Solutions for high-performance optical transmission modules Keiji Okuda Yasushi Harano Today, information communications networks are being required to handle a whole host of information, such as Internet-based animated images, and other data, in addition to traditional, analogue telephone and facsimile communications. In order to cope with the growing size and diversity of communications traffic, there have been moves to increase capacity and functionality in communications networks by adopting optical networks like that illustrated in Fig. 1. Oki Electric’s solution in this field is based on its group of optical communications modules (an ONU module for ATM-PON systems, 2.5 Gb/s MINI-DIL type LD module and 10 Gb/s PD-TIA module) which are vital elements for achieving higher capacity and functionality, and lower communications fees, in the respective access networks, metropolitan networks, and backbone networks which make up the overall optical network structure. This paper will look at these different modules in turn. Takahiro Yamada Youkou Hirose structure was chosen because it is more suited to compact integration and automated assembly than fibre coupler or space beam type designs, and therefore it is excellent for mass production and cost reduction. Polyimide filters are used for the WDM filters in order to reduce costs. By fixing a polyimide filter to one face of the PLC, 1.3 µm band wavelengths and 1.5 µm band wavelengths can be multiplexed and Pre-amp WDM filter PLC PD element Si substrate (1) Fibre Si substrate (2) Ceramic substrate LD element mPD element Plastic package MUX DEMUX Backbone DWDM Networking OLA/ ADM IP Networking Metropolitan/Access Ring Metropolitan/Access Ring Distance between PD & LD 5mm ATM-PON Systems Fig. 1 Optical network ONU optical module for ATM-PON systems The ONU optical module for ATM-PON systems provides a solution for creating high-speed, low-fee access services. Fig. 2 shows the structure and external appearance of an ONU optical module for ATM-PON used for two-way communications in an access network, and Table 1 shows the specifications of this module. The WDM (Wavelength Division Multiplex) optical circuit which multiplexes (MUX) and demultiplexes (DEMUX) 1.3 µm wavelengths and 1.5 µm wavelengths uses an optical waveguide (PLC : Planar Lightwave Circuit) fitted with a WDM filter. The PLC 82 OKI Technical Review April 2002/Issue 190 Vol.69 No.2 Fig. 2 Structural diagram and external view of ONU optical module for ATM-PON Special issue on devices ● Table 1 622.080 Mb/s ONU optical module specifications Item Receiver Conditions Min. value Average value Max. value 1.6 P0 CW Threshold current Ith — Central wavelength λc P0=1.6mW, RMS Spectral half-width ∆λ Forward voltage Vf Operating current Rise / fall time Optical output Transmitter Symbol Unit mW 40 mA 1350 nm P0=1.6mW, RMS(s) 2.1 nm P0=1.6mW 1.45 V Iop P0=1.6mW 80 mA tr/tf P0=1.6mW 0.5 ns Monitor current Im P0=1.6mW 200 1000 µA Tracking error TRE Im=const. -1 1 dB Power supply voltage Vcc — 3.0 3.3 3.6 Sensitivity R Pin=3µW, Vcc=3.3V 3.15 4.8 Frequency bandwidth BW 400 450 Minimum sensitivity Pmin -28 dBm Maximum sensitivity Pmax -3dB, Pin=3µW 622Mb/s, NRZ BER=10-10, PRBS223-1 622Mb/s, NRZ BER=10-10, PRBS223-1 λ=1550nm V kV/ W MHz Optical reflection loss ORL demultiplexed. The polyimide filter conforms to LWPF (Long Wave Pass Filter) specifications. Reception wavelengths in the 1.5 µm band input from the fibre are passed by the filter, and received by the receiver PD (photodiode), whereas the 1.3 µm band light of the transmitter LD (Laser Diode) is reflected by the filter and output via the fibre. The ATM-ONU optical module is used by driving the transmitter and receiver simultaneously, which gives rise to problems of electrical and optical cross-talk. In the module presented here, in order to avoid the problem of electrical cross-talk, a discrete mounting structure is adopted where the transmitter side LD element and the PD element in the receiver are electrically isolated. Previously, the transmitter LD and receiver PD have been mounted on the same Si substrate, which has required sufficient spacing between the LD and PD, strengthening of the GND, and other measures, to reduce electrical cross-talk. Therefore, the conventional structure obstructs advances in miniaturization and cost reduction. However, by adopting a discrete structure in which (1) a transmitter Si substrate mounted with a LD element and monitoring PD element, and (2) a receiver Si substrate mounted with a reception PD element, are connected respectively via an insulated PLC, and furthermore, by installing the Si substrate (1) and Si substrate (2) mounted on the PLC, onto a ceramic substrate and then mounting them with into a plastic package with a pre-amp, we have been able to strengthen electrical insulation, and therefore shorten the mounting distance between the LD element and PD λ=1310nm 1280 -29 -6 -7 dBm -20 dB -10 dB element, without problems of electrical cross-talk. All this has meant that we can make the PLC more compact, whilst also lowering module costs. To reduce optical cross-talk, on the other hand, it is especially important that the transmission wavelength, 1.3 µm, is shut out adequately in the receiver section, since simultaneous transmission and reception must be performed using a transmitter section of high signal power and a receiver section capable of detecting very weak signals. Our optical module reduces cross-talk by having a LWPF filter fitted in contact between the receiver side end of the PLC and the reception PD element, and by coating an optical termination resin onto the PLC chip. The discrete structure adopted also makes it possible to control the yield rate for each individual component, providing opportunities to increase the overall yield rate of the module and simplify process control. The resulting module, designed for cost savings and mass production, has external dimensions of 17.5 (L) x 12.2 (W) x 3.4 (T) mm. Characteristics of the ONU optical module for ATM-PON Fig. 3 shows the error rate characteristics of this module when receiving a 622 Mb/s signal. The minimum sensitivity during simultaneous transmission and reception is –29 dBm (BER = 10-10) or less in the temperature range –20°C ~ +75°C, and the corresponding power penalty is 2 dB or lower. OKI Technical Review April 2002/Issue 190 Vol.69 No.2 83 Table 2 Product Line-up Product Transmission rate OL3902W OL3907W Receiver Transmitter Schedule Optical output Wavelength Power supply voltage 155.52Mb/s 1.6mW 1310nm 3.3V 1550nm 120MHz 12KV/W 622.080Mb/s 1.6mW 1310nm 3.3V 1550nm 450MHz 3.15KV/W Wavelength Bandwidth Sensitivity CS 2/02 ES 12/01 CS 4/02 ES: Engineering Sample CS: Commercial Sample 10-3 LDLD not driven LDLDdriven Error Bit Rate 10-4 10-6 Class B Fig. 4 External appearance of OL3312N 10-10 FabryFabry-Perot Pelot-LD LD Spherical-lensed fibre 10-12 -34 -33 -32 -31 -30 Optical Power (dBm) -29 -28 Resin Holder -27 Fig. 3 Error rate measurement results (at 25 °C) Monitor PDPD These favourable characteristics easily satisfy the Class B standard for minimum sensitivity recommended in ITU-T G983. The sensitivity is a stable 0.85 A/W, with a total variation including the pre-amp of less than ±1 dB (between –20°C ~ +75°C). The module also has good optical output characteristics, with a tracking error of ±0.6 dB or less (–20°C ~ +75°C) corresponding to a fibre output P0 = 1.6 mW (at 25°C). The optical reflection loss inside the module is –15 dB or less at the transmission wavelength, and –25 dB or less at the reception wavelength, both representing suitable results. The product line-up and development schedule are illustrated in Table 2. Si-V substrate Si-V Fig. 5 Cross section of OL3312N I-L CHARACTERISTICS 1.50 -40˚C 25˚C 85˚C Pf mW 1mW 0.75 2.5 Gbps MINI-DIL type LD module Fig. 4 shows an external view of the 2.5G LD module OL3312N which is currently under trial manufacture. Its dimensions are 13.2 x 7.4 x 4.0 mm, which conform to the size of a MINI-DIL. Flat type leads suitable for surface mounting are used, and the pitch is set to 2.54 mm. Fig. 5 is a cross sectional view of the OL3312N. We have coupled the LD chip optically with the fibre by passive alignment, on a Si substrate mounted in a ceramic package. The optical fibre is formed with a spherical lensed process in order to achieve high output in the passive alignment. Furthermore, an AR coating is provided on the spherical lens of the fibre, to prevent light reflected 84 OKI Technical Review April 2002/Issue 190 Vol.69 No.2 0 50 If mA 100 Fig. 6 Optical characteristics of OL3312N back by the fibre from adversely affecting the optical output. In this structure, the LD chip is bare chip mounted, so it requires encapsulating. A resinencapsulated structure was adopted for the package, with the aim of simplifying the encapsulating process. Special issue on devices ● Characteristics of the 2.5 Gb/s LD module -40˚C -40˚C The 2.5 Gb/s LD module is a solution designed to increase traffic capacity in metropolitan and backbone networks. A sample module was manufactured and evaluated. Fig. 6 shows the optical characteristics of the OL3312N module. The horizontal axis represents operating current, and the vertical axis, the optical output of the fibre. Measurement temperatures of –40°C, +25°C and +85°C were used. Table 3 lists the main characteristics of the OL3312N. From Fig. 6, we can see that an optical output of 1 mW was satisfied at all measurement temperatures. Fig. 7 illustrates the rise and fall conditions and the eye pattern of the OL3312N, at respective temperatures of –40°C, +25°C and +85°C. Other measurement conditions were set to: transmission rate 2.5 Gb/s, extinction ratio 14 dB, NRZ system, random pattern 223-1, and peak power 1 mW. Figs. 7a, c & e show the rise time (tr), fall time (tf) and waveform corresponding to each measurement temperature. These figures are perfectly acceptable compared to the standard value of 150 ps. Figs. 7b, d & f show the eye pattern when an electrical filter is inserted. There were zero failed samples at any of the a tr:33.3PS, tf:73.3ps b 25˚C 25˚C d c tr:137.8ps, tf:80.0ps 85˚C 85˚C f e tr:33.3ps, tf:160.0ps •• Transmission : 2.5Gb/s rate : 2.5 Gb/s • NRZ system •NRZ power : 1 mW •• Peak : 1mW •• Extinction : 14dB ratio : 14 dB 23 •• Random : 2 -1 pattern : 223 –1 Fig. 7 OL3312N : tr, tf and eye pattern Table 3 List of OL3312N characteristics Parameter Condition Fibre output Continuous light Min. value Standard value 1.5 Start of lifespan End of lifespan 2.5 Gb/s, random pattern 223-1 Central wavelength NRZ, peak power : 1 mW Unit mW Operating current Threshold current Max. value 1 1266 150 mA 40 mA 1.5Ith-BOL mA 1360 nm nm Spectral line width Optical output : 1 mW, root mean square 4 LD forward voltage Optical output : 1 mW:1mW 1.5 V 2000 µA Monitor current Optical output : 1 mW (room temperature / start of lifespan) 300 ps 150 Optical output : 1 mW 10-90% rise/fall time Ω 25 Input impedance dB 6 Optical reflection loss Tracking error (Room temperature – worst case temperature) PD dark current Inverse voltage : 2.2V PD capacity Inverse voltage : 2.2V Thermistor resistance Measurement temperature : 25˚C -1.5 9 10 1.5 dB 1 µA 20 pF 11 kΩ K 3435 B constant of thermistor resistance Table 4 Development schedule Product OL3314L Transmission rate Optical output 10Gb/s 0.5mW 2.5Gb/s 1mW OL3312N OL3311L OL5311L Wavelength LD 1310nm DFB 1310nm 1510nm FP DFB Schedule ES 3/02 CS 5/02 ES 11/01 CS 5/02 Notes Built-in Bias T Built-in Bias T ES: Engineering Sample CS: Commercial Sample OKI Technical Review April 2002/Issue 190 Vol.69 No.2 85 30 Response(dB) 25 20 15 10 5 0 0.1 1 10 Frequency(GHz) Fig. 10 Frequency characteristics Fig. 8 External view of 10 Gb/s PIN-AMP module Si Gel Resin Pig-tailed Fiber Bare Fiber Si-V Substrate PD-chip Fiber Cover Non-Hermetic Ceramic Package Gal Wing Lead Fig. 11 Output waveform GaAs TIA 10-3 Fig. 9 Structure of 10 Gb/s PIN-AMP module 10 Gb/s PIN-TIA module The 10 Gb/s PIN-TIA module provides a solution for high speed and high capacity in metropolitan and backbone networks. Fig. 8 shows the composition and external appearance of the 10 Gb/s PIN-AMP module we have recently completed. The external dimensions of the module are 9.4 mm x 10.25 mm x 2.15 mm. The structure is illustrated in Fig. 9. The module is built by mounting a TIA and Si-V-grooved substrate, and the like, on a ceramic substrate formed with a microstrip line which is impedance matched within the package. The main feature of the module is that these elements can be mounted by a lens-less system, without self alignment, because a marker for PD mounting is provided on the Si substrate, and the 86 OKI Technical Review April 2002/Issue 190 Vol.69 No.2 10-4 10-5 Bit Error Rate measurement temperatures, and good eye openings were obtained. Here, we have manufactured samples of a 2.5 GLD module OL3312N using passive alignment, and obtained good characteristics in sample evaluation tests. We are currently developing other LD devices using this technology. Table 4 shows the corresponding product line-up and development schedule. 10-6 10-7 10-8 10-9 10-10 10-11 10-12 -20 -10 0 Optical Input Power [dBm] Fig. 12 Error rate characteristics Special issue on devices ● Table 5 10 Gb/s PIN-AMP module (OD9651N) specifications Item Symbol Wavelength Sensitivity Frequency bandwidth Transimpedance λ Max. sensitivity Conditions Min. value Average value Max. value 1280 Unit nm 1580 λ=1550nm 0.7 0.8 BW f3dB, Pin=-10dBm 7.0 7.5 GHz Zt Pin=-10dBm, RL=50Ω 57 60 dBΩ 0 2 dBm Rpd 9.953Gb/s, NRZ Pmax Min. sensitivity Pmin Input computed noise current density In PRBS2-31-1ber10-12 9.953Gb/s,NRZ, PRBS2-31-1ber10-12 Average: 0.5GHz~BW A/W -20 -18 10 12 dBm pA/ Hz Table 6 PIN-AMP product line-up Product OD9651N OD9652N System OC-192, STM-64 Transmission rate 9.95328Gb/s 10.664Gb/s Freq. band 7.5GHz 8.5GHz Max. sensitivity 0dBm -3dBm Min. sensitivity -18dBm -17dBm 12.5Gb/s 10GHz 0dBm -15dBm OD9653N Schedule CS 1/02 ES 1/02 CS 6/02 ES: Engineering Sample CS: Commercial Sample substrate is formed with a V-shaped groove for installing the fibre. This structure provides excellent advantages over a butterfly type system using lens-based coupling, from the viewpoints of compact integration, mass producibility and cost reduction. Characteristics of the PIN-TIA module Fig. 10 shows the frequency characteristics of the 10 Gb/s PIN-TIA module. Good characteristics of sensitivity η = 0.9 A/W and trans impedance 60.3 dBΩ were obtained. In Fig. 11, we can see the output waveform (Ta = +25°C, 20 mV/div, 20 processing step/div) of the PINTIA module at 10 Gb/s (NRZ, pseudo-random pattern (PRBS) 231-1), Pin = -10 dBm, extinction ratio 8.6 dB). This confirms that a good eye opening is obtained. Fig. 12 shows the measurement results for error rate (10 Gb/s, NRZ, PRBS 231-1, extinction ratio 8.6 dB). In the temperature range of Ta = +0 – +70°C ( ■ : +0°C; ▲ : +25°C; ● : +70°C), the minimum sensitivity is –19.3 dBm (BER 10-12), and the maximum sensitivity is + 3.5 dBm (BER 10-12), so results are stable with respect to temperature fluctuation, and reception over a wide dynamic range is possible. Table 5 lists the specifications of the 10 Gb/s PIN-TIA module (OD9651N). Product line-up and development programme We have released two types of PIN-TIA module : the OD9651N for 10 Gb/s signal rates (9.95328 Gb/s, OC-192, STM-64), and the OD9652N for 10.7 Gb/s communications (10.664 Gb/s, FEC). We have also completed trial manufacture of a third model, the OD9653N, designed for 12.5 Gb/s operation. This product line-up and corresponding development schedule are indicated in Table 6. From here on, in response to demands for greater functionality, we will be driving forward with development of high-gain modules (with built-in limiting amps) for the data communications market, and 40 Gb/s reception modules aimed at even faster networks, Conclusion This essay has presented the optical communications components developed by Oki to provide solutions for increasing capacity, improving functionality and reducing communications fees in optical network systems. Alongside the three key factors of low cost, high functionality and high speed, our future research will also be directed towards achieving greater functional integration. Authors Keiji Okuda: Optical Components Company, Advanced Opto Dept. Takahiro Yamada: Optical Components Company, Advanced Opto Dept. Yasushi Harano: Optical Components Company, Advanced Opto Dept Youkou Hirose: Optical Components Company, Advanced Opto Dept. OKI Technical Review April 2002/Issue 190 Vol.69 No.2 87