Output of preamplifier recorded by a storage oscilloscope. The preamplifier and the oscilloscope were connected by a RG-58 cable (20 ns delay). The small signal response could not be distinguished above the pulsed EMI.
Transmitter circuit diagram. In Sec. III D, an external signal supply was applied at V1 replacing the photovoltaic detector and preamplifier to test the frequency response and gain of the fiber-optic link.
Receiver circuit diagram.
Output voltage of the receiver plotted against the input current of the laser diode. The data was measured shortly after turning the laser on (red squares) and after having waited several minutes while the laser is on (blue triangles). The solid and dashed lines refer to linear fits in the interval 5 mA <I d < 50 mA.
The frequency response of the system was measured for different input amplitudes (peak-to-peak). The 3 dB point is approximately 35 MHz, above which response rolls off sharply.
The relation between input signal of transmitter circuit and output of receiver circuit is linear at 10 MHz. It shows an effective gain of -0.236 ± 0.002.
Layout of measurements: Case 1, steady-state; Case 2, small-signal pulsed gain test; and Case 3, high-power pulsed oscillator test.
Case 1: Steady-state laser power measurement. We used the new measurement system to measure the power level of a low-power cw laser and the result was compared with that of an absolutely calibrated power meter. The applied power range was limited by the saturation of the preamplifier in the fiber-optic measurement system.
Case 2: Time-dependent measurement of TEA amplified laser output using the fiber-optic system. The pre-amplification power level is about 0.22 W and the peak power is about 0.72 W yielding a single-pass gain of 3.3.
Case 3: Fast laser pulse measurement using the fiber-optic system. The FWHM width of the laser pulse was about 50 ns, which was as expected. The low-intensity tail was due to the momentum transfer from N2 to CO2 molecules.
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