A fast tunable dual-wavelength laser based on in-fiber acousto-optic Mach-Zehnder interferometer (AO-MZI) with new fabrication process is proposed. Not only could the center wavelength of the output laser be optimized with enhanced tuning range about 30 nm by tuning the polarization and the driving frequency of the radio frequency (RF) signal accordingly, but also the spectral spacing between the two output wavelengths could be tuned from ~0 nm to 2.65 nm by controlling the power of the RF signal. The tuning mechanism was also discussed.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In recent years, high-performance dual-wavelength lasers have attracted much attention, which can be applied in optical communication , optical spectroscopy, optical imaging , optical fiber sensing , microwave and terahertz (THz) signal generation [4–6], etc. By taking advantages of compact size, low cost and its compatibility to current optical communication and sensing networks, all fiber dual-wavelength lasers are essentially important for many applications.
Various configurations have been explored to fabricate multi-wavelength lasers. The most widely used method is to introduce a multi-wavelength filter into the laser system, for example, fiber Bragg gratings (FBG) [3,4,7], Fabry–Pérot interferometers [8,9] and Mach Zehnder interferometers (MZI) from two cascaded long period fiber gratings (LPFG) [10–13], etc. It is worth noting that unique gain medium in these systems is preferred to get good correlation of the two wavelengths. Therefore, it could generate quality beat signals for various applications. To make such applications more flexible and convenient, fast tunable dual-wavelength lasers are desired. It is well known that acousto-optic (AO) effect in fiber could provide faster tuning characteristics than that based on the strain or temperature tuning in a FBG or a LPFG pair. In our previous work, we established an in-fiber acousto-optic MZI (AO-MZI) by etching a sandwich-structured single mode fiber (SMF) via hydrofluoric acid, based on which a fast tuning dual-wavelength laser had been demonstrated . The center wavelength around 1565 nm could be fast tuned with a tuning range of about only 3.6 nm. However, the spacing of the two output laser wavelengths cannot be optimized to provide a wide tuning in the frequency of the optical beats.
In this article, a tunable two-wavelength fiber laser with the center wavelength of a larger tuning range and fast tunable spectral spacing of the two output wavelengths was demonstrated based on the AO-MZI prepared by tapering technique other than using hazardous hydrofluoric acid. The central wavelength of the tunable laser could be tuned in a much larger range of about 30 nm, and the spectral spacing of the two output wavelengths could be fast tuned within 2.65 nm in the experiment. The tuning mechanism was also presented. The laser gains advantages of simplicity, stability, and continuous and fast tuning speed, which would be more flexible in practical applications.
2. MZI and dual-wavelength laser configuration
In our previous work, a dual-wavelength all-fiber laser based on a tunable AO-MZI was proposed. The MZI was actually an in-fiber MZI with fast tuning of its spectrum by AO effect, which was also compacter than a traditional MZI with two individual arms . The sandwich-structured SMF used in the MZI was prepared by etching SMF with hazardous hydrofluoric acid. In the experiment, the center wavelength was proved to be fast tuned with a tuning range of around 3.6 nm.
The etching process plays two important roles: one is to increase the AO interaction efficiency, because the coupling efficiency between the core and cladding modes in AO tunable filter (AOTF) is increasing monotonically as the fiber diameter decreases , the other is to make sure that the resonant wavelengths of the two AOTFs in the cladding-etched fiber are entirely different from that in the fiber not etched, especially in the fiber between the two AOTFs to provide light path difference of the MZI.
It has been noticed that both tapering and etching could increase AO effect in the AOTF [16–19]. Compared to the latter, tapering technique avoids using dangerous chemicals and is much safer than etching process, which should also give a firm support to make two identical AO interaction areas. Thus we applied tapering technique to fabricate the sandwich-structured SMF used in the MZI.
By using heat-and-pull method, we tapered a piece of SMF twice to make a symmetric sandwiched structure. By gluing it to a unique acoustic-transducer (AT) as shown in Fig. 1(a), we built a tunable AO-MZI with cascaded AOTFs. Similar to our previous work, the regions L1 and L3 were of the same shape, with the thinnest diameter and length of 65 μm and 7.5 cm, respectively, in which the same refractive index modulation could be generated by acoustic wave. Due to the tapering, the core of the SMF was not of a uniform diameter and the core in the regions of L1 and L3 was much thinner than other places, which could also increase the AO effect. The region of L2 was of a 125 μm diameter and a 12.9 cm length. The acoustic wave was generated and amplified by the AT when the RF signal applied, and it would propagate along the bare sandwiched fiber structure to induce dynamic gratings as shown in Fig. 1(a). The period of the dynamic acoustic grating in the fiber can be given as 16,21].
With the dynamic gratings, coupling between the core fundamental mode () and the co-propagating cladding mode () will satisfy the phase matching condition 
3. Experimental Results
In the experiment, to optimize the center wavelength of the ring laser, an additional polarizer was added into the ring laser as shown in Fig. 1(b). In the configuration, FI was a fiber isolator, PC was a polarization controller, EDFA was an erbium doped fiber amplifier, OSA was an optical spectrum analyzer and a 90/10 coupler provided the output port of the ring laser. AO-MZI acted as the multi-wavelength comb filter in the configuration.
Without RF driving signal applied on the AO-MZI, the laser in the configuration of Fig. 1(b) could also work as a typical ring laser. By optimizing the PC to control the polarization in the laser cavity, we could control the output wavelength of the ring laser as shown in Fig. 2(a). The wavelength could be tuned from 1551.49 nm to 1581.84 nm. Dual-wavelength laser could be observed when the RF driving signal applied to the AO-MZI was of the appropriate frequency. In the further research, we found that the wavelength spacing of the dual-wavelength laser could be modified by changing the driving power of the RF signal. With the laser center wavelength of 1555.94 nm and the RF driving frequency of 517.0 kHz, the dual-wavelength laser spectra at different driving power are given in Fig. 2(b), and the dual wavelengths accordingly are presented in Fig. 2(c). The red and black dots in Fig. 2(c) are for the longer and shorter wavelength, respectively and the lines are just guidance to eyes. As the RF driving power decreased from 31.0 dBm to 19.0 dBm, the shorter wavelength could be tuned continuously from 1554.62 nm to 1555.88 nm, while the longer wavelength could be tuned from 1557.28 nm to 1555.90 nm. When the driving power decreased further, there would be only one wavelength left in the output laser spectrum. The wavelength spacing could be modified from 2.65 nm to about 0 nm by decreasing the power of the RF signal.
It is well known that increasing the RF power will enhance the AO efficiency. The wavelength spacing tunability should be related to the transmission tuning characteristics of the AO-MZI. Thus we measured the transmission spectra of the AO-MZI with different RF driving powers and presented the results in Fig. 3(a). The spectra were measured with a resolution of 0.1 nm and the driving powers being 27.0 dBm, 30.0 dBm, 33.0 dBm, 36.0 dBm and 39.0 dBm, respectively. It was noticed that both the insertion loss and the visibility of the comb filter transmission increased as the power of the RF signal increased in the experiment. The insertion losses were all of a nearly symmetric shape, which were shown in Fig. 3(b), and it is about 10 dB with a driving power of 39.0 dBm, whose bandwidth was about 30 nm. The insertion loss came from the scattering of the etched cladding surface and the two non-identical dynamic gratings because of the acoustic reflection in the tapering area. The visibility were also increased and shown in Fig. 3(c) in a linear coordinate, with each interference fringe bandwidth of around 2 nm.
From the results of Fig. 2(c), we noted that the maximum wavelength spacing was around 2.7 nm. To make it clear, the insertion loss and the visibility of the AO-MZI and gain profile of the ring laser were taken into consideration, and the simulation was presented. We assumed a Lorentz-profile gain spectrum with a bandwidth covering the whole C band (1530 nm - 1565 nm). The insertion loss was also set to be a Lorentz profile with a bandwidth of 30nm, and the interference fringe with a period of 2 nm was amplitude-modulated by the insertion loss as shown in Fig. 4(a). At the same time, both insertion loss and modulation depth were related to the applied RF power, which was determined by the structure of the AO-MZI, especially the acoustic transducer. Generally speaking, increasing the RF power applied in the AO-MZI will result in the increasing of the insertion loss and the modulation depth (visibility) of the interference fringe. Thus in the simulation, we assumed that both insertion loss and modulation depth were linearly increasing as the RF power applied increased in arbitrary unit. The superposition of the gain and loss is presented in Fig. 4(b), where the RF power applied is in arbitrary unit.
It is clear that two gain peaks tend to be symmetric when the RF power is large enough. When the RF power increases further, the wavelength spacing of the two peaks is increasing accordingly and the valley between the two peaks will get deeper. We also noticed that when the applied RF power was increasing in the experiment, the output wavelength would split with an increasing spectral spacing, which was in accordance with the simulation results. This actually shows us how to improve the tuning range of the wavelength spacing. With a boarder gain spectrum of a Lorentz profile and a broader interference fringe, the maximum spectral spacing will increase accordingly.
As a conclusion, we have demonstrated a tunable dual-wavelength laser with both tunable center wavelength and wavelength spacing based on an in-fiber tunable MZI. By increasing the driving power of the acoustic transducer and increasing insertion loss of the AO-MZI, the wavelength spacing could be tuned within 2.65 nm. Based on the experiment results, we presented the simulation based on our theory analysis and its results showed good agreement with the experiment. Compared with other configurations of dual-wavelength lasers, the unique gain medium in the system supports good correlation of the two wavelengths, which makes it possible for the practical applications such as THz beat generation.
This work is financially supported by the 973 Programs (2013CB328702 and 2013CB632703), National Natural Science Foundation of China (NSFC) (11574161, 61705024, 11174153, 11404263, 11674181, 61475161 and 11374165), the CNKBRSF (2011CB922003), the 111 Project (B07013).
Feng Gao, Xuanyi Yu, Guoquan Zhang and Jingjun Xu are also at the Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China.
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