MULTIWAVELENGTH LIGHT SOURCE WITH INTEGRATED DFB LASER ARRAY AND STAR COUPLER FOR WDM LIGHTWAVE COMMUNICATIONS

We have witnessed a rapid advancement in optical fiber telecommunications in the last decade. The bit rate of early optical transmission systems in the late 1970's was 45 Mbit/s. By the m1d4980's, the transmission speed had been increased to rates ranging from 400 Mbitls to 560Mbit/s. More recently, SONET hierarchy, from OC- 1 , OC-3, OC-12, to OC-48 at 51.84 Mbitis, 1 55.52MbitJs, 622 Mbitls to 2.488 Gbitls respectively, has been the standard chosen by CCITT. The next SONET standard rate of OC-l92 at 10 Gbitis is now under consideration. Although laser transmitters at 10 Gbitls rate are still in the research and development stage, new products will be introduced in the marketplace to fuffill systems and networking needs in the near future. The singlemode optical fiber has a potential bandwidth of over 30 THz. In order to utilize this bandwidth, high density wavelength-division-multiplexing (WDM) has been considered. WDM can offer not only the increased usage of bandwidth, but also the flexibility of routing signals in a network by means of wavelength [11. In the last few years, the transmission distances have been increased dramatically due to the use of Er-doped fiber amplifiers. High power semiconductor lasers at either 0.98 tm or 1.48 .tm wavelengths are used for the pump. The powers into the amplifier required for both pump wavelengths are about 20 mW and 50 mW respectively. On the other hand, semiconductor opticai amplifiers are attractive for monolithic integration with lasers, waveguides, modulators, wavelength multiplexers and demultiplexers. For example multi-wavelength DFB laser arrays, as many as 20 wavelengths, have been integrated monolithically with wavelength combiners (star coupler) and optical amplifiers on InP substrate [2). These integrated multi-wavelengths DFB laser arrays have the advantage of easier wavelength control and lower cost per wavelength than the discrete DFB laser transmitters for WDM applications. Such photonic integrated circuits (PlC) can increase the functionality and reduce the optical fiber-to-device interfaces that are the main factor for the high cost of optoelect.ronic components. The fiber-to-device interface is also the major concern for component reliability.

Multi-wavelength light sources are key components in wavelength division multiplexing (WDM) optical transmission systems and are indispensable in future photonic networks. Monolithically integrated distributed feed back (DFB) laser arrays are widely used for their stable and highly reliable single-mode operation in modern WDM system, which enables optical networks cost-effective, reliable and provides higher performance and more functionality. Different lasing wavelengths of a laser array can be obtained either by varying the pitch of the built-in grating or by changing the waveguide's effective refractive index from element to element.

A 16-channel electroabsorption modulated tunable multi-wavelength DFB laser array is proposed and experimentally demonstrated. The sixteen DFB lasers, each with a wavelength spacing of 100 GHz, are arranged in a 4 × 4 matrix. The wavelengths are designed using an interleaving scheme to reduce grating crosstalk. The grating is designed and fabricated using the reconstruction equivalent chirp (REC) technique to simplify the grating fabrication and ensure precise wavelength control. The electroabsorption modulator and the tunable multi-wavelength laser array share the same active layer to reduce fabrication complexity and costs. Wavelength tuning is achieved by selecting the active laser without altering the thermoelectric cooler (TEC). Consequently, 16 channels with 100 GHz spacing are obtained, each operating in good single-mode properties with SMSRs of above 45 dB. The average wavelength spacing is 106.1 GHz, and only 6.1 GHz deviated from the design. The semiconductor optical amplifier (SOA) is integrated to compensate for the frequency chirp and amplify the output power, thus enabling long-distance transmission. The output power is amplified to over 18 mW with a 150 mA current injected into the SOA. The 3-dB bandwidth of the EAM exceeds 20 GHz, producing clear 25 Gb/s back-to-back eye diagrams with mask margins over 40% for the 16 channels. With the assistance of the chirp compensation by the SOA, the eye diagrams of the 16 channels remain clear after 10 km of transmission. Due to the superior performance, the proposed device is promising for use in DWDM systems, particularly in applications requiring fast channel switching, such as optical packet switching and NG-PON2 technology.

We review the progress of multiwavelength DFB laser arrays made for multiwavelength optical networks. The goal is to reduce the per-wavelength transmitter cost in manufacturing and network element control. Using photonic integration, we have addressed and resolved several important issues related to laser arrays such as wavelength accuracy, output power and optical packaging. State of the art results are summarized and its impact on the multiwavelength optical network is assessed.

We discuss the design, fabrication, and performance of experimental multiwavelength laser array transmitters that have been used in the reconfigurable optical network testbed for the Optical Network Technology Consortium (ONTC). The experimental four-node multiwavelength network testbed is SONET/ATM compatible. It has employed multiwavelength DFB laser arrays with 4 nm wavelength spacing for the first time. The testbed has demonstrated that multiwavelength DFB laser arrays are indeed practical and reproducible. For the DFB laser arrays used in such a network the precise wavelength spacing in the array and the absolute wavelength control are the most challenging tasks. We have obtained wavelength accuracy better than /spl plusmn/0.35 nm for all lasers, with some registered to better than /spl plusmn/0.2 nm. We have also studied the array yield of our devices and used wavelength redundancy to improve the array yield. Coupling efficiencies between -2.1 to -4.5 dB for each wavelength channel have been obtained. It is achieved by using specially designed lensed fiber arrays placed on a silicon V-grooved substrate to exactly match the laser spacing. The transmitter consisted of a multichip module containing a DFB laser array, an eight-channel driver array based on GaAs IC's, and associated RF circuitry.

A multi-wavelength laser source is important for applications in areas such as wavelength-division multiplexing (WDM), interferometry, lidar and optical data storage. Simultaneous oscillation of multipe wavelengths have been demonstrated in several methods including spatially chirped Bragg reflector vertical cavity surface emitting laser (VCSEL) arrays,1 multichannel grating cavity (MGC) laser,2 multistripe array grating integrated (MAGIC) laser,3 and multi-wavelength DFB laser array by control selective area MOVPE.4 In those devices, arrays are required for the generation of multi-wavelength oscillation. Each array element is responsible for its individual wavelength oscillation. In this work, it is demonstrated that a single-stripe Fabry-Perot laser diode is able to oscillate at selected multiple wavelengths with a wavelength separation ≥1.3 nm, several times larger than the Fabry-Perot longitudinal mode spacing.

X-ray lithography was used to fabricate a first-order grating with a lambda /4 phase shift for a DFB (distributed feedback) laser array. The grating fabrication process is shown. Pulsed operation results are shown for a ridge-type laser array, which was designed with 7.5-A wavelength variation from laser to laser and a spacing of 125 mu m. >

With the significant technology advancement in photonic integration and the demands of compact light sources of multiple wavelengths, multiwavelength laser arrays (MWLAs) can play important roles in high-capacity optical signal transmission, photonic switching, and optical sensing. Manufacturing with semiconductor foundries and the integration with electronic integrated circuits bring not only advanced technologies but also new concepts for realizing MWLAs. This paper will first overview the existing technology development and then discuss the challenges and possible solutions for enabling photonic-integrated MWLAs. The overview will cover monolithically and hybrid-integrated MWLAs. The wavelength registration for both in-plane and vertical-cavity MWLAs will be discussed and compared.

The authors propose a star topology for multihop lightwave networks in which the conventional N*N passive star coupler is replaced by fixed wavelength division multiplexing (WDM) cross-connects. The proposed topology overcomes three major limitations of the conventional star topology. First, it reduces the number of wavelengths needed in a (p,k) ShuffleNet from kp/sup k+1/ wavelengths in the conventional topology to p wavelengths in the proposed one. Second, the signal power loss due to the 1/N power splitting at the star coupler no longer exists in the WDM cross-connects and, therefore, the restriction on the supported number of users by the star network is alleviated. Third, it completely eliminates the need for wavelength filtering at the input to the receivers, as is the case in the conventional star topology. The use of 2*2 WDM lapped couplers as a cheap WDM cross-connect for a (2,k) ShuffleNet is investigated. A p*p star coupler followed by p fiber Fabry-Perot filters can be used as a p*p WDM cross-connect for a (p,k) ShuffleNet at the expense of using p/sup 2/ rather than p wavelengths. >

An equivalent corrugation pitch modulated distributed-feedback semiconductor multiwavelength laser array (MLA) with two equivalently assisted phase shifts (EAPSs) based on the reconstruction-equivalent-chirp technique is theoretically studied and experimentally demonstrated. The simulated results show that the longitudinal photon density distribution of the studied MLA is much more uniform than that of the MLA without EAPSs. Accordingly, the longitudinal spatial hole burning of the proposed MLA is therefore suppressed more effectively. The experimental results show that the studied MLA has good single-longitudinal-mode performance. The highest side mode suppression ratio (SMSR) is even up to 52.63dB. Meanwhile, the tunable wavelength range of the investigated MLA is 25.94nm under the thermal tuning scheme from 15.12°C to 46.93°C. All channels are within a wavelength deviation of ±0.001n m. The SMSRs are all above 38dB.

This paper describes the performance improvement of 10-Gb/s four-channel WDM dispersion-supported transmission systems by using broadened passband arrayed-waveguide grating demultiplexers. Multimode interference couplers are assumed to be used to broaden the passaband of these demultiplexers. Performance Improvement of 40-Gb/s Capacity Four-Channel WDM Dispersion-Supported Transmission by Using Broadened Passband Arrayed-waveguide Grating Demultiplexers Mario M. Freire Department of Mathematics and Computer Science, University of Beira Interior Rua Marques d'Avila e Bolama, P-6200 Covilha, Portugal Telephone: +351 75 3101700, Fax: +351 75 26198, E-mail: mfreire@ alpha.ubi.pt Henrique J. A. da Silva Department of Electrical Engineering, Pole II of University of Coimbra Pinhal de Marrocos, P-3030 Coimbra, Portugal Telephone: +351 39 7006248, Fax: +351 39 7006247, E-mail: hjas@ci.uc.pt Summary Integrated N×N wavelength multiplexers based on arrayed-waveguide gratings (AWG) are being developed for wavelength division multiplexing (WDM) systems and networks, since they can offer four basic functions: multiplexing, demultiplexing, add/drop multiplexing, and N×N full-interconnection [1]. The use of this technology for demultiplexing in high density WDM systems may impose very stringent requirements on the alignment of signal wavelengths and demultiplexers due to their peaked (Gaussian-like) spectral passband. However, the passband of these demultiplexers may be broadened by the use of multimode interference couplers (MMI), as proposed and demonstrated in [2]. In this paper, we describe the performance improvement due to the use of broadened passband AWG demultiplexers in 40-Gb/s capacity four-channel WDM dispersion-supported transmission (DST) systems with 0.5 nm channel spacing. The block diagram of the simulated 10-Gb/s four-channel WDM-DST system is shown in Fig. 1. The system model is broadly similar to the one reported in [3] and includes a rate equation model for quantum-well lasers, and the frequency responses of a PIN photodiode and of a standard singlemode fiber (SMF) with chromatic dispersion of 17 ps/(nm.km) at 1532 nm. The frequency response of the AWG filter is assumed to be Gaussian and the frequency response of the broadened passband AWG filter is modeled by the double Gaussian approximation [2] with r = 2 , which is the ratio between the peak separation and the FWHM of each simple Gaussian profile. The optical amplifiers (EDFAs) have been considered as linear with a noise figure of 6 dB and an equivalent noise bandwidth of 1 THz. A RC lowpass filter was used to provide the 3-dB bandwidth required by the DST method [4]. The receiver sensitivity was estimated taking into account the signal-ASE and ASE-ASE beat noises, the thermal and shot noises, as well as the contribution due to frequency-to-intensity conversion of laser phase noise after propagation via dispersive fibers [5]. Fig. 2 shows the receiver sensitivity for channel 2 versus fiber length for single-channel DST, without optical filters, and for four-channel WDM-DST using an arrayed-waveguide grating demultiplexer with (BP-AWG) and without (AWG) broadened passband. For comparison purposes, the receiver sensitivity of four-channel WDM-DST using a three-mirror Fabry-Perot demultiplexer (TMF) is also shown. For distances larger than 80 km, the performance of four-channel WDM-DST systems using three-mirror Fabry-Perot demultiplexers is close to the performance of single-channel DST systems without optical filters. Compared with the performance of single-channel DST without optical filters, the use of conventional arrayed-waveguide grating filters as demultiplexers in four-channel WDMDST degrades the system performance at least 0.6 dB for link lengths larger than 11 km. However, the use of broadened passband AWG demultiplexers instead of the conventional AWG demultiplexers improves the performance of the four-channel WDM-DST system at least 1.5 dB, for link lengths larger than 11 km (see Fig. 2). Detailed BER characteristics for channel 2 are displayed in Fig. 3 for four-channel WDM-DST via 204 and 253 km SMF. For the 204 km transmission simulation, sensitivities of -23.886, -23.169 and -24.827 dBm have been estimated for TMF, AWG and BP-AWG demultiplexers, respectively. The BER curves for 253 km clearly indicate an error floor due to laser phase noise. In conclusion, the use of broadened passband AWG demultiplexers improves the performance of four-channel WDM-DST systems with 0.5 nm channel spacing, while conventional AWG demultiplexers are less suitable to operate at this channel spacing.

High performance, inexpensive light sources are needed for the advancement of wavelength division multiplexed (WDM) photonic access networks. In this paper, the unique features of multiwavelength modelocked semiconductor lasers are demonstrated with an emphasis on possible integration into access networks. An evaluation of multiwavelength passive, active, and hybrid modelocking reveals that hybrid modelocking is optimum for the suppression of mode partition noise as well as coherence tailoring in multiwavelength lasers. Low-distortion propagation of 10 channeltimes5 GHz multiwavelength pulses through multimode fiber is achieved. Intracavity gain flattening enables the simultaneous generation of 6 GHz pulse trains into 168 parallel wavelength channels for a 1 THz aggregate pulse rate from a single laser source. This work conveys the potential for semiconductor laser modelocking to produce multiple telecommunications wavelengths for data transmission and auxiliary functions in photonic networking

We propose what we believe to be a novel method for realizing monolithically integrated multi-wavelength surface-emitting laser arrays (MWSELAs) and experimentally demonstrate its feasibility. The MWSELA consists of six oxide-confined grating-coupled surface-emitting lasers (GCSELs), where the emission wavelength of each device is tuned by adjusting the period of the surface grating while maintaining an identical epitaxial structure. The fabricated six-wavelength GCSEL array achieves precise wavelength control with an average wavelength spacing of 1.6 nm. All elements in the array exhibit excellent single-mode operation, with side-mode suppression ratios exceeding 30 dB at an injection current of 12 mA and low threshold currents of approximately 1.5 mA. The proposed method offers a new pathway to realize monolithically integrated MWSELAs with flexible physical layouts through a simplified fabrication process. Its advantages-including low power consumption, stable single-mode performance, and compatibility with wafer-level manufacturing and testing-make it an ideal candidate for developing cost-effective MWSELAs.

A laser array of four DFB (distributed feedback) lasers with a wavelength spacing as small as 1 AA has been demonstrated. The wavelength spacing is coarsely set by the built-in grating periods and is fine-tuned by a heater array integrated with the laser array. A tuning range of 14 AA and a channel spacing of 1 AA have been achieved. It is shown that both thermal crosstalk and RF crosstalk can be very low. It is noted that the integrated thermally tuned DFB laser array will be very useful in optical frequency division multiplexing systems. >

Distributed feedback (DFB) semiconductor laser arrays operating at precisely controlled wavelengths are important components for wavelength division multiplexing networks. Recently, precise wavelength control has been realized by the reconstruction-equivalent-chirp (REC) technique based on sampled Bragg gratings (SBGs) [1]. However, the effective coupling coefficient, κ, of a sampled grating is necessarily reduced substantially from that of a uniform grating, compromising the single longitudinal mode (SLM) performance of DFB lasers. To overcome this, designs of SBGs with phase shifted grating sections have been proposed and demonstrated in fibre lasers. In these structures, the (not required) zeroth-order mode is suppressed while the index modulation experienced by the nonzeroth-order channels is enhanced, the ±1st order channels being of particular interest [2]. Here, for the first time, we apply a combination of π-phase shifted gratings with the REC technique to DFB diode lasers. Using a single electron beam lithography (EBL) step we have demonstrated an increased effective coupling coefficient κ and have fabricated an eight-channel laser array with a spacing of 100 GHz (∼0.8 nm @1550 nm). For the same channel spacing using conventional 0th order gratings (Fig. 1(a)), a resolution of about 0.125 nm would be required, which is beyond the typical resolution limit of 0.5 nm of EBL machines.