The present invention relates generally to multiplexing of communications signals and more particularly to the wavelength division multiplexing of optical signals in fiber optics communications.
Wavelength Division Multiplexing (WDM) is a way of increasing the capacity of an optical fiber by simultaneously operating at more than one wavelength within a single optical fiber. Multiple optical signals of different wavelength are transmitted in the same direction over one strand of fiber, and the signals are later separated by wavelength at the distant end. In order to establish some standards for WDM operations, the International Telecommunication Union (ITU) has proposed successive systems of standardized frequencies to be used as channels for optical telecommunications, with each system incorporating more and more channels, usually with smaller and smaller wavelength separation between the channels. This system of channels is spoken of as the xe2x80x9cITU gridxe2x80x9d and currently includes 80 channels utilizing a wavelength range centered around 1550 nm (193,300 GHZ ) with a channel spacing of approximately 0.4 nm (50 GHZ). An ITU grid with 50 GHZ frequency spacing is shown in FIG. 1 with the fringe order pattern from a Fabry-Perot interferometer using an etalon of appropriate parameters superimposed on the grid. There has also been a system proposed which uses channel spacing of 25 GHZ (0.2 nm). It will be easily apparent that the smaller the spacing is between channels, the more sensitive a multiplexed channel system will be to fluctuations that will cause the channel frequencies to drift away from the target grid frequencies. In a system where the frequency spacing is 100 GHz, the variation of from the grid frequency of 0.01 nm (1.2 GHz) would be undesirable, but perhaps may not be disastrous. In a 50 GHz system, a 0.01 nm variation would impair the performance of the system, and in a 25 GHz system, such a deviation would likely be disabling.
An interference filter is a type of tool that is often used to separate multiple wavelengths of light that are included in a beam of light. A Fabry-Perot Interferometer is one type of interference filter that is often used for wavelength filtering and separation. Interference filters operate by providing a pair of mirrored surfaces with a cavity formed between them. Incident light undergoes multiple reflections between the mirrored surfaces, which typically reflect greater than 95% of the light at each surface. The incident and reflected wave interfere with each other constructively or destructively depending on their phase relationship. Where there is no phase difference between successive wave, constructive interference is produced and a maximum is produced in the transmission portion. Where the phase are 180 degrees out of phase, destructive interference occurs and a minimum is transmitted. A maximum occurs when the round trip optical path is some integer multiple of whole wavelengths, and also depends on the thickness of the cavity (d), the index of refraction of the cavity material (n), and the angle of incidence (0), which are related by the formula:
2d n cosxcex8=mxcex,
where m is an integer, often termed the order number and xcex is the wavelength of the light. The parallel rays of each wavelength are often focused by a lens in order to produce a familiar ring pattern. The result is a series of transmission peaks of separated wavelength. The separation distance between adjacent peaks is equally spaced when plotted with respect to inverse wavelength, and is called the Free Spectral Range (FSR).
Etalons are special Fabry-Perot interferometers which have fixed spacing between the reflective surfaces, thus the thickness of the cavity d is therefore not subject to direct parallel variation. However, the etalon may be tilted, changing the angle of the etalon relative to the angle of incidence of the light beam, which thus increases the optical path length. This allows the etalon to be xe2x80x9ctunedxe2x80x9d over a limited range to alter the peak transmission wavelengths. FIGS. 2 and 3 show etalons as used in the prior art. FIG. 2 shows the incident light striking the etalon at an angle xcex8, and FIG. 3 shows the same etalon tuned at the same angle xcex8, thereby increasing the optical path length n to angle tune the etalon.
However, angle tuning of an etalon introduces other problems caused by the insertion loss due to the variation in angle. Besides the difficulties of producing very tiny variations in angle, when the etalon is tuned at a small angle, the output beam can become oblong in shape, with non-uniform beam intensity distribution. As this angle increases, this effect becomes more pronounced. When optics are used to collect the output light from the etalon, a large insertion loss variation is often seen. This variation is typically from 1-4 dB.
The variation of etalon insertion loss also commonly causes the operating point of the output spectrum to shift by as much as 10 pm (0.01 nm=1.2 GHz). As discussed above, errors of this magnitude can seriously interfere with operation of systems which use 25 Ghz frequency separations and even with 50 GHz systems.
Thus, there is a great need for a etalon which is usable in a multi-channel wavelength locking system which does not produce such large variations in insertion loss, beam quality, and wavelength shift.
Accordingly, it is an object of the present invention to provide a multi-channel wavelength locking system with better ITU setting repeatability.
Another object of the invention is to provide a multi-channel wavelength locking system with improved temperature stability.
And another object of the invention is to provide a multi-channel wavelength locking system which produces smaller variations in insertion loss.
A further object of the present invention is to provide a multi-channel wavelength locking system which produces smaller variations in beam quality.
An additional object of the present invention is to provide a multi-channel wavelength locking system which produces less wavelength shift.
Yet another object of the present invention is to provide a multi-channel wavelength locking system which can be used with wavelengths which are separated by as little as 25 GHz or less.
A yet further object of the present invention is to provide a tunable etalon which has reduced manufacturing costs due to more relaxed dimensional tolerances in the parts.
Briefly, one preferred embodiment of the present invention is a wavelength locker for providing wavelength-locked multi-channels signals, including a number of radiation sources providing beams of radiation at wavelengths separated by a predetermined spacing, each spaced wavelength being a channel which is connected to a multiplexer. One or more wavelength lockers produce equally-spaced spectral lines which are tunable wavelength locker includes one or more etalons which includes a gas-tunable medium having a variable optical index of refraction. The etalons may either be transmissive, or reflective with respect to which surface the major portion of the light exits. The etalons are tuned by varying the pressure or composition of the gas-tunable medium until the spectral lines of the etalon align with the ITU grid. The gas properties are then fixed by at least temporarily sealing the etalon enclosure.
In use in a multi-channel system, beam splitters divert a portion of the beams of radiation into the wavelength lockers. Detectors then receive the spectral line output of the wavelength lockers and detects shifts in wavelength of the spectral line output to generates control signals. The wavelength of each of the radiation sources is then adjusted in response to the control signals.
The output of the detector may also be normalized in order to isolate the variations which are attributable to shift in wavelength. The channels may be sequentially sampled and the outputs adjusted by a single wavelength locker or there may be separate wavelength lockers for each input channel.
A method for locking the wavelengths of a plurality of radiation sources to a wavelength reference is also included.
An advantage of the present invention is that it provides improved ITU setting repeatability.
Another advantage of the present invention is that it provides improved temperature stability.
And another advantage of the present invention is that it produces very small variations in insertion loss and beam quality.
A further advantage of the present invention is that it can be used with wavelengths which are separated by as little as 25 GHz or less.
A yet further advantage is that the present invention can provide a tunable etalon which has reduced manufacturing costs due to more relaxed dimensional tolerances in the parts.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.