A widely tunable (30 nm) fiber laser based on a double Sagnac loop mirror configuration is proposed and demonstrated. A semiconductor optical amplifier (SOA) placed between the two loop mirrors acts as the gain medium. The fiber laser has two output ports with adjustable optical power outputs. Wavelength tunability is obtained through the use of a thin film tunable filter, while optical power adjustability is accomplished by proper adjustment of each of the loop mirror reflectivity via a polarization controller. A total output power of + 9 dBm is measured and the potential for higher output powers is discussed. Optical power stability of better than ± 0.15 dB is measured for 6 hours.
© 2009 OSA
Continuously tunable wavelength fiber lasers have attracted considerable interest in recent years due to their applicability in wavelength division multiplexing (WDM) optical communications, fiber sensors, photonic component characterization, optical testing, and RF photonics and other applications. Typically, a tunable fiber laser consists of a fiber based resonator (cavity) for oscillations, a wavelength-selective component for tunability, and a gain medium for population inversion. Various cavity designs, such as linear , ring , compound fiber ring , and Fox-Smith  for single-longitudinal-mode operation has been investigated.
Several gain media approaches have been implemented either using erbium doped fiber amplifier (EDFA) , semiconductor optical amplifiers (SOA) , stimulated Raman scattering [7–9], stimulated Brillouin scattering (SBS) [10,11], or combination of the above techniques [12,13]. However, SOA has proved a better choice over EDFA due to the inhomogenous nature of the linewidth broadening. SOA based approaches have been used to provide multiwavelength fiber laser at room temperature . In addition, the use of SOA in an EDFA based ring laser has been shown to suppress noise in the fiber laser .
Different approaches have been used to obtain the required wavelength(s). Such approaches include the use of a wavelength selective element in the cavity, e.g., Fabry-Perot Etalon , fiber comb filters , interferometer filter . Other alternatives are based on the use of fiber Bragg gratings and/or birefringent fibers within single or multiple fiber ring laser configurations  or in a linear cavity . In addition, FBG approaches include a superimposed chirped fiber Bragg grating (CFBG) in a ring cavity , a multimode FBG in a linear cavity , a sampled FBG in a ring cavity . A limitation of such approaches is the use of different type of fibers (e.g., single mode, high birefringence, 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. Hence, switchable operation is used to operate at the different wavelengths .
Widely tunable single wavelength fiber lasers have also been demonstrated and they provide the flexibility of very wide tunable range (e.g., 30 nm). Yeh et.al. demonstrated an EDFA based 30 nm tunable fiber ring laser in the S-band . A triple fiber ring configuration combined with a tunable Fabry-Perot filter and isolators was used. Zhou et.al. demonstrated a 28 nm tunable fiber ring laser in the 1300 nm band , by using polarization elements and isolators in the cavity. In both of these cases the single wavelength output signal is obtained through a fixed 90/10 optical splitter. Wide variations in the optical power (e.g., >2.3 dB) are also observed.
In this paper, we present a simple, compact, inexpensive, SOA based, widely tunable fiber laser. The design is based on a dual Sagnac loop mirror that forms the cavity and a SOA as a gain medium. The system does not require any expensive isolator, circulator, high birefringence fiber or high power pumps. Its ability to use the two Sagnac loop mirrors as both output ports and variable reflectors, gives the fiber laser a unique capability to control independently its output power, unlike previous demonstrations that used a fixed split ratio optical tap at the output of the fiber laser [26,27] leading to fixed output optical power. We demonstrate operation in the C-band by using only single mode fiber (SMF) components. A tunability of 30 nm, a full width half maximum (FWHM) of 0.07 nm, and an optical power stability of better than 0.3 dB over 6 hours are demonstrated.
2. Experimental set-up
The experimental setup is presented in Fig. 1 . The linear cavity 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 (InPhenix, model IPSAD 1501) for the gain medium and (c) 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 placed into LM2. A polarization controller (PC) is used in each of the loops to control the state of polarization of the signal and hence adjust the amount of light directed to the output (OUT) of each loop or back in the cavity. The SOA is placed between LM1 and LM2. Note that there is no optical isolator in the cavity. Hence the signal can propagate in both directions. A 90:10 optical coupler is connected to each of the output ports, where 90% port is connected to the optical spectrum analyzer (OSA) via a variable optical attenuator (VOA). 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 other 10% port is connected to a power meter to measure the total power of the laser. Optical taps with split ratio of 99/1 are used in the cavity to monitor the optical signal in and out from the SOA and to be able to measure each of the the LM reflectivity. All fiber connections are performed using FC/APC connectors.
3. Characterization of the fiber laser
3.1 Fiber laser threshold
When the SOA is driven by a bias current (IB) it emits amplified spontaneous emission (ASE) that propagates in both directions. The tunable filter in LM2 allows only wavelengths within its passband to circulate in the fiber laser cavity. In addition, PC2 is adjusted to allow some portion of the laser signal to exit at OUT2, while the rest is reflected back into the cavity. Similarly, PC1 is set so that LM1 operates as a 100% mirror. In doing so, only the selected wavelengths from the TF survive in the cavity as standing waves and the fiber laser starts to lase at the center wavelength of the TF.
Since the system uses very few components, the cavity loss is kept to its minimum. 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 as well as removing the taps used for test and measurement. It is seen that the system starts to lase when the IB of the SOA is set as low as 26.8 mA. Figure 2 shows the output of the system when IB is varied from 26 mA to 50 mA. It is important to note that for currents < 26.8 mA the fiber laser does not lase and the FWHM of the output signal remains the same as the FWHM of the filter (e.g., 0.3 nm). However, as soon the IB is increased to 26.8 mA the FWHM changes from 0.3 nm to 0.07 nm and the peak power increases by ~30 dB (Fig. 2(a)). For 26.8 mA < IB <75 mA, lasing continues but gain competition dominates and there is mode hoping among the various longitudinal modes. At higher currents (e.g., > 75 mA) there is no mode hoping and the power is stabilized. Figure 2 (b) shows the evolution of the peak output power and FWHM measured at OUT2, as a function of IB. We also intentionally increased the optical losses in the fiber laser cavity, using a variable optical attenuator (VOA) and we observe that when IL in the cavity increases, the ITh increases, and the total output peak power reduces as expected.
3.2 Wavelength tunability
The output wavelength of the fiber laser can be adjusted by tuning the TF. Figure 3(a) shows the tuning range of the dual output fiber laser, which is obtained by tuning the TF. The output optical power of all signals shown in Fig. 3(a) is within 1.2 dB without any adjustments on the bias current of the SOA. Note that the optical power fluctuations is smaller than the measured TF wavelength dependent loss of 2.1 dB this is due to the fact that the SOA operates in its saturation regime. By adjusting the IB and the PC the power variation can be minimized to 0.1 dB. The lineshape of the laser depends upon the transfer function of the filter used in the loop LM2 as expected. Figure 3(b) shows the output spectrum (diamonds: OUT2, circles: OUT1) of the fiber laser when a TF with a narrow bandwidth (0.3 nm) is used and a broader bandwidth (1 nm, NEWPORT-TBF-1.0) in LM2 (OUT2, squares). A FWHM of 0.07 nm and 0.27 nm is measured, respectively. Note that both outputs, OUT1 and OUT2, have almost identical FWHM for the same TF.
3.3 Dual-output power tunability
Another unique feature of this laser is the dual port operation. Hence, we can use both ports as output ports simultaneously. There are two ways the output power of the laser can be controlled: (a) by varying the reflectivity of LM1 and/or LM2, while keeping the SOA gain constant (b) by changing the gain of the SOA while keeping the reflectivity of mirrors constant.
In the first approach, IB is set at 200 mA and the reflectivity of both mirrors is made >99.9% by adjusting PC1 and PC2. At this initial setting, the power at the two ports, OUT1 and OUT2 is measured at −27 dBm and −26 dBm, respectively. Then, PC1 is kept at its initial setting, while PC2 is slowly adjusted to gradually change the reflectivity of LM2 from 99% to 14.8%. Figure 4(a) shows that the output power OUT2 changes from −26 dBm to + 7 dBm, while OUT1 remains at ~-27 dBm. Hence, a dynamic range of ~33 dB is obtained for OUT2 of the fiber laser. The FWHM is measured by the OSA and is found to be 0.076 ± 0.004 nm. However, we observe that the FWHM gradually increases from 0.07 nm to 0.13 nm when the above experiment is repeated by changing the reflectivity of LM1 from 99.9% to 11.3% while keeping the reflectivity of LM2 at its initial setting (See Fig. 4(a)). As the output power increases at OUT1, there is less feedback signal power that circulates in the cavity which makes the ASE more prominent. LM1 does not have a filter to suppress the ASE. Hence, the FWHM of the laser becomes broader and the laser quality gradually degrades at OUT1. We do not see such broadening of FWHM at OUT2, since the TF suppresses the ASE independent of the level of the feedback signal power in the cavity. However, both outputs have similar FWHM when the reflectivity of the loop mirror LM1 is set at above 60% or in other words, when the power at OUT1 is kept at less than + 6.5 dBm. The maximum power from any output is found to be ~ + 9 dBm as shown in Fig. 4(a). It is also seen that the power at OUT2 is about 2 dBm lower than that of OUT1. This is due to the extra 1.8 dB IL of the TF in LM2.
It is important to note here that the above approach not only is applicable to single-port operation, but also for dual-port operation. Let’s assume that in an application where one would like to set the power at OUT2 to + 6 dBm while varying the signal at OUT1. The first step is to select the optimum operating SOA gain that will provide a total power of 9 dBm (equivalent to the maximum optical power of the single port operation). This can be accomplished by having the fiber laser operating with OUT1 and OUT2 optical power at + 6 dBm each. The IB is set at 200mA. By adjusting both PC1 and PC2 simultaneously we can optimize the reflectivity of both LM1 and LM2, in order to keep OUT2 at a constant power of + 6 dBm, while varying OUT1 from + 6 dBm to −6.2 dBm as shown in Fig. 4(b). In this case the FWHM of the laser at both ports remain the same (~0.076 nm) throughout the experiment.
The second approach is more straightforward. The power tunability of the fiber laser is achieved by changing the gain of the SOA while keeping the reflectivity of the loop mirrors constant. This methodology requires initialization of the laser; hence at some current setting (i.e. 200mA), arbitrary reflectivity of each loop mirror is set and then the SOA gain can be used to control the optical output power. Figure 5 shows the OUT1, OUT2 optical powers while the IB is changed from 50 mA to 400 mA. Note that the reflectivity of each mirror remain unchanged throughout the experiment. The reflectivity values are set so that OUT1 optical power is 14 dB lower than that of OUT2.
3.4 Optical power stability
The stability of the laser was measured by taking the optical spectrum analyzer (OSA) measurements for 6 hours at time intervals of 1 min. The OSA had a resolution of 0.01 nm and no averaging was used. Figure 6 shows power fluctuations within ± 0.15 dB. The main 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. The power can be stabilized by proper packaging of the system.
We have proposed and demonstrated a dual port widely tunable fiber laser that is based on a simple design of a SOA SM dual fiber loop mirror configuration. Tunability of 30 nm and dual output power of + 6 dBm at each port is obtained simultaneously. The output of each port can be adjusted by controlling the LM reflectivity and the IB. Power fluctuation of < ± 0.15 dB is observed and further improvement can be accomplished by proper packaging or implementation using photonic integrated circuit technology, also leading to compact packages. The output optical power can be increased by reducing the passive optical IL in the cavity, by replacing fiber connections with splices as well as removing the VOA used in the cavity for our experiments. In addition, reducing the passive IL, can reduce the required IB, leading to lower power consumptions and higher efficiency.
Our proposed design is based on TF and SOA technologies that can be designed to operate at different optical bands, hence tunable fiber lasers covering the S, C or L bands can be implemented. In addition, since there are no isolators, circulators, or long fiber lengths in the module and due to the simplicity of the cavity, our proposed fiber laser can also be implemented in photonic integrated circuit (PIC) platform, leading to compact modules.
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