Tunable transmission grating laser with feedback

Abstract

In tunable transmission grating lasers, alignment of the lasing cavity mode to the grating filter spectrum of the laser can be achieved using the position of the intracavity beam relative to the gain medium as feedback. In various embodiments, the displacement of the intracavity beam from the gain medium is indirectly monitored using an image of the intracavity beam created outside the cavity and an additional transmission grating. Various devices for measuring the position of this monitoring beam and for adjusting tunable components of the laser based thereon are described.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 034,268, filed June 3, 2020, which is hereby incorporated by reference in its entirety. BACKGROUND

[0003] Many optical sensing and imaging techniques, including, for example, optical frequency domain reflectometry (OFDR), gas absorption line detection, and optical coherence tomography (OCT), use high-speed laser wavelength scanning. Typically, two conditions determine the lasing wavelength of a laser. The first condition, also referred to as the phase matching condition through the laser cavity, is that one round trip of light through the cavity accumulates 2p-N radians, where N is an integer. Different N values correspond to different cavity modes, which represent possible resonance frequencies of the cavity. The second condition is that the loss through the cavity is less than the gain, or in other words, the transmission efficiency in the cavity is greater than zero. The range of wavelengths that satisfy this condition, reflected in the wavelength-dependent transmission efficiency, can be controlled by design to limit the number of cavity modes that can exist in the cavity. Thus, the wavelength-dependent transmission efficiency acts as a mode selection spectral filter. In a tunable laser used for wavelength scanning, the wavelength locations of the cavity modes and the wavelength locations of the spectral filter are typically controllable independently of each other. During such a tuning process, it is desirable to maintain some alignment between the lasing cavity mode and the filter spectrum. Otherwise, if the selected cavity mode shifts relative to the filter, mode hopping can occur - i.e., a sudden wavelength jump from one cavity mode to another. In laser wavelength scanning, such mode hopping is generally undesirable. Thus, a method for aligning the cavity mode of a laser with the filter spectrum is needed to avoid or at least reduce mode hopping. BRIEF DESCRIPTION OF DRAWINGS

[0004] The present disclosure provides a tunable transmission grating laser with sign-magnitude feedback that helps to align the cavity mode of the laser with the filter spectrum. Various embodiments are described with reference to the accompanying drawings, in which:

[0005] Figure 1 is a schematic top view of a basic configuration of an example tunable laser with a controllable mirror and a transmission grating according to various embodiments;

[0006] Figure 2A and Figure 2B schematically illustrate a spectrum of a laser cavity mode of a laser of Figure 1

[0007] Figure 3A and Figure 3B in top view Figure 1 ​different relative orientations between the tunable laser and the transmission grating of the laser, illustrating tuning of the grating filter spectrum according to various embodiments;

[0008] Figure 4A and Figure 4B is Figure 1 a schematic top view of the tunable laser of Figure 4C illustrating a way to shift the cavity mode of the laser without altering the filter spectrum according to various embodiments, and

[0009] Figure 5A and Figure 5B schematically shows Figure 2A the laser cavity mode of Figure 2B the grating filter spectrum of

[0010] Figure 6 is a schematic top view of an example tunable laser according to one embodiment, having a transmission grating and a mirror movable about a remote pivot point to facilitate mode-hop-free tuning;

[0011] Figure 7A is Figure 1 a schematic top view of the tunable laser of Figure 7B illustrating an off-center cavity mode relative to the filter spectrum of the laser, as can be indirectly monitored to facilitate alignment according to various embodiments, and

[0012] Figure 8 is a schematic side view of the tunable laser of Figure 1 illustrating a tilt angle of the controllable mirror, as can be adjusted according to various embodiments;

[0013] Figure 9 is a schematic block diagram of a tunable transmission grating laser system with feedback control according to various embodiments;

[0014] Figure 10 is a schematic top view of a portion of a tunable transmission grating laser system with feedback control according to various embodiments, including Figure 1 the tunable laser of

[0015] Figure 11A is a schematic front view of a quadrant detector for measuring a monitor beam spot according to various embodiments, and Figure 11B shows in side view a corresponding position of the intra-cavity beam spot relative to the gain medium of the laser;

[0016] Figure 12 is a schematic top view of a portion of a tunable transmission grating laser system with feedback control according to one embodiment that utilizes a position sensitive diode to measure the location of a monitor beam;

[0017] Figure 13 is a schematic top view of a portion of a tunable transmission grating laser system with feedback control according to one embodiment that utilizes an imaging array to measure the location of a monitor beam;

[0018] Figure 14 is a schematic top view of a portion of a tunable transmission grating laser system with feedback control according to one embodiment that is configured to scan a monitor beam across a receiver;

[0019] Figure 15A is a schematic top view of a portion of a tunable transmission grating laser system with feedback control according to one embodiment that is configured to measure the location of a monitor beam with a small area movable receiver, and Figure 15A is a front view of an example fiber based receiver as can be used in this embodiment;

[0020] Figure 16 is a flowchart of a method of aligning an intracavity beam to a gain medium in a tunable laser according to various embodiments.

[0021] Figure 17 is a flowchart of a method of mode hop free laser tuning according to various embodiments. DETAILED DESCRIPTION

[0022] A tunable laser is described herein that uses a controllable resonator mirror in conjunction with a mode selecting transmission grating to facilitate wavelength scanning. Also described is an approach to aligning a selected cavity mode of a tunable laser to the grating filter spectrum of the laser and minimizing the intracavity loss in the laser by the degree of alignment of the laser beam inside the resonant cavity to the gain medium of the laser. In principle, movement of the intracavity beam relative to the gain medium can provide a good feedback signal for alignment. However, it is often impractical to place a detector around the gain medium and shield it from stray light to directly measure the beam position in the laser. According to various embodiments, this constraint is circumvented by using a second transmission grating to diffract the zeroth order transmission of the mode selecting transmission grating used inside the resonant cavity to create an image of the intracavity beam (also referred to herein as the "monitor beam"). The monitor beam can be focused down to create an image of the beam spot created on the gain medium by the intracavity beam. The displacement of this monitor beam spot from the position associated with mode to filter alignment can be used as a feedback control signal to adjust the mode selecting grating and / or the controllable mirror.

[0023] The foregoing summary, as well as the following detailed description of certain embodiments of the present application, will be better understood when read in conjunction with the accompanying drawings, in which:

[0024] Figure 1 A basic configuration of an example tunable laser 100 is illustrated in accordance with various embodiments. The laser 100 (as viewed in a top view, i.e., in a projection onto a plane in which the optical beam propagates, herein also referred to as the "laser plane") comprises a resonator cavity and a gain medium 102 inside the resonator cavity, in which stimulated emission occurs. In the illustrated example, the gain medium 102 is a gain channel of a diode gain block 103. However, other types of gain media can alternatively be used, such as doped glasses (e.g., fused silica doped with erbium or ytterbium), doped crystals (YAG, titanium sapphire, ruby), gases (argon, neon, helium, fluorine) and dyes in solvents. The resonator cavity is formed between two resonator mirrors 104, 106. One of the resonator mirrors, as shown mirror 104, is controllable and serves as a tuning element for the laser wavelength; this mirror 104 is also referred to herein as the "controllable resonator mirror" or "controllable mirror". The other, fixed mirror 106, can be formed by the back surface of the diode gain block 103, as shown. Alternatively, a separate mirror can be placed on the side of the gain medium 102 opposite the controllable mirror 104. Regardless of the specific implementation, the fixed mirror 106 is typically configured to couple a portion of the laser light out of the cavity and into optics that use the laser light for some purpose. Inside the cavity, a lens 108 can focus light onto the gain medium 102 and collimate reflected light from the gain medium 102.

[0025] The laser 100 further comprises an intra-cavity transmission grating 110 (also referred to herein more simply as the "transmission grating 110" or simply "grating 110"), i.e., a diffractive grating that transmits rather than reflects diffracted light. The transmission grating 110 is placed between the controllable mirror 104 and the gain medium 102 and serves as a selective loss or optical filter that limits the wavelengths that can exist in the "cavity". The controllable mirror 104 is positioned in the path of the first (or higher) order diffraction of the transmission grating 110, such that light between the grating 110 and the controllable mirror 104 propagates at an angle (e.g., approximately a right angle) relative to light between the gain medium 102 and the grating 110.

[0026] Figure 2A The spectrum of the cavity modes 200 of the laser 100 is schematically shown, illustrating their uniform frequency spacing. The frequency (and thus, wavelength) of the cavity modes 200 depends on the cavity length, which can be varied, e.g., by moving the controllable mirror 104 closer to or further away from the grating 110.

[0027] Figure 2BThe filter spectrum 202 of the laser is schematically shown, which depends on the configuration of the transmission grating 110 and the mirror 104. Only cavity modes 200 with wavelengths within the filter spectrum 202 can exist in the cavity; therefore, the transmission grating 110 serves as a mode selection element. The spectral width Δf of the filter spectrum 202 is a function of the width of the region of the grating 110 illuminated by the beam in the cavity. The spectral positions of the filter spectrum 202 (e.g., according to their peak frequencies f) P (corresponding to peak wavelength λ) P The definition depends on the angles at which light is incident on and diffracted from grating 110, and the angle of mirror 104. More specifically, the position of the peak in the filter spectrum is determined according to the grating equation:

[0028] d(sinθ i –sinθ m )=mλ,

[0029] Where d is the periodicity of the diffraction grating (e.g., the distance between adjacent grooves in a grating), θ i θ is the angle of incidence of light from gain medium 102 on the grating. m λ is the diffraction angle of diffraction order m (m is an integer), and λ is the wavelength. For a specific incident angle (θ) i ), filter peak wavelength λ P With diffraction angle θ m The sine changes proportionally, and the diffraction angle θ m This is associated with a portion of the light reflected back into the gain medium within the cavity, and this angle can be controlled by the mirror 104. That is, among light diffracted at multiple angles, the resonator mirror 104 selectively reflects the light back into the gain medium 102 at a single angle by virtue of its orientation relative to the grating 110. Therefore, the filter spectrum 202 of the laser 100 can be shifted by rotating the mirror 104 in the laser plane.

[0030] To illustrate this concept, Figure 3A and Figure 3B The different relative orientations between the controllable mirror 104 and the transmission grating 110 of the laser 100 are depicted in a top view, illustrating the tuning of the filter spectrum 202 according to various embodiments. The grating 110 is in a fixed position and orientation relative to the gain medium 102 (resulting in a fixed incident angle θ on the grating 110). i However, the controllable resonator reflector 104 is relative to... Figure 3A and Figure 3BThe gratings 110 in the cavity are oriented at two different angles. The gratings 110 diffract light of different wavelengths at these corresponding angles (illustrated by solid and dashed lines, respectively), and regardless of which wavelength is orthogonally incident on the mirror 104, it is reflected back to the gain medium 102, thereby enhancing it within the cavity. Therefore, the tilted mirrors effectively shift the filter spectrum 202, causing its peak to appear at different frequencies f. P and wavelength λ P Place.

[0031] Figure 4A and Figure 4B The top view of the laser 100 illustrates various ways to offset the cavity mode 200 of the laser without changing the filter spectrum 202. Figure 4C The spectral shift of the cavity mode 200 is illustrated in a schematic depiction of the spectrum of cavity mode 200 relative to the filter spectrum 202. The wavelength position of the mode is determined by the following equation:

[0032]

[0033] Where N is an integer, L opt λ is the optical length of the cavity, and λ is the wavelength. It is the sum of all phase shifts occurring in the cavity (such as grating diffraction and metallic reflection). The mirror 104 is moved (e.g., along a generally perpendicular direction 400 to the mirror surface (which is the direction of the diffracted beam 402 that will be reflected back onto the grating 110)... Figure 4A As shown, this causes the optical length L of the cavity (specifically, the portion of the cavity between the grating 110 and the mirror 104) to increase. opt The optical length of the cavity can be changed by translating the grating 110 in a direction 404 parallel to the collimated beam exiting the lens 108 without changing the orientation of the grating, such as... Figure 4B As shown; in this case, the length of the cavity portion between the gain medium 102 and the grating 110 changes. The grating 110 can also be translated in direction 406 parallel to the plane of the grating 110, which similarly does not change the orientation of the grating. Translation in this direction 406 does not cause a change in cavity length, but it does cause a phase shift. The change. As will be understood, any translation of grating 110 in the laser plane can be described as a superposition of translations in directions 404 and 406, and due to the optical length L opt Phase shift in the cavity The shift in cavity mode 200 is caused by a certain combination of changes.

[0034] Thus, changing the angle of the mirror 104 within the laser plane allows tuning the location of the filter spectrum 202, while moving the controllable mirror 104 to adjust the distance between the mirror 104 and the grating 110, or shifting the grating 110 to adjust the distance between the gain medium 102 and the grating 110 or the phase shift in the cavity, allows tuning the location of the cavity mode 200 independently of the filter spectrum 202. The mirror angle and mirror distance to the grating or grating position together can be used to control the location of the cavity mode 200 relative to the filter spectrum 202, thus selecting the single mode that is present in the cavity because it is amplified more than any other mode.

[0035] Figure 5A and Figure 5B Two relative positions between the cavity mode 200 and the filter spectrum 202 are illustrated, and the respective selected modes 500, 502 corresponding to the mode pattern with the highest gain are indicated. It is generally desirable to align the selected mode 502 with the peak 504 of the filter spectrum 202, as shown in Figure 5B , to maximize the intensity of the laser, although in some cases it can be desirable to align the selected mode with a position that is offset from the filter peak 504. In either case, the alignment between the selected cavity mode 502 and the filter spectrum 202 should be maintained during laser tuning. If the selected cavity mode 500 and the filter peak 504 become misaligned, for example, if the cavity mode that was initially aligned with the filter peak 504 shifts away from the peak 504, as shown in Figure 5B , not only does the beam intensity drop, but a mode hop can occur as another mode becomes closer to the filter peak 504. This mode hop is generally undesirable and should be avoided or at least reduced in laser wavelength scans. Mode hop free laser tuning can be achieved when the tuning of the selected cavity mode and the tuning of the filter spectrum are coordinated in a way that maintains alignment between the two throughout the wavelength scan.

[0036] Figure 6 is a schematic top view of an example transmissive grating tunable laser 600 having a controllable mirror movable about a remote pivot point 602, illustrating one approach to achieving continuous alignment. The remote pivot point 602 links the position and angle of the mirror 104 through a triangular relationship. In some cases, this can enable mode hop free tuning. In other scenarios, the coordination between the mirror position and the mirror angle via the pivot point 602 by itself does not guarantee elimination of mode hops, but will reduce any misalignment, thus reducing the amount of correction needed. Unfortunately, the tolerances on this pivot point 602 are typically very tight, and the triangular relationship can only apply for a limited tuning range.

[0037] In various embodiments, spectral alignment between the cavity mode and the mode selection filter is achieved by monitoring the spatial alignment between the intracavity laser beam and the gain medium 102. Reconsidering the grating equation again, d(sinθ) i –sinθ m ) = mλ, having or not having a diffraction angle θ m The laser cavity mode with a wavelength that produces a beam perfectly orthogonal to the reflector constitutes an eccentric mode. For example... Figure 7A As shown, in this eccentric mode 700, the intracavity beam spot is spatially displaced from the center of the gain medium 102 in the laser plane. Furthermore, due to reduced coupling into the gain medium 102, the eccentric mode 700 exhibits a spectral shift from the peak of the filter spectrum 202, such as... Figure 7B As shown. Conversely, when the lasing cavity mode is at the low-loss (maximum diffraction) wavelength of the grating, the diffraction-limited beam spot is well aligned with the gain medium 102 and couples back into the gain medium 102 with maximum efficiency. Therefore, the movement of the beam spot relative to the gain medium 102 to satisfy the grating equation is a good indicator of misalignment between the cavity mode and the grating filter.

[0038] In addition to being rotatable and translatable in the laser plane, the controllable mirror 104 may have one or more additional degrees of freedom. For example, the controllable mirror 104 may have an additional degree of freedom regarding its tilt angle relative to the laser plane. The collimated and diffracted return beam from the grating 110 and focused by the lens 108 onto the gain medium 102... Figure 8 The tilt angle 800 is illustrated in a side view of the laser cavity. The tilt angle 800 is a rotation angle about an axis defined by the intersection of the mirror plane and the laser plane. This tilt angle 800 is typically chosen such that the reflected light is diffracted by grating 110 and focused back onto the gain medium 102. If the tilt angle 800 is closed, losses are introduced into the cavity. Therefore, it is desirable to adjust the tilt angle 800 to achieve alignment of the beam within the focusing cavity with the gain medium 102 in a direction orthogonal to the laser plane (e.g., vertical if the laser plane is horizontally oriented). If the laser can be manufactured such that a low loss angle can be maintained over a period of time, within a certain temperature range, and during laser tuning, the adjustment of the tilt angle can be eliminated. However, if the tilt angle 800 can be controlled, the laser can be assembled without tight tolerances, resulting in a cost advantage. This control can be achieved using the displacement of the beam spot from the gain medium 102 as a feedback signal.

[0039] Figure 9 This is a schematic block diagram of a tunable transmission grating laser system 900 with feedback control for aligning an intracavity beam in one or two dimensions (in the laser plane and / or in a direction perpendicular to the laser plane), according to various embodiments. System 900 includes, as referenced...Figure 1 The described tunable laser 100 has a transmission grating 110 inside the resonant cavity for mode selection, and has a controllable resonator mirror 104. The controllable mirror 104 and / or the transmission grating 110 are movable along various degrees of freedom, e.g., by suitable electronically driven actuators such as translation stages, pistons, electrostatically controlled microelectromechanical system (MEMS) actuators, voice coil actuators, and / or set screws, as known to those of ordinary skill in the art. In some embodiments, the controllable mirror 104 can be linearly moved in the laser plane, typically away from or towards the transmission grating 110, as well as rotated within the laser plane and tilted with respect to the laser plane (as described with reference to Figures 3A to 4B The transmission grating 110 itself can likewise be linearly movable in the laser plane (as illustrated with reference to Figure 4B

[0040] The controller 904 can control these degrees of freedom of the motion of the mirror 104 and the transmission grating 110 to tune the wavelength of the tunable laser 100 while aligning the laser beam with the gain medium. The controller can be implemented by any suitable combination of hardware and / or software. For example, in some embodiments, the controller is implemented by a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other electronic circuitry. In other embodiments, the controller is implemented in software running on a general-purpose computer, i.e., with processor-executable instructions stored in a memory and executed by one or more hardware processors of the computer. The instructions can also be stored individually on any machine-readable medium, i.e., any medium that is capable of storing, encoding, or carrying the instructions for execution by a computer machine and any data structures that are used in conjunction with such instructions, The non-limiting examples of machine-readable media include solid-state memories, as well as optical and magnetic media. Specific examples of machine-readable media include non-volatile memory, such as semiconductor memory devices (e.g., Electrically Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM)) and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; Random Access Memory (RAM); Solid State Drives (SSDs); and CD-ROM and DVD-ROM disks. In some examples, the machine-readable media includes non-transitory machine-readable media.

[0041] ​The system 900 further includes a second transmissive grating 906 (hereinafter also referred to as "extracavity transmissive grating" 906) outside the resonant cavity that diffracts the zeroth order transmission of the intracavity transmissive grating 110 to generate a monitor beam, and a position monitoring subsystem 908 for measuring the movement of the monitor beam spot and its displacement from a position corresponding to the intracavity beam being aligned with the gain medium. As noted, this creation of a monitor beam and beam spot serves to bypass the spatial limitations inside the cavity that would make it impractical if not impossible to place a detector near the gain medium 102. The measured position or displacement of the monitor beam spot is provided as a feedback signal to the controller 904, which can then adjust the resonator mirror 104 and transmissive grating 110 accordingly.

[0042] Figure 10 is a schematic top view of a tunable laser 100 according to various embodiments along with a second extracavity transmissive grating 1000 (corresponding to grating 906) to create a monitor beam 1002 that allows indirect measurement of the displacement of the intracavity laser beam from the gain medium 102. The second transmissive grating 1000 is placed outside the resonant cavity in the path of the zeroth order transmission (through the transmissive grating 110, disposed inside the cavity) of the return beam 1004 from the controllable mirror 104; this zeroth order transmission is also referred to herein as "zeroth order return beam" 1006, while the return beam 1004 is referred to as "diffracted return beam" upon diffraction into the gain medium 102.

[0043] The second transmissive grating 1000 is oriented parallel to the transmissive grating 110, or more generally, at an angle with respect to the path of the zeroth order return beam 1006 that is equal to the angle of the transmissive grating 110 with respect to the return beam 1004 (to allow the zeroth order return beam 1006 to be redirected before it encounters the second transmissive grating 1000). With this orientation of the second transmissive grating 1000, the resulting monitor beam 1002 will be an image of the diffracted return beam 1008. The monitor beam 1002 can be focused, e.g., by a lens 1010 or other focusing optics, to a monitor beam spot 1012 that is an image of the beam spot 1014 on the gain medium 102 inside the resonant cavity. If this lens 1010 has the same focal length as the lens 108, the monitor spot 1012 will be substantially the same as the beam spot 1014. However, it is advantageous to focus the monitor beam 1002 by a lens with a longer focal length, as this will make the monitor beam spot 1012 larger and the spatial deflection from its alignment position larger. It is also possible to have no lens at all and simply place a position sensor in the far field of the beam, but this is often impractical due to the large propagation distance required. The position of the monitor beam spot 1012 can be measured in various ways, as described below with reference to FIG. 10. Figures 11A to 15BThe detector and other apparatus for measuring the beam spot, along with focusing lens 1010, constitute a position monitoring subsystem 908, as illustrated.

[0044] In some embodiments, the monitoring beam spot is measured with a position sensitive detector placed at the focal plane of lens 1010. As one example of a position sensitive detector, Figure 11A The front view illustrates a quadrant detector 1100, and the position of a monitoring beam spot 1102 relative to detector 1100. For comparison, Figure 11B The side view illustrates the position of an intracavity beam spot 1104 relative to gain channels 1106 of a diode gain block 1108, which in one embodiment can be used as gain medium 102. As can be seen, intracavity beam spot 1104 is slightly displaced from the center of gain channels 1106. If, for example, quadrant detector 1100 is positioned based on a calibration process prior to measurement, so that for an aligned intracavity laser beam, the monitoring beam spot would be centered on detector 1100, Figure 11B The misalignment shown in FIG. 11B would be reflected by the off-center position of monitoring beam spot 1102 on detector 1100, as shown in FIG. 11C. Figure 11A

[0045] The amount of misalignment can be quantified by measuring the difference in the amount of light detected between the quadrants of detector 1100 (which will translate into a difference in photocurrent). Misalignment in the laser plane (as shown in FIG. 11B) (corresponding to a cavity mode that does not coincide with the filter spectral peak) will result in a non-zero photocurrent difference between quadrants Q2 and Q4. Vertical out-of-plane misalignment caused when controllable mirror 104 is tilted relative to the laser plane will result in a non-zero photocurrent difference between quadrants Ql and Q3. Advantageously, a quadrant detector (e.g., made of indium gallium arsenide (InGaAs) or germanium (Ge)) can be used at the laser operating wavelength in the near infrared region, e.g., 1550 nm, which is outside the detection band of silicon-based detectors. Furthermore, the four detectors that make up quadrant detector 1100 are inexpensive, and only a few additional acquisition channels are required, but there are no moving parts.

[0046] Figure 12 A portion of a tunable laser system is shown in schematic top view according to one embodiment, which utilizes a position sensitive diode (PSD) 1200 (as an alternative to quadrant detector 1100) at the focal plane of lens 1010 to measure the position of a monitoring beam spot 1012. A PSD provides continuous position information based on changes in local resistance measured at different points connected to electrodes of the device.

[0047] Figure 13 ​Yet another embodiment is illustrated again in a schematic top view, where an imaging array 1300 is used to measure the position of the beam spot 1012. Unlike a PSD, the imaging array 1300 provides discrete position information in its pixelated image. The imaging array 1300 can be, for example, a charge-coupled device whose sensor pixels are implemented by complementary metal-oxide-semiconductor (CMOS) capacitors. Depending on the operating wavelength range of the tunable laser, a CMOS-based imaging array can or can not be cost effective. For example, a CMOS-based imaging array that can detect light at wavelengths around 1550 nm (commonly used in OFDR systems) can cost more than $10,000. However, for wavelengths shorter than 1000 nm, a silicon-based CMOS imaging array can be used that costs as little as about $1. Thus, for short wavelengths, a CMOS imaging array is a suitable option for a position sensitive detector to measure the position of the monitoring beam spot.

[0048] Figure 14 A portion of a tunable laser system is illustrated in a schematic top view, where a single small area receiver is used in conjunction with a scanning mirror 1402 (or other beam scanner) in the path between the focusing lens 1010 and the receiver 1400 to determine the focused monitoring beam spot 1012 location according to various embodiments, the scanning mirror 1402 scans the focused monitoring beam 1404 across the receiver 1400. In this position monitoring system, the receiver 1400 remains at a fixed location, and the scanning mirror 1402 moves linearly, e.g., towards or away from the focusing lens 1010, or rotates in the laser plane, to move the monitoring beam spot 1012. The receiver 1400 can be, for example, an individual photodetector, or as shown, the input face of an optical fiber coupled to a photodetector at the other end.

[0049] The mirror position and / or orientation can be calibrated by aligning the intra-cavity beam with the gain medium, and then translating or rotating the mirror 1402 to scan the focused monitoring beam 1404 across the area including the receiver 1400, and determining the position and / or orientation of the mirror 1402 in the laser plane at which the focused monitoring beam 1404 is detected at the receiver 1402 and the measured intensity is maximized. When the laser is subsequently misaligned, the monitoring beam spot 1012 will generally no longer coincide with the receiver 1400. The misalignment of the laser can then be quantified by determining the distance or angle of rotation by which the mirror 1042 needs to be moved relative to the calibrated position or orientation in order to redirect the focused monitoring beam 1404 onto the receiver 1400. Alternatively, the mirror 1402 can also be used in a manner similar to a quadrant detector by moving the mirror 1402 to four (or more) different predetermined linear positions or orientations, and measuring the intensity detected at the receiver 1400. Figure 11A ​Figure 12 position monitoring system of the type shown in FIG. 1. In this case, the mirror motion is selected to be small enough to keep the monitoring beam spot 1012 overlapping the receiver 1400, allowing the receiver 1400 to measure the intensity at different points within the monitoring beam spot 1012. The benefit of using a scanning mirror 1402 to measure the monitoring beam is that only a single detection channel is needed, however, this comes at the cost of moving parts and typically multiple scans.

[0050] Figure 15A a portion of a tunable laser system is shown in schematic top view, again using a small area receiver 1500, but moving the receiver itself, rather than the monitoring beam as in Figure 14 Here, the receiver 1500 can also be, for example, an individual photodetector, or an input face of an optical fiber 1502 (as shown) coupled to a photodetector at the other end. Figure 15B is a front view of an example fiber-based receiver that can be used in this embodiment, illustrating the relative sizes of the monitoring beam spot 1012, the fiber 1502, and the fiber core 1504. In one example, the fiber is a 125 μιη standard optical fiber with a 9 μιη fiber core. The input face of the fiber 1502 can be moved, for example, up to 100 μιη in both the vertical (1506) and horizontal (1508) directions. The fiber movement can be accomplished, for example, using an electrostatic field generated between a pair of capacitor plates 1510 or by magnetic or piezoelectric means.

[0051] Figure 16 is a flowchart of a method 1600 of aligning an intracavity beam with a gain medium in a tunable laser 100 according to various embodiments. The method 1600 involves creating a monitoring beam outside of the laser cavity by diffracting a zeroth order return beam that is transmitted through an intracavity grating through a second extracavity transmission grating (1602). The monitoring beam serves as an image of the intracavity beam, and its focused beam spot or some other location associated with the monitoring beam can be used to determine whether the intracavity beam is aligned. To this end, for example, in a calibration step (1604), the alignment position of the monitoring beam spot is determined, i.e., the position of the beam spot when the intracavity beam is aligned with the gain medium (in the laser plane, by virtue of the alignment of the lasing cavity mode with the filter peak of the laser, or in a direction perpendicular to the laser plane).

[0052] Calibration (at 1604) can be performed by scanning the controllable resonator mirror 104 of the tunable laser over a range of angles and linear positions while monitoring the output power of the laser and monitoring the spot position. From this data, a map of spot position versus laser output power can be generated. The aligned position of the monitoring spot is generally the position of maximum laser output power. In addition to measuring and recording the position of the monitoring beam spot itself, the position of other things that are otherwise associated with the monitoring beam spot can be monitored, such as the angular position (in other words, orientation) of the scanning mirror when the mirror redirects the monitoring beam onto a fixed receiver, as Figure 14 illustrated. The aligned position of the scanning mirror is the angular position at which the mirror directs the monitoring beam onto the receiver when the intracavity beam is aligned with the gain medium. Calibration can be performed, for example, upon start-up of the laser, and thereafter as needed, for example to compensate for drift of the laser over time and / or temperature.

[0053] After calibration, the displacement of the beam spot or other position associated with the monitoring beam from the aligned position can be measured (1606) and used as feedback to control the physical configuration of the controllable resonator mirror 104, the transmission grating 110, or both (1608). Control parameters of this configuration include, for example, the position of the controllable mirror along the direction of the return beam and the position of the transmission grating 110 in the laser plane, both of which affect the cavity mode, and the tilt angle of the controllable mirror 104, which determines the out-of-plane alignment of the focused intracavity beam with the gain medium 102.

[0054] Adjustments to the control parameters can in principle be made in real time as the laser is being tuned, for example using a control signal output by a proportional-integral-derivative (PID) controller. However, in many systems and applications the laser scan rate is too high for real-time adjustments to be made. For example, if a CMOS or similar imaging array is used in the position monitoring subsystem, the readout rate from the array can be significantly lower than the scan rate. However, the scanning laser is typically operated in a quasi-steady state, with the drive signals to the controllable mirror 104 and / or gain diode providing the gain medium being repetitive and typically occurring above 100 Hz. In such systems, the scan can be characterized over multiple scans as a function of the monitored beam position as a function of the drive signal or wavelength, and then adjustments made at a rate lower than the scan rate. For example, for an imaging array that determines the position of the beam monitoring spot, a fast global shutter can be used to expose the array to only a brief slice of the total scan during each scan, and then the slice is advanced relatively slowly through the scan. For example, if a scan takes 3 ms and the exposed slice covers 5 ps, then after 600 samples the total scan is characterized. Furthermore, if the imaging array can be read at a rate of 30 frames per second, then it will take 20 seconds to fully characterize the scan. In practice, the spot can be very localized so that it is sufficient to read out a very small set of pixels. Such region of interest (ROI) readout of the imaging array is very common for CMOS devices, and can allow for an effective frame rate higher than, for example, 600 Hz, corresponding to an update rate of approximately 1 Hz.

[0055] Figure 17 is a flowchart of a mode-hop-free laser tuning method 1700 according to various embodiments, as an example application of the general alignment method of Figure 16 . The mode-hop-free tuning method 1700 includes creating a monitoring beam outside the laser cavity (1702), and measuring the displacement of the position associated with the monitoring beam in the laser plane from its alignment position as the laser wavelength is scanned (1704). Performing the laser scan (1704) involves simultaneously tuning the cavity mode and the filter spectrum of the laser, using the linear position of the controllable mirror 104 or grating 110 to tune the mode and the rotation angle of the controllable mirror 104 to tune the filter. If the mode and filter spectrum do not remain aligned throughout the scan, then a non-zero displacement will be measured (in 1706). The displacement measurements can be measured across the wavelength tuning range over multiple scans, and then assembled into a curve that characterizes the laser scan in its entirety (1708). The controller 904 uses this characterization to adjust the drive form to tune the controllable mirror 104 and / or transmissive grating 110 to better align the cavity mode with the filter spectrum (1710). The characterization and adjustment process can be repeated throughout subsequent scans.

[0056] While various aspects of the present application have been described in terms of preferred embodiments, it is to be understood that the application is fully capable of being practiced with a full range of equivalents of those preferred embodiments.

Claims

1. A method for aligning a diffracted return beam with a gain medium of a tunable laser system, the tunable laser system including a mode-selecting first transmissive grating disposed in a resonant cavity between the gain medium and a controllable resonator mirror, the diffracted return beam resulting from a return beam from the controllable resonator mirror being diffracted by the first transmissive grating, the method comprising: creating a monitor beam outside the resonant cavity by diffracting a zero-order return beam transmitted through the first transmissive grating through a second transmissive grating disposed outside the resonant cavity; measuring a displacement of a position associated with the monitor beam relative to an alignment position; and controlling a physical configuration based on the measured displacement, the physical configuration being a physical configuration of the controllable resonator mirror, or a physical configuration of the first transmissive grating, or a physical configuration of both the controllable resonator mirror and the first transmissive grating.

2. The method of claim 1, wherein the physical configuration includes at least one parameter selected from the group consisting of: a position of the controllable resonator mirror along a direction of the return beam, a position of the first transmissive grating in a laser plane defined by the return beam and the diffracted return beam, and a tilt angle of the controllable resonator mirror relative to the laser plane.

3. The method of claim 2, wherein: measuring the displacement includes measuring a displacement of the position associated with the monitor beam relative to the alignment position in the laser plane; and controlling the physical configuration includes controlling the position of the controllable resonator mirror along the direction of the return beam or the position of the first transmissive grating in the laser plane to align a cavity mode of a laser of the tunable laser system and a filter spectrum associated with the first transmissive grating and the controllable resonator mirror.

4. The method of claim 2, wherein: measuring the displacement includes measuring a displacement of the position associated with the monitor beam from the alignment position out of the laser plane; and controlling the physical configuration includes controlling the tilt angle of the resonator mirror based on the measured displacement out of the laser plane to align the diffracted return beam with the gain medium in a direction orthogonal to the laser plane.

5. The method of claim 2, wherein: measuring the displacement includes measuring the displacement both in the laser plane and out of the laser plane; and controlling the physical configuration includes controlling the position of the controllable resonator mirror relative along the direction of the return beam or the position of the first transmissive grating in the laser plane and controlling the tilt angle of the controllable resonator mirror.

6. The method of claim 2, further comprising: calibrating the tunable laser system by measuring the position associated with the monitoring beam and an output power of a laser of the laser system at a range of positions of the controllable resonator mirror along the direction of the return beam or a range of positions of the first transmission grating in the laser plane and a range of tilt angles of the resonator mirror to create a mapping between the position associated with the monitoring beam and the output power; and determining the alignment position based on the mapping.

7. The method of any one of claims 1 to 6, further comprising focusing the monitoring beam onto a position sensitive detector, wherein the position associated with the monitoring beam is a position of the focused monitoring beam on the position sensitive detector.

8. The method of any one of claims 1 to 6, further comprising focusing the monitoring beam and scanning the focused monitoring beam across an area containing a small area receiver using a scanning mirror, wherein the position associated with the monitoring beam corresponds to an orientation of the scanning mirror when the focused monitoring beam is incident on the small area receiver.

9. The method of any one of claims 1 to 6, further comprising focusing the monitoring beam and scanning a small area receiver across an area intersected by the focused monitoring beam, wherein the position associated with the monitoring beam corresponds to a position of the small area receiver within the scanned area when the focused monitoring beam is incident on the small area receiver.

10. A tunable laser system comprising: a first resonator mirror and a second resonator mirror, the first resonator mirror and the second resonator mirror forming a resonant cavity, the first resonator mirror being controllable; a gain medium disposed inside the resonant cavity; a mode selecting first transmission grating disposed inside the resonant cavity between the gain medium and the controllable resonator mirror, the first transmission grating being configured to generate a diffracted return beam and a zero order return beam from a return beam received from the controllable resonator mirror; a second transmission grating disposed outside the resonant cavity in a path of the zero order return beam, the second transmission grating being configured to diffract the zero order return beam to generate a monitoring beam; a position monitoring subsystem configured to measure a displacement of a position associated with the monitoring beam relative to an alignment position; and a controller configured to control a physical configuration based on the measured displacement, the physical configuration being a physical configuration of the controllable resonator mirror, or a physical configuration of the first transmission grating, or a physical configuration of both the controllable resonator mirror and the first transmission grating.

11. The tunable laser system of claim 10, wherein the second transmission grating is angularly oriented relative to the path of the zero order return beam, the angle being equal to an angle of the first transmission grating relative to the path of the return beam.

12. The tunable laser system of claim 10, wherein the physical configuration comprises at least one parameter selected from the group consisting of: a position of the controllable resonator mirror along a direction of the return beam, a position of the first transmission grating in a laser plane defined by the return beam and the diffracted return beam, and a tilt angle of the controllable resonator mirror relative to the laser plane.

13. The tunable laser system of claim 12, wherein: the position monitoring subsystem is configured to measure the displacement by measuring a displacement of the position associated with the monitoring beam relative to the alignment position in the laser plane; and the controller is configured to control the physical configuration by controlling the position of the controllable resonator mirror along the direction of the return beam or the position of the first transmission grating in the laser plane to align a cavity mode of a laser of the tunable laser system and a filter spectrum associated with the first transmission grating and the controllable resonator mirror.

14. The tunable laser system of claim 12, wherein: the controller is further configured to tune a wavelength position of the filter spectrum associated with the first transmission grating and the controllable resonator mirror by adjusting an orientation of the controllable resonator mirror in the laser plane; the position monitoring subsystem is configured to measure the displacement in the laser plane by measuring the displacement in the laser plane across a tuning range of the wavelength position of the filter spectrum; and the controller is further configured to control the position of the controllable resonator mirror along the direction of the return beam or the position of the first transmission grating in the laser plane based on the measured displacement in the laser plane for an instantaneous wavelength position of the filter spectrum to enable mode-hop-free wavelength tuning while tuning the filter spectrum.

15. The tunable laser system of claim 14, wherein measuring the displacement in the laser plane across the tuning range comprises: measurements of the displacement in the laser plane made over a plurality of tuning cycles of the wavelength position of the filter spectrum.

16. The tunable laser system of claim 14, wherein: the position monitoring subsystem is configured to measure the displacement in the laser plane while tuning the filter spectrum; and the controller is configured to control the position of the controllable resonator mirror by controlling in a feedback control loop.

17. The tunable laser system of claim 12, wherein: the position monitoring subsystem is configured to measure the displacement by measuring a displacement of the position associated with the monitoring beam out of the laser plane from the alignment position; and the controller is configured to control the physical configuration by controlling the tilt angle of the resonator mirror based on the measured displacement out of the laser plane to align the diffracted return beam with the gain medium in a direction orthogonal to the laser plane.

18. The tunable laser system of claim 12, wherein: the position monitoring subsystem is configured to measure the displacement by measuring the displacement both in the laser plane and out of the laser plane; and the controller is configured to control the physical configuration by controlling the position of the controllable resonator mirror relative along the direction of the return beam or the position of the first transmission grating in the laser plane, and controlling the tilt angle of the controllable resonator mirror.

19. The tunable laser system of claim 12, wherein the controller is further configured to: create a mapping between the position associated with the monitoring beam and an output power of a laser of the laser system based on the position associated with the monitoring beam and the output power over a range of positions of the controllable resonator mirror relative along the direction of the return beam or the position of the first transmission grating in the laser plane and a range of tilt angles of the resonator mirror; and determine the alignment position based on the mapping.

20. The tunable laser system of claim 10, further comprising: first focusing optics disposed inside the resonant cavity for focusing the diffracted return beam onto the gain medium; and second focusing optics disposed outside the resonant cavity for focusing the monitoring beam.

21. The tunable laser system of claim 20, wherein a focal length of the second focusing optics is greater than a focal length of the first focusing optics.

22. The tunable laser system of any one of claims 10 to 20, wherein the position monitoring subsystem comprises a position sensitive detector disposed in a path of the monitoring beam, and wherein the position associated with the monitoring beam corresponds to a position of the monitoring beam on the position sensitive detector.

23. The tunable laser system of claim 22, wherein the position sensitive detector comprises a detector selected from the group consisting of: a quadrant detector, a CMOS imaging array, a charge-coupled device (CCD), and a position sensitive diode (PSD).

24. The tunable laser system of any one of claims 10 to 20, wherein the position monitoring subsystem comprises a small area receiver and a beam scanner configured to scan the monitoring beam across an area containing the small area receiver, wherein the position associated with the monitoring beam corresponds to an orientation of the beam scanner when the monitoring beam is incident on the small area receiver. ​ 25. The tunable laser system of any one of claims 10 to 20, wherein the position monitoring subsystem comprises a movable small-area receiver and an actuation mechanism configured to move the small-area receiver across an area intersected by the monitoring beam, wherein the position associated with the monitoring beam corresponds to a position of the small-area receiver when the monitoring beam is incident on the small-area receiver.

26. The tunable laser system of any one of claims 10 to 20, wherein the position monitoring subsystem comprises a small-area receiver configured to receive incident light from the monitoring beam, the small-area receiver comprising a photodetector or an input face of an optical fiber coupled at an output end of the optical fiber to a photodetector.

27. A tunable laser system comprising: a laser comprising a resonator cavity having one or more cavity modes and a first transmission grating as a mode-selective spectral filter introducing wavelength-dependent cavity loss; tuning means for tuning the one or more cavity modes and a wavelength of the spectral filter; and monitoring means for creating an image of an intracavity beam outside the resonator cavity and for measuring a position associated with the image, the monitoring means comprising a second transmission grating for diffracting a zeroth order transmission of the first transmission grating; and control means for controlling the tuning means based on a displacement of the measured position from an alignment position to wavelength-align the one or more cavity modes with the mode-selective spectral filter.

28. The tunable laser system of claim 27, wherein: the resonator cavity is formed between a first mirror and a second controllable resonator mirror; and wherein the tuning means comprises means for rotating and translating the controllable resonator mirror in a laser plane.

29. The tunable laser system of claim 27, wherein the monitoring means further comprises means for focusing the image of the intracavity beam and means for detecting a resulting focal spot.

30. The tunable laser system of claim 27, wherein: the resonator cavity is formed between a first mirror and a second controllable resonator mirror and comprises a gain medium; the system further comprises tuning means for tuning a tilt angle of the controllable resonator mirror relative to a laser plane; and the control means further controls the tuning means for tuning the tilt angle based on a displacement of the measured position from an alignment position in a direction perpendicular to the laser plane to align an intracavity laser beam with the gain medium in the direction perpendicular to the laser plane.

31. A machine-readable medium comprising a plurality of machine-readable instructions that, when executed by one or more processors associated with a tunable laser system comprising a mode-selective first transmission grating disposed in a resonator cavity between a gain medium and a controllable resonator mirror, cause the one or more processors to perform a method comprising: by diffracting the zeroth order return beam transmitted through the first transmission grating through a second transmission grating disposed outside the resonant cavity, creating a monitor beam outside the resonant cavity; measuring a displacement of a position associated with the monitor beam relative to an alignment position; and controlling a physical configuration based on the measured displacement, the physical configuration being a physical configuration of the controllable resonator mirror, or a physical configuration of the first transmission grating, or a physical configuration of both the controllable resonator mirror and the first transmission grating.

32. The machine readable medium of claim 31, wherein the physical configuration comprises at least one parameter selected from the group consisting of: a position of the controllable resonator mirror along a direction of the return beam, a position of the first transmission grating in a laser plane defined by the return beam and the diffracted return beam, and a tilt angle of the controllable resonator mirror relative to the laser plane.

33. The machine readable medium of claim 32, wherein: measuring the displacement comprises measuring a displacement of the position associated with the monitor beam relative to the alignment position in the laser plane; and controlling the physical configuration comprises controlling the position of the controllable resonator mirror along the direction of the return beam or the position of the first transmission grating in the laser plane to align a cavity mode of a laser of the tunable laser system with a filter spectrum associated with the first transmission grating and the controllable resonator mirror.

34. The machine readable medium of claim 32, wherein: measuring the displacement comprises measuring a displacement of the position associated with the monitor beam from the alignment position out of the laser plane; and controlling the physical configuration comprises controlling the tilt angle of the resonator mirror based on the measured displacement out of the laser plane to align the diffracted return beam with the gain medium in a direction orthogonal to the laser plane.

35. The machine readable medium of claim 32, wherein: measuring the displacement comprises measuring the displacement both in the laser plane and out of the laser plane; and controlling the physical configuration comprises controlling the position of the controllable resonator mirror relative along the direction of the return beam or the position of the first transmission grating in the laser plane and controlling the tilt angle of the controllable resonator mirror.

36. The machine readable medium of claim 32, wherein the method further comprises: calibrating the tunable laser system by measuring the position associated with the monitor beam and an output power of a laser of the laser system over a range of positions of the controllable resonator mirror along the direction of the return beam or a range of positions of the first transmission grating in the laser plane to create a mapping between the position associated with the monitor beam and the output power; and determining the alignment position based on the mapping.

37. The machine-readable medium of any one of claims 31 to 36, wherein the method further comprises: focusing the monitoring beam onto a position sensitive detector, wherein the position associated with the monitoring beam is a position of the focused monitoring beam on the position sensitive detector.

38. The machine-readable medium of any one of claims 31 to 36, wherein the method further comprises: focusing the monitoring beam and scanning the focused monitoring beam across a region containing a small area receiver using a scanning mirror, wherein the position associated with the monitoring beam corresponds to an orientation of the scanning mirror when the focused monitoring beam is incident on the small area receiver.

39. The machine-readable medium of any one of claims 31 to 36, wherein the method further comprises: focusing the monitoring beam and scanning a small area receiver across a region intersected by the focused monitoring beam, wherein the position associated with the monitoring beam corresponds to a position of the small area receiver within the scanned region when the focused monitoring beam is incident on the small area receiver.

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