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Broad-band absorption bleaching in erbium-doped fiber by in-band narrow-linewidth pumping is reported. For the first time, spectra of absorption bleaching of erbium-doped fiber is measured and the refractive index changes inferred through the Kramers–Kronig relations in this true two-level system in order to understand its operation in the external cavity of a laser.

The increasing demand for broadband mobile communications has generated interest in exploring new frequency bands and modifying network structures. In such systems, photonic technologies can bring both cost reduction as well as an increase in performance, mainly due to the low-loss properties of optical fibers. An optical source capable of producing tunable, high-quality microwave/mm-wave signals would be of great interest not only in such communications systems, but in fiber sensors and numerous other applications as well. One potentially cost-effective method to fabricate such a system is via optical heterodyning. In this approach, the difficulties in generating a high-quality signal are two-fold. The first issue is in maintaining a specific frequency difference (i.e. microwave signal) between the lasers for an extended period of time. The second is in narrowing the inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to values practical for a communications system. Both of the above requirements are facilitated by the newly developed doped-fiber, external cavity laser (DFECL), which offers relatively stable single-longitudinal-mode operation in addition to narrow linewidth operation. This paper will demonstrate frequency locking of a DFECL using a delay-line discriminator. The RF linewidth, initially 10-15MHz, is reduced to levels conducive to optical PLL locking. Optical power levels are approximately -3 dBm and unamplified microwave power output levels are typically -35 dBm, depending on photodetector responsivity. Carrier-tonoise ratios are generally 40-45 dB. The physical mechanisms underlying the observed laser dynamics are discussed, including laser-to-fiber alignment and thermal fluctuations.

A doped fiber external cavity laser (DFECL) exhibits high power, narrow linewidth and stable wavelength which may be tuned. We demonstrate IEEE 802.11 signal transmission by direct modulating the ultra long DFECL at ~2.5GHz.

The doped fiber external cavity semiconductor laser (DFECL) has been reported with a simple structure, high power, narrow linewidth, and stable wavelength. The DFECL is mostly suitable to be an optical carrier generator for external modulation or microwave optical generation. Because of mode locking, the DFECL, with saturable absorber in its external cavity, has the possibility to be direct modulated at its multiples of cavity resonant frequency. The useful modulation frequency of the laser can be increased significantly. In this paper, we present experimental results about the transmission response of direct modulation of an ultra-long DFECL, and the modulated microwave signal transmission at the frequency of the 22nd. multiple of the cavity resonant frequency. Modulated narrow bandwidth microwave signals at 2.4GHz were transmitted by this DFECL. The received RF spectrum has no obvious distortion for a 10MHz narrow band microwave signal and, all the resonant and harmonic frequencies in the 0~2.5GHz region are 50 dB lower than the transmitted wave. The results show that narrowband modulated microwave can be transmitted at high frequency by the long DFECL; even through the cavity round-trip frequency is very low. We conclude that this ultra-long doped fiber external cavity semiconductor laser can be used for narrowband wireless communication with direct modulation.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on the first experiments of optical microwave generation by heterodyne beating of two doped-fiber semiconductor external-cavity lasers (DFECL). Our measurements show that minor side-mode beating in DFECL affect the optical heterodyne beat signal. Using the RF methods we study the side-mode suppression in the DFECL.

We present an experimental study of tuning characteristics and wavelength stability of a 1490 nm doped fiber external cavity laser. The standing wave in the erbium-doped fiber in an external-cavity laser causes spatial hole burning and absorption modulation and forms a dynamic grating. The dynamic grating can effectively suppress side modes and eliminate wavelength fluctuations in the laser. The operating wavelength of this hybrid laser is shown for the first time to our knowledge to be smoothly tunable by changing the semiconductor laser drive current or temperature.

We report on the spectral analysis of the first 1490 nm external cavity laser with an intra-cavity erbium-doped fiber. Self-mode beating and heterodyne beating methods are employed to measure the side mode suppression ratios and to study the effect of the rare-earth doped fiber on the spectrum of the external-cavity laser. Experimental and simulated results show a substantial decrease in the number of the external-cavity modes and a side-mode suppression ratio of >15 dB.