Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2003-01-21
2004-11-16
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S048000, C372S029021
Reexamination Certificate
active
06819832
ABSTRACT:
The invention relates to arrangements for monitoring the emission wavelength of laser sources.
Commercial WDM (Wavelength Division Multiplex) transmission systems, such as “dense” WDM (DWDM) systems provide high transmission capacity by using reduced channel spacing (e.g. 100-50 GHz). Real time wavelength monitoring and control is thus necessary in order to ensure the wavelength stability required for the optical sources used in such systems.
A number of devices adapted for that purpose are based on the arrangement currently referred to as “wavelength locker”. This usually consists of two photodiodes sampling two portions of the laser beam. Specifically, one of the diodes, used as a reference, samples an unfiltered portion of the laser beam. Another portion of the laser beam is passed through an optical filter and caused to impinge onto the second photodiode. The response (i.e. the photocurrent) of the second diode is thus a function of the possible displacement of the actual wavelength of beam generated by the laser source with respect to the wavelength of the filter and can thus be used as a signal to stabilise the emission wavelength of the laser source.
Various arrangements are known in order to effect stabilisation. For instance, in the case of diode lasers a Peltier element can be used as a stabilising element by controlling the temperature of the laser diode.
A beam splitter is used to split the laser beam into a main beam to be used for the intended application (e.g. for launching into a fiber) and one or more secondary beam or beams to be directed towards the photodiodes of the wavelength locker arrangement. An optical filter or etalon can be used with one of the photodiodes.
Exemplary of such prior art arrangements are U.S. Pat. No. 4,309,671 and U.S. Pat. No. 6,144,025.
In European patent application No. 01 3 04 026.6, included in the prior art under the provisions of Art. 54(3) EPC, an arrangement is disclosed wherein the radiation lobe emitted from the back facet of a laser is directed partly—usually by means of direct propagation in air—toward a first photodiode used as a reference and partly—through a semiconductor slice acting as a collimating member and a wavelength-sensitive interference filter—toward a second photodiode to generate a control signal to stabilize the laser wavelength.
The possibility is also contemplated of exploiting optical radiation derived from the main radiation beam emitted from the first facet of the laser source: this is again done by splitting the necessary radiation out of the main radiation beam by means of a beam splitter.
Consequently, a reduction in the “useful” or usable power has been heretofore considered as inevitable in all wavelength locker arrangements intended to operate on the main laser beam generated by a laser source.
The main object of the present invention is thus to provide an arrangement for monitoring the emission wavelength of a laser source giving rise to a smaller reduction in terms of usable power, while including fewer components and providing improved thermal-mechanical stability.
According to the present invention, that object is achieved by means of an arrangement having the features called for in the claims which follow.
The present invention is based i.a. on recognising that those arrangements where the (main) beam of the laser source is launched into an optical fiber with a very small core size, such as a single mode fibre, a part of the power generated by the laser cannot be effectively launched into the fiber.
This phenomenon (i.e. at least some optical power being “wasted” or “lost” during launching into the fiber) is primarily due to two different factors, namely aberration in the optical system and laser beam quality and is particularly evident in the case of laser diode sources.
Due to the small spot size of laser diodes, beam launching optics for use with such radiation sources normally require large numerical aperture (NA) component. Glass spherical (ball) lenses are a popular choice for that purpose due to their large NA and low cost. Their inherent drawback is large beam aberration, particularly near the marginal part (edge) of the beam.
Due to the nature of mode matching in launching a beam into a single mode fibre, the marginal part of the beam, that is subject to aberration, contributes much less than the otherwise un-aberrated beam.
Also, the spatial distribution of the optical power generated by a laser diode may deviate from a Gaussian beam, which is regarded as the ideal beam profile for launching into a single mode fibre. The spatial distribution in question may in fact correspond to a Gaussian beam having superposed a certain amount of higher order beams. In comparison with a truly Gaussian beam, any such higher order beam exhibits a higher power level near the edge of the beam and contributes much less to coupling into the fiber.
A large proportion of the power eventually not launched into the fiber is often in the marginal portion of the beam at certain positions of the beam path, e.g. after the first lens in the optical system provided for launching the optical radiation into the fiber. The amount of power lost can be as high as 20-40% (and even higher) of the total power depending on optical design and laser source characteristics.
Also, in some cases, in order to achieve the desired bandwidth, the laser diode is driven at a current higher than required in terms of power requirement for launching into the fiber. Power must be taken out through either spatial filtering (i.e. cut off the marginal part of the beam) or by “detuning” the fibre launching arrangement.
The present invention is thus based on the recognition of the possibility of exploiting—in order to generate the sample beam used for stabilisation purposes—that marginal part of the main beam generated by the laser source that would not be properly used for launching into the fiber.
The basic advantages offered by the invention are related to the possibility of dispensing with the beam splitter provided in current wavelength locker arrangements, with the ensuing reduction of the number of components in the arrangement.
Also, the power available for detection and monitoring the emission wavelength is potentially higher for the same fibre coupling efficiency penalty and easier to control through the positioning of detectors in comparison to an optical configuration with a beam splitter that requires a new beam splitter of different reflectivity.
The beam splitter being dispensed with, the optical path from the laser source to the detector can be reduced. A reduction in the optical path length and the number of components is thus achieved that improves the thermal-mechanical stability of the wavelength dependent signal.
REFERENCES:
patent: 4309671 (1982-01-01), Malyon
patent: 6144025 (2000-11-01), Tei et al.
patent: 6704334 (2004-03-01), Yokoyama
patent: 6711188 (2004-03-01), Ito et al.
patent: 0881633 (1998-12-01), None
patent: 0967698 (1999-12-01), None
Patent Abstracts of Japan publication No. 55126208.
Patent Abstracts of Japan publication No. 03091283.
Patent Abstracts of Japan publication No. 03286574.
Patent Abstracts of Japan publication No. 62058689.
Agilent Technologie,s Inc.
Rahll Jerry T.
Ullah Akm Enayet
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