Coherent light generators – Particular resonant cavity
Reexamination Certificate
2002-03-22
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
C372S045013
Reexamination Certificate
active
06711203
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lasers emitting single longitudinal and traverse mode radiation at selected wavelengths defined by a frequency comb, in particular stepwise tunable external cavity surface emitting semiconductor lasers pumped optically or electronically for use in spectroscopy, process control and optical communications. Particularly disclosed are laser designs, and manufacturing and assembly processes for frequency stable and rapidly tunable lasers for optical communications.
BACKGROUND OF THE INVENTION
The use of lasers as part of a system for optical channel switching in a fiber optic transport network is known. However, existing systems utilize a multiplicity of individual lasers, each of which emits at a single frequency. A further problem is that current switching systems using single frequency lasers require extremely complex circuitry to transform a set of input signals to a set of output signals of different frequencies.
A significant enhancement of such laser based optical channel switching systems would be achieved by use of a laser having the following operating characteristics:
1) the laser is rapidly tunable to specific desired output frequencies, e.g. the frequencies of the ITU grid.
2) The laser provides random access to any particular output frequency (i.e. transmission channel).
3) The laser is reliable and consistent in the output frequency to which it is tunable over a long service life without requiring extensive servicing or a carefully controlled operating environment.
4) The laser is consistently receptive to an input signal since it must always tune to the correct output frequency (i.e. channel number).
5) The laser provides substantially uniform output power independent of the particular output frequency selected.
We have discovered a laser system which fulfills the above indicated performance requirements.
Typical lasers oscillate at a number of frequencies (i.e. wavelengths) that correspond to modes of their optical cavities. In certain applications, it is known to select a single such mode and to adjust its frequency by varying the cavity length or some other laser parameter. Methods of doing this are described in text books such as A. Yariv, Quantum Electronics, John Wiley and Sons, New York, 2
nd
edition, 1975 and M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy Revised Edition, Academic Press, San Diego, 1988 and Demtroder, Laser Spectroscopy, Springer, Berlin, 1996.
However, the task of quickly switching the laser output frequency from one externally selected value to another externally selected frequency—without undergoing laser action at intermediate frequencies—poses significant technical difficulties. Known means of doing this alter some other significant laser parameter, such as output power, create an instability in the output frequency either before or after the frequency switch, and/or switch to an unwanted value which must then be homed-in on (actually adjusted to) the desired frequency, which requires time. In many optical communications and spectroscopy applications, delay in obtaining stable operation after a frequency switch is certainly undesirable and is frequently unacceptable.
Most semiconductor lasers generally follow two basic architectures. The first laser type has an in-plane cavity, and the second laser type has a vertical cavity, a so-called vertical-cavity surface-emitting laser or “VCSEL”. If the optical resonance cavity is formed externally of the semiconductor structure (active region) i.e. one of the reflecting surfaces is physically separated from the active region, the laser is known as a vertical external cavity surface-emitting laser or “VECSEL”.
Electrically pumped diode lasers are most frequently of the in-plane cavity type. Necessary optical feedback with the in-plane type is most frequently provided by simple cleaved-facet mirrors at each end of the optical cavity. The reflectance of such cleaved mirrors, while generally sufficient is not very high, and laser energy is thus emitted through the cleaved mirrors to the external ambient at opposed edges of the structure, giving rise to “edge-emitting” diode lasers. Such relatively simple structures are sometimes referred to as Fabry-Perot diode lasers. Epitaxial patterning of a grating pattern along a top surface of an edge-emitting diode laser can be provided to set a design wavelength, resulting in a so-called distributed feedback diode (“DFB”) laser.
In-plane electrically pumped lasers, such as DFB lasers, are typically single mode, and are also typically tunable continuously across some wavelength band from near-infrared into the visible light spectrum. Rapid tuning may be carried out by controlling the electrical pumping current, while slower tuning may be carried out by controlling the temperature of the laser via a heat sink and a thermal cooler/heater arrangement. Such in-plane lasers have known uses including optical wavelength absorption spectroscopy, storage, printing and telecommunications. In-plane lasers are frequently employed within telecommunications systems using optical fiber as the information transfer medium. Conventionally, multiple channels are carried through a single optical fiber, and it is therefore necessary when using a Fabry-Perot laser or DFB laser as the illuminating source to regulate the wavelength of the transmitting laser in order to stay on a selected channel.
In order to keep an in-plane diode laser tuned to a desired wavelength, current and thermal control loops must be provided to stabilize the laser at a desired wavelength, particularly as the laser ages during usage. Also, since there is no absolute wavelength stabilization within these in-plane lasers, the emission wavelength may drift, absent careful feedback control, during usage and over the lifetime of the laser. This tendency to drift or change emission characteristics with temperature and over time puts stringent conditions on the materials and control systems used to make the laser.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that they manifest a tendency to mode-hop. Mode-hopping basically means that for a given pumping current, the laser can unexpectedly hop to a completely different mode (wavelength). As the current is increased, there are wavelengths at which the mode hopping (wavelength jumping) becomes uncontrollable. Moreover, diode lasers may manifest a hysteresis, in that mode hopping may occur at different wavelengths as the control current is increased as compared to when the control current is decreased. Another drawback of in-plane diode lasers is that output power is inextricably intertwined with active region temperature and pumping current. Another issue with in-plane diode lasers is that the transverse optical beam profile is typically elliptical rather than circular and has high divergence, increasing the complexity of coupling the laser energy into an optical fiber, or coupling the laser waveguide mode to an external cavity mode.
Dense wavelength division multiplexing (DWDM) for optical fiber telecommunications applications requires optical transmitters that can be tuned to any frequency in the standard ITU grid (wavelength comb) with a relative frequency error not greater than ten percent of the ITU channel spacing. This requirement implies that an optical transmitter laser has excellent frequency stability as well as broad tunability. For a 12.5 GHz channel spacing, the transmitter must have 1.25 GHz of absolute accuracy and frequency (wavelength) stability. Such control of the laser frequency cannot be achieved with existing DFB lasers without complex electronic control and frequently carried out diagnostics. Furthermore, compensation algorithms must be developed in the laser control system to handle the DFB's known aging processes, which is often unpredictable.
A desirable characteristic of an ideal DWDM optical transmitter is that a single laser can cover all of the DWDM channels, and that it can be reliably and reproduci
Garnache Arnaud
Katchanov Alexandre
Knippels Guido
Levenson Marc
Lodenkamper Robert
BlueLeaf, Inc.
Burkand Herbert
Schipper John F.
Vy Hung T
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