Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-08-30
2002-06-11
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06404793
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of optoelectronics, including semiconductor laser diodes and semiconductor laser diode based optical amplifiers. In particular, the present invention relates to a novel apparatus and a method for signal modulation and control of signal coupling in semiconductor laser diodes and semiconductor laser diode optical amplifiers.
BACKGROUND OF THE INVENTION
Fiber optics and semiconductor lasers are now essential telecommunications technologies. Given the drastic increase in demand for high speed information transfer, the increased use of high bandwidth optical communications is a natural solution to increasing bandwidth demands. Fiber optic systems are capable of transferring high symbol rate signals over long distances with low attenuation using dielectric waveguides in the form of optical fibers. Optical fibers are cylindrical dielectric waveguides and offer a better bitrate times distance product than copper wires for a given attenuation or signal to noise ratio. However, in order to derive maximum usefulness from optical sources and ensure the highest data transfer capabilities, high bandwidth, high extinction ratio modulation techniques and device structures are critical. Such techniques and structures must be directed toward maximizing the signal to noise ratio at the receiver or receivers.
Methods for modulating the beams of laser diodes, optical amplifiers, and other optical devices are instrumental for facilitating the high-speed transfer of information across optical conduits. Improved modulation methods further are useful in minimizing factors such as signal loss due to poor signal coupling from one device to another. Prior art modulation of semiconductor laser diodes typically involves direct modulation of the injection luminescence current or external modulation of the optical beam usually through a Mach-Zehnder (MZ) interferometer or device of like kind.
Integrated MZ interferometers or related devices use a single mode input waveguide which is split into two branches of equal or nearly equal length and then recombined into a single mode waveguide. When a single bias voltage or multiple bias voltages are applied to one or both of the branches, a phase difference occurs between the two optical signals. A controllable amount of constructive or destructive interference occurs when these two optical signals are combined in such a manner. By controlling the amount of bias voltage applied to the branches, and thus the amount of constructive or destructive interference between signals, the amplitude of the recombined output signal can be modulated.
MZ based modulators must be constructed from an electro-optic material in which the refractive index is a function of applied voltage such as lithium niobate. Because such materials tend to be relatively costly, external MZ or related modulators are extremely expensive compared to direct modulation.
Direct modulation of a semiconductor laser by varying the magnitude of the applied injection luminescence current imposes an amplitude variation in the laser element. Controlled amplitude variations constitute the signal. While direct modulation is effective at signal constitution, it does not affect nor control the direction of the optical beam. Direct modulation may also cause transient, undesired wavelength shifts commonly referred to as “chirp” and capable of degrading pulse shape when the optical signal travels in a dispersive media such as optical fiber.
Direct modulation also restricts dynamic range and reduces the extinction ratio of the signal since the modulation current is varied about a bias point nearly midway between the laser threshold current and the maximum current for safe operation. Dynamic range refers to the ratio of maximum output signal power to the minimum output signal power subject to a signal fidelity criteria such as 1 dB compression and usually applied to analog transmission. Extinction ratio refers to the ratio of peak output with an applied input signal set at a maximum level to the minimum output power with no input signal applied. Thus the limited dynamic range and extinction ratio of prior art modulation techniques limit performance when used to conduct optical telecommunications.
For example, if the sum of the bias and modulation currents are below laser threshold, as they typically are during low signal level constitution, the optical signal is extinguished. During high signal level generation, the signal may be “clipped”. Clipping is problematic for analog optical signals requiring signal fidelity. Conversely, if the bias point is selected closer to the maximum safe current to avoid low-level clipping, device reliability may be impaired and “idle channel” noise may be increased. Idle channel noise may be particularly problematic in multichannel optical communications.
If many such high-biased lasers are interconnected over an optical network for example, the aggregate optical noise degrades the performance of signal processing devices such as signal demultiplexers and detectors supporting the operation of the network. High bias levels however do increase the intrinsic speed of the semiconductor lasers since the relaxation frequency increases with optical power. Moreover, direct modulation at high bit rates or high frequencies requires fast driver circuits capable of handling relatively large currents since the entire device is “pumped” with current. Providing both high speed and high bias levels within a driver circuit complicates the design of the driver. High speed operation using direct modulation may also be limited by device capacitances.
Capacitances inherently present as an unavoidable consequence of the physical design and construction of a semiconductor device may affect the operation of the device especially at high speeds. High speed operation therefore requires optimization of junction design to reduce parasitic device capacitances. Such optimization usually requires, among other measures, minimizing the length of the device.
Prior art methods for optimizing transmission include beam control for focusing and steering. Such methods are disclosed, for example in U.S. Pat. No. 5,524,013 issued to Nakatsuka, et al on Jun. 4, 1996. Nakatsuka, et al discloses a Beam Scannable Laser Diode wherein the position and emitting direction of a laser beam can be varied by controlling injection currents. By modulating injection currents, the electron distribution and refractive index profile for the optical media may be modified. Nakatsuka, et al relies on changing the refractive index profile to bend an incumbent beam in a manner similar to a gradient index lens by lateral diffraction. One disadvantage of Nakatsuka, et al is that the device is gain guided in the lateral dimension giving rise to astigmatism leading to poor beam quality. Moreover, gain guiding diminishes the beam steering benefits when such a device is operated at low power. Another disadvantage of Nakatsuka, et al is the need for an integral lens. Not only does such a lens require additional dry etching steps to construct, but the use of such a lens leads to optical aberrations including undesirable “coma”.
U.S. Pat. No. 5,319,659 issued to Hohimer on Jun. 7, 1994 discloses a Semiconductor Diode Laser Having an Intra cavity Spatial Phase Controller for Beam Control and Switching. In such prior art laser devices, steering and switching of a single mode signal are accomplished by means of integrating Intra cavity controllers disposed within an electrical contact metallization layer during laser fabrication. Hohimer however does not accommodate multiple lateral waveguide modes.
To best appreciate method and apparatus in the claimed invention disclosed hereinafter, an understanding of the fundamental characteristics of the dielectric waveguide are essential. For a complete understanding of the underlying theory of dielectric optical waveguides, “Theory of Dielectric Optical Waveguides”, Dietrich Marcuse, Academic Press 1974, incorporated herein by reference, may be referred to. In
Leung Quyen
Scott, II Robert L.
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