The following articles show it's not having a DBR laser but what you do with it that makes you competitive:
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Step-wise tunability of a DBR laser with a superimposed fiber grating external cavity
Mattias Andomat, Pierre-Yves Fonjallaz, Fredrik Olofsson
ACREO AB Isafjordsgatan 22, 164 44 Kista, Sweden
Tel: + 46 8 632 7753, fax: 46 8 632 7710, email: matado@imc.kth.se
Fiber Bragg gratings (FBG) have been used in many different ways in combination with semiconductor laser diodes to improve their emission properties [1-3]. One motivation for using FBGs is their good wavelength stability in comparison with simple Fabry-Perot cavities or even with DFB or DBR diode lasers. FBG can be used to stabilize the emission wavelength of diode lasers both actively and passively. In the first case, part of the laser emission is detected after reflection on and transmission through a fiber grating. The relative intensity of both signals is used to adjust the currents settings of the diode to maintain the wavelength constant [4]. In the second case, a pigtailed FBG is used as the out-coupling reflector of the cavity. In that situation, the laser cavity suffers from a relatively high loss due to the imperfect coupling from the diode to the single-mode fiber. Nevertheless, since the cavity is made considerably longer (factor 10 to several 100), and thanks to the spectral selectivity of the grating which allows for the selection of a single longitudinal mode, spectral linewidths of the order of few hundreds of kHz are achievable [1]. In addition to the wavelength stability and reduced linewidth obtained with FBG external cavities, there is an increasing interest in being able to switch the wavelength between standardized channels. This has recently been reported and a sampled FBG was used in combination with a simple FP cavity allowing for the switching between eight 100 GHz-spaced channels [1]. In this paper, we report on the wavelength stabilization and step-wise tunablity of a DBR laser with the help of a superimposed fiber grating. We believe it is a promising configuration to obtain robust and high quality step-wise tunable external-cavity semiconductor lasers (ECSL).
Superimposed FBG (SIFBG) are fabricated by exposing the fiber with subsequent interference pattern of varying modulation period [5]. It corresponds to fabricating a first grating and re-exposing the same portion of fiber to generate other resonance wavelengths. When the second exposition is realized, the Bragg reflectivity of the first grating is reduced. When the third exposition occurs, the two first resonances are reduced and so on. By adjusting the dose of the subsequent expositions, it is possible to get constant reflectivity peaks without side lobes. Superimposed gratings are much easier to fabricate than sinc-sampled gratings [6]. The main problem is the wavelength control since the frequency separation is not automatically constant, as it is the case for sampled gratings. For this experiment, a SIFBG has been fabricated, with three 40% reflectivity peaks and 100GHz spacing, using the multiple fringe printing technique [7]. The length is 40 mm and the apodization was chosen to be gaussian.
This SIFBG was combined with a DBR laser whose front facet was AR-coated to a residual reflectivity of about 1% (with a single layer of zirconium dioxide). The rear facet was left as cleaved. In order to increase the coupling efficiency to the DBR , the fiber had a lensed shaped extremity but was not AR-coated. The coupling efficiency was measured to be about 35%. The distance between the DBR and the center of the SIFBG was 25 mm.
In order to investigate the tunability of the proposed external cavity laser, the current of the Bragg (IBragg) and phase (IPhase) sections of the DBR semiconductor laser were both swept using a computer controlled current supply. During the currents sweep, at every new value of (IBragg, IPhase) the values of power and wavelength were measured. The results can be seen in Figures 1 and 2 (Igain = 50mA). Fig.1 wavelength mapping Fig.2 power mapping Fig.3 Cross-section of the wavelength mapping Fig.4 Cross-section of the wavelength mapping In figure 1, regions of constant wavelength (represented with grayscale) can be seen. This is even clearer in figures 3 and 4, which present sweeps of the Bragg current only. This shows that the wavelength tuning of the laser is robust with respect to tuning currents, which in turn makes it insensitive to degradation. Simultaneously, Figure 2 shows that those regions of constant emission wavelength also correspond to maximum output power. Lasing at other tuning currents is due to insufficient AR-coating. The maximum output power is 0.3mW, which is an order of magnitude lower than the power at the same working point for the DBR-laser alone before AR-coating. This is due to the 35% coupling efficiency between DBR and SIFBG.
Fig.5 Laser spectrum ( Ibragg = 46.2mA Igain = 40mA Iphase = 2.5mA T = 20.2 øC ) In this experiment side mode supression ratio (SMSR) was measured for a number of operating points. Values above 30dB were easily obtained. The linewidth was determined to be 0.5MHz in a self homodyne delayline measurement. The main benefit of this combination of a DBR and a SIFBG is the low sensitivity of the emission
wavelength to current variations. In particular, since the phase can vary rather much, such a device can probably be directly modulated with ultra low chirp [3]. The parasitic reflections inside the cavity should be reduced significantly in order to get a better insight of the maximal SMSR achievable. For direct modulation of such a device, the laser cavity has to be relatively short. Since the index profile of the SIFBG is not limited by the spatial resolution of the fabrication process, short multiple-peak FBGs can easily be fabricated. However, the maximum number of reflection peaks with a superimposed grating is only limited by the fiber?s photosensitivity to get reasonable reflectivities. Therefore, such a device might be improved considerably with respect to the number of stabilisation channels, output power and SMSR.
An external cavity DBR laser has been built with a superimposed FBG. Step-wise tunability over three wavelength channels has been demonstrated with excellent wavelength stability. Such a combination of a DBR and a SIFBG seems extremely promising regarding the relatively low quality of both fiber-DBR coupling and AR coatings in the case presented here.
[1] J.-F. Lemieux et.al., ?Step-tunable (100GHz) hybrid laser based on Vernier effect between Fabry-Perot cavity and sampled fibre Bragg grating?, Electronics Letters, vol. 35, no. 11, 1999
[2] D.M. Bird et.al., ?Narrow line semiconductor laser using fibre grating?, Electronics Letters, vol. 27, no. 13, pp. 1115-1116,
1991
[3] R. Paoletti et.al., ?15-GHz Modulation Bandwidth, Ultralow-Chirp 1.55-æm Directly Modulated Hybrid Distributed Bragg
Reflector (HDBR) laser Source, IEEE Photonics Technology Letters, vol. 10, no. 12, 1998
[4] M. Adomat et.al. ?Wavelength Stabilization of DBR Laser Using a Fiber With Multiple Gratings?, pp. 161-162, ECOC?98,
1998
[5] A. Othonos et al., ?Superimposed multiple Bragg gratings?, Electron. Lett. , vol. 30, pp. 1972-1974, 1994.
[6] M. Ibsen et al, ?Sinc-sampled fiber Bragg grating for identical multiwavelength operation?, OFC?98, pp. 5-6, 1998.
[7] P.-Y. Fonjallaz et.al. ?New Interferometer for Complete Design of UV-written Fibre Bragg Gratings?, pp. 36 38, OSA
Conference on Bragg Gratings, Photosensitivity, and Poling in Glass Fibers and Waveguides: Applications and
Fundamentals, 1997
Part of this work was financed by the Foundation for Knowledge and Competence Development. We would like to thank Henrik hlfeldt and Adela Saavedra for useful discussions.
And, finally, a paper by Siemens, "Reconfigurable all-optical networking at terabit transmission rates"
P. Leisching, H. Bock, A. Richter, D. Stoll, and G. Fischer
SIEMENS ICN TR ON E T, Advanced Transport Systems, 81379 Mnchen, Germany
Phone: 0049-89-722-23062, FAX: -24510, patrick.leisching@icn.siemens.de
Future all-optical telecommunication networks for terabit transmission rates require the use of optical routing to cope with the increasing capacity demand due to growing internet traffic. Laboratory wavelength division multiplexing (WDM) demonstrations have shown up to 3.2 Tbit/s (80x40 Gbit/s) transmission rate [1]. In contrast to long-haul WDM systems, metropolitan networks will be optimized to offer flexible and scalable bandwidth for new services using transparent optical transmission. The highest static drop capacity reported so far for a metropolitan network is 80 Gbit/s [2]. The SIEMENS subnetwork of KOMNET (innovative communication networks for the future information society) investigates dynamic optical routing of up to 80 WDM channels. In this paper we are reporting on transparent all-optical metropolitan network experiments at 10 Gbit/s
line capacity for each channel, i.e. 0.8 Tbit/s in one direction. The system limitations imposed by the 50 GHz ITU grid at 10 and 20 Gbit/s for reconfigurable optical add/drop multiplexers (OADMs) are evaluated. The OADMs are composed of tunable fiber Bragg gratings (FBGs), an optical frequency interleaver (OFI) and optical switching matrices.
1. Experimental set-up
The experimental configuration is shown in Fig. 1. The WDM ring consists of three 20 km spans of standard single mode fiber (SSMF) and matches the upcoming field-trial KOMNET in Berlin. Dispersion compensating fiber (DCF) at the end of each span ensures fully compensated segments with a residual dispersion of less than 50 ps/nm. Channels can be added/dropped at each of the three OADMs. Fig. 1. Schematic experimental set-up of the field trial in Berlin. Fig. 2.: Wavelength comb of 80 channels added at the hub node at the input of OADM I. All OADM sites will be connected to adjacent networks: There is a long-haul transmission system and an access network supplied by SIEMENS. The optical cross connect is used to switch traffic to another ring network. The reconfigurable OADMs are composed of either tunable FBG technology using drop and park position [3,4] or a combination of an OFI and tunable FBGs [5]. Following the wavelength-selective drop process there is an optical switching matrix to enable for optical routing to different tributary equipment and/or optical interconnections. Thus, independent wavelength channels can be dropped an routed by remote software control steering all-optical components.To investigate the system performance, the signal wavelengths were all added at 50 GHz spaced wavelength comb after transmitting one span is shown in Fig. 2. In order to create the datastream of 800 Gbit/s, 80 laser wavelengths in the C-band (1530.66 to 1562.16 nm) are combined and modulated at 10 Gbit/s (PRBS: 231-1) by a LiNbO3 Mach-Zehnder modulator. Gain flattened boosterEDFAs are used to amplify the signals to a maximum channel power level of 0 dBm.2.
Results and Discussion
The optical parameters of a piezo-tunable FBG module optimized for the 100 GHz ITU grid are shown i Fig. 3. Tuning the inner FBG from 192.6 to 192.55 THz switches this channel from drop to park position,i.e., to a through state. Inter- and intrachannel crosstalk values are well below 25/35dB indicating the good quality of the fiber gratings. The interchannel crosstalk of 20 to 25 dB additionally required for a 80 channel system is supplied by a conventional dielectric demultiplexer. Tuning the center frequency of the gratings does not change the shape of reflection and transmission spectra. The group delay (not shown) is also unchanged in park position. In addition to their good optical properties, the piezo-tunable FBGs are passively temperature compensated. A center wavelength stability of about 2 pm/øC can be achieved [4] thus enabling OADMs without active temperature stabilization. The piezo switching time from add to drop position is below 1 ms thus enabling optical protection purposes.
Fig. 3. Reflection and transmission of a FBG module in drop and park position.Fig. 4. BER versus received optical power of channels dropped at OADM I and OADM II. To set up a reconfigurable OADM for the 50 GHz ITU grid employing 100 GHz fiber grating technology an OFI is used [5]: The channels with 50 GHz spacing are converted into four streams of channels separated by 200 GHz. To demonstrate the system performance, the single channel power penalty is determined by a 10 Gbit/s and/or 20 Gbit/s bit error rate (BER) measurement (not shown). The
measurements at 10 Gbit/s of both piezo-tunable and thermooptically tunable FBGs reveal a power penalty in through position of < 0.5 dB. Penalties below 0.5 dB are also measured for the OFI. These results rule out excessive dispersion penalties and in combination with simultations demonstrate the cascadability of up to 7 OADMs in a metropolitan fiber ring system. Increasing the line rate up to 20 Gbit/s increases the power penalties to 2-3 dB indicating an upper limit for a system based on FBGs and 50/100 GHz technology. Typical results for the 50 GHz grid OADM network realized experimentally are shown in Fig. 4. An accumulated capacity of 0.8 Tbit/s is fed into the ring system. Error-free transmission is obtained for all wavelength channels that are re-routed by remote control. Dropping channel 7/11 out of 80 channels at OADM I leads to a penalty of about 1 dB. Electronically switching channel 7 or channel 11 from drop at OADM I to drop at OADM II increases the penalty to about 2 dB. Increasing the single channel power by 3 dB (channel 7/channel 11) decreases the penalty slightly at OADM I and significantly at OADM II.
These results and the single channel measurements confirm that the penalty increase from OADM I to OADM I is solely due to accumulation of EDFA noise and can be avoided using optimized EDFAs. To allow for optical routing following the wavelength selection process, the OADMs supplied in the field-trial are composed of a four FBG unit and a 4x4 optical switching matrix. System requirements for these key components are a low insertion loss (< 4 dB), high crosstalk values (> 40 dB), fast optical switching time (1 ms) for optical protection needs and an option for a latching state. The thermooptic 4x4 based on polymers fulfills all criteria besides the latching option. The microoptic solution is driven by
piezos with low power consumption, therefore enabling the latching state. The transmission and group delay properties of both alternatives do not affect a transmitted signal up to 10 Gbit/s. The stability of the traffic in the optically closed fiber ring loop is not affected by circulation of amplified spontaneous emission (ASE) from the EDFAs. The free spectral range of the demultiplexing component in the hub node sufficiently suppresses the ASE below 1530 nm and above 1562 nm. Thus out-of-band ring laser action is avoided. There is no in-band laser action, as long as there is one or more active channels present in the ring.
3. Conclusion
We demonstrated system operation of a 60 km metropolitan fiber-ring composed of reconfigurable OADMs capable of routing up to 80 optical channels using the 50 GHz ITU grid. Dynamic optical networking at an aggregate line capacity of 0.8 Tbit/s has been shown in this transparent WDM ring. Error free transmission was obtained for wavelength channels that were routed by remote control. The key components are an OFI, tunable FBGs and optical switching matrices. These components did not cause any significant signal distortions at 10 Gbit/s, i.e., FBG dispersion does not hamper cascadation of up to 7 OADMs. The distribution of dropped signals to a final destination can be performed by an optical switching matrix stage without affecting the signal quality. Limitations of the system operation are currently mainly due to the accumulation of EDFA noise. The optimum reconfigurable OADM for metropolitan purposes in the KOMNET field-trial includes an OFI stage to allow for 25/50 GHz channel spacing. To enable future reliable optical protection switching within 1 ms, piezo-tunable FBGs in combination with microoptic matrices are preferred. Using these OADMs, flexible drop capacities of several hundred Gbit/s are achievable, an order of magnitude larger than that of conventional SDH/SONET network elements.
4. References
[1] C. Scheerer, C. Glingener, A. F„rbert, J.-P. Elbers, A. Sch”pflin, E. Gottwald, and G. Fischer: ?3.2 Tbit/s (80 x 40 Gbit/s)
bidirectional WDM/ETDM transmission over 40 km standard single-mode fibre?, Electron. Lett., to be published, 1999.
[2] R.S. Vodhanel, F. Shehadeh, J.-C. Chiao, G.-K. Chang, C. Gibbons, and T. Suzaki: ?Performance of an 8-wavelength 8-node
WDM ring network experiment with 80 Gbit/s capacity?, OFC?96, postdeadline paper PD20, pp. 439 ? 442.
[3] P. Leisching, H. Bock, A. Richter, D. Stoll, and G. Fischer: ?Optical add/drop multiplexer for dynamic channel routing?,
Electron. Lett., vol. 35, pp. 591-592, 1999.
[4] A. Richter , T. Andritschke, H. Bock, P. Leisching, D. Stoll, L. Qu‚tel, and S. Aguy: ?Passive temperature compensation of
piezo-tunable fibre Bragg gratings?, Electron. Lett, vol. 35, pp. 1269-1270, 1999.
[5] H. Bock, A. Richter, P. Leisching, C. Glingener, D. Stoll, G. Fischer, P. Pace, J. Philipson, and Mark Farries: ?0.8 Tbit/s all-optical-
networking in a transparent WDM ring network?, submitted to ECOC?99
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