Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-19
2001-08-28
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S254000
Reexamination Certificate
active
06282005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical communications systems, such as those used in telecommunications, and more particularly, to broadband or high speed digital communications systems of the type carrying information by the optical modulation of a light source via a surface plasmon wave device.
2. Description of Background Art
Optical communication systems are used with regularity today for many telecommunications systems. One such system of interest is the cable television system which is essentially a wired broadcast system, or point to multipoint system, which uses lightwaves over optical fiber cables to carry a broadband signal modulation. These systems modulate the laser light with 80-110 channels of programming with a signal which extends from about 50 MHz to about 1000 MHz or greater. In many systems, there is a reverse set channels in the band from 5 MHz to 50 MHz.
The optical communication systems of the cable television systems comprise a plurality of laser transmitters with a source frequency of either 1310 nm or 1550 nm which is modulated with the broadband signal which can be made up form a number of sources, such as satellite downlinks, terrestrial links, local programming, and UHF and VHF rebroadcasts. The modulated light is carried over optical fibers with perhaps some regeneration or amplification before being demodulated by an optical receiver at the other end of the optical fiber and thereafter distributed to a number of subscribers in a local geographic area around the receiver.
Because of the complexity of the broadband signal and its bandwidth, a common method for modulating the laser light is linear modulation of the intensity or amplitude of the light. This can be generally accomplished in one of two ways, either by direct modulation or by external modulation.
In direct modulation, a laser diode is biased with a current which is then mixed with the broadband signal to produce the intensity modulation. There are some problems with this technique including finding laser diodes with a linear current to light transfer characteristic. A very small percentage of predicable laser diodes can meet the stringent requirements for linearity, noise and output power which are needed in modern cable television systems. One solution has been to use less linear laser diodes but to compensate their nonlinearities by predistorting the broadband signal before direct modulation. While this concept has proven to be relatively effective, there are limits to the compensation as wider temperature range, operational power and bandwidth are required. Alternatively, external modulators, particularly of the Mach Zehnder (interferrometric) type have been utilized, where after a laser source signal has been generated, its optical output is modulated with a broadband signal applied to the external modulator. These systems however also produce nonlinearities in their transfer characteristics. In addition, there are inherent losses in the coupling of the source laser light to the modulator and its transmission through the modulator, thereby reducing effective operational power.
A surface plasmon wave (SPW) is an electromagnetic wave which propagates along the interface between two materials having dielectric constants of opposite signs, e.g., a metal and a dielectric layer. The polarization of a SPW is transverse magnetic (TM) and its electric field is perpendicular to the propagation interface. SPWs can be analyzed by techniques used for TM optical modes since they obey identical field equations and satisfy the same boundary conditions. Unique features of SPWs are that almost all of their energy is concentrated at the dielectric/metal interface and their propagation characteristics are very sensitive to environmental changes in their proximity.
Optical plasmon wave structures can be made by employing a device which couples an optical guided lightwave into a SPW. By controlling the amount of power coupling between the optical signal and the SPW, optical power may be dissipated producing an attenuation of the optical signal. If the amount of coupling is controlled electrically, a variable attenuator (modulator) device can be provided. Because of the extremely small interaction lengths needed, the optical plasmon wave modulator can be a very compact device which can be implemented as an integrated optics structure. This small size and other advantages of the device can produce substantial benefits with its use in the electronics and communications areas. One particularly advantageous plasmon wave modulator is disclosed in co-pending application Ser. No. 09/048,489, filed Mar. 26, 1998 in the name of E. Anemogiannis, and entitled “ ”, which application is assigned commonly with this present application. The disclosure of Anemogiannis is hereby incorporated by reference.
SUMMARY OF THE INVENTION
The invention provides novel broadband or high speed digital optical communications systems including a SPW modulator device.
In a first embodiment, a broadband communications system includes an optical transmitter coupled to an optical fiber communications link which carries an optically modulated light information signal to an optical receiver. In a first form, the optical transmitter includes a laser light source which is optically coupled to a SPW modulator which has been particularly adapted for analog broadband communications by selecting its transfer characteristic. A broadband electrical signal containing a plurality of information channels, for example CATV channels, is applied to the SPW modulator electrodes as the information signal. The modulation signal varies the power coupling of the guided laser light source signal into a SPW in the modulator and results in an intensity modulated optical signal which is output to the optical fiber for transmission to the optical receiver of the system.
Important advantages of the SPW modulator in this embodiment include its inherent linearity over previous devices, particularly external modulators such as the Mach Zehnder modulator. This provides a less complex system by lessening the necessity for predistortion circuits and other compensation techniques in some cases and reducing the complexity and precision of most others. In many applications, the external compensation for the device can be eliminated, or at least significantly reduced in most applications to provide less distortion and better over all performance. The low internal losses of the device permit average power lasers to effectively drive a SPW modulator and provide optical signals above the required communications link budgets. Additionally, the SPW modulator produces a low return signal because of its high directionality. This reduces the complexity of the laser transmitter by eliminating the need for an isolator between it and the light source to stabilize the laser. In addition, The small size of the SPW modulator is advantageous and allows it to be provided as integrated optical structures of relatively high densities.
In a second embodiment, an optical communications system includes a high power laser coupled to an optical splitter to divide its output power in two or more optical source outputs. Each optical source output is then used to drive an associated analog SPW modulator. Each of the modulators modulates its optical source signal with an information signal. After optical modulation, each modulated lightwave is coupled to a corresponding one of a plurality of optical fibers for transmission to one of a plurality of optical receivers. In this manner, several information signals can be communicated over the system using only one laser source. A net benefit from using one higher power laser, rather than several lower power ones, is one of cost, purity and similarity of the several signals. This configuration is also enhanced by the lower loss, higher linearity and other attendant advantages of the SPW modulators.
Additionally, for either the first or second embodiment a high speed digital optical communicati
Anemogiannis Emmanuel
Thompson Leo J.
Morris Manning & Martin LLP
Pascal Leslie
Phan Hanh
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