We propose and demonstrate a simple dual port tunable from the C- to the L-band multi-wavelength fiber laser based on a SOA designed for C-band operation and fiber loop mirrors. The laser incorporates a polarization maintaining fiber in one of the fiber loop mirrors and delivers multi-wavelength operation at 9 laser lines with a wavelength separation of ~2.8 nm at room temperature. We show that the number of lasing wavelengths increases with the increase of the bias current of the SOA. Wavelength tunability from the C to L-band is achieved by exploiting the gain compression of a SOA. Stable multi-wavelength operation is achieved at room temperature without temperature compensation techniques, with measured power and the wavelength stability within < ±0.5 dB and ±0.1 nm, respectively.
© 2011 OSA
Multi-wavelength fiber lasers have considerably advanced in recent years due to their variety of applications, such as dense wavelength division multiplexing (DWDM) optical communications [1, 2], optical testing , fiber optic sensors , and RF photonics [5, 6].
Various resonator design concepts for multi-wavelength fiber laser operation have been demonstrated. These include linear cavities , ring cavities [8-9], compound fiber ring cavities , and Fox-Smith cavities . The wavelength selectivity can be achieved by inserting various wavelength selective elements in the cavity, such as an Opto-VLSI processor , a Fabry-Perot etalon , fiber comb filters  or an interferometer filter . A superimposed chirped fiber Bragg grating (CFBG) or a polarization-maintaining fiber (PMF) in a ring cavity [16, 17] are also alternative wavelength-selective components that are commonly used in the implementation of fiber lasers. Finally, different gain media have been proposed and demonstrated. Previous works have used, erbium doped fiber amplifiers (EDFA), nonlinear processes, such as, stimulated Brillouin and semiconductor optical amplifiers (SOA) as gain media. Zhao et al.  designed a switchable multi-wavelength erbium-doped ring laser using two cascaded FBGs in a highly-birefringence fiber. Though a few wavelengths (e.g., 3 or 4) lasing oscillations have been obtained, the laser seems unstable at room temperature. Using a spatially distributed Fabry-Perot resonator and an erbium doped fiber, Brochu et. al.  achieved multi-wavelength lasing over 16 channels. Shirazi et. al  exploited the stimulated Brillouin nonlinear process and used 25 km of single mode fiber (SMF) as a gain medium, while Al-Mansoori et. al  developed a gain media by combining Brillouin and Erbium gain in the same laser cavity to achieve multi-wavelength fiber laser operation. However, the construction complexity, high cost and low flexibility were serious drawbacks. Stability becomes a major issue for the multi-wavelength laser, especially, when the spacing between the neighboring wavelengths becomes small. Various techniques for the reduction of wavelength competition have been proposed. One such approach is to cool the EDFA to 77K with liquid nitrogen [22, 23]. However, this approach is not practical. The second approach is to insert a frequency shifter in the cavity  but it makes the cavity more lossy and expensive.
Semiconductor fiber ring lasers have been applied in tunable laser [25, 26], multiwavelength optical sources [27–29], short-pulse generation [28–30], and all-optical clock recovery [31,32]. Dutta et. al  used a combination of two EDFAs and a SOA with a delay interferometer acting as a comb filter to generate more than 60 wavelengths. Nevertheless, the hardware intensive design (e.g., three amplifiers along with circulators) leads to a relative expensive system. SOA has proved a better choice over EDFA due to following advantages: it has good short-term stability, it can generate multiwavelength oscillation at room temperature, and it can be directly mode locked at high bit-rate by optical injection. Furthermore, due to the inhomogenous nature of the linewidth broadening, the spacing of the wavelengths in multiwavelength (comb like) operation can be much narrower in case of the SOA as oppose to EDFA.
In this paper, we present a simple compact, inexpensive, SOA based, multi-wavelength widely tunable fiber laser. A C-band design SOA is used, yet multi-wavelength operation either at the C- or L-band region is accomplished by exploiting the gain compression phenomena of the SOA. Unlike previous designs, the system does not require any expensive isolator, circulator or high power pumps, thus simplifying its implementation. Its ability to use the fiber loop mirrors to implement dual-output ports, longitudinal mode selection and variable cavity lifetime, gives the fiber laser a unique design feature. In addition, a tunability of 50 nm, with a wavelength separation of ~2.8 nm was observed. Finally, wavelength stability of ±0.1nm and optical power stability ±0.5 dB is measured for over one hour.
2. Laser configurations
The experimental setup is presented in Fig. 1 . The linear cavity fiber laser consists of four main components:
- 1) Broabdand reflection fiber mirrors formed by a Sagnac interferometer (LM1) and Lyot-Sagnac (LM2). These also act as output ports.
- 2) A commercial SOA (Kamelian, model OPA-20-N-C-SU) for the gain medium
- 3) A 2 m commercially available Panda polarization maintaining fiber (PMF), and
- 4) A variable optical attenuator (VOA).
The SOA is placed between the two loop mirrors with a polarization controller is placed in each of the loops. The other port of the loop mirrors is used as an output, namely OUT1 and OUT2 (Fig. 1). Note that there is no optical isolator in the cavity; hence the signal propagates in both directions. At one of the output ports (OUT1), the laser power is split into two parts using a 90:10 spliter . The 90% port is connected to a variable optical attenuator and then to an optical spectrum analyzer (OSA) for monitoring the spectral content of the fiber laser. The 10% port is connected to a power meter (PM) to measure the total power of the output port. The OSA is set at a resolution of 0.01 nm. The 2 m PMF fiber is placed in the LM2 loop for longitudinal mode selection.
3. Characterization of the fiber laser
3.1 Gain medium (SOA)
To characterize the gain medium of the proposed laser system, we examine the amplified spontaneous emission (ASE) and the linear and nonlinear gain response of the SOA. The ASE at two different bias current (IB) settings of the SOA is shown in Fig. 2 . The wavelength at which we observe the highest power spectral density of the ASE is at 1504 nm with a 3 dB bandwidth of ~51 nm when the SOA is operated at 100 mA. We also found that the 3dB bandwidth remained > 50 nm when the bias current was increased to 200mA.
The gain response of the SOA at both the linear and the saturated regions are examined at IB=200 mA (Fig. 2). The small-signal gain (open circles) and the ASE curves, at 200 mA, have similar profile. However, when the SOA operates at its saturation region, the profiles do not agree with each other. The maximum small-signal gain of ~21 dB occurs at 1545 nm for the input power of −9.2 dBm. On the other hand, the maximum gain of 10 dB and the peak saturated output power of 11 dBm are measured when the SOA is operated in the saturation region. Note that in order to make sure that the SOA operates in satuation region, we need to use at least 5 dBm of input power to the SOA. However, the maximum output power of the tunable laser source used for the characterization experiment was less than 2 dBm, for wavelengths below 1545 nm, hence, we could not make gain measurements for the entire C-band.
The gain flatness plays an important role in supporting many laser line oscillations simultaneously. Both the small-signal and the large-signal gain spectra are investigated in the C-band region. A gain fluctuation of 2 dB is noticed for the small-signal gain. However we observe a gain variation of 0.08 dB, when the SOA is operated at the saturation level. The gain ripple and the polarization dependent gain (PDG) are 1 dB and 0.5 dB, respectively. Thus, one must carefully adjust the state of polarization of light in the laser cavity to obtain a flat laser spectrum.
3.2. Longitudinal mode selectivity
The resonant cavity consists of two loops, a Sagnac loop (LM1) and a Lyot-Sagnac (LM2), which are connected to the two ends of the fiber pigtail of the SOA (L3) as shown in Fig. 3 . In Fig. 3, we have lumped the length of the SOA within the L3 distance. Thus one round trip for the laser cavity is:
Kim et. al.  derived the transmission function of a Lyot-Sagnac loop with two sections of PM fiber of different birefringence and the equation is expressed as:Eq. (3) by setting L 2=0 and θ 2=0 and the above equation can be written as
Using trigonometric identities Eq. (4) can be simplified and the filter transmission function can be written as:
The first term of the transmission function T(λ,θ 1) is a function of the wavelength dependent phase difference between the two axes of the PM fiber that produces a sinusoidal output and it is responsible of the space between the two wavelengths. On the other hand, the second term is a function of θ 1, which depends upon the position of the polarization controller PC2. However, we will treat it to be a constant value since we do not change the position of the PC2 during the experiment and so we can express Eq. (5) as:
Note that the sin(θ 1) in the Eq. (6) determines the amplitude of the transmission function and the phase term is not influenced by the angle of θ 1. Since we are only interested in the phase term we choose θ 1=90° to disregard the amplitude dependency in our simulation. We set the length of PMF, L PMF~2 m and varied Δn 1, until the phase difference in the peaks between the experimental and the simulated data becomes negligible. In Fig. 4 , we plot two representative peaks of the experimental data and compare them with the simulated data. The spacing between two adjacent peaks, Δλ and the birefringence, Δn 1 are found to be 2.79 nm and 4.31×10−4, respectively. The linewidth (Full Width Half Max-FWHM or 3 dB) of the emitted wavelengths is measured at 0.3 nm.
Note that the Lyot-Sagnac filter is placed inside the cavity of the laser and the photon undergoes number of trips before it is emitted. Therefore, the transmission function in the laser cavity with unit gain, T(λ)LC can be expressed as:Eq. (7) is increased, the linewidth of the transmission function becomes narrower and we notice a good fit between the simulation (squares) and the experimental data (solid line) for n=10 (Fig. 4(a)). Also, taking into account the gain profile the experimental data and measurements are in a very good agreement as shown in Fig. 4(b). We observe that the 3 dB linewidth for the experimental and the simulation curve for ten passes is 0.3 nm, while the theoretical 3 dB linewidth for the single pass is 1.4 nm.
3.3 Wavelength tunability
The wavelength tunability is realized by the following methods:
- a) varying the reflectivity of the Sagnac loop mirror (LM1) and
- b) changing the cavity loss with a VOA positioned in the cavity as shown in Fig. 1.
Since our first approach, to wavelength tunability, is reflectivity dependent, we made sure that the cavity loss was kept to its minimum. Hence, in this approach we do not use the VOA. The total cavity loss is found to be less than 10 dB. However, this loss can be further reduced by replacing 8 FC/APC fiber connectors (not shown in Fig. 1) with splices. The reflectivity of each loop mirror is kept at ~90% and the output is measured by the OSA.
The graph in Fig. 5 shows that the lasing wavelengths are shifted to longer wavelengths although the gain of the SOA is small in that region. We also repeated the experiment without the PMF and observed that the lasing operation produces a strong output signal in the same region (e.g., >1580 nm)
We studied the gain characteristic of the SOA, which is designed for C-band operation, at two different wavelengths. One of them is chosen in the C band region (e.g., 1554 nm) while the other at a longer wavelength in the L band (e.g., 1597nm). A tunable C and L-band laser source (Santec-T415) was connected to the SOA and its output was monitor wit and OSA. First we set the laser source at 1554 nm and vary the input power to the SOA from −10.9 dBm to 6.2 dBm and monitored the output power at the OSA. The experiment was repeated by setting the wavelength of the laser source at 1597 nm. The gains at those two wavelengths are shown in Fig. 6 . We notice that when the input power to the SOA is set at about −10.2 dBm the gain at 1554 nm and 1597 nm are 18 dB and 10.3 dB, respectively. However, as we increase the input power to 6.2 dBm the gain at 1554 nm decreases to 5.5 dB, while the gain at 1597nm reduces to 4.1 dB. Clearly, at high input power (e.g., 6.2 dBm), gain compression at the shorter wavelength (e.g., 1554 nm) is very high compared to the longer wavelength (e.g., 1597nm). Hence, the gain difference between the two wavelengths is merely 1.48 dB in the saturation region, whereas, the gain difference remain high (e.g., 8.2 dB) when the SOA is operated in the linear region. As a result, for high input power the SOA shows a peak gain shift toward longer wavelength . We can conclude that under lasing conditions at high feedback power, the system starts emitting in the L-band region instead of the C-band.
We can regulate the feedback power to the SOA by varying the reflectivity of the loop mirror LM1 and hence exploit the gain compression for wavelength tunability. Figure 7 demonstrates that the wavelength tunability can be achieved by simply changing the reflectivity of the Sagnac loop (LM1). Note that during the aforementioned experiment, the reflectivity of the Loyt-Sagnac loop (LM2) was constant (~90%). We investigated the system performance under two different SOA bias currents. For IB =100 mA, when the reflectivity of LM1 was changed from 6.9% to 89.1%, the center of the lasing wavelength band was tuned from 1545 nm to 1579 nm (Fig. 7 (a)). As the reflectivity is increased so does the feedback (input) power to the SOA, which forces the SOA to operate at saturation and hence at the longer wavelength band. A similar trend is observed, when the SOA is operated at higher bias currents. Nevertheless, at higher bias currents (e.g., 200 mA), we observed that a larger number of multi-longitudinal-mode lasing oscillations are generated (Fig. 7(b)). We observe that the number of lasing wavelengths increased from 7 to 9 as the bias current changed form 100 mA to 200 mA. At 100 mA as the wavelength tuned from L-band to C-band we observed that the ASE level also increased, thereby, decreasing the signal-to-spontaneous-noise ratio form 31 dB to 20 dB for IB= 100 mA. However, at higher bias current (IB= 200 mA) the signal-to-spontaneous-noise ratio remained ~30 dB throughout C and L band and the total output power of 9 dBm was measured.
The second method for wavelength tunability is achieved by inserting a VOA in the laser cavity as shown in Fig. 1. The VOA is set at 0 dB and then the polarization controller of the loop LM1 is adjusted so that laser oscillations take place at the longer wavelengths. After this initialization process and after reaching a steady state, the settings of the two polarization controllers, PC1 and PC2 are kept unchanged throughout the experiment. At a VOA setting that provides minimum cavity loss, the feedback power to the SOA is maximized and hence induces gain compression of the SOA and shift the gain peak towards the L-band. As the insertion loss through the VOA is gradually increased to 10 dB, we notice that the lasing wavelength shifts from the L-band to C-band as shown in Fig. 8 . Note that the number of lasing wavelengths also increases as the SOA bias current changed from 100 mA (Fig. 8(a)) to 200 mA (Fig. 9(b) ). The number of wavelengths also depends upon the length of the PM fiber and the fiber birefringence. In our experiments, we chose 2 m long PM fiber because it was the longest PM fiber available in our laboratory.
3.4 Wavelength and power stability
The stability of the laser was evaluated by measuring the output wavelength using the OSA, for 1 hour at time intervals of 1 min with a resolution bandwidth of 0.01 nm (no averaging was used). Figure 9 shows power and wavelength fluctuations of ±0.5 dB and 0.1 nm respectively for one of the wavelengths (e.g., 1549.85 nm) of the laser output. We believe that this power fluctuation is caused by small temperature variations in the laboratory that affect the state of polarization of the propagating signals and hence the output of the laser. This drift can be minimized by proper packaging of the system and if the length of the cavity is further reduced.
We have proposed and demonstrated a simple fiber laser with dual port tunable multi-wavelength operation by using a SOA and two fiber loop mirrors. Due to the simple design and the limited number of fiber-optic components in the cavity, we are able to keep the cavity loss to low levels (e.g., <10 dB). Two different methods were implemented for wavelength tunability. The first one is based on adjustment of the bias current, while the second one is based on increasing the cavity loss. In the latter case, an additional component (e.g., VOA) is used. We measure the maximum total power of +9 dBm when the system was operated using the first method, while, the second method output maximum power was measured at 6 dBm.
Simultaneous lasing operation of 9 wavelengths with a wavelength separation space of ~2.8 nm was observed. The number of lasing wavelengths increases with the bias current of the SOA. We were able to accomplish tunability from C to L band, about ~50nm, by using a C-band designed SOA. Power and wavelength fluctuation of <± 0.5 dB and ±0.1 nm is observed and further improvement can be accomplished by proper packaging. Using longer length of PM fiber will allow for a larger number of output wavelengths. Note also, that since no isolators and/or circulators are used in the system, the design can be implemented on a photonic integrated circuit platform allowing us to have better temperature stability control and hence better optical power and wavelength stability. Finally, the potential of this fiber laser is that it can be tailored to provide tunable wavelengths at a variety of optical bands, since it is based on the PMF and the SOA that can be designed to operate at different optical bands.
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