External Cavity Diode Laser Arrangement

Abstract

A laser arrangement including a laser element; a volume Bragg grating, VBG, for providing optical feedback to the laser element along a beam path; a polarizing beam splitter, PBS, arranged in the beam path between the laser element and the VBG; and a polarization-modifying element arranged in the beam path between the PBS and the VBG element; wherein the polarization-modifying element is structured and arranged to alter a polarization state of light reflected from the VBG such that the PBS is operative to divide said light reflected from the VBG into a first portion that provides the optical feedback to the laser element and a second portion that provides an output beam from the laser arrangement. Embodiments may be useful for low wavenumber Raman spectroscopy.

Claims

1. A laser arrangement, comprising a laser element; a volume Bragg grating, VBG, for providing optical feedback to the laser element along a beam path; a polarizing beam splitter, PBS, arranged in the beam path between the laser element and the VBG; and a polarization-modifying element arranged in the beam path between the PBS and the VBG element; wherein said polarization-modifying element is structured and arranged to alter a polarization state of light reflected from said VBG such that said PBS is operative to divide said light reflected from said VBG into a first portion that provides the optical feedback to the laser element and a second portion that provides an output beam from the laser arrangement.

2. The laser arrangement of claim 1, wherein the polarization-modifying element is a quarter wave-plate.

3. The laser arrangement of claim 1, wherein the laser element is an edge-emitting diode laser.

4. The laser arrangement of claim 1, wherein the laser element is a vertical-cavity surface-emitting laser, VCSEL.

5. The laser arrangement of claim 1, wherein the PBS and the polarization-modifying element are structured and arranged to provide the output beam by the PBS transmitting light reflected from the VBG and to provide the feedback to the laser element by the PBS reflecting light reflected from the VBG.

6. The laser arrangement of claim 1, wherein the PBS and the polarization-modifying element are structured and arranged to provide the output beam by the PBS reflecting light reflected from the VBG and to provide the feedback to the laser element by the PBS transmitting light reflected from the VBG.

7. The laser arrangement of claim 1, further comprising a second polarization-modifying element positioned between the laser element and the PBS.

8. The laser arrangement of claim 1, further comprising beam shaping optics positioned between the laser element and the PBS.

9. The laser arrangement of claim 8, wherein said polarization-modifying element is structured and arranged to alter the polarization state of light reflected from said VBG such that said first portion providing optical feedback to the laser element includes 5%-20%, of the power of light reflected from said VBG.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Further details and embodiments of the invention will be given in the below detailed description, in which reference is made to the accompanying drawings on which:

[0018] FIG. 1 schematically shows a prior art diode laser arrangement in which optical feedback is provided by means of a VBG;

[0019] FIG. 2 schematically shows a diode laser arrangement similar to that shown in FIG. 1, but in which the output beam is filtered using a dielectric filter;

[0020] FIG. 3 schematically shows a diode laser arrangement similar to that shown in FIG. 1, but in which the output beam is filtered using an additional VBG;

[0021] FIG. 4 schematically shows an embodiment of the present invention, in which an output beam is extracted by reflection from the PBS;

[0022] FIG. 5 schematically shows an embodiment of the present invention, in which an output beam is extracted by transmission through the PBS;

[0023] FIG. 6 shows spectral characteristics of the diode laser arrangements shown in FIGS. 1-3; and

[0024] FIG. 7 shows spectral characteristics of an embodiment of the inventive diode laser arrangement.

DETAILED DESCRIPTION

[0025] Referring to FIG. 1, there is schematically shown a prior art diode laser arrangement of the kind disclosed in WO 2005/013439. A diode laser element 110 has a spectral gain profile within which optical radiation is generated. Light emitted by the diode laser element is collected by beam-shaping optics 120 and then directed towards a volume Bragg grating, VBG, 130. The VBG reflects part of the light within a predetermined spectral band back towards the diode laser element 110, such that the laser element is caused to generate light within that spectral band. Since only part of the light within the predetermined spectral band is reflected by the VBG, a substantial fraction of light within the predetermined spectral band is transmitted through the VBG and can be used as an output 140 from the diode laser arrangement.

[0026] It should also be noted that the VBG is generally transmissive for light having a wavelength outside the predetermined spectral band. This means that the peak wavelength in the output, as determined by the spectral band of the VBG, is surrounded by side-modes excited by the laser element.

[0027] FIG. 2 shows another diode laser arrangement. The arrangement of FIG. 2 is similar to that of FIG. 1, except that the output is filtered using a dielectric filter 232 in order to suppress side modes (i.e. improve the SMSR). A diode laser element 210 has a spectral gain profile within which optical radiation is generated. Light emitted by the diode laser element is collected by beam-shaping optics 220 and then directed towards a volume Bragg grating, VBG, 230. The VBG reflects part of the light within a predetermined spectral band back towards the diode laser element 210, such that the laser element is caused to generate light within that spectral band. Since only part of the light within the predetermined spectral band is reflected by the VBG, a substantial fraction of light within the predetermined spectral band is transmitted through the VBG and can be used as an output 240 from the diode laser arrangement. The dielectric filter 232 is placed in the beam path after the VBG with the purpose of cleaning the output from unwanted side-modes. While the arrangement of FIG. 2 provides some improvement in terms of side-mode suppression, it is limited by the comparatively broad spectral width of dielectric filters. Again, this means that the peak wavelength in the output, as determined by the spectral band of the VBG, is surrounded by side-modes excited by the laser element.

[0028] FIG. 3 shows another diode laser arrangement in which the output is filtered in order to suppress side-modes. A diode laser element 310 has a spectral gain profile within which optical radiation is generated. Light emitted by the diode laser element is collected by beam-shaping optics 320 and then directed towards a volume Bragg grating, VBG, 330. The VBG 330 reflects part of the light within a predetermined spectral band back towards the diode laser element 310, such that the laser element is caused to generate light within that spectral band. Since only part of the light within the predetermined spectral band is reflected by the VBG, a substantial fraction of light within the predetermined spectral band is transmitted through the VBG 330 and can be used as an output 340 from the diode laser arrangement. In the arrangement shown in FIG. 3, a second VBG 332 is used for providing the filtering, in addition to the first VBG 330 providing the optical feedback to the laser element. By carefully matching the spectral band reflected by the first VBG 330 to the spectral band reflected by the second VBG 332, very high side-mode suppression can be obtained. The first VBG 330 provides feedback to the laser element in order to condition the generation of light therein, and even though the first VBG will transmit the side-modes excited by the laser element, the second VBG 332 effectively filters out those side-modes by only reflecting, as an output 340 from the diode laser arrangement, light within the predetermined spectral band.

[0029] While the arrangement of FIG. 3 has the potential of providing an excellent output in terms of side-mode suppression, it has some associated drawbacks that could make it less interesting in practice. As will be understood, in order for the arrangement to perform as desired, the spectral bands of reflection for the first and the second VBG must be carefully matched. If the second VBG does not reflect precisely the same spectral band as that provided by the first VBG as optical feedback to the laser element, the arrangement will fail or underperform. Hence, the arrangement of FIG. 3 has the absolute constraint that both VBGs must reflect the same spectral band. However, it is difficult to fulfil this constraint over typical operating conditions. Firstly, the second VBG operates under non-normal incidence, and there is typically a very narrow angle acceptance in order to maintain reflectivity for the predetermined spectral band. Moreover, the angle acceptance decreases as the spectral bandwidth of the VBG is made narrower and as the angle of incidence increases. In practice, this very limited angle acceptance also leads to the requirement that the output beam should be transversally single mode and well-collimated. Secondly, the spectral band of a VBG is temperature sensitive, which means that both VBGs must be carefully and jointly temperature controlled. Furthermore, VBGs are comparatively costly and the use of a second unit adds undesirably to the overall cost of the arrangement. To some extent, the above challenges can be addressed by tuning the angle of incidence on the VBG. For a collimated and transversally single-mode laser beam and an angle of incidence on the VBG of about 20-30 degrees, the reflected wavelength can be tuned slightly by changing the angle of incidence. The two VBGs could then be allowed to differ slightly with respect to the nominal wavelength for which they are designed. However, once the components have been fixed in position during assembly, such angle tuning will no longer be available.

[0030] FIG. 6 shows the spectral characteristic of the optical output of the arrangements shown in FIGS. 1-3, respectively. The peak output, which is the same for all three arrangements, can be seen at reference numeral 600. While the output has a narrow peak centered at the same wavelength for all three arrangements of FIGS. 1-3, it is evident that the side-mode behavior is very different between the arrangements. The side-modes are shown at reference numerals 601, 602 and 603, respectively.

[0031] The side-modes of the arrangement shown in FIG. 1, for which there is no additional filtering but the output is merely conditioned by optical feedback into the laser element from the VBG, there is a broad background of side-mode radiation shown at 601. This background corresponds to the gain profile of the laser element.

[0032] Some of this side-mode background is removed in the arrangement of FIG. 2, which has a side-mode background as shown at 602 in FIG. 6. The width of the side-mode background 602 corresponds to the transmission window of the dielectric filter 232 shown in FIG. 2. Even though the width of the side-mode background has been reduced, the level of the background is still the same around the peak 600. It is also evident from FIG. 6 that the side-mode background is still rather wide compared to the peak 600.

[0033] The spectral characteristics of the arrangement of FIG. 3 is also shown in FIG. 6. The peak 600 is still the same as for the arrangements of FIGS. 1 and 2, but the side-mode background is drastically reduced, as shown at 603. In fact, any side-mode background is almost completely eliminated by the use of the second VBG 332.

[0034] The spectral characteristics of an embodiment of the present invention is shown in FIG. 7. The set-up used for measuring the spectral characteristics was the one shown in FIG. 4. As evident, the side-mode suppression is just as effective as in the arrangement shown in FIG. 3. A narrow peak of the output is shown at reference numeral 700 without any side-modes around the same. There is only some low-level noise present outside the peak 700, as shown at 701.

[0035] A first embodiment of the invention is schematically shown in FIG. 4. A laser element 410 has a gain profile within which optical radiation is generated. Light emitted by the laser element is collected by beam-shaping optics 420 and directed towards a polarizing beam splitter, PBS, 422. In the embodiment shown, the polarization of light generated by the laser element 410 is linear and in such orientation that it passes through the PBS 422. However, in other embodiments there may be one or more polarizing-modifying elements provided between the laser element 410 and the PBS 440 in order to provide a polarization such that the generated light passes through the PBS 422.

[0036] After having passed, i.e. transmitted through, the PBS 422, light passes a polarizing-modifying element in the form of a quarter wave-plate 424. The quarter wave-plate modifies the linearly polarized light generated by the laser element 410 and transmitted through the PBS 422 into generally elliptically polarized light. After having passed the quarter wave-plate, the light impinges upon a volume Bragg grating, VBG, 430. The VBG 430 has a Bragg structure that is effective to reflect light within a narrow and predetermined spectral band back towards the quarter wave-plate. Light outside of the predetermined spectral band passes through the VBG 430 and is lost from the arrangement. It is thus typically desired to structure the VBG for high reflectivity at the desired wavelength peak, and in this case the reflectivity is about 95%. Yet higher reflectivity, approaching 100%, may undesirably lead to side-modes appearing in the reflected light from the VBG and should normally be avoided.

[0037] The light reflected from the VBG 430 then passes the quarter wave-plate 424 again, and is modified back into linearly polarized light but in another orientation compared to the light originally generated by the laser element 410. This means that when the light impinges upon the PBS 422 after having been reflected from the VBG 430 and modified by the quarter wave-plate 424, some light will be transmitted towards the laser element 410 and some light will be reflected out as an output beam 440. It should be noted that the light that is transmitted back towards the laser element 410 then has the same polarization state as the light originally generated by the laser element. This transmitted light returns into the laser element 410 and conditions it to generate light having similar spectral properties, i.e. spectral properties similar to the reflection band of the VBG.

[0038] In particular, the light reflected by the PBS 422 into the output beam 440 is entirely made up of light that has just been reflected from the VBG 430 and thus contains no light that comes directly from the laser element 410. In this manner, the output beam 440 is free from side-modes generated by the laser element 410 outside the reflection band of the VBG 430. In other words, the spectral properties of the output beam are determined by the properties of the VBG.

[0039] The embodiment shown in FIG. 4 thus provides a spectrally very clean output beam 440 using only a single VBG, while at the same time allowing spectrally selective feedback into the laser element.

[0040] Another embodiment of the present invention is schematically shown in FIG. 5. A laser element 510 has a gain profile within which optical radiation is generated. Light emitted by the laser element is collected by beam-shaping optics 520 and directed towards a polarizing beam splitter, PBS, 522. In the embodiment shown, the polarization of light generated by the laser element 510 is linear and in such orientation that it is reflected by the PBS 522. However, in other embodiments there may be one or more polarizing-modifying elements provided between the laser element 510 and the PBS 522 in order to provide a polarization such that the generated light is reflected by the PBS 522.

[0041] After having passed, i.e. been reflected by, the PBS 522, light passes a polarizing-modifying element in the form of a quarter wave-plate 524. The quarter wave-plate modifies the linearly polarized light generated by the laser element 510 and reflected by the PBS 522 into generally elliptically polarized light. After having passed the quarter wave-plate, the light impinges upon a volume Bragg grating, VBG, 530. The VBG 530 has a Bragg structure that is effective to reflect light within a narrow and predetermined spectral band back towards the quarter wave-plate. Light outside of the predetermined spectral band passes through the VBG 530 and is lost from the arrangement. It is thus typically desired to structure the VBG for maximum reflectivity at the desired wavelength peak, and in this case the reflectivity is about 95%. Yet higher reflectivity, approaching 100%, may undesirably lead to side-modes appearing in the reflected light from the VBG and should normally be avoided.

[0042] The light reflected from the VBG 530 then passes the quarter wave-plate 524 again, and is modified back into linearly polarized light but in another orientation compared to the light originally generated by the laser element 510. This means that when the light impinges upon the PBS 522 after having been reflected from the VBG 530 and modified by the quarter wave-plate 524, some light will be reflected towards the laser element 510 and some light will be transmitted out as an output beam 540. It should be noted that the light that is reflected back towards the laser element 510 then has the same polarization state as the light originally generated by the laser element. This reflected light returns into the laser element 510 and conditions it to generate light having similar spectral properties, i.e. spectral properties similar to the reflection band of the VBG.

[0043] In particular, the light transmitted through the PBS 522 into the output beam 540 is entirely made up of light that has just been reflected from the VBG 530 and thus contains no light that comes directly from the laser element 510. In this manner, the output beam 540 is free from side-modes generated by the laser element 510 outside the reflection band of the VBG 530. In other words, the spectral properties of the output beam are determined by the properties of the VBG.

[0044] The embodiment shown in FIG. 5 thus provides a spectrally very clean output beam 540 using only a single VBG, while at the same time allowing spectrally selective feedback into the laser element.

[0045] An advantage of the present invention is that the polarization-modifying element, i.e. the quarter wave-plates 424 and 524 in FIGS. 4 and 5, can be oriented during assembly of the laser arrangement to provide a desired balance between feedback and output. By rotating (the optical axis of) the quarter wave-plate, the polarization state of light returning to the PBS after having been reflected by the VBG can be varied, leading to a correspondingly varied fraction of light being transmitted/reflected by the PBS. In embodiments that comprise beam-shaping optics 420/520, about 5%-20% and preferably about 10% of the light reflected back from the VBG (reflectivity 95%) is preferably provided as optical feedback towards the laser element. If there is no beam-shaping optics, beam divergence will cause less of the optical feedback to actually enter into the laser element, thus typically requiring more light to be fed back towards the laser element. This possibility to vary the fraction of light being transmitted/reflected by the PBS is particularly advantageous considering the fact that magnitude of the reflectivity of the VBGs cannot be altered once they have been made.

[0046] The laser element in embodiments of the present invention is preferably an edge emitting single- or multimode diode laser. However, embodiments using vertical-cavity surface-emitting lasers, VCSELs, are also contemplated.

CONCLUSION

[0047] A laser arrangement is disclosed, comprising a laser element; a volume Bragg grating (VBG) for providing optical feedback to the laser element along a beam path, a polarizing beam splitter (PBS) arranged in the beam path between the laser element and the VBG; and a polarization-modifying element arranged in the beam path between the PBS and the VBG element; wherein said polarization-modifying element is structured and arranged to alter a polarization state of light reflected from the VBG such that an output beam can be provided by the PBS comprising light that has been reflected from the VBG. Thereby, the spectral properties of the output beam are determined by the properties of the VBG.

[0048] In some embodiments, the PBS is oriented to maximize transmission of linearly polarized light that is emitted from the laser element, as illustrated in FIG. 4. In other embodiments, the PBS is oriented to maximize reflection of linearly polarized light that is emitted from the laser element, as illustrated in FIG. 5.

[0049] By providing an output beam comprising light that has been reflected from the VBG, a spectrally clean output with an excellent side-mode suppression ratio is obtained. The spectral properties of the output beam are determined by the properties of the VBG. Since side modes outside of the spectral band of the VBG are not present in the output beam, embodiments of the present invention are expected to be particularly attractive for use in low wavenumber Raman spectroscopy.

[0050] Embodiments of the present invention could thus provide a spectrally narrow-band output using a single VBG. The requirement in other arrangements of having two separate VBGs of different reflectivities that are spectrally matched is eliminated. Furthermore, the amount of optical feedback to the laser element can be freely selected by rotating the quarter wave-plate into a desired orientation before fixing it during assembly of the laser arrangement.