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Abstract
Light manipulation for all-fiber devices has played a vital role in controllable photonic devices. A graphene-based metasurface is proposed to realize light manipulation. A row of VCSEL-based optical engines with low crosstalk is used as the control light to modulate the signal transmitted in the microstructured fiber. In this configuration, the proposed device can work independently of the wavelength division multiplexing (WDM) system. With an insertion loss of only 0.28 dB, evanescent wave coupling to graphene layers is polarisation-insensitive. The device could be effectively manipulated for a few days (not less than 72 hours), which possesses the capacity to dynamically modulate the signal light with both low-temperature sensitivity and low-wavelength sensitivity. The 35 nm wavelength interval results in a change of only about 0.1 dB in the output light intensity of the microstructured fiber when the wavelength changes from 1530 nm to 1565 nm. Moreover, the modulation depth is approximately 2 dB when the modulating voltage is 2.2 V, which may open avenues for channel detection techniques and have deep implications in top tuning applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light manipulation has played a vital role in controllable photonic devices [1,2]. In contrast to light manipulation devices based on planar waveguides [3–10], the device based entirely on optical fibers can be conveniently integrated into current optical systems, such as fiber-optic communications and sensing systems. Taking advantage of the mature fiber-based platform, leakage of the optical field can be achieved by machining tiny structures on the surface of the optical fiber. Microstructured fibers such as side-polished [11–13] and tapered fibers [14,15] have a controllable evanescent field, providing an ideal platform to realize an all-fiber light manipulation device. Mohanraj et al. demonstrate an all-fiber temperature sensor using molybdenum disulfide (MoS2) nanosheets coated side-polished fiber (SPF) [16]. The spectral output of this device changes as a function of temperature. In 2019, Yang et al. present a novel compact all-optical fiber phase shifter [17]. The refractive index around the tapered microfiber was varied to reversibly shift the interference dips. Most recently, all-optical light manipulation arising in a graphene-embedded SPF was designed and investigated, which has achieved all-optical signal processing [18]. Although the previously proposed devices can achieve dynamic light manipulation, their optical performances rely on wavelength division multiplexing (WDM) systems. The introduction of one or more WDM systems for the operation of the device will complicate the optical system. The actual operating loss of the optical system will also be much greater than the insertion loss of the device.
The potential for a compact and low insertion loss controllable photonic device is expected to be implemented using 2D materials. Being atomically thin, graphene [19,20], black phosphorus (BP) [21,22], MXene [23,24], and other two-dimensional materials are highly flexible for incorporation into different photonic structures. Graphene is a very promising material for photonic and optoelectronic applications due to its unique electronic structure. One of the fascinating features of graphene is that its optical transition properties can be significantly modified by the application of external fields (e.g., optical carrier excitation leading to Pauli blocking [14,25] or electrically biased Fermi level tuning [26–29]). Taking advantage of the Pauli blocking effect, a graphene monolayer exhibits a constant absorption coefficient of 2.3% over a wide spectral range from the visible to the infrared [30]. Compared to pure graphene, graphene-based metasurfaces can generate special electromagnetic responses [31,32]. With high field confinement, wavefront shaping can be achieved by the design of the control light and metasurface structures. Without a WDM system, the control light to excite the graphene carrier can no longer be used with a conventional fiber laser system. Vertical cavity surface-emitting lasers (VCSELs) are premium optical engines with high-density electrical and optical assembly [33,34]. Incoherent VCSEL arrays provide tunable output beam profiles without beam quality degradation due to the extremely weak intrinsic optical coupling. As a result, the applications of VCSEL arrays can be further extended to fields of beam steering, free space optical interconnection, and multichannel communication [35,36].
Herein, we propose a graphene-based metasurface to realize light manipulation via the Pauli blocking effect arising in an all-fiber device. The metasurface composed of graphene-poly(methyl methacrylate) (PMMA) composite film enables significant light−graphene interaction by creating a tightly confined evanescent field guided along the surface of the microfiber. A row of VCSEL-based optical engines with low crosstalk is used as the control light to modulate the signal transmitted in the microstructured fiber. In this configuration, the proposed device can work independently of the WDM system. Two-dimensional finite-element method (FEM) is utilized to calculate and investigate the characteristics of the phase distribution in the transmission of control light. The results show that the sub-wavelength (approximately half of the wavelength) thickness of the metasurface ensures the high quality of graphene and minimizes the influence of graphene wrinkles on modulation. In particular, the study concerns a device that could be effectively manipulated for a few days (not less than 72 hours), which possesses the capacity of both low-temperature sensitivity and low-wavelength sensitivity. The 35 nm wavelength interval results in a change of only about 0.1 dB in the output light intensity of the microstructured fiber when the wavelength changes from 1530 nm to 1565 nm. With an insertion loss of only 0.28 dB, our design ensures good polarisation-insensitive characteristics. The modulation depth is approximately 2 dB when the modulating voltage is 2.2 V, which may open avenues for optical logic gates [37,38], channel detection [39,40], and top tuning applications [41,42].
2. Structure and design of the light manipulation device
The experiment setup is schematically illustrated in Fig. 1. A linear array of VCSELs driven by an electrical signal input from the radio frequency (RF) coaxial connector. The RF module used in this application is compliant with the Sub Miniature Version A (SMA) standard. Illustrations in Fig. 1 display the microscope image of the VCSEL array (#1) and microstructured fiber (#2), respectively. As shown in inset #1, the length L of each VCSEL and the spacing P between the VCSELs are 200 µm and 50 µm, respectively. A wire bonding technique is used to connect the gold wires that provide the operating voltage to the VCSEL array, and the diameter of the wires is limited to 5 ± 0.2 microns to avoid bond collapse or pad breakage. In our experiment, the microfiber was obtained by tapering a section of a single-mode fiber (SMF 28, Corning Inc.) into 8 µm in diameter. The length L0 of the uniform diameter tapered section is 10 mm, and the graphene-based metasurface is directly wrapped around the tapered section of the microfiber, as displayed in inset #2. Flame pulling of the fiber can cause the signal light to leak out of the core without disruption of the fiber path. The weak evanescent wave generated by the signal light propagates along the graphene-based metasurface and undergoes significant absorption attenuation.
Fig. 1. Schematic of the light manipulation device, which consists of a linear array of VCSELs and a graphene-based metasurface coated microfiber. Illustrations display the microscope image of the VCSEL array (#1) and microstructured fiber (#2), respectively.
At the defined molecular weight, PMMA can be used as a highly efficient e-beam resist for nanolithography. It adheres strongly to silicon, metals, and two-dimensional materials. In the present experiment, the PMMA (AR-P 672.045, Allresist, Germany) was spin-coated on high-quality copper-based large-area monolayer graphene (Cu-S2, Beijing Graphene Institute, China) with a speed of 800 rpm, corresponding to a film thickness of about 0.6 µm. After drying at 70°C, for 10 minutes in a convection oven, the copper foils were etched using a ferric trichloride (45%FeCl3 basis, I299165, Allresist, Germany) solution. The graphene-PMMA composite film was directly transferred onto the tapered region of the microfiber after being cleaned with deionized water (ACS 036645, Alfa Aesar, UK). To prevent damage to the tapered area of the fiber, a specific concentration of ethanol was added to deionized water so that microfiber naturally sinks to the bottom of the vessel, but graphene-PMMA composite film floats on the surface of deionized water. A section of the as-fabricated microstructured fiber is shown in Fig. 2(a), and the graphene-PMMA composite film area was visible under an optical microscope. According to the inset in Fig. 2(a), the graphene films can be directly obtained by the dissolution of PMMA in acetone solution. A total of five points on the microstructured fiber were selected for graphene Raman characterization, from the transition area in the fiber taper to the region of uniform diameter. Figures 2(b) and 2(c) display the microscope image and the Raman spectrum of point 1, respectively. The representative microscope image in Fig. 2(b) shows a high-quality uniform graphene layer at the point 1 position, and the Raman spectrum in Fig. 2(c) shows sharp peaks in the G and 2D modes, demonstrating the high quality of the monolayer graphene. As can be seen from Fig. 2(d), the Raman features for the remaining four points in Fig. 2(a) are measured. The observable D peak is used to characterize structural defects in graphene samples and mainly comes from the defective grain boundaries between the graphene domains [43]. The main reason for the high D peak is the presence of collapse and folding of monolayer graphene films. After the PMMA film support effect is lost, the graphene layer on the fiber surface inevitably collapses. This collapse is concentrated in the area of the tapered fiber with a uniform diameter, while the transition area of the microfiber has a larger diameter so that the graphene retains its intact monolayer properties. The G peak of the point 2 to 5 positions did not move to the low frequency, indicating that the layers of the four positions of the microstructured fiber are the same.
Fig. 2. (a) A section of the as-fabricated microstructured fiber. The illustration in (a) displays the microstructured fiber after the PMMA film has been removed with acetone. (b) and (c) The microscope image and Raman spectrum of point 1 position in (a). (d) The Raman spectrum of points 2 to 5 positions in (a).
It can be concluded that the number of graphene layers in points 2 to 5 positions is between 3 and 4 layers. Indeed, in areas where the diameter of the tapered fiber is uniform, the number of layers of graphene film is less than that measured in Fig. 2(d) when the graphene-PMMA composite film is applied to the tapered fiber.
3. Results and discussion
The spacing P between the VCSELs is optimized to decrease the crosstalk while maintaining a significant light manipulation effect. For the completed light manipulation device shown in Fig. 3(a), we removed the electrodes of one VCSEL every four VCSELs. Whether the current compact size causes crosstalk is determined by the extent to which the modulation waveform changes before and after the removal of the electrodes. The smaller the change in modulation waveform before and after removing the electrode, the smaller the crosstalk between adjacent VCSELs. Experimental results show that for an array of 1 × 40 VCSELs, cross-talk between adjacent VCSELs is almost negligible when $P \ge 35$ µm. In this experiment, P is set to 50 µm to strictly limit crosstalk between adjacent VCSELs.
Fig. 3. (a) The completed structure of the light manipulation device. (b) The diagram of VCSELs irradiating a graphene-PMMA composite film at different angles, where the angles varied from 30° to 0° in steps of -10°. (c)-(f) Phase distribution of the control light incident on the metasurface for deflection angles of 30°, 20°, 10°, and 0°, respectively. The blue lines in (c)-(f) represent the positions of graphene-PMMA composite films.
Here we utilize the metasurface concept and employ a gradient index metasurface to realize the modulation of optical signals in optical fibers. Two main factors affect the modulation effect, one is the absorption attenuation of the graphene and the other is the refractive index mutations in the metasurface. Both of these factors are associated with the folding of the monolayer graphene. The metasurface structure ensures the high quality of graphene and minimizes the influence of graphene wrinkles on modulation. According to the illustration in Fig. 3(a), when the control light has passed through the metasurface on the side close to the VCSEL, it will transmit through the microfiber and reach the side away from the VCSEL.
Artificially folded graphene-PMMA composite films lead directly to large phase discontinuities in the control light transmission process. By engineering a phase discontinuity along an interface of the microfiber, the general relation of anomalous reflection and refraction can be described as [44,45]:
Similar to the gradient refractive index metasurface given in Ref. [46], the graphene-PMMA composite structure proposed in this work has a certain refractive index gradient in the axial direction of the optical fiber. Equation (1) implies that dϕ/dx can modify the direction of the rays refracted and reflected. In this scheme, the control light incident on an ultrathin metasurface can be bent by an angle using phase discontinuity. As a result, the refractive index of graphene film on the side away from the VCSEL will have a sudden change due to the non-uniformity pump light energy received. The addressable incoherent VCSEL array can emit different far-field patterns and are an excellent solution for the precise design of the gradient refractive index on microfiber surfaces. An interesting finding is that a metasurface with a specific gradient in the refractive index can prevent light from passing through the waveguide [46]. In this way, an addressable incoherent VCSEL array can effectively control the modulation depth of the device and can even prevent optical signal transmission in optical fibers. However, the solution for addressable VCSEL arrays comes at the cost of power stability and wavelength stability due to variable current injection. In addition, the wiring of the addressable electrodes will be extremely complex to avoid short circuits. We have therefore connected the VCSEL arrays in parallel to the same positive and negative terminals to ensure the stability of the control light.
Note that dϕ/dx is essentially an additional momentum contribution, so the presence of film folds can form a gradient refractive index on the surface of fiber. Due to the good viscosity and elasticity of the PMMA film, the number of graphene layers covering the microfiber remains essentially constant. The folding angle of the metasurface can be estimated from the graphene-PMMA composite film wrapped around the microfiber shown in Fig. 2(a). The angle α between the folded metasurface and the x-direction does not exceed 30 degrees in the current process. Figure 3(b) shows a diagram of the gradient refractive index metasurface induced by folding graphene-PMMA composite film. VCSELs irradiating a graphene-PMMA composite film at different angles, where the angles varied from 30°to 0°in steps of -10°. The phase distribution of the control light incident on the metasurface for deflection angles of 30°, 20°, 10°, and 0° is illustrated in Fig. 3(c)-3(f), showing good continuity in the x direction. In the simulation, the y-direction in Fig. 3(c)-3(f) represents the control light emission direction, which is perpendicular to the VCSEL plane. Changes in the phase of the control light transmission are always accompanied by large variations in the modulation signal. However, this effect is very low and negligible for metasurface structures under current processes. This is mainly due to the fact that the phase variation relies on the optical path accumulation, which can be altered by changing the geometry parameters (thickness and deflection angles) of the metasurface. Although both the refractive index of the graphene and the deflection angle of the metasurface is tunable, the sub-wavelength (approximately half of the wavelength) thickness of the graphene-PMMA composite films does not provide sufficient optical paths to achieve a wide range of phase changes.
To see how the metasurface structure affects light transmission through a microfiber, a row of VCSEL with a wavelength of 850 nm is used as a control light for manipulating the absorption of the 1550 nm signal light. The test system is demonstrated in Fig. 4(a). The power of the signal light had to be kept below -20 dBm, which was not strong enough to change the absorption of graphene. The light source (Golight, OS-TL-M2-C2-1-10-0-S-FA) in this experiment is power-controlled at -21.8 dBm with a wavelength of 1550 nm. A set of polarization controllers (PC) were applied for the input signal light. The arbitrary waveform generator (Hantek, HDG2082B) provides the VCSEL array with low-frequency, high-frequency, and direct current (DC) signals via the RF module shown in Fig. 1. Photodetectors (Thorlabs, DET01CFC/M) and oscilloscopes (RIGOL, DS1102Z-E) observe the modulated optical and electrical signal, respectively.
Fig. 4. (a) Experimental setup of all-fiber light manipulation device test system. (b) The measured modulated waveform of the signal pulse with pulse duration Δτ of 300 µs. (c) The details of the third pulse in (b).
In Fig. 4(a), the yellow line and the black line represent the electrical signal and the optical signal, respectively. The signal pulses are switched according to the intensity of the control light, which could be readily observed at the oscilloscopes. To explore the dynamic light manipulation characteristic of the microstructured fiber, the measured modulated waveform of the signal pulse is shown in Fig. 4(b), where the pulse duration Δτ is 300 µs. The pulse signals generated by VCSEL arrays were stably maintained during the experiment for a few days. Allowing the system shown in Fig. 4(a) to operate without interruption, the modulation waveform recorded in the oscilloscope is stable for 72 hours, after which the peak value of the modulated signal becomes unstable. This instability was caused by an unstable signal light entering the microstructured fiber. The modulated waveform was returned to its initial state by switching the light source off and on again after 10 minutes, while the VCSEL array remained pulsed. This operational stability benefits from the extremely long operating life of the VCSEL (over 15,000 hours), which is superior to conventional all-fiber graphene modulators based on WDM systems. The details of the third pulse from Fig. 4(b) can be seen in Fig. 4(c). In this case, the observed long tail of the signal pulses is not intrinsic but due to the slow recovery time of the temperature.
Heat, which is difficult to quantify, is generated near the gold wires feeding the VCSEL array when the RF module is operating in alternating current (AC). Operating voltage and output power at different temperatures as a function of supply current for a single VCSEL are shown in Fig. 5(a) and Fig. 5(b), respectively. Although the VCSEL is driven by a current source, the optical output power versus applied voltage is a key parameter for evaluating the modulation performance. Therefore, the relationship between the VCSEL output power and the applied voltage is given here as shown in the inset of Fig. 5(a).
Fig. 5. (a) and (b) Operating voltage and output power at different temperatures as a function of supply current for a single VCSEL. The illustration in (a) displays the relationship between the VCSEL output power and the applied voltage. (c) and (d) Output spectrum of the microstructured fiber at various modulation voltages, and the voltage range during the measurement is 0 V to 2.2 V.
As it is difficult to measure the ambient temperature in the vicinity of the gold wire, we lowered the temperature in the laboratory to 10°C for more than an hour and then placed the printed circuit board (PCB) with only a single VCSEL on a thermostable heating table. By fixing the spatial optical power meter probe (Thorlabs, PM100D) to the light output of the VCSEL, the temperature of the heating table is changed to obtain the correspondence between the supply current and output power at different temperatures. It can be seen that the presented proposed light manipulation devices have excellent temperature-insensitive properties. A room temperature of 20°C is chosen here to allow the VCSEL to complete the experiment in good working condition, as the results show that the output power of the VCSEL decreases slightly with increasing temperature.
We characterize the modulation depth by measuring the output power of the microstructured fiber. The modulation depth is defined as the difference in the optical signal intensity at the microstructured fiber output when the VECSEL array is switched “ON” and “OFF”. The optical transmission of the all-fiber light manipulation device was actively tuned by applying a voltage to the VCSEL array. As different voltages correspond to different modulation depths, the optical power at the output of the microstructured fiber versus the supply voltage is measured here under DC conditions. The output spectrum of the light manipulation device, shown in Fig. 5(c), is measured with the spectrometer (Yokogawa, AQ6370D). As the voltage increases, the light intensity at the output of the microstructured fiber increases, as detailed in Fig. 5(d). To avoid damage to the VCSEL, the voltage range during the measurement is 0 V to 2.2 V. The modulation depth will increase further as the voltage is increased to drive the VCSEL into an overload condition. In theory, the modulation depth of the device can be increased by increasing the output light intensity of the VCSEL. However, this is only possible if the graphene has not yet reached saturation absorption. From Fig. 5(d), the output intensity of the microstructured fiber will be -22.77 dBm with VCSEL array set to “OFF” and -20.89 dBm with VCSEL array set to “ON”, and the supply voltage set to 2.2 V. This means that the modulation depth is approximately 2 dB when the modulating voltage is 2.2 V. As well as increasing the modulation voltage, increasing the leakage of the evanescent wave is also an effective way for achieving high modulation depth. However, this will undoubtedly increase the insertion loss of the light manipulation device proposed here. The insertion loss of the entire light manipulation system is 0.28 dB in this work. To qualify the polarisation-insensitive performance of this all-fiber device with low insertion loss, we traversed all polarisation states of the incident light signal using the PC in Fig. 4(a). Spectrometer observations show that changes in the polarisation state of the incident light signal have almost no effect on the output spectrum of the modulated light signal when the VCSEL array is operated in the DC state. When the VCSEL array is operated in the AC state, oscilloscope observation shows that changing the polarisation state of the incident light signal does not lead to a change in the modulation waveform.
The light manipulation device in this work is based on an all-fiber system and a graphene-based metasurface. It is wavelength-insensitive, relying on the ultra-broadband absorption of graphene. For measurements, the VCSEL array is set to the “ON” state and 2.2 V DC is applied. An optical power meter (Ceyear, 6337D) is connected to the output of the microstructured fiber. The variation in output light intensity of the microstructured fiber as a function of wavelength has been recorded, and the result is presented in Fig. 6(a). Due to the limited tunable range of the light source, the test is only performed between 1530 nm and 1565 nm in 5 nm increments. The output light intensity of the microstructured fiber increases with increasing wavelength, while this trend gradually decreases. As can be seen from the results in Fig. 6(a), the 35 nm wavelength interval results in a change of only about 0.1 dB in the output light intensity of the microstructured fiber. In recent years, channel detection techniques have relied on top tuning. This involves overlaying a low amplitude, low-frequency sinusoidal signal as a marker on the signal at one end of the transmitter [41,42]. The modulation effect of the light manipulation device for sinusoidal signals with periods of 250 µs and 300 µs is shown in Fig. 6(b) and 6(c), respectively. Results show that there is little distortion in the modulated waveform when a standard sinusoidal signal from an arbitrary waveform generator is applied to the VCSEL array. Figures 6(b) and 6(c) were tested at a wavelength of 1550 nm.
Fig. 6. (a) Output light intensity of the microstructured fiber as a function of wavelength. (b) and (c) The modulation effect of the light manipulation device for sinusoidal signals with periods of 250 µs and 300 µs, respectively.
Table 1 compares the all-fiber light manipulation device proposed in this paper with the existing research. Among these dynamic light manipulation devices, most of their optical performances rely on WDM systems. Importantly, the majority of these devices can only provide low insertion loss and cannot simultaneously satisfy polarisation-insensitive and temperature-insensitive properties. After these comparisons, the device with VCSEL array and graphene-based metasurface can be fully competent for dynamic light manipulation. In addition, the VCSEL array has certain advantages over conventional lasers when used as a pump source. For example, when using a VCSEL array as the pump source, the operating loss of the entire optical system is only determined by the manufacturing process of the microstructured fiber. This makes the loss of the entire optical system very controllable. Importantly, the interaction length of a VCSEL array with graphene can theoretically be extended indefinitely. The lifetime and stability of VCSELs will also be greater than that of conventional lasers.
Table 1. Comparison between similar reported all-fiber light manipulation devices and this study
4. Conclusions
In conclusion, a general scheme is proposed to implement effective light manipulation for all-fiber devices. With the advantage of fiber compatibility, our device easily integrates into an all-fiber system. The proposed light manipulation device consists of a VCSEL array and a graphene-based metasurface, which can work independently of the WDM system. The control light composed of the VCSEL array modulates the signal transmitted in the microstructured fiber, and this phenomenon is proven to be quick to respond. In the current process, the thickness of the metasurface is achieved at sub-wavelength (approximately half of the wavelength) levels, which means it cannot provide sufficient optical paths to achieve a wide range of phase changes. As a result, metasurface wrinkles and controlling light-induced changes in the refractive index of the graphene hardly destabilize the modulated waveform. With an insertion loss of only 0.28 dB, evanescent wave coupling to graphene layers is polarisation-insensitive due to the geometrical symmetry of the device. The device could be effectively manipulated for a few days (not less than 72 hours), and the actual life is mainly limited by the stability of the signal light source. The device possesses the capacity to dynamically modulate the signal light with both low-temperature sensitivity and low-wavelength sensitivity. The 35 nm wavelength interval results in a change of only about 0.1 dB in the output light intensity of the microstructured fiber when the wavelength changes from 1530 nm to 1565 nm. Moreover, the modulation depth is approximately 2 dB when the modulating voltage is 2.2 V, which may open avenues for channel detection techniques and have deep implications in top tuning applications.
Funding
National Key Research and Development Program of China (2019YFB2204003); Fundamental Research Funds for the Central Universities (2022YJS006); National Natural Science Foundation of China (61827817, 62221001, 62235003); State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University (RCS2019ZZ007).
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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