JP2002528902A - Continuous wavelength tunable external cavity laser - Google Patents

Abstract

(57) Abstract The present invention provides a continuously tunable external cavity laser (ECL) having a compact shape factor and accurate wavelength selection (tuning). A novel interference filter that can be used to tune the ECL eliminates mode hops and reduces feedback from both spurious interference (210) and spurious reflections (214) in the external resonator. A novel tuning mechanism is disclosed that allows for mechanical FM tuning of wide-range ECL tuning elements, such as interference filters, diffractive elements, and retroreflectors (122). A novel feedback circuit is disclosed that enables closed loop feedback to select the output wavelength in the laser.

Description

DETAILED DESCRIPTION OF THE INVENTION

[Delegation Regarding Copyright] A part of the disclosure content in the present patent document includes a copyrighted work. The copyright owner shall not object to any person viewing the U.S. Patent and Trademark Office files or records and facsimile copying patent documents.
In all other respects, it reserves the right to enforce copyright.

FIELD OF THE INVENTION [0002] The present invention relates generally to tunable external cavity lasers, and more particularly, to an improved tuning system that reduces mode hopping and reduces unwanted feedback.

Description of Related Art Wavelength tunable external cavity diode lasers (ECDLs) are widely used in lightwave test and measurement equipment and are rapidly developing wavelength division multiplexing (WDM).
It has been recognized as an essential component in optical voice and data communication. Many applications belonging to these fields have many different sets of performance specifications. However, in order to reduce the size of the opto-mechanical assembly, to enable continuous phase tuning of the oscillation wavelength, to enable servo control of the wavelength, and to increase the line width. Audio rate (for example, 100 Hz to 10
It is a general requirement that controllable frequency modulation (FM) at (kHz) and the ability to produce a highly linear wavelength sweep are provided.

[0004] Commercially available ECDLs have various wavelength tunable configurations. Generally, either a retroreflector, a diffraction grating or an interfering element provides a feedback wavelength to the gain medium,
Thus, it is used for controlling the output wavelength of the laser. In the first of these tunable configurations, such as the "littman" type, tuning (wavelength selection) is achieved by moving either a diffraction grating or a retroreflector mounted on an arcuate pivot arm. . To perform continuous phase tuning requires a fairly long and massive arm. This arm is driven by either a rotary stepping motor or a rotary DC motor. Stepper motors allow long distance travel and self-contained position encoding, but their bandwidth is too small for FM, and they are discontinuous in nature, making them unsuitable for linear FM sweeps. DC motors can produce a linear sweep if they have sufficient torque, but cannot be used for FM. In a second configuration, tuning is performed by rotating the interference filter with respect to the optical path. The laser in this second configuration is described in a paper by Dr. P. Zorabedian et al.
ilter-tuned, alignment-stabilized, semiconductor-cavity laser ”13 Optic
s Letters, pp. 826-828 (1988). This laser structure also utilizes an actuator, such as a stepper motor, which
It is not suitable for the M modulation method. In both the first and second configurations, FM modulation is typically performed by modulating the injection current into the semiconductor optical gain medium, which results in amplitude modulation (AM) on the laser output. It has a very undesirable effect.

There is a need for a new type of external cavity laser that provides a wide range of continuous tuning, has a compact, robust, inexpensive shape factor, and is capable of selecting an FM wavelength.

SUMMARY OF THE INVENTION The present invention provides a continuously tunable external cavity laser (ECL) with a compact shape factor and accurate tuning. A novel interference filter that can be used to tune the ECL eliminates mode hopping and reduces feedback from both spurious interference and spurious reflections in the external resonator. ECL tuning elements, such as interference filters,
A novel tuning mechanism is disclosed that allows for mechanical FM tuning of diffraction gratings and retroreflectors. A novel feedback circuit is disclosed that enables closed loop feedback to select the output wavelength in the laser.

In an embodiment of the present invention, a tunable external cavity laser including a gain medium, a retroreflector, first and second reflectors, and a positioner is disclosed. The gain medium emits a beam. A retroreflector is provided in the path of the beam. The first and second reflectors are positioned opposite each other and at an angle to each other in a beam path between the gain medium and the retroreflector. The positioner changes the spacing between the first and second reflectors that feedback the selected wavelength to the gain medium to tune the laser and the reflector along the beam path, thereby tuning the laser. At least one of the first reflector and the second reflector is positioned.

In an embodiment of the present invention, a tunable external cavity laser including a gain medium, a tuning element, and a voice coil actuator is disclosed. The gain medium emits a beam. The tuning element is movably positioned in the beam path to feed back the selected wavelength to the gain medium. A voice coil actuator is coupled to the tuning element and responsive to the electrical signal positions the tuning element in the path of the beam and feeds the selected wavelength back to the gain medium, thereby tuning the tunable laser.

In a modified embodiment of the present invention, a tunable external cavity laser including a gain medium, a tuning element, an encoder, a reader, and a positioner is disclosed. The gain medium emits a beam. The tuning element is movably positioned in the beam path to feed back the selected wavelength to the gain medium. The encoder records along its length an index corresponding to at least one of the position of the tuning element and the selected wavelength of the beam. The reader reads the indicator and records a feedback signal corresponding to the indicator read from the encoder. One of the encoder and the reader is coupled to a tuning element. A positioner is coupled to the tuning element and positions the tuning element in the beam path to feed a selected wavelength back to the gain medium to tune the tunable laser. The positioner positions the tuning element such that the indices read by the reader substantially correspond to the selected wavelength.

In a modified embodiment of the present invention, a method for controlling an output wavelength of an external cavity laser is disclosed. The method includes moving the tuning element to a continuous position from end to end of the position range, and measuring a corresponding output wavelength at each of the continuous positions from end to end of the position range in response to the moving step. Recording a corresponding output wavelength indication along the length of the encoding medium coupled to the selected one of the base and the tuning element in response to the measuring step. Reading the corresponding output wavelength indication recorded on the encoding medium in the storage step from one of the elements not selected, and indicating the corresponding output wavelength read in the reading step for the tuning element. Moving the value to a selected position that matches the selected wavelength. The above and other features of the invention and the above and other advantages will now be described in detail with reference to the accompanying drawings. If you read Will be clear.

DETAILED DESCRIPTION OF THE EMBODIMENTS The continuously tunable external cavity laser (ECL) of the present invention has a compact shape factor and accurate tuning. A novel interference filter that can be used to tune the ECL eliminates mode hopping (also called mode hop) and reduces feedback from both spurious interference and spurious reflections in the external resonator. A novel tuning mechanism is disclosed that allows for mechanical FM tuning of tuning elements (also called wavelength selective elements) such as interference filters, diffraction gratings and retroreflectors. A novel feedback circuit is disclosed that enables closed loop feedback to select the output wavelength in the laser.

In some embodiments of the present invention, a “degenerate resonator” type resonator structure is utilized. This structure is very tolerant of the various angular and lateral misalignments that the laser may experience. This property simplifies assembly and gives the laser a robust tolerance for environmental changes during transport, storage and use. The principle of degenerate resonators is well known. In another embodiment of the invention, a Littman structure is shown.

FIGS. 1, 2 and 4 to 7 are shown with respect to a three-axis (X, Y, Z) coordinate system. The optical components of all embodiments except for the embodiment shown in FIGS. 7A and 7B are themselves arranged along an optical axis 100 parallel to the Z axis.

FIGS. 1A and 1B are an isometric view and a plan view, respectively, of an embodiment of an external cavity laser of the present invention. The laser external cavity is defined by a partially reflective rear facet 104 of the gain medium / laser amplifier 102 and an external retroreflector 122. Tunable feedback for controlling the laser emission wavelength is obtained by an external resonator optically coupled to the anti-reflection (AR) side of the gain medium. The effective reflectivity of the external cavity is much larger than the residual reflectivity of the AR-coated front facet 106, so the tuning element, ie, the interference filter / etalon, is sufficient to put the laser into a "strong feedback" configuration. You can send out some kind of feedback.

The external cavity laser includes a laser amplifier 102, a lens system 164, a 、 wavelength plate /
It includes optical retarders 112, 114, interference filters / etalons 162, correction elements 120, translation mechanisms 144, tuners 160, retroreflectors 122, bases 138, and mounting blocks 140, 142, 146, 148. In the illustrated embodiment, the laser amplifier 102 is a laser diode with a rear facet 104 and a front facet 106. The lens system is collimated /
It has focusing lenses 108 and 110. The etalon has a first reflector 116
And a second reflector 118.

Structurally, the tunable laser is shown positioned along a longitudinal axis 100. In an embodiment of the present invention, the laser amplifier comprises a partially reflective dielectric coating applied to the rear facet and an AR applied to the front facet.
A conventional Fabry-Perot laser diode with a coating (laser diodes are also called semiconductor lasers). The front facet 106 and the rear facet 104 of the laser diode are aligned with the longitudinal axis 100. The laser diode is coupled to base 138 via mounting block 148. The proximal end of the external resonator is provided at the front facet 106 of the laser diode. The distal end of the external resonator is
It is constituted by. The retroreflector is attached to the base via the mount 140. A collimating / focusing lens pair 108, 110 is provided coaxially with the optical axis 100 near the front facet 106. These lenses collimate and focus the light beam 124 from the laser diode 102. The collimating and focusing lens is attached to the base 138 via the mount 146. Quarter wave plate / optical retarder 112 is coaxially positioned with and between the collimating and focusing lenses, and is mounted to the base via mount 146. A second one of the optical retarders 114 is located at its distal end in the external resonator in proximity to the retroreflector 122 and also has a base 138 via a mounting block 140 shared with the retroreflector. Attached to. The etalon 162 may be a gas space type or a solid type. In the illustrated embodiment, the etalon is of the gas space / air gap type. The etalon constituted by the first and second flat reflectors 116, 118 located opposite each other is of the wedge type air gap type. The reflectors are mounted on the inward facing surfaces of the transmissive substrate 130 and the correction element, respectively. In the illustrated embodiment, the reflector comprises a transmissive substrate 130 and a highly reflective, eg, R>
It is preferably formed by applying a dielectric coating to the inner surface of the correction element 120 having a 90% dielectric layer. The substrate and the correction element are mounted on the mount 14
2 and to the base 138 via the translation mechanism 144.

In operation, the embodiments shown in FIGS. 1A and 1B can both tune the wavelength in the external resonator and adjust the resonator length to avoid mode hopping.

In FIG. 1A, beam 124 is applied to front facet 106 of laser amplifier 102.
It is shown out of the box. In one embodiment, the laser amplifier comprises a laser diode that preferentially amplifies linearly polarized light in one direction. The beam passes through the lens 108
The polarization state is changed from linearly polarized light to circularly polarized light by the に よ っ て wavelength plate 112 and focused by the laser 110. Beam 110
The waist portion of the beam, i.e., the portion where the wavefront is substantially flat, is located within the air gap portion of the etalon 162. The filtered beam passes through a second one of the transparent substrate 130 and the quarter-wave plate 114 to the retroreflector 122. Retro-reflector 122 and rear facet 104
And / or both may be fully or partially reflective.
In the illustrated embodiment, output beams 150 and 152 are each
Are shown passing through a partially reflective retroreflector 122 and a partially reflective rear facet 104, respectively.

Tuning of the resonator is achieved by translating wedge-shaped etalon 162 across the optical path of beam 124. In the embodiment shown in FIG. 1, translation of the etalon 162 is provided by a translation mechanism 144 that translates the etalon along a translation axis 190. As the etalon translates along the translation axis 190, the laser beam intersects the increasingly narrow portion of the wedge-shaped air gap, thereby tuning to the progressively shorter wavelength that tunes the resonator.

The required operating range of the translation mechanism 144 along the translation axis 190 may actually be in the centimeter range. Using various compact designs of motors, such as thermodynamic motors, ultrasonic motors, linear stepping motors, etc., this displacement can be obtained. In an embodiment of the present invention, translation mechanism 144 operates under the control of tuner 160. The tuner is operable with or without a wavelength feedback element to select a particular wavelength to tune the resonator.

The etalon 162 has a repetition loss characteristic called “order”. For interference filters / etalons, the orders are very closely spaced. The filter / etalon can tune the laser ± 1 / 2.FSR away from the gain peak, where the FSR is the free spectral range (free spectral range) or frequency spacing of the peak. Thus, for example, for a tuning range of 50 nm, F
SR must be on the order of 6,250 GHz for a wavelength of 1.55 μm.

Tuning the laser will result in mode hopping if integer multiples of half a wavelength in the external cavity are not maintained at a constant value. To this end, a correction element 120 is provided. In the illustrated embodiment, the correction element is made of a light transmissive material having a uniform refractive index. This correction element has a wedge-shaped cross section in the YZ plane. The outer surface of the correction element 120 facing the laser amplifier 102 is preferably provided with an AR coating.

By translating the correction element 120 to change the optical path length of the resonator in synchronization with the filtering wavelength of the etalon 162, a constant number of modes is maintained while tuning the laser in the external resonator. A constant number of modes is obtained by increasing the thickness of the correction element as the etalon gap widens during translation and thereby tunes to longer wavelengths. Since the refractive index of the correction element is greater than the refractive index of air, the overall average refractive index in the external resonator increases. This reduces the beam propagation velocity and effectively lengthens the optical path length of the resonator.

Thus, by cascading a correction element 120, for example, a glass wedge, with a wedge-shaped interference filter / etalon 162 and changing the optical path length of the resonator in synchronization with the filtering wavelength, the laser is tuned into the resonator during tuning. A constant number of modes is maintained.

FIG. 2A is a side view of a wedge-shaped air gap type etalon and a correction element portion of the wavelength tunable external cavity laser shown in FIGS. 1A and 1B. FIG. 2B is an exploded side sectional view of the air gap and the correction element portion of the wavelength tunable laser. The etalon 162 and the correction element 120 are sandwiched between the focusing lens 110 and the quarter-wave plate 112 on the base side, and the quarter-wave plate 114 and the retroreflector 122 on the far side.
Sandwich. In this embodiment, each of the components is aligned with and intersects the optical axis 100. In this embodiment, the translation axis 190 is generally orthogonal to and intersects the optical axis 100. However, this is not necessary.

The maximum angle of the etalon wedge formed by the first reflector 116 and the second reflector 118, that is, the tuning angle, is obtained by dividing the center deviation by one beam diameter, and the full width at half maximum (FWHM) of the etalon / filter. ).

In this embodiment of the present invention, the correction element 120 is made of an optical element having a uniform refractive index. The correction element has a wedge-shaped profile with generally flat outer and inner surfaces that converge at a correction angle 202. In the embodiment shown, the correction element 120
And the air gap etalon 162 are both coupled to a mount 142, which is coupled to a translation mechanism 144. The translation mechanism 144 is attached to the base 138 (see FIG. 1A). The translation mechanism is coupled to and controlled by tuner 160. The tuner may include a feedback element, eg, a sensor, that monitors a laser operating parameter, eg, the emission wavelength of the laser. In embodiments of the present invention where the translation mechanism is a piezoelectric element (also referred to as an electrostrictive element), the tuner generates an electrical impulse that controls expansion and contraction of the piezoelectric element along the translation axis 190. This has the effect of synchronizing and translating both the air gap etalon and the correction element across the optical axis.

In FIG. 2A, the combined etalon and correction element assembly is shown in a retracted position 204 and an extended position 206 linearly displaced from each other along a translation axis 190. FIG. 2B shows the air gap etalon 1 at each of these locations.
FIG. 3 is an exploded cross-sectional view of optical paths 224, 226 and 220, 222 passing through both 62 and the correction element 120. In the retracted position 204, the beam traverses the relatively thick portions 224, 226 of the air gap etalon and the correction element, respectively. Extension position 206
In, the beam traverses the relatively thin portions 220, 222 of the air gap etalon and the correction element, respectively. Thus, in the extended position of the etalon and the correction element, the etalon supports short wavelengths or high frequencies of lasing, where constructive interference occurs between the reflective surfaces of the etalon. In the retracted position, the etalon supports long wavelengths and constructive low frequencies where constructive interference occurs. Etalon gap "G
The required relationship between the change in the optical path length “P” and the change in the optical path length “P” is dP / P = dG / G.

Without the correction element 120, as a result of the translational tuning of the air gap etalon,
Mode hopping may occur. This is because both the optical length and the actual length of the external resonator are constant. The optical length “P” of the resonator is the refractive index “P”.
n "and the product of the thickness" L "along the optical axis of each of the elements" i "through which the beam passes.
Specifically, the optical length of the external cavity is the sum of each of these products, or P = Σn _{i ·} L i. In the embodiment shown in FIG.
0 is a material made of a substrate with a uniform refractive index, which varies in thickness,
This is activated by being attached to the same translation mechanism 144 that positions the etalon 162. Thus, the translation of the correction element 120 can change the average refractive index of the optical path. By synchronizing the etalon and the correction element,
The average index of refraction along the beam path is directly proportional to the tuning wavelength, thus maintaining the same integer multiple of half a wavelength in the external resonator and eliminating mode hopping.

In addition to the above-described mechanism for tuning the resonator by translating the etalon and for correcting the optical path, the embodiment of the present invention shown in FIGS. The amount of reflection 214 is reduced. These spurious reflections occur when the laser beam hits the first reflector 116. In the embodiment shown in FIGS. 1 and 2, the normal line of each of the first reflector and the second reflector is located in the ZY reference plane. Because of the narrow wedge (also known as tuning) angle between the reflectors, the beam intersects each of the reflectors substantially orthogonally. As a result, out-of-band unfiltered spurious reflections and interference are sent directly back to the gain medium (see 214 and 210 in FIG. 2). This unwanted feedback is detrimental to the operation of the laser.

The quarter-wave plates 112 and 114 reduce feedback from the false reflection 214. The filter / etalon 162 operates in a double-pass transmission scheme that sends narrow bandwidth light back to the gain medium. High finesse interference filters / etalons have sharp transmission peaks. Light not transmitted by the filter / etalon 162 is reflected back toward the gain medium. This embodiment of the present invention utilizes the principle that the gain medium preferentially amplifies substantially linearly polarized light along with the polarization conversion characteristics of the quarter wave plate. The filtered light passes through both quarter-wave plate 112 and quarter-wave plate 114 twice, thus returning to the gain medium with the same polarization state as at the start. The reflected light 214 passes twice through the quarter wave plate 112 only, and thus returns to the gain medium in the orthogonal polarization state at the start, and is thus not amplified by the gain medium.

FIGS. 2C and 2D illustrate alternative embodiments of etalons and correction elements having each of the features described above, including translational tuning and suppression of both mode hopping and false reflection.

In the first of these embodiments, shown in FIG. 2C, the second reflector of the air gap etalon is mounted directly on the inner surface of the quarter wave plate 114 and the retroreflector 122 is mounted on the quarter wave plate 114. Directly attached to the outer surface of the The entire assembly, including the associated wedge correction element 120, comprises a mount 142 and a translation mechanism 14
4 to a base 138 (not shown). These components are moved along a translation axis 190 (see FIG. 2A) by a translation mechanism, whereby
As described above in connection with FIG. 2A, in maintaining an integer multiple of half a wavelength in the resonator,
The same effect can be obtained in terms of tuning the external resonator and adjusting the refractive index. In addition, the quarter-wave plate has substantially the same effect as that described above, that is, the effect of suppressing the false reflection of the beam from the first reflector 116.

In another embodiment of the present invention shown in FIG. 2D, many of the features shown in FIG. 2B having a combination of a retro-reflector 122 with a quarter-wave plate 114 and a second reflector 118 are retained. In this embodiment of the invention, the correction element 230 is a substrate of uniform thickness but with a refractive index that is graded along the translation axis.
Gradient index of refraction can be achieved by various vacuum dopant diffusion techniques that increase the index of refraction along the translation axis. The correction element 230 includes a mount 142 and a translation mechanism 144.
Is connected to the base through. The translation mechanism expands and contracts the correction element across the optical path 100 when the air gap is expanding and contracting. This has the effect of causing the beam to cross the cross-section of the correction element, thereby increasing or decreasing the refractive index, thus affecting the overall optical path length of the resonator. As described above, the quarter-wave plates 112 and 114 can suppress false reflection of the beam from the first reflector 116.

FIG. 2E shows an alternative embodiment of the present invention where both the etalon correction element and the first reflector 116 are stationary. In this embodiment, only the etalon second reflector 118, quarter-wave plate 144, and retroreflector 122 assembly undergo translation along translation axis 190 by translation mechanism 144 (see FIG. 2A). The opposing first reflector 116 and second reflector 118
The same angular relationship described above in connection with FIGS. 2A-2D is maintained, that is, the tuning angle 200. The second reflector 118 is a generally flat surface that intersects the optical axis 100. Translation of the second reflector 118 along the translation axis results in a change in the thickness of the air gap between the reflectors with respect to the optical axis. This allows tuning of the external resonator described above. Further, the correction element 240 is fixed. In this embodiment, the correction element is made of a material whose refractive index can be changed by an electrical stimulus provided by the mode controller 260.
The mode controller is coupled to both correction element 240 and tuner 160. The tuner extends or retracts the etalon second reflector 116 to increase or decrease the frequency at which external resonator lasing occurs. In parallel, the mode controller applies appropriate electrical energy to the correction element 240 to increase or decrease the refractive index of the correction element, thereby changing the optical path length of the resonator and suppressing mode hop during laser tuning. I do.

FIGS. 3A and 3B show FIGS. 1A and 1B with and without the correction element 120.
FIG. 2B is a graph showing the operating state of the wavelength tunable laser shown in FIG. 2B and FIGS. 2A to 2E (see also reference numerals 230 and 240 in FIGS. 2C to 2E).

The wavelength of the longitudinal mode of a laser resonator is related to the length of the resonator by the relationship that L _ext is approximately equal to m · λ _m / 2, where m is an integer called the number of modes. is there. Typically, m> 10 ³ . The laser only oscillates at the wavelength of the longitudinal mode. The size of the mode hop is about λ _c ² /
2L _ext , where λ _c is the center of the tuning range. Thus, if the cavity length remains constant when tuning the filter / etalon peak, the laser output will change intermittently, producing a tuning characteristic called mode hop tuning (see FIG. 3).

As shown in FIG. 3A, tuning of the resonator without the correction element includes a change in the wavelength of the tuned resonator over the entire operating range and a change of an integer multiple of a half wavelength. This results in an intermittent plot 300 of the laser wavelength against the feedback wavelength obtained by the etalon / filter.

On the other hand, if the cavity length can be changed so that L _ext = M · λ / 2 for a constant integer M, the laser always oscillates with the same mode number when the laser wavelength λ is changed. Will do. In other words, no mode hops will occur and the laser wavelength will follow the filter wavelength continuously. This type of tuning is called a mode-hop-free or phase-continuous tuning scheme, and is shown in FIG. 3B. To ensure single longitudinal mode operation with a high side mode suppression ratio, the filter needs to create a differential loss between the lasing mode and the neighboring longitudinal mode. Raging will occur at the peak of the net gain. Whether or not the side mode can be removed depends on the full width at half maximum (FWHM) of the filter loss characteristic compared to L _ext . L _ext is the resonator length of the external resonator. Ideally, FWHM is approximately equal to c / L _ext , but this is not a requirement. This condition can be addressed with techniques that utilize interference filters / etalons.

As shown in FIG. 3B, the plot 302 of lasing wavelength versus wavelength tuned by the etalon / filter is substantially continuous, ie, there is no discontinuity due to mode hopping. This phase-continuous tuning is achieved by a correction element, which in each case of the embodiments shown in FIGS. 2A to 2E, etc., as the wavelength of tuning of the resonator increases as the wavelength of the resonator increases. It has the property of increasing length.

FIGS. 4A and 4B show a modified embodiment of the present invention that enables suppression of both spurious reflection and spurious interference without using an in-resonator waveplate. FIG. 4A is an isometric side view of the tuning and correction element portion of the laser, and FIG. 4B is a plan view thereof. The laser is
It has a lens system 440, which focuses the waist region of the laser beam on the etalon, so that it is elongated on the long axis and compressed along the short axis, so that it is substantially An elliptical profile 410 will be obtained. The major axis and the minor axis where beam expansion and compression are performed are located in a plane parallel to the X, Y plane. The short axis associated with the compression of the beam is substantially collinear with the translation axis 190 described above, into which the wedge-shaped etalon is translated. The wedge or tuning angle 200 lies substantially in the Z, Y plane.

As shown in FIG. 4B, in this embodiment, the entire tuning and correction assembly including the translation mechanism 144, the mount 142, the etalon 162, the correction element 120, and the substrate 130 has a tight tilt angle 402 about the translation axis. It is only shifted. This tilt, of the order of 6 °, is sufficient to direct the false reflection 214 away from the optical axis 100. Further, this embodiment can move the false interference 210 away from the optical axis. As a result,
The result is that the fidelity of the laser tuning is increased and a quarter wave plate or its equivalent is not required.

FIGS. 5-7 show alternative embodiments of the anamorphic / astigmatic lens system 440. 5A and 5B are an isometric view and a plan view, respectively, of an astigmatic / anamorphic lens system 440. FIG. In this embodiment, laser diode 102 is fabricated with a waveguide that produces a highly elliptical output beam 126. The lenses 108 and 110 each help collimate and focus the elliptical beam 126 into an elliptical cross-sectional profile 410, the waist region of which falls within the etalon 162. Compression of the beam along both the Y axis and the translation axis 190 increases the accuracy with which the etalon can tune the laser. The expansion of the beam along the X-axis is controlled by the second reflector 118 of the etalon and the retroreflector 12.
2 increases the walk-off loss in the spurious interference 210 that occurs. Thereby, the feedback from the spurious interference to the gain medium is reduced, and the operating characteristics of the laser are improved.

FIGS. 6A and 6B are an isometric view and a plan view, respectively, of a modified embodiment of the astigmatic / anamorphic lens system 440. In the embodiment shown in FIG. 6A, the astigmatic lens system distorts beam 124 along both the X and Y axes. The beam is compressed along the Y axis and expanded along the X axis. This results in the desired elliptical cross-sectional profile 410 with an elliptical cross-sectional beam waist region within the etalon 162.
The astigmatic lens system 440 has two cylindrical lenses 608, 610 positioned between the front facet of the laser 106 and the correction element 120. The longitudinal axes of each of these cylindrical lenses are orthogonal to each other.

FIGS. 7A and 7B are isometric side and plan views, respectively, of another embodiment of an anamorphic / astigmatic lens system 440. In this embodiment, anamorphic prisms 708, 710 and focusing lens 706 compress beam 124 along an axis (ie, the Y axis) so that the beam has a desired elliptical profile 410 and its waist region is It is located in the etalon 162.

Each of the above configurations shows a translation axis that is generally parallel to the reference Y-axis in order to facilitate disclosure. However, this is not necessary. In an alternative embodiment, the translation axis may not be parallel to the Y axis. However, the only condition for this is that the first
Or, any of the second reflectors can be appropriately displaced. Voice coil tuned external cavity laser voice coil actuators (VCA) can be used to tune the ECDL of Littman or interference filter tuned cavity resonators, resulting in linear or arcuate motion. In the Littman configuration, tuning is performed by selecting the feedback wavelength to the gain medium and by adjusting the ECD
This is achieved by an arcuate movement of a diffraction grating or retroreflector attached to the proximal end of the arcuate voice coil actuator that tunes L. The arcuate voice coil actuator allows for quick positioning corresponding to FM tuning or linear FM sweep. If tuning is achieved by an interference filter, the linear or arcuate movement of the voice coil actuator can be used to select the feedback wavelength to the gain medium to tune the ECDL.

FIGS. 8A, 8B, 9A, 9B, 10A and 10B show an interference filter tuned ECDL. 8A and 8B, the interference filter is linearly operated by the VCA. 9A, 9B, 10A, and 10B, the interference filter is operated in an arc by the VCA. In any type of VCA,
The magnetic field exerts a force on the coil as current flows through the coil, causing linear or circular motion. The force is given by Equation 1 below, where F is the force generated in the coil, N is the number of turns in the coil, L is the length of the coil under the action of the magnetic field, and i is the applied force. It is a current.

[0048]

## EQU00001 ## F = NLBi VCA generally has a substantially linear control characteristic, a hysteresis of substantially zero, and a resolution with respect to very fine positions. Positional resolution may be limited primarily by current noise in the drive electronics. As a practical matter, the position resolution is less than 0.1 μm. Further, the VCA drive system should have a low electrical and mechanical time constant and a high power to weight and volume ratio compared to conventional ECDL actuators, such as stepper motor and DC motor systems.

FIGS. 8A, 8B, 9A, 9B, 10A and 10B show a combination of an interference element and a linear and arcuate voice coil actuator. In alternative embodiments of the invention, these actuators can also be used to tune other ECDL tuning elements. These tuning elements include interference elements, diffraction elements, retroreflectors, and the like. Interference elements include air gaps and solid etalons. Diffraction elements include gratings and reflectors. Retroreflectors include two-sided prisms, corner cubes and mirrors. Another advantage of either linear or arcuate voice coil actuators is that they can be made relatively lightweight as compared to conventional ECDL actuators, such as stepper motors or DC motors. This weight reduction improves the ECDL response time, wavelength sweep speed, and FM range.

The interferometer shown in FIGS. 8 to 12, for example, an air gap etalon, exhibits both a function of reducing false feedback and a function of compensating for a change in optical path length, and thus performs ECDL.
A substantially constant integer multiple of half a wavelength can be maintained in the cavity resonator of These functions can be performed with either an air gap or a solid etalon without departing from the scope of the present invention. When the novel etalon disclosed herein is combined with a voice coil actuator (which may be linear or arcuate), the tuning assembly, i.e., the tuning element and actuator, is compared to a conventional ECDL actuator, such as a stepper motor or DC motor. And relatively lightweight, for example 1
It can be manufactured in less than g. This weight saving improves the ECDL response time, wavelength sweep, and FM range.

The straight and arc etalons shown in FIGS. 8-12 are configured such that the spacing between opposing reflective surfaces varies linearly across the optical aperture. Thus, when a small diameter light beam is incident on the etalon, the transmission wavelength will change when the etalon is displaced in a plane perpendicular to the beam path. The trajectory of this displacement is a straight line segment (line segment) and an arc. The etalon is moved by a linear or rotary VCA, depending on what kind of movement is desired. Small and inexpensive high bandwidth V
CA is utilized in optical and magnetic data storage systems. The stroke (travel distance) of the straight line VCA ranges from a few microns to 1/4 of an inch. Appropriate V
CA is available from Aura Systems, Inc., BEI Sensors and Systems Company, or Planning Systems, Inc. 8A and 8B show a modified embodiment of the actuator used in the etalon. In this embodiment, the actuator is an electrically actuated voice coil or solenoid 800, 802 that extends or retracts etalon 162 across optical path 100 to create a feedback wavelength to gain medium 102 and tune the laser.

FIG. 8A is an isometric side view of an embodiment of an interferometer tuned external cavity diode laser. A cavity resonator is formed between the gain medium 102 and the external retroreflector 122, both of which are mounting blocks 148, 14 respectively.
0 to the base 138. The opposite end of the cavity resonator
It is constituted by an external retroreflector 122, which is also mounted on the base via a mounting block 140. An interfering element, for example, an air gap etalon 162, is located in the cavity. The interfering element is coupled to the base via voice coils / solenoids 800,802. Solenoid base portion 800 is attached to base 138. The piston portion 802 of the solenoid can extend and retract along the axis 190 to move away from and approach the base 138, thus providing a wedge-shaped air gap etalon 162 between the gain medium and the external retroreflector 122. Move across the extended light path 100. A collimating and focusing optical system 440 is shown between the air gap etalon 162 and the gain medium (see FIGS. 4 to 7).

FIG. 8B is a cross-sectional side view of the voice coil / solenoid 800, 802 shown in FIG. 8A when coupled to tuner 160. The piston portion 802 of the voice coil is securely coupled to electrical windings 804, which extend downwardly from the piston in the form of a cylinder. At the center of the piston, a positioning element 806 is shown extending vertically downward. The piston 802 is
It is extendably coupled to the base 800 via a permanent magnet 808 and a laminated stack 810, which constitutes a tubular annulus surrounding the coil 804 therebetween. A central hole provided in the stack stack surrounds the position sensor. Within the tuner 160, a processor 820 and a storage device 822 are shown. The storage device has both the program code and the look-up table 824. The look-up table has data or formulas that correlate wavelength with position. In the illustrated embodiment, the look-up table has a plurality of records, each with a wavelength column 840 and a position column 842. These columns record the correlation between the amount of displacement of the piston 802 with respect to the base 800 and the wavelength of the tunable laser corresponding thereto.

In operation, the processor 820 receives a request to tune a cavity resonator to a particular wavelength. The processor utilizes the look-up table 824 to determine the appropriate displacement / position of the piston relative to the base. The processor then applies the appropriate electrical energy to coil 804. The current in the coil exerts a magnetic force on the coil in a positive or negative direction substantially perpendicular to the magnetic flux created by the permanent magnet 808 in the piston. As a result, the piston is displaced with respect to the base. Using the feedback provided by the position detector 806, the processor adjusts the energy delivered to the coil to maintain the relative displacement of the piston and base at the required level corresponding to the level indicated in the look-up table.

The piston and base can be dynamically positioned relative to each other using only electrical energy for both extension and retraction of the piston. Alternatively, a return spring (not shown) can be used to move piston 800 toward or away from the base, and the energy applied to coil 804 offsets the biasing force of the spring to move the piston. Help. Closed-loop control of the output wavelength Tuning elements, such as interference filters, diffractive elements or closed-loop feedback of retroreflectors, can be achieved using a number of devices, including:
Encoders (e.g., electrical, optical or magnetic), position sensors and optical sensors may be used, and their use does not depart from the scope of the invention. ECDL actuators, such as voice coils, can operate in several modes with the aid of certain position feedback devices, including positioning modes. The tuning elements shown in FIGS. 8A and 8B, such as the VCA, use a position feedback device that is an encoder or sensor. Suitable position sensors include inductive and glass scale position feedback devices. Some of the sensor technologies described above operate based on proximity measurements and do not actually require physical contact with the etalon. Voice coil actuators can also operate in a wavelength feedback control loop fashion using sensors or encoders.

As a variant, the ECDL actuator, eg, VCA, may be an optical, magnetic or other read or read / write head and associated encoder, eg, FIG.
0A, 10B and 11 may be provided. The encoder may be mounted on a tuning member, such as an etalon, with the head mounted on a stationary member, such as the base, or vice versa. If the encoder is optical, the reader may have a light source and an optical detector. In a modified embodiment of the present invention,
During calibration at the factory, the calibration data may be written on a fixed storage medium or the encoder itself. If the encoder is write-programmable, each tunable ECDL can be individually calibrated throughout the tuning range using an external wavelength meter during device fabrication.

FIGS. 9A and 9B are isometric side and rear views, respectively, of a modified embodiment of the present invention that tunes an external cavity diode laser using a precisely positioned interferometer 920. In this embodiment of the invention, the arcuate actuator has mechanical commonality with the voice coil arcuate actuator used in computer disk drives. The tuning arm 940 has a support 93 at the center.
The support column is rotatably connected to the base 900 through the base 0, and the column extends upward at a right angle from the base. In this mounting configuration, both the proximal end 946 and the distal end 942 of the tuning arm move in an arc in a plane parallel to the top surface of the base 900. A shaped electric coil assembly 950 is mounted at the proximal end of the tuning arm. The coil is biased via conductor 952 and has radially opposite sides. Between the coil and the base are two adjacent permanent magnets 960, 962 of opposite polarities, with the dipole generally perpendicular to the top surface of the base 900. Supplying electrical energy of a first polarity to coil 950 via conductor 952 creates a temporary magnetic field such that the distal ends of the pivot arms have permanent magnets 960, 962 of opposite polarities. Will move toward one of them. Conversely, when an electrical signal of the opposite polarity is applied to actuator coil 950 via signal line 952, a magnetic field of the opposite polarity is created, which causes the distal end of the pivot arm to move to the opposite permanent magnet. Will do. In a modified embodiment of the present invention, the magnet and the actuator coil may be arranged in a reverse position, and the magnet may be mounted on the tuning arm and the coil may be mounted on the base.

At the opposite end of the tuning arm, ie at the far end, a wedge-shaped interference element or etalon 920 is mounted. In the illustrated embodiment, the interference element is an air gap etalon with a transparent substrate 922 and a correction element 924 mounted on the top and bottom surfaces of the distal end of the tuning arm. The air gap formed between the reflective inner surfaces of the etalon is wedge-shaped. The etalon 920 is connected to the housing 902
In the orthogonal cavity formed between the gain medium and the rear retro-reflector 914, an arc-shaped motion is obtained. The housing 902 is attached to the base in a direction that directs the laser beam 904 to the corner cube 908, and the corner cube is also attached to the base. Laser beam is corner cube 9
When hitting 08, perpendicular to the base deflects upward along optical path 972. This optical path intersects the wedge-shaped air gap of the air gap etalon 920. As the etalon is moved in an arc by the tuning arm 940 in a plane intersecting the beam path, the thickness between the opposing reflective surfaces on each side of the air gap increases or decreases depending on the direction of motion. This selects a particular wavelength at which lasing occurs in the cavity via feedback of the selected wavelength to the gain medium by reflection from the corner cube 908. Light that has passed through the transparent substrate of the etalon 922 impinges on the retroreflector 914. The output beam is obtained through a rear facet of the gain medium along axis 970 or a retroreflector 914 along axis 972.

In an alternative embodiment of the present invention, the upper surface of transmissive substrate 922 is coated with a reflective material, thus acting as a retroreflector.

Many variations of the arc-tuned etalon 920 can be envisioned without departing from the scope of the present invention. The illustrated etalon has a wedge-shaped air gap. In an alternative embodiment of the invention, a wedge-shaped solid etalon with reflective outer surfaces obliquely facing each other is mounted on the distal end of the actuator. In yet another embodiment of the present invention using an air gap etalon 920, only one of the etalon components, ie, the transmissive substrate 922 or the correction element 924, is attached to an actuator or tuning arm 940, and the other to the base. Fixed. An etalon component mounted on the actuator tunes the etalon and provides tunable feedback to the gain medium, ie, the laser diode. However,
The condition is that the etalon component attached to the actuator has a reflective surface located in a plane that is offset from the plane of rotation of the tuning arm. Thus, when the component is swept by the actuator across its stationary counterpart,
The spacing between these two along the optical path, ie the thickness, will vary. This changes the feedback wavelength to the gain medium, thereby tuning the laser to the selected wavelength.

In the illustrated embodiment, the selection of the output wavelength is made by an optical reader, ie, a laser, provided in the housing 902. The laser emits a beam 906, which strikes a second corner cube 910 which deflects it vertically upwards into an optically encoded strip (FIG. 1).
0 and FIG. 11). The reflected beam includes information stored in the encoder, which information is also read by an optical reader housed within housing 902 and correlates the arc position of etalon 920 with the output wavelength of the cavity. Is determined. In an alternative embodiment of the invention, the optically encoded strip is mounted on a base and the reader is coupled to a tuning arm, which passes over the strip when tuning the laser.

As will be apparent to those skilled in the art, many variations on the illustrated geometries are possible without departing from the scope of the invention. In embodiments of the present invention, the arcuate movement of the tuning element may lie in a plane perpendicular to the base. In this embodiment, the cavity resonator may be linear rather than orthogonal.

In the embodiment shown in FIGS. 9A and 9B, the proximal end 946 of the tuning arm 940 is shown.
If the device is designed such that the moments of inertia of the distal end 942 are substantially equal to each other, a certain degree of impact resistance can be obtained. In yet another embodiment of the present invention, positioning the laser between the tuning arm and the base allows the shape factor to be reduced. To further reduce the form factor, both the actuator coil 950 and the magnets 960, 962 are mounted at the distal end 942 of the tuning arm, thereby eliminating the need for the proximal end of the tuning arm (impact balance). Is a different consideration. ECDL programmable encoder view 10A and 10B for the closed-loop control of the tuning elements are respectively a plan view and a side view of a precisely tunable interference filter described above in connection with FIGS. 9A and 9B. Further, in both FIGS. 10A and 10B, associated calibration and control circuitry for utilizing a precisely tuned interference filter is shown. In both FIG. 10A and FIG.
000 is shown coupled to the laser, actuator and read / write encoder head of the interferometer tuned ECDL. The tuner is processor 1
002, storage 1004, positioning logic 1008, and read or read / write logic 1010. The program code 1006 stored in the storage device 1004 controls the operation of the processor. In the illustrated embodiment, the positioning of the etalon 920 is controlled by a drive signal sent to the actuator 950 by the positioning logic 1008 in connection with the closed-loop feedback of the output wavelength provided by the encoder 1030. Encoder 1030
Is attached to the lower surface of the lower plate 924 of the air gap etalon.
The encoder has an output wavelength corresponding to each position of the encoder within the cavity. An optical reader housed within the housing 902 emits a laser beam that deflects upwards on a corner cube 910 and impinges on an encoder. The reflected signal from the encoder has wavelength information corresponding to each position of the etalon, and this wavelength information is read by an optical reader housed in the housing 902. This information is read by the read logic 1010.
Where it is used as part of a feedback control loop to enable processor 1002 and positioning logic 1008 to properly energize actuator coil 950.

In the illustrated embodiment, the actuator arm 940 is biased by the actuator and permanent magnet assembly in both clockwise and counterclockwise directions.
In alternative embodiments of the invention, the return force may be generated in either a clockwise or counterclockwise direction, without departing from the scope of the invention. Applying the above technique, ie, a linear / arc shaped voice coil tuner, to an ECDL tuning element, such as a diffractive element and a retroreflector, achieves the same advantages without departing from the scope of the present invention.

In the illustrated embodiment, position feedback is achieved by an optical encoder. As will be apparent to those skilled in the art, the same advantages can be achieved using other types of encoders, including electrical and magnetic encoders. In yet another embodiment of the present invention, position feedback may be achieved by electrical, optical or magnetic position sensors.

In embodiments of the invention utilizing an encoder, various degrees of accuracy between a given selected output wavelength of the ECDL and the actual output wavelength may be obtained depending on how the encoding is performed. Can be. In a first embodiment of the invention, a standard encoder is attached to all of the ECDLs in a model or production line. In this embodiment, the encoder is a read-only medium on which certain indices and / or data are stored, from which the position and / or output wavelength can be determined for a statistically averaged ECDL. The accuracy of this method will vary from standard with the statistical deviation of each ECDL. In addition, even if the ECDL has tuning properties approaching statistical standards, it is still necessary to accurately mount encoders and readers. Its purpose is to obtain the required relationship between the actual output wavelength and the encoded wavelength / position.

If high accuracy is required between the selected output wavelength and the actual output wavelength,
It may be advantageous to attach a blank read / write programmable encoder to a tuning element, such as an air gap etalon. In this embodiment of the invention,
The tunable data is produced with the encoder attached to the tuning element without any stored data on wavelength or position. Within the housing 902, a read / write encoder head is provided. The head may utilize an electrical, magnetic or optical writing function that encodes the information on the encoder strip 1030. To calibrate the ECDL, the encoder is programmed with data obtained by the wavelength meter 1020 coupled to the output beam of the ECDL. The wavemeter is only used during the next calibration phase and does not need to be transported with the ECDL itself. Instead, information about the output wavelength or position information that can be correlated with the output wavelength is stored directly on the encoder. As a result, the cost and complexity of the shape factor in the device is reduced.

To calibrate the ECDL, the positioning logic 1008 sweeps the air gap etalon over the tuning range, pauses along the tuning range, allowing the wavelength meter to measure the output wavelength and reading and writing. Allows the encoder to write the relevant wavelength on the appropriate radial or straight segment of the encoder strip 1030. At the end of this stage, the encoder strip has device specific output wavelength information along its length. With the wavelength information embedded in the encoder, only the tuner 1000 is provided, and the wavelength tunable laser is transported with the wavelength meter 1020 omitted. In operation, device-specific information on the encoder strip allows for high accuracy between the selected wavelength and the output wavelength.

In yet another embodiment of the present invention, the read-only function can be prioritized, eliminating the costs associated with providing a read-write function in the unit. Provided that during calibration at the factory, an optical, electrical or magnetic encoding function suitable for incorporating information obtained by the wavelength meter onto the programmable encoder strip 1030 is performed.

As will be apparent to those skilled in the art, the encoding techniques described above can be utilized in conjunction with alternating tuning elements of an external cavity laser, which provide equivalent advantages. Does not depart from the scope of the invention. Such tuning elements include, but are not limited to, retroreflectors and diffractive elements. Arc Tuned Interference Filter for ECDL FIG. 11 is a detailed side view of the housing 902 and etalon 920 shown in FIGS. The gain medium contained within housing 902 emits beam 904, which strikes corner cube 908. This beam is the etalon 9
22 and deviates upwards along path 972 at right angles. The air gap etalon is configured between opposing reflectors 1110 and 1112, which are mounted on opposing inner surfaces of the transmissive substrate 922 and the correction element 924, respectively. Light that has passed through the etalon impinges on the reflection element 1100 of the retroreflector 914. As the wedge-shaped etalon travels exactly through beam path 972, the thickness of the air gap changes, thereby selecting a particular wavelength at which feedback to the gain medium in housing 902 occurs. This results in tuning of the cavity resonator. In the illustrated embodiment, the encoder 1030 is mounted on the bottom surface of the correction element 924. In the illustrated embodiment, the encoder is
It is optically readable by an output and return beam 906, which exits an optical detector in housing 902. This encoder has the above-mentioned position and / or wavelength information at which the closed-loop feedback described above with reference to FIG. 10 is performed.

In an alternative embodiment of the present invention, the lens system includes a gain medium, such as a laser diode and an etalon 920, in the housing 902 to focus the beam on the etalon.
Are provided in the optical path between them. As described above in connection with FIGS. 4-7, the lens system focuses the waist region of the laser beam on the etalon, which is elongated on the long axis and compressed along the short axis, so that substantially As a result, an elliptical cross-sectional profile can be obtained. The short axis at which the compression of the beam takes place is substantially tangent to the etalon arc path.

12A and 12B are side end views of a modified embodiment of an interferometric element, for example, an air gap etalon 920, viewed inward from the laser. The corner cubes 908, 910, the base 900, the retroreflector 914, and the distal end of the tuning arm 942 are shown in both of these figures. In the first embodiment shown in FIG. 12A, the transmissive substrate 920 is orthogonal to the optical path 972. Correction element 924
And the inner reflective surface of the transmissive substrate 922 are separated from each other at a tuning angle. Both the thickness 1200 of the air gap at one end of the etalon and the thickness 1202 of the correction element are smaller than the thickness 1204 of the air gap at the other end of the etalon and the thickness 1206 of the correction element. The tuning angle determines the change in the air gap over the length of the etalon. This correlates with the tunable range of the ECDL. The principles described above in connection with FIGS. 1-3 can be embodied in the precisely tunable interferometric element shown in FIG. 12A, and the advantages obtained are substantially the same.

In FIG. 12A, the correction element 924 may have a uniform refractive index. In this embodiment of the invention, the thickness of the correction element increases correspondingly with the thickness of the air gap,
Thus, a uniform optical path length is obtained over the entire tuning range. As a variant, the correction element may have a uniform thickness and a linearly varying index of refraction, so that a uniform optical path length is obtained over the entire tuning range.

In a modified embodiment of the present invention shown in FIG. 12B, the transparent substrate 922 and the correction element 92
The etalon 920 containing both of them is oriented at a tilt angle that indicates the degree of rotation about the Z axis. This orientation of the etalon reduces spurious interference and feedback to the gain medium, as described above in connection with FIGS.

In an alternative embodiment of the present invention, the etalon is an arcuate solid wedge with reflective outer surfaces that extend in relation to one another.

FIG. 13 is an isometric side view of an embodiment of an arcuate actuator with a grating and a retroreflector for tuning an external cavity diode laser. The retroreflector 1302 is connected to the distal end 942 of the arcuate actuator 940.
Is joined to. Laser 1306 and diffraction element, eg, grating 130
4 are coupled to the base 900. The laser emits a beam 1310 at a grazing angle to the grating. This beam is reflected as shown at 1312 and directed to the retroreflector. The wavelength feedback to the laser depends on the orientation of the laser, grating and retro-reflector. The actuator pivot is, in embodiments of the present invention, positioned to maintain a substantially constant integer multiple of half-wavelengths in the external resonator over the entire tuning range. This reduces mode hopping. A useful output beam 1314 is obtained. In an alternative embodiment of the invention, tuning is performed by a diffractive element instead of a retroreflector. The arcuate voice coil actuator allows for rapid positioning corresponding to FM tuning or linear FM sweep. An optical read / write element may be used in conjunction with a recording medium mounted on plate 924 to control the tuning of the laser as described above.

The above description of a preferred embodiment of the invention has been presented for purposes of illustration. The present invention is not limited to the disclosed form itself. Obviously, many modifications and alterations in the art will occur to those skilled in the art. The scope of the present invention is defined by the scope of the present invention described in the claims and the equivalents thereof.

[Brief description of the drawings]

FIG. 1A is an isometric view of a tunable external cavity laser of the present invention.

FIG. 1B is a plan view of a wavelength-tunable external cavity laser of the present invention.

FIG. 2A is an exploded side view of the etalon and the correction element shown in FIGS. 1A and 1B.

FIG. 2B is a diagram showing a modified embodiment of the etalon and the correction element shown in FIG. 2A.

FIG. 2C is a diagram showing a modified embodiment of the etalon and the correction element shown in FIG. 2A.

FIG. 2D is a diagram showing a modified embodiment of the etalon and the correction element shown in FIG. 2A.

FIG. 2E is a diagram showing a modified embodiment of the etalon and the correction element shown in FIG. 2A.

FIG. 3A is a graph illustrating mode hopping of the illustrated external cavity laser.

FIG. 3B is a graph showing a continuous phase tuning state of the illustrated external cavity laser.

FIG. 4A is an isometric view of a modified embodiment of an etalon and a correction element that suppresses both spurious interference and spurious reflection.

FIG. 4B is a plan view of a modified embodiment of an etalon and a correction element that suppress both spurious interference and spurious reflection.

FIG. 5A is a detailed isometric view of an external cavity diode laser having an embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 5B is a plan view of an external cavity diode laser having an embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 6A is a plan view of an external cavity diode laser having another embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 6B is a plan view of an external cavity diode laser having another embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 7A is a plan view of an external cavity diode laser having yet another embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 7B is a plan view of an external cavity diode laser having yet another embodiment of a lens system suitable for use in the novel etalons and correction elements shown in FIGS. 4A and 4B.

FIG. 8A is an isometric side view of an embodiment of the present invention including a voice coil actuator for an interference filter tuned external cavity diode laser.

FIG. 8B is a detailed sectional side view of a voice coil actuator, a position sensor, and a control circuit of the tunable laser of FIG. 8A.

FIG. 9A is an isometric side view of an embodiment of an arcuate actuator for a tunable external cavity diode laser.

FIG. 9B is an isometric side view of an embodiment of a tunable external cavity diode laser arc actuator.

10A is a plan view of the arc-tuned external cavity diode laser of FIGS. 9A and 9B including associated calibration and control circuitry.

10B is a side view of the arc-tuned external cavity diode laser of FIGS. 9A and 9B including associated calibration and control circuitry.

11 is a detailed side view of an embodiment of the resonator portion of the tunable external cavity diode laser of FIGS. 9 and 10. FIG.

FIG. 12A is a side view of an end face of a modified embodiment of the interference filter tuning element of the cavity resonator shown in FIG. 11;

FIG. 12B is a side view of an end face of a modified embodiment of the interference filter tuning element of the cavity resonator shown in FIG. 11;

FIG. 13 is an isometric side view of an embodiment of an arcuate actuator with a grating and a retroreflector to tune an external cavity diode laser.

────────────────────────────────────────────────── ─── Continued on the front page (31) Priority claim number 09 / 342,342 (32) Priority date June 29, 1999 (June 29, 1999) (33) Priority claim country United States (US) ( 81) Designated countries EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, SD, SL, SZ, TZ, UG, ZW), EA (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), AE, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, R, CU, CZ, DE, DK, DM, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK , SL, TJ, TM, TR, TT, UA, UG, US, UZ, VN, YU, ZA, ZW (72) Inventor Chapman William B United States 94086 Sunnyvale South Pastria Rear Avenue 421 (72) Invention Jennifer Michael Y. United States of America 95070 Saratoga Clemson Avenue 18339 F-term (reference) 5F072 AB13 JJ20 KK01 KK06 KK08 KK18 KK26 KK30 PP07 YY15 5F073 AA67 AA83 AB25 AB29 BA01 EA29

Claims (35)

[Claims] 1. A tunable external cavity laser, comprising: a gain medium for emitting a beam; a tuning element movably positioned in a beam path to feed back a selected wavelength to the gain medium; A boss coil actuator coupled to the tuning element and responsive to the electrical signal for positioning the tuning element in the path of the beam and feeding back the selected wavelength to the gain medium, thereby tuning the tunable laser. A tunable external cavity laser, comprising: 2. The tunable external cavity laser according to claim 1, wherein the tuning element includes at least one of an interference element, a diffraction element, and a retroreflector. 3. The tunable external device according to claim 1, wherein said tuning element comprises an etalon having a first reflector and a second reflector which are coupled at an angle to each other and are opposed to each other. Cavity laser. 4. The tunable external cavity laser according to claim 3, wherein said first and second reflectors have a wedge-shaped gap formed therebetween. 5. A wedge-shaped substrate forming a wedge between a first surface and a second surface located on opposite sides of each other, wherein the first and second reflectors each include the first and second reflectors. 4. The tunable external cavity laser according to claim 3, wherein the laser is attached to the surface and the second surface. 6. The optical length of the resonator coupled to at least one of the first reflector and the second reflector to reduce mode hopping during tuning of the tunable external cavity laser. 4. The tunable external cavity laser according to claim 3, further comprising a correction element for changing the wavelength. 7. The correction element further comprising a substrate having a uniform refractive index and a linearly varying thickness defined along an axis parallel to the beam path. 7. The tunable external cavity laser according to 6. 8. The tunable external cavity laser of claim 5, wherein the correction element further comprises a substrate having a graded refractive index defined across the beam path. 9. The tunable external cavity laser according to claim 5, wherein the correction element further includes an optical element whose refractive index changes according to the received electric signal. 10. The tunable external cavity laser according to claim 1, wherein said gain medium is a laser diode. 11. A voice coil that generates a magnetic field in response to an electric signal, a voice circuit coupled to the voice coil,
A controller for controlling an electrical signal to position the voice coil with respect to the magnetic circuit, wherein one of the magnetic circuit and the voice coil movably positions the tuning element in a beam path. 2. The tunable external cavity laser of claim 1, wherein said tunable external cavity laser is coupled to said tuning element. 12. The magnetic circuit, wherein the magnetic circuit comprises an annular body at least partially surrounding the voice coil such that an electrical signal linearly positions the voice coil relative to the magnetic circuit. The tunable external cavity laser according to claim 11. 13. An arc-shaped tuning arm to which both the tuning element and one of the magnetic circuit and the voice coil are mounted, wherein the electric signal causes the voice coil to be arcuate with respect to the magnetic circuit. The tunable external cavity laser according to claim 11, wherein the laser is positioned. 14. A feedback circuit for sending a feedback signal corresponding to at least one of a position of the tuning element and a selected wavelength of a beam to the controller, wherein the controller controls the positioning of the tuning element. The tunable external cavity laser of claim 11, further comprising a logic circuit that uses a feedback signal to maintain the selected wavelength of the beam. 15. A controller responsive to a feedback signal controlling the positioning of the tuning element to maintain a selected wavelength, and controlling an electrical signal to position the tuning element; and a position of the tuning element and a beam position. 2. The tunable external cavity laser according to claim 1, further comprising a feedback circuit that sends a feedback signal corresponding to at least one of the selected wavelengths to the controller. 16. The feedback circuit further comprises an encoder, the encoder recording, along its length, an indicator corresponding to at least one of a position of the tuning element and a selected wavelength of the beam. , The feedback circuit comprises:
A reading device that reads the indicator and records a feedback signal corresponding to the indicator read from the encoder, wherein one of the encoder and the reading device is coupled to the tuning element. The tunable external cavity laser according to claim 15. 17. The encoder according to claim 16, wherein the encoder includes at least one of an optical encoder, a magnetic encoder, and an electric encoder.
A tunable external cavity laser as described. 18. The wavelength tunable external cavity laser according to claim 15, further comprising a writing device for writing an index into said encoder. 19. The writing device according to claim 18, wherein the writing device includes at least one of an optical writing device, a magnetic writing device, and an electric writing device.
A tunable external cavity laser as described. 20. A tunable external cavity laser, comprising: a gain medium for emitting a beam; a tuning element movably positioned in a beam path to feed back a selected wavelength to the gain medium; and an encoder. And the encoder records, along its length, an index corresponding to at least one of the position of the tuning element and a selected wavelength of the beam, and the wavelength-tunable external cavity laser includes: A reading device for reading the index and recording a feedback signal corresponding to the index read from the encoder, wherein one of the encoder and the reading device is coupled to the tuning element; The resonator laser is further coupled to the tuning element to position the tuning element in the path of the beam and to gain a selected wavelength. A positioner that feeds back to the medium to tune the tunable laser, wherein the positioner positions the tuning element such that the index read by the reader substantially matches the selected wavelength. Characteristic tunable external cavity laser. 21. The encoder according to claim 20, wherein the encoder includes at least one of an optical encoder, a magnetic encoder, and an electric encoder.
A tunable external cavity laser as described. 22. The tunable external cavity laser according to claim 20, further comprising a writing device for writing an index into said encoder. 23. The writing device according to claim 19, wherein the writing device includes at least one of an optical writing device, a magnetic writing device, and an electric writing device.
A tunable external cavity laser as described. 24. A method for controlling an output wavelength of an external cavity laser, comprising: a base, a gain medium for emitting a beam, and a selected output wavelength fed back to the gain medium to provide an external wavelength. A tuning element variably positioned in the path of the beam to tune the resonator laser, the method comprising: moving the tuning element to a continuous position across a position range; Measuring a corresponding output wavelength at each successive position from end to end of the position range in response to said moving step; and, in response to said measuring step, to a selected one of a base and a tuning element. Recording an indication of the corresponding output wavelength along the length of the combined encoding medium; and, from the unselected one of the base and the tuning element, recorded on the encoding medium in the storing step. versus Reading the corresponding output wavelength indication and moving the tuning element to a selected position where the corresponding output wavelength indication read in the reading step matches the selected wavelength. A control method characterized in that: 25. The control method according to claim 24, further comprising the step of suppressing mode hopping during tuning of the external cavity laser by changing an optical path length. 26. A tunable external cavity laser, comprising: a gain medium that emits a beam; a retroreflector provided in a beam path; and a beam path between the gain medium and the retroreflector facing each other in the beam path. And changing the spacing between the first and second reflectors, which are positioned at an angle to each other and feed back the selected wavelength to the gain medium to tune the laser, and the reflectors along the beam path. A tunable external cavity laser comprising: a positioner for positioning at least one of said first reflector and said second reflector so as to tune the laser. 27. The positioner according to claim 27, wherein at least one of the first reflector and the second reflector is accurately positioned in a plane intersecting a beam path to tune the gain medium. 27. The tunable external cavity laser according to 26. 28. The tunable external cavity laser according to claim 27, wherein said first reflector and said second reflector are coupled to each other. 29. The apparatus further comprises a wedge-shaped transmissive substrate having first and second surfaces at opposite angles that are coupled to the first reflector and the second reflector, respectively. A tunable external cavity laser according to claim 27. 30. The tunable external cavity laser according to claim 27, wherein the first and second reflectors have a wedge-shaped gap formed therebetween. 31. A tuning device coupled to a positioner in the path of the beam to suppress mode hopping during tuning of the external cavity laser, the positioner further changing the optical length of the laser. 28. The tunable external cavity laser according to claim 27, wherein the correction element is positioned so as to suppress mode hopping during tuning of the external cavity laser. 32. The correction element further comprises a substrate having a uniform refractive index and a linearly varying thickness defined along an axis parallel to the beam path, the substrate comprising a beam path. 32. The tunable external cavity laser of claim 31, wherein the tunable external cavity laser is coupled transversely to a positioner to suppress mode hopping during tuning of the laser amplifier. 33. A substrate having a graded index of refraction defined along a translation axis extending across a path of the beam, wherein the substrate is translated across the path of the beam during tuning of the laser amplifier. 32. The tunable external cavity laser of claim 31, wherein the tunable external cavity laser is coupled to a translation mechanism to suppress mode hopping. 34. The first and second reflectors are positioned to intersect the path of the beam at an oblique angle, thereby reducing spurious interference and spurious reflection of the beam. The tunable external cavity laser according to claim 26. 35. The positioner linearly positions at least one of the first reflector and the second reflector in a plane intersecting a beam path, thereby tuning a gain medium. 27. The tunable external cavity laser according to claim 26.