Coded data generation or conversion – Analog to or from digital conversion – Using optical device
Reexamination Certificate
2002-04-23
2003-12-09
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Using optical device,
C341S155000
Reexamination Certificate
active
06661361
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved optical analog to digital (A/D) converter which is able to achieve a high conversion rate and to a digital optical wavemeter which is able to generate a direct digital output of optical frequency at high measurement rates.
2. Discussion of Prior Art
The processing of signals is preferably carried out in the digital domain because digital processing is fast, accurate and reliable. However, where a system involves the reception of analogue waveforms, such as radio or microwave systems, radar systems and current television systems, it is necessary to employ an A/D converter to convert signals from the analogue to the digital domain. Currently, the performance, in particular the speed and resolution, of A/D converters is the limiting factor in the performance of certain systems, in particular radar.
The majority of known AND converters use electronic components which operate at radio frequencies (rf). Monolithic electronic A/D converters which can achieve speeds of 1 GSa/s at 8 bit accuracy are just becoming available. It has long been recognised that higher sampling rates are potentially achievable using A/D converters employing optical components.
One approach to optical A/D conversion is outlined in a paper entitled “Fibre and integrated optic devices for signal processing” which was published by H. F. Taylor in Proc SPIE Vol 176 (1979) pp 17-27. This approach uses mode locked lasers to provide fast sampling of the input analogue signal, uses optical detectors as sample and hold circuits and exploits the periodic response of integrated optic Mach-Zehnder modulators. The analogue input voltage to be digitised is applied simultaneously to the electrodes of each of a plurality of modulators and the lengths of the electrodes on successive modulators vary in a binary sequence. The response of each modulator varies sinusoidally with input voltage, with a period dependent on the length of the electrode. The outputs of each of the modulators is fed via photodetectors to an electronic comparator in order to generate a binary output varying periodically with input voltage. If the input voltage is fed to an array of such devices whose periodicities are arranged in a binary sequence, the output from the array of comparators forms a parallel binary representation of the input voltage in conventional or Gray Code format. A variation of this type of device is disclosed in U.S. Pat. No. 4,947,170.
A simplified A/D converter of this type is disclosed in EP 319, 286, in which a single Mach-Zehnder interferometer is used and signals are tapped off from different parts of the arms of the interferometer between successive stages of electrodes. U.S. Pat. No. 4,694,276 utilises a multiple wavelength optical source as a sampling source in order to provide higher bit resolution for a set number of interferometers. U.S. Pat. No. 5,381,147 uses a double frequency optical source to input a different frequency of light into the two arms of the interferometers, in order to provide an inherent frequency down conversion.
One problem with these types of device is the difficulty of making the array of modulators have sufficiently reproducible responses using the electro-optic effect to generate accurate relative phase changes, especially if an array of 8 of them is required to provide 8 bit resolution. Another difficulty concerns the power level of the input analogue voltage signal which has to be high enough to drive a plurality of electro-optic devices.
SUMMARY OF THE INVENTION
A digital optical wavemeter will generate a digital representation of the wavelength or equivalently the frequency of an input optical signal. They are used, for example to test the wavelength stability of a laser or to measure the frequency variation or CHIRP of an amplitude modulated laser diode. One type, uses a scanning Michelson interferometer and has a measurement rate of the order of 1 Hz. There is a requirement for an optical wavemeter that can operate at higher measurement rates.
According to a first aspect of the present invention, there is provided an optical analogue to digital (A/D) converter for converting an input analogue voltage signal into a digital output, comprising:
means for frequency modulating an optical signal with an input analogue voltage signal,
delay means for splitting the frequency modulated optical signal and generating at least one differentially delayed signal and at least one reference signal,
at least one combining means arranged to combine the or each differentially delayed signal with the or one of the reference signal(s), and
at least one converter means for converting a combined signal output from an associated one of the combining means to a digital output.
The amplitude of the or each combined signal will vary sinusoidally with the frequency of the frequency modulated optical signal which is input into the delay means and so will vary sinusoidally with the input analogue voltage, provided the optical frequency varies linearly with analogue voltage. The periodicity of this sinusoidal response is dependent on the length of the relative delay between the delayed optical signal and the reference optical signal with which it is combined. For example, by using the delay means to generate a plurality of differentially delayed signals and the combining means to combine them with a reference signal, a plurality of input analogue voltage dependent sinusoidal responses is generated with differing periodicities. Converter means convert each sinusoidal response into a digital electrical response. The differential delays can be chosen so that the output from each converter means represents at least one bit of a multi-bit output representative of the input analogue voltage so that the combined output of the converter means provides a direct digital representation of the input analogue voltage. In this way the present invention utilises the frequency modulation of optical signals to provide a high speed A/D converter. Conversion rates and resolutions shown in the table in
FIG. 10
can be achieved, assuming a 1% laser tuning range and other issues which are covered later. The delay means and the combining means should preferably preserve the polarization of the delayed and reference signals so that on combination of the delayed and reference signals either destructive or constructive interference occurs.
Preferably, the means for frequency modulating an optical signal with an input analogue voltage signal comprises a frequency tunable laser, preferably a singlemode semiconductor laser diode, which generates an optical signal with a frequency related, preferably linearly, to an input analogue voltage tuning signal. The input analogue voltage signal to be converted to digital form is used to frequency tune the laser and so generate a frequency modulated optical signal with a frequency directly related to the input voltage level. Thus, according to this preferred embodiment of the present invention the input voltage only has to drive the tuning of the laser and not a plurality of electro-optic phase shifters, as in known devices.
Preferably the converter means comprises at least one comparator means for comparing a combined signal output from an associated one of the combining means to a reference value and generating a 0 or a 1.
An advantage of the present invention over some known electrical analogue to digital converters is the need for relatively few comparators. This is especially true relative to state of the art flash analogue to digital converters.
Preferably, the delay means comprises at least one interferometer having a first arm for generating a reference signal and a second arm for generating at least one differentially delayed signal. More preferably, the delay means comprises a plurality of interferometers each having a first arm for generating a reference signal and a second arm for generating a differentially delayed signal. The interferometer structure can easily be implemented using optical fiber
Johnstone Alan
Lewis Meirion F.
Nixon & Vanderhye P.C.
Qinetiq Limited
Wamsley Patrick
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