Transcript
Injection locking of Fabry-Pérot lasers and the temperature dependence of their emission in selfseeded WDM passive optical networks Marko Šprem Applied Optics Laboratory, Department of Wireless Communications Faculty of Electrical Engineering and Computing, University of Zagreb Unska 3 / XII, HR-10000 Zagreb, Croatia e-mail:
[email protected]
Abstract—The influence of temperature on coated (2%) and uncoated Fabry-Pérot laser output in a proposed colorless WDM system with self-seeding via modulation averaging reflector is studied. In particular, the connection between the information about the optical laser output from the laser integrated monitor photodetector and optical power output of the entire system with temperature change of the laser diode is investigated.
broadband light source (BLS) to provide seeding light to identical transceivers connected to the WDM ports on the AAWG in the remote node. The BLS is located in the central office. The light emitted from the BLS is filtered by the AAWG (also referred to as spectral slicing) and brought to the client end where it is used to injection lock the FP-LD or reflective semiconductor optical amplifier [11].
Keywords—Fabry-Pérot; colorless; WDM-PON.
Among the disadvantages of the self-seeded network architecture is intensity variation of the optical wave returning to the gain element (due to the presence of modulation in the seeding light) and fragile link stability. The first problem reduces the link margin but can be reduced by saturating the gain element (laser or reflective semiconductor optical amplifier) [7] or by using modulation averaging technique [8]. The gain elements are generally very polarization dependent, while the birefringence in the fiber and other optical elements degrade, add noise and can destabilize the link. Stabilization of self-seeded systems has been one of its difficult problems. Time reversal of the accrued polarization is one approach [9], while depolarizing the beam and stabilizing modulation using modulation-averaging reflectors (MAR) located in the remote node is another [8][10] (used in this work). The transmitting part of a self-seeded WDM-PON system is illustrated in Fig. 1 when a MAR is connected at the output of the AAWG. Light oscillates between the Fabry-Pérot laser diode (FP-LD) and the MAR while it is being filtered with the AAWG and coupled out (as P4) of the cavity using a coupler.
self-seeding;
I. INTRODUCTION Self-seeded colorless wavelength-division-multiplexed passive optical networks (WDM-PON) have been investigated as a part of New Generation PON phase 2 development efforts on access networks. They may give growth to access networks with the lowest cost bidirectional link per Gbps per user [1-4]. The widespread adoption of self-seeded WDM-PON is still limited by high transmitter cost and so there is a market need for cost-effective colorless WDM transmitters. This work is focused on creating an adaptable low-cost multi-wavelength transmitter for WDM-PON. Self-seeding WDM-PON architecture comprises an extended resonator with an optical gain and modulation source at the client end and reflector and a wavelength-selecting filter in the remote node. The extended cavity can be many kilometers long. The multiplexer is a wavelength-selecting filter, specifically an athermal array-waveguide grating (AAWG) with any number of channels adapted to the dense WDM ITU grid. AAWG with 16 - 96 channels with 200, 100, 50 and 25 GHz channel separation are available today. The target data rate per wavelength today is 10Gbps at channel separations 100 GHz and smaller. The filtered amplified spontaneous emission light emitted by self-seeded WDM-PON transmitters exhibits a fundamental noise and rate limits owing to the statistical properties of thermal light it emits [5], but up to date transmission rates over 10 Gbps have been reported in selfseeded systems [6], hence target is achievable. Self-seeding is an alternative approach to the previous generation WDM-PON architectures which employ a
The longitudinal modes of FP-LDs shift with temperature around 0.1 nm/°C. When using uncooled FP-LDs for injection locking, the intention is to ensure that the seeding light matches the wavelength of one of the FP-LD resonant modes. If the wavelengths did not match, for example with temperature change of FP-LD, the output power of the FP-LD 100 GHz
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Figure 1. Measurement setup for self-seeding technique via MAR.
This work first presents the temperature dependence of the light-emission characteristics of a short-cavity FP-LD with 2% front-facet reflectivity as groundwork for temperature tuning of FP-LD performance in WDM-PON systems. A FP-LD which was extracted from a 16-channel 200-GHz BLS-seeded WDM-PON system is used and applied to a denser 100 GHz athermal AWG by self-seeding. The second part investigates how effective injection locking can be achieved using uncoated FP-LDs in the same self-seeding technique, in compared to injection locking with BLS-seeding as in [12]. The results show that the front facet reflectivity of uncoated FP-LD is too high for injection locking. In other words, the FP-LDs have to be coated to reduce the front-facets reflectivity to establish a stable link II. EXPERIMENTAL SETUP The test setup (Fig. 1) forms an extended cavity composed of injection-locked FP-LD, a 32-channel AAWG with 100 GHz channel spacing, and a MAR. By this a test setup forms a self-seeded multi-wavelength optical source. The optical couplers provide measurement of optical powers: incident on the FP-LD (P1), emitted by the FP-LD (P2), reflected by the MAR (P3), and emitted by the multi-wavelength optical source (P4). The test setup imitates real-life WDM-PON system with the exception of having lengths of distribution fiber between client end and the remote node, and the feeder fiber between remote node and central office equal to the patch cords of optical couplers used. To drive the FP-LD and control the temperature a microcontroller based setup was formed (Fig. 2). It was used to set laser drive current ILD, the power to the heater (10 Ω resistor), to monitor the integrated photo diode (IMPD) current IPD, and read the temperature T using the LM35 thermometer. An optical power meter, with analog output which was fed to the microcontroller, was used to measure the optical powers (P1-P4). In this way, all the experiments, data acquisition and export were conducted using the microcontroller. The advantage of this approach was its simplicity and it was a preliminary test for integrating the required functionality into a single fiber-optic module that would contain the FP-LD and a microcontroller. Data in port (Fig. 2) was not used in any experiments, hence the FP-LD was operated in continuous-wave (CW) mode. A feedback control loop was implemented on the microprocessor to adjust the laser drive current ILD in order to maintain either constant photodetector current IPD or optical powers (P1 or P2) as we vary the FP-LD temperature. These
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could not be sufficient for successful transmission. One way for eliminating this risk is to use AWG with channel frequency width wider than the frequency mode spacing of FP-LD, so that at least two mode wavelengths fit within the channel bandwidth. In this way, the seeding light always excites at least one mode regardless of the temperature of the FP-LD. A problem occurs when using AAWGs with channel spacing that is smaller than the longitudinal mode separation in the FP-LD. However, manufacturing of long FP-LDs is problematic because of higher cost. For 100 GHz wavelength separation, the FP-LD length has to approach 1 mm.
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Figure 2. FP-LD driver scheme. Driver current (ILD) was driven by the DAC port and monitored as voltage on a 20 kΩ resistor by the ADC port. The photodetector current (IPD), temperature from LM35 temperature sensor and optical detectors (P1 - P4) were all sensed by the available ADC ports on the microcontroller. The heater (10Ω) was driven by pulse width modulated output. DATA IN port was not used.
measurements are referred as constant IPD mode, constant P1, or P2 mode. The effective coupling of optical energy from the cavity formed by the FP-LD and the AAWG (and possibly MAR) to the outside world (P4) is referred to as output coupling. III. MEASUREMENTS WITH 2% COATED FP-LD The FP-LD was extracted from an optical network terminal (ONT) of a commercial BLS-seeded WDM-PON system (ADC PONy Express 16 DWDM PON). The laser is packaged in a Transmitter Optical Subassembly (TOSA) along with an integrated monitor photodiode (IMPD) and mounted on a temperature-controlled stage. The FP-LD laser came with a ~2% front-facet coating and about 60 GHz mode separation. Fig. 3(a) shows the free-running (no optical feedback) emission spectra of the FP-LD. When no optical feedback is provided, the laser does not reach threshold within the operating current range planned by the control circuit, hence the free-running spectra (Fig. 3(a)) never shows any dominant wavelengths. When the external cavity is terminated with a MAR, the AAWG provides optical feedback peaking at only one wavelength thus injection locking the FP-LD. This is manifested with a strong and narrow emission shown in Fig.
Figure 3. Emission spectrum of 2% coated FP-LD measured at P2 when (a) there is no extended cavity formed (“free-running emission”) and (b) the emission spectrum measured when MAR is connected, namely, the FP-LD is injection-locked. The laser drive current in both cases was ILD=35 mA.
3(b) and reduced laser gain (shallower resonance fringes in the emission spectrum). Channel 3 of AAWG with wavelength 1535.1 nm was used for all experiments. The temperature change of the FP-LD in the proposed injection locking system should cause a change in photodetector current (IPD), output laser power (P2), and the system output power (P4). Longitudinal modes of FP-LD shift towards longer wavelengths with increase of temperature. Since every channel of AAWG is an intra-cavity filter it is clear that when any one of the FP-LD longitudinal modes coincides with the selected channel, the output power (P4) should increase. In addition, since the photodetector current IPD is proportional to the optical power emitted from the laser, the IPD can be used to determine when FP-LD mode coincides with the externally asserted filter center wavelength (with temperature change). For example, when using the feedback loop to adjust the ILD to maintain constant IPD, the ILD should show a minimum when extended cavity has lowest loss, i.e. when the FP-LD mode coincides with the AAWG channel. The results reported in this paper show that this simple analysis is generally true, but that the relationship between laser drive current ILD, photodetector current IPD and the outputs of the optical system (P1, P2 and P4) is a bit more complex and not entirely understood. All the measurements were made with both heating and cooling temperature-scan directions. There is a small (<1°C) shift in temperature between the data for those two scans, but the relationships between currents (IPD, ILD) and optical outputs (P1, P2, P4) were the same. Therefore only results for the cooling scans are shown. A. Constant IPD In this measurement, the laser temperature was varied from 43°C to 24°C while the ILD was adjusted to keep IPD constant. The laser emission spectrum at P2 and the output powers from P1, P2, and P4 were monitored. The spectra shows that wavelengths of multiple modes of the FP-LD all drift towards longer wavelengths (Fig. 4), but that there is substantial modepulling when any of the FP-LD modes come close to the externally asserted low loss wavelength. This is visible by the peak wavelength jumping to or away from the constant value of 1535.1 nm. The measured currents and optical powers (P1, P2 and P4) are normalized and plotted on the same graph (Fig. 5). It can be seen that the drive current ILD oscillates as the longitudinal modes of the laser go in and out of resonance with the externally asserted AAWG channel. There is also growth of ILD on the average with the increase of temperature, as a result of laser gain reduction with temperature. When at some temperature the laser mode matches with the AAWG channel, the losses of the extended resonator (between FP-LD and MAR) are at minimum. With the lower resonator losses the laser current is expected to be at minimum as well. The oscillations in the optical power P2 are synchronized with the laser current ILD and exhibit extrema that coincide with the extrema in the ILD current. That is expected as more current through the gain causes the laser to emit more spontaneous emission.
Figure 4. Plot of the wavelengths of distinguishable local maxima in the emission spectrum P2 of the injection locked FP-LD as temperature varies.
The emitted power P4 and the power returning to seed the FP-LD also exhibit an oscillation versus temperature. The variation in the seeding power P1 is proportional to P4 since P1 is effectively a filtered and reflected version of P4. It was expected that the minima in the laser-emitted power P2 would coincide with the maxima of the emitted and seeding powers P4 and P1. However, the results show that this is not the case. There is a small, but noticeable temperature offset between the P2 minimum and P4 maximum (shown in Fig. 5). The temperature offset is between 1.2°C and 1.6°C. Optical power P3 was the same as power P1 with small power difference due to the AAWG channel attenuation. B. Constant P2 In this measurement, the ILD current was driven to maintain constant power P2. The results, shown in Fig. 6 show behavior consistent with what was observed in Constant IPD measurement previously: P2 is proportional to ILD, except for the growth in average ILD with temperature, and a small offset between the temperatures of maximum P4 and IPD. Relative to temperature change, the change of ILD current seems to be almost linear, with very small variations. Compared with results in Fig. 5, it can be confirmed that variations in laser power P2, with temperature change, depend mostly on
Figure 5. Driving ILD current for constant IPD current. All values are plotted with linear units; P2≈[-12.94, -11.66] dBm; P4≈[-15.95, -14.67] dBm; P1≈[21.73, -24.28] dBm; ILD≈[20, 30] mA; IPD≈250 µA.
Figure 6. Driving ILD current for constant power P2. All values are plotted with linear units; P2≈ -9.5 dBm; P4≈[-14.56, -13.40] dBm; IPD≈[300,370] µA; ILD≈[25, 34] mA.
variations of ILD current. C. Constant P1 In this experiment, the ILD current was driven to maintain constant power P1. The results show different behavior from what was observed in Constant P2 measurement previously: IPD, ILD and P4 all seem to be proportional with one another, except for the growth in average ILD with temperature (Fig. 7). The temperature offset between maxima is very small. Relative to P4, the ILD is in offset by ~ 0.2°C, and the IPD is by ~ - 0.2°C. Compared with results in Fig. 6, the slope of the curves is to the right. IV. MEASUREMENTS WITH UNCOATED FP-LD The goal of these measurements was to evaluate whether it was possible to achieve the same injection-locking temperature dependent results with an uncoated FP-LD. The FP-LD driver scheme was the same as in the previous measurements, but the test setup had a difference of having the optical coupler of 99% between FP-LD and AAWG. Since this FP-LD had larger front-facet reflectivity, the greater coupling ratio would ensure better injection locking. Channel 23 of AAWG with wavelength 1550.9 nm was used in this measurement. Only results for spectrum analysis of emitted laser power P2 are given, the free running spectra and spectra with injection-locking using MAR (Fig. 8). The temperature of the FP-LD was set so that one longitudinal mode would match with the channel wavelength of the AAWG. It can be seen that at that channel wavelength one mode dominates, but also that the adjacent modes are also fairly dominant. The gain of the laser did not reduce as it was the case with 2% coated FP-LD. The FP-LD with this spectrum is not efficient for usage in communications, because there is substantial energy stored in the adjacent resonant modes. It would have had higher biterror-rate (BER) and shorter transmitting distance in comparison with 2% coated FP-LD. The main problem is that the front-facet reflector is now comparable to the externally provided feedback (by self-seeding) and hence with a small
Figure 7. Driving ILD current for constant power P1. All values are plotted with linear units; P1≈ -24.40 dBm; P4≈[-15.89, -14.65] dBm; ILD≈[23, 38] mA; IPD≈[150, 265] µA.
increase in ILD the Fabry-Perot laser starts lasing regardless of whether self-seeding is present. This means that Fabry-Perot lasers should be coated to reduce this effect in order to realize a stable link, controllable by the external reflector. V. CONCLUSION Injection locking of FP-LDs by self-seeding allows one to use the available optical and electrical variables at the client end (IPD, ILD, P1 and P2) to gain information about the interaction between the modes of the FP-LD and the AAWG at the remote node which is hundreds of thousands of meters away. In this work, a simple microprocessor-driven control system is presented, featuring feedback control loops for stabilizing the laser output depending on either monitor photodiode current, or any other optical power measured in the system. The developed algorithms will be implemented in a complete temperature and output optical power stabilizing system – the goal of this project.
Figure 8. Emission spectrum of 30% coated FP-LD measured at P2 when (a) there is no extended cavity formed (“free-running emission”) and (b) the emission spectrum measured when MAR is connected.
The origin of the temperature offset between temperaturedependence of light emitted from the laser (P2) and the light emitted from the multi-wavelength optical source (P4) is not entirely understood. The offset is related to the mode pulling and the mutual interaction of the laser with the AAWG mode. Finally, in this work the result is given for injection locking an uncoated FP-LD. It is shown that the injection locking with only one dominant resonant longitudinal mode of this FP-LD cannot be achieved with the proposed self-seeding scheme. Therefore, FP-LDs with front-facet reflectivity lower than on the uncoated devices will be used for future development for an adaptable low-cost multi-wavelength transmitter for WDM-PON. ACKNOWLEDGMENT This work was funded by HRZZ research project “EXCAR - Sustainable Optical Communication and Sensor Systems based on Extended-Cavity Resonators” IP-11-2013-3425. REFERENCES [1]
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