Phase noise in an actively modelocked semiconductor laser system

The pulse sequence from an actively modelocked InGaAsP semiconductor laser was

focussed onto an InGaAs PIN photodiode with a 3dB bandwidth in excess of 22GHz. The output signal was recorded using either a Hewlett-Packard HP8562B spectrum analyser with a frequency range of 0.1-22GHz and a minimum resolution bandwidth of lOOHz, or a higher resolution (lOHz) type HP70900 analyser with a frequency range of 2.7GHz. The phase noise content was initially studied when the RF drive source to the laser was a tunable Marconi 2019

frequency synthesiser. The data represented in figure 5.1 show the close-in sidebands attending

the 1®^ 20^, and 50^ harmonics of the pulse train Fourier comb.

Indeed, the noise sidebands are seen to increase with the harmonic number n. This is indicative of a substantial amount of timing jitter in the optical pulse train. Also shown in figure 5.1 is the spectrum of the RF drive signal. It can be seen that the noise of the drive signal has

been transferred directly onto the laser pulse train and no other noise components are evident in

this case. The data obtained from these spectrum analyser records were plotted in figure 5.2 in

terms of single-sideband noise power (relative to the power of the cairier). The data thus obtained violate the small deviation regime, on which the theoretical development was based, at frequencies below IkHz and so the extraction of quantitative information from this noise

distiibution plot is severely limited. However, from knowledge gained using the streak camera the relative timing jitter in the range 30Hz to IMHz was estimated to be ~2-3ps.

Marconi Drive

1st Harmonic

SPAN lO- OOKHs: SWP 3 _ Osac

CENTER B. 02S3 5AOOGHa: SPAN 1 O, QDKHa; ► RSW lOOHss VBW 1 OOHz SWP 3.

50th Harmonic

CENTER 15. 0743BS05GHZ SPAN 1O.OOHHz ►RBW lOOHz VBW 1 oaHac SWP 3. Osao

Figure 5.1 Close-in noise sidebands of (a) (b) 20* and (c) 50* harmonics of the Fourier comb derived from the modelocked semiconductor laser, where the RF modulation source was a Marconi 2019 frequency synthesiser.

■ fundamental ■ 9 th Harmonic " 50th Harmonic i -30 4 -40“ S -50“ -60“ -70- -80" -90" -100“ -1 1 0“ -120“ frequency offset (Hz)

Figure 5.2 Single sideband noise content of a modelocked InGaAsP semiconductor laser with Marconi 2019 synthesised RF drive source. [The solid line indicates the small signal

deviation criterion]

5.4 Optimisation of phase noise performance of the semiconductor laser

system

The relatively poor noise performance of the laser pulse sequence detailed above was seen

to be due mainly to the quality of the current modulation signal applied to the laser. It is perhaps worth explaining here that the Marconi synthesiser was not of especially poor quality and it is

merely the fact that synthesisers in general tend to have inferior noise performance compared with fixed frequency crystal oscillators. This arises essentially because an enor signal is required to

sustain tlie phase-locking between the free-running, tunable, voltage-controlled oscillator and the

internal crystal oscillator derived frequency reference. Indeed, of all the synthesisers tested the Marconi was found to have the lowest phase-noise content.

In an attempt to improve the laser noise performance an inhouse-designed and constructed fixed frequency crystal oscillator was employed. The crystal oscillator used had a frequency of 150MHz and its output power was split into two components. One was used to drive the streak camera diagnostic and the other was frequency doubled and amplified before being applied to the

semiconductor laser. The corresponding noise sidebands measured for this laser configuration are

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1=

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4

%

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CENTER I B . OOOOS2SGH* SPAN 5 0 0 . O R H *