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Abstract
Microwave photonics is an important application for mode-locked lasers and requires the lasers to have high repetition rate and relatively low timing jitter. Although many mode-locked lasers based on two-dimensional (2D) materials have been reported, their applications in high-quality microwave photonics have not been achieved yet. We demonstrate a linear-cavity mode-locked Erbium-doped fiber laser with tungsten disulfide (WS2) saturable absorber (SA). By optimizing the WS2 SA fabrication, SA with low loss and moderate modulation depth is obtained. The laser based on this WS2 SA is demonstrated to have a fundamental repetition rate of 523 MHz, a central wavelength of 1558.6 nm, a 3-dB bandwidth of 7.4 nm and a timing jitter of 28 fs. With this laser, microwave photonic application of frequency synthesis is obtained with a tuning range up to 10 GHz and an extinction ratio greater than 60 dB in the whole range. We believe this is the first work that utilizes a 2D material SA based mode-locked laser for high-quality microwave photonic applications. This work indicates the potential of 2D material SA for high-speed and high-precision photonic and microwave systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, novel two-dimensional (2D) materials have attracted intense interest for their abundant photonic properties [1,2]. Saturable absorption is one of the most widely investigated properties. Many works of mode-locked and Q-switched lasers based on 2D material saturable absorber (SA) have been reported [2,3]. Briefly, these 2D materials include graphene [2,4–14], graphene oxide [15,16], topological insulators [17–19], transition metal dichalcogenides [20–31], black phosphorus [32–37] and many other new nanomaterials [38–43]. The laser operation wavelengths have been extended from visible to mid-IR region [33,37,44,45]. Gigahertz-level high repetition rates of mode-locked lasers have been demonstrated [7,18]. Moreover, all-optical signal processing based on various nanomaterials have also been investigated. Optical phase shifter [46–48], modulator [49–51], switch [47,48,52] and autocorrelator [53] are reported using thermo-optic effect, saturable absorption and other nonlinear effect.
Meanwhile, for many high-speed and high-precision photonic applications such as photonic analog-to-digital convertor, a mode-locked fiber laser with high fundamental repetition rate and low timing jitter is required [54]. High repetition rate enables high processing speed [55] and low timing jitter allows high processing precision [56]. Compared with harmonic mode locking, fundamental mode locking has a single pulse propagating in the cavity and avoids the discrepancy among multiple pulses in the cavity, which usually leads to better spectral purity and lower noise level. Also, operation wavelength near 1550 nm is preferred because mature optical devices for telecommunications can be directly used to build the system. Therefore, a fundamentally mode-locked fiber laser near 1550 nm with high repetition rate and low jitter is desired. A few such lasers with nanomaterial SA have been reported including carbon nanotubes [57], graphene [7] and MoS2 [29]. Other works based on III-V semiconductor SA and nonlinear polarization evolution have reached 1 GHz [58] and 517 MHz [59] near 1550 nm, respectively. Although graphene has been shown a successful SA for mode locking operation. It has not been utilized for microwave photonic application yet. Also it is still unclear whether other 2D material SAs can achieve comparable performance in high-repetition-rate and low-jitter mode locking. Demonstration of high-performance laser with 2D materials other than graphene can extend the applications of various novel 2D materials for high-speed and high precision microwave photonics systems.
In this paper, we report a 523 MHz mode-locked Erbium-doped fiber laser with tungsten disulfide (WS2) saturable absorber and its application in microwave photonics. The WS2 nanosheets are embedded in polyvinyl alcohol (PVA) thin film to form an SA. The SA has a modulation depth of 3.4% and a saturation intensity of ∼200 MW/cm2. The WS2-PVA SA is incorporated into a fiber linear cavity and the fundamental mode locking operation is achieved. The laser has a repetition rate of 523.1 MHz, a central wavelength of 1558.6 nm, a bandwidth of 7.4 nm and a timing jitter of 28 fs (1 kHz – 1 MHz). Moreover, microwave photonic application of high-purity microwave frequency synthesis using this laser has been demonstrated. Frequency up to 10.46 GHz is obtained and all the generated frequencies have an extinction ratio ≥60 dB. We believe this is the first work that utilizes a 2D material saturable absorber based mode-locked laser for high-quality microwave photonic application. This work indicates the potential of 2D material SA for high-speed and high-precision photonic and microwave systems.
2. Saturable absorber fabrication
The WS2 saturable absorber is a key component for the high-repetition-rate fiber laser. Our previous work has shown that saturable absorber with fast decay time can improve the noise properties of mode-locked lasers by suppressing the noise coupling inside the cavity [60]. Compared with conventional III-V semiconductor based saturable absorber, 2D materials typically exhibit faster decay process and thus have the potential to enable low-noise mode locking operation. However, due to the fabrication process, the transmission loss of 2D material SA (e.g., absorption, scattering, coupling, etc.) is not very good compared with the III-V semiconductor SA. This drawback counteracts the advantage of fast decay of 2D materials. In this work, we further optimize the fabrication process of WS2 SA to reduce the transmission loss while maintaining a moderate modulation depth.
The fabrication of WS2 SA includes two steps: preparation of WS2 nanosheets and fabrication of WS2-PVA thin-film SA. Liquid-phase exfoliation (LPE) method can produce large-volume nanosheets with high quality. Therefore, we choose LPE method to prepare WS2 nanosheets from commercial WS2 powders. The WS2 powders are exfoliated to few-layer nanosheets by ultrasonic processing in deionized water with sodium cholate (SC) as surfactant. Typically, 5 mg/mL WS2 powders are dispersed in 1.5 mg/mL SC aqueous solution and sonicated for 1 hour using a horn probe sonic tip (VibraCell CVX; 750 W) with 38% output power. Then the mixture is centrifuged at 3000 rpm for 90 minutes to separate nanosheets and un-exfoliated flakes. The upper part of the dispersion is collected by a pipette and the WS2 nanosheet dispersions are obtained. The un-exfoliated flakes are the main reason for scattering loss, so the centrifugation step is processed very carefully to remove these flakes. The existence of WS2 nanosheets in the dispersions is confirmed by transmission electron microscope (TEM), as shown in Fig. 1(a). The concentration of WS2 nanosheet dispersions is ∼0.016 mg/ml. WS2 nanosheets with the size of a few hundreds of nanometers can be observed. It is then mixed with PVA aqueous solution and dried on a glass plate at room temperature to form thin film, as shown in Fig. 1(b). The WS2-PVA thin film is finally cut into ∼2×2 mm pieces and transferred onto fiber end to function as an SA, as shown in Fig. 1(c). The Raman spectrum of WS2-PVA SA is measured at an excitation wavelength of 532 nm and shown in Fig. 1(d) to confirm the existence of WS2. The frequency difference between the in-plane vibrational mode E12g (354.7 cm−1) and the out-of-plane vibrational mode A1g (420.7 cm−1) is ∼66.0 cm−1. It is known that this frequency difference is related to layer number of WS2 nanosheets due to the anomalous lattice vibrations of the layered 2H-WS2. The value of 66.0 cm−1 indicates the number of monolayers to be 3-5 [61]. The results demonstrate that high quality 2D WS2-PVA thin film is fabricated.
Fig. 1. (a) TEM image of WS2 nanosheets. (b) Photo of WS2-PVA thin film. (c) Photo of WS2-PVA SA transferred on fiber end. (d) Raman spectrum. (e) Saturable absorption.
The saturable absorption of the WS2-PVA SA has also been characterized, as shown in Fig. 1(e). A typical two-arm system is used. The output of a homemade 1550-nm mode-locked laser (37 MHz repetition rate, ∼250 fs pulse duration and ∼1 kW pulse peak power) is divided to two arms with 99:1 power split ratio. Light with 1% power in one arm is directly measured by a power meter as reference. Light with 99% power in the other arm is injected in to the SA and then measured by a second power meter. By comparing the readings in two power meters, the nonlinear transmission property of SA, i.e. saturable absorption, can be obtained. More details of the two-arm system can refer to our previous work [3]. In Fig. 1(e), the transmission (absorption) of the SA increases (decreases) with the increase of input optical intensity, which clearly indicates the saturable absorption of the sample. The modulation depth is ∼3.4%. The saturation intensity is ∼200 MW/cm2. The nonsaturable loss is ∼24% which is due to the scattering and absorption of the residual WS2 flakes and PVA thin film.
3. High-repetition-rate mode-locked laser
The experimental setup of the mode-locked fiber laser is shown in Fig. 2(a). The linear cavity design is adopted to allow short cavity length and high fundamental repetition rate. The cavity consists of ∼11 cm Erbium-doped fiber (EDF, LIEKKI Er80 8/125) and ∼8.5 cm standard single mode fiber (SMF). The EDF has an anomalous dispersion of ∼−22 ps2/km. The net cavity dispersion is ∼−0.0043 ps2 and the laser is designed to operate in the soliton mode-locking regime. On the left end of the cavity, the EDF is spliced with a short piece of SMF and glued to a standard ceramic ferrule, as shown in Fig. 2(b). The ferrule end facet is polished and connected with a coated fiber FC/PC connector using a fiber adapter. This FC/PC connector has a dichroic coating with 10% transmission near 1550 nm as output coupler and high transmission at 980 nm for pump coupling. On the right end of the cavity, the WS2-PVA SA is sandwiched between a fiber ferrule and a dielectric mirror, as shown in Fig. 2(c). The fiber ferrule has a high reflection coating at 980 nm (∼95%) to improve the pumping efficiency and protect the SA from being damaged by the residual pump. The dielectric mirror has a high reflection near 1550 nm (∼98%). The cavity birefringence is adjusted by a compact fiber polarization controller (PC). Outside the cavity, a 980/1550 wavelength division multiplexer (WDM) is used to combine the 976-nm pump light and the laser output near 1556 nm. The laser output is characterized by optical spectrum analyzer (Yokogawa AQ6370C), autocorrelator (Femtochrome 103XL) and signal source analyzer (Rohde & Schwarz FSUP50), etc.
Fig. 2. (a) Laser design. (b) Detailed structure of the left end of the cavity. (c) Detailed structure of the right end of the cavity with WS2-PVA SA. WDM: wavelength division multiplexer; EDF: Erbium-doped fiber; PC: polarization controller; SA: saturable absorber.
The mode locking operation is obtained when the injected pump power is increased to ∼250 mW. The mode locking operation becomes unstable when the pump power is further increased up to 390 mW and beyond. The output optical spectrum at the pump power of 350 mW is shown in Fig. 3(a). The central wavelength is 1558.6 nm and the 3-dB bandwidth is 7.4 nm. Kelly sidebands are not observed in the spectrum which is a typical feature for the lasers with short cavity and weak cavity perturbation [29,57,58]. The relation between the injected pump power and the laser output power is provided in Fig. 3(b). The pump power threshold of mode locking operation is ∼250 mW which is higher than conventional 2D materials based mode-locked fiber lasers with long cavity. This is because the cavity is short and the cavity dispersion and nonlinearity are both small. As a result, the cavity requires a higher intra-cavity power to initiate the mode locking operation. The slope efficiency is ∼2.5%. At a pump power of 350 mW, the output power is 8.7 mW. The maximum output power of mode locking state is ∼9 mW, corresponding to a maximum output pulse energy of ∼17.2 pJ.
The time-domain properties are then investigated, as shown in Fig. 4. The laser output is measured by a 10-GHz photodetector (EOT 3500F) and a 2-GHz oscilloscope (Agilent DSO9254A). The measured waveform is shown in Fig. 4(a). The period is ∼1.9 ns, corresponding to a repetition rate of ∼523 MHz. This period is consistent with the round-trip time of the 19.5-cm laser cavity. In the oscilloscope trace, there are some ringing parts after the falling edge of the pulses. They are due to the slight impedance mismatch among the photodetector, cable and oscilloscope. The autocorrelation trace of the output pulse is also measured, as shown in Fig. 4(b). Only a single pulse profile is observed and no bound-state pulses are observed. The autocorrelation trace has a width (FWHM) of 711 fs and is well fitted by a soliton profile (sech2). The corresponding soliton pulse width is 461 fs. The time-bandwidth product is 0.42. The pulse has a negative chirp due to the pigtail fiber outside the laser cavity. In the laser, the WS2 SA can sustain a few months during the whole experiment, and in each single experiment of ∼3 hours the mode locking state can be successfully maintained, which also indicates the good stability of the saturable absorber.
The frequency-domain properties are also studied, as shown in Fig. 5. The RF spectrum is measured by an RF spectrum analyzer (Keysight N9000B). Figure 5(a) shows the RF spectra near the fundamental frequency of 523 MHz with a span of 10 MHz and a resolution bandwidth (RBW) of 100 Hz. An extinction ratio of ∼89 dB is obtained. The inset shows the same spectra with a smaller span of 10 kHz and an RBW of 10 Hz. The RF spectrum up to 5 GHz is presented in Fig. 5(b). In the whole spectrum, no noise spurious peak is observed which indicates that the laser operates in a stable and low-noise state and has good spectral purity. To further study the timing noise of the laser, the phase noise is measured by a signal source analyzer, as shown in Fig. 6. At offset frequency 1 kHz, the phase noise spectrum value (power spectral density) is ∼−115 dBc/Hz. The integrated timing jitter is also calculated based on the phase noise spectrum. It can be found that the timing jitter from 1 kHz to 1 MHz is ∼28 fs. Further improvement on the SA nonsaturable loss and coupling efficiency may help to reduce the timing jitter.
Fig. 5. (a) RF spectrum near 523 MHz with 10 MHz span and 100 Hz RBW. Inset: RF spectrum near 523 MHz with 10 kHz span and 10 Hz RBW. (b) RF spectrum up to 5 GHz.
4. Microwave photonic application
Fundamentally mode-locked lasers provide a high-purity RF spectrum after photodetection, which has the potential in microwave photonic application to generate tunable microwave frequency with high extinction ratio in a wide span. In this section, we use our 523 MHz mode-locked laser to obtain high-purity microwave frequency synthesis from 0.52 GHz to 10.46 GHz.
The experimental setup is shown in Fig. 7(a). A fiber Mach-Zehnder interferometer (MZI) with slightly unequal arm length is constructed. A fiber spool of 16 km fiber is used to provide high net dispersion and stretch the optical pulses. In the upper arm of MZI, a tunable delay line (TDL) is used to precisely adjust the delay between two arms. In the lower arm, a variable optical attenuator (VOA) and a PC are inserted to balance the loss and control the polarization state between two arms. This structure is also known as a chirp-mixing structure, which performs as a narrow band-pass filter in the microwave domain [62,63].
Fig. 7. (a) Experimental setup for microwave generation with 532-MHz mode-locked laser. (b) Principle of microwave generation
The principle can be described by Fig. 7(b). Fiber spool linearly stretches the optical pulses. Then the pulses are split and delayed in the MZI. The left panel in Fig. 7(b) shows the frequency-time map of pulses in the two arms. When they are re-combined after MZI, two optical frequency components overlap in time domain. After photodetection, as shown in the right panel in Fig. 7(b), the beating microwave frequency fc is equal to the difference between two optical frequencies and its value is determined by the delay between two arms. Therefore fc is the center frequency of the band-pass filter in microwave frequency domain and is approximately given by [62]
where Δt is the delay between two arms, β2 = −22 ps2/km is the second-order dispersion parameter of fiber and L = 16 km is the fiber length. By changing the delay, different frequency components can be chosen, shown as the dashed lines in Fig. 7(b). The filter profile (power) is given by [62]
Table 1. Extinction ratio values for the generated microwave frequencies
For the mode-locked laser with a repetition rate fR, the phase noise of the generated microwave frequency at kfR (k is a positive integer) is related to the phase noise of the laser by [64]
where Sph,m and Sph,l are the phase noise spectra of the microwave frequency and laser, respectively. Δf is the offset frequency. The measured phase noise spectra of the generated microwave are shown in Fig. 9(a) following the relation of Eq. (3). The integrated timing jitters (1 kHz – 1 MHz) are shown in Fig. 9(b).5. Conclusions
We have demonstrated a mode-locked Erbium-doped fiber laser with WS2 saturable absorber and its application in microwave photonic application. The WS2 SA has a modulation depth of 3.4% and a saturation intensity of ∼200 MW/cm2. The laser operates at fundamental mode locking state with a repetition rate of 523 MHz. The laser has a central wavelength of 1558.6 nm, a 3-dB bandwidth of 7.4 nm and an output power of 8.7 mW. The laser operates at a low-noise state with an extinction ratio of 89 dB in the RF spectrum and a timing jitter of 28 fs (1 kHz – 1 MHz). In the demonstration of high-quality microwave photonic application, high-purity microwave frequencies up to 10.46 GHz have been synthesized with more than 60 dB extinction ratio. Our work indicates the potential of novel 2D material devices such as WS2 saturable absorber for high-speed and high-precision microwave photonic applications.
Funding
National Natural Science Foundation of China (61535006, 61675217, 61875122, 61922056); Program of Shanghai Academic Research Leader (17XD1403900); Chinese Academy of Sciences Key Technology Talent Program (XDB16030700); Chongqing Research Program of Basic Research and Frontier Technology (QYZDB-SSW-JSC041).
Disclosures
The authors declare no conflicts of interest.
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