Coherent light generators – Particular component circuitry
Reexamination Certificate
1998-08-21
2001-03-20
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
C372S026000, C372S096000, C372S097000
Reexamination Certificate
active
06205161
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for operating a laser diode, whereby the intensity of the laser beam produced by the diode is adjusted with an adjustment signal and directed along a path to stock, such as printer stock or photographic stock, that is to be illuminated. Under certain conditions unstable reflections from the stock, acting as a reflector, act on the diode with respect to time and/or space, thus interfering with the laser intensity.
Laser diodes are employed in many fields of communications. They are particularly practical because the intensity of the light they emit can be modulated. Since they are so small and easy to control electronically, laser diodes can be employed to transmit data very rapidly. This feature is of significant advantage not only for transmitting but also for reproducing information.
Laser diodes, however, do have one drawback in that they tend to shift spontaneously from one mode to another at specific ranges of intensity. This tendency results in the low-frequency intensity fluctuations called mode hopping. Mode hopping is particularly annoying when laser diodes are employed to switch statically or dynamically, especially for signal modulation.
Several ways to avoid mode hopping have already been suggested. The U.S. Pat. No. 4,817,098 for example proposes a system that involves various means of maintaining laser outputs constant. The temperature of the diode can be measured for instance and regulated to minimize temperature fluctuations and accordingly maintain its output as constant as possible.
The U.S. Pat. No. 5,283,793 suggests superposing a high-frequency signal over an imaging signal being applied to printer stock.
US Pat. Nos. 4,799,069 and 5,386,124, finally, suggest preventing mode hopping by briefly turning the laser diode off between every pair of pixel signals.
These approaches to the prevention of mode hopping are either technically rather complicated or lead to relatively unsatisfactory results. One object of the present invention is accordingly a method and device for preventing the sudden low-frequency disruptions in laser intensity that mode hopping causes. A more specific object is to stabilize the light emitted by a laser diode scanning a surface with respect to mode hopping when reflections from the surface interfere with the diode.
These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by producing intentional and controlled feedback of enough of the light emitted by the laser diode back to the diode's effective surface, via another path, to maintain the laser diode in a state of coherence collapse.
SUMMARY OF THE INVENTION
The present invention is based on the awareness that mode hopping can be prevented by establishing as many different modes as possible inside the laser diode. This approach generates high-frequency noise and suppresses the low-frequency noise generated by mode hopping.
The light emitted from the laser diode is fed back to its effective surface by way of an optical component that is stable with respect to time, and, in terms of the diode, with respect to space, maintaining the diode in a state of coherence collapse. A little of the emitted light is accordingly reflected by an external mirror and hence to the laser resonator by appropriate optical components. The light is fed back over a different path that can be at least partly separate from the utility path. The diode's light-emitting area constitutes, in conjunction with the light-feedback component, an external resonator that is considerably longer than the diode's resonator alone. The resonance frequencies (equivalent to modes) of the external resonator are accordingly much closer together than those of the laser resonator itself. The mode intervals can for example be 500 MHz for an external resonator 30 cm long and 170 GHz for a GaAlAs laser diode 0.2 mm long.
The light fed back to the laser diode over the utility path will always exhibit slight fluctuations in amplitude and phase provoked not only by optical noise (spontaneous emission) from the diode itself but also by time-variant optical reflections from the various moving components (pivoting mirrors and film).
Once the output of the light being intentionally fed back over the second path exceeds a prescribed minimum, the fluctuations in amplitude and phase at the effective surface will be amplified to the point of static inherent modulation in the light output (AM) and in the laser frequency (FM). The period T of this inherent modulation will be dictated by the time 2·L
extern
/c the lightwave takes to travel in the additional external resonator, whereby L
extern
is the length of the external resonator and c the speed of light (3·10
10
cm/sec). The reciprocal of period T, however, is precisely the frequency interval
df=
1/
T=c/
(2·
L
extern
)
of the external resonator's modes.
The combined AM and FM inherent modulation generates a large number of sidebands, that coincide precisely with the external resonator's modes. A large number of external-resonator modes can accordingly be simultaneously established in a laser system comprising a laser diode and an external resonator, and the emitted laser energy can be uniformly distributed over a large number of modes and maintained constant in the time-based means.
One effect that can be intentionally achieved and maintained time-constant in accordance with the present invention is that the large number of modes will considerably decrease the coherence length of the resulting radiation. This state of the laser diode is called coherence collapse because the radiation will convert abruptly from a state of higher coherence (monomodal operation with static mode hopping) to one of lower coherence (multimodal operation without mode hopping) once the beam has exceeded a critical threshold in the fed-back light output.
In systems intended to stabilize the frequency of the laser beam, the optical coupling is intended to ensure that the optical path is at least as short as the light's coherence. Systems of this type are described in German Patents Nos. 3,442,188 A1 and 3,410,729 A1 l for example. Feeding the light back in the system in accordance with the present invention, which exploits coherence collapse, is intended on the other hand to ensure that the path of the fed-back light is actually longer than the coherence, and the feedback in such a system is accordingly called incoherent.
How much optical-feedback output is need to achieve coherence collapse, depends on the type of laser diode and on how much light the external resonator can reflect. It is on the order of a small percentage of the emitted output. R. W. Tkach, Regimes of Feedback Effects in 1.5 □&mgr;m Distributed Feedback Lasers,
Journal of Lightware Technology,
LT-4, 11 (November 1986), 1655-61 and K. Petermann, “Laserdiode Modulation and Noise”, Kluwer Academic Publishers, Dordrecht (NL), 1988, 251-90 describe studies of laser diodes operating subject to coherence collapse and discuss appropriate levels of reflection. K. Petermann, External Optical Feedback Phenomena in Semiconductor Lasers,
IEEE Journal of Selected Topics in Quantum Electronics,
1, 2 (June 1996) 480-89, describes further studies of laser diodes operating in the same state.
The present invention can be employed to particular advantage in what are called monomodal lasers, the internal resonators of which normally generate light of a single wavelength. Monomodal lasers usually exhibit abrupt mode hopping at specific points when the current strength changes. The laser's activity, in other words, jumps from one mode to an adjacent mode. It was discovered in accordance with the present invention that intentionally inducing a coherence collapse can keep the light output in the low-frequency range much more constant than is possible in monomodal operation. The emitted light will also be much less sensitive to undesired
Agfa-Gevaert Aktiengesellschaft
Font Frank G.
Milde Hoffberg & Macklin, LLP
Rodriguez Armando
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