Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1999-01-14
2001-09-11
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S032000, C372S034000
Reexamination Certificate
active
06289028
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lasers and more particularly to apparatus for monitoring and controlling the wavelength of the laser radiation.
BACKGROUND OF THE INVENTION
In Dense Wavelength Division Multiplexing (DWDM), multiple light beams, each of a different wavelength and representing a distinct channel for the transmission of data, are combined (multiplexed) to propagate as a beam along a single optical beam path, such as a beam path defined by an optical fiber. The amount of information that can be carried along the beam path, e.g., by the fiber, is thus greatly increased. At the receiving end of the beam path the channels are de-multiplexed and appropriately demodulated. Each channel employs a laser light source, typically a semiconductor laser, such as a distributed feedback (DFB) laser or a distributed back reflection (DBR) laser, that produces a beam at the wavelength of that channel. A modulator modulates the beam to carry the channel's data. The development of a practical wide band amplifier that can be inserted in the optical beam path, such as the erbium doped fiber amplifier, has made DWDM a reality and spurred much technical innovation in related devices, such as multiplexers, demultiplexers, modulators, etc.
One important concern with DWDM systems is achieving higher data rates, such as by increasing the number of channels. The wavelength stability of the laser sources limits number of channels. The wavelength of a laser light source typically drifts over time, and the channels cannot be so closely spaced such that the wavelength of one channel laser source drifts too close to the wavelength at which another channel light source is operating. Information will be lost. Accordingly, the better the stabilization of the wavelength of the laser sources, the more densely the channels can be packed within a particular wavelength range.
For example, the wavelength of a DFB laser is known to be affected by several factors, such as laser source current, laser temperature, and aging of the laser. In most practical applications, the wavelength of the laser is stabilized by regulating the temperature of the laser, because changing the current affects the overall system power budget and provides a more limited range of wavelengths over which the laser can be tuned. DFB lasers are typically temperature stabilized using a thermal control loop consisting of a thermistor to sense the device temperature, an electronic feedback loop, and a thermoelectric cooler (TEC) that responsive to feedback adjusts the temperature of the laser. Thermal regulation is employed because it also protects the DFB laser from overheating, and helps to stabilize power output of the laser. However, laser drift is still a concern and limits the density of channels. Improvement is required to more densely pack channels, and hence obtain higher data rates, in DWDM systems.
Another important concern in implementing a DWDM system is wavelength management and optimization. System designers face difficult problems when optimizing a DWDM link. They need to minimize losses, yet maintain adequate channel isolation and consider other parameters relating to wavelength. Several components within a DWDM system, such as optical amplifiers (e.g. an erbium doped fiber amplifier), multiplexers, demultiplexers, optical isolators, add/drop multiplexers and couplers, are sensitive to wavelength. Fiber dispersion is also a consideration. Control, e.g., tuning, of the wavelength of individual channels within available channel bandwidths is not typically fully realized as an optimization tool.
Yet another concern in operating such systems involves monitoring the laser radiation used for some, or all, of the channels. As noted above, the wavelength is known to vary with the electrical current supplied to the laser, the temperature of the laser, and with the aging of the laser. Monitoring of the wavelengths can be useful in maximizing performance of the overall information transmission system.
The problems of wavelength regulation, control, and monitoring have not been satisfactorily resolved. Better wavelength monitoring, regulation and control will allow higher performance laser information systems that are more readily designed, maintained and modified, and denser packing of channels, and hence higher data rates. Fewer types of lasers could achieve a given number of communication channels. Existing methods and apparatus are not entirely adequate.
Accordingly, it is an object of the invention to address one or more of the aforementioned disadvantages and drawbacks of the prior art.
Other objects of the invention will in part be apparent and in part appear hereinafter.
SUMMARY OF THE INVENTION
The present invention achieves these and other objects by providing an apparatus for monitoring the wavelength of laser radiation to produce an error signal representative of the deviation of the wavelength from a set-point wavelength. The error signal can be used as part of a feedback loop to monitor, stabilize, tune, or otherwise control the wavelength of the laser, for example, by controlling the temperature of the laser or the current supplied to the laser. Modifying the manner in which the error signal is produced biases, or varies, the set-point wavelength, hence tuning the wavelength of the laser.
In one aspect, the invention provides an apparatus for monitoring the wavelength of laser radiation, including an optical beam splitting apparatus for splitting first and second split beams from a beam to be monitored. A first optical filter is responsive to the first split beam for producing therefrom a first filtered beam in accordance with a first spectral filter function; a second optical filter is responsive to the second split beam in accordance with a second spectral filter function for producing a second filtered beam therefrom.
The first filter includes a substrate having a filter disposed therewith, and a surface of the substrate is disposed for receiving the first split beam at a non-zero angle of incidence. The non-zero angle can be selected such that the first and second spectral filter functions cross at a selected crossing wavelength, and such that they can define a capture range of wavelengths that includes at least a portion of the bandwidth of a channel of a DWDM system. The beam comparison element compares the first and second filtered beams for producing an error signal representative to the deviation of the wavelength of the beam from a set-point wavelength, which can correspond to the crossing wavelength.
In another aspect, the invention provides a wavelength-stabilized laser system that includes a laser for producing a beam of laser radiation having a selected wavelength, and an element for splitting first and second laser light beams from the beam of laser radiation. A first optical filter is disposed for receiving at least a portion of the first split beam. The first filter produces a first filtered beam, and a photodetector is arranged for receiving at least a portion of the first filtered beam and for producing a first detected signal. A second optical filter is disposed for receiving at least a portion of the second split beam and for producing a second filtered beam, and a second photodetector receives at least a portion of the second split beam and produces a second detected signal. The first filter includes a substrate having a filter layer, the filter layer including at least one film layer having a dielectric constant differing from that of the substrate. An error signal circuit is in electrical communication with the first and second photodetectors for generating an error signal responsive to the first and second detected signals and representative of the deviation of the wavelength of the laser radiation from a set-point wavelength. A laser wavelength control element is in electrical communication with the error circuit for adjusting the operating temperature of the laser in response to the error signal such that the wavelength of the laser tends toward the set-point wavelength.
Allie David J.
Dunn Paul E.
Munks Timothy C.
Davie James W.
McCormick Paulding & Huber LLP
Uniphase Telecommunications Products, Inc.
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