1. Introduction

Nanosecond pulses are attractive for potential use in generating high energy pulsed lasers for many industrial and scientific applications, such as micromachining, metrology, bio-medical, and telecommunication [1–3]. Nanosecond pulse fiber lasers have normally low repetition rate in a range of few MHz and their energies can be easily increased to microjoule levels through external amplification. Conventionally, nanosecond pulses lasers were achieved by active modulation approach utilizing electro-optic and acousto-optic modulators, which is bulky and high cost. The nanosecond laser can also be realized by passive technique based on Q-switching and mode-locking. Passively Q-switched lasers mainly use crystal based saturable absorber, such as Cr:YAG, as a Q-switcher. However, a stable pulse train is difficult to achive by this approach. On the other hand, stable nanosecond pulses can be obtained by extension of the cavity length in a passively mode-locked fiber laser using nonlinear polarization rotation (NPR) technique [4,5] or semiconductor saturable absorber mirror (SESAM) [6]. However, NPR based mode-locked fiber lasers are relatively disadvantages in term of environmental stability and reliability since it requires the adjustment of the polarization state of the oscillating light in the cavity. SESAMs are widely used, but they are still quite costly for purposes of mass production.

Recently, two-dimensional (2D) nanomaterials such as graphene, black phosphorus, transition metal dichalcoganides (TMDs), topological insulators (TI) have fascinated many interests for saturable absorber (SA) applications [7–9]. Most of these works were focused on obtaining Q-switching pulses with microsecond pulse width and mode-locking pulses with femtosecond or picosecond pulse width. Only a few works have been reported on generating nanosecond pulse. For instance, Xu et al demonstrated a mode-locked nanosecond EDFL using a graphene SA [10]. On the the other hand, despite many new materials and techniques are discovered in developing new SA for Q-switching and mode-locking pulse generations, Gallium based materials are rarely being investigated. Previously, Ahmad et al. proposed and demonstrated a 2D layered metal monochalcogenide material so called gallium selenide (GaSe) as a Q-switcher to achieve Q-switching pulses in a Thulium Holmium co-doped fiber laser system [11].

As of nanosecond generation, it is particular difficult for realizing mode-locked fiber laser with the passive methods due to the short recovery time of the saturable absorber. Normally, the cavity requires special design to obtain pulse train with decent amplitude and time stability. Extension of the cavity length in a passively mode-locked fiber laser appears to be an attractive way to get stable nanosecond pulse. For instance, Mao et al. reported 24 ns pulse generation from a passively mode-locked erbium-doped fiber laser (EDFL) with nonlinear polarization rotation (NPR) technique in a ring cavity [5]. In this paper, we report the use of new material, Gallium nitride, known as GaN to generate stable nanosecond pulses from an EDFL in a long ring cavity. GaN is the semiconductor material, which has been widely used in for defense and commercial applications. It also has high bandwidth and high thermal conductivity at room temperature, which make it a good material for high-performance radio-frequency microdevices and optoelectronics devices. For this purpose, we prepare Gallium Nitride (GaN) in the form of a crystal structure. The SA device is designed by sandwiching together the polished GaN substrate between two fiber ferrules connectors to produce mode-locking pulses train with a pulse width of 492.3 ns as well as a repetition rate, average output power and pulse energy of 966.8 kHz, 2.797 mW and 3.10 nJ, respectively. The proposed mode locked operates at 1559 nm region which is desirable for optical communications and other applications that require all-fiber system. Even though the price of GaN material is almost similar with the SESAM materials, the preparation of the SA is much simpler and thus the SA device cost should be cheaper.

2. Experimental arrangement

In this experiment, C-plane undoped Gallium Nitride (GaN) substrate fabricated via hydride vapour phase epitaxy (HVPE) by Suzhou Nanowin Science and Technology Co. Ltd was used. The size of the GaN substrate was 2 inch with thickness of around 350 µm. The substrate was polished until the thickness of 100 µm and the GaN layer was around 20 µm on the substrate. Figure 1 illustrates the scanning electron microscopy (SEM) image of the GaN surface where the Ga face dislocation density was less than 5 × 10⁸ cm^-2. Inset figure shows the GaN substrate, becomes more transparent after polishing. Even though the polished surface shown in Fig. 1 seems to be slightly contaminated, a mode-locked operation was easily obtained by incorporation a small piece of the polished cystal film inside the laser cavity. The polished GaN crystal film was analyzed using the energy-dispersive X-ray (EDX) method to identify the existence of the Ga and N elements. The result is shown in Fig. 2. The resulting spectrum reveals that Ga, N and C are present in the film. It was also traced that about 67.61 Wt% of Ga and 20.13% of N elements exist on the surface of the film.

 

Fig. 1. SEM image of the GaN surface. Inset shows the polished GaN substrate with a thickness 100 µm.

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Fig. 2. EDX analysis data for the polished GaN substrate.

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Figure 3 illustrates the absorption characteristic of the GaN SA, which has an absorption loss of ∼5.5 dB at vicinity of 1.5-micron region. Another vital parameter for SA is a nonlinear optical response. The amplified mode-locked fiber laser is used in this work to measure nonlinear optical response for our GaN substrate film and confirm its saturable absorption ability. We employed a laser source operating at 1560 nm with 3 ps pulse width, and 1.0 MHz repetition rate as a light source to conduct a balanced twin-detector measurement. In this experiment, the absorption power is recorded as a function of incident intensity on the GaN film by varying the input laser power. As illustrated in Fig. 4, the polished GaN film exhibits the saturable absorption property that the absorption decreases with optical intensity. The modulation depth, saturable intensity and nonsaturable absorption of the film are obtained as 2%, 0.46 MW/cm² and 63%, respectively.

 

Fig. 3. The absorption characteristic of the polished GaN film.

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Fig. 4. The non-linear optical properties of the polished GaN film by balance twin-detector mesurement technique.

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The prepared GaN film crystal SA was added into an Erbium doped fiber laser (EDFL) ring cavity, which is schematically shown in Fig. 5. The ring cavity consists of a wavelength division multiplexer (WDM), a 1.5 m long EDF, a 200 m long standard single mode fiber (SMF), an isolator, a 10 dB output coupler and a SA device. The EDF has a cut-off wavelength of 900 nm, numerical aperture of 0.24 and 23.9 dB/m absorption at 980 nm. It has core and cladding diameters of 5.1 and 125.4 µm respectively and the peak absorption of 41.1 dB/m at 1530 nm. The fiber was pumped by a 980 nm laser diode through the WDM. The 200 m long SMF was added into the cavity to tailor the dispersion characteristic and nonlinearity of the cavity and allow a nanosecond pulses generation. The isolator was used to force the unidirectional operation in the fiber ring cavity. The laser was coupled out from the cavity through 10% port of the 10 dB coupler. The SA device was constructed by breaking the polished GaN thin glass into a tiny piece by using a standard scissor so that it can be attached onto a fiber ferrule surface as shown in the inset of Fig. 5. The ferrule was then connected with another new ferrule via a fiber adaptor to form an all-fiber SA device. A small amount of index matching gel was used at the connection to minimize parasitic reflections. The insertion loss of the SA is measured to be around 1.5 dB. The output spectrum of the laser was measured using an optical spectrum analyzer (Ando AQ-6370C) while the pulse train was observed by a 500-MHz digital oscilloscope (Tektronix, TDS3054B) in conjunction with high speed photodetector. 7.8 GHz Radio Frequency (RF) spectrum analyzer (Anritsu MS2683A) was used to investigate the repetition rate and stability of the Q-switched laser. The average output power of the pulse laser is measured by the power meter (Thorlabs PM 100D) coupled with its power head (S145C Integrating sphere Photodiode Power Sensor, InGaAs, 800-1700nm @3W). The overall length of the laser cavity was approximately 213.5 m.

 

Fig. 5. The schematic diagram of the mode-locked laser with GaN SA.

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3. Results and discussion

The performance of the proposed GaN based EDFL was investigated by varying the power of the 980 nm pump. A stable self-started mode-locked pulse is obtained as the 980 nm pump power is increased gradually above the threshold of 149.51 mW. This mode-locking threshold is relatively high due to the high insertion loss of the GaN film crystal based SA and long cavity. The GaN film crystal is also observed to maintain stable without thermal damage as the pump power was further increased to the maximum available pump power of 182.34 mW. The laser also produces a stable repetition rate of 967 kHz, which shows that the mode-locking operation is maintained up to the maximum pump power. Figure 6(a) shows the output optical spectrum measured under a pump power of 149.51 mW, which operated at the centre wavelength of 1562 nm with 3 dB spectral bandwidth of 0.8 nm. We observed the optical spectrum broadening, which was most probably due to the self-phase modulation effect in the laser cavity. Figure 6(b) shows the oscilloscope trace of the mode-locked pulse train. The time interval between the pulses is about 1 µs, that matches with cavity length of 213.5 m. The pulse width of laser was kept to be 485 ns, when adjusting the value of pump power from 149.51 to 182.34 mW.

 

Fig. 6. Mode-locking performance of the GaN based EDFL (a) The optical spectrum (b) The pulse train recorded by ossciloscope (c) The Radio frequency spectrum of the pulsed, (d) the relationship between pump power with output power and pulse energy.

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The stability of the laser is investigated based on the radio frequency (RF) spectrum, which was obtained by using a RF spectrum analyzer. The result is plotted in Fig. 6(c) at input pump power of 165 mW, which indicates the fundamental frequency of 967 kHz. This frequency matches very well to the peak to peak duration (pulse period) of the oscilloscope trace. The obtained RF spectrum has a high SNR up to 59.03 dB, further indicating the stability of the pulses. We also observe the long-term stability of the mode-locked EDFL by monitoring the output spectrum of the EDFL for 48 hours. We obtained an almost similar shape of spectra with nearly the same output power with a fluctuation of less than 2%. This indicaties that the mode-locked EDFL is considerably stable over the time. Figure 6(d) shows the mode-locked output power and single pulse energy against the input pump power. The figure displays, the output power increased from 2.514 mW to 3.068 mW with the corresponding pump power input from 149.51 mW to 182.34 mW. The slope efficiency from the graph is calculated as 1.69% which is slightly lower than [12,13]. The efficiency of the Q-switched laser is considerably low and is expected due to high insertion loss from the SA, which in turn increase the intra-cavity loss. The optical loss can be further improved through a cavity optimization such as reduction of the total cavity length, reducing the SA loss, rearrangement of the optical devices and enhancing the fusion-spliced connection spots. From the graph of Fig. 6(d), it also observed that the maximum pulse energy is 3.10 nJ is achieved and the pulse energy increases linearly with the pump power. Further investigating the behavior of pulse train, the GaN based SA was temporarily removed, and no pulses were observed at the oscilloscope and the RF spectrum analyzer. This indicates that the SA device is responsible for the mode-locking operation.

Finally, the performance of the proposed nanosecond mode-locked EDFL based on GaN SA was compared with recently published nanosecond EDFL using other types of SA materials as summarized in Table 1. Those results higtlighted that the demonsttrated mode-locked laser based on GaN has the highest pulse energy. However, the threshold pump power was higher compared to other works due to the insertion loss of the SA device. The performance of the laser can be improved by reducing the substrate film loss and optimizing the EDFL cavity especially in realizing the high power operation of the laser. These results verify the mode-locking ability and shows that the GaN material could be used in optoelectronics device and communication device in the C band region.

Table 1. Several nanosecond mode-locked fiber lasers performances using different SAs

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4. Conclusion

We have experimentally demonstrated the generation of nanosecond pulses in an EDFL mode-locked by a GaN in a form of polished crystal structure as a SA. The fabricated film shows a modulation depth of 2% and a saturable optical intensity of 0.46 MW/cm². Stable mode-locked nanosecond pulses at 1562 nm with spectral width of 0.8 nm, pulse duration of 485 ns and a fundamental repetition rate of 967 kHz have been obtained. The average output power was 3.068 mW, corresponding to single pulse energy of 3.1 nJ at pump power of 182.34 mW. The experimental results suggest that GaN is a promising material for photonics applications and ultrafast light generation that operating in C-band region.