Coherent light generators – Optical fiber laser
Reexamination Certificate
2002-06-18
2004-02-10
Ip, Paul (Department: 2828)
Coherent light generators
Optical fiber laser
C385S010000, C385S037000
Reexamination Certificate
active
06690685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method for producing a fiber laser.
2. Discussion
A fiber laser is an optical device comprising a doped optical fiber (active fiber) and a pump source adapted to provide a pump radiation to the doped optical fiber in order to excite the dopant. Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), Thulium (TM) and Praseodymium (Pr). The particular rare earth element or elements used is determined in accordance with the wavelength of the laser emission and the wavelength of the pump light.
The excited dopant tends to generate, as a consequence of its de-excitation, a stimulated emission radiation. The fiber laser also includes reflecting elements suitable to confine the stimulated emission radiation inside the optical fiber and to allow, when predetermined amplification conditions are reached, the output of part of this radiation. The reflecting elements may be, for example, Bragg gratings written on opposite ends of the doped optical fiber. A Bragg grating includes an alternation of zones with high refraction index and zones of low refraction index, mutually spaced at a distance that establishes the reflection wavelength (Bragg wavelength).
Bragg gratings are typically written in the core of standard transmission fibers, i.e. fibers not doped for stimulated emission purposes, in order to define reflecting elements for the transmission signals. Prior to the writing process on a standard fiber, a photosensitizer is usually added to the core of the fiber in the region predisposed to host the grating. The writing process then includes the exposure of the photosensitized region to a UV radiation, which generates an interferential pattern in the said core region inducing a refraction index variation. The interferential pattern may be obtained by means of different techniques, of which the most used are the “phase mask” technique and the technique consisting in focusing on the said core region two interfering UV beams.
Real time information on the grating characteristics during writing may be obtained by feeding to one end of the optical fiber the radiation of a wide spectrum source (e.g. a white light lamp or a LED) and by detecting, by means of a spectrum analyzer, the reflection spectrum at the same end of the fiber or the transmission spectrum at the opposite end of the fiber. This setup offers information on the peak wavelength, the intensity and the shape of the grating, and these information can be used to control process parameters like the UV intensity, the writing duration and the grating length.
A further technique to get information related to the writing process comprises feeding to one end of the fiber a wavelength tuned laser radiation and detecting the reflection optical power (at the same fiber end) or the transmission optical power (at the opposite fiber end) by means of a power meter. This further technique is slower but allows a higher resolution with respect to the previous one. Then, the first technique (wide band radiation feeding) is preferably used for real time monitoring of the writing process, while the second (tuned laser radiation feeding) is preferably used for grating characterization at the end of the writing process.
An easy technique to realize a resonant cavity on an active fiber comprises joining the active fiber with two stretches of not doped optical fiber each including a Bragg grating having a Bragg wavelength at the predetermined laser wavelength. However, the unavoidable insertion losses at the fiber joining region induces a laser power reduction and undesired reflections inside the resonant cavity which degrade the laser performances.
A different solution consists in writing the Bragg gratings directly in the core of the active fiber. In this case, the high absorption of the active fiber at the wavelengths of the radiation fed to the fiber (i.e. the wavelengths used for the real time monitoring or for grating characterization) makes the above mentioned monitoring techniques impracticable.
Typically, before writing a grating on an active fiber an evaluation of the required exposure time is made by considering data previously collected on identical but not doped fibers. However, no information is available in this way on the effective grating reflectivity obtained at the end of the writing process and, consequently, on the effective laser efficiency.
The document of Mikael Svalgaard, “
Ultraviolet light induced refractive index structures in germanosilica
”, Ph. D. thesis, March 1997, Mikroelektronik Centret, Published by Mikroelektronik Centret, Technical University of Denmark, Building 345 east, DK-2800 Lyngby, Denmark, depicts in Chapter 4 a work addressed to investigate the frequency stability of Er-doped fiber lasers that incorporate Bragg fiber gratings as the end mirrors. Svalgaard indicates that the dynamics of forming Bragg gratings involves spectral shifts of the same order of magnitude as the grating bandwidth (typically a fraction of a nanometer) and that such small changes during UV writing critically affect the performance of the resulting fiber laser. A method is proposed for real time monitoring of the laser performance based on simultaneous UV grating fabrication and pumping of the Er doped fiber.
SUMMARY OF THE INVENTION
In the description of the experimental setup (paragraph 4.2), a 10 cm Er-doped fiber is considered, whose ends are spliced to standard telecommunication fiber. The (first) grating formation dynamics is monitored in transmission using a broadband 1550 nm LED source. The first grating is exposed until the transmittance at the Bragg wavelength is 0.028±0.001. During the writing of the second grating, Er-doped fiber is pumped by a 980 nm multimode diode laser through a 1530/980 nm wavelength-division multiplexing fiber coupler (WDM), and the laser output (near 1530 nm) is monitored on a spectrum analyzer. When the exposure time of the second grating approached that of the first, a maximum lasering power is reached. To prevent feedback optical isolators are used after both the diode and fiber lasers, and all fiber ends are angled. As reported in paragraph 4.3, to obtain robust single-frequency operation, the cavity must be very short. In the specific case, the cavity is 12.5±1 mm long. Furthermore, according to Svalgaard, it is critical that the second gratings Bragg wavelength matches that of the first for lasering to occur.
The Applicant has observed that the fiber laser considered in the above document is a single-longitudinal-mode wavelength stabilized doped fiber laser, which includes an active fiber having a relatively low absorption at the Bragg wavelength, mainly due to the fact that the fiber is very short. This feature allows using a standard technique (feeding a wide band radiation to the fiber and detecting the related fiber output spectrum) for monitoring the characteristics of the first grating during the writing process.
The Applicant has noticed that, if a fiber laser has to be realized which includes an active fiber having a high absorption at the Bragg wavelength, the above method is no more suitable.
For the aim of the present invention, with “high absorption” it is intended an absorption of at least 15 dB in a range of about ±10 nm centered at the Bragg wavelength. The absorption of the fiber depends mainly on its geometry, on its length and on the dopant concentration.
The Applicant has in particular noticed that the first grating writing monitoring by means of a LED source or another wide band source would not be possible in case of a high absorption fiber, due to the excessive signal loss inside the fiber which would avoid correct spectrum detection.
Fiber lasers including a high absorption active fiber may be used, for example, as pump sources for optical amplifiers in optical transmission systems. For this kind of application, it is nor required to have a single-mode stabilized laser radiation and the active fiber is preferably
Oliveti Guido
Rossi Giacomo
Tormen Maurizio
Agon Juliana
Al Nazer Leith
Corning O.T.I. SpA
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