Patent Application
20080018988
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
Referring initially to
In
As an overview, in certain embodiments, the light source assembly 10 is uniquely designed so that the output beam 20 has a specific, relatively narrow spectral width and a specific center wavelength, with sufficient power for use in precision measurement systems. Further, in certain embodiments, the light source assembly 10 generates an output beam 20 with sufficient spectral width to eliminate unwanted interference effects. The light source assembly 10 can be used in systems with relatively tight spectral-width and/or center wavelength requirements. Further, the light source assembly 10 disclosed herein can eliminate the need for a phase modulator. Accordingly, the light source assembly 10 can be easier to control.
The light generator 12 generates a generator beam 22 that is directed at the circulator 18. In one embodiment, the light generator 12 is a diode that functions as a superluminescent diode (“SLD”). In certain embodiments, the light generator 12 is designed so that the generator beam 22 has a center wavelength of between approximately 630 and 640 nm, and/or an output power of between approximately 1 and 20 milliwatts. For example, the generator beam 22 can have a center wavelength of approximately 633 nm, and/or an output power of approximately 10 milliwatts. Alternatively, the light generator 12 can be designed so that the generator beam 22 has a center wavelength and/or an output power that is greater or lesser than the values detailed above.
In certain embodiments, the light generator 12 is a broadband light source and the generator beam 22 has a spectrum of greater than approximately 1 nm.
One non-exclusive example of a suitable light generator 12 is a 635-nm, AlGaInP, multiple quantum well active layer laser diodes with output facet AR coated.
In one embodiment, the light generator 12 includes a first side 12A that is coated with a high reflection (“HR”) coating 12B and a second side 12C that is coated with an anti-reflection (“AR”) coating 12D. The HR coating 12B reflects light that is directed at the first side 12A back into the gain medium. The AR coating 12D allows light directed at the second side 12C to easily exit the light generator 12. With this design, the AR coating 12D inhibits lasing. One or both of the sides 12A, 12C can be angled to enhance performance of the light generator 12.
Additionally, the light source assembly 10 can include a first optical element 23 that collimates and focuses the first generated beam 22. For example, the first optical element 23 can include one or more optical lens.
The amplifier 14 generates an amplified beam 24 that is directed at the optical filter 16. In one embodiment, the amplifier 14 is a diode that functions as a semiconductor optical amplifier. In certain embodiments, the amplifier 14 is designed so that the amplified beam 24 has a center wavelength of between approximately 630 and 640 nm, and/or an output power of between approximately 10 and 50 milliwatts. For example, the amplified beam 24 can have a center wavelength of between approximately 633 nm, and/or an output power of approximately 20 milliwatts. Alternatively, the amplifier 14 can be designed so that the amplified beam 24 has a center wavelength and/or an output power that is greater or lesser than the values detailed above.
In one embodiment, the amplifier 14 includes a first side 14C that is coated with an anti-reflection (“AR”) coating 14D and a second side 14A that is coated with a high-reflection (“HR”) coating 14B. The HR coating 14B reflects light that is directed at the first side 14A back into the gain medium. The AR coating 14D allows light directed at the second side 14C to easily exit the amplifier 14. With this design, the AR coating 14D inhibits lasing. One or both of the sides 14A, 14C can be angled to enhance performance of the amplifier 14.
Additionally, the light source assembly 10 can include a second optical element 25 that collimates and focuses the amplified beam 24. For example, the second optical element 25 can include one or more optical lens.
In one embodiment, the amplifier 14 is a double pass amplifier. For example, the amplifier 14 can include a waveguide. In the embodiment illustrated in
One non-exclusive example of a suitable amplifier 14 is a 635-nm, AlGaInP, multiple quantum well active layer laser diodes with output facet AR coated.
In
The filter assembly 15 filters the generator beam 22 from the light generator 12 that is directed at the amplifier 14, and filters the amplified beam 24 from the amplifier 14 that is directed towards the circulator 18. The portion of the light generator beam 12 that passes through the filter assembly 15 is referred to as a filtered generator beam 22A. Further, the portion of the amplified beam 24 that passes through the filter assembly 15 is referred to herein as the transmitted beam 26. The transmitted beam 26 is subsequently directed at the circulator 18. In one embodiment, the filter assembly 15 includes only one optical filter 16 and the generator beam 22 and the amplified beam 24 are both directed at the optical filter 16. With this design, (i) the same optical filter 16 filters both the generator beam 12 and the amplified beam 24 and (ii) the generator beam 12 and the amplified beam 24 follow the same path through the optical filter 16 but in opposite directions. This can reduce the size, cost, and/or complexity of the light source assembly 10.
In one embodiment, the filter assembly 15 precisely controls both the center wavelength and the spectral width of the transmitted beam 26. Further, the design of the filter assembly 15 can be precisely tailored to achieve the desired center wavelength and spectral width of the output beam 20. In one embodiment, the optical filter 16 is a band pass type filter that transmits a band of wavelengths (“the passband”) and blocks wavelengths outside of the passband. The passband has a center wavelength that is at the center of the passband. For example, in alternative non-exclusive embodiments, the filter is a narrow band pass filter with a passband having a bandwidth of approximately 0.1, 0.5, 1, 1.5, 2, or 5 nanometers. As a result thereof, the transmitted beam 26 has a precisely controlled, relatively narrow spectral width.
Further, the transition from transmitting to rejection can be sharp. In alternative, non-exclusive embodiments, the filter 16 has a spectral slope capable of transitioning between 10% and 90% transmission in less than 0.2 nm, 0.5 nm, 1 nm, or 5 nm.
Additionally, the optical filter 16 is designed so that the center wavelength of the passband is near the desired wavelength for operation of the precision apparatus 11. With this design, the optical filter 16 passively controls the transmitted beam 26 to have the desired center wavelength and a relatively narrow spectral width. Moreover, the filtering of the amplified beam 24 reduces any noise originating in the amplifier 14 that lies outside of the passband. Further, the input, e.g. the filtered generator beam 22A to the amplifier 14 has the desired spectral width. This concentrates the available power of the amplifier 14 in the desired spectrum to improve the efficiency of the amplifier 14. Stated in another fashion, in-band light does not compete with out-of band light for gain from the amplifier 14.
In certain embodiments, the optical filter 16 can be moved, e.g. rotated, to precisely fine tune the center wavelength. One non-exclusive example of a suitable, optical filter 16 is model number LL01-633 sold by Semrock, located in Rochester, N.Y.
It is noted that other, possibly more complicated filter 16 types could be used instead of a bandpass filter. An example is that of a filter 16 to compensate the naturally peaked gains of the generator 12 and amplifier 14 so that the spectrum of the transmitted beam is flatter and/or broader than that if the filter 16 were not present. It is also noted that the filter 16 could have a variable, controllable shape. Such variable filters 16 are achieved by various means, such as by spatially modulating a spectrally dispersed beam, and would allow the selection of a desired spectrum with arbitrary shape.
The circulator 18 (i) receives the generator beam 22 from the light generator 12, (ii) directs the generator beam 22 at the optical filter 16, (iii) receives the transmitted beam 26 from the optical filter 16, (iv) directs the output beam 20 toward the device 21, (v) provides isolation between the amplifier 14 and the generator 12, and (vi) provides isolation between the amplifier 14 and the device 21. One embodiment of the circulator 18 is described in more detail below.
In certain embodiments, the temperature of light generator 12, the amplifier 14 and/or other components of the light source assembly 10 can be actively controlled.
Each of the first, second and center Polarizing Beamsplitter cubes 230, 234 split randomly polarized beams into two orthogonal, linearly polarized components. Each of the first, second and center Polarizing Beamsplitter cubes 230, 234, 238 can consist of a pair of precision high tolerance right angle prisms cemented together with a dielectric coating on the hypotenuse of one of prisms.
The center Polarizing Beamsplitter cube 238 is at 45 degrees. Waveplates could be used to manipulate polarization if this 45 degree angle is undesirable. Each of the faraday rotators 232, 236 is an optical device that rotates the polarization of light due to the Faraday effect.
In one embodiment, one of the light generator 12 and the amplifier 14 operates in a TM mode and produces an elliptical beam and the other one of the light generator 12 and the amplifier 14 operates in a TE mode and also produces an elliptical beam. In this embodiment, the faraday rotators 232, 236 are configured so that each rotate polarization 45 degrees in the same direction to produce a total of 90 degrees of rotation. This allows mode matching with respect to both ellipse orientation and polarization, helping to optimize coupling of light from the light generator 12 to the amplifier 14.
If the light generator 12 and the amplifier 14 have similar polarization (both TE or both TM), the faraday rotators 232, 236 can be configured to rotate polarization in opposite directions to produce a total of zero degrees of rotation. This could eliminate the need for a waveplate to achieve mode matching.
The implementation shown in
With the light source assembly 10 illustrated in
In
In certain embodiments, a possible benefit of amplifying the generator beam 22 is the ability to run the light generator 12 at lower drive levels. This can result in less ripple in the spectrum of the output beam 20. At higher drive levels of the light generator 12, the high gain combined with the residual reflection from the AR coating can produce significant ripple that produces peaks in the contrast vs. optical-path-length-difference plots. Because the amplifier is largely saturated, its small signal gain is low and it is less prone to producing ripple. Thus, it is the light generator 12 that dictates the level of ripple. Accordingly, in certain embodiments, the present invention provides a way to produce enough power while operating the light generator 12 at relatively low drive levels.
Additionally, the light source assembly 710 can include one or more optical isolators 790A, 790B. In the embodiment illustrated in
While the particular apparatus 10 as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
