiber (SMF-28)). A tunable bandpass filter (TBF) was incorporated into the cavity in the second part of the experiment in order to check the tunability of the developed SA. The TBF was inserted between the SA device and the optical coupler. An optical spectrum analyzer (OSA), radio-frequency spectrum analyzer, oscilloscope (GW INSTEK GDS-3352, 350 MHz bandwidth with 1.2 GHz photodetector) and power meter were used to monitor the optical spectrum, radio-frequency spectrum, temporal profile and average output power, respectively.

3. Results and discussion

3.1 Q-switched laser

The laser cavity produced a continuous-wave lasing at LD power of 20 mW. While the Q-switching was observed at LD power of 56 mW. Figure 3 summarizes the characteristics of the pulsed laser. The laser had a wavelength at 1560.1 nm with a 3-dB bandwidth of 1.8 nm, see Fig. 3(a). The pulse repetition rate was tuned from 66.3 kHz to 109 kHz whereas the pulse width shrank from 6.3 µs to 2.2 µs as the LD power was gradually increased up to 262 mW, see Fig. 3(b). We confirmed throughout the experiment that the HOSM was responsible for the pulsing phenomenon, as it was only observed when the HOSM PVA thin film was inside the cavity. However, the work in Ref. [43] has produced mode-locking operation by using anhydrous alcohol (ethanol) as SA. In order to make sure that the PVA we used did not contribute to the pulsing operation, we inserted pristine PVA thin film inside the cavity. Similarly to work in Ref. [44] no pulsing operation was observed. Figure 3(c) depicts the trends of average output power and pulse energy versus LD input power. The output power increased linearly from 0.9 mW to 7.4 mW and the output pulse energy raised from 13.57 nJ to 67.9 nJ. The linear correlation between output pulse energy and the input LD power reflects an excellent performance of the Q-switching operation [45]. Figure 3(d) shows oscilloscope traces at LD power of 262 mW.

 

Fig. 3. Q-switched laser: (a) output optical spectra, (b) pulse width duration and pulse repetition rate versus LD power, (c) output power and pulse energy versus LD power, (d) output pulse train at maximum LD powers and (e) RF spectrum.

Download Full Size | PDF

The figure shows that the pulse train has a uniform intensity distribution, which indicates a stable operation. However, when a higher input power was launched into the cavity (above 262 mW and up to 350 mW), an amplitude fluctuation occurred, which is due to the full saturation of the SA. It should be noted that the SA produced the pulsing operation again when the LD power was between 56 mW and 262 mW, this shows that the threshold of the optical damage is higher than 350 mW. To further investigate the stability of the laser, the radio frequency spectrum was recorded at a span of 1.5 MHz, as shown in Fig. 3(e). The signal-to-noise ratio (SNR) of 53.54 dB is observed, which reveals a very stable operation.

3.2 Tunable wavelength Q-switched laser

In this part of the experiment, a TBF was incorporated into the cavity setup in order to check the tunability of the developed SA. Figure 4 summarize the wavelength tuning operation at LD power of 262 mW. As the TBF was tuned, the laser wavelength was shifted continuously from1519.6 nm to 1562.8 nm covering almost all C-band region. The TBF has a resolution of 0.1 nm and a tuning range of 43.2 nm. Figure 4(a) shows the output spectrum sampled at every ${\sim} $ 1 nm within the given range. As shown in the figure, the samples have the same shape and almost the same amplitude of $- $ 3.4 dBm (${\pm} \; $2.24 dB), which suggest a very stable tuning operation. The repetition rate and pulse width versus wavelengths are shown in Fig. 4(b). The laser produced a repetition rate of 92.7 kHz at a wavelength of 1519.6 nm, while the maximum pulse repetition rate of 124 kHz was observed at 1530.5 nm. As the wavelength was shifted to 1562.8 nm, the repetition rate was gradually decreased to its minimum value of 82.3 kHz. The pulse duration of the tunable laser behaved inversely to repetition rate, in which the minimum pulse width of 1.9 µs was recorded at 1530.5 nm. This because of the EDF gain is much higher around 1530 nm than other wavelengths. The higher gain will increase the number of electrons that are transferred to the upper energy level in the EDF, which will reduce the falling and rising time of the pulses, which in turn increases the repetition rate and reduces the pulse width [46]. We expect that further optimization of the cavity and increasing pump power should lead to a mode-locking operation. Figure 4(c) shows the average output power and pulse energy versus wavelength. The average output power was almost constant at ${\sim} $6.1 mW as the wavelength was tuned. The pulse energy, as it is determined by dividing output power by the pulse repetition rate, behaved inversely to the repetition rate.

 

Fig. 4. Tunable wavelength Q-switched laser: (a) output spectrum at different wavelengths between 1520 - 1563.3 nm, (d) pulse width duration and repetition rate vs wavelength (e) pulse energy and output power vs wavelength, and (f) shows the SNR value at different wavelengths

Download Full Size | PDF

The minimum and maximum pulse energy values of 48.9 nJ and 76.67 nJ were recorded at 1530.5 nm and 1562.8 nm, respectively. In order to investigate the stability of the tunable laser, the SNR was recorded at different wavelengths. As shown in Fig. 4(d), the SNR was around 51 dB for all the sampled wavelengths. This confirms the stability of the tunable fiber laser. We believe that a wider tunability can be achieved if TBF with a wider tuning range is used. This experiment demonstrated that HOSM can be used as effective a broadband passive Q-switcher, and showed that the SA can be utilized for applications that require a wide tunability range along with highly stable performance

4. Conclusion

A stable Q-switched laser was demonstrated by using organic small molecules (HOSM) as SA in an EDFL cavity. As the LD power was tuned 56 mW to 262 mW, the pulse width reduced from 6.3 to 2.2 µs, and the pulse repetition rate was tuned from 66.3 to 109 kHz. Then in the second part of the experiment, a tunable bandpass filter was inserted into the cavity to produce tunable-wavelength pulsing. As the TBF was tuned, the wavelength was shifted continuously from 1519.6 nm to 1562.8 nm. The SNR of both experiments was above 51 dBm, which suggest a very stable pulsing operation. The results presented in this work suggest that HOSM is a very good candidate as a SA in pulsed fiber laser technology.