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as lif carrier noise [2] and injection-recombina. tion shot noise (31, generated in the tuning ECCtions which are biased below threshold. The efficiency of the PM-to-IM conversion depends on frequency [I], and it is quite important to identify how each frequency noise degrades the transmission characteristics of lasers. In this paper, the phase noise of widely Nnable Sampled Grating Distributed Bragg Reflector (SG-DBR) lasers [4] is measured. Then, we investigate which frequency component more affects the transmission perfarmance by measuring RIN after transmission through fiber and identifying additional intensity noise caused by each frequency component of the phase noise. We also find out the way to define a laser linewidth, which most accurately represents the transmission eharacterintics of lasers. ExperimentsPhase noise measurement is performed using an optical frequency discriminator [SI which is incorporated in the ADVANTEST chirp measurement system (Q7606B). Since optical frequency fluctuations resulting from laser phase noise are in the range of a few MHz or less, the FSR of the frequency discriminator has been modified to 15GHz to realize enough high resolution. me optical output from the frequency discriminator is analyzed with a Lightwave Signal Analyzer (LSA, Agilent 71400C). Two types of widely-tunable transminers based on a SG-DBR laser monolithically integrated with a semiconductor optical amplifier areiested. One is a laser diode module (LDM), which consists of a standard 24-pin bmtemv oackaee. The other is a tunable laser assembly '(?LA)-packaged in a small-form factor containing the LDM plus current and temperature cantrol circuits.Rw is measured after transmission through a standard single-mode fiber (SMF-28) for these transmitters. The fiber length is 50 km so that the optical power received by the LSA is large enough to detect the RIN after fiber without additional amplification from an EDFA, preventing the noise-floor increase caused by the amplified spontaneous emission of EDFA. The TLA has a capability of dithering its optical frequency in order to suppress Stindated Brillouin Scaniring (SBS), so that the TLA can launch high power (> 10 dBm) into fiber without causina OXECSS noise due to SBS. Figure I shows the phase noise spectrum of the LDM. The nhase noise exhibits white freauencv w6ite frequency noise similar to the LDM, but more non-white components are observed below 500 MHz in the TLA, and they are attributed to the digital control circuits in the TLA. Figure 2 shows the RIN after 50 km fiber of the TLA for various fiber launch power. The peaks and dips observed in the RIN spectrum result from the PM-to-IM conversion. As can be seen in Figure 3, the TLA also shows a large RIN after fiber at low frequency, which is equal to or larger than the first peak at high frequency. The tone observed at I1 GHz results from SBS, and its mapirude increases with fiber launch power, The RIN after fiber at low frequency is larger for higher launch power as well, indicating that it might be also c...