在很多利用光控制的應用中，維持?琠w的光強度至關重要。有些系統採用簡單的LED或雷射二極體作為光源，但是隨著時間的推移，即便最初校準得很好的光源也會變差。隨著LED的老化，其電流－發光轉換比率會降低，發光強度也會變弱。要長期保持工廠設定的發光強度需要一個控制電路來監控發光狀況，並控制供給光發射元件的電流以保持輸出?琠w。這種配置適合以下應用：用於精確光強度測量的光強度測定應用、針對伺服系統精確光定位的控制應用，以及光參考測試設備等。圖1所示為這種系統的組成示意圖。
光電二極體特性
矽光電二極體跟PN結二極體的結構類似，只不過前者的P層有點薄。P層厚度可進行調整以使光的波長能被檢測到。跟其它二極體一樣，光電二極體也有電容器，電容器大小與加在其上的反偏壓成正比，典型值範圍為2-20pF。光電二極體有正極和負極，可工作於正向模式(電流從正極流向負極)或反向模式(電流從負極流向正極)。當光電二極體工作於反向模式(正極為負)時，在某一特定頻率上其發光線性特別好，這在設計控制電路時尤其有用。
原型設計
在圖2中，一個原型電路用於分析帶運算放大器的控制環。該電路驅動一個PNP電晶體，電晶體再給LED提供電流來產生光源。LED所發出的部份光會照到光電二極體上，再轉換成很小的電流，一般只有10μA左右。在這種情況下，光電二極體工作於反向模式。因此，當沒有光照時，光電二極體內除漏電流(也稱‘暗電流’)外什麼也沒有，而放大器處於過載狀態。這時將從基極吸收被連接於基極的電阻限制的電流，該電阻使電晶體最初處於飽和狀態。一旦電流開始流經電晶體，LED或雷射二極體就開始發光。光電二極體將部份光能轉換為電流，流過RG。隨著電流的增加，RG兩端的電壓降也隨之升高。當該電壓接近VBIAS(圖2中接到地)時，控制環就封閉以保持正確的電晶體驅動電壓以及?琠w的LED電流，因而維持?琠w的光強度(或光電二極體電流)。這就是基本的電路DC分析。
圖3給出了該電路的一個實例，它採用美國國家半導體公司的LMV2011精密運算放大器。參考電壓由該公司的LM4041-1.2元件產生，它提供固定的1.225V參考電壓，該元件的電流設定為約10mA，正處於其工作範圍的中間。
VBIAS由兩個精密度為1％的電阻產生，電壓值設定為約1V。VREF和VBIAS之間的差值除以RG，就可得出控制環封閉的光電二極體電流。要注意的是，VBIAS須小於VREF，否則電路不能工作。如果光電二極體電流為10mA，那麼RG應該為0.2 × 10E-6或20.0K。
採用一個4.7K的電阻來限制PN200A PNP電晶體基極電流，該電阻將電流限定在1mA左右。該電晶體的(值約為100，因此它所提供的最大電流約為100mA，這已經超出小型SOT-23封裝的散熱範圍。為防止電晶體的過高熱量，將一個限流電阻與LED或雷射二極體串聯，使該二極體達到最大工作負載，因而限定電晶體集電極的電流。如果需要更大的電流，則須選用集電極電流大的電晶體，而且要採用SOT-223等較大的封裝。為了限定電路頻寬以維持穩定性，選用一個15pF的電容器與光電二極體電容器並聯(在1.2VBIAS時其電容器大小也約為15pF)，因而將放大器的工作頻率限定在250KHz左右。
作者：Richard F. Zarr
在利用光來控制一個過程的應用中，要長期保持工廠設定的發光強度需要一個控制電路來監控發光狀況，並控制供給光發射器件的電流以保持輸出恒定，採用一個簡單的運算放大器電路就可為許多應用提供精確的光強度。即便發光器件老化，通過調整LED電流，一個控制環也可維持恒定的光強度。
在很多利用光來控制一個過程的應用中，維持恒定的光強度至關重要。有些系統採用簡單的LED或鐳射二極體作為光源，但是隨著時間的推移，即便最初校準得很好的光源也會變差。隨著LED的老化，其電流-發光轉換比率會降低，發光強度也會變弱。要長期保持工廠設定的發光強度需要一個控制電路來監控發光狀況，並控制供給光發射器件的電流以保持輸出恒定。這種配置適合以下應用：用於精確光強度測量的光強度測定應用、針對伺服系統精確光定位的控制應用，以及光參考測試設備等。圖1所示為這種系統的組成示意圖。
光電二極體特性
矽光電二極體跟PN結二極體的結構類似，只不過前者的P層有點薄。P層厚度可進行調整以使光的波長能被檢測到。跟其他二極體一樣，光電二極體也有電容，電容大小與加在其上的反偏壓成正比，典型值範圍為2-20pF。光電二極體有正極和負極，可工作于正向模式(電流從正極流向負極)或反向模式(電流從負極流向正極)。當光電二極體工作於反向模式(正極為負)時，在某一給定頻率上其發光線性特別好，這在設計控制電路時尤其有用。
矽光電二極體跟PN結二極體的結構類似，只不過前者的P層有點薄。P層厚度可進行調整以使光的波長能被檢測到。跟其他二極體一樣，光電二極體也有電容，電容大小與加在其上的反偏壓成正比，典型值範圍為2-20pF。光電二極體有正極和負極，可工作于正向模式(電流從正極流向負極)或反向模式(電流從負極流向正極)。當光電二極體工作於反向模式(正極為負)時，在某一給定頻率上其發光線性特別好，這在設計控制電路時尤其有用。
原型設計
在圖2中，一個原型電路用於分析帶運算放大器的控制環。該電路驅動一個PNP電晶體，電晶體再給LED提供電流來產生光源。LED所發出的部分光會照到光電二極體上，再轉換成很小的電流，一般只有10μA左右。在這種情況下，光電二極體工作於反向模式。因此，當沒有光照時，光電二極體內除漏電流(也稱“暗電流”)外什麼也沒有，而放大器處於超載狀態。這時將從基極吸收被連接於基極的電阻限制的電流，該電阻使電晶體最初處於飽和狀態。一旦電流開始流經電晶體，LED或鐳射二極體就開始發光。光電二極體將部分光能轉換為電流，流過RG。隨著電流的增加，RG兩端的電壓降也隨之升高。當該電壓接近VBIAS(圖2中接到地)時，控制環就封閉以保持正確的電晶體驅動電壓以及恒定的LED電流，從而維持恒定的光強度(或光電二極體電流)。這就是基本的電路DC分析。
在圖2中，一個原型電路用於分析帶運算放大器的控制環。該電路驅動一個PNP電晶體，電晶體再給LED提供電流來產生光源。LED所發出的部分光會照到光電二極體上，再轉換成很小的電流，一般只有10μA左右。在這種情況下，光電二極體工作於反向模式。因此，當沒有光照時，光電二極體內除漏電流(也稱“暗電流”)外什麼也沒有，而放大器處於超載狀態。這時將從基極吸收被連接於基極的電阻限制的電流，該電阻使電晶體最初處於飽和狀態。一旦電流開始流經電晶體，LED或鐳射二極體就開始發光。光電二極體將部分光能轉換為電流，流過RG。隨著電流的增加，RG兩端的電壓降也隨之升高。當該電壓接近VBIAS(圖2中接到地)時，控制環就封閉以保持正確的電晶體驅動電壓以及恒定的LED電流，從而維持恒定的光強度(或光電二極體電流)。這就是基本的電路DC分析。
圖3給出了該電路的一個實例，它採用國家半導體公司的LMV2011精密運算放大器。參考電壓由該公司的LM4041-1.2器件產生，它提供固定的1.225V參考電壓，該器件的電流設定為約10mA，正處於其工作範圍的中間。
VBIAS由兩個精度為1%的電阻產生，電壓值設定為約1V。VREF和VBIAS之間的差值除以RG，就可得出控制環封閉的光電二極體電流。要注意的是，VBIAS須小於VREF，否則電路不能工作。如果光電二極體電流為10mA，那麼RG應該為0.2×10E-6或20.0k。
採用一個4.7k的電阻來限制PN 200A PNP電晶體基極電流，該電阻將電流限定在1mA左右。該電晶體的(值約為100，因此它所提供的最大電流約為100mA，這已經超出小型SOT-23封裝的散熱範圍。為防止電晶體的過高熱量，將一個限流電阻與LED或鐳射二極體串聯，使該二極體達到最大工作負荷，從而限定電晶體集電極的電流。如果需要更大的電流，則須選用集電極電流大的電晶體，而且要採用SOT-223等較大的封裝。為了限定電路帶寬以維持穩定性，選用一個15pF的電容與光電二極體電容並聯(在1.2VBIAS時其電容大小也約為15pF)，從而將放大器的工作頻率限定在250kHz左右。
可視鐳射驅動器有數位控制功率調製
Visible-Laser Driver Has Digitally Controlled Power Modulation
| |||
Many laser diodes include a photodiode that generates a current proportional to the intensity (optical power) of the laser beam. Most of these photodiodes, however, have relatively slow response times and cannot track the peak optical power of a typical modulated laser diode. Instead, the driver circuits for these devices control the laser by monitoring a relative average optical power. The circuit in the figure below includes a 10-bit digital-to-analog converter (DAC) with 3-wire serial input that operates and maintains a visible-light laser diode at constant average optical output power. A separate digital input line (MOD) enables a comparator with open-drain output (IC4) to implement digital communications by pulsing the laser-diode through Q1. Circuit components were chosen to minimize the layout are and cost.  For Larger Image |
This circuit provides digital control of the modulation and power output of a visible-light laser diode.Resistor R6 converts the photodiode current to a usable voltage, which is applied to the inverting input of a "leaky" integrator based on the high-speed op amp IC3. The integrator smoothes out variations in the modulation and prevents the feedback loop from trying to regulate the laser pulses. The integrator is made leaky (by R10) to ensure compensation of downward as well as upward variations in the average power.
Thus, the integrator creates an error signal and base drive for Q1 by monitoring the voltage across R6 and comparing it to the DAC's preset voltage. The DAC's reference voltage (from IC1) is 2.5V, but its output-voltage buffer has a gain of 2V/V, giving the DAC output an adjustment range of 0 to 5V. With its nominal base voltage set by the DAC output, Q1 controls the optical power by regulating current through the laser diode.
R9 provides isolation and helps to stabilize IC3 when the base of Q1 is being shorted and released by a signal from the MOD input. By maintaining a small laser-diode current during the "off" periods of digital modulation, R1 preempts another problem: Startup time for a laser diode increases tremendously if the forward current goes to zero. R1 ensures that the laser current is below the threshold for lasing, but high enough to allow an acceptable turn-on time for communication and modulation.
A similar version of this article appeared in the March 23, 1998 issue of Electronic Design magazine.
http://pdfserv.maximintegrated.com/en/an/AN1811.pdf
Thus, the integrator creates an error signal and base drive for Q1 by monitoring the voltage across R6 and comparing it to the DAC's preset voltage. The DAC's reference voltage (from IC1) is 2.5V, but its output-voltage buffer has a gain of 2V/V, giving the DAC output an adjustment range of 0 to 5V. With its nominal base voltage set by the DAC output, Q1 controls the optical power by regulating current through the laser diode.
R9 provides isolation and helps to stabilize IC3 when the base of Q1 is being shorted and released by a signal from the MOD input. By maintaining a small laser-diode current during the "off" periods of digital modulation, R1 preempts another problem: Startup time for a laser diode increases tremendously if the forward current goes to zero. R1 ensures that the laser current is below the threshold for lasing, but high enough to allow an acceptable turn-on time for communication and modulation.
A similar version of this article appeared in the March 23, 1998 issue of Electronic Design magazine.
http://pdfserv.maximintegrated.com/en/an/AN1811.pdf
| Related Parts | ||
| MAX4124 | Single/Dual/Quad, Wide-Bandwidth, Low-Power, Single-Supply Rail-to-Rail I/O Op Amps | Free Samples |
| MAX5354 | 10-Bit Voltage-Output DACs in 8-Pin µMAX | Free Samples |
| MAX6125 | SOT23, Low-Cost, Low-Dropout, 3-Terminal Voltage References | Free Samples |
| MAX988 | High-Speed, Micropower, Low-Voltage, SOT23, Rail-to-Rail I/O Comparators | Free Samples |
Behind the Light Show in Optical TransceiversOct 21, 2002 |
Abstract: This article explains, in laymen's terms, the growing importance of optic-based telecommunications and describes the various standards (SDH, SONET, and FDDI) that are being used to implement this technology. As a young technology, improvements can be made to both improve the performance of the technology and reduce costs in manufacturing process. Digital potentiometers provide a means to set critical laser driver and feedback circuits for optimal performance. Temperature sensing mechanisms inside these pots can be used to monitor and compensate these circuits for any parametric changes that may occur as a function of temperature. These digital potentiometers can also be used to automate production calibration procedures once performed manually using mechanical potentiometers or handpicked fixed resistors. This not only reduces the production cost of these laser drivers, but also can be used to improve the reliability and quality of the devices by removing the element of human error from the calibration process. Dallas Semiconductor potentiometers discussed include the DS1845, DS1846, DS1847, and DS1848.
In recent years, news about communications networks technology always seems to involve some pronouncement on the urgent need for more bandwidth. The facts still bear repeating: A growing number of people with telephones, faxes, modems, and computers are, through the exchange of terabytes of digitized information (videos, images, modeling procedures, as well as data and voice), demanding a larger share of the carrier spectrum. In response, high-tech communications companies who thrive on growth are competing to feed this appetite for bandwidth. Over the past decade, major resources have gone into developing fiber-optic networks in which light waves transport information at rates of gigabits per second through optical fibers finer than the human hair.
The stakes are very high. In a May 2000 news release, The Aberdeen Group (Boston, MA), an IT consulting firm, predicted that "The optical network market, excluding SONET elements, will grow to $17.7 billion by 2003. The suppliers that can deliver the technologies that solve the problems that carriers face, will be the ones to succeed."
The use of the plural in "suppliers" and "technologies" highlights a key issue in this article. The burgeoning communications network is highly complex. While a few large companies tend to dominate the global deployment of the optical network, behind the scenes there is a fusion of technologies developed by multiple companies, each with specialized technological expertise. Maxim falls into this latter category; we have designed a family of variable resistors especially for optical transceiver modules. A look at where Maxim's resistors fit into the grand scheme of communications networks reveals something about the way the communications industry develops solutions.
Identifying the Big Picture
Optical transceiver modules are designed and built by a variety of manufacturers. Applications for the modules include Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Fiber Distributed Data Interface (FDDI), Fiber Channel, Fast Ethernet and Gigabit Ethernet. The names of these systems reflect the range of internationally defined transmission protocols and standards. On the other hand, the modules themselves were initially developed without definitive physical characteristics.
Recognizing the need for conformity, if their products were to succeed, a group of manufacturers banded together and developed a multi-source agreement (MSA) for transceiver modules in 1998. The group consisted of AMP Incorporated, Hewlett-Packard Company, Lucent Technologies Microelectronics Group, Nortel (Northern Telecom), Siemens AG-Fiber Optics, and Sumitomo Electric Lightwave Corp. These parties agreed to cut the size of their modules in half (to 0.535 inch in width) and specified a set of module packages and pin-outs that would be interchangeable among the variety of RJ-45-style (including duplex LC, MT-RJ, and SC/DC) optical connectors used in high-speed fiber-channel applications.
Currently, a new consortium is drafting a new MSA for transceiver modules, reflecting a larger contingent of manufacturers and a new generation of modules. These multi-source manufacturers now include Agilent Technologies, Glaze Network Products, E2O Communications, Finisar, Fujikura Technology America, Hitachi Cable, Infineon Technologies, IBM, Lucent Technologies, Molex, OCP, Picolight, Stratos Lightwave, Sumitomo Electric Lightwave, and Tyco Electronics. The module specification is now called small form-factor pluggable (SFP) and covers expected transmission rates of up to 5.0Gb/s. The specifications reflect the industry's drive for high-density signal transmission in hot-pluggable modules of smaller size and higher speed.
To find where our resistors come into the picture, it helps to understand some basics about the transceiver module. The module converts incoming light waves to electrical signals and outgoing electrical signals back to light. Of fundamental significance, the optical transceiver is based on semiconductor laser technology. The module is a printed circuit board (PCB), and the optical source for the coveted bandwidth is a tiny semiconductor chip: a light-emitting or laser diode. At frequencies in the near-infrared spectrum, the laser's output can be modulated in tens of GHz, a capacious bandwidth.
The following briefly summarizes a signal path through the transceiver module. The receiving port connects to incoming light fibers. A photodetector diode converts the light to electrical signals, which are then amplified so that clock and data signals can be recovered, de-multiplexed, and sent out through the electrical interface. The photodetector requires an automatically power-controlled bias circuit to provide a constant operating voltage (see Figure 1). Meanwhile on the transmitting side of the module, electrical clock and data-bit signals are synthesized and latched and sent to the laser driver. Finally, the laser driver sends the signal as electrical current to the laser diode, which converts electron energy to light.
Figure 1. Typical average power control circuit using a monitor photodiode and adjustable resistor to set bias current.
In some designs that use laser diodes, a photodetector monitors the laser diode output and, in a feedback loop, reconverts the light back to electrical circuits that measure the laser's actual output power. This feedback stabilizes the laser output power. The optical feedback is a complicating drawback to this design. However, the latest laser technology, vertical cavity surface emitting lasers (VCSELs), often do not require a photodetector because of exceedingly low current.
The laser driver must do two things: it must maintain a consistent DC-bias current to set the laser operating point, and it must maintain a modulation current to carry the signal. As manufacturers strive to increase signal throughput in transceivers, the laser source must be carefully characterized for operating constants in order to control the light output.
Whether laser diode or VCSEL, the laser emitter in any optical transceiver is a semiconductor whose photoelectric effects depend on the interplay of current, voltage, and resistance. Some of the following factors affect safety and performance:
From the big-picture viewpoint, we can delineate the accumulated facts as follows:
In modules that aim to shrink into SFPs, more densely integrated components that combine memory and two separately configured potentiometers save space by replacing multiple parts. Furthermore, the DS1845's 2-wire interface meets the transceiver producers' requirement for in-circuit programmability and is compatible with existing 2-wire EEPROM.
To meet more a specialized need, Dallas Semiconductor developed the DS1846, which combines three linear taper potentiometers with nonvolatile memory and a CPU supervisor in reduced TSSOP packaging. This level of integration in such a small chip saves board space and cuts cost and procurement delays, expediting product development. As with the DS1845, the nonvolatile memory is used to configure and store application-specific calibration data. And, to control wiper settings for each potentiometer, there's also memory space available for user-specific data.
The DS1846's on-chip micromonitor tracks voltages. On detecting an out-of-tolerance voltage level, the micromonitor initiates and holds a system reset until safe operating conditions return. The micromonitor is programmable for various voltage levels and includes a manual reset.
The third potentiometer can be used to monitor another variable or to provide a coarse trim for one of the other resistors.
Intended for demanding laser applications, the DS1847 and DS1848 compensate for the laser's thermal characteristics over temperature ranges (see Figure 2). The DS1848 has an extra 128 bytes of general-purpose EEPROM; otherwise the two are alike. The chips store resistance characteristics relative to temperature in an onboard look-up table (LUT). An integrated temperature sensor constantly measures and reports temperatures during laser operation. The DS1847 or DS1848 compares the reading to the value stored in the LUT and adjusts resistance according to the designer's defined resistance characteristics. The value determined by the temperature sensor is also stored in EEPROM (updated every 10msec), and is available to the user over the 2-wire bus. It should also be noted that the DS1847 and DS1848 operate automatically. As a temperature change is detected, control circuits automatically adjust resistance to achieve the compensating current value without any user intervention.
Figure 2. Variable resistors, designed for optical transceivers like this one, automatically calibrate each diode more accurately than the older, mechanical-trim potentiometers.
As a whole, all Maxim circuits employ a low-power CMOS technology that contributes to holding down a sensitive power budget. All circuits operate throughout the industrial temperature range and with both 3V and 5V power supplies.
Clearly, in a huge, complex, driven market like optical communications networks, many players operate at many levels. The success of the laser, the star in a new and exotic technology, can depend on a comparatively old and familiar player, the humble resistor. Of course, Maxim's caveat to the moral of this fable would be, "It's not your grandfather's resistor any more."
http://pdfserv.maximintegrated.com/en/an/AN1769.pdf
Recognizing the need for conformity, if their products were to succeed, a group of manufacturers banded together and developed a multi-source agreement (MSA) for transceiver modules in 1998. The group consisted of AMP Incorporated, Hewlett-Packard Company, Lucent Technologies Microelectronics Group, Nortel (Northern Telecom), Siemens AG-Fiber Optics, and Sumitomo Electric Lightwave Corp. These parties agreed to cut the size of their modules in half (to 0.535 inch in width) and specified a set of module packages and pin-outs that would be interchangeable among the variety of RJ-45-style (including duplex LC, MT-RJ, and SC/DC) optical connectors used in high-speed fiber-channel applications.
Currently, a new consortium is drafting a new MSA for transceiver modules, reflecting a larger contingent of manufacturers and a new generation of modules. These multi-source manufacturers now include Agilent Technologies, Glaze Network Products, E2O Communications, Finisar, Fujikura Technology America, Hitachi Cable, Infineon Technologies, IBM, Lucent Technologies, Molex, OCP, Picolight, Stratos Lightwave, Sumitomo Electric Lightwave, and Tyco Electronics. The module specification is now called small form-factor pluggable (SFP) and covers expected transmission rates of up to 5.0Gb/s. The specifications reflect the industry's drive for high-density signal transmission in hot-pluggable modules of smaller size and higher speed.
To find where our resistors come into the picture, it helps to understand some basics about the transceiver module. The module converts incoming light waves to electrical signals and outgoing electrical signals back to light. Of fundamental significance, the optical transceiver is based on semiconductor laser technology. The module is a printed circuit board (PCB), and the optical source for the coveted bandwidth is a tiny semiconductor chip: a light-emitting or laser diode. At frequencies in the near-infrared spectrum, the laser's output can be modulated in tens of GHz, a capacious bandwidth.
The following briefly summarizes a signal path through the transceiver module. The receiving port connects to incoming light fibers. A photodetector diode converts the light to electrical signals, which are then amplified so that clock and data signals can be recovered, de-multiplexed, and sent out through the electrical interface. The photodetector requires an automatically power-controlled bias circuit to provide a constant operating voltage (see Figure 1). Meanwhile on the transmitting side of the module, electrical clock and data-bit signals are synthesized and latched and sent to the laser driver. Finally, the laser driver sends the signal as electrical current to the laser diode, which converts electron energy to light.
Figure 1. Typical average power control circuit using a monitor photodiode and adjustable resistor to set bias current.
In some designs that use laser diodes, a photodetector monitors the laser diode output and, in a feedback loop, reconverts the light back to electrical circuits that measure the laser's actual output power. This feedback stabilizes the laser output power. The optical feedback is a complicating drawback to this design. However, the latest laser technology, vertical cavity surface emitting lasers (VCSELs), often do not require a photodetector because of exceedingly low current.
The laser driver must do two things: it must maintain a consistent DC-bias current to set the laser operating point, and it must maintain a modulation current to carry the signal. As manufacturers strive to increase signal throughput in transceivers, the laser source must be carefully characterized for operating constants in order to control the light output.
The Laser Diode and VCSEL
The Fabry-Perot type of laser diode emits a coherent light beam from the narrow, beveled edge of the chip, with reflecting mirrors incorporated at the edges or stationed outside the chip. For the future of the communications industry however, a more promising laser source is the VCSEL. As its name suggests, the VCSEL vertically emits the laser beam from a circular cavity 5 to 25 micrometers in diameter at the top (the bottom is a future possibility) of the chip. The mirrors are incorporated as an integrated array on both ends of the cavity—a design known as a "distributed Bragg reflector." In the future, parallel optical interconnects using multi-element VCSEL arrays could enable terabyte throughput.
Academic and corporate institutions are vigorously developing VCSEL designs for more widespread deployment. Compared to edge-emitters, the VCSEL requires less current and has a lower lasing threshold (1mA or 2mA versus 30mA). At this level, simple current control is often sufficient without the extra photodetector to monitor output. The VCSEL's emitting aperture is measurably larger, which means that the output beam's angle of divergence (a measure of dispersion) is significantly smaller. There are several manufacturing and processing advantages, as well. The die is much smaller, allowing more VCSELs to be packed on a wafer with more interconnects; all the VCSELs on an entire wafer can be tested at once. Lastly, the VCSEL is more robust in operation than a laser diode, with a longer life expectancy and lower failure rates.Whether laser diode or VCSEL, the laser emitter in any optical transceiver is a semiconductor whose photoelectric effects depend on the interplay of current, voltage, and resistance. Some of the following factors affect safety and performance:
- Laser output is exceedingly sensitive to temperature.
- Laser power output tends to change over the life of the laser and this aging increases with temperature.
- As VCSELs operate at significantly lower power and temperature than diodes, the failure rate over time is proportionately lower.
- The laser emitter needs to be protected from random power transients as well as transients during power-on and power-off.
- Even though a laser's near-infrared light is invisible to humans, a beam entering the eye is still focused on the retina and can cause permanent damage. Because of potentially serious effects on personal safety and laser function, regulations require that laser power output be limited to a few hundred microwatts.
From the big-picture viewpoint, we can delineate the accumulated facts as follows:
- Exploding demand for bandwidth leads to the development of optical networks.
- Optical networks use optical transceiver modules to physically convert optical and electrical signals.
- Manufacturers of transceiver modules are driven to decrease physical size and increase signal throughput to multiple gigabits per second.
- Transceiver modules use photodiodes to receive light signals and laser diodes or VCSELs to send light signals.
- As data rates continually increase, modules' photo-active components require ever more precise, reliable power control in order to prevent laser failure, prolong life expectancy, and/or operate within desired output parameters.
A Niche Industry
While not in any strict sense one of the communications companies, Maxim brings expertise to the table in several pertinent technologies: digitally controlled variable resistors and potentiometers, EEPROM, temperature sensors, and extremely low-power CMOS methods. In response to the needs of gigabit optical technology, Maxim has produced a product family with a new range of features.
With the DS1845 Dual Potentiometer with EEPROM, Dallas Semiconductor (a wholly owned subsidiary of Maxim Integrated) designed the semiconductor industry's first potentiometer with integrated memory, specifically for service in pluggable gigabit transceiver modules. The DS1845 combines two linear-taper potentiometers with 256 bytes of EEPROM, which is required by the MSA standards. The higher-resolution, 256-position potentiometer can be used to control modulation current and the 100-position potentiometer can be used to control bias current. Users configure both outputs and store the wiper settings and required serial ID data in the on-chip, nonvolatile EEPROM memory for reference during operation.In modules that aim to shrink into SFPs, more densely integrated components that combine memory and two separately configured potentiometers save space by replacing multiple parts. Furthermore, the DS1845's 2-wire interface meets the transceiver producers' requirement for in-circuit programmability and is compatible with existing 2-wire EEPROM.
To meet more a specialized need, Dallas Semiconductor developed the DS1846, which combines three linear taper potentiometers with nonvolatile memory and a CPU supervisor in reduced TSSOP packaging. This level of integration in such a small chip saves board space and cuts cost and procurement delays, expediting product development. As with the DS1845, the nonvolatile memory is used to configure and store application-specific calibration data. And, to control wiper settings for each potentiometer, there's also memory space available for user-specific data.
The DS1846's on-chip micromonitor tracks voltages. On detecting an out-of-tolerance voltage level, the micromonitor initiates and holds a system reset until safe operating conditions return. The micromonitor is programmable for various voltage levels and includes a manual reset.
The third potentiometer can be used to monitor another variable or to provide a coarse trim for one of the other resistors.
Intended for demanding laser applications, the DS1847 and DS1848 compensate for the laser's thermal characteristics over temperature ranges (see Figure 2). The DS1848 has an extra 128 bytes of general-purpose EEPROM; otherwise the two are alike. The chips store resistance characteristics relative to temperature in an onboard look-up table (LUT). An integrated temperature sensor constantly measures and reports temperatures during laser operation. The DS1847 or DS1848 compares the reading to the value stored in the LUT and adjusts resistance according to the designer's defined resistance characteristics. The value determined by the temperature sensor is also stored in EEPROM (updated every 10msec), and is available to the user over the 2-wire bus. It should also be noted that the DS1847 and DS1848 operate automatically. As a temperature change is detected, control circuits automatically adjust resistance to achieve the compensating current value without any user intervention.
Figure 2. Variable resistors, designed for optical transceivers like this one, automatically calibrate each diode more accurately than the older, mechanical-trim potentiometers.
As a whole, all Maxim circuits employ a low-power CMOS technology that contributes to holding down a sensitive power budget. All circuits operate throughout the industrial temperature range and with both 3V and 5V power supplies.
Clearly, in a huge, complex, driven market like optical communications networks, many players operate at many levels. The success of the laser, the star in a new and exotic technology, can depend on a comparatively old and familiar player, the humble resistor. Of course, Maxim's caveat to the moral of this fable would be, "It's not your grandfather's resistor any more."
http://pdfserv.maximintegrated.com/en/an/AN1769.pdf
| Related Parts | ||
| DS1845 | Dual NV Potentiometer and Memory | Free Samples |
| DS1846 | NV Tri-Potentiometer, Memory and MicroMonitor | Free Samples |
| DS1847 | Dual Temperature-Controlled NV Variable Resistor | Free Samples |
| DS1848 | Dual Temperature-Controlled NV Variable Resistor & Memory | Free Samples |
沒有留言:
張貼留言
注意：只有此網誌的成員可以留言。