Abstract:
A novel wavelength measurement method wherein an optical pulse is launched into a dispersive medium with known dispersion properties, such as a dispersion compensating fibre or a dispersion compensating Bragg grating. The specific wavelength of the dispersion-induced light beam is obtained by measuring the propagation time delay through the dispersive medium and relating that propagation time to the calibrated dispersive medium.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to monitoring and measuring optical sources, and more particularly, to wavelength measurement of an optical source. 
     BACKGROUND OF THE INVENTION 
     The widespread and exponential growth of the communication network systems during the past few years has provided for extensive proliferation of optics-based designs and systems. Due to the higher bandwidth and lower losses associated with optical signal transmission, optoelectronic systems are increasingly becoming the prevalent platforms for the implementation of many high speed network communication systems. 
     Because future high capacity services will have many different optical channels, methods that use the same equipment to monitor and measure the wavelength of multiple channels are preferable since they share the total cost among numerous channels. 
     The development of a fast and accurate wavelength measurement scheme is also desirable for a number of applications, such as distributed feedback lasers used in wavelength division multiplexed (WDM) communication systems or wavelength tunable lasers, wherein one or more wavelengths need be measured and monitored in order to adjust and stabilize the wavelength of the source. 
     A customary method of determining the wavelength of a light source is to split the signal into two paths and observe the interference pattern between the signal and a delayed version of itself, a technique commonly referred to as the Michelson interferometer technique. The wavelength of the input signal can then be obtained by carefully comparing the period of the zero-crossings of the resulting waveform with the waveform of a known standard. 
     The Michelson interferometer method requires a highly accurate laser wavelength reference and has moving mechanical parts that are quite bulky and necessitate fastidious alignment and calibration. Also, in the Michelson interferometer, the ratio of the index of refraction at the reference wavelength to the index of refraction at the unknown wavelength is a function of the ambient environment (such as humidity, gas content, temperature, etc.) which ultimately affects the accuracy of the measurement. In addition, erroneous results may be obtained if a modulated signal is present at the input of the Michelson interferometer. 
     Various other existing wavelength measurement techniques, such as the Static Fabry-Perot, the Frizeau interferometer and wavelength discriminators have been proposed, but they fail to provide a practical solution to alignment and detection problems commonly associated with accurate wavelength measurement. For instance, the Static Fabry-Perot interferometer has filters with repeated bandpass effect. The repeated bandpass phenomenon causes inaccuracy in the measurement since ascertaining which bandpass is being used in the repeated filter response is difficult. The Fizeau interferometer is only optimum for measuring light sources with low frequency modulation and suffers from poor sensitivity. Although wavelength discriminators offer a simple and cost effective solution to wavelength measurement, this comes at the cost of reduced performance and accuracy. 
     Hence, there is a need for a simple and accurate wavelength measuring technique. Preferably, the new method would eliminate the alignment and calibration requirements associated with mechanical parts used in many existing wavelength measurement set-ups and would be insensitive to changes in the testing environment. As well, the new technique would desirably be functionally precise when used for measuring modulated signals or in combination with optical filters. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these key issues by providing a robust, fast, accurate, compact and potentially low cost method and apparatus for determining the wavelength of a particular light source which can be easily integrated into existing optoelectronics systems. 
     The invention can be used (a) as a means for determining the wavelength of a light beam, (b) as a wavemeter used for calibrating the wavelength steps of a tunable laser. A person skilled in the art may readily devise various other applications u sing the principles of th e invention. 
     The present invention involve s th e measurement of the time it takes for a pulse of light to propagate through an optical dispersive medium having known dispersion properties relative to wavelength. This propagation delay time can then be translated to wavelength using the calibrated properties of the dispersive medium. 
     In the wavelength measurement method using dispersion timing, the entire system consists of static components that are less bulky and less sensitive to inaccurate measurement than mechanically moving parts. This in turn offers great flexibility in design and provides for excellent structural integration in the design and implementation of a complex optical architecture. Also, another interesting aspect of the current invention is its design simplicity, providing for a robust, easily implementable and wavelength measurement system. Because this novel approach provides for precise optical monitoring of the wavelength, it becomes an optimal platform for wavelength stabilization to reduce cross-talk for use, for instance, in a WDM environment. 
     Yet another added advantage of the proposed method is that increasing the resolution only comes at the cost of reduced sensitivity, and not increased complexity, as with mechanical wavemeters. 
     Another advantage over the Michelson wavemeter is that, because it has no moving parts, calibration is easily maintained and variations of dispersion with temperature can also be calibrated. 
     Other aspects and advantageous features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a wavelength measurement system according to the invention using a dispersion compensating fiber (DCF) in transmission mode; 
     FIG. 2 is a block diagram of a wavelength measurement system according to the invention using a dispersion compensating Bragg grating (DCG); and 
     FIG. 3 is a block diagram of a wavelength measurement system according to the invention using a DCF in reflection mode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an optical signal source  10  has an output connected through a fast optical switch  12  to one end of a dispersive medium in the form of a DCF  14 , the other end of which is connected to a fast detector  16 . An output of the fast detector  16  is connected to a first input  19  of process electronics circuitry  20  which may be a high speed oscilloscope from Hewlett Packard or Tektronix. A timing circuit  18  which may be a programmable clock/pulse generator, has a first output  21  which is connected to the optical switch  12  and a second output  22  which is connected to a second input  23  of the process electronics  20 . 
     In operation, an optical signal from source  10  is gated by the fast optical switch  12  in response to a trigger signal from the timing circuit  18 , thereby creating an optical pulse composed of the wavelength which is to be determined. The time-of-flight (start time) of the pulse is essentially the time the trigger signal fires and that is stored in the process electronics  20 . As the optical pulse propagates through the DCF  14 , its propagation speed depends upon the chromatic dispersion of the DCF at the particular wavelength of the pulse. The fast detector  16  detects the arrival of the dispersion-affected pulse and signals the time-of-arrival to the process electronics  20 . As the time-of-flight is already stored in the process electronics  20 , the time for the pulse to propagate through the DCF is obtained in the process electronics  20  by subtracting the time-of-flight from the time-of-arrival. If the dispersion vs wavelength of the DCF is calibrated then the propagation time translates to a measurement of the wavelength. The sensitivity of this technique expressed as the wavelength resolution, Δγ can be obtained from the expression:          Δ                 λ     =         Δ                 t                    FOM   ×   DR                              
     where Δt is the resolution of the time measurement measured in seconds, DR is the dynamic range of the detector (maximum allowable loss in dB), and FOM is the figure of merit of the dispersive device measured in ps/nm/dB. The FOM can be used to characterize the measurand quality from the losses&#39; point of view. The FOM specifies how far the fast detector is from the quantum light performance, or in other words, how much the noise overwhelms the minimum possible output signal. 
     The optical signal source  10  may emit optical signals of different wavelengths rather than a single wavelength. The operation of the circuit is unchanged except that the fast detector would detect different times of arrival for the different wavelengths and each time-of-arrival would be compared with a single time-of-flight to obtain the time for each wavelength to propagate through the DCF. In this way each wavelength can be determined. 
     Referring to FIG. 3 this shows a system identical to that of FIG. 1 except that the DCF  14  is arranged to operate in the reflection mode instead of the transmission mode which effectively doubles the length of the DCF. Thus, an optical splitter orcirculator  30  is located between the first optical switch  12  and the input end of the DCF  14  and a mirror  40  is positioned at the other end of the DCF. In this configuration the fast detector  16  is fed by an output  31  of the circulator  30 . Referring to FIG. 2, this is similar to FIG. 3 except that a dispersion compensating Bragg grating (DCG)  14 ′ replaces the DCF and mirror of FIG.  3 . 
     The operation of the systems of FIGS. 2 and 3 is similar to the transmission mode of FIG. 1 except that the optical pulses emanating from the fast optical switch  12  and passing into the dispersive medium are reflected back out the same end and passed from the circulator  30  to the fast detector  16 . 
     Commercially available dispersion compensating fibres (DCFs) or dispersion compensating Bragg grating fibres (DCGs) have very low insertion loss (typically 5 dB), thereby making them an attractive choice for use as the dispersive medium. For example, for a DCF with a linear dispersion of 100 ps/nm/km in the 1550 nm wavelength window and loss of approximately 0.5 dB/km, the FOM is about 200 ps/nm/dB. If the fast detector has sensitivity of 20 dB below its input power, then it is possible to obtain a dispersion value of 4000 ps/nm. Hence, in order to measure a wavelength with 0.025 nm resolution, the propagation delay needs to be measured with a 100 ps resolution. This is well within the realistic measurement range of the currently available optoelectronic detectors and timing circuits. 
     DCGs are expected to have higher FOMs than DCFs but they are presently limited to a narrower wavelength range than DCFs and they do not yet have a smooth dispersion profile. These problems are related to present manufacturing limitations and are not expected to be fundamental limitations. 
     It is noted that the resolution of the method described above can be increased by increasing the dispersion or by shortening the pulse duration (width). 
     Once dispersion is calibrated with a known reference source, it will not vary significantly over time. In an alternative embodiment, an on board reference source that is calibrated to a gas absorption line, is used to check the accuracy of the technique. 
     Persons skilled in the art will appreciate that there are yet more alternative implementations and modifications possible to implement a wavelength measurement technique by dispersion timing, and that the above implementation is only an illustration of this embodiment of the invention. The scope of the invention, therefore, is only to be limited by the claims appended hereto.