Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-01-29
2003-12-16
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S006000, C372S068000
Reexamination Certificate
active
06665320
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to laser systems. More specifically, it relates to a novel class of tunable lasers that can provide effectively continuous tuning in a tuning range spanning multiple gain spectra in a simple, versatile, and economical way. Such novel tunable lasers are particularly suited for fiber-optic networks and telecommunication component testing applications.
BACKGROUND OF THE INVENTION
As fiber-optic networks employing wavelength division multiplexing (WDM) become increasingly pervasive as the backbone of modern communications systems, there is a growing demand for tunable laser sources that can provide a wide range of wavelengths in a simple, versatile, and economical way. Such tunable laser sources are desired, for instance, in swept-wavelength testing of passive and active telecommunication components. Tunable laser sources are also employed in multi-channel coherent communication systems, spectroscopic measurements, and optical amplifier characterizations.
Extended (or external) cavity diode lasers (ECDLs) are conventionally employed in the art to provide tunable laser sources for swept-wavelength testing in telecommunications and other applications. For purpose of elucidating the principle and the distinct features of the present invention, the underlying principle of operation of ECDLs is briefly described below. A more detailed description of external cavities is well documented in the art, for example, in “Spectrally Narrow Pulsed Dye Laser without Beam Expander” by Littman et al., Applied Optics, vol.17, no.14, pp.2224-2227, Jul. 15 1978; “Novel geometry for single-mode scanning of tunable lasers” by Littman et al., Optics Letters, vol.6, no.3, pp.117-118; “External-Cavity diode laser using a grazing-incidence diffraction grating” by Harvey et al., Optics Letters, vol.16, no.12, pp.910-912; and “Widely Tunable External Cavity Diode Lasers” by Day et al., SPIE, Vol. 2378, pp.35-41.
In a tunable ECDL, as the name suggests, the wavelength selection and tuning functions are external to the gain element where the laser action takes place. Such a system typically utilizes an external cavity of variable length in conjunction with a diffraction grating and a movable mirror (or simply-a movable diffraction grating), all external to a semiconductor diode (serving as a gain element). An incident laser beam is diffracted by the grating. A diffracted beam with the desired wavelength is selected by the movable mirror, further reflected back onto the diffraction grating, and subsequently transmitted back to the semiconductor diode where further amplification takes place. Rotation and/or translation of the movable mirror enables the system to be tuned to different wavelengths. (Alternatively, the movable diffraction grating is rotated/translated, to provide tunability in wavelength.) The ultimate limit to the tuning range is set by the gain spectrum of the semiconductor diode.
In an ECDL, the number of nodal points of the standing wave in the laser cavity is proportional to L/&lgr;, where &lgr; is the operating wavelength and L is the total optical length of the laser cavity (primarily provided by the length L
ext
of the external cavity). Therefore, if the wavelength tuning takes place while L is maintained constant, the number of nodal points in the laser cavity changes discontinuously. That is, the wavelength cannot be continuously varied, but rather, it leaps in discrete steps—termed as mode-hops. As a result, it is often difficult to tune in a desired wavelength, and there may also be substantial fluctuations in the output power of the laser. Mode-hops can be avoided by varying the length L of the laser cavity as the wavelength tuning takes place (such that as the tuning passband of the diffraction grating shifts in response to the tuning, the underlying axial modes of the laser cavity follow accordingly), in a coordination that requires great accuracy and stringent tolerance. Coordinating the wavelength tuning and the cavity-length changing in ECDLs has been a rather arduous and expensive undertaking.
Efforts have been made in the art to preventing mode-hops and thereby providing more continuous tuning, as exemplified by U.S. Pat. Nos. 5,172,390, 5,319,668, 5,347,527, 5,491,714, 5,493,575, 5,594,744, 5,862,162, 5,867,512, 6,026,100, 6,038,239, 6,115,401, and 6,134,250. For example, U.S. Pat. No. 5,319,668 describes an external cavity semiconductor laser, comprising a semiconductor laser diode, a diffraction grating and a movable mirror. The movable mirror is mounted on a pivot so positioned to provide simultaneous rotation and linear translation, thereby enabling continuous single-mode tuning. The mirror pivot point is determined by a detailed calculation which takes into account a number of factors in the laser cavity, so as to maintain a precise control of the length of the laser cavity. U.S. Pat. No. 5,347,527 discloses a tunable external cavity laser source and a process for adjusting the laser source, such that continuous tunability can be provided. U.S. Pat. No. 5,867,512 describes an external cavity semiconductor laser and an elaborate tuning arrangement for avoiding mode-hops. Particular effort is made in this patent to correct the chromatic dispersion effects in the laser cavity. The appearance of these prior art patents (along with many others) in fact serve as a testimony of the difficulty with combating mode-hops in ECDLs.
U.S. Pat. No. 6,115,401 discloses a laser system, in which a gain medium (such as a semiconductor diode) is optically coupled to an external cavity containing a monolithic prism assembly. The monolithic prism assembly, including a transparent substrate carrying a Fabry-Perot thin film interference filter, plays the role of the diffraction grating in a conventional ECDL (as described above). Translation of the monolithic prism assembly provides continuous mode-hop-free tuning of the laser operating wavelength. The intent of this invention is to make a compact single-frequency tunable laser with very narrow linewidths, primarily aiming at dense wavelength division multiplexing (DWDM) applications.
U.S. Pat. No. 6,134,250 describes a single-mode wavelength selectable ring laser, which operates at a single wavelength selectable from any channel passband of a multiple-channel wavelength multiplex/demultiplex element (e.g., an arrayed waveguide grating router (AWGR)). A Fabry-Perot semiconductor optical amplifier (FP-SOA) is connected to AWGR to form a ring laser structure, where FP-SOA is used as an intra-cavity narrow-band mode-selecting filter to stabilize the laser oscillation to a single axial mode. As such, this ring laser system can only provide discrete tuning from one wavelength passband of the wavelength filter to another. That is, continuous tuning cannot be achieved in this system. It should be noted that although this prior art patent discloses a configuration in which several semiconductor optical amplifiers (SOAs) are implemented in a demultiplexer AWGR, each of the SOAs is dedicated to amplify an optical signal with a particular wavelength. That is, these SOAs function merely as wavelength switches. And more important is the fact that the overall tuning range of this laser system is limited by the single gain spectrum of FP-SOA. Hence, this prior art laser system is suited as providing a wavelength-selectable laser, as opposed to a tunable laser.
Daneu et al. in “Spectral beam combining of a broadstrip diode laser in an external cavity”, CLEO 2000, describes a “spectral beam combining” laser system, in which several diode lasers placed in parallel are being used simultaneously, each having the identical gain spectrum but lasing at a different wavelength. By having several lasers operating simultaneously, the collective output beam of the system has many wavelengths superimposed, thereby providing a higher power. However, the overall tuning range of this system is no more than what a single diode is able to provide. Moreover, the external cavity in this case does not employ an optical
Arbore Mark A.
Harb Charles
Kmetec Jeffrey D.
Ip Paul
Lightwave Electronics
Lumen Intellectual Property Services Inc.
Rodriguez Armando
LandOfFree
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