State-of-thgart Fiber Optics for Reference Frequency Distribution over Short Distances*

We have characteri~ed a number of recently developed fiber optic components that hold the promise of unprecedented stability for pas~ively etabilired fkequency distribution l i s . These components include a fiber optic transmitter, an optical isolator and a new type of flber optic cable. A novel laser transmitter exhibits wtremely low sendtivity to intensity and polarixation changes of rdected light due to cable flexure. This virtually eliminates one of the ehortcom$gs in previous laeer transmitters. A high isolation, low lose optical isolator has been developed whieh also virtually elminatee laser sensitivity to changes in intensity and polarisation of reflected light. A newly developed fiber ha6 been teeted. This 5ber ha^ a thermal c o ~ c i e n t of less than 0.5 ppml°C, nearly 20 times lower than the beet coaxial bardlime cable and 10 times lower than any previous flber optic cable. The uee of this fiber in fkequency distribution systems will greatly enhance the stability. Theee and other components are highly suitable for,distribution syetems with short extent, such as within a building. A test was performed to demonstrate the insensitivity to cable flexure when theee new components are used in a fiber optic link. The etandard loose tube 5ber optic cable was installed between the control room and the cone of an antenna in a NASA/SPL Deep Space Station, The round trip cable length was 850 meters. Delay variations in the fiber optic cable were measured and compared to delay variations in an aqjacent coaxial cable. Phase delay was monitored while antenna movement flexed the cables. The measured stability of the fiber optic cable was found to be subetantially superior to that of the coaxial cable. In this paper, we will present reeults of our testa and provide the design for a stable distribution link, together with the projection of the stabilities achievable with the present statmf-theart. Distribution systems degrade the phase and frequency stability of transmitted frequency reference signals[l]. A reduction in Signal-to-Noise Ratio (SNR) of a transmitted signal and delay changes in the transmission path are the primary causes of degradation. These effects are caused by distribution system noise which reduces the SNR, and variations in the environmental temperature, which cause delay changes. The degree of delay change is dependent on the Thermal Coefficient of Delay (TCD) of the distribution system componenta, Fiber optic systems have several major advantages over conventional distribution systems, which usually employ coaxial cables. Having all dielectric cable, fiber optic systems are not subject to ground loops and are generally immune to pick-up of Electromagnetic Interference (EMI) and Radio 'This work represents the results of one phaae of reaearch carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautia and Space Administration. Frequency Interference (RFI). The very low loss at high frequencies in a fiber optic system helps preserve the SNR of a transmitted signal. Also, new fiber optic cables with very low TCD, recently developed and available commercially, can greatly reduce temperature induced delay changes. A major disadvantage of fiber optic systems in frequency distribution applications has been their sensitivity to changing reflections back into the laser diode[2]. Changes in reflection are caused by cable flexure and vibration and result in delay changes across the laser diode. Recent developments, which will be described in this paper, have virtually eliminated this problem. Because of the potential improvements in performance, the Jet Propulsion Laboratory (JPL) is developing stable short distance fiber optic links to diatribute local frequency references within the Deep Space Networks (DSN's) Deep Space Stations (DSSYs). These links will distribute the frequency reference signals from the frequency and timing interface in each station to the users within the station. The requirements for the short links axe quite different from the requirements for the long links used for frequency reference distribution between stations a t the Goldstone, California Deep Space Communications Complex (DSCC). Because of cost-performance tradeoffs between short fiber optic links and coaxial cable links, the short fiber optic links must be relatively inexpensive, simple and reliable. The long distance fiber optic links are more expensive and more complex with optical and electronic feedback to stabilize the delay of a transmitted signal. The long links enable the use of a centralized frequency and timing facility thus reducing the number of expensive frequency standards needed in a DSCC. Therefore, a higher cost for these links can be justified. The performance of short fiber optic links is expected to be considerably better than the performance of an equivalent coaxial cable link. The fiber optic link will eliminate ground loops and provide considerable improvement in the thermal stability of the cable. The optical fiber's superior thermal stability will reduce the need to add mass and insulation to the cable to increase its time constant. It will also reduce the temperature stability requirement for the air conditioning systems in certain areas of the stations. The cables used in short fiber optic links within a station may be exposed to temperature variations that can exceed 6 "C in 20 minutes and 30 OC in 12 hours over some portion of their length. They obviously cannot be buried like long links at Goldstone to benefit from temperature isolation provided by burial. These links may also be subjected to vibration from equipment such as air conditioners. For some applications the cables will be routed through the antenna wrap-up where they will be flexed when the antenna is moved. This relatively dynamic environment requires that the links be insensitive to cable vibration and flexure and that cables with low TCD be utilized. In the remainder of this paper new technology that can be used to meet the special requirements of the short distance fiber optic frequency reference distribution links will be discussed. Test results on an experiment that demonstrates an optically isolated laser's insensitivity to cable flexure and vibration will also be presented. Finally, a state-of-the-art fiber optic frequency distribution link for short distance applications will be described. Reducing Instabilities Caused by Reflections Cable flexure can cause group delay changes as large as 200 ps across a fiber optic link if no means is used to desensitize the laser diode to reflections. Optical isolation of the semiconductor laser diode can reduce such changes to less than 0.03 ps. The optical isolation can be obtained by the use of bulk optical isolators using the Faraday principle. Optical isolators of this type consist of a polarizer to fix the polarization of the laser light, followed by a Faraday rotator which rotates the polarization vector by 45'. The light at the output of the rotator enters an output polarizer with its axis 4S0 rotated with respect to the polarization axis of the first polarizer. Therefore, the light passes through the output polarizer unimpeded. Because the Faraday principle is nonreciprocal in the forward and reverse directions, reflected light back into the isolator assembly experiences a rotation angle which is crossed with the axis of the input polarizer. The reflected light is therefore blocked providing the reverse isolation. The degree of isolation achieved by this type of isolator strongly depends on the amount of light scattering within the isolator. Once polarized reflected light scatters within the isolator, the polarization is lost, and components that do not have their axis crcwed with the exit polarizer pasa through and degrade the isolation. Optical isolators of this type are manufactured by several companiee. The isolation afforded is typically 35 to 40 dB and the forward loss is typically less than 2 dB. Although this level of isolation is very good it is not adequate for precise fiber optic frequency distribution. In order to improve laser isolation, one company in Japan has developed a laser diode with an integral dual (two isolators in series) optical isolator[3]. This approach provides high isolation at the expense of an additional isolator and additional forward loss. An optical isolator system developed at JPL to be used in frequency distribution links has up to 70 dB isolation and 1.3 dB forward loss[4]. The JPL isolator system was assembled from a commercial bulk isolator, as described above, and expanded beam single-mode fiber connectors (Fig. 1). The first expanded beam connector expands and collimates the optical beam emitted by the fiber. The highly collimated beam passes through the isolator elements and is collected by the second connector. The total loss is only 1.3 dB in the forward direction. The improvement in isolation for the JPL aystem is due to the narrow acceptance angle of the expanded beam connectors. The collimated reflected light with the appropriate polarization is rejected by the exit polarizer. While the narrow acceptance angle of the input connector rejects the scattered reflected light exiting the isolator because it is not parallel to the axis of the isolator. Low Thermal Coefficient of Delay Optical Fiber Sumitomo Electric Industries, Ltd. of Japan has developed a low Thermal Coefficient of Delay (TCD) single-mode optical fiber[S]. This is an elegant means for reducing frequency instabilities in a reference frequency distribution system. It affords considerable improvement in transmision stability without adding to the complexity or reducing the reliability of the transmission system. The TCD of this fiber has been measured at JPL and found to be less than 0.5 parts per million per "C (ppml0C) from 0 "C to 30 OC. At around 0 OC the TCD is zero. It rises slowly as the temperature rises and is 0.5 ppm at about 30 "C. The curve in Fig. 2 shows the TCD for this fiber in ppm/"C versus temperature. Figure 3 compares the TCD of this fiber with the TCD of standard single-mode fiber and 718 inch diameter coaxial hardline # 64-875 RG254/U. This coaxial hardline has the lowest average TCD for any coaxial cable measured by the Time and Frequency Systems Group at JPL. It can be seen that the TCD of the fiber at 25 "C is 20 times lower than that of the coaxial cable. Use of the low TCD optical fiber would result in an Allan deviation 20 times lower than a system using the RG254/U coaxial cable. The TCD of a standard optical fiber results from two effects, the temperature dependence of the index of refraction of the fiber material, and the thermal coefficient of expansion of the fiber. An increase in temperature causes the index of refraction to decrease which in turn decreases the group delay through the fiber. An increase in temperature also causes expansion of the fiber which results in an increase in the group delay through the fiber. These two effects partially cancel resulting in a TCD for standard singlemode fiber of about +7pprn/OC[6]. Sumitorno achieves a low TCD fiber by coating a standard fiber with an inner layer of elastic material and an outer layer of liquid crystal material having a negative thermal coefficient of expansion. This liquid crystal material compresses the fiber longitudinally with rising temperature. The compression of the fiber increases the index of refraction of the fiber material which increases the group delay through the fiber. Compression of the fiber also decreasea the change in length of the fiber which decreases the group delay through the fiber. The result of these two effects is to impart a negative TCD to the fiber. The thermal coefficient of expansion of the liquid crystal material is too high and would result in a net negative TCD for the fiber if it were applied directly to it. The layer of elastic material between the fiber and the liquid crystal coating couples the right amount of force from the liquid crystal material to the fiber to result in a near zero TCD for the fiber.