CN1700540A - External resonator semiconductor laser and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a design for an external-cavity single-mode laser in which a short optical path length for the optical cavity (e.g., -3-25 mm) provides sufficient space to allow a longitudinal mode of a single wavelength selective element, such as a micro-machined etalon, to provide single mode operation and optionally a selectable mode of operation.

Description

External resonator semiconductor laser and manufacturing method thereof

related application

This application claims U.S. Provisional Application No. 60/472914, filed May 23, 2003; U.S. Provisional Application No. 60/472873, filed May 23, 2003; and U.S. Provisional Application No. Priority of Application No. 60/472692, which is incorporated herein by reference in its entirety.

technical field

The present invention generally relates to lasers optically coupled to an external optical resonator, and in particular the invention relates to a laser diode capable of tuning by selecting a specific emission wavelength from a discrete set of emission wavelengths, characterized in that the coupled laser and external optical cavity. More specifically, the present invention relates to microfabricated Fabry-Perot etalons (etalons) for use in external cavity laser systems to select desired lasing modes, demonstrated for use in wavelength division multiplexing (WDM) fiber optic communications system.

Background technique

Fiber optic communication systems, such as those commonly used in wireless communications, consist of three basic elements: a transmitter/source, a fiber optic link or channel, and a detector/receiver. If lasers - as opposed to light-emitting diodes (LEDs) - are used as light sources, such systems achieve absolute advantages in terms of data transmission capacity, speed and distance; and if specifically semiconductor laser diodes are used as transmitter light sources, in terms of cost, compactness , reliability and power consumption will be further beneficial.

Although lasers are nominally monochromatic sources, in fact lasers typically produce light at several emission wavelengths (called modes) unless steps are taken to suppress all modes rather than just one, in which case the laser is called a single-mode laser . Such single-mode lasers offer superior performance in fiber optic systems due to reduced scattering losses, which still allow for high data speeds and long transmission distances.

The transmission capacity and functions of optical fiber links can be significantly increased through wavelength division multiplexing (WDM) technology. In general, multiplexing refers to the simultaneous transmission of several signals or information in the same circuit or channel. For example, frequency division multiplexing is accomplished in coaxial fiber optic systems by providing several independent carrier signals, each having a specific frequency assigned to it, and modulating each carrier signal with an independent information signal. At the receiving end of the coaxial cable, a selected low-pass filter separates several carrier frequencies so that each carrier is demodulated to produce the original information signal. As another example, in digital data transmission systems, time-division multiplexing is achieved by interleaving bit streams from several signal sources to form a composite high-speed bit stream. At the receiving terminal, the byte stream is decomposed with appropriate time frames and synchronization signals.

An analog multiplexing technique known as wavelength division multiplexing (WDM) is used in fiber optic communication systems. WDM is based on the simultaneous transmission of optical signals assigned to different carrier wavelengths. Several lasers with discrete and well-separated emission wavelengths are independently modulated by several information signals. The carrier wavelengths assigned by the WDM system are called "channels". All modulated laser outputs are launched into a common fiber and demultiplexed by wavelength-sensitive filters at the receive end of the link. Thus, the signals resolved according to their carrier wavelengths are coupled to detectors for specific channel wavelengths. At present, the fiber optical system using WDM has well established the basic structure of some wireless communications.

WDM systems require at least several - sometimes up to 50-100 laser transmitters, each operating at a different emission wavelength. Since the emission wavelength is largely an inherent property of conventional lasers, WDM systems require several or more different types of laser diodes, each with a specific emission wavelength. In contrast, a preferred example of a WDM system employs only one type of laser, but the laser is easily modulated and set by the user to operate at any one of the predetermined emission wavelengths. Such lasers are known as "tunable," where the emission wavelength can be tuned according to specific application and system requirements. In a WDM fiber optic system with multiple channels, each channel transmitter should employ the same type of laser, but with its individual transmit wavelength tuned and set to the wavelength assigned for that particular channel. That is, this approach uses only one type of tunable laser, rather than many different types of single-mode lasers with separate emission wavelengths, which is generally known to be problematic because it significantly simplifies the overall size of the WDM system , configuration, assembly and maintenance. For example, instead of inventorying, installing and maintaining many different types of lasers, a conventional tunable laser is used as the light source for all transmitters, and its emission wavelength is selected based on where it is inserted into the WDM system. The use of tunable lasers also allows for easy retrofitting of WDM systems, and the reassignment of transmitters to new channels is easy by selecting new emission wavelengths. Accordingly, the present invention provides a device for realizing mode conversion of squeezed external resonator lasers and applies a particularly simple structure to WDM optical communication systems, thereby providing an important advance in the field of laser technology.

Contents of the invention

The present invention also provides an external resonant cavity semiconductor laser, which includes: a laser gain medium, a light source for providing optical radiation; an external optical resonator arranged in optical communication with the gain medium, and the size of the resonant cavity is short enough to allow a single A wavelength selective device selects and maintains a single longitudinal laser mode; and a single wavelength selective device disposed in the external optical resonant cavity selects and maintains a single longitudinal laser mode. In one desired configuration, the wavelength selective device comprises a micromachined etalon. For example, the present invention provides a micromachining etalon comprising: a crystalline substrate having first and second opposing surfaces; a spacer layer disposed over the first surface of the substrate, the spacer layer having an outer surface; The first surface extends through the substrate to a through hole of the second surface, the through hole has a base adjacent to the exposed portion of the spacer layer; and a first interference filter disposed on the outer surface of the spacer layer and in the A second interference filter is disposed on the exposed portion of the spacer layer to provide an etalon between the interference filters.

The present invention also provides a method for fabricating a micromachining etalon, comprising: providing a crystal substrate having first and second opposing surfaces; providing a spacer layer over the first surface of the substrate, the spacer layer having a outer surface; forming a through hole extending from the first surface through the substrate until the second surface, the through hole has a base adjacent to the exposed portion of the spacer layer; and providing a first interference filter on the outer surface of the spacer layer sheet and a second interference filter is provided on the exposed portion of the spacer layer to provide an etalon between the interference filters.

Description of drawings

Figures 1a-1c schematically illustrate different embodiments of external cavity lasers using etalons as mode filtering or mode selecting devices according to the present invention, including: Figure 1a with a static etalon, Figure 1b with a tiltable etalon as well as Figure 1c etalon with variable gap.

Figure 2 schematically illustrates a conventional etalon consisting of thin dielectric layers coated on both sides with a multilayer dielectric stack that acts as an interference reflector.

Figure 3 shows typical spectral transmission characteristics of etalons.

Figures 4a-4e show the relevant spectral properties of external resonator edge emitting lasers, particularly for relatively short external optical cavity lengths and the resulting widely spaced modes, and include: 4a is the laser gain bandwidth , 4b is the optical resonator mode, 4c is the allowed laser emission mode, 4d is the spectral response of the mode-selective device for a particular setting, and 4e is the selected laser emission output for the mode selection shown in Fig. 4d.

Figures 5a-5h show the relevant spectral properties of external resonator edge-emitting lasers, particularly for the case of relatively short external optical resonator lengths and the resulting near-space modes, and include: 5a is the laser gain bandwidth, 5b is Optical resonator mode, 5c is the allowed laser emission mode, 5d is the mode filter to reduce the number of modes; 5e is the spectral response for a specially set mode-selective device with moderate resolution, 5f is the mode from Fig. 5e The selected laser in the selected device emits output and shows the unintentional selection of adjacent modes, 5g is the spectral response of the mode-selected device for a particular setup with improved resolution relative to that shown in Fig. 5e, and 5h is Fig. Mode-selective laser emission output shown in 5g.

Figure 6a schematically illustrates a micromachined etalon consisting of a substrate with grooves conformally coated with a multilayer dielectric stack on both sides.

Figures 6b-6h schematically illustrate the sequence of fabrication steps used to fabricate the micromachined etalon shown in Figure 6a, comprising: 6b the substrate on which the spacer layer is formed, 6c the substrate Grooves formed, 6d is a conformally coated multilayer dielectric stack formed over both sides of the etalon, 6e is a via for etching spacer formation, 6f is etching spacer to form an air gap , 6g is a top view of the micromachined etalon showing the area removal of vias and spacer layers, and 6h is the application of electrodes to tune by changing the air gap.

Figures 7a-7d schematically illustrate a substrate (7a) coated with a thin multilayer dielectric stack on one side thereof and a substrate (7a) having a beveled recess and also coated with a multilayer dielectric stack on one side thereof An alternative embodiment of a micromachined etalon consisting of another substrate (7b), a combined structure (7c) formed by bonding said substrates together and forming on both sides of said combined structure groove (7d).

Figure 8a illustrates a simulation showing a 24-mm Fabry-Perot optical resonator mode.

Figure 8b illustrates a simulation showing a 12-mm Fabry-Perot optical resonator mode.

Figure 8c illustrates a simulation showing the spectral transmission of a micromachined etalon.

Figure 8d illustrates in more detail the simulation showing the spectral transmission peak of the microfabricated etalon.

Figure 8e illustrates a simulation showing the spectral transmission of a microfabricated etalon, where the air-gap space is variable, resulting in a shift in the transmission peak.

Figure 9a schematically illustrates a tunable external optical resonator laser system with mode counting and selection capabilities.

Figure 9b schematically illustrates a tunable external optical resonator laser system with mode counting and selection capabilities and means for reading out the reference frequency to establish the laser emission mode.

Detailed ways

The present invention relates to the design of an external resonator single mode laser 100, where the short optical path length (~3-25mm) of the optical resonator 103 provides sufficient space for the longitudinal modes, allowing single wavelength selective elements such as micromachined etalons ( etalon) 120 provides a single-mode work, and can freely select the work mode. The total path length of the laser optical resonator 103 is the path length between the reflectors 108, 110 of the laser resonator 103, typically one reflector 110 comprising a coating on the outer polyhedral face of the lasing element 102 and as The path length between one reflector 108 of the outer lens. The micromachined etalon 120 is a wavelength sensitive device comprising a pair of reflective surfaces, such as thin film multilayer dielectric stacks 202, 204 separated by a thin transparent layer or air gap 124, which can be used to selectively transmit or reflect specific wavelengths of light . Thus, the etalon 120 is used to preferentially suppress or maintain the lasing mode in the optical cavity 103 .

External cavity laser 100 is configured to cooperate with etalon 120 such that micromachined etalon 120 can switch the laser output between several or more discrete wavelength emission modes defined by the total path length of laser optical cavity 103 . The tunable etalon 120 can modify its wavelength-dependent transmission characteristics by changing the angle of light incident on the etalon, for example, tilting the etalon 120 . Alternatively, the etalon can be designed as a variable air gap space that can be adjusted by changing the etalon gap between the multilayer dielectric stacks 202 , 204 . For example, etalon 120 may be tuned by voltage control signals applied to electrodes formed on etalon 120 to vary the air gap space. The small size of the micromachined etalon 120 allows a relatively reduced external optical cavity length, which in turn can generate more widely spaced laser modes, making mode selection easier to accomplish with a simpler etalon design. Since the mode spacing is wider, the filtering requirements on the etalon 120 are reduced (the passband width does not have to be too narrow). It is thus further easy to use the micromachined etalons disclosed herein for laser mode selection.

The invention also relates to a method that enables the use of wavelength selective elements for discrete mode transitions between longitudinal modes of the laser cavity 103, thus allowing counting of transitions between excited and non-excited states from a reference or "local" wavelength. As simple a device as it is to determine a change in operating wavelength.

Referring now to the drawings, in which like elements are numbered identically, and in particular, three exemplary configurations of the present invention are shown in FIGS. 1A-1C . An external optical resonator laser 100 is provided comprising optical and optoelectronic elements distributed along a common optical axis A-A'. The light source comprises a laser emitting element 102, which may be an optical gain material/medium or combination of optical materials exhibiting optical gain, for example in an optically defined waveguide mode or resonant cavity mode, which is beneficial for stimulating luminescence. For example, the lasing element 102 may comprise an edge emitting semiconductor diode laser under bias for a WDM fiber optic system. Other types of lasers may also be used, such as surface emitting lasers including vertical cavity surface emitting lasers (VCSELs). Suitable laser media may include various compound semiconductors including alloys of InP, GaAs and InAs. The lasing element 102 is optically coupled to an external optical cavity 105 longitudinally along the optical axis A-A'. The sum of the length of the laser element 102 and the length of the outer cavity 105 defines a total optical cavity length 103 . The optical path length determines the longitudinal mode space if the optical path length is determined by summing the product of the physical path length along A-A' of each physical element and its effective emission coefficient at its associated wavelength. Gaps between elements are also included in the calculations with a factor of -1.0 for air or otherwise if the optical package consumes that space. The lasing element 102 is defined by a first end surface 104 and a second end surface 106 perpendicular to the optical axis A-A'. If the lasing element 102 is formed on a single crystal sample, the end surfaces 104 and 106 can be formed by cutting the crystal to expose flat, parallel surfaces called edge facets.

The optical resonant cavity 103 is formed by two emitters 110, 108 located at the optical axis A-A' and in which the lasing element 102 is arranged. Optical resonator 103 includes an inner resonator portion 102 of lasing element 102 and an outer resonator portion 105 . The end emitter 108 may be an external separate mirror of the lasing element 102 to define one end of the cavity. The plane of reflector 108 is desirably oriented so that it is perpendicular to the standard optical axis A-A'. Opposite this end, the other end of the optical cavity defined by the reflector 108 is defined by a reflector arranged as a reflective coating 110 formed on the first surface 104 of the lasing element 102 . The second surface 106 of the laser element, which is also the edge facet, can be coated with an anti-reflection coating 112 to prevent reflections at the second surface 106 so that at the first and second ends of the laser emitting element 102 No resonant cavity is provided between the surfaces 104,106.

In operation, the laser medium is pumped electronically or optically by injection of minority carriers in the p-n junction or by absorption of light, either of which results in a non-equilibrium distribution of charge carriers. In this case, the lasing material exhibits an optical gain such that the absorption of photons leads to the stimulated emission of photons within a defined spectral gain bandwidth. Lasing is the emission of photons as a result of recombination of excess charge carriers associated with non-equilibrium conditions, some of which are confined within optical cavity 103 by reflector 108 and reflector 110 . A portion of this emitted light has a defined wavelength and direction of propagation (collectively referred to as modes) such that the constructive interference of the optical resonator 103 provides sufficient feedback and thus maintains the lasing mode. In practice, reflective coating 110 is not reflective so much that part of the photons generated by stimulated emission are transmitted through end surface 104 and coating 110 outside optical cavity 103 . This light constitutes the laser output beam 114 . Other structures of known external cavity lasers can be used, for example, the two surfaces of the semiconductor lasing element 102 can be AR coated and two external mirrors can be used, alternatively, a facet lens can reflect most of the light , and the outer lens 108 can be coupled out of the system with a smaller reflector.

Because optical resonator 103 typically supports multiple modes and lasers have a finite gain bandwidth, laser 100 will emit light at several wavelengths. The allowable multimode laser emission wavelength is determined by the effective optical length of the resonator 103: as the optical resonator length is reduced, the laser 100 exhibits fewer more widely spaced emission wavelength modes. The optical resonator mode is the mode of the total optical resonator length between the reflectors 108 , 110 on one end being the facet coating 110 and the other end being the outer resonator mirror 108 . Thus, the optical cavity mode comprises the superposition of the optical path length 103 of the lasing element 102 and the optical path length of the external cavity 105 . For WDM applications, it is used to reduce laser shots to a single mode, and further has the ability to select a specific single shot mode.

External optical resonator 105 may contain other optical elements, such as lens 116, to sufficiently collimate or focus the light from the semiconductor laser element so that it propagates through etalon 120, reflects from reflector 108, and allows the laser to operate to obtain a clean lasing A suitably low loss of gain is returned to the lasing element 102 . Another structure can be obtained, for example a concave lens or a combination of a lens and a concave mirror for the reflector 108 . Additional optional elements may include filters, gratings, prisms, and the like. Other optical components can still be placed outside the optical resonator 103, for example, a collimating lens 118 to collimate the output of the laser.

For example, FIG. 1a shows a micromachined etalon 120 disposed along optical axis A-A' in an external optical cavity according to the present invention. Micromachined etalons 120 are used to eliminate a subset of allowable laser modes. In essence, the micromachined etalon 120 perturbs the resonance conditions of some optical cavity modes that are characterized in that the etalon 120 does not transmit the wavelengths for the resonance conditions. Instead, modes with wavelengths transmittable by the micromachined etalon 120 are maintained. Desirably, the micromachined etalon 120 is configured to allow only a single (selected) laser cavity mode to resonate, which structure can be fabricated by the inventive fabrication process set forth below.

Returning to the mode selection aspect of the present invention, as shown in Figure 1b, the microfabricated etalon 120 can be tilted, thereby modifying the angle of incidence and thus its spectral transmission characteristics with respect to the resonator modes, and the result can be changed in order to maintain the emission The mode (or modes) selected for laser operation. Alternatively, FIG. 1c illustrates that the micromachined etalon 120 can include an air or vacuum gap 124 that can be changed to modify the spectral transmission characteristics of the micromachined etalon 120, and that optical resonances can be selected through the air or vacuum gap 124. The specific laser mode of the cavity 103.

By thus elaborating, the external cavity laser 100 can function as a light source by which the user can select a single, narrowband emission wavelength from a multitude of suitable discrete emission wavelength bandwidths limited primarily by the length of the resonator.

According to the present invention, the component complexity of the tunable laser 110 is substantially reduced because the optical resonator 103 has a relatively short optical path length so that the longitudinal modes are relatively widely spaced. In turn, tuning of laser light with mode-selective output can be accomplished using relatively simple wavelength-discriminating devices such as micromachined etalons 120, which can include relatively fewer dielectric layers than typical interference filters, for example. Furthermore, due to the short external optical cavity length, the simplicity afforded by selecting and filtering widely spaced emission wavelengths enables the elimination of auxiliary components and devices typically found in conventional WDM laser systems. For example, a typical tunable laser typically requires several complex feedback mechanisms including piezoelectrically adjustable cavity length, thermal trimming, and tunable gratings or other filters, each of which has the ability to provide information to maintain a single Known frequency control loop. The present invention provides a much simpler feedback mechanism, e.g. if the temperature of the system is fixed, then the position of the longitudinal mode can be known with sufficient precision, strictly controlled by the modulation in the optical power observed during tuning from longitudinal mode to longitudinal mode. By accurately calculating the number of transitions, it is easy to make the tunable element select the desired mode.

Now, returning to the etalon structure, a schematic diagram of the etalon 102 is shown in FIG. 2 . The etalon 102 includes a pair of reflective thin film dielectric stacks 202, 204 separated by a thin sheet of transparent material 206 of dimension _d0 or an interposed air gap or gap filled with a gaseous material. Stacks 202, 204 represent thin metallized dielectric reflectors or desired broadband dielectric reflectors based on alternating 1/4 wavelength thick layers of high and low coefficient materials. The center wavelength for a dielectric reflector design should typically be chosen to be close to the center of the desired range of laser operating frequencies. The wavelength discrimination capability of the etalon 102 increases with the number of layers inserted in the multilayer dielectric stack. In each stack 202, 204, the number of layers can be up to 50 or more as desired.

Depending on the wavelength of the incident light and the incident angle θ, the etalon 120 will reflect or transmit high contrast incident rays. This effect can be used to select the wavelength of the incident radiation by transmitting certain wavelength ranges and reflecting other wavelength ranges. A typical transmission spectrum of etalon 120 is shown in FIG. 3 . The wavelength separation and width of the transmission bandwidth, as well as the difference between the transmission peak 302 and the reflection baseline 304, can be tailored to meet the specific needs of a given application. The transport characteristics of the micromachined etalon 120 can be accurately simulated, and design optimization can be readily made based on computer simulations. Tuning of the micromachined etalon 120, ie changing the wavelength dependence of the transmission peak of FIG. 3, can be achieved in several ways. The spacing do between the dielectric stacks can be _varied . In the case of etalons with air gaps, as shown in Figure 1C, this can be achieved by varying the separation distance of the air gaps. A second method of tuning is to tilt the etalon 120 relative to the optical axis of the beam, which changes the angle of incidence and modifies the transmission of the beam through the micromachined etalon 120 accordingly. Another way of tuning the etalon is to fill the gap 206 with a material that can change its reflection coefficient, for example by applying an electric field.

Now, returning to the operation of the external cavity laser 100 , the total cavity optical path length illustrates the significantly calculated performance of the laser 100 . The effective optical path length λ _eff of the device can be defined as:

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}$

Here, d _i and _ni are the thickness and refractive index of elements including the device.

In the case of the etalon 120, this requires the product of the index of refraction _ni and the layer thickness _d for each element layer i of the dielectric stack with the index of refraction n of the transparent sheet or air containing each gap between the physical elements ₀ (n=1) and the product of the thickness d ₀ of the sheet or air gap (or multiple air gaps) is summed. In this case, gel or encapsulation is used to fill the space between the elements, and then the refractive index of this material is used instead of n=1. In more detail, for an etalon consisting of two dielectric stacks having a total number of N layers, the effective optical path length λ _eta,eff of the etalon 120 can be defined as:

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; eta</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; eff</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; di</span>}{<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}$

As will be explained with respect to the present invention, the motivation to better reduce such optical path lengths exists and can be implemented by the micromachined micromachined etalon 120 of the present invention.

The total optical path length λ _cav,eff of the optical resonator 103 determines the mode spacing of the laser. For a nominal emission wavelength λ ₀ , the wavelength separation Δλ between adjacent emission modes is roughly given by:

$\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}<span\; class="notranslate">\; \&cong;</span>\frac{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; cav</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; eff</span>}}}$

Therefore, shortening the value of the optical resonator length λ _cav,eff produces more widely spaced modes, ie Δλ increases. Mode spacing presents and satisfies method and device constraints on mode selectivity, where closely spaced laser emission modes require more mode selectivity with high spectral resolution.

Regarding the present invention and with reference to Fig. 1a-1c, the effective length λ _{cav of optical cavity, eff} is:

λ _cav,eff =d _las n _las +d ₁ +d _lens n _lens +d ₂ +λ _eta,eff +d ₃

where _dlas is the laser length measured between the first and second end surfaces 104, 106 of the lasing element 102, _nlas is the refractive index of the laser material, _dlens is the thickness of the lens 116 on the optical axis, and n _lens is the refractive index of the lens. By using microfabricated elements of lenses (or multi-lenses) and etalons (or multi-etalons), the effective optical cavity length can be kept small. In this way, the components and the system are miniaturized, which, in addition to their attractiveness for compact system integration, also provides greater latitude in changing the mode spacing of the external cavity laser 100 .

Figures 4A-4C and Figures 5A-5H illustrate the spacing of optical cavity modes and their relationship to laser tuning. FIG. 4 a shows the gain spectrum 402 of the semiconductor laser 102 . Photons generated by radiative transitions at wavelengths within the gain spectrum 402 are lased, and if optical feedback is provided, the internal gain of the lasing medium exceeds the loss of light. The optical cavity 103 provides this feedback in order to suppress a discrete arrangement or "comb" of wavelength modes 404 (Fig. 4b) with a mode spacing Δλ, in relation to the optical path of the lasing element 102. The optical resonator mode 404 overlapping the gain bandwidth 402 of the laser 102 determines the resonator laser emission wavelength. Figure 4c shows the output spectrum of the external cavity laser 100, which is a convolution of the spectra of Figures 4a and 4b. The solid line of FIG. 4d shows the transmission band spectral response 403 of an etalon, such as the microfabricated etalon 120 in FIG. The single mode laser output 405 is shown. It should be noted that it is desirable that the free spectral range (FSR), ie, the distance between the transmission peaks 403 of the wavelength-discriminating micromachined etalon 120 is greater than the gain bandwidth of the laser (as shown in Figure 4a), or that more than one lasing mode will be maintained. By the same token, the determined bandwidth measured by the full-width half-maximum (FWHM) of the micromachined etalon 120 must be narrow enough to avoid selection of the adjacent laser mode 401 supported by the optical cavity 103 if compared to Neighboring laser mode 401 occurs when the desired mode of operation cannot achieve sufficient bandwidth to suppress mode 401 to produce the desired mode of selective gain. While the side mode rejection ratio and acceptable values are application dependent, the power ratio between the desired mode and the side mode can be measured. For many WDM applications, they are from 20 to 45dB.

Furthermore, as explained with reference to FIGS. 1B and 1C , by tilting the micromachined etalon 120 or adjusting the slit spacing, the transmission passband of the micromachined etalon 120 can be shifted to a new wavelength range. Such conversion is represented, for example, by dashed line 403a in Figure 4d, where selection of a different mode of laser 100 results in a laser emission output converted to the corresponding emission wavelength 403a (dashed line in Figure 4e). In this way, the laser system can be tuned discontinuously, wherein distinct, well-separated wavelengths can be selected from a well-defined set of stable transmit wavelengths assigned to WDM channels. It should be noted that a certain degree of temperature stabilization is required and determination of a stable temperature is made by allowing transitions of possible operating modes. The allowed transitions depend on the channel spacing and differ from CWDM and DWDM, and also differ according to raster narrowness. For example, a DWDM system that allocates 50 GHz spaced channels typically requires a more stable wavelength of operation than a DWDM system that allocates 200 GHz. The temperature drift is primarily determined by the variation in the temperature coefficient of the lasing element 102 along the physical length of the temperature substrate (CTE) on which the element is mounted, with the optical elements employed in the system. Various methods of unheated wavelength combing can be employed, such as building on low CTE substrates, inserting low dN/dT optical elements, or inserting compensation elements with negative dN/dT or negative CTE. If temperature control limits are exceeded, for example, if a thermoelectric cooler is required for the desired application, then this construction technique can optionally be employed.

From the embodiment described with respect to Figure 4, the need for an etalon with sufficient spectral resolution to select single cavity laser modes is demonstrated. As a corresponding example, Figure 5 shows an external cavity laser with more closely spaced emission modes. Since the mode spacing is approximately inversely proportional to the cavity length, more compact modes are achieved as the length of the outer cavity 105 is increased from that shown in FIG. 4 . By comparing the allowed resonator modes of Fig. 4b with Fig. 5b, an external optical resonator 105 having a length exceeding that of Fig. 4 is suitable to exhibit more closely spaced modes. As the resonator modes become more compact, ie, reduce Δλ, the etalon 120 must have greater discrimination, ie, significantly lower FWHM, so as not to inadvertently select adjacent modes when selecting a target mode. More concisely, an etalon transmission response 503 with a FWHM wider than the optical cavity mode spacing will select more than one mode, represented by the solid output emission peaks 502 and 504 of Fig. 5f. Similarly, when the etalon transmission band 503a switches to select another laser mode, additional adjacent peaks are inadvertently selected, represented by the two dashed laser output peaks 506 and 508 in Figure 5f. This problem should be corrected by adopting an etalon with a larger discriminative, i.e. narrow FWHM, illustrated by an ideal etalon with the transmission bandwidth spectral response shown in Fig. 5g, which achieves the Single mode laser output. As mentioned earlier, the FSR of the wavelength selector must also be greater than the gain bandwidth of the laser to avoid selecting more than one mode that does not identify the application.

Another purpose of addressing closely spaced modes is to insert interference filters, etalons or similar wavelength discriminating devices into the external optical resonant cavity 105 for the purpose of removing a subset of modes. As a result, a small density comb of resonator modes is thus achieved. Figure 5d shows the attenuated interference filter passband response, eg, for each of the other resonator modes. Additional components reduce the resolution requirements of the wavelength-discriminating mode selector, but where the additional complexity is significant, an increase in the external optical resonator length should also be required in order to allow sufficient space to insert the resonator. The present invention provides the ability to obtain a tunable etalon with sufficient resolution to select a single laser mode, and the etalon without the need to lengthen the external optical cavity in order to accommodate larger or additional mode-selective elements (thereby creating more compact spatial patterns, exacerbating the pattern resolution problem).

Another aspect related to the effective optical path length of the laser emission mode involves etalon tuning. The process of tilting the microfabricated etalon 120 or changing its slit distance d ₀ modifies its spectral transmission characteristics for changing the effective optical path length of the external cavity 105 . As a result, the laser emission wavelength also changes. Ideally, this effect would be minimized in order to stabilize the emission mode spectrum, and the design criteria for the micromachined etalon 120 would de-emphasize the effect of etalon tuning on the effective optical path length of the external cavity 105 . This is easily achieved by designing an inserted air gap instead of a solid dielectric in the etalon 120, since the gap 124 comprises a substantial portion of the etalon's optical path, whereby when the etalon 120 is tilted, the gap 124 does not change the overall resonance Cavity path length. Feedback control, additional wavelength locking etalons, or geometric adjustments can optionally be introduced to ameliorate these perturbation effects.

The present invention brings several important features to the etalon 120 used to tune the external laser. First, the micromachined etalon 120 can be fabricated by micromachining methods, whereby the micromachined etalon 120 is relatively compact and compatible with micro-optical assemblies. Second, the micromachined etalon 120 can be tuned by electromechanical action, where a voltage signal modulates the transfer characteristics of the micromachined etalon 120 . Third, the optical path length of the micromachined etalon 120 is relatively small compared to conventional etalons. Fourth, the effect of tuning on the total resonator optical path length of the micromachined etalon 120 is small by comparison with conventional etalons. These features, discussed below, have significant advantages in employing them in mode selection for external resonator lasers.

In accordance with the present invention, it is desired that the micromachined Fabry-Perot etalon 120 be compact with high precision, reproducible optical transmission characteristics, and in addition, provide some functionality to tune the spectral transmission characteristics of the micromachined etalon 120 so that laser mode selection can be achieved . It is desirable that the tuning of the micromachined etalon 120 be under electrical control. In conclusion, the transmission characteristics of the micromachined etalon 120 are modulated, for example, by one or more electro-mechanical effects, so that the wavelength at the center of the transmission passband (or multiple passbands) of the tunable micromachined etalon 120 is function of applied voltage.

Figure 6a shows a cross-sectional view of a specific etalon structure 600 according to the present invention. The etalon 600 may be fabricated in a flat plate or substrate 602, such as a silicon wafer typically conventionally employed in the microelectronics industry. A groove 604 having a sloped sidewall 605 may be formed in one side of the substrate 602 . This particular structure is typical of resonant cavities fabricated in <100> silicon by wet anisotropic etching, where the etch stops on an etch stop layer such as silicon oxide or heavily doped silicon. Another etching method can be used, such as deep reactive ion etching. The side of the substrate 602 in which the recess 604 is formed will be referred to as the "back side" 606 of the etalon. A spacer layer 608 made of a material different from the substrate 602 is formed on the side of the substrate 602 opposite to the groove 604 . The outer surface of the spacer layer 608 will be referred to as the “front surface” 607 of the etalon 600 . Dielectric coated multilayer thin film stacks 612, 614 cover the front and back surfaces 607, 606, respectively, of the etalon. The front side coating 612 formed on the flat front surface of the substrate 602 is very flat. Backside coating 614 is conformal and uniformly covers groove sidewalls 605 , base 603 and backside 606 . An air gap 616 may be provided between the two dielectric stacks 612 , 614 at the center above the groove 604 . Air gaps 616 may be created by etching spacers 608 through etch holes formed in top dielectric stack 612 . The multilayer dielectric coatings 612, 614 are configured according to the number of layers, the order of the layers, the refractive indices of the layers, and the thickness of the layers to function as dielectric reflectors, such as thin film optical elements known in the art and As a reflective and anti-reflective coating manufactured routinely on optical components. For example, coatings 612, 614 may consist of alternating layers of silicon dioxide (refractive index of approximately 1.46) and silicon nitride (refractive index of approximately 2.0). The optical thickness (ie, the actual physical thickness multiplied by the refractive index of the material layer at the wavelength of interest) of the component layers of the interference filter can be fabricated to correspond to a quarter wavelength of the laser emission wavelength or multiples thereof. The front and back dielectric coatings 612, 614 may be deposited in the same process so that the front and back side coatings 612, 614 have very similar optical and transmission properties. Suitable methods of depositing the dielectric layer include thermal evaporation, electron beam evaporation, sputter deposition and chemical vapor deposition. Chemical Vapor Deposition (CVD) includes Low Pressure CVD and Plasma Enhanced CVD, ALD is photon-attracting because they are conformal and used to deposit front and back coatings 612, 614 simultaneously. It should be seen that the structure of the micromachined etalon 120 of Fig. 6a exhibits the basic features of the etalon shown in Fig. 2, namely the separation of the two interference filters by an air gap or thin sheet of transparent material.

Now, returning to the manufacture of the etalon 600, the present invention also provides a method of manufacturing the etalon 600, the sequence of steps is illustrated in Figures 6b-6h. Figures 6a-6f and Figure 6h are cross-sectional views, and Figure 6g is a top view. It should be understood that the techniques described herein illustrate a sequence of steps that can be adapted as desired to establish and use conventional microfabrication techniques, but that various changes to and modifications to these steps will be apparent to those of ordinary skill in the art. Various substitutions may also produce structures having substantially the same properties and functions.

In FIG. 6b, a layer 608 of spacer material is deposited or otherwise formed on a substrate 602, such as a silicon wafer. A single crystal silicon wafer polished on both sides is a better choice for substrate 602 because of its smooth and flat surface and controlled etch properties. Other suitable substrates include glass, ceramic, fused silica, quartz, or other types of semiconductor wafers. Typically, the silicon substrate 602 has a thickness of about 0.5 millimeters, and the spacer layer 608 has a thickness in the range of 2 microns to 30 microns. Alternatively, the substrate 602 may be a silicon-on-insulator (SOI) structure with a 0.1-2 micron layer separating a ∽0.5 mm thick silicon wafer from a deposited 2-30 micron thick silicon layer. Thick buried silicon dioxide layer. It should be noted that the substrate need not be transparent to the laser wavelength, since the formation of the grooves 604 removes all substrate material from the optical path of the laser beam. The rationale for material selection is that the substrate material can be etched selectively with respect to the spacer material. Many combinations of materials meet this requirement, including a boron-doped or other impurity-doped silicon spacer layer 608 deposited on an undoped or lightly doped silicon substrate as the substrate 602, or A silicon spacer layer 608 is deposited on the sapphire substrate 602 . In the former case, the spacer layer 608 is doped to make it resistant to etching with undoped or lightly doped silicon that would otherwise be etched.

In one desired embodiment, substrate 602 may be a single crystal silicon wafer, and spacer material 608 is silicon heavily doped with boron. Boron-doped silicon layer 608 may be formed by epitaxial growth or ion implantation of a boron-doped layer into silicon wafer 602 .

As shown in FIG. 6c, a recess 604 can be etched through the substrate 602 in the form of a tapered hole formed through the substrate 602 having a spacer layer 608 as an etch stop barrier. (or an etch stop provided by the oxide, which is in contact with the spacer layer in the case of SOI silicon wafers). In the case of a silicon substrate 602 such as (100) silicon, potassium hydroxide (KOH) solution may be used as an anisotropic etchant to provide the sidewall taper of the recess 604 . Other etchants that can be used for this purpose include hydrazine and EDP (ethylene diamine pyrocatechol). Furthermore, isotropic etchants may also be employed as is typically the case with other substrate materials such as ceramics. A desirable property of the etchant is that it removes the substrate material faster than the spacer material. Alternatively, in the case of SOI wafers, selectivity of the etchant to the buried oxide layer is required. A typical diameter of the groove 604 at the spacer layer 608 is 100-500 microns, although this size can vary entirely depending on the application and the beam-narrowing portion size of the lasing element 102 in the optical cavity 103 .

As shown in Figure 6d, a multilayer thin film dielectric stack 612 forming an optical interference filter is formed on the front surface 607 of the substrate, and a dielectric stack 614 is formed on the backside 606 of the substrate. Multilayer structures with alternating high and low indices of refraction and quarter-wavelength thicknesses were designed and deposited according to methods widely used in thin film optics. The layers may consist of, for example, silicon dioxide, silicon nitride or other dielectric materials including metal oxides and wide bandgap semiconductors such as gallium nitride or silicon carbide. Depending on the number of layers and the center wavelength of the design, a typical total thickness of either the front side dielectric stack or the back side dielectric stack can be 2-15 microns.

As shown in FIG. 6e , a set of vias 618 may be formed in the front dielectric stack 612 . The via holes expose the underlying spacer layer 608, allowing the spacer layer 608 to be easily etched. Vias 618 may be defined by photolithography combined with wet or dry etching (eg reactive ion etching) or by other methods such as laser drilling. Vias 618 may be located above recess 604 , or around recess 604 and above substrate 602 . Typical via diameters may be 10-50 microns. As shown in FIG. 6f, the portion of the spacer layer 608 directly above the groove 604 is removed by etching, leaving an air gap 616 between the front side dielectric stack 612 and the back side dielectric stack 614, and wherein the air gap 616 Align with groove 604.

The etching of the spacer layer 608 is easy to use several material combinations for the substrate 602 and the spacer layer 608 and is easy to use an etchant represented by a macromolecular formula. For example, if the spacer layer 608 is undoped silicon or boron-doped silicon and the substrate 602 is silicon, then the spacer layer 608 may be etched with xenon difluoride. When the substrate 602 is covered with a dielectric layer such as nitride and oxide, xenon difluoride gas tends to etch silicon isotropically, but leaves the substrate 602 undamaged. Liquid etchants may also be employed. The etchant should not significantly etch or roughen the dielectric stacks 612, 614, although some slight etching of the substrate 602 can be tolerated. As a result of etching a portion of spacer layer 608 , a portion of front side dielectric stack 612 is suspended just above air gap 616 .

A top view of an exemplary via pattern for etching spacer layer 608 is shown in FIG. 6g as the case of 8 vias 618 arranged around a square pattern. The dashed rectangle of FIG. 6f shows the approximate extent of the area where a gap 616 etched from the spacer layer 608 separates the two parallel dielectric stack reflectors 612 , 614 . In FIG. 6g, the dashed rectangle also roughly corresponds to a top view of a portion of the front side thin film dielectric stack 612, the portion of which is suspended above the air gap 616 and is indicated by 620 in FIG. 6f. This region serves as the optical function part of the etalon 600 and provides the optical interference effect of the laser beam. The portion of the unetched substrate surrounding the groove 604 (i.e., the area outside the area of the dashed rectangle in the top view of FIG. 6g, or corresponding to the portion of the substrate indicated by (622) in the cross-sectional view of FIG. The mechanical stability of the tool 600 also facilitates the operation and installation of the etalon 600 in the optical cavity 103 . The substrate 602 also supports mounting and aligning reference surfaces to facilitate precise positioning and orientation of the etalon 600 . Due to the geometric design of the micromachined etalon 600 , the portion 622 surrounding the groove 604 represents a bulk relative to the substrate 602 that does not contribute to the optical path length of the etalon 600 . In this design, the net stress required by the coating should be controlled by the typically small net tensile stress required.

The etalon 600 may be provided with additional structures to allow its tuning. Electrostatically actuated etalon 600 is shown in cross-section in FIG. 6h , with conductive electrodes 624 formed on the outside of front dielectric stack region 620 , suspended above air gap 616 . This set of electrodes is a common electrode, ie electrically connected. If the substrate 602 is sufficiently conductive, it can act as the counter electrode. Optionally, a counter electrode 626 is deposited on the outer surface of the back dielectric stack 614 as counter electrode 626 in the form of a second conductive pad, eg, composed of indium tin oxide (ITO). Tin oxide and ITO have the advantage of being transparent at the wavelengths used in fiber optic communications, thereby avoiding blurring of the optical path. An applied voltage between electrode 624 and opposing electrode 626 (or electrodes) provides an electrostatic force that stretches or resists suspended dielectric stack 620, thereby altering air gap 616 between dielectric stacks 614 and 620 interval. This changes the transmission characteristics of the etalon 600, in particular, shifting the wavelength of the narrow transmission band (shown at 302 in FIG. 3). Thus, this etalon structure provides tuning of the transmission frequency band and, consequently, wavelength tuning by the applied electrical signal.

An alternative embodiment of an etalon 700 for micromachined etalon 120 in external cavity laser 100 is shown in Fig. 7d, and Figs. 7a-7d show an exemplary sequence of fabrication steps thereof. FIG. 7 a shows a substrate 702 with a dielectric stack 704 formed on one side 703 thereof. The dielectric stack 704 may be structurally, compositionally, and functionally similar to the dielectric stacks 612 , 614 described with respect to FIG. 6 . Figure 7b shows a separate substrate 706 in which, by masking and controlled etching or drilling, a shallow recess 708 has been formed in one side of the substrate and the same side 705 of the substrate has been conformally covered There is a dielectric stack 710 . As shown in Figure 7c, the two substrates 702, 706 are aligned and mounted or bonded together such that a recess 708 is provided on both sides by the dielectric stack 704 and 710 bonded The internal space or air gap 712. As shown in Figure 7c, this structure provides an etalon 700 with an air-gap space d. As shown in FIG. 7d , portions of both substrates 702 , 706 are removed to form aligned recesses 714 and 716 that reveal two unsupported dielectric stacks 718 and 720 bonded air gaps 712 . As explained with respect to FIG. 6g , electrodes in the form of conductive pads or conductive oxide films can be applied to the unsupported dielectric stack for electrostatic actuation and electrically controlling the tuning of the etalon gap 712 .

Example

A notable feature of the present invention is the compactness of the micromachined etalon and the correspondingly shortened optical path length of the optical cavity 103 allows for a simpler method of more optimized mode spacing and mode selection. These aspects of the invention are further exemplified in the following modeled and simulated examples.

Figure 8a shows the calculated comb shape of the longitudinal wavelength mode for an effective total optical resonance cavity length l _cav,eff of 24 mm in the spectral range centered at 1550 nm wavelength. The mode spacing is about 0.5 nm. Figure 8b shows the results of similar simulations shortening the total optical path length of the laser resonator to 12 mm. It is expected that the mode spacing increases to about 1 nm from that of the first embodiment. Figures 8a and 8b illustrate the effect of the length of the optical resonator on the mode spacing, and a shorter optical resonator leads to a wider spaced mode, which requires less stringent solution requirements for wavelength discrimination and mode selection devices.

Figure 8c (rough illustration) and Figure 8d (more detailed illustration) show the calculated spectral transmission of an etalon consisting of two dielectric stacks separated by a 5.8 μm air gap, the dielectric stack being silicon nitride and di 20 alternating layers of silicon oxide, each quarter wavelength thick (=λ/4n, where λ=1550nm, n is the refractive index applicable to silicon dioxide or silicon nitride). The FWHM of the transmission passband is about 0.3nm. The FSR (free spectral range), ie the wavelength separation between adjacent transmission passband peaks, of the etalon is approximately 90 nm. This FSR is significantly larger than the gain bandwidth of typical 1550-nm wavelength emitting lasers, a requirement specified in the related discussion in Figures 4 and 5 to prevent maintenance of more than one lasing mode. Furthermore, the resolution of the exemplary micromachined etalon is suitable for resolving adjacent peaks in the case of a 12-mm long optical cavity. As seen in Fig. 8d, there is approximately 15 dB attenuation for modes within 1 nm of peak transmission. All of these embodiments are single pass calculations, and the double pass essentially increases the attenuation between the pass passes. The attenuation required for a given application will depend on various variables, including the gain curve of the laser element to be used. The suppression obtained is easily tuned by increasing or decreasing the number of dielectric reflectors.

The potential tuning capability of the above etalon adjusted with reference to the slit is represented by the three spectral transmission curves shown in Fig. 8e. Shortening the 5.8 micron air gap in 8 nm increments produces an appreciable shift in the transmission peak to shorten to wavelengths as shown by curves 802, 804, and 806, respectively. Curve 802 is for an etalon with a 5.820 micron gap. Curve 804 is for a gap shortened by 8 nm to 5.812 microns, and curve 806 is for a gap shortened further to 5.804 microns. Thus, the simulations show that a relatively modest modification of the air gap, and thus easily obtained by electrostatic excitation effects, yields useful wavelength discrimination for a mode separation of 1 micrometer, which occurs in an optical resonant cavity with an effective optical length of 12 mm 103 in. Tilting the micromachined etalon 120 by an angle of 1-35 degrees will obtain a similar correction of the transmission characteristics.

In a specific embodiment of the invention, the significant influence on the optical path length is:

i. An edge emitting laser 102 with a physical length _dlas = 250 microns and an optical length of about 850 microns.

ii. 400 micron diameter anti-reflection coated spherical lens 116 manufactured by Spinel and having an optical path length of approximately 720 microns.

iii. For example, a tunable etalon 120 composed of 10-20 layers of silicon nitride and silicon dioxide, supported by a frame formed outside the peripheral area of the wafer forming the etalon 120 as shown in FIG. 6g. As a result, the etalon optical path length is approximately 500 microns, and it should be noted that this thickness is based on the space allowed for insertion of the silicon wafer into which etalon 120 is formed. When the laser beam path is limited to grooves formed in the silicon substrate, the optical path is not augmented by transmission through the silicon.

Overall, the space between these three elements (laser emitting element 102, etalon 120, and lens 116) can be fabricated to be less than 1 mm.

Assuming these optical paths are as follows: 850 microns (laser) + 720 microns (lens) + 515 microns (etalon) + 1 mm (gap space) = ~3 mm, giving a minimum total optical path length of about 3 mm. Thus, depending on the desired comb spacing, elements can be arranged with more loose or open spacing.

The present invention also provides simple and precise control of the laser reflected wavelength using a micromachined tunable etalon by monitoring the laser output with a photodetector, an exemplary embodiment of which is schematically shown in Figure 9a. The system shown in Figure 9a comprises a laser 902, a collimating lens 904, and a lens 906 with a highly reflective coated facet 908 defining an optical resonant cavity 910 therebetween. Laser beam output 912 is transmitted through coated facet 908 . The system also includes a beam splitter 914 disposed on the output beam 912, a photodetector 916 for receiving the split portion of the output beam 912, a pulse counter circuit 918, a controller 920, and a circuit 922 for exciting the micromachined etalon 924 . A photodetector 916 so positioned continuously monitors the laser output. When a laser switches or hops between modes, there are short breaks in its light output that can be detected and registered as pulses. When the controller 920 clears the electro-active etalon for the entire spectral range, the counter circuit 918 senses the occurrence of a mode transition and counts the number of mode transitions that include the reflection wavelength of the system with a fixed wavelength reflection mode. The detector 916 then provides feedback to optimize the positioning of the etalon 924 to optimize power in the desired mode of operation. The absolute position of the comb pattern can be set during the assembly process or by adjusting the temperature adjustment system on the TEC to a temperature at which the pattern comb of the longitudinal mode is centered.

In the system depicted in Fig. 9a, some misoperations, noise effects or other failures may cause mode hopping and lose the track of the mode, resulting in mode shift error. Alternatively, fabricating a variable total optical path length can produce variations in the starting mode. Remedy can be achieved by establishing a local channel wavelength, which is used by the system, as shown in Figure 9b, in order to establish a fixed reference point. The system of FIG. 9b is similar to the system of FIG. 9a except for the second beam splitter 926, the filter 928 for tuning the "local" channel, and the second photodetector 930. Diffraction gratings, prisms, etalons, or similar devices may be used in place of filter 928 . The second beam splitter 926 continuously samples the laser output and filters the laser output through a low-pass optical interference filter 928 , which transmits only the laser output at the reference wavelength to the second photodetector 930 . The signal registered by the second photodetector 930 indicates that the laser 902 is operating in a mode corresponding to the reference wavelength. Therefore, the controller 920 excites the etalon 924 by tripping voltages to clear the entire pattern, and counts the number of transitioned patterns in clearing by the photodetector 916 and the counting circuit 918 . Thus, the laser 902 can be tuned to any mode desired and the number of mode hops can be counted from the reference mode.

These and other advantages of the invention will be apparent to those of ordinary skill in the art from the foregoing description. Accordingly, those of ordinary skill in the art will recognize that changes and modifications can be made to the above-described embodiments without departing from the broad inventive concept of the invention. It is therefore to be understood that this invention is not limited to the particular embodiments described herein, but it is intended to cover all changes and modifications within the scope and spirit of the invention as set forth in the appended claims.

Claims (11)

1. A method for manufacturing a micromachined etalon, comprising: providing a crystal substrate having first and second opposing surfaces; providing a spacer layer over the first surface of the substrate, the spacer layer having an outer surface; forming a via extending through the substrate from the first surface to the second surface, the via having a base adjacent to the exposed portion of the spacer layer; and A first interference filter is provided over the outer surface of the spacer layer and a second interference filter is provided over the exposed portion of the spacer layer such that an etalon is provided between the interference filters. 2. A method for fabricating a micromachined etalon according to claim 1, comprising removing a portion of the spacer layer adjacent said through hole to provide a gap between the interference filters. 3. A method for fabricating a micromachining etalon according to claim 2, comprising providing a conductive electrode adjacent to said gap, the electrode configured to provide a repulsive or attractive force that alters the gap space. 4. The method for fabricating a micromachined etalon according to any one of claims 1-3, wherein forming the via comprises anisotropically etching the substrate. 5. A micromachining etalon comprising: A crystalline substrate having a first surface and a second surface; a spacer layer disposed over the first surface of the substrate, the spacer layer having an outer surface; a via extending through the substrate from the first surface to the second surface, the via having a base adjacent to the exposed portion of the spacer layer; and a first interference filter disposed over the outer surface of the spacer layer and a second interference filter disposed over the exposed portion of the spacer layer so as to provide an etalon between the interference filters . 6. An external resonator type semiconductor laser, comprising: a laser gain medium for providing a light source of optical radiation; an external optical resonator disposed in optical communication with the gain medium, the resonator having dimensions short enough to allow a single wavelength selective device to select and maintain a single longitudinally emitting lasing mode; and A single wavelength selective device placed in an external optical resonator to select and maintain a single longitudinally emitting lasing mode. 7. The external resonator semiconductor laser of claim 6, wherein the wavelength selective device comprises a micromachined etalon. 8. The external resonator semiconductor laser according to claim 7, wherein the etalon comprises a variable spatial gap adjustable from a first size to a second size to provide for use in the first longitudinal mode or in the second longitudinal mode Shoot lasers. 9. An external resonator semiconductor laser according to claim 7 or 8, wherein the etalon includes electrodes disposed adjacent to the slit for providing a repulsive or attractive force to modify the space of the slit. 10. An external cavity semiconductor laser according to any one of claims 6-9, wherein the wavelength selective device comprises single crystal silicon. 11. The external resonator semiconductor laser according to any one of claims 6-9, wherein the wavelength selective device comprises: a crystalline substrate having first and second opposing surfaces; a spacer layer disposed over the first surface of the substrate, the spacer layer having an outer surface; a via extending through the substrate from the first surface to the second surface, the via having a base adjacent to the exposed portion of the spacer layer; and A first interference filter is disposed over the outer surface of the spacer layer and a second interference filter is disposed over the exposed portion of the spacer layer to provide an etalon between the interference filters.

Images