Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2000-12-20
2002-12-10
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C359S199200, C359S240000, C372S032000
Reexamination Certificate
active
06493131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the wavelength-locking of optical sources such as lasers, and more specifically to locking the sources at wavelengths which are separated from each other by a fixed amount.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as SONET have been developed for the transmission of data over optical fibers.
Fiber optic communications systems utilize a number of basic building blocks. For example, almost all, if not all, fiber optic systems include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and transmits the resulting optical data signal across the optical fiber to the receiver. The receiver recovers the original data from the received optical data signal. Other basic building blocks include optical amplification, add-drop multiplexing, wavelength filtering, and wavelength stabilization and wavelength locking of sources.
As one example of wavelength locking, heterodyne receivers typically will require the wavelength locking of two optical signals. These receivers typically have better noise performance than direct detection receivers but they require the use of an optical local oscillator. The optical local oscillator is mixed with the incoming optical data signal. This effectively frequency shifts the optical data signal from its original optical carrier frequency down to the difference frequency between the optical data signal and the local oscillator. Typically, the result is an electrical RF signal, which is then processed to recover the data. For efficient operation, the difference frequency between the two optical signals should be held fairly constant. In other words, the optical local oscillator should be wavelength-locked to the optical data signal with an approximately constant frequency offset.
In some of the examples given below, each of the two optical signals has a wavelength of approximately 1.55 micron with a frequency offset of approximately 11.55 GHz between the two signals. In these examples, the 1.55 micron wavelength is selected because the example application is a fiber optic communications system which uses this wavelength. The 11.55 GHz offset is selected because the example application is designed to use this frequency offset in order to recover the data transmitted over the fiber. For further details on such a system, see co-pending U.S. patent application Ser. No. 09/474,659, “Optical Communications System Using Heterodyne Detection”, by Ting K. Yee and Peter H. Chang, filed Dec. 29, 1999; and U.S. patent application Ser. No. 09/728,373, ““Optical Communications System Using Multiple Heterodyne Detection”, by Ting K. Yee and Peter H. Chang, filed Nov. 28, 2000. However, these examples are not meant to limit the invention. For example, heterodyne receivers may also be used for free space optical communications (e.g., in satellite communications), optical data storage and recognition, or coherent imaging and holography. The wavelengths and frequency offsets required from the wavelength locker in each of these applications will depend on the specifics of the application.
One approach to wavelength locking uses conventional phase locked loop principles to phase lock the actual frequency difference between the two optical signals to a stable reference sinusoid. This approach typically requires phase/frequency detectors and a source for generating the stable reference sinusoid but these components can be both complex and expensive. In addition, phase locked loops are used primarily to lock signals which are fairly pure sinusoids and to lock the signals with a high degree of accuracy (e.g., to actually lock them in phase). Many wavelength-locking applications, including the heterodyne receiver application discussed above, simply do not fit this profile. For example, in the heterodyne application, the incoming optical data signal is an optical carrier which has been modulated with data so it will not be a pure sinusoid and, as a result, the difference frequency signal to be locked to the reference sinusoid likely also will not be a pure sinusoid. In addition, the local oscillator in heterodyne receivers typically does not have to be locked to the optical data signal with the accuracy intended by phase locked loops. Thus, the super-accurate locking simply adds unnecessary expense and complexity. This is compounded by the fact that many fiber optic systems will require large numbers of the wavelength locking device.
Another approach to wavelength locking which has been attempted is based on gas cells. Many of these approaches try to take advantage of the physical properties of various gases. Thus, through non-linear optical or other physical interactions in the gas, two optical signals are locked to each other or two optical signals at slightly different wavelengths are generated from a common source. However, these approaches typically rely on the physical properties of gases and, therefore, are limited in their applicability. For example, if a particular gas has physical properties which allow the locking of &lgr;
1
to &lgr;
2
, this is fine if the two wavelengths of interest are &lgr;
1
and &lgr;
2
but is irrelevant if any other wavelengths are desired. It can be especially problematic if it is desirable to tune the wavelength locking over a range of wavelengths and a range of frequency separations, as is often the case. In addition, these approaches require the use of a gas cell. The handling of these cells and the gases used to fill them adds expense and complexity.
Thus, there is a need for approaches to wavelength locking of optical signals which are simple, inexpensive and matched in performance to the requirements of the application, particularly since many applications will require a large number of wavelength-locking devices.
SUMMARY OF THE INVENTION
In accordance with the present invention, a device is used for wavelength locking a wavelength-variable optical signal to a target wavelength. The target wavelength is offset from a wavelength of an optical reference signal by a preselected frequency offset. The device includes the following components coupled in series: a photomixer section, a frequency filter and comparison circuitry. The photomixer section includes a square law detector, such as a photodiode. The photomixer section mixes the wavelength-variable optical signal with the optical reference signal to produce a beat component and also produces a frequency test signal from the beat component. When the wavelength-variable optical signal is at the target wavelength, the frequency test signal will be located at a target frequency. The frequency filter has a transfer function which varies monotonically (i.e., either increasing or decreasing) around the target frequency. The frequency filter a
Chang Peter H.
Tarng Shin-Sheng
Yazhgur Slava
Yee Ting K.
Fenwick & West LLP
Kestrel Solutions, Inc.
Lee John D.
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