Larry A. Coldren
University of California, Santa Barbara, CA 93106
and Agility Communications, Santa Barbara, CA 93117
Email: coldren@ece.ucsb.edu

Over the past two decades or so, InP-based materials have been well developed for optical communications components in the 1300-1600 nm wavelength range. This paper will review some recent developments in these materials using two rather different laser examples based upon two somewhat different alloy systems lattice- matched to InP. The quaternary materials systems are chosen to provide more or less optimal device characteristics in both cases.
The first example is a lattice-matched vertical-cavity surface-emitting laser (VCSEL) that can be created to operate at any wavelength within the entire 1300 — 1600 nm range[1]. A schematic is shown in the upper-left side of Fig. 1. This figure also shows materials choices for this range. Figure 2 shows example results. To obtain the desired characteristics of low-threshold currents, high-output powers, and high-operating temperatures, these devices are constructed with MBE-grown undoped A1GaAsSb DBR mirrors, A1GaInAs quantum-well active regions, and n-doped InP intracavity contacts. A tunnel junction is used on the p-side of the active for hole injection. The high index contrast of the A1GaAsSb DBRs results in wide stopbands in both cases; i.e., >150nm @ 1550nm and >100 nm @ 1310 nm. This is useful for WDM arrays. Although relatively good results are illustrated, the 1310 device suffers from an improper gain/mode alignment and poor mirror morphology, and the 1550 device used only a crude aperture consisting of an etched out active region; i.e., more to come. The most recent research results include the development of high-quality apertures and electron blocking layers in this material system. By adding a small percentage of Ga to the AlAsSb stable oxidation with low optical loss is possible. The quality and benefits of using these are illustrated by the edge-emitter results in Fig. 3 [2].

Fig. 1 VCSEL schematic and materials choices for 1300-1550nm range.

Fig. 2: Experimental results for a Sb-based VCSEL.

Fig. 3: Experimental edge-emitter results at 1.55 microns.

Fig. 4: Schematic and results for sampled grating tunable lasers.

Fig. 5: CW performance of SGDBR-SOA-EAM.

Fig. 6: Results of Quantum Well Intermixing (QWI).

Fig. 7: Quantum Well Intermixing (QWI) transmitter results.

The second example actually includes two versions of a photonic integrated circuit technology to generate widely-tunable sampled-grating lasers integrated with amplifiers (SOAs) and modulators. In the first case, the approach for active/passive integration is the removal of offset quantum wells, and ridge waveguides are utilized for lateral photon confinement [3]. Figures 4 and 5 show the advantages of monolithic integration using the sampled-grating lasers and the cw performance of the SGDBR lasers integrated with SOAs and EAMs. In the second case (Figs. 6 and 7), a new quantum-well intermixing (QWI) technique is used for active/passive integration, and a buried rib structure is used for lateral carrier and photon confinement [4]. Both illustrate relatively high device performance. The materials in these cases are MOCVD-grown InGaAsP/InP alloys for the quantum-well active regions as well as the transverse confinement waveguide layers.
Figures 4 and 5 include results from Agility Communications, while Figs. 6 and 7 summarize UCSB work. The results indicate that any wavelength across > 40nm around 1550 nm can be accessed, that fiber-coupled output powers up to 20 mW are available in a cw version, and that integrated laser-modulator versions can provide reaches > 200 km @ 2.5 Gbs. New research on monolithically integrated modulators promises 10 Gbs transmitters with reaches > 80 km without dispersion compensation. The new QWI work shows that multiple bandgaps can be integrated in a high-performance integration platform for improved efficiency and performance. In this case a shallow P implant is placed in a sacrifical layer of InP above the active quantum-wells to create vacancies, these are diffused, the InP is removed, and the cladding is regrown. Multiple bandgaps are possible by alternately selectively removing the InP layer and annealing in between.
References
[1] S. Nakagawa, et al, S of Selected Topics in Quantum Electronics, 7, (2), p224, April, 2001
[2] M.H.M. Reddy, et al, Proc. 2002 MBE Conf, San Francisco; Also to appear S Crystal Growth, 2003.
[3] V. Akulova, et al, S of Selected Topics in Quantum Electronics, 8, Dec., 2002; Also, 0. Fish, et al, 2002 Proc. NFOEC
[4] B. Skogen, et al, Proc. 2002 Integrated Photonics Research, Vancouver; Also .1.5. T.Q.E., 8, Dcc, 2002

The Future of Optical Communications

Shigeyuki Akiba
KDDI Submarine Cable Systems Inc.
3-7-1 Nishi-Shinjuku, Shinjuku-ku
Tokyo 163-1033, Japan
TEL: +81-3-5908-3701 FAX: +81-3-5908-3711 E-mail: akibascs@kddiscs.co.jp

Abstract: While high-end transmission technology is approaching an inherent limit, switching/processing technology should play an important role as a driving element in future optical communication businesses. The evolution of bandwidth-hungry new services and business applications such as distant learning/education, telemedicine and other applications can be enabled by the growth of metro and access optical infrastructure and will eventually promote the growth in the whole photonic industry.

Fig. 1: Traffic growth did not catch up to the increase in capacity. This resulted in an over-build situation. New applications in education, telemedicine, entertainment, and others should evolve with the emerging bandwidth services in core networks and with the advancement of technology in metro and access networks.

1. Introduction
Internet demand together with WDM technology has driven a tremendous growth in the optical fiber infrastructure. This has resulted in an over-build or so-called glut situation as depicted in Fig.1. It is unfortunate that the fiber-optic communications business now faces a most difficult environment after the abrupt collapse of the market. The disrupted optical communication business can only recover by the demand created from the evolution of new services. These services such as video-based services through IP networks in education, medical, entertainment and shopping tend to be hungry for capacity. This paper discusses some scenarios in the revival of the optical network industry.

Fig. 2: A diagram of a future residence adopting home network technology.

Fig. 3: A diagram of services through the internet and home network.

Fig. 4: A diagram of distance learning and telemedicine through high-speed audio-visual remote systems.

Fig. 5: An example of video distribution through IP-multicast.

Fig. 6: Future trends of the trunk network.

2. Business Drivers
The telecommunications industry grew steadily as telephone services penetrated throughout the world. Users were charged based on the minutes for a call and the number of calls increased gradually over the years. The advent of the Internet has drastically changed the business circumstance. An average phone bill to users has decreased over time due to de-regulation and competition. In many countries the telephone services business is shrinking rather than staying constant. While the mobile phone business is growing its business in the traditional phone business model, the fixed phone business has either collapsed or is being replaced by Internet services.
It is our view that future switching services will shift from the conventional phone switching services to bandwidth switching services. Also, the need for control in e-homes and e-businesses will drive a huge demand for bandwidth. At an e-home, all electric appliances as well as the phone, the Fax, the PC, the PDA, the printer and the mobile phone will be connected based on IP infrastructure. At the services level distant learning/education, telemedicine, entertainment, video, data, and archive services will be in demand throughout the network. Some examples are shown in Figs. 2-5. This will inevitably cause a growth in traffic and allow the fiber glut situation demonstrated in Figure 1 to recover.

3. Technology Drivers
(1) Trunk Network
Optical fiber technology has so far provided the capability of transmitting huge amounts of information. We expect that new technology such as 40 Gb/s transmission will be pursued and will result in even more terabits on a fiber pair.
From the business point of view, the simple increase in the transmission of bits may not produce the high growth in new business and service opportunities that we had experienced in the past. Additional function in the form of intelligence is required to stimulate future growth. This intelligence will be the natural progression to wavelength switching or photonic packet switching /processing.
Given this change we expect that the new technical requirements for the transport and network, especially in the trunk and core would be as follows:

• A soft permanent (leased line) connection or switched (on demand) connection
• The adoption of an automatic switched connection through the control plane
• An optical network domain connection through NNI using a switched connection paradigm
• Protection and restoration for five 9 availability
• The mixture of fast protection premium type traffic and best effort type traffic
• Adoption of OAM functions of SDH/SONET in optical cross-connect
• The accommodation of various signal formats with optical wavelength level granularity
• End-to-end transparency of the control channel and supervisory channel
• Network scalability that is scalable to a large mesh network
• Easy to upgrade system capacity including in-service upgrade capability
• Figure 6 summarizes the future trend of the trunk network.

(2) Metro and Access
More dramatic progress will be seen in the metro and access areas as depicted in Figure 7. Ethernet based PON (E-PON) as well as G-PON architecture is widely adopted and will be further proliferated in the FTTH and metro networks. FTTH will compete with high-speed xDSL and will obtain a firm business foundation in the future as end users start to find themselves in need of bandwidth as large as 100 Mb/s or more. Figure 8 shows examples of the cost of broadband services in Japan.
Optical technology may also find application in connection with wireless services. Mobile communications companies are increasing their service bit rate and fixed wireless access companies are trying to incorporate mobility and expand their service category. This will result in a high-speed wireless metro area networks that will require large-area and large-bandwidth data links. Only optical communications technology can provide the solution for such requirements.

Fig. 7: A diagram of metro and access networks.

Fig. 8: An example of FTTH (Broadband) Services in Japan.

(3) Equipment and Devices
In the core transport and networks, 40Gb/s transmission technologies including chromatic dispersion and PMD compensation devices combined with OXC element devices and equipment should go forward to enhance flexibility and economic advantage. It seems that we are now facing a barrier due to the high cost of both 40 Gb/s and OXC technology. Unless devices and equipment costs decrease to a reasonably low level, it will be difficult to expect a sizable growth.

4. Perspective and Conclusion
It seems clear that expanding the capacity per fiber will not by itself create a new business opportunity in the future. Optical fiber communications are now finding a new growth scenario within metro and access networks as well as in wireless backbone networks. In time, bandwidth switching and processing services in the core transport and network areas will also become the new business opportunity. It is important that the industry provide unique intelligent technology to utilize the capacity for new and better services in an economical way.