Laser wavelength stabilizing device

Abstract

A wavelength sensor for stabilizing the wavelength of a laser beam, comprising an etalon placed in the laser beam and inclined with respect to the laser beam. The reflected beam from the etalon forms an interference pattern on a segmented photodetector having two detector segments. Output signals from the two detector segments are used to derive an error signal for a closed control loop to achieve wavelength stabilization.

Description

[0001] Priority

[0002] This application claims priority to U.S. Patent Application Serial No. 16 / 518,689, filed on July 22, 2019, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present invention generally relates to stabilizing the output wavelength of a laser. The present invention particularly relates to stabilizing the output wavelength of a laser operating in a single longitudinal mode. Background Art

[0004] In certain laser applications, it is necessary to select a single longitudinal mode to operate the laser and maintain the undisturbed operation of the selected mode for a relatively long period of time. For example, laser spectroscopy or trace gas detection. A single longitudinal mode has a corresponding wavelength within the gain bandwidth of the laser. Typically, a laser resonator operates on longitudinal modes near the peak in the gain spectrum. In a laser with a resonator length of approximately 130 millimeters (mm), the wavelength separation between adjacent longitudinal modes is only 4.3 picometers (pm) in wavelength or equivalently 1130 megahertz (MHz) in frequency.

[0005] The result of this small spacing is that even a slight perturbation in the resonator length can cause the selected longitudinal mode to drift to a wavelength with less gain. This drift causes a "mode-hop" when the resonator stops operating in the original mode and resumes operating in another mode closer to the gain peak. The mode-hop can be sudden when at least two longitudinal modes compete for the available gain, or there may be a period of multimode operation during the transition. A sudden mode-hop can cause a discontinuity, a drop, or a spike in the laser power. An extended mode-hop can cause an increase in the amplitude noise of the laser power. For example, the resonator length can change due to changes in the ambient temperature around the laser, changes in the temperature of the internal components of the resonator, or mechanical disturbances. Typically, the noise imposed on the laser output power is more undesirable than the resulting gradual shift in the output wavelength. Not surprisingly, certain methods for actively limiting mode-hops have been developed over the years.

[0006] In an early method, an uncoated etalon was included in the laser resonator and was slightly tilted with respect to the longitudinal axis of the resonator. One of the end mirrors of the resonator was mounted on a piezoelectric (PZT) transducer which had an electrical driver for adjusting the resonator length. When laser operation was initiated, the etalon initially operated at the longitudinal mode of the resonator at the transmission peak (reflection minimum) of the etalon. A photodetector was arranged to receive any laser radiation reflected from the etalon. Drift of the initial longitudinal mode wavelength caused an increase in the total reflection of the etalon received by the photodetector. The photodetector and the PZT-actuated mirror were combined in a feedback loop which actively adjusted the resonator length to keep the total reflection radiation received by the detector to a minimum, thus maintaining operation at the initial longitudinal mode.

[0007] In a paper “Stabilization of Single-Longitudinal Mode Operation in a Q-Switched Nd:YAG Laser”, by I.D. Carr et al., Optics Communications, Vol. 55, No. 3, pp. 179-184, the authors discussed the so-called drawbacks of the above stabilization method. These included unequal surface reflectivities of the etalon, walk-off effects caused by etalon tilt, and diffraction broadening between successive reflections in the etalon. As a solution, the authors described an arrangement in which the etalon was located between two quarter-wave plates in the resonator. The etalon and the quarter-wave plates were in turn located between two polarization beam splitters. The etalon was oriented at an angle of incidence perpendicular to the resonator axis. Any radiation reflected from the etalon was directed out of the resonator by the innermost polarization beam splitter and reached the photodetector through another polarizer. Again, the resonator included a PZT-actuated end mirror. Again, the photodetector and the PZT-actuated mirror were combined in a feedback loop to keep the radiation received by the photodetector to a minimum.

[0008] Another polarization-related method is described in the paper “A Birefringent Etalon as a Single-Mode Selector in a Laser Cavity” by K.S. Gardner et al. in Optics Express, Vol. 12, No. 11, pp. 2365-2370. The birefringent etalon is placed in the laser cavity (resonator) and is slightly tilted with respect to the resonator axis. The polarization axis of the birefringent etalon is slightly tilted with respect to the polarization plane of the laser radiation circulating in the resonator. The laser radiation reflected from the etalon is directed to a polarization beam splitter that is rotated 45 degrees with respect to the polarization plane of the reflected radiation. The polarization beam splitter decomposes the reflected light into two components that are plane-polarized, orthogonally polarized, and detected separately. The zero-crossing error signal is generated from the sum and difference of the detected components and is used to adjust the resonator length to stabilize the operation on a single longitudinal mode.

[0009] A polarization-independent stabilization method is described in the paper “Frequency Locking a Laser to an Optical Cavity using Spatial Mode Interference” by D.A. Shaddock et al. in Optics Letters, Vol. 24, No. 21, pp. 1499-1501. In this method, a single longitudinal mode beam having the TEM 00 transverse mode of the laser is directed onto one of the mirrors of a slightly misaligned three-mirror ring cavity. The TEM 00 eigenmode of the ring cavity is coupled into the ring cavity, and the higher-order TEM 01 eigenmodes of the ring cavity are rejected by the ring cavity. A portion of the TEM 00 eigenmode beam and the rejected TEM 01 eigenmode beam are directed onto a split photodetector having two independent detection elements. The two beams interfere optically on the split photodetector. The phase difference between the TEM 00 mode beam and the TEM 01 mode beam varies as the TEM 00 mode drifts in wavelength with respect to the TEM 01 mode. The error signal generated by subtracting the outputs of each detection element is used to correct any wavelength drift in the laser.

[0010] Regardless of how effective any of the above-discussed stabilization methods may be compared to the original tilted-cavity etalon, all methods involve adding two or more optical components to the laser being stabilized, correspondingly increasing the cost of the laser. In the methods of Gardner et al. and Carr et al., at least one birefringent optical element and at least one polarization beam splitter are required. Birefringent optical elements made of single crystals are much more expensive than non-birefringent optical elements made of optical glasses such as fused silica. The method of Shaddock et al. does not require a birefringent element, but requires at least three coated mirrors, with the corresponding costs of assembling and aligning the ring cavity.

[0011] A laser stabilization method is needed that is at least comparable in effectiveness to existing technology methods, but with a minimum number of additional optical components. Preferably, none of the additional optical components are birefringent optical elements. Summary of the Invention

[0013] In one aspect, a wavelength sensing device for a laser beam includes a non-birefringent etalon having first and second parallel surfaces. The etalon is positioned in the laser beam. A segmented photodetector has first and second detector segments. The etalon is tilted with respect to the laser beam such that first and second reflected beams from the first and second parallel surfaces, respectively, are directed onto the segmented photodetector to form an interference pattern. The segmented photodetector is located at a far-field distance from the first and second reflected beams. The first and second detector segments generate first and second output signals, respectively. Changes in the first and second output signals correspond to changes in the wavelength of the laser beam. Description of the Drawings

[0014] The drawings, which are included in and form a part of this specification, schematically illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, are used to explain the principles of the invention.

[0015] Figure 1A and 1B schematically illustrates a preferred embodiment of a wavelength sensing device for tracking wavelength shifts in a laser beam, the device including a non-birefringent etalon tilted with respect to the laser beam, resulting in first and second reflected beams from the first and second surfaces of the etalon forming an interference pattern on a segmented photodetector including first and second detector segments.

[0016] Figure 2A and 2B is a graph schematically showing, as a function of the distance from the center of the first reflected beam in the near field, the electric field amplitude calculated across Figure 1B the first and second reflected beams, with the reflected beams being completely out of phase and completely in phase, respectively.

[0017] Figure 3A , Figure 3B and Figure 3C are images schematically showing the two - dimensional spatial intensity distribution of the calculation of the interference pattern on the detector section Figure 1B where the first and second reflected light beams are completely out of phase, slightly out of phase, and completely in phase, respectively.

[0018] Figure 4 is a graph schematically showing the difference, sum error signals derived from the output signal of the detector section Figure 1B as a function of the wavelength drift of the laser beam.

[0019] Figure 5 Schematically shows a distributed feedback (DFB) diode laser stabilized using Figure 1B a wavelength sensing device, where the wavelength sensing device is used in a closed control loop for changing the drive current of the DFB diode laser.

[0020] Figure 6 is a graph schematically showing the error signal as a function of frequency for an embodiment of a DFB diode laser Figure 5 with and without feedback from the wavelength sensing device of the present invention.

[0021] Figure 7 Schematically shows an optically pumped semiconductor (OPS) laser stabilized using Figure 1B a wavelength sensing device, where the wavelength sensing device is used in a closed control loop for changing the resonator length of the OPS laser.

[0022] Figure 8 is a graph schematically showing the error signal as a function of frequency for an embodiment of an OPS laser Figure 7 with and without feedback from the wavelength sensing device of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] Now turning to the drawings, where like features are denoted by like reference numerals. Figure 1ASchematically shown is a preferred embodiment 10 of a wavelength sensing (frequency sensing) device according to the present invention for tracking wavelength shifts (frequency shifts) in a laser radiation beam 12. Cartesian axes are described for reference, where the x-axis and y-axis are arbitrarily assigned. The z-axis is the propagation axis of the laser beam 12. The device 10 includes a non-birefringent etalon 14 located in the laser beam 12 and inclined with respect to the laser beam 12. The normal 16 of the etalon 14 is inclined at an angle θ with respect to the propagation axis of the laser beam 12. The etalon 14 has parallel planar first surface 18 and planar second surface 20. The etalon can be made of any glass or crystal that is transparent to laser radiation, provided that the etalon does not exhibit birefringence. For example, the etalon 14 has a thickness T in the range of approximately 0.1 mm to approximately 2.0 mm. The angle θ is in the range of approximately 1 degree to approximately 10 degrees. The exact position of the etalon 14 in the laser beam 12 is not critical, although the etalon is preferably located near the waist of the beam so that the laser beam is collimated or nearly collimated in the etalon.

[0025] Inclining the etalon 14 with respect to the laser beam 12 causes a first beam 22 and a second beam 24 to be reflected from the first surface 18 and the second surface 20, respectively. As shown, these two reflected beams propagate parallel to each other and form an interference pattern 26 on the receiving surface 28 of the segmented photodetector 30. The interference pattern 26 is discussed in detail below. The segmented photodetector 30 has a first detector segment 32 and a second detector segment 34. In the device 10, the detector segments 32 and 34 are electrically separated by an isolation barrier 36. A suitable segmented photodetector is the model SD066-24-21-011-ND two-cell photodiode, available from Digi-Key Electronics, Thief River Falls, Minnesota.

[0026] It will be apparent to those skilled in the art that there will be other reflected beams from the etalon. In Figure 1A is depicted a third such reflected beam 38 and a corresponding transmitted beam 40. The radiation intensity in the third reflected beam 38 is at least two orders of magnitude less than the radiation intensity in the first reflected beam 22 and the second reflected beam 24. Thus, the third reflected beam 38 and subsequent reflected beams do not significantly contribute to the interference pattern. Accordingly, only the reflected beams 22 and 24 are depicted in further figures of the wavelength sensing device of the present invention. However, the calculations described and discussed herein substantially include all reflected beams.

[0027] In Figure 1A for simplicity of illustration, each beam is depicted by the propagation axis of the beam. However, the etalon operates by optical interference between beams having a cross-section. Figure 1B Schematically shown is Figure 1AFor the wavelength sensing device 10, the laser beam 12 (shaded) and the reflected beams 22 and 24 (diagonally hatched) are depicted as extending spatially between their respective peripheral rays. The reflected beams 22 and 24 spatially overlap in the volume 42 (cross-hatched) and form an interference pattern 26 on the receiving surface 28. The beams in the figure are highly collimated. However, it should be noted that when the etalon is near the waist of the laser beam, diffraction will further propagate through the reflected beams of the segmented photodetector.

[0028] The segmented photodetector 30 is located at a distance D from the etalon, and the etalon is in the far field of the reflected beam. Preferably, the distance D is at least one quarter of the Rayleigh range of the reflected beam. Optionally, a positive lens can be used to focus the reflected beam, thereby shortening the distance D. Those skilled in the art can easily determine the optimal distance D empirically according to the description of the present invention herein.

[0029] Figure 2A is a schematic illustration of the electric field amplitude calculated across Figure 1B the first reflected beam 22 and the second reflected beam 24 as a function of the distance x from the center of the first reflected beam in the near field when the reflected beams are completely out of phase. In the calculation, the etalon is located at the waist of the laser beam. The etalon is made of fused silica, has a thickness T of 1 mm, and has a free spectral range of 103 GHz (equivalent to 390 pm) at a laser wavelength of 1064 nm (nm). The tilt angle θ is 5°. Due to the slight offset between the centers of the reflected beams caused by the tilt of the etalon, the electric field distributions do not completely overlap. Due to the additional loss when the second beam encounters the two surfaces of the etalon, the amplitude of the negative peak of the second reflected beam 24 is about 4% smaller than the amplitude of the positive peak of the first reflected beam 22.

[0030] Figure 3A is a schematic illustration of when as Figure 2A shown the reflected beams are completely out of phase, the calculated intensity image of the interference pattern 26 formed by the reflected beams on the receiving surface 28 of Figure 1B . In the calculation, the segmented photodetector 30 is located at a position approximately one Rayleigh range from the etalon 14. The interference pattern includes two relatively dark lobes 44A and 44B. The lobes are visible due to the imprecise overlap of the reflected beams. Between the lobes 44A and 44B are dark fringes 46. The segmented photodetector is preferably arranged such that the barrier 36 (indicated by the dashed line) is aligned along the center of the dark fringe 46. That is, the lobes of the interference pattern are incident on different detector segments, and the barrier 36 is aligned in the middle between the lobes. The wavelength of the laser beam 12 for which the reflected beams are completely out of phase corresponds to the minimum total intensity of the interfering reflected beams.

[0031] Figure 3B is an image of the interference pattern 26, similar toFigure 3A , but the reflected light beams are slightly out of phase. Here, the laser beam 12 has a wavelength of approximately 1064 nm, which is displaced by approximately 0.0085 nm from the wavelength of the completely out-of-phase wave. As the wavelength of the laser beam drifts away from the completely out-of-phase state, one of the lobes 44A and 44B will become larger and brighter, while the other will become smaller and darker. The dark fringe 46 will drift across the interference pattern in the x direction and will disappear when the reflected light beams are completely in phase. In the figure. In Figure 3A , lobe 44B is larger and brighter, and the fringe 46 has drifted to the left. Figure 3B

[0032] If the wavelength of the laser beam continues to drift, the interference pattern becomes a large bright spot 48 centered on the barrier 36. Figure 2B And 3C depict this completely in-phase situation. Figure 2A And Figure 2B And Figures 3A - 3C together illustrate how the interference pattern 26 depends on the phase relationship between the reflected light beams 22 and 24.

[0033] The detector segments 32 and 34 respectively generate first and second output signals. The error signal is obtained by dividing the difference between these two output signals by the sum of these two output signals. This error signal is a function of the difference between the wavelength of the laser beam and the wavelength (etalon wavelength) corresponding to the two completely out-of-phase reflected light beams. These signals are shown in Figure 4 , Figure 4 is a graph of the calculated difference signal (scaled by 10), the total signal, and the error signal as a function of the wavelength of the laser beam. In the calculation, the laser beam 12 has a power of 1 watt (W). In the graph, the difference signal and the total signal are depicted as the optical power in watts incident on the detector segments of the photodetector 30. This graph indicates the displacement of the laser beam wavelength from the etalon wavelength.

[0034] The change in the output signal corresponds to the change in the wavelength of the laser beam. A signal processor ( Figure 1A and 1B not shown) can be arranged to receive the output signals and determine the error signal associated with the output signals. The error signal represents the phase relationship between the reflected light beams 22 and 24. The error signal can be used in a closed control loop to change the parameters of the wavelength response of the laser beam 12 to keep the error signal at or close to zero, thereby stabilizing the wavelength of the laser beam to the etalon wavelength. For example, the laser parameters can be the resonator length, the pump power, or a tuning element in the laser that generates the laser beam.

[0035] Figure 5 Schematically shows the use of Figure 1BThe wavelength stabilization diode laser device 50 of the wavelength sensing device 10. The device 50 includes a distributed feedback (DFB) diode laser 52. The DFB diode laser can be commercially procured as a package and has a periodic structure that provides distributed feedback incorporated into the laser resonator (not shown). A drive current is supplied to the DFB diode laser 52 by a current source (laser driver) 54. The DFB diode laser excited by the drive current transmits an output beam designated as the laser radiation beam 12, Figure 1A which is consistent with the description of the wavelength sensing device 10. The wavelength sensing device 10 is clearly outside the resonator of the DFB diode laser 52.

[0036] A current bypass connected in parallel with the DFB diode laser 52 is formed by a PNP bipolar transistor T1 in series with a load resistor R1. The bypass current flows through the resistor R1 and through T1 between the emitter E and the collector C. The device 10 is used in a closed control loop that changes the drive current to the DFB diode laser 52. The interfering reflected beams 22 and 24 from the etalon 14 are incident on the segmented photodetector 30, where the detector segments 32 and 34 of the segmented photodetector 30 are symbolically represented as separate photodiodes. The detector segments 32 and 34 are electrically connected to a controller 56, which includes a signal processor that determines an error signal, as described above with reference to Figure 4 the above. Here, the error signal is the voltage applied to the base B of the transistor T1, which correspondingly changes the current through the current bypass. This in turn changes the complementary drive current through the DFB diode laser 52. As is known in the art, changing the drive current through the DFB diode changes the wavelength of the laser beam 12 by changing the temperature at the diode laser junction.

[0037] Figure 6 Schematically shows the measurement error signal in one embodiment of the Figure 5 wavelength stabilization diode laser device 50 as a function of frequency. The error signal here is scaled to 20 volts (V) and assumes an electrical load of 50 ohms. The 20 V corresponds to a hypothetical interference pattern that irradiates only one photodetector segment with all the optical power. These are essentially power spectra that illustrate the noise in the wavelength (frequency) of the laser beam. For example, a sinusoidal modulation of the wavelength will appear as a peak in the power spectrum at the sinusoidal modulation frequency. For the DFB diode laser 52, using the wavelength sensing device 10, the wavelength of the laser beam is measured relative to the etalon wavelength. In Figure 6 this, the relative noise spectra are measured with and without feedback from the wavelength sensing device of the present invention; that is, the wavelength stabilization and instability cases of the DFB diode laser. When unstable, the powered DFB diode laser operates freely.

[0038] In this embodiment, the DFB diode laser 52 is of the model CMDFB1064A available from II-VI LaserEnterprise GmbH, Zurich, Switzerland. The nominal output wavelength of such a DFB diode laser is 1064 nm. The segmented photodetector 30 is of the model SD066-24-21-011-ND as illustrated above. The etalon 14 of the device 10 is a fused silica etalon with a thickness T of 0.3 mm. The etalon is tilted at an angle of approximately 5° with respect to the laser beam 12. The distance D between the etalon 14 and the segmented photodetector 30 is approximately 500 mm.

[0039] It can be seen that for frequencies less than approximately 10 Hz, the relative noise with stabilization (with feedback) is approximately 25 decibels (dBm) less than the relative noise without stabilization (without feedback). At frequencies between approximately 10 Hz and 100 Hz, the difference between the noise spectra with and without stabilization gradually becomes smaller due to the thermal response time at the diode laser junction being several hundred milliseconds. In essence, the thermal inertia of the diode laser inhibits the required high-frequency feedback modulation.

[0040] Figure 7 Schematically shown is an optically pumped semiconductor (OPS) laser 60 stabilized using Figure 1B the wavelength sensing device 10. The OPS laser 60 includes an OPS chip 62 having a semiconductor gain structure 64 covering a high reflector structure 66. The OPS chip is bonded to a heat sink 68. The gain structure 64 is optically pumped, here by pump radiation P supplied by a diode laser array (not shown). The OPS laser 60 has a linear resonator 70 formed between the high reflector structure 66 of the OPS chip 62 and a partially transmissive output coupler mirror 72 mounted on a PZT 74. In response to the optical pumping, a laser radiation beam having a fundamental wavelength circulates in the resonator 70 as shown by the arrow F. The output laser radiation beam 76 is transmitted through the output coupler mirror 72. Typically, a birefringent filter 78 provides coarse wavelength selection (frequency selection).

[0041] Here, the etalon 14 of the device 10 is located in the resonator 70 between the birefringent filter 78 and the output coupler mirror 72. The circulating laser beam in the resonator is comparable to Figure 1B the laser beam 12, except that the circulating laser beam in the resonator is bidirectional. The reflected beams 22 and 24 from the etalon 14 form an interference pattern 26 on the detector segments 32 and 34 of the segmented photodetector 30 as above referenced Figure 1A 、 Figure 1B 、 Figure 3A 、 Figure 3B and Figure 3CThe optional lens 80 may be located in the reflected beam between the etalon and the photodetector to shorten the distance to the detector, as described above, and to narrow the interfering beams incident on the photodetector.

[0042] The detector segments 32 and 34 are electrically connected to a controller 56 which includes a signal processor to derive an error signal from the output signals of the detector segments, as described above. Here, the controller applies a voltage responsive to the error signal to the PZT 74. The applied voltage changes the length of the resonator and thus the wavelength (frequency) of the circulating laser beam F. This closed control loop holds the error signal at or near zero.

[0043] It should be noted that only a sufficient description of the OPS laser 60 is provided here for understanding the principles of the present invention. A detailed description of the OPS laser, including several different resonator configurations, is provided in U.S. Patent No. 6,097,742 assigned to the assignee of the present invention, the entire disclosure of which is incorporated herein by reference.

[0044] Figure 8 is schematically shown as a function of frequency Figure 7 the measured error signal in one embodiment of the OPS laser 60. Again, these are essentially the power spectra of the noise in the laser beam wavelength, measured with and without feedback from the wavelength sensing means of the present invention. In this embodiment, the OPS laser is arranged to deliver an output laser beam 76 at a wavelength of approximately 1064 nm. The resonator length is 130 mm. The PZT 74 is a circular PZT stack of type PK25LA2P2 available from Thorlabs of Newton, NJ. The segmented photodetector 30 is a model SD066-24-21-011-ND photodiode as illustrated above. The etalon 14 is made of fused silica and has a thickness of 1 mm. The etalon is tilted at an angle of approximately 5 degrees with respect to the circulating laser beam. The distance from the etalon to the segmented photodetector 30 is approximately 500 mm. The lens 80 has a focal length of approximately 250 mm and is included to fit the incident beam within the physical dimensions of the segmented photodetector 30. If a larger photodetector is used, the lens may be omitted.

[0045] From Figure 8 it can be seen that, compared to no feedback, the relative noise is reduced by up to 25 to 30 dB with feedback, similar to Figure 6 However, for Figure 8For the OPS laser, this reduction is maintained at a few kilohertz. At higher frequencies, due to the reduced mechanical agility of the PZT-actuated mirror, the noise spectrum begins to converge. In essence, the mechanical resonance of the PZT-actuated mirror distorts the feedback modulation at higher frequencies. In a separate experiment, using a different OPS laser with a resonator length of approximately 340 mm, the maximum reduction in noise was maintained at approximately 10 KHz.

[0046] In the description provided above, the wavelength sensing device and method of the present invention are applied to DFB diode lasers and OPS lasers with linear resonators. However, the wavelength sensing device of the present invention is effective for other types of solid-state lasers as well as lasers with ring resonators. For solid-state lasers that produce sufficient output power, the wavelength sensing device of the present invention can be deployed outside the resonator, as Figure 5 shown for the DFB diode laser. The wavelength sensing device of the present invention can also be used in intracavity frequency-converted lasers.

[0047] Due to the temperature dependence of the coefficient of thermal expansion and the refractive index of the transparent material used to fabricate the etalon, the etalon wavelength is typically temperature-dependent. A change in temperature in the etalon will change the thickness T of the etalon as well as the optical length in the etalon. To maintain a constant wavelength, it may be necessary to regulate the temperature of the etalon. Alternatively, to adjust the output wavelength of the laser, the temperature of the etalon can be intentionally changed. The wavelength sensing device in the closed control loop of the present invention will tune the wavelength by continuously minimizing the error signal.

[0048] In the above embodiments, the segmented photodetector is a photodiode having two photocells separated by an isolation barrier, which is a convenient commercial device incorporated into the wavelength sensing device of the present invention. Alternatively, the segmented photodetector can be a photodiode array or a pair of discrete photodiodes arranged such that each photodiode intercepts only one lobe of the interfering reflected beam. As Figure 2B shown in and FIG. 2D, the segmented photodetector can be a camera or other imaging device, and the detector segments are the two halves of the image. Two output signals will be derived by integrating each half of the image.

[0049] In summary, it has been found that placing an etalon in a laser beam and tilting it with respect to the laser beam forms an interference pattern in the far field of the beam reflected from the etalon. This interference pattern can be directed onto a segmented photodetector to obtain an error signal corresponding to the wavelength of the laser beam. This enables the etalon and the segmented photodetector to be used as a wavelength sensor. The error signal can be used to close a control loop to minimize wavelength drift of the laser beam. This is described above and is deployed differently in two types of lasers. Other types of lasers to which the present invention is applicable are suggested. Unlike prior art wavelength stabilization devices that require several relatively expensive optical components, the wavelength sensor of the present invention in its simplest form requires only one additional optical element, namely an uncoated and non-birefringent etalon. Such an etalon can be manufactured in relatively large batches at relatively low cost.

[0050] The present invention is herein described as a preferred embodiment deployed in embodiments of two types of lasers. However, it should be noted that the present invention is not limited to the preferred embodiments and examples described and depicted herein. Instead, the present invention is limited only by the appended claims.

Claims

1. A wavelength sensing device for a laser beam, comprising: A non-birefringent etalon having first and second parallel planar surfaces, the etalon being located in the laser beam; And A segmented photodetector having first and second detector segments, the etalon being tilted with respect to the laser beam such that first and second reflected beams from the first and second parallel surfaces are respectively directed onto the segmented photodetector to form an interference pattern, the segmented photodetector being located at a far-field distance from the first and second reflected beams, the first and second detector segments respectively generating first and second output signals, and changes in the first and second output signals corresponding to changes in the wavelength of the laser beam.

2. The device according to claim 1, further comprising a signal processor that receives the first and second output signals and derives an error signal representative of the phase relationship between the first and second reflected beams therefrom.

3. The device according to claim 2, wherein, The error signal is derived by dividing the difference between the first and second output signals by the sum of the first and second output signals.

4. The device according to claim 3, wherein, When the first and second reflected beams are completely out of phase, the error signal is zero.

5. The device according to claim 1, wherein The laser beam is the output beam of a laser.

6. The device according to claim 1, wherein The laser beam circulates within a laser resonator.

7. The apparatus according to claim 1, wherein, The interference pattern includes two lobes that impinge on different detector segments when the first and second reflected beams are out of phase.

8. An optical device, comprising: A laser that transmits a laser radiation beam; A non-birefringent etalon having first and second parallel planar surfaces, the etalon being located in the laser radiation beam; A segmented photodetector having first and second detector segments, the etalon being tilted with respect to the laser radiation beam such that first and second reflected beams from the first and second parallel surfaces are respectively directed onto the segmented photodetector to form an interference pattern, the segmented photodetector being located at a far-field distance from the first and second reflected beams, the first and second detector segments respectively generating first and second output signals; And A controller for receiving the first and second output signals and deriving an error signal representative of the phase relationship between the first and second reflected beams therefrom; Wherein a parameter of the laser is changed in response to the error signal to stabilize the wavelength of the laser radiation beam.

9. The apparatus according to claim 8, wherein, The laser is a diode laser.

10. The device according to claim 8, wherein, The parameter is the drive current that excites the laser.

11. The device according to claim 8, wherein, The error signal is derived by dividing the difference between the first and second output signals by the sum of the first and second output signals.

12. The apparatus according to claim 8, wherein, When the first and second reflected beams are completely out of phase, the error signal is zero.

13. The device according to claim 8, wherein, The interference pattern includes two lobes that impinge on different detector segments when the first and second reflected beams are out of phase.

14. An optical device, comprising: A laser, including a resonator in which a laser beam circulates, the resonator having a resonator length; A non-birefringent etalon having first and second parallel planar surfaces, the etalon being located in the circulating laser beam; A segmented photodetector having first and second detector segments, the etalon being tilted with respect to the circulating laser beam such that first and second reflected beams from the first and second parallel surfaces are directed onto the segmented photodetector and form an interference pattern, the segmented photodetector being located at a far-field distance from the first and second reflected beams, the first and second detector segments generating first and second output signals respectively; and a controller for receiving the first and second output signals and deriving therefrom an error signal representative of the phase relationship between the first and second reflected beams; wherein a parameter of the laser is changed in response to the error signal to stabilize the wavelength of the circulating laser beam.

15. The apparatus according to claim 14, wherein, The laser is an optically pumped semiconductor laser.

16. The apparatus according to claim 14, wherein, The parameter is the resonator length.

17. The apparatus according to claim 16, wherein, The laser resonator is formed between a high reflector and a partially transmissive output coupler mirror, the output coupler mirror being mounted on a piezoelectric transducer, and wherein a voltage applied to the piezoelectric transducer is responsive to the error signal and changes the length of the resonator.

18. The apparatus according to claim 14, wherein, The error signal is zero when the first and second reflected beams are completely out of phase.

19. The device according to claim 14, wherein, A lens is located in the first and second reflected beams between the etalon and the photodetector.

20. The device according to claim 14, wherein The interference pattern includes two lobes incident on different detector segments when the first and second reflected beams are out of phase.

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