10 Gbit/s VCSELs for datacom: devices and applications

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1 1 Gbit/s VCSELs for datacom: devices and applications Dominique Vez ab*, Sven Eitel a, Stephan Hunziker a, Grant Knight a, Michael Moser a, Rainer Hoevel a, Hans-Peter Gauggel a, Marcel Brunner a, Albert Hold a, and Karlheinz Gulden a a Avalon Photonics Ltd., Badenerstrasse 569, CH-848 Zurich, Switzerland b Electromagnetic Fields and Microwave Electronics Laboratory Department of Information Technology and Electrical Engineering Swiss Federal Institute of Technology Zurich, Switzerland ABSTRACT High data-rate communication links are placing increasing demands on the performance and cost of semiconductorlaser diodes. Vertical-cavity surface-emitting lasers (VCSELs) are ideal light sources for 1 Gbit/s applications. At Avalon Photonics Ltd., high-performance multimode VCSELs and VCSEL arrays are developed and fabricated for applications in low-cost fiber-optic communication links. An overview of static and dynamic characteristics of oxideconfined 85 nm VCSELs with data rates of 1 Gbit/s is presented. These 1 Gbit/s VCSELs are developed for the next generation 1 Gigabit Ethernet standard. Results show low threshold, high temperature operation, high modulation efficiency, short rise and fall times, and well-open eye-diagrams at different temperatures. Transmission over 6 m high-bandwidth multimode fiber at 1 Gbit/s is demonstrated. Mainly due to their low noise level and high linearity, these state-of-the-art devices are also well suited for transparent fiber-optic links using subcarrier multiplexed modulation schemes. Spurious-free dynamic ranges greater than 1 dbhz 2/3 are reported. Keywords: 1 Gbit/s VCSEL, surface-emitting laser, 1 Gigabit Ethernet, datacom, high-speed modulation, highbandwidth multimode fiber, transparent link, analog modulation. 1. INTRODUCTION The rapid growth in the use of fiber optics in data communications in recent years has been brought about by the continued exponential growth in the bandwidth demand of computing and communication systems. The Internet has emerged as a global medium for commerce, entertainment, and communications. It is the fastest growing segment of the communications market: Some experts predict that, over the next four years, the Internet will grow by as much as 1 to 2 times. The key enabling technologies responsible for this information and communication revolution are a result of very large-scale integrated circuits for computation and, more recently, semiconductor-laser and fiber-optic technology for communications. Fiber-optic technology is the ideal technology of choice for the realization of communication links with very large bandwidth capacities that widely exceed those available with copper links 1. As this technology has progressed over the last few years, a wide range of data rates and a variety of specific protocol standards have been investigated and developed for a broad range of communication applications 2. * Correspondence: dominique.vez@avap.ch; Phone ; Fax ; Avalon Photonics Ltd., Badenerstrasse 569, CH-848 Zurich, Switzerland

2 The applications where the data-rate breakthroughs are expected to make the most impact in terms of cost and volume are local area networks (LANs), multiprocessor links, and central office switching areas. In most cases the incremental increases in interconnection capacity are not sufficient to address this demand. It is this fact that has led the fiber-optic community to jump from the 1 Gbit/s optical links to development of 1 Gbit/s technologies. VCSELs and VCSEL arrays operating at 85 nm are key components for short-haul, multi-gigabit interconnects. VCSELs offer advantages over conventional diode lasers. These well-known advantages 3 include performance advantages like surface-normal emission, circular output beam, low beam divergence, small active volume, low threshold current, high efficiency, single longitudinal mode, wide temperature operation, inherent high-speed modulation, two-dimensional array, and manufacturing advantages like on-wafer testing, high-volume and high-density production, fabrication based on inexpensive microelectronics technology, and ease of integration. VCSELs were first commercialized in volume production in In data communications, VCSELs are today widely used in Gigabit Ethernet and Fibre Channel applications. The use of VCSELs and VCSEL arrays in 1 Gbit/s applications such as 1 Gigabit Ethernet (1 GbE) and 1 Gigabit Fibre Channel is progressing. The prediction that VCSELs will continue to play a dominant role is now well accepted. The remaining uncertainty is which type of VCSEL implementation will the market favour for 1 Gbit/s standards. In this paper, different optical technologies for 1 Gbit/s links and basic characteristics of the new 1 GbE will be first reviewed. Static and dynamic properties of the 1 Gbit/s VCSELs fabricated at Avalon Photonics will be then presented. 1 Gbit/s transmission over 6 m of high-bandwidth MMF will also be demonstrated, which shows that with optimized fibers, link length of several hundred meters are possible. Finally, the use of the 1 Gbit/s VCSELs for analog applications will be investigated. Spurious-free dynamic ranges (SFDRs) greater than 1 dbhz 2/3 indicate that these VCSELs are well suited for low-cost transparent links. 2. PROPOSED OPTICAL TECHNOLOGIES FOR 1 GBIT/S LINKS A number of different optical sources are used for fiber-optic communications 5. Several factors determine which is used for each specific application, but one general guideline is reliable: The source that allows the system to attain expected performance goals with the overall lowest cost impact is heavily preferred. Fiber-optic communication links can be divided into three broad classes that are characterized primarily by the distance between source and receiver. Each of these classes generally uses different optical sources and fiber to address unique requirements. Long-haul telecommunications can be considered to consist of links of greater than about 2 km and transmission is made over single-mode fibers (SMFs). Links between 5 m and 2 km also transmit over SMFs for metropolitan area networks (MANs). For long distances, dispersion in SMFs limits high data-rate transmission, while low data-rates are limited by losses in SMFs. Below 5 m multimode fibers (MMFs) are used for LANs. Prior to the availability of VCSELs, edgeemitting lasers were used predominantly for longer distances and light-emitting diodes (LEDs) for shorter distances. These choices were primarily based upon cost-performance trade-offs. VCSELs now change the landscape significantly. VCSELs are an ideal source for short and medium-distance high-speed data communications. Overviews of multi-gigabit optical technologies can be found in references 2, 5, and 6. A brief overview of serial-optical link, wide wavelength-division multiplexing (WWDM), and parallel-optical link is presented below. Serial-optical link: A number of applications, such as central offices and private links do not rely on the installed fiber base, but are open to using any cost effective solution, even if it requires a new fiber installation. The serial solution requires inexpensive fiber and known connector technology. The hidden cost is in the sources (lasers) and the receiver modules. In this approach one laser is modulated at 1 Gbit/s data rate and transmits the signal over a single fiber. The laser may operate in a single-mode regime, in which case the signal is transmitted over SMF and the wavelength in use is either 131 nm or 155 nm. Shorter distances use directly modulated and uncooled Fabry-Perot lasers (FPs) and distributed-feedback lasers (DFBs), while longer distances use externally modulated (EM) and temperature controlled DFBs. If a multimode laser is used, such as 85 nm VCSEL, then the signal is transmitted over MMF. Traditional MMFs (5 and 62.5 µm core diameters) have very limited bandwidth. At data rate of 1 Gbit/s the distances are limited to approximately 3 m. When a signal modulated at 1 Gbit/s is transmitted over such a fiber, a low bit error rate (BER) may only be achieved over several tens of meters. In recent years, much progress has been made in the

3 development of a new type of MMF, which with a special VCSEL output-beam launching technique, can be used to propagate high-speed signals over substantially larger distances than the conventional MMFs 7. These distances would be sufficient for most enterprise LANs. Wide wavelength-division multiplexing (WWDM): Currently, the majority of installed building backbones use 62.5 µm MMFs and there is a strong desire to use the installed base of fiber in moving towards 1 Gbit/s data rate. As already mentioned, these legacy MMFs suffer from a very high dispersion at 1 Gbit/s data rate. One of the ways to avoid this dispersion problem and still use MMFs is to code the signal into four or more data streams each transmitted over a different wavelength, and then have all of the wavelengths independently propagate over the same MMF. Each data stream is now made slower so that it can propagate over the legacy MMF. In this technology the number of wavelengths is small while the wavelength separation is wide (contrary of dense WDM). WWDM technology is used with SMFs and MMFs. This technology requires a transmitter using outputs from four or more lasers modulated at lower data rate, that are combined into a single fiber yielding an aggregate 1 Gbit/s data rate. Each laser operates at a different wavelength. If single-mode lasers are used, then all four wavelengths are combined and transmitted over one SMF. Similarly, if a multimode laser is used, then wavelengths are transmitted over a MMF. It has been also proposed that with additional coupling equipment, single-mode outputs can be coupled into a MMF. Parallel-optical link: A 1 Gbit/s data rate places demanding technological advances on the driving and receiving circuitry, optoelectronic devices, and thermal and high-speed electrical design in respect to the 1 Gbit/s technology. Since the development of new technology is always associated with higher initial device cost and typically longer time to availability, aggregating slower, less costly devices to realize links with higher throughput were investigated as the pathway to lower overall cost per unit bandwidth. Link aggregation in principle means using any number of independent channels in parallel so that the individual bandwidth required of channel is proportionally lower while the overall bandwidth is constant. This technology requires thus an array of four, eight, twelve or more lasers modulated at a lower rate. All channels are transmitted over a fiber ribbon containing four, eight, twelve or more fibers yielding an aggregate data rate of 1 Gbit/s or higher. For example, four channels operating at 2.5 Gbit/s result in an aggregate bandwidth of 1 Gbit/s. Both single-mode and multimode solutions have been investigated. Today, several companies are currently working on 1 Gbit/s VCSELs for serial-optical links: Honeywell VCSEL Products 8, AXT Optoelectronics 9, Emcore Optical Devices 1, U-L-M Photonics GmbH 11, Zarlink Semiconductor AB 12, and Avalon Photonics Ltd. 13, OPTICAL ETHERNET STANDARDS 3.1 Different Ethernet standards 15 From its origin more than 25 years ago, Ethernet has evolved to meet the increasing demands of packet-switched networks. Due to its proven low implementation cost, its known reliability, and relative simplicity of installation and maintenance, its popularity has grown to the point that today nearly all traffic on the Internet originates or ends with an Ethernet connection. Further, as the demand for ever-faster network speeds has grown, Ethernet has been adapted to handle these higher speeds. Ethernet has become the dominant network technology in LANs. The Gigabit Ethernet standard is already being deployed in large numbers in both corporate and public data networks, and has begun to move Ethernet from the realm of LANs out to encompass MANs. The technology exists to take Ethernet to the next level. The time is ripe for establishing a 1 Gigabit Ethernet standard that moves the goal posts for 1 Gbit/s further into MANs and, ultimately, into wide area networks (WANs) as well. Table shows different characteristics of predecessor Ethernet standards.

4 Standard Rate [Gbit/s] Length [km] Fiber Core Dia. [µm] Optical Sources 1 Ethernet nm - 86 nm LEDs 2 Fast Ethernet nm LEDs 3 Gigabit Ethernet nm - 86 nm lasers nm FPs 6 single-mode 13 nm FPs Table Predecessor optical Ethernet standards Gigabit Ethernet standard (IEEE 82.3ae) Overviews of 1 Gigabit Ethernet (1 GbE) standard can be found in references 15 and 16. This standard was ratified on June The key goals were: To preserve the Ethernet frame format, to support full-duplex operation only over fiber-optic media, to provide physical layer (PHY) specifications that support link distances of at least 65 m over MMFs, 3 m over installed MMFs, 2 km over SMFs, 1 km over SMFs, and 4 km over SMFs, and finally to define two families of physical interfaces LAN PHY (operating at a data rate of 1 Gbit/s) and WAN PHY (operating at a data rate compatible with STS-192c/VC-4-64c). There are two differences between 1 GbE and other speeds of Ethernet. First is the inclusion of a long-haul (4 km) optical transceiver or physical medium dependent (PMD) interface for SMFs that can be used with either the LAN PHY or WAN PHY for building MANs. The second is the WAN PHY option, which allows 1 GbE to be transparently transported across existing synchronous optical network (SONET) OC-192c or synchronous digital hierarchy (SDH) VC-4-64c infrastructures. PMDs, optical fibers, and distances used in 1 GbE are summarized in Table Fiber Type 62.5 µm MMF 62.5 µm MMF 5 µm MMF 5 µm MMF 5 µm MMF 1 µm SMF Optical Sources Modal Bandwidth 16 a b - [MHz*km] SR/SW 85 nm serial 2 to 26 m 2 to 33 m 2 to 66 m 2 to 82 m 2 to 3 m - 85 nm VCSELs LR/LW 131 nm serial m to 1 km 13 nm DFBs ER/EW 155 nm serial m to 4 km 155 nm EM DFBs LX4 131 nm WWDM d 2 to 3 5 MHz*km c 2 to 24 m 2 to 3 m - 2 m to 1 km 1275, 13, 1325, 135 nm DFBs Table Gigabit Ethernet PMDs, fibers, and distances 18 : a Commonly referred to as FDDI grade fiber ; b Sometimes referred to as 1 GbE MMF ; c 62.5 µm MMF has a modal bandwidth of 5 MHz*km at 13 nm as opposed to 16 or 2 MHz*km at 85 nm; d WWDM PMD is only defined for use in a LAN PHY. Other PMDs may be used in either a LAN or WAN PHY. (Table reproduced from IEEE 82.3ae). The following table only presents requirements for the 1GBASE-S media types since this paper focuses on 85 nm 1 Gbit/s VCSELs. Table gives the specifications that the 1GBASE-S transmitter shall meet. It shall also meet the center wavelength, maximum RMS spectral width, and minimum optical modulation amplitude (OMA) triple trade-off.

5 This triple trade-off defines the minimum transmit OMA required for a given center wavelength and a certain range of spectral widths. Description 1GBASE-SW 1GBASE-SR Unit Coding (physical coding sublayer (PCS)) 64B/66B + WIS WAN 64B/66B LAN Signaling speed (nominal) Gbit/s Signaling speed variation from nominal (max) +/- 2 +/- 1 ppm Center wavelength (range) 84 to 86 nm RMS spectral width a (max) Average launch power (max) See footnote b See footnote c Average launch power (min) dbm Launch power (min) in OMA See footnote b Extinction ratio (min) 3 3 db RIN 12 OMA (max) db/hz Optical return loss tolerance (max) db Encircled flux See footnote d Transmitter and dispersion penalty (TDP) e (max) db Rise time, fall time ps Bit error rate (BER) (encoded) Total jitter peak-peak (Tx out) ps Deterministic jitter peak-peak (Tx out) ps Table GBASE-S transmit characteristics 18 : a RMS spectral width is the standard deviation of the spectrum; b Trade-offs are available between spectral width, center wavelength, and minimum OMA; c The 1GBASE-S launch power shall be the lesser of the class 1 safety limit or 1 dbm; d The encircled flux at 19 µm shall be greater than or equal to 86% and at 4.5 µm less than or equal to 3% when measured into 5 µm MMF; e TDP (max) and OMA (min) are at the respective wavelength and spectral width as specified in the triple trade-off. (Table reproduced from IEEE 82.3ae) GBIT/S TECHNOLOGY AT AVALON PHOTONICS LTD. 4.1 Device design and fabrication Avalon Photonics 1 Gbit/s VCSELs and VCSEL arrays are fabricated using selective oxidation for efficient electrical and optical confinement. Oxide-confined devices have previously proved to be superior to proton-implanted VCSELs in terms of most performance characteristics such as threshold current 19, wallplug efficiency 2, and modulation bandwidth 21. The VCSEL structures presented in this paper are grown on (1) GaAs semi-insulating (SI) substrates by metalorganic vapor phase epitaxy (MOVPE) technique in one single-epitaxial run. Figure shows a schematic of this structure. The distributed Bragg reflectors (DBRs) consist of λ/4 Al x Ga 1-x As alternating high and low refractive index layers. The top p-doped DBR (lower reflectivity) and bottom n-undoped DBR are typically constructed using 27 and 37 mirror pairs respectively. The active region, placed in the middle of a λ cavity, consists of three GaAs quantum wells (QWs) embedded in Al.3 Ga.7 As barriers. The high-aluminum content layer for oxidation is positioned above the active

6 region in the top p-dbr close to the cavity. Oxidation is performed by exposing the wafer, after mesa etching, to water vapor at high temperature. Mesa is formed by wet chemical etching down to a n-contact layer, which is placed in the bottom n-dbr, thus enabling an intra-cavity n-contact. A ring-contact on top of the VCSEL mesa provides electrical p- contact to the p-doped layers of the top-dbr. Both electrical contacts are thus placed on the top epitaxial side of the wafer. The bondpads are designed as an electrically short symmetric coplanar waveguide (CPW), which allows on-wafer microwave probing, n and p-drive, and is suitable for wire and flip-chip bonding. CPW allows the wiring impedance to be precisely controlled. The chip design is therefore optimized for very low pad capacitance of the coplanar line, so that remaining device capacitance is only due to the small contribution from the oxidation layer and the diode junction. These minimized parasitics serve to increase the VCSEL bandwidth. coplanar line metallization oxide layer coplanar line metallization p-doped DBR n-contact n-contact layer n-undoped DBR GaAs SI-substrate Figure Cross-sectional schematic of 1 Gbit/s top-emitting VCSELs. Avalon Photonics provides single VCSELs, 1x4 and 1x12 VCSEL arrays at a data rate of 1 Gbit/s per VCSEL. Both common-cathode (Figure (a)) and common-anode (Figure (b)) ground configurations are available. 25 µm 25 µm cathode anode cathode anode (a) (b) Figure x4 VCSEL arrays: (a) Common-cathode ground and (b) common-anode ground.

7 4.2 Static characteristics One of the key issues to achieve cost-effective production of VCSEL devices, which have the inherent advantage of being tested directly on-wafer, is the control of the epitaxial growth and the mapping of the processed wafer. This wafer mapping enables the production of statistics relating to the processing yield. Figure shows a map of the threshold current for half of a 2-inch wafer containing three different VCSEL structures. Small dark dots indicate failures. Large black squares represent alignment marks. A repetitive set of three rows (three different threshold currents) indicates excellent growth and processing uniformity over the wafer. Table gives the main characteristics of the 1 Gbit/s VCSELs fabricated at Avalon Photonics for data communication applications. Row Column I th [ma] Figure Wafer map of threshold current Parameter Min. Max. Unit Parameter Min. Max. Unit Threshold current ma Differential 6 ma Ω Threshold voltage V Optical output 6 ma mw Slope efficiency mw/ma Thermal impedance K/mW Table Typical characteristics of 1 Gbit/s VCSELs. For practical applications, a small dependence of slope efficiency and threshold current on temperature is important to avoid increasing complexity of the driver circuit. Figure shows typical light-current (L-I) and current-voltage (I- V) curves for a 8 µm oxide aperture 1 Gbit/s VCSEL over a 3-1 C substrate temperature range. A maximum output power just above 3 mw is obtained at 3 C before the laser thermally rolls over, and a slope efficiency of.37 mw/ma is measured at this temperature (Figure 4.2.3). The optical output power decreases from 3.11mW down to 1.33 mw over this temperature range. This is more than a factor of 2. The slope efficiency decreases linearly from.37 mw/ma to.24 mw/ma with increasing substrate temperature. The reduction in power signifies that the laser gain and the cavity resonance align according to the temperature of the VCSEL. The cavity resonance/laser gain alignment dominates the temperature dependence of VCSEL operation. Since both laser gain (through band-gap) and cavity resonance (through refractive index) depend on temperature, both red-shift as the temperature increases. However, the laser gain shifts to longer wavelengths faster than the cavity resonance, causing spectral misalignment between the cavity resonance and peak gain, leading to degradation of the laser performance. Thus VCSELs at different temperatures have different optical output powers. The laser continues to operate above 1 C. Note that other effects related to injection efficiency, carrier confinement, and decreased gain with increasing temperature limit the operating temperature of VCSELs. The minimum threshold current of.84 ma occurs at a substrate temperature of 4.5 C as shown in Figure Maximum change of about 13% relative to the minimum threshold current occurs in the 3-7 C temperature range, which is suitable for the driver circuit. The minimum threshold current occurs at approximately the temperature where the cavity resonance is aligned with the peak gain: This is an important design specification. Because of this, the cavity resonance is intentionally designed to be at a slightly longer wavelength relative to the peak laser gain at room temperature, so that at higher current injection and thus, higher operating temperature, the peak gain shifts

8 Optical Output Power [mw] C C 1. 1 C.5 7 C 5 C 6 C 8 C C 4 C Drive Current [ma] Forward Voltage [V] Threshold Current [ma] Substrate Temperature [ C] Slope Efficiency [mw/ma] Figure L-I and I-V characteristics at various substrate temperatures for a 8 µm oxide aperture 1 Gbit/s VCSEL. Figure Corresponding threshold current and slope efficiency variations with substrate temperature. into alignment with the cavity resonance to yield minimum threshold current at a particular temperature. Typical optical spectra measured at room temperature (RT) and obtained by butt-coupling into a 62.5 µm MMF are presented in Figure The laser is driven with currents of 4, 6, and 8 ma. Emission wavelength is centered around 85 nm and shifts towards longer wavelengths with increasing levels of dissipated power. The device has a 8 µm oxide aperture and.8 ma threshold current. The internal temperature of a device is a critical parameter for device lifetime. Thermal impedance is measured by monitoring thermally induced wavelength shift with dissipated power. The thermal impedance of VCSEL is plotted as a function of oxide aperture diameter in Figure The measured thermal impedances of a 8 µm oxide aperture VCSEL is 2.69 K/mW. Figure shows that very small oxide aperture diameters lead to high thermal impedances, which degrade the thermal properties of VCSELs. 12 RT 6 Relative Optical Power [db] ma 6 ma 4 ma Wavelength [nm] Figure Butt-coupling optical spectra taken at RT for various DC drive currents of a 8 µm oxide aperture VCSEL with.8 ma threshold current. Thermal Impedance [K/mW] ~ 1/(d ox ) Oxide Aperture Diameter [µm] Figure Thermal impedance versus oxide aperture diameter.

9 Figure shows two spectra measured at room temperature and obtained by optically focusing the beam into a 62.5 µm MMF. The bottom spectrum corresponds to a 8 µm oxide aperture VCSEL (I th =.9 ma) driven at 6 ma and the upper spectrum to a 1 µm oxide aperture VCSEL (I th = 1.2 ma) driven at 8 ma. Due to multimode emission, the RMS spectral width is.422 nm (within a 2 db drop with respect to maximum optical power) for the bottom spectrum and.447 nm for the 1 µm oxide aperture VCSEL. Figure represents the far-field pattern at RT of a 8 µm oxide aperture VCSEL (threshold current of.7 ma) for various drive currents. The dependence of the beam divergence on current is clearly shown. Divergence angle becomes larger with increasing current and at 2 ma (~ 3x I th ), a thermally induced lensing mechanism causes the VCSEL to deviate from its Gaussian-mode Relative Optical Power [db] Wavelength [nm] profile. The beam divergence with 1/e 2 intensity profile is 26.7 deg at 6 ma and thus allows a good coupling into 62.5 µm RT Oxide aperture diam. = 1 µm Drive current = 8 ma λ RMS =.447 nm Oxide aperture diam. = 8 µm Drive current = 6 ma λ RMS =.422 nm Common-anode ground Figure RMS spectral width within a 2 db drop relative to the maximum optical power for VCSELs of 8 and 1 µm oxide aperture diameters at RT. The coupling efficiency into a standard 62.5 µm MMF of a 8 µm oxide aperture VCSEL (threshold current of.6 ma) is shown in Figure The coupling efficiency is plotted as a function of the distance between mesa surface and fiber end. For butt-coupling and at 8 ma drive current, an excellent coupling efficiency of 83% is achieved. This coupling efficiency drops by 3 db at a distance of ~ 14 µm. Intensity [a.u.] RT 1 ma ma ma ma ma ma Beam Divergence Angle [deg] Coupling Efficiency [%] Coupling into a standard 62.5/125 GI MMF Drive current = 8 ma Distance VCSEL Surface / Fiber End [µm] Figure Far-fields for various drive currents of a 8 µm oxide aperture VCSEL with.7 ma threshold current. The beam divergence with 1/e 2 intensity profile is 26.7 deg at 6 ma. Figure Coupling efficiency into a standard 62.5 µm MMF versus distance between mesa surface and fiber end. The VCSEL has 8 µm oxide aperture and.6 ma threshold current. RT measurement.

10 4.3 Small-signal modulation Thanks to their small active volume and high photon density, VCSELs are capable of providing high speed at low drive currents. To analyze the dynamic properties of 1 Gbit/s VCSELs, small-signal modulation measurements are performed on-wafer using a calibrated network analyzer and a high-speed silicon photodetector. The emitted light is butt-coupled into a 62.5 µm MMF (back-to-back). Measurements are performed at room temperature. Results of smallsignal modulation response measurements are shown in Figure The frequency relative response functions shown in Figure (a) for a VCSEL with 7.7 µm oxide aperture (I th =.7 ma) demonstrate a fast increasing 3 db bandwidth with increasing current. A bandwidth of 9.3 GHz at only 4 ma is obtained and a bandwidth just above 11 GHz is achieved at 6 ma. The maximum bandwidth is 12.4 GHz at 8 ma and is indicated by the almost flat response at this drive current. Devices with smaller oxide apertures achieve a higher bandwidth at a lower current than the devices with larger apertures. This behavior is shown in Figure (b) by determining the so-called modulation current efficiency factor (MCEF) for VCSELs with different oxide apertures. The extracted 3 db bandwidth frequency is plotted as a function of the square root of the current above threshold for each of the three oxide apertures. For low drive current the bandwidth increases linearly and the slope of the linear fits give the respective MCEFs. Higher MCEF for VCSEL with small oxide aperture is clearly seen. A MCEF of 5.38 GHz/mA 1/2 is achieved for a 7.7 µm oxide aperture, while the MCEF drops down to 3.72 GHz/mA 1/2 for 11.7 µm oxide aperture. Due to a low threshold current of.7 ma and a relatively high MCEF, the 7.7 µm oxide aperture VCSEL achieves 1 GHz at only 4.2 ma. Relative Response [db] 1 RT ma Frequency [GHz] 3 db Bandwidth [GHz] 14 RT Oxide aperture diam. = 7.7 µm MCEF = 5.38 GHz/mA 1/2 8 7 Oxide aperture diam. = 9.7 µm MCEF = 4.34 GHz/mA 1/ Oxide aperture diam. = 11.7 µm MCEF = 3.72 GHz/mA 1/ (I-I th ) 1/2 [ma 1/2 ] Figure (a) Small-signal response for a 7.7 µm oxide aperture 1 Gbit/s VCSEL with.7 ma threshold current and (b) extracted 3 db bandwidth for different oxide aperture diameters. Measurements are made at RT. VCSELs achieve the required minimum bandwidth for 1 Gbit/s operation (approximately 8 GHz or higher) in a wide range of oxide aperture diameters, but devices with larger apertures generally need higher drive currents than the small ones. CMOS VCSEL drivers, which are less expensive than SiGe drivers, require lower currents. Thus a resulting trend towards VCSELs with smaller oxide apertures compared to Gbit/s VCSELs is appeared. But smaller VCSELs suffer from reliability problems because higher current-density operation and higher thermal impedance. Thus, very small oxide apertures are excluded, since an optimized design must provide both the required bandwidth at lower current and a high reliability.

11 4.4 Large-signal modulation The large-signal response is of paramount importance for digital systems, and it is distinguished by a requirement to maximize the optical modulation amplitude. This allows the largest signal-to-noise ratio (SNR) for a given optical power level, which is generally limited by eye safety regulations. To maximize the SNR, the optical power of the one logic state must be large and the other must be very small. The degree to which this is successful is measured by the extinction ratio (ER), the ratio of on to off power. The ER is specified in standards such as 1 GbE for example, where the ER must be at least 3 db for the 1GBASE-S media type. Large-signal modulation measurements are presented in Figure for a 7.5 µm oxide aperture 1 Gbit/s VCSEL with a threshold current of.7 ma. The eye-diagram shown in Figure (a) is measured on wafer at RT. Electrical contact is made with microwave coplanar probes linked to a pulsed pattern generator (PPG). The PPG provides the modulation by generating a long pseudorandom bit sequence (PRBS) nonreturn-to-zero (NRZ) pattern at 1 Gbit/s. The VCSEL is driven at 7 ma through a bias-t and modulated with 5 db ER. The emitted light is collected by butt-coupling to a 2 meter long 62.5 MMF and detected by a 12 GHz bandwidth fast photoreceiver. The signal is then filtered through an OC-192 low pass 4 th -order Bessel-Thomson filter, which has a 3dB bandwidth frequency of 7.46 GHz. The 1 Gbit/s eye-diagram is well open and fulfills the OC-192 SONET mask-test. The broadened rising and falling edges are characteristic of pattern-dependent jitter, where the laser turn-on delay depends upon the driving signal recent history. Pattern-dependent jitter is a dynamic form of duty-cycle distortion that reduces the time window during which a discrete level decision can be made with confidence. Figure (b) shows the rise and fall times of the same VCSEL. They are measured using a periodic signal of 2 Gbit/s. The device is again driven at 7 ma and modulated with 5 db ER. The 2% and 8% rise/fall times (including rise and fall times from PPG and photoreceiver) are 31 and 44 ps respectively. (a) (b) Figure (a) Filtered eye-diagram at 1 Gbit/s for a 7.5 µm oxide aperture 1 Gbit/s VCSEL with.7 ma threshold current and (b) 2% rise time and 8% fall time at 2 Gbit/s for the same device. The laser is driven at 7 ma and modulated with 5 db ER in both pictures. Measurements are made at RT. For practical applications, a small variation of eye-diagrams shapes over a certain temperature range is required. In order to investigate this temperature behavior, eye-diagrams for a 8 µm oxide aperture 1 Gbit/s VCSEL (I th =.7 ma) are measured over substrate temperatures ranging from 5 C to 85 C. Experimental conditions are the same as the ones used to measured the eye-diagram shown in Figure (a), except variable drive currents to keep the optical power almost constant for all temperatures. The results shown in Figure demonstrate well-open eyes at substrate temperatures ranging from 5 C to 85 C.

12 (a) (b) (c) Figure Filtered eye-diagrams at 1 Gbit/s for a 8 µm oxide aperture VCSEL with.7 ma threshold current. The eye-diagrams are measured (a) at 5 C and 6.1 ma, (b) at 4 C and 7 ma, and (c) at 85 C and 9 ma. The optical power (at the fiber end) is.87 mw at 5 C and 4 C, and.79 mw at 85 C. Apart back-to-back testing with standard 5 or 62.5 µm MMF, transmission over 6 m of high-bandwidth 5 µm MMF (Lucent LazrSPEED TM ) is demonstrated. The 1 Gbit/s unfiltered eye-diagrams for a VCSEL with 6.7 µm oxide aperture (I th =.6 ma) are shown in Figure The laser is biased at 6 ma and modulated with 6 db ER. The eyediagram for back-to-back measurement (a) is well open at 1 Gbit/s with only slight overshoot. After 6 m transmission (b) the eye opening is slightly reduced but the eye remains open and symmetric. The small reduction of the eye opening is due to fiber attenuation and dispersion. 1 m standard 5 µm MMF 6 m high-bandwidth MMF (a) (b) Figure Unfiltered eye-diagrams measured at 1 Gbit/s (PRBS NRZ pattern) for a 6.7 µm oxide aperture VCSEL with.6 ma threshold current. The eye-diagrams are measured (a) back-to-back with a standard 5 µm MMF and (b) after 6 m transmission over a high-bandwidth MMF. Measurements are made at RT. In both cases, bit error rates (BERs) below 1-12 are achieved without indications of a BER floor. An example of such measurements is given in Figure 4.4.4, where the measured BER is plotted as a function of the received average optical power. The measured VCSEL has a 8.6 µm oxide aperture (I th =.9 ma), is biased at 6 ma, and modulated with 6 db ER. The power penalty for 6 m transmission is 3 db for a BER of 1-12 due to fiber dispersion. Figure BER as a function of the received optical power at 1 Gbit/s. Measurements are made at RT. Bit Error Rate 1E-3 1E-5 1E-7 1E-9 1E-11 I bias =6 ma, ER=6 db 1E Received Optical Power [dbm] back-to-back 6 m LazrSpeed (5 µm MMF)

13 5. ANALOG FIBER-OPTIC LINKS BASED ON VCSEL TECHNOLOGY VCSELs are also of interest for optical distribution of microwave signals, for example in wireless communication and radar systems operating in the low GHz range. VCSELs are low-cost, high-performance optical sources for data communication links. For analog modulation, characteristics such as the small-signal modulation bandwidth, intermodulation distortion, and noise are important parameters determining the maximum modulation frequency and the dynamic range. In order to investigate the suitability of the 1 Gbit/s VCSELs for these analog applications, the spurious-free dynamic range (SFDR) is determined at different drive currents and frequencies. The SFDR is a useful measure to characterize how the third-order intermodulation distortion (IMD3) caused by nonlinearities in the modulated VCSEL limits system performance 22. Two-tone measurements are performed to measure the IMD3 23. For these measurements the signals from two RF signal generators are combined and used to modulate the VCSEL with two signals of equal power and a frequency separation of 2 khz. The laser emission is butt-coupled into a 5 µm MMF and detected by a 12 GHz receiver. The electrical signal is then fed to an electrical spectrum analyzer. An example of a measured spectrum is given in Figure 5.1 (a) for a 7.6 µm oxide aperture VCSEL biased at 2 ma. It shows the fundamental input signals and the third-order intermodulation (IM3) peaks. The measured output power levels of the fundamental and the IM3 signals are plotted as a function of the input power (Figure 5.1 (b)) to determine the third-order intercept points (IIP3/OIP3) and the SFDR. The SFDR is the maximum separation between the fundamental and the IM3 signals at the points where the IM3 signals just appear above the noise level. To enhance the accuracy of the laser noise measurement, the modulation is switched off and the noise amplified with an additional low-noise amplifier. The laser noise level in Figure 5.1 (b) is then calculated from the measured noise, by correcting for the measurement system noise floor (laser bias current turned off) and the amplifier gain, and assuming a bandwidth of 1 Hz. Output Power [dbm] Input Signals 3 rd order Intermodulation (IM3) Frequency [MHz] (a) Output Power [dbm] Fundamental IM3 SFDR -12 IIP3 Noise level Input Power [dbm] (b) Figure 5.1 (a) Measured spectrum showing two input signals and third-order intermodulation peaks and (b) plot of output power of the fundamental and IM3 signals versus input power used to determine the SFDR. The SFDR values for three 1 Gbit/s VCSELs with different oxide aperture diameters are plotted in Figure 5.2 as a function of the drive current. The measurements are performed at 9 and 18 MHz. The SFDR values for 5.6 and 7.6 µm oxide apertures at 9 MHz peak at a medium current of 5 ma, with values of 13.6 and 15.9 dbhz 2/3 respectively. This reflects the highest linearity and the lowest noise levels of these VCSELs at 5 ma. Moreover, the resonant distortions are small at 5 ma, since the resonance frequency is far above the frequencies of the RF signals. The largest device, on the other hand, has to be driven at a higher current of 8 ma for the maximum SFDR of 12.8 dbhz 2/3 (9 MHz). This is mainly due to a shift of the most linear region towards a higher drive current and the lower intrinsic modulation efficiency. The latter results in a higher resonant distortion at 5 ma. A very similar current dependence of

14 the SFDRs is also observed at 18 MHz. The peak SFDR values are comparable to the highest reported values for 85 nm VCSELs 22 and are close to those measured for FP and DFB edge-emitters, which is due to the generally low intermodulation distortions and low noise levels. For optimum drive currents the third-order input intercept points (IIP3) are between 18 and 2 dbm for all devices sizes and at both frequencies. The measured SFDRs meet the requirements of important applications like remote antenna addressing in mobile phone and local cable television (CATV) distribution. SFDR [db*hz 2/3 ] oxide aperture 5.6 µm 7.6 µm 9.6 µm 9 MHz SFDR [db* Hz 2/3 ] oxide aperture 5.6 µm 7.6 µm 9.6 µm 18 MHz Current [ma] Current [ma] Figure 5.2 Measured SFDRs versus drive current for 1 Gbit/s VCSELs with different oxide aperture diameters, at 9 and 18 MHz. 6. CONCLUSION In this paper, different optical technologies for 1 Gbit/s links and basic characteristics of the new 1 GbE were first briefly reviewed. Then static and dynamic properties of the 1 Gbit/s VCSELs fabricated at Avalon Photonics were presented in detail. Results showed low threshold current, small temperature sensitivity, suitable spectral width, small divergence beam allowing good coupling efficiency into MMFs, MCEF of 5.38 GHz/mA 1/2 and 3 db bandwidth frequency of 1 GHz at only 4.2 ma for a 7.7 µm oxide aperture VCSEL, short rise and fall times, and good temperature stability of 1 Gbit/s eye-diagrams. Then 1 Gbit/s transmission over 6 m of high-bandwidth MMF was demonstrated, which shows that with optimized fibers, link length of several hundred meters are possible. Finally, the used of the 1 Gbit/s VCSELs for analog applications was investigated. SFDR values greater than 1 dbhz 2/3 were measured at 9 and 18 MHz, which indicates that these VCSELs are well suited for low-cost transparent links. ACKNOWLEDGEMENTS The authors gratefully acknowledge support from the Epitaxy, Process Technology, and Packaging & Testing groups at Avalon Photonics. This work could not have been presented without their dedicated efforts. The authors are also indebted to Prof. Dr. Werner Bächtold from the Electromagnetic Fields and Microwave Electronics Laboratory of the Swiss Federal Institute of Technology Zurich.

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