Key parameters and certification of optical SFP modules

Knowing certain principles easily compensates for the lack of knowledge of certain facts.
Helvetius

Optical Transceivers

Currently, the use of optical technology in the construction of telecommunication networks has become almost ubiquitous. Anyone who had to deal with optical switching or transmitting equipment faced the work of optical transceivers - transceivers (eng. Transceiver = transmitter + receiver).

Transceivers are designed to convert electrical signals into optical signals for subsequent transmission over a fiber-optic line and subsequent optoelectronic conversion at the reception. At the initial stage of development of fiber optics, transceivers were mounted on printed circuit boards of active equipment. Subsequently, with the growth of the range of such devices (switches, routers, multiplexers, media converters), it became necessary to separate the parts responsible for processing information and transmitting it (in fact, pairing with an optical line).

In the past 10-15 years, optical transceivers are compact plug-in modules designed for various parameters of transmission lines and installed in standardized electrical ports of active equipment. This allows you to optimize costs in the design, and especially - the reconstruction of optical networks. For example, it is possible to increase the speed, transmission distance, increase the amount of information transmitted through the use of spectral multiplexing systems (WDM, CWDM, DWDM). Or, say, use different types of transceivers for remote subscribers in one switch.

Now the most popular standard for interchangeable optical transceivers are SFP modules (small Form-factor Pluggable). They are small-sized structures in a metal case (for mechanical protection and electromagnetic shielding) with leads for connection to active equipment slots. Also in the module there are two optical ports: a radiator (Tx) and a photodetector (Rx) for operation in a two-fiber mode. In mono-fiber SFP there is only one optical port, and the direction of transmission and reception is divided inside the module using an integrated WDM multiplexer (BOSA, Bidirectional Optical Sub-Assemblies). In this case, transceivers operate in a pair at two wavelengths.
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In addition to the emitter and photodetector, the circuit board of the module contains circuits for providing the emitter pumping current, conversion to a linear code, bias on the photodetector, thermal stabilization, etc.


Fig.1. Block diagram of a removable optical transceiver

• TIA - transimpedance amplifier;
• LimA - limiting amplifier;
• DDM - digital diagnostic module;
• EEPROM - ROM with module parameters;
• O / E - opto-electronic converter;
• E / O - electro-optical converter.

All modules support hot swap mode (HotSwap) during operation. In most modern constructions, the DDM (Digital Diagnostics Monitoring) digital monitoring function is implemented, which allows monitoring the internal temperature, the power supply voltage, the laser bias current, the laser output power and the level of the received optical signal from an external terminal.

Geometrical dimensions, mechanical parameters, power supply, parameters of electrical interfaces and other data of the modules are specified in the MSA SFF-8704i specification.

As for the parameters of the optical interface, they are described in a fairly generalized form in the standards for Ethernet networks: 802.3u (100BASE-X), 802.3ae (1000BASE-X), 802.3ae (10GBASE-X) and others.



Tab.1. Ethernet optical interface standards

* The interface is not standardized, but is actively used in the market.
** According to some sources - up to 100 km

The SFP standard provides for the transfer of information at a speed of 1 Gbit / s with the possibility of transmitting 100 Mbit / s or only 100 Mbit / s. For the transmission of higher-speed streams, SFP + (10 Gbit / s), XFP (10 Gbit / s), QSFP + (40 Gbit / s), CFP (100 Gbit / s) were further developed. However, at higher speeds, signals are processed at higher frequencies. This requires a larger heat sink and, accordingly, large dimensions. Therefore, in fact, the SFP form factor has been preserved only in the SFP + modules.

In this article, we will only talk about the parameters of the most popular now SFP, SFP + and XFP modules, since transceiver models at speeds above 10 Gbit / s are a separate and rather interesting question.

Here we are, without claiming to be complete and without giving mathematical calculations, consider, first of all, the system of parameters of the optical interfaces of the receiving-transmitting modules. Understanding the essence of the parameters will allow to correctly design segments of optical networks: choose the optimal parameters of the emitter and photodetector at the lowest cost.

Optical emitter parameters

Transmitter type.
As a rule, laser diodes are used as emitters, the type of which depends on the type of fiber, as well as the required power and narrowband. Fabry-Perot (FP) lasers are characterized by an average power, a wide spectrum of radiation and a relatively low cost (Fig. 2). They are used with single-mode (at a wavelength of 1310 nm, less often - 1550 nm) and multimode fibers (at wavelengths of 850 nm and 1300 nm) with line lengths from a few hundred meters to several kilometers and transmission speeds of 100 Mbit / s and 1 Gbit / with. Vertical-emitting lasers (VCSEL) have been developed for local optical networks. They are distinguished by low cost, narrow spectrum and work, as a rule, with multimode fibers at a wavelength of 850 nm with transmission of 1 Gbit / s and 10 Gbit / s over distances of several hundred meters. Dynamic single-mode lasers with distributed feedback (DFB) have a narrow spectrum with medium and high power. The production technology with suppression of side modes of radiation determines the cost greater than that of the two previous types of lasers. They are designed to work with single-mode fibers at wavelengths of 1310 nm and 1550 nm, while transmitting information at a speed of 1 Gbit / s, 10 Gbit / s and more over distances of tens of kilometers (with amplifiers several hundred kilometers). Such radiators are used in CWDM systems. The most complex and expensive lasers with an external resonator (EML) have an extremely narrow spectrum. This is fundamentally important when transmitting high-speed flows (10 Gbit / s, 40 Gbit / s, 100 Gbit / s) over long distances, especially at a wavelength of 1550 nm, where there is a sufficiently large chromatic dispersion in the fibers. Narrow-band EML lasers are also used in CWDM and DWDM spectral multiplex systems. It should be noted that manufacturers do not always indicate in the specifications the type of radiator.

Fiber type (Fiber type).
For the transmission of optical signals, as a rule, two main types of fibers are used: multimode (MM) and single mode (SM). Accordingly, the emitter and photodetector of the optical transceiver should be designed to work with one of these two types of fibers. This is usually reflected in their labeling and technical specification. Features of fiber types (for example, OM3, OM4 - for multimode or DS, NZFSF, BIF - for single-mode) should not be taken into account. Another thing is that the attenuation coefficient, chromatic dispersion coefficient, broadband ratio (only for MM) and other parameters of the fiber types used must be taken into account when calculating the power budget, total dispersion, line length, etc.

The number of optical ports.
Two-fiber optical transceivers use two ports: an optical emitter (Tx, Transmitter) and a photodetector (Rx, Receiver). Such modules are used to transmit in two different directions two fibers and one working wavelength. Recently, single-fiber transceivers with a single optical port are much more common. They work what is called “paired”: transmission in two different directions in one fiber goes on two working wavelengths. Transmit and receive signals are divided inside the module using an integrated WDM multiplexer.

Type of optical connector (Connector type).
To connect to the optical line can be used a variety of types of connectors. Nowadays, small-size LC connectors (in dual-fiber and single-fiber modules), and SC (only in single-fiber modules) are the most popular on Ethernet networks.

The width of the spectral line (Max. Spectral Width).
This rather important parameter depends on the type of radiator. The greater the width of the spectral line, the greater the total chromatic dispersion in the line. For multimode fiber communication systems, intermode dispersion prevails, therefore less expensive and more broadband radiators such as FP or VCSEL are often used there. Since they have a line spectrum (Fig. 2), the root-mean-square spectrum width (RMS) is normalized for them, which is approximately 3 ... 5 nm for FP and 0.5 ... 1 nm for VCSEL. The DFB and EML lasers have one pronounced lobe (one longitudinal mode) in the spectrum and the internal structure of the suppression of the other (side) modes. Therefore, their spectrum is determined by the central lobe at -20 dB. For DFB it is 0.1 ... 0.5 nm, and for EML - about 0.01 ... 0.08 nm.

Side Mode Suppression Ratio (SMSR).
This parameter applies only to DFB and EML lasers. It shows how much dB the amplitude of the first side mode (petal) is less than the amplitude of the central longitudinal mode (see figure [Emitters Spectra]). Thus, a numerical characteristic of the quality of the selectivity of the radiator cavity is given. Usually the minimum value of the SMSR is normalized to 30 dB.



Fig.2 Typical spectra of laser emitters of various types

Transmitter Central Wavelength.
This is the wavelength at which the highest radiation power is transmitted. For DFB and EML lasers, it almost coincides with the peak wavelength. Usually, the wavelengths of local attenuation minima (“transparency windows”) in optical fibers are used to transmit signals: 850 nm or 1310 nm for multimode fibers; 1310 nm or 1550 nm - for single mode. For CWDM and DWDM optical transceivers, the wavelength corresponds to the frequency grid specified in ITU-T Recommendations G.694.2 and G.694.1, respectively (see Table 2).



Tab. 2. CWDM Optical Transceiver Wavelengths

Maximum and minimum emitter power (Max./Min Average output power, Mean power).
The average power level at the radiator output, i.e. power introduced into the fiber. Medium - I mean not the peak level. As a rule, the specifications give two values: maximum and minimum. The production technology of optical emitters (TOSA, Transmitter Optical Sub-Assemblies) implies some variation of parameters. The actual output power will be between the maximum and minimum value. But when calculating the power budget in the line, it is necessary to take into account the minimum value of the average power.



Fig.3. The power levels of optical signals when transmitting them on line

Eye chart (Eye pattern).
It is a graphical representation of a digital signal, allowing to evaluate the quality of transmission. It is the result of the imposition of all the pulses of the real sequence on the clock interval. The overlapping of impulses "1" and "0" and forms, in fact, the "eye" (Fig. 4). Its vertical opening is determined by the levels of single and zero, and the horizontal stretch - by the rise time (Rise Time) and recession (Fall Time) of pulses. Since the shape of the output signals is probabilistic in nature, the resulting eye is always somewhat "blurred." To normalize the eye-diagrams, a special pattern (Eye pattern mask) is provided in which all variations must fit.

International standards (ITU-T G.957, IEEE 802.3) spelled out formalized parameters of type X and Y, defining the boundaries of the template elements. It is crucial to maintain the correct waveform at the receiving side. However, the presence of interference in the transmission of signals along the line leads to a reduction in the area of ​​opening the eyes. Distortion in amplitude is determined by the resulting distortion due to intersymbol transitions, superimposed power of re-reflected pulses, non-ideal characteristics of amplifiers, etc. Aperture reduction occurs due to dispersion distortion, phase jitter (jitter) and other factors affecting the distortion of the pulse fronts. Amplitude and temporal distortion can also lead to the moment when the receiving device will not optimally select the time and level of the decision on compliance of "1" or "0". Numerical eye chart is characterized by OMA and ER parameters, which are discussed further.



Fig.4. Eye diagram of the output optical signal

The amplitude of the optical modulated signal (Optical Modulation Amplitude, OMA) and the pulse quenching ratio (Extinction Ratio, ER).
Both of these parameters characterize the magnitude of the disclosure of the "eye" in the eye chart. The difference is that OMA characterizes the difference in optical power levels "1" and "0" in relation to their absolute values ​​(in dB or mW), and ER characterizes the ratio of these levels to each other (as a dimensionless quantity or in dB). After the signal passes through the optical transmission line, the amplitude of the signal decreases, and the OMA decreases. And since the ratio levels and “1” and “0” decrease, their ER ratio practically does not change. These parameters are important for estimating receive error rate. With their help, such a characteristic is calculated as a deterioration in the quality of the signal at the reception due to a decrease in the power of the pulse (Power Penalty). Real minimum ER values ​​are usually 8.2… 10 dB for 100 Mbit / s and 1 Gbit / s transceivers.

For high speeds and short distances, lower values ​​are specified - 3.5 ... 5.5 dB. Despite the fact that a larger ER value implies better recognition conditions for receiving signals, it is quite difficult technically to provide a large difference in the levels of “1” and “0” at the transmitter output. A higher upper level is limited by the temperature regime of the radiation source. And lowering the level of "0" will complicate its recognition at the reception.



Fig.5. Power levels and amplitude of the output optical signal

Photo receiver sensitivity (Receiver Sensitivity).
Sensitivity characterizes the minimum power level received by a photodetector, which still provides the specified value of the error rate. A lower sensitivity level naturally allows an increase in the dynamic range of the entire system (Fig. 3). However, at low detectable powers, the own shot and thermal noises of the photodetector can affect. Typically, the sensitivity of the photodetector is in the range of -15 ... -21 dB for SFP, calculated on the line with a length of several kilometers, -14 ... -28 dB for lines 20 - 40 km, -32 ... -35 dB for lines 80 - 160 km and -40 ... -45 dB for lines of about 200 km. It should be borne in mind that the sensitivity of the receiver depends on the transmission rate. For example, for a speed of 10 Gbit / s there is almost no sensitivity below -24 dB. At low levels of the received signal, avalanche photodiodes are usually used, which, however, make quite large noises. To increase the sensitivity, an increase in the sensitive area of ​​the photodetector is required. On the other hand, this limits the speed of the photodiode, as the charge resorption time increases, and avalanche multiplication delays increase.

Photo receiver overload level (Receiver overload).
Shows the maximum power level that can be fed to the photodetector. Exceeding this level will lead to a nonlinear mode of operation and a sharp increase in the error rate at the reception, and with greater power - to the destruction of the sensitive area of ​​the photodetector. That is, an elementary breakdown of the reverse-shifted photodiode occurs. Some manufacturers even separate these two states by specifying “distortion level” (receiver overload saturation) and “level of destruction” (receiver overload damage). In any case, it is not necessary to experiment with photodetector overloads. Special attention should be paid to this when assembling the line layout “on the table”. If the receiver overload level according to the specification is above the permissible minimum transmitter power, it is strictly forbidden to connect the emitter directly to the photo detector with a patchcord. In this case, it is imperative to use an insert-attenuator with attenuation of at least the difference between the two parameters. Typically, the overload level of the photo detector is within -3 ... + 2 dBm. However, for some modules it can be -8 ... -10 dBm. In itself, this value says nothing about the quality of the receiver. The only need is to take care not to burn the expensive module.

Total output jitter (Total Jitter).
The jitter of the phase (jitter) of the optical transmitter manifests itself in a pulse shift at a clock interval or a pulse edge shift. As a rule, the cause of jitter is in the imperfection of the master oscillator and phase locked loop systems. Subsequently, at the reception, this may lead to a shift in the point in time at which the decision about the level of the signal takes place. This out of sync is especially unpleasant for networks and systems operating in synchronous mode. Ethernet networks are less sensitive to jitter on transmission. Total jitter is normalized either in units of time (ps) or as part of the clock interval (UI), at which the peak shifted relative to another peak (pp). A typical requirement is 0.24 UI or 0.35UI for Gigabit Ethernet and 0.21 UI for 10G Ethernet. Some manufacturers still separately specify the jitter caused by the data content (Data Dependent Jitter, DDJ) and its own non-signaling jitter (Uncorrelated Jitter, UJ), but these refinements are not so significant.



Fig.6. Transmit jitter

Minimum relative noise power density (Relative Intensity Noise, RIN).
The parameter characterizing the emitter's own noise in a given frequency band. They arise as a result of spontaneous radiation of the source and depend on the temperature mode, the ratio of the bias current and the threshold current. Noise power decreases in proportion to the square of the average radiation power. Acceptable value is - 120 ... 130 dB / Hz. The greater the range and speed of transmission, the lower the noise density (i.e., the greater the absolute value with the minus sign) is desirable to have. For reference, you can add that the emitters for transmitting analog signals (for example, in cable television networks) have 20 - 30 dB lower.

Loss of reflection from the receiver (Receiver Reflectance, Return Loss, RL).
This parameter shows how much dB the signal reflected from the receiver port is below the level of the signal fed to this port. Accordingly, the more attenuated the reflected (not useful) signal, the better. Then the parameter becomes larger in absolute value with a minus sign. As a rule, RL is specified at the level of -21 ... -28 dB. However, for interfaces designed for small lengths of lines (type S), in the connector on the photodetector side there can be an open area of ​​the photodetector, not the receiving fiber in the ferule. Then the reflection losses are normalized at the level of -12 ... -14 dB. That is, in essence, the magnitude of the reflected power is indicated in the Fresnel reflection at the glass / air interface. This allows you to reduce the cost of an optical SFP module with acceptable transmission parameters. A similar parameter is sometimes specified for the transmitter port (Transmitter Reflectance), with approximately the same values ​​in dB. However, it is difficult to measure it, and there is no need to take into account in the calculations, since we may be interested only in the power of the radiator actually introduced into the fiber.

Dynamic range (Attenuation range, AR, Optical link loss).
Shows in dB, which signal power losses can be assumed without loss of the quality of the transmitted information, i.e. without increasing the error rate above a given. The dynamic range is not always specified in the manufacturers specifications, but is easily calculated as the difference between the minimum permissible power of the optical emitter and the sensitivity of the photodetector. For small transmission speeds and / or low dispersion in the line, the dynamic range of the transceivers is the key parameter determining the maximum transmission distance or the length of the regeneration / amplification section. For example, for transceivers operating at a wavelength of 1550 nm, AR is ~ 14 dB for a 40 km line, ~ 23 ... 24 dB for 80 km, ~ 28 ... 29 dB for 100 km, ~ 32 ... 34 dB for 120 km In general, it is possible to choose the approximate dynamic range of the transceiver by multiplying the average line losses taking into account welding (~ 0.25 dB / km for λ = 1550 nm and ~ 0.38 dB / km for λ = 1310 nm) by the line length and adding to The quality of the operational margin of 2-3 dB.

Permissible Dispersion (Dispersion Tolerance, DT).
Shows the maximum value of the dispersion, which is allowed on the transmission line (or regeneration section), without significant deterioration in the quality of information. Deterioration occurs due to intersymbol interference (partial imposition of adjacent clock pulses) when transmitting a digital signal sequence. This can lead to both transient effects between channels and synchronization noise at the reception. The allowable dispersion is specified for transmission over single mode fibers. In principle, the rms sum of the chromatic and polarization dispersions should be taken into account as permissible. But in practice, at speeds up to 10 Gbit / s and line lengths up to 100 km, only the first component is essential. Firstly, it is much larger, especially in the 1550 nm wavelength range. And secondly, the total chromatic dispersion grows in proportion to the length of the line, and the polarization dispersion in proportion to the square root of the length. Permissible dispersion is indicated in ps / nm.If the specified value is divided by the coefficient of chromatic dispersion of the fiber in ps / (nm · km), then it is possible to approximately determine the allowable length of the transmission line, limited by dispersion distortions. This parameter is not always indicated in the manufacturer’s specifications, more often for single-wave transceivers operating in the 1550 nm range or CWDM transceivers in the 1470 - 1610 nm range. Typical DT values ​​are 800 ps / nm (for lines up to 80 km), 1600 ps / nm - up to 80 km, 2400 ps / nm - up to 120 km. For smaller distances, the dispersion is usually not normalized.

Transmission quality deterioration due to dispersion (Dispersion Penalty, DP).
This parameter characterizes the deterioration of the signal-to-noise ratio at the reception due to the influence of dispersion on the transmitted signal. The effect is to reduce the amplitude of the signal and stretch the fronts to adjacent clock intervals. Accordingly, the deterioration will be greater, the greater the total dispersion in the line and the smaller the interval. Numerically, DP is determined by the logarithm of a quantity inversely proportional to the product of the chromatic dispersion coefficient, the width of the source spectral line, the line length and the linear information transmission rate in a square.

Typically, the DP value is specified for high-speed interfaces designed for long transmission lines. Acceptable parameter value is within 4 dB. Otherwise, you need to make a more accurate calculation of the project on the resulting noise and take some technical measures. For example, the use of optical or electronic compensation of chromatic dispersion.



Fig. 7. Dependence of the transmission quality degradation due to dispersion on the line length at different transmission speed and spectral width of the emitter.

Optical transceiver certification

First, a few words about the principles of certification. It is widely believed that certification is the quality control of products. In fact, certification is a procedure for confirming certain parameters of a product, the requirements of certain standards. No more and no less.
The certificate itself contains a list of standards, compliance with which was confirmed by tests, documents, calculations. On the other hand, if, for example, in a certain AB chain you have a certificate as proof of the compliance of element “B” with its corresponding standards, you can be sure that if standardized AB and VB joints are used A ”, then the whole chain will work. And this is already important, for example, for telecommunications, where multicomponent networks and systems are commonly used.

Another important useful quality of certification is to conduct laboratory tests in an accredited independent laboratory. Even with a very high level of production and your full confidence in the manufacturer it is always useful to carry out tests "on the side". Especially if it is really a test, not a formal reply. First of all, even the “most branded brands” were repeatedly observed in non-compliance with standards, although not as often as “handicrafts” of various stripes. And secondly, the behavior of tests often allows not only to actually measure the values ​​of the parameters, but also to analyze their margin (margin) in relation to the limits (limit) provided by the standards. From this stock, in part, one can judge the reliability of the device or system.

That was our goal. Conduct real tests with good calibrated devices, get results on the basic parameters of transmitting FoxGate optical transceivers and get a certificate of conformity to provide it to our customers.

Of course, SFP-modules do not belong to the list of mandatory certification, since they are not household devices or devices with increased risk of operation. Therefore, voluntary certification was carried out. However, to obtain the certificate of UkrSEPRO with confirmation of the possibility of using the equipment on public networks of Ukraine, we had to fulfill two conditions. Firstly, the standards used were to comply with the “List of standards and norms to which the technical means of wired telecommunications, which are intended for use in the public telecommunications network of Ukraine, must conform.” And, secondly, the certification body and testing laboratory must be accredited by the Communications Administration of Ukraine. We chose the testing laboratory "Energosvyaz" (the beginning - Kolchenko AV)Knowing her good equipment with measuring devices for SDH and Ethernet networks, as well as high professionalism of the staff, most of whom have been working with fiber optics for more than 10-15 years.

It is quite natural that during the certification tests not all the parameters specified in the technical specifications of the manufacturers or in the standards are checked. Some parameters are difficult to measure. And this requires specialized and expensive equipment. Moreover, the higher the frequency band (or transmission rate) - the more expensive equipment is required. And the costs of certification and verification, and even considerable funds for continuous confirmation of accreditation, do not contribute to the good equipment of our laboratories with modern measuring instruments.

Sometimes, the optical measuring equipment of acceptable accuracy is quite large and bulky. Rather, it is suitable for factory conditions, where there is a place for its installation and the expediency of its use for control on the stream.

Therefore, when choosing the scope of tests / measurements, there is always a rational approach:

• conduct as many tests as possible to confirm the norms (and most important principles!) of technical standards;
• Do not go into complex, costly and long-term tests;
• if possible, conduct tests (even those that are not included in the standards, but specified in the manufacturer's documents) that would enable the customer to evaluate the performance and reliability of the devices.

On this basis, we coordinated with the certifiers a test program, which, in general, included the main energy parameters of the optical interface (since it is important for calculating lines), as well as spectral characteristics and an eye diagram that allow you to check the quality of the output signal (important for line work at extreme distances and over time). The characteristics of electrical transceiver interfaces were also checked, but we will not dwell on these results, since they, as a rule, do not cause problems with the operation of optical modules.

The following small-size optical transceivers of the FoxGate brand were put up for certification:

• 19 SFP (100 / … 1 /): , , 1310/1490/1550/1570 , 3 120 ;
• 15 SFP+ (1 / … 10 /): , , 850/1310/1550 , 0,3 80 ;
• 15 XFP (10 /): , , 850/1270/1310/1550 , 0,3 80 ;
• 3 SFP CWDM (100 / … 1 /): , , 18 ITU-T G.694.1, 40/80/120 ;
• 3 types of SFP + CWDM modules (10 Gbit / s): two-fiber, for single-mode fibers, for 18 wavelengths according to the grid of frequencies ITU-T G.694.1, at distances of 20/40/70 km;
• 3 types of XFP CWDM modules (10 Gbit / s): two-fiber, for single-mode fibers, for 18 wavelengths according to the grid of frequencies ITU-T G.694.1, at a distance of 20/40/70 km.

Fig. 8. FoxGate Optical Transceivers

The measurement results of the energy parameters of the optical interface

* The average output radiation power was determined at working wavelengths using an OT-2-5 optical power meter.



Fig. 9. Measurement of the output optical power of the transceivers.

The result was within the range between the specified maximum and minimum values. On average, the measured power exceeded the minimum by 3 ... 5 dB. The minimum margin is 2.3 dB.

* It was very interesting to check the stability of the radiation level of the optical transmitter in time. As a result, it can be noted that when turned on, the emitter enters the mode in a few minutes. After that, the average output power can vary by no more than 0.01 dB within 10 minutes (no longer waited). Interestingly, the XFP emitters and all three lines of CWDM sources (especially SFP), which were enough for half a minute, entered the mode most quickly.

* The sensitivity of the photodetector was measured using an OT-2-5 optical power meter and a PHOTOM-7081ZA variable attenuator.



Fig. 10. Measurement of the sensitivity of the optical receiver

* The reflection loss on the photodetector was determined using an IVPo instrument (see Fig. 11). In the process of testing from an internal calibrated source, the unmodulated signal was fed to a photodetector. The reflected signal returns to the same port, and through the internal coupler enters the recording photo detector. The results were within the specified values: about 14 dB for modules with open photodetectors and 34 ... 37 dB for detectors with a ferula.



Fig. 11. Measurement of the attenuation of the reflection of the optical receiver

* The analysis of the eye-charts was performed using a Tektronix CSA803C telecommunications signal analyzer. This is a rather complicated measurement in itself, since a specialized analyzer (oscilloscope) with huge bandwidth is needed - up to several GHz and several tens GHz depending on the speed of the transmitted data stream. In addition, it is important to synchronize this signal and minimize the effect of high-frequency noise and interference. Taking into account the hardware capabilities of the laboratory, the analysis was carried out only for 1 Gb / s modules. As expected, the eye-diagrams at the exit of the emitters fit the mask perfectly.



Fig.12. Eye Chart Measurement at Optical Transmitter Output

* The overload level of the photodetector of most samples was not measured in order not to burn the module. In those cases when the overload level was higher than the maximum output level of the radiator, we were convinced of the transceiver operability when connecting directly “from the output to the input”.

The results of measurements of spectral parameters

Certification laboratories are extremely rarely equipped with measuring instruments that allow viewing the spectral characteristics of components in the optical range. Not even new devices, quite expensive. And if we add to this the problems with their metrological certification, the costs of certification and periodic verification, it becomes clear why no one is keen to have such devices on the balance sheet.
Images of spectra of all types of optical modules, as well as their spectral parameters in the testing laboratory "Energosvyaz" were obtained using the Acterna ONT-50 network analyzer in the 1310 nm and 1550 nm ranges, as well as Yokogawa AQ6370 in the 850 nm range.

* The general view of the obtained spectra corresponds to the theoretical (described above) for emitters of the types FP, VCSEL, DFB, EML.



Fig. 13. Results of measurements of the spectra of optical emitters

* We were pleased with the results of measuring the width of the spectral line of radiation sources. For FP lasers, the root-mean-square width of the spectrum (RMS) was 1.5 ... 1. nm with the specified 3.5 ... 4 nm. In addition, the spectrum analyzer automatically calculates the half-full maximum spectrum width (FWHM), which for the Gaussian distribution is defined as the 2.35 width of the RMS spectrum. DFB lasers showed values ​​of 0.12 ... 0.45 nm at a rate of 1 nm. And the narrowest spectrum was expected for lasers with an external modulator (EML) - 0.02 ... 0.08 nm. This allows you to provide a large transmission distance even at a speed of 10 Gbit / s without fear of the influence of chromatic dispersion.

* The central wavelength for DFB and EML lasers is determined quite easily from the peak value of the fundamental mode of the emitter. For FP and VCSEL lasers, the weighted average wavelength is taken into account, taking into account all the fundamental modes, which may differ somewhat from the peak mode. The results obtained for all modules met the specifications. The difference of the wavelength from the nominal was ± 3 ... 8 nm (with a norm from ± 10 nm to ± 40 nm) for conventional SFP, SFP +, XFP without further optical compaction. The CWDM optical transceivers are more stringent requirements: a tolerance of -6 / + 7.5 nm from the nominal wavelength corresponding to the frequency grid G.694.2. When measuring the spread was only ± 0.4 ... 2.4 nm.



Fig.14. Measurement of spectra at the output of optical transmitters

* The stability of the central wavelength (in time) is not a normalized parameter. However, it, like the stability of the output power, to some extent characterizes the quality of the transmitting part of the module. The drift of the central wavelength stopped for about 20 ... 30 seconds. For CWDM transceivers, stabilization took 3… 5 seconds.

* The measured sideband suppression ratio significantly exceeded the norm - 30 dB. DFB lasers mattered SMSR within -37 ... -32 dB, and EML - within -39 ... -50 dB. This indicates a good selectivity of the radiator, i.e. on the quality of fabrication of an internal periodic lattice in the semiconductor structure.

Conclusion

Certification tests of optical transceivers FoxGate confirmed the compliance of electrical and optical interface parameters with the requirements of international standards. The results obtained for the normalized optical characteristics were within the specified limits, and the value of the margin made it possible to judge the long-term reliable operation of the modules, as well as the possibility of working on transmission lines slightly exceeding the calculated ones. Additionally, the studied characteristics allowed us to indirectly talk about the high quality of the components and the assembly, which ensures good reliability of the modules.
The testing laboratory "Energosvyaz" demonstrated good equipment with modern measuring devices of the optical range, as well as a high technical and methodological level of training of test engineers.