We demonstrate an all-optical tunable microfiber knot resonator (MFKR) by direct light-graphene interaction using external vertical incidence pump laser. The 1530 nm CW pump source is employed to irradiate the sample, which can achieve the performance modulation of MFKR including transmission loss, extinction ratio, and resonant wavelength by the saturable absorption, photo-thermal, and optical Kerr effects, respectively. Compared with the MFKR with only the bottom graphene film, the tunable ranges of transmission loss and extinction ratio are increased by 69 and 125 times, respectively, which can induce a remarkable amplitude tuning. The resonant wavelength of MFKR occurs a red-shift under the irradiation of the pump light, and the red-shift range can exceed one free spectral range (FSR), which means the resonant wavelength could be tuned in the full wavelength range of the transparent window of optical fiber. It is promising for the device to be applied as an all-optical modulator, tunable optical filter, etc.
© 2017 Optical Society of America
Recently, the resonant devices have been extensively used as optical filter, optical modulator, logic gate, and all-optical devices in the field of fiber optics and integrated optics [1–4]. The tunable microfiber knot resonator (MFKR) has attracted widespread attention from academic community, and it is promising to be applied in optical signal processing and communication filed [5–7].
The light-control-light microfiber device, utilizing the optical nonlinear effects including saturable absorption, optical Kerr effect, stimulated Brillouin scattering and stimulated Raman scattering, has become the research hotspot [8–10]. However, a deadly defect of the relatively low nonlinear coefficient (n2≈10−16 cm2W−1) lies in the optical fiber, which hinders the realization of the remarkable tuning efficiency; therefore, it is a great challenge to achieve the high effective all-optical tuning . In order to improve the nonlinear coefficient of optical fiber, one of the possible approach is doping high nonlinearity materials such as Bi2O3 (n2≈10−14 cm2W−1) in the optical fiber , in addition, high nonlinearity 2D materials such as black phosphorus (n2≈10−13 cm2W−1) , MoS2 (n2≈10−7 cm2W−1)  or graphene (n2≈10−7 cm2W−1)  can also be covered on the surface of microfiber to enhance the interaction between light and nonlinearity matter and improve tuning efficiency [16, 17].
Graphene, an atomic-layer thick 2D material, is popular in optical communication field due to its outstanding stability, unique band structure, and excellent properties of electricity and optics [18, 19]. The 2.3% linear absorption per layer and high nonlinearity effect are widely used in all-optical tunable microfiber devices [20, 21]. The microfiber fabricated by flame-heated taper drawing technology has strong evanescent field, which can notably enhance the light-graphene interaction [22, 23], and the graphene decorated microfiber for ultrafast optical modulation has been reported by Tong. et al., in which the in-fiber ultrafast signal processing using femtoseconds pulse laser has been realized .
MFKR fabricated by twisting the microfiber into a knot is an attractive optical device due to its outstanding performance and compact size [25,26]. As a constructive interference device , the interaction between light and graphene can be greatly enhanced by resonant recirculation in optical micro-cavity . Recently, Gan. et al. reported an all-optical controlled microfiber resonator featured by graphene’s photo-thermal effect generated by in-fiber pump laser , which realized a significant modulation for the resonant wavelength. However, the generated photo-thermal effect gives rise to the change of resonant wavelength, which means the pump laser cannot meet the resonant condition of MFKR. Consequently, the pump laser cannot effectively couple into the knot region to interact with graphene, and thus the nonlinear effect cannot be induced . Due to the inexistence of resonant cavity’s enhancement effect, the lower pump power also cannot provide an opportunity to employ nonlinear effect to tune the MFKR.
In this paper, we demonstrate an efficient all-optical tunable MFKR with the tunable transmission loss, extinction ratio, and resonant wavelength. An external-fiber pump laser is used to irradiate the graphene-assisted MFKR to realize all-optical tuning. In this case, the limitation that pump laser cannot be sustainably kept in the knot region caused by the resonant wavelength red-shift is overcome. Two pieces of graphene film are covered on the top and bottom of MFKR to form graphene-assisted sandwich structure in order to improve the interaction between the light and graphene. The commercial polydimethylsiloxane (PDMS) film (Gel-Pak Inc.) is used to protect the device, extend the service life, and ensure the integrity of graphene. In this way, both a strong light-matter interaction and a reduced transmission loss induced by graphene are obtained. By employing the direct interaction between pump laser and graphene instead of using evanescent field, many optical characteristics could be generated, including photo-thermal effects, saturable absorption, and optical induced refractive index change. Compared with the MFKR with only the bottom graphene film, the tunable ranges of transmission loss and extinction ratio are increased by 69 and 125 times, respectively. In addition, the red-shift of the MFKR’s resonant wavelength under the irradiation of the pump laser can reach at 0.78 nm (over 4 FSR), which means the fabricated graphene-assisted MFKR has good performance of tuning wavelength.
The microfiber is fabricated from a standard single mode fiber (Corning Inc.) using the flame-heated technique. The standard fiber is drawn into bi-conical tapers with a diameter of ~3.11 µm in the uniform waist region, and the length of tapered region is about 3 cm, which is shown in Fig. 1(a) with the SEM image of microfiber. The loss using this fabricated method is approximately 0.01 dB/mm .
The microfiber is cut from the tail end of the waist region, and formed into a ring resonator by tying a knot. In order to collect the light transmitting out of the knot, the cut area is reconnected by van der Waals and electrostatic forces, and then fixed by low-index Teflon glue adhesive. The optical microscope image of the microfiber knot resonator with a radius of 1.54 mm is shown in Fig. 1(b).
The detailed manufacturing operation of MFKR with graphene-assisted sandwich structure (MFKRWG) is shown in Fig. 2. As shown in Fig. 2(a), the graphene film fabricated by chemical vapor deposition (CVD) is transferred to the MgF2 glass substrate after exfoliating from the copper foil. The fabricated microfiber knot is located on a graphene-covered MgF2 glass substrate (Fig. 2(b)). The another graphene film is transferred to the PDMS film by the same method, and then covered above the MFKR, as shown in Fig. 2(c), the desired MFKRWG is finally finished. The lateral view of the device is shown in Fig. 2(d), two pieces of graphene film clamp the device tightly to form a sandwich structure to ensure graphene and MFKR are in close contact. The sandwich structure region is measured as ~12.6 mm.
3. Characteristic of MFKR
In order to evaluate the influence of graphene on the microfiber, we use beam propagation method to simulate the mode field distribution of the microfiber with graphene-assisted sandwich structure. In this simulation environment, a microfiber with the diameter of 3 µm is sandwiched by two piece graphene films, and then placed in the air. The refractive index of graphene, microfiber, and air at 1550 nm are 2.52 + 2.24i, 1.47, and 1, respectively. Figure 3(a) shows the computed fundamental mode profile of a graphene-sandwiched microfiber (GMF) at 1550 nm. The evanescent field provides a strong near-field interaction between the microfiber and graphene. The calculated effective index (neff) is about 1.417. The horizontal cut of the mode profile at Y = 0 is shown in Fig. 3(b). Compared with the mode of the bare microfiber (the black line in Fig. 3(b)), the mode amplitude of GMF at the boundary decreases greatly (the red line in Fig. 3(b)). The main reason is the graphene has imaginary part of refractive index and the light at the boundary can be absorbed by graphene.
The characteristic of the MFKR can be developed combining with the theory of ring resonators. In the case of broad spectrum light incidence, the spectrum can be obtained using transfer matrix method. The output intensity of the MFKR can be described as
where α is the optical loss coefficient of the MFKR in knot region; t represents the transmission coefficient; is the phase accumulation of the optical signal in knot region for each propagation period, neff, R, and λ are the real part of the effective index, radius of the knot, and wavelength, respectively.
The spectrum from 1.539 µm to 1.540 µm simulated by Eq. (1) is shown in Fig. 3(c). The simulated spectrum (red line) agrees with the measured spectrum (blue line) well. According to the calculated transmission spectrum, the quality factor (Q) is about 19253, the finesse factor (F) is about 2.35, and the FSR is about 0.2 nm. Considering the spectrum of output intensity under the resonant condition, i.e. (m is an integer), the output intensity can be described asFigure 3(d) shows the relationship between the output intensity and the loss coefficient α when t = 0.7. Employing the advantage of the saturable absorption of graphene, the extinction ratio of the device can be tuned to rise by increasing the loss coefficient at the left side of critical coupling point, thereby the amplitude of the output spectrum can be modulated. In addition, the transmission loss of the device can also be reduced under the irradiation of the pump laser due to saturable absorption characteristic of graphene.
4. Experimental results and discussions
The photo-thermal, saturable absorption, and optical Kerr effects of MFKRWG are simultaneously demonstrated in this paper. In order to characterize the direct interaction between light and graphene, we use a continuous pump laser to irradiate graphene film from external, as shown in Fig. 4, the pump laser illuminates on the whole graphene film uniformly. It is the better method for MFKRWG to finish an all-optical tunable control compared with others . Because if the pump laser couples into the GMFKR through the wavelength division multiplexing (WDM) and interact with graphene by stronger evanescent field, the pump laser with the fixed wavelength cannot couples into the device or interacts with the graphene due to the variation of resonant wavelength caused by the graphene’s photo-thermal effect, and the limitation can be overcome efficiently by external irradiating demonstrated in this paper.
To quantitatively study the all-optic tunable MFKRWG, we employ a beam of pump laser to illuminate the device vertically from the external. The pump power is sequentially changed from 0 mW to 97 mW, 197 mW, 308 mW, 418 mW, 518 mW, 603 mW, respectively. The experimental setup consists of a tunable laser (TL) near 1550 nm, an amplified spontaneous emission (ASE) source, an erbium-doped fiber amplifier (EDFA), an optical power meter (OPM), an optical spectrum analyzer (OSA), a variable optical attenuator (VOA), and a 10/90 splitter, which is shown in Fig. 5. The pump laser at 1530 nm generated by the TL is coupled into an EDFA in order to obtain the high pump power, and then coupled into the VOA to tune the pump power. The ASE light is firstly coupled into the DUT (the device under test), and then the output light from DUT is coupled into the OSA for the spectrum observation. The 10/90 optical power splitter is employed to divide the light into two parts: the 10% port is used to monitor the output power of pump laser, while the 90% port is used to irradiate the DUT as pump laser. The MFKR is sandwiched tightly by two pieces of graphene film and the top graphene film with PDMS is placed on the MFKR (Fig. 3). Since the transmittance of PDMS film is about 90% , it hardly affects the interaction between pump laser and DUT.
The transmission spectrum of the MFKRWG from 1540 nm to 1541 nm is shown in Fig. 6. We observe the variation of transmission spectrum with different pump power. The transmission spectrum without pump laser is obtained as shown in Fig. 6(a). The transmission loss and extinction ratio are −24.7 dB and 1.5 dB, respectively. The relatively higher transmission loss and lower extinction ratio can be attributed to the increased loss of knot region induced by the discontinuous refractive index and the absorption of graphene. The discontinuous refractive index causes the light scatter at the interface between graphene film and microfiber, which leads to the transmission loss increase. And the other reason is that the evanescent light is absorbed by the graphene film. The absorption is related to the number of graphene layers and the contact area between microfiber and graphene film. A great number of graphene layers and large contact area could enhance the interaction between light and graphene efficiently, but the transmission loss can be increased simultaneously. Besides, a smaller diameter microfiber could also enhance the interaction of light and graphene, but it leads to an increasing transmission loss as well. In order to achieve the optimal balance between the transmission loss and light-graphene interaction, we choose the 3-µm-diameter microfiber and multi-layer graphene to increase the interaction of light and graphene. The use of graphene-sandwiched method (Fig. 2) reduces the contact area, thereby reducing the transmission loss. Compared with other graphene-covered microfiber methods , this method not only ensures the graphene and light have sufficient interaction, but also reduces the transmission loss. We could also improve the quality of graphene to decrease the optical loss such as improving the purity of graphene.
As the pump power increasing, the transmission loss and extinction ratio result in obvious variations as shown in Fig. 6(b)-(g), which are caused by the Pauli blocking induced by saturable absorption of graphene. The black arrow shows the resonant wavelength under the different pump power. The resonant wavelength exhibits gradual red-shift which results from the change of refractive index of the microfiber induced by photo-thermal effect and optical Kerr effect.
The relationship between the transmission loss variation and pump power is shown in Fig. 7, which indicates the transmitted power of the MFKRWG can be enhanced with the pump power increasing. The red solid line is the fitting curve of the transmission loss variation with the pump power increasing. The transmission loss is continually reduced along with the increasing pump power, and it reaches its minimum at 603 mW. The reduced transmission loss is about 7.4 dB with the pump power of 603 mW, which is a great tuning range for optical loss. The transmission loss variation with the pump power increasing can be attributed to the characteristic of saturable absorption. Due to the Pauli blocking induced by the saturable absorption (Fig. 3), the pumped laser (1530 nm) with a higher power and shorter wavelength could be first absorbed by the graphene during the irradiation. In the meanwhile, the carriers in the graphene undergo inter-band transitions and the graphene maintains the saturable absorption for the remaining photon, thus the signal light (near 1540 nm) from ASE passes through the covered graphene directly without being absorbed. We can also investigate that the transmission loss approximately remains the same value at 0 mW-197 mW (the value of loss variation is only ~0.4 dB).
The evident decrease occurs after the pump power exceeds 197 mW. Because threshold requirement for saturable absorption exists, the lower pump power makes the transmission power keep a constant until the pump power reaches 197 mW. The tuning efficiency of the transmission loss for MFKRWG (Δ loss variation/Δ pump power) is about 0.02 dB/mW (the red solid line shown in Fig. 7(a)). The black solid line in Fig. 7(a) is the fitting curve of the transmission loss variation only with bottom graphene, and the tuning efficiency of the transmission loss is only about 0.00029 dB/mW, which means the contact between microfiber and the bottom graphene film is quite weak before covering the top graphene film due to the fact that the contact area of the bottom graphene film and the microfiber is very small. After covering the top graphene film, the microfiber could be in close contact with the graphene film as a result of the effect of pressure. Comparing the two curves (the black and red solid line shown in Fig. 7(a)), the ratio of the tuning efficiencies for two different graphene structure is about 69 times. Therefore, the graphene film generates an excellent effect on improving transmission loss for the MFKRWG.
Due to the reduction of the transmission loss of the microfiber knot area, the loss coefficient (α) could increase, as shown in Fig. 4(d). We define αc is the loss coefficient when the MFKR is at critical coupling condition. When the α<αc, the output power at the resonant wavelength could decrease along with the increasing of the value of α, and the extinction ratio of MFKR will be increased. The extinction ratio variation along with the pump power increasing is shown in Fig. 7(b). Owning to the reduction of the transmission loss of knot region, the extinction ratio is continually increased. The extinction ratio of the GMFKR responds with a relative increase of ~5 dB when the pump power changes from 197 mW to 518 mW, which strongly benefits to modulating the amplitude of the output spectrum. Combined with the reduced transmission loss and increased extinction ratio of MFKR, the tuning efficiency of the amplitude can be further improved.
A significant phenomenon is found out that the extinction ratio reduces when the pump power changes from 518 mW to 603 mW. The main reason is that the transmission loss of knot region increases due to the destruction of the partial graphene film. The sufficient pump power and exposure time could produce thermal damage by the absorption of photons and energy dissipation through phonons, which breaks the quality of graphene film and introduces the new loss for microfiber, which leads to the decrease of the extinction ratio.
The black solid line in Fig. 7(b) is the extinction ratio variation for the pump power increasing process before covering the top graphene film. The value of the extinction ratio variation with the increasing pump power from 0 mW to 603 mW is 0.076 dB, which remains almost unchanged. The tuning efficiency of the extinction ratio (Δ extinction ratio variation/Δ pump power) is only about 0.00013 dB/mW. After covering the top graphene film, the tuning efficiency of the extinction ratio increases to 0.016 dB/mW from 197 mW to 518 mW. Comparing these two different efficiencies, we can find it enhances by 125 times after covering the top graphene film, which indicates the external irradiation could offer a better tunable performance for MFKRWG.
During the experiment, we also observe an evident red-shift phenomenon simultaneously. Figure 8 shows the resonant wavelength variation of the MFKRWG with the pump power increasing. The resonant wavelength of MFKRWG without top graphene film appears linear increasing with the pump laser changing from 0 mW to 603 mW (the blue solid line), which results from the variation of the microfiber’s refractive index induced by the photo-thermal effect of graphene. Due to the irradiation of out-fiber pump laser, the graphene film produces a great deal of Joule heat that increases the temperature of microfiber to increase the refractive index of microfiber. The variation of the resonant wavelength is only about 0.116 nm for the pump power increasing from 0 mW to 603 mW on account of the untight contact between microfiber and the bottom graphene.
As shown in Fig. 8, compared with the MFKR only with the bottom graphene film, the resonant wavelength of the MFKRWG exhibits sharper red shift for the pump power increasing from 0 mW to 603 mW due to the combination of photo-thermal and optical Kerr effects simultaneously. Under the high pump power exposure, multi-layer graphene film appears a nonlinear phase shift induced by photo-induced refractive index change, which is also called optical Kerr effect. The variation of the graphene’s Kerr refractive index can also change the effective refractive index of microfiber, and change the resonant wavelength of MFKR. Taking into account the photo-thermal effect and the Kerr effect simultaneously, the nonlinear red-shift for the resonant wavelength of MFKRWG occurs as shown in Fig. 8 (red line), and the variation of the resonant wavelength can reach about 0.78 nm (exceeding 4 FSR ranges). Therefore, the tunable range of the resonant wavelength could cover all the transparent wavelength window of optical fiber.
In addition, in order to verify the thermal response is mainly attributed to graphene rather than other factors such as PDMS, MFKR, etc. The thermal response characterization of the DUT without graphene has also been measured (the black line in Fig. 8), from which we can see the red-shift value is very small when the top and bottom grapheme films are removed. In other words, the effective tuning of the resonant wavelength is mainly attributed to photo-thermal and optical Kerr effects of the graphene rather than other factors.
In conclusion, we have experimentally demonstrated an external vertical optical tunable MFKRWG by direct interaction between pump laser and graphene. The fabrication process that MFKR is sandwiched by top and bottom graphene film has been optimized to enhance the light-graphene interaction. Three kinds of optical effects including, photo-thermal, saturable absorption, and optical Kerr effects occur in this experiment simultaneously. By external irradiation of pump laser, the drawback that pump laser cannot accumulate in the knot region due to the variation of the MFKR’s resonance induced by the photo-thermal effect can be avoided. Due to strong light-graphene interaction, the maximum variation of the transmission loss is 7.4 dB, and the extinction ratio is improved to be 5.0 dB. Compared with the structure of MFKR only with the bottom graphene film, the tuning efficiencies of transmission loss and extinction ratio are improved to be 69 times and 125 times for MFKRWG, respectively. By combining the reduced transmission loss with increased extinction ratio, the tuning range of the amplitude enhances. The resonant wavelength of GMFKR could be tuned from 0 nm to 0.78 nm, which exceeds 4 FSRs. The all-optical tunable range of the resonant wavelength can cover the all wavelength in the transparent window of optical fiber. The performance of this device such as the quality factor, insert loss, and tunable range of amplitude would be further optimized by improving the fabrication process of the microfiber and enhancing the quality of graphene film.
National Natural Science Foundation of China (NSFC) (61405082); Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2016KF14); Fundamental Research Funds for the Central Universities (lzujbky-2015-k08, lzujbky-2016-k05).
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