1. Introduction

Single or multi-wavelength fiber lasers [1–5] have attracted considerable research interest in recent years due to their applicability in optical communications, optical instrument and fiber sensors [6–8]. Moreover, a switchable dual wavelength fiber laser is a desirable candidate for frequency-tunable, high-power, and low phase noise microwave or THz-wave generation [9–14] as wavelengths generation using photoconductive mixing technology does not require a high-quality microwave source, which can be very complex and expensive.

Several gain media approaches have been implemented either using erbium doped fiber amplifier (EDFA) [15], semiconductor optical amplifiers (SOA) [16], stimulated Raman scattering [17–19], stimulated Brillouin scattering (SBS) [20,21], or combination of the above techniques [22,23]. However, EDFA has limitations due to the strong homogenous line broadening and cross-gain saturation in the EDF that leads to an unstable oscillation. Different techniques such as hybrid gain medium [24], an external light injection [25], and an unpumped EDF as a saturable absorber (SA) based narrow bandwidth filter [26] are implemented to overcome this difficulty. In addition, the use of SOA in an EDFA based ring laser has been shown to suppress noise in the fiber laser [27]. Hence, SOA approaches are of interest.

Wavelength selection in the cavity can be accomplished with different techniques. The most common ones are Fabry-Perot Etalon [28,29], fiber comb filters [30] and interferometer filter [31]. Other alternatives are based on the use of phase shifted fiber Bragg gratings (FBGs) [10], saturable absorber based Sagnac loop [29] and birefringent fibers within single or multiple fiber ring laser configurations [32] or in linear cavity [33]. However, limitation of such approaches is the use of different type of fibers (e.g., single mode, high birefringent, polarization maintaining etc) that can jeopardize the robustness of the fiber laser, as well as, the necessary use of isolators and circulators in the fiber ring configuration. Furthermore, most of these FBG based approaches are limited to a few (e.g., 2-4) wavelengths and the wavelength tunability is limited, due to the limited tunability of FBGs.

In this paper, we demonstrate a simple compact, inexpensive, SOA-based, dual-wavelength tunable fiber laser, that can potentially be used for photoconductive mixing and generation of waves in the microwave or THz regions. A C-band commercially available SOA is used as the gain medium. Sagnac loop mirrors form the reflector and output ports in the cavity. Finally the wavelength selection is achieved through the use of a FBG and a tunable thin film-based filter. The proposed design does not require any expensive isolator, circulator, high birefringent fiber or high power pumps, thus leading to a less expensive design. The paper is structured as follows. In Section 2, we describe the experimental set-up and highlight the advantages. In Section, 3 we present the measurements and discuss the results. Finally in Section 4, we conclude and summarize the advantages of the approach and the potential applications.

2. Experimental set-up

The experimental setup is presented in Fig. 1 . The linear cavity (L_c~11.5 m) fiber laser consists of three main components: (a) two Sagnac interferometers, LM1 and LM2, used simultaneously as broadband reflection mirrors and output ports (b) a commercial SOA for the gain medium and (c) a FBG with a reflection peak at 1544.28 nm and a 3-dB bandwidth of 0.28 nm is placed in the midway of the LM1, whereas a thin film based tunable filter (TF) (SANTEC OTF-655-03D) with FWHM of 0.3 nm and an insertion loss (IL) of ~1.8 dB is also placed in the same loop. We have looked into SANTEC OTF-320, which is also a thin film tunable filter but has a sharper roll off with a 3-dB and 20 dB bandwidths of 0.25 nm and 1.2 nm respectively. The transmission spectrum of the FBG and the tunable filters with an un-polarized spontaneous amplified emission (ASE) are shown in Fig. 2(a) and Fig. 2(b) respectively. A polarization controller (PC) is used in each of the loops to control the state of polarization of the signal to adjust the amount of light directed to the output of each loop or back in the cavity. The SOA is placed between LM1 and LM2. The reflectivity of loop LM2 is kept at 99.9%. Note that there is no optical isolator in the cavity. Hence the signal can propagate in both directions. A 90:10 optical tap is connected to the output port (OUT1), where the 10% port is connected to an optical spectrum analyzer (OSA). The spectrum analyzer is set at a resolution of 0.01 nm to monitor both the peak power and linewidth (full width half maximum-FWHM) of the laser. The 90% port is connected to the power meter to measure the total power of the laser. An optical isolator is placed at the output of the fiber laser to minimize back-reflections from the instrumentation. All fiber connections are performed using FC/APC connectors.

 

Fig. 1 Experimental setup of the SOA based dual loop mirror fiber laser.

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Fig. 2 Transmission spectrum of (a) Bragg Grating and (b) thin film filters.

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3. Characterization of the dual-wavelength fiber laser

3.1 Switchable operation: single and dual mode operation

When the SOA is driven by a bias current (I_B) it emits amplified spontaneous emission (ASE) that propagates in both directions. The loop LM2 reflects almost 99.9% of the ASE back to the LM1. The two wavelengths that are selected to propagate and lase in the cavity are selected by the two filters in the cavity. In particular, one is selected by the FBG and the other by the thin film tunable filter. The two wavelength generations occur as follows: the ASE at the 3-dB coupler of the Sagnac loop, LM1, splits into two equal counter-propagating beams as shown in Fig. 1. In the middle of loop LM1, the counterclockwise (ccw) propagating beam encounters the FBG, which reflects the wavelength that falls within the FBG resonance bandwidth, λ_BG (1544.32 nm), back towards the 3-dB coupler. The rest of the wavelengths after passing through the FBG, pass through the tunable filter, which allows only the passband wavelength, λ_TF, to circulate fully in the loop, and all other wavelengths are rejected. Similarly, the clockwise (cw) propagating ASE encounters first the TF, which once again allows only the λ_TF to propagate towards the FBG. Since this selected wavelength (e.g., λ_TF) does not fall within the FBG resonance bandwidth, it is transmitted through the FBG. The cw λ_TF is then recombined with the ccw λ_TF at the 3-dB coupler. Hence, only the selected wavelength λ_TF from the TF and the λ_BG reflected by the FBG survive in the cavity. Hence, the fiber laser starts to lase at these two wavelengths. Note that the wavelength λ_BG, which is reflected from FBG and the incoming incident λ_BG (reflected by LM2) combine at the 3 dB coupler (at LM1) to produce Michelson-like interference at the output (OUT1). Simultaneously, the cw and ccw wavelengths selected by the tunable filter, λ_TF will propagate through the loop and interfere according to their phase difference, which is proportional to the birefringence of the fiber of the Sagnac loop. The polarization controller in LM1 (PC1) is used to control the fiber birefringence and hence a continuous control of the Sagnac loop reflectivity can be achieved. This fine adjustment of the reflectivity is used to balance the gain and loss of the cavity. Because of the polarization dependent loss in the cavity, the laser can be designed to operate in stable dual-wavelength or single-wavelength modes, at room temperature, by simply adjusting the PC1. In order to operate as a dual laser source, the PC1 is adjusted so that the cavity loss equals the overall gain of the SOA for both wavelengths. Figure 3 shows the dual-wavelength operation of the laser with the lasing wavelengths at 1544.32 nm and 1549.5 nm corresponding to the reflection peak of the FBG and the passband wavelength of the tunable filter, respectively. The SOA is driven by a bias current of 100 mA.

 

Fig. 3 Dual wavelength operation of the laser when the SOA is driven by a bias current of 100 mA.

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The Optical Signal-to-Noise Ratio (OSNR) is measured to be about 39 dB for the dual wavelength operation. Note that the resolution of the OSA is set at 0.01nm for the measurement of OSNR for both the single and dual operation and we treat the suppressed second wavelength as the noise at the wavelength of interest in the single-wavelength operation (see Fig. 4 ). The 3-dB bandwidth at each lasing wavelengths, 1544.32 nm and 1549.5 nm are ~0.11 nm and ~0.08 nm, respectively (Fig. 3). We also investigate the characteristics of the dual-wavelength laser at three different SOA gain settings obtained by bias currents of I_B = 150 mA, 200 mA and 300 mA. The 3-dB bandwidth, OSNR and output power for each of the wavelengths are shown in Table 1 . The 3-dB bandwidth is found to be the narrowest at the low gain setting (0.127 nm for 1544.32 nm and 0.104 nm for 1549.5nm) and the bandwidth increases as the SOA bias current increases to 300 mA as we expected. The maximum power of 3.9 dBm is measured for each wavelength at I_B = 300 mA. The total cavity passive loss is found to be ~10 dB. However, this loss can be further reduced by replacing eight FC/APC connectors (not shown in Fig. 1) with splices.

 

Fig. 4 Single-wavelength operation of the fiber laser (a) wavelength selected by the FBG (b) wavelength selected by the thin film based tunable filter.

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Table 1. 3-dB bandwidth, OSNR and output power at different bias current for dual-wavelength operation selected by the Bragging grating (λ_BG) and the tunable filter (λ_TF).

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The laser system can be easily switched to single wavelength operation from dual-wavelength operation by just adjusting the setting of PC1. For a particular setting of PC1 the cavity loss for one of the wavelengths becomes much higher than the loss of the other one. Hence, only one wavelength survives, whereas the other one, which exhibits the higher loss, is suppressed. The single-wavelength operation of the laser when the SOA is driven with I_B = 100 mA is shown in Fig. 4. We adjust PC1 such that the wavelength selected by the tunable filter undergoes destructive interference, therefore, the cavity loss for λ_TF is increased and allows only wavelength λ_BG to resonate within the cavity. We are able to suppress the wavelength λ_TF significantly and achieve OSNR of 41 dB, as shown in Fig. 4(a). Similarly, for a different PC1 setting, the cavity loss for the λ_BG can be increased (Michelson-like interference) and hence, the wavelength selected by the tunable filter survives in the cavity. An OSNR of 43 dB is achieved as shown in Fig. 4(b).

3.2 Wavelength tunability

Another unique feature of the laser system is its capability of tuning the wavelength separation, Δλ, between the two operating wavelengths. The wavelength selected by the FBG, 1544.32 nm, is kept constant, whereas the wavelength selected by the tunable filter is tuned over the entire C-band as shown in Fig. 5 . The minimum separation of 0.87 nm is achieved with the present setup, however, if narrower bandwidth filters are used the minimum separation can be further reduced. This capability of tuning the wavelength separation can be exploited to generate wavelengths in THz and microwave region [14].

 

Fig. 5 Variable wavelength difference Δλ obtained by tuning the tunable thin film filter. (a) 5.3 nm, (b) 10.28 nm, (c) 15.28 nm. .

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3.3 Optical power and wavelength stability

The stability of the laser was measured by taking optical spectrum analyzer (OSA) measurements for one hour, at time intervals of two minutes. The OSA was set at a resolution of 0.01 nm and no averaging was used. Figure 6(a) shows power fluctuations of <0.32 dB. The power fluctuations can be caused by the presence of multiple longitudinal modes in the laser source. In this work, no effort was made to curtail the laser cavity length, all the components were connected using relatively long pigtails for a total cavity length of about 11.5 m. This corresponds to a mode spacing of about 8.8 MHz. In order to experimentally determine the mode spacing the output of the laser was sent to a u²t photodetector and the beating signal analyzed with a spectrum analyzer. The observed mode spacing was about 8.5 MHz, in reasonable agreement with the cavity length measurement. Note that several methods have been recently proposed to achieve single-longitudinal-mode oscillation such as by using Sagnac filters and/or a saturable absorber [34]. Another limitation in our experiment is the temperature fluctuation in the laboratory and the polarization drift due to the long fiber lengths of the fiber pigtailed components in our set-up. However, the power can be stabilized by proper packaging of the system. Similarly, the wavelength fluctuations were also measured for 60 minutes at time interval of one. We operated the laser in dual mode and recorded the fluctuation of both wavelengths. We noticed a wavelength variation of <0.024 nm as shown in Fig. 6(b).

 

Fig. 6 Power and wavelength fluctuations of the laser at dual mode operation.

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4. Conclusions

We proposed and experimentally demonstrated a simple, compact, and inexpensive, SOA-based, dual-wavelength tunable fiber laser. A unique characteristic of this design is the simultaneous selectivity of dual wavelengths and the tunability of the wavelength difference (Δλ) between them. This is accomplished by placing a narrow bandwidth (e.g., 0.3 nm) tunable thin film filter and a FBG (with bandwidth 0.28 nm) inside the loop mirror that also operates as the output port. The total optical power for the two wavelengths is 6.9 dBm and a power fluctuation of <0.32 dB is observed. Wavelength fluctuation of 0.024 nm was measured. Further improvement in the stability of the laser can be achieved by proper packaging. Note that since our proposed design does not require any isolator and/or circulator, photonic integrated circuit technology approach can be applied to lead to further miniaturization and power and wavelength stability. The output optical power can be increased by reducing the passive optical IL in the cavity, by replacing fiber connections with splices. Unlike previous dual-wavelength fiber laser designs the system does not require thermal control to achieve wavelength tunability. The wavelength tunability is achieved by simply tuning the band-pass filter and, in doing so, the wavelength difference (Δλ = λ_TF - λ_FB) can also be adjusted.