Coherent light generators – Optical fiber laser
Reexamination Certificate
1999-11-15
2001-02-06
Lee, John D. (Department: 2874)
Coherent light generators
Optical fiber laser
C372S029011, C372S029020, C372S096000, C372S102000
Reexamination Certificate
active
06185230
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optical signal processing and, more particularly, to the generation of a low-noise, single frequency laser output.
BACKGROUND OF THE INVENTION
In optical communications, it is desirable to use a communication wavelength in the wavelength range of approximately 1.5 micrometers (&mgr;m). This is because 1.5 &mgr;m is the lowest loss wavelength of conventional single-mode glass fibers. As the number of desired channels and demand for high video quality increases, the corresponding demands on an optical transmission source also increase. Among these is the demand for a low-noise, single-frequency 1.5 &mgr;m laser source. Fiber laser sources have been created which use an erbium (Er) doped optical fiber as a gain medium. However, such fiber lasers typically suffer from a tradeoff between gain and power, and mode discrimination.
It is commonly understood that, within a fiber laser, longitudinal mode spacing is inversely proportional to the length of the laser cavity. That is, for a longer resonator cavity, the frequency spacing between adjacent resonance frequencies is smaller. As a result, more longitudinal modes are within the (necessarily finite) width of the gain spectrum, and mode discrimination is poor. Even if the effective gain spectrum of the fiber laser is narrowed through the use of narrow band reflectors, such as Bragg gratings written directly into the core of the fiber, many modes may exist with that effective gain bandwidth. For the designer of a single-frequency laser, this causes several difficulties. Firstly, the wide spectrum emitted from the laser interferes with other laser sources transmitted at different wavelengths in wavelength division multiplexed systems. Secondly, dispersion in the wide spectrum results in pulse spreading. Finally, mode competition between longitudinal modes causes amplitude instability.
By making the laser cavity shorter, the mode spacing is increased and, if the cavity is short enough, the gain spectrum may be limited to a single resonance frequency. However, the length of the gain medium (i.e. the doped fiber surrounding the cavity) also affects the optical gain of the laser. With a relatively short length of fiber, the gain of the laser is low since there is less distance across which the pump energy may be absorbed within the fiber.
It would be desirable to have an optical source that has an output power significantly higher than previously available, and which has the mode discrimination benefits of a short fiber gain medium.
SUMMARY OF THE INVENTION
The present invention provides a single-frequency laser source in which a high output power is achieved by developing a resonant condition for the pump energy wavelengths within the laser cavity. In general, the laser source has an optical fiber gain medium and a signal reflection apparatus that reflects light at the signal wavelength within the gain medium, so as to establish a resonance condition at the signal wavelength. The signal reflection apparatus is substantially entirely reflective when it comes to optical energy in the cavity traveling toward an input side of the cavity, and partially reflective to optical energy in the cavity traveling toward an output side of the cavity. This partial reflectivity provides the desired output coupling for the laser. The gain medium is pumped by a pump source that couples light at an appropriate pumping wavelength into the laser cavity. Once coupled into the laser cavity, the pump energy is retained within the cavity using a pump resonator that reflects light back and forth within a resonant cavity that includes the gain medium. This maximizes the pumping efficiency, particularly for the short fiber laser, by retaining the pump energy within the gain medium.
The signal reflection apparatus may take different forms, as may the pump resonator. For example, the signal reflection apparatus may have a distributed feedback (DFB) type configuration in which a periodic grating is built into a pumped part of the gain medium. As known in the art, a DFB grating has a shift in its structure that results in a
{fraction (1/4+L )}-wavelength phase shift in light at the chosen wavelength (e.g., the signal wavelength), thus giving the desired resonance condition. Alternatively, the signal reflection apparatus may also have a distributed Bragg reflector (DBR) type configuration, in which an input periodic grating is located relatively close to an input side of the gain medium and an output periodic grating is located relatively close to an output side of the gain medium. The input grating is highly reflective to keep the optical energy at the signal wavelength within the cavity, while the output grating is partially reflective to allow output coupling of the laser energy.
The configuration described above for the signal reflection apparatus may be combined with different configurations for the pump resonator. The pump resonator may be a DFB type structure that is reflective at the pump wavelength and that is built into a part of the gain medium occupied by a DFB type signal reflection apparatus grating. The pump resonator may also be a DBR type structure having two highly reflective periodic gratings, one on either side of the signal reflection apparatus, that reflect optical energy at the pump wavelength back and forth within the gain medium. The DBR type pump resonator may be used with either a DFB type or a DBR type signal reflection apparatus. In either circumstance, the gratings of the pump resonator surround the grating or gratings of the signal reflection apparatus, and provide “resonant pumping” of the gain medium across the area occupied by the signal reflection apparatus. Also, when a DBR type signal reflection apparatus is used with a DBR type pump resonator, a Q-switch may also be used to generate a high-intensity, short-duration pulse.
In one variation of the invention, a DBR-type arrangement of pump gratings may be used to construct a short fiber amplifier similar to the short fiber lasers discussed above. However, rather than using a signal reflection apparatus which develops the desired signal wavelength such that it originates in the gain medium, only the pump resonator is used. The signal to be amplified is then introduced into the pumped gain medium, where it is amplified before being output. The resonant pumping of the gain medium maintains a desired level of population inversion, and allows the signal passing through the gain medium to be amplified by stimulated emission. Also, with the use of only a single, broadband signal reflector, an amplified spontaneous emission (ASE) source may be formed that provides a broadband optical output.
In one embodiment of the invention, a plurality of resonant-pumped lasers are arranged in series and allow an optical signal to be generated at any of a plurality of different signal wavelengths. Each of the laser cavities may have a different combination of DFB and DBR type signal reflection apparatus and pump resonator. For each laser, a DFB type signal reflection apparatus may be combined with either a DFB or a DBR type pump resonator, or a DBR type signal reflection apparatus may be combined with a DBR type pump resonator. Each of the lasers in series has a signal reflection apparatus constructed to oscillate at a signal wavelength different than the signal wavelengths of the other lasers. Furthermore, each of the lasers has a pump resonator constructed to oscillate at a pump wavelength different than the pump wavelengths of the other lasers. Thus, when a first pump wavelength is directed into the series arrangement of lasers, it resonates only in a first laser cavity that has its pump resonator constructed to oscillate at the first pump wavelength. This results in the gain medium of the first laser cavity being resonant pumped, while the other laser cavities achieve no such pumping. The pumping of the first laser cavity causes the generation of signal energy within the first cavity, which resonates at a first desired signal wavelength, bei
Kudirka & Jobse LLP
Lee John D.
SDL Inc.
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