Coherent light generators – Optical fiber laser
Reexamination Certificate
1998-11-12
2001-02-20
Font, Frank G. (Department: 2877)
Coherent light generators
Optical fiber laser
C376S249000, C376S249000, C376S108000, C376S249000, C376S249000, C376S249000, C376S249000, C376S249000, C376S249000, C376S249000, C376S249000, C376S102000, C359S341430, C359S199200, C359S199200, C359S199200, C385S037000, C385S032000
Reexamination Certificate
active
06192058
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiwavelength actively mode locked external cavity semiconductor laser useful in fiber optic telecommunications systems and in other applications employing wavelength division multiplexing of high speed digital or analog optical signals and, more particularly, to a multiwavelength actively mode-locked external cavity semiconductor laser for the simultaneous reliable generation of multiple optical carrier signals for use in such systems.
2. Description of the Prior Art
Prior art multiwavelength optical signal processing and transmission systems rely upon optical sources to generate optical carriers for modulation of digital, or analog, data by a multiplicity of modulators. The modulated optical carriers propagate within substantially identical single transverse spatial modes occupying substantially the same position in a transmission medium as may be provided by single-mode waveguides, such as optical fibers or planar optical waveguides, or by free-space optics. Each modulated optical carrier can be separately distinguished from all others by means of an optical filter designed to pass, to one or more output ports, a given optical carrier wavelength while rejecting all others presented at its input port. Such a modulated optical carrier can be demodulated to convert information carried thereupon to electronic form. Systems employing multiple optical carriers distinguished by wavelength are designated as dense wavelength division multiplexing (DWDM) systems.
The optical sources employed in prior art DWDM systems are discrete laser source components, one for each wavelength. Reliability of these sources is assured by practices such as extended life-testing and thermal, electrical, and mechanical tests known to those skilled in the art, as exemplified by Bellcore TR-NWT-000468. Such laser sources are typically continuous wave, single frequency, single spatial mode diode lasers, conventionally of the distributed feedback (DFB) design, such that, e.g., a 40-wavelength system employs 40 separate DFB laser sources and 40 separate manufactured subsystems containing additional fiber optic, electronic, and electromechanical components and their interconnections including a single fiber optic output port to carry each modulated optical carrier for further combination with all others. Therefore, each such subsystem includes overhead in the form of additional components that are duplicated for each separate optical carrier wavelength.
It is the reliance upon single wavelength optical sources which requires prior art multiwavelength system architects to design an entire subsystem, not merely one source, for each optical carrier wavelength in prior art systems. The major sources of complexity and cost of a multiwavelength transmissions systems product are electronic, optical, and electromechanical components, their assembly, and related costs for assembly and manufacturing processes. These costs all accrue separately for each optical source. Therefore, single wavelength optical sources represent a serious disadvantage for the manufactured system because their use requires that the latter costs must be aggregated to the extent of the number of optical carrier wavelengths employed. Systems relying upon such aggregation are large in size, consume excessive power, and suffer from reliability concerns or require undesirably frequent maintenance associated with the use of a large number of optical source components.
The prior art contains examples of continuous wave (cw) optical source components capable of emitting multiple optical carriers simultaneously. In general, cw sources exhibit inherent shortcomings as compared to pulsed sources. A technique capable of generating multiple cw optical carriers simultaneously, put forth by Zah et al., requires a monolithic chip containing multiple DFB lasers. For practical purposes, the complexity of such optoelectronic integrated circuits results in low yield and high cost. In addition, the difficulty of packaging and qualifying for reliability assurance, as required for telecommunication applications, a source module including 20 or more separate lasers, as these devices must, involves significant challenges over and above those of demonstrating the devices' functionality.
Unlike pulsed lasers, individual cw lasers, whether tunable or single-wavelength, are unable to emit multiple optical carriers simultaneously. In contrast to cw schemes, pulsed operation bestows a particular advantage with respect to the dynamics of net gain, defined as the difference between the roundtrip gain provided by an amplifying medium and round-trip propagation loss within the laser resonator expressed in dB. The net gain transiently exceeds threshold for the highest net gain mode by a margin sufficient to permit the net gain for a multiplicity of additional modes to exceed threshold and thus for them, too, to lase. Such a margin cannot be maintained in steady state cw operation within a single gain medium because there the steady-state phenomenon of net gain clamping ensures that only a single mode is able to reach threshold.
Mode-locked pulsed laser operation, by contrast, offers significant advantage over cw operation for multiwavelength operation, as shown by Delfyett et al. A conventional mode-locked pulsed laser operates with substantial equality between round-trip travel time of an optical signal within the laser resonator and the pulse period divided by an integer factor greater than or equal to unity. When substantially transform-limited, a mode-locked pulsed laser emits Gaussian pulses occupying a full-width half maximum frequency spectrum &Dgr;&ngr; determined by &Dgr;t&Dgr;&ngr;=0.4413, where &Dgr;t is the full-width half maximum pulse duration.
An actively mode-locked laser is one in which a periodic variation is impressed upon net gain. In an actively mode-locked semiconductor external cavity laser (AMSECL) of the type described by Delfyett et al., optical gain is provided by an angled-stripe semiconductor optical amplifier (SOA). The minimum fundamental period of pulses emitted by an AMSECL, i.e., for which the aforementioned integer factor is unity, is limited by the practical cavity size. For free-space optical components, an approximate practical lower bound would 167 psec for fundamental mode-locked pulses, corresponding to approximately a 2.5 cm cavity. In general, other considerations may indicate a design of substantially larger cavity AMSECLs. Either harmonic mode-locking in which the integer factor exceeds unity or pulse interleaving subsequent to the AMSECL can be employed to attain shorter periods than generated by the AMSECL, as taught in the prior art.
The AMSECL has a number of advantages over other mode-locked lasers. Because it is based on the angled-stripe SOA, it can be made small and relatively inexpensive. Only a single angled-stripe SOA is required as the optical gain element of the AMSECL. As a two-terminal device, the angled-stripe SOA is simple to design. It is similar to, but simpler in design than, a DFB laser. Additionally, there is no need to invoke the complexity involved in separate pumping and amplifying components as required for active fiber amplifiers. Beyond feedback and collimation optics, no additional components are required.
In a conventional AMSECL net optical gain in the angled-stripe SOA is varied by direct application of periodic electrical bias to the angled-stripe SOA at an RF frequency substantially equal to the laser pulsation rate. The frequency spectrum of pulses emitted by a conventional AMSECL is accordingly modified from that of a cw laser. Additional frequency components are emitted at discrete frequency intervals from the fundamental component, corresponding to the pulsation rate in the case of fundamental mode-locking. The number of substantial wavelength components associated with a given fundamental component is such that the total frequency spectrum occupied by a fundamental and associated additional wave
Burke William J.
Flores Ruiz Delma R.
Font Frank G.
Sarnoff Corporation
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