EP0899178A1

EP0899178A1 - Laser-based railroad signal light

Info

Publication number: EP0899178A1
Application number: EP98306410A
Authority: EP
Inventors: Joseph T. Verdeyen; Ronald W. Snee
Original assignee: Safetran Systems Corp
Current assignee: Siemens Industry Inc
Priority date: 1997-08-18
Filing date: 1998-08-11
Publication date: 1999-03-03
Also published as: HRP980428A2; CA2243809A1; ZA987324B; AU7184798A

Abstract

A railroad light signal fixture, either for railway/highway grade crossing application or right-of-way use, includes a housing (40 or 76), each having a signal light aperture (42). There is a lens structure (44,46 or 78,80) covering the aperture and there is a source of illumination within a housing opposite the lens structure. The source is provided using a laser (48,50,52,54,56 or 70) which provides for light within a specific colour spectrum as required for the specific application.

Description

The present invention relates to railroad signal lights.
Such a signal light may be for use at railroad highway and pedestrian grade crossings, as flashing warning lights, and for railroad right-of-way control signals. For many years, incandescent lamps have provided the light source for all railroad signal applications. Conventionally, the incandescent lamp will have essentially uncoloured illumination, with the desired colour being provided by a colour filter. For example, for crossing gates and other warning devices there is typically a red colour filter, a light collecting device and a diverging lens. For right-of-way signals in which red, yellow and green are the conventional colours, there will be filters to ensure that the proper colour of illumination is provided through the proper opening.
A laser light source has many advantages over an incandescent lamp as a source of light. Longer life is perhaps the most significant advantage. For example, present incandescent signal lights used by the railroad have a rated lifetime of approximately 2,000 hours, with 1,000 hours being a more common, realistic period of use. A laser light source may be rated at least as long as 10,000 hours and the accompanying electronics will have a much longer life. Moreover, a laser light source may be composed of an array of multiple lasers or multiple sources and since it is extremely unlikely that there would be a simultaneous failure of all such lasers, a more realistic situation would involve a slow degradation of the light source which can be monitored internally and a signal sent to railroad personnel and, when the light source reaches a predetermined level of illumination, the laser array can be replaced.
Another important advantage is that the coupling of a laser to a fibre is far more efficient than such a coupling to an incoherent source such as an incandescent lamp or an LED (light emitting diode).
Further, a laser light source is more easily directionalised and its amplitude can be easily controlled with low cost available semiconductor electronics. Laser light sources will operate at substantially higher electrical-to-light efficiencies than current, present-day incandescent lamps.
According to the present invention, there is provided a railroad signal light fixture including: a housing; a signal light aperture in said housing; lens means covering said aperture; and a source of illumination within said housing to provide light directed through said lens means, said source of illumination being provided from a laser which provides for light within a specific colour spectrum.
The laser may provide for light in the red colour spectrum at a wavelength within the range of 630-680 nanometres; or light in the green colour spectrum at a wavelength in the range of 555 nanometres; or light in the yellow colour spectrum at a wavelength in the range of 588 nanometres.
The laser may be connected by a light conduit to a location positioned opposite said lens means. In this case, the laser may be located remote from said housing, for example using a light conduit which is a fibre optic cable (for example a fibre optic cable doped with a rare earth material such as praseodymium).
Alternatively, said laser may be located within said housing.
The laser may be a laser diode which provides light within a specific colour spectrum. In this case, the laser diode may be one of an array of such laser diodes. Each of the laser diodes may be connected by a respective light conduit to a common location positioned opposite said lens means.
The signal light fixture could function as a crossing warning light or as a train control signal, for example.
There may be a plurality of such signal light housings, each having a signal light aperture and lens means covering its aperture, there being such a source of illumination within each of said housings to provide light directed through the lens means of the housing, the source of illumination in each housing being provided from at least one laser which provides for light within a colour spectrum specific to that housing.
In the case of a laser diode, since its wavelength is virtually independent of the drive current, its input or output power, it chromaticity coordinates (or colour) will not be a function of the current. An incandescent lamp produces light by virtue of the high temperature of the filament according to the blackbody radiation law and thus its spectral content is a strong function of the power used to heat the filament.
As a further advantage, it is not necessary to use a filter in front of a light source using laser, as the source itself will provide the desired colour.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-
Fig. 1 shows a typical prior art railroad crossing warning light;
Fig. 2 is a diagrammatical illustration of a typical prior art railroad signal housing using an incandescent lamp source;
Fig. 3 is a perspective view illustrating an array of laser diodes connected to fibre optic cables;
Fig. 4 is a section illustrating a right-of-way railroad signal using laser diodes as a light source;
Fig. 5 is a section, similar to Fig. 4, showing a bundle of laser diodes directly adjacent a diffusing lens;
Fig. 6 is a perspective view of the bundle of laser diodes shown in Fig. 5; and
Fig. 7 illustrates a railroad right-of-way signal having three adjacent laser-based signal lights to provide light at three different wavelengths.
In embodiments to be described of the present invention, laser diodes are utilised as the light source for railway signals, both highway crossing flashing signal fixtures and railroad right-of-way train control fixtures, in the form of an array of such diodes as the light source. The array may be located directly at the point of illumination or it may be remotely located and coupled by fibre optic cables to the point of illumination within the signal housing.
Focusing on red as the colour used in highway crossing gate flashing lights and as one of the signals for right-of-way train control, such an array of laser diodes may operate within the red colour spectrum of 630-680 nanometres (nm). Within this spectrum, the laser diodes will not all emit light at the same wavelength, but may emit a plurality of different wavelengths within this specific spectrum. For railroad right-of-way signals the array of laser diodes may operate near a wavelength of 588 nm for a yellow light and 555 nm for a green light. In the case of a "white" light requirement, one could use combinations of these colours to produce "white" or use two "complementary" colours, e.g. those wavelengths connected by a straight line through the white point on the "Commission Internationale d'Eclairage" (CIE) chromaticity diagram. As an example, 494 and 670 nm, as well as 474 and 568 nm are complementary colours.
In addition to using an array of laser diodes at the specific wavelengths set forth above, it is also within the scope of the invention to utilize a fibre optic cable doped with a rare earth which can be excited by very efficient near-infrared lasers to produce visible light of the appropriate wavelengths. This is accomplished by the sequential absorption of two infrared photons from the pumping lasers and the emission of visible photons from the excited rare earth atoms. A particular appropriate rare earth is praseodymium. A fibre optic cable doped with this material would utilize lasers at 1.01µm and 835 nm to generate either 635 nm (red), 520 nm (green-blue) or 492 nm (blue) wavelengths of light. Other rare earths and different fibres can be utilized to produce different colours. What is important is that a doped fibre optic cable used in combination with lasers operating at particular wavelengths can provide any necessary colour which would find utility in the railroad environment. For further details on the use of lasers in combination with doped fibre optic cable, see The Journal of Quantum Electronics, Vol. 33, No. 6, June 1977, and the article at page 905, entitled "Theory of Pr³⁺ -- Doped Fluoride Fiber Upconversion Lasers" by Yuxing Zhao and Simon Fleming.
Fig. 1 shows a typical prior art highway crossing flashing light signal fixture. There is an outer housing 10 which may have an inner mirror coating 12. The coating is placed on a parabolic reflector 14 and there is a colour filter and diverging lens 16 closing the aperture of the housing 10. An incandescent light source 18 is positioned within the housing and will direct light along the paths of the arrows 20 out through the filter and diverging lens 16. Such a light source is conventional and can be found throughout the United States wherever there are highway or pedestrian crossings of railroad tracks.
Fig. 2 illustrates a typical railroad right-of-way signal fixture. There is a housing 30 which may be blackened on the inside so as to avoid light reflection. The housing 30 has an aperture 32 and a light source 34 positioned within the housing. There is a lens system which consists of an inner lens 36 and an outer lens 38, with the combination of the two lenses providing a means to direct the light from the signal source along a predetermined path, with only slight divergence in the beam over its useful length. This is important in that the signal light must be visible from a great distance and this can only come about if the light from the source is concentrated along a desired path.
A laser diode railroad signal light source according to an example of the present invention is shown in Figs. 3 and 4. It may be useful in either a warning light environment or as a right-of-way signal. There is a housing 40 which has a signal light aperture 42. A lens system comprising an inner lens 44 and an outer lens 46, similar to the prior art lens structure of Fig. 2 is used in Fig. 4.
The source of illumination is provided from a plurality of laser diodes indicated at 48, 50, 52, 54 and 56 energised via a line 90 connected to a power source. The number is only illustrative and specifically there may be anywhere from 15 to 20 such laser diodes and they may be spread in the wavelength spectrum, using red as an example of the desired colour, from 630-680 nanometres. For a green or yellow signal light, laser diodes in other spectrums would be utilised. The laser diodes indicated at 48 and following are all connected into a light source array such as illustrated in Fig. 3 and located at an opening 58 within a wall 60 of the housing 40. Fibre optic light transmissive conduits indicated at 62 connect each of the laser diode light sources into the bundle of light sources illustrated in Fig. 3. One of the advantages of using laser diodes is that they can be located remotely from the signal housing by taking advantage of the superior coupling of a laser to a fibre optic cable and by the low loss characteristics of readily available fibre optic cables. For example, the loss for a ten metre length of fibre optic cable may be as low as 0.04 dB yielding a transmission factor of 99.08%.
There may be an enclosure 64 which binds the light sources together in a closed array so that the effect is effectively a point source of light, which emits with a characteristic cone of radiation, as indicated by its numerical aperture, and emits at one or multiple wavelengths and which can be located at the focal point of the collimating optics. Although the laser diodes as illustrated in Fig. 4 are within the signal housing, they may be remotely located therefrom, such as in a control box along the signal right-of-way or adjacent to a railroad highway crossing.
Of particular advantage in the use of laser diodes as a light source is the longer life attributed both to the diodes themselves as a source and the fact that even if one or more of the individual diodes within an array should go out, there is still adequate light from the array as a whole. Moreover, solid state sources for the electronic controls will contribute to the long life of such a signal light. A further advantage to laser diodes is that each laser diode is specific to a particular wavelength and thus all of the light which emanates from the laser diode is at that wavelength. In contrast, in an incandescent lamp the light is spread throughout a wide portion of the light spectrum. As a specific example of laser diodes which may be satisfactorily used in the present invention, laser diodes are available with nominal wavelengths of 635, 650, 670, 640 nanometres.
Fig. 5 illustrates the use of a bundle of laser diodes at 70, with appropriate electrical connections 72, positioned in an aperture 74 within a signal light housing 76. There are no fibre optic cables in this embodiment of the invention, but a diverging lens 78 is placed in front of the bundle 70 of laser diodes, after which the beam emanating from the diverging lens will be collimated by a lens 80. The end result will be a light beam along the path of arrows 82. Fig. 6 is a perspective view of the laser bundle 70 and shows a plurality of individual laser diode light sources 84, all within a sheath or sleeve 86.
Fig. 7 shows a multiple of the housing 76 of Fig. 5 so as to provide a three-signal light source suitable for railroad right-of-way train control. For example, in a three-signal light matrix, there may be the typical red, green and yellow lights provided by the individual bundles of laser diodes, with the laser diodes functioning at the wavelengths described above. Clearly, the invention can encompass more than three specific signal lights and colours other than red, green and yellow, as required for train control.
The use of laser diodes as a light source also permits the light source to be amplitude-modulated with simple and reliable electronics. This would have particular advantage in train control, as a receiver on a locomotive could be tuned to a particular modulation rate providing an electronic back up connection for train control in addition to the engineer's sighting of the visible signal.

Claims

A railroad signal light fixture including: a housing; a signal light aperture in said housing; lens means covering said aperture; and a source of illumination within said housing to provide light directed through said lens means, said source of illumination being provided from a laser which provides for light within a specific colour spectrum.
A signal light fixture according to claim 1, wherein the laser provides for light in the red colour spectrum at a wavelength within the range of 630-680 nanometres.
A signal light fixture according to claim 1, wherein the laser provides for light in the green colour spectrum at a wavelength in the range of 555 nanometres.
A signal light fixture according to claim 1, wherein the laser provides for light in the yellow colour spectrum at a wavelength in the range of 588 nanometres.
A signal light fixture according to any preceding claim, wherein the laser is connected by a light conduit to a location positioned opposite said lens means.
A signal light fixture according to claim 5, wherein said laser is located remote from said housing.
A signal light fixture according to claim 6, wherein said light conduit is a fibre optic cable.
A signal light fixture according to claim 7, wherein said fibre optic cable is doped with a rare earth material.
A signal light fixture according to claim 8, wherein said rare earth material is praseodymium.
A signal light fixture according to any of claims 1 to 5, wherein said laser is located within said housing.
A signal light fixture according to any of claims 1 to 7 or claim 10, wherein the laser is a laser diode which provides light within a specific colour spectrum.
A signal light fixture according to claim 11, wherein the laser diode is one of an array of such laser diodes.
A signal light fixture according to claim 12 as dependent on any of claims 5, 6 and 7, wherein each of the laser diodes is connected by a respective light conduit to a common location positioned opposite said lens means.
A signal light fixture according to any preceding claim, wherein the fixture functions as a crossing warning light.
A signal light fixture according to any of claims 1 to 13, wherein the fixture functions as a train control signal.
A signal light fixture according to any preceding claim, wherein there are a plurality of such signal light housings, each having a signal light aperture and lens means covering its aperture, there being such a source of illumination within each of said housings to provide light directed through the lens means of the housing, the source of illumination in each housing being provided from at least one laser which provides for light within a colour spectrum specific to that housing.