Abstract-A new method for measuring the linewidth enhance ment factor of semiconductor lasers using the injection-locking technique is presented. This idea is based on the relation between the upper and lower bounds of the locked and unlocked regimes when the detuning of the pump and slave laser is plotted as a function of the injection power. Our results are confirmed with an independent measurement using amplified spontaneous emission (ASE) spectroscopy as well as our theory, which takes account of the realistic quantum-well (QW) band structure and many-body effects. This method provides a new approach to measure the linewidth enhancement factor above laser threshold.Index Terms-Linewidth enhancement factor, optical injection, semiconductor laser.T HE LINEWIDTH enhancement factor ( ) is one of the key parameters for semiconductor lasers. The linewidth enhancement factor characterizes the linewidth broadening and chirp due to fluctuation in the carrier density, which are detri mental sources for high-speed performance. Several methods have been proposed to measure , such as interferometric measurement [1], RF-modulation measurement [2], injec tion-locking method [3], and amplified spontaneous emission (ASE) method [4]. In this letter, a new method for the determi nation of the linewidth enhancement factor of a semiconductor Fabry-Perot (FP) laser is performed by measuring the injection locking range of the laser. In the past, it was proposed that can be obtained by measuring the optical power change of an injection-locked laser [3]. In this method, the injection level should be low to satisfy the assumption for the calculation. At the same time, it is difficult to injection-lock the laser and measure the power variation at low injection levels accurately. Our method does not require any fitting parameters, which would bring uncertainty for different laser systems. Meanwhile, our results are in excellent agreement with our independent measurement based on ASE spectroscopy. Both experimental results are confirmed with our theory, which takes account of realistic band structure with the valence band-mixing effect and many-body effects.In an optical injection-locked laser system, which consists of a master laser (stable laser oscillation) and a test laser, the locking properties of the test laser depend on its linewidth en hancement factor . The locking range is asymmetric when , and the value of the linewidth enhancement factor can be evaluated from this asymmetry. Fig. 1 shows the experimental setup. The test laser is a buried heterostructure FP laser with five compressively strained quantum wells (QWs) made of materials with a lasing wavelength at 1550 nm [5]. The test laser has a threshold current of 11.7 mA, and is operated at a bias current of 25 mA and a heat sink temperature of 25 C. The master lasers are selected from a group of single wavelength DFB lasers with sidemode suppression ratios greater than 40 dB and lasing wavelengths ranging from 1520 nm to 1580 nm. The master laser is mounted on a heat sink...