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We investigate bidirectional optical wave propagations in a dual-pumped erbium doped fiber ring laser without isolator, and observe optical bistabillity behaviors. Consequently, we propose and construct a NOLM-NALM fiber ring laser to demonstrate and exploit this bidirectional optical bistability phenomenon in optical switching by introducing two tunable variable ratio couplers in the system. Numerical analyses based on the proposed laser structure have also been demonstrated corroborated with the experimental results.
©2004 Optical Society of America
1. Introduction
Unidirectional ring lasers provide good lasing efficiency, less sensitive to back reflections and good potential for single longitudinal mode operation [4]. Hence, most of the lasers built todate satisfy this criterion. This unidirectional operation is often achieved by incorporating an isolator within the laser cavity. Several research works are done based on this unidirectional behavior. Recently [1] has reported the strong optical bistability phenomenon in a unidirectional erbium doped fiber ring laser (EDFRL) operating in L-band. They have achieved more than 100mW wide bistable region, and proposed that the bistable region controllability by lasing wavelengths and erbium doped fiber lengths. Other than the continuous wave operation, this bistability behavior has also been observed by [5], however with small pump modulation. This optical bistability effect is the basic operation principle for optical logic gates, optical memories and optical switches.
In this paper, we investigate the bidirectional lightwave propagations in an erbium doped fiber ring laser without the use of isolator and bidirectional optical bistability is observed. Having identified this bidirectional bistability of the laser, we propose to turn this commonly known as undesirable signal into a useful one. Hence, a nonlinear optical loop mirror — nonlinear amplifying loop mirror (NOLM-NALM) fiber ring laser is proposed and constructed. In addition, we developed a numerical model based on the laser constructed. Optical switching capability has been achieved both numerically and experimentally.
2. Theoretical analysis
We refer readers to [2] for the theory of optical bistability. Here, we focus on developing a model for our constructed laser. For simplicity, we assume a noiseless, lossless and linearly polarized system in our NOLM-NALM fiber laser model. As the cavity length is relatively short, the dispersion effect can be ignored.
Let T0n and T1n be the through-amplitude transmission coefficient and cross-amplitude transmission coefficient of coupler n respectively, and are defined as T0n=√κn and T1n=j√(1-κn), where j2=-1, and κn is the coupling ratio of coupler n. For a coupler with coupling ratio κn of and inputs of E1 and E2, the outputs can be written as
Nonlinear optical loop mirror (NOLM) is formed by connecting the two output ports of the coupler together, as shown in Fig. 1(a). The structure of nonlinear amplifying loop mirror (NALM) is somehow similar to NOLM, however with an additional gain element in between the output ports, as depicted in Fig. 1(b). With input injected into port1, the outputs for both structures at port2 are [6,7]
where n2 is the nonlinear coefficient, L is the loop length, λ is the operating wavelength. G is the gain of the amplifying element, and is defined as G=g0/{1+(Ii/Is)}, where g0 is the unsaturated gain coefficient, Ii is the input intensity and Is is the saturation intensity of the amplifier. In NALM, additional nonlinear phase shift has been introduced into the system, depending on the preceding input power. The inter-connection between the VRCs acts as a feedback path for the laser. With this feedback and the nonlinear mechanisms within the cavity such as saturable absorption of EDFA and nonlinear phase shift, optical bistability is observable. We numerically simulate the laser behavior by combining the effects mentioned above.
3. Experimental setup
3.1 Dual-pumped erbium doped fiber ring laser
The experimental setup of the simple dual-pumped erbium doped fiber ring laser is shown in Fig. 2(a). It is basically a simple optical closed loop with a dual-pumped EDFA, which is forward pumped by a 1480nm laser diode, with maximum pumping power of 200mW and backward pumped by a 980nm laser diode, with maximum pumping power of 120mW, and some other associated optical couplers. The pump signals are coupled into the cavity via 1480/1550 nm and 980/1550 nm WDM couplers. The erbium doped fiber used in the setup is about 20m and the Er3+ concentration is about 200ppm. The maximum gain and saturation power obtained from this EDFA is about 20dB and 16dBm respectively. A 10dB optical coupler is used to tap out the signals in both clock-wise and counter clock-wise directions, i.e. Output 1 (counter clock-wise) and Output 2 (clock-wise). All the connections are spliced together to eliminate and minimize the reflection losses. All the fibers used in the setup are standard single mode fiber, Corning SMF-28, with dispersion value of ~16.7ps/nm/km. The total cavity insertion losses are estimated to be about 4dB. It is noted that the laser cavity does not contain any optical isolator and optical bandpass filter. This is simply because we want to investigate the bidirectional lightwaves propagations under free lasing condition, i.e. without any spectral constraint.
3.2 NOLM-NALM fiber laser
The second part of the experiment is done by introducing a few passive photonic components into the laser cavity as shown in Fig. 2(b), which is used to investigate the switching behavior of the proposed laser. Two variable ratio optical couplers (VRCs), with coupling ratios ranging from 20% to 80% are inserted into the cavity. One of the outputs of VRC1 is connected to one of the inputs of VRC2. An optical bandpass filter, with 2nm 3dB bandwidth and 10nm tunable range centered at 1560nm, is used to reduce the amplified spontaneous emission (ASE) noise within the cavity and for lasing wavelength tunability. No isolator is used in this setup as well since we are turning the bidirectional propagating lighwaves into our advantage. The outputs are extracted and observed at Output1 and Output2 from VRC1 and VRC2 respectively.
Similar laser structure has been demonstrated by [3] in obtaining the unidirectional lightwave propagation, whereby the lightwave was passed through in one direction and suppressed in the other. The work was concentrating on the power difference between the two outputs, and did not extend further into unconventional and nonlinear regions. Similar unidirectional results are obtained in our laser too when the ratios of the VRCs are adjusted to the desirable values as discussed in [3].
Fig. 2. Experimental setup for (a) dual-pumped erbium doped fiber ring laser; (b) NOLM-NALM fiber laser
4. Results & discussion
4.1 Dual-pumped erbium doped fiber ring laser
The ASE spectrum of the laser is observed and shown in Fig. 3(a), which covers the wavelength range from 1530nm to 1570nm. By increasing both the 980nm and 1480nm pumping currents to their maximum allowable values, we obtained bidirectional lasings, which is shown in Fig. 3(b). The upper plot of the figure is the lightwave propagating in the clock-wise (Output2) direction while the other one in the opposite direction. The lasings of the laser in both directions are not very stable due to the disturbance from the opposite propagating lightwave and the ASE noise contribution in the absence of filter. The Output1 is mainly contributed by the back reflections from the fiber ends and connectors, as well as some back-scattered noise. This observation serves as a basic understanding of the erbium doped fiber ring laser under no directionality and spectra constraints prior to the NOLM-NALM fiber laser construction.
Fig. 3. (a) ASE spectrum of the laser; (b) Lasing characteristics in both directions (upper trace: Output 2, lower trace: Output1)
The bistable characteristics at a lasing wavelength of about 1562.2nm for both clock-wise and counter clock-wise directions are shown in Fig. 4 and Fig. 5, with log and linear scales for the vertical axes of Fig. 4(a) & Fig. 5(a) and Fig. 4(b) and Fig. 5(b) respectively. We obtain about 15dBm difference between the two propagating lightwaves. We maintain 980nm pump current at 100mA and adjust the 1480nm pump current upwards and then downwards to examine the bistability behavior of the laser. The bistable region obtained is about 30mA of 1480nm pump current at a fixed value of 100mA of 980nm pump current. No lasing is observed in the counter clockwise (CCW, Output1) lightwave propagation. This bistable region can be further enhanced by increasing the 980nm pump current to a higher level. By maintaining 980nm pump current at ~175mA, we obtain a bistable region as wide as ~70mA 1480 nm pump current. When the 980nm pump current is maintaining at a higher level (>175mA), the lasing of Output2 (CW) remains in the 1480 nm pump current decreasing interval even when the 1480 nm is switched off, as shown in Fig. 5(a) and 5(b). This bistable behavior is mainly due to the saturable absorption of the EDF [5].
Fig. 4. Hysteresis loops obtained from the EDFRL for 980 nm pump current=100 mA, (a) log scale; (b) linear scale
Fig. 5. Hysteresis loops obtained from the EDFRL for 980 nm pump current=200 mA, (a) log scale; (b) linear scale
4.2 NOLM-NALM fiber laser
One interesting phenomenon observed before the bandpass filter is inserted into the cavity is the wavelength tunability. The lasing wavelength is tunable from 1530 to 1560 nm (almost the entire EDFA C-band), by changing the coupling ratios of the VRCs. We believe that this wavelength tunability is due to the change in the traveling lightwaves’ intensities, which contributes to the nonlinear refractive index change, and in turn modifies the dispersion relations of the system, and hence the lasing wavelength. Therefore, the VRCs within the cavity not only determine the directionality of the lightwave propagation, but also the lasing wavelength. The details of the wavelength tunability of this laser involve significant works, both analytically and experimentally. The main interest of this paper is the bistable state of the laser and not the wavelength tunability, therefore the physics and investigations of the wavelength tunability are excluded here.
For a conventional erbium doped fiber ring laser, bistable stable is not observable when the pump current is far above the threshold value, where saturation starts to take place. However, a small hysteresis loop has been observed in our laser setup even with high pump current, i.e. near saturation region, when changing the coupling ratio of one VRC while that of the other one remains unchanged, as shown in Fig. 6. Changing the coupling ratio of the VRC, is directly altering the total power within the cavity, and hence modifying its gain and absorption behavior. As a result, a small hysteresis loop is observable even with a constant high pump power, which can be an added advantage to the existing bistable state.
Fig. 6. Hysteresis loop observed while changing the coupling ratio of one VRC while maintaining that of the other, when operating at high pump current.
The power distribution of Output1 and Output2 of the NOLM-NALM fiber laser obtained experimentally is depicted in Fig. 7(a). We are able to obtain the switching between the outputs by tuning the coupling ratios of VRC1 and VRC2. The simulation results for transmisivities for various coupling ratios under linear operation are shown in Fig. 7(b), since the available pump power of our experiment setup is insufficient to create high power within the cavity. Both the experimental and numerical results have come to some agreements, but not all, since the model developed is simple and does not consider the polarization, dispersion characteristics etc. of the propagating lightwaves.
Fig. 7. (a) Experimental results for Output1 (Op1) and Output2 (Op2) for various coupling ratios (k1 — coupling ratio of VRC1, k2 — coupling ratio of VRC2); (b) Simulation results for transmisivities of Output1 (solid line) and Output2 (dotted line) for various coupling ratios of VRC1 and VRC2
6. Conclusions and future prospects
Bidirectional optical bistability in a dual-pumped erbium doped fiber ring laser without isolator has been studied. A ~70mA 1480 nm pump current bistable region has also been obtained. With this bidirectional bistability capability, we experimentally constructed and numerically simulated a NOLM-NALM fiber laser for switching operation. The VRCs used in the setup not only control the lightwave directionality, but also its lasing wavelength. Having understood the basic operation of this type of laser, we foresee its potential in applications, such as photonic flip-flops, optical buffer loop, photonic pulse sampling devices, etc.
Acknowledgments
This research is partially supported by the Nanyang Technological University academic research fund.
References and links
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