Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser

Journal Pre-proofs Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser M.Z. Zulkifli, F.D. Muhammad, M.F Mohd Azri, MK. Mohd Yusof, K.Z. Hamdan, S.A Samsudin, M. Yasin PII: DOI: Reference:

S2211-3797(19)33332-7 https://doi.org/10.1016/j.rinp.2020.102949 RINP 102949

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Results in Physics

Received Date: Revised Date: Accepted Date:

12 November 2019 30 December 2019 14 January 2020

Please cite this article as: Zulkifli, M.Z., Muhammad, F.D., Mohd Azri, M.F, Mohd Yusof, MK., Hamdan, K.Z., Samsudin, S.A, Yasin, M., Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser, Results in Physics (2020), doi: https://doi.org/10.1016/j.rinp.2020.102949

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Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser M. Z. Zulkifli1, F. D. Muhammad2, M.F Mohd Azri1, M K. Mohd Yusof1,K. Z. Hamdan2, S.A Samsudin3, and M. Yasin4 1Department

of Physics, Kulliyyah of Science, International Islamic University of Malaysia, Bandar Indera Mahkota, 25200 Kuantan, Pahang, Malaysia 2Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 3 Department of Bioprocess and Polymer Engineering, School of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Johor Bahru, Johor, Malaysia 4Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya, Indonesia

Corresponding authors: M.Z. Zulkifli and M. Yasin (e-mail: [email protected]; [email protected])

Abstract: A tunable passively Q-switched erbium-doped fiber laser with ultranarrow linewidth is proposed and demonstrated. With the single wall carbon nanotubes (SWCNTs) as the passive Q-switch, the ultranarrow linewidth laser operation is realized based on spectral filtering effect using an ultranarrow tunable bandpass filter (UNTBF). The Q-switched laser spectrum is tunable from 1525 to 1561 nm, covering a wavelength range of 36 nm. At 1545 nm, the highest repetition rate of ~ 38 kHz and the lowest pulse width of 1.15 μs are obtained, with the corresponding pulse energy of approximately 0.18 nJ. The linewidth measurement using selfheterodyne technique yields a linewidth value of 17.5 kHz, which may be the narrowest linewidth for a Q-switched fiber laser to the best of our knowledge. Keywords: Ultra-narrow linewidth, tunable wavelength, passively Q-switched and erbiumdoped fiber laser

Introduction Investigation on Q-switched fiber laser has been widely exploited by the researchers due to its ability to produce high-energy pulses at relatively low repetition rate in a compact design for a variety of applications [1-5]. Generally, Q-switched fiber lasers can be achieved via active

or passive approaches. Compared to those fabricated using the active technique, passive Qswitching are more favorable in terms of compactness, simplicity, and flexibility in design. Passively Q-switched fiber lasers have been intensively studied using various kinds of saturable absorbers (SAs) such as transition metal doped crystals [6] and semiconductor saturable absorber mirrors (SESAMs) [7]. Recently, single-wall carbon nanotubes (SWCNTs) have shown promising potential in mode-locked and Q-switched fiber laser systems due to its intrinsic saturable absorption properties, ultrafast recovery time and wide absorption wavelength bandwidth [8,9]. A significant interest has also emerged on single-longitudinal-mode (SLM) or singlefrequency Q-switched lasers since they can offer additional advantage such as low phase noise which is useful for specialized applications such as radar, telecommunication, spectroscopy and optical coherent detection [10-12]. As in the case of SLM generation in fiber laser, the issue of multimode output that rises due to mode hopping, homogeneous gain broadening, long cavity length, and very narrow longitudinal mode spacing results in formation of noises in frequency domain, thus restricting the SLM operation in the cavity [13,14]. Many approaches pertaining to SLM generation have been implemented such as by using multiple ring cavity structures [15], tunable ring resonators [16], external light injection [17], unidirectional loop mirrors [18] and acousto-optic tunable filters [19]. However, most of the techniques employ complex configuration and could not guarantee their suitability to work in tandem with the Q-switched operation. In principle, to generate the SLM Q-switched fiber laser, a Q-switching element is introduced into an SLM fiber laser cavity. One of the attemps to realize the SLM Q-switched fiber laser operation includes the insertion of Q-switch in an SLM distributed Bragg reflection (DBR) fiber laser [20,21], however, the deficiency of this method is that the output pulse energy is limited by the low energy storage of short length of the active medium [10]. On the other hand, the Q-switched generation in SLM ring cavity fiber laser normally suffers from disruption in the SLM oscillation due to the rapid modulation in the Q-switching process [10]. This issue is then overcome by implementing the injection seeding technique [10, 22-25]. However, the narrowest linewidth reported so far by using the injection seeding technique is ~1.5 MHz [10], which is rather wide. As such, it still remains a big challenge to narrow down the linewidth of the SLM Q-

switched fiber laser to kHz range. Apart form that, it is also worth to note that most of the Qswitching mechanisms in the aforementioned works are demonstrated using the active method and there are still limited studies on the SLM Q-switched fiber laser based on passive approach. As an improved scheme in developing the SLM Q-switched fiber laser, the addition of spectral filters in Q-switched ring laser cavity to impede the building-up of multimode oscillation as well as the incorporation of saturable absorber as the passive Q-switching element could be highly attractive. On top of that, particular feature such as wavelength tuning ability of the spectral filter would be a plus point towards providing the flexibility in selecting the lasing wavelength. Previously, the tunable Q-switched fiber laser are achieved using variety of tuning elements, with the tuning range of 33 nm [5], 10 nm [26], and 7 nm [27]. It is necessary to further increase the tuning range of the Q-switched fiber laser to cater a wider applications. In this paper, we introduce an ultranarrow tunable bandpass filter (UNTBF) to realize the SLM oscillation in an EDFL based on spectral filttering effect with an additional feature of wavelength tunability while enabling the Q-switched operation by using SWCNT as the passive Q-swicthing element. An ultranarrow linewidth of 17.5 kHz is obtained from the SLM EDFL as measured by the self-heterodyne technique. Stable Q-switched pulses with the repetition rate and the pulse width from 5.8 kHz to ~ 38 kHz, and 8.12 µs to 1.15 µs are achieved. The Q-switched laser spectrum is tunable from 1525 to 1561 nm, covering a wavelength range of 36 nm. The proposed system is able to yield a narrow 3 dB bandwidth of the Q-switched spectrum, with a value of 0.017 nm. To the best of our knowledge, this is the narrowest bandwidth obtained with a passively Q-switched ring-cavity fiber laser. Such results indicate the capability of the proposed system to yield a wavelength tunable passively Q-switched laser operation from an ultranarrow linewidth SLM EDFL. Experimental set up The experimental set up for the proposed tunable narrow linewidth passively Q-switched EDFL is shown in Figure 1. A ~3.0 m long MetroGain-12-type erbium-doped fiber (EDF) is used as the gain medium. The erbium ions concentration of the EDF is 960 ppm, which contributes to the absorption coefficient of approximately 12 dB/m at 980 nm and about 18 dB/m at 1550 nm. The EDF is pumped by a 980 nm laser diode with a maximum output power of 83.2

mW via a 980 nm port of a 980/1550 nm wavelength-division multiplexer (WDM), with the common output of the WDM is fusion spliced to the EDF. The other end of the EDF is connected to an input of an optical isolator to impose unidirectional propagation of light within the ring cavity. Then, the output signal from the optical isolator is coupled to the SWCNT-based SA, which is responsible for generating the Q-switched pulses. The SA fabrication process as well as its parameters characterization has been described in detail in Ref. [28]. The other end of patch cord that sandwiches the SWCNT-based SA is coupled to the input port of the XTM-50 Yenista ultra narrow tunable bandpass filter (UNTBF). The filter consists of bulk optics in combination with diffraction gratings, which leads to high selectivity. Then, the output signal from the filter passes through to the input port of an optical coupler. The light from the 90% port of the coupler is coupled to the 1550-nm port of the WDM, thus forming a ring laser cavity. The portion of the signal extracted by the 10% port of the coupler is fed to an optical spectrum analyzer (OSA Yokogawa AQ63703) with a resolution of 0.02 nm for spectral analysis. A LeCroy 352A oscilloscope attached with a Thorlabs D400 FC InGas photo detector with a bandwidth of 1 GHz is used to measure the properties of the produced Q-switched pulse train. This experiment is repeated by adjusting the controller to tune the wavelength across the tunable range.

Figure 1: Experimental set up of the ultranarrow linewidth tunable Q-switched EDFL. WDM: wavelength division multiplexer, EDF: erbium-doped fiber, CNT: carbon nanotube, SA: saturable absorber, OSA: optical spectrum analyzer.

Results and discussion To characterize the ultranarrow linewidth EDFL, the laser is firstly examined without incorporating SWCNT as the SA in the laser cavity. Figure 2 (a) shows a continuous wave (CW) laser from the ring cavity without the insertion of the UNTBF, whereas Figure 2 (b) is the ultranarrow linewidth laser spectrum achieved with the use of the UNTBF. Both spectra are taken at 100 nm wavelength span and the resolution is fixed to 0.02 nm at a pump power of 83.2

mW. As depicted in the figure, the ultranarrow linewidth laser spectrum is represented by a very sharp thin line with a 3-dB bandwidth of 0.013 nm. On the other hand, the free running laser attained without the use of the UNTBF has a wider 3 dB bandwidth of 0.341 nm at a central wavelength of 1567.85 nm.

Figure 2: (a) Normal continuous wave (CW) laser spectrum without UNTBF and (b) ultranarrow linewidth laser spectrum by employing UNTBF

In order to verify the SLM operation in the ultranarrow linewidth laser, the laser output is examined under radio frequency spectrum analyser (RFSA) via the 1 GHz bandwidth photo detector. Figure 3 shows the laser output as observed from the RFSA across a frequency span of 0–500 MHz. From the observation, the absence of beat frequency verifies that the laser is operating in SLM. Due to the limitation of the bandwidth of the photo detector, observation of the beat frequency within higher frequency range could not be performed. In addition, it can be seen from the RF spectrum that the relaxation oscillation signal is hardly visible. This is due to

the high amplified spontaneous emission (ASE) produced by the high pump power in this system which suppresses the relaxation oscillation signal, resulting in a very low peak to noise ratio of the signal. Further verification of the SLM operation is done by heterodyne technique using a local oscillator for linewidth measurement purpose. In this technique, to detect the beat signal in the RF spectrum, it is necessary to have a smaller RFSA resolution than the laser linewidth to be measured. Thus, the resolution of the RFSA is set to be as small as 30 Hz in this measurement. The output RF spectrum is shown in Figure 4, whereby the presence of a beat signal at 44.80 MHz is detected. The bandwidth of the beat signal, ∆fb is taken at 20-dB point to avoid any frequency jitter, which is measured to be 350 kHz. Based on the equation of ∆fb = 2 99 ∆v, the value of the laser linewidth, ∆v is estimated to be 17.5 kHz.

Figure 3: RF spectrum of the output laser across 500 MHz frequency span

Figure 4: RF spectrum of delayed self-heterodyne signal The CW operation of the proposed setup with the SWCNT and UNTBF starts at a pump power of 42.7 mW while the power threshold for the Q-switched operation is recorded at 52.4 mW. Figure 5 (a) shows the Q-switched output spectrum taken at the maximum pump power of 83.2 mW with a central wavelength of 1561 nm. The inset in the figure shows the zoom in view of the output spectrum with the measured 3 dB bandwidth of approximately 0.017 nm, which is narrower than the reported value of 0.04 nm and 0.073 nm in Ref. [29] and Ref. [30] respectively. By tuning the UNTBF, the central wavelength of the output spectrum can be tuned from 1561 nm to 1525 nm and the stable Q-switched pulse trains on the oscilloscope still remain. Figure 5 (b) shows the output spectra of the ultranarrow linewidth Q-switched EDFL for 10 tuned wavelengths at the wavelength interval of 4 nm over a wavelength range of 36 nm. It is interesting to note that the tuning range of 36 nm achieved in this work is much wider compared to those achieved in most previous works on tunable Q-switched fiber laser, such as in Ref. [31] and [32], with the reported tuning range of ~10 nm and ~8.4 nm respectively. Moreoever, since the UNTBF used in this work can be tuned manually, the wavelength shift is dependent on the

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Figure 5: (a) Q-switched EDFL output spectrum with a 3 dB bandwidth of 0.017 nm; (b) Spectra of tunable Q-switched EDFL at a wavelength interval of 4 nm.

The variation of average output power and repetition rate at different wavelengths by tuning the UNTBF is investigated at the fixed pump power of 83.2 mW and the result is shown in Figure 6. The measurement is made from the wavelength of 1525 nm to 1561 nm, corresponding to the wavelength range of the output spectrum as shown in Figure 5 (b). The average output power and the repetition rate are observed to vary in a similar trend across the tuned wavelength. The variation of the average output power is attributed to the wavelengthdependence of EDF gain spectrum and cavity loss which in turn influences the variation of the repetition rate. This is because there is an interrelation between the intracavity laser and the repetition rate, such that with a larger output power, the intracavity laser is stronger, and consequently the optical transition bleaching of SWCNT SA is faster under a faster population inversion or depletion, leading to faster repetition rates. The highest average output power and repetition rate are both achieved at 1545 nm with a value of ~6.9 mW and ~38 kHz respectively. Both the average output power and repetition rate can vary from ~1.7 to ~6.9 mW and from ~14.8 to ~38 kHz respectively with the different operation wavelength.

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Figure 6: Variation of average output power and repetition rate at different wavelengths within tuning range

To further investigate the variation of the Q-switched performance during the wavelength-tuning process, measurement on the pulse width and pulse energy is carried out against wavelength, as shown in Figure 7. The behavior of the pulse width variation against wavelength is opposite to that of the repetition rate, with the narrowest pulse width of 1.15 µs attained at 1545 nm and the largest pulse width of 5.48 μs at 1541 nm. On the other hand, the value of the pulse energy is given by the value of the average output power divided by pulse repetition rate. The pulse energy varies across the range of wavelength tuning from 0.105 nJ at 1521 nm highest

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Figure 8 (a) and (b) shows the pulse train of the Q-switched pulse as taken from the oscilloscope at the wavelength of 1545 and 1541 nm respectively. The pulse train at 1545 nm has a pulse interval of 26.42 μs, which corresponds to a repetition rate of ~38 kHz. As for the pulse train at 1541 nm, the pulse interval is 67.39 μs, corresponding to a repetition rate of ~14.8 kHz. Only small fluctuation in the pulse intensity is observed in both pulse trains. This slight fluctuation occurs due to the loss that the pulse endures which is related to the tiny fluctuations of the stored energy, and this consequently results in the fluctuations of the gain [33]. Such behavior normally happens in passively Q-switched laser. The small fluctuation of the pulse intensity and the even temporal spacing between the pulses in this proposed system indicates that the pulse operation is stable. (a)

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Figure 8: Q-switched output pulse train taken at (a) 1545 nm and (b) 1541 nm

The evolution of pulse repetition rate and pulse width of the ultranarrow linewidth Qswitched EDFL against the pump power at the wavelength of 1561 nm is presented in Figure 9. Based on the figure, the pulse repetition rate increases agaisnt the pump power and the maximum repetition rate attained is 13.3 kHz at the pump power of 83.2 mW. On the other hand, the pulse width of the system gets narrower as the pump power is increased, with the narrowest value of

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Figure 10 shows the development curve of the average output power and pulse energy against pump power. As can be seen from the figure, the average output power increases almost linearly with respect to the pump power with an increment of about 0.11 to 0.28 μW for every ~4 mW rise in pump power. The highest output power is 2.51 μW, which is attained at the highest pump power of 83.2 mW. The reason for the low average output power obtained in this work is due to the high cavity loss which mainly originates from the high insertion loss of the UNTBF, with a value of approximately 20 dB. As for the case of pulse energy, the value evoles from 0.11 nJ to a maximum value of 0.21 nJ in response to the change of pump power from 52.4 mW to 79.4 mW. However, the pulse energy drops slightly to 0.19 nJ as the pump power reaches

its maximum value of 83.2 mW. Similar trend has also been reported in Ref. [34], which infers that the system possibly exceeds its optimal operating point above a certain intracavity power. 3.00

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Conclusion A tunable passively Q-switched in an SLM ultranarrow linewidth EDFL has been proposed and demonstrated. The ultranarrow linewidth EDFL is realized by using UNTBF for creating the spectral filtering effect while the Q-switched operation is enabled by the SWCNT as the saturable absorber. Based on self-heterodyne technique, the linewidth of the EDFL is measured to be 17.5 kHz. The Q-switched laser spectrum is tunable from 1525 to 1561 nm, covering a wavelength range of 36 nm. The repetition rate of the Q-switched pulses generated in this work ranges from 5.8 kHz to ~ 38 kHz, and the pulse width ranges from 1.15 µs to 8.12 µs. Such results verify that the Q-switched pulses can be locked to the single cavity mode in this proposed SWCNT-based Q-switched ultranarrow linewidth SLM EDFL. Acknowledgements We would like to thank the Ministry of Higher Education of Malaysia (MOHE) for providing the grant of FRGS/1/2019/STG02/UPM/02/4

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AUTHORSHIP STATEMENT Manuscript title: Tunable Passively Q-Switched Ultranarrow Linewidth Erbium-Doped Fiber Laser All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Result in Physics Authorship contributions Please indicate the specific contributions made by each author.The name of each author must appear at least once in each of the three categories below. Conception and design of study: M.Z.Zulkifli, F.D Muhammad ; acquisition of data: M.F Mohd Azri, M. K Mohd Yusof, K.Z Hamdan analysis and/or interpretation of data: M.Z.Zulkifli, S.A Samsudin, F.D Muhammad, Drafting the manuscript:M .Z Zulkifli, F.D Muhammad; revising the manuscript critically for important intellectual content: M Yasin,

Approval of the version of the manuscript to be published M. Z. Zulkifli, F. D. Muhammad, M.F Mohd Azri, M K. Mohd Yusof, K. Z. Hamdan, S.A Samsudin, and M. Yasin

Acknowledgements All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors.

Highlights

1. We demonstrated a passively Q-switched erbium-doped fiber laser with ultranarrow linewidth 2. Ultranarrow linewidth laser operation is realized based on spectral filtering effect 3. A linewidth value of 17.5 kHz, which may be the narrowest linewidth for a Q-switched fiber laser to the best of our knowledge.