CA2817472A1 - Swept source and optical coherence tomograph using same - Google Patents

Abstract

The swept source can be used in a optical coherence tomograph and have a laser cavity having a light source for generating a light beam along an optical axis, a filter having a Fabry-Perot cavity having (i) a substrate having a substrate plane parallel to the optical axis; (ii) two reflectors including a mobile reflector, each of the two reflectors comprising layers extending normal from the plane of the substrate and transversely across the optical axis, the reflectors having air spacings between the layers, the two reflectors being spaced from one another with an air gap formed between the two reflectors, the air gap having a thickness along the optical axis; and (iii) an electrostatic actuator operable to move the mobile reflector along the optical axis to change the thickness of the air gap and thereby tune the Fabry-Perot cavity allowing selection of different laser wavelengths;
and an output coupler.

Description

SWEPT SOURCE AND
OPTICAL COHERENCE TOMOGRAPH USING SAME
BACKGROUND
[0001] Swept sources having fast repetition rates are of particular interest for optical coherence tomography (OCT) and other biomedical optics imaging applications.
OCT is an imaging technique, which consists in acquiring a series of parallel depth-scans within a semi-transparent medium by optical interrogation. The scans are then processed in a two or three dimensional model from which cross sectional views can be extracted using signal processing.

[0002] OCT is used as a medical imaging and diagnosis tool, and is also used in non-destructive testing (NOT) applications. The main features considered when selecting OCT
over competing X-ray and ultrasound based imagery are resolution, acquisition rate, size and cost (and, in the case of X-rays, the absence of ionizing radiations).
Often times, when selecting a specific technology, one trades off one or the other of these features for some of the others.

[0003] OCT is also a tool for nonbiological applications, even if less commonly use. In fact, as the technology has a contactless and noninvasive operating method it is attractive for nondestructive testing and evaluation (NDT, NDE) of manufactured parts and remote conduits such as tubes and pipes. Numerous industrial applications include surface and thickness measurements; NDT, NDE and noncontact material characterization for ceramics, glass and optical components, polymers, fiber composites and paper; and quality evaluation of data storage devices.

[0004] Although former OCT techniques were satisfactory to a certain degree, there remained room for improvement.
SUMMARY

[0005] There is provided a swept source for SS-OCT (Swept Source-OCT) having a fast repetition rate for its cost. The swept source can have a micro electromechanical system based filter that can be actuated in a push-pull configuration with a linear relation between the spectral scan and the actuation voltage.

[0006] In accordance with one aspect, there is provided an optical coherence tomograph comprising : a swept source having a light source for generating a light beam along an optical axis, a filter having a Fabry-Perot cavity having (i) a substrate having a substrate plane parallel to the optical axis; (ii) two reflectors including a mobile reflector, each of the two reflectors comprising layers extending normal from the plane of the substrate and transversely across the optical axis, the reflectors having air spacings between the layers, the two reflectors being spaced from one another with an air gap formed between the two reflectors, the air gap having a thickness along the optical axis; and (iii) an electrostatic actuator operable in a push-pull configuration with an actuation voltage, in which push-pull configuration the mobile reflector is electrostatically pushed into one direction along the optical axis and the mobile reflector is electrostatically pulled in the same direction along the optical axis, thereby allowing to tune the Fabry-Perot cavity in a selection of different wavelengths with a linear relation between the resulting thickness of the air gap and the actuation voltage of the electrostatic actuator, and an output coupler; an interferometer coupling the output coupler of the swept source to a reference arm and a sample arm, and configured to produce optical interference between the reference arm and the sample arm; a detector connected to the interferometer so as to detect the optical interference; and a signal processor for processing the optical interference to produce optical coherence tomography images.

[0007] In accordance with another aspect, there is provided a swept source for generating laser at different wavelengths, the swept source comprising: a laser cavity having a semiconductor optical amplifier for generating a laser beam along an optical axis; a filter having a Fabry-Perot cavity having (i) a substrate having a substrate plane parallel to the optical axis; (ii) two reflectors including a mobile reflector, each of the two reflectors comprising layers extending normal from the plane of the substrate and transversely across the optical axis, the reflectors having air spacings between the layers, the two reflectors being spaced from one another with an air gap formed between the two reflectors, the air gap having a thickness along the optical axis; and (iii) an electrostatic actuator operable to move the mobile reflector along the optical axis to change the thickness of the air gap and thereby tune the Fabry-Perot cavity allowing selection of different laser wavelengths; and an output coupler.

[0008] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.
DESCRIPTION OF THE FIGURES

[0009] In the figures, [0010] Fig. 1 is a schematic diagram of a swept source optical coherence tomograph;

[0011] Fig. 2 is an example of the swept source with a cavity in ring configuration;

[0012] Fig. 3 is an example of the emission spectrum of the swept source for six different actuation voltages;

[0013] Fig. 4 is a Scanning Electron Microscope (SEM) image of an example of micro electromechanical filter having an electrostatic actuator where the inset emphasizes the Fabry-Perot cavity;

[0014] Fig. 5 is graph illustrating the relation between the displacement of the mobile reflector of the Fabry-Perot as a function of the actuation voltage wherein a quadratic relation is observed for a conventional actuation technique and a linear relation is observed for the push-pull configuration;

[0015] Fig. 6 is an example of a finite element analysis (FEM) simulation showing that the bi-directional comb-drive actuator has a natural modal displacement limited to an axis, the x axis, for instance; and [0016] Fig. 7 is an example of the amplitude of the mobile reflector of the Fabry-Perot cavity where the natural resonance frequency of the bi-directional comb-drive actuator is approximately 35 kHz.

DETAILED DESCRIPTION

[0017] Fig. 1 show a typical swept source optical coherence tomograph. In SS-OCT, the swept source (SS) 70 has an interferometer 80 which produces optical interference with a reference arm (RA) 74 and a sample arm (SA) 76. The optical interference between the two arms 74 and 76 of the interferometer 80 are used to measure a reflectivity profile as a function of depth. After the optical interference is detected by a detector (PD) 72, the optical coherence tomography three dimensional images are obtained after processing using a signal processor 78.

[0018] The schematic of the swept source 70 cavity used in the optical coherence tomograph disclosed is shown in Fig. 2. In this specific embodiment, the components are optically connected using optical fiber 66 (Corning SMF-28) in a ring configuration cavity.
This embodiment comprises a gain medium which is a semiconductor optical amplifier (BOA) 52 from Thorlabs (BOA1017S, Newton, NJ) centered at 1310 nm with a spontaneous emission 3dB bandwidth of 70 nm, operable with a semiconductor optical amplifier controller.
54. The performance of the SOA is highly dependent on polarization. This component is therefore surrounded by two polarization controllers (PC) 56 which were used to optimize the power of a laser output measured at an output coupler 58. Two circulators 60 or isolators can be used as unidirectional devices in order to avoid back reflections from the different interfaces to propagate in the cavity to the gain medium, which could otherwise stimulate emission at unwanted frequencies and alter the laser emission spectrum.
Finally, a MEMS-based filter 62 has a Fabry-Perot cavity (FP) 10 that is actuated with a filter controller 64.
The MEMS-based filter 62 will be detailed extensively below in this disclosure.

[0019] The gain medium emits a spontaneous emission spectrum, which propagates through the cavity in the direction determined by the two isolators 60. Then, the MEMS-based filter 62 is used to filter this spontaneous emission. It only allows wavelengths near its resonance to complete multiple roundtrips in the cavity, stimulate emission in the gain medium and consequently produce a laser emission. A tap coupler 58 (50/50) was inserted in the ring cavity, connected to the filter 62, and used as an output coupler.
With this tap coupler 58, half of the signal is redirected into the cavity while the other half of the signal is characterized using an optical spectrum analyzer 68 (typically referred to in the art as an OSA).

[0020] Using the laser cavity shown in Fig. 2, the fabricated MEMS-based device was used as an intra-cavity filter to form a wavelength swept source. The laser, centered at 1310 nm, was tuned over more than 20 nm as shown in Fig. 3. The full-width at half-maximum is lower than 0.06 nm. Its measurement was limited by the resolution bandwidth of the OSA. At this stage, the main limitation of the laser for OCT application is its small tuning range. It is believed that the tuning range could be increased with the optimization of the fabrication process since previous work demonstrates that such performances can indeed be reached.
Another approach would be to use multiple filters in parallel, covering specific and complementary spectral region. For example, one filter could cover the spectral range from 1280 nm to 1300 nm while another filter could cover the spectral range from 1300 nm to 1320 nm. With this embodiment, the whole spectrum of the SOA could be covered.
The low cost, small size and easy integration of the filter can render such an approach very attractive.

[0021] Fig. 4 shows a SEM image of a MEMS-based filter 62. The filter has a Fabry-Perot cavity 10 and a bi-directional comb-drive actuator 12. The Fabry-Perot cavity comprises a substrate having a plane 14 parallel to the optical axis 16, two reflectors 18 and 26 (that can also be referred to as Bragg mirrors) where each reflector comprises layers 20 of monocrystalline silicon extending normal from the plane 14 of the substrate and transversely across the optical axis 16, air spacings 22 between the layers 20, and an air gap between the two reflectors 18 and 26, the air gap having a thickness 24 along the optical axis 16, one of the two reflectors being mobile; and an electrostatic actuator 12 operable, in the plane of the substrate, to move the mobile reflector 26 along the optical axis 16 to change the thickness 24 of the air gap and thereby tune the Fabry-Perot cavity 10 allowing selection of different laser wavelengths. In this specific embodiment, it will be noted that each reflector has two layers 20, though it will be understood that the number of layers can change in alternate embodiments (e.g. 3 layers, 4 layers, etc).

[0022] The inset of Fig. 4 shows a close-up view of the two reflectors 18 and 26 of the Fabry-Perot cavity 10. This type of cavity, inside the reflection bandwidth of the reflectors, reflects light except at its resonance frequencies where the transmission is maximal Henceforth, this Fabry-Perot cavity 10 can be used as a wavelength selection filter. Of course, various parameters influence the optical response and the performances of such filters. In particular, the design proposed was optimized using a model recently published in "Advances in Modeling, Design, and Fabrication of Deep-Etched Multilayer Resonators" by R. St-Gelais, A. Poulin, and Y.-A. Peter, Journal of Lightwave Technology, doi:
10.1109/JLT.2012.2191136, taking into account not only the dimensions and refractive indices of the layers but also the limitations due to Gaussian beam divergence and surface roughness.

[0023] More specifically, the electrostatic actuator 12 has two fixed portions 28 extending from the plane 14 of the substrate and a mobile portion 32 between the two fixed portions, the mobile portion 32 is elastically connected to the substrate and rigidly connected to the mobile reflector 26. In a typical embodiment, the mobile reflector extends from the mobile portion in the plane of the substrate in order for the mobile reflector to be perpendicular with the optical axis. Moreover, in the illustrated embodiment, the connection of the mobile portion 26 to the substrate is achieved using four springs 34, each having the same resonance frequency. More emphasized in Fig. 6, the mobile portion 32 has edges 38 each facing an edge 40 of the fixed portion 28, wherein the edges 38 of the mobile portion 32 each have a plurality of teeth 42 interspaced with a plurality of teeth 44 protruding from the edges 40 of the two fixed portions 28 thereby forming the bi-directional comb-drive actuator 12. To control this electrostatic actuator, a controller is used to control the electric potential onto an electrode 46 of one fixed portion, onto an electrode 48 of the mobile portion, and onto an electrode 50 of the other fixed portion, wherein the electrode 48 is grounded, for instance.

[0024] The use of the bi-directional comb-drive actuator 12 was enabled by the in-plane configuration. Unlike parallel plate actuators, whose plates collapse for displacements larger than one third of the initial gap, these later allow large displacements and do not exhibit a pull-in voltage (sometimes referred to as `pull-in instability' in the art).

[0025] Conventional actuators are composed of two electrodes, one being fixed while the other is movable, which leads to a quadratic relation between the displacement and the actuation voltage, just as shown in Fig. 5. In order to maximize the performance for OCT
imaging systems, a device providing a linear spectral scan over a spectral range of the swept source is desirable. In this embodiment, the linear behavior can be obtained from a bi-directional comb-drive actuator 12 by operating it in push-pull configuration using an additional electrode 50. It is interesting to note that the in-plane configuration of this filter is well adapted to the implementation of this third electrode 50.

[0026] In the MEMS-based filter using the push-pull configuration, a bias source Vb of 200 V, for instance, and an alternative source Va can be used to actuate the comb-drive actuator.
Consequently, applied voltages on the electrodes 46 and 50 are respectively Vb+Va and Vb-1 0 Va, while electrode 48 is grounded. Doing so, the mobile reflector 26 is electrostatically pushed along the optical axis 16 toward the electrode 46. Asymmetry between the electric potentials linearizes the electrostatic actuator response. The value of the alternative source determines the amplitude of the reflector displacement. The value of the bias source determines the slope of the linear spectral scan relation observed between the displacement and the actuation voltage, as shown in Fig. 5. Moreover, it has to be noted that the spectral scan is proportional to the displacement of the mobile reflector 26 of the Fabry-Perot cavity 10, consequently, when the relation between the displacement and the actuation voltage is linear, so is the relation between the spectral scan and the actuation voltage.

[0027] Optimization of the device is not limited to the optical performances of the Fabry-Perot cavity. The electromechanical response of the device was simulated and optimized using a finite element method (FEM). One of the aspects studied was the shape and resonance mode at a first resonance frequency. The springs 34 are designed to achieve an oscillation in the plane 14 of the substrate, parallel to the optical axis of the cavity in a natural modal displacement at a natural resonance frequency of 40 kHz, for instance.

[0028] Fig. 6 presents the natural resonance mode of the device obtained through the FEM simulation, which shows that the shape obtained was the one desired.
Furthermore, the shape of the first mode was mainly determined by the symmetry of the device with respect to the x axis and by the ratio between the spring constants along the z and the x axis. Of course, the stiffness of the mobile portion has to be high enough to avoid an unwanted deformation when actuated at high frequencies.

[0029] In the context of the instant disclosure, it is considered sufficient to mention that the device can be structured by bulk micromachining of a SO1 substrate, which can be done with a single photolithography step. The in-plane configuration design is well adapted to such a simple fabrication process. in addition, it allows the integration of optical fiber grooves 36 which greatly simplify the optical alignment. In an embodiment, the substrate has two optical fiber grooves 36 each extending from the Fabry-Perot cavity 10 normally to the reflectors and parallel with the plane 14 of the substrate along the optical axis 16. For the optical fiber aligned to be satisfactory, the optical fiber groove 36 can be configured and adapted to snugly receive the optical fiber.

[0030] A white light interferometer (such as manufactured by Fogale Nanotech for instance) was used to characterize the actuator 12. Its in-plane configuration having a displacement parallel to the optical axis was monitored for different actuation voltages and frequencies. First, an alternative voltage is applied between the electrode 46 and 48. The amplitude of the mobile reflector 26 displacement was measured as a function of the alternative voltage frequency, where the results are reported in Fig. 7. The natural resonance frequency was observed at 35 kHz. It is approximately 13% lower than the expected value of 40 kHz. This shift can be explained by slight variations between the expected and the experimental dimensions of the springs 34. These variations are likely largely imparted by the etching step where we can observe some under-etch and a slight non-verticality of the trenches. It is considered likely that these geometrical parameters have a greater influence than the air damping, which have been taken into account during the optimization of the device geometry. It was confirmed by visual inspection that the vibration mode corresponding to this frequency is the one shown in Fig. 6. The resonance peak of this vibration mode is narrow and does not affect frequencies lower than 30 kHz.
The device should consequently be able to follow arbitrary signal waveform with frequency components lower than 30 kHz. It is considered likely that this limit on the actuation frequency will be overcome by stiffening the springs connecting the mobile portion to the substrate.

[0031] It is believed that the concept presented herein can be used in to make embodiments which provide a useful combination of small size, low cost, design simplicity, robustness, and linearity. Linearity of the spectral scan and the 35 kHz natural resonance frequency of the actuator were demonstrated. Furthermore, it was demonstrated that the swept source can follow arbitrary signal waveforms with frequency components lower than 30 kHz. This frequency could be easily increased by simple modifications of the geometry of the device. It is considered that the achieved laser tuning range of 20 nm will be further improved by working on the fabrication process or by using multiple filters in parallel. Finally, it is believed that the development of a source exhibiting high repetition rates, linearity and low cost is a significant step toward broader uses of OCT for clinical and NDT
applications.

[0032] As will be understood, the examples described above and illustrated are intended to be exemplary only. In other embodiments, the optical axis could be guided by air in free space, or the signal can be guided by an optical fiber or by integrated silicon waveguides to name examples. The pumping mechanism could be a laser diode or any other laser or flash lamps capable of exciting atoms in the gain medium. The swept source could be used in applications other than swept source optical coherence tomography, including but not limited to spectrally encoded confocal microscopy, spectrally encoded endoscopy, extended-focus chromatic confocal microscopy, etc. The SS-OCT could be used in medical, NOT
or any other field where it is needed. Furthermore, although in the embodiment detailed above, the spectral range of the swept source was around 1310 nm, it will be understood that alternate embodiments can work at other wavelengths where the light is guided by the substrate, e.g.
between 1000 nm and 10 pm. Also, the gain medium could be a semiconductor wafer or rare-earth doped fiber such as an erbium-doped fiber or the like, semi-conductor amplifier, or another light source than a laser source can be used altogether. Alternate embodiments using erbium-doped fibers can sweep in wavelengths between 1460 nm and 1640 nm for instance. The scope is indicated by the appended claims.

Claims (38)

1. An optical coherence tomograph comprising :
a swept source having a light source for generating a light beam along an optical axis, a filter having a Fabry-Perot cavity having (i) a substrate having a substrate plane parallel to the optical axis; (ii) two reflectors including a mobile reflector, each of the two reflectors comprising layers extending normal from the plane of the substrate and transversely across the optical axis, the reflectors having air spacings between the layers, the two reflectors being spaced from one another with an air gap formed between the two reflectors, the air gap having a thickness along the optical axis; and (iii) an electrostatic actuator operable to move the mobile reflector in a push-pull configuration upon receiving an actuation voltage, thereby allowing to tune the Fabry-Perot cavity in a selection of different wavelengths, and an output coupler;
an interferometer coupling the output coupler of the swept source to a reference arm and a sample arm, and configured to produce optical interference between the reference arm and the sample arm;
a detector connected to the interferometer so as to detect the optical interference; and a signal processor for processing the optical interference to produce optical coherence tomography images.

2. The optical coherence tomograph of claim 1, wherein the light source is a laser source, and the swept source further has a laser cavity coinciding with the optical axis, the laser cavity having a gain medium; a pumping mechanism operable to pump the gain medium for laser generation along the optical axis.

3. The optical coherence tomograph of claim 2, wherein the laser cavity is in a ring configuration.

4. The optical coherence tomograph of claim 2, wherein the optical axis of the laser cavity is guided by an optical fiber.

5. The optical coherence tomograph of claim 4, wherein the gain medium has a rare-earth doped fiber and wherein the pumping mechanism has a laser diode coupled to the gain medium.

6. The optical coherence tomograph of claim 4, further comprising a semiconductor optical amplifier assuming the functions of the laser gain media and the pumping mechanism.

7. The optical coherence tomograph of claim 1 wherein, in the push-pull configuration the mobile reflector is electrostatically pushed into one direction along the optical axis and the mobile reflector is electrostatically pulled in the same direction along the optical axis, with a linear relation between the resulting thickness of the air gap and the actuation voltage of the electrostatic actuator.

8. The optical coherence tomograph of claim 1, wherein the electrostatic actuator has two fixed portions extending normal from the plane of the substrate and a mobile portion between the two fixed portions, the mobile portion being elastically connected to the substrate and connected to the mobile reflector, the mobile portion having opposite edges each facing a corresponding edge of the fixed portion.

9. The optical coherence tomograph of claim 8, wherein the edges of the mobile portion each have a plurality of teeth interspaced with a plurality of teeth protruding from the edges of the two fixed portions thereby forming a bi-directional comb-drive actuator.

10. The optical coherence tomograph of claim 8, wherein the fixed portions and the mobile portion are connected to separate electrodes that are mounted in the plane of the substrate.

11. The optical coherence tomograph of claim 8, wherein the mobile portion of the electrostatic actuator is elastically connected to the substrate by springs.

12. The optical coherence tomograph of claim 5, wherein the substrate has two optical fiber grooves each extending from the Fabry-Perot cavity normal to the reflectors, each optical fiber groove being parallel with the plane of the substrate along the optical axis, the optical fiber groove being configured and adapted to snugly receive the optical fiber.

13. The optical coherence tomograph of claim 1, wherein the electrostatic actuator is operable by a controller.

14. The optical coherence tomograph of claim 1, wherein each reflector has two layers.

15. The optical coherence tomograph of claim 1, wherein each reflector is made of monocrystalline silicon.

16. The optical coherence tomograph of claim 1, wherein the output coupler is optically connected to the Fabry-Perot cavity.

17. The optical coherence tomograph of claim 2, wherein the laser cavity comprises at least two filters in parallel thereby enabling selection of laser wavelengths on specific and complementary spectral ranges.

18 The optical coherence tomograph as defined in claim 1, wherein polarization controllers surround the swept source in the laser cavity, along the optical axis thereby controlling a polarization of a signal.

19. The optical coherence tomograph as defined in claim 1, wherein unidirectional devices are optically connected in the laser cavity, along the optical axis, thereby controlling a direction of a signal in the laser cavity and avoiding reflections back to the swept source.

20. A swept source for generating laser at different wavelengths, the swept source comprising :
a laser cavity having a semiconductor optical amplifier for generating a laser beam along an optical axis;
a filter having a Fabry-Perot cavity having (i) a substrate having a substrate plane parallel to the optical axis; (ii) two reflectors including a mobile reflector, each of the two reflectors comprising layers extending normal from the plane of the substrate and transversely across the optical axis, the reflectors having air spacings between the layers, the two reflectors being spaced from one another with an air gap formed between the two reflectors, the air gap having a thickness along the optical axis; and (iii) an electrostatic actuator operable to move the mobile reflector along the optical axis to change the thickness of the air gap and thereby tune the Fabry-Perot cavity allowing selection of different laser wavelengths;
an output coupler.

21. The swept source of claim 20, wherein the laser cavity is in a ring configuration.

22. The swept source of claim 20, wherein the optical axis of the laser cavity is guided by an optical fiber.

23. The swept source of claim 20, wherein the electrostatic actuator is operable to move the mobile reflector in a push-pull configuration upon receiving an actuation voltage.

24. The swept source of claim 23 wherein, in the push-pull configuration the mobile reflector is electrostatically pushed into one direction along the optical axis and the mobile reflector is electrostatically pulled in the same direction along the optical axis, with a linear relation between the resulting thickness of the air gap and the actuation voltage of the electrostatic actuator.

25. The swept source of claim 23, wherein the electrostatic actuator has two fixed portions extending normal from the plane of the substrate and a mobile portion between the two fixed portions, the mobile portion being elastically connected to the substrate and connected to the mobile reflector, the mobile portion having opposite edges each facing a corresponding edge of the fixed portion.

26. The swept source of claim 25, wherein the edges of the mobile portion each have a plurality of teeth interspaced with a plurality of teeth protruding from the edges of the two fixed portions thereby forming a bi-directional comb-drive actuator.

27. The swept source of claim 25, wherein the fixed portions and the mobile portion are connected to separate electrodes that are mounted in the plane of the substrate.

28. The swept source of claim 25, wherein the mobile portion of the electrostatic actuator is elastically connected to the substrate by springs.

29. The swept source of claim 22, wherein the substrate has two optical fiber grooves each extending from the Fabry-Perot cavity normal to the reflectors, each optical fiber groove being parallel with the plane of the substrate along the optical axis, the optical fiber groove being configured and adapted to snugly receive the optical fiber.

30. The swept source of claim 20, wherein the electrostatic actuator is operable by a controller.

31. The swept source of claim 20, wherein each reflector has two layers.

32. The swept source of claim 20, wherein each reflector is made of monocrystalline silicon.

33. The swept source of claim 20, wherein the output coupler is optically connected to the Fabry-Perot cavity.

34. The swept source of claim 20, wherein the laser cavity comprises at least two filters in parallel thereby enabling selection of laser wavelengths on specific and complementary spectral ranges.

35. The swept source as defined in claim 20, wherein polarization controllers surround the swept source in the laser cavity, along the optical axis thereby controlling a polarization of a signal.

36. The swept source as defined in claim 20, wherein unidirectional devices are optically connected in the laser cavity, along the optical axis, thereby controlling a direction of a signal in the laser cavity and avoiding reflections back to the swept source.

37. The optical coherence tomograph of claim 2, wherein the optical axis of the laser cavity is guided by a silicon waveguide.

38. The optical coherence tomograph of claim 20, wherein the optical axis of the laser cavity is guided by a silicon waveguide.