Field-programmable optical source

Details Field-programmable optical source Field-programmable optical source

2000-11-08

2003-04-01

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

C372S020000

Reexamination Certificate

active

06542534

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to optical devices and, more particularly, to optical sources with optical outputs, the specific output wavelengths of which are user selectable.
With the development of dense wavelength-division multiplexing (DWDM) technology for telecommunications, there is a growing interest in an optical source with an optical output that is wavelength-tunable and is also stable once the optical output has been tuned to a desired wavelength. In a multi-channel, DWDM telecommunication system, a distinct wavelength is assigned to each channel and the number of available channels in a telecommunication bandwidth is dependent on the linewidth (i.e., a narrow range of wavelengths around the assigned wavelength) of the optical signal used at each channel. In order to increase the number of channels which can be fitted into the available bandwidth, the range of wavelengths at each channel must be decreased. The standard channel range used in the telecommunications industry is less than 0.8 nm per channel (corresponding to 100 GHz at 1550 nm) and is further decreasing. In the DWDM system, any instability in the optical source larger than the channel range will result in communication error. To avoid such error, the optical output of the optical source used at each channel must be stable in wavelength within the assigned channel range.
The increase in the number of channels also creates a problem in that the number of optical sources forming a DWDM transmitter is also increased. In general, optical sources generate heat as well as light and the optical output of optical sources tend to depend on temperature. Therefore, instability in certain optical sources is exacerbated by the increased number of optical sources in close proximity within the DWDM transmitter. Supplementary components, packaging, and circuitry are needed in order to effectively control the temperature of each optical source, thus adding to the cost of the DWDM transmitter.
Many of the existing DWDM systems use a series of distributed feedback (DFB) lasers as the optical sources in the DWDM transmitter. A DFB laser is normally designed and manufactured for a specific optical wavelength. Its output wavelength is partly stabilized by a temperature control apparatus using feedback circuitry which monitors the output wavelength of the optical source and regulates the temperature control apparatus accordingly. Since the output wavelength of a DFB laser is further dependent on the input current, the feedback circuitry may regulate the current supply of the DFB laser as well. Essentially, the feedback circuitry serves as a frequency locker that locks the output frequency, which corresponds to the output wavelength, of the DFB laser at a particular value. Due to its dependence on temperature and input current, the output wavelength of the DFB laser can be tuned over a narrow wavelength range of 5 to 8 nm by controlling the temperature of the DFB laser and the current supplied to the laser. Once tuned to a specific wavelength, the wavelength stability of a DFB laser output is approximately −12.5 GHz/° C. (or correspondingly +0.1 nm/° C.) with respect to case temperature and −1.25 GHz/mA (or correspondingly +0.01 nm/mA) with respect to bias current.
There are, however, drawbacks to the use of DFB lasers in a DWDM system. The fabrication of a DFB laser is a lengthy process requiring the formation of a sub-micron, periodic structure within a multilayer semiconductor structure to act as a wavelength-selective grating element. The output wavelength of the DFB laser is heavily dependent on the shape and periodicity of the periodic structure, hence precision of the fabrication process is crucial. Although many essentially identical DFB laser chips can be produced during a single fabrication run, DFB laser chips for different output wavelengths are normally fabricated separately. Consequently, the production of a DFB laser for a given output wavelength often necessitates a long lead time once the output wavelength has been specified to the manufacturer. The production of a series of DFB lasers for a complete DWDM system can take even longer, requiring many production runs since each channel of a DWDM system requires its own DFB laser.
Furthermore, due to the relatively high temperature coefficient of semiconductor laser materials, the feedback circuitry and temperature and current controllers discussed in the above paragraph are required to control the actual output wavelength even after the DFB laser chip has been fabricated using high precision processes. For the DFB laser to be useful in a DWDM context, an external wavelength reference must also be supplied to accurately regulate the output wavelength. Additionally, since the power output of the DFB laser is proportional to the input current and the feedback circuitry regulates the input current in order to control the output wavelength of the laser, the actual power output of a particular DFB laser is limited by the need for output wavelength stabilization. Due to such difficulty in directly controlling output power, an external attenuator is often needed at each channel in order to achieve uniform optical power output across the channels in a WDM transmitter system using a series of DFB lasers. In addition to input current control, the DFB laser requires the use of active heating and cooling measures using the aforementioned temperature control apparatus. Hence, a separate output wavelength regulation mechanism, which adds to the power consumption of the DFB laser operation, is needed for each laser used in the DWDM system with respect to temperature and input current. Moreover, in order to reduce frequency chirp often produced by the direct modulation mechanism, the DFB laser output must be modulated externally. Therefore, although each DFB laser chip is relatively inexpensive, the peripheral equipment such as the temperature control apparatus, controllable current supply, external attenuators, feedback circuitry and external modulator significantly add to the complication and total cost of a multi-channel DWDM system using such lasers.
Another commercially-available device which could be used as an optical source in a DWDM system is a tunable diode laser. For example, one type of tunable laser is based on a mechanical tuning scheme where one of the mirrors which form the laser cavity is physically moved to change the grazing angle at which an optical input from a separate diode laser is incident on a bulk grating in the laser cavity, thus changing the wavelength of the optical output of the tunable laser. Tunable lasers can generally be tuned over a wavelength range of 40 to 80 nm and are often used in optical component testing in a scanning mode where the output of the tunable laser is scanned over a part of or the entire wavelength range to test the wavelength-dependent response of an optical device. However, the precision actuators and components within a tunable laser as well as the laser controller mechanism and software are generally expensive. For example, tunable lasers currently on the market cost tens of thousands of dollars each at the time of this writing (typically $35,000 to $63,000 for laboratory instruments). Furthermore, since each channel in a DWDM system is preassigned to a specific wavelength, the optical source used at each channel needs to be tuned only to that specific wavelength at time of installation. The wavelength of a given channel may be re-assigned on occasion, but, on the whole, the optical source is made to operate at a single wavelength without the need for wavelength scanning. Therefore, the precision actuators and other tuning components of the tunable laser are generally superfluous once the laser has been tuned to the specific wavelength for a given channel. Moreover, currently available tunable lasers are relatively large compared to compact semiconductor lasers. For these reasons, it is submitted to be impractical to provide a tunable laser for each channel of a

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