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Tunable passively Q-switched thulium-doped fiber laser operating at 1.9 μm using arrayed waveguide grating (AWG)
Optics Communications 380 (2016) 195–200
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable passively Q-switched thulium-doped fiber laser operating at 1. 9 μm using arrayed waveguide grating (AWG) M.Z. Samion a, M.F. Ismail a,n, A. Muhamad a, A.S. Sharbirin a, S.W. Harun a,b, H. Ahmad a a b
Photonics Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2016 Received in revised form 2 June 2016 Accepted 5 June 2016
Thulium-doped fiber lasers (TDFLs), operating in the 1.8–2.0 mm wavelength region, have been viewed as an important research topic, due to their potential in various fields of applications. However, the growing need to advance the development of applications in various fields for instance medicine and environment sensor, has led to a deeper and specific study of Q-switched TDFLs with wavelength tunability. In this paper, a stable, tunable Q-switched TDFL operating in a wavelength range near to 1.9 mm by exploiting the use of a multiwall carbon nanotube (MWCNT)-based thin film as a saturable absorber (SA), and the use of an arrayed waveguide grating (AWG) for wavelength tunability, is presented. The tuning range of the Q-switched pulses generated covered a wavelength range that spanned from 1871.6 nm to 1888.8 nm. The repetition rate of the generated Q-switched pulses covers a range of frequency starting from 41.19 kHz to 68.3 kHz with a change in pump power from 242.2 mW until 360.9 mW. & 2016 Elsevier B.V. All rights reserved.
Keywords: Tunable Saturable absorber Q-switching Thulium-doped fiber laser AWG MWCNT/PVA
1. Introduction There have been great interests in fiber lasers as compared to bulk optics systems due to their compactness and also ease of alignment. Generally, fiber lasers are being constructed from a gain medium, which provides the required amplification, together with a pump laser diode connected through a wavelength division multiplexer (WDM). An optical isolator is also used to force the unidirectional propagation in a ring configuration. There have been numerous reports of amplification in the S- (1480–1520 nm) [1–4], C- (1540–1560 nm) [5–7] and L- (1560–1600 nm) [8–10] bands which can be configured easily into a fiber laser. Due to the rapid increase of the demand of data traffic, there has been interest in the 2 mm band region as to complement the existing bands, whereby thulium is the active medium. Besides continuous wave (CW) operation, there are interests to pursue the development in pulsed fiber laser system based on Q-switching techniques that have wide application in sensing and medical sectors. Q-switched fiber lasers, which are lasers that emit energetic pulses that can be obtained through active [11] or passive [12] Q-switching techniques, have gained a lot of attention due to their capacity to produce pulses of light with high energy at relatively low repetition rates [13]. Such laser pulses can subsequently be used in many advanced applications – e.g. in range-finding, remote-sensing n
Corresponding author. E-mail address: [email protected] (M.F. Ismail).
http://dx.doi.org/10.1016/j.optcom.2016.06.012 0030-4018/& 2016 Elsevier B.V. All rights reserved.
and in the field of medicine [14]. Though Q-switching can be produced through active systems, the technique usually requires the application of an electric signal, applied to an acousto-optic or electro-optic modulator [15–18] - and can be rather complicated and costly as compared with passive Q-switching systems. Passively Q-switched fiber lasers offer the appealing advantages of compactness, simplicity, and flexibility in design. The technique can be implemented by using a more direct approach, which is the incorporation of an SA in the laser cavity. Numerous different types of SA have been demonstrated to achieve Q-switched operation, of which one example is the semiconductor saturable-absorber mirror (SESAM) [19,20]. Although this method works well in both bulk-laser and fiber-laser systems, SESAMs have several drawbacks that include high cost of fabrication, fragility, and a narrow tuning range that is in the region of only a few tens of nanometers [21]. Many saturable absorbers have been introduced as an alternative to overcome the limitation of SESAMs. Most recently, graphene and CNT-based SAs have been introduced because they could offer simplicity and compatibility in terms of system designs. In addition, they also have low saturation intensities and ultrafast recovery times [22–24], making them a more attractive alternative as compared with other SAs. The reason why graphene is less preferable than CNT-based SAs is that the saturation intensity of graphene depends greatly on the wavelength. At nearinfrared wavelengths, graphene exhibits large saturation intensities – while, in the mid-infrared region, its saturation intensity becomes equal to or even less than that of CNT-based SAs [25], making it less appealing for some applications.
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Recently, passively Q-switched thulium-doped fiber lasers (TDFLs) operating in the 1.9 mm region have gained much interest because they could be utilized in various ‘eye-safe’ applications, especially in the medical field. This is due to their emission being strongly absorbed by water [26], making them suitable to be applied in ophthalmic surgery. Q-switched TDFLs are also able to generate higher pulse energies at their output [27], making them important in the field of material processing and they are more efficient and are more simple as they do not require the subtle balancing of losses associated with mode-locking. Although Q-switched TDF lasers operating at a single wavelength have many applications, extensive interest in generating Q-switched TDF lasers with wavelength tunability exists for areas such as spectroscopy, material processing, and sensing [28]. In addition, tunability of basic TDF lasers in the 2.0 mm wavelength region has already been widely demonstrated by using volume Bragg gratings (VBGs) [29–31], and also by the use of diffraction gratings [18,32]. However, these methods are quite complex due to the use of bulky optic components in the laser cavity. This paper presents a method for producing a passively Q-switched output pulses that provide wavelength tunability for a thulium-doped fiber by incorporating a MWCNT-based SA and by utilizing the use of a silica-based arrayed waveguide grating (AWG). The use of the AWG has been previously demonstrated in achieving switchable-wavelength fiber lasers operating in the C-band up until the L-band wavelength region [33–35]. The low propagation loss of the AWG, combined with its high fiber-coupling efficiency [36], has made it quite popular to be used to precisely de-multiplexing a high number of optical signals. Hence, AWG was chosen in this demonstration due to its low insertion loss, low power consumption and more importantly, it could provide a simple, yet effective way to obtain wavelength tunability. By interchanging the channels of the AWG, discrete wavelength tuning of the Q-switching operation can be realized over a span, ranging from 1871.6 nm to 1888.8 nm without using any complex modulation techniques or special filters. The experimental results show that the generated Q-switched microsecond pulses have a wide repetition rate range, commencing from 41.19 kHz up to a maximum value of 68.3 kHz. Fig. 1.
2. Experimental setup The experimental setup of the proposed tunable passively Q-switched TDFL using AWG is shown in Fig. 1. The fiber ring consisted of two 1562.4 nm central wavelength pump laser diodes (Princeton Lightwave) with 350 mW output powers at 2.6 A of drive current in pulsed mode and 300 mW at 1.75 mW in CW mode. In our experiment, one acted as a forward-pump source and the other as a backward-pump source. However, the output power of each laser diode is limited to 240 mW in CW mode due to the limitation in the laser diode driver that could only deliver up to 1.5 A of current. Each of the laser diodes was then connected to a 1550 nm wavelength isolator, which was then connected to a 1550/2000 nm wavelength division multiplexing (WDM) coupler. One port of each of the two WDMs was fusion spliced to a gain medium comprising of a 4 m long thulium-doped fiber (TDF, OFS), which has a peak absorption of 200 dB/m at 790 nm with a cutoff wavelength at 1350 nm and core and cladding diameters of 5 mm and 125 mm respectively. The other port of the backward-pump-connected WDM was connected to a polarization insensitive isolator, operating at 2.0 mm to enforce unidirectional propagation of light within the ring cavity. The isolator was then linked to a 90:10 optical coupler where the 10% port of the coupler was used to extract a portion of the signal oscillating in the cavity for further analysis. Meanwhile, the 90% port was connected to an AWG, a silica-based waveguide that acts as a wavelength selective element. The mentioned AWG was used as a splitter to diffract the incident beam into different wavelengths. In principle, an AWG consists of an array of waveguides and two couplers. When an incident beam consisting of multiple wavelengths enters the input coupler, the beam is coupled into an array of waveguides. The beam subsequently propagates through the individual waveguides with different path lengths, towards the second coupler, which is the output. The beam travelling at different path lengths along different array waveguides will arrive at the output coupler with equal phase, resulting in the diffraction and interference at the output coupler. As a result, each wavelength is only focused into only one of the output channels. To obtain fiber lasers at several different wavelengths, 10 input channels of the AWG were used to provide wavelength selectivity inside the cavity. The output of the AWG was connected to an SA device where the device consisted of a FC/PC adapter containing two FC/PC connector ends that were separated by an MWCNT-PVA composite that acted as the host material. Physical characteristics of the MWCNT/PVA-based SA included a distributed diameter range of 10–20 mm, length of 1–3 mm, and thickness of about 50 mm. The other end of the SA device was connected to the 2000 nm port of the forward-pump-connected WDM, thus forming and completing the ring laser cavity. The portion of the signal extracted by the 10% port of the 90:10 coupler was connected to an optical spectrum analyzer (OSA Yokogawa AQ6375) with a resolution of 0.05 nm for spectral analysis or to an oscilloscope (Yokogawa DLM2054) through a 12.5 GHz InGaAs photodetector (Newport).
3. Results and discussions
Fig. 1. Schematic diagram for wavelength tuning in passively Q-switched fiber laser using an MWCNT/PVA SA.
A Q-switched pulsed laser with variable wavelengths was obtained from this proposed system. The output spectra of the tunable Q-switched fiber laser taken at pump power of 312.4 mW are shown in Fig. 2. Ten channels from the AWG were used to tune the fiber laser to several distinct and sharp laser lines, ranging from 1871.6 nm to 1888.8 nm, with an approximately 2.0 nm interchannel wavelength spacing. By just switching the channels of the
M.Z. Samion et al. / Optics Communications 380 (2016) 195–200
Fig. 2. Output spectra of the tunable Q-switched operation for ten tuned wavelengths at pump power of 312.4 mW.
AWG from the first channel to the tenth channel, the wavelength of the Q-switched fiber laser were tuned from longest wavelength of 1888.8 nm to the lowest wavelength of 1871.6 nm. The Q-switching was observed when the pump power reached 242.2 mW, up until a maximum power of 360.9 mW. When the pump power was raised above this value, the Q-switching operation started to destabilize, whereby fluctuation in the pulse amplitude became apparent. To avoid thermal damage on the saturable absorber, the pump power was limited at this value.
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The stability measurements of the tuned wavelengths when the AWG was placed in the cavity are shown in Fig. 3. Three channels from the AWG were selected to represent the whole tuning spectrum. The first channel was chosen as it corresponds to the longest wavelength, which is 1888.8 nm, the fifth channel corresponds to the center wavelength, 1881.2 nm, and the tenth channel corresponds to the shortest wavelength, 1871.6 nm. The stability test was conducted for a period of sixty minutes with results taken over a five-minute interval. The cavity was placed in a stable environment i.e. laboratory condition with low thermal variation and minimal noise disturbance. These results prove the proposed system is relatively stable over time, since the average variation of the peak power was only about 1 dBm and the peak wavelength did not vary. The measurements of pulse repetition rate, pulse width, output power and pulse energy against pump power at different operating wavelength of 1871.6 nm, 1881.2 nm, and 1888.8 nm are shown in Fig. 4. Fig. 4a indicates the linear dependence between the pulse repetition rate and the pump power. The pulse repetition rate increased from 44.91 kHz to 68.3 kHz at 1871.6 nm, 46.21 kHz to 67.11 kHz at 1881.2 nm, and 41.19 kHz to 65.85 kHz at 1888.8 nm, with a change in pump power from 242.2 mW until 360.9 mW. There was an approximately 3–7 kHz increase in repetition rate for every increase of 10 mW in the pump power. In contrast to the pulse repetition rate having an approximately linear relationship with the pump power, the pulse width decrease almost linearly as can be observed in Fig. 4b. The pulse width of the MWCNT-based tunable Q-switched TDFL decreased from a value of 2.3–1.6 μs at output wavelength of 1871.6 nm, 2.5–1.6 μs
Fig. 3. Stability measurement of Q-switched operation for 60 minutes with output wavelength of (a) 1871.6 nm, (b) 1881.2 nm and (c) 1888.8 nm at pump power of 312.4 mW.
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Fig. 4. Characterization of the MWCNT-based tunable Q-switched TDFL parameters as function of input pump power, with the varying parameters being; (a) repetition rate, (b) pulse width, (c) output power, (d) pulse energy.
at 1881.2 nm, and 3.4–1.7 μs for 1888.8 nm, during the same increase in pump power, which was from 242.2 mW until 360.9 mW. Fig. 4c and d shows the output power and the pulse energy of the three channels against the input pump power. As seen in Fig. 4c, the output power taken at wavelengths of 1871.6 nm, 1881.2 nm and 1888.8 nm increased linearly beginning at input pump power of 242.2 mW until to the maximum pump power of 360.9 mW. In Fig. 4d, it can be seen that the pulse energies obtained at the maximum pump power of 360.9 mW are 11.8 nJ, 11.7 nJ and 10.2 nJ, which correspond to the wavelength of 1888.8 nm, 1881.2 nm and 1871.6 nm respectively. The variation of the repetition rates and pulse widths at different wavelengths while the pump power was fixed at 312.4 mW is shown in Fig. 5. It can be seen from this figure that the pulse repetition rate becomes lower as the wavelength is tuned from the shortest wavelength to the wavelength of 1877.4 nm but then to increase at
Fig. 5. Variation of repetition rate and pulse width for the tuned wavelengths at pump power of 312.4 mW.
1879.2 nm. The repetition rate then decreases again until the longest wavelength of 1888.8 nm. From the results, it is obvious that the change in the repetition rates as a function of wavelength does not follow a specific pattern. The variation is caused by different amount of losses that occur in different AWG channels. The highest repetition rate achieved at 312.4 mW is 61.3 kHz, with a corresponding pulse width of 1.7 ms at 1871.6 nm, whereas the minimum repetition rate at the mentioned pump power is 55.5 kHz, corresponding to a pulse width of 1.7 ms at 1877.4 nm. Fig. 6 is a plot of the output pulse train at pump power of 312.4 mW, where it can be seen that the Q-switched pulse train has uniform pulse shapes, with minimal fluctuations from peak to peak for all three wavelengths as can be seen in Fig. 6a, b and c. The observed pulse trains on the oscilloscope shown in those figures have very minimal fluctuations in the peak intensity, which indicate that the Q-switched operation was free from self-mode locking effects and pulse jitter effects. The laser output exhibited spectral broadening across a wider wavelength, whereby the shortest wavelength of 1871.6 nm resulted in the highest pulse repetition rate of 61.3 kHz compared to the other wavelength of 1881.2 nm and 1888.8 nm, each with a lower repetition rate of 60.6 kHz and 56.2 kHz respectively. The Q-switching operation was stable at every wavelength without any distinct amplitude modulations in each Q switched envelop of the spectrum, and this finding verified that the self-mode locking effect on the Q-switching was suppressed. As can be seen in Fig. 6, the difference in the pulse intervals for the three wavelengths are not that significant, where the pulse intervals for each of the three wavelengths from the shortest to the longest wavelength are 16.31 ms, 16.50 ms and 17.79 ms respectively. In contrast with an earlier demonstration by H. Ahmad et al. and M. T. Ahmad et al. [37,38], the demonstration of the passively Q-switched output using graphene and MWCNT at operating wavelength of 2.0 mm has a lower maximum repetition rate of around
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Fig. 6. Output pulse train at pump power of 312.4 mW at different output wavelengths of (a) 1871.6 nm, (b) 1881.2 nm, and (c) 1888.8 nm.
the use of the channels of the AWG, the Q-switched fiber laser could be tuned to several different wavelengths ranging from 1871.6 to 1888.8 nm, with an approximately 2.0 nm inter-channel wavelength spacing. The set-up is also capable of generating Q-switched microsecond pulses with a wide repetition rate range, ranging from 41.19 kHz, up to a maximum value of 68.3 kHz with a minimum pulse width as low as 1.9 ms. The set-up that we have demonstrated also offers simplicity and cost-effectiveness in its design.
Acknowledgement
Fig. 7. RF spectrum of the Q-switched laser.
16.0 kHz and 21.0 kHz respectively compared to our experiment which has a maximum value of 68.3 kHz at 1871.6 nm. Moreover, all the tuned wavelengths in this demonstration have also higher maximum repetition rates compared to the mentioned works, which is about an average of 60 kHz. The radio frequency (RF) spectrum of the Q-switched pulse was taken to determine the stability of the generated Q-switched operation after the insertion of the AWG. The output RF spectrum in Fig. 7 was taken at 56.15 kHz with pump power of 312.4 mW at longest operating wavelength of 1888.8 nm where the signal-tonoise ratio of the pulse is 63.23 dB which is reasonably high, indicating that the generated Q-switched operation is stable. 4. Conclusions A passive MWCNT-based Q-switched fiber laser with wavelength tunability by the use of an AWG has been presented. By maximizing
We would like to acknowledge the generous funding from Ministry of Higher Education of Malaysia (MOHE) through the grant of LRGS(2015)/NGOD/UM/KPT and University of Malaya under the grant of ROGS (BR003-2016).
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable passively Q-switched thulium-doped fiber laser operating at 1. 9 μm using arrayed waveguide grating (AWG) M.Z. Samion a, M.F. Ismail a,n, A. Muhamad a, A.S. Sharbirin a, S.W. Harun a,b, H. Ahmad a a b
Photonics Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2016 Received in revised form 2 June 2016 Accepted 5 June 2016
Thulium-doped fiber lasers (TDFLs), operating in the 1.8–2.0 mm wavelength region, have been viewed as an important research topic, due to their potential in various fields of applications. However, the growing need to advance the development of applications in various fields for instance medicine and environment sensor, has led to a deeper and specific study of Q-switched TDFLs with wavelength tunability. In this paper, a stable, tunable Q-switched TDFL operating in a wavelength range near to 1.9 mm by exploiting the use of a multiwall carbon nanotube (MWCNT)-based thin film as a saturable absorber (SA), and the use of an arrayed waveguide grating (AWG) for wavelength tunability, is presented. The tuning range of the Q-switched pulses generated covered a wavelength range that spanned from 1871.6 nm to 1888.8 nm. The repetition rate of the generated Q-switched pulses covers a range of frequency starting from 41.19 kHz to 68.3 kHz with a change in pump power from 242.2 mW until 360.9 mW. & 2016 Elsevier B.V. All rights reserved.
Keywords: Tunable Saturable absorber Q-switching Thulium-doped fiber laser AWG MWCNT/PVA
1. Introduction There have been great interests in fiber lasers as compared to bulk optics systems due to their compactness and also ease of alignment. Generally, fiber lasers are being constructed from a gain medium, which provides the required amplification, together with a pump laser diode connected through a wavelength division multiplexer (WDM). An optical isolator is also used to force the unidirectional propagation in a ring configuration. There have been numerous reports of amplification in the S- (1480–1520 nm) [1–4], C- (1540–1560 nm) [5–7] and L- (1560–1600 nm) [8–10] bands which can be configured easily into a fiber laser. Due to the rapid increase of the demand of data traffic, there has been interest in the 2 mm band region as to complement the existing bands, whereby thulium is the active medium. Besides continuous wave (CW) operation, there are interests to pursue the development in pulsed fiber laser system based on Q-switching techniques that have wide application in sensing and medical sectors. Q-switched fiber lasers, which are lasers that emit energetic pulses that can be obtained through active [11] or passive [12] Q-switching techniques, have gained a lot of attention due to their capacity to produce pulses of light with high energy at relatively low repetition rates [13]. Such laser pulses can subsequently be used in many advanced applications – e.g. in range-finding, remote-sensing n
Corresponding author. E-mail address: [email protected] (M.F. Ismail).
http://dx.doi.org/10.1016/j.optcom.2016.06.012 0030-4018/& 2016 Elsevier B.V. All rights reserved.
and in the field of medicine [14]. Though Q-switching can be produced through active systems, the technique usually requires the application of an electric signal, applied to an acousto-optic or electro-optic modulator [15–18] - and can be rather complicated and costly as compared with passive Q-switching systems. Passively Q-switched fiber lasers offer the appealing advantages of compactness, simplicity, and flexibility in design. The technique can be implemented by using a more direct approach, which is the incorporation of an SA in the laser cavity. Numerous different types of SA have been demonstrated to achieve Q-switched operation, of which one example is the semiconductor saturable-absorber mirror (SESAM) [19,20]. Although this method works well in both bulk-laser and fiber-laser systems, SESAMs have several drawbacks that include high cost of fabrication, fragility, and a narrow tuning range that is in the region of only a few tens of nanometers [21]. Many saturable absorbers have been introduced as an alternative to overcome the limitation of SESAMs. Most recently, graphene and CNT-based SAs have been introduced because they could offer simplicity and compatibility in terms of system designs. In addition, they also have low saturation intensities and ultrafast recovery times [22–24], making them a more attractive alternative as compared with other SAs. The reason why graphene is less preferable than CNT-based SAs is that the saturation intensity of graphene depends greatly on the wavelength. At nearinfrared wavelengths, graphene exhibits large saturation intensities – while, in the mid-infrared region, its saturation intensity becomes equal to or even less than that of CNT-based SAs [25], making it less appealing for some applications.
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Recently, passively Q-switched thulium-doped fiber lasers (TDFLs) operating in the 1.9 mm region have gained much interest because they could be utilized in various ‘eye-safe’ applications, especially in the medical field. This is due to their emission being strongly absorbed by water [26], making them suitable to be applied in ophthalmic surgery. Q-switched TDFLs are also able to generate higher pulse energies at their output [27], making them important in the field of material processing and they are more efficient and are more simple as they do not require the subtle balancing of losses associated with mode-locking. Although Q-switched TDF lasers operating at a single wavelength have many applications, extensive interest in generating Q-switched TDF lasers with wavelength tunability exists for areas such as spectroscopy, material processing, and sensing [28]. In addition, tunability of basic TDF lasers in the 2.0 mm wavelength region has already been widely demonstrated by using volume Bragg gratings (VBGs) [29–31], and also by the use of diffraction gratings [18,32]. However, these methods are quite complex due to the use of bulky optic components in the laser cavity. This paper presents a method for producing a passively Q-switched output pulses that provide wavelength tunability for a thulium-doped fiber by incorporating a MWCNT-based SA and by utilizing the use of a silica-based arrayed waveguide grating (AWG). The use of the AWG has been previously demonstrated in achieving switchable-wavelength fiber lasers operating in the C-band up until the L-band wavelength region [33–35]. The low propagation loss of the AWG, combined with its high fiber-coupling efficiency [36], has made it quite popular to be used to precisely de-multiplexing a high number of optical signals. Hence, AWG was chosen in this demonstration due to its low insertion loss, low power consumption and more importantly, it could provide a simple, yet effective way to obtain wavelength tunability. By interchanging the channels of the AWG, discrete wavelength tuning of the Q-switching operation can be realized over a span, ranging from 1871.6 nm to 1888.8 nm without using any complex modulation techniques or special filters. The experimental results show that the generated Q-switched microsecond pulses have a wide repetition rate range, commencing from 41.19 kHz up to a maximum value of 68.3 kHz. Fig. 1.
2. Experimental setup The experimental setup of the proposed tunable passively Q-switched TDFL using AWG is shown in Fig. 1. The fiber ring consisted of two 1562.4 nm central wavelength pump laser diodes (Princeton Lightwave) with 350 mW output powers at 2.6 A of drive current in pulsed mode and 300 mW at 1.75 mW in CW mode. In our experiment, one acted as a forward-pump source and the other as a backward-pump source. However, the output power of each laser diode is limited to 240 mW in CW mode due to the limitation in the laser diode driver that could only deliver up to 1.5 A of current. Each of the laser diodes was then connected to a 1550 nm wavelength isolator, which was then connected to a 1550/2000 nm wavelength division multiplexing (WDM) coupler. One port of each of the two WDMs was fusion spliced to a gain medium comprising of a 4 m long thulium-doped fiber (TDF, OFS), which has a peak absorption of 200 dB/m at 790 nm with a cutoff wavelength at 1350 nm and core and cladding diameters of 5 mm and 125 mm respectively. The other port of the backward-pump-connected WDM was connected to a polarization insensitive isolator, operating at 2.0 mm to enforce unidirectional propagation of light within the ring cavity. The isolator was then linked to a 90:10 optical coupler where the 10% port of the coupler was used to extract a portion of the signal oscillating in the cavity for further analysis. Meanwhile, the 90% port was connected to an AWG, a silica-based waveguide that acts as a wavelength selective element. The mentioned AWG was used as a splitter to diffract the incident beam into different wavelengths. In principle, an AWG consists of an array of waveguides and two couplers. When an incident beam consisting of multiple wavelengths enters the input coupler, the beam is coupled into an array of waveguides. The beam subsequently propagates through the individual waveguides with different path lengths, towards the second coupler, which is the output. The beam travelling at different path lengths along different array waveguides will arrive at the output coupler with equal phase, resulting in the diffraction and interference at the output coupler. As a result, each wavelength is only focused into only one of the output channels. To obtain fiber lasers at several different wavelengths, 10 input channels of the AWG were used to provide wavelength selectivity inside the cavity. The output of the AWG was connected to an SA device where the device consisted of a FC/PC adapter containing two FC/PC connector ends that were separated by an MWCNT-PVA composite that acted as the host material. Physical characteristics of the MWCNT/PVA-based SA included a distributed diameter range of 10–20 mm, length of 1–3 mm, and thickness of about 50 mm. The other end of the SA device was connected to the 2000 nm port of the forward-pump-connected WDM, thus forming and completing the ring laser cavity. The portion of the signal extracted by the 10% port of the 90:10 coupler was connected to an optical spectrum analyzer (OSA Yokogawa AQ6375) with a resolution of 0.05 nm for spectral analysis or to an oscilloscope (Yokogawa DLM2054) through a 12.5 GHz InGaAs photodetector (Newport).
3. Results and discussions
Fig. 1. Schematic diagram for wavelength tuning in passively Q-switched fiber laser using an MWCNT/PVA SA.
A Q-switched pulsed laser with variable wavelengths was obtained from this proposed system. The output spectra of the tunable Q-switched fiber laser taken at pump power of 312.4 mW are shown in Fig. 2. Ten channels from the AWG were used to tune the fiber laser to several distinct and sharp laser lines, ranging from 1871.6 nm to 1888.8 nm, with an approximately 2.0 nm interchannel wavelength spacing. By just switching the channels of the
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Fig. 2. Output spectra of the tunable Q-switched operation for ten tuned wavelengths at pump power of 312.4 mW.
AWG from the first channel to the tenth channel, the wavelength of the Q-switched fiber laser were tuned from longest wavelength of 1888.8 nm to the lowest wavelength of 1871.6 nm. The Q-switching was observed when the pump power reached 242.2 mW, up until a maximum power of 360.9 mW. When the pump power was raised above this value, the Q-switching operation started to destabilize, whereby fluctuation in the pulse amplitude became apparent. To avoid thermal damage on the saturable absorber, the pump power was limited at this value.
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The stability measurements of the tuned wavelengths when the AWG was placed in the cavity are shown in Fig. 3. Three channels from the AWG were selected to represent the whole tuning spectrum. The first channel was chosen as it corresponds to the longest wavelength, which is 1888.8 nm, the fifth channel corresponds to the center wavelength, 1881.2 nm, and the tenth channel corresponds to the shortest wavelength, 1871.6 nm. The stability test was conducted for a period of sixty minutes with results taken over a five-minute interval. The cavity was placed in a stable environment i.e. laboratory condition with low thermal variation and minimal noise disturbance. These results prove the proposed system is relatively stable over time, since the average variation of the peak power was only about 1 dBm and the peak wavelength did not vary. The measurements of pulse repetition rate, pulse width, output power and pulse energy against pump power at different operating wavelength of 1871.6 nm, 1881.2 nm, and 1888.8 nm are shown in Fig. 4. Fig. 4a indicates the linear dependence between the pulse repetition rate and the pump power. The pulse repetition rate increased from 44.91 kHz to 68.3 kHz at 1871.6 nm, 46.21 kHz to 67.11 kHz at 1881.2 nm, and 41.19 kHz to 65.85 kHz at 1888.8 nm, with a change in pump power from 242.2 mW until 360.9 mW. There was an approximately 3–7 kHz increase in repetition rate for every increase of 10 mW in the pump power. In contrast to the pulse repetition rate having an approximately linear relationship with the pump power, the pulse width decrease almost linearly as can be observed in Fig. 4b. The pulse width of the MWCNT-based tunable Q-switched TDFL decreased from a value of 2.3–1.6 μs at output wavelength of 1871.6 nm, 2.5–1.6 μs
Fig. 3. Stability measurement of Q-switched operation for 60 minutes with output wavelength of (a) 1871.6 nm, (b) 1881.2 nm and (c) 1888.8 nm at pump power of 312.4 mW.
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Fig. 4. Characterization of the MWCNT-based tunable Q-switched TDFL parameters as function of input pump power, with the varying parameters being; (a) repetition rate, (b) pulse width, (c) output power, (d) pulse energy.
at 1881.2 nm, and 3.4–1.7 μs for 1888.8 nm, during the same increase in pump power, which was from 242.2 mW until 360.9 mW. Fig. 4c and d shows the output power and the pulse energy of the three channels against the input pump power. As seen in Fig. 4c, the output power taken at wavelengths of 1871.6 nm, 1881.2 nm and 1888.8 nm increased linearly beginning at input pump power of 242.2 mW until to the maximum pump power of 360.9 mW. In Fig. 4d, it can be seen that the pulse energies obtained at the maximum pump power of 360.9 mW are 11.8 nJ, 11.7 nJ and 10.2 nJ, which correspond to the wavelength of 1888.8 nm, 1881.2 nm and 1871.6 nm respectively. The variation of the repetition rates and pulse widths at different wavelengths while the pump power was fixed at 312.4 mW is shown in Fig. 5. It can be seen from this figure that the pulse repetition rate becomes lower as the wavelength is tuned from the shortest wavelength to the wavelength of 1877.4 nm but then to increase at
Fig. 5. Variation of repetition rate and pulse width for the tuned wavelengths at pump power of 312.4 mW.
1879.2 nm. The repetition rate then decreases again until the longest wavelength of 1888.8 nm. From the results, it is obvious that the change in the repetition rates as a function of wavelength does not follow a specific pattern. The variation is caused by different amount of losses that occur in different AWG channels. The highest repetition rate achieved at 312.4 mW is 61.3 kHz, with a corresponding pulse width of 1.7 ms at 1871.6 nm, whereas the minimum repetition rate at the mentioned pump power is 55.5 kHz, corresponding to a pulse width of 1.7 ms at 1877.4 nm. Fig. 6 is a plot of the output pulse train at pump power of 312.4 mW, where it can be seen that the Q-switched pulse train has uniform pulse shapes, with minimal fluctuations from peak to peak for all three wavelengths as can be seen in Fig. 6a, b and c. The observed pulse trains on the oscilloscope shown in those figures have very minimal fluctuations in the peak intensity, which indicate that the Q-switched operation was free from self-mode locking effects and pulse jitter effects. The laser output exhibited spectral broadening across a wider wavelength, whereby the shortest wavelength of 1871.6 nm resulted in the highest pulse repetition rate of 61.3 kHz compared to the other wavelength of 1881.2 nm and 1888.8 nm, each with a lower repetition rate of 60.6 kHz and 56.2 kHz respectively. The Q-switching operation was stable at every wavelength without any distinct amplitude modulations in each Q switched envelop of the spectrum, and this finding verified that the self-mode locking effect on the Q-switching was suppressed. As can be seen in Fig. 6, the difference in the pulse intervals for the three wavelengths are not that significant, where the pulse intervals for each of the three wavelengths from the shortest to the longest wavelength are 16.31 ms, 16.50 ms and 17.79 ms respectively. In contrast with an earlier demonstration by H. Ahmad et al. and M. T. Ahmad et al. [37,38], the demonstration of the passively Q-switched output using graphene and MWCNT at operating wavelength of 2.0 mm has a lower maximum repetition rate of around
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Fig. 6. Output pulse train at pump power of 312.4 mW at different output wavelengths of (a) 1871.6 nm, (b) 1881.2 nm, and (c) 1888.8 nm.
the use of the channels of the AWG, the Q-switched fiber laser could be tuned to several different wavelengths ranging from 1871.6 to 1888.8 nm, with an approximately 2.0 nm inter-channel wavelength spacing. The set-up is also capable of generating Q-switched microsecond pulses with a wide repetition rate range, ranging from 41.19 kHz, up to a maximum value of 68.3 kHz with a minimum pulse width as low as 1.9 ms. The set-up that we have demonstrated also offers simplicity and cost-effectiveness in its design.
Acknowledgement
Fig. 7. RF spectrum of the Q-switched laser.
16.0 kHz and 21.0 kHz respectively compared to our experiment which has a maximum value of 68.3 kHz at 1871.6 nm. Moreover, all the tuned wavelengths in this demonstration have also higher maximum repetition rates compared to the mentioned works, which is about an average of 60 kHz. The radio frequency (RF) spectrum of the Q-switched pulse was taken to determine the stability of the generated Q-switched operation after the insertion of the AWG. The output RF spectrum in Fig. 7 was taken at 56.15 kHz with pump power of 312.4 mW at longest operating wavelength of 1888.8 nm where the signal-tonoise ratio of the pulse is 63.23 dB which is reasonably high, indicating that the generated Q-switched operation is stable. 4. Conclusions A passive MWCNT-based Q-switched fiber laser with wavelength tunability by the use of an AWG has been presented. By maximizing
We would like to acknowledge the generous funding from Ministry of Higher Education of Malaysia (MOHE) through the grant of LRGS(2015)/NGOD/UM/KPT and University of Malaya under the grant of ROGS (BR003-2016).
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