Multiwavelength fiber laser sources with bragg-grating ... - IEEE Xplore

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 19, NO. 4, APRIL 2001

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Multiwavelength Fiber Laser Sources with Bragg-Grating Sensor Multiplexing Capability L. Talaverano, S. Abad, S. Jarabo, and M. López-Amo, Senior Member, IEEE

Abstract—Two erbium-doped fiber ring lasers (EDFRLs) with simultaneous emission at four different wavelengths are demonstrated. Both systems employ fiber Bragg gratings (FBGs) to select the operation wavelengths within the ring. The sensing capability of the FBGs has been taken advantage of, allowing for the sources to be used as sensor multiplexing schemes. The first system employs four FBGs in a tree filter topology, achieving four output channels with 5 dBm power each. The second system comprises an in-line filtering topology with active fiber segments within the filter. This second source yields 2-dBm output signals and allows for a higher number of lines to be easily added to the system. A comparison between both topologies is carried out, and their capability for sensor multiplexing is demonstrated. Index Terms—Erbium-doped fiber (EDF), fiber Bragg grating (FBG), optical fiber amplifiers, optical fiber lasers, optical fiber sensors, ring lasers, sensor interrogation, wavelength division multiplexing (WDM).

I. INTRODUCTION

E

RBIUM-doped fiber ring lasers (EDFRLs) have been known light sources for quite a long time. Their main benefits over nonfiber-based sources are the high output powers and the small losses involved in their connection to fiber-optic networks. In the last few years and due to the huge development of wavelength division multiplexing (WDM) technology, EDFRL sources emitting in multiple wavelengths simultaneously have been investigated. The selection of their operation wavelengths has been achieved by using different optical filtering devices, like WDMs and arrayed waveguide gratings (AWGs) [1]–[3], Mach–Zehnder filters [4]–[6], in-fiber comb filters [7], or fiber Bragg gratings (FBGs) [8], [9]. Also, a variety of methods have been employed to try to maximize the number of emission lines and to improve their stability, like fiber cooling with liquid nitrogen [4], [5], [7], [10]. Regarding FBGs, their use as optical fiber sensors has also been a very important research field. The advantages of fiberoptic sensors over electronic sensors on certain applications, like harsh environments or as embedded sensors in composite materials, have already been proven [11]–[13]. Among fiber-

Manuscript received October 3, 2000; revised January 16, 2001. This work was supported by CICYT under Project CICYT-TIC98-0397-C03-01 and by the Gobierno de Navarra, Spain. L. Talaverano, S. Abad, and M. López-Amo are with the Departamento de Ingeniería Eléctrica y Electrónica, Universidad Pública de Navarra, Pamplona E-31006, Spain. S. Jarabo is with the Departamento de Física Aplicada, Universidad de Zaragoza, Zaragoza E-50009, Spain. Publisher Item Identifier S 0733-8724(01)02760-8.

optic sensors, FBGs have a growing importance given their simplicity, small size, low losses, and flexible design. FBGs are usually employed as strain and/or temperature sensors, encoding the information of the measured parameter on their operation wavelength. Combining these two fields, i.e., EDFRL sources and FBG sensors, multiwavelength sources and multiplexing systems for fiber sensor networks can be obtained. As an example, in [8], an EDFRL was employed for sensor interrogation in a four FBG strain sensor network. However, the source on that system operated on a single wavelength by including an additional tunable Fabry–Pérot filter within the ring, and therefore the information from the four sensors could not be obtained simultaneously. In this paper, two different multiwavelength EDFRLs are shown, both of them employing FBGs to perform the selection of the operation wavelengths. The sensing capacity of the FBGs gives these sources the capability to be also used as sensor-network multiplexing schemes, simultaneously obtaining the information from all the sensors. This multiplexing technique provides high signal powers, thus improving the signal-to-noise ratios (SNRs) achieved by using conventional broadband interrogation schemes, which are usually restricted by the limited power of this type of sources. Also, by exploiting the FBG wavelength dependence with temperature and strain, tunability of the accomplished multiwavelength sources can be achieved. II. EXPERIMENTAL SETUPS AND RESULTS Both topologies comprise a standard single-mode fiber (SMF) ring in which EDF is incorporated, acting as the active medium. For these setups, a commercial EDFA was used (Photonetics, model BT 1300), providing 13-dBm output saturation power and a maximum 35-dB small signal gain. This EDFA already contains the 980-nm pump source, a wavelength division coupler, and two optical isolators, which ensure unidirectional operation and therefore avoid the spatial hole-burning effect. As already stated, the wavelength selection will be carried out by means of FBGs. These gratings will also be used for wavelength tuning of the sources and as transducer elements when using the systems as sensor multiplexing schemes. The operation wavelengths of the FBGs have been located in the flat region of the erbium gain profile in order to achieve good equalization between channels. Gratings of 1539.6, 1547.7, 1552.2, and 1557.7 nm have been used, each one showing 0.5-nm bandwidth and 99% reflectivity. One of the major problems in multiwavelength ring lasers is correctly adjusting the cavity losses on each wavelength in order to achieve oscillation of the system in all the desired channels.

0733–8724/01$10.00 ©2001 IEEE

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Fig. 1. Experimental setup for the tree filter topology. = 1539:6, = 1547:7, = 1552:2, and = 1557:7-nm.

The oscillation threshold power for each wavelength is different due to the nonflat shape of the erbium fiber gain profile; thus individual loss control for each wavelength is required. To solve this problem, a variable attenuator (VA) has been employed on each grating. In our setups, these VAs have been built by fixing SMF to micrometric screws, and losses are adjusted by changing the curvature radius of the fiber. This provides a simple and nonexpensive attenuator while avoiding insertion losses [9] . All the free terminations on both systems have been immersed in refractive-index-matching gel to avoid undesired reflections. A. Tree Topology The experimental setup for the first EDFRL is shown in Fig. 1. Three-decibel couplers are used to incorporate the FBGs into the laser cavity, creating a tree topology. Optical power inside the ring is thus divided into four branches of approximately equal power. Each of these branches is composed of the FBG and a VA, used to finely adjust the losses of the corresponding wavelength in order to achieve oscillation in all the channels. A 90/10 coupler is used to extract 10% of the EDFA output power from the ring. The output spectrum of this fiber laser for a 90-mW pump power is shown in Fig. 2. The power of each of the four output channels is around 5 dBm, and the power differences between them are within 0.5 dB. For every channel, the signal power is more than 35 dB higher than the ASE noise floor. Fig. 3 shows the total output power as a function of the pump power. The lasing threshold has been found to be 6.5 mW, and the efficiency is 1.45%. Still other two- and three-wavelength sources could be found on the free branches of the 3-dB couplers and on the gel-matched terminations of the Bragg gratings. These supplementary high-power sources could be used as additional monitoring points for the sensor network, increasing its reliability. Fig. 4 shows an example of these (a) two- and (b) three-wavelength outputs. Note that in the two-channel outputs [Fig. 4(a)], along with the expected two signals there is remnant power emission at the other operating wavelengths in the ring. This power is due to nonideal response of the

Fig. 2. Multiwavelength output for the tree filter topology as measured by the optical spectrum analyzer for a 90-mW pump power.

FBG and to unwanted reflections on the system. The effect of this residual power will depend on the application given to these additional sources. If they are used as light sources, the remnant power from the rest of the FBGs might be considered a noise source, which could introduce severe crosstalk along the communication system. On the other hand, when used for sensor interrogation, these lines will also carry the information of the rest of the sensors in the system, and might be used for monitoring. Signal power values for all the sources in the system are shown in Table I. These values correspond to 90-mW pump power on the EDFA. The main drawback of this topology is the difficulty involved in expanding the number of channels in the structure. Increasing the number of channels to eight would involve adding four more

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Fig. 3. Efficiency curve for the tree filter topology.

couplers to the three already used, and therefore raising the cavity losses by at least 6 dB. This increase in losses would reduce the system’s efficiency and make difficult the simultaneous oscillation of the structure in all the desired wavelengths.

(a)

B. Series Topology The second source demonstrated is based on an in-line filtering topology, as shown in Fig. 5. In this case, the FBGs are placed consecutively and inserted into the ring through a circulator, and again a VA is included with every FBG. However, the individual control of each channel is not straightforward this time, since modifications made to the first channels also have an effect on the following FBGs. An added difficulty is the higher attenuation suffered by the channels as we advance into the filtering structure, due to the transmission through the gratings, splices, connectors, and VAs. For our setup, these losses restricted the number of oscillating channels to just two. In order to overcome this limitation, optical amplification is included within the filter in the form of EDF segments pumped by a 980-nm source. By combining active fiber segments and the fine attenuation control provided by the VAs, the oscillation conditions for the wavelengths in the system can be achieved. As shown in Fig. 5, an amplifying stage has been included after the second FBG, allowing four-channel oscillation with good power equalization between output channels (Fig. 6). Given the low cavity losses in this configuration, the coupling constant ( ) of the power-extraction coupler can be increased. This yields higher power output signals, thus improving the efficiency of the system. Table II summarizes the results obtained for five different values of . As expected, higher values of clearly improve the system’s efficiency. Still another advantage of increasing is that the signals remaining on the filter branch are weaker, and lower pump power is needed to compensate for losses within the filter. As a drawback, leaving weaker signals inside the ring starts driving the amplifier out of the saturation regime. This increases competition between channels and reduces stability. As shown in Table II, for coupling ratios higher than 75%, stable simultaneous oscillation of the four channels could not be achieved. Bearing all this in mind, the optimum

(b) Fig. 4. (a) Two-channel and (b) three-channel outputs on the tree filter topology as measured by the optical spectrum analyzer for a 90-mW pump power.

value of must reach a compromise between efficiency and stability. The laser output for a 50% coupler and 90-mW pump power on the ring is shown in Fig. 6. All the channels have powers greater than 2 dBm, and the optical SNR values are higher than 35 dB. The differences of power among the four channels have been kept under 1.5 dB by using the VAs. The main advantage of this system is its capability to be easily expanded to a higher number of channels. To prove this advantage, a fifth FBG is added to the filter, having 1559.6-nm cen-

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POWER VALUES

FOR

EACH CHANNEL

IN

ALL

THE

TABLE I OUTPUTS OF THE EDFRL WITH TREE FILTERING TOPOLOGY. * INDICATES POWER VALUE UNDER ASE NOISE FLOOR LEVEL

Fig. 5. Experimental setup for the in-line filter topology. = 1539:6, = 1547:7, = 1552:2, and = 1557:7-nm.

tral wavelength. In this case, no more amplifying segments have been required to achieve oscillation, although further amplification could be included in the filter if necessary. Although strong gain competition appears between gratings at 1557 and 1559 nm, simultaneous oscillation on the five wavelengths has been accomplished, as shown in Fig. 7. III. COMPARISON BETWEEN TREE AND SERIES STRUCTURES

Fig. 6. Multiwavelength output for the in-line filter topology as measured by the optical spectrum analyzer. 90-mW pump power and 50% coupling ratio on the output coupler.

Given their common nature, the two structures exhibit quite parallel features. They are both ring lasers with erbium-doped fiber acting as the active medium, and therefore their wavelength range limitations are similar. Since also the FBGs employed are equal, most of the emission-line characteristics are identical. The differences between both systems arise from the unlike filtering topologies. Attention will be now paid to the particular advantages and drawbacks of each of the demonstrated structures. As already mentioned, the main drawback of the tree topology is the difficulty involved in expanding the number of channels, which would imply a higher number of couplers with the associated increase in cavity losses. Even with just four channels, the cavity losses of this structure are much higher than the in-line structure losses. These high cavity losses force

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TABLE II FEATURES OF THE EDFRL WITH IN-LINE FILTERING TOPOLOGY AS A FUNCTION OF POWER EXTRACTION RATIO

IV. APPLICATION AS SENSOR MULTIPLEXING SYSTEMS

Fig. 7. Five-wavelength laser oscillation on the in-line filter topology.

keeping the of the output coupler low, yielding a very low efficiency of the system. As an advantage, since the FBGs are located in independent branches, individual channel attenuation control is uncomplicated and simultaneous oscillation for all the channels can be achieved in a fairly simple way. Also interesting is the fact that the filter is completely passive, making the structure more reliable. Still another strong point of this network is the presence of multiple monitoring points for the sensor multiplexing system, which further reinforce the reliability of the network. The in-line filter structure’s major weakness is its complexity. Active devices are included within the filter, and attenuation control is hard to carry out, making it difficult to achieve simultaneous oscillation of the channels. As for its benefits, the main advantage of the structure is its capability to be expanded, which has already been demonstrated by adding a fifth channel without major difficulty. Also, the efficiency and output powers of this second structure are much better than those obtained with the tree topology due to the lower losses of the cavity.

The possibility of using the same FBGs employed for filtering as temperature or strain sensors has already been pointed out. We have chosen to use the FBGs as temperature sensors, measuring a sensitivity of 0.01 nm C. In order to demonstrate the multiplexing capability of the already presented structures, the 1539-nm FBG on the in-line filter topology has been placed inside a climatic chamber, and temperature cycles from room temperature (24 C) up to 100 C have been carried out. Fig. 8 shows the output signals of the system at (a) 55 C and (b) 75 C. As shown, the 1539-nm FBG changes its central wavelength from 1539.81 to 1540.06 nm (0.25 nm), while the rest of the channels in the system remain unchanged. The sensitivity and response of the FBG sensors are not modified when the sensor is placed inside the EDFRL system. Given that the FBGs’ wavelength variations are small, the simultaneous oscillation of all the channels is maintained, and no additional attenuation control through the VAs is needed. Care must be taken when choosing the operation wavelengths of the system, so that the wavelength domains of the FBGs do not overlap. Considering a 2-nm domain for each sensor (200 C temperature variation) and a 30-nm erbium gain bandwidth (1530–1560 nm), 15 sensors could be included in the system. Nevertheless, the limiting factor will probably be the maximum number of simultaneous oscillating channels on the ring without deterioration of the system stability due to gain competition between channels. V. CONCLUSION The multiple-wavelength emission of two EDFRL sources has been achieved by using FBGs to perform wavelength selection on the systems. Two different filter topologies have been demonstrated, accomplishing four-channel sources with high output powers. The first system comprises a tree topology FBG filter, yielding output powers around 5 dBm for each of the four channels and displaying additional two- and three-line sources. The second system uses an in-line FBG filter topology, which involves additional optical amplification to overcome losses and allows for an increase in the number of emission lines. The efficiency of this second system is higher than the first one, and signals over 2 dBm have been obtained, although a compromise exists between efficiency and stability on the system. Further

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hand, the in-line structure allows for an easy expansion of the number of channels, although the inclusion of active devices within the filter increases the complexity of the system. Finally, the capability of these systems for sensor interrogation by using the FBGs as transducers has been demonstrated. Multiplexing of temperature FBG sensors has been proven, maintaining the simultaneous oscillation of all the channels. REFERENCES

(a)

(b) Fig. 8. EDFRL with in-line filter topology as temperature sensor network interrogation scheme. output at (a) 55 and (b) 75 C.

work will be done in order to improve the stability, especially focusing on the optimization of the filter amplifying sections to reduce the amount of fluorescence introduced into the ring by the filter’s EDF. A comparison between both structures has been carried out, and the major drawbacks and strong points of both topologies have been reported. The first structure provides simple individual channel control but lacks flexibility when increasing the number of channels and shows a low efficiency. On the other

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L. Talaverano, photograph and biography not available at the time of publication.

S. Abad, photograph and biography not available at the time of publication.

S. Jarabo, photograph and biography not available at the time of publication.

M. López-Amo (M’91–SM’98), photograph and biography not available at the time of publication.