Design of 10G Optical module at the hottest extrem

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Design of extreme temperature 10G Optical module

although 2.5gbit/s and 1/2/4gbit/s optical communication modules can be applied in a wide temperature range (usually -20 ℃ to +85 ℃), 10gbit/s modules fall behind

the need to use such a high-speed optical module in such a wide temperature range is obvious. The line card density has increased to 80gbi to help you further understand the equipment t/s. The 10gbit/s line rate is gradually approaching the local access point where the environment is not easy to control. In addition, the new pluggable transceiver multi-source protocol (MSA), such as XFP, puts forward higher requirements for temperature adaptability

these three factors have prompted device manufacturers to develop 10gbit/s modules with wider temperature performance (-20 ℃ to +85 ℃). However, technologies that simultaneously meet the requirements of ambient temperature and transmission rate have only recently emerged

comparison between EML and DML at a wider temperature

uncooled direct modulation laser (DML) is a good choice for 1310nm short distance (10km) transmission system of single-mode fiber. Distributed feedback (DFB) DMLS are particularly popular. They have been successfully used in short-distance transmission, especially 10gbit/s Ethernet for a long time

however, if DML is applied at a wider temperature or reaches the performance of SONET, there will be great challenges. The photoelectric conversion curve (L-I curve) of all DMLS will change with temperature and time. This effect is caused by the asymmetry of laser

Figure 1 shows the difference between laser rise time and laser fall time. This asymmetry also changes with temperature. The most common way to compensate for these effects is to use a lookup table, which records the data of many temperature tests carried out during the production of the laser

the eye diagram of typical 10gbit/sdml illustrates the inherent asymmetry of laser rise time and fall time. This asymmetry changes with temperature

another way to overcome these laser effects and asymmetries is to make the laser always output light and modulate the output light with an external modulator, which is the so-called externally modulated laser (EML). In this case, EML based on electric absorption (EA) modulator is the most suitable due to its size and advantages in modulation drive. The RF performance of EML is determined by the fast absorption capacity of EA components and has nothing to do with the dynamic characteristics of laser, so it can obtain stable RF performance independent of operating temperature

EML is usually used for transmission of 1550nm40 km or more. Optimizing the operation of EA modulator can make the chirp very small or negative, so that low modulation voltage can also obtain a linear extinction curve. When the modulation current changes, the extinction curve of DML is also linear. However, the modulation range of DML is still a problem, because when the "0" level is close to the luminescence threshold, DML will produce adverse excitation effect. For both lasers, 1 Test standard: the gas spring performance test standard qc/t 207 ⑴ 996 and jb/t 8064.1 ⑼ 6 are now linear electro-optical conversion functions, which require strict RF Design of the entire RF channel from the driver to the module. That is, the design must maintain strict control over the modulation amplitude in order to obtain the appropriate output performance

the introduction of 1310nmeml without refrigeration has changed this situation. The structural feature of this kind of transmitter is that the two functions of laser generation and modulation are integrated in separate vertically coupled waveguide elements. By vertically coupling these components, each function can be optimized independently to meet the requirements of refrigeration free operation, without the expensive regeneration steps common in EML production. DFB laser elements are aluminum based, which can produce good output optical power performance when operating without refrigeration. EA is designed as a transformation curve with anti-s shape, which is different from the linear curve mentioned above. Because EA works in the limited states of light "1" and light "0", EA can be driven to the saturated state

the advantage of limiting the EA modulation curve is that the uncooled 1310nmeml can operate stably in the whole temperature range under open-loop control (see Figure 2). In this way, the RF output drift of the modulation driver does not need to be adjusted with the change of temperature and life, so as to ensure the expected extinction ratio (ER). Since the RF configuration is independent of temperature and life, using these uncooled emltosa can simplify the module characteristic test and production test to DC test

from the 10GBASE LR eye diagram, the uncooled EML has good performance when operating in the range of -20 ℃ (left) to +95 ℃ (right)

the ER obtained here is much larger than the 6dB requirement of SONET link, so that module manufacturers can install a set of transmitting components in the module, which can be used for 10gbit/s Ethernet and sonet/sdh. If manufacturers are concerned about power consumption, they can take advantage of the fact that the Er of uncooled EML increases with the increase of temperature, and use the solution of small RF drift at high temperature: we can add lubricating oil between the gaps to reduce the friction coefficient between them to reduce power consumption

for 1310nm10km transmission system, especially 10GBASE LR and oc-192sr1 system, the structure of 1310nmeml Tosa without refrigeration is very simple due to the use of open-loop control; Open loop control can also improve performance. Uncooled 1310nm EML also enables similar transmitter technology to be applied to longer distance 1550nm transmission, and allows designers to make full use of a modulation driver in a variety of structures

drive design without refrigeration EML under a wider temperature

correct selection of modulation driver without refrigeration EML can further reduce the design workload and shorten the time to market. If the average output power of the laser still changes with temperature, the DC control loop used in DML is required for 1310nmeml without refrigeration. However, at present, most drivers only have RF modulation function, and external devices are needed to support the DC control loop required by the laser and meet the needs of eye protection. Although these peripheral support circuits are very simple, due to the large number, many module manufacturers are difficult to package the previous modules into small modules (such as XFP)

in order to solve this problem, a "Qu Jinping" actuator with integrated automatic power control (APC) loop is needed. This can reduce the number of devices, so that the occupied space can meet all the commercial 10gbit/s module packaging. The integration of fault prevention and laser control timing function can further reduce the number of chips and occupied space

as mentioned above, uncooled EML with constant modulation amplitude under open circuit operation requires a modulation driver that maintains RF drift stability when temperature, power supply voltage and data rate change. This performance can be achieved by carefully setting the control voltage with a simple resistive voltage divider. In this way, the designer can ensure sufficient output drift in any case and reduce the number of product tests

Figure 3 shows the layout of 1310nm uncooled EML and its related modulation drive circuit. The layout is based on a commercial reference design. In the past five years, many design teams have relied on reference design to demonstrate the usability of devices before integrating devices into their own products, so as to shorten the time to market and reduce cost pressure and workload. These reference designs are usually complete circuit board structure diagrams and layouts, including laser drivers and Tosa control circuits connected to microcontrollers, as well as key designs of RF interfaces from drivers to Tosa circuits. The three-dimensional model of RF interface is the key to design a correct RF interface circuit. A reasonable reference design combined with test data provides a good starting point for designers to quickly transition from laboratory evaluation to the packaging of 10gbit/s module final products, including XFP, X2, xenpak, and 300 pin packaging

this block diagram is based on the joint reference design of vsc7982eml driver of Vitesse company and 1310nm uncooled emltosa of 10t3081 of apogeephotonics company with a wider temperature range, and provides a design example in line with XFP and similar small package (SFP)

from the sonet/sdhoc-192 rate optical data eye diagram (Figure 4) and 10GBASE LR optical data eye diagram (Figure 2), we can clearly see the appropriate RF matching of the device and the benefits of the module bandwidth independent of electrical and optical devices, that is, the module performance can well meet the application specifications

uncooled EML is used for OC-192 transmission and has good performance in the range of -20 ℃ (left) to +90 ℃ (right)

single mode optical fiber modules need to achieve high-capacity transmission over a distance of 10km. In fact, system designers are increasing the density of line cards to reduce costs, and operators are gradually opening 10gbit/s links outside the central office, which promotes the demand for devices with a temperature adaptation range of more than 0 ℃ to 70 ℃. This is the natural process of industry development. Just like the demand for 2.5gbit/s modules in the 1990s, the demand for 10G modules will become more obvious in the next few years. Now there are technologies to develop these modules, which pave the way for module designers to meet this new demand. (end)

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