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A Fiber-optic-based Programmable Delay Line
Princeton Electronic Systems Inc. (PES)
An important application of optoelectronic devices in microwave signal processing is optical delay lines. Optoelectronic devices with wide bandwidth, high dynamic range and small size play a crucial role in the performance improvement of signal processing systems. Extremely flat response of the delay elements at microwave frequencies is achieved because the optical fiber loss is independent of RF. This performance advantage has resulted in the design and manufacture of a line of fiber-optic-based programmable delay lines. The model PESDL4045 programmable delay line provides six bits of true time delay from 0 to 12.6 ms in 200 ns steps over the frequency range of 4 to 4.5 GHz. A block diagram of the unit is shown in Figure 1 . The basic and critical components of the optical delay line are low noise and high dynamic range optical transmitters, delay elements, a front-end optical receiver and low loss, low crosstalk optical switches.
A 4 to 4.5 GHz optical transmitter module has been designed and fabricated using a distributed feedback laser module operating at 1300 nm with a 3 dB bandwidth of 7.5 GHz. The high performance optical transmitter module was realized using a reactive matching technique to efficiently transfer the microwave energy into the laser diode. The use of a reactive matching network reduces the overall noise of the system. The optical transmitter matching circuit was realized on a 0.032"-thick substrate with a relative dielectric constant of 3.38.
The 4 to 4.5 GHz optical receiver module was designed and fabricated using a Fermionics Opto-Technology type FD50S7-F InGaAs PIN photodiode as the photodetector. The high performance optical receiver modules were realized using an active matching technique to efficiently transfer the microwave energy out of the photodiode to the module output. For the low noise optical receiver, a Fujitsu type FHx35LG low noise pseudomorphic heterojunction field effect transistor was used as a gain element. The optical receiver circuit was designed and fabricated using a 0.032"-thick substrate with a relative dielectric constant of 3.38.
Single-mode fiber switches are used to switch the modulated optical signal. The 2 × 2 switch is capable of bypassing the delay elements in the cascaded delay line. The switches have 900 mm buffered fiber with an operating optical wavelength of 1250 to 1600 nm. The insertion loss of the switches in the on position and in the cross connection corresponding to no delay range from 0.43 to 0.83 dB and 0.5 to 0.77 dB, respectively. Crosstalk is less than -60 dB and optical return loss is better than -52 dB.
The six-bit delay system operating over the frequency range of 4 to 4.5 GHz utilizes si× binary optical delay elements from 200 to 6400 ns corresponding to a fiber length of 40 to 1280 m. The six optical fibers are housed in a single box, which measures 7.00" × 10.25" × 4.00", and the fibers are terminated with FC/UPC connectors. Optical insertion loss of fiber elements is less than 1 dB.
A minicomputer controls the operation of the optical delay line. The transmitter and receiver are switched on and off via a small keypad, which can also be used to select different time delays. The computer activates the switches corresponding to the required length of the optical delay line and sets up proper gain in the optical receiver for the uniform frequency response at different delays. The miniature display in front of the unit indicates the state of the transmitter/receiver modules as well as the selected delay time.
A prototype of the programmable optical delay line was assembled in a 16.5" × 10.0" × 5.5" enclosure. The unit includes the 4 to 4.5 GHz optical transmitter module, six single-mode 2 × 2 optical switches, six delay elements, the 4 to 4.5 GHz optical receiver module, a minicomputer controller and related fiber-optic connectors, and electronic circuitry.
The performance of the fiber-optic-based delay line was measured using a network analyzer and spectrum analyzer. A plot of the insertion loss of the programmable delay line over the frequency range of 0.5 to 5.5 GHz is shown in Figure 2 . The data indicate the fiber-optic-based delay line has an insertion loss of 9.5 dB with flatness of better than +/-0.3 dB over the bandwidth of 4 to 4.5 GHz.
Specifications of the model PESDL4045 programmable delay line include delay settings of 0 to 12,600 ns in 200 ns steps. Residual delay to be added to all settings is approximately 70 ns. The unit's bandwidth is 4 to 4.5 GHz, and gain is -9.5 dB +/-0.5 dB (nom) at all delay settings (+/-0.3 dB (nom) and +/-0.5 dB (worst case) over the 4 to 4.5 GHz band). The delay line's noise floor is -119 dBm/Hz at 12,600 ns and -120 dBm/Hz at other settings, and the group delay distortion over the band is 100 ps at all settings. Input impedance is 50 W, input return loss is -15 dB and output return loss is -13 dB. Applications for the programmable delay line include radar testing, RF memory in electronic warfare systems, multipath fading emulation, temporary RF memory and microwave signal processing.
Princeton Electronic Systems Inc. (PES),