Optical receivers in cable networks.

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An optical receiver is an electro-optical device for converting optical signals into electrical signals. It consists of an optical detector and intermediate connecting components between the optical input and the coaxial output. The input of the optical receiver is supplied with an optical signal from the output of the fiber optical line. The receiver processes the received electrical signal, amplifying it and converting current pulses into voltage pulses so that the signal from the receiver's output is compatible with the radio frequency transmission system that is connected to its output. It is the parameters of the optical receiver that largely determine technical capabilities distribution system, including the length of the regeneration section, the operating frequency band of the reverse channel and the quality of the output signal.

Key factors when choosing optical receivers

1. Photodetector sensitivity. It is measured by the ratio of its output voltage to the input optical power.

2. Quantum efficiency. This is a characteristic that is similar to the sensitivity of a diode, expressed as the ratio of the number of photons incident on the diode to the number of electrons generated by them, forming a current in the external circuit. An efficiency of 1 (or 100%) means that each photon increases the current in the external circuit by one electron.

3. Dark current. Even in the absence of incident light, some current flows through the diode due to the thermal generation of electron-hole pairs. This current, the magnitude of which depends on the temperature of the device, is called dark or leakage current.

4. Equivalent or average noise power (NEP). It is the rms signal power required to produce unity signal-to-noise ratio, or the minimum optical power required to produce a current equal to the instrument's intrinsic rms noise current, which is analogous to the thermal detection threshold of the receiver.

5. Rise time (response time). This is the time it takes for the detector to increase its electrical output level from 10 percent to 90 percent of peak value. This time can be of the order of 1 for avalanche diodes, about 3 - 4 for pin diodes and depends on the bias voltage.

6. Bias voltage. When working with current, the detector requires bias into the work area by applying a bias voltage to it. Typically, pin diodes require less than 100 V of bias, while avalanche diodes require the application of several thousand volts. The fact that applying a bias voltage increases the temperature of the photodetector explains its effect on response time, dark current, and sensitivity of the device. As the bias increases, the performance characteristics of the photodiode change.

The main element of the receiver is a photodetector, which converts incoming light energy into electrical energy of the output signal. There are mainly two types of photodetectors in use today: PIN diodes and APD avalanche diodes. Let us consider in general terms the structure of these devices.

A PIN diode is a semiconductor structure that includes a region of positive charges (positive), a region of negative charges (negative) and a neutral region separating them (intrinsic), depleted of charge carriers. The depletion region is created by reverse biasing the junction, in which very weak light flows through the device. reverse current. During reverse bias, electrons tend to leave the n-region into the external circuit and form holes in the p-region, depleting the transition region of charge carriers.

When light hits the surface of a diode, the absorbed photons create electron-hole pairs in the depletion region. The electrons and holes then separate under the reverse bias of the junction and flow towards their respective regions. Each electron-hole pair produces a current of one electron in the external circuit. The structure of the PIN diode and the field strength diagram in it are shown in Fig. 11.5.

In an ideal PIN diode, each photon creates one electron-hole pair. If a weak light flux falls on the diode, then the produced electricity may not be sufficient to detect it against the background of the internal noise of the pin diode itself and the external circuit.

The PIN diode has the following characteristics:

relatively simple structure compared to avalanche diodes;
relatively weak sensitivity to changes in the temperature of the device;
quantum efficiency is usually less than or equal to 1;
limited dynamic range;
low cost;
Compared to avalanche diodes, low sensitivity at in this regard signal/noise

Avalanche Photo Diode or APD (Avalanche Photo Diode) is an alternative to PIN diode based photo detector. Compared to the latter, it has a number of advantages. If a weak light flux falls on the surface of the PIN diode, then the output signal of the detector is also weak, so I would like to increase its level before further processing and amplification in the electronic part of the photodetector. This is provided by a structure called APD, which is shown in Fig. 11.6.

A strong electric field is created inside the depletion region of the avalanche diode, the strength of which is shown as a peak in the figure. The main charge carriers generated by photons incident on the diode (as in pin diodes) when entering this strong field capable of increasing the output energy by several electron volts. Colliding with the crystal lattice, the majority carrier gives off enough energy to propel an electron from the valence band to the conduction band. This process is called impact ionization. As a result, minority carriers can create even more charge carriers. The result is a phenomenon known as avalanche breakdown, which explains the internal gain in the diode.

The number of electrons forming a current in the external circuit of the diode is equal to the product of the number of incident photons and the avalanche multiplication factor of the device. Therefore, APDs have a quantum efficiency of about 4 (i.e., more than 100%), although this can also lead to increased noise at the device output. Avalanche diodes are sensitive to temperature changes, so an APD photodetector typically includes an AGC (automatic gain control) circuit that maintains a stable bias voltage. Avalanche diodes have the following characteristics:

more complex structure compared to PIN diodes;
The sensitivity of the device depends on its temperature;
quantum efficiency is between 3 and 4;
wider dynamic range;
high strength and long service life;
higher cost compared to PIN diodes;
sensitivity is usually 5 - 6 dB higher than that of PIN diodes.

Minimal information about the design of receivers is necessary for both the developer of the optical transmission system and the maintenance technical personnel in order to monitor the performance of the system and the correctness of detection. In Fig. 11.7 shown structural scheme optical receiver. Typically, the optical receiver is a sensitive broadband photodetector with an input spectral range corresponding to the operating wavelength (for example, 1200 - 1600 nm for a wave of 1550 nm), which is combined in one housing with a powerful two-stage radio frequency amplifier with high linearity. For reliable detection, the optical signal level at the receiver input must be at least twice the receiver's own noise level. To ensure the required signal-to-noise ratio or, in the case digital transmission, the required BER value, a stronger input optical signal is desirable. This requirement is similar to the acceptable input signal level to instrument noise figure ratio in a conventional high-frequency analog transmission system. To reduce noise, some optical receiver circuits include a transimpedance amplifier (a voltage amplifier controlled by current through a field-effect transistor).

Such receivers that use PIN diode detectors are sometimes called PIN-FET (PIN field-effect transistor) devices. Field-effect transistor V in this case used to amplify the detector output. Since the active areas of the detector surface are relatively large, efficient introduction light signal from the fiber exit to the detector is not a difficult task. Sometimes, to minimize losses when introducing light into the detector, fibers with a core size larger than those used in the transmission link are used in the form of short lengths of flexible fiber. Typically, PIN diode-based receivers are simpler in design than APDs. The latter, especially in combination with a thermoelectric control (TEC) device, are more complex devices.

Currently, many models of optical receivers with different design features. It is impossible to talk about all the features, but we will try to highlight the main ones. It is usually based on a modular design with wide choice modules for various purposes. Depending on the technical requirements requirements for the network, at the choice of the developer in different models The following components can be installed: AGC module, optical transmitter of the return channel, diplexer of the forward and reverse channels, additional replaceable output signal dividers. The presence of AGC is very important in networks with varying loads or in conditions of poor stability of trunk parameters, in particular, with a low class headend. The radio frequency amplifier is built according to the same basic principles and the schemes that were described in the previous chapter. The output stage must have high linearity and is created using a Push-Pull or Power Doubler scheme; an interstage equalizer and an attenuator with smooth or step adjustment are switched between the amplifier stages.

Although it is possible to transmit light along a fiber in both directions, the return channel is often organized on a separate fiber using return channel transmitters built into some models of optical receivers and return channel optical receivers installed at the headend. The basis of the reverse channel optical transmitter is also a semiconductor laser diode with a system for temperature stabilization of output radiation power. The modulation coefficient is adjusted by changing the signal level supplied to the emitter modulator, for which an attenuator is installed at the input of the optical transmitter. The operating frequency band of reverse channel transmitters and receivers may vary depending on its load. The operating bandwidth of the optical receiver of the direct channel at the output must correspond to the bandwidth of the subsequent distribution network(50 - 862 MHz or 900 - 2150 MHz).

Additional functionality provided by optical receivers

Possibility of power supply from local power supply or via coaxial cable.
Possibility of connection using various optical connectors (FC, SC, E2000) and radio frequency connectors (RG-11, RG-11M).
Availability of test points for monitoring parameters of forward and reverse channels.
The presence of replaceable diplexers that allow you to stepwise change the upper frequency of the return channel to 30, 55 or 65 MHz.
Availability of an additional optical input for optical backbone redundancy.
The presence of power dividers that allow you to organize two radio frequency outputs.
Availability of a built-in pilot frequency generator for equipment control by the NMS (Network Management System) network management system at the head station.

The described characteristics and features of the receivers allow you to create extensive hybrid interactive networks cable television large channel and subscriber capacity with fairly long highways without optical repeaters. One optical receiver can serve a coaxial distribution segment, including from 500 to 2000 subscribers while broadcasting up to 80 digital and analog signals. For example, the input optical signal level of the OR-8601A TVBS receiver is -8...+2 dBM, and the output signal level is 112 - 116 dBµV with a C/N ratio of more than 51 dB. Its input dynamic range is thus at least 10 dB with a sensitivity of -8 dBm at 1550 nm. When using the OT8620SQ TVBS optical transmitter with an output power of 13 dBm at a wavelength of 1330 nm, the length of the optical backbone will be more than 40 km with simultaneous broadcast of 40 television channels and an input signal level of 2 dBm, taking into account that the loss in the fiber will be 0.4 dB/km. The STV and CSO ratings of this receiver are more than 65 and 61 dB, respectively.

EN-50083 requirements for a set of indicators published by the manufacturer in the specification of an optical receiver and amplifier

Operating wavelength range in nm.
Range of input optical levels.
C/N ratio at a specified optical modulation index and input power (for analog transmission).
Input power for a specified number of errors in a data stream (for digital transmission).
Maximum equivalent noise power NEP.
Maximum input equivalent noise current density.
Optical return loss coefficient over the wavelength range (recommended value should exceed 40 dB).
Supply voltage and current.
Type of optical connectors or splices.
Fiber type.
Mean time between failures (MTBF).
Demodulation characteristics.
Voltage sensitivity and its tolerance in V/W - the range of automatic level control.
Nominal operating output level - output frequency range.
Uneven amplitude-frequency characteristics.
Intermodulation at stated output levels.

The receiver can be equipped with indicators of input optical level deviations. The electrical output port of the device must have a nominal impedance of 75 ohms (in some special cases specified in the standard, an impedance of 50 ohms is acceptable). The return loss factor must correspond to one of the categories given in EN 50083.

Manufacturers must report the following optical amplifier parameters:

saturated power depending on input wavelength;
saturation output power in dBm as a function of input wavelength;
noise figure as a function of input power at a specified wavelength;
nonlinear distortion indicators;
optical return loss coefficient in the input wavelength range (recommended value should exceed 40 dB);
minimum optical return loss coefficient caused by reflection dispersion;
supply voltage and current;
type of fiber connector or splice;
fiber type;
mean time between failures (MTBF).

The amplifier must be equipped with an "on" output power indicator to indicate the emission of light.

Summary

The construction of large cable television networks is impossible without the use of optical fiber as a transport or backbone transmission line from head equipment to distribution subscriber segments, which are usually carried out on the basis of coaxial cable. At the beginning of the optical line, at the head station, an optical transmitter is installed. The final device of an optical line is an optical receiver. If the trunk or transport line is long, it is possible to connect an optical amplifier between the transmitter and the successor, but in conventional cable television networks this is not necessary. In many ways, the quality of transmission in an optical line is determined by the quality of the fiber.

Distortion and noise for digital and analog optical systems are determined by different indicators and are measured in different units. Conversion of rise time to frequency band is possible. Specifications for analog active equipment and optical fiber are usually expressed in the same terms as specifications for active equipment in coaxial systems. This allows you to find comprehensive quality indicators hybrid system a combination method using diagrams or analytical expressions and thereby simplifies the calculations when designing a system including fiber optic segments and a coaxial structure. As in coaxial systems, in analog optical systems the magnitude of intermodulation distortion depends on the number television signals and the level of the output optical signal of the transmitter. The amount of noise depends on the receiver device and the level of the optical signal at the receiver input.

The first optical receivers for CATV networks appeared a little over ten years ago. Since their appearance and development, they have undergone significant changes - both technical specifications, and in terms of cost. Let me remind you that the first receivers - optical nodes - as a rule, included an optical return channel transmitter and were intended for interactive networks with support for the DOCSIS protocol. The presence of a return channel transmitter (and, accordingly, the possibility of installing such an optical node in a network with DOCSIS support) was explained primarily by the fact that in the USA and in most European countries by that time (approximately 2000) a very powerful cable infrastructure had already been built, which no one wanted to significantly change and rebuild. This was precisely the main reason for the rapid development of DOCSIS technology - it made it possible to short time modernize already existing network and make it interactive. Those first optical nodes, as a rule, served large coaxial clusters (up to 2-5 thousand subscribers) and were very expensive devices - the price of such “pieces of iron” often reached two thousand dollars or even higher!

Russian operators have lagged behind their more “advanced” American and Western colleagues - due to noise ingression (especially in the return channel band); on the other hand, active equipment (in particular, amplifiers) was not designed to organize a return channel.

But, as they say, every cloud has a silver lining - the boom in the construction of cable networks in Russia occurred precisely at the time when optical receivers without support for a return channel began to appear, which made it possible to significantly simplify their design, and, consequently, significantly reduce the price. A more rapid drop in prices for receivers also became possible for two more main reasons - the technology for the production of optical equipment has significantly developed and, in addition, Russian market, in addition to equipment from well-known American and Western companies, a flow of cheap equipment poured in from the countries of the Asian region, primarily from China. At the same time, the first models of domestically produced receivers began to appear. The appearance on the market of such inexpensive optical receivers ultimately allowed Russian operators to begin building fiber-to-the-home networks (FTTB/FTTH technologies). As for interactivity and data transmission, they began to use network hardware– parallel to the CATV network (on other fibers), a data transmission network (optical Metro-Ethernet) was deployed.

In addition to the abandonment of return channel transmitters, two main trends emerged in the development of optical receivers for CATV networks - firstly, receivers with high output levels (about 107–110 dBµV and even higher) began to appear on the market. This was the reason for the emergence and development of FTLA technology - Fiber To the Last Active (the last active element in the network). The name of the technology speaks for itself - after such receivers with high output levels (during the construction of “optics into the house”), the need to install home coaxial amplifiers disappeared. The use of relatively expensive receivers with high output levels compared to cheap receivers but with low output levels has a number of technical advantages and is very often justified economically (see the list of publications for the article).

Subsequently, there was a desire to obtain an optical receiver of the FTTH class (i.e., with a high output level), and at the same time, such a receiver could operate at reduced levels of input optical power. Let me remind you that the first receivers serving large coaxial clusters, in order to achieve a good signal-to-noise margin, had to have an optical signal at the input with a level of about 1 mW (0 dBm). The development of fiber-to-the-home (FTTB/FTTH) technology has significantly reduced the requirements for the signal-to-noise parameter at the receiver output - the level of input optical power has become possible to reduce to a level of -3 - -4 dBm (and sometimes lower). But here’s the problem: as the input optical power decreased, there was also a significant decrease in the level of the RF signal at the receiver output. This reduction follows the one-to-two rule - when the input optical signal level is reduced by 1 dBm, the output RF signal is reduced by 2 dBµV. To avoid such a decrease in the output level and generally make it independent of possible changes in the level of the optical signal in the network, the idea arose to use an AGC system as part of an optical receiver.

So, about three or four years ago, in general terms, the formation of the market for optical receivers took place, the main types of which we see now:

– optical receiver units with the ability to install return channel transmitters. Cost - from approximately 200–300 USD for “immigrants” from the countries of the Asian region, to 700–1000 USD for their more noble “brothers”;

– cheap optical receivers without a return channel, relatively simple in design and with output levels of 100–110 dBµV (origin, as a rule, China);

– receivers specifically designed for fiber-to-the-home networks – without a return channel, with a high output level (107–115 dBµV) and a built-in AGC function. Such receivers often have additional bells and whistles, which we will talk about a little later. The cost indicator is from 120–130 to 230–250 USD.

I would like to note that the above gradation is arbitrary and does not aim to strictly systematize all those models that are on the market. First class of receivers in currently has become relatively rare - as a rule, optical nodes are used only in networks that were designed to support the DOCSIS protocol (either already built or being modernized).

As for the second class of receivers, the majority of the market for these receivers is currently occupied by receivers from the countries of the Asian region, although there are models of both domestic production and well-known foreign brands. These receivers are the simplest and cheapest, the price of some of them drops to 70–80 USD. Receivers of this class remained the most popular for a long time, until the next generation of receivers appeared, which gave them significant competition.

The first known receiver of this new generation was the Lambda Pro 50 (Vector) receiver. The high output level, the presence of an AGC function, as well as convenient functionality made this receiver actually a market favorite for a couple of years - the aspirations of other manufacturers (including the Asian region) to make a cheaper analogue for a long time did not have significant success.

However, life does not stand still, and in the last year and a half, several new models from this class of receivers have appeared, which I would like to talk about in more detail.

Receivers CXE800/CXE880 (TELESTE)

One of the well-known manufacturing companies working on the creation of effective “optics to the home” receivers was the European company Teleste. The products of this company have long been known not only throughout the world, but also on the Russian market. Teleste equipment has gained great popularity not only due to its excellent technical characteristics, but also due to its exceptional reliability. I would like to note that Finland is a country with a harsh climate, and its equipment is perfectly suited for those Russian operators who have increased demands on climatic conditions. Appearance The CXE800 receiver and its block diagram are presented respectively in Fig. 1 and 2.

The CXE800 (Teleste) receiver has one RF output (a second output can be easily used by installing a special divider insert or coupler). This typical receiver FTTH class, which does not have a return channel and is relatively simple in concept. The output stage of the receiver is organized using GaAs MESFET technology, which achieves a high output level (up to 118 dBµV). The CXE800 has a built-in AGC system based on the input optical power level, which ensures constant high level RF signal when the input optical signal changes (AGC depth is -7–0 dBm). The metal cast housing significantly increases heat transfer during operation of the receiver and reduces the risk of overheating. The receiver has local power supply (165–255 V) and has a very wide range of operating temperatures - from -40 to +55 ° C - few manufacturers can boast of such values! In addition, I would like to note the high protection of the CXE800 from electromagnetic interference and lightning discharges - Teleste guarantees resistance to impulse interference with a potential of up to 6 kV!

For those operators who use DOCSIS technology, a specially released version of the receiver based on the CXE800 is the CXE880 optical node, which has a built-in FP return channel transmitter. This unit is distinguished by its relative simplicity of design compared to many competitive models from other well-known manufacturers and, accordingly, a lower price. The CXE880 node can be locally or remotely powered, depending on customer requirements.

I would like to note that CXE800 receivers are already successfully used in many Russian networks. It was this receiver that was chosen by the Stream TV group of companies as the main receiver for the construction of optical networks in many cities of Russia.

Receivers OD002 and OD100 (TERRA)

Terra equipment is also well known cable operators– it is distinguished by an optimal price-quality ratio, European level of performance and high reliability. The OD002 and OD100 receivers were developed by Terra specifically as receivers for fiber-to-the-home networks that do not use the DOCSIS protocol and where data transmission is carried out on parallel fibers (typically Metro-Ethernet). Models OD002 and OD100 (Fig. 3 and 4) - with local power supply, have almost the same functionality and, to a first approximation, differ only in different output levels of the RF signal. As practice has shown, not all operators need an output level of 113 dBµV (this is exactly the operating level of the OD100 with AGC turned on) - you can often get by with a lower output level, and the cost of the receiver can be significantly reduced (less powerful output stage, lower power consumption and heat transfer, respectively, and a simpler body). Therefore, the operating level of the OD002 receiver is up to 107 dBµV, which made it possible to reduce its cost by more than one and a half times! The housings of OD receivers are cast, which improves their heat transfer and reduces the risk of overheating. Receivers OD002 and OD100 have one RF output and have a built-in AGC system based on the level of the input optical signal. The operating range of the AGC is very wide – from -7 to +2 dBm. In addition, these receivers have very good noise parameters - as practice has shown, it is possible to use these receivers at input signal levels near the lower limit of the AGC range (for example, -6 dBm) without significant “noise” of the signal.

I would like to especially note the presence of such a built-in option in the OD receiver family as the presence of a liquid crystal digital indicator, which can be used to display the optical power level at the receiver input using a built-in measurement system. In addition, the same indicator serves to display the RF signal parameters in setup mode. It is also interesting that the output parameters are configured without the help of insert modules, using a built-in microprocessor and push-button control. When the power is turned off, the settings are saved in the receiver's memory. All this allows you to significantly simplify the installation and configuration of the receiver and do without additional measuring equipment, which is especially important when building networks with deep optical penetration, when the costs of installing and maintaining a large number of receivers become very significant.

Receiver OD120 (TERRA)

Although the OD120 receiver is based on the OD100 model, I think it makes sense to highlight it especially since it is on this moment one of the most modern and functional devices on the market today. This receiver is interesting because it implements the ability remote monitoring and control of its main parameters through the use of the integrated Ethernet adapter UD210. Another one interesting feature model OD120 is that a digital interface board has been added to the receiver design (Fig. 5), which interacts the receiver with external devices. So, in particular, on this board there are contacts for controlling the power relay network switch(switch), and if the switch freezes, it can be restarted. In addition, the digital interface board is used to retrieve information from an external alarm sensor (for example, a sensor for opening a box in which the equipment is located), as well as information about the operating mode of the unit uninterruptible power supply(UPS). Maximum amount OD120 receivers on the network are limited only by the number of free IP addresses in the operator’s network. The OD120 receiver comes with a unique parameter description (set of MIB files) for SNMP protocol(version v2c). In these MIB files, parameters are divided into three categories:

– read only,
– readable and customizable,
– transmitted messages (TRAP).

However, some descriptions in the MIB file (such as the name of the receiver and its location) can be specified by the network operator, which is very convenient when maintaining the network.

The parameters read include the receiver serial number, input optical power level, working temperature, voltage at the output of the power supply, etc. The second group of parameters are the values of the attenuators and interstage corrector, activation of the AGC system, threshold values of parameters at which signals about emergency situations(alarms). Transmitted messages (TRAP) are the alarms themselves, indicating malfunctions or deviations of parameters from the maximum permissible set values. A set of MIB files allows you to integrate the OD120 optical receiver into the network operator's monitoring and management system. UD210 adapter parameters such as IP address, network mask, username, password, etc. can be easily set by connecting a computer with network card. A Telnet client for Windows is used to connect.

The ability to remotely monitor and control parameters implemented in the OD120 receiver makes it an extremely attractive solution for those operators who care about the reliability of their services and use modern technical means to monitor and maintain your network.

In conclusion, I would like to add that the wide variety of optical receiver models currently on the market can satisfy almost any operator’s requirements.

			In this article, I would like to continue the conversation about modern trends in the development of fiber-optic equipment for CATV networks, in particular, optical receivers. A wide variety of manufacturers of this equipment segment, as well as a wide range of receiver models currently presented on the Russian market, on the one hand, can satisfy the most sophisticated needs of cable operators, and on the other hand, can create a choice problem for those cable operators who are modernizing their optical network or are building it for the first time.

	The article was published in the magazine "Cable Guy" No. 3 2009.

The first optical receivers for CATV networks appeared a little over ten years ago. Since their appearance and development, they have undergone significant changes - both in technical characteristics and in cost indicators. Let me remind you that the first receivers - optical nodes - as a rule, included an optical return channel transmitter and were intended for interactive networks with support for the DOCSIS protocol. The presence of a return channel transmitter (and, accordingly, the possibility of installing such an optical node in a network with DOCSIS support) was explained primarily by the fact that in the USA and in most European countries by that time (approximately 2000) a very powerful cable infrastructure had already been built, which no one wanted to significantly change and rebuild. This was precisely the main reason for the rapid development of DOCSIS technology - it made it possible to modernize an existing network and make it interactive at minimal cost and in the shortest possible time. Those first optical nodes, as a rule, served large coaxial clusters (up to 2-5 thousand subscribers) and were very expensive devices - the price of such “pieces of iron” often reached two thousand dollars or even higher!

But, as they say, every cloud has a silver lining - the boom in the construction of cable networks in Russia occurred precisely at the time when optical receivers without support for a return channel began to appear, which made it possible to significantly simplify their design, and, consequently, significantly reduce the price. A more rapid drop in prices for receivers also became possible for two more main reasons - the technology for the production of optical equipment has significantly developed and, in addition, in addition to the equipment of well-known American and Western companies, a flow of cheap equipment from the countries of the Asian region has poured into the Russian market, primarily from China. At the same time, the first models of domestically produced receivers began to appear. The appearance on the market of such inexpensive optical receivers ultimately allowed Russian operators to begin building fiber-to-the-home networks (FTTB/FTTH technologies). As for interactivity and data transmission, network equipment began to be used for this - in parallel with the CATV network (on other fibers), a data transmission network (optical Metro-Ethernet) was deployed.

So, about three or four years ago, in general terms, the formation of the market for optical receivers took place, the main types of which we see now:

– optical receiver units with the ability to install return channel transmitters. Cost - from approximately 200–300 USD for “immigrants” from the countries of the Asian region, to 700–1000 USD for their more noble “brothers”;

– cheap optical receivers without a return channel, relatively simple in design and with output levels of 100–110 dBµV (origin, as a rule, China);

– receivers specifically designed for fiber-to-the-home networks – without a return channel, with a high output level (107–115 dBµV) and a built-in AGC function. Such receivers often have additional bells and whistles, which we will talk about a little later. The cost indicator is from 120–130 to 230–250 USD.

I would like to note that the above gradation is arbitrary and does not aim to strictly systematize all those models that are on the market. The first class of receivers has now become relatively rare - as a rule, optical nodes are used only in networks that were designed to support the DOCSIS protocol (either already built or being modernized).

However, life does not stand still, and in the last year and a half, several new models from this class of receivers have appeared, which I would like to talk about in more detail.

Receivers CXE800/CXE880 (TELESTE)

One of the well-known manufacturing companies working on the creation of effective “optics to the home” receivers was the European company Teleste. The products of this company have long been known not only throughout the world, but also on the Russian market. Teleste equipment has gained great popularity not only due to its excellent technical characteristics, but also due to its exceptional reliability. I would like to note that Finland is a country with a harsh climate, and its equipment is perfectly suited for those Russian operators who have high demands on climatic conditions. The appearance of the CXE800 receiver and its block diagram are shown in Fig. 1, respectively. 1 and 2.

The CXE800 (Teleste) receiver has one RF output (a second output can be easily used by installing a special divider insert or coupler). This is a typical FTTH receiver that has no return channel and is relatively simple in concept. The output stage of the receiver is organized using GaAs MESFET technology, which achieves a high output level (up to 118 dBµV). The CXE800 has a built-in AGC system based on the input optical power level, which ensures a constant high RF signal level as the input optical signal changes (AGC depth is -7-0 dBm). The metal cast housing significantly increases heat transfer during operation of the receiver and reduces the risk of overheating. The receiver has local power supply (165–255 V) and has a very wide range of operating temperatures - from -40 to +55 ° C - few manufacturers can boast of such values! In addition, I would like to note the high protection of the CXE800 from electromagnetic interference and lightning discharges - Teleste guarantees resistance to impulse interference with a potential of up to 6 kV!

Receivers OD002 and OD100 (TERRA)

Terra's equipment is also well known to cable operators - it is distinguished by its optimal price-quality ratio, European level of performance and high reliability. The OD002 and OD100 receivers were developed by Terra specifically as receivers for fiber-to-the-home networks that do not use the DOCSIS protocol and where data transmission is carried out on parallel fibers (typically Metro-Ethernet). Models OD002 and OD100 (Fig. 3 and 4) - with local power supply, have almost the same functionality and, to a first approximation, differ only in different output levels of the RF signal. As practice has shown, not all operators need an output level of 113 dBµV (this is exactly the operating level of the OD100 with AGC turned on) - you can often get by with a lower output level, and the cost of the receiver can be significantly reduced (less powerful output stage, lower power consumption and heat transfer, respectively, and a simpler body). Therefore, the operating level of the OD002 receiver is up to 107 dBµV, which made it possible to reduce its cost by more than one and a half times! The housings of OD receivers are cast, which improves their heat transfer and reduces the risk of overheating. Receivers OD002 and OD100 have one RF output and have a built-in AGC system based on the level of the input optical signal. The operating range of the AGC is very wide – from -7 to +2 dBm. In addition, these receivers have very good noise parameters - as practice has shown, it is possible to use these receivers at input signal levels near the lower limit of the AGC range (for example, -6 dBm) without significant “noise” of the signal.

I would like to especially note the presence of such a built-in option in the OD receiver family as the presence of a liquid crystal digital indicator, which can be used to display the level of optical power at the receiver input using a built-in measurement system. In addition, the same indicator serves to display the RF signal parameters in setup mode. It is also interesting that the output parameters are configured without the help of insert modules, using a built-in microprocessor and push-button control. When the power is turned off, the settings are saved in the receiver's memory. All this allows you to significantly simplify the installation and configuration of the receiver and do without additional measuring equipment, which is especially important when building networks with deep optical penetration, when the costs of installing and maintaining a large number of receivers become very significant.

Receiver OD120 (TERRA)

Although the OD120 receiver is based on the OD100 model, I think it makes sense to highlight it as it is currently one of the most modern and functional devices on the market today. This receiver is interesting because it implements the ability to remotely monitor and control its main parameters through the use of an integrated UD210 Ethernet adapter. Another interesting feature of the OD120 model is that a digital interface board has been added to the receiver design (Fig. 5), which interacts the receiver with external devices. So, in particular, this board contains contacts for controlling the power relay of the network switch, and if the switch freezes, it can be restarted. In addition, the digital interface board is used to retrieve information from an external alarm sensor (for example, an opening sensor for a box in which the equipment is located), as well as information about the operating mode of the uninterruptible power supply (UPS). The maximum number of OD120 receivers in the network is limited only by the number of free IP addresses in the operator’s network. The OD120 receiver comes with a unique parameter description (set of MIB files) for the SNMP protocol (version v2c). In these MIB files, parameters are divided into three categories:

– read only,
– readable and customizable,
– transmitted messages (TRAP).

However, some descriptions in the MIB file (such as the name of the receiver and its location) can be specified by the network operator, which is very convenient when maintaining the network.

The read parameters include the serial number of the receiver, the level of input optical power, operating temperature, voltage at the output of the power supply, etc. The second group of parameters are the values of attenuators and interstage corrector, activation of the AGC system, threshold values of parameters at which signals about abnormal situations are generated ( alarms). Transmitted messages (TRAP) are the alarms themselves, indicating malfunctions or deviations of parameters from the maximum permissible set values. A set of MIB files allows you to integrate the OD120 optical receiver into the network operator's monitoring and management system. UD210 adapter parameters such as IP address, network mask, username, password, etc. can be easily set by connecting a computer with a network card. A Telnet client for Windows is used to connect.

In conclusion, I would like to add that the wide variety of optical receiver models currently on the market can satisfy almost any operator’s requirements.

Optical receivers in cable networks.

Key factors when choosing optical receivers

The PIN diode has the following characteristics:

Additional functionality provided by optical receivers

EN-50083 requirements for a set of indicators published by the manufacturer in the specification of an optical receiver and amplifier

Manufacturers must report the following optical amplifier parameters:

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