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Abstract
All-optical modulator is a crucial device in next generation of all-optical communications, interconnects, and signal processing. Here, we demonstrate an all-optical phase modulator with graphdiyne (GDY)-deposited microfiber structure. The phase shift of the signal light can be readily controlled by pump light by thermo-optic effect. This all-optical modulator can achieve a phase shift slope of 0.0296 π·mW−1 and a rising time of 5.48 ms at 25 Hz (3 ms, 50 Hz). Modes distributions in GDY-deposited microfiber at different wavelength are numerical analyzed and the normalized phase conversion efficiency of GDY are calculated. The results show that GDY has a considerable normalized phase conversion efficiency of 0.1644 π·mW−1·mm−1, which is higher than that of graphene, MXene and WS2 based all-optical modulators. This work proves the potential of GDY in all-optical modulator device at telecommunication band and provides a support to all-optical signal processing systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical modulator is an essential device in optical communication systems and optical information processing [1,2]. The optical modulators can be divided into different types, such as magneto-optical modulators, electro-optical modulators, acousto-optic modulators and all-optical modulators, which are widely used in different fields. In optical fiber communication systems, electro-optical modulators as the mainstream modulator have been fully investigated and developed, such as lithium niobate waveguide modulators, silicon-based modulators and so on. However, due to the ‘electronic bottleneck’, these modulators meet huge challenge for the explosive bandwidth and rate requirements in future optical communication systems. Otherwise, the large size of the device is disadvantageous for more intensive device integration. Thus, developing small size all-optical modulation devices may be a key to solve this problem.
2D materials have advantages of mechanical flexibility, easy fabrication, integration, and robustness [3], which is a strong candidate for many integrated devices. Besides, the novel electrical, thermal and optical properties such as ultrafast carrier relaxation time, excellent thermal-optic conversion efficiency, and large third nonlinearity also verify its potentiality in all-optical modulation. Recently, by taking advantage of the evanescent field in microfiber covered with 2D materials, a simple and effective way to realize all-optical modulation are demonstrated [4–11]. For example, due to the ultrafast carrier relaxation time (∼2.2 ps) and large third nonlinearity, graphene-based ultrafast all-optical modulator with high response time have been reported [7,10]. Also, because of the excellent thermo-optic conversion efficiency and thermal conductivity [12], graphene has been already widely used in thermal-optic modulators. In 2015, the first all-optical thermal phase modulators based on graphene were reported [9]. Due to the considerable thermo-optical conversion efficiency, the modulator achieved a phase conversion efficiency of 0.091 π·mW−1 and 3.2 ms response time at 20 Hz. Although the all-optical thermal phase modulators have slow response time due to low heat accumulation and dissipation speed, it owns relatively high phase shift and modulation depth origin from the interference structure, which may be hopefully used in next generation low-speed information switching areas. In addition to graphene, all-optical thermal phase modulator based on other 2D materials like black phosphorus [6], WS2 [8], and antimonene [4] have also been reported and show considerable performance.
Graphdiyne as a novel carbon allotrope with graphene, joins in the 2D materials family in recent years and owns excellent physical and chemical properties. It has been reported abundant applications including in separation, purification, energy storage and transfer, catalysts, biomedicine and therapy, and optical detectors [13–18]. Also, GDY has a natural direct bandgap of 0.44-1.47 eV, high carrier mobility at room temperature [19] and excellent nonlinear optical properties, which endow its potential to realize broadband all-optical devices. In 2018, Guo et al. measured a large nonlinear absorption coefficient that more than β > –1 cm·GW−1 with the Z-scan method and demonstrated a mode-locked fiber laser at 1070, 1557 and 1880 nm bands [20,21]. In 2019, Wu et al. experimentally measured nonlinear refractive index n2 ≈ 1×10−5 cm2·GW−1 of GDY with the SSPM method [22]. Additionally, the comparable linear absorption of GDY (1.7% for monolayer) with graphene (2.3% for monolayer), high thermo-optic conversion efficiency and thermal conductivity [23] make GDY a great thermo-optic material compared with graphene. It has been reported that GDY can be used as photo thermal therapy for tumors [24,25]. Similarly, GDY-based all-optical thermal modulator may have an excellent performance. However, as we know, GDY-based all-optical thermal modulator has not been reported up to now and deserved to investigate.
In this work, we demonstrate a GDY-based all-optical thermal phase modulator. The modulator realizes a phase shift of 0.0296 π·mW−1 and a rising time of 5.48 ms at 25 Hz (3 ms at 50 Hz). The maximum modulation frequency of our modulator can reach 1000 Hz. We numerically analyze the realization principle by a finite element analysis technique and observe the heating change by thermal imaging, which verify the modulation is caused by thermo-optic effect of GDY. Our modulator has a normalized phase conversion efficiency of 0.1644 π·mW−1·mm−1, which is higher than that of graphene, MXene and WS2 based all-optical modulators and can be expected to be utilized in next generation of all-optical communication systems.
2. Experimental
The GDY nanosheets are exfoliated from large area GDY film grow on copper foil [26] and prepared by using the standard liquid-phase exfoliation (LPE) method [27]. Add 10 mL ethanol solution to the centrifuge tube containing 5 mg GDY powder to prepare 0.5 mg·mL−1 GDY dispersions. The dispersions are then sonicated for 3 hours by bath sonication method and centrifuged at 1500 rpm and 3000 rpm for 90 minutes to remove the unexfoliated flakes. The top 1/2 of the dispersions are collected by another tube. The prepared GDY dispersions has a concentration of 0.05 mg·mL−1. As shown in Fig. 1(a), nanosheets are measured with a transmitted electron microscope (TEM). The crystalline phase of exfoliated nanosheets can be confirmed by the high-resolution transmission electron microscope (HRTEM, Fig. 1(b)) and electron diffraction pattern (inset of Fig. 1(b)), which accord with the characteristics of GDY. Through measuring the Raman spectrum of GDY nanosheets (Fig. 1(c)), we can further confirm the characteristic peaks of GDY, which is compatible with Ref. [19]. Figure 1(d) shows the atomic force microscope (AFM) image of GDY nanosheets, in which we can see that the size and the thickness of GDY nanosheets are about 1∼2 µm and 100∼200 nm, respectively. We plot the thickness curves of randomly selected nanosheet 1 and nanosheet 2 in inset of Fig. 1(d), in which the thickness are 151 nm and 154 nm, respectively. Furthermore, we count thickness of 100 GDY nanosheets and get the statistics distribution as shown in Fig. 1(e). The average thickness of the nanosheets is about 150 nm. UV-vis absorption spectroscopy is employed to characterize the optical property of GDY, as shown in Fig. 1(f). Here, we use NMP (N-Methylpyrrolidone) as dispersion solvent to prepare GDY dispersions. It is clearly shows that GDY have a broadband absorption ranging from 600 to 1600 nm, implying their potentials for applications in visible, near-infrared and communication devices.
Fig. 1. Characterizations of GDY. (a) TEM image, (b) HR-TEM images, the inset in (b) shows the FFT pattern of a square-enclosed area, (c) Raman spectrum, (d) AFM topographic image and (e) Thickness distribution of GDY flakes measured in AFM, (f) UV-vis absorption spectrum of GDY dispersions.
The GDY-deposited microfiber is the key element in the all-optical modulator. We prepare GDY-deposited microfiber by using the optical deposited method [28], as shown in Fig. 2(a). A single mode fiber (SMF-28) is tapered to microfiber with a diameter of 7.5 µm and then immersed in GDY dispersions. The length of taper is about 2.2 mm. The output power of the microfiber is monitored by an optical power meter to control the deposition process. We inject 20 mW light into the microfiber. According to experience, when the light power drops to 20%, stop the deposition process immediately and dry the sample for 24 hours. After drying for one day, the transmittance of the deposited microfiber in 1550 nm band down to 11%. An optical microscope image of prepared GDY-deposited fiber is shown in Fig. 2(a). We count the area that depositing GDY and obtain an effective interaction length, which is approximately about 180 µm.
Fig. 2. (a) Schematic diagram of optical deposition method and optical microscopic image of the GDY-deposited microfiber, (b) Experimental setup of the all-optical modulator.
The experimental setup of a Mach-Zehnder all-optical modulator is shown in Fig. 2(b). Because GDY has a broadband absorption from 600 to 1600 nm and the most mature fiber laser technology in near-infrared wavelength, we choose 980 nm laser as pump light and 1550 nm laser as signal light. The signal light at 1550 nm (linewidth of 80 nm) with partial polarization and pump light at 980 nm (linewidth of 10 nm) with no polarization are combined by a wavelength division multiplexer (WDM) and input to an MZI system. In the MZI system, the combined lights are split into two beams by a 90:10 coupler to generate a high contrast spectrum. The length difference of two arms in our interferometer is about 2 mm, corresponding free spectrum range (FSR) of 0.8 nm at 1550 nm. The light with 90% intensity goes through in GDY-deposited microfiber, which will absorb the pump light and signal light. Due to the loss of WDM and 90:10 coupler, the maximum pump power entering microfiber can achieve 80 mW (has WDM1 and 90:10 coupler) /130 mW (only has 90:10 coupler). Two beams will interfere in a 50:50 coupler and generate interference spectrum, and pump light is filtered by WDM2. The fiber type used in this system is all common single-mode fiber (SMF-28). The interference spectrum is detected by an optical spectrum analyzer (Yokogawa AQ6375B). To record the modulated signals, we use a 1550 nm (bandwidth of 0.3 nm) bandpass filter to narrow the bandwidth of signal light. The modulated signal is transferred to an electrical signal with an avalanche diode (APD) and detected by an oscilloscope (Tektronix TDS 2022C).
3. Results and discussions
As demonstrated in Fig. 3(a), we observe a π shift of the interference spectrum when input pump power from 0 to 34 mW. The phase shift increases linearly with a slope efficiency of 0.0296 π·mW−1 as shown in Fig. 3(b), confirming an all-optical phase modulation. The deviation from linear dependence when pump powers larger than 60 mW mainly originate from pump source. When the pump power on samples ups to 60-80 mW, our pump source reaches the maximum power area and become a little instability. Accordingly, the measured interference spectrums generate some vibration and cause influence in measurement results. Thus, we calculate the mean squared error for our phase shift experiment and for the obtain value of the phase shift slope are 0.0125 π and 0.0012 π·mW−1, respectively, which shows a small value and verify the credibility of slope. The maximum phase shift ∼ 3π is obtained. We also do a comparison experiment using the bare microfiber as shown in Fig. 3(b), which doesn’t show apparent phase shift. This result indicates the signal light at output modulated by the varied pump light. Limited by the maximum output power of our laser diode and system loss of 40%, we can’t achieve a higher phase shift. But the result still proves the realization of GDY-deposited fiber as a phase shifter. Furthermore, all-optical information processing is demonstrated. A modulated pump light (34.8 mW average power) and unmodulated signal light (5 mW) are input into the proposed system together. The repetition rate of the square wave is 25 Hz. A clear on-off operation of the switcher is realized, and the frequency and duty cycle of the signal light is matched with the pump light, as shown in Fig. 3(c). The extinction ratio of our MZI system is ∼ 2 dB, which is due to the difference of output power in the two interference arms. By adding a variable attenuator (VOA) into one arm, we can achieve a maximum extinction ratio of 10 dB. The rising time of modulator measured by oscilloscope is 5.48 ms at 25 Hz and 3 ms at 50 Hz, respectively. The rising edge and falling edge of signal light are smooth to some extent in comparison to the square wave of pump light, which can be contributed to slow heat generation and dissipation. As gradually improving the modulation frequency from 25 Hz to 1000 Hz, the waveform becomes sharper and modulation amplitude is smaller, as shown in the Fig. 3(d). Because the noise of APD is about 1 mV, the max modulation frequency is 1000 Hz, and the higher modulation frequency can’t be detected over than 1000 Hz.
Fig. 3. (a) Measured interference spectrum with increasing pump power, (b) Phase shift versus the pump power, (c) Measured output light modulated by control light, (d) Various signal amplitude at different modulation frequency.
Because of the relative slow modulation speed, this may be an all-optic modulator based on thermo-optic effect like Ref. [9] reported. According to the thermo-optic phase modulator model, the phase change Δϕ is determined by the temperature change ΔT, which can be described as [9,29]:
where Δϕ is the phase that we want to modulate. dn/dT = 1.1×10−5 K−1 is thermo-optic coefficient of silica, L is the interacted length in materials, λ is the wavelength of modulated light, ΔT is the temperature change of modulator. The temperature distribution T(r) in the microfiber can be described with the steady-state heat conduction equation $-\textrm{k}{\nabla ^\textrm{2}}\textrm{T}(\textrm{r} ){\; = \; }{\textrm{Q}_\textrm{s}}$ [9], where k ≈ 1.38 W/m/K is the thermal conductivity of silica, ${\textrm{Q}_\textrm{s}}{\; = \; }\frac{\textrm{1}}{\textrm{2}}{\mathrm{\varepsilon }_\textrm{0}}\mathrm{\omega }\textrm{Im}\{{{\mathrm{\varepsilon }_\textrm{r}}} \}\textrm{|E}{\textrm{|}^\textrm{2}}\; $is the heat source per unit volume generated by GDY absorption [9,30], ɛ0 is the dielectric permittivity of vacuum, ω is the angular frequency of light, |E|2 is light intensity interacted with materials and ɛr is the relative permittivity of GDY. To solve this equation, we need to introduce a boundary condition ${-}\textrm{k}\nabla \textrm{T}(\textrm{r} ){\textrm{|}_{\textrm{r = }{\textrm{r}_\textrm{0}}}}{\; = \; h(T)(T(}{\textrm{r}_\textrm{0}}\textrm{) - }{\textrm{T}_\textrm{0}}\textrm{)}$ [9], where h(T) is the heat transfer coefficient of air, T(r0) is the temperature at the fiber edge and T0 is the room temperature. From the heat transfer equation and boundary condition, we find that the change of temperature ΔT is correlated with the diameter of fiber, h(T), ɛr and λ.We compare some typical performance indicators of thermal-optic phase modulators based on different 2D materials, as demonstrated in Table 1. In Table 1 we can obviously see that our modulator has a higher phase shift slope than that of WS2 and Boron based modulator, but relative lower than that of Graphene and MXene based modulators. That is mainly owe to short deposited length of materials. When considering the normalized phase conversion efficiency of materials, our modulator shows a higher value than that of graphene, WS2 and MXene, which indicates GDY is an excellent thermo-optic material that can be used in the next generation all-optical modulators.
Table 1. Typical 2D materials all-optical modulators based on thermo-optic effect.
To further understand the physical mechanism of the modulation process, we numerically analyze the laser-pumped thermal process in the GDY-deposited microfiber by using a finite element analysis technique (COMSOL Multiphysics). In the simulation model, a 150 nm thick GDY layer is located at the surface of a 7.5 µm thick microfiber. For GDY’s refractive index, we use a value of 2.01 + 0.9i [33]. The two-dimensional model distribution of 980 nm is shown in Fig. 4(a). The mode distribution conforms the Gaussian distribution of single-mode fiber and white line means location of GDY. Obviously, GDY is interacted with 980 nm light. In Fig. 4(b), we plot the radial distributions of modes at wavelengths of 980 and 1550 nm. It can see an abrupt variation due to the index discontinuity at two different wavelengths, which is owed to light absorption of GDY. We calculate the propagation attenuations of the two modes from their complex propagation constants, which are α980nm = 14.9 dB/mm and α1550nm = 51.2 dB/mm for the modes at 980 and 1550 nm, respectively. The calculated values are close to the attenuations we measure (11% transmission at 1550 nm is almost ∼ 53.2 dB/mm), which verify correctness of our model. The effective interacted length between GDY and fiber can be calculated as Leff = log(T1550nm)/α1550nm ≈ 0.187 mm, which is also close to our deposited length. It is worth mentioning that the discontinuity at 1550 nm is more obvious than that at 980 nm, which is mainly due to the larger mode field diameter at 1550 nm than that at 980 nm and will cause a higher loss in 1550 nm. The light intensity of the GDY surface accounts for 3% and 10% of the total light intensity at 980 nm and 1550 nm, respectively. According to the distribution of intensity, the light intensity on the GDY surface is roughly calculated as 6.7×104 W·cm−2 at the maximal pump power (80 mW/980 nm). That means a weak light intensity interacted with GDY and can’t stimulate the intrinsic third nonlinearity (usually acquire millions of MW·cm−2). Therefore, we consider the main modulation origin from that GDY absorbs light energy and converts it into heat, which is called the thermo-optic effect, and leads to the thermal phase change of the fiber core.
Fig. 4. (a) Mode distribution of 980 nm light when covering GDY (GDY is indicated by white line), (b) Mode distributions in the middle line, (c) Infrared thermograms of the microfiber at 10 mW and 130 mW respectively, (d) Temperature change versus pump power.
We also explore the temperature change to verify the thermo-optic effect, and the thermal images are shown in the Fig. 4(c). The ambient temperature is 25.7 °C, and the temperature of 26.0 °C at 10 mW and 31.8 °C at 130 mW that measured by thermograms. Figure 4(d) shows the temperature change versus pump power measured by thermal image. A slope of 0.05 °C·mW−1 is fitted. The result shows that a significant temperature increased when the high pump power input, which can’t be found in bare microfiber. It tells that the thermal generation of modulator is owed to GDY, which acts as a heater to heat the fiber and modulate refractive index of the fiber core.
4. Conclusions
In conclusion, we realize an all-optical thermal phase modulator by using GDY-deposited microfiber structure. By absorbing the pump light at 980 nm, a strong light-matter interaction happened in deposited field, which heats the core of microfiber and shift phase. We obtain a phase shift slope of 0.0296 π·mW−1 and normalized phase conversion efficiency of 0.1644 π·mW−1·mm−1. The modulator also has a considerable speed of response, showing a rising time of 5.48 ms (3 ms) in 25 Hz (50 Hz). The theoretical and experimental results show that our modulator has a comparable performance with graphene, MXene and WS2 based modulators. Our work demonstrates that GDY is an excellent 2D material that using in all-optical thermal phase modulators.
Funding
National Natural Science Foundation of China (8196130108); Shenzhen Science and Technology Innovation Program (JCYJ20170302153323978, JCYJ20170410171958839).
Acknowledgements
The authors thank to Dr. Feng Zhou for his valuable discussion.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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