Modulation performance

The output signal waveforms were recorded under various drive signal combinations (see

figure 7.8). With small RF signal levels the output resembles that of the injection signal. On increasing the RF component of the drive signal the output becomes more intense and temporally narrowed, eventually resulting in a pulse-like output with strong RF pumping. It can thus be observed that on a single-pass the device is capable of amplifying and gating to <200ps an initially CW input signal (see figure 7.9).

By using low DC cunent levels coupled with large RF signal powers the output waveform duration is minimised, and this feature will result in fast ultrashort pulse build-up times when used in a mode-locked laser configuration. In figure 7.10 a sequence of oscillogr ams of the device output recorded for increasing modulation frequency is reproduced. The drive signals were ananged such that the peak intensities were roughly equal in all cases. It can be seen that (apart from a somewhat noisy region on the trailing edge of the 312MHz waveform) the

s

Figure 7.9 Gain window of C.A.M. device at 624MHz.

I

U

II

I

n

Figure 7.10 Modulation responses at 312, 624, and 936MHz.

The response of the device was recorded with increasing launched power from the DFB laser, and a plot of peak output intensity of the resultant modulated waveform is shown in figure 7.11. For these measurements the modulation frequency was 624MHz and the amplifier was

biased at 150mA with an applied signal power from the signal generator of -IdBm. It can be

seen that the peak intensity saturates with increasing input optical signal power. The details of this

wavelength and the saturation output power level. Optimisation of this characteristic should lead to higher output powers from conti'olled amplifier modelocked lasers but this was not attempted

in this phase of the work. ^ 35 I 30 i

8 20 i

1 13^

0 — I— I— I— I— I— I I 1— 1— 1 I— I— I— I— I— I I I I I

0 2 4 6 8 10

Input power from DFB laser (arb. units) Figure 7.11 Peak intensity of output waveform versus input optical power. 7.3 Summary

The modulator described here is based on a semiconductor amplifier which essentially

tiansfoims a slow sinusoidal modulation signal into a fast pulsed response. This is advantageous since the electronic drive circuitry and matching networks can be simplified. Also, for modelocked lasers the system performance could be likened to that achieved with the

synchronous pumping technique^^^ and hence the deleterious features normally associated with

sinusoidal modulation can be avoided. Moreover, the low input signal power for the onset of the nonlinear response to input optical power should aid mode-locked pulse production in low average power systems such as fibre lasers. Schemes have been devised for the application of the C.A.M. technique to high power laser systems and these will be addressed in a subsequent chapter.

CHAPTER 8