Optical component design for a correlation-based optical time-domain reflectometer - in reference to the HP 8145A Optical Time-Domain Reflectometer - includes related article on signal-to-noise ratio for detection using a PIN diode - technical

Hewlett-Packard Journal, Dec, 1988 by Jurgen Beck, Siegfried Gross, Robin Giffard

Optical Component Design for a Correlation-Based Optical Time-Domain Reflectometer

THREE PRINCIPAL COMPONENTS of an optical time-domain reflectometer are the laser diode and its driver, the optical system, and the receiver, as shown in Fig. 1. The laser diode emits the probe light pulses. This light is guided and coupled into the fiber under test by the optical system. The reflected and backscattered light coming out of the fiber under test is split out of the forward ray path and guided to the receiver by the optical system. The output of the receiver, a sampled digital signal, is fed to the digital signal processor.

This article discusses the design of the laser driver, the optical system, and the receiver. The digital processor is discussed in the article on page 29.

Optical System

The key component of the optical system is the splitting device. The backscattered light can be split in various ways. Basically, three groups of splitting devices can be distinguished:

* Bulk optical systems with passive components (e.g., splitting cubes with dielectric or metal coatings, or polarization prisms)

* Bulk optical systems with active components (e.g., acousto-optic deflectors)

* Fiber-guided systems (e.g., directional couplers).

The choice of a splitting technique depends on the requirements of the instruments. For the HP 8145A OTDR, two important requirements are single-mode operation and the two-wavelength capability (1300 or 1540 nm, upgradable to both). Additionally, the measurement process requires low insertion loss and very low polarization sensitivity. The cost and availability of the components and the complexity and difficulty of the assembly process are other important factors.

The optical system of the HP 8145A OTDR is based on a fiber-guided design. Fig. 2 shows a block diagram of the optical system. Fig.2a shows the basic version, operating at a wavelength of 1300 nm. the probe light emitted by the laser diode passes through a fiber connector to a 3-dB single-mode directional coupler. This coupler, built using the fused biconical taper technique, serves as a splitter for both wavelength bands, 1280 to 1320 and 1520 to 1560 nm. Half of the probe light is coupled to the fiber under test through the connector interface at the instrument front panel. The interface accepts adapters for the various connector types. The half of the probe light not coupled to the output of the wavelength independent coupler is lost in the second coupler output port, which is terminated to avoid backreflections into the system. The backscatter signal coming out of the fiber under test passess through the coupler in the backwards direction and is again split with a coupling ratio of 50% (3 dB). One half of this signal is conducted by a pigtail to a photodiode at the receiver input. The pigtail is connected to the wavelength independent coupler by a fusion splice.

The system is converted to dual-wavelength operation by inserting an upgrade subassembly between the 1300-nm laser diode and the wavelength independent coupler, as shown in Figs. 2b and 2c. In the two-wavelength version, the lgiht waves of both laser diodes are combined into one fiber by means of a second directional coupler. In this case, the coupler has a wavelength dependent coupling ratio, which yields a high combining efficiency and low loss. This coupler is called the wavelength division multiplexer. The remaining ray path of the upgraded version is the same as in the basic version. Besides the enhanced optical system, the two-wavelength version requires an additional laser diode driver circuit for the 1540-nm laser diode. Fig. 3 shows the complete upgrade module including the optical system, the 1540-nm laser diode, and the driver circuitry.

Loss Mechanisms

The power efficiency of the optical system is characterized by its two-way insertion loss. The two-way insertion loss is the loss in the forward direction between the laser diode pigtail and the the fiber under test, plus the loss in the backward direction between the fiber under test and the pigtail of the photodiode. Fig 4 shows typical two-way insertion loss and its origins for the basic an two-wavelength versions of the optical system.

The main loss component is the coupling loss of the wavelength independent coupler, that is, the amount of light that is loss in the terminated end of the coupler. Since the coupler is used bidirectionally, a broad tolerance ([plus or minus]8%) for the coupling ration can be specified. The difference in two-way coupling loss for a 50% coupler and a 42% coupler is only 0.1 dB, because the backward coupling ratio is simply 100% minus the forward coupling ratio.

Besides coupling loss, the wavelength independent coupler also exhibits excess loss, which is the light that is lost to the surroundings within the coupling region.

For the wavelength division multiplexer, an overall insertion loss is specified. It includes coupling loss, excess loss, and variations in coupling ratio caused by polarization effects. The wavelength division multiplexer has a high coupling ratio for 1300 nm and a low coupling ratio for 1540 nm. the specified insertion loss is 0.5 dB maximum for both nominal wavelengths.