Optics: measuring and testing – By shade or color – With color transmitting filter
Reexamination Certificate
2002-03-28
2004-12-28
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By shade or color
With color transmitting filter
C356S416000, C250S225000, C250S226000
Reexamination Certificate
active
06836330
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a wavelength detector and a polarization sensor that can be used to measure the wavelength or the polarization state of a monochromatic optical signal. Specifically, it provides a means to adjust the properties of a beamsplitter so that it is better suited to either one or the other task. This invention can be used in the fields of optics, telecommunications, and laser spectroscopy.
BACKGROUND OF THE INVENTION
Wavelength measurement devices that are used to detect, monitor and control a laser's wavelength and polarization are emerging as integral components of laser optical systems. There are growing demands for wavelength and polarization measurement devices because the telecommunications, spectroscopic, and analytical chemistry industries have grown to the point where accurate management of these properties is essential. The development of dense wavelength division multiplexing (DWDM) systems in telecommunications, and high sensitivity spectroscopic systems in analytical chemistry has led to a demand for wavelength and polarization measurement and control systems that are fast, accurate, give real time wavelength readouts, and are inexpensive.
In optical telecommunications systems, for example, the laser light sources have to be held to a wavelength that moves by less than 1 GHz and at a constant polarization if they are to operate in DWDM systems that have wavelength spacings of 100 GHz or less. This process is achieved using wavelength and polarization measurement devices that samples the output of a laser and provide a signal that can be used to adjust the laser's properties to the correct value and limit its deviation from that value. Additionally, wavelength measurement devices are used to accurately switch the laser wavelength from one telecommunications channel (ITU channel) to another while maintaining the polarization state. Rapid and accurate wavelength switching (microseconds to nanosecond switching times) over a wide wavelength range (40 nm at 1550 nm) is emerging as a new, and essential requirement for DWDM systems.
In spectroscopy and analytical chemistry, concentrations of chemical constituents or molecular and atomic components can be easily and inexpensively measured with tunable diode lasers provided there exists an accurate and reasonably rapid means to tune the laser to the required wavelength.
Furthermore, the polarization of the laser light on entrance and exit of an optical system needs to be monitored and controlled so that its trajectory through the system can be optimized.
Historically, wavelength measurements have been performed in several ways.
1. A prism or diffraction grating is used to disperse different wavelengths into different directions, each direction corresponding to a unique wavelength. By scanning the directions with a slit and a detector sensitive to light intensity, the wavelength properties of an optical signal can be determined.
2. Alternatively, a scanning optical interferometer can be employed, typically a Michelson interferometer. The wavelength of an optical signal is determined by changing the length of the interferometer by a known amount, and counting the number of interference fringes (a narrow bandwidth optical signal is assumed).
To determine the wavelength of an optical signal with very high accuracy, these measurement techniques must be augmented by a calibration measurement where:
a) the unknown wavelength is compared to a spectroscopic signal; or
b) the unknown wavelength is compared to stabilized laser signal.
Both measurement methods required mechanical movement. Therefore, considerable time is required for the measurement to be performed, typically on the order of seconds. Also, due to requirements for stable mechanical accuracies at micron dimensions, expensive and bulky mechanical components are required. With accurate calibration, great accuracies can be achieved with these techniques, better than 1*10
−13
meter resolution, however the time to take the measurements limited their usage in many applications. The need for mechanical stability and repeatability, combined with the complexity of calibration measurements, ensures that the measurement devices are bulky and costly, restricting their usage.
Methods have been developed that partially solve some of these problems. If measurements are to be performed on a monochromatic signal (i.e., one where the bandwidth is very small compared to the center frequency) at a specific wavelength, a dielectric bandpass filter or a Fabry-Perot etalon can be used in place of a wavelength reference. When an optical signal is incident on a dielectric filter or a Fabry-Perot etalon at or near a resonance, transmission through (or reflection from) the filter or etalon is determined by the wavelength of the optical signal and the resonant characteristics of the dielectric filter or etalon. This is illustrated in
FIGS. 1 and 2
which show a dielectric filter
11
a
,
FIG. 1
, and an etalon
11
b
,
FIG. 2
, in a wavelength measurement system. The light beam is applied to a beamsplitter
12
. Light from the beamsplitter is applied directly to photodiode
13
and to a second photodiode
14
after it has been transmitted (or reflected) by the dielectric filter
11
a
or etalon
11
b
. The outputs of the photodiodes are then compared. The use of a beamsplitter and photodiodes eliminates any error due to changes in beam intensity. A measurement of the transmission (or reflection) therefore determines the wavelength of the light, except that the same transmission (or reflection) can correspond to different wavelengths. When the wavelength of the source is approximately known, as is sometimes the case, this is not a problem. More generally, it means that the wavelength has been determined to be one of several values. For the case of a dielectric bandpass filter with a single transmission peak, the wavelength is determined to be one of two values. In the case of a Fabry-Perot etalon with multiple resonances, the wavelength can be one of multiple values.
These methods make possible wavelength locking of a monochromatic laser light source to a desired wavelength. When the wavelength of the source differs from the desired wavelength, that selected by the filter or etalon, the transmission of the light through the filter or etalon differs from the desired transmission as determined by the output of the two photodiodes. The difference is used to provide an error correction signal that can be used to adjust the wavelength of the laser source to the correct value.
If a bandpass dielectric filter is used for wavelength measurement or wavelength locking, the disadvantage is that it works over a limited wavelength range (typically <2 nm). This means that it cannot be used for applications such a tunable telecommunications lasers where wide wavelength tunability is an important requirement. It also has the disadvantage that the wavelength resolution is dependent on the quality of the thin film coating, which often has ripple or etalon effects that limit the accuracy with which the transmission can be measured.
If a Fabry-Perot etalon is used for wavelength measurement or wavelength locking, it allows wavelength measurement and wavelength locking at a number of different wavelengths corresponding to different resonances of the Fabry-Perot. A disadvantage is that measurement accuracies are restricted by mechanical and thermal stability. Another disadvantage is that Fabry-Perot etalon have multiple resonant transmission peaks and therefore multiple wavelengths that give the same transmission. Thus, the absolute wavelength of the measurement cannot be determined. An additional disadvantage is that the measurement accuracy obtainable with an etalon is very poor at wavelengths situated halfway between its resonances. For telecommunications systems, Fabry-Perot etalon dimensions are inversely proportional to channel spacings.
Polarization sensors have been either devices that preferentially direct one polarization of light along o
Friberg Stephen R.
Harb Charles C.
Dorsey & Whitney LLP
Lambda Control, Inc.
Punnoose Roy M.
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