1. Field of The Invention
The invention relates generally to semiconductor lasers. In particular, the invention relates to a laser having a resonant cavity including both a semiconductor amplifier and an optical fiber upon which is written a Bragg grating providing the frequency and mode discrimination of the laser.
2. Technical Background
Optical amplifiers, in particular doped fiber amplifiers, have become increasingly incorporated into the backbone of the telecommunications network when it is necessary to amplify optical signals without the need to first reconvert them to electrical form. Instead, the optical signal itself is amplified regardless of its coding format. As illustrated schematically in FIG. 1, a digital optical signal being transmitted across the telecommunications network is received on a single-mode optical fiber 10 and is combined in an optical combiner 12 with an optical pump signal from an optical pump source 14. The combined signals traverse an optical amplifier 16, preferably constituted of an erbium-doped fiber amplifier (EDFA), that is, a silica fiber doped with optically active erbium ions Er3+. In an EDFA, an optical pump signal excites the erbium ions, which can then amplify another optical signal at a lower frequency than the pump frequency. In a telecommunications system, the pump signal amplifies the input data signal, and the EDFA outputs the amplified data signal to a single-mode output fiber 18. Other configurations are possible, for example, the data and pump signal may counter-propagate, and further components such as isolators and stabilization circuits may be included. These variations are not important to the invention and will not be further discussed.
For optical communications, the optical carrier for data signals is typically in the 1300 to 1350 nm or 1525 to 1610 nm ranges while the pump signal is advantageously in the 950 to 1000 nm range, but other wavelength ranges are possible.
For long-range transmission, it is greatly desired that not only the transmission fibers 10, 18 be single mode but that the elements associated with the EDFA 16 also be single mode so as to maintain the integrity of the data signal being amplified. In particular, it is advantageous that a single-mode fiber 19 link the optical pump source 14 and the optical combiner 12 and other parts of the EDFA.
Modern optical fiber communication systems have placed increasingly difficult requirements on the systems illustrated in FIG. 1, in particular the need for increasing levels of amplification. Not unexpectedly, it is desired to amplify a single optical signal as much as possible so as to maximize the distance between required amplification stations. Not so apparent is the increased power requirement necessitated by wavelength division multiplexing (WDM). In this fiber optic transmission technology, a number of closely space optical channels carry different data channels and are simultaneously amplified by the single EDFA 16. The number of WDM channels is increasing from 4 and 16 to 64 and considerably beyond. Each of the multiple channels requires its own fraction of optical pump power. Furthermore, for channel separation, high data rates, and efficiency, it is greatly desired that the data signal be carried only on a single-mode fiber and be combined with a pump signal carried on a single mode fiber. That is, the optical pump source 14 is being expected to provide ever higher amounts of optical power while maintaining its single-mode characteristics.
The prevalent types of single-mode fiber (SMF) systems include semiconductor laser sources based on either narrow single-stripe laser diodes or flared master oscillator-power amplifier (MOPA) configurations. Single-stripe lasers have been reported with output powers of 700 mW, and MOPA with output powers in excess of 1 W. The best currently commercially available SMF coupled pump source provide only about 200 mW of optical pump power at 980 nm. The apparent reasons for this low number include stability and reliability problems produced by the very high optical power densities on the laser mirrors.
A broad-area laser diode has a width considerably larger than that associated with single-mode operation, for example, a stripe width of 100 xcexcm, so that broad-area laser diodes produce multi-moded optical outputs. Their large size allows them to generate higher optical power while still operating at a fairly low power density. However, it is extremely difficult to achieve stable operation with a fundamental (zero-order) transverse mode, the mode of use in pumping a single-mode fiber amplifier. Adequate mode selectivity can be achieved with a narrow slit and an external mirror defining one end of the cavity. However, stability is achieved only by the continued precise alignment of the external elements.
Another approach in the design of high-power lasers uses a multiple quantum-well (MQW) structure in a surface-emitting configuration (that is, a vertical-cavity surface-emitting laser, VCSEL) with an external spherical mirror defining one end of the cavity. This approach produces high performance, but the alignment of the external mirror must be precisely maintained, and at higher pump power the mode selectivity may not be strong enough to ensure operation only in the zero-order mode. These structures typically rely upon either semiconductor interference mirrors formed on a face of the VCSEL or upon the length of the vertical optical cavity formed on the chip to provide the frequency discrimination required of a laser. Such mirrors, particularly when formed of semiconductors, require a large number of layers whose thicknesses must be precisely controlled, contributing to both increased cost and optical loss. Frequency-discrimination based upon cavity length of a semiconductor structure introduces large thermal drift in the lasing frequency because of the large thermal dependence of the effective dielectric constants in semiconductors.
All these solutions further do not effectively address the added problem of coupling the lasing light into a single-mode optical fiber, as required for many applications including the preferred optical pump source for an EDFA.
The invention includes a semiconductor optical amplifier coupled to a single-mode optical fiber upon which is written a Bragg grating having a period corresponding to the desired lasing wavelength. The Bragg grating defines one end, preferably the output end, of an optical cavity which includes the semiconductor amplifier.
The semiconductor laser of the invention is advantageously used as an optical pump source for a doped fiber amplifier, such as an erbium-doped fiber amplifier requiring pump light around 980 nm.
In one set of embodiments of the invention, the semiconductor amplifier includes a wide-area stacked structures of semiconductor layers generally corresponding to the structure of a vertical-cavity surface-emitting laser. Preferably, the active region includes a plurality of strained quantum wells spaced at the nodes the desired lasing wavelength.
The semiconductor stack can be optically pumped either from the front side or the back side.
A plurality of optical pump sources can be directed at inclined angles toward the beam area of the semiconductor stack.
Advantageously, a partially transmissive mirror may be coated on the side of the semiconductor stack facing the fiber so as to create a coupled cavity laser in which the amplification occurs in one cavity and the wavelength selection predominantly occurs in the other cavity.
In another set of embodiments of the invention, the semiconductor amplifier includes an edge-emitting diode laser operating as an amplifier with a mirror on one side and which is electrically pumped.
In yet a further set of embodiments, the optical amplifier includes an solid-state laser rod optically pumped by a diode laser.