A net normal dispersion all-fiber laser using a hybrid mode-locking mechanism

The improved understanding and management of the various processes that govern the dynamics of pulse propagation in optical fibers have led to remarkable improvements in the performance of mode-locked fiber lasers in recent years. This is particularly evident when we look at the rise in achievable pulse energies and peak powers from fiber resonators. In fact, recent reports show that fiber lasers are now capable of competing on level terms with their solid-state counterparts, in terms of maximum attainable pulse energies [1, 2]. In addition, fiber lasers are compact, cost-effective devices with passive cooling and fiber compatibility, all of which are key technological advantages over solid-state lasers which make them desirable for many commercial and scientific applications in fields as diverse as metrology, material processing and imaging.

Prominent among the various mode-locking regimes due to their potential to deliver pulses with energies in the tens of nJ are the so-called dissipative soliton (DS) fiber lasers. In such dissipative systems, the nonlinearity is not only balanced by dispersion, as is the case in the classic soliton configuration, but also by the gain, gain saturation, saturable absorption, spectral filtering and losses [3, 4]. The majority of reports discussing dissipative soliton regimes with high pulse energies employ nonlinear polarization.

The NPE has evolved as an artificial saturable absorber (SA) [5, 6]. The NPE is an excellent mode-locking device with deep, adjustable modulation depth. However, a nonlinear polarization evolution (NPE) typically depends on bulk free-space optics that can suffer from long term reliability and reproducibility issues due to external perturbations. Furthermore, an NPE often requires realignment of the laser cavity polarization states, and it is incompatible with the use of polarization maintaining (PM) fiber components, which are beneficial for commercial laser products. Semiconductor saturable absorption mirrors (SESAMs) have often been used in all-PM fiber laser configurations [7, 8], but the integration of SESAMs into fiber configurations is not straightforward and damage at high powers can still be an issue.

In order to achieve mode-locked operation in a PM compatible, all-fiber configuration, we can use a nonlinear amplifying loop mirror (NALM). Developed in the late 1980s, NALMs employ beam splitters that separate the incoming beam into counter-propagating components that reach the gain fiber and are amplified at different times, see figure 1. In this way, when the two beams recombine in the splitter they have accumulated different nonlinear phase shifts. This interference creates the effect of a fast, artificial saturable absorber which favors pulsed operation over continuous wave (CW) [9, 10]. NALMs are capable of generating sub-100 fs pulses, but achieving self-starting mode-locked operation with NALMs is often challenging. The recent growing interest in NALMs can be justified by their all-fiber format and PM compatibility. For example, Aguergaray et al have demonstrated an all-PM, all-normal dispersion (ANDi) fiber laser mode-locked by an NALM generating short pulses in an environmentally stable and self-starting configuration and went on to achieve pulse energies as high as 16 nJ [11, 12].

Zoom In Zoom Out Reset image size

In this letter, we propose a hybrid mode-locking mechanism to generate dissipative solitons in a PM compatible, all-fiber configuration. With our approach, we ensure self-starting operation and remarkable environmental stability by combining the NALM with a carbon nanotube saturable absorber (CNT-SA), as shown in figure 1. The role of the CNT-SA in this configuration is to assist the pulse formation and increase the laser stability. CNTs exhibit nonlinear saturable absorption and a combination of a fast, sub-picosecond response in bundles of CNTs with inter-tube recombination and a slower recovery of isolated semiconducting CNTs [13]. This characteristic makes CNTs excellent for the purpose of initiating the pulse formation.

The use of two SA mechanisms has been demonstrated in previous literature; those reports include a stretched pulse fiber laser at 1.55 μm based on a semiconductor SA assisting the initial pulse formation and an NALM assisting in the stabilization of the pulses [14], and a wave breaking free fiber laser where self-starting operation was ensured by an SESAM while an NPE acted as an additional pulse shaper [15].

In this letter, we use the hybrid CNT–NALM mode-locking scheme described above to demonstrate an all-fiber, self-starting, dispersion-mapped, dissipative soliton laser. The laser exhibits excellent long term stability and emits pulses with 2.75 nJ pulse energy. Damage of the CNTs in high power operation was prevented by sealing the CNT-SA in a nitrogen-rich environment.

The experimental set-up consists of 10 m of erbium-doped fiber (EDF) pumped by a 980 nm diode laser and a 50/50 splitter that couples the NALM to a second loop containing an isolator to ensure uni-directionality and an output coupler (OC) with 30% output. The CNT-SA was fabricated by spraying a CNT thin film into a fiber-end and butt-coupling to a second fiber-end as shown in the inset of figure 1. The CNT-SA was sealed in a nitrogen-gas chamber to avoid oxidation of the CNTs, thus increasing the long term stability of the SA at high intracavity powers [16]. A polarization controller (PC) in the NALM was used to optimize the laser output but was not crucial to the self-starting of the laser. Here, the 10 m of EDF had normal dispersion at 1550 nm; the rest of the cavity consisted of standard single mode fiber (SMF) with anomalous dispersion at 1550 nm. The total cavity length was 19 m with a net normal dispersion of 0.152 ps².

Highly stable, self-started mode-locking was achieved at a pump power of approximately 68 mW. Figure 2(a) shows a typical dissipative soliton optical spectrum with steep edges. The spectral bandwidth at full-width-half-maximum (FWHM) is 38 nm which is the broadest we could achieve. The spectral characteristics of the laser output could be modified by adjusting the polarization states using the PC, figure 2(b). The pulse duration and chirped properties were measured by an optical pulse analyzer (HR 150 C-band Optical Pulse Analyser, Southern Photonics), and the pulse duration before compression was estimated to be approximately 590 fs, figure 3. We used 3 m of SMF fiber to compensate the positive linear chirp and de-chirped pulse duration to 230 fs, inset of figure 3. The measurement of the compressed pulse was limited by the resolution of the autocorrelator. Based on the spectral bandwidth of its spectral profile and assuming a Gaussian pulse shape, we should be able to compress the pulses to less than 100 fs.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The laser operates at its fundamental repetition rate at a frequency of 10.22 MHz with a signal-to-noise ratio (SNR) of approximately 70 dB for the fundamental RF signal and 65 dB for the 20th order RF signal, both measured with a span of 5 kHz, and a resolution bandwidth of 30 Hz (figure 4).

Zoom In Zoom Out Reset image size

Pulsed operation of the laser without the CNT-SA was also possible by creating an external perturbation to initiate the mode-locking at pump powers exceeding 240 mW (compared to 68 mW with the CNT-SA). However, the laser stability and reproducibility of the spectral and temporal characteristics were jeopardized by the absence of the CNT-SA.

One of the main challenges when using CNT-SAs in this fiber laser configuration is their relatively low damage threshold. CNT-SAs are known to be easily damaged when operating at high pulse energies. We circumvented this problem by sealing the CNT-SA in a nitrogen-rich environment, which is schematically shown in the inset of figure 1. In this way, we can prevent what we consider to be the primary cause for degradation of the device, oxidation [16].

We operated the laser at the highest available pump power of 500 mW over a 25 h period with the nitrogen-sealed CNT-SA as an experimental illustration. Not only did we not observe any damage but, more significantly, the laser operated at its fundamental repetition rate at all pump powers from 68 to 500 mW and showed remarkable stability at its highest output power as shown in figure 5(a). The average output power was 28.84 mW corresponding to a pulse energy of 2.75 nJ. Here, we are only limited by the available pump power, therefore further power scaling should, in principle, be possible. For further investigation of long term stability in the laboratory conditions, we switched off the nitrogen supply system and reduced the pump power; one week of operation without any deterioration was observed, figure 5(b).

Zoom In Zoom Out Reset image size

In addition, we also demonstrated an analogous configuration, a hybrid NALM, CNT-SA mode-locked fiber laser using an all-PM configuration [17]. While the PM laser proposed operated in the anomalous dispersion regime producing soliton-like pulses, the use of PM fiber enhanced the environmental stability of the laser by suppressing the uncontrolled rotation of the polarization states and allowed us to eliminate the PC (in figure 1).

In conclusion, we demonstrated a hybrid mode-locking mechanism in a dispersion-mapped, dissipative soliton. Mode-locking was achieved by the combined effect of a CNT-SA and an NALM. While the pulse energy achieved using this fiber laser is still modest when compared to some previous reports using ytterbium-doped fiber and an NPE configuration [5, 6], the approach here proposed offers turnkey operation in a PM compatible, all-fiber design with excellent stability and single pulse operation over a wide range of output powers. Thus, this laser design offers a simple solution for real applications.

This work was supported by the NEXT Program by JSPS under Grant No. LR012.