1.  Field of the Invention
The invention is generally related to the area of optical data communications, and more particularly, related to add/drop optical devices or subsystems. 
2.  The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols.  Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities.  Wavelength division multiplexing (WDM) is a technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength.  Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber.  To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements. 
A tunable WDM add/drop system is one of such elements that are designed to add or drop one or more specific wavelengths or channel signals.  In a fiber optic network, there are cases of exchanging wavelengths among optical signals on xe2x80x9cloopsxe2x80x9d within networks.  The exchanges occur at points where two or more loops intersect for the purpose of exchanging wavelengths.  In particular, the exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network.  In other words, a signal would drop a wavelength to the other loop while simultaneously adding the matching wavelength from the other loop.  The adding and dropping of wavelengths often occur together.  Each wavelength is commonly referred to as a channel or data channel.  A tunable WDM add/drop system exists at the points to facilitate these exchanges. 
In general, tunable WDM add/drop systems often utilize fixed or tunable fiber Bragg gratings to provide the necessary wavelength selectivity for the add/drop function.  To add or drop a specific wavelength, the accurate control of the signal at an absolute wavelength is of high requirement.  Any deficient design in the tuning wavelength accuracy could lead to problems that include optical cross talk, signal fluctuation and numerous other undesirable effects.  These problems could be compounded if additional channels are added or dropped as part of a service upgrade.  Specifically, add/drop devices in a typical optical communications system are designed to accommodate a predetermined number of channels for adding and dropping, because the losses associated with adding and dropping must be accounted for in each of the output paths of the add/drop system.  Hence, more loss could be introduced as more channels are added and dropped. 
There have been many efforts in design absolute wavelength selectivity.  One exemplary technology is to fabricate wavelength selective elements based on recording an index of refraction grating in the core of an optical fiber, for instance, disclosed in U.S. Pat.  No.  4,474,427 to Hill et al.  and U.S. Pat.  No.  4,725,110 to Glenn et al.  However, there are many considerations with absolute wavelength selectivity.  The cost and complexity elevate considerably when the requirement on the wavelength selectivity is stringent.  There is thus a need for techniques that lead to efficient designs of add/drop devices without compromising performance thereof. 
On the other hand, in most tunable systems, during a tuning cycle, express channels or wavelengths (i.e., the channels or wavelengths not to be dropped) are dropped as the system sweeps through its free spectral range (FSR) before stopping at the desired wavelength to be dropped.  This is disadvantageous as the data information in the express wavelengths suffers a hit or momentary increase in bit error rate (BER), thus there is another need for techniques that eliminate or at least minimize such effects that occurs on the express channels during the tuning cycle. 
This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments.  Simplifications or omissions may be made to avoid obscuring the purpose of the section.  Such simplifications or omissions are not intended to limit the scope of the present invention. 
The present invention pertains to methods and apparatus for adding and dropping a desired channel or wavelength by calibrating optical tunable devices precisely for the desired wavelength.  According to one aspect of the present invention, an optical path is formed and includes two tunable devices and a circulator coupled between the two tunable devices so as to prevent possible interference to channels and wavelengths passing through the two tunable devices.  A small portion of a dropped signal is used as a reference signal to be introduced into the optical path.  A reflected signal of the reference signal is coupled out of the optical path for determination of control signals most suitably for the two tunable devices so as to achieve an accurate control over an absolute wavelength of the tunable devices. 
According to one embodiment, the reference signal is modulated by frequency modulation.  The reflected reference signal is converted to an electrical signal, wherein the electrical signal thus carries a DC component and a FM component.  Further the FM modulated reference signal is modulated by amplitude modulation such that the DC component can be lifted above a noise floor and has a better signal-to-noise ratio.  According to another embodiment, an optical slope filter is used.  The optical slope filter is a linear device and has specific reflectance and transmission characteristics for each wavelength in the passing band thereof.  Utilizing the photocurrents from two detectors associated with the optical slope filter, the wavelength of the dropped signal can be determined.  If there are any discrepancies with respect to the desired wavelength, the control signals to the tunable filters can be determined accordingly or iteratively in a signal processor controller unit receiving the electrical signal. 
The present invention also pertains to methods and apparatus for eliminating or at least minimizing interference to those channels or wavelengths (hereinafter referring to as transmitted or express channels or wavelengths) passing through tunable devices.  For most tunable systems, during a tuning cycle, express channels or wavelengths may be dropped as the system sweeps through its free spectral range (FSR) before stopping at the desired wavelength to be dropped.  This is disadvantageous as the data information in the express wavelengths suffers a hit or momentary increase in bit error rate (BER).  According to one aspect of the present invention, the wavelength selection to be dropped is done xe2x80x9coff-linexe2x80x9d and does not affect the express channels. 
According to one embodiment of the present invention, two individual optical blocks are used to provide two individual optical paths.  The two individual optical blocks are configured to maintain one path active while the other is in stand-by setup or tuning mode.  While one optical path is adding and dropping a wavelength, the other optical path can be calibrated for a new wavelength.  When an optical switch switches from one path to another, a new wavelength to be added and dropped is effectuated. 
There are many benefits, advantages and features in the present invention.  One of them is to achieve the stringent requirement on accurate controls over an absolute wavelength commonly used in the tunable devices. 
Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings.