Passively mode-locked laser using an entirely centred erbium-doped fiber

The generation and manipulation of high-speed optical pulses has become a topic of great interest in the fiber laser research community within the past few years. The main advantage of these very high-speed optical pulses lies in their wide range of applications, such as in high-speed communication [1] as well as in sensor applications [2, 3], which are made possible in a fiber laser by using an active laser medium. This active laser medium, also known as a gain medium, is a source of optical gain within a laser and is used in a laser cavity to amplify the light power and to compensate for the cavity loss. An optical fiber doped with rare-earth elements such as erbium, thulium and ytterbium [4] is commonly used as an active gain medium. Energy must be pumped into the active laser medium for it to be able to add energy to the amplified light, and the resulting amplified light can be further exploited for means of obtaining ultra-short pulse output. The generation of ultra-short pulses is also called a mode-locking operation. A laser ring cavity requires a gain medium such as erbium-doped fiber (EDF) and also a 'mode-locker' component in order to obtain stable mode-locked soliton pulses. The most widely utilized mode-locking technologies employ a nonlinear effect, such as nonlinear polarization rotation (NPR) as reported in [5] or a nonlinear optical loop mirror (NOLM) as proposed in [6, 7], along with the use of a fast saturable absorber (SA) element such as semiconductor saturable absorber mirrors (SESAMs) [8], carbon nanotubes (CNT) [9] or graphene [10].

The effectiveness of the nonlinear effect technique is strongly dependent on the polarization and phase evolution of the optical pulse in the laser cavity; one example is pulses in a long cavity being very susceptible to the environment-induced fiber birefringence. On the other hand, SESAM technology is superior to the nonlinear technique especially in ultra-long cavity mode-locking because its saturable absorption is independent of the cavity length [11]. SESAMs have become a dominant mode-locking means for SA-based technology in fiber lasers, and have been deployed widely as a SA within commercial applications within the last decade [12], although SESAMs are associated with several drawbacks that limit usage with respect to mode-locking techniques. Fabrication of SESAMs is expensive and requires clean room facilities, with incorporation into complicated systems proving to be a time-consuming process [13]. Furthermore, integration in fiber ring cavity configuration is not straightforward and is restricted to particular types of linear cavity topology due to an inherent reflective property. Mode-locking techniques for SAs are gradually being updated with SESAMs replaced by CNT and graphene, and this process has received considerable attention within the research community. In recent years, the discovery of the ultra-fast phenomenon in CNT [14] has led to wide investigation of CNT for mode-locking optimization, especially with regards to erbium-doped fiber lasers (EDFL). This attentiveness is due to CNT advantages of ultra-fast recovery time in the picosecond region [15], polarization insensitivity [16], and easy integration with fiber connectors in comparison to SESAMs [17]. The applicability of CNT has been demonstrated in various configurations and has a satisfactory performance for wavelengths ranging from 1 to 2 μm [16]. Additionally, numerous mode-locking fiber lasers have been reported, including stretched pulse mode-locked [18], dissipative solitons [19, 20], wideband-tunable fiber lasers [21] and multi-wavelength ultra-fast fiber lasers [22]. This paper details a passively mode-locked fiber laser using a two-core optical fiber, with an undoped outer region of 9.38 μm diameter that surrounds a 4.00 μm diameter central core region doped with erbium ions at 400 ppm concentration. This optical fiber, abbreviated as TC-EDF, was developed in-house, with a designated application as a bend-sensitive active optical fiber sensor [23, 24]. This same fiber was also used to demonstrate mode-locked behaviour using CNT as a saturable absorber. Unlike the standard EDFL systems that normally lase at around 1550 to 1560 nm, TC-EDFL lasing occurs at 1533 nm, which consequently allows for further possibilities relating to applications. The work described here demonstrates the realization of stable soliton pulses via a simple ring cavity mode-locked by a CNT-based mode-locker.

The remainder of the paper is organized as follows: section 2 describes the doping profile of TC-EDF, section 3 details the experimental setup and configuration of a TC-EDFL, section 4 covers results and an ensuing discussion, and the findings of the work are concluded in section 5.

TC-EDF is a new type of fiber in which the fiber core comprises two annular regions, these being a concentrated erbium-doped inner core region and a surrounding undoped outer core region. The concentrated inner core acts as the source mechanism for the amplified spontaneous emission (ASE) spectrum, and the high concentration of erbium dopant in this region results in the propagating Gaussian beam achieving maximum intensity at the core, causing excitation of erbium ions within this inner core of the fiber [23, 24]. Recent experiments reported in [24] demonstrate the ASE output is relatively stable with negligible variation in output power over a measurement period of one hour.

The doping profile of TC-EDF and a cross-sectional view of TC-EDF are shown in figures 1(a) and (b) respectively. Figure 1(a) details the outer region as consisting of SiO₂ co-doped with GeO₂ and having a diameter of 9.38 μm, while the inner core of 4.00 μm diameter contains SiO₂ co-doped with Er₂O₃ at a concentration of 400 ppm and additionally Al₂O₃. Thus the diameter ratio between the total core and the erbium-doped region is 0.426.

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The experimental setup for the proposed mode-locked TC-EDF laser is shown in figure 2. A TC-EDF 3 m in length was inserted within a ring cavity and pumped via a conventional 980 nm laser diode (LD) in conjunction with a 980/1550 nm wavelength division multiplexer (WDM). ASE generated from this pumped TC-EDF subsequently propagated through an isolator operating at 1550 nm, which ensured a unidirectional input to a following 99:1 fused coupler that had the purpose of extracting a small portion of the signal via the 1% port for further analysis. The 99% port of the coupler was connected to a thin film of CNT-SA, a polarisation controller (PC) and the WDM 1550 port in series. The CNT-SA was fabricated by Cheap Tubes Inc using 99% pure single-walled CNTs (SWCNTs) between 3 and 30 μm in length with average diameters between 1 and 2 nm. Fabrication details of these CNT-SAs can be found in [25].

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The fraction of the propagating signal tapped out by the 1% port of the coupler was analysed using multiple types of analysing instruments: the optical characteristic of the signal was captured by a Yokogawa AQ6370C optical spectrum analyser (OSA), while the generated pulse output was examined with an Agilent 834400 light-wave detector connected to a LeCroy 352A oscilloscope with a 500 MHz bandwidth, an Anritsu MS2683A radio frequency spectrum analyzer (RFSA) possessing a 7.8 GHz bandwidth and an Alnair Labs HAC-200 auto-correlator.

Figure 3(a) shows the ASE spectrum of the TC-EDF when pumped with a 980 nm laser diode of 30 mW output power, and observation of this spectrum reveals this type of EDF is prone to lasing at 1533 nm, as shown in the inset of figure 1. Figure 3(b) demonstrates the tunable lasing wavelength of TC-EDF with various spool diameters in regard to the ASE of EDF, in which it can be seen that the TC-EDF lasing is tuned to a shorter wavelength using a smaller spool diameter of 10 cm. Since the lasing occurred at the center peak of the EDF, this fiber had an advantage of absorption at the highest concentration of the erbium ions. A suitable mode-locked fiber laser of TC-EDF thus can be designed based around the ASE spectrum shown in figure 3(b), wherein sufficient ASE power will cause the ring laser to oscillate in the locked frequencies.

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The output spectrum of the soliton mode-locked EDFL was obtained as shown in figure 4 with use of the OSA. A broad spectrum with a 3 dB bandwidth of 4.5 nm at the central wavelength of 1532.7 nm was attained at the optimum polarization state. This result is comparable with the 3 dB spectral bandwidth of a similar cavity configuration using the SWCNT-based saturable absorber [26, 27]. Figure 5 shows the trace of the mode-locked pulse train, which is obtained from the output coupler, in the oscilloscope with a repetition rate of 10.37 MHz. The generated pulses yielded an average output power of approximately 0.33 mW with a pulse energy of around 31.8 pJ and a peak power of 42.7 W.

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The characteristics of the mode-locked pulse are further investigated via an Alnair HAC-200 auto-correlator. Figure 6 shows the calibrated auto-correlator pulse trace measured at its full width at half maximum (FWHM) at 0.7 ps and a time-bandwidth product (TBP) of 0.40 alongside a Fourier-transform-limited sech² pulse profile, which is slightly higher than the expected transform limited value of 0.315 for the standard temporal profile of a sech² pulse. The high TBP obtained from the experiment is due to the presence of pulse chirping as a result of dispersion in the laser cavity [28].

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Figure 7 shows the output pulse train in frequency domain obtained from the RFSA. The spectrum confirms that the fundamental harmonic has a frequency of 10.37 MHz and has a clearly defined pulse with a peak-to-pedestal ratio of 36 dB. This spectrum of subsequent harmonics at intervals of 10.37 MHz is to be expected from the RFSA output. The inset of figure 7 shows the fundamental harmonic frequency of the mode-locked laser output measured with 100 Hz frequency span and 300 Hz resolution.

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A passively mode-locked fiber laser using a new TC-EDF as the linear gain medium was proposed and demonstrated as described in this work. The TC-EDF in question has significant potential as a source for very short pulses and also represents a good alternative for mode-locking at the particular wavelength of 1532.7 nm where lasing occurs. This wavelength is at the center peak of the EDF, which also has the highest intensity of the erbium. The demonstration reported here of the unique features offered by TC-EDF in combination with an easy CNT preparation and high ASE stability output raises anticipation of further substantive developments in relation to the generation of stable ultrafast pulses in a simple laser cavity setting. The authors believe that a higher repetition rate is achievable if the cavity length is reduced, and as such will improve the output characteristics of the TC-EDF discussed in this paper.

We would like to thank the Ministry of Education (MOHE) and the university of Malaya for the research funding, under grant number, UM.C/625/1/HIR/MOHE/SC/29/01 and Dr M Chandra Paul from the fiber Optics and Photonics Division, Central Glass and Ceramic Research Institute, Kolkata, India for providing the dual-core erbium doped fiber used in the experiment.