Medium optical power meters for automated operating standards and OFTS parameters verification systems
We distinguish two main principles underlying the measurement of optical power:
• the principle of converting the energy of optical radiation into thermal energy, and then thermal energy into electrical voltage or current;
• the principle of photoelectric energy conversion of optical radiation directly into an electrical signal.
According to these principles, optical power meters are divided into calorimetric wattmeters based on methods for measuring temperature rise (caused by the optical radiation under study) and wattmeters based on methods for measuring photocurrent caused by the presence of photons of optical radiation. The photo cells (photodiodes, photoresistors, photomultipliers) are used in the devices of the second type.
Calorimetric wattmeters can operate in a wide range of wavelengths (from UV to IR and microwave ranges). They have a large inertia (depending on the design of the absorber) – tens of seconds and a sensitivity of the order of a few µW with a measurement error no worse than ± (0.1–2)%.
Calorimetric optical power converters are used more often when solving problems of the metrological support of OFTS as part of the State and operating standards of units of average optical radiation power. The design of the equipment including these meters is highly complex, and the degree of automation of such a verification process is low.
Currently, the means of measuring the average optical power of steel wattmeters with photoelectric conversion of the energy of optical radiation based on photodiodes are the most common ones. Their main advantages are high sensitivity (10–12–10–14 W), small inertia, ease of use [1, 2].
The first samples of optical wattmeters to automate the process of measuring optical power, appeared abroad in the mid‑70s. The structure of these devices included the GP-IB trunk interface, created based on IEEE‑488 standard (corresponding to the international standard IEC625.1 and its domestic equivalent – GOST 26.003–80). For example, in Japan, Ando has advertised the AQ‑1111 wattmeter with photodiode converters (the wavelength range is 0.6–1.1 µm and 1.0–1.7 µm; the range of measured values of the average power is 10–9–10–2 W with basic relative measurement error ± 5%), as well as the AQ‑1112 wattmeter with a calorimetric converter, (wavelength range 0.6–1.7 µm; dynamic range 10–5–10–2 W with basic relative measurement error ±3%). Photodiode wattmeters with similar characteristics were also manufactured by Anritsu (Japan) ML93A (wavelength range 0.38–1.15 µm and 0.75–1.8 µm) and by Hewlett Packard (USA) HP8140A (wavelength range 0.4–1.11 µm).
Currently, there are quite a lot of optical power meters from different companies on the market of instrumentation. Basically, these are photodiode portable wattmeters with standard external interfaces.
Such devices include optical power meters manufactured by: Agilent Technologies (USA) N7745A; EXFO (Canada) EPM 50; Fiber Instrument Sales (USA) OV-PM, OV2; Fluke network (USA) Multi Fiber Pro; E. Hioki (Japan) 3664, etc. [3–5] The technical characteristics of wattmeters allow their use in wide spectral range of 0.4–1.7 µm (overlapped by one or more photodiode converters).
The dynamic range of instruments is 60–70 dB, the measurement error is ±5% at the calibration wavelength. The difference between devices mainly consists in their dimensions, functions and availability of external interfaces (RS232, USB, GP-IB, Ethernet).
In the USSR, in the late 70s, a calorimetric wattmeter M3–49 (with a wavelength range of 0.4–11 µm, dynamic range 10–4–10–2 W with a basic relative error of measurement ±3%). The device did not have a full-fledged external interface, but had access to an alphanumeric printing device that allows it to be included in automated measuring systems (AMS) and used to take measurements, without implementing remote control functions.
In the mid‑80s, the first commercially available optical power meter OMK3–79 with a COP interface (GOST 26.003–80) was developed. A wattmeter made it possible to measure the average optical power, as well as determine the radiation wavelength in the spectral range of 0.6–1.6 µm. The measurement range of the average power is 10–8–10–2 W, the measurement error of the optical power is 7% at wavelengths of 0.85 µm, 1.3 µm, 1.5 µm and 10% in the working spectral range. The error in measuring the wavelength of radiation is 2%. The integrated microprocessor fully provided the necessary interface functions, which made it easy to use the wattmeter as part of various AMSs.
A number of average optical power meters with high technical characteristics, designed to operate as part of automated operating standards [6, 7], has been created at Nizhny Novgorod Research Instrument Engineering Institute "Quartz". One of them is an OM3–109 optic fiber wattmeter. Structurally, the wattmeter is made in the form of a base unit in which power sources are located, a display device with a control panel and two seats for replaceable transducer units operating in the spectral ranges of 0.8–1.1 µm and 1.0–1.65 µm, respectively. The structural scheme of the optic fiber wattmeter is shown in fig. 1.
At the inputs of the converters there are optic fiber connectors that allow you to connect both multi-mode and single-mode optical cables. Microlenses, located in the connectors, form a parallel beam of radiation necessary for the operation of various optical nodes of the plug-in unit: dispersive elements and attenuators.
The installation of the dispersion element in the optical path of the device serves to accurately determine the wavelength of the input radiation and reduce measurement errors. The functional dependence of the spectral absorption characteristic of the incident radiation of the dispersive element is monotonic and has a large slope within the working range of the wavelength of the wattmeter.
The introduction to the optical path of the attenuator extends the range of linearity of the photodetector to 100 mW. The attenuator includes a flap that closes the optical channel (in the "Zero setting" mode). Control of the electromagnetic drives of the dispersion element, discrete attenuators, attenuator and damper is performed by the controller of discrete dampers.
A photodiode (a photodiode based on InGaAs structures) is used as a photodetector in the wattmeter. The photodetector current is converted to a voltage that is fed to the input of a programmable amplifier. A programmable amplifier allows you to change the transmission coefficient in accordance with the level of the optical signal power supplied. The amplifier also includes a unit of calibration resistors, which is used to achieve an exact voltage value proportional to the optical signal. In the ADC, the voltage is converted into a binary code and transmitted via the in-device serial synchronous SPI interface to the base unit. The plug-in unit includes an identification EEPROM, which stores correction factors and calibration characteristics of optical and electrical path nodes.
The control of replaceable units of converters is carried out by micro-controller software and hardware built into the base unit. Information exchange between the units is made via communication channels. Standard synchronous I2C and SPI interfaces with a serial data transmission format are used as such channels.
There are factors that limit the accuracy of measurements of photodiode wattmeters: the nonlinearity of the photodetector conversion function; non-uniformity of attenuation function of the attenuator in the spectral range; transmission coefficient error. The last factor is caused by the inaccuracy of the installation of the calibration resistors of the programmable amplifier. To reduce the measurement error of the average optical power, the device implements an algorithmic method for correcting measurement error. It consists in the experimental determination of the calibration characteristics of the components of the optical and electrical paths of the instrument using exemplary measuring instruments.
Characteristic values at the calibration points (corresponding to a series of wavelengths in a given spectral range) in the form of arrays of nodes of approximation are recorded in the EEPROM of the replacement units. During measurements in the automatic mode, the microcontroller performs an algorithm for panoramic viewing and searching for the elements of the arrays (approximation nodes) necessary to determine the exact value of the measured power. The values of the calibration characteristics between calibration points are calculated by the piecewise linear approximation method. Based on the obtained values of the calibration functions, correction factors are calculated to convert the measurement results to the final form.
The application of such a solution in the OM3–109 average optical power meter (Fig. 2) allowed us to reduce the measurement error to ±2% (at fixed wavelengths) and to ±3.5% in the operating spectral range of 0.8–1.7 microns. The measurement range of the average optical power is 10–12–10–1 W (provided by two replacement units: 10–9–10–1 and 10–12–10–6 W).
The availability of three external standard interfaces (RS232, USB, COP) allows for the efficient use of the measuring device in modern AMS as well as in previously developed measuring systems.
To expand the possibilities of using the wattmeter and reduce its cost, a version of the device was developed (Fig. 3) with integrated optical converters in the Propac PRO package by Schroff GmbH (Germany).
The device has the following technical characteristics: the range of measured values of the average power is 10–9–10–1 W, the working spectral range is 0.8–1.7 µm, the main relative error of measurement of the average power is ± 5%.
The devices considered above are aggregated in AMS by an instrument and module principle. According to this principle, the basis for combining software-controlled devices (modules), each of which can operate both autonomously and as part of the AMS, is the presence of a standard interface. An interface is a combination of electrical, structural, and software tools for connecting a control computer with measuring instruments necessary for monitoring and studying the parameters of a measurement object. Moreover, each of the devices included in the AMS contains an integrated interface module used for connection to a standard trunk.
The development of modern compact and mobile AMS with high volumes of transmitted data used, in particular, in the development and operation of weapons and special equipment both in our country and abroad led to the creation of a new class of measuring instruments: modular test and monitoring equipment (MTME). Similar AMS are based on the unit and modular principle of construction. The contained modules cannot operate independently, outside the system trunk, based, as a rule, on the system computer buses. The system trunk is maintained by a computer built into the modular platform (crate) with a control panel and display or by an external computer through the trunk controller. These trunk interfaces include VME / VXI, ISA / PC104, PCI / PXI, and some others.
Measuring modular platforms for measuring parameters of optic fiber nodes and, therefore, modules for measuring the average optical power are manufactured by: Ando (Japan) AQ8201–22; Agilent Technologies (USA) modular platform 8163 / 64 / 66, modules 8163x A / B and 8162x A / B; Yokogawa (Japan) modular platform AQ7280, modules of power meters AQ2780 / 81; EXFO (Canada) IOS605P modular platform, IOS1500 module, Wandel & Golterman (FRG) OMS‑150 / 200, wattmeter modules OLP‑110 / 130 / 150, etc. [1, 3–5].
The technical characteristics of the measuring modules correspond to the characteristics of the average meters power above, but, often, the modules operate in narrower spectral and dynamic ranges.
A set of modules for solving measurement problems of optic fiber equipment at a wavelength of 1.3 microns was developed at Nizhny Novgorod Research Instrument Engineering Institute "Quartz". The structure of these modules includes a wattmeter optic fiber VM1002.
The device is built on a two-channel system, which allows, in addition to measuring power, continuously and without switching, measure the attenuation of optical radiation in optic fiber nodes and elements. The first channel measures the radiation power fed to the input of the object being measured, the second channel measures the radiation power from the output of this object. The structural scheme of the wattmeter is shown in fig. 4.
The device’s conversion unit includes optical nodes similar to those used in the OM3–109 wattmeter. The attenuator consists of one discrete damper 20 dB and is manufactured by vacuum spraying a metal layer on a glass substrate; the dispersion filter is a vacuum-sprayed multilayer interference coating. The attenuator and filter surfaces free of absorbing and dispersing coatings, as well as the microlens surfaces, are cleared in the operating wavelength range to reduce Fresnel losses.
The digital processing unit for measuring information controls the optoelectronic components of the device and interacts with the VXI bus. The technologies used in the modular power meter make it easy to create AMS of any complexity, while providing a simple and user-friendly interface, a balance of electromagnetic compatibility and power supply.
The VM1002 device (Fig. 5) has a developed video panel (virtual panel) for interactive control of the device in the Windows2000 / XP / 7 environment and a driver, which is the link between the device and the user application. The virtual panel allows the users to use the driver functions without developing their own software to control the module. It is implemented in the Visual C environment as a standalone executable application.
The device has the following technical characteristics: the wavelength of the measured optical radiation power is 1.3 ± 0.05 µm, the range of measured values of the average power is 10–11–10–1 W, the main relative error of measurement of the average power is ±5%, the type of optic fiber is single-mode fiber. The developed meters of average optical power are used as part of automated operating standards (OK6–13 and RESM-QUARTZ) of units of average power and attenuation of optical radiation in OFTS.