WO2015018352A1

WO2015018352A1 - Wavelength-tunable external cavity laser

Info

Publication number: WO2015018352A1
Application number: PCT/CN2014/083881
Authority: WO
Inventors: Wei-Long Lee; Hong Li; Yuzhou SUN; Xiangzhong Wang; Sheng Liu
Original assignee: Innolight Technology Corporation
Priority date: 2013-08-07
Filing date: 2014-08-07
Publication date: 2015-02-12
Also published as: CN103515840A

Abstract

A wavelength-tunable external cavity laser comprises a laser gain chip, one or more lens units, a tunable etalon, and a mirror. Together, the lens unit(s), tunable etalon, and mirror form an external cavity feedback area that provides feedback to laser gain chip. The laser may also include a temperature regulator, temperature controller, and temperature sensor and/or a voltage source and voltage controller that can be used to change the tunable etalon' s transmission wavelength, e.g., to overlap with the wavelength of a longitudinal mode of the laser gain chip. It can also include a temperature regulator, temperature controller, and temperature sensor that can be used to control the output wavelength(s) of the laser gain chip. Compared with the other external cavity lasers, the present external cavity laser has a larger tuning range, higher reliability, lower cost, and so on.

Description

WAVELENGTH-TUNABLE EXTERNAL CAVITY LASER

CROSS-REFERENCE TO RELATED PATENT APPLICATION

[0001] This application claims the priority benefit of Chinese Application No. 201310341504.5, filed August 7, 2013, and entitled "A Wavelength-tunable External Cavity Laser," which application is hereby incorporated herein by reference in its entirety.

BACKGROUND

[0002] A wavelength division multiplexing passive optical network (WDM-PON) is the most promising , scalable solution for delivering high bandwidth to an end user. It combines the advantages of WDM technology and the PON topological structure. The WDM technology refers to a technology that combines several paths of modulated optical signals together through an optical multiplexer (or a wavelength division multiplexer) according to a certain wavelength interval and transmits them within a single optical fiber. In a WDM-PON system, when an optical network unit (ONU) performs uplink data transmission, different wavelengths must be used.

[0003] Nowadays, a distributed feedback (DFB) laser is widely applied due to its friendly operability and high reliability, but the DFB laser bandwidth is not wide enough to realize the wavelength tuning in the wide range. Thus, the DFB laser cannot meet the wavelength requirements of a plurality of ONUs. The existing technology of applying the DFB laser technology in the WDM-PON system not only causes high wavelength management cost, but also brings serious storage problems.

[0004] In order to solve this problem, up to this point, an arrayed waveguide gratin (AWG) technology has been predominantly utilized. An external cavity laser, comprised of an uncoated Fabry-Perot (FP) laser and a grating, can output single mode light with different wavelengths by controlling laser current and temperature and tuning grating Bragg wavelength. However, the side-mode suppression ratio (SMSR) and reliability of this technology needs to be further improved. SUMMARY

[0005] Embodiments of the present invention include wavelength-tunable external cavity lasers and methods of operating wavelength-tunable external cavity lasers. At least one example of the , [0006] According to one aspect of this invention, there is provided a

wavelength-tunable external cavity laser, comprising a laser gain chip, one or more lens units, an etalon, and a mirror. The lens unit(s), etalon, and mirror are located on one side of the laser gain chip and form an external cavity feedback area. The

wavelength-tunable external cavity laser further comprises an anti -reflective coating on the facet of the laser gain chip facing to the external cavity feedback area, and a partially transmissive and partially reflective coating on the other facet of the laser gain chip.

[0007] The lens unit(s), etalon, and mirror are arranged in sequence from the side of the laser gain chip having the anti-reflective coating so that the external cavity feedback area and the laser gain chip form a laser resonant cavity.

[0008] Compared with other external cavity lasers, examples of this

wavelength-tunable external cavity lasers has a larger range of multi-longitudinal mode laser light output wavelengths, higher reliability, lower cost, and so on. The anti-reflective coating on the facet of the laser gain chip facing to the external cavity feedback area is equivalent to extending the cavity length of the laser by the external cavity feedback area; thus, light generated by the laser gain chip does not directly perform gain oscillation within the laser gain chip, but is directly transmitted into the external cavity feedback area for filtering and then returned back into the laser gain chip for amplification (gain). In other words, the light generated by the laser gain chip resonates within a laser resonant cavity formed by the external cavity feedback area and the laser gain chip, thereby generating narrow-bandwidth multi-longitudinal mode laser light. An example laser may also include a first temperature regulation apparatus located on the laser gain chip and/or a second temperature sensor or a voltage regulation apparatus located on the tunable etalon to regulate the wavelength(s) of the laser source and/or the wavelength(s) of the transmission peak transmitted by the etalon for realizing laser regulation. Thus, an example laser can provide the different wavelength channels used by multiple ONUs at lower cost and lower network complexity. [0009] It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0010] The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar and/or structurally similar elements).

[0011] FIG. 1 shows a structural diagram (top view) of a wavelength-tunable external cavity laser according to one embodiment of the present invention.

[0012] FIG. 2 shows a structural diagram of a wavelength-tunable external cavity laser according to another embodiment of the present invention. [0013] FIG. 3 shows a comb spectrum diagram of a laser source.

[0014] FIG. 4 shows a transmission comb spectrum diagram of an etalon.

[0015] FIG. 5 shows a structural diagram (top view) of a wavelength-tunable external cavity laser according to another aspect of the present invention.

[0016] FIG. 6 shows a structural diagram (front view) of a wavelength-tunable external cavity laser according to another aspect of the present invention.

[0017] FIG. 7 shows a spectrum diagram of a wavelength-tunable external cavity laser.

[0018] FIG. 8 shows a structural diagram of an etalon whose peak transmission wavelength(s) can be tuned by heating or cooling.

[0019] FIG. 9 shows a structural diagram of another etalon whose peak transmission wavelength(s) can be tuned by heating or cooling. [0020] FIG. 10 shows a structural diagram of a tunable etalon comprising a liquid crystal material whose refractive index changes in response to an applied voltage.

[0021] FIG. 11 shows a structural diagram of a tunable etalon comprising a piezoelectric element whose length and/or shape changes in response to an applied voltage. [0022] FIG. 12 shows a structural diagram of another tunable etalon comprising a piezoelectric ceramic, the shape of which changes in response to an applied voltage;

[0023] FIG. 13 shows a structural diagram (top view) of a wavelength-tunable external cavity laser according to a further aspect of the present invention.

[0024] FIG. 14 shows a structural diagram (front view) of a wavelength-tunable external cavity laser according to a further aspect of the present invention.

[0025] FIG. 15 shows a structural diagram of a wavelength-tunable external cavity laser according to another embodiment of the present invention.

[0026] FIG. 16 shows a structural diagram of a wavelength-tunable external cavity laser according to yet another embodiment of the present invention. DETAILED DESCRIPTION

[0027] Disclosed herein are external-cavity lasers and methods of generating and tuning the wavelength of light using external cavity lasers. An example external-cavity laser can be used in a passive optical network (PON) for wavelength-division multiplexed (WDM) communications, e.g., in a manner similar to distributed feedback (DFB) lasers. Although external-cavity lasers have historically been more larger, more complex, and more expensive than the lasers used in most telecommunications applications, an exemplary external-cavity laser can be packaged in a relatively compact package at relatively low cost. This packaged external-cavity laser can be used a WDM-PON system to lower wavelength management cost and address complex storage problems. [0028] FIG. 1 shows a structural diagram of a wavelength-tunable external cavity laser comprising a laser gain chip 1, one or more lens units (lenses) 2, an etalon 3, and a mirror 4. The lens units 2, etalon 3, and mirror 4 are located on one side of the laser gain chip 1 and form an external cavity feedback area. An anti-reflective coating 6 on the facet of the laser gain chip 1 faces the external cavity feedback area. The other facet of the laser gain chip 1 is coated with a partially transmissive and partially reflective coating 7 The lens units 2, etalon 3, and mirror 4 are arranged in sequence from the side of the laser gain chip 1 having the anti -reflective coating 6 so that the external cavity feedback area and the laser gain chip 1 form a laser resonant cavity.

[0029] FIG. 2 shows a structural diagram of a wavelength-tunable external cavity laser according to another embodiment of the invention. This laser also comprising a laser gain chip 1, one or more lens units 2, an etalon 3 and a mirror 4. Again, the lens units 2, etalon 3, and mirror 4 are located on one side of the laser gain chip 1 and form an external cavity feedback area. The facet of the laser gain chip 1 facing the external cavity feedback area is coated with an anti -reflective coating 6 and the other facet of the laser gain chip 1 is coated with a partially transmissive and partially reflective coating 7. The lens units 2, etalon 3, and mirror 4 are arranged in sequence from the side of the laser gain chip 1 having the anti -reflective coating 6 so that the external cavity feedback area and the laser gain chip 1 form a laser resonant cavity. Here, the lens unit 2 is installed on a lens bracket 8. [0030] In FIGS. 1 and 2, the laser gain chip 1 may support emission of multiple longitudinal modes whose wavelengths are determined by the laser gain chip's gain band and its optical path length, which depends on the refractive index and length. The etalon 3, whose transmission spectrum peaks at discrete wavelengths as shown in FIG. 4, transmits light at one (or more) of the wavelengths emitted by the laser gain chip 1. The mirror 4 reflects the transmitted light back through the etalon 3 to the laser gain chip 1 via the lens units 2. This feedback causes the laser gain chip 1 to emit light in one or more longitudinal modes, e.g., as plotted in FIG. 3.

[0031] Put differently, the etalon 3 in FIGS. 1 and 2 is used to lock the output frequency (or frequencies) of the external cavity laser. That is, the etalon 3 performs transmission filtering of the laser gain chip emission so as to produce a laser output generated by the interference of light produced by the laser gain chip and light reflected by the external cavity feedback area. For example, the etalon 3 transmits light at particular wavelengths, permitting transmission of light at certain wavelengths within emission spectrum range of the laser source, while filtering light at other wavelengths. The etalon' s comb-type transmission spectrum is shown in FIG. 4.

[0032] As explained below, the output wavelength of the wavelength-tunable external cavity laser of FIGS. 1 and 2 can be tuned by changing the temperature of the laser gain chip 1, the etalon 3, or both, e.g., as shown in the FIGS. 8 and 9. [0033] As shown in FIGS. 1 and 2, the laser gain chip 1 emits a beam of light towards the lens unit 2, which may include one or more lenses, after passing through the anti-reflective coating 6. The lens unit 2 may collimate or loosely focus the beam, which is incident on the etalon 3, which transmits at least a portion of the beam towards the mirror 4. The mirror 4 reflects the transmitted beam back to the etalon 3, which transmits the beam to the lens unit 2. The lens unit 2 couples the reflected beam back into the laser gain chip 1, which amplifies the beam.

[0034] In this way, the beam, after being reflected by the laser resonant cavity many times, can form a desired multi-mode laser output that is transmitted via the facet of the laser gain chip 1 that is coated with a partially transmissive, partially reflective coating 7. The anti -reflective coating 6 on the facet of the laser gain chip 1 facing the external cavity feedback area prevents light generated by the laser gain chip 1 from experiencing gain oscillation within the laser gain chip 1. But directly transmitting emitted light into the external cavity feedback area for selecting desired longitudinal modes (wavelengths) and feeding back the desired modes into the laser gain chip 1 yields a multi -longitudinal mode laser output.

[0035] FIGS. 5 and 6 show a structural diagram of a wavelength-tunable external cavity laser according to another aspect of the invention, wherein the etalon 3 is an etalon that is temperature tunable. The external cavity laser shown in FIGS. 5 and 6 are both based on the external cavity laser shown in FIG. 2. FIG. 5 shows a top view of the external cavity laser, while FIG. 6 shows a front view of the external cavity laser. As shown in FIGS. 5 and 6, this laser includes a temperature regulation apparatus 5 located on the tunable etalon 3, which can be controlled to regulate temperature. For instance, if the tunable etalon 3 comprises an air-filled cavity, changing the etalon' s temperature may causes the cavity's length to change, thus changing the etalon' s transmission wavelengths.

[0036] The temperature regulation apparatus 5 shown in FIG. 5 and FIG. 6 may includes, but is not limited to, a thermo-electric cooler (TEC), a fan cooler, a liquid cooler, a semiconductor cooler, or various combinations thereof. Here, the shapes of the temperature regulation apparatus 5 include, but are not limited to, a base-plate shape, an outer housing shape, etc. Those skilled in the art should understand that the aforesaid shapes of the temperature regulation apparatus 5 are only exemplary, and other existing shapes of the temperature regulation apparatus 5 also fall within the scope of the present disclosure. [0037] Here, the temperature regulation apparatus 5 can realize temperature regulation through manual control and automatic control, including, but not limited to, control with a manual temperature regulator, an intelligent temperature regulator, a human intelligence temperature regulator, etc. Suitable manual controls include, but are not limited to: 1) realizing temperature regulation by manually rotating a temperature button of the temperature regulator; 2) realizing temperature regulation by manually setting a temperature switch; 3) performing temperature regulation through

human-machine interaction manners, such as a keyboard, a touch pad, or a

voice-control device. Suitable automatic control manner includes, but is not limited to: 1) proportional -integral-derivative (PID) control and 2) microcomputer control. Those skilled in the art should understand that the aforesaid control manners of the temperature regulation apparatus are exemplary, and other existing control manners of the temperature regulation apparatus also fall within the scope of the present invention.

[0038] Specifically, in the external cavity laser as shown in FIG. 5 and FIG. 6, controlling the output temperature of the temperature regulation apparatus 5 can change the length of the air-filled cavity in the tunable etalon 3. This change in length shifts the wavelength(s) of the tunable etalon' s transmission peak(s). For instance, the temperature change can be selected to cause one or more of the wavelengths of the tunable etalon' s transmission peaks to overlap with one or more of the laser gain chip's longitudinal modes. In other words, changing the output temperature of the temperature regulation apparatus 5 to change the transmission wavelength of the tunable etalon 3 causes the transmission peaks of the etalon 3 to overlap with peaks in the output of the laser gain chip 1. Thus, tuning the etalon temperature tunes the laser's output wavelength(s). The spectrum diagram outputted by the external cavity laser is shown in FIG. 7, wherein the laser light source is generated by interference of light produced by the laser gain chip 1 and light reflected by the mirror.

[0039] In some cases, the external cavity laser further comprises a control apparatus (not shown). Specifically, the control apparatus can control the temperature regulation apparatus 5 to regulate the etalon' s temperature. For example, the control apparatus may cause a change in the etalon' s cavity length, which in turn causes one or more peaks in the laser light source's output spectrum to overlap with respective transmission peaks of the tunable etalon at one or more predetermined wavelengths. As a result, photons achieve the largest feedback at the overlapping wavelength and the laser is generated at the overlapping wavelength through the laser resonant cavity, and then realize laser tuning.

[0040] If desired, a temperature sensor 10 can be located closely to the tunable etalon 3 for detecting the temperature of the tunable etalon 3. The temperature sensor 10 may include, but is not limited to, a thermistor, a thermocouple, a resistance temperature detector (RTD), an integrated circuit (IC) temperature sensor, or various combinations thereof. Those skilled in the art should understand that the temperature sensor 10 can be integrally or detachably coupled to the temperature regulating device 5.

[0041] FIG. 8 shows a structural diagram of a tunable etalon 3 suitable for use in the external cavity laser shown in FIGS. 5 and 6. The length of the tunable etalon' s air-filled cavity changes with change of temperature. As shown in FIG. 8, the tunable etalon 3 comprises a first component 31 and a second component 32 made of materials with different thermal expansion coefficients. The first component 31 is hollow and has at least one end with an opening as shown in FIG. 8. The second component 32 is placed completely within the first component 31. Here, the shapes of the first component 31 and the second component 32 include, but are not limited to, various shapes, such as cylinder, cone, truncated cone, cuboid, square, etc. The second component 32 may be a solid component or a component with two sealed ends.

[0042] The tunable etalon 3 further comprises one or more substrates 33 which are used to cover the openings of at least one end of the first component 31; for example, substrate 33 covers the opening of the first component 31. Here, the sealed volume formed by the first component 31, the second component 32, and the substrates 33, which cover the openings of the ends of the first component 31, is the air-filled cavity of the tunable etalon 3. [0043] Since the materials of the first component 31 and the second component 32 have different thermal expansion coefficients, controlling the output temperature of the temperature regulation apparatus 5 causes the size of the sealed volume formed by the first component 31, the second component 32, and the substrate 33(s) to change. That is, the size (and length) of the air-filled cavity of the tunable etalon 3 changes according to the change of temperature.

[0044] FIG. 9 shows a structural diagram of another tunable etalon 3 suitable for use in the external cavity laser described with reference to FIGS. 5 and 6. Again, the length of the tunable etalon' s air-filled cavity changes with the change of temperature. As shown in FIG. 9, the tunable etalon 3 comprises a first component 31 and a second component 32 made of materials with different thermal expansion coefficients. The first component 31 is hollow and has two ends with respective openings. The second component 32 is placed completely within the first component 31. The tunable etalon 3 further comprises substrate 33 and substrate 34 which are used to respectively cover the openings of the two ends of the first component 31.

[0045] At least one surface of the air-filled cavity of the tunable etalon 3 shown in FIGS. 8 and 9 in the optical path can be coated with a film which has a high reflectance (e.g., 90%, 95%, 99%), or greater) for a specific wavelength range so as to implement the frequency- selective function of the tunable etalon 3. For example, the film can have a high reflectance for wavelengths filtered by the tunable etalon 3. The tunable etalon 3 shown in FIG. 8 may have a film which has a high reflectance for a specific wavelength range coated on the facet of the substrate 33 and/or on a facet of the second component 32.

[0046] In another embodiment (see FIG. 1 or FIG. 2), the etalon' s transmission wavelength(s) can be tuned by applying a voltage to the etalon 3. In these cases, the tunable etalon 3 comprises an voltage regulation apparatus (or voltage source; not shown), which can be controlled to regulate the voltage. Here, the voltage regulation apparatus includes, but is not limited to a touch point voltage regulator, a transistor regulator, a integrated circuit regulator, a computer regulator, and various combinations thereof. Those skilled in the art should understand that these voltage regulation apparatuses are only exemplary, and other existing voltage regulation apparatus may fall within the scope of the present disclosure. A voltage-tunable etalon can include, but is not limited to including: 1) a liquid crystal whose refractive index changes in response to a change in applied voltage; and 2) a piezoelectric element whose shape (length) changes in response to a change in applied voltage.

[0047] FIG. 10 shows a structural diagram of a voltage-tunable etalon suitable for use in an exemplary external cavity laser. As shown in FIG. 10, the tunable etalon 3 comprises a liquid crystal material 41, the refractive index of which changes in response to the change of voltage. The tunable etalon 10 also includes a first piece of glass 42, a second piece of glass 43, and at least one spacer (as shown in FIG. 10, there are two spacers— spacer 44 and spacer 45), which are used to form an enclosed volume to hold the liquid crystal material. More specifically, the first piece of glass 42 and the second piece of glass 43 are located opposite each other and separated by spacer 44 and spacer 45. The surfaces of the first piece of glass 42 and the second piece of glass 43 facing the liquid crystals 41 can be coated with a film 46 which has a high reflectance 46 for a specific wavelength range and a electric conduction film 47.

[0048] The output voltage of voltage regulation apparatus (voltage source) can be applied to the liquid crystals 41 through the electric conduction film on the first piece of glass 42 and the electric conduction film on the second piece of glass 43. Applying a voltage changes the refractive index of the liquid crystals 41, which causes the transmission wavelength(s) of the tunable etalon 3 to change accordingly. For instance, the transmission wavelength(s) may be changed to overlap with respective transmission peaks of the tunable etalon 3 at one or more predetermined wavelengths. This increases feedback at the overlapping wavelength(s), thereby tuning the laser's output wavelength(s) as described above and understood in the art.

[0049] Those skilled in the art should understand that the sequence of coating the film which has a high reflectance for a specific wavelength range and coating the electric conduction film for electric conducting on the same piece of glass can be done in any particular order. For example, the electric conduction film can be coated first, and the high-reflectance film can be coated later, or vice versa.

[0050] FIG. 11 shows a structural diagram of another voltage-tunable etalon. As shown in FIG. 11, the tunable etalon 3 comprises a piezoelectric ceramic (piezoelectric element) 51, the shape of which changes in response to a change of voltage. The etalon also includes a first mirror 52 and a second mirror 53, which are used to form an air-filled cavity, the length of which changes due to changes in the shape of the piezoelectric ceramic 51. This actuation of the cavity length can be achieved by fixedly connecting the first mirror 52 and/or the second mirror 53 to the piezoelectric ceramic 51.

[0051] Applying an output voltage of a voltage regulation apparatus (voltage source) to the piezoelectric ceramic 51 polarizes the piezoelectric ceramic 51 under the applied voltage ( electric field). This causes the piezoelectric ceramic' s shape to change such that the transmission wavelength(s) of the tunable etalon 3 change. For example, the applied voltage may be chosen such that the etalon's transmission wavelength(s) overlap with respective transmission peaks in the laser gain chip's output in order to provide increased feedback. The increased feedback tunes the laser's output wavelength(s), which depend on interference between light produced by the laser gain chip 1 and light transmitted by the etalon 3 and reflected by the mirror 4. [0052] The tunable etalon 3 shown in FIG. 11 can be fixed by a fastener 54, as shown in FIG. 12.

[0053] FIG. 13 and FIG. 14 shows a structural diagram of a wavelength-tunable external cavity laser according to a further aspect of the invention. Here, the external cavity laser shown in FIG. 13 and FIG. 14 are both based on the external cavity laser shown in FIG. 2. FIG. 13 shows a top view of the external cavity laser, while FIG. 14 shows a front view of the external cavity laser. As shown in FIGS. 13 and 14, a temperature regulation apparatus 9 is located on the laser gain chip 1. This temperature regulation apparatus 9 can be controlled to regulate the laser's temperature so as to cause the refractive index of the material of the laser gain chip 1 to change. Suitable implementations of the temperature regulation apparatus 9 shown in FIGS. 13 and 14 include, but are not limited to, a fan cooler, a liquid cooler, a semiconductor cooler, or various combinations thereof. Here, the shapes of the temperature regulating device 9 include, but are not limited to, a base-plate shape, an outer housing shape, etc. Those skilled in the art should understand that the aforesaid shapes of the temperature regulation apparatus are only exemplary, and other shapes may fall within the scope of the present disclosure.

[0054] The temperature regulation apparatus 9 can be controlled through manual control or automatic control (e.g., using a manual temperature regulator, an intelligent temperature regulator, a human intelligence temperature regulator, etc.). Suitable manual control manners include, but are not limited to: 1) manually rotating a temperature button of the temperature regulator; 2) manually setting a temperature switch; and 3) performing temperature regulation through human-machine interaction manners, such as a keyboard, a touch pad, or a voice-control device. Suitable automatic control manners include, but not limited to: 1) PID control; and 2) microcomputer control. Those skilled in the art should understand that the aforesaid control manners of the first temperature regulation apparatus are only exemplary, and other control manners may fall within the scope of the present invention.

[0055] In the external cavity laser shown in FIGS. 13 and FIG. 14, controlling the output temperature of the temperature regulation apparatus 9 changes the refractive index of the optical waveguides inside of the laser gain chip 1. This change in refractive index causes the wavelength(s) of the emission peak(s) to shift, e.g., such that the light emitted by the laser gain chip 1 is transmitted by the tunable etalon 3 and reflected by the mirror 4 back into the laser gain chip 1. The laser gain chip 1 amplifies the reflected light, thereby achieving increased feedback at wavelengths of overlapping peaks in the laser gain chip's emission spectrum and the etalon' s transmission spectrum. Put differently, changing the output temperature of the temperature regulation apparatus 9 changes the spectral distribution of the comb peaks emitted by the laser gain chip 1. For instance, the output temperature may be tuned such that the peaks in the frequency comb emitted by the laser gain chip overlap with transmission peaks of the tunable etalon 3. The spectrum of the laser's output is shown in FIG. 7.

[0056] The external cavity laser may further comprise a laser gain chip temperature control apparatus (not shown) operably coupled to the temperature regulation apparatus 9. Specifically, the laser gain chip temperature control apparatus can control the temperature regulation apparatus 9 so as to regulate the laser's temperature. For example, the laser gain chip temperature control apparatus can heat or cool the laser gain chip 1 so as to change the laser gain chip's refractive index (and hence its optical path length). In certain cases, the change in the laser gain chip's refractive index may cause one or more peaks in the laser gain chip's emission spectrum to overlap with respective peaks in the tunable etalon' s transmission spectrum. This increases feedback at the overlapping wavelength(s) and causes the light at the overlapping wavelength(s) to resonate within the laser cavity. It also enables laser tuning through feedback by the reflection back into the laser gain chip 1. [0057] The external cavity laser may also include a laser gain chip temperature sensor 11 located close to the laser gain chip 1 for detecting the temperature of the laser gain chip 1. This laser gain chip temperature sensor 11 may include, but is not limited to, a thermistor, thermocouple, resistance temperature detector (RTD), IC temperature sensor, or various combinations thereof. Those skilled in the art should understand that the laser gain chip temperature sensor 11 can be integrated into the laser gain chip temperature regulating device 9 or detachably coupled to the laser gain chip 1 and/or the laser gain chip temperature regulating device 9.

[0058] Those skilled in the art should understand that the laser gain chip temperature control apparatus and the etalon temperature control apparatus can be integrated together or separate from each other.

[0059] Those skilled in the art should also understand that the controllers can synchronously or asynchronously control the output temperatures of the laser gain chip temperature regulation apparatus 9 and the etalon temperature regulation apparatus 5 so as to overlap a particular peak in the laser gain chip's emission spectrum with a particular peak in the tunable etalon' s transmission spectrum in order to enable laser tuning.

[0060] Similarly, those skilled in the art should also understand that the controllers can synchronously or asynchronously control the output temperature of the laser gain chip temperature regulation apparatus 9 and the voltage source so as to overlap a particular peak in the laser gain chip's emission spectrum with a particular peak in the tunable etalon's transmission spectrum in order to enable laser tuning.

[0061] FIG. 15 shows a structural diagram of a wavelength-tunable external cavity laser according to another embodiment. In this example, the external cavity laser further comprises an optical waveguide 13 for adjusting the cavity length of the external resonant cavity and a filter 12 for filtering the wavelength of light. The optical waveguide 13 and the filter 12 can be located at any position within the external cavity feedback area. Here, the filter 12 is used to filter the light emitted by the laser gain chip 1, and the optical waveguide 13 is an optical device made of a the material (for example, quartz glass) whose refraction index is bigger than that of the air. Here, those skilled in the art should understand that the illustrated etalon 3 (shown here with a structure of FIGS. 8 or 9) is only exemplary, but not limiting.

[0062] FIG. 16 shows a structural diagram of a wavelength-tunable external cavity laser according to another embodiment of this invention. In this case, the external cavity laser includes a laser gain chip 1, one or more lens units 2, an etalon 3, and a mirror 4, all of which are located on one side of the laser gain chip 1 and together form an external cavity feedback area. The external cavity laser also includes an

anti-reflective coating 6 on the facet of the laser gain chip 1 facing the external cavity feedback area and a partially transmissive, partially reflective coating 7 on the other facet of the laser gain chip 1. As shown in FIG. 16, the lens unit(s) 2, etalon 3, and mirror 4 are arranged in sequence from the facet of the laser gain chip 1 with the anti -reflective coating 6 so that the external cavity feedback area and the laser gain chip 1 form a laser resonant cavity.

[0063] Here, the lens unit 2 installed on a lens bracket 8 is composed of multiple lenses, labelled here as lens 14, lens 15, and lens 16; a first temperature regulation apparatus 9, which can be controlled to regulate temperature, is located on the laser gain chip 1, which can be controlled to regulate the laser gain chip's temperature; a second temperature regulation apparatus 5, which can be controlled to regulate the etalon's temperature, is located on the tunable etalon 3; a first temperature sensor 11 is located close to the laser gain chip 1; and a second temperature sensor 10 is located close to the tunable etalon 3. The first temperature regulation apparatus 9, the second temperature regulation apparatus 5, the first temperature sensor 11, and the second temperature sensor 10 may be identical or similar to the laser gain chip temperature regulation apparatus 9, the etalon temperature regulation apparatus 5, the laser gain chip temperature sensor 11, and the etalon temperature sensor 10 shown in FIGS. 5 and 6.

[0064] FIG. 16 shows a wavelength-tunable external cavity laser according to another embodiment of this invention. This exemplary laser includes an etalon 3 that can be temperature tuned as shown in the FIGS. 8 and 9. As shown in FIG. 16, a beam generated by the laser gain chip 1 propagates through the anti -reflective coating 6 to a lens unit 2, which includes lenses 14, lens 15, and lens 16, which collimates the beam or focuses it to a plane coincident with the mirror 4. The collimated or focused beam is incident on the etalon 3, which filters the beam as described above, and reflects off the mirror 4 back through the etalon 3. The etalon 3 transmits the reflected beam to the lens unit 2, which couples the beam back to the laser gain chip 1. The beam is then amplified within the laser gain chip 1. In this way, the beam, after propagating through the laser resonant cavity many times, forms a desired multi-mode laser that is emitted via the facet of the laser gain chip 1 coated with the partially transmissive, partially reflective coating 7. [0065] If desired, the output of an external -cavity laser (including those shown in the attached figures) can be stabilized for WDM communications using a real-time calibration system. Such a real-time calibration system may comprise two

photodetectors (e.g., photodiodes), an optical etalon or other wavelength- selective element, and any other suitable optoelectronic components. In operation, the real-time calibration system may sense wavelength of the external-cavity laser's output and provide a feedback signal to control the temperature or voltage applied to the tunable etalon, the temperature of the laser gain chip, or some combination thereof. For example, the real-time calibration system may determine the feedback signal based on the amplitudes and/or phases of the photocurrents emitted by the photodetectors. [0066] Conclusion

[0067] While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive

embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0068] The above-described embodiments can be implemented in any of numerous ways. For example, embodiments of designing and making the coupling structures and diffractive optical elements disclosed herein may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.

[0069] Further, it should be appreciated that a computer may be embodied in any of a number of forms, such as a rack-mounted computer, a desktop computer, a laptop computer, or a tablet computer. Additionally, a computer may be embedded in a device not generally regarded as a computer but with suitable processing capabilities, including a Personal Digital Assistant (PDA), a smart phone or any other suitable portable or fixed electronic device. [0070] Also, a computer may have one or more input and output devices. These devices can be used, among other things, to present a user interface. Examples of output devices that can be used to provide a user interface include printers or display screens for visual presentation of output and speakers or other sound generating devices for audible presentation of output. Examples of input devices that can be used for a user interface include keyboards, and pointing devices, such as mice, touch pads, and digitizing tablets. As another example, a computer may receive input information through speech recognition or in other audible format.

[0071] Such computers may be interconnected by one or more networks in any suitable form, including a local area network or a wide area network, such as an enterprise network, and intelligent network (EST) or the Internet. Such networks may be based on any suitable technology and may operate according to any suitable protocol and may include wireless networks, wired networks or fiber optic networks.

[0072] The various methods or processes (e.g., of designing and making the coupling structures and diffractive optical elements disclosed above) outlined herein may be coded as software that is executable on one or more processors that employ any one of a variety of operating systems or platforms. Additionally, such software may be written using any of a number of suitable programming languages and/or programming or scripting tools, and also may be compiled as executable machine language code or intermediate code that is executed on a framework or virtual machine.

[0073] In this respect, various inventive concepts may be embodied as a computer readable storage medium (or multiple computer readable storage media) (e.g., a computer memory, one or more floppy discs, compact discs, optical discs, magnetic tapes, flash memories, circuit configurations in Field Programmable Gate Arrays or other semiconductor devices, or other non-transitory medium or tangible computer storage medium) encoded with one or more programs that, when executed on one or more computers or other processors, perform methods that implement the various embodiments of the invention discussed above. The computer readable medium or media can be transportable, such that the program or programs stored thereon can be loaded onto one or more different computers or other processors to implement various aspects of the present invention as discussed above.

[0074] The terms "program" or "software" are used herein in a generic sense to refer to any type of computer code or set of computer-executable instructions that can be employed to program a computer or other processor to implement various aspects of embodiments as discussed above. Additionally, it should be appreciated that according to one aspect, one or more computer programs that when executed perform methods of the present invention need not reside on a single computer or processor, but may be distributed in a modular fashion amongst a number of different computers or processors to implement various aspects of the present invention.

[0075] Computer-executable instructions may be in many forms, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically the functionality of the program modules may be combined or distributed as desired in various embodiments.

[0076] Also, data structures may be stored in computer-readable media in any suitable form. For simplicity of illustration, data structures may be shown to have fields that are related through location in the data structure. Such relationships may likewise be achieved by assigning storage for the fields with locations in a

computer-readable medium that convey relationship between the fields. However, any suitable mechanism may be used to establish a relationship between information in fields of a data structure, including through the use of pointers, tags or other mechanisms that establish relationship between data elements. [0077] Also, various inventive concepts may be embodied as one or more methods, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

[0078] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. [0079] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one" .

[0080] The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. [0081] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0082] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or,

equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0083] In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," "composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

1. A wavelength-tunable external cavity laser comprising:

a laser gain chip, having a first facet coated with a partially transmissive coating and a second facet coated with an anti -reflective coating, to emit light at a plurality of wavelengths via the second facet, the second facet partially defining an external cavity that provides feedback to the laser gain chip;

a lens, in optical communication with the second facet, to collimate light emitted by the laser gain chip via the second facet;

a tunable etalon, in optical communication with the at least one lens, to transmit light at a first wavelength in the plurality of wavelengths; and

a mirror, in optical communication with the tunable etalon and partially defining the external cavity, to reflect light transmitted by the etalon towards the laser gain chip so as to cause the laser gain chip to emit light at the first wavelength via the first facet.

2. The wavelength-tunable external cavity laser of claim 1, further comprising: a temperature regulation apparatus, in thermal communication with the tunable etalon, to change a temperature of the tunable etalon so as to vary a wavelength of a transmission peak of the tunable etalon.

3. The wavelength-tunable external cavity laser of claim 2, wherein the tunable etalon comprises an air-filled cavity whose length changes in response to the change in the temperature of the etalon.

4. The external cavity laser according to claim 6, wherein at least one surface of the air-filled cavity is coated with a film having a high reflectance over a predetermined wavelength range.

5. The wavelength-tunable external cavity laser of claim 2, further comprising: a temperature control apparatus, operably coupled to the temperature regulation apparatus, to control the change in the temperature of the tunable etalon provided by the temperature regulation apparatus so as to overlap the transmission wavelength of the tunable etalon with the first wavelength in the plurality of wavelengths.

6. The wavelength-tunable external cavity laser of claim 5, further comprising: a temperature sensor, operably coupled to the temperature control apparatus and in thermal communication with the tunable etalon, to measure the temperature of the tunable etalon,

wherein the temperature control apparatus is configured to control the change in temperature based on the temperature measured by the temperature sensor.

7. The wavelength-tunable external cavity laser of claim 1, wherein the tunable etalon comprises:

a first component comprising a first material having a first thermal expansion coefficient; and

a second component comprising a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient, the second component disposed at least partially within the first component.

8. The wavelength-tunable external cavity laser of claim 7, wherein the first component defines a cavity having an aperture to receive the second component.

9. The wavelength-tunable external cavity laser of claim 8, wherein the tunable etalon further comprises:

a substrate disposed over the aperture.

10. The wavelength-tunable external cavity laser of claim 1, further comprising: a voltage source, in electrical communication with the tunable etalon, to apply a voltage to the tunable etalon so as to vary a wavelength of a transmission peak of the tunable etalon.

11. The wavelength-tunable external cavity laser of claim 10, wherein the tunable etalon comprises:

a first substrate having a first surface coated with (i) a first film having a high reflectance over a predetermined wavelength range and (ii) a first electrically conductive film;

a second substrate having a second surface coated with (i) a second film having a high reflectance over a predetermined wavelength range and (ii) a second electrically conductive film, the second surface being opposite the first surface; and

a liquid crystal material, disposed between the first substrate and the second substrate, having a refractive index that changes in response to a change in the voltage applied to the tunable etalon by the voltage source.

12. The wavelength-tunable external cavity laser of claim 10, wherein the tunable etalon comprises:

a piezoelectric element, in electrical communication with the voltage regulation apparatus, to vary an optical path length of the tunable etalon in response to a change in voltage so as to overlap the transmission wavelength of the tunable etalon with the first wavelength in the plurality of wavelengths.

13. The wavelength-tunable external cavity laser of claim 1, further comprising: a temperature regulation apparatus, in thermal communication with the laser gain chip, to change the temperature of the laser gain chip.

14. The wavelength-tunable external cavity laser of claim 13, further comprising: a temperature control apparatus, operably coupled to the temperature regulation apparatus, to control the change in the temperature of the laser apparatus provided by the temperature regulation apparatus so as to overlap the first wavelength in the plurality of wavelengths with a transmission wavelength of the tunable etalon.

15. The wavelength-tunable external cavity laser of claim 14, further comprising: a temperature sensor, operably coupled to the temperature control apparatus and in thermal communication with the laser gain chip, to measure the temperature of the laser gain chip,

16. The wavelength-tunable external cavity laser of claim 1, further comprising: an optical waveguide, in optical communication with at least one of the lens, the tunable etalon, or the mirror, to guide at least a portion of the light emitted by the laser gain chip.

17. The wavelength-tunable external cavity laser of claim 1, further comprising: a filter, in optical communication with at least one of the lens, the tunable etalon, or the mirror, to filter at least a portion of the light emitted by the laser gain chip.