Generation of 27 nm Spectral Bandwidth, Two-Port Output Pulses Directly from a Yb-Doped Fiber Laser

Abstract

We reported on a generation of 27 nm spectral bandwidth, two-port output ultrashort pulses directly from an all-normal-dispersion passively mode-locked Yb-fiber laser. Based on the nonlinear polarization rotation (NPR) mode-locking technique, high pump power and optical devices with high damage thresholds were introduced to achieve broad spectral bandwidth and strong output power. The dual wavelengths were emitted from the clockwise and counterclockwise ports, respectively, and self-started mode-locking was achieved. The bidirectional output laser generates stable pulses with up to 223.5 mW average power at a 46.04 MHz repetition rate, corresponding to a pulse energy of 5 nJ. The bidirectional ultrashort outputs of the laser provide potential applications in supercontinuum generation and medical and biological applications.

1. Introduction

Fiber lasers have been favored due to their high beam quality, low manufacturing cost, and ease of maintenance, and have been widely applied in fields such as laser communication, optical metrology, and medical treatment [1,2,3]. Compared with solid-state lasers, fiber lasers are compact in size, more stable for operation, and more capable of handling tasks beyond laboratory experiments. Among the currently available mode-locking mechanisms, 2D materials [4] are easily self-started but have a low damage threshold. NALM [5] offers excellent stability but faces challenges in achieving high repetition rates. Although the figure-nine fiber laser cavity offers a fast response time and large modulation depth, it necessitates the incorporation of numerous optical components, consequently leading to an increase in both system complexity and implementation costs [6,7]. Due to their low damage threshold, SESAMs often have a limited operational lifetime under high power and cannot maintain stable long-term operation in such conditions [8]. The Mamyshev oscillator holds significant potential for generating high-energy ultrashort pulses; however, its large modulation depth prevents self-starting, necessitating an external seed source to assist in mode-locking [9,10]. In contrast, nonlinear polarization rotation (NPR) has been experimentally demonstrated to generate pulses with high peak power [11]. NPR [12,13,14] mode-locking utilizes the birefringence effect, where under the combined action of self-phase modulation (SPM) and cross-phase modulation (XPM), orthogonal components of elliptically polarized light undergo different nonlinear phase shifts, giving rise to continuous rotation of the polarization state in the fiber. Pulses are achieved through the fast saturable absorption mechanism, realizing mode-locking and pulse narrowing. Therefore, high repetition rates and wide spectral coverage can be easily achieved by fiber lasers using the NPR mode-locking technique. Simultaneously, due to the instantaneous absorption of light by the fast saturable absorber, it is applicable in multiple areas such as optical coherence tomography, fiber sensing, and biophotonics [15,16].

With the advancement of lasers, characteristics of fiber lasers in terms of fundamental repetition rate, output energy, etc., have been continuously improving. In order to increase communication capacity, dual-wavelength mode-locked fiber lasers have become a research hotspot [17,18]. Traditional fiber lasers deliver a single wavelength, whereas dual-wavelength fiber lasers simultaneously emit ultrashort pulses at two wavelengths with a broad spectrum. To achieve dual wavelength operation, it is often necessary to add auxiliary optical components into the laser cavity or to exploit higher-order nonlinear effects in highly nonlinear fibers, such as four-wave mixing effects [19,20], and fiber gratings [21,22]. Despite significant progress, these auxiliary optical devices increase the cavity length and system complexity, while also limiting the output power. The fiber lasers based on NPR experiments not only have a simple and flexible cavity structure but can also deliver higher power through two output ports. In recent years, for example, the maximum dual-port output power reported for the following lasers is 120 mW: Yb-doped fiber lasers based on NALM mode-locking [23], figure-nine mode-locking lasers [24], and dual-axis independent mode-locking lasers [25]. Among the aforementioned lasers and linear cavity lasers with dual-port output based on NPR mode-locking [26], the maximum output spectral bandwidth is 20.37 nm. At low power levels, their power fluctuation is approximately 0.15% root-mean-square (RMS). In contrast, our laser, under the condition of high pump power and high energy output, achieves an output power of 223.5 mW, a spectral bandwidth of 27 nm, and a power fluctuation of less than 0.45% RMS.

Bidirectional noise-like pulses (NLPs) were generated in an all-normal dispersion ring cavity with NPR. The bidirectionally operated laser has the advantages of high integration and low cost. Since the pulses in both directions share the same cavity, common-mode noise can be effectively suppressed. At the same time, with the development of noise-like pulses, by continuously increasing the pump power and nonlinear amplification technology, its pulse energy and spectral bandwidth are also constantly improving, making the laser applicable in a wider range of fields, such as low coherence interferometry and supercontinuum generation [27,28].

In this paper, we constructed a two-port-output YDF laser that generated bidirectional pulses based on the NPR. The center wavelengths of the spectra in both directions are located at 1040 nm and 1026 nm, respectively. The laser has two output ports, with the emission from PBS1 corresponding to the clockwise (CW) direction and that from PBS2 to the counterclockwise (CCW) direction. The spectral bandwidths are 27 nm and 15 nm, respectively. The pulse energy in the CW direction is 5 nJ, and that in the counterclockwise (CCW) direction is 1.7 nJ. The fundamental repetition frequency is 46.04 MHz. Compared with the aforementioned NPR mode-locking fiber lasers, the laser constructed in this paper adopts a half-fiber, half-space structure, with a flexible and adjustable cavity structure. The spatial structure can be further adjusted and the fiber length shortened according to actual demand, making it possible for the laser to achieve a high repetition frequency. Benefiting from high energy and good beam quality, NLPs are suitable for applications such as low spectral coherence interference and supercontinuum generation.

2. Experimental Setup and Principles

Figure 1 shows the structure of the free-running two-port-output fiber laser. A half-fiber, half-space ring cavity design was employed, which can flexibly change the cavity length and add optical components as needed. Four semiconductor lasers (laser diodes, LDs) with a wavelength of 976 nm provided bidirectional pumping for the entire laser. After being combined in pairs through an optical coupler, the pump light was reflected into the 20 cm YDF by the wavelength division multiplexing collimator (col-WDM), thereby achieving population inversion. To avoid damage to the pump module by returning light, an optical isolator (ISO) was placed after the optical couplers. The remaining fiber consisted of a 3 m standard single-mode fiber (SMF) to provide a sufficient nonlinear phase shift. In addition, in order to increase the asymmetry of the cavity, the gain fiber was positioned as close as possible to one end of the WDM collimator (WDM-col). Two half-wave plates (HWP) and two quarter-wave plates (QWP) were deployed for polarization control in the cavity in the free-space. Two polarization beam splitters (PBSs) were deployed for coupling and output in the free-space optical path. Horizontal linearly polarized light was directly output through the PBS, while vertical linearly polarized light was reflected back into the cavity to form a loop. The polarization-dependent components used for NPR mode-locking form an equivalent Lyot filter within the cavity. The transmission of this filter exhibits periodicity with respect to wavelength, which can create multiple gain peaks within the broad gain spectrum of the Yb-doped fiber, thus supporting simultaneous lasing at two different wavelengths. Although the Lyot filter allows oscillation at multiple wavelengths, these wavelengths still have to share the same gain medium, resulting in gain competition. Due to the broad gain spectrum of the Yb-doped fiber, the CW and CCW directions typically select different wavelengths for oscillation to minimize gain competition.

The YDF provided a group velocity dispersion of 23 fs²/mm and had an absorption coefficient of 1200 dB/m at 976 nm. Both the Yb-doped fiber and other passive devices in the cavity provided positive dispersion at 1030 nm, and no dispersion management device was introduced. In this sense, the laser in the experiment operated entirely in the positive dispersion region.

3. Results and Discussion

Figure 2 shows the output powers in both directions under different pump powers, measured by a power meter (Thorlabs, PM 100, S121C, Newton, NJ, USA). As the pump power exceeded 100 mW, chaotic waveforms were observed on the oscilloscope in Figure 2. The output power increased synchronously with the pump power (indicated by the cyan region in Figure 2). When we increased the pump power to 673.6 mW, unidirectional mode-locking was achieved by rotating the wave plates, with mode locking occurring in the CCW direction but not in the CW direction. During this stage (indicated by the blue region in Figure 2), the laser output power in the CCW direction was at the level of hundreds of milliwatts, while the output power in the CW direction was only at the level of hundreds of microwatts.

The repetition rate of the laser was 46.04 MHz. When the pump power reached 1.44 W, the CW-direction power increased abruptly, while the CCW-direction power decreased. By rotating the wave plate to adjust the polarization in the laser cavity, stable mode-locking was achieved in both output directions (indicated by the red region in Figure 2), with the CW direction dominating the output. By carefully rotating the wave plate to adjust the intracavity polarization state, the bidirectional output laser was able to achieve self-started mode-locking. Under the stable bidirectional mode-locking, the output power in the CW direction was 223.5 mW, and the output power in the CCW direction was 79.9 mW. During stable bidirectional mode-locking, the output power in the CW direction is higher than that in the CCW direction. Higher output power indicates stronger nonlinearity. The stronger nonlinearity in the CW direction enables the formation of mode-locking pulses. Once the CW pulses are established, they induce mode-locking in the CCW direction through XPM. As a result, both directions can sustain mode-locking operations, leading to a bidirectional mode-locking state.

Gain position differences cause the asymmetric amplification of two beams, leading to differing nonlinear phase shifts in bidirectional pulses. Intracavity components like wave plates have directional polarization selectivity, and the cavity’s spatial structure inherently has asymmetry, resulting in different losses for pulses in two directions. These cumulative effects amplify the power difference between CW and CCW directions. Collectively, these factors lead to higher CW output power than CCW.

Figure 3a shows the spectrum in both directions by connecting the laser to the optical spectral analyzer (Thorlabs, OS201C, Newton, MA, USA). On a linear scale, the center wavelength of the CW direction in the linear scale was 1040 nm with a full width at half maximum (FWHM) spectrum bandwidth of 21 nm, while the center wavelength of the CCW direction was 1026 nm. It was observed that by changing the curvature radius of the fiber, the CCW direction spectrum appeared at multiple symmetrically distributed wavelength peaks. As the intracavity polarization state was adjusted by the wave plates, energy transfer occurred between different peaks. As shown in Figure 3a, the output spectrum in the CCW direction had higher energy in the short wavelength band and lower energy in the long wavelength band, which was due to the gain asymmetry of the YDF at 1030 nm. Wavelength radiation was also observed near 1070 nm. This was because shorter fibers favored shorter wavelengths near 1030 nm, while longer fibers favored longer wavelengths near 1070 nm. This was also related to the intensity of the pump power [29]. The spectrum exhibited irregular peaks and a broadened bandwidth, which was caused by the intra-cavity high power nonlinearity [30].

The pulse trains were detected with a 2 GHz bandwidth photodetector and recorded by a digital oscilloscope (RIGOL, MSO2302A, Suzhou, China) with a bandwidth of 300 MHz and a sampling rate of 2 GSa/s, as shown in Figure 3b. Due to the slight difference in the repetition frequency of the synchronously generated pulses in both directions, pulse walk-off occurs in the output pulse trains. It can be observed that the pulses are evenly distributed and stable, with essentially identical intensities. The time intervals between adjacent pulses were 21.7213 ns and 21.7215 ns in the two directions, respectively, corresponding to a fundamental repetition frequency of 46.04 MHz, which matches to the cavity length of 4.2 m.

During the detection process, it was observed that the radio frequency signal from the spectrometer exhibited random jitter. The internal structure of NLPs exhibits pronounced randomness and unpredictability, resulting in particularly prominent phase noise. Meanwhile, fluctuations in their temporal and intensity profiles lead to intrinsic timing jitter. These inherent properties of NLPs are likely the main cause of the observed random jitter. In addition, NPR is sensitive to environmental noise. Subsequently, these mechanical structures can be dismantled and replaced with silicon glass blocks integrated on the so-called “optical cubes” [31]. Systematic packaging can then be carried out to minimize the impact of mechanical jitter and temperature changes on frequency stability.

The autocorrelation traces of the output pulses in both directions were measured by the autocorrelator (APE, pulseCheck SM Type 2, Berlin, Germany), respectively, as shown in Figure 4. Both the CW and CCW directions output NLPs. In Figure 4a, the output pulse in the CW direction had a pedestal of 62 ps and a peak of 201 fs, and the inset shows an enlarged view of the femtosecond peak in the CW direction. In Figure 4b, the output pulse in the CCW direction had a pedestal of 52 ps and a peak of 212 fs, and the inset shows an enlarged view of the femtosecond peak in the CCW direction. A spike on the femtosecond time scale is usually located on the top of a broad pedestal in the picosecond range. The former represents the actual duration of the entire NLP packet. Although NLPs were generated or distributed randomly, they always ultimately form quasi-stable pulses. According to previous reports, most fiber output-based NLPs were generated in ring cavities. Based on the principle of NPR, Y. Jeong et al. [32,33,34] provided the transfer function $T_{NPR}$ of laser transmission characteristics in fibers.

$T_{NPR}$ is represented by the sine function, which indicates that by adjusting parameters such as gain, loss, and dispersion, the entire system is in a positive feedback state (the system’s transmittance increases with the increase in instantaneous power $P$) or a negative feedback state (the system’s transmittance decreases with the increase in instantaneous power $P$). D. Tang et al. [35] demonstrated via numerical calculations that in a net negative dispersion cavity, NLPs are formed from soliton collapse and positive cavity feedback. In a net positive dispersion cavity, under the modulation of high pump power, positive dispersion, and background noise, NLPs are generated by multiple round trips of the pulse through the ring cavity [36].

Although the coherence of NLPs is not as good as that of traditional negative dispersion dissipative solitons, their ultra-wide spectrum characteristics have broad application prospects in fields such as optical asynchronous sampling, optical coherence tomography, and sensor detection based on low coherence spectral interference. In subsequent work, by balancing the intracavity parameters, the laser will operate in a dissipative soliton state in both CW and CCW directions. The coherence and optical measurement resolution of the laser will be improved, and it will be applied to fields such as pump probes and spectral detection.

Figure 5 shows the evolution process of the spectra in two directions at a power of 2.2 W, with different polarization states. The intra-cavity polarization state was adjusted by rotating the wave plate in a certain order and direction. The spectral bandwidths under different polarization states are marked in Figure 5a,b. Both the spectral bandwidth and output power in the two directions showed a trend of first decreasing, then increasing, and finally decreasing. It was due to the fact that the pulses in the two directions shared a cavity. After experiencing a similar intra-cavity environment, the pulses exhibited coherence. Eventually, the output characteristics of the laser thus showed similar trends.

The output pulses from both directions were detected by two optical power meters and connected to a host computer for power monitoring, without any temperature or vibration control. The average output power of the laser for 8 h was shown in Figure 6. The RMS stability in both directions was 0.42% and 0.44%, respectively. The power drift in both directions was less than 1 mW over an 8 h measurement period. The slight power drift was attributed to diurnal temperature variations. It demonstrated the good stability of the bidirectional mode-locked output.

To further assess the stability of the laser output at 1037.1 nm and 1030.1 nm, the laser spectrum was repeatedly scanned at 10 min intervals over one hour, as shown in Figure 7a,c. As demonstrated in Figure 7b, the peak power fluctuation at 1037.1 nm was less than 0.14 dB, and the wavelength shift was less than 0.011 nm. As depicted in Figure 7d, the peak power fluctuation at 1030.1 nm was less than 0.33 dB, and the wavelength shift was less than 0.032 nm. These results ultimately indicate that the dual-port laser outputs exhibit a high degree of stability.

4. Conclusions

In this experiment, a two-port-output Yb-doped fiber laser was constructed based on the NPR, with an average output power reaching 223.5 mW, spectrum spanning 27 nm, and pulse energy greater than 5 nJ. The laser adopted a half-fiber, half-space ring cavity structure design, which allows the flexible adjustment of the repetition frequency and increases the possibility of achieving a high-repetition frequency single-cavity dual-wavelength fiber laser in the future. Going forward, the mechanical structure can be removed, and the optical components can be integrated on silicon glass blocks. Through systematic packaging and temperature control devices, the repetition frequency can then be stabilized using piezoelectric ceramic sheets to minimize noise. The fiber optic components in the cavity can also be replaced as needed, and the cavity structure optimized to alter the current output pulse state. The laser possesses a simple fiber and spatial structure, and exhibits excellent performance in terms of power stability, central wavelength, and spectral shape. Our work provides new perspectives for high-power, wide-spectrum, bidirectional emission fiber lasers, which are advantageous for applications in spectral measurement and precision optical metrology.