SATISFACCIÓN LABORAL

By considering the streak camera intensity record of figure 4.1 subpulse detail is clear ly in evidence. These features were fir st observed by Chen during an extensive study of mode-locked, Brewster-angled GaAlAs laser systems in 1985^^. The probable origin for the trailing subpulse

features is suggested using figure 4.2. It is assumed that a short optical pulse is incident on the gain medium at a time t^. At this point the carrier density (or gain) is substantial although it has not reached its peak value. Stimulated emission leads to a reduction of the mverted population of carriers, and this gain saturation performs the primary pulse shaping function. The pulse then exits the gain medium leaving a markedly reduced carrier density behind. Because the modulation applied to the semiconductor gain medium is in the form of a slow sinusoid, the carrier density can recover after the passage of the first pulse to a value capable of supporting a secondary pulse.

The temporal extent of the overall pulse eirvelope (including the subpulse) therefore indicates the gain window produced by the applied modulation signal.

66ps

Direction of propagation

Figure 4.1 Stieak intensity record of the output from a modelocked symmetrical extemal- cavity semiconductor laser. fi^=600MHz. The dotted line indicates the mean noise level.

I

Gain _{Modulation signal}

I_th

t_{o Time}

Figure 4.2 Origin of subpulsing in modelocked semiconductor lasers. [The shaded region ndicates the output pulse intensity spectrum.]

This indicates that there is sufficient gain recovery following the gain saturation due to the main pulse to support a secondary pulse although it does not imply how this pulse is formed. It could be due to the emission of spontaneous noise (and its subsequent amplification) during this

time window which becomes the source of the subpulse. Alternatively, and more probably the

initial long burst of noise created when the laser is turned on actually determines the detail of the modelocked pulse profile (see section 3.4). The leading part of the noise burst (or gain switched pulse) is amplified and shaped by gain saturation to form the main ultrashort pulse created by the laser. The trailing part is sustained due to the presence of residual gain but is not significantly

shaped because both the light intensity and the available gain are low.

Another source (which will be described further in section 4.4) has been identified where a substantial subpulse is created on a single-pass through the semiconductor gain medium. This latter phenomenon can only occur however under the particular conditions of very high pumping. 4.3 Gain window reduction

It follows, therefore, that it is necessary to reduce the temporal extent of the gain window to eliminate the deleterious effects of gain recovery during the pump cycle. There are several methods in which this can be affected and these will be described in the following three

subsections.

4.3.1 Short-pulse electrical drive

The most straightforward technique to provide a reduced gain window is to use a short- pulse electrical drive for the laserl^l. This can be achieved quite easily by using a commercially available step recovery diode (also known as a comb generator) which can produce short electrical pulses of ~100ps duration. These devices are unfortunately limited in operating

frequency range and require broadband power matching into the diode package to avoid degradation of the short pulse characteristics of the electrical drive signal. The diode packages

used in this work did not feature broadband power matching and so effort was made to achieve

features equivalent to those of comb generators using single frequency RF signals. 4.3.2 High-frequency modulation

An alternative approach using sinusoidal modulation to reduce the gain window is based along similar lines to that of the comb generator, and this is to increase the modulation frequency to such an extent that the gain window can be reduced to any value desired, eg a lOGHz RF

signal is equivalent to a 50ps pulsed signal. Figure 4.3 shows three sinusoidal modulation signals

gain window reduction by increasing the modulation frequency in such a manner can be clearly

seen. Using an InGaAs PIN photodiode as the detector, sampling oscilloscope traces of pulse profiles from the modelocked InGaAsP semiconductor laser with modulation frequencies of 300, 600 and 900MHz as shown in figure 4.4 were recorded for comparison purposes.

I.OW

300MHz

900MHz

600MHz

l.too ].I00 I.lOO l.JOO 1.400 .«00 1.700 .100 Figure 4.3 Gain window reduction by increasing the modulation frequency

fm = 300MHz fm = 600MHz fm = 900MHz

Figure 4.4 Subpulse suppression by increased modulation frequency. The time base is 500ps in all cases.

As expected, for increased modulation frequency the proportion of total pulse energy in

the main pulse becomes greater and the duration of the subpulse is reduced in accordance with the gain window constriction. The drawback with increasing the modulation frequency to affect

amplifier and the associated electronics become both bulky and expensive. Also, and more

importantly, the peak pulse power is significantly reduced as the modulation frequency is increased. The peak power can be found from the expression; -

Peak power = Duty factor x Average power

~ T j/T p X P^Y

= Pay/

where the modulation frequency fm=l/Tp, and Tp is the pulse duration.

The pulse durations from semiconductor lasers have been shown to be a weak function of frequency, tending to become constant at sufficiently high frequencies!^^. This is because the modulation depth decreases with frequency due to the bandwidth limitations imposed by the laser and its package. Moreover, the average output power is essentially constant as a function of frequency. This then leads to an approximately linear decrease in peak pulse power with

increasing modulation frequency;

Ph ~ Pi/H

where Pjj is the peak power at the H^h harmonic of the fundamental frequency f^.

It may therefore be concluded that for optimum peak power pulse generation the modulation frequency must not be be excessively high, but the chosen frequency must be

sufficiently high to limit the effect of subpulse generation. To this end it is attractive to develop a technique for eliminating subpulsing at relatively low modulation frequencies (eg 300-600MHz). 4.3.3 Zero DC bias operation

In this section a technique is described where the mode-locked pulse temporal envelope could be cleaned up to the extent tliat it consists of a single picosecond feature while employing

relatively low-frequency modulation (see also section 3.6.4). This technique has been termed here as zero DC bias level operation or ZBO.

Consider the modulation signal of figure 4.5(a) which is comprised of a sinusoidal RF frequency modulation superposed on a subthreshold DC bias current level as is normally used for the mode-locking of semiconductor lasers. The gain window produced by this combination of bias signals as indicated in the figure can be seen to be quite long compared to the pulse durations expected (<10ps). It is therefore not surprising that gain recovery is significant and a large

subpulse is generated following the desired ultrashort pulse. Figure 4.5(b) and (c) shows how the gain window behaves on reducing the DC bias current level used and at the same time

increasing the RF signal intensity to give the same peak drive signal. Therefore the shortest possible gain window at any modulation frequency occurs when the DC current level is at a minimum. In practice tlie minimum DC bias level possible for the semiconductor laser is not in

fact zero because no current is drawn on the negative part of the drive signal (the diode rectifies

the signal). Hence the DC Fourier component of the drive signal is always positive. A DC cunent level which maintained a positive resultant modulation signal has always been traditionally applied because it was thought that biasing the diode amplifier negative would lead to damage or at least lifetime degradation. This has not been observed in this work where the InGaAsP semiconductor amplifiers have been operated under zero DC bias and large modulation signals for many hours with no failures and no observable decrease in perfoimance.

I(th)

t Figure 4.5 Subpulse suppression by reducing the DC component of the laser drive signal.

This technique was applied to the balanced, grating-tuned cavity configuration illustrated

in figure 4.6(a). The pulse profiles recorded using a synchroscan streak camera anangement are reproduced in figures 4.6(b) and (c) where figure 4.6(b) indicates the temporal calibration of the recorded data (obtained over an integration time of 3.3s). The single pulse profiles of figure 4.6(c) show that clean, single-feature pulses can readily be obtained using the ZBO technique. Although it is noted that the traihng edge of the pulse has a slower gradient than the leading edge.

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