Cavity formation in semiconductor lasers

A semiconductor laser is often considered as an optical gain medium inside a Fabry-Perot resonant cavity. Lasing emission into cavity modes requires that photons experience at least one round trip within the resonator. A natural question concerns the effect multiple round trips in the resonator (i.e., cavity formation) has on temporal evolution of lasing light intensity and spectra. Under normal conditions the influence of cavity formation is obscured by (nonlinear) coupling of optical field with gain. In addition, such effects are usually difficult to measure in semiconductor laser diodes due to brevity of cavity round-trip time and charge carrier lifetime. In this letter we describe a fiber external cavity semiconductor laser system which effectively decouples cavity formation from charge carrier dynamics, allowing us to time resolve the intensity and spectral development of lasing light emission. A schematic diagram of our laser system is shown in Fig. 1 (a). An InGaAsP/InP strained layer multiple ( 10) quantum well laser (Ref. 1) has one antireflection (AR) coated (R < 0.1% > facet which is coupled to a fiber external cavity. The external cavity consists of an approximately 100 m length of single mode fiber, one end of which is lensed and antireflection coated,. the other end is cleaved and has a highly reflective gold coating. The cavity roundtrip time is accurately determined to be T,,,=O.9951 f 0.0001 pus by measuring the laser mode locking resonance. The as-cleaved solitary laser diode had a threshold current of 10.5 mA prior to AR coating. In Fig. l(b) we show the static (dc) light-current (L vs j) curve and optical spectra of the device when the diode’s AR coated facet is coupled to the external cavity and when the fiber is removed. In the absence of optical feedback the solitary device does not lase as evidenced by the broad emission spectrum and lack of a sharp transition in the light-current characteristic. However, in the presence of the external cavity, the dc emission versus current is characteristic of lasing action. The dc laser threshold current isjthE 11 mA indicating that the fiber external cavity is strongly coupled to the diode active region with an effective reflectivity comparable to a cleaved facet. Furthermore, emission above threshold is concentrated in a narrow spectral region around wavelength n = 1.3 pm. Introducing large bending losses in the fiber cavity results in the emission level returning to that of the isolated diode while the emission spectrum becomes broad band (similar to the case when the fiber is removed). We note, despite the high quality of the AR coating on the diode facet, above-threshold spectra are modulated by the residual diode subcavity. It is nevertheless apparent from the light-current curves that, even with a narrow external cavity mode spacing of approximately 1 MHz, the external cavity couples efficiently to the diode gain region and predominantly determines emission. The large value of TV,, facilitates study of lasing action with increasing number of cavity round trips n. In Fig. 2 (a) we show normalized pulsed light-current (L/r vs j)